An indirectly clamped capacitive micromachined ultrasonic transducer with a high electromechanical coupling factor

Abstract

An indirectly clamped capacitive micromachined ultrasonic transducer (CMUT) with a high electromechanical coupling factor is reported. An indirectly clamped diaphragm includes a circular bottom plate hanging from a ring-type middle plate below the inner end of an annular top plate. This design allows the bottom plate to vibrate in a piston-like manner. In this way, the average gap variation between electrodes can be made larger than the average diaphragm displacement. The deflection shape of the proposed diaphragm mainly depends on two geometric parameters: the relative radial position of the middle plate and the ratio of the top and bottom plate thickness. Parameter simulation results show that the optimal inner-to-outer radius ratio of the annular top plate ranges from 0.4 to 0.6 for a top and bottom plate thickness ratio of 0.5–1.5. Simulations of 8×8 CMUT arrays show that the ratio of the gap variation to the diaphragm displacement and the coupling factor were 1.62 and 2.49 times, respectively, than that for a conventional CMUT array. Measurements of static displacements of fabricated indirectly clamped diaphragms showed that the circular bottom plate moved uniformly as expected. In addition, immersion test results showed that the output pressure of a fabricated indirectly clamped CMUT array is 1.72 times higher than that of a flat clamped CMUT array under the same conditions. In conclusion, high transmission and reception efficiencies for a CMUT can be achievable using the proposed indirectly clamped structure.