This is achieved with off-axis illumination. Figure 2 shows several examples of such modes. The NILS determines, for example, the mask error factor and exposure latitude. For k 1 values below 0.53, off-axis illumination yields higher NILS than conventional illumination. From left to right, annular, quasar and dipole illumination are shown.
Zeroth order is in blue, and first orders are red and green.
For the dipole illumination, there is almost total overlap between zeroth and first orders. The use of extreme off-axis illumination i.e. very small poles with minimal overlap between zeroth and first diffraction orders makes the contrast of the image sensitive to pole balances and pupil positioning and asymmetries. Because only a small part of the pupil is used, the aberration sensitivities will also increase, an issue that is related to the projection optics. 2 ASML's lens supplier, Carl Zeiss, is improving lens technology such that the maximum RMS levels are below 0.025 l for advanced scanners. Whereas this is a sufficient metric to rank lenses overall, it does not give sufficient information on the quality of the actual imaging.