vs. vertical bias changed across slit with dipole illumination. The rotating plane of incidence (azimuthal range within -25° to 25°) is confirmed in the SHARP actinic review microscope at CXRO which mimics the optics for EUV projection lithography systems. The reason for this is a mirror is used to transform straight rectangular fields into arc-shaped fields. In order to preserve a fixed plane of incidence, the reflection from the previous mirror would be from a different angle with the surface for a different slit position; this causes non-uniformity of reflectivity. To preserve uniformity, rotational symmetry with a rotating plane of incidence is used. More generally, so-called "ring-field" systems reduce aberrations by relying on the rotational symmetry of an arc-shaped field derived from an off-axis annulus. This is preferred, as reflective systems must use off-axis paths, which aggravate aberrations. Hence identical die patterns within different halves of the arc-shaped slit would require different OPC. This renders them uninspectable by die-to-die comparison, as they are no longer truly identical dies. For pitches requiring dipole, quadrupole, or hexapole illumination, the rotation also causes mismatch with the same pattern layout at a different slit position, i.e., edge vs. center. Even with annular or circular illumination, the rotational symmetry is destroyed by the angle-dependent multilayer reflectance described above. Although the azimuthal angle range is +/- ~20° (NXE3400 field data indicate 18.2°) on 0.33 NA scanners, at 7nm design rules (36-40 nm pitch), the tolerance for illumination can be +/-15°, or even less. Annular illumination nonuniformity and asymmetry also significantly impact the imaging.