氮化矽微碟共振腔之濕度感測應用

Abstract

在本論文中我們製作出氮化矽微碟共振腔，探討其光學特性並在不同濕度環境下觀察其模態的變化。微碟共振腔的迴音廊模態具有良好的光場侷限性，因此存在許多優勢，例如品質因子 (Q值) 高、體積小，並且較容易與錐形光纖耦合，也因此非常適合用在光積體電路上。 製程方面，我們在矽 (100) 基板上沉積氮化矽 (SiN) 來做為共振腔材料，利用微影製程以及蝕刻製程將圖形轉移到氮化矽薄膜上，最後再利用氫氧化鉀 (KOH) 濕式蝕刻對矽各晶格面有不同的蝕刻率的特性，直接於矽基板上製作出支撐微碟的柱子。 量測方面，我們使用U型錐形光纖進行耦合，並以中心波長 1330nm 的超輻射二極體 (Super Luminescent Diode, SLD) 做為光源來進行傳輸頻譜量測，以此來驗證微碟共振腔的共振特性以及確定迴音廊模態的位置，再根據公式計算出Q值。我們也用模擬軟體 (COMSOL) 來分析，以便能夠快速分辨出頻譜上的迴音廊模態。我們在直徑為 10μm 的氮化矽微碟共振腔觀察到TE一階模態以及其他的高階模態，其中一階模態的Q值可以達到 2700。 為了展示氮化矽微碟共振腔在濕度感測的應用，我們在不同濕度下量測，利用環境的折射率改變使光的模態發生變化，進而得知濕度的變化。我們觀察到在濕度為 55~75% 時，模態的Q值會隨著濕度上升而下降，模態的深度也會隨著濕度上升而變得不明顯。我們的研究展示了氮化矽微碟共振腔對於環境濕度的改變是敏感的，因此具有成為濕度感測器的潛力。

In this thesis, we fabricated silicon nitride microdisk resonators, discussed their optical properties and observed mode changes under different humidity environments. The whispering gallery modes of microdisk resonators exhibit excellent optical field confinement, providing several advantages, such as high Q-factor, small volumes, and easier coupling with tapered fibers, and therefore making them highly suitable for use in photonic integrated circuits. Regarding the device fabrication process, first we deposited a silicon nitride (SiN) film as the resonator material on a silicon (100) substrate. Then we used photolithography and etching processes to transfer the pattern to the SiN film, and finally used potassium hydroxide (KOH) wet etching to directly form the pedestal of microdisks on the silicon substrate by means of anisotropic etching rate in different lattice planes. For device characterization, we used a U-shaped tapered fiber coupling method to measure the transmission spectrum with a super luminescent diode as the light source whose center wavelength is 1330nm. This measurement can be used to verify the resonance properties of the microdisk resonators and determine the positions of the whispering gallery modes, and calculate the Q-factor based on the formula. We also use COMSOL simulation software to analyze and quickly identify the whispering gallery modes in the spectrum. In the 10μm diameter silicon nitride microdisk resonators, we observed first-order TE modes and other higher-order modes, with the Q-factor of the first-order mode reaching up to 2700. In order to demonstrate the device application in humidity sensing, we performed transmission spectrum measurement under different humidity conditions to observe mode changes and thus determine the humidity variations. We observed that at humidity levels between 55~75%, the Q-factor of the modes decreases as the humidity increases, and the transmission dip also becomes less pronounced with increasing humidity. Our research demonstrates that silicon nitride microdisk resonators are sensitive to environmental humidity changes, showing their potential as humidity sensors.