Efficiency aspects in processing of metals with high-repetition-rate ultra-short-pulse lasers

Abstract

Production time and quality are main concerns in competition of laser technologies in microfabrication with other diverse methods. Ultra-short pulse lasers can offer extraordinary precision on a nanometer scale when laser fluence is close to the ablation threshold. High pulse energies or high repetition rate are usually proposed to be used for an increase in productivity of the system. We discuss experimental and modeling results on effects of both laser and material parameters to the ablation threshold, energy coupling and heating of the workpiece in case of metals. Optimal focusing of the laser beam facilitates an enhancement in the material removal rate but the comparatively large beam spot does not allow achieving a high lateral accuracy. High repetition rate is favorable for the ablation threshold. The threshold decreases with irradiation due to defect formation and modification of the surface causing better energy coupling to material. On the other hand, considerable portion of energy from the laser pulse is converted to heat even when ultra-short pulses are applied. The heat is dissipated in surrounding material causing rise in temperature of the workpiece. Plasma which is formed in picosecond time domain well absorbs laser energy in the whole spectral range and affects an energy coupling to the workpiece. Thermal management of the specimen could be a problem at high repetition rates because of the laser energy wasted in the bulk. Balancing between the rate of energy introduction and accompanying alterations in material and surrounding requires new strategies for effective use of the laser energy.Production time and quality are main concerns in competition of laser technologies in microfabrication with other diverse methods. Ultra-short pulse lasers can offer extraordinary precision on a nanometer scale when laser fluence is close to the ablation threshold. High pulse energies or high repetition rate are usually proposed to be used for an increase in productivity of the system. We discuss experimental and modeling results on effects of both laser and material parameters to the ablation threshold, energy coupling and heating of the workpiece in case of metals. Optimal focusing of the laser beam facilitates an enhancement in the material removal rate but the comparatively large beam spot does not allow achieving a high lateral accuracy. High repetition rate is favorable for the ablation threshold. The threshold decreases with irradiation due to defect formation and modification of the surface causing better energy coupling to material. On the other hand, considerable portion of energy from the laser...