| 摘要: | 矽光子技術由於其高頻寬、低損耗、可結合互補式金屬氧化物半導體製程，使其有廣泛的應用，包括濾波器、檢測器、調變器、非線性光學等，在這些應用上，直波導與共振腔的耦合扮演了重要的角色，在臨界耦合模式可以為線性元件提供好的消光比以及靈敏度，在過耦合模式可為非線性元件提供更好的轉換效率。過往使耦合增強的方法有縮短間隙距離，或是增加耦合區域的範圍，但這些方法需要更好的製程控制以及更多的波導設計，因此在波導設計的靈活性受到限制。本論文透過在直波導耦合區域的錐形波導設計，使氮化矽微環形共振腔的耦合增強，並且探討不同幾何結構的錐形波導對於耦合效應的影響。
首先以時域有限差分法對於不同結構的錐形波導進行模擬，觀察共振腔內的能量，並計算出不同錐形波導的耦合強度變化。我們利用單側漸進式錐形波導的設計，在與無錐形波導設計相比，其耦合強度最高可提升 347 倍。除此之外，利用錐形波導，可在共振腔內激發不同腔體模態，藉以達到模態轉換之能力。
再來介紹元件的製程，本論文以低壓化學氣相沉積所沉積之氮化矽作為波導，並以 i-line 步進機和電子束直寫系統進行微影製程，在優化其製程後，其品質因子皆可達到10^5。並且因為錐形波導的設計，使得間隙為 0.4 μm的微環形共振腔，符合在步進機上可達到之最小解析度，因此可以在i-line步進機上實現大量生產，而不必依靠電子束直寫系統繼續縮短間隙寬度。在量測方面，由元件在 1550 nm 光通
訊穿透光譜進行分析，可得到在錐形波導與無錐形波導的設計相比，其耦合強度在i-line步進機與電子束直寫系統上，分別可增強 7 倍與 15 倍。
本論文提供一個靈活的設計方式，利用單側漸進式錐形波導可有效提升直波導至共振腔的耦合，在不用縮減間隙寬度的情況，可望達到臨界甚至到過耦合模式。並且透過 i-line 步進機，能實現大量生產的可能性。;Silicon photonics technology has a wide range of applications, including filters, detectors, modulators, nonlinear optics, etc., due to its high bandwidth, low loss, and compatibility with complementary metal oxide semiconductor processes. In these applications, the coupling between the bus waveguide and the resonator plays an important role, which can provide a strong extinction ratio and high sensitivity for linear components in the critical coupling regime, and also provide better conversion efficiency for nonlinear components in the over coupling regime. In the past, there are several methods to optimize the coupling, including gap distance reduction or elongation of the coupling region, but these methods require better process control and complicated waveguide designs. Therefore, the flexibility in waveguide design is limited. In this thesis, the coupling of the silicon nitride micro-ring resonator is investigated with the design of the tapered waveguide in the coupling region, and the effect of the tapered waveguide with different geometric dimensions is discussed. Coupling enhancement can be identified with coupling of a straight bus.
Firstly, the tapered waveguides with different dimensions are simulated by the finite difference-time-domain (FDTD) method, the energy in the resonator is studied, and the
changes in coupling strength of different tapered waveguides are investigated. We utilize the design of the single-side tapered waveguide to achieve coupling up to 347 times higher strength in comparing to that without a tapered design.
Then, the author introduces the fabrication process of the device. In this paper, a silicon nitride layer deposited by low pressure chemical vapor deposition is used as the
waveguide core, and an i-line stepper lithography and an electron beam writing system are both utilized for the lithography process. After optimizing the process, the quality factor of both systems can be achieved with the order of 10^5. Because of the proposed design of the tapered waveguide, the micro-ring resonator with a gap of 0.4 μm complies the available (minimum) resolution of the stepper lithography, mass production with strong coupling can be achieved by the i-line stepper without relying on the costly
electronic beam writing system. In terms of measurements, the optical transmission spectra at 1550 nm are analyzed. The coupling strength of the fabricated resonators with
i-line stepper and the electron beam writing system can be compared with or without the design of the tapered waveguide, showing an enhancement by 7 times and 15 times,
respectively.
The work in this thesis provides a flexible design method, which can effectively improve the coupling between the bus waveguide and the resonator using a single-side tapered waveguide. In addition, it provides critical coupling without the need of a narrow gap width (< 400 nm) at a moderate quality factor. This design paves the way for the
mass-productive and cost-effective process of microresonator fabrication through the i line stepper lithography. |
