Characteristics of Reflected Bulk Acoustic Waves in Rotated Y-Cut Quartz

Abstract

An analysis of the reflection of hulk acoustic waves from the hottom surface in a 90-propagating rotated Y-cut quartz reflected bulk wave (KBW) device is presented. It is shown that the reflected waves from the bottom surface consist of a shear bulk acoustic wave and an inhomogeneous electric surface wave. It is demonstrated that the inhomogeneous electric surface wave radiates energy away from the region where the hulk wave is incident. This energy is then radiated into another set of shear hulk acoustic waves. As a result, the totally reflected beam is smeared and becomes wider than the receiving trans- ducer. The actual energy loss associated with the inhomogeneous elec- tric surface wave is calculated using power balance equations. Good agreement between experimental and theoretical device insertion loss is demonstrated. HE REFLECTION and refraction of elastic waves from a piezoelectric crystal-vacuum boundary is a problem of recurring interest. Very recently, there has been a renewed interest ( l)-(5) in utilizing reflected bulk acoustic waves (RBW) to transmit energy in quartz mi- crowave acoustic devices. RBW's are excited by an in- terdigital transducer (IDT), beamed into the substrate re- flected off the crystal's bottom surface. Since they are redirected to the upper surface they can be intercepted by an output IDT. In crystal orientations that support the sur- face skimming bulk wave (SSBW), RBW's are considered (5) to be a higher frequency extension of SSBW's. They are obtained by exciting the IDT at a frequency higher than that required for SSBW operation and therefore rep- resent a simple series of the complex plate mode spectrum (2). 131. Studies (l), 141, (5) show that RBW's offer the possibility of creating a new class of planar bulk wave devices which have some distinct advantages over SAW or SSBW devices, or both. These advantages include a higher frequency of operation, a broader bandwidth, and the capability of tailoring the device frequency-tempera- ture characteristics by changing transducer separation and crystal thickness.