Femtosecond laser generation of microbumps and nanojets on single and bilayer Cu/Ag thin films

Abstract

The formation mechanisms of microbumps and nanojets on films composed of single and doubleCu/Ag layers deposited on a glass substrate and irradiated by a single 60 fs laser pulse are investigatedexperimentally and in atomistic simulations. The composition of the laser-modified bilayers is probedwith the energy dispersive X-ray spectroscopy and used as a marker for processes responsible for themodification of the film morphology. For the bilayer with the top Ag layer facing the laser, the increasein fluence is found to result in a sequential appearance of a Ag microbump, the exposure of the Cuunderlayer by removal of the Ag layer, a Cu microbump, and a frozen nanojet. The Cu on Ag bilayerexhibits a partial spallation of the top Cu film, followed by the generation of surface structures thatmainly consist of Ag at higher fluences. The experimental observations are explained with atomisticsimulations, which reveal that the stronger electron-phonon coupling of Cu results in the confinementof the deposited laser energy in the top Cu layer in the Cu on Ag case and channelling of the energyfrom the top Ag layer to the underlying Cu layer in the Ag on Cu case. This difference in the energy(re)distribution directly translates into differences in the morphology of the laser-modified bilayers. In allsystems, the generation of microbumps and nanojets occurs in the molten state. It is driven by thedynamic relaxation of the laser-induced stresses and, at higher fluences, the release of vapor at theinterface with the substrate. The resistance of the colder periphery of the laser spot to the ejection ofspalled layers as well as the rapid solidification of the transient molten structures are largely defining thefinal shapes of the surface structures.