JP3260204B2 - Thermal field emission cathode - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an electron microscope, a length measuring machine,
The present invention relates to a thermal field emission cathode used for an electron beam exposure machine, an electron beam tester, and the like.

[0002]

2. Description of the Related Art In recent years, a thermal field emission cathode using a tungsten single crystal needle electrode has been developed in order to obtain an electron beam with higher brightness. Tungsten chips whose axis orientation is <100> (hereinafter referred to as W chips)
In a so-called ZrO / W thermal field emission cathode provided with a coating layer made of zirconium and oxygen, the work function of the (100) plane is selectively changed from 4.5 eV to 2.8 by the ZrO coating layer.
Since it is reduced to eV, it has features such as higher brightness and longer life than a conventional hot cathode, and is more stable and easier to use than a cold field emission cathode.

FIG. 2 shows a sectional view of a thermal field emission cathode. 1
Is a W chip, 2 is a suppressor electrode for applying a voltage for forming an electric field for suppressing the emission of thermoelectrons, 3 is a tungsten wire serving as a heater for heating the W chip, and 4 is an insulator. FIG. 1 is an enlarged view of a portion of the W tip 1 and the tungsten wire 3 in FIG. 2, and a part of the W tip is provided with a zirconium and oxygen supply source 6. Although not shown, the surface of the W chip is covered with a ZrO coating layer.

The ZrO / W thermal field emission cathode W
The tip 1 is energized and heated to the tungsten wire 3 for 180
Although used at a temperature of about 0 degrees K, it is consumed by evaporation, but zirconium and oxygen are continuously supplied from the zirconium and oxygen supply source 6 to the surface of the W chip 1 through surface diffusion to form a ZrO coating layer. to continue. When the zirconium or oxygen in the zirconium and oxygen supply source 6 is completely depleted, the formation of the ZrO coating layer is completed, the work function is increased, the function as a thermal field emission cathode is lost, and the life is ended.

A conventional method of forming a ZrO coating layer for lowering the work function is described below (US Pat. No. 4,324,999). First step: An organic solvent or the like is added to a powder of zirconium hydride (ZrH ₂ ) as a precursor of a zirconium-containing substance to form a slurry, and the slurry is attached to the <100> -oriented W chip 1 to form a pool of zirconium hydride. I do. Second step: The W chip 1 is heated under a high vacuum to decompose zirconium hydride into zirconium and hydrogen, and to diffuse zirconium into the W chip 1. Third step: W chip 1 in an oxygen atmosphere of about 10 ^-6 Torr
Is heated to form a ZrO coating layer on the W chip 1. At the same time, the whole or a part of zirconium becomes zirconium oxide, and a source of zirconium and oxygen is formed (hereinafter, this step is referred to as oxygen treatment).

[0006]

However, there are some problems when manufacturing a ZrO / W thermal field emission cathode by a conventional method. The first problem when manufacturing a ZrO / W thermal field emission cathode by a conventional method is that the life thereof greatly varies, and the rate of occurrence of failure such as fusing of the tungsten wire 3 or damage of the W chip 1 due to discharge. And the unstable behavior of the electron beam is often observed. For example, if the zirconium and oxygen supply source 6 is provided near the heater side of the W chip 1, the temperature is increased, the evaporation rate of zirconium and oxygen is increased, and the life is shortened. In particular, when the oxygen treatment is insufficient and the amount of unoxidized zirconium is large, the vapor pressure of zirconium is 1800 K higher than that of zirconium oxide.
This effect is remarkable because it is about 50 times higher at Further, if a zirconium and oxygen supply source 6 is provided on the junction between the W chip 1 and the tungsten wire 3 or on the tungsten wire, the heating characteristics change, and a larger heating current is required. In some cases, the tungsten wire 3 is melted.

On the other hand, the supply source 6 of zirconium and oxygen is W
It is preferable to provide the chip near the tip side of the chip 1 because the temperature is low, the evaporation rate is low, and the life is short. However, there has been a problem that the performance as an electron emission cathode is remarkably reduced, such as instability, but no study has been made on these.

[0008] The second problem is that the source of zirconium and oxygen peels off from the W chip during the manufacturing process, so that no product can be obtained, or even if the product is obtained, peeling occurs during use. In some cases, disadvantages such as loss of the function of the thermal field emission cathode may occur.

A third problem is that desired electron beam characteristics cannot be obtained. FIG. 3 is a schematic diagram of an electric circuit when a thermal field emission cathode is used. A thermal field emission cathode generates an electron beam with a spatial spread, but in an end use such as an electron microscope, the electron beam at the center of the axis is used.
Ip7 is mainly used. In order to secure the amount of electron beam in this part, the temperature of the thermal field emission cathode and the extraction voltage Ve
Adjust x8. However, an increase in the extraction voltage increases the electron beam Ip7 in the central portion of the shaft and also increases the amount of the electron beam in the peripheral portion. Accordingly, the total electron beam It9, which is the sum of the electron beam Ip7 at the center of the axis and the electron beam at the periphery, is also increased. If the total electron beam amount is larger than necessary, a large power supply is required to operate the thermal field emission cathode, which is economically disadvantageous. For the devices using thermal field emission cathodes already designed and manufactured,
There is a problem that a thermal field emission cathode having a characteristic that the electron beam amount in the peripheral portion is larger than the electron beam amount Ip7 in the axial center portion cannot be used. Further, when the total electron beam amount It9 is large, the amount of gas released from the electrode member increases due to the electron beam bombardment effect.
It tends to be spatially unstable. As long as the desired electron beam amount at the center of the axis can be obtained, it is desirable that the electron beam amount at the peripheral portion, that is, the total electron beam amount is smaller.

The present invention has been made in view of the above problems, and has as its object to provide a thermal field emission cathode having a stable, long life, low failure rate, and stable electron beam characteristics.

[0011]

A feature of the present invention is that a needle-like electrode of tungsten single crystal having an axial orientation of <100> is provided with a coating layer made of zirconium and oxygen and a supply source thereof. In the thermal field emission cathode composed of the electrode and the suppressor electrode, the supply source of zirconium and oxygen is located at a point 200 μm in the tip direction of the needle electrode from the junction between the needle electrode and the heater and outside the suppressor electrode. located be located between the surface and is to the zirconium by weight in the source below 3.0x10 ^-6 g or 10x10 ^-6 g. Here, in order to determine the amount of zirconium, the W chip portion of the thermal field emission cathode obtained by changing the amount of application of the precursor of the source of zirconium and oxygen in a previous experiment is subjected to elemental analysis, and the amount of application is determined. By obtaining a relationship with the amount of zirconium, the amount of zirconium can be set to a desired value without breaking the thermal field emission cathode.

Another feature of the present invention is that the axial orientation is <100>.
In a thermal field emission cathode composed of a needle electrode provided with a coating layer made of zirconium and oxygen and a supply source on a needle electrode made of zirconium and oxygen on a tungsten single crystal needle electrode having an orientation, and from the tip of the needle electrode The conic profile in the region between 10 μm and 200 μm is a straight or inwardly curved arc. Specifically, the cone angle in the region from the tip of the needle electrode to 10 μm is 10 ° to 25 °, and the cone angle in the region from the tip of the needle electrode to 200 μm is 10 ° to 35 °. Degrees.

Still another feature of the present invention is that a needle electrode of tungsten single crystal having an axis orientation of <100> is provided with a coating layer made of zirconium and oxygen and a supply source thereof. In the thermal field emission cathode comprising the electrode and the suppressor electrode, there is no reaction between the supply source of zirconium and oxygen and the tungsten needle electrode, or the thickness of the reaction layer is 15 μm or less. Specifically, in the process of forming a coating layer of zirconium and oxygen on the needle-shaped electrode of tungsten single crystal, heat treatment is performed at a luminance temperature of 1200 ° C to 1500 ° C in a vacuum with an oxygen partial pressure of 1x10 ^-7 Torr or more. , Using a precursor containing zirconium oxide as a source of zirconium and oxygen, and forming a coating layer of zirconium and oxygen on the needle electrode of tungsten single crystal, ZrW ₂
And oxidizing zirconium at a temperature lower than the eutectic temperature of Zr and Zr.

[0014]

The depletion of zirconium and zirconium or oxygen in the oxygen supply source is one factor that determines the lifetime of the ZrO / W thermal field emission cathode. The rate of consumption by evaporation is strongly dependent on temperature. Further, since the position of the source of zirconium and oxygen in the needle electrode affects the electric field distribution and the heating characteristics, the position of the source of zirconium and oxygen has an optimum position in consideration of both effects.
The present inventors have examined this point, and as a result, from the joining point of w chip 1 to the heater, the tip direction of w chip 20
It became clear that the position between the point of 0 μm and the outer surface of the suppressor electrode (B in FIG. 1; including both ends) was the optimum range of the position where the zirconium and oxygen supply source 6 was provided. It is the time until the zirconium or oxygen in the source of zirconium and oxygen is depleted, but the mechanism of its depletion is not clear in many ways, but it has been confirmed that a life of at least 5,000 hours is secured at least. But 3.
If it is less than 0x10 ^-6 g, its life is short, and the desired product cannot be obtained. If it exceeds 10x10 ^-6 g, in part, factors that govern the lifetime of the thermal field emission cathode are factors other than zirconium or oxygen depletion, such as exhaustion of the heater.
Becomes dominant, so that a remarkable effect cannot be expected in terms of life, and it becomes difficult to apply a stable coating to a W chip when manufacturing a thermal field emission cathode.

The present inventors have studied the phenomenon that the supply source of zirconium and oxygen peels off.
The source of zirconium and oxygen constitutes a needle electrode W
It has been found experimentally that the above phenomenon occurs when the reaction with the chip occurs and the reaction layer exceeds 15 μm. Although the reason is not clear, it is considered that zirconium and zirconium in the supply source of oxygen react with the W chip and diffuse to the tip of the W chip through the reaction layer and the surface of W itself due to heating in the oxygen treatment step. . FIG. 4 is a schematic view of a cross section of the W chip at a contact portion of the W chip 1 with the zirconium and oxygen supply source 6 when the reaction layer is present. A layered reaction product was observed at the contact portion, and it was found by elemental analysis that this product was composed of ZrW ₂ and Zr. When the reaction layer 10 is present, the reaction product has a lower melting point than W itself because it contains ZrW ₂ and Zr. Therefore, when the reaction layer 10 becomes thicker, the weight of the zirconium and oxygen supply source cannot be supported, and the zirconium and oxygen supply source will fall off.

To prevent this, oxygen treatment, that is, oxidation of zirconium may be performed at a temperature lower than the eutectic temperature of ZrW ₂ and Zr (about 1660 ° C.). As another preventive measure, the condition of the oxygen treatment may be limited to a temperature of 1200 ° C. to 1500 ° C. in a vacuum having an oxygen partial pressure of 1 × 10 ⁻⁷ Torr or more. Furthermore, zirconium containing a precursor containing zirconium oxide in advance and an oxygen supply source may be used. If the oxygen partial pressure is less than 1 × 10 ⁻⁷ Torr, the time required for lowering the work function is long, which is not a good idea. Regarding the temperature, if the luminance temperature is lower than 1200 ° C., it is not preferable because it takes time to lower the work function, and if the luminance temperature exceeds 1500 ° C., the temperature exceeds the eutectic temperature of ZrW ₂ and Zr. The reaction layer becomes thicker, and the source of zirconium and oxygen is more easily peeled off than the W chip.

Next, the inventors of the present invention have studied in detail the problem that it is not easy to obtain a thermal field emission cathode having a characteristic that the electron beam amount in the peripheral portion is smaller than the electron beam amount in the axial center portion. As a result, it became clear that the shape of the conical portion of the W tip 1 controls the spatial distribution of the electron beam amount. Particularly, when the shape of the conical portion of the W tip has a straight or inwardly curved arc shape in the region from the tip to 10 μm and 200 μm, the total electron beam amount hardly depends on the radius of curvature of the tip. It has been found to be a solution. This is presumed to be because the electron beam in the peripheral portion is emitted from the conical portion in the thermal electric field. For the conical shape of the W tip, 10μ from the tip
The cone angle in the region up to m is preferably 10 degrees or more and 25 degrees or less. If it is less than 10 degrees, it is not preferable because it is difficult to manufacture, and the tip shape is easily deformed during long-term use, and the long-term stability of the electron beam is lacking. On the other hand, if the angle exceeds 25 degrees, the total electron beam amount becomes large, and is not practically useful. Regarding the cone shape of the W tip, in addition, the cone angle in the region from the tip to 200 μm is preferably 10 degrees or more and 35 degrees or less. The reason for the numerical limitation is that, as described above, if it is less than 10 degrees, the difficulty in manufacturing increases and long life cannot be achieved.If it exceeds 35 degrees, the total electron beam amount is large and has no practical value That's why.

[0018]

Examples and Comparative Examples Hereinafter, examples of the present invention will be specifically described with reference to the drawings. As shown in Figure 1, the length 1.
A 2 mm <100> oriented tungsten single crystal is welded to a tungsten wire heater, and the tip of the tungsten single crystal has a tip radius of curvature of 0.3 to 0.3 mm by electrolytic polishing.
A 0.5 μm W chip 1 is formed. As an electrolytic polishing method, as shown in FIG.
With the W chip 1 as the anode and the electrolyte as N
Using aOH, the tip of the W chip 1 was immersed in the electrolytic solution while applying a DC voltage of 6 V. At this time, W
By adjusting the vertical movement of the tip 1, a W tip 1 having a different conical shape was prepared.

Next, zirconium hydride powder having a particle size of 0.5 μm to 5 μm was slurried with isoamyl acetate, and a precursor of a source of zirconium and oxygen was applied by a brush. In some cases, zirconium oxide was added to zirconium hydride so that the mole fraction of zirconium was 50%. With respect to the application position of the precursor, the positions of A, B, and C in the tip direction from the welding point of the W tip 1 and the tungsten wire 3 shown below were set. Usually, the length to the tip of the needle electrode is about 1000 to 1500 μm, and the distance of B is about 900 μm.

A: W chip 1 and tungsten wire 3
From the welding point to 200 μm in the tip direction of the W tip 1 (excluding both ends). B: A range from a welding point of the W tip 1 and the tungsten wire 3 toward a tip of the W tip 1 from a position of 200 μm to an outer surface of the suppressor electrode 2 (including both ends, provided that the zirconium and oxygen supply sources are A range that does not protrude outward from the outer surface). C: A range protruding outward from the outer surface of the suppressor electrode 2.

The applied area (zirconium and oxygen supply source 6) is about 300 μm in the longitudinal direction of the W chip 1.
After the isoamyl acetate was completely evaporated, the suppressor electrode 2 was attached, and the tip of the W tip was adjusted to protrude 300 μm from the surface of the suppressor electrode 2. Thereafter, vacuum evacuation was performed from 1 × ^{10 −9} to 1 × ^{10 −10} Torr, and current was applied to heat to decompose zirconium hydride into hydrogen and zirconium at a luminance temperature of 1200 to 1550 ° C., thereby diffusing zirconium on the W chip surface. Next, introduce oxygen to 9x10 ^{-8 to} 1x10 ^-5 To
Oxygen treatment was carried out by holding for 20 hours under an oxygen partial pressure of rr. In the production of the thermal field emission cathodes described above, a total of 27 types of thermal field emission cathodes having different types of zirconium and oxygen source precursors and zirconium weights, coating positions, oxygen treatment conditions, and electropolishing conditions were produced. And compared their properties. 1.0X10 ^-10 T for 27 kinds of prepared thermal field emission cathodes
The device was heated to a brightness temperature of 1400 ° C. under a vacuum of orr, a voltage of 4 kV was applied, and the electron beam characteristics were compared and evaluated. In addition, the shape of the tip was observed by SEM, and for a part, the joint between the supply source of zirconium and oxygen and the W chip was cut and its surface was observed. The above results are shown in Tables 1 and 2.

[0022]

[Table 1]

[0023]

[Table 2]

As described above, the thermal electric field is such that the zirconium and oxygen supply source is located in the range (B) from the junction of the W chip and the heater to the tip of the W chip from 200 μm to the outer surface of the suppressor electrode. The emission cathode can maintain stable operation without discharge, especially the weight of zirconium in zirconium and oxygen source is 3.0x10 ^-6 g
By setting it to 10 × 10 ⁻⁶ g or less, a long life of 7000 hours or more was obtained.

In addition, from 10 μm to 20 μm from the tip of the needle electrode
When the conical shape in the region up to 0 μm is a straight line or an inwardly curved arc, the total electron beam amount is not significantly affected by the tip radius of curvature, so the power supply needs to be increased even when the tip radius of curvature is small. That there is no Also, there is no reaction between the source of zirconium and oxygen and the tungsten needle electrode, or when the thickness of the reaction layer is 15 μm or less, the source of zirconium and oxygen peels off, making it difficult to obtain a thermal field emission cathode. It is clear that no problem occurs.

[0026]

According to the thermal field emission cathode of the present invention, since the supply source of zirconium and oxygen of the needle electrode is provided at an appropriate position, stable electron beam characteristics without discharge phenomenon can be obtained. It has a long life and has a great effect as a thermal field emission cathode for electron microscopes and electron beam exposure apparatuses.

[Brief description of the drawings]

FIG. 1 is an enlarged view showing a structure of a needle electrode and a suppressor electrode of a thermal field emission cathode of the present invention.

FIG. 2 is a sectional view of a thermal field emission cathode.

FIG. 3 is an electric circuit diagram when a thermal field emission cathode is used.

FIG. 4 is a schematic diagram showing an enlarged cross section of a W chip at a portion where a supply source of zirconium and oxygen is attached.

FIG. 5 is a schematic view showing a method of electrolytically polishing the tip of a W chip.

[Explanation of symbols]

 1: Tungsten chip (W chip) 2: Suppressor electrode 3: Tungsten wire 4: Insulator 5: Metal support 6: Zirconium and oxygen supply source 7: Electron beam (Ip) at the center of shaft 8: Extraction voltage (Vex) 9: Total electron beam (It) 10: Reaction layer 11: Ring electrode

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-2-27643 (JP, A) JP-A-56-82538 (JP, A) Adachi, "ZrO / W (100) thermal field emission cathode", Electron microscope, Japan Society of Electron Microscopy, 1986, (58) Field surveyed (Int. Cl. ⁷ , DB name) H01J 1/30 H01J 9/02 H01J 37/06 H01J 37/073

Claims (1)

(57) [Claims] 1. A thermal field emission cathode comprising a needle electrode of a tungsten single crystal having a <100> direction and a suppressor electrode provided with a coating layer of zirconium and oxygen,
Between 10 μm and 200 μm from the tip of the needle electrode
Conical contour in the area of
It is arc-shaped and has a zirconium weight of 3.0 × 10 ⁻⁶ g
A supply source of zirconium and oxygen containing 10 × 10 ⁻⁶ g or less is placed between a point 200 μm in the tip direction of the needle electrode from the junction of the needle electrode and the heater and the outer surface of the suppressor electrode. Located in front of
Between the needle-shaped electrode and the source of zirconium and oxygen
The reaction layer containing Zr and ZrW ₂ generated at 15 μm or less
Thermal field emission cathode which is a bottom.