Nonequilibrium Phases and Phase Transitions in the Surface Melt Morphology of Laser Irradiated Silicon

Abstract

The increased use of lasers in material processing is directly related to the coherence of the laser beam. It is the spatial coherence property which allows one to direct and/or focus the modest intensity of laser beams onto small areas of semiconductors, metals or insulators, in such high temperature1 processing applications as annealing and melting, or low temperature2 processing applications, such as catalysis, etching or chemical vapor deposition. The temporal coherence or quasi-monochromaticity of the beam, permits one to initiate state specific effects. The coherence aspect of laser-material processing is often overlooked by many researchers who would wish to regard the laser as little more than a directed energy source or an ultrafast heat-gun. It has become apparent3–5, however, that the coherence of the laser beam is responsible for development of surface patterns in the processed material. These patterns range from random morphologies to highly periodic structures whose spacing is related to the wavelength of light. It is clear that the development of these structures is related to the interference of the incident beam with surface scattered fields associated with surface polaritons or radiation remnants (lateral waves)5. The generation of surface periodic structures or “ripples” can also be viewed4,6 as a stimulated surface scattering process or a stimulated Wood’s anomaly. The specific types of patterns are determined4 by material properties such as surface roughness and dielectric constant and beam properties such as wavelength, coherence, polarization and angle of incidence. Because of the interference effects there is often a significant variation in the surface fields and beam intensities leading to inhomogeneous melting, vapor deposition, etc. In some cases, such as in the production of grating structures, this effect is seen as desirable, but more often, it is regarded as deleterious and efforts are taken to suppress it, usually with a sacrifice of some other aspect of the process. It is clearly desirable to have a fundamental understanding of the influence of beam coherence and polarization on surface processing and to a large extent we have developed such an understanding over the past five years.