Temperature Compensated Bulk Acoustic Thin Film Resonators

Thin film resonators have been made that exhibit a high degree of temperature compensation. These resonators are composed of piezoelectric aluminum nitride films, aluminum top and bottom electrodes, and are compensated with layers of silicon dioxide within the resonator. The resonators are fabricated with the solidly mounted resonator (SMR) configuration using a sequence of aluminum nitride and silicon dioxide reflector layers. Silicon dioxide has a positive temperature coefficient and can be used to offset the -25 ppm per degree C coefficient of aluminum nitride. Results are reported on hermetic packaging, temperature cycle testing, temperature coefficient measurements, and preliminary ageing. Thin film resonators may be composed of materials fabricated by methods that do not result in perfect crystal forms. Consequently the materials do not necessarily have properties that closely resemble bulk materials in magnitude or stability. Nevertheless there is considerable interest in the thin film resonator technology because devices fabricated in thin film form have shown high performance at UHF and microwave frequencies