Capacitive micromachined ultrasonic transducer (cMUT) made by a novel "reverse fabrication process"

We report a novel fabrication process of a cMUT array based on the electrostatic effect and realized by silicon micromachining technique. Several fabrication technologies for 1D and 2D cMUT have been presented in the last ten years, differing from each other in the materials used and the process steps involved. They all have in common the presence of micro-holes on the surface of the transducer necessary to evacuate the cavities under the membranes or, in the 2D array, to electrically connect upper to lower pads and to allow the electrical connection to external circuits. The authors of the present work designed and realized a cMUT transducer using a new concept. In the standard process, successive layers are deposited on the silicon wafer up to the silicon nitride structural layer of the micro membranes; our different approach consists in inverting the function of each layer and to build the cMUT capacitive cell starting from the membrane, made of LPCVD silicon nitride coating the silicon wafer, up to the bottom electrode and the backplate. By working on the back of the device, there is no need to use holes in the structural silicon nitride layer to evacuate the cavities and the pads for the electrical connections are on the bottom surface of the device.