Nanostructure and microstructure of laser-interference-induced dynamic patterning of Co on Si

We have investigated the nanostructure and microstructure resulting from simultaneous ns laser irradiation with deposition of Co films on Si(001) substrates. The spatial order and length scales of the resulting nanopatterns and their crystalline microstructure were investigated as a function of film thickness h and laser energy density E using a combination of atomic force, scanning electron and transmission electron microscopies. The results could be classified into two distinct categories based on the laser energy density used. It was observed that the thickness-dependent E required to melt the Co film (ECo) was lower than for Si (ESi) primarily because of the higher reflectivity of Si. Consequently, for energy densities ECo < E1 < ESi that preferentially melted the Co film, spatially ordered nanoparticles were formed and were attributed to capillary-driven transport in the liquid phase. The ordering length scale corresponded to the interference fringe spacing Λ and the microstructure was primarily Co metal and a metal-rich silicide phase. The native oxide layer played an important role in minimizing the Co-Si reaction. For laser energy densities E2 ≥ ESi, spatially ordered patterns with periodic length scales L < Λ were observed and resulted from interference of an incident laser beam with a beam transmitted into the Si substrate. These nanopatterns showed one- as well as two-dimensional spatial ordering of the nanostructures. The microstructure in this laser energy regime was dominated by silicide formation. These results suggest that various nanostructures and microstructures, ranging from nearly pure metal to a Si-rich silicide phase, can be formed by an appropriate choice of the laser energy simultaneously with film growth