Theory and experimental analysis of scratch resistant coating for ultrasonic fingerprint sensors

Ultrasonic imaging for fingerprint applications offers better tolerance of external conditions and high spatial resolution compared to typical optical and solid state sensors respectively. Similar to existing fingerprint sensors, the performance of ultrasonic imagers is sensitive to physical damage. Therefore it is important to understand the theory behind transmission and reflection effects of protective coatings for ultrasonic fingerprint sensors. In this work, we present the analytical theory behind effects of transmitting ultrasound through a thin film of scratch resistant material. Experimental results indicate transmission through 1 μm of Al₂O₃ is indistinguishable from the non-coated cover substrate. Furthermore, pulse echo measurements of 5 μm thick Al₂O₃ show ultrasound pressure reflection increases in accordance with both theory and finite element simulation. Consequently, feasibility is demonstrated of ultrasonic transmission through a protective layer with greatly mismatched acoustic impedance when sufficiently thin. This provides a guide for designing sensor protection when using materials of vastly different acoustic impedance values.