Contamination determination

Abstract

A method for determining contamination of an optical sensor of a detection system in a lithographic apparatus, the method comprising: directing EUV radiation onto a patterning device through an opening in a reticle masking blade; projecting reflected EUV radiation onto the detection system to cause accumulation of a contaminated area; measuring heights of the contaminated area and heights of areas of the detection system that have not received the reflected EUV radiation; and using the measured heights to determine an amount of contamination on the optical sensor of the detection system.

Description

[Technical Field]

[0001] [CROSS REFERENCE TO RELATED APPLICATIONS] This application claims the benefit of priority from European Application No. 22187852.3, filed July 29, 2022, which is incorporated herein by reference in its entirety.

[0002] [Technical field] The present invention relates to determining contamination of an optical sensor of a detection system forming part of a lithographic apparatus. [Background technology]

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

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

[0005] The lithographic substrate is supported by a substrate table when it is exposed. The substrate table may include a substrate clamp and a base that holds the substrate clamp. The base may include a reflective surface that allows interferometric measurements of the position of the substrate table. Over time, contamination may accumulate on the substrate table, particularly on areas of the substrate table that are not covered by a substrate during lithographic exposure. The substrate table is provided with a sensor system. The sensor system may be provided at the base of the substrate table (e.g., radially outward of the substrate clamp). Contamination accumulation on the sensors of the sensor system may adversely affect the performance of the sensors. The substrate table may be cleaned, for example, using hydrogen radicals. However, such cleaning is time-consuming and interrupts the exposure of lithographic substrates during cleaning. For this reason, it is desirable not to clean the substrate table too frequently. Conversely, if cleaning is postponed for too long, contamination may accumulate on the sensors, causing the performance of the lithographic apparatus to degrade outside of desired parameters.

[0006] 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

[0007] According to a first aspect of the present invention, there is provided a method for determining contamination of an optical sensor of a detection system in a lithographic apparatus, the method comprising directing EUV radiation onto a patterning device through an opening in a reticle masking blade, projecting reflected EUV radiation onto the detection system to cause a buildup of contaminated areas, measuring heights of the contaminated areas and heights of areas of the detection system not receiving the reflected EUV radiation, and using the measured heights to determine an amount of contamination on the optical sensor of the detection system.

[0008] Embodiments of the present invention have the advantage that the amount of contamination can be determined through measurement rather than simply estimating the amount of contamination. Because measurement rather than estimation is used, cleaning of the optical sensor can be performed when cleaning actually is required (instead of when estimation indicates that cleaning may be required).

[0009] The EUV radiation may be directed through multiple openings in the reticle masking blade, and the heights of multiple contaminated regions may be measured.

[0010] The EUV radiation may be directed to one or more alignment marks provided on the patterning device.

[0011] The EUV radiation may be directed to one or more areas of the patterning device that are less than 50% covered by the absorber.

[0012] The opening in the reticle masking blade may be a slot.

[0013] The measured height of the contaminated area may be compared to the model height.

[0014] The model may be generated using the measured height of the area not receiving reflected EUV radiation.

[0015] The model may comprise a line or curve extending between the measured heights of the areas not receiving reflected EUV radiation.

[0016] The optical sensor may be an image sensor.

[0017] According to a second aspect of the present invention, there is provided 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 detection system comprising an optical sensor, the reticle masking blade being provided with at least one opening, the level sensor being configured to measure heights of at least one contaminated area of the detection system and at least one non-contaminated area of the detection system, and a processor configured to determine an amount of contamination on the optical sensor by determining the height of the at least one contaminated area of the detection system.

[0018] The reticle masking blade may be provided with a plurality of openings.

[0019] One or more of the openings in the reticle masking blade may be slots.

[0020] The optical sensor may be an image sensor.

[0021] The processor may be configured to use the model to determine the height of any peaks or valleys caused by contamination.

[0022] The model may be generated using the measured heights of multiple uncontaminated regions of the optical sensing system.

[0023] 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.

[0024] According to a fourth aspect of the present invention, there is provided a computing device comprising a processor and a memory, the memory storing instructions which, when executed by the processor, cause the computing device to perform the method of the first aspect of the present invention.

[0025] The features of the different aspects of the invention may be combined with each other. [Brief explanation of the drawings]

[0026] Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying schematic drawings in which: [Figure 1] 1 depicts a schematic representation of a lithography system comprising a lithographic apparatus and a radiation source; [Figure 2] 1 illustrates a reticle masking blade system that may be used in embodiments of the present invention. [Figure 3] 1 illustrates schematically the use of a sensor system in a lithographic apparatus; [Figure 4] 1 illustrates schematically the accumulation of contamination on a sensor system of a lithographic apparatus; [Figure 5A] 10 illustrates height measurements and model heights for an uncontaminated area according to one embodiment of the present invention. [Figure 5B] 10 illustrates height measurements and model heights for contaminated and uncontaminated areas according to an embodiment of the present invention. [Figure 6] 5B shows the results of subtracting the height measurement of FIG. 5A from the height measurement of FIG. 5B according to one embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0027] 1 shows a lithography system comprising a radiation source SO and a lithography apparatus LA. The radiation source SO is configured to generate a beam of EUV radiation B and to provide the beam of EUV radiation B to the lithography apparatus LA. The lithography 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.

[0028] The illumination system IL is configured to condition the EUV radiation beam B before it is incident on the patterning device MA. Furthermore, the illumination system IL may include a facetted field mirror device 10 and a facetted pupil mirror device 11. The facetted field mirror device 10 and the facetted pupil mirror device 11 provide the EUV radiation beam B with a desired cross-sectional shape and a desired intensity distribution. The illumination system IL may include other mirrors or devices in addition to or instead of the facetted field mirror device 10 and the facetted pupil mirror device 11.

[0029] 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 produced. 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 comprise a plurality of mirrors 13, 14 configured to project the patterned EUV radiation beam B' onto the substrate W held by a substrate table WT. The projection system PS applies a demagnification factor to the patterned EUV radiation beam B' to form an image having smaller features than corresponding features on the patterning device MA. For example, a demagnification factor of 4 or 8 may be applied. Although the projection system PS in Figure 1 is shown as having only two mirrors 13, 14, the projection system PS may include a different number of mirrors (e.g., 6 or 8 mirrors).

[0030] A reticle masking blade system 20 is used to selectively coat areas of the patterning device MA, so that only the desired portion of the patterning device receives EUV radiation at any one time. During a scanning exposure, the patterning device MA and support structure MT move in the y direction, and the substrate W and substrate table WT move in the opposite y direction. In this way, a band of EUV radiation passes through an exposure field on the substrate W.

[0031] The lithographic apparatus may be a dual stage lithographic apparatus (as shown). In a dual stage lithographic apparatus, properties of the substrate W are measured in a dedicated measurement area M. The substrate is supported by a substrate table WT during the measurement (the lithographic apparatus has two substrate tables WT). The measurement may include measurement of the topography (height profile) of the substrate W, which is performed using a level sensor LS, and measurement of the positions of alignment marks on the substrate. In addition, the positions and topography (height profile) of the alignment marks on the substrate table WT are also measured. This provides a measurement of the positions of the alignment marks on the substrate within the reference frame of the substrate table WT.

[0032] Once the measurements are complete, the substrate table WT is moved to a so-called exposure position below the projection system PS and exposed to the pattern EUV radiation. When the substrate table WT is in this position, the positions of the 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 which includes the alignment marks. Aligning the patterning device MA with the substrate table WT allows the patterning device to be aligned with the substrate W. The patterning device MA may be provided with, for example, six alignment marks R1 to R6 (which may each comprise a set of gratings), as shown schematically in Figure 1.

[0033] 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 this grating 22 on the substrate table WT. An image sensor forming part of the sensor system 24 provides an output characterizing the image of the grating 22. This output allows an accurate determination of the focal plane of the projection system PS. This allows the substrate W to be positioned so that the projected image of the patterning device MA is focused on the substrate.

[0034] Substrate W may include a previously formed pattern, in which case lithographic apparatus LA aligns the image formed by patterned EUV radiation beam B' with the previously formed pattern on substrate W.

[0035] A relative vacuum, that is to say a small amount of gas (eg hydrogen) at a pressure much less than atmospheric pressure, may be provided within the source SO, illumination system IL and / or projection system PS.

[0036] The radiation source SO shown in FIG. 1 is of a type that may be referred to as a laser-produced plasma (LPP) source. A laser system 1, which may include a CO laser, is configured to impart energy via a laser beam 2 to a fuel, such as tin (Sn), supplied from a fuel emitter 3. While the following description refers to tin, any suitable fuel may be used. The fuel may be, for example, a liquid, or may be, for example, a metal or 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 in the plasma formation region 4. When laser energy is deposited in the tin, a tin plasma 7 is generated in the plasma formation region 4. Radiation, including EUV radiation, is emitted from the plasma 7 during de-excitation and recombination of electrons and ions of the plasma.

[0037] EUV radiation from the plasma is collected and focused by a collector 5. Collector 5 may, for example, comprise a near-normal incidence radiation collector 5 (more commonly referred to as a normal incidence radiation collector). Collector 5 may have a multi-layer mirror structure arranged to reflect EUV radiation (e.g., EUV radiation having a desired wavelength, such as 13.5 nm). Collector 5 may have an elliptical configuration with two foci. As described below, the first of the two foci may be at the plasma formation region 4 and the second focus may be at an intermediate focus 6.

[0038] The laser system 1 may be spatially separated from the radiation source SO. In this case, the laser beam 2 may be passed from the laser system 1 to the radiation source SO with the aid of a beam delivery system (not shown) comprising appropriate directing mirrors and / or beam expanders and / or other optics. The laser system 1, the radiation source SO and the beam delivery system may collectively be considered as a radiation system.

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

[0040] Although FIG. 1 illustrates the source SO as a laser-produced plasma (LPP) source, EUV radiation may be generated using any suitable source, such as a discharge-produced plasma (DPP) source or a free-electron laser (FEL).

[0041] Contamination can build up on the sensors of the sensor system 24. In one example, contamination building up on the image sensor of the detection system can result in an inaccurate output of the image sensor. This can cause the projected pattern to not be properly focused on the substrate W (the substrate can be mispositioned relative to the focal plane due to the inaccurate output provided by the image sensor). Embodiments of the present invention address this issue.

[0042] 2 shows a reticle masking blade system 20 that may be used in embodiments of the present invention. System 20 includes a primary blade 26, a secondary blade 27, and two side blades 28 and 29. Primary blade 26 and secondary blade 27 are movable in the Y direction. Side blades 28 and 29 are movable in the X direction.

[0043] During exposure of an exposure field of a substrate W, the opposing planar edges 26a, 27a of the primary blade 26 and secondary blade 27 are spaced apart in the Y direction relative to each other to form a rectangular opening through which EUV radiation can pass. The side blades 28, 29 are used to form the side walls of the rectangular opening depending on the desired size of the exposure field in the X direction. In Figure 2, the side blades 28, 29 have different positions, which are used during alignment measurements, as will be described below.

[0044] Also shown schematically in Figure 2 is a beam of EUV radiation B. During lithographic exposure, the beam of EUV radiation B is located between the opposing planar edges 26a, 27a of the primary blade 26 and secondary blade 27. This controls how the EUV radiation is incident on the patterning device MA and substrate WT.

[0045] During alignment measurement, the EUV radiation beam B is directed onto a different portion 30 of the primary blade 26. This portion 30 has a curved edge 31 with a curvature that corresponds to the curvature of the EUV radiation beam B in the XY plane.

[0046] The primary blade 26 is provided with three generally rectangular slots 32-34. Each slot 32-34 extends inward 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 shown schematically). The slots 32-34 may extend, for example, 4 to 6 mm in the Y direction. The slots may extend, for example, 10 to 15 mm in the X direction.

[0047] During alignment measurement, the EUV radiation beam B passes through slots 32-34 and is reflected by alignment mark 22 on the patterning device MA before being incident on sensor system 24 on the substrate table WT. Other parts of primary blade 26 block the EUV radiation beam, preventing it from being incident on the patterning device and also preventing it from being incident on the substrate table WT.

[0048] In this document, 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 along the plane of the substrate table WT and / or patterning device support structure MT.

[0049] FIG. 3, in combination with FIGS. 1 and 2, schematically illustrates the alignment of a patterning device MA relative to a substrate table WT. During operation of the lithographic apparatus LA, when the position of the patterning device MA relative to the substrate table WT needs to be determined, alignment marks R1-R6 on the patterning device MA22 are illuminated by a beam of EUV radiation B (through slots 32-34). Each of the illustrated alignment marks R1-R6 may comprise a set of gratings or may comprise other structures. This illumination is shown schematically in FIG. 3 for one alignment mark R1 (shown schematically as a single grating). For ease of illustration only, only EUV radiation reflected from the patterning device MA is shown (i.e., radiation incident on the patterning device is not shown). As can be seen, the radiation is patterned by the alignment mark R1 and passes through one of the slots 32 provided in the primary reticle masking blade 26. Some radiation B is reflected by absorbing regions on either side of the alignment mark R1 (the absorber does not completely absorb all of the radiation).

[0050] Radiation reflected from other alignment marks passes through other slots (not shown) provided in the primary reticle masking blade 26 .

[0051] The detection system 24 is provided on the substrate table. The detection system 24 includes an image sensor 25, which may be embedded in a surface of the detection system 24. The image sensor receives the pattern radiation B' and provides an output signal that enables alignment of the patterning device MA with respect to the substrate table WT (and therefore the substrate W). Radiation B reflected from the absorbing region is incident on a top surface 49 of the detection system 24.

[0052] Contamination accumulates not only on the image sensor 25 but also on the upper surface 49 of the detection system 24 (on both sides of the recess in which the image sensor is provided). In FIG. 3, contamination accumulated due to reflected pattern radiation B' is shown as a central contamination region 60. Contamination accumulated due to radiation B reflected from the absorbing region is shown as peripheral contamination regions 62, 64. A first peripheral contamination region 62 is located above the image sensor 25 and surrounds the central contamination region. Another peripheral contamination region 64 is located above the upper surface 49 of the detection system and surrounds the recess in which the image sensor 25 is provided. FIG. 3 is schematic. In reality, the peripheral contamination region 64 on the upper surface 49 of the detection system 24 is much larger than the contamination regions 60, 62 within the recess. It is the thickness of the peripheral contamination on the upper surface 49 of the detection system 24 that is measured by embodiments of the present invention.

[0053] During alignment measurements, EUV radiation passes through each of the slots 32-34 in the primary reticle masking blade 26. Alignment measurements may be performed simultaneously for two alignment marks using two image sensors. In some cases, EUV radiation passing through at least one of the slots 32-34 does not impinge on the image sensor, but instead impinges on a portion of the detection system 24 that does not include an image sensor (or other optical sensor). The EUV radiation may also impinge on the top surface of the detection system 24.

[0054] As shown in Figure 2, during measurements using the image sensor 24, the primary reticle masking blades 26 are positioned so that the EUV radiation beam B passes only through the slots 32-34. EUV radiation at other positions is blocked by the primary reticle masking blades 26. EUV radiation passing through the slots 32-34 will cause contamination to accumulate on the image sensor system. This contamination occurs due to the interaction of the EUV radiation with molecules nearby the substrate table WT. Thus, contamination such as carbon (and other materials) accumulates in the areas where the EUV radiation is incident.

[0055] 4 shows a schematic of the sensor system 24 from a top view. The image sensor 25, represented by the black square, occupies only a small percentage of the area of the sensor system 24. Areas where contamination accumulates are represented by gray blocks 42a-f. As discussed elsewhere above, the accumulation of contamination on the image sensor 25 adversely affects the performance of the image sensor.

[0056] According to one embodiment of the present invention, the buildup of contamination on the image sensor 25 is determined by measuring a profile of the contamination across the top surface of the sensor system 24. The profile is measured using a level sensor LS (see FIG. 1 ). The level sensor LS may be, for example, as described in U.S. Pat. No. 8,842,293, which is incorporated herein by reference.

[0057] The level sensor is configured to project a set of radiation beams that illuminate areas 44 (some of which are labeled in FIG. 4). The illumination areas 44 may be rectangular and may be arranged in a line as shown, although other arrangements may be used.

[0058] The substrate table WT is positioned so that the illumination areas 44 formed by the level sensor LS are incident on portions of the surface of the sensor system 24. In the illustrated embodiment, some of the illumination areas 44 are areas where contamination accumulation is not expected, while other illumination areas 44 correspond to four areas 42b-e where contamination accumulation is known to occur. For each illumination area 44, the level sensor LS provides a height measurement as an output. The height measurement from the level sensor LS for the uncontaminated illumination area 44 may be used to measure the Z height of the sensor system 24. The measured height may be compared to a previously measured calibration height. The result of this comparison may be used to adjust the height of the substrate table WT during a lithography exposure.

[0059] The output from the level sensor LS for the illumination area 44 containing the contamination 42b-e may be used to determine the contamination of the image sensor 24.

[0060] 5A and 5B show height measurements of the sensor system 24 obtained by the level sensor LS. Also shown in FIGS. 5A and 5B are the X-direction positions of the slots 32-34 in the primary reticle blade 26. Measurements may be performed by the level sensor LS using multiple wavelengths of light. For this reason, multiple measurement lines are shown in FIGS. 5A and 5B.

[0061] Figure 5A shows the output of the level sensor LS used to measure the uncontaminated areas. These areas are not contaminated because they do not receive EUV radiation (because they are in the shadow of the primary reticle blades 26). Figure 5A does not show the height measurements of the contaminated areas; instead, the height of the contaminated areas is modeled.

[0062] The upper surface of the sensor system 24 typically has significant height variations, on the order of 1 μm. This height variation is measured as an initial calibration measurement to obtain a reference feature. The reference feature is subtracted from subsequent height measurements to obtain the difference in height of the sensor system compared to the reference feature. This difference is typically in the order of nanometers or tens of nanometers. The measured height of the uncontaminated area of the sensor system after subtracting the reference feature is shown in FIG. 5A.

[0063] Height values at the contaminated area are generated using a model. In the embodiment shown in FIG. 5A, the model comprises a straight line extending between the measured height values (this may be called linear interpolation). In other embodiments, more complex models may be used. For example, a polynomial curve extending between the measured height values may be used. The polynomial curve may correspond to a nominal shape of the top surface of the sensor system 24.

[0064] In other embodiments, the linear interpolation model may be combined with measurements taken after the sensor system 24 is cleaned (e.g., using a weighted combination of linear interpolation and measurements). In general, the model may take into account measurements taken after the sensor system 24 is cleaned.

[0065] As can be seen, there is a height variation of about 2 nm in Figure 5A. The value of 0 nm on the vertical axis of Figure 5 is not particularly significant; what is important is the range of height values.

[0066] 5B shows the result of measuring the height of the sensor system 24, including the contaminated area (i.e., the area that was exposed to EUV radiation), with a level sensor. Due to the effects of the contamination, the measured height of the sensor system 24 includes two peaks and one dip.

[0067] Figure 6 shows the result of subtracting the height values of Figure 5A from the height values of Figure 5B. As may be seen, the result is two protrusions 52, 54 and one recess 53, with the protrusions connected to the recesses by substantially flat portions 55, 56.

[0068] The first protrusion 52 is located at a position corresponding to the first opening 32 of the reticle masking blade 26. It has a height of about 3 nm. The second protrusion 54 is located at a position corresponding to the third opening 34 of the reticle masking blade 26. This protrusion has a height of about 2.5 nm.

[0069] The central depression 53 in Figure 6 corresponds to the second opening 33 of the reticle masking blade 26. This depression 53 is an artifact caused by the Y-position of the contamination on the sensor system 24. As can be seen in Figure 4, the illuminated area 44 overlaps with a contamination area 42c at its top and with another contamination area 42d at its bottom. The output of this level sensor is sensitive to the Y-offset of the contamination 42c, 42d, which is why a depression is seen instead of a protrusion. However, the depression indicates the presence of contamination; that is, the contamination is identified because the measured height deviates from the model height (see Figure 5A).

[0070] The size of the protrusions 52 , 54 and recesses 53 may be used to determine the amount of contamination present on the sensor system 24 .

[0071] The level sensor LS may be used to periodically determine the height of contaminated areas of the image sensing system 40. As can be seen from FIG. 4, areas of the sensor system 24 that are not exposed to EUV radiation experience little or no contamination buildup. Thus, peaks and / or valleys of measured height are established, separated by areas free of contamination. This configuration of contamination is suitable for measurement by the level sensor LS, thus enabling the measurement of contamination buildup. That is, the presence of the peaks and / or valleys can be determined. The size of the peaks and / or valleys indicates the amount of contamination on the sensor system 24.

[0072] When the buildup of contamination on the sensor system 24 reaches a predetermined level, cleaning of the sensor system 24 (and optionally other detection systems of the substrate table WT) may be performed. Cleaning may be performed using a hydrogen radical generator.

[0073] In one example, cleaning may be performed when the measured height of a single peak exceeds 2 nm (or a different value). In another example, cleaning may be performed when the average height of multiple peaks and valleys (i.e., the size of the valleys, but corrected for the negative sign of the valleys) exceeds 2 nm (or a different value). In general, cleaning may be performed when the height of one or more peaks or valleys exceeds a predetermined threshold.

[0074] 3, contamination 60, 62 may be incident on image sensor 25 of sensor system 24, and contamination 64 may also be incident on top surface 49 of the sensor system. The thickness of contamination experienced by the central region of image sensor 25 may be greater than the thickness of contamination experienced by top surface 49 of sensor system 24 (see FIG. 3). Calibration may be used to determine the relationship between the thickness of contamination on top surface 49 of sensor system 24 and the thickness of contamination on image sensor 25. This may be taken into consideration when determining whether to clean the image sensor system.

[0075] In some embodiments, the height of a contaminated sensor system 24 may be directly compared to height measurements taken after the sensor system 24 has been cleaned (i.e., before significant accumulation of contamination and without using a model such as those described above). However, this may be less desirable because there may be some variation (drift) in the overall height of the sensor system 24.

[0076] In another embodiment, height measurements taken after the sensor system 24 has been cleaned may be taken into account when generating the model height value for the contaminated area.

[0077] In one embodiment, contamination regions can be formed on the surface of the image sensing system 40 without using patterning device alignment marks. For example, substantially reflective regions of the patterning device can be illuminated with an EUV beam and used to form contamination regions on the surface of the image sensing system. These regions can be isolated from the image sensor to prevent contamination buildup on the image sensor. This illumination of the substantially reflective regions of the patterning device can be performed every time a patterning device alignment is performed, or it can be performed less frequently. Because substantially reflective regions of the patterning device (e.g., less than 50% of the area is covered by absorber) are used, contamination builds up more quickly than contamination caused by alignment marks (e.g., more than 50% of the area is covered by absorber). Because the contamination builds up more quickly, it can be easier to measure. Calibration can be used to correlate this contamination height with the contamination height on the image sensor 25. A disadvantage of this embodiment is the extra time required to illuminate the substantially reflective regions of the patterning device.

[0078] The illustrated 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.

[0079] In the illustrated embodiment of the invention, slots are provided in the reticle masking blade. However, openings of any shape (e.g., holes rather than slots) may be provided. Although multiple slots are shown, 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.

[0080] Although the present embodiment is described in relation to a sensor system having an image sensor, the invention may also be used in relation to sensor systems having other optical sensors.

[0081] Methods according to embodiments of the present invention may be performed by a computing device. The device may include a central processing unit ("CPU") coupled to a memory. The methods described herein may be implemented in code (software) stored in a memory comprising one or more storage media and configured to be executed by a processor comprising one or more processing units. The storage medium may be integrated with the CPU and / or may be separate from the CPU. The code, which may also be referred to as instructions, is retrieved from the memory and executed by the processor, which is configured to perform operations according to the embodiments described herein. Alternatively, it is not excluded that some or all of the functionality of the CPU may be implemented by dedicated hardware circuits or configurable hardware circuits such as FPGAs.

[0082] The computing device may comprise an input configured to allow a user to input data into the software program executing on the CPU. The input device may comprise a mouse, keyboard, touch screen, microphone, etc. The computing device may further comprise an output device configured to output results of the measurement to a user.

[0083] Although specific reference may be made in this document to the use of lithographic apparatus in the manufacture of ICs, it will be appreciated that the lithographic apparatus described herein may have other applications, including the manufacture of integrated optical systems, induction and detection patterns for magnetic domain memories, flat panel displays, liquid crystal displays (LCDs), thin film magnetic heads, etc.

[0084] Although this document may specifically refer to embodiments of the invention in the context of lithography apparatus, embodiments of the invention may also be used in other apparatus. Embodiments of the invention may form part of a mask inspection apparatus, a metrology apparatus, or any apparatus that measures or processes objects such as wafers (or other substrates) or masks (or other patterning devices). These apparatus may be generally referred to as lithography tools. Such lithography tools may use vacuum conditions or ambient (non-vacuum) conditions.

[0085] 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, where the context permits, the invention is not limited to optical lithography and may be used in other applications, such as imprint lithography.

[0086] Where the context allows, 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, 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 readable by a machine (e.g., a computing 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, electrical, optical, acoustic, or other forms of propagated signals (e.g., carrier waves, infrared signals, digital signals, etc.), and others. Furthermore, firmware, software, routines, and instructions may be described herein as performing particular actions. However, it should be noted that such description is merely for convenience, and that such actions may actually occur when a computing device, processor, controller, or other device executes the firmware, software, routines, instructions, etc., causing actuators or other devices to interact with the physical world.

[0087] While specific embodiments of the present invention have been described above, it will be understood that the invention may be practiced otherwise than as described. The foregoing description is intended to be illustrative and not limiting. Thus, it will be apparent to one skilled in the art that modifications may be made to the invention as described above without departing from the scope of the claims set forth below.

[0088] (Item 1) A method for determining contamination of an optical sensor of a detection system in a lithographic apparatus, comprising: directing EUV radiation onto a patterning device through an opening in a reticle masking blade; projecting reflected EUV radiation onto the detection system to cause a buildup of a contaminated area; measuring the height of the contaminated area and the height of an area of the detection system that is not receiving the reflected EUV radiation; and using the measured height to determine an amount of contamination on the optical sensor of the detection system. (Item 2) The method of item 1, wherein the EUV radiation is directed through multiple openings in the reticle masking blade and the heights of multiple contaminated regions are measured. (Item 3) A method according to item 1 or 2, wherein the EUV radiation is directed to one or more alignment marks provided on the patterning device. (Item 4) The method of item 1 or item 2, wherein the EUV radiation is directed to one or more areas of the patterning device that are less than 50% covered by an absorber. (Item 5) The method described in Item 1, wherein the opening in the reticle masking blade is a slot. (Item 6) A method according to any one of the preceding items, wherein the measured height of the contaminated area is compared with a model height. (Item 7) The method of item 6, wherein a model is generated using the measured heights of areas not receiving the reflected EUV radiation. (Item 8) The method described in Item 7, wherein the model comprises a line or curve extending between the measured heights of the area not receiving the reflected EUV radiation. (Item 9) The method described in Item 7, wherein the model takes into account the measured height of the area that received the reflected EUV radiation, the measured height being obtained after cleaning the detection system. (Item 10) The method according to any one of the above items, wherein the optical sensor is an image sensor. (Item 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 comprising an optical sensor, At least one opening is provided in the reticle masking blade; the level sensor is configured to measure a height of at least one contaminated area of an optical detection system and at least one non-contaminated area of the optical detection system; A lithographic apparatus comprising: a processor configured to determine a height of at least one contaminated area of the optical detection system, thereby determining an amount of contamination on the optical sensor. (Item 12) A lithographic apparatus according to item 11, wherein the reticle masking blade is provided with a plurality of openings. (Item 13) A lithographic apparatus according to Item 11, wherein one or more openings in the reticle masking blade are slots. (Item 14) A lithographic apparatus according to any one of items 11 to 13, wherein the optical sensor is an image sensor. (Item 15) A lithographic apparatus according to any one of items 11 to 14, wherein the processor is configured to use a model to determine the height of a protrusion or recess caused by contamination. (Item 16) A lithographic apparatus according to Item 15, wherein the model is generated using measured heights of multiple non-contaminated areas of the optical detection system. (Item 17) A computer-readable storage medium comprising instructions that, when executed by a processor of a computing device, cause the computing device to perform the method of any one of items 1 to 10. (Item 18) A computing device comprising a processor and a memory, the memory storing instructions that, when executed by the processor, cause the computing device to perform the method of any one of items 1 to 10.

Claims

1. 1. A method for determining contamination of an optical sensor of a detection system in a lithographic apparatus, comprising: directing EUV radiation onto a patterning device through an opening in a reticle masking blade; projecting reflected EUV radiation onto the detection system to cause a buildup of a contaminated area; measuring the height of the contaminated area and the height of an area of the detection system that is not receiving the reflected EUV radiation; and using the measured height to determine an amount of contamination on the optical sensor of the detection system. method.

2. the EUV radiation is directed through a plurality of openings in the reticle masking blade, and heights of a plurality of contaminated regions are measured; The method of claim 1.

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

4. the EUV radiation is directed onto one or more areas of the patterning device that are less than 50% covered by an absorber. The method according to claim 1 or 2.

5. the opening in the reticle masking blade is a slot; The method of claim 1.

6. The measured height of the contaminated area is compared with a model height, or / and the optical sensor is an image sensor; The method according to claim 1 or 2.

7. a model is generated using the measured heights of the areas not receiving the reflected EUV radiation; The method of claim 6.

8. the model comprises a line or curve extending between the measured heights of the area not receiving the reflected EUV radiation; The method of claim 7.

9. the model takes into account a measured height of the area that received the reflected EUV radiation, the measured height being obtained after cleaning of the detection system. The method of claim 7.

10. 1. 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 detection system comprising an optical sensor, At least one opening is provided in the reticle masking blade; the level sensor is configured to measure the height of at least one contaminated area of the detection system and at least one uncontaminated area of the detection system; a processor configured to determine a height of at least one contaminated area of the detection system, thereby determining a contamination amount on the optical sensor; Lithography equipment.

11. a plurality of openings are provided in the reticle masking blade; The lithographic apparatus of claim 10.

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

13. the optical sensor is an image sensor, and / or the processor is configured to use the model to determine the height of any peaks or valleys caused by contamination; A lithographic apparatus according to any one of claims 10 to 12.

14. the model is generated using measured heights of a plurality of uncontaminated regions of the detection system; The lithographic apparatus of claim 13.

15. 1. A system for determining an amount of contamination on an optical sensor of a lithographic apparatus, comprising: The system includes a level sensor, a detection system including an optical sensor, and a processor; the level sensor is configured to measure the height of at least one contaminated area of the detection system and at least one uncontaminated area of the detection system; the processor is configured to determine the amount of contamination on the optical sensor by determining the height of at least one contaminated area of the detection system. system.

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