Temperature Compensation of Solidly Mounted Bulk Acoustic Wave Resonators

This work deals with temperature compensation of solidly mounted bulk acoustic wave resonators operating in the frequency range 2-3GHz. Passive temperature compensation was achieved through the utilization of thin layers of silicon dioxide. Silicon dioxide has the unique property of getting stiffer with the increase of temperature which leads to an increase in its sound velocity with elevated temperatures. This positive temperature coefficient of sound velocity compensates the negative coefficient of the rest of the materials inside the resonator structure. This work introduces a new way of temperature compensation. By adding very thin (30-70 nm) SiO2 layers at high stress regions within the resonator, temperature coefficients ranging from -10ppm/_C till +12ppm/_C have been successfully demonstrated. SiO2 is non piezoelectric and is softer than the piezoelectric material. And hence, its addition inside the BAW resonating structure degrades both the resonator's coupling coefficients and Q-value. The placement of SiO2 at high stress regions enabled us to reach full temperature compensation of less than 1ppm/_C with the use of only 32nm of SiO2. Thanks to this reduction in the amount of SiO2 needed for full compensation, resonators with coupling coefficient of 4% and Q-values of up to 1500 have been successfully fabricated with the proposed process. Both values are the highest reported in literature so far for solidly mounted resonators with more than a 200% improvement in the coupling coefficient in comparison with the state of the art. The thermal properties of thin film SiO2 are found to be different from their bulk corresponding values. A value of +110ppm/_C is experimentally extracted for the temperature coefficient of elasticity (TC33), which is significantly different from the +239ppm/_C of the bulk SiO2. Additional experimental data suggest that the value of the TC33 decreases with thinner SiO2 layers.