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.
Laser-Induced Forward Transfer (LIFT) of aluminum films was performed using a 7 ns Nd:YAG laser operating at 1064 nm. Aluminum films of 200 nm and 1 micron thickness were supported on glass substrates prior to transfer. Films were irradiated at the interface between the film and donor substrate using contact and non-contact configurations. Direct contact occurred between film and acceptor in contact mode, and non-contact mode used a gap of the order of tens of micrometers. Three transfer regimes were observed in contact mode- 1) single droplets or a ring-like structure with dimensions similar to or smaller than the laser spot size at low fluences, and 2) localized material transfer near the center of the laser spot above a threshold value of laser fluence, and 3) spattered material spreading outside of the laser spot. In non-contact mode at fluences below the contact mode spatter threshold, the transferred spot consisted of small droplets until reaching the spatter regime. The ring structure in contact mode is interpreted in terms of flow of molten aluminum resulting from Marangoni flow. The LIFT process observed in non-contact mode is interpreted in terms of evaporation at low fluences and phase explosion at high fluences.