Page 353 - Microsensors, MEMS and Smart Devices - Gardner Varadhan and Awadelkarim

ACOUSTIC WAVE PROPAGATION 333

I mode II mode + 1 T III mode
Particle displacement for the first three Love modes

Frequency variation of the particle displacement for the first three modes

Figure 10.9 Displacement modes for Love wave devices (note that z corresponds to x 2 and y
corresponds to X 3, in Figure 10.8)

10.5.3.2 Discussions of the characteristics of the Love waveguiding
materials

It may be noticed from earlier discussions, that the two most important parts of a Love
wave sensor are the overlayer material and the piezoelectric substrate. Our discussion now
focuses on the salient points of the waveguide, particularly with respect to the properties
of the material.
Love waves propagate near the surface of a suitable substrate material when the
surface is overlaid by a thin film with appropriate properties for a guiding layer. An
essential condition for the propagation of a Love wave is that the shear velocity in
the film is less than that in the substrate. Sensitivity to mass-loading is enhanced by
the low density of the film as well as a large difference between the shear velocities.
For a particular guiding-layer material, an optimum layer thickness exists, which results
in maximum acoustic energy density close to the surface and maximum sensitivity to
mass-loading.
Love wave devices incorporating guiding layers of poly(methyl methacrylate) (PMMA)
and sputtered SiO 2 overlaid on single-crystal quartz have been successfully demon-
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strated (Du et al. 1996). PMMA has a density of about 1.18 kg/m and has a shear
acoustic velocity of 1100 m/s (Kovacs et al. 1993; Jakoby and Vellekoop 1998; Du et al.
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1996), whereas sputtered silicon dioxide has a density of about 2.3 kg/m and a shear