A surface acoustic wave (SAW) is a type of mechanical wave motion which travels along the surface of a solid material. The wave was discovered in 1885 by Lord Rayleigh, and is often named after him. Rayleigh showed that SAWs could explain one component of the seismic signal due to an earthquake, a phenomenon not previously understood. These days, these acoustic waves are often used in electronic devices. At first sight it seems odd to use an acoustic wave for an electronic application, but acoustic waves have some particular properties that make them very attractive for specialized purposes. And they are not unfamiliar—many wristwatches have a quartz crystal used for accurate frequency generation, and this is an acoustic resonator though it uses bulk acoustic waves rather than surface waves.

A basic SAW device Fig. 1 consists of two interdigital transducers (IDTs) on a piezoelectric substrate such as quartz. The IDTs consist of interleaved metal electrodes which are used to launch and receive the waves, so that an electrical signal is converted to an acoustic wave and then back to an electrical signal.

A basic advantage is that acoustic waves travel very slowly (typically 3000 m/s), so that large delays are obtainable. The IDT geometry is capable of almost endless variation, leading to a wide variety of devices. Starting around 1970, SAW devices were developed for pulse compression radar, oscillators, and band-pass filters for domestic TV and professional radio. In the 1980s the rise of mobile radio, particularly for cellular telephones, caused a dramatic increase in demand for filters. New high-performance SAW filters emerged and vast numbers are now produced, around 3 billion annually.

Fig. 2 shows a SAW travelling along the plane surface of a solid material. As the wave passes, each atom of the material traces out an elliptical path, repeating the path for each cycle of the wave motion.

The atoms move by smaller amounts as one looks farther into the depth, away from the surface. Thus, the wave is guided along the surface. In the simplest case (an isotropic material), the atoms move in the so-called sagittal plane, i.e. the plane which includes the surface normal and the propagation direction.

For electronic devices, we need to generate the SAWs from an electrical input signal, and then use the SAW to generate an electrical output signal. The conversion process (electric to acoustic, or acoustic to electric) is called ‘transduction.’ To explain this, we first have to consider piezoelectricity, which is a property of many solid materials. In a piezoelectric material there is a mechanism which offers coupling between electrical and mechanical disturbances. Hence, application of an electric field sets up mechanical stresses and strains. Conversely, a mechanical stress due to pressure, for example, gives an electric field, and hence a voltage.

Piezoelectricity occurs in many materials but there is a primary requirement that the material must be anisotropic, so that its properties depend on the orientation relative to the internal arrangement of the atoms. Usually, this means that crystalline materials must be used. The commonest materials for SAWs are crystals of quartz, lithium niobate or lithium tantalate, which are all piezoelectric. In these crystals the SAW motion is similar to that of the isotropic case described earlier, though with the difference that the wave now has an electric field associated with it. Another important factor is because the material is anisotropic, the SAW properties depend on the orientation at which the substrate has been cut from the original material, so this must be specified. Examples of cuts are shown in Fig. 3.

SAW designers normally use ‘standard’ orientations known to give good SAW properties. An example is 34° Y-X quartz, meaning that the SAW propagates in the crystal X-direction, on a plate with the surface normal rotated 34° from the Y-axis. Rotated Y cuts of quartz in this region give parabolic frequency temperature characteristics, and hence provide excellent temperature stability. The turnover temperature may be varied by adjusting the cut angle. Many of these rotated Y cuts are given special names such as ST for 42.75°, CT for 38°, AT for 35.25°. As for isotropic materials, the waves are non-dispersive (velocity independent of frequency), and the attenuation can be very low.

Piezoelectricity is a great help for transduction. If an electric field is applied to the surface, corresponding stresses are set up which travel away from the source in the form of SAWs. The easiest method is to use a set of interleaved electrodes alternately connected to two bus bars, as in Fig. 1. The left transducer is launching the waves. When a voltage is applied, the gaps between electrodes have electric fields and, via the piezoelectric effect, mechanical stresses.

The fields and stresses alternate in sign because of the alternating connections of the electrodes, and the stresses act as sources of surface waves. If the frequency is chosen such that the SAW wavelength equals the transducer pitch, the waves generated by subsequent gaps are all in phase and therefore reinforce each other. For a given voltage, a longer transducer will give a larger wave amplitude. The transducer on the right is the same structure but used to receive the waves, i.e. to give an output voltage in response to an incident wave. It operates in a reciprocal manner to the launching transducer, so a longer transducer will give a larger voltage for a given SAW amplitude.

Performance is largely governed by the choice of substrate material. Quartz has weak piezoelectricity, which limits the fractional bandwidth to around 4%. On the other hand, it has excellent temperature stability, with ST-X orientation giving 10 ppm frequency stability over a +/- 20°C range. Lithium niobate is the opposite, giving larger bandwidths (20 %) but poorer temperature stability. The third common material is lithium tantalate, which is intermediate in both respects. The operating frequency is limited by fabrication techniques.

In production, the narrowest linewidth possible is around 0.3 µm with the i-line (365nm) UV photolithography typically used in the industry; this corresponds to a quarter-wavelength, giving a maximum frequency of typically 3 GHz. The effectiveness of these devices is also due to the fact that the wave propagates in an almost ideal manner, with very little attenuation, dispersion or diffraction. Table 1 below shows some basic characteristics of popular materials used in SAW device fabrication.

Fig. 5 shows a scanning electron microscope (SEM) photo of typical IDT fine geometry detail. In this case, the width of each line is approximately 0.5 µm.