Modulation of Temperature Coefficient of Frequency of Surface Acoustic Wave Devices by Phase Velocity Dispersion

The temperature coefficient of phase velocity (TCV) and thermal expansion coefficients (CTE) were usually treated as two major terms of temperature coefficient of frequency (TCF). In this work, owing to the dispersion of phase velocity ( V-p), a coupling mechanism of TCV and CTE in surface acoustic wave (SAW) resonators based on directly bonded LiTaO3 /SiC heterosubstrate was investigated. V-p of the shear horizontal SAW (SH-SAW) in LiTaO3 /SiC exhibited strong dispersion characteristics with the variation in LiTaO3 film thickness ( h(LT) ) to interdigital transducer pitch ( lambda ) ratios ( h(LT)/lambda ). Then, when hLT/lambda changed due to thermal expansion, V-p would change synchronously, indicating a variation in TCV resulted from CTE. Considering this coupling effect, a revised term ( TCFd ) containing the first deviation of V-p to h(LT)/lambda as a measure of dispersion was derived and introduced into the conventional TCF formula. Moreover, the negative TCF could be compensated by the coupling term if TCFd is positive. In experiments, the SH-SAW resonators with small h(LT)/lambda (0.085-0.18) exhibited well-compensated TCF values at anti-resonance frequency ( TCFa ). Particularly, the one with h(LT)/lambda of 0.11 exhibited a TCFa of - 4.1 ppm/K and an extremely high Q of 8050. The filter with a center frequency of 2.76 GHz showed the TCF values of - 8.0 and - 15.6 ppm/K at the left and right edges of passbands, respectively. This work extended a more thorough view of mechanism of TCF in heterosubstrate-based SAW resonators and demonstrated the potential to realize near-zero drift SAW devices on heterogeneous substrates with strong dispersive phase velocities.