Electroacoustic interaction between SAW and vibration modes of high-aspect-ratio electrodes built using LIGA-UV techniques on singly rotated lithium niobate wafers

Abstract

Abstnrcl-It has been shown theoretically and demonstrated experimentally that shear horizontal surface waves can exist when the surface of an isotropic substrate is perturbed by a strong corrugation, for instance consisting of deep grooves etched in the substrate, whereas these waves cannot exist without this perturbation. In this work, we analyze the behavior of surface acoustic waves with general Polarization propagating under a periodic array of metallic electrodes with large aspect ratio, deposited over a piezoelectric substrate. In this case, we had previously predicted the existence of generally polarized higher order surface modes. We have constructed high-aspectratio interdigital transducers (IDT's) on Y+128 lithium niobate using electroplating techniques. We have observed the existence of higher order surface modes using electrical measurements of the admittance of the IDT's. The predicted and experimental results agree reasonably well, although more work is required to fully understand the properties of these higher order surface modes of high-aspect -ratio IDT's. I. INTRODIJCXIONSurface Acoustic Wave (SAW) devices are widely used in telecommunicabon for frequency filtering operations. Most of these devices uqes thin electrodes to excite and guide surface waves at the top of piezoelectric substrates such as Quartz, Lithium Tantalate or Lithium Niobate. It is well known that surface waves (Rayleigh, Leaky-SAW, etc.) are affected by thc mass loading due to the-electrode gratings deposited atop the propagation surface. Previous works [ 1,2] have shown the possibility to excite and guide on an isotropic substrate more than one surface mode (shear horizontal) under very thick aluminum gratings (electrode heighv'acoustic wavelength ratio greater than 25%), allowing a multimodal response of SAW devices based on such principle. In a recent theoretical work [3,4], we have analyzed the interaction betwecn eigenmodes of thick electrodes and surface wave propagation. A generally polarized wave was assumed to propagate under metallic periodic gratings, and two different calculation approaches were implemented to simulate the electroacoustic response of such devices, respectively based on the combination of Finite Element Analysis (FEA) and Boundary Integral Methods (BIM) [5,6] and on pure FEA computations. No other assumptions than propagation in the sagittal plane are being made. Various piezoelectric substrates have been considered as potential candidates, including Y+128 lithium niobate, Y+36 lithium tantalate and ST quartz. As the electrode height is gradually increased, different waves are found to be guided under the grating, exhibiting either shear horizontal, sagittal or even general elastic polarization.In this paper, we report on experimcnts that have been performed using Y+128 lithium niobate wafcrs in the intent to show the existence of these higher order modcs. Periodic nickel gratings have been realized using deep etch UV lithography and electroplating techniques. Single and two port SAW resonators hav...