Sealed Silicon Cavity Piezoelectric Micromachined Ultrasonic Transducers with High Fill Factor

This paper presents a wafer-level sealed silicon cavity-piezoelectric micromachined ultrasonic transducer (SSC-PMUT) platform, highlighting its capability to support simultaneous fabrication of PMUT arrays with varied air cavity dimensions <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$(40-130\ \mu \mathrm{m})$</tex>, small spacings between transducers <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$(8-20\ \mu \mathrm{m})$</tex>, and controllable thickness of suspended silicon film <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$(1-1.8\ \mu \mathrm{m})$</tex>. The proposed SSC-PMUT platform offers a simplified, cost-efficient, high-filling-factor manufacturing method. Using just one photolithography step, deep reactive ion etching, and a brief thermal anneal, high-density sealed silicon cavities are formed prior to piezoelectric transducer fabrication steps. The fabricated SSC-PMUT arrays show a maximum fill factor of 77.9% with a frequency range from 1.81 to 18.45 MHz, demonstrating great potential for high fill factors, wide frequency range, and highly design-flexible PMUT arrays in integrated ultrasonic applications.