Instability-triggered transformations in thin chromium film on glass under laser irradiation

Metal films on transparent substrates are widely applied for mask production in lithography, and lasers are frequently used for their patterning. Quality of the patterning is limited by fundamental phenomena taking place close to edges of the laser ablated area. We experimentally and numerically investigated transformations in metal films during their irradiation with the nanosecond laser beam with fluences above the ablation threshold. Ridges of the resolidified metal with not uniform thickness were always formed on edges of the cleaned area. Instabilities during the ablation process forced the molten metal in the ridges to break up into droplets with the periodicity predicted by the Plateau-Rayleigh instability. The droplets on ridges were starting points for formation of ripples of metal film by irradiation with partially overlapping laser pulses. The initial droplets and later the self-organized parallel lines of chromium metal were heat sinks that cooled down the metal in their close proximity. Temperature modulation along the laser irradiation spot was high enough to initiate the Marangoni effect which resulted in movement of the molten metal from hot to colder areas.