Projection system characterisation system and method

Abstract

Disclosed herein is a computing system configured to perform a method for determining aberrations of an optical system in response to patterned light, the method comprising: determining a combination of test features that substantially corresponds to an arrangement of product features on a patterning device arranged to pattern light before the light passes through the optical system; and determining aberrations caused by the response of the optical system to the patterned light in dependence on the known response of each test feature in the determined combination.

Description

PROJECTION SYSTEM CHARACTERISATION SYSTEM AND METHODCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of EP application 23199147.2 which was filed on 22 September 2023 and which is incorporated herein in its entirety by reference.FIELD

[0002] The present invention relates to a method of characterising a projection system of a lithographic apparatus. In a lithographic exposure processes, patterned light shines through a projection system. The method of characterising a projection system according to embodiments allows the thermal response of the projection system to be determined for any pattern of the light.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. including part of a die, one die, 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. These target portions are commonly referred to as “fields”.

[0004] The lithographic apparatus comprises a projection system. The projection system comprises optical elements such as, for example, lenses. Optical aberrations may arise from imperfections in the optical elements. Optical aberrations may also arise from projection effects such as, for example, the heating of optical elements that occurs during a lithographic exposure. Projection system models are used to determine one or more adjustments that may be made to the optical elements of the projection system. The determined adjustments may have the effect of reducing optical aberrations within the projection system.

[0005] It may be desirable to provide, for example, to improve on aspects of calibration, tuning and / or use of a projection system modelling method which obviates or mitigates one or more of the problems of the prior art, whether identified herein or elsewhere.SUMMARY

[0006] According to a first aspect of the invention, there is provided a computing system configured to perform a method for determining aberrations of an optical system in response to patterned light, the method comprising: determining a combination of test features that substantially corresponds to anarrangement of product features on a patterning device arranged to pattern light before the light passes through the optical system; and determining aberrations caused by the response of the optical system to the patterned light in dependence on the known response of each test feature in the determined combination.

[0007] According to a second aspect of the invention, there is provided a characterisation system for characterising an optical system, the characterisation system comprising: an illumination apparatus configured to perform a plurality of illumination processes; a wavefront sensor configured to measure the wavefront of light after the light has travelled through an optical system in each of the plurality of illumination processes; and a computing system; wherein: each illumination process comprises patterning light with a different one of the test features in a set comprising a plurality of test features; each test feature in the set has a single pitch; each test feature in the set has a different single pitch from the other test features in the set; each test feature in the set has a single orientation; and the computing system is configured to generate data that characterises the optical system in dependence on determined changes of the measured wavefronts in the plurality of illumination processes.

[0008] According to a third aspect of the invention, there is provided a computer-implemented method of determining aberrations of an optical system in response to patterned light, the method comprising: determining a combination of test features that substantially corresponds to product features on a patterning device arranged to pattern light before the light passes through an optical system; and determining aberrations caused by the response of the optical system to the patterned light in dependence on the known response of each test feature in the determined combination.

[0009] According to a fourth aspect of the invention, there is provided a method of characterising an optical system, the method comprising: performing a plurality of illumination processes; wherein each illumination process comprises: patterning light with a different one of the test features in a set comprising a plurality of test features; and measuring the wavefront changes of the patterned light after the patterned light has travelled through an optical system; wherein: each test feature in the set has a single pitch; each test feature in the set has a different single pitch from the other test features in the set; and each test feature in the set has a single orientation.BRIEF DESCRIPTION OF THE DRAWINGS

[0010] 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 schematic overview of a lithographic apparatus; and- Figures 2A and 2B schematically show diffraction orders and diffraction angles of patterned light.DETAILED DESCRIPTION

[0011] In the present document, the terms “radiation” and “beam” are used to encompass all types of electromagnetic radiation, including ultraviolet radiation (e.g., with a wavelength of 365, 248, 193,157 or 126 nm) and EUV (extreme ultra-violet radiation, e.g., having a wavelength in the range of about 5-100 nm).

[0012] The term “reticle”, “mask” or “patterning device” as employed in this text may be broadly interpreted as referring to a generic patterning device that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate. The term “light valve” can also be used in this context. Besides the classic mask (transmissive or reflective, binary, phase-shifting, hybrid, etc.), examples of other such patterning devices include a programmable mirror array and a programmable LCD array.

[0013] To clarify the invention, a Cartesian coordinate system is used. The Cartesian coordinate system has three axis, i.e., an x-axis, a y-axis and a z-axis. Each of the three axis is orthogonal to the other two axis. A rotation around the x-axis is referred to as an Rx-rotation. A rotation around the y- axis is referred to as an Ry-rotation. A rotation around about the z-axis is referred to as an Rz -rotation. The Cartesian coordinate system is not limiting the invention and is used for clarification only.

[0014] Figure 1 schematically depicts a lithographic apparatus LA. The lithographic apparatus LA includes an illumination system (also referred to as illuminator) IL configured to condition a radiation or light beam B (e.g., UV radiation, DUV radiation or EUV radiation), a mask support (e.g., a mask table) MT constructed to support a patterning device (e.g., a mask ) MA and connected to a first positioner configured to accurately position the patterning device MA in accordance with certain parameters, a substrate support (e.g., a wafer table) WT constructed to hold a substrate (e.g., a resist coated wafer) W. A second positioner configured to accurately position the substrate support in accordance with certain parameters may be arranged. A projection lens system (e.g., a refractive projection lens system) PS configured to project a pattern imparted to the radiation beam B by patterning device MA (a patterned projection beam) onto a target portion (e.g., comprising one or more dies) of the substrate W.

[0015] In operation, the illumination system IL receives a radiation beam from a radiation source SR, e.g., via a beam delivery system. The illumination system IL may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic, and / or other types of optical components, or any combination thereof, for directing, shaping, and / or controlling radiation. The illuminator IL may be used to condition the radiation beam B to have a desired spatial and angular intensity distribution in its cross section at a plane of the patterning device MA.

[0016] The term “projection lens system” PS used herein should be broadly interpreted as encompassing various types of projection systems, including refractive, reflective, catadioptric, anamorphic, magnetic, electromagnetic and / or electrostatic optical systems, or any combination thereof, as appropriate for the exposure radiation being used, and / or for other factors such as the use of an immersion liquid or the use of a vacuum. Any use of the term “projection lens” herein may be considered as synonymous with the more general term “projection system” PS.

[0017] The lithographic apparatus LA may be of a type wherein at least a portion of the substrate may be covered by a liquid having a relatively high refractive index, e.g., water, so as to fill a space between the projection system PS and the substrate W - which is also referred to as immersion lithography. More information on immersion techniques is given in US6952253, which is incorporated herein by reference.

[0018] The lithographic apparatus LA may also be of a type having two or more substrate supports WT (also named “dual stage”). In such “multiple stage” machine, the substrate supports WT may be used in parallel, and / or steps in preparation of a subsequent exposure of the substrate W may be carried out on the substrate W located on one of the substrate support WT while another substrate W on the other substrate support WT is being used for exposing a pattern on the other substrate W.

[0019] In addition to the substrate support WT, the lithographic apparatus LA may comprise a measurement stage. The measurement stage is arranged to hold a sensor and / or a cleaning device. The sensor may be arranged to measure a property of the projection system PS or a property of the radiation beam B. The measurement stage may hold multiple sensors. The cleaning device may be arranged to clean part of the lithographic apparatus LA, for example a part of the projection system PS or a part of a system that provides the immersion liquid. The measurement stage may move beneath the projection system PS when the substrate support WT is away from the projection system PS.

[0020] In operation, the radiation beam B is incident on the patterning device, e.g. mask, MA which is held on the mask support MT, and is patterned by the pattern (design layout) present on patterning device MA. Having traversed the mask MA, the radiation beam B passes through the projection system PS, which focuses the beam onto a target portion of the substrate W. With the aid of the second positioner and a position measurement system, the substrate support WT can be moved accurately, e.g., so as to position different target portions in the path of the radiation beam B at a focused and aligned position. Similarly, the first positioner and possibly another position sensor (which is not explicitly depicted in Figure 1) may be used to accurately position the patterning device MA with respect to the path of the radiation beam B. Patterning device MA and substrate W may be aligned using mask alignment marks and substrate alignment marks. Although the substrate alignment marks occupy dedicated target portions, they may be located in spaces between target portions. Substrate alignment marks are known as scribe-lane alignment marks when these are located between the target portions.

[0021] A masking device MD is provided for selectively shielding parts of the patterning device MA from the radiation beam B. In other embodiments, not illustrated by Fig. 1, the masking device MD may be provided to selectively shield parts of the patterned projection beam. As is illustrated in Fig. 1, relay optics (or relay lens) RL is provided for projecting the radiation that passes through the masking device to the patterning device MA.

[0022] Among the factors affecting overlay and focus drift, lens heating is a major factor in deterioration of image quality, particularly with the very localized illumination sources often used, such as dipole illumination. These localized heat distributions on the lens can cause undesired effects on animaging wavefront. Aberration drift during lot exposure is unavoidable due to the heating of the lens during the exposure. The accumulated absorption of light in the lens can eventually cause imaging imperfections, which are the direct results of lens heating-induced wavefront errors. Lens heating control is important for maintaining imaging quality. An important step in lens heating control the ability to predict lens heating accurately.

[0023] The lithographic apparatus LA may further comprise one or more wavefront sensors; e.g., on the substrate support WT. A wavefront sensor may measure the quality of the wavefront image, as well as any aberrations and therefore enable (e.g., parallel) measurement of optical aberrations throughout the projection slit, thus enabling, for example, a more accurate alignment, improved patterning device MA heating correction and on the fly lens heating correction. The wavefront sensor may comprise an interferometric wavefront measurement system and may perform static measurements on lens aberrations up to high order. It may be implemented as an integrated measurement system used for system initialization and calibration. Alternatively, it may be used for monitoring and recalibration “on- demand”. As such, the wavefront sensor may be able to measure the lens aberrations on a per lot or per- wafer basis depending on its design.

[0024] It is known to use a wavefront sensor to measure wavefront errors or aberrations. These are typically described as coefficients in a series of 2D mathematical functions called Zernike polynomials. These can be regrouped to describe some well-known optical aberrations, such as spherical, coma, and astigmatism.

[0025] As described above with reference to Figure 1, during the operation of the lithographic apparatus LA light, i.e., the radiation beam B, is patterned by the pattern (design layout) present on a patterning device MA. The patterning device MA comprises an arrangement of product features. The arrangement, i.e., the pattern, of the product features imparts a pattern on the light. The patterned light is projected, via a lens, onto a substrate W during an exposure process.

[0026] The wavefront of light used in an exposure process is dependent on how the light is patterned by the patterning device MA. Specifically, the wavefront is dependent on the types and arrangements of the product features on the patterning device MA.

[0027] It is known that, for a lens that has been heated by an exposure process, the wavefront signal will carry a “memory” or “ghost” of the exposed product features (a patterning device MA, illumination and dose specific wavefront contribution). This is because, when product features are exposed, the lens (i.e., the projection system PS) heats up. The heated area is governed by the diffraction pattern induced by the illumination mode combined with the product features on the patterning device MA. Immediately after the exposure ends, the wavefront data may be determined using a wavefront sensor. The wavefront sensor will determine a change in the wavefront with respect to the situation before the lens was heated. This change is resulting from and describing the shape of the product features and illumination settings. This change will gradually fade away, as the heat diffuses in the lens material.

[0028] US2023 / 0084130 Al discloses techniques in which such wavefront data is used to improve lens heating control and is used to reduce adverse effects caused by lens heating. In US2023 / 0084130 Al aberrations that are induced by lens heating effects are determined for a specific arrangement of product features.

[0029] An actual product will typically be formed from a large number of layers with each layer having a different arrangement of product features. There may also be variation of the arrangement of product features within each layer.

[0030] A limitation of the techniques disclosed in US2023 / 0084130 Al is that wavefront data is required for a large number of arrangements of product features. The time required to obtain the required wavefront data may be prohibitive. In addition, whenever a new arrangement of product features is used, further measurements of wavefront data may be required.

[0031] Embodiments provide a new technique for characterising lens heating and / or cooling effects. The lens may be the projection system PS of a lithographic apparatus. Accordingly, embodiments provide a new technique for characterising projection system PS heating and / or cooling effects.

[0032] Embodiments determine lens heating and / or cooling responses for each of a plurality of specific pitches of test features on a patterning device MA. Combinations of the lens heating responses may be used to determine an overall lens heating response for an arrangement of test features. Advantageously, embodiments may model a lens heating response for any arrangement of the test features without the lens heating response for the specific arrangement being measured.

[0033] Embodiments are described in more detail below.

[0034] Light, with a pre-determined illumination condition, is patterned by a test feature on a patterning device MA. The test feature may have one critical dimension and a single orientation. The test feature may be, for example, a linear line pattern with the pattern having a constant pitch. It may be a dense linear line pattern.

[0035] The patterned light passes through a lens that may be an objective lens (i.e., projection system PS). The light diffracts by the pattern upon entering the lens and the diffraction causes the light to shine through different parts of the lens.

[0036] Figure 2A schematically shows the diffraction orders of patterned light. The light may have been patterned by a patterning device, for example, a patterning device MA as provided by an illumination system IL. The patterned light may therefore be before the lens. Figure 2B schematically shows, in a side view, the diffraction angles. Different orders for o,,, / oout are shown, where o,,, and ooutrespectively refer to the inner and outer angles of the light cone distribution as provided by the illumination system IL. The o,,, and ooutvalues are normalised to the numerical aperture of the optical arrangement, for example, the numerical aperture of the projection system PS.

[0037] The wavefront of the light after passing through the lens may be measured by one or more wavefront sensors. Each wavefront sensor may be as described above. Accordingly, the wavefrontsensor may be on the substrate support WT and may comprise an interferometric wavefront measurement system.

[0038] The wavefront sensor may comprise a camera that captures a pixelated image. The pixelated image is converted into a wavefront sensor signal. In this example, the wavefront sensor signal carries specific wavefront information contributed or induced by the test feature. Such a wavefront sensor signal may be a mapped representation of wavefront error across the exit pupil plane of the lens. Additionally, or alternatively, processing may be performed for the determining the Zernike polynomials that correspond to the wavefront sensor signal in accordance with known techniques.

[0039] In embodiments, wavefront data (or information) for a specific test feature and illumination condition may comprise the wavefront sensor signal and / or the corresponding Zernike polynomials for the wavefront sensor signal.

[0040] Embodiments repeatedly use the above described-process to determine the wavefront data for a plurality of test features with different pitches and illumination conditions. The wavefront data characterises the lens heating response of the lens, i.e., the projection system PS. The characterisation process may determine the full angular response of the lens, i.e., projection system PS.

[0041] The characterisation of a lens heating response according to embodiments may include the following illumination processes.

[0042] In a first illumination process, a lens may be illuminated by light with a low energy. The light used in the first illumination process is preferably unpatterned light. The purpose of the first illumination process is to fill the pupil to enable interferometric wavefront measurements. Although patterned light may alternatively be used in the first illumination process, by using unpatterned light, the pupil will be filled homogeneously and full wavefront characterisation will be possible. The lens may be an objective lens, such as the projection system PS. The first illumination process may be performed when the lens is cold. A lens is cold if it is at the ambient temperature because it has not been recently heated by an exposure process or alike. By using illumination light at a low light intensity, the light does not substantially heat the lens. The light, that has passed through the lens, may be used to illuminate a substrate W and a wavefront sensor may be used to measure the wavefront data of the light after passing the lens. This wavefront data is an initial characterisation of the cold lens with light.

[0043] A first set of a plurality of illumination processes may then be performed with the light patterned by the same test feature on a patterning device MA. The test feature may have one critical dimension and a single orientation. The test feature may be, for example, a first linear line pattern with the pattern having a first constant pitch. The first constant pitch may be referred to as a first pitch Pl.

[0044] The light patterned by the pattern with first pitch Pl may be used to illuminate a substrate W so that the lens is heated. The wavefront data may be measured at the start and at the end of the illumination process. The change, i.e., difference, in the wavefront data is a measure of the lens heating effect during the illumination of the substrate W with light patterned by the pattern with first pitch Pl.

[0045] The light patterned by the pattern with first pitch Pl may then be used to illuminate one or more further substrates W. This further heats the lens. An expected amount of lens heating may be determined given the number of substrates that have been illuminated. For each substrate W, the wavefront data may be measured at the start and at the end of the illumination process. The change in the wavefront data is a measure of the lens heating effect during the illumination of each substrate W. The process of heating substrates W, and in turn determining changes in wavefront data, may be repeated until there is substantially no change in wavefront data when a substrate is illuminated. This indicates that the thermal behaviour of the lens is saturated for light patterned by the pattern with first pitch Pl.

[0046] The lens may then be allowed to cool down so that it returns to a cold state. The cooling time of the lens may be measured so as to determine the cooling properties of the lens. The factors that define the cooling properties of the lens are expected to be the same as those that define the heating properties of the lens. The heating and cooling responses of the lens are therefore related to each other.

[0047] A second set of processes of illuminating a plurality of substrates W, and measuring wavefront data, may then be performed with the light patterned by a different test feature on a patterning device MA. The test feature may have one critical dimension and a single orientation. The test feature may be, for example, a second linear line pattern with the pattern having a second constant pitch. It may be a second dense linear line pattern. The second constant pitch may be referred to as a second pitch P2. The second pitch P2 may have a different pitch than the first pitch Pl. For example, second pitch P2 may be a larger pitch than first pitch Pl. The second pitch P2 may be a smaller pitch than first pitch Pl The orientation of the second linear line pattern may be the same as the first linear line pattern. The orientation of the second linear line pattern may be different than the orientation of the first linear line pattern.

[0048] The illumination processes on substrates with light patterned by the pattern with second pitch P2 may be performed until there is no substantial change in wavefront data when a substrate is illuminated. This indicates that the thermal behaviour of the lens is saturated for light patterned by the pattern with second pitch P2.

[0049] The lens may then be allowed to cool down so that it returns to a cold state. As described above, the cooling properties of the lens may be determined.

[0050] One or more further sets of illumination processes may then be performed on the lens, with the lens allowed to cool to the cold state between each set of illumination processes. In each set of illumination processes, a plurality of substrates W are illuminated and wavefront data is measured, as described above with first and second line patterns having first and second pitch Pl and P2, respectively.

[0051] In addition, a set of illumination processes may performed with a test feature with one critical dimension, i.e., pitch, and the same single orientation. The pitch used in any one of the sets of illumination processes may differ from the pitch used in all of the other sets of illumination processes. Accordingly, the total number of sets of illumination processes that are performed may be the same asthe total number of pitches that are used. When the number of sets of illumination processes that are performed is N, the respective pitches that are used may be referred to as Pl, P2, . . . PN. The first pitch Pl may be the smallest pitch that is used and the sizes of the pitches may sequentially increase so that the N* pitch PN is the largest pitch that is used.

[0052] The different pitches Pl, P2, . . . PN may cover the full range of pitches that may be used with the lens. The pitches may therefore cover the full angular response of lens.

[0053] To pattern light with the different pitches Pl, P2, ... PN, a separate patterning device MA may be used for providing each pitch and or pattern orientation. Each of N (or 2N for different pattern orientations) patterning devices MA may comprise a line pattern with a constant pitch that is one of Pl, P2, . . . PN. A different one of the N patterning devices MA may be used in each set of illumination processes.

[0054] Alternatively, a single patterning device MA that comprises N separate regions may be used. Each region may comprise a (dense) line pattern with a constant pitch that is one of Pl, P2, ... PN. Twice as much regions may be provided at the patterning device MA in case two regions with same pitch, but with different pattern orientation, for each pitch are considered A different region of the patterning device MA may be used in each set of illumination processes.

[0055] The above sets of illumination processes provide a dataset of wavefront data that characterises the lens heating effects for light patterned with pitches Pl to PN. Embodiments include using this dataset to model the lens heating response to any arrangement of product features on a patterning device MA.

[0056] Product features are features on a patterning device MA that are used in the manufacture of features on a substrate W in the manufacturing of an actual product. Test features differ from product features in that test features are used for characterising the thermal properties of a lens. Test features are not used in the manufacturing of the features of an actual product.

[0057] Lens heating is a linear effect and so different individual heating responses may be combined to determine an overall response. Embodiments include analysing an arrangement of product features on a patterning device MA. The product features may have any arrangement and include a mixture of patterns having different pitches. The analysis determines a combination of one or more patterns, each pattern having a pitch Pl to PN and orientation that fully, or approximately, corresponds to the arrangement of product features. Any arrangement of product features on a patterning device MA may therefore be substantially modelled as a combination of one or more patterns, each individual pattern having a pitch Pl to PN and orientation.

[0058] Embodiments model the overall lens heating response for the arrangement of product features in dependence on the individual heating responses for the combination of patterns, selected from a set of patterns covering a pitch range from a first pitch Pl to an Nlhpitch PN determined by the analysis. The model may combine, such as sum, the heating responses for the determined combination of patterns having different pitches (and or orientation). The heating response for all of the patterns having different pitches Pl to PN and orientation are known from the determined dataset of wavefront data thatcharacterises the lens heating effects for light patterned with patterns having different pitches Pl to PN (and orientation).

[0059] During a series of exposure processes with a lens, the lens may be illuminated by light that is patterned by different arrangements of product features on one or more patterning devices MA. For each arrangement of product features that is used, the arrangement of product features may be analysed to determine a substantially corresponding combination of selected patterns, selected from a set of patterns comprising pitches Pl to PN. For each determined combination of patterns, the model may combine, such as sum, the heating responses for the patterns (pitches) to model the effect on the overall lens heating.

[0060] Embodiments may therefore model the overall lens heating for a lens that is illuminated with light that is patterned by different arrangements of product features on one or more patterning devices MA. The dataset of wavefront data that characterises the lens heating effects may be used to model the lens heating resulting from any arrangement of product features.

[0061] Embodiments also include modelling the overall lens heating response in dependence on any other factors that may affect the lens heating. For example, the model may use timing data on when each exposure process is performed to determine lens cooling effects.

[0062] Any arrangement of product features may be analysed to determine how the arrangement of product features substantially corresponds to combinations of the different line patterns. Embodiments use such an analysis to determine the lens heating response. This is clearly advantageous over techniques that require wavefront measurements of every arrangement of product features is used.

[0063] Embodiments may be used to make process corrections to reduce errors caused by lens heating. For example, embodiments may be used to predict lens heating evolutionary behaviors. The predicted lens heating behavior then can be corrected by one or more of numerous lens heating correction options which are available, such as (for example) image tuner lens adjustment control. For example, a lens heating feed forward (LHFF) dataset may be generated at the end of a calibration procedure for a device layer on a scanner. This LHFF dataset may be applied to the target apparatus and layer to correct the lens heating induced wavefront drifts.

[0064] Embodiments may also be used to improve the selection and / or design of lenses. In particular, embodiments may be used to determine the circumstances that cause the strongest lens heating response. The options for mitigating a large lens heating response may therefore be considered during the lens design.

[0065] A process for characterising a lens according to an embodiment is described below.

[0066] The process comprises performing a plurality of illumination processes. Each illumination process comprises: patterning light with a different one of the test features in a set comprising a plurality of test features; and measuring the wavefront changes of the patterned light after the patterned light has travelled through a lens. Each test feature in the set has a single pitch. Each test feature in the set has a different pitch. Each test feature in the set has a single orientation.

[0067] A computer-implemented method of determining the heating response of a lens to patterned light according to an embodiment is described below.

[0068] The computer-implemented method determines a combination of test features that substantially corresponds to that on a patterning device arranged to pattern light before the light passes through a lens. The computer-implemented method then determines the heating response of the lens to the patterned light in dependence on a combination of known heating responses of the test features.

[0069] Embodiments include a number of modifications and variations to the above described techniques.

[0070] Embodiments include performing the sets of illumination processes for each of a plurality of different orientations of the test features. That is to say, the test features may be comprised by a plurality of sets of test features. Each set of test features may comprise a plurality of test features with the different pitches Pl to PN having the same orientation. The orientation of the test features in each set may be different. By using the plurality of sets of test features to generate the wavefront data, the wavefront data characterises the lens heating effects for light patterned with each of pitches Pl to PN having a plurality of different orientations. This allows more accurate modelling of any lens heating effects that are dependent on the orientation of the product features.

[0071] Embodiments have been mostly described with reference to the determination of a heating response of a lens. Embodiments more generally include the determination of the thermal response of a lens. Accordingly, embodiments may determine the heating and / or cooling responses of the lens. The lens may be the projection system PS, or any other type of optical system.

[0072] Embodiments generally include the determination of aberrations in an optical system. The aberrations are caused by thermal effects that are dependent on the pattern of patterned light.

[0073] Although specific reference may be made in this text to the use of a lithographic apparatus LA in the manufacture of ICs, it should be understood that the lithographic apparatus LA 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.

[0074] Although specific reference may be made in this text to embodiments of the invention in the context of a lithographic apparatus LA, 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. Such a lithographic tool may use vacuum conditions or ambient (non- vacuum) conditions.

[0075] 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 ortransmitting 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.

[0076] Embodiments include the following numbered clauses:1. A computing system configured to perform a method for determining aberrations of an optical system in response to patterned light, the method comprising: determining a combination of test features that substantially corresponds to an arrangement of product features on a patterning device arranged to pattern light before the light passes through the optical system; and determining aberrations caused by the response of the optical system to the patterned light in dependence on the known response of each test feature in the determined combination.2. The computing system according to clause 1 , wherein each test feature is a linear line pattern.3. The computing system according to clause 1 or 2, wherein: the test features are selected from a set of test features; each test feature in the set has a single pitch; each test feature in the set has a different single pitch from the other test features in the set; and each test feature in the set has a single orientation.4. The computing system according to clause 3, wherein: the test features are selected from a plurality of sets of test features; each set of test features comprises a plurality of test features with different pitches and the same orientation; and the orientation of the test features in each set is different.5. The computing system according to clause 3 or 4, wherein the pitches of the test features in each set cover the angular response that the optical system is designed for.6. The computing system according to any preceding clause, wherein the method further comprises determining the aberrations of the optical system in dependence on the illumination conditions of the patterned light.7. The computing system according to any preceding clause, wherein the optical system is the projection system of a lithographic apparatus.8. The computing system according to clause 7, wherein the method further comprises: determining potential aberrations that may be caused by the thermal response of the optical system to the patterned light; and determining changes to one or more processes performed by the lithographic apparatus so as to reduce the aberrations caused by the thermal response.9. The computing system according to any preceding clause, wherein determining the aberrations comprises determining the heating response of the optical system to the patterned light in dependence on the known heating response of each test feature in the determined combination.10. A characterisation system for characterising an optical system, the characterisation system comprising: an illumination apparatus configured to perform a plurality of illumination processes; a wavefront sensor configured to measure the wavefront of light after the light has travelled through an optical system in each of the plurality of illumination processes; and a computing system; wherein: each illumination process comprises patterning light with a different one of the test features in a set comprising a plurality of test features; each test feature in the set has a single pitch; each test feature in the set has a different single pitch from the other test features in the set; each test feature in the set has a single orientation; and the computing system is configured to generate data that characterises the optical system in dependence on determined changes of the measured wavefronts in the plurality of illumination processes.11. The characterisation system according to clause 10, wherein each illumination process comprises patterning light with a different one of the test features in one of a plurality of sets of test features; each set of test features comprises a plurality of test features with different pitches and the same orientation; and the orientation of the test features in each set is different.12. The characterisation system according to clause 10 or 11, wherein the illumination apparatus is further configured to perform an illumination process with low intensity unpatterned light.13. A computer-implemented method of determining aberrations of an optical system in response to patterned light, the method comprising: determining a combination of test features that substantially corresponds to product features on a patterning device arranged to pattern light before the light passes through an optical system; and determining aberrations caused by the response of the optical system to the patterned light in dependence on the known response of each test feature in the determined combination.14. The computer-implemented method according to clause 13, wherein each test feature is a linear line pattern.15. The computer-implemented method according to clause 13 or 14, wherein: the test features are selected from a set of test features; each test feature in the set has a single pitch; each test feature in the set has a different single pitch from the other test features in the set; and each test feature in the set has a single orientation.16. The computer-implemented method according to any of clauses 13 to 15, wherein the test features are selected from a plurality of sets of test features; each set of test features comprises a plurality of test features with different pitches and the same orientation; and the orientation of the test features in each set is different.17. The computer-implemented method according to clause 16, wherein the pitches of the test features in the set cover the angular response that the optical system is designed for.18. The computer-implemented method according to any ofclauses l3 to l7, wherein the method further comprises determining the aberrations of the optical system in dependence on the illumination conditions of the patterned light.19. The computer-implemented method according to any of clauses 13 to 18, wherein the optical system is the projection system of a lithographic apparatus.20. The computer-implemented method according to any of clauses 13 to 19, wherein the method further comprises determining potential aberrations that may be caused by the thermal response of the optical system to the patterned light; and determining changes to one or more processes performed by the lithographic apparatus so as to reduce the aberrations caused by the thermal response.21. The computer-implemented method according to any of clauses 13 to 20, wherein determining the aberrations comprises determining the heating response of the optical system to the patterned light in dependence on the known heating response of each test feature in the determined combination.22. A method of characterising an optical system, the method comprising: performing a plurality of illumination processes; wherein each illumination process comprises: patterning light with a different one of the test features in a set comprising a plurality of test features; and measuring the wavefront changes of the patterned light after the patterned light has travelled through an optical system; wherein: each test feature in the set has a single pitch; each test feature in the set has a different single pitch from the other test features in the set; and each test feature in the set has a single orientation.23. The method according to clause 22, wherein: each illumination process comprises patterning light with a different one of the test features in one of a plurality of sets of test features; each set of test features comprises a plurality of test features with different pitches and the same orientation; and the orientation of the test features in each set is different.24. The method according to clause 22 or 23, further comprising performing an illumination process with low intensity unpatterned light.25. A method for determining aberrations of an optical system, comprising: illuminating with at least one illumination setting a plurality of diffractive targets provided at a patterning device to generate a plurality of diffractive orders, the patterning device being arranged at an object plane of the optical system; measuring aerial images of the diffractive orders at the image side of the optical system; calculating aberrations from each aerial image for each diffractive order; storing the calculated aberrations in a database; and determining aberrations for a feature having an expected diffraction order pattern from the stored calculated aberrations.

[0077] 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 computing system configured to perform a method for determining aberrations of an optical system in response to patterned light, the method comprising: determining a combination of test features that substantially corresponds to an arrangement of product features on a patterning device arranged to pattern light before the light passes through the optical system; and determining aberrations caused by the response of the optical system to the patterned light in dependence on the known response of each test feature in the determined combination.

2. The computing system according to claim 1 , wherein each test feature is a linear line pattern.

3. The computing system according to claim 1 or 2, wherein: the test features are selected from a set of test features; each test feature in the set has a single pitch; each test feature in the set has a different single pitch from the other test features in the set; and each test feature in the set has a single orientation.

4. The computing system according to claim 3, wherein: the test features are selected from a plurality of sets of test features; each set of test features comprises a plurality of test features with different pitches and the same orientation; and the orientation of the test features in each set is different.

5. The computing system according to any preceding claim, wherein the method further comprises determining the aberrations of the optical system in dependence on the illumination conditions of the patterned light.

6. The computing system according to any preceding claim, wherein the method further comprises: determining potential aberrations that may be caused by the thermal response of the optical system to the patterned light, wherein the optical system is a projection system of a lithographic apparatus; and determining changes to one or more processes performed by the lithographic apparatus so as to reduce the aberrations caused by the thermal response.

7. The computing system according to any preceding claim, wherein determining the aberrations comprises determining the heating response of the optical system to the patterned light in dependence on the known heating response of each test feature in the determined combination.

8. A characterisation system for characterising an optical system, the characterisation system comprising: an illumination apparatus configured to perform a plurality of illumination processes; a wavefront sensor configured to measure the wavefront of light after the light has travelled through an optical system in each of the plurality of illumination processes; and a computing system; wherein: each illumination process comprises patterning light with a different one of the test features in a set comprising a plurality of test features; each test feature in the set has a single pitch; each test feature in the set has a different single pitch from the other test features in the set; each test feature in the set has a single orientation; and the computing system is configured to generate data that characterises the optical system in dependence on determined changes of the measured wavefronts in the plurality of illumination processes.

9. The characterisation system according to claim 8, wherein: each illumination process comprises patterning light with a different one of the test features in one of a plurality of sets of test features; each set of test features comprises a plurality of test features with different pitches and the same orientation; and the orientation of the test features in each set is different.

10. The characterisation system according to claim 8 or 9, wherein the illumination apparatus is further configured to perform an illumination process with low intensity unpatterned light.

11. A computer-implemented method of determining aberrations of an optical system in response to patterned light, the method comprising: determining a combination of test features that substantially corresponds to product features on a patterning device arranged to pattern light before the light passes through an optical system; anddetermining aberrations caused by the response of the optical system to the patterned light in dependence on the known response of each test feature in the determined combination.

12. The computer-implemented method according to claim 11, wherein each test feature is a linear line pattern.

13. The computer-implemented method according to claim 11 or 12, wherein: the test features are selected from a set of test features; each test feature in the set has a single pitch; each test feature in the set has a different single pitch from the other test features in the set; and each test feature in the set has a single orientation.

14. The computer-implemented method according to any of claims 11 to 13, wherein the method further comprises determining the aberrations of the optical system in dependence on the illumination conditions of the patterned light.

15. A method of characterising an optical system, the method comprising: performing a plurality of illumination processes; wherein each illumination process comprises: patterning light with a different one of the test features in a set comprising a plurality of test features; and measuring the wavefront changes of the patterned light after the patterned light has travelled through an optical system; wherein: each test feature in the set has a single pitch; each test feature in the set has a different single pitch from the other test features in the set; and each test feature in the set has a single orientation.

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