西元1915年Albert Einstein廣義相對論中提出重力波的相關理論，於1974年Hulse與Taylor藉由觀測脈衝雙星系統Binary pulsar system，間接證實重力波的存在，直到最近2015年9月14日，雷射干涉重力波組織(LIGO)位於Hanford,WA,及Livingston, LA,的兩座觀測站皆偵測到了重力波訊號，終於重力波的存在被證實，此成果除了驗證了廣義相對論之外，由於重力波可應用於星體和天文現象的觀測，因此對於宇宙天文學研究也有相當大的貢獻。而重力波的訊號很微弱，故需要盡可能地降低雜訊來源，以提高系統偵測的靈敏度，其中在頻率為40 ~ 400 Hz的範圍內是重力波偵測系統中雜訊最小的區間，此區間內的主要雜訊來源為Quantum noise與Coating Brownian noise，本實驗室致力於研究其中的Coating Brownian noise，是反射鏡上光學薄膜所產生的thermal noise，根據fluctuation-dissipation theorem得知thermal noise 跟dissipation呈正比關係，因此只要量測薄膜的機械損耗便可知薄膜熱擾動雜訊的程度。故研究目的為探討低機械損耗之高反射鏡鍍膜材料，以提高重力波偵測之靈敏度。
本實驗室欲開發以電漿輔助化學氣相沉積系統(PECVD)製程鍍製重力波偵測之高反射鏡薄膜，製程規格需要可以鍍製大面積且薄膜均勻度佳的鍍膜技術，其中PECVD具有製程溫度低、薄膜沈積速度快、且有好的階梯覆蓋能力及均勻度佳等優點，而到目前為止已使用PECVD鍍製並研究過a-Si、SiO2及SiNx (其中包括SiN0.40、SiN0.49、SiN0.65、SiN0.79、與SiN0.87五種成分比例)等材料，當中的SiNx薄膜可作為高折射率材料與SiO2(作為低折射率材料)堆疊高反射鏡，其機械損耗非常低之外，於長波段的光學消散係數也相當的小，是相當適合作為鍍製高反射鏡的材料。於先前的研究結果顯示SiNx隨著Si、N比例的改變，楊氏係數、應力與折射率都有很大的差異，比較過各成分比例之折射率與應力大小之後，本研究選擇以SiN0.40與SiO2堆疊光學薄膜，因其折射率差異較大且應力可以相互抵消，故使用電漿輔助化學氣相沉積系統(PECVD)鍍製SiO2 / SiN0.40堆疊1-pair、2-pair、3-pair及4-pair之膜層結構，並探討其機械損耗為何。
本實驗使用電漿輔助化學氣相沉積的方式鍍製四分之一1550 nm波長厚度的SiN0.40與SiO2之堆疊膜層，1-pair、2-pair、3-pair、4-pair在室溫量測頻率於100Hz之機械損耗均為10-5 order，此結果低於目前重力波觀測所使用的高反射鏡材料：600℃退火下之Ta2O5摻雜14.5%的TiO2與SiO2兩者堆疊(the 600℃ annealed 14.5% TiO2 - doped Ta2O5 / SiO2)，且SiO2 / SiN0.40之製程溫度均為300℃，其退火後之機械損耗還會再降低，因此SiO2 / SiN0.40非常值得再繼續深入研究，是相當適合作為重力波偵測之高反射鏡鍍膜的材料。

In 1915, gravitational waves in the theory of general relativity has been propounded by Albert Einstein, after that, in 1974, Hulse and Taylor had indirectly observed the existence of gravitational waves in a binary pulsar system. However , no one had directly confirmed its existence until 14th January 2015: the two detectors located at Hanford, WA and Livingston, LA of the Laser Interferometer Gravitational-Wave Observatory (LIGO) had simultaneously observed a gravitational wave. The finding has not only confirmed the existence of gravitational wave in the theory of general relativity, but also contributed greatly to astronomical observations.
However, the signal of gravitational wave is extremely weak, so the noise should be reduced as low as possible. Much research in our laboratory has been devoted to investigating the coating Brownian noise, which is the thermal noise caused by the optical coating on the high reflective mirror. According to Fluctuation-Dissipation theorem, the thermal fluctuation (thermal noise) is proportional to mechanical dissipation (mechanical loss) in the material. Therefore, as long as we measure the mechanical loss of films, we can deduce the thermal noise of films as well.
We investigated plasma-enhanced chemical vapor deposition (PECVD) method aimed for LIGO to deposit HR mirror coating. It has many advantages such as low processing temperature, fast deposition, and good uniformity for film. So far, amorphous silicon (a-Si), silicon dioxide (SiO2), and silicon nitride (SiNx) deposited by PECVD method had been studied. The different compositions of silicon nitride (SiNx) films can be deposited by PECVD with different gas flow ratio between SiH4 and NH3 while the stress and refractive index varied with nitrogen concentration. In this research, we selected SiO2 to be low index material and SiN0.40 to be high index material for optical coating because of the high index difference and stress compensation between these two materials. Therefore, we investigated the mechanical loss of SiN0.40/SiO2 quarter-wave (QW) stacks from 1 to 4 pairs at room temperature which were deposited by PECVD system furthermore the QW were the thickness of a quarter wavelength at 1550nm.
The results of this study, the mechanical loss of SiNx/SiO2 stacks with 1 to 4 pairs were investigated at room temperature, and the loss were in 10-5 order at 100 Hz, lower than the material used in current gravitational wave detector. The most important thing is that the coating loss would more reduce by thermal annealing and thus SiNx/SiO2 is worthy of further study as HR mirror coating for LIGO.

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