Abstract:
A device includes a driver laser arrangement including a beam source for generating a laser beam and an amplifier arrangement for amplifying the laser beam. The device also includes an apparatus for monitoring the laser beam that includes a transmissive optical element having a normal direction oriented at a tilt angle with respect to a beam axis of the laser beam. The apparatus also includes a spatially resolving detector for registering laser radiation reflected backwards by the transmissive optical element. The transmissive optical element has first and second sides that are oriented at a wedge angle with respect to one another and through which the laser beam passes. The first and second sides reflect first and second partial beams of the incident laser beam. The apparatus has an optical filter that prevents one of the reflected first and second partial beams from reaching the detector.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation and claims priority under 35 U.S.C. §120 to PCT Application No. PCT/EP2014/061816, filed on Jun. 6, 2014. The contents of this priority application is incorporated herein by reference in its entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    The present disclosure relates to devices have a driver laser arrangement including a beam source for generating a laser beam and an amplifier arrangement for amplifying the laser beam, and an apparatus for monitoring the laser beam. The apparatus for monitoring the laser beam includes an optical element (e.g., a planar plate) that transmits the laser beam and a spatially resolving detector for registering laser radiation reflected back at the optical element. A normal direction of the optical element is oriented at a tilt angle with respect to a beam axis of the laser beam. The disclosure also relates to an associated method for monitoring a laser beam. 
       BACKGROUND 
       [0003]    In some embodiments of EUV light sources, a monitoring laser beam for testing an alignment of optical components is reflected at a planar plate aligned at an angle to the monitoring laser beam and registered by a detector. The wavelength of the monitoring laser beam differs from the wavelength of a further laser beam (e.g., a CO 2  laser beam) that is transmitted by the planar plate, which forms a window in a vacuum chamber. 
         [0004]    A small portion of radiation power incident on the planar plate (e.g., which has a tilt) that has been introduced into a beam path is typically reflected to a location outside of the beam path, even in the case of a laser beam that is transmitted by the planar plate. For example, the laser radiation is not only reflected back from one side of the planar plate, but rather, each side of the planar plate reflects a partial beam of the incident laser beam. It has been found that interference strips emerge in recorded images when monitoring or analyzing a laser beam by observing the laser radiation reflected at a transmissive optical element on a spatially resolving detector (e.g., a camera), and so only a few details of a beam cross section of the laser beam to be monitored and imaged on the detector may still be identifiable. 
         [0005]    The difference between the degrees of reflection of the two sides of the planar plate can be increased by a reflecting coating applied to one of the sides in order to avoid such interference strips. However, the reflectivity of such a coating should not be selected to be too high, particularly in the case of laser beams with a high laser power (e.g., of multiple kilowatts), such as laser beams generated by a driver laser arrangement. Moreover, a partial beam reflected at one of the two sides, the power of which only has a few percent of the power of the partial beam reflected at the other side, can already lead to clearly visible interference strips. Alternatively, attempts can be made to remove the interference strips in the recorded images with the aid of numerical image processing algorithms, but this technique generally does not adequately remove the interference strips. 
         [0006]    In some embodiments, a device for focusing a laser beam on a workpiece includes a transmissive optical element in the form of a planar plate that is arranged at a tilt angle in relation to a beam axis of the laser beam in a convergent beam path of the laser beam and includes a spatially resolving detector for registering laser radiation reflected back at the transmissive optical element. Assigned to the detector are means for distinguishing laser radiation reflected back at a first side of the optical element from laser radiation reflected back at a second side of the optical element. The means can be an image evaluation apparatus or a diaphragm that masks the laser radiation reflected back from one of the sides of the transmissive optical element. Methods for monitoring laser processing may be associated with such a device. 
         [0007]    In certain embodiments, methods exist for coaxial beam analysis at optical systems in which a defined percentage of a beam is reflected back coaxially or at a small angle at an optical face that is perpendicular to a chief ray. The partial beam reflected back is separated from the chief ray by a beam splitter, where the partial beam is available for beam analysis. An optical element with a back-reflecting surface can have a wedge angle in order to be able to separate the reflection of the two surfaces on the image side. 
       SUMMARY 
       [0008]    The embodiments disclosed herein include devices of the type set forth at the outset and methods for monitoring a laser beam without the occurrence of spurious interference strips. 
         [0009]    In some embodiments, a device includes an optical element that has a first side and a second side that are aligned in relation to one another at a wedge angle and through which the laser beam passes. The first side of the optical element reflects a first partial beam of an incident laser beam, and the second side of the optical element reflects a second partial beam of the incident laser beam. The apparatus has an optical filter that prevents one of the two reflected partial beams from reaching a detector. 
         [0010]    The optical element is typically arranged in a collimated beam path of the laser beam. As a result of the generally very small wedge angle between the first side (e.g., through which the laser beam enters the optical element) and the second side (e.g., through which the laser beam emerges from the optical element), the two partial beams are reflected in slightly different directions such that the two partial beams can be separated from one another by an optical filter. For example, optical filtering can be implemented in a plane in which the different angles or the different directions of the laser beams (e.g., which are generated in the plane of the optical element) are converted into a spatial distribution. That is, optical filtering can be implemented in the Fourier space. 
         [0011]    Typically, the plane in which the transmissive optical element is arranged (e.g., the object plane) is imaged on an image plane of the detector (e.g., a CCD chip) by an imaging optical unit. In this case, optical filtering can be undertaken in the imaging optical unit. The tilt angle at which the normal direction of the transmitting optical element is aligned in relation to the beam axis of the laser beam (and which corresponds to the angle of incidence of the laser beam on the optical element) is large enough to decouple the reflected partial beams out of the beam path of the laser beam and is typically more than 10°, (e.g., approximately 20°). The incident laser beam and the two reflected partial beams form a common plane in which the tilt angle extends. The wedge angle of the optical element typically does not extend in the plane of the tilt angle. 
         [0012]    In some embodiments, the wedge angle is less than 10 mrad (e.g., less than 5 mrad). The transmissive optical element should have the smallest possible influence on the transmitted laser beam. This is the case for a plate that is completely planar (i.e., without a wedge angle) since such a plate does not have refractive power and merely brings about a minimal lateral offset of the laser beam according to a small thickness of the plate. As a result of the wedge angle with values in the range specified above, the laser beam only experiences a negligibly small deflection when passing through the optical element. When desired, such deflection can be taken into account without problems when aligning subsequent optical elements in the beam path. 
         [0013]    The optical filter advantageously has a focusing apparatus for focusing the reflected partial beams and a separation element for separating the two partial beams. The separation element is arranged in the region of a focal plane or in the focal plane of the focusing apparatus. As a result of the comparatively small wedge angle of the optical element, there is only a minimal deviation between the directions of reflection of the reflected partial beams. For example, the reflected partial beams separate very slowly from one another and remain spatially superposed over a long distance. The two laser beams incident on the focusing apparatus with different angles of incidence are focused at different locations in the focal plane of the focusing apparatus. The two partial beams can be separated from one another rather easily in the focal plane by deflecting one of the two partial beams with a mirror acting as a separation element such that the deflected partial beam is not incident on the detector. Downstream of the focal plane, the partial beam passed to the detector becomes larger again and generates an image of the laser beam without interference strips on the detector. 
         [0014]    In some embodiments, the separation element is embodied as a diaphragm that blocks one of the two partial beams. Via a diaphragm arranged in the focal plane or in the vicinity of the focal plane, it is possible to separate the two partial beams by blocking one of the two partial beams. The diaphragm can be a pinhole diaphragm, a slot diaphragm, or a one-sided diaphragm (e.g., a diaphragm that is only arranged on one side of the reflected partial beam). 
         [0015]    In some embodiments, the focusing apparatus is a converging lens. The reflected, collimated partial beams are incident on the lens with different angles of incidence and are therefore focused at different locations in the focal plane of the lens. The lens can serve simultaneously as an imaging optical unit for imaging the plane with the transmitting optical element on the image plane in the detector. The lens form should be selected in such a way that the aberrations during imaging are as small as possible. For example, the lens can be a biconvex lens. It has been found to be advantageous if the lens has a comparatively large focal length of more than 50 mm (e.g., more than 100 mm) in order to keep aberrations as small as possible during imaging. The lens can also be a meniscus lens, which should generally have a focal length of approximately 100 mm or more (e.g., approximately 200 mm or more). 
         [0016]    In some embodiments, the two foci of the partial beams have a spacing A in the focal plane. The spacing A corresponds to X multiplied by the diameter of the foci in the focal plane, where X&gt;2 (e.g., X&gt;5). The spacing A between the two foci therefore corresponds to at least two times (e.g., at least five times) the diameter of one of the two foci in the focal plane. The spacing A between the two foci is measured between the centers of the two foci. The diameters D of the two foci are typically of the same size in the focal plane. 
         [0017]    In some embodiments, the following relationship applies to the wedge angle γ, the wavelength λ of the laser beam, the refractive index n of the transmissive optical element, the beam radius w L  of the laser beam on the lens, the beam quality factor M 2  of the laser beam, and a multiple X of the diameter D of the foci in the focal plane: γ&gt;X λ/π M 2 /(n w L ). Here, n denotes the real part of the complex refractive index of the material of the transmissive optical element at the wavelength λ of the transmitted laser beam. The product M 2 λ/π is also referred to in the literature as the beam parameter product (BPP). In the paraxial approximation for a large, collimated laser beam, the following applies for the diameter D of the focus in the focal plane of a lens with a focal length f: D=2f/w L  λ/π M 2 . The following applies for the separation or the spacing A of the two foci in the focal plane: A=f2nγ. The relationship specified above, which is independent of the focal length f of the lens, emerges from the criterion for the separability of the two partial beams, A&gt;X*D. 
         [0018]    Preferably, X (i.e., a multiple of the diameter of the foci or of one of the foci in the focal plane) and the wedge angle satisfy the following condition: γ&gt;X*0.18 mrad (corresponding to γ&gt;X*0.6 arcmin). This condition emerges from the equation above for a wavelength λ of the laser beam of 10.6 μm (e.g., a CO 2  laser beam), a beam dimension w L =8 mm, M 2 =1, and n=2.4 as refractive index of the material of the transmissive optical element (e.g., ZnSe). 
         [0019]    In some embodiments, a beam splitter apparatus for dividing the radiation intensity of the two partial beams into first and second observation beam paths is arranged upstream of the focal plane or downstream of the focal plane in the beam path of the focused partial beams. For example, in the first observation beam path, one of the two partial beams can be separated or blocked in the focal plane with the aid of the separation element, while the second partial beam reaches the detector, as described above. For example, the second observation beam path can be used to guide the partial beams to a fast power detector (e.g., a pyroelectric power detector). The beam splitter apparatus may advantageously be arranged upstream of the focal plane for observing both the near field and the far field of the laser beam. If the beam splitter apparatus is arranged downstream of the focal plane, the near field of the laser beam is typically observed with both observation beam paths. 
         [0020]    In certain embodiments, an imaging optical unit for imaging the focus of a partial beam generated by the focusing apparatus or the foci of both partial beams on the detector or on a further detector is arranged in the second observation beam path. The other partial beam of the second observation beam path can be blocked or separated in the focal plane, like in the first observation beam path, in order to avoid artifacts. However, optical filtering or separating of the two partial beams is not mandatory in this case since two foci or focal points are present, which are imaged and generally do not overlap. If the imaging of the focus (i.e., focal point) or of the foci (i.e., focal points) is implemented by the second observation beam path on one and the same detector, the imaged partial beam(s) of the second observation beam path is/are incident at a different location on the detector than the imaged partial beam of the first observation beam path. In this case, both the far field and the near field of the laser beam are detectable (offset from one another) on one and the same detector. The imaging optical unit can be a lens or a focusing mirror. 
         [0021]    In order to detect the shift of the focal position out of the focal plane in the propagation direction of the partial beam and thus optionally to measure changes in the divergence of the laser beam, it is advantageous to image not only the focal plane itself but also planes in the vicinity of the focal plane on the detector. This can be achieved by lengthening or shortening the beam path of the second observation beam path between the imaging optical unit and the focal plane, which can be achieved by displacing deflection mirrors arranged in the second observation beam path. The focal length of the imaging optical unit typically substantially corresponds to the distance between the imaging optical unit and the focal plane of the focusing apparatus. 
         [0022]    In some embodiments, the device is advantageously configured to image the partial beam or the partial beams of the second observation beam path through the beam splitter apparatus on the detector or onto a further detector. In this case, the beam splitter apparatus is typically embodied as a transmissive optical element, at the typically parallel sides of which a first portion of the radiation power is reflected and a second portion is transmitted. The transmitted portion of the radiation power can be guided back to the beam splitter apparatus, for example by deflection mirrors, such that said beam splitter apparatus is passed through a second time by the transmitted radiation portion. In this manner, the non-blocked partial beam or the two partial beams of the second observation beam path can be imaged on the detector in a manner substantially parallel to the non-blocked partial beam of the first observation beam path. 
         [0023]    In certain embodiments, the beam splitter apparatus has a first side for reflecting both partial beams of the first observation beam path and a second side for reflecting both partial beams of the second observation beam path. In this case, the separation of the two observation beam paths at the beam splitter apparatus can be implemented by arranging the first side and the second side of the beam splitter apparatus at a wedge angle in relation to one another and/or by selecting the distance between the two sides of the beam splitter apparatus to be so large that the partial beams reflected at the respective side have a sufficiently large distance from one another and are spatially separated to such an extent that they can be detected separately from one another on the detector or on a further detector. Typically, different respective separation elements (e.g., different (pinhole) diaphragms) are required for blocking a respective partial beam of the observation beam paths, provided that optical filtering is even implemented in the second observation beam path. 
         [0024]    In some embodiments, at least one side of the beam splitter apparatus advantageously has a coating for modifying at least one optical property of both reflected partial beams of the first or of the second observation beam path. The coating(s) evaluate different properties of the laser beam on the detector. For example, the coating(s) can be polarization-selective coatings that modify the polarization direction of the partial beams or possibly only reflect one polarization direction (e.g., the s-polarized radiation portion or the p-polarized radiation portion). The coating(s) can also be wavelength-selective coatings. For example, a wavelength-selective coating can block wavelengths in the region of the fundamental wavelength of the laser beam such that only radiation with wavelengths possibly superposed on the laser beam is reflected at the coating. 
         [0025]    In certain embodiments, the device additionally includes a vacuum chamber in which a target material is arrangeable in a target region for generating EUV radiation. The device may also include a beam guiding apparatus for guiding the laser beam from the driver laser apparatus in the direction of the target region. The beam guiding apparatus guides the laser beam to a focusing element or to a focusing arrangement, which serves to focus the laser beam in a target region. In the target region, provision is made for a target material (e.g., tin) that transitions into a plasma phase when irradiated by the laser beam and emits EUV radiation in the process. The device therefore serves for generating EUV radiation (i.e., the device is embodied as an EUV light source). 
         [0026]    Methods are also disclosed for monitoring a laser beam that passes through a first side and a second side of a transmissive optical element (e.g., a planar plate). The second side of the optical element is oriented at a wedge angle with respect to the first side. A normal direction of the optical element is oriented at a tilt angle with respect to the beam axis of the laser beam. The method includes reflecting a first partial beam of the incident laser beam from the first side of the optical element, reflecting a second partial beam of the incident laser beam from the second side of the optical element, blocking one of the two partial beams before it reaches a detector by optical filtering, and recording an image of the laser beam using the detector on which the non-blocked partial beam is incident. 
         [0027]    The method can be used for the beam analysis of a laser beam that is generated by a driver laser arrangement, such as that described above. Such a laser beam typically has a high radiation power that may be greater than 10 kW. The optical element can be arranged at the output of the amplifier arrangement of the driver laser arrangement in order to analyze or monitor the amplified laser beam. However, it is also possible to arrange the optical element in the beam path between individual amplifier stages of the amplifier arrangement or at a different location within the radiation guiding apparatus in order to monitor the laser beam. It is understood that two or more transmitting optical elements can also be arranged in the beam path of the laser beam in order to monitor the laser beam in the manner described above. 
         [0028]    Further advantages will be apparent from the description and from the drawings. Likewise, the features mentioned above and still to be specified below can respectively be used separately or together in any combination. The shown and described embodiments should not be understood to be an exhaustive listing, but rather have an exemplary character. 
     
    
     
       DESCRIPTION OF DRAWINGS 
         [0029]      FIG. 1  shows a schematic illustration of a device for generating EUV radiation; 
           [0030]      FIG. 2  shows an illustration of a planar plate that has a wedge angle and two partial beams reflected at sides of the planar plate; 
           [0031]      FIGS. 3A, 3B, and 3C  show illustrations of apparatuses for monitoring a laser beam, including the planar plate of  FIG. 2 , a detector, and an optical filter; 
           [0032]      FIGS. 4A, 4B, and 4C  show illustrations of foci of the two reflected partial beams in a focal plane of a lens without and with a pinhole diaphragm and a one-sided diaphragm for blocking one of the partial beams; 
           [0033]      FIGS. 5A, 5B, and 5C  show three illustrations an apparatus similar to the apparatuses of  FIG. 3B , including a beam splitter apparatus for splitting radiation intensity of the two partial beams in order to image both a near field and a far field of the laser beam on the detector; and 
           [0034]      FIG. 6  shows an illustration of an apparatus similar to the apparatus of  FIG. 5  with a wedge-shaped beam splitter apparatus that has two polarization-selective coatings. 
       
    
    
     DETAILED DESCRIPTION 
       [0035]    In the following description of the drawings, identical reference signs are used for equivalent or functionally equivalent components. 
         [0036]      FIG. 1  schematically shows a device  1  for generating extreme ultraviolet (EUV) radiation, which has a beam source  2 , an amplifier arrangement  3  with three optical amplifiers or amplifier stages  4   a ,  4   b ,  4   c , a beam guiding apparatus  5  (depicted without detail), and a focusing lens  6 . A laser beam  7  is generated by the beam source  2  and amplified by the amplifier arrangement  3 . The focusing lens  6  serves to focus the laser beam  7  onto a target region T at which a target material  8  has been introduced. When irradiated by the laser beam  7 , the target material  8  transitions into a plasma phase and emits EUV radiation in the process. The EUV radiation is focused by a collector mirror  9 . 
         [0037]    In the example shown in  FIG. 1 , the collector mirror  9  has an opening for passing the laser beam  7 . The focusing lens  6  separates a vacuum chamber  10 , in which the target material  8  is arranged, from the beam guiding apparatus  5 . The beam source  2  has two CO 2  lasers in order to generate a pre-pulse and a main pulse, which are together amplified in the amplifier arrangement  3  and focused on the target material  8 . Together with the amplifier arrangement  3 , the beam source  2  forms a driver laser arrangement  11  of the device  1 , which forms an EUV light source. 
         [0038]    A laser power P O  of the laser beam  7  at the output of the amplifier arrangement  3  (e.g., located downstream of the third amplifier stage  4   c ) is greater than approximately 10 kW. In order to perform a beam analysis on the laser beam  7 , it is necessary to decouple a small portion of radiation power from the beam path of the laser beam  7 , which portion may be on the order of multiple watts. To this end, a plate-shaped optical element  12  (e.g., a planar plate  12 ) can be introduced into the beam path of the laser beam  7 . The planar plate  12  is embodied as shown in  FIG. 2 . 
         [0039]    The planar plate  12  is formed from material that is transparent to the laser beam  7  at a laser wavelength λ of 10.6 μm. By way of example, the material can be zinc selenide or diamond, which has a refractive index n of approximately 2.4 for the laser wavelength λ. The planar plate  12  has a first side  14   a  and a second side  14   b  lying opposite to the first side, through which the laser beam  7  passes (propagating from right to left in the illustration shown in  FIG. 2 ). The first side  14   a  and the second side  14   b  are aligned in relation to one another under a wedge angle γ, which is actually very small (e.g., γ&lt;10 mrad, or preferably, &lt;5 mrad), but which is depicted as relatively large in  FIG. 2  for clarification purposes. Therefore, the two sides  14   a ,  14   b  of the planar plate  12  are aligned almost parallel to one another such that referring to the optical element  12  as a planar plate  12  is accurate, despite the wedge angle γ. 
         [0040]    The laser beam  7  passes perpendicularly through the first side  14   a  of the planar plate  12 . A small portion of the radiation power of the laser beam  7  is reflected back (e.g., in a backward direction) as a first partial beam  13   a  at the first side  14   a  and propagates backward against the direction of propagation of the laser beam  7 . The laser beam  7  emerges at the opposing second side  14   b  of the planar plate  12 , wherein a small part of the radiation power is reflected back into the planar plate  12  as a second partial beam  13   b  at the second side  14   b . The second partial beam  13   b  propagates in the material of the planar plate  12  at twice the wedge angle 2γ relative to the angle at which the first reflected partial beam  13   a  propagates. 
         [0041]    The second partial beam  13   b  is refracted when passing through the first side  14   a  of the planar plate  12  and propagates at an angle α relative to the first partial beam  13   a , where n sin(2γ)=sin(α). Consequently, α=2nγ for small angles γ. For a separation distance s (e.g., a distance between the two partial beams  13   a ,  13   b ), the following applies: s=L/α, where α denotes the path difference between the two partial beams  13   a ,  13   b . If both partial beams  13   a ,  13   b  are incident for beam analysis on a detector  16  (as shown in  FIGS. 3A, 3B, and 3C ) of an apparatus  15  for monitoring the laser beam  7 , interference strips with a spacing which approximately corresponds to the separation s of the partial beams  13   a ,  13   b  of  FIG. 2  are generated in an image recorded by the detector  16 . 
         [0042]    In order to enable the decoupling of the partial beams  13   a ,  13   b  from the laser beam  7 , the normal direction of the planar plate  12  is aligned at a tilt angle β in relation to a beam axis  7   a  of the laser beam  7 . The tilt angle β is approximately 20° in the examples shown, but β can also be larger or smaller. The tilt angle β corresponds to the angle of incidence of the laser beam  7   a  on the planar plate  12 . The illustration of the planar plate  12  in  FIG. 2  constitutes a section along the ZY-plane of the apparatus  15  shown in  FIGS. 3A, 3B, and 3C . The tilt angle β lies in the plane of the drawing (i.e., the ZX-plane) in which the laser beam  7  and the reflected partial beams  13   a ,  13   b  propagate. Consequently, the wedge angle γ and the tilt angle β do not lie in a common plane. As shown in  FIG. 2 , the wedge angle γ, rather, extends in a plane aligned perpendicular to the plane of the drawing and extends perpendicular to the practically parallel sides  14   a ,  14   b  of the planar plate  12 . However, an alignment of the wedge angle γ in a plane perpendicular to the plane of the drawing is not mandatory; rather, the plane with the wedge angle γ can have any orientation in relation to the plane with the tilt angle β. 
         [0043]    In the apparatus  15 , the partial beams  13   a ,  13   b  reflected by the planar plate  12  are initially incident on a deflection mirror  17  and subsequently incident on a partially transmissive optical element  18 . At the partially transmissive optical element  18 , a radiation portion of the partial beams  13   a ,  13   b  is deflected to a power detector  19 , which measures and monitors the radiation power of the incident radiation portion of the partial beams  13   a ,  13   b.    
         [0044]    In order to prevent the occurrence of interference strips, the apparatus  15  has an optical filter  20 , which, in the examples shown in  FIGS. 3A, 3B, and 3C , has a converging lens  21  and a diaphragm  23  arranged in an image-side focal plane  22  of the converging lens  21 . The diaphragm  23  is embodied as a pinhole diaphragm in the examples shown, but the diaphragm  23  can also be embodied as a slot diaphragm or as a one-sided diaphragm. The two partial beams  13   a ,  13   b  are incident on the converging lens  21  with slightly different alignments and hence angles of incidence (as illustrated in  FIG. 2 ), leading to the two partial beams  13   a ,  13   b  being focused at different locations in the focal plane  22  of the converging lens  21 . Therefore, the two partial beams  13   a ,  13   b  can be separated in the focal plane  22  by blocking of one of the two partial beams  13   b , while the other partial beam  13   a  passes through the diaphragm  23  and is incident on the detector  16 . 
         [0045]    In the examples shown in  FIGS. 3A, 3B, and 3C , a further partially transmissive optical element  24  is arranged in the apparatus  15 . The partially transmissive optical element  24  deflects a radiation portion of the two partial beams  13   a ,  13   b  onto a pyroelectric detector  25 . The apparatuses  15  shown in  FIGS. 3A, 3B, 3C  substantially differ in that the first partial beam  13   a , which is allowed through the diaphragm  23 , propagates coaxially with the beam axis  7   a  of the laser beam  7  in the apparatus  15  shown in  FIG. 3A , while the first partial beam  13   a  is incident on the detector  16  perpendicular to the beam axis  7   a  of the laser beam  7  in  FIGS. 3B and 3C . 
         [0046]    The apparatuses  15  shown in  FIGS. 3B and 3C  substantially differ from one another in that the two partial beams  13   a ,  13   b  are deflected to the detector  16  at the partially transmissive optical element  24  in the apparatus  15  of  FIG. 3B , while the deflection of the two partial beams  13   a ,  13   b  to the detector  16  is implemented at a further deflection mirror  17   a  in the apparatus  15  shown in  FIG. 3C . 
         [0047]      FIGS. 4A, 4B, and 4C  show, in an exemplary manner, the focal plane  22  of the apparatus  15  of  FIG. 3A , wherein the two circular foci F a , F b  of the two partial beams  13   a ,  13   b  with an (identical) diameter D are identifiable in  FIG. 4 a    and are arranged with a spacing A from one another in the Y-direction (i.e., perpendicular to the plane of the drawing of  FIG. 3A ).  FIG. 4B  shows the focal plane  22  with the pinhole diaphragm  23 , which allows the first partial beam  13   a  to pass to the detector  16  but blocks the second partial beam  13   b . As an alternative to the pinhole diaphragm  23  shown in  FIG. 4B , a slot diaphragm  23  or a one-sided diaphragm  23  for blocking the second partial beam  13   b  can be used, as depicted in  FIG. 4C . In order to separate the two partial beams  13   a ,  13   b  cleanly from one another, it is advantageous for the two foci F a , F b  to have a spacing A in the focal plane  22  in which A&gt;X*D, where X&gt;2 or preferable &gt;5. 
         [0048]    In order to achieve this relationship, the optical filter  20  can be adapted. For example, a radius w L  of the partial beams  13   a ,  13   b  incident on the converging lens  21 , the wedge angle γ, and the parameters of the laser beam  7  (e.g., the wavelength λ and a beam quality factor M 2 ) can be adapted to one another in a suitable manner, such as where the following condition is satisfied: γ&gt;X λ/π M 2 /(n w L ). 
         [0049]    For instance, in the present example, the laser beam  7  has a wavelength λ of 10.6 μm, the beam radius w L  of the lens  21  is 8 mm, the laser beam  7  is diffraction limited (i.e., M 2 =1), and the refractive index n of the material of the planar plate  12  is n=2.4, such that the condition γ&gt;X*0.18 mrad (or γ&gt;X*0.6 arcmin) emerges from the above formula, where X&gt;2 or X&gt;5. Given these parameter values, the partial beams  13   a ,  13   b  can generally be separated virtually without problems in the focal plane  22 , and so the occurrence of interference strips on the image of the laser beam  7  recorded by the detector  16  can be avoided. 
         [0050]    In the examples shown in  FIGS. 3A, 3B, and 3C , the converging lens  21  also serves to image the laser beam  7  or the plane with the planar plate  12  on an image plane on the detector  16  (e.g. a pyroelectric detector matrix). In such examples, the converging lens  21  serves as an imaging optical unit. A distance between an object plane in which the planar plate  12  is arranged and the converging lens  21 , and the distance between the converging lens  21  and the detector  16  are typically adapted to the focal length f of the converging lens  21  in such a way that the laser beam  7  (e.g., abeam cross section of the laser beam  7 ) is imaged on the detector  16  with a reduced scale. In order to avoid aberrations, it has been found to be advantageous if the focal length f of the converging lens  21  is comparatively large and lies at approximately f&gt;50 mm (for w L =8 mm) or more. In some embodiments, other lenses (e.g., meniscus lenses or other focusing optical elements, such as focusing mirrors) can be used instead of the converging lens  21 . 
         [0051]    In addition to analyzing the beam cross section of the laser beam  7  in the manner described above, it is also possible to image or register a far field of the laser beam  7  on the same detector  16  or on a further detector. To this end, the apparatus  15  can be modified in a manner described below with respect to  FIGS. 5A, 5B, and 5C . In the apparatus  15  of  FIG. 5A , a beam splitter apparatus  27  in the form of a partially transmissive element for dividing radiation intensities of both partial beams  13   a ,  13   b  onto first and second observation beam paths  26   a ,  26   b  is arranged upstream of the focal plane  23  of the focusing lens  21  in the beam path of the focused partial beams  13   a ,  13   b . The first observation beam path  26   a  corresponds to the beam path shown in  FIG. 3B  and serves to image a near field of the laser beam  7  onto the detector  16 . 
         [0052]    The second observation beam path  26   b  guides the two partial beams  13   a ′,  13   b ′ via a partially transmissive optical element  24  to a further diaphragm  23 ′, at which the second partial beam  13   b ′ of the second observation beam path  26   b  is blocked. Unlike what is shown in  FIG. 5A , it is also possible to forgo blocking of the second partial beam  13   b ′ since the two partial beams  13   a ′,  13   b ′ of the second observation beam path  26   b  are separated on the detector  16  even without blocking. 
         [0053]    In the example shown in  FIG. 5A , the first partial beam  13   a ′ of the second observation beam path  26   b  is deflected by a further deflection mirror  29  to an imaging optical unit  28  in the form of a further lens, and the focus F a  of the first partial beam  13   a ′ of the second observation beam path  26   b  is imaged through the beam splitter apparatus  27  on the detector  16  by this further lens  28 . Alternatively, imaging of the first partial beam  13   a ′ of the second observation beam path  26   b  can be implemented past the beam splitter apparatus  27 , as depicted in  FIG. 5B . In this case, the further deflection mirror  29  of  FIG. 5A  can be omitted. It is likewise possible to undertake imaging of the first partial beam  13   a ′ past the beam splitter apparatus  27  by virtue of the further deflection mirror  29 , together with the lens  28 , being displaced further in the direction toward the focusing lens  21  in the apparatus  15 , as shown in  FIG. 5A . In this case, the imaging of the first partial beam  13   a ′ of the second observation beam path  26   b  is implemented not downstream, but upstream, of the beam splitter apparatus  27 . 
         [0054]      FIG. 5C  shows an embodiment of the apparatus  15  in which a focusing mirror  28 ′ serves as an imaging optical unit. The two partial beams  13   a ′,  13   b ′ of the second observation beam path  26   b  are, in this case, reflected back to the beam splitter apparatus  27  by the partially transmissive optical element  24 , which is aligned perpendicular to the direction of propagation of the two partial beams  13   a ′,  13   b ′. A respective radiation portion of the partial beams  13   a ′,  13   b ′ is deflected or reflected at the front side of the beam splitter element  27  in the direction of the focusing mirror  28 ′. The partial beams  13   a ′,  13   b ′ of the second observation beam path  26   b  are reflected back to the focusing mirror  28 ′, focused in the process, and once again, pass through the beam splitter element  27  and are incident on the detector  16 . Consequently, the two partial beams  13   a ′,  13   b ′ pass through the beam splitter element  27  a total of three times in the example shown in  FIG. 5C . No filtering of one of the two partial beams  13   a ′,  13   b ′ of the second observation beam path  26   b  was undertaken in the example shown in  FIG. 5C , as this is not mandatory. It is understood that the optical filtering in the second observation beam path  26   b  can be omitted from the apparatuses  15  shown in  FIGS. 5A and 5B . As an alternative to the renewed reflection of the two partial beams  13   a ′,  13   b ′ at the beam splitter element  27 , as is shown in  FIG. 5C , it is also possible to deflect the two partial beams  13   a ′,  13   b ′ onto the focusing mirror  28 ′ by one or more deflection mirrors. The setup of the apparatus  15  shown in  FIG. 5C  is advantageously compact and easy to manage. 
         [0055]    Via the apparatuses  15  shown in  FIGS. 5A, 5B, and 5C , it is possible to observe both the near field of the laser beam  7  and the far field thereof on the detector  16 , where the image of the near field and the image of the far field are imaged offset from one another on the detector  16 . By observing the focus F a  of the first partial beam  13   a ′ (or of both partial beams  13   a ′,  13   b ′) of the second observation beam path  26   b , it is possible to monitor the angle distribution (e.g., the divergence) of the laser beam  7  in the plane of the planar plate  12 . As indicated by an arrow shown in  FIGS. 5A, 5B, and 5C , the beam path between the further lens  28  or the focusing mirror  28 ′ and the focus F a  can be modified by displacing the further deflection mirror  29 , the partially transmissive optical element  24 , or possibly, other deflection mirrors. In this manner, it is possible to identify a change that may be occurring in the focal position of the imaged partial beam  13   a ′ in the propagation direction (i.e., perpendicular to the focal plane  22 ) and deduce a change in the divergence of the laser beam  7 . 
         [0056]    In an alternative embodiment of the apparatus  15 , which is shown in  FIG. 6 , both the partial beams  13   a, b  of the first observation beam path  26   a  and the partial beams  13   a ′, b′ of the second observation beam path  26   b  are reflected at the beam splitter apparatus  27  (e.g., at a first side  30   a  and at a second side  30   b  of the beam splitter apparatus  27 ). The beam splitter apparatus  27  is formed from a material that is transparent to the laser beam  7  and is provided with a reflecting coating  31  at the second side  30   b . The first side  30   a  and the second side  30   b  of the beam splitter apparatus  27  include a comparatively large wedge angle δ. The wedge angle δ is typically of the order of degrees and is therefore significantly larger than the wedge angle γ such that the two observation beam paths  26   a ,  26   b  are reflected at significantly different angles and are incident on two detectors  16 ,  16 ′ arranged next to one another. Diaphragms  23 ,  23 ′ are arranged respectively in the first observation beam path  26   a  and in the second observation beam path  26   b  in order to mask the respective second partial beam  13   b ,  13   b ′. It is understood that the two observation beam paths  26   a ,  26   b  can also be incident on a common detector in the apparatus  15  of  FIG. 6  and that two detectors can be provided in the apparatus  15  of  FIG. 5  in order to separately detect the near field and the far field. 
         [0057]    In the apparatus  15  shown in  FIG. 6 , the reflecting coating  31   b  of the beam splitter apparatus  27  is embodied as a polarization-selective layer. For example, the reflecting coating  31   b  only reflects one polarization direction (e.g., s-polarization) of the incident partial beams  13   a ,  13   b . Accordingly, a (reflecting) coating  31   a  is likewise applied to the first side  30   a  of the beam splitter apparatus  27  and reflects a polarization direction (e.g., p-polarization) that is orthogonal to the polarization direction reflected by the second coating  31   b . In this manner, different polarization directions of the laser beam  7  can be monitored separately from one another on the two detectors  16 ,  16 ′. It is understood that one or two coatings  31   a ,  31   b  that are selective for other properties of the laser beam  7  can also be applied to the beam splitter apparatus  27 . For example, a wavelength-selective coating  31   a, b  can be applied to one or both sides  30   a, b  of the beam splitter apparatus  27  in order to suppress specific wavelengths of the laser beam  7 . In this way it is possible, to detect wavelengths superposed onto the fundamental wavelength λ of the laser beam  7  separately. In order to separate the two observation beam paths  26   a ,  26   b  from one another and as an alternative or in addition to the use of a wedge angle δ, the material of the beam splitter apparatus  27  can have a thickness that is sufficient to spatially separate the two reflected partial beams  13   a ,  13   b  of the first observation beam path  26   a  from the two partial beams  13   a ′,  13   b ′ of the second observation beam path  26   b  such that these two observation beam paths  26   a ,  26   b  can be detected separately. 
         [0058]    The parameters of the laser beam  7  established during the beam analysis of the laser beam  7  can be transmitted to an open-loop or closed-loop control apparatus (not shown here), which acts on the driver laser arrangement  11  or on further components (e.g., optical elements in the beam guidance  5  of the laser beam  7 ) in order to generate a laser beam  7  with parameters optimized for generating the EUV radiation in the target region T. 
         [0059]    Unlike what was described in the context of  FIGS. 3A, 3B, and 3C , the apparatus  15  can be arranged not only downstream of the third amplifier  4   c  of the amplifier arrangement  3  but also between two of the optical amplifiers  4   a ,  4   b ,  4   c , between the first optical amplifier  4   a  and the beam source  2 , or in the beam guiding apparatus  5 . In particular, multiple apparatuses  15  described above can be used to monitor the laser beam  7  at different positions on the laser beam path into the target region T. Monitoring of the laser beam  7  can be performed in real time as described above without interference strips occurring in the images of the laser beam  7  recorded by the detector  16 . 
         [0060]    A number of embodiments have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the disclosure. Accordingly, other embodiments are within the scope of the following claims.