臺灣博碩士論文加值系統

本研究針對互補式金氧半導體微機電系統(CMOS MEMS)電容式加速度計提出創新的設計概念，以期達到高敏感度、低噪聲、及熱穩定之設計。一個於標準的0.35 μm CMOS製程中實現的創新微機電製程被提出，用以整合了一熱穩定之微機電加速度偵測器結構與一斬波穩零(Chopper Stabilization)電路。實驗結果不僅符合要求，所呈現出來的效能亦優於目前已發表的文獻。
熱穩定的效果係藉由在多層薄膜(multi-layer)微機電結構下結合一層厚的單晶矽(thick single-crystal silicon)來實踐，此高深寬比的多層膜微機電結構因包含一層厚的單晶矽而具有對殘留應力及溫度變化不敏感的特性。此外，本微機電製程避免了在乾蝕刻(dry-etching)時的電荷損害的問題，因此在微機電製程後，並未在樣品中發現因電荷損害而導致漏電流的現象。除此之外，本微機電製程也降低了單晶矽膜的切底(undercut)現象。
本研究中之斬波穩零電路係用以偵測互補式金氧半導體微機電系統電容式加速度偵測器之訊號，其設計強調於控制噪聲、結構偏移量、及電路輸入端之直流偏压。其中，斬波穩零電路與電容式加速度偵測器結構間之直流偏压問題已特別被討論，並且提出一個有效的解決方法。
本論文實現了一個CMOS MEMS電容式加速度計單晶片。實驗結果顯示其中的電路具有良好的噪声效能、有效的補償結構的偏移量、並穩定了電路輸入端之直流偏压。加速計的敏感度為595 mV g-1，背景噪聲為50 μg Hz-1/2，對應等效電容背景噪聲為0.024 aF Hz-1/2。在0 oC 到70 oC間，0-g下的加速計之電壓輸出溫度係數為1 mV oC-1，對應等效加速度的溫度變異為1.68 mg oC-1。

This thesis proposes a design concept to achieve high-sensitivity, low-noise, and thermally stable CMOS MEMS capacitive accelerometers. A novel process was demonstrated with a capacitive accelerometer, which contains a thermally stable MEMS sensor and an on-chip CMOS sensing circuit with chopper stabilization scheme, in 0.35 μm CMOS technology. Good performance was accomplished. It is superior to the results in other published works.
The thermal stability of the accelerometer was achieved by forming a thick single-crystal silicon (SCS) layer at the bottom of the multi-layer MEMS structure. A novel post-CMOS process was proposed for this purpose. The resultant MEMS structures by using this method have high aspect ratios and show the property of being insensitive to residual stress and temperature variation. Moreover, the proposed process avoids the charge damage problem usually encountered during dry-etching. No leakage current due to charge damage was ever observed in the sample chips. The proposed process also led to minimal undercut of the SCS layer after the MEMS structure release.
A sensing circuit with chopper stabilization scheme for CMOS MEMS capacitive accelerometers has been designed with emphasis on managing noise, sensor offset, and the DC bias at input terminals. The issue of DC bias was particularly addressed and an efficient, new method for obtaining reliable DC bias was proposed.
For demonstration, a CMOS MEMS accelerometer with the sensor fabricated by the proposed process and with an integrated on-chip sensing circuit was realized and characterized. Experimental results showed that the proposed circuit led to good noise performance, the random offset in the sensors was efficiently compensated, and the input DC bias voltage was well maintained. The sensitivity of the accelerometer is 595 mV g-1, and the overall noise floor is 50 μg Hz-1/2 which corresponds to an effective capacitance noise floor of 0.024 aF Hz-1/2. The zero-g temperature coefficient of the accelerometer output voltage is only 1 mV oC-1 in the temperature range from 0 oC to 70 oC, which corresponds to an effective acceleration variation rate of 1.68 mg oC-1.

ABSTRACT 1
摘 要 3
誌 謝 4
FIGURE CAPTIONS 5
TABLES LIST 9
Chapter 1 Introduction 12
1.1 Motivation 12
1.2 Review of Previous Works 14
1.3 Approach of Current Work 18
Chapter 2 Fundamentals of CMOS MEMS Micromachined Capacitive Accelerometers 20
2.1 Accelerometer Operation and Metrics 20
2.1.1 Mechanical Sensing Principle 20
2.1.2 Damping and Brownian Noise 24
2.1.3 Structural Curling and Its Compensation 31
2. 2 Capacitive Sensing and Electrostatic Actuation 35
2.2.1 Capacitive Sensing Principle 35
2.2.2 Electrostatic Actuation 40
2.2.3 Electrostatic Spring Forces 41
Chapter 3 Mechanical Design of CMOS MEMS Capacitive Accelerometers 44
3.1 Design of Robust CMOS MEMS Capacitive Accelerometers 47
3.1.1 Processes Variation and Its Compensation 47
3.1.2 Thermal Stress and Its Cancellation 53
3.1.3 Brownian Noise 58
3.2 Fabrication Process 61
Chapter 4 Electrical Design of CMOS MEMS Accelerometers 72
4.1 Chopper Stabilized Capacitive Readout Circuit 75
4.2 Circuit Design 77
4.2.1 Front-End Amplifier 77
4.2.2 DC Bias Stabilization 81
4.2.3 Demodulator 88
4.2.4 Instrumentation Amplifier and Low-Pass Filter 89
4.2.5 Offset Trimming Circuit 92
4.3 Low Noise Design Considerations 94
4.3.1 Mechanical (Brownian) Noise 94
4.3.2 Circuit Noise 96
Chapter 5 Experimental Results 101
5.1 Experiment System 104
5.2 Time-Domain Measurements 105
5.3 Noise Measurements 107
5.4 Offset Measurements 107
5.5 Thermal Stabilization Measurements 110
5.6 Comparison to Previous Work 114
Chapter 6 Conclusions 116
REFERENCES 117

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