Cryopump

Abstract

Cryopump 10 comprising a housing 12 having an inlet 16 to be connected to a vacuum chamber 18, a cold head 20 arranged in the housing for generating low temperatures for condensation, cold panel(s) 30

Description

The present invention relates to a cryopump or cryogenic pump generating a vacuum due to trapping of gases and vapers by condensation on cold surfaces. Conventional cryopumps comprise a cold head extending into a housing of the cryopump. The housing comprises an inlet, wherein the inlet of the cryopump is connected to a vacuum chamber or vacuum apparatus. The cold head is usually cooled by liquid helium or liquid nitrogen. The cold panel is connected to the cold head in order to increase the surface area of the cold head for cryo-con-densation pumping process. The cold head cools the cold panel down to temperatures usually below 20 K for pumping N2, Ar, O2 (other gases such as Xe, Kr, CO are pumped at the cold panel at temperatures below 55K; H2O vapor, CO2, are usually pumped at the first stage around 65K). Conventional cryopumps are usually combined with sorption pumps, in general activated charcoal (AC) is used as sorption material and coated on the cold panel inside the pump which is cooled by the cold head. The H2, He, and Ne are cryosorbed on the cooled AC. However, AC can be easily irreversibly contaminated by hydrocarbons including methane. Therein, AC cannot be cleaned or reactivated and must be replaced. Replacing AC is very complicated and expensive and requires the pump to be sent back to the manufacturer, thus halting operations for potentially long periods. In addition, usually the AC is glued to the cold panel by an epoxy resin. However, the epoxy resin can be degraded by radioactive materials for example of tritium. Furthermore, common epoxy resins may contaminate the pumped environment also. Replacing the commonly used epoxy resin for attaching the AC is expensive. It is an object of the present invention to provide a cryopump with improved pump performance and reliable operation. The problem is solved by a cryopump according to claim 1. The cryopump according to the present invention comprises a housing having an inlet to be connected to a vacuum chamber. A cold head is arranged in the housing for generating low temperatures for condensation. At least one cold panel is connected to the cold head, wherein one or more non-evaporable getter elements (NEG elements) are connected to the at least one cold panel. Thus, according to the present invention the activated charcoal (AC) used in the prior art is replaced by an NEG element. Therein, NEG elements have the advantage that they cannot be contaminated irreversibly with hydrocarbons. NEG elements can be reactivated by heating. Thus, a more reliable and stable operation of the cryopump is enabled. At the same time, it has been shown that NEG elements at lower temperature have increased pump performance. Thus, at the same time the low temperature of the cold head is used by the NEG element to increase their pump performance. In particular, compared to operating the NEG element under room temperature, an increase of the pumping capacity by a factor of more than 10, preferably more than 100 is feasible depending on the temperature. Preferably, the cryopump comprises a plurality of cold panels, wherein to more than one and preferably each cold panel, an NEG element is connected. Thus, by the plurality of NEG elements a sufficient surface of the NEG element is provided for efficiently pumping gases. Preferably, the cold panels are built as baffles or plates, wherein the NEG element is connected to a surface of the baffle or plates facing away from the inlet. Thus, the NEG elements are protected by the shape of the baffle and blocking the NEG elements by condensed gases on the opposite surface of the baffle facing towards the inlet is prevented. Preferably, the NEG element is cooled down to a temperature below 20 K and preferably below 10 K. At this temperatures cryo-condensation in the cryopump is possible. At the same time pump performance of the NEG elements is increased by a factor of more than 100 compared to operating the NEG element at room temperature. Preferably, the NEG element comprises one of Zr, V, Ti, Fe, Al, Ta or a combination thereof. Thus, the NEG element may comprise one of the aforementioned elements or an alloy of more than one of the aforementioned elements. Preferably, the cryopump comprises activated charcoal connected to one of the cold panels. Thus, the cryopump may comprise a combination of AC and NEG elements in order to combine the advantages together. Alternatively, the cryopump comprises no activated charcoal. Thus, afore-mentioned disadvantages of using AC are avoided. Preferably, the NEG material of the NEG element is coated onto the cold panels. Thus, due to direct coating of the NEG element to the cold panels no additional adhesive or connection element is necessary anymore. Contamination of the NEG material due to any adhesive or degradation of the adhesive are prevented. Since the NEG element can be reactivated, no replacement is necessary and permanent coating of the NEG elements to the cold panels is feasible. Preferably, at least one NEG element, more preferably more than one NEG element and most preferably all of the NEG elements are glued to the cold panels preferably by a radiation resistant epoxy resin. Preferably, at least one NEG element, more preferably more than one NEG element and most preferably all of the NEG elements are mechanically connected to the cold panels. Therein, the NEG element may be built as a separate piece and clamped or otherwise mechanically connected to the cold panels. Thus, replacement of the NEG element is enabled if necessary. Preferably, the NEG elements have a total surface of 300 cm2 or more, more preferably 500 cm2 or more and most preferably 600 cm2 or more, depending on the size of the cryopump in which the NEG element is placed. Thus, the combined surface of all the NEG elements is sufficiently large in order to provide sufficient pumping speed and capacity. Preferably, the pump capacity of the NEG element for H2 is larger than 1000 l / sec at room temperature, for cryopump with a nominal N2 pumping speed of 1500 l / s. The H2 pumping speed of the NEG material below 20 K is improved by a a factor of >10. Due to the combined effects of the large surface and the low temperature increasing the pump performance of the NEG element, high pump speeds can be achieved. Preferably, a common heating element is provided in the cryopump to reactivate the cold panels and the one or more NEG elements. Therein, the heating element may be connected to the cold head. By the heating element the temperature of the cold panels as well as the NEG elements can be increased. On the one hand condensate gases on the cold panels are evaporated and removed and at the same time the NEG elements are reactivated. Preferably, the heating element provides a temperature of above 100 °C, preferably more than 150 °C and most preferably above 250 °C in order to reactivate the NEG elements preferably at pressure below IE-5 mbar. NEG regeneration / reactivation would necessitate the use of a supporting demountable turbomolecular pump. In the following the present invention is described in detail with reference to the accompanying figure. The figure shows: Figure a schematic cryopump according to the present invention. The cryopump 10 according to the present invention comprises a housing 12 defining a pump chamber 14 and an inlet 16. The inlet 16 is connected to a vacuum chamber 18 in order to remove gases from the vacuum chamber 18 by the cryopump 10. The cryopump 10 comprises a cold head 20, which may be for example a two stage Gifford-McMahon refrigerator. Therein, the cold head 20 comprises a first stage 22 and a second stage 24. The cold head 20 may be cooled by liquid He. Therein, the first stage 22 may have a temperature of around 80 Kelvin. The second stage 24 comprises a temperature of less than 20 Kelvin and preferably less than 10 Kelvin. In particular, the temperature can be between 7 K and 9 K. The cold head 20 is surrounded by a radiation shield 26 in order to reduce heat transfer by radiation to the cold head 20. The radiation shield 26 is complemented by an inlet baffle 28. Baffle 28 and radiation shield 26 may have a temperature of around 100 Kelvin. At the baffle 28 gas compounds from the vacuum chamber 18 having a high condensation temperature are already removed. Similar, those compounds condensate at the inner surface of the radiation shield 26 as well. To the second stage 24 of the cold head 20 a plurality of cold panels 30 are connected. The cold panels 30 have a first surface 32 facing towards the inlet 16 and an opposite surface 34 facing away from the inlet 16. In particular, the cold panels 30 may be shaped as baffles. On the first surface 32 gas molecules may condensate. Thereby, the first surface 32 will be covered during pump action by frozen gas compounds. At the second surface 34 of the cold panels 30 NEG elements 36 are attached. Attaching of the NEG elements 36 to the cold panels 30 may be performed by either coating the NEG material with the NEG elements 36 directly onto the second surface 34. This is possible since replacement of the NEG element 36 is not necessary anymore. Alternatively, the NEG elements 36 may be glued or otherwise mechanically connected to the cold panels 30. Due to the shape of the cold panels 30 coverage of the NEG elements 36 by frozen or condensed gas compounds is minimized maintaining the pump action of the NEG elements 36 over a longer time of operation. Due to connection of the NEG elements 36 to the cold head 20, the NEG elements 36 are at the same low temperature as the cold panels 30. Due to the low temperature, it has been surprisingly shown that pump performance of the NEG elements 36 is strongly increased by a factor of more than 10 and in particular, more than 100. At the same time, contamination of the NEG elements due to hydrocarbons such as methane is not possible anymore compared to the prior art using activated charcoal (AC). Similar, degradation of any adhesive used to attach the AC to the cold panels in the prior art is prevented due to directly coating the NEG elements onto the second surface 34 of the cold panels 30. Since it has been found by the present invention that the pump performance of NEG materials at low temperatures increases, beneficial combination of a cryopump with a NEG element is provided by the present invention, thereby avoiding the disadvantages of using activated charcoal in the pump. Thus, the cryopump can be operated over a longer time, having an increased pump performance. Afterwards reactivation of the NEG elements 36 is possible without the need of replacing them. Reference List: 10 cryopump 12 housing 14 pump chamber 16 inlet 18 vacuum chamber 20 cold head 22 first stage 24 second stage 26 radiation shield 28 baffle 30 cold panels 32 first surface 34 second surface 36 NEG element 19 03 25

Claims

1. Cryopump comprisinga housing having an inlet to be connected to a vacuum chamber,a cold head arranged in the housing for generating low temperatures for condensation,at least one cold panel connected to the cold head andone or more non-evaporable getter, NEG, elements connected to at least one cold panel.

2. Cryopump according to claim 1, comprising a plurality of cold panels, wherein to each cold panel an NEG element is connected.

3. Cryopump according to claim 1 or 2, wherein the cold panels are built as baffles, wherein the NEG element is connected to a surface of the baffles facing away from the inlet.

4. Cryopump according to any of claims 1 to 3, wherein the NEG element is cooled to a temperature of below 20K and preferably below 10K.

5. Cryopump according to any of claims 1 to 4, wherein the NEG element comprises one of Zr, V, Ti, Fe, Al, Ta or a combination thereof.

6. Cryopump according to any of claims 1 to 5, comprising activated charcoal connected to one of the cold panels.19 03 257. Cryopump according to any of claims 1 to 5, comprising no activated charcoal.

8. Cryopump according to any of claims 1 to 7, an NEG material of at least one NEG element is coated onto the cold panels.

9. Cryopump according to any of claims 1 to 8, wherein at least one NEG element is glued to the cold panels.10.Cryopump according to any of claims 1 to 9, wherein at least one NEG element is mechanically connected to the cold panels.11.Cryopump according to any of claims 1 to 10, wherein the NEG elements have a surface of 300 cm2 or more, preferably 500 cm2 or more and more preferably 600 cm2 or more.12.Cryopump according to any of claims 1 to 11, wherein a common heating element is provided to re-activate the cold panels and the one or more NEG elements.

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