Printing dense and low-resistance copper microstructures via highly directional laser-induced forward transfer

Laser-induced forward transfer (LIFT) can be used to print micrometer-scale metallic three-dimensional (3D) structures. However, the structures produced by this method exhibit high porosity and poor electrical properties due to the non-vertical ejection and loose stacking of transfer particles. In this study, we replace the conventional copper (Cu) monolayer donor film with a chromium-copper (Cr-Cu) bilayer film. We demonstrate that this bilayer enhances laser absorption and improves glass-metal adhesion through the spontaneous formation of a CrO_x interlayer. The improved laser absorption reduces the optimal pulse energy required for transfer, while the interlayer stabilizes the transfer process, promoting more vertical ejection of material. This enhanced directionality leads to denser structures, even when the donor and receiver are placed at a larger distance. The resulting structures exhibit a porosity of 4.8 % and a specific resistance 2.9 times that of bulk copper. Cross-sectional electron microscopy is employed to investigate the microstructure and elucidate the mechanisms behind the reduced resistance. Additionally, we demonstrate the application of this 3D printing method in creating high aspect ratio microstructures and repairing open defects on printed circuit boards (PCBs).