Optimal cut of lithium niobate with suppressed Rayleigh-type mode for application in resonator SAW filters

Abstract

Results of resonators constructed on rotated YX lithium niobate (LiNbO 3 ), to provide minimum coupling of the spurious Rayleigh-type mode are reported. The resonator employs non-leaky waves which result when the metal grating's electrodes are sufficiently heavy to suppress attenuation of leaky waves. The coupling of two SAW modes propagating in rotated YX cuts was estimated as function of cut angle and electrode thickness, with Al or Cu as electrode material. The optimal orientation, 19°YX for Al grating or 20°YX for Cu grating, is compared with previously reported 15°YX cut. If the electrode thickness is optimized to suppress the spurious mode, in 15°YX the coupling of this mode grows fast with variation of frequency or electrode thickness because of close proximity between the velocities of two modes. In 19°YX cut the resonance of the spurious mode is located in the middle of the interval between the resonance and anti-resonance of non-leaky SAW mode. The spurious mode is made less sensitive to variations of frequency or metal thickness and provides better performance of resonator filters. This conclusion is confirmed experimentally.

Fractional Bandwidth up to 24% and Spurious Free SAW Filters on Bulk 15°YX-LiNbO3 Substrates Using Thickness-Modulated IDT Structures

“…Alternatively, shear-horizontal (SH) type SAWs on the well-known rotated bulk YX cuts of lithium niobate (YX-LiNbO 3 ) substrates are promising for enabling the low-cost mass production of wideband SAW filters due to their ultra-large K 2 of up to 30% and commercial availability from wafer vendors [ 19 ]. K. Hashimoto and coworkers have reported on SAW devices on a Cu-grating/15°YX-LiNbO 3 structure [ 20 , 21 , 22 ], setting a milestone for fabricating ultra-wideband SAW filters that are low-cost and that have a simple fabrication process.…”