Cryopump monitoring

Abstract

A sensor for determining an absorption state of cryopanels within a cryopump. The sensor comprises: an absorbent material; a detector for measuring a property of the absorbent material, the property changing as matter is absorbed by the material; and a heating element for heating the absorbent material to release at least some of the absorbed matter. The absorbent material of the sensor is configured to be mounted thermally coupled to a second stage of the cryopump, such that during operation of the cryopump the absorbent material is within 5K of a temperature of the cryopanels of the cryopump.

Description

[0001] CRYOPUMP MONITORING

[0002] FIELD OF THE INVENTION

[0003] The field of the invention relates to cryopumps and to how to determine their absorption state and when they might benefit from regeneration.

[0004] BACKGROUND

[0005] Cryopumps generate a high vacuum by trapping volatile species such as Hydrogen and Argon within highly porous media, such as granular activated carbon (GAC). During operation the trapping media will eventually become saturated with trapped species and pumping action will cease until the pump is regenerated. The pumps may be exposed to other species that may not necessarily leave the media during regeneration, leading to progressive loss of pumping speed during its lifetime.

[0006] In the case of hydrogen absorption, when the pump is near the hydrogen capacity it is no longer able to maintain the needed temperature and begins to lose control of the process. Once the pump is no longer able to maintain control the internal safety features engage and warm up the pump and purge the hydrogen in a safe manner. Any product in the chamber when this happens is rendered defective.

[0007] It would be desirable to be able to determine when the amount of matter absorbed is getting close to capacity, such that regeneration may occur in a timely and controlled manner.

[0008] SUMMARY

[0009] A first aspect provides a sensor for determining an absorption state of cryopanels within a cryopump, said sensor comprising: an absorbent material; a detector for measuring a property of said absorbent material, said property changing as matter is absorbed by said material; a heating element for heating said absorbent material to release at least some of said absorbed matter; wherein said absorbent material of said sensor is configured to be mounted thermally coupled to a second stage of said cryopump, preferably such that during operation of said cryopump said absorbent material is within 5K of a temperature of said cryopanels of said cryopump.

[0010] It was recognised that the amount of matter absorbed on a cryopanel is indicative of the state of the panel and of when the next regenerative cycle might be required. It was also recognised that it would be useful to be able to know this during operation of the pump, such that regeneration can be managed in a controlled way and at a time that is convenient for the process that the cryopump is being used to evacuate. In order to address these issues, a sensor has been devised that comprises its own absorption material and that is configured to be located within the cryopump such that it experiences the same or similar conditions to those experienced by the absorbent on the cryopanels. In this way the amount of matter absorbed on the absorbent material of the sensor provides an indication of the amount of material that the absorbent on the cryopanels will have absorbed. A detector associated with the sensor is used to measure a property of the absorbent material that changes as matter is absorbed and in this way the changes in property are indicative of the matter absorbed on the sensor and this can be used to estimate the matter absorbed on the cryopanels.

[0011] The sensor is configured to be mounted within the cryopump at a position that is close to and is exposed to the same environment as the cryopanels are. The sensor may be mounted such that during operation of the cryopump the absorbent material is at a temperature that is within 5K of the temperature of the cryopanels, preferably within 1 K. In some embodiments the sensor is configured to be mounted on one the cryopanels while in others it may be configured to be mounted on the second stage of the refrigerator.

[0012] In some embodiments, said sensor absorbent material is a same material as said absorbent material of said cryopanels. In other embodiments, said absorbent material is formed of a material that has the same or similar absorption properties to that of the cryopanel absorbent material and / or has absorbent properties that can be reproducibly calibrated to the absorption performance of the cryopanel absorbent material.

[0013] Although, it may be convenient to use the same absorption material as that on the panels, in some cases this material may not lend itself well to the type of sensor used and a material with similar absorption properties or ones that can be reproducibly calibrated with respect to the absorption properties of the cryopanel absorbent material may be selected.

[0014] In some embodiments, said detector comprises a capacitor, said absorbent material being mounted to form the dielectric of said capacitor, said detector being configured to detect a change in capacitance of said capacitor as matter is absorbed by said absorbent material.

[0015] One property that is affected by the amount of matter that is absorbed and that is convenient to detect is the dielectric properties of a material. A capacitor may be used with the absorbent forming the dielectric, enabling an accurate measure of the amount of matter absorbed in a space efficient and cost-effective manner.

[0016] In some embodiments, said absorbent material comprises a porous glass or other absorbent dielectric material.

[0017] One absorption material that may be particularly effective is a porous glass such as unconsolidated vycor. This material is easy to design with particular absorption properties and can be formed as a planar element making it particularly space efficient and convenient for some types of detectors.

[0018] Furthermore, its conductivity is very low, which where the sensor is a capacitor may make the response easier to link to changes in absorption, particularly where different input frequency signals are used to distinguish different absorbed materials. In some embodiments, said detector further includes a reference capacitor, said reference capacitor comprising a reference material mounted as the dielectric material of said reference capacitor, said reference material comprising similar dielectric properties to said absorbent material prior to said absorbent material absorbing any matter, said detector being configured to detect said change in capacitance of said capacitor relative to said reference capacitor.

[0019] A reduced noise arrangement may be one where a reference capacitor is used and where, rather than the absorption material forming the dielectric, a material with similar dielectric properties to the absorption material prior to it absorbing any matter is used. The reference material should have low absorption properties. Using a reference capacitor in this way allows changes in the capacitance between the sensor and reference capacitor to be detected in a way that eliminates or at least reduces the effects of noise and drift.

[0020] In some embodiments, said capacitor and reference capacitor are formed as an interdigital array, said electrodes of said capacitors comprising electrodes of said interdigital array, said capacitor and said reference capacitor comprising a common output electrode.

[0021] An interdigital array is an efficient and convenient way of forming the capacitor and where a reference capacitor is used then one electrode of the capacitors may be common to both. In some embodiments, the common electrode may output a signal to an amplifier. The other two plates of the respective capacitor may be fed by signals that are out of phase with each other and possibly different in amplitude. In this way a half bridge type arrangement is formed and exogenous factors that may affect the measurements are reduced.

[0022] In some embodiments, said absorbent material comprises activated carbon and said reference material comprises non-activated carbon. Where a reference capacitor is used then where the absorption material is activated carbon, which is often the absorption material used in a cryopump, then the reference capacitor may have non-activated carbon as its dielectric material, this having very similar properties to the activated carbon but not having the absorption properties.

[0023] In some embodiments, said sensor comprises control circuitry configured to change a frequency of a signal input to said capacitor and to determine an amount and a type of matter absorbed by said absorbent material in response to said detected changes in capacitance at different signal frequencies.

[0024] A further advantage of using a capacitor is that the type of matter that is absorbed by the material as well as the amount of matter affects the capacitance and thus, changes in capacitance at different frequencies may be used to determine both amount and type of material. In this regard different types of material may affect the capacitance differently at different frequencies and thus changing the frequency of the input signal and measuring the changes in capacitance allows the nature as well as amount of the matter absorbed by the absorption material to be determined. This may be advantageous where not only the amount of matter is useful to know but where the amount of a particular type of matter is also important. For example, where hydrogen is absorbed then it is important that this does not rise above a particular safety limit and thus, hydrogen can be determined separately to and in some cases in addition to other matter absorbed by the absorption material and both sets of information can be used in any regeneration calculation.

[0025] In some embodiments, said absorbent material comprises pores having an average diameter of between 1 and 10nm, preferably between 5 and 7nm, such that hydrogen is preferentially absorbed and larger molecule gases are inhibited from being absorbed. A further way of determining an amount of hydrogen is to configure the absorption material with pores of a size such that they are too small for the molecules of most gasses but can absorb hydrogen. In this way, the amount of matter absorbed on the material that is detected by the detector will be substantially equivalent to the amount of hydrogen absorbed. An alternative way of doing this is to place the sensor within the pump in a location where there is no line of sight between the first stage array or heat shield and the absorbent material of the sensor, such that all or most of the type I and type II gases do not reach the sensor but are trapped by the other surfaces.

[0026] In some embodiments, said detector comprises an oscillating circuit comprising a piezoelectric element having absorbent material mounted thereon, said piezoelectric element and absorbent material determining an oscillating frequency of said detector, said detector being configured to determine changes in frequency of said oscillations as matter is absorbed by said absorbent material.

[0027] An alternative or addition to a capacitive detector may be a piezoelectric element that generates oscillations within the absorbent material. The frequency of the oscillation is dependent on the mass of the absorbent material and thus as the mass of the material increases as it absorbs matter, the frequency of oscillations will change. For example, the use of a quartz crystal with an absorbent layer may provide a cost effective and accurate measurement means.

[0028] In some embodiments, said sensor comprises control circuitry to control said heater to periodically heat said absorbent material to release at least some of said absorbed matter and to determine said absorption state of said cryopanels from said detected changes in said property and said number of times said heater has been activated.

[0029] The sensor has a heater in order to regenerate the sensor in a similar way to the way the cryopanels are periodically regenerated. In some cases, the sensor may have control circuitry that controls the heater to regenerate the sensor more often than the cryopanels themselves are regenerated. This may be done to increase the accuracy and the capacity of the sensor. Changes in properties of the material as matter is absorbed will in some cases cause sensitivity to additional loading to fall as the absorbent material of the sensor becomes loaded. Thus, periodically regenerating the material may increase the sensitivity and provided that the amount of matter absorbed previously and the number of times the sensor is regenerated are recorded, the detector can more accurately determine absorbed matter and can increase its measurement capacity.

[0030] In some embodiments, a surface area of said absorbent material of said sensor is less than 5 mm2, preferably less than 1 mm2.

[0031] Where the absorbent material of the sensor is periodically regenerated by heating, then it may be advantageous if it is very small, such that any heating of the sensor absorbent material has a negligible effect on the temperature of the cryopanels and on the amount of gas within the cryopump.

[0032] In some embodiments, the periodic heating may be at a frequency of a fraction of the frequency of the input signal to the capacitor, for example it may be between 1 / 100thand 1 / 1000000thof the frequency of the input signal to the capacitor.

[0033] A further aspect provides a cryopump comprising a two stage cryogenic refrigerator; a first stage array coupled to a first stage of said two stage refrigerator; cryopanels thermally coupled to a second stage of said two stage refrigerator, said cryopanels comprising surfaces at least a portion of said surfaces being coated with an absorbent material; and a sensor according to a one aspect thermally coupled to said second stage of said cryopump, such that during operation of said cryopump said absorbent material is within 5K of a temperature of said cryopanels of said cryopump. In some embodiments, said sensor is mounted at a location with no direct line of sight between said inlet or first stage array and a surface of said absorbent material of said sensor.

[0034] Where the sensor is to be used to detect the amount of type III gasses such as hydrogen that have been absorbed, then it may be advantageous to place it at a location where type I and type II gasses are trapped by other surfaces for the most part, before they can reach the absorbent material of the sensor.

[0035] In some embodiments, said cryopump comprises control circuitry configured to receive signals from said sensor, and to determine from said signals an amount of material absorbed on said cryopanels, and in response to said amount being greater than a first predetermined amount to output an indication that said pump should be regenerated soon. The control circuitry of the sensor may be part of the cryopump control circuitry, or it may be separate and linked to it.

[0036] In some embodiments, said control circuitry is configured in response to said amount being greater than a second critical amount to regenerate said pump.

[0037] A yet further aspect provides a method of determining an amount of matter absorbed on a second stage cryopanel within a cryopump, said second stage cryopanel comprising absorbent material on at least one surface, said method comprising: mounting a sensor according to one aspect thermally coupled to a second stage of said cryopump; and measuring changes in a property of an absorbent material of said sensor as matter is absorbed by said absorbent material; and determining from said measured changes an amount of matter absorbed by said cryopanels.

[0038] 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.

[0039] BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Embodiments of the present invention will now be described further, with reference to the accompanying drawings, in which:

[0041] Figure 1 shows a cryopump according to an embodiment;

[0042] Figure 2A and 2B schematically show different configurations of a capacitive sensor;

[0043] Figure 3 shows a capacitive sensor according to an embodiment; and Figure 4 shows a flow diagram illustrating steps in a method of determining an amount of absorbed material within a cryopump.

[0044] DESCRIPTION OF THE EMBODIMENTS

[0045] Before discussing the embodiments in any more detail, first an overview will be provided.

[0046] An aspect provides a cryopump comprising: a pump inlet; a two stage refrigerator; a first stage array thermally coupled to a first stage of the two stage refrigerator; and a cryopanel structure coupled to a second stage of the two stage refrigerator; wherein at least a portion of at least some surfaces of the cryopanel structure comprise portions coated with an adsorbent material. A sensor is provided mounted on the cryopanels or second stage refrigerator. The sensor comprises an absorbent material which may have the same or similar absorption properties to that of the absorbent material coating the cryopanels. Thus, the absorption properties of the sensor may mimic those of the absorbent material of the cryopanels and by using a detector configured to detect changes in properties of the absorbent material of the sensor as it absorbs matter, the amount of absorbed matter and in some cases the type of absorbed matter may be determined. With suitable calibration this amount may be linked to the amount and possibly type of matter absorbed by the cryopanels which in turn can be linked to a timing for regeneration of these panels.

[0047] Currently, cryopumps may have no or few internal diagnostic capabilities, meaning that fill level and absorbent material performance must be deduced from the performance of the process chamber that they are evacuating. Embodiments provide a way of determining fill level and absorbent material performance by directly measuring an absorbent material within the pump with absorbent properties that simulate or can be calibrated with respect to those of the cryopump’s absorbent materials.

[0048] Detectors that detect the changes in properties of the absorbent material of the sensor as it absorbs matter may be used to determine the amount of matter absorbed on the sensor absorbent material and from that the amount of material absorbed by the cryopanels may be derived.

[0049] Figure 1 schematically shows a two stage cryogenic vacuum pump 30 comprising a sensor 50 according to an embodiment. The first stage 14 of the refrigerator is configured to cool to a first temperature T1 and the second stage 16 to cool to a lower temperature T2. There is a first stage array 12 that reflects incoming gas molecules towards a heat shield that forms the outer wall of the pumping chamber and is also cooled by the first stage of the refrigerator. This means that there is no direct line of sight between incoming molecules and the cryopanels 18. Furthermore, the sensor 50 is arranged with no direct line of sight to the first stage array 12 or heat shield, such that type I and type II gasses are preferentially trapped by the first stage array or heat shield.

[0050] The second stage of the refrigerator is thermally coupled to cryopanels 18. The internal surfaces of the cryopump that are cryogenically cooled to the first and second stage temperatures will capture type I and type II gases respectively to create a vacuum. Type III gases such as hydrogen are also captured by an absorbent material on the surfaces of the second stage cryopanel 18. As this is a capture type pump, it will require periodic regeneration to release the captured molecules and sensor 50 is there to provide an indication of the amount of material that has been captured by the absorbent material on the surface of the cryopanels 18 and from this to determine when regeneration may be required.

[0051] In this embodiment sensor 50 is connected to control circuitry 20 which in this embodiment is part of the pump control circuitry but may be separate to it. Control circuitry 20 controls the signals sent to the sensor, and processes the signals received from it and uses them to determine the amount of material absorbed on the sensor absorbent and from this the amount of material absorbed on the cryopanels 18. The control circuitry then compares this to one or more predetermined levels and determines whether regeneration of the pump is required soon. If so, it may indicate this to the process being pumped or determine from the characteristics of the pumping process whether it may be appropriate to regenerate the pump now. In either case, where it determines that regeneration is appropriate the control circuitry 20 may control heaters on the pump (not shown) to perform a regeneration cycle. If regeneration is not currently appropriate, the control circuitry 20 continues to monitor the amount of material absorbed and when a regeneration possibility arises it will trigger regeneration. If such a possibility does not arise and it determines the absorbed material level to have reached a critical amount it will trigger regeneration at this point.

[0052] In this embodiment sensor 50 is small compared to the size of the cryopanels 18 and comprises its own independent heating element 57 configured to independently regenerate the sensor 50 either periodically or when the detected amount of material absorbed on the sensor absorbent reaches a certain level. There may be a counter within control circuitry 20 to determine the number of times the sensor has been regenerated and to use this in the calculation of amount of material absorbed by the cryopanels. In this embodiment the sensing element or detector within sensor 50 comprises a capacitor where the absorbent material of the detector is used as the dielectric between the plates or electrodes of the capacitor, the dielectric constant of the absorbent material changing with matter absorbed. As the dielectric constant of a capacitor in combination with its geometric properties define its electrical impedance at a given frequency co, this change in capacitance can be simply measured from the current-voltage response of the capacitor.

[0053] Making such measurements in a site where noise and parasitics are present imposes certain requirements on the type of measurement circuitry that can be used. The existence of large parasitic elements are unavoidable in circumstances where the measurement instrument must be kept at some distance from the capacitor, which is the case where the capacitor is within the cryopump. Classical methods use capacitance bridges and shielding techniques to reduce the effects of parasitics, but such instruments are bulky, expensive, and can be quite challenging to null. A more compact and inexpensive solution is to use a transimpedance amplifiers (TIA) to generate a virtual ground on one side of the Device Under Test (DUT) 52 while simultaneously converting the current across the DUT into an easily measurable voltage. As can be seen in Figure 2A, such techniques effectively eliminate or at least reduce the influence of parasitic capacitances from the intended measurement, as stray capacitances Cp are connected to ground at each node and have no or very limited effect because both of their terminals are at zero volts.

[0054] A supplementary technique that may be employed in impedance measurements is the so-called “half-bridge” method, wherein a second out of phase excitation signal is applied to a reference capacitor 54 whose output node is then connected to the same output node as the DUT 52 see Figure 2B.

[0055] Provided the magnitudes of the in-phase and out-of-phase voltages are the same, then measured current into the transimpedance amplifier will become the difference current between the DUT 52 and reference capacitor 54, which will be proportional to the difference in capacitance between them.

[0056] A half-bridge topology has the effect of nulling off the baseline level of the reference element, making the measurement much more sensitive to changes in the DUT 52. Changing the relative amplitudes of the in-phase and out-of-phase voltages can also be used to extend the range of the measurement, by effectively dividing down, or multiplying up the value of the reference element.

[0057] Where a topology such as shown in Figure 2B is used in the sensor of an embodiment then the reference capacitor may be formed in essentially the same way as the sensor capacitor but with a dielectric material that is not absorbent to gases within the cryopump. For example, activated carbon may be used in the sensor capacitor or DUT 52 and non-activated carbon may be used in the reference capacitor 54.

[0058] Where a capacitance cell such as a half-bridge capacitance cell is used in the sensor 50 for monitoring adsorption in a cryocooler then it should preferably be configured to have one or more of the following characteristics:

[0059] (1 ) a similar view factor per unit adsorbate to the rest of the absorbent material on the cryopanels, or an absorbent that can be reproducibly calibrated to the performance of the absorbent of the cryopanels

[0060] (2) It should preferably be substantially planar so as to ease mass production and integration, potentially of several sensors across the cooler platen.

[0061] (3) It should preferably have a well calibrated reference so as to better distinguish between real adsorption phenomena and unrelated drifts such as those commonly encountered during cryogenic cooldown.

[0062] (4) Its signal lines should preferably be electrically shielded.

[0063] (5) It should have enough thermal connection to the cryoplaten to cool effectively, but be small enough and sufficiently isolated to allow for independent regeneration e.g. it may be configured to have a thermal relaxation time of between 10-20s, this being determined by the heat capacity of the sensor and the thermal conductivity of the link to the cryoplaten or cryopanels. For two typical GAC absorbers, this would imply a Kunk of about 0.005 mW-K’1-M’1. Such low thermal conductance at ~10K is easily obtainable with thin G10 supports. Isolation could be improved by using Vespel SP1 , which is 10x lower in thermal conductivity.

[0064] (6) Preferably it should be able to detect changes in fill level on a sub-second time scale so as to track performance during a single process cycle, requiring low noise levels in the signal path.

[0065] (7) Preferably, it should use real granulated active carbon for its test cell, so as to best measure the real behaviour of the system, and in particular to account for changes in GAC properties as they are exposed to process materials.

[0066] Points (1 ) and (2) can be covered by using 3 element interdigital array (figure 3), with channels milled in between elements to reduce stray parallel capacitance and also act as alignment structures for the GAC.

[0067] Figure 3 shows a capacitor of a detector according to an embodiment that is configured in a similar way to the embodiment of Figure 2B. There is a shared output electrode or plate 10 for the reference capacitor 54 and the sensor capacitor or DUT 52. In this embodiment, the dielectric for the reference capacitor is non-activated carbon 55, while that of the sensor capacitor 52 is activated carbon 53. A sensor configured in this way may be made to be very small with a surface area of the absorbent material of a few square millimetres, this allows the absorbent of the sensor to be regenerated quickly with very little heat energy. This allows the sensor to be periodically regenerated without unduly affecting the cryopump.

[0068] The above capacitance method of sensing uses an absorbent material. Changes in capacitance due to material absorption is used to indicate how much capacity is remaining in the pump.

[0069] Although the above embodiments describe a sensor that uses changes in capacitance of an absorbent material to determine the amount of material absorbed, alternative methods where another property of a material changes with absorbed material would also be appropriate. For example, a piezoelectric material with absorbent properties, or one coated with an absorbent material could be used and changes in the frequency of oscillation of the crystal could be used to determine the amount of material absorbed. Such a method would incorporate a piezoelectric element driven by an electronic circuit at a specific frequency. As the absorbent material increases in mass due to stored material the frequency of the oscillation would change thus allowing measurement of the amount of material absorbed. This sensor may be scalable into a MEMS (micro electro-mechanical system) device.

[0070] Figure 4 shows a flow diagram showing steps of a method performed when determining an amount of material absorbed on cryopanels within a cryopump.

[0071] In an initial step S10 a sensor is mounted thermally coupled to the cryopanels of the cryopump. Then at step S20 operation of the cryopump is started and changes in the properties of an absorbent material within the sensor are measured to determine the amount of material absorbed. At step S30 this determined amount is used to determine the amount of material absorbed on the cryopanels. Steps S20 and S30 may include periodic regeneration of the absorbent material of the sensor with the number of times it is regenerated being used in the calculation to determine the amount of material absorbed on the cryopanels.

[0072] At step D5 it is determined if the amount absorbed on the cryopanels is greater than a first predetermined amount and if not the steps S20, S30 and D5 are repeated until the amount does reach the first predetermined amount. At this point, at step S40 a ‘regenerate soon’ indication is output and this may be output to the process being evacuated by the pump and / or it may be output as a warning indication to a user and / or it may be output to control circuitry controlling the pump, each of which may use it as indication that it needs to determine when regeneration might be possible. At step D15 it determined whether the process being evacuated is at an appropriate point in its cycle to be regenerated. This may be done in response to a signal from the process itself, perhaps in response to the warning indication output to the process at D5, or it may in response to detected characteristics of the pumping procedure. If it is at an appropriate point to be regenerated then the pump is regenerated at step S60 and in this way, regeneration of the pump occurs at an appropriate point without any harm to the process being pumped and before the vacuum pump reaches a critical point.

[0073] If it is not at an appropriate point to be regenerated, then step S50 is performed, and the pump continues pumping and performs further measurements to determine the amount of material absorbed on the cryopanel. It is then determined at D25 if the amount of matter absorbed on the cryopanels is greater than a second predetermined amount that may be considered a critical amount, if it is then the pump is regenerated at step S60 irrespective of the current status of the process being evacuated. If not, then the pump continues pumping and further measurements to determine an amount of material absorbed on the cryopanels are performed. The step of determining whether the process is being pumped is at an appropriate point to be regenerated is performed periodically along with the determination as to whether the amount absorbed has reached a critical amount and if either is true, then the pump is regenerated at step S60.

[0074] In summary embodiments provide:

[0075] • A sensor on the second stage array or heat station of a cryopump which can equate the amount of stored material in some cases hydrogen to an electrical charge or other distinguishable signal.

[0076] • Such a sensor allows improved process control in the vacuum chambers evacuated by the cryopump.

[0077] • Such a sensor potentially reduces the manufacturing waste or improves the product quality of articles manufactured in the vacuum chambers. Although illustrative embodiments of the invention have been disclosed in detail herein, with reference to the accompanying drawings, it is understood that the 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.

[0078] REFERENCE SIGNS

[0079] 10 output electrode

[0080] 12 first stage array

[0081] 14 first stage refrigerator 16 second stage refrigerator

[0082] 18 cryopanels

[0083] 20 control circuitry

[0084] 30 cryopump

[0085] 50 sensor 52 sensing capacitor

[0086] 53 sensing absorbent material

[0087] 54 reference capacitor

[0088] 55 reference absorbent material

[0089] 57 sensor heater

Claims

CLAIMS1 . A sensor for determining an absorption state of cryopanels within a cryopump, said sensor comprising: an absorbent material; a detector for measuring a property of said absorbent material, said property changing as matter is absorbed by said material; and a heating element for heating said absorbent material to release at least some of said absorbed matter; wherein said absorbent material of said sensor is configured to be mounted thermally coupled to a second stage of said cryopump, such that during operation of said cryopump said absorbent material is within 5K of a temperature of said cryopanels of said cryopump.

2. A sensor according to claim 1 , wherein said absorbent material of said sensor is a same material as said absorbent material of said cryopanels.

3. A sensor according to claim 1 , wherein said absorbent material comprises a porous glass or other absorbent dielectric material.

4. A sensor according to any preceding claim, wherein said detector comprises a capacitor, said absorbent material being mounted to form the dielectric of said capacitor, said detector being configured to detect a change in capacitance of said capacitor as matter is absorbed by said absorbent material.

5. A sensor according to claim 4, wherein said detector further comprises a reference capacitor, said reference capacitor comprising a reference material mounted to form the dielectric of said reference capacitor, said reference material comprising similar dielectric properties to said absorbent material prior to said absorbent material absorbing any matter, said detector being configured to detect said change in capacitance of said capacitor relative to said reference capacitor.

6. A sensor according to claim 5, wherein said capacitor and reference capacitor are formed from an interdigital array, said electrodes of said capacitors comprising electrodes of said interdigital array, said capacitor and said reference capacitor comprising a common output electrode.

7. A sensor according to claim 5 or 6, wherein said absorbent material comprises activated carbon and said reference material comprises non-activated carbon.

8. A sensor according to any one of claims 4 to 7, wherein said sensor comprises control circuitry configured to change a frequency of a signal input to said capacitor and to determine an amount and a type of matter absorbed by said absorbent material in response to said detected changes in capacitance at different signal frequencies.

9. A sensor according to any preceding claim, wherein said detector comprises an oscillating circuit comprising a piezoelectric element having absorbent material mounted thereon, said piezoelectric element and absorbent material determining an oscillating frequency of said detector, said detector being configured to determine changes in frequency of said oscillations as matter is absorbed by said absorbent material.

10. A sensor according to any preceding claim, said sensor comprising control circuitry to control said heater to periodically heat said absorbent material to release at least some of said absorbed matter and to determine said absorption state of said cryopanels from said detected changes in said property and said number of times said heater has been activated.

11. A sensor according to claim 10, wherein a surface area of said absorbent material of said sensor is less than 5 mm2, preferably less than 1 mm2.

12. A cryopump comprising:a two stage cryogenic refrigerator; a first stage array coupled to a first stage of said two stage refrigerator; cryopanels thermally coupled to a second stage of said two stage refrigerator, said cryopanels comprising surfaces at least a portion of said surfaces bine coated with an absorbent material; and a sensor according to any preceding claim mounted thermally coupled to said second stage of said cryopump.

13. A cryopump according to claim 12, wherein said sensor is mounted at a location with no line of sight between an inlet or said first stage array and a surface of said absorbent material of said sensor.

14. A cryopump according to claim 12 or claim 13, wherein said cryopump comprises control circuitry configured to receive signals from said sensor, and to determine from said signals an amount of material absorbed on said cryopanels, and in response to said amount being greater than a first predetermined amount to output an indication that said pump should be regenerated soon; and in response to said amount being greater than a second critical amount to regenerate said pump.

15. A method of determining an amount of matter absorbed on a second stage cryopanel within a cryopump, said second stage cryopanel comprising absorbent material on at least one surface, said method comprising: mounting a sensor according to any one of claims 1 to 11 thermally coupled to a second stage of said cryopump; and measuring changes in a property of an absorbent material of said sensor as matter is absorbed by said absorbent material; and determining from said measured changes an amount of matter absorbed by said cryopanels.

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