JP2001516128A - Cathode made of getter material - Google Patents

Abstract

  (57) [Summary] The cathode for a vacuum electronic device is made of a material that acts as a getter. The cathode may include a mixture of getter material and diamond powder.

Description

DETAILED DESCRIPTION OF THE INVENTION

TECHNICAL FIELD The present invention relates to the field of cathodes, and more particularly, to cathodes used in vacuum electronic applications.

BACKGROUND ART In electronics, cathodes are required for many different applications. A cathode is an electrode into which electrons enter a system such as an electrolytic cell or an electron tube. Cathodes include X-ray equipment, flat panel display systems, microwave sources, radar,
It is also used in communications equipment, high-power high-speed switches, electron beam processing of materials, steep acceleration electrodes, and many other applications.

[0002] Cathodes are generally divided into four types. That is, a thermionic cathode, a laser-excited photocathode, a field emission cathode, an explosion or plasma field emission cathode. The present invention relates to field emission cathodes used in vacuum applications.

[0003] Vacuum field emission cathodes are Fo that penetrate electrons from near the Fermi level to vacuum.
An electron beam is generated according to a specified amount of Wler-Nordheim. Relatively large electric fields are required compared to other cathode types. The required large electric field is
It results from the enhancement of the applied electric field due to surface irregularities.

One example of a device that employs a field emission cathode is a small X-ray device. Such X-ray devices are designed for use in the body and the cathode operates in a vacuum chamber. Obviously, minimal X-ray equipment is required for use in living organisms. The minimal device is easily guided to the treatment site. It is important to minimize occlusion of blood vessels to allow maximum blood flow. Thus, there is a need for small devices to combine multiple components to reduce overall size requirements.

[0005] Flat panel displays also require a small effective cathode in a vacuum environment, and field emission cathodes may be used for flat panel displays. It will be appreciated that there is a need for an effective vacuum field emission cathode that can be used for space-sensitive applications.

SUMMARY OF THE INVENTION The present invention is directed to a cathode for a vacuum electronic device, the cathode comprising a material acting as a getter. Any feature of the cathode surface allows it to function as a cathode and getter. The cathode preferably has a surface that is smooth or granular.

[0007] An additional embodiment of the present invention is a cathode for a vacuum electronic device, comprising a diamond powder mixed with a getter material, the cathode being made of a material that is capable of acting as a getter. Another additional embodiment is an X-ray device configured with a cathode acting as a getter. The X-ray device includes a small vacuum housing with an anode and a cathode-getter located inside. The X-ray device may further include an electrical connector for generating an electric field between the anode and the cathode.

BEST MODE FOR CARRYING OUT THE INVENTION A cathode can be applied to various devices, and a manufacturing method thereof employs a field emission cathode. The present invention is particularly advantageous in an apparatus using a field emission cathode used in a vacuum. Examples of vacuum electronics utilizing electron radiation include vacuum tubes, flat panel displays, and microwave tube generators. While the present invention is not so limited, an appreciation of the various concepts of the present invention is best obtained through a review of applications that operate in such an environment.

One example of such an application is a miniature X-ray device suitable for insertion into the body for local X-ray emission. Such an apparatus includes a miniature vacuum housing and an anode and a cathode. The cathode also acts as a getter housed within the housing. For use in the body, such as in a body conduit, the device includes a connector to the anode and cathode. The connector generates an electric field between the anode and the cathode. The device with the connector may be placed in a cathode for insertion into the body. Typically, the miniaturized design of the device is less than or equal to about 5 mm, preferably less than or equal to 3 mm, particularly preferably less than or equal to 1-2 mm (e.g., cylindrical, oval, oblong shaped devices). Give the outer cross-sectional dimensions. Such dimensions allow access to small body conduits. The connector is
High voltage can be applied while maintaining a small structure suitable for a small body conduit in an insulated state.

The goal of any electron emitting element, particularly the miniaturized X-ray device described above, is, for example,
It is effectively increased by lowering the required electric field. Small irregularities on the surface of the cathode cause an increase in the magnitude of the electric field with respect to the applied voltage, thereby increasing the chance of electrical flashover. The weaker the required electric field at the cathode, the more the imperfections of the cathode surface are tolerated without the risk of flashover.

According to the invention, the getter acts as an electron emitter when a potential difference is applied. Thus, getters may be used as cathodes. This combination of elements
Create smaller devices with a very simple design.

According to the invention, the cathode made of a metallic material does not require any further requirements for emitting electrons in a moderate electric field for X-ray applications. Often, additional coatings such as diamond are applied to the surface of the cathode to improve the cathode field emission characteristics. The physical properties of the cathode surface may be arbitrary so that the cathode functions in both capacities. This property ranges from smooth to granular. A smooth surface cathode promotes uniform emission of electrons and avoids high local electric field densities. The granulated surface of the cathode provides a number of fine irregularities that allow efficient field emission of electrons at moderate electric fields.

As mentioned above, diamond coatings are sometimes used on the surface of the cathode to improve field emission performance. The diamond coating may be formed by a laser sputtering technique as described in U.S. Pat. No. 4,987,007, issued Jan. 22, 1991, invented by Wagal. Laser sputtering techniques create diamond-like carbon or amorphous diamond. Amorphous diamond is less stable when exposed to heat from about 500 ° C. to 600 ° C. or more than other types of diamond, such as natural diamond, high pressure high temperature diamond or high quality CVD diamond. Therefore, if the cathode is subjected to a high temperature treatment, it is preferable to use this type of diamond for the cathode. For example, the resistance of amorphous diamond is affected after being heated from about 500 ° C to 600 ° C. Natural diamond, high pressure high temperature diamond and high quality CVD diamond exhibit more stable thermal behavior.

The present invention can improve the field emission by incorporating these diamond materials into the cathode material. FIG. 7 shows a general process for forming the cathode of the present invention. Granular diamond consisting of natural diamond, high pressure high temperature diamond and high quality CVD diamond may be mixed with getter material according to one embodiment of the present invention shown in FIG. Next, the mixed material is annealed in a vacuum furnace. Acceptable annealing methods include: mixing the cathode material 20; placing the mixed material in the cathode mold 22; and thermally bonding the mold 24 at a temperature of from about 1000 ° C. to 1200 ° C. for about 1 hour; Removing 26 the cathode from the mold.

Because the getter material is brittle, a step 28 of attaching the cast getter to the base is preferred. In addition, the cathode must be placed in an environment that is as clean as possible. Dust contamination must be avoided.

Cathodes with diamond films exhibit different emission characteristics for different geometrical shapes of the coating. Therefore, precision manufacturing techniques are used to obtain the desired cathode shape. This method is expensive, difficult and time consuming. In contrast, the field emission characteristics of the present invention can be adjusted by changing the concentration of the diamond material. The concentration of diamond in the cathode is easier to adjust than the diamond coating geometry.

A typical cathode is formed to be spaced from the anode. A triode shape may be used if there is enough space in the device to allow for a third or gate electrode. The third electrode may be located beside the cathode, anode-cathode current and anode-
Allows independent control of cathode voltage. Thus, more changed performance characteristics become apparent from the cathode when the gate electrode is used. An example of the gate electrode 10 is shown in FIG. The cathode 14 is arranged in a space inside the gate electrode 10. The emitted electrons enter the anode 12. This type of gate electrode is particularly relevant for flat panel display applications where the anode 12 is a screen.

Using a granular getter material, a field emission density of about 0.5-5 mA / mm ² was observed on the cathode in an electric field of 10 −15 V / μm. The level of this emission current is X
Similar to that found in moderately working diamond coated cathodes for wire production. However, devices that use granular getter materials as cathodes are not very complicated to manufacture. The cathode may be formed by thermal diffusion bonding of a powder getter material or, if a granular surface is required, by use of a getter powder having a granule size of 0.5-50 μm in diameter. . A smooth cathode surface can be achieved by the use of getter material and mixing with irregular diamonds where the granules protrude over the entire surface of the cathode.

According to one embodiment of the present invention, low field X-ray radiation is generated from a cathode created by mixing diamond powder and a granular getter material prior to forming the cathode of the present invention. Diamond materials exhibit variable properties as field emitters and easily lose electrons when an electric field is applied. When diamond powder is included in the cathode, an electric field of 10-50 V / μm generates a current density in the range of 1-10 mA / mm ² .

[0020] Diamond material can be purchased on the market from a number of sources. For example, the diamond powder material may be obtained from a CVD process, although it may be made from natural diamond or high pressure diamond.

The getter cathode 14 may be made from many different types of getter materials. The getter may include zirconium, aluminum, vanadium, iron and / or titanium. In one embodiment, the getter material may be made from an alloy including vanadium, iron, and zirconium. One successful choice of getter cathode is
SAES Getters in Gallarate 215, Milan 20151, Italy
S. p. A. Through ST. Getter alloy ST707 is made by thermal diffusion bonding and has 24.6% vanadium and 5.
It is made of 4% iron and 70% zirconium. The getter is, for example, 10
It is a sufficient conductor of electron emission capable of acting as an effective cathode in a moderate electric field of -60 V / μm.

Before the getter is operated, it is coated with a passivation layer such as an oxide. The oxide film shields the getter material from the atmosphere in a clean state. When the getter is heated to operating temperature in vacuum, the oxide diffuses inside the getter, revealing an active getter surface,
This reacts with the molecule and binds. In a vacuum, the active getter surface reacts with the most suspended molecules, binding the molecules to the getter, thereby improving the amount of vacuum. SA
The ES ST707 alloy getter has an activation temperature of 400-500C.

The shape of the cathode affects the electron emission of the cathode and may be selected to be most suitable for a particular cathode application. In one embodiment of the present invention, the field emission characteristics of the cathode may be modified by an adjustment step 30 that changes the surface of the cathode to achieve predetermined field emission parameters. For example, in spark adjustment, multiple applications of high voltage are performed at different distances and at relative positions of the electrodes, causing an electrical discharge between the electrodes. The discharge can remove unstable radiation spots on the cathode surface and can drive field emission parameters to a desired range. The adjustment must not eliminate all electron emission points. However, for conical cathodes, radiating points located away from the top, more than 40-45 ° off the central axis of the cathode, may be eliminated to reduce the risk of flashover. .

The adjustment of the cathode is described in Academic Press, R.A. V. Lath
Am, "High Voltage Vacuum Insulation: Basic Considerations and Technical Implementation". For example, current regulation involves energizing the cathode and a series of resistors. The applied voltage is increased in a small step so that the "pre-breakdown" current of the cathode is stabilized before processing. The regulation procedure typically continues until the intended operating voltage is reached.

As an example of an anode, “glow discharge” conditioning involves sputtering of low energy gas ions to remove impurities from the cathode surface. A low voltage AC glow discharge allows an alternating current to occur between the cathode and anode by increasing the pressure in the vacuum chamber while being applied to the cathode and anode.

Yet another example is gas conditioning, which involves progressively increasing the electric field in the gap near the cathode at a current of a few microamps and a pressure of about −10 ⁻⁵ mbar. The discharge current is allowed to extinguish for 20 minutes to their asymptotic limit. The procedure is repeated at increasing electric field levels until the intended operating electric field of the cathode is reached.

As a final example, a spark adjustment known as “spot knocking” may be used. Although a reduced series of resistances is necessary to increase the rate of energy dissipation within the spark, the method may be implemented substantially as a current regulation method. In using this method, the external capacitance associated with the gap must be minimized so that only a limited amount of energy (≦ 10 J) is lost in the gap during sparking. A more subtle form of spark conditioning involves using a discharge in the nanosecond range under ultra-high vacuum conditions.

[0028] Current regulation may preferably be used to improve the cathode in one embodiment of the present invention. Slow application of the voltage associated with the current adjustment is the most preferred method of adjusting the cathode of the present invention. As mentioned above, current regulation involves a slow increase in the application of voltage across the cathode and anode. The voltage application starts at about 1 kV / mm and is gradually increased to about 60 kV / mm, possibly to 100 kV / mm. The procedure for adjusting one cathode takes about 30 minutes. The conditioning procedure may be performed before the cathode is assembled to the housing.

The slow application of gradually increasing the voltage dissolves the fine irregularities that appear on the cathode to smooth the surface. The degree of smoothing may be controlled such that some rounded micro-roughness is maintained, or such that the surface actually lacks such micro-roughness. In this way, the sharpest voltage-emitting fine irregularities are thermally dulled, following the excessive electron emission provided by the current regulation. FIG. 1 shows a cathode 14-1 formed with a granular material. When a voltage is applied to the cathode 14-1 in FIG. 1, an extremely large electric field is formed on sharp irregularities. The electron emission becomes very large at these locations, causing overhanging and melting of the fine irregularities. Before the micro-roughness melts, a very large current is generated, causing a spike in the emission curve. FIG. 3 shows a linear application of voltage to the anode and cathode. When the voltage of FIG. 3 is applied to the anode and cathode configurations of FIG. 1, the current shown in FIG. 4 results. There are many spikes in the current plot of FIG.

After adjusting the surface of the cathode 14-1, the surface of the cathode 14-2 shown in FIG. 2 results. Sharp spikes are smoothed. If the cathode 14-2 is used at about 20 keV, the adjustment may be about 0-200 V to ensure a smooth rate of electron emission at the operating voltage.
Implemented at 25 keV. FIG. 5 shows a plot of current versus time for the cathode of FIG. 2, assuming the voltage application shown in FIG. When the cathode is operated to generate a current of 100 mA, it is preferable to regulate the cathode with a current of about 200-300 mA.

It is preferable to balance the effect of the regulation, which improves the reproducibility and stability of the emitted current and the desired effect of the fine irregularities, which reduces the electric field required for the emission. The smooth surface shown in FIG. 2 is an example of how this equilibrium was achieved. In performing this adjustment method, the test cathode is used to determine the exact time and adjustment method, and these parameters apply to other adjustment methods.

The materials mixed to form the cathode are about 0.5-50 μm, especially 5-15 μm.
Contains getter material in the form of granules of μm. This material may include diamond granule material of approximately the same granule size. Diamond powder is about 0.5-20wt
% Of the cathode material. More specifically, the diamond powder is made up about 1-10 wt% of the cathode material.

[Brief description of the drawings]

FIG. 1 is a side view of one embodiment of a cathode according to the present invention.

FIG. 2 is a side view of another embodiment of the cathode according to the present invention.

FIG. 3 is a graph showing the time of the applied voltage plot versus the current plot shown in FIGS. 4 and 5 for the cathode of the present invention.

FIG. 4 is a plot of current versus time for the cathode of FIG.

FIG. 5 is a plot of current versus time for the cathode of FIG.

FIG. 6 is a perspective view of another embodiment of a cathode and a gate electrode according to the present invention.

FIG. 7 is a block diagram of a manufacturing process of an embodiment according to the present invention.

[Procedural Amendment] Submission of translation of Article 34 Amendment of the Patent Cooperation Treaty

[Submission date] February 17, 2000 (2000.2.17)

[Procedure amendment 1]

[Document name to be amended] Statement

[Correction target item name] Claims

[Correction method] Change

[Correction contents]

[Claims]

13. The cathode, wherein the getter material is attached directly to the conductor material so as to apply an electric field across said cathode.

[Procedure amendment]

[Submission date] November 20, 2000 (2000.11.20)

[Procedure amendment 1]

[Document name to be amended] Drawing

[Correction target item name] Fig. 7

[Correction method] Change

[Correction contents]

FIG. 7

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Claims (17)

[Claims] 1. A cathode for a vacuum electronic device, comprising a material acting as a getter. 2. The cathode of claim 1, wherein said material comprises diamond powder. 3. The cathode according to claim 1, wherein the surface of the cathode is smooth or granular. 4. The cathode according to claim 1, wherein the surface of the cathode is granular, and has a large number of irregularities that allow electron field emission when an electric field is applied to the cathode. 5. The cathode of claim 3, wherein said cathode surface is granular and comprises a granular material having a granular size in the range of about 0.5-50 μm. 6. The cathode of claim 1, wherein said cathode is formed by thermal diffusion bonding of a getter material. 7. The cathode according to claim 1, wherein said material is selected from the group consisting of vanadium, iron, titanium, aluminum, and zirconium. 8. The cathode according to claim 1, wherein said cathode is an alloy comprising vanadium, iron and zirconium. 9. A cathode for a vacuum electronic device, comprising a mixture of a material acting as a getter and diamond powder. 10. The cathode according to claim 9, wherein the surface of the cathode is granular, and has a large number of irregularities that allow electron field emission when an electric field is applied to the cathode. 11. A cathode for a vacuum electronic device, the cathode comprising a material acting as a getter, wherein the cathode is prepared by adjustment. 12. The cathode according to claim 11, wherein the surface of the cathode is smooth or granular. 13. The cathode of claim 11, wherein the adjustment comprises spark adjustment by applying a high voltage multiple times to the cathode at different locations. 14. The cathode according to claim 11, comprising current adjustment by applying a current to the cathode across the anode and the cathode. 15. The cathode according to claim 11, wherein the surface of the cathode is smooth. 16. An X-ray apparatus comprising a small housing, in which an anode and a cathode made of a material acting as a getter are arranged. 17. A connector for connecting the anode and the cathode,
An X-ray apparatus according to claim 16.