Methods and systems for determining reticle deformation

Abstract

Disclosed herein is a computer system configured to: generate a set of models including, for each of a plurality of reticle supports of a reticle, one or more models of the effects of the reticle support on a substrate illuminated with light patterned by the reticle, wherein each model is based on initial tilt data of the reticle support; and determine scale factors of each model by fitting each model to measured effects of the reticle supports on a substrate; wherein the set of models with the determined scale factors model the reticle supports and / or the effect of the reticle supports on a substrate illuminated with light patterned by the reticle.

Description

METHODS AND SYSTEMS FOR DETERMINING RETICLE DEFORMATIONCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of US application 63 / 702,146 which was filed on 1 October 2024, and which is incorporated herein in its entirety by reference.FIELD

[0002] The present disclosure relates to techniques for improving the accuracy of models of the effect of reticle supports and deformations of reticle scales on lithographic processes.BACKGROUND

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

[0004] To project a pattern on a substrate a lithographic apparatus may use electromagnetic radiation. The wavelength of this radiation determines the minimum size of features which can be formed on the substrate. A lithographic apparatus, which uses extreme ultraviolet (EUV) radiation, having a wavelength within the range 4-20 nm, for example 6.7 nm or 13.5 nm, may be used to form smaller features on a substrate than a lithographic apparatus which uses, for example, deep ultraviolet (DUV) radiation with a wavelength of 157 nm or 193 nm or 248 nm.

[0005] A lithographic apparatus can include a reticle stage to hold a patterning device (e.g., a reticle) to transfer a pattern to a substrate. Reticle heating and / or cooling can cause changes in reticle properties that can affect the radiation beam path (e.g., focus) and cause distortions in the patterned substrate (e.g., overlay errors). The shape of a reticle may also be deformed by other effects, such as the applied clamping force to the reticle and drifts in the properties of the hardware for supporting the reticle. Drifts in the properties of the hardware for determining the position of the reticle may also occur and, if not compensated for, increase overlay errors.SUMMARY

[0006] There is a general need to improve on known techniques for modelling the states and effects of all hardware that affect the performance of a reticle. Process corrections may then be determined and applied to reduce the overlay error that would otherwise occur. This may avoid the rework of production substrates, and / or increase the fabrication throughput and yield of lithographic processes.

[0007] According to a first aspect, there is provided a computer system configured to: generate a set of models comprising, for each of a plurality of reticle supports of a reticle, one or more models of the effects of the reticle support on a substrate illuminated with light patterned by the reticle, whereineach model is based on initial tilt data of the reticle support; and determine scale factors of each model by fitting each model to measured effects of the reticle supports on a substrate; wherein the set of models with the determined scale factors model the reticle supports and / or the effect of the reticle supports on a substrate illuminated with light patterned by the reticle.

[0008] According to a second aspect, there is provided a computer system configured to generate a model for determining the deformations of reticle scales, and / or the effect of deformations of reticle scales, for an arrangement comprising a reticle supported by reticle supports, wherein the computer system is configured to: obtain a measurement of both the effects of deformations of reticle scales and the effect of reticle supports on a substrate illuminated with light patterned by the reticle; determine the effect of the reticle supports in the obtained measurement; determine the effect of deformations of reticle scales as the difference between the determined effect of the reticle supports and the obtained measurement; and determine the parameters of a model of the effect of deformations of reticle scales by fitting the parameters of the model to the determined effect of deformations of reticle scales.

[0009] According to a third aspect, there is provided a computer system configured to: generate, according to the first aspect, a model of reticle supports for a reticle, wherein the model has first scale factors determined in dependence on first measured effects of the reticle supports on a substrate; obtain second measured effects of the reticle supports on a substrate; and determine changes to the reticle supports in dependence on changes to the scale factors of the model that fit the model to the second measured effects of the reticle supports.

[0010] According to a fourth aspect, there is provided a computer system configured to: generate, according to the second aspect, a model of the deformations of reticle scales with first parameters in dependence on a first determination of the effect of deformations of reticle scales; obtain a second determination of the effect of deformations of reticle scales; and determine changes to the deformations of reticle scales in dependence on changes to the parameters of the model that fit the model to the second determination of the effect of deformations of reticle scales.

[0011] According to a fifth aspect, there is provided a method comprising: generating a set of models comprising, for each of a plurality of reticle supports of a reticle, one or more models of the effects of the reticle support on a substrate illuminated with light patterned by the reticle, wherein each model is based on initial tilt data of the reticle support; and determining scale factors of each model by fitting each model to measured effects of the reticle supports on a substrate; wherein the set of models with the determined scale factors model the reticle supports and / or the effect of the reticle supports on an illuminated substrate by the reticle.

[0012] According to a sixth aspect, there is provided a method of generating a model for determining the deformations of reticle scales, and / or the effect of deformations of reticle scales, for an arrangement comprising a reticle supported by reticle supports, wherein the method comprises: obtaining a measurement of both the effects of deformations of reticle scales and the effect of reticle supports on a substrate illuminated with light patterned by the reticle; determining the effect of thereticle supports in the obtained measurement; determining the effect of deformations of reticle scales as the difference between the determined effect of the reticle supports and the obtained measurement; and determining the parameters of a model of the effect of deformations of reticle scales by fitting the parameters of the model to the determined effect of deformations of reticle scales.

[0013] According to a seventh aspect, there is provided a method comprising: generating, according to the fifth aspect, a model of reticle supports for a reticle, wherein the model has first scale factors determined in dependence on first measured effects of the reticle supports on a substrate; obtaining second measured effects of the reticle supports on a substrate; and determining changes to the reticle supports in dependence on changes to the scale factors of the model that fit the model to the second measured effects of the reticle supports.

[0014] According to an eighth aspect, there is provided a method comprising: generating, according to the sixth aspect, a model of the deformations of reticle scales with first parameters in dependence on a first determination of the effect of deformations of reticle scales; obtaining a second determination of the effect of deformations of reticle scales; and determining changes to the deformations of reticle scales in dependence on changes to the parameters of the model that fit the model to the second determination of the effect of deformations of reticle scales.

[0015] According to a ninth aspect, there is provided a non-transitory computer readable medium program comprising computer readable instructions configured to cause a processor to perform the method of the fifth aspect.

[0016] According to a tenth aspect, there is provided a system comprising: a computer system; and a lithographic apparatus; wherein the computer system is further configured to control the operation of lithographic apparatus in dependence on a model generated according to the method of the fifth aspect.

[0017] Implementations of any of the techniques described herein may include an EUV light source, a DUV light source, a system, a method, a process, a device, and / or an apparatus. The details of one or more implementations are set forth in the accompanying drawings and the description below. Other features will be apparent from the description and drawings, and from the claims.

[0018] Further features and example embodiments of the aspects, as well as the structure and operation of various aspects, are described in detail below with reference to the accompanying drawings. It is noted that the aspects are not limited to the specific aspects described herein. Such aspects are presented herein for illustrative purposes only. Additional aspects will be apparent to persons skilled in the relevant art(s) based on the teachings contained herein.BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The accompanying drawings, which are incorporated herein and form part of the specification, illustrate the aspects and, together with the description, further serve to explain the principles of the aspects and to enable a person skilled in the relevant art(s) to make and use theaspects.

[0020] FIG. 1 is a schematic illustration of a lithographic apparatus, according to an example aspect.

[0021] FIG. 2A is a schematic illustration of a lithographic cell, according to an example aspect.

[0022] FIG. 2B is a schematic illustration of holistic lithography including a computer system to enhance a lithographic process, according to an example aspect.

[0023] FIG. 3A is a schematic bottom perspective illustration of a reticle stage and a reticle, according to an example aspect.

[0024] FIG. 3B is a schematic bottom plan illustration of the reticle stage shown in FIG. 3A.

[0025] FIG. 4 schematically shows a side on view of an arrangement of a reticle stage 400 and a lens 403.

[0026] FIG. 5 schematically shows a reticle 405 supported by the reticle stage 400.

[0027] The features and exemplary aspects of the aspects will become more apparent from the detailed description set forth below when taken in conjunction with the drawings, in which like reference characters identify corresponding elements throughout. In the drawings, like reference numbers generally indicate identical, functionally similar, and / or structurally similar elements. Additionally, generally, the left-most digit(s) of a reference number identifies the drawing in which the reference number first appears. Unless otherwise indicated, the drawings provided throughout the disclosure should not be interpreted as to-scale drawings.DETAILED DESCRIPTION

[0028] This specification discloses one or more aspects that incorporate the features of this present invention. The disclosed aspect(s) merely exemplify the present invention. The scope of the invention is not limited to the disclosed aspect(s). The present invention is defined by the claims appended hereto.

[0029] The aspect(s) described, and references in the specification to “one aspect,” “an aspect,” “an example aspect,” “an exemplary aspect,” etc., indicate that the aspect(s) described may include a particular feature, structure, or characteristic, but every aspect may not necessarily include the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same aspect. Further, when a particular feature, structure, or characteristic is described in connection with an aspect, it is understood that it is within the knowledge of one skilled in the art to effect such feature, structure, or characteristic in connection with other aspects whether or not explicitly described.

[0030] Spatially relative terms, such as “beneath,” “below,” “lower,” “above,” “on,” “upper” and the like, may be used herein for ease of description to describe one element’s or feature’s relationship to another element(s) or feature(s) as illustrated in the figures. The spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. The apparatus may be otherwise oriented (e.g., rotated 90 degrees or at otherorientations) and the spatially relative descriptors used herein may likewise be interpreted accordingly.

[0031] The terms “about,” “substantially,” and “approximately” as used herein indicates the value of a given quantity that can vary based on a particular technology. Based on the particular technology, the terms “about,” “substantially,” and “approximately” can indicate a value of a given quantity that varies within, for example, 1-15% of the value (e.g., ±1%, ±2%, ±5%, ±10%, or ±15% of the value).

[0032] The term “parasitic thermal effects” as used herein indicates induced or internal stresses and / or deformations of a reticle, for example, due to heating and / or cooling the reticle (e.g., by resistive heating, gas flow cooling, exposing the reticle to a dose of radiation, etc.) or mechanical pressures and / or deformations from clamping and / or holding the reticle on the reticle stage.

[0033] The term “non-production substrate” as used herein indicates a substrate (e.g., a wafer) that is not part of a production lot and is not fabricated by a lithographic process into a device (e.g., an IC chip). For example, a non-production substrate can be a chuck temperature conditioning (CTC) wafer or calibration wafer for a reticle calibration method, for example, to calibrate a reticle heating model and to acclimate the reticle by exposing the reticle and the CTC wafer to a dose of radiation and measuring a reticle alignment and / or a reticle temperature.

[0034] The term “production substrate” as used herein indicates a substrate (e.g., a wafer) that is part of a production lot and is fabricated by a lithographic process into a device (e.g., an IC chip). For example, a production substrate can be a wafer (e.g., silicon) for fabrication and inline real-time calibration of a reticle heating model, for example, by exposing the reticle and the wafer to a dose of radiation and measuring a reticle alignment and / or a reticle temperature.

[0035] The term “reticle heating model” as used herein indicates a modal deformation approach (e.g., analysis of different reticle mode shapes) to determine reticle heating effects based on reticle alignment and / or reticle shape deformations and a finite element model (FEM) (e.g., COMSOL). For example, the reticle heating model can include deterministic (e.g., no random future states) or non- deterministic (e.g., including random future states) reticle heating effects. Further, the reticle heating model can be deemed a reticle heating execution algorithm (RHEA) that uses inline modal calibrations to determine the baseline reticle heating dynamics. The reticle heating model can be calibrated by exposing a reticle and a non-production substrate to a dose of radiation for inline realtime calibration of the reticle heating model. In some aspects, for example, the reticle heating model can be calibrated by exposing a reticle and a production substrate to a dose of radiation for inline realtime calibration of the reticle heating model. Other reticle heating models utilize a sensor-based approach (e.g., using RTS measurements) to calibrate the reticle heating model. This is described in further detail in U.S. Patent No. 10,429,749, U.S. Patent No. 10,281,825, and U.S. Patent Application Publication No. 2020 / 0166854, the contents of which are incorporated by reference herein in their entireties.

[0036] Reticle heating causes changes in reticle properties that can affect the radiation path and causefabrication errors (e.g., overlay). Reticle mechanical deformations (e.g., based on reticle temperature) can be calculated and decomposed into k-parameters. Each thermo-mechanical mode (e.g., eigenvector) can be modeled in time using modal participation factor p and time constant r. Measured overlay and / or alignment can be used to model the related k-parameter drifts, which can be used to calculate adjustments to the feed-forward parameters p and r. The reticle heating model can also include adjusting feed-forward parameters p and r. This is described in further detail in U.S. Patent No. 10,429,749, U.S. Patent Application Publication No. 2020 / 0166854, and WIPO Patent Application Publication No. 2021 / 043519, the contents of which are incorporated by reference herein in their entireties.

[0037] The term “finite element model” (FEM) as used herein indicates a method for numerically solving differential equations arising in the reticle heating model (e.g., heat transfer equations, structural analysis equations, fluid flow equations, etc.). For example, baseline reticle heating dynamics can be analyzed with the FEM through finite element analysis. This is described in further detail in U.S. Patent No. 10,429,749, U.S. Patent No. 10,281,825, and U.S. Patent Application Publication No. 2020 / 0166854, the contents of which are incorporated by reference herein in their entireties.

[0038] The term “key performance indicators” (KPIs or k-parameters) as used herein indicates coefficients of polynomials that are fit to distortions of reticle alignment marks and / or edge alignment marks. The k-parameters parameterize the distortion of the imaging across the field of each substrate. For example, each k-parameter can describe a certain image distortion component (e.g., scaling error, barrel distortion, pincushion distortion, etc.). For example, two important k-parameters are k4 that represents distortion in Y-axis magnification and k 18 that represents distortion in Y-axis barrel shape. The k-parameters can be used as input to a lithographic process (e.g., lithographic apparatus LA, lithographic cell LC, control system CL) to correct the distortion. This is described in further detail in U.S. Patent No. 10,429,749, U.S. Patent Application Publication No. 2020 / 0166854, and WIPO Patent Application Publication No. 2021 / 043519, the contents of which are incorporated by reference herein in their entireties.

[0039] Aspects of the disclosure may be implemented in hardware, firmware, software, or any combination thereof. Aspects of the disclosure may also be implemented as instructions stored on a machine-readable medium, which may be read and executed by one or more processors. A machine- readable medium may include any mechanism for storing or transmitting information in a form readable by a machine (e.g., a computing device). For example, a machine-readable medium may include read only memory (ROM); random access memory (RAM); magnetic disk 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, and / or instructions may be described herein as performing certain actions. However, it should be appreciated that such descriptions are merely for convenience and thatsuch actions in fact result from computing devices, processors, controllers, or other devices executing the firmware, software, routines, instructions, etc.

[0040] Before describing such aspects in more detail, however, it is instructive to present example environments in which aspects of the present disclosure may be implemented.

[0041] Example Lithographic System

[0042] FIG. 1 shows a lithographic system comprising a radiation source SO and a lithographic apparatus LA. The radiation source SO is configured to generate an EUV and / or a DUV radiation beam B and to supply the EUV or DUV radiation beam B to the lithographic apparatus LA. The lithographic apparatus LA comprises an illumination system IL, a support structure MT (e.g., a mask table, a reticle table, a reticle stage) configured to support a patterning device MA (e.g., a mask, a reticle), a projection system PS, and a substrate table WT configured to support a substrate W.

[0043] The illumination system IL is configured to condition the EUV or DUV radiation beam B before the EUV or DUV radiation beam B is incident upon the patterning device MA. Thereto, the illumination system IL may include a faceted field mirror device 10 and a faceted pupil mirror device 11. The faceted field mirror device 10 and faceted pupil mirror device 11 together provide the EUV or DUV radiation beam B with a desired cross-sectional shape and a desired intensity distribution. The illumination system IL may include other mirrors or devices in addition to, or instead of, the faceted field mirror device 10 and faceted pupil mirror device 11.

[0044] After being thus conditioned, the EUV or DUV radiation beam B interacts with the patterning device MA. This interaction may be reflective (as shown), which may be preferred for EUV radiation. This interaction may be transmissive, which may be preferred for DUV radiation. As a result of this interaction, a patterned EUV or DUV radiation beam B’ is generated. The projection system PS is configured to project the patterned EUV or DUV radiation beam B’ onto the substrate W. For that purpose, the projection system PS may comprise a plurality of mirrors 13, 14 which are configured to project the patterned EUV or DUV radiation beam B’ onto the substrate W held by the substrate table WT. The projection system PS may apply a reduction factor to the patterned EUV or DUV radiation beam B’, thus forming an image with features that are smaller than corresponding features on the patterning device MA. For example, a reduction factor of 4 or 8 may be applied. Although the projection system PS is illustrated as having only two mirrors 13, 14 in FIG. 1, the projection system PS may include a different number of mirrors (e.g., six or eight mirrors).

[0045] The substrate W may include previously formed patterns. Where this is the case, the lithographic apparatus LA aligns the image, formed by the patterned EUV or DUV radiation beam B’, with a pattern previously formed on the substrate W.

[0046] Example Lithographic Cell

[0047] FIG. 2A shows a lithographic cell LC, also sometimes referred to as a lithocell or cluster. Lithographic apparatus LA may form part of lithographic cell LC. Lithographic cell LC may also include one or more apparatuses to perform pre- and post-exposure processes on a substrate.Conventionally these include spin coaters SC to deposit resist layers, developers DE to develop exposed resist, chill plates CH, and bake plates BK. A substrate handler, or robot, RO picks up substrates from input / output ports I / O 1 , 1 / O2, moves them between the different process apparatuses and delivers them to the loading bay LB of the lithographic apparatus LA. These devices, which are often collectively referred to as the track, are under the control of a track control unit TCU which is itself controlled by a supervisory control system SCS, which also controls the lithographic apparatus LA via lithography control unit LACU. Thus, the different apparatuses can be operated to maximize throughput and processing efficiency.

[0048] In order for the substrates W exposed by the lithographic apparatus LA to be exposed correctly and consistently, it is desirable to inspect substrates to measure properties of patterned substrates, for example, overlay errors between subsequent layers, line thicknesses, critical dimensions (CD), etc. Lor this purpose, inspection tools (e.g., metrology tool MT) may be included in lithographic cell LC and / or lithographic apparatus LA. If errors are detected, adjustments, for example, may be made to exposures of subsequent substrates or to other processing steps that are to be performed on the substrates W, especially if the inspection is done before other substrates W of the same batch or lot are still to be exposed or processed.

[0049] An inspection apparatus, which may also be referred to as a metrology apparatus or metrology tool MT, is used to determine properties of the substrates W, and in particular, how properties of different substrates W vary or how properties associated with different layers of the same substrate W vary from layer to layer. The inspection apparatus may alternatively be constructed to identify defects on the substrate W and may, for example, be part of lithographic cell LC, integrated into lithographic apparatus LA, and / or be a stand-alone device. The inspection apparatus may measure the properties on a latent image (e.g., image in a resist layer after the exposure), on a semi-latent image (e.g., image in a resist layer after a post-exposure bake step), on a developed resist image (e.g., image in which the exposed or unexposed parts of the resist have been removed), or on an etched image (e.g., image after a pattern transfer step, such as etching).

[0050] Example Computer System

[0051] FIG. 2B shows a computer system CL, also referred to as a controller or processor. Computer system CL may be part of lithographic cell LC, integrated into lithographic apparatus LA, and / or be a stand-alone device. Computer system CL is configured to enhance a lithographic process, for example, calibrate a reticle heating model. Typically the patterning process in lithographic apparatus LA is one of the most critical steps in the processing, which requires high accuracy of dimensioning and placement of structures on the substrate W. To ensure this high accuracy, three systems can be combined in a so-called “holistic” control environment as schematically depicted in FIG. 2B. As shown in FIG. 2B, the “holistic” environment can include lithographic apparatus LA, computer system CL, and metrology tool MT. For example, lithographic apparatus LA (a first system) can be connected to computer system CL (a second system) and metrology tool MT (a third system).

[0052] The key of such holistic lithography is to optimize the cooperation between these three systems to optimize a lithographic process, for example, to enhance the overall process window and provide tight controls loops to ensure that the patterning performed by lithographic apparatus LA stays within a process window. The process window defines a range of process parameters, for example, dose, focus, overlay, etc., within which a specific manufacturing process yields a defined result, for example, a functional semiconductor device — typically within which the process parameters in the lithographic process or patterning process are allowed to vary.

[0053] Computer system CL may, for example, use (e.g., part of) the design layout to be patterned to predict which resolution enhancement techniques to use and to perform computational lithography simulations and calculations, for example, to determine which mask layout and lithographic apparatus settings achieve the largest overall process window of the patterning process (shown in FIG. 2B by the double arrow in the first scale SCI). Typically, the resolution enhancement techniques are arranged to match the patterning possibilities of lithographic apparatus LA. Computer system CL may also be used to detect where within the process window lithographic apparatus LA is currently operating (e.g., using input from metrology tool MT) to predict whether defects may be present, for example, due to sub-optimal processing (shown in FIG. 2B by the arrow pointing “0” in the second scale SC2).

[0054] Metrology tool MT may provide input to computer system CL, for example, to enable accurate simulations and predictions. For example, metrology tool MT may provide alignment information. Metrology tool MT may provide feedback (e.g., via computer system CL) to lithographic apparatus LA to identify possible drifts, for example, in a calibration status of lithographic apparatus LA (shown in FIG. 2B by the multiple arrows in the third scale SC3). In lithographic processes, it is desirable to make frequent measurements of the structures created, for example, for process control and verification. Different types of metrology tools MT can be used, for example, to measure one or more properties relating to lithographic apparatus LA, a substrate W to be patterned, and / or reticle alignment. This is described in further details in U.S. Patent No. 11,099,319 and WIPO Patent Application Publication No. 2021 / 043519, the contents of which are incorporated by reference herein in their entireties.

[0055] Example Reticle Stage and Reticle

[0056] FIGS. 3A and 3B show schematic illustrations of reticle stage 200, according to example aspects. FIG. 3 A is a schematic bottom perspective illustration of reticle stage 200 and reticle 300, according to an example aspect. FIG. 3B is a schematic bottom plan illustration of reticle stage 200 and reticle 300 shown in FIG. 3A.

[0057] Reticle stage 200 (e.g., support structure MT) can be used in a lithographic apparatus (e.g., lithographic apparatus LA) to hold a patterning device (e.g., patterning device MA). Reticle stage 200 can include bottom stage surface 202, top stage surface 204, side stage surfaces 206, clamp 250, reticle cage 224, and / or reticle 300. In some aspects, reticle stage 200 with reticle 300 can beimplemented in lithographic apparatus LA. For example, reticle stage 200 can be the support structure MT in lithographic apparatus LA. In some aspects, reticle 300 can be disposed on bottom stage surface 202 and held by clamp 250. For example, as shown in FIGS. 3A and 3B, reticle 300 can be disposed on clamp 250 (e.g., an electrostatic clamp) at a center of bottom stage surface 202 with reticle frontside 302 facing perpendicularly away from bottom stage surface 202. In some aspects, reticle cage 224 can be disposed on bottom stage surface 202. For example, as shown in FIGS. 3A and 3B, reticle 300 can be disposed at a center of bottom stage surface 202 and secured by reticle cages 224 adjacent to each comer of reticle 300.

[0058] In some lithographic apparatuses, for example, lithographic apparatus LA, reticle stage 200 with clamp 250 can be used to hold and position reticle 300 for scanning or patterning operations. In some aspects, as shown in FIGS. 3A and 3B, reticle stage 200 can include first encoder 212 and second encoder 214 for positioning operations. For example, first and second encoders 212, 214 can be interferometers. First encoder 212 can be attached along a first direction, for example, a transverse direction (i.e., X-direction) of reticle stage 200. Second encoder 214 can be attached along a second direction, for example, a longitudinal direction (i.e., Y-direction) of reticle stage 200.

[0059] As shown in FIGS. 3 A and 3B, reticle 300 can include reticle frontside 302, alignment mark 310, and / or edge alignment mark 320. Alignment mark 310 is configured to measure a reticle alignment between reticle 300 and a substrate (e.g., substrate W, non-production substrate, production substrate). In some aspects, as shown in FIGS. 3A and 3B, one or more alignment marks 310 can be disposed in the comers and / or the center of reticle 300 for an RA measurement. Edge alignment mark 320 is configured to measure a reticle shape deformation of reticle 300 due to thermal expansion when reticle 300 is not within a predetermined temperature (e.g., at 22 °C ± 0.2 °C). In some aspects, as shown in FIGS. 3A and 3B, one or more edge alignment marks 320 can be disposed along the perimeter edges (e.g., horizontal and vertical edges) of reticle 300 for a reticle shape deformation (RSD) measurement. In some aspects, the results of the RA measurement and / or the RSD measurement can be converted to a reticle temperature, for example, by a FEM that solves for temperature based on reticle alignment and / or reticle deformation.

[0060] Reticle scales and clamping by reticle supports

[0061] Figure 4 schematically shows a side on view of an arrangement of a reticle stage 400 and a lens 403. Figure 5 schematically shows a reticle 405 supported by the reticle stage 400. The reticle stage 400 may be substantially the same as the earlier described reticle stage 200. The reticle 405 may be substantially the same as the earlier described reticle 300, and alternatively referred to as a patterning device MA.

[0062] As shown in Figure 4, the reticle stage 400 is located relative to, such as above, the lens 403. The reticle stage 400 may comprise reticle scales 402. There may be encoders 401 next to the lens 403. There may be at least two reticle scales 402 aligned orthogonally in the x-y plane. The x-y plane may correspond / translate to the movement plane of a substrate W during an exposure process withlight patterned by the reticle 405. There may be two reticle scales 402 aligned in the y-direction and one reticle scale 402 aligned in the x-direction. For each reticle scale 402, there is a respective encoder 401 that uses the reticle scale 402 to obtain position measurements. The reticle scales 402 may be read by the encoders 401 so that the position of the reticle stage 400 may be accurately measured. The encoders 401 may be, for example, interferometers. The encoders 401 may use laser light paths 404a, 404b to obtain measurements of the reticle scales 402.

[0063] There is a certain amount of deformation of the reticle scales 402 from an ideal shape. The manufacturing of the reticle scales will not be perfect and so there will be initial shape deformations. The deformations of the reticle scales 402 may also drift, that is to say the deformations may change over time due to heating and other effects. If not compensated for, the deformations of the reticle scales 402 may reduce the accuracy of the measured location of the reticle stage 400 and thereby cause at least overlay error. It is preferable for the deformations of the reticle scales 401 to be accurately determined so that their effects may be at least partially compensated for. This may reduce the overlay error that would otherwise occur. The determined deformations of the reticle scales 402 may be expressed as a translation error in the y-direction, a translation error in the x-direction, and rotation error about the z-direction.

[0064] As shown in Figure 5, the reticle 405 may be clamped to the reticle stage 400 by three reticle supports 501a, 501b, 501c. The reticle supports 501a, 501b, 501c are all substantially aligned in the z- direction. The clamping force applied by the reticle supports 501a, 501b, 501c may deform the shape of the reticle 405 and cause at least overlay error. Any misalignment of the reticle supports 501a, 501b, 501c to the z-direction may also deform the shape of the reticle 405 and cause at least overlay error. Another source of overlay error is gravity causing the parts of the reticle 405 that are not in contact with a reticle support 501a, 501b, 501c to sag. The deformation caused by the clamping may also drift, that is to say it may change over time. It is preferable for the clamping deformations to be accurately determined so that their effects may be at least partially compensated for. This may reduce the overlay error that would otherwise occur.

[0065] There are a number of problems with existing models for determining the deformations of the reticle scales and the effects of the reticle supports 501a, 501b, 501c (i.e., the clamping deformations). In known techniques, the modelling of the deformation of the reticle scales is not decoupled from the modelling of the clamping deformations. A change of hardware of the reticle stage 400 may therefore adversely affect the modelling of the hardware that is not changed. Another problem with known modelling techniques is that non-actuatable content may be created that is amplified by updates to the models. This may make the control loops noisy and unstable. The monitoring of drifts is therefore difficult and inaccurate.

[0066] Embodiments provide a new approach to modelling the deformations of the reticle scales and clamping deformations. Embodiments model the deformation contributions from the clamping effects and reticle scales based on known physical data of the reticle supports 501a, 501b, 501c and measureddata of patterned features on a substrate W (i.e., in resist data). The contributions to the measured error of features on the substrate W is separately determined for the clamping deformations and the reticle scale deformations. Embodiments provide a more accurate approach to modelling clamping deformations and reticle scale deformations than known techniques.

[0067] In embodiments, finite element method (FEM) models are used to determine the effect of the reticle supports 501a, 501b, 501c (i.e., the clamping deformations). Although the reticle supports 501a, 501b, 501c are aligned substantially vertically, manufacturing imperfections will result in each reticle support 501a, 501b, 501c having a tilt. Although the tilts of the reticle supports 501a, 501b, 501c may drift during the lifetime of the reticle supports 501a, 501b, 501c, the initial tilt of each reticle support 501a, 501b, 501c is known data supplied by the manufacturers of reticle supports 501a, 501b, 501c, and / or is directly measurable data.

[0068] To accurately model the effect of deformations of the reticle supports 501a, 501b, 501c, the FEM models of embodiments use the known initial tilt data of the reticle supports 501a, 501b, 501c as the initial clamping deformations. The tilt of each reticle support 501a, 501b, 501c may be defined as a rotation about the y-axis (ry) and a rotation about the x-axis (rx). For each reticle support 501a, 501b, 501c, embodiments generate two separate FEM models. For each reticle support 501a, 501b, 501c, a FEM model is generated of the effect of the deformation arising from the ry tilt, and a FEM model is generated of the effect of the deformation arising from the rx tilt. Accordingly, when there are three reticle supports 501a, 501b, 501c, six separate FEM models are generated of the effect of the deformations arising from the tilts of each reticle support 501a, 501b, 501c. A further FEM model is also generated of the expected effects, such as due to sag under gravity of the parts of the reticle 405 that are not touching a reticle support 501a, 501b, 501c, if none of the reticle supports 501a, 501b, 501c had any tilt.

[0069] In embodiments, an initial calibration procedure for the reticle supports 501a, 501b, 501c is performed in which a lithographic process is performed on a test substrate. Metrology techniques are used to measure a fingerprint on the test substrate. This is an in-resist measurement of all contributions to overlay error and may be determined using known techniques.

[0070] The effects of other sources of overlay than the clamping deformations and reticle scale deformations may be known and / or modelled. There are a number of known models for determining the effects of other sources of overlay than the clamping deformations and reticle scale deformations. The effects of the other sources of overlay may be removed (e.g., subtracted) from the actual measurement to generate a measured fingerprint of substantially only the clamping deformations and reticle scale deformations. This measured fingerprint is an in-resist measurement of substantially only the effect of the clamping deformations and reticle scale deformations and is used to determine models of the clamping deformations and reticle scale deformations.

[0071] Embodiments then determine the contribution of each of the FEM models to the measured fingerprint. As described earlier, there are two FEM models for each reticle support 501a, 501b, 501cand a further FEM model for modelling the expected fingerprint with perfectly vertical reticle supports 501a, 501b, 501c. When there are three reticle supports 501a, 501b and 501c, there are therefore seven FEM models. The contribution of each of the FEM models to the measured fingerprint may be determined but adjusting a scale factor of each FEM model so as to best fit the measured fingerprint.

[0072] When three reticle supports 501a, 501b, 501c are used, the effect of the clamping deformations has a known pattern in the measured fingerprint that may be referred to a bird beak pattern. As shown in Figure 5, the three reticle supports 501a, 501b, 501c are arranged with one reticle support 501a centrally positioned on one side of the reticle 405 and two reticle supports 501b, 501c located toward opposite ends on the other side of the reticle 405. There is a local change in the fingerprint at the centrally positioned reticle support 501a and this effect may be referred to as a bird beak pattern. The contribution of each of the FEM models to the measured fingerprint may be determined by adjusting the scale factor of each FEM model so as to minimize the measured bird beak pattern in the fingerprint. The minimization processes may determine a best fit of the FEM models to the measured fingerprint.

[0073] The determined scale factors and any other parameters of the FEM models provide a model of the effect of the clamping deformations. So long as the initial calibration procedure is performed soon after the reticle supports 501a, 501b, 501c are obtained, it is appropriate to use the manufacturer provided tilt data that was used in the FEM models. The tilt data of each reticle support 501a, 501b, 501c provided by manufacturers is accurate and any drift of the tilts of the reticle supports 501a, 501b, 501c occurs over a relatively long period of time.

[0074] To determine any drift in the tilts of the reticle supports 501a, 501b, 501c, embodiments include performing a second calibration procedure for the reticle supports 501a, 501b, 501c in which another lithographic process is performed on a test substrate to obtain a second measured fingerprint. The clamping deformations of the existing FEM models may then be changed to better fit the modelled effect of the clamping deformations to those in the second measured fingerprint. The determined changes to the clamping deformations may be a determination of the drifts of the tilts of the reticle supports 501a, 501b, 501c.

[0075] The FEM models, their scale factors and any other parameters may all be stored in a library that may be referenced whenever it is necessary to determine the clamping deformations and / or model the effect of clamping deformations.

[0076] The effect of deformations of the reticle scales 402 may be determined as the difference between the modelled effect of clamping deformations and the measured fingerprint. A residual fingerprint may be determined by subtracting the modelled effect of the clamping deformations, as determined by the FEM models of embodiments, from the measured fingerprint. Embodiments may then determine the parameters of a model of the effect of reticle scale deformations by fitting the expected effect of the reticle scale deformations to the residual fingerprint.

[0077] Embodiments may use any of a number of models for determining the deformations in the reticle scales 402 from the residual fingerprint. A preferred model according to embodiments is a smooth 2D polynomial. The parameters of the smooth 2D polynomial may be determined by the model determining a best fit of the effect of deformations in the reticle scales 402 to the residual fingerprint. The determined deformations of the reticle scales 402 by the model may be expressed as a translation error in the y-direction, a translation error in the x-direction, and rotation error about the z- direction.

[0078] The drift of the deformations in the reticle scales 402 may be determined by a performing a second calibration procedure in which another lithographic process is performed on a test substrate to obtain a second measured fingerprint. As described earlier, the clamping deformations of the existing FEM models may be changed to better fit the modelled effect of the clamping deformations to those in the second measured fingerprint. A second residual fingerprint may then be determined by subtracting the effect of the changed modelled clamping deformations from the second measured fingerprint. The properties of the reticle scales 402 in the model for determining the effect of deformations in the reticle scales 402 may then be changed to better fit the second residual fingerprint. The determined changes to the properties of the reticle scales 402 may be a determination of the drifts of the properties of the reticle scales 402.

[0079] Advantageously, embodiments provide separate models of the effects of clamping deformations and the deformations of the reticle scales 402.

[0080] The model of the clamping deformations may be used to determine any drift in the clamping deformations. The effect of the drift may then be at least partially compensated for so as to reduce the overlay error that would otherwise occur.

[0081] The model of the deformations of the reticle scales 402 may be used to determine any drift in the deformations of the reticle scales 402. The effect of the drift may then be at least partially compensated for so as to reduce the overlay error that would otherwise occur.

[0082] Advantageously, the models according to embodiments are based on actual physical data of reticle supports 501a, 501b, 501c and actual measured data of the effects of deformations on a substrate W. Advantages over known techniques may include one or more of: improved accuracy of the models, improved stability of the models, and a reduced computation requirement.

[0083] Embodiments include a number of modifications and variations to the above -de scribed techniques.

[0084] The generation of the model of the effect of the reticle supports 501a, 501b, 501c may be a multi-step process. For example, a first step may comprise determining a model of the effect of the clamping force and / or gravity on the reticle 405. Subsequent step(s) may tune the model parameters determined in the first step to model the effect of tilts of the reticle supports 501a, 501b, 501c and / or gravity on the reticle 405.

[0085] Embodiments may generally be used to determine the properties, and / or drifts of theproperties, of other hardware than the reticle supports 501a, 501b, 501c and reticle scales 402.

[0086] The models of the clamping deformations and / or the effects of clamping deformations are not restricted to being FEMs. Embodiments include using any type of model for the clamping deformations.

[0087] The models according to embodiments may be used on their own or in conjunction with any other technique for determining a deformation of hardware or the effect of a deformation of hardware.

[0088] Embodiments may be applied in any type of lithographic system. For example, embodiments may be applied in both EUV and DUV systems.

[0089] Although specific reference can be made in this text to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications, such as the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, LCDs, thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms “wafer” or “die” herein may be considered as synonymous with the more general terms “substrate” or “target portion,” respectively. The substrate referred to herein may be processed, before or after exposure, in for example a track unit (a tool that typically applies a layer of resist to a substrate and develops the exposed resist), a metrology unit and / or an inspection unit. Where applicable, the disclosure herein may be applied to such and other substrate processing tools. Further, the substrate may be processed more than once, for example in order to create a multi-layer IC, so that the term substrate used herein may also refer to a substrate that already contains multiple processed layers.

[0090] Although specific reference may have been made above to the use of aspects in the context of optical lithography, it will be appreciated that aspects may be used in other applications, for example imprint lithography, and where the context allows, is not limited to optical lithography. In imprint lithography, a topography in a patterning device defines the pattern created on a substrate. The topography of the patterning device may be pressed into a layer of resist supplied to the substrate whereupon the resist is cured by applying electromagnetic radiation, heat, pressure, or a combination thereof. The patterning device is moved out of the resist leaving a pattern in it after the resist is cured.

[0091] It is to be understood that the phraseology or terminology herein is for the purpose of description and not of limitation, such that the terminology or phraseology of the present specification is to be interpreted by those skilled in relevant art(s) in light of the teachings herein.

[0092] The term “substrate” as used herein describes a material onto which material layers are added. In some aspects, the substrate itself may be patterned and materials added on top of it may also be patterned, or may remain without patterning.

[0093] The following examples are illustrative, but not limiting, of the aspects of this disclosure. Other suitable modifications and adaptations of the variety of conditions and parameters normally encountered in the field, and which would be apparent to those skilled in the relevant art(s), are within the spirit and scope of the disclosure.

[0094] Although specific reference may be made in this text to the use of the apparatus and / or system in the manufacture of ICs, it should be explicitly understood that such an apparatus and / or system has many other possible applications. For example, it can be employed in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, LCD panels, thin- film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms “reticle,” “wafer,” or “die” in this text should be considered as being replaced by the more general terms “mask,” “substrate,” and “target portion,” respectively.

[0095] While specific aspects have been described above, it will be appreciated that the aspects may be practiced otherwise than as described. The description is not intended to limit the scope of the claims.

[0096] It is to be appreciated that the Detailed Description section, and not the Summary and Abstract sections, is intended to be used to interpret the claims. The Summary and Abstract sections may set forth one or more but not all example aspects as contemplated by the inventor(s), and thus, are not intended to limit the aspects and the appended claims in any way.

[0097] The aspects have been described above with the aid of functional building blocks illustrating the implementation of specified functions and relationships thereof. The boundaries of these functional building blocks have been arbitrarily defined herein for the convenience of the description. Alternate boundaries can be defined so long as the specified functions and relationships thereof are appropriately performed.

[0098] The foregoing description of the specific aspects will so fully reveal the general nature of the aspects that others can, by applying knowledge within the skill of the art, readily modify and / or adapt for various applications such specific aspects, without undue experimentation, without departing from the general concept of the aspects. Therefore, such adaptations and modifications are intended to be within the meaning and range of equivalents of the disclosed aspects, based on the teaching and guidance presented herein.

[0099] Various example embodiments include the following numbered clauses:1. A computer system configured to: generate a set of models comprising, for each of a plurality of reticle supports of a reticle, one or more models of the effects of the reticle support on a substrate illuminated with light patterned by the reticle, wherein each model is based on initial tilt data of the reticle support; and determine scale factors of each model by fitting each model to measured effects of the reticle supports on a substrate; wherein the set of models with the determined scale factors model the reticle supports and / or the effect of the reticle supports on a substrate illuminated with light patterned by the reticle.2. The computer system according to clause 1, wherein, for each reticle support, the computer system is configured to: generate a first model of the effect of the tilt of the reticle support about a first axis; and generate a second model of the effect of the tilt of the reticle support about a second axis that is orthogonal to the first axis.3. The computer system according to clause 1 or 2, wherein the initial tilt data of the reticle support is known data from measurements of the reticle support.4. The computer system according to any preceding clause, wherein the computer system is further configured to generate a model, in the set of models, that is a model of the effects of the reticle supports on a substrate illuminated with light patterned by the reticle when none of the reticle supports are tilted; and determine a scale factor of the model by fitting the model to measured effects of the reticle supports on a substrate.5. The computer system according to any preceding clause, wherein the set of models is for determining clamping deformations and / or the effect of clamping deformations.6. The computer system according to any preceding clause, wherein the models in the set of models are finite element method, FEM, models.7. The computer system according to any preceding clause, wherein the measured effects of the reticle supports on a substrate have been pre-processed to substantially remove modelled and / or known overlay effects from sources other than the reticle supports and reticle scale deformations.8. A computer system configured to generate a model for determining the deformations of reticle scales, and / or the effect of deformations of reticle scales, for an arrangement comprising a reticle supported by reticle supports, wherein the computer system is configured to: obtain a measurement of both the effects of deformations of reticle scales and the effect of reticle supports on a substrate illuminated with light patterned by the reticle; determine the effect of the reticle supports in the obtained measurement; determine the effect of deformations of reticle scales as the difference between the determined effect of the reticle supports and the obtained measurement; and determine the parameters of a model of the effect of deformations of reticle scales by fitting the parameters of the model to the determined effect of deformations of reticle scales.9. The computer system according to clause 8, wherein the computer system is configured to use a model to determine the effect of the reticle supports in the obtained measurement; wherein the used model has been generated according to the method of any of clauses 1 to 7.10. The computer system according to clause 8 or 9, wherein the model of effect of deformations of reticle scale is a smooth 2D polynomial model.11. A computer system configured to: generate, according to any of clauses 1 to 7, a model of reticle supports for a reticle, wherein the model has first scale factors determined in dependence on first measured effects of the reticle supports on a substrate; obtain second measured effects of the reticle supports on a substrate; and determine changes to the reticle supports in dependence on changes to the scale factors of the model that fit the model to the second measured effects of the reticle supports.12. A computer system configured to: generate, according to any of clauses 8 to 10, a model of the deformations of reticle scales with first parameters in dependence on a first determination of the effect of deformations of reticle scales; obtain a second determination of the effect of deformations ofreticle scales; and determine changes to the deformations of reticle scales in dependence on changes to the parameters of the model that fit the model to the second determination of the effect of deformations of reticle scales.13. A method comprising: generating a set of models comprising, for each of a plurality of reticle supports of a reticle, one or more models of the effects of the reticle support on a substrate illuminated with light patterned by the reticle, wherein each model is based on initial tilt data of the reticle support; and determining scale factors of each model by fitting each model to measured effects of the reticle supports on a substrate; wherein the set of models with the determined scale factors model the reticle supports and / or the effect of the reticle supports on a substrate illuminated with light patterned by the reticle.14. The method according to clause 13, wherein, for each reticle support, the method comprises: generating a first model of the effect of the tilt of the reticle support about a first axis; and generating a second model of the effect of the tilt of the reticle support about a second axis that is orthogonal to the first axis.15. The method according to clause 13 or 14, wherein the initial tilt data of the reticle support is known data from measurements of the reticle support.16. The method according to any of clauses 13 to 15, further comprising: generating a model, in the set of models, that is a model of the effects of the reticle supports on an illuminated substrate by the reticle when none of the reticle supports are tilted; and determining a scale factor of the model by fitting the model to measured effects of the reticle supports on a substrate.17. The method according to any of clauses 13 to 16, wherein the set of models is for determining clamping deformations and / or the effect of clamping deformations.18. The method according to any of clauses 13 to 17, wherein the models in the set of models are finite element method, FEM, models.19. The method according to any of clauses 13 to 18, wherein the measured effects of the reticle supports on a substrate have been pre-processed to substantially remove modelled and / or known overlay effects from sources other than the reticle supports and reticle scale deformations.20. A method of generating a model for determining the deformations of reticle scales, and / or the effect of deformations of reticle scales, for an arrangement comprising a reticle supported by reticle supports, wherein the method comprises: obtaining a measurement of both the effects of deformations of reticle scales and the effect of reticle supports on a substrate illuminated with light patterned by the reticle; determining the effect of the reticle supports in the obtained measurement; determining the effect of deformations of reticle scales as the difference between the determined effect of the reticle supports and the obtained measurement; and determining the parameters of a model of the effect of deformations of reticle scales by fitting the parameters of the model to the determined effect of deformations of reticle scales.21. The method according to clause 20, wherein the method comprises using a model todetermine the effect of the reticle supports in the obtained measurement; wherein the used model has been generated according to the method according of any of clauses 13 to 19.22. The method according to clause 20 or 21, wherein the model of effect of deformations of reticle scale is a smooth 2D polynomial model.23. A method comprising: generating, according to any of clauses 13 to 19, a model of reticle supports for a reticle, wherein the model has first scale factors determined in dependence on first measured effects of the reticle supports on a substrate; obtaining second measured effects of the reticle supports on a substrate; and determining changes to the reticle supports in dependence on changes to the scale factors of the model that fit the model to the second measured effects of the reticle supports.24. A method comprising: generating, according to any of clauses 20 to 22, a model of the deformations of reticle scales with first parameters in dependence on a first determination of the effect of deformations of reticle scales; obtaining a second determination of the effect of deformations of reticle scales; and determining changes to the deformations of reticle scales in dependence on changes to the parameters of the model that fit the model to the second determination of the effect of deformations of reticle scales.25. A non-transitory computer readable medium program comprising computer readable instructions configured to cause a processor to perform the method of any of clauses 13 to 24.26. A system comprising: a computer system; and a lithographic apparatus; wherein the computer system is further configured to control the operation of lithographic apparatus in dependence on a model generated according to the method of any of clauses 13 to 24.

[0100] The breadth and scope of the aspects should not be limited by any of the above -de scribed example aspects, but should be defined only in accordance with the following claims and their equivalents.

Claims

CLAIMS1. A computer system configured to: generate a set of models comprising, for each of a plurality of reticle supports of a reticle, one or more models of the effects of the reticle support on a substrate illuminated with light patterned by the reticle, wherein each model is based on initial tilt data of the reticle support; and determine scale factors of each model by fitting each model to measured effects of the reticle supports on a substrate, wherein the set of models with the determined scale factors model the reticle supports and / or the effect of the reticle supports on a substrate illuminated with light patterned by the reticle.

2. The computer system according to claim 1, wherein, for each reticle support, the computer system is configured to: generate a first model of the effect of the tilt of the reticle support about a first axis; and generate a second model of the effect of the tilt of the reticle support about a second axis that is orthogonal to the first axis.

3. The computer system according to claim 1 or 2, wherein the initial tilt data of the reticle support is known data from measurements of the reticle support.

4. The computer system according to any preceding claim, wherein the computer system is further configured to: generate a model, in the set of models, that is a model of the effects of the reticle supports on a substrate illuminated with light patterned by the reticle when none of the reticle supports are tilted; and determine a scale factor of the model by fitting the model to measured effects of the reticle supports on a substrate.

5. The computer system according to any preceding claim, wherein the models in the set of models are finite element method (FEM) models.

6. The computer system according to any preceding claim, wherein the measured effects of the reticle supports on a substrate have been pre-processed to substantially remove modelled and / or known overlay effects from sources other than the reticle supports and reticle scale deformations.

7. A computer system configured to: generate, according to any of the preceding claims, a model of reticle supports for a reticle,wherein the model has first scale factors determined in dependence on first measured effects of the reticle supports on a substrate; obtain second measured effects of the reticle supports on a substrate; and determine changes to the reticle supports in dependence on changes to the scale factors of the model that fit the model to the second measured effects of the reticle supports.

8. A method comprising: generating a set of models comprising, for each of a plurality of reticle supports of a reticle, one or more models of the effects of the reticle support on a substrate illuminated with light patterned by the reticle, wherein each model is based on initial tilt data of the reticle support; and determining scale factors of each model by fitting each model to measured effects of the reticle supports on a substrate, wherein the set of models with the determined scale factors model the reticle supports and / or the effect of the reticle supports on a substrate illuminated with light patterned by the reticle.

9. The method according to claim 8, wherein, for each reticle support, the method comprises: generating a first model of the effect of the tilt of the reticle support about a first axis; and generating a second model of the effect of the tilt of the reticle support about a second axis that is orthogonal to the first axis.

10. The method according to any of claims 8 or 9, further comprising: generating a model, in the set of models, that is a model of the effects of the reticle supports on an illuminated substrate by the reticle when none of the reticle supports are tilted; and determining a scale factor of the model by fitting the model to measured effects of the reticle supports on a substrate.

11. The method according to any of claims 8 to 10, wherein the models in the set of models are finite element method (FEM) models.

12. The method according to any of claims 8 to 11, wherein the measured effects of the reticle supports on a substrate have been pre-processed to substantially remove modelled and / or known overlay effects from sources other than the reticle supports and reticle scale deformations.

13. A method comprising: generating, according to any of claims 8 to 12, a model of reticle supports for a reticle, wherein the model has first scale factors determined in dependence on first measured effects of the reticle supports on a substrate;obtaining second measured effects of the reticle supports on a substrate; and determining changes to the reticle supports in dependence on changes to the scale factors of the model that fit the model to the second measured effects of the reticle supports.

14. A non-transitory computer readable medium program comprising computer readable instructions configured to cause a processor to perform the method of any of claims 8 to 13.

15. A system comprising: a computer system; and a lithographic apparatus, wherein the computer system is further configured to control the operation of lithographic apparatus in dependence on a model generated according to the method of any of claims 8 to 13.

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