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 resist (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. The interest for PMMA is considerable also because it has been reported in literature that beyond a given threshold fluence (that in turn depends on the ion) the resist turns negative: i.e. the cross linking rate becomes higher than the scission rate. No explanation has been reported 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 fluences, the second ones at high fluences. ii) While at low fluence there is a good agreement between the experimental results and a simple equation based on random scission model, with increasing fluence one observes larger and larger deviations from this equation. These results will be discussed and compared with the existing literature reports.