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Inter-element coupling effects in pulse-echo ultrasonic fingerprint sensors

Ultrasonic fingerprint sensors based on micromachined ultrasound transducers require very fine pitch to achieve high resolution imaging. We investigate a 110×56 PMUT array based on monolithic eutectic bonding to CMOS. To achieve high fill-factor, the PMUTs are anchored through small eutectic pillars, resulting in a high degree of mutual coupling between neighboring PMUTs. As a result, the mode shape is more complicated than a single-MUT dynamic model. We created a finite element model that better models the mode shapes exhibited by the entire array. Experimental measurement of the mode-shape via laser Doppler vibrometry allows us to compute the volume velocity, which correlates to the measured far field pressure.