An optically transparent air-coupled capacitive micromachined ultrasonic transducer (CMUT) fabricated using adhesive bonding

Transparent transducers are desired in the applications where optics and acoustics are combined, such as integrating ultrasound sensing or parametric arrays for directional sound in display, combined optical and acoustical microparticle manipulation, and backward-mode photoacoustic imaging. Some piezoelectric materials, e.g., PVDF, LNO, and PLZT, have been investigated for such applications. Besides, a Fabry-Perot (Etalon) optical ultrasound sensor can be used as a transparent ultrasound receiver. Implementing a transparent transducer in CMUT technology is desired to take advantage of wide bandwidth, ease of fabrication, and broad selection of processing materials. We have demonstrated a CMUT with ITO bottom electrode for improved transparency fabricated using anodic bonding. The main hurdle for the transparency in the visible wavelength range was a 2-μm silicon plate. In this work, we have designed and fabricated a CMUT with a glass plate using adhesive bonding and achieved improved transparency in the full visible wavelength range.