Surface morphology and porosity induced by swift heavy ions of low and high stopping power on PMMA thin films

Abstract

We report on surface modification of poly(methyl methacrylate) (PMMA) thin films deposited onto silicon wafers induced by fast ions in a wide range of electronic stopping powers [14 keV/nm (2.1 GeV Bi), 2.2 keV/nm (17 MeV Au) and 0.019 keV/nm (2 MeV H)]. Marked differences were observed on surface morphology induced by low and high dE/dx ions. The effect of the 2 MeV H beam on the PMMA surface was a weak smoothing, whilst for the heavier Bi and Au ions, pronounced roughening occurs with development of porosity and other surface features in a broad range of spatial frequencies. For the heavy ions, surface changes develop in basically two regimes. At the single ion regime at low fluences, most of the changes can be attributed to sputtering and flow of material in individual ion impacts. At the track overlap regime, porosity emerge in a pattern of holes, with an areal density not directly related to the ion fluence, but still spread uniformly across the film. A good correlation between the input of energy and the changes in surface morphology was obtained when (dE/dx)1.85 was used as the scaling factor.