AlN films sputtered on iridium electrodes for bulk acoustic wave resonators

We report on the growth of highly c-axis-textured aluminum nitride (AlN) films sputtered on iridium layers for thin film bulk acoustic microresonator applications. Iridium films were evaporated on oxidized silicon covered with Ti or Mo/Ti seed layers to improve crystal quality and adhesion. The crystal quality of the AlN films as a function of the crystal structure of the iridium electrode and its pre-conditioning by Ar+ bombardment was analyzed by X-ray diffraction, atomic force microscopy and wet chemical etching. Solidly mounted resonators using a single low-impedance layer of silicon dioxide for acoustic isolation were fabricated for a preliminary assessment of the piezoelectric activity of AlN films and the influence of the AlN/Ir stack on the performance of the devices. The electromechanical coupling factor of the AlN films was derived by fitting the electrical response of the resonators to Mason's physical model. AlN films exhibited very high coupling factors (7.5%) barely dependent on the width of the rocking-curve around the AlN 00·2 reflection. The high acoustic impedance of iridium electrodes provided resonators with quality factors higher than those of similar resonators built on lighter electrodes, such as molybdenum.