Propagation Characteristics of Surface Waves in Quartz and Their Influence on Device Performance

Abstract

Of major importance in the design of Surface Acoustic Wave (SAW) delay lines or resonators for high precision applications is the selection of crystalline orientation and direction of propagation of surface waves in quartz. A proper selection of the orientation and propagation direction of surface waves in quartz depends upon various propagation characteristics, such as surface wave velocity, electromechanical coupling factor, power flow angle, surface wave attenuation in vacuum and in presence of gaseous loading, diffraction and beam steering losses, temperature and stress dependences of surface wave velocity (or phase delay). Both static and dynamic temperature induced effects are of importance. In addition, various types of stress distributions in the resonator volume which can affect the device performance include intrinsic and thermal stresses in electrode films, bonding and mounting stresses and externally applied acceleration (or vibration) induced stresses in the crystal substrate. While most of the wave propagation characteristics are determined by solving linear equations of motion and are well understood, temperature and stress induced effects on the surface wave propagation are studied from the solution of equations of motion for small dynamic fields superposed on a bias and important advances in our understanding are currently being made. A review will be presented of the recently obtained results and comparison of propagation characteristics of several orientations with the standard ST-cut will be made. It will be shown that there are compelling evidences to infer that some of these singly and doubly rotated orientations will lead to improved performance of SAW devices, albeit, will require tighter tolerance on the orientation angles than we are used to with the widely used ST-cut.