Influence of composition and glass transition temperature on the diffusion and solubility behaviour of methyl ethyl ketone-isopropyl alcohol mixtures in poly(methyl methacrylate)

Abstract

The process of removal by a solvent mixture of low molar mass polymer generated as a consequence of chain scission is a critical step in electron beam lithography. The development of the image depends on a number of factors, including the composition of the solvent and the nature of the polymer. In this paper the effects of change in the composition of mixtures of methyl ethyl ketone and isopropyl alcohol, a common development solvent used in lithography, and the glass transition temperature of poly(methyl methacrylate) films on the mutual diffusion coefficient are reported. The mutual diffusion coefficient decreases and becomes asymmetric towards low volume fraction of solvent as the proportion of isopropyl alcohol in the mixture is increased. The higher glass transition temperature films are prone to exhibiting crazing on exposure to solvent. The diffusion of the mixture into the polymer film is selective and preferential for methyl ethyl ketone. Diffusion becomes complex as the content of the mixture moves towards a higher isopropyl alcohol composition. Also, there is evidence for both lowering of the glass transition temperature and re-precipitation of the polymer by the non-solvent (isopropyl alcohol). Change in the initial T_p of the films leads to small changes in the swelling rate. In the development process of electron beam resist films used in semiconductor lithography, crazing probably plays as important a role in the overall development process as simple solvent driven dissolution.

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