Field emission electron source, electron optical device, and manufacturing method

Abstract

To realize a highly bright and practical field emission electron source.SOLUTION: A field emission electron source for use in an electron optical device includes a needle-shaped shunt whose tip end portion narrows toward the tip, a substantially spherical convex portion formed at the tip of the shunt, a coating which covers the shunt and the substantially spherical convex portion, and an opening which exposes a portion of the substantially spherical convex portion, and the shunt and the substantially spherical convex portion are formed from a first metal, the coating is formed from a second metal, and the second metal has a work function greater than that of the first metal.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a field emission electron source, an electron optical device, and a manufacturing method. [Background technology]

[0002] 2. Description of the Related Art Field emission electron sources are used in electron optical devices such as transmission electron microscopes and scanning electron microscopes.

[0003] The field emission electron source applies a voltage between a sharpened needle electrode (hereafter referred to as the tip) and the opposing electrode (extraction electrode), and the electric field at the tip is set to 5×10 9 It is an electron source that emits electrons by increasing the input current to about [V / m].

[0004] Field emission electron sources are characterized by high brightness (amount of current emitted from a unit area into a unit solid angle) and a narrow energy spread of emitted electrons, and are widely used in applications such as high-resolution observation and electron beam interference measurement. W (tungsten) single crystals with (310) or (111) orientations are usually used as materials for field emission electron sources.

[0005] Currently, there is a demand for further improvement in the brightness of field emission electron sources in electron microscopes for the purposes of high-resolution observation, short-time exposure, improved observation throughput, electron beam interference measurement, etc. A known method for improving the brightness of field emission electron sources is to reduce the field emission source diameter (precisely the virtual source diameter, since it is an imaginary light source) to the atomic order to increase the brightness.

[0006] Regarding technology for reducing the light source diameter, for example, Patent Document 1 describes "an electron beam source for an electron optical device, characterized in that it comprises a microstructure in which a countable number of atoms of a second metal different from the first metal are arranged in a manner that allows them to be repaired by heating, covering the tip surface of a first metal substrate having a needle-like shape with a sharpened tip." [Prior art documents] [Patent documents]

[0007] [Patent Document 1] JP 2006-134638 A Summary of the Invention [Problem to be solved by the invention]

[0008] Conventional field emission electron sources such as the electron beam source for an electron optical device described in Patent Document 1 have the following problems and are therefore difficult to say are practical.

[0009] It is difficult to reproducibly create a light source diameter on the atomic order. It is difficult to obtain a stable emission current due to collisions of ions caused by ionization of the gas around the tip. When trying to obtain a probe current of several nA to several tens of nA required for practical equipment, the current density in the emission region becomes very high, and orbital displacement due to Coulomb interaction increases the source diameter, reducing brightness and increasing the energy spread of the emitted electrons.

[0010] The present invention has been made in view of the above-mentioned points, and an object of the present invention is to realize a field emission electron source that has high brightness and is practical. [Means for solving the problem]

[0011] The present application includes a number of means for solving at least some of the above problems, examples of which are as follows.

[0012] In order to solve the above problems, a field emission electron source according to one embodiment of the present invention is a field emission electron source for use in an electron optical device, wherein a tip end portion of the field emission electron source has a needle-shaped shunt whose diameter tapers toward the tip, a substantially spherical convex portion formed at the tip of the shunt, a coating covering the shunt and the substantially spherical convex portion, and an opening exposing a portion of the substantially spherical convex portion, wherein the shunt and the substantially spherical convex portion are formed from a first metal, the coating is formed from a second metal, and the second metal has a work function greater than that of the first metal. Effect of the Invention

[0013] According to the present invention, a highly bright and practical field emission electron source can be realized.

[0014] Problems, configurations, and effects other than those described above will become apparent from the following description of the embodiments. [Brief description of the drawings]

[0015] [Figure 1] FIG. 1 is a diagram showing an example of the configuration of the tip end portion of a field emission electron source according to an embodiment of the present invention. [Diagram 2] FIG. 2 shows an example of a magnified scanning electron microscope image of the tip end portion. [Diagram 3] FIG. 3 is a diagram for explaining the step of providing the openings. [Figure 4] FIG. 4(A) is a schematic diagram showing an example of an electric field radiation pattern when no coating or opening is provided, and FIG. 4(B) is a schematic diagram showing an example of an electric field radiation pattern corresponding to the tip end portion according to this embodiment. [Diagram 5] FIG. 5 is a diagram showing a simulation result of the total emission current with respect to the probe current. [Figure 6] FIG. 6 is a diagram showing a simulation result of the energy width with respect to the probe current. [Figure 7]FIG. 7 shows the simulation results of the ratio of the virtual source diameter when the Coulomb interaction with respect to the probe current is taken into account and when it is not taken into account. [Figure 8] FIG. 8 shows the simulation results of the off-peak degree versus the probe current. [Figure 9] FIG. 9 is a diagram showing an example of the configuration of the tip end of a conventional field emission electron source. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. The embodiment is an example for explaining the present invention, and appropriate omissions and simplifications are made in order to clarify the explanation. The present invention can be implemented in various other forms. Unless otherwise limited, each component may be singular or plural. The position, size, shape, range, etc. of each component shown in the drawings may not represent the actual position, size, shape, range, etc. in order to facilitate understanding of the invention. In all drawings for explaining the embodiment, the same members are generally given the same reference numerals, and repeated explanations are omitted. In addition, in the following embodiments, the components (including element steps, etc.) are not necessarily essential, except when specifically stated or when it is clearly considered essential in principle. In addition, when it is said that "consists of A", "consists of A", "has A", or "includes A", other elements are not excluded, except when it is specifically stated that only that element is included. Similarly, in the following embodiments, when referring to the shape, positional relationship, etc. of a component, etc., it includes those that are substantially similar or similar to that shape, etc., except when specifically stated or when it is clearly considered not to be the case in principle. In addition, the term "obtain" includes, as specific examples, at least the subject generating, calculating, and receiving from the outside.

[0017] <Example of the configuration of the tip end portion 100 of a conventional field emission electron source> A field emission electron source according to one embodiment of the present invention is used in electron optical devices such as a transmission electron microscope, a scanning electron microscope, etc. First, before describing a tip end portion 10 (FIG. 1) of a field emission electron source according to one embodiment of the present invention, a tip end portion 100 of a conventional field emission electron source will be described.

[0018] 9 shows a configuration example of a tip end portion 100 of a conventional field emission electron source. The tip end portion 100 is formed, for example, from a material such as W (tungsten), and has a needle-shaped shunt 11 whose diameter decreases toward the tip (the lower side of the drawing), and a substantially spherical convex portion 12 at the tip of the shunt 11.

[0019] The shunt 11 is formed by electrolytically polishing a rod-shaped material. The roughly spherical protrusion 12 is provided for the purpose of exposing a clean surface at the tip of the shunt 11 and preventing the tip of the shunt 11 from being destroyed by static electricity or the like. When the material of the shunt 11 is W, the roughly spherical protrusion 12 is formed by repeating flashing at a temperature of about 1800°C.

[0020] Between the tip end portion 100 and an extraction electrode (not shown), an aperture 15 having the same potential as the extraction electrode is disposed.

[0021] In the field emission electron source, electrons are emitted from the surface of the approximately spherical convex portion 12 by applying a voltage between the tip end portion 100 and the extraction electrode. A virtual light source 18 exists at a position where the trajectory of electrons (hereinafter referred to as probe current) 17 that have passed through the aperture of the aperture 15, among the electrons (hereinafter referred to as total emission current) 16 emitted from the surface of the approximately spherical convex portion 12, is extended in the reverse direction.

[0022] In the tip end 100 of a conventional field emission electron source (where the curvature radius of virtual light source 18 is, for example, about 100 to 200 [nm]) that is not an atomic-order microelectron source like the tip end 100, a total emission current 16 with a solid angle of about 1 [sr] is emitted from the virtual light source 18, and a probe current 17 that passes through an aperture 15 is used for scanning the sample surface, etc. The probe current 17 is about 1 / 1000 of the total emission current 16.

[0023] That is, the majority of the total emission current 16, other than the probe current 17, is blocked by the aperture 15 and is not actually used. In addition, since the Coulomb interaction between electrons emitted from the field emission electron source occurs not only between the electrons forming the probe current 17 but also between electrons other than the probe current 17, it is desirable to reduce the current 16 other than the probe current 17.

[0024] Therefore, in the tip end portion 10 (FIG. 1) of the field emission electron source according to one embodiment of the present invention, the current other than the probe current 17 of the total emission current 16 is reduced.

[0025] <Example of the configuration of the tip end portion 10 of the field emission electron source according to one embodiment of the present invention> 1 shows a configuration example of a tip end portion 10 of a field emission electron source according to an embodiment of the present invention. Note that common parts between the tip end portion 10 and a conventional tip end portion 100 (FIG. 9) are given the same reference numerals and their explanations are omitted as appropriate.

[0026] The tip distal end portion 10 has a shunt 11 that tapers in diameter toward the tip (the lower side in the drawing), and a substantially spherical protrusion 12 at the tip of the shunt 11.

[0027] For example, the material of the tip front end portion 10 is W(310). However, the material is not limited to W(310) orientation, and W(111) orientation or other metals may be used. W(310) orientation corresponds to the first metal of the present invention.

[0028] The shunt 11 is made by electrolytically polishing the tip of a rod-shaped single crystal with a diameter of about 0.1 mm in the W (310) orientation into a needle-like shape. The roughly spherical convex portion 12 is formed into a sphere or drop shape with a radius of curvature of about 100 to 200 nm.

[0029] Figure 2 shows an example of a magnified scanning electron microscope image of the tip end 10, which uses a material with a W (310) orientation. However, the approximately spherical protrusion 12 is much smaller in size than the tip of the shunt 11, so it cannot be seen in Figure 2.

[0030] Returning to Figure 1, a coating 13 having a thickness of several nm is formed on the shunt 11 and the approximately spherical convex portion 12 at the tip distal end portion 10. The coating 13 is formed by chemical vapor deposition using a material (e.g., Os (osmium)) having a higher work function than the material (e.g., W (310) orientation) of the shunt 11 and the approximately spherical convex portion 12.

[0031] The work function of the W(310) surface, which is the material of the shunt 11 and the substantially spherical convex portion 12, is 4.3 [eV]. The work function of Os, which is the material of the coating 13, is 5.2 [eV]. Os is a metal with a melting point of 3045 [°C] and can form a uniform amorphous coating with little granularity, so it is used for coatings provided on electron optical devices that require heat resistance and are sensitive to contamination. Os corresponds to the second metal of the present invention. Note that, in addition to Os, Pt (platinum) or Pd (palladium) may be used as the material of the coating 13.

[0032] However, of coating 13 covering substantially spherical convex portion 12, coating 13 on an axis (hereinafter referred to as the emission axis) passing through virtual light source 18 and the center of aperture 15 is removed to provide opening 14, exposing the material (e.g., W) of substantially spherical convex portion 12. Opening 14 has a radius of, for example, about 1 [nm].

[0033] <Step of Providing Openings 14 in Approximately Spherical Convex Parts 12> FIG. 3 is a diagram for explaining a process of providing the opening 14 in the substantially spherical convex portion 12. As shown in FIG.

[0034] First, a tip distal end 10 having no opening 14 and in which the shunt 11 and the substantially spherical protrusion 12 are entirely covered with a coating 13, and a tip distal end 10' before being entirely covered with the coating 13 are prepared.

[0035] Next, the tip end 10 and the tip end 10' are placed facing each other in a vacuum or in an imaging gas such as He (helium) or Ne (neon). Then, a scanning tunneling microscope is constructed by driving the approximately spherical convex portion 12' of the tip end 10' as a probe by a piezoelectric driving mechanism (not shown).

[0036] Next, the surface of the approximately spherical convex portion 12 of the tip end portion 10 is scanned to identify the W(310) plane. Then, within a circular region with a radius of about 1 [nm] of the identified W(310) plane, a high voltage is applied by the power source 31 between the tip end portion 10 and the tip end portion 10' to generate a strong electric field of several tens of [V / nm]. Then, in the electric field, field evaporation is generated on the surface on the emission axis of the approximately spherical convex portion 12 of the tip end portion 10, and the atoms of the coating 13 are ionized and sequentially removed to form the opening 14. Note that a field emission microscope or a field ion microscope may be used to confirm whether the desired crystal plane as the opening 14 has been obtained.

[0037] <Regarding the electric field radiation pattern of electrons emitted from the tip end portion 10> 4 shows a schematic diagram of the electric field radiation pattern of electrons emitted from the tip end 10 in the W(310) direction. Fig. 4(A) shows an example of the case where the approximately spherical protrusion 12 is not covered with the coating 13, and Fig. 4(B) shows an example of the case where the approximately spherical protrusion 12 is covered with the coating 13 and has an opening 14 (in the case of this embodiment).

[0038] In the figure, the white circles inside represent the orientations with low work functions and large amounts of electron emission, while the diagonal-lined circles represent the dark orientations with high work functions and small amounts of electron emission. In the case of (A) in the figure, the actual electric field emission pattern is such that the brightness of the (310) orientation overlaps, so each circle does not appear as an independent circle, and the boundaries between each orientation are unclear.

[0039] On the other hand, in the case of FIG. 1B corresponding to this embodiment, all directions other than the W(310) plane on the emission axis are covered with the coating 13, so the total emission current is less likely to be emitted. As a result, it is possible to brighten only the radiation from the W(310) plane on the emission axis.

[0040] If the W(310) surface on the emission axis is contaminated by gas adsorption or the like and the probe current decreases, weak flashing (short-term current heating) at about 1000°C can remove the adsorbed gas and return the probe current to its original state. Os, which is used as the material for the coating 13, can withstand heating at least at about 1000°C.

[0041] <Simulation results of emitted electron characteristics> Next, Figures 5 to 8 show the results of a Monte Carlo simulation of the emitted electron characteristics taking into account Coulomb interactions in a case where the approximately spherical convex portion 12 is not covered with the coating 13 (shown by the dashed line in the figure, hereinafter referred to as the W case) and a case where the approximately spherical convex portion 12 is covered with the coating 13 and an opening 14 is provided (shown by the solid line in the figure, hereinafter referred to as the Os case).

[0042] The simulation was performed under the conditions that the radius of the approximately spherical convex portion 12 was 130 nm, the work function of the W(310) surface was 4.3 eV, the work function of Os was 5.2 eV, and the radius of the opening 14 was a circular area of ​​1 nm.

[0043] The horizontal axis in each of FIGS. 5 to 8 is common to all of the graphs and indicates the probe current Ip [nA].

[0044] The vertical axis of Figure 5 shows the total emission current It [μA]. As is clear from the figure, when trying to obtain the same probe current Ip, the total emission current It in the case of Os is about 1 / 10 of that in the case of W.

[0045] The vertical axis of Fig. 6 shows the energy width ΔE [eV], which is the result of the effect on the emission axis direction of the Coulomb interaction between the electrons in the probe current and the electrons in the total emission current. As is clear from the figure, the energy width ΔE when trying to obtain the same probe current Ip is smaller in the case of Os than in the case of W. For example, the energy width ΔE when not considering the Coulomb interaction is calculated to be 0.28 [eV] using the Fowler-Nordheim theory of field emission, so in the case of Os, even when obtaining a probe current Ip of 40 [nA], the energy width ΔE is 0.34 [eV], which is not much increased.

[0046] The vertical axis of Fig. 7 shows the ratio of the diameter dwc of the virtual light source 18 when the Coulomb interaction is taken into account to the diameter dwoc of the virtual light source 18 when the Coulomb interaction is not taken into account. It is known that the effect of the Coulomb interaction between electrons in the direction perpendicular to the emission axis appears as an increase in the diameter of the virtual light source 18. As is clear from the figure, the increase ratio of the diameter of the virtual light source 18 due to the Coulomb interaction in the case of Os is suppressed to about 4 / 7 of that in the case of W.

[0047] The vertical axis in Fig. 8 indicates converted luminance β. The positions of virtual light source 18 in the cases of Os and W are approximately the same. As is clear from the figure, the converted luminance β in the case of Os is approximately three times that in the case of W. Moreover, in the case of W, the converted luminance β plateaus as the probe current Ip increases, whereas in the case of Os, the converted luminance β maintains an increasing tendency as the probe current Ip increases. This is because in the case of W, the rate of increase in the diameter of virtual light source 18 is greater than the rate of increase on the side with the larger probe current Ip.

[0048] <Summary> Compared with the tip tip 100 (Figure 9), the tip tip 10 of this embodiment can reduce the parts of the total emission current emitted from the part of the tip tip 10 covered with the high work function material (coating 13) other than the probe current while maintaining the amount of probe current when the same electric field strength is applied.

[0049] As a result, the Coulomb interaction between the electrons in the probe current and the electrons in the surrounding total emission current other than the probe current is greatly reduced, and the orbital displacement is also reduced, so that the increase in the diameter and energy width of the virtual light source 18 is suppressed, making it possible to emit an electron beam with high brightness, large current, and low energy width.

[0050] In addition, the amount of electron-stimulated desorbed gas emitted when the total emission current collides with the aperture in the electron optical device or the inner wall of the device is reduced, and the frequency with which positive ions generated by ionizing the gas collide with the electron source is reduced, so that stable electron emission can be maintained.

[0051] The present invention is not limited to the above-described embodiment, and various modifications are possible. For example, the above-described embodiment has been described in detail to clearly explain the present invention, and is not necessarily limited to those having all of the configurations described. In addition, it is possible to replace a part of the configuration of one embodiment with a configuration of another embodiment, or to add a part of the configuration of another embodiment. [Explanation of symbols]

[0052] 10 tip end, 11 shunt, 12 roughly spherical convex portion, 13 coating, 14 aperture, 15 aperture, 16 total emission current, 17 probe current, 18 virtual light source, 31 power supply

Claims

1. 1. A field emission electron source for use in an electron optical device, comprising: The tip end of the field emission electron source is A needle-shaped shunt that tapers toward the tip. A substantially spherical protrusion formed at the tip of the shunt; a coating covering the shunt and the substantially spherical convex portion; an opening exposing a part of the substantially spherical convex portion; the shunt and the substantially spherical convex portion are formed from a first metal; the coating is formed from a second metal; The second metal has a work function greater than that of the first metal. Field emission electron source.

2. 2. The field emission electron source according to claim 1, The opening is Formed on the emission axis of the virtual light source Field emission electron source.

3. 2. The field emission electron source according to claim 1, the first metal is tungsten; The second metal is osmium, platinum, or palladium. Field emission electron source.

4. 1. An electron optical device using a field emission electron source, comprising: The tip end of the field emission electron source is A needle-shaped shunt that tapers toward the tip. A substantially spherical protrusion formed at the tip of the shunt; a coating covering the shunt and the substantially spherical convex portion; an opening exposing a part of the substantially spherical convex portion; the shunt and the substantially spherical convex portion are formed from a first metal; the coating is formed from a second metal; The second metal has a work function greater than that of the first metal. Electro-optical device.

5. A method for manufacturing a tip of a field emission electron source for use in an electron optical device, comprising the steps of: preparing a tip of the field emission electron source having a needle-shaped shunt made of a first metal and tapering toward a tip thereof, and a substantially spherical protrusion formed at the tip of the shunt; covering the shunt and the substantially spherical convex portion with a coating made of a second metal having a work function greater than that of the first metal; The substantially spherical convex portion covered with the coating is disposed opposite another substantially spherical convex portion not covered with the coating, A voltage is applied between the approximately spherical convex portion and the other approximately spherical convex portion to electrochemically evaporate the coating covering the approximately spherical convex portion, thereby forming an opening that partially exposes the approximately spherical convex portion. A manufacturing method comprising the steps of:

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