Correction of the field curvature in SCALPEL projection systems

The field curvature and axial chromatic aberration, 2 of 13 aberrations in an electron optical system, make the major contributions to the image blur of a SCALPEL projector. We first give a simple yet quantitative evaluation of the point spread function from the field curvature and chromatic aberrat ion and show the merit of keeping a comparatively low field curvature. Then we describe two ways to reduce the field curvature in an electron projecti on system. One is to use an immersion magnetic doublet with a constant fiel d along most of the optical column with opposite signs at two ends which ar e connected by a sharp field transition region. The other way is to use spa ce charge foils. By putting a space charge foil with homogeneously distribu ted space charge density on either the object or image plane, we show the f ield curvature can be eliminated in principle without affecting any other a berrations. (C) 2000 American Vacuum Society. [S0734-211X(00)12506-5].