Review of resist-based flare measurement methods for extreme ultraviolet lithography

Flare (stray light) is an important effect impacting extreme ultraviolet lithography (EUVL) imaging system performance. Four flare measurement methods including Kirk, modulation transfer function, double exposure, and zonal ring approximation method are reviewed and analyzed theoretically. The point spread function of an EUV NXE:3100 exposure tool is extracted from the measured Kirk flare (KF) and fitted with a double-fractal model. The KF for this NXE:3100 tool is determined to be 8.5% for a 2-μm diameter absorber pad placed in a 12-mm outer radius bright field, which is larger than the previous 5% KF data measured by ASML and IMEC in 2011. The observation of the increased flare level in the NXE:3100 tool suggests that contamination of EUV optics may be a potential problem for EUVL manufacturing.