Optimization on benzocyclobutene-based CMUT fabrication with an inverse structure

It has been demonstrated that the adhesive wafer bonding technique benefits the fabrication of capacitive micromachined ultrasonic transducers (CMUTs) in the aspects of structural flexibility, CMOS process compatibility, and fabrication yield. Recently, the feasibility of fabricating CMUTs using benzocyclobutene (BCB) based polymer as both the adhesive and structural material was reported. However, the issues of voids at the wafer bonding interface, material deformation and electrode distance control need to be addressed. In this paper, we present an improved fabrication process that allows a void-free device with reduced polymer deformation. The chemical reactions during the BCB curing process were revealed using Fourier transform infrared spectroscopy. Microscopy and cross-sectional SEM images were employed to illustrate the structure improvement in terms of voids and polymer deformation. In addition, an inverse structure that allows an accurate control of the effective electrode distance was presented. The cross-sectional SEM images and the impedance measurement showing the dynamic response of the CMUT demonstrated the feasibility of this inverse structure. The differences between the inverse structure and the conventional structure were also discussed and compared.