A Fingerprint Sensor with Impedance Sensing for Fraud Detection

A fingerprint sensor that integrates fraud detection and fingerprint sensing to prevent spoofing with a fake (artificial) finger is presented. Fingerprint identification using capacitive fingerprint sensing provides small user-authentication systems. For systems that need a higher level of security, fraud detection, which determines whether the sensed finger is alive or not, is necessary. Integrating fraud detection capability into a capacitive sensor is important because attempted fraud has to be detected at the same time that the fingerprint is captured. Various methods that use information about a finger, such as its electrical characteristics, optical characteristics or elastic characteristics, have been tried. Impedance-sensing is suitable from the viewpoint of using electrical signals. An impedance-sensing circuit should not increase the chip size nor degrade the quality of the captured fingerprint image. To meet these requirements, we propose an impedance-sensing scheme built into a capacitive sensor and implemented as a circuit and electrode without changing the chip size.