Wide bandwidth Bragg mirrors for multi-band filter chips

We investigate the performance of Bragg mirrors containing iridium and porous silicon dioxide layers as acoustic reflectors for aluminum nitride solidly mounted resonators. The very large mismatch between the acoustic impedances of these materials allow to broaden the bandwidth of the mirrors to values larger than 2 GHz. Bragg mirrors consisting of five alternating layer of Ir and porous SiO₂ are designed and fabricated. Resonators of AlN with Ir electrodes with resonant frequencies ranging between 1.6 GHz and 2.7 GHz are built on top of the same Bragg mirror. Their coupling coefficients around 6% are roughly independent of the resonant frequency, whereas their quality factors vary from 1200 to 300 as the resonant frequency increases.