US3542488A - Method and apparatus for producing alloyed getter films in sputter-ion pumps - Google Patents

Description

United States Patent Lewis D. Hall Palo Alto, California 771,240

Oct. 28, 1968 Nov. 24, 1970 Andar/ITI, Inc.

Palo Alto, California a corporation of California lnventor Appl. No. Filed Patented Assignee METHOD AND APPARATUS FOR PRODUCING ALLOYED GETTER FILMS IN SPUTTER-ION PUMPS 16 Claims, 7 Drawing Figs.

U.S.Cl.... 1 417/49 Int. Cl F041) 37/02 Field of Search 230/69;

[56] References Cited UNITED STATES PATENTS 3,198,422 8/1965 Kienel 230/69 3,398,879 8/1968 James et a1. 230/69 Primary Examiner Robert M. Walker Attorney-Harvey G. Lowhurst EVACUABLE PUMP CHAMBER Patented Nov. 24, 1979 EVACUABLE CHAMBER PUMP EEEEEE EEEEEE Fig. 4

INVENTOR LEWIS D. HALL mack Z wm3wwmma TIME IN MlNUTES ATTORNEY METHOD AND APPARATUS FOR PRODUCING ALLOYED GETTER FILMS IN SPUTTER-ION PUMPS STATEMENT OF THE INVENTION This inventionrelates to sputter-ion pumps and more particularly to a sputterion pump and pumping method in which the getter film is formed by combining two or more different reactive metals, one of which is a transition metal such as titanium.

DESCRIPTION OF THE PRIOR ART tive metals such as silver, copper or gold. Accordingly, this pump describes .a cathode structure comprised of a relatively reactive metal cathode and a relatively nonreactive metal cathode, the nonreactive cathode having a high sputtering yield and the reactive cathode having a low sputtering yield. As the result of such a configuration, the nonreactive metal is sputtered at high rates over the other portions of the cathode, such as the titanium, and, of course, over the anode to thereby prevent the reevolution of the previously buried ions. One unfortunate limitation of the described pump is that during the sputtering operation the titanium portion of the cathode has its surface covered over with a relatively inert metal which causes aconsiderable decrease in the pumping capability for the active gases such as nitrogen, oxygen, water'vapor and the like. In addition to the impaired ability of'the pump to handle very low pressures, the pump also is very difficult to start.

. Another prior art pump which is markedly improved over the Kienel pump is described in US. Pat. No. 3,398,879 and in an article entitled Inert Gas Pumping Using Differential Sputter Yield Cathode, by Tom and James, Trans. l3th A,V.S. Symposium I966. The pump described in this reference is also of the diode" type configuration and utilizes a flat solid titanium cathode. Even though this pump has an increased ability to handle noble gases, when compared to the standard diode pump, this is believed to be due to the fact that tantalum sputters more readily than titanium. However, its ability to handle reactive gases is appreciably less.

Yet another two-metal cathode sputter-ion pump is described in an article entitled Use of Molybdenum In Getter Ion Pumps by Jackson and Haas, Inc., J. VAC, SCI. TECH. 3, 80, 1966. In this pump, a flat solid molybdenum cathode faces an identically shaped solid flat titanium cathode in diode" type configuration. This combination of opposed reactive metals is capable of extreme vacuums, in excess of l Torr, but its pumping speed is low, particularly after an appreciable volume of gas is introduced into the pump which increase the pressure to above 10- Torr.

OBJECTS OF THE INVENTION It is therefore a primary object of the present invention to provide an apparatus and a method for increasing the pump ing capability of a sputter-ion pump for reactive gases.

It is a further object of the present invention to improve the starting characteristics ofsputter-ion pumps.

It is still a further object of the present invention to provide a sputter-ion pump which pumps reactive gases more rapidly than was heretofore possible, particularly in the pressure range from 0,2 to Torr, and which has an improved start ing characteristic particularly after the introduction of a volume of gas into the previously highly evacuated pump.

It is still a further object of the present invention to utilize a new type of gettering material, and, more particularly, to form a getter alloy which comprises, as one of its components, no more than 90 percent and no less than 10 percent transition metal such as titanium and in which the remaining phases are highly reactive metals or metal alloys.

schematic block diagram chamber 22-is an anode structure so SUMMARY OF THE llNVENTION I In accordance with a preferred embodiment of the present invention the-cathodes, in an otherwise conventional type of sputter-ion pump, are made to sputter a gettering substance which is composed of at least two reactive metals, one of which is a transition metal such as titanium. Each cathode is constructed of at least two different reactive metals which are preferably arranged by alternating one reactive metal with another reactive metal in grid fashion to form the cathode. The two reactive metals may be arranged whereby one metal forms the columns and the other metal forms the rows of a grid structure or, in the alternative, columns of one metal alternating with columns of the other metal. In the preferred embodiment of the present invention the cathodes are transparent" to form what is generally known as a "triode" pump.

The present invention is also disclosed in a paper by Dr. Lewis D. Hall published in Transactions of the 15th A.V.S. Symposium, Oct. 30. I968. and the disclosure made in this paper is expressly included by reference into this specificatron.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 diagrammatically illustrates a sputtering system having a pump with a pair of cathode-s constructed in accordance with the presentinvention;

FIG. 2 is an end view of a cathode constructed in accordance with the present invention;

FIGS. 3-6 illustrate, diagrammatically, a number of em bodiments of the cathode structure of the present invention, and

FIG. 7 is a graph showing the vacuum achieved as a function of pumping time, i.e., pumping speeds, for triode type pumps having conventional cathodes and cathodes constructed in ac cordance with the present invention.

DESCRIPTION OFTHE PREFERRED EMBODIMENTS l of the drawing, there is shown in form a sputter-ion pumping system of the type well known in the prior art but having a novel cathode structure in accordance with the present invention, The system illustrated includes an evacuable chamber 10, of which it is desired that the pressure be reduced to I X 10- Torr or below. A mechanical pumping means I2 is typically provided in the system for pumping the chamber down to approximately lO- Torr, at which time the valve means 14 is closed and the sputter-ion pump is actuated to further reduce the pressure in the chamber 10 to a vacuum well in excess of 10" Torr.

The sputter-ion pump is essentially comprised of a vacuum Referring now to FIG.

- enclosure I6 of nonmagnetic metal positioned between a pair of magnetic poles l8 and 20 for establishing a magnetic field across the pumping chamber 22 Disposed within the pumping as to provide an improved electrostatic field, and two transptirent cathode structures 26 and 28 fromwhich thesputtering action is generated. Transparc-nt,;as used herein, is defined as having at least one aperture therein such that the sputtered material can pass through the cathode. I

Whereas in the prior art structures, thecathode plates were usually constructed from one of a number of reactive metal substances, the novel structure in accordance with the present invention employs two or more reactive metals in order to produce an alloy vapor" and cause the formation of alloy deposits over as much of the internal pump area as possible. In this way, a considerable improvement over the results obtainable with the single substance cathodes perhaps the best of which is titanium is obtained.

In the illustrated structure the anode 24 is electrically connected to the walls of the enclosure 16 defining the chamber 22, and the two are maintained at ground potential. The cathodes 26 and 28 are charged to a high negative potential with respect to ground. The cathode structure, as illustrated in the several embodiments shown in FIGS. 2 through 6, is comprised of various arrays of two or more different reactive metals in strip form. The cathodes may be comprised of any suitable configuration of metallic strips which may be positioned in gridlike form or louver form, or in any other suitable configuration.

The principal feature of the present invention is that the strips comprising the array are constructed of at least two different types of reactive metal and are arranged in a predetermined manner, so as to provide a mixing of the vaporized materials, either before or after they have condensed upon the surfaces of the pumping chamber.

Thin vapor alloy producing techniques have been found to produce pumping characteristics which greatly exceed those of the prior art. In operation. a high negative potential is applied to the cathodes 26 and 28 via a conductive lead 30, which passes through a conducting block 32, positioned in the wall of the enclosure housing 16. Since the housing 16 and the anode 24 are grounded, and thus operate at zero potential, the effect of applying the high negative potential to cathodes 26 and 28 is to produce a region of intense electric field between the anode 24 and the cathode plates 26 and 28. Within the region between the anode 24 and the cathode plates 26 and 28 a stray electron will be attracted towards the positive anode 24. As it travels toward anode 24, the electron will gain kinetic energy and may collide with a neutral gas molecule within the chamber 22.

Assuming the electron has gained sufficient energy, it will ionize the neutral molecule and produce a free electron and a positive ion. The positive ion will then be attracted to the cathode plate because of its high negative charge. Upon striking the gridlike elements of the cathode the ion will disintegrate portions thereof, and also cause secondary electrons and protons to be emitted which may further enhance the ionization of the gas within the chamber 22. The vaporized particles of the cathode materials which are dislodged by the ion bombardment form a vapor within the interior of the chamber 22 which is then caused to condense upon the interior surfaces thereof. Since the cathode materials are of a reactive metal, the sputtered material serves to entrap other gaseous molecules that it happens to come in contact'with.

By utilizing two or more different metals in the cathode structure, physically positioned closely proximate one another, a pumping action is achieved which greatly exceeds the pumping rate ofprior art structure. This is believed to be a result of a mixing of the vapors of the two different reactive materials, so as to, in effect, provide an alloy vapor" which is substantially the same as would be obtained from sputtering an alloy of the separate materials. The cathode plates may be made of a combination of any of the metallic substances such as molybdenum, chromium, tungsten, tantalum, niobium, iron, titanium, zirconium, nickel, barium, aluminum, thorium, magnesium, vanadium, hafnium, or other transition elements of the fourth, fifth or sixth groups of the periodic table including the rare earths, as well as the semiconductors boron, silicon and germanium. Silicon is of particular interest because ofits known tendency to form oxides and nitrides.

One of the principal requirements of a good sputter-ion pumping means is that it have high gas handling capability. That is. it must be able to pump rapidly down to a high vacuum gain after an appreciable volume of gas is introduced into the system which is being maintained at a high vacuum. In accordance with the present invention, a considerable improvement over the results obtainable with single-metal titanium cathodes is obtained.

In order to illustrate the improved characteristics of the novel cathode structure of the present invention and, in particular, the increased pumping rate, reference is made to FIG.

7 of the drawing which is a graphical comparison between a slatted grid structure .of titanium only and a physically similar slatted structure wherein the alternate titanium slats were replaced with zirconium slats. The cathodes consist of rows of thin metal slats of approximately .025 inches X inches X 2- /2 inches, and are spaced about one-eighth inch center to center. The titanium structure was chosen as a reference since it is universally used in commercial triodc pumps, and in most commercial diode" pumps. Furthermore, the literature to date indicates that for pumps in which a single metal cathode is used no metal has given better results than titanium, although in some of the tests, zirconium alone was used and was found to give essentially identical results to titanium.

The tests. exemplary results of which are illustrated in FIG. 7 of the drawing, were made by connecting two sputter-ion pumps in parallel in a system such as is illustrated in FIG. I. With the system initially pumped down to about 4 X ltl- Torr, all pumps were turned off. The pressure was then allowed to rise and at l X lt) Torr the B-A gauge was turned off, since this is its upper limit. After a specified period oftime the pump under test was turned on and the values of the pressure v. time were recorded. This procedure was followed repeatedly for the all-titanium cathode and the experimental cathode in turn. The results were found to be quite reproducible. It is estimated that the maximum pressure at the start of each pump-down was about 30 microns when 40 minutes were allowed to elapse after turning offthe B-A gauge.

In FIG. 7, the solid lines represent the results obtained using the all-titanium cathode on one hand, and the titanium-zirconium cathode on the other hand, each being turned on 20 minutes after the B-A gauge was turned off. The dashed lines represent corresponding tests initiated 42 minutes after the B- A gauge was turned off. These curves dramatically illustrate the outstanding superiority of the novel cathode structure using alternate slats of titanium and zirconium over the prior art structure using titanium alone.

In FIGS. 2 through 5, various alternative configurations of dual material cathode structures are illustrated. The slats designated A are of one type of material, such as titanium, for example, and slats designated B are of another type of material, such as zirconium or vanadium, for example. To sum marize, material A may be of any element from atomic number 2l (Sc) through 28 (Ni); 39 (Y) through 42 (Mo); 57 through 74; or 89 (Ac) through 92 (U), Material B (C, D, etc.) may be from the same set as A, but further including Be, Mg, Ca, Sr, Ba, 8, Al,C,Si or Ge.

In FIG. 6 a trimetal cathode structure is illustrated which includes a first metal A, such as titanium, a second metal B, such as zirconium, and a few strips of a third metal C, such as aluminum, which were arranged across the titanium and zirconium strips A and B. This structure was found to give one of the fastest pump-downs obtained during the initial tests of the novel cathode structure.

It is, of course, to be understood that the illustrated configurations of the dual-metal and trimetal cathode structures are merely illustrative in'nature, and are not to be construed as limiting the forms of cathode structure contemplated for use in carrying out the invention. As an example, the cathode structure may be comprised of an interwoven mesh of two or more different types of metals, or may consist of a honeycombed like structure comprised of a plurality of cylindrical segments of different types of metal. Moreover, the structure may take any other suitable form in which the two or more different types of metals are positioned proximate one another, so that the vapors created by the sputtering action will become intermixed during their flight to condense on an interior surface of the structure. Furthermore, it is contemplated that in the case of the diode" type pumps, two or more metals might be laminated together 'to form a striplike surface wherein alternate stripes are of one type of metal and the remaining stripes are of another type of metal.

After having read the above disclosure, many more altera tions and modifications of the invention will be apparent to those of skill in the art. It is to be understood that this description of preferred embodiments is for purposes of illustration only and is in no manner intended to be limiting inany way.

Accordingly, I intend that the appended claims be interpreted as covering all modifications which fall within the true spirit and scope of my invention.

Iclaim:

l. In a sputter-ion pump apparatus using the principle of cathode disintegration by ion bombardment and including a vacuum envelope enclosing a evacuable volume. an anode structure, a pair of substantially identical cathodes for disintegrationby ion bombardment and, means for establishing substantially identical electric fields between said anode structure and each of said cathodes, the improvement comprising: cathodes comprised of at least two different types of reactive gettering materials distributed in a substantially uniform manner so as to form the operativesurface ofsaid cathodes 2. in a sputter-ion pump apparatus as recited in claim 1 wherein said cathodes are formed of a plurality of members in which groups of members of one type of reactive gettering material are alternativelypositioned with respect to groups of another type of reactive gettering material.

3. In a sputter-ion pump apparatus as recited in claim 2 wherein one of said reactive gcttcring materials consists of titanium and the remaining gettering materials are selected from the group consisting of zirconium. cobalt, tungsten, haf- 5. A sputter-ion pump apparatus as recited in claim 2 wherein one of said reactive gettering materials consists of aluminum and the remaining gettering materials are selected from the group consisting of titanium, zirconium, cobalt, tung sten, hafnium, iron, niobium, tantalum, molybdenum, silicon, boron and germanium. I

6, In a sputter-ion pum'p apparatus as recited in claim 2 wherein said members are spacially separated so as to render said cathodes transparent.

7. ln a sputter-ion pump apparatus as recited in claim 1 wherein a selected portion of each cathode is of one reactive gettering material and remaining portion of the cathode is of another reactive gettering'material.

8. In a sputter-ion pump apparatus as recited in claim lin which the operative surface of each cathode is formed of at least two different reactive gettering materials which are interspersed with one another.

9. In a sputter-ion pump apparatus as recited in claim 8 wherein portions of one type of reactive gettering material alternatc with portions of, another type of reactive gettering material. r

, 10. In a sputter-ion pump apparatus as recited in claim 9 wherein said portions are spaced apart closely proximate to one another.

ll. ln a sputter-ion pump apparatus as recited in claim 1 wherein each cathode is formed of one or more alloys of the reactive gettering materials.

12. A method of producing an alloy of two or more materials by sputtering, comprising the steps of:-

providing an anode structure within an evacuable housing;

providing a cathode structure within said housing having an operative portion which is constituted ofa plurality ofdiscrete members of which at least one of said members is selected from the group consisting of titanium, aluminum and magnesium, and the balance of said members is of at least one other reactive material selected from the group consisting of zirconium, tungsten, hafnium, niobium, tantalum, molybdenum, chromium. iron, cobalt, silicon,

boron and germaniung producingan electric field between said anode structure 13. A method of producing an alloy of two or more materi als as recited in claim 12. which further includes the step of spacing said one members fromsaid other members so that the vapors produced by particle collision with the respective cathode members are caused to intermix prior to condensing upon a surface within said housing 14. A sputter-ion pump comprising: an evacuable enclosure; an anode structure disposed within said evaeuable enclosure; i i

a pair of substantially identical cathodes within said enclosure comprised of a plurality of members of at least two different reactive getter materials;

potential supply means connected between said anode structure and'said cathodes for rendering said cathodes more negative than said anode structure; and

a pair of pole pieces for establishing a magnetic field within said evacuable enclosure.

15. A sputter-ion pump as recited in claim 14 wherein said members are elongated and disposed closely proximate one another in spaced apart relationship.

16. A sputter-ion pumpas recited in. claim 15 wherein alternate ones of said members are of one type of reactive gettcring material and the remaining members are of another type of reactive gettering material.

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