2 - Basics of Acoustic Waves and Piezoelectricity

Abstract

This chapter highlights the basic properties of surface acoustic waves and their generation by an interdigital transducer (IDT) located on the free surface of a piezoelectric substrate. The excitation of an IDT on a piezoelectric substrate can lead to the generation of bulk acoustic waves as well as surface waves. Bulk waves incident on the receiving IDT induce voltages. These interfere with the voltage signals due to received surface waves. The resultant voltage due to both sources can degrade the in-band performance specifications on amplitude, phase, or group delay response. Surface acoustic waves can be generated at the free surface of an elastic solid. In the SAW devices, the generation of such waves is achieved by the application of a voltage to a metal-film interdigital transducer (IDT) placed on the surface of a piezoelectric solid. Bulk acoustic waves can be characterized in terms of three modal types of mechanical excitation. One of these is a compressional wave, termed the longitudinal bulk wave, that is polarized in the direction of the acoustic wave propagation vector. The other two wave components, termed transverse or shear waves, have their vibrational modes. The chapter also outlines the acoustic bulk wave excitation by an IDT as a highly undesirable effect in SAW filter design that can cause severe degradation of filter performance and can also limit the fractional bandwidth obtainable with normal IDT structures.