Systematic Development of Integrated Capacitance Measurement System With Sensitivity Tuning

Modern sensor systems integrate microelectromechanical system (MEMS)-based sensors with signal conditioning circuits. MEMS sensors are fabricated through micromachining technology, which is relatively new compared with the integrated circuit technology, used since last few decades to fabricate circuits. As a result, these microsensors experience significant variation after fabrication. Besides, packaging of the sensors and integration with the circuit are also nontrivial as the sensors are of micrometer dimensions and they may have movable structures. In this paper, we have developed an integrated capacitance measurement system with a fully on-chip signal conditioning circuit. A systematic method of integration and testing with a silicon-on-insulator (SOI) MEMS capacitive accelerometer sensor is also presented. The signal conditioning circuit incorporates an array of on-chip capacitors to nullify the sensor capacitance mismatch and a tunable square-wave generator to tune the sensitivity of the system to cope up with the variations after fabrication of the devices. The complete circuit is designed and fabricated in United Microelectronics Corporation 0.18-$\mu \text{m}$ CMOS process technology. This circuit is integrated with an SOI MEMS sensor structure and various test methods are discussed. The integrated system is then mounted on a vibrating shaker along with a reference accelerometer and the measurement results are provided.