Pollutant determination

Abstract

A method of determining contamination of an optical sensor of a sensing system in a lithographic apparatus, the method comprising: directing EUV radiation through an opening in a reticle masking blade (26) and onto a patterning device; projecting the reflected EUV radiation onto a sensing system, thereby causing accumulation of a contaminant region; measuring a height of the contaminant region and a height of a region in the sensing system that does not receive the reflected EUV radiation; and determining an amount of contaminant on an optical sensor of the sensing system using the measured height.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to EP application 22187852.3 filed on July 29, 2022, which is incorporated herein by reference in its entirety. Technical Field

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

[0004] A lithographic apparatus is a machine configured to apply a desired pattern onto a substrate. A lithographic apparatus may be used, for example, in the manufacture of integrated circuits (ICs). A lithographic apparatus may, 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. Compared to a lithographic apparatus that may use radiation with a wavelength of 193 nm, for example, a lithographic apparatus that uses extreme ultraviolet (EUV) radiation with a wavelength in the range of 4-20 nm (e.g., 6.7 nm or 13.5 nm) may be used to form smaller features on a substrate.

[0006] When the lithographic substrate is exposed, the lithographic substrate is supported by a substrate table. The substrate table may include a substrate clamping portion and a base for holding the substrate clamping portion. The base may include a reflective surface, which allows the position of the substrate table to be interferometrically measured. Over time, contaminants may accumulate on the substrate table. In particular, contaminants may accumulate on areas of the substrate table that are not covered by the substrate during lithographic exposure. The substrate table is provided with a sensor system. The sensor system may be arranged in the substrate table base (for example, radially outside the substrate clamping portion). When contaminants accumulate on the sensors of the sensor system, this may have an adverse effect on the performance of the sensor. The substrate table can be cleaned, for example, using hydrogen radicals. However, this cleaning takes time, and exposure to the lithographic substrate will be suspended during cleaning. For this reason, it is expected that the substrate table will not be cleaned too frequently. However, on the contrary, if the cleaning is delayed too long, the accumulation of contaminants on the sensor may cause the performance of the lithographic equipment to deteriorate beyond the expected parameters.

[0007] It may be 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 present invention, there is provided a method for determining contamination of an optical sensor of a sensing system in a lithographic device, the method comprising: directing EUV radiation through an opening in a masking blade of a reticle and onto a pattern forming device; projecting reflected EUV radiation onto the sensing system, thereby causing accumulation of an area of ​​contamination; measuring the height of the area of ​​contamination and the height of an area of ​​the sensing system that does not receive the reflected EUV radiation; and using the measured height to determine the amount of contamination on the optical sensor of the sensing system.

[0009] Advantageously, embodiments of the present invention allow the amount of contaminants to be determined by measurement rather than just estimation. Because measurements are used rather than estimates, cleaning of the optical sensor can be performed when cleaning is actually required (rather than when estimates indicate that cleaning may be required).

[0010] The EUV radiation may be directed through a plurality of openings in the reticle masking blades.Heights of a plurality of contaminant regions may be measured.

[0011] The EUV radiation may be directed onto one or more alignment marks provided on the patterning device.

[0012] The EUV radiation may be directed onto one or more regions of the patterning device that are less than 50% covered by absorbers.

[0013] The openings in the reticle masking blades may be slots.

[0014] The measured height of the contaminant area may be compared to the modeled height.

[0015] The model may be generated using measured heights of areas that do not receive the reflected EUV radiation.

[0016] The model may comprise a line or curve extending between the measured heights of areas that do not receive the reflected EUV radiation.

[0017] The optical sensor may be an imaging sensor.

[0018] According to a second aspect of the present invention, there is provided a lithography apparatus comprising a pattern forming device support structure, a mask shielding blade, a projection system, a level sensor, and a substrate table provided with a sensor system, the sensor system comprising an optical sensor; wherein at least one opening is provided in the mask shielding blade; and wherein the level sensor is configured to measure the height of at least one contaminant area of ​​the optical sensing system and at least one uncontaminated area of ​​the optical sensing system; and a processor is configured to determine the height of at least one contaminant area of ​​the optical sensing system, thereby determining the amount of contaminants on the optical sensor.

[0019] A plurality of openings for EUV may be provided in the reticle masking blades.

[0020] The one or more openings in the reticle masking blades may be slots.

[0021] The optical sensor may be an imaging sensor.

[0022] The processor may be configured to use a model to determine the height of the peaks or dips caused by the contaminants.

[0023] The model may be generated using measured heights of a plurality of uncontaminated areas of the optical sensing system.

[0024] 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.

[0025] According to a fourth aspect of the present invention, there is provided a computing device, comprising a processor and a memory, wherein the memory stores instructions, which, when executed by the processor, enable the computing device to perform the method of the first aspect.

[0026] Features of different aspects of the invention may be combined together. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:

[0028] Figure 1 schematically depicts a lithographic system including a lithographic apparatus and a radiation source;

[0029] Figure 2 depicts a reticle masking blade system that may be used with embodiments of the present invention;

[0030] Figure 3schematically depicts the use of a sensor system for a lithographic apparatus;

[0031] Figure 4 Schematic depiction of contaminant buildup on a sensor system of a lithographic apparatus;

[0032] Figure 5A depicts height measurements and modeled heights for an uncontaminated area according to an embodiment of the present invention;

[0033] Figure 5B depicts height measurements and modeled heights for contaminated and uncontaminated areas according to an embodiment of the present invention; and

[0034] Figure 6 Describes an embodiment of the present invention from Figure 5B Subtract the height measurement from Figure 5A The results of the height measurement results. DETAILED DESCRIPTION

[0035] 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 provide 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.

[0036] The illumination system IL is configured to condition the EUV radiation beam B before it is incident on the patterning device MA. Thereby, the illumination system IL may comprise a faceted field mirror arrangement 10 and a faceted pupil mirror arrangement 11. The faceted field mirror arrangement 10 and the faceted pupil mirror arrangement 11 together provide a desired cross-sectional shape and a desired intensity distribution for the EUV radiation beam B. The illumination system IL may comprise other mirrors or arrangements in addition to or instead of the faceted field mirror arrangement 10 and the faceted pupil mirror arrangement 11.

[0037] After this conditioning, 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, which are 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', thereby forming 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 in Figure 1 The projection system PS is shown in FIG. 2 with only two mirrors 13 , 14 , but the projection system PS may comprise a different number of mirrors (eg six or eight mirrors).

[0038] The reticle masking blade system 20 is used to selectively cover an area of ​​the patterning device MA so that only a desired portion of the patterning device receives EUV radiation at any given time. 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 will pass through the exposure field on the substrate W.

[0039] The lithographic apparatus may be a dual stage lithographic apparatus (as shown). In a dual stage lithographic apparatus, properties of a substrate W are measured in a dedicated measurement region M. During the measurement, the substrate is supported by a substrate table WT (there are two substrate tables WT in the lithographic apparatus). The measurement may include a measurement of the topography (height profile) of the substrate W, and a measurement of the position of alignment marks on the substrate, performed using a level sensor LS. In addition, the position and topography (height profile) of the alignment marks on the substrate table WT are measured. This provides a measurement of the position of the alignment marks on the substrate in the reference frame of the substrate table WT.

[0040] Once the measurements are completed, the substrate table WT is then moved to a so-called exposure position below the projection system PS where it is exposed by patterned EUV radiation. When the substrate table WT is in this position, the positions of alignment marks on the substrate table are measured relative to the positions of alignment marks 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. For example, the patterning device MA may be provided with six alignment marks R1-R6 (each alignment mark may comprise a set of gratings), such as Figure 1 Schematically depicted in .

[0041] In addition, a grating 22 provided on the support structure MT is illuminated by the EUV beam B. The projection system PS forms an image of the grating 22 at the substrate table WT. An imaging sensor forming part of the sensor system 24 provides an output characterizing the image of the grating 22. The output allows the focal plane of the projection system PS to be accurately determined. This in turn allows the substrate W to be positioned so that the projected image of the patterning device MA is focused on the substrate.

[0042] The substrate W may include a previously formed pattern. In this case, the lithographic apparatus LA aligns an image formed by the patterned EUV radiation beam B′ with a pattern previously formed on the substrate W.

[0043] 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.

[0044] For example, Figure 1 The radiation source SO shown in is of a type that may be referred to as a laser produced plasma (LPP) source. A laser system 1 (which may include a CO2 laser, for example) is arranged to deposit energy via a laser beam 2 into a fuel, such as tin (Sn), provided by, for example, a fuel emitter 3. Although described below with tin, 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 deposition of laser energy into the tin causes a tin plasma 7 to be generated in the plasma formation region 4. During de-excitation and recombination of electrons with ions of the plasma, radiation including EUV radiation is emitted from the plasma 7.

[0045] EUV radiation from the plasma is collected and focused by a collector 5. For example, the collector 5 comprises a near normal incidence radiation collector 5 (sometimes more generally referred to as a normal incidence radiation collector). The collector 5 may have a multilayer mirror structure arranged to reflect EUV radiation (e.g., EUV radiation having a desired wavelength such as 13.5 nm). The collector 5 may be an elliptical configuration having two foci. As described below, a first focus of the two foci may be at the plasma formation region 4, and a second focus of the two foci may be at the intermediate focus 6.

[0046] The laser system 1 may be spatially separated from the radiation source SO. In this case, the laser beam 2 may be delivered from the laser system 1 to the radiation source SO with the aid of a beam delivery system (not shown) comprising, for example, suitable directing mirrors and / or a beam expander 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.

[0047] 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 in the plasma formation region 4. The image at the intermediate focus 6 serves 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 adjacent to an opening 8 in an enclosure 9 of the radiation source SO.

[0048] 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.

[0049] Contaminants will accumulate on the sensors of the sensor system 24. In one example, contaminants accumulated on the imaging sensors of the sensing system may cause the imaging sensors to provide inaccurate outputs. This in turn may cause the projected pattern to be poorly focused at the substrate W (due to the inaccurate outputs provided by the imaging sensors, the substrate may be mispositioned relative to the focal plane). Embodiments of the present invention address this problem.

[0050] Figure 2 A reticle masking blade system 20 is depicted that may be used with embodiments of the present invention. System 20 includes a primary blade 26, a secondary blade 27 and two side blades 28, 29. Primary blade 26 and secondary blade 27 are movable in the Y direction. Side blades 28, 29 are movable in the X direction.

[0051] During exposure of an exposure field of a substrate W, the facing planar edges 26a, 27a of the primary blade 26 and the secondary blade 27 are spaced apart from each other in the Y direction to form a rectangular opening through which EUV radiation can pass. Depending on the desired X-direction size of the exposure field, side blades 28, 29 are used to form the side walls of the rectangular opening. Figure 2 In the embodiment shown in FIG. 1 , the side blades 28, 29 have different positions. This position is used during the alignment measurement, as explained below.

[0052] Figure 3 Also schematically depicted in FIG. 1 is an EUV radiation beam B. During lithographic exposure, the EUV radiation beam B is positioned between opposing planar edges 26a, 27a of the primary and secondary blades 26, 27. This controls the way in which the EUV radiation is incident on the patterning device MA and the substrate WT.

[0053] During alignment measurements, the EUV radiation beam B is directed towards a different portion 30 of the main blade 26. This portion 30 has a curved edge 31, the curvature of which corresponds to the curvature of the EUV radiation beam B in the XY plane.

[0054] The main blade 26 is provided with three generally rectangular slots 32-34. Each slot 32-34 extends inwardly relative to the curved edge 31. Each slot 32-34 extends in the Y direction by an amount greater than or equal to the width of the EUV radiation beam B in the Y direction (as schematically depicted). For example, the slots 32-34 may extend 4 mm to 6 mm in the Y direction. For example, the slots may extend 10 mm to 15 mm in the X direction.

[0055] During alignment measurement, the EUV radiation beam B passes through the slots 32-34, is reflected by the alignment marks 22 on the patterning device MA, and is incident on the sensor system 24 on the substrate table WT. Other parts of the main blade 26 block the EUV radiation beam so that it does not impinge on the patterning device MA and does not impinge on the substrate table WT.

[0056] Herein, the Y direction is used to indicate the scanning direction of the lithographic apparatus LA and the X direction is used to indicate the direction perpendicular to the scanning direction and lying in the plane of the substrate table WT and / or the patterning device support structure MT.

[0057] Figure 3 Combination Figure 1 and Figure 2 The alignment of the patterning device MA relative to the substrate table WT is schematically depicted. During operation of the lithographic apparatus LA, when it is desired to determine the position of the patterning device MA relative to the substrate table WT, alignment marks R1-R6 on the patterning device MA 22 are illuminated by the EUV radiation beam B (passing through the slots 32-34). Each of the depicted alignment marks R1-R6 may comprise a set of gratings, and may comprise other structures. Figure 3 Schematically depicted in FIG. 1 is an illustration of illumination for one alignment mark R1 (which is schematically depicted as a single grating). For ease of illustration, only EUV radiation reflected from the patterning device MA is depicted (i.e., radiation incident on the patterning device is not depicted). As can be seen, the radiation is patterned by the alignment mark R1 and passes through one of the slots 32 provided in the master reticle masking blades 26. Some radiation B is reflected by the absorbing regions on both sides of the alignment mark R1 (the absorber may not completely absorb all of the radiation).

[0058] Radiation reflected from other alignment marks passes through other slots (not depicted) provided in master reticle masking blades 26 .

[0059] The sensing system 24 is arranged on the substrate table. The sensing system 24 comprises an imaging sensor 25 which may be concealed into the surface of the sensing system 24. The imaging sensor receives the patterned radiation B' and provides an output signal which allows alignment of the patterning device MA relative to the substrate table WT (and hence relative to the substrate W). The radiation B reflected from the absorbing regions will be incident on the upper surface 49 of the sensing system 24.

[0060] Contaminants can accumulate on the imaging sensor 25, as well as on the upper surface 49 of the sensing system 24 (either side of the recess where the imaging sensor is located). Figure 3 , the contaminants accumulated due to the reflected patterned radiation B' are shown as a central contaminant region 60. The contaminants accumulated due to the radiation B reflected from the absorption region are shown as peripheral contaminant regions 62, 64. The first peripheral contaminant region 62 is located on the imaging sensor 25 and surrounds the central contaminant region. The other peripheral contaminant region 64 is located on the upper surface 49 of the sensing system and surrounds the recess in which the imaging sensor 25 is disposed. Figure 3 In practice, the peripheral contaminant area 64 on the upper surface 49 of the sensing system 24 is much larger than the contaminant areas 60 and 62 in the recess. The embodiment of the present invention measures the thickness of the peripheral contaminant on the upper surface 49 of the sensing system 24 .

[0061] During alignment measurement, EUV radiation will pass through each of the slots 32-34 in the master reticle masking blades 26. Alignment measurement can be performed simultaneously for two alignment marks using two imaging sensors. In some cases, EUV radiation that passes through at least one of the slots 32-34 will not be incident on the imaging sensor, but will be incident on a portion of the sensing system 24 that does not include the imaging sensor (or other optical sensor). The EUV radiation may be incident on the upper surface of the sensing system 24.

[0062] like Figure 2 As shown, during measurement using imaging sensor 24, main reticle masking blades 26 are positioned so that EUV radiation beam B only passes through slots 32-34. EUV radiation at other locations is blocked by main reticle masking blades 26. EUV radiation passing through slots 32-34 can cause contaminant accumulation on the imaging sensor system. Such contaminants are generated due to the interaction between EUV radiation and molecules present near substrate table WT. Therefore, contaminants such as carbon (and other materials) will accumulate at the area where EUV radiation is incident.

[0063] Figure 4The sensor system 24 is schematically depicted as viewed from above. The imaging sensor 25, depicted as a black square, occupies only a small area of ​​the sensor system 24. Areas where contaminant accumulation occurs are represented as gray blocks 42a-f. As described above, when contaminants accumulate in the imaging sensor 25, this will adversely affect the performance of the imaging sensor.

[0064] According to an embodiment of the present invention, contaminant accumulation on the imaging sensor 25 is determined by measuring the contaminant profile across the upper surface of the sensor system 24. The profile is measured using a level sensor LS (see Figure 1 ). The level sensor LS may be, for example, as described in US8842293, which is incorporated herein by reference.

[0065] The level sensor is configured to project a radiation beam set that illuminates area 44 (at Figure 4 ). Some of the areas 44 are labeled in FIG. 4 . The illumination areas 44 may be rectangular and may be arranged in rows as depicted. Other arrangements may be used.

[0066] The substrate table WT is positioned so that the illumination area 44 formed by the level sensor LS is the incident r portion of the surface of the sensor system 24. In the depicted embodiment, some of the illumination areas 44 are areas where contaminants are not expected to accumulate, and other illumination areas 44 correspond to four areas 42b-e, where it is known that contaminants will accumulate in the four areas 42b-e. For each illumination area 44, the level sensor LS provides a height measurement as output. The height measurement result from the level sensor LS for the uncontaminated illumination area 44 can be used to measure the height of the sensor system 24 in the Z direction. The measured height can be compared with the previously measured calibration height. The result of this comparison can be used to adjust the height of the substrate table WT during lithography exposure.

[0067] The output from level sensor LS for illuminated area 44 including contaminants 42 b - e may be used to determine contamination of imaging sensor 24 .

[0068] Figure 5A and Figure 5B The measurement results of the height of the sensor system 24 obtained by the level sensor LS are depicted. Figure 5A and Figure 5B Also shown are the positions in the X direction of the slots 32-34 provided in the master reticle blades 26. Measurements can be performed by the level sensor LS using light of multiple wavelengths. Figure 5A and Figure 5B The reason for multiple measurement lines is shown in .

[0069] exist Figure 5A, the output of the level sensor LS is depicted when it is used to measure uncontaminated areas. These areas are uncontaminated because they do not receive EUV radiation (they are located in the shadow of the master reticle blades 26). Figure 5A The measurements of the height of the area receiving the pollutants are not depicted in . Instead, the height of the area receiving the pollutants is modeled.

[0070] The upper surface of the sensor system 24 has a significant height variation, typically on the order of 1 μm. This height variation is measured as an initial calibration measurement to obtain a reference shape. The reference shape is subtracted from subsequent height measurements to obtain the difference in height of the sensor system compared to the reference shape. Typically, this difference is in the range of nanometers or tens of nanometers. The measured height of an uncontaminated area of ​​the sensor system after subtracting the reference shape is Figure 5A As depicted in .

[0071] The height values ​​of the contaminated areas are generated using the model. Figure 5A In the illustrated embodiment, the model includes straight lines extending between the measured height values ​​(which may be referred to as linear interpolation). In other embodiments, a more complex model may be used. For example, a polynomial curve extending between the measured height values ​​may be used. The polynomial curve may correspond to a reference shape of the upper surface of the sensor system 24.

[0072] In other embodiments, the linear interpolation model may be combined with measurements obtained after cleaning sensor system 24 (eg, using linear interpolation and a weighted combination of the measured values). In general, the model may take into account measurements obtained after cleaning sensor system 24.

[0073] As can be seen, Figure 5A There is a height variation of about 2 nm. Figure 5A The 0 nm value on the vertical axis has no particular significance - it is the range of height values ​​that is important.

[0074] Figure 5B Depicted are horizontal sensor measurements of the height of the sensor system 24 including a contaminated region (ie, a region that received EUV radiation). Due to the effects of the contaminants, the measured height of the sensor system 24 includes two peaks and one dip.

[0075] Figure 6 Describes from Figure 5B Subtract the height value from Figure 5A As can be seen, the result is two peaks 52, 54 and a concave portion 53, the peaks being connected to the concave portion by substantially flat portions 55, 56.

[0076] The first peak 52 is located at a position corresponding to the first opening 32 of the reticle masking blade 26. Its height is about 3 nm. The second peak 54 is located at a position corresponding to the third opening 34 of the reticle masking blade 26. The height of this peak is about 2.5 nm.

[0077] Figure 6 The concave portion 53 at the center corresponds to the second opening 33 of the mask masking blade 26. The concave portion 53 is an artifact caused by the position of the contaminant on the sensor system 24 in the Y direction. Figure 4 As can be seen in FIG. 4 , the upper end of the illuminated area 44 overlaps with the contaminant area 42c, and the lower end of the illuminated area 44 overlaps with a different contaminant area 42d. The level sensor output is sensitive to the Y-direction offset of the contaminants 42c, 42d, which is why a concave portion is seen instead of a peak. However, the concave portion indicates the presence of contaminants. That is, since the measured height deviates from the modeled height (e.g., Figure 5A ), thus identifying the contaminant.

[0078] The size of the peaks 52 , 54 and the depression 53 may be used to determine the amount of contaminants present on the sensor system 24 .

[0079] The level sensor LS may be used to periodically determine the height of the contaminated area of ​​the image sensing system 40. Figure 4 As can be seen in FIG. 1 , there is little or no contamination accumulation in areas of the sensor system 24 that do not receive EUV radiation. As a result, peaks and / or depressions of the measured height are established, which are separated by areas without contamination. This contamination configuration is suitable for measurement by the level sensor LS and therefore allows the contamination accumulation to be measured. That is, the presence of peaks and / or depressions can be determined. The size of the peaks and / or depressions is indicative of the amount of contamination on the sensor system 24.

[0080] When contaminant accumulation on the sensor system 24 reaches a predetermined level, the sensor system 24 (and optionally other sensing systems of the substrate table WT) may be cleaned. The cleaning may be performed using a hydrogen radical generator.

[0081] In one example, cleaning can be performed when the measured height of one of the peaks exceeds 2 nm (or a different value). In another example, cleaning can be performed when the average height of the peaks and the magnitude of the depressions (i.e., the size of the depressions, but corrected for the negative sign of the depressions) exceed 2 nm (or a different value). In general, cleaning can be performed when the height of one or more peaks or depressions exceeds a predetermined threshold.

[0082] like Figure 3As depicted, contaminants 60, 62 may be incident on imaging sensor 25 of sensor system 24, and contaminant 64 may also be incident on upper surface 49 of the sensor system. The thickness of contaminants received by the center region of imaging sensor 25 may be greater than the thickness of contaminants received by upper surface 49 of sensor system 24 (see FIG. Figure 3 ). Calibration can be used to determine the relationship between the thickness of contaminants on the upper surface 49 of the sensor system 24 and the thickness of contaminants on the imaging sensor 25. This can be taken into account when determining whether to clean the imaging sensor system.

[0083] In one embodiment, the height of the contaminated sensor system 24 can be directly compared to the height measurement obtained after the sensor system 24 is cleaned (i.e., before contaminants accumulate significantly, and without using the model mentioned above). However, this may not be preferred because the overall height of the sensor system 24 may have some drift.

[0084] In another embodiment, height measurements obtained after the sensor system 24 has been cleaned may be taken into account when generating the model height value for the contaminated area.

[0085] In an embodiment, without using patterning device alignment marks, contaminant regions may be formed on the surface of the imaging sensing system 40. For example, generally reflective regions in the patterning device may be irradiated by the EUV beam and used to form contaminant regions on the surface of the imaging sensing system 40. These regions may be separated from the imaging sensor 24 so that contaminant accumulation is not caused on the imaging sensor. Such irradiation of generally reflective regions of the patterning device may be performed each time alignment of the patterning device is performed, or may be performed less frequently. Because generally reflective regions of the patterning device are used (e.g., less than 50% of the region is covered by absorbers), contaminants will accumulate faster than contaminants caused by alignment marks (e.g., more than 50% of the region is covered by absorbers). Because contaminants accumulate faster, they may be easier to measure. Calibration may be used to correlate the height of this contaminant with the height of the contaminant on the imaging sensor 25. The disadvantage of this embodiment is that additional time is used to irradiate generally reflective regions of the patterning device.

[0086] The depicted embodiment uses a level sensor to measure the height of four contaminated areas 42b-e using a single measurement. However, in other embodiments, other combinations of contaminated areas may be measured.

[0087] In the embodiments depicted herein, the masking blades of the reticle are provided with slots. However, any form of opening may be provided (e.g., holes may be provided instead of slots). Although multiple slots are depicted, a single slot (or other opening) may be provided. Two slots (or other openings) may be provided. Other numbers of slots or other openings may be provided.

[0088] Although this embodiment is described in conjunction with a sensor system having an imaging sensor, the invention may also be used in conjunction with sensor systems having other optical sensors.

[0089] The method according to an embodiment of the present invention may be performed by a computing device. The device may include a central processing unit ("CPU") connected to a memory. The method described herein may be implemented in code (software), which is stored on a memory including one or more storage media and is arranged to be executed 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 (which may be referred to as an instruction) is configured to be extracted from the memory and executed on the processor to perform operations consistent with the embodiments discussed herein. Alternatively, it is not excluded that some or all of the functions of the CPU are implemented in a dedicated hardware circuit or a configurable hardware circuit (such as an FPGA).

[0090] The computing device may include an input portion configured to enable a user to input data into a software program running on the CPU. The input device may include a mouse, a keyboard, a touch screen, a microphone, etc. The computing device may also include an output device configured to output measurement results to the user.

[0091] Although specific reference may be made herein to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications. Other possible 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.

[0092] Although specific reference may be made herein to embodiments of the invention in the context of lithographic equipment, embodiments of the invention may be used in other equipment. Embodiments of the invention may form part of a mask inspection equipment, a metrology equipment, or any equipment that measures or processes an object such as a wafer (or other substrate) or a mask (or other pattern forming device). These equipment may generally be referred to as lithographic tools. Such lithographic tools may use vacuum conditions or ambient (non-vacuum) conditions.

[0093] Although specific reference may have 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 such as imprint lithography where the context permits.

[0094] Where context permits, 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 by instructions stored on a machine-readable medium, which may be read and executed by one or more processors. A machine-readable medium may include any mechanism for storing or transmitting information in a form that can be read 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.), and the like. In addition, firmware, software, routines, instructions may be described in the present invention as performing certain actions. However, it should be understood that such descriptions are merely for convenience, and that these actions are in fact generated by a computing device, a processor, a controller, or other devices that execute firmware, software, routines, instructions, etc., and that performing such operations may cause an actuator or other device to interact with the physical world.

[0095] Although specific embodiments of the present invention have been described above, it should be understood that the present invention may be practiced in a manner other than that described. The above description is intended to be illustrative rather than restrictive. Therefore, it will be clear 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.

[0096] Items

[0097] 1. A method of determining contamination of an optical sensor of a sensing system in a lithographic apparatus, the method comprising:

[0098] directing EUV radiation through openings in the reticle masking blades and onto the patterning device;

[0099] projecting reflected EUV radiation onto the sensing system, thereby causing accumulation of areas of contamination;

[0100] measuring the height of the contaminant region and the height of regions of the sensing system that do not receive reflected EUV radiation; and

[0101] The measured height is used to determine an amount of contaminant on the optical sensor of the sensing system.

[0102] 2. The method of clause 1, wherein the EUV radiation is directed through a plurality of openings in the reticle masking blades, and wherein heights of a plurality of contaminant regions are measured.

[0103] 3. A method according to clause 1 or clause 2, wherein the EUV radiation is directed onto one or more alignment marks disposed on the pattern forming device.

[0104] 4. A method according to clause 1 or clause 2, wherein the EUV radiation is directed onto one or more areas of the patterning device that are less than 50% covered by absorbers.

[0105] 5. The method of clause 1, wherein the openings in the reticle masking blades are slots.

[0106] 6. A method according to any of the preceding clauses, wherein the measured height of the contaminant area is compared to a modelled height.

[0107] 7. A method as described in clause 6, wherein the model is generated using measured heights of areas that do not receive the reflected EUV radiation.

[0108] 8. A method as described in clause 7, wherein the model includes a line or curve extending between the measured heights of areas that did not receive the reflected EUV radiation.

[0109] 9. The method of clause 7, wherein the model takes into account a measured height of an area receiving the reflected EUV radiation, the measured height being obtained after cleaning the sensing system.

[0110] 10. A method according to any of the preceding clauses, wherein the optical sensor is an imaging sensor.

[0111] 11. A lithographic apparatus comprising a patterning device support structure, a reticle masking blade, a projection system, a level sensor, and a substrate table provided with a sensor system, the sensor system comprising an optical sensor; wherein

[0112] The masking blade of the reticle is provided with at least one opening; and

[0113] wherein the level sensor is configured to measure the height of at least one contaminated area of ​​the optical sensing system and at least one uncontaminated area of ​​the optical sensing system; and

[0114] The processor is configured to determine a height of at least one contaminated region of the optical sensing system to thereby determine an amount of contamination on the optical sensor.

[0115] 12. The lithographic apparatus of clause 11, wherein a plurality of openings for EUV are provided in the reticle masking blades.

[0116] 13. The lithographic apparatus of clause 11, wherein the one or more openings in the reticle masking blades are slots.

[0117] 14. A lithographic apparatus according to any one of clauses 11 to 13, wherein the optical sensor is an imaging sensor.

[0118] 15. A lithographic apparatus according to any of clauses 11 to 14, wherein the processor is configured to use a model to determine the height of the peaks or dips caused by the contaminants.

[0119] 16. A lithographic apparatus according to clause 15, wherein the model is generated using measured heights of a plurality of uncontaminated areas of the optical sensing system.

[0120] 17. 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 10.

[0121] 18. 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 any one of clauses 1 to 10.

Claims

1. A method of determining contamination of an optical sensor of a sensing system in a lithographic apparatus, the method comprising: directing EUV radiation through openings in the reticle masking blades and onto the patterning device; projecting reflected EUV radiation onto the sensing system, thereby causing accumulation of areas of contaminants; measuring a height of the contaminant region and a height of a region of the sensing system that does not receive reflected EUV radiation; as well as The measured height is used to determine an amount of contaminant on the optical sensor of the sensing system. 2 . The method of claim 1 , wherein the EUV radiation is directed through a plurality of openings in the reticle masking blades, and wherein heights of a plurality of contaminant regions are measured.

3. A method according to claim 1 or claim 2, wherein the EUV radiation is directed onto one or more alignment marks provided on the patterning device.

4. A method according to claim 1 or claim 2, wherein the EUV radiation is directed onto one or more areas of the patterning device that are less than 50% covered by absorbers. The method of claim 1 , wherein the openings in the reticle masking blades are slots.

6. A method according to any one of the preceding claims, wherein the measured height of the contaminant area is compared with a modelled height, and / or wherein the optical sensor is an imaging sensor. The method of claim 6 , wherein the model is generated using measured heights of areas that did not receive the reflected EUV radiation. 8 . The method of claim 7 , wherein the model comprises a line or curve extending between measured heights of areas that did not receive the reflected EUV radiation.

9. The method of claim 7, wherein the model takes into account a measured height of an area receiving the reflected EUV radiation, the measured height being obtained after cleaning the sensing system.

10. A lithographic apparatus comprising a patterning device support structure, a reticle masking blade, a projection system, a level sensor, and a substrate table provided with a sensing system, the sensing system comprising an optical sensor; in The masking blade of the reticle is provided with at least one opening; and wherein the level sensor is configured to measure the height of at least one contaminated region of the sensing system and at least one uncontaminated region of the sensing system; and The processor is configured to determine a height of at least one contaminated region of the sensing system to thereby determine an amount of contamination on the optical sensor.

11. The lithographic apparatus of claim 10, wherein a plurality of openings for EUV are provided in the reticle masking blades.

12. The lithographic apparatus of claim 10, wherein the one or more openings in the reticle masking blades are slots.

13. The lithographic apparatus according to any one of claims 10 to 12, wherein the optical sensor is an imaging sensor, and / or The processor is configured to use a model to determine the height of the peaks or dips caused by the contaminants.

14. The lithographic apparatus of claim 13, wherein the model is generated using measured heights of a plurality of uncontaminated areas of the sensing system.

15. A system for determining an amount of contamination on an optical sensor for a lithographic apparatus, the system comprising: a level sensor, a sensing system including the optical sensor, and a processor, wherein the level sensor is configured to measure the height of at least one contaminated region of the sensing system and at least one uncontaminated region of the sensing system; as well as The processor is configured to determine a height of at least one contaminated region of the sensing system to thereby determine an amount of contamination on the optical sensor.

Images