Materials for bulk acoustic wave (BAW) resonators and filters

Thin film bulk acoustic wave (BAW) resonators and filters are appropriate for mobile communication systems operating at high frequencies between 1–10 GHz. The resonance frequency is mainly determined by the thickness of the piezoelectric layer. Piezoelectric films used for this application are, therefore, several 100 nm in thickness (up to approx. 2 μm) depending on frequency. 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 RF micro-electronic applications. High quality AlN films with strong c-axis orientation are achieved by optimum sputter deposition conditions and 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 highly c-axis oriented AlN thin films. The relationship between sputter deposition conditions, AlN films structure on the one hand and electromechanical coupling factor k and relevant electrical parameters on the other hand will be discussed. A one-dimensional physical model is used to describe the bulk acoustic wave resonatorʹs electrical impedance data accurately. Thin PZT films are grown via sol–gel processing. These films show high electromechanical coupling factor k of 0.3–0.6 and are therefore attractive for wide bandwidth filter applications.