Sputter ion pump cathode

Abstract

A sputter ion pump cathode (106) comprising: a sheet of material (200) having an aperture (202) therethrough; and a protrusion (204) extending along an axis (206) from an edge (208) of the aperture (202) inwards towards a central portion (210) of the aperture (202); wherein the protrusion (204) is twisted about the axis (206), for example, by approximate 90°.

Description

[0001] SPUTTER ION PUMP CATHODE

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to sputter ion pump cathodes, sputter ion pumps, and the fabrication of sputter ion pump cathodes.

[0004] BACKGROUND

[0005] Sputter ion pumps operate by sputtering a metal getter.

[0006] 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.

[0007] SUMMARY OF THE INVENTION

[0008] In an aspect, there is provided a sputter ion pump cathode comprising a sheet of material having an aperture therethrough, a protrusion extending along an axis from an edge of the aperture inwards towards a central portion of the aperture. The protrusion is twisted about the axis.

[0009] The sputter ion pump cathode may comprise a plurality of protrusions, each protrusion of the plurality of protrusions extending along a respective axis from the edge of the aperture inwards towards the central portion of the aperture. Each protrusion may be twisted about its respective axis.

[0010] The sputter ion pump cathode may comprise a plurality of apertures through the sheet of material, and, for each aperture of the plurality of apertures, one or more protrusions extend along a respective axis from an edge of that aperture inwards towards a central portion of that aperture. Each protrusion may be twisted about its respective axis.

[0011] The axis may lie within a plane defined by the sheet of material.

[0012] The protrusion may be twisted such that a plane defined by a portion of the protrusion is nonparallel with a plane defined by the sheet of material.

[0013] The protrusion may be twisted about its axis by approximately 90°.

[0014] The protrusion may comprise a proximal end at the edge of the aperture, a distal end opposite the proximal end, and a twisted portion between the proximal end and the distal end. The twisted portion may be closer to the proximal end than the distal end.

[0015] The sputter ion pump cathode may comprise a further sheet of material having a further aperture therethrough, and a further protrusion extending along a further axis from an edge of the further aperture inwards towards a central portion of the further aperture. The further protrusion may be twisted about the further axis. The further sheet of material may be attached to the sheet of material and positioned such that the further aperture is aligned with the aperture and such that further protrusion is spaced apart from the protrusion.

[0016] The sputter ion pump cathode may comprise a plurality of further protrusions, each further protrusion of the plurality of protrusions extending along a respective axis from the edge of the further aperture inwards towards the central portion of the further aperture. Each further protrusion may be twisted about its respective axis. The further sheet of material may be positioned such that each of the plurality of further protrusions are spaced apart from the plurality of protrusions.

[0017] The sputter ion pump cathode may comprise a plurality of further apertures through the further sheet of material, and, for each further aperture of the plurality of apertures, one or more further protrusions extend along a respective axis from an edge of that further aperture inwards towards a central portion of that further aperture. Each further protrusion may be twisted about its respective axis. The further sheet of material may be positioned such that each further aperture of the plurality of further apertures is aligned with a respective aperture of the plurality apertures.

[0018] In a further aspect, there is provided a triode sputter ion pump comprising a sputter ion pump cathode according to any preceding aspect.

[0019] In a further aspect, there is provided a method of forming a sputter ion pump cathode, the method comprising providing a sheet of material, cutting an aperture through the sheet of material including forming a protrusion extending along an axis from an edge of the aperture inwards towards a central portion of the aperture, and twisting the protrusion about the axis.

[0020] The cutting the aperture through the sheet of material may include forming a plurality of protrusions, each protrusion of the plurality of protrusions extending along a respective axis from the edge of the aperture inwards towards the central portion of the aperture. The method may further comprise twisting each protrusion about its respective axis.

[0021] The method may further comprise cutting one or more further apertures through the sheet of material including, for each further aperture, forming a further protrusion extending along an axis from an edge of that further aperture inwards towards a central portion of that further aperture, and twisting the further protrusions about their respective axes.

[0022] The method may further comprise providing a further sheet of material, cutting a further aperture through the further sheet of material including forming a further protrusion extending along an axis from an edge of the further aperture inwards towards a central portion of the further aperture, twisting the further protrusion about the axis, positioning the further sheet of material against the sheet of material such that the further aperture is aligned with the aperture and such that further protrusion is spaced apart from the protrusion, and attaching the further sheet of material to the sheet of material.

[0023] 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;

[0024] Figure 2 is a schematic illustration (not to scale) of a perspective view of a portion of a cathode of the sputter ion pump showing an aperture element;

[0025] Figure 3 is a schematic illustration (not to scale) of a front view of the portion of the cathode;

[0026] Figure 4 is a process flow chart showing certain steps of an embodiment of a method for forming the cathode;

[0027] Figure 5 is a schematic illustration (not to scale) of a front view of a portion of the sheet of material at a step of the method of Figure 4;

[0028] Figure 6 is a schematic illustration (not to scale) of a front view of a portion of a further cathode;

[0029] Figure 7 is a schematic illustration (not to scale) of a perspective view of a portion the further cathode; and

[0030] Figure 8 is a schematic illustration (not to scale) of a side view of the portion the further cathode.

[0031] DETAILED DESCRIPTION

[0032] 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.

[0033] 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.

[0034] 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. spot 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.

[0035] 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.

[0036] The cathodes 106 comprise a plurality of aperture elements 108. In this embodiment, the aperture elements 108 are substantially identical e.g., in size, shape, and / or orientation. In this embodiment, the aperture elements 108 are arranged as an array.

[0037] The cathodes 106 and the aperture elements 108 are described in more detail later below with reference to Figures 2 and 3.

[0038] 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. 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.

[0039] Figure 2 is a schematic illustration (not to scale) of a perspective view of a portion of a cathode 106 showing one of the aperture elements 108 of the plurality of aperture elements 108.

[0040] Figure 3 is a schematic illustration (not to scale) of a front view of a portion of a cathode 106 showing one of the aperture elements 108 of the plurality of aperture elements 108.

[0041] The cathode 106 comprises a sheet of material 200. In this embodiment, the material is a metal or alloy, such as titanium, a titanium alloy, tantalum, aluminium, or an aluminium alloy such as aluminium mixed rare-earth alloy (AIRE).

[0042] The aperture element 108 further comprises an aperture 202 formed through the sheet of material 200, and a plurality of protrusions 204. In this embodiment, there are six protrusions 204. Each protrusion 204 extends along a respective axis 206 from an edge 208 of the aperture 202 inwards towards a central portion 210 of the aperture 202.

[0043] The protrusions 204 are spaced apart around the edge 208 of the aperture 202, at different radial locations. The protrusions 204 may be radially spaced apart uniformly around the edge 208 of the aperture 202.

[0044] In this embodiment, the aperture 202 is a substantially circular aperture. The central portion 210 may be the centre of the circular aperture 202.

[0045] Each protrusion 204 is twisted about its respective axis 206.

[0046] In this embodiment, the sheet of material 200 may be substantially planar and may define a plane. For example, the plane defined by the sheet of material 200 may be substantially parallel with the plane of the page of Figure 3. The axes 206 of the protrusions 204 may lie within, or be parallel with, the plane defined by the sheet of material 200.

[0047] Each protrusion 204 comprises a proximal end 214 located at the edge 208 of the aperture 202, and a distal end 212 opposite the proximal end 214. The distal end 214 is located at or proximate to the central portion 210 of the aperture 202.

[0048] In this embodiment, each protrusion 204 comprises a fin portion 216 and a connection portion 218. The fin portion 216 is an octagonal fin, panel, or plate which is attached at one edge to the connection portion 218. The fin portion 216 is located at and defines the distal end 212 of the protrusion 204. The connection portion 218 is located at and defines the proximal end 214 of the protrusion 204. The connection portion 218 is disposed between and connects or attaches together the fin portion 216 and the edge 208 of the aperture 202.

[0049] A length of a protrusion 204 may be defined as the radial distance (i.e., the distance along the axis 206) between the distal end 212 and the proximal end 214 of that protrusion 204. In some embodiments, the length of the connection portion 218 (i.e., along the axis 206) may be between 10% and 40% of the length of the protrusion 204, for example about 25%. In some embodiments, the length of the fin portion 216 (i.e., along the axis 206) may be between 60% and 90% of the length of the protrusion 204, for example about 75%.

[0050] A width of a protrusion 204 may be defined as the circumferential distance across the protrusion 204.

[0051] In this embodiment, for each protrusion 204, the connection portion 218 is twisted about its respective axis 206. In contrast, the fin portion 216 is untwisted (e.g., is a substantially flat or planar fin, panel, or plate) that is rotated relative to the plane defined by the sheet of material 200 by virtue of the twisted connection portion 218. The twisted portion of the protrusion 204 (i.e., the connection portion 218) is closer to the proximal end 214 than the distal end 212. Thus, a plane defined by the fin portion 216 is nonparallel with a plane defined by the sheet of material 200. In this embodiment, the connection portion 218 is twisted or rotated about its axis 206 by approximately 90°. Thus, the plane defined by the fin portion 216 is approximately perpendicular to the plane defined by the sheet of material 200.

[0052] Figure 4 is a process flow chart showing certain steps of an embodiment of a method 400 for forming, fabricating, or manufacturing the sputter ion pump cathode 106.

[0053] It should be noted that certain of the process steps depicted in the flowchart of Figure 4 and described below may be omitted or such process steps may be performed in differing order to that presented below and shown in Figure 4. Furthermore, although all the process steps have, for convenience and ease of understanding, been depicted as discrete temporally-sequential steps, nevertheless some of the process steps may in fact be performed simultaneously or at least overlapping to some extent temporally.

[0054] At step s402, a sheet of material 200 is provided.

[0055] At step s404, the sheet of material 200 is cut (e.g., by laser cutting or stamping) to form the plurality of apertures 202 therethrough, including forming, for each aperture 202, the plurality of protrusions 204. At this stage, the protrusions 204 are cut in an “untwisted” state, i.e. the connection portions 218 of the protrusions are not twisted or rotated about their axes 206.

[0056] Figure 5 is a schematic illustration (not to scale) of a front view of a portion of the sheet of material 200 at step s404 showing one of the apertures 202 formed in the sheet of material 200.

[0057] In this embodiment, at step s404, both the connection portion 218 and the fin portion 216 are untwisted, e.g., are substantially flat or planar members that lie within the plane defined by the sheet of material 200.

[0058] A width of a protrusion 204 may be defined as the circumferential distance across the protrusion 204. In this embodiment, for each protrusion, a width of the connection portion 218 is less than a maximum width of the fin portion 216. This advantageously tends to facilitate the twisting of the connection portions 218 about their axes 206. At step s406, the protrusions 204 are twisted about their respective axes 206. The twisting of a protrusion may be performed by a machine, tool, or human gripping the fin portion 216 and rotating said fin portion 216 about the axis 206, thereby to cause the corresponding connection portion 218 to twist about the axis 206. In this embodiment, the fin portion 216 are rotated about their respective axes 206 by approximately 90°.

[0059] Twisting a protrusion 204 about its axis 206 may comprise rotating that protrusion 204 about its axis 206 such that a first portion of the protrusion 204 at a first side of the axis 206 is moved out of the plane of the sheet of material 200 to a position at a first side of that plane, and a second portion of the protrusion 204 at a second side of the axis 206 opposite to the first side of the axis 206 is moved out of the plane of the sheet of material 200 to a position at a second side of that plane opposite to the first side of the plane. For example, twisting a protrusion 204 about its axis 206 may comprise rotating its fin portion 216 about the axis such that a first portion of the fin portion 216 at a first side of the axis 206 is moved above the plane of the sheet of material 200, and a second portion of the fin portion 216 at a second side of the axis 206 opposite to the first side of the axis 206 is moved below the plane of the sheet of material 200, and such that the connection portion 218 is twisted about the axis 206.

[0060] Thus, the method 400 for forming, fabricating, or manufacturing the sputter ion pump cathode 106 is provided.

[0061] In the above embodiments, a cathode 106 comprises a single sheet of material 200 having a plurality of aperture elements. However, in other embodiments, a single cathode may comprise multiple sheets of material attached together. Advantageously, by forming a single cathode from multiple sheets of material attached together, the density of complex geometry features (i.e. , the protrusions) for each aperture element may be increased.

[0062] By way of example, Figure 6 is a schematic illustration (not to scale) of a front view of a portion of a further embodiment of a cathode, herein after referred to as the “further cathode” 600. Figure 6 shows one aperture element that the further cathode 600 comprises a plurality of such aperture elements 108 arranged as an array, for example as described above in more detail with respect to Figure 1 .

[0063] Figure 7 is a schematic illustration (not to scale) of a perspective view of a portion the further cathode 600 showing one of the aperture elements 108.

[0064] Figure 8 is a schematic illustration (not to scale) of a side view of a portion the further cathode 600 showing one of the aperture elements 108.

[0065] In this embodiment, the cathode 600 comprises two cathodes, namely a first cathode 106a and a second cathode 106b. The two cathodes 106a, 106b are stacked together, i.e. arranged in a side-by-side relationship in abutment. The two cathodes 106a, 106b are arranged such that major faces of the sheets of material 200a, 200b of the cathodes 106a, 106b contact each other. The two cathodes 106a, 106b may be attached together in any appropriate way, such as by spot welding.

[0066] In this embodiment, the two cathodes 106a, 106b are arranged such that each aperture 202 of the first cathode 106a is aligned with a respective aperture 202 of the second cathode 106b.

[0067] In this embodiment, for each aperture element 108, the protrusions 204a of the first cathode 106a (which are hereafter referred to as the first protrusions 204a) have different radial positions about the aperture 200 compared to the protrusions 204b of the second cathode 106b (which are hereafter referred to as the second protrusions 204b). More specifically, the first protrusions 204a and the second protrusions 204b are arranged about respective apertures 202 on the first and second cathodes 106a, 106b such that, when the apertures are aligned and the first and second cathodes 106a, 106b brought into abutment, the first protrusions 204a and the second protrusions 204b are substantially uniformly distributed about the aligned apertures, e.g. when viewed from the front as shown in Figure 6.

[0068] The first cathode 106a and the second cathode 106b may be substantially as described earlier above with respect to Figures 2 and 3. The first cathode 106a and the second cathode 106b may be formed using the method 400 described earlier above with respect to Figures 4 and 5. The further cathode 600 may then be formed from the first cathode 106a and the second cathode 106b by the positioning the sheets of material 200a, 200b of the cathodes 106a, 106b in abutment such that the apertures 202 of the cathodes 106a, 106b are aligned and such that the protrusions 204a, 204b are spaced apart. The cathodes 106a, 106b may then be attached together.

[0069] In this embodiment, each aperture element 108 of the further cathode 600 comprises 12 protrusions (i.e., 6 first protrusions 204a of the first cathode 106a and 6 second protrusions 204b of the second cathode 106b).

[0070] The first cathode 106a may be substantially identical or similar to the second cathode 106b, except that the aperture elements 108 of the first cathode 106a are rotated about their central axes (i.e., the centres 210) compared to the aperture elements 108 of the second cathode 106b, such that the first protrusions 204a occupy different radial positions about the apertures 202 compared to the second protrusions 204b. In this embodiment, where each aperture element 108 of each cathode 106a, 106b comprises 6 protrusions 214a, 204b, the aperture elements 108 of the first cathode 106a are rotated about their central axes compared to the aperture elements 108 of the second cathode 106b by approximately 30°.

[0071] Thus, complex-geometry cathodes for triode sputter ion pumps and the production thereof are provided.

[0072] In some embodiments, each cathode may be constructed from two plates. Each plate has shapes cut or stamped out of the plate in association with each cell of the pump’s anode. The shapes are a pattern about the central axis of the associated anode cell. The shape is a pattern of symmetric features including an open centre. After the shapes have been cut, the complex geometry of each plate is formed by symmetrically twisting, e.g. by approximately 90°, each of the positive features patterned within the plane of the plate. The cut-out patterns are designed such that two similar or identical plates may be stacked flush together to double the density of complex geometry features associated with each anode cell. One of the plates may be rotated about one of its major axes to do so. The anode cell array may be symmetric about one of the major axes.

[0073] Advantageously, multiple (e.g., two) of the above-described cathodes can be stacked in abutment to create a single cathode. When two cathodes are stacked together as described above, the twisted protrusions of the different cathodes tend only to be offset relative to each other by the thickness of one sheet of material.

[0074] Advantageously, the above-described cathodes tend to reduce or eliminate the use of extra components or features that are conventionally used to join two plates together into one cathode, such as frame hardware to hold slats in place, or cuts and bends around the edges of each plate to create a frame and standoff spacing between the plates for interfacing.

[0075] Advantageously, the above-described cathodes tend to be easy to manufacture, using only cutting and twisting operations.

[0076] Advantageously, the geometry of deformation tends to provide for uniform stress in the twisted / deformed region. The stress tends to be a continuous shear around the neutral axis, as opposed to both compression and tension stresses on opposite sides of the neutral axis in an angled out-of-plane bend.

[0077] In the above embodiments, the sputter ion pump is a triode sputter ion pump. The sputter ion pump may be, for example, a standard diode or a noble diode 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.

[0078] 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.

[0079] In some embodiments, multiple different sizes of anode cell are used in the same anode array. In such embodiments, a cathode may comprise multiple different sizes of apertures corresponding to the different sizes of anode cell.

[0080] 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.

[0081] 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.

[0082] In the above embodiments, the apertures of the cathode aperture elements are substantially circular in shape. However, in other embodiments, the aperture of one or more of the cathode aperture elements is non-circular in shape.

[0083] In the above embodiments, each aperture element of a cathode comprises 6 or 12 protrusions. However, in other embodiments, one or more aperture elements of a cathode comprises a different number of protrusions.

[0084] In the above embodiments, for each aperture elements, the protrusions are arranged uniformly about the edge of the aperture. However, in other embodiments, one or more of the aperture elements comprises protrusions arranged differently, i.e. non-uniformly about the edge of the aperture.

[0085] In the above embodiments, each protrusion comprises a substantially octagonal fin portion, and connection portion having smaller width than the fin portion. However, in other embodiments, one or more of the protrusions has a different shape. In the above embodiments, each protrusion is twisted by approximately 90° about its axis to be out of plane with, and substantially perpendicular to, the sheet of material. However, in other embodiments, one or more of the protrusions is twisted about its axis to a different degree, other than 90°, such as between 85° and 95°, or between 80° and 100°, or between 75° and 105°, or between 70° and 110°, or between 65° and 115°, or between 60° and 120°, or between 55° and 125°, or between 50° and 130°, or between 45° and 135°.

[0086] Reference numeral list

[0087] 100 - sputter ion pump element

[0088] 102 - envelope

[0089] 104 - anode assembly

[0090] 106, 106a, 106b - cathode

[0091] 108 - cathode aperture elements

[0092] 200, 200a, 200b - sheet of material

[0093] 202 - aperture

[0094] 204, 204a, 204b - protrusions

[0095] 206 - axes

[0096] 208 - aperture edge

[0097] 210 - aperture centre

[0098] 212 - distal end

[0099] 214 - proximal end

[0100] 216 - fin portion

[0101] 218 - connection portion

[0102] 400 - method s402 - s406 - method steps

[0103] 600 - further cathode

Claims

CLAIMS1 . A sputter ion pump cathode comprising: a sheet of material having an aperture therethrough; and a protrusion extending along an axis from an edge of the aperture inwards towards a central portion of the aperture; wherein the protrusion is twisted about the axis.

2. The sputter ion pump cathode of claim 1 , wherein the sputter ion pump cathode comprises: a plurality of protrusions, each protrusion of the plurality of protrusions extending along a respective axis from the edge of the aperture inwards towards the central portion of the aperture; wherein each protrusion is twisted about its respective axis.

3. The sputter ion pump cathode of any preceding claim, wherein the sputter ion pump cathode comprises: a plurality of apertures through the sheet of material; and, for each aperture of the plurality of apertures, one or more protrusions extend along a respective axis from an edge of that aperture inwards towards a central portion of that aperture; wherein each protrusion is twisted about its respective axis.

4. The sputter ion pump cathode of any preceding claim, wherein the axis lies within a plane defined by the sheet of material.

5. The sputter ion pump cathode of any preceding claim, wherein the protrusion is twisted such that a plane defined by a portion of the protrusion is nonparallel with a plane defined by the sheet of material.

6. The sputter ion pump cathode of any preceding claim, wherein the protrusion is twisted about its axis by approximately 90°.

7. The sputter ion pump cathode of any preceding claim, wherein the protrusion comprises: a proximal end at the edge of the aperture; a distal end opposite the proximal end; and a twisted portion between the proximal end and the distal end; wherein the twisted portion is closer to the proximal end than the distal end.

8. The sputter ion pump cathode of any preceding claim, wherein the sputter ion pump cathode comprises: a further sheet of material having a further aperture therethrough; and a further protrusion extending along a further axis from an edge of the further aperture inwards towards a central portion of the further aperture; wherein the further protrusion is twisted about the further axis; and the further sheet of material is attached to the sheet of material and positioned such that the further aperture is aligned with the aperture and such that further protrusion is spaced apart from the protrusion.

9. The sputter ion pump cathode of claim 8 when dependent on claim 2, wherein the sputter ion pump cathode comprises: a plurality of further protrusions, each further protrusion of the plurality of protrusions extending along a respective axis from the edge of the further aperture inwards towards the central portion of the further aperture; wherein each further protrusion is twisted about its respective axis; andthe further sheet of material is positioned such that each of the plurality of further protrusions are spaced apart from the plurality of protrusions.

10. The sputter ion pump cathode of claim 8 or 9 when dependent on claim 3, wherein the sputter ion pump cathode comprises: a plurality of further apertures through the further sheet of material; and, for each further aperture of the plurality of apertures, one or more further protrusions extend along a respective axis from an edge of that further aperture inwards towards a central portion of that further aperture; wherein each further protrusion is twisted about its respective axis; and the further sheet of material is positioned such that each further aperture of the plurality of further apertures is aligned with a respective aperture of the plurality apertures.

11. A triode sputter ion pump comprising a sputter ion pump cathode according to any preceding claim.

12. A method of forming a sputter ion pump cathode, the method comprising: providing a sheet of material; cutting an aperture through the sheet of material including forming a protrusion extending along an axis from an edge of the aperture inwards towards a central portion of the aperture; and twisting the protrusion about the axis.

13. The method of claim 12, wherein the cutting the aperture through the sheet of material includes forming a plurality of protrusions, each protrusion of the plurality of protrusions extendingalong a respective axis from the edge of the aperture inwards towards the central portion of the aperture; and the method further comprises twisting each protrusion about its respective axis.

14. The method of claim 12 or 13, further comprising: cutting one or more further apertures through the sheet of material including, for each further aperture, forming a further protrusion extending along an axis from an edge of that further aperture inwards towards a central portion of that further aperture; and twisting the further protrusions about their respective axes.

15. A method of any of claim 12 to 14, further comprising: providing a further sheet of material; cutting a further aperture through the further sheet of material including forming a further protrusion extending along an axis from an edge of the further aperture inwards towards a central portion of the further aperture; twisting the further protrusion about the axis; positioning the further sheet of material against the sheet of material such that the further aperture is aligned with the aperture and such that further protrusion is spaced apart from the protrusion; and attaching the further sheet of material to the sheet of material.

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