Observation device for EUV light systems and corresponding EUV light systems

Abstract

The present invention relates to an observation device (3) for an EUV light system (1) for observing material droplets (2) for EUV light generation. The observation device (3) comprises an optical system (6) for directing a measurement laser beam (4) along a predetermined beam path to each material droplet (2) and the corresponding reflected light (7), and a sensor system (9) positioned at the end of the beam path for directing the reflected light (7). Shading elements (12, 20, 22) are positioned between the optical system (6) and the sensor system (9) in at least the central region of the beam path to prevent interference reflections (11, 14, 17, 21) of the measurement laser beam (4) generated in the optical system (6) from entering the sensor system (9), so that at least a portion of the reflected light (7) can reach the sensor system (9) regardless of the shading elements (12, 20, 22). The present invention further relates to an EUV light system (1) having such an observation device (3).

Description

Technical Field

[0003] ,

[0004] , , , , ,

[0001] The present invention relates to an observation device for an EUV laser system for determining the position of a material droplet for EUV light generation. The present invention further relates to an EUV laser system comprising the same.

[0002] EUV light, particularly light having wavelengths in the range of about 10 nm to about 121 nm, may be useful for various applications. For example, EUV light having a central wavelength of 13.5 nm can be used for EUV lithography. A common method for generating such EUV light is to irradiate a suitable material droplet with a high-intensity laser pulse. This causes the material droplet to at least partially evaporate or then convert it into a plasma that emits EUV light. In order to enable efficient and reliable EUV light generation, it is expedient, for example, first to determine the position of each material droplet. For this purpose, a weak pre-pulse or a measurement laser pulse can be emitted, which does not vaporize the material droplet but only illuminates and / or pre-aligns the material droplet for the stronger main laser pulse. The light reflected from the droplet of the material can then be analyzed, for example, to determine its position.

[0003] However, several problems and challenges can arise in this regard. For positioning, the laser pulse typically has to be directed or focused using an optical system, which can then generate interfering reflections. These can obscure or overlap with the actual measurement signal, i.e., the light reflected by the droplet of the material, thus impairing the accuracy and reliability of the positioning. The use of a polarization-based filter mechanism typically only filters out some of the potentially interfering reflections and can also ultimately reduce the brightness or intensity of the reflected light measured for positioning in an undesirable manner.

Background Art

[0004] The objective of the present invention is to enable the precise, reliable, and efficient positioning of material droplets for EUV light generation.

[0005] Summary of the Invention This objective is achieved by the subject matter of the independent claims, or by the main and secondary claims. Further possible configurations of the invention are expressed in the dependent claims, specification, and drawings. Features, advantages, and possible embodiments described in the specification for one of the subject matters of the independent claims shall be considered at least similarly as features, advantages, and possible embodiments of each of the other subject matters of the independent claims, and, optionally, in combination with one or more of the dependent claims, for any possible combination of the subject matters of the independent claims.

[0006] The observation device according to the present invention can be used in an EUV light system to observe a photo-generating region, i.e., an interaction zone where a droplet of material may evaporate or be converted into a plasma to generate EUV light. This means that the observation device can be used, for example, to determine the position of each material droplet for EUV light generation. The observation device can also be used for further or other analyses. Such material droplets may consist at least partially of a material such as tin, which generates EUV light under laser irradiation of sufficient intensity to cause at least partial evaporation of the material droplet. During operation of the corresponding EUV light system, such material droplets can be generated in or fall through a vacuum chamber. The observation device according to the present invention can be provided for coupling with or positioned in such a vacuum chamber, or can comprise such a vacuum chamber.

[0007] The observation device according to the present invention has an optical system for directing a measurement laser beam or measurement laser pulse onto a material droplet or to an intended droplet position, i.e., focusing it, or, for example, pre-parallelizing it, and for directing the light reflected in operation from each material droplet along a predetermined beam path, i.e., focusing and focusing or parallelizing it. This beam path may travel through the optical system in the opposite direction to the measurement laser beam or measurement laser pulse, or may be emitted from this optical system. Furthermore, the observation device according to the present invention has a sensor located at the other end of this beam path for receiving or directing the reflected light. This detected and reflected light can then serve as a basis for determining, for example, the position of each material droplet, and / or determining or measuring wavefront distortion and / or aberrations, etc. Such a sensor may include, for example, a camera, a CCD chip, or another optical sensor. The sensor may be equipped with or coupled to appropriate evaluation electronic equipment. This allows for the analysis, or evaluation, of measured values ​​or sensor signals generated by a sensor system when receiving or detecting reflected light in order to determine predetermined parameter values, such as the position of each material droplet. For this purpose, for example, intensity distribution and / or wavefront and / or phase distribution can be evaluated.

[0008] According to the present invention, the observation device also has at least one light-shielding element. This is positioned between the optical system and the sensor, as seen along the beam path, to prevent any interference reflections of the measurement laser beam or each measurement laser pulse emitted from the optical system from entering the sensor in the direction of the sensor during the intended operation of the observation device or the EUV light system comprising it. This is also referred to as interference reflection blocking. In particular, the light-shielding element can be positioned in a region of the beam path where interference reflections emitted from each material droplet and / or optical system are at least partially parallelized. The light-shielding element can therefore prevent interfering reflections from reaching the sensor in at least the central area of ​​the beam path. For this purpose, the light-shielding element can, for example, absorb the light incident thereon, or, in the case of a reasonably wavelength-selective design, absorb only the interference reflections and not the light reflected by each material droplet, or reflect or deflect the interference reflections out of the beam path. For this purpose, the light-shielding element is positioned at least in the central region, i.e., in or centered on the radial cross-sectional plane of the beam path, perpendicular to the direction of light propagation given along the beam path. Therefore, the beam path describes the spatial area through which the light reflected by each droplet of material is guided or propagated, at least from the optical system to the sensor, as intended. The light-shielding elements are designed and positioned within the beam path so that, regardless of the light-shielding elements, at least a portion of the light reflected by each material droplet can reach the sensor system along the beam path.

[0009] For this purpose, in possible embodiments of the present invention, the size or diameter of the light-shielding element in the radial direction, i.e., as viewed in the cross-sectional plane, can be smaller than the diameter of the beam path or light distribution of light reflected by each material droplet, which is given there, i.e., the position of the light-shielding element and is directed or guided along the beam path during the operation of the observation device or the EUV light system comprising it. Thus, the light-shielding element can be positioned and sized such that at least a portion of the reflected light can pass through the light-shielding element along the beam path to the sensor. In other words, the light-shielding element can then be positioned to block only the central region of the beam path, or to affect the propagation of interference reflection only there, but not affect the portion of the beam path surrounding this central region, e.g., an annular portion, or the light distribution of light reflected by each material droplet. The center or central position of the central region, i.e., this central placement of the light-shielding element, here refers to its position in a cross-sectional plane perpendicular to the radial direction or the local longitudinal extension direction of the beam path or the central longitudinal axis of the beam path, and not to the longitudinal position, i.e., not to a position in the longitudinal direction along the beam path or the direction of light propagation. Therefore, the light-shielding element does not need to be located midway between the optical system and the sensor in the longitudinal direction or the direction of light propagation. In this respect, the central longitudinal axis of the beam path can correspond to the optical axis, in particular, in its intended alignment of the optical system, i.e., to tolerances or unintended deviations. The design and dimensions of the light-shielding elements proposed herein allow any influence or obstruction of light reflected by each material droplet by the light-shielding element to be kept particularly low. This means, for example, that the intensity of light reflected by each material pod, which is ultimately detected by the sensor, can be kept particularly high, and that particularly precise and accurate analysis of this reflected light can be made possible.

[0010] The light-shielding element can be, for example, simply a coating or coated area of ​​a larger element or component, or an area of ​​a component functionally designed to block interfering reflections that may extend beyond the light-shielding element in a cross-sectional plane. Such a component can therefore be designed as a light-shielding element in some areas, or can carry or enclose a light-shielding element. However, in other areas of such a component, particularly in areas surrounding the light-shielding element, this component may be designed differently from the light-shielding element, or may not be coated by the light-shielding element. In these other areas, at least reflected light can therefore pass through the light-shielding element to reach the sensor system.

[0011] At least one light-shielding element can therefore be a component of an optical element, such as a lens, a parallel plane plate, or, for example, a plate that is partially and / or wavelength-selectively transparent. The light-shielding element can be designed as a coating that is at least partially opaque to interference reflection. The light-shielding element can be formed from a material and / or coating that is at least partially opaque to interference reflection having a first wavelength and / or first polarization, and at least partially transparent to light having a second wavelength and / or second polarization reflected by each droplet of the material. The light-shielding element can extend radially, i.e., perpendicular to the local longitudinal direction of the beam path, and in particular completely, across the beam path or light distribution of interference reflection, and across the beam path or light distribution of light reflected by each droplet of the material.

[0012] The corresponding wavelength-selective and / or polarization-selective light-shielding elements can be combined with one or more further light-shielding elements that can make the light completely opaque or at least partially opaque, regardless of the wavelength and / or polarization of the light.

[0013] Light-shielding elements can be made from the same material, that is, they can be designed as homogeneous. This can enable particularly simple and cost-effective production.

[0014] Similarly, the light-shielding elements can also be made from different materials, i.e., they can be designed as heterogeneous. A portion of the light-shielding elements positioned within the beam path or light distribution of interference reflection may consist entirely or partially of a material and / or coating, or may include a material or coating that is at least partially, preferably at least almost completely, opaque to interference reflection. Another portion of the light-shielding elements, i.e., particularly the portion positioned outside the beam path or light distribution of interference reflection, may consist entirely or partially of a different material and / or coating, or may include a material or coating that is at least largely transparent to the light reflected by each material droplet. This allows for particularly simple and robust placement and mounting of the light-shielding elements within the beam path. This means, for example, that a retaining arm for holding the light-shielding elements within the central area of ​​the beam path does not need to protrude into the beam path, which could otherwise have an undesirable effect on the light reflected by each material droplet.

[0015] In general, the arrangement and design of the light-shielding elements can prevent interference reflections from reaching the sensor. In this regard, the light-shielding elements can easily block at least those interfering reflections that occur when the measuring laser beam strikes the optical system perpendicularly, i.e., at an incident angle of 0°. Such interference reflections may also be referred to as perpendicular or linear interference reflections. The corresponding light-shielding elements may also be referred to here as the first light-shielding elements. One or more further light-shielding elements may be provided and / or one or more other types of interference reflections may be blocked, as will be described in more detail elsewhere.

[0016] The present invention is based on the discovery that the measuring laser beam actually has a significantly smaller numerical aperture, i.e., an effectively significantly smaller beam diameter, than the reflected light that can be used for positioning. Therefore, for example, a light-shielding element can effectively filter out or eliminate a central area, or at least the interference reflections, i.e., their wavelengths, from the light distribution of the reflected light or the light reaching the sensor, including the interference reflections of the measuring laser beam. This prevents positioning from being affected by interference reflections. At the same time, some of the reflected light that passes through the light-shielding element and reaches the sensor system still allows the position of each material droplet to be determined, for example, without corresponding analysis of the wavefront of the reflected light, in order to determine the focal position from constraints, such as wavefront curvature. Specifically, for example, analysis to determine the position of each material droplet is not impaired or significantly impaired by the shielding element, because the perfect numerical aperture, i.e., the perfect maximum size or perfect maximum diameter of the reflected light distribution, is preserved and therefore available for analysis, due to the central placement or effect of the shielding element, e.g., a smaller effective diameter compared to the light distribution of reflected light or the numerical aperture or diameter of the corresponding beam path. The placement of shielding elements in the beam path may result in the loss of information on higher frequency aberrations, but this does not actually represent a significant limitation or impairment.

[0017] As a result of the embodiments described in the present invention, at least interference reflections parallelized by the optical system or interference reflections with relatively small aperture or divergence angles can be easily blocked by the light-shielding element. Depending on the application, the optimal longitudinal position and / or diameter of the light-shielding element can be determined empirically, or by calculation or simulation. Specifically, the light-shielding element can be positioned between the coupling point where the measurement laser beam is coupled into the beam path and the sensor system. As a result, the irradiation of each material droplet by the measurement laser beam is not affected by the light-shielding element.

[0018] The present invention enables, for example, the near-complete elimination of interference reflections along the path to the sensor, even in cases of less-than-ideal anti-reflective coatings on the optical system. This, in turn, enables particularly precise, reliable, and efficient analysis, such as determining the position of a material droplet. Specifically, for example, it eliminates the need to increase the output of the measurement laser beam to compensate for intensity losses due to polarization-based filtering mechanisms, such as λ / 4 wave plates.

[0019] In one possible embodiment of the present invention, at least one light-shielding element is designed in a rod-like manner and is positioned to extend longitudinally along the longitudinal axis of the beam path, particularly across several different interference reflection focal areas, so that interference reflections propagating obliquely along the longitudinal axis can strike the outer surface of the light-shielding element. The light-shielding element can be designed, for example, as a solid rod or a hollow rod or tube. The longitudinal axis of the beam path can correspond to the optical axis of the optical system at least substantially or at least in sections, or extend parallel to the optical axis in the intended ideal arrangement. The light-shielding element designs proposed herein allow interference reflections to be blocked not only at one end face of the light-shielding element facing the optical system. Alternatively, a larger surface area is available in the form of a side surface to block interference reflections, for example, to absorb interference reflections, or to reflect interference reflections out of the beam path, or to deflect or diffract interference reflections, so that interference reflections cannot reach the sensor. The sides of a rod-shaped shielding element can extend, for example, at least substantially parallel to the local longitudinal direction of the beam path. The sides can therefore surround the corresponding central beam path extending in the longitudinal direction. Due to the longitudinal extension of the rod-shaped shielding element, it can be positioned particularly easily and with reduced precision or tolerance requirements to block as much of the unwanted reflection as possible. The rod-shaped shielding element can extend, for example, across the focal point or focal area of ​​several unwanted reflections or several types of unwanted reflections that originate from or may be caused by different locations or features of the optical system. This can eliminate the need for further shielding elements or enable the construction of a particularly simple and cost-effective observation device.

[0020] In further possible embodiments of the present invention, the observation device has an adjustment device for adjusting or displacing a light-shielding element. This means that the light-shielding element can be displaced, for example, manually or by a motor or automated means, particularly along the beam path, i.e., longitudinally. This allows the observation device to be used particularly flexibly and to adapt to different conditions or characteristics of the optical system over time, or to changes therein. This allows interference reflections to be blocked particularly reliably and permanently, thus ensuring particularly reliable, consistent, and coherent performance of the observation device.

[0021] In one possible embodiment of the present invention, at least one aperture diaphragm surrounding the beam path is positioned between the optical system and the sensor system along the beam path. In other words, the diaphragm surface can limit the local cross-section, i.e., the maximum diameter of the beam path, at least at the location of the diaphragm surface, i.e., the longitudinal position. The diaphragm surface can therefore have a central aperture or a central semi-transparent area through which reflected light can pass, but at the same time, the opaque area of ​​the diaphragm surface surrounding the central aperture or central area blocks stray light to and from the beam path. This allows further interference reflections to be blocked and therefore prevented from reaching the sensor. The diaphragm surface can be positioned in particular at the longitudinal position of a light-shielding element. The diaphragm surface can then surround the light-shielding element or be mounted on or within a component common to the light-shielding element. This allows particularly effective blocking or filtering of interference reflections occurring at different angles to be achieved at this point. Overall, aperture diaphragms can allow for further improvements in positioning. For this purpose, several diaphragm surfaces can also be positioned at different longitudinal positions along the beam path. These can have different sizes or aperture widths, i.e., light-transmitting inner diameters, for example, if the beam path has a beam waist or a larger numerical aperture or a larger diameter at the corresponding position in each case. This can also result in more effective blocking of interference reflections.

[0022] In further possible embodiments of the present invention, the observation device has at least one optical coupling element for coupling a measurement laser beam into the beampath in the direction of the optical system, i.e., in the direction of the intended droplet position. This coupling element can be, in particular, an optical beam splitter, for example, a partially transparent mirror. The coupling element can be positioned at an angle particularly between 0° and 90°, particularly 45°, with respect to the optical axis of the optical system or the local central longitudinal axis of the beampath. Viewed along the beampath, the coupling element is positioned here between the optical system and the sensor. In this way, the optical system can be used both for focusing the measurement laser beam onto the material droplet and for directing the corresponding reflected light from the material droplet to the sensor. This allows for a particularly simple and compact design of the observation device. Similarly, the measurement laser beam can be directed onto the material droplet along the final beampath of the reflected light, thereby achieving particularly high efficiency, i.e., a particularly high optical yield of the reflected light, which is ultimately detected by the sensor in comparison to the intensity or brightness of the original measurement laser beam.

[0023] In further possible embodiments of the present invention, the observation device has at least one additional shielding element for blocking interference reflections, i.e., for preventing interference reflections from entering the sensor system at least within or from the central area. This at least one additional shielding element can therefore be positioned in the beam path in addition to the first shielding element. However, this at least one additional shielding element is positioned differently from the first shielding element in this respect, along the beam path, i.e., when viewed longitudinally or in the direction of light propagation. The additional shielding element may also be referred to herein as a second shielding element or additional shielding element. The use of several shielding elements positioned at different longitudinal positions in the beam path, but effective with respect to interference reflections positioned at the center or within the central area, can enable particularly effective and efficient blocking of different interference reflections. Such different interference reflections are, for example, interference reflections generated by different surfaces or different features or details of the optical system, and therefore may emanate from the optical system at different angles and / or have different divergence angles. Thus, different interference reflections may, for example, have different focal positions. Each light-shielding element can then be positioned at its respective focal position. This allows different interference reflections to be blocked by light-shielding elements of the smallest effective diameter in each case. This ultimately allows interference reflections to be blocked completely, as well as allowing a particularly large portion of the reflected light to pass through the light-shielding elements or through the transparent area of ​​the light-shielding elements to reach the sensor system. This means that a reasonably high level of efficiency can be achieved. If the corresponding positions where several light-shielding elements must be positioned are relatively close to each other, these positions can also be covered or blocked by a single reasonably elongated, for example, rod-shaped light-shielding element. Such an elongated light-shielding element can then extend, for example, to several focal positions of different types of interference reflections, or across several such focal positions. This can result in even more complete blocking of interference reflections and / or a particularly simple structure for the observation device.

[0024] In further possible embodiments of the present invention, the first or additional shielding element, which may be a further or second shielding element described elsewhere, or another such further or additional shielding element, is positioned within the region of the focal point of interference reflection caused or generated by a concave surface of the optical system as viewed from the direction of the sensor system. Such a focal point may be located between the coupling point of the measurement laser beam into the beampath and the sensor system. In such cases, the corresponding interference reflection originating from the concave surface, also referred herein as concave interference reflection, can be blocked particularly effectively and efficiently by the additional shielding element positioned within the corresponding focal point. In this regard, the concave surface may be a surface that is shaped as intended, such as the inside of the surface of a focusing or focusing lens of the optical system facing the intended location of a material droplet. Similarly, there may be concave interference reflections that may be caused by a concave defect or unintended deformation of the optical system or its components. These can be blocked by appropriately positioning the additional shielding element within the corresponding focal point, so that effective and reliable determination of the material droplet location is possible despite the relatively less-than-ideal design of the optical system.

[0025] In further possible embodiments of the present invention, the observation device comprises at least one imaging optical system for imaging, i.e., for directing or focusing interference reflections, for example, on or within a specific imaging plane or a specific focal point. The imaging optical system can be positioned between the optical system and a first light-shielding element, particularly when viewed along the beam path. Specifically, the imaging optical system can be positioned between the first light-shielding element and a coupling point or coupling element, as otherwise described, for coupling the measurement laser beam into the beam path. However, in principle, the imaging optical system can be positioned entirely or partially in front of and / or behind the coupling point of the measurement laser beam, when viewed along the beam path. Specifically, the imaging optical system can be designed and positioned to focus one or more interference reflections in a region on the side of the imaging optical system facing the sensor system, i.e., facing away from the optical system, particularly from the coupling point of the measurement beam, when viewed along the beam path. Similarly, the imaging optical system can be designed and positioned to at least partially parallelize the light and / or interference reflections reflected by each material droplet. Specifically, the parallel state of light reflected by each material droplet can be restored by the imaging optical system while the light is away from the imaging optical system or before it re-enters the imaging optical system. For example, the distance measured along the beam path between the two lenses of the imaging optical system can correspond to the sum of their focal lengths for this purpose.

[0026] The imaging optical system is, in particular, at least one relay telescope, or may comprise such a telescope. Such a transmitting telescope may comprise, for example, at least or just two lenses. If the observation device or imaging optical system comprises several such transmitting telescopes, they may be positioned all before or all after the coupling point or coupling element of the measuring laser beam, or partially before and partially after. Whether one transmitting telescope or several transmitting telescopes are used, and / or how they are constructed or designed in detail, i.e., their focal lengths, can be determined in each individual case, for example, depending on the complexity or variety of interference reflections. The imaging optical system, in particular at least one transmitting telescope, can therefore be designed as a lens optical system. However, it is also possible to design the imaging optical system as a mirror optical system. In a particularly simple design, for example, two lenses with the same focal length can be used in the imaging optical system, arranged so that their respective imaging distances correspond to their respective object distances. In principle, other designs and / or arrangements with different lens focal lengths are also possible, for example.

[0027] When using such an imaging optical system or such a transmitting telescope, the aperture surface or further such aperture surfaces described elsewhere can be positioned, for example, between its lenses or optical elements, within the region of the beam waist that gives rise to the imaging optical system. There, the diameter of the inner aperture or second transmissive aperture, i.e., the hollow or transmissive aperture aperture, can be made particularly small without blocking the light reflected by each material droplet that will be detected by the sensor during operation. This allows undesirable scattered light around or from the beam path to be blocked particularly effectively without any other constraints, i.e., prevented from reaching the sensor.

[0028] In a possible further development of the invention, the imaging optical system has an adjustment device by means of which the position of the focus of the imaging optical system and / or the distance between optical elements of the imaging optical system, for example lenses or mirrors, can be adjusted, particularly along the longitudinal direction of the beam path. In this way, as described in the section regarding the adjustment device for displacing the light-shielding element, the observation device can be improved in terms of its flexibility and robustness and, in particular, in terms of its particularly simple and effective usability.

[0029] In further possible embodiments, the imaging optical system is configured to image a plane, where interference reflections on the optical system, particularly when viewed along the beampath, result from the non-orthogonal incidence of the measurement laser beam on the optical system onto a light-shielding element or further light-shielding element located between the coupling point of the measurement laser beam into the beampath and the sensor system. In other words, the imaging optical system can be used to image interfering reflections onto, and here on, the plane or within, the plane, that occur when the measurement laser beam does not strike the optical system, i.e., the surface or local surface area of ​​the optical system, at an incidence angle other than 0°. This may occur, for example, due to imperfect coaxial alignment of the measurement laser beam with respect to the central longitudinal axis of the intended beampath or the optical axis of the optical system, and / or due to an intended or unintended tilt or inclination of the optical system or at least one element of the optical system that causes interference reflections with respect to this longitudinal axis of the intended beampath. The corresponding interference reflections may also be referred to here as oblique interference reflections. Oblique interference reflections considered herein can be emitted from the optical system at an acute angle with respect to the longitudinal axis of the intended beam path. Therefore, in principle, there may be at least a partial radial or transverse offset between the oblique interference reflection or the corresponding interference-reflecting laser beam and the shielding element intended to block them. This, in principle, could then lead to these oblique interference reflections passing or radiating at least partially past the shielding element. However, this offset can be compensated by the imaging optical system so that such oblique interference reflections can be imaged at least essentially completely onto the shielding element, for example, without the need to increase the effective or efficient diameter of the shielding element.

[0030] In a possible further development of the invention, the imaging optical system is configured to image the focus of a so-called convergent interference reflection onto or into a second focus between the point of coupling of the measurement laser beam into the beam path and the sensor system, i.e., is designed or constructed accordingly. In this case, such a convergent interference reflection can, whether intentionally or not, be caused by the concave surface of the optical system as seen from the direction of the sensor. This concave surface can then focus any interference reflection occurring at the focus determined by the curvature of the concave surface, which can also be referred to as the natural or first focus. In this regard, this first focus can be located within the beam path between the optical system and the point of coupling where the measurement laser beam is coupled into the beam path. An additional light-blocking element arranged at this first focus will thus, on the one hand, effectively block the convergent interference reflection, but on the other hand, will at least partially block the measurement laser beam and thus ultimately effectively reduce the amount of light available for determining the position of each material droplet.

[0031] To avoid this, in a further development of the invention proposed herein, the first light-blocking element or a further light-blocking element is arranged within the region of the second focus. This further light-blocking element can be the further or second light-blocking element described elsewhere, or another further light-blocking element. Specifically, if the corresponding focal lengths are the same or the light-blocking element is sufficiently small depending on the size of the light-blocking element or the diameter of the light distribution of the interference reflection, the same light-blocking element can be used to block several different types of interference reflections. Alternatively, several light-blocking elements can be used at different longitudinal positions along the beam path.

[0032] In further developments of the invention proposed herein, the imaging optical system can therefore be configured to image or focus convergent interference reflections in the area between the coupling point of the measurement laser beam and the sensor system onto a light-shielding element or an additional light-shielding element. The imaging optical system or at least a corresponding portion thereof can be positioned particularly between the coupling point of the measurement laser beam and the sensor system. This prevents any obstruction or attenuation of the measurement laser beam on the optical system or its path to each material droplet by the corresponding light-shielding element. Further developments of the invention proposed herein may be particularly useful when a natural or first focal point, arising directly from the concave surface of the optical system, is located between the optical system and the coupling point of the measurement laser beam. The imaging optical system can then efficiently and completely block the corresponding convergent interference reflections without interfering with the irradiation of the measurement laser beam to each material droplet. By appropriately adapting or designing the imaging optical system, the second focal point can be flexibly positioned, allowing for flexible adaptation to specific installation space conditions or other requirements, for example. This can, for example, enable a particularly flexible and / or particularly simple structure of the observation device, as well as particularly complete blocking of convergent interference reflections by particularly small light-shielding elements. This, in turn, can lead to or contribute to a particularly high photoacquisition rate for determining the position of each material droplet, thereby which can be achieved in a particularly accurate, reliable, and efficient manner.

[0033] In a possible further development of the present invention, the imaging optical system comprises at least two lenses. In this regard, the focal length and the focal length of at least one lens on the optical system side, i.e., the lens facing the optical system along the beam path, correspond to its distance from the first focal point. The convergent interference reflection is then guided between the two lenses of the imaging optical system as a parallelized beam, i.e., a beam that is at least essentially neither converged nor divergent. In other words, the convergent interference reflection between the lenses of the imaging optical system can be guided as a parallel beam or a parallel beam bundle. For example, the two lenses of the imaging optical system can be identical, and the distance between the two lenses can then correspond to twice their focal lengths. Depending on the design, the first or further shading element, or another further shading element, as described elsewhere, can then be positioned between the sensor-side lens and the sensor of the imaging optical system at a distance corresponding to this focal length or a focal length. In this way, the convergent interference reflection can be effectively and efficiently blocked in a particularly simple and cost-effective design of the observation device. However, in principle, other designs of the imaging optical system or other arrangements of lenses in the imaging optical system and / or corresponding light-shielding elements are also possible.

[0034] In further possible embodiments of the present invention, the imaging optical system is configured to image or focus so-called convergent interference reflections onto a light-shielding element or further light-shielding element, or another further light-shielding element, which is positioned along the beampath between the coupling point of the measurement laser beam into the beampath and the sensor system, as described elsewhere. Such convergent interference reflections may here arise, intentionally or unintentionally, from the convex surface of the optical system as viewed from the direction of the sensor. This is another type of interference reflection. Without the imaging optical system, the diameter of these convergent interference reflections or their light distributions would increase with increasing distance from the optical system. Thus, such convergent interference reflections can then reach the sensor system, for example, after uncontrolled reflections inside the housing and / or at least one other component of the observation device surrounding the beampath, for example, by passing at least partially through the first light-shielding element. This can be avoided by the design of the imaging optical system proposed herein for focusing or concentrating such convergent interference reflections onto a light-shielding element centrally positioned in the beampath, or into the corresponding plane. Therefore, corresponding interferences to the analysis of light reflected by each material droplet, such as the positioning of each material droplet, can be effectively and efficiently avoided or at least reduced by such convergent interference reflections.

[0035] When several light-shielding elements are used to block different types of interfering reflections, they can be designed to be the same or different, for example, they can have the same or different sizes or diameters. This may depend on the possibilities or limitations given in individual cases regarding the substantially achievable focal points for different interference reflections, and / or the positioning of the light-shielding elements at each focal point or focal plane. By individually minimizing the size of the light-shielding elements, the blocking of light reflected unintentionally by each material droplet can also be minimized. By using identical light-shielding elements, i.e., corresponding common-part strategies, a particularly simple and cost-effective construction of the observation device can be made possible.

[0036] The present invention also relates to an EUV light system having or comprising an observation device according to the present invention. The EUV light system according to the present invention can be set up or designed, for example, for EUV lithography. Thus, the EUV light system according to the present invention can also comprise further components, for example, at least one laser source for a measurement laser beam and a main laser beam for evaporating each material droplet and / or a corresponding beam guide, and / or a beam guide or optical system for focusing or guiding the generated EUV light, etc. However, the EUV light system according to the present invention can also be designed or configured, for example, for other applications.

[0037] Further features of the present invention may be found by the following description and drawings. The features and combinations of features described above herein, as well as the features and combinations of features shown below in the description and / or drawings alone, can be used in other combinations or on their own, not only in the combinations shown in each case, but also in other combinations, without departing from the scope of the present invention. [Brief explanation of the drawing]

[0038] [Figure 1] This is a partial schematic diagram of an EUV light system having an observation device for determining the position of material droplets for EUV light generation. [Figure 2] This is a schematic diagram of a portion of the observation device in the second modified example. [Figure 3] This is a schematic diagram of a portion of the observation device in the third modified example. [Figure 4] This is a schematic diagram of a portion of the observation device in the fourth modified example.

[0039] Elements that are identical or functionally identical are indicated by the same reference numeral in the drawings.

[0040] Figure 1 shows a partial schematic diagram of the EUV light system 1. In this device, for example, a droplet of material 2, such as a tin droplet, can be at least partially evaporated or converted into a plasma by a laser or one or more laser pulses, which then leads to the emission of extreme ultraviolet radiation.

[0041] In order to precisely target each material droplet 2, it is useful to first determine its corresponding position. For this purpose, the EUV light system 1 has an observation device 3, which here functions or can be used as a positioning device, at least among others. To determine the position of each material droplet 2, it can be illuminated with pulses of the measurement laser beam 4 shown here. The measurement laser beam 4 or corresponding laser pulse is emitted before the main laser pulse for evaporating the material droplet 2 and is significantly weaker than this main laser pulse, and is therefore also called a preceding pulse, so that the material droplet 2 can be illuminated or pre-adjusted, for example, expanded, by the measurement laser beam 4 but not evaporated. The measurement laser beam 4 here can be coupled via a coupling element 5 and deflected in the direction of the material droplet 2. The measurement laser beam 4 then passes through the optical system 6 of the observation device 3, thereby being focused onto the material droplet 2.

[0042] The material droplet 2 can then reflect the light from the measurement laser beam 4 as reflected measurement light 7. Here, this reflected measurement light 7 also passes through the optical system 6 in the opposite direction. In this regard, the reflected measurement light 7 can be parallelized, for example, as shown here. The reflected measurement light 7 can then be guided along the corresponding beam path and then focused onto the sensor 9 of the observation device 3, for example, via the detection optical system 8. For further illustration and clarification, the central beam path axis 10 of the intended beam path of the reflected measurement light 7 is schematically shown here.

[0043] The sensor system 9 may, for example, include a camera and corresponding measuring and evaluation electronic equipment. In general, the sensor system 9 may therefore include at least one position-sensing sensor. This means that the position of each material droplet 2 can ultimately be determined using reflected measuring light 7. For this purpose, for example, beam path parameters, or optical elements or other configurations of the observation device 3 may be specified and considered.

[0044] In the method described so far, a portion of the light from the measurement laser beam 4 may be reflected along its path to each material droplet 2, for example, by defects and / or damage on the surface of the optical system 6 and / or the material. This leads to corresponding interference reflections, which may also be called ghost reflections. Where such interference reflections occur and how strong they are may depend, for example, on the configuration of the optical system 6, as well as their materials, quality, surface properties, coatings, etc. However, in principle, such interference reflections may be strong enough to cover the reflection of the measurement laser beam 4, i.e., the reflected measurement light 7, which is actually desirable for determining the position of each material droplet 2. In such cases, if the interference reflection reaches the sensor 9, i.e., is detected by the sensor 9, the positioning of the material droplets 2 may be impaired.

[0045] To address this problem, the measurement laser beam 4 can have a significantly small numerical aperture, i.e., a diameter significantly smaller than the reflected measurement light 7 or its beam path used for positioning. Therefore, when the measurement laser beam 4 strikes the surface of the optical system 6 perpendicularly, and the optical system 6 is not tilted with respect to the beam path axis, i.e., aligned with an optical axis that is at least substantially coaxial or identical to the beam path axis 10, a linear interference reflection 11 is generated, which propagates at least substantially to the center, i.e., only in the central region of the beam path and thus in the direction of the sensor 9, where the light distribution of the reflected measurement light 7 is located. Like the reflected measurement light 7, this linear interference reflection 11 can pass through the coupling element 5 and propagate further along the beam path in the direction of the sensor 9. However, in this case, the observation device 3 includes a first light-shielding element 12 positioned within the corresponding central region of the beam path. This first light-shielding element 12 therefore blocks further propagation of the linear interference reflection 11. In this regard, the diameter of the first light-shielding element 12 is smaller than the diameter of the beam path or the light distribution of the reflected measurement light 7. As a result, some, and especially most, of the reflected measurement light 7 can pass through the first light-shielding element 12 to the sensor system 9.

[0046] However, the diameter of the first light-shielding element 12 can be made larger here than the diameter of the light distribution of the linear interference reflection 11. As a result, the linear interference reflection 11 can be prevented from reaching the sensor 9 completely or at least largely.

[0047] The central area around the beam path axis 10 can also be referred to as the near field. The area away from the beam path axis 10 can also be referred to as the far field. The first light-shielding element 12 can therefore block not only the linear interference reflection 11 but also the near field area of ​​the reflected measurement light 7. However, this does not change the imaging of the far field reflected measurement light 7 onto the sensor system 9, and thus, regardless of the first light-shielding element 12, the position determination of each material droplet 2 is still possible based on the position of the reflected measurement light 7 detected by the sensor system 9.

[0048] Here, in order not to affect the measurement laser beam 4, the first light-shielding element 12 is positioned between the back of the coupling element 5, i.e., the coupling element or the corresponding coupling point where the measurement laser beam 4 is coupled into the beam path, and the sensor system 9, when viewed along the beam path.

[0049] In addition, the outer aperture surface 13 is positioned here, for example, at the location or level of the first light-shielding element 12. This aperture surface 13 surrounds, for example, a ring-shaped beam path. As a result, an area remains between the aperture surface 13 and the first light-shielding element 12 through which the reflected measurement light 7 can pass and reach the sensor 9. The aperture aperture 13 can therefore block, i.e., shield, external stray light.

[0050] The linear interference reflection 11 is shown here as a parallelized beam or parallelized beam bundle. However, there may also be non-parallelized spurious reflections or spurious reflection components that extend obliquely to the beam path axis 10 and / or diverge or converge. Such interference reflections can be focused or concentrated, for example, by suitable optical elements positioned in front of and / or behind the coupling element 5. Further shielding elements can be positioned, provided that they do not coincide with the position of the first shielding element 12 at the corresponding focal plane or focal position. With a properly adapted arrangement, various types of interference reflections can also be blocked, i.e., eliminated. In this way, interference reflections outside the near-field or central area covered by the shielding elements can be captured, directed, or imaged within this central area. Thus, for example, the corresponding angular deviation between the propagation direction or central longitudinal axis of the corresponding interference reflection radiated or emanating from the optical system 6 at an angle with respect to the beam path axis 10 can also be corrected. An example of this is shown in the remaining drawings and described below. The differences and additions to observation device 3 shown in Figure 1 will be the main focus of this discussion.

[0051] Figure 2 shows a schematic diagram of the observation device 3 in a second modification. Here, for example, one of the elements of the optical system 6 is tilted, i.e., inclined with respect to the beam path axis 10. As a result, the measurement laser beam 4 strikes this element of the optical system 6 at an angle of inclination different from 0°. This then creates an oblique interference reflection 14 that propagates at a certain angle, i.e., not parallel to the beam path axis 10. This oblique interference reflection 14 can, in principle, radiate past the first light-shielding element 12. To prevent such oblique interference reflection 14 from reaching the sensor system 9, the observation device 3 also includes an imaging optical system 15, here positioned in the beam path. This can be designed, for example, to have two telescopic lenses 16 as a transmitting telescope. The imaging optical system 15 directs or images oblique interference reflections 14 that strike the telescopic lens 16 facing the area of ​​the material droplet 2 within the intended installation position of the optical system 6, either on the optical system side or the droplet side, into the central area at least partially outside the central area covered by the first light-shielding element 12. Thus, the oblique interference reflections 14 can also be imaged onto the first light-shielding element 12 and therefore blocked by the first light-shielding element 12. For example, the telescopic lenses 16 here each have a focal length f and are positioned at an object distance g from the plane of the optical system 6 or the first light-shielding element 12.

[0052] Within the imaging optical system 15, that is, between its telescopic lenses 16, a beam waist is consequently created that has the minimum diameter of the light distribution of the reflected measurement light 7. A diaphragm 13 or another diaphragm 13 is positioned within this area. By positioning them within the beam waist area, the inner aperture or transmittance area of ​​the aperture diaphragm 13 can be selected to be particularly small without blocking the reflected measurement light 7. This allows disruptive stray light to be blocked particularly effectively by the aperture diaphragm 13. However, ultimately, the diaphragm 13 is optional and flexible in its positioning along the beam path, i.e., its longitudinal position. Similarly, several diaphragm 13 can be used at different longitudinal positions along the beam path, which may be the same or different.

[0053] Figure 3 shows the observation device 3 in a third modification. Here, a concave interference reflection 17 is illustrated as another type of interference reflection. This may originate from or be generated by the surface of the optical system 6, which is concave from the viewpoint of the sensor system 9. The concave interference reflection 17 can be focused to a focal point 18 by the correspondingly concave curved surface of the optical system 6. However, this focal point 18 may be located between the coupling element 5 and the optical system 6. Therefore, blocking the concave interference reflection 18 at its own focal point 18 is impractical, as this would then also block the measurement laser beam 4 at least partially. Instead, an imaging optical system 15 is provided here as well. This first parallelizes the concave interference reflection 17 and then focuses it to a second focal point 19 located between the coupling element 5 and the sensor 9. If this second focal point 19 is in the area of ​​the first light-shielding element 12, the concave interference reflection 17 can also then be blocked by the first light-shielding element 12.

[0054] However, in this example, the second focus 19 is located in a different plane, i.e., in a longitudinal position along the beam path or beam path axis 10, different from the first light-shielding element 12. To effectively and efficiently block concave interference reflection 17, the second light-shielding element 20 is positioned in the area of ​​the second focus 19. To ensure the position of the second light-shielding element 20 at the second focus 19, the second light-shielding element 20 can be made adjustable along the beam path axis 10, for example, by an adjustment device. Additionally or alternatively, the imaging optical system can be designed to be deformable, i.e., adjustable. For this purpose, at least one of the telescopic lenses can be tilted with respect to the beam path axis 10 and / or displaced along the beam path axis 10, for example, by a corresponding adjustment device. Thus, for example, the second focus 19 can be shifted on and / or along the beam path axis 10 to coincide with the second light-shielding element 20. Similarly, the first light-shielding element 12 and / or further light-shielding elements, and / or the optical elements of the observation device 3, can be adjustable or displaceable. The corresponding adjustment or setting device may have, for example, one or more adjustment screws and / or servo motors, particularly electric ones, and / or at least one cable pull for adjustment or setting or displacement. If the imaging optical system 15 has a sufficiently large adjustment range and / or the first light-shielding element 12 is sufficiently adjustable, the second focal point 19 can be made to coincide with the position of the first light-shielding element 12. In other words, the first light-shielding element 12 can then be positioned within the second focal point 19 and replace the second light-shielding element 20. This may allow for the omission of a separate second light-shielding element 20.

[0055] As usual, the telescopic lenses 16 of the imaging optical system 15 are designed and positioned here such that they have the same focal length f, and these focal lengths f also correspond to the distance of the optical system or droplet-side telescopic lens 16 to the focal point 18, and the distance of the sensor-side telescopic lens 16 to the second focal point 19 or second light-shielding element 20. However, other arrangements may be possible, and / or other configurations may result from, for example, shifting at least one of the telescopic lenses 16, particularly along the beam path axis 10.

[0056] Figure 4 shows a schematic diagram of the observation device 3 in the fourth modification. In addition to linear interference reflections 11 and concave interference reflections 17, convex interference reflections 21 are also shown here as an example. These may originate from or arise from the surface of the optical system 6, which is convex from the viewpoint of the sensor system 9. Thus, the convex interference reflections 21 in the area between the coupling element 5 and the optical system 6 may have a larger cross-section or diameter therein than the actual measurement laser beam 4, for example, so that the convex interference reflections 21 cannot be substantially blocked there. Here again, the observation device 3 includes an imaging optical system 15, which is designed here to focus the convex interference reflections 21 in the area between the coupling element 5 and the sensor 9. At the longitudinal position of the corresponding focal point, a third light-shielding element 22 is positioned here, as an example, to block the convex interference reflections 21.

[0057] For clarity, although not explicitly shown here, one or more aperture surfaces 13 can also be provided in the modifications shown in Figures 3 and 4. Similarly, several transmitting telescopes can be used in combination with each other, and / or the imaging optical system 15 can comprise more than two telescopic lenses 16 and / or other optical elements.

[0058] The light-shielding elements 12, 20, and 22 can be designed, for example, as parallel plane plates with a central region that selectively absorbs and / or reflects at least or simply the wavelengths of interference reflections 11, 14, 17, and 21. The central regions of these plates, or at least the remaining regions surrounding them, i.e., extending into the far-field region, can each be transparent to at least the reflected measurement light 7. This allows for particularly simple mounting or fastening of the light-shielding elements 12, 20, and 22. The light-shielding elements 12, 20, and 22 can be positioned, for example, perpendicular to the beam path axis 10. Similarly, the light-shielding elements 12, 20, and 22 can be tilted with respect to the beam path axis 10 or the propagation plane of the reflected measurement light 7 at an angle, for example, in the range of 2° to 45°. In particular in this case, the light-shielding elements 12, 20, and 22 can be designed, for example, as reflective mirrors. Undesirable interference reflections 11, 14, 17, and 21 can then be deflected out of the beam path by shielding elements 12, 20, and 22 so that they cannot reach the sensor 9. In this case, different variations and / or arrangements of the shielding elements can also be combined so that the shielding elements 12, 20, and 22 can be designed and / or arranged in the same or different ways. Similarly, some of the shielding elements 12, 20, and 22 shown individually here can be combined or replaced by absorbing or reflecting rods or cylinders, etc., that extend longitudinally along the beam path axis 10. Such rod-shaped shielding elements 12, 20, and 22, i.e., those that extend longitudinally along the beam path axis 10, may be easier to position to achieve the desired effect and, according to their small diameter, can keep a particularly large portion of the interference reflections 11, 14, 17, and 21 away from the sensor system 9. For example, such rod-shaped light-shielding elements 12, 20, 22 do not need to be precisely positioned on one of the focal planes of the interference reflectors 11, 14, 17, 21, because they can extend along the beam path axis 10 across or beyond one or even several corresponding focal areas.For example, if the position of the second focal point 19 is not precisely known, or if a shift occurs during operation, the rod-shaped light-shielding elements 12, 20, 22 can continue to prevent the corresponding interference reflection 17 from reaching the sensor 9. In this case, for example, light from at least one of the interference reflections 11, 14, 17, 21 can fall onto the outer surface of the rod-shaped light-shielding elements 12, 20, 22. The sides can be designed to be absorbent for at least one of the corresponding wavelengths of the interference reflections 11, 14, 17, 21, for example. Similarly, the surfaces of the light-shielding elements 12, 20, 22 can be structured or microstructured in at least some areas so that wavelength-selective incident light for at least one of the interference reflections 11, 14, 17, 21 is reflected away from the sensor system 9. Overall, the rod-shaped design of at least one of the light-shielding elements 12, 20, 22 can reduce the number of light-shielding elements required and / or simplify their arrangement. Similarly, observation devices can be designed to be particularly robust against changes in optical properties and / or geometric shape caused, for example, by vibration or relative motion of components, or by, for example, the effects of heat.

[0059] For each individual application, the optimal longitudinal position and size of the described components can be determined, for example, using standard optical formulas.

[0060] In addition to the modifications shown herein as examples, other modifications or variations are also possible. For example, the measuring laser beam 4 may be emitted or coupled obliquely with respect to the beam path axis 10, or offset parallel to it. In the latter case, the beam path axis 10 of the reflected measuring light 7 does not need to coincide with the central longitudinal axis of the measuring laser beam 4. Similarly, the imaging optical system 15 or further imaging optical systems 15 may be positioned entirely or partially between the coupling element 5 and the optical system 6. Similarly, the reflected measuring light 7 may be guided to the sensor 9 along a differently designed beam path, for example, an angled or inclined beam path. These and / or other possible variations or adaptations can allow for particularly flexible adaptation of the observation device 3 to different requirements and / or installation space conditions, etc.

[0061] Overall, the examples described demonstrate how, in the context of EUV light generation, undesirable reflections during the positioning of the final photogenerating material can be effectively and efficiently avoided or at least reduced compared to conventional approaches. [Explanation of Symbols]

[0062] 1 EUV light system 2. Droplets of material 3. Observation equipment 4. Measuring laser beam 5. Connecting elements 6 Optical Units 7. Reflected measurement light 8. Detection Optics 9. Sensor System 10 Beam path axis 11 Linear Interference Reflection 12. First light-shielding element 13. Aperture diaphragm 14. Oblique interference reflection 15' Imaging Optical Unit 16 Telescopic Lenses 17. Concave Interference Reflection 18 focus 19. Second Focus 20. Second light-blocking element 21 Convex Interference Reflection 22 Third light-shielding element f focal length g object distance

Claims

1. An observation device (3) for an EUV light system (1) for observing a material droplet (2) for generating EUV light, comprising: an optical system (6) for focusing a measurement laser beam (4) onto the material droplet (2) and for directing the light (7) reflected by the material droplet (2) along a predetermined beam path; and a sensor system (9) positioned at the end of this beam path for directing the reflected light (7), wherein at least one light-shielding element (12, 20, 22) is positioned in at least the central region of the beam path to prevent interference reflections (11, 14, 17, 21) of the measurement laser beam (4) emitted from the optical system (6) in the direction of the sensor system (9) when viewed along the beam path, so that at least a portion of the reflected light (7) can reach the sensor system (9) along the beam path regardless of the light-shielding element (12, 20, 22).

2. The observation device (3) according to claim 1, characterized in that the diameter of at least one light-shielding element (12, 20, 22) is smaller than the diameter of the light distribution of the reflected light (7) guided along the beam path.

3. The observation device (3) according to claim 1 or 2, characterized in that the light-shielding elements (12, 20, 22) are rod-shaped and, in particular, are arranged in such a manner that they extend longitudinally along the longitudinal axis (10) of the beam path across the focal areas of several different interference reflections (11, 14, 17, 21), so that interference reflections (14, 17, 21) propagating obliquely to the longitudinal axis (10) can collide with the outer surfaces of the light-shielding elements (12, 20, 22).

4. The observation device (3) according to any one of claims 1 to 3, characterized in that the observation device has an adjustment device, and the light-shielding elements (12, 20, 22) are arranged to be displaceable by the adjustment device, particularly along the beam path.

5. The observation device (3) according to claim 1, characterized in that, when viewed along the beam path, the aperture diaphragm (13) surrounding the beam path is positioned between the optical system (6) and the sensor system (9), particularly in the longitudinal direction of the light-shielding elements (12, 20, 22).

6. The observation device (3) according to any one of claims 1 to 5, wherein the observation device (3) has a coupling element (5), particularly an optical beam splitter, for coupling the measurement laser beam (4) to the beam path, and the coupling element (5) is positioned between the optical system (6) and the sensor system (9) when viewed along the beam path.

7. The observation device (3) according to any one of claims 1 to 6, wherein the observation device (3) has at least one further light-shielding element (20, 22) to prevent interference reflections (17, 21) in at least the central region of the beam path from entering the sensor system (9), and the further light-shielding element (20, 22) is located at a different location from the light-shielding element (12) when viewed along the beam path.

8. The observation device (3) according to any one of claims 1 to 7, wherein the light-shielding element (12) or further light-shielding elements (20, 22) are positioned within the region of the focal point (19) of the interference reflection (17), particularly between the coupling point (5) of the measurement laser beam (4) into the beam path and the sensor system (9), and the focal point (19) is determined by the concave surface of the optical system (6) when viewed from the direction of the sensor system (9).

9. The observation device (3) according to any one of claims 1 to 8, characterized in that the observation device (3) has an imaging optical system (15) which includes at least one transmitting telescope (15, 16) in particular for imaging the interference reflections (11, 14, 17, 21) positioned between the optical system (6) and the light-shielding element (12) as viewed in the beam path, particularly along the beam path.

10. The observation device (3) according to claim 9, characterized in that the imaging optical system (15) has an adjustment device, the adjustment device can adjust the position of the focal point (19) of the imaging optical system (15) and / or the distance between the optical elements (16) of the imaging optical system (15), particularly along the longitudinal direction (10) of the beam path.

11. The observation device (3) according to claim 9 or 10, characterized in that the imaging optical system (15) is planar and is designed to image a plane, in which interference reflections (14) on the optical system (6) are generated, particularly when viewed along the beam path, due to the non-orthogonal incidence of the measurement laser beam (4) on the optical system (6) onto the light-shielding element (12) or further light-shielding elements (20, 22) positioned between the coupling point (5) of the measurement laser beam (4) into the beam path and the sensor system (9).

12. The imaging optical system (15) is configured to produce a focal point (18) from the concave surface of the optical system (6) when viewed from the direction of the sensor system (9), and the focal point (18) is configured to image within a second focal point (19) between the coupling point (5) and the sensor system (9) for convergent interference reflections (17) in the beampath between the optical system (6) and the coupling point (5) of the measurement laser beam (4) in the beampath, wherein the light-shielding element (12) or further light-shielding elements (20, 22) are arranged within the region of the second focal point (19), characterized in that the observation device (3) according to any one of claims 9 to 11.

13. The observation device (3) according to claim 12, wherein the imaging optical system (15) comprises two lenses (16), and the focal length (f) of the optical system-side lens (16) corresponds to its distance (f) from the focal point (18) such that the converging interference reflection (17) between the two lenses (16) of the imaging optical system (15) is guided as a parallel beam.

14. The observation device (3) according to any one of claims 9 to 13, characterized in that the imaging optical system (15) is designed to image diverging interference reflections (21) occurring on the light-shielding element (12) or further light-shielding elements (20, 22) positioned between the coupling point (5) of the measurement laser beam (4) into the beam path and the sensor system (9), as viewed from the direction of the sensor system (9) from the convex surface of the optical system (6).

15. An EUV light system (1) having an observation device (3) according to any one of claims 1 to 14.

Images