TRACK OVERLAP REGIME IN ION-IRRADIATED PMMA

Chemical and physical effects induced by bombardment of nearly monodis perse Poly(methyl methacrylate) (PMMA) with energetic ions (He+) at 20 0 keV have been studied. Bond scission is the main results of low flue nce ion irradiation but, at high fluence there is a regime transition of PMMA from positive tone resist behavior to negative tone. The ion-i nduced chemical reactions in the macromolecular film give rise to gas evolution and chemical changes in the solid residue. The gas formed du ring the bombardment has been studied by means of a mass spectrometer, while the solid polymer has been characterized by measuring the molec ular weight distribution (MWD) and the absorption coefficient in the N ear Infrared (NIR), visible, and ultraviolet regions. X-ray Photoelect ron Spectroscopy (XPS) has been performed to learn what happens at the surface of the polymer after bombardment. The main result of these st udies is that at fluence higher than 10(13) cm(-2) both chemical and p hysical effects follow a nonlinear behavior due to the transition from a single-track regime to a track overlap regime. This transition is e xplained in terms of progressive chemical change of the irradiated pol ymer from the pristine chemical structure to a new one depleted of the ester pendent groups. (C) 1998 John Wiley & Sons, Inc.