Effects of multi-wavelength irradiation of polymers on the ablation threshold and ablation rate

The structuring of materials by pulsed UV-lasers is often characterised by a certain ablation threshold and a minimum ablation rate, which both depend on the applied wavelength and the selected material. Such minimum ablation rate can be, parameter dependent, in the range of up to some hundred nm. Below and around the ablation threshold, effects like back-side ablation, compaction, melting, material decomposition or cracking have been observed for the ablation process. For further reduction of the ablation rate, while maintaining a controlled and well defined ablation pattern, multi-wavelength excitation of the material has been applied. With a first laser pulse with a wavelength of λ = 157 nm, the material has been excited. The high photon energy is capable of breaking the molecular bonds. With a second laser pulse, wavelength λ = 193 nm, lattice vibrations and material evaporation will be induced, exploiting the increased absorption within the exited state. The relevant parameter here is the time delay between the two laser pulses, which has been varied in the range of 0 to 150 ns. The experiments have been carried out with the polymers PMMA and PTFE. For the ablation rate it has been observed that ablation rate changes with the time delay between the laser pulses. PMMA shows a minimum between 20 and 80 ns, while PTFE has a local maximum between 40 and 80 ns. The surface temperature has been identified as the effect, causing this dependency. The ablation threshold can be reduced by this coincident irradiation for both materials, when comparing it to single 193 nm processing.