Preparation of ArF Photoresist Polymers based on (Meth)acylate Derivatives Containing Adamantyl and Lactyl Groups via Reversible Addition-Fragmentation Chain Transfer (RAFT) Polymerization

Abstract

ArF (immersion) photolithography is a dominant technology for fabrication of nano-size patterns. ArF photoresist (PR) polymers are usually composed of more than two monomers and prepared by free radical polymerization (FRP). Due to the fast radical cycles of FRP, the copolymers should be inhomogeneous in molecular weight and composition. The inhomogeneity of the copolymers can induce the roughness on the patterns and irregular pattern results. For reduced pattern roughness and uniform lithographic results, well-defined polymers should be prepared. Hence, in this study, ArF PR polymers composed of three (meth)acrylates were prepared via reversible addition-fragmentation chain transfer (RAFT) polymerization. At first a series of polymers were prepared via RAFT polymerization and FRP. Three methacrylates with lithographic functionalities, such as 2-ethyl-2-adamantyl methacrylate (EAdMA), α-gamma-butyrolactone methacrylate (GBLMA), and 3-hydroxy-1-adamantyl methacrylate (HAdMA) were used as monomer components. 2-Cyanoprop-2-yl-1-dithionaphthalate (CPDN) was synthesized and used as the chain transfer agent (CTA). The compositions and the conversion ratios of monomers were measured by proton nuclear magnetic resonance (1H NMR) spectroscopy, and the molecular weight and polydispersity index values were measured by gel permeation chromatography (GPC). In both polymerizations, the order of the monomer reactivity was GBLMA>HAdMA>>EAdMA. This caused composition gradient in RAFT polymerization and composition inhomogeneity in FRP. The polymers prepared using RAFT polymerization had lower molecular weight distributions and more uniform compositions. In case of FRP, the compositions of the polymers were different with the conversion ratios of monomers due to the loss of small chains during precipitation. The improvement in molecular weight distribution and composition uniformity of the PR polymers should be beneficial for lithography. A preliminary lithography test showed that the line width roughness (LWR) of patterns fabricated from a PR containing a RAFT-prepared polymer was lower than that of patterns from a PR containing FRP-prepared polymers. Moreover the lithography results from the PRs containing RAFT prepared polymers were regular. Hence I further studied about the preparation of PR polymer via RAFT polymerization. A series of acid-cleavable branched polymers mainly composed of EAdMA, GBLMA, and HAdMA were synthesized via RAFT polymerization. An acid-cleavable dimethacrylate, 2,5-dimethyl-2,5-hexanediol dimethacrylate (DMHDMA) was used as a brancher. Since DMHDMA contains two methacrylate groups, it induced branched structures of the polymers. The degree of branching could be controlled by the feed ratio of DMHDMA/CPDN, and the molecular weight of primary chains was controlled by the feed ratio of monomer/CPDN. The size and structure of the polymers obtained after hydrolysis were very close to those of linear polymers prepared by RAFT copolymerization using the same amount of reagents only without the acid-cleavable monomer. Preliminary lithography test using an ArF source was conducted. The LWR values of the branched polymers composed of similar primary chains were very similar. Moreover, the LWR value of a branched polymer composed of the smallest primary chains was the smallest. For more homogenous polymers, ArF lithography polymers were prepared from a mixture of a methacrylate and two acrylates. Since the reactivities of GBLMA and HAdMA were same whereas those of the monomers and that of EAdMA were quite different, the polymers composed of the three methacrylates have gradient composition. If the reactivities of monomers are similar, a polymer having homogeneous composition can be prepared. Since the reactivity of acrylate monomer is lower than that of methacrylate monomer having same functionality, more active monomers, GBLMA and HAdMA, were exchanged with acrylates monomers having same functionalities, α-gamma-butyrolactone acrylate (GBLA), and 3-hydroxy-1-adamantyl acrylate (HAdA). Since these two monomers have similar reactivities with that of EAdMA, polymers having homogeneous composition were expected. A problem of a polymer composed of acrylates is a low glass transition temperature. However, this was overcome by increasing molecular weight. A series of acid-cleavable branched polymers and linear polymer were prepared via RAFT polymerization. The molecular weight of primary chains and the degree of branching of the polymers were controlled by the feed ratio of monomer/CPDN and DMHDMA/CTA, respectively. Glass transition temperatures of the polymers increased as the degree of branching grew due to an increase in molecular weight. When the branched polymers were hydrolyzed, molecular weight and polydispersity index value were similar to those of linear polymer prepared by RAFT polymerization with the same molar feed ratio of monomer/CPDN. However, better lithography results were not obtained probably due to an improper lithography condition. The lithography conditions in this research was optimized for methacrylate based polymers which has high Tg value (>180 oC), while the Tg value of the polymers composed of methacrylate and acrylate were less than 125 oC. Since the polymers composed of methacrylate and acrylates are more homogeneous in composition than the polymers composed of methacrylates, the polymers composed of methacrylate and acrylates should be more beneficial as the PR polymer under proper lithography condition.