PMUT Structure Design with a Scar-Free Minimally Invasive Surgery Process on (111) Silicon Wafer

The scar free "MIS" (micro-holes inter-etch and sealing) process has been developed successfully to form pressure sensors, flow sensors and accelerometers. This work extends this fabrication technique to the PMUT device field and make optimization on the number and position of micro-holes for fast cavity release and less side effect. Since hexagon cavity under bulk silicon can be realized on (111) wafer, it is possible to design close-packed hexagonal array structure for strong focused acoustic output. Based on a diaphragm-on-cavity structure which formed by anisotropic wet etch on (111) wafer in 80°C TMAH solution, molybdenum and aluminum nitride are then sputtered to construct the driving layers of PMUT device. In the cavity releasing part, we develop two types of configurations of micro-holes for comparison. For the first type, the micro-holes are rotating symmetrically arranged every 120° along each side within the hexagon membrane to hasten etching process. To eliminate lateral over etching along the (111) crystal plane and form regular hexagon membrane above the cavity, micro-holes on the three specific sides are indented by an inter-etch distance in the second type. In addition, the number of holes has been minimized to maintain structural stability. Both FEM models are simulated with the same boundary and damping condition by COMSOL software. Figure of merit (defined by the product of resonance frequency and average displacement of membrane) here shows that the optimized type has even better utilization of piezoelectric performance. With the help of this fabrication design, IC-compatible and low-cost mass production of PMUT array devices with high fill factor in high resonance frequency can be achieved.