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Dielectric thin films for MEMS-based optical sensors
Microelectronics Reliability 47 (2007) 733–738 www.elsevier.com/locate/microrel
Dielectric thin films for MEMS-based optical sensors M. Martyniuk *, J. Antoszewski, C.A. Musca, J.M. Dell, L. Faraone School of Electrical, Electronic and Computer Engineering, The University of Western Australia, Crawley, WA 6009, Australia Available online 13 March 2007
Abstract Nanoindentation and optical measurements have been employed in order to investigate the mechanical properties of low-temperature (50–330 C) plasma-enhanced chemical vapour deposited (PECVD) SiNx, as well as thermally evaporated SiOx and Ge thin films for applications in micro-electro-mechanical systems (MEMS) fabricated on temperature sensitive, non-standard substrates. The temperature of the SiNx deposition process is found to strongly influence Young’s modulus, hardness, and stress, with a critical deposition temperature in the 100 C to 150 C range which depends on the details of other deposition conditions such as chamber pressure and RF-power. The properties of PECVD SiNx films deposited above this critical temperature are found to be suitable for MEMS applications, whereas films deposited at lower temperatures exhibit low Young’s modulus and hardness, as well as environment-induced stress instabilities. The investigated thin films have been incorporated into a monolithic integrated technology comprising low-temperature (125 C) MEMS and HgCdTe IR detectors, in order to realize successful prototypes of tuneable IR microspectrometers. 2007 Elsevier Ltd. All rights reserved.
1. Introduction The salient feature of next generation infrared (IR) onchip integrated sensors is likely to be sensitivity in a narrow wavelength band that is tuneable over a selected range of the IR spectrum. It is proposed that this can be achieved by the integration of present-day IR detectors with thinfilm based micro-electro-mechanical systems (MEMS) optical mirror technology, as shown schematically in Fig. 1. Narrow-band sensitivity is obtained by optical resonance phenomena within a Fabry–Perot (FP) cavity, that is created by two Bragg reflectors and is monolithically integrated with the IR detector. Electrostatic actuation of the thin-film membrane supported top mirror is the means of providing wavelength discrimination of the incident IR photons which, for example, could be used for target discrimination or remote sensing of various chemical/biological species via identification of narrow spectral features. The critical element in the realization of such a proposed infrared ‘‘microspectrometer’’ device is the suspended membrane/mirror actuator, which is a multilayered struc*
Corresponding author. E-mail address: [email protected] (M. Martyniuk).
0026-2714/$ - see front matter 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.microrel.2007.01.060
ture consisting of SiNx, SiOx, and Ge individual layers. This suspended, flexible, and IR reflecting structure needs to be free of any warping, and exhibit a high-degree of parallelism relative to the bottom mirror. The presence of excessive stress levels, stress gradients, or overall stress imbalance in the suspended structure leads to tilt, warp, collapse, rupture and/or deformation of the top membrane/mirror actuator. In addition, the suspended membrane/mirror structure needs to be sufficiently flexible to allow the FP cavity length to be electrostatically varied at reasonably low-voltages; however, the Bragg mirror itself needs to be stiff enough so that it is self-supporting and does not deform under electrostatic actuation conditions. Any mirror non-parallelism or deformation will lead to undesirable degradation of the FP cavity finesse or, in extreme cases, could lead to optical instability of the cavity. The mechanical properties of the individual SiNx, SiOx, and Ge layers, and in particular the Young’s moduli, thin film stresses, as well as stress gradients, are critical parameters that determine the mechanical response of the suspended structure to the actuating electrostatic field. Moreover, in a monolithic integrated approach, high-quality thin-film layers need to be attained under the stringent requirement of low-temperature processing (<125 C) for
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M. Martyniuk et al. / Microelectronics Reliability 47 (2007) 733–738 Table 1 The investigated thin film samples Sample
Preparation parameters thickness
Film thickness
a-series PECVD SiNx
SiH4 – 5 sccm Gas chemistry: NH3 – 45 sccm N2 – 100 sccm RF: 100 W at 13.6 MHz Pressure: 450 mTorr Temperature: 50–300 C Substrate: Si and/or GaAs
500 nm
b-series PECVD SiNx
SiH4 – 5 sccm Gas chemistry: NH3 – 45 sccm N2 – 100 sccm RF: 75 W at 13.6 MHz Pressure: 875 mTorr Temperature: 55–330 C Substrate: Si and/or GaAs
700 nm
Thermally evaporated SiOx
Source material: silicon monoxide ˚ /s Rate: 10 A Pressure: 4 · 10 6 mbar Temperature: 25 C Substrate: Si
280 nm
Thermally evaporated Ge
Source material: polycrystalline Ge ˚ /s Rate: 10 A Base pressure: 1 · 10 7 mbar Temperature: 25 C Substrate: Si
150 nm
Fig. 1. General concept of Fabry–Perot interferometric filter using Ge/ SiOx/Ge mirrors formed on an IR detector. Wavelength tuning is achieved through electrostatic actuation of the optical cavity length.
HgCdTe-based IR technology, where exposure to higher temperatures has the potential to cause irreversible device damage. This paper presents an investigation of the mechanical properties of low-temperature (50–330 C) plasma-enhanced chemical vapour deposited (PECVD) structural SiNx thin films, as well as thermally evaporated Ge and SiOx thin films utilized in the Bragg IR mirrors. The employed characterization techniques consist of depth sensing indentation (DSI), as well as optical measurements of stress-induced substrate curvature. 2. Experimental details The investigated samples consisted of two series of PECVD SiNx samples, labeled a- and b-series, that were deposited in an Oxford Instruments PECVD 80 system. In addition, thermally evaporated SiOx and Ge were also prepared. The deposition temperature of both a- and b-series PECVD SiNx was varied from 50 C to 330 C and all other deposition parameters were held constant for all samples within each series. In comparison to the deposition conditions of the a-series PECVD SiNx, the RF power was decreased and the chamber pressure was increased for the b-series samples. See Table 1 for details of the sample preparation. All thin films were deposited on 100 lm thick Si or GaAs substrates. The nanoindentation experiments were performed using a Hysitron TriboScope DSI system in conjunction with a Digital Instruments DimentionTM 3000 NanoScope IIIa Scanning Probe Microscope (SPM). The indenter load function (load versus time) that was adopted for the indentation experiments was the same as published in Ref. [1], where the final unloading cycle was used for the extraction of the elastic modulus and hardness of the thin film/substrate bi-layers [2]. The Young’s modulus and hardness of the thin film material was extracted according to the procedures presented in Ref. [3] which were further developed in Ref. [4]. The thin film stress was determined through measurements of stress induced substrate curvature, which was
obtained from changes in the angle of reflection from coated substrates of parallel incident laser beams according to the setup and procedures published in Ref. [5]. The measurement apparatus allowed for stress measurements to be performed under controlled ambient conditions. 3. Results and discussion Fig. 2 presents the Young’s modulus and hardness as a function of thin film deposition temperature for a-series (open diamonds) and b-series (open circles) PECVD SiNx. The Young’s modulus (hardness) values of thermally evaporated SiOx and Ge were determined to be 102 GPa (4.5 GPa) and 92 GPa (6.6 GPa), respectively, and are represented by dashed and dotted lines. A Poisson ratio of 0.25 was assumed in the determination of Young’s modulus values for all films. Considering the results obtained for PECVD SiNx thin films, it is evident that both Young’s modulus and hardness are strongly influenced by the deposition temperature. Decreasing the temperature of the PECVD process causes Young’s modulus and hardness to decrease in a similar fashion. For the a-series films deposited at temperatures above 225 C, a constant Young’s modulus (hardness) value of about 150–160 GPa (14–15 GPa) was obtained. A moderate and approximately linear decrease from 150 GPa to 115 GPa (14 GPa to 10 GPa) in Young’s modulus (hardness) was found to be associated with decreasing deposition temperature from
M. Martyniuk et al. / Microelectronics Reliability 47 (2007) 733–738
deposition temperature [ο C] 200
50
100
150
200
250
300
450
β −series PECVD SiNx
350
thermally evaporated SiO x thermally evaporated Ge
residual stress [MPa]
Young's modulus [GPa]
300 150
100
50
0
735
150
0 -150 α −series PECVD SiNx
-300
α−series PECVD SiNx
4 months after deposition 5 months after deposition 18 months after deposition
β − series PECVD SiNx thermally evaporated SiOx thermally evaporated Ge
-450 50
18
100
150
200
250
300
350
deposition temperature [ ο C]
12 9 6
hardness [GPa]
15
3
Fig. 3. Deposition temperature dependence of residual stress of a-series PECVD SiNx as measured four months (open diamonds), five months (open circles), and 18 months (open squares) after thin film deposition, as well as of b-series PECVD SiNx (full triangles) as measured within one day of deposition. The solid lines joining the data points are simply a guide for the eye. Positive stress values denote tensile stress and negative stress values denote compressive stress.
0 50
100
150
200
250
300
350
ο
deposition temperature [ C] Fig. 2. Young’s modulus and hardness as a function of deposition temperature for a-series (open diamonds) and b-series (open circles) PECVD SiNx. The solid lines joining the data points are simply a guide for the eye. The dashed and dotted lines represent the parameters obtained for thermally evaporated SiOx and Ge, respectively.
200 C to 100 C. Further decrease in deposition temperature to below 100 C caused an additional and abrupt decrease in both Young’s modulus and hardness. The Young’s modulus and hardness of the b-series SiNx films were found to exhibit a similar deposition temperature dependence to the a-series samples, although the values of Young’s modulus and hardness for the b-series were significantly lower. Furthermore, whereas the abrupt decrease in Young’s modulus and hardness occurred around a deposition temperature of 100 C for the a-series, it was observed to be more gradual and at a higher deposition temperature for the b-series SiNx samples. The a- and b-series SiNx thin films were found to be characterized by similar material properties only for deposition temperatures above 300 C. In comparison with results at higher temperatures, a-series and b-series SiNx films deposited at 50 C were characterized by dramatically lower Young’s modulus (hardness) values of 50 GPa (4 GPa) and 3.5 GPa (0.1 GPa), respectively. Furthermore, it has been observed that material properties (including but not limited to modulus, hardness, and stress) of thin films investigated in this work are independent of the substrate
(for Si and GaAs substrates), which suggests that the thin film properties are unaffected by the details of the local bonding environment at the thin film/substrate interface [6]. The room temperature residual stress of PECVD SiNx thin films measured in a laboratory atmosphere is shown in Fig. 3 as a function of the deposition temperature. After thin film deposition the samples were stored in laboratory atmosphere, and the results presented in Fig. 3 consist of data for a-series PECVD SiNx that were obtained four months (open diamonds), five months (open circles), and 18 months (open squares) after deposition, as well as of data for b-series samples that were obtained within one day of deposition. The results clearly indicate that the stress values measured for films deposited at and above 150 C do not change with time, and that a linear dependence of residual stress on deposition temperature is observed, which is consistent with previously reported results [7–10]. For a-series PECVD SiNx as the deposition temperature is decreased from 300 C to 150 C the magnitude of the thin film residual stress changes linearly from 330 MPa tensile stress to 160 MPa compressive stress. Films deposited at 100 C and below were found to deviate from the linear relationship observed for depositions above 100 C, and exhibit stress values that become increasingly more tensile with extended storage time. It is evident from Fig. 3 that, similar to the residual stress values of a-series PECVD SiNx, the stress of b-series PECVD SiNx is highly tensile (330 MPa) at high-deposition temperatures (300 C) and decreases with decreasing deposition temperature. However, whereas the a-series
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SiNx films exhibit near-zero stress for 200–225 C deposition temperatures, the stress magnitude of b-series SiNx is minimized at the much lower deposition temperature of 125 C. Thermally evaporated Ge and SiOx thin films were characterized by a nominal 70 MPa tensile stress as measured in laboratory conditions and 40 MPa compressive stress as measured in vacuum, respectively. These values are indicated in Fig. 3 by dotted and dashed lines for Ge and SiOx thin films, respectively. The generally tensile nature and high Young’s modulus and hardness of SiNx films deposited at high-temperatures (300 C) using a SiH4/NH3/N2 chemistry can be correlated with hydrogen incorporation and the amount of energy supplied to constituent species during the thin film formation process [7,10]. In essence, increasing deposition temperature results in increasingly more energetic species on the surface of the thin film being formed, such that any condensing constituent hydrogen species or other loosely bound particles would exhibit an increasing probability of desorption. This results in the formation of additional favourable bonding sites for the silicon and nitrogen constituent atoms, as well as stimulating bonding and crosslinking of already adhered species. The end result is a denser and more tightly bonded structure that contains less hydrogen species, resulting in a built-in tensile stress, and a highYoung’s modulus and hardness. The decrease in energy of condensing particles associated with a decrease in deposition temperature results in an increase in hydrogen incorporation in the thin film structure and a correspondingly lower degree of interatomic cross-linking, thus decreasing the tensile nature of the thin film and lowering Young’s modulus and hardness. At even lower deposition temperatures (100 C), adhering constituent particles do not have sufficient energy to rearrange themselves into their preferred positions/orientations in order to form a dense film and, due to the high-concentration of incorporated hydrogen, there is a tendency to form a compressive thin film. The above explanation is in agreement with Heavy-Ion Elastic Recoil Detection Analysis (HI-ERDA) experiments [11,12], which were performed on a-series PECVD SiNx in order to identify thin film constituent species. The resultant thin film atomic compositions have been published in Ref. [5]. Similar to previously reported results [7,13,14], the thin films were found to contain considerable concentrations of hydrogen, which decrease from 38 at.% for films deposited at 100 C to 20 at.% for films deposited at 300 C. Associated with the decrease in hydrogen content with increasing deposition temperature is an increase in silicon and nitrogen species, and an increase in the silicon-tonitrogen ratio from Si/N 0.55 for 100 C deposited films to Si/N 0.65 for 300 C deposited films [5]. These findings are consistent with previous reports [7] indicating that decreasing the deposition temperature of SiNx results in a higher concentration of weaker N–H bonds [15], lower density [16], higher coefficient of thermal expansion [5] and, as observed in this work, lower values of Young’s modulus and hardness.
The results of this study of PECVD SiNx shows that films deposited with higher RF power at lower pressures (a-series SiNx) were found to be, in general, characterized by higher compressive or less tensile values (see Fig. 3), as well as higher Young’s modulus and hardness (see Fig. 2) in comparison to films deposited with lower RF power at higher pressures (b-series SiNx). Since all other deposition parameters were held constant, this observation is a direct consequence of the differences in RF power and chamber pressure between a- and b-series PECVD SiNx. Increasing the RF power generates more reactive and energetic species, as well as increasing the potential difference in the electric field (DC bias) between the plasma and the sample surface. These conditions tend to produce more compact and more compressive films characterized by higher modulus and hardness. The formation of more compact and compressive films is also enhanced with decreasing deposition pressure, since the mean free path of gaseous species in the chamber will increase, resulting in a decreasing probability of interspecies collisions and hence more energetic species on the surface of the thin film. Irrespective of the deposition pressure or RF power, the a- and b-series PECVD SiNx films deposited at temperatures 300 C have all been found to share a state of similar tensile stress (see Fig. 3) as well as the same Young’s modulus and hardness (see Fig. 2). This suggests that temperature-stimulated desorption of loosely adhered species, rather than compacting of the thin film structure by energetic ions, is the dominant mechanism in the formation of SiNx thin films at high-temperature. On the other hand, the thermal energy supplied to the thin film formation process at deposition temperatures below 300 C may be insufficient to produce a dense film, and the stress state of the thin film becomes sensitive to the degree of thin film densification. The HI-ERDA measurements also indicated that atmospheric oxidation has occurred for films deposited below 100 C [17,18]. The atomic composition obtained for films deposited at 50 C (Si 30 at.%, O 60 at.%, N 0 at.%, and H 10 at.%) is consistent with a silicon dioxide film with 10 at.% of incorporated hydrogen, and implies complete conversion into SiOx. The 80 C deposited film was found to be only partially converted to SiOx [5]. In addition, SiNx films deposited at temperatures below 100 C were found to be porous, which was inferred by the presence of etch-pits in thin film coated silicon substrates etched in 40 wt.% potassium hydroxide (KOH) aqueous solution at 80 C, indicating the presence of pinholes in the SiNx thin films. Films deposited below 100 C were characterized by a pinhole density that was more than one order of magnitude greater than that found in films deposited above 100 C. This explains the very different hardness and Young’s modulus values obtained for films deposited below and above 100 C. The increasing tensile stress associated with long-term atmospheric storage of low-temperature deposited SiNx films (see Fig. 3) correlates well with the observed oxida-
M. Martyniuk et al. / Microelectronics Reliability 47 (2007) 733–738
difference between stress at atmospheric pressure and stress in vacuum for
change in residual stress [MPa]
150
PECVD SiNx of
α−series β −series
100
50
0 50
100
150
200
250
300
350
deposition temperature [ ο C] Fig. 4. The difference between stress measured at atmospheric pressure to that measured under vacuum for a-series (open diamonds) and b-series (full circles) PECVD SiNx thin films as a function of the deposition temperature.
0.8
7.5V
0V
0.7
normalized responsivity
tion of these films. This correlation is interesting since asdeposited SiOx films are generally compressively stressed [19,20], and the trend towards increasing tensile stress with oxidation is in contradiction with the observed increase in compressive stress for silicon monoxide films exposed to an oxidizing ambient [21,22]. However, the increasingly tensile nature of the oxidized SiNx films can be related to the transformation of SiNx into SiOx. At the same time as oxygen is incorporated into the film matrix, N–H groups are removed from the film structure, resulting in film shrinkage and tensile stress development. It has been observed that low-temperature PECVD SiNx films, whose stress state changes with prolonged exposure to atmosphere, also exhibit significantly more tensile stress values when measured in vacuum as compared to their stress measured at atmospheric pressure (see Fig. 4). The data is surprisingly consistent, given that the data for b-series SiNx were obtained on the day of the deposition event, whereas the displayed data for a-series SiNx were obtained 18 months after deposition. The observed results can be readily attributed to the increasingly porous nature of SiNx films deposited at lower temperatures, such that any reduction in the ambient pressure will drive out any loosely incorporated species from the film network, leading to film shrinkage and a more tensile film state. It has been found that the observed stress change is reversible upon re-exposing the film to, not only atmosphere, but also to high-purity nitrogen, helium, argon, or oxygen. This rules out any change associated with film oxidation, or incorporated species dipole interactions [23], as likely processes that may be responsible for the different stress values measured between atmospheric pressure and in vacuum. Technological applications where thin films serve as structural layers of suspended structures may require more diligent stress control. Additional processing and the intro-
737
0.6 0.5 0.4 0.3 0.2 0.1 0 1.6
1.8
2
2.2
2.4
wavelength (micrometers) Fig. 5. Optical transmission of a Fabry–Perot tunable filter fabricated monolithically on a HgCdTe photoconductor, as determined from detector responsivity. Applied filter electrostatic actuation voltages range from 0 V to 7.5 V [24].
duction of sacrificial layers involved in fabricating free standing structures could in effect influence the stress of the thin films as compared to thin film layers deposited directly on substrates. 4. Conclusions The extensive characterization of structural properties of SiNx, SiOx, and Ge thin films presented here indicates that a multilayered suspended IR reflector structure, that is free of stress-induced deformations can be realized via fabrication processes which do not exceed 125 C. Young’s modulus and hardness of thin films which form the multilayered suspended structure satisfy the mechanical requirements of MEMS actuation devices. The thermally evaporated SiOx thin film is nominally compressively stressed, which is compensated by sandwiching it between two thermally evaporated Ge layers characterized by nominally tensile stress. The IR reflecting Ge/SiOx/Ge tri-layer is in turn embedded in (b-series) PECVD SiNx deposited at 125 C, which has been shown to be essentially stress free. The presented results were used to develop an integrated MEMS filter/IR detector structure, which has been shown to be viable with successful prototypes having been fabricated [24]. Fig. 5 shows the measured optical response of a monolithically integrated tunable filter and IR detector structure, which has been fabricated using the presented mirror/membrane structure on an HgCdTe photoconductor. The structural properties of the SiNx, SiOx, and Ge layers encompassed in the suspended IR reflector allow for short-wave IR (SWIR) photon detection in a narrow wavelength band of full-width at half-maximum of 100 nm that is tunable over a range of 2.2–1.85 lm with a maximum tuning voltage of only 7.5 V. This restricted
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tuning range is a consequence of snap-down, which occurs at a comparatively low voltage for voltage-tuned devices in comparison to charge tuning. Since long-wave IR (LWIR) detectors need to operate at cryogenic temperatures for ultimate performance, stresses in the suspended actuator need to be controlled over a wide temperature range. Therefore, investigations aimed at gaining a precise knowledge of the coefficient of thermal expansion for all materials used in device fabrication are currently underway, with some preliminary results already published [5]. Although the objectives of this work have been focused on a specific application related to multi-spectral IR detection technology, the findings presented are directly applicable to any MEMS technologies which need to be merged with temperature-sensitive substrates/materials. Acknowledgements The authors would like to thank Robert G. Elliman at the Australian National University for determining thin film composition via HI-ERDA experiments, and the Australian Research Council (ARC) for financial support of this work. References [1] Martyniuk M, Sewell RH, Musca CA, Dell JM, Faraone L. Nanoindentation of HgCdTe prepared by molecular beam epitaxy. Appl Phys Lett 2005;87:251905-3. [2] Oliver WC, Pharr GM. Measurement of hardness and elastic modulus by instrumented indentation: advances in understanding and refinements to methodology. J Mater Res 2004;19:3–20. [3] Jung Y-G, Lawn BR, Martyniuk M, Huang H, Hu XZ. Evaluation of elastic modulus and hardness of thin films by nanoindentation. J Mater Res 2004;19:3076–80. [4] Fischer-Cripps AC. Critical review of analysis and interpretation of nanoindentation test data. Surf Coat Technol 2006;200:4153–65. [5] Martyniuk M, Antoszewski J, Musca CA, Dell JM, Faraone L. Environmental stability and cryogenic thermal cycling of lowtemperature plasma-deposited silicon nitride thin films. J Appl Phys 2006;99:53519-1. [6] Martyniuk M. Low-Temperature Micro-Opto-Electro-Mechanical Technologies for Temperature Sensitive Substrates, PhD thesis, University of Western Australia, 2006. [7] Smith DL, Alimonda AS, Chen C-C, Ready SE, Wacker B. Mechanism of SiNxHy deposition from NH3–SiH4 plasma. J Electrochem Soc 1990;137:614–23. [8] Claassen WAP, Valkenburg WGJN, Willemsen MFC, Van De Wijgert WM. Influence of deposition temperature, gas pressure, gas
[9]
[10]
[11]
[12]
[13]
[14]
[15]
[16]
[17]
[18]
[19]
[20]
[21] [22] [23] [24]
phase composition, and RF frequency on composition and mechanical stress of plasma silicon nitride layers. J Electrochem Soc 1985;132:893–9. Winchester KJ, Dell JM. Tunable Fabry–Perot cavities fabricated from PECVD silicon nitride employing zinc sulphide as the sacrificial layer. J Micromech Microeng 2001;11:589–94. Winchester KJ. Tunable Microelectromechanical Fabry–Perot Filters Using Plasma Enhanced Chemical Vapour Deposited Silicon Nirtide Membranes. PhD Thesis, University of Western Australia, 2003. Elliman RG, Timmers H, Weijers TDM. Hydrogen detection with a gas ionization elastic recoil detector. Nucl Instrum Methods Phys Res B: Beam Interact Mater Atoms 2004;219–220:410–4. Weijers TDM, Elliman RG, Timmers H. Heavy ion elastic recoil detection analysis of silicon-rich silica films. Nucl Instrum Methods Phys Res B: Beam Interact Mater Atoms 2004;219–220:680–5. Thurn J, Cook RF, Kamarajugadda M, Bozeman SP, Stearns LC. Stress hysteresis and mechanical properties of plasma-enhanced chemical vapor deposited dielectric films. J Appl Phys 2004;95: 967–76. Hughey MP, Cook RF. Stress development kinetics in plasmaenhanced chemical-vapor-deposited silicon nitride films. J Appl Phys 2005;97:114914-1. Soh MTK, Savvides N, Musca CA, Martyniuk M, Faraone L. Local bonding environment of plasma deposited nitrogen-rich silicon nitride thin films. J Appl Phys 2005;97:93714-1. Walmsley BA, Liu Y, Hu XZ, Bush MB, Winchester KJ, Martyniuk M, et al. Effects of deposition temperature on the mechanical and physical properties of silicon nitride thin films. J Appl Phys 2005;98:44904-1. Liao W-S, Lin C-H, Lee S-C. Oxidation of silicon nitride prepared by plasma-enhanced chemical vapor deposition at low temperature. Appl Phys Lett 1994;65:2229–31. Chang H-Y, Meng C-Y, Huang C-W, Lee S-C. The low-temperature a-SiNx films with high impermeability and high optical gap with application to organic light-emitting diode. J Appl Phys 2005;98: 84501–5. Thurn J, Cook RF. Stress hysteresis during thermal cycling of plasma-enhanced chemical vapor deposited silicon oxide films. J Appl Phys 2002;91:1988–92. Haque MS, Naseem HA, Brown WD. Residual stress behavior of thin plasma-enhanced chemical vapor deposited silicon dioxide films as a function of storage time. J Appl Phys 1997;81:3129–33. Novice MA. Effect of atmospheric exposure on stress in evaporated silicon monoxide films. Br J Appl Phys 1962;13:561–3. Hill AE, Hoffman GR. Stress in films of silicon monoxide. Br J Appl Phys 1967;18:13–22. Hirsch EH. On the origin of adsorption stress in thin porous films. J Phys D (Appl Phys) 1982;15:1991–2002. Musca CA, Antoszewski J, Winchester KJ, Keating AJ, Nguyen T, Silva KKMBD, et al. Monolithic integration of an infrared photon detector with a MEMS-based tunable filter. IEEE Electron Dev Lett 2005;26:888–90.
Dielectric thin films for MEMS-based optical sensors M. Martyniuk *, J. Antoszewski, C.A. Musca, J.M. Dell, L. Faraone School of Electrical, Electronic and Computer Engineering, The University of Western Australia, Crawley, WA 6009, Australia Available online 13 March 2007
Abstract Nanoindentation and optical measurements have been employed in order to investigate the mechanical properties of low-temperature (50–330 C) plasma-enhanced chemical vapour deposited (PECVD) SiNx, as well as thermally evaporated SiOx and Ge thin films for applications in micro-electro-mechanical systems (MEMS) fabricated on temperature sensitive, non-standard substrates. The temperature of the SiNx deposition process is found to strongly influence Young’s modulus, hardness, and stress, with a critical deposition temperature in the 100 C to 150 C range which depends on the details of other deposition conditions such as chamber pressure and RF-power. The properties of PECVD SiNx films deposited above this critical temperature are found to be suitable for MEMS applications, whereas films deposited at lower temperatures exhibit low Young’s modulus and hardness, as well as environment-induced stress instabilities. The investigated thin films have been incorporated into a monolithic integrated technology comprising low-temperature (125 C) MEMS and HgCdTe IR detectors, in order to realize successful prototypes of tuneable IR microspectrometers. 2007 Elsevier Ltd. All rights reserved.
1. Introduction The salient feature of next generation infrared (IR) onchip integrated sensors is likely to be sensitivity in a narrow wavelength band that is tuneable over a selected range of the IR spectrum. It is proposed that this can be achieved by the integration of present-day IR detectors with thinfilm based micro-electro-mechanical systems (MEMS) optical mirror technology, as shown schematically in Fig. 1. Narrow-band sensitivity is obtained by optical resonance phenomena within a Fabry–Perot (FP) cavity, that is created by two Bragg reflectors and is monolithically integrated with the IR detector. Electrostatic actuation of the thin-film membrane supported top mirror is the means of providing wavelength discrimination of the incident IR photons which, for example, could be used for target discrimination or remote sensing of various chemical/biological species via identification of narrow spectral features. The critical element in the realization of such a proposed infrared ‘‘microspectrometer’’ device is the suspended membrane/mirror actuator, which is a multilayered struc*
Corresponding author. E-mail address: [email protected] (M. Martyniuk).
0026-2714/$ - see front matter 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.microrel.2007.01.060
ture consisting of SiNx, SiOx, and Ge individual layers. This suspended, flexible, and IR reflecting structure needs to be free of any warping, and exhibit a high-degree of parallelism relative to the bottom mirror. The presence of excessive stress levels, stress gradients, or overall stress imbalance in the suspended structure leads to tilt, warp, collapse, rupture and/or deformation of the top membrane/mirror actuator. In addition, the suspended membrane/mirror structure needs to be sufficiently flexible to allow the FP cavity length to be electrostatically varied at reasonably low-voltages; however, the Bragg mirror itself needs to be stiff enough so that it is self-supporting and does not deform under electrostatic actuation conditions. Any mirror non-parallelism or deformation will lead to undesirable degradation of the FP cavity finesse or, in extreme cases, could lead to optical instability of the cavity. The mechanical properties of the individual SiNx, SiOx, and Ge layers, and in particular the Young’s moduli, thin film stresses, as well as stress gradients, are critical parameters that determine the mechanical response of the suspended structure to the actuating electrostatic field. Moreover, in a monolithic integrated approach, high-quality thin-film layers need to be attained under the stringent requirement of low-temperature processing (<125 C) for
734
M. Martyniuk et al. / Microelectronics Reliability 47 (2007) 733–738 Table 1 The investigated thin film samples Sample
Preparation parameters thickness
Film thickness
a-series PECVD SiNx
SiH4 – 5 sccm Gas chemistry: NH3 – 45 sccm N2 – 100 sccm RF: 100 W at 13.6 MHz Pressure: 450 mTorr Temperature: 50–300 C Substrate: Si and/or GaAs
500 nm
b-series PECVD SiNx
SiH4 – 5 sccm Gas chemistry: NH3 – 45 sccm N2 – 100 sccm RF: 75 W at 13.6 MHz Pressure: 875 mTorr Temperature: 55–330 C Substrate: Si and/or GaAs
700 nm
Thermally evaporated SiOx
Source material: silicon monoxide ˚ /s Rate: 10 A Pressure: 4 · 10 6 mbar Temperature: 25 C Substrate: Si
280 nm
Thermally evaporated Ge
Source material: polycrystalline Ge ˚ /s Rate: 10 A Base pressure: 1 · 10 7 mbar Temperature: 25 C Substrate: Si
150 nm
Fig. 1. General concept of Fabry–Perot interferometric filter using Ge/ SiOx/Ge mirrors formed on an IR detector. Wavelength tuning is achieved through electrostatic actuation of the optical cavity length.
HgCdTe-based IR technology, where exposure to higher temperatures has the potential to cause irreversible device damage. This paper presents an investigation of the mechanical properties of low-temperature (50–330 C) plasma-enhanced chemical vapour deposited (PECVD) structural SiNx thin films, as well as thermally evaporated Ge and SiOx thin films utilized in the Bragg IR mirrors. The employed characterization techniques consist of depth sensing indentation (DSI), as well as optical measurements of stress-induced substrate curvature. 2. Experimental details The investigated samples consisted of two series of PECVD SiNx samples, labeled a- and b-series, that were deposited in an Oxford Instruments PECVD 80 system. In addition, thermally evaporated SiOx and Ge were also prepared. The deposition temperature of both a- and b-series PECVD SiNx was varied from 50 C to 330 C and all other deposition parameters were held constant for all samples within each series. In comparison to the deposition conditions of the a-series PECVD SiNx, the RF power was decreased and the chamber pressure was increased for the b-series samples. See Table 1 for details of the sample preparation. All thin films were deposited on 100 lm thick Si or GaAs substrates. The nanoindentation experiments were performed using a Hysitron TriboScope DSI system in conjunction with a Digital Instruments DimentionTM 3000 NanoScope IIIa Scanning Probe Microscope (SPM). The indenter load function (load versus time) that was adopted for the indentation experiments was the same as published in Ref. [1], where the final unloading cycle was used for the extraction of the elastic modulus and hardness of the thin film/substrate bi-layers [2]. The Young’s modulus and hardness of the thin film material was extracted according to the procedures presented in Ref. [3] which were further developed in Ref. [4]. The thin film stress was determined through measurements of stress induced substrate curvature, which was
obtained from changes in the angle of reflection from coated substrates of parallel incident laser beams according to the setup and procedures published in Ref. [5]. The measurement apparatus allowed for stress measurements to be performed under controlled ambient conditions. 3. Results and discussion Fig. 2 presents the Young’s modulus and hardness as a function of thin film deposition temperature for a-series (open diamonds) and b-series (open circles) PECVD SiNx. The Young’s modulus (hardness) values of thermally evaporated SiOx and Ge were determined to be 102 GPa (4.5 GPa) and 92 GPa (6.6 GPa), respectively, and are represented by dashed and dotted lines. A Poisson ratio of 0.25 was assumed in the determination of Young’s modulus values for all films. Considering the results obtained for PECVD SiNx thin films, it is evident that both Young’s modulus and hardness are strongly influenced by the deposition temperature. Decreasing the temperature of the PECVD process causes Young’s modulus and hardness to decrease in a similar fashion. For the a-series films deposited at temperatures above 225 C, a constant Young’s modulus (hardness) value of about 150–160 GPa (14–15 GPa) was obtained. A moderate and approximately linear decrease from 150 GPa to 115 GPa (14 GPa to 10 GPa) in Young’s modulus (hardness) was found to be associated with decreasing deposition temperature from
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deposition temperature [ο C] 200
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Fig. 3. Deposition temperature dependence of residual stress of a-series PECVD SiNx as measured four months (open diamonds), five months (open circles), and 18 months (open squares) after thin film deposition, as well as of b-series PECVD SiNx (full triangles) as measured within one day of deposition. The solid lines joining the data points are simply a guide for the eye. Positive stress values denote tensile stress and negative stress values denote compressive stress.
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deposition temperature [ C] Fig. 2. Young’s modulus and hardness as a function of deposition temperature for a-series (open diamonds) and b-series (open circles) PECVD SiNx. The solid lines joining the data points are simply a guide for the eye. The dashed and dotted lines represent the parameters obtained for thermally evaporated SiOx and Ge, respectively.
200 C to 100 C. Further decrease in deposition temperature to below 100 C caused an additional and abrupt decrease in both Young’s modulus and hardness. The Young’s modulus and hardness of the b-series SiNx films were found to exhibit a similar deposition temperature dependence to the a-series samples, although the values of Young’s modulus and hardness for the b-series were significantly lower. Furthermore, whereas the abrupt decrease in Young’s modulus and hardness occurred around a deposition temperature of 100 C for the a-series, it was observed to be more gradual and at a higher deposition temperature for the b-series SiNx samples. The a- and b-series SiNx thin films were found to be characterized by similar material properties only for deposition temperatures above 300 C. In comparison with results at higher temperatures, a-series and b-series SiNx films deposited at 50 C were characterized by dramatically lower Young’s modulus (hardness) values of 50 GPa (4 GPa) and 3.5 GPa (0.1 GPa), respectively. Furthermore, it has been observed that material properties (including but not limited to modulus, hardness, and stress) of thin films investigated in this work are independent of the substrate
(for Si and GaAs substrates), which suggests that the thin film properties are unaffected by the details of the local bonding environment at the thin film/substrate interface [6]. The room temperature residual stress of PECVD SiNx thin films measured in a laboratory atmosphere is shown in Fig. 3 as a function of the deposition temperature. After thin film deposition the samples were stored in laboratory atmosphere, and the results presented in Fig. 3 consist of data for a-series PECVD SiNx that were obtained four months (open diamonds), five months (open circles), and 18 months (open squares) after deposition, as well as of data for b-series samples that were obtained within one day of deposition. The results clearly indicate that the stress values measured for films deposited at and above 150 C do not change with time, and that a linear dependence of residual stress on deposition temperature is observed, which is consistent with previously reported results [7–10]. For a-series PECVD SiNx as the deposition temperature is decreased from 300 C to 150 C the magnitude of the thin film residual stress changes linearly from 330 MPa tensile stress to 160 MPa compressive stress. Films deposited at 100 C and below were found to deviate from the linear relationship observed for depositions above 100 C, and exhibit stress values that become increasingly more tensile with extended storage time. It is evident from Fig. 3 that, similar to the residual stress values of a-series PECVD SiNx, the stress of b-series PECVD SiNx is highly tensile (330 MPa) at high-deposition temperatures (300 C) and decreases with decreasing deposition temperature. However, whereas the a-series
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SiNx films exhibit near-zero stress for 200–225 C deposition temperatures, the stress magnitude of b-series SiNx is minimized at the much lower deposition temperature of 125 C. Thermally evaporated Ge and SiOx thin films were characterized by a nominal 70 MPa tensile stress as measured in laboratory conditions and 40 MPa compressive stress as measured in vacuum, respectively. These values are indicated in Fig. 3 by dotted and dashed lines for Ge and SiOx thin films, respectively. The generally tensile nature and high Young’s modulus and hardness of SiNx films deposited at high-temperatures (300 C) using a SiH4/NH3/N2 chemistry can be correlated with hydrogen incorporation and the amount of energy supplied to constituent species during the thin film formation process [7,10]. In essence, increasing deposition temperature results in increasingly more energetic species on the surface of the thin film being formed, such that any condensing constituent hydrogen species or other loosely bound particles would exhibit an increasing probability of desorption. This results in the formation of additional favourable bonding sites for the silicon and nitrogen constituent atoms, as well as stimulating bonding and crosslinking of already adhered species. The end result is a denser and more tightly bonded structure that contains less hydrogen species, resulting in a built-in tensile stress, and a highYoung’s modulus and hardness. The decrease in energy of condensing particles associated with a decrease in deposition temperature results in an increase in hydrogen incorporation in the thin film structure and a correspondingly lower degree of interatomic cross-linking, thus decreasing the tensile nature of the thin film and lowering Young’s modulus and hardness. At even lower deposition temperatures (100 C), adhering constituent particles do not have sufficient energy to rearrange themselves into their preferred positions/orientations in order to form a dense film and, due to the high-concentration of incorporated hydrogen, there is a tendency to form a compressive thin film. The above explanation is in agreement with Heavy-Ion Elastic Recoil Detection Analysis (HI-ERDA) experiments [11,12], which were performed on a-series PECVD SiNx in order to identify thin film constituent species. The resultant thin film atomic compositions have been published in Ref. [5]. Similar to previously reported results [7,13,14], the thin films were found to contain considerable concentrations of hydrogen, which decrease from 38 at.% for films deposited at 100 C to 20 at.% for films deposited at 300 C. Associated with the decrease in hydrogen content with increasing deposition temperature is an increase in silicon and nitrogen species, and an increase in the silicon-tonitrogen ratio from Si/N 0.55 for 100 C deposited films to Si/N 0.65 for 300 C deposited films [5]. These findings are consistent with previous reports [7] indicating that decreasing the deposition temperature of SiNx results in a higher concentration of weaker N–H bonds [15], lower density [16], higher coefficient of thermal expansion [5] and, as observed in this work, lower values of Young’s modulus and hardness.
The results of this study of PECVD SiNx shows that films deposited with higher RF power at lower pressures (a-series SiNx) were found to be, in general, characterized by higher compressive or less tensile values (see Fig. 3), as well as higher Young’s modulus and hardness (see Fig. 2) in comparison to films deposited with lower RF power at higher pressures (b-series SiNx). Since all other deposition parameters were held constant, this observation is a direct consequence of the differences in RF power and chamber pressure between a- and b-series PECVD SiNx. Increasing the RF power generates more reactive and energetic species, as well as increasing the potential difference in the electric field (DC bias) between the plasma and the sample surface. These conditions tend to produce more compact and more compressive films characterized by higher modulus and hardness. The formation of more compact and compressive films is also enhanced with decreasing deposition pressure, since the mean free path of gaseous species in the chamber will increase, resulting in a decreasing probability of interspecies collisions and hence more energetic species on the surface of the thin film. Irrespective of the deposition pressure or RF power, the a- and b-series PECVD SiNx films deposited at temperatures 300 C have all been found to share a state of similar tensile stress (see Fig. 3) as well as the same Young’s modulus and hardness (see Fig. 2). This suggests that temperature-stimulated desorption of loosely adhered species, rather than compacting of the thin film structure by energetic ions, is the dominant mechanism in the formation of SiNx thin films at high-temperature. On the other hand, the thermal energy supplied to the thin film formation process at deposition temperatures below 300 C may be insufficient to produce a dense film, and the stress state of the thin film becomes sensitive to the degree of thin film densification. The HI-ERDA measurements also indicated that atmospheric oxidation has occurred for films deposited below 100 C [17,18]. The atomic composition obtained for films deposited at 50 C (Si 30 at.%, O 60 at.%, N 0 at.%, and H 10 at.%) is consistent with a silicon dioxide film with 10 at.% of incorporated hydrogen, and implies complete conversion into SiOx. The 80 C deposited film was found to be only partially converted to SiOx [5]. In addition, SiNx films deposited at temperatures below 100 C were found to be porous, which was inferred by the presence of etch-pits in thin film coated silicon substrates etched in 40 wt.% potassium hydroxide (KOH) aqueous solution at 80 C, indicating the presence of pinholes in the SiNx thin films. Films deposited below 100 C were characterized by a pinhole density that was more than one order of magnitude greater than that found in films deposited above 100 C. This explains the very different hardness and Young’s modulus values obtained for films deposited below and above 100 C. The increasing tensile stress associated with long-term atmospheric storage of low-temperature deposited SiNx films (see Fig. 3) correlates well with the observed oxida-
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difference between stress at atmospheric pressure and stress in vacuum for
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tion of these films. This correlation is interesting since asdeposited SiOx films are generally compressively stressed [19,20], and the trend towards increasing tensile stress with oxidation is in contradiction with the observed increase in compressive stress for silicon monoxide films exposed to an oxidizing ambient [21,22]. However, the increasingly tensile nature of the oxidized SiNx films can be related to the transformation of SiNx into SiOx. At the same time as oxygen is incorporated into the film matrix, N–H groups are removed from the film structure, resulting in film shrinkage and tensile stress development. It has been observed that low-temperature PECVD SiNx films, whose stress state changes with prolonged exposure to atmosphere, also exhibit significantly more tensile stress values when measured in vacuum as compared to their stress measured at atmospheric pressure (see Fig. 4). The data is surprisingly consistent, given that the data for b-series SiNx were obtained on the day of the deposition event, whereas the displayed data for a-series SiNx were obtained 18 months after deposition. The observed results can be readily attributed to the increasingly porous nature of SiNx films deposited at lower temperatures, such that any reduction in the ambient pressure will drive out any loosely incorporated species from the film network, leading to film shrinkage and a more tensile film state. It has been found that the observed stress change is reversible upon re-exposing the film to, not only atmosphere, but also to high-purity nitrogen, helium, argon, or oxygen. This rules out any change associated with film oxidation, or incorporated species dipole interactions [23], as likely processes that may be responsible for the different stress values measured between atmospheric pressure and in vacuum. Technological applications where thin films serve as structural layers of suspended structures may require more diligent stress control. Additional processing and the intro-
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wavelength (micrometers) Fig. 5. Optical transmission of a Fabry–Perot tunable filter fabricated monolithically on a HgCdTe photoconductor, as determined from detector responsivity. Applied filter electrostatic actuation voltages range from 0 V to 7.5 V [24].
duction of sacrificial layers involved in fabricating free standing structures could in effect influence the stress of the thin films as compared to thin film layers deposited directly on substrates. 4. Conclusions The extensive characterization of structural properties of SiNx, SiOx, and Ge thin films presented here indicates that a multilayered suspended IR reflector structure, that is free of stress-induced deformations can be realized via fabrication processes which do not exceed 125 C. Young’s modulus and hardness of thin films which form the multilayered suspended structure satisfy the mechanical requirements of MEMS actuation devices. The thermally evaporated SiOx thin film is nominally compressively stressed, which is compensated by sandwiching it between two thermally evaporated Ge layers characterized by nominally tensile stress. The IR reflecting Ge/SiOx/Ge tri-layer is in turn embedded in (b-series) PECVD SiNx deposited at 125 C, which has been shown to be essentially stress free. The presented results were used to develop an integrated MEMS filter/IR detector structure, which has been shown to be viable with successful prototypes having been fabricated [24]. Fig. 5 shows the measured optical response of a monolithically integrated tunable filter and IR detector structure, which has been fabricated using the presented mirror/membrane structure on an HgCdTe photoconductor. The structural properties of the SiNx, SiOx, and Ge layers encompassed in the suspended IR reflector allow for short-wave IR (SWIR) photon detection in a narrow wavelength band of full-width at half-maximum of 100 nm that is tunable over a range of 2.2–1.85 lm with a maximum tuning voltage of only 7.5 V. This restricted
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tuning range is a consequence of snap-down, which occurs at a comparatively low voltage for voltage-tuned devices in comparison to charge tuning. Since long-wave IR (LWIR) detectors need to operate at cryogenic temperatures for ultimate performance, stresses in the suspended actuator need to be controlled over a wide temperature range. Therefore, investigations aimed at gaining a precise knowledge of the coefficient of thermal expansion for all materials used in device fabrication are currently underway, with some preliminary results already published [5]. Although the objectives of this work have been focused on a specific application related to multi-spectral IR detection technology, the findings presented are directly applicable to any MEMS technologies which need to be merged with temperature-sensitive substrates/materials. Acknowledgements The authors would like to thank Robert G. Elliman at the Australian National University for determining thin film composition via HI-ERDA experiments, and the Australian Research Council (ARC) for financial support of this work. References [1] Martyniuk M, Sewell RH, Musca CA, Dell JM, Faraone L. Nanoindentation of HgCdTe prepared by molecular beam epitaxy. Appl Phys Lett 2005;87:251905-3. [2] Oliver WC, Pharr GM. Measurement of hardness and elastic modulus by instrumented indentation: advances in understanding and refinements to methodology. J Mater Res 2004;19:3–20. [3] Jung Y-G, Lawn BR, Martyniuk M, Huang H, Hu XZ. Evaluation of elastic modulus and hardness of thin films by nanoindentation. J Mater Res 2004;19:3076–80. [4] Fischer-Cripps AC. Critical review of analysis and interpretation of nanoindentation test data. Surf Coat Technol 2006;200:4153–65. [5] Martyniuk M, Antoszewski J, Musca CA, Dell JM, Faraone L. Environmental stability and cryogenic thermal cycling of lowtemperature plasma-deposited silicon nitride thin films. J Appl Phys 2006;99:53519-1. [6] Martyniuk M. Low-Temperature Micro-Opto-Electro-Mechanical Technologies for Temperature Sensitive Substrates, PhD thesis, University of Western Australia, 2006. [7] Smith DL, Alimonda AS, Chen C-C, Ready SE, Wacker B. Mechanism of SiNxHy deposition from NH3–SiH4 plasma. J Electrochem Soc 1990;137:614–23. [8] Claassen WAP, Valkenburg WGJN, Willemsen MFC, Van De Wijgert WM. Influence of deposition temperature, gas pressure, gas
[9]
[10]
[11]
[12]
[13]
[14]
[15]
[16]
[17]
[18]
[19]
[20]
[21] [22] [23] [24]
phase composition, and RF frequency on composition and mechanical stress of plasma silicon nitride layers. J Electrochem Soc 1985;132:893–9. Winchester KJ, Dell JM. Tunable Fabry–Perot cavities fabricated from PECVD silicon nitride employing zinc sulphide as the sacrificial layer. J Micromech Microeng 2001;11:589–94. Winchester KJ. Tunable Microelectromechanical Fabry–Perot Filters Using Plasma Enhanced Chemical Vapour Deposited Silicon Nirtide Membranes. PhD Thesis, University of Western Australia, 2003. Elliman RG, Timmers H, Weijers TDM. Hydrogen detection with a gas ionization elastic recoil detector. Nucl Instrum Methods Phys Res B: Beam Interact Mater Atoms 2004;219–220:410–4. Weijers TDM, Elliman RG, Timmers H. Heavy ion elastic recoil detection analysis of silicon-rich silica films. Nucl Instrum Methods Phys Res B: Beam Interact Mater Atoms 2004;219–220:680–5. Thurn J, Cook RF, Kamarajugadda M, Bozeman SP, Stearns LC. Stress hysteresis and mechanical properties of plasma-enhanced chemical vapor deposited dielectric films. J Appl Phys 2004;95: 967–76. Hughey MP, Cook RF. Stress development kinetics in plasmaenhanced chemical-vapor-deposited silicon nitride films. J Appl Phys 2005;97:114914-1. Soh MTK, Savvides N, Musca CA, Martyniuk M, Faraone L. Local bonding environment of plasma deposited nitrogen-rich silicon nitride thin films. J Appl Phys 2005;97:93714-1. Walmsley BA, Liu Y, Hu XZ, Bush MB, Winchester KJ, Martyniuk M, et al. Effects of deposition temperature on the mechanical and physical properties of silicon nitride thin films. J Appl Phys 2005;98:44904-1. Liao W-S, Lin C-H, Lee S-C. Oxidation of silicon nitride prepared by plasma-enhanced chemical vapor deposition at low temperature. Appl Phys Lett 1994;65:2229–31. Chang H-Y, Meng C-Y, Huang C-W, Lee S-C. The low-temperature a-SiNx films with high impermeability and high optical gap with application to organic light-emitting diode. J Appl Phys 2005;98: 84501–5. Thurn J, Cook RF. Stress hysteresis during thermal cycling of plasma-enhanced chemical vapor deposited silicon oxide films. J Appl Phys 2002;91:1988–92. Haque MS, Naseem HA, Brown WD. Residual stress behavior of thin plasma-enhanced chemical vapor deposited silicon dioxide films as a function of storage time. J Appl Phys 1997;81:3129–33. Novice MA. Effect of atmospheric exposure on stress in evaporated silicon monoxide films. Br J Appl Phys 1962;13:561–3. Hill AE, Hoffman GR. Stress in films of silicon monoxide. Br J Appl Phys 1967;18:13–22. Hirsch EH. On the origin of adsorption stress in thin porous films. J Phys D (Appl Phys) 1982;15:1991–2002. Musca CA, Antoszewski J, Winchester KJ, Keating AJ, Nguyen T, Silva KKMBD, et al. Monolithic integration of an infrared photon detector with a MEMS-based tunable filter. IEEE Electron Dev Lett 2005;26:888–90.