A pulsed excimer laser or flashlamp-pumped dye laser is used to melt 1 to 5 µm Au layers. The size of feature which can be filled by laser planarization is limited because of the finite melt duration, but the basic goal of levelling local surface topography is accomplished.
Laser reflow of thicker films reveals fundamental differences between the excimer laser and the flashlamp-pumped dye laser, resulting from the order-of-magnitude difference in pulse duration. The excimer pulse is short compared to the thermal diffusion time of the metal layer and this can create voids and craters in the metal above discontinuities in the underlying structure. For Au thicknesses greater than about 3 µm, void formation occurs at fluences at or below that necessary to planarize the film. The dye laser has a planarization threshold similar to that of the excimer but it takes over 60% greater energy to create voids than to planarize a 5 µm film.
A second damage mode, common to both laser sources, occurs where adjacent planarization zones overlap in a patterned area: thin spots can be created by the first planarization pulse which are vaporized by the second pulse.