|
11 June 08 |
Requirements to Piezoelectric Thin Films for RF SAW/BAW Devices
Abstract
Various types of radio frequency (RF) surface and bulk acoustic wave (SAW/BAW) devices are commercialized as electronic components, and some of them serve as key elements in modern communication systems. Since their achievable performances are inherently limited by piezoelectric materials used for the electromechanical transduction, we device designers are always keen to have new piezoelectric materials and their deposition techniques, which offer outstanding features. This lecture is aimed at discussing what properties are necessary in piezoelectric thin films for the realization of high performance RF SAW/BAW devices. First, RF acoustic wave devices employing piezoelectric thin films are surveyed, and it is revealed how properties of piezoelectric thin films influence to device performances. We also clarify additional requirements imposed for realization of desired device configuration and for compatibility with the other fabrication processes. Since the conditions often conflict with those desirable for the film growth, degradation of crystal qualities is often necessary so as to fulfill all the requirements. Next, it is shown that superior crystalline and/or dielectric properties do not always bring superior acoustic and piezoelectric properties. This is due to the fact that acoustic wave properties are governed by mesoscopic and macroscopic properties of materials, and are not directly influenced by microscopic structures. Detailed discussions will be given on impact of the binding strength at grain boundaries to the excitation and propagation characteristics of acoustic waves. It should be noted that influence of the grain boundaries is considerably dependent on the type of propagating acoustic wave and the propagation direction.
Authors