METHOD AND APPARATUS FOR CONTROLLING AN ELECTROSTATIC LENS ABOUT A CENTRAL RAY TRAJECTORY OF AN ION BEAM

摘要

A method of controlling deflection of a charged particle beam in an electrostatic lens (700) includes establishing a symmetrical electrostatic lens configuration comprising a plurality of electrodes (714, 716) disposed at unadjusted positions (L1) that are symmetric with respect to the central ray trajectory (702) with applied unadjusted voltages that create fields symmetric with respect to the central ray trajectory. A symmetric electric field is calculated corresponding to the set of unadjusted voltages. A plurality of lower electrodes (716) is arranged at adjusted positions (L2) that are asymmetric with respect to the central ray trajectory. A set of adjusted voltages is obtained for the plurality of lower electrodes, wherein the set of adjusted voltages corresponds to a set of respective potentials of the symmetric electric field at respective adjusted asymmetric positions. The adjusted voltages are applied to the asymmetric lens configuration when the charged particle beam passes therethrough.