Controlled CO₂ laser melting of silicon

The application of a CO₂ laser to the controlled melting of doped silicon surfaces was studied theoretically and corroborated with previous experimental results. Because of the lack of interband transition at 10.6 μm, energy deposition was dominated by free-carrier absorption, which could be affected by the doping concentration and the dopant spatial distribution. It was shown that for undoped samples, avalanche ionization played an important role in the initiation of the melting process. In most cases, melting could be induced on the surface without concomitant laser breakdown. The resolidified surface remained smooth under scanning electron microscopy examination. The effect of pulse duration on the controlled melting of silicon was also studied in detail. It was found that ultrashort CO₂ laser pulses could melt silicon without breakdown damage over a wide range of intensities and sample impurities. For nanosecond duration pulses, similar intensity ranges were not available, except for highly doped samples.