The influence of acoustic reflectors on the temperature coefficient of frequency of solidly mounted resonators

For many applications, compensation of the temperature coefficient of the resonant frequency in solidly mounted resonators is needed. The influence of the different materials in such a complex structure upon this coefficient is not easily understandable. The material characteristics of each layer influence this coefficient but the relative position of each layer into the whole stack with respect to the piezoelectric material (AlN) can be even more influential. Study of this situation shows that the first two layers of the stacked reflector are most influential in the temperature coefficient. Several kinds of resonators with asymmetry (with layers of non-λ/4 thicknesses) are tested obtaining resonators with nearly zero temperature coefficient of the frequency and with optimized response.