Patent Number: 062884018
Section: summary

FIELD OF THE INVENTION The present invention relates to a field emission source used, for example, in an electron beam microcolumn, and in particular to the electrostatic alignment of a charged particle beam. BACKGROUND Miniature electron beam microcolumns ("microcolumns") are based on microfabricated electron "optical" components and field emission sources operating under principles similar to scanning tunneling microscope ("STM") aided alignment principles. Field emission sources are bright electron sources that are very small, making them ideal for use in microcolumns. One type of field emission source is a Schottky emitter, such as the type discussed in "Miniature Schottky Electron Source," H. S. Kim et al., Journal of Vacuum Science Technology Bulletin 13(6), pp. 2468-72, November/December 1995 incorporated herein by reference. For additional field emission sources and for information relating to microcolumns in general, see the following publications and patents: "Experimental Evaluation of a 20.times.20 mm Footprint Microcolumn," by E. Kratschmer et al., Journal of Vacuum Science Technology Bulletin 14(6), pp. 3792-96, November/December 1996; "Electron Beam Technology-SEM to Microcolumn," by T. H. P. Chang et al., Microelectronic Engineering 32, pp. 113-130, 1996; "Electron-Beam Microcolumns for Lithography and Related Applications," by T. H. P. Chang et al., Journal of Vacuum Science Technology Bulletin 14(6), pp. 3774-81, November/December 1996; "Electron Beam Microcolumn Technology And Applications," by T. H. P. Chang et al., Electron-Beam Sources and Charged-Particle Optics, SPIE Vol. 2522, pp. 4-12, 1995; "Lens and Deflector Design for Microcolumns," by M. G. R. Thomson and T. H. P. Chang, Journal of Vacuum Science Technology Bulletin 13(6), pp. 2445-49, November/December 1995; U.S. Pat. No. 5,122,663 to Chang et al.; and U.S. Pat. No. 5,155,412 to Chang et al., all of which are incorporated herein by reference. FIG. 1 shows a schematic cross sectional view of a conventional field emission source 10, which includes an electron emitter 12 and an extraction electrode 14. The electron emitter 12 is a Schottky emitter with a tungsten tip 16 serving as a cathode, which is spot welded on a filament 18. The filament 18 is mounted on two rods 20, which are held by a base 22, and is surrounded by a suppressor cap 24. The extraction electrode 14 defines a center aperture 15. The aperture 15 and following (downstream) lenses (not shown) in the microcolumn define the optical axis 26 for the field emission source 10. By applying a voltage Vc to the tip 16 and a voltage Ve to the extraction electrode 14, a resulting electric field causes the emission of electrons from tip 16. A voltage Vs applied to the suppressor cap 24 suppresses undesired thermionic electrons. An important consideration in the field emission source 10 is that the electron emitter 12 is aligned with the optical axis 26. Because the diameter of aperture 15 is typically 1-2 .mu.m (micrometers), a small misalignment, e.g., 1 .mu.m, will result in a large off-axis aberration and an undesirable increase in the total spot size. Thus, a small misalignment can severely deteriorate the performance of a microcolumn. Conventionally, to ensure proper alignment, the electron emitter 12 is mechanically aligned with the optical axis 26. Thus, electron emitter 12 is physically moved, as indicated by arrows 28, by the use of, e.g., alignment screws, a micrometer x-y stage, a piezoelectric stage, or a scanning tunneling microscope (STM) like positioner to align position electron emitter 12 with optical axis 26. Unfortunately, mechanical alignment is difficult to achieve and is difficult to maintain over extended periods of time due to drift problems. Thus, there is a need for a field emission source that can be easily aligned with the optical axis. SUMMARY A field emission source in accordance with the present invention produces a charged particle beam that is electrostatically aligned with the optical axis. The field emission source includes a charged particle emitter, such as a Schottky or cold-field emitter. Centering electrodes define an aperture through which a beam of charged particles from the emitter passes and which is approximately centered on the optical axis. The centering electrodes provide an electrostatic deflection field near the optical axis that aligns the beam of charged particles with the optical axis, i.e., the axis of the electron beam passes through the center of the next lens down stream. Thus the emitter need not be precisely aligned mechanically with the optical axis. The center electrodes may be, for example, a quadrupole (or higher multipole) arrangement of electrodes placed between the emitter and an extraction electrode. By applying centering potentials of equal amplitude and opposite polarity on opposing elements of the centering electrodes, an electrostatic deflection field is created near the optical axis. The electrostatic deflection field aligns the charged particle beam with the optical axis thereby obviating the need to mechanically align the emitter with the optical axis. A second set of centering electrodes may be used to further deflect the charged particle beam and to ensure that the charged particle beam is approximately parallel with the optical axis. The centering electrodes may be integrally formed on the extraction electrode with an insulating layer between the extraction electrode and the centering electrodes and between the first set of centering electrodes and the second set of centering electrodes if a second set is used. In another embodiment, the extraction electrode is split into a quadrupole (or higher multipole) arrangement. The extraction potential and the centering potentials are then superimposed.