Yasunori Iwasaki

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.

A hybrid substrate for a temperature-compensated surface acoustic wave filter

A hybrid substrate that is appropriate for temperature compensation of a surface acoustic wave (SAW) filter is developed. A one-port SAW resonator was fabricated on the hybrid substrate to study its electronic characteristics and thermal stability. The spurious response originating from bulk wave reflection at the layer interface was found to be sufficiently small with a relatively thin piezoelectric layer. The minimum temperature coefficient of frequency (TCF) was −8.4 ppm/°C.