A novel method for fabricating capacitive micromachined ultrasonic transducers with ultra-thin membranes

Production of ultra-thin membranes facilitates the development of miniature capacitive micromachined ultrasonic transducers (CMUTs), which have great potential in biomedical imaging applications. We introduce a novel process, incorporating atomic layer deposition (ALD) and diffusion bonding, for the fabrication of CMUTs with ultra-thin membranes. First, an Al₂O₃ layer is deposited on an upper silicon wafer by ALD. Next, a gold layer is deposited on the Al₂O₃ layer and patterned to create circular cavities. Then the whole structure is transferred to a bottom wafer by diffusion bonding and the upper silicon wafer is etched away to release the Al₂O₃ membrane. Finally, another gold layer is deposited on the membrane for wiring and membrane excitation. Initial results show high quality membranes can be produced using this process with highly conformal surface qualities and extremely thin dimensions (<300 nm). Based on the dimensional characteristics created by this process, we simulate the performance of these transducers using equivalent circuit analysis. The results show that this new fabrication method provides another avenue for optimizing CMUT performance, especially in power savings, sensitivity and potentially increased reliability. Work to test the fabricated elements is currently under way.