Surface displacements of a PMUT array: Interferometric characterization and mechanical crosstalk evaluation

Piezoelectric micromachined ultrasonic transducers (PMUTs) enable substantial electrically induced strains at low voltages. In this work a combination of Laser Doppler Vibrometry (LDV) and Mapping Single Beam Laser Interferometry (SBLI) was used to characterize PMUT displacement shapes at frequencies of 600 kHz and 1 kHz, respectively. Finite element modeling demonstrated excellent agreement with the displacements, once the device geometry was corrected to measured (rather than nominal) dimensions and appropriate boundary conditions were applied. Mapping SBLI was also used to characterize mechanical crosstalk at low frequencies, which is critical for evaluating PMUT performance. It was also demonstrated that driving one element induces bending of the PMUT substrate over dimensions on the 3 mm scale. The analysis of displacement profiles near the edges of released parts of PMUT structure revealed 150 μm transition regions between clamped and released areas. This reduced the active length (where displacement reaches its maximum values) of the PMUT by ∼4%.