Schottky electron source, method for producing schottky electron source, and electron beam device

Abstract

The present invention provides an electron beam device such as an electron microscope which is stable and which has high resolution and a high operation rate. A Schottky electron source according to the present invention comprises: a tungsten (W) single crystal that is joined to a tip part of a filament; a diffusion source that contains at least zirconium and oxygen and that is sintered on a portion of the W single crystal; and a suppressor electrode, said electron source emitting electrons by applying, to a lead-out electrode that faces the suppressor electrode, an electric field in which the W single crystal has a negative polarity. The W single crystal includes a substantially cylindrical body portion, a substantially conical shank portion, a columnar crystal portion of an octagonal prism that extends from the shank portion, and a facet of a W (100) crystal face at a top part of the tip of the columnar crystal portion. The diffusion source is a sintered body which is on the generatrix of the shank portion or a part of which is on the generatrix of the shank portion. The diffusion source, which is the sintered body, is surrounded by the suppressor electrode, and a part of the shank portion beyond a part where the sintered body is formed projects from an opening at a central part of the tip of the suppressor electrode.

Description

Schottky electron source, method of manufacturing the Schottky electron source, and electron beam apparatus

[0001] The present invention relates to a Schottky electron source, a method for manufacturing a Schottky electron source, and an electron beam apparatus, and more particularly to a Schottky electron source (thermal field emission electron source) used in an electron beam apparatus such as an electron microscope.

[0002] Electron microscopes have spatial resolution beyond the optical limit, allowing observation of microstructures and composition analysis on the nm to pm order. For this reason, they are widely used in engineering fields such as materials, physics, medicine, biology, electricity, and mechanics. Among electron microscopes, the scanning electron microscope (SEM) is an instrument that can easily observe the surface of a sample.

[0003] Electron sources used in electron beam instruments such as scanning electron microscopes include thermionic emitters (TE), cold field emitters (FE), and Schottky emitters (also known as thermal field emitters (TFE)). Among these electron sources, thermionic emitters (TE), cold field emitters (FE), and Schottky emitters (SE, also known as thermal field emitters (TFE)) emit electron beams that are slightly less monochromatic and brighter than cold field emitters. However, their monochromaticity and brightness are significantly higher than those of thermionic emitters, enabling the realization of high-spatial-resolution electron microscopes. Furthermore, because Schottky emitters are heated to high temperatures, they are less susceptible to gas adsorption and have a more stable emission current than cold field emitters operated at room temperature. Furthermore, Schottky emitters are also easier to extract large currents than cold field emitters. For this reason, Schottky electron sources are widely used in a variety of electron microscopes, as they are capable of achieving both high spatial resolution and highly stable operation in electron microscopes. These include length-measuring electron microscopes, which measure semiconductor microcircuits at high resolution using low current in semiconductor factories, and analytical electron microscopes, which perform elemental analysis at high speed using large current.

[0004] The most practical Schottky electron source (thermal field emission electron source) currently in use is the Zr / O / W electron source, which is a tungsten (W) single crystal equipped with a diffusion source of zirconium (Zr) atoms and oxygen (O) atoms, such as zirconium oxide (ZrO2). Figure 1 shows a schematic diagram of the structure of a Zr / O / W electron source as a Schottky electron source. The Schottky electron source 100r consists of a V-shaped filament 3 welded to two electrode pins 2 that penetrate a ceramic insulator 1, and a W single crystal 4 with a long axis oriented in the

[0100] direction welded to its tip. The body 4-1 of the W single crystal 4 is equipped with a diffusion source 5, made of a sintered compact such as zirconium oxide, which serves as a source of Zr atoms and O elements, and is surrounded by a suppressor electrode 6 to suppress unwanted thermionic emission. The suppressor electrode 6 has an opening 6-1 at the center of its tip, and a part of the W single crystal 4 protrudes from the opening 6-1. When an electric field that makes the W single crystal 4 negative is applied between the suppressor electrode 6 and the extraction electrode 7, electrons are emitted from the W single crystal 4.

[0005] Figure 2 is an enlarged schematic diagram of the tip of the W single crystal 4. The W single crystal 4 is formed with a shank 4-2 toward the tip by electrolytic polishing, and the tip is then built up by applying a high electric field while heating in a vacuum, forming a columnar crystal structure 8. A flat facet 9 of the W(100) crystal plane is formed at the tip of the columnar crystal structure 8, and this facet 9 serves as the electron emission surface of the Zr / O / W electron source.

[0006] In the Zr / O / W electron source, Zr and O atoms diffuse from a diffusion source 5 on the body 4-1 of a W single crystal 4 heated to approximately 1800 K by a filament 3 through the shank 4-2 to the facet 9 at the tip of the columnar crystal structure 8, forming a monolayer coating of Zr and O. This selectively lowers the work function of the facet 9 from 4.5 eV to 2.8 eV, allowing the facet 9 to function as an electron emission surface. To emit electrons from the facet 9, a high voltage is applied to the extraction electrode 7, making the W single crystal 4 negative, and a strong electric field is applied to the tip facet 9. At this time, the effective work function of the tip facet 9 is further lowered by the Schottky effect, allowing electrons primarily thermally excited within the W single crystal 4 (which may also include tunneling electrons passing through the upper vacuum barrier) to be extracted into vacuum.

[0007] This Zr / O / W electron source generally exhibits very little fluctuation or noise in the emission current, enabling stable electron beams to be obtained over several months to several years. However, when used for long periods under relatively low electric field and low current density conditions, such as those used for high-resolution imaging, the emission current of the Zr / O / W electron source can become unstable. This occurs because the buildup effect of the columnar crystal structure 8 weakens during long-term electron emission under low electric field conditions, causing the end faces of the facets 9 to collapse, resulting in the formation of ring-shaped step structures called dark rings 10 within the facets 9. Figure 3 shows an enlarged schematic diagram of the tip of such an unstable W single crystal 4. The step portions of these dark rings 10 are shielded from the electric field and inhibit the diffusion of Zr and O, preventing a decrease in the work function. Therefore, electron emission is more difficult than with a normal facet 9. Furthermore, since this dark ring 10 moves over time within the facet 9, when the dark ring 10 crosses the probe portion used in the electron microscope, the amount of current that can be extracted decreases, causing fluctuations in the probe current and hindering stable operation of the electron microscope.

[0008] Non-Patent Document 1 reveals that effective means for stably operating a Schottky electron source (thermal field emission electron source) such as a Zr / O / W electron source are to reduce the current flowing through the filament 3 to lower the operating temperature of the W single crystal 4 and to increase the extraction voltage to increase the electric field strength at the tip facet 9.

[0009] However, lowering the operating temperature of the W single crystal 4 reduces the number of electrons thermally excited within the W single crystal 4, resulting in a decrease in the amount of current that can be extracted. Furthermore, the diffusion coefficient of Zr and O atoms from the diffusion source 5 when they diffuse onto the surface of the facet 9 at the tip of the W single crystal 4 decreases, reducing the coverage of the monolayer of Zr and O atoms on the facet 9 (to a coverage of 1 or less). This increases the work function of the facet 9 above 2.8 eV, reducing the amount of electrons emitted.

[0010] As a countermeasure, if the extraction voltage is increased to maintain the amount of current that can be extracted, the effective work function will be further reduced due to the Schottky effect, and electrons will be emitted over a wider energy range, which will broaden the energy spread of the emitted electrons and reduce their monochromaticity. This will increase the chromatic aberration of the electron lens of the electron beam device and lead to a decrease in the spatial resolution of the electron microscope.

[0011] Ultramicroscopy 110 (2010) 1243-1254 ('Collapsing rings' on Schottky electron emitters, MS Bronsgeest, P. Kruit, Ultramicroscopy, Volume 110, Issue 9, August 2010, Pages 1243-1254)

[0012] The problem to be solved by the present invention is to obtain stable electron emission for a long period of time while maintaining high monochromaticity and minimizing current reduction even when using a Schottky electron source (thermal field emission electron source) at low temperatures below 1800 K.

[0013] SUMMARY OF THE INVENTION An object of the present invention is to solve the above problems and to provide an electron beam apparatus such as an electron microscope that has high resolution, high availability, and stability.

[0014] The above problems can be solved by the following means.

[0015] The Schottky electron source has the following features: (1) The Schottky electron source includes two electrodes 2 provided on a ceramic substrate 1, a filament 3 joined to both electrodes, a W single crystal 4 joined to the tip of the filament 3, a diffusion source 5 containing at least Zr atoms and O atoms sintered onto a portion of the W single crystal 4, and a suppressor electrode 6, and emits electrons by applying an electric field that makes the W single crystal 4 negative to an extraction electrode 7 facing the suppressor electrode 6 via a vacuum. The W single crystal 4 includes a substantially cylindrical body portion 4-1, a substantially conical shank portion 4-2, an octagonal columnar crystal portion 8 extending from the shank portion 4-2, and a facet 9 of a W(100) crystal plane at the tip apex of the columnar crystal portion. The diffusion supply source 5 is a sintered body formed on the generatrix of the conical shank portion 4-2, or a portion thereof formed on the generatrix of the shank portion 4-2, and the sintered diffusion supply source 5 is surrounded by the suppressor electrode 6, and a portion of the approximately conical shank portion 4-2 beyond the sintered body forming portion protrudes from an opening 6-1 at the center of the tip of the suppressor electrode 6. (2) The diameter D of the approximately cylindrical body portion 4-1 is in the range of 100 to 150 μm, the height L1 of the approximately conical shank portion 4-2 protruding from the suppressor electrode 6 is in the range of 200 μm to 300 μm, and the opening angle α (°) of the approximately conical shape of the shank portion 4-2 is as follows: 2 tan －１ (D / 2L1)>(π / 180)×α(°) (3) The length L3 of the columnar crystal portion 8 of the octagonal columnar portion is 0.0252α, where α(°) is the opening angle of the approximately conical shank portion 4-2. 1.74 ≧L3(μm)≧0.0008α 3.024 The range is.

[0016] (4) The distance L2 between the suppressor electrode 6 and the extraction electrode 7 is in the range of 2.0 to 2.4 times the height L1 of the approximately conical shank portion protruding from the opening 6-1 at the center of the tip of the suppressor electrode 6. (5) The sum of the length L4 of the generatrix beyond the sintered body forming portion of the diffusion source 5 and the length L3 of the columnar crystal portion 8 of the octagonal column portion is 600 μm or less.

[0017] (6) The total area of ​​four {100} crystal faces on the side surface of the octagonal columnar crystal portion 8 is smaller than the total area of ​​four {110} crystal faces. (7) A method for manufacturing a Schottky electron source for forming the octagonal columnar crystal portion 8 includes applying a high voltage of positive polarity to an extraction electrode 7 on a W single crystal 4 heated to a high temperature of 1800 K or higher. (8) In the method for manufacturing a Schottky electron source described in (7) above, a small amount of oxidizing gas is introduced during manufacturing. (9) The Schottky electron source described in (1) above is used as an electron source for an electron beam device.

[0018] The effect of the present invention is that a Schottky electron source (thermal field emission electron source) can emit electrons stably for a long period of time while maintaining high monochromaticity and with little current reduction even when used at temperatures below 1800 K. This makes it possible to provide a stable electron beam device such as an electron microscope with high resolution and high operating efficiency.

[0019] 1 is a structural schematic diagram of a Schottky electron source (thermal field emission electron source); 2 is an enlarged schematic diagram of the tip of a W single crystal 4 of a Schottky electron source (thermal field emission electron source); 3 is an enlarged schematic diagram of the tip of a W single crystal 4 of a Schottky electron source (thermal field emission electron source) on which a dark ring is formed; 4 is a schematic diagram of a W single crystal 4, where (a) is a schematic diagram of a body 4-1 and a shank 4-2 of a W single crystal 4 processed by DC electrolytic polishing, and (b) is a schematic diagram of a body 4-1 and a shank 4-2 of a W single crystal 4 processed by AC electrolytic polishing; 5 is a schematic diagram of a Schottky electron source according to the present invention, where (a) is a schematic diagram of a Schottky electron source having a diffusion source 5 sintered on the generatrix of a substantially conical shank 4-2, and (b) is a schematic diagram of a Schottky electron source having a diffusion source 5 sintered on the generatrix of the body 4-1 and the shank 4-2. 6 is a schematic diagram of the structure of a Schottky electron source in which a suppressor electrode 6 of the present invention surrounds the diffusion source 5 of FIG. 5. This graph shows the relationship between the diameter D of a W single crystal used in a Schottky electron source of the present invention and the range of the opening angle α of the approximately conical shank portion 4-2, with the height L1 of the approximately conical shank portion 4-2 protruding from the suppressor electrode 6 as a parameter. This graph shows the ratio of the electric field strength at the tip of a W single crystal 4 produced by DC electrolytic polishing to the electric field strength at the tip of a W single crystal 4 having an opening angle α of the approximately conical shank portion 4-2 produced by AC electrolytic polishing. This is a schematic diagram illustrating the height L1 of the shank portion protruding from the suppressor electrode 6 and the distance L2 between the suppressor electrode 6 and the extraction electrode 7. This graph shows the relationship between the electric field strength at the tip of the W single crystal 4 and the ratio of the height L1 of the shank portion protruding from the suppressor electrode 6 to the distance L2 between the suppressor electrode 6 and the extraction electrode 7. 1 is a schematic diagram of a method for manufacturing a Schottky electron source according to the present invention, in which a high voltage is applied to a W single crystal 4 heated to a high temperature of 1800 K or higher by passing current through a filament 3, so that the W single crystal 4 has positive polarity relative to an extraction electrode 7, as a means for forming the octagonal columnar crystal portion 8 of the present invention. FIG. 2 is a schematic diagram showing the structure of the octagonal columnar crystal portion 8 of the Schottky electron source according to the present invention. FIG. 3 is a graph showing the range of the length L3 of the columnar crystal portion 8 relative to the cone angle α of the shank portion 4-2 of the Schottky electron source according to the present invention, as indicated by the upper and lower limits of the L2 / L1 ratio. FIG. 4 is a schematic diagram of the structure of the Schottky electron source according to the present invention. FIG. 5 is a schematic diagram of an electron microscope equipped with the Schottky electron source according to the present invention.1A and 1B are graphs showing the current stability of a Schottky electron source fabricated by a conventional AC electrolytic polishing method and a Schottky electron source according to the present invention.

[0020] Hereinafter, examples will be described with reference to the drawings. However, in the following description, the same components will be assigned the same reference numerals, and repeated explanations may be omitted. Note that the drawings may be more schematic than the actual embodiment in order to clarify the description, but they are merely examples and do not limit the interpretation of the present invention.

[0021] The present inventors have discovered a structure and a manufacturing method of a Schottky electron source (thermal field emission electron source) 100 that can maintain high monochromaticity and operate stably even when used at low temperatures below 1800 K (1600 K or higher but lower than 1800 K). The following describes an embodiment of the invention.

[0022] First, the Schottky electron source (thermal field emission electron source) 100 is constructed as in the conventional case. Two electrode pins 2 penetrate a ceramic insulator 1 serving as a base, and a V-shaped filament 3 is welded to the electrode pins 2. A W single crystal 4 with a long axis oriented in the

[0100] direction is welded to the tip of the V-shaped filament 3. The W single crystal 4 has a body 4-1 that is approximately cylindrical, and the diameter of the W single crystal 4 is typically 100 to 150 μm. If the diameter of the body 4-1 is too small, the strength is insufficient, and the W single crystal 4 is prone to bending during use. If the diameter of the body 4-1 is too large, it becomes difficult to sharpen the tip of the W single crystal 4 by electropolishing, and the power required to heat the W single crystal 4 increases. Therefore, the optimum diameter D of the body 4-1 of the W single crystal 4 is, for example, 100 to 150 μm.

[0023] Next, the W single crystal 4 is processed by electrolytic polishing to form a sharpened shank portion 4-2. There are two types of electrolytic polishing techniques: DC electrolytic polishing is used when the tip curvature radius r of the W single crystal 4 is to be 1 μm or less, and AC electrolytic polishing is often used when the tip curvature radius r of the W single crystal 4 is to be 1 μm or more. A schematic diagram of the W single crystal 4 will be explained using Figure 4. Figure 4 is a schematic diagram of the W single crystal 4, where (a) is a schematic diagram of the body portion 4-1 and shank portion 4-2 of the W single crystal 4 processed by DC electrolytic polishing, and (b) is a schematic diagram of the body portion 4-1 and shank portion 4-2 of the W single crystal 4 processed by AC electrolytic polishing.

[0024] As shown in Figure 4(a), DC electropolishing allows for the tapered shank portion 4-2 to be machined thinly. This technique facilitates the creation of a small cone-shaped opening angle of 5-10° near the tip of the shank portion 4-2, with a tip curvature radius r of 1 μm or less. This facilitates the concentration of an electric field at the tip of the W single crystal 4, allowing a high electric field to be applied even with a low extraction voltage, resulting in a high-brightness electron source. This technique is therefore suitable for realizing high-resolution electron microscopes. Furthermore, the small opening angle of the cone-shaped opening angle results in a high electric field strength at the end of the facet 9 at the tip of the W single crystal. This prevents the facet 9 from collapsing and dark rings from forming, resulting in stable electron emission for a long period of time.

[0025] On the other hand, in AC electropolishing, the shank portion 4-2 is machined into an almost conical shape, as shown in Figure 4(b). Therefore, the opening angle of the tip of the shank portion 4-2 is typically large, approximately 10 to 25°, and the tip curvature radius r of the W single crystal 4 is often 1 μm or greater. This makes the tip of the W single crystal 4 less susceptible to electric field concentration. When used at low current density and low voltage, the end facet 9 at the tip of the W single crystal 4 is prone to collapse, resulting in the formation of dark rings. This results in unstable current flow, making it unsuitable for high-resolution observations or length measurements requiring stable operation over long periods of time. On the other hand, the tip curvature radius r of the W single crystal 4 is large, at 1 μm or greater, and the area of ​​the tip facet 9 is also large, making it easy to extract a large current when a high electric field is applied. Therefore, this method is suitable for elemental analysis and other applications.

[0026] The objective of the present invention is to provide a Schottky electron source 100 with high resolution and stable emission current. In this invention, however, we deliberately used AC electrolytic polishing instead of DC electrolytic polishing to form the shank 4-2. This is because, even when the Schottky electron source 100 is operated at temperatures lower than the conventional 1800 K, the surface diffusion of Zr and O atoms is not suppressed, the coverage of the Zr and O monolayer coating on the facet 9 at the tip of the W single crystal is maintained, and an increase in the work function is prevented. In the DC electrolytic polishing shown in Figure 4(a), the shank 4-2 is curved and tapers toward the tip of the W single crystal. This results in a long diffusion distance and a narrow diffusion path for the surface-diffusing Zr and O atoms. In contrast, in the AC electrolytic polishing shown in Figure 4(b), the shank 4-2 is nearly conical, with a linear and short diffusion distance and a wide diffusion path. Therefore, the surface diffusion of Zr and O atoms is easily maintained even when the Schottky electron source 100 is operated at temperatures lower than 1800 K.

[0027] In the present invention, as a device for maximizing the surface diffusion of Zr atoms and O atoms, the formation location of the diffusion source 5 such as zirconium oxide that is usually sintered to the body portion 4-1 of the W single crystal 4 will be described with reference to Fig. 5. Fig. 5 is a schematic diagram of a Schottky electron source according to the present invention, in which (a) is a schematic diagram of a Schottky electron source having the diffusion source 5 sintered on the generatrix of the approximately conical shank portion 4-2, and (b) is a schematic diagram of a Schottky electron source having the diffusion source 5 sintered on both the body portion 4-1 and the generatrix of the shank portion 4-2.

[0028] As shown in Figure 5(a), the W single crystal 4 was formed by sintering the W single crystal 4 to the generatrix of the approximately conical shank 4-2, which had been formed by AC electropolishing. Alternatively, as shown in Figure 5(b), the W single crystal 4 was formed by sintering the W single crystal 4 to a portion of the body 4-1 located on the shank 4-2 side and at least a portion of the generatrix of the shank 4-2 adjacent to the portion of the body 4-1. This technique minimizes the diffusion path from the W single crystal 4 to the tip of the W single crystal 4, maximizing the surface diffusion of Zr and O atoms. Furthermore, the surface of the electropolished W single crystal 4 is smoother and less irregular than the surface of the non-electropolished W single crystal 4, further promoting the surface diffusion of Zr and O atoms.

[0029] Next, as shown in FIG. 6 , a suppressor electrode 6 is attached. Here, the suppressor electrode 6 surrounds the sintered diffusion source 5, and only the tip of the shank portion 4-2, where no sintered body is formed, protrudes from the suppressor. This prevents the tip of the W single crystal 4 from being damaged by discharge due to disturbance of the electric field at the tip of the W single crystal 4 or the generation of foreign matter from the sintered body of the diffusion source 5. As shown in FIG. 6 , the Schottky electron source 100 has a V-shaped filament 3 welded to two electrode pins 2 that penetrate a ceramic insulator 1, and a W single crystal 4 welded to the tip of each filament. As described in FIG. 5 , the shank portion 4-2 of the W single crystal 4 is provided with a diffusion source 5, which is a sintered body such as zirconium oxide and serves as a source of Zr atoms and O elements, and is surrounded by a suppressor electrode 6 to suppress unnecessary thermionic emission.

[0030] FIG. 7 examines the specific range of the opening angle α of the shank portion 4-2 of the present invention. Three examples are shown in FIG. 7 (Example 1: upper limit of the opening angle α (°) (L1 = 200 μm), Example 2: upper limit of the opening angle α (°) (L1 = 250 μm), and Example 3: upper limit of the opening angle α (°) (L1 = 300 μm)). As will be described later, the height L1 of the approximately conical shank portion 4-2 protruding from the suppressor electrode 6 is preferably in the range of 200 μm to 300 μm, for example, to apply an appropriate electric field required for electron emission to the tip of the W single crystal 4. Furthermore, as already mentioned, the optimum range for the diameter D of the W single crystal 4 is, for example, 100 to 150 μm. Therefore, the upper limit of the opening angle α (°) of the approximately conical shank portion 4-2 protruding from the opening 6-1 at the center of the tip of the suppressor electrode 6 is expressed by the following formula (1):

[0031] 2 tan －１ (D / 2L1)>(π / 180)×α(°) (Formula 1) The opening angle α of the shank portion 4-2 is preferably in the range of, for example, 19° to 42°. There is no particular lower limit, but the opening angle α of the shank portion 4-2 that can be produced by AC electrolytic polishing is, for example, in a realistic range of 10° or more.

[0032] As described above, in the present invention, a nearly conical shank portion 4-2 is formed. However, the problem with this structure is that the opening angle α of the shank portion 4-2 is larger than when produced by DC electrolytic polishing, making it difficult for the electric field to concentrate at the tip of the W single crystal 4. As a result, the electric field strength near the tip of the W single crystal 4 decreases, the end surface of the facet 9 is easily broken, dark rings occur, and the emission current tends to become unstable. Figure 8 shows the dependence of the opening angle α of the shank portion 4-2 on the electric field strength at the tip of the W single crystal 4 produced by AC electrolytic polishing compared to the electric field strength at the tip of the W single crystal 4 produced by DC electrolytic polishing when the same extraction voltage is applied. In Figure 8, the first range RA1 indicates the range of the opening angle (tip opening angle) α of the shank portion 4-2 produced by typical DC electrolytic polishing. The second range RA2 indicates the range of the opening angle (tip opening angle) α of the shank portion 4-2 produced by typical AC electrolytic polishing. Compared to a W single crystal 4 produced by DC electrolytic polishing (shown here as an example with an opening angle (α) of 8°), the electric field intensity ratio at the tip of a W single crystal 4 produced by general AC electrolytic polishing with an opening angle (α) of 10 to 25° decreases to 97.5 to 81%. Furthermore, when the opening angle α is increased to 42°, which is the upper limit of the opening angle α (°) of the conical shank portion 4-2 of the present invention, the electric field intensity decreases to 62%.

[0033] Therefore, compared with Schottky electron sources produced by DC electrolytic polishing, Schottky electron sources produced by AC electrolytic polishing are difficult to use because the nine facet edges are prone to collapse and dark rings are likely to occur, particularly in the low electric field and low current operation used for high-resolution observation and length measurement.

[0034] In the present invention, this problem was solved by combining the following two techniques (1) and (2). (1) The ratio of the height L1 of the shank portion 4-2 protruding from the suppressor electrode 6 to the distance L2 between the suppressor electrode 6 and the extraction electrode 7 was adjusted within the range of 2 to 2.4, allowing a higher electric field to be applied to the tip of the W single crystal 4. The height L1 is also referred to as the tip protrusion height L1, and the distance L2 is also referred to as the inter-electrode distance L2. (2) A new manufacturing method was discovered that reasonably extends the length L3 of the columnar crystal structure 9 at the tip of the W single crystal 4, making it possible to apply a higher electric field to the tip of the W single crystal 4 even when the opening angle α of the shank portion 4-2 is large.

[0035] First, we will explain (1). FIG. 9A is a schematic diagram illustrating an example of the configuration of a Schottky electron source 100, illustrating the height L1 of the shank portion 4-2 protruding from the suppressor electrode 6 and the distance L2 between the suppressor electrode 6 and the extraction electrode 7. FIG. 9B is a graph showing the change in electric field strength as a function of the ratio of the height L1 of the shank portion 4-2 protruding from the suppressor electrode 6 to the distance L2 between the suppressor electrode 6 and the extraction electrode 7. Typically, the extraction electrode 7 and the suppressor electrode 6 are disposed opposite each other. The height L1 of the approximately conical shank portion 4-2 protruding from the suppressor electrode 6 is set between 200 μm and 300 μm to apply an appropriate electric field required for electron emission to the tip of the W single crystal 4. Furthermore, the distance L2 between the suppressor electrode 6 and the extraction electrode 7 is typically set to, for example, approximately 2.4 times the height L1. As can be seen from the graph in FIG. 9B, the electric field strength at the tip of the W single crystal 4 can be increased by reducing the L1 / L2 ratio. For example, by reducing the L1 / L2 ratio by 2x, the electric field strength at the tip of the W single crystal 4 can be improved by 19%, compensating for 97.5-81% of the decrease in electric field strength at the tip of a W single crystal 4 with an opening angle (α) of 10-25° created by AC electropolishing. However, if L1 / L2 is reduced too much below 2, the distance L2 between the W single crystal 4 and the extraction electrode 7 becomes too close, increasing the risk of the W single crystal 4 being melted by discharge. Therefore, it is difficult to reduce the lower limit of L1 / L2 below 2, and this is insufficient for W single crystals 4 with opening angles α of 25° or greater. Therefore, to further increase the electric field strength at the tip of the W single crystal 4, we combined the method (2) of increasing the length L3 of the columnar crystal structure to increase the electric field strength at the tip of the W single crystal 4. This method is effective not only for W single crystals 4 with opening angles α of 25° or greater, but also for W single crystals 4 with opening angles α of 10-25°.

[0036] Next, we will explain (2). Typically, the columnar crystal structure 9 is created by heating the W single crystal 4 in a vacuum, applying a negative voltage to the W single crystal 4 relative to the extraction electrode 7 to emit electrons, and building up a W(100) single crystal by applying an electric field to the tip of the W single crystal 4. However, with this method, if the temperature of the W single crystal 4 or the extraction electric field is too high to promote the buildup, the emission current becomes too large, generating a large amount of electron beam impact desorption gas and increasing the risk of the W single crystal 4 being melted by discharge. This imposes limitations on the process temperature and electric field, and extending the length of the columnar crystal structure 9 requires a long process time of several tens to several hundreds of hours, which is not practical.

[0037] Therefore, in the present invention, a positive voltage is applied to the W single crystal 4 relative to the extraction electrode 7, i.e., an electric field of the opposite polarity to that used for electron emission is applied, as shown in Figure 10. By applying an electric field of the opposite polarity, electron emission from the W single crystal 4 is prevented, and the W single crystal 4 is heated to a higher temperature of 1800K or more, and even if an electric field higher than that used for electron emission is applied, the risk of damage to the W single crystal 4 due to discharge can be avoided.

[0038] Furthermore, when a small amount of oxidizing gas such as air or oxygen is introduced, the total area of ​​the four {100} crystal faces on the side of the octagonal columnar crystal portion 8 becomes smaller than the total area of ​​the four {110} crystal faces, as shown in Figure 11. This suppresses unwanted electron emission (side emission) from the {100} crystal faces on the side of the W single crystal. Therefore, even if the length L3 of the columnar crystal structure 9 is extended, the total current from the W single crystal 4 can be suppressed, eliminating the need to increase the current capacity of the high-voltage power supply operating the W single crystal 4. As shown in Figure 11, the octagonal columnar crystal portion 8 extends from the shank portion 4-2. A flat facet 9 of a W(100) crystal face is formed at the tip (apex) of the columnar crystal structure 8. This facet 9 at the tip serves as the electron emission surface of the Zr / O / W electron source.

[0039] In this way, by using a new manufacturing method that reasonably extends the length L3 of the columnar crystal structure 9 at the tip of the W single crystal 4, it has become possible to apply a high electric field to the tip of the W single crystal 4 even if the opening angle α of the shank portion 4-2 is large.

[0040] 12 is a graph showing the range of the length L3 of the columnar crystal structure 9 at the tip of the W single crystal 4 required to obtain the same electric field strength at the tip of the W single crystal 4 as in DC electrolytic polishing, relative to the opening angle α of the shank portion 4-2 in AC electrolytic polishing. This is combined with adjusting the ratio of the height L1 of the shank portion 4-2 protruding from the suppressor electrode 6 to the distance L2 between the suppressor electrode 6 and the extraction electrode 7 within the range of 2 to 2.4. This results in the length L3 of the columnar crystal portion 8 of the octagonal column being 0.0252α when the opening angle of the approximately conical shank portion 4-2 is α (°). 1.74 ≧L3(μm)≧0.0008α 3.024 By doing so, it is possible to obtain the same electric field strength at the tip as that of the W single crystal 4 produced by DC electrolytic polishing. This solves the problem that even when AC electrolytic polishing is used, the end surface of the facet 9 collapses, causing a dark ring and resulting in an unstable emission current.

[0041] The distance from the sintered body of the diffusion source 5 to the tip of the W single crystal 4 is preferably 600 μm or less, and is preferably as short as possible within the range where it is not exposed to the outside through the opening 6-1 at the center of the tip of the suppressor electrode 6. This is because the empirical diffusion distance within the operating temperature range of the Schottky electron source (1600 to 1800 K) is approximately 600 μm. Therefore, in the present invention, if the length of the generatrix of the shank portion 4-2 beyond the sintered body of the diffusion source 5 is L4, then it is desirable that L3 + L4 ≦ 600 μm.

[0042] FIG. 13 is a schematic diagram showing the structure of the Schottky electron source of the present invention described above.

[0043] In Example 2, a Schottky electron source fabricated by conventional AC electropolishing and a Schottky electron source according to the present invention were actually mounted on an electron microscope as an electron beam device, and current stability at low current densities in high-resolution mode was compared. Figure 14 is a schematic diagram of a scanning electron microscope 1000 according to Example 2. The Schottky electron source (thermal-field emission electron source) 100 is heated by a filament current supplied by a heating power supply 104 floating on an acceleration power supply 103 controlled by a computer 101 and a controller 102. A positive voltage is applied to the extraction electrode 106 by an extraction power supply 105 relative to the Schottky electron source (thermal-field emission electron source) 100, causing electrons to be emitted by Schottky emission. A negative voltage is applied to the suppressor electrode 108 surrounding the Schottky electron source (thermal-field emission electron source) 100 by a suppressor power supply 107, suppressing unwanted electron emission from the lateral surfaces of the W single crystal of the Schottky electron source (thermal-field emission electron source) 100.

[0044] The emitted electron beam 109 is accelerated toward a grounded anode 110 by a negative high voltage applied by an acceleration power supply 103. The accelerated electron beam 109 is focused by a first condenser lens 111, an aperture 112, a second condenser lens 113, an objective lens 115, and an astigmatism correction coil 114, and scanned by a deflection scanning coil 116 to irradiate an observation area on a sample 117 placed on a sample stage. The secondary electrons thus generated are detected by a secondary electron detector 118. When measuring the angular radiation current density, a Faraday cup 119 is inserted in place of the sample 117 to measure the probe current. In other words, the electron beam device 1000 includes an electron source 100, a sample stage on which the sample 117 is placed, and an electron optical system that converges electrons emitted from the electron source 100 into a beam and irradiates the electrons on the sample 117 on the sample stage.

[0045] Fig. 15A is a graph showing the current stability of a Schottky electron source fabricated by a conventional AC electrolytic polishing method. Fig. 15B is a graph showing the current stability of a Schottky electron source of the present invention. Fig. 15A is a graph showing the current stability of a Schottky electron source fabricated by a conventional AC electrolytic polishing method (shank opening angle α = 20°) when operated at a low current density during high-resolution observation at a low temperature of 1600 K.

[0046] 15B is a graph showing the current stability when a Schottky electron source 100 is operated at a low current density during high-resolution observation at a low temperature of 1600 K, in which the length L3 of the columnar crystal structure 9 at the tip of the W single crystal 4 is extended by applying a high temperature and an electric field of reverse polarity as shown in FIG. 10 to the same Schottky electron source. The current stability of the conventional Schottky electron source (FIG. 15A) can be compared with that of the Schottky electron source 100 of the present invention (FIG. 15B).

[0047] As shown in Figure 15A, the Schottky electron source fabricated by the conventional AC electropolishing method repeatedly undergoes large current fluctuations of approximately ±30% due to the formation and disappearance of dark rings, and also generates a large amount of small shot noise because the emission current is mixed with field emission components other than Schottky electron emission due to an increase in the work function.

[0048] In contrast, as shown in FIG. 15B, the Schottky electron source 100 of the present invention exhibited stable electron emission without large current fluctuations due to the formation and disappearance of dark rings and without current noise.

[0049] The invention made by the inventor has been specifically described above based on examples, but it goes without saying that the present invention is not limited to the above-described embodiments and examples, and various modifications are possible.

[0050] 1: insulator, 2: electrode pin, 3: filament, 4: W single crystal, 4-1: body part of W single crystal, 4-2: shank part of W single crystal, 5: diffusion supply source, 6: suppressor electrode, 7: extraction electrode, 8: columnar crystal structure, 9: facet, 10: dark ring, 100: Schottky electron source (thermal field emission electron source), 101: computer, 102: controller, 103: acceleration power supply, 104: heating power supply, 105: Extraction power supply, 106: extraction electrode, 107: suppressor power supply, 108: suppressor electrode, 109: electron beam, 110: anode, 111: first condenser lens, 112: aperture, 113: second condenser lens, 115: objective lens, 114: astigmatism correction coil, 116: deflection scanning coil, 117: sample, 118: secondary electron detector, 119: Faraday cup, 1000: scanning electron microscope

Claims

1. A Schottky electron source comprising: two electrodes provided on a ceramic substrate; a filament joined to the two electrodes; a tungsten (W) single crystal joined to a tip of the filament; a diffusion source containing at least zirconium and oxygen sintered onto a portion of the W single crystal; and a suppressor electrode, wherein electrons are emitted by applying an electric field that makes the W single crystal negative to an extraction electrode facing the suppressor electrode, wherein the W single crystal includes a substantially cylindrical body portion, a substantially conical shank portion, and an octagonal columnar crystal portion extending from the shank portion, and a facet of a W (100) crystal plane at the tip apex of the columnar crystal portion, wherein the diffusion source is a sintered body formed on a generatrix of the shank portion or a portion thereof on the generatrix of the shank portion, and the diffusion source of the sintered body is surrounded by the suppressor electrode, and a portion of the shank portion beyond the portion where the sintered body is formed protrudes from an opening in the center of the tip of the suppressor electrode.

2. A Schottky electron source according to claim 1, wherein the diameter (D) of the body portion is in the range of 100 to 150 μm, the height (L1) of the shank portion protruding from the suppressor electrode is in the range of 200 μm to 300 μm, and the opening angle α (°) of the approximately conical shank portion is 2 tan -1 A Schottky electron source, characterized in that (D / 2L1)>(π / 180)×α(°).

3. In the Schottky electron source according to claim 1, the length (L3) of the columnar crystal portion is 0.0252α when the opening angle of the shank portion is α (°). 1.74 ≧L3 (μm) ≧0.0008α 3.024 The Schottky electron source is characterized in that:

4. A Schottky electron source according to claim 1, wherein the distance (L2) between the suppressor electrode and the extraction electrode is in the range of 2.0 to 2.4 times the height (L1) of the shank portion protruding from the opening at the center of the tip of the suppressor electrode.

5. A Schottky electron source according to claim 1, wherein the sum of the length (L4) of the generatrix of the shank portion from the sintered body to the tip side of the diffusion supply source and the length (L3) of the columnar crystal portion is 600 μm or less.

6. A Schottky electron source according to claim 1, wherein the total area of ​​the four {100} crystal faces on the side surfaces of the columnar crystal portion is smaller than the total area of ​​the four {110} crystal faces.

7. A method for manufacturing a Schottky electron source according to claim 1, wherein the columnar crystal portion is formed by applying a high voltage of positive polarity to the W single crystal heated to a high temperature relative to the extraction electrode.

8. The method for manufacturing a Schottky electron source according to claim 6, wherein an oxidizing gas is introduced when the columnar crystal portion is formed.

9. An electron beam apparatus comprising: an electron source; a sample stage for placing a sample thereon; and an electron optical system for converging electrons emitted from the electron source into a beam and irradiating the electrons onto the sample on the sample stage, wherein the electron source comprises: two electrodes provided on a ceramic substrate; a filament joined to the two electrodes; a tungsten (W) single crystal joined to the tip of the filament; a diffusion source containing at least zirconium and oxygen sintered onto a portion of the W single crystal; and a suppressor electrode, and is a Schottky electron source that emits electrons by applying an electric field that makes the W single crystal negative to an extraction electrode facing the suppressor electrode, wherein the W single crystal includes a substantially cylindrical body portion, a substantially conical shank portion, an octagonal columnar crystal portion extending from the shank portion, and a W(100) crystal facet at the tip apex of the columnar crystal portion, the diffusion supply source is a sintered body formed on a generator line of the shank portion or a part of the sintered body is formed on the generator line of the shank portion, the diffusion supply source of the sintered body is surrounded by the suppressor electrode, and a part of the shank portion beyond the part where the sintered body is formed protrudes from an opening in the center of the tip of the suppressor electrode.

Images