One-port SAW resonators fabricated on single-crystal diamond

Diamond has the highest known surface acoustic wave (SAW) phase velocity, sufficient for applications in the gigahertz range. In addition, diamond can be synthesized from methane gas by chemical vapor deposition (CVD) and is also free from rare earth and rare metal materials. Although numerous studies have demonstrated SAW devices on polycrystalline diamond thin films, all of these devices have a much larger propagation loss than single-crystal materials such as LiNbO₃, LiTaO₃, and quartz. Hence, we fabricated and characterized one-port SAW resonators on single-crystal diamond substrates synthesized using a high-pressure high-temperature method and microwave plasma CVD to identify and minimize sources of propagation loss. A series of one-port resonators were fabricated with an interdigital transducer (IDT)/AlN/diamond structure, and their characteristics were measured. The best performing device using a type-Ib (100) diamond single crystal exhibited a resonance frequency f of 5.2 GHz, and the equivalent circuit model gave a quality factor Q of 8346. Thus, a large fQ product of 4.4 × 10¹³ was obtained, and the propagation loss was found to be only 0.004 dB/wavelength. These excellent properties are attributed to the large group velocity, lack of grain boundaries in the single-crystal diamond, smooth surface of the AlN thin film and diamond substrate, and inclusion of energy-trapping gratings in the IDT. These results show that single-crystal diamond SAW resonators have great potential for use in low-noise super-high-frequency oscillators as sustainable SAW devices.