Low-temperature coefficient bulk acoustic wave composite resonators

Abstract
This letter reports on an acoustic bulk wave resonator structure having an absolute temperature coefficient of series or parallel resonant frequency of less than 4 ppm/°C over the temperature range −20 to +120 °C comparing favorably with that of conventional AT cut quartz. In addition, the resonant Q was 7600 for parallel and 7300 for series resonance at frequencies near 330 MHz for the fundamental mode. The resonator is composed of a unique composition of an AlN piezoelectric film sputtered on a single-crystal silicon membrane for a total thickness of less than 10 μ. The resonator coupling coefficient was found to be 2.5% corresponding to film coupling of 12%. The AlN films were prepared in a dc planar magnetron reactive sputtering system.