Next Generation Photoresists Using Block Copolymer Self-Assembly

Next generation photoresist systems using block copolymer self-assembly have been developed and characterized within the Ober Research Group. Block copolymers containing methacrylate polymer backbones have been designed for low absorption in 193 nm lithography. The polymers synthesized are composed of two polymer chains that can be selectively crosslinked or removed, respectively, from the polymer film. Additionally, a new process has been developed that avoids expensive synthesis by using the self-assembling properties of an inexpensive, commercially available triblock copolymer surfactant. We have shown that selective blending and crosslinking of a molecular glass photoresist inside one of the phases of the self-assembled template yields well defined lines only 15 nanometers in diameter. Summary of Research: Block copolymer lithography relies on a self-assembly process to form periodic structures such as sheets, cylinders and spheres that are composed of two or more polymer chains. One or more of these polymers, or “blocks,” can be subsequently removed from the thin film to form nanoscale stencils that allow the replication of the self-assembling pattern in the underlying substrate [1]. In the past, our group has demonstrated a block copolymer patterning system, poly(a-methylstyrene)-block-poly(4hydroxystyrene), PaMS-b-PHOST, and the ability to control the morphology and long-range ordering of the microdomains [2]. The majority component of the block copolymer, PHOST, is a well-known negativetone photoresist, allowing traditional photolithographic patterning of the film. Additionally, the PaMS minority component of the block copolymer can be selectively removed, which leads to sub-lithographic resolution within the crosslinked areas. These nanoporous templates can then be used to pattern underlying silicon structures. In this past year, we have extended this approach to other block copolymer systems. The latest generation of block copolymer photoresist systems is designed for use in 193 nm lithography. Well defined poly(methylmethacrylate-block-2-hydroxethyl methacrylate) (PMMA-b-PHEMA) copolymers were polymerized via sequential anionic polymerization with molecular weights ranging from 38K to 72K. Similar to the PaMS-b-PHOST system, PHEMA can be used as a negative-tone photoresist, and PMMA can be selectively removed with high doses of UV light through chain scission processes. We have demonstrated the ability to use the block copolymer as a photoresist for topdown patterning with CNF exposure tools, as shown in Figure 1. Thin films of PMMA-b-PHEMA containing triphenylsulfonium triflate photoacid generator and tetrakis (methoxylmethyl) glycoluril crosslinking units were spin-coated onto SiO2 substrates and used as a Figure 1: Optical microscopy shows top-down lithographic patterns present in PMMA-b-PHEMA, demonstrating its ability as a photoresist.