General condition for moving objective lens (MOL) with combined electrostatic and magnetic fields

Abstract

This paper presents a theoretical study on the Moving Objective Lens (MOL) for general focusing and deflection system with combined electrostatic and magnetic fields. A general MOL condition is derived from the paraxial ray equation instead of from conventional field expansions. Conditions are predicated for hybrid-field MOLs in which the focusing and deflection fields are of different type. A test system consisting of a magnetic with bell-shaped axial field distribution and in-lens electrostatic deflectors is used to analyze the characteristics of the hybrid-field MOL. Third order geometrical and first order chromatic aberrations are also studied in comparison with its pure-field counterpart. Based upon these results, some applications of the hybrid-field MOL concept are recommended.