Surface-acoustic-wave devices

(electronics) Any device, such as a filter, resonator, or oscillator, which employs surface acoustic waves with frequencies in the range 10⁷-10⁹ hertz, traveling on the optically polished surface of a piezoelectric substrate, to process electronic signals.

View more Technology videos

Devices which employ surface acoustic waves (SAW) in the analog processing of electronic signals with frequencies in the range 10⁷–⁹ Hz.

Surface acoustic waves which contain both compressional and shear components in phase quadrature, propagating nondispersively along and bound to solid surfaces, were discovered by Lord Rayleigh in the 1880s. As an example, earthquakes furnish sources for propagating these waves on the Earth's surface. It is of importance for electronic applications that if the solid is a piezoelectric material, the surface acoustic energy is complemented by a small amount of electric energy. This electric energy provides the physical mechanism for the coupling between conventional electromagnetic signals and propagating SAW. The coupling is attained by means of inter-digital transducers (IDT). SAW devices have led to a versatile microminiature technology for analog signal processing in the frequency range 10⁷–10⁹ Hz. Notable devices include bandpass filters, resonators, oscillators, pulse compression filters, and fast Fourier transform processors. Application areas include the color television consumer market, radar, sonar, communication systems, and nondestructive testing. See also Piezoelectricity.

In the basic arrangement, a piezoelectric substrate, often crystalline quartz, has a polished upper surface on which two transducers are deposited. The input transducer is connected, via fine bonded leads, to the electric source through an electrical matching network. The output transducer drives the load, usually 50 ohms, through another electrical matching network. Because these transducers are bidirectional, they lead to devices with at least 6 decibels (dB) loss even in the passband. The unwanted acoustic waves are absorbed by terminations at the ends of the piezoelectric substrate. A metal baffle serves to isolate electromagnetically the two transducers.

The transducers consist of a set of metal interdigital electrodes, each a few hundred nanometers thick, fed from two bus-bars (see illustration). For this transducer arrangement the period p of the interdigital electrode structure is constant and equals one surface acoustic wavelength λ₀ at the center of frequency ƒ₀ of the response. The width of the metal electrodes is typically p/4, being 100 micrometers at 10⁷ Hz and 1 μm at 10⁹ Hz. The electrode overlap distance w is also constant and defines the acoustic beam width, which is typically 40 wavelengths. See also Transducer.

Interdigital construction of surface acoustic-wave transducer. x is a band-pass filter characteristic.

*Mg = magnesium; Al = aluminum; He = helium.

The 100-μm electrodes are readily fabricated by using techniques standard to the semiconductor integrated circuit industry of metallization: photoresist, masking, and chemical etching. The 1-μm electrodes require more sophisticated processing techniques. These include conformable optical masks and x-ray lithography coupled with sputter etching by radio-frequency and ion-beam methods for even finer resolutions. See also Integrated circuits.