IMPROVED THERMAL FIELD EMISSION ALIGNMENT

Abstract: An electron-beam microcolumn alignment method and system in situ includes a split suppressor cap (124) for a miniature Schottky electron (16) or other field emission source. The split suppressor cap is segmented into four or more electrically separate electrode elements (124...130) which are independently driven and controlled by separate deflection voltages (Vs+/-Vx, Vs+/-Vy) to scan the electron beam without requiring mechanical movement.

Description: IMPROVED THERMAL FIELD EMISSION ALIGNMENT BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to charged particle imaging using electron beams and in particular a method and system for aligning the field emission beam. Description of Related Art In recent years, there has been a significant increase in interest in low voltage scanning electron beam systems for applications in surface inspection of materials, metrology, testing and lithography. Conventional scanning electron beam systems are large immobile devices. Although scanning electron beam systems have many applications, such as semiconductor related inspection and testing, conventional scanning electron beam systems are limited in their usefulness because of their size, immobility, and associated costs. For instance, because the sample being observed, as opposed to the electron microscope, must be moved during the inspection process, a conventional scanning electron microscope requires the use of a vacuum chamber that is much larger than the sample. Further, the sample must be positioned at an angle relative to a conventional scanning electron microscope to produce a beam incidence angle required for three-dimensional-like surface feature imaging, which makes handling large or delicate samples difficult. Moreover, throughput of a conventional electron microscope is limited because only one electron microscope can observe a sample at a time. An effort to improve electron-beam systems has resulted in miniature electron-beam microcolumns ("microcolumns"). Microcolumns .ire based on microfabricated electron "optical" components and field emission sources operating under principles similar to scanning tunneling microscope (STM) aided alignment principles, also called STM aligned field emission (SAFE). The alignment principles used by microcolumns are similar to STMs in that a precision X-Y-Z positioner is used to control a sharp tip, and to utilize the emission from the tip to measure the position of the tip. Microcolumns are discussed in general in the publication "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, Nov./Dec. 1996, which is incorporated herein by reference. The microcolumns are formed of high aspect ratio micromechanical structures, including microlenss and deflectors. Although the STM positioner is feasible for positioning the field emission electron beam, it is most useful in prototype systems. The STM mechanical alignment requires a complex mechanical structure and the operation is not as precise as desired. Further, it is desirable to dynamically correct for positional drift of the emission electron beam during operation of the field emission system. It thus would be desirable to achieve alignment of the field emission beam during operation of the microcolumn without utilizing mechanical structures. SUMMARY OF INVENTION A method and system for aligning a source in an electron-beam microcolumn in situ includes a split suppressor cap for a miniature Schottky electron or other field emission source. The split suppressor cap is segmented into four or more separate electrode elements, which can be independently controlled with separate deflection voltages to scan the electron beam without mechanical movement. The electronic control also allows for dynamic correction of the field emission beam drift during operation of the microcolumn. BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is an exploded perspective view of a microcolumn which can incorporate the present invention. FIG. 2 is an exploded perspective view of a microcolumn source and microlens which can incorporate the present invention. FIG. 3 is a side sectional view of a miniature Schottky electron source. FIG. 4 is a side view of an embodiment of a split suppressor cap for the electron source of the present invention. FIG. 5 is a top view of the suppressor cap of FIG. 4. Utilization of the same reference numerals in different FIGS, indicates similar or identical elements, structurally and/or functionally. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1, a prior art microcolumn is designated generally by the reference numeral 10, illustrated with a grid sample 12 and a channeltron electron detector 14 which is utilized to generate scanning transmission electron microscope (STEM) images from electron transparent samples. The microcolumn includes an electron source (not illustrated), which can be a miniature cold-field or Schottky emitter having a field emitter tip 16. The tip 16 can be a Zr/O/W Schottky-type emitter tip or if a cold emitter tip, could be a single crystal tungsten, hafnium carbide or diamond tip. The tip 16 preferably is mounted on a positioner 18, such as a three axis STM type X-Y-Z positioner. The positioner 18 has a range of movement on the order of tens of microns up to about one (1) millimeter (mm) in each axis. The positioner 18 has a nanometerscale positioning accuracy capability and is utilized to align the tip 16 with an electron optical column 20. The column 20 can have a length on the order of three and one-half (3.5) mm. The tip 16 is aligned with a five (5) micron aperture 22 for example purposes, in an extractor 24. The extractor 24 is combined with an anode 26, having an aperture 28 on the order of one hundred (100) microns to form a selectively scaled dual electrode source lens 30 . The resulting electron beam 32 is directed to a beam limiting aperture 34 in an aperture member 36. The aperture 34 is on the order of several microns, illustrated as two and one-half (2.5) microns, in diameter. The spacing and aperture size selected determine the convergence of the resulting e-beam 38 at the grid 12. From the aperture 34, the beam 38 is passed through a beam deflector 40 which can be a single unit or a multiple unit octupole scanner/stigmator. The deflector 40 is utilized to deflect or scan the beam 38 across the sample 12. A multiple electrode Einzel lens 42 focuses the beam 38 onto the sample 12 at a working distance 44 of one (1) to two (2) mm. The lens 42 can, for example, include three electrodes 46, 48, 50, each having an aperture 52 with a diameter on the order of two hundred (200) microns. The microcolumn 10 also can include an electron detector 54, which can be a microchannel plate electron detector for secondary and backscattered electrons or a metal- semiconductor detector for low energy backscattered electrons. The microcolumn 10 can be operated to produce a 1 KeV beam 38. It is understood that FIG. 1 illustrates merely one example of many possible field emission sources and electron optical columns that may be utilized in the microcolumn 10. For additional field emission sources and electron optical columns that may be used in the microcolumn 10 in general, see the following articles and patents: "Experimental Evaluation of a 20x20 mm Footprint Microcolumn", by E. Kratschmer et al., Journal of Vacuum Science Technology, Bulletin 14 (6), pp. 3792-96, Nov./Dec. 1996; "Electron Beam Technology - SEM to Microcolumn," by T.H.P. Chang et al., Microelectronic Engineering 32, pp. 113-130, 1996; "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. 2245-49, Nov./Dec. 1995; "Miniature Schottky Electron Source," by H.S. Kim et al., Journal of Vacuum Science Technology, Bulletin 13 (6), pp. 2468-72, Nov./Dec. 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. Referring to FIG. 2, one example of the construction of the source lens 30 and the Einzel lens 42 is illustrated. For additional fabrication details, see "High Aspect Ratio Aligned Multilayer Microstructure Fabrication" by K.Y. Lee, S.A. Rishton, and T.H.P. Chang, Journal of Vacuum Science Technology, Bulletin 12 (6), pp. 3425-30, Nov./Dec. 1994, also incorporated herein by reference. The source 30 includes a plurality of silicon wafers or chips 60, 62 and 64, which are spaced apart by one hundred (100) to five hundred (500) micron thick insulating layers 66 and 68. The layers 60 to 68 are not drawn to scale. The layers 66 and 68 are generally formed from glass, such as glass sold under the trademark, Pyrex. The layers 60 to 68 then are precisely aligned and bonded together to form the source 30, typically by electrochemical anodic bonding. Prior to the bonding process, the silicon chips are processed with electron beam lithography and reactive-ion etching to form a respective silicon membrane 70, 72, and 74 in each of the chips 60, 62 and 64. The required beam apertures, such as the respective apertures 22, 28 and 34, then are formed in the membranes 70, 72 and 74. The membranes 70, 72 and 74 are on the order of one (1) to two (2) microns thick. The membranes 70, 72 and 74 and the apertures 22, 28 and 34 form the elements 24, 26 and 36 of the lens 30. In a like manner, the electrodes 46, 48 and 50 of the lens 42 are formed with central silicon membranes 76, 78 and 80, in which are formed respective apertures 52. Again, the lens 42 includes a plurality of Pyrex insulating layers 84 and 86, which also include apertures 88 and 90, which are larger in diameter than the apertures 52. The layers 46, 48, 50, 84 and 86 again are aligned and typically bonded together to form the lens 42. Referring to FIG. 3 a miniature Schottky electron source 100 is illustrated, which can be the emitter source for the microcolumn 10 and can include the emitter tip 16. As more fully described in "Miniature Schottky electron source" by H.S. Kim et al., Journal of Vacuum Science Technology, Bulletin 13(6), pp. 2468-72, Nov./Dec. 1995, which is incorporated by reference, the electron source 100 includes a suppressor cap 102. The cap 102 is mounted over an insulator body 104, which encompasses and insulates a pair of conductors 106, 108 which support and supply the power to the tip 16. The tip 16, preferably can be a Zr/O/W alloy tip and is mounted on a miniature filament 110 enclosed within the suppressor cap 102. The tip 16 extends from the suppressor cap 102 through an opening 112. A conventional Schottky electron source, typically would have length L of about twenty (20) millimeters (mm) and a width (or diameter) W of about seventeen (17) mm. The electron source 100 has a length L of thirteen and seven tenths (13.7) mm and a width W of four (4) mm. A conventional source would have a one hundred and twenty-five (125) micron filament, but the electron source 100 has a seventy five (75) micron tungsten filament. The smaller filament size enables the electron source 100 to operate at a heating power of one and one half (1.5) to one and eight tenths (1.8) watts of heating power versus two and one-half (2.5) to three and one half (3.5) watts for a conventional electron source. As illustrated in FIG. 1, the STM positioner 18 can be utilized to mechanically adjust the positioning of the electron source 100 and hence the tip 16. This physical alignment is not optimal for a number of reasons. Referring to FIG. 4, one embodiment of a suppressor cap of the present invention is designated by the reference numeral 114. The electron source 100 through the tip 16, emits an electron beam 116, see FIG. 1. The beam 116 is illustrated as being perfectly aligned with the aperture 22 in the extractor 24, but in fact this required optimum alignment can be difficult to achieve. The aperture 22 has a diameter on the order of a few microns and the tip 16 has an operating distance 118 from the aperture 22 on the order of one hundred (100) microns. The suppressor cap 102 or 114 reduces the number of thermionically emitted electrons from the filament 110 and from the shank 120 (see FIG. 3) of the tip 16. In other conventional electron sources, (not illustrated) the tip is mechanically centered at the working distance 118 and aligned with the aperture 22. However, the electron source 100 cannot be mechanically aligned in the same manner in the microcolumn 10, because of the assembly procedure required for the microcolumn 10, the significantly smaller dimensions of the electrodes in the source 30 and the overall dimensions of the elements in the microcolumn 10. While utilization of the STM positioner 18 is feasible, it adds complexity and size to the microcolumn 10, is not as reliable as desired and requires more time than desirable to scan the beam 116 over the large areas, which can be required. Although the beam 116 is illustrated as aligned with the aperture 22, it more likely will be misaligned, such as illustrated by a beam 116'. The misaligned beam 116' is totally (as illustrated) or partially directed away from the aperture 22. This causes a total or partial failure of the beam 116' passing through the aperture 22, which when aligned forms a maximum power beam 122 which will become the beam 38 (see FIG. 1). The suppressor cap 114 of the present invention solves this alignment/scanning focus problem by utilizing four (4), as illustrated in FIGS. 4 and 5, or more segmented or separated electrodes 124, 126, 128 and 130. Although illustrated and described as a miniature Schottky electron source, the suppressor cap 114 of the present invention can be utilized with any type of thermal field emission source. The suppressor voltage, Vs, which is utilized to reduce the number of thermionically emitted electrons from the filament 110 and the tip shank 120 is summed with the desired deflection voltages, +V y and +V X applied to each of the electrode segments 124, 126, 128 and 130 such as coupled from a power source (not illustrated) to steer the beam 116 as desired. Ramped electrical signals can be applied to the electrodes 124, 126, 128 and 130 to quickly scan the beam 116 over the extractor 24 to locate the optimum operating position of the beam 116. The deflection voltages ±V X and ±V y , also can be utilized to dynamically correct for drift of the location of the source beam 116, during operation of the electron source 100, such as during operation of the microcolumn 10. The deflection voltages can be generated, for example, by a scan generator, essentially as two synchronized ramp waves. The position of the beam 116 can be monitored by measuring the beam current incident on the extractor 24, the anode 26 or the limiting aperture 34. Because the deflection required are small and the beam voltage is low, high speed electronics can be utilized to generate the required scan signals. Although the present invention has been described with reference to particular embodiments, the described embodiments are examples of the present invention and should not be taken as limitations. Although four electrodes 122, 124, 126 and 128 have been illustrated and described, additional electrode segments could be utilized, if desired. As will be appreciated by those skilled in the art, various other adaptations and combinations of the embodiments described herein are within the scope of the present invention as defined by the attached claims.

Claims: CLAIMS: 1. A method of aligning an emission beam for a thermal field emission source, comprising: dividing a suppressor cap into at least four electrode segments, electrically isolated from one another and driving each said electrode with a suppressor voltage and a deflection voltage individual to each said electrode to move and align the emission beam. 2. The method of claim 1 , including providing the same deflection voltage to opposite pairs of electrodes, the deflection voltage being of opposite polarity on each one of the electrodes of each pair of said electrodes. 3. The method of claim 2, including ramping said deflection voltages on said electrode pairs to scan said emission beam over an area. 4. The method of claim 2, including providing said suppressor cap in a microcolumn emission source and maximizing an output beam for said emission source by aligning said emission beam in said microcolumn by varying said deflection voltages. 5. The method of claim 4, including dynamically correcting for drift of said emission beam alignment during operation of said microcolumn by varying said deflection voltages. 6. An improved thermal field emission source, comprising: means for aligning an emission beam for the thermal field emission source, including a divided suppressor cap having at least four electrode segments, electrically isolated from one another and means for driving each said electrode with a suppressor voltage and a deflection voltage individual to each said electrode for moving and aligning for said emission beam. 7. The source of claim 6, including means for providing the same deflection voltage to opposite pairs of electrodes, the deflection voltage being of opposite polarity on each one of the electrodes of each pair of said electrodes. 8. The source of claim 7, including means for ramping said deflection voltages on said electrode pairs for scanning said emission beam over an area. 9. The source of claim 7, including said suppressor cap forming part of a microcolumn emission source and means for maximizing an output beam for said emission source by including means for aligning said emission beam in said microcolumn by varying said deflection voltages. 10. The source of claim 9, including means for dynamically correcting for drift of said emission beam alignment during operation of said microcolumn by varying said deflection voltages. 11. An improved thermal field emission source, comprising: a thermal field emission source, including an emission beam, a divided suppressor cap having at least four electrode segments which encompass said source, said electrode segments electrically isolated from one another and a power source coupling a suppressor voltage and a deflection voltage to individually drive each said electrode to move and align said emission beam. 12. The source of claim 11, including the same deflection voltage coupled to opposite pairs of electrodes, the deflection voltage being of opposite polarity on each one of the electrodes of each pair of said electrodes. 13. The source of claim 12, including said deflection voltages ramped on said electrode pairs to scan said emission beam over an area. 14. The source of claim 12, including said suppressor cap forming part of a microcolumn emission source, said microlens including an extractor with an aperture therein, said output beam maximized for said emission source, said emission beam aligned in said microcolumn with said extractor aperture by said deflection voltages. 15. The source of claim 14, including said emission beam alignment dynamically connected with said extractor aperture during operation of said microcolumn with said deflection voltages.

Inventors: CHANG TAI-HON PHILIP, KIM H S, MURAY L P, CHANG, TAI-HON, PHILIP, KIM,, H., S., MURAY, L., P.
Applicants: ETEC SYSTEMS INC, ETEC SYSTEMS, INC.
Application year: 1999
Main classification: H01J37
IPC codes: H01J37