Page 334 - Microsensors, MEMS and Smart Devices - Gardner Varadhan and Awadelkarim

314 INTRODUCTION TO SAW DEVICES

that are coupled into the guiding layer and then propagate in the waveguide at angles to
the surface. These waves reflect between the waveguide (which is usually deposited from
a material whose density would be lower than that of the material underneath) surfaces
as they travel in the guide above the IDTs. The frequency of operation is determined by
the thickness of the guide and the IDT finger-spacing (Tournois and Lardat 1969). Love
wave devices are mainly used in liquid-sensing and offer the advantage of using the same
surface of the device as the sensing active area. In this manner, the loading is directly
on top of the IDTs, but the IDTs can be isolated from the sensing medium that could,
as stated previously, negatively affect the performance of the device (Du et al. 1996). It
is again important that interfaces (guiding layer, substrate) be kept undamaged and care
taken to see that the deposition process used gives a fairly uniform film at a constant
density over the thickness (Kovacs et al. 1993).
Love wave sensors have been put to diverse applications, ranging from chemical
microsensors for the measurement of the concentration of a selected chemical compound
in a gaseous or liquid environment (Kovacs et al. 1993; Haueis et al. 1994; Gizeli et al.
1995) to the measurement of protein composition of biologic fluids (Kovacs et al. 1993;
Kovacs and Venema 1992; Grate et al. 1993a,b). Polymer (e.g. PMMA) layer-based
Love wave sensors (Du et al. 1996) are used to assess experimentally the surface mass-
sensitivity of the adsorption of certain proteins from chemical compounds. It has also
been shown recently that a properly designed Love wave sensor is very promising for
(bio)chemical sensing in gases and liquids because of its high sensitivity (relative change
of oscillation frequency due to a mass-loading); some of the sensors with the aforemen-
tioned characteristics have already been realised (Kovacs et al. 1993). As is discussed in
the next chapter, the main advantage of shear Love modes applied to chemical-sensing in
liquids derives from the horizontal polarisation, so that they have no elastic interactions
with an ideal liquid. It is also sometimes noticed that viscous liquid loading causes a
small frequency-shift that increases the insertion loss of the device (Du et al. 1996).

9.5 CONCLUDING REMARKS

This chapter should provide the reader with the necessary background to the basic prin-
7
ciples governing waves and SAW devices .
Figure 9.10 summarises the different types of waves that can propagate through a
medium. These are waves that travel through the bulk of the material (Figure 9.10 (a)
and (b)). The compressive (P) wave is sometimes called a longitudinal wave and is well
known for the way in which sound travels through air. On the other hand, the S wave is a
transverse bulk wave and looks like a wave traveling down a piece of string. In contrast,
waves can travel along the surface of a media, (Figure 9.10 (c) and (d)). These waves are
named after the people who discovered them. The Rayleigh wave is a transverse wave that
travels along the surface and the classic example is the ripples created on the surface of
water by a boat moving along. The Love wave is again a surface wave, but this time the
waves are SH or vertical. This mode of oscillation is not supported in gases and liquids,
and so produces a poor coupling constant. However, this phenomenon can be used to a
great advantage in sensor applications in which poor coupling to air results in low loss
(high Q-factor) and hence a resonant device with a low power consumption.

7
Some of the material presented here may also be found in Gangadharan (1999).