Microdroplet deposition by laser-induced forward transfer

A unique. form of Laser-Induced Forward Transfer (LIFT) has been developed that is capable of depositing single micrometer-sized droplets. LIFT was performed using a 7 ns Nd:YAG laser operating at 1064 nm. One micron films of aluminum and nickel supported on glass donor substrates were used as samples. Films were irradiated at the interface between the film and donor substrate using the standard LIFT technique. At fluences slightly above the melting threshold, single droplets were transferred to the acceptor substrate, with deposit sizes between 1 and 2 microns. This is significant since the laser beam diameter (> 12 microns) is much larger than the deposited droplets. SEM images of the original donor films after laser irradiation indicated a re-solidified melt pool with a raised bump at the center, the point of ejection of the transferred droplets. The physical origins of the droplet formation and transfer are unclear, but appear to be a result of the combined effects of surface tension and volumetric expansion during the solid-liquid phase change process.