Understanding and controlling the structure of thin polymer films used in photolithography

Abstract

Line edge roughness (LER) is one of the major impediments to the semiconductor industry achieving the desired device performance and satisfying the goals set by International Technology Roadmap for Semiconductor (ITRS). LER is defined as any sort of unwanted roughness in semiconductor features, and among many other factors, structural heterogeneity of the photoresist materials is one of the sources of LER in photolithographic devices. This gives the motivation to study a new class of copolymer named ‘gradient copolymer’, which is unique in terms of the copolymer chain structure. The copolymer composition of gradient copolymers changes from one end of the chain to the other gradually, while the copolymer composition is on average constant along the chains for conventional statistical copolymers. In contrast, block copolymers show an abrupt switch in the polymer composition. In this study, statistical, block and gradient copolymers have been synthesized for two different monomer pairs (styrene – acrylonitrile and hydroxystyrene – t-butyl acrylate) using reversible addition-fragmentation chain transfer (RAFT) and conventional radical polymerization (CvRP) method, and their surface properties extensively studied.The kinetics of copolymerization of styrene (St) and acrylonitrile (AN) by RAFT method was studied in detail. Reactivity ratios of St and AN were determined for nine different feed compositions for RAFT, and the effect of solvent on reactivity ratios was discussed based on the bootstrap model. Batch polymerization of styrene (St) and acrylonitrile (AN) yielded statistical and spontaneous gradient copolymers depending on the monomer feed ratio, and the chain structures were confirmed from the change in the copolymer composition. St- and AN-centered triad distributions were determined using quantitative 13C NMR. Using the continuous feeding approach, forced gradient St-AN copolymers were prepared, and St-block-AN copolymers synthesized by the chain extension method. Statistical, gradient and block copolymers have also been synthesized for the acetoxystyrene – t-butyl acrylate monomer system. The statistical and gradient structures were confirmed from the change in the composition since this pair does not provide any well resolved peaks for different triad sequences in 13C NMR. Hydroxystyrene – t-butyl acrylate (HOST-tBA) copolymers were prepared by selective hydrolysis of 4-acetoxystyrene – t-butyl acrylate (AOST-tBA) copolymers.The properties of the both copolymer systems were found to be strongly dependent on the copolymer structure. A unique broad glass transition was observed for the gradient copolymers, which is different from behaviour of the statistical and block copolymers. Surface properties of St-AN statistical, gradient and block copolymer thin films were found to be very different due to the differences in their structure. X-ray photoelectron spectroscopy (XPS) and surface free energy studies suggest strong phase segregation for the block copolymer and the statistical copolymers do not show any phase segregation. However, the extent of phase segregation in gradient copolymer thin films was found to be intermediate of those of block and statistical copolymer thin films. However, the effect of structural differences on thin film properties was not as significant for the HOST-tBA system. Grazing angle attenuated total reflectance Fourier transform infra-red (GATR-FTIR) studies provided detailed information on extent of hydrogen bonding in the HOST-tBA copolymer thin films. The proportion of ‘free’ and hydrogen bonded carbonyl groups was quantified from the IR spectra and found to be in good agreement with the structure of HOST-tBA statistical, gradient and block copolymers.Overall, the study provided important information on the effect of chain sequence distribution on the properties of thin polymeric films. The implications for lithographic applications is discussed.