Piezoelectric boundary acoustic waves: Their underlying physics and applications

This paper reviews physical properties of piezoelectric boundary acoustic waves (PBAWs) and their application to miniature and high performance RF filters. First, basic properties of PBAWs are discussed. It is shown that PBAWs are supported in various structures provided that highly piezoelectric material(s) are employed as structural member(s). For example, PBAWs are trapped near the electrode region in the SiO₂/heavy grating electrode/rot. YX-LN structure. This means that PBAW properties in the structure are independent of the SiO₂ layer thickness. This is a significant advantage for mass production. Rot. YX-LN possesses piezoelectric coupling with the Rayleigh-type PBAW as well as the SH-type one. It is shown that the coupling can be negligible when the device structure is properly designed. The PBAW devices employing SiO₂/Au electrodes/0-15degYX-LN are now being mass-produced. Because of the removed cavity from the chip surface, the packaged device size can be reduced dramatically. The minimum insertion loss achieved is comparable to that of conventional SAW filters in a relatively large device size. The SiO₂ layer is effective in achieving the improved temperature coefficient of frequency.