Microstructure formation on metal surface by femtosecond laser and its molecular dynamics simulation

It is generally known that femtosecond laser processing has distinct characteristics such as slight heat affected zone and sharp edge. In the present study, we carried out femtosecond laser irradiation to chromium binary photomasks in air with a rectangular aperture. Some types of surface microstructure (ripples, columns, etc.) were observed in the laser-irradiated area. Microcolumns protruding above the original surface were formed after repetitive irradiation of pulses with energy close to the ablation threshold. In order to investigate the mechanism of microstructure formation, we adopted analytical approach. Laser diffraction through an optical system with a rectangular aperture was analyzed for the estimation of intensity distribution at the focal point. Molecular dynamics (MD) simulation was also conducted for the examination of laser ablation of metal by considering the electron-phonon relaxation. As a result, it was shown that porosity is formed in the vicinity of surface. This porosity deformation is caused by the insufficient energy to remove the material from the surface. It was estimated through the MD simulation that the porosity formation proceeds by repetitive irradiation of pulses with energy close to the ablation threshold and their surrounding grows to microcolumns. The microstructure formation is significantly influenced by irradiation condition such as laser energy and intensity distribution.