Cold field emitter electron gun with self-cleaning extractor using reverse e-beam current.

Abstract

The e-beam device includes a cold field emission source 102 that emits electrons and an extraction electrode 104 that is positively biased with respect to the cold field emission source 102 to extract the electrons from the cold field emission source 102. The extraction electrode 104 has a first opening 112 for the electrons. The e-beam device further includes a mirror electrode 116 with a second opening 118 for the electrons. The mirror electrode 116 is configurable to be positively biased with respect to the extraction electrode 104 during a first mode of operation and negatively biased with respect to the extraction electrode 104 during a second mode of operation. The extraction electrode 104 is disposed between the cold field emission source 102 and the mirror electrode 116. The e-beam device further includes an anode 120 that is positively biased with respect to the extraction electrode 104 and the cold field emission source 102. The mirror electrode 116 is disposed between the extraction electrode 104 and the anode 120.

Description

[Technical field]

[0001] The present disclosure relates to electron beam ("e-beam") devices, and more particularly, to electron beam ("e-beam") devices for cleaning extraction electrodes of electron guns in e-beam devices. [Background technology]

[0002] Cold field emission sources may be a good choice for high throughput and high spatial resolution e-beam applications. However, their use in such applications suffers from residual gas pressure that can contaminate the cold field emission source, shorten its lifetime, and cause emission noise. Residual gas pressure in a typical extreme ultra-high vacuum environment (≈1e-11 Torr) is created by outgassing of vacuum chamber components. One such component is the extraction electrode (sometimes simply referred to as the extractor). Once the cold field emission source is installed and a nominal vacuum is achieved, the extraction electrode is baked out to reduce subsequent outgassing. However, bake-out alone is insufficient to completely desorb molecules from the surface of the extraction electrode in the vicinity of the cold field emission source. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] International Publication No. 2021 / 061500 Summary of the Invention [Problem to be solved by the invention]

[0004] Therefore, there is a need for a method and system for cleaning extraction electrodes. [Means for solving the problem]

[0005] In some embodiments, the e-beam device includes a cold field emission source that emits electrons and an extraction electrode that is positively biased with respect to the cold field emission source to extract the electrons from the cold field emission source. The extraction electrode has a first opening for the electrons. The e-beam device further includes a mirror electrode with a second opening for the electrons. The mirror electrode is configurable to be positively biased with respect to the extraction electrode during a first mode of operation and negatively biased with respect to the extraction electrode during a second mode of operation. The extraction electrode is disposed between the cold field emission source and the mirror electrode. The e-beam device further includes an anode that is positively biased with respect to the extraction electrode and the cold field emission source. The mirror electrode is disposed between the extraction electrode and the anode.

[0006] In some embodiments, the method includes providing an e-beam apparatus including a cold field emission source, an extraction electrode with a first opening for electrons from the cold field emission source, a mirror electrode with a second opening for the electrons, and an anode. The extraction electrode is disposed between the cold field emission source and the mirror electrode. The mirror electrode is disposed between the extraction electrode and the anode. The method further includes cleaning the extraction electrode, where cleaning the extraction electrode includes positively biasing the extraction electrode relative to the cold field emission source and the mirror electrode. The method further includes using the e-beam apparatus after cleaning the extraction electrode. Using the e-beam apparatus includes positively biasing the extraction electrode relative to the cold field emission source and positively biasing the mirror electrode and the anode relative to the extraction electrode.

[0007] For a better understanding of the various described implementations, reference should be made to the following detailed description taken in conjunction with the following drawings, in which: [Brief description of the drawings]

[0008] [Figure 1A] FIG. 2 is a cross-sectional side view of an e-beam device including a cold field emission source, an extraction electrode, a mirror electrode, and an anode when the e-beam device is configured for a normal operating mode in accordance with some embodiments. [Figure 1B]FIG. 1B is a cross-sectional side view of the e-beam apparatus of FIG. 1A configured for a mode of operation for cleaning an extraction electrode in accordance with some embodiments. [Diagram 2] FIG. 1C illustrates an example of the e-beam apparatus of FIGS. 1A-B, in which switches are used to configure the mirror electrodes, according to some embodiments. [Diagram 3] 1 is a flowchart illustrating a method of operating an e-beam device in accordance with some embodiments. [Figure 4] 1 is a graph showing the reduction in steady-state outgassing achieved through e-beam irradiation of a surface. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] Like numbers refer to corresponding parts throughout the figures and specification.

[0010] Reference will now be made in detail to various embodiments, examples of which are illustrated in the accompanying drawings. In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the various described embodiments. However, it will be apparent to those skilled in the art that the various described embodiments may be practiced without these specific details. In other instances, well-known methods, procedures, components, circuits, and networks have not been described in detail so as not to unnecessarily obscure aspects of the embodiments.

[0011] Cold field emission, sometimes simply called field emission, refers to the emission of electrons from a sharp-tipped emitter that is at a relatively negative potential (e.g., several kilovolts) with respect to a nearby electrode. Cold field emission can occur at room temperature (considered cold compared to thermionic emission) and can result in an e-beam that is several orders of magnitude more dense than the e-beam produced by thermionic emission.

[0012] 1A and 1B are cross-sectional side views of an e-beam device 100 including a cold field emission source 102, an extraction electrode 104, a mirror electrode 116, and an anode 120 arranged in that order, according to some embodiments. The cold field emission source 102 may also be referred to as a field emission source, a field emitter, or a cold field emitter. The extraction electrode 104 is disposed between the cold field emission source 102 and the mirror electrode 116, which is disposed between the extraction electrode 104 and the anode 120. The cold field emission source 102 has a pointed tip pointing towards the anode 120. In this configuration, the cold field emission source 102 functions as a cathode. These components are disposed within a vacuum chamber 122. The vacuum chamber 122 can achieve ultra-high vacuum (UHV). (UHV is defined as a vacuum chamber that is approximately 1000 sq. m.) -9 (E-beam device 100 is a standard, well-known technical term that refers to a degree of vacuum having a pressure on the order of 1000 MPa (3000 MPa) or less.) In some embodiments, e-beam device 100 is an electron microscope (eg, a scanning electron microscope (SEM)).

[0013] 1A illustrates an e-beam device 100 configured for a first mode of operation, according to some embodiments. The first mode of operation is a normal operating mode. A first voltage -V beam is applied to the cold field emission source 102. A second voltage +V ext -V beam is applied to the extraction electrode 104 to positively bias the extraction electrode 104 with respect to the cold field emission source 102. beam is 10kV, V ext Since V is 3 kV, −10 kV is applied to the cold field emission source 102 and −7 kV is applied to the extraction electrode 104. Other examples are possible. For example, ext ) can be 3 kV to 5 kV. The anode 120 and mirror electrode 116 are positively biased with respect to the cold field emission source 102 and the extraction electrode 104. For example, the anode 120 and mirror electrode 116 are grounded, with the mirror electrode 116 electrically connected to the anode 120.

[0014] With the components biased in this manner, the cold field emission source 102 emits electrons 114; the extraction electrode 104 extracts the electrons 114 from the cold field emission source 102. In this manner, the cold field emission source 102 and the extraction electrode 104 function as an electron gun for the e-beam device 100. Some electrons 114-1 pass through an aperture 112 in the extraction electrode 104 (e.g., in the center of the extraction electrode 104). The electrons 114-1 also pass through an aperture 118 in the mirror electrode 116 (e.g., in the center of the mirror electrode 116). The aperture 112 in the extraction electrode 104 is positioned below the cold field emission source 102. The aperture 118 in the mirror electrode 116 is positioned below the aperture 112 in the extraction electrode 104. In some embodiments, aperture 118 is wider than aperture 112 (e.g., such that substantially all of the electrons 114-1 that pass through aperture 112 also pass through aperture 118). In some embodiments, extraction electrode 104 and / or mirror electrode 116 are radially symmetric about an axis that extends through cold field emission source 102, aperture 112 (e.g., the center of aperture 112), and / or aperture 118 (e.g., the center of aperture 118).

[0015] Other electrons 114-2 emitted by the cold field emission source 102 miss the aperture 112 and impinge on a portion 106 of the top surface of the extraction electrode 104 that surrounds the aperture 112. The top surface of the extraction electrode 104 is the surface that faces the cold field emission source 102. The current from the electrons 114-2 impinging on the top surface portion 106 removes chemisorbed radicals (i.e., desorbs radicals) from the portion 106 by Desorption Induced by Electronic Transitions (DIET). Other surfaces of the extraction electrode 104, including the bottom surface 110 (i.e., the surface that faces the mirror electrode 116) and the portion 108 of the top surface beyond (e.g., surrounding) the portion 106, are impinged with a much smaller current than the portion 106. Thus, the portion 108 and the bottom surface 110 tend to have a chemisorbed monolayer that occasionally releases cations through induced desorption when struck by electrons. The emitted positive ions will follow the reversed electron path and reach the cold field emission source 102, reducing the emission lifetime and creating emission noise.

[0016] To clean and anneal the top surface portion 106 as well as the portion of the bottom surface 110 near the opening 112, including the inner bore of the extraction electrode 104, the electric field in the area between the extraction electrode 104 and the mirror electrode 116 is reversed relative to the normal operating mode. By reversing the electric field in this area, the e-beam (i.e., electrons 114-1) is directed around and strikes the portion 124 (FIG. 1B) of the bottom surface 110 (including the portion defining the inner bore of the extraction electrode 104), thereby desorbing radicals from the monolayer on the portion of the bottom surface 110 (which faces the mirror electrode 116). This step may be performed after baking out the extraction electrode 104 and / or flashing the cold field emission source 102. Ions released by this desorption will follow the reversed electric field and be directed away from the cold field emission source 102.

[0017] 1B illustrates the e-beam device 100 configured for a second mode of operation (i.e., a mode for cleaning the extraction electrode 104) according to some embodiments. As in the first mode of FIG. 1A, the extraction electrode 104 is positively biased with respect to the cold field emission source 102. For example, a first voltage -V beam is applied to the cold field emission source 102, and a second voltage +V ext -V beam is applied to the extraction electrode 104 to extract electrons 114 from the cold field emission source 102 (i.e., cause the cold field emission source 102 to emit electrons). As in the first mode, the anode 120 may be positively biased with respect to the extraction electrode 104 and the cold field emission source 102. For example, the anode 120 is grounded. However, the mirror electrode 116 is configured to be negatively biased with respect to the extraction electrode 104. As shown in FIG. 1B, in some embodiments, the mirror electrode 116 is electrically connected to the cold field emission source 102. For example, a first voltage -V beam is applied to both the cold field emission source 102 and the mirror electrode 116, while a second voltage +V ext -V beam is applied to the extraction electrode 104.

[0018] In this manner, the mirror electrode 116 can be configured to be positively biased with respect to the extraction electrode 104 during a first mode of operation and negatively biased with respect to the extraction electrode 104 during a second mode of operation. In some embodiments, the mirror electrode 116 can be configured to be electrically connected to the anode 120 during the first mode of operation (as shown in FIG. 1A) and to the cold field emission source 102 during the second mode of operation (as shown in FIG. 1B).

[0019] Make the mirror electrode 116 sufficiently negative with respect to the extraction electrode 104 (e.g., at least V extBy biasing the aperture 112 (by itself), electrons 114-1 passing through the aperture 112 are directed back towards the extraction electrode 104 where they collide with the bottom portion 124, thus cleaning and annealing the portion 124. The mirror electrode 116 effectively acts as a mirror that reflects the electrons 114-1 back to the extraction electrode 104, causing the extraction electrode 104 to self-clean.

[0020] FIG. 2 is a cross-sectional side view of an e-beam device 200, which is an example of the e-beam device 100 (FIGS. 1A-1B), according to some embodiments. The e-beam device 200 includes a switch 202 that is used to configure (i.e., bias) the mirror electrode 116. The switch 202 is used to electrically connect the mirror electrode 116 to the anode 120 during a first mode of operation (i.e., during a normal operating mode) and to the cold field emission source 102 during a second mode of operation (i.e., during an extraction electrode cleaning mode). The mirror electrode 116, the cold field emission source 102, and the anode 120 are electrically connected to respective terminals of the switch 202. The switch 202 is controlled by a control signal 204. In the first mode of operation, the control signal 204 causes the switch 202 to connect the terminal for the mirror electrode 116 to the terminal for the anode 120. In the second mode of operation, the control signal 204 causes the switch 202 to connect the terminal for the mirror electrode 116 to the terminal for the cold field emission source 102. In some embodiments, the control signal 204 may be asserted for the first mode of operation and deasserted for the second mode of operation, or vice versa.

[0021] FIG. 3 is a flow chart illustrating a method 300 of operating an e-beam apparatus, according to some embodiments. In the method 300, an e-beam apparatus (e.g., apparatus 100, FIGS. 1A-1B; apparatus 200, FIG. 2) is provided (302) including a cold field emission source (e.g., cold field emission source 102), an extraction electrode (e.g., extraction electrode 104) with a first opening (e.g., opening 112) for electrons from the cold field emission source, a mirror electrode (e.g., mirror electrode 116) with a second opening (e.g., opening 118) for the electrons, and an anode (e.g., anode 120). The extraction electrode is disposed between the cold field emission source and the mirror electrode. The mirror electrode is disposed between the extraction electrode and the anode. In some embodiments, the first opening is positioned below the cold field emission source and the second opening is positioned below the first opening. The second opening may be wider than the first opening. The extraction electrode and the mirror electrode may be radially symmetric about an axis passing through the cold field emission source, the first opening (e.g., the center of the first opening), and / or the second opening (e.g., the center of the second opening).

[0022] To reduce contamination and achieve a high vacuum, the extraction electrode is baked out (304) and the cold field emission source is flushed (304). The bakeout may be performed at approximately 200° C. by direct heating of the extraction electrode via a resistor. Flashing refers to briefly heating the cold field emission source (e.g., for a few seconds) to clean its surface. The bakeout and flashing are performed before turning on the e-beam.

[0023] To further reduce contamination, the extraction electrode is cleaned (306). This cleaning may occur after bakeout and flashing. Cleaning the extraction electrode includes positively biasing the extraction electrode relative to the cold field emission source and the mirror electrode. Positively biasing the extraction electrode relative to the cold field emission source extracts electrons from the cold field emission source, thereby turning on the e-beam. By positively biasing the extraction electrode relative to the mirror electrode, the mirror electrode acts as a mirror that reflects the electrons back to the bottom surface of the extraction electrode.

[0024] In some embodiments, the mirror electrode is electrically connected (308) to a cold field emission source (e.g., as shown in FIG. 1B) to clean the extraction electrode. For example, a switch (e.g., switch 202, FIG. 2) is configured to electrically connect (310) the mirror electrode to the cold field emission source. In some embodiments, a first negative voltage (e.g., −V beam ) is applied to the cold field emission source and the mirror electrode (312) (e.g., applied to the cold field emission source while the mirror electrode remains electrically connected to the cold field emission source), and a second negative voltage (e.g., +V ext -V beam ) is applied to the extraction electrode (312). The magnitude of the second negative voltage is greater than the magnitude of the first negative voltage (e.g., by an amount V ext For example, the first negative voltage is −10 kV and the second negative voltage is −7 kV. In another example, the difference between the second negative voltage and the first negative voltage is 3 kV to 5 kV.

[0025] After the extraction electrode is cleaned, the e-beam device is used (314). To use the e-beam device, the extraction electrode is positively biased with respect to the cold field emission source, and the mirror electrode and the anode are positively biased with respect to the extraction electrode. In some embodiments, the mirror electrode is electrically connected to the anode (316) (e.g., as shown in FIG. 1A). For example, a switch (e.g., switch 202, FIG. 2) is configured to electrically connect the mirror electrode to the anode (318). The mirror electrode and the anode may be grounded (320) (e.g., the anode may be grounded and the mirror electrode may be connected to the anode, thereby also grounding the mirror electrode). A first negative voltage (e.g., −V beam ) may be applied to the cold field emission source (322), and a second negative voltage (e.g., +V ext -V beam ) may be applied to the extraction electrode (322).

[0026] The method 300 improves the cleaning of the surface of the extraction electrode by utilizing electron transfer induced desorption in the cleaning step 306. Thus, the performance of the cold field emission source during use of the e-beam tool (i.e., during step 314) is improved, as is the lifetime of the cold field emission source.

[0027] FIG. 4 is a graph 400 illustrating the reduction in steady-state outgassing achieved through e-beam irradiation of a surface. To produce graph 400, the anode 120 in the configuration of FIG. 1B was replaced with a microchannel plate that functions as an ion detector. The bias 402 of the ion detector is varied and the ion current 404 measured by the ion detector is recorded for different values ​​of bias 402. The bias 402 is relative to the potential of the extraction electrode 104. The vacuum pressure was 1e-8 Torr (i.e., 10 -8Torr). When bias 402 is lowered to -1000V, current 404 is in region 406, which corresponds to energetic ions created at the extraction electrode by desorption (i.e., by DIET). In region 406, current 404 is substantially constant and independent of bias 402. As bias 402 drops below -1000V and becomes increasingly more negative, current 404 enters region 408, which becomes more negative, i.e., the absolute value of the current increases. Region 408 corresponds to lower energy ions produced by electron ionization (i.e., free space ionization) along the beam path, and the additional current in region 408 compared to region 406 is due to ions created by free space ionization. The increasing magnitude of current 404 as bias 402 becomes more negative indicates that ions produced by free space ionization outnumber ions produced by desorption from the extraction electrode 104; i.e., the surface of the extraction electrode 104 produces fewer ions than free space electron ionization. These results indicate that constant electron bombardment of the extraction electrode 104 removes chemisorbed contaminants from the extraction electrode 104 and minimizes steady-state desorption to the extraction electrode 104 by preventing the formation of a monolayer of chemisorbed contaminants.

[0028] The previous description for purposes of illustration has been described with reference to specific embodiments. However, the above exemplary discussion is not intended to be exhaustive or to limit the scope of the claims to the precise form disclosed. Many modifications and variations are possible in light of the above teachings. The embodiments have been selected to best explain the principles underlying the claims and their practical application, so that those skilled in the art can best use the embodiments with various modifications as appropriate for the particular use envisaged.

Claims

1. a cold field emission source for emitting electrons; an extraction electrode positively biased with respect to the cold field emission source for extracting the electrons from the cold field emission source, the extraction electrode having a first opening for the electrons; a mirror electrode having a second opening for the electrons and configurable to be positively biased with respect to the extraction electrode during a first mode of operation and negatively biased with respect to the extraction electrode during a second mode of operation, the extraction electrode being disposed between the cold field emission source and the mirror electrode; and an anode positively biased with respect to said extraction electrode and said cold field emission source, said mirror electrode being disposed between said extraction electrode and said anode.

2. 2. The apparatus of claim 1 , wherein the mirror electrode is configurable to be electrically connected to the anode during the first mode of operation and to be electrically connected to the cold field emission source during the second mode of operation.

3. 3. The apparatus of claim 2, further comprising a switch for electrically connecting the mirror electrode to the anode during the first mode of operation and to the cold field emission source during the second mode of operation.

4. 3. The apparatus of claim 2, wherein the anode is grounded.

5. 2. The apparatus of claim 1, the first opening is positioned below the cold field emission source; The apparatus, wherein the second opening is positioned below the first opening.

6. 6. The apparatus of claim 5, wherein the second opening is wider than the first opening.

7. 6. The apparatus of claim 5, wherein the extraction electrode and the mirror electrode are radially symmetric about an axis passing through the cold field emission source, the first opening, and the second opening.

8. 2. The apparatus of claim 1, the first mode of operation is a normal operating mode; 4. The apparatus according to claim 3, wherein said second mode of operation is a mode for purifying said extraction electrode.

9. Providing an electron beam device comprising a cold field emission source, an extraction electrode with a first opening for electrons from the cold field emission source, a mirror electrode with a second opening for the electrons, and an anode; the extraction electrode is disposed between the cold field emission source and the mirror electrode; providing, the mirror electrode being disposed between the extraction electrode and the anode; cleaning the extraction electrode, the cleaning comprising positively biasing the extraction electrode with respect to the cold field emission source and the mirror electrode; using the electron beam device after cleaning the extraction electrode, positively biasing the extraction electrode with respect to the cold field emission source; positively biasing the mirror electrode and the anode with respect to the extraction electrode. The method according to claim 1, further comprising:

10. 10. The method of claim 9, wherein positively biasing the extraction electrode with respect to the cold field emission source and the mirror electrode comprises electrically connecting the mirror electrode to the cold field emission source.

11. 10. The method of claim 9, wherein positively biasing the mirror electrode and the anode with respect to the extraction electrode comprises electrically connecting the mirror electrode to the anode.

12. 12. The method of claim 11, wherein positively biasing the mirror electrode and the anode with respect to the extraction electrode further comprises grounding the mirror electrode and the anode.

13. 10. The method of claim 9, positively biasing the extraction electrode relative to the cold field emission source comprises applying a first negative voltage to the cold field emission source and applying a second negative voltage to the extraction electrode, the magnitude of the second negative voltage being less than the magnitude of the first negative voltage; positively biasing the extraction electrode relative to the cold field emission source and the mirror electrode comprises applying the first negative voltage to the cold field emission source and the mirror electrode and applying the second negative voltage to the extraction electrode; 4. The method of claim 3, wherein positively biasing the mirror electrode and the anode with respect to the extraction electrode comprises grounding the mirror electrode and the anode.

14. 10. The method of claim 9, the electron beam device further comprising a switch for selectively electrically connecting the mirror electrode to the cold field emission source or the anode; positively biasing the extraction electrode with respect to the cold field emission source and the mirror electrode includes configuring the switch to electrically connect the mirror electrode to the cold field emission source; 4. The method of claim 3, wherein positively biasing the mirror electrode and the anode relative to the extraction electrode comprises configuring the switch to electrically connect the mirror electrode to the anode.

15. 15. The method of claim 14, positively biasing the extraction electrode relative to the cold field emission source and the mirror electrode further comprises applying a first negative voltage to the cold field emission source and the mirror electrode, and applying a second negative voltage to the extraction electrode, the magnitude of the second negative voltage being less than the magnitude of the first negative voltage; positively biasing the extraction electrode relative to the cold field emission source comprises applying the first negative voltage to the cold field emission source and applying the second negative voltage to the extraction electrode; 13. The method of claim 12, wherein positively biasing the mirror electrode and the anode with respect to the extraction electrode further comprises grounding the mirror electrode and the anode.

16. 10. The method of claim 9, further comprising baking out the extraction electrode and flushing the cold field emission source prior to cleaning the extraction electrode.

17. 10. The method of claim 9, the first opening is positioned below the cold field emission source; The method of claim 1, wherein the second opening is positioned below the first opening.

18. 20. The method of claim 17, wherein the second opening is wider than the first opening.

19. 20. The method of claim 17, wherein the extraction electrode and the mirror electrode are radially symmetric about an axis passing through the cold field emission source, the first opening, and the second opening.

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