Study Of Small Capacitance Readout Technology For MEMS Sensors

Recently rapid progress has been continuously made on micro-electro-mechanical systems (MEMS) technology with the developing micro-nano fabrication techniques. MEMS sensors have been more and more widely used in automotive, biomedical, electronic and national defence areas. Piezoresistive, piezoelectric and capacitive sensing are three main types of sensing mechanisms for MEMS sensors. Among them, the capacitive sensing has the advantages of low power dissipation, high temperature stability and compatibility with CMOS process. As a result, capacitive MEMS sensors are most widely used. However, the precise readout of small capacitance is one key technology for designing capacitive MEMS sensors because they usually have smaller sensing capacitance and smaller mechanical sensitivity.Continuous time voltage readout technology, continuous time current readout technology and switched capacitor charge readout technology are three mostly common small capacitance readout technoloies for capacitive sensing. Among them, the continuous time voltage sensing is widely used in capacitive MEMS sensors due to its superior noise performance. In this work, the continuous time voltage sensing has been employed to realize small capacitance readout. The design idea has considered the traditional IC design methods as well as the specific micromachining technology.Firstly, the optimization analysis of noise performance, power dissipation and carrier signal has been performed. For the noise performance, based on the analysis of noise sources in small capacitance readout circuit, the method of capacitance matching has been employed to optimize the noise performance under new CMOS-MEMS technology. For the power dissipation, based on models of single structure and multi-stage structure, the gain of each stage and the number of stages have been optimized by normalizing the power dissipation of multi-stage structure. As far as the carrier signal is concerned, the simplified one-dimensional model of electrostatic actuator has been applied to optimize the signal's amplitude and frequency. The optimization of noise performance, power dissipation and carrier signal can provide guidance to the design of small capacitance readout circuit.Secondly, based on the performance parameter optimization analysis, each submodule of small capacitance readout circuit has been designed on the circuit level. The optimization of circuit performance, as well as the effect of temperature on main circuit parameters, has been studied. The complete circuit has been successfully verified by using Cadence Spectre in CSMC 0.5μm CMOS process. The simulation results indicate that the capacitance detection resolution can reach the order of aF, and the capacitance variation shows a good linear relation with the output voltage variation. The output voltage offset is only 0.23 mV, the equivalent input-referred noise is about17 nV/Hz^1/2, and the resolution is 0.56 mV/aF. Under the 5-V power supply, the power consumption is only 2.145 mW, and the output delay is 9.563μs. The overall performance of the designed small capacitance readout circuit can meet the requirement of MEMS sensors, and its power dissipation performance is even better than reported data in similar work.Finally, based on simulation and verification of the whole circuit, the sensing capacitance mismatch caused by manufacturing errors has been preliminarily studied under the new CMOS-MEMS technology. The effect of device mismatch in IC process on the reference voltage has been analyzed and discussed for evaluating indirectly the effect on carrier signal. The tentative discussions of manufacturing errors and device mismatch presented in this thesis could also be an improvement compared to other similar researches.