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 mu m by 200 mu m 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 mu m and a capacitance change of 63 fF, averaging a capacitive sensitivity of 222 kHz/fF. The measured spring constant is 923 N/m, producing a force sensitivity of 27.1 kHz/mu N.