Optimization of reflector and transducer properties for surface acoustic wave devices on 128° LiNbO₃

摘要

Aluminium grating structures used for reflecting surface acoustic waves (SAW) on 128° LiNbO3 are studied in this dissertation. The properties of gratings are obtained from simulated and measured frequency responses of tailored test structures. In the simulations, Green's functions are used to characterize the piezoelectric substrate, the finite element method (FEM) is applied for calculating the fields in the metallic electrodes, and the boundary element method (BEM) is employed at the interface. The majority of the simulated responses are obtained with a rigorous two-dimensional tool for finite structures.The reflection and transmission coefficients of the grating are extracted applying time gating to the test structure responses. Both their amplitudes and phases are evaluated. The former is used for obtaining the reflectivity of the reflectors. Furthermore, energy comparisons serve to address the attenuation inside the gratings, and, for short reflectors, provide the means to estimate the performance of the gratings. A number of analysis methods are used for the extraction of parameters. The quantitative results, presented in the framework of the coupling-of-modes terminology, are given as a function of the electrode geometry.The observed great difference in the reflectivity of narrow short-circuited electrodes and that of wide floating electrodes is exploited in novel unidirectional structures introduced. The great advantage of the unidirectional schemes proposed is the large critical dimension of the structures, allowing the use of standard optical lithography up to the frequencies 2.5-3 GHz. In particular, low-loss and wideband unidirectional transducers are demonstrated for 2.45 GHz.