Acoustic wave sensors and their technology

In the past two decades, acoustic-wave devices have gained enormous interest for sensor applications. The delay line device, where a transmitting and a receiving interdigital transducer are realized on a (piezoelectric) substrate is the most common structure used. The sensitive part is the surface between the two transducers. By placing the device in the feedback loop of an amplifier, an acoustic-wave oscillator is formed with properties such as inherent high sensitivity, high resolution, high stability and a frequency output signal which is easy to process. A very interesting development is the large amount of wave types now available for sensor applications. Sensors have been published using Rayleigh waves, Lamb waves, Love waves, acoustic plate modes, and surface transverse waves (STW). Each of these wave types have their special advantages and disadvantages with respect to sensitivity, stability, usability in liquids or gases, and fabrication complexity. For the fabrication of the acoustic-wave devices, planar technologies are used, which will be discussed in the paper. Examples will be given of gas sensors, biochemical sensors in liquids, viscosity and density sensing and high-voltage sensing. A comparison of the usability of the different wave types will be presented. (C) 1998 Elsevier Science B.V.