Generation mechanism of distortion aberration in a symmetric magnetic doublet for an electron beam projection system

Radial and azimuthal distortion aberrations are increasingly a function of the image side lens bore radius in the range from 1.25 to 5 times as large as the maximum image field radius. This phenomenon is inconsistent with our previous understanding. An assumption is made that these large distortions for the large bore radii come from the influence of the magnetic field of one on the other, thereby destroying the symmetry. This assumption is confirmed from the following simulation. When these distortions are calculated for the ideal case where the magnetic fields are calculated in the condition without the other lens, they are decreased to around 1/10 of those for the case where the magnetic fields are calculated in the real condition with the lenses in proximity. When the object endash image distance is 800 mm and the bore radii of lens 1 are 100 and 20 mm, the residual radial and azimuthal distortions are 1.5 and 0.7 nm, the beam blur is smaller than 45 nm for the beam semiangle from 0.05 to 0.7 mrad, where the main-field and subfield sizes in the image plane are 20 mm and 250 μm, respectively, the beam energy is 100 keV, and the space charge effects are neglected. copyright 1996 American Vacuum Society