Wideband material detection for spoof resistance in GHz ultrasonic fingerprint sensing

One of the primary motivations for using ultrasound reflectometry for fingerprint imaging is the promise of increased spoof resistance over conventional optical or capacitive sensing approaches due to the ability for ultrasound to determine the elastic impedance of the imaged material. A fake 3D printed plastic finger can therefore be distinguished in impedance compared to tissue. However, ultrasonic sensors are still vulnerable to materials that are similar in impedance to tissue, such as water or rubber. In this work, the possibility of differentiating between materials similar in acoustic impedance by utilizing the large bandwidth of GHz ultrasonic fingerprint sensors to measurement of frequency dependence of acoustic reflection coefficient is investigated. The results indicate that differences in frequency dependence can be seen for some materials but overall the acoustic impedances are sufficiently different such that frequency dependence is not required for material differentiation.