IEEE TRANSACTIONS ON ULTRASONICS, FERROELECTRICS, AND FREQUENCY CONTROL, . 60, . 5, MAY 2013 986 0885–3010/$25.00 © 2013 IEEE Low Propagation Loss in a One-Port SAW Resonator Fabricated on Single-Crystal Diamond for Super-High- Frequency Applications Satoshi Fujii, Tatsuya Odawara, Haruya Yamada, Tatsuya Omori, Ken-ya Hashimoto, Hironori Torii, Hitoshi Umezawa, and Shinichi Shikata Abstract—Diamond has the highest known SAW phase ve- locity, sufficient for applications in the gigahertz range. Howev- er, although numerous studies have demonstrated SAW devices on polycrystalline diamond thin films, all have had much larger propagation loss than single-crystal materials such as LiNbO 3 . Hence, in this study, we fabricated and characterized one-port SAW resonators on single-crystal diamond substrates synthe- sized using a high-pressure and high-temperature method to identify and minimize sources of propagation loss. A series of one-port resonators were fabricated with the interdigital trans- ducer/AlN/diamond structure and their characteristics were measured. The device with the best performance exhibited a resonance frequency f of 5.3 GHz, and the equivalent circuit model gave a quality factor Q of 5509. Thus, a large fQ product of approximately 2.9 × 10 13 was obtained, and the propaga- tion loss was found to be only 0.006 dB/wavelength. These excellent properties are attributed mainly to the reduction of scattering loss in a substrate using a single-crystal diamond, which originated from the grain boundary of diamond and the surface roughness of the AlN thin film and the diamond sub- strate. These results show that single-crystal diamond SAW resonators have great potential for use in low-noise super-high- frequency oscillators. I. I S - substrates generally have a low SAW phase velocity and allow only thin metal interdigital transducers (IDTs), making their propagation loss imprac- tically large above 3 GHz. Diamond substrates, however, have a SAW phase velocity more than two times those of single-crystal LiNbO 3 , LiTaO 3 , and quartz, thereby mak- ing them suitable for much higher operating frequencies [1]. Indeed, numerous studies have demonstrated SiO 2 / IDT/ZnO/diamond devices with an operating frequency range of 2 to 10 GHz and with outstanding temperature stability [2]–[6]. Aside from devices employing ZnO thin film, there are several reports on SAW devices that employ AlN thin film as the piezoelectric material to achieve phase velocities of over 10 km/s on diamond [7]–[10]. We also reported a SiO 2 /IDT/AlN/diamond SAW device that allowed a high- er metal thickness for the IDT and exhibited a tempera- ture coefficient of frequency (TCF) of zero at room tem- perature. Such one-port SAW resonators fabricated with AlN instead of ZnO showed a quality factor Q of 660 at a 5.4-GHz anti-resonance frequency with a frequency drift of only about 90 ppm between -25°C and 80°C—less than that of SAW devices based on ST-quartz substrates—and a propagation loss of only ~0.03 dB/wavelength at 6 GHz [11]–[13]. However, because of the polycrystalline nature of the diamond thin film substrate, this propagation loss is still larger than that for single-crystal substrates, which is a significant factor in practical use. Several factors contribute to the loss of a SAW reso- nator: transmission loss, which is caused by energy leak- age to outside the system by incomplete reflection at a reflector; native loss, which is caused by the viscosity of the substrate material; scattering loss, which is caused by the scattering of the SAW by surface asperities or grain boundaries in polycrystalline materials; diffraction loss, which is caused by beam spreading during two-dimen- sional propagation of the SAW; and ohmic loss, which is caused by the resistivity of the IDT electrodes. Each of these factors should be individually mitigated to achieve good performance of the resonators. In previous studies, we achieved low propagation loss of the SAW in a polycrystalline diamond thin film by con- trolling the crystalline quality of its thin film: we achieved smaller grains, a narrower grain distribution, and a pre- ferred grain orientation. These results suggest that the dependence of phase velocities on crystal directions should be reduced and that the effect of the grain boundaries should be minimized for ensuring low propagation loss in polycrystalline diamond thin films [14], [15]. Indeed, Manuscript received June 8, 2012; accepted February 5, 2013. This research was funded by a grant from the Japan Society for the Promo- tion of Science (JSPS) through the Funding Program for World-Leading Innovative R&D on Science and Technology (FIRST Program), initiated by the Council for Science and Technology Policy (CSTP). This research was also supported by Grants-in-Aid for Scientific Research and the Murata Science Foundation. S. Fujii is with the Organization for Academic–Industrial Collabora- tion and Intellectual Property, Chiba University, Chiba, Japan (e-mail: s_fujii@faculty.chiba-u.jp). T. Odawara, H. Yamada, T. Omori, K. Hashimoto, and S. Shikata are with the Department of Electrical and Electronic Engineering, Chiba University, Chiba, Japan. H. Torii is with MES AFTY Corporation, Tokyo, Japan. H. Umezawa and S. Shikata are with the Diamond Research Labora- tory, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Japan. DOI http://dx.doi.org/10.1109/TUFFC.2013.2656