Mechanism of laser ablation – Ablation process and debris formation

In this paper the mechanism of excimer laser ablation of Si3N4 ceramics is analyzed. An image intensifier with minimum gate time of 3 ns was used for observing time resolved behavior of the plasma plume and the particles produced during ablation process in various ambient gas species and pressures. Absorption and scattering at wavelength of 248 nm and 532 nm were also measured in the plasma plume, respectively.High temperature plasma was produced at delay time around 4~20 ns, and propagate spherically at speeds around 5~15 km/s. The absorption coefficient of the plasma plume at 248 nm was about 3~10 mm−1, which corresponds to the electron densities of the order of 1019 cm−3. It was shown that the removal rate per unit incident laser fluence saturates at higher fluences because the laser-induced plasma absorbs the incident laser beam due to inverse Bremsstrahlung. Raleigh scattering of 532 nm from the decomposed particles of 5~20 nm in diameter initiated at delay time around 1 μs. At delay time later than 10 μs, the particle flow backwards towards the specimen surface was observed, which was considered to be caused by a cooling wave to produce the debris, and then were followed by bouncing at the surface and staying above the surface for several hundreds micro seconds. Deposition rate of the debris was as large as 20 nm/pulse in O2 atmosphere. The debris formation is shown to be closely related with the blast wave theory. The amount of the debris was reduced in lighter gases like He or reduced atmospheric pressure.In this paper the mechanism of excimer laser ablation of Si3N4 ceramics is analyzed. An image intensifier with minimum gate time of 3 ns was used for observing time resolved behavior of the plasma plume and the particles produced during ablation process in various ambient gas species and pressures. Absorption and scattering at wavelength of 248 nm and 532 nm were also measured in the plasma plume, respectively.High temperature plasma was produced at delay time around 4~20 ns, and propagate spherically at speeds around 5~15 km/s. The absorption coefficient of the plasma plume at 248 nm was about 3~10 mm−1, which corresponds to the electron densities of the order of 1019 cm−3. It was shown that the removal rate per unit incident laser fluence saturates at higher fluences because the laser-induced plasma absorbs the incident laser beam due to inverse Bremsstrahlung. Raleigh scattering of 532 nm from the decomposed particles of 5~20 nm in diameter initiated at delay time around 1 μs. At delay time later than ...