Low Temperature Coefficient Shear Wave Thin Films for Composite Resonators and Filters

Abstract

Low temperature coefficient thin film composites have been synthesized using both ZnO/Si and AlN/Si. Using these composites, resonators have been fabricated which exhibit an absolute temperature coefficient of series resonant frequency of less than 1 ppm/C around room temperature and an overall coefficient better than ATcut quartz over the measured temperature of -20°C to 120°C. Shear waves were generated by ZnO or A1N films having c-axes oriented up to 45 from the s ubstrate normal in order to produce quasi-shear waves in the films and pure shear waves in the p+ Si membrane substrate. The oriented films were produced in a modified dc planar magnetron sputtering system having an auxiliary anode biased to control film orientation. By independently measuring the temperature coefficient of i sol ated piezoelectric films, t he temperature coefficient o f the boron doped p+ Si was inferred to be approximately 810 ppm/C as an average over the membrane thickness. Theoretical results suggest that the p+ Si membrane should have a temperature coefficient ranging from -10 ppm/C to +30 ppm/C over the membrane thickness.