Coincident multiwavelength irradiation of polymers with UV and VUV laser

Abstract

In this paper the material ablation of polymers with a cascade of two laser pulses of different UV-wavelengths, exploiting the effects of the excited state absorption (ESA), is described. With a first vacuum ultraviolet (VUV) laser pulse with a wavelength of λ = 157 nm, the material is excited at low energy densities. A second, ultraviolet (UV) laser pulse, either λ = 193 nm or λ = 248 nm, is applied with a defined time delay between Δτ = 0 and 200 ns to the first pulse to induce the ablation process. Experiments have been carried out on polymethyl methacrylate (PMMA) and polytetrafluorethylene (PTFE). The effects on the ablation quality, ablation threshold and the ablation rate have been investigated. The focus has been set on the range of low energy densities, aiming for minimum ablation by each single laser pulse. By applying the multi wavelength pulse trains, the ablation quality can be improved, compared to the conventional UV laser machining of such materials. The machining of PMMA with a cascade of λ = 157 and λ = 248 nm does not show the material foaming, which can be observed when applying only λ = 248 nm laser pulses in the low energy regime. Applied on PTFE, the multi wavelength ablation shows improved results when using λ = 157 nm and λ = 193 nm, compared to the pure λ = 193 nm processing. The excited state increases the absorption of the UV laser light in the material, allowing a minimised ablation rate or higher depth resolution. This can be reduced down to the range of some nm (PMMA) or a few ten nm (PTFE). Besides the fluence, the time delay Δτ has been identified as a process parameter to vary the ablation rate. Also this dependency is qualitatively not unique for all wavelength, material and fluence combinations, but most of the experimental data point out an increased sensitivity for time delays between Δτ = 30 and Δτ = 60 ns. This sensitivity correlates with the time resolved temperature course on the surface, which is induced by the excitation pulse.