Ejection Regimes in Picosecond Laser-Induced Forward Transfer of Metals

Abstract

Laser-induced forward transfer (LIFT) is a 3D direct-write method suitable for precision printing of various materials, including pure metals. To understand the ejection mechanism and thereby improve deposition, here we present visualizations of ejection events at high-spatial (submicrometer) and hightemporal resolutions, for picosecond LIFT of copper and gold films with a thickness 50 nm ≤ d ≤ 400 nm. For increasing fluences, these visualizations reveals the fluence threshold below which no ejection is observed, followed by the release of a metal cap (i.e., a hemisphere-shaped droplet), the formation of an elongated jet, and the release of a metal spray. For each ejection regime, the driving mechanisms are analyzed, aided by a two-temperature model. Cap ejection is driven by relaxation of thermal stresses induced by laser-induced heating, whereas jet and spray ejections are vapor driven (as the metal film is partly vaporized). We introduce energy balances that provide the ejection velocity in qualitative agreement with our velocity measurements. The threshold fluences separating the ejection regimes are determined. In addition, the fluence threshold below which no ejection is observed is quantitatively described using a balance between the surface energy and the inertia of the (locally melted) film. In conclusion, the ejection type can now be controlled, which allows for improved deposition of pure metal droplets and sprays.

Impact of a high-speed train of microdrops on a liquid pool

“…Only recently has the focus shifted to the impact of much smaller drops, with a radius of a few tens of micrometres, corresponding to the typical size of drops coming from, e.g., an inkjet nozzle (Basaran, Gao & Bhat 2013;Driessen et al 2013). Microdrop impact is highly relevant for many rapidly developing applications, such as immersion lithography (Keij et al 2013), extreme ultraviolet (EUV) lithography (Klein et al 2015) and 3D printing (Gibson, Rosen & Stucker 2010;Pohl et al 2015), spray painting and spray coating. Very recently it has been found that the phenomena for microdrops impacting on a solid surface are similar to those of larger (millimetre-sized) impacting drops .…”

[INVITED] Laser-induced forward transfer: A high resolution additive manufacturing technology

“…Other groups used thicker films [17] or longer pulse durations [76,77] to perform the transfer of liquid metal droplets from a solid film. They successfully printed conductive lines, but the thermal effects induced by the high energy deposition required to melt such thin film lead to the generation of large amount of melted debris around the printed structures.…”

Additive and Photochemical Manufacturing of Copper

“…The ejection momentum is the most important factor for transferring the target material from the donor layer to the acceptor layer in LIFT34. A sacrificial layer, called the dynamic release layer, is usually applied between the carrier and the donor layer to increase the momentum during LIFT.…”