Surface acoustic waves (SAW) have been employed in a large number of different devices mainly for frequency control applications. Today the annual production of SAW devices world-wide is more than 200 million pieces. Every TV-set manufactured today uses at least one SAW filter in the IF stage of the receiver. Moreover, there are a lot of other applications as well, and the number of devices produced is increasing rapidly. The main reasons for that are their small sizes, low costs and high performance due to utilising the technological improvements of the well-developed microelectronic technique. Furthermore, they are highly reliable and reproducible due to their solid-state properties and the relatively good understanding of the basic physical processes. From a physical point of view, SAW are mechanical waves travelling along the surface of a solid. So called interdigital transducers (IDT) overlaying as a specially shaped metallic structure on the surface of a piezoelectric crystal usually are employed for the excitation. An IDT is made by a photolithographic process and is the key element for all SAW devices. By using the inverse piezoelectric effect such a structure excites SAW with a velocity of about 10(5) times smaller than the velocity of electromagnetic waves in vacuum. For this reason it is possible to produce SAW devices in a frequency range from 10 MHz up to some GHz. Depending on the crystal cut and the direction of the travelling wave different types of waves do exist. Thus SAW can be distinguished in different types depending on the particle displacement (linear, elliptical or circular polarisation), on the attenuation function from the surface into the volume of the substrate, on the coupling to an electric field or not and the material system where waves are travelling (monocrystaline, crystal with or without piezoelectric behaviour, layered structures, membrane). For frequency controlling devices there are almost always used Rayleigh-type waves with an elliptical particle displacement. The most frequently used materials are Quartz (SiO2) or Lithiumniobate (LiNbO3), respectively, in monocrystaline form. The mainly used type of frequency filters is a delay line with two IDT's and a travelling path for the SAW between. The behaviour of such a construction is determined by material parameters, the shape of the two IDT's an the physical conditions for the travelling SAW. There is a great variety of different filter types meeting the demands of the electronic industry. The limits of today's filters are approximately in a range from 10 MHz up to 2 GHz with a fractional bandwidth up to 20% and bandpass characteristics corresponding to the customers specifications. The second main type of SAW devices is the resonator with one or two IDT's and reflecting gratings next to them. The case of resonance means that only standing waves are existing and the sharp resonance peak in the frequency is only depending on the geometrical layout of the exciting IDT and the gratings in connection with the propagation conditions for the SAW. Devices used for signal processing must have a high stability and have to be insensitive to variations of environmental conditions. However, this is not possible. It is known that SAW devices react to changes in physical conditions in the propagation path by changes both their amplitude and phase velocity. A variation of the phase velocity leads to a frequency shift and the change in amplitude can be measured immediately. So it is possible to make use of such changes in the electric characteristics for "feeling" changes in physical environmental conditions. SAW sensors have the advantages of extremely high sensitivity and an output signal in the form of a frequency shift which enables them to be directly connected to a digital signal processing without any analogue/digital conversion. Moreover, there is a possibility to construct passive sensors where the sensor element itself does not need any energy supply. The principle is based on the big dynamic range for SAW excitation and detection. If one of the IDT's of a SAW device is connected to an antenna then it possible to excite SAW via a RF signal transmitted from an interrogating equipment and received by the antenna. The SAW excited in this way travels along the surface and is influenced by environmental conditions. Thus the response of the device retransmitted to the interrogation equipment by the same antenna contains an information about the environment (e.g. temperature, pressure, etc.). As an example, the SAW structure can be a resonator. An interrogating RF signal can create a standing wave in this resonator element. The resonance frequency in the moment of measurement is influenced by the environmental conditions. After the interrogating signal is switched off the resonator oscillates with attenuated oscillations with the actual resonance frequency. This attenuated oscillation is retransmitted via the antenna to the interrogation equipment and can be processed. Another possibilities are to use reflectors on the surface of the element or to design the delay line in the form of a tapped delay line. In a similar manner it is possible to construct passive identification tags. They also do not need any own energy source and can be interrogated in a wireless manner via a RF signal as well. When the principles are combined in integrated or hybrid form there are a lot of new possibilities to get information from places which are not or very hard accessible otherwise. Hereby the sensor together with the amount of a physical parameter simultaneously gives an identification signal and so the place can be identified where the measurement occurs. A whole string of examples shows the capability of this concept.
