Temperature characteristics of solidly mounted piezoelectric thin film resonators

Solidly mounted piezoelectric thin film resonators (SMRs), which are consisting of a piezoelectric film on alternating laminated low- and high-acoustic-impedance quarter-wave layers on a substrate, are now receiving attention as bulk acoustic wave devices suitable for use in a super-high-frequency range. For practical applications, the temperature dependence of frequency is desirable to be very low. This paper reports the theoretical analysis on the temperature characteristics of the SMR using zinc oxide (ZnO) as the piezoelectric material, silicon dioxide (SiO₂) as the low impedance material, and ZnO as the high impedance material. The calculated results show that the temperature coefficient of frequency (TFC) varies from -60 ppm/°C to -27 ppm/°C with increasing the total layer number. This improvement of TCF results from the positive TCF of SiO₂. It is also shown that the temperature compensation of the SMR can be achieved by increasing the thickness of the first λ/4 SiO₂ layer just below the piezoelectric film or by depositing a thin SiO₂ capping layer on the piezoelectric film, although the effective electromechanical coupling factor K² and the electric Q decrease a little. Similar theoretical analysis has been performed on the SMRs using ZnO or aluminum nitride (AlN) as the piezoelectric material, molybdenum (Mo) or aluminum nitride (AlN) as the high impedance material, and SiO₂ as the low impedance material.