An optically transparent capacitive micromachined ultrasonic transducer (CMUT) fabricated using SU-8 or BCB adhesive wafer bonding

摘要

This paper describes an optically transparent capacitive micromachined ultrasonic transducer (CMUT) fabricated with two indium tin oxide (ITO) coated glass wafers bonded together using adhesive wafer bonding. Both SU-8 photoresist and photosensitive benzocyclobutene (BCB) were investigated as the adhesive layer, which also helps define the cavities and form the insulation layer in the CMUT structure. The CMUT was designed as a single transducer on a 100-mm-diameter glass wafer. The key feature of the device is a thin glass top plate with an ITO electrode, coated with an SU-8 or BCB insulation layer, adhesively bonded onto cavities that are formed by patterning SU-8 or BCB on an ITO-coated glass substrate. The fabricated CMUTs using both bonding materials demonstrated an optical transmittance of 70%-80% in the full visible wavelength range. Vacuum sealing of the device was confirmed by the atmospheric deflection of 2.8 urn in the center of a CMUT cell. The electrical input impedance was measured after establishing the electrical connections using silver epoxy. The fabricated CMUTs showed a resonance frequency of approximately 62 kHz in air and the series resistance was measured as approximately 30 Ω.