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研究生： 許育禎
Hsu, Yu-Chen
論文名稱： 高密度電漿化學氣相沉積法製作低應力氮化矽薄膜於微機電應用之研究
Fabrication of Low Stress HDP-CVD Silicon Nitride Films in MEMS Application
指導教授： 高騏
Cau, Chi
學位類別： 碩士
Master
系所名稱： 工學院 - 航空太空工程學系
Department of Aeronautics & Astronautics
論文出版年： 2002
畢業學年度： 90
語文別： 中文
論文頁數： 88
中文關鍵詞： 殘留梯度應力 、懸臂樑 、殘留應力 、微橋 、微機電系統
外文關鍵詞： MEMS, residual stress, residual gradient stress, microbridge, cantilever beams
相關次數： 點閱：97  下載：4 
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    • 中文摘要
    對於微機電系統(MEMS)而言，高密度電漿化學氣相沉積法(HDP-CVD)是一種較新的薄膜沉積技術，此沉積法可得到較PECVD法性質更為出色的薄膜。然而，薄膜材料均難以避免殘留應力的存在。有別於半導體元件的是，薄膜製成的微懸浮結構需要對底下的犧牲層進行蝕刻以使微結構能夠懸浮，存在於薄膜中的殘留應力和殘留梯度應力因此得以釋放，而導致微結構產生彎曲和挫曲等結構形變。
    本論文研究旨在以HDP-CVD製作低殘留應力之氮化矽薄膜，並探討沉積參數、微懸浮結構與殘留應力之相關性。實驗結果顯示，適當的調整沉積參數，可使氮化矽殘留應力值分佈於-200~200MPa之間。此外，文中亦探討退火溫度為400℃~600℃時，對薄膜化學鍵結與殘留應力之影響。當退火溫度升高至75℃~300℃時，薄膜中之殘留氣體會減少，導致薄膜中之NH3以氣態方式向外擴散。同時，氮化矽薄膜殘留應力隨退火溫度增加逐漸轉變成拉伸應力型態。
    為觀察殘留應力於薄膜中之分佈，實驗中設計了懸臂樑結構。結果顯示，氮化矽懸臂樑因承受一正向的殘留梯度應力，使結構體呈現向上翹曲。此研究結果顯示，適當的調整HDP-CVD沉積參數，則可調降薄膜殘留應力以及製作出微懸浮結構。

    Abstract
    High density plasma chemical vapor deposition (HDP-CVD) is an newly thin film process for the fabrication of microelectromechanical systems (MEMS). The HDP film has many excellent film properties than the PECVD film. However, thin film materials are normally under residual stresses as a result of fabrication processes. Unlike microelectronic devices, a microstructure is no longer constrained by its underlying sacrificial layer after anisotropic etch undercutting; therefore, residual stresses and residual gradient stress may result in bending and buckling of a microstructure.
    The objective of this present work is to use HDP-CVD technology to fabricate a low stress silicon nitride films and study the effects of deposition parameters on microstructure and stress distribution of there films in as-deposited state. The experimental results showed that the residual stress in the silicon nitride films can be reduced to the range of -200~200MPa by adjusting proper deposition parameters. The chemical contents and stress distribution were studied as a function of the annealing temperatures in the range from 400℃ to 600℃. The residual gases decreases with the increase in annealing temperatures from 75℃ to 300℃, causing the out-diffusion of NH3. Meanwhile, the stress in silicon nitride film could be transformed from the compressive region to the tensile region.
    In addition, self-deformed micromachined cantilevers are fabricated to exhibit the residual gradient stress from curvature of the bending beams. The silicon nitride cantilever beams bent upward by a positive residual gradient stress. The present work shows that adjusting the HDP-CVD deposition parameters, the residual stress of the deposited films can be significantly reduced and a flat microstructures can be fabricated.

    目錄 致謝 摘要 英文摘要 目錄…………………………………………………………………I 表目錄…………………………………...........................III 圖目錄……..……………….....………………..………..IV 符號說明……………………………..……...………..…...IX 第一章、緒論 1-1 簡介…….…………………………………………………1 1-2 文獻回顧……………………………….………………2 1-3 研究動機………………………………...……………..7 第二章、薄膜沉積現象與薄膜應力生成原理 2-1 薄膜沉積現象……………………….…………………....9 2-2 薄膜殘留應力生成之原理………………..……………..11 第三章、低應力氮化矽實驗參數設計與實驗設備 3-1 低應力氮化矽之實驗參數設計………...……………...16 3-2 低應力氮化矽薄膜實驗與微懸浮結構之製程設備…….18 第四章、微懸浮結構之設計與製作 4-1 微懸浮結構之設計...…………...……………………..23 4-2 微懸浮結構製作………………………………………...23 第五章、實驗結果與討論 5-1 HDP-CVD氮化矽薄膜殘留應力分析 5-1-1感應耦合電漿功率(ICP Power)……….……………...…...27 5-1-2基板溫度………...……………………………………………..29 5-1-3反應氣體流量比(SiH4/NH3)………………………………..30 5-1-4氮化矽薄膜應力隨沉積時間變化之穩定性……….………….31 5-1-5熱循環退火與應力關係……………………………….…….33 5-1-6 HDP-CVD氮化矽薄膜之抗蝕刻特性……………………….….35 5-2 微懸浮結構製作 5-2-1微懸浮結構之製作方法………………………….…………….37 5-2-2犧牲層移除與深蝕刻…………………………………………39 5-5-3微懸浮結構之形變現象…………………………………….40 第六章、結論……………………………….…………………….42 6-1 氮化矽薄膜殘留應力方面………………….………….42 6-2 微懸浮結構方面……………………………….…….43 參考文獻………………………………………….45 LIST OF TABLES Table 3-1 Properties and operator range of HDP-CVD……..49 Table 4-1 Standard Clean……………….…………………….50 Table 5-1 Wet etch rate characteristics of HDP-CVD silicon nitride films…...51 Table 5-2 HDP-CVD, PECVD and LPCVD silicon nitride films properties..........51 Table 5-3 Parameters of HDP-CVD deposition process for microstructure…......52 LIST OF FIGURES Figure 1-1 (a) Bending of cantilever beam by residual gradient stress along vertical direction. This gradient stress is modeled as bending moment M. (b) Hypothetical state that the beam with gradient stress is straightened by liquid tension during drying. (c) The beam is pulled down and stuck to the substrate as the rinse liquid dried off[10]………………………….………….……53 2-1 Step of thin film deposition. (a) Nucleation, (b) Grain growth, (c) Grain coalescence, (d) Filling of channels, (e) Film growth. Where 1 is adatoms and 2 is adatoms of desorb【19】…………………………………............….…54 2-2 Thermal stress. Tension and compression are determined by the relative size of thermal expansion coefficients of film and substrate. Suppose a strain-free film at deposition temperature, Td, is cooled to room temperature, Tr, on a substrate with a different coefficient of thermal expansion……..……..... 55 3-1 Outline of the experiment steps……….…………………56 3-2 Outline of the downstream HDP-CVD reactor used...........57 3-3 Design principle of the inductive coupling plasma, δs is the skin depth…58 4-1 Outline of the microstructure of photomask design……………......………..59 4-2 Fabrication procedures of the cantilever beams primary design………......61 4-3 Fabrication procedures of the improved design………………………...……….62 5-1 FTIR spectra of silicon nitride films deposited at different ICP power (process parameters: T=350℃, SiH4:NH3=50:150(sccm), P=15mTorr)….…………….63 5-2 Film stress of deposited silicon nitride films versus deposition rate (process parameters of HDP-CVD： SiH4:NH3=50:150(sccm), RF=13.56MHz, Power density =0.0283W/cm2, 0.0396W/cm2, 0.0509W/cm2, P=15mTorr; process parameters of VHF PECVD：SiH4:NH3:N2=120:600:2800(sccm), RF=13.56MHz, Power density=0.32~0.64W/cm2, T=300℃)………64 5-3 FTIR spectra of silicon nitride films deposited at different ICP Power (process parameters: T=350℃, SiH4:NH3=15:150(sccm), P=15mTorr)…...………….65 5-4 (a) Film stress of deposited silicon nitride films versus temperature and (b) Deposition rate of deposited silicon nitride films versus temperature (process parameters: SiH4:NH3=15:150(sccm), ICP Power=500W, 700W, 900W, P=15mTorr)………………........66 5-5 Film stress of silicon nitride films deposited at different SiH4-contents in the SiH4/NH3 gas composition ( process parameters of HDP-CVD: T=250℃、300℃、350℃, ICP Power=900W, P=15mTorr ; process parameters of MW-ECR: T=200℃, PMW=400W, PRF=50W, MW=2.45GHz) ……………………..67 5-6 FTIR spectra of silicon nitride films deposited at different SiH4-contents in the SiH4/NH3 gas composition (process parameters of HDP-CVD: T=350℃, ICP Power=900W, P=15mTorr)….………………..........…..68 5-7 Film stress of deposited silicon nitride films versus total gas flow rate (process parameters: SiH4/NH3=1, T=200℃, ICP Power=900W, P=15mTorr)…….…..69 5-8 (a) Film stress of deposited silicon nitride films versus time after deposition; (b) FTIR spectra of silicon nitride films versus time after deposition (process parameters: SiH4:NH3=50:150(sccm), T=350℃, ICP Power=900W, P=15mTorr)……………………….………...………….70 5-9 TDS-APIMS analysis result of ion intensity of silicon nitride films versus temperature (process parameters: SiH4:NH3=50:150(sccm), T=350℃, ICP Power=900W, P=15mTorr)………………………….71 5-10 Residual stress variation with temperature for 2μm thick HDP-CVD nitride films: (a) Annealing at 400℃; (b) Annealing at 500℃(process parameters: SiH4:NH3=50:150(sccm), T=350℃, ICP Power=900W, P=15mTorr)……………….......72 5-11 Residual stress variation with temperature for silicon nitride films: (a) 0.85μm thick PECVD nitride films (process parameters: SiH4:NH3=20:80(sccm), T=300℃, RF Power=50W); (b) 0.16μm thick LPCVD nitride films (process parameters: 3SiH2+7NH3, T=780℃)……………………...….....73 5-12 Cracks of 2μm thick HDP-CVD silicon nitride films due to tensile stress of LPCVD annealing at 600℃………….……74 5-13 (a) H3PO4 etching rate of deposited silicon nitride films versus temperature (process parameters: SiH4:NH3=15:150(sccm), SiH4:NH3=50:150(sccm), ICP Power=700W, 900W, P=15mTorr) and (b) Film stress and BOE etching rate of deposited silicon nitride films versus temperature (process parameters: SiH4:NH3=12:12(sccm), ICP Power=900W, P=5mTorr). The indicated are as-refractive index……………………………......….75 5-14 FTIR spectra of silicon nitride films deposited at different temperature (process parameters: SiH4:NH3=12:12(sccm), ICP Power=900W, P=5mTorr)………….76 5-15 Microscopic picture of 2μm thick silicon nitride films after BOE wet etching: (a) Swelling due to particles effect; (b) Broken due to stress concentration………………………………….…..........…...77 5-16 Nano-indenter analysis result of Young’s Modulus of silicon nitride films versus displacement into surface. (process parameters: SiH4:NH3=50:150(sccm), T=350℃, ICP Power=900W, P=15mTorr)………………………78 5-17 Diaphragm deformation of the sacrificial layer due to compressive stress. (a) Micro-sensor, (b) Cantilever beams, (c) Micro-bridge……..............….79 5-18 Diaphragm deformation of microstructure after the sacrificial layer removal due to residual gradient stress................80 5-19 Microscopic views of microstructures after the sacrificial layer removal for fabrication procedures of the improved design……….........…81 5-20 Diaphragm of micro-pressure sensor after the sacrificial layer removal. (a) Before wet etching, (b) Using KOH solution without addition of IPA leading to formation of silicate on wafer surface, (c) Using KOH solution 3M with IPA 12w.t % at 60℃………........82 5-21 Microscopic views of microstructure after the sacrificial layer removal. (a) Cantilever beams, (b) Micro-bridge. In this case, the prescription of etchant is (KOH solution 3 mole/l with IPA 12w.t % at 60℃)……..….83 5-22 Particles contamination effect of microstructure after the sacrificial layer removal, were made from silicon nitride as deposited by HDP-CVD. (a) Cantilever beams, (b) Micro-bridge. In this case, the prescription of etchant is (KOH solution 2 mole/l at 60℃)………….……84 5-23 Microscopic views of microstructure after the sacrificial layer removal and measurement results of surface profiler. (a) Cantilever beams, (b) Micro-bridge………………………..........…………………….85 5-24 Cantilever beams with a gradient stress before and after release. A hypothetical state that the beam with gradient stress is straightened by liquid tension during drying is shown, after release but before bending, to illustrate the change in bending moment results from release. After bending, the stress is zero throughout the beam……………….....86 5-25 Silicon nitride cantilever beams bent upward by values of residual gradient stress. Upwardly bending beams were made from silicon nitride as deposited by HDP-CVD at 350℃ without succeeding heat treatments, and the gradient stress is 36 MPa/μm………………..........………..87 符號說明 英文字母 E 楊氏模數(Young's Modulus) h 矽晶圓厚度(Thickness of silicon wafer) L 懸臂樑之設計長度(Length of cantilever beams) R 矽底材的曲率半徑變化量(Curvature of silicon wafer) Td 薄膜沉積溫度(Deposition temperature of thin film) Tr 室溫(Room temperature) t 薄膜厚度(Thickness of thin films) umax 懸臂樑之最大變形量(Maximum deflection of cantilever beams) 希臘字母 Γ 殘留梯度應力 (Residual Gradient Stress) σtot 總應力(Total stress of thin film) σint 本質應力(Intrinsic stress) σext 外應力(Extrinsic stress) σth 熱應力(Thermal stress) σf 殘留應力(Residual stress) νf 薄膜波森比(Poisson’s ratio of thin film) αf 薄膜熱膨脹係數(Thermal expansion coefficient of thin film) αs 底材熱膨脹係數(Thermal expansion coefficient of substrate) δs Skin Depth

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