Low-Loss Diamond Surface Acoustic Wave Devices Using Small-Grain Poly-Crystalline Diamond

We have developed a poly-crystalline diamond film whose grain size is about 0.5 µm, which is smaller than that of our previously studied diamond films applied in surface acoustic wave (SAW) devices. Using the small-grain diamond, three kinds of narrow-band SAW filters were fabricated with the SiO2/interdigital-transducer (IDT)/ZnO/diamond/Si structure at the center frequencies from 2.488 to 5.0 GHz. The insertion losses of these filters were lower by 1.3 to 3.2 dB and the Q values were 200 higher than those obtained using the standard diamond. The Q value is defined as the center frequency divided by the bandwidth at a loss level 3 dB lower than the minimum insertion loss. "Standard diamond" refers to our previously studied diamond. The propagation loss, electro-mechanical coupling coefficient (K2), phase velocity and temperature coefficient of frequency (TCF) were investigated on the basis of the measured characteristics of these filters. It was found that the propagation loss was about two thirds of that obtained using the standard diamond film while TCF, K2 and phase velocity were almost the same as those obtained using the standard diamond.