雷射干涉重力波組織 (LIGO, Laser Interferometer Gravitational wave Observatory)建造雷射干涉重力波偵測器作為天文觀測站，欲透過大型麥克森干涉儀直接偵測到重力波訊號，以觀測星體變化等天文現象。然而，重力波的訊號非常微弱，必須盡可能降低系統與環境雜訊才能進行觀測。此偵測系統整體雜訊的最低處約為頻率40~400 Hz區間，在此頻率區間的雜訊主要由Quantum noise與Coating Brownian noise主導，其中的Coating Brownian noise是反射鏡上光學薄膜產生的一種熱擾動雜訊(thermal noise)，主要受薄膜材料與環境溫度影響。此外，根據統計力學中的Fluctuation-dissipation theorem可知，熱擾動雜訊與機械損耗為正比關係，透過量測機械損耗便可得知熱擾動雜訊的程度。因此本實驗室針對偵測系統最靈敏的頻率區間(40~400 Hz)，研究低機械損耗的薄膜材料，以降低反射鏡上光學薄膜的熱擾動雜訊，達到提高偵測系統靈敏度之目的。
參考文獻指出高應力氮化矽薄膜具有非常低的機械損耗，且氮化矽薄膜降至低溫環境下能夠得到更低的機械損耗。然而，這些文獻所使用的試片共振頻率高達百萬赫茲(MHz)以上，且應用目的與我們不同，因此我們針對雷射干涉重力波偵測器最靈敏的頻率區間探討高應力氮化矽薄膜的機械損耗與薄膜特性。
於本實驗室先前研究中，藉由調整電漿輔助化學氣相沉積系統(PECVD)的製程氣體流量比(NH3/SiH4)改變薄膜組成成分，將五種不同成分的氮化矽薄膜(SiN0.40、SiN0.49、SiN0.65、SiN0.79、SiN0.87)沉積於矽懸臂基板上並分析薄膜特性與機械損耗。結果顯示隨著氣體流量比增加，薄膜折射率降低、應力增加且機械損耗也逐漸降低。然而，在薄膜成分為SiN0.87時矽懸臂的彎曲模態(bending mode)機械損耗出現反轉現象(inversion)，導致其薄膜機械損耗無從得知。我們針對實驗結果進行分析，推測這種現象是因為薄膜的張應力(tensile stress)過高使矽懸臂彎曲，導致矽懸臂基板本身的thermoelastic loss降低，影響彎曲模態的機械損耗。因此，本論文將利用雙面鍍膜的方式改善高應力SiN0.87薄膜造成矽懸臂彎曲問題，並取得SiN0.87薄膜的機械損耗為主要研究方向。另一部分，在本論文中將針對本實驗室的室溫機械損耗量測系統仍存在之可能損耗來源進行研究與分析，以改善並減少系統量測誤差。

In order to detect the gravitational wave (GW) directly to confirm the general theory of relativity from Albert Einstein and for future gravitational wave astronomy, the international Laser Interferometer Gravitational Wave Observatory (LIGO) utilize Michelson interferometer to detect GW signals. However, the signal of gravitational waves is extreme weak, so the system noise and the environment noise should be reduced as low as possible. In all of the background noise, the coating Brownian noise which caused by optical coating is a limiting source of thermal noise at 40-400 Hz. It depends on the properties of coating materials and surrounding temperature. Thus, investigating a low mechanical loss material to reduce the coating Brownian noise is necessary.
In our previous work, the stress of the SiNx increased with value of x. Moreover, it was found that high stress SiNx film has lower mechanical loss. Even mechanical loss of the high stress coated cantilever was lower than that of the uncoated. This unusual phenomenon is referred to as "inversion" for high stress SiN0.87 film. The inversion appears in most of the bending modes, but none of the torsion modes. It may attribute to reduction of the thermoelastic loss of the silicon substrate, which were warped due to the tensile stress of the SiN0.87 film. Because the torsion modes of the silicon cantilever are not susceptible to thermoelastic loss. In this paper, we studied the fabrication, material properties and mechanical loss of SiN0.87 film which is deposited by PECVD in detail. Finally, we developed a formula by considering the energy dissipation, and then substitute the results of measurement into the formula to obtain the loss angle of high stress SiN0.87 film.

Key word：silicon nitride film, stress, mechanical loss, double-side coating, Young’s modulus, refractive index, extinction coefficient.

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