Void-free direct bonding of CMUT arrays with single crystalline plates and pull-in insulation

The implications on direct bonding quality, when using a double oxidation step to fabricate capacitive micromachined ultrasonic transducers (CMUTs), is analyzed. The protrusions along the CMUT cavity edges created during the second oxidation are investigated using simulations, AFM measurements, and a proposed analytical model, which is in good agreement with the simulated results. The results demonstrate protrusion heights in the order of 10 nm to 40 nm, with higher oxidation temperatures giving the highest protrusions. Isotropically wet etched cavities exhibit significantly smaller protrusions than anisotropically plasma etched cavities after the second oxidation. It is demonstrated that the protrusions will prevent good wafer bonding without subsequent polishing or etching steps. A new fabrication process is therefore proposed, allowing protrusion-free bonding surfaces with no alteration of the final structure and no additional fabrication steps compared to the double oxidation process. Two identical CMUT arrays with circular and square cavities having diameter/side lengths of 72 μm/65 μm and a 20 μm interdistance are fabricated with the two processes, demonstrating void-free bonding and 100 % yield from the proposed process compared to poor bonding and 7 % yield using the double oxidation process.