A CMOS Micromachined Capacitive Tactile Sensor With High-Frequency Output

This paper describes the design and characterization of a CMOS-micromachined tactile sensing device that can be utilized for fingerprint recognition. The complete post micromachining steps are performed at die level without resorting to a wafer-level process, providing a low-cost solution for production. The micromechanical structure has an area of 200 mum by 200 mum and an initial sensing capacitance of 153 fF. An oscillator circuit is used to convert the pressure induced capacitance change to a shift in output frequency. The circuit has a measured initial frequency at 49.5 MHz under no applied force. The total frequency shift is 14 MHz with a corresponding mechanical displacement of 0.56 mum and a capacitance change of 63 fF, averaging a capacitive sensitivity of 222 kHz/fF. The measured spring constant is 923N/m, producing a force sensitivity of 27.1 kHz/muN