Filamentless electron source

Abstract

To provide a Schottky emitter in which an operation temperature and a power consumption are reduced by heating the Schottky emitter by a resistance heating of an emitter itself.SOLUTION: Electron sources can include an electron source crystal 106 coupled in series between opposing electrically conductive supports to form an electrically conductive path, and electrical resistance of each of the electrically conductive supports is lower than electrical resistance of the electron source crystal 106. Electron source crystals 106 can include an emitting end 108 and opposing shank end 110, and the shank end 110 includes opposing leg portions 114a, 114b. Electrically conductive supports can include foil supports 104a, 104b spaced apart across a gap, and the opposing leg portions are attached respectively to the foil supports 104a, 104b such that the foil supports 104a, 104b are electrically connected to form the electrically conductive path.SELECTED DRAWING: Figure 1A

Description

Technical Field

[0001] The field is an electron source.

Background Art

[0002] Electron microscopes and other precision measurement and imaging devices typically include a cathode electron source for generating an electron source beam. Electron sources such as Schottky emitters typically include a filament heated by an electric current. The filament extends between two electrical contacts and is bent to form a symmetric "A" shape. When an electric current flows through the filament, the tip of the filament is rapidly heated, and the temperature rises above 1000K. An electron emitter crystal (e.g., tungsten <100>) is attached to the tip of the bend and receives the heat generated by the filament through its attachment to the filament. During operation of the source, the filament is heated, an electrostatic potential is applied between the emitter crystal and an adjacent anode, and the work function of the emission surface on the emitter crystal is reduced by the application of ZrO. The increase in temperature and the decrease in the work function cause a rapid increase in electron emission from the crystal tip, thereby generating an electron source beam.

[0003] Unfortunately, electron sources with wire filaments are inefficient when operating at high temperatures (e.g., 1800K in the case of Schottky). All the power required to maintain a high temperature at the emission surface is generated in the wire filament and is dissipated by emission and heat conduction. Operating materials at high temperatures causes various problems such as significant thermal expansion and excessive heat to surrounding components. To reduce the operating power of the electron source, emission and heat conduction must be reduced. A typical solution to this problem is to change the dimensions and material of the filament wire, but this solution is limited by the mechanical properties of the filament wire. Therefore, improved electron sources and related methods are needed.

Summary of the Invention

[0004] According to one aspect of the disclosed art, the electron source includes an electron source crystal coupled in series between opposing conductive supports to form a conductive path, wherein the electrical resistance of each conductive support is lower than that of the electron source crystal. In some examples, the electron source crystal includes an emission end and opposing shank ends, the shank ends including opposing leg portions. In some examples, the conductive supports comprise foil supports spaced apart by a gap, with each of the opposing leg portions attached to the respective foil support so as to be electrically connected to form a conductive path. In some examples, the electron source further includes a base and a pair of electrical contacts extending from the base, with the ends of each foil support opposite the gap attached to the respective electrical contacts of the pair. In some examples, the foil supports extend outward perpendicular to the longitudinal direction from each leg portion. In some examples, the foil supports extend in a height direction parallel to the longitudinal direction of the electron source crystal by at least 10 times the thickness of the electron source crystal. In some examples, the foil supports have a thickness less than their respective length and height, and their thickness is less than twice the thickness of the electron source crystal. In some examples, the dimensions and material of the foil support are configured relative to the dimensions and material of the leg portion so that the emission end is primarily heated to the emission temperature by Joule heating generated in the leg portion. In some examples, the foil support has milled edges facing each other across a gap. In some examples, the foil support is made from tungsten, rhenium, hafnium, tantalum, molybdenum, or an alloy thereof. In some examples, a reservoir of material configured to reduce the work function of the emission surface located on the electron source crystal is included. In some examples, the gap separates at least a portion of the reservoir. In some examples, the conductive support includes a wire member. In some examples, the conductive support includes a filament support.In some examples, the shank end includes a pair of lead faces configured to bond to their respective conductive supports, and the bonding of the crystal to the conductive supports causes current to flow from one lead through the crystal out to the other lead, thereby heating the crystal primarily from within the crystal rather than by heat conduction from heat generated within each support member. Some examples include electron particle focusing systems that include one of the electron sources described herein.

[0005] According to another aspect of the disclosed technology, the electron source includes an electron source crystal having an emission end and an opposing shank end, the shank end being formed from a pair of opposing leg portions.

[0006] According to further aspects of the disclosed technology, the apparatus includes an electron source crystal and means for supporting the electron source crystal and generating Joule heat primarily within the crystal during operation.

[0007] According to further embodiments of the disclosed technology, a method for forming an electron source includes arranging an electron source crystal having an emission end and an opposing shank end, and removing a portion of the emitter through a portion of the shank end of the electron source crystal to form opposing crystal legs separated by a gap. In some examples, arranging the electron source crystal includes arranging the electron source crystal in relation to a conductive support member, and this method may further include attaching the shank end of the electron source crystal to the conductive support member to form an emitter assembly, and removing the portion of the emitter includes (i) removing a portion of the emitter assembly through a portion of the shank end of the electron source crystal to form opposing crystal legs separated by a gap, and (ii) removing a portion through the thickness of the support member and across the height of the support member to form a separate opposing support that is spaced apart by a gap and connected across the gap through the opposing crystal legs. In some examples, attaching the shank end of the electron source crystal to the conductive support member includes spot welding the shank portion of the electron source crystal to the conductive support member. In some examples, removing the portion of the emitter includes laser milling the portion of the emitter. In some examples, removing the emitter portion involves a focused ion beam (FIB) milling the emitter portion. In some examples, before positioning the electron source crystal, a conductive support member is attached to a pair of electrical contacts extending from a base. In some examples, positioning the electron source crystal relative to the conductive support member involves marking the position on the support member corresponding to the aligned position of the electron source crystal while the conductive support member is fixed. In some examples, the marking involves forming an optical mark or pattern on the conductive support member. In some examples, a reservoir of material configured to reduce the work function of the emission surface on the electron source crystal is positioned. In some examples, removing the emitter portion through the shank end portion of the electron source crystal to form opposing crystal legs separated by a gap involves removing the portion of the crystal where the reservoir of material is located. In some examples, the conductive support member includes a conductive foil strip.

[0008] The aforementioned and other purposes, features, and advantages of the disclosed technology will become apparent from the following detailed description, which will proceed with reference to the attached figures. [Brief explanation of the drawing]

[0009] [Figure 1A] This is a side view of the electron source assembly, with a significant portion of the foil length removed to illustrate the features of the split shank emitter. [Figure 1B] Figure 1A is a side view of the electron source assembly, enlarged to show the base and electrical contacts. [Figure 1C] Figures 1A to 1C are perspective views of the electron source assembly. [Figure 2] This is a flowchart showing the method for fabricating an electron source assembly. [Figure 3] This is a side view of an electron particle focusing system. [Figure 4] This is a side view of another electron source assembly with a gap extending through a zirconium reservoir. [Figure 5] This is a side view of another electron source assembly equipped with a foil support having a platform shape. [Figure 6] This is an electrical circuit diagram of an electron source assembly. [Figure 7] This is a side view of another electron source assembly using a cut filament. [Figure 8] Figure 7 shows a microscopic image of an exemplary electron source assembly. [Modes for carrying out the invention]

[0010] A Schottky emitter is a type of electron source that uses both electrostatic potential and temperature to form an emission. Disclosed examples of Schottky electron sources provide resistive heating of the crystal at the crystalline portion of the emitter crystal, rather than using filaments / wires to support and heat the crystal as in conventional designs. Disclosed examples also include other electron field emission sources, such as cold electron sources (e.g., CFEs, thermionic electrons, nanowires, etc.). In various examples, such as those shown in Figure 1A and other figures, the majority of the heat generated directly within the crystal, rather than being transferred to the crystal by a crystal support structure, can provide various advantages, including improved thermomechanical stability of existing filament structures, lower power input and lower operating temperatures for suppressors and extractors, and insulators close to the electron source. Such improvements allow for improved local vacuum environments, improved manufacturing reproducibility, reduced manufacturing steps (e.g., elimination of pyrometer measurements), and lower operating temperatures for crystal reservoirs (e.g., zirconium oxide), which can extend the emitter's lifespan.

[0011] In filament-based emitters, the primary purpose of the filament is to provide a thermal energy source from Joule heating, enabling the chip to operate at high temperatures such as 1800K. While filaments are convenient, they require excessive operating power, cause large thermal expansion, and tend to induce thermomechanical drift. Furthermore, at such high temperatures, evaporation of the resistively heated filament can cause changes in filament temperature, worsening the mean time between failures. Disclosed examples show that current can be conducted through an electron source crystal (e.g., a field emitter crystal) having a resistance higher than the conductive paths entering and leaving the crystal, where the electron source crystal can be formed from a single crystal, a polycrystalline crystal, or a combination thereof. In this specification, the term crystal can represent any type of electron emission source that can be attached to a filament or foil. Series connection can reduce the dependence on the filament's heat generation capacity when heating the crystal to a suitable operating emission temperature.

[0012] For example, in the case of A-shaped or other existing filaments, the filament is cut at the crystal connection, forcing the current to pass through the crystal and causing most of the Joule heat that would normally be provided by the filament rather than being generated in the crystal itself. Thus, existing filament structures such as wires can continue to be used. Some disclosed examples also allow for the effective removal of filaments from electron source structures. For example, a foil strip or other support structure can be used to replace the bent filament typically used in commercially available electron sources. The foil strip in the example may be straight over the distance between electrical contacts, and the emitter single crystal can be spot-welded to the foil strip. The emitter and foil strip can form an emitter assembly, which can then be plasma-milled or laser-milled to produce a split-emitter configuration.

[0013] In some cases, the emitter can be milled to create a split-emitter configuration (e.g., without foil strips or other support structures). Using a single crystal of appropriate length, virtually all heating forces are formed at the two crystal legs. If the single crystal is oriented along the optical axis for emission, virtually all thermal expansion of the emitter assembly is in that direction. The dimensions of the foil, single crystal, and milled section can be selected to limit the temperature of the foil and the support to which the foil is attached.

[0014] Figures 1A and 1B show an exemplary electron source assembly 100, including a foil strip 102 laser-milled to form first and second foil supports 104a and 104b. In a typical example, the electron source assembly 100 is a Schottky electron source assembly. In further examples, the electron source assembly may be a cold electron source assembly, a thermionic electron source assembly, a nanowire-based electron source assembly, and the like. The foil supports 104a and 104b can be fabricated from a variety of materials such as tungsten, rhenium, hafnium, tantalum, molybdenum, another metal, or a metal alloy. The selected material typically has a high melting point and can hold the electron source crystal 106, for example, via spot welding or laser welding attachments. Generally, as used herein, the emitter is referred to as a “crystal” and can encompass a variety of electron source structures that emit electrons, including nanowires. The electron source crystal 106 includes a body 107 with an emission end 108 and an opposing shank end 110. For example, a zirconium oxide crystal reservoir 109 is located in the main body 107. The crystal 106 is positioned along the optical axis 111. The optical axis 111 is typically aligned with the optical axis of the associated system in which the assembly 100 is installed, such as an electron microscope. The crystal 106 is conventionally a single crystal such as tungsten, but other materials such as rhenium can be used in other examples, as will be further described below. The emission end 108 includes an emission surface 112 from which electrons are emitted. The emission surface 112 is generally aligned with the optical axis 111, so that electrons are emitted from the emission surface 112 in the direction of the optical axis 111. The shank end 110 includes a pair of crystal legs 114a, 114b facing each other across a gap 115. The crystal legs 114a and 114b are joined at vertices 116 (top of Figure 1) spaced apart from the foil strip 102, so that the foil supports 104a and 104b are not directly connected to each other, and the gap 115 extends in the height direction (e.g., z direction) of the foil strip 102. The presence of the gap 115 causes the foil supports 104a and 104b to be electrically and mechanically coupled through the crystal legs 114a and 114b and the undivided portion of the body 107 of the crystal 106.In various examples, the gap can be approximately 0.1 μm, 1 μm, 2 μm, 5 μm, 10 μm, 15 μm, 20 μm, 40 μm, or larger.

[0015] The foil supports 104a, 104b can have, for example, a small thickness extending in the x-direction within the plane of Figure 1A, and a large height extending in the z-direction, which is, for example, perpendicular in Figure 1A. In some examples, the thickness can range from 10 μm to 100 μm and the height can range from 0.5 mm to 10 mm, but other thicknesses and heights are also possible, including uniform or non-uniform ones. The height and thickness of the supports 104a, 104b near the attachment points to the crystal legs 114a, 114b generally define the foil cross-sectional area. The exemplary foil cross-sectional areas described herein are typically larger than those of an "A"-shaped cylindrical filament-based Schottky emitter. A larger foil cross-sectional area can provide more structural support to the electron source crystal 106. In a typical example, the foil height is large enough so that the ends of the crystal legs 114a, 114b do not extend beyond the lower edge 118 of the foil strip 102. The foil height can also be chosen to be large enough to be manipulated with tweezers or other tools. In some examples, the thickness of the foil support is chosen in relation to the selected foil material and machining parameters of the milling process used to form the gap 115. For example, some foil materials and milling processes can have a maximum material removal depth, or a maximum removal depth for a particular set of processed crystal legs, gaps, or foil support attributes. Examples of attributes include material burrs, splatters, thermal weakening zones, warping, localized melting and re-solidification, etc. Laser milling is sometimes superior to FIB milling, as it can achieve greater machining depths and therefore greater foil thicknesses.

[0016] Generally, the larger cross-sectional area of ​​the foil supports 104a, 104b (compared to a bent wire filament) provides lower electrical resistance. This is advantageous because it allows a larger portion of the Joule heating to occur in the crystal legs 114a, 114b rather than in the foil supports 104a, 104b. Thus, assembly 100 can be called a filamentless electron source. Compared to an "A" shaped filament-based emitter, Joule heating occurs only within the filament because virtually all current passes through the filament. When current flows and heats the filament, the emitter crystal is heated only by conduction. In contrast to the split crystal legs 114a, 114b at the shank end 110, the current path forces the current to flow through the crystal legs 114a, 114b and the body 107, thereby I 2 A significant portion of the R heating occurs within crystal 106. As the lengths of the crystal legs 114a and 114b increase, the energy formed in the upper partitioned sections of the foil support 104a and 104b also increases.

[0017] Furthermore, since the crystal legs 114a and 114b are formed by splitting crystal 106, the cross-sectional area of ​​the crystal legs 114a and 114b is reduced by more than half. For example, the cross-sectional area of ​​each crystal leg of a split crystal with a diameter of 0.127 mm (5 mil) is approximately 0.006 mm². 2It is less than . Since resistance is inversely proportional to area, the division of crystal 106 and the corresponding reduction in the cross-sectional area of ​​the current path significantly increases electrical resistance. Another important advantage of the disclosed example due to higher resistance is the corresponding reduction in supply current and therefore the supply power to heat the emitter tip to the operating temperature (e.g., 1800K). While a Schottky electron source of a single bent filament may not consume as much power as other components of an electron microscope, large industrial systems using arrays of electron sources can achieve significant energy savings and lower operating temperatures of adjacent components due to the low-power attributes of the disclosed example using foil supports and divided crystal legs. In some examples, additional energy savings can be obtained by the increase in resistance provided by the reduction in crystal diameter and by the selection of higher-resistance crystal materials. For example, since area is proportional to the square of the diameter, the resistance of the crystal legs increases sharply as the diameter changes linearly. Examples of crystal diameters can include less than 5 mils, such as 4 mils, 3 mils, or less. In some examples, the crystal diameter can be greater than 5 mils, such as 6 mils, 8 mils, or more. In some cases, other materials with higher resistivity than tungsten, such as rhenium, can be used.

[0018] Figures 1B-1C show the electron source assembly 100 with electrical contacts 120a and 120b attached. The electrical contacts 120a and 120b may typically be in the form of pillars extending from a base 122 made of ceramic or another insulator. Foil supports 104a and 104b extend to the contacts 120a and 120b and define the length of the foil strip. In many examples, the electrical contacts 120a and 120b are arranged radially symmetrically opposite the center of the base 122, so that the emitter assembly 100 can be installed with the longitudinal spread of the emission surface 112 and the electron source crystal 106 aligned with the optical axis 111 and the center of the base 122. In typical examples, one or more jigs can be used to position the crystal at the center of the base 122. The aligned position of the electron source crystal 106 is typically less than 600 μm radially offset from the center of the base 122. During operation, a power supply (not shown) is coupled to contacts 120a and 120b, forming an electric current via the foil supports 104a and 104b, the crystal legs 114a and 114b, and the crystal body 107.

[0019] As discussed above, by using the conductive foil supports 104a, 104b, a cylindrical bent filament can be dispensed with. To heat the emitter chip to an operating temperature of 1800K, a cylindrical bent filament typically operates at 2000K or higher. When the filament temperature is high, a significant amount of thermal expansion also occurs. Thus, even the placement of the filament relative to the electrical contacts, the placement or orientation of the crystals attached to the filament, or even a slight asymmetry in the filament itself can result in various shifts in the X, Y, and / or Z directions. For example, 1 to about 10 μm or more in X and / or Y can become up to about 40 μm in the Z direction, or due to the angle with respect to the Z direction. In the disclosed example, due to the primary amount of Joule heat generated within the crystal 106, the foil supports 104a, 104b can remove the unwanted energy generated by conductively directing heat to the posts of the electrical contacts 120a, 120b. The Joule heat can mainly be within the crystal 106 and can include an arrangement where the temperature within the crystal 106 is higher than the temperature of the foil supports 104a, 104b. Examples of the temperature difference include 50K, 100K, 200K, 400K, etc. The dimensions of the foil supports 104a, 104b and the crystal 106 can also be selected to reduce power and thermal expansion. Preferably, the foil supports 104a, 104b remain below 1000K during operation, and the posts of the electrical contacts 120a, 120b can be at other temperatures but are in the range of 450K to 700K.

[0020] Thus, the foil supports 104a, 104b generally remain substantially at a low temperature and are accompanied by a reduced response to thermal expansion compared to an overheated "A" - shaped bent filament. In a further example, a filament such as a bent filament can be held, and the assembly can be milled or machined to form split crystal leg portions, and the filament can be split to provide sufficient electrical resistance within the crystal to the filament, whereby Joule heat is mainly generated within the crystal.

[0021] FIG. 2 shows an exemplary method 200 of manufacturing an electron source assembly, such as a Schottky electron source assembly. In 202, a support end portion of an electron source crystal is attached to a conductive foil strip to form an emitter assembly. For example, the crystal can be attached via welding, an adhesive, or another process. In 204, a portion of the emitter assembly is removed, for example, by laser milling or FIB milling, to form a pair of opposing crystal leg portions that face the support end portion of the crystal, and to form a separate foil support or other conductive support. For example, the assembly can remove a portion of the shank end portion of the electron source crystal by milling or machining to form a pair of opposing crystal legs separated by a gap. The milling or machining can be performed across the entire thickness of the foil strip to fully form the gap. The removal is typically continued across the entire height of the foil strip to form separate opposing foil supports separated by the gap, such that the supports are electrically connected across the gap via the opposing crystal legs. In some examples, an additional non-conductive base can be attached to the foil to provide additional structural support, and since the current path passes through the crystal legs, there is no need to remove material to penetrate the non-conductive base. In a further example, the crystal can be held and the opposing crystal leg portions removed without milling through the foil strip or forming a foil support. In such an example, a foil support or other conductive support can then be attached, or the separated crystal can be fixed to another structure.

[0022] In some cases, the foil strip can be attached to a pair of electrical contacts on a support extending from the base, for example by spot welding, at 206, before the electron source crystal is attached to the foil strip at 202. Attaching the foil strip to the contacts before attaching the electron source crystal to the foil strip avoids additional bending and other stresses on the crystal associated with positioning and alignment after removal of the strip and emitter assembly. Furthermore, since the electron source crystal is not yet attached to the foil strip, the accuracy of foil strip alignment when attaching the foil strip to the contacts may be lower because the foil strip may lack bends or vertices associated with bent wire filaments.

[0023] Before proceeding with mounting the electron source crystal to the conductive foil strip in 202, the electron source crystal can be carefully aligned to the center of the foil strip, coinciding with the center of the base. For example, various jigs can be used on the crystal and / or base and contacts to bring the crystal into alignment before mounting in 202. By mounting the crystal after the foil strip is attached, and avoiding the use of filaments, additional steps associated with bent wire filaments, such as aligning the filament vertices to the base and aligning the crystal to the filament vertices, can be avoided, resulting in more accurate centering. In some examples, in 208, the position can be marked on the foil strip to place the electron source crystal in a centrally aligned position. For example, optical alignment markers, such as crosshairs, lines, or other optical shapes, can be projected onto the foil strip using, for example, an LED or laser, and the crystal can be positioned relative to the alignment markers before mounting in 202. In some examples, the markers may be permanent marks, notches, grooves, etc. With the position marked, the crystal can be mounted to the foil in 202.

[0024] Figure 3 shows an electron particle focusing system 300 including an electron emitter source 302, which can be one of the filamentless arrangements described herein as an electron source assembly 304. The electron emitter source 302 typically includes a power supply and controller 306 coupled to the electron source assembly 304, which provides a current primarily for heating the crystal within the crystal rather than supporting foil legs. In a typical example, the power supply and controller 306 can also be coupled to a cathode 308 and an anode 310, which provide a potential to increase field emission from the electron emitter source crystal tip of the assembly 304. The electron beam generated by the electron emitter source 302 is then directed through a particle focusing column 312 to a sample 314 placed in a sample chamber 316. In some examples, the system 300 can include a focused ion beam column 318 for ions directed to the sample 314. Examples of the system 300 may include a transmission electron microscope, a scanning electron microscope, a scanning transmission electron microscope, and the like. Further examples may include critical dimension analysis systems (such as CD-SEM), defect review systems, and lithography systems.

[0025] Figure 4 shows an exemplary electron source assembly 400, which includes an electron source crystal 402 having two legs 404a, 404b, each having a portion fixed to a foil support 406a, 406b. A crystal reservoir 408, such as zirconium, is positioned on the crystal 402 adjacent to the emitter tip 410. A gap 412 between the legs extends at least partially through the zirconium oxide reservoir 408.

[0026] Figure 5 shows an exemplary electron source assembly 500, which includes an electron source crystal 502 having two crystal legs 504a, 504b attached to respective foil supports 506a, 506b. The foil supports 506a, 506b are attached to respective electrical contact posts 508a, 508b, which are fixed to a base 510. The foil supports 506a, 506b can be formed from a foil strip by machining through the crystal 502 and the foil strip, and the legs 504a, 504b are formed through the same process. The foil strip may include a non-rectangular shape. The foil strip may include a platform section 512 to which the crystal 502 is fixed before machining, and opposing leg members 514a, 514b that can be fixed to the electrical contact posts 508a, 508b.

[0027] The figure shows an exemplary electron source assembly 600, which includes an electron source crystal 602 having resistance R1 and conductive supports 604 and 606, each having resistances R2 and R3, respectively, where R1 is also low resistance. In a typical example, the electron source crystal 602 emits electrons in relation to the temperature of the crystal 602. An energy source 608 is coupled to contacts 610 and 612 of the conductive supports 604 and 606. The heat generated to raise the temperature of the crystal 602 is mainly generated within the crystal 602 by the current flowing through the crystal 602 and its larger resistance R1 compared to the smaller resistances of R2 and R3.

[0028] Figure 7 shows an example of an electron source assembly 700, which includes a filament 702 cut by FIB milling to form opposing filament supports 704a and 704b. The assembly 700 includes an electron source crystal 706, which includes leg portions 708a and 708b, respectively, cut by FIB milling to form a gap 710. As the current is forced to flow through the crystal 706, heat is generated primarily within the crystal 706, raising the crystal temperature for emission from the emitter tip 712, without relying on thermal conduction of heat resistively formed within the filament 702. Figure 8 is a microscopic image of an exemplary electron source crystal attached to a filament, with the cut filament apex and the cut crystal shank.

[0029] Similar to the other examples above, by splitting the emitter shank and passing the current through the shank, the heating force can be localized within the crystal 706, for example, near the tip 712, which can cause the supporting hairpin filament 702 to be approximately 200K cooler compared to a typical emitter that does not have a series electrical path through the crystal 706. In certain examples, including a split shank configuration of the crystal 706 can be made more efficient by reducing emission from the filament 702 due to the lower temperature. Heating of the suppressor and alumina base can also be reduced due to the reduced total heating force. Reduced heating reduces waste heat transfer to all surrounding emitter elements, improving the vacuum environment of the electron source as the operating temperature and gas emission rate decrease.

[0030] Various shapes of foil strips, filament wires, and other related supports can be used in various examples. In some examples, notches or other markings can be presented to indicate alignment for fixing to electrical contacts or for fixing crystal support ends or crystal legs. During operation, the foil support is generally heated to a lower temperature than the electron source crystal so that thermal expansion is attenuated or has a minimal effect. In some examples, the foil strip may be triangular, for example, with vertices corresponding to crystal positions. Because heating is reduced, even thermal expansion from a large triangular structure will generally be in the Z direction rather than the X or Y direction, if the cross-section and other properties of the foil lead to heating that occurs mainly in the crystal. A rectangular strip configured to extend directly from contact to contact usually results in less Z-direction thermal expansion than other shapes, and substantially less expansion compared to an A-shaped filament type emitter. Another advantage of the disclosed examples is that the reduction in X and Y shifts is accompanied by a reduction in Z-direction expansion.

[0031] General Considerations As used in this application and claims, the singular forms "a," "an," and "the" also include the plural form unless otherwise explicitly indicated by the context. In addition, the term "includes" means "equipped with." Furthermore, the term "combined" does not exclude the existence of intermediate elements between combined items.

[0032] The systems, apparatus, and methods described herein should not be construed as restrictive in any way. Rather, this disclosure covers all novel and non-obvious features and aspects of the various disclosed embodiments, both individually and in various combinations and partial combinations. The disclosed systems, methods, and apparatus are not limited to any particular aspect or feature or combination thereof, nor do they require any one or more particular advantages or problems to be solved. Any operating theories provided are for ease of explanation, but the disclosed systems, methods, and apparatus are not limited to such operating theories.

[0033] Some operations of the disclosed methods are described in a particular order for convenience of presentation; however, it should be understood that this style of description is inclusive of reordering unless a specific order is required by the specific terminology used below. For example, operations described sequentially may, in some cases, be reordered or performed simultaneously. Furthermore, for simplification, the accompanying diagrams may not show the various ways in which the disclosed systems, methods, and apparatus can be used with other systems, methods, and apparatus. In addition, this specification sometimes uses terms such as “generate” and “provide” to describe the disclosed methods. These terms are high-level abstractions of the actual operations performed. The actual operations corresponding to these terms vary depending on the particular embodiment and are readily recognizable to those skilled in the art.

[0034] In some examples, values, procedures, or devices are referred to as “lowest,” “best,” “minimum,” etc. Such descriptions are intended to indicate that a choice is possible from many usable functional options, and it will be understood that such a choice does not need to be superior, smaller, or otherwise desirable compared to other choices.

[0035] In view of the numerous possible embodiments to which the principles of the disclosed technology may be applied, it should be recognized that the illustrated embodiments are merely representative examples and should not be construed as limiting the scope of this disclosure. The alternatives specifically discussed in these sections are merely illustrative and do not constitute all possible alternatives to the embodiments described herein. For example, the various components of the systems described herein may be combined in function and use. Accordingly, we claim all that falls within the scope of the appended claims.

Claims

1. An electron source comprising an electron source crystal coupled in series between opposing conductive supports to form a conductive path, wherein the electrical resistance of each of the conductive supports is lower than the electrical resistance of the electron source crystal.

2. The electron source according to claim 1, wherein the electron source crystal includes an emission end and an opposing shank end, and the shank end includes an opposing leg portion.

3. The electron source according to claim 2, wherein the conductive support includes foil supports spaced apart by a gap, and each of the opposing leg portions is attached to each of the foil supports such that the foil supports are electrically connected to form the conductive path.

4. Bass and, The electron source according to claim 3, comprising a pair of electrical contacts extending from the base, wherein the ends of each foil support opposite the gap are attached to the respective electrical contacts of the pair.

5. The electron source according to claim 3, wherein the foil support extends outward perpendicularly in the longitudinal direction from each leg portion.

6. The electron source according to claim 3, wherein the foil support extends in a height direction parallel to the longitudinal direction of the electron source crystal by a distance of at least 10 times the thickness of the electron source crystal.

7. The electron source according to claim 3, wherein the foil support has a thickness less than its respective length and height, and the thickness is less than twice the thickness of the electron source crystal.

8. The electron source according to claim 3, wherein the dimensions and material of the foil support are configured with respect to the dimensions and material of the leg portion such that the emission end is heated to the emission temperature mainly by Joule heating generated in the leg portion.

9. The electron source according to claim 3, wherein the foil support comprises milled edges facing each other across the gap.

10. The electron source according to claim 3, wherein the foil support is made of tungsten, rhenium, hafnium, tantalum, molybdenum, or an alloy thereof.

11. The electron source according to claim 3, comprising a reservoir of a material configured to reduce the work function of an emission surface located on the electron source crystal.

12. The electron source according to claim 11, wherein the gap separates at least a portion of the reservoir.

13. The electron source according to claim 2, wherein the conductive support includes a wire member.

14. The electron source according to claim 2, wherein the conductive support includes a filament support.

15. The electron source according to claim 2, wherein the shank end includes a pair of lead surfaces configured to be coupled to the respective conductive support, and the coupling of the electron source crystal to the conductive support causes current to flow from one lead through the electron source crystal to the other lead, thereby heating the electron source crystal mainly from within the electron source crystal rather than by heat conduction from heat generated within the respective support members.

16. An electron particle focusing system comprising the electron source described in claim 1.

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