Abstract:
A field emission source produces a charged particle beam that can be electrostatically aligned with the optical axis. Quadrupole (or higher multipole) centering electrodes approximately centered on the optical axis are placed between the emitter and the 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 deflect the charged particle beam back and to ensure that the charged particle beam is parallel with the optical axis. Further, the extraction electrode may be split into a quadrupole arrangement with the extraction and centering potentials superimposed.

Description:
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 20×20 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 μm (micrometers), a small misalignment, e.g., 1 μm, 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. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a schematic cross sectional view of a conventional field emission source, which includes an electron emitter that is mechanically aligned with the optical axis as shown by arrow  28 . 
     FIG. 2 shows a schematic cross sectional view of a field emission source including centering electrodes to electrostatically aligned an electron beam the optical axis in accordance with an embodiment of the present invention. 
     FIG. 3 shows a top view of the extraction electrode and centering electrodes. 
     FIG. 4 shows a schematic cross sectional view of the correction of an electron beam produced by a misaligned field emission source in accordance with the present invention. 
     FIG. 5 shows a schematic cross sectional view of a field emission source with two sets of centering electrodes in accordance with another embodiment of the present invention. 
     FIG. 6 shows a schematic cross sectional view of the correction of an electron beam produced by a misaligned field emission source in accordance with another embodiment of the present invention. 
     FIG. 7 shows a schematic cross sectional view of a field emission source with centering extraction electrodes in accordance with another embodiment of the present invention. 
     FIG. 8 shows a top view of the centering extraction electrodes from FIG.  7 . 
    
    
     DETAILED DESCRIPTION 
     FIG. 2 shows a schematic cross sectional view of a field emission source  100 , with an electron emitter  102  and an extraction electrode  120  and including centering electrodes  130  to electrostatically align an electron beam with the optical axis  101  in accordance with an embodiment of the present invention. 
     The electron emitter  102  is a Schottky emitter with an etched single crystal tungsten tip  104 , approximately 50-100 μm in diameter, that is spot-welded on a filament  106  such as a tungsten wire, approximately 50-100 μm in diameter. The filament  106  is mounted on a support structure, which includes a base  108 , two rods  110 , and a suppressor cap  112 . The filament  106  is connected to the rods  110 , which is supported by the base  106 . Electron emitter  102  may also be a cold-field emitter as is well known in the art. 
     The electron emitter  102  is mounted in front (upstream) of the extraction electrode  120 . The extraction electrode  120  defines a center aperture  122 , which is approximately 1-2 μm diameter. Following extraction electrode  120  are the conventional lens structures of the microcolumn, which for the sake of simplicity are shown as a single lens electrode  140  defined by a lens aperture  142 . The optical axis  101  is centered on the extraction electrode aperture  122  and the lens aperture  142 . 
     The field emission source  100  electrostatically corrects any misalignment between the electron emitter  102  and the optical axis  101 . Thus, the electron emitter  102  may be rigidly mounted with respect to optical axis  101  and only a coarse physical prealignment of the electron emitter  102  with the extraction electrode  120  is necessary. The prealignment is mechanically performed, for example, using a conventional flexure stage or inertial walker during assembly. Advantageously, the electrostatic alignment in accordance with the present invention aligns the electron beam with the optical axis with the same or greater precision as with the conventional mechanical alignment. Thus, the necessity of extremely precise mechanical alignment is obviated. 
     In accordance with one embodiment of the present invention, the electrostatic alignment is achieved by electrostatic centering electrodes  130  positioned between the electron emitter  102  and the extraction electrode  120 . FIG. 3 shows a top (plan) view of the extraction electrode  120  and electrostatic centering electrodes  130 . As shown in FIG. 3, the centering electrodes  130  are in a quadrupole arrangement with electrode elements  130   a ,  130   b ,  130   c , and  130   d  and approximately centered on optical axis  101 . It should be understood that centering electrodes  130  may be a higher number multipole arrangement, e.g., an octopole or dodecapole. 
     The centering electrodes  130  are fabricated using the same micromachining technology used to fabricate lens components in a microcolumn, as is well understood by those of ordinary skill in the art. An electrically insulating layer  132  is deposited over the extraction electrode  120 . The insulating layer  132  is for example silicon dioxide, pyrex, or a similar material and is 0.5 to 20 μm thick. A conductive layer, such as aluminum, gold, silicon (that is heavily n doped), copper, platinum, or other conductive material, is then deposited over the insulating layer  132  to a thickness of 1-100 μm. The conductive layer is then lithographically patterned and etched to form the desired centering electrodes  130 . The deposition, patterning and etching of a conductive layer is well understood by those of ordinary skill in the art. 
     To cause the emission of electrons, a voltage Vc is applied to the rods  110  of the electron emitter  102 , while a voltage Vs is applied to the suppressor cap  112 , and a voltage Ve is applied to the extraction electrode  120 . The difference in potentials between the electron emitter  102  and the extraction electrode  120  (Vc-Ve) creates a strong electric field in the area of the tip  104 , causing the emission of electrons. The temperature of the tip  104  is regulated to approximately 1700 to 1800 degrees K by a current passing through the filament  106 , and the average power is 1.5-2.0 W. 
     Potentials are applied to the individual centering electrode elements  130   a ,  130   b ,  130   c , and  130   d  to form a deflection field near the optical axis  101 . The deflection field approximately centers the emitted electron beam with respect to the optical axis, i.e., the axis of the electron beam passes through the center of the next lens down stream. Potentials of equal amplitude and opposite polarity are applied to opposite electrodes. Thus, for example, electrode element  130   a  will be at a voltage Vdx while electrode element  130   c  will be at a voltage −Vdx. Similarly, electrode element  130   b  will be at a voltage Vdy while electrode element  130   d  will be at a voltage −Vdy. The typical voltages used on the electrode elements range from a few tens of volts to a few hundred volts. If the electron emitter  102  is properly aligned with optical axis  101  and thus no centering potential is necessary, a uniform bias potential Vb may be applied to all individual electrode elements so that a uniform extraction field is preserved. 
     FIG. 4 shows a schematic cross sectional view of a misaligned field emission source  100  producing an electron beam  103  while centering electrodes  130  electrostatically align the electron beam  103  with the optical axis  101 . As shown in FIG. 4, without the centering potential produced by centering electrodes  130 , an electron beam would be misaligned with the optical axis (as indicated by the broken lines  103   a ). By application of centering potential on centering electrodes  130 , an electrostatic deflection field is generated (as indicated by arrow  131 ), which deflects the electron beam  103  so that it is in approximate alignment with the optical axis  101 , i.e., the axis of the electron beam passes through the center of the next lens down stream (not shown in FIG.  4 ). 
     The centering process may result in a small tilt of the electron beam  103  with respect to the optical axis  101 , as shown in FIG.  4 . The centering systems in the lenses that follow the extraction electrode  120 , e.g., lens  140  shown in FIG. 2, may compensate for any residual tilt. 
     FIG. 5 shows a schematic cross sectional view of a field emission source  200  in accordance with another embodiment of the present invention. Field emission source  200  is similar to field emission source  100 , shown in FIG. 2, like designated elements being the same, however, field emission source  200  includes a second set of electrostatic centering electrodes  210  follow centering electrodes  130 . The second set of centering electrodes  210  are similar in fabrication and operation to centering electrodes  130 . The second set of centering electrodes  210  are used to allow simultaneous beam translation and parallelism to the optical axis thereby removing the residual tilt generated by centering electrodes  130  (which is illustrated in FIG.  4 ). 
     Centering electrodes  210  are fabricated in a manner similar to centering electrodes  130 . An insulating layer  212  of approximately 0.5 to 20 μm is deposited over the extraction electrode  120 . A conductive layer that forms the second set of centering electrodes  210  is deposited over the insulating layer  212 . Another insulating layer  130 , similar to insulating layer  212  is then deposited followed by another conductive layer that forms the first set of centering electrodes  130 . The stack of conductive layers and insulating layers is then lithographically patterned and etched to define the desired centering electrodes  130  and second set of centering electrodes  210 . Of course, if desired additional sets of centering electrodes may be produced in a similar manner. 
     FIG. 6 shows a schematic cross sectional view of a misaligned field emission source  200  producing an electron beam  203  while centering electrodes  130  and a second set of centering electrodes  210  electrostatically align the electron beam  203  with the optical axis  101 . As shown in FIG. 6, by application of centering potential on centering electrodes  130 , a first electrostatic deflection field is generated (as indicated by arrow  231 ), which deflects the electron beam  203  so that it is in approximate alignment with the optical axis  101 , i.e., the axis of the electron beam  203  passes through the center of the centering electrodes  210 . The applied centering potentials are of equal amplitude and opposite polarity for opposite electrodes, i.e., ±Vdx 1  and ±Vdy 1  (which is applied to the centering electrode elements not shown in the cross sectional view of FIGS.  5  and  6 ). 
     By application of a second centering potential on the second set of centering electrodes  210 , a second electrostatic deflection field is generated (as indicated by arrow  232 ), which deflects the electron beam  203  in a direction opposite to the direction that the electron beam  203  was deflected by centering electrodes  130 . The second set of centering potentials are applied to opposite electrodes of the second set of centering electrodes  210 , i.e., ±Vdx 2  and ±Vdy 2  (which is applied to the centering electrode elements not shown in the cross sectional view of FIGS.  5  and  6 ). As shown in FIG. 6, the orientations of the deflection fields generated by the two sets of deflection electrodes  130  and  210  are opposite in direction. The second set of potentials applied to centering electrodes  210  removes residual tilt created by centering electrodes  130 , thereby deflecting the electron beam  203  to be approximately parallel with the optical axis  101 , e.g., within 3 milliradians. A bias potential Vb may be applied to one or both sets of centering electrodes  130  and  210  so that a uniform extraction field is preserved if no electrostatic alignment is necessary. 
     FIG. 7 shows a schematic cross sectional view of a field emission source  300  in accordance with another embodiment of the present invention. Field emission source  300  is similar to field emission source  100 , shown in FIG. 2, like designated elements being the same, however, the extraction electrode  120  and the centering electrodes  130  are replaced with a centering extraction electrode  310 . 
     FIG. 8 shows a top view of the centering extraction electrode  310 . As shown in FIG. 8, the centering extraction electrodes  310  is an extraction electrode split into a quadrupole arrangement having electrode elements  310   a ,  310   b ,  310   c , and  310   d . Of course, centering extraction electrode  310  may have a higher multipole arrangement if desired. 
     The centering extraction electrodes  310  operate as both the extraction electrode and the centering electrode. As shown in FIGS. 6 and 7, the extraction potential Ve and the centering potentials ±Vdx and ±Vdy are superimposed on the individual elements of the centering extraction electrodes  310 . 
     The centering extraction electrodes  310  are fabricated using the same micromachining silicon technology used to fabricate lens components in a microcolumn, as is well understood by those skilled in the art. If desired, centering extraction electrodes  310  may be fabricated on a substrate (not shown), such as a silicon substrate, which may aid in the prevention of warping or mechanical breakdown of the centering extraction electrodes  310 . 
     While the present invention has been described in connection with specific embodiments, variations of these embodiments will be obvious to those of ordinary skill in the art in light of the present disclosure. Thus, for example, while the present disclosure describes a field emission source in accordance with the present invention as including an electron emitter, it should be understood that any charged particle, including positive ions may be emitted and electrostatically aligned in accordance with the present invention. Therefore, the spirit and scope of the appended claims should not be limited to the foregoing description.