Surface acoustic wave (SAW) characteristics of the SiO2/interdigital-transducer(IDT)/ZnO/diamond
structure were studied theoretically and experimentally. It was found that the 2nd
mode in this structure can provide a zero temperature coefficient of frequency (T
C
F) as well as
high velocity of 10,000 m/s and a large electromechanical coupling coefficient of 1.4%. Utilizing
this wave, a 2.5 GHz narrow-band SAW filter was successfully fabricated with IDTs of 0.9 µm
finger width, which resulted in superior characteristics for a retiming filter with a small insertion
loss of 6.7 dB, Q value of 660 and small frequency deviation of 100 ppm in the temperature range
from -40 to 85°C. The insertion loss is smaller than any other retiming filters which have ever been
reported with quartz, a conventional SAW material, and also the frequency deviation with
temperature is smaller than that in the case of ST-cut quartz.
Diamond films with various crystal qualities were grown by chemical vapor deposition on silicon wafers. Their crystallinity was characterized by Raman scattering and electron backscattering diffraction. By fabricating a device structure for surface acoustic wave (SAW) using these diamond films, the propagation loss was measured at 1.8 GHz and compared with the crystallinity. It was found that the propagation loss was lowered in relatively degraded films having small crystallites, a narrow distribution in the diamond crystallite size, and preferential grain orientation. This experiment clarifies diamond film characteristics required for high-frequency applications in SAW filters.
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