Sputter-Deposited PMUTs: Resonance Analysis and Validation with an Analytical Solution

The desire for compact devices in ultrasonic-based applications requires the miniaturization of both the electronic components and the ultrasonic transducers themselves. It is for this reason that the focus shifts from traditional manufacturing of ultrasonic transducers to micromachined ultrasonic transducers (MUTs), which allow for micrometer feature sizes and still maintain their resonance frequencies in the kilohertz and lower megahertz region. Commonly, PMUTs are manufactured with a piezoelectric lead zirconate titanate (PZT) layer deposited via the sol-gel method. In comparison, our process deposits the PZT layer via sputtering, which eliminates one additional etching step and ensures a strong adhesion to the bottom electrode. A subsequent annealing process in a furnace oven induces the structural change of PZT from the amorphous phase to the pyrochlore phase to the final perovskite phase. The measured resonance frequency <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$f_{\text{rm}}$</tex> for the <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$(\mathbf{0, 1})$</tex>-mode of our PMUT yielded <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\boldsymbol{f}_{\text{rm}}=\mathbf{6 1 9. 0 4} \text{kHz}$</tex>. The analytic solution results in a resonance frequency <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\boldsymbol{f}_{\text{ra}}=618.9 \text{kHz}$</tex> deviating by less than 0.1% from the measured value. Therefore, the analytic approach proved to accurately estimate the resonance frequency, which will facilitate future PMUT designs.