Surface rippling induced in thin films by a scanning laser

The variation in the thickness of thin films produced by a scanning laser was calculated and compared with experiment. Fluid flow in the molten-metal film was induced by surface-tension gradients generated by the local heating of the laser beam. The surface of the film became rippled, limited by capillary effects and the constraint of the solid substrate. A Nd:YAG scanning laser was used to melt and alloy 2-μm-thick films consisting of Pb and Sn layers. The shape of the thickness profile measured with a two-beam interferometer agreed with the calculated shape. Measurements of the surface composition indicated that alloying occured and influenced the magnitude of the surface relief. The surface topography of laser alloying and recrystallizing thin films as discussed in the recent literature, is reviewed on terms of the surface-tension-gradient mechanism.