Exposure apparatus and associated method of determining a correction for an exposure process

Abstract

There is provided a method of correcting an exposure process for exposing a pattern on a substrate. The method comprises obtaining imaging effect data describing variation of an imaging effect on said pattern on the substrate at a plurality of different substrate locations along a scan direction along which the illumination region is scanned during an exposure, said imaging effect arising from a variation of an illumination pupil shape in at least the scan direction, the illumination pupil shape describing a spatial distribution of illumination radiation in an illumination pupil plane; and determining a correction for said exposure process which improves said imaging effect.

Description

EXPOSURE APPARATUS AND ASSOCIATED METHOD OF DETERMINING A CORRECTION FOR AN EXPOSURE PROCESSCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of EP application 24192040.4 which was filed on July 31, 2024 and which is incorporated herein in its entirety by reference.FIELD

[0002] The present invention relates to An exposure apparatus (e.g., a lithographic apparatus). The present invention also relates to a corresponding method of forming a pattern on a target region of a substrate. The lithographic apparatus and method may use extreme ultraviolet (EUV) radiation.BACKGROUND

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

[0004] To project a pattern on a substrate a lithographic apparatus may use electromagnetic radiation. The wavelength of this radiation determines the minimum size of features which can be formed on the substrate. A lithographic apparatus, which uses extreme ultraviolet (EUV) radiation, having a wavelength within the range 2-20 nm, for example 6.7 nm or 13.5 nm, may be used to form smaller features on a substrate than a lithographic apparatus which uses, for example, radiation with a wavelength of 193 nm.

[0005] The angular distribution of the radiation beam in a plane of the patterning device may be referred to as an illumination mode or illumination pupil and may be chosen in dependence on the pattern being imaged to increase image contrast on the substrate. However, a known mask shadowing or mask 3D effect can result in the illumination pupil varying during an exposure scan.

[0006] It is desirable to mitigate and / or correct for this mask shadowing or mask 3D effect.SUMMARY

[0007] In a first aspect there is provided a method of determining a correction for an exposure process for exposing a pattern on a substrate, the method comprising: obtaining imaging effect data describing variation of an imaging effect on said pattern on the substrate at a plurality of different substrate locations along a scan direction along which the illumination region is scanned during an exposure, said imaging effect arising from a variation of an illumination pupil shape in at least the scan direction, the illumination pupil shape describing a spatial distribution of illumination radiation in anillumination pupil plane; and determining a correction for said exposure process which improves said imaging effect.

[0008] In a second aspect of the invention, there is provided a computer program comprising program instructions operable to perform the method of the first aspect when run on a suitable apparatus.

[0009] In a third aspect of the invention, there is provided a manufacturing apparatus configured to provide product structures to a substrate in a manufacturing process, said manufacturing apparatus comprising the computer program of the second aspect.

[0010] Further aspects, features and advantages of the invention, as well as the structure and operation of various embodiments of the invention, are described in detail below with reference to the accompanying drawings. It is noted that the invention is not limited to the specific embodiments described herein. Such embodiments are presented herein for illustrative purposes only. Additional embodiments will be apparent to persons skilled in the relevant art(s) based on the teachings contained herein.BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:Figure 1 depicts a lithographic system comprising a lithographic apparatus and a radiation source;Figure 2A schematically shows a generally circular portion of the field facet mirror device of the lithographic system shown in Figure 1, showing a central obscuration portion and two portions which receive radiation;Figure 2B shows an example shape of a field facet for a faceted field mirror device in a known EUV lithographic apparatus of the form shown in Figure 1;Figure 3 illustrates conceptually the different illumination pupil shapes along a scan direction of an illumination slit;Figure 4A illustrates conceptually the resultant pattern shift at substrate level due to the variation in illumination pupil shape along the scan direction; andFigure 4B illustrates conceptually a calibration concept for determining corrections per pupil measurement location.DETAILED DESCRIPTION

[0012] Figure 1 shows a lithographic system comprising a radiation source SO and a lithographic apparatus LA. The radiation source SO is configured to generate an EUV radiation beam B and to supply the EUV radiation beam B to the lithographic apparatus LA. The lithographic apparatus LA comprises an illumination system IL, a support structure MT configured to support a patterning deviceMA (e.g., a mask), a projection system PS and a substrate table WT configured to support a substrateW.

[0013] The illumination system IL is configured to condition the EUV radiation beam B before the EUV radiation beam B is incident upon the patterning device MA. Thereto, the illumination system IL may include a facetted field mirror device 10 and a facetted pupil mirror device 11. The faceted field mirror device 10 and faceted pupil mirror device 11 together provide the EUV radiation beam B with a desired cross-sectional shape and a desired intensity distribution. The illumination system IL may include other mirrors or devices in addition to, or instead of, the faceted field mirror device 10 and faceted pupil mirror device 11.

[0014] The lithographic apparatus LA further comprises a controller CN that is operable to control a configuration of the faceted field mirror device 10 and faceted pupil mirror device 11. To achieve this, the controller CN may be operable to send a suitable control signal si, S2 to each of the faceted pupil mirror device 11 and the faceted field mirror device 10.

[0015] After being thus conditioned, the EUV radiation beam B interacts with the patterning device MA. As a result of this interaction, a patterned EUV radiation beam B’ is generated. The projection system PS is configured to project the patterned EUV radiation beam B’ onto the substrate W. For that purpose, the projection system PS may comprise a plurality of mirrors 13, 14 which are configured to project the patterned EUV radiation beam B’ onto the substrate (or “wafer”) W held by the substrate table (or “wafer table”) WT. The projection system PS may apply a reduction factor to the patterned EUV radiation beam B’, thus forming an image with features that are smaller than corresponding features on the patterning device MA. For example, a reduction factor of 4 or 8 may be applied. Although the projection system PS is illustrated as having only two mirrors 13, 14 in Figure 1, the projection system PS may include a different number of mirrors (e.g. six or eight mirrors).

[0016] The substrate W may include previously formed patterns. Where this is the case, the lithographic apparatus LA aligns the image, formed by the patterned EUV radiation beam B’, with a pattern previously formed on the substrate W.

[0017] A relative vacuum, i.e. a small amount of gas (e.g. hydrogen) at a pressure well below atmospheric pressure, may be provided in the radiation source SO, in the illumination system IL, and / or in the projection system PS.

[0018] The radiation source SO shown in Figure 1 is, for example, of a type which may be referred to as a laser produced plasma (LPP) source. A laser system 1, which may, for example, include a CO2 laser, is arranged to deposit energy via a laser beam 2 into a fuel, such as tin (Sn) which is provided from, e.g., a fuel emitter 3. Although tin is referred to in the following description, any suitable fuel may be used. The fuel may, for example, be in liquid form, and may, for example, be a metal or alloy. The fuel emitter 3 may comprise a nozzle configured to direct tin, e.g. in the form of droplets, along a trajectory towards a plasma formation region 4. The laser beam 2 is incident upon the tin at the plasma formation region 4. The deposition of laser energy into the tin creates a tin plasma 7 at the plasmaformation region 4. Radiation, including EUV radiation, is emitted from the plasma 7 during deexcitation and recombination of electrons with ions of the plasma.

[0019] The EUV radiation from the plasma is collected and focused by a collector 5. Collector 5 comprises, for example, a near-normal incidence radiation collector 5 (sometimes referred to more generally as a normal-incidence radiation collector). The collector 5 may have a multilayer mirror structure which is arranged to reflect EUV radiation (e.g., EUV radiation having a desired wavelength such as 13.5 nm). The collector 5 may have an ellipsoidal configuration, having two focal points. A first one of the focal points may be at the plasma formation region 4, and a second one of the focal points may be at an intermediate focus 6, as discussed below.

[0020] The laser system 1 may be spatially separated from the radiation source SO. Where this is the case, the laser beam 2 may be passed from the laser system 1 to the radiation source SO with the aid of a beam delivery system (not shown) comprising, for example, suitable directing mirrors and / or a beam expander, and / or other optics. The laser system 1, the radiation source SO and the beam delivery system may together be considered to be a radiation system.

[0021] Radiation that is reflected by the collector 5 forms the EUV radiation beam B. The EUV radiation beam B is focused at intermediate focus 6 to form an image at the intermediate focus 6 of the plasma present at the plasma formation region 4. The image at the intermediate focus 6 acts as a virtual radiation source for the illumination system IL. The radiation source SO is arranged such that the intermediate focus 6 is located at or near to an opening 8 in an enclosing structure 9 of the radiation source SO.

[0022] Although Figure 1 depicts the radiation source SO as a laser produced plasma (LPP) source, any suitable source such as a discharge produced plasma (DPP) source or a free electron laser (FEL) may be used to generate EUV radiation.

[0023] The faceted field mirror device 10 and the faceted pupil mirror device 11 are arranged to provide a desired angular distribution of the radiation beam B, at the patterning device MA, as well as a desired uniformity of radiation intensity at the patterning device MA. The angular distribution of radiation at the patterning device MA may be referred to as the illumination mode. It may also be referred to as the pupil or pupil fill. The illumination system IL may be arranged to provide Kohler illumination of an illumination region IR (that the patterning device MA may be moved through during exposure of a substrate W) such that the plasma at the plasma formation region 4 is out of focus (and therefore does not influence properties of the radiation beam) at the patterning device MA and in the conjugate plane of the substrate W. As used herein, the illumination region IR may also be referred to as the illumination slit or the slit. Said illumination region may comprise the only region of the patterning device illuminated by said at least a portion of the EUV radiation beam B at any instance of time.

[0024] In lithography, the illumination of the patterning device MA is very important. In particular, it is desirable to control the angular distribution of the radiation at the illumination region IR where thepatterning device MA is exposed to radiation. This angular distribution of the radiation is conveniently described in terms of the spatial distribution of the radiation in an illumination pupil plane, which describes how a cone of light that is incident on each point on the patterning device MA is filled. As such, an illumination pupil plane may comprise any angularly resolved plane between the illumination source and the substrate (e.g., within the illumination branch). This spatial distribution of the radiation in an illumination pupil plane may be referred to as the illumination pupil profile or simply the illumination pupil. In a conventional illumination mode, the radiation uniformly fills a circular region of the illumination pupil plane centred on the optical axis such that each point on the patterning device is illuminated by a solid cone of light. In dipole illumination mode, the radiation fills two regions of the illumination pupil plane that are spaced apart from, and on opposite sides to, the optical axis. Many other illumination modes are known. In principle, an optimum illumination mode can be defined to image a given pattern under given conditions. Therefore, it is desirable to provide flexibility in the illumination mode.

[0025] The uniformity of the illumination is also important. The uniformity of illumination affects the uniformity of dose to which the target portion of the substrate W is exposed, which affects critical dimension uniformity (CDU), an important measure of the uniformity of the dimension of features formed on the substrate W. For example, it may be desirable to maintain a desired spatial intensity distribution of radiation across the illumination region IR. As used herein the spatial intensity distribution of radiation across the illumination region IR may be referred to as the slit profile.

[0026] The collector 5 is generally of the form of a concave mirror, which is arranged to collect the radiation which is emitted from the plasma formation region 4 into a solid angle subtended by the collector 5. This radiation is reflected and focused at the intermediate focus 6. As a result, within the housing, the radiation beam B is generally of the form of a converging cone of radiation, which converges at the intermediate focus 6, an outer edge of this cone being indicated in Figure 1 by two lines. Downstream of the intermediate focus 6, the radiation beam B is generally of the form of a diverging cone of radiation, which is incident on the generally circular field facet mirror device 10. However, the radiation source SO may comprise an obscuration that will block a portion of this radiation cone such that there will be some portion of the diverging cone of radiation that which will not receive radiation from the collector 5. For example, the radiation source SO may comprise a shield (not shown) which may be arranged to prevent the laser beam 2 from propagating through the opening 8 and into the lithographic apparatus LA (where it may damage optical components). This shield may be supported by the enclosing structure via a support (not shown). Together, the shield and the support form an obscuration of the radiation source SO. Therefore, as indicated schematically in Figure 2A a generally circular portion 20 of the field facet mirror device 10 may comprise a central portion 22, which coincides with the obscuration of the radiation source SO and does not receive any radiation, and two portions 24, 26 which do receive radiation. It will be appreciated that this is schematic and that the obscuration may have any shape or configuration.

[0027] A known faceted field mirror device 10 used in an EUV lithographic apparatus of the type shown in Figure 1 is now described with reference to Figures 2 A and 2B.

[0028] In known EUV lithographic apparatus, the faceted field mirror device 10 comprises a plurality of field facets. In particular, in such known EUV lithographic apparatus, the portions 24, 26 of the faceted field mirror device 10 that receive radiation from the radiation source SO are provided with a plurality of field facets. For example, the portions 24, 26 of the faceted field mirror device 10 that receive radiation from the radiation source SO may be provided with of the order of 100 field facets (for example 300 field facets). In a plane of the faceted field mirror device 10 each field facet may have a curved shape 28, as shown in Figure 2B. In general, in the plane of the faceted field mirror device 10 each field facet may have an elongate shape having a longer dimension in an x-direction and a shorter dimension in a y-direction. Each field facet comprises a mirror that may be arranged to image the intermediate focus 6 onto the faceted pupil mirror device 11. To achieve this, the field facets may, for example, be concave.

[0029] The faceted pupil mirror device 11 comprises a plurality of pupil facets. The faceted pupil mirror device 11 is arranged to project an image of each field facet of the faceted field mirror device 10 onto an illumination region IR (also referred to as the slit or illumination slit) at the patterning device MA. The illumination system IE is configured so that each field facet is imaged on the illumination region IR in an overlapping manner. Desirably, in such known lithographic apparatus, each image of a field facet fills the illumination region IR. The overlap of the images of the field facets at least partially evens out irregularities in the radiation beam B provided by the radiation source SO.

[0030] The illumination region IR may be curved or straight. In general, in a scanning lithographic apparatus LA, the illumination region IR is elongate having a longer dimension and a shorter dimension. The shorter dimension may coincide with a scanning direction or y-direction of the support structure MT and the longer dimension may coincide with a non-scanning direction or x-direction. The illumination region IR may be curved or straight. The illumination region IR is indicated in Figure 1, which shows the patterning device in cross section. The longer dimension (in the x-direction) of the illumination region IR is perpendicular to the plane of Figure 1 and the shorter dimension (in the y- direction) of the illumination region IR lies in the plane of Figure 1.

[0031] It will be appreciated that the illumination region IR may have a shape in the plane of the patterning device MA that generally corresponds to that of each of the field facets, for example being generally of the form of the shape 28 shown in Figure 2B. Note that the shape and size of the projection of each field facet onto the plane of the patterning device MA may not be exactly the same as the illumination region IR and will depend on the magnification factors applied in the x and y directions by the field facet, the pupil facets (and any other optics in upstream of the illumination region IR). The edges of the illumination region IR may be defined by two sets of patterning device masking blades (not shown), which may truncate the radiation such that radiation is not incident on the patterning device outside of the illumination region IR. Therefore, in such known lithographic apparatus, each of thefield facets may be arranged such that in the absence of such patterning device masking blades they would each overfill the illumination region IR. The illumination region IR may receive radiation from a central portion of each field facet, as indicated by the dotted line 29 inside the example shape 28 of a field facet shown in Figure 2B.

[0032] Therefore, in some existing EUV lithographic apparatus, a region surrounding the illumination region IR is also provided with EUV radiation (although the illumination region at the patterning device MA is masked from this portion of the EUV radiation, which is not used for exposing the substrate W). This region surrounding the illumination region IR may receive EUV radiation from an edge portion of each field facet mirror (which may be the portion disposed between the dotted line 29 and the edge of shape in the schematic illustration in Figure 2B). At least a part of this portion of the EUV radiation that is not used for exposing the substrate W may be incident on an energy sensor, which may be used to monitor and control a dose of radiation that is delivered to the substrate W.

[0033] It has been proposed to use a faceted field mirror device 10 and / or a faceted pupil mirror device 11 having a larger number of individually directable or movable reflective optical elements so as to provide better control over the illumination modes of the lithographic apparatus. For example, it has been proposed to use a faceted field mirror device 10 and / or a faceted pupil mirror device 11 comprising a micro-electromechanical system (MEMS) micro-mirror array.

[0034] In a typical lithographic system, a pattern on a patterning device is illuminated and the resulting diffraction is collected and projected to form an aerial image at the substrate level. The aerial image may trigger a photochemical reaction in a photoresist layer on the substrate. Thus, a pattern may be transferred from the patterning device to the substrate. An EUV lithography tool may use an EUV reflective patterning device that may consist of a multilayer stack having EUV reflection portions and EUV absorption portions to define a pattern. A typical EUV mask in high volume manufacturing scenarios may, for example, use a tantalum (Ta) absorber. In some cases, for example when a pattern is highly regular and / or dense, an EUV mask may comprise an attenuated phase shift mask.

[0035] Typical EUV systems may optimize a pupil to increase printability and for high throughput for a given pattern on a patterning device, however, typical EUV illuminators may not have flexibility to control an angular distribution from layer to layer (i.e. different pupils from patterning device to patterning device). Thus, there may not be control over a pupil in an expose field. This lack of flexibility may be intrinsic to an optical design of the illumination system.

[0036] A reflective patterning device (e.g., an EUV patterning device) may have an incident / diffraction angle dependent phase shift between diffraction orders. This may lead to miss- alignment of an aerial image at substrate level between different incidence angles. For a patterning device based on absorption and reflection (e.g., a Ta patterning device) this effect is known as mask shadowing effects, “mask 3D effects”, or” M3D fading”. M3D fading may also persist in attenuated phase shift masks. The main disadvantage of M3D fading is contrast loss. For example, an optimal pupil for a particular use case may be a y-dipole. An aerial image at substrate level may be a superposition ofa north pole of the y-dipole and a south pole of the y-dipole. Experimentally, the aerial images from the north and the south pole may be aligned at substrate level to achieve optimal contrast. In practice, owing to patterning device induced fading, the images from the north and south poles may be mis-aligned which may result in a sub-optimal contrast, or contrast loss.

[0037] Duo-monopole exposures (as described in W02020221556, incorporated herein by reference) may in some cases overcome the above-mentioned disadvantages. However, when implementing a duo-monopole exposures method, by exposing one monopole the light in the complimentary monopole may be mostly lost, which may result in a throughout penalty. Two separate exposures may be used with a lower dose, one for the north pole and one for the south pole, but this may incur a penalty of additional overhead between exposures and low doses may lead to stage limited throughput. To reproduce the same dose as the dipole exposure, the two monopole exposures should not provide the full dose of the dipole in the single poles, but only the dose that would have exposed in each pole of the full dipole. The dose tuning is achieved by decreasing the stage speed. This is stage limited throughput.

[0038] Current EUV illuminators generate a slit intensity profile which has a trapezoidal shape in the scan direction (y direction). The slopes of the trapezoidal shape cause the pupil through scan to be unstable which leads to “sunset” and “sunrise” mask shadowing or mask 3D effects and consequent y- location (scan direction location) dependent image fading. The “sunset” and “sunrise” mask shadowing results in a loss of illumination at angles corresponding to an upper part of the illumination pupil (at a first end of the slit in the y-direction) or lower part of illumination pupil (at a second end of the slit in the y-direction). This mask shadowing effect is one of the most important contributors to image contrast degradation. The physical effect at substrate level is a pattern shift of the imaged pattern having a magnitude which varies with y-location along the scan.

[0039] More generally, the y-direction dependent pattern shift variation on the substrate results from a y-direction dependent illumination pupil, which changes at substrate level along the scan due to non-ideality of the illuminator.

[0040] Although this effect is based on a physical mechanism (mask shadowing), an improper evaluation typically results in attributing the effect to other causes such as projection system aberrations (lens aberrations or aberrations of optical elements within the projection system). This means that it may presently be addressed by inducing a change of the projection system state which is not ideal, and results in less projection system correction potential for correcting true projection system aberration. This approach has limited solution space as the projection system has little correction potential. Other methods have been proposed, all based on a fixed set of settings (static correction), exploiting the current hardware, such as illumination pupil optimization and / or different illuminator improvements.

[0041] A proper attribution of the mask shadowing effect allows more appropriate and effective strategies to correct or mitigate for this effect. Currently, there is no dedicated strategy for determining the fading impact and separating this from the projection system aberration impact. The conceptsdisclosed herein also provide for a dynamic correction, as opposed to many of the aforementioned methods which are static.

[0042] Additionally, existing correction interfaces such as, for example for overlay correction, do not take into account possible fading degradation. The concepts proposed herein enables an improved fading-aware algorithm which determines the correction to correct for both overlay and contrast / fading (e.g., via a merit function which takes into account both overlay and contrast / fading).

[0043] It is therefore proposed to obtain imaging effect data describing an imaging effect resultant from variation of an illumination pupil shape along at least a scan direction within an illumination region (hereafter an illumination slit) on imaging patterns on a substrate at a plurality of different substrate locations along the scan direction; and determine a correction to improve said imaging effect, e.g., to improve the imaging effect at each of the plurality of different substrate locations along the scan direction.

[0044] The imaging effect data may be determined directly from a comparison of a first wavefront measurement which is insensitive to the illumination pupil shape and a second wavefront measurement which is sensitive to the illumination pupil shape. Alternatively, illumination pupil data may be obtained which describes the spatial distribution of the radiation in an illumination pupil plane (hereafter the illumination pupil shape) for a plurality of illumination locations within the illumination slit, said plurality of illumination locations comprising different locations in at least a scan direction of the illumination region and the imaging effect data may then be determined from the illumination pupil data (e.g., simulating the imaging effect based on the different illumination pupil shapes of the illumination pupil data.

[0045] The imaging effect may comprise any pattern shift, contrast loss / degradation or other effect due to mask shadowing and / or fading.

[0046] An important aspect of the concepts disclosed herein is that the corrections are determined (and applied) in dependence of scan direction or y-location in a deterministic, predictable way, rather than being (directly) time dependent. This enables the corrections to be calibrated in advance, offline, based on measured, modelled or otherwise known pupil shape variation through-slit. These corrections can be determined and applied per scan location without time dependence.

[0047] The method may comprise measuring or modelling the pupil shape at different y locations. From this information the impact on imaging degradation for different locations of the substrate stage and patterning device stage during the scan can be computed. The substrate stage is configured to position the substrate table WT in one or more directions with respect to the patterned illumination beam. Similarly, the patterning device stage is configured to position the support structure MT (which supports the patterning device) in one or more directions with respect to the illumination beam. The relation to time is only indirect. As a calibration strategy, the correction may be pre-calculated as any suitable exposure correction such as positioning corrections for the substrate stage and / or patterning device stage along their movement during the scan of an exposure field. Other corrections may compriseprojection system corrections (e.g., corrections for one or more projection system optical elements such as one or more lenses or mirrors within the projection system). Such projection system corrections may be co-optimized with the substrate stage and / or patterning device stage corrections.

[0048] Figure 3 is a schematic representation of the pupil variation in the y-direction in the slit 300. Within slit 300 is shown measured (or modelled) illumination pupils 310a - 310e, e.g., as may be measured using a wavefront sensor at these locations. These illumination pupils 310a - 310e substantially vary in only (or mainly) the y-direction, and as such corresponding y-locations in the slit have substantially the same illumination pupil for different x-locations.

[0049] Figure 4A is a schematic illustration of example pattern shift variation through slit 400 in the scan direction resultant from illumination pupil variation such as illustrated in Figure 3. Each arrow 410a-410e conceptually represents a respective different y-dependent pattern shift or contrast degradation magnitude.

[0050] As has been mentioned, the proposed calibration is fully deterministic, in the sense that the computed corrections (e.g., substrate stage and / or patterning device stage trajectories and optionally projection system corrections) depend only on the y-locations of the scan, and not on absolute time or other possible drifting criteria. The full scan can be decomposed in steps, for example these steps may correspond with the number of available illumination pupil measurements (e.g., or at least the number of different measurement locations in the y-direction),

[0051] Figure 4B conceptually illustrates such a calibration concept for exposure of exposure field 420. In the specific example of Figure 3, the measurement set comprises five pupil measurements [yO, yl, y2, y3, y4], one for each y-location 310a-310e. Of course, the number of y-locations measured may be more or fewer than five. The method may comprise computing the pattern shift expected at each of these y-locations (e.g., using an exposure simulation, such exposure simulations being well known). Alternatively, a difference of two wavefront measurement sets (each at corresponding different y- locations e.g., y-locations 310a-310e), with only one set having pupil dependence, may be used to directly compute the pattern shift. Based on the computed pattern shift a respective correction [C(y0), C(yl), C(y2), C(y3), C(y4)] for each of these calculated pattern shifts is determined. For example, each of these corrections may be expressed as adjustments to be applied to at least the patterning device and / or substrate stage set-points; optionally also as adjustments of the projection system setpoints. Such a correction set, properly discretized, represent the trajectories of the substrate stage and patterning device stages (and optionally projection system optical elements) over time, with time being parametrized only by the geometric coordinate y.

[0052] The correction may comprise implementing the correction via only the substrate and patterning device stages, while keeping the projection optical elements (e.g., lenses or mirrors) fixed at a pre-optimized position (e.g., based on a conventional projection system calibration). Alternatively, a co-optimized correction may be determined for the stages and projection optical elements, taking into account range and constraints for other correction strategies, such as patterning device alignment orscanning lens model corrections (typically performed per exposure or every few exposures). The scanning lens model is a projection system (lens) model which is used during exposure. It makes very fast adjustments to reduce aberration using a limited set of manipulators compared to more comprehensive models used offline. For example, such an optimization may be constrained such that acceptable correction potential (e.g., projection system correction potential and / or substrate stage and patterning device stage correction potential) is maintained, and / or weighted in favor of substrate stage and / or patterning device stage correction over projection system correction.

[0053] Patterning device alignment may comprise the alignment of the patterning device to the substrate, e.g., for a lithographic apparatus which comprises a measurement side and an exposure side. At the measurement side, the position of marks on the substrate may be measured relative to a common reference or fiducial. At the expose side, the position and shape of the patterning device is measured relative to the fiducial. In this manner, a common reference frame between measure side and expose side is established. This reference frame may be used to align the measure side substrate measurements to the required expose side exposure actuations.

[0054] A significant benefit of the deterministic (purely location dependent) nature of the proposed correction is that the corrections may be determined offline and treated as functionally independent from other runtime corrections such as the aforementioned patterning device alignment and scanning lens model corrections which have inputs that are strongly time varying and not fully predictable upfront. In this regard, it can be appreciated that the illumination pupil effect is, at least to a first approximation, not time varying and can be fully and accurately known / characterized prior to exposure of a lot (e.g., a group or number of substrates being exposed in sequence), such as at runtime or in a node feasibility phase (e.g., “an initial calibration during which exposure settings are determined for a particular use-case or “node”, such as inter alia illumination pupil shape, best focus / dose, preliminary tests on scanners and in resist).

[0055] This means that the optimization used to determine the corrections can be based on any advanced algorithm and constraints for a projection system model (e.g., a “lens model”). Purely as examples, such advanced algorithms may comprise evolutionary algorithms (such as genetic optimization), quadratic programming, Newton’s method, quadratically constrained quadratic program (QCQP), Quadratic Programming Active Set, Pareto front and / or any other algorithm which is typically too computational demanding to perform when timing is critical. This is fully compliant with usage on the lithographic apparatus without any timing constraint, as the only real time part of the proposed methods is the actuation itself; the computation of the corrections, being only y-location dependent, is performed offline or at least outside of the time-critical path. This additionally provides full freedom on co-optimization with projection system constraints, and / or taking into account other clients of other algorithms, such as those independently used to compute substrate stage and patterning device stage and projection system setpoints to correct for drifts and other transient effects.

[0056] It is possible that the concepts disclosed herein are implemented as an inline correction, e.g., computed and applied inline. In such as case, as the pupil information is available at lot start, e.g., after lot correction, the algorithm could run in parallel after lot correction and before exposure, when other actions are in the critical path.

[0057] The concepts disclosed herein may also be used to improve overlay correction and control, e.g., to make the overlay correction “fading-aware”, such that fading is taken into account. Presently, overlay corrections are determined as coefficients (sometimes referred to as k-parameters) for a polynomial which is optimized (e.g., in a best fit method) such that a correction based on the polynomial minimizes the respective measured overlay data. Each of the k-parameters typically describes a particular shape (distortion component) of the overlay (e.g., over a substrate or over a field), in a manner similar to the way Zernike coefficients are used for describing projection system aberration. As such, the k-parameters describe placement error or offset in the substrate plane. The determined coefficients may be fed back to the lithographic apparatus in the form of a sub-recipe characterized by the coefficients.

[0058] This overlay correction optimization presently does not take into account imaging fading. The determined overlay correction (realized via substrate stage and / or patterning device stage and projection system correction) optimizes for the k-factors only (e.g., sets their magnitude), and therefore can degrade other factors to which it is unaware, causing parasitic fading. It is proposed therefore that a combined correction may comprise a combination of the presently performed overlay corrections with the fading corrections determined according to the concepts disclosed herein, so as to jointly minimize overlay and compensate for the parasitic fading.

[0059] The present overlay correction is determined using a predetermined merit function with Zernike weights from the projection system model. Speed requirements do not allow the correction determination to deviate from a simple (e.g., least-square) algorithm. However, there may be a significant fading degradation encountered when enabling such overlay corrections. The potential to use the concepts described herein to pre-compute a set of fading corrections provides new strategies to improve lithographic apparatus performance. For example, to provide for an overlay correction method which takes into account fading such that the determined overlay correction has reduced fading degradation compared to present methods.

[0060] In all the examples described, the shift induced at different locations (e.g., different y- locations and optionally different x-locations) due to the illumination pupil and object of the correction (e.g., the particular use-case or pattern being imaged) may be determined from measurements already performed using a first sensor (first wavefront sensor) and second sensor (second wavefront sensor or transmission image sensor (TIS)). In particular, the first wavefront measurement performed with the first sensor uses a diffusive part of the fiducial (or the patterning device) and therefore the measurement is pupil-independent while the second wavefront measurement performed with the second sensor uses a reflective part of the fiducial (or the patterning device) and therefore is pupil-dependent (sensitive tothe pupil shape). Therefore the shift change through y (fading effect) due to the illumination pupil can be determined from a comparison of measurements using the first sensor and second sensor at different y-locations (and optionally x-locations), making it possible to directly determine the image shift (and therefore pattern shift) due to the pupil variation.

[0061] In an alternative method the shift induced at different locations may be determined via simulation. For example, the illumination pupil may be measured and / or simulated at different y- locations to obtain illumination pupil data. For example, the illumination pupil may be measured using the first sensor described above via a pinhole rather than a diffusive surface, or otherwise. The illumination pupil data comprising these pupil measurements and / or simulated pupils can be input to a physical simulation which simulates the expected image pattern shifts at the slit locations corresponding to the measurements.

[0062] Due to the offline nature of the calibration, other methods for determining the fading effect may comprise a user determination e.g., based on in-resist measurements. The pattern shift, and therefore fading correction may be determined as parametrized in terms of fading parameters e.g., fadingY, fadingX, with possible variation in the slitX. The correction may be for example linear or quadratic in y. Such an approach may be similar to that already described for overlay correction based on k factors. In that case an external method (e.g., in-resist measurements), which may include trial and error, is used to determine the magnitude of each k-parameter by which the correction is described, and the on-product result. In the present example, the magnitude of fading (e.g., the pattern shift due to fading in terms of at least the fading parameters) can be determined, for example, based on in-resist measurements. Once the magnitude of each fading parameter is determined, a desired fading correction may be selected by a user and an algorithm can use this to compute the required corrections. This concept is particularly relevant to the fading aware overlay optimization described above; here the overlay and fading corrections may be defined in an analogous manner.

[0063] The concepts disclosed can optionally be used in the x-direction, to determine an x- direction illumination pupil variation, resultant pattern shift and subsequent x-location dependent correction. As such, the method may comprise measuring illumination pupils and determining corrections for a two-dimensional (x,y) array of locations. While the illumination pupil has much less variation with x-location in the slit, there is potentially some advantage in determining corrections for this x-location variation. With the correction calculations being able to be performed offline, and the required measurements already being performed, the additional inline time and overhead is negligible.

[0064] The calibration may be performed for each use-case (e.g., for each particular pattern / layer being exposed).

[0065] The concepts proposed therefore provide an opportunity to dynamically counteract contrast loss, in addition to what can be corrected statically with other methods.

[0066] The concepts proposed herein can also be combined with the illumination optimization (e.g., source-projection system optimization). Illumination optimization may be used to optimize theillumination pupil (and projection system) for a particular use-case. In such a method, the illumination pupil optimization may take into account the new correction capability provided by the concepts disclosed herein. As such, and purely for example, an optimized illumination pupil may be asymmetric or more asymmetric than is typical to take into account the correction capability concepts disclosed herein.

[0067] It is often desirable to be able to computationally determine how a patterning process would produce a desired pattern on a substrate. Computational determination may comprise simulation and / or modeling, for example. Models and / or simulations may be provided for one or more parts of the manufacturing process. For example, it is desirable to be able to simulate the lithography process of transferring the patterning device pattern onto a resist layer of a substrate as well as the yielded pattern in that resist layer after development of the resist, simulate metrology operations such as the determination of overlay, and / or perform other simulations. The objective of a simulation may be to accurately predict, for example, metrology metrics (e.g., overlay, a critical dimension, a reconstruction of a three dimensional profile of features of a substrate, a dose or focus of a lithography apparatus at a moment when the features of the substrate were printed with the lithography apparatus, etc.), manufacturing process parameters (e.g., edge placements, aerial image intensity slopes, sub resolution assist features (SRAF), etc.), and / or other information which can then be used to determine whether an intended or target design has been achieved. The intended design is generally defined as a pre-optical proximity correction design layout which can be provided in a standardized digital file format such as GDSII, OASIS or another file format.

[0068] In some embodiments, an optimization process of a system may be represented as a cost function. The optimization process may comprise finding a set of parameters (design variables, process variables, inspection operation variables, etc.) of the system that minimizes the cost function. The term “design variables” as used herein comprises a set of parameters of a lithographic projection apparatus or a lithographic process, for example, parameters a user of the lithographic projection apparatus can adjust, or image characteristics a user can adjust by adjusting those parameters. It should be appreciated that any characteristics of a lithographic projection process, including those of the source, the patterning device, the projection optics, or resist characteristics can be among the design variables in the optimization. The cost function can have any suitable form depending on the goal of the optimization. For example, the cost function can be weighted root mean square (RMS) of deviations of certain characteristics (evaluation points) of the system with respect to the intended values (e.g., ideal values) of these characteristics. The cost function may often be a non-linear function of the design variables. Then standard optimization techniques may be used to minimize the cost function.

[0069] Although specific reference may be made in this text to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications. Possible other applications include the manufacture of integrated opticalsystems, guidance and detection patterns for magnetic domain memories, flat-panel displays, liquidcrystal displays (LCDs), thin-film magnetic heads, etc.

[0070] Although specific reference may be made in this text to embodiments of the invention in the context of a lithographic apparatus, embodiments of the invention may be used in other apparatus. Embodiments of the invention may form part of a mask inspection apparatus, a metrology apparatus, or any apparatus that measures or processes an object such as a wafer (or other substrate) or mask (or other patterning device). These apparatus may be generally referred to as lithographic tools. Such a lithographic tool may use vacuum conditions or ambient (non- vacuum) conditions.

[0071] Where the context allows, embodiments of the invention may be implemented in hardware, firmware, software, or any combination thereof. Embodiments of the invention may also be implemented as instructions stored on a machine-readable medium, which may be read and executed by one or more processors. A machine-readable medium may include any mechanism for storing or transmitting information in a form readable by a machine (e.g., a computing device). For example, a machine-readable medium may include read only memory (ROM); random access memory (RAM); magnetic storage media; optical storage media; flash memory devices; electrical, optical, acoustical or other forms of propagated signals (e.g. carrier waves, infrared signals, digital signals, etc.), and others. Further, firmware, software, routines, instructions may be described herein as performing certain actions. However, it should be appreciated that such descriptions are merely for convenience and that such actions in fact result from computing devices, processors, controllers, or other devices executing the firmware, software, routines, instructions, etc. and in doing that may cause actuators or other devices to interact with the physical world.

[0072] While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. The descriptions above are intended to be illustrative, not limiting. Thus it will be apparent to one skilled in the art that modifications may be made to the invention as described without departing from the scope of the claims set out below.

Claims

CLAIMS1. A method of determining a correction for an exposure process for exposing a pattern on a substrate, the method comprising: obtaining imaging effect data describing variation of an imaging effect on said pattern on the substrate at a plurality of different substrate locations along a scan direction along which an illumination region is scanned during an exposure, said imaging effect arising from a variation of an illumination pupil shape in at least the scan direction, the illumination pupil shape describing a spatial distribution of illumination radiation in an illumination pupil plane; and determining a correction for said exposure process which improves said imaging effect.

2. A method as claimed in claim 1, wherein said step of determining a correction comprises determining a respective correction for each of said plurality of different substrate locations along the scan direction.

3. A method as claimed in claim 1 or 2, wherein the imaging effect comprises an effect of fading resulting from said variation in the illumination pupil shape.

4. A method as claimed in any preceding claim, wherein the imaging effect comprises a pattern shift of said pattern and / or a degradation in contrast of said pattern.

5. A method as claimed in any preceding claim, wherein said correction comprises determining at least a position correction for one or both of a substrate stage and a patterning device stage.

6. A method as claimed in any preceding claim, wherein said correction comprises determining at least a projection system correction for one or more projection system optic elements used to expose the pattern on the substrate.

7. A method as claimed in any preceding claim, wherein said correction is a co-optimized correction for substrate stage, patterning device stage and projection system optic elements.

8. A method as claimed in any preceding claim, wherein at least said steps of obtaining illumination pupil data, determining an imaging effect and determining a correction are performed offline, prior to performing said exposure process.

9. A method as claimed in any preceding claim, wherein said corrections are dependent directly on location in at least the scan direction with no direct time dependency.

10. A method as claimed in claim 9, comprising determining and storing said correction as a function of said location in at least the scan direction prior to performing said exposure process; and applying said correction as a function of said location in at least the scan direction during said exposure process.

11. A method as claimed in any preceding claim, wherein said imaging effect data is determined by: obtaining illumination pupil data comprising a respective illumination pupil shape for a plurality of illumination locations within the illumination region, said plurality of illumination locations comprising different locations in at least the scan direction; and simulating an exposure based on said illumination pupil data to obtain said imaging effect data.

12. A method as claimed in any of claims 1 to 10, wherein said imaging effect data is determined by performing: a first wavefront measurement which is insensitive to the illumination pupil shape; a second wavefront measurement which is sensitive to the illumination pupil shape; and comparing the first wavefront measurement and second wavefront measurement to determine said imaging effect data.

13. A method as claimed in claim 12, wherein said first wavefront measurement is performed using a first sensor and the second wavefront measurement is performed using a second sensor.

14. A method as claimed in any of claims 1 to 10, wherein said imaging effect data is determined by performing in-resist measurements to determine said imaging effect in terms of one or more fading parameters.

15. A method as claimed in any preceding claim, wherein said imaging effect data further describes said variation of an imaging effect in a slit direction of the illumination region perpendicular to said scan direction; and wherein said step of determining a correction is performed for each of said different locations in the slit direction.

16. A method as claimed in any preceding claim, comprising obtaining overlay data relating to overlay on the substrate; and determining said correction to co-optimize said overlay and said imaging effect.

17. A method as claimed in any preceding claim, comprising optimizing said illumination pupil so as to take into account the correction capability resultant from said determined correction.

18. A method as claimed in any preceding claim, comprising performing an exposure on said substrate using said determined correction.

19. A computer program comprising program instructions operable to perform the method of any preceding claim when run on a suitable apparatus.

20. A non- transient computer program carrier comprising the computer program of claim 19.

21. A manufacturing apparatus as claimed in any preceding claim, wherein the manufacturing apparatus comprises an exposure apparatus having: a substrate stage for holding a substrate; a patterning device stage for holding a patterning device; a computer program carrier comprising the computer program of claim 19; and a processor operable to control an exposure process using said correction.

22. A manufacturing apparatus as claimed in claim 21, further comprising: a fading correction interface being configured to receive a fading correction request describing a desired fading correction to be imposed in terms of coefficients for one or more fading parameters; wherein said processor is operable to determine a fading correction from said a fading correction request.

23. An integrated circuit manufactured by a manufacturing apparatus as claimed in claim 21 or

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