Piezoelectric Microspeaker on Corrugated Parylene Diaphragm

This paper presents a microspeaker driven by a set of four slit-decoupled piezoelectric ZnO unimorph cantilevers suspended by a corrugated Parylene-D diaphragm. Four trapezoidal unimorphs on the edges of a$6 \times 6 \text{mm}^{2}$diaphragm drive a flat Parylene-D diaphragm at the center, as SU-8-templated Parylene corrugations around the diaphragm seal the gaps between the unimorphs while providing a compliant boundary that relieves edge stress and promotes piston-mode motion without acoustic leakage. The$\text{Al} / \text{ZnO} / \text{SiO}_{2} / \text{Al} /$Parylene stack is dimensioned to minimize bending stiffness and place the neutral axis slightly above the ZnO top surface. A DRIEfree process on a SiN membrane, combined with singlemask patterning of the$\text{Al} / \text{SiO}_{2} / \text{ZnO}$stack and a lowtemperature Parylene step, yields a batch-fabricable, lowcost microfabrication process that is compatible with polymer diaphragms. Three fabricated microspeakers with the Parylene corrugation are characterized in a$2-\text{cc}$coupler under$20 \mathrm{V}_{\mathrm{p}-\mathrm{p}}$drive, consuming$\sim 43 \mu \mathrm{W}$over$0.1-20 \text{kHz}$. All exceed 80 dB SPL and substantially outperform otherwise identical devices without corrugations. The fundamental resonance around 1.3 kHz shows a mechanical quality factor$>80$. Total harmonic distortion remains below 1% (minimum 0.5%) over$0.1-1 \text{kHz}$, and device-to-device variation is within$\pm 5 \text{dB}$. Qualitative listening to the music, including piano playing, produced by the microspeaker confirms clean, undistorted output up to 12.5 kHz.