Resonant holographic interferometry measurements of laser ablation plumes in vacuum, gas, and plasma environments

Resonant holographic interferometry and dye-laser-resonance-absorption photography have been utilized to investigate the expansion of the laser ablation plumes produced by a KrF excimer laser beam (248 nm) focused onto an aluminum target (-0.1 cm’, 2-6 J/cm’). Plume expansion was studied in vacuum and in background argon gas pressures of 14 mTorr, 52 mTorr, 210 mTorr, 1 Torr, and 35 Ton: The existing theory for the interpretation of resonant interferograms has been extended to account for Doppler shift effects, the diagnostic laser bandwidth, and the selective absorption of the laser beam. Absolute line densities in the range 4.3X1O13-1.OX1O15 cme2 have been measured in the ablation plumes, which imply measured Al neutral densities of up to 1 X 1015 cmm3. The total number of Al neutral atoms in a plume has been measured to be -3 X 1014, which corresponds to a surface etch rate of -1 mn/pulse. Expansion velocities in the range 1.1-1.4 cm/w were measured for the pressures s210 mTorr, while -0.3 cm/p was measured for 1 Torr and -0.08 cm/p was measured for 35 Tom Ablation plume expansion into a 1 Torr rf argon plasma environment was compared with the expansion into a 1 Torr argon gas. The ablation plume appeared to expand and dissipate slightly faster in the plasma.