Patent Number: 045256292
Section: claims

1. A deflective focusing system for a charged particle beam comprising: a magnetic lens for focusing a charged particle beam;  a plurality of rings made of magnetic material arranged substantially concentric with said magnetic lens inside of said magnetic lens, said rings being arranged dividedly in the direction of the central axis of said magnetic lens so as to form a predetermined magnetic focusing field distribution; and  a one-stage electrostatic deflector having a plurality of deflection electrodes which are divided in a circumferential direction of said magnetic lens, are arranged substatially concentric with said magnetic lens inside of said magnetic lens and are extended in the direction of said central axis so as to form a predetermined electrostatic deflection field distribution, the charged particle beam passing through said concentrically arranged deflection electrodes to be deflected in accordance with a voltage applied to said deflection electrodes. 2. A deflective focusing system for a charged particle beam as claimed in claim 1, further comprising ring-like grounding electrodes which are disposed substantially concentric with said magnetic lens on the object and image plane sides of said electrostatic deflector along the passage of said charged particle beam. 3. A deflective focusing system for a charged particle beam as claimed in claim 2, wherein ring-like spacers made of non-magnetic material and having substantially the same diameter as said ring are inserted between said rings, so that the magnetic focusing field distribution is adjusted by the thicknesses of said rings in the direction of said central axis and the thicknesses of said ring-like spacers. 4. A deflective focusing system for a charged particle beam as claimed in claim 2 or 3, wherein a gap for adjusting the electrostatic deflection field is provided between said ring-like grounding electrodes and said electrostatic deflector, so that said electrostatic deflection field distribution is adjusted in accordance with the length of said gap. 5. A deflective focusing system for a charged particle beam as claimed in claim 4, wherein the inner diameter of said ring-like grounding electrode is more reduced than the inner diameter of said deflection electrode to have an abrupt fringe in the field distribution. 6. A deflective focusing system for a charged particle beam as claimed in claim 4, wherein said electrostatic deflector has the end portion on said image plane side shifted from the end portion of said ring on said image plane side toward said object plane side in the direction of said central axis. 7. A deflective focusing system for a charged particle beam as claimed in claim 2, wherein said electrostatic deflector has the inner diameter substantially equal to that of said ring-like grounding electrode. 8. A deflective focusing system for a charged particle beam as claimed in claim 7, further comprising a case having a first room, which is formed by said case and a sealing member made of nonmagnetic material, for accommodating the coil of said magnetic lens, a second room, which is formed by said sealing member, said electrostatic deflector and said grounding electrodes, for accommodating said rings, and a third room whose boundary is defined by said electrostatic deflector and said grounding electrodes and through which said charged particle beam passes, said sealing member sealing said second room vacuum-tightly and fixing said rings at predetermined positions. 9. A deflective focusing system for a charged particle beam as claimed in claim 8, wherein said case has a flange extended inwardly to cover said ring-like grounding electrode disposed on said object plane side of said electrostatic deflector. 10. A deflective focusing system for a charged particle beam as claimed in claim 2, wherein said ring-like grounding electrode on said image plane side has a flange for supporting said electrostatic deflector. 11. A deflective focusing system for a charged particle beam as claimed in claim 6, wherein a stigmator coil is wound on the periphery of a portion of said electrostatic deflector which is protruded from said rings toward said object plane side. 12. A deflective focusing system for a charged particle beam as claimed in claim 11, wherein a dynamic focusing coil is wound around said stigmator coil. 13. A deflective focusing system for a charged particle beam as claimed in claim 1 or 2, further comprising a shielding electrode around the outer periphery of said electrostatic deflector. 14. A deflective focusing system for a charged particle beam as claimed in claim 2, wherein said electrostatic deflector has the end portion on said image plane side shifted from the end portion of said ring on said image plane side toward said object plane side in the direction of said central axis. 15. A deflective focusing system for a charged particle beam as claimed in claim 14, wherein a stigmator coil is wound on the periphery of a portion of said electrostatic deflector which is protruded from said rings toward said object plane side. 16. A deflective focusing system for a charged particle beam as claimed in claim 15, wherein a dynamic focusing coil is wound around said stigmator coil.