Pump systems

Abstract

A cryopump system 10 comprises a cryochiller 22 which cools a refrigerant to a first cryogenic temperature T1 for supply to a single stage cryocooler 13 of a cryopump 11. The cryocooler cools the refr

Description

FIELD OF THE INVENTION The field of the invention relates to pump systems and methods of operating pump systems, in particular but not limited to, a cryopump system and a turbomolecular pump system. BACKGROUND Cryopumps comprise a cryocooler for cooling a refrigerant, such as helium, to cryogenic temperatures. One or more cryogenic surfaces cooled by the refrigerant capture gases to create a vacuum. Cryopumps may be used to create the vacuum required in semiconductor manufacturing processes for example and generally a cryopump with a two-stage cryocooler is used to pump different types of gases and create the desired vacuum. The first stage cools refrigerant initially at an ambient temperature to a first cryogenic temperature capable of capturing certain types of gas and the second stage further cools the refrigerant to a second lower cryogenic temperature for capturing gases not captured at the first cryogenic temperature. It would be desirable to provide an improved pump system. SUMMARY According to an aspect, there is provided, a cryopump system comprising: a cryochiller configured to cool refrigerant to a first cryogenic temperature for supply to a single-stage cryocooler of a cryopump; and the cryopump comprising: the single-stage cryocooler configured to cool the refrigerant from the first cryogenic temperature to a second cryogenic temperature lower than the first cryogenic temperature, the single-stage cryocooler being configured to operate using a different refrigeration cycle to the cryochiller; and a cryogenic surface configured to be cooled by the single-stage cryocooler to the second cryogenic temperature for capturing gas. Refrigerant is typically supplied to a cryocooler of a cryopump at an ambient temperature. Therefore, to achieve the low cryogenic temperatures necessary for capturing certain types of gas, two-stage cryocoolers are used. The first stage cools the ambient temperature refrigerant to a first cryogenic temperature and the second stage further cools the refrigerant to a second lower cryogenic temperature. By supplying refrigerant which has been pre-cooled by a cryochiller, a single stage cryocooler may have sufficient cooling power to cool the refrigerant to the second lower cryogenic temperature. As a result, a two stage cryocooler is not required. The single-stage cryocooler may allow for a more compact cryopump having a reduced bill of materials (BOM). The cryopump may also be more reliable as fewer seals are required compared to a cryopump comprising a two-stage cryocooler. Cryopumps are often used in semiconductor manufacturing where space in the clean room is at a premium. Therefore, providing a more compact cryopump is desirable. Whilst embodiments require a separate cryochiller in place of the first stage of a two stage cryocooler, the cryochiller may be located remote from the cryopump, for example, in a subfab. Accordingly, the space required within the processing chamber or clean room for the cryopump may be reduced. Further, the cryochiller is configured to operate using a different refrigeration cycle to the cryocooler. Whilst the cryocooler may be limited in the type of refrigeration cycle it can use due to space requirements in the processing chamber, the cryochiller may not be limited in this way. For example, this flexibility to select different refrigeration cycles may enable the cryochiller to have a greater cooling power and capacity compared to the cryocooler. In some embodiments, the cryopump system comprises a radiation shield configured to be cooled by the refrigerant to the first cryogenic temperature, the radiation shield being arranged to at least partially surround the cryogenic surface. In some embodiments, the radiation shield is arranged to completely enclose the cryogenic surface. Conventionally, a radiation shield is attached to a two-stage cryocooler at an end of the first stage such that it is cooled to the intermediate cryogenic temperature. In embodiments, the radiation shield is cooled by the refrigerant arriving at the cryopump which has been precooled by the cryochiller. The radiation shield acts to shield any cryogenic surfaces at the second lower cryogenic temperature from radiation which may reduce the pumps capacity to absorb gas. In some embodiments, the radiation shield is arranged to at least partially surround the single-stage cryocooler. In some embodiments, the radiation shield is arranged to surround the entire single-stage cryocooler. In some embodiments, the cryopump system comprises another cryogenic surface configured to be cooled by the refrigerant to the first cryogenic temperature for capturing gas. Conventionally, a cryogenic surface is attached to a two-stage cryocooler at the end of the first stage and is configured to capture gases at the intermediate cryogenic temperature. In embodiments, the another cryogenic surface is cooled by refrigerant arriving at the cryopump which has been precooled by the cryochiller such that it can capture gases at the first cryogenic temperature. Gases which may be captured at the first cryogenic temperature include, for example, water vapour. In some embodiments, the radiation shield comprises the another cryogenic surface. In other words, the radiation shield and the another cryogenic surface are formed from the same component. In some embodiments, the single-stage cryocooler is configured to operate using a Gifford-McMahon, GM, refrigeration cycle. This is a commonly used refrigeration cycle for a cryocooler of a cryopump. ln some embodiments, the cryochiller is not configured to operate using a GM refrigeration cycle. In some embodiments, the cryochiller is configured to operate using a Stirling refrigeration cycle, a Joule-Thompson refrigeration cycle, or a reverse Brayton cycle. Such cooling cycles may enable the cryochiller to have a greater capacity compared to cryocoolers which typically use the GM cooling cycle. In some embodiments, the cryochiller is remote from the single-stage cryocooler. This provides flexibility as to where the cryochiller may be located. For example, in some embodiments, the cryopump is positioned inside a clean room, processing chamber or fab and the cryochiller is positioned in a subfab such that it does not take up space in the fab. As a result, embodiments offer greater flexibility and may take up less room inside the clean room compared to a cryopump comprising a two-stage cryocooler without loss of functionality. In some embodiments, the cryochiller is configured to provide a cooling power of more than 1 KWatts. In some embodiments, the cryocooler is configured to provide a cooling power of less than 200W. In some embodiments, the cryochiller has variable temperature control such that the first cryogenic temperature can be selectively changed. In this way, by changing the settings of the cryochiller, refrigerant can be supplied to the cryopump at a selected temperature. By comparison, conventional two-stage cryocoolers do not have the ability to change the cryogenic temperature of the refrigerant between the first and second stages. As a result, a two-stage cryocooler often over cools the refrigerant which is subsequently warmed up to achieve the desired temperature for capturing gases. Embodiments avoid this inefficiency by providing control over the first cryogenic temperature being generated by the cryochiller. In some embodiments, the first cryogenic temperature is within the range 60 to 125K. ln some embodiments, the second cryogenic temperature is within the range 4 to 25K. In some embodiments, the refrigerant comprises helium. In some embodiments, the cryopump system further comprises a compressor configured to supply pressurised refrigerant cooled by the cryochiller to the cryopump. In some embodiments, the cryochiller is integrated with the compressor to supply pressurised refrigerant to the cryopump at the first cryogenic temperature. In this way, the system comprises a single refrigerant stream which is cooled by the cryochiller. In other embodiments, the cryochiller is configured to cool a second refrigerant stream and a heat exchanger interconnecting the pressurised refrigerant from the compressor and the second refrigerant stream is used to cool the pressurised refrigerant to the first cryogenic temperature. In some embodiments, the cryopump system further comprises a supply line for carrying the refrigerant at an ambient temperature from the compressor to the cryopump, the refrigerant being cooled to the first temperature by the cryochiller between the compressor and the cryopump; a return line for returning refrigerant from the cryopump to the compressor; and a heat exchanger configured to exchange heat between refrigerant in the supply line which has not yet been cooled by the cryochiller and refrigerant in the return line. In this way, the cold refrigerant returning from the cryocooler may be used to help begin cooling the ambient refrigerant from the compressor, thereby reducing the burden on the cryochiller. In some embodiments, the cryopump system comprises a plurality of cryopumps, each cryopump comprising a single-stage cryocooler arranged to receive refrigerant cooled by the cryochiller to the first temperature. In this way, the cryochiller may serve a plurality of cryopumps each comprising a single-stage cryocooler. Cryochillers have greater cooling power than cryocoolers and therefore have a greater capacity than the first stage of a typical two-stage cryocooler. This means the cryocooler can serve many cryopumps having a single-stage cryocooler which would otherwise have to be larger two-stage cryocoolers. Thus, no additional space in the sub fab may be required when additional cryopumps are added to the system. Furthermore, using a single cryochiller in place of many first stages of a two-stage cryocooler may improve the energy efficiency of the system. In some embodiments, the cryopump system comprises a respective heat exchanger for each of the plurality of cryopumps. For example, the return line from each pump may have a separate heat exchanger coupled with the supply line. In other embodiments, the plurality of cryopumps are served by a single heat exchanger. For example, the return lines from each cryopump may merge and a single heat exchanger couples the combined return line to the supply line. According to another aspect, there is provided a method of operating a cryopump system, the method comprising: cooling a refrigerant to a first cryogenic temperature using a cryochiller; supplying the refrigerant to a single-stage cryocooler of a cryopump, the single-stage cryocooler being configured to operate using a different refrigeration cycle to the cryochiller; and cooling the refrigerant to a second cryogenic temperature lower than the first cryogenic temperature using the single-stage cryocooler; capturing gas using a cryogenic surface cooled by the single-stage cryocooler to the second cryogenic temperature. By cooling refrigerant to the first cryogenic temperature using a cryochiller which is distinct and separate from the cryopump, the cryopump may not need a two-stage cryocooler in order to reach the low cryogenic temperatures necessary for capturing certain types of gases. Accordingly, a single-stage cryocooler may be used which can be cheaper, more reliable and more compact than a two-stage cryocooler. In some embodiments, the cryopump system comprises a cryopump system according to the aspect. Also described is a turbomolecular pump system comprising: a cryochiller configured to cool refrigerant to a cryogenic temperature; a cryogenic surface configured to be cooled by the refrigerant to the cryogenic temperature for capturing gas; and a turbomolecular pump configured to pump gas from an inlet to an outlet of the turbomolecular pump, the inlet being arranged to receive gas that has passed over the cryogenic surface. Turbomolecular pumps may struggle to pump gases which are captured or adsorbed by the cryogenic surfaces at the temperatures achieved by the second stage of a two-stage cryocooler. However, it was identified that these gases may be slowed down by a cryogenic surface at the first intermediate cryogenic temperature allowing a turbomolecular pump to be used to evacuate gases that have not been captured by the cryogenic surface. Rather than using a cryopump comprising a single-stage cryocooler to slow down the gases, embodiments remove the need for a cryocooler by using pre-cooled refrigerant from a cryochiller to directly cool a cryogenic surface. The turbomolecular pump is arranged to evacuate the gases which are slowed but not captured by the cryogenic surface. In some embodiments, the cryogenic surface defines a passage for gas leading to the inlet of the turbomolecular pump. In some embodiments, the cryogenic surface defines an annulus, the passage being defined through the annulus. ln some embodiments, the cryochiller is configured to operate using a refrigeration cycle that is not a GM refrigeration cycle. In some embodiments, the cryochiller is configured to operate using a Stirling refrigeration cycle, a Joule-Thompson refrigeration cycle, or a reverse Brayton cycle. Such cooling cycles may allow for an increased capacity compared to cryocoolers which typically use the GM cooling cycle. In some embodiments, the cryochiller has variable temperature control such that the cryogenic temperature can be selectively changed. In this way, the cryogenic temperature may be controlled. In some embodiments, the cryogenic temperature is within the range 60 to 125K. In some embodiments, the refrigerant comprises helium. In some embodiments, the turbomolecular pump system does not comprises a cryocooler. In some embodiments, the turbomolecular pump system comprises a plurality of turbomolecular pumps and a plurality of respective cryogenic surfaces, each cryogenic surface being configured to be cooled to the cryogenic temperature by refrigerant from the cryochiller. In this way, the large capacity of the cryochiller may be used to cool refrigerant for many cryogenic surfaces. This avoids the need for a dedicated cryochiller for each cryogenic surface leading to a more efficient and cost-effective system. Also described is a method of operating a turbomolecular pump system, the method comprising: cooling a refrigerant to a cryogenic temperature using a cryochiller; capturing gas using a cryogenic surface cooled by the refrigerant to the cryogenic temperature; and pumping gas that has been cooled but not captured by the cryogenic surface using a turbomolecular pump. In this way, there is no need for a cryocooler. ln some embodiments, the turbomolecular pump system comprises a turbomolecular pump system according to the further aspect. Further particular and preferred aspects are set out in the accompanying independent and dependent claims. Features of the dependent claims may be combined with features of the independent claims as appropriate, and in combinations other than those explicitly set out in the claims. Where an apparatus feature is described as being operable to provide a function, it will be appreciated that this includes an apparatus feature which provides that function or which is adapted or configured to provide that function. BRIEF DESCRIPTION OF THE DRAWINGS Embodiments of the present invention will now be described further, with reference to the accompanying drawings, in which: Figure 1 shows a cryopump system according to an embodiment; Figure 2 shows a cryopump system according to an embodiment; Figure 3 shows a turbomolecular pump system; Figure 4 shows a turbomolecular pump system; and Figure 5 shows a legend for Figures 1 to 4. DESCRIPTION OF THE EMBODIMENTS Before discussing the embodiments in any more detail, first an overview will be provided. Cryocoolers are used in cryogenic devices, such as cryopumps, to cool a refrigerant to cryogenic temperatures. Typically, the refrigerant is helium and the cryocooler is configured to operate using a Gifford-McMahon (GM) refrigeration cycle. A two stage cryocooler comprises a first stage serving two purposes: to provide cooling capacity at a higher temperature and to pre-cool the helium gas for the second stage. The second stage is used to further cool the helium to a lower cryogenic temperature. Embodiments aim to simplify a two stage cryocooler by reducing the current two stage GM-cryocooler to a single stage cryocooler which would reduce the bill of materials (BOM) cost of the refrigerator and simplify the manufacturing process. Moreover, a single stage cryocooler requires fewer seals which are a leading cause of failure so reliability may be improved. The single stage cryocooler may also have a more compact design which would take up less space in the processing chamber / clean room. However, single stage cryocoolers only provide cooling capacity for one temperature whereas two stage cryocoolers offer a cooling capacity at their first and second stages. Since different gasses condense at different temperatures, the two stages of a conventional two-stage cryogenic vacuum pump are set to temperatures that can capture different gasses. The first and second stages will cryogenically cool any surfaces attached to them and are known as first stage surfaces and second stage surfaces respectively. For a two stage cryocooler, the first stage cooling capacity is used to cool a radiation shield that protects the second stage from any radiant heat. This allows the second stage’s cooling capacity to be used for the application’s process instead of dealing with radiant heat. The cryopump may further utilise the first stage cooling capacity to cool a surface at a cryogenic temperature for capturing certain types of gas. In some cases, the radiation shield and the cryogenic surface are formed as one component. Type 1 gasses may be defined as gases which can be condensed at the higher temperature first stage surfaces and the lower temperature second stage surfaces, while type 2 gasses may be defined as gases which can only be captured at the cooler second stage surfaces. The second stage surfaces often have porous materials attached to them to capture the gasses that condense at temperatures even lower than the second stage. These are referred to as type 3 gasses. The first stage surfaces are designed to capture the majority of type 1 gasses before they reach the second stage surfaces. This helps to keep the second stage surfaces free from type 1 gasses so more type 2 and type 3 gasses can be captured by the second stage surfaces. Helium gas refrigerant typically enters a two-stage cryocooler at ambient temperature and is cooled by the first stage to an intermediate first cryogenic temperature before being cooled by the second stage to its lowest cryogenic temperature. It would be desirable to obtain the advantages associated with using a single-stage cryocooler without sacrificing the functionality of the two-stage cryogenic having a higher and a lower cryogenic temperature. It was realised that if the helium gas enters the cryocooler pre-cooled to the intermediate cryogenic temperature, the first stage is no longer needed and a single-stage cryocooler would be sufficient to cool the refrigerant to the lower cryogenic temperature comparable to that achieved by the second-stage of a two-stage cryocooler. Therefore, some embodiments provide a cryopump system comprising a cryogenic chiller or cryochiller which is used to effectively replace the first stage of a two-stage cryocooler. The cryochiller precools the helium gas refrigerant to a temperature comparable to the temperature produced by the first stage of a two stage cryocooler. Cryochillers do not use the same GM refrigeration cycle as the cryocoolers so they may have greater cooling power and capacity. As a result, the cryogenic chiller may act as the first stage for multiple cryocoolers. In this way, the first stage of many cryocoolers may be replaced with one large capacity central cryogenic chiller. Furthermore, the cryochiller may be remotely located such that it does not take up space in the clean room. In some embodiments, the cooling capacity of the pre-cooled helium gas can be used for processes such as cooling the radiation shield before passing through a single stage cryocooler effectively acting as the second stage of a two-stage cryocooler. Pre-cooling the helium gas maintains the functionality associated with the first stage cooling capacity of a two-stage cryocooler that is needed for many applications. In other embodiments, the cryogenic chiller replaces the entire cryocooler and is used in combination with a turbomolecular pump. For example, the pre-cooled refrigerant from the cryochiller may be used to cool a cryogenic surface for capturing type 1 gases. Rather than using a single-stage cryocooler to obtain the temperatures necessary for capturing type 2 and type 3 gases, embodiments employ a turbomolecular vacuum pump to pump the remaining type 2 and type 3 gasses which have been cooled but not captured by the type 1 gas capturing cryogenic surfaces. In summary, a cryopump system and a turbomolecular vacuum pump system are described as alternatives to a conventional cryopump comprising a two-stage cryocooler. In particular, they may replace the functionality of the first stage of a two stage cryocooler or a two stage cryogenic vacuum pump, with a remote large capacity cryochiller. Some advantages include but are not limited to: • replace the first stage of multiple cryocoolers with a central cryogenic chiller potentially leading to a space saving, a cost saving and improved reliability; • maintain the first stage cooling capacity lost when switching from a two stage cryocooler to a single stage cryocooler; and • replace a cryopump with a cryogenically cooled surface and a turbomolecular vacuum pump. Figure 1 shows a cryopump system according to an embodiment in which a refrigerant, such as helium, is pre-cooled by a cryochiller to a temperature comparable to the first stage temperature of a two-stage cryocooler. The cryopump system 10 comprises a single-stage GM cryocooler 13 of a cryopump 11 being supplied pressurised refrigerant from a compressor 16 via a supply line 18. The refrigerant leaves the compressor 16 at an ambient temperature and is cooled to a first cryogenic temperature, T1, by the cryochiller 22. Specifically, the cryochiller 22 cools a separate refrigerant stream and a heat exchanger 21 is used to indirectly cool the refrigerant along the supply line 19 to the cryopump 11. It will be appreciated that the cryochiller 22 may instead directly cool the refrigerant being supplied to the cryopump 11, thereby obviating the need for a second refrigerant stream. In such embodiments, the cryochiller 22 may be integrated with the compressor 16. The cryochiller 22 is configured to use a different refrigeration cycle other than a GM cycle, for example, the cryochiller 22 may be configured to operate using a Stirling refrigeration cycle, a Joule-Thompson refrigeration cycle, ora reverse Brayton cycle. These cycles enable the cryochiller 22 to have significantly more cooling power and increased capacity compared to a single or a double stage GM cryocooler. As a result, the cryochiller 22 may serve multiple cryocoolers 13 as discussed in more detail below. The refrigerant at the first cryogenic temperature, T1, is used to cool a cryogenic surface 14. The cryogenic surface is configured to capture type 1 gases and also act as a radiation shield for the second cryogenic surface 15 which is coupled to an end of the single stage cryocooler 13. The single stage cryocooler 13 receives the refrigerant at the first cryogenic temperature, T1, and cools the refrigerant to a second colder cryogenic temperature, T2. Since the high-pressure refrigerant is pre-cooled by the cryochiller to a temperature, T1, comparable to the first stage temperature of a two-stage cryocooler, the refrigerant can be cooled to a temperature, T2, comparable to the second stage temperature of a two-stage cryocooler. Hence, the cryochiller can replace the functionality of the first stage in a two-stage cryocooler. The second cryogenic surface 15 is then cooled to the second cryogenic temperature, T2, such that it can capture type 2 gases. The surfaces 14, 15 cryogenically cooled to the first and second stage temperatures therefore capture type 1 and type 2 gasses to create a vacuum. It will be appreciated that the first and the second cryogenic surfaces 14, 15 are optional and including either one of them will depend on the desired application of the system. In some embodiments, the first cryogenic temperature, T1, is between 60 and 125K inclusive. In some embodiments, the second cryogenic temperature, T2, is between 4 to 25K inclusive. It will be appreciated that the temperatures T1 and T2 may be selected based on the temperature required to pump certain types of gases. In some embodiments, the cryochiller 22 has variable temperature control such that the first cryogenic temperature, T1, can be selectively changed. In this way, the cryochiller may be set to different temperatures depending on the desired gas capturing properties of the first and second stage surfaces. By comparison, conventional two-stage cryocoolers do not have the ability to selectively change the cryogenic temperature of the refrigerant between the first and second stages. The refrigerant is returned to the compressor 16 via a return line 19. The refrigerant in the return line is still colder than the refrigerant at ambient temperature leaving the compressor 16. Therefore, a heat exchanger 17 allows the colder returning refrigerant to cool the warmer pressurised refrigerant, thereby reducing the burden of the cryochiller. It will be appreciated that the heat exchanger 17 is optional. Note that Figure 5 shows a legend for the symbols used in Figures 1 to 4. Figure 2 shows a cryopump system 110 according to an embodiment. The system 110 comprises a plurality of cryopumps 13a-c, each comprising a single stage cryocooler, such as the type shown in Figure 1, whose refrigerant is precooled by the same cryochiller 22. The cryochiller 22 has a sufficiently large capacity to act as the first stage for each of the plurality of cryocoolers 13a-c. The refrigerant is supplied to the cryocoolers 13a-c by a compressor 16 and cooled to a first cryogenic temperature, T1, in similar fashion to the system of Figure 1. However, it will be appreciated that each cryocooler 13a-c may instead have a dedicated heat exchanger for coupling to the refrigerant stream of the cryochiller 22. Similarly, on the return line, each cryocooler 13a-c may have a dedicated heat exchanger 17. It will further be appreciated that the cryocoolers 13 may or may not have the cryogenically cooled surfaces 14, 15 shown in Figure 1 depending on the application. Figure 3 shows a turbomolecular pump system. The system comprises a cryochiller 22 configured to supply refrigerant at a cryogenic temperature, T1, to a cryogenic surface 31 for capturing type 1 gases 33. The temperature, T1, may be comparable to the first stage temperature of a two stage cryocooler. The surface 31 may define an annulus having a passage defined therethrough for allowing gases no captured by the surface to enter an inlet to a turbomolecular pump 32 as shown by the arrows 34. The turbomolecular vacuum pump 32 is used to pump the remaining type 2 and type 3 gasses 34 which are slowed but not captured by the surface 31. In this system, instead of pre-cooling refrigerant for a cryocooler, the large capacity chiller 22 is used to directly cool the surface 31 to the first stage temperature, T1, for capturing type 1 gases. It will be appreciated that the precise geometry of the system may differ from what is shown. Figure 3 shows gas flowing through the surface 31 and into a turbomolecular vacuum pump 32. The surface 31, being cooled to a cryogenic temperature by the cryogenic chiller 22, captures type 1 gases 33. The remaining type 2 and type 3 gasses are usually difficult to pump using a turbomolecular pump. However, as the remaining type 2 and 3 gas will be cooled and slowed, the efficacy of the turbomolecular pump to pump such gases will be improved, thus enabling the type 2 and 3 gases to be more effectively evacuated by the turbomolecular vacuum pump. Accordingly, the cryochiller 22 and cryogenic surface 31 serve to replace the first stage of a two-stage cryocooler and the turbomolecular vacuum pump 32 replaces the functionality of the second stage by pumping the type 2 and 3 gasses that it would typically capture. 5 Figure 4 shows a turbomolecular pump system 130 including a large capacity cryogenic chiller 22 and multiple pairs of cryogenic surfaces 31a-c and turbomolecular pumps 32a-c, such of the pair shown in Figure 3. In this system, the cryochiller 22 is serving as the first stage for each of the devices by io cryogenically cooling the surfaces 31 a-c, while the turbomolecular vacuum pumps 32a-c serve as their second stages. Although illustrative embodiments of the invention have been disclosed in detail herein, with reference to the accompanying drawings, it is understood that the 15 invention is not limited to the precise embodiment and that various changes and modifications can be effected therein by one skilled in the art without departing from the scope of the invention as defined by the appended claims and their equivalents. REFERENCE SIGNS 10 Cryopump System 11 Cryopump 13 Single-stage cryocooler 5 14 1st Cryogenic Surface and Radiation Shield 15 2nd Cryogenic Surface 16 Compressor 17 Heat exchanger 18 Supply line io 19 Return line 20 Cryogenic refrigerator 21 Heat Exchanger 22 Cryochiller 30 Turbomolecular pump system 15 31 Cryogenic surface 32 Turbomolecular vacuum pump 33 Type 1 gas 120 Cryopump system 130 Turbomolecular pump system 20

Claims

1. A cryopump system comprising:a cryochiller configured to cool refrigerant to a first cryogenic temperature for supply to a single-stage cryocooler of a cryopump; andthe cryopump comprising:the single-stage cryocooler configured to cool the refrigerant from the first cryogenic temperature to a second cryogenic temperature lower than the first cryogenic temperature, the single-stage cryocooler being configured to operate using a different refrigeration cycle to the cryochiller; anda cryogenic surface configured to be cooled by the single-stage cryocooler to the second cryogenic temperature for capturing gas.

2. A cryopump system according to claim 1, further comprising a radiation shield configured to be cooled by the refrigerant to the first cryogenic temperature, the radiation shield being arranged to at least partially surround the cryogenic surface.

3. A cryopump system according to claim 1 or claim 2, further comprising another cryogenic surface configured to be cooled by the refrigerant to the first cryogenic temperature for capturing gas.

4. A cryopump system according to any preceding claim, wherein the single-stage cryocooler is configured to operate using a Gifford-McMahon, GM, refrigeration cycle.

5. A cryopump system according to any preceding claim, wherein the cryochiller is configured to operate using a Stirling refrigeration cycle, a Joule-Thompson refrigeration cycle, or a reverse Brayton cycle.

6. A cryopump system according to any preceding claim, wherein the cryochiller is remote from the single-stage cryocooler.

7. A cryopump system according to any preceding claim, wherein the first cryogenic temperature is within the range 60 to 125K.

8. A cryopump system according to any preceding claim, wherein the second cryogenic temperature is within the range 4 to 25K.

9. A cryopump system according to any preceding claim, wherein the refrigerant comprises helium.

10. A cryopump system according to any preceding claim, further comprising a compressor configured to supply pressurised refrigerant cooled by the cryochiller to the cryopump.

11. A cryopump system according to claim 10, further comprising: a supply line for carrying the refrigerant at an ambient temperature from the compressor to the cryopump, the refrigerant being cooled to the first temperature by the cryochiller between the compressor and the cryopump;a return line for returning refrigerant from the cryopump to the compressor; anda heat exchanger configured to exchange heat between refrigerant in the supply line which has not yet been cooled by the cryochiller and refrigerant in the return line.

12. A cryopump system according to any preceding claim, wherein the system comprises a plurality of cryopumps, each cryopump comprising a single-stage cryocooler arranged to receive refrigerant cooled by the cryochiller to the first temperature.

13. A method of operating a cryopump system, the method comprising:cooling a refrigerant to a first cryogenic temperature using a cryochiller;supplying the refrigerant to a single-stage cryocooler of a cryopump, the single-stage cryocooler being configured to operate using a 5 different refrigeration cycle to the cryochiller; andcooling the refrigerant to a second cryogenic temperature lower than the first cryogenic temperature using the single-stage cryocooler;capturing gas using a cryogenic surface cooled by the single-stage cryocooler to the second cryogenic temperature.10

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