US3824039A - Sublimable targets - Google Patents

Sublimable targets Download PDF

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Publication number
US3824039A
US3824039A US00237335A US23733572A US3824039A US 3824039 A US3824039 A US 3824039A US 00237335 A US00237335 A US 00237335A US 23733572 A US23733572 A US 23733572A US 3824039 A US3824039 A US 3824039A
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United States
Prior art keywords
shield
source
sublimable
sublimable material
perforations
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Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Expired - Lifetime
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US00237335A
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English (en)
Inventor
B Power
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BOC Group Ltd
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British Oxigen Ltd
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Publication date
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J7/00—Details not provided for in the preceding groups and common to two or more basic types of discharge tubes or lamps
    • H01J7/14—Means for obtaining or maintaining the desired pressure within the vessel
    • H01J7/18—Means for absorbing or adsorbing gas, e.g. by gettering
    • H01J7/186—Getter supports
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J41/00—Discharge tubes for measuring pressure of introduced gas or for detecting presence of gas; Discharge tubes for evacuation by diffusion of ions
    • H01J41/12—Discharge tubes for evacuating by diffusion of ions, e.g. ion pumps, getter ion pumps

Definitions

  • 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.
  • 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.
  • known sources with high titanium storage bulk sublimators
  • 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.
  • 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.
  • 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.
  • a reflector plate 12 Extending into the interior of the housing 4 from beyond a reflector plate 12 are supports 14 for filaments 16.
  • 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.
  • 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.
  • a charge 20-of sublimable material such as titanium
  • 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.
  • tantalum is preferred, it would be possible to use molybdenum or other suitable refractory material as the non-sublimable material.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • the non-uniform rate of attrition experienced in known unshielded targets is 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.
  • 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.
  • 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.
  • a stack of washers is mounted on anode 2, being kept in place by two stops 30, of which the details are immaterial.
  • large washers 32 of non-sublimable material such as tungsten or tantalum
  • 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.
  • 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.
  • the sublimable material is heated by any suitable means.
  • 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.
  • 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.
  • 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.
  • the functioning of the source is otherwise as described above.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • a shield as claimed in claim 1 in which the shield is in the form of a hollow cylinder having planar, imperforate, end portions.

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  • Compressors, Vaccum Pumps And Other Relevant Systems (AREA)
US00237335A 1971-03-24 1972-03-23 Sublimable targets Expired - Lifetime US3824039A (en)

Applications Claiming Priority (1)

Application Number Priority Date Filing Date Title
GB774571A GB1386251A (en) 1971-03-24 1971-03-24 Source of sublimable material

Publications (1)

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US3824039A true US3824039A (en) 1974-07-16

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US00237335A Expired - Lifetime US3824039A (en) 1971-03-24 1972-03-23 Sublimable targets

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US (1) US3824039A (https=)
DE (1) DE2214234A1 (https=)
FR (1) FR2130600B1 (https=)
GB (1) GB1386251A (https=)
IT (1) IT950690B (https=)

Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US20130078112A1 (en) * 2011-09-28 2013-03-28 The Boeing Company Sublimation pump and method

Citations (11)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US1712370A (en) * 1926-04-27 1929-05-07 Gen Electric Electric discharge device
US2100045A (en) * 1935-10-12 1937-11-23 Alexander Paul Deposition of metallic films from metal vaporized in vacuo
US2107945A (en) * 1934-11-20 1938-02-08 Gen Electric Cathode structure
US2146374A (en) * 1934-09-10 1939-02-07 King Lab Inc Getter
US2469626A (en) * 1946-06-20 1949-05-10 Philips Lab Inc High vacuum getter
US2951170A (en) * 1958-12-04 1960-08-30 Machlett Lab Inc Getter for electron tubes
US3229147A (en) * 1961-09-01 1966-01-11 Gen Electric Thermionic emitter and method of making same
US3324331A (en) * 1966-01-28 1967-06-06 Eg & G Inc Gaseous reservoir and heater for hydrogen thyratrons
US3357634A (en) * 1966-02-28 1967-12-12 Nat Res Corp Orbiting electron vacuum device and anode-getter apparatus therefor
US3388290A (en) * 1964-04-15 1968-06-11 Wisconsin Alumni Res Found Electron orbiting device including a flat,ribbon-type,thermionic filament
US3391303A (en) * 1965-01-25 1968-07-02 Lewis D. Hall Electronic vacuum pump including a sputter electrode

Patent Citations (11)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US1712370A (en) * 1926-04-27 1929-05-07 Gen Electric Electric discharge device
US2146374A (en) * 1934-09-10 1939-02-07 King Lab Inc Getter
US2107945A (en) * 1934-11-20 1938-02-08 Gen Electric Cathode structure
US2100045A (en) * 1935-10-12 1937-11-23 Alexander Paul Deposition of metallic films from metal vaporized in vacuo
US2469626A (en) * 1946-06-20 1949-05-10 Philips Lab Inc High vacuum getter
US2951170A (en) * 1958-12-04 1960-08-30 Machlett Lab Inc Getter for electron tubes
US3229147A (en) * 1961-09-01 1966-01-11 Gen Electric Thermionic emitter and method of making same
US3388290A (en) * 1964-04-15 1968-06-11 Wisconsin Alumni Res Found Electron orbiting device including a flat,ribbon-type,thermionic filament
US3391303A (en) * 1965-01-25 1968-07-02 Lewis D. Hall Electronic vacuum pump including a sputter electrode
US3324331A (en) * 1966-01-28 1967-06-06 Eg & G Inc Gaseous reservoir and heater for hydrogen thyratrons
US3357634A (en) * 1966-02-28 1967-12-12 Nat Res Corp Orbiting electron vacuum device and anode-getter apparatus therefor

Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US20130078112A1 (en) * 2011-09-28 2013-03-28 The Boeing Company Sublimation pump and method
US9057362B2 (en) * 2011-09-28 2015-06-16 The Boeing Company Sublimation pump and method

Also Published As

Publication number Publication date
DE2214234A1 (de) 1972-10-12
IT950690B (it) 1973-06-20
FR2130600A1 (https=) 1972-11-03
FR2130600B1 (https=) 1976-08-06
GB1386251A (en) 1975-03-05

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