CONFORMATIONS OF POLY(METHYL METHACRYLATE) AND ITS DEGRADED FORMS UPON RADIATION

The radiation-induced decomposition of syndiotactic (st) and isotactic (it) poly(methyl methacrylate) (PMMA), containing CH2C(COOCH3)(CH3) units, to yield irradiation products (PMMA(d)) resulting from hydrogen abstraction from alpha-methyl or methylene groups to form main chain (M=) or side chain (S=) double bonds is studied by conformational analysis. Relative minimum energy conformations of the st- and it-isomers of PMMA as free molecules are studied. The isomers of PMMA(d) have relative minima in an overall linear (L) chain (assumed to be the form retained in the solid) as well as a global minimum in a bent (B) form (assumed to contribute to forms in solution). The impact of the change of a tetrahedral carbon atom to a trigonal carbon atom upon double bond formation on the conformational energy is fundamental to the understanding of the results. The experimental results are explained by the following theoretical observations. In reactions involving only overall linear conformations, side chain double bond formation is favored because of the resulting large increase in steric repulsion between polymer units in main chain double bond formation. In reactions proceeding to bent conformations, however, the decreased steric interaction yields both main and side chain double bond formation within 6.3 kcal/mol for the st-PMMA(d) isomer and 2.2 kcal/mol for the it-PMMA(d) isomer. Reactions on surfaces are assumed to be constrained to the overall linear conformation of the starting material, PMMA. Prevention of both M= and S= degradation products is desired for microlithographic applications. Increased solubility arising from main chain scission (MCS) is desired, and both M= and S= for that reason are undesirable with respect to resist sensitivity. It is proposed that the troublesome S= process can be attenuated or eliminated by replacing alpha-CH3 with alpha-CF3, thus enhancing the distribution of products toward MCS.