Silicon-On-Nothing ScAlN pMUTs

This work presents a promising microfabrication technique employing the silicon-on-nothing (SON) process to form a $2\ \mu\mathrm{m}$ thick continuous monocrystalline silicon membrane over a vacuum cavity of $1\ \mu\mathrm{m}$ 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 $170\ \mu\mathrm{m}$ down to $38\ \mu\mathrm{m}$ 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 $Q$ -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.