Novel surface structure and its fabrication process for MEMS fingerprint sensor

This paper describes a microelectromechanical systems (MEMS) fingerprint sensor that has novel protrusions on the sensor surface in order to detect clear fingerprint images for various finger surface conditions. A preliminary experiment clarified the importance of contact between soft finger ridges and the sensor surface in producing fingerprint images. Based on this, novel T-shaped protrusions on the sensor surface are proposed as an interface to mediate the contact. In order to fabricate the overhung form of the T-shaped structure, photosensitive polyimide and a copper sacrificial layer were used. Using the CMOS-compatible MEMS fabrication process, the arrayed T-shaped protrusions of polyimide are stacked on a CMOS large-scale integrated circuit. The sensor produced clear fingerprint images for various kinds of fingers. Measurement of the relationship between applied force and sensor output showed that the T-shaped protrusions achieved high sensitivity regardless of finger elasticity. Reliability tests confirmed that the sensor with the T-shaped protrusions has sufficient mechanical and electrical strength for wide applications.