Theoretical calculation of photoabsorption of various polymers in an extreme ultraviolet region

We have calculated the linear absorption coefficients of various resist polymers using the mass absorption coefficients at 13 nm and the density obtained from the graph-theoretical treatment derived by Bicerano. The values indicate that the transmittance at 13 nm of conventional resists used in 193-nm, 248-nm and 365-nm lithography is about 30% when the thickness is 3000 Angstrom and 60-70% when it is 1000 Angstrom. This shows that conventional resists are suitable for an EUVL (extreme ultraviolet lithography) thin-layer resist (TLR) process using a hard-mask layer, but their large photoabsorption makes them unsuitable for a single-layer resist (SLR) process. To design polymers that are suitable for an SLR process, we further calculated the absorption of about 150 polymers. The results suggest that the introduction of aromatic groups into a polymer not only reduces the absorption at 13 nm but also increases the etching resistance.