Michael Zenou

Laser sintering of copper nanoparticles

Copper nanoparticle (NP) inks serve as an attractive potential replacement to silver NP inks in functional printing applications. However their tendency to rapidly oxidize has so far limited their wider use. In this work we have studied the conditions for laser sintering of Cu-NP inks in ambient conditions while avoiding oxidation. We have determined the regime for stable, low-resistivity copper (< × 3 bulk resistivity value) generation in terms of laser irradiance and exposure duration and have indicated the limits on fast processing. The role of pre-drying conditions on sintering outcome has also been studied. A method, based on spectral reflectivity measurements, was used for non-contact monitoring of the sintering process evolution. It also indicates preferred spectral regions for sintering. Finally, we illustrated how selective laser sintering can generate high-quality, fine line (<5 µm wide) and dense copper circuits.

Laser jetting of femto-liter metal droplets for high resolution 3D printed structures

Laser induced forward transfer (LIFT) is employed in a special, high accuracy jetting regime, by adequately matching the sub-nanosecond pulse duration to the metal donor layer thickness. Under such conditions, an effective solid nozzle is formed, providing stability and directionality to the femto-liter droplets which are printed from a large gap in excess of 400u2009μm. We illustrate the wide applicability of this method by printing several 3D metal objects. First, very high aspect ratio (A/Ru2009>u200920), micron scale, copper pillars in various configuration, upright and arbitrarily bent, then a micron scale 3D object composed of gold and copper. Such a digital printing method could serve the generation of complex, multi-material, micron-scale, 3D materials and novel structures.

Laser Transfer of Metals and Metal Alloys for Digital Microfabrication of 3D Objects

3D copper logos printed on epoxy glass laminates are demonstrated. The structures are printed using laser transfer of molten metal microdroplets. The example in the image shows letters of 50 µm width, with each letter being taller than the last, from a height of 40 µm (s) to 190 µm (l). The scanning microscopy image is taken at a tilt, and the topographic image was taken using interferometric 3D microscopy, to show the effective control of this technique.

Digital laser printing of aluminum micro-structure on thermally sensitive substrates

Aluminum metal is of particular interest for use in printed electronics due to its low cost, high conductivity and low migration rate in electrically driven organic-based devices. However, the high reactivity of Al particles at the nano-scale is a major obstacle in preparing stable inks from this metal. We describe digital printing of aluminum micro-structures by laser-induced forward transfer in a sub-nanosecond pulse regime. We manage to jet highly stable molten aluminum micro-droplets with very low divergence, less than 2 mrad, from 500 nm thin metal donor layers. We analyze the micro-structural properties of the print geometry and their dependence on droplet volume, print gap and spreading. High quality printing of aluminum micro-patterns on plastic and paper is demonstrated.

Supersonic laser-induced jetting of aluminum micro-droplets

The droplet velocity and the incubation time of pure aluminum micro-droplets, printed using the method of sub-nanosecond laser induced forward transfer, have been measured indicating the formation of supersonic laser-induced jetting. The incubation time and the droplet velocity were extracted by measuring a transient electrical signal associated with droplet landing on the surface of the acceptor substrate. This technique has been exploited for studying small volume droplets, in the range of 10–100 femto-litters for which supersonic velocities were measured. The results suggest elastic propagation of the droplets across the donor-to-acceptor gap, a nonlinear deposition dynamics on the surface of the acceptor and overall efficient energy transfer from the laser beam to the droplets.

Digital laser printing of metal/metal-oxide nano-composites with tunable electrical properties

We study the electrical properties of aluminum structures printed by the laser forward transfer of molten, femtoliter droplets in air. The resulting printed material is an aluminum/aluminum-oxide nano-composite. By controlling the printing conditions, and thereby the droplet volume, its jetting velocity and duration, it is possible to tune the electrical resistivity to a large extent. The material resistivity depends on the degree of oxidation which takes place during jetting and on the formation of electrical contact points as molten droplets impact the substrate. Evidence for these processes is provided by FIB cross sections of printed structures.

Temporal pulse shaping for smoothing of printed metal surfaces

Abstract. The surfaces of laser-induced forward transfer (LIFT) printed metal structures show typical roughness characteristic of the metal droplet size (3 to 10u2009u2009μm). Submicron voids are often observed in the bulk of such printed metal structures with consequences on the mechanical strength, chemical resistivity, and electrical conductivity. We present the results of our efforts to reduce surface roughness and bulk voids by controlled laser melting. We have used temporally shaped pulses from a fiber laser tunable in the range from 1 to 600 ns in order to improve the quality of LIFT printed copper and aluminum structures. For the best case shown, roughness was improved from RRMS=0.8u2009u2009μm to RRMS=0.2u2009u2009μm and the relative percentage of the voids was reduced from 7.3% to 0.9%.

Additive manufacturing of metallic materials

Abstract Digital printing of metals is probably the most important element for “functional” additive manufacturing. Printed structures made up of polymers usually lack the required mechanical, electrical, and thermal properties for functional structures and devices. Metal deposition by conventional methods, either by evaporation or by electrochemistry, is quite inadequate for fast built-up, multilayered 3D structures. In this chapter, we describe methods and materials for additive manufacturing of metal by classifying them into two categories. The first category deals with the differences between 2D and 3D additive manufacturing of metal. The second category deals with the metallic phase used for the manufacturing process. The metallic phase includes the following three main phases: metallic particle, metallic complex, or molten metallic droplet.