WO1999009580A1

WO1999009580A1 - Cathode from getter material

Info

Publication number: WO1999009580A1
Application number: PCT/US1998/017064
Authority: WO
Inventors: Victor I. Chornenky; Dale L. Schreiner
Original assignee: Xrt Corp.
Priority date: 1997-08-18
Filing date: 1998-08-18
Publication date: 1999-02-25
Also published as: AU8911898A; EP1005702A1; JP2001516128A

Abstract

A cathode for a vacuum electronic device is described which is made of a material that also allows the cathode to act as a getter. The cathode may include a mixture of a getter material and a diamond powder.

Description

CATHODE FROM GETTER MATERIAL

Field Of The Invention The invention relates to the field of cathodes, and more particularly, cathodes that are used for vacuum electronic applications. Background Of The Invention

In the electronic arts, cathodes are required for many diverse applications. A cathode is an electrode by which electrons enter a system, such as an electrolytic cell or electron tube. Cathodes are also employed in x-ray devices, flat panel display systems, microwave sources, radar, communications, high power fast switches, electron beam processing of materials, high gradient accelerators, and many other applications.

Cathodes are generally divided into four types: thermionic cathodes, laser driven photo-cathodes, field emission cathodes, and exploding or plasma field emission cathodes. The present invention relates to field emission cathodes, which are used in vacuum applications.

Vacuum field emission cathodes produce an electron beam by Fowler- Nordheim quantum tunneling of electrons from near the Fermi level into the vacuum. A relatively large electric field is required compared to other cathode types. The large electrical field that is required can be obtained from enhancements of the applied field due to surface irregularities.

One example of a device that may employ a field emission cathode is a miniature x-ray device. Such x-ray device is designed for use inside a body, and the cathode operates inside a vacuum chamber.

It will be appreciated that the smallest possible x-ray device is needed for use inside living beings. A smaller device will be more easily guided to the site of treatment. It is also important to minimize the occlusion of the blood vessel, in order to allow blood flow to the greatest extent possible. Therefore there is a need in small devices to combine multiple components in order to reduce the overall size requirements. Flat panel displays also require small, effective cathodes 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 in applications that are sensitive to space restraints.

Summary of the Invention The present invention is directed to a cathode for a vacuum electronic device, the cathode is composed of a material that also allows the cathode to act as a getter. Any form of the cathode surface will allow it to function as a cathode and getter. The cathode preferably has a surface that may be smooth or granular.

One additional embodiment of the invention is a cathode for a vacuum electronic device, the cathode composed of a material that also allows it to act as a getter and containing diamond powder mixed with a getter material. Another additional embodiment is an x-ray device constructed with the cathode which also acts as a getter. The x-ray device includes a miniature vacuum housing with an anode and the cathode-getter disposed inside. The x-ray device may also include an electrical connector for generation of an electric field between the anode and cathode.

Brief Description of the Drawings The invention may be more completely understood in consideration of the detailed description of various embodiments of the invention which follows in connection with the accompanying drawings. Fig. 1 is a side view of one embodiment of a cathode of the present invention.

Fig. 2 is a side view of another embodiment of a cathode of the present invention.

Fig. 3 is a plot of voltage applied versus time for the current plots shown in Fig. 4 and Fig. 5, for the cathode of the present invention.

Fig. 4 is a plot of current versus time for the cathode of Fig. 1. Fig. 5 is a plot of current versus time for the cathode of Fig. 2. Fig. 6 is a side view of another embodiment of a cathode and gate electrode of the present invention. Fig. 7 is a block diagram of a manufacturing process of one embodiment of the present invention. While the invention is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example in the drawings and will be described in detail. It should be understood, however, that the intention is not to limit the invention to the particular embodiments described. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.

Detailed Description of the Various Embodiments The cathode is applicable to a variety of devices, and methods of fabrication thereof, which employ field emission cathodes. The invention is particularly advantageous in devices using field emission cathodes in vacuum uses. Examples of vacuum electronics that utilize electron emission include vacuum tubes, flat-panel displays, and microwave tube generators. While the present invention is not so limited, an appreciation of various aspects of the invention is best gained through a discussion of application examples operating in such environments. One such application example is a miniature x-ray device which is suitable for insertion into a body to deliver localized x-ray radiation. Such a device includes a miniature vacuum housing and an anode and cathode, which also acts as a getter, contained within the housing. For use within a body such as within vessels of the body, the device also includes a connector to the anode and cathode. The connector enables generation of an electric field between the anode and cathode. The device with its connector may be disposed within a catheter for insertion into the body. Typically, the miniaturized design of the device provides an outer cross-sectional dimension of the device (for example, the cross-sectional diameter of a cylinder, ellipse or spheroid shaped device) of less than or equal to about 5 millimeters, preferably no more than about 3 millimeters and especially preferably no more than about 1-2 millimeters. Such dimension enables access to the small vessels of the body. The connector preferably is capable of carrying high voltage under insulated conditions yet maintaining a miniature construction suitable for the small vessels of the body. A goal for any electronic emission component, and especially for the above described miniaturized x-ray device, is increased efficiency, for example, by reducing the required electric field. Small irregularities on the surface of the cathode result in an increase in the magnitude of the electrical field for an applied voltage, thereby increasing the chance of electrical flashover. The weaker the required electrical field at the cathode, the more imperfections can be tolerated on the cathode surface without risking flashover.

Another goal in many electronic devices and especially for the above- described miniature x-ray device is to maintain a vacuum condition within the volume surrounding the cathode. In order to create the vacuum, frequently, the components are assembled within a vacuum or the chamber is pumped out by conventional methods. Further, a getter may also be disposed within the volume. The getter has an activation temperature and once activated it will react with stray gas molecules in the vacuum. The getter eliminates stray gas molecules, improving the quality of the vacuum within the volume.

In accordance with this invention, a getter may serve as an electron emitter when a voltage differential is applied. Therefore, the getter may be used as the cathode. This combination of elements results in a smaller electronic device with a very simple design.

According to the invention, a cathode made of metal materials may not require further processing in order to emit electrons at moderate electrical fields for x-ray applications. Frequently, an additional coating, such as diamond, is used on the surface of a cathode to improve the cathode's field emission characteristics. The physical characteristic of the cathode surface may be any that will allow the cathode to function in both capacities. This characteristic will range from smooth to granular. A smooth surfaced cathode promote uniform emission of electrons and avoid high local field density. A granulated surface of the cathode provides multiple microprotrusions capable of efficient field emission of electrons at moderate electrical fields.

As mentioned above, a diamond coating is sometimes used on the surface of a cathode to improve field emission qualities. This diamond coating may be produced by a laser sputtering technique, as described in U.S. Patent 4,987,007, issued January 22, 1991, invented by Wagal. The laser sputtering technique produces diamond-like carbon, or amorphous diamond. Amorphous diamond is less stable when exposed to heat above about 500°C to 600°C than other types of diamond, such as natural diamond, high pressure high temperature diamond or high quality CVD diamond. Therefore, it is preferable to use these types of diamond in a cathode, if that cathode is to be subjected to a high temperature process. For example, the resistivity of amorphous diamond is affected after being heated to 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 is able to use the more stable types of diamond to improve field emission qualities by incorporating these diamond materials into the cathode material. Fig. 7 shows the general steps of forming the cathode of the present invention. Granular diamond from natural diamond, high pressure high temperature diamond or high quality CVD diamond may be mixed in with getter material, according to one embodiment of the present invention in step 20 of Fig. 7. Then the mixed material is annealed in a vacuum furnace. An acceptable annealing procedure can include the steps of mixing the cathode material, filling cathode molds with the mixed material in step 22, thermally bonding the molds at approximately 1000°C to 1200°C for about one hour in step 24, and removing the cathodes from the molds in step 26.

It is desirable to attach a base to the getter after molding, step 28, because the getter material is fragile. Further, the cathode should be kept in as clean an environment as possible. Dust contamination should be avoided.

Cathodes with diamond films may exhibit different emission characteristics for different geometrical configurations of the coating. Therefore, precise masking techniques are used to achieve the desired cathode configuration. This process is expensive, difficult and time-consuming. In contrast, the field emission characteristics of the present invention may be adjusted by changing the concentration of diamond material. The concentration of diamond in a cathode is simpler to adjust than the geometrical configuration of diamond coatings.

A typical cathode is configured to be spaced apart from an anode. This configuration is called a diode. A triode configuration may also be used when there is enough space in the device to permit a third or gate electrode. The third electrode may be positioned near the cathode and allows independent control of the anode- cathode current and the anode-cathode voltage. Therefore, more varied performance characteristics may be elicited from a cathode when a gate electrode is used. One example of a gate electrode 10 is shown in Fig. 6. The cathodes 14 are positioned in spaces in the gate electrode. The emitted electrons are incident on the anode 12. This type of gate electrode may be particularly relevant to flat panel display applications where the anode 12 will be the screen.

Using granular getter materials, field emission current densities of approximately 0.5-5 milliamps per square millimeter are observed at the cathode at electrical fields of 10-50 volts per micron. This level of emission current is similar to that found in diamond-coated cathodes operating at moderate electrical fields for x-ray production. However, a device using a granular getter material as the cathode is considerably less complicated to manufacture. The cathode may be formed by thermal diffusion bonding of powdered getter material, or if a granulated surface is desired, use of getter powder having granular sizes of .5 to 50 micrometers in diameter. A smoother cathode surface may be achieved by using conditioning processes and coating processes. A granulated cathode surface may be achieved by use of granulated getter material and its mixture with diamond in proportions that allow the granules to protrude above the overall surface of the cathode.

In accordance with one embodiment of the invention, low-electrical field x- ray radiation is produced from a cathode made by mixing diamond powder with the granulated getter materials before forming the cathode of the present invention. Diamond materials display valuable properties as field emitters, losing electrons easily as a field is applied. Where a diamond powder is included in the cathode, an electrical field of 10-50 volts per micron will produce current densities in the range of 1 - 10 milliamps per square millimeter.

The diamond material may be purchased commercially from numerous sources. For example, diamond powder material may be obtained from CVD processes, although it may be produced from natural beds of diamond or high temperature high pressure diamond. The getter cathode 14 may be composed of many different types of getter materials. The getter may include zirconium, aluminum, vanadium, iron, and/or titanium. In one embodiment, the getter materials may be composed of an alloy including vanadium, iron and zirconium. One successful choice for the getter cathode is a material produced by SAES Getters, S.p.A., via Gallarate 215, 20151 Milano, Italy and referred to as ST707. Getter alloy ST707 is produced by thermal diffusion bonding and is composed of 24.6% vanadium, 5.4% iron, and 70%) zirconium. The getter will be sufficiently conductive and capable of electron emission to serve as an effective cathode at moderate electrical fields, for example, 10 volts per micron to 60 volts per micron.

Before the getter is activated, it is covered with a passivating layer, such as a layer of oxidation, that shields the getter material from the atmosphere at normal conditions. When the getter is heated to an activation temperature in a vacuum, the oxidation layer diffuses into the interior of the getter, revealing the active getter surface, which will react and bond with molecules. Under vacuum conditions, the active getter surface reacts with most stray gas molecules and bonds them to the getter, thereby improving the quality of the vacuum. The SAES ST707 alloy getter has an activation temperature of 400-500°C.

The shape of the cathode affects the electron emission qualities of the cathode, and may be chosen to best suit the particular cathode application.

In one embodiment of the invention, the field emission properties of the cathode may be modified by a conditioning procedure in step 30 that alters the surface of the cathode to achieve predetermined field emission parameters. For example, in spark conditioning, multiple applications of high voltage are carried out at different distances and relative positions of the electrodes, causing electrical discharges between electrodes. The discharges can remove unstable emitting sites at the cathode surface and drive the field emission parameters into the desired range. Conditioning should not eliminate all electron-emitting sites. However, for conical cathodes, emitting sites that are located far way from the tip, such as more than 40- 45° off of the central axis of the cathode, may be eliminated to reduce the risk of flashover. A conditioned cathode is capable of more consistent performance.

Conditioning of the cathode may be carried out according to the concepts set forth in High Voltage Vacuum Insulation: Basic Concepts and Technological Practice, R.V. Latham, Editor, Academic Press, 1995. For example, current conditioning includes applying a voltage over the cathode and a series resistor. The applied voltage is increased in small steps such that a "pre-breakdown" current in the cathode stabilizes before proceeding. The conditioning procedure is typically continued until the intended operating voltage is reached.

As another example, "glow-discharge" conditioning includes using the sputtering action of low-energy gas ions to remove contaminants from the cathode surfaces. A low- voltage AC glow discharge is allowed to occur between the cathode and an electrode by raising the pressure in the vacuum chamber while alternating current is provided to the cathode and the electrode.

Yet another example is gas conditioning, which includes progressively increasing the field at the gap near the cathode at currents of a few microampere and a pressure of about -10^"5 mbar. The emission currents are allowed to quench for a 20 minute period to their asymptotic limit. The procedure is repeated at increasing field levels until the intended operating field of the cathode is reached.

As a final example, spark conditioning may be used, also known as "spot- knocking". The method may be carried out essentially as the current conditioning method; although a reduced series resistor may be necessary to increase the rate of dissipation of energy in the spark. In using this method, the external capacitance associated with the gap should be minimized so that only a limited amount of energy (< 10 J) is dissipated in the gap during sparking. A more sophisticated form of spark conditioning includes using discharges in the nanosecond range under ultra high vacuum conditions.

Current conditioning may preferably be used to improve a cathode in one embodiment of the present invention. The slow application of voltage involved in current conditioning may be the most preferable method for conditioning the cathode of the present invention. As discussed above, current conditioning involves slow increases in the application of voltage across the cathode and an anode. Voltage application may begin at about 1 kilovolt per millimeter and gradually increase to about 60 kilovolts per millimeter, and perhaps as high as 100 kilovolts per millimeter. The voltage application may increase in increments of 1 kilovolt per millimeter in one embodiment of the present invention. The procedure for conditioning one cathode may take about thirty minutes. The conditioning process may be carried out before the cathode is assembled with the housing. The slow application of increasing voltage gradually melts the microprotrusions present on the cathode so as to smooth the surface. The degree of smoothing may be controlled so that some rounded microprotrusions are maintained or the surface is practically devoid of such microprotrusions. In this manner, the sharpest field-emitting microprotrusions may be thermally blunted following excessive electron emission brought on by the current conditioning. Fig. 1 shows a cathode 14-1 surface as formed with granular materials. When a voltage is applied to cathode 14 of Fig. 1, an extremely large electrical field is formed at the sharp microprotrusions. The electron emissions is also very large at these locations, resulting in the overheating and melting of the microprotrusions. Before the microprotrusion melts, a very large current is generated, causing spikes 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 configuration of Fig. 1, the current shown in Fig. 4 results. There are many spikes in the current plot of Fig. 4.

After conditioning the cathode 14-1, the cathode 14-2 shown in Fig. 2 results. The sharp spikes are smoothed out. If the cathode 14-2 is intended for use at about 20 keV, then conditioning may be carried about from 0 keV to 25 keV to ensure a smooth rate of electron emission at the performance voltage. Fig. 5 shows a plot of current versus time for the cathode of Fig. 2, assuming the voltage application shown in Fig. 3. If the cathode will be operated to produce a current of 100 microamps, then it may be desirable to condition the cathode at currents of about 200 to 300 microamps.

It is preferred to balance the effect of the conditioning, which improves the reproducibility and stability of the current emitted, and the desired effect of the microprotrusions, which reduces the electrical field required for emission. The smoother cathode surface shown in Fig. 2 is one example of how this balance is achieved. In implementing this conditioning process, test cathodes may be used to determine the exact time and method of conditioning, and these parameters will be applied to other conditioning processes.

The material mixed to form the cathode will include getter material in granular shapes of about .50 to 50 microns, or more particularly about 5 to 15 microns. This material may include diamond granular material of approximately the same granular size. The diamond powder may be present in the cathode material at about 0.5% to 20%) by weight. More particularly, the diamond powder may make up approximately 1%> to 10% of the cathode material by weight. The various embodiments described above are provided by way of illustration only and should not be construed to limit the invention. Those skilled in the art will readily recognize various modifications and changes which may be made to the present invention without strictly following the exemplary embodiments and applications illustrated and described herein, and without departing from the true spirit and scope of the present invention which is set forth in the following claims.

Claims

WE CLAIM:

1. A cathode for a vacuum electronic device, the cathode being comprised of a material that also allows the cathode to act as a getter.

2. The cathode of claim 1, wherein the material includes diamond powder.

3. The cathode of claim 1, wherein its surface is smooth or granular.

4. The cathode of claim 1 , wherein the cathode surface is granular and provides multiple protrusions capable of field emission of electrons when an electrical field is applied to the cathode.

5. The cathode of claim 3, wherein the cathode surface is granular and is formed from a granular material including granular sizes in a range of about 0.5 to 50 micrometers .

6. The cathode of claim 1 , wherein the cathode is formed by thermal diffusion bonding of getter material.

7. The cathode of claim 1 , wherein the material is selected from the group consisting of vanadium, iron, titaninm, aluminum and zirconium.

8. The cathode of claim 1 , wherein the cathode is an alloy comprising vanadium, iron and zirconium.

9. A cathode for a vacuum electronic device, the cathode being comprised of a mixture of a material and diamond powder wherein the material also allows the cathode to act as a getter.

10. The cathode of claim 9, wherein the surface of the cathode is granular and provides multiple protrusions capable of field emission of electrons when an electrical field is applied to the cathode.

11. A cathode for a vacuum electronic device, the cathode being comprised of a material that also allows the cathode to act as a getter, and the cathode being prepared by conditioning.

12. The cathode of claim 11, wherein a surface of the cathode is smooth or granular.

13. The cathode of claim 11 , wherein the conditioning comprises spark conditioning by subjecting the cathode to a plurality of applications of high voltage at different positions.

14. The cathode of claim 11, wherein the conditioning comprises current conditioning by subjecting the cathode to a current across an anode and the cathode.

15. The cathode of claim 11 wherein the cathode surface is smooth.

16. An x-ray device comprising a miniature housing and disposed therein an anode and a cathode which is comprised of a material that also allows the cathode to act as a getter.

17. The x-ray device of claim 16 further comprising a connector to the anode and cathode.