Temperature Compensation in SAW Filters by Tri-Layer Wafer Engineering

Bonded wafer concept is modified by introduction of a third layer that is used for compensation (or even overcompensation) of wafer warping thus increasing the amount of stress at the upper surface of the piezoelectric LiTaO₃ layer resulting in significant improvement of temperature coefficient of frequency (TCF), that may become zero or even positive. We have successfully demonstrated variants of structure where a third layer with relatively higher coefficient of thermal expansion (CTE) is deposited or bonded on the back surface thus compensating or introducing opposite warping of the combined tri-layer structure. The experimental results and the modeling show that with appropriate choice of supporting substrate materials the unwanted warping can be eliminated and the TCF closer to zero is routinely obtained in LiTaO₃/Si/Cu tri-layer structures with thick Al electrodes.