In-situ and ex-situ ripples formation on copper thin films induced by nano and picosecond pulsed lasers

Abstract

Laser induced periodic surface structure (LIPSS) process on copper thin films by nanoand picosecond pulsed lasers was analyzed by scanning electron microscope (SEM) and timeresolved reflectivity (TRR) methods. The 500nm and 1000nm thickness of copper thin filmswere deposited on silicon and glass substrates by magnetron sputtering technique. Differentperiodic surface micro and nanostructures of copper thin films (thickness of 1000nm) on siliconsubstrate were observed post-mortem by SEM. Namely, a wrinkling microstructure and classicalripple with period spatial near wavelength of laser (approximately 266nm) were induced bynanosecond pulse laser at fluence 100 mJ/cm2. The conical nanostructure and ripple microstructure were induced by picosecond pulse laser. TRR method permits the real timemonitoring of melting and/or surface morphology changes. The reflectivity signals have shownthe increase of the removal copper thin film as increasing the laser fluence and of the number oflaser shots. Under the same laser conditions (fluence and number of shots), copper thin films onglass were removed easier than on silicon case due to the intrinsic thermal conductivity ofsubstrate. An obtained TRR signal of wrinkling formation was induced by nanosecond pulselaser. TRR method is suitable for monitoring LIPPS in the nanosecond but not in the picosecond. However, SEM analyses give more information and details of the structure changes in the nanoand picosecond. We can assume that mainly thermal 'drift' forces are responsible for wrinkling microstructure formation in the nanosecond regime, and photonic forces for periodic surface nanostructure formation in the picoseconds one.