| Summary: | 碩士 === 國立交通大學 === 電控工程研究所 === 101 === 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.
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