Abstract:
Methods, devices, and systems for protecting a vacuum environment from leakage are provided. The devices include an optical component for gas-tight closure of the vacuum environment, a retention device configured to retain the optical component and including a cooling region separated from the vacuum environment in a gas-tight manner and configured to receive a cooling medium to cool the optical component, a first part-region of the optical component being arranged in the cooling region, and a reduced-pressure region configured to have a reduced pressure and separated in a gas-tight manner from the vacuum environment and from the cooling region, a second part-region of the optical component being arranged in the reduced-pressure region, and a detector configured to detect a leakage in the optical component when the cooling medium flows from the cooling region into at least one of the reduced-pressure region or the vacuum environment.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation and claims priority under 35 U.S.C. §120 to PCT Application No. PCT/EP2014/052798, filed on Feb. 13, 2014. The contents of this priority application is incorporated herein by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     The present invention relates to methods, devices, and systems for protecting a vacuum environment from leakage. 
     BACKGROUND 
     Some extreme ultraviolet (EUV) radiation production devices have a vacuum chamber in which a target material can be introduced to produce EUV radiation. A beam guiding chamber serves to guide a laser beam from a laser source into the vacuum chamber. Between the vacuum chamber and the beam guiding chamber an intermediate chamber is provided. A first window that closes the intermediate chamber in a gas-tight manner serves to introduce the laser beam from the beam guiding chamber into the intermediate chamber. A second window that closes the intermediate chamber in a gas-tight manner serves to discharge the laser beam from the intermediate chamber into the vacuum chamber. 
     The vacuum chamber is separated from the environment in a sealing manner by the second window. If the second window is destroyed or the seal of the second window is faulty, gas from the environment can flow into the vacuum chamber since in the vacuum chamber there is a lower pressure than in the environment, for example, in the beam guide. The second window or the seal thereof consequently constitute potential sources of leakage. Owing to a non-tight window, it is possible not only for gas, but also where applicable for fluid materials, for example, cooling water for cooling the window, to reach the vacuum environment. In some cases, the primary sealing of the vacuum chamber by the second window is supplemented with a secondary sealing by the first window so that, in the event of damage to or breakage of the second window, only the (small) gas volume present in the intermediate chamber can reach the vacuum environment. 
     In some systems, a device has an optical component having a non-planar surface which includes an optically active region. A flow guide is formed on the non-planar surface to orientate a gas in order at least in a part-region of the optically active region of the non-planar surface of the optical component to produce a turbulent flow to cool the optical component. The optical component may, for example, be a convex lens which serves to introduce a laser beam into the vacuum chamber of an EUV radiation production device and which focuses the laser beam in a target region within the vacuum chamber. 
     SUMMARY 
     One aspect of the invention features a device of protecting a vacuum environment from leakage, including: an optical component for the gas-tight closure of the vacuum environment, a cooling device for cooling the optical component by supplying a fluid or gaseous cooling medium to a cooling region which is separated from the vacuum environment in a gas-tight manner and in which a first part-region of the optical component is arranged, a vacuum production device for producing a reduced pressure in a reduced-pressure region which is separated in a gas-tight manner from the vacuum environment and from the cooling region and in which a second part-region of the optical component is arranged, and a detection device for detecting a leakage in the optical component when the cooling medium flows from the cooling region into the reduced-pressure region and/or into the vacuum environment. 
     A closure of the vacuum environment by the optical component in the context of this application is intended to be understood to mean that the optical component separates the vacuum environment that can be formed in a vacuum chamber from a surrounding space in a gas-tight manner which may be located, for example, in another chamber and in which there is a higher pressure than in the vacuum chamber. The surrounding space can be separated in a gas-tight manner both from the reduced-pressure region and from the cooling region. The cooling region can have an inlet opening for the cooling medium and an outlet opening for the cooling medium, that is to say, there is an exchange of the cooling medium. 
     In the event of a defect in the optical component, for example, the formation of a crack, the fluid or gaseous cooling medium from the cooling region can reach the reduced-pressure region via the crack or defect in the optical component, that is to say, the first and second part-regions of the optical component are no longer separated from each other in a gas-tight manner. The reduced pressure is a pressure which is lower than atmospheric pressure under normal conditions. The reduced-pressure region forms a separate vacuum, that is to say, the reduced-pressure region is not connected to the vacuum chamber or to the vacuum environment. In the reduced-pressure region, the cooling medium may be discharged or drawn away so that the cooling medium does not reach the vacuum environment. If a flow of the cooling medium from the cooling region into the reduced-pressure region and/or into the vacuum environment is detected, counter-measures can be taken which prevent the cooling medium or relatively large amounts of the cooling medium from being introduced into the vacuum environment. The second part-region of the optical component, which is arranged in the reduced-pressure region, can be located at the side of the optical component facing away from the vacuum environment. 
     In an embodiment, the device is constructed, in the event of a leakage in the optical component, to block a supply line for supplying the cooling medium to the cooling region. In this manner, it is possible to prevent additional cooling medium from flowing via the inlet opening into the cooling region, which could otherwise reach the vacuum environment. The blocking of the supply line may, for example, be carried out via a switchable valve, for example, a directional valve which is switched by the detection device into an appropriate switching position. 
     In another embodiment, the device is constructed, in the event of a leakage in the optical component to supply a typically gaseous flushing medium to the cooling region. Owing to the flushing of the cooling region using a medium which is inert with respect to the components which are arranged in the vacuum environment, typically a cooling gas, in particular a noble gas, for example, argon or optionally hydrogen, directly after the detection of the leakage, the cooling medium can be removed from the cooling region so that the optical components which are located in the vacuum environment are protected from contamination or damage by the cooling medium. The flushing with the flushing medium can be carried out under pressure to flush out the cooling medium, e.g., in the most rapid manner possible. Of course, the flushing with the flushing medium can be carried out in a time-limited manner and until the cooling medium has been completely removed from the cooling region. 
     In a development, the device is adapted to supply the flushing medium to the cooling region via an outlet opening or an inlet opening for the cooling medium. A flushing of the cooling region using the counter-flow principle, that is to say, counter to the flow direction of the cooling medium, has been found to be advantageous, but the flushing medium can also be supplied to the cooling region via the inlet opening for the cooling medium or optionally via another opening. 
     In a development, the device has a switchable valve for selectively connecting the outlet opening to a supply line for the flushing gas or to a discharge line for the cooling medium. The valve may in this instance be switched from a first switching position (for cooling operation), in which the discharge line for the cooling medium is connected to the outlet opening, into a second switching position, in which the supply line is connected to the outlet opening (for flushing operation). The switching from cooling operation to flushing operation is carried out when the detection device detects a leakage in the optical component. Alternatively, a switchable valve may be provided to selectively produce a connection of an inlet opening of the cooling region for the cooling medium to a supply line for the cooling medium or to a supply line for the flushing medium to switch between cooling operation and flushing operation. 
     In another embodiment, the device is adapted to produce a fluid connection between the cooling region and the reduced-pressure production device to draw the flushing medium from the cooling region. In this manner, the flushing medium may be drawn from the cooling region in a particularly effective manner. In particular, residual molecules which have remained in the line system after the flushing operation can also be drawn away by the reduced-pressure production device. 
     In another development, the device has a switchable valve for selectively connecting an inlet opening of the cooling region to a supply line for the cooling medium or to a connection line for connection to the reduced-pressure production device. The switchable valve may optionally be produced in a structural unit with the switchable valve described above. The switchable valve may in particular be resiliently loaded, that is to say, it has in the state with no power applied a defined valve position or switching position. The switching position in the state without any electrical power is typically used for cooling operation. In this case, the switchable valve is only acted on with electric current and electrically switched into the switching position for flushing operation when a leakage is detected. 
     Alternatively, the device may have a switchable valve for selectively connecting the outlet opening of the cooling region to a discharge line for the cooling medium or to a connection line for connection to the reduced-pressure production device. Such a valve may also be resiliently loaded, the switching position of the valve in the state with no electrical power applied typically being used for cooling operation. 
     In some embodiments, the detection device has a pressure sensor for determining a (reduced) pressure produced by the reduced-pressure production device and/or a pressure sensor for determining a pressure in the vacuum environment. The pressure sensor is constructed to measure a pressure or a temporal pressure change, in particular a pressure increase, which is present in the reduced-pressure region or in a line formed between the reduced-pressure production device and the reduced-pressure region or an inlet opening of the reduced-pressure region or in the vacuum environment. 
     In a development, the detection device is constructed, when a pressure threshold value of the reduced pressure and/or the pressure in the vacuum environment is exceeded or when a threshold value of a pressure increase of the reduced pressure and/or the pressure in the vacuum environment is exceeded, to detect a flow of the cooling medium from the cooling region into the reduced-pressure region. As a result of the flow of the cooling medium, the pressure detected by the pressure sensor typically increases. If the pressure detected by the pressure sensor or the pressure increase, that is to say, the pressure change per time unit (gradient), is above a threshold value, this is indicated to the detection device or optionally to a control device, which takes the corresponding steps to switch the device from cooling operation to flushing operation (see above). 
     In another embodiment, the detection device has a residual gas analyzer for detecting the cooling medium in the vacuum environment. The residual gas analyzer has a mass spectrometer to analyze the residual gas atmosphere in the vacuum environment or in the vacuum chamber in a qualitative and quantitative manner. The detection of molecules of the cooling medium, for example, water molecules, may indicate that cooling medium has flowed into the vacuum environment. 
     In a development, the residual gas analyzer is constructed, when a threshold value for the partial pressure of the cooling medium in the vacuum environment is exceeded or when a threshold value of an increase of the partial pressure of the cooling medium in the vacuum environment is exceeded, to detect a flow of the cooling medium from the cooling region into the vacuum environment. The residual gas analyzer can produce a mass spectrum, based on which the number of molecules of materials contained in the vacuum environment, more specifically the number of molecules of a mass-to-charge ratio characteristic of the respective material, can be determined. The number of molecules of a respective material is typically proportional to the partial pressure of this material in the vacuum environment and can therefore be used to determine whether the cooling medium has flowed into the vacuum environment. As long as a small proportion of a material which is identical to the cooling medium, for example, water vapor, is in any case present in the vacuum environment, that is to say, without the presence of a leakage, it may be advantageous to detect the presence of a leakage with reference to a threshold value of the increase of the partial pressure being exceeded (that is to say, the change of the partial pressure per time unit). 
     In another embodiment, the device further includes a retention device for retaining the optical component, in which the cooling region and the reduced-pressure region are formed. The retention device may in particular be a holder in which the optical component is inserted. Both the first part-region and the second part-region may be constructed in the form of hollow spaces in the retention device or in the holder which are separated from each other in a gas-tight manner by the optical component inserted into the retention device and optionally by way of additional seals. Both the first and the second part-region are typically located in the portion of the optical component received by the holder, that is to say, outside an optically used part-region of the optical component. 
     In another embodiment, the device further includes at least one seal for separating the cooling region from the reduced-pressure region in a gas-tight manner. The at least one seal is arranged between the cooling region and the reduced-pressure region and serves to close any gap which may be present between the optical component and the retention device in a gas-tight manner so that the cooling medium cannot reach the reduced-pressure region from the cooling region when the optical component is intact. The seal may be constructed, for example, as an O-ring, a molded seal, etcetera. The device or the holder may have at least one additional seal which is arranged between the reduced-pressure region and the optically used part-region of the optical component and which serves to separate the reduced-pressure region from the vacuum environment or from the space surrounding the optical component in a gas-tight manner. 
     In another embodiment, the reduced-pressure production device is constructed as a Venturi nozzle. In a Venturi nozzle, the reduced pressure is produced in a removal pipe which is typically arranged at the narrowest location of a pipe through which a pressure medium, typically a fluid, for example, water, flows. At the narrowest location of the pipe, the static pressure is minimal so that in the removal pipe there is a reduced pressure which can be used to draw a fluid or gaseous medium and which in this instance is used to draw the cooling medium or the flushing medium. Of course, it is optionally also possible to use other reduced-pressure production devices to produce a reduced pressure in the reduced-pressure region, for example, suction pumps or the like. 
     In another embodiment, the optical component has an optically used part-region and the first part-region is further away from the optically used part-region than the second part-region. This is advantageous since, in the event of a formation of a crack in the optical component, the cooling medium first reaches the second part-region before reaching the optically used part-region and can be drawn off by the reduced-pressure production device. The second part-region may in particular be constructed in an annular manner so that any cooling water discharged from the cooling region can reach the second part-region regardless of the type of crack formed. 
     In another embodiment, the optical component is constructed as a plane-parallel plate or window, that is to say, the optical component has no beam-shaping function. As described further above, a laser beam can enter the vacuum environment through such a window. Owing to the high level of thermal conductivity thereof, (synthetically produced) diamond is often used as a material for such a window, in particular when the device is used in an EUV radiation production device having high laser powers (&gt;1 kW). The production costs for such a window of diamond material are comparatively high so that the thickness of such a window is generally selected so as not to be too large. When a window is used with a comparatively large thickness, insufficient cooling may further result in destruction of the diamond material (burnout). 
     Another aspect of the invention features an EUV radiation production device including: a vacuum chamber having a vacuum environment in which a target material can be arranged to produce EUV radiation, and a device as described above. The optical component serves in this instance to separate the vacuum environment in the vacuum chamber from another chamber, for example, a beam guiding chamber, in a gas-tight manner, or an intermediate chamber. The beam guiding chamber serves to guide a laser beam from a beam source, which may have, for example, one or more CO 2  lasers, into the vacuum chamber. The laser beam is focused in a target region which is located in the vacuum chamber and on which the target material is arranged. 
     A further aspect of the invention features a method of protecting a vacuum environment from a leakage in an optical component that closes the vacuum environment in a gas-tight manner. The method includes the steps of: supplying a cooling medium to a cooling region which is separated from the vacuum environment in a gas-tight manner and in which a first part-region of the optical component is arranged, producing a reduced pressure in a reduced-pressure region which is separated from the vacuum environment and from the cooling region in a gas-tight manner and in which a second part-region of the optical component is arranged, and detecting a leakage in the optical component when the cooling medium flows from the cooling region into the reduced-pressure region. As has been described above, the flow can be detected, for example, with reference to an increase of the pressure in the reduced-pressure region or in lines which are connected thereto. 
     In a variant, after the detection of the leakage, the supply of the cooling medium to the cooling region is prevented to prevent introduction of the cooling medium via the optical component into the vacuum environment. 
     In another variant, after the detection of the leakage, a flushing medium is supplied to the cooling region. The flushing medium is typically inert with respect to the optical components arranged in the vacuum environment, that is to say, it does not lead to damage or contamination of these components. 
     In a development, a fluid connection is produced between the cooling region and a reduced-pressure production device to draw off the flushing medium from the cooling region. Using the reduced-pressure production device, it is also possible to draw off residual molecules which have remained in the line system after the flushing operation. The flushed-out cooling medium or flushing medium can be collected. 
     Other advantages of the invention will be appreciated from the description and the drawings. The features which have been mentioned above and those which are set out in greater detail below can also be used individually or together in any combination. The embodiments which are shown and described are not intended to be understood to be a definitive listing, but instead are of exemplary character in order to describe the invention. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic illustration of an example EUV radiation production device that has a beam guiding chamber, a vacuum chamber and an optical component for the gas-tight separation of the two chambers. 
         FIG. 2A  is an illustration of an example device for protection of a vacuum environment formed in a vacuum chamber from a leakage of an optical component during cooling operation. 
         FIG. 2B  is an illustration of the device from  FIG. 2A  during flushing operation. 
         FIGS. 3A and 3B  are illustrations similar to  FIGS. 2A and 2B  with a reversed flow direction of a cooling medium in comparison with  FIGS. 2A and 2B , respectively. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description of the drawings, identical reference numerals are used for components which are the same or which have the same function. 
       FIG. 1  shows an example EUV radiation production device  1  which has a driver laser device  2 , a beam guiding chamber  3  and a vacuum chamber  4 . In a vacuum environment  4   a  which is formed in the vacuum chamber  4 , a focusing device in the form of a focusing lens  6  is arranged to focus a CO 2  laser beam  5  in a target region B. The EUV radiation production device  1  shown in  FIG. 1  substantially corresponds to the structure as described in US 2011/0140008 A1, the subject matter of which is incorporated in this application by reference in its entirety. The illustration of measuring devices for monitoring the beam path of the laser beam  5  has been omitted for reasons of clarity. 
     The driver laser device  2  includes a CO 2  beam source and a plurality of amplifiers to produce a laser beam  5  with a high beam power (&gt;1 kW). For a detailed description of examples of possible embodiments of the driver laser device  2 , reference may be made to US 2011/0140008 A1. From the driver laser device  2 , the laser beam  5  is redirected via a plurality of redirection mirrors  7  to  11  of the beam guiding chamber  3  and another redirection mirror  12  in the vacuum chamber  4  onto the focusing lens  6  which focuses the laser beam  5  in the target region B on which tin is arranged as a target material  13 . 
     The target material  13  is hit by the focused laser beam  5  and in this instance converted into a plasma state, which serves to produce EUV radiation  14 . The target material  13  is supplied to the target region B using a preparation device (not shown) which guides the target material  13  along a predetermined path which intersects with the target region B. For details of the provision of the target material, reference may also be made to US 2011/0140008 A1. 
     In a beam guiding space  3   a  of the beam guiding chamber  3 , there is provided a device  15  for enlarging a beam diameter of the laser beam  5  which has a first off-axis parabolic mirror  16  having a first convex-curved reflecting surface and a second off-axis parabolic mirror  17  having a second concave-curved reflecting surface. The reflecting surfaces of an off-axis parabolic mirror  16 ,  17  each form the off-axis segments of an (elliptical) paraboloid. The term “off-axis” means that the reflecting surfaces do not contain the rotation axis of the paraboloid (and consequently also not the apex of the paraboloid). 
     A vacuum pump  18  serves to produce in the vacuum chamber  4  an operating pressure p 2  which is in the fine vacuum range (generally substantially lower than 1.0 mbar). The operation of the vacuum chamber  4  under reduced-pressure conditions is required, since, in a residual gas environment with an excessively high pressure, there would be excessive absorption of the EUV radiation  14  produced. In contrast, the beam guiding chamber  3  or the inner space  3   a  which is formed therein is operated at a substantially higher pressure p 1 , which may be, for example, in the order of magnitude of approximately 5 mbar above atmospheric pressure (e.g., 1013 mbar). The beam guiding chamber  3  is consequently placed under an excess pressure with respect to the environment of the EUV radiation production device  1  in a targeted manner in order to protect the optical elements which are arranged in the beam guiding chamber  3  from contamination. 
       FIG. 1  also shows a device  20  for protecting the vacuum environment  4   a  in the vacuum chamber  4  from leakage in an optical component  21  in the form of a planar plate or a window which serves to close the vacuum environment  4   a  in a gas-tight manner, i.e., to separate the vacuum chamber  4  and the beam guiding chamber  3  as a gas-tight sealing. The window  21  is integrated in a holder  22 . Through the window  21 , the laser beam  5  can enter the vacuum environment  4   a . The material of the window  21 , in this instance (synthetically produced) diamond, becomes powerfully heated as a result of the high radiation power of the laser beam  5 . 
     As can be seen in  FIG. 2A , the holder  22  has a hollow space which forms a cooling region  23  and to which a cooling medium  24 , for example, in the form of cooling water, can be supplied via an inlet opening  23   a . As can also be seen in  FIG. 2A , a first part-region  21   a  of the surface of the window  21  is arranged in the cooling region  23  and comes into contact with the cooling medium  24 . The cooling region  23  forms a substantially annular hollow space and has at a side diametrically opposed to the inlet opening  23   a  an outlet opening  23   b  to discharge the cooling medium  24 . The supply of the cooling medium  24  is carried out in the example shown in  FIG. 2A  via a supply line  26  which extends from a cooling device (cooling unit or cooler)  25 . The heated cooling medium  24  which is discharged at the outlet opening  23   b  of the cooling region  23  is supplied via a discharge line  27  to the cooling device  25  which discharges the heat absorbed by the cooling medium  24  to the environment or to another medium. The cooling device  25 , the supply line  26  and the discharge line  27  form a closed cooling circuit for the cooling medium  24 . Of course, this is not necessarily the case, that is to say, the heated cooling medium  24  may, for example, when cooling water is used—be discharged to the environment. 
     The device  20  further has a reduced-pressure production device (or a reduced-pressure generator) in the form of a Venturi nozzle  28 . The Venturi nozzle  28  is supplied via a supply line  29  with a fluid pressure medium which is taken from a reservoir which is not shown. The Venturi nozzle  28  has a removal pipe  28   a  in which the pressure medium flowing through the Venturi nozzle  28  produces a reduced pressure. The removal pipe  28   a  is connected by means of a connection line  30  to a reduced-pressure region  31  which is formed in the holder  22  of the window  21  and which is constructed in the example shown as a hollow space. The reduced-pressure region  31  is constructed in the manner of a circular ring in the region of the window  21  so that a second annular part-region  21   b  of the window  21 , more specifically of the surface thereof, is located in the reduced-pressure region  31  and is acted on with a reduced pressure p u  which may, for example, be only slightly below atmospheric pressure (e.g., 1013 mbar). To separate the reduced-pressure region  31  from the cooling region  23 , a first seal  32   a  in the form of an O-ring is fitted in the holder  22 . 
     In order to separate the reduced-pressure region  31  from the beam guiding chamber  3 , a second seal  32   b  in the form of another O-ring is fitted in the holder  22 . 
     The reduced pressure p u  in the reduced-pressure region  31  is monitored by means of a detection device (e.g., a detector) which is constructed in the example shown as a pressure sensor  33 . The pressure sensor  33  is used, in the event of a predetermined threshold value p s  of the reduced pressure p u  being exceeded (p u &gt;p s ) or in the event of a predetermined threshold value Δp s  of an increase Δp u  of the pressure p u  (per time unit) being exceeded, to detect a flow of the cooling medium  24  from the cooling region  23  into the reduced-pressure region  31 , which flow may occur in the event of damage, for example, in the event of the formation of a crack, in the window  21 . Fig. When the threshold value Δp s  of the pressure increase Δp u  is exceeded, the device  20  switches from cooling operation which is illustrated in  FIG. 2A  (e.g., Δp u &lt;Δp s ) to flushing operation which is illustrated in  FIG. 2B  (e.g., Δp u &gt;Δp s ). 
     Alternatively or in addition to detecting the flow of the cooling medium  24  from the cooling region  23  into the reduced-pressure region  31 , a detection of the flow of the cooling medium  24  into the vacuum environment  4   a  may also be carried out. This is possible, for example, using a pressure sensor  37  (cf.  FIG. 1 ) which is connected to the vacuum environment  4   a  and which serves to determine the operating pressure p 2  in the vacuum chamber  4 . If the operating pressure p 2  in the vacuum chamber  4  is above a pressure threshold value p 2,s , a flow of the cooling medium  24  into the vacuum environment  4   a  is detected and the device  20  is switched from cooling operation to flushing operation. Alternatively or in addition, when a threshold value Δp 2,s  for the pressure increase Δp 2  of the operating pressure p 2  in the vacuum environment  4   a  is exceeded, the device  20  can also be switched from cooling operation to flushing operation. 
     Additionally or alternatively, a residual gas analyzer  38  which is illustrated in  FIG. 1  can also be used as a detection device. The residual gas analyzer  38  is constructed for analysis of the residual gas atmosphere present in the vacuum environment  4   a  and enables quantitative determination of the partial pressure or the number of molecules of the gaseous materials contained in the residual gas atmosphere. In this instance, the residual gas analyzer  38  serves to determine the partial pressure p H20  of the cooling medium  24  in the form of cooling water, more specifically of water vapor, in the residual gas atmosphere or in the vacuum environment  4   a.    
     If the partial pressure p H20  of the cooling medium  24  is above a threshold value p H20,s , the device  20  is switched from cooling operation to flushing operation. Alternatively or additionally, when a threshold value p H20,s  for the pressure increase Δp H20  of the partial pressure p H20  of the cooling medium  24  in the vacuum environment  4   a  is exceeded, the device  20  can also be switched from cooling operation into flushing operation. The latter (i.e., using the comparison of the pressure increase ΔP H20  with a threshold value Δp H20,s ) is particularly advantageous when, without the flow of the cooling medium  24  from the cooling region  23 , a small quantity of the cooling medium  24  is already present in the vacuum environment  4   a . Typically, in the vacuum environment  4   a , a small quantity of water vapor is present so that, when water is used as a cooling medium  24 , the detection of the leakage using the comparison of the pressure increase Δp H20  with a threshold value Δp H20,s  may where applicable be more favorable than the detection using the comparison of the partial pressure p H20  with a threshold value P H20,s . 
     In order to switch the device  20  from cooling operation to flushing operation, the pressure sensor  33 , the additional pressure sensor  37  and/or the residual gas analyzer  38  act(s) directly or optionally via a control device on two switchable valves  34   a ,  34   b . The control device can be implemented as a suitable software and/or hardware, e.g., a microcontroller, an application-specific integrated circuit (ASIC), and/or a field programmable gate array (FPGA). In this instance, the first valve  34   a , which is constructed as a directional valve having two switching positions, is switched from a first switching position, which is shown in  FIG. 2A  and in which the supply line  26  for the cooling medium  24  is connected to the inlet opening  23   a  of the cooling region  23 , into a second switching position, which is shown in  FIG. 2B  and in which the connection between the supply line  26  and the inlet opening  23   a  is separated, and the inlet opening  23   a  is instead connected to the connection line  30  between the intake pipe  28   a  of the Venturi nozzle  28  and the reduced-pressure region  31 . At the same time, the second valve  34   b  is also switched from a third switching position (cf.  FIG. 2A ) into a first switching position (cf.  FIG. 2B ) in which the discharge line  27  for the cooling medium  24  is separated from the outlet opening  23   b . In this manner, the cooling device  25  or the cooling circuit  25 ,  26 ,  27  is completely separated from the optical component  21 . 
     In the first switching position of the second valve  34   b  as shown in  FIG. 2B , a supply line  35  for a flushing medium  36  is simultaneously connected to the outlet opening  23   b  of the cooling region  23 . The flushing medium  36  is an inert gas, for example, argon, or optionally hydrogen. The flushing medium  36  is taken from a reservoir which is not illustrated graphically and in which the flushing medium  36  is kept under pressure. The flushing medium  36  which is introduced into the cooling region  23  via the outlet opening  23   b  urges the cooling medium  24  which is discharged from the cooling region  23  through the inlet opening  23   a . The inlet opening  23   a , in the second switching position of the first valve  34   a  as shown in  FIG. 2B , is connected to the connection line  30  and consequently the Venturi nozzle  28  via which the cooling medium  24  and also the flushing medium  36  is drawn off. The drawn-off cooling medium  24  and also the drawn-off flushing medium  36  reach the Venturi nozzle  28  via the removal pipe  28   a , are carried by the pressurized fluid flowing in the Venturi nozzle  28  and may optionally be collected. Using the Venturi nozzle  28 , it is also possible to draw off residual molecules of the cooling medium  24  which still remain in the line portions between the inlet opening  23   a  of the cooling region  23  or between an inlet opening  31   a  of the reduced-pressure region  31  and the Venturi nozzle  28 . 
     If the flushing operation is completed, the device  20  can be switched into a rest mode, in which the second valve  34   b  assumes a second switching position thereof (and/or in which the first valve  34   a  keeps at the second switching position thereof), in which the outlet opening  23   b  is completely blocked so that no more flushing medium  36  reaches the cooling region  23  and the reduced-pressure region  31 . In addition, the supply line  29  of the Venturi nozzle  28  can be blocked so that the Venturi nozzle  28  no longer produces reduced pressure p u . In the rest mode produced in this manner, the damaged window  21  can be removed from the holder  22  and be replaced by a new window. 
     The new window closes the reduced-pressure region  31  in a gas-tight manner with respect to the cooling region  23 , that is, the new window tightly seals the reduced-pressure region  31  from the cooling region  23 , so that the device  20  can be operated in cooling operation again. The switching of the second valve  34   b  into the second switching position can be carried out automatically after a predetermined period of time which is optionally predetermined by a time-delay member or a time-delay circuit. Of course, the switching of the valves  34   a ,  34   b  does not necessarily have to be carried out by the detection device or the pressure sensor  33 , but there may optionally be provided in the device  20  an additional control device to which the measurement signal supplied by the pressure sensor  33  is transmitted and which, depending on the reduced pressure p u  measured carries out the switching of the valves  34   a ,  34   b  or the switching between cooling operation, flushing operation and rest mode. Of course, in place of two controllable valves  34   a ,  34   b , only a single controllable valve may be provided in the device  20 . In the example shown, the first controllable valve  34   a  is resiliently loaded so that the first controllable valve  34   a  assumes the first switching position (i.e., cooling operation of the device  20 ) shown in  FIG. 2 a    without a voltage being applied or without being acted on with electric current. 
       FIGS. 3A and 3B  show an embodiment in which the flow direction of the cooling medium  25  is reversed in the cooling circuit  25 ,  26 ,  27  with respect to the embodiment shown in  FIGS. 2A and 2B . Accordingly, the function of the inlet opening  23   a  and the outlet opening  23   b  is transposed. The second valve  34   b  is therefore constructed for the selective connection of the inlet opening  23   a  to the supply line  26  for the cooling medium  24  or to the supply line  35  for the flushing medium  36 . Accordingly, the first switchable valve  34   a  is constructed for selectively connecting an outlet opening  23   b  of the cooling region  23  to a discharge line  27  for the cooling medium  24  or to a connection line  30  for connection to the reduced-pressure production device  28 . Optionally, the supply and discharge of the flushing medium  36  may be carried out not via the inlet opening  23   a  or the outlet opening  23   b  (or vice versa), but instead via additional openings. 
     In summary, it is possible to produce in the manner described above effective protection of a vacuum environment  4   a  from the introduction of a cooling medium  24  in the event of a leakage in an optical component  21 , which closes the vacuum environment  4   a  in a gas-tight manner. Of course, the optical component does not necessarily have to be constructed as a window  21  but may optionally also be constructed as a lens or in another manner. The optical component  21  also does not necessarily serve to separate the vacuum chamber  4  from the beam guiding chamber  3  as shown in  FIG. 1 , since, between the vacuum chamber  4  and the beam guiding chamber  3 , an intermediate chamber (not shown) may optionally be arranged. 
     A number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the following claims.