Method for use with a sensor system of a lithographic apparatus and a lithographic apparatus comprising a sensor system

Abstract

The present disclosure relates to a method for use with a sensor system. The sensor system comprises a reflective mark comprising a reflective diffraction grating and a plurality of sensors comprising a first sensor and a second sensor. The first sensor comprises a first transmissive diffraction grating and a first radiation detector, and the second sensor comprises a second transmissive diffraction grating and a second radiation detector. The method comprises measuring radiation reflected from the reflective mark with the first sensor; measuring radiation reflected from the reflective mark with the second sensor; and determining an error component associated with a change in the relative position of the first transmissive diffraction grating and the second transmissive diffraction grating based on the measurement of radiation reflected from the reflective mark with the first sensor and the measurement of radiation reflected from the reflective mark with the second sensor.

Description

METHOD FOR USE WITH A SENSOR SYSTEM OF A LITHOGRAPHIC APPARATUS AND A LITHOGRAPHIC APPARATUS COMPRISING A SENSOR SYSTEMCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of EP application 24191537.0 which was filed on 29 July 2024 and which is incorporated herein in its entirety by reference.FIELD

[0002] The present invention relates to a method for use with a sensor system of a lithographic apparatus, a lithographic apparatus comprising a sensor system, and a computer program for a lithographic apparatus comprising a sensor system.BACKGROUND

[0003] A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., comprising part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned.

[0004] Lithography is widely recognized as one of the key steps in the manufacture of ICs and other devices and / or structures. However, as the dimensions of features made using lithography become smaller, lithography is becoming a more critical factor for enabling miniature IC or other devices and / or structures to be manufactured.

[0005] A theoretical estimate of the limits of pattern printing can be given by the Rayleigh criterion for resolution as shown in Equation / :Equation 1 where X is the wavelength of the radiation used, NA is the numerical aperture of the projection system used to print the pattern, kl is a process-dependent adjustment factor, also called the Rayleigh constant, and CD is the feature size (or critical dimension) of the printed feature. It follows from Equation 1 that reduction of the minimum printable size of features can be obtained in three ways: by shortening the exposure wavelength X, by increasing the numerical aperture NA or by decreasing the value of kl.

[0006] In order to shorten the exposure wavelength and, thus, reduce the minimum printable size, it has been proposed to use an extreme ultraviolet (EUV) radiation source. EUV radiation is electromagnetic radiation having a wavelength within the range of 10-20 nm, for example within the range of 13-14 nm. It has further been proposed that EUV radiation with a wavelength of less than 10 nm could be used, for example within the range of 5-10 nm such as 6.7 nm or 6.8 nm. Such radiation is termed extreme ultraviolet radiation or soft x-ray radiation. Possible sources include, for example, laser-produced plasma sources, discharge plasma sources, or sources based on synchrotron radiation provided by an electron storage ring. In other types of lithographic apparatus (e.g. a deep ultraviolet lithographic apparatus), the exposure wavelength may be larger (e.g. in the deep ultraviolet range). For example, the exposure wavelength may be between 100 nm and 400 nm, e.g. 365, 248, 193, 157 or 126 nm.

[0007] In a lithographic apparatus, the patterning device may be supported on a patterning device support, and the substrate may be supported on a substrate support. The lithographic apparatus may further comprise a projection system configured to o project a beam of radiation from the patterning device to the substrate.

[0008] The lithographic apparatus may comprise one or more interferometric sensor systems configured to be used in the alignment of various components / systems (e.g. in the alignment of the patterning device, the patterning device support, the substrate, and the substrate support). Additionally or alternatively, the one or more interferometric sensor systems may be used to characterize aberrations of the projection system.

[0009] An interferometric sensor system may comprise a reflective mark associated with the patterning device, and a plurality of sensors associated with the substrate. The reflective mark may comprise a reflective diffraction grating. Each of the plurality of sensors may comprise a transmissive diffraction grating and a radiation detector. The transmissive diffraction gratings of the plurality of sensors may be formed in a sensor plate. During use, the sensor system may perform measurements using different ones of the plurality of sensors simultaneously, i.e. in parallel.

[0010] Over time, the distance between each of the transmission diffraction gratings formed in the sensor plate may change. This may be because of thermal deformation of the sensor plate and / or due to changes in materials used in conjunction with the sensor plate, such as adhesives. Changes in the materials used in conjunction with the sensor plate may occur due to the conditions (e.g. vacuum pressure) inside the lithographic apparatus.

[0011] Changes in the distance between each of the transmissive diffraction gratings formed in the sensor plate may lead to the introduction of an error component to the measurements made by the interferometric sensor system. This error component may mean that some interferometric sensor systems are not suitable for measuring low order aberrations. Additionally or alternatively, the error component may mean make it necessary to provide the sensor plate with additional features so that deformation of the sensor plate can be mitigated through calibration processes.SUMMARY OF THE INVENTION

[0012] An aim of the present invention is to provide an improved method for use in calibrating an interferometric sensor system of a lithographic apparatus. One particular aim of the present invention is to provide a method which allows errors arising from deformation of the sensor plate to be fully quantified without the introduction of new structural features to the interferometric sensor system.

[0013] According to an aspect of the present disclosure, there is provided a method for use with a sensor system of a lithographic apparatus. The sensor comprises a reflective mark comprising a reflective diffraction grating and a plurality of sensors comprising a first sensor and a second sensor. The first sensor comprises a first transmissive diffraction grating and a first radiation detector, and the second sensor comprises a second transmissive diffraction grating and a second radiation detector. The method comprises measuring radiation reflected from the reflective mark with the first sensor; measuring radiation reflected from the reflective mark with the second sensor; and determining an error component associated with a change in the relative position of the first transmissive diffraction grating and the second transmissive diffraction grating based on the measurement of radiation reflected from the reflective mark with the first sensor and the measurement of radiation reflected from the reflective mark with the second sensor.

[0014] According to another aspect of the present disclosure, there is provided a lithographic apparatus comprising a sensor system. The sensor system comprising a reflective mark comprising a reflective diffraction grating, and a plurality of sensors comprising a first sensor and a second sensor. The first sensor comprising a first transmissive diffraction grating and a first radiation detector, and the second sensor comprising a second transmissive diffraction grating and a second radiation detector. The lithographic apparatus further comprises a controller configured to control the lithographic apparatus to perform the method of the present disclosure.

[0015] According to another aspect of the present disclosure, there is provided another method for use with a sensor system of a lithographic apparatus. The sensor system comprises a plurality of reflective marks comprising a first reflective mark and a second reflective mark, the first reflective mark comprising a first reflective diffraction grating and the second reflective mark comprising a second reflective diffraction grating. The sensor system further comprises a sensor comprising a transmissive diffraction grating and a radiation detector. The method comprises: measuring radiation reflected from the first reflective mark with the sensor; measuring radiation reflected from the second reflective mark with the sensor; and determining an error component associated with a change in the relative position of the first reflective diffraction grating and the second reflective diffraction grating based on the measurement of radiation reflected from the first reflective mark with the sensor and the measurement of radiation reflected from the second reflective mark with the sensor.BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which corresponding reference symbols indicate corresponding parts.

[0017] Figure 1 schematically depicts a lithographic apparatus.Figure 2 schematically depicts a more detailed view of the lithographic apparatus.Figure 3 schematically depicts an interferometric sensor system.Figure 4 schematically depicts the relative positions of reflective marks and transmissive marks during a method in accordance with an embodiment.Figure 5 schematically depicts the relative positions of reflective marks and transmissive marks during a method in accordance with an embodiment.Figure 6 schematically depicts the relative positions of transmissive marks and images of reflective marks during a method in accordance with an embodiment.Figure 7 schematically depicts the relative positions of transmissive marks and images of reflective marks during a method in accordance with an embodiment.Figure 8A schematically depicts the relative positions of transmissive marks and images of reflective marks during a method in accordance with an embodiment.Figure 8B schematically depicts the relative positions of transmissive marks and images of reflective marks during a method in accordance with an embodiment.The features shown in the Figures are not necessarily to scale, and the size and / or arrangement depicted is not limiting. It will be understood that the Figures include optional features which may not be essential to the invention. Furthermore, not all of the features of the apparatus are depicted in each of the figures, and the Figures may only show some of the components relevant for describing a particular feature. Like parts in different figures are indicated by like references.DETAILED DESCRIPTION

[0018] Figure 1 schematically depicts a lithographic apparatus 100 including a source collector module SO according to one embodiment of the invention. The apparatus 100 comprises: an illumination system (or illuminator) IL configured to condition a radiation beam B (e.g., EUV radiation).- a support structure (e.g., a mask table) MT constructed to support a patterning device (e.g., a mask or a reticle) MA and connected to a first positioner PM configured to accurately position the patterning device;- a substrate table (e.g., a wafer table) WT constructed to hold a substrate (e.g., a resist-coated wafer) W and connected to a second positioner PW configured to accurately position the substrate; and- a projection system (e.g., a reflective projection system) PS configured to project a pattern imparted to the radiation beam B by patterning device MA onto a target portion C (e.g., comprising one or more dies) of the substrate W.

[0019] The illumination system IL may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic or other types of optical components, or any combination thereof, for directing, shaping, or controlling radiation.

[0020] The support structure MT holds the patterning device MA in a manner that depends on the orientation of the patterning device, the design of the lithographic apparatus, and other conditions, such as for example whether or not the patterning device is held in a vacuum environment. The support structure MT can use mechanical, vacuum, electrostatic or other clamping techniques to hold the patterning device MA. The support structure MT may be a frame or a table, for example, which may be fixed or movable as required. The support structure MT may ensure that the patterning device MA is at a desired position, for example with respect to the projection system PS.

[0021] The term “patterning device” should be broadly interpreted as referring to any device that can be used to impart a radiation beam B with a pattern in its cross-section such as to create a pattern in a target portion C of the substrate W. The pattern imparted to the radiation beam B may correspond to a particular functional layer in a device being created in the target portion C, such as an integrated circuit.

[0022] Examples of patterning devices include masks, programmable mirror arrays, and programmable liquid-crystal display (LCD) panels. Masks are well known in lithography, and include mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. An example of a programmable mirror array employs a matrix arrangement of small mirrors, each of which can be individually tilted so as to reflect an incoming radiation beam in different directions. The tilted mirrors impart a pattern in a radiation beam, which is reflected by the mirror matrix.

[0023] The projection system PS, like the illumination system IL, may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic or other types of optical components, or any combination thereof, as appropriate for the exposure radiation being used, or for other factors such as the use of a vacuum. It may be desired to use a vacuum for EUV radiation since other gases may absorb too much radiation. A vacuum environment may therefore be provided to the whole beam path with the aid of a vacuum wall and vacuum pumps.

[0024] As here depicted, the lithographic apparatus 100 is of a reflective type (e.g., employing a reflective mask).

[0025] The lithographic apparatus 100 may be of a type having two (dual stage) or more substrate tables WT (and / or two or more support structures MT). In such a “multiple stage” lithographic apparatus the additional substrate tables WT (and / or the additional support structures MT) may be used in parallel, or preparatory steps may be carried out on one or more substrate tables WT (and / orone or more support structures MT) while one or more other substrate tables WT (and / or one or more other support structures MT) are being used for exposure.

[0026] Referring to Figure 1, the illumination system IL receives an extreme ultraviolet radiation beam from the source collector module SO. Methods to produce EUV light include, but are not necessarily limited to, converting a material into a plasma state that has at least one element, e.g., xenon, lithium or tin, with one or more emission lines in the EUV range. In one such method, often termed laser produced plasma (“LPP”) the required plasma can be produced by irradiating a fuel, such as a droplet, stream or cluster of material having the required line-emitting element, with a laser beam. The source collector module SO may be part of an EUV radiation system including a laser, not shown in Figure 1, for providing the laser beam exciting the fuel. The resulting plasma emits output radiation, e.g., EUV radiation, which is collected using a radiation collector, disposed in the source collector module. The laser and the source collector module SO may be separate entities, for example when a CO2 laser is used to provide the laser beam for fuel excitation.

[0027] In such cases, the laser is not considered to form part of the lithographic apparatus 100 and the radiation beam B is passed from the laser to the source collector module SO with the aid of a beam delivery system comprising, for example, suitable directing mirrors and / or a beam expander. In other cases the source may be an integral part of the source collector module SO, for example when the source is a discharge produced plasma EUV generator, often termed as a DPP source.

[0028] The illumination system IL may comprise an adjuster for adjusting the angular intensity distribution of the radiation beam. Generally, at least the outer and / or inner radial extent (commonly referred to as o-outer and o-inner, respectively) of the intensity distribution in a pupil plane of the illumination system IL can be adjusted. In addition, the illumination system IL may comprise various other components, such as facetted field and pupil mirror devices. The illumination system IL may be used to condition the radiation beam B, to have a desired uniformity and intensity distribution in its cross-section.

[0029] The radiation beam B is incident on the patterning device (e.g., mask) MA, which is held on the support structure (e.g., mask table) MT, and is patterned by the patterning device MA. After being reflected from the patterning device (e.g., mask) MA, the radiation beam B passes through the projection system PS, which focuses the radiation beam B onto a target portion C of the substrate W. With the aid of the second positioner PW and position sensor PS2 (e.g., an interferometric device, linear encoder or capacitive sensor), the substrate table WT can be moved accurately, e.g., so as to position different target portions C in the path of the radiation beam B. Similarly, the first positioner PM and another position sensor PSI can be used to accurately position the patterning device (e.g., mask) MA with respect to the path of the radiation beam B. The patterning device (e.g., mask) MA and the substrate W may be aligned using mask alignment marks Ml, M2 and substrate alignment marks PI, P2.

[0030] A controller 500 controls the overall operations of the lithographic apparatus 100 and in particular performs an operation process described further below. Controller 500 can be embodied as a suitably-programmed general purpose computer comprising a central processing unit, volatile and non-volatile storage means, one or more input and output devices such as a keyboard and screen, one or more network connections and one or more interfaces to the various parts of the lithographic apparatus 100. It will be appreciated that a one-to-one relationship between controlling computer and lithographic apparatus 100 is not necessary. In an embodiment of the invention one computer can control multiple lithographic apparatuses 100. In an embodiment of the invention, multiple networked computers can be used to control one lithographic apparatus 100. The controller 500 may also be configured to control one or more associated process devices and substrate handling devices in a lithocell or cluster of which the lithographic apparatus 100 forms a part. The controller 500 can also be configured to be subordinate to a supervisory control system of a lithocell or cluster and / or an overall control system of a fab.

[0031] Figure 2 shows the lithographic apparatus 100 in more detail, including the source collector module SO, the illumination system IL, and the projection system PS. An EUV radiation emitting plasma 210 may be formed by a plasma source. EUV radiation may be produced by a gas or vapor, for example Xe gas, Li vapor or Sn vapor in which the radiation emitting plasma 210 is created to emit radiation in the EUV range of the electromagnetic spectrum. In an embodiment, a plasma of excited tin (Sn) is provided to produce EUV radiation.

[0032] The radiation emitted by the radiation emitting plasma 210 is passed from a source chamber 211 into a collector chamber 212.

[0033] The collector chamber 212 may include a radiation collector CO. Radiation that traverses the radiation collector CO can be focused in a virtual source point IF. The virtual source point IF is commonly referred to as the intermediate focus, and the source collector module SO is arranged such that the virtual source point IF is located at or near an opening 221 in the enclosing structure 220. The virtual source point IF is an image of the radiation emitting plasma 210.

[0034] Subsequently the radiation traverses the illumination system IL, which may include a facetted field mirror device 22 and a facetted pupil mirror device 24 arranged to provide a desired angular distribution of the unpattemed beam 21, at the patterning device MA, as well as a desired uniformity of radiation intensity at the patterning device MA. Upon reflection of the unpattemed beam 21 at the patterning device MA, held by the support structure MT, a patterned beam 26 is formed and the patterned beam 26 is imaged by the projection system PS via reflective elements 28, 30 onto a substrate W held by the substrate table WT.

[0035] More elements than shown may generally be present in the illumination system IL and the projection system PS. Further, there may be more mirrors present than those shown in the Figures, for example there may be 1- 6 additional reflective elements present in the projection system PS than shown in Figure 2.

[0036] Alternatively, the source collector module SO may be part of an LPP radiation system.

[0037] As depicted in Figure 1, in an embodiment the lithographic apparatus 100 comprises an illumination system IL and a projection system PS. The illumination system IL is configured to emit a radiation beam B. The projection system PS is separated from the substrate table WT by an intervening space. The projection system PS is configured to project a pattern imparted to the radiation beam B onto the substrate W. The pattern is for EUV radiation of the radiation beam B.

[0038] The space intervening between the projection system PS and the substrate table WT can be at least partially evacuated. The intervening space may be delimited at the location of the projection system PS by a solid surface from which the employed radiation is directed toward the substrate table WT.

[0039] The lithographic apparatus depicted in Figures 1 and 2 is an EUV lithographic apparatus, i.e. a lithographic apparatus configured to project patterned EUV radiation onto the substrate W. EUV radiation is electromagnetic radiation having a wavelength within the range of 10-20 nm, for example within the range of 13-14 nm. EUV radiation with a wavelength of less than 10 nm could be used, for example within the range of 5-10 nm such as 6.7 nm or 6.8 nm. Such radiation is termed extreme ultraviolet radiation or soft x-ray radiation.

[0040] It will be appreciated that other types of lithographic apparatus may use radiation outside of this EUV range. For example, a deep ultraviolet (DUV) lithographic apparatus may be configured to project DUV radiation onto the substrate W. DUV radiation may have a wavelength of between 100 nm and 400 nm, e.g. 365, 248, 193, 157 or 126 nm.

[0041] The techniques of the present disclosure may be implemented in any type of lithographic apparatus, e.g. an EUV lithographic apparatus or a DUV lithographic apparatus. The following description focuses on the case that the lithographic apparatus is an EUV lithographic apparatus. However, the techniques of the present disclosure are not limited thereto.

[0042] Figure 3 is a schematic diagram illustrating the principle of operation of a sensor system 300. The sensor system 300 may be a shearing interferometric sensor system. An image of a reflective mark 301, 302 is projected onto a sensor of a sensor device 310 at substrate level by the projection system (represented by reflective elements 28, 30). The reflective mark 301, 302 may comprise a diffraction grating, e.g. a reflective diffraction grating. The sensor of the sensor device 310 comprises a transmissive mark 311 and a radiation sensor 312. The transmissive mark 311 may comprise a diffraction grating, e.g. a transmissive diffraction grating. The radiation sensor 312 is configured to detect radiation that has passed through the transmissive mark 311. The image of the reflective mark 301, 302 may be formed by a beam of radiation that is projected onto the reflective mark 301, 302, e.g. a beam of radiation generated by the source SO of the lithographic apparatus, e.g. a beam of EUV radiation.

[0043] In the sensor system 300 depicted in Figure 3, there are a plurality of (i.e. two or more) sensors, each sensor comprising a transmissive mark 311 and a corresponding radiation detector 312. The transmissive marks 311 may be formed in, or coupled to, a sensor plate 312.

[0044] In some embodiments, one radiation detector 312 may be configured to detect radiation that passes through different ones of the transmissive marks 311. That is, one radiation detector 312 may correspond to a plurality of the transmissive marks 312. For example, a single radiation detector 312 may be provided for all of the transmissive marks 312. In this case, it may be said the sensor device 310 comprises a plurality of sensors, wherein each of the plurality of sensors comprises one specific transmissive mark and the common radiation detector 312.

[0045] In the sensor device 310 depicted in Figure 3, the plurality of sensors (and, therefore, the plurality of transmissive marks 311) are distributed across the surface of the sensor device 310 in a two-dimensional array. In some embodiments, a plurality of sensors (and, therefore, a plurality of transmissive marks 311) are arranged linearly, i.e. along a straight line. In some embodiments, a plurality of sensors (and, therefore, a plurality of transmissive marks 311) are arranged along a curved line. The straight line or the curved line may correspond to an exposure slit (i.e. an area which is illuminated by the projection system). Alternatively, the sensors may be arranged in a different pattern, e.g. a zig-zag pattern.

[0046] The reflective mark 301, 302 may be disposed at the support structure MT. Reflective marks 301, 302 are associated with the patterning device, in that they are positionable at the same location in the path of the radiation beam 21 as a patterning device is during exposure of substrates, i.e. at the object plane of the projection system.

[0047] The sensor device 310 is associated with the substrate in that it is positionable at the same location in the path of the radiation beam 21 as is a substrate during the exposure of substrates, i.e. at the image plane of the projection system. The sensor device 310 may be mounted on, disposed at or incorporated in the substrate support WT. The sensor device 310 may be the substrate support WT. The sensor device 310 may be referred to as a measurement block.

[0048] The reflective mark 301 may be provided on a calibration device 303. The calibration device 303 may be physically similar to a patterning device MA used in an exposure process and can be held by the patterning device support (support structure MT). In some embodiments, the lithographic apparatus may comprise a plurality of reflective marks 301. The plurality of reflective marks 301 may be arranged linearly. Alternatively, the plurality of reflective marks 301 may be distributed across a surface of the calibration device 303, e.g. in a two-dimensional array. In some embodiments, the reflective mark 301 is disposed on a patterning device MA that is used in the production of ICs (i.e. a patterning device MA which has a pattern formed thereon, wherein the pattern is a pattern that is to be imparted to the substrate W by the lithographic apparatus).

[0049] The reflective mark 302 may be provided on a fiducial 304 mounted to the support structure MT. The fiducial 304 may be used while the patterning device MA used in the exposure process issupported by the support structure MT. The fiducial 304 may comprise a plurality of reflective marks 302 distributed around the support structure MT. The plurality of reflective marks 302 on the fiducial 304 may be arranged linearly, i.e. along a straight line.

[0050] The lithographic apparatus may comprise one or both of the reflective mark 301 (i.e. a reflective mark 301 disposed on a calibration device 303) and the reflective mark 302 (i.e. a reflective mark disposed on the support structure MT).

[0051] In some embodiments of the sensor system 300, the reflective marks 301 and the sensors (formed of a transmissive mark 311 and a detector 312) may be provided in pairs. For example, a first pair may comprise a first reflective mark and a first sensor, and a second pair may comprise a second reflective mark and a second sensor. During use of the sensor system 300, the first sensor may measure radiation reflected from the first reflective mark and the second sensor may measure radiation reflected from the second reflective mark. In other words, the sensor system 300 may be configured such that each sensor of the sensor device 310 measures radiation reflected from a corresponding reflective mark.

[0052] The diffraction grating on the reflective mark 301 may be a one -dimensional diffraction grating. The reflective mark 301 may comprise two orthogonal diffraction gratings e.g. an X-grating and a Y-grating.

[0053] The reflective mark 301 may comprise a multilayer reflector. The multilayer reflector may be configured to reflect EUV radiation therefrom. The multilayer reflector may comprise a distributed Bragg reflector. For EUV radiation, the multilayer reflector may have a reflectance of greater than 60% and preferably greater than 70%. The multilayer reflector may comprise a plurality of alternating layers. For example, the multi-layer stack may comprise alternating layers of molybdenum (Mo) and silicon (Si).

[0054] The reflective mark 301 may comprise an absorber layer provided on a surface of the multilayer reflector. The absorber layer may be formed of a material which absorbs EUV radiation. The absorber layer may be provided to only a portion of the surface of the multilayer reflector, so as to form the diffraction grating of the reflective mark.

[0055] The multilayer reflector may further comprise a capping layer. In the case that the multilayer reflector comprises a capping layer, the absorber layer may be provided on a surface of the capping layer.

[0056] The transmissive mark 311 comprises a diffraction grating. The diffraction grating of the transmissive mark 311 may be a two-dimensional diffraction grating. The diffraction grating may comprise a plurality of transmissive circular portions arranged in a two-dimensional array. In other embodiments, the transmissive portions may be a plurality of square portions arranged in a two- dimensional array, e.g. in a checkerboard pattern or a Gingham pattern.

[0057] The process for determining aberrations of the projection system using an interferometric sensor system of the type described above is described in more detail in WO2019149468A1 and WO2022248154, which are hereby incorporated by reference in their entirety.

[0058] The sensor system may 300 may be used in the alignment of various components / sy stems (e.g. in the alignment of the patterning device MA, the patterning device support MT, the substrate W, and the substrate support WT). Additionally or alternatively, the one or more sensor systems may be used to characterize aberrations of the projection system PS.

[0059] The sensor system 300 referred to in the present disclosure may be a transmission image sensor (TIS). Detailed disclosure of a TIS may be found in US 6888151 B2, WO 2022207259 Al and WO 2017207512 A2, for example, all of which are incorporated herein by reference.

[0060] Additionally or alternatively, the sensor system 300 referred to in the present disclosure may be a shearing interferometer phase-stepping measurement sensor (PARIS). Detailed disclosure of PARIS may be found in: WO 2021069147 Al, WO 2022248154 Al and WO 2022268679 Al, all of which are incorporated herein by reference.

[0061] Interferometric sensor systems such as the TIS and the PARIS are configured to perform measurements using different ones of a plurality of sensors simultaneously, e.g. in parallel. Over time, the distance between each of the transmission diffraction gratings 311 formed in (or disposed on) the sensor plate 313 may change. This may be because of thermal deformation of the sensor plate 313 and / or due to changes in materials used in conjunction with the sensor plate 313, such as adhesives. Changes in the materials used in conjunction with the sensor plate 313 may occur due to the conditions (e.g. vacuum pressure) inside the lithographic apparatus.

[0062] Changes in the distance between each of the transmission diffraction gratings 311 formed in the sensor plate 313 may lead to the introduction of an error component to the measurements made by the sensors of the sensor system 300.

[0063] Additionally or alternatively, the error component may make it necessary to provide the sensor plate 313 with additional features so that deformation of the sensor plate can be mitigated through calibration processes. For example, a sensor plate 313 which comprises the transmissive diffraction gratings 311 of a TIS may comprise a plurality of calibration marks formed thereon. The calibration marks on the sensor plate 313 of the TIS may be used to measure deformation of the sensor plate 313 of the TIS, such that the deformation of the sensor plate 313 of the TIS can be compensated for in future measurements performed by the TIS. However, measuring the deformation of the sensor plate 313 of the TIS may require moving the TIS from an exposure station (i.e. a region of the lithographic apparatus in which exposure of the substrate W occurs) to a measurement station. This may be time-consuming, and thus reduce the availability of the lithographic apparatus.

[0064] Further, it may not be possible to provide calibration marks on the sensor plates 313 of some types of sensor systems 300. For instance, it may not be possible to provide calibration marks on the sensor plate 313 of a PARIS, because proving such calibration marks on the sensor plate 313 of thePARIS would risk damaging the transmissive marks 311 on the sensor plate 313 of the PARIS. Consequently, it may not be possible to calibrate the PARIS in such a way that error arising from deformation of the sensor plate 313 of the PARIS is mitigated. This means that the PARIS may not be suitable for measuring low order aberrations of the projection system (e.g. aberrations corresponding to Zemike coefficients 2, 3 and 4 (Noll’s index)). Even if calibration marks can be used, a calibration of the sensor plate is time consuming.

[0065] The present disclosure is directed to providing a method of characterizing changes in the distance between transmissive diffraction gratings on a sensor plate of an interferometric system. The sensor system may be the sensor system 300 described above with reference to Figure 3. That is, the sensor system upon which the method of the present disclosure is performed comprises a reflective mark 301 and a plurality of sensors, wherein each sensor comprises a transmissive diffraction grating 311 and a radiation detector 312. The sensor system is configured such that radiation (e.g. EUV radiation) reflected from the reflective mark 301 is measured by one or more of the plurality of sensors 312.

[0066] The reflective mark 301 comprises a reflective diffraction grating, as described above. The plurality of sensors comprises at least a first sensor and a second sensor. The first sensor comprises a first transmissive diffraction grating 311 and a first radiation detector 312 (as described above), and the second sensor comprises a second transmissive diffraction grating 311 and a second radiation detector 312 (as described above). In some embodiments, a common radiation detector 312 is provided for some or all of the sensors, such that the first radiation detector 312 is the same as the second radiation detector 312.

[0067] The method of the present disclosure comprises measuring radiation reflected from the reflective mark 301 with the first sensor, and measuring radiation reflected from the reflective mark 301 with the second sensor. That is, two separate sensors are used to measure radiation reflected from one reflective mark 301. To do this, two separate sensors may be aligned with the image of the reflective mark at substrate level. In the case that a single radiation detector 312 is provided for all of the transmissive diffraction gratings 311 formed on the sensor plate 313, measurements of the radiation reflected from the reflective mark 301 are performed using two separate transmissive diffraction gratings 312 (which equates to performing measurements of the radiation reflected from the reflective mark 301 with two separate sensors).

[0068] The method further comprises determining an error component associated with a change in the relative position of the first transmissive mark 311 and the second transmissive mark 311 (or, in some examples, determining the change in the relative position of the first transmissive mark 311 and the second transmissive mark 311 itself). The determination of the change in the relative position of the first transmissive mark 311 and the second transmissive mark 311 is based on the measurement of radiation reflected from the reflective mark 301 with the first sensor and the measurement of radiation reflected from the reflective mark 301 with the second sensor. The measurement of radiationreflected from one reflective mark with two different sensors allows an error component associated with each sensor (e.g. an error component arising from a change in the relative position of the sensors) to be extracted from the measurements made by the sensor system 300. In other words, the measurement of radiation reflected from one reflective mark with two different sensors allows the error component associated with the sensors to be separated from other components of the measurements, e.g. a baseline component of the measurement, and a component corresponding to aberrations of the projection system PS.

[0069] The method of the present disclosure allows errors arising from deformation of the sensor plate 313 to be accurately quantified. Further, the method of the present disclosure allows errors arising from deformation of the sensor plate 313 to be quantified without the introduction of new structural features to the interferometric sensor system (i.e. structural features which are provided solely for the purpose of quantifying errors arising from deformation of the sensor plate 313). Further, the present method allows for the quantification of errors arising from deformation of the sensor plate 313 in interferometric sensor systems which are unsuitable for structural features provided solely for the purpose of quantifying errors arising from deformation of the sensor plate 313, e.g. the PARIS. Consequently, as a result of the method of the present disclosure, it may be possible to measure low order aberrations of the projection syst using interferometric sensor systems which have previously been unsuitable for the measurement of low order aberrations of the projection system.

[0070] The radiation reflected from the reflective mark 301 may be projected onto a certain position at the level of the substrate W (i.e. a certain position relative to the frame of the lithographic apparatus). Thus, to measure the radiation reflected from the reflective mark 301 using the first sensor and the second sensor, the sensor device 310 in which the first sensor and the second sensor are provided may be moved from a first position to a second position. The first position may be a position at which the first sensor (and thus the first transmissive mark 311) is aligned with the radiation reflected from the reflective mark 301. The second position may be a position at which the second sensor (and thus the second transmissive mark 311) is aligned with the radiation reflected from the reflective mark 301. The moving of the sensor device 310 from the first position to the second position may be performed after the the measurement of radiation reflected from the reflective mark 301 with the first sensor. The moving of the sensor device 310 from the first position to the second position may be performed before performing the measurement of radiation reflected from the reflective mark 301 with the second sensor.

[0071] As explained above, the sensor device 310 may be supported by the substrate support WT. Thus, the step of moving the sensor device 310 from the first position to the second position may comprise moving the substrate support WT from a first position to a second position.

[0072] In some embodiments, the method further comprises characterizing a deformation of the sensor plate 313 based on the change in the relative position of the first transmissive mark and thesecond transmissive mark determined based on the measurement of radiation reflected from the reflective mark 301 with the first sensor and the measurement of radiation reflected from the reflective mark 301 with the second sensor.

[0073] In some embodiments, the method further comprises calibrating the sensor system 300 based on the change in the relative position of the first transmissive mark 311 and the second transmissive mark 311. The calibrating of the sensor system 300 may comprise applying a correction to measurements performed by the sensor system 300, wherein the correction accounts for the error introduced by the change in the relative position of the first transmissive diffraction grating 311 and the second transmissive diffraction grating 311.

[0074] In some embodiments, the method further comprises calibrating the sensor system 300 based on the deformation of the sensor plate 313 as characterized based on change in the relative position of the first transmissive mark and the second transmissive mark determined.

[0075] In some embodiments, the sensor system may comprise a plurality of reflective marks 301 disposed at the patterning device (i.e. in the object plane of the lithographic apparatus). In this case, the reflective mark 301 described above may be one of a plurality of the plurality of reflective marks 301. The plurality of reflective marks may comprise a first reflective mark and a second reflective mark.

[0076] In some embodiments, the first reflective mark 301 may correspond to the first sensor (i.e. during normal use of the sensor system, radiation reflected from the first reflective mark 301 may be measured by the first sensor), and the second reflective mark 301 may correspond to the second sensor (i.e. during normal use of the sensor system, radiation reflected from the second reflective mark 301 may be measured by the second sensor). In some embodiments, the sensor system 300 may be configured such that radiation reflected from the first reflective mark 301 is measured by the first sensor at the same time as radiation reflected from the second reflective mark 301 is measured by the second sensor. For example, the first sensor may detect radiation reflected from the first reflective mark in parallel with the second sensor detecting radiation reflected from the second reflective mark so that the sensor system 300 can obtain a desired measurement output. The desired measurement output may be based on a combination of the measurement made by the first sensor and the measurement made by the second sensor. A position of the sensor device 310 in which each sensor is aligned with its corresponding reflective mark may be referred to as a standard position.

[0077] The method of the present disclosure may comprise measuring two reflective marks 301 with two sensors. That is, the method of the present disclosure may comprise:(a) performing a measurement of radiation reflected from the first reflective mark 301 with the first sensor;(b) performing a measurement of radiation reflected from the second reflective mark 301 with the second sensor;(c) performing a measurement of radiation reflected from the first reflective mark 301 with the second sensor;(d) performing a measurement of radiation reflected from the second reflective mark 301 with the first sensor; and(e) determining an error component associated with a change in the relative position of the first transmissive diffraction grating and the second transmissive diffraction grating based on the measurements obtained in (a) to (d).

[0078] In some embodiments, steps (a) and (b) may be performed simultaneously. This may be because the lithographic apparatus is configured such that, when the first sensor is aligned with radiation reflected from the first reflective mark 301, the second sensor is aligned with radiation reflected from the second reflective mark 301. Performing some measurements simultaneously may increase the speed with which the method of the present disclosure can be performed.

[0079] In some embodiments, steps (c) and (d) may be performed sequentially. This may be because, when second sensor is aligned with the radiation reflected from the first reflective mark 301, the first sensor is not aligned with the radiation reflected from the second reflective mark 301.

[0080] In some embodiments, the sensor device 310 may be in a standard position (i.e. a position in which the first sensor is aligned with the radiation reflected from the first reflective mark and the second sensor is aligned with the radiation reflected from the second reflective mark) for steps (a) and (b). After step (b), the sensor device 310 may be moved to a different position, in which the second sensor is aligned with the radiation reflected from the first reflective mark 301. After step (c), the sensor device 310 may be moved to another position, in which the first sensor is aligned with the radiation reflected from the second reflective mark 301.

[0081] The method of the present disclosure is not limited with regard to the number of reflective marks 301 or the number of sensors. In general, increasing the number of measurements performed (i.e. performing more measurements with different combinations of reflective marks and sensors) may improve the extent to which the error components introduced by the sensors can be characterized.For example, increasing the number of measurements performed (i.e. performing more measurements with different combinations of reflective marks and sensors) may increase the accuracy with which the change in the relative positions of the first and second transmissive marks can be determined.

[0082] As described above, the method of the present disclosure comprises performing measurements with the sensor system 300 (as opposed to performing measurements on a component of the sensor system 300). Further, the method can be performed while the sensor device 310 (and the substrate support on which the sensor device 310 may be provided) are in an exposure station of the lithographic apparatus. The exposure station of the lithographic apparatus may be a portion of the lithographic apparatus within which the patterned beam of radiation is projected onto the substrate W.

[0083] Further, by the fact that the method of the present disclosure makes use of measurements performed by the sensor system 300 itself, it is not necessary to modify the components of the sensorsystem 300 so that calibration of the sensor system 300 can be performed. For example, it is not necessary for calibration marks to be provided on the sensor plate 313.

[0084] Figure 4 schematically depicts the relative positions of a plurality of reflective marks 301 and transmissive marks 311 of a plurality of sensors during a method in accordance with the present disclosure.

[0085] In Figure 4, there is shown a fiducial 304 having a plurality of reflective marks 3011, 3012,3013, 3014, 3015 disposed thereon. The fiducial 304 is shown in a single position. This is because the fiducial may not move during the method of the present disclosure. As will be recognized, the description of the method of the present disclosure with reference to Figure 4 applies equally to the case where the reflective marks 301 are disposed on a calibration device 303, rather than the fiducial 304.

[0086] Figure 4 also shows, for reference, a sensor plate 313 having a plurality of transmissive marks 311 disposed thereon in an undeformed state and in the standard position (see “R” in Figure 4).

[0087] Further, Figure 4 shows the sensor plate 313 in a case where the sensor plate 313 is deformed in a plurality of positions (see “A” to “C” in Figure 4). Specifically, Figure 4 shows the sensor plate 313 in a case where the sensor plate 313 is in the standard position, and is deformed (see “A” in Figure 4). Figure 4 also shows the sensor plate 313 in a case where the sensor plate 313 has been shifted to the right, and is deformed (see “B” in Figure 4). Figure 4 also shows the sensor plate 313 in a case where the senor plate has been shifted to the left, and is deformed (see “C” in Figure 4). The deformation of the sensor plate 313 in “A” to “C” may be substantially the same. “A” to “C” may represent positions of the sensor plate 313 in different steps of the method of the present disclosure, as will be described in further detail below.

[0088] As shown in Figure 4, the fiducial 304 comprise five reflective marks 3011, 3012, 3013,3014, 3015. That is, the fiducial comprises a first reflective mark 3011, a second reflective mark 3012, a third reflective mark 3013, a fourth reflective mark 3014, and a fifth reflective mark 3015. The first to fifth reflective marks 3011, 3012, 3013, 3014, 3015 are arranged linearly, i.e. along a straight line. For the purposes of explanation, the first to fifth reflective marks 3011, 3012, 3013, 3014, 3015 are arranged along a left / right direction.

[0089] As shown in Figure 4, the sensor plate 313 comprise five transmissive marks 3111, 3112, 3113, 3114, 3115 (and thus 5 sensors). That is, the fiducial comprises a first transmissive mark 3111, a second transmissive mark 3112, a third transmissive mark 3113, a fourth transmissive mark 3114, and a fifth transmissive mark 3115. For the purposes of explanation, the first to fifth transmissive marks 3111, 3112, 3113, 3114, 3115 are arranged along the left / right direction.

[0090] The first to fifth transmissive marks 3111-3115 (or first to fifth sensors) may each have an index i. The index i of the first transmissive mark 3111 is 1, the index i of the second transmissive mark 3012 is 2, the index i of the third transmissive mark 3113 is 3, the index i of the fourth transmissive mark 3114 is 4, and the index i of the fifth transmissive mark 3015 is 5. The first to fifthtransmissive marks 3111, 3112, 3113, 3114, 3115 may correspond to the first to fifth reflective marks 3011, 3012, 3013, 3014, 3015, respectively. As will be recognized, the description of the method of the present disclosure with reference to Figure 4 may be applied to sensor systems in which there are more or fewer reflective marks 301 than are depicted in Figure 4, and / or more or fewer transmissive marks 311 (i.e. more or fewer sensors) than are depicted in Figure 4.

[0091] A measurement, m, made by a sensor may comprise several components, as shown in Equation 2. m = s + e + IEquation 2Where .v is a baseline measurement for the sensor (i.e. the measurement expected by the sensor in initial conditions, or when there are substantially no aberrations introduced in the projection system and substantially no error introduced by the sensor itself); e is an error component introduced by the sensor (e.g. as a result of a deformation of the sensor plate 313 causing the relative positions of the transmissive marks 3111, 3112, 3113, 3114, 3115 to change); and I is a component corresponding to aberrations of the projection system.

[0092] When the sensor plate 313 is in the standard position (see “A” in Figure 4), radiation reflected from the first to fifth reflective marks 3011, 3012, 3013, 3014, 3015 is measured with the corresponding sensors. That is, radiation reflected from the first reflective mark 3011 is measured using the first transmissive mark 3111, radiation from the second reflective mark 3112 is measured using the second transmissive mark, etc. When the sensor plate 313 is in the standard position, the measurement for a sensor at index i is given by Equation 3. mt = st + et + li ( = 1, ...5)Equation 3 where m. is the measurement made by the sensor at index i; s, is the baseline measurement for the sensor at index i; is the error component introduced by the sensor at index i; and 4 is the component corresponding to aberrations of the projection system at the position of the sensor at index i (when the sensor plate 313 is in the standard position).

[0093] When the sensor plate 313 is in position “B” in Figure 4, the sensor plate 313 has been shifted to the right by a distance of twice the spacing of the transmissive marks 311 on the sensor plate 313 (relative to the standard position). As used in the present disclosure, the spacing of the transmissive marks refers to a distance between the center of one transmissive mark and the center of an adjacent transmissive mark. Thus, the first transmissive mark 3111 is aligned with radiation reflected from the third reflective mark 3013, the second transmissive mark 3112 is aligned with theradiation reflected from the fourth reflective mark 3014 and the third transmissive mark 3114 is aligned with the radiation reflected from fifth reflective mark 3015. Thus, the sensor comprising the first transmissive mark 3111 measures the radiation reflected from the third reflective mark 3013, the sensor comprising the second transmissive mark 3112 measures the radiation reflected from the fourth reflective mark 3014 and the sensor comprising the third transmissive mark 3114 measures the radiation reflected from fifth reflective mark 3015. When the sensor plate 313 has been shifted to the right as shown in Figure 4, the measurement for a sensor at index i is given by Equation 4.Equation 4 where m, is the measurement made by the sensor at index i; si+2 is the baseline measurement for the sensor at index i+2; e, is the error component introduced by the sensor at index i; and h+2 is the component corresponding to aberrations of the projection system at the position of the sensor at index i+2 (when the sensor plate 313 is in the standard position). For example, when the sensor plate 313 is in position “B”, the measurement made by the first sensor is formed of the baseline measurement of the third sensor (because the first sensor is positioned where the third sensor is positioned when the sensor plate 313 is in the standard position), a component corresponding to the error arising from the first sensor, and a component corresponding to the aberrations of the projection system at the position of the third sensor when the sensor plate 313 is in the standard position (because the first sensor is positioned where the third sensor is positioned when the sensor plate 313 is in the standard position).

[0094] When the sensor plate 313 is in position “C” in Figure 4, the sensor plate 3113 has been shifted to the left by a distance of twice the spacing of the transmissive marks 311 on the sensor plate 313 (relative to the standard position). Thus, the third transmissive mark 3113 is aligned with radiation reflected from the first reflective mark 3011, the fourth transmissive mark 3114 is aligned with the radiation reflected from the second reflective mark 3012 and the fifth transmissive mark 3115 is aligned with the radiation reflected from third reflective mark 3013. Thus, the sensor comprising the third transmissive mark 3113 measures the radiation reflected from the first reflective mark 3011, the sensor comprising the fourth transmissive mark 3114 measures the radiation reflected from the second reflective mark 3012 and the sensor comprising the fifth transmissive mark 3015 measures the radiation reflected from third reflective mark 3013. When the sensor plate 313 has been shifted to the left as shown in Figure 4, the measurement for a sensor at index i is given by Equation 5. mi= si-2+ et+ lt-2(j = 3, ... 5)Equation 5where m, is the measurement made by the sensor at index i; Sj.2 is the baseline measurement for the sensor at index i-2; e, is the error component introduced by the sensor at index i; and 1,-2 is the component corresponding to aberrations of the projection system at the position of the sensor at index i-2 (when the sensor plate 313 is in the standard position). For example, the measurement made by the third sensor is formed of the baseline measurement of the first sensor (because the third sensor is positioned where the first sensor is positioned when the sensor plate 313 is in the standard position), a component corresponding to the error arising from the third sensor, and a component corresponding to the aberrations of the projection system at the position of the first sensor when the sensor plate 313 is in the standard position (because the third sensor is positioned where the first sensor is positioned when the sensor plate 313 is in the standard position).

[0095] From the measurements performed while the sensor plate 313 is in positions “A”, “B” and “C”, a system of equations is obtained (each equation being of the form of Equation 3, Equation 4 or Equation 5). The system of equations is a system of simultaneous equations. Solving this set of equations allows the error component for each sensor (e;) to be determined. From the error components for each sensor, the change in the relative position of the transmissive marks 3111, 3112, 3113, 3114, 3115 can be determined.

[0096] In the above description, a move of twice the spacing of the transmissive marks 311 is described. It will be recognized that any multiple of the spacing of the transmissive marks 311. For instance, the sensor plate 313 may be moved to the right by one spacing of the transmissive diffraction gratings, 3 times the spacing of the transmissive diffraction gratings. In general, where the sensor plate 313 is moved to the right by a distance of n times the spacing of the transmissive diffraction gratings 313, the measurement made by a sensor is given by Equation 6. mi i+n d” Si + Zj+jjEquation 6

[0097] Where the sensor plate 313 is moved to the left by a distance of n times the spacing of the transmissive diffraction gratings 313, the measurement made by a sensor is given by Equation 7Equation 7

[0098] Figure 5 schematically depicts the relative positions of a plurality of reflective mark 301 and transmissive marks 311 of a plurality of sensors during a method in accordance with another embodiment of the present disclosure. As depicted in Figure 5, the sensor the sensor system 300 comprises three reflective marks 301 (i.e. a first reflective mark 3011, a second reflective mark 3012 and athird reflective mark 3013) on the fiducial 304, and three sensors (i.e. a first sensor, a secondsensor and a third sensor). The first sensor comprises a first transmissive mark 3111, a second transmissive mark 3112, and a third transmissive mark 3113, each disposed on the sensor plate 313. Figure 5 depicts the fiducial 304 in a single position, and the sensor plate 312 in five positions (“A” to “E”). The second reflective mark 301 is interposed between the first reflective mark 301 and the third reflective mark 301. The second sensor may be interposed between the first sensor and the third sensor.

[0099] For the sensor system depicted in Figure 5, the method of the present disclosure may comprise:(a) performing a measurement of radiation reflected from the first reflective mark 3011 with the first sensor (i.e. using the first transmissive mark 3111);(b) performing a measurement of radiation reflected from the second reflective mark 3012 with the second sensor (i.e. using the second transmissive mark 3112);(c) performing a measurement of radiation reflected from the third reflective mark 3013 with the third sensor (i.e. using the third transmissive mark 3113);(d) performing a measurement of radiation reflected from the first reflective mark 3011 with the third sensor (i.e. using the third transmissive mark 3113);(e) performing a measurement of radiation reflected from the third reflective mark 3013 with the first sensor (i.e. using the first transmissive mark 3111);(f) performing a measurement of radiation reflected from the second reflective mark 3012 with the third sensor (i.e. using the third transmissive mark 3113);(g) performing a measurement of radiation reflected from the second reflective mark 3012 with the first sensor (i.e. using the first transmissive mark 3111); and(h) determining an error component associated with a change in the relative position between two or more of the first transmissive mark 3111, the second transmissive mark 3112 and the third transmissive mark 3113 based on the measurements obtained in (a) to (g).

[0100] In some embodiments, steps (a) to (c) are performed simultaneously. This may be possible because, in a standard position of the sensor device 310 (position “A” in Figure 5), the first transmissive mark 3111 is aligned with radiation reflected from the first reflective mark 3011, the second transmissive mark 3112 is aligned with radiation reflected from the second reflective mark 3012, and the third sensor 3113 is aligned with radiation reflected from the third reflective mark 3013.

[0101] In the step (d) of performing a measurement of radiation reflected from the first reflective mark 3011 with the third sensor (i.e. using the third transmissive mark 3113), the sensor plate 313 may be in the position “D”. In the step (e) of performing a measurement of radiation reflected from the third reflective mark 3013 with the first sensor (i.e. using the first transmissive mark 3111), the sensor plate 313 may be in the position “E” In the step (f) of performing a measurement of radiation reflected from the second reflective mark 3012 with the third sensor (i.e. using the third transmissive mark 3113), the sensor plate 313 may be in the position “B”. When the sensor plate is in the position“B”, a measurement of radiation reflected from the first reflective mark 3011 may be performed using the second sensor (i.e. using the second transmissive diffraction grating 3112). In the step (g) of performing a measurement of radiation reflected from the second reflective mark 3012 with the first sensor (i.e. using the first transmissive mark 3111), the sensor plate 313 may be in the position “C” When the sensor plate is in the position “C”, a measurement of radiation reflected from the third reflective mark 3013 may be performed with the second sensor (i.e. using the second transmissive diffraction grating 3112).

[0102] The step (h) of determining the change in the relative position between two or more of the first transmissive mark 3111, the second transmissive mark 3112 and the third transmissive mark 3113 based on the measurements obtained in (a) to (g) may comprise forming a system of equations based on the measurements performed in steps (a) to (g) (and any other measurements performed), as described above with reference to Figure 4.

[0103] At the bottom of Figure 5, there is depicted a sensor plate 313 which has been rotated by an angle a, e.g. rotated by an angle a in a horizontal plane (i.e. in a plane in which the sensor plate 313 is disposed). It will be appreciated that rotation of the sensor plate 313 may be performed as part of the method of the present disclosure. After rotation of the sensor plate 313, the sensor plate may be moved along its longitudinal axis (where the longitudinal axis is at the angle a relative to, e.g. an X- direction). Rotating and then subsequently translating the sensor plate 313 in this way may allow deformation of the sensor plate in two perpendicular directions (e.g. an X-direction and a Y-direction) to be characterized. Rotating and then subsequently translating the sensor plate 313 may be preferable (e.g. faster) than moving the sensor plate in the X-direction and then moving the sensor plate 313 in the Y-direction.

[0104] Figure 6 schematically depicts the relative positions of transmissive marks 3111, 3112, 3113, 3114, 3115, 3116 on a sensor plate 313, and images of reflective marks, during a method in accordance with an embodiment. The transmissive marks 3111, 3112, 3113, 3114, 3115, 3116 may be arranged on a curved line. When the reflective marks are exposed to radiation, images of the reflective marks at the substrate level may be arranged on a curved line, e.g. a curved line which corresponds to the curved line on which the transmissive marks 3111, 3112, 3113, 3114, 3115, 3116 are arranged.

[0105] To perform the method of the present disclosure using a sensor system 300 comprising the sensor plate 313 depicted in Figure 6, the sensor plate 313 may be rotated such that the radiation reflected from a given reflective mark is measured by different sensors (i.e. is measured using different transmissive marks 3111, 3112, 3113, 3114, 3115, 3116). In Figure 6, images of the reflective marks arranged on the dashed line “A” are images before a rotation of the sensor plate 313, and images of the reflective marks arranged on the dotted line “B” are images after a rotation of the sensor plate 313. The amount of rotation of the sensor plate may be equal to an angular spacing of the transmissive marks on the sensor plate, or equal to any multiple of the angular spacing of thetransmissive marks on the sensor plate. Equations 3 to 7 above may be straightforwardly adapted to this situation. This is because Equations 3 to 7 are not dependent on the nature of the movement of the sensor device 310 to align different transmissive marks with the image projected from a reflective mark.

[0106] In cases where the transmissive marks 3111, 3112, 3113, 3114, 3115, 3116 are not arranged with spatial periodicity, it may be necessary to measure combinations of reflective marks and transmissive marks 3111, 3112, 3113, 3114, 3115, 3116 individually. Figure 7, 8A and 8B schematically depicts the relative positions of transmissive marks and images of reflective marks during a method in accordance with such an embodiment. The images of reflective marks on the lines, “A”, “B” and “C” are images are for different positions of the sensor plate 313. In the methods represented by Figures 7, 8A and 8B, a combination of rotational and translational movement of the sensor device 310 may be performed so that different transmissive marks are sequentially aligned with the image projected from a reflective mark. Equations 3 to 7 above may be straightforwardly adapted to this situation. This is because Equations 3 to 7 are not dependent on the nature of the movement of the sensor device 310 to align different transmissive marks with the image of a reflective mark.

[0107] In some cases, the reflective marks 3011-3015 may be disposed on a plate in a similar way to how the transmissive marks 3111-3115 are disposed on the sensor plate 313. Deformation of the plate on which the reflective marks 3011-3015 are disposed (and subsequent changes in the distance between reflective diffraction gratings) may lead to the introduction of an error component in measurements made by the sensor system. To quantify this error component, a modified version of the method described above may be performed, where the modification is that the plate on which the reflective marks are disposed is moved such that a single sensor is used to measure radiation reflected from a plurality of the reflective marks.

[0108] A method of quantifying an error component arising from a deformation of a plate on which the reflective marks are disposed may comprise measuring radiation reflected from a first reflective mark with a sensor, and measuring radiation reflected from a second reflective mark with the sensor (i.e. the same sensor as is used to measure the radiation reflected from the first reflective mark). The method may further comprise determining an error component associated with a change in the relative position of the first reflective diffraction grating and the second reflective diffraction grating based on the measurement of radiation reflected from the first reflective mark with the sensor and the measurement of radiation reflected from the second reflective mark with the sensor. As will be appreciated by the skilled person, features of the method described herein for determining an error component associated with a change in the relative position of a first transmissive diffraction grating and a second transmissive diffraction grating may be applied to the method of determining an error component associated with a change in the relative position of the first reflective diffraction grating and the second reflective diffraction grating.

[0109] In some cases, deformation of the plate on which the reflective marks are disposed is negligible or deformation of the sensor plate 313 is negligible. Additionally or alternatively, one of the error component associated with deformation of the plate on which the reflective marks are disposed and the error component associated with the sensor plate 313 may be quantified and / or eliminated before performing the method of the present disclosure to determine the other of the error component associated with deformation of the plate on which the reflective marks are disposed and the error component associated with the sensor plate 313. Additionally or alternatively, the method of the present disclosure may be performed to determine one of the error component associated with deformation of the plate on which the reflective marks are disposed and the error component associated with the sensor plate 313 while neglecting any error component introduced by the other of the error component associated with deformation of the plate on which the reflective marks are disposed and the error component associated with the sensor plate 313.

[0110] Although specific reference may be made in this text to the use of a 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.

[0111] 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 by instructions stored on a machine-readable medium, which may be read and executed by one or more processors. A machine -readable medium may include any mechanism for storing or transmitting information in a form 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.

[0112] 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 apparatus may be generally referred to as lithographic tools.

[0113] 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.

[0114] Aspects of the invention are described in the following numbered clauses.1. A method for use with a sensor system of a lithographic apparatus, the sensor system comprising: a reflective mark comprising a reflective diffraction grating; and a plurality of sensors comprising a first sensor and a second sensor, the first sensor comprising a first transmissive diffraction grating and a first radiation detector, and the second sensor comprising a second transmissive diffraction grating and a second radiation detector, wherein the method comprises: measuring radiation reflected from the reflective mark with the first sensor; measuring radiation reflected from the reflective mark with the second sensor; and determining an error component associated with a change in the relative position of the first transmissive diffraction grating and the second transmissive diffraction grating based on the measurement of radiation reflected from the reflective mark with the first sensor and the measurement of radiation reflected from the reflective mark with the second sensor.2. The method of clause 1, wherein the lithographic apparatus comprises: a patterning device having a patterning surface configured to impart a pattern to a beam of radiation, wherein the patterning surface of the patterning device is in an object plane; a substrate support configured to support a substrate such that a surface of the substrate is in an image plane; and a projection system configured to project the beam of radiation from the patterning surface of the patterning device to the surface of the substrate, wherein the reflective mark is disposed at the object plane, and the first transmissive diffraction grating and the second transmissive diffraction grating are disposed at the image plane.3. The method of clause 1 or 2, wherein the substrate support comprises the first sensor and the second sensor.4. The method of any of clauses 1 to 3, wherein the patterning device comprises the reflective mark.5. The method of any of clauses 1 to 3, wherein the patterning device is supported by a patterning device support, and the patterning device support comprises the reflective mark.6. The method of any of the preceding clauses, further comprising moving the substrate support from a first position to a second position.7. The method of clause 6, wherein moving the substrate support from the first position to the second position is performed after the performing the measurement of radiation reflected from thereflective mark with the first sensor and before the performing the measurement of radiation reflected from the reflective mark with the second sensor.8. The method of clause 6 or 7, wherein the first position is a position at which the first sensor is aligned with radiation reflected from the reflective mark, and the second position is a position at which the second sensor is aligned with radiation reflected from the reflective mark.9. The method of any of the preceding clauses, further comprising determining a change in the relative position of the first transmissive diffraction grating and the second transmissive diffraction grating.10. The method of any of the preceding clauses, wherein the first transmissive diffraction grating and the second transmissive diffraction grating are disposed on a sensor plate.11. The method of clause 10, further comprising characterising a deformation of the sensor plate based on the change in the relative position of the first transmissive diffraction grating and the second transmissive diffraction grating.12. The method of any of the preceding clauses, further comprising calibrating the sensor system based on the change in the relative position of the first transmissive diffraction grating and the second transmissive diffraction grating.13. The method of any of the preceding clauses, wherein the reflective mark is one of a plurality of reflective marks comprising a first reflective mark and a second reflective mark.14. The method of clause 13, wherein the method comprises:(a) performing a measurement of radiation reflected from the first reflective mark with the first sensor;(b) performing a measurement of radiation reflected from the second reflective mark with the second sensor;(c) performing a measurement of radiation reflected from the first reflective mark with the second sensor;(d) performing a measurement of radiation reflected from the second reflective mark with the first sensor; and(e) determining an error component associated with a change in the relative position of the first transmissive diffraction grating and the second transmissive diffraction grating based on the measurements obtained in (a) to (d).15. The method of clause 14, wherein steps (a) and (b) are performed simultaneously.16. The method of clause 14 or 15, wherein the sensor system is configured such that, in use, the first sensor performs a measurement of radiation reflected from the first reflective mark and the second sensor performs a measurement of radiation reflected from the second reflective mark.17. The method of clause 16, wherein the sensor system is configured such that, in use, the first sensor performs the measurement of radiation reflected from the first reflective mark at the sametime as the second sensor performs the measurement of radiation reflected from the second reflective mark.18. The method of any of clauses 1 to 12, wherein the reflective mark is one of a plurality of reflective marks comprising a first reflective mark, a second reflective mark and a third reflective mark, and the sensor system further comprises a third sensor, the third sensor comprising a third transmissive diffraction grating and a third radiation detector.19. The method of clause 18, wherein the method comprises:(a) performing a measurement of radiation reflected from the first reflective mark with the first sensor;(b) performing a measurement of radiation reflected from the second reflective mark with the second sensor;(c) performing a measurement of radiation reflected from the third reflective mark with the third sensor;(d) performing a measurement of radiation reflected from the first reflective mark with the third sensor;(e) performing a measurement of radiation reflected from the third reflective mark with the first sensor;(f) performing a measurement of radiation reflected from the second reflective mark with the third sensor;(g) performing a measurement of radiation reflected from the second reflective mark with the first sensor; and(h) determining an error component associated with a change in the relative position of the first transmissive diffraction grating and the second transmissive diffraction grating and / or the third transmissive diffraction grating based on the measurements obtained in (a) to (g).20. The method of clause 18 or 19, wherein the second reflective mark is interposed between the first reflective mark and the third reflective mark, and / or wherein the second sensor is interposed between the first sensor and the third sensor.21. The method of clause 19 or 20, wherein steps (a) to (c) are performed simultaneously.22. The method of any of the preceding clauses, wherein the sensor system is configured to be used in the alignment of the substrate support and the patterning device.23. The method of any of the preceding clauses, wherein the sensor system is configured to be used to determine aberrations of the projection system.24. The method of any of the preceding clauses, wherein the method is performed while the substrate support is in an exposure station of the lithographic apparatus.25. A method of manufacturing a device, the method comprising the method for use with a sensor system of a lithographic apparatus of any of the preceding clauses.26. A lithographic apparatus comprising:a sensor system, the sensor system comprising a reflective mark comprising a reflective diffraction grating, and a plurality of sensors comprising a first sensor and a second sensor, the first sensor comprising a first transmissive diffraction grating and a first radiation detector, and the second sensor comprising a second transmissive diffraction grating and a second radiation detector; and a controller configured to control the lithographic apparatus to perform the method of any of clauses 1 to 25.27. A computer program for the lithographic apparatus of clause 26, the computer program comprising instructions to cause the controller to control the lithographic apparatus to perform the method of any of clauses 1 to 25.28. A method for use with a sensor system of a lithographic apparatus, the sensor system comprising: a plurality of reflective marks comprising a first reflective mark and a second reflective mark, the first reflective mark comprising a first reflective diffraction grating and the second reflective mark comprising a second reflective diffraction grating; and a sensor comprising a transmissive diffraction grating and a radiation detector, wherein the method comprises: measuring radiation reflected from the first reflective mark with the sensor; measuring radiation reflected from the second reflective mark with the sensor; and determining an error component associated with a change in the relative position of the first reflective diffraction grating and the second reflective diffraction grating based on the measurement of radiation reflected from the first reflective mark with the sensor and the measurement of radiation reflected from the second reflective mark with the sensor.

[0115] While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described.

[0116] 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 for use with a sensor system of a lithographic apparatus, the sensor system comprising: a reflective mark comprising a reflective diffraction grating; and a plurality of sensors comprising a first sensor and a second sensor, the first sensor comprising a first transmissive diffraction grating and a first radiation detector, and the second sensor comprising a second transmissive diffraction grating and a second radiation detector, wherein the method comprises: measuring radiation reflected from the reflective mark with the first sensor; measuring radiation reflected from the reflective mark with the second sensor; and determining an error component associated with a change in the relative position of the first transmissive diffraction grating and the second transmissive diffraction grating based on the measurement of radiation reflected from the reflective mark with the first sensor and the measurement of radiation reflected from the reflective mark with the second sensor.

2. The method of claim 1, wherein the lithographic apparatus comprises: a patterning device having a patterning surface configured to impart a pattern to a beam of radiation, wherein the patterning surface of the patterning device is in an object plane; a substrate support configured to support a substrate such that a surface of the substrate is in an image plane; and a projection system configured to project the beam of radiation from the patterning surface of the patterning device to the surface of the substrate, wherein the reflective mark is disposed at the object plane, and the first transmissive diffraction grating and the second transmissive diffraction grating are disposed at the image plane.

3. The method of claim 1 or 2, wherein the substrate support comprises the first sensor and the second sensor.

4. The method of any of claims 1 to 3, wherein the patterning device comprises the reflective mark.

5. The method of any of claims 1 to 3, wherein the patterning device is supported by a patterning device support, and the patterning device support comprises the reflective mark.

6. The method of any of the preceding claims, further comprising moving the substrate support from a first position to a second position.

7. The method of any of the preceding claims, further comprising determining a change in the relative position of the first transmissive diffraction grating and the second transmissive diffraction grating.

8. The method of any of the preceding claims, wherein the first transmissive diffraction grating and the second transmissive diffraction grating are disposed on a sensor plate.

9. The method of any of the preceding claims, further comprising calibrating the sensor system based on the change in the relative position of the first transmissive diffraction grating and the second transmissive diffraction grating.

10. The method of any of the preceding claims, wherein the reflective mark is one of a plurality of reflective marks comprising a first reflective mark and a second reflective mark.

11. The method of claim 10, wherein the method comprises:(a) performing a measurement of radiation reflected from the first reflective mark with the first sensor;(b) performing a measurement of radiation reflected from the second reflective mark with the second sensor;(c) performing a measurement of radiation reflected from the first reflective mark with the second sensor;(d) performing a measurement of radiation reflected from the second reflective mark with the first sensor; and(e) determining an error component associated with a change in the relative position of the first transmissive diffraction grating and the second transmissive diffraction grating based on the measurements obtained in (a) to (d).

12. The method of any of claims 1 to 9, wherein the reflective mark is one of a plurality of reflective marks comprising a first reflective mark, a second reflective mark and a third reflective mark, and the sensor system further comprises a third sensor, the third sensor comprising a third transmissive diffraction grating and a third radiation detector.

13. The method of claim 12, wherein the method comprises:(a) performing a measurement of radiation reflected from the first reflective mark with the first sensor;(b) performing a measurement of radiation reflected from the second reflective mark with the second sensor;(c) performing a measurement of radiation reflected from the third reflective mark with the third sensor;(d) performing a measurement of radiation reflected from the first reflective mark with the third sensor;(e) performing a measurement of radiation reflected from the third reflective mark with the first sensor;(f) performing a measurement of radiation reflected from the second reflective mark with the third sensor;(g) performing a measurement of radiation reflected from the second reflective mark with the first sensor; and(h) determining an error component associated with a change in the relative position of the first transmissive diffraction grating and the second transmissive diffraction grating and / or the third transmissive diffraction grating based on the measurements obtained in (a) to (g).

14. A lithographic apparatus comprising: a sensor system, the sensor system comprising a reflective mark comprising a reflective diffraction grating, and a plurality of sensors comprising a first sensor and a second sensor, the first sensor comprising a first transmissive diffraction grating and a first radiation detector, and the second sensor comprising a second transmissive diffraction grating and a second radiation detector; and a controller configured to control the lithographic apparatus to perform the method of any of claims 1 to 13.

15. A method for use with a sensor system of a lithographic apparatus, the sensor system comprising: a plurality of reflective marks comprising a first reflective mark and a second reflective mark, the first reflective mark comprising a first reflective diffraction grating and the second reflective mark comprising a second reflective diffraction grating; and a sensor comprising a transmissive diffraction grating and a radiation detector, wherein the method comprises: measuring radiation reflected from the first reflective mark with the sensor; measuring radiation reflected from the second reflective mark with the sensor; and determining an error component associated with a change in the relative position of the first reflective diffraction grating and the second reflective diffraction grating based on the measurement of radiation reflected from the first reflective mark with the sensor and the measurement of radiation reflected from the second reflective mark with the sensor.

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