CMOS-MEMS壓阻式頻率輸出加速度計

Abstract

加速度計被廣泛的採用於各個領域，包括智慧型手機、汽車安全氣囊、GPS導航系統等。具備數位輸出的加速度計由於非常容易與後端數位信號處理整合在同一顆IC，因此更受歡迎。本論文採用CMOS-MEMS製程設計並實作出一顆具備數位輸出的三軸加速度計。此加速度計的機械結構部分使用單一質量塊搭配三組置放於四根懸吊樑上的感測電阻來偵測三軸加速度，這些電阻被配置於三個RC振盪器之中，不同的加速度將改變這些感測電阻的電阻值進而改變振盪器的頻率，最後我們使用三個計數器來測量振盪器頻率，如此可判別該瞬間的加速度大小。模擬軟體Coventorware以及Hspice顯示，在此結構下當Z軸有1G加速度時，感測電阻會產生ΔR/R ≈ 0.93 %的變化量，對應的振盪器也會產生11 MHz的頻率變化。 元件製作部分，在收到CMOS晶片後，我們先使用研磨拋光機減小晶片的厚度，接著對晶片背面作黃光製程以定義出蝕刻區域，搭配所設計的保護環後便可對晶片的背面作深度蝕刻，再對晶片的正面蝕刻二氧化矽層後即完成結構釋放。 本論文製作出兩種質量塊大小不同的加速度計，質量塊較重的在測量完自然共振頻率後因懸吊樑過於脆弱而損毀，其量測到的自然共振頻率為103 Hz；質量塊較輕的則完成所有測試，此元件的自然共振頻率為464 Hz；靜態量測結果顯示，因為質量塊過輕而無法判別X、Y軸方向+-1G的加速度，只能判別Z軸方向+-1G的加速度，其解析度為10.11 mG/Hz；動態量測結果顯示，振盪器的頻率會隨著加速度而改變，其靈敏度為315.8 kHz/G，解析度為21.6 mG/Hz。

Accelerometers has been applied in a lot of fields recently which concludes smart phone, air bags in car, GPS system, etc. Because most of the accelerometers are used in electrical device, accelerometers with digital output would be more popular. Thus, we will design and fabricate an accelerometer with digital output by CMOS-MEMS process in this thesis. First, detecting 3-axis accelerations by single proof mass will be introduced. Then we can use a pair piezoresistors to control the oscillator in the sensing circuit. After measuring the frequency of the oscillator, we will know the acceleration. The Coventorware and Hspice simulations show the change of the sensing resistor is ΔR/R ≈ 0.93 % if the device is under 1G acceleration in the Z-axis. The frequency of the oscillator will respond to the acceleration for 11 MHz variation. About the fabrication of the accelerometer, the CMOS chip is first thinned by a polisher. Then photoresist is used to define the etching zone on CMOS chip backside. The chip can be etched on backside perfectly by using the guard ring. Finally, the structure of CMOS chip can be released after removing the silicon oxide layer by front side etch. There are two accelerometers in this thesis. One has a bigger proof mass and the other has a smaller proof mass. The bigger one was crashed after measurement because of the fragile structure. The nature frequency of the bigger one is 103 Hz. The smaller one was more robust than the bigger one and be tested completely. The nature frequency of the smaller one is 464 Hz. However, the stactic measurement result shows that it is unable to dectect +-1G acceleration in the X-axis and Y-axis due to the light proof mass. Fortunatelly, it can still detect +-1G acceleration in the Z-axis and the resolution is 10.11 mG/Hz. The dynamic measurement shows that the frequency of oscillator would change with acceleration. The sensitivity is 315.8 kHz/G and the resolution is 21.6 mG/Hz.