7E-6 Aluminum Nitride Bulk Acoustic Wave Devices with Iridium Bottom Electrodes

Bulk acoustic wave (BAW) test resonators in the frequency range between 1.8 to 1.9 GHz were fabricated with piezoelectric aluminum nitride (AIN) films sputtered on iridium (Ir) bottom electrodes. The crystal structure and piezoelectric response of AIN films grown on Ir were as good as those of the best AIN films grown on other metallic electrodes, like platinum (Pt), molybdenum (Mo) or tungsten (W). Solidly mounted resonators (SMR) test devices with a single low-impedance layer of silicon dioxide (SiO₂) for acoustic isolation were used for the preliminary assessment of both the piezoelectric activity of AIN and the influence of the iridium layer on the performance of the devices. The transversal electromechanical coupling factor of the AIN films was derived by fitting the electrical response of the resonators to Mason´s physical model, which allowed us to obtain a material dependent coupling factor. AIN films exhibited very high coupling factors (7.5 %) barely dependent on the width of the rocking curve (RC) around the AIN 00-2 reflection. The high acoustic impedance of the Ir bottom electrode reduced the mechanical losses of the BAW resonators, which exhibited higher quality factors than resonators built on lighter Mo bottom electrodes. The influence of the thickness of Ir and Mo bottom electrodes in the performance of the devices was also compared.