SMALL AREA PULSED LASER PROCESSING OF ALUMINUM ON SILICON

Single pulse Q-switched Nd:YAG laser irradiation, typically of 100ns pulse duration, has been used to locally alloy small areas (5 (mu)m (--->) 15 (mu)m diameter) of n-type silicon covered with a thin (1000(ANGSTROM)) aluminum film. The primary emphasis of this research was to understand the metallurgical aspects of the laser induced reaction. The alloy regions have been investigated with scanning electron microscopy (SEM) and with an ion microanalyzer operated in the scanning ion microscope mode and in the secondary ion mass spectrometry (SIMS) mode. A simple analytic thermal model was developed to aid in the interpretation of the results by correlating physical properties of the alloy regions to the conditions and pulse parameters of the laser irradiation.; SEM investigations showed that at low incident power a well defined melt pool is formed on the aluminum. As the laser power is increased, the melt pool eventually forms an annular ring. Correlation of the SEM micrographs with SIMS analysis showed that aluminum did not penetrate into the silicon unless the melt puddle showed the annular ring structure.; The ion microanalyzer studies show that the aluminum penetration is, for the most part, bounded by a paraboloid of revolution. Approximately 200(ANGSTROM) below the surface there is a right circular cylindrical region that cuts through the paraboloidal volume. It is approximately 700(ANGSTROM) thick, and has a radius that ranges from slightly less than the melt radius to asymptotically approaching the eutectic radius with increasing power. The right circular cylinder region is shown to be an aluminum-silicon alloy of close to eutectic composition. For the remaining volume bounded by the paraboloid of revolution aluminum penetrates relatively flatly into the silicon at concentrations higher than the equilibrium solid solubility of a few times 10('20) atoms/cm('3). After the flat penetration the aluminum concentration falls off abruptly with a grade constant of the order of 5 x 10('23) atoms/cm('4). The maximum aluminum penetration (typically 1 (mu)m) is much deeper than equilibrium alloying would predict.; Results of this study show that to form the melts required for the observed aluminum penetration, a reflection coefficient change with surface melting must occur. This causes the melt temperature of silicon to be reached throughout the volume. For the time scale of the laser pulses melt temperature is not reduced as predicted by the phase diagram. Aluminum is transported in the melts by liquid diffusion, with evidence that is aided by convection in the melts formed at higher laser power. Cooling of the melt is slowed by the liberation of latent heat energy. This allows the solidification to take place in a quasi-equilibrium state that shows characteristics similar to equilibrium cooling.; Current-voltage measurements show that pn junctions are formed in the laser alloyed regions. The reverse breakdown voltage is in reasonable agreement with calculations based on the diode grade constant and junction curvature.