Temperature compensation of Longitudinal Leaky SAW with silicon dioxide overlay

Longitudinal leaky surface acoustic wave (LLSAW) is attracting considerable attention for its high velocity and reasonable coupling coefficient. The intrinsic temperature coefficient of frequency (TCF) for these waves for different longitudinal cuts with metal gratings is in the range of -110 to -90 ppm/degC. However, for certain filter applications the TCF of the LLSAW waves must be quite low in the range of ~ 20 to -20 ppm/degC. The introduction of a positive TCF overlay material (generally SiO₂) alters the LLSAW wave characteristics, and also changes the coupling coefficient. The modified LLSAW mode retains the same high velocity characteristics with an estimated improved TCF. Thus, a systematic study has been attempted by us to evaluate the effect of SiO₂ on some interesting longitudinal cuts, such as, YZ and 128deg lithium niobate. In this paper, we show the possibility to realize a good TCF value for LLSAW mode using a three dimensional (3-D) periodic finite element (FE) model. The predicted results show an improvement in the TCF value to -20 ppm/degC for the modified LLSAW mode which is in excellent agreement with the measurement results.