Acoustoelectric interaction

Figure 3.24 Equivalent-circuit model to describe the acoustoelectric interaction between a SAW and charge-carriers in a film overlay. (Reprinted with permiuion, See Ref. [48]. e 1989 IEEE.)...

Figure 3J6 Measured changes in SAW velocity and attenuation vs metal film thickness, illustrating the acoustoelectric interaction.

It is apparent from Equation 3.36 that the acoustoelectric interaction takes on a particularly simple form when Aa/k is plotted vs Av/vo, with sheet conductivity as the variable parameter as oi increases from zero, a semicircle centered at (Av/Vo, Aa/k) = (-f /4,0) and having radius K /A is traced out [48]. The angular position along the semicircle corresponding to a, is [48]... 

Acoustoelectric interactions enable solution electrical properties to be probed with AW devices. It should be noted that these acoustoelectric interactions can be shorted out using a conductive (e.g., metal) layer between the substrate and the solution for APM and FPW devices. Similarly, for TSM devices, if the grounded electrode is placed in contact with the solution, no acoustoelectric effect should be present. The key parameter that has been monitored is solution conductivity. For example, measurements of AW responses vs conductivity have been reported using TSMs [11,15,31,32] and APMs (see Figure 3.36) [16,33-35]. 

The mechanical and electrical displacements for metalized and free surfaces at liquid-36 YX LiTa03 interface are shown in Figure 4.4a and 4.4b [22]. Most of the acoustic energy is conhned to within one wavelength from the surface of the substrate. When the surface is metalized and electrically shorted, the potential on the surface is zero. In this case, only the normalized displacement (U2) interacts with the liquid loading, and the phenomenon is called mechanical perturbation. If the surface is free and electrically open, then both U2 and normalized electric potential (d>) interact with the adjacent liquid medium (Figure 4.4b). Interactions of d> and the electrical properties of the liquid constitute the acoustoelectric interaction. The influence of both the mechanical and acoustoelectrical interactions on sensor response and material characterization is discussed in the subsequent sections. 

The sensor sensitivity equation for the acoustoelectric interaction can be derived from an extension of the perturbation method of Auld (Section 4.6.1). The electrical properties of liquid are represented by the relative permittivity and conductivity, a. An approximate theory for the acoustoelectric interaction has been derived by Kondoh et al. [21,22,38,48,54-56] assuming a nonconductive liquid as reference. 

Figure 4. Perturbations in a) plate mode velocity and b) attenuation due to liquid entrainment by the APM device surface. Data are for glycerol/water mixtures which only contact the surface between transducers. The solid lines are calculated from a viscoelastic model for the liquid the dashed lines are calculated using a Newtonian model, c) Perturbation in oscillator frequency due to acoustoelectric interactions between APM and ions and dipoles in solution. Solid lines are calculated using best-fit values for the dielectric coefficient of each solvent.

A number of interactions can affect plate-mode propagation characteristics, particularly in a liquid environment. In the following sections, models of several of the important modes of interaction will be outlined and compared to experimental results. These include (1) mass accumulation on the device surface, (2) viscous entrainment of the contacting liquid medium by the oscillating device surface, and (3) acoustoelectric coupling between evanescent plate mode electric fields and the liquid. 

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