Materials for bulk acoustic wave (BAW) resonators and filters

Thin film bulk acoustic wave (BAW) resonators and filters are appropriate f or mobile communication systems operating at high frequencies between 1-10 GHz. The resonance frequency is mainly determined by the thickness of the p iezoelectric layer. Piezoelectric films used for this application are, ther efore, several 100 nm in thickness (up to approx. 2 mum) depending on frequ ency. Piezoelectric thin film materials used for bulk acoustic wave devices include AlN, ZnO thin films for small bandwidth applications and also PZT films for wide bandwidth applications. Within Philips piezoelectric AlN and PbZrxTi1-xO3 (PZT) layers are investigated with respect to their potential for RIF micro-electronic applications. High quality AIN films with strong c-axis orientation are achieved by optimum sputter deposition conditions an d by applying suited nucleation layers. Electromechanical coupling factors k of 0.25 +/- 0.03, which are close to the bulk data, have been found in hi ghly c-axis oriented AlN thin films. The relationship between sputter depos ition conditions. AlN films structure on the one hand and electromechanical coupling factor k and relevant electrical parameters on the other hand wil l be discussed. A one-dimensional physical model is used to describe the bu lk acoustic wave resonator's electrical impedance data accurately. Thin PZT films are grown via sol-gel processing. These films show high electromecha nical coupling factor k of 0.3-0.6 and are therefore attractive for wide ba ndwidth filter applications. (C), 2001 Elsevier Science Ltd. All rights res erved.