Anisotropic elastic characterization of surfaces from 2 MHz to 20 GHz

A range of techniques is now available to characterise the elastic properties of surfaces and surface layers. Applications include measurement of stress, subsurface damage, layer thickness and bonding, and the elastic properties of thin layers. At the bottom of the frequency range, group velocity measurements can be made by timing the transit of pulses between knife edges. Over a range of higher frequencies acoustic microscopy can be used to measure the interference between specular and surface waves, using the V(z) technique with cylindrical (line-focus-beam) lenses or transducers. For dispersive surfaces V(f) techniques can be used, either in an acoustic microscope with V-groove lenses, or in an ultrasonic microspectrometer with a spherical-planar pair. Surface Brillouin spectroscopy enables the frequency range to be extended to 20 GHz, with acoustic wavelengths less than 300 nm and sensitivity to surface layers considerably thinner than that. All of these techniques can give azimuthal resolution on anisotropic surfaces. The near field technique of ultrasonic force microscopy is able to give information about anisotropic structures on surfaces with nanometre resolution. The user is now in a strong position to choose the technique most appropriate to the material which he wishes to study or characterise, and a growing range of applications is becoming established. (C) 1998 Elsevier Science B.V.