(593f) Finite Element Modeling of Surface Acoustic Wave (Saw) Sensor Responses

Surface acoustic wave sensors detect chemical and biological species by monitoring the shifts in frequency of surface acoustic waves generated on piezoelectric substrates. These devices are conveniently small, relatively inexpensive and quite sensitive. Considerable attention has been focused on the development of response models to understand the characteristics of surface acoustic waves generated in SAW devices. Recent advances in sensors and wireless communication systems indicate the need for high performance SAW devices often operating in high frequency (GHz) range. Most of the analytical techniques require simplification of second order effects such as backscattering, charge distribution, diffraction and mechanical loading. However, these effects become significant for SAW devices operating in the high frequency range. Finite element approach has proven to be a viable option to model wave propagation in SAW devices operating in MHz-GHz range (Ippolito, 2003 and Xu, 2002).

A 3-D finite element model has been developed to investigate the device characteristics such as frequency response using transient analysis of a two port delay line SAW device. A micron sized piezoelectric substrate with dimensions (400μm width x 1600μm propagation length x 500μm depth) was simulated to gain insights into the sensor response. Two IDT finger pairs in each port were defined at the surface of X-cut, Y-propagating LiNbO3 substrate. The fingers were defined with periodicity of 40 μm and aperture width of 200 μm. The IDT fingers were modeled as mass-less conductors and represented by a set of nodes coupled by voltage degrees of freedom (DOF). A total of approx. 80,000 elements (more than 100000 nodes) were generated. The model was created to ensure higher node density at the surface and throughout the middle of the device to study the different modes of surface acoustic waves and the use of tetragonal elements with 4 DOF ensured the same. Three DOF's provided the displacements in the longitudinal (x), normal (y), and the shear horizontal (z) directions and a fourth for the voltage. The structure was simulated for a total of 100 nanoseconds (ns), with a time step of 1 ns. The excitation of the structure was provided by applying an impulse function on the transmitter IDT fingers.

Different substrates of LiNbO3, LiTaO3, and quartz were modeled to study the effect of material properties on wave propagation in SAW devices. Contour plots of displacements in time domain show the propagation of surface waves and also illustrate the mechanical effect of the receiving port. The presence of electromagnetic feed-through is verified by comparing models with and without the receiving IDT's. Relaxing the assumption of using a mass-less conductor for the inter-digitated transducer (IDT) fingers can give insights into the effect of electrical and mechanical loading, which are usually significant for devices operating in the high frequency range.

The use of SAW devices in sensors can involve the use of various novel sensing layers such as metal nanowires, nanotubes and carbon nanotubes for detection of target analytes. With the incorporation of such sensing layers, the frequency response of the SAW devices becomes dramatically different. A priori knowledge of the frequency response can lead to improved designs and considerable time reduction in fabricating efficient devices. Modification of the delay path to include different arrangements of arrays of nanowires/nanotubes indicates a shift in frequency response of the SAW device. Optimizing the arrangements of the same can result in higher frequency shifts leading to improved sensor response. Similarly, using different arrangements of IDT's (eg. hexagonally placed IDT's) to generate waves simultaneously along different delay paths can lead to superposition/amplitude change, the magnitude of which depends on the exact IDT location and the number of finger pairs in each IDT. 3-D and simplified finite element models involving manipulation of the design parameters can be used to maximize the sensor response. Overall, our modeling work results in improved understanding of sensor response.

References:

Finite element analysis of second order effects on the frequency response of a SAW device. Guanshui, Xu. Ultrasonics Symposium, 2000 IEEE 1 (22-25) 2000,187 ? 190.

A 3-dimensional finite element approach for simulating acoustic wave propagation in layered SAW devices. Ippolito, S.J.; Kalantar-Zadeh, K.; Powell, D.A. and Wlodarski, W. Ultrasonics Symposium, 2003 IEEE 1 (5-8) 2003, 303 ? 306.