Sub-Resolution Assist Feature in Attenuated Phase-Shift Mask for Extreme Ultraviolet Lithography

A sub-resolution assist feature (SRAF) is a non-printing auxiliary pattern used to improve the DOF and to correct the critical dimension (CD) of an isolated pattern. Recently, the application SRAF of in extreme ultraviolet lithography is being considered since the depth of focus (DOF) of sub-18 nm isolated line and space pattern is less than 100 nm. A larger SRAF is more effective in improving DOF than the bias optical proximity correction, but it is prone to be printed. An 6% attenuated phase shift mask (PSM) allows a larger SRAF compared to the conventional binary intensity mask (BIM) because of the higher reflectivity in SRAF region. Therefore, DOF of isolated pattern is improved by using PSM. And also, the margin in the SRAF CD and position accuracy is wider for PSM. This is because the printed CD error depending on SRAF width and center position variation is reduced by lower change in diffraction efficiency.