A study on temperature-compensated hybrid substrates for surface acoustic wave filters

A hybrid substrate, comprising a thin LiTaO₃ layer bonded to a low-coefficient-of-thermal-expansion support, can be used instead of a conventional LiTaO₃ bulk substrate to improve the temperature coefficient of frequency (TCF) of surface acoustic wave (SAW) devices. The use of a thinner LiTaO₃ layer will clearly reduce the TCF, but on the other hand, will also lead to large spurious responses in the filter characteristics. However, we report here that if the bottom of the LiTaO₃ layer is made sufficiently rough, the problem can be effectively mitigated. In addition, numerical calculations indicate that correct choice of the support material can further mitigate the spurious response. We found that glass was superior to silicon as the support material. A SAW filter comprising a rough-bottom LiTaO₃ layer only six-wavelengths thick bonded to a glass substrate exhibited spurious responses of just 0.5 dB, which is comparable to that obtained with a conventional LiTaO₃ bulk substrate. Thus, the combination of an appropriate substrate material and a rough-bottomed LiTaO₃ layer affords a SAW filter with small TCF and yet good response characteristics.