Patent Number: 062884018
Section: description

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 .mu.m in diameter, that is spot-welded on a filament 106 such as a tungsten wire, approximately 50-100 .mu.m 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 .mu.m 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 130a, 130b, 130c, and 130d 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 .mu.m 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 .mu.m. 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 130a, 130b, 130c, and 130d 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 130a will be at a voltage Vdx while electrode element 130c will be at a voltage -Vdx. Similarly, electrode element 130b will be at a voltage Vdy while electrode element 130d 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 103a). 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 .mu.m 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., .+-.Vdx1 and .+-.Vdy1 (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., .+-.Vdx2 and .+-.Vdy2 (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 310a, 310b, 310c, and 310d. 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.