Silicon-On-Nothing ScAlN pMUTs

Abstract

This work presents a promising microfabrication technique employing the silicon-on-nothing (SON) process to form a <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$2\ \mu\mathrm{m}$</tex> thick continuous monocrystalline silicon membrane over a vacuum cavity of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$1\ \mu\mathrm{m}$</tex> in depth. Utilizing the SON process, high fill-factor piezoelectric micromachined ultrasonic transducer (pMUT) arrays on an 8-inch silicon wafer with cavity widths ranging from <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$170\ \mu\mathrm{m}$</tex> down to <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$38\ \mu\mathrm{m}$</tex> have been demonstrated. Devices are designed with 15% scandium-doped aluminum nitride as the piezoelectric layer of the pMUT for both air-coupled and water-coupled applications. The air-coupled pMUTs show a peak displacement frequency from 0.8 to 1.6 MHz with a <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$Q$</tex> -factor between 120 to 194. The water-coupled pMUT arrays show a transmit pressure measured by a needle hydrophone, in DI water at a distance of 20 mm, ranging between 0.4 to 6.9 kPa/V with peak frequency between 5 to 13.4 MHz and fractional bandwidth 56 to 36%, respectively. The piezoelectric-over-SON process proposed here has the potential to gain traction in low-cost and high-yield pMUT manufacturing.