ION-BEAM EFFECTS ON THE SURFACE AND ON THE BULK OF THIN-FILMS OF POLYMETHYLMETHACRYLATE

Poly(methyl methacrylate) (PMMA) is a prototype polymer for positive r esist (scission rate higher than cross linking rate). Notwithstanding its importance the literature studies on the chemical effects induced by energetic beam irradiation on this polymer are relatively scarce. T he interest for PMMA is considerable also because it has been reported in literature that beyond a given threshold fluence (that in turn dep ends on the ion) the resist turns negative: i.e. the cross linking rat e becomes higher than the scission rate. No explanation has been repor ted till now in literature for this behaviour. In this paper we report a study on the molecular weight distribution (MWD) of thin PMMA films irradiated with energetic beams. In addition we have also performed a study of the effects induced by the bombardment on the surface of the polymer by using the XPS (X-ray photoelectron spectroscopy) technique . The main results are: i) There is a strong evidence of contemporary scission and cross linking events. The first ones predominate at low f luences, the second ones at high fluences. ii) While at low fluence th ere is a good agreement between the experimental results and a simple equation based on random scission model, with increasing fluence one o bserves larger and larger deviations from this equation. These results will be discussed and compared with the existing literature reports.