NEG element

Abstract

A non-evaporable getter (NEG) element 10 comprises a substrate 14, wherein the substrate is an electrical insulator, and an NEG material layer 12 on the substrate, wherein the NEG material layer is ap

Description

The present invention relates to a non-evaporable getter (NEG) element, a NEG pump comprising such a NEG element and a method for fabricating such a NEG element. In the prior art a NEG element can be provided as a thin film coating which is applied to a surface of a vacuum chamber to provide distributed pumping. Alternatively, the NEG element is formed as pressed or sintered discs from powder to provide discrete elements for use in cartridge pumps. To condition and activate the NEG material it must be heated to temperatures between 150°C and 700°C depending on the used material. Conventional NEG cartridge pumps typically use a stack of discs of NEG elements which are heated by placing them near or in some cases in direct contact with a discrete heater cartridge such as a ceramic heater. Therein, the heater cartridge may be formed as a cylindrical rod, wherein the disc-shaped NEG elements are arranged around this rod in a stacked manner. Hence, heat provided by the heater cartridge is only generated in the center of the NEG elements and must distributed to the outer parts of each NEG element either by heat conduction or radiation. Thus, the NEG elements are prone to inhomogeneous heating causing either local overheating or underheating. At the same time, due to heating via heat conduction across pressed or sintered elements which provide poor heat conduction, the time for activation or the necessary temperature is increased. When the NEG material is provided by a thin film coating, these thin films are usually applied to the walls of the vacuum chamber and are heated during the chamber bake out using traditional methods to externally heat the vacuum chamber from the outside. However, this causes difficulties when the NEG material needs to be regenerated since the whole vacuum chamber needs to be heated for reactivation of the NEG material. Therein, conventionally, the NEG material is applied by sputtering or evaporation depositing enabling only thin layers, thereby reducing bulk effects of the NEG material and the storage capacity of, for example, hydrogen in the bulk of the NEG material. It is an object of the present invention to provide an NEG element enabling improved heating for activation and regeneration. The problem is solved by a NEG element according to claim 1, a NEG pump according to claim 12 and a method for fabricating a NEG element according to claim 16. The non-evaporable getter (NEG) element according to the present invention comprises a substrate, wherein the substrate is an electrical insulator and in particular a dielectric material. Further, the NEG element comprises a NEG material layer on the substrate. Therein, the NEG material layer is directly connected or applied to a surface of the substrate and in direct contact with the substrate. Therein, the NEG material layer is applied to the substrate by thermal spray coating. Due to use of thermal spray coating, thicker layers can be generated compared to evaporation deposition or sputtering. In particular, the NEG material layer may have a thickness of more 100pm and preferably more than 1mm. In particular, the thickness is between 100pm and 1000pm, preferably between 100pm and 500pm, and more preferably between 150pm and 250pm. Although these thicknesses may also be feasible with conventional deposition techniques, thermal spray coating is more efficient and provides a dense NEG material layer. Thus, compared to NEG material provided by a thin film coating in the conventional manner, by the NEG material layer applied by thermal spray coating, a thicker layer of NEG material is provided more efficiently, enhancing bulk effects such as depositing of hydrogen in the bulk of the NEG material. At the same time compared to the NEG elements of NEG cartridge pumps, the NEG element only comprises an NEG material layer at the surface of the substrate, reducing the amount of NEG material used. Consequently, the NEG element is not completely formed by the NEG material. In addition, by the substrate structural stability is improved compared to the pressed or sintered NEG discs in the conventional art which are usually brittle and prone to damage in particular during transport, installation and due to thermal stress. At the same time due to thermal spray coating the bulk effect provided by a convention NEG disc is still maintained. Preferably, the NEG material layer comprises one or more of titanium (Ti), zirconium (Zr), aluminum (Al), vanadium (V), hafnium (Hf), tantalum (Ta) or iron (Fe). In particular, the NEG material layer comprises an alloy of one or more of these elements. In particular, the NEG material layer is made of ZrVFe or TiZrV. Preferably, the NEG material layer is electrically conductive. In particular, the NEG element may comprise a first electrical contact and a second electrical contact both connected to the NEG material layer to supply a current to the NEG material layer. Thus, by the current, the NEG material layer itself serves as resistive heater and thus can be self-heated or self-regenerated. The need for an additional heater element either as conductive thin film or even as discrete heater cartridge in the case of an NEG cartridge pump can be omitted. Thereby, heat is directly produced in the NEG material layer, providing a homogenous heat distribution to the NEG material and reducing the necessary power and time for activation or regeneration of the NEG material. In addition, in combination with the thickness of the NEG material layer provided by the thermal spray coating, the voltage required to supply a sufficient current through the NEG material layer is reduced. By the increased thickness of the NEG material layer, the resistance of the NEG material layer is reduced, thereby reducing the required voltage in order to provide the same heating power or apply a similar current compared to thin film coatings and thin film heaters. Thus, the need of high-voltage power supplies is omitted. Preferably, the NEG material is shaped in a tortuous path or track. Thus, by the tortuous path the length of the conductor of the NEG material is increased, thereby increasing the heating efficiency in the NEG material layer when applying a current to the NEG material layer. Therein, preferably the first electrical contact and the second electrical contact are connected to the tortuous path at its beginning and its end. Consequently, the current is fed via the NEG material layer along the complete tortuous path in order to provide resistive heating to the NEG material. In another aspect the present invention provides an NEG element comprising an electrically insulating substrate and electrically conductive NEG material located on the substrate, the NEG material being shaped in a tortuous path. Preferably, the tortuous path is created by laser ablation. Preferably, the tortuous path is at least partially and preferably completely covering a surface of the substrate and preferably one or more of a top surface, back surface and at least one side face of the substrate. Hence, the tortuous path may extend over the complete surface of the substrate thereby increasing on the one hand the area of the NEG material exposed to the vacuum chamber and at the same time providing the ability to heat all NEG material at all surfaces of the substrate homogenously. Preferably, the substrate comprises a top surface, an opposite back surface and at least one side surface, wherein the NEG material layer covers the top surface. Alternatively, the NEG material layer covers the top surface and the opposite back surface. Alternatively, the NEG material layer covers the top surface, the back surface and the at least one side surface. Hence, the NEG material may cover all surfaces of the substrate which may exclude those surfaces necessary for mounting the NEG element. Preferably, the first electrical contact and the second electrical contact are arranged on the same side of the substrate or arranged on opposite sides of the substrate. Consequently, the electrical contacts are accessible from the same side of the NEG element or are accessible from opposite sides of the NEG element allowing, in particular, connecting of the NEG elements in series when the NEG elements arranged in a stacked manner as in conventional NEG cartridge pumps. Preferably, the NEG material layer on the top surface is electrically connected to the NEG material layer on the back surface by vias through the substrate and / or by the NEG material layer on the side face of the substrate. Hence, if the NEG material also covers the side faces of the substrate, the NEG material layer on the top surface and the NEG material layer on the back surface may be electrically connected via the NEG material layer on the side face. In particular, if the NEG material layer is patterned to comprise a tortuous path, the tortuous path may extend from the top surface to the back surface across the side face of the substrate. Preferably, the substrate is disc shaped. Consequently, the overall NEG element may be disc shaped and the NEG element can be implemented in conventional NEG cartridge pumps replacing conventional NEG elements formed as pressed or sintered discs. In another aspect of the present invention, an NEG pump is provided comprising a plurality of NEG elements as aforementioned. Preferably, the NEG elements are arranged in a stacked manner along an axial axis. Preferably, the NEG material layers of the plurality of NEG elements are electrically connected to each other, in particular, connected in series. Consequently, the NEG material layers of the plurality of NEG elements can be activated or regenerated at the same time. Alternatively, the NEG elements are connected in parallel. Thus, upon failure of one of the NEG elements, the other NEG elements can be still operated. Preferably, the NEG elements are arranged in a stacked manner, wherein the second electrical contact of one NEG element is at the same time the first electrical contact of an adjacent NEG element. In particular, the first electrical contact of NEG element is arranged on the opposite side of the second electrical contact of the same NEG element. Hence, when stacking the NEG elements together, electrical contact between adjacent NEG elements is provided, connecting these NEG elements in series or in parallel to each other. Preferably, the NEG pump comprises no additional heater elements such as a discrete heater cartridge or ceramic heater as in the conventional art or may not comprise a thin film heater in addition. Heating is completely provided by the NEG material layer itself without the need of additional heating elements reducing the complexity of the NEG elements and the NEG pump itself. In another aspect the present invention provides an NEG pump comprising a stack of NEG elements electrically connected to each other, each element comprising an electrically insulating substrate and NEG material shaped in a tortuous path. In another aspect of the present invention a method for fabricating an NEG element as described before is provided. The method includes: providing an electrically insulating substrate; and applying an NEG material layer onto the substrate by thermal spray coating. Preferably, after applying the NEG material layer onto the substrate by thermal spray coating, the NEG material layer may be patterned in order to shape a tortuous path or track to provide a heating element by the NEG material layer. Patterning of the NEG material layer maybe performed by laser ablation, i. e. removing parts of the NEG material layer on the substrate in order to prevent electrical conductivity between these separated sections of the NEG material layer. Features described above in relation to one aspect of the invention are equally applicable to the other aspects of the invention, and vice versa. In the following the present invention is described in more detail with reference the accompanying figures, which illustrate: Figure 1 an NEG element in side view, Figure 2 an NEG element in top view, Figure 3 a first embodiment of an NEG vacuum pump, Figure 4 another embodiment of an NEG pump, Figure 5 another embodiment of an NEG element, Figure 6 a specific application scheme of the NEG pump, and Figure 7 another embodiment of an NEG element. Figure 1 is a detailed view of an NEG element 10. The NEG element 10 comprises a substrate 14. The substrate 14 may be provided by a dielectric substrate or may be provided by another electrically insulating material. Therein, the substrate 14 may be provided by a dielectric coating of a part of the vacuum chamber itself. On top of the substrate 14 an NEG material layer 12 is applied by thermal spray coating. The thermal spray coating provides thicker films of NEG material compared to the conventional methods of deposition of NEG material in thin film applications in a more efficient and reliable way. Therein, the NEG material layer 12 is electrically conductive such that by an electrical contact 16 a current can be supplied to the NEG material layer 12 in order to provide a resistive heating effect directly by the NEG material layer 12 itself without the need of any additional heating element. Figure 2 illustrates the NEG element 10 in a top view, wherein in the example of Figure 2, the NEG element 10 is disc shaped. The NEG material layer 12 in the example of Figure 2 is patterned into a tortuous path or track. Therein, the surface of the substrate 14 may be fully covered by thermal spray coating with the NEG material in order to form the NEG material layer 12. Subsequently, intermediate parts or sections of the NEG material may be removed for example by laser ablation or any other technique to form the tortuous path as shown in Figure 2. Of course, Figure 2 only shows one example of the tortuous path that covers the substrate of the NEG element. Other shapes are also possible and the present invention is not limited to the illustrated example of Figure 2. A beginning of the tortuous path may be connected to a first electrical contact element 16A, wherein the end of the tortuous path may be connected to a second electrical contact element 16B in order to supply a current along the path of the NEG material layer across substantially the whole surface of the substrate 14. The geometry (particularly the aspect ratio) of the NEG track or tortuous path of the NEG material layer 12 is selected to give a suitable value for resistance between the two electrical contact element 16A, 16B while providing good coverage of the substrate surface and maximizing surface area for pumping. This allows a current to be passed through the NEG material layer which will dissipate heat uniformly throughout and raise its temperature to the point of conditioning or activation. Conventional NEG discs are about 2 mm thick, whereas the layer thickness achievable with the thermal spray process is more than 10pm, preferably more than 100pm. In particular, the thickness is between 100pm and 1000pm, preferably between 100pm und 500pm and more preferably between 150pm and 250pm. This means that the surface area to volume ratio of the material can be increased. As only the exposed surface of the material contributes to pumping speed, this allows less NEG material to be used while achieving the same surface area and hence pumping speed. However, at the same time, the overall volume of material is still increased compared to thin film NEG elements which will provide sufficient total capacity for volume absorbed gases, specifically hydrogen. If applied to the chamber surface in the form of a NEG coating, the NEG element has the advantage of the ability to heat and activate in inaccessible areas and localized heating of the NEG material in environments where the chamber cannot be heated to the required temperature for activation. As the heating effect is generated within the NEG material, it does not need to be transmitted from a discrete heater to the material. This means that the heating could be made smaller and more localized to where it is needed and kept away from any temperature sensitive components within the system. Direct heating of the NEG material also facilitates a method to determine the temperature of the NEG material without the need for any discrete thermal measurement devices (e.g. thermocouples, IR cameras, etc.). The voltage and current supplied to the NEG material can be used to measure the change in resistivity of the material which corresponds to a certain temperature. Current NEG pumps use separate heater elements meaning that this method can only be used to determine the heater filament resistivity. While it is possible to correlate this to a NEG material temperature at steady state, it is sensitive to changes in thermal transfer and gives no indication of real-time transient temperatures. The NEG track or the tortuous path of the NEG material layer 12 generates the heat uniformly throughout, as opposed to a discrete heater which will transfer the heat from the center outwards. This improves uniformity of temperature across the NEG element 10 and allows for better utilization of the NEG material. Further, energy efficiency of the pump for a given pumping speed is improved. In current NEG pumps the heater filament is the hottest part of the system (possibly hotter than 750°C) however when the NEG is heated directly, this may reduce the temperature of the hottest part of the system to approximately 400°C. To supply power an electrically and thermally conductive path exists from the filament to outside the vacuum, resulting in conduction losses that do not contribute to NEG heating. As the NEG element according to the present invention is cooler due to direct heating, this will reduce the conduction losses along this path. Figure 3 illustrates an NEG pump 20 comprising a plurality of NEG elements 10 stacked along an axial direction. Therein, each NEG element 10 may be disc shaped and, according to the embodiment of Figure 3, each NEG element 10 comprises a substrate 14 having a top surface and a bottom surface. Therein, an NEG material layer 12 is applied to both the top surface and the bottom surface of each substrate 14. Therein, the electrical contact elements 16A, 16B are connected to a respective common terminal contacts 17A, 17B providing a current to each of the electrical contact elements 16A, 16B of the NEG elements 10 such that the individual NEG material layers 12 of the respective NEG elements 10 are connected in parallel. Therein, the NEG material layer 12 of each NEG element 10 may be patterned to comprise a tortuous path or track along which a current can flow from the first electrical contact element 16A to the second electrical contact element 16B, respectively. Therein, the NEG material layer 12 of one or more and preferably all NEG elements 10 may be patterned identically or at least two of them are patterned differently. For example, one or more and in particular each NEG material layer 12 of each NEG element 10 may be patterned as shown in the example of Figure 2. This embodiment has the advantage that if a single element should fail (burn out) then electrical contact is retained for the remaining elements. This is in particular advantageous in UHV / XHV systems, where exposure of the chamber to air for repair of broken elements must be avoided. Figure 4 illustrates an NEG pump 20. The NEG pump 20 of Figure 4 is formed of a stack of NEG elements 10 along an axis similar to the embodiment as shown in Figure 3. Each NEG element 10 has a first electrical contact element 16B on the back surface of the respective substrate 14 and a second electrical contact 16A on the top surface of the respective NEG element 10 which are electrically connected via the NEG material layer 12 also extending across the side faces of the substrate 14. The intermediate electrical contact elements 18 are positioned such that when the NEG elements 10 are stacked, the electrical contact element of one NEG element 10 is touching the electrical contact element of the next or adjacent NEG element 10 in the stack. This efficiently connects the respective tortuous path of the NEG elements 10 in series and allow a current to be passed through the whole stack using terminal contacts 17A, 17B at each end. Therein, the NEG material layer 12 of each NEG element 10 may be patterned to comprise a tortuous path or track along which a current can flow from the first electrical contact element 16A to the second electrical contact element 16B, respectively. Therein, the NEG material layer 12 of one or more and preferably all NEG elements 10 may be patterned identically or at least two of them are patterned differently. For example, one or more and in particular each NEG material layer 12 of each NEG element 10 may be patterned as shown in the example of Figure 2. This embodiment has advantage of reducing contact between the NEG material and the supply wire which will reduce thermal conduction along the supply wire and simplifies the structure and contact scheme of the NEG elements 10. Figure 5 illustrates an NEG element applied to the surface and in particular an insulating surface of a substrate 14’. Subsequently, the NEG material layer 12 may be patterned to introduce a track like structure or a tortuous path similar to this shown in Figure 2. This enables the simple NEG element to be constructed on any generic, insulating surface even when the surface is not perfectly flat. By having a non-flat surface of substrate 14’ it is also possible to further increase the surface area of this NEG element 10. Electrical contacts 16A, 16B may be provided on the surface of the insulating substrate 14' or may be provided by vias extending through the insulating substrate 14'. Figure 6 illustrates an NEG pump 20 similar to the embodiment of Figure 4, wherein some of the intermediate electrical contacts 26,28 are connected via switches 22, 24 to ground or live. In particular, the intermediate contacts 26, 28 connected to the respective switches 22, 24 are evenly distributed. When the switches 22, 24 are left open, all the NEG elements 10 are in series and the voltage is divided equally across all of them. When 5 the switches 22, 24 are closed each of the evenly sections is connected to the supply voltage in parallel and the voltage across each NEG element 10 increases. As power is proportional to voltage squared, two discrete power levels can be produced such as conditioning or activation levels - without the need for external voltage supply control. Additional switches could be used to add more than two discrete power settings. 10 The embodiment of Figure 7 comprises a resistive heater layer 36 which is for example formed by a patterned resistive heating layer. Therein, the resistive heating layer may form a tortuous path similar to the example of Figure 2 for the NEG material 12. Electrical contact elements 16A', 16B' contact this track or tortuous path of the resistive heater layer 36 to a current supply in order to provide heat to the NEG material layer 12 on top of the substrate 14. 15 REFERENCE NUMERAL LIST 10 12 5 14, 14’ 16A, 16A’ 16B, 16B’ 17A 17B 10 20 22, 24 26, 28 36 NEG element NEG material layer substrate first electrical contact element second electrical contact element terminal contact terminal contact NEG pump switches intermediate contacts resistive heater layer

Claims

1. Non-evaporable getter (NEG) element comprising,a substrate, wherein the substrate is an electrical insulator;an NEG material layer on the substrate;wherein the NEG material layer is applied to the substrate by thermal spray coating.

2. NEG element according to claim 1, wherein the NEG material layer has a thickness of more than 100pm and preferably more than 1mm, in particular, the thickness is between 100pm and 1000pm, preferably between 100pm und 500pm and more preferably between 150pm and 250pm.

3. NEG element according to claim 1 or 2, wherein the NEG material layer comprises one or more of titanium (Ti), zirconium (Zr), aluminum (Al) vanadium (V), Hafnium (Hf), Tantal (Ta) and iron (Fe).

4. NEG element according to any of claims 1 to 3, wherein the NEG material layer is electrically conductive.

5. NEG element according to any of claims 1 to 4, further comprising a first electrical contact and a second electrical contact, both connected to the NEG material layer to supply a current to the NEG material layer.

6. NEG element according to any of claims 1 to 5, wherein the NEG material layer is shaped in a tortuous path, wherein preferably the first electrical contact and the second electrical contact are connected to the tortuous path at its beginning and its end.

7. NEG element according to claim 6, wherein the tortuous path is at least partially and preferably completely covering a surface area of the substrate.

8. NEG element according to any of claims 1 to 7, wherein the substrate comprises a top surface, an opposite back surface and at least one side face, wherein the NEG material layer covers the top surface or covers the top surface and the back surface or covers the top surface, the back surface and the at least one side face.

9. NEG element according to claim 8, wherein the first electrical contact and the second electrical contact are arranged on the same side of the substrate or are arranged on opposite sides of the substrate.

10. NEG element according to claim 9, wherein the NEG material layer on the top surface is electrically connected to the NEG material layer on the back surface by vias through the substrate and / or by NEG material layer on the side face of the substrate.

11. NEG element according to any of claims 1 to 10, wherein the substrate is disc shaped.

12. NEG pump comprising a plurality of NEG elements according to any of claims 1 to 11.

13. NEG pump according to claim 12, wherein the NEG material layers of the plurality of NEG elementsare electrically connected to each other, in particular connected in parallel.

14. NEG pump according to claim 12 or 13, wherein the NEG elements are arranged in a stacked manner, wherein the second electrical contact of one NEG element is at the same time the first electrical contact of an adjacent NEG element.

15. NEG pump according to any of claims 12 to 14, comprising no additional heater element.

16. Method for fabricating an NEG element, the method comprising:providing an electrically insulating substrate; andapplying an NEG material layer onto the substrate by thermal spray coating.s

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