MEMS fingerprint sensor immune to various finger surface conditions

This paper describes a new fingerprint sensor that detects the topography of finger ridges and valleys. We propose a microelectromechanical systems (MEMS) cavity structure for the pixels arrayed on the sensor surface and a fabrication process that stacks the cavity structures on a CMOS LSI. A thin film on top of the cavity structures is bent by finger ridges mechanically, which is detected by the sensing circuits below them electronically. Based on an analytical model, we designed a cavity structure suitable for fingerprint detection. We fabricated the cavity structures on the sensing circuits by a fabrication process that entails gold electroplating, sacrificial layer etching for cavities, and a sealing technique. The fabricated fingerprint sensor consists of about 57 334 pixels in the area of 11.2 mm × 12.8 mm, which yields a high spatial resolution of 508 dots-per-inch (dpi). With these pixels working together, fingerprint images were obtained. The sensor produces clear fingerprint images regardless whether the finger was dry or wet, which confirms its potential for various practical applications.