Experimental Measurements and Modeling of Aluminum Reflection Gratings on YZ LiNbO3 for OFC SAW Sensors

Abstract

Lithium niobate has been recently used for orthogonal frequency coded (OFC) SAW temperature sensors (Puccio, D, et. al., 2006), due to its high sensitivity to temperature and high reflectivity. The OFC technique uses multiple reflector banks; each bank having a different local center frequency determined by OFC design. Devices are currently fabricated with aluminum reflectors having frac12 wavelength period at the local reflector of a given chip. In order to increase the device operating frequency for a given electrode line resolution, harmonic operation of the reflector has been studied. Because of lithium niobate's high coupling coefficient, efficient reflection can be obtained for 1-wavelength period electrodes, corresponding to second harmonic operation. When used in conjunction with harmonically operated transducers, the device operating frequency can be increased for a given photolithographic line width resolution. In order to accurately predict fundamental and second harmonic behavior of these sensors at varying normalized metal thicknesses and varying mark to pitch ratios, the extraction of reflectivity and grating velocity is essential. Research has been conducted on fundamental and second harmonic reflectivity on YZ LiNbO3, using analysis and data extraction techniques similar to that presented by P.V. Wright (Wright, PV, 2000). Data has been obtained over normalized metal thickness ranging from 0.4% and 4% and mark to pitch ratios between 0.2 and 0.9. The data has been studied in both the time and frequency domain and has yielded reflectivity comparable to fundamental operation. Experimental results of grating reflection and velocity on YZ-LiNbO3 is presented in this paper. Fundamental and second harmonic reflectivity is reported versus mark to pitch ratio and normalized metal thickness, and these results are used to define the equivalent circuit parameters used in a transmission line model. Given the extracted reflectivity data, the COM model can then be used to predict reflector performance used in OFC devices. Fundamental and second harmonic OFC SAW devices are fabricated at 500 MHz and results of predicted and measured device performance are compared