WO2020073506A1 - 电子源和电子枪 - Google Patents

电子源和电子枪 Download PDF

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Publication number
WO2020073506A1
WO2020073506A1 PCT/CN2018/123951 CN2018123951W WO2020073506A1 WO 2020073506 A1 WO2020073506 A1 WO 2020073506A1 CN 2018123951 W CN2018123951 W CN 2018123951W WO 2020073506 A1 WO2020073506 A1 WO 2020073506A1
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Prior art keywords
substrate
electron source
emission
needle tip
field strength
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PCT/CN2018/123951
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English (en)
French (fr)
Inventor
刘华荣
王俊听
王学慧
戚玉轩
胡贤斌
靳学明
黄钊
陈迪志
李艺晶
邓友银
Original Assignee
中国电子科技集团公司第三十八研究所
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Priority to JP2020542131A priority Critical patent/JP6952907B2/ja
Priority to KR1020207022412A priority patent/KR102539958B1/ko
Priority to US16/966,907 priority patent/US11189453B2/en
Priority to EP18936834.3A priority patent/EP3736848B1/en
Publication of WO2020073506A1 publication Critical patent/WO2020073506A1/zh

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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J37/00—Discharge tubes with provision for introducing objects or material to be exposed to the discharge, e.g. for the purpose of examination or processing thereof
    • H01J37/02—Details
    • H01J37/04—Arrangements of electrodes and associated parts for generating or controlling the discharge, e.g. electron-optical arrangement, ion-optical arrangement
    • H01J37/06—Electron sources; Electron guns
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J37/00—Discharge tubes with provision for introducing objects or material to be exposed to the discharge, e.g. for the purpose of examination or processing thereof
    • H01J37/02—Details
    • H01J37/04—Arrangements of electrodes and associated parts for generating or controlling the discharge, e.g. electron-optical arrangement, ion-optical arrangement
    • H01J37/06—Electron sources; Electron guns
    • H01J37/073—Electron guns using field emission, photo emission, or secondary emission electron sources
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J1/00—Details of electrodes, of magnetic control means, of screens, or of the mounting or spacing thereof, common to two or more basic types of discharge tubes or lamps
    • H01J1/02—Main electrodes
    • H01J1/30—Cold cathodes, e.g. field-emissive cathode
    • H01J1/304—Field-emissive cathodes
    • H01J1/3048—Distributed particle emitters
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J3/00—Details of electron-optical or ion-optical arrangements or of ion traps common to two or more basic types of discharge tubes or lamps
    • H01J3/02—Electron guns
    • H01J3/021—Electron guns using a field emission, photo emission, or secondary emission electron source
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J37/00—Discharge tubes with provision for introducing objects or material to be exposed to the discharge, e.g. for the purpose of examination or processing thereof
    • H01J37/02—Details
    • H01J37/04—Arrangements of electrodes and associated parts for generating or controlling the discharge, e.g. electron-optical arrangement, ion-optical arrangement
    • H01J37/06—Electron sources; Electron guns
    • H01J37/065—Construction of guns or parts thereof
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J37/00—Discharge tubes with provision for introducing objects or material to be exposed to the discharge, e.g. for the purpose of examination or processing thereof
    • H01J37/02—Details
    • H01J37/04—Arrangements of electrodes and associated parts for generating or controlling the discharge, e.g. electron-optical arrangement, ion-optical arrangement
    • H01J37/06—Electron sources; Electron guns
    • H01J37/07—Eliminating deleterious effects due to thermal effects or electric or magnetic fields
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J2201/00—Electrodes common to discharge tubes
    • H01J2201/30—Cold cathodes
    • H01J2201/304—Field emission cathodes
    • H01J2201/30403—Field emission cathodes characterised by the emitter shape
    • H01J2201/30426—Coatings on the emitter surface, e.g. with low work function materials
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J2201/00—Electrodes common to discharge tubes
    • H01J2201/30—Cold cathodes
    • H01J2201/304—Field emission cathodes
    • H01J2201/30403—Field emission cathodes characterised by the emitter shape
    • H01J2201/30438—Particles
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J2201/00—Electrodes common to discharge tubes
    • H01J2201/30—Cold cathodes
    • H01J2201/304—Field emission cathodes
    • H01J2201/30446—Field emission cathodes characterised by the emitter material
    • H01J2201/30449—Metals and metal alloys
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J2237/00—Discharge tubes exposing object to beam, e.g. for analysis treatment, etching, imaging
    • H01J2237/002—Cooling arrangements
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J2237/00—Discharge tubes exposing object to beam, e.g. for analysis treatment, etching, imaging
    • H01J2237/06—Sources
    • H01J2237/063—Electron sources
    • H01J2237/06325—Cold-cathode sources
    • H01J2237/06341—Field emission
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J2237/00—Discharge tubes exposing object to beam, e.g. for analysis treatment, etching, imaging
    • H01J2237/06—Sources
    • H01J2237/063—Electron sources
    • H01J2237/06325—Cold-cathode sources
    • H01J2237/06341—Field emission
    • H01J2237/0635—Multiple source, e.g. comb or array

Definitions

  • the present disclosure relates to the field of electron source technology, and more particularly, to a field emission electron source and electron gun.
  • the free electrons in the metal can be emitted under certain conditions. If the cathode is made of metal and made into a very fine needle shape, and thousands of volts are applied in a vacuum, the electrons in the metal can be emitted from the cold metal of the cathode. This method of emitting electrons is called field emission and belongs to cold cathode emission.
  • the most important indicator is brightness, which directly determines its beam quality.
  • the brightness can be shown as formula (1):
  • B is the brightness
  • I is the emission current
  • S is the equivalent emission area
  • d is the equivalent diameter
  • ⁇ is the space emission angle
  • ⁇ is the emission half angle.
  • the luminance B is proportional to the acceleration voltage V a, as shown in Equation (2).
  • the most ideal electron source is cold field emission electron sources (CFE).
  • CFE cold field emission electron sources
  • the brightness of CFE is about one order of magnitude higher than that of other types of electron sources, and there is little energy dissipation ( ⁇ 0.3 eV).
  • atomic-level electron sources with low work functions have become a research hotspot, that is, the emission point is composed of only one or several atoms.
  • Schottky thermal-field emission source Schottky thermal-field emission source
  • the present disclosure provides a stable electron source and electron gun that have a large field emission current and can operate under poor vacuum.
  • An aspect of the present disclosure provides an electron source, including: one or more needle tips, wherein at least one needle tip includes one or more fixed emission points, and the emission points include metal atoms formed on the surface of the needle tip and gas molecules reaction product. Because the emission point is a reaction product formed by the metal atoms fixed on the surface of the needle tip and the gas molecules, it is rooted on the surface of the needle tip, not the gas molecules or free particles that are free on the surface of the needle tip, and will not gather together due to the free emission point. The formation of a new emission point leads to overcurrent burning, which effectively improves stability.
  • the emission point includes the reaction product formed by the metal atoms and gas molecules on the surface of the needle tip, relative to the metal atoms or other metal compounds (such as metal borides, etc.) ), In the working environment (the presence of gas molecules) has better stability, such as less likely to interact or react with the working environment such as hydrogen, etc., further improving the stability of the electron source.
  • the emission point of the electron source provided by the present disclosure may be a reaction product formed by one or several metal atoms and gas molecules, that is, an atomic-level electron source with a low work function can be formed.
  • the reaction product significantly reduces the surface work function, and the formation of surface emission point cones also significantly improves the emission ability.
  • the current value of the field emission current can be increased by increasing the number of emission points. In this way, a stable electron source with a large field emission current can be formed.
  • the emission point includes a reaction product formed by metal atoms and gas molecules under an electric field. This facilitates the formation of an emission point at a specified position of the needle tip, especially at a position where the electric field has an advantage, such as the protrusion of the needle tip.
  • At least one needle tip includes a substrate and one or more high field strength structures on the substrate that are higher than the field strength of other parts of the substrate, wherein, At least one outer surface of the high-field strength structure includes metal atoms, and the metal atoms on the surface of the high-field strength structure are more likely to form reaction products with gas molecules in the same environment by virtue of the field strength to preferentially generate in the high-field strength structure Launch point.
  • at least one needle tip includes a substrate and one or more active regions on the substrate that are more reactive than other parts of the substrate, wherein at least one outer surface of the active region includes Metal atoms.
  • At least one needle tip includes a substrate and one or more high field strength structures on the substrate that are higher than the field strength of other parts of the substrate, at least one of the high field strength structures Part of the surface is an active area with high reactivity, wherein the outer surface of the active area includes metal atoms, and the metal atoms on the surface of the active area are more likely to form reaction products with gas molecules in the same environment by virtue of field strength advantages and activity advantages.
  • the active area includes metal atoms, and the metal atoms on the surface of the active area are more likely to form reaction products with gas molecules in the same environment by virtue of field strength advantages and activity advantages.
  • the electric field is an electric field generated by applying a positive bias voltage, a negative bias voltage, or a combination of a positive bias voltage and a negative bias voltage to the needle tip, wherein, for the application of a positive bias voltage, the electric field
  • the field strength includes 1-50V / nm, and when a negative bias is applied, the field strength includes 1-30V / nm. In this way, the atoms on the surface of the needle tip will not be evaporated, and the tip will not be burned due to overcurrent during the formation of the emission point.
  • the high field strength structure includes protrusions.
  • the size of the protrusion is on the order of sub-nanometer to 100 nanometers.
  • the protrusion is formed by any one or more of the following methods: heat treatment, electric field application, thermal-electric field treatment, etching or nano-processing, or the following method: for example, plating a single crystal on the tip of a single crystal metal Layers of metal atoms are reshaped by heat treatment to form protrusions.
  • a tip including a protrusion in a reaction with a gas molecule under vacuum conditions, at least part of the surface of the protrusion has metal atoms that have the same or greater reaction than other surface parts of the substrate Activity, that is, at least a part of the surface of the protrusion (such as a designated area) has a metal atom with a larger reactivity than other areas.
  • the metal atoms on the surface of the active area of the substrate have a greater reactivity than other surface portions of the substrate during the reaction with gas molecules under vacuum conditions.
  • the substrate material is a conductive material
  • the high field strength structure material is a conductive material
  • the substrate and / or high field strength structure surface is a metal atom
  • the high-field-strength structure material is the same as or different from the substrate material
  • the metal atomic material on the substrate and / or high-field-strength structure surface is the same or different from the high-field strength structure material, and when different, the substrate And / or metal atoms on the surface of the high-field-strength structure are formed by evaporation or electroplating
  • the material of the metal atoms on the substrate and / or the surface of the high-field-strength structure is the same as or different from the substrate material, and when different,
  • the metal atoms on the surface of the substrate and / or high-field-strength structure are formed by evaporation or electroplating.
  • the gas molecules are introduced gas molecules and / or gas molecules remaining in a vacuum environment, wherein the gas molecules include hydrogen-containing element gas molecules, and any one or more of the following gas molecules : Nitrogen-containing gas molecules, carbon-containing gas molecules or oxygen-containing gas molecules.
  • the hydrogen element-containing gas molecule includes hydrogen gas.
  • the disappearance temperature of the emission point is lower than the small value of the disappearance temperature of the substrate, the high field strength structure, and the metal atom, and the disappearance temperature of the emission point is higher than The working temperature of the electron source, or the disappearance temperature of the emission point is lower than the small value of the disappearance temperature of the substrate, the high field strength structure and the metal atom, and the disappearance temperature of the emission point is higher than The large value of the working temperature of the electron source and the desorption temperature of the gas molecules adsorbed on any of the needle tips facilitates desorption, repair, etc. by heat treatment, so as to extend the working life.
  • the substrate material is a conductive material and the melting point is higher than 1000K; and / or; the high field strength structure material is a conductive material and the melting point is higher than 1000K; and / or; the substrate and
  • the material of the metal atoms on the surface of the high-field-strength structure is a metal material with a melting point higher than 1000K, and the reaction product of the metal atom and the gas molecule includes the reaction product of the metal atom and the gas molecule with a melting point higher than 1000K under vacuum conditions.
  • the tip of the needle is preferably made of a conductive material with a melting point higher than 1000K, which has better stability, and is convenient for cleaning the tip of the needle by using the heat treatment method as described above.
  • the metal material includes any one or more of the following: tungsten, iridium, tantalum, molybdenum, niobium, hafnium, chromium, vanadium, zirconium, titanium, rhenium, palladium, platinum, rhodium, osmium, Metal materials with melting points higher than 1000K, such as platinum, ruthenium, gold or hexaboride metal.
  • the metal material is tungsten
  • the emission point is a hydrogen tungsten compound.
  • the size and shape of the substrate and / or high field strength structure of the needle tip are adjusted to adjust the size of the electron beam beam angle, or the size of the emission point is adjusted by adjusting the size of the high field strength structure
  • the quantity, or, by adjusting the structure of the substrate and / or the structure of the high field strength structure adjusts the magnitude or consistency of the voltage of the electron source emission current, or adjusts the shape of the tip of the needle tip to adjust the direction of the emission current.
  • the lead-out voltage may be lower than -0.5KV, for example, the lead-out voltage is -0.4KV.
  • a high-field-strength structure or a region with high reactivity is located at the center of the surface of the substrate, or a high-field-strength structure is located on a substrate whose size is greater than a set threshold, or the metal atoms It is located at the top of the high field strength structure or at the center of the surface of the substrate.
  • the working conditions of the needle tip include: when the needle tip temperature ⁇ 1000K, the working pressure ⁇ 10 -3 Pa, or, when 500K ⁇ needle tip temperature ⁇ g00K, the working pressure ⁇ 10 -6 Pa, or, when When the tip temperature is ⁇ 150K, the working pressure is ⁇ 10 -6 Pa. Due to the formation of the emission point and the low operating temperature, the structure of the electron source does not change. During operation, the structure of the electron source does not change, and the value of the applied voltage does not change.
  • the size of the emission point is nano-scale or sub-nano-scale, that is, the size of the emission point can be ⁇ the size of the protrusion, and by adjusting the operating voltage, the emission current value of the needle-point emission point can reach the order of 10 mA.
  • the electron source has cold field emission characteristics, and the magnitude of the emission current is adjusted by adjusting the extraction voltage.
  • an electron gun including an electron source as described above for emitting electrons, and a cooling device and a gas introduction device, wherein the cooling device is used to dissipate heat from the electron source, the The electron source is fixed on the cooling device through an electrically insulated thermal conductor.
  • a heating device is used to heat the electron source to adjust the temperature.
  • the gas introduction device is used to introduce a gas containing hydrogen. Due to the formation of the emission point and the low operating temperature, the structure of the electron source does not change during operation, the applied voltage value does not change, and the stable voltage value makes the design of the electron gun simpler.
  • the electron source may further remove at least one emission point by forming one or more fixed emission points on at least one needle tip, and applying gas field to adsorb gas molecules on the emission point, To remove at least one launch point.
  • FIG. 1 schematically shows the typical field emission behavior of the electron source of the tungsten (310) single crystal tip in the prior art
  • FIG. 2 schematically shows a schematic diagram of an electron source according to an embodiment of the present disclosure
  • FIG. 3 schematically shows the preparation and use environment of an electron source according to an embodiment of the present disclosure
  • FIG. 4 schematically shows a schematic diagram of a preparation process of an electron source according to an embodiment of the present disclosure
  • FIG. 5A schematically shows a schematic diagram of a high field strength structure on a substrate according to an embodiment of the present disclosure
  • 5B schematically shows a schematic diagram of metal atoms on the surface of a high-field-strength structure according to an embodiment of the present disclosure
  • 5C schematically illustrates a schematic diagram of an active area on a substrate according to an embodiment of the present disclosure
  • 5D schematically shows a schematic diagram of an active region on a high field strength structure according to an embodiment of the present disclosure
  • FIG. 6 schematically illustrates a schematic diagram of an emission area with an emission point according to an embodiment of the present disclosure
  • FIG. 8 schematically shows a schematic diagram of an emission point forming process according to another embodiment of the present disclosure.
  • At least one of A, B, and C in general, it should be interpreted according to the meaning generally understood by those skilled in the art (for example, “having A, B, and C "At least one of the structures” shall include, but not limited to, structures having A alone, B alone, C alone, A and B, A and C, B and C, and / or A, B, C, etc. ).
  • FIG. 1 schematically shows the typical field emission behavior of the electron source of the tungsten (310) single crystal tip in the prior art.
  • the existing electron source such as the tungsten (310) single crystal tip electron source
  • the existing electron source will go through the following three stages during use.
  • the aforementioned problems become more serious at larger emission currents.
  • the total emission current that can work stably for a long time is ⁇ 10 microamperes, and the utilization rate is very low.
  • the Schottky field emission electron source Schottky thermal-field emission source dominates the field of high-brightness electron sources.
  • the current field emission electron source (generally refers to the metal needle tip) can only work in ultra-high vacuum ( ⁇ 10 -8 Pa), which severely restricts the scope of application of CFE. Further in-depth research was conducted here, and the following characteristics were found.
  • the residual gas components in the vacuum are H 2 , CO, CO 2 , and the main component is H 2 .
  • the adsorption of H 2 causes the emission ability of the clean surface to gradually deteriorate. It can be said that in this vacuum range, the influence of H 2 fundamentally determines the field emission performance of the needle tip. Therefore, how to cope with the influence of H 2 has become the key to achieving a highly stable needle point.
  • this free substance is not fixed.
  • Another problem is that when exposed to the atmosphere, due to the size of this substance and the comparison of gas molecules, it is extremely susceptible to gas interference.
  • the inventor Based on the above analysis, reasoning and experiments, the inventor provides the electron source of the present disclosure to achieve long-term stable operation, can provide a large field emission current, can work in a poor vacuum environment, and is exposed to the atmosphere It is also less susceptible to gas interference.
  • the embodiments of the present disclosure provide an electron source.
  • the electron source may include one or more needle tips, wherein at least one needle tip includes one or more fixed emission points, and the emission point includes a reaction product formed by metal atoms on the surface of the needle tip and gas molecules, due to the metal atoms on the surface of the needle tip Fixed on the surface of the needle tip, the reaction product formed with the gas molecules is the reaction product rooted on the surface of the needle tip, and since the reaction product is a reaction product formed by the reaction of metal atoms and gas molecules under similar working conditions, the reaction product is again The activity of the gas molecule reaction is not large, so the stability is high. In addition, since the number of reaction products can be controlled, it is possible to provide a larger field emission current by increasing the number of reaction products. In addition, since the reaction product is a reaction product formed by the reaction of metal atoms and gas molecules, it is not susceptible to gas interference even when exposed to the atmosphere.
  • FIG. 2 schematically shows a schematic diagram of an electron source according to an embodiment of the present disclosure.
  • the electron source may include one or more needle tips.
  • the following uses one needle tip as an example for description.
  • the needle tip may include one or more emission points fixed on the surface of the needle tip, and the emission point may be a reaction product formed by metal atoms and gas molecules on the surface of the needle tip.
  • the reaction product formed by metal atoms and gas molecules on the surface of the needle tip by applying an electric field can be implemented in various ways, for example, directly applying a voltage to the needle tip to form a higher field strength on the surface of the needle tip to promote The reaction product formed by the reaction of metal atoms and gas molecules on the surface of the needle tip; it can also be a voltage applied to the field strength generating structure (such as electrodes, etc.) near the needle tip to form an electric field, and then form a higher field strength on the surface of the needle tip to promote the surface of the needle tip The metal atoms and gas molecules react to form reaction products.
  • the field strength generating structure such as electrodes, etc.
  • the field formed on the surface of the needle tip and the formation method of the field are not limited, as long as a field (such as an electric field) can be formed on the surface of the needle tip that can promote the reaction of metal atoms on the surface of the needle tip with surrounding gas molecules to form a reaction product.
  • a field such as an electric field
  • the electric field is generated by applying a positive bias voltage, a negative bias voltage, or a combination of positive and negative bias voltages to the needle tip
  • the electric field wherein, when a positive bias is applied, the field strength of the electric field includes 1 to 50 V / nm, and when a negative bias is applied, the field strength of the electric field includes 1 to 30 V / nm.
  • the emission point may be formed at a specified position of the needle tip, for example, within a certain range where the needle axis intersects the surface of the needle tip, or may be formed on a specific structure, such as protrusions, etc., which have the advantage of field strength to preferentially form metal atoms and
  • the structure of the reaction product of the gas molecule can also be formed in a specific area with reactive activity, such as a specific metal atom area that is more likely to react with the gas molecule. Of course, it can also be used in combination of the above two situations. No limitation here.
  • the metal atom may be a metal atom on the surface of the tip body, that is, the type of the metal atom is the same as the type of the tip body, or may be a different type of metal atom formed on the surface of the tip by means of evaporation, plating, or the like.
  • the metal atom is a metal material with a melting point higher than 1000K, which has better stability, and is convenient for cleaning the tip of the needle by using the heat treatment method as described above.
  • the metal material having a melting point higher than 1000K may include any one or more of the following: tungsten, iridium, tantalum, molybdenum, niobium, hafnium, chromium, vanadium, zirconium, titanium, rhenium, palladium, platinum, rhodium, osmium, Ruthenium, gold or hexaboride metal, etc.
  • one of the metal atoms is used as the metal atom on the surface of the needle tip alone, or a stack of several metal atoms, such as a stack of titanium layer ⁇ platinum layer ⁇ tungsten layer Etc., or a non-elementary metal layer formed by mixing several kinds of metal atoms, which is not limited herein.
  • the gas molecules may be introduced by a gas introduction device, such as a specific gas molecule introduced through a gas flow valve or the like, or may be gas molecules remaining when the chamber is evacuated, and of course, the above two methods may also be used
  • the gas molecules include hydrogen-containing gas molecules and any one or more of the following gas molecules: nitrogen-containing gas molecules, carbon-containing gas molecules or oxygen-containing gas molecules.
  • the above gas molecules may be introduced gas molecules. Therefore, the amount of gas introduced can be dynamically adjusted.
  • the vacuum degree is ⁇ 10 -4 Pa.
  • the main residual gas in the vacuum chamber is hydrogen.
  • the hydrogen element-containing gas molecules include hydrogen gas.
  • the temperature at which the emission point disappears for example, the decomposition temperature can be lower than the temperature at which the tip body disappears to remove the emission point.
  • the emission temperature of the emission point can be higher than the working temperature and the temperature at which the adsorbed gas molecules are desorbed. It is convenient for desorption by simple heat treatment (such as Flash heat treatment) to restore the electron source to a stable state.
  • the electron source provided by the present disclosure has a low extraction voltage of the emission point, a fixed and non-free emission point, a long life, and can work under a poor vacuum. Due to the formation of the emission point and the low operating temperature, the structure of the electron source does not change during operation, and the applied voltage value does not change. For example, it can work under the working conditions shown below, for example, when the tip temperature is ⁇ 1000K, the working pressure ⁇ 10 -3 Pa, or, when 500K ⁇ needle tip temperature ⁇ 800K, working pressure ⁇ 10 -6 Pa, or, when needle tip temperature ⁇ 150K, working pressure ⁇ 10 -6 Pa.
  • the size of the emission point is on the order of nanometers or sub-nanometers.
  • the emission current value of the emission point of the needle tip can reach the order of 10 mA.
  • the electron source has the characteristics of cold field emission, and the magnitude of the emission current can be adjusted by adjusting the drawn voltage.
  • the metal material is tungsten, and correspondingly, the emission point is a hydrogen tungsten compound.
  • FIG. 3 schematically shows the preparation and use environment of an electron source according to an embodiment of the present disclosure.
  • the preparation and use device may include a vacuum chamber 111, a cooling head 107 (the cooling head 107 includes a heating device, the heating device is not shown), a sample holder 105, an electron source 101, and a high-voltage power supply 115 , Particle beam 113, fluorescent screen assembly 103 and gas introduction device 109, wherein the right figure is an enlarged schematic view of the dotted frame in the left figure, the sample stage 105 may include a hair fork 120, the hair fork 120 is used to fix the electron source 101, above
  • the power supply is the one that heats the tip of the needle. In this way, the electron source 101 can be heated and / or pressurized, the gas in the vacuum chamber can be controlled, and the electron emission from the electron source can be observed through the fluorescent screen assembly 103.
  • FIG. 4 schematically shows a schematic diagram of a preparation process of an electron source according to an embodiment of the present disclosure.
  • the right picture is an enlarged schematic view of the image in the dashed box in the left picture, and the dashed box below the left picture is schematic.
  • the gas molecules in the environment (the small gray dots in the left picture) will gradually move toward the tip of the needle tip.
  • the gas molecules move to the tip of the electron source, the gas molecules and the metal atoms (white dots) on the surface of the needle tip form a reaction product (black Dots, or launch points, referred to as Ma1), the black dots will be rooted on the surface of the needle tip, rather than free on the surface of the needle tip.
  • black Dots or launch points, referred to as Ma1
  • FIG. 5A schematically shows a schematic diagram of a high field strength structure on a substrate according to an embodiment of the present disclosure.
  • At least one needle tip includes a substrate and one or more high field strength structures on the substrate that are higher than the field strength of other parts of the substrate, wherein at least one of the high field strength structures
  • the outer surface of the metal includes metal atoms.
  • the number of protrusions may be 1, 3, 5, 10, etc., which is not limited herein.
  • the high field strength structure material and the substrate material may be the same or different.
  • the metal atom material on the surface of the substrate and / or high field strength structure is the same as or different from the material of the high field strength structure.
  • the metal atom on the surface of the substrate and / or high field strength structure may be It is formed by evaporation or electroplating, which is not limited here.
  • the metal atom material on the surface of the substrate and / or high field strength structure and the substrate material may be the same or different, and when different, the metal atom on the surface of the substrate and / or high field strength structure may be It is formed by plating or electroplating, which is not limited here.
  • the material of the substrate and the material of the high field strength structure are the same as or different from the material of the body of the needle tip, which is not limited herein.
  • metal atoms on at least part of the surface of the protrusion have the same or greater reactivity than other surface parts of the substrate.
  • the high field strength structure may include protrusions corresponding to the Lo1 position.
  • the size of the protrusion may be on the order of sub-nanometer to 100 nm. Due to the advantage of high field strength at the protrusion, when a voltage is applied to the tip of the needle, under the effect of the field strength, at least part of the metal atoms on the surface of the protrusion will form a reaction product with gas molecules. In this way, the launch point can be formed at a specified position on the surface of the needle tip simply and quickly.
  • the number of emission points on the surface of the protrusion can be controlled by adjusting the size of the protrusion, the size and length of the applied bias, and other parameters, for example, more can be formed by increasing the duration of the applied bias or increasing the size of the protrusion To increase the emission current.
  • the protrusion is formed by any one or more of the following methods: heat treatment, electric field application, thermal-electric field treatment, etching or nano-machining, etc., or the following method: for example, a layer of metal atoms is deposited on the tip of the single crystal metal needle, and the heat treatment Reshaping to form protrusions; It should be noted that as long as the method of forming protrusions on the surface of the needle tip is applicable, it is not limited here.
  • the substrate material is a conductive material and the melting point is higher than 1000K
  • the high field strength structure material is a conductive material and the melting point is higher than 1000K
  • the substrate and / or the metal atoms on the surface of the high field strength structure The material is a metallic material with a melting point above 1000K
  • the reaction product of the metal atom and the gas molecule includes the reaction product of the metal atom and the gas molecule with a melting point higher than 1000K under vacuum conditions.
  • FIG. 5B schematically shows a schematic diagram of metal atoms on the surface of a high-field-strength structure according to an embodiment of the present disclosure.
  • the material of the substrate, the high field strength structure and the body of the needle tip may be metallic materials, or not metallic materials (such as conductive materials), when the electron source does not include high field strength In the structure, as long as the surface of the substrate includes metal atoms and the substrate can introduce current; when the electron source includes a high field strength structure, as long as the surface of the high field strength structure includes metal atoms, In addition, the high field strength structure can only introduce current.
  • the high field strength structure is located at the center of the surface of the substrate, or the high field strength structure is located on a substrate with a size greater than a set threshold, such as a substrate with a larger size, or,
  • the metal atom is located at the center of the surface of the top of the high field strength structure.
  • the size and shape of the substrate and / or high field strength structure of the needle tip can be adjusted to adjust the beam angle of the electron beam
  • the number of emission points can also be adjusted by adjusting the size of the high field strength structure
  • the size and consistency of the voltage of the electron source emission current can be adjusted by adjusting the structure of the substrate and / or the structure of the high field strength structure
  • the direction of the emission current can be adjusted by adjusting the shape of the tip of the needle tip. For example, when the substrate size is increased, the beam angle is reduced; the field evaporation reduces the height of the protrusion, and the beam angle is reduced; the smaller the size of the protrusion, the smaller the number of emission points under the same electric field.
  • At least one needle tip includes a substrate and one or more high field strength structures on the substrate that are higher than the field strength of other parts of the substrate, wherein at least one of the high field strengths
  • the outer surface of the structure includes metal atoms, and the metal atoms on the surface of the high-field-strength structure are more likely to form reaction products with gas molecules in the same environment by virtue of the field strength to preferentially generate emission points in the high-field-strength structure.
  • FIG. 5C schematically illustrates a schematic diagram of an active area on a substrate according to an embodiment of the present disclosure.
  • At least one needle tip includes a substrate and one or more active regions on the substrate that are more reactive than other parts of the substrate, corresponding to the Lo2 position, wherein at least one of the active regions
  • the surface includes metal atoms.
  • the active area is shown in the shaded area of FIG. 5C.
  • the number of active regions may be 1, 3, 5, 10, etc., which is not limited herein, and the emission point may be preferentially formed in the active region located in the top region of the needle tip.
  • the material of the substrate and the material of the body of the needle tip may be the same or different.
  • the metal atoms in the active area may be formed by evaporation or electroplating.
  • a metal atom layer of a certain area is formed at the intersection of the axis of the needle tip and the surface by electroplating.
  • Materials on other surfaces have higher reactivity with gas molecules. Accordingly, in the reaction with gas molecules under vacuum conditions, the metal atoms on the surface of the active area of the substrate have a greater reactivity than other surface portions of the substrate.
  • the region with high reactivity is located at the center of the surface of the substrate, or the metal atom is located at the center of the surface of the substrate.
  • At least one needle tip includes a substrate and one or more active regions on the substrate that are more reactive than other parts of the substrate, wherein at least one outer surface of the active region includes metal Atoms, the metal atoms on the surface of the active area are more likely to form reaction products with gas molecules in the same environment by virtue of the activity advantage, so as to preferentially generate emission points in the active area.
  • FIG. 5D schematically shows a schematic diagram of an active region on a high field strength structure according to an embodiment of the present disclosure.
  • At least one needle tip includes a substrate and one or more high field strength structures on the substrate that are higher than the field strength of other parts of the substrate, and at least part of the surface of the high field strength structure It is an active area with high reactivity, corresponding to the Lo3 position, wherein the outer surface of the active area includes metal atoms.
  • the metal atoms on at least part of the surface of the protrusion have the same or greater reactivity than other surface parts of the substrate in the reaction with the gas molecule under vacuum conditions In this way, it is possible to more accurately control the emission point to be formed on the specified area of the protrusion, for example, the shadow area on the protrusion of FIG. 5D forms the emission point.
  • the method of forming metal atoms on the surface of the active region reference may be made to the method of forming the active region in the previous embodiment, which will not be described in detail here.
  • At least one needle tip includes a substrate and one or more high field strength structures on the substrate that are higher than the field strength of other parts of the substrate, at least part of the high field strength structure
  • the surface is an active area with high reactivity, wherein the outer surface of the active area includes metal atoms, and the metal atoms on the surface of the active area are more likely to form reaction products with gas molecules in the same environment by virtue of the field strength advantage and the activity advantage. Priority is given to generating emission points in the active area.
  • FIG. 6 schematically shows a schematic diagram of an emission area with emission points according to an embodiment of the present disclosure.
  • a needle tip with a protrusion is used as an example for description.
  • the top view of the emission point can refer to the right figure of FIG. 6.
  • the area with the higher field strength is more likely to form the emission point first, but the emission point is not necessarily formed only in the area with the highest field strength, of course, as shown in Figure 6 Since the apex position of the protrusion shown is the strongest, the probability of forming the emission point first at the apex is also the highest.
  • the shape of the protrusion in FIG. 6 is hemispherical, and it may also be square, polygonal, mesa-shaped, semi-elliptical, and other geometric structures, which are not limited herein.
  • the disappearance temperature of the emission point is lower than the small value of the disappearance temperature of the substrate, the high field strength structure and the metal atom, and the disappearance temperature of the emission point is higher than the electrons The working temperature of the source; or, the disappearance temperature of the emission point is lower than the small value of the disappearance temperature of the substrate, the high field strength structure, and the metal atom, and the disappearance temperature of the emission point is higher than The maximum value of the working temperature of the electron source and the desorption temperature of the gas molecules adsorbed on any needle tip.
  • the disappearance temperature of the emission point is higher than the desorption temperature of the gas molecules adsorbed on any of the needle tips, it is convenient to desorb the gas molecules by heating, and it is convenient to improve the stability of the electron source through simple processing.
  • the background vacuum degree of the vacuum chamber 111 is ⁇ 10 -3 Pa (generally should be better than 10 -6 Pa).
  • the cooling head cold head 107 has an insulated sample holder 105, which can realize a heating device (such as a heating sheet, a heating rod, etc.) on the sample holder, and the temperature can be adjusted between 10 and 500K.
  • the pre-treated needle tip (such as a tungsten single crystal needle tip, which can be a needle tip with protrusions, the size of the protrusion is nm or sub-nm, or it can be a needle tip with a larger reactive area)
  • a voltage is applied to the needle tip, and the voltage may be a positive high voltage V P or a negative high voltage V N.
  • the power supply 115 may be a dual-output high-voltage power supply with an output range of ⁇ 0 to 30 kV.
  • the reaction gas molecules 109 for introducing a gas introduction means, such as H 2, other reaction gases may be, for example, a gas containing H elements, water, CH 4, etc., through the gas intake can be dynamically adjusted, when introduced into the vacuum degree is generally ⁇ 10 - 4 Pa (It should be noted that the residual gas molecules in the chamber can also be used directly, the main component of which is hydrogen).
  • the fluorescent screen assembly 103 is used to convert the particle beam image into a light image. When the signal is small, the fluorescent screen-multi-channel plate assembly can be used to amplify the signal. When a voltage is applied to the needle tip, the particle beam can be drawn out, and the applied voltage can be positive pressure or negative pressure; when the positive pressure is, there is an imaging gas, the positive ion beam is output; when the negative pressure, the electron beam .
  • FIG. 7 schematically shows a schematic diagram of an emission point forming process according to an embodiment of the present disclosure.
  • the formation of the emission point Ma1 is achieved by the reaction of gas molecules and surface metal atoms under the action of a strong electric field, and the formation of Ma1 proceeds at a certain temperature.
  • the formation process of the emission point is based on the preparation method determined after in-depth study of the hydrogen adsorption behavior in a small area on the surface of the needle tip.
  • a tungsten single crystal (111) needle tip and form a protrusion on the needle tip by the method described above, such as after flash treatment (heating to 1200K for 3s, during which can also be supplemented by bias, etc.), this can A nano-level protrusion is formed in the middle of the surface of the needle tip, and its surface is clean.
  • FIG. 7a when a negative pressure is applied to the needle tip to -2KV, a field electron emission mode is formed. To avoid the impact of ion bombardment, the formation temperature of Ma1 is ⁇ 50K.
  • the emission current (I E ) has been controlled within 5nA and the vacuum degree is 10 -7 Pa throughout the formation of the emission point.
  • the gas adsorption first causes the emission capability to decrease, the emission pattern on the display interface of the fluorescent screen assembly 103 gradually becomes darker, and the emission current gradually decreases, that is, the emission capability of the existing clean surface of tungsten gradually decreases.
  • the emission point described in this disclosure begins to form.
  • the composition of the emission point and the previous emission substance are different.
  • the previous emission The substance is a tungsten atom of a tungsten single crystal, and the emission point at this time is a reaction product of tungsten atoms and gas molecules on the surface of the needle tip, such as tungsten atoms and hydrogen molecules, and the reaction product is fixed on the surface of the needle tip.
  • the gas such as H 2
  • the gas undergoes adsorption, dissociates under the electric field, and further combines with the surface metal atoms to form a certain HW reaction product (compound), which belongs to a type of Ma1.
  • the compound seems to bond directly with the surface and does not move.
  • similar materials can be formed at other locations, but they do not move, and have always been a stable launch point.
  • the emission capability of a single emission point can reach more than 30uA. If a dense emission area is formed, the emission patterns are connected together, and the total current can reach the order of 100uA. If the emission area is increased (the number of emission points is increased), an emission current in the order of 10 mA can be achieved, far exceeding the stable emission capability of the existing CFE ( ⁇ 10 ⁇ A). Recently, Keigo Kasuya et al. Can achieve the ultimate emission current of about 3000 microamperes by increasing the vacuum degree of the cavity to 4 ⁇ 10 -10 Pa, but this vacuum degree is difficult to achieve.
  • the high emission current can be maintained at high vacuum, and the maximum emission current can decay rapidly at low vacuum.
  • FIG. 8 schematically shows a schematic diagram of an emission point forming process according to another embodiment of the present disclosure. It can be seen from the figure that the formed emission area is small and concentrated.
  • the tip of the formed emission point (The tip terminated by emission site) has field emission consistency, that is, the voltage of the emission current is consistent, for example, when the emission current is 1 microampere, the voltage is -1.2 ⁇ 0.1KV.
  • the structure of the electron source does not change during operation, the applied voltage value does not change, and the stable voltage value makes the design of, for example, an electron gun simpler.
  • the emission point is formed on the tip protrusion formed by the method of field-induced etching, field evaporation, etc.
  • the emission current When it is 1 microampere, the voltage can be lower than -0.5KV (for example, the extraction voltage is -0.4KV), so that the structure design of the electron gun is simpler.
  • the electron source has good stability.
  • the emission point (refer to the solid black circle in Figure 4) can be stably rooted at a specific position on the surface of the needle tip. Multiple emission points can give a total current of mA level, and can Work under poor vacuum (10 -5 Pa), and easy to save.
  • Tip tip The tip contains the substrate Base and the protrusion protrusion of the nm level on the substrate Base.
  • the size of the protrusion is nm or sub-nm.
  • An emission point is formed on the surface of the protrusion.
  • the formation process of the emission point Ma1 is shown below. Under a strong electric field (such as in the range of 1-50V / nm, it should be noted that the electric field will be affected by the voltage polarity and material properties). Metal atoms form a gas-metal compound, which will be described below by taking an HW compound as an example. At higher vacuum, the main residual gas molecule is H 2 . Since the surface electric field of the protrusion is the strongest, it is preferentially formed here.
  • Ma1 is a reaction product that is rooted on the surface of the needle tip and formed by the reaction of gas molecules and metal atoms on the surface of the needle tip. The positions of these emission points are fixed and do not move, but a single Ma1 may flicker under different emission currents.
  • Ma1 has a surface protection effect: Since Ma1 is formed by the reaction of the most surface atoms of metals with gas molecules, that is, reaction products, it is more difficult for them to continue to interact with space gas molecules, such as adsorption, etc .; even after gas adsorption It is difficult to continue to affect Ma1 on the surface of the needle tip, and the chemical reaction continues to occur immediately.
  • the adsorbed gas on Ma1 can be effectively desorbed by means such as low temperature heating ( ⁇ 1000K), and the existence of Ma1 can be kept. Even if the heating part removes Ma1 and exposes the metal atom part, during the working process (the presence of the electric field E), Ma1 can be further formed there.
  • Ma1 adds a protective layer to the needle tip, effectively blocking the reaction between the gas and the needle tip under a strong field. Where Ma1 is not formed on other surfaces, due to the weak local electric field and weak emission capability, the comparison is ignored. This greatly improves the working stability and stable period of the electron source.
  • Ma1 has strong environmental adaptability and can work under a vacuum of 10 -5 Pa. If supplemented with low temperature, it can work under a vacuum of 10 -3 Pa.
  • the higher the vacuum the longer the continuous working time; and the smaller the emission current, the longer the continuous working time.
  • Different emission current ranges can be selected according to specific requirements, such as the vacuum conditions to be used.
  • the emission ability is significantly enhanced compared to the existing CFE, there may be two mechanisms: the first is the significant reduction of the work function; the second is the formation of the surface-level emission point cone. The combined effect of these two actions makes the lead-out voltage V 0 much lower (the reduction ratio is> -30%).
  • the size of the emission current can be directly controlled by the leading voltage, and the output of pulse current (pulse voltage) and constant current (constant voltage) can be realized.
  • the emission area of the upper surface of the protrusion can be controlled and changed by, for example, adjusting the diameter of the protrusion of the tip of the needle tip.
  • the emission area can reach the atomic level or the 10nm level.
  • the size of the emission area can be controlled by changing the area of the active area.
  • the position (Lo) where Ma1 is formed can be pre-selected, for example, by forming an electric field predominant area Lo1 on the surface of the needle tip (see FIG. 5A, for example, forming a nm-level protrusion), so that the electric field is higher than the others Area; or an active area Lo2 is formed on the surface of the needle tip, and this area reacts with the gas to have greater activity (refer to FIG. 5C, relative to other areas); or a certain area Lo3 (refer to FIG. 5D) has the above two characteristics.
  • the protrusions can be formed by heat treatment, thermal-electric field treatment, etching, nano-processing, etc .; the active atom region can be formed by various atom evaporation methods.
  • the size of the emission area can also be controlled by the above-mentioned means, and its scale is nm or atomic level.
  • Lo1 is located above a larger substrate at the tip of the needle.
  • the size of the electron beam beam angle can be achieved by controlling the size of the needle tip substrate and protrusions.
  • the needle tip has a geometry with a protrusion in the center.
  • the electric field intensity in the area around the needle tip is weaker than the electric field intensity in the center of the surface of the needle tip.
  • At least one layer or several layers on the outermost surface of the needle tip are metal atoms, which can be various metal materials with melting points higher than 1000K.
  • the body material of the needle tip may be metal or other conductive materials, such as metal-hexaboride, etc. Preferably, it has a higher melting point (eg,> 1000K).
  • Needle tip conditions Taking a needle tip with a protrusion as an example, a nano-scale protrusion is first formed on the area where the above substance needs to be formed on the needle tip surface, so that when the needle tip is biased (V S ), a strong surface is generated on the protrusion surface
  • V S needle tip is biased
  • E S electric field
  • E S is greater than the electric field around it.
  • E S the surface electric field protruding from the needle tip
  • E S the surface electric field protruding from the needle tip
  • E S can be positive, and its intensity needs to ensure that the atoms on the surface of the needle tip do not evaporate; in addition, E S (or V S ) can also be negative, to ensure that the needle tip does not burn down due to overcurrent.
  • Gas molecules The main component is H 2 , or gas molecules containing H element. It can be achieved through the introduction of external gas molecules, or it can directly use the residual gas molecules in the vacuum.
  • Gas molecules or ions are adsorbed on the surface of the needle tip, and under strong fields, they gradually react with atoms on the surface of the needle tip, and eventually form a stable emission point.
  • the temperature range of the preparation process should be less than the decomposition temperature of the emission point Ma1. According to the observation, it should be less than 1000K.
  • the reaction temperature is below 150K at a low temperature, and in another preferred embodiment, the working temperature is between 500 and 800K. Generally, this temperature should also ensure that the surface morphology of the needle tip is not changed, such as below the deformation temperature of the tip of the needle tip.
  • Removal method of forming the emission point if the emission capability of the formed emission point is degraded, the aged emission point can be removed.
  • the field emission can be removed by field evaporation, that is, applying a positive high pressure. It can also be directly removed by heating to greater than 1000K. After removal, Ma1 can be formed again in situ.
  • the electron source provided by the present disclosure since the emission point of the electron source is a reaction product formed by metal atoms and gas molecules fixed on the surface of the needle tip, rather than gas molecules or free particles free on the surface of the needle tip, will not be
  • the emission points are grouped together to form a new emission point, which leads to overcurrent burning, which effectively improves the stability.
  • the emission point includes the reaction product.
  • metal atoms or other metal compounds such as metal borides, etc.
  • the emission point of the electron source may be a reaction product formed by a single metal atom and a gas molecule, that is, the emission point is composed of only one or several atoms, and can form an atomic-level electron source with a low work function.
  • the emission point is a reaction product formed by gas molecules and metal atoms on the surface of the needle tip, and is rooted on the surface of the needle tip, it is relatively less susceptible to ion bombardment and the gas in the environment.
  • the field emission current can be improved by increasing the number of emission points.
  • the electron source provided by the present disclosure has the advantages of being stable, having a large field emission current, being able to work under a poor vacuum, and being easy to store.
  • an electron gun including the electron source, the cooling device, the heating device, and the gas introduction device as described above.
  • the electron source is used to emit electrons
  • the cooling device is used to dissipate heat to the electron source
  • the electron source is fixed on the cooling device through an electrically insulated thermal conductor
  • the heating device is used to The electron source is heated to adjust the temperature
  • the gas introduction device is used to introduce a gas containing hydrogen element. Because the formation temperature and working temperature of the emission point on the electron source surface of the electron gun are low, the structure of the electron source does not change during operation, the applied voltage value does not change, and the voltage value is also more stable, making the design of the electron gun simpler.

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Abstract

一种电子源(101)，该电子源(101)包括：一个或多个针尖，其中，至少一个针尖包括一个或多个固定的发射点(Ma1)，所述发射点(Ma1)包括针尖表面的金属原子与气体分子形成的反应产物，还提供了一种电子枪。

Description

电子源和电子枪 技术领域
本公开涉及电子源技术领域，更具体地，涉及一种场发射的电子源和电子枪。
背景技术
金属中的自由电子在特定条件下可以发射出来，若用金属构成阴极并做成极细的针尖状，在真空中施以数千伏电压，金属中的电子即可从阴极冷金属中发射，这种发射电子的方法称为场发射，属于冷阴极发射。
对于电子源而言，最重要的指标就是亮度，直接决定了其束流品质。在引出电压V0下，亮度可如公式(1)所示：
Figure PCTCN2018123951-appb-000001
其中，B为亮度，I为发射电流，S为等效发射面积，d为等效直径，Ω为空间发射角，α为发射半角。此外，亮度B正比于加速电压V a，如公式(2)所示。
Figure PCTCN2018123951-appb-000002
由公式(1)可知，为得到高亮度，需要尽可能得提升I，以及减小α和d。此外，获得一定的发射电流所需要的引出电压V0越低越好，而这需要针尖的发射面具有较低的功函数以及较尖锐的头部结构。另外，电子源的另外一个关键指标就是单色性，可用能散δE表示。
综合上述考虑，最理想的电子源就是冷场发射电子源(cold field emission electron sources，简称CFE)，CFE的亮度高过其它种类电子源约一个量级以上，且有很小的能散(～0.3eV)。此外，为了尽量追求极限直径，近年来具有低功函数的原子级电子源成为研究热点，即发射点仅有一个或者数个原子组成。
在实现本公开构思的过程中，发明人发现现有技术中CFE至少存在如下问题：第一，CFE稳定性往往很差，需在极高真空下(10 -9～10 -8Pa)才能工作，这严重限制了其使用范围。而即使在该环境下，也需定期处理以获得较稳定的工作状态。第二，受离子轰击的影响，CFE较容易烧毁。第三，前述问题在较大发射电流下变得更为严重，现有的CFE一般可长时间稳定工作的总发射电流为～10微安，且利用率很低。鉴于前述弊端，在高亮度电子源领域占据主导地位的是肖特基式场发射电子源(Schottky thermal-field emission  source)。
发明内容
有鉴于此，本公开提供了一种稳定的、具有较大的场发射电流、可以在较差真空度下工作的电子源和电子枪。
本公开的一个方面提供了一种电子源，包括：一个或多个针尖，其中，至少一个针尖包括一个或多个固定的发射点，所述发射点包括针尖表面的金属原子与气体分子形成的反应产物。由于该发射点是固定在针尖表面的金属原子与气体分子形成的反应产物，根植于针尖表面，而非游离在针尖表面的气体分子或者游离颗粒物等，不会因游离状的发射点聚集在一起形成新的发射点而导致过电流烧毁，有效提升了稳定性，此外，该发射点包括针尖表面的金属原子与气体分子形成的反应产物，相对于金属原子或其它金属化合物(如金属硼化物等)而言，在工作环境中(存在气体分子)具有更好的稳定性，如更加不容易与工作环境中的例如氢气等发生作用或反应，进一步提升了电子源的稳定性。另外，本公开提供的电子源的发射点可以为一个或数个金属原子与气体分子形成的反应产物，即能形成具有低功函数的原子级电子源。此外，该反应产物使表面功函数显著降低，表面发射点尖锥的形成也使发射能力显著提高。另外，可以通过增加发射点的数量来提升场发射电流的电流值。这样就可以形成稳定的、具有较大的场发射电流的电子源。
根据本公开的实施例，所述发射点包括金属原子与气体分子在电场下形成的反应产物。这样便于在针尖的指定位置，尤其是具有电场优势的位置形成发射点，如针尖的突起处。
根据本公开的实施例，在一个具体实施例中，至少一个针尖包括衬底和所述衬底上的一个或多个比所述衬底其它部位的场强高的高场强结构，其中，至少一个所述高场强结构的外表面包括金属原子，该高场强结构表面的金属原子凭借场强优势在相同的环境中更容易与气体分子形成反应产物，以优先在高场强结构生成发射点。在另一个具体实施例中，至少一个针尖包括衬底和所述衬底上的一个或多个比所述衬底其他部位反应活性大的活性区域，其中，至少一个所述活性区域外表面包括金属原子，该活性区域表面的金属原子凭借活性优势在相同的环境中更容易与气体分子形成反应产物，以优先在活性区域生成发射点。在另一个具体实施例中，至少一个针尖包括衬底和所述衬底上的一个或多个比所述衬底其它部位的场强高的高场强结构，所述高场强结构的至少部分表面是反应活性大的活性区域，其中，所述活性区域外表面包括金属原子，该活性区域表面的金属原子凭借场强优势和活性优势在相同的环境中更容易与气体分子形成反应产物，以优先在活性区域生成 发射点。
根据本公开的实施例，所述电场为通过给所述针尖施加正偏压、负偏压或者正偏压和负偏压的结合而产生的电场，其中，对于施加正偏压时，电场的场强包括1～50V/nm，对于施加负偏压时，电场的场强包括1～30V/nm。这样可以避免针尖表面的原子不被蒸发，也可以避免发射点形成过程中不会因过流而导致针尖被烧毁。
根据本公开的实施例，所述高场强结构包括突起。
根据本公开的实施例，所述突起的尺寸为亚纳米至100纳米量级。
根据本公开的实施例，所述突起通过以下任意一种或多种方法形成：热处理、施加电场、热-电场处理、刻蚀或者纳米加工，或者如下方法：例如在单晶金属针尖上面镀一层金属原子，通过热处理重塑形成突起。
根据本公开的实施例，对于包括突起的针尖，在真空条件下与气体分子的反应中，所述突起的至少部分表面的金属原子比所述衬底的其他表面部分具有相同或更大的反应活性，即突起表面至少部分区域(如指定区域)上有反应活性相对其他区域大的金属原子。对于不包括突起的针尖，在真空条件下与气体分子的反应中，所述衬底的活性区域的表面的金属原子比所述衬底的其他表面部分具有更大的反应活性。
根据本公开的实施例，所述衬底材料是导电材料，或者，所述高场强结构材料是导电材料，或者，所述衬底和/或高场强结构表面是金属原子，或者，所述高场强结构材料与衬底材料相同或者不同，或者，所述衬底和/或高场强结构表面的金属原子材料与高场强结构材料相同或者不同，当不同时，所述衬底和/或高场强结构表面的金属原子通过蒸镀或者电镀等方式形成，或者，所述衬底和/或高场强结构表面的金属原子材料与衬底材料相同或者不同，当不同时，所述衬底和/或高场强结构表面的金属原子通过蒸镀或者电镀等方式形成。
根据本公开的实施例，所述气体分子为引入的气体分子和/或真空环境中残留的气体分子，其中，所述气体分子包括含氢元素气体分子，以及以下任意一种或多种气体分子：含氮元素气体分子、含碳元素气体分子或者含氧元素气体分子。
根据本公开的实施例，所述含氢元素气体分子包括氢气。
根据本公开的实施例，所述发射点的消失温度低于所述衬底、所述高场强结构和所述金属原子的消失温度的小值，且所述发射点的消失温度高于所述电子源的工作温度，或者所述发射点的消失温度低于所述衬底、所述高场强结构和所述金属原子的消失温度的小值，且所述发射点的消失温度高于所述电子源的工作温度和吸附于任一针尖上的气体分子的 脱吸附温度的大值，这样便于通过加热处理进行脱吸附、修复等，以延长工作寿命。
根据本公开的实施例，所述衬底材料是导电材料并且熔点高于1000K；并且/或者；所述高场强结构材料是导电材料并且熔点高于1000K；并且/或者；所述衬底和/或高场强结构表面的金属原子的材料为熔点高于1000K的金属材料，所述金属原子与气体分子的反应产物包括在真空条件下熔点高于1000K的金属原子与气体分子的反应产物。针尖优选地采用熔点高于1000K的导电材料，稳定性更好，且便于利用如上所述的热处理等方法对针尖进行清洁。
根据本公开的实施例，所述金属材料包括以下任意一种或多种：钨、铱、钽、钼、铌、铪、铬、钒、锆、钛、铼、钯、铂、铑、锇、铂、钌、金或者六硼化金属等熔点高于1000K的金属材料。
根据本公开的实施例，所述金属材料为钨，所述发射点为氢钨化合物。
根据本公开的实施例，通过调节所述针尖的衬底和/或高场强结构的尺寸和形状以调节电子束束流角的大小，或者，通过调节高场强结构的尺寸调节发射点的数量，或者，通过调节衬底的结构和/或高场强结构的结构调节电子源发射电流的电压的大小或一致性，或者，通过调节针尖顶部的形状以调节发射电流方向。其中，引出电压可低于-0.5KV，如引出电压为-0.4KV。
根据本公开的实施例，高场强结构或者反应活性大的区域位于所述衬底的表面中心位置，或者，高场强结构位于尺寸大于设定阈值的衬底上，或者，所述金属原子位于所述高场强结构顶端或者所述衬底的表面中心位置。
根据本公开的实施例，所述针尖的工作条件包括：针尖温度≤1000K时，工作压强≤10 -3Pa，或者，500K≤针尖温度≤g00K时，工作压强≤10 -6Pa，或者，当针尖温度≤150K时，工作压强≤10 -6Pa。由于发射点形成和工作温度较低，电子源结构不改变，在工作时电子源结构不改变，施加电压值不发生变化。
根据本公开的实施例，所述发射点的尺寸为纳米级或亚纳米级，即发射点的尺寸可以≤突起的尺寸，通过调节工作电压，针尖发射点发射电流值可达10mA量级。
根据本公开的实施例，所述电子源具有冷场发射特点，通过调节引出电压调节发射电流大小。
本公开的另一个方面提供了一种电子枪，包括如上所述的电子源，用于发射电子，以及冷却装置和气体引入装置，其中，所述冷却装置用于给所述电子源散热，所述电子源通过电绝缘热导体固定在所述冷却装置上，加热装置，用于给所述电子源加热调节温度；所 述气体引入装置，用于引入包含氢元素的气体。由于发射点形成和工作温度较低，在工作时电子源结构不改变，施加电压值不发生变化，电压值稳定使电子枪的设计更简单。
根据本公开的实施例，所述电子源还可以通过如下操作去除至少一个发射点：在至少一个针尖上形成一个或多个固定的发射点之后，通过施加电场使得气体分子吸附在发射点上，以去除至少一个发射点。
附图说明
通过以下参照附图对本公开实施例的描述，本公开的上述以及其他目的、特征和优点将更为清楚，在附图中：
图1示意性示出了现有技术中的钨(310)单晶针尖的电子源的典型的场发射行为；
图2示意性示出了根据本公开实施例的电子源的示意图；
图3示意性示出了根据本公开实施例的电子源的制备和使用环境；
图4示意性示出了根据本公开实施例的电子源的制备过程示意图；
图5A示意性示出了根据本公开实施例的衬底上的高场强结构的示意图；
图5B示意性示出了根据本公开实施例的高场强结构表面的金属原子示意图；
图5C示意性示出了根据本公开实施例的衬底上的活性区域的示意图；
图5D示意性示出了根据本公开实施例的高场强结构上的活性区域的示意图；
图6示意性示出了根据本公开实施例的具有发射点的发射面积的示意图；
图7示意性示出了根据本公开实施例的发射点形成过程示意图；以及
图8示意性示出了根据本公开另一实施例的发射点形成过程示意图。
具体实施方式
以下，将参照附图来描述本公开的实施例。但是应该理解，这些描述只是示例性的，而并非要限制本公开的范围。在下面的详细描述中，为便于解释，阐述了许多具体的细节以提供对本公开实施例的全面理解。然而，明显地，一个或多个实施例在没有这些具体细节的情况下也可以被实施。此外，在以下说明中，省略了对公知结构和技术的描述，以避免不必要地混淆本公开的概念。
在此使用的术语仅仅是为了描述具体实施例，而并非意在限制本公开。在此使用的术语“包括”、“包含”等表明了所述特征、步骤、操作和/或部件的存在，但是并不排除存在或添加一个或多个其他特征、步骤、操作或部件。
在此使用的所有术语(包括技术和科学术语)具有本领域技术人员通常所理解的含义， 除非另外定义。应注意，这里使用的术语应解释为具有与本说明书的上下文相一致的含义，而不应以理想化或过于刻板的方式来解释。
在使用类似于“A、B和C等中至少一个”这样的表述的情况下，一般来说应该按照本领域技术人员通常理解该表述的含义来予以解释(例如，“具有A、B和C中至少一个的结构”应包括但不限于单独具有A、单独具有B、单独具有C、具有A和B、具有A和C、具有B和C、和/或具有A、B、C的结构等)。在使用类似于“A、B或C等中至少一个”这样的表述的情况下，一般来说应该按照本领域技术人员通常理解该表述的含义来予以解释(例如，“具有A、B或C中至少一个的结构”应包括但不限于单独具有A、单独具有B、单独具有C、具有A和B、具有A和C、具有B和C、和/或具有A、B、C的结构等)。本领域技术人员还应理解，实质上任意表示两个或更多可选项目的转折连词和/或短语，无论是在说明书、权利要求书还是附图中，都应被理解为给出了包括这些项目之一、这些项目任一方、或两个项目的可能性。例如，短语“A或B”应当被理解为包括“A”或“B”、或“A和B”的可能性。
以下首先对现有技术中的场发射行为进行说明以便更好地理解本公开的技术方案。
图1示意性示出了现有技术中的钨(310)单晶针尖的电子源的典型的场发射行为。
如图1所示，现有的电子源，如钨(310)单晶针尖的电子源在使用过程中会经历如下三个阶段，首先，是较清洁的电子源，随着气体的吸附，进入稳定期(Stability)，然而，随着气体的进一步吸附，电流噪声逐渐出现，进入不稳定期(Instability)，电子源稳定性变差，需Flash处理(短时间加热至约2000℃)，以重新回到稳定状态。如不及时处理，表面会逐渐出现污染物，发射电流开始剧烈波动，最终导致烧毁。
关于上述烧毁，发明人进一步的研究表明其与离子轰击密切相关。这是由于电子发射后会电离周围空间的气体分子，进而轰击到针尖。一个可能就是针尖表面被轰击形成多个突起，多个突起分别作为发射点，最后导致过多电流，造成烧毁。还有一种机制，就是吸附到针尖表面的气体分子或其和其它物质的结合物，在电场作用下不断移动，最终在一个表面的缺陷点(如由离子轰击产生)汇集成一个纳米级突起作为发射点，发射点的迅速长大导致了过流，最终导致了针尖的烧毁。
进一步地，前述问题在较大发射电流下变得更为严重。一般可长时间稳定工作的总发射电流为～10微安，且利用率很低。鉴于前述弊端，在高亮度电子源领域占据主导地位的是肖特基式场发射电子源(Schottky thermal-field emission source)。
本质上讲，对CFE而言，任何材料都不能避免气体吸附和离子轰击的影响。但 是，如工作在大电流下(＞10微安)，电子激励脱气(特别在电子轰击引出极产生)，就会进一步劣化真空度，使得针尖发射稳定性很差，波动幅度极大，更加无法长期稳定工作。因而，如何提供出稳定的、较大的场发射电流，一直是冷场发射电子源发展历程中最主要的挑战。
为了避免上述气体吸附和离子轰击的影响，当前场发射电子源(一般指金属针尖)仅能在超高真空中工作(＜10 -8Pa)，这严重制约了CFE的适用范围，发明人针对于此又进行了进一步深入研究，发现以下特点，真空中残留气体成分有H 2，CO，CO 2，而主要成分则是H 2。H 2的吸附会导致清洁表面的发射能力逐渐变差。可以说在该真空范围，H 2的影响从根本上决定了针尖的场发射性能。因此，如何应对H 2的影响成为实现高稳定度针尖的关键。现有技术中也存在一些技术方案可以缓解气体吸附的问题，例如，通过进一步提高腔体真空度至1×10 -9Pa量级，Keigo Kasuya等发明了一种技术(mild flashings at 700℃)，使得W(310)面一直处于一个较清洁的发射状态，延长了其使用时间并获得了高发射能力。该专利的技术方案目前已广泛应用于Hitachi的电子显微镜产品中。
此外，还存在一些技术方案直接利用一些针尖表面游离颗粒物(atomic clusters)作为发射点，也是尝试的一种解决方式。这些游离物，可以是通过在较差真空度下长时间放置形成的污染物，电场作用使得这些游离物质可以移动到针尖某处。这种发射点发射角很小(～5°)，引出电压极低，亮度可以达到传统W(310)的10倍以上。尽管不能够形成较大的发射电流(一般可以稳定地给出～10nA)，但是展现出了极好的稳定性(＜1×10 -7Pa)。一个可能推论就是，极小的束流角和发射面积可以有效降低离子轰击的影响。然而，如前所述，这种游离物质是不固定的，发明人发现，在电流较大时(＞1uA)，这种电子源容易烧毁，且在工作工程中，还会有这种物质不断出现，逐渐改变其发射状态，很难长时间维持。另外一个问题就是在暴露大气时，由于这种物质的尺寸和气体分子可以比拟，极易受到气体的干扰。
基于以上种种分析、推理及实验，发明人提供了本公开的电子源以实现可长期稳定的工作，能提供较大的场发射电流，能在较差的真空环境下工作，且在暴露在大气中时也不易受到气体的干扰。
本公开的实施例提供了一种电子源。该电子源可以包括一个或多个针尖，其中，至少一个针尖包括一个或多个固定的发射点，所述发射点包括针尖表面的金属原子与气体分子形成的反应产物，由于针尖表面的金属原子固定在针尖表面，其与气体分子形成的反应产 物为根植于针尖表面的反应产物，且由于该反应产物是金属原子与气体分子在类似于工作条件下反应形成的反应产物，该反应产物再次与气体分子反应的活性不大，因此稳定性较高，此外，由于可以控制反应产物的数量，因此，可以通过增加反应产物的数量来实现提供较大的场发射电流。另外，由于反应产物是金属原子与气体分子反应形成的反应产物，即使暴露在大气中也不易受气体的干扰。
图2示意性示出了根据本公开实施例的电子源的示意图。
如图2所示，该电子源可以包括一个或多个针尖，为了便于进行说明，以下均以一个针尖为例进行说明。
针尖可以包括一个或多个固定在针尖表面的发射点，该发射点可以为位于针尖表面的金属原子与气体分子形成的反应产物。
在一个实施例中，通过施加电场而使得针尖表面的金属原子与气体分子形成的反应产物，具体可以采用多种实现方式，例如，直接给针尖施加电压在针尖表面形成较高的场强以促使针尖表面的金属原子和气体分子发生反应形成的反应产物；也可以是给针尖附近的场强产生结构(如电极等)施加电压形成电场，进而在针尖表面形成较高的场强以促使针尖表面的金属原子和气体分子发生反应形成反应产物。总之，在针尖表面形成的场，及该场的形成方式不做限定，只要能在针尖表面形成能促使针尖表面金属原子与周围气体分子反应形成反应产物的场(如电场)即可。
在通过给针尖施加电压以形成场的实施例中，示意性的例子中，所述电场为通过给所述针尖施加正偏压、负偏压或者正偏压和负偏压的结合而产生的电场，其中，对于施加正偏压时，电场的场强包括1～50V/nm，对于施加负偏压时，电场的场强包括1～30V/nm。
所述发射点可以形成在针尖的指定位置，例如，针轴线与针尖表面相交处的一定范围内，也可以是形成在特定的结构上，如突起等具有场强优势，以优先形成金属原子与气体分子的反应产物的结构，还可以是形成在特定的具有反应活性的区域，如具有更容易与气体分子发生反应的特定的金属原子区域，当然，也可以是上述两种情形的组合使用，在此不做限定。
其中，所述金属原子可以是针尖本体表面的金属原子，即金属原子的种类和针尖本体的种类相同，还可以是通过蒸镀、电镀等方式在针尖表面形成的不同种类的金属原子。优选地，该金属原子为熔点高于1000K的金属材料，稳定性更好，且便于利用如上所述的热处理等方法对针尖进行清洁。例如，该熔点高于1000K的金属材料可以包括以下任意一种或多种：钨、铱、钽、钼、铌、铪、铬、钒、锆、钛、铼、钯、铂、铑、锇、钌、金或者 六硼化金属等，例如，其中某一种金属原子单独作为针尖表面的金属原子，或者其中几种金属原子形成的叠层，如钛层\铂层\钨层形成的叠层等，又或者其中几种金属原子混合形成的非单质的金属层，在此不做限定。
所述气体分子可以是由气体引入装置引入，如通过气体流量阀等引入的特定的气体分子，也可以是在对腔室抽真空时残留的气体分子等，当然也可以是上述两种方式的组合，在此不做限定。其中，所述气体分子包括含氢元素气体分子，以及以下任意一种或多种气体分子：含氮元素气体分子、含碳元素气体分子或者含氧元素气体分子。上述气体分子可以是引入的气体分子，因此，气体引入量可以动态调整，一般在引入时，真空度＜10 -4Pa。当直接利用真空腔室中的残余气体时，真空腔室中主要残留气体为氢气。优选地，所述含氢元素气体分子包括氢气。
需要说明的是，发射点消失的温度，如分解的温度可以低于针尖本体消失的温度以便去除发射点，发射点的消失温度可以高于工作温度和使得吸附的气体分子脱吸附的温度，这样便于通过简单的热处理进行脱吸附(如Flash等热处理)使得电子源恢复稳定状态。
本公开提供的电子源由于其结构特点，发射点的引出电压低、发射点固定不游离，具有较长的寿命，还可以在较差的真空度下工作。由于发射点形成和工作温度较低，在工作时电子源结构不改变，施加电压值稳定不发生变化，例如，可以在如下所示的工作条件下工作，例如，针尖温度≤1000K时，工作压强≤10 -3Pa，或者，500K≤针尖温度≤800K时，工作压强≤10 -6Pa，或者，当针尖温度≤150K时，工作压强≤10 -6Pa。所述发射点的尺寸为纳米级或亚纳米级，通过调节工作电压，针尖发射点发射电流值可达10mA量级。该电子源具有冷场发射特点，可以通过调节引出电压调节发射电流大小。在一个优选的实施例中，所述金属材料为钨，相应地，所述发射点为氢钨化合物。
图3示意性示出了根据本公开实施例的电子源的制备和使用环境。
如图3所示，制备和使用装置可以包括真空腔111、制冷头107(该制冷头107包括加热装置，加热装置未示出)、样品台(sample holder)105、电子源101、高压电源115、粒子束113、荧光屏组件103和气体引入装置109，其中，右图是左图中虚线框中的放大示意图，样品台105可以包括发叉120，该发叉120用于固定电子源101，上方的电源是加热针尖的电源。这样就可以实现给电子源101加热和/或施压，还可以控制真空腔室中的气体，通过荧光屏组件103可以观察电子源发射电子的情况。
图4示意性示出了根据本公开实施例的电子源的制备过程示意图。
如图4所示，右图是对左图中虚线框中图像的放大示意图，左图下方的虚线框为示意 性的。从左图可以看到，在给电子源施加偏压后，由于针尖处的场强最强，环境中的气体分子(左图中的灰色小点)会逐渐向针尖顶端移动，这也是造成现有技术中的钨单晶的电子源发射能力下降的原因之一。从右图可以看出，随着气体分子移动到电子源的针尖处，通过在针尖处形成场(如电场等)以使气体分子与针尖表面的金属原子(白色小点)形成反应产物(黑色圆点，即发射点，简称Ma1)，该黑色圆点会根植于针尖表面，而非游离在针尖表面。
图5A示意性示出了根据本公开实施例的衬底上的高场强结构的示意图。
如图5A所示，至少一个针尖包括衬底和所述衬底上的一个或多个比所述衬底其它部位的场强高的高场强结构，其中，至少一个所述高场强结构的外表面包括金属原子。突起的数量可以为1个、3个、5个、10个等，在此不做限定。
需要说明的是，所述高场强结构材料与衬底材料可以相同或者不同。此外，所述衬底和/或高场强结构表面的金属原子材料与高场强结构材料相同或者不同，当不同时，所述衬底和/或高场强结构表面的金属原子可以是通过蒸镀或者电镀等方式形成，在此不做限定。另外，所述衬底和/或高场强结构表面的金属原子材料与衬底材料可以相同或者不同，当不同时，所述衬底和/或高场强结构表面的金属原子可以是通过蒸镀或者电镀等方式形成，在此不做限定。此外，所述衬底的材料、所述高场强结构的材料与所述针尖的本体的材料相同或不同，在此不做限定。相应地，在真空条件下与气体分子的反应中，所述突起的至少部分表面的金属原子比所述衬底的其他表面部分具有相同或更大的反应活性。
如图5A所示，所述高场强结构可以包括突起，对应Lo1位置。其中，所述突起的尺寸可以为亚纳米至100纳米量级。由于突起处具有高场强优势，当给针尖施加电压时，在场强的作用下，突起表面的至少部分金属原子会与气体分子形成反应产物。这样就可以简单快捷地在针尖表面的指定位置处形成发射点。此外，通过调节突起的尺寸、施加的偏压大小及时长等参数可以调节突起表面的发射点的数量可控，例如，可以通过增加施加的偏压的时长或者增大突起的尺寸来形成更多的发射点以增大发射电流等。
所述突起通过以下任意一种或多种方法形成：热处理、施加电场、热-电场处理、刻蚀或者纳米加工等，或者如下方法：例如在单晶金属针尖上面镀一层金属原子，通过热处理重塑形成突起；需要说明的是，只要能在针尖表面形成突起的方法都适用，在此不做限定。
可选地，所述衬底材料是导电材料并且熔点高于1000K，所述高场强结构材料是导电材料并且熔点高于1000K，所述衬底和/或高场强结构表面的金属原子的材料为熔点高于1000K的金属材料，以及
所述金属原子与气体分子的反应产物包括在真空条件下熔点高于1000K的金属原子与气体分子的反应产物。
熔点高于1000K的材料的示例如上所述，在此不再赘述。针尖优选地采用熔点高于1000K的导电材料，稳定性更好，且便于利用如上所述的热处理等方法对针尖进行清洁或恢复。图5B示意性示出了根据本公开实施例的高场强结构表面的金属原子示意图。
需要强调的是，所述衬底、所述高场强结构和所述针尖的本体的材料都可以是金属材料，或不是金属材料(如导电材料即可)，当电子源不包括高场强结构时，只要保证所述衬底的表面包括金属原子，且所述衬底可以引入电流即可；当电子源包括高场强结构时，只要保证所述高场强结构的表面包括金属原子，且所述高场强结构可以引入电流即可。
在一个优选的实施例中，高场强结构位于所述衬底的表面中心位置，或者，高场强结构位于尺寸大于设定阈值的衬底上，如尺寸较大的衬底上，或者，所述金属原子位于所述高场强结构顶端的表面中心位置。
此外，可以通过调节所述针尖的衬底和/或高场强结构的尺寸和形状以调节电子束束流角的大小，还可以通过调节高场强结构的尺寸调节发射点的数量，也可以通过调节衬底的结构和/或高场强结构的结构调节电子源发射电流的电压的大小和一致性，并且可以通过调节针尖顶部的形状以调节发射电流方向。例如，增大衬底尺寸，则束流角减小；场蒸发使突起高度降低，则束流角减小；突起的尺寸越小，相同的电场下则发射点数量越少等。
本公开提供的电子源，至少一个针尖包括衬底和所述衬底上的一个或多个比所述衬底其它部位的场强高的高场强结构，其中，至少一个所述高场强结构的外表面包括金属原子，该高场强结构表面的金属原子凭借场强优势在相同的环境中更容易与气体分子形成反应产物，以优先在高场强结构生成发射点。
图5C示意性示出了根据本公开实施例的衬底上的活性区域的示意图。
如图5C所示，至少一个针尖包括衬底和所述衬底上的一个或多个比所述衬底其他部位反应活性大的活性区域，对应Lo2位置，其中，至少一个所述活性区域外表面包括金属原子。活性区域如图5C的阴影区域所示。活性区域的个数可以为1个、3个、5个、10个等，在此不做限定，发射点可以优先形成在位于针尖顶部区域的活性区域中。
需要说明的是，衬底的材料和所述针尖的本体的材料可以相同或不同。所述活性区域 的金属原子可以是通过蒸镀或者电镀等方式形成，例如，通过电镀在针尖的轴线与表面相交处形成一定面积的金属原子层，该金属原子层的材料相较于衬底的其它表面的材料与气体分子具有更高的反应活性。相应地，在真空条件下与气体分子的反应中，所述衬底的活性区域的表面的金属原子比所述衬底的其他表面部分具有更大的反应活性。
在一个优选的实施例中，反应活性大的区域位于所述衬底的表面中心位置，或者，所述金属原子位于所述衬底的表面中心位置。
本公开提供的电子源，至少一个针尖包括衬底和所述衬底上的一个或多个比所述衬底其他部位反应活性大的活性区域，其中，至少一个所述活性区域外表面包括金属原子，该活性区域表面的金属原子凭借活性优势在相同的环境中更容易与气体分子形成反应产物，以优先在活性区域生成发射点。
图5D示意性示出了根据本公开实施例的高场强结构上的活性区域的示意图。
如图5D所示，至少一个针尖包括衬底和所述衬底上的一个或多个比所述衬底其它部位的场强高的高场强结构，所述高场强结构的至少部分表面是反应活性大的活性区域，对应Lo3位置，其中，所述活性区域外表面包括金属原子。
在本实施例中，对于包括突起的针尖，在真空条件下与气体分子的反应中，所述突起的至少部分表面的金属原子比所述衬底的其他表面部分具有相同或更大的反应活性，这样可以更加精准的控制发射点在突起的指定区域上形成，如在图5D的突起上的阴影区域形成发射点。活性区域表面的金属原子的形成方式可以参考上一实施例中形成活性区域的方式，在此不再详述。
本公开提供的电子源，至少一个针尖包括衬底和所述衬底上的一个或多个比所述衬底其它部位的场强高的高场强结构，所述高场强结构的至少部分表面是反应活性大的活性区域，其中，所述活性区域外表面包括金属原子，该活性区域表面的金属原子凭借场强优势和活性优势在相同的环境中更容易与气体分子形成反应产物，以优先在活性区域生成发射点。
图6示意性示出了根据本公开实施例的具有发射点的发射面积的示意图。
如图6所示，以具有突起的针尖为例进行说明，当整个突起表面都形成发射点时，发射点的俯视图可以参考图6的右图所示。需要说明的是，在突起表面形成发射点的过程中，场强越高的区域首先形成发射点的概率越大，但发射点并不是一定只形成在场强最高的区域，当然，如图6所示的突起的顶点位置由于场强最强，在顶点处首先形成发射点的概率也最大。图6中突起的形状为类半球状，此外，还可以是方形、多边形、台形、半椭圆形 以及其它几何结构等，在此不做限定。
需要说明的是，所述发射点的消失温度低于所述衬底、所述高场强结构和所述金属原子的消失温度的小值，且所述发射点的消失温度高于所述电子源的工作温度；或者，所述发射点的消失温度低于所述衬底、所述高场强结构和所述金属原子的消失温度的小值，且所述发射点的消失温度高于所述电子源的工作温度和吸附于任一针尖上的气体分子的脱吸附温度的大值。当所述发射点的消失温度高于吸附于任一针尖上的气体分子的脱吸附温度时，便于通过加热使得气体分子脱吸附，便于通过简单的处理来提升电子源的稳定性。
对电子源的测试环境可以参考图3所示，真空腔111的本底真空度≤10 -3Pa(一般应优于10 -6Pa)。制冷头cold head 107上有一个绝缘的样品台(sample holder)105，可以实现在sample holder上设置一加热装置(如加热片、加热棒等)，温度可以在10～500K之间调整。将预处理好的针尖(如钨单晶针尖，该钨单晶针尖可以是具有突起的针尖，突起的尺寸是nm或亚nm级，也可以是具有较大反应活性的区域的针尖)放置在样品台105上，对该针尖加电压，电压可以是正高压V P，也可以是负高压V N。电源115可以为双路输出高压电源，输出范围±0～30kv。气体引入装置109用于引入反应气体分子，如H 2，也可以是其它反应气体，例如含H元素气体，水，CH 4等，气体通入量可以动态调整，一般引入时真空度＜10 -4Pa(需要说明的是，也可以直接利用腔室内残余气体分子，其主要成分为氢气)。荧光屏组件103用于将粒子束图像转换成光图像，当信号很小时，可以采用荧光屏-多通道板组件将信号进行放大。当对针尖加电压时，可以引出粒子束，该施加的电压可以是正压、也可以是负压；当正压时，有成像气体时，输出正离子束；当负压时，输出电子束。
以下以一个具体实施例来对电子源的制备过程进行演示。
图7示意性示出了根据本公开实施例的发射点形成过程示意图。
通过在强电场作用下的气体分子和表面金属原子的反应实现形成发射点Ma1，Ma1的形成在一定温度下进行。发射点的形成过程是基于对针尖表面的一个小区域内氢气吸附行为的深入研究后确定的制备方法。
首先，可以提供一个钨单晶(111)的针尖，经过如上所述的方法在针尖上形成突起，如经过Flash处理(加热至1200K，持续3s，期间还可以辅以偏压等)，这样可以在针尖的表面中间位置形成一个纳米级的突起，其表面是清洁的，参考图7a所示，当对针尖加负压至-2KV时，形成了场发射模式(field electron emission mode)。为避免离子轰击的造成的影响，Ma1形成温度在～50K。在发射点形成的全程，发射电流(I E)一直控制在5nA 以内，真空度在10 -7Pa。
如图7b至图7e所示，气体吸附首先导致发射能力下降，荧光屏组件103的显示界面上的发射图案逐渐变暗，发射电流逐渐降低，即现有的钨清洁表面的发射能力逐渐下降。
如图7f所示，随着时间推移，发射图案近乎完全消失，此时，传统的钨清洁表面发射能力几乎全部消失。
如图7g所示，随着继续给电子源施加偏压，本公开所述的发射点开始形成，此时的发射点和之前的发射物质的构成不同，以钨为例进行说明，之前的发射物质为钨单晶的钨原子，此时的发射点为针尖表面的钨原子与气体分子，如钨原子与氢气分子的反应产物，该反应产物固定在针尖表面。
如图7h至图7k所示，随着继续给电子源施加偏压或者持续等待，最终该发射点变得更亮。
如图7l所示，最终，形成高发射能力的发射点，发射电流进一步电流增大，通过对比图7a和图7l，可以清楚看到，发射点的发射能力明显提升，且发射点更加集中在中间突起位置。
在上述过程中，由于电流一直控制的很小，且荧光屏与针尖离得很远，且真空度很好，可排除离子轰击的影响。另外，加正高压可以形成相同的场发射特性的物质。而此时完全没有发射电流，说明，离子轰击导致的游离原子级颗粒物没有参与发射点Ma1的形成过程。
在发射点Ma1的形成过程中，气体(如H 2)经过了吸附，在电场下离解，进一步与表面金属原子产生结合，形成某种H-W反应产物(compound)，属于Ma1的一种，这种compound似乎直接与表面结合，并不移动。如其它位置也可以形成类似物质，但是其并不发生移动，一直是位置稳定的发射点。
与发射能力相关的参数中，单个发射点的发射能力可以达到30uA以上，若是形成一个密集的发射区域，发射图连成一片，总电流可达100uA量级。若增大发射面积(发射点数量增多)，可以实现10mA量级的发射电流，远远超过现有的CFE的稳定发射能力(～10微安)。近来Keigo Kasuya等通过提高腔体真空度至4×10 -10Pa可达到极限发射电流约3000微安，但是该真空度很难达到。
在不同的真空度下，具有不同的发射能力。一般来说，高真空时，可以维持发射电流大，低真空时，最大发射电流迅速衰减。
图8示意性示出了根据本公开另一实施例的发射点形成过程示意图。从图中可以看到， 形成的发射区域小且集中。
形成的发射点的针尖(The tip terminated by emission site)具有场发射一致性，即发射电流的电压具有一致性，例如，发射电流为1微安时，电压为-1.2±0.1KV。
由于发射点形成和工作温度较低，在工作时电子源结构不改变，施加电压值不发生变化，电压值稳定使例如电子枪的设计更简单。
另外，通过调节衬底的结构和/或高场强结构调节电子源发射电流的电压的大小和一致性，例如通过场致刻蚀、场蒸发等方法形成的针尖突起上形成发射点，发射电流为1微安时，电压可以低于-0.5KV(例如引出电压为-0.4KV)，从而使电子枪结构设计更简单。
以下进一步对通过上述方法制备的电子源的结构及性能参数等进行说明。
(一)关于发射点的物质相关
该电子源具有良好的稳定性，发射点(参考图4中的实心黑圆所示)能够稳定地根植于针尖表面的特定位置，多个发射点最大可以给出mA级别的总电流，并且可以工作在较差真空度下(10 -5Pa)，且易于保存。
以具有突起的电子源为例进行说明：
针尖tip：针尖包含衬底Base，以及衬底Base上的nm级别的突起protrusion，突起的尺寸是nm或亚nm级。在突起表面形成了发射点。
发射点Ma1的形成过程如下所示，在强电场下(如在1～50V/nm范围内，需要说明的是，电场会受到电压极性以及材料属性影响)，由气体分子直接与针尖表面的金属原子形成结合物(Gas-metal compound)，以下以H-W compound为例进行说明。在较高真空下，主要残留气体分子是H 2。由于突起的表面电场最强，优先在此形成。
Ma1是根植于针尖表面，由气体分子与针尖表面金属原子反应形成的反应产物，这些发射点的位置都是固定的，并不移动，但是在不同发射电流下单个Ma1可能会闪烁。
Ma1具有表面保护作用：由于Ma1是金属最表面原子与气体分子反应形成的，即反应生成物，因此使得其较难继续与空间气体分子发生作用，如吸附等等；即使气体吸附后，它们也很难继续对针尖表面的Ma1产生影响，即刻继续发生化学反应。可通过低温加热(＜1000K)等手段有效脱附Ma1上的吸附气体，且可以保持Ma1存在。即使加热部分去除了Ma1，暴露出了金属原子部分，在工作过程中(有电场E的存在)，该处还可以进而再形成Ma1。因而，Ma1相当对针尖加了一层保护层，有效隔断了气体与针尖在强场下的反应。而其它表面未形成Ma1的地方，由于当地电场很弱，发射能力很弱，忽略对比。 这大大提升了电子源的工作稳定性和稳定周期。
关于工作条件，Ma1具有很强的环境适应性，可以工作在10 -5Pa的真空下。如果辅以低温，其可以工作在10 -3Pa的真空下。当然，真空度越高，持续工作时间越长；而发射电流越小，持续工作时间越长。可以根据具体需求，例如需使用的真空条件，选用不同的发射电流范围。
关于发射能力相较于现有的CFE显著增强的原因探讨，可能存在两种机制的作用：第一种，功函数的显著降低；第二种，表面原子级的发射点尖锥的形成。这二者作用的共同效果使得引出电压V 0降低很多(降低比例＞～30％)。
关于场发射特征：发射电流大小，可以直接通过引出电压控制，可以实现脉冲电流(脉冲电压)，恒定电流(恒压)的输出。
关于发射面积的说明，例如，突起的上表面的发射区域面积，可以通过，例如可以通过调节针尖头部的突起直径来控制和改变发射面积。发射面积可以达到原子级别，也可以达到10nm级别。对于具有活性区域的例子，可以通过改变活性区域的面积来控制发射面积的大小。
(二)关于发射点形成位置相关
Ma1形成的位置(Lo)可以通过预先选定，例如，通过如下方法，在针尖表面形成一个电场优势区域Lo1(参考图5A，例如形成一个nm级突起)，使得该处电场高于针尖的其它区域；或者在针尖表面形成一个活性区域Lo2，该区域与气体反应的具有更大活性(参考图5C，相对于其它区域)；或者某区域Lo3(参考图5D)具备上述两种特征。其中突起可以通过热处理、热-电场处理、刻蚀、纳米加工等手段形成；活性原子区域可以通过各种原子蒸镀等方式形成。
发射面积的大小也可以通过上述手段控制，其尺度为nm或者原子级别。优选地，Lo1位于针尖一个较大衬底之上。
电子束束流角的大小可以通过控制针尖衬底、突起的尺寸来实现。
优选地，该针尖具有一个中心具备突起的几何结构。优选的，针尖的周围区域的电场强度弱于针尖的表面中心的电场强度。
针尖最表面至少有一层或数层是金属原子(metal atom)，其种类可以是各种熔点高于1000K的金属材料。此外，针尖的本体材料可以是金属，也可以是其它导电材料，如金属六硼化物(metal-hexaboride)等，优选地，具有较高熔点(如＞1000K)。
(三)关于制备条件相关
针尖条件：以具有突起的针尖为例，在针尖表面需形成上述物质的区域先形成一个纳米级突起，以便使得在对针尖加偏压(V S)时，在突起表面产生一个较强的表面电场E S，E S大于其周边处电场。
电场条件：对针尖加偏压V S时，针尖上突起的表面电场为E S，为～1V/nm量级。E S(或V S)可为正，其强度需保证针尖表面原子不产生蒸发；此外，E S(或V S)也可为负，需保证发射不因过流而导致针尖烧毁。
气体分子：主要成分是H 2，或者含H元素的气体分子。可以通过外界气体分子引入实现，也可以直接利用真空里面的残余气体分子。
反应过程：气体分子(或离子)吸附到针尖表面，在强场下，逐渐与针尖表面的原子发生反应，最终形成稳定的发射点。
制备过程的温度范围：温度应小于发射点Ma1的分解温度。根据观察，应小于1000K。优选地，在一个优选实施例中，反应温度在低温150K以下，在另一优选实施例中，工作温度在500～800K。一般地，该温度还应保证不改变针尖的表面形貌，如低于针尖顶部的变形温度。
形成发射点的去除方式：若形成的发射点发射能力老化(degrade)，可去除该老化的发射点。例如，可以通过场蒸发，即加一正高压，去除表面的发射点。也可以直接通过加热至大于1000K，该发射点Ma1直接去除。去除后可以原位再次形成Ma1。
本公开提供的电子源，由于该电子源的发射点是固定在针尖表面的金属原子与气体分子形成的反应产物，而非游离在针尖表面的气体分子或游离颗粒等，不会因游离状的发射点聚集在一起形成新的发射点而导致过电流烧毁，有效提升了稳定性，此外，该发射点包括反应产物，相对于以金属原子或其它金属化合物(如金属硼化物等)而言，在工作环境中(存在气体分子)具有更好的稳定性，如更加不容易与工作环境中的氢气等发生反应，进一步提升了电子源的稳定性。另外，电子源的发射点可以为单个金属原子与气体分子形成的反应产物，即发射点仅有一个或数个原子组成，能形成具有低功函数的原子级电子源。此外，由于该发射点是气体分子与针尖表面的金属原子形成的反应产物，根植于针尖表面，相对不易受到离子轰击的影响，也不易受环境中的气体影响。进一步地，可以通过增加发射点的数量来提升场发射电流。综上，本公开提供的电子源具有稳定、较大的场发射电流、可以在较差真空度下工作以及易于保存等优点。
本公开的另一个方面提供了一种电子枪，包括如上所述的电子源、冷却装置、加热装置和气体引入装置。其中，所述电子源用于发射电子，所述冷却装置用于给所述电子源散 热，所述电子源通过电绝缘热导体固定在所述冷却装置上，所述加热装置用于给所述电子源加热调节温度，所述气体引入装置用于引入包含氢元素的气体。由于该电子枪的电子源表面的发射点的形成温度和工作温度较低，在工作时电子源结构不改变，施加电压值不发生变化，电压值也更加稳定，使电子枪的设计更简单。
本领域技术人员可以理解，本公开的各个实施例和/或权利要求中记载的特征可以进行多种组合或/或结合，即使这样的组合或结合没有明确记载于本公开中。特别地，在不脱离本公开精神和教导的情况下，本公开的各个实施例和/或权利要求中记载的特征可以进行多种组合和/或结合。所有这些组合和/或结合均落入本公开的范围。
以上对本公开的实施例进行了描述。但是，这些实施例仅仅是为了说明的目的，而并非为了限制本公开的范围。尽管在以上分别描述了各实施例，但是这并不意味着各个实施例中的措施不能有利地结合使用。本公开的范围由所附权利要求及其等同物限定。不脱离本公开的范围，本领域技术人员可以做出多种替代和修改，这些替代和修改都应落在本公开的范围之内。

Claims (21)

  1. 一种电子源，包括：
    一个或多个针尖，其中，至少一个针尖包括一个或多个固定的发射点，所述发射点包括针尖表面的金属原子与气体分子形成的反应产物。
  2. 根据权利要求1所述的电子源，其中，所述发射点包括金属原子与气体分子在电场下形成的反应产物。
  3. 根据权利要求1所述的电子源，其中：
    至少一个针尖包括衬底和所述衬底上的一个或多个比所述衬底其它部位的场强高的高场强结构，其中，至少一个所述高场强结构的外表面包括金属原子，并且/或者
    至少一个针尖包括衬底和所述衬底上的一个或多个比所述衬底其他部位反应活性大的活性区域，其中，至少一个所述活性区域外表面包括金属原子，并且/或者
    至少一个针尖包括衬底和所述衬底上的一个或多个比所述衬底其它部位的场强高的高场强结构，所述高场强结构的至少部分表面是反应活性大的活性区域，其中，所述活性区域外表面包括金属原子。
  4. 根据权利要求2所述的电子源，其中，所述电场为通过施加正偏压、负偏压或者正偏压和负偏压的结合而产生的电场；
    对于施加正偏压时，电场的场强包括1～50V/nm；
    对于施加负偏压时，电场的场强包括1～30V/nm。
  5. 根据权利要求3所述的电子源，其中，所述高场强结构包括突起。
  6. 根据权利要求5所述的电子源，其中，所述突起的尺寸为亚纳米至100纳米量级。
  7. 根据权利要求5所述的电子源，其中，所述突起通过以下任意一种或多种方法形成：热处理、施加电场、热-电场处理、刻蚀或者纳米加工。
  8. 根据权利要求5所述的电子源，其中：
    对于包括突起的针尖，在真空条件下与气体分子的反应中，所述突起的至少部分表面的金属原子比所述衬底的其他表面部分具有相同或更大的反应活性；以及
    对于不包括突起的针尖，在真空条件下与气体分子的反应中，所述衬底的活性区域的表面的金属原子比所述衬底的其他表面部分具有更大的反应活性。
  9. 根据权利要求3所述的电子源，其中：
    所述衬底材料是导电材料；并且/或者
    所述高场强结构材料是导电材料；并且/或者
    衬底和/或高场强结构表面是金属原子；并且/或者
    所述高场强结构材料与衬底材料相同或者不同；并且/或者
    所述衬底和/或高场强结构表面的金属原子材料与高场强结构材料相同或者不同，当不同时，所述衬底和/或高场强结构表面的金属原子通过蒸镀或者电镀形成；并且/或者
    所述衬底和/或高场强结构表面的金属原子材料与衬底材料相同或者不同，当不同时，所述衬底和/或高场强结构表面的金属原子通过蒸镀或者电镀形成。
  10. 根据权利要求2所述的电子源，其中：
    所述气体分子为引入的气体分子和/或真空环境中残留的气体分子，
    其中，所述气体分子包括含氢元素气体分子，以及以下任意一种或多种气体分子：含氮元素气体分子、含碳元素气体分子或者含氧元素气体分子。
  11. 根据权利要求10所述的电子源，其中，所述含氢元素气体分子包括氢气。
  12. 根据权利要求3所述的电子源，其中：
    所述发射点的消失温度低于所述衬底、所述高场强结构和所述金属原子的消失温度的小值，且所述发射点的消失温度高于所述电子源的工作温度；或者
    所述发射点的消失温度低于所述衬底、所述高场强结构和所述金属原子的消失温度的小值，且所述发射点的消失温度高于所述电子源的工作温度和吸附于任一针尖上的气体分子的脱吸附温度的大值。
  13. 根据权利要求9所述的电子源，其中：
    所述衬底材料是导电材料并且熔点高于1000K；并且/或者
    所述高场强结构材料是导电材料并且熔点高于1000K；并且/或者
    所述衬底和/或高场强结构表面的金属原子的材料为熔点高于1000K的金属材料；以及
    所述金属原子与气体分子的反应产物包括在真空条件下熔点高于1000K的金属原子与气体分子的反应产物。
  14. 根据权利要求13所述的电子源，其中，所述金属材料包括以下任意一种或多种：
    钨、钽、铌、钼、铼、铪、铱、锇、铑、钌、铂、钯、金、铬、钒、锆、钛、或者六硼化金属。
  15. 根据权利要求14所述的电子源，其中，所述金属材料为钨；以及
    所述发射点包括氢钨化合物。
  16. 根据权利要求3所述的电子源，其中，
    通过调节所述针尖的衬底和/或高场强结构的尺寸和形状以调节电子束束流角的大小；并且/或者
    通过调节高场强结构和/或活性区域的尺寸调节发射点的数量；并且/或者
    通过调节衬底的结构和/或高场强结构的结构调节电子源发射电流的电压的大小或一致性；并且/或者
    通过调节针尖顶部的形状以调节发射电流方向。
  17. 根据权利要求16所述的电子源，其中：
    高场强结构或者反应活性大的区域位于所述衬底的表面中心位置；并且/或者
    高场强结构位于尺寸大于设定阈值的衬底上；并且/或者
    所述金属原子位于所述高场强结构顶端或者所述衬底的表面中心位置。
  18. 根据权利要求2所述的电子源，其中，所述针尖的工作条件包括：
    针尖温度≤1000K时，工作压强≤10 -3Pa；或者
    500K≤针尖温度≤800K时，工作压强≤10 -6Pa；或者
    当针尖温度≤150K时，工作压强≤10 -6Pa。
  19. 根据权利要求2所述的电子源，其中：
    所述发射点的尺寸为纳米级或亚纳米级；以及
    通过调节工作电压，针尖发射点发射电流值可达10mA量级。
  20. 根据权利要求1所述的电子源，其中，所述电子源具有冷场发射特点，通过调节引出电压调节发射电流大小。
  21. 一种电子枪，包括：
    如权利要求1至20任一项所述的电子源，用于发射电子；
    冷却装置，用于给所述电子源散热，所述电子源通过电绝缘热导体固定在所述冷却装置上；以及
    加热装置，用于给所述电子源加热调节温度；
    气体引入装置，用于引入包含氢元素的气体。
PCT/CN2018/123951 2018-10-12 2018-12-26 电子源和电子枪 WO2020073506A1 (zh)

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