Assessment of solidly mounted resonators with wide-band asymmetric acoustic reflectors

We investigate the influence of the structure of acoustic reflectors on the quality factors of solidly mounted bulk acoustic wave resonators made of aluminum nitride. We compare the quality factors of resonators built on conventional λ/4 Bragg reflectors with those of resonators fabricated on asymmetric acoustic reflectors composed of layers of thicknesses different from λ/4. The two kinds of reflectors are made of porous silicon dioxide and iridium as low and high acoustic impedance materials, respectively. We assess the effective electromechanical coupling factor and the quality factor of various resonators tuned at different frequencies within the reflector band. The effective electromechanical coupling factor is roughly constant at around 6.3% for all the devices. For the symmetric reflectors the quality factor at the antiresonant frequency reaches a minimum at the centre of the band, whereas it remains roughly constant in the whole band for the asymmetric reflectors. This is attributed to the greater reflection of shear waves in the center of the band provided by the asymmetric reflectors.