Experimental observation of higher order surface acoustic modes in high aspect ratio electroplated nickel electrodes on Y+128 lithium niobate

The interaction of eigenmodes of thick electrodes and surface waves is analyzed. A generally polarized surface wave is assumed to propagate under a metallic periodic grating with a large thickness, on a piezoelectric substrate. Two different approaches have been implemented to simulate the electroacoustic response of such devices, respectively a combination of finite element analysis (FEA) and boundary integral methods, and pure FEA computations. No other assumptions than propagation in the sagittal plane were made, and the polarization of the modes was identified. A multimodal surface wave behavior is observed, and either transversally, vertically or generally polarized modes are found. Experiments have been performed using Y+128 lithium niobate wafers. Periodic nickel gratings have been constructed using deep etch UV lithography and electroplating techniques. SAW resonators have been designed and implemented, exhibiting a period of 20 μm (acoustic wavelength close to 40 μm). Nickel electrodes have been grown up to 15 μm thick, and the response of the devices have been measured using both a network analyzer and a heterodyne optical sensor. The responses show several contributions with large amplitude arising under the SSBW frequency, as predicted