Absorber dependence of M3D overlay errors in high-NA and hyper-NA EUV lithography

The M3D effect causes telecentricity errors at the illumination slit edges, when the dipole illumination is used for vertical L/S. The telecentricity errors at the right edge and left edge have opposite signs, so they cannot be compensated by changing the sizes of the two poles. The M3D overlay error is defined as the product of the telecentricity error at the illumination slit edge and half the depth of focus. The M3D overlay error depends on the scanner NA and the absorber material. When the conventional Ta absorber is used, the error is negligible for NA 0.33 scanners. It is slightly below the overlay budget for NA 0.55 scanners. However, it exceeds the budget for NA 0.75 scanners. The M3D overlay error becomes larger when PSMs are used. The root cause is the large phase distortion at the absorber sidewalls. The M3D overlay error can be reduced by using binary masks. Binary masks are required for hyper-NA lithography.