Large-Scale Periodic Modification of Reactive Multilayer Morphology by Direct Laser Interference Patterning of Growth Substrates

Reactive materials are a combination of two or more phases that can react exothermically in response to an external stimulus. The reaction behavior is determined by the transport of mass and heat and by the release of the latter. Therefore, the micro- and nanoscale morphology of the reactant phases plays a crucial role. A common arrangement of reactants is the sputter-deposited multilayer stack, where the bilayer thickness at a given overall composition is the main design parameter.<br/>Deviations from the ideally flat binary layer stacking would provide an opportunity to modify the reaction behavior, but are usually not achievable by simply modifying the deposition process parameters. In this work, we present a method to modify the micro- and nanoscale morphology of magnetron sputtered Ni/Al multilayers using micrometer-scale, large area periodic surface patterning of the growth substrate.<br/>Line structures with different periodicities have been created on thin Copper substrates by means of picosecond Direct Laser Interference Patterning (DLIP). Electropolishing has been utilized in a subsequent process to remove sharp asperities inherent to the laser processing, resulting in nearly ideal, smooth sinusoidal surface profiles. In the deposited Ni/Al multilayer stack, which retains the line structure even after detachment from the substrate, we found different changes in the layer morphology, depending on the structure period and depth. Shadowing effects during deposition caused by the substrate topography resulted in pores at distinct locations across the samples. In addition, the substrate pretreatment led to a local change in the interface roughness between the individual layers of the multilayer stack as well as to a periodic modulation of the bilayer thickness. It is discussed how the well-defined large-scale distribution of defects and the modified morphology can be used to quantitatively understand how pores inhibit heat and mass transport and thus affect the self-propagating reaction.