Pollution measurement

Abstract

A method for determining contamination of an optical sensor in a lithographic apparatus, the method comprising: illuminating a pattern onto a patterning device using EUV radiation; projecting the patterned and reflected EUV radiation towards the optical sensor, thereby forming an aerial image of the pattern; and moving the optical sensor relative to the patterned and reflected EUV radiation such that the intensity of the EUV radiation measured by the optical sensor varies as a function of the position of the optical sensor. The intensity measured by the optical sensor passes through a minimum value, and the method further comprises measuring the contamination of the optical sensor using the measured intensity.

Description

Technical Field

[0001] Cross-reference to related applications

[0002]

[0001] This application claims the priority of European Patent Application No. 22188166.7 filed on August 1, 2022, and incorporates the entire content thereof by reference into this application.

[0003]

[0002] The present invention relates to measuring the contamination of an optical sensor that forms part of a lithographic apparatus.

Background Art

[0004]

[0003] A lithographic apparatus is a machine constructed to apply a desired pattern onto a substrate. The lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). The lithographic apparatus can project a pattern in a patterning device (e.g., a mask) onto a layer of radiation-sensitive material (resist) provided on a substrate.

[0005]

[0004] To project a pattern onto a substrate, the lithographic apparatus can use electromagnetic radiation. The wavelength of this radiation determines the minimum size of the features that can be formed on the substrate. Using a lithographic apparatus that uses extreme ultraviolet (EUV) radiation having a wavelength in the range of 4 - 20 nm, for example 6.7 nm or 13.5 nm, smaller features can be formed on the substrate than using a lithographic apparatus that uses radiation having a wavelength of, for example, 193 nm.

[0006]

[0005] When a lithography substrate is exposed, the substrate is supported by a substrate table. The substrate table may comprise a substrate clamp and a base for holding the substrate clamp. The base may comprise a reflective surface that enables an interferometric measurement of the position of the substrate table. Over time, contamination can accumulate on the substrate table. In particular, contamination can accumulate in regions of the substrate table that are not covered by the substrate during lithography exposure. A sensor system is provided on the substrate table. The sensor system can be provided within the substrate table base (e.g., radially outside the substrate clamp). When contamination accumulates on the optical sensor of the sensor system, it can adversely affect the performance of the optical sensor. The substrate table may be cleaned, for example, using hydrogen radicals. However, such cleaning is time-consuming and lithography substrate exposure is interrupted during cleaning. For this reason, it is desirable not to clean the substrate table too frequently. Conversely, if the cleaning is delayed too much, the performance of the lithography apparatus may deviate from the desired parameters due to the accumulation of contamination on the sensor.

[0007]

[0006] It may be desirable to provide a method and apparatus that overcome or mitigate one or more problems associated with the prior art.

SUMMARY OF THE INVENTION

[0008]

[0007] According to one aspect of the invention, a method for determining contamination of an optical sensor in a lithography apparatus is provided, the method comprising illuminating a pattern on a patterning device using EUV radiation, projecting the patterned and reflected EUV radiation towards the optical sensor, thereby forming an aerial image of the pattern, and moving the optical sensor relative to the patterned and reflected EUV radiation such that the intensity of the EUV radiation measured by the optical sensor varies as a function of the position of the optical sensor, the intensity measured by the optical sensor passing through a minimum value, and the method further comprising measuring the contamination of the optical sensor using the measured intensity.

[0009]

[0008] Advantageously, the method provides a direct measurement of contamination in a straightforward manner. Since the contamination is directly measured, instead of estimating the contamination, cleaning can be carried out only when there is contamination at a level such that cleaning is required. This avoids cleaning being performed when it is not necessary (which occurs when cleaning is carried out based on an estimate of contamination). Thus, the frequency of cleaning can be reduced. Thereby, the productivity of the lithographic apparatus is improved. Embodiments of the invention can correct the measurement taking into account the influence of contamination, for example using a known relationship between the thickness of the contamination and the signal output from the optical sensor. This relationship can be determined by performing calibration measurements for different thicknesses of contamination.

[0010]

[0009] The depth of the minimum can be used to measure the contamination of the optical sensor.

[0011]

[0010] The movement of the optical sensor may be substantially parallel to the direction of the projected EUV radiation.

[0012]

[0011] The method may further comprise obtaining information regarding the cross-sectional shape of the contamination using the shape of the measured intensity.

[0013]

[0012] The movement of the optical sensor may be substantially parallel to the direction of the projected EUV radiation.

[0014]

[0013] The optical sensor may have an area larger than the area of the aerial image of the alignment pattern used to perform alignment measurements in relation to the optical sensor.

[0015]

[0014] The pattern illuminated for the measurement of contamination may be an alignment pattern.

[0016]

[0015] The measurement of contamination can take into account the intensity of the background radiation incident on the optical sensor.

[0017]

[0016] The measured contamination can be compared to a threshold value.

[0018]

[0017] According to a second aspect of the invention, there is provided a lithographic apparatus comprising a patterning device support structure, a projection system, and a substrate table provided with an optical sensor, the lithographic apparatus comprising a processor configured to move a sensing system relative to a projected and patterned EUV radiation beam such that the intensity of the EUV radiation measured by the optical sensor varies as a function of the sensor position and includes a minimum value, the processor being further configured to use the measured intensity to measure contamination of the optical sensor.

[0019]

[0018] Advantageously, the lithographic apparatus obtains a direct measurement of contamination in a straightforward manner.

[0020]

[0019] The processor may be configured to measure contamination of the optical sensor using the depth of the minimum value.

[0021]

[0020] The movement of the optical sensor may be substantially perpendicular to the direction of the projected and patterned EUV radiation beam.

[0022]

[0021] The processor may be configured to obtain information regarding the cross-sectional shape of the contamination using the shape of the measured intensity.

[0023]

[0022] The movement of the optical sensor may be substantially parallel to the direction of the projected EUV radiation.

[0024]

[0023] The optical sensor may have an area larger than the area of the aerial image of an alignment pattern used to perform alignment measurements associated with the optical sensor.

[0025]

[0024] The measurement of contamination may take into account the intensity of background radiation incident on the optical sensor.

[0026]

[0025] The measured contamination can be compared to a threshold value.

[0027]

[0026] According to a third aspect of the 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 invention.

[0028]

[0027] According to a fourth aspect of the invention, there is provided 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 of the first aspect of the invention.

[0029]

[0028] The features of different aspects of the invention can also be combined together.

Brief Description of the Drawings

[0030]

[0029] Some embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings.

[0031]

Figure 1

Figure 2A

Figure 2B

Figure 3

Figure 4A

Figure 4B

Figure 5A

Figure 5B

Figure 6A

Figure 6B

Figure 7

Figure 8

Figure 9

Figure 10

Figure 11

Figure 12

DETAILED DESCRIPTION OF THE INVENTION

[0032]

[0030] FIG. 1 shows a lithography system comprising a radiation source SO and a lithography apparatus LA. The radiation source SO is configured to generate an EUV radiation beam B and supply the EUV radiation beam B to the lithography apparatus LA. The lithography apparatus LA includes 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.

[0033]

[0031] The illumination system IL is configured to condition the EUV radiation beam B before the EUV radiation beam B is incident on the patterning device MA. In addition thereto, the illumination system IL may include a facet field mirror device 10 and a facet pupil mirror device 11. The facet field mirror device 10 and the facet pupil mirror device 11 together provide an EUV radiation beam B having a desired cross-sectional shape and a desired angular distribution. The illumination system IL may include other mirrors or devices in addition to or instead of the facet field mirror device 10 and the facet pupil mirror device 11.

[0034]

[0032] After being conditioned in this way, the EUV radiation beam B interacts with the patterning 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. For that purpose, the projection system PS can 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 can apply a reduction factor to the patterned EUV radiation beam B’ to form an image having features smaller than the corresponding features on the patterning device MA. For example, a reduction factor of 4 or 8 can be applied. In FIG. 1 the projection system PS is shown as having only two mirrors 13, 14, but the projection system PS can also include a different number of mirrors (e.g., six or eight mirrors).

[0035]

[0033] During the scan exposure, the patterning device MA and the support structure MT move in the y direction, and the substrate W and the substrate table WT move in the opposite y direction. In this way, the band of EUV radiation passes through the exposure field on the substrate W.

[0036]

[0034] The substrate W can include a pre-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 position of the alignment marks on the substrate table is measured relative to the position of the alignment marks 22 on the patterning device MA. This is done using a sensor system 24 that includes alignment marks. By aligning the patterning device MA with the substrate table WT, the patterning device can be aligned with the substrate W. The alignment marks 22 can include a variety of different shapes and / or patterns. At least some of the patterns may be grids.

[0037]

[0035] A relative vacuum, i.e., a small amount of gas (e.g., hydrogen) at a pressure much lower than atmospheric pressure, may be provided within the radiation source SO, the illumination system IL, and / or the projection system PS.

[0038]

[0036] The radiation source SO shown in FIG. 1 is of a type that can be referred to, for example, as a laser-produced plasma (LPP) source. A laser system 1, which may include, for example, a CO2 laser, is arranged to deposit energy via a laser beam 2 onto a fuel such as tin (Sn) provided, for example, from a fuel dispenser 3. Although tin is referred to in the following description, any suitable fuel may be used. The fuel can be, for example, in liquid form or, for example, a metal or alloy. The fuel dispenser 3 can comprise a nozzle configured to direct, for example, tin in the form of droplets along a trajectory towards the plasma formation region 4. The laser beam 2 is incident on the tin in the plasma formation region 4. The deposition of laser energy onto the tin generates a tin plasma 7 in the plasma formation region 4. During the de-excitation and recombination of electrons with ions of the plasma, radiation including EUV radiation is emitted from the plasma 7.

[0039]

[0037] EUV radiation from the plasma is collected and focused by the collector 5. The collector 5 includes, for example, a near-normal incidence radiation collector 5 (which may also be more generally referred to as a normal incidence radiation collector). The collector 5 may have a multilayer mirror structure arranged to reflect EUV radiation (for example, EUV radiation having a desired wavelength such as 13.5 nm). The collector 5 may have an ellipsoidal configuration with two foci. As will be described below, the first of the foci may be in the plasma formation region 4 and the second of the foci may be in the intermediate focus 6.

[0040]

[0038] The laser system 1 may be spatially separated from the radiation source SO. In this case, the laser beam 2 may be sent from the laser system 1 to the radiation source SO using, for example, a beam delivery system (not shown) including suitable guiding mirrors and / or beam expanders, and / or other optical systems. The laser system 1, the radiation source SO, and the beam delivery system can be regarded as a radiation system as a whole.

[0041]

[0039] The radiation reflected by the collector 5 forms an EUV radiation beam B. The EUV radiation beam B is focused on the intermediate focus 6 and forms an image of the plasma existing in the plasma formation region 4 at the intermediate focus 6. The image of the intermediate focus 6 functions as a virtual radiation source for the illumination system IL. The radiation source SO is arranged such that the intermediate focus 6 is arranged at or near the opening 8 of the enclosure structure 9 of the radiation source SO.

[0042]

[0040] In FIG. 1, the radiation source SO is shown as a laser-produced plasma (LPP) source, but EUV radiation can also be generated using any suitable radiation source such as a discharge-produced plasma (DPP) source or a free electron laser (FEL).

[0043]

[0041] Contamination accumulates in the sensors of the sensor system 24. In one example, contamination accumulates in the optical sensor of the sensing system, which may cause the optical sensor to give an inaccurate output. As a result, the alignment of the projection pattern with respect to the pattern already present on the substrate W may deteriorate. Embodiments of the present invention address this problem.

[0044]

[0042] FIGS. 2A and 2B schematically show the sensor system 24 provided on the substrate table W. FIG. 2A is a top view of the sensor system, and FIG. 2B shows the sensor system in cross section. The sensor system 24 is composed of optical sensors 25 to 27 of different sizes. The sensor system 24 may include other optical sensors.

[0045]

[0043] The sensor system 24 includes a substrate 40 in which detectors 28 to 30 are formed. An opaque material layer 42 is provided on the substrate, and openings 31 to 33 are formed in the opaque material above the detectors 28 to 30. The amount of EUV radiation incident on each detector 28 to 30 is controlled by the size of each opening 31 to 33. The sensor system 24 may include a semiconductor and may be formed using lithography.

[0046]

[0044] The first optical sensor 25 has a square shape when viewed from above and includes a detector 28 (e.g., a photodiode) surrounded by the opening 31. The first optical sensor 25 can have an area of, for example, about 50×50 μm 2 . The shape of the optical sensor 25 can be defined by the opening 31. As will be further described below, the optical sensors of this embodiment are square, but the optical sensors can have any shape.

[0047]

[0045] The second optical sensor 26 includes a detector 29 (e.g., a photodiode) surrounded by the opening 32. A grating 34 is formed in the opaque material layer 42. The grating 34 extends in the X direction.

[0048]

[0046] The third optical sensor 27 includes a detector 30 (e.g., a photodiode) surrounded by the aperture 33. A grating 35 is formed in the opaque material layer 42. The grating 35 extends in the Y direction. This grating 35 is not shown in FIG. 2B in order to avoid complicating the drawing.

[0049]

[0047] In use, the optical sensors 25 - 27 can be used to ensure alignment of the patterning device MA with the substrate table W (see FIG. 1). The method of achieving this is schematically illustrated in FIG. 3. 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 by the projection system PS (see FIG. 1) to form an aerial image 44 of the alignment mark 22. The projection system PS typically includes a reduction factor, and as a result, the aerial image 44 may 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 only one alignment mark and optical sensor are shown in FIG. 3, the same method is used for the other alignment marks and optical sensors.

[0050]

[0048] When aligning the patterning device MA with respect to the substrate WT, the alignment can be performed in two parts. In the first part, so-called coarse alignment is performed. In the coarse alignment, the substrate table WT is moved a considerable distance, usually several hundred microns (e.g., 200 microns) by scanning movement, whereby the aerial image 44 of the first alignment mark completely crosses the first detector 28. The alignment mark may also be called a capture mark in order to enable "capturing" the rough positioning of the substrate table WT with respect to the patterning device MA. The capture mark 40 may be square. However, the shape of the alignment mark is arbitrary. Similarly, the first detector 28 is square, but may have any shape. The detector 28 needs to have an area larger than the area of the aerial image of the alignment mark. Simultaneously with the capture mark aerial image passing through the first detector 28, a plurality of periods of the aerial image of the X-direction grating pass through the X-direction grating detector 29.

[0051]

[0049] Next, the substrate table WT is moved in an oblique direction (+X direction and -Y direction). Subsequently, the substrate table WT is moved in a scanning operation in the Y direction, usually by several hundred microns (e.g., 200 microns) at a time. As a result, the capture mark aerial image 44 completely passes through the first detector 28 in the Y direction. The Y-direction grating aerial image passes through a plurality of periods of the Y-direction grating detector 30.

[0052]

[0050] Coarse alignment makes it possible to determine the position of the substrate table with an accuracy of about several tens of nanometers or more. Subsequently, a second alignment process is performed. The second alignment process may be referred to as fine alignment. In fine alignment, the substrate table is positioned such that the aerial image 44 of the capture mark is centered on the first detector 26, and the X- and Y-direction grating aerial images are also centered with respect to the X and Y grating detectors 29, 30. Next, a small movement of usually up to 2 microns in the diagonal direction is performed. The signals obtained as a result from the X and Y grating detectors 29, 30 indicate the relative positions of the grating aerial images and the X and Y grating detectors 29, 30. This signal can be used to align the substrate table WT to the patterning device MA with an accuracy of nanometers or less than nanometers.

[0053]

[0051] The sensor system 24 can also be used to ensure that the substrate table W is in the correct Z-direction position with respect to the focal plane of the projection system PS of the lithographic apparatus. For this purpose, the substrate table WT and the sensor system 24 can be moved by a distance of about 100 microns in the Z direction. The position in the Z direction can be determined with an accuracy of several nanometers or more.

[0054]

[0052] During coarse alignment and fine alignment, the alignment marks on the patterning device MA are illuminated by the EUV radiation beam B, as a result of which EUV radiation is incident on the sensor system 24. Thereby, contamination occurs due to the interaction between the EUV radiation and gas molecules or other contaminants in the vicinity of the sensor system 24. As described above, during fine alignment, the aerial image of the capture mark 44 is centrally located above the first detector 28. During coarse alignment, the substrate table WT and the sensor system 24 are moved in a scanning operation under 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 occurs. As a result, the EUV radiation is consistently incident on the central region of the first detector 28, and contamination accumulates in the central region. The contamination has the shape of the aerial image of the capture mark 44 projected during fine alignment. The same applies to 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. As a result, contamination in the shape of the aerial image 44 does not accumulate in a part of the region of the first detector 28. The same applies to the other marks and detectors of the sensor system.

[0055]

[0053] Embodiments of the present invention measure the thickness of the contamination formed in the center (i.e., the contamination accumulated in the central region of the detector). By combining the contaminated region and the non-contaminated region on the detector, it becomes possible to measure the thickness of the contamination. The combination of the contaminated region and the non-contaminated region occurs because the aerial image of the alignment mark is smaller than the region of the detector that detects the alignment mark. By measuring the thickness of the contamination, it is advantageously possible to clean the sensor system 24 when the thickness of the contamination requires cleaning. This is superior to estimating the thickness of the contamination based on the cumulative dose of EUV radiation projected onto the sensor system 24 and cleaning the contamination based on that estimate. This is because if the estimate is used, there is a possibility that cleaning will be performed when it is not necessary.

[0056]

[0054] In some embodiments, contamination may have a known effect on the output signal provided by the sensor of the sensor system. For example, for position measurements in the Z direction, a certain thickness of contamination may cause a known offset between the measured position and the actual position. This offset may be taken into account to determine the actual position. Thus, in some embodiments, the measurement error can be corrected by measuring the thickness of the contamination. In some embodiments, instead of correcting the measurement error, the effect of the measurement error can be taken into account when adjusting the lithographic apparatus. For example, if it is known that the output of the detector is offset by a certain amount when there is a contamination of a specific thickness, the adjustment of the lithographic apparatus can be performed based on the combination of the output of the detector and the known offset.

[0057]

[0055] FIG. 4A schematically shows the scan operation of the first detector 28 under the aerial image of the capture mark 44 during coarse alignment, and schematically shows the signal S output from the first detector 28 when this is completed. Other movements of the first detector 28 may be used to acquire the output signal (further described below). The signal S is proportional to the intensity I of the light detected by the detector 28 as a function of time t. As shown, the aerial image 44 starts 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 indicated by the dashed arrow. When the aerial image 44 overlaps the detector 28, the output signal increases linearly. The output signal reaches a maximum when the aerial image 44 of the capture mark is fully positioned over the detector 28. The output signal remains at the maximum until the aerial image of the capture mark 44 starts to move away from the detector 28 (i.e., until the overlap between the aerial image and the detector 28 starts to decrease). The signal drops to zero. Next, the detector 28 is moved diagonally (as indicated by the dashed arrow) until the aerial image 44 is aligned with the detector 28 in the X direction and away from the detector in the Y direction. Next, a scan in the Y direction is performed. This results in the same output signal being generated.

[0058]

[0056] FIG. 4B schematically shows the signal output from detector 28 when there is contamination in the detector. FIG. 4B also schematically shows the contamination on detector 28, which is shown 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 capture mark. This is because, as further explained above, during fine alignment, some movement of detector 28 with respect to aerial image 44 occurs, causing the contamination 45 to spread slightly.

[0059]

[0057] In FIG. 4B, the coarse alignment scan again starts with a scan in the X direction, followed by a scan in the Y direction. During the scan in the X direction, the signal starts at zero and reaches a maximum when the entire aerial image 44 is over detector 28. However, when the aerial image 44 starts to overlap with the contamination 45, the output signal decreases. This is because the contamination 45 absorbs EUV radiation, reducing the amount of EUV radiation incident on detector 28. The output signal continues to decrease and reaches a minimum when the aerial image 44 is completely over the contamination 45. As the aerial image 44 moves away from the contamination 45, the signal starts to increase and reaches a maximum again when the aerial image is completely over the non - contaminated part of detector 28. Thereafter, as the aerial image moves away from detector 28, the signal gradually decreases to zero.

[0060]

[0058] Next, a coarse alignment scan is performed in the Y direction, generating the same output signal (as shown in the figure).

[0061]

[0059] In an embodiment of the present invention, the presence of a minimum value 50 in the output signal is used to identify whether contamination is present. The depth of the minimum value 55 indicates the thickness of the contamination 45 present on detector 28. Using the depth of the minimum value 55, it is possible to determine when the thickness of the contamination present on detector 28 reaches a threshold amount and the detector needs to be cleaned. This threshold can be identified, for example, by determining that the depth of the minimum value 50 has reached the threshold.

[0062]

[0060] In the embodiments shown in FIGS. 4A and 4B, the size of the detector is such that the entire aerial image 44 of the alignment mark (capture mark in this case) can enter a region of the detector 28 that is not significantly contaminated. However, embodiments of the present invention can also handle situations where this is not the case. Such embodiments are shown in FIGS. 5A and 5B. As shown in the figures, the detector 51 is smaller than the detectors shown in FIGS. 4A and 4B. The aerial image 44 of the capture mark is the same size as that shown in FIGS. 4A and 4B.

[0063]

[0061] Referring to FIG. 5A, when there is no contamination, the signal output from the detector 24 is equivalent to the signal seen in FIG. 4A. In particular, the signal has a flat central portion for X - direction scanning and Y - direction scanning.

[0064]

[0062] FIG. 4B schematically shows the contamination on the detector 28, which is shown as a dark region 51 in the center of the detector. For the reasons explained above, the contaminated region 51 is slightly larger than the aerial image 44 of the capture mark (however, embodiments of the present invention will function even if the contaminated region is the same size as or smaller than the aerial image).

[0065]

[0063] In FIG. 5B, the maximum signal output from the detector 28 is lower than the maximum signal seen in FIG. 5A. This is because there is no position of the aerial image 44 of the capture mark relative to the detector 51 that gives a complete signal from the detector (there is always an overlap with the contamination or a part of the aerial image does not overlap with the detector). However, as schematically shown in FIG. 5B, when the aerial image passes through the contaminated region 51 of the detector 28, the output signal passes through a minimum value 52. Similar to the embodiment of FIG. 4B, the presence of the minimum value 52 can be used to determine whether contamination is present. The depth of the minimum value 52 can be used to measure the thickness of the contamination 51. A threshold minimum depth, or other threshold characteristics, can be set. When this threshold is exceeded, cleaning of the detector 28 can be performed.

[0066]

[0064] In the above embodiment, the vertical scanning operation of the detector 28 with respect to the aerial image is used. However, in other embodiments, the contamination of the detector can also be measured using the single-direction scanning operation of the detector 28.

[0067]

[0065] References in this document to the movement of the detector 28 can be interpreted as being related to the movement of the sensor system 24.

[0068]

[0066] In the above embodiment, the scanning operation of the detector 28 with respect to the aerial image 44 is used. However, the detector 28 may instead be stepped between different positions with respect to the aerial image 44. This can be performed, for example, when it is desirable to obtain more accurate individual measurement values. When the detector 28 remains in its position for a certain period of time instead of moving continuously like in a scan, more photons will be incident on the detector. This improves the signal-to-noise ratio and enables more accurate measurements. Since the general shape of the signal, seen as a function of the X and Y positions of the detector 28, does not change significantly, it is already known. This means that the positions of the step measurements (also called discrete measurements) can be selected to provide sufficient data to accurately characterize the shape of the signal output from the detector 28. For example, the measurement positions may be selected such that they can characterize the dips 50, 52.

[0069] [

[0067] ] FIGS. 6A and 6B schematically illustrate another embodiment of the present invention. Referring first to FIG. 6A, the effect of moving detector 28 in the Z direction when the aerial image is centered on the detector is schematically shown. Detector 28 is schematically shown within the focal plane of the projection system. The patterned EUV radiation beam B' carrying the aerial image is also schematically shown. The patterned radiation beam B' has a minimum diameter at the focal plane of the projection system. When detector 28 is in the focal plane, the aerial image of the alignment mark is in focus on the detector. As detector 28 moves in the Z direction away from the focal plane, the focus of the alignment mark image becomes increasingly blurred. That is, the alignment mark image spreads over a wider distance.

[0070] [

[0068] ] FIG. 6A schematically shows the signal S output from detector 28 as detector 28 is moved in the Z direction (from below the focal plane to above the focal plane). Since detector 28 starts below the focal plane, the projected image of the capture mark is out of focus. As a result, the EUV radiation is scattered and part of the EUV radiation will be outside detector 28. As detector 28 moves towards the focal plane of the projection system, the aerial image of the capture mark is brought into focus and becomes smaller. The amount of EUV radiation incident on detector 28 increases and signal S increases. As the aerial image of the capture mark approaches focus, all of the EUV radiation is incident on the detector and output signal S reaches a maximum value. Signal S remains at the maximum value while the image of the alignment mark is fully overlapping detector 28. When detector 28 moves away from the focal plane and part of the EUV radiation falls outside the detector, the signal decreases again.

[0071]

[0069] Referring to FIG. 6B, when there is contamination 45 on detector 28, a minimum value 54 can be seen in the signal S output from the detector. The presence of the minimum value 54 indicates that there is contamination 45 on detector 28. The depth of the minimum value 54 indicates the thickness of the contamination present on detector 28. The threshold depth of the minimum value 54 can be set. When the threshold is reached, detector 28 can be cleaned. Similar to other embodiments, this embodiment can be used to measure the thickness of the contamination regardless of whether the contaminated area is larger than, smaller than, or the same size as the aerial image area. The contaminated area needs to be smaller than the area of the detector.

[0072]

[0070] FIG. 7 is a graph showing the signal output from detector 28 when the method shown in FIG. 4B is executed. The vertical axis of the graph indicates the power S measured by the detector, and the horizontal axis indicates the position of the detector in the X direction. The line shows the result of a scan measurement (i.e., continuous measurement while detector 28 is moving). The circles show the results of individual (discrete) measurements. As can be seen from FIG. 7, the discrete measurements are sufficient to characterize the shape of signal S.

[0073]

[0071] Use mathematical formulas to characterize the signal, thus enabling the measurement of the thickness of the contamination on the detector. In the mathematical formula, it is assumed that the detector is square and the alignment mark is also square. The following symbols are used. S: Power measured by the detector D: Length of the side of the detector m: Length of the side of the aerial image of the alignment mark k: Length of the side of the contamination (assuming the contamination is generally square for simplicity, even if its shape deviates from this later. That is, k is within a few percent of m) I: EUV power per unit area incident on the patterning device MA (since the value is relative, the power does not need to be in watts) R M : Reflectivity of the alignment mark (e.g., about 65%) R abs : Reflectivity of the absorber region surrounding the alignment mark (which can be, for example, 2%) b: Maximum signal measured by the sensor c: Dip in the signal during the scan (the aerial image of the alignment mark is completely on top of the contamination) a: Background signal when the aerial image does not overlap with the detector cont: Signal loss due to contamination (can be a few percent)

[0074]

[0072] Here, for the sake of clarity, it is assumed that the contamination layer has a uniform thickness (constant profile) across the contamination region. However, embodiments of the present invention also function when the contamination is non-uniform (further explained below).

[0075]

[0073] The EUV power I per unit area represented above is the power incident on the patterning device MA. The transmission of EUV radiation by the projection system PS (see FIG. 1) of the lithographic apparatus reduces the power per unit area. However, this transmission loss is constant and is canceled out in the calculations, so it can be ignored in the calculations used in the present invention (similarly for I).

[0076]

[0074] When the relationship between the contamination thickness and the signal loss is known, the signal loss due to contamination cont can be converted into a contamination thickness. For example, there may be a signal loss of about 0.5% per nanometer of contamination thickness. In such a case, when the signal loss cont is 60%, this corresponds to a contamination thickness of about 120 nm.

[0077]

[0075] FIG. 8 shows measurements when the detector 28 is large enough such that the aerial image 44 of the alignment mark can completely enter the non-contaminated region of the detector 28. The equation characterizing the power S measured by the detector for each position of the aerial image is shown below. In FIG. 8, the reflectivity R of the absorber abs is set to zero, and as a result, no background signal a is seen. The equation is as follows.

Equation

[0078]

[0076] For the scenario shown in FIG. 8, the equation is simplified as follows.

Number

[0079]

[0077] In Equation 1, the size of dip c is normalized by dividing by the maximum signal b. If there is no dip, the fraction c / b is 1, so the value of cont is 0. If dip c exists, 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 gives a direct characterization of the thickness of the contamination.

[0080]

[0078] In the modified scenario, the detector 28 is large enough such that the aerial image 44 of the alignment mark can fully enter the non - contaminated region of the detector 28, but the reflectivity R abs of the absorber is not zero. Since R abs is not zero, a background signal a is seen (in fact, it is common to see some background signal).

[0081]

[0079] For this scenario, the following equation characterizes the contamination cont.

Number

Number

[0082]

[0080] Figure 9 shows the measurement when the detector 28 is small, whereby the aerial image 44 of the alignment mark cannot fully enter the non-contaminated area of the detector 28. In Figure 9, the reflectivity R of the absorber abs is zero, and as a result, the background signal a is not seen.

[0083]

[0081] For the scenario shown in Figure 9, the following equation characterizes the contamination cont.

Equation

[0084]

[0082] The value of cont gives a direct characterization of the thickness of the contamination, with the approximation that there is no background signal.

[0085]

[0083] The size of the dip 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 dip can also be called a minimum value.

[0086]

[0084] Embodiments of the present invention use a square alignment mark and a square detector. However, the shapes of the alignment mark and the detector are arbitrary. The aerial image of the alignment mark needs to be smaller than the detector. Thereby, when the movement of the detector is small or negligibly small during the alignment measurement, a part of the detector area is not illuminated by the alignment mark. As a result, a part of the detector area is contaminated and a part of the detector area is not contaminated. Embodiments of the present invention advantageously use this combination of the contaminated area and the non-contaminated area to measure the thickness of the contamination. The detector and the alignment mark do not have to be of a single solid shape (e.g., square, rectangular, circular). The detector and the alignment mark may have complex shapes such as a checkerboard arrangement (i.e., an arrangement of squares).

[0087]

[0085] In the described embodiments of the present invention, the aerial image used when measuring the thickness of the contamination is the same shape and size as the aerial image used during the alignment measurement. However, the aerial image used when measuring the thickness of the contamination may be different in shape and / or size from the aerial image used during the alignment measurement. The aerial image used when measuring the thickness of the contamination may have an area larger than the area of the detector. However, the aerial image used when measuring the thickness of the contamination still needs to pass through positions that provide different amounts of overlap between the aerial image and the detector.

[0088]

[0086] In the above embodiments, it is assumed that the thickness of the contamination is uniform. However, the thickness of the contamination may be non-uniform. Embodiments of the present invention can characterize changes in the thickness of the contamination.

[0089]

[0087] FIG. 10 schematically shows a scenario where the thickness of the contamination is not uniform. In FIG. 10, the aerial image initially partially overlaps the contamination. In this situation, the signal output from the detector is represented as follows.

Equation

Equation

[0090]

[0088] FIG. 11 shows contaminations with four different cross-sectional shapes. These are called uniform, linear, inverse linear, and inverse parabolic. FIG. 12 shows the signals seen for each contamination cross-sectional shape (calculated using the above equations). As can be seen from FIG. 12, the signals have different curve shapes for each contamination cross-sectional shape (although the curve shapes of linear and inverse parabolic are very similar).

[0091]

[0089] Embodiments of the present invention can analyze the shape of a signal to obtain information regarding the cross-sectional shape of contamination. The shape of the signal can be determined, for example, using conventional curve fitting. Knowing the cross-sectional shape of the contamination advantageously enables the selection of a method for performing cleaning accordingly.

[0092]

[0090] When it is desirable to obtain information regarding the cross-sectional shape of contamination, the measurement of the detector output can be performed with higher accuracy (and / or more measurements can be performed) than when it is only desirable to measure the overall thickness of the contamination. In other words, when the characterization of the shape of the signal is desirable, more accurate measurements and / or more measurements can be performed compared to when it is desirable to determine the depth of the dip of the signal.

[0093]

[0091] When embodiments of the present invention are used, the signal output from the detector is self-consistent. That is, the signal depends on the geometric shape of the alignment mark (and may also depend on the geometric shape of the detector). The signal does not depend on parameters such as the reflectivity of the alignment mark, which may change over time (and may also differ between different lithographic apparatuses). Thus, embodiments of the present invention enable comparison between measurements performed with the same optical sensor at different times. Embodiments of the present invention also enable comparison of contamination measurements with different optical sensors within the same or different lithographic apparatuses.

[0094]

[0092] Embodiments of the present invention can measure contamination using a plurality of different detectors within the same lithographic apparatus. This enables independent measurement of the thickness of the contamination for each detector.

[0095]

[0093] Embodiments of the present invention also propose a measurement method that enables measurement of the light transmittance of a grating detector, thereby enabling determination of the transmittance drift caused by contamination growing on the surface over time.

[0096]

[0094] In this method, the SS-SLS fiducial is used as a reticle stage reflector so that EUV light is projected onto the sensor. The SS-SLS fiducial mainly consists of a large rectangular portion with a multilayer blank, ensuring the highest and uniform reflection of most of the EUV slit. Thus, the slit is projected uniformly at the wafer level.

[0097]

[0095] Here, the TIS plate can be positioned under the slit so that its detector can be fully illuminated. In particular, for reticle alignment, for example, the normal set 2 (Txh,Tyh) grid detector is fully illuminated by light (in technical terms, "flooding"), so its signal is independent of the alignment conditions. Therefore, the signal S grating measured by the illuminated detector can be described as follows.

Equation

[0098]

[0096] First, by "flooding" the detector, 100% of the area is illuminated, so term A grating can be ignored. Of course, when "flooding" the detector, there is a risk that the detector signal will saturate. This can be corrected by reducing the gain of the sensor, but this is not always possible. In such cases, it may be possible to compensate for the risk of clipping in the signal by using different illumination settings or using pupils with a low pupil filling ratio. Without sacrificing performance, the total intensity can be easily reduced to 20% or less. Therefore, once the desired pupil is determined, it can be reused for each measurement.

[0099]

[0097] To avoid systematic errors due to the asymmetry of the slit or fiducial reflectivity, or the long-term drift of the fiducial reflectivity itself, measurements are always performed at the same nominal positions of the slit and fiducial, and terms C slit and R fiducial of the slit and fiducial can be ignored. Furthermore, note that the drift of the fiducial reflectivity occurs on a time scale of several months, which is much longer than the drift due to sensor contamination, and can therefore be ignored in principle.

[0100]

[0098] To neutralize the effect of defocus, the alignment position of the beam is measured in advance, and then the intensity at the focus is measured, or the measurement of the grating intensity is performed either while scanning the beam focus plane vertically or through a sequence of stationary scans along the same Z-axis to determine the peak intensity at the focus.

[0101]

[0099] For example, to suppress noise due to source fluctuations, the signal of any detector to be used for measurement can be recorded and averaged over a certain period of time to suppress noise in the signals of all sources. Furthermore, the signal can be normalized not only to neutralize source fluctuations, but also to neutralize a more important measurement system. Various methods are possible. For example, the signal of a nearby ratio detector that also performs the same normalization function during reticle alignment can be used. In this case, the ratio detector signal can be described by the same formula as the grating, i.e.,

Number

[0102] [000100] Of course, the ratio detector is also affected by drift due to contamination. In this case, since the term T ratio can be independently estimated by the method shown in the previous embodiment, it can also be corrected by the corresponding value C ratio . Therefore, assuming no focus shift and the detector area is fully illuminated, the following can be calculated.

Number

Number

[0103] [000101] T oc It should be noted that the drift of T is expected to occur on a time scale much longer than the characteristics of sensor contamination in principle, so it may be negligible at least in the short term. On the other hand, by utilizing this characteristic and comparing two normalization methods, it is possible to derive a KPI that represents the drift over time of the transmission loss of the optical column, especially focusing on POB. [Number]

[0104] [000102] By expanding this method and repeating the measurement at multiple locations across the entire slit, it is possible to map the transmission loss of the entire POB over time. For further cancellation of any drift in the POB transmittance, the ES readout is matched with the measurement of a spot slit sensor (SS-SLS) that addresses it at the wafer level. Since the SS-SLS is also affected by contamination, the inter-sensor matching of the SS-SLS between the measurement sensor and the anchor sensor can be utilized. Matching the ES to the SS-SLS is already an available measurement and is known by the name of FSLIE (Full Slit Integrated Energy) measurement, and the matching of the sensor and the anchor is the same. Therefore, the following can be obtained. [Number] Finally, by reversing the above KPI, it is also possible to alternatively determine the contamination of the SS-SLS itself. [Number]

[0105] [000103] The method according to an embodiment of the present invention can be executed by a computing device. The device may include a central processing unit ("CPU") to which a memory is connected. The method described herein may be implemented in code (software) stored on a memory including one or more storage media and configured to be executed on a processor including one or more processing units. The storage media may be integrated with the CPU and / or separated from the CPU. The code, which can be referred to as instructions, is fetched from the memory and executed on the processor to perform operations in accordance with the embodiments described herein. Alternatively, it is not excluded that some or all of the functions of the CPU are implemented in dedicated hardware circuits or configurable hardware circuits such as FPGAs. Generally, this method can be executed by a processor.

[0106] [000104] The computing device may include an input configured to enable a user to input data into a software program being executed on the CPU. Input devices include a mouse, keyboard, touch screen, microphone, etc. The computing device may further include an output device configured to output measurement results to the user.

[0107] [000105] Although the lithographic apparatus is specifically referred to herein for use in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications. Possible other applications include the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat panel displays, liquid crystal displays (LCDs), thin film magnetic heads, etc.

[0108] [000106] Although embodiments of the present invention may be specifically referred to in relation to a lithographic apparatus herein, embodiments of the present invention may be used in other apparatuses. Embodiments of the present invention may form part of an apparatus for measuring or processing an object such as a mask inspection apparatus, a metrology apparatus or a wafer (or other substrate) or a mask (or other patterning device). These apparatuses may collectively be referred to as lithographic tools. Such lithographic tools may use vacuum conditions or ambient (non-vacuum) conditions.

[0109] [000107] Although specific reference has been made above to the use of embodiments of the present invention in the context of optical lithography, it will be appreciated that the present invention may be used in other applications, such as imprint lithography, and is not limited to optical lithography if the circumstances allow.

[0110] [000108] If the circumstances allow, embodiments of the present invention may be implemented in hardware, firmware, software, or any combination thereof. Embodiments of the present invention may also be implemented as instructions stored on a machine-readable medium and read and executed 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 computer device). For example, a machine-readable medium may include read only memory (ROM), random access memory (RAM), magnetic storage media, optical storage media, flash memory devices, or electrical, optical, acoustic, or other forms of propagated signals (e.g., carrier waves, infrared signals, digital signals, etc.). Also, in this specification, firmware, software, routines, instructions may sometimes be described as performing some action. However, such descriptions are for convenience only and such actions are actually performed by a computing device, processor, controller, or other device executing firmware, software, routines, instructions, etc., and it should be understood that an actuator or other device may interact with the physical world in that case.

[0111] [000109] Although the specific embodiments of the present invention have been described so far, it goes without saying that the present invention can be implemented in ways other than those described. The above description is intended as an illustration rather than a limitation. Therefore, as will be apparent to those skilled in the art, the invention described herein may be modified without departing from the scope of the appended claims.

[0112]

[0110] Clause 1. A method for 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 EUV radiation that has been patterned and reflected towards the optical sensor, thereby forming an aerial image of the pattern; Moving the optical sensor with respect to the EUV radiation that has been patterned and reflected such that the intensity of the EUV radiation measured by the optical sensor varies as a function of the position of the optical sensor; The intensity measured by the optical sensor passes through a minimum value, and the method further comprises measuring the contamination of the optical sensor using the measured intensity. 2. The method according to clause 1, wherein the depth of the minimum value is used to measure the contamination of the optical sensor. 3. The method according to clause 1 or 2, wherein the movement of the optical sensor is substantially parallel to the direction of the projected EUV radiation. 4. The method according to clause 3, wherein the method further comprises obtaining information regarding the cross-sectional shape of the contamination using the shape of the measured intensity. 5. The method according to clause 1 or 2, wherein the movement of the optical sensor is substantially parallel to the direction of the projected EUV radiation. 6. The method according to any one of clauses 1 to 5, wherein the optical sensor has an area larger than the area of the aerial image of the alignment pattern used to perform alignment measurements in relation to the optical sensor. 7. The method according to clause 6, wherein the pattern illuminated for the measurement of the contamination is an alignment pattern. 8. A method according to any one of claims 1 to 7, wherein the measurement of contamination takes into account the intensity of the background radiation incident on the optical sensor. 9. A method according to any one of claims 1 to 8, wherein the measured contamination is compared with a threshold value. 10. 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 configured to move a sensing system relative to a projected and patterned EUV radiation beam such that the intensity of the EUV radiation measured by the optical sensor varies as a function of the sensor position and includes a minimum value, and the processor is further configured to measure the contamination of the optical sensor using the measured intensity. 11. A lithographic apparatus according to claim 10, wherein the processor is configured to measure the contamination of the optical sensor using the depth of the minimum value. 12. A lithographic apparatus according to claim 10 or 11, wherein the movement of the optical sensor is substantially perpendicular to the direction of the projected and patterned EUV radiation beam. 13. A lithographic apparatus according to claim 12, wherein the processor is configured to obtain information regarding the cross-sectional shape of the contamination using the shape of the measured intensity. 14. A lithographic apparatus according to claim 10 or 11, wherein the movement of the optical sensor is substantially parallel to the direction of the projected EUV radiation. 15. A lithographic apparatus according to any one of claims 10 to 14, wherein the optical sensor has an area larger than the area of the aerial image of an alignment pattern used to perform alignment measurements in relation to the optical sensor. 16. A lithographic apparatus according to any one of claims 10 to 15, wherein the measurement of contamination takes into account the intensity of the background radiation incident on the optical sensor. 17. A lithographic apparatus according to any one of claims 10 to 16, wherein the measured contamination is compared with a threshold value. 18. A computer-readable storage medium comprising instructions that, when executed by a processor of a computing device, cause the computing device to perform a method according to any one of claims 1 to 9. A computing device including a processor and a memory, the memory storing instructions that, 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 for 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 EUV radiation that has been patterned and reflected towards the optical sensor, thereby forming an aerial image of the pattern; Moving the optical sensor relative to the patterned and reflected EUV radiation such that the intensity of the EUV radiation measured by the optical sensor varies as a function of the position of the optical sensor; The intensity measured by the optical sensor passes through a minimum value, and the method further comprises measuring the contamination of the optical sensor using the measured intensity.

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

3. The method according to claim 2 further comprises obtaining information regarding the cross-sectional shape of the contamination using the shape of the measured intensity.

4. The movement of the optical sensor is substantially parallel to the direction of the projected EUV radiation, according to the method of claim 1.

5. The optical sensor has an area larger than the area of the aerial image of an alignment pattern used to perform alignment measurements associated with the optical sensor, according to any one of claims 1 to 4.

6. The pattern illuminated for the measurement of the contamination is the alignment pattern, according to the method of claim 5.

7. The measurement of the contamination takes into account the intensity of background radiation incident on the optical sensor, and / or the measured contamination is compared to a threshold value, according to any one of claims 1 to 6.

8. A lithographic apparatus comprising a patterning device support structure, a projection system, and a substrate table provided with an optical sensor, the lithographic apparatus comprising a processor configured to move the optical sensor relative to the projected and patterned EUV radiation beam such that the intensity of the EUV radiation measured by the optical sensor varies as a function of the sensor position and includes a minimum value, the processor being further configured to measure the contamination of the optical sensor using the measured intensity.

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

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

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

12. The optical sensor has an area larger than the area of the aerial image of the alignment pattern used to perform alignment measurements in relation to the optical sensor, or / and The lithographic apparatus according to any one of claims 8 to 11, wherein 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 with 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, the lithographic apparatus including a processor.

15. A system for measuring the contamination of an optical sensor for a lithographic apparatus, the system comprising the optical sensor and a processor configured to move the optical sensor relative to a projected and patterned EUV radiation beam such that the intensity of the EUV radiation measured by the optical sensor varies as a function of the sensor position and includes a minimum value, the processor being further configured to measure the contamination of the optical sensor using the measured intensity.

Images