以電漿輔助化學氣相沉積法鍍製於矽懸臂之氮化矽其熱退火後對於室溫機械損耗之影響

Annealing effect on the room temperature mechanical loss of the silicon nitride films deposited with PECVD on silicon cantilever

陳信傑(Hsin-Chieh Chen)

指導教授 ： 趙煦

國立清華大學/電機資訊學院/光電工程研究所/碩士(2017年)

摘要

雷射干涉重力波偵測組織(LIGO ,Laser Interferometer of Gravitational wave Observatory)為一重力波觀測組織，其利用大型麥克森干涉儀量測重力波訊號，由於重力波訊號相當微弱，難以偵測，故量測時必須減少額外的雜訊，由雜訊頻譜中可知，在100Hz左右，其雜訊來源主要為coating Brownian noise，此雜訊為薄膜產生且難以直接量測得到，但經由fluctuation dissipation可知該雜訊與薄膜本身之機械損耗成正比關係，因此量測薄膜機械損耗為本實驗室主要的研究之一。
在之前的研究中，本實驗室所開發之高反射鏡SiN0.40H0.79/SiO2堆疊膜之機械損耗已低於目前LIGO正在使用之600℃ annealed 14.5% TiO2 - doped Ta2O5 / SiO2堆疊膜。為克服鍍膜時，因應力使得基板彎曲導致量測到之機械損耗不準確性，必須利用雙面鍍膜方式平衡應力，由於傳統矽懸臂基板有一粗糙面使得兩薄膜鍍膜情形不同，容易造成誤判，故使用SOI wafer製作量測用基板，其受的box layer之保護，故製作完成時並無粗糙面。此外從文獻探討中得知薄膜之機械損耗可能與薄膜內之氫含量有關，故本文針對本實驗室所鍍製之五種氮化矽薄膜材料(SiN0.40H0.79、SiN0.49H0.68、SiN0.65H0.60、SiN0.79H0.62、SiN0.87H0.93)利用傅立葉轉換紅外光譜儀(FTIR, Fourier Transform Infrared Spectroscopy)之量測結果進行氫含量計算，在氫含量計算結果中，Si-H鍵會隨著SiH4/NH3之氣體通量比增加而上升，N-H鍵因此下降，將氫含量與機械損耗進行交叉比對後，得Si-H鍵與室溫機械損耗有一正相關性。本研究中利用退火方式，觀察本實驗室所製作之材料(SiN0.40H0.79)是否可以因此有效降低機械損耗，退火條件為大氣環境、450℃、30分鐘，但SiN0.40H0.79薄膜易受熱處理而產生表面損傷，因此利用SiO2薄膜將SiN0.40H0.79薄膜包覆其中形成三層結構進行退火，不但使得表面損傷消失，而且此結構因退火處理使得退火前後機械損耗從1.23×〖10〗^(-4) 降至4.13×〖10〗^(-5)。

關鍵字

退火 ； 氮化矽 ； 機械損耗 ； 氫含量 ； 電漿輔助化學氣相沉積法

並列摘要

Large Michelson interferometer set up by Laser Interferometer of Gravitational Wave Observatory (LIGO) is conducted to detect gravitational wave. Owing that the signal of gravitational wave is extremely weak, it’s necessary to reduce the noise caused by the interferometer to approach accurate result. According to the noise spectrum, the sensitivity of interferometer is mostly limited by coating Brownian noise and Quantum noise at approximately 100Hz. The coating Brownian noise, which is proportional to mechanical loss stated by fluctuation-dissipation theorem, comes from high reflective coating on the mirror while Quantum noise is laser related. In research of reducing noise, deducting the mechanical loss of optical thin films becomes critical topic as well. Based on our recent study, the mechanical loss of SiN0.40/SiO2 stacks is lower than “600℃ annealed 14.5% TiO2 - doped Ta2O5 / SiO2 stacks” which is the material used in current gravitational wave detectors.
In order to eliminate the stress effect on silicon cantilever, a double-side coating process is designed. Due to the wet-etching process, conventional silicon cantilever is one-side roughened. Thus, the thin films are coated under the different surface condition with conventional silicon cantilever which the roughened side contributed extra mechanical loss. Our team fabricated a double-side smooth silicon cantilever from Silicon-on-Insulator (SOI), hence the thin films could be coated under both smooth surface condition.
Discovered in our study, applying different gas flow rate ratio leads to unique IR absorption spectra of silicon nitride films by using Fourier Transform Infrared Spectroscopy (FTIR). From literatures, the peak at 2150 cm-1 wavenumber is given by Si-H bonds when the other one is at 3350 cm-1 wavenumber given by N-H bonds. These two peaks are observed in our IR spectra and we can evaluate the bond density of each bond by related equation. As SiH4 /NH3 gas flow rate ratio increasing, the Si-H bond density increases while the N-H bond density decreases.
The SiN0.40H0.79 film is annealed in air for 30 min at 450℃. As a result of thermal anneal would induce damages, we investigate a three-layer structure which the SiN0.40H0.79 film is sandwiched between SiO2 films to improve the surface without appearance of damages and the mechanical loss of three-layer structure is reduced from 1.23×〖10〗^(-4) to 4.13×〖10〗^(-5).

並列關鍵字

annealing ； mechanical loss ； silicon nitride ； hydrogen content ； pecvd

參考文獻

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