Laser-induced surface structures at polymer surfaces irradiated by ns-UV-laser in water confinement and in air

Laser-induced ripple structures formation at photoresist films (thickness of 700 nm) is investigated in water and in air using a partial polarized Excimer laser beam (wavelength of & lambda; = 248 nm, pulse duration of & tau; = 25 ns). Unlike the LIPSS (Laser-induced periodic surface structures) generated in air, the laser-induced corrugated ripples have a nondirectional structure. A minimal laser fluence of - 11 mJ/cm2 is required for the laser-induced ripples' formation in water which is two times higher than that of - 5 mJ/cm2 in air. The laser fluence required for the ripples' formation on the photoresist is decreasing with the increased laser pulse numbers that indicates the incubation effects. The height of the ripples increases with the applied laser pulse numbers. The maximal height of the ripples can reach to about 180 nm. The width of the ripples increases with the laser pulse numbers and saturates at about 450 nm. FIB (Focused ion beam) cross sections indicate that the ripples are still developed on the photoresist substrate, and the residual thickness of the substrate is about 500 nm. The bubbles generated by laser beam ablation change the laser bean path and then affect the laser transmission. The morphological characteristics of the corrugated structures suggest that the mechanisms are related to the hydrodynamic instabilities of the melted photoresist during the thermal process of laser irradiation, the bubbles' pulsation and the scattering of the fast moving small bubbles induced by the collapsing of big bubbles. This study may help to understand the mechanisms of laser fabrication under water confinement better and promote the potential applications in laser micromachining techniques.