Test system for camera, mask inspection system, and method for testing camera

Abstract

To provide a test system for a camera, a mask inspection system, and a method for testing a camera, which are able to be used to estimate the longevity of cameras in a photomask inspection system.SOLUTION: A test system for a camera comprises a camera (23) and a first capture plate (101). The camera (23) comprises a camera housing (25) and a vacuum flange (31) formed on the camera housing (25). The vacuum flange (31) is adapted for attaching the camera (23) to a vacuum chamber (41). The camera housing (25) supports an image sensor (24). The image sensor (24) comprises an image sensor surface layer (28). The first capture plate (101) comprises a first capture plate surface layer (102). The first capture plate surface layer (102) corresponds to the image sensor surface layer (28). The invention also relates to a mask inspection system and to a method for testing a camera, in particular, an extreme ultraviolet (EUV) camera.SELECTED DRAWING: Figure 3

Description

Technical Field

[0001] The present invention relates to a test system for a camera, a mask inspection system, and a method for testing a camera. In particular, the camera can be an EUV camera.

Background Art

[0002] A photomask is used in a microlithography projection exposure system for manufacturing integrated circuits having particularly small structures. The photomask is illuminated with extreme ultraviolet (EUV) radiation of a very short wavelength and imaged onto a lithography object, so that the mask structure is transferred onto the lithography object.

[0003] In order to improve the quality of the image generated on the lithography object, it is necessary that the photomask be dimensionally accurate and not affected by impurities. It is known to inspect the photomask before operation or during an interruption of operation in a microlithography projection exposure system. For this purpose, a so-called aerial image of the photomask or a part of the photomask is taken, for which the photomask is projected not onto the lithography object but onto the EUV image sensor of a camera. Based on the image on the EUV image sensor, it is possible to evaluate whether there are defects or impurities in the photomask.

[0004] The inspection is usually performed in a photomask inspection system equipped with a vacuum housing, and a vacuum is applied inside the vacuum housing during the inspection. During the inspection, components of a camera such as an image sensor are exposed to the vacuum.

[0005] The image sensor includes an image sensor surface layer. The image sensor surface layer is a protective layer configured to protect the image sensor from the harmful effects of EUV light.

[0006] During the inspection process, EUV light degrades the surface of the camera, particularly the surface layer of the image sensor. The degradation causes particles to be released from the surface layer of the image sensor, which can lead to contamination (e.g., by carbon) of not only the image sensor itself but also other parts of the photomask inspection system such as the projection mirror.

[0007] To prevent this, the inspection system including the camera is purged with a purge gas such as hydrogen or helium. The purge helps prevent EUV-induced contaminants, but the purge gas also erodes the surface layer of components within the inspection system, such as the surface layer of the image sensor. The camera, particularly the image sensor, can be subject to the gas release effects caused by the vacuum and the purge gas.

[0008] Therefore, the camera, particularly the image sensor, is expected to gradually degrade over time due to exposure to EUV light and the purge gas. As a result, the image sensor's image performance deteriorates over time, and it needs to be replaced after a while to maintain the inspection performance of the inspection system.

[0009] The use of cameras in EUV inspection systems for photomasks is a recent development. Therefore, data on the long-term behavior of cameras used in photomask inspection systems is not yet available. However, it is desirable to be able to estimate the lifespan of cameras in such systems, for example, to estimate long-term operating costs. SUMMARY OF THE INVENTION

[0010] Accordingly, an object of the present invention is to provide a test system for a camera, a mask inspection system, and a method for testing a camera that can be used to estimate the lifespan of a camera in a photomask inspection system. This object is achieved by the features of the independent claims. Advantageous embodiments are defined in the dependent claims.

[0011] According to the present invention, a test system for a camera comprises a camera and a first capture plate. The camera has a camera housing and a vacuum flange formed on the camera housing. The vacuum flange is adapted to attach the camera to a vacuum chamber. The camera housing supports an image sensor. The image sensor comprises an image sensor surface layer, and the first capture plate comprises a first capture plate surface layer. The first capture plate surface layer corresponds to the image sensor surface layer.

[0012] The present invention is based on the idea that during the exposure phase, in a test system, a higher EUV intensity can be used for a shorter period of time, and during operation in a photomask inspection system, a test target camera can be exposed to a lower EUV intensity over a longer period of time, to predict the long-term behavior of the test target camera. In the evaluation phase, the surface of the components of the test system, for example, the first capture plate surface layer, can be analyzed.

[0013] The first capture plate can capture EUV contaminants such as particles and emissions generated by exposing the image sensor to EUV light in the test system. Since the first capture plate surface layer corresponds to the image sensor surface layer, the evaluation of the image sensor surface layer can be performed independently of the image sensor itself. In other words, the first capture plate has the advantage that the image sensor surface layer exists again separately from the image sensor in the test system (i.e., in the form of the first capture plate surface layer) and is exposed to the same test conditions (e.g., EUV light intensity and purge gas environment) of the test operation during the exposure phase. Therefore, the test system according to the present invention enables a combination of both the imaging performance evaluation of the image sensor and the independent surface analysis of the image sensor surface layer by using the camera and the first capture plate to analyze the first capture plate surface layer. By combining both, conclusions regarding the lifespan of the camera can be drawn.

[0014] The image sensor surface layer may comprise silicon and boron. In one embodiment, the image sensor surface layer is a layer of back-thinned silicon doped with boron implant. The first capture plate surface layer may comprise silicon and boron. In one embodiment, the first capture plate surface layer is a layer of back-thinned silicon doped with boron implant. The composition of the image sensor surface layer may be the same as the composition of the first capture plate surface layer. In certain embodiments, the entire first capture plate is made of a material corresponding to the material of the image sensor surface layer.

[0015] The camera can be an EUV camera. The image sensor is thus an EUV image sensor, i.e., an image sensor sensitive to EUV light. EUV light is light in the extreme ultraviolet spectral range with a wavelength from 5 nm to 100 nm, particularly from 5 nm to 30 nm. In particular, the EUV light can have a wavelength of 13.5 nm. The EUV camera can be adapted for use in a photomask inspection system where a photomask is projected onto the EUV image sensor of the EUV camera.

[0016] The test system may comprise an evaluation unit for reading and analyzing electrical signals from the image sensor. The electrical signals from the image sensor can be used to evaluate the imaging performance of the image sensor not only during the evaluation phase but also during the exposure phase. For this purpose, the EUV light intensity can be linked to the electrical signal output by the image sensor, thereby allowing conclusions to be drawn about the state of the image sensor surface layer. The evaluation unit may comprise a memory for storing the measurement signals from the image sensor. The measured values can thus be read from the memory later for evaluation. The evaluation may include providing a correlation between the signal from the image sensor and the corresponding data from the analysis data of the first capture plate surface layer.

[0017] The vacuum flange surrounds the illumination area inside or on the camera housing. The illumination area can be exposed to EUV light through the vacuum flange during the exposure phase. The image sensor and the first capture plate are disposed inside the illumination area. In other words, both the image sensor and the first capture plate can be exposed to EUV light through the vacuum flange during the exposure phase.

[0018] The test system may include a second capture plate. The second capture plate includes a second capture plate surface layer. The second capture plate surface layer corresponds to the material surface layer used in another part of the photomask inspection system. In particular, the second capture plate surface layer may correspond to the mirror surface layer. The mirror surface layer is the material surface layer used in the projection mirror of the photomask inspection system. The second capture plate surface layer may be a ruthenium layer. The second capture plate surface layer may also be a multilayer in which layers of molybdenum and silicon are alternately laminated.

[0019] The second capture plate can be disposed inside the illumination region together with the image sensor and the first capture plate. In other words, the second capture plate can also be exposed to EUV light via the vacuum flange during the exposure phase. Similar to the first capture plate, the second capture plate can capture EUV-induced contaminants such as particles and emissions generated by exposing the image sensor to EUV light in the test system. When the surface layer of the second capture plate corresponds to the mirror surface layer, it is possible to evaluate the long-term behavior of the projection mirror of the photomask inspection system. When the surface layer of the second capture plate corresponds to the material layer used in another part of the photomask inspection system, it is possible to evaluate the long-term behavior of each part of the photomask inspection system. Using the second capture plate, it is possible to collect supplementary information regarding the long-term behavior of the photomask inspection system, for example, through surface analysis of the surface layer of the second capture plate. In particular, when the surface layer of the second capture plate is a ruthenium layer, it is possible to collect supplementary information regarding the long-term behavior of the ruthenium-coated optics and mirrors used in the photomask inspection system. The second capture plate can be a substrate coated with a ruthenium layer.

[0020] The image sensor and the first capture plate are preferably disposed on a plane perpendicular to the incident axis of the EUV light incident on the illumination region. When the test system includes the second capture plate, the image sensor and the second capture plate are also preferably disposed on a plane perpendicular to the incident axis of the EUV light incident on the illumination region. By disposing the capture plate on the same plane as the image sensor, it is ensured that the image sensor and the capture plate are exposed to the same test conditions such as EUV light intensity.

[0021] The capture plate can be curved or flat.

[0022] The test system may comprise a vacuum chamber, to which a camera is attached by a vacuum flange. The vacuum chamber and the camera housing together form a vacuum housing. When a vacuum is applied to the vacuum housing, the camera housing may be subjected to a pressure difference between the interior of the camera housing and the external environment. The test system may comprise means for generating a vacuum within the vacuum housing. The image sensor and the capture plate may be exposed to the vacuum.

[0023] The structure of the test system may support a first capture plate and / or a second capture plate. In one embodiment, the camera housing supports the first capture plate and / or the second capture plate. In another embodiment, the vacuum chamber supports the first capture plate and / or the second capture plate.

[0024] During the evaluation phase, it is possible to perform surface analysis "in-situ" while the first and second capture plates are supported by the structure of the test system. In other embodiments, it may be desirable to remove the capture plate from the test system before performing the surface analysis. For this purpose, the capture plate, i.e., the first or second capture plate, may be supported by the structure of the test system by a removable connection for removing the capture plate from the test system. It is also possible for both the first and second capture plates to be supported by said removable connection. The removable connection may comprise a capture plate holder and / or an EUV-resistant adhesive. If the capture plate is removable, it can be removed from the test system before surface analysis of the capture plate surface layer. This is advantageous because the capture plate surface layer can be evaluated separately from the camera using surface analysis methods that would further damage or destroy the image sensor. For example, it is possible to evaluate an image sensor surface layer spatially separated from the actual image sensor. The surface analysis may include X-ray photoelectron spectroscopy (XPS) or other known surface analysis methods.

[0025] The test system may include an EUV light source. The EUV light source may be disposed within a vacuum housing and may have a line of sight to an illumination area. The line of sight may be a straight line or may be deflected, for example, by directing an EUV beam emitted from the EUV light source toward the illumination area via an illumination system composed of one or more projection mirrors. The illumination system may form EUV light into an EUV beam that illuminates the illumination area with uniform intensity. The EUV beam hitting the image sensor may have a diameter of at least 100 mm, preferably at least 300 mm. When the test system includes an EUV light source, the test system may include a control unit for controlling the illumination settings of the EUV light source, such as EUV light intensity or illumination pattern.

[0026] The test system may include a first gas inlet for introducing a purge gas into the vacuum housing. The purge gas can be introduced during the exposure phase to create a purge gas environment that similarly spreads during the operation of the photomask inspection system. Next, the camera can be tested in the purge gas environment. The purge gas can be hydrogen or helium.

[0027] The test system may include a second gas inlet for introducing a dilution gas. After the purge gas is introduced into the test system during the exposure phase, the dilution gas can be introduced into the vacuum housing during the cleaning phase. The dilution gas can be mixed with the purge gas to dilute the purge gas to a concentration that can be safely discharged. The dilution gas can be nitrogen or another inert gas, such as a noble gas.

[0028] The test system may include a purge gas reservoir connectable to the first gas inlet. The test system may include a dilution gas reservoir connectable to the second gas inlet.

[0029] The test system may comprise a first gas outlet for discharging gas from the test system, in particular from the vacuum housing. It is possible to supply the exhaust gas, in particular the exhaust gas mixture of the purge gas and the dilution gas, to a recycling system to reuse the purge gas and / or the dilution gas in the test system. For this purpose, the test system may comprise a recycling reservoir connected to the first gas outlet.

[0030] The test system may comprise a second gas outlet. The second gas outlet is useful for flushing the test system with a gas, such as extreme clean dry air (XCDA). In this context, flushing means, in particular during the cleaning phase, rapidly introducing and discharging a gas.

[0031] The test system may comprise a modular chamber forming part of the vacuum housing. The modular chamber may be attached to the vacuum flange of the camera housing. The gas inlet and / or the gas outlet may be components of the modular chamber. The modular chamber can be attached to the camera as a modular adapter for providing a purge gas and / or diluting the purge gas with a dilution gas. When the modular chamber is attached to the camera housing, the camera housing can still be attached to the vacuum chamber via the modular chamber, forming the vacuum housing. For this purpose, the modular chamber may comprise two vacuum flanges, a first vacuum flange for attaching to the camera housing and a second vacuum flange for attaching to the vacuum chamber. Thus, the modular chamber can be arranged between the camera housing and the vacuum chamber. The vacuum chamber, the camera housing, and the modular chamber then form a vacuum housing exposed to a vacuum, and EUV light is directed through the modular chamber towards the illumination area in the camera.

[0032] The test system may comprise a pressure sensor for monitoring the pressure inside the test system, in particular inside the vacuum housing.

[0033] The test system may comprise an adapter plate for providing lighting options. The adapter plate is arranged between the EUV light source and the image sensor in order to vary the incident EUV beam. The adapter plate may be arranged within the camera housing or may be attached to the camera housing. The adapter plate may also be arranged within the modulation chamber or within the vacuum chamber. The adapter plate may comprise an aperture, in particular a pinhole aperture. Only a certain portion of the EUV light reaches the image sensor through the pinhole aperture, thereby enabling the control of the intensity of the incident EUV light on the illumination area. Different sizes of pinhole apertures may be provided for use in the evaluation of the dynamic range of the image sensor. The pinhole aperture may be adjustable in size or a set of interchangeable pinhole apertures may be provided. It is also possible to provide adapter plates for dark field imaging and bright field imaging. Dark field imaging means the measurement of the dark current of the image sensor, i.e., the capture of so-called dark frames. This is achieved by blocking all incident EUV light from reaching the image sensor. Bright field imaging (sometimes also called flat field imaging) means illuminating the image sensor uniformly and with high intensity and measuring the dynamic range of the image sensor. Both the dark field image and the bright field image can be used to perform so-called flat field correction.

[0034] The test system may comprise two modulation chambers, one for the gas inlet and outlet and one for the adapter plate. It is also possible for the gas inlet and outlet and the adapter plate to be provided together within one modulation chamber.

[0035] The image sensor can be composed of a single sensor or a plurality of sub-sensors. The plurality of sub-sensors can be arranged in a matrix arrangement or a puzzle arrangement. The matrix arrangement means that the sub-sensors are arranged in a rectangle or a square in adjacent rows and columns. The puzzle arrangement means that adjacent sub-sensors are arranged with horizontal or vertical offsets from each other, and steps are formed within the arrangement.

[0036] In particular, when the sub-sensors are arranged in a puzzle arrangement, although not limited to this arrangement, due to the formed steps and symmetry constraints, not all parts of the image sensor are fully used for mask inspection in the photomask inspection system. In other words, not all parts of the image sensor are fully used for the imaging performance of the image sensor. Therefore, in one embodiment of the present invention, the unused part of the image sensor that is not used for mask inspection in the photomask inspection system can be used for energy monitoring of incident light, particularly EUV light. In other words, the image sensor can be provided with an energy monitoring part for monitoring the energy of incident light.

[0037] The calibrated photodiode can be arranged within the illumination area inside the camera housing. The image sensor and the photodiode are preferably arranged on a plane perpendicular to the incident axis of the EUV light incident on the illumination area. The signal from the calibrated photodiode can be used for calibration of the signal from the image sensor and for energy monitoring of the incident light.

[0038] The present invention also relates to a mask inspection system comprising a camera, a positioning device for a photomask, and a projection lens for imaging the photomask onto an image sensor of the camera. The camera has a camera housing and a vacuum flange formed on the camera housing. The vacuum flange is attached to a vacuum chamber of the mask inspection system. The camera housing supports an image sensor. The image sensor has an image sensor surface layer. The mask inspection system further comprises a first capture plate having a first capture plate surface layer. The first capture plate is adapted to capture EUV contaminants such as particles and emissions generated by exposing the image sensor to EUV light in a test system. The first capture plate surface layer corresponds to the image sensor surface layer. As a result, the composition of the image sensor surface layer is identical to the composition of the first capture plate surface layer.

[0039] The present invention also relates to a method for testing a camera, in which the camera comprises a camera housing and a vacuum flange formed on the camera housing. The vacuum flange is adapted to attach the camera to a vacuum chamber. The camera housing supports an image sensor. The image sensor comprises an image sensor surface layer, and the first capture plate comprises a first capture plate surface layer. The first capture plate surface layer corresponds to the image sensor surface layer. The method comprises a preparation phase in which the image sensor surface layer and the first capture plate surface layer are exposed to a vacuum, an exposure phase in which the image sensor surface layer and the first capture plate surface layer are exposed to EUV light, and an evaluation phase in which the first capture plate surface layer is inspected.

[0040] The vacuum is formed within a vacuum housing, and the image sensor and the first capture plate may be exposed to the vacuum. For this purpose, the camera housing may be attached to the vacuum chamber by the vacuum flange to form the vacuum housing.

[0041] A purge gas may be introduced into the test system so that the image sensor and the first capture plate are exposed to the purge gas.

[0042] The formation of a vacuum and the introduction of a purge gas can be performed in a preparation phase before an exposure phase.

[0043] During a cleaning phase, the purge gas can be mixed with a dilution gas. Thereafter, the gas mixture can be discharged from the test system.

[0044] During an evaluation phase, the first capture plate can be removed from the camera before the surface layer of the first capture plate is inspected.

[0045] If the test system comprises a second capture plate, the method steps applicable to the first capture plate can also be applied to the second capture plate.

[0046] The exposure phase can continue for at least one day, preferably at least one month, more preferably at least one year.

[0047] This disclosure encompasses the evolution of methods with features described in the context of a test system according to the present invention. This disclosure encompasses the evolution of test systems described in the context of a method according to the present invention.

[0048] The present invention will be described by way of example below using advantageous embodiments with reference to the accompanying drawings.

Brief Description of the Drawings

[0049]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

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Figure 7

Figure 8

Figure 9

Figure 10

Figure 11

Mode for Carrying Out the Invention

[0050] The photomask inspection system shown in FIG. 1 can be used to inspect the microlithography photomask 17.

[0051] The microlithography photomask 17 is generally intended to be used in a microlithography projection illumination system (not shown). In the microlithography projection illumination system, the photomask 17 is illuminated with extreme ultraviolet (EUV) radiation having a wavelength of, for example, 13.5 nm, and the pattern formed on the photomask 17 is imaged onto the surface of a lithography object in the form of a wafer. The wafer is coated with a photoresist that reacts to EUV radiation. The photomask inspection system is used to inspect whether the photomask conforms to the specifications and is free of impurities.

[0052] The photomask 17 may have an aspect ratio of from 1:1 to 1:3, preferably from 1:1 to 1:2, more preferably 1:1 or 1:2. The photomask 17 may have a substantially rectangular shape. The photomask 17 preferably has a length and width of from 5 to 7 inches (12.7 to 17.78 cm), more preferably a length and width of 6 inches (15.24 cm). Alternatively, the photomask 17 has a length of from 5 to 7 inches (12.7 to 17.78 cm), a width of from 10 to 14 inches (25.4 to 35.56 cm), preferably a length of 6 inches (15.24 cm) and a width of 12 inches (30.48 cm).

[0053] In the photomask inspection system, as shown in FIG. 1, the photomask 17 is arranged such that the EUV beam 15 emitted from the EUV light source 14 is directed towards the photomask 17 via the illumination system 16. The illumination system 16 is used to form the EUV radiation into a beam for illuminating the inspection field 20 on the surface of the photomask 17 with uniform intensity. The small inspection field 20 relative to the surface of the photomask 17 is shown in FIG. 2 not to scale. For example, the inspection field 20 may have dimensions of 0.5 mm × 0.8 mm. A field stop is arranged in the illumination system 16 to limit the illumination area to the inspection field 20 on the surface of the photomask 17. The X-Y stage 37 is used to move the photomask within the X-Y plane so that various inspection fields 20 can be brought within the range of the EUV beam path.

[0054] The EUV beam 15 reflected from the photomask 17 passes through the projection optical system 22 and reaches the EUV camera 23 equipped with the image sensor 24. The projection optical system forms the inspection field 20 of the photomask 17 on the image sensor 24 of the EUV camera 23. The EUV light source 14, the illumination system 16, the photomask 17, the projection optical system 22, and the EUV camera 23 are arranged in a vacuum housing 40 to which a negative pressure is applied during the operation of the inspection system. The EUV camera 23 includes a camera housing 25 in which the image sensor 24 is arranged. The rear part of the camera housing 25 protrudes from the vacuum housing 40 while the image sensor 24 is exposed to the vacuum inside the vacuum housing 40. Thus, the camera housing 25 forms a part of the vacuum housing 40 and is exposed to the same pressure difference as the other parts of the vacuum housing.

[0055] The EUV light source 14 is a plasma light source in which an EUV beam with a wavelength of 13.5 nm is emitted from a plasma. Tin or xenon is a medium that can be used to generate a plasma suitable for emitting such an EUV beam. To generate the plasma, droplets of the medium can be exposed to a laser beam.

[0056] Not only the mirrors of the illumination system 16 but also the mirrors of the projection optical system 22 are designed as EUV mirrors having a particularly high reflectivity for the EUV beam. The optical surface of the EUV mirror can be formed by a high-reflection coating. This can be a multilayer coating, particularly a multilayer coating having alternating layers of molybdenum and silicon. With such a coating, about 70% of the incident EUV beam can be reflected. In the case of so-called grazing-incidence mirrors, the optical surface can be formed by a ruthenium layer.

[0057] The projection optical system 22 has a magnification exceeding 100 times. To completely capture the image generated by the inspection field 20 of the photomask 17, the area of the image sensor 24 is larger than the area of the inspection field 20 according to the magnification. For example, the image sensor 24 can have dimensions of about 100 mm to 200 mm.

[0058] During the inspection process, EUV light 15 deteriorates the surface of the image sensor 24, and particles from the image sensor surface layer not only contaminate the image sensor 24 itself but also contaminate other parts of the photomask inspection systems 16, 22. To address this, the vacuum housing 40 is purged with hydrogen as the purge gas. The purge gas helps prevent EUV-induced contaminants, but it also erodes components within the inspection system, such as the surface layer of the image sensor 24. The image sensor 24 is subject to the gas release effects caused by the vacuum and the purge gas within the vacuum housing 40.

[0059] Figures 3 and 4 show a test system according to the present invention for testing the EUV camera 23. The test system includes an EUV camera 23, a flat first capture plate 101, and a flat second capture plate 201. The EUV camera 23 has a camera housing 25 and a vacuum flange 31 formed on the camera housing 25. The vacuum flange 31 is adapted to attach the EUV camera 23 to a vacuum chamber 41. The camera housing 25 supports the image sensor 24, the first capture plate 101, and the second capture plate 201. Since the camera 23 is an EUV camera, the image sensor 24 is sensitive to EUV light.

[0060] The image sensor 24 includes an image sensor surface layer 28, the first capture plate includes a first capture plate surface layer 102, and the second capture plate includes a second capture plate surface layer 202. The first capture plate surface layer 102 corresponds to the image sensor surface layer 28. Both the image sensor surface layer 28 and the first capture plate surface layer 102 have a thinned back surface and are boron-implanted silicon. The second capture plate surface layer 202 is a ruthenium layer applied to a substrate and corresponds to the material surface layer used in the mirror surface layer, i.e., the projection mirror of the projection optical system 22 in the inspection system according to FIG. 1.

[0061] During the exposure phase, a higher EUV intensity than in the inspection system (Figure 1) is used in the test system for a shorter period to predict the long-term behavior of the camera. The exposure phase is shorter than the lifetime of the camera in the inspection system, but can still last for more than a year.

[0062] In the embodiments illustrated in FIGS. 3 and 4, the first capture plate 101 and the second capture plate 201 capture EUV contaminants such as particles and emissions generated by exposing the image sensor 24 to EUV light. The first capture plate surface layer 102 corresponds to the image sensor surface layer 28, so the image sensor surface layer 28 appears twice within the test system. Thus, the image sensor surface layer 28 can be evaluated independently of the image sensor 24 itself by the first capture plate 101.

[0063] The test system includes an evaluation unit 27 for reading and analyzing the electrical signals from the image sensor 24. For this purpose, the evaluation unit 27 is electrically connected to the image sensor 24.

[0064] The vacuum flange 31 surrounds the illumination area 32 on the camera housing 25. During the exposure phase, the illumination area 32 is exposed to EUV light. The image sensor 24 and both the first and second capture plates 101, 201 are arranged inside the illumination area 32 and are thus exposed to EUV light during the exposure phase.

[0065] The image sensor 24 and both the first and second capture plates 101, 201 are arranged on a plane 33 perpendicular to the incident axis of the EUV light incident on the illumination area 32. Thus, not only the image sensor surface layer 28, but also the first and second capture plate surface layers 102, 202 are exposed to the same EUV light intensity.

[0066] The first and second capture plates 101, 201 are supported by the camera housing by a removable connection. In the illustrated embodiment, the removable connection is an EUV-resistant adhesive.

[0067] During the evaluation phase, the EUV light is not active and thus the illumination area is not exposed to the EUV light. The first and second capture plates 101, 201 are removed from the camera housing 25 by loosening the adhered connection. Thereafter, the surface layers 102, 202 of the first and second capture plates are analyzed separately from the camera 23 using XPS, i.e., surface analysis method.

[0068] As shown in FIG. 5, according to one embodiment, the test system includes a vacuum chamber 41, and the camera 23 is attached to the vacuum chamber 41 by a vacuum flange 31. The vacuum chamber 41 and the camera housing 25 together form a vacuum housing 40. The vacuum chamber includes means for generating a vacuum inside the vacuum housing 40. Not only the image sensor 24 but also both capture plates 101, 201 are exposed to the vacuum inside the vacuum housing 40, i.e., there is a pressure difference between the camera housing 25 and the external environment. Inside the vacuum housing 40, an EUV light source 14 is disposed. The EUV light source 14 has a line of sight to the illumination area 32, and this line of sight is deflected by a plurality of projection mirrors (not shown) also disposed inside the vacuum chamber 41. The EUV light reaching the illumination area uniformly illuminates the illumination area 32.

[0069] FIG. 6 illustrates another embodiment of the test system. A modular chamber 51 is disposed between the camera 23 and the vacuum chamber 41. The modular chamber 51 is attached to the vacuum chamber 41 by a vacuum flange 52, and the camera housing 25 is attached to the modular chamber 51 by a vacuum flange 31. The vacuum chamber 41, the modular chamber 51, and the camera housing 25 form a vacuum housing 40, i.e., there is a pressure difference with respect to the external environment between the modular chamber 51 and the camera housing 25. The EUV light emitted from the EUV light source 14 is directed through the modular chamber 51 towards the camera 23.

[0070] A first gas inlet 53 is arranged in the modulation chamber 51. During the exposure phase, hydrogen is introduced as a purge gas into the vacuum housing 40 through the modulation chamber 51 to form a purge gas environment. The camera 23 is exposed to a purge gas environment similar to that in the inspection system according to FIG. 1.

[0071] A second gas inlet 54 for introducing nitrogen as a dilution gas into the vacuum housing 40 during the cleaning phase is arranged in the modulation chamber 51. Nitrogen dilutes the hydrogen inside the vacuum housing 40 to a concentration at which it can be safely discharged.

[0072] First and second gas outlets 55, 56 are arranged in the modulation chamber 51 to discharge gas from the vacuum housing 40. In one step during the cleaning phase, the vacuum housing 40 is flashed with XCDA by rapidly introducing and discharging gas through all the gas inlets and gas outlets.

[0073] The adapter plate 57 is arranged between the modulation chamber 51 and the vacuum chamber 41, that is, between the EUV light source 14 and the image sensor 24 on the camera housing 25. The adapter plate 57 is provided with a pinhole aperture, that is, only a certain portion of the EUV light reaches the image sensor 24 through the adapter plate 57.

[0074] According to the embodiment illustrated in FIG. 7, the image sensor 24 consists of a single sensor 60. FIG. 8 illustrates an embodiment in which the image sensor 24 consists of a plurality of sub-sensors 61 arranged in a matrix arrangement. FIG. 9 illustrates an embodiment in which the image sensor 24 consists of a plurality of sub-sensors 62 arranged in a puzzle arrangement. The sub-sensors 62 are arranged such that adjacent sub-sensors 62 are offset from each other in a perpendicular direction, and a step 63 is formed within the arrangement. FIG. 10 illustrates the puzzle arrangement according to FIG. 9 and shows the active area 71 of the image sensor 24 that is actually used for imaging performance. The portion 70 outside the active area 71 does not contribute to the imaging performance but is used to monitor the energy of the incident EUV light. The portion 70 is called an energy monitoring portion.

[0075] FIG. 11 illustrates another embodiment of a test system in which a photodiode 80 is arranged in the illumination area 32 near the image sensor 24. The photodiode 80 is arranged on the plane 33 (FIG. 4) so as to be exposed to the same EUV light intensity as the image sensor 24 and the capture plates 101, 201. The photodiode 80 is calibrated before being arranged in the camera 23. Therefore, the signal from the calibrated photodiode 80 calibrates the signal from the image sensor 24 and is used to monitor the incident EUV light intensity.

Explanation of Reference Numerals

[0076] 14 Light source 15 Light, beam 16 Illumination system, photomask inspection system 17 Photomask 20 Inspection field 22 Projection lens, projection optical system 23 Camera 24 Image sensor 25 Camera housing 26 Positioning device 27 Evaluation unit 28 Image sensor surface layer 31 Vacuum flange 32 Illumination area 33 Plane 37 Positioner 40 Vacuum Housing 41 Vacuum Chamber 51 Modulation Chamber 52 Vacuum Flange 53 First Gas Inlet 54 Second Gas Inlet 55 First Gas Outlet 56 Second Gas Outlet 57 Adapter Plate 60 Sensor 61 Sub - sensor 62 Sub - sensor 63 Step 70 Outer Part 71 Active Region 80 Calibrated Photodiode 101 First Capture Plate 102 First Capture Plate Surface Layer 201 Second Capture Plate 202 Second Capture Plate Surface Layer

Claims

1. A test system for a camera comprising a camera (23) and a first capture plate (101), the camera (23) having a camera housing (25) and a vacuum flange (31) formed on the camera housing (25), the vacuum flange (31) adapted for mounting the camera (23) to a vacuum chamber (41), the camera housing (25) supporting an image sensor (24), the image sensor (24) comprising an image sensor surface layer (28), the first capture plate (101) comprising a first capture plate surface layer (102), the first capture plate surface layer (102) corresponding to the image sensor surface layer (28).

2. The test system of claim 1 , wherein the camera housing (25) supports the first capture plate (101).

3. 3. A test system according to claim 1 or 2, comprising a second capture plate (201), said second capture plate (201) comprising a second capture plate surface layer (202).

4. The test system of claim 3 , wherein the second capture plate surface layer (202) is a ruthenium layer.

5. 5. The test system of claim 3 or 4, wherein the camera housing (25) supports the second capture plate (201).

6. Test system according to any one of the preceding claims, comprising an evaluation unit (27) for reading signals from the image sensor (24).

7. 7. The test system of claim 1, further comprising a vacuum chamber (41), the camera housing (25) and the vacuum chamber (41) forming part of a vacuum housing (40), and a pressure differential being provided between the inside of the vacuum housing (40) and an external environment.

8. 8. The test system of claim 7, further comprising an EUV light source (14) disposed inside the vacuum housing (40) and having a line of sight to an illumination area (32) for exposing the image sensor (24) and the first capture plate (101) to EUV light.

9. 9. The test system of claim 1, further comprising a first gas inlet (53) for introducing a purge gas and exposing the image sensor (24) and the first capture plate (101) to the purge gas, and a first gas outlet (55) for exhausting the gas.

10. 10. The test system of claim 9, wherein the purge gas is hydrogen and / or helium.

11. 11. A test system according to claim 9 or 10, comprising a second gas inlet (54) for introducing a diluent gas, said diluent gas being nitrogen or an inert gas.

12. A test system according to any one of claims 7 to 11, comprising a modular chamber (51) arranged between the vacuum chamber (41) and the camera housing (25) and forming part of the vacuum housing (40).

13. 13. The test system of claim 12, wherein the first gas inlet (53) and the second gas inlet (54) are disposed within the modular chamber (51).

14. 14. A test system according to any one of claims 8 to 13, comprising an adapter plate (57) for providing illumination options, said adapter plate (57) comprising a pinhole aperture or adapted for dark field imaging or adapted for white field imaging.

15. The test system according to any one of claims 1 to 14, wherein the image sensor (24) consists of a single sensor (60), or of multiple sub-sensors (61) in a matrix arrangement, or of multiple sub-sensors (62) in a puzzle arrangement.

16. A test system according to any one of claims 6 to 15, comprising a calibrated photodiode (80) for calibrating the signal from the image sensor (24).

17. The test system according to any one of the preceding claims, wherein the camera (23) is an EUV camera.

18. A test system as described in any one of claims 1 to 17, wherein the first capture plate surface layer (102) corresponds to the image sensor surface layer (28), such that the composition of the image sensor surface layer (28) is identical to the composition of the first capture plate surface layer (102).

19. A mask inspection system comprising a camera (23), a positioning device (26) for a photomask (17), and a projection lens (22) for imaging the photomask (17) onto an image sensor (24) of the camera (23), the camera (23) having a camera housing (25) and a vacuum flange (31) formed on the camera housing (25), the vacuum flange (31) being attached to a vacuum chamber (41) of the mask inspection system, the camera housing (25) supporting an image sensor (24), the image sensor (24) having an image sensor surface layer (28). and wherein the mask inspection system further comprises a first capture plate (101), the first capture plate (101) having a first capture plate surface layer (102), the first capture plate (101) adapted to capture EUV induced contaminants, such as particles and emissions, resulting from exposing the image sensor (24) to EUV light in a test system, the first capture plate surface layer (102) corresponding to the image sensor surface layer (28), such that a composition of the image sensor surface layer (28) is identical to a composition of the first capture plate surface layer (102).

20. A method for testing a camera, the camera (23) comprising a camera housing (25) and a vacuum flange (31) formed on the camera housing, the vacuum flange (31) adapted for mounting the camera (23) to a vacuum chamber (41), the camera housing (25) supporting an image sensor (24), the image sensor (24) comprising an image sensor surface layer (28), a first capture plate (101) for mounting the first capture plate (101) to a vacuum chamber (41), 13. A method for testing a camera comprising: a first capture plate surface layer (102) corresponding to the image sensor surface layer (28); in a preparation phase, the image sensor surface layer (28) and the first capture plate surface layer (102) are exposed to a vacuum; in an exposure phase, the image sensor surface layer (28) and the first capture plate surface layer (102) are exposed to EUV light; and in an evaluation phase, the first capture plate surface layer (102) is inspected.

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