高折射係數比且高微縮之微環型共振腔製作與分析

High Index Contrast and Ultra Compact Microring Resonator

曾智偉(Chih-Wei Tseng)

指導教授 ： 李明昌 ； 陳永睿

國立清華大學/電機資訊學院/光電工程研究所/博士(2014年)

https://doi.org/10.6843/NTHU.2014.00167

摘要

由於以矽為基板的微環形共振腔元件因為其小尺寸、可因應各式各樣的應用、具有量產的能力，以及與現存IC製程技術相容，使得此元件正吸引許多研究團隊的注意;因此，應用於高折射率比(high-index-contrast)之微環形濾波器的奈米製作技術，元件參數萃取與特性分析能力尤其重要。
這篇論文的主要目的是利用電子束之奈米製作技術開發高品質因子(quality factor)與極微縮(ultra compact)之微環形共振腔(microring resonator)於矽基板上提供未來高自由頻譜範圍(large FSR)與狹小通道寬度(narrow pass-band)的高密度分波多工技術(DWDM)之應用。實驗的設計利用單一波導耦合共振腔的結構，搭配時域波導耦合理論(coupled mode theory in time domain)萃取元件光強度損耗參數與光強度耦合參數，提供於多環結構(multi-ring)之光電轉換開關(optical switch)之元件設計參數來符合高密度分波多工技術規格。搭配等效折射率法(EIM)與时域有限差分法(FDTD)執行穿透頻譜分析方法，正確的分析並預測元件特性，同時驗證實驗結果。
藉由分析穿透頻譜的方法，我們發現波導耦合微環形共振腔之結構會產生光強度之輻射損耗(radiation loss)，隨著耦合間隙(gap width)的縮減，輻射損耗劇烈的上升，光強度耦合係數伴隨著共振腔之不同光學損耗，此結果為波導耦合共振腔元件提供了新的設計思維，根據在高折射比且半徑2.75微米(μm)的矽環形共振腔的模擬結果中顯示，其功率耦合係數在間隙100奈米時僅4%，但光強度輻射損耗達到0.0625dB/周長，這嚴重的影響多環光開關的設計與特性。利用間隙耦合誘發輻射損耗的現象，當間隙大小符合微擾耦合(weak coupling)條件時，可以求出微環形濾波器的本質(intrinsic)光學損耗，此方法為目前最正確且精準的方式來計算出波導耦合之共振腔其光學損耗。同時也發現環形共振腔模態與耦合區域(coupling region)產生之模態干涉產生法諾共振 (fano resonance)造成穿透頻譜的不對稱。為了避免因狹小間隙引發輻射損耗的發生，文中提出以錐形波導(taper waveguide)鑲入光強度耦合區域及保角耦合結構(conformal racetrack)來增加光強度耦合係數並降低輻射損耗。
透過改變圖形切割與電子束曝寫方式來減少因曝光製程所引起的波導側邊粗糙度，提升共振腔品質因子，且在矽基板上實現高品質因子與極微縮之微環形濾波器，其寬度0.5微米、半徑僅1.75微米、半高寬(FWHM)0.143奈米(nm)、品質因子在TE偏振模態下為10,938。實驗結果中觀察出高折射率比(high-index-contrast)之波導耦合共振腔元件具有因狹小間隙(gap width)誘發輻射損耗之效應，以半徑2.75微米、寬度0.5微米的微環形濾波器為例，本質光功率損耗係數為0.01382dB/周長，在間隙200奈米時，光功率損耗係數為0.03455dB/周長、輻射損耗係數達0.02dB/周長。利用量測新加坡IME公司所製作之試片，顯示在248奈米光源製程設備下，微環形濾波器因製程因數共振波長的變動量標準差約0.4 nm、群速度的變動量標準差約2×10-3 。

關鍵字

高折射係數比 ； 微環型共振腔 ； 微環形濾波器 ； 輻射損耗 ； 法諾共振 ； 電子束

並列摘要

Silicon-based microring devices are attracting much research attention due to their small footprint, versatility in various applications, and compatibility with the existing IC process for mass production. Hence, there is a strong demand for the nanofabrication and analysis method of high-index-contrast and ultra-compact microring resonator.
The objectives of this dissertation are to utilize electron beam lithography, focusing on nanofabrication technology, to develop high quality factor and ultra-compact microring resonator on the SOI substrate with a large free spectrum range (FSR) and narrow pass-band for DWDM application. The configuration of single waveguide coupled microring resonator and couple mode theory in a time-domain method are utilized to extract ring resonator loss and coupling coefficient. These two parameters are important for designing the multi-ring-based optical switch devices, which the optical switch is a basic element in DWDM system. We use a quasi-3D-FDTD method, a two dimensional Finite-difference time-domain method (FDTD) plus an effective index method (EIM), to analyze the transmittance spectrum of single waveguide-coupled microring resonator, and to verify the experimental results.
By analyzing the transmittance spectrum, we find that the structure of waveguide-coupled microring resonator induces the radiation loss in the coupling region. As the gap width shrinks, the radiation loss increases rapidly. The gap width dependent optical loss gives us new insights into waveguide coupled resonator-based devices. With the simulation results of a silicon microring resonator with a radius of 2.75 μm and gap width of 100 nm, the coupling coefficient is just 4%, but the optical ring loss achieves 0.0625dB/circumference. That seriously affects the design and performance of multi-ring-based devices. By employing the effect of narrow gap-inducing radiation loss, as the gap width is large enough for weak coupling condition, it can obtain the intrinsic loss of a microring resonator; this is an effective and accurate method to estimate the optical loss of a waveguide-coupled ring resonator. Furthermore, we find that the Fano resonance phenomenon creates a little asymmetry on the transmission spectrum of ring. This could be due to multiple modes interacting with a single eigen-mode ring resonator for different relative phases, for example, slot mode and higher order mode brought from the neighbor bus waveguide and ring resonator. To avoid the narrow gap width inducing radiation loss, we use the configuration of a tapered waveguide embedded, and a conformal racetrack to enhance the coupling efficiency and reduce radiation loss.
By changing the pattern segmented method and electron beam writing strategies to reduce the sidewall roughness on the waveguide, we demonstrated a high quality, ultra-compact microring filter with waveguide width of 0.5μm, radius of 1.75μm, FWHM of 0.143nm and quality factor of 10,938 at TE-like mode. We show that single waveguide coupled silicon microring resonator provides the radiation loss due to narrow gap width. For example, the silicon microring resonator with radius of 2.75μm and waveguide width of 0.5μm has the intrinsic loss of 0.01382 dB/circumference. At gap width of 200 nm, the optical ring loss is 0.03455 dB/circumference and radiation loss is 0.2 dB/circumference. Through IME’s sample, we can see that the fabrication variation on the performance of silicon microring resonator involves the drifting of resonance wavelength in standard deviation of 0.4 nm and group velocity in standard deviation of 2×10-3.

並列關鍵字

High Index Contrast ； Microring Resonator ； large free spectrum range (FSR) ； DWDM ； Fano resonance ； radiation loss

參考文獻

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(Doctoral dissertation, National Tsing Hua University,2010).