Elimination of landing errors in electron-optical system of mixed field type

In an electron-optical system having an envelope with an axis, a target structure in the envelope extending normal to said axis, an electron gun structure in the envelope directing a beam of electrons toward the target structure and including means defining an object of the electron optical system which is spaced in the direction along said axis from said target structure, magnetic field means for generating a constant magnetic field along said axis within a cavity of the envelope between said object and said target structure, and electric field means for generating a variable electric field within said cavity orthogonal to said magnetic field and capable of causing deflection of said beam along two coordinates in a plane orthogonal to said axis so that said magnetic and electrical fields are respectively operative in said cavity to project a focused image of said object upon said target structure and to simultaneously scan said image across the surface of said target structure:

• a method of eliminating beam landing errors comprising dimensioning and locating said electric field means to provide said electric field with field-free regions proximate to said means defining said object and said target structure, respectively, and dimensioning and locating said magnetic field means to provide said magnetic field with flare field regions at its opposite ends producing landing errors which have vector representations substantially cancelling the vector representations of landing errors due to said field-free regions, respectively.