DE102022200726A1 - Method for determining a reflection property of an optical element to be examined, device for determining a reflection property of an optical element to be examined, computer program product and lithography system - Google Patents

Abstract

The invention relates to a method for determining a reflection property (2) of an optical element (3) to be examined in an optical system (5) with one or more optical elements (3, 6), after which the reflection property (2) is determined using a radiation source (7) emitted by the optical system (5) guided and shaped radiation is determined. It is also provided that a thermal load (9) is determined for the optical element (3) to be examined and the reflection property (2) of the optical element (3) to be examined is inferred from the thermal load (9) determined.

Description

The invention relates to a method for determining a reflection property of an optical element to be examined in an optical system with one or more optical elements, according to which the reflection property is determined using radiation emitted by a radiation source that is guided and shaped through the optical system.

The invention also relates to a device for determining a reflection property of an optical element to be examined, which has a thermal load measuring device, in an optical system with one or more optical elements by means of radiation emitted by a radiation source and guided and shaped through the optical system.

The invention also relates to a computer program product with program code means.

Furthermore, the invention relates to a lithography system, in particular a projection exposure system for microlithography with an illumination system and illumination optics, which has at least one optical element.

In a known manner, optical elements influence the properties of the light waves interacting with them. In order to avoid undesired structures of the resulting wave fronts, the surface of the optical elements must have a flawless design. Examples of optical elements that can be mentioned are planar mirrors, concave mirrors, web mirrors, facet mirrors, in particular field facet mirrors, convex lenses, concave lenses, convex-concave lenses, plano-convex lenses and plano-concave lenses. Glass and silicon, among others, are known as materials for optical elements, in particular mirrors.

Lithography systems, in particular projection exposure systems, have a large number of optical elements. The nature of the optical elements is of particular importance when using the optical elements in a microlithographic DUV (Deep Ultraviolet) projection exposure system and very particularly when using them in a microlithographic EUV (Extreme Ultraviolet) projection exposure system.

The optical elements are exposed to a variety of harmful influences that change their nature and thus impair their functionality, since the light modulated by the optical elements, for example an EUV mirror, has a very small wavelength and the resulting wave fronts do be disturbed by the slightest impairment of the nature of the optical element. On the other hand, the structures shown on the projection surface are very small and therefore also susceptible to the slightest changes in the nature of the optical element. In particular, a reflectivity of the optical elements for the light is very sensitive to contamination and/or changes in coatings applied to the optical elements.

It is known from practice that, in order to generate EUV light, tin drops are ionized in such a way that a plasma is created which emits EUV radiation in all directions.

The EUV radiation is caught, for example, by collector mirrors which, in addition to the EUV light emitted by the plasma, are also exposed to the damaging effects of tin ions and tin drops. For example, drops of tin deposited on a collector mirror can change its optical properties. In particular, a blinding of the mirror can occur due to a degradation of the reflection properties, which is caused by a tin layer applied by tin drops. In particular, this blinding of the mirror can only occur in certain areas.

A deterioration in the reflection properties, for example of the collector mirror, but in particular also of components arranged after the collector mirror, in particular a field facet mirror, can be caused by the fact that light is no longer directed by the applied tin layer but is reflected diffusely. Furthermore, the tin layer can also lead to a deterioration in the reflection property in that an absorptivity is increased. As a result, both the reflectivity is worsened and thermal energy is introduced into the optical elements, which have an increased degree of absorption. This can lead, for example, to temporary or permanent distortions and/or damage to the optical element and thus to a deterioration in the imaging quality in projection optics.

In order to ensure error-free reflection properties, for example, according to the prior art known from practice, the collector mirror of an EUV projection exposure system is regularly exchanged and replaced by a collector mirror of error-free design.

In the WO 2019/063254 A1 is also a method for determining at least one property of an EUV source in a projection exposure processing system for semiconductor lithography, the property being determined based on the electromagnetic radiation emanating from the EUV source by determining a thermal load for a downstream component of the projection exposure system and using the thermal load determined to determine the property of the EUV source.

It is therefore known from the prior art to draw conclusions about the properties of the source located in front of the monitored component of the projection exposure system or its optical properties on the basis of thermal energy or heat output, in particular a thermal load, applied to a monitored component of the projection exposure system.

A disadvantage of the prior art is that to determine a reflection property of a first component, according to the WO 2019/063254 A1 the EUV source, a second, monitored component is required for which a thermal load is determined and which is arranged downstream in the beam path of the projection exposure system. This makes it difficult to reliably determine the performance of an optical system that includes the first and second components, in particular the performance of a projection exposure system, since, for example, the second, monitored component itself can also be affected by the entry of tin drops and thus by degradation can.

The present invention is based on the object of creating a method for determining a reflection property of an optical element to be examined which avoids the disadvantages of the prior art, in particular enabling a reliable determination of the reflection property of the optical element under examination.

This object is achieved by a method for determining a reflection property of an optical element to be examined in an optical system with one or more optical elements in that the reflection property is determined using radiation emitted by a radiation source and guided and shaped by the optical system, wherein according to the invention, a thermal load is determined for the optical element to be examined and conclusions are drawn about the reflection property of the optical element to be examined on the basis of the thermal load determined.

The present invention is also based on the object of creating a device for determining a reflection property of an optical element to be examined, which avoids the disadvantages of the prior art, in particular enables a reliable determination of the reflection property of the optical element under examination.

This object is achieved by a device for determining a reflection property of an optical element to be examined, which has a thermal load measuring device, in an optical system with one or more optical elements by means of radiation emitted by a radiation source and guided and shaped through the optical element, characterized in that that according to the invention a device is provided and set up to infer the reflection property of the optical element to be examined on the basis of a thermal load determined by the thermal load measuring device.

The present invention is also based on the object of creating a computer program product which avoids the disadvantages of the prior art, in particular enabling a reliable determination of a reflection property of an optical element to be examined which can be detected during operation.

According to the invention, this object is achieved by a computer program product with program code means for carrying out a method according to the invention when the program is executed on a device of a device according to the invention.

The present invention is also based on the object of creating a lithography system which avoids the disadvantages of the prior art, and in particular enables its functionality to be determined reliably and during operation.

This object is achieved by a lithography system, in particular a projection exposure system for microlithography, with an illumination system that has a radiation source, illumination optics and projection optics, the illumination optics and/or the projection optics having at least one optical element, in that a reflection property of a of the optical elements of the lithography system is determined by means of the method according to the invention and/or is determined by means of the device according to the invention.

In the method according to the invention for determining a reflection property of an optical element to be examined in an optical system with one or more optical elements, the reflection property is determined using of a radiation emitted by a radiation source, guided and shaped by the optical system. According to the invention, a thermal load is determined for the optical element to be examined and the reflection property of the optical element to be examined is inferred from the thermal load determined.

In the context of the invention, the thermal load is to be understood as thermal energy and/or thermal output introduced into a body and/or one or more partial regions of the body, in particular into the optical element and/or partial regions of the optical element. Such an input of heat energy and/or heat output can lead to an increase in temperature of the body and/or parts of the body. If, for example, the temperature of the body and/or parts of the body is to be kept constant, it is advantageous if thermal energy and/or thermal output is dissipated from the body and/or parts of the body in the amount of the thermal load. In particular, it is advantageous if a cooling capacity of a cooling system that cools the body and/or the partial areas of the body corresponds to the thermal load.

The method according to the invention offers the advantage that by evaluating information about the thermal load, in particular by evaluating information about a temperature to which the optical element to be examined is exposed, its optical properties, in particular its reflection property, can be inferred. This means that by detecting information that does not directly match the optical properties of the optical element to be examined, it is possible to draw a conclusion about the optical properties of the optical element to be examined.

This has the particular advantage that no devices for the direct detection of the radiation, in particular light, have to be brought into the optical system. This avoids an unfavorable disturbance of the course of radiation in the optical system.

This is particularly advantageous when the optical element to be examined is not the first optical element in the radiation path of the optical system.

The determination of the reflection property of the optical element to be examined can advantageously be carried out without intervention in a position of the optical element to be examined at its installation site, ie in situ.

In an advantageous development of the method according to the invention, it can be provided that the reflection property of the optical element to be examined is inferred by comparing the thermal load with a thermal reference value to be expected.

It is particularly advantageous to determine the reflection property of the optical element to be examined if the thermal load determined is considered in relation to a thermal load to be expected, the thermal comparison value.

If, for example, the thermal load deviates from the thermal comparison value in such a way that the thermal load is higher than the thermal comparison value, this can be an indication that the surface of the optical element to be examined has an absorption that deviates beyond expectations and, for example, from a norm radiation striking the surface of the optical element takes place.

Increased absorption, for example caused by contamination of the optical element to be examined, in particular by tin from an EUV source, can be a clear indication of a reduction in reflectivity and thus a deterioration in the reflection properties.

Accordingly, the reflection property can in particular be a reflectivity of the optical element to be examined. In addition to or instead of the reflection property, other optical properties of the optical element to be examined, for example transparency in the case of a lens, can also be determined.

It can be advantageous if the thermal comparison value to be expected is determined by a surface intensity of the radiation on the optical element to be examined by a source intensity of the radiation source and by a simulation taking into account optical properties of a part located upstream of the optical element to be examined of the optical system is determined.

Determining the thermal comparison value by simulating the surface intensity can advantageously take into account the optical properties of those parts of the optical system which are upstream of the optical element to be examined.

Upstream in this context means against the propagation direction of the light through the optical system. downstream in this context means along the direction of propagation of the light through the optical system.

A simulation is particularly advantageous because no physical intervention in the optical system for measuring the surface intensity is necessary.

In this case, it is advantageous if the source intensity is measured, since reliable knowledge about an actual state of the radiation source can thereby be obtained.

The simulation becomes particularly reliable if the optical properties of the part of the optical system located upstream of the optical element to be examined are known. As a result, the surface intensity can be determined reliably and precisely, for example by wave modeling or ray tracing, which represent established methods from the prior art.

A determination of an intensity in an intermediate focus or intermediate focus, in particular an intermediate focal plane or intermediate focal plane, can correspond to a determination of the surface intensity, provided that the radiation from the intermediate focus to the surface of the optical element to be examined is not influenced by any other optical elements. As a result, the intensity in the intermediate focus is directly linked to the surface intensity.

It is particularly advantageous if the reflection property of the optical element to be examined is inferred from the determined thermal load and the thermal comparison value by comparing an exit intensity of the radiation guided and shaped by the optical system with the thermal load.

If the thermal comparison value is known, e.g. through a simulation, and if there is also a determined value for the thermal load, it is advantageous, particularly when the surface intensity actually present on the surface of the optical element to be examined is not known, to draw conclusions about its reflection property, albeit an exit intensity of the radiation guided and shaped by the optical system is detected.

If this exit intensity is compared with the thermal load, the comparison reveals whether deviations in the thermal load from the thermal comparison value are due to a change in the reflection property or to a change in the surface intensity. In particular, a change in the surface intensity can be caused by changes in the reflection properties of a part of the optical system located upstream of the optical element to be examined.

For example, a thermal load that is higher than the thermal reference value can be determined at a specific point on the optical element, while at a certain position at the end of the optical system, which corresponds to the point on the optical element, i. H. lies on the same light path, a reduced exit intensity is determined. This can indicate, particularly in the case that only a single mirror interacts with the radiation, that the increased thermal load is associated with an increased degree of absorption and thus with a reduced reflection property at this point of the optical element.

A generic conclusion on the reflection property of the upstream part of the optical system, but without considering the exit intensity, is from WO 2019/063254 A1 known.

An advantageous simultaneous detection of the thermal load and the exit intensity can ensure that no time-dependent effects, such as heating of the optical system, impair the comparability of the exit intensity and the thermal load.

A particularly advantageous embodiment of the method according to the invention can consist in determining an exit intensity comparison value and comparing the exit intensity with the exit intensity comparison value. As a result, a loss of intensity, which the radiation experiences, for example due to degraded optical elements in the optical system, can be recognized immediately and, in particular, quantified. If, for example, an exit intensity is measured which is only half of the exit intensity comparison value, then it is obvious that significantly more radiation than expected was absorbed in the optical system.

In particular, a non-uniformity of the escape intensity and/or the thermal load and/or the escape intensity comparison value and/or the thermal comparison value and/or ratios of the aforementioned variables can be determined. In this way, for example, normalization to a reference can be made possible.

An amplitude of the aforementioned variables and/or the ratios derived from them can be compared, for example, between several optical systems. Such a comparison between optical systems also allows the detection of uniform degradations. An indication of a uniform degradation can be a higher temperature rise of an optical element compared to other systems after normalization to the incident power.

The exit intensity comparison value can be determined here, for example, by a simulation that takes into account the source intensity and the optical properties of all optical elements of the optical system.

Furthermore, the optical system is understood to mean the entirety of all elements influencing the radiation, which are arranged on the radiation path between the radiation source and the point at which the exit intensity is measured. For example, only a part of a projection exposure system can be considered as an optical system by a suitable position of the measurement of the exit intensity.

Of particular advantage, however, is a measurement of the exit intensity in a projection exposure system at a position at which a wafer can be arranged, or is typically arranged. This position is often particularly advantageously easily accessible for measuring the exit intensity.

It can be advantageous if a reflection property of the downstream part of the optical system is inferred from the determined reflection property and/or the thermal load of the optical element to be examined and/or the exit intensity.

It can also be advantageous if a reflection property of the upstream part of the optical system is inferred based on the determined reflection property and/or the thermal load of the optical element to be examined and/or the exit intensity.

If the reflection property and/or the thermal load of the optical element to be examined is known, the reflection property of the upstream part and/or the downstream part of the optical system can advantageously be reconstructed by comparing them with the exit intensity.

For example, deviations between the exit intensity and the reflection property of the optical element to be examined indicate deviating reflection properties of the part lying downstream, provided that it is certain and/or known which surface intensity the radiation forms on the optical element.

In this context, a conclusion about a property is not to be understood exclusively as a strict and/or complete determination of the property, but rather also as the addition of further information which can be used to determine the property.

It can be advantageous if that part of the optical system which is located upstream of the optical element to be examined, in particular a collector mirror, is designed in such a way that its actual optical properties correspond to its target optical properties.

If the target properties of the part located upstream are known and it is ensured that the actual optical properties of the part located upstream correspond to the target optical properties, a simulation including these properties can provide information about the surface intensity. In particular, the simulation can be used to determine the actual surface intensity at the time of measurement, which is consistent with the thermal reference value.

Knowledge of the surface intensity can be ensured, for example, by the fact that the upstream part of the optical system, for example a collector mirror, is new and its actual optical properties correspond to the target optical properties.

After this, the surface intensity can be determined by means of a simulation.

Likewise, if there is knowledge and/or a justified assumption that the reflection property of the part of the optical system located downstream is unrestricted and does not deviate from the target properties, if the reflection property of the optical element to be examined deviates from the exit intensity, it can be concluded that, for example, a deviation from the standard in the Reflective properties of the upstream part may be present.

Since information is now available about the formation, for example, of a wavefront of the radiation immediately before it hits the optical element to be examined, further changes in the wavefront can affect the reflection properties of the optical element to be examined and/or the reflection properties of the other optical elements which form the downstream part of the optical system.

In particular, if the upstream part is formed by a collector mirror, the actual optical properties will always match the target optical properties of the collector mirror, for example, whenever the collector mirror is replaced due to degradation and replaced by a new, correctly specified collector mirror . At this point in time, the aforesaid knowledge of the optical properties of the upstream part occurs and the described conclusions about the downstream part of the optical system can be drawn.

It should be noted here that a reduction in the emission intensity and/or reduced thermal load compared to the thermal comparison value can also be caused by absorption of the radiation, in particular EUV radiation, by gases located in the optical system or in the area of the radiation source. If the exit intensity is reduced, for example, and the thermal load and the source intensity are normal, absorption can take place in the downstream part of the optical system on the optical elements there and/or on the gases present there.

In the case of an upstream part of the optical system, the actual properties of which correspond to the target properties, an inconsistency between the surface intensity and the exit intensity can be identified, assuming that the actual optical properties of the downstream part of the optical system meet the target optical properties. Properties correspond to indicate that the radiation source does not form proper radiation. This can manifest itself, for example, in an undesired spectral range or gas transmissions.

It is advantageous if the surface intensity and/or the thermal comparison value and/or the thermal load and/or the exit intensity and/or the source intensity and/or the reflection property are determined in a spatially resolved manner as flat maps.

A particularly advantageous feature of a planar, two-dimensional determination of the surface intensity and/or the thermal reference value and/or the exit intensity and/or the source intensity and/or the reflection property of the optical element to be examined is that the reflection property of the optical element to be examined is also only on partial areas of the surface of the optical element can be determined.

Furthermore, it is advantageous here that the patterns of the expression of the properties that result and occur from the areal mapping can be compared.

For example, a dissimilar pattern in thermal load and leak intensity may indicate that the downstream portion is reducing an imaging fidelity of the system, even without comparison to the thermal benchmark and/or the leak intensity benchmark. This can be caused by an inhomogeneous deterioration in the reflection properties.

For example, a comparison value, in particular the thermal comparison value and/or the exit intensity comparison value, can be determined by determining a mean value of the measured variable and comparing the expression of the measured variable in a pixel of the mapping and with the mean value.

It is advantageous if the mapping of the escaping intensity is compared with the mapping of the thermal load, in that a mapping of a cross-correlation and/or a correlation coefficient of the mapping of the escaping intensity with the mapping of the thermal load is determined.

A comparison using a cross-correlation or a correlation coefficient has the advantage that a measure of a match between the discharge intensity and the thermal load can be determined. In particular, this degree of agreement can be determined either in a spatially resolved manner, for example by cross-correlation, and/or as a single numerical value, for example by the correlation coefficient. In this way, a reproducible and easily interpretable value of the agreement and the comparison can be determined in a computationally simple and efficient manner.

It is advantageous if the optical element to be examined is a facet mirror, in particular a field facet mirror, or a mirror of projection optics or a collector mirror.

Of particular interest for the method and of particular advantage, particularly when operating a projection exposure system, is knowledge of the reflection properties of the field facet mirror. A mirror of a projection lens system or a collector mirror are also important for the operation of a projection exposure system, and knowledge of their reflection properties can provide information about the functionality of the projection exposure system. So it is advantageous if these optical Elements represent the optical element to be examined in the method according to the invention.

It is advantageous if the thermal load is determined from a change in geometry of at least one part of the optical element to be examined, preferably from a change in the distance between a position sensor and a sensor target.

The thermal load can be determined in particular based on a change in geometry of at least one part of the optical element. Position sensors that are present in any case can also be used in an advantageous manner for this purpose, it being possible, for example, to determine the distance or a change in the distance of a position sensor from a sensor target.

A facet mirror, in particular the field facet mirror, has a large number of individual reflecting optical elements, so-called mirror facets, which can usually be manipulated mechanically, which are mounted in a defined manner on a typically cooled support body and can be actuated by means of so-called plungers.

Plungers are usually rod-shaped extensions on the side facing away from the reflective surface of the mirror facet. A sensor system is usually also present for each facet, which is used to determine current geometric parameters of the mirror facet being considered in each case. In particular, position sensors can be present, by means of which a displacement or deformation of components of the mirror facet or the associated kinematics can be determined. In this case, for example, a distance between a position sensor and a sensor target arranged on the reflective surface of the mirror facet facing away from a sensor target can be measured, for example in the z-direction; this distance is commonly referred to as the z-gap. The z-direction is to be understood as meaning that spatial direction which runs essentially perpendicularly to the reflecting surface of a field facet mirror or perpendicularly to the lateral extension of its carrier body.

The sensor target is a reference element, based on which the position sensor can determine a parameter, for example the z-gap; it can be a reflective element for optical measurements, for example.

If deviations that occur with regard to the z-gap are detected compared to a reference state in which all mirror facets see a defined reference thermal load, a change in the distribution of the thermal load for the region in question can be detected from this. From this, for example, local contamination on the field facet mirror and also its position can be inferred.

Since the z-gap has to be determined simply to determine and regulate the alignment of the mirror facets during operation of the system, the method according to the invention allows, without intervention in the operation of the system, i.e. in situ, simply by evaluating those that are available anyway To determine parameters occurring changes in the reflection property of the field facet mirror, caused for example by local contamination. This eliminates the need to interrupt production for measurement. Furthermore, no structural modifications of the projection exposure system to be monitored are required to carry out the method according to the invention; it is sufficient to evaluate parameters that are already available.

Analogously, the condition of a collector mirror of an EUV projection exposure system can also be monitored spatially resolved in situ. Temperatures on the field facet mirror, which represents a first optical element after the collector mirror, which are low relative to a source power at a corresponding plasma position, indicate a loss of light on the collector mirror.

Particularly advantageous in a situation in which the field facet mirror represents the optical element to be examined is the fact that a very precise and very well reproducible measurement of the thermal load can be carried out directly from a thermal expansion of the plunger of the field facet mirror and without the attachment of further measurement technology.

In an advantageous development of the method according to the invention, it can be provided that the reflection property is determined by a ratio of a determined temperature change of the optical element to be examined to an expected and/or measured radiation intensity of the radiation, in particular EUV radiation.

As a result, the reflection property can advantageously be determined precisely, in particular since the temperature change can be caused by the thermal load.

It can be provided that the areal mapping or the two-dimensional measurement data of the ascertained discharge intensity on the basis of the areal mapping of the one or more ascertained thermal loads in the manner of a factorization tion is analyzed with regard to the light losses leading to the measured exit intensity. Such a procedure is based on the idea that the two-dimensional mapping of the exit intensity determined at the end of the optical system is caused by a multiplication of a series of light loss characteristics or reflection properties of the optical elements interacting with the radiation connected in series. Since only the product of the reflection properties is visible as the exit intensity, additional, preferably orthogonal, information about the individual reflection properties can advantageously be obtained for factoring. Such additional information can represent the areal mapping of the thermal load.

By determining the areal mapping of the thermal loads of the individual optical elements, the contributions of the reflection properties of the individual optical elements to the emergence of the measured exit intensity can be determined one after the other using a sequential method or an iterative process.

It can be provided that measurement data of the thermal load or thermal measurements are normalized with measurements of the exit intensity of the radiation, in particular the EUV radiation. Provision can be made here for a reconstruction of the exit intensity to be determined on the basis of radiation pupils in a first step.

It can also be provided that an irregularity or the non-uniformity of two-dimensional measurement datasets of the thermal load and the discharge intensity is evaluated. As a result, courses of a loss of light can be determined, for example.

It can also be provided that an absolute value or a uniformity of the measured values of the thermal load and the discharge intensity is used. A uniform loss of light can also be determined in this way. A uniform loss of light or a uniformly reduced reflection property or uniform offset can be caused, for example, by a homogeneous degradation of a mirror. As a result, the same radiation intensity can be lost through absorption in all regions of the mirror. A sample comparison of the areal maps of the discharge intensity and the thermal loads can therefore advantageously be supplemented by a comparison of the absolute values.

In an advantageous embodiment of the method, it can be provided that only measured values relating to the thermal load are used. Provision can be made here for changes in the measured values to be evaluated over time and/or by means of an, in particular relative, comparison to a design.

In a further advantageous embodiment of the method, it can be provided that the measured values of the thermal load are normalized, preferably to the values of the exit intensity. Such a method can be used over time, providing an opportunity for trend analysis.

It can be provided here that the two-dimensional distributions of the exit intensity or the light intensities and/or a pupil measured on a wafer plane are measured at one end of an optical column or of the optical system. The two-dimensional distributions of the discharge intensity measured in this way can be used to standardize the two-dimensional data sets of the measurements of the thermal values or thermal load.

For this purpose it is advantageous if it is assumed that any light loss takes place at locations where there are detectors or sensors for the thermal load. In particular, it can be assumed that the light loss takes place at one or more mirrors.

Provision can be made for the data sets of the exit intensity to be used to carry out a reconstruction. In this case, a group of mirrors at which, according to the assumption, any as yet unknown loss of light takes place, is systematically reduced in that an influence of individual mirrors in the group can be ruled out. To this end, it is advantageous if some of the mirrors in the group have thermal load sensors. The reconstructed radiation intensities at the respective mirrors can be used for the purpose of normalizing the thermal load data for said mirrors, which have thermal load measuring devices or thermal load sensors, or for the two-dimensional measurement data of these thermal load measuring devices or thermal load sensors.

Alternatively or additionally, it can be provided that an entry intensity is determined at the beginning of the optical column or the optical system. Such an entry intensity can indicate, for example, which total thermal load from the radiation source reaches the optical system. If this matches the measured thermal values, for example, then an influence of those mirrors in which the thermal load values correspond to the impinging radiation can be determined will be deleted from the group of mirrors with unknown reflection properties.

In an advantageous embodiment of the method, it can be provided that at least one reference data record of the optical system is taken into account. In this way, for example, the determined reflection properties can be checked.

It can be provided that the reference data record is a data record of the design of the optical system. Such a data set can be determined in particular by simulation. In particular, methods such as the finite element method or analytical methods can be used here. In particular, it can be provided that such simulation data records are determined in a time-resolved manner, in particular under a variable thermal load. Here, the individual optical elements can be assigned their respective contribution to the exit intensity by simulation and compared with the actually determined thermal load.

In particular, the normalization of the thermal load to assigned radiation intensity distributions can take place iteratively. In each iteration step, the assigned radiation intensity distribution can be determined more precisely and a renewed, more precise normalization can thus be carried out. Radiation intensity distributions on one or more further optical elements can then be inferred. A variation group formed from these optical elements can in turn be assigned a share in the emergence of the exit intensity by determining the thermal loads.

Provision can be made for the reference data record to be recorded within an optical system. Here, in particular, a referencing can be provided via standard ranges of optical properties which have a low tendency to degradation. In other words, this means that a recorded thermal load or the recorded escape intensities are compared with expected standard values at locations affected by little degradation.

Provision can be made for the reference data set to be determined from a pool of reference systems which have little or no light loss. Reference systems can, for example, represent new or previously little used optical systems with a low expected degradation of the optical elements or the reflection properties. In this case, the use of design values can advantageously be avoided.

In an advantageous embodiment of the method, it can be provided that a sensitivity of the method is maximized. As a result, the reflection properties can be determined particularly reliably.

A targeted determination of the thermal load can be provided. Such a targeted determination has the advantage of high sensitivity. For this purpose it can be provided that one or more thermal time constants of the optical system are initially determined without a thermal load and then for the same period of time with a constant thermal load that is as high as possible. Provision can be made here for the exit intensity to be determined at least once, in particular at the wafer level. Changes occurring in a time response function can be used here. In addition to this, an amplitude per unit power of a response function on the thermal load can be used.

Provision can be made for the thermal load to be measured during operation of the optical system or in-line.

For this purpose, it can be provided that an evaluation of the thermal load data is at least approximately determined by filtering, for example by means of filter algorithms or other post-processing methods, which is simulated similarly to the data of a targeted thermal load determination, as described above. Here, for example, a focus can be on trend analysis, preferably with similar workloads. This allows conclusions to be drawn about the reflection properties without requiring a deeper ad hoc understanding of the system. This is particularly advantageous when a thermal load that varies over time makes simple evaluation difficult during operation.

In an advantageous embodiment it can be provided that the reflection property is determined by introducing an additional variability of the radiation interacting with the optical element. Provision can be made here for switching different settings and/or using a reticle with specifically selected structures. Such specifically selected structures of the reticle can have closely spaced lines in an x-direction and/or a y-direction orthogonal to the x-direction. As a result, an illumination of a part of the mirror surface of the mirror can be changed in a targeted manner and the thermal load applied to it can thus possibly also be changed.

The invention also relates to a device for determining a reflection property of an optical element to be examined.

The device according to the invention is set up to determine a reflection property of an optical element to be examined, which has a thermal load measuring device, in an optical system with one or more optical elements by means of radiation emitted by a radiation source and guided and shaped through the optical system. According to the invention, a device is provided and set up to infer the reflection property of the optical element to be examined on the basis of a thermal load determined by the thermal load device.

The device according to the invention has the advantage that the thermal load measuring device provided can be used to determine the reflection property of the optical element to be examined without intervening in the course of the radiation. In this way, the optical properties of the optical system can advantageously be retained.

In this case, the thermal load measuring device can be designed in particular as a temperature sensor. Furthermore, it can be provided that the thermal load measuring device is at least partially arranged and/or formed directly on the optical element to be examined.

Furthermore, it can be provided that the thermal load measuring device is designed at least partially as a temperature sensor for detecting a temperature change in a cooling medium cooling the optical element and/or is arranged in the cooling medium.

Furthermore, it can be provided that the thermal load measuring device is set up at least partially for detecting temperature-induced changes in properties of the optical element. The actual temperature and thus the thermal load on the optical element can then be inferred from temperature-induced changes in properties of the optical element. The temperature-induced changes in properties of the optical element can be, for example, changes in color and/or bending and/or changes in a resistance, preferably an electrical resistance.

It is of particular advantage if the thermal load measuring device is arranged in such a way that there is no interaction with the radiation modulated by the optical system when used as intended. In particular, the thermal load measuring device can be arranged outside of a beam path of the radiation.

By detecting the thermal load by means of the thermal load measuring device on the optical element to be examined, a complex, indirect determination of the reflection property, for example by thermal loads of other optical elements measured upstream and/or downstream, can also be avoided.

In an advantageous development of the device according to the invention, it can be provided that the device compares the thermal load with a thermal reference value to be expected.

The device can advantageously compare the thermal load with an expected thermal reference value, for example by storing typical empirical thermal reference values in the device, the numerical value of which is compared by the device with the value determined by the thermal load measuring device.

If the thermal load measuring device determines, for example, a thermal load value which the device classifies as greater than the expected thermal comparison value, this can indicate that the reflection property of the optical element is reduced. A reduction in the reflection property of the optical element to be examined can occur, for example, in that the degree of absorption on the surface of the optical element is increased. Due to increased absorption of the radiation on the surface of the optical element to be examined, thermal energy is introduced into it. This increases the thermal load compared to an expected thermal reference value. This applies in particular if the thermal comparison value was determined for a desired degree of reflection, in particular one that is within given specifications.

In an advantageous further development of the device according to the invention, it can be provided that one is provided in order to determine an exit intensity of the radiation after it has exited the optical system.

To obtain additional information about the reflection property, it can be advantageous to determine the exit intensity of the radiation at the end of the optical system. The intensity of the radiation transmitted through the optical system is influenced by the reflection property of the optical element to be examined. A determination of the radiation intensity at the exit, i.e. the exit intensity, therefore allows an even more detailed determination of the reflection property of the optical element to be examined, especially in combination with the thermal load.

In an advantageous development of the device according to the invention, it can be provided that a source radiation measuring device is provided in order to determine a source intensity of the radiation from the radiation source before it enters the optical system.

In order to enable an even more detailed determination of the reflection property, it is advantageous to determine the source intensity of the radiation from the radiation source. For this purpose, a source radiation measuring device can advantageously be provided in front of the optical system. In particular, a comparison of the thermal load with a thermal load to be expected based on the measurement of the source intensity provides information about the reflection property of the optical element. The measurement of the source intensity is made possible by the source radiation measuring device and thus also indirectly the reliable determination of the thermal comparison value.

EUV sources in particular emit radiation in all spatial directions. As a result, the source radiation measuring device can advantageously be arranged in such a way that the entry of radiation into the optical system is not impaired by the source radiation measuring device, since it captures a different solid angle of the radiation than the optical system.

In an advantageous development of the device according to the invention, it can be provided that the device is set up to calculate a surface intensity of the To determine radiation on a surface of the optical element to be examined by means of a simulation.

A determination of the thermal reference value from the determined source intensity and from the optical properties of the part of the optical system upstream of the optical element to be examined, i.e. that part of the optical system which lies between the radiation source and the optical element to be examined, can advantageously be carried out by a Simulate the surface intensity on the optical element.

If the source intensity and the optical properties of the part of the optical system located between the source and the optical element to be examined are taken into account and, in particular, are used as the basis for a simulation, the simulation can calculate an expected surface intensity and thus an expected thermal load and thus an expected Determine thermal comparison value.

In this case, for example, the desired optical properties, in particular those properties which would be expected for optical elements designed according to their specifications, can be taken into account. In this case, the simulation results in a surface intensity to be expected in the case of a part of the optical system that is, for example, brand new and/or has a defect-free design and is unchanged compared to its specifications and is located upstream.

This occurs in particular when, for example, the upstream part of the optical system, in particular a collector mirror, has been introduced into the optical system in a new way and in particular with known specifications. With the onset of operation, degradation of the upstream part, in particular of the collector mirror, begins, and thus a deviation of the actual optical properties from the target optical properties.

Provision can also be made for the simulation to be based on the actual optical properties. For example, by measuring a thermal load on the optical elements of the upstream part of the optical system, knowledge of the reflection properties and/or other optical properties of the optical elements in the upstream part of the optical system can be gained. If this information is known, a surface intensity on the optical element to be examined can be determined by simulation and a thermal comparison value adapted to the degradation or the actual state of the upstream part of the optical system can thereby be determined.

With these measures, the reflection property of the optical system to be examined can advantageously be precisely determined, since an advantageously precise thermal comparison value can be determined.

In an advantageous development of the device according to the invention, it can be provided that the device is set up to determine the expected thermal comparison value from the surface intensity and/or to calculate the thermal to compare malload to exit intensity and/or to compare thermal load to exit intensity and surface intensity.

The device is advantageously set up to compare the various items of information, in particular the surface intensity, the thermal comparison value to be expected, the thermal load, the exit intensity and the surface intensity.

A comparison of the aforementioned variables can provide information about the reflection property of the optical element to be examined and/or about the optical properties of the upstream and/or downstream part of the optical system in different constellations.

In a first function, the device can be set up to determine the thermal comparison value from the surface intensity by using the intensity and the reflection and absorption properties of the optical element that correspond to the specifications to calculate thermal energy and/or thermal output introduced into the optical element and thus a to determine the expected thermal load that is present in the case of the specifications and thus the thermal comparison value.

In a second function, the device is set up to compare the actually measured and existing thermal load with the exit intensity. If, for example, the emergence intensity is unexpectedly low compared to the thermal load, this can indicate degradation and/or a reduction in the reflection properties of the part of the optical system located downstream.

In a third function, the device can be set up to compare the thermal load with the exit intensity and the surface intensity. This makes it possible to balance a radiation input and a radiation output of the optical element to be examined and that part of the optical system located downstream from the optical element to be examined.

In particular, the reflection property of the optical element to be examined can be determined by detecting the thermal load, whereby a performance of the downstream part of the optical system to be examined can be precisely determined by balancing the radiation input and the radiation output. For example, a high surface intensity and at the same time a very low thermal load in the presence of a low exit intensity can indicate that the radiation is lost in the part of the optical system located downstream, for example due to absorption processes.

Various sections of the optical system can be examined by a suitable combination of the above information, namely the surface intensity, the thermal load, the thermal comparison value, the exit intensity and the surface intensity.

In a fourth function, the device can be set up to determine the optical properties of the upstream part of the optical system from the surface intensity and the exit intensity and the thermal load.

For example, a reduced output intensity at a thermal load corresponding to the thermal reference value may indicate a reduced reflectance property of both the upstream part of the optical system and the optical element under test, provided that the downstream part of the optical system can be assumed to meet specifications and not is demoted. In this case, insufficient light transmission through the upstream part, which can be caused by a reduction in the reflection properties of the upstream part, is compensated for by increased absorption and thus a reduced reflection property of the optical element to be examined in such a way that the input of thermal energy into the optical element to be examined corresponds to the thermal reference value. Including the information about the exit intensity allows conclusions to be drawn about a degradation both of the optical element to be examined and of the upstream part of the optical system, in particular the collector mirror.

If the actual properties of the part located upstream correspond to the target properties, then a conclusion as to a performance of the radiation source is possible from the aforementioned consideration.

In an advantageous development of the device according to the invention, it can be provided that the device and/or the thermal load measuring device and/or the source radiation measuring device and/or the exiting radiation measuring device are set up to measure the surface intensity and/or the thermal comparison value and/or the exiting intensity and/or the source intensity and/or determine the reflectance property as areal mappings.

Areal detection of the aforementioned properties enables the examination of partial areas of the surface of the optical element to be examined and/or other elements of the optical system and/or the examination of an emission characteristic of the radiation source.

Furthermore, it can be provided that the thermal load measuring device is designed as a thermal imaging camera. The thermal imaging camera is preferably set up to record infrared radiation, preferably caused by the thermal load, emanating from the optical element being examined. In this way, the areal mapping of the thermal load can be recorded in a simple manner.

For areal mapping, it can be advantageous if the thermal load measuring device extends in a position-resolving manner, for example under an optically relevant surface of the optical element to be examined. The device can be set up to determine the surface intensity or the thermal comparison value by means of a simulation, also over an area and in particular with spatial resolution.

Furthermore, a spatially resolved determination of the exit radiation by the exit radiation measuring device can advantageously be carried out areally, in particular by means of areally configured radiation sensors, for example CCD sensors.

A planar determination of an emission characteristic of the radiation source by the source radiation measuring device also enables a planar determined source intensity to be included in the simulation, which also includes the reflection property of the subsequent upstream part of the optical system. In particular, it can advantageously be taken into account that the upstream part of the optical system leads to a change in the intensity distribution radiated into the optical system and thus to a characteristic surface intensity.

In order to process and analyze the acquired planar mappings, it is particularly advantageous if the device is set up to carry out matrix algebraic arithmetic operations.

In order to advantageously, reliably and precisely determine the functionality of the optical system, it can be provided that the method according to the invention is carried out in parallel and/or in succession on several, in particular also all, optical elements of the optical system that lie in the radiation path. The more optical elements of the optical system the method according to the invention is carried out on, the more precisely and with fewer assumptions the reflection properties of the examined optical elements and/or the other optical elements of the optical system can be determined.

Provision can also be made for the device according to the invention to be implemented on a plurality, in particular a majority or on all, of the optical elements of the optical system that lie in the radiation path. It is advantageous here if the device is set up to integrate the information recorded from, for example, all optical elements, in particular with regard to the thermal load.

The invention also relates to a computer program product with program code means to carry out a method according to the invention according to the above and following statements, when the program is executed on a device of an inventive device according to the above and following statements.

The device can be designed as a microprocessor. Instead of a microprocessor, any other device can also be provided for implementing the device, for example one or more arrangements of discrete electrical components on a printed circuit board, a programmable logic controller (PLC), an application specific integrated circuit (ASIC) or another programmable circuit, for example also a field programmable gate array (FPGA), a programmable logic array (PLA), and/or a commercial computer.

Execution of the computer program product on a device which is provided for operation and for checking the functionality in a projection exposure system and is already implemented in this according to the prior art is particularly advantageous.

The invention also relates to a lithography system, in particular a projection exposure system.

The lithography system according to the invention, in particular a projection exposure system for microlithography, comprises an illumination system which has a radiation source, illumination optics and projection optics, the illumination optics and/or the projection optics having at least one optical element.

The reflective property of at least one of the optical elements of the lithography system determined with the method according to the invention and/or using a device according to the invention.

Features that have been described in connection with one of the objects of the invention, namely given by the method according to the invention, the device according to the invention, the computer program product according to the invention and the lithography system according to the invention, can also be advantageously implemented for the other objects of the invention. Likewise, advantages that were mentioned in connection with one of the objects of the invention can also be understood in relation to the other objects of the invention.

In addition, it should be noted that terms such as "comprising", "having" or "with" do not exclude any other features or steps. Furthermore, terms such as "a" or "that" which indicate a singular number of steps or features do not exclude a plurality of features or steps - and vice versa.

In a puristic embodiment of the invention, however, it can also be provided that the features introduced in the invention with the terms “comprising”, “having” or “with” are listed exhaustively. Accordingly, one or more listings of features may be considered complete within the scope of the invention, e.g. considered for each claim. The invention can consist exclusively of the features mentioned in claim 1, for example.

It should be mentioned that designations such as “first” or “second” etc. are primarily used for reasons of distinguishing the respective device or method features and are not necessarily intended to indicate that features are mutually dependent or related to one another.

Exemplary embodiments of the invention are described in more detail below with reference to the drawing.

The figures each show preferred exemplary embodiments in which individual features of the present invention are shown in combination with one another. Features of an exemplary embodiment can also be implemented separately from the other features of the same exemplary embodiment and can accordingly easily be combined with features of other exemplary embodiments by a person skilled in the art to form further meaningful combinations and sub-combinations.

Elements with the same function are provided with the same reference symbols in the figures.

• 1 eine EUV-Projektionsbelichtungsanlage im Meridionalschnitt;
• 2 eine DUV-Projektionsbelichtungsanlage;
• 3 eine schematische Darstellung eines Ausführungsbeispiels der erfindungsgemäßen Vorrichtung;
• 4 eine blockdiagrammmäßige Darstellung eines Ausführungsbeispiels des erfindungsgemäßen Verfahrens;
• 5 eine blockdiagrammmäßige Darstellung eines weiteren Ausführungsbeispiels des erfindungsgemäßen Verfahrens;
• 6 eine blockdiagrammmäßige Darstellung eines weiteren Ausführungsbeispiels des erfindungsgemäßen Verfahrens;
• 7 eine schematische Darstellung flächiger Kartierungen;
• 8 eine exemplarische Messung einer Thermallast weiterer optischer Elemente; und
• 9 eine schematische Darstellung eines exemplarischen Feldfacettenspiegels, bei dem die Erfindung realisiert werden kann.

Show it:

• 1 an EUV projection exposure system in the meridional section;
• 2 a DUV projection exposure system;
• 3 a schematic representation of an embodiment of the device according to the invention;
• 4 a block diagram representation of an embodiment of the method according to the invention;
• 5 a block diagram representation of a further embodiment of the method according to the invention;
• 6 a block diagram representation of a further embodiment of the method according to the invention;
• 7 a schematic representation of two-dimensional mapping;
• 8th an exemplary measurement of a thermal load of further optical elements; and
• 9 a schematic representation of an exemplary field facet mirror in which the invention can be implemented.

The following are first with reference to 1 the essential components of an EUV projection exposure system 100 for microlithography are described by way of example. The description of the basic structure of the EUV projection exposure system 100 and its components should not be understood as limiting here.

In addition to a radiation source 102 , an illumination system 101 of the EUV projection exposure system 100 has illumination optics 103 for illuminating an object field 104 in an object plane 105 . In this case, a reticle 106 arranged in the object field 104 is exposed. The reticle 106 is held by a reticle holder 107 . The reticle holder 107 can be displaced via a reticle displacement drive 108, in particular in a scanning direction.

In 1 a Cartesian xyz coordinate system is drawn in for explanation. The x-direction runs perpendicularly into the plane of the drawing. The y-direction is horizontal and the z-direction is vertical. The scan direction is in 1 along the y-direction. The z-direction runs perpendicular to the object plane 105.

The EUV projection exposure system 100 includes projection optics 109. The projection optics 109 are used to image the object field 104 in an image field 110 in an image plane 111. The image plane 111 runs parallel to the object plane 105. Alternatively, there is also an angle other than 0° between the object plane 105 and the image plane 111 is possible.

A structure on the reticle 106 is imaged onto a light-sensitive layer of a wafer 112 arranged in the region of the image field 110 in the image plane 111. The wafer 112 is held by a wafer holder 113. The wafer holder 113 can be displaced via a wafer displacement drive 114, in particular along the y-direction. The displacement of the reticle 106 via the reticle displacement drive 108 on the one hand and the wafer 112 on the other hand via the wafer displacement drive 114 can be synchronized with one another.

The radiation source 102 is an EUV radiation source. The radiation source 102 emits in particular EUV radiation 115, which is also referred to below as useful radiation or illumination radiation. The useful radiation 115 has, in particular, a wavelength in the range between 5 nm and 30 nm a DPP (Gas Discharged Produced Plasma) source. It can also be a synchrotron-based radiation source. The radiation source 102 can be a free-electron laser (FEL).

The illumination radiation 115 emanating from the radiation source 102 is bundled by a collector 116 . The collector 116 can be a collector with one or more ellipsoidal and/or hyperboloidal reflection surfaces. The at least one reflection surface of the collector 116 can be used in grazing incidence ("Grazing Incidence", GI), i.e. with angles of incidence greater than 45°, or in normal incidence ("Normal Incidence", NI), i.e. with angles of incidence smaller than 45° of the illumination radiation 115 are applied. The collector 116 can be structured and/or coated on the one hand to optimize its reflectivity for the useful radiation 115 and on the other hand to suppress stray light.

After the collector 116, the illumination radiation 115 propagates through an intermediate focus in an intermediate focal plane 117. The intermediate focal plane 117 can represent a separation between a radiation source module, comprising the radiation source 102 and the collector 116, and the illumination optics 103.

The illumination optics 103 comprises a deflection mirror 118 and a first facet mirror 119 downstream of this in the beam path. The deflection mirror 118 can be a planar deflection mirror or alternatively a mirror with an effect that influences the beam beyond the pure deflection effect. Alternatively or additionally, the deflection mirror 118 can be designed as a spectral filter, which separates a useful light wavelength of the illumination radiation 115 from stray light of a different wavelength. If the first facet mirror 119 is arranged in a plane of the illumination optics 103 which is optically conjugate to the object plane 105 as a field plane, it is also referred to as a field facet mirror. The first facet mirror 119 includes a multiplicity of individual first facets 120, which are also referred to below as field facets. Of these facets 120 are in the 1 only a few shown as examples.

The first facets 120 can be embodied as macroscopic facets, in particular as rectangular facets or as facets with an arcuate or part-circular edge contour. The first facets 120 can be embodied as planar facets or alternatively as convexly or concavely curved facets.

Like for example from the DE 10 2008 009 600 A1 is known, the first facets 120 themselves can each also be composed of a multiplicity of individual mirrors, in particular a multiplicity of micromirrors. The first facet mirror 119 can be embodied in particular as a microelectromechanical system (MEMS system). For details refer to the DE 10 2008 009 600 A1 referred.

The illumination radiation 115 runs horizontally between the collector 116 and the deflection mirror 118, ie along the y-direction.

A second facet mirror 121 is arranged downstream of the first facet mirror 119 in the beam path of the illumination optics 103. If the second facet mirror 121 is arranged in a pupil plane of the illumination optics 103, it is also referred to as a pupil facet mirror. The second facet mirror 121 can also be arranged at a distance from a pupil plane of the illumination optics 103 . In this case, the combination of the first facet mirror 119 and the second facet mirror 121 is also referred to as a specular reflector. Specular reflectors are known from US 2006/0132747 A1 , the EP 1 614 008 B1 and the U.S. 6,573,978 .

The second facet mirror 121 includes a plurality of second facets 122. In the case of a pupil facet mirror, the second facets 122 are also referred to as pupil facets.

The second facets 122 can also be macroscopic facets, which can have round, rectangular or hexagonal borders, for example, or alternatively facets composed of micromirrors. In this regard, also on the DE 10 2008 009 600 A1 referred.

The second facets 122 can have plane or alternatively convexly or concavely curved reflection surfaces.

The illumination optics 103 thus forms a double-faceted system. This basic principle is also known as the "Fly's Eye Integrator".

It can be advantageous not to arrange the second facet mirror 121 exactly in a plane which is optically conjugate to a pupil plane of the projection optics 109 .

The individual first facets 120 are imaged in the object field 104 with the aid of the second facet mirror 121 . The second facet mirror 121 is the last beam-forming mirror or actually the last mirror for the illumination radiation 115 in the beam path in front of the object field 104.

In a further embodiment of the illumination optics 103 that is not shown, transmission optics can be arranged in the beam path between the second facet mirror 121 and the object field 104 , which particularly contribute to the imaging of the first facets 120 in the object field 104 . The transmission optics can have exactly one mirror, but alternatively also have two or more mirrors, which are arranged one behind the other in the beam path of the illumination optics 103 . The transmission optics can in particular comprise one or two mirrors for normal incidence (NI mirror, "normal incidence" mirror) and/or one or two mirrors for grazing incidence (GI mirror, "gracing incidence" mirror).

The illumination optics 103 has the version in which 1 shown, exactly three mirrors after the collector 116, namely the deflection mirror 118, the field facet mirror 119 and the pupil facet mirror 121.

In a further embodiment of the illumination optics 103, the deflection mirror 118 can also be omitted, so that the illumination optics 103 can then have exactly two mirrors downstream of the collector 116, namely the first facet mirror 119 and the second facet mirror 121.

The imaging of the first facets 120 by means of the second facets 122 or with the second facets 122 and transmission optics in the object plane 105 is generally only an approximate imaging.

The projection optics 109 includes a plurality of mirrors Mi, which are numbered consecutively according to their arrangement in the beam path of the EUV projection exposure system 100 .

At the in the 1 example shown, the projection optics 109 includes six mirrors M1 to M6. Alternatives with four, eight, ten, twelve or another number of mirrors Mi are also possible. The penultimate mirror M5 and the last mirror M6 each have a passage opening for the illumination radiation 115. The projection optics 109 are doubly obscured optics. The projection optics 109 has an image-side numerical aperture which is greater than 0.5 and which can also be greater than 0.6 and which can be 0.7 or 0.75, for example.

Reflection surfaces of the mirrors Mi can be designed as free-form surfaces without an axis of rotational symmetry. Alternatively, the reflection surfaces of the mirrors Mi can be designed as aspherical surfaces with exactly one axis of rotational symmetry of the reflection surface shape. Just like the mirrors of the illumination optics 103, the mirrors Mi can have highly reflective coatings for the illumination radiation 115. These coatings can be designed as multilayer coatings, in particular with alternating layers of molybdenum and silicon.

The projection optics 109 has a large object-image offset in the y-direction between a y-coordinate of a center of the object field 104 and a y-coordinate of the center of the image field 110. This object-image offset in the y-direction can be something like this be as large as a z-distance between the object plane 105 and the image plane 111.

The projection optics 109 can in particular be anamorphic. In particular, it has different image scales βx, βy in the x and y directions. The two image scales βx, βy of the projection optics 109 are preferably at (βx, βy)=(+/−0.25, +/-0.125). A positive image scale β means an image without image reversal. A negative sign for the imaging scale β means imaging with image inversion.

The projection optics 109 thus leads to a reduction in the ratio 4:1 in the x-direction, ie in the direction perpendicular to the scanning direction.

The projection optics 109 lead to a reduction of 8:1 in the y-direction, ie in the scanning direction.

Other imaging scales are also possible. Image scales with the same sign and absolutely the same in the x and y directions, for example with absolute values of 0.125 or 0.25, are also possible.

The number of intermediate image planes in the x-direction and in the y-direction in the beam path between the object field 104 and the image field 110 can be the same or, depending on the design of the projection optics 109, can be different. Examples of projection optics with different numbers of such intermediate images in the x and y directions are known from U.S. 2018/0074303 A1 .

In each case one of the pupil facets 122 is assigned to precisely one of the field facets 120 in order to form a respective illumination channel for illuminating the object field 104 . In this way, in particular, lighting can result according to Köhler's principle. The far field is broken down into a large number of object fields 104 with the aid of the field facets 120 . The field facets 120 generate a plurality of images of the intermediate focus on the pupil facets 122 assigned to them.

The field facets 120 are each imaged onto the reticle 106 by an associated pupil facet 122 in a superimposed manner in order to illuminate the object field 104 . In particular, the illumination of the object field 104 is as homogeneous as possible. It preferably has a uniformity error of less than 2%. Field uniformity can be achieved by superimposing different illumination channels.

The illumination of the entrance pupil of the projection optics 109 can be geometrically defined by an arrangement of the pupil facets. The intensity distribution in the entrance pupil of the projection optics 109 can be set by selecting the illumination channels, in particular the subset of the pupil facets that guide light. This intensity distribution is also referred to as an illumination setting.

A likewise preferred pupil uniformity in the area of defined illuminated sections of an illumination pupil of the illumination optics 103 can be achieved by redistributing the illumination channels.

Further aspects and details of the illumination of the object field 104 and in particular the entrance pupil of the projection optics 109 are described below.

The projection optics 109 can in particular have a homocentric entrance pupil. This can be accessible. It can also be inaccessible.

The entrance pupil of the projection optics 109 cannot regularly be illuminated exactly with the pupil facet mirror 121 . When imaging the projection optics 109, which telecentrically images the center of the pupil facet mirror 121 onto the wafer 112, the aperture rays often do not intersect at a single point. However, a surface can be found in which the distance between the aperture rays, which is determined in pairs, is minimal. This surface represents the entrance pupil or a surface conjugate to it in position space. In particular, this surface shows a finite curvature.

The projection optics 109 may have different positions of the entrance pupil for the tangential and for the sagittal beam path. In this case, an imaging element, in particular an optical component of the transmission optics, should be provided between the second facet mirror 121 and the reticle 106 . With the help of this optical component, the different positions of the tangential entrance pupil and the sagittal entrance pupil can be taken into account.

At the in the 1 In the arrangement of the components of the illumination optics 103 shown, the pupil facet mirror 121 is arranged in a surface conjugate to the entrance pupil of the projection optics 109 . The first field facet mirror 119 is arranged tilted to the object plane 105 . The first facet mirror 119 is tilted relative to an arrangement plane that is defined by the deflection mirror 118 .

The first facet mirror 119 is tilted relative to an arrangement plane that is defined by the second facet mirror 121 .

In 2 an exemplary DUV projection exposure system 200 is shown. The DUV projection exposure system 200 has an illumination system 201, a device known as a reticle stage 202 for receiving and precisely positioning a reticle 203, by means of which the later structures on a wafer 204 are determined, a wafer holder 205 for holding, moving and precisely positioning the wafer 204 and an imaging device, namely a project tion optics 206, with a plurality of optical elements, in particular lenses 207, which are held in a lens housing 209 of the projection optics 206 by means of sockets 208.

As an alternative or in addition to the lenses 207 shown, various refractive, diffractive and/or reflective optical elements, including mirrors, prisms, end plates and the like, can be provided.

The basic functional principle of the DUV projection exposure system 200 provides that the structures introduced into the reticle 203 are imaged onto the wafer 204 .

The illumination system 201 provides a projection beam 210 in the form of electromagnetic radiation that is required for imaging the reticle 203 onto the wafer 204 . A laser, a plasma source or the like can be used as the source for this radiation. The radiation is shaped in the illumination system 201 via optical elements in such a way that the projection beam 210 has the desired properties in terms of diameter, polarization, shape of the wave front and the like when it strikes the reticle 203 .

An image of the reticle 203 is generated by means of the projection beam 210 and transmitted to the wafer 204 in a correspondingly reduced size by the projection optics 206 . The reticle 203 and the wafer 204 can be moved synchronously, so that areas of the reticle 203 are imaged onto corresponding areas of the wafer 204 practically continuously during a so-called scanning process.

Optionally, an air gap between the last lens 207 and the wafer 204 can be replaced by a liquid medium that has a refractive index greater than 1.0. The liquid medium can be, for example, ultrapure water. Such a structure is also referred to as immersion lithography and has an increased photolithographic resolution.

The use of the invention is not limited to use in lithography systems and also not to use in projection exposure systems 100, 200, in particular also not with the structure described. Nevertheless, the invention is particularly suitable for lithography systems, in particular projection exposure systems, in particular with the structure described. Furthermore, the invention and the following exemplary embodiments are not to be understood as being restricted to a specific design. The following figures represent the invention only by way of example and in a highly schematic manner.

3 shows a schematic representation of a device 1 for determining a reflection property 2 (see FIG 7 ) of an optical element 3 to be examined, which has a thermal load measuring device 4 . The optical element 2 to be examined is part of an optical system 5 with one or more optical elements 3, 6. The reflection property 2 is determined by means of radiation emitted by a radiation source 7 and guided and shaped by the optical system 5. In this case, a device 8 is provided and set up, based on a thermal load 9 determined by the thermal load measuring device 4 (see 7 ) on the reflection property 2 (see 7 ) of the optical element 3 to be examined.

in the in 3 illustrated embodiment, an exit radiation measuring device 10tems 5 is provided to measure an exit intensity 11 (see 7 ) of the radiation after exiting the optical system 5.

Furthermore, in the in 3 illustrated embodiment, a source radiation measuring device 12 is provided in order to determine a source intensity of the radiation from the radiation source 7 before it enters the optical system 5 .

The device 8 is in the in 3 In the exemplary embodiment shown, a surface intensity 13 (see 7 ) of the radiation on a surface of the optical element 3 by means of a simulation.

Furthermore, in the in 3 illustrated embodiment, the device 8 is set up to determine an expected thermal comparison value from the surface intensity 13 and to compare the thermal load 9 with the exit intensity 11 and to compare the thermal load 9 with the exit intensity 11 and the surface intensity 13 .

Furthermore, in the in 3 In the illustrated embodiment, the device 8 and the thermal load measuring device 4 and the source radiation measuring device 12 and the exit radiation measuring device 10 are configured to determine the surface intensity 13 and the thermal reference value and the exit intensity 11 and the source intensity and the reflection property 2 and the thermal load 9 as a two-dimensional mapping.

The device 8 also compares the thermal load 9 with a thermal reference value to be expected.

4 shows a block diagram representation of an embodiment of a method for determining the reflection property 2 of the optical element 3 to be examined in the optical system 56, after which the reflection property 2 is determined using the radiation emitted by the radiation source 7 and guided through the optical system 5 and shaped. A thermal load 9 for the optical element 3 to be examined is determined in a thermal load block 14 and the determined thermal load 9 is used in a conclusion block 15 to conclude the reflection property 2 of the optical element 3 to be examined represented by the reflection property block 16 .

in the in 4 In the exemplary embodiment shown, the reflection property 2, represented by the reflection property block 16, of the optical element 3 to be examined is inferred by comparing the thermal load 9 with a thermal reference value to be expected. The thermal comparison value to be expected, represented by a thermal comparison value block 17, is included in the conclusion, represented by the conclusion block 15, in the determination of the reflection property 2. The involvement is in the in 4 illustrated embodiment illustrated by an arrow.

in the in 4 The exemplary embodiment of the method illustrated in the exemplary embodiment of the method to be expected is the thermal comparison value to be expected, represented by the thermal comparison value block 17, in that the surface intensity 13 of the radiation on the optical element 3 to be examined is determined by a source intensity 13 of the radiation source 7, represented by a source intensity block 18, of the radiation source 7 and by a simulation, represented by a simulation block 19, is determined taking into account optical properties of a part of the optical system 5 located upstream of the optical element 3 to be examined, represented by an upstream block 20.

Furthermore, in the in 4 illustrated embodiment, the method based on the determined thermal load 9, represented by the thermal load block 14, and the thermal comparison value, represented by the thermal comparison value block 17, on the reflection property 2, represented by the reflection property block 16, of the optical element 3 closed by the exit intensity 11, represented by an exit intensity block 21 which is compared to the thermal load 9 by the radiation guided and shaped by the optical system 5 .

Furthermore, in the in 4 illustrated embodiment of the method based on the determined reflection property 2 and the thermal load 9 of the optical element to be examined 3 and the exit intensity 11 on a reflection property of the upstream part, represented by the upstream block 20, of the optical system 5 closed. In addition, 5 a second conclusion block 22 is shown, in which the information about the determined reflection property 2 from the reflection property block 16 and the determined thermal load 9 from the thermal load block 14 and the exit intensity 11 from the exit intensity block 21 are integrated in order to conclude the reflection property 2 of the upstream block 20 .

Said integration of the aforesaid information is in the in 5 illustrated embodiment illustrated by arrows between the blocks.

in the in 3 In the method illustrated, the reflection property 2 is determined by a ratio of a determined temperature change of the optical element 3 to be examined to an expected and/or measured radiation intensity of the radiation, in particular EUV radiation.

in a 6 From the exemplary embodiment of the method shown, a reflection property of a part of the optical system 5 located downstream can also be inferred. Here, the optical properties of the part of the optical system 5 located downstream, represented by a downstream block 23, are taken into account when determining an exit intensity comparison value, represented by the exit intensity comparison value block 24. In a third conclusion block 25, the information is integrated and then the reflection property of the part of the optical system 5 located downstream is inferred.

7 shows an exemplary embodiment of the method, according to which the surface intensity 13 and the thermal load 9 and the exit intensity 11 and the reflection property 2 are determined as spatially resolved maps 26 .

In the illustrated embodiment, a two-dimensional mapping of the surface intensity 13 and a two-dimensional mapping 26 of the thermal load 9 and a two-dimensional mapping 26 of the exit intensity 11 are shown. The values of the aforesaid quantities are 27 at a first location and two at one th place 28 shown pictorially. A high value is represented here by a black coloring and a low value by a white coloring of an inner area of the pixels depicting the first location 27 and the second location 28 .

An assignment, in particular a mapping of the positions on the respective flat maps 26 to one another, or a transformation rule of position coordinates of the maps 26 to one another, in particular the positions of the first location 27, the second location 28 and a third location 29 on the flat maps 26, can in this case take place through elementary geometric considerations and/or through optical measurements, for example beam scanning.

From the illustrated flat maps 26 of the surface intensity 13, the thermal load 9 and the exit intensity 11, a flat map 26 of the reflection property 2 of the optical element 3 to be examined can be inferred.

From a comparison of the surface intensity 13, the thermal load 9 and the exit intensity 11, it can be concluded that the reflection property 2 is degraded at the first location 27, since only a low exit intensity 11 is determined at high surface intensity 13 and high thermal load 9.

It can be determined that the reflection property 2 at the second location 28 is undisturbed and undegraded, since a low thermal load and a high exit intensity can be determined with a high surface intensity.

At the third location 29 there is a high surface intensity 13 , a reduced thermal load 9 and a reduced exit intensity 11 . This can be interpreted, for example, as an indication of an intensity loss, which is caused by a degradation of the part of the optical system 5 lying downstream.

In the 7 The mapping 26 of the exit intensity 11 shown can be compared in the illustrated embodiment with the mapping 26 of the thermal load 9 by a mapping 26 of a cross correlation or a correlation coefficient of the mapping 26 of the exit intensity 11 with the mapping 26 of the thermal load 9 being determined.

in the in 3 illustrated embodiment of the device 1 according to the invention, the optical element 3 to be examined can be a facet mirror, in particular a field facet mirror 50 shown in more detail below (see 9 ), in particular a field facet mirror 119, 121 of a projection exposure system 100, or a mirror Mi of a projection optics 109, 206 or a collector mirror 116.

The invention is particularly suitable for use with optical elements that are mirrors of a projection exposure system 100, 200, in particular one of the 1 and 2 mirrors shown (116, 118, 119, 120, 121, 122, Mi, 207).

8th 10 shows an exemplary measurement of the thermal load of mirrors M1 to M6, which can be arranged in ascending order along the beam path in the projection optics 109. A temperature deviation dT from an initial temperature T0 is plotted on a vertical y-axis. The mirrors M1 to M6 are plotted on a horizontal x-axis. In this example, the T0 value forms the basis of the thermal comparison value.

In the present example, a temperature change dT results from the fact that a thermal load, which is above or below the thermal comparison value, is entered on the mirrors M1 to M6. If one understands the thermal load as heat output, which is applied by light absorbed at the mirrors M1 to M6, then an equilibrium temperature of the mirrors M1 to M6 results from the fact that a heat dissipation comes into equilibrium with the heat output introduced, with the heat dissipation in turn being influenced by the Temperature of the mirrors M1 to M6 depends.

For example, a higher temperature of the mirrors leads to an increased heat dissipation, which is due to a heat transport equation. An increased absorption of light and thus an increased heat output, which is applied to the mirror, will therefore initially lead to an increase in temperature until the heat dissipation in turn corresponds to the input heat output. From that point on, the temperature of the mirror will remain at this new, elevated, equilibrium temperature.

The difference between this new equilibrium temperature and the equilibrium temperature T0 to be expected, for example on the basis of a specification, is represented by the temperature difference dT.

in the in 8th In the exemplary measurement shown, no deviation from the thermal comparison value can be determined for a mirror M1, no deviation from the thermal comparison value can be determined for a mirror M2, an increase in the thermal load im for a mirror M3 compared to the thermal benchmark, for a mirror M4 a reduction in thermal load compared to the thermal benchmark is determined, for a mirror M5 a reduction in thermal load compared to the thermal benchmark, but an increased thermal load compared to the mirror M4, and a reduced thermal load compared to mirror M5 and a reduced thermal load compared to the comparative thermal value can be determined at mirror M6.

These observations may indicate that mirror M3 has degraded, reducing its reflectivity. As a consequence, the radiation forms a reduced intensity at the surface of the mirror M4, which leads to a reduced thermal load on the mirror M4. Despite a reduced thermal loading of the upstream mirror M4, the mirror M5 shows a reduced thermal loading to a lesser extent compared to the thermal benchmark.

This can be interpreted as an indication that the mirror M5 is also degraded, which is why a larger proportion of the intensity already reduced by the degradation of the mirror M3 is still absorbed on the mirror M5, as a result of which the mirror M5 heats up more than starting from the would be expected to heat up the mirror M4. This interpretation is supported by the fact that the mirror M6 following the mirror M5 has a further reduced thermal load and again a larger deviation of the thermal load from the thermal comparison value. It should be noted here that, as already described, an absolute value of a temperature deviation of a mirror, which is caused by an increased thermal load, depends on the size of the heat dissipation from the mirror.

In particular, it can be provided that the temperature deviation dT is determined using temperature sensors and mirror preheating devices (not shown) integrated in the mirrors M1, M2, M3, M5. It can be observed here that 1% degradation leads to a 0.1 K temperature change dT. The temperature change dT can be measured either with the mirror preheater switched off or by the power consumption of the mirror preheater.

In order to be able to compare the temperature differences of the mirrors M1 to M6, it must be ensured that the heat dissipation follows the same temperature dependence for all mirrors M1 to M6. For example, it would be less advantageous if the mirror M3 only allows a very small amount of heat to be dissipated, while the mirror M4 allows a very large amount of heat to be dissipated. In this case, only a very small increase in the thermal load in mirror M3 would lead to a large increase in temperature, while a reduction in the thermal load on mirror M4 would lead to a significantly smaller reduction in temperature.

In particular, it should be noted that in the in 8th In the exemplary measurement shown, the thermal loads are shown as one-dimensional values and not as maps. Therefore, comparison methods based on a spatially resolved characteristic of the thermal load are not applicable in this case.

9 1 shows, schematically and by way of example, the field facet mirror 50 in which the invention can be implemented. In an EUV projection exposure system, as in 1 shown, the field facet mirror 50, 119 is the first element in the light path of an illumination system for illuminating a reticle 106. This means that changes in the local distribution of the intensity of the light emanating from the radiation source 7 reach it almost immediately, without effects that could have an impact on other optical elements 6 in the light path, except for the deflection mirror 118, go back, are added.

Alternatively, the optical element 3 to be examined can also be the deflection mirror 118 which, due to its spatial proximity to the radiation source 7, can be exposed to an increased input of tin onto its surface.

The field facet mirror 50 comprises a plurality of movable, in particular displaceable mirror facets 51, 122, which are movably arranged by means of kinematics 52, for example flexure joints, on a carrier body 54 that can be cooled by cooling channels 58. By moving or tilting the mirror facets 51, a specific, case-related intensity distribution, a so-called setting, can be set for the following light path. As shown in the example shown, the desired movement can be achieved in that a magnetic force is exerted by means of electric actuator coils 57 on a permanent magnet 56 arranged on a rod-shaped extension 55 on the mirror facet 51, which is also referred to as a facet plunger. The actuator coils 57 are arranged in a carrier body 54 that can also be cooled by means of cooling channels 58 . The carrier body 54 has an opening 62 for each facet ram 55, through which a facet ram 55 engages.

Contactless actuation of the mirror facets 51 can be achieved by the arrangement shown. Also arranged on the carrier body 54 or on the actuator coils 57 are the Position sensors 60, by means of which the spatial relationship between their sensor targets 61 and the position sensors 60 and in particular also the distance between the position sensors 60 and the sensor targets 61, the so-called sensor gap or Z-gap, can be determined.

As already mentioned, the sensor targets 61 can be, for example, mirrors or reticle plates if optical sensors are used. In this case, the sensor targets 61 are arranged on the permanent magnets 56 of the facet tappets 55 .

It is advantageous if the sensor enables a three-dimensional position measurement. The value of the Z gap is included as a parameter in the determination of the position for the mirror facet 51 under consideration. How from the 9 recognizable, with a thermally induced change in length of the plunger 55, the value for the Z-gap also changes. Such a thermally induced change in length can be caused in particular by the fact that the local distribution of the radiation-induced thermal load on the mirror facet 51 changes. This change can result in particular from local changes in the reflection property 2 of the mirror facet 51 and from local contamination of this mirror facet 51—in this case the value for the Z gap would decrease due to the expansion of the plunger 55 caused thereby. This opens up the possibility of using the already existing position sensors 60 to detect newly occurring local contamination of the facet mirror 50 or newly occurring changes in the reflection properties 2 of the upstream part and /or to close the downstream part of the optical system 5. Furthermore, the known spatial relationship between individual mirror facets 51 and the locations on the reflecting surfaces of the optical elements of the optical system 5 can be used to determine where contamination has occurred on the optical elements of the optical system. Typical field facet mirrors 50 contain more than three hundred individual mirror facets 51, so that a high spatial resolution can be achieved in an advantageous manner.

in the in 9 In the illustrated exemplary embodiment of the optical element 3 to be examined as a field facet mirror 50, 119, 121, the thermal load 9 results from a change in geometry of at least part of the field facet mirror 50, 119, 121, preferably from a change in a distance (Z-gap) of a position sensor 60 from a sensor target 61 definitely.

reference list

1

contraption

2

reflection property

3

optical element to be examined

4

thermal load measuring device

5

optical system

6

optical element

7

radiation source

8th

furnishings

9

thermal load

10

exit radiation measuring device

11

Exit Intensity

12

source radiation measuring device

13

surface intensity

14

thermal load block

15

conclusion block

16

reflection property block

17

Thermal Comparator Block

18

source intensity block

19

simulation block

20

upstream block

21

Exit Intensity Block

22

second conclusion block

23

downstream block

24

exit intensity comparison value block

25

third conclusion block

26

areal mapping

27

first place

28

second place

29

third place

50

Facet mirror / field facet mirror

51

mirror facet

52

kinematics

54

carrier body

55

pestle

56

permanent magnet

57

Kitchen sink

58

cooling channel

60

position sensor

61

sensor target

62

breakthrough

100

EUV projection exposure system

101

lighting system

102

radiation source

103

lighting optics

104

object field

105

object level

106

reticle

107

reticle holder

108

reticle displacement drive

109

projection optics

110

image field

111

picture plane

112

wafers

113

wafer holder

114

Wafer displacement drive

115

EUV / useful / illumination radiation

116

collector

117

intermediate focal plane

118

deflection mirror

119

first facet mirror / field facet mirror

120

first facets / field facets

121

second facet mirror / pupil facet mirror

122

second facets / pupil facets

200

DUV projection exposure system

201

lighting system

202

reticle stage

203

reticle

204

wafers

205

wafer holder

206

projection optics

207

lens

208

version

209

lens body

210

projection beam

wed

mirror

QUOTES INCLUDED IN DESCRIPTION

This list of documents cited by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Patent Literature Cited

• WO 2019/063254 A1 [0012, 0014, 0045]
• DE 102008009600 A1 [0175, 0179]
• US 2006/0132747 A1 [0177]
• EP 1614008 B1 [0177]
• US6573978 [0177]
• US 2018/0074303 A1 [0196]

Claims (21)

Method for determining a reflection property (2) of an optical element (3) to be examined in an optical system (5) with one or more optical elements (3, 6), according to which the reflection property (2) is based on a radiation source (7) emitted radiation guided and shaped by the optical system (5) is determined, characterized in that a thermal load (9) is determined for the optical element (3) to be examined and based on the determined thermal load (9) on the reflection property (2) of the examined optical element (3) is closed. procedure after claim 1 , characterized in that conclusions are drawn about the reflection property (2) of the optical element (3) to be examined by comparing the thermal load (9) with a thermal reference value to be expected. procedure after claim 2 , characterized in that the thermal comparison value to be expected is determined in that a surface intensity (13) of the radiation on the optical element (3) to be examined is determined by a source intensity of the radiation source (7) and by a simulation taking into account optical properties of one of the to be examined optical element (3) is determined upstream part of the optical system (5). procedure after claim 2 or 3 , characterized in that the reflection property (2) of the optical element (3) to be examined is concluded on the basis of the determined thermal load (9) and the thermal comparison value, in that an exit intensity (11) of the radiation guided and shaped through the optical system (5). is compared with the thermal load (9). Procedure according to one of Claims 1 until 4 , characterized in that based on the determined reflection property (2) and / or the thermal load (9) of the optical element to be examined (3) and / or the exit intensity (11) on a reflection property (2) of the downstream part of the optical system ( 5) is closed. Procedure according to one of Claims 1 until 5 , characterized in that a reflection property (2) of the upstream part of the optical system ( 5) is closed. Procedure according to one of Claims 1 until 5 , characterized in that a part of the optical system (5) located upstream of the optical element (3) to be examined, in particular a collector mirror (116), is designed in such a way that its actual optical properties correspond to its target optical properties. Procedure according to one of Claims 1 until 7 , characterized in that the surface intensity (13) and/or the thermal comparison value and/or the thermal load (9) and/or the exit intensity (11) and/or the source intensity and/or the reflection property (2) as flat maps (20) be determined spatially resolved. procedure after claim 8 , characterized in that the mapping (26) of the exit intensity (11) is compared with the mapping (26) of the thermal load (9) by a mapping (26) of a cross correlation and/or a correlation coefficient of the mapping (26) of the exit intensity ( 11) is determined with the mapping of the thermal load (9). Procedure according to one of Claims 1 until 9 , characterized in that the optical element (3) to be examined is a facet mirror, in particular a field facet mirror (50, 119, 121), or a mirror (Mi) of projection optics (109, 206) or a collector mirror (116). Procedure according to one of Claims 1 until 10 , characterized in that the thermal load (9) is determined from a change in geometry of at least one part of the optical element (3) to be examined, preferably from a change in a distance (Z-gap) of a position sensor (60) from a sensor target (61). . Procedure according to one of Claims 1 until 11 , characterized in that the reflection property (2) is determined by a ratio of a determined temperature change of the optical element to be examined (3) to an expected and/or measured radiation intensity of the radiation, in particular EUV radiation. Device (1) for determining a reflection property (2) of an optical element (3) to be examined, which has a thermal load measuring device (4), in an optical system (5) with one or more optical elements (3, 6) by means of one Radiation emitted by the radiation source (7) and guided and shaped by the optical system (5), characterized in that a device (8) is provided and set up to infer the reflection property (2) of the optical element (3) to be examined using a thermal load (9) determined by the thermal load measuring device (4). Device (1) after Claim 13 , characterized in that the device (8) compares the thermal load (9) with an expected thermal reference value. Device (1) after Claim 13 or 14 , characterized in that an exit radiation measuring device (10) is provided in order to determine an exit intensity (11) of the radiation after it has exited the optical system (5). Device (1) according to one of Claims 13 until 15 , characterized in that a source radiation measuring device (12) is provided in order to determine a source intensity of the radiation from the radiation source (7) before it enters the optical system (5). Device (1) after Claim 16 , characterized in that the device (8) is set up to from the source intensity and from optical target properties and / or optical actual properties of the optical element to be examined (3) upstream part of the optical system (5). To determine surface intensity (13) of the radiation on a surface of the optical element to be examined (3) by means of a simulation. Device (1) after Claim 17 , characterized in that the device (8) is set up to determine the thermal reference value to be expected from the surface intensity (13) and/or to compare the thermal load (9) with the exit intensity (11) and/or to compare the thermal load ( 9) to be compared with the exit intensity (11) and the surface intensity (13). Device (1) according to one of Claims 13 until 18 , characterized in that the device (8) and/or the thermal load measuring device (4) and/or the source radiation measuring device (12) and/or the exit radiation measuring device (10) are set up to measure the surface intensity (13) and/or the thermal comparison value and/or or to determine the exit intensity (11) and/or the source intensity and/or the reflection property (2) and/or the thermal load (9) as flat maps (26). Computer program product with program code means to implement a method according to one of Claims 1 until 12 perform if the program on a device (8) of a device (1) according to one of Claims 13 until 19 is performed. Lithography system, in particular projection exposure system (100, 200) for microlithography, with an illumination system (101, 201) which has a radiation source (102), illumination optics (103) and projection optics (109, 206), the illumination optics (103) and/or the projection optics (109, 206) has at least one optical element (3, 6, 116, 118, 119, 120, 121, 122, Mi, 207), characterized in that a reflection property (2) of one of the optical elements (3, 6116, 118, 119, 120, 121, 122, Mi, 207) of the lithography system by a method according to any one of Claims 1 until 12 is determined and / or by means of a device (1) according to one of Claims 13 until 19 is determined.

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