Piezoelectric MEMS based acoustic sensors: A review

Sensors and Actuators A 301 (2020) 111756

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Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna

Piezoelectric MEMS based acoustic sensors: A review Washim Reza Ali a,b , Mahanth Prasad a,b,∗ a b

CSIR-Central Electronics Engineering Research Institute, Pilani (Rajasthan)-333031, India Academy of Scientific and Innovative Research (AcSIR), Ghaziabad-201002, India

a r t i c l e

i n f o

Article history: Received 15 August 2019 Received in revised form 18 October 2019 Accepted 13 November 2019 Keywords: Acoustic sensor Piezoelectric Sputtering Sol-gel Reliability Characterization Testing

a b s t r a c t This paper discusses piezoelectric acoustic devices based on widely used piezoelectric materials. Commonly used piezoelectric thin film deposition techniques and the influence of process parameters on the growth, texture and orientation of the films are discussed. Etching techniques are also outlined. A comparative study of different devices developed previously is given. Also, applications of developed devices in aero-acoustic and medical fields have been briefly discussed. Flow charts of various techniques of deposition along with a combined one for full acoustic device fabrication are given. Various techniques, frequently used for thin film characterization have been discussed. The testing and measurement techniques to determine the responses of acoustic devices such as sensitivity, resonance frequency, frequency response, piezoelectric co-efficient etc. have been briefly illustrated. This paper discusses common failure modes with respect to the field of use of acoustic devices. It also concisely discusses various reliability tests done in industries to assess the quality of the developed devices. This review has also suggested directions for future development of thin film acoustic sensors. © 2019 Elsevier B.V. All rights reserved.

Contents 1. 2.

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4.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 1.1. Piezoelectric materials for MEMS acoustic sensor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Common deposition techniques of piezoelectric layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2.1. Sputtering technique . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2.1.1. Aluminum nitride (AlN) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 2.1.2. Zinc oxide (ZnO) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 2.1.3. Lead zirconate titanate (PZT) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 2.2. Sol-gel technique . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 2.2.1. Zinc oxide (ZnO) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 2.2.2. Lead zirconate titanate (PZT) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Etching of the piezoelectric materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 3.1. Aluminum nitride (AlN) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 3.2. Zinc oxide (ZnO) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 3.3. Lead zirconate titanate (PZT ) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Developed piezoelectric based acoustic devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 4.1. PZT based devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 4.2. ZnO based devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 4.3. AlN based devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 4.4. Comparative study of developed devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 4.4.1. Fully clamped vs cantilever structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 4.4.2. Circular vs square structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

∗ Corresponding author at: CSIR-Central Electronics Engineering Research Institute, Pilani (Rajasthan)-333031, India. E-mail address: [email protected] (M. Prasad). https://doi.org/10.1016/j.sna.2019.111756 0924-4247/© 2019 Elsevier B.V. All rights reserved.

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5. 6.

7.

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4.4.3. d31 vs d33 -modes of transduction and use of IDT electrodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 Applications of acoustic devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 Characterization and testing of piezoelectric based acoustic devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 6.1. Characterization of piezoelectric thin films . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 6.1.1. Commonly used techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 6.1.1.4. Scanning electron microscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 6.1.1.5. Field emission scanning electron microscopy (FESEM) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 6.1.2. Utilization of techniques for characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 6.2. Testing & measurement techniques of the performance parameters of acoustic device . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 6.2.1. Frequency response & sensitivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 6.2.2. Dynamic range (DR) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 6.2.3. Resonance frequency and displacement of the diaphragm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 6.2.4. Measurement of piezoelectric properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 Reliability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 7.1. HTOL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 7.2. THB . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 7.3. LTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 7.4. HTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 7.5. TC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 7.6. MTC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 7.7. MS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 7.8. ESD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 Conclusion and future trends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 Biography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

1. Introduction High sensitivity and wireless capability make acoustic sensors one of the most commonly used MEMS devices. Some of the major areas applications of sensors includes, biosensors [1,2] sound pressure [3–8] humidity, temperature and mass sensor [9] etc. The piezoelectric material-based sensors offer two main advantages [9]: (i) there is no requirement of the input power and (ii) it offers a broad dynamic range. Broad dynamic range is an essential requirement for the measurement of wide range of sound pressure levels (SPL) in harsh environments such as the battlefield [10]. In aerospace applications, the measurement of high sound pressure level is an important requirement since sound pressure generated by launch vehicle and large booster rocket can result in the fatigue of metal panels and structures. Current research is oriented towards technologies with less size, monolithic integration and superior performance due to the demand of small size, low cost and low power consumption. Piezoelectric acoustic sensor based on the micro-electromechanical system (MEMS) technology is the most appropriate solution for the next generation wireless communication systems [11]. Generally, SAW devices are fabricated in two ways–one is using bulk piezoelectric substrates such as quartz, LiNbO3 or LiTaO3 and the other one is using piezoelectric thin films such as PZT, ZnO or AlN [12]. Conventional substrates such as glass or silicon (Si) are used for depositing piezolayer [13,14]. Thin film SAW devices offer several advantages over the bulk ones such as device design flexibility, production cost etc. However, the most important advantage is the feasibility of integration in complex MEMS systems or CMOS electronics which is not possible in bulk devices [15]. 1.1. Piezoelectric materials for MEMS acoustic sensor Piezoelectric polycrystalline thin films such as lead zirconate titanate (PZT), zinc oxide (ZnO) and aluminum nitride (AlN) are widely used for high frequency surface acoustic wave (SAW) devices and thin film bulk acoustic wave resonators (TFBARs) [11]. PVDF is another piezoelectric material widely used in piezoelec-

tric polymer-based sensors due to excellent features like flexibility, workability and fast dynamic response. However, PVDF is difficult to involve in conventional microfabrication processes as it needs mechanical stretching to convert the non-polar ␣ -phase into piezoelectric polar ˇ-phase [16]. Although its co-polymer P(VDF-TrFE) does not require stretching but it does require high voltage poling [16] to force the alignment of the internal dipoles like PZT. Therefore, except ZnO and AlN, none of these materials are fully compatible with CMOS fabrication process. ZnO is one of most commonly used piezoelectric thin films for various applications in MEMS such as film-bulk acoustic-wave resonators (FBAR) [17] SAW resonators [18], acousto-optic devices [19] and acousto-electric devices [20]. PZT has also been used in ultrasonic transducer [21–24], acoustic sensor [25,26], pressure microsensor [27] and accelerometers [28,29]. However, PZT -based sensor provides lower sensitivity [30] than ZnO based sensor. Solgel PZT has a transverse piezoelectric coefficient d31 of around 27-274pC/N [25] which is much higher than that of for ZnO but at the same time it also has a relative dielectric constant which is about 100 times larger than that for ZnO. As a result, the beneficial effect of the PZT’s larger d31 is canceled in the case of sensing [26]. Thus, ZnO has emerged as one of famous piezoelectric materials and has been widely used as ultrasonic transducers and surface-acoustic wave (SAW) devices [18]. In last few years, many researchers have reported the use of ZnO based SAW devices for microfluidic applications [31,32], but ZnO films create serious issues for biomedical and microfluidic applications because of its instability in liquid solutions. This renders ZnO less ideal material for microfluidic applications than AlN. Moreover, ZnO is also prone to form oxygen vacancies, which act as donor-like impurities. This makes the material conductive which results in the destruction of its piezoelectric activity [33]. The higher conductivity of ZnO films compared to AlN results in higher power loss [34]. Moreover, Zn is a fastdiffusing ion which results in contamination problems for CMOS manufacturing [35]. Furthermore, the low processing temperature of ZnO film and its vulnerability towards attack by almost all acids and bases render it less compatible with the complementary metal oxide semiconductor (CMOS) processing [36]. Temperature of the

W.R. Ali and M. Prasad / Sensors and Actuators A 301 (2020) 111756

processes should be maintained below 430o C [36] for high piezoelectric performance of ZnO layer which is a major constraint in the processing of ZnO based devices. Aluminum nitride is different from rest of the III-V group compound semiconductor and wide band gap materials as it has hexagonal close packed wurtzite structure and higher piezoelectric coefficient [37,38]. Aluminum nitride (AlN) exhibits many useful properties such as high dielectric constant 8.5, high melting point (3273 K) [39], high electrical resistivity (approx. 1014 .cm), wide energy gap (6.2 eV), high acoustic velocity (approx. 5500 m/s and 11,354 m/s for transversal and longitudinal bulk waves, respectively, and 5607 m/s for surface waves), high piezoelectricity, high thermal conductivity (2.85W cm−1 K −1 at room temperature), chemical stability, transparency in the visual and infrared regions, low thermal expansion coefficient (4.2 × 10−6 K −1 and 5.3 × 10−6 K −1 for the direction along and perpendicular to the c-axis, respectively) [40–43]. In contrast to ZnO which is partially compatible, AlN deposition and processing are fully compatible with CMOS and MEMS processes [44] and all these properties of AlN result in its better c -axis orientation. AlN films have better chemical and thermal stability than ZnO films. c-axis oriented aluminum nitride (AlN) has been considered as a promising piezoelectric material for high temperature applications. It has a very high Curie temperature of over 2000o C, and can maintain its usable piezoelectric properties up to 1150o C in Ar [45] It also demonstrates excellent physical and chemical stability in atmosphere at temperatures exceeding 700o C [46] and exhibits rather broad frequency operating bandwidth when used as a thin film piezoelectric transducer. Also AlN based devices have much larger SAW propagation velocity than that of in the ZnO based devices. Moreover, their much larger elastic constant causes propagation losses to be lower than that of in ZnO based devices [47]. In comparison with PZT, the lower piezoelectric coefficients of AlN is mitigated by a significantly reduced dielectric constant (e.g. e31,f = − 1.05C 2 , ε33,f = 10.7 for AlN versus e31,f = − 9.6C m2

m

and ε33,f = 650 − 1300 for PZT [48]). The piezoelectric coefficient of AlN is one-tenth that of PZT but at the same time its dielectric constant is also less than one hundredth of that of PZT. This results in lower parallel plate capacitance of AlN thin filmbased devices which leads to an improvement in the signal-to-noise ratio in these devices compared to those that use PZT thin film [49]. Among all its properties, the high acoustic velocity of AlN makes it a material of choice for thin film based high-frequency acoustic devices to be used in mobile phones, alarm and security systems, military equipment, sensors, etc. [43¸50–53]. AlN is also advantageous in terms of its higher stability in harsh environment which results in better sensor performance [54]. Therefore, the use of AlN thin films facilitates the development of acoustic devices that can be operated at higher frequencies with improved sensitivity and in higher temperature and harsher environments compared to ZnO thin film-based devices [43,55,56]. Table 1 compares the properties of commonly used thin film piezoelectric materials. Fu et al. [57] have discussed design and fabrication aspects SAW devices using AlN and ZnO for application in biosensors. Similarly, Fei et al. [58] has illustrated the sputtering technique for depositing AlN thin film for energy harvester applications. Polcawich et al. [59] have discussed deposition and patterning techniques of PZT. However, none of these papers have discussed the use of these materials in the context of microphone and aero-acoustic application. Also, the use of different simulation tools for the development of acoustic sensors are not well documented. Moreover, reliability aspect of the acoustic sensors has not been properly addressed till date. The basic idea of this paper is to give a detailed illustration of different fabrication process steps followed by different researchers for the development of acoustic sensors using AlN, ZnO and PZT along with a brief discussion on reliability. Therefore, this article

3

Table 1 Properties of commonly used piezo materials [25,26,31,35,39,40,42,43,51,55,57,60]. Materials

PZT

ZnO

AlN

Density (103 kg/m3 ) Young’s Modulus (GPa) Poisson’s Ratio Piezoelectric constant d33 (pC/N) Effective Coupling Co-efficient k2 (%) Acoustic Velocity of longitudinal waves (m/s) Acoustic Velocity of transverse waves (m/s) Dielectric Constant Co-efficient of thermal O expansion (CTE) ×10−6 C

7.6 56-98 0.27-0.3 60-223

5.61-5.72 208 0.36 5.9-12.4

3.25-3.3 300-395 0.22-0.29 3.4-6.4

20-35

1.5-1.7

3.1-8

4500

6333

10150 -11000

3900

2700

5500-5670

300-1300 1.75-2

10.9 4-6.5

8.5-10.5 5.3

has been organized into following parts: (i) a detailed description of the commonly used deposition techniques like sputtering and solgel used for AlN, ZnO and PZT. Each deposition technique has been considered individually for each material and the effects of various deposition parameters on film properties are discussed in details; (ii) a detailed illustration of fabrication process steps followed in previously developed devices. Also, a comparison has been shown among various developed devices; (iii) a brief overview of various techniques like XRD, SEM, FESEM, AFM and EDX along with proper examples to elucidate their utility in thin film characterization; (iv) an illustration of various measurement techniques used for testing of developed devices and (v) reliability aspect of acoustic devices has been discussed in view of device’s field use and the possible failure mode. A brief overview of various reliability tests like HTOL, THB, LTS etc. has been given. 2. Common deposition techniques of piezoelectric layer Many deposition methods such as physical vapour deposition (PVD) methods [61], evaporation [62], pulsed laser deposition (PLD) [63], sol-gel processing [64,65], ion beam deposition [66], molecular beam epitaxy (MBE) [67], plasma enhanced CVD (PECVD) [68,69], metal-organic chemical vapour deposition (MOCVD) [70], and atomic layer deposition (ALD) [71] have been used to develop commonly used piezo-layers. Among these sputtering [72,73] have been widely used to develop c-axis oriented AlN and ZnO layers and sol-gel technique has been used for PZT layer [74]. Primary factors considered for the selection of a particular deposition technique include crystalline quality of films, deposition throughput, process temperatures and cost as well as the availability of equipment. However, sputtering is preferred among all these established techniques due to its simplicity, good reproducibility, possibility of operation at low temperature, and compatibility with standard Sifabrication technology. In addition, sol-gel technique has also been used widely in the development of PZT and ZnO layers. Therefore, we will primarily focus on the sputtering and sol-gel technique only. 2.1. Sputtering technique DC/RF sputtering is used to deposit films of ZnO and AlN from high-purity metallic Zn or Al targets [72,73,75–77] or by RF sputtering from ceramic ZnO or AlN targets at room or elevated temperatures, all in an Ar and O2 or N2 gas mixture [78]. Many authors also reported sputter deposition of PZT thin films using ceramic target [79–81]. Although RF deposition can produce films with a smoother surface, the sputtering rate of the RF process is much lower than that of DC sputtering at the same target power. A metallic target of Zn or Al is used in DC sputtering so optimization of

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This region is very unstable due to hysteresis nature of the transition process parameters from metal to poison and from poison to metal modes. Although poison mode is slower, it can produce highly piezoelectric films that are repeatable and have controllable stress [92]. Best piezoelectric activity can be obtained in polycrystalline thin films when all the microcrystals are aligned with the c-axis perpendicular to the film surface. Since energy is needed to form the preferred c-axis orientation so its formation is dependent upon process parameters. Therefore, sputtering conditions such as target power, gas pressure, gas flow rates, bias voltage, substrate materials, deposition temperature and annealing temperature influence the microstructure, texture and piezoelectric properties of sputtered thin films [94–98]. Following sections illustrate the sputtering technique used for the deposition of AlN, ZnO and PZT. Fig. 1. Hysteresis curve of reactive sputtering showing different modes (Reprinted with permission from Ref. [93]. Copyright © 2018, MDPI (Basel Switzerland)).

the process parameters is needed to prevent charge build-up on the target. This phenomenon significantly affects the deposition rate. It causes surface roughness which may lead to poor crystal quality of the films. Another problem associated with DC magnetron sputtering is its low ion energy and low ion flux. However, use of a pulsed DC system rectifies all these problems. With pulsed DC power, the polarity of the power delivered to the target is alternated between negative and positive at a frequency in the range from 20−350 kHz [82]. During a negative pulse, which is the normal condition for sputtering, ions are attracted to the target to eject target atoms. On the contrary, electrons from the plasma are attracted to the target to discharge any charged regions during the positive pulse [83–87]. Barshilia et al. [88] reported the use of an unbalanced magnetic field to increase ion bombardment which improved the deposition process and film texture of AlN films. A balanced magnetron type deposition process enhances the production of AlN which facilitates the synthesis of AlN films with different preferential orientations such as (100), (002) and even (101). On the contrary, a strongly unbalanced magnetron provides a limited production of AlN species in the plasma phase. There is a strong increase in the ratio of ions to metal atom flux on the growing films. An ion energy, typically in the range of 20–30 eV is provided to the growing film by the ion flux which facilitates the growth of dense (002) oriented films with high crystalline quality without any substrate heating. Here, the magnetic field at the target edge is normally modified to concentrate the plasma onto substrates [88,89]. In all these systems, whether DC/pulsed DC or RF, reactive sputtering is a must to deposit a film of compound material like ZnO or AlN. However, the performance of reactive sputtering process ¨ is greatly influenced by the effect of target poisoning ¨. Target poisoning is the formation of the desired compound film not only on the substrate but also on the sputter target [90]. This significantly reduces sputter yield which in turn reduces deposition rate with the increase in the supply of the reactive gas [90,91]. Therefore, AlN can be deposited in two modes: “metallic” mode and “poisoned (or compound) mode.” In metallic mode, partial pressure of nitrogen is kept low which prevents the formation of a thick insulating layer of AlN on the target. As a result, Al sputtered from the target reacts with N2 on the substrate surface. On the other hand, in the poisoned mode, target surface reacts with N2 to form a layer of AlN on the target itself which is then sputtered off the target as AlN molecule. These molecules then condense on the surface of the surface to form AlN film [92]. A consequence of poisoning is the hysteresis behaviour of the target voltage variation with the changes in reactive gas flow. Therefore, there exists a transition mode between metal and poison modes as shown in Fig. 1 [93].

2.1.1. Aluminum nitride (AlN) Deposition parameters can be optimized to control the surface morphology of the AlN thin films. Chu et. al reported that application of a negative bias voltage to the substrate at low temperature resulted in the (002) orientation of wurtzite AlN [99]. It has also been reported that increasing substrate temperature and nitrogen concentration increases the surface roughness of AlN films [100]. Therefore, the deposition conditions play a major role in deciding the orientation of AlN thin films. 2.1.1.1. Effect of deposition parameters on the sputter rate and orientation of AlN thin film. Ababneh et al. [101] investigated the influence of plasma power, pressure level and gas composition on the orientation and stress of the deposited film. It has been reported that the sputter rate decreases with the increase in nitrogen concentration as shown in Fig. 2.a. Same behaviour was observed at different values of powers ranging between 300 and 1000 W as well as pressures between 2 × 10−3 mbar and to 6 × 10−3 mbar. Authors argued that the lower mass of the the ionized molecule (i.e. N 2+ ) compared to single-charged ion (i.e. Ar+ ) resulted in a lower energy transfer which in turn led to a lower sputter rate when N2 gas was used as the sputtering agent [101]. The sputter rate increases at higher values of sputtering power and at lower sputtering pressures, respectively as shown in Fig. 2.b. When the plasma power is increased, the positively charged ions are accelerated with higher kinetic energy to the target. As a result, there is an increase in the transfer of the momentum to the target which in turn leads to a higher sputter rate. On the other hand, increasing the pressure will result in the decrease of the mean free path in the gas atmosphere which enhances the collision probability among the particles. This leads to a decrease in the sputter rate [101]. Although impurity incorporation into the film from background gases can be reduced by using a high deposition rate but it also results in a decrease in the ion/atom flux ratio which in turn leads to a poor texture as well as tensile stress. Moreover, due to reaction and diffusion limitations, extremely high deposition rates may result in non-stoichiometric films as well as structural defects. However, background gas incorporation will result in poor film quality at a low deposition rate [86]. The intensity of the (002) peak decreased when sputtering pressure is increased from 2 × 10−3 mbar to 6 × 10−3 mbar, as depicted in the Fig. 2.c. Authors argued that with the increase of sputtering pressure, the number of atoms increased in the chamber which effectively shortened the mean free path of the sputtered particles. As a result, the sputtered Al atoms experienced more gas scattering before arriving at the substrate surface with most of their kinetic energy dissipated. Hence, the adatoms will have less time to rearrange on top of the surface to form close-packed (002) plane with higher formation energy, before the next layer forms which in turn enhances the probability of formation of undesired orientations [72,102]. Liu et al. [72]

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5

reported the effect of increasing the sputtering power, on the texture of AlN films. He argued that at higher power, the ejected Al atoms had higher kinetic energy when they arrive on the substrate. As a result, these atoms arrange themselves to form close packed (002) plane. Also, Iriarte et al. [86] observed that the degree of c -axis orientation is dependent on the film thickness up to a value of 1 m. Beyond this thickness, there is no influence of the layer thickness on the degree of orientation. Also, the base pressure (pressure inside the vacuum chamber prior to the process start) should be below 10−7 mbar which is very much necessary to prevent the incorporation of oxygen into the growing AlN thin film. Since Al has a strong chemical affinity towards oxygen for the formation of Al2 O3 which will degrade the surface topography and will lead to a poor preferred orientation [86,103]. The effect of oxygen content on the degree of c-axis orientation will be severe even if the concentration is as low as 5%. It has also been reported that highly c-axis oriented AlN thin film can be deposited on various substrates irrespective of its crystallinity [86,104]. Substrate temperature is another parameter which greatly influences the quality of the deposited AlN thin film. Yang et al. [105] carried out deposition of AlN thin film on top of Molybdenum (Mo) at substrate temperatures of 20, 200, 400 and 600o C. They observed that the crystal orientation changed from (101) to (002) when substrate temperature was increased from 20o C to 400o C. Authors argued that with the increase in temperature, adtoms get more energy to arrange themselves on the surface of the substrate. So, crystallinity increases with substrate temperature. However, beyond 400o C, the intensity of the (002) peak decreases. It is due to the degradation of the crystal structure which arises from the increased thermal stress due to the bigger CTE differences between AlN film and its substrate [106]. Similar phenomenon has also been reported by Jin et al. [107]. 2.1.1.2. Effect of deposition parameters on film stress. Deposition of AlN or any piezo-material on a substrate results in mechanical stress in thin films. Basically, stress arises from two factors–one is due to the differences in coefficients of thermal expansion (CTE) of piezoelectric thin film and the underlying layer and the other one is due to the presence of structural imperfections such as grain boundaries, vacancies etc. in the piezo-layer. Stress due to structural imperfections arise due to incomplete structural ordering when the film is deposited at a substrate temperature less than 20% of the melting point of the thin film material [108]. The biaxial film stress f can be measured with a stress analyzer based on the wafer bow technique. f can be calculate from the difference of the wafer bow, before and after the thin film deposition using Stoney’s equation [109–111]: Es ts2 f = 6 (1 − ϑs ) tf where

Fig. 2. Influence of sputtering parameters: (a) Sputter rate vs gas composition at power Pp = 300W and pressure pb = 4 × 10−3 mbar, (b) Sputter rate vs sputtering pressure at different plasma powers in pure N2 environment and (c) X-ray diffraction patterns of AlN films sputter deposited at 300 W in 100% N2 atmosphere (Reprinted with permission from Ref. [101]. Copyright © 2010, Elsevier).

Es (1−ϑs )



1 1 − Rf Ro



(1)

denotes the biaxial Young’s modulus of the substrate, Es

is the Young’s modulus and ϑs is the Poisson’s ratio of the substrate), ts is the thickness of the substrate, tf is the thickness of the film, Ro and Rf are the radii of curvature before and after film deposition respectively. It has also been reported that the tensile film stress increases with increasing sputtering power and with decreasing nitrogen content respectively. Liu et. al also studied the effect of sputtering power, nitrogen concentration and sputtering pressure. But they used lower power levels from 45-200 W which is significantly lower than that used in [101]. They observed that nearly unstressed film was obtained at sputtering pressure of 0.6 Pa with nitrogen concentration of 40% and sputtering power of 145 W. At pressure lower than 0.6 Pa, the films exhibit compressive stress, while at pressure higher than 0.6 Pa, the films show a tensile stress [72].

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W.R. Ali and M. Prasad / Sensors and Actuators A 301 (2020) 111756

Table 2 Deposition parameters of sputtered AlN. Sputtering process

Substrate Temperature (o C)

Substrate to target distance (cm)

Sputtering Power (W)

Gas Sputtering Composition Pressure (Ar/N2 )% (mbar)

DC magnetron sputtering DC magnetron sputtering DC magnetron sputtering Rf magnetron sputtering

80 (Self-heating) 150 (Self-heating) Below 80

6

145

60/40

6.5

1000

0/100

3-7

900

25/75

4 × 10

No heating

5

300

50/50

6.67 × 10−3

The compressive stress in these AlN films which are sputtered at low temperature is due to the presence of defects, e.g. interstitials, Al or N vacancies, dislocations which are formed due to the energetic bombardment during the film growth [112]. Table 2 lists the optimized deposition condition used by various authors. 2.1.1.3. Effect of post deposition annealing. Kar et al. [114] studied the effect of rapid thermal annealing (RTA) crystallinity of sputter deposited AlN thin films with thickness 500 nm. RTA was carried out in nitrogen ambient from 400o C to 1000o C for 90 s with ramp up and ramp down temperatures of 60o C and 40o C respectively. Authors observed that the intensity of c-axis orientation increases from 400o C to 800o C but RMS surface roughness increases from 2.1 nm to 3.68 nm. Increasing the temperature further upto 1000o C leads to the appearance of microcracks [113,114]. Gillinger et al. [115] also studied the effects of post deposition annealing temperature on the sputtered AlN thin films with thicknesses of 100 nm and 500 nm in the temperature from 250o C to 1000o C. Authors observed that at 1000o C, the sample with 100 nm thickness are completely oxidized after 2 h in oxygen atmosphere but that with 500 nm layer still showed the characteristic (002) wurtzite peak. It is because the oxidation process was not fully completed for 500 nm film. They also observed that the film stress for both film thicknesses remained constant up to an annealing temperature of 600o C independent of the atmosphere. But above 700o C, authors reported a shift towards compressive stress. Authors argued that it is attributed to the diffusion of oxygen or nitrogen into the AlN thin film [115]. Vergara et al. have also reported that post deposition annealing improves the preferred (002) orientation due to increase in grain size of (002) oriented crystallites, formation of new grains with (002) preferred orientation and reduction of grains with orientations other than (002) [116]. 2.1.2. Zinc oxide (ZnO) Deposition conditions such as the RF power [117,118], deposition pressure, substrate temperature [119–121], deposition gas composition (Ar: O2 ) [51,117,122,123] and bottom electrode materials [117,118,124] strongly influence the properties of the ZnO thin films. Deposition of ZnO using sputtering technique can be done from either ZnO ceramic target or Zn metallic target. ZnO deposition using Zn metallic target employs reactive process, hence control in phase and stoichiometry is not easy. The main disadvantage is the existence of an unstable transition region between the metallic and oxidic deposition mode [125] whereas if the sputtering is done using ZnO ceramic target, the resulting stoichiometry is near ideal and phase is hexagonal as it is governed by target properties itself [118,124,126–132]. But still, sputtering using Zn metallic target [133–135] is preferred because of its cost-effective deposition over large coating area, higher deposition rate compared to that of using ZnO ceramic target [125].

Thickness (m)

Intrinsic stress developed

Surface roughness (nm)

FWHM of the rocking curve of XRD for c-axis orientation

Ref.

4 × 10−3

0.4

0.93

3.1o

[72]

4 × 10−3

0.4

Compressive (1000 MPa) Tensile (426.7 MPa)

1.7

0.33o

−3

2.2 − 5 1

7.7

[101] o

[86]

[104] 0.32o − 0.5o (depending on type of substrate)

2.1.2.1. Effect of deposition parameters on ZnO thin film orientation. It has been reported that high deposition rates usually result in randomized polycrystalline structures within the thin film which deteriorates the piezoelectric performance. The deposition rate can be controlled by the RF power. The growth rate increases with increasing RF power. It is due to the fact that higher RF power facilitates more argon ions in the plasma which causes an increased bombardment on the target [127]. Even if a ZnO target is used then also O2 is required in the atmosphere, otherwise depletion of oxygen in the ZnO film may lead to conductivity and consequent poor piezoelectricity. The ratio of the two feed gases, argon and oxygen, also affects the stoichiometry of the deposited film [120]. The deposition rate gets reduced by a higher oxygen content [128]. It has been reported that the preferred orientation of ZnO is enhanced due to the formation of large crystalline structures at a higher deposition pressure although surface roughness increases [75,136]. However, the growth rates are decreased with increasing sputter pressure due to decreased mean-free path of sputtered atoms. More collisions occur on the path of travel of particles from the target to the substrate and the resulting scattering causes the decrease of the deposition rate. However, for low deposition pressure, deposited atoms do not get sufficient time to settle down on substrate which increases film roughness. Therefore, surface roughness is mainly decided by the growth rate which is again dependent on sputtering power, pressure and target to substrate distance [75,121]. That is, the surface roughness increases as the growth rate of ZnO film increases [129,130]. Kang et al. [129] also reported that the surface roughness of the ZnO film is influenced by the surface condition of the metal bottom electrode on which it is grown. Authors also observed that the best results in terms of the crystallinity of ZnO films will be obtained if target-sample distances were 50 and 70 mm for Au and Al bottom electrodes respectively. Also, considerable residual stress is accumulated after deposition and cooling to room temperature when ZnO is deposited at an elevated temperature. Lin et al. [126] used a ceramic target of ZnO, with purity 99.99%, reaction gases argon and oxygen with purity of 99.999%, the substrate was LPCVD-deposited low-stress Si3 N4 on a single crystal silicon substrate and sputtered with Pt and Mo. Authors reported that when sputter power increases, the intensity of preferred (002) peak of ZnO film on Mo and Pt increases. Therefore, the number of high energy particles increase with the increase of power which in turn increases the sputter rate. As a result, the impinged atoms have more energy and momentum. Thus, these atoms have more energy to rearrange into a crystal lattice to obtain c-axis (002) crystallographic direction with minimum surface energy and maximum atom density [137]. Also, with the increase of sputter power, ZnO thin film surface grain size increases which indicates the enhancement of thin film crystallinity. Authors also pointed out that with the increase of ZnO thickness, its piezoelectric property improves so it has less energy loss and hence higher electromechanical cou-

W.R. Ali and M. Prasad / Sensors and Actuators A 301 (2020) 111756

pling constant. However, after a certain thickness, it degrades due to the fact that surface roughness also increases with film thickness. R. Ondo-Ndong et al. [121] used a cylindrical metallic zinc target with 99.95% purity for fabrications of zinc oxide films utilizing magnetron sputtering in an oxygen (20%) and argon mixed gas atmosphere with an RF power of 50 W at a frequency of 13.56 MHz. High-purity mixed oxygen-argon introduced during deposition raised the pressure in the chamber to 3.35 × 10−3 Torr. The target to substrate distance was 7 cm and was kept fixed for each deposition runs. The films were deposited on (100) silicon substrate (380m thick). The substrate temperature was 100o C. Ferblantier et al. [133] also used same deposition condition as mentioned in [121]. In both studies, the authors used pre-sputtering of the metal zinc target to remove any contamination on the surface. They reported to obtain high c-axis orientation [121,133]. Substrate also influences the quality of the sputtered ZnO thin film. Golebiowski compared the qualities of different ZnO thin films sputtered on glass, (100) silicon, silicon nitride (Si3 N4 ), borosilicate glass (BSG), aluminum (Al) and chromium (Cr). He reported that high quality ZnO films can be obtained on the amorphous substrates such as glass, BSG and Si3 N4 [118,123,127]. It was also reported that while using gold (Au) as bottom electrode, titanium (Ti) could facilitate a better ZnO film formation than Cr as the adhesion layer for gold. ZnO deposited on Au/Ti gives smaller FWHM of (002) peaks compared to that of in case of Au/Cr. Also, Au (111) peak gives smaller FWHM when deposited on Ti. This implies that Ti facilitates growth of better-quality Au layer which in turn facilitates the growth of well oriented ZnO film [131]. Also, it has been reported that a thin layer of Pt on Al could lead to the growth of a high quality ZnO film [131]. Sharma et al. [138] studied the effect of film thickness on orientation. From the XRD patterns of the three deposited ZnO films on Si with thicknesses 222 nm, 250 nm and 342 nm, they observed high intensity (002) peak along with low intensity (103) peaks in all the three thicknesses. Also, the (002) peak intensity increases with increase in thickness for (002) crystallographic plane. According to the kinetics of crystal growth, a growth competition takes place among the neighbouring crystals as per their crystal orientation when the thickness of the film increases. The faster growing crystals will grow over the slower growing ones. This competition leads to the growth of same type crystal faces to form a free surface which improves the crystalline quality of the deposited films. When crystals exhibiting the same type of crystal faces proceed to the free surface then only this competition gets terminated [138–140]. If the lower layer is crystallized, the further crystallization of the upper films can occur epitaxially [141]. When ZnO films are thicker then longer deposition time and thickness give enough room to the crystal faces for the competition to occur. That is the reason why (102) and (103) peaks were not observed when ZnO films were thicker than 500 nm [138]. Three peaks of (002), (102) and (103) appeared in the XRD for 200 nm and 400 nm thicknesses. However, when the films were thicker than 500 nm, only (002) diffraction peak of ZnO was observed. Therefore, it can be inferred that when the ZnO films are thick enough, the surface of film tends to be the (002) plane, which has the minimum surface free energy if there is no effect from the substrate [141]. Fujimara et al. [141] also reported that the same tendencies concerning the texture formation of ZnO films were observed when Zn metal target instead of ZnO was used, although the gas compositions to obtain each texture were a little bit changed. 2.1.2.2. Effect of deposition pressure, temperature and power on film stress. The stress affects the mechanical and electrical properties of the resulting films [131,133]. Stress in zinc oxide films is due to a thermal component and an intrinsic component. The difference between film deposition temperature and the device operating temperature is the main cause of thermal component. The thermal

7

expansion coefficient of zinc oxide (a-axis, 4 × 10−6 K −1 ) is larger than of silicon (2.33 × 10−6 K −1 ) which results in a tensile lateral stress in the zinc oxide due to cooling after deposition. The imperfections which occur in the crystallites during film growth cause the tensile stress. It has been reported that ZnO exhibits compressive intrinsic stress and its magnitude can be much larger than the (tensile) thermal component. Therefore, the resultant residual stress in ZnO film is compressive in nature. Several parameters influence the stress in the ZnO film which include deposition temperature, deposition pressure, deposition power, gas mixture and also the material on which the ZnO film is deposited [133,134,142,143]. Films with a high melting point which have been deposited at a low temperature, suffer from large intrinsic stress. The reason being, at low deposition temperatures, the adsorbing atoms have very little energy so they cannot arrange themselves in their lowest energy state which leads to the build of intrinsic stresses [144]. However, if the deposition temperature is made too high then other negative effects such as large grain growth, can take place. Intrinsic stress may also result from high deposition pressure. At high pressure, the particles suffer from larger number of collisions so they arrive at the substrate with a lower kinetic energy which makes their arrangement more difficult. However, stress may occur at very low pressure too. It is because, the energy of the adsorbed atoms can become too high which would result in the bombardment of the substrate. This obstructs film formation which in turn leads to intrinsic stress. A higher sputter power facilitates a higher deposition rate which causes a higher particle energy at the substrate. This results in a smoother arrangement of the adsorbed atoms [142]. When a thin film grown on a silicon wafer exhibits stress, the wafer will bend. The curvature of the silicon wafer both before and after zinc oxide deposition can be measured to calculate the stress f in the zinc oxide layer by using Stoney’s equation which has already been shown in equation (1) in the previous section [109–111]. The thermal stress is given by [142]:



th = ˛f − ˛s f = i + th





Ef 1 − ϑf



(Td − T )

(2) (3)

where ˛f and ˛s are the coefficients of thermal expansion of the film and substrate respectively, Td is the film deposition temperature and T is the device operating temperature. The amount of intrinsic stress can be calculated from equations 1, 2 and 3. Cimpoiasu et al. [142] observed that the stress in zinc oxide layers grown directly on silicon show a much larger value than that of grown on aluminum-covered silicon. They also observed that the stress in zinc oxide films grown on highly textured (111) Al film is smaller compared to the stress of zinc oxide grown on poorly (111) textured aluminum film or on SiO2 . Therefore, the texture of the underlying layer also influences the intrinsic stress in ZnO film. 2.1.2.3. Effect of post deposition annealing. Stress reduction in the ZnO film can be accomplished by post deposition annealing at temperatures of 300–550o C [134,142,143,145]. Also, boundary defects are eliminated by heat treatment which improves the preferred c-axis orientation of the polycrystalline zinc oxide film [134,143]. However, if the underlying layer is Al then annealing at this temperature greatly reduces the impedance of the film due to diffusion of the underlying Al [142]. However, in case of ZnO deposited on Ti/Pt/Si structure or only Si, it is found to greatly enhance the preferred c-axis orientation [134]. Table 3 lists the sputtering deposition conditions used by previous researchers. 2.1.3. Lead zirconate titanate (PZT) In the past, PZT thin films have been successfully deposited by sputtering techniques with a strong focus on RF sputtering [79,81].

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Table 3 : Sputtering deposition parameters for ZnO. Sputtering process

Substrate Temperature (o C)

Substrate to target distance (cm)

Sputtering Power (W)

Sputtering Pressure (mTorr)

Gas Composition Ar/O2 (%)

Thickness (m)

FWHM of the rocking curve of XRD for c -axis orientation

Ref.

DC sputtering (Zn target) Rf sputtering (Zn target) Rf magnetron sputtering (ZnO target) Rf sputtering (Zn target)

375

11

1500

6

only O2

—

—

[142]

100

7

50

3.35

80/20

3-5

0.28o

[133]

300

7

200

10

40/60

—

[126]

100

—

550

20

40/60

3-3.5

FWHM value not given (c -axis oriented with 2 = 34o ) FWHM value not given (c -axis oriented with 2 = 34.4o )

When Si is used as substrate, formation of lead silicide at the substrate interface is prevented by using buffer layers and/or metal layers. Normally, a thermally grown layer of silicon dioxide is used as a buffer layer in combination with a bilayer of Pt/Ti as a bottom electrode for PZT films. Pt is primarily selected because of its high thermal conductivity, good stability in a high temperature oxygen environment. Ti is used to promote the adhesion of Pt films to the substrate [146]. It has been reported that when a Pt/Ti bilayer is used as a bottom electrode for PZT films, Ti atoms diffuse out through the grain boundaries of the Pt layer onto the Pt surface, and the out diffused Ti enhances the formation of a perovskite PZT film [146,147].There are several other metal/conducting oxides such as Ru/RuO2 and Ir/IrO2 that can be used as bottom electrode in place of Pt electrodes exhibit a higher resistance to Pb inter-diffusion and thus can be a better barrier [59]. PZT thin films can be deposited using either reactive sputtering or through RF sputtering. In reactive deposition, three metal targets (Pb, Zr and Ti) are used, whereas in RF sputtering ceramic targets are utilized [59]. Kim et al. [148] reported that the tetragonality of sputtered PZT thin films reduces with the increase in thickness of the sputtered layer but piezoelectric coefficient increases [59,148]. The piezoelectric coefficient of PZT is inherently linked to its crystalline quality [149]. It has been reported that the highest value of piezoelectric coefficients is obtained at the morphotropic phase boundary (MPB) which is the region where the crystal structure changes abruptly between the tetragonal and rhombohedral symmetry [150]. The MPB is located approximately at PbZr 0.52 Ti0.48 O3 or PZT (52/48). Both the dielectric permittivity and piezoelectric coefficients reach a maximum at this composition [151]. Vasantkumar et. al reported on rapid-thermal-annealing behavior of PZT films that were prepared by reactive dc sputtering. They used a multi-element metal target consisting of zirconium and titanium sectors. Substrate was sputtered at a temperature of 200o C and a power of 50 W. They reported that the pyrochlore phase transforms into perovskite at 750o C in the lead-deficient case and that the perovskite phase forms at 600o C from intermediate compounds such as lead titanate and a zirconium-rich compound in the leadrich case. The cause of the appearance of the pyrochlore phase was a deficiency in lead due to evaporation at the substrate temperature employed. The perovskite PZT formed through the pyrochlore phase has a defect structure due to excessive lead vacancies [152]. Bi et al. [146] reported the deposition of PZT thin film (800 nm thick) on Pt/Ti/SiO2 /Si in Ar gas by Radio Frequency sputtering and RTA (rapid thermal annealing) in 700o C for 20 min and authors successfully obtained single perovskite structure and no pore and crack on the surface of thin film. Hu et al. [153] studied the effect of heating rate on the crystallization behaviour of amorphous PZT thin films. They demonstrated that if conventional furnace annealing (CFA) is employed then there would be two-stage transition (amorphous state to metastable pyrochlore phase then to stable

[6]

perovskite phase). As a result, the process would be much slower. On the contrary if rapid thermal annealing (RTA) is utilized then due to the very fast heating rate, there would be a single-stage transition from amorphous state to perovskite phase [153]. Wilke et al. [154] reported a very low temperature ∼25o C deposition of PZT thin film on glass substrate using Ar as sputtering gas. The use of glass substrate restricted the maximum allowable process temperature to 550o C so that it is below the glass transition temperature. Authors deposited 1 m thick PZT by sputtering using a ceramic PZT target. Targets included 10% excess PbO in order to compensate for Pb loss during the deposition and crystallization step. The film was subjected to rapid thermal annealing (RTA) at 550o C for 60 s for crystallization and then to a 24 h annealing in box furnace to remove excess PbO. The films are reported to show good piezoelectric properties but suffer from huge thermal stress of the order of a few GPa [154]. One major challenge with sputter deposition is accurate control of the Pb and O content within the films. The high volatility of PbO above ∼500o C requires incorporation of excess PbO in the starting films to compensate for lead loss during crystallization. In systems using multielement targets, the stoichiometry of the as-deposited sputtered film can be controlled through adjusting the chamber pressure during deposition [155]. Another important parameter is the substrate temperature during deposition. Loss of Pb content will be higher if the deposition temperature is made too high which in turn necessitates the use of higher excess lead content targets. Pure perovskite phase can be obtained for PZT thin films by maintaining the substrate at a temperature higher than 600o C. However, if substrate temperature is low then an annealing step is required on the sputtered film to convert it into the desired perovskite phase. Since crystallization of PZT 52/48 is typically nucleation limited [156] so using a template layer would significantly lower the crystallization temperature. PZT films with higher Ti content crystallize at lower temperatures [157]. Kwok and Desu [158] reduced the deposition temperature of MPB PZT compositions to 500o C using a Ti-rich seed layer. The advantage of using the seed layer is that it increases the number of active sites for perovskite phase nucleation and also provides control over PZT film texture [149,159]. Perovskite nucleation density in PZT film is reported to be 60 times higher on TiO2 than on Pt. It is also reported that the PZT film grown on a TiO2 template layer crystallizes at a relatively lower temperature than that for the PZT film grown without a TiO2 template layer [160]. Sputtering process itself provides energy to the growing film by ionic bombardment [161]. Growth temperatures as low as 450o C has been reported for crystallization of MPB PZT films using sputter deposition [153,154]. It was reported in literature that when a target with excess lead is used then growth of perovskite PZT requires a deposition or annealing temperature in the range of 550 − 750o C. In the low-temperature deposition case, post-deposition anneal is normally carried out at 550o C or above for typically 1 h or longer.

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Table 4 : Rf magnetron deposition parameters for PZT. Zr/Ti ratio

Sputtering Pressure (Pa)

Composition

52/ 48 52/ 48 53/ 47

53.3 0.4 1−2

O2 only Ar/O2 (92/8)

Sputtering Power/ Power density

Substrate temperature (o C)

Annealing Temperature (o C)

Thickness (m)

Piezo-electric co-efficient (pm/V )

Ref.

600 200 530 − 630

Not used 650 (for 30 minutes)

0.4 − 4

d33 = 330

3W/cm2 100 W

1

d31 = −28

[148] [159] [162]

The sample’s prolonged stay at such elevated temperature, either during deposition or during annealing, usually causes deterioration of the film–substrate interface. Table 4 lists the RF magnetron deposition conditions used by previous researchers. 2.2. Sol-gel technique In the sol–gel process, the main process steps are: (a) hydrolysis of the molecular precursor; (b) polymerization via successive molecular additions of ions, forming oxo-hydroxyl, or aquabridges; (c) condensation by dehydration; (d) nucleation and (e) growth [163,164]. Deposition of PZT and ZnO using sol-gel route requires a maximum temperature of 700o C during annealing process. However, in case of AlN, it requires a very high nitridation temperature of 1250 − 1350o C which will damage the underlying electrode layers present in a microdevice structure [165,166]. Therefore, sol-gel route is not practically feasible in case AlN based micro-device. That is why, the sol-gel processes for only ZnO and PZT have been elaborated in the following section. 2.2.1. Zinc oxide (ZnO) Two sol–gel routes are currently used: metal alkoxides in organic solvents or metal salts in aqueous solutions [167]. ZnO films are obtained from inorganic salts or organic salts like acetates and acetyl-acetonates, dissolved in alcoholic media. Once the salt is dissolved in the media, there will be an in-situ formation of alkoxide or alkoxy-complexes and after that, the complex undergoes transformation through hydrolysis and polymerization which result in the growth of the oxide [65]. Among zinc precursors, zinc acetate dihydrate (ZAD) is the most widely used [168,169]. Metal alkoxides are not suitable as precursor because they are very sensitive to moisture and highly reactive. Metal salts are widely used because of their low cost and commercial availability. The main drawback of using inorganic salts like nitrates as precursors for sol–gel ZnO films is the inclusion or difficult removal of anionic species in the final product [170]. Similarly, if alkoxides are used then additives like ethylene glycol, diamines or triamines are added in high concentrations in the starting solution to yield stable sols and also to obtain homogeneous layer. But it leads to purity problems can arise in the final film. On the other hand, if zinc acetate is used as a starting compound then acetic acid, one of the products of the hydrolysis reaction, is very soluble in the solvent medium and the also acetate groups decompose under annealing, thereby producing combustion volatile byproducts [170]. Therefore, purity of the final ZnO film is maintained when ZAD is used. Also, Ohya et al. [171] reported that ZnO films synthesized from ZAD exhibited a strong orientation of the c-axis compared to other precursors. Alcohols are normally used as solvents for ZAD precursor. Among all the alcohols, the most used ones are ethanol, 2-propanol and 2-methoxyethanol(2-ME). Hosono et al. [172] has made a comparative study of different alcoholic solvents of ZnO and reported that Zinc acetate dehydrate (ZAD) is more soluble in methanol (MeOH) than in ethanol (EtOH) or 2-methoxyethanol. Authors argued that the higher the dielectric constant of the alcoholic solvent, the more it is capable of dissolving ZAD. Also, it has been reported that the reflux time required for the formation of ZnO will be more for a solvent

with lesser dielectric constant i.e. 2-ME (72 h)>EtOH (48 h)>MeOH (12 h) [172]. Additives facilitate zinc salt dissolution in alcoholic solvents. Solubility of ZAD is limited in alcohols like ethanol and 2propanol in the absence additives or heating [173] so additives, like (mono- to tri-) ethanolamines or lactic acid are used for complete dissolution so as to form a stable and clear sol [168,169,174,175]. Additives also function as chelating and stabilizing ligands [176]. The formation of ZnO colloidal particles in an alcoholic solvent takes place in two stages. Small clusters called oligomers are continuously formed during the early stage of phase transformation. Aggregation of these oligomers leads to crystalline wurtzite which is the primary colloidal particle at later stages. The primary particles then aggregate and form a third family, the secondary colloidal particles. Tokumoto et al. [177] suggested that the growth of the colloidal particles is a stepped, discontinuous, process. Therefore, mechanism of aggregation is heterogeneous coagulation which leads to a hierarchical structure. Properties of the solvent strongly influence the kinetics of nucleation and growth of thin films. Nucleation and growth are much slower for shorter chain length alcohols (ethanol, 1-propanol) than that of for longer chain length alcohols (1-butanol to 1-hexanol). Also increasing temperature increases particle size for all alcoholic solvent. Moreover, particle size also increases with alcohol chain length. Thus, the alcohols help to control the morphology and particle size of ZnO [178,179]. Moreover, it has been reported that the use of solvents with higher boiling point like 2-methoxyethanol/methanol/ethanol with MEA results in a strong preferred c-axis orientation [176,180]. On the other hand, films prepared with low boiling point solvent like 2-propanol with DEA gives imperfect preferred orientation [176]. Furthermore, among the additives, films with MEA as additive is reported to show a very high intensity (002) peak whereas films with DEA as additive show a weak (002) peak [176,181,182]. Freshly prepared or aged sols are deposited onto a substrate using either dip- [29,183] or spin-coating [173,184–186]. The crystallographic orientation of ZnO films also depends on the coating method [187]. Authors reported high c-axis orientation with (002) peak only for films deposited with dip-coating method (withdrawal speed of 1 cm/min). On the contrary, for films deposited by spincoating (3000 rpm), three peaks with high intensity for (002) peak was observed. Shivaraj et al. [188] also reported that dip coated films show a smoother surface with lower surface roughness compared to spin coated films although both films are annealed at the same temperature. Also, in case of dip coating, films prepared with lower WS’s (withdrawal speed) (1.2−2 cm/min) are reported to show intense preferred (002) orientation. Also increasing the WS, decreases the grain size and hence decreases crystallinity but surface roughness increases [183,189]. The thickness of the deposited film also plays a crucial role in deciding the degree of crystal orientation. As reported in [190], two approaches were adopted in literature to produce dense films (method A) or colloid-based films (method B). In method A, first ZAD was dissolved in a 2-methoxyethanolMEA solution with the ratio [MEA] / [Zn (II)] fixed at 1 or 2. The final solution concentration of ZAD was fixed at 0.75 mol/L. The solution mixture was then aged at 60o C for 120 min under stirring to obtain clear and colorless solution. Thereafter, spin-coating

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(speed: 3000 rpm & duration: 30seconds) was used to deposit the solution on silica glass. Preheating of these films were done at 300o C for 10 min after each coating and this procedure could be repeated several times to get the desired thickness. The films were finally post-heated at 550o C (1 h) in order to obtain crystallized wurtzite ZnO. In method B, ZAD was refluxed for 3 h at 80o C in dry ethanol to prepare a solution of Zn (II) with a concentration of 0.2 mol/L. MEA and ethanol were then added to adjust the final concentration (0.05 mol/L of Zn2+ ). The solution aged at 60o C for 72 h gives a translucent yellow colored sol (colloidal dispersion). After filtering, the sol was deposited on glass substrates by spincoating (speed: 3000 rpm & duration: 30seconds). These films were preheated (100o C, 5 min) between successive spin-coating steps. Thereafter, they were pre-treated at 135o C for 36–72 h, and further post-treated at 450 − 550o C for 2 h [65,190]. Also, Malek et al. [191] studied the effect of precursor solution concentration on the intrinsic stress of the sol-gel deposited film. Authors prepared the sol by dissolving ZAD in 2-methoxyethanol and MEA. The concentration of zinc acetate was varied from 0.2 M to 1.0 M, keeping molar ratio of ZAD and MEA constant at 1:1. The solution was stirred and heated at 80o C for 1 h until it became clear and homogeneous. The deposited films were annealed for 1 h at 500o C. Authors found that for the 0.4 M precursor concentration, the stress was minimum (∼420MPa). Authors attributed the formation of higher stress levels at higher precursor molar concentrations (>0.4 M) to the decrease in crystallinity which in turn contributes to a higher occurrence of defects and lattice distortions in the crystal structure [191]. 2.2.1.1. Role of pre-heat treatment and post-heat treatment. Generally, the heat treatment of the deposited films is carried out in two steps. In the first step, a pre-heat treatment (with temperature 40 − 300o C) is applied for a short duration. It is necessary for solvent evaporation and organic compounds removal. In the second step, a post-heat treatment (with temperature 300 − 700o C depending on substrate type) is carried out to grow crystallized thin films [29,173,174,184–186,190,192–195]. Pre-heat temperature is one of the major factors determining the preferred orientation of ZnO films. It affects the solvent vaporization, zinc acetate decomposition and zinc oxide crystal growth [170,189,192]. The thin films should be subjected to a heat treatment with a temperature higher than the boiling point of the solvent and the additives as well as near to the crystallization temperature of ZnO. It has been reported that in the case of 2-methoxyethanol and MEA as solvent and additive, respectively when the preheating temperature is too high (> 300o C), there will be an abrupt vaporization of the solvents and the thermal decomposition of the zinc acetate, simultaneously with the crystallization which will disturb the unidirectional crystal growth. On the other hand, when the preheating temperature is too low (< 300o C), the complete vaporization and the thermal decomposition of the zinc acetate occurs abruptly at the postheating step at postheating temperatures over 500o C which may also disturb the unidirectional crystal growth. Therefore, for a 2methoxyethanol/MEA sol, pre-heat temperature of around 300o C was reported to be the most appropriate to produce (002) oriented films [189,196]. Yahiya et al. [197] and Aydemir et al. [183] also reported a preheating temperature of 300o C for sol prepared using ZAD/ethanol. Furthermore, Natsume et al. [184] reported a low preheating temperature of 80o C for ZAD/methanol sol without any additives. It has also been reported that the preferential orientation of (002), increases with increasing post heating temperature [183,187,188,198]. There is an increase in grain size with annealing temperature due to grain coalescence at higher temperatures [198]. Moreover, surface roughness also increases with the increase in annealing temperature [188]. It has been reported in literature that a temperature range of 500 − 600o C is the most appropriate one [183]. However, it also has been observed that there exists an

upper limit in temperature for each system above which there will be a loss of orientation [184,189,196,197]. Fig. 4.a is showing the flow chart of the spin coating method adopted in literature. Since, the solubility of ZAD is highest in methanol while solubility in methoxymethanol is lowest. That is why when methanol is used, no basic additive is required [184,199]. On the contrary, in case of methoxymethanol or ethanol, MEA/DEA is used to enhance the solubility of ZAD [182,197,200]. However, Sagar et. al [200] showed that the addition of MEA to the methanolic solution does improve the c-axis orientation. The process flow for depositing ZnO using ZAD and ethanol with MEA as additives will be same as that of shown for ZAD and methoxymethanol (Fig. 3.a). Also Fig. 3.b is showing the flowchart for the dip coating method. 2.2.2. Lead zirconate titanate (PZT) The first reports of sol-gel deposition of Pb (Zr, Ti) O3 (PZT) thin films were given by Budd et al. [201]. For PZT films, Budd used lead acetate trihydrate, titanium isopropoxide, zirconium n-propoxide and 2-methoxyethanol as solvent. Ledermann et al. [202] reported a modified sol-gel method. They used five precursor solutions of rhombohedral, MPB and tetragonal PZT composition with x = 0.3, 0.4, 0.45, 0.5 and 0.6 which were synthesized using an improved 2-methoxyethanol route of Budd et al. Authors prepared the precursor solution by dissolving lead acetate reagent in 2-methoxyethanol and then water is removed through distillation. The dehydrated powder was redissolved in dry 2-methoxyethanol for a duration of 1 h at 110o C. Titanium and zirconium alkoxides were then added. Thereafter, the solution was refluxed for a duration of 1 h at 120o C. Finally, authors used vacuum distillation at 300 mbar and 125o C for removing reaction by-products and also for adjusting the concentration of the precursors to 0.4 M for all compositions. A 4 vol.% formamide was added to the solution to improve the drying behaviour of the sol-gel. Formamide acts as an inhibitor for hydrolysis. The solution was then filtered and stored under dry argon. Authors prepared two precursor solutions so as to compensate for the lead loss during the rapid thermal annealing (RTA). The standard one has 10% lead excess and the solution used for the last layer before the RTA has 30% lead excess. Calame et. al also reported to use similar route to obtain the precursor solution [203]. PZT thin films were then deposited on Pt coated silicon wafers by multiple spin coating of the solution precursors. After each coating step, the layer was pyrolyzed slowly (ramp of 20o C/s) at 350o C for 15 s. Four single layers (three layers with 10% lead excess, one layer with 30% lead excess) were deposited. Finally, RTA was done at 650o C for 1 min (standard). As film thickness of a single spin is about 60 nm, 16 and 64 single layers were required to form 1 and 4 m PZT films, respectively [202]. Fig. 4 is showing the flowchart for PZT thin film deposition. 2.2.2.1. Role of annealing temperature on crystallization of sol-gel PZT. Annealing facilitates a controlled conversion of the asdeposited structure to a PZT crystalline phase [204]. Annealing at temperature of 500 − 700◦ C, facilitates the formation of the perovskite phase. Heating transforms the amorphous phase into oxygen deficient pyrochlore/fluorite structure at a temperature range of 350 − 600◦ C. Above ∼470◦ C, the perovskite phase starts crystallizing. However, for temperature above 700◦ C and long exposures, volatilization of PbO occurs which results in the formation of a new lead deficient phase forms at the surface of the film [205–207]. This will degrade the film quality [204]. Zhu et al. [204] also observed that films annealed in O2 atmosphere showed better surface morphology, microstructure and polarization compared with the films annealed in air atmosphere. Furthermore, Yaseen et al. [208] observed that higher annealing temperature improves ferroelectric and dielectric properties.

W.R. Ali and M. Prasad / Sensors and Actuators A 301 (2020) 111756

11

Fig. 3. Sol-gel deposition techniques for ZnO: (a) Spin coating technique and (b) Dip coating technique.

2.2.2.2. Role of seed layer and bottom electrode on crystallization of sol-gel PZT. Nucleation controlled growth allows also to choose the texture of the film by using suitable combination of seeding layer and electrode. Choice of the bottom electrode greatly influences the crystalline texture, quality and properties of the piezoelectric film. Muralt et al. [149] demonstrated the controlled growth of (111)-textured PZT thin films using few nm thick titania (TiO2 ) seed layer on platinized electrode [59,149]. It was shown that this very thin titania affinity layer reduces the effective activation energy for nucleation, even though it exhibits another crystalline phase than ZT . The role of titania consists essentially in pinning PbO to avoid excess desorption before nucleation. The interdiffusion of Ti on top of the Pt electrode can be prevented by using the TiO2 as a passivate layer over Ti adhesion layer [209,210]. Therefore, control of the composition and crystalline texture are very much necessary while depositing PZT using sol-gel technique in order to achieve excellent structural and piezoelectric properties. For PZT (52/48), the highest coefficients are reported for a (001) crystalline texture [35]. Garino and Harrington [211] observed that dielectric constant increases with the decrease of the tensile stress in the film. Also, Lian et al. [212] observed that the piezoelectric constants and dielectric constants increase with film thickness for films with preferred (111) and (100) orientations. Authors argued that the increase in the piezoelectric property with thickness could be due to the changes in the residual stress. Same authors also reported that the residual stresses in films with (100) preferred orientation was smaller than those in films with (111) preferred orientation, which resulted in higher piezoelectric response, lower dielectric dissipation factor, and higher value of saturation polarization [212]. A lot of work has been done to optimize the properties of PZT to suit the needs of specific device performance in the form

of substrate optimizations [213,214]. Also, studies have been done to improve the electrical properties of PZT composites [215]. The advantages of CSD method over vacuum deposition processes lie in a better control of the material composition in multicomponent systems, the ability to produce uniform large area coating and the relative low level of investments required for the equipment [216]. The major inconvenience of this spincoating deposition method lies in a need of an absolute planar substrate without any prior microstructuration. In case of prefabricated three-dimensional structures, this will be impossible. In those cases, vacuum deposition processes are preferred. 3. Etching of the piezoelectric materials While pattering thin film, etch selectivity to the mask layer and undercut (lateral etching) are the critical aspects that need to be carefully considered. Dry [217–220] as well as wet etching processes [221–224] have been used to pattern piezo films. However wet etching process is more popular because of low cost and simple set-up. 3.1. Aluminum nitride (AlN) For etching AlN, many authors have reported the use of Ti/Cr as masking layer as well as top electrode [221,222,225,226]. It has been reported that Ti is preferred over Al because it has less lattice mismatch (5%) with AlN than Al (23%) which reduces the strain at the interface [221]. Also, when TMAH is used as an etchant, it does not etch Si/Ti at room temperature but it does etch Al so Al is not preferred [225]. Moreover, Ti is used because of its high cohesion to AlN and stability in AlN etchants [222]. Saravanan et al. [225] used

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generated hydronium ions (H3 O+ ) which attack the oxygen in ZnO. Etching ZnO in a controlled manner requires the use of a weak acid with a proper level of ion. When various acids and bases such as HCl, H2 SO4 , HBr, HF, HNO3 , H3 PO4 etc. are used to pattern the ZnO film then it has been observed that due to the preferential orientation of the grains, the lateral etch rate is several times higher than the vertical etch rate. This results in a large undercut under the photoresist etch mask which leads to negative slope or hanging structures at the edge of ZnO [231]. NH4 Cl being a weak acid facilitates etching of ZnO film in a controlled manner [232]. However, etch rate will be high at an elevated temperature because reaction gets accelerated due to the energy from heat. Moreover, etching rate will also be high when concentration is high. Therefore, large undesired undercuts will be produced at high etch rate which may result from either high temperature or high concentration [231]. Kwon et al. [231] proposed an electrolysis mechanism for etching in which they used two parallel copper plates as anode and cathode for electrical current through a 20% NH4 Cl solution and dipped the sample in the solution. Authors observed highly anisotropic etching effect due to the addition of electrolytically added Cu ions since Cu ions controls the H3 O+ formation [231]. Prasad et al. [233] also reported to obtain highly anisotropic etch profiles and positive slope at the edge of ZnO layer using similar set-up. Liu et al. [234] observed the effect of annealing on etching and found that annealing results in smoother side face. Also, O2 annealed samples show a lower etching rate than N2 annealed ones. Also, Ching et al. [228] observed that etching has no effect on the crystallinity of the ZnO film. Lim et al. [218] compared two dry etching methods based on plasma of CH4 (orC2 H6 ) /H2 /Ar and observed that using C2 H6 instead of CH4 increases the ZnO etch rate by approximately a factor of 2. 3.3. Lead zirconate titanate (PZT )

Fig. 4. Sol-gel process flow for PZT deposition.

25%TMAH as etchant at room temperature to etch AlN with Cr patterned to function as both top electrode and mask. Doll et al. [221] used Ti as top electrode as mask for AlN etching with 25% TMAH as etchant. Chen et al. [222] used 10% at 50o C as etchant with 200 nmTi as masking layer. He observed that room temperature deposited AlN has higher undercutting than AlN deposited at temperatures higher than 300o C.Yang et al. [217] reported a dry etching method using Cl2 /BCl3 /Ar at 700 W power and 0.5 Pa pressure. They used SiO2 as a masking layer with Pt bottom electrode acting as an etch stop. The selectivity of AlN/Pt was 3:1 [217]. However, compared to wet etching technique using Ti, it needs an extra oxide masking layer which needs to be stripped after AlN etching so it increases process steps. Although dry etching is highly anisotropic and can produce vertical profiles [227], it is not preferred because highly active Cl2 plasma has low etched selectivity between materials and can cause the underlying damage by ion bombardment. 3.2. Zinc oxide (ZnO) The etching rate in ZnO is dependent on the crystallographic orientation of the ZnO [228]. The etching rate of ZnO (002) is higher than ZnO (103) [229]. Also, Sun et al. [230] have demonstrated that O-terminated ZnO is etched more rapidly and uniformly compared to Zn-terminated ZnO. Most acids (even at a very low concentration of (0.4%) dissolve ZnO. The dissolution is carried out by the

For patterning PZT thin films, ion beam etching (IBE) is the most commonly used dry etching technique. It produces patterns with little under cutting. However, it is difficult to get satisfactory PZT pattern because of the difference in sputtering efficiency of Pb, Zr and Ti [235]. It is also difficult to get reliable PZT pattern since the etch selectivity of PZT over photoresist and Pt has been found to be low in reactive ion etching (RIE) processes as well as high-density plasma assisted techniques such as inductively coupled plasma (ICP) and electron cyclotron resonance (ECR). Moreover, all these dry etching processes are only suitable for films less   than 1m thick because of the slow etch rate 10 − 32nm/min . These processes would become extremely time consuming and expensive for thicker films [223,224,235]. Kim et al. [220] investigated the effect of dry plasma etching on the PZT thin film. Authors used two gas mixtures- Cl2 /CF4 (8/2) with Ar or O2 . It was observed that although Cl2 /CF4 /Ar plasma provides superior etch rate but it also deteriorates the remnant polarization more compared with Cl2 /CF4 /O2 plasma. However, for both gas mixtures, there was surface contamination due to ZrClx . Although good recovery of remnant polarization is possible in case of Cl2 /CF4 /O2 plasma utilizing post-etching annealing in O2 environment but it is not possible when Cl2 /CF4 /Ar plasma is used [220]. On the other hand, wet etching is an effective and popular technique for MEMS owing to its high etching rate, low cost, low surface damage and high selectivity [223,224,235]. PZT can be regarded as a compound of PbO, ZrO2 and TiO2 , that is why etchants containing several compositions are required for PZT thin-film etching. Zheng et al. [235] reported a two-step process in which authors used a mixture BHF: HCl: NH4 Cl : H2 O = 1 : 2 : 4 : 4 as the etchant and an additional dip of 15seconds in HNO3 : H2 O = 2:1 solution to remove the residues produced in the etching. He also observed that due to the addition of NH4 Cl, undercutting decreased signifi-

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Table 5 Techniques for etching Piezo-layers. Piezo-Material

AlN

Etching method

Wet

Dry

ZnO

Wet Dry

PZT

Wet Dry

Etchant

Etch rate(nm/min)

Masking Layer

25% TMAH

22

Cr (60nm) < 2nm/min

[225]

Ti (166 nm) Ti/Cr Ti SiO2 (1m)

[221] [226] [222] [217] [230] [231] [233] [228] [218] [235] [236] [224] [26] [219]

25% TMAH 25% TMAH at 82o C 10% KOH Cl2 /BCl3 /Ar 1 − 15%NH4 Cl 20% NH4 Cl 20% NH4 Cl 10% KOH C2 H6 /H2 /Ar BHF: HCl: NH4 Cl: H2 O (1:2:2: 4:4) HNO3 : BHF: H2 O (4.5:4.55:90.95) BHF + CH3COOH + HNO3 + HCl + NH4 Cl + EDTA + H2 O H2 O : HCl : HF (2 : 1 : 0.02) SF 6 plasma at 300W

600 77.5 1.4-19 190-400 400





Si3 N4 Photoresist

20-25 16 3300 200

Photoresist Photo resist (Shipley 1830) Photoresist Photoresist (AZP 4620)

65

Pt

Ref.

cantly from 5.5:1 to 1.5:1 which showed that NH4 Cl is an effective additive is in decreasing the undercutting of the PZT pattern. Furthermore, the etching rate of the modified etchant increased from 0.013m/s to 0.016m/s [235]. Ezhilvalavan et al. [224] used mixture BHF + CH3COOH + HNO3 + HCl + NH4 Cl + EDTA + H2 O as the etching solution with the etching rate of 200nm/min ute and the side etching ratio was 1.5 : 1. Table 5 is showing a detail of etching methods used for piezolayers. 4. Developed piezoelectric based acoustic devices Piezoelectric thin films offer the advantages of low power, low noise, high frequency and large output.  Most widely  used piezoelectric materials are AlN, ZnO and Pb Zr (1−x) Tix O3 (PZT) [237]. These piezoelectric thin film materials have either the wurtzite (AlN and ZnO) or the perovskite (PZT) crystal structure. The major difference between the two crystal structures is that there is a possibility of reorienting the polarization direction between multiple crystallographic directions (ferroelectricity) in case of perovskite structure. AlN and ZnO are non-ferroelectric materials so the growth parameters of the films need to controlled to align it along the polar direction. On the other hand, in ferroelectric materials, reorientation of the polarization vector along different directions is possible through the application of high electric field along the desired direction. This process is called poling. PZT ceramics being ferroelectric, require poling after deposition, to exhibit a net piezoelectric effect [238]. 4.1. PZT based devices In paper [239], a piezoelectric microphone based on PZT-coated silicon cantilever is presented. The cantilever is composed of the Pt/ PZT / Pt/ Ti/ SiO2 / Si3 N4 / SiO2 / Si multilayer structure. The sensitivity of the fabricated microphones has been measured with a standard microphone and it was found to be 40mV/Pa. Zhu et al. [240] reported an in-plane polarized PZT film based ultrasonic microacoustic device. Here, the piezoelectric PZT films are polarized along in-plane direction by interdigitated (IDT) electrodes. Therefore, the device transduces in d33 -mode. Also, the capacitance of IDT electrodes is much smaller compared to that in conventional sandwich structures. As a result, the voltage reduction because of the large dielectric constant of PZT can be avoided. The device consists of the layers Pt/ PZT / TiO2 / SiO2 . The maximum sensitivity has been reported to 11mV/Pa at the resonance frequency of 44 kHz [240]. Stephen B. Horowitz et al. [241] reported a micromachined piezoelectric microphone used for aero-acoustic applications. It consists of a circular 3m -thick diaphragm of silicon. A thin annular ring of piezoelectric PZT with thickness of 267nm was deposited

Fig. 5. Piezoelectric Microphone: (a) an optical photograph of the piezoelectric microphone and (b) Cross sectional view of the device showing Si diaphragm with circular piezoelectric composite ring (Reprinted with permission from Ref. [241]. Copyright © 2007, Acoustical Society of America).

at the edge of the diaphragm. The PZT layer was sandwiched between a 220nm Ti/Pt layer and a 180nm Pt layer that serve as the bottom and top electrodes, respectively. A 100nm -thick TiO2 layer is used to separate the entire ring structure from the Si membrane. It acts as a diffusion barrier for the PZT during processing. The piezoelectric ring was placed near the clamped boundary as shown in Fig. 5 since this is the region of the largest stress concentrations during deflection which in turn leads to a higher transduction capability and hence higher sensitivity. The Si diaphragm was released by DRIE and the PZT layer was deposited by Sol-gel technique.

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The sensor shows a sensitivity of 0.75V/Pa, a dynamic range of 35.7.8 − 169dB (ref. 20Pa) and a resonant frequency of 59kHz. 4.2. ZnO based devices The earliest microphone reported by Kim et al. [242] consists of ZnO layer with multiple annular electrodes deposited on a 1.4m -thick square diaphragm composed of LPCVD silicon nitride. The sidelength of the reported microphone was 2mm. To minimize residual stress to a low value, authors used Si-rich silicon nitride for the diaphragm instead of stoichiometric material. CVD SiO2 was used to cover the ZnO layer. Buried layers of Poly-Si was used as bottom electrode and Al deposited on SiO2 functioned as top electrode. The sensitivity at 3kHz is about 800V/Pa with the amplifier gain of 20dB (i.e. the gain is 10). The first mechanical resonance of the diaphragm occurs near 30kHz and there is almost no response below 800Hz. Authors argued that the low sensitivity was due to apparent residual compressive strain in the diaphragm structures which arose because of processing [242]. Seung et al. [243] reported a micromachined piezoelectric cantilever transducer, which functions both as a microphone and as a microspeaker. The size of the cantilever is 2000 × 2000 × 4.5m3 . It consists of a zinc oxide (ZnO) piezoelectric thin film on a supporting layer of low-pressure chemical-vapor-deposited (LPCVD) silicon nitride. The measured microphone sensitivity is fairly constant at 30mV/Pa in the low frequency range and rises to 200mV/Pa at the lowest resonant frequency of 890Hz. The 30mV/Pa sensitivity is one of the highest reported value for a microphone with a micromachined diaphragm. Sang et al. [244] also reported a device, which works both as a microphone and a microspeaker. It has a 0.5m thick zinc oxide (ZnO) piezoelectric thin film on a 1.5m thick silicon nitride membrane which is made of LPCVD. The reported maximum deflection in the center of membrane is 1m at 7.3kHz with input drive with a 15V0−P (zero-peak). The reported sensitivity of the microphone was 0.51mV/Pa at 7.3kHz with noise level of 18dBSPL. In [245], authors reported a piezoelectric microphone built on a circular diaphragm (CD). The circular diaphragm was fabricated using the boron etch-stop method. The diameter and thickness of the circular diaphragm are 2mm and 1m, respectively. A 7.4m -thick boron-doped layer was used as a support layer for the circular diaphragm. The resultant wafer stack after fabrication is Si/ SiO2 / Ti/ Pt/ ZnO/ Pt. The reported sensitivity of the device over the range of 400Hz − 10kHz was reported to be 39V/Pa. Arya et al. [246] proposed an acoustic device consisting of a piezoelectric ZnO layer of 2.4m -thick was sandwiched between two Al-top and bottom electrodes. The top electrode was segmented to enhance the sensitivity of the device. Furthermore, a microtunnel of 100m wide and 21m deep is designed to achieve the lower cut-on frequency of 5Hz. The theoretical results show that the sensor has sensitivity (RMS) of 126.3V/Pa and 96.6V/Pa in case of central and outer electrodes respectively. The resonant frequency of 85kHz is obtained from simulation. 4.3. AlN based devices Williams [247] has developed an AlN based microphone in which the diaphragm is composed of passivation, electrode (molybdenum), piezoelectric (aluminum nitride), and structural layers as shown in Fig. 6.a. The fabrication steps as shown in Fig. 6.b can be summarized as- (A) etching of the cavity in silicon wafer, (B) deposition of sacrificial oxide, (C) deposition and pattering of the structural layer, Mo/AlN/Mo stack, and passivation layer, (D) DRIE through backside to form the back cavity and finally and (E) release of the diaphragm via sacrificial oxide etch. The device was reported

Fig. 6. AlN based MEMS acoustic sensor: (a) Microphone structure with a circular diaphragm with an annular electrode/piezoelectric/electrode ring and (b) Fabrication process steps (Reprinted with permission from Ref. [247]. Copyright © 2011, PhD Thesis by M.D. Williams).

to show a sensitivity of 39V/Pa, a dynamic range of 40.4 – 171.6 dB and a bandwidth of 69 Hz – 20 kHz. Another paper [248] reported a monolithic piezoelectric Aluminum Nitride (AlN) MEMS-CMOS microphone for high-sensitivity, low power applications. The MEMS microphone consists of a circular-shaped AlN- SiO2 unimorph membrane. Wafer-scale eutectic bonding of a 0.18 m CMOS buffer to the MEMS microphone was done to fabricate the integrated device. The piezoelectric microphone was formed by 1 m -thick AlN and 1 m -thick SiO2 layers. It provides an average sensitivity of 0.68mV/Pa, a resonance frequency of 11.2 kHz, and a noise floor of 0.03V/Hz. Miles et al. [249] developed an innovative, biomimetic approach utilizing an inherently directional sensing structure inspired by the hearing mechanism of the parasitoid fly Ormia ochracea. This species of fly is capable of detecting a 5kHz mating calls of its cricket host with a 2-degree resolution using two tympana separated by 500 m beneath its head. The enhanced sensitivity is achieved from the mechanical coupling produced by an intertympanal bridge connecting the two tympana. Moreover, the hearing organ when works in rocking mode (i.e. two membranes moving out of phase) then it functions as a pressure gradient microphone. On the other hand,

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floor of 31.35 dB is the lowest reported value for any acoustic device till date [252]. Based on the previous literature, the basic process flow of an acoustic device can be summarized as- (i) selection of Si wafers and cleaning using RCA1 (NH 3 (29%) : H2 O2 (30%) : DI water = 1:1:5) and RCA2 (HCl (37%) : H2 O2 (30%) : DI water = 1:1:6); (ii) fabrication of micro-hole/tunnel for pressure compensation and thereafter, bulk micromachining to release diaphragm; thermal oxidation to grow a thin insulating oxide layer (a layer of TiO2 is needed to act as a Pb diffusion barrier if PZT is used as the piezoelectric layer); (iii) deposition of piezo-layer using sputtering/sol-gel and then annealing, patterning using photolithography and subsequent etching if required; (iv) electrode (Ti/Pt) deposition by sputtering, annealing, patterning using lithography and then etching to create inter-digitated electrodes (IDTs); (v) thin PECVD oxide layer deposition is done for passivation, followed by RIE to open the pads and (vi) then anodic bonding with pyrex glass to seal the cavity. After anodic bonding, dicing, wire bonding and packaging are done at the final stages. The representative fabrication process flow is shown in Fig. 8. 4.4. Comparative study of developed devices

Fig. 7. Labeled SEM of the device and the inset micrograph showing details of pivot structure. (Reprinted with permission from Ref. [8]. Copyright © 2013, American Institute of Physics).

when it works in bending mode (i.e. two membranes moving in phase) then it functions as a pressure microphone (i.e. an omnidirectional microphone). However, the hearing organ performs as a combination of these two basic microphone models. The basic model of microphone mimicking the that of in the fly is realized with a fully released polysilicon plate and a T-shaped torsional beam placed at its center, perpendicular to the plate’s long axis. The torsional beam performs two functions-one is it acts as a pivot to facilitate the model to perform a see-saw rocking mode and the other one is to divide the plate into two diaphragms with the same surface area so that they represent the two identical tympana of the fly Ormia. This facilitates the bending mode operation [250]. Kuntzman et al. [251] developed a PZT sensing microphone with the first resonance frequencies at 13 kHz. The basic principle of operation can be explained in terms of the rotation of a semi-rigid beam about torsional pivots under the influence of pressure gradients along the length of the beam. This results in the fundamental rotational mode which does not respond to uniform pressure. Also, a second, symmetric bending mode results from the finite bending stiffness of the beam. This mode responds to uniform pressure loading. The transduction mechanism is performed by four piezoelectric cantilever springs connected to the ends of the rotational beam with integrated PZT thin films. Fig. 7 is showing the labeled micro graph of the device [8,251]. Recently, Rahaman et al. [252] reported a modified version of the sensor reported in [8,251] by utilizing AlN and inter-digitated electrodes which facilitates d33 mode of operation. The use of AlN lowers the thermomechanical noise which results in better SNR as well as lower noise floor (minimum detectable pressure) measurements in addition to the higher sensitivity because of the d33 mode of operation. Authors here used four layers of piezoelectric materials on the diaphragm instead of using four springs as in the case of Refs. [8,251] and the diaphragm was supported by using two torsional beams. This caused both diaphragms to vibrate freely which resulted in higher sensitivity. Authors reported a sensitivity of 5.43 mV/Pa at an SPL level of 94 dB or 1 Pa. The measured noise

Piezoelectric transduction offers several advantages such as durability, high sensitivity and low noise device which does not require any external power to operate. Capacitive sensors are capable of high sensitivity and dynamic range; however, their high impedance necessitates the use of an impedance buffer close to the sensor. Piezoresistive techniques generally have lower sensitivity and require temperature compensation for accurate measurement. Unlike capacitive techniques, they have a relatively low impedance. However, in contrast to piezoelectric transduction, both capacitive and piezoresistive transduction require some form of external power for operation. For aero-acoustic applications, a microphone with no external power requirement has a key advantage for widespread deployment [241]. Most of today’s MEMS microphones are based on capacitive transduction but they suffer from high risk of stiction between diaphragm and back-plate [248]. Also, the inclusion of off-chip CMOS electronics in these microphones results in the degradation of its sensitivity owing to high parasitic capacitance of the wire bonds [248]. Similarly, the CMOS-MEMS acoustic devices have the advantage of on-chip electronics, but it requires high DC bias volt [244]. On the contrary, piezoelectric microphones are mechanically more robust. There is no requirement of a DC voltage for operation which results in the reduction of the overall power consumption. All these facilitate a more linear response in piezoelectric microphones compared to capacitive ones [253]. Also, many authors have demonstrated the possibility of fabricating the piezoelectric MEMS microphone and CMOS amplifier on the same chip, which lowers the interconnect parasitic [248,254]. Moreover, piezoelectric microphones offer a wider dynamic range and are easier to fabricate [241,255,256]. However, they exhibit relatively low sensitivity. So some researchers tried to increase the sensitivity by reducing the tensile residual stress of a transducer diaphragm using a cantilever with a nitride film [245]. Therefore, diaphragms of piezoelectric microphones can be made circular [26,241,245,248,255,256] or square [242,244,246,253,254,257–259] and cantilever [239,243,260]. Also, Table 6 gives a comparison of different developed devices. 4.4.1. Fully clamped vs cantilever structure Piezoelectric diaphragm-based devices are formed by sandwiching the piezoelectric material between two metals [26] The sensitivity of diaphragm-based MEMS acoustic transducers is a function of the diaphragm deflection which is significantly affected by the residual stress in the diaphragm [242,254,258]. Freeing up

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Fig. 8. Process flow for acoustic sensor fabrication.

Table 6 Comparison of acoustic devices (* -at resonance, ** -at 1 kHz). Piezo-layer

Deposition Technique

Type

Sensitivity

Dynamic Range

Res. Freq. (kHz)

Bandwidth

Year

Ref.

0.3m - Thick ZnO 0.5m -Thick ZnO 0.5m -Thick ZnO 0.5m -Thick ZnO 0.8m -Thick PZT 0.5m-Thick PZT 0.5m- Thick ZnO 1m - Thick PZT 0.8 ␮ m -Thick ZnO 267nm -Thick- PZT 0.8m -Thick ZnO 0.7 ␮ m -Thick PZT 1m -Thick AlN 1.8m -Thick AlN AlN 900nm -Thick PZT 2.4m-Thick ZnO 1m -Thick AlN 0.5m -Thick AlN

— Sputtering Magnetron Sputtering RF magnetron sputtering — Sol-gel Sputtering Sol-gel Sputtering Sol-Gel Sputtering Sol-gel Sputtering — Sputter deposition Sol-gel Sputtering — Sputtering

Square diaphragm Square diaphragm Square diaphragm Cantilever Cantilever Cantilever Square diaphragm Circular diaphragm Circular Diaphragm Circular Diaphragm Circular diaphragm Circular diaphragm Cantilever Circular diaphragm Circular Diaphragm Rectangular shape Square diaphragm Circular diaphragm Rectangular shape

80 V/Pa 1mV/Pa 0.92 mV/Pa 30 mV/Pa 18 − 51mV/Pa 40 mV/Pa ∗ 0.51mV/Pa 97.9 − 920 nV/Pa — 1.66 V/Pa 39.6VPa∗∗ 490 V/Pa 0.588 mV/Pa 27 V/Pa 39 V/Pa 0.61 mV/Pa 96.6− 126.3V/Pa 0.68 mV/Pa 5.45 mV/Pa

— — — Max 100 — — — — — 47.8 − 169 — Noise floor is 34dB upto 128 36-157 40 − 171.6 — — — Noise floor is 31.35 dB

30kHz 16kHz 18kHz 890Hz — — 7.3kHz — 8.6kHz 59kHz 54.8kHz 24.45 kHz 18.4 kHz 176 kHz 129.5 kHz 13kHz 85kHz 11.2 kHz 14.55

100Hz − 40kHz 200Hz − 20kHz — 100Hz − 1kHz — 20Hz − 20kHz — — — up to 6.7 kHz 400Hz − 10kH up to 6.4kHz — 20Hz − 10kH 69Hz − 20kH — ∼22kHz 20Hz − 12kHz 20 Hz-20 kHz

1989 1991 1993 1996 2000 2002 2003 2003 2005 2007 2008 2009 2010 2010 2012 2013 2015 2017 2019

[242] [258] [254] [243] [266] [239] [244] [26] [267] [241] [245] [268] [260] [256] [255] [8] [246] [248] [252]

three edges, thereby forming a cantilever improves the deflection greatly [253]. Lee et al. proposed the cantilever concept for an audio microphone [243]. ZnO was replaced with PZT as the piezoelectric

layer of the cantilever structure so as to improve the sensitivity of the cantilever microphone from 30mV/Pa in [243] to 40mV/Pa in Ref. [239] which is evident from data in Table 6. Microphone

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diaphragms that are clamped on all four edges suffer from residual stress. In comparison to clamped-clamped structure, the cantilever diaphragms are much more compliant. The cantilever is free from the residual stress found in clamped-clamped diaphragms, which decreases the transducer response [243]. Cantilever microphone because of higher deflections produce high sensitivity. However, compared to cantilevers and two-edge clamped diaphragms, fouredge clamped diaphragms have higher operating frequency and enhanced reliability. 4.4.2. Circular vs square structure Due to the anisotropic etch property of most etchants, micromachined Si-wafer based acoustic devices bear an intrinsic limitation regarding its shape. That is why, most of diaphragm designs adopted for microphones are quadrangular. However, Polcawich et al. [26] and Williams et al. [255] reported CD microphones using deep RIE process. Deep RIE produces no stiction/contamination, and structures with very high aspect ratio and smooth sidewall can be obtained. Authors [245] used the boron etch stop method is used to fabricate a circular diaphragm. Higher sensitivity of the circular diaphragms is due to the uniform distribution of the maximum stress along the circumference but in the square diaphragms, the maximum stress distribution is along a part of the edges [245]. Also, from LDV (Laser Doppler Vibrometry) measurement results, authors [245] found that the displacement of the CD microphone was twice as large as that of the SD microphone which resulted in a sensitivity in CD microphone which is 197% higher than that of in SD microphone. However, despite having so many advantages, few works have been reported for circular diaphragms. It is because the release of circular diaphragm is difficult and it is not possible using traditional wet etching techniques. Deep RIE is the possible but it is much costlier compared to wet etching process and also has more operating parameters than wet etching. 4.4.3. d31 vs d33 -modes of transduction and use of IDT electrodes Authors in [240] adopted the interdigitated electrode approach to increase the sensitivity of the square diaphragm sensor for ultrasonic applications. Polarization of the PZT films are done through film thickness to make them piezoelectric. So, they operate in d31 -mode of transduction. However, the large dielectric constant of PZT films results in large capacitance between the top and bottom electrodes which in turn limits the voltage sensitivity. To increase the sensitivity, the piezoelectric PZT films are in-plane polarized by interdigitated (IDT) electrodes which facilitates transduction in d33 -mode. Also, the voltage reduction because of the large dielectric constant of PZT can be evaded since the capacitance of IDT electrodes is much smaller than that of in conventional sandwich structures. Furthermore, the d33 piezoelectric coefficient of PZT film is 2 – 3 times larger than that of the d31 -mode PZT. All of these result in a larger sensitivity in d33 -mode devices than that of in d31 -mode devices. Another advantage of using d33 -mode devices is that by varying the top IDT electrodes configuration, the output capacitances of d33 -mode devices can be varied to facilitate the impedance matching of various interface circuits, without affecting the sensitivity of transducer. On the other hand, in case of d31 -mode devices, thickness of the PZT film needs to be changed to vary the capacitance for impedance matching with interface circuits which would in turn affect the sensitivity. It is evident that among all the devices whether it is cantilever or diaphragm-based structure or square or circular structure, the PZT based devices have the highest sensitivity owing to PZT’s high piezoelectric co-efficient but the major problem with these devices is that it is not compatible with conventional Si fabrica-

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tion technology. ZnO and AlN are quite similar because both are non-ferroelectric [35]. However, AlN has the advantage of being fully compatible with silicon semiconductor technology while ZnO is not. AlN is a large band gap (6 eV) material with a large resistivity, whereas ZnO is really a semiconductor (3.0 eV) with the inherent risks that off-stoichiometry might lead to doping (as e.g. Zn interstitials) during deposition. This may result in an increased conductivity. The dc conductivity results in a high dielectric loss at low frequencies which is harmful for sensors and actuators working at frequencies below about 10 kHz [35]. Moreover, it has been demonstrated in literature that doping with metals such as Sc, yttrium (Y), Cr is an effective way to increase the piezoelectric response of AlN films by almost 500% [261–265].

5. Applications of acoustic devices Majority of piezoelectric microphones reported in the literature [26,242–245,248,252,266] focused in hearing aid applications. Most of them reported maximum SPL sensing of up to 100 dB with a maximum operating frequency of 12 kHz. However, authors in [241,246,255] designed the acoustic sensors primarily for aeroacoustic applications. The sensor developed by Horowitz et al. [241] was to be utilized in the localization and detection of noise produced by aircrafts so as to limit the noise emission from them. The microphone developed by Williams et al. [255] can sense an SPL level of up to 171.1 dB and is intended for aero-acoustic measurements. Similarly, the reported microphone by Arya et al. [246] can sense an SPL level of up to 140 dB and it is intended for both aero-acoustic and auditory applications. The use of piezoelectric based auditory systems has garnered attention in the field of Cochlear Implants (CI). Cochlear implants are done to treat severe hearing loss which results from the destruction of hair cells of the organ of corti in cochlea. Hair cells are present all along the basilar membrane (BM) and they are responsible for converting sound-induced vibration of BM into electricity to stimulate auditory nerves [269]. Although, the current implant technology recovers the patient from deafness by using external electrodes implanted in Cochlea but it suffers from several drawbacks such as the need for frequent battery charging/replacement, need for wearing external components and the risk of damage when exposed to an aqueous medium [270]. To eliminate these drawbacks, piezoelectric materials have been recently used for creating artificial basilar membrane (ABM). The membrane performs the function of frequency selectivity for the cochlea. The ABM is realized by using several thin film piezoelectric cantilever beams, each of which resonates at a specific frequency within the daily acoustic band. The signal generated by the piezoelectric thin film will be processed by the interface circuitry to simulate the auditory nerves in the cochlea [270,271]. Jang et al. [271]reported the use of an AlN based cantilever array to realize the ABM. Author used different sizes of cantilever beams in the array to provide frequency selectivity. The sensor provides a frequency selectivity in the audio range of 2.92–12.6 kHz with an incoming pressure sensitivity of 75−95 dB SPL. Jang et. al [272] also reported another ABM realized by 10-channel SU-8 based AlN cantilever array. Different lengths of cantilever beams are used to mimic the tonotopic characteristics of the cochlea. The sensor’s frequency selectivity is in the range of 659.4−2375 Hz which is very close to the normal human communication frequency range of 300−3000 Hz. Ilik et. al. [270] reported a PZT based array consisting of 8 cantilever beams. Each of these beams resonates at a specific frequency within the acoustic band (250–5000 Hz). The author reported a voltage output of 114 mV when excited at 110 dB Sound Pressure Level (SPL) at 1325 Hz. Recently, Zhao et al. [273] reported another AlN based acoustic transducer which consists of an array of 4 cantilever

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beams. The device has been reported to show a linear voltage variation when excited by an SPL of 80−95 dB within the frequency band of 1−14 kHz. 6. Characterization and testing of piezoelectric based acoustic devices 6.1. Characterization of piezoelectric thin films Thin films can be characterized using XRD, AFM, EDX, SEM and FESEM. 6.1.1. Commonly used techniques Different techniques are utilized to determine the composition, grain size, surface roughness, thickness etc. of thin films. Brief Illustrations of different techniques are given below. 6.1.1.1. X-ray diffraction technique. X-ray diffraction (XRD) is nondestructive characterization technique which can be used to determine the crystal structure of solids, lattice constant, crystallite size. It can also be used to identify unknown materials, study orientation of single crystals and analyze the defects and strain in the lattice etc. [274]. This technique is based on the Bragg’s equation [275]: m = 2dsin

(4)

where ’’ is the monochromatic X-ray wavelength,  is the angle of diffraction from the lattice plane which is spaced with a distance d and m is an integer which indicates the order of reflection. The penetration depth of X-rays is often found in the 10 – 100 m range. In most thin film investigations, since the thickness is substantially lower so it causes a large fraction of the diffractogram to stem from the substrate. Therefore, for the analysis of thin films, the primary X- ray beam should enter the sample under very small angles of incidence. This small angle of incidence known as grazing incidence X-ray diffraction angle causes significant increase in the path traversed by the X-rays so that the structural information obtained by the diffractogram stems primarily from the thin film. Typically, X-ray diffraction scans are made on the sample at grazing angles of 0.1 − 0.5o , depending on the thickness of the sample. The prepared test samples can be scanned between a range say 20 – 60◦ in the range in step scanning mode with steps of say 0.05o with a scan speed of 4sec/step. The intensity of diffracted X-ray from the sample is recorded against 2 values and graphs of intensity vs 2 are plotted by the diffractogram. The recorded curves are matched with the ICDD (International Centre for Diffraction Data) to confirm the formation of the desired crystalline phase. The grain size of the crystallites can be calculated from the diffraction peaks using Scherer’s formula [275]: D=

0.9 Bcos

(5)

Here, B is the full width at half maximum (FWHM) of the peak of the intensity vs 2 curve. 6.1.1.2. Atomic force microscopy. The basic operating principle of atomic force microscopy (AFM) is based on the measurement of interacting forces between a probe and the sample. The probe is a sharp tip which is coupled to the end of a cantilever. The cantilever is made of silicon or silicon nitride. The cantilever gets deflected due to the attractive or repulsive forces between the tip and the sample surface whenever AFM scans a sample [276]. Typical end radius of the tip is in the range of 2−20 nm. The scanning motion is conducted by a piezoelectric tube scanner which scans the tip in a raster pattern with respect to the sample or scans to the sample with respect to the tip. The bending is detected by using a laser beam

that reflects on the cantilever back side into a split photodetector. Changes in the cantilever deflection or oscillation and amplitude are determined by detecting the difference in the photodetector output voltages [277]. AFM can scan under three different modes: contact, non-contact and tapping mode, also known as intermediate or oscillating mode [276]. In contact-mode AFM, the change in cantilever deflection is monitored using the split photodiode detector when the scanning probe scans the sample surface. A constant cantilever deflection is maintained through a feedback loop by vertically moving the scanner so as to generate a constant photodetector difference signal. A computer stores the distance moved by the scanner vertically at each x, y data point to form the topographic image of the sample surface. This feedback loop maintains a constant force in the of 0.1–100 nN during imaging. On the contrary, when the sample is very sensitive and can be influenced by tip-sample interactive forces then non-contact mode is preferred [278]. Tapping mode AFM is a hybrid between contact mode and non-contact mode (NC -mode). In this mode, the cantilever is made to oscillate at its resonance frequency which is in kHz and the surface is lightly tapped during scanning [279]. The advantage of the Tapping mode over contact mode is that it eliminates the lateral shear forces present in contact mode. Therefore, image of soft, fragile and adhesive surfaces can be obtained in Tapping mode without damaging them which is not possible in contact-mode AFM [277]. It is also more effective than NC-AFM in coping with topographies with greater variations in height [278]. Major advantage of AFM is that it images the sample in three dimensions and thus allows the characterization of height of samples even in nano-range. AFM can be used to calculate changes in roughness, surface area variations due to differences in deposition parameters and also the thickness of the thin films. On the other hand, SEM can be used to image high aspect ratio structures such as trenches, via holes, cavities and also undercuts which is not possible to image using AFM. Another difference between the two types of microscopy is the environment of their operation. SEM is operated in a vacuum environment [280] whereas AFM is conducted in an ambient or fluid environment. SEM requires conductive coatings to image poorly conductive surfaces without sample charging [257,280] which may deteriorate the sample. In this respect, AFM is advantageous compared to SEM since it can image insulating surfaces at high resolution in fluid. The image and scanning mechanism is not disturbed by the presence of the fluid in case of AFM since AFM technique is not based on conductivity. However, using environmental electron scanning microscopy, non-conducting samples can also be measured without any requirement of the conducting coating [277].

6.1.1.3. Energy dispersive X-ray spectroscopy. Detection of elemental composition of substance can be performed by EDX spectroscopy utilizing a scanning electron microscope. It is capable of detecting elements that possess an atomic number which is higher than that of boron [281]. In a typical scanning electron microscope, the samples interact with the primary electron (PE) beam and produce characteristic X-rays. Due to the impact of the PEs, electrons are knocked out of the inner electron shells of an atom. Since the atom enters in an excited energy state, the empty electron orbital is filled by an electron from the outer shell (higher energy) of the atom [280]. This electron releases the energy difference through the emission of a photon. This emitted characteristic X-ray can be considered as a “finger print” of the atom that takes part in the interaction since the energy difference depends on the electron configuration of the atom. Since no two elements in the sample can have the same X -ray emission spectrum so they can be differentiated and measured for their respective concentration in the sample [282]. Therefore, the collection of characteristic X-ray spec-

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tra allows the analysis of the elemental composition of the sample at the spot of investigation. 6.1.1.4. Scanning electron microscopy Scanning electron microscopy (SEM) is used to study the features of a specimen at very high magnification by focusing a beam of highly energetic electrons on a spot of the specimen. The interactions of the electron beam and specimen produce signals which are acquired to form the image [281]. These interactions are divided into elastic and inelastic interactions. The deflection of the incident electron by the specimen atomic nucleus or by outer shell electrons of similar energy give rise to elastic scattering. There is a negligible energy change due to collision and scattered electrons undergo a wide-angle directional change. Incident electrons that are elastically scattered through an angle of more than 90o are called back-scattered electrons (BSE). These electrons produce a useful signal for imaging the sample [257]. The signal is usually detected by a semiconductor BSE detector which consists of two or four semiconductor diodes. On the other hand, inelastic scattering is the result of a variety of interactions between the incident electrons of the primary focused beam and the electrons and atoms of the sample. The primary beam electron transfers considerable amount of energy to an atom of the specimen. This leads to the generation of secondary electrons (SE) due to the excitation of specimen electrons during the ionization of the specimen atoms. These secondary electrons possess energies of less than 50 eV and can be used to image or analyze the sample [283]. SEs are usually collected and amplified by means of an Everharte Thornley detector [280]. A number of other signals are also produced when an electron beam strikes a sample in addition to those signals that are utilized to form an image. These signals include the emission of characteristic X-rays, Auger electrons, and cathodoluminescence [280]. The electron source used to generate the electron beam is usually either tungsten (W ) filament or solid-state crystal. The tungsten electron filament is a widely used electron source owing to its low price, high reliability, and suitability for low magnification imaging [257]. The solid-state crystal sources include lanthanum hexaboride (LaB6 ) or cerium hexaboride (CeB6 ) [280]. These sources have higher brightness output compared to the tungsten filament. Another advantage of using solid state sources is that they give longer life hours than the tungsten filament due to their lower work function and can produce a higher rate of emission than the tungsten filament with the same amount of supplied current [281]. A conductive sample or sample surface is a prerequisite in all investigations using a standard SEM setup (i.e., high vacuum in the microscope chamber). For imaging non-conductive samples such as polymers, biological materials etc., a conductive coating is necessary to avoid charging of the sample which would otherwise make SEM observation impossible. Typically, a coating of 5−20 nm layers of carbon, gold, platinum, or chromium is sputter deposited [280]. 6.1.1.5. Field emission scanning electron microscopy (FESEM) FESEM images are used to generate higher resolution with a clearer image quality compared to SEM. The major difference between SEM and FESEM is that SEM uses thermionic electron sources whereas FESEM uses field emission gun (FEG) which emits electron beams using a potential gradient. The FEG consists of a single filament of W with a pointed sharp tip as the electron source [281]. A resolution of up to 2−3 nm can be achieved due to decrease in the interaction volume. The decrease in interaction volume is attributed to the low-energy electron beam. This results in better resolution with enhanced surface characters [284]. In field emission guns (FEGs), free electrons are generated by extraction from the material by quantum tunneling. When a material is subjected to a sufficiently strong electric field under high vacuum, it will emit electrons in the region of maximum field strength [280]. FEG can be

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Table 7 Summary of applications of characterization techniques. Characterization Technique

Applications

XRD

Done to confirm the phase, preferred orientation of the solids, grain size of the crystallites Done to measure the surface roughness, thickness of thin films etc. Composition of the compound thin film i.e. ratios of each element present in the compound. Inspection of the surface topography of the film, fabricated cavities, undercuts, trenches etc. Resolution and brightness, thousand times higher than SEM, high resolution images are used to study cross-sectional morphology of multilayer structure, texture of thin films etc.

AFM EDX SEM FESEM

categorized into cold field emission (CFE) and thermal field emission (Schottky emission (SE) sources) [282]. In CFE, the electron emission depends purely on the electric field between the electrodes and it occurs at room temperature. High brightness can be achieved due to the small diameter of the electron beam and emission area, despite the low current of the beam. However, unstable current emission can occur due to the formation of layers of absorbed gases on the tip of the FEG after prolonged use [281]. Schottky emission gun utilizes the Schottky effect which causes the emission of electron from the heated (∼1800K) tungsten tip due to the lowering of potential barrier under a strong electric field. The surface of the tip is covered with a thin layer of ZrO to decrease of the work function of the tip (∼2.7eV ) so as to facilitate easy emission of electrons. In comparison to the thermionic sources, FEG can produce much higher brightness (1000 times higher), much lower energy spread (∼0.7eV only compared to 3eV in case of thermionic sources) and a much lower spot size which in turn facilitate higher source resolution FESEM images [285]. However, the electron   must be placed in ultrahigh vacuum environment ∼10−8 Pa during operation to ensure electron stability and inhibit cathode contamination. Table 7 below summarizes various techniques that can be used to characterize the thin films. 6.1.2. Utilization of techniques for characterization We have deposited a 1.8 m -thick ZnO thin film using reactive RF magnetron sputtering. We have characterized the thin film using XRD, EDX, AFM and FESEM to elucidate the utility of these techniques. 6.1.2.1. XRD characterization. As can be seen from the Fig. 9, the diffraction peak is located around 34.4◦ is indicative of highly caxis-preferred orientation. Authors has obtained full width at half maximum (FWHM) of the ZnO (002) diffraction from the XRD pattern and it is 0.525◦ for the deposited film. The grain size in the ZnO thin film has been calculated using the Debye-Scherer formula using equation (9) and for , equal to 0.154056 nm for the Cu target used in the generation of the X-ray. The obtained ZnO grain size was 16.54 nm. 6.1.2.2. EDX characterization. It was done to confirm the chemical composition of the ZnO film. It revealed that the ZnO thin film consists of Zn and O elements only. The EDX spectrum indicated that the atomic ratio of Zn/O in the ZnO film is 42.6:57.4. Fig. 10 is showing the EDX spectrum. 6.1.2.3. AFM characterization. The columnar texture of ZnO thin film is evident from 2D image [Fig. 11.a)] as well as the 3D AFM image [Fig. 11.b]. Also, it shows that the surface is dense. The roughness analysis from the AFM showed that the surface roughness (RMS value) is 4.19 nm.

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Since externally applied electric field is zero in piezoelectric devices so equation (11) can be modified toD = d31 

(8)

The induced electrical charge (Q ) can be related to the electrical displacement (D) as: Q = DA

(9)

Where A is the area of the square diaphragm. Substituting Eq. (12) into Eq. (13) will give [286]:

a  Q = d31

a

dxdy 0

(10)

0

Here a is the side length of the square diaphragm. Voltage (V ) is related to induced charge (Q ) as: V=

Fig. 9. XRD pattern of the ZnO thin film.

6.1.2.4. FESEM characterization. The dense texture of ZnO thin film as evident from Fig. 12 indicates high quality c-axis-oriented film. Therefore, the findings of AFM and XRD have been confirmed by FESEM. 6.2. Testing & measurement techniques of the performance parameters of acoustic device

6.2.1. Frequency response & sensitivity The sensitivity (S) reflects the ability of a diaphragm to convert a pressure (P) to an electric voltage (V ). V P

(6)

For piezoelectric composite diaphragm, the electrical field displacement D in Z -direction (the thickness direction) is related to the Z -direction electrical field (E) and average stress (), which is generated by the piezoelectric composite diaphragm bending as follows: D = εE + d31 

(7)

where ε and d31 are the permittivity and piezoelectric (strain) coefficient of the piezoelectric film, respectively.

(11)

Here a and t are the side length and thickness of the square piezoelectric film respectively and εo is the permittivity of vacuum and εr is the relative permittivity of the piezoelectric material Therefore, sensitivity can be obtained from equations (6), (10) and (11) as [286]: d31 t S= εo εr AP

To test the performance of a fabricated microphone, various metrics are considered which include frequency response and sensitivity, operation bandwidth, dynamic range, resonance frequency.

S=

Qt εo εr A

a 

a

dxdy 0

(12)

0

Equation (17) shows that voltage sensitivity is directly proportional to the stress,  (expressed here as function of diaphragm dimensions), piezoelectric film thickness, t; piezoelectric co-efficient, d31 and inversely proportional to the dielectric constant of piezoelectric film, εr . That is why even though PZT has a very high d31 , its sensitivity is reduced because of its even higher dielectric constant (maximum∼1300). The stress  in the d31 mode diaphragm and the d33 mode diaphragm are similar.  t  in the d31 mode diaphragm is the thickness of the diaphragm. Although increasing thickness can produce a larger voltage but it also reduces the flexibility of the diaphragm. On the other hand,  t  in the d33 mode diaphragm is the distance between two neighboring electrodes. It can be configured to obtain a value much larger than the thickness of the diaphragm. That is why, d33 mode diaphragm has a sensitivity much larger than that of in d31 mode diaphragm [287]. Variation of this sensitivity with the frequency of the applied input pressure gives the frequency response of the acoustic device. The region of the frequency response that is approximately flat is

Fig. 10. EDX analysis of ZnO thin film.

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Fig. 12. The FESEM image of the ZnO thin film.

Fig. 13. Frequency and phase response of an acoustic sensor.

Fig. 11. AFM characterization of ZnO thin film: (a) AFM image and roughness analysis of the ZnO thin film and (b) 3D - AFM image of the ZnO thin film.

known as flat band as shown in Fig. and its corresponding magnitude value is termed as sensitivity, measured in Volt/Pa or often dB (ref. 1 V/Pa). It relates the output voltage to the input pressure. The total frequency range corresponding to the flat band within some tolerance, usually ±3 dB (or sometimes ±2 dB), is known as the bandwidth [288]. The lower end of the bandwidth at f−3dB is the cut-on frequency, while f+3dB is the cut-off frequency. Lower cut off frequency is dictated by the presence of acoustic holes in the structure, transduction mechanism, and/or interface electronics [246]. The lower cut-off frequency is dictated by the resonance behavior of the diaphragm. In practice, microphones can exhibit an infinite number of resonances because they are continuous system with infinite degrees of freedom [289] but in the Fig. 13 only the first or fundamental resonance is shown. The phase of an ideal microphone in the flat band is zero which means there is no lag between input and output. Authors in [26,241,243,255] reported the measurement of the frequency response of piezoelectric microphones (in volts per Pascal) over the audio range. It was done through the comparison of the fabricated microphone with a reference microphone in an acous-

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Fig. 14. Representative PWT set up for the acoustic characterization of a microphone.

tic plane wave tube (PWT). A PWT is a rigid waveguide which is designed to allow the propagation of planar waves propagate below its cutoff frequency fc , which is dependent on cross-sectional geometry and the isentropic speed of sound co of the associated gas which is normally air [241,243], although use of helium has also been reported [255]. The representative set up for the acoustic measurement of a microphone is shown in Fig. 14. Both the reference microphone and test microphone are mounted at the end of the PWT so they are simultaneously exposed to the same pressure for drive frequencies less than fc [241,243,255]. The reference microphone which is normally used in acoustic measurement is a Brüel & Kjær-4138 1/8-inch pressure field microphone [288]. A Brüel & Kjær Type 3560D multichannel portable PULSE system was used to generate the test signal as well as to acquire data. A Techron7540 power amplifier amplified the pseudorandom test signal before it reached a BMS 4590 compression driver. The frequency response was measured using periodic random noise at around 75 dB, ref 20 Pa and 500 spectral averages over a bandwidth from 0 to 6.4 kHz with a 1 Hz bin width which is the considered frequency resolution for the measurement [241]. However, the lower frequency limit of the excitation signal is dictated by the response of compression driver [255]. It is reported that the frequency response of the test microphone is determined as the frequency response function relating the output of the test microphone (in volts) and the output of the calibrated reference microphone (in Pa) and the calculation is performed in the PULSE software [241,243]. To check the linearity of the microphone sensitivity, a single tone signal at 1 kHz has been used to drive the BMS4590 compression driver [290]. Starting from the lowest value, the SPL was increased in steps up to a maximum testable level. However, the measurement of maximum detectable pressure is limited by the output capacity of the compression driver [241]. 6.2.2. Dynamic range (DR) It extends from the minimum detectable pressure pmin to the maximum pressure pmax of the device. It is measured in the units of dB. DR = pmax − pmin

(13)

These sound pressure levels (pmax or pmin ) are normally quantified in the logarithmic scale since they can vary over a wide range. Sound pressure level is defined in the units of dB as [291]:



SPL = 20log10

prms pref



(14)

where prms is the RMS pressure level and pref is a reference pressure. In air, pref is taken as 20 Pa which is the approximate threshold of hearing in the 1–4 kHz range for young persons [291]. Therefore, typical sound pressure levels vary from 0 dB (at the threshold of hearing) to 120 dB (at the threshold of pain) [291]. However, sound

pressure levels associated with, for example, aircraft engines can exceed this threshold by several orders of magnitude up to 160 dB. 6.2.2.1. Maximum detectable pressure. Characterization of microphone’s linearity refers to the pattern of changes in the voltage output of the microphone with SPL. Nonlinearities in the microphone, whether geometric or electronic sets the maximum detectable pressure and is quantified using the total harmonic distortion (THD). It is a ratio between total power of a signal and power of the fundamental frequency, it is expressed in percentage. The definition of THD can be given as [292]: THD =

Power Total − Power Fundamental Power Total

(15)

THD is most commonly measured along with noise (THD + N). In this measurement, the microphone is first excited with a sine wave signal and then power of harmonics is measured by applying notch filter on the fundamental frequency [292]. Therefore, maximum pressure pmax for a microphone is the pressure at which the THD reaches a prescribed value, for example 3% [255]. 6.2.2.2. Minimum detectable pressure. In a microphone, an input pressure that yields an output voltage lower than the noise floor of the microphone cannot be easily detected. Therefore, a microphone’s minimum detectable pressure is defined as the pressure that produces an output signal equivalent to the noise floor. Fig. 15 gives a representative measurement set-up used in [241,255] to obtain noise floor measurements. The Faraday cage is used to triple shield the sensor from potential electromagnetic interference [293]. The output signal of the test microphone was sent to a preamplifier (SRS560) placed in the middle Faraday cage. The preamplifier was set to a gain setting of 1000, with a high pass filter cutoff of 0 .03 Hz and a low pass filter cutoff of 1 MHz. The output of the preamplifier was then routed, via feed-through adapters, out of the Faraday cage to an SRS785 dynamic signal analyzer. By shorting the inputs to the preamplifier and recording the resulting output voltage noise PSD, the noise spectrum of the experimental setup alone was obtained. For a 1 Hz bin width centered at 1 kHz, the output voltage with no acoustic signal applied is 2.02 nV due to just the sensor, which corresponds to an equivalent acoustic pressure of 2.69 mPa or 35.7 dB [241]. 6.2.3. Resonance frequency and displacement of the diaphragm Analytically resonance frequency of a square diaphragm can be calculated as [294]:

  fr =

1

D 2 T + a4 2a2

 (16)

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23

Fig. 15. Noise floor measurement set-up.

where, T , and a are flexural rigidity of the diaphragm, tensile force per unit length (T = r h) where r is the residual stress and h is the total thickness of the diaphragm), diaphragm mass per unit area, and sidelength of the diaphragm, respectively. The flexural rigidity D can be calculated as: D=

∼Y h3



12 1 − ϑ2



(17)

The effective young’s modulus ∼Y can be calculated as: ∼Y =

Y 1 − ϑ2

(18)

Here, ϑ is the Poisson’s ratio and Y is the Young’s Modulus of the diaphragm material. However, limitation of the equation (18) is that it considers diaphragm of a single material but in practice, the diaphragm is of composite in nature. 6.2.3.1. Use of Laser Doppler vibrometer to determine resonance frequency and displacement. To measure the displacement as well as resonant frequency, the packaged microphone is placed under a Polytec scanning laser vibrometer system [295] and is electrically excited with a periodic chirp signal with a bandwidth from say 0 to 200 kHz and with an amplitude of say 5 V. The frequency response was obtained for a single point measurement at the diaphragm center. The resonant frequency will be the frequency at which diaphragm displacement will be highest [26,241,244,245,255]. Fig. 16 shows a representative set-up to measure the displacement of the test sample. 6.2.4. Measurement of piezoelectric properties The piezoelectric co-efficient of the piezoelectric thin  transverse  film d33f can be calculated by using LDV [221]. Doll et al. [221] measured the d33f coefficient at the wafer scale by biasing the AlN film across its thickness and measuring the induced deflection of the top surface as shown in Fig. 17. Authors used high frequencies (800 kHz – 1.2 MHz) to drive the wafer. High frequencies are used to avoid the excitation of the bending modes of the wafer and also to improve the displacement resolution of the LDV. A spectrum analyzer (HP3562A) was used to calculate the amplitude of the bias voltage. The change in film thickness was calculated from the measured velocity and driving frequency using LDV. d33f =

ε3 t = E3 Vbias

(19)

Fig. 16. Representative LDV set-up to measure Displacement & Resonance frequency.

Fig. 17. Set up for measuring d33f .

where ε3 is the the strain and E3 is the electric field in the piezoelectric film in the out-of-plane direction, t is the change in the thickness of the piezoelectric film and Vbias is the electric poten-

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tial applied across it. The clamping effect of the thick Si substrate results in a thin film piezoelectric coefficient (d33f ), smaller than the actual piezoelectric coefficient of the material (d33 ) [51]. Therefore, the actual d33 coefficient can be obtained using the relation [51]: d33 = d33f +

E 2S13 E S11

E + S12

(20)

E , S E and S E are where the elastic compliance parameters are S13 12 11 396 GPa, 108 GPa and 137 GPa respectively and d31 = 2.265pm/V which are taken from [51].

Table 8 Possible failure modes according to field use conditions. Field use condition

Failure Mode

Humidity/Liquid contamination. Particles or dust Temperature Chemicals Shock and Vibration Electrostatic discharge Combined Environment

Sticking membrane Particles between diaphragm & backplate Broken wires, lifted ball bond Corrosion of metallic pads Broken Membrane or Back plate Shorts and non-functional device Sticking membrane, Ruptured membrane/back plate and damaged wires.

7. Reliability Reliability is the probability of operating a device for a given time period under specified conditions without failure. In other words, it reflects the physical performance of the device over time and is considered as a measure of its future dependability and trustworthiness [296]. Reliability issues of microsystems can be categorized into issues related to microstructure itself and issues related to assembly and packaging. Reliability aspects concerning the microstructure arise from inherent mechanical and electrical material properties but it can also result from effects caused by the combination of different materials in a common device [297]. As for example, different coefficients of thermal expansion of different materials used in a microdevice can result in mechanical stress; also, chemical reactions at interfaces can lead to oxidation of metal layers etc. The design of movable structures like membranes should take into consider the maximum deflection and the possibility of surface contact, which may result in sticking or fracture [298]. Tadigadapa et al. [299] has divided the overall reliability of MEMS into reliability associated with the micromachined parts and electronics and the reliability considerations of package and packaging processes. Since most MEMS invariably contain micromachined mechanical structures of some kind (e.g. diaphragm or cantilever beam) for their operation, the thermo-electro-mechanical interaction of the package with the micromachined part greatly affects their overall performance and reliability. The performance of MEMS degrades due since they have normally been omitted in the initial system design considerations and can lead to long term drift and reliability problems. When designing any MEMS device, one must keep in mind the environment for which it is designed to withstand. During the field use, MEMS microphones are exposed to challenging environment. Common environmental conditions which can degrade the performance of a device include temperature, humidity, high-G shock, vibration, electro-magnetic forces, and corrosion. Material issues which can arise from these environmental conditions include stress, fatigue, creep, wave propagation, thermal coefficient of expansion, and grain orientation and size. For example, PZT film is preferred in piezoelectric applications due to its high electromechanical coupling coefficient (k2 ) and piezoelectric coefficient (dij ). However, there is a high probability of contamination in the clean room during its processing due to the presence of lead in this material. Similarly, the performance of ZnO degrades significantly at high temperature due to its low Curie point. However, AlN can retain its properties even at high temperatures. That’s why AlN is a more reliable material compared to the other two in harsh environment. Electrical issues arise from temperature sensitivity and packaging issues which result in shorted or broken leads [300]. Mechanical shock can cause fracture cracks in the structures [301] and it can also cause stiction problems [302], short circuiting [303], wire bond failure [304], and package failure [303]. Wire bond corrosion and subsequent failures are caused by the corrosive gasses and humidity. Although many commonly utilized MEMS devices require hermetic sealing but MEMS microphones need to inter-

act with incident sound pressure waves. Therefore, isolation of the active piezoelectric sensing element from the ambient cannot be done in case of MEMS acoustic devices. It is exposed to airborne impurities and humidity which can harm the normal operation of the microphone [305]. Iyer et. al classified the common mode failures in microphones according to different field-use conditions as shown in Table 8 [301,306]. These failure modes lead to lower sensitivity and hence performance degradation over a long period. Reliability of MEMS microphones have been studied by few authors. Fang and Huang [307] demonstrated the effect of high impact loading. They studied the mechanical reliability of MEMS microphones subjected to very high g shocks and demonstrated the characteristics of diaphragm under shock loadings of up to 30,000 g. Li et al. [308] studied the impact of shock loading on a MEMS microphone using Finite element simulations to identify the potential failure sites on the diaphragm. Authors also observed the effect of mechanical shock by applying shock levels from 1500 g to 80,000 g. Cracks were reported in the diaphragm and backplate at the acceleration level of 65,000 g in the Z+ and Z- directions [301,308]. The effect of airborne impurities and humidity on MEMS microphone has been also been studied by Broas et al. [305] who observed galvanic corrosion in thin film Si-MEMS. Li. et. al also studied the reliability of MEMS microphones under a mixed flowing gas (MFG) environment. MFG testing is done to assess the resistance of the device against corrosion caused by different gasses in the atmosphere. Authors observed cracks on both the membrane and backplate after 90 days of MFG test [308]. Lall et al. [309] studied the effect of extreme operating environmental stresses on characteristics of MEMS microphones. Authors used test vehicles with MEMS microphones under three different harsh environmental conditions: high temperature operating life (HTOL) at 125o C at 3.3 V, low temperature storage (LTS) at −35o C and temperature humidity 85o C/ 85% RH at 3.3 V. Authors measured the incremental shift and degradation in output parameters namely distortion, frequency response, power supply rejection capability of IC, frequency vs pressure characteristics and analog output voltage of the MEMS microphone [309]. Various reliability tests like HTOL, THB, LTS etc. can performed to assess the quality of the fabricated and packaged device. Table 9 is showing the different test parameters used in the industries to check the reliability of MEMS microphone under harsh environment [309–311]. Data acquisition duration of each test may vary as per the designed parameters of the fabricated device and field requirements. Details of different tests are given below:

7.1. HTOL High temperature operating life (HTOL) test is done to test the endurance of the microphone sensor in harsh environment. The device is stressed dynamically by subjecting it to the maximum junction temperature, load current and internal power dissipation. The test is performed to investigate typical failure modes like oxide

W.R. Ali and M. Prasad / Sensors and Actuators A 301 (2020) 111756 Table 9 Reliability matrix of a microphone under harsh environment [306,309–311]. Test Condition

Test Parameters

Data Acquisition duration (Hrs.)

HTOL 3.3 V THB 3.3 V LTS HTS TC

125◦ C 8 5◦ C/85% −40◦ C 120◦ C −40◦ C/+125◦ C (850 cycles) cycling: +30 ◦ C/+65 ◦ C RH= +90% 10,000 g / 0.1 ms, 5 shocks for each axis ± 2000 V (3 discharges on all pins)

100-1000 100-1000 100-1000 100-1000 100-1000

MTC MS ESD

100-1000 100-1000 –

faults, metal degradation and wire bond aging under worst case device application stress conditions [309,310].

25

7.7. MS In mechanical shock test, the device is subjected to high g shock levels (10,000 g / 0.1 ms, 5 shocks for each axis). It is done to assess the robustness of the components against moderately severe shocks which arise from suddenly applied forces or abrupt change in motion which may result from handling, transportation or field operation [310,311].

7.8. ESD In electrostatic discharge (ESD) test, the device is subjected to a high-voltage peak on all pins and the resulting ESD stress is simulated according to different simulation models (human body model, machine model, charged device mode). This test is done to classify the device according to its susceptibility to damage or degradation by exposure to electrostatic discharge [306,310].

8. Conclusion and future trends 7.2. THB Temperature humidity bias (THB) test is done to at examine failure mechanisms especially in the die-package surroundings which is caused by harsh wet conditions and electric potential. It investigates typical failure modes such as electro-chemical corrosion [310].

7.3. LTS Low temperature storage (LTS) test is done to investigate the failure mechanisms of the device at extremely cold environments for prolonged time. The device is subjected to a minimum temperature allowed by the package materials which is even lower than the minimum operating temperature [306,310].

7.4. HTS This test is done to investigate the failure mechanisms due to high temperature. The failure mechanisms include wire-bond solder joint aging, data retention faults, and metal stress-voiding. In high-temperature storage (HTS) test, the device is stored in a maximum temperature environment allowed by the package materials. This temperature, sometimes can be even higher than the maximum operative temperature [310,311].

7.5. TC In the temperature cycling (TC) test, the device is subjected to cycled temperature excursions, between a hot and a cold chamber in air atmosphere. This test investigates failure modes related to the thermomechanical stress. This stress arises due to the different thermal expansion of the materials interacting in the die-package system which may result in typical failure modes such as metal displacement, dielectric cracking, wire bond failure etc [310].

7.6. MTC In moisture and temperature cycling (MTC) test, the endurance of the device against the combined effect of moisture and temperature cycles. The device is subjected to cycled temperature and moisture exposure [310].

This review discusses three widely used piezoelectric materialsAlN, ZnO and PZT. It discusses the properties of these three materials as a potential candidate for the development of acoustic sensor for microphone as well as aero-acoustic applications. It also discusses various deposition techniques commonly used for these three materials. Sputtering is preferred among all these established techniques due to its simplicity, good reproducibility, low operating temperature, and compatibility Si-fabrication processing. For AlN and ZnO, RF magnetron sputtering is the most widely used deposition technique whereas for PZT, sol-gel is the most widely used technique. AlN sputtered at low sputtering pressure shows not only excellent crystallographic properties but also the lowest values for both surface roughness and oxygen contamination. In case of PZT, sol-gel is more advantageous compared to sputtering because better control of the material composition in thin film layer is possible in multi-component systems (PZT is a combination of Pb, Zr and Ti). Reliability is an important aspect which needs to be considered for operation of an acoustic sensor in harsh environment such as high temperature, humidity and high-g shocks. Reliability tests like HTOL, THB, LTS, MS and ESD are very much necessary to assess the endurance of the device against different failure modes which may arise from different field use conditions. Among the three materials, AlN has emerged as the most suitable and reliable one for use in harsh environment such as high temperature and humidity environment since it remains chemically stable. However, its piezoelectric response is much lower compared to PZT. Doping is an effective way to increase its piezoelectric response. However, lot of work needs to be done to further improve its piezoelectric output by exploring new doping materials so as to make it comparable to PZT based devices. For its deposition, most commonly used technique is sputtering which is an expensive process. For cost effective development of AlN based acoustic sensors, sol-gel route needs to be explored for its deposition. Techniques should be evolved for low temperature deposition of sol-gel film. Moreover, AlN has a great potential for use in flexible acoustic sensor using substrates such as SU-8, polyimide. However, reported piezoelectric co-efficient is quite low. Substrate material greatly influences the crystallinity and hence the piezoelectric response of thin films. Therefore, new flexible substrate materials need to be explored to improve the piezoelectric response as well as the crystallinity of the films. Many authors also reported high sensitivity, directional acoustic with low noise floor by using the winged diaphragm structure. However, all of them concentrated on low frequency hearing aid

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applications. Therefore, this structure needs to be explored and modified for a high SPL aero-acoustic measurement. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgment The authors acknowledge the research project (Ref. no. EMR/2017/005107) funded by Science & Engineering Research Board (SERB), DST, Govt. of India for the financial support in carrying out this work. The authors thank Director, CSIR-CEERI, Pilani, for encouragement and guidance. They are also thankful to all members of Smart Sensor Area for helpful discussion and support. References [1] W. Pang, H. Zhao, E.S. Kim, H. Zhang, H. Yu, X. Hu, Piezoelectric microelectromechanical resonant sensors for chemical and biological detection, Lab on a Chip 12 (1) (2012) 2944. [2] W. Xu, X. Zhang, S. Choi, J. Chae, A high-quality-factor film bulk acoustic resonator in liquid for biosensing applications, J. Microelectromechanical Syst. 20 (1) (2011), 213220. [3] L. Rufer, G.D. Pasquale, J. Esteves, F. Randazzo, Micro-acoustic source for hearing applications fabricated with 0.35␮m CMOS-MEMS process, Procedia Eng. 120 (2015) 944–947. [4] K. Shin, J. Jeon, J.E. West, W. Moon, A micro-machined microphone based on a combination of electret and field-effect transistor, Sensors (Switzerland) 15 (8) (2015) 20232–20249. [5] J. Lee, J.H. Jeon, C.H. Je, Y.G. Kim, S.Q. Lee, W.S. Yang, J.S. Lee, S.G. Lee, A concave patterned TiN/PECVD-Si3 N4 /TiN diaphragm MEMS acoustic sensor based on a polyimide sacrificial layer, J. Micromechanics Microengineering 25 (12) (2015). [6] M. Prasad, V. Sahula, V.K. Khanna, ZnO etching and micro-tunnel fabrication for high-reliability MEMS acoustic sensor, Ieee Trans. Device Mater. Reliab. 14 (1) (2014) 545–554. [7] R.S. Fazzio, T. Lamers, O. Buccafusca, A. Goel, W. Dauksher, Design and performance of aluminum nitride piezoelectric microphones, TRANSDUCERS and EUROSENSORS’ 07-4th International Conference on Solid-State Sensors, Actuators and Microsystems (2007) 1255–1258. [8] M.L. Kuntzman, J. Gloria Lee, N.N. Hewa-Kasakarage, D. Kim, N.A. Hall, Micromachined piezoelectric microphones with in-plane directivity, Appl. Phys. Lett. 102 (5) (2013). [9] P.R. Scheeper, A.G.H. Van der Donk, W. Olthuis, A review of silicon microphones, Sens. Actuators A Phys. 44 (1994) 111. [10] W.R. Ali, M. Prasad, Design and fabrication of microtunnel and Si-diaphragm for ZnO based MEMS acoustic sensor for high SPL and low frequency application, Microsyst. Technol. 21 (6) (2015) 1249–1255. [11] Y. Hou, M. Zhang, G. Han, C. Si, Y. Zhao, J. Ning, A review: aluminum nitride MEMS contour mode resonator, J. Semicond. 37 (10) (2016). [12] J. Zhou, M. Demiguel-Ramos, L. Garcia-Gancedo, E. Iborra, J. Olivares, H. Jin, J.K. Luo, A.S. Elhady, S.R. Dong, D.M. Wang, Y.Q. Fu, Characterisation of aluminium nitride films and surface acoustic wave devices for microuidic applications, Sens. Actuators B Chem. 202 (2014) 984–992. [13] J. Reboud, Y. Bourquin, R. Wilson, G.S. Pall, M. Jiwaji, A.R. Pitt, A. Graham, A.P. Waters, J.M. Cooper, Shaping acoustic fields as a toolset for microfluidic manipulations in diagnostic technologies, Proc. Natl. Acad. Sci. 109 (38) (2012), 1516215167. [14] X. Du, M. Swanwick, Y. Fu, J. Luo, A. Flewitt, D. Lee, S. Maeng, W. Milne, Surface acoustic wave induced streaming and pumping in 128◦ y-cut LiNbO3 for microfluidic applications, J. Micromechanics Microengineering 19 (3) (2009). [15] Y.F. Li, Y.Q. Fu, S.D. Brodie, M. Alghane, A.J. Walton, Enhanced micro-droplet splitting, concentration, sensing and ejection by integrating electro-wetting-on-dielectrics and surface acoustic wave technologies, 2011 16th International Solid-State Sensors, Actuators and Microsystems Conference, TRANSDUCERS’11 (2011) 2936–2939. [16] R.S. Dahiya, M. Valle, G. Metta, L. Lorenzelli, S. Pedrotti, Deposition, processing and characterization of p(vdf-trfe) thin films for sensing applications, in: Sensors, 2008 IEEE, 2008, pp. 490–493. [17] M. Driscoll, R. Moore, J. Rosenbaum, S. Krischnaswamy, J. Szedon, Recent advances in monolithic film resonator technology, Ultrasonic Symposium (1986) 365–370. [18] F.S. Hickernell, The micro-structural characteristics of thin-film zinc oxide for SAW transducers, Ultrasonic Symposium (1984) 239–242.

[19] M. Shiang Wu, A. Azuma, T. Shiosaki, A. Kawabata, Low-loss ZnO optical waveguides for SAWAO applications, IEEE Trans. Ultrason. Ferroelectr. Freq. Control 36 (4) (1989) 442–445. [20] F.S. Hickernell, Zinc oxide films for acoustoelectronic device applications, IEEE Trans. Ultrason. Ferroelectr. Freq. Control 32 (1985) 621–629. [21] J. Peng, C. Chao, H. Tang, Piezoelectric micromachined ultrasonic transducer based on domeshaped piezoelectric single layer, Microsyst. Technol. 16 (10) (2010) 1771–1775. [22] P. Muralt, S. Member, N. Ledermann, J. Baborowski, A. Barzegar, S. Gentil, B. Belgacem, S. Petitgrand, A. Bosseboeuf, N. Setter, Transducers Based on PZT Thin Films, IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control 52 (12) (2005) 2276–2288. [23] C. Wang, Z. Wang, T.L. Ren, Y. Zhu, Y. Yang, X. Wu, H. Wang, H. Fang, L. Liu, A micromachined piezoelectric ultrasonic transducer operating in d33 mode using square interdigital electrodes, IEEE Sens. J. 7 (7) (2007) 967–975. [24] K. Yamashita, H. Murakami, T. Fukunaga, M. Okuyama, S. Aoyagi, Y. Suzuki, Ultrasonic micro array sensor using piezoelectric PZT thin film and resonant frequency tuning by poling, in: Proceedings of the 13th IEEE International Symposium on Applications of Ferroelectrics, 2002. ISAF 2002, 2002, pp. 487–490. [25] Z. Wang, C. Wang, L. Liu, Design and analysis of a PZT-based micromachined acoustic sensor with increased sensitivity, IEEE Trans. Ultrason. Ferroelectr. Freq. Control 52 (10) (2005) 1840–1850. [26] R.G. Polcawich, M. Scanlon, J. Pulskamp, J. Clarkson, J. Conrad, D. Washington, R. Piekarz, S. Trolier-Mckinstry, M. Dubey, Design and fabrication of a lead zirconate titanate (PZT) thin film acoustic sensor, Integr. Ferroelectr. 54 (November 2014) (2003) 595–606. [27] E. Defay¸, C. Millon, C. Malhaire, D. Barbier, PZT thin films integration for the realisation of a high sensitivity pressure microsensor based on a vibrating membrane, Sens. Actuators A Phys. 99 (1–2) (2002) 64–67. [28] P. Scheeper, J.O. Gullov, L.M. Kofoed, A piezoelectric triaxial accelerometer, J. Micromechanics Microengineering 6 (1) (1996). [29] X.H. Wang, J. Shi, S. Dai, Y. Yang, A sol-gel method to prepare pure and gold colloid doped ZnO films, Thin Solid Films 429 (1-2) (2003) 102–107. [30] H. Jafe, D.A. Berlincourt, Piezoelectric transducer materials, Proc. IEEE 53 (10) (1965) 1372–1386. [31] X.Y. Du, Y.Q. Fu, S.C. Tan, J.K. Luo, A.J. Flewitt, W.I. Milne, D.S. Lee, N.M. Park, J. Park, Y.J. Choi, S.H. Kim, S. Maeng, ZnO film thickness effect on surface acoustic wave modes and acoustic streaming, Appl. Phys. Lett. 93 (9) (2008). [32] H.F. Pang, Y.Q. Fu, L. Garcia-Gancedo, S. Porro, J.K. Luo, F. Placido, J.I. Wilson, A.J. Flewitt, W.I. Milne, X.T. Zu, Enhancement of microfluidic efficiency with nanocrystalline diamond interlayer in the ZnO-based surface acoustic wave device, Microfluid. Nanofluidics 15 (3) (2013) 377–386. [33] V. Lughi, Aluminum Nitride Thin Films for MEMS Resonators: Growth and Characterization, UCSB Doctoral Thesis (June), 2006. [34] S. Shelton, M.L. Chan, H. Park, D. Horsley, B. Boser, I. Izyumin, R. Przybyla, T. Frey, M. Judy, K. Nunan, F. Sammoura, K. Yang, CMOS-compatible AlN piezoelectric micromachined ultrasonic transducers, Proceedings - IEEE Ultrasonics Symposium (2009) 402–405. [35] S. Troiler-Mckinstry, P. Muralt, Thin film piezoelectrics for mems, J. Electroceramics 12 (2004) 7–17. [36] T. Xu, G. Wu, G. Zhang, Y. Hao, The compatibility of ZnO piezoelectric film with micromachining process, Sens. Actuators A Phys. 104 (1) (2003) 6167. [37] T. Adam, J. Kolodzey, C.P. Swann, M.W. Tsao, J.F. Rabolt, The electrical properties of capacitors with AlN gate dielectrics, Appl. Surf. Sci. 175-176 (2001) 428–435. [38] D. Liufu, K.C. Kao, Piezoelectric, dielectric, and interfacial properties of aluminum nitride films, J. Vac. Sci. Technol. A Vac. Surf. Films 16 (4) (1998) 2360–2366. [39] C. Mirpuri, S. Xu, J.D. Long, K. Ostrikov, Low-temperature plasma-assisted growth of optically transparent, highly oriented nanocrystalline AlN, J. Appl. Phys. 101 (2) (2007). [40] M. E. Levinshtein, S. L. Rumyantsev and M. S. Shur, Properties of Advanced Semiconductor Materials GaN, AlN, InN, BN, SiC, SiGe, John Wiley & Sons, Inc (2001). [41] H. Okano, N. Tanaka, Y. Takahashi, T. Tanaka, K. Shibata, S. Nakano, Preparation of aluminum nitride thin films by reactive sputtering and their applications to GHz-band surface acoustic wave devices, Appl. Phys. Lett. 64 (2) (1994) 166–168. [42] H. Takikawa, K. Kimura, R. Miyano, T. Sakakibara, A. Bendavid, P.J. Martin, A. Matsumuro, K. Tsutsumi, Effect of substrate bias on AlN thin film preparation in shielded reactive vacuum arc deposition, Thin Solid Films 386 (2) (2001) 276–280. [43] C. Caliendo, P. Imperatori, E. Cianci, Structural, morphological and acoustic properties of AlN thick films sputtered on Si (001) and Si (111) substrates at low temperature, Thin Solid Films 441 (1-2) (2003) 32–37. [44] Kuo-Sheng Kao, Chung-Jen Chung, Ying-Chung Chen, Tien-Fan Ou, T.-K. Shing, The influence of varied sputtering condition on piezoelectric coefficients of AlN thin films, 14th IEEE International Symposium on Applications of Ferroelectrics (2004) 181–184, 2004. ISAF-04. 2004, Vol. 00. [45] R.C. Turner, P.A. Fuierer, R.E. Newnham, T.R. Shrout, Special Issue on Transducers Materials for high temperature acoustic and vibration sensors: a review, Appl. Acoust. 41 (4) (1994) 299–324. [46] R. Hou, D. Hutson, K.J. Kirk, Y. Qing Fu, AlN thin film transducers for high temperature non-destructive testing applications, J. Appl. Phys. 111 (7) (2012).

W.R. Ali and M. Prasad / Sensors and Actuators A 301 (2020) 111756 [47] Y.Q. Fu, J.K. Luo, X.Y. Du, A.J. Flewitt, Y. Li, G.H. Markx, A.J. Walton, W.I. Milne, Recent developments on ZnO films for acoustic wave-based bio-sensing and microfluidic applications: a review, Sens. Actuators B Chem. 143 (2) (2010) 606–619. [48] P. Muralt, PZT thin films for microsensors and actuators: where do we stand? IEEE Trans. Ultrason. Ferroelectr. Freq. Control 47 (4) (2000) 903–915. [49] R.J. Przybyla, S.E. Shelton, A. Guedes, I.I. Izyumin, M.H. Kline, D.A. Horsley, B.E. Boser, In-air rangefinding with an AlN piezoelectric micromachined ultrasound transducer, IEEE Sens. J. 11 (11) (2011) 2690–2697. [50] M.B. Assouar, O. Elmazria, M. Elhakiki, P. Alnot, Study of structural and microstructural properties of AlN films deposited on silicon and quartz substrates for surface acoustic wave devices, J. Vac. Sci. Technol. B Microelectron. Nanometer Struct. 22 (4) (2004) 1717. [51] F. Martin, P. Muralt, M.-A. Dubois, A. Pezous, Thickness dependence of the properties of highly c-axis textured AlN thin films, J. Vac. Sci. Technol. A Vac. Surf. Films 22 (2) (2004) 361–365. [52] R. Lanz, P. Muralt, Solidly mounted BAW filters for 8 GHz based on AlN thin films, IEEE Symposium on Ultrasonics (2004) 178–181, http://dx.doi.org/10. 1109/ultsym.2003.1293383, 2003, no. 1. [53] A.E. Wickenden, L.J. Currano, T. Takacs, J. Pulskamp, M. Dubey, S. Hullavarad, R.D. Vispute, The effect of microstructure on AlN MEMS resonator response, Integr. Ferroelectr. 54 (2003) 565–574. [54] F. T. Goericke, Aluminum Nitride Sensors for Harsh Environments, Ph.D. thesis, University of California, Berkeley (2013). [55] H.P. Loebl, M. Klee, C. Metzmacher, W. Brand, R. Milsom, P. Lok, Piezoelectric thin AlN films for bulk acoustic wave (BAW) resonators, Mater. Chem. Phys. 79 (2-3) (2003) 143–146. [56] W.C. Shih, Z.X. Zoh, Fabrication of AlN films by RF magnetron sputtering for surface acoustic wave applications, Ferroelectrics 459 (1) (2014) 52–62. [57] Y.Q. Fu, J.K. Luo, N.T. Nguyen, A.J. Walton, A.J. Flewitt, X.T. Zu, Y. Li, G. McHale, A. Matthews, E. Iborra, H. Du, W.I. Milne, Advances in piezoelectric thin films for acoustic biosensors, acoustofluidics and lab-on-chip applications, Prog. Mater. Sci. 89 (2017) 31–91. [58] C. Fei, X. Liu, B. Zhu, D. Li, X. Yang, Y. Yang, Q. Zhou, AlN piezoelectric thin films for energy harvesting and acoustic devices, Nano Energy 51 (2018) 146–161. [59] R.G. Polcawich, J.S. Pulskamp, Lead Zirconate Titanate (PZT) for M/NEMS, Springer International Publishing, Cham, 2017, pp. 39–71. [60] U.C. Kaletta, C. Wipf, M. Fraschke, D. Wolansky, M.A. Schubert, T. Schroeder, C. Wenger, AlN/SiO2 /Si3 N4 /Si (100)-Based CMOS compatible surface acoustic wave filter with - 12.8 dB minimum insertion loss, IEEE Trans. Electron Devices 62 (3) (2015) 764–768. [61] F.U. Hamelmann, Thin film zinc oxide deposited by CVD and PVD, J. Phys. Conf. Ser. 764 (1) (2016) 08. [62] J.P. Atanas, R. Al Asmar, A. Khoury, A. Foucaran, Optical and structural characterization of ZnO thin films and fabrication of bulk acoustic wave resonator (BAW) for the realization of gas sensors by stacking ZnO thin layers fabricated by e-beam evaporation and rf magnetron sputtering techniques, Sens. Actuators A Phys. 127 (1) (2006) 4955. [63] W. Wang, Z. Liu, W. Yang, Y. Lin, S. Zhou, H. Qian, H. Wang, Z. Lin, G. Li, Effect of nitrogen pressure on the properties of AlN films grown on nitrided Al (1 1 1) substrates by pulsed laser deposition, Mater. Lett. 129 (2014) 39–42. [64] H. Fazmir, Y. Wahab, A. Anuar, M. Zainol, M. Najmi, M. Mazalan, M. Md Arshad, Properties of piezoelectric layer deposited by sol-gel method for MEMS sensor applications, Appl. Mech. Mater. 780 (2015) 23–27. [65] L. Znaidi, Sol-gel-deposited ZnO thin films: a review, Mater. Sci. Eng. B Solid. Mater. Adv. Technol. 174 (1-3) (2010) 18–30. [66] T.L. Hu, S.W. Mao, C.P. Chao, M.F. Wu, H.L. Huang, D. Gan, Deposition characteristics of AlN thin film prepared by the dual ion beam sputtering system, J. Electron. Mater. 36 (1) (2007) 81–87. [67] Y.H. Kim, J.H. Lee, Y.K. Noh, J.E. Oh, S.J. Ahn, Microstructural characteristics of AlN thin layers grown on Si (110) substrates by molecular beam epitaxy: transmission electron microscopy study, Thin Solid Films 576 (2015) 61–67. [68] G. Ecke, G. Eichhorn, J. Pezoldt, C. Reinhold, T. Stauden, F. Supplieth, Deposition of aluminium nitride films by electron cyclotron resonance plasma-enhanced chemical vapour deposition, Surf. Coat. Technol. 98 (1-3) (1998) 1503–1509. [69] C.-H. Chao, D.-H. Wei, Synthesis and characterization of high c-axis ZnO thin film by plasma enhanced chemical vapor deposition system and its UV photodetector application, J. Vis. Exp. 104 (2015) 1–15. [70] B.P. Zhang, K. Wakatsuki, N.T. Binh, N. Usami, Y. Segawa, Effects of growth temperature on the characteristics of ZnO epitaxial films deposited by metalorganic chemical vapor deposition, Thin Solid Films 449 (2004) 12–19. [71] A. Yamada, B. Sang, M. Konagai, Atomic layer deposition of ZnO transparent conducting oxides, Appl. Surf. Sci. 112 (1997) 216–222. [72] H.Y. Liu, G.S. Tang, F. Zeng, F. Pan, Influence of sputtering parameters on structures and residual stress of AlN films deposited by DC reactive magnetron sputtering at room temperature, J. Cryst. Growth 363 (2013) 80–85. [73] H.E. Cheng, T.C. Lin, W.C. Chen, Preparation of [0 0 2] oriented AlN thin films by mid frequency reactive sputtering technique, Thin Solid Films 425 (1-2) (2003) 85–89. [74] N. Izyumskaya, Y.I. Alivov, S.J. Cho, H. Morkoc¸, H. Lee, Y.S. Kang, Processing, structure, properties, and applications of PZT thin films, Crit. Rev. Solid State Mater. Sci. 32 (3-4) (2007) 111–202.

27

[75] J.W. Hoon, K.Y. Chan, J. Krishnasamy, T.Y. Tou, D. Knipp, Direct current magnetron sputter deposited ZnO thin films, Appl. Surf. Sci. 257 (7) (2011) 2508–2515. [76] X.D. Gao, E.Y. Jiang, H.H. Liu, G.K. Li, W.B. Mi, Z.Q. Li, P. Wu, H.L. Bai, Fabrication and characterization of orientated grown AlN films sputtered at room temperature, Physica Status Solidi (A) Applications and Materials Science 204 (4) (2007) 1130–1137. [77] A. Kumar, M. Prasad, V. Janyani, R.P. Yadav, Fabrication and simulation of piezoelectric aluminium nitride based micro electro mechanical system acoustic sensor, J. Nanoelectron. Optoelectron. 14 (9) (2019) 1267–1274, 8. [78] Z. Vashaei, T. Aikawa, M. Ohtsuka, H. Kobatake, H. Fukuyama, S. Ikeda, K. Takada, Influence of sputtering parameters on the crystallinity and crystal orientation of AlN layers deposited by RF sputtering using the AlN target, J. Cryst. Growth 311 (3) (2009) 459–462. [79] M. Okuyama, Y. Matsui, H. Nakano, T. Nakagawa, Y. Hamakawa, Preparation of PbTiO3 ferroelectric thin film by RF sputtering, Japanese Journal of Applied Physics, Part 1: Regular Papers and Short Notes and Review Papers 18 (8) (1979) 1633–1634. [80] R. Frunza, D. Ricinschi, F. Gheorghiu, R. Apetrei, D. Luca, L. Mitoseriu, M. Okuyama, Preparation and characterisation of PZT films by RF-magnetron sputtering, J. Alloys. Compd. 509 (21) (2011) 6242–6246. [81] S.B. Krupanidhi, N. Maei, M. Sayer, K. El-Assal, Rf planar magnetron sputtering and characterization of ferroelectric Pb(Zr, Ti)O3 films, J. Appl. Phys. 54 (11) (1983) 6601–6609. [82] P.J. Kelly, J.W. Bradley, Pulsed magnetron sputtering - process overview and applications, Journal of Optoelectronics and Advanced Materials 11 (9) (2009) 1101–1107. [83] W.D. Sproul, D.J. Christie, D.C. Carter, Control of reactive sputtering processes, Thin Solid Films 491 (1-2) (2005) 1–17. [84] J. Lin, W.D. Sproul, J.J. Moore, Z. Wu, S. Lee, R. Chistyakov, B. Abraham, Recent advances in modulated pulsed power magnetron sputtering for surface engineering, Jom 63 (6) (2011) 48–58. [85] K. Sarakinos, J. Alami, S. Konstantinidis, High power pulsed magnetron sputtering: a review on scientific and engineering state of the art, Surf. Coat. Technol. 204 (11) (2010) 1661–1684. [86] G.F. Iriarte, J.G. Rodríguez, F. Calle, Synthesis of c-axis oriented AlN thin films on different substrates: a review, Mater. Res. Bull. 45 (9) (2010) 1039–1045. [87] F. Martin, M.E. Jan, B. Belgacem, M.A. Dubois, P. Muralt, Shear mode coupling and properties dispersion in 8 GHz range AlN thin film bulk acoustic wave (BAW) resonator, Thin Solid Films 514 (1-2) (2006) 341–343. [88] H.C. Barshilia, B. Deepthi, K.S. Rajam, Growth and characterization of aluminum nitride coatings prepared by pulsed-direct current reactive unbalanced magnetron sputtering, Thin Solid Films 516 (12) (2008) 4168–4174. [89] C. Duquenne, P.Y. Tessier, M.P. Besland, B. Angleraud, P.Y. Jouan, R. Aubry, S. Delage, M.A. Djouadi, Impact of magnetron conguration on plasma and film properties of sputtered aluminum nitride thin films, J. Appl. Phys. 104 (6) (2008). [90] S. Berg, T. Nyberg, Fundamental understanding and modeling of reactive sputtering processes, Thin Solid Films 476 (2005) 215–230. [91] D. Guttler, B. Abendroth, R. Grotzschel, W. Moller, D. Depla, Mechanisms of target poisoning during magnetron sputtering as investigated by real-time in situ analysis and collisional computer simulation, Appl. Phys. Lett. 85 (25) (2004) 6134–6136. [92] K. ya Hashimoto (Ed.), RF Bulk AcousticWave Filters for Communications, Artech House, Boston, 2009. [93] Q. Wang, L. Fang, Q. Liu, L. Chen, Q. Wang, X. Meng, H. Xiao, Target voltage hysteresis behavior and control point in the preparation of aluminum oxide thin films by medium frequency reactive magnetron sputtering, Coatings 8 (04) (2018). [94] O. Kluth, G. Schöpe, J. Hüpkes, C. Agashe, J. Müller, B. Rech, Modified Thornton model for magnetron sputtered zinc oxide: film structure and etching behaviour, Thin Solid Films 442 (1-2) (2003) 80–85. [95] Q. Wang, D.C. Ba, Y.C. Zhang, R.H. Jiang, An investigation on hysteresis process of ZnO film deposition and changes of surface morphology and wettability, Mater. Lett. 68 (2012) 320–323. [96] P. Prepelita, R. Medianu, B. Sbarcea, F. Garoi, M. Filipescu, The influence of using different substrates on the structural and optical characteristics of ZnO thin films, Appl. Surf. Sci. 256 (6) (2010) 1807–1811. [97] J.-B. Lee, M.-H. Lee, C.-K. Park, J.-S. Park, Effects of lattice mismatches in ZnO/substrate structures on the orientations of ZnO films and characteristics of saw devices, Thin Solid Films 447 (448) (2004) 296–301. [98] N. Ekem, S. Korkmaz, S. Pat, M.Z. Balbag, E.N. Cetin, M. Ozmumcu, Some physical properties of ZnO thin films prepared by RF sputtering technique, Int. J. Hydrogen Energy 34 (12) (2009) 5218–5222. [99] A.K. Chu, C.H. Chao, F.Z. Lee, H.L. Huang, Influences of bias voltage on the crystallographic orientation of AlN thin films prepared by long-distance magnetron sputtering, Thin Solid Films 429 (1-2) (2003) 1–4. [100] Q.X. Guo, M. Yoshitugu, T. Tanaka, M. Nishio, H. Ogawa, Microscopic investigations of aluminum nitride thin films grown by low-temperature reactive sputtering, Thin Solid Films 483 (1-2) (2005) 16–20. [101] A. Ababneh, U. Schmid, J. Hernando, J.L. Sánchez-Rojas, H. Seidel, The influence of sputter deposition parameters on piezoelectric and mechanical properties of AlN thin films, Mater. Sci. Eng. B Solid. Mater. Adv. Technol. 172 (3) (2010) 253–258.

28

W.R. Ali and M. Prasad / Sensors and Actuators A 301 (2020) 111756

[102] F.S. Ohuchi, P.E. Russell, AlN thin films with controlled crystallographic orientations and their microstructure, Journal of Vacuum Science & Technology A 5 (4) (1987) 1630–1634. [103] L. Vergara, M. Clement, E. Iborra, A. Sanz-Hervás, J. García López, Y. Morilla, J. Sangrador, M.A. Respaldiza, Influence of oxygen and argon on the crystal quality and piezoelectric response of AlN sputtered thin films, Diam. Relat. Mater. 13 (4-8) (2004) 839–842. [104] A.V. Singh, S. Chandra, G. Bose, Deposition and characterization of c-axis oriented aluminum nitride films by radio frequency magnetron sputtering without external substrate heating, Thin Solid Films 519 (18) (2011) 5846–5853. [105] J. Yang, X. Jiao, R. Zhang, H. Zhong, Y. Shi, B. Du, Growth of AlN films as a function of temperature on Mo films deposited by different techniques, J. Electron. Mater. 43 (2) (2014) 369–374. [106] A. Iqbal, G. Walker, L. Hold, A. Fernandes, P. Tanner, A. Iacopi, F. Mohd-Yasin, The sputtering of AlN films on top of on- and of-axis 3C-SiC (111)/Si (111) substrates at various substrate temperatures, J. Mater. Sci. Mater. Electron. 29 (3) (2018) 2434–2446. [107] H. Jin, B. Feng, S. Dong, C. Zhou, J. Zhou, Y. Yang, T. Ren, J. Luo, D. Wang, Influence of substrate temperature on structural properties and deposition rate of AlN thin film deposited by reactive magnetron sputtering, J. Electron. Mater. 41 (7) (2012) 1948–1954. [108] L.I. Maissel, R. Glang, Handbook of Thin Film Technology, McGraw-Hill, New York, 1970. [109] G.G. Stoney, The tension of metallic films deposited by electrolysis, proceedings of the royal society a: mathematical, Physical and Engineering Sciences 82 (553) (1909) 172–175. [110] G.C.A.M. Janssen, M.M. Abdalla, F. van Keulen, B.R. Pujada, B. van Venrooy, Celebrating the 100th anniversary of the Stoney equation for film stress: Developments from polycrystalline steel strips to single crystal silicon wafers, Thin Solid Films 517 (6) (2009) 1858–1867. [111] M. Ohring, The Materials Science of Thin Films, Academic Press, 1992. [112] S.-H. Lee, K.H. Yoon, D.-S. Cheong, J.-K. Lee, Relationship between residual stress and structural properties of AlN films deposited by r.f. Reactive sputtering, Thin Solid Films 435 (1) (2003) 193–198. [113] J. Kar, G. Bose, S. Tuli, Influence of rapid thermal annealing on morphological and electrical properties of rf sputtered AlN films, Mater. Sci. Semicond. Process. 8 (6) (2005) 646–651. [114] J. Kar, S. Mukherjee, G. Bose, S. Tuli, J. Myoung, Impact of post-deposition annealing on surface, bulk and interface properties of rf sputtered AlN films, Mater. Sci. Technol. 25 (8) (2009) 1023–1027. [115] M. Gillinger, M. Schneider, A. Bittner, P. Nicolay, U. Schmid, Impact of annealing temperature on the mechanical and electrical properties of sputtered aluminum nitride thin films, J. Appl. Phys. 117 (6) (2015), 065303. [116] L. Vergara, J. Olivares, E. Iborra, M. Clement, A. Sanz-Hervas, J. Sangrador, Effect of rapid thermal annealing on the crystal quality and the piezoelectric response of polycrystalline AlN films, Thin Solid Films 515 (4) (2006) 1814–1818. [117] S. Chandra, V. Bhatt, R. Singh, RF sputtering: a viable tool for MEMS fabrication, Sadhana -Academy Proceedings in Engineering Sciences 34 (4) (2009) 543–556. [118] Y.H. Hsu, J. Lin, W.C. Tang, RF sputtered piezoelectric zinc oxide thin film for transducer applications, J. Mater. Sci. Mater. Electron. 19 (7) (2008) 653–661. [119] M.-C. Pan, T.-H. Wu, B. Tuan Anh, W.-C. Shih, Fabrication of highly c -axis textured ZnO thin films piezoelectric transducers by rf sputtering, Journal of Materials Science-materials in Electronics- J Mater Sci-Mater Electron 23 (02) (2012). [120] W.J. Jeong, G.C. Park, Electrical and optical properties of ZnO thin film as a function of deposition parameters, Sol. Energy Mater. Sol. Cells 65 (1) (2001) 37–45. [121] R. Ondo-Ndong, G. Ferblantier, F. Pascal-Delannoy, A. Boyer, A. Foucaran, Electrical properties of zinc oxide sputtered thin films, Microelectronics J. 34 (11) (2003) 1087–1092. [122] R.C. Lin, Y.C. Chen, K.S. Kao, Two-step sputtered ZnO piezoelectric films for film bulk acoustic resonators, Appl. Phys. A Mater. Sci. Process. 89 (2) (2007) 475–479. [123] J. Golebiowski, Fabrication of piezoelectric thin film of zinc oxide in composite membrane of ultrasonic microsensors, J. Mater. Sci. 34 (19) (1999) 4661–4664. [124] J. Molarius, J. Kaitila, T. Pensala, M. Ylilammi, Piezoelectric ZnO films by r.f. Sputtering, J. Mater. Sci. Mater. Electron. 14 (5-7) (2003) 431–435. [125] O. Kappertz, R. Drese, M. Wuttig, Correlation between structure, stress and deposition parameters in direct current sputtered zinc oxide films, J. Vac. Sci. Technol. A Vac. Surf. Films 20 (6) (2002) 2084. [126] Y.C. Lin, C.R. Hong, H.A. Chuang, Fabrication and analysis of ZnO thin film bulk acoustic resonators, Appl. Surf. Sci. 254 (13) (2008) 3780–3786. [127] Y.M. Lu, W.S. Hwang, W.Y. Liu, J.S. Yang, Effect of RF power on optical and electrical properties of ZnO thin film by magnetron sputtering, Mater. Chem. Phys. 72 (2) (2001) 269–272. [128] Y. Yoshino, T. Makino, Y. Katayama, T. Hata, Optimization of zinc oxide thin film for surface acoustic wave filters by radio frequency sputtering, Vacuum 59 (2-3) (2000) 538–545. [129] D.J. Kang, J.S. Kim, S.W. Jeong, Y. Roh, S.H. Jeong, J.H. Boo, Structural and electrical characteristics of R.F. Magnetron sputtered ZnO films, Thin Solid Films 475 (2005) 160–165, 1-2 SPEC. ISS.

[130] F.C. van de Pol, F.R. Blom, T.J. Popma, R.f. Planar magnetron sputtered ZnO films I: structural properties, Thin Solid Films 204 (2) (1991) 349–364. [131] Y. Yoshino, Y. Ushimi, H. Yamada, M. Takeuchi, Zinc Oxide Piezoelectric Thin Films for Bulk Acoustic Wave Resonators, Murata Manufacturing Co., Ltd (100), 2003, pp. 2–26. [132] S.S. Lin, J.L. Huang, Effect of thickness on the structural and optical properties of ZnO films by r.f. Magnetron sputtering, Surf. Coat. Technol. 185 (2-3) (2004) 222–227. [133] G. Ferblantier, F. Mailly, R. Al Asmar, A. Foucaran, F. Pascal-Delannoy, Deposition of zinc oxide thin films for application in bulk acoustic wave resonator, Sens. Actuators A Phys. 122 (2) (2005), 184188. [134] W.L. Dang, Y.Q. Fu, J.K. Luo, A.J. Flewitt, W.I. Milne, Deposition and characterization of sputtered ZnO films, Superlattices Microstruct. 42 (1-6) (2007) 89–93. [135] K. Kandpal, N. Gupta, Study of structural and electrical properties of ZnO thin film for Thin Film Transistor (TFT) applications, J. Mater. Sci. Mater. Electron. 28 (21) (2017) 16013–16020. [136] S.-H. Park, B.-C. Seo, G. Yoon, H.-D. Park, Two-step deposition process of piezoelectric ZnO film and its application for film bulk acoustic resonators, J. Vac. Sci. Technol. A Vac. Surf. Films 18 (5) (2000) 2432. [137] E.M. Bachari, G. Baud, S. Ben Amor, M. Jacquet, Structural and optical properties of sputtered ZnO films, Thin Solid Films 348 (1) (1999) 165–172. [138] S. Sharma, C. Periasamy, P. Chakrabarti, Thickness dependent study of RF sputtered ZnO thin films for optoelectronic device applications, Electron. Mater. Lett. 11 (6) (2015) 1093–1101. [139] G. Knuyt, C. Quaeyhaegens, J. D’Haen, L.M. Stals, A model for thin film texture evolution driven by surface energy effects, Physica Status Solidi (B) Basic Research 195 (1) (1996) 179–193. [140] P.B. Barna, M. Adamik, Fundamental structure forming phenomena of polycrystalline films and the structure zone models, Thin Solid Films 317 (1-2) (1998) 27–33. [141] N. Fujimara, T. Nishihara, S. Goto, J. Xu, T. Ito, Control of preferred orientation for ZnOx films: control of self-texture, J. Cryst. Growth 130 (1993) 269–279. [142] A. Cimpoiasu, N.M. van der Pers, T.H. de Keyser, A. Venema, M.J. Vellekoop, Stress control of piezoelectric ZnO films on silicon substrates, Smart Mater. Struct. 5 (6) (1996) 744–750. [143] F. Hickernell, Post-deposition annealing of zinc oxide films, Ultrasonics Symposium (1981) 489–492. [144] F. Hickernell, T. Hickernell, Stress relaxation in triode sputtered zinc oxide films, Ultrasonics Symposium (1983) 311–315. [145] A. Kumar, M. Prasad, V. Janyani, R.P. Yadav, Fabrication and annealing temperature optimization for a piezoelectric ZnO based mems acoustic sensor, J. Electron. Mater. 48 (9) (2019) 5693–5701. [146] Z. Bi, Z. Zhang, P. Fan, Characterization of PZT ferroelectric thin films by RF-magnetron sputtering, J. Phys. Conf. Ser. 61 (1) (2007) 120–124. [147] B. Vilquin, R. Bouregba, G. Poullain, M. Hervieu, H. Murray, Orientation control of rhomboedral PZT thin films on Pt/Ti/SiO2 /Si substrates, Eur. Phys. J. Appl. Phys. 15 (3) (2003) 153–165. [148] D.M. Kim, C.B. Eom, V. Nagarajan, J. Ouyang, R. Ramesh, V. Vaithyanathan, D.G. Schlom, Thickness dependence of structural and piezoelectric properties of epitaxial Pb (Zr 0.52 Ti 0.48 ) O3 films on Si and SrTiO3 substrates, Appl. Phys. Lett. 88 (14) (2006) 10–13. [149] P. Muralt, T. Maeder, L. Sagalowicz, S. Hiboux, S. Scalese, D. Naumovic, R.G. Agostino, N. Xanthopoulos, H.J. Mathieu, L. Patthey, E.L. Bullock, Texture control of PbTiO3 and Pb (Zr, Ti)O3 thin films with TiO2 seeding, J. Appl. Phys. 83 (7) (1998) 3835–3841. [150] B. Jafe, R.S. Roth, S. Marzullo, Piezoelectric Properties of Lead Zirconate-lead Titanate Solid Solution Ceramics, 1954. [151] D. Damjanovic, Ferroelectric, dielectric and piezoelectric properties of ferroelectric thin films and ceramics, Rep. Prog. Phys. 61 (9) (1998) 1267–1324. [152] C.V. Vasant Kumar, R. Pascual, M. Sayer, Crystallization of sputtered lead zirconate titanate films by rapid thermal processing, J. Appl. Phys. 71 (2) (1992) 864–874. [153] H. Hu, C.J. Peng, S.B. Krupanidhi, Effect of heating rate on the crystallization behavior of amorphous PZT thin films, Thin Solid Films 223 (2) (1993) 327–333. [154] R.H.T. Wilke, R.L. Johnson-Wilke, V. Cotroneo, W.N. Davis, P.B. Reid, D.A. Schwartz, S. Trolier-McKinstry, Sputter deposition of PZT piezoelectric films on thin glass substrates for adjustable X-ray optics, Appl. Opt. 52 (14) (2013) 3412. [155] M. Migliuolo, R.M. Belan, J.A. Brewer, Absence of negative ion effects during on-axis single target sputter depositions of Y-Ba-Cu-O thin films on Si (100), Appl. Phys. Lett. 56 (25) (1990) 2572–2574. [156] C.K. Kwok, S.B. Desu, Formation kinetics of PbZrx Ti1-x O3 thin films, J. Mater. Res. 9 (7) (1994) 1728–1733. [157] A.P. Wilkinson, J.S. Speck, A.K. Cheetham, S. Natarajan, J.M. Thomas, In situ X-ray diraction study of Crystallization Kinetics in PbZr1-x Tix O3 , (PZT, x = 0.0, 0.55, 1.0), Chem. Mater. 6 (6) (1994) 750–754. [158] C.K. Kwok, S.B. Desu, Low temperature perovskite formation of lead zirconate titanate thin films by a seeding process, J. Mater. Res. 8 (2) (1993) 339–344. [159] A. Bose, M. Sreemany, S. Bysakh, Role of TiO2 seed layer thickness on the nanostructure evolution and phase transformation behavior of sputtered PZT thin films during post-deposition air annealing, J. Am. Ceram. Soc. 94 (11) (2011) 4066–4077.

W.R. Ali and M. Prasad / Sensors and Actuators A 301 (2020) 111756 [160] C.H. Park, Y.H. Oh, M.S. Won, Y.G. Son, Ferroelectric properties of PZT/TiO2 structure according to the deposition condition of TiO2 interlayer, Integr. Ferroelectr. 75 (November 2014) (2005) 131–137. [161] A. Karuppasamy, A. Subrahmanyam, Studies on the room temperature growth of nanoanatase phase TiO2 thin films by pulsed dc magnetron with oxygen as sputter gas, J. Appl. Phys. 101 (6) (2007). [162] K. Tsuchiya, T. Kitagawa, Y. Uetsuji, E. Nakamachi, Fabrication of smart material PZT thin films by RF magnetron sputtering, method in Micro actuators, Trans. Jpn. Soc. Mech. Eng. Ser. A 71 (701) (2011) 66–72. [163] A.C. Pierre, Introduction to sol-gel processing, in: The International Series in Sol-Gel Processing: Technology & Applications, 1st ed., Springer Science+Business Media, New York, 1998. [164] M.Z. Hu, E.A. Payzant, C.H. Byers, Sol-gel and ultrafine particle formation via dielectric tuning of inorganic salt-alcohol-water solutions, J. Colloid Interface Sci. 222 (1) (2000) 20–36. [165] M. Ghosh chaudhuri, J. Basu, G.C. Das, S. Mukherjee, M. Mitra, T. Ohji, A novel method of synthesis of nanostructured aluminum nitride through sol-gel route by in situ generation of nitrogen, J. Am. Ceram. Soc. 96 (02) (2013). [166] M.A. Nurfahana, Z.Y. Lee, C.Y. Fong, S. Shiong Ng, Growth and characterization of AlN thin film deposited by sol-gel spin coating techniques, in: Solid State Science & Technology Towards an Immersive Breakthrough, Vol. 1107 of Advanced Materials Research, Trans Tech Publications, 2015, pp. 667–671. [167] J. Livage, D. Ganguli, Sol-gel electrochromic coatings and devices: a review, Sol. Energy Mater. Sol. Cells 68 (3-4) (2001) 365–381. [168] Y. Ohya, H. Saiki, T. Tanaka, Y. Takahashi, Microstructure of TiO2 and ZnO films fabricated by the sol-gel method, J. Am. Ceram. Soc. 79 (4) (1996) 825–830. [169] W. Tang, D.C. Cameron, Aluminum-doped zinc oxide transparent conductors deposited by the sol-gel process, Thin Solid Films 238 (1) (1994) 83–87. [170] L. Armelao, Sol-gel synthesis and characterization of ZnO-based nanosystems, Thin Solid Films 394 (1-2) (2001) 90–96. [171] Y. Ohya, H. Saiki, Y. Takahashi, Preparation of transparent, electrically conducting ZnO film from zinc acetate and alkoxide, J. Mater. Sci. 29 (15) (1994) 4099–4103. [172] E. Hosono, S. Fujihara, T. Kimura, H. Imai, Non-basic solution routes to prepare ZnO nanoparticles, J. Solgel Sci. Technol. 29 (2) (2004) 71–79. [173] D. Bao, H. Gu, A. Kuang, Sol-gel-derived c-axis oriented ZnO thin films, Thin Solid Films 312 (1998) 37–39. [174] S. Chakrabarti, D. Ganguli, S. Chaudhuri, Substrate dependence of preferred orientation in solgel-derived zinc oxide films, Mater. Lett. 58 (30) (2004) 3952–3957. [175] T. Tsuchiya, T. Emoto, T. Sei, Preparation and properties of transparent conductive thin films by the sol-gel process, J. Non. Solids 178 (C) (1994) 327–332. [176] M. Ohyama, T. Yoko, S. Sakka, Preparation of ZnO films with preferential orientation by sol-gel method, J. Ceram. Soc. Jpn. 104 (4) (1996) 296–300. [177] M.S. Tokumoto, S.H. Pulcinelli, C.V. Santilli, A.F. Craievich, SAXS study of the kinetics of formation of ZnO colloidal suspensions, J. Non. Solids 247 (1-3) (1999) 176–182. [178] Z. Hu, G. Oskam, P.C. Searson, Influence of solvent on the growth of ZnO nanoparticles, J. Colloid Interface Sci. 263 (2) (2003) 454–460. [179] D. Sun, M. Wong, L. Sun, Y. Li, N. Miyatake, H.J. Sue, Purification and stabilization of colloidal ZnO nanoparticles in methanol, J. Solgel Sci. Technol. 43 (2) (2007) 237–243. [180] K. Foo, M. Kashif, U. Hashim, W.-W. Liu, Effect of different solvents on the structural and optical properties of zinc oxide thin film for optoelectronic applications, Ceram. Int. 40 (1, Part A) (2014) 753–761. [181] K. Nishio, S. Miyake, T. Sei, Y. Watanabe, T. Tsuchiya, Preparation of highly oriented thin film exhibiting transparent conduction by the sol-gel process, J. Mater. Sci. 31 (14) (1996) 3651–3656. [182] S.H. Yoon, D. Liu, D. Shen, M. Park, D.J. Kim, Effect of chelating agents on the preferred orientation of ZnO films by sol-gel process, J. Mater. Sci. 43 (18) (2008) 6177–6181. [183] S. Aydemir, S. Karakaya, Effects of withdrawal speed on the structural and optical properties of sol-gel derived ZnO thin films, J. Magn. Magn. Mater. 373 (2015) 33–39, recent Advances in Nanomagnetism and Spintronics. [184] Y. Natsume, H. Sakata, Zinc oxide films prepared by sol-gel spin-coating, Thin Solid Films 372 (1) (2000) 30–36. [185] Y.S. Kim, W.-P. Tai, S.-J. Shu, Effect of preheating temperature on structural and optical properties of ZnO thin films by sol-gel process, Thin Solid Films 491 (1–2) (2005) 153–160. [186] K.L. Foo, M. Kashif, U. Hashim, S.M. Usman Ali, M. Willander, Study of ZnO thin film on silicon substrate by sol-gel spin coating method for bio-medical application, 2012 International Conference on Biomedical Engineering (ICoBE) (2012) 223–226. [187] M.H. Habibi, M. Khaledi Sardashti, Structure and morphology of nanostructured zinc oxide thin films prepared by dip vs. spin-coating methods, J. Iran. Chem. Soc. 5 (4) (2008) 603–609. [188] B. Shivaraj, H.N. Murthy, M. Krishna, B. Satyanarayana, Effect of annealing temperature on structural and optical properties of dip and spin coated ZnO thin films, procedia materials science 10, 2nd International Conference on Nanomaterials and Technologies (CNT 2014) (2015) 292–300. [189] M. Ohyama, H. Kozuka, T. Yoko, Sol-gel preparation of ZnO films with extremely preferred orientation along (002) plane from zinc acetate solution, Thin Solid Films 306 (1) (1997) 78–85.

29

[190] L. Znaidi, G.J. Soler Illia, R. Le Guennic, C. Sanchez, A. Kanaev, Elaboration of ZnO thin films with preferential orientation by a soft chemistry route, J. Solgel Sci. Technol. 26 (2003) 817–821. [191] M.F. Malek, M.H. Mamat, M.Z. Sahdan, M.M. Zahidi, Z. Khusaimi, M.R. Mahmood, Influence of various sol concentrations on stress/strain and properties of ZnO thin films synthesised by sol-gel technique, Thin Solid Films 527 (2013) 102–109. [192] R. Brenier, L. Ortéga, Structural properties and stress in ZnO films obtained from a nanocolloidal sol, J. Solgel Sci. Technol. 29 (3) (2004) 137–145. [193] M. Dutta, S. Mridha, D. Basak, Effect of sol concentration on the properties of ZnO thin films prepared by sol-gel technique, Appl. Surf. Sci. 254 (9) (2008) 2743–2747. [194] R.D. Yang, S. Tripathy, Y. Li, H.J. Sue, Photoluminescence and micro-Raman scattering in ZnO nanoparticles: the influence of acetate adsorption, Chem. Phys. Lett. 411 (1-3) (2005) 150–154. [195] T. Saidani, M. Zaabat, M.S. Aida, R. Barille, M. Rasheed, Y. Almohamed, Influence of precursor source on solgel deposited ZnO thin films properties, J. Mater. Sci. Mater. Electron. 28 (13) (2017) 9252–9257. [196] J.H. Lee, K.H. Ko, B.O. Park, Electrical and optical properties of ZnO transparent conducting films by the sol-gel method, J. Cryst. Growth 247 (1-2) (2003) 119–125. [197] S.B. Yahia, L. Znaidi, A. Kanaev, J.P. Petitet, Raman study of oriented ZnO thin films deposited by sol-gel method, Spectrochimica Acta-Part A: Molecular and Biomolecular Spectroscopy 71 (4) (2008) 1234–1238. [198] M. Arif, A. Sanger, P.M. Vilarinho, A. Singh, Effect of annealing temperature on structural and optical properties of solgel-derived ZnO thin films, J. Electron. Mater. 47 (7) (2018) 3678–3684. [199] V.B. Patil, S.G. Pawar, S.L. Patil, S.B. Krupanidhi, ZnO nanocrystalline thin films: a correlation of microstructural, optoelectronic properties, J. Mater. Sci. Mater. Electron. 21 (4) (2010) 355–359. [200] P. Sagar, P.K. Shishodia, R.M. Mehra, Influence of pH value on the quality of sol-gel derived ZnO films, Appl. Surf. Sci. 253 (12) (2007) 5419–5424. [201] K.D. Budd, S.K. Dey, D.A. Payne, Sol-gel processing of PT, PZ, PZT, and PLZT thin films, Br. Ceram. Proc. 36 (April) (1985) 107. [202] N. Ledermann, P. Muralt, J. Baborowski, S. Gentil, K. Mukati, M. Cantoni, A. Seifert, N. Setter, {1 0 0}-textured, piezoelectric Pb(Zrx Ti1-x )O3 thin films for MEMS: integration, deposition and properties, Sens. Actuators A Phys. 105 (2) (2003) 162–170. [203] F. Calame, P. Muralt, Growth and properties of gradient free sol-gel lead zirconate titanate thin films, Appl. Phys. Lett. 90 (6) (2007) 1–4. [204] C. Zhu, Z. Yong, Y. Chentao, Y. Bangchao, Investigation the effects of the excess Pb content and annealing conditions on the microstructure and ferroelectric properties of PZT (52-48) films prepared by sol-gel method, Appl. Surf. Sci. 253 (3) (2006) 1500–1505. [205] B.A. Tuttle, R. Schwartz, D. Doughty, J. Voigt, A. Carim, Characterization of chemically prepared PZT thin films, Mrs Proc. 200 (1990) 159–165. [206] M.J. Lefevre, J.S. Speck, R.W. Schwartz, D. Dimos, S.J. Lockwood, Microstructural development in sol-gel derived lead zirconate titanate thin films: the role of precursor stoichiometry and processing environment, J. Mater. Res. 11 (8) (1996) 2076–2084. [207] M. Klee, A.D. Veirman, P.V.D. Weijer, U. Mackens, H.V. Hal, Analytical Study of the growth of polycrystalline titanate thin film, Philips J. Res. 47 (1993) 263–285. [208] M. Yaseen, X. Chen, W. Ren, Y. Feng, P. Shi, X. Wu, W. Zhu, Effect of annealing temperature on ferroelectric electron emission of sol-gel PZT films Ceramics International, Vol. 39, Elsevier, 2013, pp. 471–474. [209] T. Maeder, L. Sagalowicz, P. Muralt, Stabilized platinum electrodes for ferroelectric film deposition using Ti, Ta and Zr adhesion layers, J. Appl. Phys. 37 (Part 1, No. 4A) (1998) 2007–2012. [210] G. Willems, D. Wouters, H. Maes, R. Nouwen, Nucleation and orientation of sol-gel PZT thin films on Pt electrodes, Integr. Ferroelectr. 15 (1997) 19–28. [211] T.J. Garino, M. Harrington, Residual stress in PZT thin films and its effect on ferroelectric properties, Mrs Proc. 243 (1992) 341–347. [212] L. Lian, N.R. Sottos, Effects of thickness on the piezoelectric and dielectric properties of lead zirconate titanate thin films, J. Appl. Phys. 87 (8) (2000) 3941–3949. [213] C.T. Shelton, P.G. Kotula, G.L. Brennecka, P.G. Lam, K.E. Meyer, J.P. Maria, B.J. Gibbons, J.F. Ihlefeld, Chemically homogeneous complex oxide thin films via improved substrate metallization, Adv. Funct. Mater. 22 (11) (2012) 2295–2302. [214] L. Trupina, C. Miclea, L. Amarande, M. Cioangher, Growth of highly oriented iridium oxide bottom electrode for Pb(Zr, Ti)O3 thin films using titanium oxide seed layer, J. Mater. Sci. 46 (21) (2011) 6830–6834. [215] D.W. Wang, M.S. Cao, J. Yuan, Q.L. Zhao, H.B. Li, D.Q. Zhang, S. Agathopoulos, Enhanced piezoelectric and ferroelectric properties of Nb2 O5 modified lead zirconate titanate-based composites, J. Am. Ceram. Soc. 94 (3) (2011) 647–650. [216] L.M. Sanchez, D.M. Potrepka, G.R. Fox, I. Takeuchi, K. Wang, L.A. Bendersky, R.G. Polcawich, Optimization of PbTiO3 seed layers and Pt metallization for PZT-based piezo MEMS actuators, J. Mater. Res. 28 (14) (2013) 1920–1931. [217] J. Yang, C. Si, G. Han, M. Zhang, L. Ma, Y. Zhao, J. Ning, Researching the aluminum nitride etching process for application in MEMS resonators, Micromachines 6 (2) (2015) 281–290. [218] W. Lim, L. Voss, R. Khanna, B.P. Gila, D.P. Norton, S.J. Pearton, F. Ren, Comparison of CH4 /H2 and C2 H6 /H2 inductively coupled plasma etching of ZnO, Appl. Surf. Sci. 253 (3) (2006) 1269–1273.

30

W.R. Ali and M. Prasad / Sensors and Actuators A 301 (2020) 111756

[219] G. McLane, R. Polcawich, J. Pulskamp, B. Piekarski, M. Dubey, E. Zakar, J. Conrad, R. Piekarz, M. Ervin, M. Wood, Reactive ion etching of sol-gel deposited lead zirconate titanate (PZT) thin films in SF6 plasmas Integrated Ferroelectrics, Vol. 37, 2001, pp. 67–74. [220] K.T. Kim, M.G. Kang, C.I. Kim, Study on the etching damage characteristics of PZT thin films after etching in Cl-based plasma, Microelectron. Eng. 71 (3-4) (2004) 294–300. [221] J.C. Doll, B.C. Petzold, B. Ninan, R. Mullapudi, B.L. Pruitt, Aluminum nitride on titanium for CMOS compatible piezoelectric transducers, J. Micromechanics Microengineering 20 (2) (2010). [222] D. Chen, J. Wang, D. Xu, Y. Zhang, The influence of the AlN film texture on the wet chemical etching, Microelectronics J. 40 (1) (2009) 15–19. [223] R.A. Miller, J.J. Bernstein, A novel wet etch for patterning lead zirconate-titanate (PZT) thin-films, Integr. Ferroelectr. 29 (3-4) (2000) 225–231. [224] S. Ezhilvalavan, Z. Zhang, J. Loh, J.Y. Ying, Microfabrication of PZT force sensors for minimally invasive surgical tools, J. Phys. Conf. Ser. 34 (1) (2006) 979–984. [225] S. Saravanan, E. Berenschot, G. Krijnen, M. Elwenspoek, A novel surface micromachining process to fabricate AlN unimorph suspensions and its application for RF resonators, Sens. Actuators A Phys. 130-131 (2006) 340–345, SPEC. ISS. [226] S. Marauska, T. Dankwort, H.J. Quenzer, B. Wagner, Sputtered thin film piezoelectric aluminium nitride as a functional MEMS material and CMOS compatible process integration Procedia Engineering, Vol. 25, Elsevier B.V., 2011, pp. 1341–1344. [227] S.J. Pearton, C.R. Abernathy, F. Ren, J.R. Lothian, P.W. Wisk, A. Katz, C. Constantine, Dry etching of thin-film InN, AlN and GaN, Semicond. Sci. Technol. 8 (2) (1993) 310–312. [228] C.G. Ching, P.K. Ooi, S.S. Ng, M.A. Ahmad, Z. Hassan, H. Abu Hassan, M.J. Abdullah, Fabrication of porous ZnO via electrochemical etching using 10% wt. Potassium hydroxide solution, Mater. Sci. Semicond. Process. 16 (1) (2013) 70–76. [229] J.N. Ding, F. Ye, N.Y. Yuan, C.B. Tan, Y.Y. Zhu, G.Q. Ding, Z.H. Chen, The relationship of etching behavior and crystal orientation of aluminum doped zinc oxide films, Appl. Surf. Sci. 257 (5) (2010) 1420–1424. [230] J. Sun, J. Bian, H. Liang, J. Zhao, L. Hu, Z. Zhao, W. Liu, G. Du, Realization of controllable etching for ZnO film by NH4 Cl aqueous solution and its influence on optical and electrical properties, Appl. Surf. Sci. 253 (11) (2007), 51615165. [231] Jae Wan Kwon, Eun Sok Kim, Fine ZnO patterning with controlled sidewall-etch front slope, J. Microelectromechanical Syst. 14 (3) (2005) 603–609. [232] S.-C. Chang, D.B. Hicks, R.C.O. Laugal, Patterning of zinc oxide thin films, in: Technical Digest IEEE Solid-State Sensor and Actuator Workshop, 1992, pp. 41–45. [233] M. Prasad, R.P. Yadav, V. Sahula, V.K. Khanna, C. Shekhar, Controlled chemical etching of ZnO film for step coverage in MEMS acoustic sensor, J. Microelectromechanical Syst. 21 (3) (2012) 517–519. [234] L.K. Liu, C. Shi, G.Q. Wang, Y. Wang, L. Sun, Comparative surfacing studies of sputtered and thermal annealed ZnO films wet etched with NH4 Cl and acidic etchants, Surf. Coat. Technol. 331 (October) (2017) 85–89. [235] K. Zheng, J. Lu, J. Chu, A novel wet etching process of Pb(Zr, Ti)O3 thin films for applications in microelectromechanical system Japanese Journal of Applied Physics, Part 1: Regular Papers and Short Notes and Review Papers, Vol. 43, 2004, pp. 3934–3937. [236] L. Che, E. Halvorsen, X. Chen, An optimized one-step wet etching process of Pb(Zr0.52 Ti0.48 )O3 thin films for microelectromechanical system applications, J. Micromechanics Microengineering 21 (10) (2011). [237] N. Bassiri-Gharb, Piezoelectric MEMS: Materials and Devices, Springer, Boston, MA, 2008, pp. 413–430, Ch. 20. [238] S. Watanabe, T. Fujiu, T. Fujii, Effect of poling on piezoelectric properties of lead zirconate titanate thin films formed by sputtering, Appl. Phys. Lett. 66 (12) (1995) 1481–1483. [239] L.T. Zhang, T.L. Ren, J.S. Liu, L.T. Liu, Z.J. Li, Fabrication of a cantilever structure for piezoelectric microphone, Japanese Journal of Applied Physics, Part 1: Regular Papers and Short Notes and Review Papers 41 (11 B) (2002) 7158–7159. [240] Y.P. Zhu, T.L. Ren, C. Wang, Z.Y. Wang, L.T. Liu, Z.J. Li, Novel in-plane polarized PZT film based ultrasonic micro-acoustic device, TRANSDUCERS and EUROSENSORS’ 07-4th International Conference on Solid-State Sensors, Actuators and Microsystems (2007) 1291–1294. [241] S. Horowitz, T. Nishida, L. Cattafesta, M. Sheplak, Development of a micromachined piezoelectric microphone for aeroacoustics applications, J. Acoust. Soc. Am. 122 (6) (2007) 3428–3436. [242] E.S. Kim, R.S. Muller, P.R. Gray, Integrated microphone with CMOS circuits on a single chip, in: Technical Digest - International Electron devices Meeting, 1989, pp. 880–883. [243] S.S. Lee, R.P. Ried, R.M. White, Piezoelectric cantilever microphone and microspeaker, J. Microelectromechanical Syst. 5 (4) (1996) 238–242. [244] S.C. Ko, Y.C. Kim, S.S. Lee, S.H. Choi, R.K. Sang, Micromachined piezoelectric membrane acoustic device, Sens. Actuators A Phys. 103 (2003) 130–134. [245] W.S. Lee, S.S. Lee, Piezoelectric microphone built on circular diaphragm, Sens. Actuators A Phys. 144 (2) (2008) 367–373. [246] D. Arya, C.C. Tripathi, M. Prasad, Design and modeling of a ZnO-based MEMS acoustic sensor for aero-acoustic and audio applications, The 2015 2nd

[247] [248]

[249]

[250]

[251]

[252]

[253]

[254]

[255]

[256]

[257]

[258]

[259]

[260] [261] [262]

[263]

[264]

[265]

[266]

[267] [268]

[269]

[270]

[271]

[272]

International Symposium on Physics and Technology of Sensors, No. March, IEEE, Pune (2015) 278–282. M.D. Williams, Development of a Mems Piezoelectric Microphone for Aero-acoustic Applications, Ph.D. thesis, University of Florida (2011). J. Segovia-Fernandez, S. Sonmezoglu, S.T. Block, Y. Kusano, J.M. Tsai, R. Amirtharajah, D.A. Horsley, Monolithic piezoelectric aluminum nitride MEMS-CMOS microphone, TRANSDUCERS 2017 - 19th International Conference on Solid-State Sensors, Actuators and Microsystems (2017) 414–417. R. Miles, D. Robert, R. Hoy, Mechanically coupled ears for directional hearing in the parasitoid fly ormia ochracea, J. Acoust. Soc. Am. 98 (1996) 3059–3070. Y. Zhang, R. Bauer, J.C. Jackson, W.M. Whitmer, J.F.C. Windmill, D. Uttamchandani, A low frequency dual-band operational microphone mimicking the hearing property of ormia ochracea, J. Microelectromechanical Syst. 27 (4) (2018), 667676. M.L. Kuntzman, N.N. Hewa-Kasakarage, A. Rocha, D. Kim, N.A. Hall, Micromachined in-plane pressure-gradient piezoelectric microphones, IEEE Sens. J. 15 (3) (2015) 1347–1357. A. Rahaman, A. Ishfaque, H. Jung, B. Kim, Bio-inspired rectangular shaped piezoelectric mems directional microphone, IEEE Sens. J. 19 (1) (2019) 88–96. H.J. Zhao, T.L. Ren, J.S. Liu, L.T. Liu, Z.J. Li, Fabrication of high-quality PZT-based piezoelectric microphone, TRANSDUCERS 2003 - 12th International Conference on Solid-State Sensors, Actuators and Microsystems, Digest of Technical Papers 1 (2003) 234–237. R.P. Ried, D.M. Hong, R.S. Muller, E.S. Kim, E.S. Kim, Piezoelectric microphone with on-chip CMOS circuits, J. Microelectromechanical Syst. 2 (3) (1993) 111–120. M.D. Williams, B.A. Grifin, T.N. Reagan, J.R. Underbrink, M. Sheplak, An AlN MEMS piezoelectric microphone for aero-acoustic applications, J. Microelectromechanical Syst. 21 (2) (2012) 270–283. M.D. Williams, B.A. Griffin, A microelectromechanical systems-based piezoelectric microphone for aeroacoustic measurements, J. Acoust. Soc. Am. 128 (2010). W. Zhou, R. Apkarian, Z.L. Wang, D. Joy, Fundamentals of scanning electron microscopy (SEM), in: Scanning Microscopy for Nanotechnology: Techniques and Applications, Springer, 2007, pp. 1–40. E. Kim, J. Kim, R. Muller, Improved IC-compatible piezoelectric microphone and CMOS process, TRANSDUCERS’ 91: 1991 International Conference on Solid-State Sensors and Actuators (1991) 270–273. Y. Yang, T.L. Ren, L.T. Zhang, N.X. Zhang, X.M. Wu, J.S. Liu, L.T. Liu, Z.J. Li, Miniature microphone with silicon-based ferroelectric thin films, Integr. Ferroelectr. 52 (2003) 229–235. R.J. Littrell, High performance piezoelectric mems microphones, PhD thesis, University of Michigan (2010). H. Liu, F. Zeng, G. Tang, F. Pan, Enhancement of piezoelectric response of diluted Ta doped AlN, Appl. Surf. Sci. 270 (2013) 225–230. M. Akiyama, T. Kamohara, K. Kano, A. Teshigahara, Y. Takeuchi, N. Kawahara, Enhancement of piezoelectric response in scandium aluminum nitride alloy thin films prepared by dual reactive cosputtering, Advanced materials (Deereld Beach, Fla.) 21 (2009) 593–596. ¨ P. Frach, T. Modes, O. Zywitzki, G. Suchaneck, S. Barth, H. Bartzsch, D. Glöy, G. Gerlach, Magnetron sputtering of piezoelectric AlN and AlScN thin films and their use in energy harvesting applications, Microsyst. Technol. 22 (7) (2016) 1613–1617. P. Mayrhofer, H. Riedl, H. Euchner, M. Stoger-Pollach, P. Mayrhofer, A. Bittner, U. Schmid, Microstructure and piezoelectric response of Yx Al1-x N thin films, Acta Mater. 100 (2015) 81–89. S. Manna, K.R. Talley, P. Gorai, J. Mangum, A. Zakutayev, G.L. Brennecka, V. Stevanovic, C.V. Ciobanu, Enhanced piezoelectric response of AlN via CrN alloying, Phys. Rev. Applied 9 (2018), 034026. T.-L. Ren, L.-T. Zhang, L.-T. Liu, Z.-J. Li, Design of a new ferroelectrics-silicon integrated microphone and microspeaker ISAF 2000. Proceedings of the 2000 12th IEEE International Symposium on Applications of Ferroelectrics (IEEE Cat. No.00CH37076), Vol. 2, 2000, pp. 885–888. W. Seob Lee, Y. Kim, J. Seung Lee, S.W. Lee, S. Lee, Fabrication of Circular Diaphragm for Piezoelectric Acoustic Devices, 2005, pp. 307–310. Z. Wang, J.M. Miao, T. Xu, G. Barbastathis, M.S. Triantafyllou, W. Fang, Micromachined piezoelectric microphone with high signal to noise ratio, TRANSDUCERS 2009 - 2009 International Solid-State Sensors, Actuators and Microsystems Conference (2009) 1553–1556. H. Lee, J. Chung, G. Hwang, C.K. Jeong, Y. Jung, J.-H. Kwak, H. Kang, M. Byun, W. Kim, S. Hur, S. Oh, J. Lee, Sensors: flexible inorganic piezoelectric acoustic nanosensors for biomimetic artificial hair cells (adv. Funct. Mater. 44/2014), Adv. Funct. Mater. 24 (11) (2014). B. Ilik, A. Koyuncuoglu, O. Sardan-Sukas, H. Kulah, Thin film piezoelectric acoustic transducer for fully implantable cochlear implants, Sens. Actuators A Phys. 280 (2018) 38–46. J. Jang, J. Lee, S. Woo, D. Sly, L. Campbell, J.-H. Cho, S. O’Leary, M.-H. Park, S. Han, J.-W. Choi, J.H. Jang, H. Choi, A microelectromechanical system artificial basilar membrane based on a piezoelectric cantilever array and its characterization using an animal model, Sci. Rep. 5 (2015) 12447. J. Jang, J. Hun Jang, H. Choi, Mems flexible artificial basilar membrane fabricated from piezoelectric aluminum nitride on an SU-8 substrate, J. Micromechanics Microengineering 27 (2017), 075006.

W.R. Ali and M. Prasad / Sensors and Actuators A 301 (2020) 111756 [273] C. Zhao, K.E. Knisely, D.J. Colesa, B.E. Pfingst, Y. Raphael, K. Grosh, Voltage readout from a piezoelectric intracochlear acoustic transducer implanted in a living guinea pig, Scientific Reports 9 (12) (2019). [274] K. Inaba, X-Ray thin-film measurement techniques, J. Phys. Ther. Assoc. 24 (1) (2008) 10–15. [275] B.D. Cullity, Elements of X ray Diffraction, 2nd ed., Addison-Wesley Publishing Company Inc., 1978. [276] S. Mourdikoudis, R.M. Pallares, N.T. Thanh, Characterization Techniques for Nanoparticles: Comparison and Complementarity Upon Studying Nanoparticle Properties, 2018. [277] P. Russell, D. Batchelor, J. Thornton, SEM And AFM: Complementary Techniques for High Resolution Surface Investigations, 2001. [278] R.E. Smallman, A.H.W. Ngan, Modern Physical Metallurgy, 8th ed., Elsevier Ltd, 2014. [279] Á. Mechler, J. Kopniczky, J. Kokavecz, A. Hoel, C.G. Granqvist, P. Heszler, Anomalies in nanostructure size measurements by AFM, Physical Review B Condensed Matter and Materials Physics 72 (12) (2005) 1–6. [280] S. Henning, R. Adhikari, Scanning electron microscopy, ESEM, and X-ray microanalysis, in: Microscopy Methods in Nanomaterials Characterization, Elsevier Inc., 2017, pp. 1–30. [281] M. Abd Mutalib, M. Rahman, M. Othman, A. Ismail, J. Jaafar, Scanning Electron Microscopy (SEM) and Energy-Dispersive X-Ray (EDX) Spectroscopy, Elsevier B.V., 2017. [282] J.I. Goldstein, D.E. Newbury, M.R. Joseph, N.W.M. Ritchie, J.Henry J. Scott, D.C. Joy, Scanning Electron Microscopy and X-Ray Microanalysis, 4th ed., Springer, 2018. [283] H. Seiler, Secondary electron emission in the scanning electron microscope, J. Appl. Phys. 54 (11) (1983) R1–R18. [284] R. Gauvin, K. Robertson, P. Horny, A.M. Elwazri, S. Yue, Materials Characterization Using High Resolution Scanning-electron Microscopy and x-ray Microanalysis, 2006. [285] http://www.jeol.co.jp/en/applications/pdf/sm/sem-atoz-all.pdf, Scanning Electron Microscope A to Z. [286] J. Li, C. Wang, W. Ren, J. Ma, ZnO thin film piezoelectric micromachined microphone with symmetric composite vibrating diaphragm, Smart Mater. Struct. 26 (5) (2017), 055033. [287] E. Hong, S.V. Krishnaswamy, C.B. Freidho, S. Trolier-Mckinstry, Micromachined piezoelectric diaphragms actuated by ring shaped interdigitated transducer electrodes, Sens. Actuators A Phys. 119 (2) (2005) 521–527. [288] https://www.bksv.com/-/media/literature/ProductData/bp2030.ashx, 1/8-inch Pressure-field Microphone Type 4138. [289] S.S. Rao, Vibration of Continuous Systems, Wiley, 2007. [290] http://www.bmsspeakers.com, 4590/4590p2 inch compression driver. [291] D.T. Blackstock, Fundamentals of Physical Acoustics, Vol. 109, Wiley, 2000. [292] C. Jaroslaw, Conception, Fabrication and Characterization of a MEMS Microphone, Ph.D. thesis, INSA Lyon (2015). [293] R. Dieme, G. Bosman, T. Nishida, M. Sheplak, Sources of excess noise in silicon piezoresistive microphones, J. Acoust. Soc. Am. 119 (5) (2006) 2710–2720. [294] P.C. Hsu, C.H. Mastrangelo, K.D. Wise, A high sensitivity polysilicon diaphragm condenser microphone, Proceedings MEMS 98. IEEE. Eleventh Annual International Workshop on Micro Electro Mechanical Systems. An Investigation of Micro Structures, Sensors, Actuators, Machines and Systems (Cat. No.98CH36176) (1998) 580–585. [295] https://www.polytec.com/us/vibrometry/technology/, Laser doppler vibrometry. [296] M. Ohring, An Overview of Electronic Devices and Their Reliability, Elsevier Inc., 1998, pp. 1–36. [297] R. Muller-Fiedler, U. Wagner, W. Bernhard, Reliability of MEMS-a methodical approach, Microelectron. Reliab. 42 (2002) 1771–1776. [298] U. Wagner, J. Franz, M. Schweiker, W. Bernhard, R. Müller-Fiedler, B. Michel, O. Paul, Mechanical reliability of MEMS-structures under shock load, Microelectron. Reliab. 41 (9-10) (2001) 1657–1662. [299] S. Tadigadapa, N. Naja, Reliability of microelectromechanical systems (MEMS), Techniques 4558 (2001) 197–205. [300] T. Brown, Harsh military environments and microelectromechanical (MEMS) devices, in: Sensors, 2003 IEEE, 2003, pp. 753–760. [301] J. Li, J. Makkonen, M. Broas, J. Hokka, T.T. Mattila, M. Paulasto-Krockel, J. Meng, A. Dasgupta, Reliability assessment of a MEMS microphone under shock impact loading, 2013 14th International Conference on Thermal, Mechanical and Multi-Physics Simulation and Experiments in Microelectronics and Microsystems, EuroSimE 2013 (2013) 1–6. [302] N. Tas, T. Sonnenberg, H. Jansen, R. Legtenberg, M. Elwenspoek, Stiction in surface micromachining, Journal of Micromechanics and MicroengineeringJ. Micromechanics Microengineering 6 (4) (1996) 385–397.

31

[303] D. Tanner, J. Walraven, K. Helgesen, L. Irwin, F. Brown, N. Smith, N. Masters, MEMS reliability in shock environments, Reliability Physics Symposium, 2000 (2000) 129–138. [304] J. Meng, T. Mattila, A. Dasgupta, M. Sillanpaa, R. Jaakkola, K. Andersson, E. Hussa, Testing and multi-scale modeling of drop and impact loading of complex MEMS microphone assemblies, 2012 13th International Thermal, Mechanical and Multi-Physics Simulation and Experiments in Microelectronics and Microsystems, EuroSimE 2012, IEEE (2012) 1–8. [305] M. Broas, J. Li, X. Liu, Y. Ge, A. Peltonen, T.T. Mattila, M. Paulasto-Kröckel, Galvanic corrosion of silicon-based thin films: a case study of a MEMS microphone, Proceedings - Electronic Components and Technology Conference 2015-July (2015) 453–459. [306] G. Iyer, W. Li, L. Gopalakrishnan, Design, reliability and manufacturing readiness for MEMS microphone, Proceedings of the ASME 2011 Pacific Rim Technical Conference & Exposition on Packaging and Integration of Electronic and Photonic Systems (2011) 369–374. [307] W. Fang, Q. Huang, A study of the mechanical reliability of a MEMS microphone, Proceedings of the International Symposium on the Physical and Failure Analysis of Integrated Circuits, IPFA (2013) 716–719. [308] J. Li, M. Broas, J. Raami, T.T. Mattila, M. Paulasto-Kröckel, Reliability assessment of a MEMS microphone under mixed owing gas environment and shock impact loading, Microelectron. Reliab. 54 (6-7) (2014) 1228–1234. [309] P. Lall, A. Abrol, D. Locker, Effects of sustained exposure to temperature and humidity on the reliability and performance of MEMS microphone, ASME 2017 International Technical Conference and Exhibition on Packaging and Integration of Electronic and Photonic Microsystems InterPACK2017, San Francisco (2017) 1–15. [310] Application Note: Best Practices in the Manufacturing Process of Mems Microphones, 2014 www.st.com. [311] http://vespermems.com/wp-content/uploads/2018/01/Vesper-VM2000Brochure.pdf, Reliability specifications.

Biographies

Washim Reza Ali received his M. Tech degree from Tezpur University, Tezpur, Assam, India in 2013. Currently, he is pursuing Ph.D. under Academy of Scientific & Innovative Research (AcSIR) at CSIR-Central Electronics Engineering Research Institute (CEERI), Pilani, India. His current research interests include deposition, characterization of piezoelectric thin film and development of MEMS based acoustic sensors for aero-acoustic and medical applications.

Mahanth Prasad (Principal Scientist) received his M.Tech. degree from Panjab University, Chandigarh, India, in 2003, and the Ph.D. degree from Malaviya National Institute of Technology (MNIT), Jaipur, India in 2013. He joined the Council of Scientific and Industrial Research–Central Electronics Engineering Research Institute (CSIR-CEERI), Pilani, India, as a Scientist in 2005, where he is currently a Principal Scientist and heading the Transducers and Actuators Group. He has filed one Indian patent and published more than 50 research papers in various reputed international and national journals and conference proceedings. He has supervised several masters and undergraduate student thesis. Recently (2016), he was awarded a BHAVAN (Building Energy Efficiency Higher & Advanced Network) Fellowship by Department of Science and Technology (DST), Govt. of India, and the Indo-U.S. Science and Technology Forum (IUSSTF). He worked as BHAVAN fellow during 2016–2017 at Rensselaer Polytechnic Institute (RPI), New York, USA. Also, he was awarded Raman Research Fellowship (RRF) for the year 2017–2018 by CSIR, New Delhi. He worked as RAMAN fellow in 2018 at Rensselaer Polytechnic Institute (RPI), New York, USA. He is currently engaged in the development of various MEMS-based devices, acoustic sensors, FBAR, Micro-hotplate-based Gas sensors, piezoelectric microphones.