Cathode and process for producing the same

Details Cathode and process for producing the same Cathode and process for producing the same

2001-10-03

2003-05-06

Patel, Ashok (Department: 2879)

Electric lamp and discharge devices

Electrode and shield structures

Cathodes containing and/or coated with electron emissive...

C313S3460DC, C313S311000, C313S352000

Reexamination Certificate

active

06559582

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a cathode operable at high temperature and a process for preparing the same. More specifically, the present invention relates to a cathode which is operable at higher temperature (for example, at least 1,400° C.) than the operational temperature for impregnated cathodes and which comprises environmentally safe material, and a process for preparing the same.
Conventionally, a cathode as shown in FIGS.
15
(
a
) and (
b
) has been used for a medium to large electron tube such as a tube for huge power supply equipment. Meanwhile, a cathode shown in FIG.
15
(
a
) has been generally used for a lamp of high power discharge tube, such as a source lamp for photolithography machines. These cathodes, operable even at high temperature of at least 1,400° C. where impregnated cathodes are inoperative, comprises a tungsten cathode
21
containing (about 2% by weight of) thorium oxide (ThO
2
) (hereinafter referred to as thoriated cathode) which is connected to an electrode
20
. Recently, impregnated cathodes are gradually applied for medium to large electron tube, since there are improvements in the degree of vacuum inside the tube and change in tube design based on environmental requirement. However, thoriated cathodes are the only practical cathode for the lamp of high power discharge tube, and cannot be easily replaced with the impregnated cathode.
Referring to the thoriated cathode, ThO
2
in tungsten W is deoxidized by tungsten or carbon C on the surface of the cathode at about 1,500 to 1,800° C., and a Th-W mono atomic layer is formed on the cathode surface. Thereby, work function of about 2.7 eV can be achieved, and electron emission characteristics of about 10 A/cm
2
can be obtained under vacuum of 10
−5
Pa at 2,000° C. The fact indicates that the electron emission characteristics is improved by about 1,000 times or more as compared with tungsten cathodes (which have work function of about 4.5 eV). However, since ThO
2
contained in the thoriated cathode is a radioactive material, strict management is required for handling. Also, there are potential health and environmental problems. Along with recent environmental approaches, there is a tendency to restrict or stop the use of thorium mainly among its providers, i.e., European countries, indicating another possible problem of lack of stable supply in future.
In addition to the thoriated cathode and the tungsten cathode, there are cathodes having a construction shown in FIG.
16
. These are used as high-intensity electron beam source for an electron beam photography machine of an electron microscope or ultra LSI micro processing. The cathode is operable at high temperature and constructed in a way that a lanthanum boride (LaB
6
) cathode
22
is connected to electrodes
20
. The cathode has metallic electrical conductivity and relatively low work function (2.68 eV). The electron emission characteristics of about 20 to 100 A/cm
2
can be obtained under vacuum of 10
−5
Pa at operational temperature of 1,600° C. In addition, the cathode has relatively high ion bombardment resistance, and the original electron emission characteristics can be easily recovered even after exposure to atmosphere. However, since LaB
6
has monocrystal structure, it is necessary to select most appropriate (
100
) or (
210
) crystal plane to draw sufficient electron emission characteristics. Relatively speaking, life time of LaB
6
is as short as 500 to 2,000 hours. This is because problems still remain with respect to the stability of LaB
6
composition. In other words, though LaB
6
is far more stable than other rare earth borides (such as YB
6
and GdB
6
), many report the problems with the stability of surface composition at high temperature. Thus, LaB
6
involves disadvantage in difficult handling due to the monocrystal structure and life time due to the stability of compound in itself.
Another but minor example is a zirconium-covered tungsten cathode
23
(monocrystal (
100
) plane) as shown in FIG.
17
. This is partially used for an electron beam photolithography machine for the micro processing of ultra LSI. In the zirconium-covered tungsten cathode, zirconium hydride is thermally decomposed in vacuum and zirconium is adsorbed on the surface of tungsten. By introducing oxygen thereafter, electric dipole moment of a Zr—O—W layer
24
is formed on the surface. This enables to reduce work function to about 2.4 eV and excellent characteristics can be achieved. As a similar construction, development of Ti—O—W (monocrystal (
100
) plane) has been reported so far. It is said that the operational temperature is about 1,500° C. and life time thereof is 5,000 hours, while vacuum of at least 10
−7
Pa is required. In any case, there are many problems such as selection of crystalline plane of tungsten monocrystal and practical reproducibility.
SUMMARY OF THE INVENTION
As mentioned above, the use of thoriated cathode operative at high temperature involves potential health and environmental problems since it contains radioactive materials. In addition, stable supply of the material is also at stake. On one hand, impregnated cathodes are generally not operable when the temperature is at least 1,400° C. And LaB
6
or zirconium-covered tungsten cathodes (monocrystal (
100
) plane) have, on the other hand, problems with handling difficulty such as plane direction adjustment, and stability.
The present invention has been carried out in order to solve the above problems. The object of the present invention is to provide a cathode which is easy to handle and harmless at the same time with a construction which is stable and capable of generating excellent electron emission characteristics even at high temperature of at least 1,400° C., and a process for preparing the same.
The cathode of the present invention comprises a polycrystalline substance or a polycrystalline porous substance of high-melting point metal and an emitter material dispersed into the polycrystalline substance or the polycrystalline porous substance in an amount of 0.1 to 30% by weight in the cathode, wherein the emitter material comprises at least one selected from the group consisting of hafnium oxide, zirconium oxide, lanthanum oxide, cerium oxide and titanium oxide.
By adopting this construction, a monatomic layer derived from hafnium oxide, zirconium oxide, lanthanum oxide, cerium oxide and titanium oxide (including Hf—W or the like without oxygen and Hf—O—W or the like through oxygen) is formed on the surface of high-melting point metal such as tungsten or molybdenum (Mo) at high operational temperature. The monatomic layer is relatively stable at high temperature, reduces work function, and serves as a cathode capable of generating excellent electron emission.
The high-melting point metal material is preferably alloy obtained by adding 0.01 to 1% by weight of Hf, Zr or Ti to tungsten or molybdenum. These added elements act as a reducing agent to improve reducing ability of the high-melting point metal element.
It is preferable to dispose a metal layer of at least one selected from the group consisting of iridium (Ir), ruthenium (Ru), osmium (Os) and rhenium (Re) at least on an electron emission surface of the polycrystalline substance or the polycrystalline porous substance. According to this, work function is further decreased.
It is also preferable to dispose a tungsten carbide layer or a molybdenum carbide layer at least on an electron emission surface of the polycrystalline substance or the polycrystalline porous substance. According to this, work function is further decreased.
Preferably, crystalline grains of the polycrystalline substance or the polycrystalline porous substance are structured fibrously in the same direction. According to this, toughness is improved and processing becomes easier. Furthermore, when carbonization takes place, a carbide layer is formed only on the outermost surface due to this high density construction.
In another embodiment of the present invention, a compound layer of at least

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