The radiation chemistry of poly(methyl methacrylate) polymer resists

Abstract

Poly(methyl methacrylate) (PMMA) films of approximately 2.5 μm have been irradiated to various doses using Al x rays. The effect of this irradiation is shown by monitoring the variation in intensity of the Raman vibrational bands of the major constituent species of PMMA, as a function of dose. It is shown that at low doses the molecular weight is reduced as a consequence of main chain scission arising from abstraction of the ester group. This substantiates the previously reported increase in dissolution rate in this dose regime. However, at larger doses it is shown that a cross‐linking reaction generated primary C=C bonds between chain molecules at end‐group sites previously activated by the scission process. This is believed to increase the molecular weight of the polymer resist at larger doses and explains the previously reported drop in the resist dissolution rate at large x‐ray doses. The Raman data suggest that scission and cross‐linking proceed concurrently in response to incident radiation. A model to predict the dissolution rate of the resist as a function of dose based on this dependence of cross‐linking on scission is presented. The results generated from this model are compared to our previously reported experimental resist dissolution data to substantiate conclusions drawn from the Raman results.