Observation device for an EUV light system, and corresponding EUV light system

Abstract

The invention relates to an observation device (3) for an EUV light system (1) for observing a droplet of material (2) for EUV light generation. The observation device (3) comprises: optics (6) for directing a measurement laser beam (4) to a droplet of material (2) and from corresponding reflected light (7) along a predefined beam path; and a sensor system (9), positioned at the end of the beam path, for detecting the reflected light (7). In order to prevent interfering reflections (11, 14, 17, 21) of the measurement laser beam (4) that occur at the optics (6), a light-blocking element (12, 20, 22) is positioned between the optics (6) and the sensor system (9) at least in a central region of the beam path, ensuring that at least some of the reflected light (7) can reach the sensor system (9) despite the light-blocking element (12, 20, 22). The invention also relates to an EUV light system (1) comprising such an observation device (3).

Description

[0001] Observation device for an EUV light system and corresponding EUV light system

[0002] 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 invention further relates to an EUV laser system equipped therewith.

[0003] EUV light, particularly light with a wavelength in the range of approximately 10 nm to approximately 121 nm, can be useful for various applications. For example, EUV light with 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 an intense laser pulse. This at least partially vaporizes the material droplet or converts it into a plasma that then emits EUV light. To enable efficient and reliable EUV light generation, it is useful, for example, to first determine the position of the respective material droplet. For this purpose, a weaker pre- or measuring laser pulse can be emitted which does not vaporize the material droplet, but rather only illuminates it and / or preconditions it for a stronger main laser pulse.Light reflected from the drop of material can then be analyzed to determine, for example, its position.

[0004] However, this can present a number of problems and challenges. For example, the laser pulse for position determination typically has to be directed or focused using optics, which in turn can generate interfering reflections. These can overshadow or overlay the actual measurement signal, i.e., the light reflected from the droplet of material, and thus impair the accuracy and reliability of the position determination. The use of a polarization-based filter mechanism can typically only filter out a portion of the potentially interfering reflections and can also undesirably reduce the brightness or intensity of the reflected light to be measured for position determination.

[0005] The object of the present invention is to enable a particularly accurate, reliable and efficient position determination of material droplets for EUV light generation.

[0006] This problem is solved by the subject matter of the independent claims, or rather, the main and secondary 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.

[0007] The observation device according to the invention can be used in an EUV light system for observing a light generation region, i.e., an interaction zone in which a material droplet can be vaporized or converted into a plasma to generate EUV light. Thus, the observation device can be used, for example, to determine the position of a respective material droplet for EUV light generation. However, the observation device can also be used for further or other analyses. Such a material droplet can consist at least partially of a material, such as tin, which generates EUV light under laser irradiation sufficiently intense for the at least partial vaporization of the material droplet. During operation of a corresponding EUV light system, such a material droplet can, for example, be generated in a vacuum chamber or can pass through a vacuum chamber.The observation device according to the invention can be provided or arranged for coupling to such a vacuum chamber or can comprise such a vacuum chamber.

[0008] The observation device according to the invention has an optical system for directing, i.e. in particular for focusing or, for example, for previously collimating a measuring laser beam or measuring laser pulse onto the material droplet or to a designated droplet position, and for directing, i.e., for example, for collecting and focusing or collimating light reflected from the respective material droplet during operation along a predetermined beam path. This beam path can, in particular, lead through the optical system in a direction opposite to that of the measuring laser beam or measuring laser pulse, or can emanate from this optical system. Furthermore, the observation device according to the invention has a sensor system arranged at the other end of this beam path for receiving or detecting the reflected light. This detected reflected light can then, for example, be used as a basis for determining the position of the respective material droplet and / or a determination orMeasurement of wavefront deformation and / or aberrations and / or the like. Such a sensor system can, for example, comprise a camera, a CCD chip, or another light sensor. The sensor system can be equipped with or coupled to appropriate evaluation electronics. This allows a measurement or sensor signal generated by the sensor system upon recording or detecting the reflected light to be analyzed, i.e. evaluated, in order to determine a predetermined parameter value, for example the position of the respective material droplet. For this purpose, an intensity distribution and / or wavefront and / or phase distribution and / or the like can be evaluated.

[0009] According to the invention, the observation device additionally comprises at least one light-blocking element. This is arranged between the optics and the sensor system, viewed along the beam path, in order to keep interfering reflections of the measuring laser beam or the respective measuring laser pulse emanating from the optics in the direction of the sensor system away from the sensor system during normal operation of the observation device or the EUV light system equipped therewith. This is also referred to herein as blocking the interfering reflections. In particular, the light-blocking element can be arranged in a region of the beam path in which the light reflected by the respective material droplet and / or the interfering reflections emanating from the optics is / are at least partially collimated. The light-blocking element can therefore prevent the interfering reflections from reaching the sensor system, at least in a central region of the beam path.For this purpose, the light-blocking element can, for example, absorb incident light or, with a correspondingly wavelength-selective design, only the interfering reflections, but not the light reflected by the respective droplet of material, or reflect or deflect it out of the beam path. For this purpose, the light-blocking element is arranged in the beam path at least in a central region, i.e., centrally in or relative to the radial cross-sectional plane of the beam path, which is perpendicular to the direction of light propagation along the beam path. The beam path thus describes a spatial region in which the light reflected by the respective droplet of material is guided or propagated as intended, at least from the optics to the sensor system.The light blocking element is designed and arranged in the beam path in such a way that, despite the light blocking element, at least a part of the light reflected by the respective material droplet can reach the sensor system along the beam path.

[0010] For this purpose, in one possible embodiment of the invention, the size or diameter of the light blocking element in the radial direction, i.e. viewed in the cross-sectional plane, can be smaller than a diameter of the beam path or the light distribution of the light reflected by the respective material droplet, which is directed or guided along the beam path during operation of the observation device or the EUV light system equipped therewith, which diameter is given there, i.e. at the position of the light blocking element. The light blocking element can therefore be arranged and dimensioned in such a way that at least a portion of the reflected light can pass past the light blocking element along the beam path to the sensor system. In other words, the light blocking element can then be arranged in such a way that it only blocks oronly influences the propagation of the interfering reflections there, but not in a part of the beam path or the light distribution of the light reflected by the respective material droplet surrounding this central region, for example an annular part. The central or central position of the central region, i.e. this central or central arrangement of the light blocking element, refers here to its position in the radial direction or cross-sectional plane perpendicular to the local longitudinal extension direction of the beam path or to the central longitudinal axis of the beam path, and not to the longitudinal position, i.e. not to the position in the longitudinal direction along the beam path or the direction of light propagation. The light blocking element is therefore not necessarily arranged halfway in the longitudinal direction or direction of light propagation between the optics and the sensor system.The central longitudinal axis of the beam path can, in particular, correspond to the optical axis of the optics in its intended alignment—i.e., subject to tolerances or unintentional misalignments. The design and dimensioning of the light-blocking element proposed here can minimize any influence or impairment of the light reflected by the respective material plug. This, for example, allows the intensity of the light reflected by the respective material plug ultimately detected by the sensor to be kept particularly high, enabling a particularly precise and accurate analysis of this reflected light.

[0011] The light-blocking element can, for example, be merely a coating or a coated region of a larger element or component, or a region of a component functionally designed to block interfering reflections, which can extend beyond it in the cross-sectional plane. Such a component can thus be configured as the light-blocking element in some regions, or carry or encompass the light-blocking element. However, in another region of such a component, in particular surrounding the light-blocking element, this component can be configured differently or not be coated with the light-blocking element. In this other region, at least the reflected light can thus pass past the light-blocking element to reach the sensor system.

[0012] The at least one light-blocking element can therefore be a component of an optical element, such as a lens, a plane-parallel plate that is transparent in certain regions and / or in a wavelength-selective manner, or the like. The light-blocking element can be designed as a coating that is at least partially opaque to interfering reflections. The light-blocking element can be made from a material and / or a coating that is at least partially opaque to interfering reflections with a first wavelength and / or a first polarization and at least partially transmissive to light with a second wavelength and / or a second polarization reflected by the respective drop of material. The light-blocking element can extend radially, i.e. perpendicular to the local longitudinal direction of the beam path, in particular completely, across the beam path orthe light distribution of the interfering reflections and extend completely or partially over the beam path or the light distribution of the light reflected by the respective material droplet.

[0013] A corresponding wavelength-selective and / or polarization-selective light-blocking element can be combined with one or more additional light-blocking elements, which can be completely opaque or at least partially opaque, regardless of the wavelength and / or polarization of the light. The light-blocking element can be made of a single material, i.e., homogeneous. This can enable particularly simple and cost-effective production.

[0014] Likewise, the light-blocking element can also be made of a non-uniform material, i.e., it can be inhomogeneous. A part of the light-blocking element arranged in the beam path or the light distribution of the interfering reflections can consist entirely or partially of a material and / or a coating, or can comprise a material or a coating that is at least partially, preferably at least almost completely, opaque to the interfering reflections. Another part of the light-blocking element, i.e., one arranged in particular outside the beam path or the light distribution of the interfering reflections, can consist entirely or partially of a different material and / or a coating, or can comprise a material or a coating that is at least largely transparent to the light reflected by the respective drop of material.This allows, for example, a particularly simple and robust arrangement and mounting of the light-blocking element in the beam path. This eliminates the need for holding arms for the light-blocking element to protrude into the central area of ​​the beam path, which could potentially undesirably influence the light reflected by the respective droplet of material.

[0015] In general, the arrangement and design of the light-blocking element can prevent the interfering reflections striking it from reaching the sensor system. In particular, the light-blocking element can easily block at least those interfering reflections that occur when the measuring laser beam strikes the optics perpendicularly, i.e., at an angle of incidence of 0°. Such interfering reflections can also be referred to as vertical or straight interfering reflections. The corresponding light-blocking element can also be referred to here as the first light-blocking element. As explained in more detail elsewhere, one or more further light-blocking elements can also be provided and / or one or more other types of interfering reflections can be blocked.

[0016] The present invention is based on the realization that, in practice, the measuring laser beam has a significantly smaller numerical aperture, i.e., effectively a significantly smaller beam diameter, than the reflected light usable for position determination. Thus, for example, the light-blocking element can effectively punch out or eliminate a central region from the light distribution of the reflected light or the light reaching the sensor, in which the interfering reflections of the measuring laser beam are located, or at least the interfering reflections, i.e., their wavelength or wavelengths. This can prevent the position determination from being impaired by the interfering reflections.At the same time, the part of the reflected light that passes the light blocking element and reaches the sensor system still enables the position of the respective material droplet to be determined without restriction. This means, for example, that the wavefront of the reflected light can be analyzed accordingly, for example to determine a focus position from a wavefront curvature or the like. In particular, the analysis, for example to determine the position of the respective material droplet, is not impaired or not significantly impaired by the light blocking element, since due to the central arrangement or effect of the light blocking element - for example, its smaller effective diameter compared to the numerical aperture or the diameter of the light distribution of the reflected light or the corresponding beam path - the full numerical aperture, i.e. the full maximum size orThe full maximum diameter of the reflected light distribution is retained and thus available for analysis. Although the placement of the light-blocking element in the beam path may result in the loss of information from higher-frequency aberrations, this does not represent a significant limitation or impairment in practice.

[0017] Due to the described embodiment of the invention, at least interfering reflections collimated by the optics or interfering reflections with a relatively small aperture or divergence angle can be easily blocked by the light-blocking element. Depending on the application, the optimal longitudinal position of the light-blocking element and / or its diameter can be determined experimentally, computationally, or by simulation. In particular, the light-blocking element can be arranged between a coupling point, at which the measuring laser beam is coupled into the beam path, and the sensor system. As a result, the irradiation of the respective material droplet with the measuring laser beam is not impaired by the light-blocking element.

[0018] The present invention enables a largely complete elimination of interfering reflections on the way to the sensor, for example, even in the case of non-ideal anti-reflective coatings on the optics. This, in turn, enables particularly precise, reliable, and efficient analysis, for example, for determining the position of material drops. In particular, the power of the measuring laser beam does not have to be adjusted to compensate for intensity losses by a polarization-based

[0019] Filter mechanism, for example A / 4 wave plates or the like, can be increased.

[0020] In one possible embodiment of the present invention, the at least one light-blocking element is rod-shaped and arranged longitudinally along a longitudinal axis of the beam path, in particular across focal areas of several different interfering reflections, so that interfering reflections propagating obliquely to the longitudinal axis can impinge on an outer surface of the light-blocking element. The light-blocking element can be designed, for example, as a solid rod or as a hollow rod or tube. The longitudinal axis of the beam path can, at least in a simple embodiment of the observation device, correspond at least substantially or at least partially to the optical axis of the optics in the intended ideal arrangement or run parallel to it.The design of the light blocking element proposed here allows interfering reflections to be blocked not only on the end face of the light blocking element facing the optics. Instead, a larger surface area is available in the form of the lateral surface to block the interfering reflections, for example, to absorb them or to reflect them out of the beam path, or to deflect or diffract them so that they cannot reach the sensor. The lateral surface of the rod-shaped light blocking element can, for example, run at least substantially parallel to the local longitudinal direction of the beam path. The lateral surface can therefore surround a corresponding central beam path axis running in the longitudinal direction. Due to the longitudinal extension of the rod-shaped light blocking element, it can be positioned particularly easily and with reduced precision or tolerance requirements to block interfering reflections as completely as possible.The rod-shaped light-blocking element can, for example, extend across focal points or focal areas of multiple interfering reflections or multiple types of interfering reflections that may originate from or be caused by different locations or features of the optics. This can potentially eliminate the need for an additional light-blocking element or enable a particularly simple and cost-effective design of the observation device.

[0021] In a further possible embodiment of the present invention, the observation device has an adjustment device for adjusting or displacing the light-blocking element. This allows the light-blocking element to be displaced, particularly along the beam path, i.e., in the longitudinal direction, for example, manually, by motor, or automatically. This allows the observation device to be used particularly flexibly and adapted to different or changing conditions or properties of the optics over time. This allows interfering reflections to be blocked particularly reliably and permanently, thus achieving a particularly reliable, consistently high performance of the observation device.

[0022] In one possible embodiment of the present invention, at least one aperture stop is arranged between the optics and the sensor system along the beam path, surrounding the beam path. In other words, the aperture stop can limit the local cross-section, i.e., the maximum diameter of the beam path, at least at the location, i.e., the longitudinal position of the aperture stop. The aperture stop can therefore have a central opening or a central translucent region, through which the reflected light can pass, but at the same time block stray light in or from the surroundings of the beam path by an opaque region of the aperture stop surrounding the central opening or the central region. This allows further interfering reflections to be intercepted, i.e., prevented from reaching the sensor system. The aperture stop can, in particular, be arranged at the longitudinal position of the light-blocking element.The aperture stop can then surround the light-blocking element or be implemented on or in a common component with it. This allows for particularly effective blocking or filtering of interfering reflections arriving at different angles. Overall, the aperture stop can enable further improvement in position determination. For this purpose, multiple aperture stops can be arranged at different longitudinal positions along the beam path. These can, for example, have different sizes or opening widths, i.e., light-permeable inner diameters, if the beam path has a beam waist or a larger numerical aperture or a larger diameter at corresponding positions. This can potentially lead to even more effective blocking of interfering reflections.

[0023] In a further possible embodiment of the present invention, the observation device has at least one optical coupling element for coupling the measuring laser beam into the beam path in the direction of the optics, i.e., also in the direction of the intended droplet position. This coupling element can, in particular, be an optical beam splitter, for example, a partially transparent mirror or the like. The coupling element can, in particular, 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 beam path. Viewed along the beam path, the coupling element is arranged here between the optics and the sensor system. In this way, the optics can be used both to focus the measuring laser beam onto the material droplet and to direct the corresponding reflected light from the material droplet to the sensor system.This enables a particularly simple and compact design of the observation device. It also allows the measuring laser beam to be directed along the final path of the reflected light onto the droplet of material, potentially achieving particularly high efficiency—i.e., a particularly high light yield of reflected light that is ultimately captured by the sensor—compared to the intensity or brightness of the original measuring laser beam.

[0024] In a further possible embodiment of the present invention, the observation device has at least one further light-blocking element for blocking interfering reflections, i.e. for keeping interfering reflections away from the sensor system at least in or from the central region. This at least one further light-blocking element can therefore be arranged in the beam path in addition to the first light-blocking element. However, this at least one further light-blocking element is arranged at a different location along the beam path, i.e. viewed in the longitudinal direction or direction of light propagation, than the first light-blocking element. The further light-blocking element can also be referred to here as a second light-blocking element or as an additional light-blocking element. The use of a plurality of centrally arranged orLight blocking elements that are effective with regard to the interfering reflections in the central region but arranged at different longitudinal positions in the beam path can enable particularly effective and efficient blocking of different interfering reflections. Such different interfering reflections can, for example, be interfering reflections that are generated by different surfaces or different features or details of the optics and accordingly emanate from the optics at different angles and / or have different divergence angles. Accordingly, different interfering reflections can, for example, have different focus positions. A respective light blocking element can then be arranged at such a respective focus position. As a result, different interfering reflections can be blocked with light blocking elements each with a minimal effective diameter.This ultimately not only allows the interfering reflections to be blocked particularly completely, but also allows a particularly large proportion of the reflected light to reach the sensor, for example past the light-blocking element or through a transparent area of ​​the light-blocking element. This makes it possible to achieve a correspondingly high level of efficiency. If the corresponding positions at which several light-blocking elements would have to be arranged are relatively close to one another, these positions can also be covered or blocked by a single, correspondingly elongated, for example rod-shaped, light-blocking element. Such an elongated light-blocking element can then extend, for example, to several focus positions for different types of interfering reflections or across several such focus positions.This may result in an even more complete blocking of the interfering reflections and / or a particularly simple structure of the observation device.

[0025] In a further possible embodiment of the present invention, the first light-blocking element or a further light-blocking element, which can in particular be the further or second light-blocking element mentioned elsewhere or another such further light-blocking element or additional light-blocking element, is arranged in the region of a focal point of interfering reflections that are caused or generated by a concave surface of the optics viewed from the direction of the sensor system. Such a focal point can in particular lie between the coupling point of the measuring laser beam into the beam path and the sensor system. In such a case, corresponding interfering reflections originating from the concave surface, which are also referred to here as concave interfering reflections, can be blocked particularly effectively and efficiently by a further light-blocking element arranged at the corresponding focal point.The concave surface can be a surface that is designed, i.e., intentionally shaped, such as the inner side of a surface of a converging or focusing lens of the optics facing the intended position of the material droplet. Likewise, there may be interfering concave reflections that can arise from concave surface defects or unintentional misformations of the optics or a component of the optics. These can be blocked by appropriately arranging the additional light-blocking element at the corresponding focal point, so that effective and reliable position determination of the material droplets is possible despite the correspondingly non-ideal design of the optics.

[0026] In a further possible embodiment of the present invention, the observation device comprises at least one imaging optic for imaging, i.e., for example, for directing or focusing, interfering reflections, for example, onto or into a specific imaging plane or a specific focal point. The imaging optic can be arranged, in particular, between the optics and the first light-blocking element, viewed along the beam path. Specifically, the imaging optic can be arranged between the first light-blocking element and the coupling point mentioned elsewhere or the coupling element mentioned elsewhere for coupling the measuring laser beam into the beam path. In principle, however, the imaging optic can be arranged entirely or partially in front of and / or behind the coupling point of the measuring laser beam, viewed along the beam path.In particular, the imaging optics can be configured and arranged to focus the one or more interfering reflections into a region which, viewed along the beam path, lies on a side of the imaging optics facing away from the optics, in particular also from the coupling point for the measuring beam, i.e. facing the sensor system. The imaging optics can also be configured and arranged to at least partially collimate the light reflected by the respective material droplet and / or the interfering reflections. In particular, by means of the imaging optics, a collimation state of the light reflected by the respective material droplet can be or be restored before entering the imaging optics during or after leaving the imaging optics. For this purpose, for example, the distance measured along the beam path between two lenses of the imaging optics can correspond to the sum of their focal lengths.

[0027] The imaging optics can in particular be or comprise at least one relay telescope. Such a relay telescope can, for example, comprise at least or exactly two lenses. If the observation device or the imaging optics comprises several such relay telescopes, these can all be arranged before or all after the coupling point or the coupling element for the measuring laser beam, or some before and some after. Whether one relay telescope or several relay telescopes are used and / or how they are constructed or designed in detail, for example with regard to their focal lengths or the like, can be determined in each individual case, for example depending on the complexity or variety of the interfering reflections. The imaging optics, in particular the at least one relay telescope, can therefore be designed as a lens optic.However, a design of the imaging optics as a mirror optics is also possible. For a particularly simple design, for example, two lenses with identical focal lengths can be used for the imaging optics, which can be arranged such that the respective image distance corresponds to the respective object distance. However, other designs and / or arrangements, for example, with different lens focal lengths, are also possible in principle.

[0028] When using such imaging optics or such a transmission telescope, the aperture stop mentioned elsewhere or another such aperture stop can be arranged particularly in the region of a beam waist resulting in the imaging optics, for example, between its lenses or optical elements. There, an inner opening or transparency second, i.e., the inner free or transparent diameter of the aperture stop, can be particularly small without blocking the light reflected by the respective material droplet to be detected by the sensor during operation. This allows unwanted stray light in or from the vicinity of the beam path to be blocked particularly effectively without any other restrictions, i.e., prevented from reaching the sensor.

[0029] In a possible further development of the present invention, the imaging optics comprises an adjustment device by means of which the position of a focal point of the imaging optics and / or a distance between optical elements, for example lenses or mirrors, of the imaging optics can be adjusted, particularly along the longitudinal direction of the beam path. This allows the observation device to be improved in terms of its flexibility and robustness, as well as in terms of particularly simple and effective usability, analogous to the adjustment device for displacing the light-blocking element.

[0030] In a further possible embodiment, the imaging optics are designed to image a plane in which interfering reflections on the optics arise due to a non-perpendicular incidence of the measuring laser beam on the optics, onto the light blocking element or a further light blocking element arranged, in particular viewed along the beam path, between a coupling point of the measuring laser beam into the beam path and the sensor system. In other words, interfering reflections that arise when the measuring laser beam does not strike the optics, i.e. a surface or a local surface area of ​​the optics, at an angle of incidence other than 0°, can be imaged onto the light blocking element or on or in its plane by means of the imaging optics. This can happen, for example, due to a non-perfect coaxial alignment of the measuring laser beam with respect to the central longitudinal axis of the intended beam path.the optical axis of the optics and / or due to an intentional or unintentional inclination or tilt of the optics or at least one element of the optics causing the interference reflection relative to this longitudinal axis of the intended beam path. Corresponding interference reflections can also be referred to here as oblique interference reflections. The oblique interference reflections considered here can emanate from the optics at an acute angle relative to the longitudinal axis of the intended beam path. Thus, in principle, there could be at least a partial radial or lateral offset between the oblique interference reflections or a corresponding interference reflection light beam and the light blocking element intended to block them. This, in turn, could in principle lead to these oblique interference reflections at least partially passing or radiating past the light blocking element.However, this offset can be compensated for by the imaging optics, so that even such oblique interference reflections can be imaged at least essentially completely onto the light blocking element, in particular without, for example, its effective or efficient diameter having to be increased.

[0031] In a possible further development of the present invention, the imaging optics are configured, i.e., correspondingly designed or configured, to image the focal point of so-called convergent interference reflections onto or into a second focal point between the coupling point of the measuring laser beam into the beam path and the sensor system. Such convergent interference reflections can arise in this case from a concave surface of the optics, viewed from the direction of the sensor system - intentionally or unintentionally. This concave surface can then focus interference reflections arising there into the focal point determined by the curvature of the concave surface, which can also be referred to here as the natural or first focal point. This first focal point can lie in the beam path between the optics and the coupling point at which the measuring laser beam is coupled into the beam path.A further light blocking element arranged at this first focal point could therefore, on the one hand, effectively block the convergent interfering reflections, but on the other hand would also at least partially block the measuring laser beam and thus ultimately reduce the amount of light effectively available for determining the position of the respective material droplet. To avoid this, in the development of the invention proposed here, the first light blocking element or a further light blocking element is arranged in the region of the second focal point. This further light blocking element can be the further or second light blocking element mentioned elsewhere or a different further light blocking element. In particular, the same light blocking element can be used to block several different types of interfering reflections if the corresponding focal length is the same or - depending on the size of the light blocking element orthe diameter of the light distributions of the interfering reflections - are sufficiently similar. Otherwise, multiple light-blocking elements can be used at different longitudinal positions along the beam path.

[0032] In the development of the present invention proposed here, the imaging optics can therefore be designed to image or focus convergent interference reflections in the area between the coupling point of the measuring laser beam and the sensor system onto the light blocking element or onto the further light blocking element. The imaging optics or at least a corresponding part of the imaging optics can be arranged in particular between the coupling point for the measuring laser beam and the sensor system. This can prevent any impairment or attenuation of the measuring laser beam on its way to the optics or the respective material droplet by the corresponding light blocking element. The development of the present invention proposed here can be particularly useful when the natural orThe first focal point, which results directly from the concave surface of the optics, lies between the optics and the coupling point of the measuring laser beam. The imaging optics can then efficiently and completely block corresponding convergent interference reflections without hindering the radiation of the measuring laser beam to the respective material droplet. By appropriately adapting or designing the imaging optics, the second focal point can be flexibly positioned, enabling flexible adaptation to, for example, specific installation space conditions or other requirements. This can, for example, enable a particularly flexible and / or simple design of the observation device as well as a particularly complete blocking of the convergent interference reflections using a particularly small light blocking element.This, in turn, can lead to or contribute to a particularly high light yield for determining the position of the respective material droplet, so that this can then be achieved in a particularly accurate, reliable and efficient manner.

[0033] In a possible development of the present invention, the imaging optics comprises at least two lenses. The focal length or focal length of at least the lens on the optics side, i.e. the lens facing the optics viewed along the beam path, corresponds to its distance from the first focal point. The convergent interference reflections are then guided between the two lenses of the imaging optics as a collimated beam, i.e. one that is at least essentially neither convergent nor divergent. In other words, the convergent interference reflections between the lenses of the imaging optics can be guided as a parallel beam or parallel beam bundle. For example, the two lenses of the imaging optics can be identical, in which case the distance between the two lenses can be twice their focal length or focal length. A beam can then be formed between the lens on the sensor side of the imaging optics and the sensor, which also corresponds to this focal length or focal length.Depending on the design, the first light-blocking element or the further light-blocking element mentioned elsewhere or another further light-blocking element can be arranged at a distance corresponding to the focal length. In this way, the convergent interference reflections can be effectively and efficiently blocked with a particularly simple and cost-effective design of the observation device. In principle, however, other designs of the imaging optics or other arrangements of the lenses of the imaging optics and / or the corresponding light-blocking element are also possible.

[0034] In a further possible embodiment of the present invention, the imaging optics are configured to image or focus so-called divergent interference reflections onto the light-blocking element or the further light-blocking element arranged elsewhere, in particular, viewed along the beam path, between a coupling point of the measuring laser beam into the beam path and the sensor system, or another further light-blocking element. Such divergent interference reflections can arise here from a convex surface of the optics - intentionally or unintentionally, viewed from the direction of the sensor system. This is therefore a further type of interference reflection. Without the imaging optics, the diameter of these divergent interference reflections or their light distribution would increase with increasing distance from the optics.Therefore, such divergent interference reflections could then, for example, at least partially bypass the first light-blocking element and reach the sensor system—for example, after an uncontrolled reflection on an inner side of a housing surrounding the beam path and / or at least one other component of the observation device. This can be avoided by the design of the imaging optics proposed here for collecting or focusing such divergent interference reflections onto a light-blocking element arranged centrally in the beam path or into a corresponding plane. Thus, a corresponding disruption of the analysis of the light reflected by the respective material droplet, for example, the position determination of the respective material droplet, caused by such divergent interference reflections can be effectively and efficiently avoided or at least reduced.

[0035] If multiple light-blocking elements are used to block the various types of interfering reflections, they can be of identical or different designs, for example, they can have identical or different sizes or diameters. This can depend, for example, on the practically achievable focus for the various interfering reflections and / or on the possibilities or limitations in the individual case regarding the positioning of the light-blocking elements at the respective focal point or focal plane, and / or similar factors. By individually minimizing the size of the light-blocking elements, the – ultimately unintentional – blocking of light reflected by the respective droplet of material can also be minimized.By using identical light blocking elements, i.e. a corresponding common parts strategy, 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 the observation device according to the invention. The EUV light system according to the invention can, for example, be configured or designed for EUV lithography. Accordingly, the EUV light system according to the invention can comprise further components, for example at least one or more laser sources for the measuring laser beam and for a main laser beam for evaporating the respective material droplet and / or a corresponding beam guide and / or, for example, also a beam guide or optics for collecting or guiding the generated EUV light and / or the like. Likewise, the EUV light system according to the invention can, for example, be configured or designed for other applications. Further features of the invention can be derived from the following description of the figures and from the drawing.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 combination specified, but also in other combinations or on their own, without departing from the scope of the invention.

[0037] The drawing shows:

[0038] Fig. 1 is a partial schematic representation of an EUV light system with an observation device for determining a position of a material droplet for EUV light generation;

[0039] Fig. 2 is a partial schematic representation of the observation device in a second variant;

[0040] Fig. 3 is a partial schematic representation of the observation device in a third variant; and

[0041] Fig. 4 is a partial schematic representation of the observation device in a fourth variant;

[0042] Identical or functionally equivalent elements are provided with the same reference numerals in the figures.

[0043] Fig. 1 shows a partial schematic representation of an EUV light system 1. In it, for example, by means of a laser or one or more laser pulses, a material drop 2, for example a tin drop, can be at least partially evaporated or converted into a plasma, which then leads to the emission of extreme ultraviolet radiation.

[0044] In order to be able to precisely target the respective material droplet 2, it is useful to first carry out a corresponding position determination. For this purpose, the EUV light system 1 has an observation device 3, which here functions or can be used, at least among other things, as a position determination device. To determine the position of the respective material droplet 2, it can be illuminated with a pulse of a measuring laser beam 4 indicated here. The measuring laser beam 4 or the corresponding laser pulse is also referred to as a pre-pulse, since it is emitted before a main laser pulse for evaporating the material droplet 2 and is significantly weaker than this main laser pulse, so that the material droplet 2 can be illuminated or pre-conditioned, for example expanded, by the measuring laser beam 4, but is not evaporated.The measuring laser beam 4 can be coupled here via a coupling element 5 and redirected toward the material droplet 2. The measuring laser beam 4 can then pass through an optics 6 of the observation device 3 and thus be focused on the material droplet 2.

[0045] The material droplet 2 can then reflect the light of the measuring laser beam 4 as reflected measuring light 7. This reflected measuring light 7 also radiates through the optics 6 in the opposite direction. The reflected measuring light 7 can be collimated, for example, as indicated here. The reflected measuring light 7 is then guided along a corresponding beam path and can then be focused, for example, via a detection optics 8, onto a sensor 9 of the observation device 3. For further illustration and clarification, a central beam path axis 10 of the intended beam path of the reflected measuring light 7 is schematically indicated here.

[0046] The sensor system 9 can, for example, comprise a camera and corresponding measurement and evaluation electronics. In general, the sensor system 9 can therefore comprise at least one position-sensitive sensor. This ultimately allows the position of the respective material droplet 2 to be determined based on the reflected measurement light 7. For this purpose, parameters of the beam path or the optical elements, or other configurations of the observation device 3, for example, can be specified and taken into account.

[0047] In the method described so far, a portion of the light of the measuring laser beam 4 can be reflected on its way to the respective material drop 2, for example on surfaces and / or material defects and / or damage and / or the like of the optics 6. This leads to corresponding interfering reflections, which can also be referred to as ghost reflections. Where such interfering reflections are generated and their intensity can depend, for example, on the configuration of the optics 6 as well as on its material, quality, surface texture, coating and / or the like. In principle, however, such interfering reflections can be so strong that they can mask the reflection of the measuring laser beam 4 actually desired for determining the position of the respective material drop 2, i.e. the reflected measuring light 7.If in such a case the interference reflections reach the sensor 9, i.e. are also detected by it, the position determination of the material drop 2 can be impaired.

[0048] To counteract this problem, it can be exploited that the measuring laser beam 4 has a significantly smaller numerical aperture, i.e., a significantly smaller diameter, than the reflected measuring light 7 used for position determination or its beam path. Accordingly, when the measuring laser beam 4 strikes a surface of the optics 6 perpendicularly and the optics 6 are aligned without tilting relative to the beam path axis, i.e., with an optical axis that is at least substantially coaxial or coincident with the beam path axis 10, it generates, for example, a straight interference reflection 11, which propagates at least substantially only in a center, i.e., a central region of the beam path and thus also of the light distribution of the reflected measuring light 7, in the direction of the sensor 9.This straight interference reflection 11, like the reflected measurement light 7, can pass through the coupling element 5 and propagate further along the beam path toward the sensor system 9. In the present case, however, the observation device 3 comprises a first light-blocking element 12, which is arranged in the corresponding central region of the beam path. This first light-blocking element 12 thus blocks the further propagation of the straight interference reflection 11. The diameter of the first light-blocking element 12 is smaller than the diameter of the beam path or the light distribution of the reflected measurement light 7. As a result, a portion, in particular a large portion, of the reflected measurement light 7 can pass past the first light-blocking element 12 to the sensor system 9.

[0049] However, the diameter of the first light-blocking element 12 can be larger than the diameter of the light distribution of the straight interference reflection 11. This can completely or at least largely prevent the straight interference reflection 11 from reaching the sensor 9.

[0050] A central or mid-area around the beam path axis 10 can also be referred to as the near field. An area further away from the beam path axis 10 can also be referred to as the far field. The first light-blocking element 12 can therefore block not only the straight interfering reflection 11, but also a near-field area of ​​the reflected measuring light 7. However, this does not change the image of the reflected measuring light 7 in the far field onto the sensor system 9, so that despite the first light-blocking element 12, a position determination of the respective material droplet 2 is still possible based on the portion of the reflected measuring light 7 detected by the sensor system 9.

[0051] In order not to influence the measuring laser beam 4 here, the first light blocking element 12 is arranged behind the coupling element 5, viewed along the beam path, i.e. between this or the corresponding coupling point at which the measuring laser beam 4 is coupled into the beam path, and the sensor system 9.

[0052] Additionally, an outer aperture stop 13 is arranged here—for example, at the location or level of the first light-blocking element 12. This aperture stop 13 surrounds the beam path, for example, in a ring shape. This leaves an area between the aperture stop 13 and the first light-blocking element 12 through which the reflected measurement light 7 can pass to reach the sensor 9. External stray light can be intercepted, i.e., blocked, by the aperture stop 13.

[0053] The straight spurious reflection 11 is shown here as a collimated beam or collimated beam bundle. However, there can also be non-collimated spurious reflections or spurious reflection components which, for example, run obliquely to the beam path axis 10 and / or diverge or converge. Such spurious reflections can, for example, be collected or focused by suitable optical elements arranged in front of and / or behind the coupling element 5. In corresponding focal planes or focus positions, provided these do not coincide with the position of the first light-blocking element 12, further light-blocking elements can be arranged. With a correspondingly adapted arrangement, various types of spurious reflections can also be blocked, i.e. eliminated, in this way. In this case, even outside the near field orInterference reflections extending from the central region covered by a light-blocking element are captured and directed or imaged into this central region. Thus, for example, corresponding angular deviations between the propagation direction or the central longitudinal axis of corresponding interference reflections emitted or emanating from the optics 6 at an angle to the beam path axis 10 can also be corrected. Examples of this are shown in the remaining figures and are explained below. The focus here is primarily on differences or additions compared to the observation device 3 shown in Fig. 1.

[0054] Fig. 2 shows a schematic representation of the observation device 3 in a second variant. Here, for example, one of the elements of the optics 6 is inclined, i.e., tilted relative to the beam path axis 10. As a result, the measuring laser beam 4 strikes this element of the optics 6 at a corresponding angle of incidence other than 0°. This, in turn, generates an oblique interference reflection 14 that propagates at an angle, i.e., not parallel to the beam path axis 10. This oblique interference reflection 14 could, in principle, thus radiate past the first light-blocking element 12. In order to also prevent such oblique interference reflections 14 from reaching the sensor system 9, the observation device 3 here also comprises an imaging optics 15 arranged in the beam path. This can, for example, be designed as a transmission telescope with two telescopic lenses 16.The imaging optics 15 directs or images the oblique interference reflection 14, which strikes the optics-side or droplet-side telescopic lens 16, i.e., facing the region of the material plug 2 in the intended installation position of the optics 6, at least partially outside the central region covered by the first light-blocking element 12, into the central region. Thus, the oblique interference reflection 14 can also be imaged onto the first light-blocking element 12 and thus blocked by it. For example, the telescopic lenses 16 here each have a focal length f and are each arranged at a distance of the object distance g from the optics 6 or the plane of the first light-blocking element 12.

[0055] Within the imaging optics 15, i.e. between its telescopic lenses 16, a beam waist with a minimal diameter of the light distribution of the reflected measuring light 7 is created. The aperture diaphragm 13 or another aperture diaphragm 13 is arranged in this area. By arranging it in the area of ​​the beam waist, an inner opening or transparency area of ​​the aperture diaphragm 13 can be selected to be particularly small without blocking the reflected measuring light 7. This allows disruptive stray light to be blocked particularly effectively by the aperture diaphragm 13. Ultimately, however, the aperture diaphragm 13 is optional and flexible in its positioning along the beam path, i.e. in its longitudinal position. Likewise, several aperture diaphragms 13 can be used at different longitudinal positions along the beam path, which can be the same or different. Fig. 3 shows the observation device 3 in a third variant.Here, a concave interference reflection 17 is illustrated as a further type of interference reflection. This can originate from or be generated by a surface of the optics 6 that is concave from the perspective of the sensor system 9. The concave interference reflection 17 can be focused at a focal point 18 by the correspondingly concavely curved surface of the optics 6. However, this focal point 18 can lie between the coupling element 5 and the optics 6. Thus, it would not be practical to block the concave interference reflection 18 at the focal point 18 itself, since the measuring laser beam 4 would then also be at least partially blocked there. Instead, the or an imaging optics 15 is provided here too. This first collimates the concave interference reflection 17 and then focuses it at a second focal point 19, which lies between the coupling element 5 and the sensor system 9.If this second focal point 19 lies in the area of ​​the first light blocking element 12, the concave interference reflection 17 can then also be blocked by the first light blocking element 12.

[0056] In the present example, however, the second focal point 19 is located in a different plane, i.e., at a different longitudinal position along the beam path or the beam path axis 10 than the first light-blocking element 12. In order to effectively and efficiently block the concave interference reflection 17, a second light-blocking element 20 is arranged in the region of the second focal point 19. To ensure the arrangement of the second light-blocking element 20 at the second focal point 19, the second light-blocking element 20 can be adjustable along the beam path axis 10, for example, by means of an adjustment device. Additionally or alternatively, the imaging optics can be designed to be variable, i.e., adjustable. For this purpose, at least one of the telescopic lenses can be tiltable relative to the beam path axis 10 and / or displaceable along the beam path axis 10, for example, by means of a corresponding adjustment device.Thus, for example, the second focal point 19 can be shifted onto the beam path axis 10 and / or along the beam path axis 10 so that it coincides with the second light-blocking element 20. Likewise, the first light-blocking element 12 and / or other light-blocking elements and / or optical elements of the observation device 3 can be adjustable or displaceable. Corresponding adjustment or setting devices can comprise, for example, one or more adjusting screws and / or a servomotor, in particular an electric one, and / or at least one cable pull and / or the like for the adjustment or setting or displacing.

[0057] If the imaging optics 15 have a sufficiently large adjustment range and / or the first light-blocking element 12 is adjustable far enough, the second focal point 19 can coincide with the position of the first light-blocking element 12. In other words, the first light-blocking element 12 can then be arranged at the second focal point 19 and replace the second light-blocking element 20. This may eliminate the need for the separate second light-blocking element 20.

[0058] By way of example, the telescopic lenses 16 of the imaging optics 15 are designed and arranged here such that they have identical focal lengths f, and these focal lengths f also correspond to the distance of the optics- or drop-side telescopic lens 16 from the focal point 18 and the distance of the sensor-side telescopic lens 16 from the second focal point 19 or the second light-blocking element 20. However, other arrangements may also be possible and / or other configurations may result, for example, by displacing at least one of the telescopic lenses 16, in particular along the beam path axis 10.

[0059] Fig. 4 shows a schematic representation of the observation device 3 in a fourth variant. Here, in addition to the straight interference reflection 11 and the concave interference reflection 17, a convex interference reflection 21 is also shown by way of example. This can be generated by or originate from a surface of the optics 6 that is convex from the perspective of the sensor system 9. Thus, the convex interference reflection 21 in the area between the coupling element 5 and the optics 6 can, for example, have a larger cross-section or diameter than the actual measuring laser beam 4 there, so that the convex interference reflection 21 cannot be practically blocked there. Here, too, the observation device 3 comprises the or an imaging optics 15. This is designed here to focus the convex interference reflection 21 in the area between the coupling element 5 and the sensor system 9.At the longitudinal position of the corresponding focus, a third light blocking element 22 is arranged here, for example, to block the convex interference reflection 21.

[0060] Although not explicitly shown here for the sake of clarity, one or more aperture stops 13 can also be provided in the variants according to Fig. 3 and Fig. 4. Likewise, several transmission telescopes can be used in combination with one another and / or the imaging optics 15 can comprise more than just two telescope lenses 16 and / or other optical elements. The light-blocking elements 12, 20, 22 can, for example, be designed as plane-parallel plates with a central region that at least or only selectively absorbs and / or reflects the wavelength or wavelengths of the interfering reflections 11, 14, 17, 21. The central region or at least a remaining region of these plates surrounding it, i.e. extending in the far-field region, can each be transparent at least to the reflected measuring light 7. This can enable particularly simple mounting or fastening of the light-blocking elements 12, 20, 22.The light-blocking elements 12, 20, 22 can, for example, be arranged perpendicular to the beam path axis 10. Likewise, the light-blocking elements 12, 20, 22 can be tilted relative to the beam path axis 10 or a propagation plane of the reflected measuring light 7, for example, at an angle in the range of 2° to 45°. In this case in particular, the light-blocking elements 12, 20, 22 can be designed, for example, as reflective mirrors. The unwanted interfering reflections 11, 14, 17, 21 can then be deflected out of the beam path by the light-blocking elements 12, 20, 22, so that they cannot reach the sensor system 9. Different variants and / or arrangements of light-blocking elements can also be combined, so that the light-blocking elements 12, 20, 22 can be designed and / or arranged in the same or different ways.Likewise, several of the light-blocking elements 12, 20, 22 shown individually here can, for example, be combined or replaced by an absorbing or reflecting rod, cylinder, or the like, in particular one that extends longitudinally along the beam path axis 10. Such a rod-shaped light-blocking element 12, 20, 22, i.e., one that extends longitudinally along the beam path axis 10, can be positioned more easily to achieve the desired effect and, with a particularly small diameter, can keep a particularly large portion of the interfering reflections 11, 14, 17, 21 away from the sensor system 9. Thus, such a rod-shaped light-blocking element 12, 20, 22 does not necessarily have to be positioned exactly in a focal plane of one of the interfering reflections 11, 14, 17, 21, since it can extend along the beam path axis 10 over one or even several corresponding focal areas or beyond.For example, even if the position of the second focal point 19 is not precisely known or if a shift occurs during operation, a rod-shaped light-blocking element 12, 20, 22 can continue to prevent the corresponding interfering reflection 17 from reaching the sensor 9. In this case, for example, light from at least one of the interfering reflections 11, 14, 17, 21 can then fall onto an outer surface of the rod-shaped light-blocking element 12, 20, 22. The surface can, for example, be designed to be absorbent—at least for the corresponding wavelength or wavelengths of at least one of the interfering reflections 11, 14, 17, 21. Likewise, a surface of the light blocking element 12, 20, 22 can be structured or microstructured at least in some areas in such a way that light incident there - optionally wavelength-selective for at least one of the interfering reflections 11, 14, 17, 21 - is reflected away from the sensor system 9.Overall, the rod-shaped design can have at least one of the light blocking elements.

[0061] 12, 20, 22 reduce the required number of light-blocking elements and / or simplify their arrangement. Likewise, the observation device can be designed to be particularly robust, for example, against vibrations or relative movements of components or changes in optical properties and / or geometries, for example due to thermal influences or the like.

[0062] In each individual application, the optimal longitudinal positions and sizes of the described components can be determined, for example, using the usual optical formulas.

[0063] In addition to the variants illustrated here as examples, further variants or variations are also possible. For example, the measuring laser beam 4 can be irradiated or coupled in at an angle to the beam path axis 10 or offset parallel to it. In the latter case, the beam path axis 10 for the reflected measuring light 7 and a central longitudinal axis of the measuring laser beam 4 would not necessarily coincide. Likewise, the or another imaging optics 15 can be arranged entirely or partially between the coupling element 5 and the optics 6. Likewise, the reflected measuring light 7 can be guided to the sensor system 9, for example, along a differently configured, for example angled or tilted, beam path. These and / or other possible variations or adaptations can enable particularly flexible adaptation of the observation device 3 to different requirements and / or installation space conditions and / or the like.

[0064] Overall, the examples described demonstrate how, in the context of EUV light generation, unwanted reflections during the positioning of the ultimately light-generating material can be avoided or at least effectively and efficiently reduced compared to previous approaches.

[0065] 1 EUV light system

[0066] 2 drops of material

[0067] 3 Observation facility

[0068] 4 measuring laser beam

[0069] 5 Coupling element

[0070] 6 Optics

[0071] 7 reflected measuring light

[0072] 8 Detection optics

[0073] 9 Sensor technology

[0074] 10 Beam path axis

[0075] 11 straight interference reflection

[0076] 12 first light blocking element

[0077] 13 Aperture diaphragm

[0078] 14 oblique interference reflection

[0079] 15 Imaging optics

[0080] 16 telescope lenses

[0081] 17 concave interference reflection

[0082] 18 focus point

[0083] 19 second focus point

[0084] 20 second light blocking element

[0085] 21 convex interference reflection

[0086] 22 third light blocking element f focal length g object distance

Claims

PATENT CLAIMS 1. Observation device (3) for an EUV light system (1) for observing a material drop (2) for EUV light generation, comprising an optical system (6) for focusing a measuring laser beam (4) onto the material drop (2) and for directing light (7) reflected by the material drop (2) along a predetermined beam path, as well as a sensor system (9) arranged at the end of this beam path for detecting the reflected light (7), wherein, viewed along the beam path, between the optical system (6) and the sensor system (9), at least one light blocking element (12, 20, 22) for keeping away interfering reflections (11, 14, 17, 21) of the measuring laser beam (4) emanating from the optical system (6) in the direction of the sensor system (9) is arranged at least in a central region of the beam path, so that at least a part of the reflected light (7) despite the light blocking element (12, 20, 22) along the beam path to the sensor (9).

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

3. Observation device according to one of the preceding claims, characterized in that the light blocking element (12, 20, 22) is designed in a rod-shaped manner and is arranged longitudinally along a longitudinal axis (10) of the beam path, in particular across focus 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 impinge on an outer surface of the light blocking element (12, 20, 22).

4. Observation device according to one of the preceding claims, characterized in that the observation device has an adjustment device and the light blocking element (12, 20, 22) is arranged to be displaceable by means of the adjustment device, in particular along the beam path.

5. Observation device (3) according to claim 1, characterized in that, viewed along the beam path, an aperture diaphragm (13) is arranged between the optics (6) and the sensor system (9), in particular at the longitudinal position of the light blocking element (12, 20, 22), which surrounds the beam path.

6. Observation device (3) according to one of the preceding claims, characterized in that the observation device (3) has a coupling element (5), in particular an optical beam splitter, for coupling the measuring laser beam (4) into the beam path, wherein, viewed along the beam path, the coupling element (5) is arranged between the optics (6) and the sensor system (9).

7. Observation device (3) according to one of the preceding claims, characterized in that the observation device (3) has at least one further light blocking element (20, 22) for keeping interfering reflections (17, 21) away from the sensor system (9) at least in the central region of the beam path, wherein the further light blocking element (20, 22) is arranged at a different location than the light blocking element (12) viewed along the beam path.

8. Observation device (3) according to one of the preceding claims, characterized in that the light blocking element (12) or a further light blocking element (20, 22) is arranged in the region of a focal point (19) of interfering reflections (17), in particular between a coupling point (5) of the measuring laser beam (4) into the beam path and the sensor system (9), wherein the focal point (19) is determined by a concave surface of the optics (6) viewed from the direction of the sensor system (9).

9. Observation device (3) according to one of the preceding claims, characterized in that the observation device (3) has an imaging optics (15) arranged in the beam path, in particular viewed along the beam path between the optics (6) and the light blocking element (12), in particular comprising at least one transmission telescope (15, 16), for imaging the interference reflections (11, 14, 17, 21).

10. Observation device according to claim 9, characterized in that the imaging optics (15) has an adjustment device by means of which the position of a focal point (19) of the imaging optics (15) and / or a distance between optical elements (16) of the imaging optics (15), in particular along the longitudinal direction (10) of the beam path, can be adjusted.

11. Observation device according to claim 9 or 10, characterized in that the imaging optics (15) are designed to image a plane in which interference reflections (14) on the optics (6) arise due to a non-perpendicular incidence of the measuring laser beam (4) on the optics (6) onto the light blocking element (12) or a further light blocking element (20, 22) arranged, in particular viewed along the beam path, between a coupling point (5) of the measuring laser beam (4) into the beam path and the sensor system (9).

12. Observation device (3) according to one of claims 9 to 11, characterized in that the imaging optics (15) are designed to image the focal point (18) into a second focal point (19) between the coupling point (5) and the sensor system (9) for convergent interference reflections (17) which arise from a concave surface of the optics (6) viewed from the direction of the sensor system (9) and whose focal point (18) lies in the beam path between the optics (6) and a coupling point (5) of the measuring laser beam (4) in the beam path, wherein the light blocking element (12) or a further light blocking element (20, 22) is arranged in the region of this second focal point (19).

13. Observation device (3) according to claim 12, characterized in that the imaging optics (15) comprises two lenses (16), wherein the focal length (f) of the optics-side lens (16) corresponds to its distance (f) from (18) the focal point (18), so that the convergent interference reflections (17) between the two lenses (16) of the imaging optics (15) are guided as a collimated beam.

14. Observation device (3) according to one of claims 9 to 13, characterized in that the imaging optics (15) are designed to direct divergent interference reflections (21), which arise from a convex surface of the optics (6) viewed from the direction of the sensor system (9), onto the light blocking element (12) or a, in particular along the Viewed along the beam path, an additional light-blocking element (20, 22) arranged between a coupling point (5) of the measuring laser beam (4) into the beam path and the sensor system (9) is to be imaged.

15. EUV light system (1) comprising an observation device (3) according to one of the preceding claims.

Images