Characterization and modeling of a piezoelectric micromachined ultrasonic transducer with a very large length/width aspect ratio

The objective of the current study was to characterize and model the performance of piezoelectric micromachined ultrasonic transducers (pMUTs) with large length/width aspect ratios. Single-element pMUTs with 20 different dimensions corresponding to aspect ratios ranging from 5:1 to 23:1 were designed. Multiple samples were fabricated for each design so that statistically meaningful data could be obtained. The pMUTs were characterized by the impedance measurement combined with an equivalent circuit analysis. A one-dimensional composite beam model was also used to correlate the equivalent circuit components with the structural parameters, and gain insight into the performance characteristics of pMUTs. The resonant frequencies were observed to decrease with the width of the membrane, but have no appreciable length dependence. With the correction of parasitic capacitance, the effective coupling coefficients were observed to increase with the width up to around 150 µm and then decrease. However, they did not show clear and consistent length dependence. The variation of the coupling coefficient as a function of width of the membrane was shown to be mainly due to the relative ratios between the electrode and membrane widths rather than membrane width itself. Although the model presented in this study was a simple one-dimensional electro-mechanical model, it did seem to offer both good qualitative and quantitative insights into the performance of pMUTs and provide a convenient tool for designing thin membrane transducers with a large aspect ratio. The model can also take into consideration the residual stress effect and offer an even more realistic prediction.