Hafnium Nitride as High Acoustic Impedance Material for Fully Insulating Acoustic Reflectors

Non-conductive high acoustic impedance materials could provide a simpler solution to some of the challenges that solidly mounted resonators display in biosensing and telecom applications. Hafnium nitride is used as a high acoustic impedance material in fully insulating acoustic reflectors. The HfN thin films display a density close to 11000 kg/m³, which is 80% of the nominal bulk value (13800 kg/m³, and a longitudinal acoustic velocity of 5400 m/s. These values result in a ZA of 59.4 Mrayl, which is considerably higher than that obtained for the most used insulating high acoustic impedance materials (AIN and Ta₂O₅., 36.3 Mrayl and 38.9 Mrayl respectively) and very close to that of Mo sputtered films (64.7 Mrayl). A five-layer acoustic reflector centered at 3.5 GHz made of HfN/SiO₂ is quantitatively compared with a standard Mo/SiO₂ reflector. The devices on the HfN/SiO₂show a quality factor Q of 750 at the resonant frequency, higher than those fabricated on the Mo/SiO₂ reflector that displayed a Q value of 300.