Electron beam emitter device

Abstract

To provide a more excellent metal boride electron emitter.SOLUTION: An emitter 101 including a metal boride material has at least a partially rounded end 105 with a radius of 1 μm or less. An electric field is applied to this emitter 101 to generate an electron beam from the emitter. Due to removal of a material from a single crystal rod to form the emitter 101, a metal boride material-containing emitter having a round end with a radius of 1 μm or less is formed.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present disclosure relates to electron emitters made from boron compounds. [Background technology]

[0002] (CROSS-REFERENCE TO RELATED APPLICATIONS) This application claims priority to U.S. Provisional Patent Application No. 62 / 356,738, filed June 30, 2016, the disclosure of which is incorporated herein by reference.

[0003] Alkaline earth and rare earth boron compounds can be used for thermionic emission (thermionic electron emission). The physical and chemical properties of these materials make boron compound cathodes superior to tungsten thermionic cathodes. Lanthanum hexaboride (LaB6) cathodes are used for thermionic electron emitters in scanning electron microscope (SEM) instruments, transmission electron microscopes, electron beam lithography systems, and other electron optical systems. Although they boost the performance of electron optical systems, especially at low beam energies, these boron compound cathodes have required cumbersome vacuum technology modifications to reduce oxidation-related damage.

[0004] Boron compounds, e.g., LaB6, are highly reactive with oxidants and water vapor at normal cathode operating temperatures. In conventional systems, the vacuum environment around the boron compound cathode is maintained by using cold traps, sputter ion pumps, vacuum seals, welded metal bellows, and sealing technologies. Boron compound thermionic emitters also require modifications to achieve emission stability. For example, a separate Wehnelt bias supply is used to reduce the active emission area to only the cathode ends, given that a dielectric film builds up on the cathode over time. Purity and cleanliness issues require specific materials to achieve thermionic operation with boron compound cathodes.

[0005] Even with these modifications, the thermionic emission gun assembly for LaB6 did not reach operating equilibrium and the operating temperature was insufficient to drive off internal water vapor. High partial pressure water vapor in the LaB6 cathode environment was causing mass loss, erosion and damage to the cathode. An improper cathode operating environment can result in accelerated mass loss.

[0006] LaB6 has not been used as a large emission area thermionic emitter in a triode gun, where source brightness is limited by space charge effects. A Wehnelt potential is used to prevent unwanted "Maltese cross" shaped electron emission, thereby partially shielding the extraction potential from the anode. When used as a thermionic emitter (e.g. at about 1800K), the cathode material undergoes mass loss through oxidation and sublimation of the boride material or forming oxides. The sublimation process causes changes in the shape and emission characteristics of the cathode, so that over time portions of the emitting surface or bulk of the emitter material may be lost. Sublimated or evaporated oxides become insulating films and may contribute to instabilities in the optical system.

[0007] Operating the cathode at a lower temperature can reduce the rate of mass loss of boride material from the cathode. Lower temperatures result in less material deposition on the back surface of the Wehnelt electrode (e.g., LaB6). However, operation of metal (e.g., La, Hf) and other rare earth boride cathode emitters at low temperatures in a high vacuum environment can easily result in contamination of the emission surface depending on the partial pressure of organic contaminants. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] US Patent Application Publication No. 2015 / 0002009 [Patent Document 2] U.S. Patent No. 6,387,717 [Patent Document 3] US Patent Application Publication No. 2011 / 0101238 [Patent Document 4] US Patent Application Publication No. 2006 / 0226753 Summary of the Invention [Problem to be solved by the invention]

[0009] Thus, what is needed are better metal boride electron emitters. [Means for solving the problem]

[0010] In a first aspect, there is provided an apparatus comprising an emitter including a metal boride material, the emitter having an at least partially rounded end having a radius of 1 μm or less.

[0011] The metal boride material may include a species selected from the list consisting of alkali metals, alkaline earth metals, transition metals, lanthanides, and actinides. The metal boride material may be a metal hexaboride material. The metal boride material may include LaB6. The crystal orientation of the metal boride material is <100> It can be said that.

[0012] The emission area of ​​the above emitter is 1mm 2 The radius of the rounded end may be less than 700 nm, less than 450 nm, or less than 100 nm. The emitting area of ​​the emitter may be less than 1 μm 2 The at least partially rounded end may have a flat emitting facet.

[0013] In a second aspect, a method is provided that includes providing an emitter including a metal boride material, applying an electric field to the emitter, and generating an electron beam from the emitter, the emitter having an at least partially rounded end with a radius of 1 μm or less.

[0014] This can occur in cryogenic field emission mode, room temperature field emission mode, warm field emission mode, thermal field mode and / or photocathode mode. Warm field emission mode operates at temperatures above ambient and below 1000K.

[0015] The above occurrence is 10 -9 Operating pressure below 10 Torr or -11 This can occur at operating pressures of up to Torr.

[0016] In a third aspect, a method is provided that includes removing material from a single crystal rod to form an emitter including a metal boride material, the emitter having an at least partially rounded end with a radius of 1 μm or less.

[0017] For a more complete understanding of the nature and objects of the present disclosure, reference should be had to the ensuing detailed description taken in conjunction with the accompanying drawings, in which: [Brief description of the drawings]

[0018] [Figure 1] FIG. 1 is a front view of one embodiment of an electron emission system according to the present disclosure. [Diagram 2] FIG. 1 illustrates the molecular structure of a metal hexaboride. [Diagram 3] 1 is a flow chart of a method according to the present disclosure. [Figure 4] FIG. 13 is a graph showing the luminance at reduced size versus extraction voltage for LaB6. [Diagram 5] This is a graph comparing the brightness when reduced with the electric field at the end vertex for LaB6. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0019] Although the claimed subject matter is described in terms of certain embodiments, other embodiments, including embodiments that do not provide all of the advantages and features described herein, are also intended to be within the scope of the present disclosure. Various structural, logical, process step, and electronic changes may be made without departing from the scope of the present disclosure. Accordingly, the scope of the present disclosure is to be defined solely by reference to the appended claims.

[0020] The emitters provided by the embodiments described herein have high brightness, good stability and long life. As described herein, the emitting area of ​​the emitter can be reduced from conventional boride cathodes to improve brightness. The emitting area of ​​the boride is typically 10-100 μm 2 The brightness of a thermionic emitting surface is determined in part by the size of the emitting area. For example, typical emitters can be 20 μm in diameter or larger. The emitters described herein are, however, smaller than 1 μm. 2 The LEDs can have an emission area of ​​less than 100 nm. The reduction in size of the emission area can increase brightness by several orders of magnitude. The embodiments described herein can be integrated into existing systems, thus reducing redesign costs.

[0021] FIG. 1 is a front view of an electron emission system 100, which includes an emitter 101 and an emitter mounting mechanism 102. The electron emission system 100 is configured to emit electrons into an environment or space surrounding the electron emission system 100. The emitter 101 has a first portion 103, a second portion 104, and a rounded end 105. The first portion 103 may be cylindrical. The second portion 104, excluding the rounded end 105, may be generally frusto-conical. The second portion 104 may be disposed on a tip of the first portion 103. The width or diameter of the second portion 104 may taper between the first portion 103 and the rounded end 105. The first portion 103 and the second portion 104 may be generally rounded, although the first portion 103 and the second portion 104 may be provided with facets due to their crystalline structure. The rounded end 105 is located on a tip of the second portion 104 and may be generally truncated. The rounded end 105 may be at least partially or completely rounded, and may have shapes other than those described herein.

[0022] The first portion 103, or the first portion 103 and the second portion 104, may be referred to as the “shaft” of the emitter 101. The shaft of the emitter 101 is held by an emitter mounting mechanism 102.

[0023] The dimensions of the emitter 101, the first portion 103, the second portion 104, or the rounded end 105 can vary. The shaft of the emitter 101, or the emitter 101 itself, can be on the nanometer or micrometer scale in both length and diameter.

[0024] The configuration of the emitter mounting mechanism 102 can vary from that depicted in Figure 1. In one example of the emitter mounting mechanism 102, the emitter end is supported using a tungsten hairpin seated on a ceramic insulator with electrodes for the hairpin legs. The hairpin can be heated to perform flash cleaning or to raise the emitter temperature to a thermal field emission (TFE) value (e.g., about 1800K). A ground reference power supply can provide a bias voltage for the emitter, e.g., about 5 kV.

[0025] An electric field is applied to or within electron emission system 100. The electric field can be applied using charged plates or other techniques.

[0026] The rounded end 105 is configured to emit free electrons into a reduced pressure space around the electron emission system 100. As shown in the inset of FIG. 1, the rounded end 105 has a radius 106. The radius 106 is measured from the center 108 of the rounded end 105 through the bulk of the rounded end 105 to the outer surface 107. The radius 106 of the rounded end 105 can be less than 1 μm and can have any value and range up to 1.0 nm. For example, the radius 106 of the rounded end 105 can be 700 nm or less, 450 nm or less, or 100 nm or less. The radius is greater than 0 μm.

[0027] The emission area of ​​the emitter 101 is 1 μm 2 The emission area may be a portion of the outer surface 107 of the rounded end 105.

[0028] The rounded end 105 may be uniformly rounded or non-uniformly rounded. The rounded end 105 may have a flat emitting facet 109. For example, a small <100> There may be a flat emission facet 109 in the form of an azimuthally nanoflat. The flat emission facet 109 can be used to generate a suitably collimated electron beam. In one example, the flat emission facet 109 has a collimated electron beam of less than 1 μm. 2 A discharge area of ​​less than 100 mm can be provided.

[0029] Alternatively, rounded end 105 may have a generally hemispherical or parabolic shape, which spreads the electron emission over a larger area, resulting in a smaller, more bright electron pool that can pass through the electron optics.

[0030] The desired rounded ends 105 can be provided by controlling the surface crystallinity.

[0031] Changing the radius of the rounded end 105, such as by reducing the diameter of the rounded end 105, increases the brightness of the emitted beam. This is the reduced brightness Br of the emitter, B r =I / (πr vs 2 ΩV ext ) Equation 1 where I is the field emission current, r vs is the radius of the virtual source, Ω is the solid angle of the beam, and V ext is the operating voltage. Two parameters in this brightness equation change as the end diameter is reduced. First, a smaller end results in a higher enhancement of the electric field at that end, which reduces the voltage required to emit a given current, V ext The value of B decreases when the image is reduced. r Second, the smaller the end diameter, the larger the virtual source size r vs Since the brightness becomes smaller, B r The value of becomes larger.

[0032] Using rounded ends 105 with radii less than 1 μm or on the nanometer scale increases brightness. Brightness generally increases as the radius decreases because electrons are emitted from a smaller surface area. Even higher brightness values ​​may be achieved with boride emitters with radii less than 100 nm. For example, radii of 5 nm, 10 nm, 25 nm, 50 nm, and 100 nm provide emitter emitting surface areas of approximately 160 nm2, respectively. 2 , 630nm 2 , 3930nm 2 , 15710nm 2 and 62830 nm2 It becomes.

[0033] The release area of ​​the round end 105 is 1 mm 2 The emitting area of ​​the rounded end 105 may correspond to the entire surface area of ​​the rounded end 105 or to a portion of the surface area of ​​the rounded end 105. For example, the emitting area may correspond to a flat emitting facet 109.

[0034] The emitter 101 can be fabricated from a metal boride material. For example, the metal boride material can be a metal hexaboride material such as that shown in FIG. 2, where the metal is represented by the large sphere M and the boron is represented by the small sphere B. The metal in the metal boride material can include species selected from the list consisting of alkali metals, alkaline earth metals, transition metals, lanthanides, and actinides. The emitter can consist of, consist of, or consist essentially of the metal boride materials described herein.

[0035] Exemplary metal boride materials include or include metal hexaborides such as LaB6, CsB6, CeB6, CaB6, and others. The metal boride materials may include other rare earth hexaborides from the lanthanide group. Many metal boride materials behave similarly in an emitter because their respective work functions are similar, they respond to surface contaminants (e.g., oxygen, water) in roughly the same way, and they all typically operate at roughly the same temperatures as thermionic emitters. However, there are differences in volatility between metal borides. For example, CeB6 is less likely to evaporate than LaB6. The effects of evaporation can be reduced by operating the emitter 101 at lower temperatures.

[0036] Metal hexaboride structures include caged structures in which transition metals, lanthanide metals, and other metals are surrounded by boron clusters (as shown in Figure 2) to provide a low work function. Disruption of this caged structure can compromise the electron density at the surface. However, metal boride structures other than the hexaboride structure can provide acceptable work functions in emitters.

[0037] The metal boride material may be a superconducting MT4B4 material, where M is Sc, Y, Ln, Th or U, and T is Ru, Os, Co, Rh or Ir. For example, the metal boride material may be CeCo4B4.

[0038] The brightness of the emitter increases or decreases inversely to the work function of the material. When the emitter material is reduced, the brightness is defined by Equation 1. The field emission current I is inversely proportional to the work function of the material. The lower the work function value, the larger the current and the higher the brightness value. Virtual emission source size r vs is inversely proportional to the work function, which is increased by a factor of 4, so the virtual source size increases at the lower work function, but the increase in current dominates, so the total luminance is higher at the lower work function.

[0039] Typically, the band gap energy of hexaborides is about 2 eV. Typically, acceptable work functions for high brightness field emission are 5 eV or less. Hexaboride materials, such as LaB6 or CeB6, have work functions below 3 eV. Another emitter material electrical property satisfied by hexaboride materials is the dielectric constant. The dielectric nature of semiconductors allows an electric field to penetrate the material, causing Fermi pinning of the energy bands. The Fermi pinning or degeneracy causes the conduction band to be below the Fermi level of the material, thereby giving the emitting surface a metallic character (high electron density).

[0040] Impurities in the metal boride material can affect the work function, and the impurity levels in the metal boride material can be selected to achieve an acceptable work function for the emitter.

[0041] The metal borides may include covalently and / or ionically bonded atoms in their chemical formula, such as alkali metal, alkaline earth metal, transition metal, lanthanide, or actinide atoms. For example, the bulk of the crystal may be covalently bonded and the surface of the crystal may be ionically bonded. Other intracrystalline bonding configurations are also possible.

[0042] According to certain examples, the metal boride material can include a combination of different metal borides.

[0043] The metal boride emitting surface at the rounded end 105 can be formed of a single crystal material to improve brightness. Amorphous or polycrystalline metal boride materials may be used. In one example, the metal boride material is fully crystalline. In another example, the metal boride material is partially crystalline.

[0044] The crystal orientation of the metal boride material in the emitter 101 is as shown in FIG. <100> It can be said that: <100> This crystal orientation is one of the orientations with the lowest work function for LaB6. Other crystal orientations that exhibit a work function less than about 3.0 eV may be used. For example, <310> The plane is thermally stable and exhibits the lowest work function of 2.50 eV of any single crystal face of LaB6. In order of increasing work function, the crystal orientations of LaB6 are (210), (100), (110), (111), and (211). For example, the LaB6 crystal orientations (100), (110), (111), and (210) would yield work functions of 2.3 eV, 2.5 eV, 3.3 eV, and 2.2 eV, respectively. Orientations exhibiting higher work functions would be more difficult to grow because they are less thermally stable and reordering may occur during growth. Noncubic faces would also be more susceptible to carbon contamination.

[0045] Electrons are generated by applying an electric field to the electron emission system 100. As shown in Figure 3, an emitter comprising a metal boride material having a rounded end with a radius of 1 μm or less is provided at 200. An electric field is applied to the emitter at 201. An electron beam is generated from the emitter at 202.

[0046] The extraction voltage used may be about 0.5 kV to 10 kV, and may be any value or range up to 0.1 kV. For example, the extraction voltage may be about 1 kV to 10 kV or about 1.5 kV to 10 kV.

[0047] The electric field for metal boride materials should be in the range of about 1 V / nm to 4 V / nm, and may be any value or range, for example down to 0.1 V / nm. Fields in the range of 1 V / nm to 4 V / nm are particularly useful for metal boride materials whose work functions reach 3 eV, although other field values ​​are possible.

[0048] In one example, the extraction voltage used with a LaB6 emitter may be in the range of about 1.5 kV to 10 kV, with an electric field in the range of about 1 V / nm to 3 V / nm, and in another example, with an electric field in the range of about 1.4 V / nm to 2.5 V / nm.

[0049] The stability of the emitter can be controlled. Although the rounded ends 105 are smaller than in conventional systems, the smaller area or radius of the rounded ends 105 means that smaller particles or errors can cause instability. Thus, by operating the electron emission system 100 in an ultra-clean high vacuum or better (e.g., an emission chamber environment with a total partial pressure of less than 1E-9 Torr), a combination of high brightness and long life can be achieved. Other vacuum parameters relating to higher pressure may be used with tolerances for stability, brightness and / or life. In Figures 4 and 5, the brightness of a LaB6 emitter with rounded ends is plotted against the applied bias voltage and the electric field at the emitter end apex at various operating temperatures. As can be seen from the modeling results, the brightness of a LaB6 emitter with rounded ends is approximately 100 nm. 8 A / (sr m 2 A reduced brightness of 0.5V can be achieved with an emitter tip having a radius of 1 μm or less.

[0050] Operation in an ultra-clean high vacuum environment provides superior results for metal boride emitters, including LaB6. Most of the mass loss in LaB6 emitters comes from poor vacuum resulting in high partial pressures of reactive gases in the emitter environment. Ultra-clean high vacuum operation reduces mass loss through evaporation or sublimation of small diameter emitter structures, resulting in more stable operation and longer emitter life.

[0051] The metal boride emitters described herein can be configured to operate in a variety of operating modes, such as cryogenic field emission mode, room temperature field emission mode, warm field emission mode, thermal field mode, or photocathode mode. Combinations of these modes can also be implemented using the metal boride emitters described herein. The metal boride emitters can be optimized for a particular mode, and the parameters used for each mode can be tailored to the metal boride emitter. For example, the dimensions of the rounded ends can be changed for each mode.

[0052] For cryogenic, low temperature, warm temperature or Schottky emission, a radius of less than 100 nm may be desirable. For photocathodes, the boride emitter could be a nanotip emitter or have a flat surface with angstrom-scale roughness. Photons absorbed by the emitter could be matched to the energy band gap of the material.

[0053] In either field emission mode, an electric field is applied to or within the electron emission system, for example with a positive high voltage using an electrode. This positive high voltage attracts electrons, causing some to leave the surface of the emitter. If the applied electric field is high enough to lower the potential barrier on the rounded end-to-vacuum interface, the electrons will tunnel through the surface barrier and travel to the biased anode (i.e., quantum mechanical tunneling).

[0054] The operating temperature in cryogenic field emission mode will be about 0K to less than 300K. The temperature of the system will be below the emitter temperature during operation. Cryogenic field emission has the potential to reduce the energy distribution of the emitted electrons. The energy spread (ΔE) can be reduced by narrowing the Fermi-Dirac distribution in the emitter. Periodic temperature flashing can be incorporated into the cryogenic field emission mode to keep the emission stable.

[0055] The use of rounded ends 105 having a radius of less than 1 μm or on the nanometer scale in cryogenic field emission can increase brightness and provide stability at typical emitter temperatures, e.g., about 100° C. to 200° C.

[0056] MT4B4 metal boride material can be used at cryogenic temperatures to provide superconducting properties.

[0057] Room temperature field emission typically operates at 65°F to 100°F (18°C to 38°C). The room temperature field emission mode of operation may not require the heat removal hardware that is required for cryogenic field emission, making it less costly to implement.

[0058] Warm field emission mode operates at temperatures above ambient and below 1000K, or above ambient and below the temperature at which thermionic emission can be detected for the system. The emitter temperature is between the temperature of the system and the temperature at which thermionic emission can be detected for the system. Warm field emission mode allows for a greater number of electrons in the conduction band, resulting in reduced current fluctuations and reduced electron-hole interactions in the bulk, and / or a shorter residence time for impacting molecules at the emitter tip. The advantage of warm field emission comes from the lower binding energy of unwanted impacting molecules (e.g., H2O, H2, CO, CO2, O2, N2, or hydrocarbons), resulting in a shorter residence time for these molecules on the emitter surface.

[0059] In thermal field emission mode, the same brightness can be achieved using lower extraction voltages, e.g., the same brightness as in room temperature field emission mode. The emitter temperature in thermal field emission mode can be about 1800 K. The radius of the rounded end of the emitter is preferably 0.5 μm to 1 μm in thermal field emission mode, but other radii can be used. Thermal field emission provides more energetic electrons for emission, but at the expense of increased energy spread due to the addition of the thermal energy spectrum. One of the advantages of thermal field emission mode is that the residence time of adsorbates can be significantly reduced, thus reducing the high frequency noise in the beam current. The energy spread can be reduced by appropriately configuring the electron optics.

[0060] In photocathode mode, light of a certain frequency falls on the emitter. Photoemission occurs, generating a current of electrons: photons are absorbed, electrons move to the surface of the emitter, and they escape into the vacuum. This can be done in conjunction with other field emission modes, or separately.

[0061] Certain reticle and wafer inspection applications require the electron source to exhibit the highest possible reduction brightness Br (brightness normalized by beam energy) and the smallest possible energy spread (ΔE) with a stability of less than 1% over the entire inspection period. One of the causes of high frequency current fluctuations in field emitters is the constant adsorption / desorption of residual gas molecules in the vacuum. The root mean square noise is inversely proportional to the 1.5 power of the emission surface area. A smaller tip radius may result in stronger noise under the same vacuum conditions. A lower vacuum reduces noise. Stable emission can also be achieved by heating the rounded tip to a certain temperature (e.g., about 380K to 1000K) during emission, thereby removing volatile species and preventing their retention on the surface (reducing molecular residence time), as well as by keeping the emission surface clean. Heating the emitter further reduces the tunneling distance and increases the electron energy, making it easier for field emission to occur. However, heating the emitter can result in a spread of energy, i.e., a larger energy spread.

[0062] In the photocathode field emission combined mode, the following process can occur: photons are absorbed, electrons collect in the conduction band, and the electrons migrate to the surface. Heat is applied to the emitter, and the electrons escape to the vacuum with an applied electric field. Typically, the photon energy is required to be high enough to excite the electrons into the conduction band, but lower than the ionization energy. The laser penetration depth may need to be considered when optimizing the laser delivery.

[0063] In either emission mode, the evaporation rate of the metal boride materials may not permit their use as small end radius field electron emitters at typical Schottky emission temperatures (e.g., operation above 1500 K). To optimize lifetime and stability, a physical balance between thermodynamic processes and sharpening by an applied extraction field can be used.

[0064] The optimum total operating pressure for boride field emitters is 10 for cryogenic field emission, room temperature field emission, warm field emission, or photocathode mode. -9 Torr or less. This operating pressure is the sum of all partial pressures of vacuum-relevant molecules (e.g. H2O, H2, CO, CO2, O2, N2 or hydrocarbons). For H2, the partial pressure limit is 10 -12 Torr, whereas for any other molecule, the partial pressure will be 10 -10 It is sufficient to set the temperature to less than Torr.

[0065] The operating pressure may vary depending on the operating mode. For example, the operating pressure may vary depending on the emission mechanism and surface activation energy. The thermal field emission mode is 10 -9 The cryogenic field emission mode can be operated at temperatures below 10 Torr. -10 The cryogenic field emission mode can also be operated at temperatures below 10 Torr. -11 It is sufficient to operate it at or below Torr.

[0066] Thermal field emission can be insensitive to pressure because the added thermal energy tends to desorb the contaminants. If the binding energy associated with the adsorbates is high, a lower pressure can be used to reduce the impact velocity of the adsorbates.

[0067] Photocathodes may provide the ability to desorb any surface contaminants with added energy from an illumination source, which may depend on the surface activation energy of the material used.

[0068] Operating pressure and other vacuum parameters can affect contamination or erosion of the emitter. An increase in the particle population in the emitter environment, e.g. caused by water vapor or other particles, can lead to accelerated mass loss. Since only high work function surfaces are exposed to the extraction field, the work function emitting area can disappear and emission can drop to near zero. Any pitting of the emitting material can disrupt the crystal orientation and affect the work function.

[0069] For example, when a thin carbon layer is formed on an electron current emitting surface, carbonaceous contamination of the emitting surface can occur, especially at lower operating temperatures. Potential sources of carbonaceous contaminants include volatile organics (e.g., oils or lubricants) associated with the vacuum system, residues from abrasives or cleaning agents, residual fibers from cotton swabs or wipes, and other sources. Carbon films can contaminate emitting surfaces with high work function layers, reducing the emission current.

[0070] Also, for example, water vapor can cause oxidation, sublimation, or evaporation of materials at the emitter, so refractory or dielectric materials may be applied on other surfaces, such as the interior surfaces, pores, and anode surface.

[0071] Further, for example, oxidation of the boride material can result in contamination of the boride material, and borate formation can occur on or within the emitter.

[0072] The vacuum environment surrounding the electron emission system is controlled to avoid carbonaceous contamination, water vapor damage, or oxidation, and the operating pressure of this environment may be dependent on the mode of operation.

[0073] Prior to use, the boride emitters can be prepared by heating to 1600 K in an ultra-clean, ultra-high vacuum environment for approximately 30 minutes to remove any oxide layer from the emitter surface.

[0074] The metal boride material may be prepared using a floating zone technique. The metal boride material may be shaped or reduced in size. The metal boride emitters may be manufactured using a variety of methods. The metal boride emitters may be manufactured by grinding or polishing a single crystal rod to form a sharp end. The metal boride emitters may be manufactured by electrochemically etching a single crystal rod to form a sharp end. The metal boride emitters may be manufactured by exposing a single crystal rod to a reactive environment to selectively remove material to form a sharp end. The metal boride emitters may be manufactured by exposing a single crystal rod to a reactive environment to selectively remove material using a patterned sidewall coating on the rod to facilitate the formation of a sharp end. The metal boride emitters may be manufactured by growing single crystal nanoparticles or nanowires for use as electron emitters. Metal boride emitters can be fabricated by attaching single crystal nanoparticles or nanowires to a mounting structure for use as an electron emitter. Metal boride emitters can be fabricated by electron beam induced deposition of boride material to form small needles. Metal boride emitters can be fabricated by thermally assisted sublimation in high pressure oxygen. Metal boride emitters can be fabricated by ion bombardment. Combinations of these various fabrication techniques can also be performed.

[0075] In one example, deposition is used to fabricate the metal boride emitters. Metal borides, such as LaB6, can form long, thin, high aspect ratio needles under certain conditions. For example, a small amount of contaminant material, such as a transition metal, is deposited on the surface of a larger seed crystal. Heating the seed crystal and metallic contaminant causes material surface migration. The metallic contaminant catalyzes the growth of the boride material into an oriented single crystal attached to the larger seed crystal. The seed crystal can act as a holder.

[0076] For example, the generation of small needles from a gaseous environment may also be performed. The support structures may be carbon nanotubes with transition metals encapsulated therein. Such structures may be generated using metal-organic chemical vapor deposition (MOCVD) or other processes.

[0077] The disclosed embodiments can be used in reticle and wafer inspection and metrology systems. The systems can be configured to achieve the desired vacuum environment specifications. Examples of such systems include electron beam wafer or reticle inspection systems using one or more electron sources, electron beam wafer or reticle review systems using one or more electron sources, electron beam wafer or reticle metrology systems using one or more electron sources, or systems where at least one electron source is required to generate X-rays using one or more electron beams for use in wafer or reticle metrology, review, or inspection. The electron stream from the emitter can be directed to a sample, such as a semiconductor wafer or other workpiece. The electron stream can be propagated through extraction and focusing electrodes to form an electron beam with a desired beam energy and beam current. One or more lenses can be used to generate a small electron beam spot on the sample. Deflectors can be used to scan the electron beam. The sample can be placed on a stage, such as a stage that can be scanned relative to the electron beam. Secondary and backscattered electrons can be emitted from the sample when the electron beam strikes it, and can be collected and accelerated towards a detector.

[0078] Although the present disclosure has been described in connection with one or more particular embodiments, it will be understood that other embodiments of the present disclosure may be made without departing from the spirit and scope of the present disclosure, which is to be deemed limited only by the appended claims and the reasonable interpretation thereof.

Claims

1. an emitter including a metal boride material; The emitter has a generally hemispherical or parabolic rounded end, the rounded end having a radius of 1 μm or less to a curved outer surface, and ２ a flat emitting facet having a diameter of less than 1 mm and generating a collimated electron beam from said flat emitting facet; The apparatus further comprises means for heating said rounded end to between 380K and 1000K while generating said electron beam, thereby removing volatile species and decreasing tunneling distance and increasing electron energy.

2. 2. The apparatus of claim 1, wherein said metal boride material comprises a species selected from the list consisting of alkali metals, alkaline earth metals, transition metals, lanthanides and actinides.

3. 2. The apparatus of claim 1, wherein said metal boride material is a metal hexaboride material.

4. 2. The apparatus of claim 1, wherein the metal boride material is LaB ６ The device includes:

5. 2. The apparatus of claim 1, wherein said metal boride material has a <100> crystallographic orientation.

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