JPH10255703A - Electron emitter negative electrode - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a practical electron emitting negative electrode in which a change with the passage of time can be controlled for a radius of a curvature of a needle shaped electrode tip and a long time operation can be performed under a certain operating condition. SOLUTION: In this electron emitter negative electrode, a covered layer, which is composed of oxygen and at least one element selected from a group consisting of titanium, micronium, magnesium, aluminum, and hafnium, is disposed to a needle shaped electrode constituted of a monocrystal of tungsten or molybdenum having an axis direction <100>. This electron emitter negative electrode has such a feature that a cone half angle in a tipped cone part of the needle shaped electrode is 5 deg. or less, and desirably, a feature that a radius of curvature of the needle shaped electrode tip is 0.6 micron meter or less.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electron emission cathode used for an electron microscope, an electron beam measuring device, an electron beam exposure device, an electron beam tester and the like.

In order to increase the precision and efficiency of electron beam-based devices such as electron microscopes, higher-luminance electron beams are required, and various cathodes such as a thermal field emission cathode or a Schottky emission cathode are used. Is being considered. Among these cathodes, a needle-shaped electrode of tungsten single crystal or a needle-shaped electrode of molybdenum single crystal whose axis direction is <100> direction is selected from a group of titanium, zirconium, magnesium, aluminum, or hafnium. Electron emission cathodes provided with a coating layer made of an element and oxygen (hereinafter referred to as surface-coated cathodes) have been receiving attention.

[0003] Among the surface-coated cathodes, ZrO / W in which a coating layer (hereinafter, referred to as a ZrO layer) made of zirconium and oxygen is provided on the surface of a tungsten single crystal needle-like electrode (hereinafter, referred to as a W chip). The thermal field emission cathode has its Zr
Due to the O layer, the work function of the (100) plane of the tungsten single crystal can be increased from about 4.5 eV to about 2.8 eV without the coating layer.
Therefore, only a small flat single (100) plane (hereinafter referred to as a micro facet) formed at the tip of the <100> -oriented W chip can be used as an electron emission region. It is characterized by the fact that it can obtain a high-brightness electron beam with a longer lifetime than conventional hot cathodes, and also because it operates stably even at a lower vacuum than the cold field emission cathode and is easy to use, etc. Is taking a bath.

[0004]

2. Description of the Related Art As shown in a sectional view of FIG. 2, a structure of a ZrO / W thermal field emission cathode is such that a W chip 1 for emitting an electron beam from a tip is welded and fixed to a tungsten filament 3, Reference numeral 3 denotes a suppressor electrode 2 fixed to the insulator 4 via a metal column 5 and further forming an electric field for suppressing unnecessary electron emission.
It is provided with. The ZrO / W thermal field emission cathode is used by energizing and heating the tungsten filament 3 to raise the temperature of the W chip to about 1800K.

Referring to the structure of the ZrO / W thermal field emission cathode in detail, FIG. 3 is an enlarged view of a portion of the W tip 1 and the tungsten wire 3, and a part of the W tip 1 has zirconium and oxygen. Is provided. Although not shown, the surface of the W chip is ZrO
Covered with layers. The tip of the W chip 1 is generally composed of a conical part having a conical half angle α as shown in FIG. 4, a parallel part, a spherical part having a radius of curvature R, and a micro facet part. When the half cone angle α or the radius of curvature R is small, the parallel portion may have a shape curved inward. The parallel portion becomes shorter as the radius of curvature R becomes larger or as the half cone angle α becomes larger, and sometimes the parallel portion does not exist (see FIG. 5). In the conventional electron emission cathode, the half angle of the cone at the tip of the needle electrode is limited to a size exceeding 5 °.
The half angle of the cone tends to increase as the radius of curvature of the tip increases.

[0006] Since surface-coated cathodes such as the ZrO / W thermal field emission cathode are operated by heating to a high temperature of about 1800 K, the surface tension acting on the needle-shaped electrode becomes large under use conditions, and the needle-shaped electrode becomes large. It is known that mass transfer occurs from the tip of the electrode toward the side face, and as a result, the radius of curvature of the tip of the needle-shaped electrode gradually increases (P
hys. Rev .. 117, 1452 (1960), Jo
urnal of Applied Physics,
31, 790 (1960)). Therefore, when the surface-coated cathode is operated under a constant extraction voltage,
The phenomenon that the radiated current density also decreases because the electric field intensity generated at the tip of the needle electrode gradually decreases due to the enlargement of the tip radius of curvature. The decrease in the emission current density, that is, the increase in the radius of curvature of the tip of the needle-shaped electrode is more remarkable as the radius of curvature of the tip is smaller.

As described above, surface-coated cathodes such as the ZrO / W thermal field emission cathode can obtain a high-brightness electron beam for a long period of time and are easy to operate. It is rapidly spreading in use equipment. In these various applications of electron beam utilizing equipment, stable electron beams with little change over time over a long period of time such as 5000 hours or 10000 hours are required. It has not yet arrived. That is, the radius of curvature of the tip of the needle-shaped electrode gradually increases with the elapse of use time according to the mechanism described above. Therefore, when the electrode is operated at a constant extraction voltage, the angular current density gradually decreases. In order to keep the voltage constant, it is necessary to gradually increase the withdrawal voltage for use. In particular, the above problem is remarkable when the radius of curvature of the tip is 0.6 μm or less.

Japanese Patent Application Laid-Open No. 6-84451 discloses a ZrO / W thermal field emission cathode having a small tip radius of curvature.
It is disclosed that 0.4 μm has a characteristic that the emission angular current density is high but the total emission current is not so high. However, the ZrO / W thermal field emission cathode also has a disadvantage that the tip radius changes remarkably and the extraction voltage must be increased frequently in order to operate at a constant angular current density. I have. Frequent changes in the extraction voltage affect the electro-optical characteristics, and large changes in the radius of the tip of the needle electrode change the source size and affect the optical characteristics, leading to a reduction in resolution. It is not preferable.

[0009]

SUMMARY OF THE INVENTION The present inventors have made various studies in view of the above circumstances, and as a result, have found that when the conical half angle of the tip conical portion of the needle electrode is made extremely small, the tip angle of the tip of the needle electrode is extremely small. It has been found that mass transfer can be suppressed, and as a result, the time-dependent change in the radius of curvature of the tip of the needle-shaped electrode can be reduced, and an electron emission cathode excellent in operability when applied to various electron beam utilizing devices can be provided. Thus, the present invention has been accomplished.

[0010] That is, an object of the present invention is to provide a practical electron emission cathode capable of suppressing a change over time in the radius of curvature of the tip of a needle-shaped electrode and operating for a long time under a constant operating condition. I do.

[0011]

SUMMARY OF THE INVENTION According to the present invention, an axial azimuth of <1 is provided.
An electron emission cathode comprising a needle electrode made of a single crystal of tungsten or molybdenum having a orientation of at least one element selected from the group consisting of titanium, zirconium, magnesium, aluminum and hafnium and oxygen. An electron emission cathode characterized in that the conical half angle of the conical tip of the needle electrode is 5 ° or less, and preferably, the radius of curvature of the needle electrode tip is 0.6 μm.
The electron emission cathode described below.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION In the present invention, the half angle of a cone
By ゜ or less, it is possible to reduce the change in radius due to mass transfer at the tip of the needle-shaped electrode, and as a result, has the effect of suppressing the decrease in radiation current density to a level that does not pose a practical problem, In particular, the effect is remarkable in the region of 0.6 μm or less where the characteristic change is remarkable in the past.

That is, FIG. 1 shows that the initial angular current density is approximately 12%.
FIG. 5 is an explanatory diagram showing a change in the radius of curvature of the tip of the needle electrode of the electron emission cathode according to the present invention at 0 μA / sr in comparison with that of a conventionally known electron emission cathode. The horizontal axis represents the radius of curvature (R0) of the tip of the needle electrode immediately before use, and the vertical axis represents the rate of change (R5000 / R0) of the radius of curvature of the tip of the needle electrode. Here, R5000 represents the radius of curvature of the tip of the needle electrode after 5000 hours of use. In the electron emission cathode according to the present invention, R5000 / R0 is small in the entire region of R0 as compared with the conventionally known electron emission cathode, and the difference is remarkable especially in the region where R0 is 0.6 μm or less.

Hereinafter, the present invention will be described in more detail with reference to Examples and Comparative Examples.

【Example】

[Examples 1 to 5, Comparative Examples 1 and 2] V-shaped tungsten filaments were attached to the tips of two metal posts brazed to the insulator by spot welding. Further, a tungsten single crystal thin wire of <100> orientation having a length of 3.0 mm and a diameter of about 0.13 mm was attached to the top of the V-shaped tungsten filament by spot welding.

In Examples 1 to 4, a 0.5N NaOH aqueous solution having a lower concentration than the conventional one was applied to the ring-shaped electrode, and the tip of the tungsten single crystal thin wire was immersed.
Further, electropolishing is performed while reciprocating a tungsten single crystal thin wire up and down with a stroke of about 150 μm on the liquid surface of the NaOH aqueous solution, and a part of the thin wire is cut off to obtain a needle electrode having a conical half angle of 5 ° or less. Was. In the above operation,
In Comparative Example 1 and Comparative Example 2, the concentration of the aqueous NaOH solution was set to a conventionally known concentration.

In Example 5, a narrow portion having a diameter of about 3.6 μm was formed in a part of the tungsten single crystal thin wire by electrolytic polishing, and then the tungsten single crystal thin wire including the narrow portion was energized to apply the narrowing. The part is locally heated, and the tip has a radius of curvature of 1.8 μm and a cone half angle of 4 due to fusing.
A needle-shaped electrode was prepared.

Next, zirconium hydride crushed in an organic solvent and slurried is applied to a position about 0.2 mm from the apex of the tungsten filament of the tungsten single crystal thin wire, and further applied in an ultrahigh vacuum apparatus. At 3 × 10 ^-9
After evacuating to Torr, the tungsten filament is heated to about 1800 K by energizing a tungsten filament through a metal support, and then oxygen is added to 3 × 10 ⁻⁶ To.
rr was introduced and maintained for 48 hours. By this operation, the zirconium hydride was thermally decomposed and oxidized to form a reservoir made of zirconium oxide.

A suppressor electrode is put on the insulator, a needle-like electrode is protruded from a hole of the suppressor, mounted on an electron gun paired with an extraction electrode, and arranged in an ultrahigh vacuum apparatus.
Vacuum was drawn to × 10 ⁻¹⁰ Torr. Energize the tungsten filament to heat the needle electrode to 1800K,
Subsequently, a negative high potential (extraction voltage) was applied to the needle electrode with respect to the extraction electrode, and electron emission was started. The high-voltage application and measurement system is as shown in FIG. 6, and the angular current density Ip ′ = Ip / ω was calculated from the on-axis current Ip passing through the extraction electrode and the solid angle ω. Example 1
Table 1 shows the results of Example 5, Comparative Example 1 and Comparative Example 2.

[0019]

[Table 1]

FIG. 7 shows Example 1 and Comparative Example 1, and FIG. 8 shows Examples 3, 4 and Comparative Example 2 regarding the electron emission characteristics during the operation up to 1000 hours. Electron emission cathode according to the present invention, compared with conventionally known electron emission cathode,
It is clear that the electron emission characteristics are much more stable.

[0021]

The electron emission cathode of the present invention has a feature that the change in the radius of curvature of the tip of the needle-shaped electrode with time is kept small and the device operates for a long time under a certain operating condition. When used for an electron beam utilizing device, there is no need to frequently adjust the voltage, and the operability is excellent, which is industrially useful.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram showing a change in a radius of curvature of a tip of a needle-like electrode of an electron emission cathode according to the present invention.

FIG. 2 is a sectional view of an electron emission cathode.

FIG. 3 is an enlarged view of a needle electrode portion of the electron emission cathode.

FIG. 4 is a shape diagram of a tip of a needle-like electrode of an electron emission cathode.

FIG. 5 is another shape diagram of the tip of the needle-like electrode of the electron emission cathode.

FIG. 6 is a schematic diagram of applying a high voltage to an electron emission cathode and measuring an electron beam.

FIG. 7 is a diagram showing a change over time in the angular current density of the electron emission shadow according to the first embodiment.

FIG. 8 is a diagram showing a change with time of the angular current density of the electron-emitting cathodes according to Examples 3 and 4.

[Explanation of symbols]

 Reference Signs List 1 W tip 2 Suppressor electrode 3 Tungsten filament 4 Insulator 5 Metal column 6 Screw 7 Reservoir α Conical half angle R Radius of curvature

Claims (2)

[Claims] 1. A coating layer comprising at least one element selected from the group consisting of titanium, zirconium, magnesium, aluminum and hafnium and oxygen on a needle-like electrode comprising a single crystal of tungsten or molybdenum having an axial orientation of <100>. Wherein the conical half angle of the conical tip of the needle-shaped electrode is 5 ° or less. 2. The electron emission cathode according to claim 1, wherein the radius of curvature of the tip of the needle-shaped electrode is 0.6 μm or less.