Development and Characterization of PMUTs using Enchanced C-SOI Wafers

This paper presents the development of Piezoelectric Micromachined Ultrasonic Transducers (PMUTs) on an Enhanced Cavity Silicon-On-Insulator (EC-SOI) wafer. An AlN (aluminum nitride) based PMUT array, designed to operate at 7 MHz in water (9.5 MHz in air) with a size of 2.05 x 2.05 mm<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup>, was developed. The development EC-SOI wafer involved adding extra steps to locally remove material from the device layer of a standard C-SOI wafer. This process enhanced the uniformity in thickness at the wafer level of the device layer. PMUT arrays were fabricated on both C-SOI and EC-SOI wafers for comparison. Wafer-level electrical characterization showed that the frequency uniformity of PMUTs on the EC-SOI wafer was more than 3 times better than that of PMUTs fabricated on standard C-SOI wafers. The wafer-level resonance frequency variation was less than 3.5%, and 95% of the chips exhibited a frequency variation of less than 2%. Acoustic characterization of the PMUT array demonstrated a transmit sensitivity of 6.9 kPa/V and a bandwidth of 2.8 MHz at resonance.