Chipscale GHz ultrasonic channels for fingerprint scanning

In this paper we present 1-3 GHz frequency ultrasonic interrogation of surface ultrasonic impedances. The chipscale and CMOS integration of GHz transducers can enable surface identification imaging for many applications. We use aluminum nitride piezoelectric thin films driven at maximum amplitudes of 4-V_pp to launch and measure pulse packets. In this paper we first use the contrast in ultrasonic impedance between air and skin to create an image of a fingerprint. As a second application we directly measure the reflection coefficient for different liquids to demonstrate the ability to measure the ultrasonic impedance and distinguish between three different liquids. Using a rubber phantom the image of a portion of a fingerprint is captured by measuring changes in signal levels at the resonance frequency of the piezoelectric transducers 2.7 GHz. Reflected amplitude waves from air and skin differ by factors of 1.8-2. The measurements for three different liquids; water, isopropyl alcohol, and acetone show that the three liquids have sufficiently different acoustic impedances to be able to identify them.