Pollution measurement

Abstract

A method of determining contamination of an optical sensor in a lithographic apparatus, the method comprising: irradiating a pattern on a patterning device with EUV radiation; projecting the patterned reflected EUV radiation towards an optical sensor, thereby forming a spatial image of the pattern; and moving the optical sensor relative to the patterned reflected EUV radiation such that the intensity of the EUV radiation measured by the optical sensor varies according to the position of the optical sensor, where the intensity measured by the optical sensor experiences a minimum, and where the method further comprises: measuring contamination of the optical sensor using the measured intensity.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to EP application 22188166.7, filed on August 1, 2022, and the entire contents of which are incorporated herein by reference. Technical Field

[0003] The present invention relates to measuring contamination of an optical sensor forming part of a lithographic apparatus. Background Art

[0004] A lithographic apparatus is a machine configured to apply a desired pattern to a substrate. A lithographic apparatus can be used, for example, to manufacture integrated circuits (ICs). A lithographic apparatus can, for example, project a pattern at a patterning device (e.g., a mask) onto a layer of radiation-sensitive material (resist) provided on a substrate.

[0005] To project a pattern onto a substrate, a lithographic apparatus may use electromagnetic radiation. The wavelength of the radiation determines the minimum size of a feature that can be formed on the substrate. A lithographic apparatus using extreme ultraviolet (EUV) radiation having a wavelength in the range of 4 nm to 20 nm (e.g., 6.7 nm or 13.5 nm) may be used to form smaller features on a substrate compared to a lithographic apparatus using radiation having a wavelength of, for example, 193 nm.

[0006] When the lithographic substrate is exposed, it is supported by a substrate table. The substrate table may include a substrate fixture and a base for holding the substrate fixture. The base may include a reflective surface that allows the position of the substrate table to be interferometrically measured. Over time, contamination may accumulate on the substrate table. In particular, during lithographic exposure, contamination may accumulate on an area of ​​the substrate table that is not covered by the substrate. The substrate table is provided with a sensor system. The sensor system may be located in the substrate table base (e.g., radially outward of the substrate fixture). When contamination accumulates on the optical sensor of the sensor system, this may have an adverse effect on the performance of the optical sensor. The substrate table may be cleaned, for example, using hydrogen radicals. However, this cleaning takes time, and during the cleaning, the exposure of the lithographic substrate may be suspended. For this reason, it is best not to clean the substrate table too frequently. However, on the contrary, if the cleaning is delayed too long, the accumulation of contamination on the sensor may cause the performance of the lithographic apparatus to degrade to be outside the ideal parameters.

[0007] It is desirable to provide a method and apparatus that overcomes or alleviates one or more problems associated with the prior art. Summary of the invention

[0008] According to a first aspect of the invention, there is provided a method for determining contamination of an optical sensor in a lithography apparatus, the method comprising: irradiating a pattern on a pattern forming device using EUV radiation; projecting the patterned reflected EUV radiation toward an optical sensor to form a spatial image of the pattern; and moving the optical sensor relative to the patterned reflected EUV radiation such that the intensity of the EUV radiation measured by the optical sensor varies depending on the position of the optical sensor, wherein the intensity measured by the optical sensor experiences a minimum value, and wherein the method further comprises: measuring contamination of the optical sensor using the measured intensity.

[0009] Advantageously, the method provides a direct measurement of contamination in a simple manner. Because the contamination is measured directly rather than estimated, cleaning can be performed to remove the contamination only when the contamination reaches a level that requires cleaning. This avoids performing cleaning when no cleaning is necessary (which would occur if the cleaning was based on an estimate of the contamination). Therefore, the frequency of cleaning can be reduced. This increases the productivity of the lithographic apparatus. Embodiments of the present invention can correct the measurement to take into account the effect of contamination (e.g., using a known relationship between the thickness of the contamination and the signal output from the optical sensor). The relationship can be determined by performing calibration measurements on contamination of various thicknesses.

[0010] The depth of the minimum can be used to measure the contamination of the optical sensor.

[0011] The movement of the optical sensor is typically perpendicular to the direction of the projected EUV radiation.

[0012] The method may further comprise using the shape of the measured intensity to obtain information about the cross-sectional shape of the contamination.

[0013] The movement of the optical sensor may be substantially parallel to the direction of the projected EUV radiation.

[0014] The area of ​​the optical sensor may be larger than the area of ​​the aerial image of the alignment pattern used to perform alignment measurements associated with the optical sensor.

[0015] The pattern illuminated for contamination measurement may be an alignment pattern.

[0016] The measurement of contamination can take into account the intensity of the background radiation incident on the optical sensor.

[0017] The measured contamination can be compared to a threshold value.

[0018] According to a second aspect of the invention, there is provided a lithography apparatus comprising a pattern forming device support structure, a projection system and a substrate table provided with an optical sensor, wherein the lithography apparatus comprises a processor configured to move the sensing system relative to the projected patterned EUV radiation beam so that the intensity of the EUV radiation measured by the optical sensor varies depending on the sensor position and has a minimum value, and wherein the processor is further configured to use the measured intensity to measure contamination of the optical sensor.

[0019] Advantageously, the lithographic apparatus obtains a direct measurement of contamination in a simple manner.

[0020] The processor may be configured to use the depth of the minimum to measure contamination of the optical sensor.

[0021] The movement of the optical sensor may be substantially perpendicular to the direction of the projected patterned EUV radiation beam.

[0022] The processor may be configured to use the shape of the measured intensities to obtain information about the cross-sectional shape of the contamination.

[0023] The movement of the optical sensor may be substantially parallel to the direction in which the EUV radiation is projected.

[0024] The area of ​​the optical sensor may be larger than the area of ​​the aerial image of the alignment pattern used to perform alignment measurements with respect to the optical sensor.

[0025] The measurement of contamination can take into account the intensity of the background radiation incident on the optical sensor.

[0026] The measured contamination can be compared to a threshold value.

[0027] According to a third aspect of the present invention, there is provided a computer readable storage medium comprising instructions which, when executed by a processor of a computing device, cause the computing device to perform the method of the first aspect of the present invention.

[0028] According to a fourth aspect of the present invention, there is provided a computing device comprising a processor and a memory, the memory storing instructions which, when executed by the processor, enable the computing device to perform the method of the first aspect of the present invention.

[0029] Features of different aspects of the invention may be combined. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Embodiments of the present invention will now be described by way of example only with reference to the accompanying schematic drawings, in which

[0031] - Figure 1 schematically depicts a lithographic system including a lithographic apparatus and a radiation source;

[0032] - Figure 2A and Figure 2B depicts a sensor system forming part of a lithographic apparatus;

[0033] - Figure 3 schematically depicts the use of an optical sensor of the sensor system of FIG2 ;

[0034] - Figure 4A and Figure 4B schematically depicts measurements performed in accordance with an embodiment of the invention and used to measure contamination on an optical sensor;

[0035] - Figure 5A and Figure 5B schematically depicts a measurement performed according to another embodiment of the invention and used to measure contamination on an optical sensor;

[0036] - Fig. 6A and Figure 6B schematically depicts a measurement performed according to another embodiment of the invention and used to measure contamination on an optical sensor;

[0037] - Figure 7 is a graph showing a signal to be output from an optical sensor when a method according to an embodiment of the present invention is used; and

[0038] - Figures 8 to 10 Schematically depicts measurements according to an embodiment of the invention;

[0039] - Fig.11 depicts contamination with four different cross-sectional shapes; and

[0040] - Fig.12 The resulting signals of an embodiment of the present invention are depicted for these four cross-sectional shapes. DETAILED DESCRIPTION

[0041] Figure 1 A lithographic system is shown comprising a radiation source SO and a lithographic apparatus LA. The radiation source SO is configured to generate an EUV radiation beam B and to supply the EUV radiation beam B to the lithographic apparatus LA. The lithographic apparatus LA comprises an illumination system IL, a support structure MT configured to support a patterning device MA (e.g. a mask), a projection system PS, and a substrate table WT configured to support a substrate W.

[0042] The illumination system IL is configured to condition the EUV radiation beam B before it is incident on the patterning device MA. Furthermore, the illumination system IL may include a faceted field mirror device 10 and a faceted pupil mirror device 11. The faceted field mirror device 10 and the faceted pupil mirror device 11 together provide an EUV radiation beam B having a desired cross-sectional shape and a desired intensity distribution. In addition to the faceted field mirror device 10 and the faceted pupil mirror device 11, or instead of the faceted field mirror device 10 and the faceted pupil mirror device 11, the illumination system IL may include other mirrors or devices.

[0043] After being so conditioned, the EUV radiation beam B interacts with the pattern forming device MA. As a result of this interaction, a patterned EUV radiation beam B' is generated. The projection system PS is configured to project the patterned EUV radiation beam B' onto the substrate W. To this end, the projection system PS may include a plurality of mirrors 13, 14 configured to project the patterned EUV radiation beam B' onto the substrate W held by the substrate table WT. The projection system PS may apply a reduction factor to the patterned EUV radiation beam B' so as to form an image having features that are smaller than corresponding features on the pattern forming device MA. For example, a reduction factor of 4 or 8 may be applied. Although Figure 1 The projection system PS is shown with only two mirrors 13, 14, but the projection system PS may comprise a different number of mirrors (eg six or eight mirrors).

[0044] During scanning exposure, the patterning device MA and support structure MT move in the y-direction, and the substrate W and substrate table WT move in the opposite y-direction. In this way, a band of EUV radiation passes through an exposure field on the substrate W.

[0045] The substrate W may comprise a previously formed pattern. In this case, the lithographic apparatus LA aligns the image formed by the patterned EUV radiation beam B' with the pattern previously formed on the substrate W. The positions of the alignment marks on the substrate table are measured relative to the positions of the alignment marks 22 on the patterning device MA. This is done using a sensor system 24 comprising alignment marks. This alignment of the patterning device MA relative to the substrate table WT allows the patterning device to be aligned with the substrate W. The alignment marks 22 may comprise a variety of different shapes and / or patterns. At least some of the patterns may be gratings.

[0046] A relative vacuum, ie, a small amount of gas (eg, hydrogen) at a pressure well below atmospheric pressure, may be provided in the radiation source SO, the illumination system IL and / or the projection system PS.

[0047] For example, Figure 1The type of radiation source SO shown may be referred to as a laser produced plasma (LPP) source. A laser system 1, which may, for example, include a CO2 laser, is arranged to deposit energy via a laser beam 2 into a fuel, such as tin (Sn), provided from, for example, a fuel emitter 3. Although tin is mentioned in the following description, any suitable fuel may be used. The fuel may, for example, be in liquid form and may, for example, be a metal or an alloy. The fuel emitter 3 may include a nozzle configured to direct the tin (for example, in the form of droplets) along a trajectory toward a plasma formation region 4. The laser beam 2 is incident on the tin at the plasma formation region 4. The laser energy is deposited into the tin, thereby generating a tin plasma 7 at the plasma formation region 4. During deexcitation and recombination of electrons and ions from the plasma, radiation including EUV radiation is emitted from the plasma 7.

[0048] EUV radiation from the plasma is collected and focused by a collector 5. The collector 5 includes, for example, an approximately normal incidence radiation collector 5 (sometimes more generally referred to as a normal incidence radiation collector). The collector 5 may have a multi-layer mirror structure arranged to reflect EUV radiation (e.g., EUV radiation having a desired wavelength such as 13.5 nm). The collector 5 may have an elliptical configuration having two focal points. A first of the focal points may be at the plasma formation region 4, and a second of the focal points may be at the intermediate focus 6, as discussed below.

[0049] The laser system 1 may be spatially separated from the radiation source SO. In this case, the laser beam 2 may be transferred from the laser system 1 to the radiation source SO with the aid of a beam delivery system (not shown) comprising, for example, suitably oriented mirrors and / or beam expanders and / or other optical devices. The laser system 1, the radiation source SO and the beam delivery system may together be considered a radiation system.

[0050] The radiation reflected by the collector 5 forms an EUV radiation beam B. The EUV radiation beam B is focused at an intermediate focus 6 to form an image at the intermediate focus 6 of a plasma present at the plasma formation region 4. The image at the intermediate focus 6 acts as a virtual radiation source for the illumination system IL. The radiation source SO is arranged such that the intermediate focus 6 is located at or near an opening 8 in an enclosure 9 of the radiation source SO.

[0051] although Figure 1 The radiation source SO is depicted as a laser produced plasma (LPP) source, but any suitable source, such as a discharge produced plasma (DPP) source or a free electron laser (FEL), may be used to generate EUV radiation.

[0052] Contamination will build up on the sensors of the sensor system 24. In one example, contamination that builds up on the optical sensors of the sensing system may cause the optical sensors to provide inaccurate outputs. This in turn may cause poor alignment of the projected pattern relative to the pattern already present on the substrate W. Embodiments of the present invention address this problem.

[0053] Figure 2A and Figure 2B A sensor system 24 provided on a substrate table WT is schematically depicted. Figure 2A Depicts the sensor system viewed from above. Figure 2B A cross section of a sensor system is depicted. The sensor system 24 comprises optical sensors 25 to 27 of different sizes. The sensor system 24 may comprise other optical sensors.

[0054] The sensor system 24 includes a substrate 40 in which the detectors 28 to 30 are formed. A layer of opaque material 42 is disposed on the substrate, and apertures 31 to 33 are formed in the opaque material over the detectors 28 to 30. The size of each aperture 31 to 33 controls the amount of EUV radiation incident on each detector 28 to 30. The sensor system 24 may include a semiconductor and may be formed using photolithography.

[0055] The first optical sensor 25 has a square shape when viewed from above, and includes a detector 28 (eg, a photodiode) surrounded by an aperture 31. The first optical sensor 25 may have, for example, a diameter of approximately 50×50 μm. 2 The shape of the optical sensor 25 may be defined by the aperture 31. As discussed further below, although the optical sensor in this embodiment is square, the optical sensor may have any shape.

[0056] The second optical sensor 26 comprises a detector 29 (eg a photodiode) surrounded by an aperture 32. A grating 34 is formed in the layer of opaque material 42. The grating 34 extends in the X direction.

[0057] The third optical sensor 27 includes a detector 30 (eg, a photodiode) surrounded by an aperture 33. A grating 35 is formed in the opaque material layer 42. The grating 35 extends along the Y direction. To avoid complicating the drawings, Figure 2B The grating 35 is not depicted.

[0058] When used, the optical sensors 25 to 27 may be used to ensure that the patterning device MA is aligned with the substrate table WT (see Figure 1 ). Figure 3The manner in which this is achieved is schematically illustrated in FIG. 1 . The EUV radiation beam B is used to illuminate the alignment mark 22 on the patterning device MA. The resulting patterned beam B′ is projected through the projection system PS (see FIG. Figure 1 ) is projected to form an aerial image 44 of the alignment mark 22. The projection system PS typically includes a reduction factor, so that the aerial image 44 can be smaller than the alignment mark 22. The aerial image 44 is formed above (or on) the detector 28 of the first optical sensor 25. Although Figure 3 Only one alignment mark and optical sensor is depicted in FIG, but the same method is used for the other alignment marks and optical sensors.

[0059] When aligning the pattern forming device MA relative to the substrate table WT, the alignment can be performed in two parts. In the first part, a so-called rough alignment is performed. In the rough alignment, the substrate table WT moves a considerable distance in a scanning movement, typically a few hundred microns (for example, 200 microns), so that the first alignment mark aerial image 44 passes completely through the first detector 28. Because the alignment mark allows the rough position of the substrate table WT relative to the pattern forming device MA to be 'captured', it can be called a capture mark. The capture mark 40 can be square. However, the alignment mark can have any shape. Similarly, although the first detector 28 is square, it can have any shape. The area of ​​the detector 28 should be larger than the area of ​​the alignment mark aerial image. While the capture mark aerial image passes through the first detector 28, multiple periods of the aerial image of the X-direction grating pass through the X-direction grating detector 29.

[0060] Then, the substrate table WT is moved in a diagonal form (in the +X direction and -Y direction). After that, the substrate table WT is moved in the Y direction in a scanning movement, typically by several hundred micrometers (e.g., 200 micrometers). As a result, the captured mark aerial image 44 passes completely through the first detector 28 in the Y direction. The Y-direction grating aerial image passes through multiple periods of the Y-direction grating detector 30.

[0061] Rough alignment allows the position of the substrate stage to be determined with an accuracy of about tens of nanometers or better. After this, a second alignment process is performed. The second alignment process can be referred to as fine alignment. In fine alignment, the substrate stage is positioned so that the captured mark aerial image 44 is centered above the first detector 28, and the X-direction grating aerial image and the Y-direction grating aerial image are also centered relative to the X-grating detector 29 and the Y-grating detector 30. Then, a small movement is performed, typically up to 2 microns in the diagonal direction. The resulting signals from the X-grating detector 29 and the Y-grating detector 30 indicate the relative position of the grating aerial image to the X-grating detector 29 and the Y-grating detector 30. This signal can be used to align the substrate stage WT with the pattern forming device MA with nanometer accuracy or sub-nanometer accuracy.

[0062] The sensor system 24 may also be used to ensure that the substrate table WT has the correct Z-direction position relative to the focal plane of the projection system PS of the lithographic apparatus. To this end, the substrate table WT and the sensor system 24 may be moved a distance of about 100 microns in the Z-direction. The Z-direction position may be determined with an accuracy of a few nanometers or better.

[0063] During coarse and fine alignment, the alignment marks on the patterning device MA are illuminated by the EUV radiation beam B, and therefore, EUV radiation is incident on the sensor system 24. This generates contamination due to the interaction between the EUV radiation and gas molecules or other contaminants near the sensor system 24. As noted above, during fine alignment, the aerial image of the capture mark 44 is centrally located on the first detector 28. During coarse alignment, the substrate table WT and the sensor system 24 are moved in a scanning motion below the aerial image of the mark. However, this movement is relatively fast, and the exposure to EUV radiation caused by this movement is relatively low, and no significant contamination is generated. Therefore, the EUV radiation is uniformly incident on the central area of ​​the first detector 28, and contamination accumulates in the central area. The contamination has the shape of the aerial image of the capture mark 44 projected during fine alignment. The same is true for the other marks and detectors of the sensor system 24. The area of ​​the aerial image of the capture mark 44 is smaller than the area of ​​the first detector 28. Therefore, a portion of the area of ​​the first detector 28 does not receive a buildup of contamination in the shape of the aerial image 44. The same is true for the other marks and detectors of the sensor system.

[0064] Embodiments of the present invention measure the thickness of contamination that is formed in a centered manner, i.e., contamination that accumulates in the center area of ​​the detector. The combination of contaminated and uncontaminated areas on the detector allows the thickness of the contamination to be measured. The combination of contaminated and uncontaminated areas results from an aerial image of an alignment mark that is smaller than the area of ​​the detector that detects the alignment mark. Measuring the thickness of the contamination advantageously allows cleaning of the sensor system 24 when the thickness of the contamination is such that cleaning is required. This is superior to estimating the thickness of the contamination based on the accumulated dose of EUV radiation projected onto the sensor system 24 and cleaning the contamination based on the estimate, because using an estimate may result in cleaning being performed when cleaning is not necessary.

[0065] In some embodiments, the effect of contamination on the output signal provided by the sensor of the sensor system may be known. For example, for Z-direction position measurement, a certain thickness of contamination may cause the measured position to be known offset from the actual position. This offset may be taken into account in order to determine the actual position. Therefore, in some embodiments, measuring the thickness of the contamination can correct for measurement errors. In some embodiments, when adjusting the lithographic apparatus, instead of correcting for measurement errors, the effect of measurement errors may be taken into account. For example, if it is known that the detector output is offset by a certain amount when a certain thickness of contamination is present, adjustments to the lithographic apparatus may be performed based on a combination of the detector output and the known offset.

[0066] Figure 4A The scanning movement of the first detector 28 under the aerial image of the capture mark 44 during the rough alignment is schematically depicted, and the signal S output from the first detector 28 when the scanning movement is completed is schematically depicted. Other movements of the first detector 28 can be used to obtain the output signal (as further explained below). The signal S is proportional to the intensity I of the light detected by the detector 28 over time t. As depicted, the aerial image 44 begins outside the area of ​​the detector 28, and the signal from the first detector 28 is zero. The aerial image 44 moves in the X direction, as schematically depicted by the dotted arrow. When the aerial image 44 overlaps with the detector 28, the output signal increases linearly. When the aerial image 44 is completely above the detector 28, the output signal reaches a maximum value. The output signal remains at a maximum value until the aerial image 44 of the capture mark begins to leave the detector 28 (i.e., the overlap between the aerial image and the detector 28 begins to decrease). The signal drops to zero. The detector 28 is then moved diagonally (as indicated by the dashed arrow) until the aerial image 44 is aligned with the detector 28 in the X direction but separated from the detector in the Y direction. A Y direction scan is then performed. This generates the same output signal.

[0067] Figure 4B The signal output from the detector 28 when contamination is present on the detector is schematically illustrated. Figure 4B Also schematically illustrated is contamination on the detector 28, depicted as a dark area 45 in the center of the detector. The dark area 45 is slightly larger than the aerial image 44 of the captured mark. This is because, as further explained above, during fine alignment, some movement of the detector 28 occurs relative to the aerial image 44. This movement causes the contamination 45 to spread a small amount.

[0068] exist Figure 4B , the coarse alignment scan again starts with an X-direction scan, followed by a Y-direction scan. During the X-direction scan, the signal starts at zero and rises to a maximum value once the entire aerial image 44 is over the detector 28. However, when the aerial image 44 begins to overlap the contamination 45, the output signal decreases. This is because the contamination 45 absorbs EUV radiation, so less EUV radiation is incident on the detector 28. When the aerial image 44 is completely over the contamination 45, the output signal continues to decrease and reaches a minimum value. When the aerial image 44 leaves the contamination 45, the signal begins to increase, and the signal reaches a maximum value again when the aerial image is completely over the uncontaminated portion of the detector 28. The signal then gradually decreases to zero as the aerial image leaves the detector 28.

[0069] Then, a coarse alignment scan is performed in the Y direction, generating the same output signal (as depicted).

[0070] In various embodiments of the present invention, the presence of a minimum 50 in the output signal is used to identify the presence of contamination. The depth of the minimum 55 indicates the thickness of the contamination 45 present on the detector 28. The depth of the minimum 55 can be used to determine when the thickness of the contamination present on the detector 28 reaches a threshold amount, thereby requiring cleaning of the detector. For example, the threshold amount can be identified by determining that the depth of the minimum 50 has reached a threshold value.

[0071] exist Figure 4A and Figure 4B In the depicted embodiment, the dimensions of the detector are such that the entire aerial image 44 of the alignment mark (in this case, capture mark) is incident on an area of ​​the detector 28 that is free of significant contamination. However, embodiments of the present invention may also accommodate situations where this is not the case. Figure 5A and Figure 5B Such an embodiment is depicted in FIG. As depicted, the detector 51 is smaller than Figure 4A and Figure 4B The size of the captured spatial image 44 of the marker is similar to that of the detector depicted. Figure 4A and Figure 4B Same dimensions as depicted.

[0072] refer to Figure 5A , when contamination does not exist, the signal output from the detector 24 is equal to Figure 4A In particular, for both the X-direction scan and the Y-direction scan, the signal has a flat central middle portion.

[0073] Figure 4B Contamination on the detector 28 is schematically illustrated, depicted as a dark region 51 in the centre of the detector. For the reasons explained above, the contaminated region 51 is slightly larger in area than the aerial image 44 of the captured mark (although embodiments of the invention would work equally well if the contaminated region were the same size or smaller than the aerial image).

[0074] exist Figure 5B The maximum signal output from the detector 28 is lower than Figure 5A This is because there is no position of the captured marker aerial image 44 relative to the detector 51 that provides a complete signal from the detector (there is always some overlap with the contamination, or a portion of the aerial image does not overlap with the detector). However, when the aerial image passes through the contaminated region 51 of the detector 28, the output signal experiences a minimum 52, as shown in FIG. Figure 5B Depicted schematically. Figure 4B As in the embodiment of the present invention, the presence of minimum 52 can be used to determine the presence of contamination. The depth of minimum 52 can be used to measure the thickness of contamination 51. A threshold minimum depth or some other threshold characteristic can be set. When the threshold is exceeded, cleaning of detector 28 can be performed.

[0075] The above described embodiments use scanning movement of the detector 28 relative to the aerial image in directions perpendicular to each other. However, in other embodiments, scanning movement of the detector 28 in a single direction may be used to measure contamination of the detector.

[0076] Herein, references to movement of detector 28 may be interpreted as referring to movement of sensor system 24 .

[0077] The above-described embodiments use scanning movement of the detector 28 relative to the spatial image 44. However, the detector 28 may instead be stepped between different positions relative to the spatial image 44. This may be done, for example, when it is desired to obtain individual measurements with greater accuracy. More photons will be incident on the detector when the detector 28 remains in position for a period of time, rather than being moved continuously as in a scan. This provides a higher signal-to-noise ratio, thereby providing a more accurate measurement. The general shape of the signal that will be seen as a function of the X and Y position of the detector 28 does not change significantly and is therefore known. This means that the location of the step measurement (which may also be referred to as a discrete measurement) may be selected to provide sufficient data to accurately characterize the shape of the signal output from the detector 28. For example, the location of the measurement may be selected to allow dips 50, 52 to be characterized.

[0078] Fig. 6A and Figure 6B Another embodiment of the present invention is schematically depicted. First, referring to Fig. 6A , schematically depicting the effect of moving the detector 28 in the Z direction when the aerial image is aligned with the center of the detector. The detector 28 is schematically depicted in the focal plane of the projection system. A patterned EUV radiation beam B' carrying the aerial image is also schematically depicted. The patterned radiation beam B' has a minimum diameter at the focal plane of the projection system. When the detector 28 is in the focal plane, the aerial image of the alignment mark is focused at the detector. As the detector 28 moves away from the focal plane in the Z direction, the alignment mark image becomes less and less focused. That is, the alignment mark image spreads over larger and larger distances.

[0079] Fig. 6A Schematically depicted is the signal S output from the detector 28 as the detector is moved in the Z direction (from below the focal plane to above the focal plane). The detector 28 starts below the focal plane, so the projected image of the captured mark is out of focus. As a result, the EUV radiation is spread out and some of the EUV radiation is outside the detector 28. As the detector 28 moves towards the focal plane of the projection system, the aerial image of the captured mark becomes focused and therefore becomes smaller. The amount of EUV radiation incident on the detector 28 increases and so does the signal S. When the aerial image of the captured mark is close to focus, all of the EUV radiation is incident on the detector and the output signal S reaches a maximum. When the image of the alignment mark completely covers the detector 28, the signal S remains at a maximum. As the detector 28 moves away from the focal plane and part of the EUV radiation falls outside the detector, the signal decreases again.

[0080] refer to Figure 6B, when there is contamination 45 on the detector 28, a minimum 54 will be seen in the signal S output from the detector. The presence of the minimum 54 indicates the presence of contamination 45 on the detector 28. The depth of the minimum 54 indicates the thickness of the contamination present on the detector 28. A threshold depth for the minimum 54 can be set. When the threshold is reached, the detector 28 can be cleaned. As with the other embodiments, this embodiment can be used to measure the thickness of the contamination regardless of whether the contamination area is larger than the area of ​​the aerial image, smaller than the area of ​​the aerial image, or the same as the area of ​​the aerial image. The contamination area should be smaller than the area of ​​the detector.

[0081] Figure 7 It shows that when executing Figure 4B The method depicted is a graph of the signal output from the detector 28. The vertical axis of the graph shows the power S measured by the detector and the horizontal axis shows the X-direction position of the detector. The line shows the result of a scanning measurement (i.e., continuous measurement while the detector 28 is being moved). The circles show the results of individual (discrete) measurements. Figure 7 , it should be appreciated that discrete measurements are sufficient to characterize the shape of the signal S.

[0082] A mathematical equation can be used to characterize the signal, allowing the thickness of the contamination on the detector to be measured. In the mathematical equation, it is assumed that the detector is square and the alignment marks are square. The following symbols are used:

[0083] S: Power measured by the detector

[0084] D: length of one side of the detector;

[0085] m: length of one side of the spatial image of the alignment mark;

[0086] k: length of one side of the contamination (for simplicity, the contamination is assumed to be generally square, even though the shape of the contamination may deviate from this square, i.e., k is within a few percent of m).

[0087] I: EUV power per unit area incident at patterning device MA (values ​​are relative, so the power need not be in Watts).

[0088] R M : Alignment mark reflectivity (which may be, for example, about 65%)

[0089] R rbs : The reflectivity of the absorber area surrounding the alignment mark (which may be, for example, 2%)

[0090] b: Maximum signal measured by the sensor

[0091] c: Signal jump in the middle of the scan (the spatial image of the alignment mark is completely above the contamination)

[0092] a: Background signal when the spatial image and the detector do not overlap

[0093] cont: signal loss caused by contamination (it may be several %)

[0094] For simplicity, it is assumed here that the contamination layer has a uniform thickness (constant profile) over the contaminated area. However, embodiments of the present invention are also applicable in the case of non-uniform contamination (as further explained below).

[0095] The EUV power per unit area I as expressed above is the power incident on the patterning device MA. The projection system PS of the lithographic apparatus (see Figure 1 ) transmission of EUV radiation would reduce the power per unit area. However, in the calculations used in the present invention, this transmission loss can be ignored because it is constant and cancels out in the calculations (as does I).

[0096] When the relationship between contamination thickness and signal loss is known, the signal loss cont due to contamination can be converted to contamination thickness. For example, approximately 0.5% signal loss may occur for each nanometer of contamination thickness. In this case, if a 60% signal loss cont is seen, this may correspond to a contamination thickness of approximately 120nm.

[0097] Figure 8 The measurement is depicted when the detector 28 is large enough so that the aerial image 44 of the alignment mark can be completely incident on an uncontaminated area of ​​the detector 28. The equation characterizing the power S measured by the detector for each position of the aerial image is given below. Figure 8 In the absorber R abs The reflectivity of is set to zero, so the background signal a is not visible. The equation is as follows:

[0098] Position (1) S(x<0)=a=0

[0099] Position (2) S(x<m)=R m ·I·(m·x)

[0100] Location (3)

[0101] Location (4)

[0102] for Figure 8 For the scenario depicted, the equation can be simplified to:

[0103]

[0104] In equation 1, the size of the jump c is normalized by dividing it by the maximum signal b. When there is no jump, the fraction c / b is 1, so the value of cont is 0. When there is a jump c, a relatively small value of c indicates a relatively large thickness of contamination (a relatively small value of c indicates a relatively small intensity of light measured by detector 28). A relatively large value of c indicates a relatively small thickness of contamination (a relatively large value of c indicates a relatively large intensity of light incident on detector 28). The value of cont directly characterizes the thickness of the contamination.

[0105] In the modified scenario, the detector 28 is large enough so that the aerial image 44 of the alignment mark can be completely incident on the uncontaminated area of ​​the detector 28, but the absorber R abs The reflectivity is not zero. abs is not zero, so the background signal a can be seen (in fact, some background signal is usually seen).

[0106] For this scenario, the following equation characterizes the contamination cont:

[0107]

[0108] in Equation 2 characterizes the transition in a similar manner to Equation 1, but takes into account the presence of the background signal a. The value of cont directly characterizes the thickness of the contamination (without the approximation that the background signal is absent).

[0109] Fig. 9 Depicted is a measurement when the detector 28 is small so that the aerial image 44 of the alignment mark is not fully incident on an uncontaminated area of ​​the detector 28. Fig. 9 In the absorber R abs The reflectivity is zero, so the background signal a is not visible.

[0110] for Fig. 9 In the scenario depicted in , the following equation characterizes the pollution cont:

[0111]

[0112] The value of cont directly characterizes the thickness of the contamination, but it approximately assumes that there is no background signal.

[0113] The size of the jump can be characterized in other ways. For example, a polynomial curve can be fitted to the output signal. Any form of curve fitting or other characterization can be used. The jump can be referred to as a minimum.

[0114] Embodiments of the present invention use square alignment marks and square detectors. However, the alignment marks and detectors can have any shape. The spatial image of the alignment mark should be smaller than the detector so that during alignment measurements, when the movement of the detector is small or negligible, some areas in the area of ​​the detector will not be illuminated by the alignment mark. Therefore, a portion of the area of ​​the detector receives contamination, while a portion of the area of ​​the detector does not receive contamination. It is this combination of contaminated areas and uncontaminated areas that is advantageously used by embodiments of the present invention to measure the thickness of contamination. The detector and alignment mark do not need to be a single physical shape (e.g., square, rectangular, circular). The detector and alignment mark can have complex shapes, such as a checkerboard arrangement (i.e., a square array).

[0115] In the described embodiments of the invention, the aerial image used when measuring the contamination thickness has the same shape and size as the aerial image used during the alignment measurement. However, the aerial image used when measuring the contamination thickness may have a different shape and / or size than the aerial image used during the alignment measurement. The aerial image used when measuring the contamination thickness may have an area that is larger than the area of ​​the detector. However, the aerial image used when measuring the contamination thickness should still be moved by providing a different amount of overlap between the aerial image and the detector.

[0116] In the embodiments described above, it is assumed that the contamination has a uniform thickness. However, the contamination may have a non-uniform thickness. Embodiments of the present invention may characterize the thickness variation of the contamination.

[0117] Fig.10 Schematic depiction of a scenario where the contamination has a non-uniform thickness. Fig.10 In this case, the aerial image initially overlaps with the contaminated part. In this case, the signal output from the detector can be expressed as:

[0118]

[0119] Then, the aerial image completely overlaps with the contamination. In this case, the signal output from the detector can be expressed as:

[0120]

[0121] Other locations where the aerial image does not overlap with the pollution can also be combined as above Figure 8 expressed.

[0122] Fig.11 Pollution with four different cross-sectional shapes is depicted. These are called uniform, linear, inverse linear, and inverse parabolic. Fig.12 The signal that would be seen for each contamination cross-sectional shape (calculated using the above equation) is shown. Fig.12 , it can be seen that for each pollution cross-section shape, the signal has a different curve shape (although the curve shapes of the linear and inverse parabolic are very similar).

[0123] Embodiments of the present invention may analyze the shape of the signal to obtain information about the cross-sectional shape of the contamination. For example, conventional curve fitting may be used to determine the shape of the signal. Knowing the cross-sectional shape of the contamination may advantageously allow the manner in which cleaning is performed to be selected accordingly.

[0124] When it is desired to obtain information about the cross-sectional shape of the contamination, measurements of the detector output may be performed (and / or more measurements may be performed) with greater accuracy than when it is desired to measure only the overall thickness of the contamination. In other words, when it is desired to characterize the shape of the signal, a higher accuracy measurement and / or more measurements may be made than when it is desired to determine the depth of a transition in the signal.

[0125] When using embodiments of the present invention, the signal output from the detector will be self-consistent. That is, the signal depends on the geometry of the alignment mark (and may also depend on the geometry of the detector). The signal does not depend on parameters such as alignment mark reflectivity, which may change over time (and may vary between different lithography apparatuses). Thus, embodiments of the present invention allow comparisons between measurements performed on the same optical sensor at different times. Embodiments of the present invention also allow comparisons of contamination measurements on different optical sensors in the same or different lithography apparatuses.

[0126] Embodiments of the present invention can use multiple different detectors in the same lithography apparatus to measure contamination. This allows the thickness of contamination of each detector to be measured independently.

[0127] Embodiments of the present invention also propose a measurement scheme that will allow the optical transmission of a grating detector to be measured, and therefore any transmission drift caused over time by contamination growing on top of its surface to be determined.

[0128] In this scheme, the EUV light is projected onto the sensor using the SS-SLS fiducial as a reticle stage reflector. The SS-SLS fiducial consists essentially of a large rectangular cross section with a multi-layer blank, thus guaranteeing an optimal and uniform reflection of a large portion of the EUV slit. Thus, the slit is projected uniformly at wafer level.

[0129] Here, the TIS plate can be located below the slit so that its detector is fully illuminated. In particular, for example, the normal group 2The grating detectors (Txh, Tyh) used for mask alignment are fully illuminated (“flooded” in the jargon) with light so that their signals do not depend on their alignment conditions. The signal Sgrating(t) measured by the illuminated detector can therefore be described as:

[0130] S grating (t) = r grating .IT grating .C slit .R fiducial .F defocus .A grating .T OC

[0131] where r grating is the responsivity of the photodiode, I is the light incident on it, and T grating is the transmission at the detector surface (possibly affected by drift over time due to contamination), C slit represents any relative change in EUV intensity within the slit, R fiducial is the reflectivity of the reference at the measurement location, F defocus A represents the decrease in light intensity at the sensor surface due to sensor defocusing. grating is the fraction of the detector surface that is illuminated, and T OC is the transmission of the optical column from the reference to the sensor. By monitoring this signal over time and converting it to an initial 'time zero' value, it is in principle possible to track this term T grating The evolution of , thus estimating the drift of detector contamination over time.

[0132] First, with a "flood" detector, 100% of its area is illuminated, so the term A grating can be neglected. Of course, a "flooded" detector carries the risk of saturating the detector signal. This can be corrected by reducing the sensor gain, however this is not always possible. If this is the case, a different illumination setting can be used to compensate for the risk of signal clipping, and using a pupil with a low pupil filling factor. A reduction in the total intensity to 20% or less can be easily achieved without loss of performance: therefore, once the desired pupil has been determined, it can be reused for each measurement.

[0133] To avoid systematic problems due to asymmetry of the slit or the reference reflectivity, or to long-term drift of the reference reflectivity itself, the measurements can always be performed at the same nominal positions of the slit and the reference, so that the slit term C can be neglected. slit and the benchmark term R fiducialFurthermore, it must be pointed out that drifts in the reference reflectivity occur on timescales of months, much longer than the timescale of sensor fouling and can therefore in principle be neglected.

[0134] To neutralize defocus effects, the alignment position of the beam can be measured in advance and then the intensity at the focus measured, or alternatively, a measurement of the grating intensity can be performed while scanning longitudinally above the beam focal plane or via a series of stationary scans along the same z-axis to determine the peak intensity at the focus.

[0135] To suppress noise caused, for example, by source fluctuations, the signal of any of the detectors used in the measurement can be recorded over a period of time and averaged, thereby suppressing noise in the signal from any source. Furthermore, to counteract source fluctuations and, more importantly, measurement system problems, the signal can be normalized. Different approaches are possible: for example, the signal of a neighboring ratiometric detector can be used, which also provides the same normalization function during reticle alignment. In this case, the ratiometric detector signal can be described using an expression similar to that of the grating, namely:

[0136] S ratio (t) = r ratio .IT ratio .C slit .R fiducial .F defocus .A ill .T OC

[0137] Of course, ratiometric detectors can also drift due to contamination, however, in this case the term T ratio can be estimated independently by the method proposed in the previous embodiment, and also by the corresponding value C ratio Therefore, assuming that the detector area is not out of focus and is fully illuminated, it can be calculated:

[0138]

[0139] where the responsivity is constant over time, the incident light is scaled, the OC transmission loss terms cancel, and only the two transmission losses due to contamination remain. Therefore, it can be derived that:

[0140]

[0141] And compare it with the time zero measurement, for example, after HRG cleaning.

[0142] An alternative option for normalization is to use the signal S measured by the energy sensor ES(t), the energy sensor is located near the reference at the reticle level and is proportional to the amount of light projected at WL on the sensor (hence, ES≈1). In this case, while the source contribution is eliminated, the optical train transmission term does not cancel (ES is located at the entrance to the optical train), so

[0143]

[0144] It is worth noting that, in principle, T OC The time scale of the drift of is expected to be much longer than that of sensor contamination and thus may be negligible at least in the short term. On the other hand, this property can be exploited and by comparing the two normalization methods, a KPI describing the drift of the transmission loss of the optical train over time can be derived, paying special attention to the POB.

[0145]

[0146] Extending this scheme and repeating the measurement at multiple locations across the slit, a graph of the transmission loss over time across the POB can be plotted. To further alternatively counteract any drift in the POB transmission, the ES readout can be matched to a corresponding spot-slit sensor (SS-SLS) measurement at wafer level, where the SS-SLS is also affected by contamination, and sensor-to-sensor matching between the SS-SLS measurement and the anchor sensor 4 can be utilized. Matching of the ES to the SS-SLS is a measurement that is already available and is named FSLIE (Full Slit Integrated Energy) measurement, as is matching of the sensor to the anchor. It can therefore be determined that:

[0147]

[0148] Finally, one may bypass the above KPIs and optionally determine contamination of the SS-SLS itself.

[0149]

[0150] The method according to an embodiment of the present invention can be performed by a computing device. The device may include a central processing unit ("CPU") connected to a memory. The method described herein can be implemented with a code (software) stored on a memory including one or more storage media, and is arranged to be implemented on a processor including one or more processing units. The storage medium may be integrated into the CPU and / or separated from the CPU. The code that may be referred to as an instruction is configured to be taken out from the memory and implemented on the processor to perform operations consistent with the embodiments discussed herein. Alternatively, some or all of the functions of the CPU are not excluded from being implemented in a dedicated hardware circuit system or a configurable hardware circuit system (such as an FPGA). In general, the method can be performed by a processor.

[0151] The computing device may include an input configured to enable a user to input data into a software program running on the CPU. The input device may include a mouse, keyboard, touch screen, microphone, etc. The computing device may also include an output device configured to output measurement results to the user.

[0152] Although specific reference may be made herein to the use of lithographic apparatus in IC manufacturing, it should be understood that the lithographic apparatus described herein may have other applications. Possible other applications include manufacturing integrated optical systems, guidance and detection patterns for magnetic domain memories, flat panel displays, liquid crystal displays (LCDs), thin film magnetic heads, etc.

[0153] Although specific reference may be made herein to embodiments of the invention in the context of a lithographic apparatus, embodiments of the invention may be used in other devices. Embodiments of the invention may form part of a mask inspection apparatus, a metrology apparatus, or any apparatus for measuring or processing an object such as a wafer (or other substrate) or a mask (or other pattern forming apparatus). These apparatuses are generally referred to as lithographic tools. Such lithographic tools may use vacuum conditions or ambient (non-vacuum) conditions.

[0154] Although specific reference has been made above to the use of embodiments of the invention in the context of optical lithography, it will be appreciated that the invention is not limited to optical lithography and may be used in other applications, for example imprint lithography, where the context permits.

[0155] Where the context permits, the embodiments of the present invention may be implemented in hardware, firmware, software, or any combination thereof. The embodiments of the present invention may also be implemented as instructions stored on a machine-readable medium, which may be read and implemented by one or more processors. A machine-readable medium may include any mechanism for storing or transmitting information in a form readable by a machine (e.g., a computing device). For example, a machine-readable medium may include: a read-only memory (ROM); a random access memory (RAM); a magnetic storage medium; an optical storage medium; a flash memory device; an electrical, optical, acoustic, or other form of propagation signal (e.g., a carrier wave, an infrared signal, a digital signal, etc.), etc. In addition, firmware, software, routines, instructions may be described herein as performing certain actions. However, it should be appreciated that such descriptions are only for convenience, and such actions are actually generated by a computing device, processor, controller, or other device that executes firmware, software, routines, instructions, etc., and in doing so, an actuator or other device may be caused to interact with the physical world.

[0156] Although specific embodiments of the present invention have been described above, it will be appreciated that the present invention may be implemented in a manner different from that described. The above description is intended to be illustrative rather than limiting. Therefore, it will be apparent to those skilled in the art that modifications may be made to the described invention without departing from the scope of the claims set forth below.

[0157] Terms

[0158] 1. A method of determining contamination of an optical sensor in a lithographic apparatus, the method comprising:

[0159] illuminating a pattern on a patterning device using EUV radiation;

[0160] projecting the patterned reflected EUV radiation toward the optical sensor to form an aerial image of the pattern; and

[0161] moving the optical sensor relative to the patterned reflected EUV radiation such that an intensity of EUV radiation measured by the optical sensor varies depending on a position of the optical sensor;

[0162] wherein the intensity measured by the optical sensor experiences a minimum, and wherein the method further comprises using the measured intensity to measure contamination of the optical sensor.

[0163] 2. The method according to clause 1, wherein the depth of the minimum is used to measure the contamination of the optical sensor.

[0164] 3. A method according to clause 1 or clause 2, wherein the movement of the optical sensor is substantially perpendicular to the direction of the projected EUV radiation.

[0165] 4. The method of clause 3, wherein the method further comprises: using the shape of the measured intensity to obtain information about the cross-sectional shape of the contamination.

[0166] 5. The method of clause 1 or clause 2, wherein the movement of the optical sensor is substantially parallel to the direction of the projected EUV radiation.

[0167] 6. A method according to any preceding clause, wherein the area of ​​the optical sensor is larger than the area of ​​the aerial image of the alignment pattern used to perform alignment measurements associated with the optical sensor.

[0168] 7. The method of clause 6, wherein the pattern illuminated for measuring contamination is the alignment pattern.

[0169] 8. A method according to any preceding clause, wherein the measurement of the contamination takes into account the intensity of background radiation incident on the optical sensor.

[0170] 9. A method according to any preceding clause, wherein the measured contamination is compared to a threshold value.

[0171] 10. A lithography apparatus comprising a pattern forming device support structure, a projection system and a substrate table provided with an optical sensor; wherein the lithography apparatus comprises a processor, the processor being configured to move the optical sensor relative to the projected patterned EUV radiation beam so that the intensity of the EUV radiation measured by the optical sensor varies according to the sensor position and has a minimum value, and wherein the processor is also configured to use the measured intensity to measure contamination of the optical sensor.

[0172] 11. The lithographic apparatus of clause 10, wherein the processor is configured to use the depth of the minimum to measure the contamination of the optical sensor.

[0173] 12. The lithographic apparatus of clause 10 or clause 11, wherein the movement of the optical sensor is substantially perpendicular to the direction of the projected patterned EUV radiation beam.

[0174] 13. The lithographic apparatus of clause 12, wherein the processor is configured to use the shape of the measured intensities to obtain information about a cross-sectional shape of the contamination.

[0175] 14. The lithographic apparatus of clause 10 or clause 11, wherein the movement of the optical sensor is substantially parallel to the direction of the projected EUV radiation.

[0176] 15. The lithographic apparatus of any of clauses 10 to 14, wherein the area of ​​the optical sensor is larger than the area of ​​an aerial image of an alignment pattern used to perform alignment measurements associated with the optical sensor.

[0177] 16. The lithographic apparatus of any of clauses 10 to 15, wherein the measurement of the contamination takes into account the intensity of background radiation incident on the optical sensor.

[0178] 17. The lithographic apparatus according to any of clauses 10 to 16, wherein the measured contamination is compared to a threshold value.

[0179] 18. A computer-readable storage medium comprising instructions which, when executed by a processor of a computing device, cause the computing device to perform the method of any one of clauses 1 to 9.

[0180] 19. A computing device comprising a processor and a memory, the memory storing instructions which, when executed by the processor, cause the computing device to perform the method according to any one of clauses 1 to 9.

Claims

1. A method of determining contamination of an optical sensor in a lithographic apparatus, the method comprising: illuminating a pattern on a patterning device using EUV radiation; projecting the patterned reflected EUV radiation toward the optical sensor to form an aerial image of the pattern; as well as moving the optical sensor relative to the patterned reflected EUV radiation such that an intensity of EUV radiation measured by the optical sensor varies as a function of a position of the optical sensor; wherein the intensity measured by the optical sensor experiences a minimum, and wherein the method further comprises using the measured intensity to measure contamination of the optical sensor.

2. The method according to claim 1, wherein the depth of the minimum is used to measure the contamination of the optical sensor, or / and Wherein the movement of the optical sensor is substantially perpendicular to the direction of the projected EUV radiation.

3. The method according to claim 2, wherein the method further comprises: The shape of the measured intensity is used to obtain information about the cross-sectional shape of the contamination. The method of claim 1 , wherein the movement of the optical sensor is substantially parallel to a direction of the projected EUV radiation.

5. A method according to any one of the preceding claims, wherein the area of ​​the optical sensor is larger than the area of ​​the aerial image of an alignment pattern used to perform alignment measurements associated with the optical sensor. The method according to claim 5 , wherein the pattern illuminated for contamination measurement is the alignment pattern.

7. The method according to any of the preceding claims, wherein the measurement of the contamination takes into account the intensity of background radiation incident on the optical sensor, or / and wherein the measured contamination is compared with a threshold value.

8. A lithography apparatus comprising a pattern forming device support structure, a projection system and a substrate table provided with an optical sensor; wherein the lithography apparatus comprises a processor, the processor being configured to move the optical sensor relative to the projected patterned EUV radiation beam so that the intensity of the EUV radiation measured by the optical sensor varies according to the sensor position and has a minimum value, and wherein the processor is also configured to use the measured intensity to measure contamination of the optical sensor.

9. The lithographic apparatus according to claim 8, wherein the processor is configured to: use the depth of the minimum value to measure the contamination of the optical sensor, or / and Wherein the movement of the optical sensor is substantially perpendicular to the direction of the projected patterned EUV radiation beam.

10. The lithographic apparatus of claim 9, wherein the processor is configured to use the shape of the measured intensity to obtain information about a cross-sectional shape of the contamination.

11. The lithographic apparatus of claim 8, wherein the movement of the optical sensor is substantially parallel to the direction of the projected EUV radiation.

12. The lithographic apparatus according to any one of claims 8 to 11, wherein the area of ​​the optical sensor is larger than the area of ​​the aerial image of the alignment pattern, the alignment pattern being used to perform alignment measurements associated with the optical sensor, or / and The measurement of the contamination takes into account the intensity of the background radiation incident on the optical sensor.

13. The lithographic apparatus according to any one of claims 8 to 12, wherein the measured contamination is compared to a threshold value.

14. A lithographic apparatus comprising a patterning device support structure, a projection system and a substrate table provided with an optical sensor, wherein the lithographic apparatus comprises a processor.

15. A system for measuring contamination of an optical sensor of a lithography device, comprising the optical sensor and a processor, the processor being configured to: move the optical sensor relative to a projected patterned EUV radiation beam so that the intensity of EUV radiation measured by the optical sensor varies according to the sensor position and has a minimum value, and wherein the processor is also configured to use the measured intensity to measure the contamination of the optical sensor.

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