Sputter ion pump cathode

Abstract

A sputter ion pump cathode 106 comprises a first plate 201 of a first material arranged facing and substantially parallel with a second plate 202 of a second material, wherein the first material is a different material to the second material. The first and second materials are metals, the first material being more malleable and / or ductile than the second. The first material is preferably aluminium or an aluminium containing alloy, while the second may be titanium, tantalum or alloys thereof. The first and / or second plates may also comprise an array of apertures 211, 212, each aperture comprising a central hole 214 surrounded by a plurality of peripheral parts 216 formed by folding portions of the plates to form fins 221, 222. The disclosed cathode may form part of a sputter ion pump (SIP), preferably of the triode type, said pump comprising an anode assembly 104 disposed between two cathodes 106.

Description

FIELD OF THE INVENTION The present invention relates to sputter ion pumps, and more particularly to cathodes of sputter ion pumps. BACKGROUND Sputter ion pumps operate by sputtering a metal getter. A sputter ion pump first ionizes gas within the vessel to which it is attached. The positive ions are then accelerated by an electric field into a cathode. Cathode particles, i.e. small bits of the cathode, are sputtered into the pumping chamber. The sputtered particles condense on surfaces of the sputter ion pump. The condensed cathode material entraps ions through a combination of chemical reactions with the surface of the sputtered material, and by physically burying them beneath that sputtered material. As a result of this entrapment, pressure within the sputter ion pump is reduced. SUMMARY OF THE INVENTION In an aspect, there is provided a sputter ion pump cathode comprising a first plate of a first material and a second plate of a second material. The first plate is arranged facing and substantially parallel with the second plate. The first material is a different material to the second material. The first and second material may be metals. The first material may be more malleable than the second material. Alternatively, the first material may be less malleable than the second material. Alternatively, the first material may have the same malleability as the second material. The first material may be more ductile than the second material. Alternatively, the first material may be less ductile than the second material. Alternatively, the first material may have the same ductility as the second material. The first material may be aluminium. Alternatively, the first material may be titanium. The second material may be titanium. Alternatively, the second material may be aluminium. The first plate may comprise a first plurality of aperture elements. One or more of the first plurality of aperture elements may comprise: a central aperture through the first plate; and one or more peripheral apertures through the first plate, the one or more peripheral apertures being located at or proximate to a periphery of the central aperture, the one or more peripheral apertures surrounding the central aperture. One or more of the first plurality of aperture elements may comprise one or more first fins, each first fin being formed from a folded portion of the first plate that has been folded to be non-parallel with a plane of the first plate. The second plate may be a solid plate comprising no apertures therethrough. Alternatively, the second plate may comprise a second plurality of aperture elements. One or more of the second plurality of aperture elements may comprise: a central aperture through the second plate; and one or more peripheral apertures through the second plate, the one or more peripheral apertures being located at or proximate to a periphery of the central aperture, the one or more peripheral apertures surrounding the central aperture. One or more of the second plurality of aperture elements may comprise one or more second fins, each second fin being formed from a folded portion of the second plate that has been folded to be non-parallel with a plane of the second plate. In a further aspect, there is provided a sputter ion pump element comprising two spaced-apart cathodes and one or more anodes disposed between the two cathodes. At least one of the cathodes is in accordance with any preceding aspect. The first plate of each of cathode may be disposed closer to the one or more anodes than the second plate. The sputter ion pump element may be for a triode sputter ion pump. In a further aspect, there is provided a triode sputter ion pump comprising a sputter ion pump element according to any preceding aspect. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic illustration (not to scale) of a sputter ion pump element of a sputter ion pump; Figure 2 is a schematic illustration (not to scale) of side view cross section of the sputter ion pump element; Figure 3 is a schematic illustration (not to scale) of a first plate of the cathode of the sputter ion pump element; Figure 4 is a schematic illustration (not to scale) of the first plate; and Figures 5 to 7 are schematic illustrations (not to scale) of alternative shapes for a plate of the cathode of the sputter ion pump element. DETAILED DESCRIPTION Figure 1 is a schematic illustration (not to scale) of a perspective view of a sputter ion pump element 100. In this embodiment, the sputter ion pump element 100 is a pump element of a triode sputter ion pump. The sputter ion pump element 100 defines a substantially cuboid or rectangular envelope 102 which is configured to be connected to a system to be evacuated. The sputter ion pump element 100 comprises an anode assembly 104 and two cathodes 106. The anode assembly 104 is sandwiched between the two cathodes 106. The anode assembly 104 is disposed within the envelope 102. In this embodiment, the anode assembly 104 is a cellular anode assembly comprising a plurality of substantially hollow cells which are attached together. More specifically, the anode assembly 104 comprises a plurality of hollow cylindrical cells, or tubes, which are joined (e.g. tack welded) together to form an array of cells. In this embodiment, the cells of the anode assembly 104 are joined together to define a triangular grid structure. The cells of the anode assembly 104 may be formed from any appropriate material such as stainless steel. The cathodes 106 are disposed on opposite sides of the anode assembly 104. The anode assembly 104 is spaced apart from the cathodes 106, and may be held in position or supported relative to the cathodes 106 by an electrically insulating support, such as a support formed from a ceramic material. The cathodes 106 comprise a plurality of apertures 108 therethrough. The cathodes 106 are described in more detail later below with reference to Figure 2. In operation of the sputter ion pump element 100, a negative potential is applied to the cathodes 106 while the anode assembly 104 is held at ground potential. Alternatively, a positive potential may be applied to the anode assembly 104 while the cathodes 106 are maintained at ground potential. The electric potential employed between the anode assembly 104 and the cathodes 106 may be application dependent, but is typically between 3 kV and 7 kV. A magnetic field is provided parallel to the axes of the hollow cylindrical cells of the anode assembly 104. This magnetic field helps to form and maintain an electron cloud within the anode assembly 104. Electrons within the electron cloud then ionize the gas molecules with which they collide. The electric field between the anode assembly 104 and the cathodes 106 then accelerates the (now positive) ions into the cathodes 106. The ions impacting the surfaces of the cathodes 106 cause sputtering of the getter / cathode material. These sputtered particles are deposited on surfaces of the sputter ion pump element 100, such as on the anode assembly 104, the cathodes 106, and / or on support structures or a casing of the sputter ion pump element 100. The deposited sputtered particles tend to entrap ions through, for example, one or a combination of: chemical combination; burial (i.e., physically trapping the ions underneath the sputtered material) and diffusion in the cathode; or burial and covering over with deposited sputtered material. As a result of such ion entrapment, pressure within the sputter ion pump element 100 is reduced. Figure 2 is a schematic illustration (not to scale) of a side view cross section of the sputter ion pump element 100. In the embodiment, each cathode 106 of the sputter ion pump element 100 comprises a first plate 201 and a second plate 202. For each cathode 106, the first and second plates 201, 202 are arranged facing each other and substantially parallel with each other. In this embodiment, the cathodes 106 are arranged such that the first plates 201 of the cathodes 106 are those closest to the anode assembly 104, and the second plates 202 of the cathodes 106 are those furthest from the anode assembly 104. The first plates 201 may be considered inner plates of the cathodes 106 in the sputter ion pump element 100, and the second plates 202 may be considered outer plates of the cathodes 106 in the sputter ion pump element 100. Said another way, the first plates 201 of the cathodes 106 are disposed between the anode assembly 104 and the second plates 202 of the cathodes 106. In each cathode 106, the spacing between the first plate 201 and the second plate 202 may be application dependent, and may be for example between about 2 mm and 20 mm, or more preferably between about 4 mm and 15 mm. The first plate 201 and the second plate 202 may be attached together in any appropriate manner, for example, by tack welding or via screws. For each cathode 106, the first plate 201 is made of or comprises a first material. In this embodiment the first material is a metal. In this embodiment, the first material is aluminium. For each cathode 106, the second plate 202 is made of or comprises a second material which is different to the first material. In this embodiment the second material is a metal. In this embodiment, the second material is titanium. In this embodiment, the first material is more malleable than the second material. In this embodiment, the first material is more ductile than the second material. This, in this embodiment, the first material is easier to deform or bend than the second material. In this embodiment, each cathode 106 comprises a plurality of aperture elements 108. Each aperture element 108 comprises a respective first aperture element 211 through the first plate 201 and a respective second aperture element 212 through the second plate 201. Thus, the first plate 201 comprises a plurality of cut-outs or first aperture elements 211. Also, the second plate 202 comprises a plurality of cut-outs or second aperture elements 212. In this embodiment, each of the first and second aperture elements 211, 212 comprises a central aperture 214 and a plurality of peripheral apertures 216. The central apertures 214 and peripheral apertures 201 are apertures through the first / second plate 201, 202. In this embodiment, the peripheral apertures 216 are located at a periphery of the respective central aperture 214. The peripheral apertures 216 are distributed around, i.e. surround, the respective central aperture 214. In this embodiment, the first and second aperture elements 211, 212 are substantially identical e.g., in size, shape, and / or orientation. In this embodiment, each of the first and second plates 201, 202 comprises 38 aperture elements 211, 212 arranged as an array. In this embodiment, the central apertures 214 are approximately circular in shape. The central apertures may have diameters of between about 3 mm and about 10 mm, or more preferably between about 5 mm and about 8 mm, or more preferably about 6 mm. For a given first and / or second aperture element 211, 212, the peripheral apertures 216 extend radially from the central aperture 214. In some embodiments, the peripheral apertures 216 may be elongate in the radial direction. The peripheral apertures 216 may be connected to the central aperture 214 at a radially inward, or proximal end. Preferably, the outer diameters of the first and / or second aperture element 211, 212 are substantially equal to the inner diameters of the cylindrical hollow cells of the anode assembly 104. The inner diameters of the cylindrical hollow cells of the anode assembly 104 and / or the outer diameters of the first and / or second aperture element 211, 212 may be, for example, between about 10 mm and 30 mm, or more preferably between about 15 mm and 25 mm, or more preferably between about 15 mm and 20 mm, e.g. about 15 mm, about 16 mm, about 17 mm, about 18 mm, about 19 mm, or about 20 mm. The inner diameters of the cylindrical hollow cells of the anode assembly 104 and / or the outer diameters of the first and / or second aperture element 211, 212 being about 15 mm tends to be most preferable. Preferably, each aperture element 108 is located opposite to an end of a respective cylindrical hollow cell of the anode assembly 104. More specifically, preferably, the first and / or second aperture elements 211, 212 are co-axial with the hollow cells of the anode assembly 104, i.e. the centres of the circular central apertures 214 are aligned along longitudinal axes of respective cylindrical hollow cells of the anode assembly 104. In this embodiment, the first plate 201, and specifically the first aperture elements 211, comprise one of more first fins 221. Specifically, as an example, in this embodiment, each first aperture element 211 comprises 6 first fins 211. Each first fin 221 is formed from a folded portion of the first plate 201 that has been folded to be non-parallel with a plane of the first plate 201. In some embodiments, the first fins 221 may be substantially perpendicular to the plane of the first plate 201. In this embodiment, the one or more first fins 221 extend in a direction from the first plate 201 towards the second plate 202. A length of the first fins 221, in the direction from the first plate 201 towards the second plate 202, may be application dependent, and may be for example between about 2 mm and 10 mm, or more preferably between about 3 mm and 7 mm, or more preferably between about 4 mm and 5 mm, for example about 4 mm. In this embodiment, the second plate 202, and specifically the second aperture elements 212, comprise one of more second fins 222. Specifically, as an example, in this embodiment, each second aperture element 211 comprises 6 second fins 212. Each second fin 222 is formed from a folded portion of the second plate 202 that has been folded to be non-parallel with a plane of the second plate 202. In some embodiments, the second fins 222 may be substantially perpendicular to the plane of the second plate 202. In this embodiment, the one or more second fins 222 extend in a direction from the second plate 202 towards the first plate 201. A length of the second fins 222, in the direction from the second plate 202 towards the first plate 201, may be application dependent, and may be for example between about 2 mm and 10 mm, or more preferably between about 3 mm and 7 mm, or more preferably between about 4 mm and 5 mm, for example about 4 mm. Figure 3 is a schematic illustration (not to scale) showing a front view of the first plate 201. In the view shown in Figure 3, the first fins 221 extend in a direction that is non-parallel with, e.g. substantially perpendicular to, a plane of the page. It will be appreciated by those skilled in the art that the second plate 202 may have a shape that is similar to or the same as that of the first plate 201 shown in Figure 3. Figure 4 is a schematic illustration (not to scale) showing a front view of the first plate 201 at a certain stage of fabrication of the first plate 201. Specifically, Figure 4 shows the first plate 201 at a stage where the first aperture elements 211 have been cut out of the plate 201, but before the first fins 221 have been folded or bent out of the plane of the first plate 201, i.e. when the first fins 221 are substantially parallel with the plane of the first plate 201. To form the first plate 201, the first fins 221 are bent or folded out of the plane of the first plate 201. Advantageously, aluminium, from which the first plate 201 is formed in this embodiment, tends to be easier to machine or cut, and fold or bend compared to, for example, titanium. Accordingly, the fabrication of the first plate 201, and thus the cathode 106, tends to be easier compared to cathodes formed only of material such as titanium. Advantageously, the use of aluminium as a plate in a cathode in a triode sputter ion pump can yield favourable pumping speeds of, e.g., nitrogen or other noble gases when compared to titanium cathode plates. Aluminium tends to be easier to machine and cheaper than, for example, titanium. Accordingly, the use of aluminium for the first plate of the above-described cathode, which may have a complex geometry, tends to beneficial. Titanium, when used for the second plate of the above-described cathode, tend to provide for good sputter yield, chemical resistance, mechanical strength, temperature stability, and binding energy. Advantageously, the above-described triode cathode which combines both aluminium and titanium plates, tends to result in improved performance compared to conventional triode, conventional diode, or noble diode sputter ion pump cathodes. Use of the aluminium plate tends to provide stable pump speeds (e.g., nitrogen pump speeds) and tends to facilitate use of geometries that are conducive to high pressure operation, like in a triode. Additionally, like in a triode, noble gases tend to hit the aluminium geometry at a glancing angle and impinge on the anode. Synergistically, the high sputter yield of the titanium plate tends to allow for these gases to be buried under a layer of titanium on the anode. As aluminium tends to be easier to machine and bend than titanium, it opens opportunities for many more shapes and complex geometries for a triode cathode. In the above embodiments, the sputter ion pump is a triode sputter ion pump. However, in other embodiments the sputter ion pump is a different type of ion pump such as a diode sputter ion pump. In the above embodiments, the anode assembly comprises a plurality of hollow cylindrical cells, or tubes. Also, the cells of the anode assembly are joined together to define a triangular grid structure. However, in other embodiments the anode assembly may have a different structure. For example, one or more of the cells of the anode assembly may have a shape other than cylindrical, such as prismatic. Also, in some embodiments, the cells of the anode assembly may be joined together to define an arrangement other than a triangular grid structure, such as a square grid structure. In the above embodiments, the cathode aperture elements are arranged to define a triangular grid structure. However, in other embodiments the cathode aperture elements have a different arrangement. The cathode aperture elements may define an arrangement other than a triangular grid structure, such as a square grid structure. In the above embodiments, the cathode aperture elements are substantially identical e.g. in size, shape, and / or orientation. However, in other embodiments, one or more of the cathode aperture elements is different to one or more of the other cathode aperture elements. In the above embodiments, the central apertures of the cathode aperture elements are circular in shape. However, in other embodiments, the central aperture of one or more of the cathode aperture elements is non-circular in shape. In the above embodiments, the first material is aluminium. However, in other embodiments, the first material may be a different material, other than aluminium, that is different to the second material. Examples of such alternative first materials include, but are not limited to, aluminium alloys, such as aluminium mixed rare-earth alloy (AIRE). AIRE tends to yield a favourable nitrogen pumping speed. In the above embodiments, the second material is titanium. However, in other embodiments, the second material may be a different material, other than titanium, that is different to the first material. Examples of such alternative second materials include, but are not limited to, titanium alloys or tantalum. In the above embodiments, both the first and second plates of each cathode comprise cutouts, apertures or aperture elements therethrough. However, in other embodiments, at least one or the first or second plates of one or more of the cathodes does not have any cutouts, apertures or aperture elements therethrough. For example, in some embodiments, the second plate of one or more of the cathodes is a solid plate (e.g., of titanium) comprising no apertures therethrough. In the above embodiments, each aperture element comprises a single central aperture and peripheral apertures extending radially therefrom. The peripheral apertures are located at a periphery of the central aperture and may be connected to the central aperture. The aperture elements may exhibit radial symmetry, rotational symmetry, and / or reflection symmetry. However, in other embodiments, one or more of the aperture elements may have a different structure or shape. Figure 5 is a schematic illustration showing a first example shape for a first and / or second plate 500 of a sputter ion pump cathode for a triode sputter ion pump. In this embodiment, the cathode plate 500 comprises a plurality of cutouts or aperture elements 502. Each aperture element 502 comprises a single central aperture 504 and a plurality of peripheral apertures 506. The aperture elements 502 are substantially identical e.g. in size, shape, and / or orientation. In this embodiment, the cathode plate 500 comprises 38 aperture elements 502 arranged as an array and defining a triangular grid. In this embodiment, the cathode plate 500, and specifically the aperture elements 502, comprise a plurality of fins 508. Specifically, in this embodiment, each aperture element 502 comprises 12 fins 508, arranged as 6 pairs. In other embodiments, there may be a different number of fins, which may be arranged differently. Each fin 508 is formed from a folded portion of the plate 500 that has been folded to be non-parallel (e.g., perpendicular) with a plane of the plate 500. Figures 6 and 7 are schematic illustrations showing a second example shape for a first and / or second plate 600 of a sputter ion pump cathode for a triode sputter ion pump. In this embodiment, the cathode plate 600 comprises a plurality of elongate aperture elements, in the form of substantially parallel slots 602. The slots 602 are transverse slots across a width of the cathode plate 600. In this embodiment, the cathode plate 600 comprises 23 slots 602 arranged side-by-side across a length of the plate 600. In this embodiment, the cathode plate 600 comprises a plurality of fins 604. Specially, adjacent to each slot 604, an elongate fin 604 extends from the 5 surface of the plate 600. Each fin 604 is formed from a folded portion of the plate 600 that has been folded to be non-parallel (e.g., perpendicular) with a plane of the plate 600. Reference numeral list 100 - sputter ion pump element 102 - envelope 104 - anode assembly 5 106 - cathode 108 - cathode aperture elements 201 - first plate 202 - second plate 211 - first aperture elements 10 212 - second aperture elements 214 - central aperture 216 - peripheral aperture 221 - first fins 222 - second fins 15 500, 600 - example alternative shapes

Claims

1. A sputter ion pump cathode comprising:a first plate of a first material; anda second plate of a second material; whereinthe first plate is arranged facing and substantially parallel with the second plate; andthe first material is a different material to the second material.

2. The sputter ion pump cathode of claim 1, wherein the first and second material are metals, and the first material is more malleable and / or ductile than the second material.

3. The sputter ion pump cathode of claim 1 or 2, wherein the first material is aluminium.

4. The sputter ion pump cathode of any of claims 1 to 3, wherein the second material is titanium.

5. The sputter ion pump cathode of any of claims 1 to 4, wherein the first plate comprises a first plurality of aperture elements.

6. The sputter ion pump cathode of claim 5, wherein one or more of the first plurality of aperture elements comprises:a central aperture through the first plate; andone or more peripheral apertures through the first plate, the one or more peripheral apertures being located at or proximate to a periphery of the central aperture, the one or more peripheral apertures surrounding the central aperture.

7. The sputter ion pump cathode of claim 5 or 6, wherein one or more of the first plurality of aperture elements comprise one or more first fins, each first fin being formed from a folded portion of the first plate that has been folded to be non-parallel with a plane of the first plate.

8. The sputter ion pump cathode of any of claims 1 to 7, wherein the second plate is a solid plate comprising no apertures therethrough.

9. The sputter ion pump cathode of any of claims 1 to 7, wherein the second plate comprises a second plurality of aperture elements.

10. The sputter ion pump cathode of claim 9, wherein one or more of the second plurality of aperture elements comprises:a central aperture through the second plate; andone or more peripheral apertures through the second plate, the one or more peripheral apertures being located at or proximate to a periphery of the central aperture, the one or more peripheral apertures surrounding the central aperture.

11. The sputter ion pump cathode of claim 9 or 10, wherein one or more of the second plurality of aperture elements comprise one or more second fins, each second fin being formed from a folded portion of the second plate that has been folded to be non-parallel with a plane of the second plate.

12. A sputter ion pump element comprising:two spaced-apart cathodes; andone or more anodes disposed between the two cathodes; whereinat least one of the cathodes is in accordance with any of claims 1 to 11.

13. The sputter ion pump element of claim 12, wherein:for the at least one of the cathodes that is in accordance with any of5 claims 1 to 11, the first plate of each of those cathodes is disposed closer to the one or more anodes than the second plate.14 The sputter ion pump element of claim 12 or 13, wherein the sputter ion pump element is for a triode sputter ion pump.o15. A triode sputter ion pump comprising a sputter ion pump element according to claim 14.15

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