Modified maleic anhydride copolymers as e-beam resists

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Copolymers of maleic anhydride with styrene, ethylene, and methyl vinyl ether were modified and evaluated for use as negative working resist materials in electron beam lithography. The copolymers used were modified by reaction with an organometallic compound, which resulted in incorporation of tin in the pendant chains of the copolymers. The polymers were blended with a reactive plasticizer, dipentaerythritol pentaacrylate (DPEPA), to enhance sensitivity and to improve resolution. The esterified copolymer of styrene and maleic anhydride was fractionated to obtain fractions with a range of molecular weights and dispersities. The fractions obtained were used to determine effects of molecular weight and dispersity of the base polymer on sensitivity and contrast of the resist. The lithographic response of the modified copolymers was compared with that of the original unmodified base copolymer. Incorporation of tin into the copolymer resulted in an increase in sensitivity to electron beam. It also improved the dimensional stability of the resist material. There was a dramatic increase in oxygen reactive ion resistance by incorporation of tin, which was a result of formation of an etch-resistant passivating barrier. The etch resistance of the tin modified copolymer containing approximately 20 weight % tin was about twenty fold higher than that of poly(methyl methacrylate) under identical etching conditions. Blending the polymer with dipentaerythritol pentaacrylate resulted in a twenty fold increase in electron beam sensitivity over the base copolymer. Tin modified polymers blended with this plasticizer yielded sensitivities in the range of 0.2 to 1 .5tC/cm2. Fractions of copolymer obtained were blended with DPEPA and evaluated for lithographic properties. Increase in molecular weight of the base polymer results in increase in sensitivity of the resist. Monodisperse polymers yielded higher contrast and better resolution. The effect of molecular weight of base copolymer on contrast was determined independent of molecular weight distribution.