US3309302A

US3309302A - Method of preparing an electron tube including sputtering a suboxide of titanium on dielectric components thereof

Info

Publication number: US3309302A
Application number: US314182A
Authority: US
Inventors: Heil Oskar
Original assignee: Varian Associates Inc
Current assignee: Varian Medical Systems Inc
Priority date: 1963-10-07
Filing date: 1963-10-07
Publication date: 1967-03-14
Anticipated expiration: 1984-03-14

Description

O. HEN. METHOD OF PREPARING -AN ELECTRON TUBE INCLUDING SPUTTERING A SUBOXIDE OF TITANIUM ON DIELECTRIC COMPONENTS THEREOF Filed Oct. '7, 1963 MERCURY DIFFUSS/ON fie PUMP INVENTOR. OSKAF? HEIL BY WX/EMZ;

-ATTORNEY United States Patent fornia Filed Oct. 7, 1963, Ser. No. 314,182 7 Claims. (Cl. 204-192) This invention relates to the reduction of secondary electron emission on the surface of a dielectric body bombarded by electrons and more particularly relates to the reduction of electron multipactor phenomena on the surface of a dielectric body exposed to electromagnetic waves.

In the electron tube industry, one of the limiting factors determining the amount of power which can be extracted from an electron tube containing electromagnetic wave energy with frequencies above 3000 megacycles involves the capability of the output transmission system. to handle the high energy output. The output transmission system often contains a window transparent to high frenquency electromagnetic energy that is formed of dielectric material. The window is generally used to maintain a vacuum seal in the tube so as to prevent the entry of water, water vapor, dust or other extraneous matter therein. The limiting factor in the output transmission system is that the amount of high frequency electromagnetic Wave energy which a window is capable of passing has not kept pace with the amount of high frequency electromagnetic wave energy which an electron tube is capable of generating. Thus, while it is possible with state-of-the-art electron tubes to generate very high frequency electromagnetic wave energy, it is difiicult to extract the energy from the electron tube because of the limitations imposed by the output transmission system, especially where such output transmission system. utilizes a dielectric window as used in microwave tubes, such as k'lystrons, for instance.

Output windows transparent to electromagnetic energy are usually fabricated from dielectric materials having a coefiicient of secondary electron emission greater than unity when subjected to bombardment by electrons whose energy levels lie within a certain range. This secondary electron emission coefiicient characteristic renders the dielectric materials suscepitible to destructive heating due to single surface multipactor phenomena which occur when the dielectric body is exposed to high frequency electromagnetic fields. This phenomenon is explained at length in an article appearing in the IRE Transactions of the Professional Group of Electron Devices, Volume ED-S, Number 4, dated July 1961.

It is an object of this invention to provide a method for reducing secondary electron emission on the surface of a dielectric body.

It is a further object of this invention to provide a method for reducing electron multipactor on the surface of a dielectric window exposed to large amounts of high frequency electromagnetic Wave energy.

Briefly described, this invention relates to a method for reducing secondary electron emission on the surface of a dielectric body. A layer consisting of a material having a secondary electron emission coeflicient whose maximum value is less than unity is sputtered onto a surface of the dielectric body. The sputtered layer on the dielectric body has a high resistance, preferably, greater than ohms per square and is firmly bonded to the surface of the dielectric body.

This invention also relates to a method for reducing electron multipactor on a dielectric window transparent to electromagnetic Wave energy which is to be used in the presence of free electrons and a high frequency electromagnetic field. A layer of a material whose secondary electron emission coefficient is less than unity is sputtered onto a surface of the window. The sputtered layer has a resistance greater than 10 ohms per square and a thickness sulficient to reduce electron multipactor on the surface of the dielectric window.

In the drawing:

The sole figure is a cross-sectional view of the sputtering apparatus used in connection with the method of the subject invention.

'Referring to the sole figure, a dielectric body 10 is mounted on three equally spaced dielectric support rods 12, preferably, of fused silica within a chanmbcr 14, such as, for example, a bell jar. A metal plate 16 serves as a base support for the three equally spaced support rods 12. The metal plate 16 is provided with a circular opening located within and defined by the three support rods 12. A target 18 is suspended at a distance from the dielectric body 10 by a lead 20 that is provided with rigid insulation 22 within the chamber 14. The lead 20 is connected through an ammeter 24 to an appropriate source of negative DC. voltage, for example, 1200 to 1500 volts, thus making the target 18 a negative electrode. A lead 26 is connected to the metal base plate 16 and to the positive side of the voltage source thus making the base plate 15 a positive electrode. A volt meter 28 is connected across the voltage source for the purpose of monitoring the applied potential.

The chamber 14 contains a gas suitable for creating a plasma in the chamber 14 when the DC. voltage source is electrically connected to the

leads

20 and 26. The positive ions of the plasma created in the chamber 14 are attracted to the target 18 and strike the target with sufiicient energy to cause the material of the target 18 to sputter off and onto the dielectric body 10.

In one example, the target 18 consisted of an elongated cylindrically shaped body of titanium monoxide that was 1.5 inches long and 3 inches in diameter and was spaced 1.5 inches from the dielectric body 10. The dielectric body 10 Was used as a circular dielectric window in an output coupling arrangement of a klystron and had a 3 inch diameter and a thickness of .15 inch. A mercury diifusion pump 30 Was connected to the bottom of metal plate 15 for the purpose of maintaining a mercury atmosphere at a pressure of 10 torr at a temperature of 25 C.

A trap was mounted below the circular opening in the base plate 16 to maintain the mercury pressure at a constant value and at an exact temperature of about 18 C. The trap used was simply a circular or elliptical shaped cylinder filled with water at room temperature.

A helical copper RF. coil 32 is Wound around the bell jar 14 and energized by an oscillator operating at a frequency of 27 megacycles and providing several hundred watts of power for the purpose of exciting or concentrating the mercury plasma in the region about the target 18 and the window 10. The copper coil 32 used in the example was approximately 1 inch wide.

The coating or layer which was sputtered onto the dielectric window 10 has, preferably, a thickness of 1000 angstrom units. This thickness provides a visible film of titanium monoxide. The mercury ion current was, preferably, held at 4 milliamperes for about a 60 minute period in order to constantly reproduce coatings having the same thickness on the dielectric window 10.

The thicknesses of the coatings produced in accordance with the sputtering operation described above are easily reproduced and the sputtering technique that is used in producing coatings of low secondary electron emissive material on the dielectric window 10 is temperature insensitive since the energy of the impinging ions is in the range of 1200 to 1500 electron volts which is far above and unaffected by thermal energies. The sputtered molecules of titanium monoxide have energies in the neighborhood of several electron volts which is also above thermal energies and hence, unetfected thereby. The sputtering operation provides coatings that are firmly bonded to the dielectric window without the necessity of heating the dielectric Window 10.

The dielectric window 10 was selected from any one of the materials of the group consisting of alumina, beryllia, and silica.

In another example, thicker coatings of titanium monoxide were deposited on the dielectric window 10 to suppress electron multipactor withopt lowering the resistance of the surface of the dieelectric window 10 which should be, preferably, greater than 10 ohms per square. A flattened nickel Wire mesh was placed on the surface of the dielectric window 10 prior to the sputtering operation. A network of dots of titanium monoxide was thus sputtered onto the dielectric window 10 covering of the area of the window 10. The dots were .75 mm. apart and provided an inherently large electrical resistance thereby permitting the thicker coatings, if desired. It was discovered that the sputtered dot coating reduced electron multipactor on the surface of the dielectric window 10.

It is to be understood that the above-described arrangements are simply illustrative of the application of the principles of the invention. Numerous other arrangements may be readily devise-d by those skilled in the art which will embody the principles of the invention and fall within the spirit and scope thereof.

I claim:

1. In the manufacture of an electron. tube having a dielectric body and in which operation of the tube causes the surface of the dielectric body to be bombarded by electrons, the improvement comprising the steps of sputtering onto saidv surface a dotted coating of a suboxide of titanium.

2. A method as claimed in claim 1 in which the dots of said dotted coating of the suboxide of titanium are spaced .75 millimeters apart and cover 20% of said surface of the dielectric.

3. In the manufacture of an electron tube having a dielectric body and in which operation of the tube causes the surface of the dielectric body to be bombarded by electrons, the improvement comprising the step of sputtering a suboxide of titanium onto said surface from a target electrode made of said suboxide material, said suboxide material having a secondary electron emission coefficient of less than unity, and said suboxide material being sputtered onto said surface to form a film which is thick enough to reduce the secondary emission coefiicient of said surface and thin enough to have a high electrical resistance.

4. In the manufacture of an electron tube having a dielectric body and in which operation of the tube causes the surface of the dielectric body to be bombarded by electrons, the improvement comprising the step of sputtering onto said surface a suboxide of titanium whose secondary electron emission coefiicient has a maximum value less than unity for a time sufiicient to form a film having a resistance greater than 10 ohms per square and a thickness on the order of about 1000 angstrom units, said sputtering being performed with a target electrode made of said suboxide of titanium.

5. In the manufacture of an electron tube having a dielectric body and in which operation of the tube causes the surface of the dielectric body to be bombarded by electrons, the improvement comprising the step of spurtering onto said surface titanium monoxide for a time sufiicient to form a film of said titanium monoxide having a resistance greater than 10 ohms per square on said surface, said sputtering being performed with a target electrode made of titanium monoxide.

6. In the manufacture of an electron tube having a dielectric body and in which operation of the tube causes the surface of the dielectric body to be bombarded by electrons, the improvement comprising the steps of placing the dielectric body within a gas filled chamber having two spaced electrodes, one of said electrodes being of a material consisting essentially of sub-oxides of titanium connecting a DC. voltage across said electrodes to create a plasma in said chamber, said one electrode being operated at a negative potential in said plasma so that positive ions in the plasma bombard said one electrode knocking off particles of the electrode which deposit on the surface of said dielectric body, and forming a coating of said low secondary electron emissive material on said surface of said dielectric body having a resistance greater than 10 ohms per square and a thickness sufficient to reduce the secondary electron emission of said dielectric body to a value less than unity.

7. In the manufacture of an electron tube having a dielectric body and in which operation of the tube causes the surface of the dielectric body to be bombarded by electrons, the improvement comprising the steps of placing the dielectric body within a chamber having two spaced electrodes and containing a mercury atmosphere which is at a pressure of 10* torr at a temperature of 25 C., one of said electrodes being of titanium monoxide material which has a coefiicient of secondary electron emission less than unity, connecting a DC. voltage having a value in the range of between 1200 and 1500 volts across said electrodes to create a mercury plasma in said chamber, applying an RF. potential across said chamber to concentrate the mercury plasma in the region between said dielectric body and said one electrode, said one electrode being operated at a negative potential in said mercury plasma so that the positive mercury ions in the plasma bombard said one electrode knocking off particles of the electrode which deposit on the surface of said dielectric body, and forming a coating of said titanium monoxide material on said surface of said dielectric body having a resistance greater than 10 ohms per square and a thickness on the order of about 1000 angstrom units.

References Cited by the Examiner UNITED STATES PATENTS OTHER REFERENCES Boot et al., I. of Electronics and Control, vol. 4, pp. 434, 435 and 449-452.

Preist et al., I.R.E. Transactions On Electron Devices, July 1961, pp. 243-251.

Wehner, Advances in Electronics and Electron Physics, (Edited by Morton), vol. VII, pp. 249-254, 267 and 268,

HOWARD S. WILLIAMS, Primary Examiner.

JOHN H. MACK, Examiner.

R. K. MIHALEK, Assistant Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No 3, 309, 302 March 14 1967 Oskar Heil certified that error appears in the above numbered pat- It is hereby hat the said Letters Patent should read as ent requiring correction and is corrected below.

Column 1, line 24, for "frenquency" read frequency column 2, line 15, for "chanmber" read chamber column 3, line 16, for "dieelectric" read dielectric column 3, line 38, for "steps" read step Signed and sealed this 26th day of December 1967.

(SEAL) Attest:

EDWARD J. BRENNER Edward M. Fletcher, Jr.

Commissioner of Patents Attesting Officer

Claims

1. IN THE MANUFACTURE OF AN ELECTRON TUBE HAVING A DIELECTRIC BODY AND IN WHICH OPERATION OF THE TUBE CAUSES THE SURFACE OF THE DIELECTRIC BODY TO BE BOMBARDED BY ELECTRONS, THE IMPROVEMENT COMPRISING THE STEPS OF SPUTTERING ONTO SAID SURFACE A DOTTED COATING OF A SUBOXIDE OF TITANIUM.