Fabrication of Sub-Micron Surface Structures on Copper, Stainless Steel and Titanium using Picosecond Laser Interference Patterning

Picosecond direct laser interference patterning is investigated theoretically and experimentally for the bulk metals copper, stainless steel and titanium. In the past, results on thermal modelling for nanosecond irradiation were reported for pitches in the range of several micrometers. When nanosecond pulses are utilized, the smallest possible pitch on metals is limited by the thermal diffusion length. In this case, the laser patterning process is predominantly determined by heat conduction mechanisms, and pitches less than 1 μm are not feasible. Picosecond laser pulses allow values below 1 μm pitch size on metallic surfaces. The modelling and simulation of DLIP is based on the two-temperature-model and was carried out for a pulse duration of 35 ps at 515 nm wavelength and a laser fluence of 0.1 J cm-2. The subsurface temperature distribution of both electrons and phonons was computed for periodic structures with a pitch of 0.8 μm. The increase in temperature rises for a lower absorption coefficient and a higher thermal conductivity (larger absorption depth) . The distance, at which the maximum subsurface temperature occurs, increases for a small absorption coefficient. High absorption and low thermal conductivity minimizes internal heating and give rise to a pronounced surface micro topography with pitches smaller than 1 μm. Periodic line-like surface structures were produced using two interfering beams on copper, stainless steel and titanium surfaces with a pitch of 0.7 μm using a Yb:YAG-Laser with 515 nm wavelength and a pulse duration of 35 ps.