A High-Density and Dual-Frequency PMUT Array Based on Thin Ceramic PZT for Endoscopic Photoacoustic Imaging

Photoacoustic imaging can be used for in vivo early cancer diagnosis due to its cross-sectional imaging capability and high contrast, but there always exists a tradeoff between resolution and imaging depth. For that piezoelectric micromachined ultrasonic transducer (pMUT) arrays with both higher-frequency and lower-frequency pMUT elements are needed for achieving high resolution and large imaging depth simultaneously. In this paper, we present the design, fabrication, and characterization of a high-density and dual-frequency pMUT array based on thin ceramic PZT, which consists of 128 pMUTs operating at 1.2 MHz and 128 pMUTs operating at 3.4 MHz. By using wafer bonding and chemical mechanical polishing techniques, ceramic PZT with high piezoelectric coefficients was thinned down to only 4 μm and employed as the piezoelectric layer of the designed pMUT array. The dual-frequency performance of the pMUT array was characterized with electrical impedance, vibration modes, cross-coupling, and acoustic sensing measurements. With a chip size of 7 ×7 mm and integration of 256 pMUTs with two operating frequencies into one array, the developed pMUT array shows a great potential for endoscopic photoacoustic imaging to achieve high-resolution and large imaging depth simultaneously.