利用傳統光學微影和i-line紫外光微影製作氮化矽微共振腔;Silicon nitride microresonators fabrication using conventional optical lithography and i-line UV lithography

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题名:	利用傳統光學微影和i-line紫外光微影製作氮化矽微共振腔;Silicon nitride microresonators fabrication using conventional optical lithography and i-line UV lithography
作者:	黃威豪;Huang, Wei-Hao
贡献者:	光電科學與工程學系
关键词:	氮化矽微共振腔;傳統接觸UV曝光;雙重曝光技術;i-line UV 步進機;Silicon nitride microresonators;Conventional contact UV lithography;Double exposure technology;i-line UV stepper lithography
日期:	2022-12-02
上传时间:	2024-09-19 15:30:09 (UTC+8)
出版者:	國立中央大學
摘要:	在過去半個世紀，矽積體光路上有巨大的發展，可利用成熟的CMOS技術進行製作低成本、大規模的積體光路。在應用上除了電信通信產業還擴及感測、非線性光學、量子光學、光機械甚至神經科學。而矽積體光路中，微共振腔主要作用為調變器、感測器、濾波器和高泵功率的腔體。微共振腔的耦合強度通常由輸入波導和微共振腔間的間距決定，間距則受曝光解析度的限制。儘管電子束微影可實現高消光比(extinction ratio)的共振，但這是昂貴且費時的方法。本論文第一部分展示在傳統的接觸式曝光機使用雙重曝光技術在微共振腔上達到次微米的耦合間距，並實現耦合其消光比為8 dB和間距< 1 μm。此方式成本低且可達到需i-line UV 步進機、電子束微影等高解析曝光技術才能實現的次微米耦合間距。製程所使用的光阻是AZ-5214E光阻，它圖案反轉的特性由於側壁的傾斜角度可使用剝離(lift-off)製程，除此之外還有較好的解析度其優勢。AZ-5214E光阻在製程中仍有一些問題需要解決，其中對於溫度的敏感性會嚴重影響間距。我們將討論軟烤、曝後烤和後續等待時間溫度變化的影響，並得到相對穩定的結果。尤其是空曝時間的調整對耦合間距相當關鍵，且透過雙重曝光技術與傳統的一次曝光比較可看出解析度明顯地改善。相較於第一部分利用製程整合方式達到微共振腔耦合。第二部分將討論使用先進i-line UV 步進機製作的微共振腔，並與接觸式曝光機比較，其最大的差別是比它有更好的解析度(~400 nm)。步進機常用於微共振腔製作，光罩可長時間使用、結果可重複性、高產量、可曝光較大的圖案為其優勢，尤其高產量在商業化的路上是吸引人的相比於電子束微影製作微共振腔。這裡會製作不同尺寸的波導觀察固有品質因子Qi的變化，其中的Qi最大可達2105，且發現在輸入波導寬度1 μm時，受背景干擾小以至於有分析分便的傳輸頻譜，而在製作完成的波導(輸入波導寬2 μm)所觀察到的傳輸損耗上達到0.08 dB/mm。透過改善退火方式，由之前的實驗包埋層二氧化矽的波導進行退火，改為包埋層為空氣進行退火觀察到Qi明顯的改變，其Qi從原本8.1104到退火後1.3105。相比於第一部分的結果，使用i-line UV 步進機製作的微共振腔其消光比> 10 dB、高Q值、結果可重複性。 ;In the past half-century, there has been tremendous growth in the area of silicon integrated circuits. Utilizing standard CMOS manufacturing processes, integrated, large-scale photonic circuits are available with low-cost fabrication. In addition to the mature of telecommunication industry, silicon photonic applications have now expanded into sensing, nonlinear optics, quantum optics, opto-mechanics, and even neuroscience. Among all the photonic functions, microresonators are mostly applied as modulators, sensors, filters, and cavities for enhancing pump power. The coupling strength of microresonator is typically determined by the available gap between the bus- and resonator-waveguide and limited by the lithography resolution for patterning. Although resonance with high extinction ratio can be achieved by advanced electron-beam lithography (EBL), it is time-consuming and costly. For the first part of this work, the author shows the capability to achieve sub-micron coupling gap between the bus- and microresonator-waveguides with the conventional contact UV lithography. Using the proposed double exposure technology, effective coupling can be achieved in which an extinction ratio 8 dB and gap size < 1 μm is demonstrated. This method exhibits low-cost lithography and yields a sub-micron gap which can be only previously achieved by high-resolution exposure techniques, such as i-line UV stepper lithography or EBL. The patterning of photoresist is based on image reversal process, providing negative slope of the patterning sidewall in the lift off process. In addition, this process gives better resolution, comparing to the direct image projection. However, this process is sensitivity to the fabrication parameters. For instance, temperature of processes will strongly affect the fabricated gap. We will discuss the effect on the temperature change of soft bake, postexposure bake, and the subsequent waiting time, and obtain the relatively stable fabrication process. We show that the adjustment of flood exposure time is especially critical to the coupling gap, and the resolution can be significantly improved by the proposed double exposure technique. In the second part, we study the capability to fabricate microresonators by i-line UV stepper lithography, which has better resolution (~400 nm) than the contact lithography. I-line Steppers are commonly used in fabrication of microresonator, due to its longer mask lifetime, the repeatability, high productivity, and large-scale manufacturing. In particular, the productivity paves the way for integrated photonics and shows comparable performance with the EBL for microresonator fabrication. Here, we will discuss different waveguide dimensions and investigate the highest available intrinsic quality factor Qi. In this work, the maximum Qi is demonstrated at 2105. Furthermore, it is found that, when the input waveguide width is set at 1 μm, the background interference can be suppressed in comparing to the 2 μm bus. Also, the observed transmission loss at 2 μm is 0.08 dB/mm. To improve the quality of the fabricated waveguides, we performed annealing with oxide cladding in the previous experiment, and then performed annealing with air cladding, and observed a significant change in Qi from 8.1104 to 1.3105. Comparing to the results in the first part, a high extinction ratio > 10 dB, high Q, repeatability can be achieved by the i-line UV stepper lithography.
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