US3824039A

US3824039A - Sublimable targets

Info

Publication number: US3824039A
Application number: US00237335A
Authority: US
Inventors: B Power
Original assignee: British Oxigen Ltd
Current assignee: BOC Group Ltd
Priority date: 1971-03-24
Filing date: 1972-03-23
Publication date: 1974-07-16
Anticipated expiration: 1991-07-16
Also published as: FR2130600A1; GB1386251A; FR2130600B1; IT950690B; DE2214234A1

Abstract

A source of sublimable material is at least partially enclosed within a body or shield of nonsublimable material and in heatexchange relation therewith. Both the sublimable material and the shield may be of various shapes and configurations, and both the sublimable and the non-sublimable material may be supported on a common support.

Description

United States Patent 1191 Power July 16, 1974 SUBLIMABLE TARGETS 2,107,945 2/1938 111111 313/178 2,146,374 21939 K 313/181 x [75] Inventor: Power Crawley 2,469,626 5/1949 13:55 417/51 x England 2,951,170 8/1960 LGfCI'SOILW 313/178 3,229,147 1/1966 Affleck 417/49 [73] Asslgnee 3 3:"? 5 3,324,331 6 1967 Reinhardt.. 313/180 x 3,357,634 12/1967 Maliakal .,417 49 [22] Filed; Mar 23,1972 3,388,290 6/1968 Herb ....417/48X 3,391,303 7/1968 Hall 315/108 Appl. No.: 237,335

[30] Foreign Application Priority Data Mar. 24, 1971 Great Britain 7745/71 [52] US. Cl. 417/48 [51] Int. Cl. F04b 37/00 [58] Field of Search 417/48, 49, 51; 3-13/178-181 [56] References Cited UNITED STATES PATENTS 1,712,370 5/1929 White 417/51 X 2,100,045 11/1937 Alexander 417/48 X Primary ExaminerWilliam L. Freeh Attorney, Agent, or Firm-Dennison, Dennison, Townshend & Meserole ABSTRACT A source of sublimable material is at least partially enclosed within a body or shield of nonsublimable material and in heat-exchange relation therewith. Both the sublimable material and the shield may be of various shapes and configurations, and both the sublimable and the non-sublimable material may be supported on a common support.

11 Claims, 7 Drawing Figures PATENTED JUL 1 6 m4 saw 2 0r 3 FIG.3

SUBLIMABLE TARGETS This invention relates to sublimable sources, by which is meant a body of material which is adapted to sublime when its temperature is raised sufficiently.

The sources of the present invention are particularly suitable for use in vacuum pumps of the sublimation type in general, or radial electric field (REF) vacuum pumps in particular, but are not limited to such a use. In sublimation pumps a body of a desired getter material is heated to cause sublimed material from the body to be deposited on a cooled housing. In known sources with high titanium storage (bulk sublimators), as the surface area available for heat loss byradiation is reduced by sublimation, the constant supply of energy heats the source to a higher temperature, at which the rate of sublimation is increased, to increase the heat loss from the smaller surface. This non-uniform rate of sublimation is undesirable, and the present invention aims at producing a sublimable source with moreuniform characteristics than known sources. Other difficulties which the present invention mitigates are discussed below.

Accordingly the present invention provides a source of sublimable material which is as claimed in the appended claims.

By non-sublimable in this specification is meant a material of high melting point having a vapour pressure, at the operating temperature of the source, which is negligible compared with that of the sublimable material.

The present invention will now be described by way of example with reference to the accompanying drawings, in which:

FIG. 1 is a perspective view of an REF pump incorporating a source of the present invention, with the housing partially cut away to show the central anode;

FIG. 2 is a view, part in section and part in elevation, and drawn to a larger scale, of the source of the present invention shown in FIG. 1;

FIG. 3 is a view, similar to FIG. 2, of an alternative form of source, and;

FIGS. 4, 5, 6, and 7 are perspective views in larger scale of alternate forms of the source illustrated in FIG. 1.

The pump shown in FIG. 1 includes an anode 2 extending along the axis of a cylindrical water-cooled housing acting electrically as an earthed cathode 4. The housing 4 is provided with an end flange 6 leading to the interior ofequipment (not shown) to be evacuated.

Extending into the interior of the housing 4 from beyond a reflector plate 12 are supports 14 for filaments 16. By means which are shown in the drawing but which are not described in further detail in this specification (because they do not form part of the subjectmatter of this application) the anode 2 is raised to an electric potential appreciably above that of the housing 4, and one or both of the filaments 16 is or are heated. The support 14 associated with each filament prevents electrons emitted from the heated element falling directly on the anode 2. The electrons which are not incident on the support 14 tend to be deflected, by the radial electrical field extending between the anode 2 and housing 4, into orbits around the anode 2. These orbits are of circular or elliptical section with an axial component, resulting in the electrons oscillating along the pump axis while travelling around the anode. This provides long electron trajectories which ensure numerous collisions between the electrons and residual gas molecules. These collisions both ionize the gas molecules and disturb the orbits of the electrons so that they are eventually captured by the anode, the greater proportion being intercepted bythe or each source 18, because of its greater diameter.

The or each source 18 includes a perforated shield 22 of a non-sublimable material, such as tantalum, which is heated by the electron bombardment and which encloses a charge 20-of sublimable material, such as titanium, which is heated mainly by radiation from the shield 22. The titanium or like metal sublimes and the resultant vapour effuses through perforations 26 in the shield 22 to become deposited on the inside surfaces of the housing 4 to form a continually-replenished layer of gettering material participating in the pumping action of the pump.

Although tantalum is preferred, it would be possible to use molybdenum or other suitable refractory material as the non-sublimable material.

When the electrons fall directly on an unshielded titanium charge (as in known pumps) there arises the disadvantage, mentioned above, that the material tends to sublime from the source at a non-uniform rate, the specific rate of sublimation being a function of the temperature of the source, which increases as its size decreases. Another disadvantage is brought about by the inclusion of carbon impurities in the material forming the source. In operation of the pump hydrocarbons such as methane are formed due to bombardment of the source (which invariably contains carbon impurities) by hydrogen or hydrogen-bearing compounds present in the residual gas. These gaseous hydrocarbons are pumped only slowly by the titanium deposit and constitute components of the pump atmosphere which are undesirable because the pumps are frequently used to evacuate equipment required to be free of hydrocarbons.

A further disadvantage of known pumps is that the sublimed material tends to be deposited more thickly on that part of housing 4 which is immediately opposite the source than in more remote locations. This localized increase can result in the sublimate flaking off or having to be cleaned off the housing to restore the pump to a desired operating condition.

In the source shown in FIGS. 1 and 2 these disadvantages are at least partially overcome by provision of the perforated shield 22, so that the body of sublimable material is heated by radiation rather than directly by electron bombardment. In FIG. 2, the source (indicated by the general reference 18) is mounted on an anode rod 2. The source includes a body 20 (or charge) of sublimable material inside a shield. The shield consists of a perforated sleeve 22 and two imperforate end caps 24 of which each is secured to rod 2 and to the sleeve 22 to keep the sleeve spaced axially and radially from body 20. The non-sublimable material from which at least the sleeve 22 is formed, and preferably also both end caps 24, has to be of high melting point, have a low vapour pressure, and be substantially inert at the high degrees of vacuum attained in the interior of the pump, and to have adequate thermal and electrical conductivities. A suitable material is tantalum which, under the operating conditions of the pump, has a negligible vapour pressure compared with that attained by the titanium forming body 20.

I As shown, the sleeve 22 is perforated so that the perforations 26 remove a known proportion of the surface of the sleeve 22. It has been found useful to use a sleeve 22 in which about to 20 percent of its area has been removed by perforations, but the amount could be increased or reduced for other applications.

The perforations 26 may be made by drilling holes through the material forming the sleeve. Alternative methods may be used for forming the perforations, and they may be shaped so that they tend to deflect material sublimed from body 20 to control the area and uniformity of the film deposited. This shaping may be done by deforming the sleeve in the region of the holes, rather in the manner of a grater as shown in FIGS. 6 and 7, so that the cross-sectional area of a perforation is greater when viewed at an acute angle to the surface of sleeve 22 than when viewed normally thereto. If such shaping is adopted it is preferable for the perforations to one side of the central plane of the shield to be directed in a direction opposite to the perforations atthe oppositeside. By adopting this expedient, it has been found that material sublimed from the source is deposited on the water-cooled housing 4 either over a larger area than with unshielded sources, or with a moreuniform deposition thickness, or with both. This thus effectively increases the time during which an REF pump using sources of the present invention can be operated before the housing has to have the deposited sublimate removed by scraping or otherwise.

The principle of operation of the source shown in FIG. 2 is based on the fact that the potential of the shield is the same as that of the body 20, so that virtually all the incident electrons fall on the shield, with onlya negligible proportion passing through the perforations 26 and falling directly on body 20. These incident electrons raise the temperature of the shield to a red-heat or white-head so that the shield then heats body 20 by radiation to a temperature at which the material of the body sublimes, i.e. it forms a vapour directly from the solid state, without the material of the body becoming fused. This emitted vapour then passes through the perforations 26 and falls on the inside surface of housing 4 so that a layer of getter material is continually deposited. This is effective both to getter and to bury ions of the gases being pumped.

Because of the restriction offered by the shield to the passage of the vapour, it might be necessary to raise the temperature of body 20 to a higher valuethan would be necessary if the shield were absent, in order to increase the vapour pressure of. body 20 to such a value that it compensates for the resistance offered by shield 18. If the perforations cover percent of the surface of the sleeve 22, then it has been found that the vapour pressure of the material can be increased some sevenfold by raising the temperature of body 20 by about 100 C. This gives rise to approximately the same mass flow of sublimed material as when using unshielded "sources, so that the operation of the pump is unaffected.

This higher temperature of body 20 is achieved by bombarding the shield with more power to raise its temperature to a higher value than an unshielded source would require to reach.

This can have the additional advantage that a higher electron current leads to a higher pumping speed for inert gas.

The non-uniform rate of attrition experienced in known unshielded targetsis at least reduced, if not removed altogether, by the faet that the external dimensions of the shield do not change appreciably in use, so that substantially the same amount of heat is radiated by the shield to the housing irrespective of the size of body 20.

In addition, the production of methane and undesired hydrocarbons is reduced by the fact that hydrogen and hydrogen-bearing.compounds in the atmosphere outside the source are largely denied access, because of the presence of the shield 22, to the carbon impurities present in body 20.

The carbon impurities present in the shield tend initially to produce hydrocarbons, but this tendency diminishes rapidly as the impurities combine chemically and are not replaced owing to the very low rate of sublimation at the temperatures reached by the shield.

It has also been found that the distribution of sublimed material on the'cathode 4 is improved over that obtained with unshielded sources, even when the perforations 26 are not given directional characteristics as discussed briefly above. The present invention is not based on a particular theory of operation, but it is thought that this desirable result is achieved by each of the perforations 26 functioning somewhat as a point source of sublimed material, which thus tends to radiate from each perforation 26 along a wider solid angle than happens when the shield is absent. This may be due, on an alternative or additional explanation, to the temperature of the surface of body 20 opposite each perforation 26 being lower than the adjacent portions. overlain by sleeve 22. This could give rise to less material being vaporised from such aligned portions of body 20 than from the surrounding areas, so that .one has a greater proportion of vaporised material passing through the perforations 26 at an angle thereto than coaxially therewith.

Although the sleeve 22 has been shown as being cylindrical, it can have other shapes. For example it can happed that a plain cylindrical shield is unevenly bombarded by electrons, so that excessive temperature variations arise along its length. Frustoconieal or other contoured shapes of shield can be employed to give a more uniform temperature.

The patterning of the perforations 26, and their number and size, might also be variables affecting operation of the source under different conditions and for different applications.

In that form of source 18 shown in FIG, 3 a stack of washers is mounted on anode 2, being kept in place by two stops 30, of which the details are immaterial. As shown, large washers 32 of non-sublimable material, such as tungsten or tantalum, are interleaved with smaller washers 34 of sublimable material, such as titanium. The source 18 can be similarly mounted in an REF pump. The thickness of the washers 34 is such that the larger washers 32 provide an electric field inhibiting incident electrons from falling on the sublimable material. Instead. the larger washers are heated by electron bombardment and heat the smaller washers to their sublimation temperature substantially solely by conduction.

As far as the indicent electrons are concerned, the larger washers define a source of substantially-constant size irrespective of the radial dimensions of the sublimable washers. They also inhibit hydrogen or hydrogenous compounds from falling on the smaller washers, so that this source possesses the major advantages of the source shown in FIG. 2.

Although the present invention has been described as being applicable to REF pumps, it is useful in sublimation pumps in general.

In such pumps the sublimable material is heated by any suitable means. It is envisaged that a source of the present invention which is intended to be heated other than by electron bombardment would be of the type shown in FIG. 2 and would use a shield adapted to be heated by Joule heat. For this purpose the support for the body of sublimable material would have to be electrically insulated from the shield, so that all the heating current would flow through the latter. Thus if the rod 2 were made of alumina or some other suitable, refractory, insulation, material, then separate conductors connected to the shield 22 would have to be provided for the heating current. Apart from the different method of heating, the functioning of the source is otherwise as described above.

If a source for a sublimation pump were to be as shown in FIG. 2, with the material of the shield 22 being electroresistive as well as non-sublimable, the geometry of the shield would require very large currents to be used to heat the shield to a hot enough temperature to sublimate body solely by radiation. In an effort to increase the electrical resistance of the shield 22 (and thus reduce the heating current) the perforations 26 would preferably be in the form of transaxial slots as seen in FIG. 5 which would overlap longitudinally so that the metal between the slots would present a serpentine, non-longitudinal, and lengthy path to the flow of heating current.

Alternatively the perforations 26 could be replaced by the gap between the adjacent edges of a strip of electroresistive material in the shape of a helix as seen in FIG. 4 with-adjacent turns spaced apart. This helical gap is intended in this specification to be covered by the term perforation.

The vapour source shown in FIG. 4 comprises an outer envelope made from a single helix 40 of nonsublimable material extending between imperforate end caps 42 secured to support rod 2. The adjacent edges of the helical strip 40 are spaced apart from each other so as to leave a helical gap through which the sublimed material can issue. Extending alongside, and in contact with, the helix 40 are two support rods 44 which are provided so as to prevent the helix from sagging in operation. Two or more support rods 44 are provided so as to make the source substantially insensitive to its particular orientation.

In that source shown in FIG. 5, the envelope is formed from a single sheet 46 of non-sublimable material formed with a pattern of rectangular apertures 48 through which the sublimed material can pass.

In those forms of the invention shown in FIGS. 6 and 7, the apertures for the sublimed material are formed so as to have a directional effect on the sublimed material. Each aperture 50 is formed by striking-out a portion of the material of the envelope 52 so as to define a somewhat crescent-shaped aperture, rather in the manner of a cheese-grater. In that source shown in FIG. 6, the portions 54 defining the apertures 50 are arranged to project outwardly of the cylindrical outer surface of envelope 52, whereas that form shown in FIG. 7 has the portions 54 projecting into the interior of the envelope. The FIG. 7 source is preferred because it presents a target to the orbiting electrons which is substantially in the shape of a right cylinder, whereas the outwardly projecting portions 54 of the FIG. 6 source act as foci for the electrons, and thereby become heated to a greater extent than the rest of the material of the envelope, which is disadvantageous.

I claim:

1. A source of sublimable material for use in sublimation pumps having a housing, a body of sublimable material, a shield of non-sublimable material encasing said body of sublimable material, said body and shield supported within the housing, a support rod mounted within the housing for supporting said body of sublimable material, means supporting said shield on said support rod, said body of sublimable material being in the form of a cylinder having an axial opening therethrough, said support rod extending through said axial opening and through said means supporting said shield, said shield of non-sublimable material having a larger axial dimension than said cylindrical body of sublimable material and supported on said support rod in spaced relationship axially and radially from said body of sublimable material and maintained in heat transfer relation therewith, said shield having spaced perforations formed along the axial and circumferential dimensions thereof.

2. A source as claimed in claim 1, in which the perforations are in the form of slots which partially overlap adjacent slots longitudinally to define a serpentine body of non-sublimable material.

3. A source as claimed in claim 1, in which the shield is in the form of a strip of electroresistive material in the shape of a helix having adjacent turns spaced apart to define a helical gap for the emission of sublimed material.

4. A source as claimed in claim 1 in which the portions of the shield defining the perforations are shaped so that material passing through the perforations has a preferred direction which is not perpendicular to the axis of the support rod.

5. A shield as claimed in claim 1 in which the shield is in the form of a hollow cylinder having planar, imperforate, end portions.

6. A source as claimed in claim 1, in which the perforations are circular in plan.

7. A source as claimed in claim 6, in which the perforations extend over 10 to 20 percent of the area of the curved surfaces of the shield.

8. A source as claimed in claim 1, in which the sublimable material is of metal.

9. A source as claimed in claim 8, in which the metal is titanium metal or alloy.

10. A source as claimed in claim 1, in which the nonsublimable material is of a refractory metal.

11. A source as claimed in claim 10, in which the metal is of tantalum metal or alloy.

Claims

1. A source of sublimable material for use in sublimation pumps having a housing, a body of sublimable material, a shield of nonsublimable material encasing said body of sublimable material, said body and shield supported within the housing, a support rod mounted within the housing for supporting said body of sublimable material, means supporting said shield on said support rod, said body of sublimable material being in the form of a cylinder having an axial opening therethrough, said support rod extending through said axial opening and through said means supporting said shield, said shield of non-sublimable material having a larger axial dimension than said cylindrical body of sublimable material and supported on said support rod in spaced relationship axially and radially from said body of sublimable material and maintained in heat transfer relation therewith, said shield having spaced perforations formed along the axial and circumferential dimensions thereof.

10. A source as claimed in claim 1, in which the non-sublimable material is of a refractory metal.