Ultrasonic transducer tuning using wafer bonding method

Abstract

This study demonstrates the wafer bonding method for performance tuning of piezoelectric Micromachined Ultrasonic Transducer (pMUT) from on-the-shelf piezoelectric disc (PZT). Polydimethylsiloxane (PDMS) and Epoxy were studied as adhesive materials. A thick bonding layer was deposited using the spin coating technique. Wafer bonding was carried out at room temperature to simulate in-situ pMUT repairing scenario. Bonding integrity is analyzed using Field Emission Scanning Electron Microscope (FESEM) images, while electrical characterization of pMUT is carried out using impedance analysis. Fabricated pMUTs have been calibrated using the pulse-echo technique in a freshwater tank. This study found that PDMS at the minimum thickness of 28 µm is preferably compatible for in-situ wafer bonding of pMUT compared to the Epoxy. PDMS has significantly reduced device impedance at 62.4 % reduction compared to 58.9 % reduction for Epoxy. Both pMUTs were able to transmit and receive short acoustic ping with the calibrated speed of sound of 1333.3 m/s. PDMS has successfully contributed to a broader operating frequency between 30 – 150 kHz for transmission and 75 – 140 kHz for the reception.