雷射干涉重力波觀測站(Laser Interferometer Gravitational-Wave Observatory, LIGO)利用大型麥克森干涉儀應用於重力波的偵測。由於重力波訊號非常微弱又容易受到各種雜訊影響，因此必須降雜訊以提升重力波偵測系統的靈敏度。本實驗室主要的研究方向為優化重力波干涉儀中高反射鏡上的薄膜材料，在各式雜訊中反射鏡薄膜材料的熱擾動雜訊為影響系統的雜訊來源之一，根據fluctuation-dissipation theorem得知此雜訊與薄膜材料的機械損耗成正比，且作為光學應用之高反射鏡也需擁有優秀的光學性質，因此本實驗室致力於研究低機械損耗與優秀光學特性之薄膜材料。
先前本實驗室在高折射率方面的薄膜材料研究為使用PECVD鍍製出低機械損耗的氮化矽薄膜SiN0.33H0.58與使用LPCVD鍍製出低光學吸收的氮化矽薄膜SiN0.91H0.02，而在低折射率則利用PECVD鍍製氮氧化矽薄膜並透過改變反應氣體N2O與SiH4的流量比來調整薄膜之特性，依照薄膜的元素組成分可分為高含氧量(silica-like)與高含氮量(nitride-like)，在高含氧量薄膜的部分雖擁有比較低的光學吸收但不足以達到LIGO的標準，且機械損耗較高，因此本研究針對此薄膜進行優化，希望能將光學吸收與機械損耗同時再降低。而高含氮量薄膜的機械損耗低但光學吸收則較高，有關此薄膜之優化請參考洪阡譯之論文。在本實驗室過往的研究中發現退火可降低光學吸收與機械損耗，因此本研究利用退火的方式來使薄膜優化。
研究結果顯示，在900度6小時的純氮退火後有最佳的薄膜參數，在經過高溫退火後影響光學吸收的主因N-H鍵大量斷鍵使得吸收下降，在1064nm之光學吸收從1.43×10^(-6)降至3.3×10^(-7);1550nm之光學吸收從5.8×10^(-6)降至5.8×10^(-7);1950nm之光學吸收則從1.5×10^(-5)降至2.6×10^(-6)，此數據為本實驗室首次有光學吸收降低至10^(-7)，為本實驗室研究的一大進步。而在機械損耗方面在退火後有也顯著的降低且比LIGO目前所使用的低折射率氧化矽薄膜相比更為下降。
至此本實驗在優化光學與機械特性的這一目標取得了非常好的結果，但使用高溫退火使得材料性質產生一缺點，即為退火減少N-H鍵使得薄膜有5~6%的減薄情形，此結果非常不利於薄膜在高反射鏡上之應用，因此期望未來能使用其他的沉積方法來讓薄膜有更好的表現。

The Laser Interferometer Gravitational-Wave Observatory (LIGO) uses the large Michelson interferometer for gravitational wave detection. Since the gravitational wave signal is feeble and easily affected by various kinds of noise, it is necessary to reduce the noise to improve the sensitivity of the gravitational wave detection system. The main research direction of this laboratory is to optimize the thin-film material on the high-refractive mirror in the gravitational wave interferometer, among all kinds of noises, the coating Brownian noise of the mirror film material is one of the noise sources affecting the system. According to the fluctuation-dissipation theorem, this noise is proportional to the mechanical loss of the thin film material, and as a high-refractive mirror for optical applications having excellent optical properties is also important. Therefore, our laboratory has been dedicated to developing thin-film materials with low mechanical loss and outstanding optical properties.
In previous research on high refractive index material in our laboratory, we used PECVD to deposition silicon nitride film,SiN033H0.58, which had a low mechanical loss and use LPCVD to deposition silicon nitride film,SiN0.91H0.02, which had a lower optical absorption. On the other hand,for the low-index material’s research we used PECVD to deposition silicon oxynitride films, and the characteristics of the films are adjusted by changing the flow ratio of the reactive gases N2O and SiH4. According to the elemental composition of the film, it can be divided into high oxygen content (silica-like) and high nitrogen content (nitride-like). Although the part of the silica-like silicon oxynitride film has a relatively low optical absorption, it is not enough to meet the standard of LIGO, and the mechanical loss of the film is relatively high. Therefore, this study is optimized for this film, hoping to reduce the optical absorption and mechanical loss at the same time. On the other hand, the nitride-like silicon oxynitride film had low mechanical loss but high optical absorption. For the optimization of this film, please refer to the thesis by Qian-Yi Hong. In previous studies in our laboratory, it was found that annealing can reduce optical absorption and mechanical loss, so this research attempted to use annealing to optimize the film.
The results show that the optimum film parameters are obtained after pure nitrogen annealing at 900°C for 6 hours, after high temperature annealing, N-H bonds that affect optical absorption was broken, resulting in a decrease in absorption. The optical absorption at 1064nm decreases from 1.43×10^(-6) to 3.3×10^(-7); the optical absorption at 1550nm decreases from 5.8×10^(-6) to 5.8×10^(-7); the optical absorption at 1950nm decreased from 1.5×10^(-5) to 2.6×10^(-6), this data is the first time that the optical absorption of the laboratory has been reduced to 10^(-7), which is a major advance in the research in this laboratory. In terms of mechanical loss, there is a significant reduction after annealing and is even lower than low-refractive-index SiO2 thin-film currently used in LIGO.
So far, this experiment has achieved superb results with the goal of optimizing optical and mechanical properties. However, the use of high temperature annealing leads to a disadvantage of material properties, that is the thin film thinning by 5~6% due to annealing because the concentration of N-H bonds is reduced during annealing. This result is unfavorable for the application of the film to high-refractive mirrors, so it is expected that other methods of deposition can make the film more perfect in the future.

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