Diamond-based capacitive micromachined ultrasonic transducers

Diamond & Related Materials 22 (2012) 6–11

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Diamond-based capacitive micromachined ultrasonic transducers Baris Bayram ⁎ Department of Electrical and Electronics Engineering, Middle East Technical University, Ankara, Turkey

a r t i c l e

i n f o

Article history: Received 14 August 2011 Received in revised form 23 October 2011 Accepted 30 November 2011 Available online 6 December 2011 Keywords: Capacitive micromachined ultrasonic transducer Ultrananocrystalline diamond Plasma-activated direct bonding Microelectromechanical devices

a b s t r a c t Capacitive micromachined ultrasonic transducers (CMUTs) employing diamond membranes are demonstrated. The design, finite element modeling, microfabrication, and experimental characterization of diamondbased CMUTs are reported. Ultrananocrystalline diamond having a chemical mechanical polished silicon dioxide interlayer deposited via high temperature oxide (HTO) process at 850 °C in a low pressure chemical vapor deposition (LPCVD) furnace is employed as the membrane to form vacuum sealed cavities using plasma-activated direct bonding technology. Electrical impedance, deflection, and transmission measurements of a fabricated CMUT are performed in air using an impedance analyzer, a white light interferometer and a hydrophone, respectively. Experimental results verify the accuracy of finite element modeling. Diamond-based CMUTs possess 3-dB fractional bandwidth of 3% at a center frequency of 1.74 MHz in air. Our experimental results demonstrate diamond as an alternate membrane material for CMUTs, and that diamond can be employed in novel microelectromechanical devices. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Capacitive micromachined ultrasonic transducers (CMUTs) are electromechanical energy conversion devices used to transmit and receive ultrasound. CMUTs fabricated by sacrificial release process mostly feature Si3N4 membranes [1]. The limitations of sacrificial release process to achieve very large membranes without breaking or very small membranes with high fill factor adversely affect the precise engineering of the transducer physical parameters. The Si3N4 membranes are also hard to present well-controlled deflection profiles due to the processdependent residual stress in the membrane material [2]. Waferbonding technology introduced single crystal silicon membranes to be utilized in CMUTS [3]. This technology has significantly reduced the complexity and the time of the processing of CMUTs additionally offering superior process control, high yield, and improved uniformity compared to the already mature sacrificial release process [2]. Best part of wafer bonding technology is to present a well-known silicon crystal material as a membrane, and to achieve vacuum sealed cavities without the need to open etch holes on the membrane, both of which directly translate into reliable operation in immersion. Ultrasound applications require the transducer surface to be in contact with the acoustic medium. Because the surface is subject to environmental conditions as well as external pressures, the durability of the membrane defined by hardness is also a major criterion for CMUT performance. Because of electrostatic forces in addition to the atmospheric pressure due to the vacuum sealed cavities, Young's ⁎ Tel.: + 90 312 210 4420; fax: + 90 312 210 2304. E-mail address: [email protected]. URL: http://www.ultramems.com. 0925-9635/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2011.11.006

modulus of the membrane plays an important role in the membrane deflection profiles as well. Compared to all potential membrane materials as well as current membrane materials of Si3N4 and silicon, diamond distinguishes itself based on high Young's modulus and exceptional hardness (see Table 1 for material properties of Si3N4, Si, and diamond). High frequency devices can be achieved with thinner membranes using diamond presenting high stiffness-to-mass ratio. Chemical inertness and surface modification of diamond are further benefits for the transducer surface. For example, hydrophilic O2-terminated diamond surface, achieved by oxygen plasma or piranha wet processing, will withstand against the detrimental cavitation shock of bubbles in immersion [4]. Because no wet chemical etchant of diamond exists, its use is best suited for extreme and harsh environments [5]. However, diamond membranes have not yet been investigated for CMUTs. In this study, we used polycrystalline diamond material featuring ultrananocrystalline diamond (UNCD) crystals with a grain size of 3–5 nm. Advanced Diamond Technologies (ADT, IL, US) provided 4-inch wafers with 1 μm thick UNCD custom grown with very low residual stress. Based on UNCD as the membrane material, we present the design, microfabrication, and initial operational characteristics of CMUTs with diamond membranes. Experimental results validate the finite element model developed, and motivate diamond as an alternate membrane material. 2. Design and finite element modeling A circular CMUT (demonstrated in this work) having a diameter of 5.2 mm is composed of 1500 circular membranes with a radius of 60 μm as shown in Fig. 1(a,b). For the design presented in this

B. Bayram / Diamond & Related Materials 22 (2012) 6–11

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Table 2 Physical parameters of the diamond-based CMUT.

Fig. 1. A single CMUT design having a circular shape. (a) Top view of single CMUT drawing. (b) Magnified, top view of a CMUT cell and its neighboring cells. (c) Schematic cross section of a CMUT cell.

study, the design parameters are summarized in Table 2, and the cross section of a CMUT cell is shown in Fig. 1(c). Finite element methods (FEM) are used to analyze the CMUT using a commercially available software package (ANSYS 11, ANSYS Inc., Canonsburg, PA). A 2-D axisymmetric finite element model is used to model the circular CMUT cell. The ANSYS standard element type, PLANE230, is used in the meshing of all elements for electrical harmonic analysis of the CMUT to determine the effect of the high resistive UNCD as a function of frequency. This analysis requires only electrical material properties (dielectric constant (ε) and resistivity (ρ)) to be specified as given in Table 1. Because the top surface of the doped silicon substrate and the bottom surface of the Al electrode constitute the ground and the top electrodes, respectively, electrical material properties of silicon and Al are not needed for this analysis. The resistivities of SiO2 and vacuum are not specified to model a perfect insulator. The resistivity of undoped UNCD used in this analysis is specified to be between 10 3 and 10 4 ohm·cm by the manufacturer. The same 2-D axisymmetric finite element model is also used for structural and electrostatic coupled field analysis. The ANSYS standard element types, PLANE121, which features charge and voltage variables and PLANE42, which features displacement and force variables, are used for calculation of the equilibrium deflection profiles of the membranes under DC bias. Based on the results of the initial

Table 1 Material properties of Si3N4, Si, diamond, UNCD, SiO2, and Al at room temperature: Young's modulus (E), density (d), Poisson's ratio (v), hardness (h), dielectric constant (ε), dielectric strength (Eds), resistivity (ρ), thermal conductivity (k), and thermal expansion coefficient (α) are given. Material properties

Si3N4

Si

Diamond

UNCD

SiO2

Al

320 3270 0.26 1580 5.7 100

160 2332 0.29 1000 11.7 3.7

1200 3520 0.2 10,000 5.7 100

850 3300 0.07

73 2200 0.17

70 2700 0.35

Inner radius of the CMUT, μm

2586

Number of cells Membrane support spacing (smem), μm Membrane radius (rmem), μm Electrode radius (rmetal), μm Electrode thickness (tmetal), μm Diamond membrane thickness (tmem), μm High temperature oxide thickness (tox), μm Oxide and gap thickness (tgap), μm Silicon substrate thickness, μm

1500 3 60 30 0.4 1.0 0.23 1.57 525

electrical harmonic analysis, the top electrode voltage extends over the bottom of the high resistive UNCD below a certain frequency. Therefore, the top electrode voltage is applied on the bottom of the diamond for DC bias deflection calculations. The substrate is supported at the bottom, and the structure is guided in vertical motion only on the left and right sides of the model due to the symmetry. Vacuum is used for electric field calculation in the gap, and it does not exist as a material in the structural model. The ANSYS macro, ESSOLV, is used to couple the electrostatic and structural solutions iteratively until a converged solution is achieved. The finite element model of a converged solution at a bias voltage of 190 V is shown in Fig. 2. 3. Fabrication The microfabrication of CMUT with diamond membranes is based on plasma-activated direct wafer bonding technology. In this technology, two wafers, a base wafer with cavity definition and a wafer with UNCD coating, are prepared separately for bonding. Following the bonding, the wafer supporting the UNCD is removed, transferring the layer as a membrane onto the patterned base wafer. The final steps involve the formation of electrical contacts for membrane actuation. The process steps for CMUT microfabrication are shown in Fig. 3. The processing was made on 4-in. n-type b100> silicon wafers (1–5 ohm·cm) suitable for industrial mass production (Fig. 3(a)). First, these wafers are doped with phosphorus at 1050 °C in an n-type doping furnace for 1 hour (Fig. 3(b)). This is because the conductive top layer is required to have good ohmic contact for ground electrode. Then, thermal SiO2 thickness of 1.57 μm is grown on the doped silicon wafer at 1000 °C in an oxidation furnace (Fig. 3(c)). This oxide layer forms the gap height (tgap) for the CMUT cavities (see Table 2). Following the oxide formation, the grown oxide is patterned with lithography and reactive ion etching (RIE) using CHF3/CF4 chemistry (STS RIE, Surface Technology Systems, Newport, UK), until the oxide is completely removed inside the cavity (Fig. 3(d)). The base wafer with cavity definition is to be one of the bonding pair. The other wafer of the bonding pair is a wafer with UNCD coating. Undoped UNCD having a resistivity of 103–104 ohm·cm and featuring low residual stress (i.e. b50 MPa) is used as the membrane material in this study. UNCD is deposited by Advanced Diamond Technologies (IL, US). A silicon wafer with UNCD of 1 μm has a surface roughness (Ra) of

Materials E (GPa) d (kg/m3) v h (kg/mm2) ε Eds (105 V/cm) ρ (ohm-cm) k (W/m-K) α (10− 6/K)

5.7 3

10 –10 30 3.3

151 2.5

2000 1.1

3.8 4

Fig. 2. The finite element model of a converged solution at a bias voltage of 190 V. An aspect ratio increase of 6 is used in the visual display of the vertical axis.

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the quality of the oxide [7]. In order to achieve a high quality direct bonding, the surface of the wafers should be as smooth as possible (Ra b 0.5 nm for surface scan of 5 μm × μm). To decrease the surface roughness of the as-deposited HTO, CMP is performed by Axus Technology (Chandler, AZ, US) using Semi-Sperse 25-E slurry (Cabot Microelectronics, IL, US) diluted with water (slurry:water) (1:1) on Westech Model 472 CMP System (Speedfam-IPEC, Japan). Wafer cleaning of the slurry remaining after the CMP is performed on OnTrack DSS-200 Post CMP Cleaner (OnTrack, CA, US). The surface roughness (Ra) of the HTO is reduced down to 0.3 nm with this chemical mechanical polishing (CMP), and the final oxide thickness is achieved after CMP (Fig. 3(f)). This completes the preparation of the bonding pair. The patterned wafer is direct bonded to this HTO/UNCD/silicon wafer (Fig. 3(g)). The wafers prepared in Fig. 3(d,f) are plasma activated prior to direct bonding. For direct bonding, the wafers are held in contact under high vacuum (10 − 4 mbar) at room temperature and the wafer pair is pressed with a piston force of 10 kN [8]. The wafer pair is heated to 550 °C, kept at this temperature for 7 hours, and then cooled down to 75 °C before the piston force is removed and the chamber is vented. To form the diamond membranes, the bulk silicon wafer of UNCD in the bonded pair is etched in tetramethylammonium hydroxide (TMAH), while the other side is protected by thermal oxide (Fig. 3(h)). For diamond etching in O2 plasma, SiO2 is used as the masking material. Plasma enhanced chemical vapor deposition (PECVD) of SiO2 on UNCD is performed (Fig. 3(i)), and is followed by patterning SiO2 via lithography, and RIE etching of SiO2/UNCD/SiO2 (Fig. 3(j)). Using the patterned oxide as the mask, the etching of UNCD is performed at a rate of 1600 Å/min using inductively coupled plasma (ICP) (RIE power = 100 W, ICP power= 1000 W, oxygen flow rate= 50 sccm, temperature = 20 °C, pressure =10 mTorr). Following the removal of the UNCD, RIE of SiO2 is followed, etching oxide under the etched UNCD region as well as the oxide on top of UNCD as shown in Fig. 3(j). Silicon and UNCD act as etch-stop for RIE of SiO2, whereas SiO2 acts as an etch-stop for RIE of UNCD, facilitating the etching process. Electrical connections are formed with aluminum (Al). Al is sputtered over the wafer surface (Fig. 3(k)). The sputtered Al is patterned via lithography, and wet etched to form the top electrode on the membrane and the ground electrode on the n+-doped silicon (Fig. 3(l)). In summary, employment of direct bonding technology for the microfabrication of diamond-based CMUTs can reduce the manufacturing cost and can be economically viable due to high device yield. Besides, as we employ plasma activation to achieve permanent bonds below a temperature of ~550 °C (compared to high temperature anneals at ~1000 °C), the yield is higher with no crack formation observed in the diamond membrane due to mismatch in thermal expansion coefficients. Furthermore, perfectly vacuum sealed cavities are achieved with silicon dioxide interlayer deposited by LPCVD at 850 °C [6] compared to PECVD at 300 °C [9] because of the high quality of the interlayer free of any trapped gas within LPCVD-SiO2. Overall, this 3-maskprocess is realized on 4-inch wafer scale, and it is comparable to the state-of-the-art fabrication techniques employed for CMUTs. Fig. 3. Process steps for microfabrication of diamond-based CMUT.

4. Results and discussion

10 nm, which is much larger than the requirement for direct wafer bonding (0.5 nm) (Fig. 3(e)). Low surface roughness can be achieved with the deposition of SiO2 layer followed by chemical mechanical polishing (CMP) [6]. The high temperature oxide (HTO) deposition is realized at 850 °C in an LPCVD furnace (TS6604, Tempress Systems, The Netherlands) with flow rates of SiH2CL2:N2O = 65 sccm:130 sccm under a total pressure of 400 mTorr [6]. The quality of HTO is very similar to thermally grown oxide as an electrical insulator. Following the oxide deposition, keeping the same temperature and total pressure, samples are annealed for 3 hours under N2 flow (200 sccm) to increase

The CMUT design features circular membranes with a fill factor of 82%, which is the ratio of the membrane area to the total area in the cell. Although a higher fill factor means a more efficient transducer design, and it is possible to achieve using hexagonal membranes instead, circular membranes are intentionally selected for the first prototype to be fabricated due to (1) computation-efficient 2-D axisymmetric finite element modeling to be used in its design, and (2) increasing contact surface area for successful direct wafer bonding process in the fabrication. In the conventional designs featuring Si3N4 or Si membranes, the insulation layer is laid on the highly doped silicon substrate, whereas in our designs featuring UNCD membranes, the insulation layer is laid

B. Bayram / Diamond & Related Materials 22 (2012) 6–11

on the bottom of the UNCD membrane as a solution to overcome the fabrication related difficulties: (1) Surface roughness of as-grown UNCD is approximately 10 nm, which is much larger than the direct wafer bonding limit of 0.5 nm. (2) Our plasma-activated direct wafer bonding experiments using chemically mechanically polished UNCD with a surface roughness of 1 nm were unsuccessful [8]. Because the SiO2 on UNCD not only serves as a smooth surface for successful direct bonding, but also as an electrical insulator to prevent shorting of the high resistive diamond acting as top electrode and the highly doped silicon surface being the ground electrode, special care on the deposition method and selection of the related process parameters is shown in the deposition of SiO2 on UNCD. Because of the relative thickness and Young's Modulus of SiO2 and diamond, effective mechanical properties are mainly determined by the diamond membrane. The phosphorus doping of the silicon substrate results in a surface sheet resistance of 1.2 ohm/sq. The thermal oxide thickness on original and phosphorus-doped substrate is 1.48 μm and 1.57 μm, respectively. Thicker thermal oxide is a result of enhanced doping density in the vicinity of the surface, facilitating the diffusion of the gases within the Si to grow thermal oxide on the Si/SiO2 interface. As a high quality dielectric insulator, SiO2 is deposited on UNCD in an LPCVD furnace using a flow rate ratio of 1:2 instead of the more conventional recipe of 1:5 for SiH2Cl2:N2O. Although excessive N2O is used to grow more uniform SiO2 films conventionally, N2O is also a high oxidizer potentially dangerous for diamond surface at high temperatures. Therefore, selecting a lower flow rate for N2O, the diamond surface is protected against reacting with N2O. Finite element modeling results based on the electrical harmonic analysis of the CMUT determine the effect of the high resistive UNCD on capacitance as a function of frequency. The capacitance–frequency relation is extracted for the CMUT with a frequency sweep from 10 kHz up to 1 GHz in Fig. 4. Half and full metallization cases are considered for UNCD featuring a resistivity of 10 3 and 10 4 ohm·cm. Full metallization case is particularly helpful to compare the FEM results with the theoretical calculations related with the resistance and the capacitance values of the membrane material at a given frequency. Below a critical frequency (fc) fc ¼

1 2πρε 0 εr

where ρ and εr are the resistivity and relative dielectric permittivity of the material respectively, the membrane will start acting as if like a conductor [10]. Using the material properties of UNCD (ρ= 104 ohm·cm, εr=5.7), the critical frequency (fc) is around 32 MHz for full metallization. The FEM result agrees with the theoretical calculation for full

Fig. 4. Finite element modeling results based on the electrical harmonic analysis of the CMUT.

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metallization, and also shows that capacitance is increasing below this critical frequency in agreement with the theoretical calculations. If the resistivity of UNCD is lower (ρ= 103 ohm·cm), then the critical frequency will be larger. If the resistivity of UNCD is so low to be considered as conductive, then the voltage applied on the Al electrode, regardless of half or full metallization, appears on the bottom of the UNCD covering the full membrane surface for the frequency range of our interest. When half metallization case is considered, the critical frequency is reduced to lower frequency values because of the distributed resistance and capacitance of the membrane with high aspect ratio. Based on our FEM results, the diamond acts as if like a conductor below 100 kHz and 1 MHz for a resistivity of 10 4 and 103 ohm·cm, respectively. Based on FEM results, undoped UNCD membrane can be safely considered to be conductive for DC bias calculations, resulting for the DC voltage applied on the Al electrode to appear on the bottom of the UNCD covering the full membrane surface. Electrical impedance measurement of CMUT in air is performed at a probe station using an impedance analyzer (4294A, Agilent Technologies, CA, USA). Fig. 5 shows the impedance measurement of a CMUT biased at 40 V. Resonance behavior at a frequency of 1.74 MHz is observed as seen in Fig. 5. Finite element modeling using ANSYS software package is employed to predict the theoretical resonance frequency. The finite element analysis (FEA) calculates a resonance frequency of 1.82 MHz for the CMUT, which is close to the experimental value. Assuming a 4% UNCD thickness variation specified by the manufacturer, 1.74 MHz is achieved giving a perfect fit to the experimental value. The CMUT capacitance at 2 MHz is extracted as 85 pF from the impedance measurement. Comparing this value on the FEM results in Fig. 4, we observe that the resistivity of undoped UNCD used in this study is slightly larger than 10 3 ohm·cm. Excellent match between experimental and FEM capacitance results is observed for a resistivity of 1.84 × 10 3 ohm·cm. At the resonance frequency of 1.74 MHz, the effective metal radius is much larger than the actual metal radius, but still smaller than the full coverage, as seen in Fig. 4. Electrical deflection measurements are realized after the wafer is diced, and the CMUT glued to the PCB is wire bonded to the conductive paths on board. White light interferometer measurement is employed to determine the deflection at the membrane center as a function of bias voltage. The experimental deflection measurements are compared to FEM results based on electrostatic-structural coupled field analysis in Fig. 6. DC voltage applied on the Al electrode is assumed to be on the bottom of the UNCD covering the full membrane surface as shown in Fig. 4. The experimental surface deflection profile over a 350 × 260 μm 2 area at Vbias=100 V is shown in the inset

Fig. 5. Electrical impedance measurement of CMUT in air at a bias voltage of 40 V.

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B. Bayram / Diamond & Related Materials 22 (2012) 6–11

Fig. 6. White light interferometer measurement and FEA results of deflection at the membrane center as a function of bias voltage. Inset shows the experimental surface deflection profile over a 350 × 260 μm2 area at Vbias = 100 V.

of Fig. 6. At a bias voltage of 0 V, the deflection at the membrane center is 0.34 μm because of the atmospheric pressure. As the bias voltage is increased, the deflection increases because of the higher electrostatic force acting on the membrane. Furthermore, the deflection curves in Fig. 6 yield a collapse voltage of 200 V based on FEM results. All these results show that the experimental data and FEM calculations are in good agreement. Transmission measurements are realized in air using a broadband hydrophone (HNA-0400, Onda Corporation, CA, USA) connected to a preamplifier (AH-2020-25, Onda Corporation, CA, USA). The hydrophone is aligned with the normal at the center of the CMUT, and is separated by 1.9 mm from the transducer surface. The CMUT, biased at 100 V, is excited with a 5 cycle burst of 35 Vp − p sine signal at 1.74 MHz in air. The large excitation amplitude is provided with a linear broadband amplifier connected to a waveform generator. The output voltage waveform of the hydrophone is shown in Fig. 7. Electromagnetic feedthrough is followed by the acoustic signal as expected [11]. The initial acoustic signal received at t = 5.6 μs is enlarged in the inset of Fig. 7. The 3-dB fractional bandwidth of 3% at a center frequency of 1.74 MHz is measured with a hydrophone, demonstrating successful operation of diamond-based CMUTs. The bandwidth is calculated using the

attenuation coefficient of the exponential decay observed in Fig. 7. Because the air transmission measurement of the first diamond-based CMUT prototype is intended to demonstrate its operation rather than its performance, the hydrophone measurement result is provided as is, without any data processing. In this study, the detailed fabrication and demonstration of the diamond-based CMUT prototype are presented. The detailed fabrication process flow can also be employed in novel microelectromechanical devices, where the exceptional material properties of diamond can be utilized. Employing the presented fabrication flow, diamond-based CMUT is fabricated, and its operation is validated with the electrical impedance and airborne ultrasound generation measurements. Although the advantages of using diamond other than other common materials such as Si and Si3N4 as the membrane material are not directly observable from the initial characterization results, the inherent superior material properties (High Young's modulus etc.) of diamond are expected to improve reliability and longevity of such transducers. The primary ultrasound area that we envision for diamond-based CMUTs is therapeutic biomedical applications requiring generation of high intensity focused ultrasound (HIFU) [12]. Therapeutic ultrasound applications require large amplitude, continuous wave (CW) excitations of the transducers in transmit unlike the imaging applications requiring small amplitude, pulse excitations in transmit and broadband spectrum in receive [13]. The advantages of diamond membranes will be highly beneficial in generation of extreme ultrasound for therapy. 5. Conclusion We have designed a diamond-based CMUT and developed microfabrication technology compatible with (4-inch wafer) industrial mass production to employ diamond as a membrane in transducers. Plasma-activated direct bonding technology employed to fabricate diamond-based CMUTs enables higher yields and reduced manufacturing costs suitable for mass fabrication. The presented process flow can be extended to a variety of devices such as biological, chemical and radiation sensors, where the exceptional material properties of diamond can be utilized. Our CMUTs with UNCD membranes are fabricated and operated airborne generating ultrasonic waves at a center frequency of 1.74 MHz with a 3-dB fractional bandwidth of 3%. The experimental findings are compared with finite element analysis and found in harmony. Our results motivate towards diamond as the alternate material for microelectromechanical devices. Acknowledgments This work is supported by The Scientific and Technological Research Council of Turkey (TÜBİTAK) (PI: Baris Bayram, project no: 107E153, 110E072) and Middle East Technical University (METU) (PI: Baris Bayram, project no: BAP-03-01-2011-001). This work is conducted in the clean room facility of METU MEMS Center (Middle East Technical University, Ankara, Turkey). The author would like to thank Advanced Diamond Technologies for providing very low stress UNCD wafers, Dan Trojan from Axus Technology for performing CMP on the wafer surfaces of high temperature oxide, and Alexander Filbert from EV Group for performing EVG301 wafer cleaning, EVG810LT plasma activation, and EVG520IS wafer bonding. References

Fig. 7. Hydrophone output voltage waveform for CMUT in air biased at 100 V and excited with a 5 cycle burst of 35 Vp − p sine signal at 1.74 MHz. Hydrophone is aligned with the normal of the center of the CMUT, and is separated by 1.9 mm from the transducer surface.

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