Process for making low optical density polymers and copolymers for

Abstract

The present invention pertains to a process for the preparation of polymers comprising poly(4-hydroxystyrene) or substituted poly(4-hydroxystyrene) or both. The polymers have low optical density (absorbance) over a wavelength range from about 240 to about 260 nm, as well as low optical density over the near UV and visible spectrum (310 to 800 nm). Such polymers are produced by polymerizing 4-acetoxystyrene and/or substituted 4-acetoxystyrene monomers, with or without other comonomers, and subsequently performing transesterification. In the most preferred embodiment of the present invention, the polymers are produced using at least one alcohol as a reaction medium, a chain transfer agent and a transesterification reactant, providing an unusually economic process. These polymers are particularly useful as photoresist components for use in combination with deep UV, X-ray, and E-Beam imaging systems. To obtain polymers having low optical density over the 240 to 260 nm radiation wavelength range, it is necessary to use an initiator which does not comprise a substantially absorbant structure over the 240 to 260 nm range or to use an initiator concentration of less than about 3 mole % of the monomer(s), to obtain a polymerization conversion of the initial polymerization monomers of at least about 90% by weight, or to remove residual monomer(s) from the reaction medium prior to transesterification, and to obtain a transesterification conversion of the polymerized monomers of at least 85% by weight. It is also necessary to avoid the use of transesterification catalyst which can cause the formation of chemical structures which can absorb over the 240 to 260 nm radiation wavelength range.