Surface acoustic wave propagation on piezoelectric substrates with thin-film PECVD silicon nitride

The SAW characteristics of thin-film silicon nitride (Si₃ N₄) on piezoelectric substrates of Y-Z and 64° Y-X lithium niobate, and 36° Y-X lithium tantalate have been measured in the frequency range from 30 MHz to above 1.0 GHz. Silicon nitride films 250, 500, and 1000 nm thick were deposited by plasma enhanced chemical vapor deposition (PECVD). The SAW velocity and propagation loss properties were measured using linear arrays of thin-film aluminum interdigital electrodes patterned on the upper film surface. The elastic constants of the films could be determined by fitting a theoretical velocity dispersion curve to the measured velocities for Rayleigh wave propagation. The SAW propagation loss values for the films on the different substrates were compared to the loss characteristics without the film layers. Temperature coefficient of frequency (TCF) measurements of the SAW velocity characteristics for the various film/substrate combinations were also made. The results of these measurements, comparative data on acoustic constants, and the performance enhancements for SAW devices which could be realized by the use of silicon nitride on piezoelectric substrates are discussed