I. Motivation/Background Focused interdigital transducers (FIDTs) based on concentric wave surfaces can excite surface acoustic waves (SAW) with high intensity, high beam-width compression ratio and small localized area. The excited waves can be utilized to actuate and process smallest possible amounts of fluids on the planar surface of a piezoelectric chip as well as detect small variations in localized regions leading to highly sensitive biosensors. In the present work, we develop a 3-D finite element model to investigate and analyze the wave propagation characteristics as well as the frequency response of a SAW device with FIDTs based on concentric wave surfaces.

II. Computational details The focused SAW (F-SAW) device was constructed by adopting a pair of concentrically shaped FIDTs on piezoelectric substrates such as LiNbO3 and LiTaO3 as shown in Fig 1. The 3-D FE model describes two-port FIDT structures on the surface of YZ LiNbO3 and consists of three finger pairs in each port. The fingers are considered as mass-less electrodes to ignore the second-order effects arising from electrode mass, thereby simplifying computation. The periodicity of the finger pairs is 40 microns and the aperture width of the fingers varies depending on their radial distance from device center as shown in Fig 2. The transmitting and receiving IDT's are spaced 90 microns or 2.25λ apart. The simulated F-SAW device dimensions are 800 microns in propagation length, 500 microns wide and 400 microns deep. The simulated models have a total of approx. 250000 nodes and are solved for four degrees of freedom (three displacements and voltage). The model was created to have the highest densities throughout the surface and middle of the substrate. Two kinds of analysis are carried out along each of the three delay lines: (1) An impulse input of 10 V over 1 ns is applied to study the frequency response of the device and (2) AC analysis with a 5 V peak-peak input and 100 MHz frequency to study the wave propagation characteristics.

III. Results/Discussion The effect of transducer design parameters such as degree of arc (Da), geometric focal length (fL), and wavelength (λ) on the propagation characteristics (displacement and voltage profiles) of F-SAW was studied. The frequency response obtained for an F-SAW was compared to a conventional SAW device with the same wavelength. In comparison with the conventional SAW devices fabricated with uniform interdigital transducers, we find that the focused SAW devices are more sensitive to variations in the focal area instead of the whole delay-line region (Fig. 2). This makes them suitable for application requiring detection or manipulation of localized variations, such as acousto-optic or acousto-electric effects. Additionally, the ability of focused SAWs to efficiently stir and mix smallest possible amounts of fluids, can be employed to enhance micro-agitation in processes such as biological hybridization assays where transport of molecules is otherwise diffusion limited and therefore notoriously slow. Efforts are underway to simulate F-SAWs based on LiTaO3 which would serve as a better biosensor element for liquid phase applications.