Influence of metallization on temperature stability of SAW devices

Most Surface Acoustic Wave (SAW) devices exhibit a very small sensitivity to thermal effects. However, even on instrinsically compensated crystal cuts, the deposition of metal strips at the surface (transducers or reflectors) induces important changes in the thermoelastic properties of the device. A theoretical approach based on the Sinha-Tiersten perturbation method is proposed to model the influence of metallization on SAW properties on (ST,X) quartz, namely the temperature stability of the phase velocity of Rayleigh waves. Since this perturbation method only gives access to the first order Temperature Coefficient of Frequency (TCF), it is combined with a conventional calculation of the second order TCF to predict the evolution of the turnover temperature. The proposed calculation also requires temperature derivatives of the elastic constants of the metal which can be calculated for different materials. More classical approaches are also considered based on the calculation of surface permittivity using thermally varied elastic constants. Finally, theoretical results are compared to experimental data measured on SAW devices on (ST,X) quartz using Aluminium, Gold or Chromium gratings.