Disturbance of adhesion upon ablation of thin films by laser pulses

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The effect of IR and UV laser pulses on thin metal and composite films on glass substrates as a function of the energy density is studied. Upon irradiation by ∼300-ns laser pulses with a nonuniform energy-density distribution over the laser-beam cross section, the characteristic regions can be distinguished on the film surface. The dimensions of these regions correlate with the energy distribution in the beam and correspond to the evaporation, melting, and damage conditions caused by thermal stresses. For a uniform energy-density distribution over the laser-beam cross section and a pulse duration of ∼20 ns, the adhesion of metal and composite films to glass was disturbed due to induced thermal stresses without substrate melting. The threshold laser-energy densities required for disturbing the adhesion of titanium, titanium nitride, zirconium, niobium, and stainless-steel films on glass substrates are measured. Numerical estimates of the surface temperature and thermal stresses caused by heating show that the film adhesion to a substrate can be overcome by expending a small fraction of the energy, while most of the energy of thermal stresses goes to the formation of cracks and the kinetic energy of escaping film fragments. It is suggested to use pulsed laser radiation to roughly estimate the adhesion of metal and composite films to glass substrates.