Multiscale simulation of extreme ultraviolet nanolithography: impact of acid–base reaction on pattern roughness

Abstract

In extreme ultraviolet lithography (EUVL), a photoacid generator (PAG) blended photoresist (PR) is used to transfer the blueprint from the mask to the wafer. The photoacids are predominantly activated by photo-triggered secondary electron attachment in EUVL, which is distinct from the direct electron excitation of PAG in conventional deep ultraviolet resists. However, uncontrolled acid dispersion causes excessive deprotection and serious damage to the residual pattern. A quencher base compound was introduced in the PR to confine the acid diffusion, but the molecular level description of the acid ↔ quencher interaction and the resulting influence on the pattern image remains elusive. We examine the quencher effect that neutralizes the acid and impedes the deprotection in the masked region, leading to the enhancement of the line edge roughness (LER) of the pattern. In addition, we investigated the loading effect of the quencher and predicted the optimal concentration of the quencher for the best LER, consistent with experimental reports. The LER optimum along the quencher concentration is interpreted by the reciprocal relationship between the acid-blocking effect at the line edge of the pattern (positive) and the acid trapping behavior in the exposure domain (negative). The variations in the deprotection homogeneity clearly demonstrate the negative impact of the acid trap phenomenon on the chemical reaction according to the quencher's loading. In multiscale modeling, PAG dissociation energy curve calculated from density functional theory (DFT) is applied to molecular dynamics (MD) simulation to reproduce the indirect photochemical activation in EUVL. The sequential multiscale framework (DFT-MD-finite difference method) also reproduces acid/base diffusion, neutralization, deprotection kinetics, and pattern morphology in EUV resist at the molecular level.