RADIATION DEGRADATION OF POLY(METHYL METHACRYLATE) IN THE SOFT-X-RAY REGION

The molecular weight distribution change has been measured for the photoresist poly(methyl methacrylate) [PMMA] after in-vacuo exposure to monochromatic soft x-rays from the Canadian Synchrotron Radiation Facility [CSRF]. The experimental changes in the molecular weight distributions derived from gel permeation chromatography [GPC], were compared to a simple Monte Carlo simulation model that assumes random main chain scission. Using this model a scission radiation chemical yield of G(x-ray)vacuo(S) = 1.28 +/- 0.10 at room temperature was found to give the best fit at a photon energy of 621 eV. This value is similar to values reported previously in the literature using electron beam and gamma-ray sources, but significantly larger than those reported for fast neutrons, alpha-particles, or energetically charged particles. It was found that in this soft x-ray energy regime, that degradation of PMMA involves primarily a random scission process of the main chain. The results of a least-squares fit of this soft x-ray G(S) data and all available literature values from other radiation sources, to the linear energy transfer [LET] dE/dx are discussed. (C) 1993 John Wiley & Sons, Inc.