Cold field emitter assembly

Abstract

To provide apparatus and methods for a mechanically stable, long-life, cold field emitter assembly which is compatible with ultra-high vacuum and occasional high-temperature flashing.SOLUTION: A metal adapter is welded between a hexaboride electrode and a metal filament. Some embodiments use a tungsten filament, a tantalum adapter, and a LaB6 microrod electrode with a nanorod emitter tip. Other material combinations are disclosed, as also usage in electron sources for electron microscopes. In variations, the adapter is deposited onto the filament and the electrode then welded to the adapter.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] The present disclosure relates to electron sources. [Background technology]

[0002] Cold field emission electron sources operating at or near room temperature offer higher brightness, better beam coherence, higher position stability, and longer lifetimes compared to sources operating at higher temperatures, making them very attractive for use in high-resolution electron microscopes such as S / TEMs, SEMs, and other applications. Lanthanum hexaboride ("LaB6") is a refractory ceramic with good electrical conductivity and a low work function. LaB6 cold field emitters have been used for approximately 10 -10 We have demonstrated stable emission at moderate vacuums of 1000 torr, showing great potential as a next-generation cold field emission source. However, material incompatibility between LaB6 and commonly used emitter support materials can lead to erosion over time. Some conventional approaches to overcome this problem use graphite paste to bond LaB6 to the support without direct contact between the LaB6 and the support. When present in large quantities, graphite paste can create trapped gas reservoirs, which can make source operation difficult in ultrahigh vacuum and contaminate the LaB6 emitter during source operation. Therefore, improved techniques for implementing LaB6 cold field sources for efficient and stable operation over a long lifetime are needed. Summary of the Invention

[0003] In summary, an example of the disclosed technology uses an adapter to connect an electrode to a support filament. Each fusion zone joins the adapter to the electrode and filament. Illustratively, a LaB6 electrode can be attached to a tungsten filament via a tantalum adapter, although other materials and combinations can be used. The fusion zone can be formed by spot welding or laser welding without filler material to obtain a mechanically stable, ultra-high vacuum compatible emitter assembly.

[0004] In other examples, welding can be used to form a fusion zone between the adapter and the electrode, and other techniques can be used to assemble the adapter and filament. Illustratively, a tantalum adapter can be formed by deposition onto the filament.

[0005] The foregoing and other objects, features, and advantages of the present invention will become more apparent from the following detailed description which proceeds with reference to the accompanying drawings. [Brief explanation of the drawings]

[0006] [Figure 1A] 1 is a diagram of an electron microscope and its electron source to which embodiments of the disclosed technology can be applied. [Figure 1B] 1 is a diagram of an electron microscope and its electron source to which embodiments of the disclosed technology can be applied. [Figure 2] 1 is an image of a conventional emitter structure. [Figure 3A] 10 is an SEM image of a LaB6 electrode with a nanorod tip, which can be incorporated into an example of the disclosed technology. [Figure 3B] 10 is an SEM image of a LaB6 electrode with a nanorod tip, which can be incorporated into an example of the disclosed technology. [Figure 4] FIG. 1 is a side view of an exemplary emitter structure in accordance with the disclosed technology. [Figure 5] 1 is a photomicrograph of an exemplary emitter structure according to the disclosed technology. [Figure 6] 1 is a photomicrograph of an exemplary emitter structure according to the disclosed technology. [Figure 7] 1 is a diagram of an exemplary emitter structure in accordance with the disclosed technology; [Figure 8] 1 is a diagram of an exemplary emitter structure in accordance with the disclosed technology; [Figure 9] 1 is a diagram of an exemplary emitter structure in accordance with the disclosed technology; [Figure 10]1 is a diagram of an exemplary emitter structure in accordance with the disclosed technology; [Figure 11] 1 is a flowchart of a first exemplary method in accordance with the disclosed technology. [Figure 12] 10 is a flowchart of a second exemplary method in accordance with the disclosed technology. [Figure 13] 1A-1C are diagrams of exemplary configurations for spot welding components of an emitter structure in accordance with the disclosed technology; [Figure 14] 1A-1C are diagrams of exemplary configurations for laser welding components of an emitter structure in accordance with the disclosed technology. DETAILED DESCRIPTION OF THE INVENTION

[0007] Introduction Electron microscopes are widely used for materials and device analysis, as well as for biological research. Various configurations are used, including SEM, TEM, and STEM. In all of these configurations, an electron beam is generated by an electron source and delivered to the sample. Electron sources commonly utilize thermionic emission, Schottky emission, or cold field emission. Cold field emission can be advantageous in providing higher brightness, lower energy spread, better coherence, and longer lifetime, primarily due to low temperature (≦300 K) operation, and appears as a smaller virtual source than other emitter types. Cold field emitters offer an electron energy spread of approximately 0.2–0.3 eV, and a 10 9 A / (sr.m 2 A practical brightness of over 1.5V can be achieved. Cold field emission sources generally have an electrode with a tip (called the "emitter") having a lateral extent of a few to a few hundred nanometers, where electron emission can occur under the influence of an externally applied electric field.

[0008] Lanthanum hexaboride (LaB6) is an attractive material for cold field emitters due to its low work function and high conductivity, especially at temperatures of ~10 -10They have demonstrated stable cold emission in a moderate vacuum of 1000 Torr. As a ceramic material, LaB6 can be fabricated into nanorod tips and can withstand high temperatures. Other rare-earth hexaborides can also be used.

[0009] However, considerations of material compatibility, vacuum compatibility, and mechanical stability can adversely affect performance. Cold field emitters, in particular, can be operated at room temperature for electron microscopy, but in-situ cleaning of the emitter is often accomplished by resistively heating the emitter assembly, also known as "thermal flashing." For effective heating, it may be desirable to mount the emitter electrode directly on the filament. Tungsten, a common emitter filament material with a high melting point, can generate and withstand temperatures exceeding 2000 °C without mechanical problems.

[0010] However, tungsten and LaB6 may not bond well directly: the bond between tungsten and LaB6 is susceptible to erosion and atomic migration (especially at high cleaning temperatures), which can lead to mechanical drift or bond failure.

[0011] Some examples of the disclosed technology use an adapter made of tantalum to join a filament (e.g., tungsten) and an electrode (e.g., LaB6). Tantalum offers good compatibility with LaB6 and a low rate of joint degradation. As a metal, tantalum is vacuum compatible and can be welded to both tungsten and LaB6 without filler. Tantalum has a high melting point, similar to LaB6 and tungsten. Therefore, a welded assembly of a tungsten filament, a tantalum adapter, and a LaB6 filament can withstand high temperatures and be welded to an ultra-high vacuum (e.g., 4×10) with low outgassing. -10 It can support temperatures below 1000kJ / s and provide excellent mechanical stability during thermal flashing in the range of 600K to 2200K.

[0012] The above-mentioned material selections are exemplary. Other material combinations can be used as well. For example, the filament material can include tungsten alloy, rhenium, or a rhenium alloy. The adapter material can include tantalum or a tantalum alloy. The electrode material can include other hexaborides. Other filament, adapter, or electrode materials can also be used with the disclosed technology. In a variation, a tungsten alloy filament material can be used to selectively control filament resistivity, mechanical filament strength, or weld reliability.

[0013] 1A and 1B are diagrams of an electron microscope 100 and electron source to which examples of the disclosed technology can be applied. An electron source 111, electron optics 121, and sample 131 can be arranged along an axis 105. The electron source 111 can generate an electron beam 115, which can be shaped by optics 121 and delivered to the sample 131. A detector 141 can detect transmitted electrons, for example, in a TEM or STEM configuration, and a detector 142 can detect secondary particles, for example, in an SEM configuration. The sample 131 is not part of the illustrated electron microscope and is shown in dashed outline. Furthermore, a given electron microscope can have only one of detectors 141 or 142.

[0014] Figure IB is a schematic diagram of an electron source 102 that can incorporate the disclosed technology. An electron source 111 can be implemented as shown in Figure IB.

[0015] The emitter 130 can be attached to the filament 110 using any of the techniques described herein. Details of the adapter and various emitters are omitted from FIG. 1B for simplicity. The emitter 130, in conjunction with the anodes 150, 160, can generate an electron beam 115 along a centerline 105. The centerline 105 passes through apertures 152, 162 in the extraction anode 150 and acceleration anode 160, respectively.

[0016] The electron source 102 can incorporate at least four electrical terminals 141-144. A power supply 172 can be coupled to terminals 141 and 142 to drive current through the filament 110 for thermal regulation of the electron source 102. A power supply 173 can apply an extraction voltage V1 between the emitter 130 (coupled to terminal 142) and a terminal 143 of the extraction anode 150. A power supply 176 can apply acceleration voltages V2-V1 between the extraction anode 150 and a terminal 144 of the acceleration anode 160.

[0017] Figure 2 shows an image 200 of a conventional emitter structure before tip formation. Dimensions are indicated by a scale 202. Because graphite is not suitable for welding, a bead of water-based graphite paste 220 can be applied to join the filament 210, and an electrode 230 can be attached to the graphite paste 220 to form the emitter assembly. As shown, a substantial mass of water-based graphite paste 220 may be required to support the mass of an electrode 230 having a thickness of 50 μm or more and a length greater than 1.2 mm. Graphite has good material compatibility with LaB6. However, as water evaporates from the paste 220 during room-temperature curing, pumping, baking, and emitter flushing, trapped gas pockets can be left behind, which can continue to outgas over time, degrading the vacuum around the electron source and potentially leading to emitter contamination. The disclosed technology can completely eliminate the graphite paste, thereby providing improved performance.

[0018] 3A and 3B are SEM images 301, 302 of exemplary LaB6 electrodes with nanorod tips that can be incorporated into the disclosed emitter assemblies. The illustrated electrodes have microrod structures with nanorod emitter tips. The illustrated electrodes have diameters that are stepped from about 100 μm to about 80 nm. Shaping of the microrods to form the nanorod tips can be accomplished by ion milling, for example, using a focused ion beam (FIB) tool.

[0019] First, a generally square microrod (see, e.g., microrod 740 in FIG. 7 ) with a diameter of approximately 100 μm can form a first step, which can be tapered at its distal end, as seen in shoulder 311 in FIG. 3A . The second step 312 can be a microrod section with a diameter of approximately 5 μm and a length of approximately 30 μm, with another shoulder leading to nanorod tip 310, which is shown in more detail in FIG. 3B . In this example, tip 310 has third and fourth nanorod layers 313 and 314. As shown, step 313 has a length of approximately 1.6 μm and a diameter of approximately 500 nm, and step 314 has a diameter of approximately 80 nm and a length of approximately 1.27 μm. Furthermore, as shown, step 314 has a sharp tip. In this example, the tip radius is approximately 10 nm.

[0020] term An "adapter" is a physical component that joins two other components. In some disclosed embodiments, the adapter is tantalum or a tantalum alloy and is used to join a filament and an electron electrode in an electron emitter assembly.

[0021] The term "atomic percent" (or "at%") refers to the percentage of atoms in a given device or material that are a particular element or set of elements. Thus, atomic percent can apply to a single element such as boron, a periodic table series such as the rare earths or lanthanides, a group of elements such as the halogens, a periodic table block such as the transition metals, or another particular set of elements. The similar term "weight percent" (or "wt%") refers to the percentage of weight contributed by a particular element or set of elements.

[0022] As a verb, "to clamp" refers to the act of holding two physical objects together by the reversible application of force. Because welding, soldering, and adhesive bonding are not reversible (requiring different, unrelated operations for assembly and disassembly), they are not considered clamping operations. As a noun, a "clamp" is a device that can reversibly apply force to two physical objects to hold them together.

[0023] As a noun, "current" refers to an "electrical current," i.e., a flow of charged particles. Current may flow through a wire or other conductive material, but this is not a requirement. An electron beam is an example of an electric current flowing in a vacuum.

[0024] The term "cylindrical" refers to an elongated object ("cylinder") having a uniform cross-sectional shape over a length of at least three times the maximum transverse extent (called the "diameter") of that cross-sectional shape. A "wire" is a conductive cylinder. While some cylindrical objects described herein have a generally circular cross-section, this is not a requirement. For example, various microrods or nanorods shown herein have a square cross-section and are also cylindrical. Rectangular microrod cylinders (e.g., 50 μm × 100 μm cross-sections) can also be used. The length of a cylinder can be measured along its "axis," which is the line connecting the centroids of successive cross-sections. The axis need not be straight; a cylinder (e.g., filament 110) can have bends. The requirement of a uniform cross-sectional shape means that the diameter and cross-sectional area can vary by up to 1.5 times over the required length. However, a cylinder can have a twist about its axis while maintaining a uniform cross-sectional shape. Some electrodes of interest herein (see Figures 3A and 3B) may have, for example, segmented cylindrical sections joined by tapered sections that may not be cylindrical.

[0025] As a verb, "depositing" refers to the act of applying a particulate material as a layer onto a physical object. In some examples, this act can be formed by physical or chemical vapor deposition, sputtering, plating, or powder-based additive manufacturing. Laminating, wrapping, or welding are not considered deposition because they apply bulk material (as opposed to particulate material) to a physical object.

[0026] An "electric field" is a manifestation of electrical energy stored within a volume, which exerts a force on charges within the volume or at the volume's boundary. Generally, applying a voltage between two objects can generate an electric field in the region between the objects. Electric fields can also exist in propagating electromagnetic energy, such as laser beams.

[0027] An "electrode" is a conductive object through which charged particles flow to or from a vacuum, gas, or another fluid. Electrodes can often be metallic, but this is not a requirement. Of interest in the disclosed example is a LaB6 electrode, which acts as a field emission source of electrons.

[0028] An "electron emitter" (or simply "emitter") is a device that can be used as an electron source in an electron microscope or other instrument. A device does not need to actually emit electrons to be considered an electron emitter. Electron emitters generally operate via cold field emission, where electrons can be pulled from the emitter by a strong electric field (e.g., at room temperature or temperatures below 200°C), or via thermionic and Schottky emission, where electrons gain sufficient energy from both heat and electricity to overcome the work function of the electrode material and thus escape from the electrode. As used herein, the terms emitter and electron emitter can refer to the portion of the electrode (e.g., 314) from which electron emission occurs; an electrode can be a monolithic object from which an emitter can be formed; and an "emitter structure" or "emitter assembly" can refer to an assembly of an electrode and one or more other objects (e.g., a filament or adapter; see Figure 4), including an unfinished assembly lacking an emitter tip.

[0029] A "filament" is an electrically conductive device that can be used to convert electrical energy (e.g., electrical current flowing through the filament) into heat. A device does not need to actually generate heat to be considered a filament.

[0030] A "fusion zone" is a region at the interface between two welded bodies where materials from the two bodies are intermixed. Thus, if the two bodies have distinct compositions, the local material composition within the fusion zone can be different from the composition of either body.

[0031] "Installation" refers to the act of incorporating an object into an assembly of multiple objects.

[0032] A "microrod" is an elongated structure having a maximum transverse dimension in the range of 1-500 μm. A "nanorod" is an elongated structure having a maximum transverse dimension in the range of 1-1000 nm. Some electrodes of interest herein are microrods with nanorod tips. Some microrods and nanorods described herein are cylindrical, e.g., having a square or rectangular cross section, although this is not a requirement.

[0033] The term "pulse" refers to a waveform that has a finite temporal range, is continuous over that range, has an average value over that range, and crosses that average value exactly twice, e.g., once on the rise and once on the fall. By way of example, electrical current, laser energy, or other forms of energy can be applied as pulses.

[0034] A "terminal" is an attachment point of an electrical component through which voltage or current signals can flow to or from the electrical component.

[0035] "Thickness" refers to the longitudinal dimension of an object in the direction of current flow, energy flow, or application of clamping force.

[0036] As a verb, "welding" refers to joining two physical objects by the application of energy, melting adjacent portions of the two objects and joining them together after the melted portions cool and solidify. The molten materials of the two objects can intermingle to form a fusion zone. Brazing and soldering are not considered welding because they do not melt portions of both objects. Welding can be performed with or without a filler material, the latter being called "autogenous." Some examples of the disclosed techniques use optical energy to cause melting, e.g., "laser welding." Other examples use electrical energy to cause melting, e.g., "spot welding." Both laser welding and spot welding can be autogenous. As a noun, "weld" refers to the area of ​​the welded objects that was in a molten state during the welding operation. At least a portion of the weld can be a fusion zone. Welds formed by spot welding or laser welding are called "spot welds" or "laser welds," respectively. In some examples, the energy deposited in a single welding operation can be distributed across multiple object boundaries, resulting in simultaneous welding of multiple object pairs.

[0037] As a verb, "wrap" refers to the act of bending a sheet object around another object, at least up to the point where the sheet object contacts itself. In some disclosed embodiments, the adapter can be implemented as a foil wrapped around the filament.

[0038] Exemplary Emitter Structures 4 is a cross-sectional view 400 of an exemplary structure in accordance with the disclosed technology. In this structure, an adapter 420 can be coupled to a filament 410 by a first fusion zone 415 and to an emitter electrode 430 by a second fusion zone 425. The material of the adapter 420 can have excellent interface properties with one or both of the filament 410 and the electrode 430, avoiding problems of erosion or mechanical instability with direct electrode-filament bonds. Furthermore, a bond in the form of a fusion zone characteristic of a weld can provide a structure that is ultra-high vacuum compatible and free of outgassing problems associated with some other types of bonds.

[0039] In some embodiments, the filament 410 can incorporate at least 50 at%, 80 at%, or 90 at% tungsten, rhenium, or a tungsten-rhenium alloy. The portion of the filament 410 adjacent the first fusion zone 415 can be a cylindrical wire having a maximum transverse extent in the range of 25 to 500 μm.

[0040] The filament 410 and the adapter 420 can have different material compositions. The adapter 420 can incorporate at least 80 at%, 90 at%, 95 at%, 98 at%, or 99 at% tantalum. Substantially flat embodiments of the adapter 420 (see, e.g., FIGS. 9-10 ) can have thicknesses ranging from 1 to 500 μm, 5 to 100 μm, 25 to 125 μm, 10 to 50 μm, or 20 to 30 μm. Other adapters 420 (see, e.g., FIGS. 7-8 ) can have distances between the centers of gravity of the first fusion zone 415 and the second fusion zone 425 in any of these ranges.

[0041] The electrode 430 can incorporate at least 60 at%, 80 at%, or 85 at% boron. The electrode 430 can incorporate a rare earth hexaboride, a lanthanide hexaboride, or a hexaboride material such as LaB6, among others. The portion of the electrode 430 adjacent the second fusion zone 425 can be a microrod that connects to a nanorod tip at the distal end of the electrode 430. Arrows 432 indicate the distal and proximal directions of the electrode 430 relative to the depicted emitter assembly.

[0042] The combination of a tungsten filament, tantalum foil adapter, and LaB6 electrode is particularly advantageous, combining the rigidity of tungsten, the material compatibility of tantalum, and the low work function of LaB6. This combination can be welded, and all components can withstand very high temperatures above 2000°C. Negligible position drift of the emitter tip can be achieved under periodic thermal flashes below 2200K.

[0043] The first fusion zone 415 can incorporate a mixture of a first material (e.g., tungsten) of the filament (e.g., 410) and a second material (e.g., tantalum) of the adapter (e.g., 420). The second fusion zone 425 can incorporate a mixture of a second material (e.g., tantalum) of the adapter (e.g., 420) and a third material (e.g., LaB6) of the electrode (e.g., 430). The first fusion zone 415 or the second fusion zone 425 can be a weld, such as a spot weld or a laser weld. Because the fusion zone can have a different composition than either of the joined bodies, the fusion zone can be a different component of the assembled structure.

[0044] An electron source similar to 102 in FIG. 1B can incorporate the structure of FIG. 4. Electron emission can occur at the (distal) tip of the electrode 430, and thus the tip is the emitter (130). The electron beam (115) can be emitted directed generally along the centerline (105) of the tip. The electron source can further include an extraction anode (150) having an aperture (152) through which the centerline of the tip passes. By applying an extraction voltage (V1) between the structure (tip) and the extraction anode, an electric field can be generated at the tip that is strong enough to extract electrons from the tip toward the extraction anode. An acceleration anode (160) can be positioned downstream of the extraction anode and also includes an aperture (162) through which the centerline of the tip passes. The voltage difference (V2-V1) between the extraction anode and the acceleration anode can generate an electric field that causes the emitted electrons passing through the aperture of the extraction anode to further accelerate toward the acceleration anode and then pass through the aperture of the acceleration anode toward the target (131).

[0045] The electron source can incorporate at least four electrical terminals (141-144) to which voltages or currents can be applied to operate the electron source, including (i) a current through the filament (110), (ii) an extraction voltage applied to an extraction anode relative to the emitter structure, and (iii) an acceleration voltage applied to an acceleration anode relative to the extraction anode.

[0046] Furthermore, this electron source incorporating the structure of FIG. 4 can itself be installed in an electron microscope similar to that of FIG. 1A.

[0047] 5 and 6 are photomicrographs 500, 600 of exemplary emitter structures, each with a scale 502, 602. In FIG. 5, a tungsten filament 510 has a generally circular cross-section and is wrapped around a tantalum foil adapter 530. The filament 510 and foil 530 can be welded together, forming a fusion zone (similar to 415 in FIG. 4) not visible in FIG. 5. A LaB6 electrode 540 can be welded to the foil 530. Some evidence of melting is visible in the contact area 554, but most of the fusion zone (425) between the electrode 540 and the foil 530 is hidden beneath the electrode 540. The electrode 540 has a tip 542 capable of emitting electrons under the influence of an applied external electric field. In variations, other material compositions can be used for the filament 510, foil 530, or electrode 540, as disclosed herein.

[0048] FIG. 6 shows another exemplary emitter assembly. While the electrode 540 in FIG. 5 is symmetrically attached to the two arms of the filament 510, the electrode 640 in FIG. 6 is asymmetrically attached to the two arms of the filament 610. In particular, the tantalum sleeve adapter 630 has an elongated cross-section. The filament 610 and sleeve 630 are folded to provide a generally flat surface to which the electrode 640 can be conveniently attached. The filament 610, sleeve adapter 630, and electrode 640 can have material compositions similar to those in FIG. 5 or other compositions. Within the contact region 654, a marginal portion of the fusion zone between the electrode 640 and the sleeve 630 can be seen. In FIG. 6, the emitter tip has not yet been formed at the distal end 643 of the electrode 640.

[0049] 7-10 are views 700, 800, 900, and 1000 of additional emitter structures. In FIG. 7, a filament 720 is welded to an adapter 730. The top of the adapter 730 can be a hollow cylinder into which an electrode 740 can be inserted. Clamping force and / or welding energy can be applied in the direction indicated by arrow 752 to bond the electrode 740 to the adapter 730. The hollow cylindrical shape of the adapter 730 can constrain the orientation of the electrode 740, which can be advantageous for fixation during assembly.

[0050] 8 shows a variation in which the top of the adapter 830 has a through slot to which an electrode 840 can be attached. The filament 820 and force / energy arrow 852 are similar to their counterparts in FIG. 7 and will not be described further. The slotted shape of the adapter 830 allows external access to the interface between the adapter 830 and the electrode 840, which may be advantageous for laser welding, for example, with a laser beam oriented at an oblique angle relative to the arrow 852.

[0051] Figures 7-8 show emitter structures in which the adapter bonds to the filament and electrode are spaced apart or not collinear, allowing the adapter-filament and adapter-electrode bonds to be formed separately. In contrast, Figures 9-10 show emitter structures in which the adapter-filament and adapter-electrode bonds are close together or collinear, allowing both bonds to be formed in a single welding operation.

[0052] 9, an adapter 930 is formed around a filament 920. The electrode 940, adapter 930, and filament 920 can all be clamped together as an assembly by applying a force in direction 952. The adapter-filament and adapter-electrode fusion zones can then be advantageously formed in a single spot welding operation by passing a current through the assembly in the direction indicated by either arrow 952. Alternatively, such an assembly can be welded by laser welding, optionally repositioning the laser between multiple welds.

[0053] FIG. 10 illustrates a variation. In FIG. 9 , the adapter 930 extends substantially along the length of the filament 920 (or over 50% of the length of the filament), while in FIG. 10 , the adapter 1030 extends along a smaller portion of the length of the filament 1020 (or less than 50% of the length of the filament). A longer sleeve on the adapter 930 may make clamping easier for manufacturing and welding and may further protect the surface of the filament 920 from degradation at high temperatures. Conversely, a shorter sleeve on the adapter 1030 may be more economical and may avoid vacuum problems with residual gas pockets between the sleeve 1030 and the filament 1020. A straight filament segment 1020 may facilitate wrapping with tantalum foil.

[0054] While arrows 952, 1052 suggest a single welding operation to form both the adapter-filament junction and the adapter-electrode junction, this is not a requirement; as in Figures 7-8, the adapter-filament junction and the adapter-electrode junction can be formed in separate operations. For example, a tantalum sleeve 930, 1030 can be clamped around the filament 920, 1020 and then welded into a subassembly, after which the electrode 940, 1040 can be clamped to the subassembly and welded to the adapter 930, 1030. Alternatively, the sleeve 930, 1030 can be formed by depositing it on the filament 920, 1020 before welding the electrode 940, 1040 onto the sleeve 930, 1030.

[0055] Electrode 740 is shown as having a square face at its distal end (e.g., at the top of diagram 700, away from adapter 730). While the nanorod or pointed tip can be formed after assembly of the structure shown, this is not a requirement; in other embodiments, the emitter tip can be formed on electrode 730 before assembly of the structure shown. Similar considerations apply to electrodes 840, 940, 1040.

[0056] The embodiments shown herein are exemplary, and numerous other configurations can be implemented within the scope of the disclosed technology. For example, a stacked assembly can include a filament-adapter-electrode-adapter-filament sequence. That is, the two outer layers can form the support arms, and the adapter layer can provide material compatibility between the electrode and both filament arms. The entire stacked assembly can be spot welded in a single welding operation.

[0057] First Exemplary Method 11 is a flowchart 1100 of a first exemplary method according to the disclosed technology. The method can be used to form an emitter assembly similar to the emitter assembly of FIG. 4, or variations thereof.

[0058] At process block 1110, an adapter (similar to 420) can be glued to the filament (410). At process block 1120, the adapter can be clamped to the electrode (430), and at process block 1130, the adapter and electrode can be welded together.

[0059] Numerous variations and extensions of this method can be implemented within the scope of the disclosed technology, some of which are described in Figures 4, 12, or elsewhere herein. The adapter can incorporate tantalum or a tantalum alloy. The electrodes can incorporate rare earth hexaborides.

[0060] Bonding of block 1110 can be accomplished in a variety of ways. The filament and adapter can be clamped and welded together to form a fusion zone (similar to 415). In some examples, the filament, adapter, and electrode can all be clamped together before any welding operations are performed. Both welding operations can be performed as a single operation. One or both welding operations can be performed by electric spot welding or laser welding. Either welding operation, or both welding operations together, can be performed with a single timed pulse of energy delivered to the clamped objects to avoid damage caused by a second energy pulse when the interface between the clamped objects is in a molten state.

[0061] As an alternative to clamping and welding, a deposition process can be used to form and adhere the adapter onto the filament. Illustratively, chemical vapor deposition (CVD) or sputtering can be used.

[0062] Exemplary Method Extensions 12 is a flowchart 1200 of a second exemplary method illustrating extensions to the first method. These extensions may be applied alone or in any combination, or may be applied with variations.

[0063] An emitter assembly incorporating the filament, adapter, and electrodes can be fabricated in process block 1230. As indicated by the dashed line, block 1230 can be performed using the method of Figure 11, although this is not a requirement.

[0064] In a first expansion, an adapter can be wrapped around the filament at block 1210 before forming the assembly at block 1230.

[0065] Next, as shown in Figures 3A and 3B, the cold field emitter can incorporate nanorod tips (310, 311) at the distal ends of the microrod electrodes. In various embodiments of the second extension, the nanorod tips can be performed in block 1220A before assembling the emitter structure in block 1230, or optionally in block 1220B after the emitter structure has been assembled. Block 1220B is shown with dashed lines to indicate that it is an alternative to block 1220A.

[0066] As a third extension, in block 1240, the emitter assembly can be placed in an electron microscope, and as a further extension, in block 1250, an electric field can be generated at the electrode tip to extract electrons from the electrode and generate an electron beam.

[0067] Welding example FIG. 13 is a diagram 1300 of an exemplary configuration for spot welding components of an emitter assembly. As shown, the filament 1310, adapter 1320, and electrode 1330 can be aligned and held between welding terminals 1341, 1342. Clamping force can be kept minimal to reduce the risk of cracking the electrode 1330. One or both of the terminals 1341, 1342 can be spring-loaded. A power source 1350 can drive current 1352 in a circuit through the wires 1345, 1346, the terminals 1341, 1342, and the aligned devices 1310, 1320, 1330, thereby forming a fusion zone similar to 415, 425 in FIG. 4. Illustratively, the terminal 1341 can be a plate on which the assembly 1360 is supported, and the terminal 1342 can be in the form of a pointed probe tip. Advantages of spot welding include a single set-up action for clamping and energy delivery, concentrated energy delivery at the interface to be welded (which can have high contact resistance), minimal access required for welding equipment (only two wires), and avoidance of filler material.

[0068] Numerous variations and extensions can be made within the scope of the disclosed technology. The current 1352 can flow in opposite directions. The clamping and spot welding operations can be performed on just the adapter 1320 and electrode 1330, or just the adapter 1320 and filament 1330. In embodiments, welding can be performed with 1-10 J of electrical energy (often 3-6 J) delivered to the assembly 1360 in current pulses having a duration of 50-500 ms (or about 150 ms) and a full width at half maximum of 10-200 ms (or about 40 ms).

[0069] FIG. 14 is a diagram 1400 of an exemplary configuration for laser welding emitter components in accordance with the disclosed technology. As shown, a filament 1410, an adapter 1420, and an electrode 1430 can be clamped together as an assembly 1460 (clamp not shown). A laser beam can be directed at select areas of the assembly from different directions, as indicated by arrows 1451, 1452. Deposited laser energy can cause adjacent portions of the adapter 1420 and the electrode 1430 to melt, resulting in the formation of a fusion zone similar to 425 in FIG. 4. Similarly, deposited laser energy can cause adjacent portions of the adapter 1420 and the filament 1410 to melt, resulting in the formation of a fusion zone similar to 415. Advantages of laser welding include the ability to focus energy delivery to a specific point, reduced clamping forces and reduced risk of mechanical damage to the assembly 1460 due to non-contact delivery of energy, and avoidance of filler materials.

[0070] Numerous variations and extensions can be made within the scope of the disclosed technology. In various embodiments, the laser energy can be scanned continuously across the area where the laser energy is deposited. Or, the laser energy can be deposited in one, two, or more discrete spots to form localized melt and fusion zones. The amount of energy deposition can be similar to that used in spot welding. Fewer or more lasers can be used.

[0071] Additional Examples Additional examples of the disclosed technology are described below.

[0072] Example 1 is a structure comprising a filament, an adapter, a first fusion zone that joins the filament to the adapter, an electrode, and a second fusion zone that joins the electrode to the adapter.

[0073] Example 2 includes the subject matter of example 1, further specifying that the filament comprises at least 50 at% tungsten.

[0074] Example 3 includes the subject matter of example 1, further specifying that the filament comprises at least 50 at% rhenium.

[0075] Example 4 includes the subject matter of any of Examples 1-3, further providing that the sum of the atomic percent of tungsten and rhenium in the filament is at least 50.

[0076] Example 5 includes the subject matter of any of Examples 1-4, further specifying that the filament comprises a cylindrical wire adjacent the first fusion zone having a diameter in the range of 25-500 μm and a length at least three times the diameter.

[0077] Example 6 includes the subject matter of any of Examples 1-5, further specifying that the first fusion zone comprises a mixture of a first material of the filament and a second material of the adapter.

[0078] Example 7 includes the subject matter of any of Examples 1-6, further specifying that the first fusion zone is at least a portion of the weld.

[0079] Example 8 includes the subject matter of any of Examples 1-7, further specifying that the filament and the adapter have different material compositions.

[0080] Example 9 includes the subject matter of any of Examples 1-8, further specifying that the adapter comprises at least 90 at% tungsten.

[0081] Example 10 includes the subject matter of any of Examples 1-9, further specifying that the adapter has a thickness in the range of 1-500 μm.

[0082] Example 11 includes the subject matter of any of Examples 1-10, further providing that the second fusion zone comprises a mixture of the first material of the electrode and the second material of the adapter.

[0083] Example 12 includes the subject matter of any of Examples 1-11, further specifying that the second fusion zone is at least a portion of the weld.

[0084] Example 13 includes the subject matter of any of Examples 1-12, further specifying that the electrode comprises at least 60 at% boron.

[0085] Example 14 includes the subject matter of any of Examples 1-13, further specifying that the electrode comprises a hexaboride material.

[0086] Example 15 includes the subject matter of any of Examples 1-14, further specifying that the electrode comprises a rare earth hexaboride material.

[0087] Example 16 includes the subject matter of example 15, further specifying that the electrode comprises lanthanum hexaboride.

[0088] Example 17 includes the subject matter of any of Examples 1-16, further specifying that the electrode comprises a microrod adjacent the second fusion zone and a nanorod tip at a distal end.

[0089] Example 18 is an electron source comprising: the structure of any one of Examples 1 to 17 having an emitter formed at a tip of an electrode; an extraction anode having a first opening around a centerline of the emitter; an acceleration anode having a second opening around a centerline of the emitter; and at least four electrical terminals configured to receive power to drive a current through the filament to apply an extraction voltage to the extraction anode and an acceleration voltage to the acceleration anode.

[0090] Example 19 is an electron microscope including the electron source of Example 18.

[0091] Example 20 is a method that includes adhering an adapter to a filament, clamping the adapter to an electrode, and welding the adapter and the electrode.

[0092] Example 21 includes the subject matter of example 20, further specifying that the adapter comprises tantalum or a tantalum alloy and the electrode comprises a rare earth hexaboride.

[0093] Example 22 includes the subject matter of any of Examples 20-21, further specifying that adhering includes clamping the filament to an adapter and welding the filament and the adapter together.

[0094] Example 23 includes the subject matter of example 22, further including wrapping an adapter around the filament before clamping the filament.

[0095] Example 24 includes the subject matter of any of Examples 22-23, further specifying that the two clamping operations clamp all of the filament, adapter, and electrode together before any welding operations are performed.

[0096] Example 25 includes the subject matter of any of examples 22-24, further providing that the two welding operations are performed as a single motion.

[0097] Example 26 includes the subject matter of any of Examples 22-25, further providing that at least one of the welding operations is electric spot welding.

[0098] Example 27 includes the subject matter of any of Examples 22-26, further providing that at least one of the welding operations is laser welding.

[0099] Example 28 includes the subject matter of any of Examples 22-27, further providing that at least one of the welding operations includes delivering a single pulse of energy.

[0100] Example 29 includes the subject matter of any of Examples 20-28, further including forming a nanorod tip on the distal end of the electrode subsequent to welding the electrode.

[0101] Example 30 includes the subject matter of any of Examples 20-28, further including forming nanorod tips on the distal ends of the electrodes prior to welding the electrodes.

[0102] Example 31 includes the subject matter of any of Examples 20-30, further specifying that the adhering includes depositing an adapter over the filament.

[0103] Example 32 is a procedure that includes forming an emitter assembly by the method of example 20, forming a nanorod tip on a distal end of an electrode before or after the welding operation, and placing the emitter assembly with the nanorod tip in an electron microscope.

[0104] Example 33 includes the subject matter of example 32, further including generating an electric field at the tip of the electrode to generate an electron beam in the electron microscope.

[0105] Example 34 is a structure including a filament having a first material composition, a rare earth hexaboride electrode including a microrod adjacent a second fusion zone, a nanorod tip at a distal end, an adapter having a thickness in the range of 5 to 100 μm and a second material composition having at least 80 at% tantalum, the second material composition being different from the first material composition, a first fusion zone joining the filament to the adapter, and a second fusion zone joining the electrode to the adapter.

[0106] Example 35 is an electron microscope including a cold field emission electron source including the structure of Example 34.

[0107] General Considerations As used in this application and the claims, the singular forms "a," "an," and "the" include the plural forms unless the content clearly dictates otherwise. Additionally, the term "includes" means "comprises." Furthermore, the term "coupled" does not exclude the presence of intermediate elements between the coupled items. Furthermore, as used herein, the terms "or" and "and / or" refer to any one item or combination of items in a phrase. Terms connected by "or" or "and / or" are not necessarily mutually exclusive.

[0108] The systems, methods, and apparatus described herein should not be construed as limiting in any way. Instead, the present disclosure is directed to all novel and non-obvious features and aspects of the various disclosed embodiments, alone and in various combinations and subcombinations with one another. The disclosed systems, methods, and apparatus are not limited to any specific aspects or features or combinations thereof, and the disclosed systems, methods, and apparatus do not require that any one or more specific advantages be present or problems be solved. Techniques of any example embodiment can be combined with techniques described in any one or more of the other examples. Any theory of operation is provided for ease of explanation, but the disclosed systems, methods, and apparatus are not limited to such theories of operation.

[0109] Although some operations of the disclosed methods are described in a particular sequential order for convenient presentation, it should be understood that this style of description encompasses reordering unless a particular ordering is required by specific language set forth below. For example, operations described sequentially may, in some cases, be reordered or performed simultaneously. Furthermore, for simplicity, the accompanying drawings may not show the various ways in which the disclosed systems, methods, and apparatuses may be used in conjunction with other systems, methods, and apparatuses. In addition, the description may use terms such as “accelerate,” “acquire,” “bond,” “analyze,” “apply,” “clamp,” “clean,” “configure,” “deliver,” “deposit,” “determine,” “direct,” “drive,” “form,” “fuse,” “produce,” “direct,” “place,” “pulverize,” “obtain,” “provide,” “receive,” “scan,” “transmit,” “use,” or “weld” to describe the disclosed techniques. These terms are high-level abstractions of the actual operations that take place. The actual operations corresponding to these terms will vary depending on the particular implementation and will be readily discernible to those skilled in the art with this disclosure at hand.

[0110] In some examples, values, procedures, or devices may be referred to as "minimum," "best," "maximum," "optimum," "extreme," etc. It will be understood that such descriptions are intended to indicate that a choice from few or many alternatives is possible, and that such a choice is not necessarily lower, better, less, or otherwise preferred over other options.

[0111] Any theories of operation, scientific principles, or other theoretical explanations presented herein with reference to the devices or methods of the present disclosure are offered for purposes of understanding and are not intended to limit the scope, and the devices and methods within the appended claims are not limited to devices and methods that function in a manner described by such theories of operation.

[0112] Any of the disclosed methods may be controlled by or implemented as computer-executable instructions or computer program products stored on one or more computer-readable storage media, such as tangible, non-transitory computer-readable storage media, and executed on a computing device (e.g., any available computing device, including a tablet, smartphone, or other mobile device containing computing hardware). A tangible computer-readable storage medium is an available, tangible medium that can be accessed within a computing environment (e.g., one or more optical media disks, such as a DVD or CD, a volatile memory component (such as DRAM or SRAM), or a non-volatile memory component (such as a flash memory or hard drive)). A computer-readable storage medium includes memory and storage. The terms computer-readable medium or computer-readable storage medium do not include signals and carrier waves. Furthermore, the terms computer-readable medium or computer-readable storage medium do not include communication ports.

[0113] Any computer-executable instructions for carrying out the disclosed techniques, as well as any data created and used during the practice of the disclosed embodiments, can be stored on one or more computer-readable storage media. The computer-executable instructions can be, for example, a dedicated software application or part of a software application accessed or downloaded via a web browser or other software application (such as a remote computing application). Such software can be executed, for example, on a single local computer (e.g., any suitable commercially available computer) or in a networked environment using one or more networked computers (e.g., via the Internet, a wide area network, a local area network, a client-server network, a cloud computing network, or other such network).

[0114] The disclosed technology is not limited to any particular computer or hardware type, and the specific details of suitable computers and hardware are well known and need not be described in detail in this disclosure.

[0115] In view of the many possible embodiments to which the principles of the disclosed subject matter may be applied, it should be recognized that the illustrated embodiments are merely preferred examples of the disclosed subject matter and should not be construed as limiting the scope of the claims. Rather, the scope of the claimed subject matter is defined by the following claims. We therefore claim all that comes within the scope and spirit of those claims.

Claims

1. A structure comprising: A filament; An adapter and a first fusion zone that joins the filament to the adapter; An electrode; a second fusion zone connecting the electrode to the adapter.

2. the sum of the atomic percent of tungsten and rhenium in the filament is at least 50; The structure of claim 1 .

3. the first fusion zone comprises a mixture of a first material of the filament and a second material of the adapter; The structure of claim 1 .

4. the atomic percent of tantalum in the adapter is at least 90; The structure of claim 1 .

5. the second fusion zone comprises a mixture of a first material of the electrode and a second material of the adapter; The structure of claim 1 .

6. the electrode comprises a rare earth hexaboride material; The structure of claim 1 .

7. The electrode is a microrod adjacent to the second fusion zone; 10. The structure of claim 1, comprising a nanorod tip at a distal end.

8. an electron source, The structure according to claim 1 , which has an emitter formed at a tip of the electrode; an extraction anode having a first opening about a centerline of the emitter; an acceleration anode having a second opening about the centerline of the emitter; Receiving electricity, Driving a current through the filament; applying an extraction voltage to the extraction anode; at least four electrical terminals configured to apply an accelerating voltage to the accelerating anode.

9. 1. An electron microscope comprising: An electron microscope comprising the electron source according to claim 8.

10. 1. A method comprising: Gluing the adapter to the filament; clamping the adapter to an electrode; welding the adapter and the electrode together.

11. the adapter comprises tantalum or a tantalum alloy; the electrode comprises a rare earth hexaboride; The method of claim 10.

12. The bonding clamping the filament to the adapter; and welding the filament and the adapter together.

13. wrapping the adapter around the filament prior to clamping the filament; The method of claim 12 further comprising:

14. The two welding operations are performed as a single motion. The method of claim 12.

15. At least one of the welding operations is electric spot welding. The method of claim 12.

16. At least one of the welding operations is laser welding. The method of claim 12.

17. at least one of the welding operations includes delivering a single pulse of energy; The method of claim 12.

18. forming a nanorod tip at a distal end of the electrode following the welding of the electrode; The method of claim 10 further comprising:

19. forming a nanorod tip on the distal end of the electrode prior to said welding of the electrode; The method of claim 10 further comprising:

20. The bonding The method of claim 10 , comprising depositing the adapter onto the filament.

21. A procedure comprising: forming an emitter assembly according to the method of claim 10; forming a nanorod tip on the distal end of the electrode before or after the welding operation; and placing the emitter assembly having the nanorod tip in an electron microscope.

22. generating an electric field at the tip of the electrode to generate an electron beam within the electron microscope; 22. The procedure of claim 21 further comprising:

23. A structure comprising: a filament having a first material composition; A rare earth hexaboride electrode, a microrod adjacent to the second fusion zone; a rare earth hexaboride electrode having a nanorod tip at its distal end; An adapter, a thickness in the range of 5 to 100 μm; an adapter having a second material composition having at least 80 at% tantalum, the second material composition being different from the first material composition; and a first fusion zone that joins the filament to the adapter; the second fusion zone coupling the electrode to the adapter.

24. 1. An electron microscope comprising:

24. An electron microscope comprising a cold field emission electron source comprising the structure of claim 23.

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