US Patent 7511799 Lithographic projection apparatus and a device manufacturing method

A lithographic apparatus wherein a dipole illumination mode used for printing a line pattern, is arranged to provide quadrupole illumination. Radiation emanating from the two additional poles and passing the mask pattern without being affected by diffraction is prevented from reaching the wafer by a radiation blocking aperture disposed in the projection system. Astigmatism aberration due to lens heating associated with the dipole illumination mode is reduced by lens heating associated with the additional poles of the quadrupole illumination mode.