Page 333 - Microsensors, MEMS and Smart Devices - Gardner Varadhan and Awadelkarim

ACOUSTIC WAVES 313

M2 layer (waveguiding layer - usually
SiO 2)

Figure 9.8 Schematic of a Love wave propagation region and relevant layers

Figure 9.9 Wave generation on Love wave mode devices

The basic principle behind the generation of the waves is quite similar to that presented
in the description of an SH-SAW sensor. The only difference would be the fact that the
Love wave mode would be the same SH-SAW mode propagating in a layer that was
deposited on top of the IDTs. This layer helps to propagate and guide the horizontally
polarised waves that were originally excited by the IDTs deposited at the interface between
the guiding layer and the piezoelectric material beneath (Du et al. 1996). The particle
displacements of this wave would be transverse to the wave-propagation direction, that is,
parallel to the plane of the surface-guiding layer. The frequency of operation is determined
by the IDT finger-spacing and the shear wave velocity in the guiding layer. These SAW
devices have shown considerable promise in their application as microsensors in liquid
media (Haueis et al. 1994; Hoummady et al. 1991).
In general, the Love wave is sensitive to the conductivity and permittivity of the
adjacent liquid or solid medium (Kondoh and Shiokawa 1995). The IDTs generate waves