In this paper, we describe a method for improving the quality of fingerprint images at long sensing distances by reducing the fringe capacitance formed between a pixel and surrounding fingerprint patterns. Air-walls were applied to the glass attached to a fingerprint sensor for reducing the edge capacitance. Fingerprints and air-wall structures were modeled using a three-dimensional capacitance analysis tool. A prototype was fabricated by stacking glass layers with air-walls with a depth of 50 μm and a pitch of 50 μm on a silicon-type capacitive sensor. Using the results of simulations and prototype experiments, we confirmed that the proposed air-wall structure achieved high enough resolution to distinguish 2.5-lp/mm fingerprint patterns at a sensing distance of 300 μm or longer, and its contrast improved from 0.59 to 0.98.
Keywords: capacitive fingerprint sensor; fingerprint image enhancement; fingerprint recognition.