Article Material Structure and Mechanical Properties of and Thin Films Deposited by Plasma Enhanced Chemical Vapor Deposition

Zhenghao Gan 1, Changzheng Wang 2 and Zhong Chen 1,*

1 School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798, Singapore; [email protected] 2 School of Materials Science and Engineering, Liaocheng University, Liaocheng 252000, China; [email protected] * Correspondence: [email protected]; Tel.: +65-6790-4256

 Received: 9 May 2018; Accepted: 27 August 2018; Published: 30 August 2018 

Abstract: Silicon nitride and silicon oxynitride thin films are widely used in microelectronic fabrication and microelectromechanical systems (MEMS). Their mechanical properties are important for MEMS structures; however, these properties are rarely reported, particularly the fracture toughness of these films. In this study, silicon nitride and silicon oxynitride thin films were deposited by plasma enhanced chemical vapor deposition (PECVD) under different silane flow rates. The silicon nitride films consisted of mixed amorphous and crystalline Si3N4 phases under the range of silane flow rates investigated in the current study, while the crystallinity increased with silane flow rate in the silicon oxynitride films. The Young’s modulus and hardness of silicon nitride films decreased with increasing silane flow rate. However, for silicon oxynitride films, Young’s modulus decreased slightly with increasing silane flow rate, and the hardness increased considerably due to the formation of a crystalline silicon nitride phase at the high flow rate. Overall, the hardness, Young modulus, and fracture toughness of the silicon nitride films were greater than the ones of silicon oxynitride films, and the main reason lies with the phase composition: the SiNx films were composed of a crystalline Si3N4 phase, while the SiOxNy films were dominated by amorphous Si–O phases. Based on the overall mechanical properties, PECVD silicon nitride films are preferred for structural applications in MEMS devices.

Keywords: silicon nitride; silicon oxynitride; thin film; PECVD; mechanical property; hardness; Young’s modulus; fracture toughness

1. Introduction

Silicon nitride (SiNx) and silicon oxynitride (SiOxNy) thin films deposited by plasma enhanced chemical vapor deposition (PECVD) are widely used in electronic device applications including passivation, isolation, insulation, and etch masking. They are also increasingly employed in microelectromechanical systems (MEMS) as functional structures in the form of beams, bridges, and membranes [1–6]. The mechanical properties of the thin films are very important to the design and reliability of the devices. Various studies have been conducted to evaluate the films’ mechanical properties including Young’s modulus, residual and hardness, etc. [7,8]. Adhesion of silicon oxide and nitride to substrate (such as copper) employed in micro- or nano-electronics has also been studied [9,10]. Compared with bulk samples made by powder sintering, data for the mechanical properties of silicon nitride thin films are limited and scattered. This is due to the fact that the

Surfaces 2018, 1, 59–72; doi:10.3390/surfaces1010006 www.mdpi.com/journal/surfaces Surfaces 2018, 1 60 properties of the films are closely related to the film composition, , and microstructure, which are dependent on the deposition condition. Among existing reports, Dong et al. [5] found that the Young’s modulus of a radio frequency (RF) magnetron sputtered amorphous silicon oxynitride film was 122 GPa. Danaie et al. used a low frequency PECVD reactor to prepare silicon oxynitride films with controllable residual stresses via different gas flow rates [6]. Yau et al. [8] also employed an RF magnetron sputtering method to prepare silicon nitride films under different flow rates. The films were amorphous and the nitrogen content was found to have decreased with increasing nitrogen flow rate. The elastic modulus showed a decreasing trend with increasing nitrogen flow rate in the range of 130–155 GPa. Clearly, the film properties strongly depend on the microstructures, which vary with the deposition method and processing parameters [5–12]. Therefore, a systematic analysis of the correlation among composition, microstructure, and mechanical properties is necessary for our chosen PECVD method. In particular, fracture strength and fracture toughness of the silicon nitride and silicon oxynitride films are very important in designing against device failure, but the work in this area is scarce. In fact, to the best of our knowledge, there has been no report available for the fracture toughness of silicon nitride and silicon oxynitride thin films. Most of the published work on fracture toughness of silicon nitride was carried out on bulk specimens using a conventional fracture mechanics test [13,14], and the focus was mainly on the effect of doping [15,16]. However, the fracture toughness of thin films can be very different from the ones measured from bulk materials due to the difference in microstructure and composition. In the available reports on thin film fracture toughness and fracture strength studies on silicon nitride, micro-bridge [17] or micro-bulge specimens [18] had to be fabricated by a lithographic approach, which is both expensive in sample making and inaccurate in result interpretation. For example, in the micro-bridge test, the residual stress in the films had to be estimated in order to calculate the fracture toughness of the thin film [19]. Due to the uncertainty on the residual stresses, the reported value for the critical stress intensity factor ranges from 1.8 ± 0.3 MPa·m1/2 for low-stress film to an upper bound value of 14 MPa·m1/2 for a high-stress film [17]. The micro-bulge test [18], however, can only yield information on the biaxial modulus and tensile fracture strength, not the fracture toughness of the film. No report is available for the fracture toughness and fracture strength of silicon oxynitride thin films. In this paper, the chemical composition, bonding state, microstructure, and mechanical properties of silicon nitride and oxynitride thin films, prepared by PECVD under varying silane flow rate, were studied. The mechanical properties include Young’s modulus, nano-indentation hardness and fracture toughness. The correlation between the mechanical properties and materials’ structural factors was established.

2. Materials and Methods Silicon nitride and oxynitride films were prepared by PECVD at 200 ◦C on Si (100) substrates. All thin film samples were prepared on the silicon substrate except for the fracture toughness measurement, which will be described later. Prior to the deposition, the substrates were pre-cleaned with acetone, alcohol, and de-ionized water, followed by a nitrogen blow-dry using a static neutralizing blow-off gun. The silicon oxynitride films were synthesized by introducing silane (SiH4) and nitrous oxide (N2O) gases into the deposition chamber. The silicon nitride films were prepared using SiH4 with ammonia (NH3) gases. In each case, the silane flow rate was varied while other deposition parameters remained unchanged. Details of the deposition parameters are given in Table1. All these deposition parameters were chosen on the basis of the available BKM (best known method) parameters in the industry. Surfaces 2018, 1 61

Table 1. Deposition parameters of PECVD silicon nitride and oxynitride films.

SiH Flow N O Flow NH Flow Pressure Radio Frequency Film 4 2 3 Rate (sccm) Rate (sccm) Rate (sccm) (mTorr) Power (W)

SiOxNy 20, 30, 50 400 – 730 100 SiNx 16, 32, 50 – 160 620 250

The thickness of the as-deposited films was measured using a Tencor P-10 surface profilometer (ClassOne Equipment, Atlanta, GA, USA). The phases of the thin films were identified by X-ray diffractometer (XRD-6000, SHIMADZU, Kyoto, Japan) using Cu Kα radiation (wavelength of 1.54 Å) at 50 kV and 20 mA with a thin film goniometer (Rigaku, Tokyo, Japan) at a glancing angle of 1◦. The chemical states of the atomic species and atomic ratio in the thin films were analyzed by XPS (Kratos AXIS spectrometer, Manchester, UK) with the monochromatic Al Kα X-ray radiation at 1486.71 eV. The base vacuum in the XPS analysis chamber was about 10−9 Torr. The samples were also analyzed using Fourier transform (FTIR) with a Perkin Elmer system 2000 FTIR spectrometer (Waltham, MA, USA). The spectra were taken in transmission mode at normal incidence. Young’s modulus and hardness of the thin films were obtained by nanoindentation (NanotestTM, Wrexham, UK). A diamond Berkovich indenter (three-faced pyramid) was used, and the maximum depth was controlled to be around 20 nm to eliminate the possible influence from the substrate. Detailed information on the determination of the Young’s modulus and hardness using the nanoindentation method is provided in the Supporting Information (Figure S1 and Equations (S1)–(S3)). The fracture toughness of the thin films was measured using a controlled buckling test. Details about the test are available in existing reports [19,20]. To prepare the controlled buckling beam specimens, polyetherimide (Ultem®, SABIC Asia Pacific Pte. Ltd., Singapore) was used as the substrate so that the test-piece has the required flexibility for the buckling test. A schematic illustration of the test is shown in Figure S2 in the Supporting Information. The dimensions of the samples were 48 mm × 3 mm × 0.175 mm. Five to ten samples were tested for each deposition condition. The Young’s modulus and Poisson ratio of the Ultem® substrate were 2.6 GPa and 0.36, respectively. The ratio of the thickness of the tested films to the thickness of the substrate was between 1:500 to 1:1000. The small ratio in thicknesses was to ensure that the one-side coated thin films were under uniform tensile stress through the thickness when the film-on-substrate beam bent during the experiment. For the fracture toughness measurement, the film was placed on the tension side of the bending beam. The test jig was placed on the platform of an optical microscope so that the initiation of cracking in the thin film could be directly observed [19]. The film fracture was expected to occur at the point of maximum curvature at the center of the test piece. Based on the lateral displacement at crack initiation, the fracture strain (or stress) and fracture toughness could be calculated [19–21]. There was a small amount of residual stress in the coated films. This stress was calibrated [22] in the fracture toughness calculation. The fracture toughness, in terms of the critical energy release rate, GIC, is given by

1 E G = ε2 g(α, β)h (1) Ic 2 c (1 − υ2) f where εc is the critical fracture strain, E is the Young’s modulus, ν is the Poisson’s ratio, and hf is the thickness of the film. g(α, β) is an elasticity mismatch factor between the film and the substrate [19].

3. Results and Discussion

3.1. Film Composition and Bonding State Analysis

Figure1 shows the atomic percentage and atomic ratio in the SiN x and SiOxNy films. The atomic percentages were average values for three independent sampling points, after surface etching for 4 min. In the SiNx films, it is clear that the Si concentration increased, and N concentration decreased with the Surfaces 2018, 1 62 increase of silane flow rate. The N:Si atomic ratio (Figure1a) decreased from 1.1 to about 0.65 when the silane flow rate increased from 16 to 50 sccm. In the SiOxNy films, the O atom concentration decreased, and the Si and N concentrations increased with the silane flow rate. In all cases, the O atomic concentration was always higher than 50%, the Si concentration was between 30% to 40%, and the N atom concentration was lower than 10%. As a result, the O:Si ratio was reduced from x = 2.07 to 1.45, while the N:Si ratio increased from Surfacesy = 0.13 2018 to, 0.25.1, x FOR PEER REVIEW 4 of 14

65 1.2 SiNx 60 1.0 55 N 1s 50 .8 Si 2p 45

.6 (=N/Si) x ratio Atom Atom percentage (%) Atom percentage 40

35 .4 10 20 30 40 50 60 SiH flow rate (sccm) 4 (a) 70 2.5 O 1s SiOxNy 60 2.0 50 x (=O/Si)

40 1.5

Si 2p 30 1.0 20 Atom ratio x and y Atom ratio

Atom percentage (%) Atom percentage .5 10 N 1s y (=N/Si) 0 0.0 15 20 25 30 35 40 45 50 55 SiH flow rate (sccm) 4 (b)

Figure 1. Atomic percentage and atomic ratio in the (a) SiNx and (b) SiOxNy films. Figure 1. Atomic percentage and atomic ratio in the (a) SiNx and (b) SiOxNy films.

Typical survey spectra of XPS for the surfaces of both SiNx and SiOxNy are presented in Figure 2, andTypical they are survey globally spectra similar of XPS to foreach the other. surfaces The of peaks both SiNcorrespondingx and SiOxN ytoare Si presented2s and 2p, in O Figure 1s and2, Augerand they were are clearly globally present. similar The to eachN 1s other. peak was The peakspronounced corresponding in the SiN tox Sisample 2s and but 2p, was O 1s weak and Augerin the SiOwerexN clearlyy sample. present. A C 1s The peak N1s was peak also was found, pronounced which wa ins the due SiN tox exposuresample but of wasthe samples weak in theto air SiO afterxNy thesample. film formation. A C 1s peak was also found, which was due to exposure of the samples to air after the film formation. Surfaces 2018, 1 63 SurfacesSurfaces 2018 2018, ,1 1, ,x x FOR FOR PEER PEER REVIEW REVIEW 55 ofof 1414

OO 1s 1s

OO Auger Auger Si 2p Si 2p CC Auger Auger SiSi 2s 2s NN 1s 1s CC 1s 1s SiOxNySiOxNy Counts (a.u.) Counts Counts (a.u.) Counts

SiNxSiNx

00 200 200 400 400 600 600 800 800 1000 1000 1200 1200 Binding energy (eV) Binding energy (eV)

Figure 2. x x y FigureFigure 2. 2. TypicalTypical Typicalsurvey survey surveyspectra spectra spectraof of ofXPS XP XPSS forfor forsurfaces surfaces surfaces of of of both both both SiN SiN SiNxx andand and SiOSiO SiOxxNNNyy films.films. films.

The high-resolution spectra of the SiNx sample corresponding to N 1s and Si 2p after etching are TheThe high-resolution spectra spectra of of the the SiN SiNx xsamplesample corresponding corresponding to toN N1s 1sand and Si 2p Si 2pafter after etching etching are aregivengiven given inin Figure inFigure Figure 3a,b,3a,b,3a,b, respectively.respectively. respectively. TheyThey They areare are plotteplotte plotteddd afterafter after thethe the correctioncorrection correction ofof of chargingcharging charging effectseffects effects usingusing using aa abindingbinding binding energyenergy energy ofof of 284.6284.6 284.6 eV,eV, eV, whichwhich waswas the the C C 1s 1s1s peak peakpeak obtained obtained from from thethe surface.surface. ItIt isis notednoted thatthat nonono peakpeakpeak of ofof O OO 1s 1s1s was waswas detected detecteddetected after afterafter etching, etching,etching, although althoughalthough it was itit waswas observed observedobserved at the atat surface thethe surfacesurface (Figure (Figure(Figure2). It was 2).2). ItIt seen waswas thatseenseen all thatthat the allall N the 1sthe peaks NN 1s1s werepeakspeaks centered werewere centeredcentered at 397.9 at eV,at 397.9397.9 which eV,eV, was whichwhich attributed waswas attributedattributed to the N-Si toto bond thethe N-SiN-Si [23]. bondbond However, [23].[23]. theHowever,However, relative thethe height relativerelative of the heightheight peak ofof decreased thethe peakpeak withdecreaseddecreased the increase withwith thethe of increaseincrease the silane ofof flow thethe silanesilane rate. Figureflowflow rate.rate.3b shows FigureFigure that3b3b showsshows the center thatthat of thethe the centercenter Si 2p of coreof thethe level SiSi 2p2p shifted corecore level fromlevel shifted 102.8shifted eV fromfrom to 101.7 102.8102.8 eV eVeV and toto to 101.7101.7 101 eV,eVeV and correspondingand toto 101101 eV,eV, tocorrespondingcorresponding a silane flow to rateto a a silane ofsilane 16, flow 32,flow and rate rate 50 of of sccm, 16, 16, 32, 32, respectively. and and 50 50 sccm, sccm, The respectively. respectively. binding energy The The binding binding shifted energy toenergy a lower shifted shifted value, to to whichaa lowerlower was value,value, consistent whichwhich with waswas theconsistentconsistent decreasing withwith of thethe the decreasidecreasi N:Si ratio,ngng of asof the shownthe N:SiN:Si in ratio,ratio, Figure asas1 shown.shown A similar inin FigureFigure trend has1.1. AA alsosimilarsimilar been trendtrend confirmed hashas alsoalso by beenbeenHirohata confirmedconfirmed et al. [ by23by ] Hirohata.Hirohata It was also etet al. observedal. [23].[23]. ItIt was thatwas alsothealso relative observedobserved height thatthat ofthethe the relativerelative Si 2p peakheightheight increased ofof thethe SiSi with 2p2p peak thepeak increase increasedincreased of the withwith silane thethe increaflowincrea ratesese of becauseof thethe silanesilane more flowflow Si atoms raterate because werebecause incorporated moremore SiSi atomsatoms into thewerewere films. incorporated incorporated into into the the films. films.

12001200 NN 1s 1s (SiNx) (SiNx) 10001000

800800 5050 sccm sccm 600600

Counts (a.u.) Counts 32 sccm Counts (a.u.) Counts 400400 32 sccm

200200 1616 sccm sccm 00 394394 396 396 398 398 400 400 402 402 404 404 Binding energy (eV) Binding energy (eV) ((aa))

Figure 3. Cont. Surfaces 2018, 1, x FOR PEER REVIEW 6 of 14 Surfaces 2018, 1 64

1000 Surfaces 2018, 1, x FOR PEER REVIEWSi 2p (SiNx) 6 of 14 800

1000 50 sccm 600 Si 2p (SiNx) 800 400 5032 sccm sccm Counts (a.u.) 600 200 400 32 sccm 16 sccm Counts (a.u.) 0 20098 100 102 104 106 108 Binding energy (eV) 16 sccm 0 98 100 102(b) 104 106 108

Figure 3. The high-resolution spectra of the BindingSiNx samples energy corresponding (eV) to (a) N 1s and (b) Si 2p after Figure 3. a b etching.The Films high-resolution were formed spectrausing varied of the silane SiNx samples flow(b) rates corresponding of 16, 32, and to 50 ( sccm.) N 1s and ( ) Si 2p after etching. Films were formed using varied silane flow rates of 16, 32, and 50 sccm. Figure 3. The high-resolution spectra of the SiNx samples corresponding to (a) N 1s and (b) Si 2p after The high-resolution spectra of the SiOxNy sample corresponding to O 1s, N 1s, and Si 2p after etching. Films were formed using varied silane flow rates of 16, 32, and 50 sccm. etchingThe high-resolutionare presented in spectraFigure 4a–c. of the The SiO OxN 1sy sampleand N 1s corresponding peaks were centered to O 1s, at N 532.1 1s, and eV Siand 2p 398 after eV, etchingrespectively, areThe presented whichhigh-resolution were in Figure not spectra 4dependenta–c. of The the O SiOon 1s x andtheNy samplesilane N 1speaks correspondingflow rate. were The centered to centers O 1s, at N 532.1of 1s, the and eV Si Si and2p 2p core after 398 eV,level respectively,shiftedetching to a whichlower are presented binding were not in energy. Figure dependent 4a–c. At the The on 20 O the sccm1s silaneand flow N 1s flow rate, peaks rate. the were Si The 2pcenteredcenters peak atwas of532.1 thecentered eV Si and 2p at core398 103.2 eV, level eV, shiftedwhichrespectively, tocorresponded a lower bindingwhich to werethe energy. SiO not2 dependentbond. At the When 20 on sccm thethe flow silanesilane rate, flow flowthe rate. rate Si The 2pincreased peakcenters was ofto the centered30 Sisccm 2p coreand at 103.2 level50 sccm, eV, shifted to a lower binding energy. At the 20 sccm flow rate, the Si 2p peak was centered at 103.2 eV, whichthe centers corresponded shifted to to 102.7 the SiO eV2 andbond. 102.6 When eV, respec the silanetively. flow It was rate increasedreported that to 30 the sccm binding and 50energy sccm, of thesilicon centerswhich oxynitride shifted corresponded todepended 102.7 to eVthe on andSiO the2 102.6bond. nitrogen When eV, respectively. contthe silaneent [24]. flow It The wasrate bindingincreased reported energyto that 30 sccm the differencebinding and 50 sccm, between energy the centers shifted to 102.7 eV and 102.6 eV, respectively. It was reported that the binding energy of ofsilicon silicon oxide oxynitride and silicon depended oxynitride on the was nitrogen between content 1.5 and [24 3.9]. The eV. bindingThus, the energy current difference study finds between that at silicon oxynitride depended on the nitrogen content [24]. The binding energy difference between siliconthe 20silicon oxide sccm oxide andsilane silicon and flow silicon rate, oxynitride oxynitride the N concen was was between trationbetween 1.5was 1.5 and soand low 3.9 3.9 eV.thateV. Thus,Thus, silicon the dioxide current current (SiOstudy study2) finds predominantly finds that that at at theformed. 20 sccmthe 20Silicon silane sccm oxynitridesilane flow flow rate, rate, thebecame Nthe concentration N significant concentration in was sampleswas so so low low with that that 30 siliconsilicon and dioxide dioxide50 sccm (SiO (SiOflow2) 2predominantly )rates. predominantly formed.formed. Silicon Silicon oxynitride oxynitride became became significant significant in in samples samples with with 30 30 andand 50 sccm sccm flow flow rates. rates. 4000 4000O 1s (SiOxNy) O 1s (SiOxNy)

3000 3000 50 sccm 50 sccm 20002000 Counts (a.u.) Counts Counts (a.u.) Counts 3030 sccm sccm 10001000

2020 sccm sccm 0 0 528 530 532 534 536 528 530 532 534 536 Binding energy (eV) Binding energy (eV) (a) (a) Figure 4. Cont. Surfaces 2018, 1 65 Surfaces 2018, 1, x FOR PEER REVIEW 7 of 14

350 N 1s (SiOxNy) 300

250

200 50 sccm 150

Counts (a.u.) 30 sccm 100 20 sccm 50

0 394 396 398 400 402 404 Binding energy (eV) (b)

1200 Si 2p (SiOxNy)

1000

800 50 sccm 600

Counts (a.u.) Counts 400 30 sccm

200 20 sccm 0 98 100 102 104 106 108

Binding energy (eV) (c)

Figure 4. The high-resolution spectra of the SiOxNy sample corresponding to (a) O 1s, (b) N 1s, and Figure(c 4.) SiThe 2p high-resolutionafter etching. Three spectra films ofwith the varied SiOx Nsilaney sample flow rate corresponding are shown. to (a) O 1s, (b) N 1s, and (c) Si 2p after etching. Three films with varied silane flow rate are shown. Figure 5 shows the FTIR spectra taken over a wavenumber range of 400–4000 cm−1 for both SiNx

and SiOxNy films. The spectrum of the silicon substrate has already been subtracted. For−1 SiNx films, Figurethe spectra5 shows revealed the FTIR the spectraSi–N bond taken with over the ach wavenumberaracteristic stretching range of mode 400–4000 present cm nearfor 810 both cm−1 SiN. x and SiOOtherxNy vibrationsfilms. The observed spectrum were of theN–H silicon bending substrate (approx. has 1120 already cm−1), beenSi–H subtracted.stretching (approx. For SiN 2100x films, −1 the spectracm−1), revealed and N–H the stretching Si–N bond (approx. with the3340 characteristic cm−1). These peaks stretching were modetypical present for low-temperature near 810 cm CVD. Other vibrationssilicon observed nitride films were [25]. N–H It was bending also observed (approx. that 1120 the absorptions cm−1), Si–H corresponding stretching to (approx. the vibrations 2100 cm of −1), and N–Hthe stretchingN–H and Si–H (approx. bonds 3340 increased cm−1 ).with These increasing peaks weresilane typicalflow rate, for indicating low-temperature an increase CVD in the silicon nitridehydrogen films [25 ].concentration. It was also observed Hydrogen that in thethe absorptionsSiNx film is corresponding usually undesi torable the vibrationsfor microelectronic of the N–H and Si–Happlications bonds increased because it causes with increasing hydrogen-induced silane flowdefects, rate, which indicating may reduce an increaseits insulating in the effect hydrogen and induce large stress in the films at high temperature [26,27]. Therefore, lower flow rates of silane would concentration. Hydrogen in the SiNx film is usually undesirable for microelectronic applications be recommended in order to control the hydrogen content in the SiNx films. because it causes hydrogen-induced defects, which may reduce its insulating effect and induce large stress in the films at high temperature [26,27]. Therefore, lower flow rates of silane would be recommended in order to control the hydrogen content in the SiNx films. Surfaces 2018, 1 66

Surfaces 2018, 1, x FOR PEER REVIEW 8 of 14 For silicon oxynitride (SiOxNy) films, the absorption corresponding to the Si–O bond appeared at about 1020For silicon cm−1 oxynitridein all three (SiO casesxNy) films, (Figure the5 b).absorption The Si–N corresponding bond could to also the Si–O be observed bond appeared at around at about 1020 cm−1 in all three cases (Figure 5b). The Si–N bond could also be observed at around 830 830 cm−1. It was clear that the absorption of the Si–N bond at the 20 sccm silane flow rate was very low, cm−1. It was clear that the absorption of the Si–N bond at the 20 sccm silane flow rate was very low, which was consistent with the low N content and the near-stoichiometric O:Si = 2 ratio, as indicated which was consistent with the low N content and the near-stoichiometric O:Si = 2 ratio, as indicated by XPSby XPS (Figure (Figure1b). 1b). However, However, the the absorption absorption strengthstrength of Si–N Si–N increased increased with with an anincreasing increasing silane silane flowflow rate, rate, demonstrating demonstrating that that more more and andmore more Si–NSi–N bonds bonds were were formed formed in inthe the films. films. This This result result was was consistentconsistent with with the highestthe highest amount amount of nitrogenof nitrogen incorporated incorporated in in the the film film at at 50 50 sccmsccm asas indicatedindicated by XPS, and alsoXPS, agreesand also well agrees with well the with formation the formation of a silicon of a silicon nitride nitride phase phase in thein the film film as as characterized characterized byby XRD resultsXRD shown results later. shown It is later. worth It is mentioning worth mentioning that the th lackat the of lack absorption of absorption at the at N–H the N–H or Si–H or Si–H locations indicatedlocations that indicated the hydrogen that the concentration hydrogen concentration of these groups of these was groups less than was 2–3less at%,than or2–3 below at%, or the below detection the detection limit for the IR spectroscopy [26,27]. Compared with the formation condition limit for the thin film IR spectroscopy [26,27]. Compared with the formation condition of SiNx films, of SiNx films, the presence of oxygen during film formation has prevented Si–H and N–H bond the presence of oxygen during film formation has prevented Si–H and N–H bond formation. formation.

300 SiNx N-H Si-H 250 N-H 50 sccm Si-N

200 32 sccm

150

16 sccm 100 Transmittance intensity (a.u.) intensity Transmittance

50 1000 2000 3000 4000 -1 Wave number (cm ) (a) 280 SiOxNy

260 N-H 240 Si-H

N-H 50 sccm

220 Si-N 200 180 30 sccm

160 Si-O 140 120 20 sccm 100

Transmittance intensity (a.u.) 80 60 1000 2000 3000 4000 -1 Wave number (cm ) (b)

Figure 5. FTIR spectra taken over a wavenumber range of 400–4000 cm−1 for (a) SiNx and (b) SiOxNy Figure 5. FTIR spectra taken over a wavenumber range of 400–4000 cm−1 for (a) SiN and films. x (b) SiOxNy films. Surfaces 2018, 1 67 Surfaces 2018, 1, x FOR PEER REVIEW 9 of 14

3.2.3.2. Crystallinity Crystallinity and and Phase Phase Identification Identification

GlancingGlancing angle angle X-ray X-ray diffraction diffraction patterns patterns of of the the SiN SiNx xand and SiO SiOxxNNyy filmsfilms are shown shown in in Figure Figure 6a,b,6a,b, respectively.respectively. The The SiN SiNx xfilms films were partly partly crystallized crystallized forfor all silane all silane flow flowrates, rates,as evidenced as evidenced by all the by all thebroadened broadened peaks. peaks. The XRD The peaks XRD belonged peaks belonged to neither to α-Si neither3N4 norα -Siβ-Si3N3N44 nor[23]; βrather,-Si3N 4they[23 ];seem rather, to match better with the peaks of cubic silicon nitride (c-Si3N4) reported by Jiang et al. [28]. However, a they seem to match better with the peaks of cubic silicon nitride (c-Si3N4) reported by Jiang et al. [28]. However,cautionary a cautionary note should note be should made that be made the cubic that si thelicon cubic nitride silicon phase nitride is usually phase prepared is usually by prepared a high- by atemperature high-temperature and high-pressure and high-pressure process. process. Therefore, Therefore, further furtherwork is work needed isneeded to confirm to confirm the exact the exactcrystalline crystalline phase phase type type and and structure structure of our of ourfilms. films. Since Since the atomic the atomic N:Si N:Siratio ratiowas always was always lower lowerthan the stoichiometric ratio of 1.33, these nitride films did not seem to share a common equilibrium than the stoichiometric ratio of 1.33, these nitride films did not seem to share a common equilibrium structure on deposition. For the SiOxNy shown in Figure 6b, it is clear that only amorphous phases structure on deposition. For the SiOxNy shown in Figure6b, it is clear that only amorphous phases were formed at both 20 sccm and 30 sccm flow rate. Combined with the XPS analysis reported earlier, were formed at both 20 sccm and 30 sccm flow rate. Combined with the XPS analysis reported earlier, the amorphous phase was predominantly SiO2 for the 20 sccm silane flow condition, and silicon the amorphous phase was predominantly SiO for the 20 sccm silane flow condition, and silicon oxynitride at 30 sccm. At a 50 sccm flow rate, simila2 r XRD peaks to the silicon nitride films in Figure oxynitride at 30 sccm. At a 50 sccm flow rate, similar XRD peaks to the silicon nitride films in Figure6a 6a were present, which meant the crystalline Si3N4 and amorphous oxynitride phase co-existed. This were present, which meant the crystalline Si N and amorphous oxynitride phase co-existed. This is is justified by the XPS spectrum where the3 Si4 2p core level binding energy at 50 sccm in the SiOxNy justifiedfilm further by the XPSshifted spectrum to the SiN wherex side the (Figure Si 2p 4c). core level binding energy at 50 sccm in the SiOxNy film further shifted to the SiNx side (Figure4c).

600 SiNx 500

400

300

50 sccm Counts (a.u.) Counts 200

100 32 sccm 16 sccm 0 10 15 20 25 30 35 40 45

0 2θ (a) 600 SiOxNy 500 50 sccm 400

300 30 sccm

Counts(a.u.) 200

100 20 sccm

0 10 15 20 25 30 35 40 45

0 2θ (b)

Figure 6. Glancing angle x-ray diffraction patterns of the (a) SiNx and (b) SiOxNy films at different Figure 6. Glancing angle x-ray diffraction patterns of the (a) SiNx and (b) SiOxNy films at different silane flow rates. silane flow rates. Surfaces 2018, 1 68 Surfaces 2018, 1, x FOR PEER REVIEW 10 of 14

3.3. Young’s Young’s Modulus and Hardness Table2 2 displays displays the the Young’s Young’s modulus modulus (E) (E) and and hardness hardness (H) (H) of both of both films. films. The Young’sThe Young’s moduli moduli were werecalculated calculated from thefrom measured the measured reduced modulireduced using moduli nanoindentation. using nanoindentation. A typical load-displacement A typical load- displacementcurve is shown curve in Figure is shown7. To in carry Figure out 7. the To calculation carry out the of the calculation Young’s modulusof the Young’s of the modulus film according of the filmto Equation according (S2) to (Supporting Equation (S2) Information), (Supporting the Information), modulus and the Poisson’s modulus ratio and for Poisson’s the diamond ratio indenter for the ν diamondtip, Edia = indenter 1100 GPa tip, and Ediadia = 1100= 0.2 GPa were and used. νdia Poisson’s= 0.2 were ratios used. for Poisson’s siliconnitride ratios for and silicon silicon nitride oxynitride and siliconfilms were oxynitride taken films as 0.27 were and taken 0.23, as respectively, 0.27 and 0.23, based respectively, on References based [on29 ,References30]. Overall, [29,30]. it was Overall, found itthat was the found hardness that the and hardness Young modulus and Young of modulus SiNx films of were SiNx greaterfilms were than greater the ones than of the SiO onesxNy .of Since SiOxN they. Sinceelasticity the elasticity modulus modulus is predominantly is predominantly dependent depe onndent the bonding on the states,bonding this states, finding this agrees finding with agrees the withknown the fact known that thefact Si–Nthat the bond Si–N is stiffer bond theis stiffer Si–O the bond. Si–O bond.

Figure 7. AA load-displacement load-displacement curve du duringring a nanoindentation test.

The reported Young’s moduli for SiNx filmsfilms were quite close to the ones obtained by Cardinale and Tustison [[18],18], whowho foundfound thethe biaxialbiaxial modulusmodulus (which(which is is E/(1 E/(1 −− νν)))) of of their their PECVD PECVD silicon silicon nitride nitride filmsfilms ranged from 110 to 160 GPa using a membrane bulge test. In general, the Young’s modulus obtained from PECVD SiN x filmsfilms is is lower lower than than the the one one formed by low-pressure chemical deposition (LPCVD). The plane strainstrain modulusmodulus (given(given byby E/(1E/(1 − νν2))2)) of of LPCVD LPCVD was was reported reported to to be be in in the the range range of 230 to 330 GPa [31,32]. [31,32]. This This is is probably probably due due to to the the higher higher synthesis synthesis temperatures temperatures used used in in LPCVD, LPCVD, typically at at 700 °C◦C or above. The The obtained obtained elastic elastic modulus modulus by by LPCVD LPCVD is is closer closer to to the sintered bulk ◦ Si3NN44 ((≈≈300300 GPa). GPa). On On the the other hand,hand, PECVDPECVD operates operates at at low low temperatures temperatures (100–300 (100–300C ),°C), therefore therefore the thefilm film density density might might be lower be lower than than the onethe one created create usingd using high-temperature high-temperature processes. processes.Huang Huang et al. et [ 33al.] [33]have have recently recently found found a correlation a correlation among depositionamong depo temperature,sition temperature, film density, film and density, Young’s and modulus Young’s in moduluslow-temperature in low-temperature PECVD silicon PECVD nitride silicon films. nitride In the currentfilms. In work, the current the Young’s work, modulus the Young’s and modulus hardness andof SiN hardnessx was found of SiN toxdecrease was found with to increasing decrease with silane increasing flow rate, silane corresponding flow rate, to corresponding a decreased atomic to a decreasedN:Si ratio atomic in the film’sN:Si ratio chemical in the composition.film’s chemical According composition. Figure According1a, all the Figure SiN 1a,x films all the were SiN Si-richx films were(i.e., N:SiSi-rich ratio (i.e., less N:Si than ratio 1.33), less thus than this 1.33), work thus finds this that work the finds further that away the fromfurther the away stoichiometric from the stoichiometriccomposition Si 3compositionN4, both the modulusSi3N4, both and the hardness modulus decreased and hardness for the decreased Si-rich nitride for films.the Si-rich This impliesnitride films.that the This elastic implies modulus that the of crystalline elastic modulus SiNx film of crystalline was mainly SiN controlledx film was by mainly the Si–N controlled bonding. by The the slight Si– Ndecrease bonding. in The the hardnessslight decrease with increasingin the hardness silane with flow increasing may also silane be affected flow may by thealsopresence be affected of by an theamorphous presence phase. of an amorphous However, in phase. this work, However, we are in not th ableis work, to quantify we are thenot amountable to quantify of amorphous the amount phase ofin amorphous the films. phase in the films. For silicon oxynitrideoxynitride films,films, Young’s Young’s modulus modulus decreased decreased slightly slightly with with increasing increasing silane silane flow flow rate, rate,and theand variation the variation is much is smallermuch smaller than for than the SiN forx series.the SiN Itx wasseries. noticed It was that noticed the chemical that the composition chemical composition(Si, N, O) only (Si, varied N, O) in only a relatively varied in narrow a relatively window. narrow The values window. (89.2–76.8 The values GPa) (89.2–76.8 were closer GPa) to silicon were closer to silicon oxide (ESiO2 = 70 GPa) than silicon nitride, indicating dominance of the Si–O bond. This agrees with the XPS composition analysis in Figure 1b, which showed a low nitrogen Surfaces 2018, 1 69

oxide (ESiO2 = 70 GPa) than silicon nitride, indicating dominance of the Si–O bond. This agrees with the XPS composition analysis in Figure1b, which showed a low nitrogen concentration (4.1–9.2 at%) for the three samples. The obtained Young’s moduli for silicon oxynitride films fell in the range of 78–150 GPa, which was reported by other researchers [1,5,6,34]. The hardness of the SiOxNy films showed a great increase from 4.7 GPa to 9.7 GPa when the silane flow rate increased from 20 to 50 sccm. Unlike Young’s modulus, hardness is sensitive to both chemical bonding and microstructure. As analyzed earlier, at a 20 sccm flow rate, the film consisted of predominantly amorphous SiO2. This structure had the lowest hardness. When the flow rate increased to 30 sccm, the amorphous SiOxNy film had an increased hardness of 7.8 GPa. Although the composition was very close between the 30 sccm and 50 sccm films, only the 50 sccm sample contained a crystalline Si–N phase. The presence of a crystalline phase clearly contributed to the increase in hardness in the 50 sccm sample.

Table 2. Young’s modulus and hardness obtained by nanoindentation.

Film Silane Flow Rate (sccm) Young’s Modulus (GPa) Hardness (GPa) Poisson’s Ratio [27,28] 16 153.0 ± 14.9 13.6 ± 2.9 SiNx 32 117.8 ± 9.5 11.8 ± 1.8 0.27 50 108.5 ± 10.6 10.2 ± 3.0 20 89.2 ± 4.8 4.7 ± 0.7 SiOxNy 30 78.9 ± 12.6 7.8 ± 0.9 0.23 50 76.8 ± 9.6 9.7 ± 4.0

3.4. Fracture Toughness

Table3 lists the critical energy release rate (G Ic) and fracture toughness (KIC) for the SiNx and SiOxNy films. The GIc value obtained here is an intrinsic property of the films, which reflects a material’s ability to absorb energy for per unit-area crack growth. For easy compassion with literature data, which are mostly given in terms of the critical stress intensity factor, KIC, we have also converted p 2 the measured GIc into KIC following the relation: KIc = GIcE/(1 − ν ). Typical values of the fracture toughness of steel, glass (amorphous silicon oxide), and a 200 nm-thick Si3N4 thin film are around 60, 0.6, and 1.8 MPa·m1/2, respectively [17]. The fracture toughness of our nitride and oxynitride films shown in Table3 was close to the reported Si 3N4 film.

Table 3. The critical energy release rate (GIC) and fracture toughness (KIc) of SiNx and SiOxNy films.

Silane Flow Film Film Thickness (nm) Fracture Strain (%) G (J/m2) K (MPa·m1/2) Rate (sccm) IC IC 16 223.4 0.948 23.73 1.90 SiNx 32 283.2 1.167 25.66 1.74 50 358.5 1.367 55.41 2.44 20 198.9 1.497 17.29 1.24 SiOxNy 30 245.6 1.072 9.16 0.85 50 227.9 1.046 7.69 0.77

The variation of the critical energy release rate with the silane flow rate is plotted in Figure8. The GIc increased with increasing silane flow rate for the SiNx films, and an opposite trend was observed for the SiOxNy films. Such variations are in line with the trend of the film hardness: it is known that a harder material is likely to be more brittle (lower fracture energy). From the microstructure’s perspective, we attribute the greater fracture energy in the SiNx film with increasing silane flow rate to the increased amount of non-stoichiometric, Si-rich nitride phase in the films (and possibly increased amount of amorphous content). In the case of SiOxNy, the increased film brittleness with increasing silane flow rate was clearly related to the harder, crystalline silicon nitride phase as analyzed before. Surfaces 2018, 1 70 Surfaces 2018, 1, x FOR PEER REVIEW 12 of 14

(a)

(b)

Figure 8. Critical energy release rate (GIc) for the (a) SiNx and (b) SiOxNy films with variation of the Figure 8. Critical energy release rate (G ) for the (a) SiN and (b) SiO N films with variation of the silane flow rate. Ic x x y silane flow rate.

From a mechanical point of view, SiNx films are preferred over SiOxNy films because the former possessesFrom a mechanical better material point stiffness, of view, hardness, SiNx films and arefracture preferred toughness. over SiOAs xfilmsNy films dominated because by theionic former possessesand covalent better materialbonds are stiffness, generally hardness, very brittle and and fracture prone to toughness. fracture when As films being dominated used as structural by ionic and covalentelements bonds in areMEMS generally devices, very having brittle a greater and prone fracture to fracturetoughness when is a clear being advantage used as structuralfor the device elements reliability. in MEMS devices, having a greater fracture toughness is a clear advantage for the device reliability. 4. Conclusions 4. Conclusions SiNx and SiOxNy thin films have been prepared using plasma enhanced chemical vapor depositionSiNx and SiOunderxN varyingy thin films silane have flow been rates. prepared Film composition, using plasma chemical enhanced bonding, chemical and microstructure vapor deposition underwere varying identified silane by X-ray flow photoele rates.ctron Film spectroscopy composition, (XPS), chemical Fourier bonding, transform andinfrared microstructure spectroscopy were identified(FTIR), by and X-ray X-ray photoelectron diffraction (XRD). spectroscopy The Young’ (XPS),s modulus Fourier and transform hardness infrared of the spectroscopythin films were (FTIR), and X-raycharacterized diffraction using (XRD). nanoindentation The Young’s test. modulusA controlled and buckling hardness test ofwas the used thin to filmsmeasure were the characterized fracture usingtoughness nanoindentation of these thin test. films. A controlled The current buckling study found test that was the used silicon to measurenitride films the had fracture better toughness overall of these thin films. The current study found that the silicon nitride films had better overall mechanical properties than the silicon oxynitride thin films. The main reason was that the SiNx films were composed predominantly of the crystalline Si3N4 phase, while the SiOxNy films were dominated by the amorphous Si–O phase. Within the SiNx films, Young’s modulus and hardness decreased with Surfaces 2018, 1 71 increasing silane flow rate, corresponding to a reduced amount of the strong Si–N bonding. For the SiOxNy films, an increasing silane flow rate has shown a minor effect on the Young’s modulus but a significant impact on the hardness. The fracture toughness of both series of thin films displayed opposite trends with their hardness, and this could be explained by considering that the ability for energy dissipation increases with increased ductility (reduced hardness) of the materials. PECVD SiNx is preferred when structural components in MEMS are to be fabricated because of its better resistance to fracture.

Supplementary Materials: The following are available online at http://www.mdpi.com/2571-9637/1/1/6/s1. Author Contributions: This work was initiated by Z.C. Experiment was carried out by Z.G., and analysis was done after discussion among all authors. Draft was prepared by Z.G., and revised by C.W. and Z.C. Funding: This research was funded by The Ministry of Education, Singapore grant number RG 14/03. Conflicts of Interest: The authors declare no conflict of interest.

References

1. Jozwik, M.; Delobelle, P.; Gorecki, C.; Sabac, A.; Nieradko, L.; Meunier, C.; Munnik, F. Optomechanical characterization of compressively prestressed silicon oxynitride films deposited plasma-enhanced chemical vapour deposition on silicon membranes. Thin Films 2004, 468, 84–92. [CrossRef] 2. Prodanovi´c,V.; Chan, H.W.; Graaf, H.V.D.; Sarro, P.M. Ultra-thin alumina and silicon nitride MEMS fabricated membranes for the electron multiplication. Nanotechnology 2018, 29, 155703. [CrossRef][PubMed] 3. Bagolini, A.; Savoia, A.S.; Picciotto, A.; Boscardin, M.; Bellutti, P.; Lamberti, N.; Caliano, G. PECVD low stress silicon nitride analysis and optimization for the fabrication of CMUT devices. J. Micromech. Microeng. 2015, 25, 015012. [CrossRef] 4. Li, D.-L.; Feng, X.-F.; Wen, Z.-Y.; Shang, Z.-G.; She, Y. Stress control of silicon nitride films deposited by plasma enhanced chemical vapor deposition. Optoelectron. Lett. 2016, 12, 285–289. [CrossRef] 5. Dong, J.; Du, P.; Zhang, X. Characterization of the Young’s modulus and residual stresses for a sputtered silicon oxynitride film using micro-structures. Thin Solid Films 2013, 545, 414–418. [CrossRef] 6. Danaie, K.; Bosseboeuf, A.; Clerc, C.; Gousset, C.; Julie, G. Fabrication of UV-transparent SixOyNz membranes with a low frequency PECVD reactor. Sens. Actuators A 2002, 99, 78–81. [CrossRef] 7. Jo, M.C.; Park, S.K.; Park, S.J. A study on resistance of PECVD silicon nitride thin film to thermal stress-induced cracking. Appl. Surf. Sci. 1999, 140, 12–18. [CrossRef] 8. Yau, B.S.; Huang, J.L. Effects of nitrogen flow on R.F. reactive magnetron sputtered silicon nitride films on high speed steel. Surf. Coat. Technol. 2004, 176, 290–295. [CrossRef] 9. Lane, M.W.; Liniger, E.G.; Lloyd, J.R. Relationship between interfacial adhesion and electromigration in Cu metallization. J. Appl. Phys. 2003, 93, 1417–1421. [CrossRef] 10. Dauskardt, R.H.; Lane, M.; Ma, Q.; Krishna, N. Adhesion and debonding of multi-layer thin film structures. Eng. Fract. Mech. 1998, 61, 141–162. [CrossRef] 11. Skjöldebrand, C.; Schmidt, S.; Vuong, V.; Pettersson, M.; Grandfield, K.; Högberg, H.; Engqvist, H.; Persson, C. Influence of Substrate Heating and Nitrogen Flow on the Composition, Morphological and Mechanical Properties of SiNx Coatings Aimed for Joint Replacements. Materials 2017, 10, 173. [CrossRef][PubMed] 12. Obrosov, A.; Gulyaev, R.; Zak, A.; Ratzke, M.; Naveed, M.; Dudzinski, W.; Weiß, S. Chemical and Morphological Characterization of Magnetron Sputtered at Different Bias Voltages Cr-Al-C Coatings. Materials 2017, 10, 156. [CrossRef][PubMed] 13. Sajgalik, P.; Dusza, J.; Hoffmann, M.J. Relationshop between microstructure, toughening mechanisms, and fracture toughness of reinforced silicon nitride . J. Am. Ceram. Soc. 1995, 78, 2619–2624. [CrossRef] 14. Becher, P.F.; Sun, E.Y.; Plucknett, K.P.; Alexander, K.B.; Hsueh, C.-H.; Lin, H.-T.; Waters, S.B.; Westmoreland, C.G.; Kang, E.; Hirao, K.; et al. Microstructural design of silicon nitride with imporved fracture toughness: I, effects of grain shape and size. J. Am. Ceram. Soc. 1998, 81, 2821–2830. [CrossRef] 15. Doblinger, M.; Winkelman, G.G.; Dwyer, C.; Marsh, C.; Kirkland, A.I.; Cockayne, D.J.H.; Hoffmann, M.J. Structural and compositional comparison of Si3N4 ceramics with different fracture modes. Acta Mater. 2006, 54, 1949–1956. [CrossRef] Surfaces 2018, 1 72

16. Satet, R.L.; Hoffmann, M.J. Influence of the rare-earth element on the mechanical properties of RE-Mg- silicon nitride. J. Am. Ceram. Soc. 2005, 88, 2485–2490. [CrossRef] 17. Fan, L.S.; Howe, R.T.; Muller, R.S. Fracture toughness characterization of brittle thin films. Sens. Actuators 1990, A21–A23, 872–874. [CrossRef] 18. Cardinale, G.F.; Tustison, R.W. Fracture strength and biaxial modulus measurement of plasma silicon nitride films. Thin Solid Films 1992, 207, 126–130. [CrossRef] 19. Chen, Z.; Cotterell, B.; Wang, W.; Guenther, E.; Chua, S.J. A mechanical assessment of flexible optoelectronic devices. Thin Solid Films 2001, 394, 202–206. [CrossRef] 20. Chen, Z.; Cotterell, B.; Wang, W. The Fracture of Brittle Thin Films on Compliant Substrate in Flexible Displays. Eng. Fract. Mech. 2002, 69, 597–603. [CrossRef] 21. Chen, Z.; Gan, Z.H. Fracture Toughness Measurement of Thin Films on Compliant Substrate Using Controlled Buckling Test. Thin Solid Films 2007, 515, 3305–3309. [CrossRef] 22. Chen, Z.; Xu, X.; Wong, C.C.; Mhaisalkar, S. Effect of Plating Parameters on the Intrinsic Stress in Electroless Nickel Plating. Surf. Coat. Technol. 2003, 167, 170–176. [CrossRef] 23. Hirohata, Y.; Shimamoto, N.; Hino, T.; Yamashima, T.; Yabe, K. Properties of silicon nitride films prepared by magnetron sputtering. Thin Solid Films 1994, 253, 425–429. [CrossRef] 24. Mao, A.Y.; Son, K.A.; Hess, D.A.; Brown, L.A.; White, J.M.; Kwong, D.L.; Roberts, D.A.; Vrtis, R.N. Annealing ultra thin Ta2O5 films deposited on bare and nitrogen passivated Si(100). Thin Solid Films 1999, 349, 230–237. [CrossRef] 25. Tolstoy, V.P.; Chernyshova, I.V.; Skryshevsky, V.A. Infrared spectroscopy of thin layers in silicon microelectronics. In Handbook of Infrared Spectroscopy of Ultrathin Films; John Wiley & Sons: Hoboken, NJ, USA, 2003; pp. 435–436. 26. Gupta, M.; Rathi, V.K.; Thangaraj, R.; Agnihotri, O.P.; Chari, K.S. Preparation, properties and applications of silicon nitride thin films deposited by plasma-enhanced chemical vapor deposition. Thin Solid Films 1991, 204, 77–106. [CrossRef] 27. Han, S.S.; Jun, B.H.; No, K.; Bae, B.S. Preparation of a-SiNx thin film with low hydrogen content by inductively coupled plasma enhanced chemical vapor deposition. J. Electrochem. Soc. 1998, 145, 652–658. [CrossRef] 28. Jiang, J.Z.; Lindelov, H.; Gerward, L.; Stahl, K.; Recio, J.M.; Mori-Sanchez, P.; Carlson, S.; Mezouar, M.; Dooryhee, E.; Fitch, A.; et al. Compressibility and thermal expansion of cubic silicon nitride. Phys. Rev. B 2002, 65, 161202(R). [CrossRef] 29. Lynch, C.T. (Ed.) CRC Handbook of Materials Science; CRC Press: Boca Raton, FL, USA, 1975; Volume II. 30. Carlotti, G.; Colpani, P.; Piccolo, D.; Santucci, S.; Senez, V.; Socino, G.; Verdini, L. Measurement of the elastic and viscoelastic properties of films used in microelectronics. Thin Solid Films 2002, 414, 99–104. [CrossRef] 31. Hong, S.; Weihs, T.P.; Bravman, J.C.; Nix, W.D. Measuring stiffness and residual stresses of silicon nitride thin films. J. Electron. Mater. 1990, 19, 903–910. [CrossRef] 32. Toivola, Y.; Thurn, J.; Cook, R.F.; Cibuzar, G.; Roberts, K. Influence of deposition conditions on mechanical properties of low-pressure chemical vapor deposited low-stress silicon nitride films. J. Appl. Phys. 2003, 94, 6915–6922. [CrossRef] 33. Huang, H.; Winchester, K.J.; Suvorova, A.; Lawn, B.R.; Liu, Y.; Hu, X.Z.; Dell, J.M.; Faraone, L. Effect of deposition conditions on mechanical properties of low-temperature PECVD silicon nitride films. Mater. Sci. Eng. A 2006, 435–436, 453–459. [CrossRef] 34. Kramer, T.; Paul, O. Postbuckled micromachined square membranes under differential pressure. J. Micromech. Microeng. 2003, 12, 475–478. [CrossRef]

© 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).