Imaging of laser-generated ultrasonic waves in silicon.

Abstract

In this paper we report the results of experiments that we have carried out on single-crystal specimens of silicon using a scanned ultrasonic point-source--point-receiver technique. Transient waves are generated thermoelastically using the focused beam of a Q-switched neodymium-doped yttrium aluminum garnet laser, which is scanned across one face of the disk-shaped sample, and the waves are detected with a small-aperture lead zirconate-titanate piezoelectric transducer mounted on the opposite face. We present the results in the form of gray-scale scan images, which display the spatial and time dependence of the radiated wave forms. These scan images contain clearly defined structures due to singularities in the wave field associated with various wave arrivals, including single-pass longitudinal and transverse waves, multipass sequences involving mode conversion on reflection and head waves between the longitudinal and one or both of the transverse modes. A pronounced anisotropy is observed in the amplitudes of the wave arrivals, which is mainly attributable to anisotropy or phonon focusing of the acoustic energy flux. Source directivity has a significant influence on the amplitudes of the longitudinal wave arrivals. The thermoelastic generation mechanism favors the radiation of vertical shear-(SV) type displacements, and for some experimental configurations this leads to a strong slow-quasitransverse-wave signal and the almost complete extinction of the fast-quasitransverse-wave signal, while in other cases the reverse is obtained. Monte Carlo simulations based on ray constructs account well for the various wave arrivals and for the focusing effects. The changes in the scan images arising from the presence of surface damage are interpreted, thereby demonstrating the potential usefulness of scan imaging as a tool for surface characterization.