Surface topography development of thin polystyrene films under low energy ion irradiation

Morphological changes of spin-coated thin polystyrene (PS) films due to low energy Xe+ bombardment in the energy range from 660 eV to 4 keV are investigated. These surface modifications are characterised by means of Atomic Force Microscopy (AFM). No significant topographical change between pristine and irradiated film surfaces is found for ion energy higher than 1.5 keV in the ion fluence range between 4x1014 and 4x1016 cm-2. An increase of surface roughness is however obtained after ion bombardment for energy lower than 2 keV at the highest ion fluence. Moreover, a strong change of the morphology is observed after an irradiation with Xe+ for energy less than 1.25 keV, and ion fluences higher than 5x1016 cm-2. The surface roughness increases and well-defined features appear. Their evolution is followed as a function of the fluence in the 5x1016-2x1017 cm-2 range for 1 keV Xe+ irradiation. ToF-SIMS analyses in area bombarded in such conditions reveal deep modifications of the surface composition and structure, indicating the formation of a Carbon Black form. The energy threshold observed for the development of surface morphology features is explained on the basis of the balance between surface damage and sputtering induced by the primary ion beam