Period dependence of laser induced patterns in metal films

Periodic fringed patterns with four periods in the range 1.8-10.2 μm have been produced in continuous Ag films that have thicknesses of 14.6 nm and 19.5 nm by exposing a phase mask to single pulses of an excimer laser operating at 193 nm. The films were patterned either as-grown or after homogeneous exposure to the same laser beam. For fluences above the threshold, the films undergo liquid-state dewetting that, from low to high fluences, leads to their break into holes, fingers or elongated features and finally to isolated nanoparticles irrespective of the period, thickness or fluence. The period determines the range of fluences to achieve the different morphologies since the temperature profile across the pattern depends on the period due to the existence of significant lateral heat flow across the pattern. The maximum temperature achieved at the intensity maxima/minima sites thus decreases/increases as the period decreases, leading to solid-state dewetting at regions around the intensity minima; the shorter the period, the higher this type of dewetting. These regions eventually overcome the melting temperature for the shortest period and intermediate fluence, leading to the complete transformation of the films. Finally, the initial film morphology (discontinuities or holes) rather than thickness plays an essential role in the level of transformation at fluences around the threshold.