Integration of microelectronics and biotechnology requires developing interfaces that are compatible with microelectronics processing methods, and also provide the requisite selectivity and stability when exposed to biological environments. Diamond is especially attractive because, in addition to having good electrical and chemical properties, it is widely considered to be biocompatible, and can be used as a waveguide in layered SAW devices. Additionally, the high density of diamond layer helps achieve higher acoustic wave propagation velocity and realize GHz frequency devices with transducer finger spacing achievable by optical lithography. In the present work, we use 3-D structural and 3-D/2-D fluid solid interaction finite element models to study the wave propagation characteristics in diamond layered piezoelectric surface acoustic wave devices under the influence of a fluid loading.

The structural model is first utilized to study the wave propagation in diamond layered SAW devices. The 3-D FE model describes two-port structures on the surface of LiNbO3 and LiTaO3 substrates. The simulated device 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 is 200 microns. The transmitting and receiving IDT's are spaced 130 microns or 3.25λ apart. The simulated 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. The simulated voltage and displacement profiles obtained at the output IDT's are used to measure wave attenuation and velocity changes and thereby optimize the thickness of the waveguide, i.e. the diamond layer. The optimized waveguide thickness is then used in the fluid-solid interaction FE model to investigate the influence of fluid loading on Rayleigh and SH wave propagation in diamond layered SAW devices and evaluate their use as potential biosensors.