Method for optical calibration

Abstract

A method of calibrating an illumination system for a lithographic system, the method comprising: applying a known misalignment to the illumination system; obtaining measured data representative of a measured illumination pupil generated by the illumination system; and determining an actual misalignment based on the measured data. The determining an actual misalignment may comprise: obtaining estimated data representative of an estimated illumination pupil generated by the illumination system including the known misalignment and calculating a difference between the estimated data and the measured data, wherein the calculated difference is representative of the actual misalignment.

Description

METHOD FOR OPTICAL CALIBRATIONCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of EP application 23190557.1 which was filed on August 9, 2023 and which is incorporated herein in its entirety by reference.FIELD

[0002] The present invention relates to methods and apparatus for calibrating an illumination system. The illumination system may be an illumination system of a lithographic apparatus.BACKGROUND

[0003] 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 (resist) provided on a substrate.

[0004] 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 extreme ultraviolet (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] To meet imaging requirements, the illumination system, which is used to condition the radiation beam before the radiation beam interacts with the patterning device, should be calibrated multiple times during manufacture, troubleshooting and repair / replacement of elements of the lithographic apparatus. The calibration measurement is used to determine and / or correct for pointing errors in the illumination system to increase image quality, dose accuracy and stability, and throughput.

[0006] Known methods require measurement and correction of a significant number of individual optical elements and do not allow the source of the error to be readily determined. This produces a considerably long and complex calibration procedure which leads to extended loss of availability of the lithographic apparatus for production. Alternatively, the calibration may not be regularly executed, resulting in optical transmission losses which adversely affect image quality and throughput.

[0007] It is an object of the invention to provide a calibration method for a lithographic apparatus which obviates or mitigates one or more problems associated with the prior art.SUMMARY

[0008] According to a first aspect of the invention, there is provided a method for calibrating an illumination system for a lithographic system. The method comprises: applying a known misalignmentto the illumination system; obtaining measured data representative of a measured illumination pupil generated by the illumination system; and determining an actual misalignment based on the measured data.

[0009] Prior to installation / commissioning and during operational use of a lithographic system any errors in the optical path (e.g. plasma pointing errors or optical element misalignment) may lead to transmission losses resulting in degradation in performance of the lithographic system. This can occur when elements of the lithographic system are manufactured, repaired, replaced or degrade over the operational life of the lithographic system. In order to determine and account for these potential errors a calibration step should be performed multiple times during manufacture, troubleshooting and repairs. Known methods for performing this calibration have prohibitively long execution times (e.g. in the order of hours) and are therefore not frequently performed, reducing, for example, throughput due to transmission losses.

[0010] Advantageously, by introducing a known misalignment into the illumination system, the source and magnitude of any actual misalignment which would introduce an offset in the measured illumination pupil can be more easily and quickly detected, e.g. with a single measurement. Reduced execution times (e.g. in the order of seconds) may result in more frequent calibration, increasing the performance and throughput of a lithographic system in which the illumination system may be integrated.

[0011] The radiation output from the illumination system has an illumination profile which differs from the input illumination profile. An illumination pupil may be considered to be a representation of the output illumination profile, or, in other words, the angular distribution of the radiation output from the illumination system.

[0012] Obtaining the measured data may comprise applying a known radiation profile at the input of the illumination system and measuring the output radiation profile or illumination pupil at the output of the illumination system. The measured data may represent the measured illumination pupil and may include both the known misalignment and any actual misalignment present in the illumination system. Alternatively, obtaining the measured data may comprise using a radiation source comprised within the illumination system to apply the input radiation profile and a sensor comprised within the illumination system to measure the subsequent output radiation profile. When the measured data is obtained using the radiation source comprised within the illumination system, a second calibration step may be required to correct for the difference between the radiation source within the illumination system and the EUV radiation beam of the lithographic apparatus.

[0013] Furthermore, the method may require no additional hardware to be installed in the illumination system as existing sensors and pupil control techniques may be used to apply the known misalignment to the illumination system.

[0014] The method may be computer implemented. For example, the method may be performed by a processor of the lithographic system.

[0015] The measured illumination pupil including the known misalignment and actual misalignment (if present) may be referred to as a fingerprint result and these terms are intended to be used interchangeably.

[0016] Determining an actual misalignment may comprise obtaining estimated data representative of an estimated illumination pupil generated by the illumination system including the known misalignment; and calculating a difference between the estimated data and the measured data. The calculated difference may be representative of the actual misalignment.

[0017] The estimated illumination pupil including the known misalignment may be referred to as a misalignment pupil or fingerprint pupil and these terms are intended to be used interchangeably. The measured illumination pupil (fingerprint result) may include radiation (light) only in areas (e.g. targets) where the known misalignment counteracts the actual misalignment. That is, this radiation will be visible to be viewed. In areas (e.g. targets) where the known misalignment does not counteract the actual misalignment then the radiation (light) is lost. That is, this radiation will not be visible to view or be detected.

[0018] Obtaining the estimated data may comprise obtaining simulated data representative of simulated radiation output from the illumination system, applying the known misalignment to the simulated data to generate estimated data representative of an estimated illumination pupil including the known misalignment. The simulated data and / or the estimated data may be pre-calculated. The use of pre-calculated simulated / estimated data enables the estimated date to be obtained relatively fast, such that calibration can be performed relatively quickly and with relatively low processing requirements. Alternatively, the simulated data can be simulated during the calibration process. The simulated data can be generated based on ray tracing through the illumination system for each of the predetermined illumination profiles. Alternative methods of generating simulated data are possible, for example using an optical model of the illumination system, for example representing the optical physics occurring in the illumination system.

[0019] The difference between the misalignment pupil (fingerprint pupil) and the measured illumination pupil (fingerprint result) may be, for example, a lateral shift in the misalignment pupil, a rotation of the misalignment pupil, a change in the scale / size / magnitude of the misalignment pupil or a combination of any of these effects or the like. Each of these differences may indicate a particular error source or combination of error sources and the magnitude of the effect may indicate the magnitude of the particular error or combination of errors.

[0020] Alternatively, determining the actual misalignment may comprise matching the measured data and / or fingerprint result to a pre-determined bank, library, database, archive etc. of illumination pupils. Each of the pre-determined illumination pupils may represent the effect of a particular actual misalignment on the estimated illumination pupil.

[0021] It should be understood that the above-described method could be carried out in a number of different sequences whilst resulting in the substantially the same effect, e.g. the measured data may be obtained prior to the estimated data.

[0022] The method may further comprise applying a correction to the illumination system based on the actual misalignment.

[0023] The illumination system may be comprised in a lithographic system. The method may further comprise applying a correction to the lithographic system based on the actual misalignment.

[0024] Once the source and magnitude of the actual misalignment has been determined the source of the actual misalignment may be corrected. Due to the relatively fast execution time, the calibration method may be repeated to verify that the actual misalignment has been reduced or eliminated. This advantageously allows the calibration to be verified without significantly increasing the downtime of the lithographic system.

[0025] The calibration method may be performed whilst the illumination system is installed in a lithographic apparatus or system (including a radiation source). This beneficially allows misalignments throughout the lithographic apparatus or system to be determined and corrected depending on the nature of the applied known misalignment.

[0026] The illumination system may comprise a faceted field device comprising a plurality of field facets, a faceted pupil device comprising a plurality of pupil facets, and a controller configured to control the orientation of the plurality of field facets. Applying the known misalignment may comprise selectively controlling the orientation of one or more of the plurality of field facets so as to apply an offset to the illumination pupil.

[0027] The illumination system may comprise an intermediate aperture between a radiation source and the faceted field device. The radiation source is arranged such that a source image is formed located at or near to the intermediate aperture. Applying the known misalignment may further comprise selectively controlling the radiation source so as to apply a position shift to the source image relative to the intermediate aperture, and / or to increase the source image at the intermediate aperture. Selectively controlling the radiation source may comprise shifting the radiation source towards the intermediate aperture and / or shifting the radiation source laterally relative to the intermediate aperture.

[0028] The faceted field device may comprise a faceted field mirror device. The faceted pupil device may comprise a faceted pupil mirror device.

[0029] The orientation of the field facets may determine which of the pupil facets are illuminated by the radiation input to the illumination system which, in turn, determines the output radiation profile or illumination pupil at the output of the illumination system. Each field facet may be manipulated in multiple orientations, making it possible to illuminate various pupil facets.

[0030] The offset may be a gradient offset, for example a radial offset.

[0031] The magnitude of the calculated difference between the estimated data and the measured data may depend on the gradient of the known misalignment applied to the illumination system.Advantageously this allows the sensitivity of the calibration measurement to be scaled to detect small misalignments.

[0032] Additionally, by using a gradient offset, the nature and magnitude of the actual misalignment may be easier to determine. For example, a rotational shift in a misalignment pupil with multiple or infinite lines of symmetry can be easily determined.

[0033] The illumination system may be configured such that the measured illumination pupil is substantially in the shape of a rectangle, circle, pentagon, hexagon, square, annulus or trident.

[0034] Advantageously, different shapes may be particularly effective at allowing the source and magnitude of actual misalignments to be determined and corrected. This may be particularly effective for faceted field devices comprising a large number of field facets as the misalignment pupil shape can be significantly smoother allowing smaller differences between the estimated and measured illumination pupil to be determined, therefore resulting in an improved calibration result.

[0035] According to a second aspect of the invention, there is provided an illumination system for a lithographic apparatus comprising: a controller configured to apply a known misalignment to the illumination system; and a processor configured to: obtain measured data representative of a measured illumination pupil generated by the illumination system; and determine an actual misalignment based on the measured data.

[0036] The processor may be further configured to: obtain estimated data representative of an estimated illumination pupil generated by the illumination system including the known misalignment. The processor may be further configured to calculate a difference between the estimated data and the measured data representative of the actual misalignment.

[0037] The illumination system may further comprise: a faceted field device comprising a plurality of field facets; and a faceted pupil device comprising a plurality of pupil facets. The controller may be further configured to control the orientation of one or more of the plurality of field facets so as to apply the known misalignment.

[0038] The illumination system may further comprise an intermediate aperture between a radiation source and the faceted field device. The radiation source is arranged such that a source image is formed located at or near to the intermediate aperture. The controller is further configured to control the radiation source so as to apply a position shift to the source image relative to the intermediate aperture, and / or to increase the source image at the intermediate aperture.

[0039] The processor may be further configured to determine a correction for the illumination system based on the actual misalignment.

[0040] According to a third aspect of the invention there is provided a lithographic system comprising: a radiation source; the illumination system of the second aspect; and a sensor configured to acquire measured data representative of the measured illumination pupil generated by the illumination system; wherein the lithographic system is configured to carry out the method of the first aspect.

[0041] The processor may be further configured to determine a correction for the lithographic system based on the actual misalignment.

[0042] According to a fourth aspect of the invention there is provided a computer program comprising computer readable instructions configured to cause a processor to carry out the method of the first aspect.

[0043] According to a fifth aspect of the invention there is provided a computer readable medium carrying the computer program of the fourth aspect.

[0044] According to a sixth aspect of the invention there is provided a computer apparatus comprising: a memory storing processor readable instructions; and a processor arranged to read and execute instructions stored in said memory; wherein said processor readable instructions comprise instructions arranged to control the computer to carry out the method of the first aspect.BRIEF DESCRIPTION OF THE DRAWINGS

[0045] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:Figure 1 depicts a lithographic system comprising a lithographic apparatus and a radiation source;Figure 2 depicts a schematic overview of an illumination system;Figure 3 depicts a method for calibrating an illumination system for a lithographic system;Figure 4 depicts an example known misalignment applied to an illumination pupil;Figure 5 depicts a measured illumination pupil including the example known misalignment of Figure 4;Figure 6 depicts a measured illumination pupil including the example known misalignment of Figure 4 and an unknown actual misalignment;Figure 7 depicts a measured illumination pupil including a further example known misalignment and an unknown actual misalignment;Figure 8 depicts an example estimated and measured illumination pupil;Figure 9 depicts a further example estimated and measured illumination pupil;Figure 10 depicts a yet further example estimated and measured illumination pupil;Figure 11 depicts the clipping effect of a misalignment on a single target of the measured illumination pupil;Figure 12 depicts an example known misalignment applied to the radiation source.DETAILED DESCRIPTION

[0046] 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 LAcomprises 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.

[0047] 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 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 faceted field mirror device 10 and faceted pupil mirror device 11.

[0048] 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).

[0049] 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.

[0050] 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.

[0051] 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 laser, is arranged to deposit energy via a laser beam 2 into a fuel, such as tin (Sn) which is provided from, e.g., a fuel emitter 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, for example, be a metal or alloy. The fuel emitter 3 may comprise a nozzle configured to direct tin, e.g. in the form of droplets, along a trajectory towards a plasma formation region 4. The laser beam 2 is incident upon the tin at the plasma formation region 4. The deposition of laser energy into the tin creates a tin plasma 7 at the plasma formation region 4. Radiation, including EUV radiation, is emitted from the plasma 7 during deexcitation and recombination of electrons with ions of the plasma.

[0052] The EUV radiation from the plasma 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.

[0053] 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.

[0054] 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.

[0055] Although Figure 1 depicts the radiation source SO as a laser produced plasma (LPP) source, any suitable source such as a discharge produced plasma (DPP) source or a free electron laser (FEL) may be used to generate EUV radiation.

[0056] The lithographic apparatus LA comprises the illumination system IL configured to condition electromagnetic radiation B. The illumination system IL comprises the facetted field mirror device 10 and the facetted field mirror device 10 comprises a plurality of field mirrors or facets. The apparatus LA comprises a controller 100 configured to selectively control the orientation of each of the field facets in order to provide the EUV radiation beam B with a desired cross-sectional shape and a desired intensity distribution.

[0057] The lithographic apparatus LA may comprise one or more sensors configured to determine the angular distribution of the radiation beam B at one or more locations in the path of the radiation beam B through the lithographic apparatus LA. That is, determine the illumination pupil at the output of the illumination system IL. The propagation of the radiation beam B through the Lithographic Apparatus LA is shown with arrows in Figure 1. The sensor may be a camera e.g. a pinhole camera. The sensor may be at reticle level (e.g. at the patterning device MA). Alternatively / additionally the one or more sensors may be at wafer level (e.g. at the substrate W). For example, the sensor may comprise a camera at wafer level in combination with a pinhole at reticle level. The sensor may be used for system initialization and calibration. Alternatively, the sensor may be used for monitoring and recalibration “on-demand”.

[0058] Figure 2 is a schematic illustration of an illumination system IL which may be used to generate an illumination pupil 210 at the output of the illumination system IL from an input radiation profile 220 applied to the input of the illumination system IL. The illumination system IL may form part of a lithographic apparatus. For example, the illumination system IL shown in Figure 2 may be the illumination system IL of the lithographic apparatus LA shown in Figure 1. The illumination system IL comprises a faceted field mirror device 10 comprising a plurality of field facets 230 and a faceted pupil mirror device 11 comprising a plurality of pupil facets 240. Each of the field facets 230 and pupil facets 240 comprise a reflective mirror. Radiation incident on each of the field facets 230 and pupil facets 240 is therefore at least partially reflected. The orientation of each of the field facets 230 can be controlled to reflect incident radiation towards one or more of the fixed pupil facets 240 which in turn reflects the radiation to the output of the illumination system IL. The output radiation may be measured to determine the angular distribution of the output radiation, in other words, the illumination pupil 210.

[0059] The orientation of each of the field facets 230 may be controlled by a controller, for example the controller 100 of the lithographic apparatus LA shown in Figure 1. The controller 100 may be comprised in the illumination system IL.

[0060] In use, the input radiation profile 220 may comprise a radiation beam B provided to the input of the illumination system IL. The radiation beam B may be, for example, provided by a radiation source such as the radiation source SO of the lithographic apparatus LA in Figure 1. The propagation of the radiation beam B through the illumination system IL is shown with arrows in Figure 2. By selectively controlling the orientation of each of the field facets 230, the angular distribution of the radiation propagated to the output of the illumination system IL can be controlled. As each of the field facets 230 and pupil facets 240 have a finite size with gaps in-between, if the radiation reflected by the field facets 230 is not centred on the desired pupil facet 240, some radiation will be lost. Therefore, any errors between the desired direction of radiation reflected by the field facets 230 and the actual direction of radiation reflected by the field facets 230 creates a misalignment or offset. That is, radiation reflected from the field facets 230 does not reach the desired location on the pupil facets 240. This results in a degradation of the illumination pupil 210, leading to reduced image quality, dose accuracy and stability, and throughput of the lithographic apparatus LA.

[0061] In use, any misalignment or offset can be reduced or eliminated by removing any error between the desired and actual direction of radiation reflected by the field facets 230. The adjustment, whether e.g. an adjustment of the orientation of the field facets 230 or a component external to the illumination system IL, may be in the form of a calibration or real-time adjustment. A calibration may be performed prior to the use of the illumination system IL for an intended purpose, for example a series of lithographic exposures. A calibration may be performed periodically, for example once a day or week. A calibration may be performed following the repair and / or replacement of a component of the illumination system IL or a component of the lithographic apparatus LA external to the illumination system IL. A real-time adjustment may be performed more regularly, for example once an hour orminute. As described in more detail below, the processes described herein have significantly smaller execution times than other methods of determining adjustments and as such are more likely to be performed more often and may be well suited to real-time adjustment.

[0062] Figure 3 illustrates a method 300 for calibrating an illumination system IL, for example the illumination system IL of Figure 2.

[0063] In a first step 310, a known misalignment is applied to an illumination system, such as the illumination system IL of Figure 2. The known misalignment comprises controlling the orientation of each of the field facets 230 to introduce a known error or offset between an optimised and actual direction of radiation reflected by the field facets 230. The optimised direction of radiation is that which would be observed in an optimised illumination system IL (i.e. a well aligned system). In an example, the field facets 230 close to the center of the faceted field mirror device 10 may have a small or no error or offset introduced. The error or offset may then be increased towards the edge of the faceted field mirror device 10, away from the center such that at the edge of faceted field mirror device 10 the radiation reflected from the field facets 230 does not reach the corresponding pupil facets 240 and does not form part of the illumination profile at the output of the illumination system IL. In this way a gradient offset is created from the center of the faceted field mirror device 10 to the edge of the faceted field mirror device 10.

[0064] In a second step 320, measured data representative of a measured illumination pupil is obtained. In use, the illumination system IL may also have an actual unknown misalignment introducing an additional error or offset. As such the measured illumination pupil will include the error or offset introduced by both the known and actual misalignment. The measured data may be obtained at the output of the illumination system IL. Alternatively, when the illumination system IL is installed in a lithographic apparatus, such as the lithographic apparatus LA of Figure 1, the measured data may be obtained at the patterning device MA or substrate table WT or any other suitable position. The measured data may be obtained using existing sensors thereby not requiring any additional specialist hardware to be installed in order to carry out the calibration.

[0065] The measured data can then be used to determine the actual misalignment present in the illumination system IL or the lithographic apparatus LA. In the example shown in Figure 2, in a third step 330, estimated illumination pupil data is obtained representative of an estimated illumination pupil. That is, representative of the illumination profile at the output of the illumination system IL if no actual misalignment were present. This may be obtained using modelling, simulation or the like. A database of estimated illumination pupil data for particular known misalignment profiles can be created to further decrease the calibration execution time.

[0066] In a fourth step 340, the estimated and measured data is compared to calculate a difference representative of the actual misalignment present in the illumination system IL. The actual misalignment may, for example, result in a lateral shift of the estimated illumination pupil, a rotation in the illumination pupil or a change in size or shape of the illumination pupil. The nature and magnitudeof the calculated difference indicates the source and magnitude of the actual misalignment. The known misalignment can be designed in order to detect particular actual misalignments, for example the shape of the estimated pupil or the scale of the gradient offset can be changed such that the calibration becomes more sensitive to smaller actual misalignments.

[0067] In a fifth step 350, a correction is applied to the illumination system IL or other component of the lithographic apparatus LA based on the calculated difference. Following application of the correction, the method 300 illustrated in Figure 2 may be repeated to determine if the calibration has been successful. As each calibration requires only one measurement to obtain the measured data, the execution time is significantly less than other methods of calibration and results in less downtime of the illumination system IL or other components of the lithographic apparatus LA.

[0068] While the steps of the method for calibrating the illumination system IL are denoted first, second, third etc., this does not necessarily imply a particular order required. While some steps should be performed sequentially (for example, the fourth step 340 of calculating the difference must be performed after the estimated and measured data is obtained), other steps may be performed in any order. In particular, obtaining measured data 320 and obtaining the estimated data 330 may be performed in any order including simultaneously.

[0069] Figure 4 is an illustration of the radiation incident on a faceted pupil mirror device 400 with an example known misalignment applied to a faceted field mirror device from which the incident radiation is reflected. The faceted pupil mirror device 400 comprises a plurality of facets 410 representing the pupil facets of a faceted pupil mirror device, such as the pupil facets 240 of the faceted pupil mirror device 11 of the illumination system IL of Figure 2. The incident radiation is shown as radiation spots 430. In an optimised illumination system IL (i.e. a well aligned system), the spots 430 would be positioned in the center 415 of the facets 410. This represents no difference between the desired direction of radiation reflected by the faceted field mirror device 10 and the actual direction of radiation reflected by the faceted field mirror device 10. In the faceted pupil mirror device 400 a radial gradient offset has been applied to the faceted field mirror device 10 such that, at the center of the faceted pupil mirror device 400, the spots 430 are substantially centred on the facets 410 and, at the edge of the faceted pupil mirror device 400, the spots 430 are offset from the center 415 of the facets 410. The offset is shown as lines 440 between the center 415 of each facet 410 and the actual position of the spot 430. This results in a radial gradient offset across the faceted pupil mirror device 400 of the spots 430 from the center 415 of the facets 410.

[0070] Figure 5 is an illustration of a measured illumination pupil 500 from an illumination system having had the known misalignment illustrated in Figure 4 applied. The measured illumination pupil 500 comprises a plurality of targets 510 where the offset 440 illustrated in Figure 4 is large enough that the radiation is lost and cannot therefore be measured (represented by spots 533), and a plurality of targets 520 where the offset 440 illustrated in Figure 4 is small enough that reflected radiation can be measured (represented by spots 534). The plurality of targets 520 comprise a plurality of targets 521where the spot is substantially centralized on the target 521 and substantially all of the radiation incident on the corresponding facet 410 is reflected and can therefore be measured. The targets 510, 520, 521 correspond to facets 410 of the faceted pupil mirror device 400. As the radial gradient offset of the known misalignment increases towards the outer edge of the illumination pupil 500 the spot is increasingly shifted from the center of the target. As a result, the plurality of targets 520 further comprise a plurality of targets 522 where only a proportion of the radiation incident on the corresponding facet 410 is reflected and subsequently measured. The proportion of the radiation incident on the corresponding facet 410 which is not reflected and not subsequently measured is lost. This effect is also referred to as clipping, as detailed below with reference to Figure 11. This results in a different magnitude of radiation being measured at each target 520. A representation of the measurement of the reflected radiation for this example is illustrated in Figure 8.

[0071] The shape formed by the plurality of targets 520 where the reflected radiation can be measured can be adjusted to the source and magnitude of the misalignment that needs to be calibrated out.

[0072] Figure 6 is an illustration of a measured illumination pupil 600 including the known misalignment illustrated in Figure 4 applied in addition to an unknown actual misalignment. The actual misalignment may be an error or offset in the illumination system IL or another component of the lithographic apparatus LA. An arrow shows the lateral movement of the measured illumination pupil 600 which may, for example, indicate a lateral pointing error in the electromagnetic radiation beam B applied to the input of the illumination system IL. If there was no unknown actual misalignment (for instance a plasma shift) present then the middle of the illumination pupil would ‘light up’ (similar to the illumination pupil 500 shown in Figure 5). This is because the resulting measurement in a well aligned system only shows light in the middle of the illumination pupil 500 as these spots are still near to their design spot positions. However, since there is an unknown actual misalignment in Figure 6, then a place nearer to the edge (i.e. towards the right as shown) of the illumination pupil 600 ‘lights up’ . Therefore, this illumination pupil 600 is a specific fingerprint pupil to detect plasma misalignment. By fitting the position of the targets 620 that are illuminated, the exact plasma misalignment can be found in a single measurement step. The known misalignment (e.g. known gradient / (radial) offset) applied to the spots allows the unknown misalignment to be much easier to “see” (i.e. be detected). This is in contrast to other methods which may try to direct spots to centers of the facetted pupil mirror device. It may be considered that the plurality of targets 620 where the reflected radiation can be measured represent targets where the known misalignment effectively counteracts the actual misalignment. In contrast, areas (e.g. targets 610) where this is not the case then the light (radiation) is lost. This loss mechanism may be referred to as clipping.

[0073] Figure 7 is an illustration of a measurement illumination pupil 700 including the same unknown actual misalignment of Figure 6 in addition to an adjusted known misalignment. The known misalignment of Figure 7 has been adjusted with respect to the known misalignment applied in theexample of Figure 4 (and Figure 6). The known misalignment has been adjusted such that the radial offset gradient has been increased, i.e. the offset of the spots 430 from the center of the targets 410 increases at a greater rate. An arrow shows the lateral movement of the measured illumination pupil 700. Similar to Figure 6, it may be considered that a plurality of targets 720 where the reflected radiation can be measured represent targets where the known misalignment effectively counteracts the actual misalignment and the areas (e.g. targets 710) where this is not the case then the light (radiation) is lost. In the example of Figure 7 there is a smaller lateral movement than that of the example of Figure 6 for the same actual misalignment. The increase of the gradient is one example of an adjustment that can be made to the known misalignment to tailor the calibration to a particular type and magnitude of actual misalignment. It will be appreciated that, in some examples, decreasing the gradient may result in the actual misalignment being easier to “see” (i.e. detect). Further example adjustments are illustrated in Figures 8 to 10.

[0074] Figures 8 to 10 illustrate example shapes that the plurality of targets 520 of Figure 5 (where the reflected radiation is not lost due to offset) may be configured to form. Illumination pupils 800, 900, 1000 represent a measurement of an illumination pupil with a known misalignment applied to create an illumination pupil with a known shape and / or gradient. Illumination pupils 810, 910, 1010 represent a measurement of an illumination pupil with an actual misalignment in addition to the applied known misalignment. The change (or difference) in the illumination pupil 810, 910, 1010 due to the actual misalignment is indicated by arrows. Specific shapes (such as a rectangle - not shown) - may be inserted in gradient pupils which are easily fitted.

[0075] Figure 8 is an illustration of a circular misalignment pupil 800 with a radial gradient offset applied, such as the misalignment pupil 500 of Figure 5. Illumination pupil 810 shows a lateral movement (e.g. plasma x shift) due to the actual unknown misalignment. In other examples, if the circle shape was found to be e.g. offset perpendicularly to the arrow shown in Figure 8, then this may indicate plasma xy shift.

[0076] Figure 9 is an illustration of a pentagonal misalignment pupil 900 with a linear gradient offset applied. Illumination pupil 910 shows a rotational movement due to the actual unknown misalignment. In this example the linear gradient offset allows the rotational movement to be more easily determined.

[0077] Figure 10 is an illustration of an annulus misalignment pupil 1000 with no gradient offset applied. Illumination pupil 1010 shows a change in size due to the actual unknown misalignment. This may illustrate a plasma z shift.

[0078] Figure 11 is an illustration of the clipping effect of a misalignment on a single target of the measured illumination pupil. The target 1104 may be one of the plurality of targets 521 of Figure 5 where the spot 1108 is substantially centralized on the target 1104. In this example, substantially all of the radiation incident on the corresponding pupil facet is reflected and can therefore be measured. The targets 1114, 1124 may be one of the plurality of targets 522 where the spot 1118, 1128 is shifted(indicated by an arrow in Figure 11). In this example only a proportion of the radiation incident on the corresponding pupil facet is reflected and subsequently measured. The proportion of the incident radiation which is not reflected and not subsequently measured is lost or ‘clipped’. In systems which support clipping, the magnitude of radiation measured from targets 1114, 1124 is less than the magnitude of radiation measured from target 1104.

[0079] It would be understood that one or more of the applied shapes and / or gradients may be applied simultaneously. It would be understood that one or more of the observed movements may be observed due to errors in the lithographic apparatus of various sources and magnitudes.

[0080] Figure 12 is an illustration of applying a known misalignment to the radiation source, such as the radiation source SO of Figure 1. As shown in Fig. 12 (a), when no misalignment is applied to the radiation source, the radiation source is arranged such that a source image 1200, e.g. a plasma image, is positioned in the center of an intermediate aperture 1201, such as the opening 8 of Figure 1. The intermediate aperture 1201 is arranged between the radiation source and the faceted field device. The radiation source misalignment may be applied towards the intermediate aperture 1201, i.e. in the direction of the optical path. Alternatively or additionally, the radiation source misalignment may be applied in a lateral direction relative to the intermediate aperture 1201, i.e. orthogonal to the optical path. In this example, the radiation source is controlled by applying the radiation source misalignment in the lateral direction, e.g. in the x-direction (Fig. 12 (b)) or in the y-direction (Fig. 12 (c)), relative to the intermediate aperture 1201 (indicated by an arrow in Figure 12 (b) and Figure 12(c)), such that the position of the source image 1200 is shifted relative to the intermediate aperture 1201. When the position of the source image 1200 is sufficiently shifted relative to the intermediate aperture 1201, the source image 1200 intersects with the edge 1202 of the intermediate aperture 1201, allowing to reconstruct the intermediate aperture edge position when observing the source image 1200. To reconstruct the intermediate aperture edge position, it is required to combine the known radiation source misalignment with a known misalignment to the illumination pupil, such as the example shown in Figure 4, allowing to observe the source image 1200. Alternatively, the radiation source may be controlled by applying the radiation source misalignment in the direction of the intermediate aperture 1201, as shown in Fig. 12 (d). Consequently, the size of the source image 1200 is increased at the intermediate aperture 1201. By sufficiently increasing the source image, the intermediate aperture may be filled, or overfilled, by the source image, allowing to reconstruct the complete intermediate aperture edge when observing the source image 1200. Alternatively, the source image 1200 may be increased by e.g. adapting the recipe to generate the plasma.

[0081] 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.

[0082] 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 substrate) or mask (or other patterning device). These apparatuses may be generally referred to as lithographic tools. Such a lithographic tool may use vacuum conditions or ambient (non- vacuum) conditions.

[0083] Where the context allows, embodiments of the invention may be implemented in hardware, firmware, software, or any combination thereof. Embodiments of the 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, acoustical or other forms of propagated signals (e.g. carrier waves, infrared signals, digital signals, etc.), and others. Further, firmware, software, routines, instructions may be described herein as performing certain actions. However, it should be appreciated that such descriptions are merely for convenience and that such actions in fact result from computing devices, processors, controllers, or other devices executing the firmware, software, routines, instructions, etc. and in doing that may cause actuators or other devices to interact with the physical world.

[0084] 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. A method of calibrating an illumination system for a lithographic system, the method comprising: applying a known misalignment to the illumination system; obtaining measured data representative of a measured illumination pupil generated by the illumination system; and determining an actual misalignment based on the measured data.

2. The method of claim 1, wherein determining an actual misalignment comprises: obtaining estimated data representative of an estimated illumination pupil generated by the illumination system including the known misalignment; calculating a difference between the estimated data and the measured data, wherein the calculated difference is representative of the actual misalignment.

3. The method of claim 1 or claim 2, further comprising applying a correction to the illumination system based on the actual misalignment.

4. The method of claim 1 or claim 2, wherein the illumination system is comprised in a lithographic system and the method further comprises applying a correction to the lithographic system based on the actual misalignment.

5. The method of any preceding claim, wherein the illumination system comprises a faceted field device comprising a plurality of field facets, a faceted pupil device comprising a plurality of pupil facets, and a controller configured to control the orientation of the plurality of field facets, and wherein applying the known misalignment comprises selectively controlling the orientation of one or more of the plurality of field facets so as to apply an offset to the illumination pupil.

6. The method of claim 5, wherein the illumination system comprises an intermediate aperture between a radiation source and the faceted field device, the radiation source is arranged such that a source image is formed located at or near to the intermediate aperture, wherein applying the known misalignment further comprises selectively controlling the radiation source so as to apply a position shift to the source image relative to the intermediate aperture, and / or to increase the source image at the intermediate aperture.

7. The method of claim 6, wherein selectively controlling the radiation source comprises shifting the radiation source towards the intermediate aperture and / or shifting the radiation source laterallyrelative to the intermediate aperture.

8. The method of any of the preceding claims 5-7, wherein the faceted field device comprises a faceted field mirror device and / or the faceted pupil device comprises a faceted pupil mirror device.

9. The method of any of the preceding claims 5-8, wherein the offset is a gradient offset, for example a radial offset.

10. The method of any preceding claim, wherein the illumination system is configured such that the measured illumination pupil is substantially in the shape of a rectangle, circle, pentagon, hexagon, square, annulus or trident.

11. An illumination system for a lithographic apparatus comprising: a controller configured to apply a known misalignment to the illumination system; and a processor configured to: obtain measured data representative of a measured illumination pupil generated by the illumination system; and determine an actual misalignment based on the measured data.

12. The illumination system of claim 11, wherein the processor is further configured to: obtain estimated data representative of an estimated illumination pupil generated by the illumination system including the known misalignment; and calculate a difference between the estimated data and the measured data, wherein the calculated difference is representative of the actual misalignment.

13. The illumination system of claim 11 or claim 12, further comprising: a faceted field device comprising a plurality of field facets; and a faceted pupil device comprising a plurality of pupil facets; wherein the controller is further configured to control the orientation of one or more of the plurality of field facets so as to apply the known misalignment.

14. The illumination system of claim 13, further comprising an intermediate aperture between a radiation source and the faceted field device, the radiation source is arranged such that a source image is formed located at or near to the intermediate aperture, wherein the controller is further configured to control the radiation source so as to apply a position shift to the source image relative to the intermediate aperture, and / or to increase the source image atthe intermediate aperture.

15. The illumination system of any one of claims 11 to 14, wherein the processor is further configured to determine a correction for the illumination system based on the actual misalignment.

16. A lithographic system comprising: the illumination system of any one of claims 11 to 15; and a sensor configured to acquire measured data representative of the measured illumination pupil generated by the illumination system; wherein the lithographic system is configured to carry out the method of any one of claims 1 to 10.

17. The lithographic system of claim 16, wherein the processor is further configured to determine a correction for the lithographic system based on the actual misalignment.

18. A computer program comprising computer readable instructions configured to cause a processor to carry out a method according to any one of claims 1 to 10.

19. A computer readable medium carrying a computer program according to claim 18.

20. A computer apparatus comprising: a memory storing processor readable instructions; and a processor arranged to read and execute instructions stored in said memory; wherein said processor readable instructions comprise instructions arranged to control the computer to carry out a method according to any one of claims 1 to 10.

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