Laser ablation and materials response effects in semiconductors and metals for subnanosecond pulses

Ablation of materials by short laser pulses is of interest in the areas of thin film deposition of semiconductors and superconductors, polymer etching, and in micro-machining. Ablation by excimer nanosecond pulses has been successfully employed in industrial and scientific applications. However, the need for greater control over the ablated material has spawned interest in the effects of shorter pulses. We have conducted a systematic investigation of ablation in the picosecond and femtosecond regimes. Experiments and theoretical calculations have been performed on the effects of short pulse, sub-nanosecond, laser irradiation of silicon and gold. Thresholds and rates for material removal are determined using a numerical simulation code. Experimentally, damage thresholds and material removal rates are determined by scanning electron microscopy and optical emission from the irradiated surface. Good quantitative and qualitative agreement is attained between simulated and experimental results for pulse durations ranging from nanoseconds to femtoseconds. Simulations show systematic behavior, with maxima, in the thickness of material removed as a function of pulse duration at fixed energy. The numerical code is demonstrated to be stable down to a 5-fs pulse duration. Vaporized material loss, liquid thicknesses, and maximum surface temperatures are interpreted in terms of thermal energy partitioning between solid, liquid, and vapor during the absorption and redistribution of the laser pulse as a function of time. Simulations show a significant change in material response at 100 fs as a consequence of thermal partitioning. The lasers used for the experiments are Ti:sapphire based systems having a fundamental wavelength at 770 nm and second harmonic at 385 nm. Pulse durations range from 55 fs to 7 ns.