A capacitive micromachined ultrasonic transducer (CMUT) array with nickel membranes in a convex shape

This paper reports an improved method for fabricating CMUT array using nickel electroplating over reflowed photoresist. Previously, a CMUT array using nickel on glass process which is used Doppler velocity sensor is reported [1]. The reported device use TI/Cu as sacrificial layer and nickel electroplating to form the membrane. In the newly reported CMUT, we use nickel electroplating over reflowed photoresist to form the membrane, and the cavity is formed by removing the reflowed photoresist sacrificial layer with acetone via the pre-formed released hole through the nickel membrane. The new device fabricated showed the advantage of having the membrane in a convex shape in terms of higher sensitivity [2]. A convex membrane can reduce the gap distance on the membrane edge to increase the effective capacitance. As a result, comparing the particular device reported with nickel on glass process which operates at 180 kHz for low frequency application, the preliminary experimental result of the newly reported device shows a resonant frequency at around 7.7MHz by using an impedance analyzer.