Relaxation of craters produced by ion bombardment on PMMA as a function of temperature

We report on measurements of relaxation times of craters and rims induced by 20 MeV Au ions bombarding PMMA targets at temperatures near the glass transition. PMMA thin films were bombarded at grazing angles and at low fluences (∼109 cm-2). The targets were kept at the irradiation temperature for a certain period of time before quenching to room temperature, where imaging of the surface was performed ex situ by scanning force microscopy (SFM). The average dimensions of the craters and rims as a function of annealing time t were extracted from SFM images and fitted to an exponential law [∝ exp(-t/τ)], where τ is identified as a characteristic relaxation time. Values of τ for the hillock tails were around 33 ± 7 s at 94 deg. C and increased to around 18 h at 76 deg. C, reflecting the great changes in viscosity near the glass transition temperature of the polymer. Some parts of the rims surrounding the middle of craters and the crater walls relax much more slowly, because of the severe chemical modification of the chains in the core of the impact