Device and method for determining a transmission and / or reflection behavior of an optical element

Abstract

The invention relates to a device (15) for determining a transmission and / or reflection behavior of at least one optical element, in particular of mirrors (M1-M10) of a lens (7), in particular an EUV lens, having a plurality of optical elements, an image plane (9) and an object plane (5), the device having at least one light source (2) and having at least one sensor (16, 17), and having a used beam path (N). According to the invention, at least one optical test element (18, 19) of the device (15) is arranged or can be arranged in the object plane (5) or in the image plane (8), and the test element (18, 19) is designed such that it generates a test beam path (T1, T2) in the object plane (5) or the image plane (9), with light emitted by the light source (2) and influenced by the at least one optical element being guided along said test beam path to the at least one sensor (16, 17), which is located outside the used beam path (N).

Description

[0001] DESCRIPTION

[0002] Device and method for determining the transmission and / or reflection behavior of an optical element

[0003] This application claims priority over the German patent application with file number DE 10 2024 211 236.4, filed on November 22, 2024, the contents of which are hereby incorporated in full into the present application by reference.

[0004] The present invention relates to a device for determining the transmission and / or reflection behavior of at least one optical element of a lens comprising several optical elements, an image plane, an object plane and a useful beam path, in particular an EUV projection lens, with at least one light source and with at least one light sensor.

[0005] Furthermore, the present invention relates to a projection exposure system, in particular for a lithography system, comprising an optical element, an image plane, an object plane and a useful beam path, in particular an EUV projection lens, with at least one light source and with at least one light sensor and with the device mentioned above.

[0006] Furthermore, the invention relates to a method for determining the transmission and / or reflection behavior of at least one optical element of a lens comprising several optical elements, an image plane, an object plane and a useful beam path, in particular an EUV projection lens, and in particular with the device described above.

[0007] Devices and methods of the type mentioned above are known from the prior art. State-of-the-art lithography systems, especially microlithography systems, utilize projection lenses that incorporate a multitude of optical elements, such as mirrors, lenses, diffraction gratings, and / or elements, to project or image a pattern, particularly an electrical circuit, provided in the object plane of the lens, onto the image plane in a highly reduced form. Modern lithography systems operate with light in the EUV wavelength range, particularly 13.5 nm. Both the illumination system and the projection lens employ mirror systems whose high reflectivity directly influences the overall system transmission of the lens and thus affects the economically important light throughput through the lens.This reflectivity can be impaired by contamination during operation, as well as by aging and other drift phenomena over time. In particular, mirrors oriented perpendicular to the light path or useful beam path are regularly coated with layers of over 100 individual layers, in which changes in lifetime can occur, for example, due to diffusion or hydrogen incorporation. Hydrogen, together with the EUV light, combats the otherwise occurring carbon contamination via a photocleaning process and is therefore ubiquitous in such systems. Contamination of the mirror's reflective surface can lead to increased surface roughness and thus to a higher proportion of stray light in the reflected light, resulting in reduced reflectivity.

[0008] To prevent impairment or deterioration of the performance of such a lens or a lighting device, especially a projection exposure system incorporating such a lens, for example due to cleaning and / or replacement of individual optical elements, regular monitoring of the lens's and individual optical elements' reflection and transmission behavior is important. While measuring the light intensity in the useful beam path allows monitoring of the overall lens performance, it does not provide information about the performance of individual optical elements within the lens.

[0009] In the prior art, tomographic monitoring methods are proposed for this purpose. These methods utilize the wavefront measurement data, which are regularly generated in conjunction with the field-resolved pupil intensity distribution, to identify locally resolved reflection losses. While smaller contaminants and similar substances provide clearly distinguishable signals in these methods, it is inherently difficult to calculate long-wavelength reflection changes across the entire element surface, such as tilt and constant contributions. Due to the superimposed transmission effect, however, it is not possible to determine which optical element, for example, has suffered a uniform reflectivity loss due to a contamination layer.

[0010] The present invention is therefore based on the objective of providing an improved device and an improved method for selectively determining the transmission and / or reflection behavior of an optical element within a lens comprising several optical elements. The objective of the invention is achieved by a device with the features of claim 1. This device has the advantage that the reflection or transmission behavior of a single optical element of the lens can be monitored by simple means, whereby, in particular, the aim is not to determine an absolute value of the reflection behavior, but merely a change in the reflection behavior. This is sufficient for evaluating the selected optical element in order to, for example, initiate countermeasures such as replacing or cleaning the optical element.According to the invention, an optical test element of the device is arranged or can be arranged in the object plane and / or in the image plane, and the test element is designed such that, when located in the object plane or the image plane, it generates a test beam path through which the light from the light source, influenced by the at least one optical element, is directed to the at least one sensor located outside the useful beam path. Thus, the sensor is arranged outside the useful beam path of the lens to detect the reflection behavior. This does not impair the operating behavior of the lens and allows for an in-situ examination of the lens or the at least one optical element of the lens.The optical test element, positioned in the object plane or the image plane, redirects at least some of the light from the useful beam path into the test beam path, so that it strikes both the optical element under investigation and the sensor. By performing, for example, a reflection measurement using the test element and the sensor located outside the useful beam path at different times, the behavior of the optical element over time can be determined. Comparing the measurements taken at different times allows for the detection of changes in the reflection behavior, which might be characterized, for example, by the formation of a contamination layer.With the aid of the test element and the sensor, monitoring of the optical element is possible even during operation of the lens, i.e., without having to disassemble the lens into its individual components. The optical test element is characterized in particular by its shape, coating, and / or surface finish, which ensures that the light, or at least a portion of the light from the light source, is directed to the sensor along the useful beam path. Preferably, the light source is designed as the light source of a lighting device, in particular a projection exposure system, which includes the lens. Alternatively, the light source is separate from the light source of the lighting device. Most preferably, the lens is the lens of a lighting device, in particular a projection exposure system, more preferably a lithography system, which includes the lens and at least one controllable light source.

[0011] According to a preferred embodiment of the invention, the test element is designed such that it generates at least one further test beam path through which light influenced by the at least one optical element is directed to at least one further sensor, in particular one located outside the useful beam path. This means that the light influenced by the optical element is detected not by one, but by several sensors. This offers the advantage that, by comparing the measurement results of the sensors, it can be determined whether the optical element is actually impaired in its reflective behavior, or whether one of the sensors has been impaired over time. Thus, by comparing the measurement results, it is possible to ascertain whether both sensors are functioning correctly, and the measurement result of the respective sensor can be used accordingly.

[0012] It is further preferably provided that the sensor, or at least one additional sensor, has a protective element that can be opened or moved to create a light entry window for use of the sensor. This protective element protects the respective sensor from contamination and dirt when not in use. This ensures that the sensor is only exposed to dirt and contamination when the reflection or transmission behavior of at least one of the optical elements of the lens is being checked. This results in a long service life for the sensors used in this monitoring process. In particular, each protective element is associated with a controllable actuator which, when activated, removes the protective element from a sensor surface, opens a light entry window, or covers the sensor surface to protect it.

[0013] Furthermore, it is preferably provided that the sensor and / or at least one additional sensor is relocatable, so that it can be moved, for example, into the test beam path or the useful beam path of the lens as needed. This allows the respective sensor to be stored in a retracted, unused position, protected from contamination and / or dirt, and only moved into the useful beam path or the test beam path for monitoring purposes. This also offers the advantage that the sensor can be used to monitor not only the light intensity or dosage in the test beam path, but also that of the useful beam path. According to a preferred embodiment of the invention, a controllable actuator for relocating the sensor is assigned to the sensor or the at least one additional sensor. This enables automated relocation of the sensor as needed, particularly for monitoring purposes.

[0014] Furthermore, it is preferably provided that the same optical elements of the lens are located in the test beam path as in the other test beam path. This ensures that the measurement results of the two test beam paths are directly comparable. If the measurement results differ, it can be assumed that one of the sensors is impaired in its functionality, for example, because a layer of contamination or dirt has accumulated on the sensor surface.

[0015] According to a preferred embodiment of the invention, the test beam path differs from the other test beam path by means of an optical element, in particular only one optical element, preferably by means of the at least one optical element. In this case, the test beam paths only partially share the same optical elements of the lens. Rather, one of the test beam paths also passes through or over an optical element separate from the other test beam path, in particular over the at least one optical element whose reflection behavior is to be investigated. This advantageously allows the influence of the one optical element on the overall reflection and transmission behavior of the lens to be determined.

[0016] Furthermore, it is preferably provided that at least one additional optical element is located outside the useful beam path in the test beam path. This additional optical element outside the useful beam path advantageously allows the test beam path to be steered or guided in order to direct the test beam via a selected optical element to the respective sensor. Optionally, the at least one additional optical element is designed to be movable, so that the test beam path can also be pivoted during operation from one optical element under test to another. This allows, for example, the number of optical elements to be tested to be advantageously increased using fewer sensors.

[0017] Preferably, the test element is a mirror, a lens, a reticle, or a diffusing element. In particular, the test element is designed as an EUV test element so that it operates optimally in the EUV wavelength range. Most preferably, the sensor is permanently located outside the useful beam path. This means that the sensor plays no role in the operation of the lens and is largely protected from contamination during normal operation. Optionally, the sensor can also be repositioned so that it can be oriented towards different optical elements to detect the light from different test beam paths.

[0018] The method according to the invention, with the features of claim 11, is characterized in that the optical test element is placed in the object plane or in the image plane to generate a test beam path that directs the light from the light source, influenced by the at least one optical element, to the at least one sensor located outside the useful beam path, wherein, depending on the light detected by the sensor, a change in the transmission and / or reflection behavior of the optical element is determined at at least two different times. This results in the advantages already mentioned above.

[0019] According to a preferred embodiment of the method according to the invention, the test element directs the light influenced by the optical element into at least two test beam paths to at least two sensors, and the light detected by the sensors is compared to distinguish between a change in the transmission and / or reflection behavior of the optical element and a change in one of the sensors. This results in the advantages already mentioned above.

[0020] Preferably, the test element is introduced into the object plane or the image plane during a break in the lens's use, allowing in-situ monitoring or testing of at least one optical element. This ensures that projection operation of the lens is not affected.

[0021] Preferably, the light output of the light source is monitored and taken into account when determining the transmission and / or reflection behavior. This particularly prevents a change in the transmission and / or reflection behavior of the selected optical element from being erroneously attributed to the optical element, even though, for example, the light intensity of the light source has decreased. Preferably, at least one sensor is used to measure the light output. Furthermore, it is preferably provided that the test beam path differs from the other test beam path by one, and preferably only one, optical element, whereby the transmission and / or reflection behavior of this one optical element is determined by comparing the measurements. This results in the advantages already mentioned above.

[0022] Further advantages and preferred features and combinations of features will become apparent in particular from the foregoing and from the claims. The invention will now be explained in more detail with reference to the drawing. To this end, we show...

[0023] Figure 1 shows a simplified representation of a projection exposure system.

[0024] Figure 2 shows a meridional section of a lens of the projection exposure system with an advantageous device for monitoring its reflection and / or transmission behavior, and

[0025] Figure 3 shows a meridional section of the lens of the projection exposure system with an advantageous device for monitoring its reflection and / or transmission behavior according to a further embodiment.

[0026] Figure 1 shows a simplified representation of an advantageous projection exposure system 1 for microlithography, comprising a light source 2 for illumination light and imaging light 3. The basic structure of such a projection exposure system 1 is already known, for example, from DE 10 2019 202 759 A1. The light source 2 is, in particular, an EUV light source that generates light in a wavelength range of, in particular, 5 nm to 30 nm, especially between 5 nm and 15 nm. It is, in particular, a plasma-based light source, gas-discharge-generated plasma, or a synchrotron-based light source, such as a free electron laser light source. The useful beam path of the generated light is shown in simplified form by arrows in Figure 1.

[0027] An illumination optic 6 guides the illumination light 3 along the useful beam path from the light source 2 to an object field 4 in an object plane 5. The projection exposure system 1 further comprises a lens 7 or projection optic through which the object field 4 is projected onto an image field 8 in an image plane 9 at a predetermined reduction scale. The object field 4 and the image field 8 of the lens 7 can be curved and, in particular, partially annular. Alternatively, the object field 4 and image field 8 are rectangular.

[0028] The lens 7 images a section of a reticle 10, which can also be called a reflection mask, that coincides with the object field 4. The reticle 10 is supported by a reticle holder 10A. The reticle holder 10A can optionally be moved by a reticle displacement drive 10B.

[0029] The lens 7 projects an image onto the surface of a substrate 11 in the form of a wafer, which is supported by a substrate holder 12. The substrate holder 12 can be repositioned as needed by an optional substrate repositioning drive 12A.

[0030] Figure 1 schematically depicts a beam 13 of the illumination light 3 entering the reticulum 10 and the lens 7, and a beam 14 of the illumination light exiting the lens 7 and the substrate 11. The image-side numerical aperture (NA) of the projection optics 7 is not shown to scale in Figure 1. The projection exposure system 1 shown here is of the scanner type; both the reticulum 10 and the substrate 11 are scanned during operation of the projection exposure system. However, a stepper-type configuration is also possible.

[0031] Figure 2 shows the lens 7 in a simplified meridional section, where the lens 7 has several mirrors M1 to M10. Mirrors M1, M9, and M10 are designed for perpendicular or normal incidence of the ray, respectively, and mirrors M2 to M8 are designed for grazing incidence of the illumination light 3, i.e., for angles of incidence greater than 60 degrees. Mirrors M1 to M10 have a coating that optimizes their reflectivity for the imaging light 3, such as a ruthenium coating or a molybdenum-silicon layer stack, in particular with a top layer of ruthenium.

[0032] The projection exposure system 1 further comprises a device 15 which, according to the present embodiment, includes several sensors 16, 17 and optical test elements 8 and 19. The test element 18, which lies in the object plane 5 and, for example, replaces the reticulum 10, is designed to direct the light (not shown in Figure 2), provided in particular by the light source 2, by means of dedicated reflective and / or scattering properties, such that it reaches the sensor 17 after being reflected by at least one of the mirrors M1 to M10. For this purpose, the test element 18 is shaped, coated, and / or otherwise configured such that the light is directed towards the sensor 17 or other sensors outside the useful beam path. The sensor 17 lies outside a useful beam path N, which is simplified in Figure 2 by a solid line.The useful beam path N is the one that, in normal operation, leads from the reticle 10 to the image plane 9. The test element 18 now deflects the light from the useful beam path into a test beam path TI, as shown by further solid lines in Figure 2.

[0033] The test element 19 is arranged in the image plane 9, or image field 8. It reflects the light beam of the useful beam path N, which strikes the image plane 9, into a further test beam path T2 in such a way that the reflected light strikes the sensor 16 located outside the useful beam path N. In principle, only one of the test elements 18, 19 may be present, but additional test elements may also be present to deflect the light accordingly. As an alternative to using the light source 2, a further embodiment provides that the device 15 has its own controllable light source.

[0034] During a break in the operation of the projection exposure system 1, the test elements 18, 19 are placed one after the other or simultaneously into the object plane 5 or the image plane 9 and illuminated one after the other or simultaneously by the light source 2, so that light is thrown along the test beam paths TI and T2.

[0035] The light intensities or doses detected by sensors 16, 17 are advantageously stored. For this purpose, a control unit 20 is provided, which is connected to the sensors of the light source 2 to carry out the test procedure. At a later time, during an operational break of the projection exposure system 1, at least one of the test elements 18, 19 is again placed in the object plane 5 or image plane 9 in order to direct the light according to the test beam path TI or T2 to one of the sensors 16, 17. The light intensity is again detected and stored. Subsequently, the light intensity detected at the two times is compared to determine whether the reflectivity of, for example, the mirror Ml has changed over time, in particular whether it has deteriorated. This can occur as a result of contamination or soiling.This relative comparison thus allows conclusions to be drawn about the reflectivity of the mirrors involved in the test beam path. The radiation incident on the measuring reticle 10, or in the object plane 5 and the image plane 9, can be determined for referencing or calibration using conventional dose measurement methods.

[0036] Preferably, for monitoring the effectiveness of the affected mirrors, no absolute measurement of reflectivity is performed because a relative drop in reflectivity due to measurements taken at different times is sufficient to determine the reflectivity behavior of the affected mirrors. According to a further embodiment, redundancy through the use of multiple sensors 17, 16 allows for comparison to verify whether a transmission and / or reflection loss is actually caused by the single affected mirror M1, or whether, for example, one of the sensors 16, 17 has become contaminated. For this purpose, the test element 18 is designed, for example, such that the test beam path T is directed not only to one sensor 17 but to two sensors 17, 16 to enable a direct comparison, as shown by way of example in Figure 3.

[0037] In the case of beam path TI, the reflectivity of mirror M1 is directly measured because it is the only mirror in the test beam path TI. If several mirrors M1 to M10 are located in the test beam path TI or T2, the measured light dose corresponds to the state of all mirrors in the test beam path TI or T2. To measure the reflectivity of a specific mirror within a series of mirrors, the test element 18 is designed, for example, to generate two test beam paths that include the same mirrors except for one. Only in one of the test beam paths is there an additional mirror before the light reaches the corresponding sensor of that test beam path. This allows for a direct determination of how the reflectivity of this one mirror changes over time compared to the other mirrors.

[0038] Optionally, each sensor 17, 16 is assigned a protective element, for example in the form of a protective cover, which is designed such that a light entry window is opened at a specific test time. For this purpose, a controllable actuator is assigned to the protective element. During normal operation or during operating phases of the projection exposure system 1, the light entry window is closed, thus protecting the sensor from contamination during operation. Preferably, the respective test element 18, 19 is designed as a diffusing element, with the advantage that the light is provided universally for all sensor positions, thereby reducing the light efficiency. While test beam paths to all sensors 16, 17 can be implemented with one test element, the integration time for light detection is then longer because the overall light dosage is reduced.To project a higher proportion of the light along the respective test light path, a specially diffracting test element 18 or measuring reticle is preferably used, which directs or diffractes the light specifically in the desired directions.

[0039] A high dose of light can be supplied to selected sensors 16, 17 by means of a suitably shaped mirror as a test element 18. This is shown by way of example in Figure 2, in which a clear test light path through the test elements 18, 19 is ensured. The entire object and / or image field 5, 9 can be covered with such a reflective test element 18, 19, the shape of which is designed such that incident light is focused onto one of the sensors 16, 17. According to an alternative embodiment, a first and at least one second region are present in the object plane 5 and / or in the image plane 9, in which the shape of the test element 18, 19 is designed such that light is reflected via the first region to the first sensor 16 and via the second region to the second sensor 17.This reduces the total dose of light reaching both sensors 16, 17, but provides more information overall. The adapted shape of the test elements 18, 19 results in more efficient light utilization compared to scattering elements.

[0040] Preferably, the device 15 contains enough sensors and / or test elements or test element variants that, by replacing all test elements, all sensors can be exposed to light and all affected optical elements or mirrors of the projection exposure system 1 can be detected. In the case of a justified suspicion, for example due to previous changes or a particular incident that presumably affected only one or a few mirrors, a reduced number of measurements are preferably taken to specifically target the affected mirror.

[0041] Because the sensors 16, 17 are preferably located outside the useful beam path N, the reflection points are preferably designed to deviate from the useful field and / or the beam directions to deviate from the useful aperture. Accordingly, the useful field can be overexposed, although this is only possible to a limited extent due to the design of the illumination system. Because EUV systems operate in a vacuum, the mirrors are usually surrounded by enclosures, which are often provided with smooth surfaces to reduce contamination and run roughly parallel to the edge of the useful light volume. These enclosure surfaces can reflect over-aperture light back into the system with high efficiency, as can occur in grazing incidence, thereby transforming over-aperture light in the useful field into useful aperture light in the over-field. These rays are not stopped at the aperture diaphragm and can also reach subsequent sensors.Preferably, therefore, specially shaped and optionally coated elements are attached to such reflective points or integrated directly into the housing to effect further focusing on one of the sensors 16, 17.

[0042] Figure 3 shows, by way of example, a section of a projection exposure system 1 according to a further embodiment, in which the device 15 has additional reflective elements 21 that ensure that all the over-aperture light emanating from the reticle 10 or from the test element 18 is focused. The device in the embodiment of Figure 3 also includes, by way of example, a plurality of sensors 16 located outside the useful beam path N.

[0043] If it is ensured that all the over-aperture light emanating from the test element 18 is focused onto only one sensor 16, it is also guaranteed that these reflective areas on the housings do not impair the image quality of the projection exposure system 1 during normal operation. The over-aperture light generated by the test element 19 can also be taken into account. In principle, the test beam path can also be routed via one or more mirrors outside the useful aperture. If the mirrors located outside the useful aperture are automatically movable or activatable, multiple test paths can be implemented via these mirrors using different subsets of the mirrors M1 to MIO of the objective 7. Thus, by activating one of the mirrors located outside the useful aperture, the test beam path can be modified so that a different selection of mirrors is included in the test beam path.

[0044] Optionally, at least one of the sensors 16, 17 is designed to be pivotable or repositionable so that, if necessary, it can be swivelled into the useful beam path during the exposure intervals to determine the transmission and / or reflection behavior of the lens 7. Advantageously, the sensors are then illuminated by the useful beam path, so that plane mirrors in the object plane 5 and image plane 9 are sufficient as test elements 18, 19. For measuring the illumination system, the plane mirror is positioned perpendicularly in the illumination beam path from the reticulum plane, whereby slight variations in the angle of incidence can be compensated for by limited deviations from a perfectly flat shape. The aperture diaphragm does not interfere with this.

[0045] If only a portion of the surface of the affected mirror(s) is measured (M1 to MIO), measurement errors can occur if the mirror's reflectivity does not change homogeneously. Therefore, it is preferable to determine the spatial variation of the mirror's reflectivity using conventional methods, such as tomography. This allows for the correction of any deviations in the mirror's reflectivity and, in particular, enables the determination of the average reflectivity of the affected mirror.

[0046] REFERENCE MARK LIST

[0047] 1 Projection exposure system

[0048] 2 light sources

[0049] 3 Image light

[0050] 4 object field

[0051] 5 Object level

[0052] 6 Lighting optics

[0053] 7 Lens

[0054] 8 image field

[0055] 9 Image plane

[0056] 10 reticles

[0057] 11 substrates

[0058] 12 substrate holders

[0059] 13 beams

[0060] 14 beams

[0061] 15 Device

[0062] 16 Sensor

[0063] 17 Sensor

[0064] 18 Test element

[0065] 19 Test element

[0066] 20 Control unit

[0067] 21 Reflection element

Claims

REQUIREMENTS 1. Device (15) for determining the transmission and / or reflection behavior of at least one optical element, in particular a mirror (M1-MIO) of a lens (7) having several optical elements, an image plane (9) and an object plane (5), in particular an EUV lens, with at least one light source (2) and with at least one sensor (16, 17), and with a useful beam path (N), characterized in that at least one optical test element (18, 19) of the device (15) is arranged or can be arranged in the object plane (5) or in the image plane (8), and that the test element (18, 19) is designed such that it generates a test beam path (T1, T2) in the object plane (5) or the image plane (9), through which the light from the light source (2) influenced by the at least one optical element is directed to the at least one sensor (16, 17) located outside the useful beam path (N).

2. Device according to claim 1, characterized in that the test element (18,19) is designed such that it directs the light influenced by the at least one optical element in at least one further test beam path (T1,T2) to at least one further sensor (16,17), in particular located outside the useful beam path (N).

3. Device according to one of the preceding claims, characterized in that the sensor (16,17) or the at least one further sensor (16,17) has a protective element that can be opened or moved for use of the sensor (16,17).

4. Device according to one of the preceding claims, characterized in that the sensor (16,17) or the at least one further sensor (16,17) is relocatable, in particular relocatable into the useful beam path (N) and / or into the test beam path (TI, T2).

5. Device according to one of the preceding claims, characterized in that the same optical elements of the lens (7) are located in the test beam path (T1,T2) as in the further test beam path (T1,T2).

6. Device according to one of the preceding claims, characterized in that the test beam path (T1,T2) differs from the further test beam path (T1,T2) by an optical element, in particular by the at least one optical element.

7. Device according to one of the preceding claims, characterized in that at least one further optical element is located outside the useful beam path (N) in the test beam path (T1,T2).

8. Device according to one of the preceding claims, characterized in that the test element (18, 19) is a mirror, a lens, a reticulum or a scattering element.

9. Device according to one of the preceding claims, characterized in that the sensor (16,17) is permanently located outside the useful beam path (N).

10. Projection exposure system (1), in particular for a lithography system, comprising a lens (7) having several optical elements, in particular mirrors (M1-M10) or lenses, an image plane (9), an object plane (5) and a useful beam path (N), and a device (15) according to any one of claims 1 to 9.

11. Method for determining the transmission and / or reflection behavior of at least one optical element of a lens (7) comprising several optical elements, an image plane (9) and an object plane (5), in particular an EUV projection lens, with a device (15) according to one of claims 1 to 9, or a projection exposure system according to claim 10, characterized in that the optical test element (18, 19) is placed in the object plane (5) or the image plane (9) in order to guide the light of the light source (2) influenced by the at least one optical element in a test beam path (T1, T2) to the at least one sensor (16, 17) located outside the useful beam path (N), wherein, depending on the light detected by the sensor (16, 17), a change in the transmission and / or reflection behavior of the optical element is determined at at least two different times.

12. Method according to claim 11, characterized in that the test element (18, 19) directs the light influenced by the optical element in at least two test beam paths (T1, T2) to at least two sensors (16, 17), and that the light detected by the sensors (16, 17) is compared with each other in order to distinguish between a change in the transmission and / or reflection behavior and a change in one of the sensors (16, 17).

13. Method according to one of the preceding claims, characterized in that the test element (18,19) is introduced into the object plane (5) or the image plane (9) during a break in use of the lens (7).

14. Method according to one of the preceding claims, characterized in that a luminous flux of the light source (2) is monitored and taken into account when determining the transmission and / or reflection behavior.

15. Method according to one of the preceding claims, characterized in that the test beam path (T1,T2) differs from the further test beam path (T1,T2) by one, in particular only one, optical element, wherein the transmission and / or reflection behavior of this optical element is determined by comparing the measurements.

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