Design and Fabrication of a Coupled Dual-Coil for Fingerprint Sensor Application

This paper presents a novel concept that uses the mutual inductance of a spiral dual-coil to identify fingerprint patterns. Unlike commercial optical and capacitive sensing technologies, this unique dual-coil design enables the measurement of a fingerprint by the effect of ridges and valleys on electrical impedance. In this study, a typical sensing chip with a 3×128 dual-coil array was fabricated by microfabrication process. The variations of coupling capacitance and mutual inductance were then used to verify the properties of different mediums. The measurement results show that the resonant frequency is produced at 8 MHz, which results in a maximum induced current of 1 mA and a phase shift of 41.3 degrees. Higher conductivity mediums cause larger phase shifts. Sample data resulting from a dual-coil array were successfully used to reconstruct a fingerprint image.