An investigation of the influence of thermal process on the electrical conductivity of LIFT printed Cu structures

The electrical properties of copper tracks printed by laser-induced-forward-transfer (LIFT) are typically inferior to the bulk values. Several limiting factors have been indicated such as oxidation, droplet boundaries and grain boundaries. However, the relative contribution of each of these factors has not been determined. To improve the electrical properties of LIFT printed structures, an analysis of various factors is essential. Here, we concentrate on the effect of post-printing thermal treatment on LIFT printed copper structures. A reduction in electrical resistivity by a factor of similar to 3 is obtained resulting in a value lower than 3 times the bulk copper value (1.68 mu Omega.cm). Real time resistance measurements indicate that an efficient thermal annealing can be achieved at 200 degrees C-300 degrees C within a couple of minutes. The morphological changes associated with such thermal treatment were analyzed using HR-SEM and XRD and highlight the role of stress relief, grain growth and formation of new grains primarily at the inter-droplet interfaces.