- Conference date: 30 April–3 May 2012
- Location: New Mexico, USA
Metal films on transparent substrates are widely used in industry for mask production, and lasers are frequently applied for their patterning. The quality of patterning is limited by fundamental phenomena taking place close to edges of the laser ablated area. We investigated transformations in metal films experimentally and numerically during their irradiation with the laser beam with fluences above the ablation threshold. The beam of a nanosecond pulse laser tightly focused by aspheric cylindrical lens to a stripe ( ) was applied for the back-side ablation of the chromium thin film on a glass substrate. Two phases, the initiation and growth of periodical structures in a thin metal film, were distinguished. The prolonged holes were ablated with a single laser pulse. Ridges of the resolidified metal with increased but not uniform thickness were always present on edges of the cleaned area. Instabilities during the ablation process forced the molten metal in the ridges to agglomerate into droplets with the predictable periodicity. The average ratio of the droplet formation period to the radius of the resolidified ridge was found to be close to 9.0, which was a clear evidence of the Rayleigh-Plateau instability along the molten ridge. Use of overlapping laser pulses to extend the cleaned area initiated self-organization of chromium on the glass substrate. The droplets on ridges were starting points for formation of ripples from the metal film by irradiation with partially overlapping laser pulses. Growth of a highly periodical structure of metal on glass was observed in a certain range of process parameters, depending on the pulse energy and the beam overlap. The ripples were located with a period of 2.5-4 μm parallel to the direction of the laser spot shift. The initial droplets and later the ripples were acting as heat sinks that cooled down the metal in their close proximity. The COMSOL finite element modeling revealed that 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. After each laser pulse the process lasted as long as the liquid phase existed. The time when chromium remained in liquid phase was estimated to be as long as 100 ns. The distance that chromium in molten film traveled because of Marangoni convection before it solidified has been evaluated by multiplying Marangoni speed and lifetime of the liquid phase and it corresponded to the half of a ripple period in a chromium thin film. In principle, it is possible to achieve long and smooth edges of laser ablated areas in metal films if droplet formation due to Rayleigh-Plateau instability can be suppressed.
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