Dual frequency PECVD silicon nitride for fabrication of CMUTs' membranes

Dual frequency plasma-enhanced chemical-vapour deposition (DF-PECVD) of silicon nitride has been investigated for the fabrication of membranes for micromechanical applications, and in particular for capacitive micromachined ultrasonic transducers (CMUTs). The use of high and low frequency plasma excitations, at 13.56 MHz and 340 KHz, respectively, allows to adjust the intrinsic stress of the thin silicon nitride films, varying the ratio between the time intervals during which the two RF power supplies are active in one cycle during the deposition process. In addition, silicon nitride films with high compactness and homogeneity, high resistivity, low porosity and conformally deposited on patterned substrates have been obtained. The optimized DF-PECVD silicon nitride has been used as structural layer and protection layer of the CMUTs.