Temperature-compensated surface-acoustic-wave devices with SiO2 film overlays

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Abstract
Temperature‐stable surface‐acoustic‐wave (SAW) devices have been fabricated with an rf‐sputtered SiO3. film overlay on YZ LiTaO2. The material properties of this composite structure relevant to surface‐acoustic‐wave propagation have been thoroughly studied. For an SiO2 film thickness of approximately one‐half of an acoustic wavelength, a piezoelectric coupling coefficient of 0.014 is present and the temperature stability of phase delay is similar in magnitude to that of bulk acoustic waves in AT‐cut quartz. A variation in delay of less than one part in 105 is observed over the temperature range −40 to 80 °C. The film‐overlay structure is dispersive and the nature of this dispersion has been fully characterized. The dependence of the phase and group delay on temperature and frequency has been measured. For most characteristics, theoretical as well as experimental values have been determined. In addition to YZ LiTaO3, five other cuts of LiTaO3 have been analyzed theoretically. To demonstrate potential applications for this structure, several practical devices have been fabricated and tested. Narrow‐band delay lines have been fabricated for use in SAW‐controlled oscillators. These oscillators showed a temperature stability nearly ten times better than similar devices fabricated on ST‐cut quartz. Also, to demonstrate a wide‐bandwidth application, a 13‐bit Barker code, matched‐filter pair has been fabricated. These devices also performed well.