Unipotential lens assembly for charged particle beam tubes and method for applying correction potentials thereto

The method of operating an objective lens assembly for charged particle beam tubes of the electron beam type which may lack coaxial symmetry about the lens axis and wherein the objective lens assembly comprises a plurality of at least three electrostatic lens elements axially aligned along the charged particle beam path with each lens element having an array of micro lenslet apertures formed therein and with each set of axially aligned micro lenslet apertures forming a micro lenslet;

• said method comprising applying a high voltage excitation potential to a center one of the electrostatic lens elements, and applying a dynamic focus correction potential derived from deflection potentials supplied to a charged particle tube with which the objective lens assembly is used to a different electrostatic lens element of the axially aligned assembly from that to which the high voltage excitation potential is applied.

The method of operating an objective lens assembly for charged particle beam tubes of the electron beam type which may lack coaxial symmetry about the lens axis, wherein the objective lens assembly comprises three electrostatic lens elements axially aligned along the charged particle path with each lens element having an array of micro lenslet apertures formed therein and with each set of axially aligned micro lenslet apertures forming a micro lenslet;

• said method comprising maintaining one of the outer electrostatic lens elements of the axially aligned assembly at a system ground reference potential level, applying a high voltage excitation potential to the center electrostatic lens element, and applying a dynamic focus correction potential derived from the deflection potentials supplied to the charged particle tube with which the objective lens assembly is used to the remaining outer electrostatic lens element of the axially aligned assembly.

In an electron beam tube of the compound fly's eye type having a coarse deflector and a fine deflector following the coarse deflector, an electron gun, a condenser lens disposed between the electron gun and the coarse deflector, an objective lens disposed between the coarse deflector and the fine deflector and a target member following the fine deflector all within an evacuated tube or continuously evacuated demountable column;

• the improvement wherein said objective lens comprises a unipotential lens assembly formed by a plurality of at least three axially aligned electrostatic lens elements with each lens element having an array of micro lenslet apertures formed therein and with each set of axially aligned micro lenslet apertures forming a micro lenslet, means for applying a high voltage excitation potential to at least one inner electrostatic lens element located between and bounded by the two outermost lens elements, and means for applying a dynamic focus correction potential derived from the coarse and fine deflection potentials supplied to the coarse and fine deflectors to a different electrostatic lens element of the axially aligned assembly which is separate from that to which the high voltage excitation potential is applied and which is not maintained at system ground reference potential level.