JPH0676731A - Thermoelectric field emission cathode - Google Patents

Abstract

PURPOSE:To realize electron beam characteristics which are stable and have a long life and besides have a low failure rate by providing a supply source for zirconium and oxygen in a predetermined position on a needle-like electrode. CONSTITUTION:A supply source 6 for zirconium and oxygen is provided between the junction point of a needle-like electrode made of a tungsten chip (W chip) 1 and a heating heater made of a tungsten wire 3, which is located at the point of 200mum in the direction of the tip of the needle-like electrode from the junction point, and the external surface of a suppressor electrode 2. Also, a weight of zirconium in the source 6 is set at a value of not less than 3.0X10<-6>g to not more than 10X10<-6>g. In this case, in order to determine an amount of zirconium, the relation between an amount of application and an aunt of zirconium is obtained by performing the elemental analysis of the W chip part of a thermoelectric field emission cathode which is obtained by changing an amount of application to the precursor of the supply source 6 at a preliminary experiment. Thereby, since an amount of zirconium can be set at a desired value, stable electron beam characteristics may be obtained without any discharging phenomenon and the like and at the same time the life thereof may be extended.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to an electron microscope, a length measuring machine,
The present invention relates to a thermoelectric 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, in order to obtain an electron beam of higher brightness, a thermal field emission cathode utilizing a needle electrode made of a tungsten single crystal has been developed. A tungsten chip (hereinafter referred to as a W chip) whose axis direction is a <100> direction,
A so-called ZrO / W thermal field emission cathode provided with a coating layer made of zirconium and oxygen has a work function of (100) plane selectively from 4.5 eV to 2.8 by the ZrO coating layer.
Since it is reduced to eV, it has higher brightness and longer life than the conventional hot cathode, and is more stable and easier to use than the 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. Reference numeral 5 is a metal column, and FIG. 1 is an enlarged view of the portion of the W tip 1 and the tungsten wire 3 of FIG. 2. A zirconium and oxygen supply source 6 is provided in a part of the W tip. Although not shown, the surface of the W chip is covered with a ZrO coating layer.

W of such a ZrO / W thermal field emission cathode
The tip 1 is heated to 180 by energizing the tungsten wire 3.
Since it is 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 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 ends.

A conventional method for forming a ZrO coating layer for lowering the work function is shown below (US Pat. No. 4,324,999). First step: Zirconium hydride (ZrH ₂ ) powder as a precursor of a zirconium-containing powder is added with an organic solvent or the like to form a slurry, and the slurry is adhered to a W chip 1 having a <100> orientation to form a zirconium hydride pool. To do. Second step: The W chip 1 is heated under high vacuum to decompose zirconium hydride into zirconium and hydrogen, and 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 amount or part of zirconium becomes zirconium oxide and a supply source of zirconium and oxygen is formed (hereinafter, this step is referred to as oxygen treatment).

[0006]

However, there are some problems in manufacturing a ZrO / W thermal field emission cathode by the conventional method. The first problem in manufacturing the ZrO / W thermal field emission cathode by the conventional method is that the lifespan thereof varies widely, and the rate of occurrence of failures such as melting of the tungsten wire 3 or damage of the W chip 1 due to discharge. However, unstable behavior of the electron beam is often observed. For example, if the supply source 7 of zirconium and oxygen is provided near the heater side of the W chip 1, the temperature becomes higher, the evaporation rate of zirconium and oxygen becomes faster, and the life becomes shorter. Especially, when the oxygen treatment is insufficient and the amount of unoxidized zirconium is large, the vapor pressure of zirconium is 1800 degrees K compared to the vapor pressure of zirconium oxide.
This effect is remarkable because it is about 50 times higher. Further, when the 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. There are times when the tungsten wire 3 melts.

On the other hand, the zirconium and oxygen supply source 6 is W
It is preferable if it is provided near the tip side of the tip 1 in terms of low temperature and low evaporation rate and long life, but it may disturb the electric field distribution in the vicinity of the tip of the W tip 1 and the hole of the suppressor electrode 2 to cause discharge, or to generate an electron beam. However, there has been a problem that the performance of the electron emission cathode is remarkably deteriorated due to instability, but no studies have been made on these.

The second problem is that the zirconium and oxygen supply sources are peeled off from the W chip during the manufacturing process, and the product cannot be obtained, or even if the product is obtained, peeling occurs during use. As a result, inconveniences such as loss of the function of the thermal field emission cathode may occur.

The 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. The thermal field emission cathode generates an electron beam with a spatial spread, but in the final application 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 electron beam amount in this part, the temperature of the thermal field emission cathode and the extraction voltage Ve
Adjust x8. However, the increase of the extraction voltage increases the electron beam Ip7 in the central portion of the shaft and also increases the electron beam amount in the peripheral portion. Therefore, 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 peripheral portion, is also increased. When the total electron beam amount is larger than necessary, a large power source is required to operate the thermal field emission cathode, which is economically disadvantageous. For devices that use the thermal field emission cathode that have already been designed and manufactured,
There is a problem that a thermal field emission cathode having a large electron beam amount in the peripheral portion with respect to 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, so that the electron beam is
It tends to be spatially unstable. As long as a desired electron beam amount in the central portion of the shaft is obtained, it is desirable that the electron beam amount in the peripheral portion, that is, the total electron beam amount is small.

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

[0011]

A feature of the present invention is that a needle-shaped electrode made of a tungsten single crystal having an axis orientation of <100> is provided with a coating layer of zirconium and oxygen and a supply source thereof. In a thermoelectric field emission cathode comprising an electrode and a suppressor electrode, a source of zirconium and oxygen is located at a point 200 μm from the junction of the needle electrode and a heater and outside the suppressor electrode. It is located between the surface and the surface, and the weight of zirconium in the source is 3.0x10 ^-6 g or more and 10x10 ^-6 g or less. Here, in order to determine the amount of zirconium, the W tip portion of the thermal field emission cathode obtained by changing the amount of zirconium and the precursor of the oxygen supply source in an experiment in advance was subjected to elemental analysis to determine the amount of zirconium. If the relationship with the amount of zirconium is obtained, the amount of zirconium can be set to a desired value without destroying the thermoelectric field emission cathode.

Another feature of the present invention is that the axis direction is <100>.
In a thermoelectric field emission cathode composed of a needle-shaped electrode of a tungsten single crystal composed of azimuth, a needle-shaped electrode provided with a coating layer of zirconium and oxygen and a supply source thereof and a suppressor electrode, from the tip of the needle-shaped electrode. The conical contour in the region between 10 and 200 μm lies in a straight line or an inwardly curved arc. Specifically, the cone angle in the region from the tip of the needle electrode to 10 μm is 10 degrees or more and 25 degrees or less, and the cone angle in the region from the tip of the needle electrode to 200 μm is 10 degrees or more 35 It is below the degree.

Another feature of the present invention is that a needle-shaped electrode made of a tungsten single crystal having an axial orientation of <100> is provided with a coating layer of zirconium and oxygen and a supply source thereof. In the thermoelectric field emission cathode composed of the suppressor electrode and the suppressor electrode, there is no reaction between the 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 a needle electrode of tungsten single crystal, heat treatment is performed in a vacuum with an oxygen partial pressure of 1x10 ^-7 Torr or more at a brightness temperature of 1200 ° C to 1500 ° C. In the process of applying a precursor containing zirconium oxide and zirconium oxide as a source of zirconium and oxygen, and forming a coating layer of zirconium and oxygen on a needle electrode of a tungsten single crystal, ZrW ₂
And to oxidize zirconium at a temperature lower than the eutectic temperature of Zr.

[0014]

The depletion of zirconium and / or zirconium or oxygen in the oxygen source is one of the factors that control the life of the ZrO / W thermal field emission cathode. The consumption rate due to evaporation is strongly dependent on temperature. Furthermore, since the position of the zirconium and oxygen supply source in the needle electrode affects the electric field distribution and heating characteristics, the zirconium and oxygen supply source has an optimum position in consideration of the effects of both.
The present inventors have examined this point, and as a result, from the junction of the w-tip 1 to the heater, the tip direction of the w-tip 20
It was revealed that the position between the point of 0 μm and the outer surface of the suppressor electrode (B in FIG. 1, including both ends) is the optimum range of the position where zirconium and the oxygen supply source 6 are provided. It is the time to reach the depletion of zirconium or oxygen in the source of zirconium and oxygen, but it is not clear that the mechanism of the exhaustion is unclear, but it has been confirmed that a life of around 5000 hours can be secured at the minimum. However, 3.
If it is less than 0x10 ^-6 g, its life is too short to obtain the intended product. If it exceeds 10x10 ^-6 g, one of the factors that governs the life of the thermal field emission cathode is a factor other than depletion of zirconium or oxygen, such as exhaustion of the heater.
Therefore, it is difficult to expect a significant effect in terms of life, and it becomes difficult to stably apply the W field tip to the W-chip when manufacturing the thermal field emission cathode.

The present inventors have studied the phenomenon of the zirconium and oxygen sources coming off,
The source of zirconium and oxygen constitutes the needle electrode W
It has been experimentally found that the above phenomenon occurs when the reaction layer with the chip exceeds 15 μm. The reason for this is not clear, but it is considered that zirconium in the oxygen treatment step and zirconium in the oxygen source react with the W tip and diffuse to the tip of the W tip through the reaction layer and the surface of W itself. . FIG. 4 is a schematic view of a cross section of the W tip at the contact portion of the W tip 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 this reaction layer 10 exists, its reaction product has a lower melting point than W itself because it contains ZrW ₂ and Zr. Therefore, if the reaction layer 10 becomes thicker, the weight of the zirconium and oxygen sources cannot be supported, and the zirconium and oxygen sources will drop out.

To prevent this, oxygen treatment, that is, zirconium is oxidized at a temperature lower than the eutectic temperature of ZrW ₂ and Zr (about 1660 ° C.). As another preventive measure, the conditions for oxygen treatment may be limited to a brightness temperature of 1200 ° C. or more and 1500 ° C. or less in a vacuum having an oxygen partial pressure of 1 × 10 ⁻⁷ Torr or more. Furthermore, a zirconium and oxygen supply source containing a precursor containing zirconium oxide in advance may be used. If the oxygen partial pressure is less than 1x10 ^-7 Torr, it takes a long time to reduce the work function, which is not a good idea. Regarding the temperature, if the brightness temperature is less than 1200 ° C., it takes time to lower the work function, which is not preferable. If the brightness temperature exceeds 1500 ° C., the eutectic temperature of ZrW ₂ and Zr is exceeded, so that the reaction layer is rapidly formed. The progressing reaction layer becomes thicker, and the supply sources of zirconium and oxygen are more easily peeled off than the W chip.

Next, the present inventors have made a detailed study on the problem that 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 cannot be easily obtained. As a result, it was revealed that the shape of the conical portion of the W tip 1 controls the spatial distribution of the electron beam amount. In particular, when the shape of the conical portion of the W tip has a linear or arcuate shape curved inward 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, and It has been found to be a solution of. This is presumed to be because the electron beam in the peripheral portion is thermoelectrolytically radiated from the conical portion. Regarding the shape of the conical part 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 the angle is less than 10 degrees, it is difficult to manufacture, the tip shape is easily deformed during long-term use, and the electron beam lacks long-term stability, which is not preferable. On the other hand, if the angle exceeds 25 degrees, the total electron beam amount becomes large, so that it has no practical value. Regarding the shape of the conical portion 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. As mentioned above, the reason for the numerical limitation is that if it is less than 10 degrees, manufacturing difficulty increases and long-term life cannot be achieved, and if it exceeds 35 degrees, the total electron beam amount is large and has no practical value. This is because.

[0018]

EXAMPLES AND COMPARATIVE EXAMPLES Examples of the present invention will be specifically described below with reference to the drawings. As shown in Figure 1, length 1.
2 mm of <100> orientation tungsten single crystal is welded to the heater of tungsten wire and the tip radius of tungsten single crystal is 0.3 ~ by electrolytic polishing.
A 0.5 μm W chip 1 is formed. As an electrolytic polishing method, as shown in FIG. 5, a stainless steel ring electrode 11 is used.
The cathode as the W chip 1 as the anode and the electrolyte as N
It was performed by using aOH and immersing the tip of the W chip 1 in an electrolytic solution while applying a DC voltage of 6V. At this time, W
The vertical movement of the tip 1 was adjusted to prepare W tips 1 having different conical shapes.

Next, a 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 with a brush. In some cases, zirconium oxide was also added to zirconium hydride so that the zirconium content in zirconium oxide was 50%. Regarding the coating position of the precursor, the positions 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-shaped electrode is about 1000 to 1500 μm, and the distance B is about 900 μm.

A: W tip 1 and tungsten wire 3
From the welding point of up to 200 μm in the direction of the tip of the W tip 1 (both ends are not included). B: Range from the welding point of the W tip 1 and the tungsten wire 3 to the outer surface of the suppressor electrode 2 from the position of 200 μm in the tip direction of the W tip 1 (including both ends, but the outer surface of the zirconium and oxygen supplying surface electrode) Range that does not project outside). C: Range protruding outward from the outer surface of the suppressor electrode 2.

The applied area (source 6 of zirconium and oxygen) is about 300 μm in the longitudinal direction of the W tip 1.
After the isoamyl acetate was completely evaporated, the suppressor electrode 2 was attached, and the tip of the W tip was adjusted so as to protrude from the surface of the suppressor electrode 2 by 300 μm. After that, zirconium hydride was decomposed into hydrogen and zirconium at a brightness temperature of 1200 to 1550 ° C. by evacuating from 1 × ^{10 −9} to 1 × ^{10 −10} Torr and electrically heating to diffuse zirconium on the W chip surface. Next, oxygen is introduced and 9x10 ^{-8 to} 1x10 ^-5 To
Oxygen treatment was carried out by maintaining the oxygen partial pressure of rr for 20 hours. In the production of the above-mentioned thermal field emission cathodes, a total of 27 types of thermal field emission cathodes having different types of zirconium and oxygen source precursors, zirconium weight, coating position, oxygen treatment conditions, and electropolishing conditions were produced. Then, the characteristics were compared. The prepared 27 kinds of thermal field emission cathodes are 1.0X10 ^-10 T
It was heated to a brightness temperature of 1400 ° C under a vacuum of orr and a voltage of 4 kV was applied, and 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 the surface thereof was observed. The above results are shown in Tables 1 and 2.

[0022]

[Table 1]

[0023]

[Table 2]

From the above, the thermal electric field in which the zirconium and oxygen sources are located in the range (B) from the junction of the W tip and the heater to the outer surface of the suppressor electrode at a position of 200 μm in the tip direction of the W tip. The emitting cathode can maintain stable operation without discharge, especially the weight of zirconium and zirconium in the oxygen source is 3.0x10 ^-6 g.
By adjusting the amount to 10x10 ^-6 g or less, a long life of 7,000 hours or more was obtained.

In addition, from the tip of the needle electrode to 10 μm to 20
When the contour of the conical portion in the region up to 0 μm is a straight line or an arc shape curved inward, the total electron beam amount is not greatly affected by the tip curvature radius, so it is necessary to increase the power supply even when the tip curvature radius is small. There is no. Further, when there is no reaction between the zirconium and oxygen supply source and the tungsten needle electrode, or when the reaction layer has a thickness of 15 μm or less, the zirconium and oxygen supply source is peeled off, and it is difficult to obtain a thermal field emission cathode. It is clear that no problem occurs.

[0026]

In the thermoelectric field emission cathode of the present invention, since the zirconium and oxygen supply sources of the needle-shaped electrode are provided at appropriate positions, stable electron beam characteristics without a discharge phenomenon can be obtained. It has a long life and is highly useful as a thermal field emission cathode for electron microscopes, electron beam exposure apparatuses, etc.

[Brief description of 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 electrical circuit diagram when a thermal field emission cathode is used.

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

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

[Explanation of symbols]

 1: Tungsten Chip (W Chip) 2: Suppressor Electrode 3: Tungsten Wire 4: Insulator 5: Metal Struts 6: Zirconium and Oxygen Supply Source 7: Central Electron Beam (Ip) 8: Extraction Voltage (Vex) 9: Total electron beam (It) 10: Reaction layer 11: Ring electrode

Claims (2)

[Claims] 1. A thermoelectric field emission cathode comprising a needle electrode made of a tungsten single crystal with a <100> orientation and a suppressor electrode provided with a zirconium and oxygen coating layer,
Zirconium weight sources zirconium and oxygen containing less 3.0x10 ^-6 g or 10x 10 ^-6 g, distal direction 200 of the needle-like electrode from the junction of the heater and the needle-like electrode μm
Point and the outer surface of the suppressor electrode. 2. 10 μm and 200 μm from the tip of the needle electrode
2. The thermoelectric field emission cathode according to claim 1, wherein the conical contour in the region up to m is a straight line or an arc shape curved inward.