Method and apparatus for conditioning and directing a radiation beam

Abstract

A method for selecting an illumination mode for use in a lithographic apparatus comprises: selecting a target spatial distribution of radiation for illumination of a patterning device; and, separately, selecting a target angular distribution of the radiation for illumination of the patterning device. The method further comprises selecting a configuration of illumination optics of the lithographic apparatus that achieves both the target spatial distribution and the target angular distribution of the radiation in an illumination region of the lithographic apparatus.

Description

METHOD AND APPARATUS FOR CONDITIONING AND DIRECTING A RADIATION BEAMCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of EP application 23210008.1 which was filed on 15 November 2023 and EP application 23214179.6 which was filed on 5 December 2023 and which are incorporated herein in its entirety by reference.FIELD

[0002] The present invention relates to a method for selecting an illumination mode for use in a lithographic apparatus. It may have particular application to an extreme ultraviolet (EUV) lithographic apparatus. The present invention also relates to a lithographic method that uses the illumination mode. The present invention also relates to corresponding apparatus, including an illumination system for a lithographic apparatus and lithographic apparatus comprising the illumination system.BACKGROUND

[0003] A lithographic apparatus is a machine constructed to apply a desired pattern onto a substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). A lithographic apparatus may, for example, project a pattern 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] It is desirable to provide control over various properties of a radiation beam that is used in a lithographic apparatus to project a pattern onto a substrate. For example, it is desirable to control a dose of radiation that is delivered to the substrate. Furthermore, it is desirable to provide control over the shape (i.e. the spatial distribution) of the radiation beam in a plane of the patterning device. In addition, it is desirable to provide control over the angular distribution of the radiation beam in a plane of the patterning device. The angular distribution of the radiation beam in a plane of the patterning device may be referred to as an illumination mode or pupil and this may be chosen in dependence on the pattern being imaged to increase image contrast on the substrate.

[0006] It may be desirable to provide new, alternative apparatus and / or methods for conditioning, directing and / shaping a radiation beam (for example in a lithographic apparatus) that at least partially addresses one or more problems associated with prior art arrangements whether identified herein or otherwise.SUMMARY

[0007] According to a first aspect of the present disclosure there is provided a method for selecting an illumination mode for use in a lithographic apparatus, the method comprising: selecting a target spatial distribution of radiation for illumination of a patterning device; separately selecting a target angular distribution of the radiation for illumination of the patterning device; and selecting a configuration of illumination optics of the lithographic apparatus that achieves both the target spatial distribution and the target angular distribution of the radiation in an illumination region of the lithographic apparatus.

[0008] The lithographic apparatus may be an extreme ultraviolet (EUV) lithographic apparatus and the radiation may comprise EUV radiation.

[0009] The illumination region of the lithographic apparatus is intended to mean a region in a plane that, in use, the patterning device (reticle) is disposed in (or moves through for scanning exposures) that receives radiation. Therefore, a spatial distribution and the angular distribution of radiation in the illumination region of the lithographic apparatus will be the spatial distribution and angular distribution respectively with which a patterning device is illuminated. Therefore, as used herein, the “illumination region” may also be synonymous with the term “slit”.

[0010] The angular distribution of the radiation for illumination of the patterning device (reticle) may be characterized by an intensity of radiation in pupil plane of the illumination optics of the lithographic apparatus. The pupil plane is a Fourier transform plane of the plane of the patterning device (reticle). Therefore, the angular distribution of the radiation for illumination of the patterning device may alternatively be referred to as the “pupil” or “illumination pupil”. As used herein, “the angular distribution of the radiation for illumination of the patterning device" may be synonymous with the terms “pupil” or “illumination pupil”. Selection of an appropriate illumination pupil for a given lithographic process can be important for optimizing imaging performance (in particular for contrast).

[0011] The spatial distribution of radiation for illumination of a patterning device describes the intensity of radiation in the plane of the patterning device (reticle). The plane of the patterning device (reticle) has a non-scanning direction (x) and a scanning direction (y). The non-scanning direction may alternatively be referred to as the slit direction. The spatial intensity distribution integrated in the scanning direction (y) as a function of the slit direction (x) may be refereed to as the “slit profile”. The spatial intensity distribution as a function of the scanning direction is sometimes referred to as the“slit profile through scan”. As used herein, “the spatial distribution of radiation for illumination of a patterning device” may be synonymous with the terms “slit profile” or “slit profile through scan”. Selection of an appropriate “slit profile” and “slit profile through scan” can be important for dose control (i.e. ensuring that all parts of the wafer that receive radiation receive substantially the same dose of radiation). In turn, dose control is important for critical dimension uniformity (CDU).

[0012] As explained further below, in existing lithographic methods a target angular distribution of the radiation for illumination of the patterning device or pupil is typically selected based on the lithographic process. However, in known existing methods, the spatial distribution of radiation is achieved in such a way that once the pupil has been selected the spatial distribution of radiation for illumination of the patterning device is fixed. Furthermore, with known lithographic methods there is generally a variation in the pupil across the scanning direction of the illumination region (the pupil is ‘dynamic’ during scan). In particular, there is typically a correlation between angular range in pupil and position in the illumination region in a scanning direction of the lithographic apparatus. This correlation results in undesirable overlay errors. Furthermore, the correlation becomes greater (and, therefore, overlay errors become larger) if an illumination sub-system of the lithographic apparatus is optimized so as to provide greater robustness to energy variations and energy drop outs of the radiation source. Therefore, with existing lithographic methods there is a tension between achieving good overlay performance on the one hand and good robustness to energy variations and energy drop outs of the radiation source.

[0013] The method according to the first aspect is therefore advantageous, as it allows for both the pupil and the slit profile through scan to be selected separately. Advantageously, this allows for an illumination mode wherein there is no, or negligible, correlation between pupil and position in the illumination region in a scanning direction of the lithographic apparatus. Advantageously, this allows for overlay performance and robustness to energy variations and energy drop outs of the radiation source to be disentangled (or co-optimized in case fully disentanglement is not achievable).

[0014] The radiation source may be operated at a lower power than a maximum achievable power and a controller of the radiation source may be operable to compensate for drops in source energy using this reserved power capability (referred to as a “dose margin”). Therefore, by increasing the robustness of the lithographic process to energy variations and energy drop outs of the radiation source, a smaller dose margin can be used. Effectively, this increases the available power of the radiation source and therefore increases lithographic throughput.

[0015] The selection of the configuration of illumination optics of the lithographic apparatus may be made such that a variation in the angular distribution of the radiation over the illumination region is minimized.

[0016] In particular, the variation in the angular distribution of the radiation over the illumination region in a scanning direction may be minimized.

[0017] Advantageously, this allows for the target spatial distribution (for example the scan profile) to be optimized without impact (or with minimum impact) on overlay performance. In particular, the target spatial distribution (for example the scan profile) may be optimized to maximize lithographic throughput whilst allowing sufficient robustness to energy variations and energy drop outs of the radiation source with minimum impact on overlay performance. For example, a scan profile may be selected that has greater robustness to energy variations and energy drop outs of the radiation source(for example, by having larger and shallower peripheral portions) which, in turn, can allow for a smaller dose margin to be used (which increases lithographic throughput) with little or no effect on overlay performance. This is in contrast to existing arrangements wherein the provision of such larger and shallower peripheral portions would typically increase overlay errors.

[0018] Preferably, the selection of the configuration of illumination optics of the lithographic apparatus is made such that the angular distribution of the radiation is substantially uniform over the illumination region. However, it will be appreciated by the skilled person that this may not always be possible and is dependent both on (a) the selected pupil and (b) the flexibility of the illumination optics of the lithographic apparatus. Therefore, the selection of the configuration of illumination optics of the lithographic apparatus may be made such that the variation in angular distribution of the radiation through the scanning direction is minimized.

[0019] The selection of the configuration of illumination optics of the lithographic apparatus may be made such that a non-telecentricity of the radiation over the scan direction of the illumination region is minimized.

[0020] The selection of the configuration of illumination optics of the lithographic apparatus may be made so as to co-optimize overlay performance of the lithographic apparatus and robustness to energy variations and energy drop outs of a radiation source of the lithographic apparatus.

[0021] Optimizing overlay performance may mean improving overlay robustness to focus variation through scan due to, for example, wafer non-flatness or optics errors; and / or improving overlay correction by utilizing wafer stage tilt.

[0022] It will be appreciated that if the illumination optics of the lithographic apparatus does not have sufficient flexibility to decouple correlation between pupil angular range and position within the illumination slit, the method may balance the productivity of the lithographic apparatus and the overlay performance of the lithographic apparatus.

[0023] The selecting the configuration of illumination optics of the lithographic apparatus may comprise: for each of a plurality of pixels in the pupil plane that it is desired to be illuminated: determining a plurality of optical pathways of the illumination optics that contribute to that pixel; and, for each of the plurality of optical pathways, determining: an intensity and a portion of the illumination region that it contributes to; and selecting a set of the plurality of optical pathways for all of the plurality of pixels in the pupil plane that it is desired to illuminate so as to achieve the target spatial distribution and the target angular distribution of radiation in the illumination region.

[0024] It will be appreciated that the pupil plane may be divided into a two-dimensional array of pixels. It will be further appreciated that a pixel in the pupil plane that it is desired to be illuminated is a pixel that corresponds to a portion of the pupil plane for which the target pupil has a non-zero value.

[0025] The method may be for a lithographic apparatus comprising: a first optical component comprising a two-dimensional array of independently movable reflective optical elements which is arranged to receive radiation from a radiation source; and a second optical component comprising atwo-dimensional array of independently movable reflective optical elements and which is arranged to receive radiation from the first optical component and to direct it to an illumination region. For such embodiments, selecting a configuration of illumination optics of the lithographic apparatus may comprise: selecting a configuration of the first optical component and the second optical component.

[0026] The target spatial distribution of radiation may be a profile shape in a scanning direction of a lithographic apparatus.

[0027] The profile shape in the scanning direction may be referred to as the “slit profile through scan”. In some embodiments, the profile shape in the scanning direction may have a generally trapezoidal shape. That is, in some embodiments, the profile shape in the scanning direction may comprise a central portion that is generally uniform or flat and two peripheral portions. The intensity of the radiation in the peripheral portions may vary as a function of distance from the central portion from the value of the central portion to zero. A gradient, and / or extent, of the peripheral portions may characterize how robust the lithographic apparatus and method are to energy variations and energy drop outs of the radiation source. Larger, shallower peripheral portions are more robust to energy variations and energy drop outs of the radiation source whereas smaller, steeper peripheral portions are less robust to energy variations and energy drop outs of the radiation source.

[0028] Selection of the target spatial distribution may be made in dependence on a target dose.

[0029] For example, for relatively low dose layers the slit profile through scan may have shallower peripheral portions in the scanning direction. For relatively high dose layers the slit profile through scan may have steeper peripheral portions in the scanning direction.

[0030] Selection of the target spatial distribution may be made in dependence on a target scanning speed of the lithographic apparatus.

[0031] For example, for relatively high scanning speeds the slit profile through scan may have shallower peripheral portions in the scanning direction. For relatively low scanning speeds the slit profile through scan may have steeper peripheral portions in the scanning direction.

[0032] Selection of the target spatial distribution may be made in dependence on a dimension of an illumination region of the lithographic apparatus in a scanning direction.

[0033] That is, the slit profile through scan (for example in the scanning direction) may be dependent on the width of the slit (in the scanning direction).

[0034] The method according to the first aspect may form part of a method (for example executed by a computer) for improving imaging of a feature on a reticle (mask) to a substrate during a scanning exposure operation of a lithographic apparatus. The method may comprise obtaining: (a) an expected focus variation through scan; a radiation source instability; and (c) a target dose (and scan speed). The selection of the configuration of illumination optics may co-optimize a dynamic pupil and slit profile in the scanning direction to minimize an overlay error during the scanning exposure operation due to interaction of the dynamic pupil with the effective focus variation through scan while maximizing throughput (limited by robustness of source energy instability).

[0035] The method according to the first aspect of the present disclosure may further comprise illuminating a patterning device with radiation using the selected configuration of illumination optics of the lithographic apparatus.

[0036] According to a second aspect of the present disclosure there is provided a lithographic method comprising: illuminating a patterning device using an illumination mode selected using the method according to the first aspect of the present disclosure; collecting patterned radiation scattered from the patterning device; and projecting the patterned radiation onto a substrate so as to form an image of the patterning device on the substrate.

[0037] The patterning device may be a reticle or mask. The patterning device may be disposed in the illumination region. The substrate may comprise a resist-coated silicon wafer.

[0038] The method may be a scanning exposure process. That is, illuminating the patterning device may comprise illuminating an illumination region (also known as “illumination slit” or just “slit”) while the patterning device is moved (or “scanned”) through the illumination region in a scanning direction. Similarly, projecting the patterned radiation onto the substrate may comprise moving (or “scanning”) the substrate through a wafer-level illumination region. The movement of the substrate may be such that the image of the patterning device is stationary with respect to the substrate.

[0039] The lithographic method may comprise selecting the illumination mode using the method according to the first aspect of the present disclosure.

[0040] That is, in some embodiments, the method may comprise an initial step of selecting the illumination mode using the method according to the first aspect of the present disclosure.

[0041] The lithographic method may comprise retrieving the illumination mode from memory.

[0042] That is, in some embodiments, the method may comprise an initial step of retrieving the illumination mode from memory (for example storage media). It will be appreciated that the method according to the first aspect of the present disclosure may have previously been used to select the illumination mode and then it may have been stored in memory.

[0043] Illuminating a patterning device may comprise: directing a radiation beam to a first optical component comprising a two-dimensional array of independently movable reflective optical elements; using the first optical component to direct at least a portion of the radiation to a second optical component comprising a two-dimensional array of independently movable reflective optical elements; and using the second optical component to direct at least a portion of the radiation to the patterning device.

[0044] According to a third aspect of the present disclosure there is provided a computer comprising: one or more processors; and storage media, the storage media having instructions to cause the one or more processors to carrying out the method according to the first aspect of the present disclosure.

[0045] The one or more processors may be configured to store on the storage media the selected configuration of illumination optics of the lithographic apparatus that achieves both the target spatialdistribution and the target angular distribution of the radiation in an illumination region of the lithographic apparatus.

[0046] According to a fourth aspect of the present disclosure there is provided a computer-readable medium having instructions for carrying out the method according to the first aspect of the present disclosure.

[0047] The computer-readable medium may be a non-transitory computer-readable medium.

[0048] According to a fifth aspect of the present disclosure there is provided an illumination system for a lithographic apparatus, the illumination system comprising: a first optical component comprising a two-dimensional array of independently movable reflective optical elements which is arranged to receive radiation from a radiation source; a second optical component comprising a two-dimensional array of independently movable reflective optical elements and which is arranged to receive radiation from the first optical component and to direct it to an illumination region; and a controller operable to control the first and second optical components; wherein the controller is configured so as to allow for separate control over a spatial and angular distribution of the radiation in the illumination region.

[0049] The illumination system according to the fifth aspect of the present disclosure may be operable to carry out the methods of the first and second aspects of the present disclosure.

[0050] Alternatively, the illumination system according to the third aspect of the present disclosure may be operable to execute a configuration determined by the first and second aspects of the present disclosure (which may be determined by a processor that does not form part of the lithographic apparatus).

[0051] The controller may be configured so as to carry out the method according to the first aspect of the present disclosure.

[0052] The controller may be configured to retrieve a configuration of the first and second optical elements from memory.

[0053] The controller may be configured to control the first and second optical components such that a variation in the angular distribution of the radiation for illumination of the patterning device over the illumination region is minimized.

[0054] In some embodiments, the controller may be configured to control the first and second optical components such that a non-telecentricity of the radiation over the scan direction of the illumination region is minimized.

[0055] The controller may be configured to control the first and second optical components so as to co-optimize overlay performance of the lithographic apparatus and robustness to energy variations and energy drop outs of a radiation source of the lithographic apparatus.

[0056] Each of the independently movable reflective optical elements of the first optical component may comprise a micro-electromechanical system (MEMS) micro-mirror.

[0057] Therefore, the first optical component may be considered to comprise a MEMS micromirror array.

[0058] Each of the independently movable reflective optical elements of the second optical component may comprise a micro-electromechanical system (MEMS) micro-mirror.

[0059] Therefore, the second optical component may be considered to comprise a MEMS micromirror array.

[0060] The second optical component may be disposed in a plane that is not a pupil plane of the illumination region.

[0061] According to a sixth aspect of the present disclosure there is provided a lithographic apparatus or a lithographic system comprising the illumination system according to the fifth aspect of the present disclosure.

[0062] The lithographic apparatus may further comprise a radiation source operable to generate a pulsed radiation beam and to supply said pulsed radiation beam to the first optical component.

[0063] The lithographic apparatus may further comprise a sensor operable to determine an intensity of a portion of one or more pulses of the pulsed radiation beam. A pulse of the pulsed radiation beam may be generated by the radiation source in dependence on an intensity determined by the sensor upon receipt of a portion of a previous pulse of radiation.

[0064] For example, the sensor may determine, for example pulse-by-pulse, an energy of the radiation beam and this may be used as part of a feed-back loop to stabilize an energy of the radiation beam.

[0065] The lithographic apparatus may further comprise a support structure configured to support a patterning device such that the patterning device is disposed in, or is movable through, the illumination region.

[0066] The lithographic apparatus may further comprise: a substrate table configured to support a substrate; and a projection system comprising imaging optics configured to receive radiation from the illumination region and to form an image of an object disposed in the illumination region on a substrate supported by the substrate table.BRIEF DESCRIPTION OF THE DRAWINGS

[0067] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:Figure 1 depicts a lithographic system comprising a lithographic apparatus and a radiation source;Figure 2A schematically shows a generally circular portion of the field facet mirror device of the lithographic system shown in Figure 1, showing a central obscuration portion two portions which receive radiation;Figure 2B shows an example shape of a field facet for a faceted field mirror device in a known EUV lithographic apparatus of the form shown in Figure 1;Figure 3A shows an example of an intensity profile of a radiation beam in a scanning direction that has a generally trapezoidal shape;Figure 3B shows two variants of the example intensity profile shown in Figure 3A; a first variant of the example intensity profile has larger, shallower peripheral portions whereas a second variant of the example intensity profile has smaller, steeper peripheral portions;Figure 4 shows: an example slit profile; the pupil at three different positions in the scanning direction through the slit profile; and an illustration of the aerial image formed by the projection system at these three different positions (bottom) in the scanning direction through the slit profile;Figure 5 is a schematic illustration of a method according to an embodiment of the present disclosure for selecting an illumination mode for use in a lithographic apparatus;Figure 6 schematically shows a plurality of sub-steps that may form part of a step of selecting the configuration of illumination optics of the lithographic apparatus in the method shown in Figure 5;Figure 7 is a schematic illustration of a lithographic method according to an embodiment of the present disclosure that uses for an illumination mode selected using the method shown in Figure 5;Figure 8 schematically shows a plurality of sub-steps that may form part of a step of illuminating a patterning device in the lithographic method shown in Figure 7; andFigure 9 shows an illumination system according to an embodiment of the present disclosure that is for a lithographic apparatus of the type shown in Figure 1.DETAILED DESCRIPTION

[0068] Figure 1 shows a lithographic system comprising a radiation source SO and a lithographic apparatus LA. The radiation source SO is configured to generate an EUV radiation beam B and to supply the EUV radiation beam B to the lithographic apparatus LA. The lithographic apparatus LA comprises an illumination system IL, a support structure MT configured to support a patterning device MA (e.g., a mask), a projection system PS and a substrate table WT configured to support a substrate W.

[0069] The illumination system IL is configured to condition the EUV radiation beam B before the EUV radiation beam B is incident upon the patterning device MA. Thereto, the illumination system IL may include a facetted field mirror device 10 and a facetted pupil mirror device 11. The faceted field mirror device 10 and faceted pupil mirror device 11 together provide the EUV radiation beam B with a desired cross-sectional shape and a desired intensity distribution. The illumination system IL may include other mirrors or devices in addition to, or instead of, the faceted field mirror device 10 and faceted pupil mirror device 11.

[0070] 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 areconfigured 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).

[0071] 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.

[0072] 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.

[0073] The radiation source SO shown in Figure 1 is, for example, of a type which may be referred to as a laser produced plasma (LPP) source. A laser system 1, which may, for example, include a CO2 laser, is arranged to deposit energy via a laser beam 2 into a fuel, such as tin (Sn) which is provided from, e.g., a fuel emitter 3. Although tin is referred to in the following description, any suitable fuel may be used. The fuel may, for example, be in liquid form, and may, for example, be a metal or alloy. The fuel emitter 3 may comprise a nozzle configured to direct tin, e.g. in the form of droplets, along a trajectory towards a plasma formation region 4. The laser beam 2 is incident upon the tin at the plasma formation region 4. The deposition of laser energy into the tin creates a tin plasma 7 at the plasma formation region 4. Radiation, including EUV radiation, is emitted from the plasma 7 during deexcitation and recombination of electrons with ions of the plasma.

[0074] 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.

[0075] 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.

[0076] 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 virtualradiation 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.

[0077] 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.

[0078] The faceted field mirror device 10 and the faceted pupil mirror device 11 are arranged to provide a desired angular distribution of the radiation beam B, at the patterning device MA, as well as a desired uniformity of radiation intensity at the patterning device MA. The illumination system IL may be arranged to provide Kohler illumination of an illumination region IR (that the patterning device MA may be moved through during exposure of a substrate W) such that the plasma at the plasma formation region 4 is out of focus (and therefore does not influence properties of the radiation beam) at the patterning device MA and in the conjugate plane of the substrate W. As used herein, the illumination region IR may also be referred to as the illumination slit or the slit.

[0079] In lithography, the illumination of the patterning device MA is very important. In particular, it is desirable to control the angular distribution of the radiation at the illumination region IR where the patterning device MA is exposed to radiation. This angular distribution of the radiation is conveniently described in terms of the spatial distribution of the radiation in an illumination pupil plane, which describes how a cone of light that is incident on each point on the patterning device MA is filled. In a conventional illumination mode, the radiation uniformly fills a circular region of the illumination pupil plane centred on the optical axis such that each point on the patterning device is illuminated by a solid cone of light. In dipole illumination mode, the radiation fills two regions of the illumination pupil plane that are spaced apart from, and on opposite sides to, the optical axis. Many other illumination modes are known. In principle, an optimum illumination mode can be defined to image a given pattern under given conditions. Therefore, it is desirable to provide flexibility in the illumination mode.

[0080] The uniformity of the illumination is also very important. The uniformity of illumination affects the uniformity of dose to which the target portion of the substrate W is exposed, which affects critical dimension uniformity (CDU), an important measure of the uniformity of the dimension of features formed on the substrate W. For example, it may be desirable to maintain a desired spatial intensity distribution of radiation across the illumination region IR. As used herein the spatial intensity distribution of radiation across the illumination region IR may be referred to as the slit profile.

[0081] The collector 5 is generally of the form of a concave mirror, which is arranged to collect the radiation which is emitted from the plasma formation region 4 into a solid angle subtended by the collector 5. This radiation is reflected and focused at the intermediate focus 6. As a result, within the housing, the radiation beam B is generally of the form of a converging cone of radiation, which converges at the intermediate focus 6, an outer edge of this cone being indicated in Figure 1 by two lines. Downstream of the intermediate focus 6, the radiation beam B is generally of the form of adiverging cone of radiation, which is incident on the generally circular field facet mirror device 10. However, the radiation source SO may comprise an obscuration that will block a portion of this radiation cone such that there will be some portion of the diverging cone of radiation that which will not receive radiation from the collector 5. For example, the radiation source SO may comprise a shield (not shown) which may be arranged to prevent the laser beam 2 from propagating through the opening 8 and into the lithographic apparatus LA (where it may damage optical components). This shield may be supported by the enclosing structure via a support (not shown). Together, the shield and the support form an obscuration of the radiation source SO. Therefore, as indicated schematically in Figure 2A a generally circular portion 20 of the field facet mirror device 10 may comprise a central portion 22, which coincides with the obscuration of the radiation source SO and does not receive any radiation, and two portions 24, 26 which do receive radiation. It will be appreciated that this is schematic and that the obscuration may have any shape or configuration.

[0082] The total amount of radiation energy delivered by the lithographic apparatus LA to a particular target area on the surface of the substrate W is referred to as the dose. The desired dose to be delivered to target areas on a substrate W (for example a dose that achieves a change or curing of a photoresist on the surface of the substrate W) may differ depending on the sensitivity of the photoresist. For example, in the case of a highly sensitive photoresist, a relatively small dose (e.g. 20 mJ / cm2) may be desired in order to image the pattern onto the substrate W. However, in the case of a less sensitive photoresist, a larger dose (e.g. 70 mJ / cm2) may be desired.

[0083] The radiation, e.g. EUV radiation, may be delivered in pulses, for example at a frequency in the order of 50 kHz. In the case of a pulsed radiation beam, the total dose delivered to the substrate is the sum of the individual doses delivered in each pulse.

[0084] If a target area on a substrate W receives a dose which differs from the desired dose, this is referred to as a dose error. In particular, the dose error is the difference between the actual dose received by a target area and the desired dose. A dose error may be ‘positive’ (i.e. in the event that a target area receives a larger dose than the desired dose) or ‘negative’ (i.e. in the event that a target area receives a smaller dose than the desired dose). Any dose error will result in some printing errors as some parts of the resist will receive an insufficient dose and some parts will receive an excess dose. A negative dose error may be corrected by providing an additional dose in order to compensate the previous shortfall. However, it may not be possible to correct a positive dose error since an excessive dose (e.g. leading to overexposure) has already been given, the change in the photoresist generally being irreversible. This may result in irreparable damage to the substrate W.

[0085] It may be desirable, therefore, to operate the lithographic system such that none of the target areas receive an excessive dose. It will be appreciated that a certain level of positive dose error may be considered to be acceptable in accordance with manufacturing tolerances. For example, a positive dose error of up to 1%, in particular up to 0.5% may be considered to be acceptable, depending onrequirements. An excessive dose as referred to herein means a dose which falls outside of the acceptable tolerance level.

[0086] In practice, the output power of a pulsed radiation source SO will vary with time (under either open-loop or closed-loop control). If the power of an individual pulse is higher or lower than a nominal or desired output power for each pulse then this will contribute to a dose error for any parts of the substrate W that receive that pulse. The fewer pulses received by each part of the substrate W, the greater the effect of such an individual pulse power error will be on the total dose. In some cases, the available open-loop power may be instantaneously lower than the requested operating power, resulting in too low a dose being delivered at that moment. In order to reduce the likelihood of too small a dose being delivered (also referred to as a negative dose error), the nominal operating power may be set such that it is lower than the maximum output power. This may allow for the negative dose error to be at least partially corrected by increasing the power of one or more subsequent pulses.

[0087] However, if the nominal operating power is set too low, the throughput of the system may be negatively affected. It will be appreciated, therefore, that in order to improve the throughput of the lithographic apparatus it may be desirable to increase the nominal output power of the radiation source SO. Therefore, there is a tension between limiting the effect of negative dose errors on the overall total dose and the throughput of the lithographic apparatus.

[0088] Even when controlling the radiation source using a closed loop control, there is a chance that at least one target area of the substrate W receives an actual dose of radiation which is less than the desired dose. This can be compensated for by a second, subsequent exposure pass. In other words, if the system fails to deliver a desired dose to a target area on the substrate W at a given instant during a first time period, that target area may be re-exposed at a later step in order to deliver the missing dose. Such re-exposures may be referred to as die repair.

[0089] The dose margin represents a difference between a maximum available output power of the radiation source SO and a nominal operating power of the radiation source SO and may be determined in dependence on a setpoint value.

[0090] It will be appreciated that the maximum available output power (open-loop power) and / or an operating power (closed-loop power) of the radiation source SO may be not constant but rather may be subject to some variations (for example, for a pulsed radiation source SO there may be pulse-to- pulse variations). It will therefore be understood that the dose margin may represent a difference between the average or nominal maximum available output power of the radiation source SO and an average or nominal operating power of the radiation source SO (rather than being based on instantaneous powers).

[0091] For example, if a dose margin of 10% is used, the output power may be reduced by 10% relative to the average open-loop power in order to give the nominal operating power (which is therefore 90% of the average open-loop power). This allows better controllability of the amount of radiation delivered to the substrate W, in particular since, on a pulse -by-pulse basis, in general a requestedoperating power may be increased or decreased (relative to nominal operating power) according to requirements. It will, of course, be appreciated that if the radiation source SO were operated at maximum available output power as nominal operating power, in general, there would be no scope to request an increased operating power relative to the nominal operating power in any circumstance (and therefore no facility to compensate for negative dose errors).

[0092] A known faceted field mirror device 10 used in an EUV lithographic apparatus of the type shown in Figure 1 is now described with reference to Figures 2 A and 2B.

[0093] In known EUV lithographic apparatus, the faceted field mirror device 10 comprises a plurality of field facets. In particular, in such known EUV lithographic apparatus, the portions 24, 26 of the faceted field mirror device 10 that receive radiation from the radiation source SO are provided with a plurality of field facets. For example, the portions 24, 26 of the faceted field mirror device 10 that receive radiation from the radiation source SO may be provided with of the order of 100 field facets (for example 300 field facets). In a plane of the faceted field mirror device 10 each field facet may have a curved shape 28, as shown in Figure 2B. In general, in the plane of the faceted field mirror device 10 each field facet may have an elongate shape having a longer dimension in an x-direction and a shorter dimension in a y-direction. Each field facet comprises a mirror that may be arranged to image the intermediate focus 6 onto the faceted pupil mirror device 11. To achieve this, the field facets may, for example, be concave.

[0094] The faceted pupil mirror device 11 comprises a plurality of pupil facets. The faceted pupil mirror device 11 is arranged to project an image of each field facet of the faceted field mirror device 10 onto an illumination region IR (also referred to as the slit or illumination slit) at the patterning device MA. The illumination system IE is configured so that each field facet is imaged on the illumination region IR in an overlapping manner. The overlap of the images of the field facets at least partially evens out irregularities in the radiation beam B provided by the radiation source SO.

[0095] The illumination region IR may be curved or straight. In general, in a scanning lithographic apparatus LA, the illumination region IR is elongate having a longer dimension and a shorter dimension. The shorter dimension may coincide with a scanning direction of the support structure MT and the longer dimension may coincide with a non-scanning direction. The illumination region IR may be curved or straight. The illumination region IR is indicated in Figure 1, which shows the patterning device in cross section. The longer dimension (in the x-direction) of the illumination region IR is perpendicular to the plane of Figure 1 and the shorter dimension (in the y-direction) of the illumination region IR lies in the plane of Figure 1.

[0096] In some known lithographic apparatus and methods, a substrate W (for example a resist- coated silicon wafer) is exposed to patterned radiation B’ during a scanning exposure process. A radiation beam illuminates B an illumination region IR (also known as “illumination slit” or just “slit”) while a patterning device MA (for example a reticle) is scanned through the illumination region in a scanning direction. Typically the radiation comprises pulsed radiation. Radiation scattered from thepatterning device MA is collected by imaging optics PS and projected onto a substrate W so as to form an image of the patterning device MA in a plane of the substrate W. The substrate W is also scanned so that the image is stationary with respect to the substrate W.

[0097] The pulsed radiation source SO will, in general, have instabilities (pulse to pulse energy variations) and occasionally will have energy drop outs (one or more pulses with energy significantly less than a target or nominal pulse energy). As explained above, the radiation B may be operated at a lower power than a maximum achievable power and a controller of the radiation source SO may be operable to compensate for drops in source energy using this reserved power capability (referred to as a “dose margin”).

[0098] It is important for each part of the substrate W that receives radiation to receive substantially the same dose of radiation (for example to maintain critical dimension uniformity CDU). However, since the radiation beam B is pulsed, in general, not all parts of the substrate W will receive exactly the same number of pulses. Typically, the intensity profile of the radiation beam B in the scanning direction has a generally trapezoidal shape. An example of such an intensity profile 30 is shown in Figure 3A. This shape can help with accurate dose control. Furthermore, the trapezoidal shape defines the robustness of the lithographic apparatus LA dose performance to radiation source SO instabilities and energy drop outs, as now discussed. An intensity profile of the radiation beam B in the scanning direction with a shallower slope is more robust to source energy drop out and can improve the correctability of the die repair.

[0099] The example profile shape 30 in the scanning direction shown in Figure 3A has a generally trapezoidal shape. That is, the profile shape 30 in the scanning direction comprises a central portion 32 that is generally uniform or flat and two peripheral portions 34a, 34b. The intensity of the radiation in the peripheral portions 34a, 34b varies as a function of distance from the central portion 32 from the value of the central portion 32 to zero. A gradient, and / or extent, of the peripheral portions 34a, 34b may characterize how robust the lithographic apparatus LA is to energy variations and energy drop outs of the radiation source SO. Two variants of the example intensity profile 30a, 30b are shown in Figure 3B; a first variant of the example intensity profile 30a has larger, shallower peripheral portions 34a, 34b whereas a second variant of the example intensity profile 30b has smaller, steeper peripheral portions 34a, 34b. Larger, shallower peripheral portions 34a, 34b are more robust to energy variations and energy drop outs of the radiation source SO whereas smaller, steeper peripheral portions 34a, 34b are less robust to energy variations and energy drop outs of the radiation source SO.[000100] In one known EUV lithographic system, a laser-produced plasma (LPP) radiation source SO illuminates a first optical component 10 with EUV radiation. The first optical component 10 comprises a plurality of facet mirrors, each being generally concave and having a shape that is similar to that of the illumination region IR. The radiation from all of the facet mirrors is directed to the illumination region IR via a second optical component 11 such that an image of each facet mirror is formed, the plurality of images generally overlapping so as to achieve a desired intensity distribution(e.g. the trapezoidal profile). In a central portion 32 of the illumination region IR (in the scanning direction), substantially all of the field facet images overlap and intensity is at maximum. In the peripheral portions 34a, 34b of the illumination region IR (in the scanning direction), edges of the field facet images are staggered so as to achieve an intensity fall-off and realize the trapezoidal intensity profile 30.[000101] This known illumination system can achieve a trapezoidal slitprofile in the scanning direction (y), however, there is typically a variation in the pupil over the illumination region IR in a scanning direction of the lithographic apparatus LA. In particular, one peripheral portion 34a of the slit receives radiation predominantly from a first angular range; the central portion 32 receives radiation from substantially the whole angular range; the other peripheral portion 34b of the slit receives radiation predominantly from a second, complementary angular range.[000102] This is illustrated in Figure 4. Figure 4 shows an example slit profile 40 in the scanning direction (y) which is generally of the form of the example intensity profiles 30, 30a, 30b shown in Figures 3A and 3B in that it has a generally flat central portion although the two peripheral portions are not linear. Figure 4 also shows the pupil 42a, 42b, 42c at 3 different positions in the scanning direction through the profile 40 when a target illumination mode comprises two dipoles separated in a y-direction (a direction in the pupil plane that corresponds to the scanning direction in the plane of the reticle MA). The pupils 42a, 42c are from a peripheral portion of the profile 40 and pupil 42b is from a central portion of the profile 40. It can be clearly seen from pupils 42a, 42c that the peripheral portions of the illumination region IR each receives radiation predominantly from two different, complementary angular ranges. This is not telecentric illumination, i.e. a net direction of the total radiation received by these parts of the wafer-level illumination region is not perpendicular to a plane of the substrate W. It can also be seen from Figure 4, from pupil 42b, that the central portion of the illumination region IR receives radiation from substantially the whole pupil plane (i.e. dipole illumination). This is telecentric illumination, i.e. a net direction of the total radiation received by these parts of the wafer-level illumination region is generally perpendicular to a plane of the substrate W. It will be appreciated that, in general, each point in the target region receives radiation from a range of angles and may therefore be considered to receive a plurality of contributions, each contribution defined by a different element of solid angle and having a constant direction vector. The net direction of the total radiation received by a point in the target region of the substrate may be defined as a vector sum of the direction vectors of each contribution to that point, the sum being weighted by the energy received by each such contribution.[000103] Figure 5 also shows an illustration of the aerial image formed by the projection system at the three different positions in the scanning direction through the profile 40 that correspond to the three different pupils 42a, 42b, 42c. A first one of the three different positions corresponds to a position in a first peripheral portion of the profile 40; a second one of the three different positions corresponds to aposition in a central portion of the profile 40; and a third one of the three different positions corresponds to a position in a second peripheral portion of the profile 40.[000104] It can be seen that at the second position the illumination is generally dipole illumination and this is telecentric. As a result, any defocus of the aerial image should not contribute to overlay errors. It can also be seen that at the first and third positions the illumination is almost monopole illumination and this is non-telecentric such that the aerial image is rotated. As a result, any defocus of the aerial image will, in general, contribute to an overlay error in the scanning direction of the lithographic apparatus.[000105] This partial pupil provided to the peripheral portions of the slit may be referred to as a dynamic pupil. This correlation results in undesirable overlay errors. Furthermore, the correlation becomes greater (and, therefore, overlay errors become larger) if an extent of the peripheral portions is increased.[000106] It has been proposed to use a faceted field mirror device 10 having a large number of individually directable or movable reflective optical elements so as to provide better control over the illumination modes of the lithographic apparatus LA. It has been further proposed to use a faceted pupil mirror device 11 having a large number of individually directable or movable reflective optical elements so as to provide better control over the illumination modes of the lithographic apparatus LA.[000107] Each of the independently movable reflective optical elements may comprise a microelectromechanical system (MEMS). Therefore, the faceted field mirror device 10 and / or the faceted pupil mirror device 11 may be considered to comprise a MEMS micro-mirror array.[000108] Each of the reflective optical elements may, for example, be a multilayer mirror. Each of the reflective optical elements may be configured such that its orientation can be controlled about one or two axes so that a direction to which it directs radiation can be controlled. For example, each of the reflective optical elements may have one or more actuators by which the reflective optical element can be rotated about an axis or two orthogonal axes. Thereby, each of the reflective optical elements can be controlled to direct radiation in a specific direction.[000109] The faceted field mirror device 10 may comprise of the order of 100,000 independently movable reflective optical elements. These reflective optical elements may substantially cover the portions 24, 26 of the faceted field mirror device 10 that receive radiation from the radiation source SO (see Figure 2A and accompanying discussion).[000110] In some known lithographic apparatus, such a faceted field mirror device 10 comprising a MEMS micro-mirror array is used as follows. The two-dimensional array of independently movable reflective optical elements provided on the faceted field mirror device 10 may be considered to comprise a plurality of groups of reflective optical elements. Each group of reflective optical elements may comprise a plurality of adjacent independently movable reflective optical elements on the faceted field mirror device 10. Each of the plurality of groups of reflective optical elements may be referred to as a cluster of reflective optical elements.[000111] Each group of reflective optical elements may be configured generally to replace one of the field facets discussed above. For example, each group may cover a region of the faceted field mirror device 10 that generally corresponds to the shape of a field facet of the known faceted field mirror device 10 discussed above (for example a region having a shape generally the same as the shape 28 shown in Figure 2B). It will be appreciated that each of the reflective optical elements may be generally square or rectangular in shape and therefore if the group is arranged as a generally curved elongate region of the faceted field mirror device 10 (similar to the shape 28 shown in Figure 2B), the shape may have jagged or pixelated edges along the curved sides of the shape. Furthermore, the orientations of the reflective optical elements within each group may be configured so as to provide an equivalent optical power or concave shape to a field facet of an existing faceted field mirror device 10.[000112] Each of the plurality of groups may be referred to as a field facet mirror or a virtual field facet mirror. There may be of the order of 100, for example 300, groups of independently movable reflective optical elements. Each group may comprise of the order of 1000 independently movable reflective optical elements. For example, in one embodiment each group may comprise 10 rows of independently movable reflective optical elements, each row having 100 independently movable reflective optical elements.[000113] The plurality of adjacent independently movable reflective optical elements in each group substantially cover a continuous region of the faceted field mirror device 10. It will be appreciated that this may mean that any gaps between adjacent reflective optical elements may be minimal.[000114] A shape of a continuous region of the first optical component covered by the plurality of adjacent independently movable reflective optical elements in a group of independently movable reflective optical elements may be referred to as a shape of that group of independently movable reflective optical elements. The shape of each of the groups of independently movable reflective optical elements may generally correspond to a shape of the illumination region IR (in a similar manner to the shape of each of the field facets described above generally corresponding to a shape of the illumination region IR).[000115] Each of the plurality of groups of independently movable reflective optical elements may have substantially the same size and shape. In some embodiments, the shape of each of the plurality of groups may be curved. In some embodiments, each of the plurality of groups may be generally rectangular in shape.[000116] Embodiments of the present disclosure may comprise a faceted field mirror device 10 comprising a two-dimensional array of independently movable reflective optical elements and / or a faceted pupil mirror device 11 comprising a two-dimensional array of independently movable reflective optical elements. In particular, embodiments of the present disclosure may exploit the additional flexibility provided by a faceted field mirror device 10 comprising a two-dimensional array of independently movable reflective optical elements, so as to co-optimize an illumination mode of thelithographic apparatus LA for overlay performance and robustness to energy variations and energy drop outs of the radiation source SO.[000117] In particular, embodiments of the present disclosure may use a faceted field mirror device 10 and / or a faceted pupil mirror device 11 comprising a two-dimensional array of independently movable reflective optical elements. However, rather than using these as described above wherein clusters or groups of the movable reflective optical elements of the faceted field mirror device 10 replace the field facets of the known system described above, embodiments of the present disclosure may use a faceted field mirror device 10 and / or a faceted pupil mirror device 11 comprising a two-dimensional array of independently movable reflective optical elements in a more flexible way, as now described.[000118] Some embodiments of the present disclosure relate to methods for selecting an illumination mode for use in a lithographic apparatus LA, as now discussed with reference to Figures 5 and 6.[000119] Figure 5 is a schematic illustration of a method 100 according to an embodiment of the present disclosure for selecting an illumination mode for use in a lithographic apparatus LA.[000120] The method 100 comprises: a step 110 of selecting a target spatial distribution of radiation for illumination of a patterning device MA; and a step 120 of separately selecting a target angular distribution of the radiation for illumination of the patterning device MA. Note that these two steps 110, 120 may be performed in any order, either sequentially or in parallel.[000121] The method 100 further comprises a step 130 of selecting a configuration of illumination optics IL of the lithographic apparatus LA that achieves both the target spatial distribution and the target angular distribution of the radiation B in an illumination region IR of the lithographic apparatus LA.[000122] The lithographic apparatus LA may be an extreme ultraviolet (EUV) lithographic apparatus LA and the radiation B may comprise EUV radiation. The lithographic apparatus LA may be generally of the form shown in Figure 1 and discussed above.[000123] The angular distribution of the radiation B for illumination of the patterning device MA (reticle) may be characterized by an intensity of radiation in pupil plane of the illumination optics IL of the lithographic apparatus LA. The pupil plane is a Fourier transform plane of the plane of the patterning device MA (reticle). Therefore, the angular distribution of the radiation B for illumination of the patterning device MA may alternatively be referred to as the “pupil” or “illumination pupil”. As used herein, “the angular distribution of the radiation for illumination of the patterning device" may be synonymous with the terms “pupil” or “illumination pupil”. Selection of an appropriate illumination pupil for a given lithographic process can be important for optimizing imaging performance (in particular for contrast).[000124] The spatial distribution of radiation B for illumination of a patterning device MA describes the intensity of radiation in the plane of the patterning device MA (reticle). The plane of the patterning device (reticle) has a non-scanning direction (x) and a scanning direction (y). The non-scanning direction may alternatively be referred to as the slit direction. The spatial intensity distribution integrated in the scanning direction (y) as a function of the slit direction (x) may be referred to as the“slit profile”. The spatial intensity distribution as a function of the scanning direction is sometimes referred to as the “slit profile through scan” (also referred to as scan profile). As used herein, “the spatial distribution of radiation for illumination of a patterning device” may be synonymous with the terms “slit profile” or “slit profile through scan”. Selection of an appropriate slit profile and slit profile through scan can be important for dose control (i.e. ensuring that all parts of the wafer W that receive radiation receive substantially the same dose of radiation). In turn, dose control is important for critical dimension uniformity (CDU).[000125] The illumination region IR of the lithographic apparatus LA is intended to mean a region in a plane that, in use, the patterning device MA is disposed (or moves through for scanning exposures) that receives radiation. Therefore, a spatial distribution and the angular distribution of radiation in the illumination region IR of the lithographic apparatus LA will be the spatial distribution and angular distribution respectively with which a patterning device MA is illuminated. Therefore, as used herein, the “illumination region” may also be synonymous with the term “slit”.[000126] In existing lithographic methods a target angular distribution of the radiation for illumination of the patterning device or pupil is typically selected based on the lithographic process. However, as explained further above, in known existing methods the spatial distribution of radiation is achieved in such a way that once the pupil has been selected the spatial distribution of radiation for illumination of the patterning device MA is fixed. Furthermore, with known lithographic methods there is generally a variation in the pupil across the illumination region IR. In particular, there is typically a correlation between pupil and position in the illumination region IR in a scanning direction of the lithographic apparatus LA. This correlation results in undesirable overlay errors. Furthermore, the correlation becomes greater (and, therefore, overlay errors become larger) if a spatial distribution is selected that provides greater robustness to energy variations and energy drop outs of the radiation source SO (i.e. by having larger, shallower peripheral portions 34a, 34b). Therefore, with existing lithographic methods there is a tension between achieving good overlay performance on the one hand and good robustness to energy variations and energy drop outs of the radiation source SO on the other hand.[000127] The method 100 shown in Figure 5 is therefore advantageous, as it allows for both the pupil and the scan profile to be selected separately. Advantageously, this allows for an illumination mode wherein there is no, or negligible, correlation between pupil and position in the illumination region IR in a scanning direction of the lithographic apparatus LA. Advantageously, this allows for overlay performance and robustness to energy variations and energy drop outs of the radiation source SO to be co-optimized.[000128] Furthermore, as explained above, the radiation source SO may be operated at a lower power than a maximum achievable power and a controller of the radiation source may be operable to compensate for drops in source energy using this reserved power capability (referred to as a “dose margin”). Therefore, by increasing the robustness of the lithographic process to energy variations andenergy drop outs of the radiation source, a smaller dose margin can be used. Effectively, this increases the available power of the radiation source SO and therefore increases lithographic throughput.[000129] In some embodiments, the selection of the configuration of illumination optics IL of the lithographic apparatus LA (step 130) may be made such that a variation in the angular distribution of the radiation over the illumination region IR is minimized. In some embodiments, the selection of the configuration of illumination optics IL of the lithographic apparatus LA (step 130) may be made such that a non-telecentricity over the scan direction of the illumination region IR is minimized.[000130] Advantageously, this allows for the target spatial distribution (for example the scan profile) to be optimized without impact (or with minimum impact) on overlay performance. In particular, the target spatial distribution (for example the scan profile) may be optimized to maximize lithographic throughput whilst allowing sufficient robustness to energy variations and energy drop outs of the radiation source SO with minimum impact on overlay performance. For example, a scan profile may be selected that has greater robustness to energy variations and energy drop outs of the radiation source SO (for example, by having larger and shallower peripheral portions 34a, 34b) which, in turn, can allow for a smaller dose margin to be used (which increases lithographic throughput) with little or no effect on overlay performance. This is in contrast to existing arrangements wherein the provision of such larger and shallower peripheral portions 34a, 34b would typically increase overlay errors (as discussed above with reference to Figure 4).[000131] Preferably, the selection of the configuration of illumination optics IL of the lithographic apparatus LA (at step 130) is made such that the angular distribution of the radiation is substantially uniform over the illumination region IR. However, it will be appreciated by the skilled person that this may not always be possible and is dependent both on (a) the selected pupil and (b) the flexibility of the illumination optics IL of the lithographic apparatus LA. Therefore, in general, the selection of the configuration of illumination optics IL of the lithographic apparatus LA (at step 130) is made such that the variation in angular distribution of the radiation through scanning direction (y) over the illumination region IR is minimum.[000132] In some embodiments, the selection of the configuration of illumination optics IL of the lithographic apparatus LA is made so as to co-optimize: (a) overlay performance of the lithographic apparatus LA; and (b) robustness to energy variations and energy drop outs of a radiation source SO of the lithographic apparatus LA.[000133] It will be appreciated that if the illumination optics IL of the lithographic apparatus LA does not have sufficient flexibility to decouple correlation between pupil shape and position within the illumination slit IR, the method may balance the productivity of the lithographic apparatus LA and the overlay performance of the lithographic apparatus LA.[000134] In some embodiments, the pupil plane may be considered to comprise a plurality of regions or pixels. For example, the pupil plane may be considered to comprise a two-dimensional array of pixels.[000135] Selecting the configuration of illumination optics IL of the lithographic apparatus LA (at step 130) may comprise a plurality of sub-steps, as now discussed with reference to Figure 6.[000136] In some embodiments, selecting the configuration of illumination optics IL of the lithographic apparatus LA (at step 130) may comprise: for each of a plurality of pixels in the pupil plane that it is desired to be illuminated: a step 132 of determining a plurality of optical pathways of the illumination optics IL that contribute to that pixel. Selecting the configuration of illumination optics IL of the lithographic apparatus LA (at step 130) may further comprise a step 134 of: for each of the plurality of optical pathways (determined at step 132), determining: (a) an intensity and a (b) portion of the illumination region IR that it contributes to. Selecting the configuration of illumination optics IL of the lithographic apparatus LA (at step 130) may further comprise a step 136 of: selecting a set of the plurality of optical pathways for all of the plurality of pixels in the pupil plane that it is desired to illuminate so as to achieve the target spatial distribution (as determined at step 110) and the target angular distribution of radiation in the illumination region IR (as determined at step 120).[000137] It will be further appreciated that a pixel in the pupil plane that it is desired to be illuminated is a pixel that corresponds to a portion of the pupil plane for which the target pupil (selected at step 120) has a non-zero value.[000138] In some embodiments, the method 100 is for a lithographic apparatus LA comprising: a first optical component 10 comprising a two-dimensional array of independently movable reflective optical elements which is arranged to receive radiation from a radiation source SO; and a second optical component 11 comprising a two-dimensional array of independently movable reflective optical elements and which is arranged to receive radiation from the first optical component 10 and to direct it to an illumination region IR. For example, the method 100 may be for a lithographic apparatus LA generally of the form shown in Figure 1. For such embodiments, the step 130 of selecting a configuration of illumination optics IL of the lithographic apparatus LA may comprise: selecting a configuration of the first optical component 10 and the second optical component 11.[000139] In some embodiments, the target spatial distribution of radiation (selected at step 110) may be a profile shape in a scanning direction of a lithographic apparatus LA. The profile shape in the scanning direction may be referred to as the “scan profile”. In some embodiments, the profile shape in the scanning direction may have a generally trapezoidal shape, for example of the form shown in Figures 3A and 3B and described above.[000140] In some embodiments, selection of the target spatial distribution (at step 110) is made in dependence on a target dose. For example, for relatively low dose layers the scan profile may have shallower peripheral portions 34a, 34b in the scanning direction. For relatively high dose layers the scan profile may have steeper peripheral portions 34a, 34b in the scanning direction.[000141] In some embodiments, selection of the target spatial distribution (at step 110) is made in dependence on a target scanning speed of the lithographic apparatus LA. For example, for relatively high scanning speeds the scan profile may have shallower peripheral portions 34a, 34b in the scanningdirection. For relatively low scanning speeds the scan profile may have steeper peripheral portions 34a, 34b in the scanning direction.[000142] In some embodiments, selection of the target spatial distribution (at step 110) is made in dependence on a dimension of an illumination region IR of the lithographic apparatus LA in a scanning direction. That is, the scan profile (for example in the scanning direction) may be dependent on the width of the slit (in the scanning direction). As indicated in Figure 3 A, the width 36 of the slit in the scanning direction may be the sum of the widths of the central portion 32 and the two peripheral portions 34a, 34b.[000143] In some embodiments, the method 100 shown in Figure 5 may further comprise illuminating a patterning device MA with radiation using the selected configuration of illumination optics of the lithographic apparatus LA.[000144] Some embodiments of the present disclosure relate to lithographic methods, as now discussed with reference to Figures 7 and 8.[000145] Figure 7 is a schematic illustration of a lithographic method 200 according to an embodiment of the present disclosure.[000146] The lithographic method 200 comprises a step 210 of illuminating a patterning device MA using an illumination mode selected using the method 100 shown in Figure 5. The lithographic method 200 further comprises a step 220 of collecting patterned radiation B’ diffracted from the patterning device MA. The lithographic method 200 further comprises a step 230 of projecting the patterned radiation B’ onto a substrate W so as to form an image of the patterning device MA on the substrate W. [000147] The patterning device MA may be a reticle or mask. The patterning device MA may be disposed in the illumination region IR. The substrate W may comprise a resist-coated silicon wafer.[000148] The method may be a scanning exposure process. That is, illuminating the patterning device MA (at step 210) may comprise illuminating an illumination region IR (also known as “illumination slit” or just “slit”) while the patterning device MA is moved (or “scanned”) through the illumination region in a scanning direction (y). Similarly, projecting the patterned radiation B’ onto the substrate W (at step 230) may comprise moving (or “scanning”) the substrate W through a wafer-level illumination region. The movement of the substrate W may be such that the image of the patterning device MA is substantially stationary with respect to the substrate W.[000149] Optionally, in some embodiments the lithographic method 200 may comprise selecting the illumination mode using the method 100 shown in Figure 5. That is, in some embodiments, the lithographic method 200 may comprise an initial step 100 of selecting the illumination mode using the method 100 shown in Figure 5.[000150] Additionally or alternatively, the lithographic method 200 may comprise a step 205 of retrieving the illumination mode from memory. That is, in some embodiments, the method may comprise an initial step 205 of retrieving the illumination mode from memory. It will be appreciatedthat the method 100 shown in Figure 5 may have previously been used to select the illumination mode and then it may have been stored in memory.[000151] The step 210 of illuminating a patterning device MA may comprise the following sub-steps, as now described with reference to Figure 8.[000152] The step 210 of illuminating a patterning device MA may comprises: a step 212 of directing a radiation beam B to a first optical component 10 comprising a two-dimensional array of independently movable reflective optical elements; a step 214 of using the first optical component 10 to direct at least a portion of the radiation to a second optical component 11 comprising a two-dimensional array of independently movable reflective optical elements; and a step 216 of using the second optical component 11 to direct at least a portion of the radiation to the patterning device MA.[000153] Some embodiments of the present disclosure relate to an illumination system IL for a lithographic apparatus LA, as now discussed with reference to Figure 9.[000154] Figure 9 shows an illumination system 300 for a lithographic apparatus LA according to an embodiment of the present disclosure. The illumination system 300 may be generally equivalent to the illumination system IL shown in Figure 1 and discussed above. The illumination system 300 comprises: a first optical component 310; a second optical component 320; and a controller 330.[000155] The first optical component 310 comprises a two-dimensional array of independently movable reflective optical elements and is arranged to receive radiation 312 from a radiation source. The first optical component 310 may be generally equivalent to the faceted field mirror device 10 shown in Figure 1 and discussed above.[000156] The second optical component 320 comprising a two-dimensional array of independently movable reflective optical elements and is arranged to receive radiation from the first optical component 310 and to direct it to an illumination region IR. The second optical component 320 may be generally equivalent to the faceted pupil mirror device 11 shown in Figure 1 and discussed above.[000157] The controller 330 is operable to control the first and second optical components 310, 320. In particular, the controller 330 is configured so as to allow for separate control over a spatial and angular distribution of the radiation in the illumination region IR. The controller 330 may be operable to supply: a first control signal si to the first optical component 310 and / or a second control signal S2 to the second optical component 320.[000158] The illumination system 300 shown in Figure 9 may be operable to carry out the methods 100, 200 shown in Figures 5 and 7. For example, the controller 330 may be configured so as to carry out the methods 100, 200 shown in Figures 5 and 7.[000159] In some embodiments, the controller 330 may be configured to control the first and second optical components 310, 320 such that a variation in the angular distribution of the radiation over the illumination region IR is minimized.[000160] In some embodiments, the controller 330 may be configured to control the first and second optical components 310, 320 so as to co-optimize overlay performance of the lithographic apparatusLA and robustness to energy variations and energy drop outs of a radiation source SO of the lithographic apparatus LA.[000161] Each of the independently movable reflective optical elements of the first optical component 310 may comprise a micro-electromechanical system (MEMS) micro-mirror. Therefore, the first optical component 310 may be considered to comprise a MEMS micro-mirror array.[000162] Each of the independently movable reflective optical elements of the second optical component 320 may comprise a micro-electromechanical system (MEMS) micro-mirror. Therefore, the second optical component 320 may be considered to comprise a MEMS micro-mirror array.[000163] In some embodiments, the second optical component 320 may be disposed in a plane that is not a pupil plane of the illumination region IR.[000164] Some embodiments of the present disclosure relate to a lithographic apparatus or a lithographic system comprising the illumination system 300 shown in Figure 9. The lithographic apparatus or a lithographic system may be generally of the form shown in Figure 1 and as described above.[000165] The lithographic apparatus may further comprise a radiation source SO operable to generate a pulsed radiation beam B, 312 and to supply said pulsed radiation beam to the first optical component 310.[000166] In some embodiments, the lithographic apparatus may further comprise a sensor (not shown) that is operable to determine an intensity of a portion of one or more pulses of the pulsed radiation beam B, 312. Furthermore, in some embodiments, a pulse of the pulsed radiation beam B, 312 may be generated by the radiation source SO in dependence on an intensity determined by the sensor upon receipt of a portion of a previous pulse of radiation.[000167] For example, the sensor may determine, for example pulse-by-pulse, an energy of the radiation beam B, 312 and this may be used as part of a feed-back loop to stabilize an energy of the radiation beam B, 312.[000168] The lithographic apparatus may further comprise a support structure MT configured to support a patterning device MA such that the patterning device MA is disposed in, or is movable through, the illumination region IR.[000169] The lithographic apparatus may further comprise: a substrate table WT configured to support a substrate W; and a projection system PS comprising imaging optics 13, 14 configured to receive radiation B’ from the illumination region IR and to form an image of an object MA disposed in the illumination region IR on a substrate W supported by the substrate table WT.[000170] Although specific reference may be made in this text to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications. Possible other applications include the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, liquidcrystal displays (LCDs), thin-film magnetic heads, etc.[000171] 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.[000172] 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.[000173] While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. The descriptions above are intended to be illustrative, not limiting. Thus it will be apparent to one skilled in the art that modifications may be made to the invention as described without departing from the scope of the claims set out below.

Claims

CLAIMS1. A method for selecting an illumination mode for use in a lithographic apparatus, the method comprising: selecting a target spatial distribution of radiation for illumination of a patterning device; separately selecting a target angular distribution of the radiation for illumination of the patterning device; and selecting a configuration of illumination optics of the lithographic apparatus that achieves both the target spatial distribution and the target angular distribution of the radiation in an illumination region of the lithographic apparatus.

2. The method of claim 1 wherein the selection of the configuration of illumination optics of the lithographic apparatus is made such that a variation in the angular distribution of the radiation over the illumination region is minimized.

3. The method of claim 1 or claim 2 wherein the selection of the configuration of illumination optics of the lithographic apparatus is made such that a non-telecentricity of the radiation over the scan direction of the illumination region is minimized.

4. The method of any preceding claim wherein the selection of the configuration of illumination optics of the lithographic apparatus is made so as to co-optimize overlay performance of the lithographic apparatus and robustness to energy variations and energy drop outs of a radiation source of the lithographic apparatus.

5. The method of any preceding claim wherein the selecting the configuration of illumination optics of the lithographic apparatus comprises: for each of a plurality of pixels in the pupil plane that it is desired to be illuminated: determining a plurality of optical pathways of the illumination optics that contribute to that pixel; and, for each of the plurality of optical pathways, determining: an intensity and a portion of the illumination region that it contributes to; and selecting a set of the plurality of optical pathways for all of the plurality of pixels in the pupil plane that it is desired to illuminate so as to achieve the target spatial distribution and the target angular distribution of radiation in the illumination region.

6. The method of any preceding claim wherein the method is for a lithographic apparatus comprising:a first optical component comprising a two-dimensional array of independently movable reflective optical elements which is arranged to receive radiation from a radiation source; and a second optical component comprising a two-dimensional array of independently movable reflective optical elements and which is arranged to receive radiation from the first optical component and to direct it to an illumination region; and wherein selecting a configuration of illumination optics of the lithographic apparatus comprises: selecting a configuration of the first optical component and the second optical component.

7. The method of any preceding claim wherein the target spatial distribution of radiation is a profile shape in a scanning direction of a lithographic apparatus.

8. The method of any preceding claim wherein selection of the target spatial distribution is made in dependence on a target dose.

9. The method of any preceding claim wherein selection of the target spatial distribution is made in dependence on a target scanning speed of the lithographic apparatus.

10. The method of any preceding claim wherein selection of the target spatial distribution is made in dependence on a dimension of an illumination region of the lithographic apparatus in a scanning direction.

11. A lithographic method comprising: illuminating a patterning device using an illumination mode selected using the method of any preceding claim; collecting patterned radiation scattered from the patterning device; and projecting the patterned radiation onto a substrate so as to form an image of the patterning device on the substrate.

12. The lithographic method of claim 11 comprising selecting the illumination mode using the method of any one of claims 1 to 10.

13. The lithographic method of claim 11 comprising retrieving the illumination mode from memory.

14. The method of any one of claims 11 to 13 wherein illuminating a patterning device comprises: directing a radiation beam to a first optical component comprising a two-dimensional array of independently movable reflective optical elements;using the first optical component to direct at least a portion of the radiation to a second optical component comprising a two-dimensional array of independently movable reflective optical elements; and using the second optical component to direct at least a portion of the radiation to the patterning device.

15. A computer comprising: one or more processors; and storage media, the storage media having instructions to cause the one or more processors to carrying out the method of any one of claims 1 to 10.

16. The computer of claim 15 wherein the one or more processors are configured to store on the storage media the selected configuration of illumination optics of the lithographic apparatus that achieves both the target spatial distribution and the target angular distribution of the radiation in an illumination region of the lithographic apparatus.

17. A computer-readable medium having instructions for carrying out the method of any one of claims 1 to 10.

18. An illumination system for a lithographic apparatus, the illumination system comprising: a first optical component comprising a two-dimensional array of independently movable reflective optical elements which is arranged to receive radiation from a radiation source; a second optical component comprising a two-dimensional array of independently movable reflective optical elements and which is arranged to receive radiation from the first optical component and to direct it to an illumination region; and a controller operable to control the first and second optical components; wherein the controller is configured so as to allow for separate control over a spatial and angular distribution of the radiation in the illumination region.

19. The illumination system of claim 18 wherein the controller is configured so as to carry out the method of any one of claims 1 to 14.

20. The illumination system of claim 18 wherein the controller is configured to retrieve a configuration of the first and second optical elements from memory.

21. The illumination system of any one of claims 18 to 20 wherein the controller is configured to control the first and second optical components such that a variation in the angular distribution of the radiation for illumination of the patterning device over the illumination region is minimized.

22. The illumination system of any one of claims 18 to 21 wherein the controller is configured to control the first and second optical components so as to co-optimize overlay performance of the lithographic apparatus and robustness to energy variations and energy drop outs of a radiation source of the lithographic apparatus.

23. The illumination system of any one of claims 18 to 22 wherein each of the independently movable reflective optical elements of the first optical component comprises a micro-electromechanical system (MEMS) micro-mirror.

24. The illumination system of any one of claims 18 to 23 wherein each of the independently movable reflective optical elements of the second optical component comprises a microelectromechanical system (MEMS) micro-mirror.

25. The illumination system of any one of claims 18 to 24 wherein the second optical component is disposed in a plane that is not a pupil plane of the illumination region.

26. A lithographic apparatus or a lithographic system comprising the illumination system of any one of claims 18 to 25.

27. The lithographic apparatus of claim 26 further comprising a radiation source operable to generate a pulsed radiation beam and to supply said pulsed radiation beam to the first optical component.

28. The lithographic apparatus of claim 27 further comprising a sensor operable to determine an intensity of a portion of one or more pulses of the pulsed radiation beam; and wherein a pulse of the pulsed radiation beam is generated by the radiation source in dependence on an intensity determined by the sensor upon receipt of a portion of a previous pulse of radiation.

29. The lithographic apparatus of any one of claims 26 to 28 further comprising a support structure configured to support a patterning device such that the patterning device is disposed in, or is movable through, the illumination region.

30. The lithographic apparatus of any one of claims 26 to 29 further comprising: a substrate table configured to support a substrate; and a projection system comprising imaging optics configured toreceive radiation from the illumination region and to form an image of an object disposed in the illumination region on a substrate supported by the substrate table.

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