JPH1167055A - Thermoelectron emitting filament and its manufacture - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce sputtering effects and generation of whiskers to extend life by depositing a plurality of metallic films from inside to outside on the surface of polycrystal tungsten or a tungsten-rhenium alloy from the end of a filament to a supporting part, in the order of melting point, and alloying them together. SOLUTION: Using polycrystal tungsten 1 or a tungsten-rhenium alloy as a shaft, a thin film 3 of platinum and a thin film 5 of gold, both including platinum black of 500 Å or more, are formed on the shaft, with a relatively thick layer of aluminum 5 formed thereon to provide a three-layered structure. In an end thermoelectron emitting part A, the polycrystal tungsten 1 and the thin film 3 of platinum, the thin film 3 of platinum and the thin film 4 of gold, and the thin film 4 of gold and the aluminum 5, are alloyed together through slow heating. In this case, since the polycrystal tungsten is alloyed by covering with a metallic film, the tungsten is not oxidized and a filament resists breaking, leading to longer life and eliminating getter effects, so that the filament becomes usable in a low vacuum.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a thermionic emission filament which heats polycrystalline tungsten or a tungsten-rhenium alloy constituting a filament and emits thermoelectrons from the tip of the filament, and a manufacturing method.

[0002]

3 shows an example of a hairpin filament, FIG. 4 shows an example of a point filament, FIG. 5 shows an example of a sharpened filament, and FIG. 6 shows an example of a semipoint filament. And 11 is a pedestal, 1
2 denotes Kovar glass, 13 denotes a stem pole, 14 denotes a filament, and 15 denotes a chip.

[0003] Thermionic emission filaments include, for example, FIG.
There are a hairpin filament as shown in FIG. 4, a point filament as shown in FIG. 4, a sharpened filament as shown in FIG. 5, a semi-point filament as shown in FIG. 6, and a sealed tube such as a CRT or an electron microscope. It is used for open pipes such as Of these, filaments made of alloys such as barium are used for sealed tubes, while 99.95% purity is used for open tubes.
Filaments made of pure polycrystalline tungsten as described above are used.

As shown in FIG. 3A, a hairpin filament is applied to a stem pole 13 penetrating a pedestal 11.
The tip of the filament 14 having a diameter of about 150 μφ is set in a hairpin shape, a current of about 3 A is passed in a vacuum of 1 × 10 ⁻⁵ Torr or more, and the material is heated to 2600 to 2900 ° C. to emit thermoelectrons. ing. FIG. 3 shows a front view and a side view of the tip shape of the filament 14.
(B) and (C), wherein the tip whose angle is sharply turned is heated to a high temperature and emits thermoelectrons. In this case, the size of the obtained electron source is about 30 μmφ and the total current is up to 10 μm.
A current of about 0 μA is obtained.

The phenomenon of thermionic emission is described in the following W.W. Richardson
-Expressed by the Dushman equation. j = A (1-r) T ² exp [−φ / k _B T] A = 4πmk ² e / h ³ = 120 Acm ⁻² k ⁻² where j is the saturation current density, T is temperature, and φ is the work function ,
r is the zero-field reflection coefficient of incident electrons, k _B is Boltzmann's constant, and the point filament is located at the tip of the filament 14 similar to the hairpin filament as shown in FIG. 4 (A). As shown in ()), the size of the electron source is reduced by attaching a thin electron generating chip 15 and using a point cathode having a tip radius of about 1 μm.
It is used as about μm. This point filament is suitable for use with a total current of about 10 μA.

[0006]

However, in the case of the hairpin filament shown in FIG. 3 used in the form of a hairpin, the average life is 50 to 100 hours, and the point cathode shown in FIG. In this case, the average life is only 10 to 20 hours, and the life is short, so frequent replacement is required, which has a serious problem in maintenance.

Since tungsten has a large work function φ of 4.55, it is necessary to keep the temperature extremely high in order to effectively emit thermoelectrons. Therefore, the peripheral components are heated by the heat radiation and a large amount of gas is constantly released, damaging the tungsten filament and shortening its life. Although pure tungsten is a material that is fairly stable at room temperature, 280 used for thermionic emission is used.
At a temperature of about 0 ° C., the filament is very weak against oxygen and water vapor, causing the filament to break.

[0008] In the conventional filament, in addition to the thermal noise,
Noise due to the collision of positive ions of the accelerated residual gas,
There is noise due to surface gas diffusion from the pole and the support, and no consideration is given to them. Further, in the conventional filament, there is contamination of peripheral parts due to evaporation of tungsten, and much time is spent for cleaning and vacuum management. In addition, if used for a long time, the purity of tungsten becomes high, and crystal growth is promoted, so that whiskers are generated. Due to the generation of the tungsten whiskers and the erosion by the gas of the support portion, a high-pressure discharge is generated and the wire is disconnected. There is also a problem.

[0009] The life of the filament has been somewhat improved so far by improving the vacuum, but it is still not enough, and the surface of tungsten is oxidized and easily stained.
Since the conventional hot filaments that need to be frequently replaced because they cannot be stored for a long period of time, the maintenance burden such as labor and cost required for the replacement is very large.

[0010]

DISCLOSURE OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and it is intended to prevent the oxidation of polycrystalline tungsten or a tungsten-rhenium alloy, to reduce the sputtering effect and the generation of whiskers, and to extend the filament life. It becomes something.

[0011] For this purpose, the present invention relates to a thermionic emission filament for heating polycrystalline tungsten or a tungsten-rhenium alloy constituting a filament and emitting thermoelectrons from the tip of the filament, and a method for manufacturing the same. A plurality of metal films formed on the surface of the polycrystalline tungsten or tungsten rhenium alloy from the inside to the outside in ascending order of melting point and alloyed.

The metal film includes at least gold, and platinum and gold including platinum black, gold and aluminum, and a metal having a lower work function than polycrystalline tungsten or a tungsten rhenium alloy is provided on the outermost of the plurality of metal films. , Aluminum, and a gold-aluminum alloy, or a gold-aluminum-magnesium alloy, which is alloyed with a polycrystalline tungsten or a tungsten-rhenium alloy.

[0013]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a view for explaining an embodiment of a thermionic emission filament and a method for manufacturing the same according to the present invention, and FIG. 2 is a view showing a phase diagram of an aluminum-gold alloy, where 1 is polycrystalline tungsten or tungsten rhenium. Alloy, 2 is a high melting point tungsten alloy or rhenium,
3 is a platinum thin film containing platinum black, 4 is a gold thin film, 5 is aluminum, 6 is aluminum oxide, A is a thermionic emission part,
B indicates a tip function maintaining portion, and C indicates a support portion.

The thermionic emission filament according to the present invention comprises:
As shown in FIG. 1, a platinum thin film 3 containing platinum black of 500.degree.
And a layer of a gold thin film 4 and a relatively thick layer of aluminum 5 to form a three-layer structure. Note that a slight impurity layer may be formed on the surface of tungsten when left in the air, and the high melting point tungsten alloy 2 is the impurity layer. The surface of the aluminum 5 is oxidized, so that the aluminum oxide 6
Changes slowly.

In the thermoelectron emission filament thus formed, the hairpin portion is divided into two, the curved portion becomes the tip thermoelectron emission portion A, the straight portion becomes the tip function maintaining portion B, and the other portions are the support portions. C. In order to use this as a thermionic emission filament, it is heated slowly. By this heating, the tip thermoelectron emitting portion A alloys the polycrystalline tungsten 1 and the platinum thin film 3 containing platinum black, the platinum thin film 3 containing platinum black and the gold thin film 4, and the gold thin film 4 and the aluminum 5 from inside. FIG. 2 shows a phase diagram of the aluminum-gold alloy, in which the vertical axis represents temperature and the horizontal axis represents aluminum 100.
% And the right end is 100% gold. On the surface of the aluminum 5, a considerably thick film of aluminum oxide 6 of about 1 μm is formed. Since the work function φ (4.28 or less) of these aluminum 5 and aluminum oxide 6 is smaller than that of tungsten, even when used at a lower temperature than that of tungsten, the electron source is equivalent or more.

Since the temperature of the tip function maintaining section B is lower than the temperature of the tip thermoelectron emission section A, the noble metal aluminum alloy generated during heating is more oxidized than aluminum. It oxidizes slowly because it is hard to be oxidized. In the tip function maintaining section B whose temperature is lowered by the tip thermoelectron emission section A, the polycrystalline tungsten 1
Is composed of three layers of a gold-platinum alloy thin film 3, aluminum 5, and aluminum oxide 6, which are gradually moved toward the tip by thermal diffusion, and are used for replenishment of aluminum lost by oxidation. . Due to this phenomenon, the function of the tip thermoelectron emission portion A is maintained for a long time.

Since the temperature of the supporting portion C is lower than that of the tip function maintaining portion B because the temperature is lower than that of the tip function maintaining portion B, the tungsten portion covered with a metal film such as gold or platinum is not oxidized. In addition, since gold diffuses thermally against the erosion of the residual gas, it exhibits a self-healing function.

The properties of each metal constituting the three layers are as follows:
It is as follows. First, regarding the melting point, platinum is the highest at 1700 ° C., followed by 1070 ° of gold.
C and aluminum are the lowest at 670 ° C.
The formation of each layer is also related to such a melting point, and it is usually the layer of the gold thin film 4 that can be formed by either sputtering or vapor deposition. The aluminum 5 layer that can be formed only by vapor deposition instead of sputtering. Platinum does not easily emit electrons except for ultrafine platinum black, whereas gold emits electrons but less than tungsten, and aluminum emits more thermoelectrons than gold. Gold cannot be alloyed with tungsten, but platinum can be alloyed with tungsten, gold and aluminum.

By the way, in order to prevent the surface of tungsten from rusting, it is necessary to cover the surface with a metal film such as platinum or gold. However, covering with platinum has a problem in terms of electron emission. Although there is release, it does not form an alloy with tungsten, so there is a problem that when heated to 2600 to 2900 ° C., it will far exceed the melting point of gold and will melt and fly. Aluminum is also resistant to oxygen and has a work function φ smaller than that of tungsten and emits electrons, so it is desirable to cover it with aluminum, but there is a problem that the melting point is even lower. In any case, if tungsten is heated to 2600 to 2900 ° C. for thermionic emission, it will far exceed the melting point of platinum, gold and aluminum, and it is difficult to select and use only one.

According to the present invention, considering the properties of each of the above metals, the metal that is most insoluble when heated is in the middle,
The layers are laminated from the inside to the outside in ascending order of melting point, and are melted in order from the surface side to form an alloy so that it remains. As mentioned earlier, the factor that shortens the life of the filament is
There are gas collision (sputtering), oxidation, and whiskers, but in the present invention, gold and aluminum are embraced through platinum, which has a high melting point and is difficult to evaporate, so that it remains.
The above factors can be eliminated. That is, whiskers are generated when high-purity tungsten is formed. However, since tungsten forms a high melting point intermetallic compound with platinum and aluminum, whiskers cannot be formed, and discharge by the whiskers is eliminated.

The resistance value of gold is 8.6 Ω · m smaller than the resistance value of tungsten of 24 Ω · m (700 ° C.), and that of platinum is as large as 34.3 Ω · m. In consideration of the above, an alloy having a resistance value substantially equal to that of tungsten can be produced. This is an important factor which means that the device can be attached to a delivered device without a design change.

The method of use is exactly the same as in the prior art, using a current of about 5 V 3 A. When used as a Schottky, the filament can be used for a longer period of time because the filament is not thermally stressed. Since the gold-aluminum alloy at the tip has a small resistance, it is possible to reduce the generation of Joule heat during current extraction.

It should be noted that the present invention is not limited to the above embodiment, and various modifications are possible. For example, in the above-described embodiment, an example in which the invention is applied to a hairpin filament has been described. However, the invention can be similarly applied to a point filament, a sharpened filament, a semipoint filament, and other filaments. Metal structure of the support portion C Tungsten or tungsten-rhenium alloy, platinum including platinum black, gold and aluminum are common to the support portions of the cold cathode field emission gun (C-FE) and the hot cathode field emission gun (T-FE). It has a function of eliminating the getter effect of clean tungsten and stopping the surface diffusion of gas, so that it can be used at a vacuum level lower than before.

Also, if high melting point noble metals such as iridium, osmium and ruthenium are added to platinum, the high-temperature characteristics can be improved, or magnesium can be added to aluminum, and instead, any material having a work function φ lower than that of tungsten can be used. It is well known that it can be used, and such modifications can be made as appropriate. in short,
The present invention is alloyed so that the purity of the surface of tungsten does not increase, and reduces the work function φ on the entire alloyed surface, and also covers with a protective film such as an oxide film to reduce the sputtering effect. Is to do so. At that point, it is convenient to use aluminum. Alternatively, an aluminum alloy may be used instead of aluminum to be a gold aluminum alloy or a gold aluminum magnesium alloy. Further, chromium, iron, nickel or the like may be used instead of platinum or aluminum including platinum black.
Also in this case, if a plurality of metal films are laminated from the inside to the outside in the order of melting point on the surface of tungsten, they are melted from the outside when alloying, so that the surface can be finished smoothly. Further, in the above embodiment, pure gold and aluminum are used, but gold is gold wax (for example, Bau-4),
Aluminum is aluminum wax (eg, BAl-1) or nickel wax (eg, BNi-2, BN) depending on the purpose.
i-7).

[0025]

As is apparent from the above description, according to the present invention, polycrystalline tungsten is coated with a metal film and alloyed, so that tungsten is not oxidized and filaments are not cut, so that the present invention is much more remarkable than conventional products. And a long-lived thermionic emission filament can be provided. In addition, since a polycrystalline tungsten or tungsten rhenium alloy is coated with a metal film to produce a tungsten precious metal alloy, the getter effect of clean tungsten can be eliminated, and it can be used in a low vacuum, and the noise due to the gas surface diffusion coefficient can be reduced. Can be turned off. In addition, the purity of tungsten does not increase during use, so that whiskers cannot be formed, and high-pressure discharge by whiskers can be stopped. Furthermore, aluminum, aluminum oxide having a smaller work function φ than polycrystalline tungsten or tungsten rhenium alloy,
Since the thermionic emission part was manufactured, it is possible to emit thermionic electrons at low temperature, suppress the sublimation of tungsten, and the aluminum oxide containing the metal alloy of the thermionic emission part has a sputtering effect of ions accelerated by the electric field. Since it has the effect of reducing spatter noise, thermal noise can be reduced.

A tip function maintaining portion is formed for the tip thermoelectron emission portion so that the tip thermoelectron emission portion can be covered with a material having a low work function φ for a long time, and the support portion is made of an oxidation-resistant metal. Constructed of platinum and gold, and covered with a high-melting metal compound with tungsten, it can have a self-healing effect using gas surface erosion by utilizing the surface thermal diffusion of metal, and can suppress the generation of minute discharge. .

The filament according to the present invention can be used as an industrial source as an inexpensive electron source for a low vacuum scanning electron microscope, an X-ray microanalyzer for long-term quantitative measurement, ESCA, and an electron beam lithography apparatus for long-time exposure for large wafers. The meaning is great.

[Brief description of the drawings]

FIG. 1 is a view for explaining an embodiment of a thermionic emission filament and a method of manufacturing the same according to the present invention.

FIG. 2 is a diagram showing a phase diagram of an aluminum-gold alloy.

FIG. 3 is a diagram showing an example of a hairpin filament.

FIG. 4 is a diagram showing an example of a point filament.

FIG. 5 is a diagram showing an example of a sharpened filament.

FIG. 6 is a view showing an example of a semi-point filament.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Polycrystalline tungsten or tungsten rhenium alloy, 2 ... High melting point tungsten alloy, such as WC, 3 ... Platinum thin film containing platinum black, 4 ... Gold thin film, 5 ... Aluminum, 6 ...
Aluminum oxide containing various metals, A: tip thermoelectron emission section, B: tip function maintaining section, C: support section

Claims (10)

[Claims] 1. A thermionic emission filament that heats polycrystalline tungsten or a tungsten-rhenium alloy constituting a filament and emits thermoelectrons from the tip of the filament. A thermionic emission filament characterized in that a plurality of metal films are laminated on the surface of a tungsten rhenium alloy from the inside to the outside in ascending order of melting point and alloyed. 2. The thermionic emission filament according to claim 1, wherein the plurality of metal films include at least gold. 3. The thermionic emission filament according to claim 2, wherein the metal film is made of platinum containing platinum black, gold and aluminum. 4. The thermionic emission filament according to claim 1, wherein a metal or a metal oxide having a work function smaller than that of polycrystalline tungsten or a tungsten-rhenium alloy is laminated on the outermost of the plurality of metal films. 5. The method according to claim 4, wherein aluminum is laminated on the outermost side as a metal having a work function smaller than that of polycrystalline tungsten or a tungsten-rhenium alloy.
A thermionic emission filament as described. 6. Thermionic emission filament according to claim 1, wherein a gold aluminum alloy is laminated and alloyed with a polycrystalline tungsten or tungsten rhenium alloy. 7. The thermionic emission filament according to claim 1, wherein a gold aluminum magnesium alloy is laminated and alloyed with a polycrystalline tungsten or tungsten rhenium alloy. 8. A method for producing a thermionic emission filament which heats polycrystalline tungsten or a tungsten-rhenium alloy constituting a filament and emits thermoelectrons from the tip of the filament. A method for manufacturing a thermionic emission filament, comprising laminating a plurality of metal films on the surface of crystalline tungsten or a tungsten-rhenium alloy from the inside to the outside in ascending order of melting point. 9. The method for producing a thermionic emission filament according to claim 8, wherein at least gold and aluminum are laminated. 10. The method according to claim 8, wherein platinum having at least a surface of platinum black, gold and aluminum are laminated.
A method for producing the thermionic emission filament according to the above.