Usage of the impedance effect of a spiral dual-coil for fingerprint sensor application

Abstract

This paper presents a design concept that uses the impedance effect of a spiral dual-coil array to identify fingerprint patterns. Unlike commercial optical, resistive, and capacitive sensing technologies, this unique dual-coil design enables the measurement of a fingerprint using the effect of the ridges and valleys on electrical impedance. In this study, a typical sensing chip with a 3 × 128 dual-coil array was fabricated by a simple microfabrication process. Variations of the coupling capacitance and mutual inductance were then used to verify the properties of different mediums. For a typical case in ambient air, the measured resonant frequency was 8 MHz, which resulted in a maximum induced current of 1.0 mA and a phase shift of 45.5°. The measurement results for saline solutions with different concentrations showed that mediums with higher conductivity caused larger phase shifts. The sample data resulting from the dual-coil array were successfully used to reconstruct a fingerprint image, with a good match in minutiae.