A new photoresist material for 157 nm lithography-2

Time-dependent density functional theory (TD-DFT) calculations using the B3LYP hybrid functional suggested that sulfonic acid esters are transparent at around 157 nm region. Based on these findings, poly(methyl vinyl sulfonate) [poly(VSO3Me)] was prepared and found to have an extremely low absorbance (Abs.) of 2.2 μm^-1 at 157 nm. Various alkyl vinyl sulfonates (VSO3R)s were prepared from 2-chloroethanesulfonyl chloride and alcohol components in the presence of pyridine, and their radical polymerizations were conducted in bulk using 2,2′-azobis(isobutyronitrile) as an initiator. Polymerizations of primary and secondary VSO3Rs bearing small alkyl substituents gave homopolymers with high molecular weights. Among them, the Abs. of poly(2,2,2-trifluoroethyl vinyl sulfonate) reached to 1.3 μm^-1. Various copolymers from alkyl vinyl sulfonates and 4-(1,1,1,3,3,3-hexafluoro-2-hydroxypropyl)styrene (HFISt) were also prepared and the Abs. of poly(1,1,1,3,3,3-hexafluoroisopropyl vinyl sulfonate₄₀-co-HFISt₆₀) [poly(VSO3iPr6F₄₀-co-HFISt₆₀)] was found to be 2.4 μm^-1 at 157 nm. The photoresist consisting of partially t-BOC-protected poly(VSO3iPr6F₄₀-co-HFISt₂₈-co-t-BOCHFISt₃₂) (Abs. 2.6) and an photoacid generator showed the contrast and sensitivity of 10.3 and 5.0 mJ cm^-2, respectively.