Evaluation of piezoelectric ceramic substrates for ultrasonic bulk wave filters and resonators using pulse interference method

In recent years, market demand has creared a need for high reproducibility in the frequency characteristics of piezoelectric ceramic filters and resonators that use the thickness extension mode. In order to increase this reproducibility, it is necessary mainly to improve accuracy in controlling the substrate thickness, and to reduce variations in elastic properties of substrate materials prior to the completion of products. The pulse interference method using longitudinal waves has been applied to the evaluation of piezoelectric ceramic substrates. A good correlation has been obtained between the interference frequencies, measured with this method, of substrates immediately after polishing and the frequency characteristics of completed products. It has been shown that this method is extremely useful for improving productivity of ultrasonic bulk-wave filters and resonators whose frequency characteristics directly depend upon the longitudinal wave velocities of substrate materials and their thicknesses.