Method of determining contamination information and an object stage adapted to implement the method

Abstract

The present disclosure provides a method of determining contamination information of at least one first sensor of an object table, the method comprising the steps of: - irradiating a first grating of the at least one first sensor with radiation originating from a radiation source related to a second sensor; - capturing reflected radiation as reflected from the first grating; and - determining the contamination information of the first grating based on the reflected radiation.

Description

METHOD OF DETERMINING CONTAMINATION INFORMATION AND AN OBJECT STAGE ADAPTED TO IMPLEMENT THE METHODCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of EP application 24179464.3 which was filed on June 03, 2024 and which is incorporated herein in its entirety by reference.FIELD

[0002] The present invention relates to a method of determining contamination information and to an object stage adapted to implement the method. The contamination may be carbon based or resist based deposition on a sensor. The sensor may be a sensor in a lithographic apparatus, for instance relating to or positioned in the vicinity of an object stage.BACKGROUND

[0003] Light generated by means of a radiation source can be used by exposure apparatuses for semiconductor manufacturing processes. Examples of such exposure apparatuses are a lithographic apparatus, a metrology, or an inspection apparatus, more specifically a mask inspection apparatus and even more specifically an actinic mask inspection apparatus.

[0004] A lithographic apparatus is a machine constructed to apply a desired pattern onto a substrate. A lithographic apparatus can 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 (e.g., a photoresist or resist) provided on a substrate. To project a pattern on a substrate a lithographic apparatus may use electromagnetic radiation. The wavelength of this radiation determines the minimum size of features which can be formed on the substrate. A lithographic apparatus, which uses EUV radiation, having a wavelength within the range 4-20 nm, for example 6.7 nm or 13.5 nm, may be used to form smaller features on a substrate than a lithographic apparatus which uses, for example, radiation with a wavelength of 193 nm.

[0005] An (actinic) mask inspection apparatus is an apparatus that is configured for measuring dimensions or detecting defects in masks or mask blanks. EUV lithography uses a reflective surfaces instead of a lenses as optics. Mask blanks used in EUV lithography generally have a multilayer structure which functions as a Bragg reflector, the multilayers may be alternatingly Molybdenum and Silicon. If a defect exists in this structure, the projected pattern will be deformed in the lithographic process. Therefore, mask inspection to check whether a defect is present is considered a requirement for a mass-production process. EUV mask inspection may be used for several purposes and in several different stages. Firstly, it can be used for the detection of phase defects that may occur in mask blanks. Such phase defects may occur during the manufacturing of the multilayer stack of the mask blank. If undetected, these phase defects are printed on all chips printed with the part of a maskcontaining the phase defects. Such phase defects may be correctly detected by using the same or similar (13.5nm) actinic EUV wavelength as the lithography tool. Secondly, mask inspection can be used for patterned mask inspection and can be carried out for the quality control of EUV patterned masks. For example, the mask inspection can be used to measure critical dimensions on the mask blank. In addition to phase defects, absorber pattern defects on the surface can be detected. Thirdly, mask inspection can be used for simulating exposure and determining the deterioration of optical contrast of a defect detected in the actinic inspection. Forth, the mask inspection can be used for optical proximity correction (OPC) evaluation or during mask repair process so as to improve pattern transfer fidelity. Further, it can be used for inspecting optical contrast after fixing the defect. In addition to the above, mask inspection can also be used to measure small particle / amplitude effects.

[0006] In EUV machines, contamination due to evaporation and subsequent deposition of hydrocarbon based material is a particular concern. Hydrocarbons float in the substrate stage compartment after being outgassed from the resist. The hydrocarbon based resist interacts with the EUV light, which breaks some of the atomic bonds. Broken into smaller sub-parts, the carbon based materials float into the machine and deposit as carbon based contaminants on top of exposed surfaces. Recently, resists specifically designed for use with EUV light have been developed including components other then carbon based material, such as metals or oxides. Consequently, also the resulting contamination may comprise these components.

[0007] If the contaminants deposit on, for instance, sensors, such as the image align sensor(s), the deposited contamination causes focus shift errors in the measurement of the respective sensor, due to lensing of the layer of contamination on top of detectors of the sensors. Also, the signal strength of respective sensors decreases due to the contamination. Although the layer of contamination may in practice have a thickness in the order of nanometers, the effect on sensor readings is detrimental for state of the art lithographic machines having line widths of that same order of magnitude. The contamination rate typically corresponds to the EUV dose and grows over time. The contamination requires periodical cleaning, which in turn reduces machine uptime, and the related yield and turnover.

[0008] Problems resulting from uneven deposition of contamination on areas in the lithographic apparatus relevant for imaging and exposure may include, for instance, one of more of an error in the height measurement of an object surface, an error in the image align sensor reading, and an error in wave front measurements.

[0009] According to the state of the art, solutions for a direct measurement of the contamination accumulated on top of grating detectors of image align sensors are not available. Thus far, no direct and independent measurement method aimed specific at contamination of a grating of the image align detectors has been devised.

[0010] For instance, patent application WO-24028327-A1 provides a method of determining contamination of an optical sensor in a lithographic apparatus, the method comprising illuminating apattern on a patterning device using EUV radiation, projecting patterned reflected EUV radiation towards the optical sensor and thereby forming an aerial image of the pattern, and moving the optical sensor relative to the patterned reflected EUV radiation such that an intensity of EUV radiation measured by the optical sensor varies as a function of the position of the optical sensor, wherein the intensity measured by the optical sensor passes through a minimum, and wherein the method further comprises using the measured intensity to measure contamination of the optical sensor. In the method of WO-24028327-A1, a contamination measurement is a byproduct of the image align measurement (also referred to as a measurement scan, aiming at determining the 'aligned position' of the image), from which the contamination signature can be extracted.

[0011] The method of WO-24028327-A1 is suitable to alleviate errors relating to the image align sensor. However, additional measures are required to solve the other errors as described above.

[0012] Patent application WO-24022773-A1 provides a method of determining contamination of an optical sensor of a sensing system in a lithographic apparatus. During an image align measurement, the EUV light is projected on the sensor through a reticle masking, which allows light only on given areas of the sensor (typically the areas including the detectors) and not on the rest of the surface. So, contamination builds only on exposed areas. This fact is used to measure the surface height of the contaminated areas using the level sensor, as they drift with growing contamination, using the clean areas as reference.

[0013] The method of WO-24022773-A1 is suitable to alleviate errors relating to the levelling sensor. However, additional measures are required to solve the other errors as described above.

[0014] More accurate knowledge of the contamination would be desired. For instance, none of the systems and methods referenced above can predict and solve wave front measurement errors due to contamination.

[0015] It is an aim to provide a direct measurement method for quantifying the actual contamination, allowing to trigger cleaning at the right time.SUMMARY

[0016] The present disclosure provides method of determining contamination information of at least one first sensor of an object table, the method comprising the steps of:- irradiating a first grating of the at least one first sensor with radiation originating from a radiation source related to a second sensor;- capturing reflected radiation as reflected from the first grating;- determining the contamination information of the first grating based on the reflected radiation.

[0017] In an embodiment, the at least one first sensor is an image align sensor.

[0018] In an embodiment, the second sensor is an alignment sensor.

[0019] In an embodiment, the step of capturing reflected radiation comprises capturing multiple diffraction orders of the reflected radiation.

[0020] In an embodiment, the radiation originating from the radiation source related to the second sensor is visible light.

[0021] In an embodiment, the radiation has a wavelength in the range of about 380 to about 750 nanometers

[0022] In an embodiment, the method comprises the step of using the contamination information relating to the at least one first sensor as reference to determine a cleaning schedule for a third sensor.

[0023] In an embodiment, the third sensor is a wavefront sensor or (parallel) ILIAS sensor.

[0024] In an embodiment, the method comprises the step of determining the contamination information based on radiation which is transmitted to and measured by one or more photodiodes of the at least one first sensor.

[0025] According to another aspect, the disclosure provides an object stage adapted to implement the method as described above.

[0026] According to another aspect, the disclosure provides a lithographic apparatus comprising the object stage as described above.

[0027] In an embodiment, the lithographic apparatus comprises a source of radiation for exposure of a pattern on a substrate, the radiation being in the extreme ultraviolet (EUV) spectrum.

[0028] The method and object stage of the present disclosure enable to obtain relatively accurate knowledge of the contamination as deposited on equipment, such as sensors, within an apparatus related to EUV radiation. Accurate knowledge herein may include direct measurement of the quantity of contamination. Also, the method and system of the disclosure allow to directly measure the chemical composition of the contamination. Consequently, the system and method enable, for instance, to derive a more accurate cleaning schedule, to compensate for contamination, and to correct sensor measurements such as image align measurements. The method enables a direct measurement method for quantifying the actual contamination, allowing to trigger cleaning at the right time. The latter enables to optimize the balance between machine uptime, productivity, and accuracy, resulting in increased yield overall.BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:Figure 1 shows a schematical representation of a lithographic system comprising a lithographic apparatus and a radiation source;Figure 2 shows a schematical representation of a system for (actinic) mask inspection; Figure 3 shows a schematical representation of an alternative radiation source;Figures 4A and 4B show a cross-sectional side view of a substrate and a sensor of the lithographic system, with Figure 4B exemplifying deposition of contaminants on the sensor;Figure 5 shows a cross-sectional side view of an area within the lithographic apparatus during use thereof, schematically indicating potential deposition of contaminants;Figure 6 schematically shows a cross section of an exemplary sensor provided with contaminants;Figure 7 schematically shows a cross section of an exemplary grating line and detector of a sensor;Figure 8A schematically shows a top view of an exemplary sensor provided with openings for detection of radiation;Figure 8B shows the sensor of Fig. 8A including contamination;Figure 9 A shows a side view of a measurement side of a lithographic apparatus;Figure 9B shows a side view of an expose side of a lithographic apparatus;Figure 10 shows a perspective view of an exemplary method for scanning an image align sensor;Figure 11 A shows a side view in cross section of a sensor having a grating reflecting part of incident radiation;Figure 1 IB shows a side view of the sensor of Fig. 1 IB including contamination;Figure 12 shows a diagram exemplifying an amplitude ratio for different order reflections (vertical axis) as a function of the duty cycle (horizontal axis);Figure 13A shows a side view in cross section of a sensor and its output signal;Figure 13B shows a side view in cross section of the sensor of Fig. 13A including contamination; andFigure 14 shows a top view of an embodiment of a grating of a sensor for a method of the present disclosure.DETAILED DESCRIPTION

[0030] Figure 1 shows a lithographic system comprising a radiation source SO and a lithographic apparatus LA. The radiation source SO is configured to generate an EUV radiation beam B and to supply the EUV radiation beam B to the lithographic apparatus LA. The lithographic apparatus LA comprises an illumination system IL, a support structure MT configured to support a patterning device MA (e.g., a mask), a projection system PS and a substrate table WT configured to support a substrate W.

[0031] The illumination system IL is configured to condition the EUV radiation beam B before the EUV radiation beam B is incident upon the patterning device MA. Thereto, the illumination system IL may include a facetted field mirror device 10 and a facetted pupil mirror device 11. The faceted field mirror device 10 and faceted pupil mirror device 11 together provide the EUV radiation beam Bwith 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 faceted field mirror device 10 and faceted pupil mirror device 11.

[0032] After being thus conditioned, the EUV radiation beam B interacts with the patterning device MA. As a result of this interaction, a patterned EUV radiation beam B’ is generated. The projection system PS is configured to project the patterned EUV radiation beam B’ onto the substrate W. For that purpose, the projection system PS may comprise 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’, thus forming an image with features that are smaller than corresponding features on the patterning device MA. For example, a reduction factor of 4 or 8 may be applied. Although the projection system PS is illustrated as having only two mirrors 13,14 in Figure 1, the projection system PS may include a different number of mirrors (e.g., six or eight mirrors).

[0033] The substrate W may include previously formed patterns. Where this is the case, the lithographic apparatus LA aligns the image, formed by the patterned EUV radiation beam B ’ , with a pattern previously formed on the substrate W.

[0034] A relative vacuum, i.e. a small amount of gas (e.g. hydrogen) at a pressure well below atmospheric pressure, may be provided in the radiation source SO, in the illumination system IL, and / or in the projection system PS.

[0035] The lithographic apparatus LA and radiation source SO described herein can be used in method for performing a circuit layout patterning process. A circuit layout patterning method comprises receiving a substrate with a photoresist layer. The method further comprises directing EUV radiation from radiation source to the photoresist layer to form a patterned photoresist layer. The method further comprises developing and etching the patterned photoresist layer to form a circuit layout.

[0036] Figure 2 depicts a system for (actinic) mask inspection. A mask inspection system can be used to identify or inspect defects in a mask to be used in a lithographic process by means of an apparatus described in figure 1. The mask inspection system comprises a radiation source SO2 and an illumination system IL2 and a detection system DS. A mask MA2 is placed on a mask stage MT2 and illuminated by the illumination system IL2 reflecting radiation incident from the radiation source SO2. The radiation coming from the illuminated mask is reflected by the detection system DS. In this way an image is formed on a detector D.

[0037] The radiation source SO shown in Figure 1 is, for example, of a type which may be referred to as a laser produced plasma (LPP) source. A laser system 1, which may, for example, include a CO2-i laser, is arranged to deposit energy via a laser beam 2 into a fuel (i.e., a target material), such as tin (Sn) which is provided from, e.g., a fuel generator 3. Although tin is referred to in the following description, any suitable fuel may be used. The fuel may, for example, be in liquid form, and may, forexample, be a metal or alloy. The fuel generator 3 may comprise a nozzle configured to direct the fuel, e.g. in the form of droplets, along a trajectory towards a plasma formation region 4. The laser beam 2 is incident upon the fuel at the plasma formation region 4. The deposition of laser energy into the tin creates a plasma 7 at the plasma formation region 4. Radiation, including EUV radiation, is emitted from the plasma 7 during de-excitation and recombination of electrons with ions of the plasma 7.

[0038] The EUV radiation from the plasma 7 is collected and focused by a collector 5. Collector 5 comprises, for example, a near-normal incidence radiation collector 5 (sometimes referred to more generally as a normal-incidence radiation collector). The collector 5 may have a multilayer mirror structure which is arranged to reflect EUV radiation (e.g., EUV radiation having a desired wavelength such as 13.5 nm). The collector 5 may have an ellipsoidal configuration, having two focal points. A first one of the focal points may be at the plasma formation region 4, and a second one of the focal points may be at an intermediate focus 6, as discussed below.

[0039] The laser system 1 may be spatially separated from the radiation source SO. Where this is the 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, for example, suitable directing mirrors and / or a beam expander, and / or other optics. The laser system 1, the radiation source SO and the beam delivery system may together be considered to be a radiation system.

[0040] Radiation that is reflected by the collector 5 forms the EUV radiation beam B. The EUV radiation beam B is focused at intermediate focus 6 to form an image at the intermediate focus 6 of the plasma present at the plasma formation region 4. The image at the intermediate focus 6 acts as a virtual radiation source for the illumination system IL. The radiation source SO is arranged such that the intermediate focus 6 is located at or near to an opening 8 in an enclosing structure 9 of the radiation source SO.

[0041] Although Figure 1 depicts the radiation source SO as a laser produced plasma (LPP) source, any suitable source such as a free electron laser (FEL) or a discharge produced plasma (DPP) source may be used to generate EUV radiation.

[0042] Figure 3 depicts an alternative radiation source SO3. For generating plasma target material TM, like tin or xenon may be provided to a rotating element RE, like for example rotating wheels, cylinder, or a drum or variations thereof. Target material may be provided in liquified form to the one or more rotating elements, e.g. by means of a target material bath. Alternatively, target material may also be provided in solid or frozen form (e.g. Xenon). Gaseous target material may be sprayed onto the rotating element to replace target material transformed to plasma. According to embodiments the rotating element would be cold enough to solidify the target material. An excitation device ES may be used to assist in plasma formation. According to embodiments this excitation device is a laser, like a solid-state or gas laser. Reflective optics OS, like one or more mirrors, can be used to reflect the EUV light generated to the intermediate focus 6. According to embodiments, the reflective optics comprisea collector mirror. Buffer gas flow BG may be provided to mitigate debris present in the radiation source. Also a vacuum pump VP may be provided to remove gas from the radiation source SO3.

[0043] Generally referring to Figures 4A, 4B and 5, the performance of lithographic machines using EUV as a source of radiation suffers from contamination, most notably carbon based contamination. Contamination may cause drift in output of, for instance, a sensor. Examples of contamination include plasma induced carbonization, EUV induced carbonization, airborne contamination, absorbed organic molecules from photoresists, and anything that can block EUV from reaching the sensor.

[0044] Carbon based contamination is mostly caused by the carbonization of the photoresist volatile under EUV exposure. Carbonization or carbonisation is the conversion of organic matter into carbon through destructive distillation. A photoresist (also referred to as resist) is a light-sensitive material used in several processes, such as photolithography, to form a patterned coating on a surface of a substrate. The process begins by coating the substrate with the light-sensitive material, typically an organic material. As described with reference to Fig. 1, a patterned mask MA is then applied to the surface to block light, so that only unmasked regions of the material will be exposed to radiation. A solvent, called a developer, is then applied to the surface. In the case of a positive photoresist, the photo-sensitive material is degraded by light and the developer will dissolve away the regions that were exposed to light, leaving behind a coating where the mask was placed. In the case of a negative photoresist, the photosensitive material is strengthened (either polymerized or cross-linked) by radiation, and the developer will dissolve away only the regions that were not exposed to light, leaving behind a coating in areas where the mask was not placed.

[0045] Examples of photoresist include, but are not limited to, methyl methacrylate monomers, polymethyl methacrylate (PMMA), acrylate monomer, a mixture of diazonaphthoquinone (DNQ) and novolac resin (a phenol formaldehyde resin) commonly referred to as DNQ-Novolac photoresist, and epoxy-based polymer. Alternatives include metal oxide resists, for example including tin clusters of the type [(RSn)12O4(OH)6]X2.

[0046] As exemplified in Fig. 4A, the wafer W is provided with a layer of photoresist. Upon exposure with a beam B of radiation, exposed sections of the resist may turn into volatile components 30. As indicated in Figure 5, the components 30 may typically include a reactive part, i.e. the volatile components may be regarded as free radicals. The latter can deposit on any surface within the lithographic apparatus LA. The volatile components 30 may, for instance, deposit on a sensor 32. The components 30 may deposit on the sensor 32 or any surface 36 (Fig. 3), forming a patch or layer of deposit 34. Referring to Figure 4B, the contamination 34 may for instance deposit in a particular pattern, under the influence of incident radiation B.

[0047] The sensor 32 may be any sensor. The sensor 32 may be sensor which is located near the wafer W, such as an image align sensor IS for measuring and calibrating the energy of radiation in the beam B. Due to its location relatively near the wafer, the image align sensor IS is especially vulnerable to deposition of volatile components 30. However, the volatile components 30 may depositon other surfaces 36 as well, including surfaces of mirrors 5, 10, 11, 13, 14, the mask MA and associated parts, or any part of the walls of the projection system PS and illumination system IL.

[0048] The contamination is a normal and expected phenomenon. The contamination 34 can be removed. Cleaning and removing of the contamination can be done, for instance, using a hydrogen radical generator (HRG) to generate hydrogen radicals. The latter can be introduced in the respective compartment of the machine. The hydrogen radicals will react with the carbon based contaminants 34 to form hydrocarbons, typically in gas phase. The hydrocarbons can subsequently be removed using a pump, a process which may be referred to as outgassing.

[0049] If the contaminants 30 settle on the surface of a sensor, this may typically result in errors in the reading of the respective sensor.

[0050] For instance, Fig. 6 exemplifies an image align sensor IS having a top surface. Under influence of radiation, for instance including EUV light, the radicals 30 may settle on the sensor IS, forming a layer 34 of contamination. Said layer 34 is typically uneven, and as a result may cause a lensing effect. Lensing herein means that the sensor reading may differ across the sensor surface, for the same intensity of the incident beam of radiation.

[0051] Figure 6 exemplifies the error in the level sensor measurement, which measures the surface of the sensor, due to the thickness of the contamination 34. The layer of contamination shifts the height result upwards. For instance, if the area around a mark becomes contaminated, a height measurement, referred to as topology measurement, may typically be influenced. In other words, a related level sensor measuring the level or height of the top surface of the image align sensor will provide a distorted reading, thereby linking the measured intensity of the illumination to an offset position in height direction (or ‘z-direction’).

[0052] Generally referring to Figure 7, another issue may relate to a grating GR of the image align sensor IS. A grating GR may typically include one or more openings 40, allowing incident radiation to pass while the remaining radiation is blocked. The openings 40 of the grating GR may typically be shaped in a particular pattern, such as a number of parallel slits. A detector PD, such as a photo diode, may be provided at an end of the openings of the grating facing away from the radiation beam B.

[0053] Figure 7 exemplifies a lensing effect due to contamination. Contaminant growth in the openings 40 of the grating GR of the image align sensor IS may reduce the measured areal image. Without going into technical details, the result of the error in the measured areal image results in a z- error or height error when the substrate W is moved to the expose side of the lithographic apparatus for exposure and patterning. See also Figures 9A and 9B in this respect.

[0054] Generally referring to Figures 8 A and 8B, contamination may also settle on other sensors, such as on a wavefront sensor or on a dose sensor (not shown). The wavefront sensor may be referred to as ILIAS sensor or PARIS sensor. The wavefront sensor is an optical sensor having a grating 50, integrated in the lithographic apparatus LA, that measures aberrations in the optical elements, such as lenses or mirrors 10 to 14 (see Fig. 1). The wavefront sensor grating 50 enables fast and accuratemeasurement, allowing to measure aberration of the optical elements, which allows a control loop to provide corrections.

[0055] The wavefront sensor may include a grating 50, including openings 52 (Fig. 8A). The contamination 54 may settle inside of the openings 52, as exemplified in Fig. 8B. The contamination 54 thereby may reduce the reading of the wavefront sensor. In addition, as the contamination 54 may be uneven, the contamination 54 may result in erroneous connections for the radiation wavefront.

[0056] Referring to Figures 9A and 9B, the lithographic apparatus may include an object stage including one or more movable object tables WT.

[0057] In accordance with an embodiment of the present invention, the object stage may include:- at least one first sensor having a first grating;- a second sensor having a radiation source, the radiation source being configured to, during use, emit radiation to the first grating; wherein the second sensor is configured to capture reflected radiation as reflected from the first grating of the at least one first sensor;- a processing unit for determining contamination information of the first grating based on the reflected radiation as captured by the second sensor.

[0058] The at least one sensor can e.g. be mounted on an object table of the object stage, e.g. object table WT.

[0059] The object table WT is typically movable with respect to a base (not shown). In operation, the object table WT may be provided with a substrate W. Then, the object table WT first moves towards a measurement location or measurement side, exemplified in Figure 9A. Thereafter, the object tables moves to the expose side, see Figure 9B.

[0060] Although both Figures are shown as separate Figures, please note that the measurement side (Fig. 9A) and the expose aside (Fig. 9B) may typically be located on opposite sides of the same base, with one or more substrate tables being able to move from side to side.

[0061] For technical details of an object stage comprising multiple object tables, and the functionality thereof including measurement and exposure, reference is made to, for instance, patent US-6597433.

[0062] The object table is typically provided with one or more first sensors. The first sensors are image align sensors, such as a first image align sensor IS1 and a second image align sensor IS2. The first sensors are typically arranged relatively near to, or adjacent to, the intended position of the substrate W.

[0063] The measurement side of the object stage, see Fig. 9A, is typically provided with a number of other sensors. The measurement side may be provided with one or more of a second sensor and a third sensor. The second sensor may comprise an alignment sensor AS. The third sensor may comprise a level sensor LS. The level sensor LS, which also comprises a second part or detector part LS2, is adapted to measure height or topography of the object W on the object table. The alignmentsensor AS may measure the alignment, i.e. the horizontal position in one or both of the x-direction and y-direction, of the object W with respect to a predetermined setpoint position.

[0064] The alignment sensor AS may include a light source. The light source may be a source of second radiation 60, for instance visible light. The visible light frequency may be somewhere in the 400 THz to 700 THz frequency range, approximately. Alternatively worded, visible light herein may have a wavelength in the spectrum of about 380 to about 750 nanometers. In use, an alignment mark on the object W is irradiated, for instance using the visible light. The reflected light is captured to assess a position of the mark.

[0065] The level sensor comprises a first part LS and a second part LS2. The first part may comprise a source of third radiation 62. The third radiation may be any suitable radiation for scanning the top surface of the substrate W. In use, the first level sensor part LS directs the third radiation 62 at the top surface of the substrate W. Herein, the top surface is the surface of the substrate W facing the level sensor and the alignment sensor, and facing away from the object table WT. The level sensor may scan the third radiation across the surface of the substrate, in a predetermined pattern. The second level sensor part LS2 may detect the third radiation as it is reflected from the top surface, and use the detected reflection to determine the topology or height map of the top surface of the substrate W.

[0066] Figure 10 schematically depicts the general working principle of the one or more image align sensors IS. Herein, an aerial image 60 is formed at reticle level from grating marks 62 on the mask MA or one of its related components. Herein, for an EUV based application the mark may be referred to as a reticle. The functional stage comprising the reticle and its related object table MT may be referred to as the reticle stage RS.

[0067] The image 60 is then projected via the optics OPT of the lithographic apparatus onto the object stage. Herein, at least one of the image align sensors IS is positioned at the receiving end of the projection optics OPT. The image 60 is projected on the image align sensor IS, forming a projected image. The image align sensor comprises a photodetector 64 for detecting the projected image. The photodetector is provided with photodetector gratings 66.

[0068] The first sensor IS is moved across the projected beam B, for instance in a zigzag pattern 68. The patterns 68 may typically extend in horizontal direction (x-axis) and in vertical direction (z-axis), thus enabling to create an indication of radiation intensity 70 including an area of maximum radiation intensity 72. The indication of radiation intensity may be referred to as an aerial image. The image align sensor basically scans the projected image, to find a position wherein the photodetector gratings 66 are aligned with the reticle gratings 62. The image align sensor IS herein provides the areal image 70. At the respective position of the object table where the respective gratings 62, 66 align, the areal image provides the maximum 72.

[0069] Herein, the scan 68 may extend over, for instance, about + / - 800 nm in vertical direction. The scan 68 may extend in horizontal direction over about + / - 200 nm. The latter is smaller than a grating pitch of the detector grating 66. The latter may be in the order of about 1 pm. The width of thescan 70 may however exceed the linewidth of the photodetector 64, which may be in the order of about 100 nm. These dimensions may be relevant for the sake of the present invention, as the scan length determines the contaminated area, eventually changing the effective duty cycle of the respective reflective grating.

[0070] Any one the measurement errors as described above may result in patterning errors, wherein the irradiated pattern on the substrate deviates from an optimum. The latter may generally be referred to as performance drift.

[0071] The present disclosure proposes to use the alignment sensor AS to correct or predict influence of contamination on the output of the one or more image align sensors. Herein, the method assesses the presence of contamination on a grating of the image align sensor AS by irradiating said grating of the alignment sensor AS. Contamination present will result in a variation of the intensities of different reflected orders of the applied radiation.

[0072] In general, alignment marks or gratings may include a series of lines on the respective carrier, such as a section of the image align sensor, a section of the reticle, in the resist applied to a substrate W, or on a section of the wavefront sensor. Incoming alignment sensor radiation diffracts on these lines. The diffracted light is optically processed, typically by the alignment sensor AS, to make the signal sensitive to the mark position, i.e the position of the respective grating. The mark pattern is created by moving the respective mark with respect to the respective sensor.

[0073] The present disclosure proposes two exemplary methods, which can either be used in combination or independently. First, it is proposed to extract the contamination information from the intensity of light originating from the alignment sensor, and use the light as it is reflected from the surface of the image align sensor or detector. Second, it is suggested to extract the equivalent information out of light of the alignment sensor which is transmitted through and thus measured by the detector photodiodes of the image align sensor.

[0074] In an embodiment, the contamination information can e.g. be obtained by a processing unit or processor or control unit or the like that is configured to process one or more sensor signals such as signals retrieved from the first sensor and / or the second sensor.

[0075] Herein, it is assumed it is possible to use the first grating 66 of the first sensor, i.e. the image align sensor IS, as reflective marks that create a measurable intensity signal. The reflection can be measured by the second sensor, i.e. the Alignment Sensor AS. The ability to measure the reflection has been proven to be possible via ‘pictures’ of the grating 66 of the sensor IS mark.

[0076] A first embodiment of a method according to the present disclosure is based on the principle that gratings of equal pitch but different duty cycle have different diffraction efficiency. As a result, respective gratings thus reflect incident light by distributing the light intensity differently amongst respective diffraction orders. The latter effect is depicted in the sketch in Figure 12. Figure 12 herein schematically exemplifies a ratio of a 0th, 1st, and 2ndorder diffraction amplitude (vertical axis, Ao, Ai,and A2 respectively) as a function of grating duty cycle (horizontal axis). Quantitative estimates are given at the end of the description herein below.

[0077] Referring to Fig. 11 A, a relatively clean grating 66 of the image align sensor IS (having, for instance, a pitch of about 1000 nm, and a detector line width LW of about 100 nm) may have, for instance, a 10% duty cycle mark.

[0078] Generally referring to Figure 1 IB, a ‘contaminated’ grating (having the same pitch, but a wider reflecting area due to the contamination, whose reflecting surface is proportional to the scan length, for instance about 400 um) returns a signal equivalent to a sensor having an increased duty cycle, for instance having a 40% duty cycle.

[0079] The method of the disclosure includes a step of scanning over the grating 66 of the image align sensor IS, using radiation 60 originating from the Alignment Sensor AS. In a next step, the method measures the intensity of the individual reflected diffraction orders as reflected from the first grating 66. Subsequently, the method compares the magnitudes or amplitudes of the reflected light, typically for each order, as they change due to the growing contamination reflectivity.

[0080] Herein, these steps are typically repeated at regular intervals. For instance, the steps above may be repeated between stacks of substrates, or may be repeated in a number of the measurement cycles of substrates in a stack. For instance, the steps above may be repeated for the measurement cycle of every 2nd, 3rd, 5th, 8th, or 10thsubstrate in a stack. Thus, the method of the disclosure can effectively measure the amount of contamination on the photodetector grating 66 and furthermore monitor the evolution of said contamination over time.

[0081] Monitoring the ratio of the diffraction orders of the reflected light, see Figures 11A and 1 IB, might prove particularly efficient, as shown in the example graph in Figure 12 and in the detailed analysis at the end of the description herein below. The ratio strongly varies as a function of the Duty Cycle. Measuring individual orders of the reflected radiation will be technically possible to implement, for instance, by including a detector in the alignment sensor AS equipped with so called ‘intensity channels’. Thus, the second sensor will be enabled to independently monitor the different diffraction order intensities, as exemplified in Figures 11A and 11B.

[0082] In a second embodiment, the method of the disclosure utilizes the alignment sensor AS as a probe. The measurement sequence may be comparable to the first embodiment as described above. However, see Figure 13 A, the method uses a readout or output signal 80 of the photodiodes of the image align sensor IS. Monitoring the output allows to compare the output over time. Figure 13B exemplifies an image align sensor IS having a contaminated surface, with contamination 34.

[0083] In operation, the first sensor IS is illuminated using the radiation source of the second sensor, i.e. the alignment sensor AS. In effect, the method of the disclosure measures the amount of Alignment Sensor light which is transmitted through the grating 66 and to the photodiodes of the first sensor IS. For a contaminated sensor, see Fig. 13B, the method measures how much radiation is transmitted through the contamination layer 34. Herein, please note that the image align sensor is, in apreferred embodiment, suitable for detecting EUV radiation. However, the image align sensor also has a relatively small, yet sufficiently significant, responsivity to visible light. For instance, the at least one first sensor IS is responsive to, for instance, green laser light. The latter has been proven by tests.

[0084] Due to the presence of contamination, the sensor signal 82 will be attenuated by a factor proportional to the contamination transmission Tc, which, conversely to the reflection Rc, shall decrease while the thickness of the carbon contamination 34 increases. So, monitoring the output signal 82 of the first sensor IS over time will allow to quantify the carbon growth on the surface of the first sensor IS.

[0085] An important advantage of the technique as described above is it is possible to use multiple colors of light originating from of the radiation source of the alignment sensor AS. Then, it is possible to monitor the responses, i.e. the output signal 82, to incident radiation having different wavelengths. Thus, information with respect to the composition of the contamination 34 can be extracted.

[0086] Herein, the cross section or spot size of the light originating from the second sensor AS as it is projected on the first sensor IS is smaller than the dimension of the first grating 66 of the first sensor IS. The latter allows to determine the thickness and potentially the composition of the contamination 34 locally, and its variation across the surface of the first grating 66 of the first sensor.

[0087] Please note that the embodiments as described above can be implemented independently as well as in conjunction. Herein, both methods complement one another. Thus, it is possible to, on one hand, measure a decreasing intensity signal 82 over time (as the contamination 34 increases and the transmission through the contamination decreases as the layer grows) using the photodiodes of the first sensor IS. In conjunction, the method can detect an increasing signature of the duty cycle of the first grating 66, for instance up to 40% (as the reflectivity of the first grating 66 increases as the contamination 34 grows). Combining the information enables to derive more accurate estimates.

[0088] In an embodiment, the first grating 66 of the at least one first sensor IS is used as a baseline to measure contamination 34. However, alternative and ad-hoc modification to the layout of the first sensor IS can be introduced in order to facilitate the detection of contamination 34 on the surface of the at least one first sensor IS.

[0089] Generally referring to Figure 14, in an embodiment, next to the open lines of the first grating 66 of the first detector IS one or more reflective elements 90, 92 can be arranged. When contamination 34 grows on the surface of the first detector IS, the contamination will cover the reflective elements 90, 92 as well, and hide them. The latter will, in effect, result in a shift of the signal of the first embodiment described above.

[0090] As different sections of the first sensor may have different reflective features, for instance first reflective features 90 on a first section and second reflective features on a second section, the asymmetry introduced by contamination 34 will be easier to detect.

[0091] In an embodiment, the method of the present disclosure can be applied to predict a suitable timeline and / or moment to schedule cleaning of the first sensor.

[0092] Generally referring to Figures 8 A and 8B, the lithographic apparatus LA includes, for instance, a third sensor for measuring the wave front of the radiation B. The third sensor may be referred to as ILIAS sensor or parallel-ILIAS (PARIS) sensor. The third sensor may measure the wave front of the radiation B at multiple locations in an exposure slit.

[0093] The third sensor, for instance the wavefront sensor, typically includes marks. These marks may be contaminated by the similar contamination depositions as described above. The marks of the third sensor may typically include third gratings. The contamination may result in (uneven) contamination growth and deposition in the third gratings. In an embodiment, the method of the present disclosure can be applied to predict a suitable timeline and / or moment to schedule cleaning of the third sensor. Herein, the contamination deposition on the third sensor is not measured directly, but the measurements on the at least one first sensor and the resulting estimate of the contamination on the first sensor are used to also estimate the contamination of the third sensor.

[0094] The section herein below provides a basic analytical description of the method and system, for further clarity. The surface of the first grating 66 of the first sensor is considered as a reflective grating structure. The latter creates an ‘IS alignment mark’. The mark has a pitch P~lum and, due tothe photodetector line being in the order of Lw~100nm, a duty cycle DC = — = = 10%.When scanning light on such a structure using the radiation source of the Alignment Sensor AS, for a given incident beam of intensity A and given an average reflectivity R of the detector surface of the first grating 66, the light reflected by this IS mark surface will be split in multiple diffraction beams (See Figure 11 A). The amplitude anfor any given diffraction order n, (for a purposely chosen coordinate system) can be written as an= 2DC ■ sinc(nnDC). Figure 12 provide examples of diffraction order amplitudes as a function of the Duty Cycle. This section provides a quantitative estimate.

[0095] Now, carbon contamination over image align sensors IS may be caused by scanning of EUV light. Thus, the contaminated area is defined by the scanning trajectory. For the IS gratings 66, the contamination 34 is deposited over the photodetector lines and spread laterally over a surface as wide as the scan length, typically +-200nm, and centered around the detector gratings 66. As contamination increases, so does the reflectivity Rcof the contaminated area (confirmed in measurements).Eventually, carbon contamination on top of the detector grating lines 66 will thus alter the reflectivity profile of the surface, superimposing on the clean detector grating (100 nm line width, 1 pm pitch, DCg= 10%) a similar reflecting grating structure whose pitch is the same but DC is different. For 400nm instance, tor the contaminated first sensor IS, DCr= - lum = 40%.

[0096] This structure will alter the distribution of intensity across the diffraction orders. The altered distribution can be detected by the Alignment Sensor. For the case of a clean IS grating 66 (DC = 10%), the amplitudes of the 0th, the + / -lstand the + / -2ndreflection will be respectively a0= lim 2 ■ A ■ R ■ DC ■ sinc(nnDC') = 2 ■ A ■ R ■ DC, a+1= 2 ■ A ■ R ■ DC ■ sincf iDC) and a+2= 2 ■ A ■ n-»0R ■ DC ■ sinc(2n;DC). Assuming A=100 [a.u.], R=1 [a.u.], for DC = 10% , we get a0= 20 , a±1— 17 and a±2=9.The amplitude of the diffraction orders reflected from a contaminated surface (Figure1 IB) will then be instead (assuming A=100 [a.u.] and DC = 40% and an average reflectivity of the contaminated surface Rc) a0= 80 ■ Rc, a±1— 14- Rcand a±2=10- Rc. This shows that the amplitude of the different diffraction orders depends on the mark duty cycle, and so does the relative ratio of diffraction orders. In particular, it is worth noting that A RcC-0.175 0.125 ■ RcCRc—0.85 ■ Rc. In the case of a clean detector (i.e. DC = 10%), this is instead Ao(10%) = 0.53.

[0097] The above calculations are for a simple grating and contamination profile model. A more complex model can be determined without loss of validity of the measurement principle.

[0098] Although specific reference may be made in this text 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. Possible other applications include the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, liquidcrystal displays (LCDs), thin-film magnetic heads, etc.

[0099] Although specific reference may be made in this text to embodiments of the invention in the context of a lithographic apparatus, embodiments of the invention may 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 an object such as a wafer (or other substrates) or mask (or other patterning devices). These apparatus may be generally referred to as lithographic tools. Such a lithographic tool may use vacuum conditions or ambient (non- vacuum) conditions.

[0100] 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, where the context allows, is not limited to optical lithography and may be used in other applications, for example imprint lithography.

[0101] While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. The descriptions above are intended to be illustrative, not limiting. Thus it will be apparent to one skilled in the art that modifications may be made to the invention as described without departing from the scope of the claims set out below.

Claims

CLAIMS1. Method of determining contamination information of at least one first sensor of an object table, the method comprising the steps of:- irradiating a first grating of the at least one first sensor with radiation originating from a radiation source related to a second sensor;- capturing reflected radiation as reflected from the first grating;- determining the contamination information of the first grating based on the reflected radiation.

2. The method of claim 1 , wherein the at least one first sensor is an image align sensor.

3. The method of claim 1 or 2, wherein the second sensor is an alignment sensor.

4. The method of one of the previous claims, wherein the step of capturing reflected radiation comprises capturing multiple diffraction orders of the reflected radiation.

5. The method of one of the previous claims, wherein the radiation originating from the radiation source related to the second sensor is visible light.

6. The method of claim 5, wherein the radiation has a wavelength in the range of about 380 to about 750 nanometers7. The method of one of the previous claims, comprising the step of using the contamination information relating to the at least one first sensor as reference to determine a cleaning schedule for a third sensor.

8. The method of claim 7, wherein the third sensor is a wavefront sensor.

9. The method of one of the previous claims, comprising the step of determining the contamination information based on radiation which is transmitted to and measured by one or more photodiodes of the at least one first sensor.

10. An object stage, comprising:- at least one first sensor having a first grating;- a second sensor having a radiation source, the radiation source being configured to, during use, emit radiation to the first grating; wherein the second sensor is configured to capture reflected radiation as reflected from the first grating of the at least one first sensor;- a processing unit for determining contamination information of the first grating based on the reflected radiation as captured by the second sensor.

11. The object stage of claim 10, wherein the at least one first sensor is an illumination sensor and wherein the second sensor is an alignment sensor.

12. The object stage of claim 10 or 11, wherein the radiation related to the radiation source of the second sensor is visible light.

13. The object stage of one of claims 10 to 12, comprising a third sensor, such as a wavefront sensor, wherein the processing unit for determining contamination information of the first grating based on the reflected radiation is adapted to use the contamination information relating to the at least one first sensor as reference to determine a cleaning schedule for the third sensor.

14. The object stage of one of the claims 10 to 13, wherein the first sensor is configured to capture radiation transmitted through the first grating and wherein the processing unit is configured to determine the contamination information based on the transmitted radiation as captured by the first sensor.

15. A lithographic apparatus comprising the object stage according to any of the claims 10 to 14 and a source of radiation for exposure of a pattern on a substrate, the radiation being in the extreme ultraviolet (EUV) spectrum.

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