Pulse energy effects on surface structuring and oxidation of Nb/Ti multilayers

The formation mechanisms of Laser-Induced Periodic Surface Structures (LIPSS) on Nb/Ti multilayer thin films following picosecond laser surface modification were systematically investigated. The multilayers, consisting of fifteen (Nb/Ti) bilayers with a total thickness of 440 nm, were deposited on (100) Si substrates by DC ion sputtering. Dynamic laser patterning in the form of laser-inscribed lines was performed using 15 ps pulses from a Nd:YVO 4 laser operating at 355 nm, with pulse energies ranging from 1.5 to 2.8 µJ. By increasing the pulse energy at constant scanning speeds (5 mm s −1 and 1 mm s −1 ), a sequential evolution of surface morphology was observed: (i) initial melting with cluster formation, (ii) elongation of molten regions and emergence of high-spatial-frequency LIPSS (HSFL), (iii) droplet separation and alignment into low-spatial-frequency LIPSS (LSFL), (iv) sharp boundary formation corresponding to LSFL positions, (v) surface recrystallization, and (vi) material ablation at the highest fluences. Post-irradiation compositional analysis revealed an increased oxygen content in the modified zones compared to the untreated regions, indicating the formation of an ultra-thin oxide layer.