UV-laser ablation of polyimide: From long to ultra-short laser pulses

Physical mechanisms of UV-laser ablation of polyimide (Kapton H) are discussed for different laser pulse duration from long (mu s) to ultra-short (ps) laser pulses, Theoretical analysis of experimental data suggests that with long laser pulses the mechanism appears to be thermal. The activation energies for ablation are about 1.5 eV, and typical temperatures lie in the range 1400-1800 K. With ns pulses one can distinguish between the mass loss due to depletion of light volatile species from the bulk of the polymer, and the real ablation. The former is especially important near the ablation threshold. For sub-ps laser pulses ablation may proceed via preferential removal of electronically excited species, Here, decrease in delay time between two subsequent laser pulses may result in a decrease in ablation rate due to saturation and bleaching effects.