光熱共光路干涉儀系統之設置與電漿輔助化學氣象沉積法沉積之氮化矽薄膜光學吸收研究

Photothermal common-path interferometry system setup and study of the optical absorption of the silicon nitride films deposited by PECVD method

康乃中(Nai-Chung Kang)

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

本實驗室參與重力波觀測組織(LIGO, Laser Interferometer Gravitational wave Observatory)的研究計畫，致力於研究大型麥克森干涉儀裡高反射鏡的材料。由於重力波訊號極其微弱，如何降低雜訊成了一項重要的研究方向。由LIGO的理論雜訊頻譜圖可以得知，雜訊的來源大都來自Coating Brownian noise，就是反射鏡薄膜所產生的熱擾動(thermal noise)，再根據fluctuation-dissipation theorem可以得知熱擾動跟薄膜的機械損耗成正比關係，如果可以得知薄膜的機械損耗就相當於了解薄膜的熱擾動程度。本實驗室所研究用電漿輔助化學氣象沉積法沉積的氮化矽薄膜室溫的機械損耗已經到達10-5order，跟常見高反射鏡中的光學薄膜Ta2O5的室溫機械損耗2x10-4相比，已經降低許多。但要符合LIGO規格的高反射鏡材料，除了要有低的機械損耗外、還必須具有極低的光學吸收，材料光學吸收的好壞將嚴重影響高反射鏡的品質。下世代低溫LIGO的大型麥克森干涉儀所使用的基板為矽基板，因應矽基板的光學吸收，將使用矽基板吸收較低波長1550nm的連續式(CW)雷射光源，因此透過研究得知氮化矽薄膜在1550nm波段的光學吸收非常重要。本實驗室起初利用橢圓偏光儀，企圖得知氮化矽薄膜在1550nm波段光學吸收值，但是因為吸收值太低的緣故，橢偏儀解析不出實質的吸收數值。所以本實驗室設置了一台光熱共光路干涉儀(Photothermal Common-path Interferometry)，簡稱PCI。此裝置可以量測薄膜超低的光學吸收，本論文針對PCI系統的量測原理進行推導闡述、並說明架設的流程與量測步驟，再分析比較氮化矽薄膜的量測結果。得到氮化矽薄膜的光學吸收後，根據LIGO白皮書的規格，利用Macleod光學模擬軟體設計堆疊膜SiN0.40H0.79/SiO2的膜層數，再利用此膜層數計算堆疊膜SiN0.40H0.79/SiO2的thermal noise，最終再與LIGO的thermal noise理論曲線比較。

關鍵字

光學吸收 ； 機械損耗 ； 熱擾動 ； 氮化矽薄膜

並列摘要

Our laboratory has participated in the research project of the Laser Interferometer Gravitational Wave Observatory(LIGO), which focused on the study of optical coating on the high reflective mirror in large-scale Michelson interferometers. As the gravity wave signal is extremely weak, how to reduce the noise has become an important issue. According to LIGO's theoretical noise spectrum, the source of the noise comes mostly from the Coating Brownian noise, which is the thermal noise generated by the optical coating on the high reflective mirror. In the light of fluctuation-dissipation theorem, it can be seen that the thermal noise is proportional to the mechanical loss of the film. Knowing the mechanical loss of the film is equivalent to understanding the level of thermal noise. The mechanical loss of the SiNxHy produced by PECVD in our laboratory has reached 10-5 order at room temperature. It is obviously much lower than the loss of common optical films Ta2O5 (2x10-4).
To meet the LIGO specifications of the high reflector material, apart from a low mechanical loss, a extreme low optical absorption is also necessary. The absorption will make great influence on the quality of high reflector. The latest low-temperature LIGO large-scale Michelson interferometer uses silicon substrate. In response to decrease the optical absorption of silicon substrates, CW laser light sources in 1550nm will be used. Therefore, the study of optical absorption of silicon nitride film in 1550nm is very important. At first, our laboratory used an ellipsometer to study the optical absorption of silicon nitride film in the 1550nm. However, because the absorption value was too low, the ellipsometer can not resolve the accurate absorption value. As a result, our laboratory deployed a Photothermal Common-path Interferometry(PCI) that can be used to estimate extreme low optical absorption. This thesis will expound the principle of measurement of PCI system, discuss the flow and measurement steps, and analyze the measurement results of silicon nitride film. After the data of optical absorption of the silicon nitride film was obtained, the number of layers of SiN0.40H0.79/SiO2 was calculated by Macleod optical simulation software according to the specifications of LIGO white paper. Next, the number of layers was used to calculate the thermal noise of SiN0.4/SiO2 layers. Finally, the data of the noise will be compared with the LIGO thermal noise theoretical curve.

並列關鍵字

optical absorption ； mechanical loss ； thermal noise ； silicon nitride film

參考文獻

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