Bendable Piezoelectric Micromachined Ultrasound Transducer (PMUT) Arrays Based on Silicon-On-Insulator (SOI) Technology

Abstract

This paper presents the design and fabrication method to render a silicon-on-insulator (SOI)-based pMUT array bendable. The proposed method can be applied to silicon-based pMUT arrays with different acoustical specifications. A bendable array based on the proposed method is highly conformal to the target structure. The bendable structure is made out of several silicon islands containing a pMUT array and are connected to each other by silicon springs. The silicon springs are realized by deep reactive ion etching (DRIE) that was also used to create the pMUT membranes. The fabrication process requires one additional photomask step compared to a process that realizes only pMUTs. Since the pMUT arrays are fabricated on silicon islands with the thickness of the entire SOI wafer, therefore their performance is not compromised. As a case study, a bendable array was designed and fabricated to be wrapped around a $5\times 5\times 5$ mm3 3D printed cube. Therefore, each bendable array has 6 islands with a dimension of $3\times 3$ mm2 including $3\times 3$ pMUT array, of which each pMUT has a diameter of 410 $\mu \text{m}$ and a thickness of 6 $\mu \text{m}$ with a 1 $\mu \text{m}$ Lead Zirconate Titanate (PZT) layer as the piezoelectric material. The silicon springs in between each island have a spring constant of 2.3 (N/m) in their folding direction. This compliant structure enables a 90° bending with a bending radius of 1 mm. The pMUT elements in the array resulted in a displacement response of 2.1 $\mu \text{m}$ /V and a Q-factor of 104 at 426 kHz, when all arrays were excited all together. [2019-0224]