Maskless Patterning of Metal Outflow in Alternating Metal/Ceramic Multiple Nanolayers by Femtosecond Laser Irradiation

In this work, solid-state metal transport from internal metal nanolayers onto the surface of metal/ceramic nanomultilayers (NMLs) has been directed in a controlled way by femtosecond (fs) laser irradiation and subsequent low-temperature thermal annealing. Laser-irradiation-induced modifications of the NML microstructures and stress states can be limited within the first few top nanolayers due to the focused laser energy input at the metal/ceramic interface by exploiting the local plasmonic effect. Accompanied laser peening can further refine the crystallites and introduce compressive stress at the laser-irradiated region, which reduces the activation energies for vacancy formation and migration of metal atoms in the nanoconfinement. Patterned Cu surface nanostructures (outflow) appear selectively along the laser path after air annealing at temperatures down to 360 degrees C. For the solid-state diffusion of Cu in confinement, in-plane metal transport along the Cu-AlN interfaces is much faster than the outward short-circuit diffusion of Cu across the AlN barrier layers. Localized metal outflow is accompanied by the collapse and sintering of the remaining AlN barrier layers, under the influence of the acting capillary forces, which may further accelerate the metal transport. This laser-induced maskless patterning of metal outflow is applicable not only in Cu/AlN NMLs but also in Ag/AlN NMLs, assisted by subsequent low-temperature annealing.