Pulsed DC magnetron sputtered titanium nitride thin films for localized heating applications in MEMS devices

Accepted Manuscript Title: Pulsed DC magnetron sputtered titanium nitride thin films for localized heating applications in MEMS devices Authors: Jithin M.A., K.L. Ganapathi, G.N.V.R. Vikram, N.K. Udayashankar, S. Mohan PII: DOI: Reference:

S0924-4247(17)31626-6 https://doi.org/10.1016/j.sna.2017.12.066 SNA 10553

To appear in:

Sensors and Actuators A

Received date: Revised date: Accepted date:

9-9-2017 30-12-2017 30-12-2017

Please cite this article as: M.A. J, Ganapathi KL, Vikram GNVR, Udayashankar NK, Mohan S, Pulsed DC magnetron sputtered titanium nitride thin films for localized heating applications in MEMS devices, Sensors and Actuators: A Physical (2010), https://doi.org/10.1016/j.sna.2017.12.066 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Pulsed DC magnetron sputtered titanium nitride thin films for localized heating applications in MEMS devices

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Jithin M. A.a,b, K. L. Ganapathic, G. N. V.R. Vikramd, N. K. Udayashankarb and S. Mohana Center for Nano Science and Engineering, Indian Institute of Science, Bangalore 560012, India

b

Department of Physics, National Institute of Technology Karnataka, Mangalore 575025, India c

Department of Physics, Indian Institute of Technology Madras, Chennai 600036, India

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Department of Electrical and Communication Engineering, Vignan University, Vadlamudi,

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Guntur 522213, India

Research highlights

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various substrate temperatures.

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 TiN thin films have been synthesised using pulsed dc magnetron sputtering technique at

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 Structural, surface, mechanical and electrical properties have been investigated.  Correlation between process parameter, structure, electrical and mechanical properties

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has been established.

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 Microheater patterns were fabricated with lower resistive TiN film and their heating capabilities were calibrated.

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 Maximum temperature of 250 °C is achieved by applying a power of 2.8W to the microheater.

Abstract

Titanium nitride (TiN) thin films are deposited on Si/SiO2 substratesby using Pulsed DC magnetron sputtering and are characterized for their structural, mechanical and electrical properties for their application as localized heating elements in microsystem devices. The

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influence of substrate temperature on the properties of TiN films has been investigated. The correlation between the structural orientation with mechanical and electrical properties has been established. The films deposited at a substrate temperature of 300 °C have shown better structural, mechanical and electrical properties. This film has been chosen for the fabrication of microheater and its characterization. A maximum temperature of 250 °C is achieved by applying

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a power of 2.8W to the microheater.

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Keywords: Thin films, pulsed dc magnetron sputtering, titanium nitride, resistivity, microheater

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1. Introduction

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Microactuators play an important role in microelectromechanical systems (MEMS) [1].

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There are various kinds of microactuators such as electrostatic, magnetic and thermal actuators which are being used in MEMS. Among them, thermally induced microactuators have a wide

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range of applications especially in aerospace, optical sensing and in biological systems etc. because of their favorable properties like high power generation, low operating voltages and less

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susceptibility to adhesion failures compared to electrostatic actuators [1 - 4]. The microactuators based on NiTi shape memory alloys (SMAs) are the most popular candidates among the

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thermally induced microactuatorsand are being widely used in biomedical devices [4 - 6]. A localized heating component is necessary to achieve actuation in thermal SMAs and integration in MEMS. The conventional microheaters are made with noble metals such as Pt or Au [7 - 9].

However, these metals are expensive and may not be suitable for high temperature applications [10]. Hence, an alternative material is necessary to augment the conventional heating elements. Titanium Nitride (TiN) is a popular functional material and is being used in various

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applications because of its excellent properties. It is an essential material in high-speed steel cutting tools because of its high hardness (~30 GPa) and corrosion resistance behavior [11]. In the semiconductor industry, TiN is being used as a gate electrode and diffusion barrier layer in CMOS technology because of its high electrical conductivity and diffusion barrier properties. It is also being used as a protective coating on the surfaces of medical devices such as

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microneedles, microstents etc. because of its biocompatibility [12 - 13]. It has very moderate

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heat conductivity (15 Wm-1 K for bulk) and has the potential to reach high temperatures because

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conventional Pt/Au heaters in MEMS [10].

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of its high melting point (2950 °C). Thus, TiN hotplates/microheaters can outperform

Many deposition methods have been employed to grow TiN films. However, each

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method has its own pros and cons. In general, the TiN films deposited by using physical vapor

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deposition (PVD) methods such as sputtering and evaporation are of high quality as compared to

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chemical vapor deposition (CVD) methods [14 - 16]. In PVD, conventional vacuum evaporation results in poor adhesion and is difficult in stoichiometric control. RF/DC reactive magnetron

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sputter deposition techniques are widely used for producing high-quality TiN thin films; however, the deposition rates are very low. A number of explorations were made for the

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synthesis of TiN thin films by RF and DC magnetron sputter deposition techniques [17 - 19]. The pulsed dc magnetron sputter (PDC) deposition technique is one of the best PVD techniques to get high quality films [20]. The deposition rates can be controlled in a wide range by tuning power, pulse frequency and pulse reversal time. It can produce void-free, dense and high quality

thin films [21 - 22]. Hence, it could be possible to fabricate microheaters with very good properties by using TiN thin films deposited by the PDC deposition technique. In this work, the TiN thin films have been deposited by using pulsed dc magnetron

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sputtering. The influence of the substrate temperature on the structural, mechanical and electrical properties of TiN films has been investigated. The correlation between the structural orientation and mechanical and electrical properties has been established. A TiN based microheater has been fabricated and characterized with a low restive TiN film for localized heating in MEMS.

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2. Experimental Details

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TiN thin films have been deposited onto Si and Si/SiO2 substrates by reactive pulsed dc

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magnetron sputter deposition (PDC). The ratio between the sputter gas (Argon) and reactive gas

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(Nitrogen) has been kept constant. The target used is a 99.99% pure Titanium disc of 76.2 mm diameter. The depositions have been carried out at a varied substrate temperature between room

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temperature and 300 °C using a resistive heater set up. The process chamber has been evacuated

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using a cryogenic pump to a base pressure of 1.50 × 10-6 mbar. The substrate to target distance of

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90 mm has been maintained for all the depositions. Initially, the Ti target is pre-sputtered using only Argon gas to avoid oxidation at the target surface. The chamber working pressure is

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maintained constant at 5.00 × 10-3 mbar, after Argon and Nitrogen gas are passed into the chamber. The target is supplied with a power of 100 W, a pulsed frequency of 200 kHz and a

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time reversal of 1.0 µs. Prior to deposition, the Silicon (100) substrates are cleaned initially by a rinse in deionized water followed by acetone ultrasonic treatment and are then subjected to a HF dip. Later, a 200 nm thick SiO2 layer is thermally grown on Si.

The microstructure studies of TiN thin films have been carried out using an X-ray diffractometer (RigakuSmartLab X-ray diffractometer). The surface analysis and thickness measurements have been carried out by scanning electron microscopy(SEM) and cross-sectional

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scanning electron microscopy (Ultra high resolution scanning electron with monochromatic imaging spectroscopy). The surface roughness studies and topography scans have been conducted

by

atomic

force

microscopy

(BrukerHigh-Resolution

AFM

Systems).

Nanoindentation has been carried out in a load-controlled mode using a Berkovich diamond indenter in an Agilent G200 nanoindenter system. Nanoindentation has been carried out with a

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three sided Berkovich diamond tip with a tip radius of 50 nm, using three different maximum

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loads of 0.5 mN, 1.5 mN and 3 mN in an array of 3 × 3 for each load. The loads are chosen such

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that the penetration depths are less than one third the thickness of the film. The distance between

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the indents is maintained at 15 µm to avoid any possible interaction between the indents. The electrical resistivity measurements have been carried out by using a four-probe electrical

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characterization set up (Agilent Device Analyzer B1500A DC Probe Station). The microheater

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structures are patterned using focused ion beam micromachining (UHR Dual Beam FIB System:

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Helios NanoLAB 600i FEI) of TiN film deposited at 300 °C on Si/SiO2. The SiO2 layer acts as an isolation layer to preventthe possibility of heat dissipation through the bottom silicon

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substrate. The electrical contacts for the measurement of resistivity and resistive heating are made by wire bonding (tpt). The heat measurements are carried out by using an IR camera

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(Fluke thermal IR camera).

3. Results and discussion 3.1. Structural studies:

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The XRD spectra of the films (figure 1) deposited at room temperature (TiN_RT), 100 °C (TiN_100), 200 °C (TiN_200) and 300 °C (TiN_300) show that the films are polycrystalline in nature. However, as the temperature increases, the crystallinity of the films increases. This is due to the improvement of ad-atom mobility induced by thermal energy at higher temperatures [23]. TiN_RT shows only peaks corresponding to (111) and (220) orientations of TiN at 36.7° and

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61.8° respectively. The films TiN_100, TiN_200 and TiN_300 show additional peaks at 42.7°,

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which correspond to a (200) orientation of TiN, and its intensity increases with an increase in

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substrate temperature. This is not prominent in the TiN_100 film [15 - 16, 24 - 25].

3.2. Surface topography:

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The 3D topography of the TiN_200 film is shown in figure 2. The film surface has an

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RMS roughness of about 2.9 nm. The surface roughness of the films deposited at higher

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temperatures shows an increasing trend with an increase in substrate temperature (Table 1). However, the roughness decreases with an increase in the substrate temperature from room

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temperature to 100 °C. This could be because of the introduction of a (200) peak at higher deposition temperatures. From the XRD pattern, it is clear that though the room temperature film

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is crystalline in nature, as the deposition temperature increases, a (200) plane appears which results in the more crystalline film. This might be the reason for the increase in surface roughness with the deposition temperature. Further investigation neededto be carried out to understand this behavior.

The FE-SEM images of TiN films deposited at room temperature, 100 °C, 200 °C and 300 °C are shown in figure 3. Not much difference has been observed in the grain size of the films grown at different temperatures, and it is clear that all the films are crystalline in nature.

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This also is corroborated by the XRD analysis. The thickness of the TiN films measured from SEM imaging of fractured cross-sectional samples (figure 4 for TiN_RT) is about ~277±10 nm. Using this, the rate of deposition of the films can be calculated to be ~7 nm/min. 3.3. Nanoindentation studies:

Typical load-displacement (P-h) curves for the indentation carried out on a TiN_200 film

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are shown in figure 5(a). Multiple pop-ins can be observed in the loading portion of the P-h

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curves in each of the curves. The pop-in behavior in the loading portion of the P-h curves has

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been associated only with thecrack formation and propagation in brittle materials or with the

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nucleation of dislocations in the substrate during the indentation of hard films on soft substrates

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[26 - 27]. Though TiN is a hard material compared to silicon [28], as the penetration depths were

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restricted to less than 1/3rd of the film thickness, the deformation region does not reach the substrate [29]. Hence, the pop-ins, in this case,, would be only due to cracks in the films during

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indentation. The energy associated with these pop-ins can be calculated from the area under the

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pop-in regions [26]. This energy varies from 0.12 nJ to 0.24 nJ in these films. It can also be seen that the loading curves obtained for the nanoindentation carried out on

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all the loads on the TiN_200 film follow similar paths (figure 5(a)). However, a standard deviation of about 0.85±0.2 nm can be observed in the loading curves at different loads, which is comparable to an RMS roughness [30 - 32] of about 1.5 nm obtained from the AFM. The standard deviation was obtained from the displacements of the 9 indents at half the maximum load used to generate them. Similarly, on TiN_RT and TiN_300 films, standard deviations of

1.71±0.55 and 1.50±0.17 comparable to anRMS roughness of 1.7 nm and 1.9 nm respectively can be observed (figure 5(b)). This low scatter in the P-h curves would mean that other measurement errors resulting from thermal drift and instrument noise are very few. It also

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indicates that over the measured region of 105 × 105 µm2, the film is homogeneous. As can be observed from figure 5, the maximum penetration depth is lower in the TiN_200 film compared to that in the TiN_RT film. This indicates that TiN_RT films are softer compared to TiN_200 films. The hardness and Young’s modulus of the films are calculated using the Oliver-Pharr method [33], by fitting a line for the upper half portion of the unloading curve (shown in figure

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6(b)).

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The hardness of the TiN_RT and TiN_100 films increased from about 16 GPa to about

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20 GPa and from about 14 GPa to 17 GPa respectively, with an increase in load (figure 6(a)).

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The hardness of TiN_200 and TiN_300 films decreased from about 33 GPa to about 23 GPa and

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from about 26 GPa to 22 GPa respectively with an increase in the applied load. Similar to the

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trend observed in hardness, TiN_RT and TiN_100 showed an increase in Young’s modulus with an increased load while TiN_200 and TiN_300 films showed a decreased Young’s modulus with

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an increased load (figure 6(b)). A similar trend in the hardness of the TiN_RT and TiN_100 films as well as a similar trend in the hardness of TiN_200 and TiN_300 films corroborates to

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their crystallinity (figure3). The decreasing or increasing trend in the hardness of the films with

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increasing load can be termed the indentation size effect (ISE) [34 - 35]. This results from the fact that the indenter tip is more spherical at a lower penetration depth leading to non-self-similar indents at the corresponding lower penetration depths [36 - 37]. The increase in hardness with an increased load was termed the reverse indentation size effect (RISE), which was observed in a few materials and at very low loads [35]. The standard deviation in the hardness and Young’s

modulus is higher at lower loads and decreases with increasing loads. This is because of the effect of surface roughness, which is predominant for indentations carried out at lower applied maximum loads compared to higher applied maximum loads [32].

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3.4. Electrical studies:

The sheet resistance of the films TiN_RT, TiN_100, TiN_200 and TiN_300 are 30.29, 20.50, 15.94 and 11.10 Ω/□ respectively. The electrical resistivity of the TiN films is calculated by multiplying the sheet resistance with the thickness of the films. The resistivities of the films are 832.97, 563.75, 438.35 & 305.25 µΩ.cm respectively (figure 7). The resistivity decreases

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withincreasing the substrate temperature.XRD results have shown that the intensity of the TiN

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(200) peak increaseswith an increase in substrate temperature. It is reported that the film with a

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high (200) diffraction peak intensity has less electrical resistivity [16, 24]. Therefore, there is a

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correlation between the intensity of the TiN (200) diffraction peak and the resistivity of the

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TiN thin films, which is in agreement with the reported results.

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3.5. Microheater fabrication:

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Microheaters have been fabricated with low resistive (ρ~305 µΩ.cm) TiN films deposited at a substrate temperature of 300 °C on Si/SiO2 using focused ion beam micromachining. The

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FIB image of the microheater pattern is shown in figure 8.The heating capacity of the microheater was measured at various input powers. The measured heater resistance was ~ 16 Ω.

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Figure 9 shows the performance of the heater at various powers and the corresponding thermal images of the heater at various powers are shown in figure 10. From figure 9, it can be observed that the temperature of the heater increases nearly linearly with increasing power. A maximum temperature of 250 °C has been achieved by applying a power of 2.8W. This temperature is sufficient to provide actuation in NiTi based thermal SMAs as the phase change temperature in

NiTi thin films is around 200 °C [38 - 39]. However, the temperature of the heater with TiN thin films can be further enhanced by changing the design of the heater and will be useful for applications like gas sensing[40 - 41].

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4. Conclusion

In this work, TiN thin films have been deposited onto Si and Si/SiO2 substrates by the pulsed dc magnetron sputtering technique at various substrate temperatures. The influence of substrate temperature on the structural, mechanical and electrical properties has been investigated. It has been observed that the crystal orientations of TiN films vary with deposition

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temperature. The variation in the electrical resistivity of the TiN film is a result of the change in

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crystal orientations. It has also been observed that the substrate temperature during film growth

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plays a significant role in deciding the electrical resistivity of TiN thin films. The microheater

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patterns were fabricated with the lower resistivity TiN film and their heating capabilities were

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calibrated at various input powers. The heater power consumption and heating efficiency can be

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further modified based on the heater design.

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Acknowledgments

The use of the facilities in the Micro and Nano Characterization Facility (MNCF) and the

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MEMS packaging lab at Center for Nano Science and Engineering, IISc is deeply acknowledged.

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The authors thank Arun, Santhosh and Dr. V. R. Supradeepa for the heater temperature measurement studies. The authors also thank the MNCF staffs for the XRD, AFM, SEM and electrical studies. The help from the Central Manufacturing and Technology Institute (CMTI) for the nanoindentation studies are highly acknowledged. This work is supported by the Ministry of

Communication and Information Technology under a grant for the Centre of Excellence in Nanoelectronics, Phase II.

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2580–2591.

Authors Jithin M. A. received the B. Sc. degree and the M. Sc. degree in Physics from Calicut and Bharathidasan Universities respectively. Since 2007, he is

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working in the Centre for Nano Science and Engineering (CeNSE) at the Indian Institute of Science (IISc) Bangalore, India. He is also pursuing his Ph.D. degree (under external registration scheme) in the Department of Physics, National Institute of Technology Karnataka (NITK), Surathkal since 2014.

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His current research interests include thin film processes, oxide and nitride thin films, shape memory alloys, MEMS, vacuum instrumentation and

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optical filters.

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Kolla Lakshmi Ganapathi received the Ph.D. degree from the department

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of Instrumentation and Applied Physics (IAP) in collaboration with Centre for Nano Science and Engineering (CeNSE), Indian Institute of Science

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(IISc), Bangalore, India, in January 2015. From August 2014 to May 2017,

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he worked as a Research Associate at CeNSE, IISc. He is currently a DST Inspire Faculty at the department of Physics, Indian Institute of Technology,

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Madras (IIT-M), Chennai, India. His research interests include device physics, Nanoelectronic devices of 2D

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layered materials, thin films synthesis, characterization and devices. Naga Venkata Rama Vikram received the Ph.D degree from the department of Mechanical Engineering in collaboration with Centre for Nano Science and Engineering (CeNSE), Indian Institute of Science (IISc), Bangalore, India, in September 2016. From August 2013 to December 2016, he worked as Senior Research Fellow Research at RBCCPS, IISc, and from January 2016 to June 2017 he worked as Assistant Professor in Nitte Meenakshi a Institute of Technology, Bangalore. He is currently working as Assistant Professor in Department of ECE at Vignan University, Guntur. His research interests include thin-film devices, shape memory alloys, thin

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films synthesis, Mechanical characterization, MEMS and instrumentation.

N. K. Udayashankar received the M. Sc in Physics degree from the Mangalore university in 1990. He also holds M. Tech. degree from KREC (National Institute of Technology Karnataka, Surathkal, Mangalore) in 1991 and the Ph.D. degree from the Indian Institute of Science, Bengaluru, India,

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in 1999 respectively.

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In 1991, he joined the Department of Physics, NITK Surathkal, as a Faculty

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Member, where he is currently a Professor.

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S. Mohan received the Ph.D. degree in physics from Sri Venkateswara

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University, Tirupati, India, in 1974. In 1977, he joined the Indian Institute of Science (IISc), Bangalore, India,

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where he is currently a Emeritus Professor at the Centre for Nano Science

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and Engineering (CeNSE).

Figure captions Figure 1: XRD patterns of TiN films deposited at room temperature, 100, 200 and 300 °C. Figure 2: AFM image of pulsed DC sputter deposited TiN film. Figure 3: The FE-SEM images of TiN thin films deposited at different temperatures (a)

Figure 4: Fractured cross-section image of TiN_RT films.

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TiN_RT, (b) TiN_100, (c) TiN_200 and (d) TiN_300.

Figure 5: Load-Displacement (P-h) curves of (a) TiN_200 °C film at different loads (b)

Indentation carried out at maximum load of 1.5 mN on TiN_RT, TiN_100 °C, TiN_200 °C and TiN_300 °C films.

U

Figure 6: Variation of (a) Hardness and (b) Modulus in TiN_RT, TiN_100 °C, TiN_200 °C

N

and TiN_300 °C films.

Figure 7: Variation of Resistivity with substrate temperature.

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Figure 8: FIB image of TiN heater pattern after micromachining.

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Figure 9: Power Vs heater temperature measurements of TiN microheaters.

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CC

EP

TE

D

Figure 10: IR images of microheaters at (a) 0.18 W, (b) 0.7 W, (c) 1.56 W & (d) 2.84 W

SC RI PT

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CC

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TE

D

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A

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Figure 1: XRD patterns of TiN films deposited at room temperature, 100, 200 and 300 °C

Figure 2: AFM image of pulsed DC sputter deposited TiN film

SC RI PT U N A M

Figure 3: The FE-SEM images of TiN thin films deposited at different temperatures (a)

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CC

EP

TE

D

TiN_RT, (b) TiN_100, (c) TiN_200 and (d) TiN_300

Figure 4: Fractured cross-section image of TiN_RT films

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Figure 5: Load-Displacement (P-h) curves of (a) TiN_200 °C film at different loads (b)

U

Indentation carried out at maximum load of 1.5 mN on TiN_RT, TiN_100 °C, TiN_200 °C and

EP

TE

D

M

A

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TiN_300 °C films.

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CC

Figure 6: Variation of (a) Hardness and (b) Modulus in TiN_RT, TiN_100 °C, TiN_200 °C and TiN_300 °C films.

SC RI PT

EP

TE

D

M

A

N

U

Figure 7: Variation of Resistivity with substrate temperature

A

CC

Figure 8: FIB image of TiN heater pattern after micromachining

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A

CC

EP

TE

D

M

A

N

Figure 9: Power Vs heater temperature measurements of TiN microheaters

Figure 10: IR images of microheaters at (a) 0.18 W, (b) 0.7 W, (c) 1.56 W & (d) 2.84 W

Table Captions

Table 1: Surface roughness values of TiN films deposited at various temperatures by AFM

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CC

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TE

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A

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Sample TiN_RT TiN_100 TiN_200 TiN_300 RMS Roughness (nm.) 3.34 1.33 2.90 3.67 Table 1: Surface roughness values of TiN films deposited at various temperatures by AFM