Force-Sensing Scheme for Small Mechanical Signals in Complementary Metal Oxide Semiconductor Microelectromechanical System Fingerprint Sensor

This paper proposes a force-sensing scheme with a microelectromechanical systems (MEMS) structure and a complementary metal oxide semiconductor (CMOS) amplifier for small pixels where the force from the ridges of a finger surface is detected. The MEMS structure features T-shaped protrusions on parallel electrodes facing each other and converts mechanical signals of force into electrical signals of capacitance change most efficiently. The CMOS amplifier extracts the capacitance change distinguished from the original capacitance. Calculations and measurements of the fabricated sensors show that the new MEMS structure in small 50-µm-square pixels can produce as large a capacitance change as 2 fF with a force of 1200 µN/pixel, which corresponds to about 0.5 MPa of finger pressure. The CMOS amplifier cancels out the original capacitance and converts the capacitance change of 2 fF into an output voltage change of about 0.1 V. Thus, the force-sensing scheme totally enhances the sensitivity of the CMOS MEMS fingerprint sensors regardless of the small size of the pixels.