Correlation between propagation loss and silicon dioxide film properties for surface acoustic wave devices

The correlation between the propagation loss and SiO₂ film properties has been studied for temperature-compensated SAW devices using the SiO₂/LiNbO₃ structure. The SAW devices were prepared under different deposition temperatures for SiO₂ film. Although they possessed excellent temperature coefficient of elasticity characteristics, devices prepared at lower temperature showed lower Q-factors. The SiO₂ films were also deposited on a Si substrate under the same deposition conditions used for the SAW device preparation. Optical characterization was performed with Fourier transform infrared spectroscopy (FT-IR), spectrometer measurement, and Raman spectroscopy. IR absorbance spectra were almost same in the FT-IR measurement. However, optical attenuation in the UV region decreased with the deposition temperature in the spectrometer measurement. The optical attenuation is caused by the increase of the extinction coefficient in the SiO₂ layer, and its optical wavelength dependence indicated that observed excess attenuation is caused by Rayleigh scattering. The Raman scattering also decreased with the deposition temperature in the Raman spectroscopy. The scattering is caused by the distortion of the SiO₂ network. These results indicate that the Rayleigh scattering caused by the distortion of the SiO₂ network is the main contributor to the excess SAW propagation loss in this case.