高功率脈衝磁控濺鍍氮化鎵環形共振腔製程之研究;Fabrication of guallium nitride microring resonators by high-power impulsed magnetron sputtering

Please use this identifier to cite or link to this item: http://ir.lib.ncu.edu.tw/handle/987654321/92400

Title:	高功率脈衝磁控濺鍍氮化鎵環形共振腔製程之研究;Fabrication of guallium nitride microring resonators by high-power impulsed magnetron sputtering
Authors:	吳仕昕;Wu, ShiH-Hsin
Contributors:	光電科學與工程學系
Keywords:	氮化鎵;環形共振腔;高功率脈衝磁控濺鍍;波導;矽光子;gallium nitride;microring resonator;high-power impulsed magnetron sputtering;waveguide;silicon photonics
Date:	2023-08-14
Issue Date:	2024-09-19 15:50:02 (UTC+8)
Publisher:	國立中央大學
Abstract:	為了彌補現今互補式金屬氧化物半導體 (Complementary Metal Oxide Semiconductor, CMOS)製程的熱損耗、傳輸距離與成本等問題，矽光子技術逐漸蓬勃發展，其有著傳輸損耗低與高頻寬的特性，且與 CMOS 製程有良好的相容性，矽光子整合之光積體電路在光傳輸、生物感測、量子光學與非線性光學的應用上都有很大的潛力，矽光子常見使用的波段為 1310 nm 與 1550 nm，因為矽基材料在此兩波段有著較少的吸收，對於光波導的傳輸有較少的損耗，但由於矽波導的能帶小，容易產生雙光子吸收，對於非線性光學的應用有限，因此許多研究嘗試開發其他材料作為波導媒介，本論文使用氮化鎵作為波導導光層材料，氮化鎵由於有著非對稱的晶格排列，因此同時存在 χ (2)與 χ (3)非線性，更容易產生二倍頻與頻率梳，傳統上沉積氮化鎵的方式為有機金屬化學氣相沉積法(MOCVD)，若沉積在有晶格的矽基材料，會因為晶格不配對的關係而成長困難，若基板為無晶格的矽基材料，也難以長出高品質且有晶格的氮化鎵，因此本論文使用高功率脈衝磁控濺射(HiPIMS)的方式，在室溫的沉積環境也可以得到高品質的氮化鎵薄膜。首先介紹波導的基本理論與成立條件，從 Snell’s law 得知，導光層必須為光密介質才能形成波導，本論文使用導光層材料為氮化鎵與氮化矽，再介紹環形共振腔偶合條件、品質因子與群速度色散等特性，若想達到臨界耦合需要較低損耗的環形波導與較小的間隙以減少消逝波的耦合損耗。再以時域有限差分法與有限元素法分別模擬氮化鎵與氮化矽環形共振腔之不同間隙大小的耦合效率、不同波導寬度的模態數、不同絕緣層厚度之場型分布與群速度色散，由於氮化鎵的折射率較大，需要較小的間隙大小才能達到臨界耦合，而也因為較大的折射率，需要的絕緣層厚度較小。再來介紹製程，以 HiPIMS 氮化鎵與 LPCVD 氮化矽作為波導導光層材料，由於需要小於 300 nm 的間隙才能達到臨界耦合，故使用電子束微影系統，傳統方式將圖案分割成較小的曝光邊界依序曝光，但邊界接合問題會造成波導傳輸的損耗，本論文使用(Fixed- beam moving stage, FBMS)的模式，固定電子束改為移動採台的方式，可沿著波導曝光，避免邊界接合的問題，經曝光方式優化後，在空氣包覆層的情況下，氮化鎵波導傳輸損耗從 57 dB，降低到 32 dB，經過沉積二氧化矽包覆層後，傳輸損耗降低到 16 dB，且氮化鎵的品質因子達到 4×10^4，本論文為第一個實現在二氧化矽上沉積氮化鎵並做出品質因子達到 10^4的環形共振腔。本論文使用 HiPIMS 沉積氮化鎵，在晶格不配對的情況下，為氮化鎵元件提供一個更多元的沉積方式，藉由此技術，使氮化鎵能與 CMOS 製程整合，是非常有潛力的技術。;In order to address the issues of thermal loss, transmission distance, and cost in the current technologies by Complementary Metal Oxide Semiconductor (CMOS) processes, silicon photonics technology has been gradually developing. It features low transmission loss, high bandwidth, and good compatibility with CMOS processes. Silicon photonics integrated optoelectronic circuits have great potential in applications such as optical communication, biosensing, quantum optics, and nonlinear optics. The commonly used wavelength range for silicon photonics is 1310 nm and 1550 nm, as silicon-based materials exhibit less absorption in these ranges, resulting in lower losses for optical waveguides. However, due to the small bandgap of silicon waveguides, they are prone to two-photon absorption, limiting their applications in nonlinear optics. Therefore, research efforts have focused on developing alternative materials as waveguide media. This study utilizes gallium nitride (GaN) and silicon nitride (SiNx) as waveguide core materials. GaN possesses asymmetric crystal lattice arrangement, allowing for both χ(2) and χ(3) nonlinearities, making it more suitable for second harmonic generation and frequency comb generation. Traditionally, GaN is deposited using Metal Organic Chemical Vapor Deposition (MOCVD). But due to the lattice mismatch between GaN and silicon-based materials, MOCVD cannot be used for deposition of high-quality thin film on silicon or silicon oxide substrates. Hence, this study employs High-Power Impulse Magnetron Sputtering (HiPIMS), which allows for the deposition of high-quality GaN films at room temperature. Firstly, the basic theory and conditions for waveguides are introduced. According to Snell′s law, the core must be a high refractive index material to form a waveguide. In this study, GaN based waveguide resonators are introduced. The coupling conditions, quality factor, and group velocity dispersion of ring resonators are then explained. To achieve critical coupling, it is necessary to have low-loss ring waveguides and enhance coupling strength by minimizing the gap size. The coupling efficiency, mode numbers of different waveguide widths, field distribution, and group velocity dispersion of GaN and SiNx microring resonators with varying gap sizes are simulated using the FiniteDifference Time-Domain (FDTD) method and Finite Element Method (FEM). Due to the higher refractive index of GaN, a smaller gap size is required to achieve critical coupling. Additionally, a thinner insulating layer can be adopted due to the larger refractive index of GaN. Next, the fabrication process is introduced. GaN deposited using HiPIMS and silicon nitride deposited using Low-Pressure Chemical Vapor Deposition (LPCVD) are utilized as waveguide core materials. Since a gap size smaller than 300 nm is required for critical coupling, an electron beam lithography system is used. Instead of the traditional method of dividing the patterns into smaller writefields and sequentially exposing them, this study employs the Fixed-Beam Moving Stage (FBMS) mode, where the electron beam is fixed with moving stage for patterning, allowing for exposure along the waveguide and avoiding stitching error issues that cause transmission losses. After optimizing the exposure process, the transmission loss of GaN waveguides is reduced from 57 dB to 32 dB with air cladding. With the deposition of a silicon dioxide as the cladding layer, the transmission loss is further reduced to 16 dB, and the quality factor of GaN reaches 4×10^4 . This study is the first to achieve the deposition of GaN on silicon dioxide and demonstrate a quality factor of 10^4 in a microring resonator. By utilizing HiPIMS for GaN deposition, this study provides a more versatile deposition method for GaN devices onto different substrates. This technology enables the integration of GaN with CMOS processes and holds great potential in realizing photonic devices.
Appears in Collections:	[Graduate Institute of Optics and Photonics] Electronic Thesis & Dissertation

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