多晶矽電容式微加速規的研發

Abstract

本論文研究重點在於以多晶矽(polysilicon)為感測結構材料的開發中之CMOS製程，來建立起CMOS-MEMS電容式加速規相關的設計分析、製作與量測技術，並整合微感測結構與電路，以達成批次生產為目標。 結構設計上，使用差動式電容來作電容的感測，以減少耦合的影響；且提出新式的對稱懸吊結構，以解決製程尺寸誤差所造成之雙軸靈敏度不同的問題。並將透過理論分析與有限元素分析軟體ANSYSR執行特性模擬，包含懸吊系統、自我測試致動器、操作頻寬與殘留應力的模擬與驗證。性能的最佳化則以限定晶片面積內，可達到之最高靈敏度為原則，設計出的單、雙軸結構靈敏度可分別達到0.85 fF/G與1.7 fF/G。製程上則利用退火製程成功地調整多晶矽的殘留應力，並建立退火製程和殘留應力關係的資料庫。最後利用測試懸臂來建立起抗側向沾黏及基底沾黏的設計規則，以解決嚴重的沾黏問題。 量測方面，結構尺寸部分的量測將用到本實驗室之電子顯微鏡及三維輪廓量測儀。結構翹曲量部分則由國家晶片系統設計中心提供之白光干涉儀(white-light interferometer)來量測，而頻率響應部份則透過國家晶片系統設計中心的微機電系統動態分析儀(MEMS Motion Analyzer, MMA)來量測。

In this study, employing the polysilicon-based CMOS process, the related technique of design, fabrication, and measurement are built up to develop CMOS-MEMS capacitive accelerometer which integrate sensing circuit and MEMS structure to achieve batch fabrication. For the design of micro accelerometer, differential-pair sensing finger with gap-closing type is used to reduce the coupling effect; and a novel symmetric layout of suspension is proposed to eliminate the mismatch of spring constants in X and Y axes resulted from size deviation during fabrication, which would simplify the sensing circuit design. Furthermore, through the commercial finite-element-analysis (FEA) software ANSYSR, performances of micro accelerometer including stiffness, residual stress, self-test micro-actuator, and operation bandwidth are simulated and discussed for further verification. In order to achieve high sensitivity of micro accelerometer in the constrained die-area, the sizes of sensing fingers are optimized, and the performance shows that sensitivities of micro accelerometers with single axis design and two axes design can reach to 0.85 fF/G and 1.7 fF/G, respectively. For the fabrication of micro accelerometer, the residual stress of polysilicon structure that affects the deflection of sensing fingers is adjusted successfully by annealing process, and the database of the correlation between residual stress and annealing process is built up. Furthermore, an anti-stiction design rule is established successfully by finger test key to solve the severe stiction problem. For the measurement of micro accelerometer, scanning electron microscope (SEM, NCTU) and 3D profiler (ET-4000, NCTU) are used to determine the practical geometric sizes of fabricated accelerometers. The optical profiler (white-light interferometer, CIC) and MEMS motion analyzer (MMA, CIC) are used to characterize the deflection of sensing fingers and the dynamic response of accelerometers, respectively.