A method and system for use in an exposure apparatus

Abstract

A method is disclosed for use in an exposure apparatus comprising: at least one substrate support configured to support a substrate; a patterning device support configured to support a patterning device; and imaging optics arranged to form an image of a patterning device supported by the patterning device support on a substrate supported by the at least one substrate support when disposed in an exposure region. The method comprises making one or more measurements that are indicative of a shape of a patterning device supported by the patterning device support. There is at least some temporal overlap between making the one or more measurements and moving a substrate support of the at least one substrate support away from the exposure region and / or moving a substrate support of the at least one substrate support towards the exposure region.

Description

A METHOD AND SYSTEM FOR USE IN AN EXPOSURE APPARATUSCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of US application 63 / 719,413 which was filed on 12 November 2024 and which is incorporated herein in its entirety by reference.FIELD

[0002] The present invention relates to a method for use in an exposure apparatus. In particular, the exposure apparatus may comprise a lithographic apparatus or lithographic tool. The present invention also relates to a corresponding system for an exposure apparatus. The present invention also relates to an exposure apparatus comprising such a system.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 defined by a patterning device (e.g., a mask) onto a layer of radiation-sensitive material (resist) provided on a substrate.

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

[0005] Any distortion (for example deviation in flatness) of the patterning device may result in errors in the patterning of the substrate by the lithographic apparatus, which is undesirable. Therefore, it may be desirable to measure any such distortions within the lithographic apparatus and to at least partially correct for these.

[0006] It may be desirable to provide new methods and / or new systems for use in an exposure apparatus (for example a lithographic apparatus) to enable such measurements of a patterning device that at least partially address one or more problems associated with prior art arrangements, whether such problems are disclosed herein or otherwise.SUMMARY

[0007] According to a first aspect of the present disclosure there is provided a method for use in an exposure apparatus comprising: at least one substrate support configured to support a substrate; a patterning device support configured to support a patterning device; and imaging optics arranged to form an image of a patterning device supported by the patterning device support on a substratesupported by the at least one substrate support when disposed in an exposure region, the method comprising: making one or more measurements that are indicative of a shape of a patterning device supported by the patterning device support; wherein there is at least some temporal overlap between making the one or more measurements and moving a substrate support of the at least one substrate support away from the exposure region and / or moving a substrate support of the at least one substrate support towards the exposure region.

[0008] The exposure apparatus may comprise a lithographic apparatus or scanner. For example, the lithographic apparatus may comprise an extreme ultraviolet (EUV) lithographic apparatus. The substrate support may comprise a wafer stage and may be suitable for supporting a lithographic substrate (for example a silicon wafer). The patterning device support may comprise a reticle stage and may be configured to support a reticle. The patterning device defines a pattern and, in use, the imaging optics are arranged to form an image of that pattern on the substrate. The image may, for example, be a diffraction limited image.

[0009] The method according to the first aspect is advantageous over existing arrangements, especially within a lithographic apparatus, as now discussed.

[0010] A typical existing method carried out within an EUV lithographic apparatus is now described. In between exposure of two wafers a wafer stage or support is moved from an exposure region (where the wafers can be exposed by forming a pattern of a reticle on each of a plurality of target regions) to a loading / unloading region. At the loading / unloading region a first wafer that has just been is exposed removed from the wafer stage and a second wafer to be exposed next is loaded onto the wafer stage. The wafer stage is then moved from the loading / unloading region back to the exposure region and the second wafer may be exposed. The wafer stage may be moved from the loading / unloading region back to the exposure region via an intermediate metrology region.

[0011] While at the intermediate metrology region a number of measurements may be made of the second wafer that may be used during its subsequent exposure. For example, once the second wafer is on the wafer stage (and typically clamped in place) the position of one or more alignment marks on the second wafer may be measured (for example relative to a reference position). This may allow a position of the second wafer to be more accurately positioned in a plane of the wafer during its exposure. In addition, a level sensor may be used to map a shape or topography of a surface of the second wafer once clamped to the wafer stage. This may allow a position of the second wafer to be more accurately positioned perpendicular to the plane of the wafer during its exposure, for example to ensure that the surface of the wafer remains as close as possible to a plane of best focus.

[0012] Once the wafer stage is back in the exposure region (for example within a field of view of imaging optics of the exposure apparatus) some alignment measurements are made in order to accurately align the reticle to the second wafer. In order to do this an image of one or more marks (for example gratings) on the reticle may be formed on a sensor on the wafer stage. The wafer stage may be moved so as to move the sensor through this image until a desired signal is measured by the sensor.For example a position which maximizes an amplitude of a signal measured by the sensor may be found. In this way the position of these marks on the reticle stage (relative to the wafer stage) can be determined. Such a metrology system may be referred to as a transmission image sensor (TIS). In order to simply align the reticle to the wafer, in principle, it may be sufficient to measure the position of only two marks on the reticle (relative to the wafer stage). However, in practice more marks are measured in order to determine a shape (or deformation) of the reticle. For example, in some EUV lithographic systems 6 marks may be measured in order to determine some information about a shape or deformation of the reticle.

[0013] The measurement of the 6 marks on the reticle takes a finite (non-zero) amount of time. Furthermore, while these measurements are being made marks are disposed in the field at reticle level and the sensor is disposed in the field of view of the imaging optics at wafer level. Therefore, the second wafer cannot be exposed during these measurements. Therefore, a disadvantage of such a known method is that the measurement of the 6 marks on the reticle directly impacts the throughput of the lithographic apparatus. Furthermore, because of this throughput penalty, only 6 marks are typically measured on the reticle and this only yields quite limited information regarding the shape or deformation of the reticle.

[0014] The method according to the first aspect of the present disclosure is advantageous since it comprises making one or more measurements that are indicative of a shape of a patterning device (for example reticle) supported by the patterning device support (reticle stage) at substantially the same time as moving a substrate support (wafer stage) of the at least one substrate support away from the exposure region and / or moving a substrate support (wafer stage) of the at least one substrate support towards the exposure region. Advantageously, this allows for a shape or deformation of the patterning device to be measured with little or no impact on the throughput of the exposure apparatus. Furthermore, it may allow for a more accurate shape or deformation of the patterning device to be measured with the same or less impact on the throughput of the exposure apparatus.

[0015] It will be appreciated that, in general, while a substrate is being exposed it is moved relative to the imaging optics of the exposure apparatus. Therefore, it will be appreciated that the exposure region comprises a range of positions of a substrate support (wafer stage). For example, a substrate support being disposed in an exposure region may mean that at least a part of the substrate support (or a substrate supported thereby) is disposed in a field of view of imaging optics of the exposure apparatus. The field of view of imaging optics of the exposure apparatus may be referred to as a wafer-level slit or illumination slit.

[0016] The exposure apparatus may comprise two substrate supports, each configured to support a substrate. The one or more measurements that are indicative of the shape of the patterning device may be made while a first one of the substrate supports is moved away from the exposure region and a second one of the substrate supports is moved towards the exposure region.

[0017] A first substrate support (supporting a substrate that has just been exposed) being moved away from the exposure region and a second substrate support (supporting a substrate to be exposed next) being moved towards the exposure region may be referred to as a chuck swap.

[0018] In some embodiments there may be complete temporal overlap between making the one or more measurements moving a substrate support away from the exposure region and / or moving a substrate support towards the exposure region. For example, the one or more measurements may only be made during a time in which a substrate support is being moved away from the exposure region and / or a substrate support is being moved towards the exposure region.

[0019] In some other embodiments there may be partial temporal overlap between making the one or more measurements moving a substrate support away from the exposure region and / or moving a substrate support towards the exposure region. In particular, the one or more measurements may be partially made during a time in which a substrate support is being moved away from the exposure region and / or a substrate support is being moved towards the exposure region and may be partially made once a substrate support has been moved to, and remains in, the exposure region.

[0020] The exposure apparatus may comprise: two substrate supports; a first supporting structure; and a second supporting structure, each of the first and second supporting structures being operable to support a substrate support. The one or more measurements that are indicative of the shape of the patterning device may be made while a first one of the substrate supports is transferred from the first supporting structure to the second supporting structure and while a second one of the substrate supports is transferred from the second supporting structure to the first supporting structure.

[0021] The first supporting structure may be referred to as an exposure-side supporting structure and the second supporting structure may be referred to as a metrology-side supporting structure. Transferring a first one of the substrate supports from the first supporting structure to the second supporting structure and transferring a second one of the substrate supports from the second supporting structure to the first supporting structure may be referred to as a chuck swap.

[0022] There may be at least some temporal overlap between making the one or more measurements that are indicative of a shape of a patterning device supported by the patterning device support and making one or more alignment measurements once a substrate support of the at least one substrate support has been moved into the exposure region.

[0023] As discussed above, once a wafer stage is back in the exposure region (for example within a field of view of imaging optics of the exposure apparatus) some alignment measurements are made in order to accurately align the reticle to the wafer. The one or more measurements that are indicative of a shape of a patterning device supported by the patterning device support can also partially temporally overlap with these alignment measurements without negatively impacting the throughput of the exposure apparatus.

[0024] The method may further comprise generating a deformation map of the patterning device supported by the patterning device support from the one or more measurements that are indicative of a shape of a patterning device supported by the patterning device support.

[0025] The one or more measurements that are indicative of a shape of a patterning device supported by the patterning device support may be indicative of a deformation of the patterning device in at least two of three orthogonal directions.

[0026] The three orthogonal directions may be represented by a set of Cartesian axes.

[0027] A least two of the one or more measurements that are indicative of a shape of a patterning device supported by the patterning device support may be made at substantially the same time.

[0028] This may be achieved, for example, by using a sensor or an array of sensors.

[0029] The one or more measurements that are indicative of a shape of a patterning device supported by the patterning device support may comprise measuring a position of a plurality of marks provided on the patterning device.

[0030] Advantageously, by measuring a position of a plurality of marks provided on the patterning device a deformation of the patterning device can be determined at each of the plurality or marks.

[0031] The method may further comprise: forming an image of a patterning device supported by the patterning device support on at least one target region of a substrate supported by a substrate support of the at least one substrate support when disposed in an exposure region. The formation of the image may be achieved using a set of exposure parameters that are dependent on the one or more measurements that are indicative of a shape of a patterning device supported by the patterning device support.

[0032] That is, the exposure of a substrate (e.g. a wafer) may be carried out in a way that is dependent on a previous measurements of the shape (deformation) of the patterning device (e.g. a reticle). In particular, the exposure apparatus can use the measured patterning device (reticle) deformation in various ways to improve overlay performance.

[0033] The set of exposure parameters may comprise any combination of: a position of the patterning device support; a position of the substrate support; and a position of one or more optical elements of the imaging optics.

[0034] That is, the measurements of the patterning device deformation may be used by existing techniques for imaging performance such as stage and lens physical adjustments based on feedback and feed-forward models. It will be appreciated that the positions of the patterning device support; the substrate support; and one or more optical elements of the imaging optics may be controlled dynamically during each scanning exposure of a target region of the substrate.

[0035] Note that the measurements of the patterning device deformation (the one or more measurements that are indicative of a shape of a patterning device supported by the patterning device support) may be combined with any current state of the art measurements to determine a high definition map of a patterning device.

[0036] The one or more measurements that are indicative of a shape of a patterning device supported by the patterning device support may be made each time a substrate support of the at least one substrate support is moved away from the exposure region and a substrate support of the at least one substrate support is moved towards the exposure region.

[0037] That is, the measurements of the patterning device (e.g. reticle) may be made once per substrate (wafer) swap.

[0038] The one or more measurements that are indicative of a shape of a patterning device supported by the patterning device support may be made using the system according to the third aspect of the present disclosure (as discussed below).

[0039] According to a second aspect of the present disclosure there is provided an exposure apparatus configured to carry out the method according to the first aspect of the present disclosure.

[0040] In general, the exposure apparatus comprises: at least one substrate support configured to support a substrate; a patterning device support configured to support a patterning device; and imaging optics arranged to form an image of a patterning device supported by the patterning device support on a substrate supported by the at least one substrate support when disposed in an exposure region. The exposure apparatus may be configured to expose a pattern formed on a patterning device (a reticle) onto a substrate (a wafer) using radiation (e.g. EUV radiation).

[0041] According to a third aspect of the present disclosure there is provided a system for an exposure apparatus, the system comprising: a patterning device support configured to support a patterning device; and a sensing apparatus comprising: projection optics operable to project radiation onto a target on a patterning device when supported by the patterning device support; and collection optics operable to receive a portion of the projected radiation scattered from the target; wherein the projection optics and / or the collection optics comprise flat optics, mini -optics, and / or integrated optics.

[0042] Advantageously, such an arrangement can be significantly smaller than an encoder and therefore more easily accommodated within an exposure apparatus. The exposure apparatus may comprise a lithographic apparatus, for example an extreme ultraviolet (EUV) lithographic apparatus. Space within the environment around the reticle and reticle stage in known EUV lithographic apparatus is limited. The provision of such a system wherein the projection optics and / or the collection optics comprises flat optics, mini -optics, and / or integrated optics is advantageous as it can be sufficiently small that it can be provided at reticle -level.

[0043] Integrated optics may alternatively be referred to as a photonic integrated circuit (PIC), an integrated optical circuit, or “on-chip” optics.

[0044] Note that since the targets (also referred to as alignment marks) are provided on the patterning device (reticle) their properties (e.g. mark types, pitches, deformations, reflection, 6 degree of freedom error, etc.) can be accurately calibrated for each exposure apparatus / patteming device. Therefore, the sensing apparatus of the system according to the third aspect of the present disclosurecan operate at single wavelength, single polarization, and can be designed for one specific mark and pitch. These relaxed requirements significantly reduce the sensor design complexity and cost.

[0045] The projection optics and the collection optics may both comprise flat optics, mini-optics, and / or integrated optics.

[0046] The system may further comprise a radiation source operable to produce radiation that is received by the projection optics.

[0047] The radiation source may comprise a laser.

[0048] The radiation source may also be provided on a microchip, for example the same microchip as the projection optics and / or the collection optics.

[0049] Alternatively, the radiation source may be separate from a microchip that the projection optics is provided on and the radiation may be directed to the projection optics via an optical fiber or the like.

[0050] The system may further comprise a sensor arranged to receive the portion of projected radiation scattered from the target from the collection optics and to generate a signal therefrom.

[0051] The sensor may comprise an array of sensing elements.

[0052] The sensor may also be provided on a microchip, for example the same microchip as the projection optics and / or the collection optics.

[0053] Alternatively, the sensor may be separate from a microchip that the collection optics is provided on and the radiation may be directed from the collection optics to the sensor via an optical fiber or the like.

[0054] The system may further comprise a processor arranged to receive the signal from the sensor and operable to determine a position of the target therefrom.

[0055] The processor may also be provided on a microchip, for example the same microchip as the projection optics and / or the collection optics and / or the sensor.

[0056] Alternatively, the processor may be separate from a microchip that the sensor is provided on. Some embodiments may comprise a plurality of sensing apparatus and may comprise a single processor operable to determine a position of a target from each of the plurality of sensing apparatus.

[0057] The processor may be operable to determine a position of a plurality of marks provided on a patterning device supported by the patterning device support.

[0058] Advantageously, by measuring a position of a plurality of marks provided on the patterning device a deformation of the patterning device can be determined at each of the plurality or marks.

[0059] The processor may be operable to determine a deformation map of a patterning device supported by the patterning device support from the positions of one or more targets on the patterning device.

[0060] The processor may be operable to determine a position of the target in at least two of three orthogonal directions.

[0061] The three orthogonal directions may be represented by a set of Cartesian axes.

[0062] The system may comprise a plurality of sensing apparatus and each sensing apparatus may comprise: projection optics operable to project radiation onto a target on a patterning device when supported by the patterning device support; and collection optics operable to receive a portion of the projected radiation scattered from the target. The projection optics and / or the collection optics of each of the plurality of sensing apparatus may comprise flat optics, mini-optics, and / or integrated optics.

[0063] For example the system may comprise an array of sensing apparatus.

[0064] The patterning device support may be movable relative to the or each sensing apparatus and / or the or each sensing apparatus may be movable relative to the patterning device support.

[0065] Advantageously, this may allow the sensing apparatus(s) to scan over the target(s) and / or may allow the sensing apparatus to measure one target and then move to, and measure, a second target and so on.

[0066] According to a fourth aspect of the present disclosure there is provided an exposure apparatus comprising the system according to the third aspect of the present disclosure.

[0067] The exposure apparatus may further comprise: at least one substrate support configured to support a substrate; and imaging optics arranged to form an image of a patterning device supported by the patterning device support on a substrate supported by the at least one substrate support when disposed in an exposure region.

[0068] The or each sensing apparatus of the system according to the third aspect of the present disclosure may be disposed on the same side of the imaging optics as the patterning device support.

[0069] For such an arrangement, the or each sensing apparatus of the system may be considered to be at reticle -level. Advantageously, this allows for measurements of one or more targets of a patterning device to be made while the at least one substrate support is not disposed in the exposure region.

[0070] Features of different aspects of the invention may be combined together, where the person skilled in the art deems it to be appropriate.BRIEF DESCRIPTION OF THE DRAWINGS

[0071] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:Figure 1 schematically depicts a lithographic system comprising a lithographic apparatus and a substrate table according to an embodiment of the present disclosure;Figure 2 schematically shows a new method for use in an exposure apparatus according to an embodiment of the present disclosure;Figure 3 is an example schematic plane view (in the x-y plane) of a wafer environment of a lithographic apparatus of the type shown in Figure 1 ; the new method shown in Figure 2 may be for use in an exposure apparatus of the type shown in Figure 1 and comprising a wafer environment of the type shown in Figure 3;Figure 4 schematically shows relate to a new system for an exposure apparatus according to an embodiment of the present disclosure;Figure 5 shows a variant of the new system shown in Figure 4;Figure 6 shows an example layout for any of the sensing apparatus of the system shown in Figures 4 and 5; also shown is a target (disposed on a patterning device) and a scanning direction of the target relative to the sensing apparatus;Figure 7 shows a schematic enlarged view of an example target for use with the layout shown in Figure 6; andFigure 8 schematically shows an arrangement showing a patterning device, which comprises a central image formation portion surrounded by a peripheral portion (defining a plurality of targets of the form shown in Figure 7); also shown are two sensing apparatus, each of which is of the form of the example layout shown in Figure 6 which, in use, can be scanned relative to the patterning device.DETAILED DESCRIPTION

[0072] 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 device MA (e.g. a mask), a projection system PS, a measurement system MS, and a substrate loading system SL. The support structure MT may be referred to as a patterning device support MT. The lithographic system further comprises two substrate tables WT1, WT2 which are each configured to support a substrate W. Each substrate table WT1, WT2 may be referred to as a substrate support WT1, WT2. The support structure MT and the two substrate tables WT1, WT2 may each comprise a clamp, for example an electrostatic clamp.

[0073] 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. In general, the illumination system IL is configured to control an angular and spatial intensity distribution at the patterning device MA.

[0074] 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 a 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 a substrate W held by one or thesubstrate tables WT1, WT2 when that substrate table is disposed in an exposure region (in Figure 1 substrate table WT2 is positioned in the exposure region). The projection system PS is configured to form an image of the patterning device (for example a diffraction limited image) on a substrate W supported by one of the two substrate tables WT1, WT2 disposed in an exposure region. 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).

[0075] 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 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.

[0076] The illumination region IR is in a field plane in which the reticle MA is disposed during a lithographic exposure. Therefore, the illumination region IR may be referred to as a reticle-level or object-level illumination region IR. The reticle-level illumination region IR may be referred to as a first illumination region IR. It will be appreciated that projection system PS forms an image IR’ of the illumination region IR in the plane of the substrate W. The image IR’ of the illumination region IR in the plane of the substrate W may be referred to as a wafer-level or image-level illumination region IR’ . As used herein, the wafer-level illumination region IR’ may also be referred to as the slit. The waferlevel illumination region IR’ may be referred to as a second illumination region IR.

[0077] The lithographic apparatus LA may be a scanning lithographic apparatus (also referred to as a scanner) and the image (of a reticle MA) may be formed during a scanning exposure. In such an exposure, the reticle MA may be moved in a scanning direction through the reticle-level illumination region IR. It will be appreciated that, as a result of this movement of the reticle MA, the image of the reticle MA will move at wafer level. Therefore, during the scanning exposure, the wafer W is also moved through the wafer-level illumination region IR’ . In particular, the wafer W is moved so that the image of reticle MA is substantially static with respect to the wafer W. The movement of the reticle MA and the substrate W are therefore synchronized. The movement (speed and direction) of the substrate W will be dependent on the movement of the reticle and the imaging performed by the projection system PS. In some embodiments, the projection system PS may form an inverted image of the reticle MA and therefore the substrate W may be moved in an opposite direction to the reticle MA. In some embodiments, the projection system PS may apply a reduction factor to the patterned EUVradiation beam B’ in the scanning direction and therefore the substrate W may move at a speed that is smaller than the speed of the reticle MA by this reduction factor.

[0078] 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.

[0079] 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 environment of the projection system PS.

[0080] The radiation source SO shown in Figure 1 is, for example, of a type which may be referred to as a laser produced plasma (LPP) source. A laser system 1, which may, for example, include a CO2 laser, is arranged to deposit energy via a laser beam 2 into a fuel, such as tin (Sn) which is provided from, e.g., a fuel emitter 3. Although tin is referred to in the following description, any suitable fuel may be used. The fuel may, for example, be in liquid form, and may, for example, be a metal or alloy. The fuel emitter 3 may comprise a nozzle configured to direct tin, e.g. in the form of droplets, along a trajectory towards a plasma formation region 4. The laser beam 2 is incident upon the tin at the plasma formation region 4. The deposition of laser energy into the tin creates a tin plasma 7 at the plasma formation region 4. Radiation, including EUV radiation, is emitted from the plasma 7 during deexcitation and recombination of electrons with ions of the plasma.

[0081] 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.

[0082] 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.

[0083] Radiation that is reflected by the collector 5 forms the EUV radiation beam B. The EUV radiation beam B is focused at the intermediate focus 6 to form an image at the intermediate focus 6 of the plasma 7 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.

[0084] 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.

[0085] The measurement system MS of the lithographic apparatus is configured to perform measurements of properties of a substrate W held on one of the two substrate tables WT1, WT2 when that substrate table is disposed in a metrology region (in Figure 1 substrate table WT1 is positioned in the metrology region). The metrology region may alternatively be referred to as an intermediate metrology region. The measurement system MS may comprises an alignment system, which is configured to measure the positions of alignment marks on the substrate with reference to alignment marks on the substrate table WT1. Additionally or alternatively, the measurement system MS may comprise a level sensor which is configured to measure the topography of the substrate W. These measured properties are used to ensure accuracy when a pattern is subsequently projected onto the substrate W.

[0086] The two substrate tables WT1, WT2 may both be disposed in a wafer environment WE. Each of the two substrate tables WT1, WT2 may be movable within the wafer environment WE, as discussed further below. In particular, the substrate tables WT1, WT2 may each be positionable in an exposure region (see, for example, substrate table WT2 in Figure 1) and a metrology region (see, for example, substrate table WT1 in Figure 1).

[0087] The lithographic apparatus LA is a dual-stage lithographic apparatus. That is, the lithographic apparatus includes two substrate tables WT1, WT2 and is configured such that a substrate W held on one of the substrate tables WT1 is measured by the measurement system MS simultaneously with a pattern being applied to a substrate W held on the other substrate table WT2. For ease of terminology, one of the substrate tables WT1 may be referred to as a first substrate table, and the other substrate table WT2 may be referred to as a second substrate table. Dual-stage lithographic apparatus advantageously provide higher throughput (i.e. exposure of substrates per hour) than single stage lithographic apparatus. Once the substrate W held on the first substrate table WT1 has been measured, and the substrate W held on the second substrate table WT2 has been exposed, the substrate table WT1 supporting the measured substrate W is moved to be beneath the projection system PS. At the same time, the exposed substrate W supported by the second substrate table WT2 is moved to the substrate loading system SL.

[0088] The substrate loading system SL includes a substrate handler (not depicted) which is configured to remove the patterned substrate W from one of the substrate tables WT1, WT2 when that substrate table is disposed in a loading / unloading region and then load a new substrate to be patterned onto that substrate table. Once the substrate has been loaded onto the substrate table, the measurement system MS may be used to measure alignment mark positions and the topology of the substrate. Simultaneously, the substrate W held on the other substrate table (which is disposed in an exposure region) is exposed by the lithographic apparatus LA.

[0089] The above method may be repeated many times in order to expose many substrates W using the lithographic apparatus LA.

[0090] Some embodiments of the present disclosure relate to a new method for use in an exposure apparatus. An example of such a new method 100 for use in an exposure apparatus is shown schematically in Figure 2. The new method 100 comprises: at least one substrate support configured to support a substrate W; a patterning device support configured to support a patterning device MA; and imaging optics arranged to form an image of a patterning device MA supported by the patterning device support on a substrate W supported by the at least one substrate support when disposed in an exposure region. The exposure apparatus may, for example, comprise a lithographic apparatus LA or scanner of the type shown in Figure 1 and describe above. The substrate support may comprise a wafer stage WT1, WT2 and may be suitable for supporting a lithographic substrate W (for example a silicon wafer). The patterning device support may comprise a reticle stage MT and may be configured to support a reticle MA.

[0091] The new method 100 comprises a step 110 of making one or more measurements that are indicative of a shape of a patterning device MA supported by the patterning device support MT. In particular, there is at least some temporal overlap between making the one or more measurements 110 and a step 120 of moving a substrate support MT1, MT2 of the at least one substrate support away from the exposure region and / or a step 122 of moving a substrate support MT1, MT2 of the at least one substrate support towards the exposure region. This is indicated schematically by these steps 110, 120, 122 being shown in a single dashed box in Figure 2.

[0092] The new method 100 is advantageous over existing arrangements, especially within a lithographic apparatus LA, as now discussed.

[0093] A typical existing method carried out within an EUV lithographic apparatus is now described. In between exposure of two wafers W, a wafer stage WT2 or support is moved from an exposure region (where the wafers W can be exposed by forming a pattern of a reticle MA on each of a plurality of target regions) to a loading / unloading region (for example in or adjacent to the substrate loading system SL). At the loading / unloading region SL a first wafer that has just been exposed is removed from the wafer stage and a second wafer to be exposed next is loaded onto the wafer stage. The wafer stage is then moved from the loading / unloading region back to the exposure region and the second wafer may be exposed (using projection system PS). The wafer stage may be moved from the loading / unloading region back to the exposure region via an intermediate metrology region (adjacent to the measurement system MS).

[0094] While at the intermediate metrology region a number of measurements may be made of the second wafer that may be used during its subsequent exposure (for example using the measurement system MS). For example, once the second wafer is on the wafer stage (and typically clamped in place) the position of one or more alignment marks on the second wafer may be measured (for example relative to a reference position) . This may allow a position of the second wafer to be more accurately positionedin a plane of the wafer during its exposure. In addition, a level sensor may be used to map a shape or topography of a surface of the second wafer once clamped to the wafer stage. This may allow a position of the second wafer to be more accurately positioned perpendicular to the plane of the wafer during its exposure, for example to ensure that the surface of the wafer remains as close as possible to a plane of best focus.

[0095] Once the wafer stage is back in the exposure region (for example within a field of view of imaging optics PS of the exposure apparatus LA) some alignment measurements are made in order to accurately align the reticle MA to the second wafer. In order to do this an image of one or more marks (for example gratings) on the reticle MA may be formed on a sensor on the wafer stage WT1, WT2. The wafer stage WT1, WT2 may be moved so as to move the sensor through this image until a desired signal is measured by the sensor. For example a position which maximizes an amplitude of a signal measured by the sensor may be found. In this way the position of these marks on the reticle stage (relative to the wafer stage WT1, WT2) can be determined. Such a metrology system may be referred to as a transmission image sensor (TIS). In order to simply align the reticle MA to the wafer W, in principle, it may be sufficient to measure the position of only two marks on the reticle MA (relative to the wafer stage WT1, WT2). However, in practice more marks are measured in order to determine a shape (or deformation) of the reticle MA. For example, in some EUV lithographic systems 6 marks may be measured in order to determine some information about a shape or deformation of the reticle MA.

[0096] The measurement of the 6 marks on the reticle MA takes a finite (non-zero) amount of time. Furthermore, while these measurements are being made marks (on the reticle MA) are disposed in the field IR at reticle level and the sensor is disposed in the field of view IR’ of the imaging optics PS at wafer level. Therefore, the second wafer cannot be exposed during these measurements. Therefore, a disadvantage of such a known method is that the measurement of the 6 marks on the reticle directly impacts the throughput of the lithographic apparatus LA. Furthermore, because of this throughput penalty, only 6 marks are typically measured on the reticle MA and this only yields quite limited information regarding the shape or deformation of the reticle MA.

[0097] The new method 100 is advantageous since it comprises a step 110 of making one or more measurements that are indicative of a shape of a patterning device MA (for example reticle) supported by the patterning device support MT (reticle stage) at substantially the same time as a step 120 of moving a substrate support WT2 (wafer stage) of the at least one substrate support away from the exposure region and / or a step 122 of moving a substrate support WT1 (wafer stage) of the at least one substrate support towards the exposure region. Advantageously, this allows for a shape or deformation of the patterning device MA to be measured with little or no impact on the throughput of the exposure apparatus LA. Furthermore, it may allow for a more accurate shape or deformation of the patterning device MA to be measured with the same or less impact on the throughput of the exposure apparatus LA.

[0098] It is particularly advantageous to be able to measure the shape or deformation of the patterning device MA (reticle) during operation of the lithographic apparatus LA because during operation of the lithographic apparatus LA the patterning device MA is heated up (by the EUV radiation beam B) and this can cause it to deform. In turn, such deformation can, if not corrected for, lead to an overlay error (i .e . an increase in a misalignment of two or more layers formed in the substrate W) . Realtime measurement of the shape or deformation of the reticle allows for such overlay errors to be corrected for. Such correction may be achieved, for example, by moving, rotating and / or deforming the optics in the projection system PS and / or by appropriate movement, or perturbations to the movement, of the patterning device support MT and / or the substrate supports WT1, WT2 during exposure of a substrate W. An increase in the accuracy with which the shape or deformation of the patterning device MA can be measured (which may be provided by the new method 100 shown schematically in Figure 2) can allow for a more accurate correction to be implemented and therefore for overlay errors to be further reduced.

[0099] It will be appreciated that, in general, while a substrate W is being exposed it is moved relative to the imaging optics PS of the exposure apparatus LA. For example, as explained above, the exposure apparatus may comprise a scanning lithographic apparatus LA (also known as a scanner). Therefore, it will be appreciated that the exposure region comprises a range of positions of a substrate support WT2 (wafer stage). For example, a substrate support WT2 being disposed in an exposure region may mean that at least a part of the substrate support WT2 (or a W substrate supported thereby) is disposed in a field of view IR’ of imaging optics PS of the exposure apparatus LA. The field of view IR’ of imaging optics PS of the exposure apparatus LA may be referred to as a wafer-level slit or illumination slit IR’ .[000100] In some embodiments of the new method 100, the exposure apparatus LA may comprise two substrate supports WT1, WT2, each configured to support a substrate W (for example as described above with reference to Figure 1). For such embodiments, the one or more measurements that are indicative of the shape of the patterning device are made (at step 110) while a first one of the substrate supports WT2 is moved away from the exposure region (at step 120) and a second one of the substrate supports is moved towards the exposure region (at step 122).[000101] A first substrate support WT2 (supporting a substrate W that has just been exposed) being moved away from the exposure region and a second substrate support WT1 (supporting a substrate W to be exposed next) being moved towards the exposure region may be referred to as a chuck swap.[000102] In some embodiments there may be complete temporal overlap between making the one or more measurements (at step 110), moving a substrate support WT2 away from the exposure region (at step 120) and / or moving a substrate support WT1 towards the exposure region (at step 122). For example, the one or more measurements may only be made during a time in which a substrate support WT2 is being moved away from the exposure region and / or a substrate support WT1 is being moved towards the exposure region.[000103] In some other embodiments there may be partial temporal overlap between making the one or more measurements (at step 110), moving a substrate support WT2 away from the exposure region (at step 120) and / or moving a substrate support WT1 towards the exposure region (at step 122). In particular, the one or more measurements may be partially made (at step 110) during a time in which a substrate support is being moved away from the exposure region (at step 120) and / or a substrate support is being moved towards the exposure region (at step 122) and may be partially made once a substrate support has been moved to, and remains in, the exposure region.[000104] Figure 3 is an example schematic plane view (in the x-y plane) of a wafer environment WE of a lithographic apparatus LA of the type shown in Figure 1 (in which the two substrate tables WT1, WT2 are disposed). The new method 100 shown in Figure 2 may be for use in an exposure apparatus of the type shown in Figure 1 and comprising a wafer environment WE of the type shown in Figure 3.[000105] In some embodiments, the exposure apparatus LA comprises: two substrate supports WT1, WT2; a first supporting structure 202; and a second supporting structure 204, each of the first and second supporting structures 202, 204 being operable to support a substrate support WT1, WT2.[000106] Each of the first and second supporting structures 202, 204 may be supported by a main support 220. Each of the first and second supporting structures 202, 204 may be movable relative to the main support 220 in a first direction (the y-direction in Figure 2), as indicated by arrows 206, 208. For example, each of the first and second supporting structures 202, 204 may be supported by a main support 220 via a rack and pinion type arrangement.[000107] Each of the two substrate supports WT1, WT2 may be movable relative to the supporting structure 202, 204 that it is supported by in a second direction (the x-direction in Figure 2), as indicated by arrows 222, 224. For example, each of the two substrate supports WT1, WT2 may be supported by one of the supporting structures 202, 204 via a rack and pinion type arrangement. The two substrate supports WT1, WT2 may be swapped over such that a first one of the substrate supports WT2 is transferred from the first supporting structure 202 to the second supporting structure 204 and a second one of the substrate supports WT1 is transferred from the second supporting structure 204 to the first supporting structure 202.[000108] For such embodiments, the one or more measurements that are indicative of the shape of the patterning device MA may be made (at step 110) while a first one of the substrate supports WT2 is transferred from the first supporting structure 202 to the second supporting structure 204 and while a second one of the substrate supports WT1 is transferred from the second supporting structure 204 to the first supporting structure 202.[000109] The first supporting structure 202 may be referred to as an exposure-side supporting structure and may be operable to move so as to position a substrate support WT1, WT2 supported thereby in an exposure region 212 of the exposure apparatus LA. Similarly, the second supporting structure 204 may be referred to as a metrology-side supporting structure and may be operable to move so as to position a substrate support WT1, WT2 supported thereby in a metrology region 214 of theexposure apparatus LA and / or a loading / unloading region 216 of the exposure apparatus LA. Transferring a first one of the substrate supports WT1, WT2 from the first supporting structure 202 to the second supporting structure 204 and transferring a second one of the substrate supports WT1, WT2 from the second supporting structure 204 to the first supporting structure 202 may be referred to as a chuck swap.[000110] In some embodiments, of the new method there may be at least some temporal overlap between making the one or more measurements that are indicative of a shape of a patterning device MA supported by the patterning device support MT (at step 110) and a step 124 of making one or more alignment measurements once a substrate support WT1, WT2 of the at least one substrate support has been moved into the exposure region 212.[000111] As discussed above, once a wafer stage WT2 is back in the exposure region 212 (for example within a field of view IR’ of imaging optics PS of the exposure apparatus LA) some alignment measurements may be made in order to accurately align the reticle MA to the wafer W. The one or more measurements that are indicative of a shape of a patterning device MA supported by the patterning device support MT (made at step 110) can also partially temporally overlap with these alignment measurements 124 without negatively impacting the throughput of the exposure apparatus LA.[000112] Some embodiments of the new method 100 may further comprise a step 112 of generating a deformation map of the patterning device MA supported by the patterning device support MT from the one or more measurements that are indicative of a shape of a patterning device MA supported by the patterning device support MT (as determined at step 110).[000113] In some embodiments of the new method 100, the one or more measurements that are indicative of a shape of a patterning device MA supported by the patterning device support MT (as determined at step 110) may be indicative of a deformation of the patterning device in at least two of three orthogonal directions. The three orthogonal directions may be represented by a set of Cartesian axes.[000114] In some embodiments of the new method 100, at least two of the one or more measurements that are indicative of a shape of a patterning device MA supported by the patterning device support MT (as determined at step 110) may be made at substantially the same time. This may be achieved, for example, by using a sensor or an array of sensors.[000115] In some embodiments of the new method 100, the one or more measurements that are indicative of a shape of a patterning device MA supported by the patterning device support MT (as determined at step 110) may comprise measuring a position of a plurality of marks provided on the patterning device MA. Advantageously, by measuring a position of a plurality of marks provided on the patterning device MA a deformation of the patterning device MA can be determined at each of the plurality or marks.[000116] Some embodiments of the new method 100 may further comprise: a step 130 of forming an image of a patterning device MA supported by the patterning device support MT on at least onetarget region of a substrate W supported by a substrate support WT1, WT2 of the at least one substrate support when disposed in an exposure region 212. For such embodiments the formation of the image (for example using imaging optics PS and at step 130), may be achieved using a set of exposure parameters that are dependent on the one or more measurements that are indicative of a shape of a patterning device MA supported by the patterning device support MT (as determined at step 110).[000117] That is, the exposure of a substrate W (e.g. a wafer and at step 130) may be carried out in a way that is dependent on a previous measurements (at step 110) of the shape (deformation) of the patterning device MA (e.g. a reticle). In particular, the exposure apparatus LA can use the measured patterning device MA (reticle) deformation in various ways to improve overlay performance.[000118] In such embodiments, the set of exposure parameters (used at step 130) may comprise any combination of: aposition of the patterning device support MT; aposition of the substrate support WT1, WT2; and a position of one or more optical elements 13, 14 of the imaging optics PS.[000119] That is, the measurements of the patterning device MA deformation (as determined at step 110) may be used by existing techniques for imaging performance within a lithographic apparatus LA such as stage MT, WT1, WT2 and lens 13, 14 physical adjustments based on feedback and feed-forward models. It will be appreciated that the positions of the patterning device support MT; the substrate support WT1, WT2; and one or more optical elements 13, 14 ofthe imaging optics PS may be controlled dynamically during each scanning exposure of a target region of the substrate W.[000120] Note that the measurements of the patterning device MA deformation (the one or more measurements that are indicative of a shape of a patterning device MA supported by the patterning device support MT, as determined at step 110) may be combined with any current state of the art measurements to determine a high definition map of a patterning device MA.[000121] In some embodiments, the steps of moving a substrate support WT2 away from the exposure region (at step 120) and / or moving a substrate support WT1 towards the exposure region (at step 122) may be repeated a plurality of times (as indicated schematically by arrow 140.[000122] In some embodiments of the new method 100 the one or more measurements that are indicative of a shape of a patterning device MA supported by the patterning device support MT (as determined at step 110) are made each time a substrate support WT2 of the at least one substrate support is moved away from the exposure region 212 and a substrate support WT1 of the at least one substrate support is moved towards the exposure region 212. That is, the measurements of the patterning device MA (e.g. reticle) may be made once per substrate W (wafer) swap.[000123] In some embodiments of the new method 100 the one or more measurements that are indicative of a shape of a patterning device MA supported by the patterning device support MT (at step 110) may be made using the new system, which is now described with reference to Figures 4 to 8.[000124] Some embodiments of the present disclosure relate to new exposure apparatus (for example lithographic apparatus LA of the type shown ion Figure 1 and described above) which is configured to carry out the new method 100 described above.[000125] In general, such a new exposure apparatus may comprise: at least one substrate support WT 1 , WT2, the or each configured to support a substrate W ; a patterning device support MT configured to support a patterning device MA; and imaging optics PS arranged to form an image of a patterning device MA supported by the patterning device support MT on a substrate W supported by the at least one substrate support WT1, WT2 when disposed in an exposure region 212. The exposure apparatus may be configured to expose a pattern formed on a patterning device MA (a reticle) onto a substrate W (a wafer) using radiation (e.g. EUV radiation).[000126] Some embodiments of the present disclosure relate to a new system 300 for an exposure apparatus LA. An example of such a new system 300 is shown schematically in Figure 4 and is now discussed.[000127] The new system 300 comprises: a patterning device support 310, MT configured to support a patterning device 312, MA and a sensing apparatus 320. The sensing apparatus 320 comprises: projection optics 322; and collection optics 324. The projection optics 322 is operable to project radiation 326 onto a target 314 on a patterning device MA when supported by the patterning device support 310. The collection optics 324 is operable to receive a portion 328 of the projected radiation scattered from the target 314.[000128] In particular, the projection optics 322 and / or the collection optics 324 comprise flat optics, mini -optics, and / or integrated optics. Advantageously, such an arrangement can be significantly smaller than an encoder and therefore more easily accommodated within an exposure apparatus. The exposure apparatus may comprise a lithographic apparatus LA, for example an extreme ultraviolet (EUV) lithographic apparatus of the type shown in Figure 1. Space within the environment RE (see Figure 1) around the reticle MA and reticle stage MT in known EUV lithographic apparatus is limited. The provision of such a system 300 wherein the projection optics 322 and / or the collection optics 324 comprises flat optics, mini -optics, and / or integrated optics is advantageous as it can be sufficiently small that it can be provided at reticle -level.[000129] Integrated optics may alternatively be referred to as a photonic integrated circuit (PIC), an integrated optical circuit, or “on-chip” optics.[000130] Note that since the targets 314 (also referred to as alignment marks) are provided on the patterning device MA (reticle) their properties (e.g. mark types, pitches, deformations, reflection, 6 degree of freedom error, etc.) can be accurately calibrated for each exposure apparatus LA and / or patterning device MA. Therefore, the sensing apparatus 320 of the system 300 shown in Figure 4 can operate at single wavelength, single polarization, and can be designed for one specific mark and pitch. These relaxed requirements significantly reduce the sensor design 320 complexity and cost.[000131] In some embodiments, the projection optics 322 and the collection optics 324 both comprise flat optics, mini -optics, and / or integrated optics.[000132] In some embodiments, the system 300 may further comprise a radiation source 330 operable to produce radiation 332 that is received by the projection optics 322. The radiation source 330 may comprise a laser.[000133] The radiation source 330 may also be provided on a microchip, for example the same microchip as the projection optics 322 and / or the collection optics 324.[000134] Alternatively, the radiation source 330 may be separate from a microchip that the projection optics 320 is provided on and the radiation 332 may be directed to the projection optics 322 via an optical fiber or the like.[000135] In some embodiments, the system 300 may further comprise a sensor 340 that is arranged to receive the portion 328 of the projected radiation scattered from the target 314 from the collection optics 324 and to generate a signal 342 therefrom.[000136] The sensor 340 may comprise an array of sensing elements.[000137] The sensor 340 may also be provided on a microchip, for example the same microchip as the projection optics 322 and / or the collection optics 324.[000138] Alternatively, the sensor 340 may be separate from a microchip that the collection optics 322 is provided on and the radiation 328 may be directed from the collection optics 324 to the sensor 340 via an optical fiber or the like.[000139] In some embodiments, the system 300 may further comprise a processor 350 arranged to receive the signal 342 from the sensor 340 and operable to determine a position of the target therefrom.[000140] The processor 350 may also be provided on a microchip, for example the same microchip as the projection optics 322 and / or the collection optics 324 and / or the sensor 340.[000141] Alternatively, the processor 350 may be separate from a microchip that the sensor 340 is provided on. Some embodiments may comprise a plurality of sensing apparatus 320 and may comprise a single processor 350 operable to determine a position of a target from each of the plurality of sensing apparatus 320.[000142] The processor 350 may be operable to determine a position of a plurality of marks 314 provided on a patterning device 312, MA supported by the patterning device support 310, MT. Advantageously, by measuring a position of a plurality of marks 314 provided on the patterning device 312, MA, a deformation of the patterning device 312, MA can be determined at each of the plurality or marks 314.[000143] Note that the system 300 may use any type of measurement scheme for determining a position of a mark 314 relative to the sensing apparatus 320. In some embodiments, the system 300 may use a self-referencing interferometer. For example, the mark 314 may be scanned relative to the sensing apparatus 320 (or vice versa) while being illuminated with the radiation 326 while the collection optics 324 receives a plurality of diffraction orders 328 generated as the radiation 326 is scattered by the target 314. The plurality of diffraction orders 328 may be combined so as to generate a signal 342that oscillates as the mark 314 is scanned relative to the sensing apparatus 320 (or vice versa) and a position may be determined from a phase of the oscillating signal 342.[000144] In some embodiments, the processor 350 may be operable to determine a deformation map of a patterning device 312, MA supported by the patterning device support 310, MT from the positions of one or more targets 314 on the patterning device 312, MA.[000145] In some embodiments, the processor 350 may be operable to determine a position of the target 314 in at least two of three orthogonal directions. The three orthogonal directions may be represented by a set of Cartesian axes.[000146] Figure 5 shows a variant of the system 300 shown in Figure 4. As shown schematically in Figure 5, in some embodiments the system 300 may comprise a plurality of sensing apparatus 320a, 320b, 320c. Each sensing apparatus 320a, 320b, 320c may be generally of the form of the sensing apparatus 320 described above with reference to Figure 4. In particular, each sensing apparatus 320a, 320b, 320c comprises: projection optics 322a, 322b, 322c operable to project radiation 326a, 326b, 326c onto a target 314a, 314b, 314c respectively on a patterning device 312, MA when supported by the patterning device support 310, MT. In addition, each sensing apparatus 320a, 320b, 320c comprises: collection optics 324a, 324b, 324c operable to receive a portion 328a, 328b, 328c of the projected radiation scattered from the target 314a, 314b, 314c.[000147] The projection optics 322a, 322b, 322c and / or the collection optics 324a, 324b, 324c of each of the plurality of sensing apparatus 320a, 320b, 320c comprises flat optics, mini-optics, and / or integrated optics.[000148] Embodiments of the system 300 of the type shown in Figure 5 may comprise an array of sensing apparatus 322a, 322b, 322c.[000149] Although in Figure 5, each of the plurality of sensing apparatus 320a, 320b, 320c is provided with a separate radiation source 330a, 330b, 330c operable to produce radiation 332a, 332b, 332c that is received by the projection optics 322a, 322b, 322c, in alternative embodiments, the two or more of the plurality of sensing apparatus 320a, 320b, 320c may share a common radiation source 330.[000150] In Figure 5, each of the plurality of sensing apparatus 320a, 320b, 320c is provided with a separate radiation source a sensor 340a, 340b, 340c that is arranged to receive the portion 328a, 328b, 328c of the projected radiation scattered from the target 314a, 314b, 314c from the collection optics 324a, 324b, 324c and to generate a signal 342a, 324b, 324c therefrom. However, in alternative embodiments, the two or more of the plurality of sensing apparatus 320a, 320b, 320c may share a common sensor 340.[000151] In Figure 5, each of the plurality of sensing apparatus 320a, 320b, 320c is provided with a separate processor 350a, 350b, 350c arranged to receive the signal 342a, 342b, 342c from the sensor 340a, 340b, 340c and operable to determine a position of the target therefrom. However, in alternative embodiments, the two or more of the plurality of sensing apparatus 320a, 320b, 320c may share a common processor 350.[000152] In some embodiments of the system 300, the patterning device support 310, MT may be movable relative to the or each sensing apparatus 320, 320a, 320b 320c and / or the or each sensing apparatus 320, 320a, 320b 320c may be movable relative to the patterning device support 310, MT. Advantageously, this may allow the sensing apparatus(s) 320, 320a, 320b, 320c to scan over the target(s) 314a, 314b, 314c and / or may allow the sensing apparatus 320, 320a, 320b, 320c to measure one target and then move to, and measure, a second target and so on.[000153] Figure 6 shows an example layout 400 for any of the sensing apparatus 320, 320a, 320b, 320c of the system 300 shown in Figures 4 and 5. Also shown in Figure 6 is atarget 410 (disposed on a patterning device 312, MA) and a scanning direction 412 of the target 410 relative to the sensing apparatus 320 (which is the y-direction in this example). The target 410 is equivalent to any one of the targets 314, 314a, 314b, 3214c shown in Figures 4 and 5.[000154] Figure 7 shows a schematic enlarged view of an example target 410 for use with the layout 400 shown in Figure 6. The target 410 comprises a first portion 410a and a second portion 410b adj acent to the first portion 410a in the scanning direction (the y-direction). The first portion 410a comprises a one -dimensional diffraction grating with lines disposed at a first angle (for example 45°) relative to the scanning direction (the y-direction). The second portion 410b comprises a one -dimensional diffraction grating with lines disposed at a second angle (for example -45°) relative to the scanning direction (the y-direction). The first and second angles are different such that the lines of the first portion 410a are disposed at an angle relative to the lines of the second portion 410b. In this example, the lines of the first portion 410a are perpendicular to the lines of the second portion 410b.[000155] Note that the target 410 shown schematically in Figure 7 is merely an example and other embodiments may employ other types of targets. Some non-limiting examples of targets that could be used include both single directional marks and bi-directional marks. For example, single directional marks may include single directional fine wafer marks (SF). Bi-directional marks may include: bidirectional fine wafer (BF) marks; box bi-directional fine wafer (BoxBF) marks; and combined bidirectional (CB) marks. For embodiments using BoxBF and / or CB marks, the marks may be scanned at an angle to allow for measurement of the x and y position of these marks simultaneously to increase throughput.[000156] The layout 400 comprises a first emitter 420a and a second emitter 420b. Together the first emitter 420a and the second emitter 420b are equivalent to the projection optics 322 shown in Figure 4. The first emitter 420a is operable to project radiation 332, 326 onto a first portion 410a of the target 410. The second emitter 420b is operable to project radiation 332, 326 onto a second portion 410b of the target 410. The radiation 332, 326 may be delivered to the first emitter 420a and the second emitter 420b via integrated optics waveguides 422a, 422b.[000157] The layout 400 further comprises a first collector 430a, a second collector 430b, a third collector 430c and a fourth collector 430d. Together the first collector 430a, the second collector 430b,the third collector 430c and the fourth collector 430d are equivalent to the collection optics 324 shown in Figure 4.[000158] The first collector 430a and the second collector 430b are arranged to receive a portion of the projected radiation scattered from the first portion 410a of the target 410. For example, the first collector 430a may be arranged to receive at least one diffraction beam (for example at least one positive order diffraction beam) scattered from the first portion 410a of the target 410 and the second collector 430b may be arranged to receive at least one different diffraction beam (for example at least one negative order diffraction beam) scattered from the first portion 410a of the target 410. The radiation received by the first collector 430a and the second collector 430b may be combined via a system of integrated waveguides 432a and may be output as radiation 434a. This output radiation 432a may be equivalent to the radiation 328 received by the sensor 340 of the system 300 shown in Figure 4 (or part thereof). This output radiation 432a may be used (for example via a sensor 340 and / or a processor 350) to determine a position of the target 410 in a first direction (for example the v-direction, see Figure 7).[000159] The third collector 430c and the fourth collector 43 Od are arranged to receive a portion of the projected radiation scattered from the second portion 410b of the target 410. For example, the third collector 430c may be arranged to receive at least one diffraction beam (for example at least one positive order diffraction beam) scattered from the second portion 410b of the target 410 and the fourth collector 43 Od may be arranged to receive at least one different diffraction beam (for example at least one negative order diffraction beam) scattered from the second portion 410b of the target 410. The radiation received by the third collector 430c and the fourth collector 43 Od may be combined via a system of integrated waveguides 432b and may be output as radiation 434b. This output radiation 432b may be equivalent to the radiation 328 received by the sensor 340 of the system 300 shown in Figure 4 (or part thereof). This output radiation 432b may be used (for example via a sensor 340 and / or a processor 350) to determine a position of the target 410 in a second direction (for example the u-direction, see Figure 7).[000160] This radiation 432a, 432b output by all of collectors 430a, 430b, 430c, 430d may be equivalent to the radiation 328 received by the sensor 340 of the system 300 shown in Figure 4. This output radiation 432a, 432b may be used (for example via a sensor 340 and / or a processor 350) to determine a position of the target 410 in two directions (for example the v-direction and the u-direction, see Figure 7).[000161] Figure 8 schematically shows an arrangement showing a patterning device 312, MA, which comprises a central image formation portion 312a surrounded by a peripheral portion 312b. The image formation portion 312a may define a pattern that is to be imaged onto substrates W within a lithographic apparatus LA. A plurality of targets or marks 410 that are generally of the form described above with reference to Figure 7 are formed in the peripheral portion 312b. In particular, two lines of such targets or marks 410 are formed in the peripheral portion 312b.[000162] Also shown in Figure 8 are two sensing apparatus 320a, 320b, each of which is of the form of the example layout 400 described above with reference to Figures 6 and 7. Each of the two sensing apparatus 320a, 320b can be scanned in a scanning direction (the y-direction in Figure 8) so as to measure a position of the targets 410 in a different one of the two lines of targets 410. The two sensing apparatus 320a, 320b can be scanned simultaneously. This may in practice be achieved by scanning the patterning device 312, MA relative to the two sensing apparatus 320a, 320b.[000163] The arrangement in Figure 8 allows for a (two-dimensional) position to be determined for a plurality of targets 410 on the patterning device 312, MA. Note that the arrangement may comprise more sensing apparatus, which may be arranged to allow for positions of two or more targets 410 in the same line of targets to be measured simultaneously. In some embodiments, this may be achieved using a plurality of sensing elements formed on the same integrated circuit.[000164] Some embodiments of the present disclosure relation to an exposure apparatus comprising a system 300 of the type shown in Figures 4 and 5 and as described above. The exposure apparatus may comprise a lithographic apparatus LA of the type shown in Figure 1. Accordingly, the exposure apparatus may comprise any combination of the features of the lithographic apparatus LA shown in Figure 1 and described above.[000165] In some embodiments, the exposure apparatus comprising the system 300 may further comprise: at least one substrate support WT1, WT2 configured to support a substrate W; and imaging optics PS arranged to form an image of a patterning device 312, MA supported by the patterning device support 310, MT on a substrate W supported by the at least one substrate support WT1, WT2 when disposed in an exposure region 212.[000166] In some embodiments, the or each sensing apparatus 320, 320a, 320b, 320c of the system 300 may be disposed on the same side of the imaging optics PS as the patterning device support 310, MT. For such an arrangement, the or each sensing apparatus 320, 320a, 320b, 320c of the system 300 may be considered to be at reticle -level. Advantageously, this allows for measurements of one or more targets of a patterning device MA to be made while the at least one substrate support WT1, WT2 is not disposed in the exposure region 212.[000167] Although specific reference may be made in this text to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications. Possible other applications include the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, liquidcrystal displays (LCDs), thin-film magnetic heads, etc.[000168] Although specific reference has been made in this text to embodiments of the invention in the context of a lithographic apparatus, embodiments of the invention may be used in other apparatus. Embodiments of the invention may form part of a mask inspection apparatus, a metrology apparatus, or any apparatus that measures or processes an object such as a wafer (or other substrate) or mask (or other patterning device). These apparatuses may be generally referred to as lithographic tools. Such alithographic tool may use vacuum conditions or ambient (non-vacuum) conditions. A substrate clamp according to an embodiment of the invention may form part of a lithographic tool.[000169] 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 for use in an exposure apparatus comprising: at least one substrate support configured to support a substrate; a patterning device support configured to support a patterning device; and imaging optics arranged to form an image of a patterning device supported by the patterning device support on a substrate supported by the at least one substrate support when disposed in an exposure region, the method comprising:making one or more measurements that are indicative of a shape of a patterning device supported by the patterning device support;wherein there is at least some temporal overlap between making the one or more measurements and moving a substrate support of the at least one substrate support away from the exposure region and / or moving a substrate support of the at least one substrate support towards the exposure region.

2. The method of claim 1 wherein the exposure apparatus comprises two substrate supports, each configured to support a substrate, and wherein the one or more measurements that are indicative of the shape of the patterning device are made while a first one of the substrate supports is moved away from the exposure region and a second one of the substrate supports is moved towards the exposure region.

3. The method of any preceding claim wherein the exposure apparatus comprises: two substrate supports; a first supporting structure; and a second supporting structure, each of the first and second supporting structures being operable to support a substrate support; andwherein the one or more measurements that are indicative of the shape of the patterning device are made while a first one of the substrate supports is transferred from the first supporting structure to the second supporting structure and while a second one of the substrate supports is transferred from the second supporting structure to the first supporting structure.

4. The method of any preceding claim wherein there is at least some temporal overlap between making the one or more measurements that are indicative of a shape of a patterning device supported by the patterning device support and making one or more alignment measurements once a substrate support of the at least one substrate support has been moved into the exposure region.

5. The method of any preceding claim further comprising generating a deformation map of the patterning device supported by the patterning device support from the one or more measurements that are indicative of a shape of a patterning device supported by the patterning device support.

6. The method of any preceding claim the one or more measurements that are indicative of a shape of a patterning device supported by the patterning device support are indicative of a deformation of the patterning device in at least two of three orthogonal directions.

7. The method of any preceding claim wherein at least two of the one or more measurements that are indicative of a shape of a patterning device supported by the patterning device support are made at substantially the same time.

8. The method of any preceding claim wherein the one or more measurements that are indicative of a shape of a patterning device supported by the patterning device support comprise measuring a position of a plurality of marks provided on the patterning device.

9. The method of any preceding claim further comprising:forming an image of a patterning device supported by the patterning device support on at least one target region of a substrate supported by a substrate support of the at least one substrate support when disposed in an exposure region;wherein the formation of the image is achieved using a set of exposure parameters that are dependent on the one or more measurements that are indicative of a shape of a patterning device supported by the patterning device support.

10. The method of claim 9 wherein the set of exposure parameters comprise any combination of: a position of the patterning device support; a position of the substrate support; and a position of one or more optical elements of the imaging optics.

11. The method of any preceding claim wherein the one or more measurements that are indicative of a shape of a patterning device supported by the patterning device support are made each time a substrate support of the at least one substrate support is moved away from the exposure region and a substrate support of the at least one substrate support is moved towards the exposure region.

12. The method of any preceding claim wherein the one or more measurements that are indicative of a shape of a patterning device supported by the patterning device support are made using the system of any one of claims 14 to 24.

13. An exposure apparatus configured to carry out the method of any preceding claim.

14. A system for an exposure apparatus, the system comprising:a patterning device support configured to support a patterning device; anda sensing apparatus comprising:projection optics operable to project radiation onto a target on a patterning device when supported by the patterning device support; andcollection optics operable to receive a portion of the projected radiation scattered from the target;wherein the projection optics and / or the collection optics comprise flat optics, mini-optics, and / or integrated optics.

15. The system of claim 14 wherein the projection optics and the collection optics both comprise flat optics, mini -optics, and / or integrated optics.

16. The system of claim 14 or claim 15 further comprising a radiation source operable to produce radiation that is received by the projection optics.

17. The system of any one of claims 14 to 16 further comprising a sensor arranged to receive the portion of projected radiation scattered from the target from the collection optics and to generate a signal therefrom.

18. The system of claim 17 further comprising a processor arranged to receive the signal from the sensor and operable to determine a position of the target therefrom.

19. The system of claim 18 wherein the processor is operable to determine a position of a plurality of marks provided on a patterning device supported by the patterning device support.

20. The system of claim 18 or claim 19 wherein the processor is operable to determine a deformation map of a patterning device supported by the patterning device support from the positions of one or more targets on the patterning device.

21. The system of any one of claims 18 to 20 wherein the processor is operable to determine a position of the target in at least two of three orthogonal directions.

22. The system of any one of claims 14 to 21 comprising a plurality of sensing apparatus wherein each sensing apparatus comprises:projection optics operable to project radiation onto a target on a patterning device when supported by the patterning device support; andcollection optics operable to receive a portion of the projected radiation scattered from the target; andwherein the projection optics and / or the collection optics comprises flat optics, mini -optics, and / or integrated optics.

23. The system of any one of claims 14 to 22 wherein the patterning device support is movable relative to the or each sensing apparatus and / or the or each sensing apparatus is movable relative to the patterning device support.

24. An exposure apparatus comprising the system of any one of claims 14 to 23.

25. The exposure apparatus of claim 24 further comprising:at least one substrate support configured to support a substrate; andimaging optics arranged to form an image of a patterning device supported by the patterning device support on a substrate supported by the at least one substrate support when disposed in an exposure region.

26. The exposure apparatus of claim 25 wherein the or each sensing apparatus of the system of any one of claims 14 to 23 is disposed on the same side of the imaging optics as the patterning device support.

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