Properties of positive resists. I. irradiation‐induced degradation and sensitivity of certain methyl methacrylate copolymers

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Abstract
Poly(methyl methacrylate), poly(methyl methacrylate‐co‐2‐hydroxyethyl methacrylate), poly(methyl methacrylate‐co‐itaconic acid), poly(methyl methacrylate‐co‐maleic anhydride), poly(methacrylic acid‐co‐itaconic acid), poly(methacrylic acid‐co‐maleic anhydride), and poly(itaconic acid) were prepared by copolymerization of the appropriate amounts of the two monomers at 50°C for 24–40 h using benzoyl peroxide as initiator. Selected samples were γ‐irradiated with a total dose ranging from 2 to 12 Mrad. The unexposed and degraded polymers were dissolved in tetrahydrofuran and their molecular weight distribution was determined by gel permeation chromatography. It was found that M̄n decreased drastically with increasing irradiation dose. The presence of various functional groups in both unexposed and irradiated samples was confirmed by infrared spectroscopy. The copolymer thermodynamic compatibility with various solvents was evaluated on the basis of their fractional solubility parameters. The Gs values of these copolymers were determined using the gel permeation chromatography data. It was established that, among those studied, poly(methyl methacrylate‐co‐maleic anhydride) is the best material for positive resist application based on a Gs value of 8 9 scissions per 100eV.