WO2020239430A1

WO2020239430A1 - Apparatus and method for providing a substrate with radiation

Info

Publication number: WO2020239430A1
Application number: PCT/EP2020/063170
Authority: WO
Inventors: Marcus Adrianus Van De Kerkhof; Martijn GRAM; Hendricus Johannes Maria Meijer; Johannes Arie VAN DEN BROEK; Christian Wagner
Original assignee: Asml Netherlands B.V.
Priority date: 2019-05-29
Filing date: 2020-05-12
Publication date: 2020-12-03
Also published as: KR20220015396A; CN113874788A; NL2025559A

Abstract

A lithographic apparatus operable to provide a plurality of target areas of a substrate with a nominal dose of radiation and an associate method are described. The nominal dose of radiation is provided to each of the plurality of target areas of the substrate across a number of exposures. Providing the nominal dose of radiation to each target area involves exposing a first portion of the target area to radiation while a second portion of the target area is not exposed to radiation (the shape and configuration of the first and second portions being dependent on a pattern being imaged). The number of exposures for each target area is determined in dependence on data describing mechanical behaviour of that target area of the substrate in response to receipt of a thermal load. The thermal load may correspond to a thermal load received when the first portion of the target area is provided with the nominal dose of radiation.

Description

Apparatus and method for providing a substrate with radiation

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of EP application 19177154.2 which was filed on May 29, 2019 and EP application 19208574.4 which was filed on November 12, 2019 which are incorporated herein in its entirety by reference.

FIELD

[0002] The present invention relates to lithography. Particularly, the present invention relates to methods and associated apparatuses for exposing a substrate having multiple target areas.

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 from 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] Integrated circuits may be formed from a plurality of successively applied layers, each layer being formed using lithography. It is desirable to ensure that all of these layers are well aligned. Misalignment of a pattern that is projected on to the substrate relative to a pattern exposed to layers of the substrate that have already been formed (or are yet to be formed may be referred to as“overlay”.

[0006] Overlay generally has a detrimental effect on lithographic performance. An increase in overlay may correspond to a decrease in quality, or failure, of integrated circuitry produced using the lithographic apparatus. It is generally desirable to reduce overlay.

[0007] Embodiments of the present invention relate to new methods and associated apparatuses for exposing a substrate having multiple target areas.

SUMMARY

[0008] According to a first aspect of the invention there is provided a lithographic apparatus. The lithographic apparatus may be operable to provide a plurality of target areas of a substrate with a nominal dose of radiation. The nominal dose of radiation may be provided to each of the plurality of target areas of the substrate across a number of exposures. The number of exposures for each target area may be determined in dependence on data describing mechanical behaviour of that target area of the substrate in response to receipt of a thermal load.

[0009] The lithographic apparatus may be operable to project a patterned beam of radiation via projection optics onto a substrate. The patterned beam of radiation may have a pattern which is determined by a patterning device, such as a mask or reticle. The patterning device may reflect portions of radiation so as to create the patterned beam of radiation. The substrate may be supported by a substrate support. The radiation may be extreme ultraviolet (EUV) radiation. The substrate may be a photoresist-coated silicon wafer. Following exposure to radiation, the substrate may be processed. An exposed and processed substrate may be useful in producing, for example, integrated circuitry.

[00010] The substrate may be clamped to the substrate support, which may comprise an electrostatic clamp. The component of the substrate support that physically interacts with the substrate may be a wafer table. The wafer table may contact the wafer via a plurality of burls, which may comprise projections from a base plate. A surface of the substrate opposite the surface to be exposed to the radiation (referred to as the backside of the substrate) may contact the plurality of burls and may be clamped to the burls using electrodes in a known manner. One of the reasons for supporting the substrate on burls in this manner may be that it may reduce the total contact area with the backside of the substrate and, as a result, foreign particles (e.g., at the backside of the substrate) may be less likely to distort the shape of the wafer.

[00011] The lithographic apparatus may be operable to provide radiation to one or more target areas of the substrate. Particularly, the lithographic apparatus may be operable to provide radiation to a first portion of one or more target areas of the substrate. Upon receipt of a nominal dose of radiation, the photoresist may undergo some change of state. As used herein,“dose” refers to energy received per unit area. Generally, the nominal dose may be dependent on the type of photoresist used. It may be desirable to use a photoresist that undergoes a change of state upon receipt of a relatively high dose of radiation. Advantageously, this may increase the number of photons over which the dose is spread and may therefore result in a relatively low level of shot noise in a pattern that is transferred to an exposed substrate.

[00012] It will be appreciated that“providing a target area with an exposure”, as used herein, may refer to exposing a target area to radiation once. Likewise,“providing a target area with multiple exposures” may refer to exposing a target area to radiation multiple times. It will be further appreciated that, unless stated to the contrary, providing a target area with radiation may refer to providing a first portion of the target area with radiation. For example, providing a target area with a nominal dose of radiation may refer to providing a first portion of the target area with the nominal dose of radiation. A second portion of the target area may not be provided with radiation.

[00013] Exposing the substrate to radiation may transfer thermal energy to the substrate. This local heating of the substrate may cause internal stresses and may result in thermal deformation of the substrate. Deformation of the substrate may result in misalignment of a pattern that is projected on to the substrate relative to a pattern exposed to layers of the substrate that have already been formed (or are yet to be formed). Such misalignment of successively applied patterns may be referred to as “overlay”. By convention, the term“overlay” may refer to the extent to which successive layers in an integrated circuit chip (e.g., formed by a lithographic process as described above) are laterally displaced relative to one another. A decrease in overlay may generally correspond to an increase in quality of integrated circuitry produced using the lithographic apparatus. This may be described as an increase in lithographic performance. A decrease in overlay may therefore be desirable.

[00014] Overlay due to deformation of the substrate may be characterised and corrected for, to a limited extent. For example, computer models may be operable to predict deformation of the substrate (due, for example, to applied heat loads). Such predictions may, in turn, be used to modify an arrangement of projection optics of the lithographic apparatus and/or an arrangement of the substrate support in order to reduce any contribution to overlay due to such deformation of the substrate. However, under certain circumstances, the deformation may be sufficiently large that the substrate may slip relative to the burls of the substrate support. Slipping of the substrate may result in unpredictable (and hence uncorrectable) overlay in a portion of the substrate (e.g., a target area) which slipped. Slipping of the substrate may occur above a threshold level of deformation of the substrate. Slipping of the substrate may have a detrimental impact on overlay (i.e., an increase in overlay).

[00015] The lithographic apparatus according to the first aspect of the invention is advantageous since it may allow for the nominal dose of radiation to be provided to the at least a target area of the substrate either in a single exposure or across a plurality of exposures. The number of exposures over which the dose is delivered may be determined in dependence on data describing mechanical behaviour of the at least a target area of the substrate in response to receipt of a thermal load. This thermal load may correspond to a thermal load received when a certain fraction of the target area (a“first portion” of the target area, which may be defined by the pattern being imaged) is provided with the nominal dose of radiation. A“second portion” of the target area may not be provided with radiation.

[00016] The data describing mechanical behaviour of a target area of the substrate in response to receipt of a thermal load may describe whether or not a target area will deform to such an extent that said target area will slip relative to the substrate support after having received said thermal load. The data describing mechanical behaviour of a target area of the substrate in response to receipt of a thermal load may include information relating to how the target area on the substrate may deform in response to receipt of the thermal load. In turn, the information relating to how the target area on the substrate may deform in response to receipt of a thermal load may be dependent on the how the target area is supported. Such support of the target area may be dependent on the local forces which act on the target area. Such support of the target area may include the amount of support provided by a substrate support (for example, the number and configuration of burls supporting the target area, a clamping force which clamps the substrate to the burls, and or a coefficient of friction between the burls and the substrate). In addition, such information relating to the support of the target area may include the amount of support provided by surrounding areas of the substrate (and, in turn, how these surrounding areas of the substrate may be supported by a substrate support). The data describing mechanical behaviour of a target area of the substrate in response to receipt of a thermal load may include information on internal forces within the substrate. The data describing mechanical behaviour of a target area of the substrate in response to receipt of a thermal load may include information relating to the position of the target area on the substrate. For example, having received a fixed dose of radiation, some target areas of the substrate may undergo mechanical deformation to a greater extent than other target areas of the substrate. Having received a fixed dose of radiation, some target areas of the substrate may undergo mechanical deformation to such an extent that they slip relative to the substrate support, whereas other target areas of the substrate may undergo mechanical deformation to a lesser extent, such that these target areas do not slip relative to the substrate support. There may therefore exist a threshold mechanical deformation that determines whether or not a target area will slip relative to the substrate support after having received a fixed dose of radiation. Furthermore, in general, this threshold may be different for different target areas of the substrate.

[00017] The lithographic apparatus according to the first aspect of the invention may be arranged so that the nominal dose of radiation is provided to at least one of the plurality of target areas across a plurality of exposures.

[00018] With such an arrangement at least one target area may be exposed to the nominal dose using multiple exposures (which may also be referred to as“split exposures”) in the lithographic apparatus. One or more target areas of the substrate may be provided with radiation in several separate exposures. A dose of radiation that is required to be received by a target area may be described as a nominal dose. One or more target areas of the substrate may have received the nominal dose of radiation after the multiple exposures have occurred. The inventors have realised that thermal deformation of the substrate may be reduced without reducing radiation dose. Therefore, advantageously, providing the nominal dose of radiation to one or more target areas using multiple exposures may reduce or eliminate a chance of the one or more target areas slipping relative to the substrate support. Thus, providing the nominal dose of radiation to one or more target areas using multiple exposures may reduce overlay. This may improve lithographic performance.

[00019] The plurality of exposures across which the nominal dose may be provided to the at least one of the plurality of target areas of the substrate may be separated by a time delay.

[00020] Multiple exposures of radiation may be provided to a target area with a time delay between each exposure of the multiple exposures. The time delay between exposures may allow a thermal load (per exposure) to be absorbed by a suitable cooling mechanism. For example, the heat load per exposure may be dissipated through, for example, portions of the substrate outside the target area, components which the substrate contacts, and/or an environment in which the substrate is disposed. The time delay between exposures may be described as a“recovery time”. Advantageously, the recovery time may reduce a maximum temperature of the target area relative to no recovery time being provided. This may reduce thermal deformation of the target area. This may reduce or eliminate a chance of the target area slipping relative to the substrate support. This may reduce overlay. This may improve lithographic performance.

[00021] The lithographic apparatus according to the first aspect of the invention may be operable to expose at least one other target area of the substrate during the time delay.

[00022] During the time delay, a second target area may be provided with radiation. In general, exposures of radiation to one or more target areas may be provided during time delays which follow exposures of radiation to other target areas. In this way, multiple target areas on the substrate may receive the nominal dose of radiation in a shorter time than if no exposures were provided during time delays. Therefore, advantageously, some or all target areas on the substrate may receive the nominal dose of radiation in a shorter time than if no exposures were provided during time delays. This may be described as an increase in lithographic throughput (relative to providing multiple exposures, as described above, but not providing exposures during time delays).

[00023] The plurality of target areas to which the nominal dose of radiation may be provided across a plurality of exposures may be disposed at a peripheral portion of the substrate.

[00024] Target areas of the substrate which are disposed at a peripheral portion of the substrate may correspond to target areas which have an increased chance of slipping relative to the substrate support after receiving the nominal dose of radiation in a single exposure. Target areas of the substrate which are disposed at a peripheral portion of the substrate may be provided the nominal dose of radiation across multiple exposures.

[00025] The at least one of the plurality of target areas of the substrate to which the nominal dose of radiation may be provided across a plurality of exposures may receive a substantially equal radiation dose during each exposure.

[00026] For example, if the nominal dose is delivered over n exposures (n being an integer), during each exposure, the target area may receive a dose given by 1/n times the nominal dose.

[00027] Providing an equal radiation dose during each exposure of multiple exposures of a target area may minimise a maximum thermal load received by the target area across the multiple exposures. Advantageously, this may reduce thermal deformation of the target area. This may reduce or eliminate a chance of the target area slipping relative to the substrate support. This may reduce overlay. This may improve lithographic performance.

[00028] The lithographic apparatus according to the first aspect of the invention may be arranged so that the nominal dose of radiation may be provided to at least one of the plurality of target areas of the substrate in a single exposure.

[00029] It will be appreciated that different target areas of the substrate may be provided with different numbers of exposures. This may enable multiple exposures to be provided selectively to individual target areas on the substrate. Advantageously, this may allow the risk of slippage to be mitigated whilst still maintaining an optimal throughput for the lithographic apparatus.

[00030] This may be described as adapting a method of providing multiple exposures to the substrate. For example, one or more target areas may be provided with radiation in a single exposure. One or more target areas may be provided with multiple exposures. One or more target areas may be provided with radiation across two exposures. One or more target areas may be provided with radiation across more than two exposures (e.g., three exposures).

[00031] The nominal dose of radiation may be provided to all target areas of the substrate which are not used for manufacturing integrated circuitry in a single exposure.

[00032] A substrate may comprise target areas which are not used for producing integrated circuitry. It may provide little or no benefit to reduce overlay (e.g., by using multiple exposures as described above) in these target areas. Therefore, it may be desirable to expose target areas which are not used for producing integrated circuitry to the radiation only once. This may be beneficial as this may provide no detrimental effect (i.e., a decrease) in lithographic throughput.

[00033] The data may describe mechanical behaviour of the substrate in dependence on a pattern density of a pattern of the radiation.

[00034] As discussed above, the nominal dose of radiation may be determined such that, upon receipt of the nominal dose of radiation, the photoresist on the substrate may undergo some change of state. The nominal dose may therefore be dependent on the type of photoresist used. It will be appreciated that the nominal dose (for an entire target area) necessary to elicit the change of state of the photoresist in all required portions of the target area may also vary with the pattern of the patterned radiation. For example, a pattern of the patterned radiation which covers 90% of the target area (i.e., the first portion of the target area, which is exposed to radiation, corresponds to 90% of the target area) may correspond to a higher required nominal dose than a pattern of the patterned radiation which covers 50% of the target area. An amount of the target area which is covered by the pattern (i.e., a fraction of the target area to which the first portion corresponds) may be described as a pattern density. The pattern density may have a value between 0% and 100%. The pattern density may be determined by the patterning device.

[00035] A thermal load received by the target area may therefore vary with pattern density. Mechanical behaviour of the target area (in response to receipt of a thermal load) may therefore vary with pattern density. It may therefore be beneficial for the data which describes mechanical behaviour of the substrate in response to receipt of a thermal load to also take into account the pattern density of the radiation which is provided to the substrate.

[00036] The data may describe mechanical behaviour of the substrate in response to the nominal dose being received as a single exposure.

[00037] The lithographic apparatus may comprise a substrate support. The substrate support may support the substrate. [00038] The substrate support may comprise an electrostatic clamp. The component of the substrate support that physically interacts with the substrate may be a wafer table. The wafer table may contact the substrate via a plurality of burls, which may comprise projections from a base plate.

[00039] The data may describe mechanical behaviour of the substrate in dependence on a force exerted on the substrate by a substrate support.

[00040] The data may be indicative of whether or not a target area of the substrate will slip relative to the substrate support after receiving the nominal dose of radiation in a single exposure.

[00041] As discussed above, determination of the number of exposures across which a target area may be provided with the nominal dose of radiation may be based on data describing mechanical behaviour of the substrate in response to receipt of a thermal load. The data may describe thermal behaviour of the substrate. Particularly, the data may describe mechanical behaviour of the substrate in response to the thermal load. The substrate support may exert a force on the substrate. The data may describe a force exerted on the substrate by the substrate support. Particularly, the data may describe mechanical behaviour of the substrate as constrained by, or subject to, a force exerted on the substrate by the substrate support. The data may describe mechanical behaviour of the substrate in response to a thermal load and a force exerted on the substrate by the substrate support.

[00042] The data may indicate whether a target area of the substrate will slip relative to the substrate support. The data may indicate whether a target area of the substrate will slip relative to the substrate support after receiving the nominal dose of radiation in a single exposure. If the data indicates that a target area of the substrate will slip relative to the substrate support after receiving the nominal dose of radiation in a single exposure, it may be determined that said target area is provided with multiple exposures. This may limit the provision of multiple exposures to target areas which are likely to slip. Advantageously, by providing multiple exposures only to those target areas which are likely to slip, a total number of time delays introduced by providing multiple exposures may be kept relatively low. This may reduce any detrimental effect on lithographic throughput arising from providing radiation to target areas of the substrate across multiple exposures.

[00043] The number of exposures for each target area may be determined such that lithographic throughput is optimised.

[00044] Providing the nominal dose of radiation to one or more target areas across multiple exposures may result in a longer time needed to fully expose the entire substrate (relative to the nominal dose of radiation being provided to all target areas in a single exposure). This may be described as reducing lithographic throughput. However, providing the nominal dose of radiation to one or more target areas across multiple exposures may increase the number of target areas on the substrate which may be successfully incorporated into production of an integrated circuit chip (for example, by reducing overlay as described above). This may increase an overall yield per substrate. This may be described as increasing lithographic throughput. It may be desirable to maximise lithographic throughput. [00045] One or more target areas of the substrate may be provided with multiple exposures such that lithographic throughput is maximised whilst ensuring that there is no slippage relative to the substrate support. A determination of which target areas are to be provided with multiple exposures may be such that lithographic throughput is maximised. A determination of a number of exposures provided to each target area of the target areas which are provided with multiple exposures may be such that lithographic throughput is maximised. An order in which all target areas on the substrate are provided with individual exposures (also referred to as“routing”) may be such that lithographic throughput is maximised. Maximisation of lithographic throughput may be described as optimisation of lithographic throughput.

[00046] Generally each target area may receive the same nominal dose of radiation. However, for those target areas which receive the nominal dose as a split exposure, during each exposure of the split exposure a different dose (for example an integer fraction of the nominal dose) may be provided. Therefore, some mechanism for controlling the dose may be desirable.

[00047] Dose (energy received per unit surface area) received by a target area of the substrate may be controlled in several ways.

[00048] The lithographic apparatus according to the first aspect of the invention may be a scanning apparatus. A dose of radiation received by target areas of the substrate may be controlled by controlling a speed at which the substrate moves relative to the radiation.

[00049] During an exposure of a target area, it may be that only a portion of the target area is exposed to radiation at any time. A single exposure of the entire target area may be achieved by moving the target area relative to the beam of radiation. Controlling a radiation dose received by the target area may be performed by controlling a speed at which the substrate is moved relative to the beam of radiation. This speed may be referred to as a“scan speed”. A quicker scan speed may correspond to a lower radiation dose received by the target area. Advantageously, a source which provides the radiation may be kept in a state of constant radiation production when controlling dose in this way.

[00050] A dose of radiation received by target areas of the substrate may be controlled by controlling a power of the radiation.

[00051] The radiation may be pulsed. The power of the radiation may be controlled by controlling a repetition rate of the radiation.

[00052] A source which provides the radiation may be a commercially available laser-produced plasma radiation source. Such a radiation source may comprise a droplet generator which generates a train of mass-limited fuel targets (droplets) that may arrive at a location where a pulsed high-intensity laser may be incident on the droplets so as to convert the droplets into plasma. The plasma may then create radiation. This radiation may be pulsed. Each pulse may correspond to the radiation produced by a plasma from a different fuel droplet. Controlling a repetition rate of such an LPP source may be performed by controlling a pulse timing of the pulsed high-intensity laser. This may control an amount of droplets (i.e., a proportion of all available droplets) on which the pulsed high-intensity laser may be incident, thus controlling a power output of the radiation source.

[00053] It will be appreciated that a radiation dose received by the target area may be controlled by controlling a speed at which the substrate is moved relative to the beam of radiation in combination with controlling a power of the radiation (such as by controlling a pulse timing of the pulsed high- intensity laser). It will be further appreciated that a radiation dose received by the target area may be controlled in other ways to those described above.

[00054] According to a second aspect of the invention there is provided a lithographic apparatus. The lithographic apparatus may be operable to provide a plurality of target areas of a substrate with a nominal dose of radiation. The nominal dose of radiation may be provided to each of the plurality of target areas of the substrate across a number of exposures. The number of exposures for each target area may be determined in dependence on a position of that target area on the substrate.

[00055] The lithographic apparatus according to the second aspect of the invention may incorporate any features of the lithographic apparatus according to the first aspect of the invention, as described above.

[00056] The lithographic apparatus according to the second aspect of the invention may be arranged so that the nominal dose of radiation may be provided to at least one of the plurality of target areas across a plurality of exposures. At least one of the plurality of target areas may correspond to target areas of the substrate which may be disposed at a peripheral portion of the substrate.

[00057] The lithographic apparatus according to the second aspect of the invention may be arranged so that the nominal dose of radiation may be provided to target areas of the substrate which may be disposed at a central portion of the substrate as a single exposure.

[00058] The lithographic apparatus according to the first or second aspect of the invention may comprise a radiation source operable to produce the radiation for providing the plurality of target areas with the nominal dose of radiation.

[00059] The lithographic apparatus according to the first or second aspect of the invention may comprise a patterning device support structure for supporting a patterning device so as to pattern a radiation beam for providing the plurality of target areas with the nominal dose of radiation.

[00060] The lithographic apparatus according to the first or second aspect of the invention may comprise a projection system for projecting a radiation beam onto the substrate so as to provide the plurality of target areas with the nominal dose of radiation.

[00061] The lithographic apparatus according to the first or second aspect of the invention may comprise a control system configured to control the lithographic apparatus so as to provide the plurality of target areas of the substrate with the nominal dose of radiation.

[00062] The lithographic apparatus according to the first or second aspect of the invention may be operable to determine the number of exposures across which one or more of the plurality of target areas may be provided with nominal dose of radiation. [00063] A number of exposures across which one or more target areas of the substrate may be provided with radiation may be determined by the same lithographic apparatus that provides said radiation. For example, a computer system or processor which forms part of the lithographic apparatus may be used to determine the number of exposures. Alternatively, a number of exposures across which one or more target areas of the substrate are provided with radiation may be determined using an apparatus, computer system, processor or the like which may be separate to the lithographic apparatus that provides said radiation.

[00064] According to a third aspect of the invention there is provided a method of selecting a number of exposures to radiation a target area of a substrate supported by a substrate support receives.

[00065] The method according to the third aspect of the invention may comprise: simulating mechanical behaviour of the target area upon receiving a nominal dose of radiation.

[00066] The method according to the third aspect of the invention may comprise: determining whether or not the target area will slip relative to the substrate support when the nominal dose of radiation is provided to the target area in one exposure and across multiple exposures.

[00067] The method according to the third aspect of the invention may comprise: selecting the lowest number of exposures across which the nominal dose of radiation is provided for which the target area does not slip relative to the substrate support.

[00068] According to a fourth aspect of the invention there is provided a method of exposing to radiation a substrate which may comprise a plurality of target areas and which may be supported by a substrate support.

[00069] The method according to the fourth aspect of the invention may comprise: selecting, for each target area of the plurality of target areas, an optimum number of exposures. The optimum number of exposures may correspond to the lowest number of exposures across which a nominal dose of radiation may be delivered to the target area for which the target area does not slip relative to the substrate support.

[00070] The method according to the fourth aspect of the invention may comprise: exposing each target area of the plurality of target areas to the nominal dose of radiation, wherein the nominal dose of radiation is provided to each target area across the optimum number of exposures.

[00071] The method according to the fourth aspect of the invention may comprise: determining a routing for the plurality of target areas. Each target area of the plurality of target areas may receive the optimum number of exposures. A thermal recovery time between repeated exposures of the same target area may be optimised. The exposing of each target area of the plurality of target areas may use the routing determined for the plurality of target areas.

[00072] The third and/or fourth aspects of the invention may incorporate functionality of features of the lithographic apparatus according to the first and/or second aspect of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS [00073] 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 depicts a routing of a substrate having 96 target areas according to an embodiment of the present invention;

Figure 2b depicts a routing of a substrate having 68 target areas according to an embodiment of the present invention;

Figure 2c depicts a map of substrate overlay corresponding to the routing shown in Figure 2b; Figure 3a depicts an alternative routing of a substrate having 96 target areas;

Figure 3b depicts an alternative routing of a substrate having 68 target areas; and

Figure 3c depicts a map of substrate overlay corresponding to the routing shown in Figure 3b.

DETAIFED DESCRIPTION

[00074] Figure 1 shows a lithographic system comprising a radiation source SO and a lithographic apparatus FA. 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 FA. The lithographic apparatus FA comprises an illumination system IF, a support structure MT configured to support a patterning device MA (e.g., a mask), a projection system PS and a substrate table WT configured to support a substrate W.

[00075] The illumination system IF 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 IF 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 IF may include other mirrors or devices in addition to, or instead of, the faceted field mirror device 10 and faceted pupil mirror device 11.

[00076] 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 W held by the substrate 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). [00077] 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.

[00078] 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.

[00079] 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 de excitation and recombination of electrons with ions of the plasma.

[00080] 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.

[00081] 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.

[00082] 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.

[00083] 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. [00084] The lithographic apparatus LA may be operable to project the patterned radiation beam B’ via projection optics onto the substrate W. The patterned radiation beam B’ may have a pattern which is determined by the patterning device MA. The patterning device MA may selectively reflect only some portions of radiation so as to create the patterned radiation beam B’ . The patterned radiation beam B’ may comprise extreme ultraviolet (EUV) radiation. Following exposure to the patterned radiation beam B’, the substrate W may be processed. An exposed and processed substrate W may be useful in producing, for example, integrated circuitry.

[00085] The substrate W may be provided with a coating of photoresist. The lithographic apparatus LA may be operable to provide the patterned radiation beam B’ to the surface of the substrate W on which the photoresist is provided (referred to as the radiation-facing surface of the substrate W). As described above, the substrate W may be supported by the substrate table WT. The substrate table WT may also be referred to as a substrate support, and these terms may be used interchangeably. The substrate support WT may comprise a plurality of burls 15. The plurality of burls 15 may comprise a grid formation of structures which project from a base plate.

[00086] A surface of the substrate W (referred to as the backside of the substrate W) may contact the plurality of burls 15 and may be clamped to the plurality of burls. This backside of the substrate W is generally opposite the radiation-facing surface of the substrate W (on which an image of the patterning device MA is formed). The substrate W may be clamped using, for example, an electrostatic clamp (which may be referred to as a chuck). One of the reasons for supporting the substrate W on burls 15 in this manner is that it reduces the total contact area of the substrate support WT with the backside of the substrate W. As a result, foreign particles at the backside of the substrate W may be less likely to distort the shape of the substrate W.

[00087] The substrate W may comprise a plurality of target areas. The patterned radiation beam B’ may be provided to each target area on the substrate W in turn. Within each target area, a first portion of the substrate W receives radiation and a second portion of the substrate W does not receive radiation (the shapes and configurations of the first and second portions being dependent on the patterning device MA). Each part of the first portion may be provided with a nominal dose of radiation. As used herein, “dose” refers to energy received per unit area. The nominal dose of radiation may be identical across the first portion of all target areas (for a given pattern of the patterned radiation beam B’). The nominal dose of radiation may be such that, in the first portion of a target area (which receives radiation from the projection system PS and as determined by the pattern of the patterned radiation beam B’), the photoresist undergoes some change of state.

[00088] Exposing the substrate W to radiation may transfer thermal energy to the substrate W. This may be described as applying a thermal load to the substrate W. This may result in thermal deformation of the substrate W. Deformation of the substrate W may result in misalignment of a pattern that is projected on to the substrate W relative to a pattern exposed to layers of the substrate W that have already been formed (or are yet to be formed). Such misalignment of successively applied patterns may be referred to as“overlay”. By convention, the term“overlay” refers to the extent to which successive layers in an integrated circuit chip (e.g., formed by a lithographic process as described above) are laterally displaced relative to one another. A decrease in overlay may generally correspond to an increase in quality of integrated circuitry produced using the lithographic apparatus LA. This may be described as an increase in lithographic performance. A decrease in overlay may therefore be desirable.

[00089] Overlay due to deformation of the substrate W may be characterised and corrected for, to a limited extent. For example, computer models may be operable to predict deformation of the substrate W (due, for example, to applied thermal loads). Such predictions may, in turn, be used to modify an arrangement of components within the lithographic apparatus LA (for example, orientations of mirrors within the projection system PS, a position of the substrate support WT, etc.) in order to offset any contribution to overlay due to such deformation of the substrate W. However, under certain circumstances, deformation of the substrate W may be sufficiently large that the substrate W may slip relative to the burls 15 of the substrate support WT. Slipping of the substrate W may result in unpredictable (and hence uncorrectable) overlay in a portion of the substrate W (e.g., a target area) which slipped. Slipping of the substrate W may occur above a threshold level of deformation of the substrate W. Slipping of the substrate W may have a detrimental impact on overlay (i.e., an increase in overlay).

[00090] The plurality of burls 15 of the substrate support WT may be arranged such that, when the substrate support WT is supporting the substrate W, there is a low amount of friction between the plurality of burls 15 and the backside of the substrate W. This may be desirable when placing the substrate W on the substrate support WT as this may prevent deformation of the substrate W during the placement of the substrate W onto the substrate support WT. However, this low friction between the burls 15 and the substrate W may result in a relatively high likelihood of a portion of the substrate W slipping relative to the burls 15 as a result of mechanical deformation of the substrate W in response to an applied thermal load.

[00091] An embodiment of the present invention is now described with reference to Figures 2a, 2b, and 2c.

[00092] Figure 2a shows a schematic representation of a generally circular substrate 202. The substrate 202 in Figure 2a may correspond to the substrate W of Figure 1. The substrate 202 is split into a plurality of generally rectangular target areas. The position of target areas is demonstrated by a grid 206 on which the substrate 202 is overlaid in Figure 2a. Each target area is labelled with a number from “1” to“96”. These labels represent an example order in which individual target areas are exposed to radiation.

[00093] According to an embodiment of the present invention, the lithographic apparatus LA is configured such that the nominal dose of radiation is provided to a target area of the substrate 202 either in a single exposure or across a plurality of exposures. The number of exposures across which the nominal dose is provided is determined in dependence on data describing mechanical behaviour of the target area of the substrate 202 in response to receipt of a thermal load (for example a thermal load corresponding to a first portion of the target area receiving the nominal dose).

[00094] The number of exposures over which the nominal dose is delivered to each target area may be determined by the lithographic apparatus LA. For example, a computer system or processor which forms part of the lithographic apparatus LA may be used to determine the number of exposures. Alternatively, the number of exposures over which the nominal dose is delivered to each target area may be determined using an apparatus, computer system, processor or the like which is separate to the lithographic apparatus LA. The data describing mechanical behaviour of the target area of the substrate 202 in response to receipt of a thermal load (hereafter referred to as“the data”), which is used in the determination of the number of exposures the target area is to receive, may form a basis of a model of mechanical behaviour of the substrate 202.

[00095] The data describes whether or not the target area will deform to such an extent that the target area will slip relative to the substrate support WT after having received a thermal load. The data includes information relating to how the target area will deform in response to receipt of the thermal load.

[00096] The information relating to how the target area will deform in response to receipt of a thermal load is generally dependent on the how the target area is supported (e.g., clamped to the substrate support WT). Such support of the target area is generally dependent on the local forces which act on the target area. Such support of the target area includes the amount of support provided by the substrate support WT (for example, the number and configuration of burls 15 supporting the target area, a clamping force which clamps the substrate 202 to the burls 15, and/or a coefficient of friction between the burls 15 and the substrate 202, etc.). In addition, such information relating to the support of the target area may include the amount of support provided by surrounding areas of the substrate 202 (and, in turn, how these surrounding areas of the substrate 202 are supported by the substrate support WT). The data may include information relating to the position of the target area on the substrate 202.

[00097] For a given thermal load applied to the target area, the data may describe the expected mechanical deformation of the target area. This given thermal load (for which a determination of slippage or lack thereof may be made) will generally be provided to the target area during any time in which the patterned radiation beam B’ is incident on the target area (i.e., during an exposure).

[00098] Some target areas of the substrate 202, having received a given thermal load (corresponding to the first portion of the target area receiving the nominal dose of radiation), may undergo mechanical deformation to a greater extent than other target areas of the substrate 202. Having received said given thermal load, some target areas of the substrate 202 may undergo mechanical deformation to an extent that they slip relative to the substrate support WT, whereas other target areas of the substrate 202 may undergo mechanical deformation to a lesser extent, such that these target areas do not slip relative to the substrate support WT. There may therefore exist a threshold mechanical deformation (e.g., a threshold lateral force between the substrate 202 and the burls 15, a threshold distance by which a portion of the target area deforms, etc.) that determines whether or not a target area will slip relative to the substrate support WT after having received a given thermal load. Furthermore, in general, this threshold may be different for different target areas of the substrate 202.

[00099] In an embodiment of the present invention, the data describes mechanical behaviour of each target area in response to receipt of a thermal load corresponding to receiving the nominal dose of radiation in a single exposure. The data may also describe mechanical behaviour of each target area in response to receipt of a thermal load corresponding to receiving the nominal dose of radiation across multiple exposures (such as two, three, and more exposures). The data may also describe mechanical behaviour of each target area in response to receipt of a thermal load corresponding to receiving the nominal dose of radiation across said multiple exposures, for a range of time delays between each exposure of the multiple exposures.

[000100] In an embodiment of the present invention, the data describes whether or not each target area will deform to such an extent that each target area will slip relative to the substrate support WT after having received a thermal load corresponding to receiving the nominal dose of radiation in a single exposure. In the embodiment shown in Figure 2a, it is determined from the data that target areas“1” to “38” will slip relative to the substrate support WT after having received a thermal load corresponding to receiving the nominal dose of radiation in a single exposure. It is determined from the data that target areas“1” to“38” will not slip relative to the substrate support WT after having received a thermal load corresponding to receiving the nominal dose of radiation across 2 exposures. It is determined from the data that target areas“39” to“96” will not slip relative to the substrate support WT after having received a thermal load corresponding to receiving the nominal dose of radiation in a single exposure. Therefore, it is determined from the data that the lithographic apparatus LA is to provide target areas“1” to“38” with the nominal dose of radiation across two exposures and target areas“39” to“96” with the nominal dose of radiation in a single exposure.

[000101] The grid 206 represents a method of providing the substrate 202 with radiation in accordance with the data described in the previous paragraph. The order in which individual target areas of the substrate 202 are exposed may be referred to as“routing”. A number in each element (target area) of the grid 206 depicts the routing. It will be appreciated that the routing shown in Figure 2a is only an example of a possible routing. Elements (target areas) of the grid 206 which are shaded correspond to elements which receive the nominal dose of radiation across multiple exposures (particularly, in this embodiment, across two exposures). Elements (target areas) of the grid 206 which are not shaded correspond to elements which receive the nominal dose of radiation in a single exposure.

[000102] Firstly, the lithographic apparatus LA exposes target area“1” of the substrate 202 (to the patterned radiation beam B’) with half of the nominal dose of radiation. Secondly, the lithographic apparatus LA exposes target area“2” of the substrate 202 (to the patterned radiation beam B’) with half of the nominal dose of radiation. This continues for target areas“3”,“4”, etc., up to and including target area“38”. This process is then repeated, i.e., target areas“1” to“38” are sequentially exposed (to the patterned radiation beam B’) with half of the nominal dose of radiation for a second time. Target areas “1” to“38” may be described as having received multiple exposures.

[000103] The lithographic apparatus LA then exposes target area“39” of the substrate 202 (to the patterned radiation beam B’) with the full, nominal dose of radiation in a single exposure. The lithographic apparatus LA then exposes target area“40” of the substrate 202 (to the patterned radiation beam B’) with the full, nominal dose of radiation in a single exposure. This continues for target areas “41”,“42”, etc., up to and including target area“96”. Target areas“39” to“96” may be described as having received a single exposure.

[000104] In summary: target areas“1” to“38” of the substrate 202 are sequentially exposed with half of the nominal dose of radiation; then, target areas“1” to“38” of the substrate 202 are sequentially exposed with half of the nominal dose of radiation (for a second time); then, target areas“39” to“96” are sequentially exposed with the nominal dose of radiation in a single exposure. It will be appreciated that, after this procedure is completed, all target areas of the substrate 202 have been provided with the nominal dose of radiation.

[000105] With such an arrangement at least one target area is exposed to the nominal dose of radiation using multiple exposures (which may also be referred to as “split exposures”) in the lithographic apparatus LA. In the example embodiment described above with reference to Figure 2a, target areas“1” to“38” are provided with multiple exposures, whereas target areas“39” to“96” are provided with a single exposure.

[000106] For any target area which receives multiple exposures of radiation, said target area may receive each exposure of said multiple exposures with a time delay between each exposure. For example, as described above in the example embodiment with reference to Figure 2a, the second exposure of target area“1” is not provided immediately after the first exposure of target area“1”. The second exposure of any of the target areas“1” to“38” is not provided immediately after the first exposure to the same target area.

[000107] The time delay between each exposure of said multiple exposures may allow a thermal load received by a target area which receives multiple exposures of radiation (per each exposure of said multiple exposures) to be absorbed by a suitable cooling mechanism before the next thermal load is applied. For example, the thermal load per each exposure of said multiple exposures may be dissipated through: portions of the substrate 202 outside a given target area; components which the substrate 202 contacts; and/or an environment in which the substrate 202 is disposed. The time delay between exposures may be described as a“recovery time”. Advantageously, the recovery time may reduce a maximum temperature of the target area relative to no recovery time being provided. This may reduce thermal deformation of the target area. This may reduce or eliminate a chance of the target area slipping relative to the substrate support WT. This may reduce overlay. This may improve lithographic performance. [000108] As in the example embodiment described above with reference to Figure 2a, during the time delay following exposure of a first target area which receives multiple exposures of radiation (e.g., target area“4” of the substrate 202), a second target area (e.g., target area“5” of the substrate 202) may be exposed. In general, one or more target areas may be exposed during time delays between multiple exposures of each target area that receives radiation across multiple exposures. In this way, multiple target areas on the substrate 202 may receive the nominal dose of radiation in a shorter time than if no target areas were exposed during such time delays. Therefore, advantageously, some or all target areas on the substrate 202 may receive the nominal dose of radiation in a shorter time than if no target areas were exposed during such time delays. This may be described as an increase in lithographic throughput (relative to providing multiple exposures, as described above, but not exposing any target areas during said time delays).

[000109] As in the example embodiment described above with reference to Figure 2a, the lithographic apparatus LA may be configured such that different target areas of the substrate 202 are provided with different numbers of exposures. This may enable multiple exposures to be provided selectively to individual target areas on the substrate 202. Advantageously, this allows the risk of slippage to be mitigated whilst still maintaining an optimal throughput for the lithographic apparatus LA. This may be described as adapting a method of providing multiple exposures to the substrate 202. For example, one or more target areas may be provided with radiation in a single exposure. One or more target areas may be provided with multiple exposures. One or more target areas may be provided with radiation across two exposures. One or more target areas may be provided with radiation across more than two exposures (e.g., three exposures).

[000110] The inventors have realised that thermal deformation of the substrate 202 may be reduced without reducing radiation dose. Therefore, advantageously, providing the nominal dose of radiation to one or more target areas using multiple exposures may reduce or eliminate a chance of the one or more target areas slipping relative to the substrate support WT. Thus, providing the nominal dose of radiation to one or more target areas using multiple exposures may reduce overlay. This may improve lithographic performance.

[000111] Target areas of the substrate 202 which are disposed at a peripheral portion of the substrate 202 may correspond to target areas which have an increased chance of slipping relative to the substrate support WT after a given thermal load is applied (e.g., a thermal load corresponding to receiving the nominal dose of radiation in a single exposure). The data describing mechanical behaviour of the target area of the substrate 202 in response to receipt of a thermal load may indicate that target areas of the substrate 202 which are disposed at a peripheral portion of the substrate 202 are more likely to slip relative to the substrate support WT after receiving a thermal load corresponding to receiving the nominal dose of radiation in a single exposure than other, more central, target areas of the substrate 202. Therefore, such target areas in the peripheral portion of the substrate 202 may be provided with the nominal dose of radiation across multiple exposures, as in the example embodiment described above with reference to Figure 2a.

[000112] If the nominal dose is delivered to a target area across n exposures (n being an integer), during each exposure, the lithographic apparatus LA may be configured to provide the target area with a dose given by 1/n times the nominal dose. Providing an equal radiation dose during each exposure of multiple exposures of a target area may minimise a maximum thermal load that is applied across any of the multiple exposures. Advantageously, this may reduce thermal deformation of the target area. This may reduce or eliminate a chance of the target area slipping relative to the substrate support WT. This may reduce overlay. This may improve lithographic performance.

[000113] The substrate 202 may comprise target areas which are not used for producing integrated circuitry. Target areas which are not used for producing integrated circuitry may be referred to as“non- product” target areas, and target areas which are used for producing integrated circuitry may be referred to as“product” target areas. It may provide little or no benefit to reduce overlay (e.g., by using multiple exposures as described above) in non-product target areas. Therefore, it may be desirable to provide non-product target areas with the nominal dose of radiation in a single exposure. This may be beneficial as this provides no detrimental effect (i.e., a decrease) in lithographic throughput.

[000114] When determining a routing for a substrate, it may be desirable to provide a non-product target area with the nominal dose of radiation only after providing product target areas which are adjacent to the non-product target area with the nominal dose of radiation. Therefore, any slippage of the non-product target area (after receiving the nominal dose of radiation) will not adversely affect overlay within the product target areas. When determining a routing for a substrate, it may be desirable to provide all product target areas with the nominal dose of radiation first and then, subsequently, provide all non-product target areas with the nominal dose of radiation.

[000115] As used herein, a dose of radiation is intended to mean an amount of energy per unit area that each part of the first portion of the target area. As described above, the nominal dose of radiation may be determined such that, upon receipt of the nominal dose of radiation, the photoresist on the substrate 202 may undergo some change of state. The nominal dose may therefore be dependent on the type of photoresist used. The nominal dose is delivered to each part of the first portion of the target area (as defined by the patterning device MA). It will be appreciated that the total amount of energy received by an entire target area of the substrate 202 will be dependent on the pattern of the patterned radiation. For example, a pattern of the patterned radiation which covers 90% of the target area (i.e., 90% of the target area is exposed to radiation) corresponds to a higher amount of energy (and, therefore, a higher thermal load) than a pattern of the patterned radiation which covers 50% of the target area. An amount of the target area which is covered by the pattern may be described as a pattern density. The pattern density may have a value between 0% and 100%. The pattern density may be determined by the patterning device MA. [000116] A thermal load (corresponding to a required, nominal dose of radiation being delivered to the first portion of the target area) may therefore vary be dependent on the pattern density. In turn, the mechanical behaviour of the target area may therefore be dependent on pattern density. It may therefore be beneficial for the data which describes mechanical behaviour of the substrate 202 in response to receipt of a thermal load to also take into account the pattern density of the radiation which is provided to the substrate 202.

[000117] As described above, determination of the number of exposures across which a target area is provided with the nominal dose of radiation may be based on data describing mechanical behaviour of the substrate 202 in response to receipt of a thermal load. The data may describe thermal behaviour of the substrate 202. The data may describe mechanical behaviour of the substrate 202 in response to the thermal load. The substrate support WT may exert a force on the substrate 202. The data may describe a force exerted on the substrate 202 by the substrate support WT. Particularly, the data may describe mechanical behaviour of the substrate 202 as constrained by, or subject to, a force exerted on the substrate 202 by the substrate support WT. The data may describe mechanical behaviour of the substrate 202 in response to a thermal load and a force exerted on the substrate 202 by the substrate support WT.

[000118] The data may indicate whether a target area of the substrate 202 will slip relative to the substrate support WT. The data may indicate whether a target area of the substrate 202 will slip relative to the substrate support WT after the first portion of the target area receives the nominal dose of radiation in a single exposure. If the data indicates that a target area of the substrate 202 will slip relative to the substrate support WT after the first portion of the target area receives the nominal dose of radiation in a single exposure, it may be determined that the first portion of said target area is to receive the nominal dose of radiation across multiple exposures. This may limit the provision of multiple exposures to target areas which are likely to slip. Advantageously, by providing multiple exposures only to those target areas which are likely to slip, a total number of time delays introduced by providing multiple exposures may be kept relatively low. This may reduce any detrimental effect on lithographic throughput arising from providing radiation to target areas of the substrate 202 across multiple exposures.

[000119] Providing the nominal dose of radiation to one or more target areas across multiple exposures may result in a longer time needed to fully expose the entire substrate 202 (relative to the nominal dose of radiation being provided to all target areas in a single exposure). This may be described as reducing lithographic throughput. However, providing the nominal dose of radiation to one or more target areas across multiple exposures may increase the number of target areas on the substrate 202 which can be successfully incorporated into production of an integrated circuit chip (for example, by reducing overlay as described above). This may increase an overall yield per substrate 202 at the expense of lithographic throughput. It may be desirable to optimise lithographic throughput, for example to maximise throughput, whilst keeping the risk of overlay that reduces the yield to a negligible level. [000120] One or more target areas of the substrate 202 may be provided with multiple exposures such that lithographic throughput is maximised whilst ensuring that there is substantially no slippage relative to the substrate support WT. A determination of which target areas are provided with multiple exposures may be such that lithographic throughput is optimised as described above. A determination of a number of exposures provided to each target area of the target areas which is provided with multiple exposures may be such that lithographic throughput is optimised as described above. An order in which all target areas on the substrate are provided with individual exposures (i.e., a routing) may be such that lithographic throughput is maximised. Maximisation of lithographic throughput may be described as optimisation of lithographic throughput.

[000121] Figure 2b shows a schematic representation of a generally circular substrate 204. The substrate 204 in Figure 2b may correspond to the substrate W of Figure 1. The substrate 204 is split into a plurality of generally rectangular target areas. The position of target areas is demonstrated by a grid 208 on which the substrate 204 is overlaid in Figure 2b. Each target area is labelled with a number from “1” to“68”. These labels represent an example order in which individual target areas are exposed.

[000122] Figure 2b is provided to demonstrate how the exposure method described above with reference to Figure 2a may be carried out on a substrate having a different number of target areas to the substrate 202. The substrate 202 (Figure 2a) comprises 96 target areas, whereas the substrate 204 (Figure 2b) comprises 68 target areas. The grid 208 shows shaded peripheral target areas of the substrate 204 (target areas“1” to“32”) and unshaded central target areas (target areas“33” to“68”). Target areas “1” to“32” of the substrate 204 are sequentially exposed with half of the nominal dose of radiation; then, target areas“1” to“32” of the substrate 204 are sequentially exposed with half of the nominal dose of radiation (for a second time); then, target areas“33” to“68” are sequentially exposed with the nominal dose of radiation in a single exposure. It will be appreciated that, after this procedure is completed, all target areas of the substrate 204 have been provided with the nominal dose of radiation. The determination of which target areas of the substrate 204 are provided with the nominal dose of radiation in a single exposure and which target areas of the substrate 204 are provided with the nominal dose of radiation across multiple exposures is determined based on data describing mechanical behaviour of the substrate 204, as described above with reference to the substrate 202 of Figure 2a.

[000123] Figure 2c shows a map 212 of overlay of a substrate. Overlay (a measure of misalignment of successively applied patterns on a substrate, which is generally undesirable) is shown in the map 212 as a two-dimensional vector field of overlay, with a plurality of vectors each corresponding to a point on the substrate to which the map 212 corresponds. Magnitude of overlay is demonstrated in the map 212 by the length of individual vectors.

[000124] The map 212 corresponds to the substrate 204 of Figure 2b. However, as target areas of the substrate 202 and of the substrate 204 are provided with the nominal dose of radiation using a substantially similar method, the overlay shown in the map 212 is expected to also be representative of the overlay present when the substrate 202 is exposed according to the embodiment of the present invention described above with reference to Figure 2a. Overlay is generally slightly larger at peripheral portions of the map 212 than in central portions of the map 212. For example, overlay in region 214 is larger than in central portions of the map 212.

[000125] It will be appreciated that the routings described above with reference to Figures 2a and 2b are only examples of routings which may be used in accordance with principles of the present invention.

[000126] An alternative routing may be as follows: target areas in a peripheral portion (i.e. target areas“1” to“38”) of the substrate 202 (Figure 2a) are sequentially exposed with half of the nominal dose of radiation; then, target areas in a central portion of the substrate 202 (i.e. target areas“39” to “96”) are sequentially exposed with the nominal dose of radiation in a single exposure; then, target areas in a peripheral portion (i.e. target areas“1” to“38”) of the substrate 202 are sequentially exposed with half of the nominal dose of radiation (for a second time). This routing increases the recovery time (the time between repeated exposures) for peripheral target areas“1” to“38” compared with the first described example routing for the substrate 202. This may further reduce thermal deformation of target areas. This may further reduce or eliminate a chance of target areas slipping relative to the substrate support WT. This may further reduce overlay. This may further improve lithographic performance. Optionally, when the peripheral target areas“1” to“38” are exposed for the second time, the second exposure may start at“19”, then“20”, then“21”, and continue clockwise to“18”.

[000127] A separate benefit of increasing a time delay between repeated exposures for target areas “1” to“38” is that this longer time delay may enable calculations of dose errors (for example under exposure to be performed. Such dose errors may, for example, be due to radiation source instability. This is described in more detail below.

[000128] A radiation source (e.g., the source SO) in a lithographic apparatus (e.g., the lithographic apparatus LA) may have intrinsic instability (i.e., temporal fluctuations in power output). For example, there may be some pulse-to-pulse power variation. In principle, said instability may lead to under exposure or over-exposure of photoresist on a substrate. Under-exposure may be corrected for by providing a second exposure (a“top-up” exposure). Over-exposure cannot be corrected for. In practice, the output power of the radiation source may be controlled using a control loop that substantially prevents over-exposure but which may still suffer from under-exposure, as now discussed.

[000129] It is known to control an effective source power (ESP) of a radiation source in a lithographic apparatus. A suitable control loop may be used to maintain a nominal (average) operating power of the radiation source at a level which is below the maximum available output power by a chosen amount. This chosen difference between a nominal or average maximum available output power of the radiation source (also referred to as open-loop power) and an average or nominal operating power of the radiation source may be referred to as a dose margin. An ESP is lower than a maximum achievable source power, and, as a result, instances of over-exposure can be avoided. A reduction in ESP (i.e. an increase in dose margin) generally corresponds to a reduction in source instability. In addition, the dose of radiation is spread over a greater number of pulses, reducing the effect of an individual pulse which has a significant fluctuation away from the nominal operating power. A relatively high ESP may result in a relatively short required exposure time, but may also increase the need for top-up exposures due to higher instability of the radiation source. This may have a detrimental effect on lithographic throughout. However, a relatively low ESP may result in a relatively long exposure time (to the detriment of throughput). In practice, a dose margin may be selected to maintain a relatively quick exposure without requiring too many top-up exposures.

[000130] However, in the context of the present invention, one or more target areas on a substrate are provided with the nominal dose of radiation across multiple exposures (regardless of the ESP chosen). Therefore, if it is determined to provide a target area with the nominal dose of radiation across multiple exposures (e.g., across n exposures), a relatively high ESP may be chosen for all exposures prior to the n^th exposure. This may increase dose errors in the first n-1 exposures but these can be corrected for during the n^th exposure. The ESP of the n^th exposure may be relatively low (to reduce the instances of dose error in the final dose). Further, the ESP of the n^th exposure may be configured to counteract any under-exposure or over-exposure present in any exposure prior to the n^th exposure (based on calculations made during a time delay between repeated exposures, for example). On average, the power used to expose the one or more target areas on a substrate are provided with the nominal dose of radiation across multiple exposures can be higher (due to a higher operating power for the first n-1 exposures) than it would be for target areas exposed in single exposure. This may result in a relatively high lithographic throughout. In particular, this may partly, substantially, or entirely offset any reduction in lithographic throughput brought about by providing one or more target areas with the nominal dose of radiation across multiple exposures.

[000131] For comparison with the embodiments of the present invention described in detail above, Figure 3 a depicts an alternative routing and an alternative exposure method.

[000132] Figure 3a shows a schematic representation of a generally circular substrate 302. The substrate 302 in Figure 3a may correspond to the substrate W of Figure 1. The substrate 302 is split into a plurality of generally rectangular target areas. The substrate 302 is split into 96 target areas, identically to the substrate 202 of Figure 2a. The position of target areas of the substrate 302 is demonstrated by the grid 306 on which the substrate 302 is overlaid in Figure 3a. The grid 306 demonstrates an alternative method of providing the substrate 302 with radiation. A number in each element (target area) of the grid 306 depicts the routing. Each element (target area) of the substrate 302 is provided with the nominal dose of radiation in a single exposure, sequentially, in the order indicated by the numbers within the grid 306.

[000133] Firstly, the lithographic apparatus FA exposes target area“1” of the substrate 302 (to the patterned radiation beam B’) with the nominal dose of radiation. Secondly, the lithographic apparatus FA exposes target area“2” of the substrate 302 (to the patterned radiation beam B’) with the nominal dose of radiation. This continues for target areas“3”,“4”, etc., up to and including target area“96”. All target areas (“1” to“96”) may be described as having received a single exposure. [000134] Figure 3b shows a schematic representation of a generally circular substrate 304. The substrate 304 in Figure 3b may correspond to the substrate W of Figure 1. The substrate 304 is split into a plurality of generally rectangular target areas. The substrate 304 is split into 68 target areas, identically to the substrate 204 shown in Figure 2b. The position of target areas is demonstrated by a grid 308 on which the substrate 304 is overlaid in Figure 3b. Each target area is labelled with a number from“1” to “68”. These labels represent an example order in which individual target areas are exposed.

[000135] Figure 3b is provided to demonstrate how the exposure method described above with reference to Figure 3 a may be carried out on a substrate having a different number of target areas to the substrate 302. The substrate 302 (Figure 3a) comprises 96 target areas, whereas the substrate 304 (Figure 3b) comprises 68 target areas. Target areas“1” to“68” (i.e., all target areas of the substrate 304) are sequentially exposed with the nominal dose of radiation in a single exposure. It will be appreciated that, after this procedure is completed, all target areas of the substrate 304 have been provided with the nominal dose of radiation.

[000136] Figure 3c shows a map 312 of overlay of a substrate. The substrate to which the map 312 corresponds (Figure 3c) is identical to the substrate to which the map 212 corresponds (Figure 2c), but is exposed using the exposure method described above with reference to Figure 3b. Similarly to Figure 2c, overlay is shown in the map 312 of Figure 3c as a two-dimensional vector field of overlay corresponding to points on the substrate to which the map 312 corresponds. As in Figure 2c, magnitude of overlay is demonstrated in the map 312 by the length of individual vectors.

[000137] The map 312 corresponds to the substrate 304 of Figure 3b. However, as target areas of the substrate 302 and of the substrate 304 are provided with the nominal dose of radiation using a substantially similar method, the overlay shown in the map 312 is expected to also be representative of the overlay present when the substrate 302 is exposed according to the exposure method described above with reference to Figure 3a.

[000138] Overlay is significantly larger at peripheral portions of the map 312 than in central portions of the map 312. For example, overlay in region 314 is much larger than in central portions of the map 312. Overlay in central portions of map 212 (exposed using the new method) and in central portions of map 312 (exposed using the alternative method) is similar in magnitude. However, overlay in peripheral portions of map 212 (exposed using the new method) is much smaller than overlay in peripheral portions of map 312 (exposed using the alternative method). See, for example, region 214 (Figure 2b, exposed using the new method) and region 314 (Figure 3b, exposed using the alternative method).

[000139] As is therefore demonstrated when comparing Figures 2c and 3c, providing target areas of a substrate with a nominal dose of radiation according to the embodiment of the present invention described above with reference to Figures 2a and 2b reduces overlay compared with providing target areas of a substrate with a nominal dose of radiation according to the alternative method described above with reference to Figures 3a and 3b. Advantageously, embodiments of the present invention reduce overlay. This may increase lithographic performance. [000140] Discussion below may refer to the substrate 202 of Figure 2a, but it will be appreciated that comments may apply to a substrate in general and, in particular, may apply equally to the substrate 204 of Figure 2b..

[000141] As discussed in detail above, overlay may result from distortions due to the thermal load provided to the substrate 202. For a given pattern density, overlay will generally increase with dose (as a result of a corresponding increase in thermal load). One approach to reducing overlay may be to reduce the nominal dose. However, this approach brings separate disadvantages.

[000142] A total dose (e.g., the nominal dose) of radiation received by any portion of the substrate 202 (particularly, the photoresist which is disposed on the substrate 202) is proportional to the number of photons incident on the substrate 202. Shot noise is inversely proportional to the number of photons incident on the substrate 202. Therefore, lowering the nominal dose will generally result in an increase in shot noise of the pattern provided to each target area of the substrate 202 via the patterned radiation beam B'. An increase in such shot noise corresponds to an increase in line edge roughness in the pattern provided to each target area of the substrate 202 via the patterned radiation beam B'. This may lead to an increased risk of failure of the substrate 202 and/or an increase in errors in integrated circuitry produced using the substrate 202.

[000143] Attempting to reduce overlay by lowering the nominal dose may therefore have a detrimental effect on lithographic performance. Embodiments of the present invention reduce overlay without requiring the nominal dose to be reduced, and consequently without increasing line edge roughness. Advantageously, therefore, embodiments of the present invention reduce overlay whilst avoiding the disadvantages associated with lowering the nominal dose.

[000144] According to some embodiments of the present invention, the lithographic apparatus LA may be operable to provide a plurality of target areas of a substrate (for example, the substrate 202 of Figure 2a or the substrate 204 of Figure 2b) with a nominal dose of radiation, wherein the nominal dose of radiation is provided to each of the plurality of target areas of the substrate 202 across a number of exposures, the number of exposures for each target area being determined in dependence on a position of that target area on the substrate 202. That is to say, a target area of a substrate may receive the nominal dose of radiation either in a single exposure or across multiple exposures, the determination of“single” or“multiple” being based on a location of the target area on the substrate. Further, for a target area of a substrate which receives the nominal dose of radiation across multiple exposures, the total number of exposures (i.e., to how many exposures does“multiple exposures” refer) may be determined based on a location of the target area on that substrate.

[000145] According to some embodiments of the present invention, the lithographic apparatus LA may be arranged so that the nominal dose of radiation is provided to at least one target area across a plurality of exposures. At least one target area which receives a plurality of exposures may correspond to a target area of the substrate 202 which is disposed at a peripheral portion of the substrate 202. [000146] According to some embodiments of the present invention, the lithographic apparatus LA may be arranged so that the nominal dose of radiation is provided to at least one target area which is at a central portion of the substrate 202 in a single exposure.

[000147] As has been described above, according to some embodiments of the present invention, determination of the number of exposures across which the nominal dose of radiation is provided to a target area may be based on data describing mechanical behaviour of the target area in response to thermal load. According to some embodiments of the present invention, determination of the number of exposures across which the nominal dose of radiation is provided to a target area may be based on a location of the target area relative to the substrate. It will be appreciated that, according to some embodiments of the present invention, determination of the number of exposures across which the nominal dose of radiation is provided to a target area may be based on data describing mechanical behaviour of the target area in response to a thermal load and on a location of the target area relative to the substrate. It will be further appreciated that determination of the number of exposures across which the nominal dose of radiation is provided to a target area may be based on other means.

[000148] In the embodiments described above, each target area generally receives the same nominal dose of radiation (whether this is provided in a single exposure or across multiple exposures). However, for those target areas which receive the nominal dose across multiple exposures, during each exposure of the multiple exposures a different dose (for example an integer fraction of the nominal dose) should be provided. Therefore, some mechanism for controlling the dose of radiation received by a target area is desirable.

[000149] Dose (energy received per unit surface area) received by a target area of the substrate may be controlled in several ways.

[000150] During an exposure of a target area, it may be that only a portion of the target area is exposed to radiation at any time. A single exposure of the entire target area may be achieved by moving the target area relative to the patterned radiation beam B’ . Controlling a radiation dose received by the target area may be performed by controlling a speed at which the substrate 202 is moved relative to the patterned radiation beam B’. This speed may be referred to as a“scan speed”. Moving the target area relative to the patterned radiation beam B’ may be achieved by moving the substrate support WT relative to the patterned radiation beam B’. A quicker scan speed may correspond to a lower radiation dose received by the target area. Advantageously, when controlling dose in this way (by moderating a scan speed), a radiation source which provides the radiation may be kept in a state of constant radiation production.

[000151] A radiation source which provides the radiation may be a commercially available laser- produced plasma (LPP) radiation source. Such a radiation source may comprise a droplet generator which generates a train of mass-limited fuel targets (droplets) that arrive at a location where a pulsed high-intensity laser is incident on the droplets by so as to convert the droplets into plasma. The plasma may then create radiation (such as EUV radiation). This radiation is pulsed, with each pulse corresponding to the radiation produced by a plasma from a different fuel droplet. Controlling a repetition rate of such an LPP radiation source may be performed by controlling a pulse timing of the pulsed high-intensity laser. This may control an amount of droplets (i.e., a proportion of all available droplets) on which the pulsed high-intensity laser is incident, thus controlling a power output of the radiation source. This may vary the power of the patterned radiation beam B’ .

[000152] It will be appreciated that a radiation dose received by the target area may be controlled by controlling a speed at which the substrate is moved relative to the patterned radiation beam B’ in combination with controlling a power of the radiation (such as by controlling a pulse timing of the pulsed high-intensity laser). It will be further appreciated that a radiation dose received by the target area may be controlled in other ways to those described above.

[000153] Methods of providing a substrate (for example, the substrate 202 of Figure 2a or the substrate 204 of Figure 2b) with radiation according to an embodiment of the present invention are now described. The substrate 202 may be supported by a substrate support (for example, the substrate support WT of Figure 1).

[000154] Firstly, mechanical behaviour of the target area is simulated. Mechanical behaviour of the target area after receiving different thermal loads may be simulated. Mechanical behaviour of the target area after receiving a single thermal load may be simulated. Mechanical behaviour of the target area after receiving multiple thermal loads, with varying time delays between repeated thermal loads, may be simulated. Simulations will generally be based on data which describes mechanical properties of the target area. Simulations may also generally be based on data which describes forces acting on the target area by the substrate support WT (such as electrostatic clamping forces and/or frictional forces). Simulations may also generally be based on data which describes internal forces within the substrate 202 (such as forces acting on the target area from other areas of the substrate 202).

[000155] Then, for a given, nominal dose of radiation (generally corresponding to a desired dose for eliciting a change of state in photoresist which is applied on a surface of the substrate 202), a determination of whether or not the target area will slip relative to the substrate support WT is made. The determination of slippage or lack thereof is made for the nominal dose being provided in a single exposure, a process which provides a specific thermal load to the target area (for a given pattern and therefore a given pattern density). The determination of slippage or lack thereof is made for the nominal dose being provided across multiple exposures (e.g., two, three, and more exposures), processes which provide specific, smaller thermal loads to the target area. The determination of slippage or lack thereof is made based on the simulations described in the previous paragraph.

[000156] Then, the lowest number of exposures across which the nominal dose of radiation is provided to the target area (e.g., a single exposure, two exposures, etc.) which does not result in slippage of the target area relative to the substrate support WT (as determined from the simulation and determination step detailed in the previous two paragraphs) is selected. This number of exposures may be referred to as the optimum number of exposures. [000157] Then, a routing for a plurality of target areas on a substrate is determined. Each target area of the plurality of target areas receives the optimum number of exposures (as explained in the previous paragraph). The routing may be determined such that a recovery time between repeated exposures of the same target area allows for a relatively high amount of dissipation of a thermal load received by any one target area in an exposure. The routing may be determined such that there are no large temporal gaps between exposure of a target area and a subsequent target area to be exposed. This may be described as determining the routing such that a recovery time between repeated exposures is optimised.

[000158] Finally, a lithographic apparatus is configured to provide the nominal dose of radiation to each target area on the substrate 202 across the number of exposures selected according to the previous paragraph. For target areas which receive multiple exposures, a dose of radiation received during each exposure of the multiple exposures corresponds to the nominal dose divided by the number of exposures with which the target areas will be provided.

[000159] It will be appreciated that“providing a target area with an exposure”, as used herein, refers to exposing a target area to radiation once. Fikewise,“providing a target area with multiple exposures” refers to exposing a target area to radiation multiple times. It will be further appreciated that, unless stated to the contrary, providing a target area with radiation refers to providing a first portion of the target area with radiation. For example, providing a target area with a nominal dose of radiation refers to providing a first portion of the target area with the nominal dose of radiation. Generally, when providing a target area with a nominal dose of radiation a second portion of the target area will not be provided with radiation.

[000160] 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, liquid- crystal displays (FCDs), thin-film magnetic heads, etc.

[000161] 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.

[000162] 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.

[000163] 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

1. A lithographic apparatus operable to provide a plurality of target areas of a substrate with a nominal dose of radiation, wherein the nominal dose of radiation is provided to each of the plurality of target areas of the substrate across a number of exposures, the number of exposures for each target area being determined in dependence on data describing mechanical behaviour of that target area of the substrate in response to receipt of a thermal load.

2. The lithographic apparatus of claim 1 wherein the lithographic apparatus is arranged so that the nominal dose of radiation is provided to at least one of the plurality of target areas across a plurality of exposures.

3. The lithographic apparatus of claim 2 wherein the plurality of exposures across which the nominal dose is provided to the at least one of the plurality of target areas of the substrate are separated by a time delay.

4. The lithographic apparatus of claim 3 wherein the lithographic apparatus is operable to expose at least one other target area of the substrate during the time delay.

5. The lithographic apparatus of any one of claims 2 to 4 wherein the at least one of the plurality of target areas to which the nominal dose of radiation is provided across a plurality of exposures is disposed at a peripheral portion of the substrate.

6. The lithographic apparatus of any one of claims 2 to 5 wherein the at least one of the plurality of target areas of the substrate to which the nominal dose of radiation is provided across a plurality of exposures receives a substantially equal radiation dose during each exposure.

7. The lithographic apparatus of any preceding claim wherein the lithographic apparatus is arranged so that the nominal dose of radiation is provided to at least one of the plurality of target areas of the substrate in a single exposure.

8. The lithographic apparatus of claim 7 wherein the nominal dose of radiation is provided to all target areas of the substrate which are not used for manufacturing integrated circuitry in a single exposure.

9. The lithographic apparatus of any preceding claim wherein the data describes mechanical behaviour of the substrate in dependence on a pattern density of a pattern of the radiation.

10. The lithographic apparatus of any preceding claim wherein the data describes mechanical behaviour of the substrate in response to the nominal dose being received as a single exposure.

11. The lithographic apparatus of any preceding claim comprising a substrate support which supports the substrate.

12. The lithographic apparatus of claim 11 wherein the data describes mechanical behaviour of the substrate in dependence on a force exerted on the substrate by a substrate support.

13. The lithographic apparatus of claim 11 or claim 12 wherein the data is indicative of whether or not a target area of the substrate will slip relative to the substrate support after receiving the nominal dose of radiation in a single exposure.

14. The lithographic apparatus of any preceding claim wherein the number of exposures for each target area are determined such that lithographic throughput is optimised.

15. The lithographic apparatus of any preceding claim wherein the lithographic apparatus is a scanning apparatus and wherein a dose of radiation received by target areas of the substrate is controlled by controlling a speed at which the substrate moves relative to the radiation.

16. The lithographic apparatus of any preceding claim wherein a dose of radiation received by target areas of the substrate is controlled by controlling a power of the radiation.

17. The lithographic apparatus of claim 16 wherein the radiation is pulsed and wherein the power of the radiation is controlled by controlling a repetition rate of the radiation.

18. A lithographic apparatus operable to provide a plurality of target areas of a substrate with a nominal dose of radiation, wherein the nominal dose of radiation is provided to each of the plurality of target areas of the substrate across a number of exposures, the number of exposures for each target area being determined in dependence on a position of that target area on the substrate.

19. The lithographic apparatus of claim 18 wherein the lithographic apparatus is arranged so that the nominal dose of radiation is provided to at least one of the plurality of target areas across a plurality of exposures and wherein said at least one of the plurality of target areas correspond to target areas of the substrate which are disposed at a peripheral portion of the substrate.

20. The lithographic apparatus of claim 18 or claim 19 wherein the lithographic apparatus is arranged so that the nominal dose of radiation is provided to target areas of the substrate which are disposed at a central portion of the substrate as a single exposure.

21. The lithographic apparatus of any preceding claim, wherein the lithographic apparatus comprises a radiation source operable to produce the radiation for providing the plurality of target areas with the nominal dose of radiation.

22. The lithographic apparatus of any preceding claim, wherein the lithographic apparatus comprises a patterning device support structure for supporting a patterning device so as to pattern a radiation beam for providing the plurality of target areas with the nominal dose of radiation.

23. The lithographic apparatus any preceding claim, wherein the lithographic apparatus comprises a projection system for projecting a radiation beam onto the substrate so as to provide the plurality of target areas with the nominal dose of radiation.

24. The lithographic apparatus of any preceding claim, wherein the lithographic apparatus comprises a control system configured to control the lithographic apparatus so as to provide the plurality of target areas of the substrate with the nominal dose of radiation.

25. The lithographic apparatus of any preceding claim wherein the lithographic apparatus is operable to determine the number of exposures across which one or more of the plurality of target areas are provided with nominal dose of radiation.

26. A method of selecting a number of exposures to radiation a target area of a substrate supported by a substrate support receives, comprising:

simulating mechanical behaviour of the target area upon receiving a nominal dose of radiation; determining whether or not the target area will slip relative to the substrate support when the nominal dose of radiation is provided to the target area in one exposure and across multiple exposures; and

selecting the lowest number of exposures across which the nominal dose of radiation is provided for which the target area does not slip relative to the substrate support.

27. A method of exposing to radiation a substrate which comprises a plurality of target areas and which is supported by a substrate support, comprising:

selecting, for each target area of the plurality of target areas, an optimum number of exposures, wherein the optimum number of exposures corresponds to the lowest number of exposures across which a nominal dose of radiation may be delivered to the target area for which the target area does not slip relative to the substrate support; and

exposing each target area of the plurality of target areas to the nominal dose of radiation, wherein the nominal dose of radiation is provided to each target area across the optimum number of exposures.

28. The method of claim 27 further comprising:

determining a routing for the plurality of target areas, wherein each target area of the plurality of target areas receives the optimum number of exposures and wherein a thermal recovery time between repeated exposures of the same target area is optimised; and

wherein the exposing of each target area of the plurality of target areas uses the routing determined for the plurality of target areas.