Beam guiding device for laser radiation with a suppression function for ghost reflections, and laser system comprising same

Abstract

The invention relates to a beam guiding device (15) and to a laser system (13) equipped therewith. The beam guiding device (15) comprises a sensor system (7) for detecting reflected laser radiation (5), a radiation-impermeable blocking element (12) positioned in the beam path, an optical unit (17) and a transmissive lens assembly (20) having at least one lens (20) for guiding laser radiation (1) in a forward direction (2) and the laser radiation (5) reflected opposite thereto. The lens assembly (20) is designed in such a way that a first back reflection (9) produced thereon is focused onto a plane (22) which is conjugate to the plane (21) of the blocking element (12) with respect to the optical unit (17), and such a large divergence is imposed on a second back reflection (11) likewise produced on the lens assembly (20) that the second back reflection does not reach the sensor system (7).

Description

[0001] Beam guiding device for laser radiation with suppression of ghost reflections and laser system therewith

[0002] The present invention relates to a beam guiding device for laser radiation and a laser system equipped therewith.

[0003] In laser systems, unwanted reflections can occur on various beam-guiding or beam-steering components. Depending on the application, these reflections can interfere with analysis or detection, or even lead to damage at higher laser powers. To avoid or reduce these problems, there is a need for improvements to conventional approaches, which often cannot be applied effectively in all situations.

[0004] The object of the present invention is to reduce the disturbing influence of unwanted reflections in a laser system.

[0005] The problem is solved by the subject matter of the independent claims. Further possible embodiments of the invention are specified in the subclaims, the description, and the drawings. Features, advantages, and possible embodiments presented in the description for one of the subject matter of the independent claims are to be regarded at least analogously as features, advantages, and possible embodiments of the respective subject matter of the other independent claims, as well as any possible combination of the subject matter of the independent claims, optionally in conjunction with one or more of the subclaims.

[0006] The beam guiding device or beam guiding arrangement according to the invention serves to guide laser radiation, for example, a continuous or pulsed laser beam. The beam guiding device comprises a transmissive lens arrangement for guiding the laser radiation in the forward direction, i.e., for guiding a forward laser beam toward a predetermined effective focus area. This lens arrangement can be a single lens or comprise multiple lenses or partial lenses. The effective focus area is referred to here as a focus area or focal point at which the laser radiation is focused by the beam guiding device and in which a desired effect is to be achieved with the laser radiation or is achieved during normal operation.This can, for example, be processing a material using laser radiation or heating or at least partially evaporating a drop of material, for example a tin drop for EUV light generation. In the application case of EUV light generation, the respective drop of material whose position is to be determined using the laser radiation or which is to be at least partially converted into a plasma using the laser radiation would therefore be located in the effective focus area. Furthermore, there can be other focus areas within the beam guidance device in which the laser radiation is focused, but in which no material impact is to be achieved. The beam guidance device can therefore be intended or configured in particular for EUV light generation or EUV lithography or an EUV driver laser, i.e. a corresponding laser system for EUV light generation or EUV lithography.In particular, the beam guiding device can be or comprise a focusing device for an EUV driver laser. Other applications or uses of the beam guiding device according to the invention are also possible.

[0007] The beam guidance device also comprises a detector or sensor for detecting laser radiation reflected from the effective focus area in the backward direction by the lens arrangement, i.e., a corresponding backward laser beam. The forward direction thus leads along a beam path or ray path from a source of the laser radiation through the lens arrangement to the effective focus area. The backward direction, on the other hand, leads from the effective focus area in the opposite direction through the lens arrangement and further to the sensor. A corresponding beam path or ray path of the laser radiation reflected in the backward direction, i.e., the backward laser beam, can thus partially or partially overlap with the beam path or ray path of the laser radiation propagating in the forward direction, i.e., the forward laser beam, and can partially or partially differ from it.

[0008] The sensor system can, for example, comprise a camera, a CCD chip, a photodiode, or another light or radiation sensor. The sensor system can be equipped with or coupled to evaluation electronics. This allows a measurement or sensor signal generated by the sensor system upon detection or recording of the reflected laser radiation to be analyzed, i.e. evaluated, for example to determine a predetermined parameter value, such as the position of the material droplet. For this purpose, for example, an intensity distribution and / or a wavefront and / or a phase shift and / or similar factors of the reflected laser radiation can be evaluated. The reflected laser radiation can therefore, for example, serve as a basis for determining the position of the respective material droplet in the EUV light generation and / or for determining or measuring a wavefront deformation and / or aberrations and / or similar factors.

[0009] The beam guidance device also includes a radiation obstacle or blocking element that is opaque to the laser radiation. This is arranged in the beam path of the reflected laser radiation, i.e., the reverse laser beam, between the sensor and the lens arrangement. The blocking element is dimensioned such that it only blocks a central portion of the reflected laser radiation in the radial direction or keeps it away from the sensor, while another or surrounding portion of the reflected laser radiation can pass the blocking element in the reverse beam path to the sensor. The radial direction can run perpendicular to the propagation direction of the reflected laser radiation along the reverse beam path, i.e., in the cross-sectional plane of the corresponding reverse laser beam traveling to the sensor.The diameter of the blocking element in this cross-sectional plane can therefore be smaller than the diameter of the back laser beam there, i.e., the light or radiation distribution of the reflected laser radiation. The blocking element can, for example, absorb laser radiation impinging on it or—for example, with a correspondingly wavelength-selective or phase-selective design—only absorb unwanted interference or back reflections, such as from the lens arrangement, or reflect, deflect, or scatter them out of the back beam path.

[0010] The beam guiding device also comprises an optic arranged in the beam path of the reflected laser radiation between the blocking element and the lens arrangement. This optic is designed or configured such that it images the plane of the blocking element into a plane optically conjugate thereto, which lies between the optic and the lens arrangement. This optically conjugate plane between the optic and the lens arrangement is located in the backward beam path, but can in particular also be located in the forward beam path. The optic can comprise one or more optical parts or components, for example at least or exactly two lenses. These do not necessarily have to be identical. A distance between a first lens of the optic facing the effective focus area and the conjugate plane can therefore differ from the distance between the blocking element.the plane of the blocking element and a second lens of the optics facing the sensor. The plane of the blocking element refers to the position of the blocking element in the longitudinal direction or propagation direction along the return beam path.

[0011] During operation of the beam guiding device, i.e. when laser radiation is guided through it in a forward direction, interfering or back reflections, also known as ghost reflections, can arise on the lens arrangement, in particular on different sides of one or more lenses in the lens arrangement, for example on the front side and the inner back side of the lens or of a lens. These interfering back reflections can then propagate in the backward direction. According to the invention, the corresponding lens or the corresponding lenses of the lens arrangement are shaped such that, of these two interfering back reflections, a first or focused back reflection is focused into the plane conjugate to the plane of the blocking element lying between the optics and the lens arrangement, and at least a major part of the other, i.e. a second or diverging back reflection orfrom whose intensity, in particular due to the shape of the corresponding side of the corresponding lens from which this second back reflection originates, is given such a large divergence that it does not reach the sensor. The second back reflection can therefore, for example, be focused if the divergence between the lens arrangement or the corresponding side of the lens and the conjugate plane is sufficiently large, depending on the shape of the lens outside or inside the lens, or be reflected in a scattered manner on the corresponding side of the lens. The second back reflection or its main part can then be eliminated or at least greatly attenuated, for example by absorption, for example on a surrounding housing and / or on light- or radiation-opaque apertures or diaphragms within the beam guiding device.Focusing the first back reflection in the conjugate plane means that this first back reflection propagates from there through the optics and is also focused again at the position of the blocking element. This allows at least almost complete suppression of the first back reflection to be achieved, even if the position of the blocking element cannot be freely selected.

[0012] In principle, the arrangement of a blocking element in the beam path can be an effective way of blocking disruptive back reflections. To do this, however, the blocking element would ideally have to be arranged at a point where the back reflection to be blocked is in focus or has the smallest possible diameter, in particular smaller than the diameter of the backward laser beam to be detected. In real beam guidance devices or laser systems, however, such a blocking element cannot always be freely or optimally positioned, for example due to structural conditions or spatial restrictions caused by other parts or components or requirements. This problem is countered here by appropriately adapting the shape of the lens arrangement or lens system responsible for the disruptive back reflection.Lens in combination with the optics, i.e. the utilization of the different positions of the planes conjugate with respect to the optics. The corresponding adjustment of the side of the lens or lens arrangement responsible for this disruptive backreflection could lead to an equally undesirable change in the output-side focal position of the beam guidance device, for example, in comparison to an otherwise identical or similarly designed conventional beam guidance device. However, this effect is compensated in the present case by the likewise adapted shape of the other lens side, i.e. the other side of the same lens and / or one side of another lens in the lens arrangement. Thus, in comparison to a conventional lens or a conventional beam guidance device, the focal position in the effective focus area can remain unchanged and yet an improved suppression of both disruptive backreflections at the sensor system can be achieved.A portion of the second back reflection that might propagate exactly centrally, i.e. on the optical axis of the lens arrangement or the backward beam path, would also be blocked by the blocking element, i.e. kept away from the sensor system.

[0013] The present invention therefore provides that the radii of curvature of the lens or of several lens sides of one or more lenses of the lens arrangement are adapted or predetermined in such a way that the image-side focus position in the effective focus area remains unchanged or maintained, for example compared to the use of a conventional lens or lens arrangement, in particular a conventional biconvex lens or plano-convex lens as is used in conventional beam guiding devices, but one of the two ghost reflections is focused in the plane of the blocking element, i.e. in a virtual blocking element plane, and is thus ultimately suppressed in the real blocking element plane by the blocking element arranged there, while the other ghost reflection is widened or divergent to such an extent that it at least partially or at least largely runs out of the intended beam path of the backward laser beam before it reaches the sensor system.For the latter ghost reflection, the design of the lens arrangement proposed here can therefore result in the greatest possible divergence. Such a design or configuration of the lens arrangement can enable the suppression of the resulting interfering back reflections, for example, even when other suppression measures, such as polarization-based filtering of the interfering reflections, are not practical. In particular, this can be achieved by coordinating the shape of the two sides or radii of curvature of the lens or several lenses of the lens arrangement without significant further modifications to the beam guidance device.

[0014] It may also be possible to design the lens arrangement in such a way that both or several or all of the interfering back reflections arising from the lens arrangement in the conjugate plane are intercepted or blocked by means of the blocking element. To this end, these back reflections can in particular be at least substantially focused in the conjugate plane by appropriately shaping the lens arrangement. It may also be possible to design the lens arrangement in such a way that both or several or all of the interfering back reflections arising from the lens arrangement are given such a large divergence or are scattered so strongly that they at least substantially or largely exit the beam path before the sensor system, thus do not reach the sensor system.

[0015] The blocking element can be a stand-alone component. In this case, the blocking element can, for example, be plate- or disc-shaped, or rod-shaped and extend longitudinally in the backward direction, for example cylindrical or with a polygonal cross-section, or with a cross-section corresponding to the cross-section of the backward laser beam or the first back reflection in the region of the blocking element. By means of a rod-shaped design of the blocking element that extends longitudinally in the backward direction, the first back reflection or parts thereof can, if appropriate, impinge on an outer surface of the blocking element and be absorbed there, for example, or scattered or reflected out of the backward beam path. The first back reflection can thus be blocked particularly reliably or particularly completely, for example even if it occurs during operation of the beam guiding device orof the corresponding laser system, vibrations or thermally induced displacements of the lens arrangement and / or the optics or the like occur. The first back reflection can then optionally not be blocked or only be deflected out of the beam path at one end of the blocking element facing the optics. Likewise, the beam guiding device can have an adjustment or displacement device for adjusting or displacing the blocking element in and / or against the backward direction and / or perpendicular thereto. The position or orientation of the blocking element can thus be adjustable, for example, manually or by motor or automatically, in order to achieve maximum suppression of at least the first interfering reflection and / or optimization or maximization of the intensity ratio between the backward laser beam to be detected in the sensor system and any parts of the first interfering reflection arriving there.Likewise, the blocking element can be a region of another optical element or component of the beam guidance device that is appropriately coated or made of a corresponding material, in particular different from a surrounding region of the corresponding element or component, in order to realize the described effect or functionality of the blocking element. A region of the other optical element or component surrounding this region, i.e., the blocking element, can, for example, be transparent to the reflected laser radiation, i.e., transmissive, or serve as a deflecting mirror in the return beam path.

[0016] In one possible embodiment of the present invention, a coupling element is arranged between the blocking element and the lens arrangement, in particular between the blocking element and the optics. Laser radiation can be coupled into the beam guiding device in the forward direction, i.e., in the direction of the lens arrangement or in the direction of the effective focus area, via this coupling element. The reflected laser radiation can pass through the coupling element in the backward direction to the sensor system. The coupling element can, for example, be an optical beam splitter, such as a partially transparent mirror or the like. Such a beam splitter can be arranged tilted at an angle between 0° and 90°, in particular 45°, to the optical axis of the optics or to the local central longitudinal axis of the backward beam path.If the coupling element is arranged between the sensor and the optics, the latter can serve to focus, shape, or direct the forward laser beam as well as the backward laser beam. This enables a particularly simple and compact design of the beam guidance device. Likewise, the beam paths of the forward laser beam and the backward laser beam from the coupling element to the effective focus area can overlap, whereby particularly compact and efficient beam guidance can be achieved. In a further possible embodiment of the present invention, the optics are designed as relay optics with at least two lenses. This allows for particularly precise, reliable, and space-saving installation, or with limited requirements regarding the shape of the lens orLens arrangement from which the back reflections to be suppressed originate, the imaging of the first back reflection focused between the optics and the lens arrangement onto the blocking element can be realized, in particular even if the position of the blocking element cannot be freely selected or if different amounts of installation space are available in front of and behind the optics or the like.

[0017] In a further possible embodiment of the present invention, the lens arrangement is shaped such that the second back reflection generated there, or its beam path, impinges on an aperture diaphragm surrounding the return beam path and / or an inner side of a housing of the beam guiding device surrounding the return beam path upstream of the sensor system, in particular upstream of the optics, i.e., on the side of the optics facing the lens arrangement. Such an aperture diaphragm can have an opening through which the forward laser beam and the return laser beam can pass, the maximum diameter or cross-section of which is limited by the aperture diaphragm or the diameter of the central opening of the aperture diaphragm. One or more such aperture diaphragms can be arranged along the beam path of the forward laser beam and / or the return laser beam.Since such aperture diaphragms do not serve to guide the beam outside of their central radiation-permeable area, laser radiation striking them can be absorbed or blocked particularly effectively and reliably and, with appropriate design, without causing damage. The same applies to the inside of the housing. By shaping or designing the lens or lens arrangement in such a way that the second back reflection hits the inside of the housing before the optics, a particularly large portion of the second back reflection from the return beam path can be deflected before the sensor, and a reflection on the inside of the housing can potentially hit an opposite side of the housing before the sensor, thereby being further attenuated. Overall, the intensity of any portion of the disruptive back reflections reaching the sensor can be further reduced.

[0018] In a possible further development of the present invention, an absorber material for absorbing the reflected laser radiation is arranged at least in an area of ​​the inside of the housing that is impinged by the second back reflection. In other words, the inside of the housing can be covered with an absorber material at least there or completely, or the housing can be lined with the absorber material. A suitable absorber material can be selected depending on the wavelength of the laser radiation used in the respective application in order to offer a maximum absorption rate and minimum reflectivity. The housing or the corresponding sides or walls of the housing can easily be designed to be sufficiently robust or, for example, provided with cooling on the outside to prevent damage caused by the incident back reflections.To achieve the highest possible absorption rate, the absorber material can, for example, have a three-dimensional surface structure or microstructure, for example, consisting of a multitude of cones or pyramids or corresponding truncated shapes. The arrangement of the absorber material proposed here can further minimize the proportion of back reflections that may reach the sensor. This can be achieved particularly simply, effectively, and with minimal installation space.

[0019] In a further possible embodiment of the present invention, the lens arrangement is or comprises a forward-focusing lens. Such a focusing lens can focus the laser radiation guided in the forward direction, for example, in the effective focus area in order to achieve an intended effect particularly effectively. Such a focusing lens is therefore often necessary in laser systems. The present invention allows for the effective management of the almost unavoidable interfering back reflections.

[0020] In a further possible embodiment of the present invention, the lens arrangement is or comprises the lens closest to the effective focus area in or along the forward direction, i.e., one or the last or output-side lens of the beam guiding device, in particular also of a laser system equipped therewith. This lens or the entire lens arrangement can therefore be arranged after the last mirror of the beam guiding device, viewed in the forward direction. In this case, the disruptive back reflections arising at the lens or lens arrangement typically cannot be easily suppressed, for example, with polarization-based filter methods. Thus, the present invention can be applied particularly advantageously in this case.In a further possible embodiment of the present invention, the front side of the lens is concave and the back side of the lens is convex, respectively, relative to the center point of the lens of the lens arrangement from which the back reflections originate. The front side and the back side are therefore curved in the same direction, resulting in a concave-convex lens shape. The front side has a larger radius of curvature than the back side. In other words, the back side of the lens facing the effective focus area is more strongly curved than the front side facing the optics. Due to the concave shape or curvature of the front side, the first back reflection resulting therefrom can be focused into the plane or position lying between the optics and the lens that is optically conjugate to the plane or position of the blocking element. Due to the comparatively stronger convex curvature of the back side, the first back reflection resulting therefrom can be focusedThe second back reflection occurring on the inside of the lens can be focused between this conjugate plane and the back side, i.e., closer to the lens or even within the lens, thus achieving a particularly large divergence or expanding it particularly significantly. This ultimately allows the intensity of the second back reflection possibly arriving at the sensor to be significantly reduced. At the same time, such a lens shape can achieve the same focal length, i.e., the same focus position in the effective focus area, as, for example, with a conventional biconvex lens. This allows the present invention to be particularly easily integrated into existing beam guidance devices or beam guidance device designs.

[0021] In a further possible embodiment of the present invention, the beam guiding device, in addition to the lens arrangement, has at least one further transmissive optical element for guiding the laser radiation in the forward direction. This can be, for example, a further focusing lens or a window or the like. This at least one further transmissive optical element can therefore, in particular, also be different or separate from the aforementioned optics. The beam guiding device is here set up or configured to suppress interfering back reflections arising at this at least one further transmissive optical element in a polarization-based manner. For this purpose, the beam guiding device can, for example, have at least one deflecting mirror with a polarization retarder layer and a polarizer or polarization filter in the backward beam path.This makes it possible to effectively suppress the back reflections occurring at the at least one further transmissive optical element, for example without the element having to have a special shape or without the use of a further blocking element.

[0022] In a further possible embodiment of the present invention, the position or plane of the blocking element is optically conjugate to the position or plane of the sensor. In other words, the sensor or its sensor or detector surface is arranged relative to the blocking element in such a way that the light distribution present at the position or in the plane of the blocking element in the return beam path is imaged onto the sensor. For this purpose, for example, the distance of the sensor from the blocking element along the return beam path can be adjusted accordingly and / or a corresponding optical element, for example a further lens, can be arranged between the sensor and the blocking element. With regard to this optical element or this further lens, the planes of the sensor and the blocking element, for example the end face facing away from the sensor or the center point of the blocking element, can then be optically conjugate planes to one another.The embodiment of the present invention proposed here makes it possible to keep the disturbing influence of back reflections on sensor data of the sensor system or on analysis results based thereon particularly low.

[0023] The present invention also relates to a laser system comprising at least one laser radiation source and a beam guiding device according to the invention arranged downstream of the source in the forward direction, i.e., in the intended propagation direction of the laser radiation generated thereby. The laser system according to the invention can, in particular, be the laser system mentioned in connection with the beam guiding device according to the invention or correspond thereto. The laser system according to the invention can, for example, be configured or provided for generating EUV light, i.e., for example, light with a central wavelength of 13.5 nm, as used, for example, for EUV lithography. The laser system according to the invention can, for example, be configured as an EU driver laser.One method for generating such EUV light, i.e., extreme ultraviolet radiation, involves irradiating a suitable material droplet, such as a tin droplet, with an intense laser pulse. This at least partially vaporizes the material droplet or converts it into a plasma, which then emits EUV light. To enable efficient and reliable EUV light generation, it is useful to first determine the position of the respective material droplet before its vaporization. For this purpose, a weaker pre- or measuring laser pulse can be emitted in the forward direction from the laser radiation source through the beam guidance device. This pulse does not vaporize the material droplet, but rather only illuminates it, for example, and / or preconditions it, i.e., heats it, for example, for a subsequent, stronger main laser pulse.In this case, a portion of the preliminary or measuring laser pulse can be reflected backward by the material droplet as the aforementioned reflected laser radiation and then detected by the sensor system and subsequently analyzed, for example to determine the position of the respective material droplet. In such an application, the suppression of interfering back reflections can be particularly useful, since these could falsify corresponding analysis results if they reach the sensor system and overlay the laser radiation reflected by the material droplet. Likewise, interfering back reflections of the main laser pulse can be suppressed or intercepted by the beam guidance device according to the invention in the laser system according to the invention. This can then, for example, reduce or prevent stress on or damage to the sensor system.

[0024] Further features of the invention can be derived from the following description of the figures and from the drawings. The features and combinations of features mentioned above in the description, as well as the features and combinations of features shown below in the description of the figures and / or in the figures alone, can be used not only in the respective combinations specified, but also in other combinations or on their own, without departing from the scope of the invention.

[0025] The drawing shows:

[0026] Fig. 1 is a schematic representation of a laser beam guide with a disturbing back reflection according to the prior art;

[0027] Fig. 2 shows a detected reflected intensity distribution resulting from this laser beam guidance according to the prior art;

[0028] Fig. 3 is a schematic representation of an improved laser beam guidance system with suppressed back reflections; and Fig. 4 is a detected reflected intensity distribution resulting from the improved laser beam guidance system.

[0029] Identical or functionally equivalent elements are provided with the same reference numerals in the figures.

[0030] Fig. 1 shows a partial schematic representation to illustrate a laser application according to the prior art. Here, a forward laser beam 1 is focused in a forward direction 2 by a conventional biconvex lens 3 into an effective focus area 4. In the case of EUV light generation, in particular for EUV lithography, a tin droplet, for example, can be located there. The forward laser beam 1 can be at least partially reflected by this as a backward laser beam 5, which can have a higher or larger numerical aperture than the forward laser beam 1. This backward laser beam 5 then propagates in a backward direction 6 to a sensor system 7, indicated schematically here, where it is detected for analysis purposes. However, the forward laser beam 1 can also be partially reflected at a front side 8 of the biconvex lens 3, resulting in a disturbing or undesired first back reflection 9.Likewise, the forward laser beam 1 can be partially reflected at an inner rear side 10 of the biconvex lens 3, resulting in a disturbing or undesired second back reflection 11. The back reflections 9, 11 then also propagate in the backward direction 6 to the sensor 7 and can thus interfere with or distort the intensity distribution of the backward laser beam 5 measured there.

[0031] It can be seen here that, for example, between a coupling point of the forward laser beam 1 and the sensor system 7 there is a focus area in which the interfering back reflections 9, 11 are focused and could thus be blocked there. In practice, however, a corresponding blocking element 12 that can be used for this purpose cannot always be arranged exactly in this focus area, for example due to design or optical restrictions or boundary conditions. By way of example, such a blocking element 12 is arranged here between the focus area and the sensor system 7. As a result, the blocking element 12 can block a portion of the backward laser beam 5 that is actually to be detected by the sensor system 7, but also at least a portion of the interfering back reflections 9, 11. With this arrangement, however, a significant portion of the interfering back reflections 9, 11 can still reach the sensor system 7. Fig.2 shows a schematic representation of an intensity distribution resulting from the sensor system 7 according to the prior art. No intensity is measured in a central region that is blocked or masked by the blocking element 12. However, surrounding regions with different intensities result from the back laser beam 5, the first back reflection 9, and the second back reflection 11.

[0032] The disruptive influence of the back reflections 9, 11 can, however, be avoided or at least mitigated. Fig. 3 shows a partial schematic representation of a correspondingly configured laser system 13. The laser system 13 here comprises a schematically indicated laser radiation source 14, which generates and outputs the forward laser beam 1. The forward laser beam 1 is then guided within the laser system 13 in an improved beam guiding device 15. This beam guiding device 15 comprises a coupling element 16 for coupling or deflecting the forward laser beam 1 into a main beam path or onto an optical axis of the beam guiding device 15. Downstream of the coupling element 16 in the forward direction 2 is an optic, which here is designed, for example, as a relay optic 17 with two parallel focusing lenses.Furthermore, the beam guiding device 15 also comprises at least one aperture stop 18 surrounding this main beam path. An example of this is the aperture stop.

[0033] 18 here arranged on one of the lenses of the relay optics 17 or surrounding this lens.

[0034] One or more further optical elements or components can be arranged downstream of the relay optics 17 in the forward direction 2, which are schematically shown here as additional optics

[0035] 19 are summarized. This additional optics 19 can, for example, comprise one or more focusing lenses and / or windows and / or deflecting mirrors, in particular coated with polarization retarder layers, and / or the like. Likewise, as part of the additional optics 19 or, for example, at another location along the beam path or the optical axis of the beam guiding device 15, at least one polarization filter or polarization filter structure can be arranged to suppress back reflections that arise at, in particular, transmissive, optical elements of the additional optics 19. However, such a suppression method cannot be practically applied to a last or output-side lens or lens arrangement of the laser system 13 or the beam guiding device 15. The additional optics 19 is optional, and can therefore also be omitted or be empty, i.e., contain no optical elements. Such a last orThe output-side lens arrangement is represented here by a single lens, which is designed here as a focusing lens 20. This is the last lens of the laser system 13 or the beam guiding device 15, i.e. the lens closest to the effective focus area 4 in the forward direction 2. Here, too, the first back reflection 9 can be generated on the lens arrangement, specifically in the example shown here on the front side 8 of the focusing lens 20 and the second back reflection 11 on the inner rear side 10 of the focusing lens 20. In the laser system 13 or beam guiding device 15 shown here, the position of the blocking element 12, i.e. the intensity distribution there or a corresponding blocking element plane 21 there perpendicular to the local backward direction 6, is imaged by the relay optics 17 onto an optically conjugate plane 22.This conjugate plane 22 is located here between the relay optics 17 and the focusing lens 20. For the blocking element plane 21 and the plane 22 conjugate thereto, rays focused in the conjugate plane 22 are also focused in the blocking element plane 21 after passing through the relay optics 17 in the backward direction 6, and thus here on the blocking element 12 arranged there. This means, in particular, that disruptive back reflections or

[0036] Ghost reflections or ghost rays that are or will be focused in the conjugate plane 22 are at least almost completely suppressed at the real blocking element 12, i.e., depending on the design of the blocking element 12, they are absorbed, reflected, or scattered out of the main beam path of the beam guiding device 15. The conjugate plane 22 can therefore be referred to or understood as a virtual blocking element or filter plane.

[0037] This is exploited here to suppress the first back reflection 9. For this purpose, the shape or the radius of curvature of the front side 8 of the focusing lens 20 is designed or modified compared to the conventional biconvex lens 3 according to Fig. 1 such that the first back reflection 9 is focused at least substantially in the conjugate plane 22 or has there a diameter corresponding at most to the diameter of the blocking element 12. This also results in a corresponding focusing of the first back reflection 9 on the blocking element 12 by the relay optics 17 and thus an effective suppression of the first back reflection 9 in the intensity distribution detected or measured by the sensor system 7. As already indicated, the position of the blocking element 12 cannot be freely selected, but can be predetermined, for example, by structural conditions and thus be known.Likewise, the relay optics 17 can be fixed and thus known due to requirements for guiding the forward laser beam 1 in the forward direction 2 and / or also due to mechanical or structural conditions. This also results in the position of the conjugate plane 22. Based on this, and starting from the conventional beam guidance shown in Fig. 1, the shape of the front side 8 of the biconvex lens 3 can be changed or bent such that the focal position of the first back reflection 9 arising at the front side 8 falls into the conjugate plane 22. In order to avoid or minimize necessary changes to other components or resulting changes in the properties or behavior of the laser system 13, the rear side 10 can also be adjusted accordingly in order to compensate for or compensate for a change in the focal length or focal position.For this purpose, starting from the conventional biconvex lens 3, for example, the front side 8 and the back side 10 can be changed in the same direction in terms of their radius of curvature. In the present case, this results in a concave-convex shape of the focusing lens 20. For the forward laser beam 1, which passes through the thus adapted focusing lens 20 in the forward direction 2, the changes made to the focusing lens 20 compared to the conventional biconvex lens 3 can cancel each other out, at least to the first order, i.e. without taking aberrations into account. Thus, both in the conventional beam guidance shown in Fig. 1 and in the improved beam guidance shown in Fig. 3, the forward laser beam 1 can be focused at the same point without further changes, so that there is no shift of the effective focus area 4 relative to the laser system 13 or an installation location of the biconvex lens 3 orof the focusing lens 20 and thus, for example, no changed distance of the laser system 13 to the fixed effective focus area 4 has to be set or the installation location for the focusing lens 20 has to be adjusted.

[0038] Due to the greater curvature of the rear side 10 of the focusing lens 20 compared to the front side 8, the second back reflection 11 created there is correspondingly more broadened or divergent. The second back reflection 11 is therefore not focused in the conjugate plane 22. However, the second back reflection 11, or at least a major part of the second back reflection 11, runs out of the main beam path of the beam guiding device 15, so that it does not reach the sensor system 7. For example, the second back reflection 11 can be intercepted on its path in the backward direction 6 at the aperture stop 18 and / or at an inner side of a beam guiding housing 23, which is shown here purely schematically and in detail, i.e., absorbed, for example, or deflected away from the sensor system 7. Thus, both the first back reflection 9 and the second back reflection 11 can be effectively suppressed orattenuate, so that a more precise, accurate and reliable analysis of the intensity distribution or radiation of the backward laser beam 5 detected by the sensor system 7 is possible.

[0039] As an example, Fig. 4 shows a schematic representation of a possible intensity distribution at the sensor system 7. Here, too, a central region is masked or suppressed by the blocking element 12. Surrounding this region is a region that primarily corresponds to the back laser beam 5 and an interference radiation region 24, in which an intensity caused by interference back reflections is significantly attenuated compared to the back laser beam 5 and also compared to the regions of the interference back reflections 9, 11 according to Fig. 2.

[0040] Overall, the examples described show how improved ghost suppression can be achieved.

[0041] LIST OF REFERENCE SYMBOLS

[0042] 1 forward laser beam

[0043] 2 Forward direction

[0044] 3 biconvex lens

[0045] 4 Effective focus area

[0046] 5 Reverse laser beam

[0047] 6 Reverse direction

[0048] 7 Sensor technology

[0049] 8 Front

[0050] 9 first retroreflex

[0051] 10 Back

[0052] 11 second back reflex

[0053] 12 Blocking element

[0054] 13 Laser system

[0055] 14 Laser radiation source

[0056] 15 Beam guidance device

[0057] 16 coupling element

[0058] 17 Relay optics

[0059] 18 aperture diaphragm

[0060] 19 Additional optics

[0061] 20 focusing lens

[0062] 21 Blocking element level

[0063] 22 conjugate plane

[0064] 23 Beam guide housing

[0065] 24 Interference radiation range

Claims

PATENT CLAIMS 1. Beam guiding device (15) for guiding laser radiation (1, 5), in particular for EUV light generation and / or an EUV driver laser, comprising - a lens arrangement (20) with at least one lens (20) for guiding the laser radiation (1) in the forward direction (2) towards a predetermined effective focus area (4), - a sensor system (7) for detecting laser radiation (5) reflected from the effective focus area (4) in the backward direction (6) through the lens (20), - a blocking element (12) which is impermeable to the reflected laser radiation (5), which is arranged in the beam path of the reflected laser radiation (5) between the sensor system (7) and the lens arrangement (20) and is dimensioned such that it keeps only a central portion of the reflected laser radiation (5) away from the sensor system (7) in the radial direction and that another portion of the reflected laser radiation (5) can pass the blocking element (12) to the sensor system (7), - an optical system (17) arranged in the beam path of the reflected laser radiation (5) between the blocking element (12) and the lens arrangement (20), which images the plane (21) of the blocking element (12) into a conjugate plane (22) lying between the optical system (17) and the lens arrangement (20), wherein - the lens arrangement (20) is designed such that of two back reflections (9, 11) arising on different lens sides of the lens arrangement (20), in particular on a front side (8) and an inner rear side (10) of the lens (20), when the laser radiation (1) is incident in the forward direction (2) on the lens arrangement (20), a first back reflection (9) is focused into the conjugate plane (22) lying between the optics (17) and the lens arrangement (20), and at least a main part of a second back reflection (11) is given such a large divergence, in particular by the shape of the corresponding lens side (10) of the lens (20), that it does not reach the sensor system (7).

2. Beam guiding device (15) according to claim 1, characterized in that between the blocking element (12) and the lens arrangement (20), in particular between the blocking element (12) and the optics (17), a coupling element (16) is arranged, via which laser radiation (1) can be coupled into the beam guiding device (15) in the forward direction (2) and through which the reflected laser radiation (5) can reach the sensor system (7).

3. Beam guiding device (15) according to one of the preceding claims, characterized in that the optics (17) are designed as relay optics (17) with at least two lenses.

4. Beam guiding device (15) according to one of the preceding claims, characterized in that the lens arrangement (20) is shaped such that the second back reflection (11) in front of the sensor system (7), in particular in front of the optics (17), strikes an aperture stop (18) surrounding the beam path of the reflected laser radiation (5) and / or an inner side of a housing (23) of the beam guiding device (15) surrounding this beam path.

5. Beam guiding device (15) according to claim 4, characterized in that an absorber material for absorbing the reflected laser radiation (5) is arranged at least in a region of the inside of the housing (23) onto which the second back reflection (11) strikes.

6. Beam guiding device (15) according to one of the preceding claims, characterized in that the lens arrangement (20) is or comprises a lens (20) focusing in the forward direction (2).

7. Beam guiding device (15) according to one of the preceding claims, characterized in that the lens arrangement (20) is or comprises the lens (20) of the beam guiding device (15) that is closest to the effective focus area (4) in the forward direction (2).

8. Beam guiding device (15) according to one of the preceding claims, characterized in that the front side (8) of the lens (20) of the lens arrangement (20) is concave and the back side (10) of the lens (20) is convex, wherein the front side (8) has a larger radius of curvature than the back side (10).

9. Beam guiding device (15) according to one of the preceding claims, characterized in that the beam guiding device (15) has, in addition to the lens arrangement (20), at least one further transmissive optical element (19) for guiding the laser radiation (1) at least in the forward direction (2) and is designed to suppress back reflections arising therefrom in a polarization-based manner.

10. Beam guiding device (15) according to one of the preceding claims, characterized in that the plane (21) of the blocking element (12) is optically conjugated to the plane of the sensor (7).

11. Laser system (13), in particular for EUV light generation, comprising a laser radiation source (14) and a beam guiding device (15) according to one of the preceding claims arranged downstream of the laser radiation (1) generated thereby in the forward direction (2).

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