Capacitive-Sensing Circuit Technique for Image Quality Improvement on Fingerprint Sensor LSIs

Abstract

This paper describes a new capacitive-sensing circuit technique that improves the quality of images captured with capacitive fingerprint sensor LSIs. The quality of the captured image depends on the surface condition of the finger. When the finger is dry, the electrical resistance of the finger surface is high. The finger surface resistance induces a voltage drop in the electrical potential of the finger surface (which should be grounded), which leads to poor image quality. To capture clear images even when the finger is dry, the circuit technique improves the image quality using the series resistance caused at the finger surface. The potential of the finger surface is controlled by an enhancement plate and a voltage control circuit. A test chip implementing this technique was fabricated on a 0.5 ¿m CMOS process and a sensor process. The chip captures a clear fingerprint image of a dry finger, confirming the effectiveness of the circuit technique for capturing clear fingerprint images independent of the finger surface condition with the capacitive fingerprint sensor LSIs.