Improvements to lithographic methods and apparatus

Abstract

A new (lithographic) method of forming a feature on a substrate (e.g. a wafer) comprises performing a plurality of sequential exposures, each of the plurality of exposures comprising forming an image of the feature in the vicinity of the substrate. A focal position of the image relative to the substrate is different for at least two of the plurality of exposures. There is a range of focal positions of the image relative to the substrate that is not used during any of the plurality of exposures and which lies between the focal positions of the image relative to the substrate for two of the plurality of exposures. The method according to the first aspect of the present disclosure is particularly advantageous for forming isolated features on a substrate such as isolated contacts and isolated spaces.

Description

IMPROVEMENTS TO LITHOGRAPHIC METHODS AND APPARATUSCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of EP application 24158911.8 which was filed on February 21, 2024 and which is incorporated herein in its entirety by reference.FIELD

[0002] The present invention relates to new methods of forming a feature on a substrate (for example a silicon wafer). The present invention also relates to a lithographic apparatus operable to carry out one or more of these new methods. The present invention may have particular application to extreme ultraviolet (EUV) lithography.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 may be desirable to provide new, alternative apparatus and / or methods for improving imaging performance within a lithographic apparatus that at least partially addresses one or more problems associated with prior art arrangements whether identified herein or otherwise.SUMMARY

[0006] According to a first aspect of the present disclosure there is provided a method of forming a feature on a substrate, the method comprising: performing a plurality of sequential exposures, each of the plurality of exposures comprising: forming an image of the feature in the vicinity of the substrate; wherein a focal position of the image relative to the substrate is different for at least two of the plurality of exposures; and wherein there is a range of focal positions of the image relative to the substrate that is not used during any of the plurality of exposures and which lies between the focal positions of the image relative to the substrate for two of the plurality of exposures.

[0007] The method may comprise a lithographic method. The substrate may comprise a resist- coated silicon wafer.

[0008] The method according to the first aspect of the present disclosure is particularly advantageous for forming isolated features on a substrate such as isolated contacts and isolated spaces. In particular, it can improve the depth of focus of such features (relative to an exposure wherein the focal position of the image does not move significantly relative to the substrate during exposure).

[0009] One known method for improving the depth of focus of such features in scanning lithographic apparatus (also known as scanners) is known as focus drilling. Focus drilling involves a scanning motion of the substrate (wafer) such that a plane of best focus of the aerial image moves (in a direction perpendicular to the wafer) during the scanning motion. That is, it involves a single continuous exposure wherein a focal position of the image varies continuously during the scanning motion. This results in an aerial image that is smeared out in the focus direction and consequently has a larger depth of focus. One consequence of this is that the intensity of the radiation (at any focal position sampled by the substrate) is reduced and therefore a higher dose is needed. Another consequence is that the contrast of the image is reduced.

[0010] Advantageously, the method according to the first aspect, which may be referred to as discrete focus sampling, can achieve an increased depth of focus (relative to an exposure wherein the focal position of the image does not move significantly relative to the substrate) whilst resulting in less loss of contrast over a desired focus-range than traditional focus drilling. The increased depth of focus (relative to an exposure wherein the focal position of the image does not move significantly relative to the substrate) may be similar to that achieved with traditional focus drilling.

[0011] The focal position of an image may be defined as a position of a plane of best focus of the image.

[0012] It will be appreciated that there may be some (for example relatively small) variation of a focal position of the image relative to the substrate during any of the plurality of exposures. For such embodiments, the ranges of focal positions of the images relative to the substrate for at least two of the plurality of exposures are different, non-overlapping and separated.

[0013] The plurality of sequential exposures may be temporally separate (for example, there may be a time period between two such sequential exposures). This may allow for the substrate to be moved relative to a plane of best focus between two such sequential exposures. Such an arrangement may result in optimum contrast performance.

[0014] Alternatively, in some embodiments, there may be no time period between sequential exposures of the plurality of sequential exposures. For such embodiments, an intensity of the radiation may be decrease with time towards the end of one of the sequential exposures and may increase with time towards the start of the next sequential exposures. In some embodiments, an intensity of the radiation may increase with time towards the start of each of the sequential exposures and may decrease with time towards the end of each of the sequential exposures. For such embodiments, the intensity of radiation may be reduced in between two sequential exposures. The greater this reduction in intensity is the better the contrast performance may be.

[0015] Preferably, between two sequential exposures, an intensity of the radiation may fall to less than half of a maximum intensity during each of the two sequential exposures. For example, between two sequential exposures, an intensity of the radiation may fall to less 40% of a maximum intensity during each of the two sequential exposures. For example, between two sequential exposures, an intensity of the radiation may fall to less 30% of a maximum intensity during each of the two sequential exposures. For example, between two sequential exposures, an intensity of the radiation may fall to less 20% of a maximum intensity during each of the two sequential exposures. For example, between two sequential exposures, an intensity of the radiation may fall to less 10% of a maximum intensity during each of the two sequential exposures. For example, between two sequential exposures, an intensity of the radiation may fall to less 5% of a maximum intensity during each of the two sequential exposures.

[0016] A range of focal positions of the image relative to the substrate during any one of the plurality of exposures may be less than a difference between average focal positions of that exposure and an adjacent exposure when the exposures are ordered by the average focal position of the image relative to the substrate.

[0017] In some embodiments, a range of focal positions of the image relative to the substrate during any one of the plurality of exposures is less than a difference between average focal positions of that exposure and an adjacent exposure when the exposures are ordered by the average focal position of the image relative to the substrate by at least a factor of 2. In some embodiments, a range of focal positions of the image relative to the substrate during any one of the plurality of exposures is less than a difference between average focal positions of that exposure and an adjacent exposure when the exposures are ordered by the average focal position of the image relative to the substrate by at least a factor of 3. In some embodiments, a range of focal positions of the image relative to the substrate during any one of the plurality of exposures is less than a difference between average focal positions of that exposure and an adjacent exposure when the exposures are ordered by the average focal position of the image relative to the substrate by at least a factor of 4. In some embodiments, a range of focal positions of the image relative to the substrate during any one of the plurality of exposures is less than a difference between average focal positions of that exposure and an adjacent exposure when the exposures are ordered by the average focal position of the image relative to the substrate by at least a factor of 5.

[0018] Each of the plurality of exposures may comprise: illuminating a patterning device with radiation; capturing radiation scattered from the patterning device with imaging optics; and projecting the scattered radiation onto the substrate so as to form an image of the patterning device in the vicinity of the substrate.

[0019] The patterning device may be referred to as a reticle or a mask. The patterning device may be disposed in an object plane of the imaging optics. The substrate may be disposed in, or close to an image plane of the imaging optics. The object plane is optically conjugate to the image plane. Theobject plane, the image plane (and any other optically conjugate planes) may be referred to as field planes of the imaging optics.

[0020] In some embodiments, at least two of the plurality of exposures may be formed by illuminating the feature with radiation using different illumination pupils.

[0021] For example, in general, a different illumination pupil (or angular distribution of the radiation in an object plane) may be used for each of the plurality of exposures. Advantageously, such embodiments may achieve the discrete focus sampling provided by the method according to the first aspect of the present disclosure (which can achieve an increased depth of focus relative to an exposure wherein the focal position of the image does not move significantly relative to the substrate) whilst resulting in an increase of contrast. This is contrary to traditional focus drilling, which typically results in a loss of contrast.

[0022] Performing the plurality of exposures may comprise: illuminating a plurality of discrete portions of an illumination region in an object plane, the plurality of discrete portions being at different positions in a first direction; moving a patterning device having the feature through the illumination region in the first direction; and moving the substrate such that an image of the patterning device is substantially static with respect to the substrate in the first direction; wherein a surface of the substrate is disposed at a non-zero angle to a plane of best focus in a plane containing the first direction.

[0023] That is, the plurality of exposures may be performed by a scanning exposure wherein only a plurality of discrete portions of a field plane are illuminated. The first direction may be referred to as a scanning direction. The first direction may be referred to as a y-direction in a Cartesian coordinate system. For such embodiments, the surface of the substrate being disposed at a non-zero angle to a plane of best focus in a plane containing the first direction may be referred to as an Rx tilt (i.e. a tilt about the x-direction).

[0024] It will be appreciated that in order to move the substrate in the first direction such that an image of the patterning device is substantially static with respect to the substrate in the first direction, the movement will in general be dependent on the characteristics of the imaging system. For example, if the imaging system forms an inverted image of the patterning device then the substrate will be moved in the opposite direction in the first direction to the patterning device. In addition, a speed of movement of the substrate will be dependent in the speed of the patterning device and any optical magnification factor applied by the imaging system in the first direction.

[0025] As the feature moves through one of the plurality of discrete portions of the illumination region that are illuminated, an image of the feature is formed and one of the plurality of exposures is performed.

[0026] Since the surface of the substrate is disposed at a non-zero angle to a plane of best focus in a plane containing the first direction, as the substrate moves in the first direction, the plane of best focus moves relative to the surface of the substrate. As a result, a focal position of the image relative to the substrate is different for each of the plurality of plurality of exposures.

[0027] The number of sequential exposures that are performed may, in general, be dependent on: (a) an extent of the aerial image of the feature in the focus direction; and (b) a desired depth of focus.

[0028] The plurality of discrete portions may be separated in the first direction. This may result in an optimum contrast performance.

[0029] Alternatively, in some embodiments, there may not be a strict separation between the discrete portions. For example, the intensity of radiation in between two adjacent discrete portions does not need to be zero. However, in general, the intensity between two adjacent discrete portions will fall to less than half of a maximum intensity in each of the two adjacent discrete portions. For example, the intensity between two adjacent discrete portions may fall to less 40% of a maximum intensity in each of the two adjacent discrete portions. For example, the intensity between two adjacent discrete portions may fall to less 30% of a maximum intensity in each of the two adjacent discrete portions. For example, the intensity between two adjacent discrete portions may fall to less 20% of a maximum intensity in each of the two adjacent discrete portions. For example, the intensity between two adjacent discrete portions may fall to less 10% of a maximum intensity in each of the two adjacent discrete portions. For example, the intensity between two adjacent discrete portions may fall to less 5% of a maximum intensity in each of the two adjacent discrete portions.

[0030] In general, the greater the reduction in intensity is in between two adjacent discrete portions, the better the contrast performance may be.

[0031] According to a second aspect of the present disclosure there is provided a method of forming a feature on a substrate, the method comprising: illuminating a plurality of discrete portions of an illumination region in an object plane, the plurality of discrete portions being at different positions in a first direction; moving a patterning device having the feature through the illumination region in the first direction; and moving the substrate such that an image of the patterning device is substantially static with respect to the substrate in the first direction; wherein a surface of the substrate is disposed at a non-zero angle to a plane of best focus in a plane containing the first direction.

[0032] The method may comprise a lithographic method. The substrate may comprise a resist- coated silicon wafer.

[0033] The method according to the second aspect of the present disclosure is also particularly advantageous for forming isolated features on a substrate such as isolated contacts and isolated spaces. In particular, it is a specific embodiment that allows for discrete focus sampling and therefore can improve the depth of focus of such features (relative to an exposure wherein the focal position of the image does not move significantly relative to the substrate during exposure).

[0034] The plurality of discrete portions may be separated in the first direction. This may result in an optimum contrast performance.

[0035] Alternatively, in some embodiments, there may not be a strict separation between the discrete portions. For example, the intensity of radiation in between two adjacent discrete portions does not need to be zero. However, in general, the intensity between two adjacent discrete portions will fallto less than half of a maximum intensity in each of the two adjacent discrete portions. For example, the intensity between two adjacent discrete portions may fall to less 40% of a maximum intensity in each of the two adjacent discrete portions. For example, the intensity between two adjacent discrete portions may fall to less 30% of a maximum intensity in each of the two adjacent discrete portions. For example, the intensity between two adjacent discrete portions may fall to less 20% of a maximum intensity in each of the two adjacent discrete portions. For example, the intensity between two adjacent discrete portions may fall to less 10% of a maximum intensity in each of the two adjacent discrete portions. For example, the intensity between two adjacent discrete portions may fall to less 5% of a maximum intensity in each of the two adjacent discrete portions.

[0036] An extent in the first direction of any of the plurality of discrete portions of the illumination region in the object plane may be less than a separation in the first direction of that discrete portion from any adjacent discrete portions.

[0037] The method may further comprise: capturing radiation scattered from the patterning device with imaging optics; and projecting the scattered radiation onto the substrate so as to form the image of the patterning device in the vicinity of the substrate.

[0038] In some embodiments, the illumination of at least two of the plurality of discrete portions of the illumination region may be carried out using different illumination pupils.

[0039] In principle, the intensity in the illumination-pupil may be different for each of the plurality of distinct portions of the illumination region. That is, in general, a different illumination mode (or angular distribution of the radiation in an object plane) may be used for each of the distinct portions of the illumination region. Advantageously, such embodiments may achieve the discrete focus sampling provided by the method according to the second aspect of the present disclosure (which can achieve an increased depth of focus relative to an exposure wherein the focal position of the image does not move significantly relative to the substrate) whilst resulting in an increase of contrast. This is contrary to traditional focus drilling, which typically results in a loss of contrast.

[0040] In some embodiments, at least one of the different illumination pupils may result in non- telecentric illumination of the substrate that, for the focal position of the image relative to the substrate when that illumination mode is used, results in a shift of the image of the feature that at least partially compensates for an image shift due to three-dimensional mask effects.

[0041] The method may be a (photo-)lithography method. In such method, the (aerial) image of the feature formed during each of the plurality of exposures (or while the feature is moved through each of the plurality of discrete portions of the illumination region) is typically a diffraction-limited image. In particular, typically only two diffraction orders may be captured by the numerical aperture of the imaging optics: a zeroth order contribution and a first order contribution. The first order diffraction beams are shifted relative to the zeroth order diffraction beam in the pupil plane (a Fourier transform plane to that of the object plane) in a shearing direction by a distance of +A / p, where A is the wavelength of the exposure radiation and p is the pitch of the feature. The first order contribution thatis captured by the numerical aperture of the imaging optics typically only comprises one of these: either the + 1 st order diffraction beam (for parts of the illumination radiation on a first side of the pupil plane) or the -1st order diffraction beam (for parts of the illumination radiation on a second side of the pupil plane).

[0042] For an illumination pupil comprising radiation at a position of +A / 2p relative to a center of the pupil plane in the shearing direction, the first order contribution captured by the numerical aperture of the imaging optics will be at a position of +A / 2p relative to a center of the pupil plane in the shearing direction. Therefore, these two (coherent) contributions will be incident on the substrate at equal and opposite angles and the illumination of the substrate may be considered to be telecentric. However, for a more general point in the pupil plane the illumination of the substrate will be non- telecentric. As a result the aerial image will not be parallel to the optical axis and there will be a focusdependent shift of the image formed on the substrate.

[0043] As used here three-dimensional mask effects are intended to mean a phase difference that is introduced between the zeroth and first order diffraction beams due to the three-dimensional nature of the patterning device (reticle) that defines the feature. Such a phase difference results in a shift of the image of the feature formed on the substrate.

[0044] The methods according to the first and second aspects of the present disclosure comprise performing a plurality of sequential exposures, wherein a focal position of the image relative to the substrate is different for at least two of the plurality of exposures. It is now further suggested that these focal positions may be combined with an illumination pupil that results in a non-telecentric illumination of the substrate so as to cause a shift in the position of the image formed on the substrate that at least partially compensates for an image shift due to three-dimensional mask effects.

[0045] In some embodiments of the method of the first or second aspect of the present disclosure, at least one of the different illumination pupils may be such that a pattern shift for that contribution to the image of the feature is minimized.

[0046] For example, the plurality of different illumination pupils may be such that a pattern shift for all contributions to the image of the feature are minimized.

[0047] In some embodiments of the method of the first or second aspect of the present disclosure, the plurality of different illumination pupils may be such that a contrast of the image of the feature formed on the substrate is maximized.

[0048] In some embodiments, two sequential exposures may be performed.

[0049] In some embodiments, for a first one of the two sequential exposures the illumination pupil may be such that only a portion of the pupil plane less than A / 2p from a center of the pupil plane in the shearing direction is illuminated and for a second one of the two sequential exposures the illumination pupil may be such that only a portion of the pupil plane greater than A / 2p from the center of the pupil plane in the shearing direction is illuminated, where A is the wavelength of the exposure radiation and p is a pitch of the feature.

[0050] In some embodiments, for each of the first and second sequential exposures the illumination pupil may comprise a half-leaf dipole.

[0051] The number and / or relative intensities of the plurality of sequential exposures performed may be optimized so as to maximize a homogeneity of the intensity of radiation over a target depth of focus.

[0052] The feature may comprise an isolated feature.

[0053] For example, the feature may be an isolated contact or an isolated space. In general, the feature may comprise a bright feature (i.e. a feature wherein radiation is transmitted) on a dark background.

[0054] According to a third aspect of the present disclosure there is provided a lithographic apparatus operable to perform the method of the first aspect of the present disclosure or the method of the second aspect of the present disclosure.

[0055] The lithographic apparatus may comprise an 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 operable to control the first and second optical components so as to illuminate a plurality of discrete portions of the illumination region, the plurality of discrete portions being at different positions in a first direction.

[0056] The first direction may be referred to as a scanning direction.

[0057] The plurality of discrete portions may be separated in the first direction. This may result in an optimum contrast performance. Alternatively, in some embodiments, there may not be a strict separation between the discrete portions.

[0058] Each of the independently movable reflective optical elements of the first optical component may comprise a micro-electromechanical system (MEMS) micro-mirror. Therefore, the first optical component may be considered to comprise a MEMS micro-mirror array.

[0059] Each of the independently movable reflective optical elements of the second optical component may comprise a micro-electromechanical system (MEMS) micro-mirror. Therefore, the second optical component may be considered to comprise a MEMS micro-mirror array.

[0060] 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.

[0061] 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

[0062] 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 3 schematically shows a new method of forming a feature on a substrate (for example a wafer);Figure 4A is a schematic representation of an aerial image formed during a single exposure wherein the focal position of the image does not move significantly relative to the substrate;Figure 4B is a schematic representation of two aerial images which may be formed using the new method shown in Figure 3;Figure 4C shows a line which represents a region which corresponds to a dose of radiation that is sufficient to cause a change in a photoresist on a wafer when the two aerial images shown in Figure 4B are combined;Figure 5 schematically shows a plurality of sub-steps that each of a plurality of exposures of the new method shown in Figure 3 may comprise;Figure 6 is another schematic representation of a new method of forming a feature on a substrate (for example a wafer);Figure 7 schematically shows an illumination region, which may represent the radiation in a field plane of a lithographic apparatus of the type shown in Figure 1, comprises two discrete portions which are illuminated and which are separated in a first direction (i.e. the y-direction) by a portion which is not illuminated, the illumination region may be used for implementing the methods shown in Figures 3 and 6;Figures 8 A to 8E schematically show a scanning exposure using the illumination regions shown in Figure 7 and which show a substrate in a plane that is perpendicular to the substrate and which contains the scanning direction; Figures 8A to 8E show the substrate at a plurality of successive times during the scanning exposure;Figure 9 schematically shows an optimization method that may be performed in order to optimize the number and / or relative intensities of a plurality of sequential exposures (used by the new methods shown in Figures 3 and 6) so as to maximize a homogeneity of the intensity of radiation over a target depth of focus range;Figure 10A shows a distribution of optimized coefficients determined by the method shown in Figure 9;Figure 10B shows: (a) a single aerial image; and (b) a line which represents a region which corresponds to a dose of radiation that is sufficient to cause a change in a photoresist on the wafer when the aerial images corresponding to the optimized set of coefficients shown in Figure 10A are combined;Figure 11A shows the critical dimension (CD) target contours through focus are for three situations that all have the same exposure dose: (i) no focus drilling (i.e. a single exposure at a static focus level); (ii) continuous or traditional focus drilling (i.e. a continuous range of focus levels being used); and (iii) a two-level focus arrangement (for example of the type discussed above with reference to Figures 10A and 10B);Figure 1 IB shows the intensity through focus at the centre of the image for the three situations shown in Figure 11 A;Figure 11C shows the normalized image log-slope (NILS), which is a commonly used measure for contrast, for the three situations shown in Figures 11A and 11B;Figure 12 schematically shows an illumination pupil comprising a plurality of different poles (regions of the pupil that may be illuminated, for example by the illumination system; the numerical aperture of the projection system is illustrated by a circle;Figure 13 shows a portion of the aerial images of vertical lines corresponding to a single pitch of the lines (i.e. 16 nm) formed by each of the two poles of the first pair of poles shown in Figure 12 as a function of the shearing direction ( -direction) in a plane of best focus;Figure 14 shows a pattern shift of the image formed by each of the six poles shown in Figure 12 as a function of focus;Figure 15 shows an example leaf dipole illumination pupil (with a 24% pupil fill ratio) that may be used for imaging vertical lines;Figure 16 shows an example bar dipole illumination pupil (with a 12% pupil fill ratio) wherein the two bar poles are each centered on one of two telecentric lines;Figure 17A shows a first new half-leaf dipole illumination pupil (with a 12% pupil fill ratio) which corresponds to the portions of the leaf dipole illumination pupil shown in Figure 15 that are farther from the center of the numerical aperture of the projection optics than the two telecentric lines;Figure 17B shows a second new half-leaf dipole illumination pupil (with a 12% pupil fill ratio) which corresponds to the portions of the leaf dipole illumination pupil shown in Figure 15 that are closer to the center of the numerical aperture of the projection optics than the two telecentric lines;Figure 18 shows the exposure latitude as a percentage as a function of depth of focus for imaging vertical lines with a pitch, p, of 16 nm using a wavelength of 13.5 nm and a projection system with a numerical aperture of -0.55 for three different illumination modes; andFigure 19 shows the exposure latitude as a percentage as a function of depth of focus when imaging vertical lines with a combination of a first pitch, p1, of 16 nm and second pitch, p2, of 20 nm(again using a wavelength of 13.5 nm and a projection system with a numerical aperture of -0.55) for two different illumination modes.DETAILED DESCRIPTION

[0063] 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.

[0064] 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.

[0065] After being thus conditioned, the EUV radiation beam B interacts with the patterning device MA. As a result of this interaction, a patterned EUV radiation beam B’ is generated. The projection system PS is configured to project the patterned EUV radiation beam B’ onto the substrate W. For that purpose, the projection system PS may comprise a plurality of mirrors 13, 14 which are configured to project the patterned EUV radiation beam B’ onto the substrate W held by the substrate table WT. The projection system PS may apply a reduction factor to the patterned EUV radiation beam B’, thus forming an image with features that are smaller than corresponding features on the patterning device MA. For example, a reduction factor of 4 or 8 may be applied. Although the projection system PS is illustrated as having only two mirrors 13, 14 in Figure 1, the projection system PS may include a different number of mirrors (e.g. six or eight mirrors).

[0066] 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.

[0067] 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.

[0068] 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 fuelmay 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.

[0069] 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.

[0070] 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.

[0071] Radiation that is reflected by the collector 5 forms the EUV radiation beam B. The EUV radiation beam B is focused at intermediate focus 6 to form an image at the intermediate focus 6 of the plasma present at the plasma formation region 4. The image at the intermediate focus 6 acts as a virtual radiation source for the illumination system IL. The radiation source SO is arranged such that the intermediate focus 6 is located at or near to an opening 8 in an enclosing structure 9 of the radiation source SO.

[0072] 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.

[0073] 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.

[0074] The illumination region IR is in a field plane in which the reticle MA is disposed during a lithographic exposure. Therefore, the illumination region IR may be referred as a reticle-level or object-level illumination region IR. It will be appreciated that projection system PS forms an image IR’ of the illumination region IR in the plane of the substrate W. The image IR’ of the illumination region IR in the plane of the substrate W may be referred to as a wafer-level or image-level illumination region IR’ . As used herein, the wafer-level illumination region IR’ may also be referred to as the slit.

[0075] 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.

[0076] 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.

[0077] The collector 5 is generally of the form of a concave mirror, which is arranged to collect the radiation which is emitted from the plasma formation region 4 into a solid angle subtended by the collector 5. This radiation is reflected and focused at the intermediate focus 6. As a result, within the housing, the radiation beam B is generally of the form of a converging cone of radiation, which converges at the intermediate focus 6, an outer edge of this cone being indicated in Figure 1 by two lines. Downstream of the intermediate focus 6, the radiation beam B is generally of the form of a diverging cone of radiation, which is incident on the generally circular field facet mirror device 10. However, the radiation source SO may comprise an obscuration that will block a portion of this radiation cone such that there will be some portion of the diverging cone of radiation that which will not receive radiation from the collector 5. For example, the radiation source SO may comprise a shield (not shown) which may be arranged to prevent the laser beam 2 from propagating through the opening 8 and into the lithographic apparatus LA (where it may damage optical components). This shield may be supported by the enclosing structure via a support (not shown). Together, the shield and the support form an obscuration of the radiation source SO. Therefore, as indicated schematically in Figure 2A a generally circular portion 20 of the field facet mirror device 10 may comprise a central portion 22, which coincides with the obscuration of the radiation source SO and does not receive any radiation, and two portions 24, 26 which do receive radiation. It will be appreciated that this is schematic and that the obscuration may have any shape or configuration.

[0078] 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 pupilmirror 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.

[0079] 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.

[0080] 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.

[0081] 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).

[0082] 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.

[0083] 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.

[0084] 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 ofindependently movable reflective optical elements, each row having 100 independently movable reflective optical elements.

[0085] 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.

[0086] 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).

[0087] 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.

[0088] Some embodiments of the present disclosure relate to new methods of forming a feature on a substrate (for example a wafer W). An example of such a new method 100 is now described with reference to Figure 3.

[0089] The new method 100 comprises performing a plurality of sequential exposures 110a-l lOn. Performing each of the plurality of exposures 110a-l lOn comprises forming an image of the feature in the vicinity of the substrate W. A focal position of the image relative to the substrate W is different for at least two of the plurality of exposures 110a-l lOn. Furthermore, there is a range of focal positions of the image relative to the substrate W that is not used during any of the plurality of exposures 110a- 1 lOn and which lies between the focal positions of the image relative to the substrate W for two of the plurality of exposures.

[0090] The new method 100 may comprise a lithographic method. The substrate W may comprise a resist-coated silicon wafer.

[0091] The new method 100 shown in Figure 3 is particularly advantageous for forming isolated features on a substrate W such as isolated contacts and isolated spaces. In particular, it can improve the depth of focus of such features (relative to an exposure wherein the focal position of the image does not move significantly relative to the substrate W during exposure).

[0092] One known method for improving the depth of focus of such features in scanning lithographic apparatus (also known as scanners) is known as focus drilling. Focus drilling involves a scanning motion of the substrate W (wafer) such that a plane of best focus of the aerial image moves (in a direction perpendicular to the wafer W) during the scanning motion. That is, it involves a single continuous exposure wherein a focal position of the image varies continuously during the scanningmotion. This results in an aerial image that is smeared out in the focus direction and consequently has a larger depth of focus. One consequence of this is that the intensity of the radiation (at any focal position sampled by the substrate W) is reduced and therefore a higher dose is needed. Another consequence is that the contrast of the image is reduced.

[0093] Advantageously, the new method 100, which may be referred to as discrete focus sampling, can achieve an increased depth of focus (relative to an exposure wherein the focal position of the image does not move significantly relative to the substrate W) whilst resulting in less loss of contrast over a desired focus-range than traditional focus drilling. The increased depth of focus (relative to an exposure wherein the focal position of the image does not move significantly relative to the substrate W) may be similar to that achieved with traditional focus drilling.

[0094] It will be appreciated that there will be at least two sequential exposures 110a, 110b although there may be more than two sequential exposures 110a, HOb-llOn. As described further below (with reference to Figures 9 to 10B), the number of sequential exposures may be selected using an optimization method. The general principle of the new method 100 will now be discussed with reference to Figures 4A to 4C for an example with just two sequential exposures 110a, 110b.

[0095] In general, features may be formed on a substrate, for example a resist-coated silicon, by exposing the substrate to patterned radiation. Some regions on the substrate receive radiation whereas some regions do not (or receive a relatively small amount of radiation). In regions of the substrate which receive radiation in excess of a threshold value, the resist undergoes a change such that during subsequent processing of the substrate these regions will behave differently to those regions which did not receive radiation in excess of the threshold value. For example, either the regions of the substrate which did or did not receive radiation in excess of the threshold value may be more susceptible to etching. In this way, the pattern can be transferred to the substrate W. The size and position of features formed on the wafer W in this way is dependent on the position of the aerial image formed during the exposure process relative to the wafer W.

[0096] Figure 4A is a schematic representation of an aerial image 200 formed during a single exposure wherein the focal position of the image does not move significantly relative to the substrate W. The aerial image 200 is shown as a function of both focus position (which may be referred to as z- position) and position on the wafer (which may be referred to as an x or y position). The aerial image 200 is represented by an elliptical line which corresponds to a dose of radiation that is sufficient to cause a change in a photoresist on the wafer W. All regions within the line 200 receive a sufficient dose of radiation to cause the resist to undergo a change whereas the regions outside of this do not. It will be appreciated that this is a two dimensional representation for ease of understanding and that the aerial image 200 may be represented by an ellipsoid.

[0097] Also shown in Figure 4A are two lines 202, 204, which represent a target dimension or size of the feature represented by the aerial image 200. As can be seen from Figure 4A, when the focus is at 0 (i.e. when a focal position of the aerial image 200 relative to the substrate W is aligned with theresist) a dimension of the region of the wafer that falls within the line 200 representing the aerial image matches the target dimension or size of the feature (as represented by lines 202, 204). However, if the focus coincides with line 206 (i.e. the focal position of the aerial image 200 relative to the substrate W is misaligned relative to the resist by an amount 208) then the a dimension of the region of the wafer that falls within the line 200 representing the aerial image now matches lines 210, 212. This dimension is smaller than the target dimension, representing a printing error.

[0098] Figure 4B is a schematic representation of two aerial images 200a, 200b, which may be formed using the new method 100 shown in Figure 3. In this simple example, each of the two aerial images 200a, 200b is formed by a single exposure wherein the focal position of the image does not move significantly relative to the substrate W. A focal position of a first one of the two aerial images 200a relative to the substrate W is represented by a line 214a and a focal position of a second one of the two aerial images 200a relative to the substrate W is represented by a line 214b. As can be seen from Figure 4B, the focal positions of the two aerial images 200a, 200b (which may be formed by two successive exposures 110a, 110b) relative to the substrate W are different. Furthermore, there is a range 216 of focal positions of the image relative to the substrate W that is not used during any of the two exposures 110a, 110b and which lies between the focal positions 214a, 214b of the image relative to the substrate W for the two exposures 110a, 110b.

[0099] Figure 4C shows a line 218 which represents a region which corresponds to a dose of radiation that is sufficient to cause a change in a photoresist on the wafer W when the two aerial images 202a, 202b shown in Figure 4B are combined. All regions within the line 218 receive a sufficient dose of radiation to cause the resist to undergo a change whereas the regions outside of this do not. From a comparison of Figures 4C and 4A it is clear that the two exposures represented by the two aerial images 200a, 200b can achieve an increased depth of focus (relative to the single exposure represented by aerial image 200).[000100] The focal position of an image may be defined as a position of a plane of best focus of the image.[000101] It will be appreciated that there may be some (for example relatively small) variation of a focal position of the image relative to the substrate W during any of the plurality of exposures 11 Gall On. For such embodiments, the ranges of focal positions of the images relative to the substrate W for at least two of the plurality of exposures 110a- 11 On are different, non-overlapping and separated.[000102] The plurality of sequential exposures 110a- 11 On may be temporally separated (for example, there may be a time period between two such sequential exposures). This may allow for the substrate W to be moved relative to a plane of best focus between two such sequential exposures 110a- 11 On. Such an arrangement may result in optimum contrast performance.[000103] Alternatively, in some embodiments, there may be no time period between sequential exposures of the plurality of sequential exposures HOa-l lOn. For such embodiments, an intensity of the radiation may be decrease with time towards the end of one of the sequential exposures 110a-l lOnand may increase with time towards the start of the next sequential exposures 110a- 11 On. In some embodiments, an intensity of the radiation may increase with time towards the start of each of the sequential exposures 110a- 11 On and may decrease with time towards the end of each of the sequential exposures 110a-l lOn. For such embodiments, the intensity of radiation may be reduced in between two sequential exposures 110a- 11 On. The greater this reduction in intensity is the better the contrast performance may be.[000104] Preferably, between two sequential exposures 110a-l lOn, an intensity of the radiation may fall to less than half of a maximum intensity during each of the two sequential exposures HOa-l lOn. For example, between two sequential exposures 110a-l lOn, an intensity of the radiation may fall to less 40% of a maximum intensity during each of the two sequential exposures 110a- 11 On. For example, between two sequential exposures HOa-l lOn, an intensity of the radiation may fall to less 30% of a maximum intensity during each of the two sequential exposures 110a- 11 On. For example, between two sequential exposures HOa-l lOn, an intensity of the radiation may fall to less 20% of a maximum intensity during each of the two sequential exposures 110a-l lOn. For example, between two sequential exposures 110a-l lOn, an intensity of the radiation may fall to less 10% of a maximum intensity during each of the two sequential exposures 110a-l lOn. For example, between two sequential exposures 110a- 11 On, an intensity of the radiation may fall to less 5% of a maximum intensity during each of the two sequential exposures 110a- 11 On.[000105] In some embodiments of the new method 100 shown in Figure 3, a range of focal positions of the image relative to the substrate W during any one of the plurality of exposures 110a- 1 lOn may be less than a difference between average focal positions of that exposure and an adjacent exposure when the exposures are ordered by the average focal position of the image relative to the substrate. For example, to use the simple two exposure example shown in Figures 4B and 4C and discussed above, a range of focal positions of the image relative to the substrate W during either of the two of exposures 110a, 110b is less than a difference 216 between average focal positions of that exposure 214a, 214b and an adjacent exposure 214b, 214a. As mentioned above, in the simple example shown in Figures 4A to 4C (to illustrate the principle), a range of focal positions of the image relative to the substrate W during each of the two exposures 110a, 110b is effectively zero. However, as discussed below (with reference to Figures 8 and 10), in practice a range of focal positions of the image relative to the substrate W during each of the plurality of exposures 110a-l lOn of the method 100 may be non-zero.[000106] In some embodiments, a range of focal positions of the image relative to the substrate W during any one of the plurality of exposures 110a-l lOn is less than a difference between average focal positions of that exposure and an adjacent exposure when the exposures are ordered by the average focal position of the image relative to the substrate W by at least a factor of 2. In some embodiments, a range of focal positions of the image relative to the substrate W during any one of the plurality of exposures HOa-llOn is less than a difference between average focal positions of that exposure and an adjacent exposure when the exposures are ordered by the average focal position of the image relativeto the substrate W by at least a factor of 3. In some embodiments, a range of focal positions of the image relative to the substrate W during any one of the plurality of exposures 110a-l lOn is less than a difference between average focal positions of that exposure and an adjacent exposure when the exposures are ordered by the average focal position of the image relative to the substrate W by at least a factor of 4. In some embodiments, a range of focal positions of the image relative to the substrate W during any one of the plurality of exposures 110a- 1 lOn is less than a difference between average focal positions of that exposure and an adjacent exposure when the exposures are ordered by the average focal position of the image relative to the substrate W by at least a factor of 5.[000107] Each of the plurality of exposures 110a- 11 On of the new method 100 shown in Figure 3 may comprises a plurality of sub-steps, as now discussed with reference to Figure 5.[000108] Each of the plurality of exposures 110a- 11 On may comprise a step 120 of illuminating a patterning device MA with radiation B. Each of the plurality of exposures 110a- 11 On may further comprise a step 122 of capturing radiation B’ scattered from the patterning device MA with imaging optics PS. Each of the plurality of exposures 110a- 11 On may further comprise a step 124 of projecting the scattered radiation B’ onto the substrate W so as to form an image of the patterning device MA in the vicinity of the substrate W.[000109] The patterning device MA may be referred to as a reticle or a mask. The patterning device MA may be disposed in an object plane of the imaging optics PS. The substrate W may be disposed in, or close to an image plane of the imaging optics PS. The object plane is optically conjugate to the image plane. The object plane, the image plane (and any other optically conjugate planes) may be referred to as field planes of the imaging optics PS.[000110] Some embodiments of the new method 100 shown in Figure 3 comprise the following steps (which may be equivalent to the plurality of exposures 110a-l lOn), as now described with reference to Figure 6. In particular, in the new method 100 performing the plurality of exposures HOa-l lOn may comprise the following steps (which may achieve all of the plurality of exposures), as now described with reference to Figure 6.[000111] First, the new method 100 may comprise a step 130 of illuminating a plurality of discrete portions of an illumination region IR in an object plane, the plurality of discrete portions being at different positions in a first direction (which may be referred to as a y-direction). The plurality of discrete portions may be separated in a first direction. This may result in an optimum contrast performance. An example of such an illumination process is shown schematically in Figure 7, which shows illumination region 300 which generally corresponds to the illumination region IR shown in Figure 1 and as described above with reference to Figures 1 to 2B. The illumination region 300 comprises two discrete portions 302a, 302b which are illuminated and which are separated in the first direction (i.e. the y-direction) by a portion 304 which is not illuminated.[000112] It will be appreciated that the illumination region 300 shown in Figure 7 can be considered to represent both the reticle-level illumination region IR and wafer-level illumination region IR’ (i.e.the image IR’ of the illumination region IR in the plane of the substrate W). It will also be appreciated that in practice the wafer-level illumination region IR’ will be a modified version of the reticle-level illumination region IR, modified by the pattern of the patterning device MA. However, in the following, for ease of understanding of the presently disclosed methods, the wafer-level illumination region IR’ will be depicted as a copy of the reticle-level illumination region IR (as shown in Figure 7).[000113] The new method 100 may further comprise a step 132 of moving a patterning device MA having the feature through the illumination region IR in the first direction (i.e. the y-direction).[000114] The new method 100 may further comprise a step 134 of moving the substrate W such that an image of the patterning device MA is substantially static with respect to the substrate W in the first direction (i.e. the y-direction). This involves moving the substrate W through the wafer-level illumination region IR’ in the first direction (i.e. the y-direction).[000115] It will be appreciated that in order to move the substrate W in the first direction such that an image of the patterning device MA is substantially static with respect to the substrate in the first direction (i.e. the y-direction), the movement will in general be dependent on the characteristics of the imaging system (i.e. the projection system PS). For example, if the imaging system PS forms an inverted image of the patterning device MA then the substrate W will be moved in the opposite direction in the first direction to the patterning device MA. In addition, a speed of movement of the substrate W will be dependent in the speed of the patterning device MA and any optical magnification factor applied by the imaging system PS in the first direction.[000116] That is, in the embodiments shown in Figures 6 and 7, the plurality of exposures 110a-l lOn may be performed by a scanning exposure wherein only a plurality of discrete portions of a field plane are illuminated.[000117] Although shown as separate steps, it will be appreciated that steps of moving the patterning device MA (step 132) and moving the substrate W (step 134) may be performed simultaneously.[000118] Furthermore, during this scanning exposure, a surface of the substrate W may be disposed at a non-zero angle to a plane of best focus in a plane containing the first direction (a plane which may be referred to as a y-z plane). The first direction may be referred to as a y-direction in a Cartesian coordinate system. The first direction (i.e. the y-direction) may be referred to as a scanning direction. For such embodiments, the surface of the substrate W being disposed at a non-zero angle to a plane of best focus in a plane containing the first direction may be referred to as an Rx tilt (i.e. a tilt of the wavefront about the x-direction).[000119] This scanning exposure is also shown schematically in Figures 8A to 8E, which shows the substrate W in a plane that is perpendicular to the substrate W and which contains the scanning direction (indicated by arrow 306). A position 308 on the substrate W at which the feature is formed is also marked. Figure 8A to 8E show the substrate W a plurality of successive times during the scanning exposure. Also shown is the plane of best focus 310 and the two discrete portions 302a, 302b of the of the illumination region 300 which are illuminated.[000120] At the beginning of the scan the feature is outside of the illumination region 300 at reticlelevel and, as shown in Figure 8A, the position 308 on the substrate W at which the feature is formed is outside of the illumination region IR’ .[000121] At reticle-level (i.e. in the plane of the patterning device MA), as the feature moves through one of the plurality of discrete portions 302a of the illumination region 300 that are illuminated, an image of the feature is formed (at position 308) and one of the plurality of exposures is performed.[000122] Figure 8B corresponds to a part of the scanning exposure at which the feature is illuminated by a first one of the discrete portions 302a of the illumination region 300 which is illuminated and therefore the position 308 on the substrate W at which the feature is formed is shown within this discrete portion 302a in Figure 8B.[000123] Figure 8C corresponds to a part of the scanning exposure at which the feature is disposed in the portion 304 of the illumination region 300 that is not illuminated and therefore the position 308 on the substrate W at which the feature is formed is shown within this discrete portion 304 in Figure 8C.[000124] Figure 8D corresponds to a part of the scanning exposure at which the feature is illuminated by a second one of the discrete portions 302b of the illumination region 300 which is illuminated and therefore the position 308 on the substrate W at which the feature is formed is shown within this discrete portion 302b in Figure 8D.[000125] As shown in Figure 8E, at an end of the scanning exposure the feature is no longer disposed in the illumination region 300 and therefore the position 308 on the substrate W at which the feature is formed is shown outside of the illumination region 300 Figure 8E.[000126] Since the surface of the substrate W is disposed at a non-zero angle to a plane of best focus 310 in a plane containing the first direction (i.e. the y-z plane), as the substrate moves in the first direction (the y-direction), the plane of best focus 310 moves relative to the surface of the substrate W. As a result, a focal position of the image relative to the substrate W is different for each of the plurality of plurality of exposures. For example, it can be seen from Figures 8B and 8D that for the two exposures performed in this embodiment, the position 308 on the substrate W at which the feature is formed is on opposite sides of the plane of best focus 310. It will be appreciated that a focal position of the images formed during these two successive exposures (which are partially depicted by Figures 8B and 8D) relative to the substrate W may be given by a distance (or average distance) between the plane of best focus 310 and the position 308 during each of the two exposures. It will be further appreciated that the focal position of the images formed during these two successive exposures (which are partially depicted by Figures 8B and 8D) relative to the substrate W may, for example, correspond to the lines 214a, 214b shown in Figure 4B and described above.[000127] The number of sequential exposures that are performed by the method 100 shown in Figures 3 and 6 may, in general, be dependent on: (a) an extent of the aerial image of the feature in the focus direction; and (b) a desired depth of focus.[000128] In principle, the intensity in the illumination-pupil may be different for each of the distinct portions 302a, 302b of the illumination region 300 in the y-direction. That is, in general, a different illumination mode (or angular distribution of the radiation in an object plane) may be used for each of the distinct portions 302a, 302b of the illumination region 300. Examples of embodiments wherein a different illumination mode (or angular distribution of the radiation in an object plane) is used for each of the distinct portions 302a, 302b of the illumination region 300 are discussed further below with reference to Figures 12 to 19.[000129] Optionally, an extent in the first direction (i.e. the y-direction) of any of the plurality of discrete portions 302a, 302b of the illumination region 300 in the object plane may be less than a separation in the first direction of that discrete portion from any adjacent discrete portions.[000130] For example, for the embodiment described above with reference to Figure 7 an extent 312a in the first direction (i.e. the y-direction) of a first discrete portion 302a of the illumination region is less than a separation 314 in the first direction between the two discrete portions 302a, 302b. Similarly, an extent 312b in the first direction (i.e. the y-direction) of a second discrete portion 302b of the illumination region is less than a separation 314 in the first direction between the two discrete portions 302a, 302b.[000131] In an alternative embodiment, an extent in the first direction (i.e. the y-direction) of each of the discrete portions 302a-302n may be generally equal to a separation between each pair of adjacent discrete portions.[000132] Although in the example discussed above, the plurality of discrete portions 302a, 302b are separated in a first direction, alternatively, in some embodiments, there may not be a strict separation between the plurality of discrete portions 302a-302n. For example, the intensity of radiation in between two adjacent discrete portions 302a-302n does not need to be zero. However, in general, the intensity between two adjacent discrete portions 302a-302n will fall to less than half of a maximum intensity in each of the two adjacent discrete portions 302a-302n. For example, the intensity between two adjacent discrete portions 302a-302n may fall to less 40% of a maximum intensity in each of the two adjacent discrete portions 302a-302n. For example, the intensity between two adjacent discrete portions 302a- 302n may fall to less 30% of a maximum intensity in each of the two adjacent discrete portions 302a- 302n. For example, the intensity between two adjacent discrete portions 302a-302n may fall to less 20% of a maximum intensity in each of the two adjacent discrete portions 302a-302n. For example, the intensity between two adjacent discrete portions 302a-302n may fall to less 10% of a maximum intensity in each of the two adjacent discrete portions 302a-302n. For example, the intensity between two adjacent discrete portions 302a-302n may fall to less 5% of a maximum intensity in each of the two adjacent discrete portions 302a-302n. In general, the greater the reduction in intensity is in between two adjacent discrete portions 302a-302n, the better the contrast performance may be.[000133] In some embodiments of the methods 100 shown in Figures 3 and 6, two sequential exposures 110a, 110b may be performed.[000134] In some embodiments of the methods 100 shown in Figures 3 and 6, the feature may comprise an isolated feature. For example, the feature may be an isolated contact or an isolated space. In general, the feature may comprise a bright feature (i.e. a feature wherein radiation is transmitted) on a dark background.[000135] In some embodiments of the methods 100 shown in Figures 3 and 6, the number and / or relative intensities of the plurality of sequential exposures 110a- 11 On performed may be optimized so as to maximize a homogeneity of the intensity of radiation over a target depth of focus for a fixed overall dose. An example of how this may be performed is now described with reference to Figures 9, 10A and 10B.[000136] Figure 9 schematically shows an optimization method 400 that may be performed in order to optimize the number and / or relative intensities of the plurality of sequential exposures so as to maximize a homogeneity of the intensity of radiation over a target depth of focus range, FR.[000137] The method 400 comprises a first step 410 of generating (for example simulating) a plurality of aerial images, A / ,, each corresponding to a different one of the plurality of exposures 110a- 1 lOn. The plurality of aerial images AItmay be identical except for having a different focal position relative to a reference position.[000138] For example, the step 410 of generating the plurality of aerial images Altmay comprise the sub-steps of: 412 generating a first aerial image as a function of focus, A / 0(F); and 414 generating at least one (and preferably a plurality of) focus shifted versions of the first aerial image, A / 0(F + AFj).[000139] The method 400 further comprises a step 420 of constructing a combined or total aerial image, / total, given by:where xLare a set of coefficients (or amplitudes) to be optimized. In some embodiments xL6 [0,1] (i.e. the coefficients are all greater than or equal to 0 and less than or equal to 1). For embodiments wherein the step 410 of generating the plurality of aerial images AItcomprises sub-steps 412 and 414, the combined or total aerial image, A / total, is given by:[000140] The method 400 further comprises a step 430 of finding a set of coefficients { ,} that result in as homogenous an intensity distribution as possible over a given focus range FR. It will be appreciated that step 430 may use any type of optimization process (for example to minimize a meritfunction) as desired or required. Examples of such optimization methods may include, for example, any of the following: genetic optimization algorithms; Newton’s method in optimization (also known as the Newton-Raphson method); simulated annealing; a least squares method; and / or a quadratic programming active set method.[000141] Optionally, the optimization of the set of coefficients {(} may be subject to the constraint = 1 so that the total intensity is the same for all solutions.[000142] Figures 10A and 10B show the results of an example of the optimization method 400 shown in Figure 9. Figure 10B shows a single aerial image 440 (which may correspond to first aerial image as a function of focus, A / 0(F), generated at sub-step 412). As with Figures 4A to 4C, the aerial image 440 is represented by an elliptical line which corresponds to a dose of radiation that is sufficient to cause a change in a photoresist on the wafer W. All regions within the line 440 receive a sufficient dose of radiation to cause the resist to undergo a change whereas the regions outside of this do not. It will be appreciated that this is a two dimensional representation for ease of understanding and that the aerial image 440 may be represented by an ellipsoid. Also shown in Figure 10B is the a target depth of focus range, FR.[000143] Figure 10A shows the distribution of (41) optimized coefficients x(- determined at step 430 of the method 400 shown in Figure 9. As can be seen, most of the coefficients are zero (which is in contrast to traditional focus drilling) and there are two peaks, each of which may correspond to a single one of the sequential exposures 110a- 11 On. Each of the peaks does have a non-zero range of focus values, 442a, 442b. Furthermore, there is a range of focal positions 444 of the image relative to the substrate W for which the optimized coefficients x(- are zero between the two peaks. Therefore, if these two peaks correspond to two sequential exposures 110a, 110b then there will be a range of focal positions 444 of the image relative to the substrate W that is not used during either of the plurality of exposures 110a, 110b and which lies between the focal positions 442a of the image relative to the substrate W for the first exposures 110a and the focal positions 442b of the image relative to the substrate W for the second exposure 110b.[000144] Figure 10B also shows a line 450 which represents a region which corresponds to a dose of radiation that is sufficient to cause a change in a photoresist on the wafer W when the aerial images corresponding to the optimized set of coefficients {x(} shown in Figure 10A are combined. All regions within the line 250 receive a sufficient dose of radiation to cause the resist to undergo a change whereas the regions outside of this do not.[000145] The optimization shows that, for a given total intensity consumption, in this example, the best solution comprises two focus level exposures. In order to achieve a homogenous intensity distribution over a larger focus range FRadditional focus levels may also be used. For example, for a larger focus range FRthree focus levels may be used and so on.[000146] As mentioned above, the new methods 100 (shown in Figure 3 and 6), which may be referred to as discrete focus sampling, can achieve an increased depth of focus (relative to an exposure wherein the focal position of the image does not move significantly relative to the substrate W) whilst resulting in less loss of contrast that traditional focus drilling. The increased depth of focus (relative to an exposure wherein the focal position of the image does not move significantly relative to the substrate W) may be similar to that achieved with traditional focus drilling. These results are now presented with reference to Figures 11 A to 11C.[000147] A use-case of an isolated contact-hole has been compared for three situations that all have the same exposure dose: (i) no focus drilling (i.e. a single exposure at a static focus level); (ii) continuous or traditional focus drilling (i.e. a continuous range of focus levels being used); and (iii) a two-level focus arrangement (for example of the type discussed above with reference to Figures 10A and 10B).Figure 11 A shows the critical dimension (CD) target contours through focus are compared; Figure 1 IB shows the intensity through focus at the centre of the image; and Figure 11C shows the normalized image log-slope (NILS). NILS is a commonly used measure for contrast and is defined as the log-slope of the aerial-image at threshold, normalized with the target-CD. That is, NILS is given by:where th is the threshold (which in Figure 11A is represented by the contour lines).[000148] From Figure 11 A it can be seen that the threshold contours are straighter through focus for the two focus-level arrangement (case (iii) above) than for either the no drilling or continuous drilling arrangements. This means that a larger defocus is allowed before the CD deviates 10% from specification. From Figure 1 IB it is clear that the intensity is more spread out (i.e. flatter) through focus for the two focus-level arrangement (case (iii) above). Finally from Figure 11C it can be seen that, for example, for a specification that contrast is at a level of NILS>2.5 (which is a very common specification for aerial images) the largest range is obtained by the two focus-level arrangement (case (iii) above).[000149] Case (iii) has been optimized to achieve an intensity -profile that is as flat as possible in the focus-range of -42 to 42nm. The best arrangement found by optimization was the two-level arrangement shown in Figures 10A and 10B (the two exposures centered at the focus-levels +36nm and -36nm). This is compared to a continuous drilling-case (i.e. case (ii)) where the defocus varies linearly over the domain from -60 to 60nm). Note that using an even larger domain for the continuous focusdrilling (i.e. case (ii)) does not achieve the same contrast performance (minimum-NILS) over the target focus range (-42nm to 42nm) and, in addition, this would require a significant increase in dose.[000150] As discussed above, the new methods 100 shown in Figures 3 and / or 6 may use a plurality of different illumination pupils, as now discussed further. In particular, in some embodiments of the method 100 shown in Figure 3, at least two of the plurality of exposures 110a- 11 On may be formed by illuminating the feature with radiation using different illumination pupils (so as to form the image of the feature in the vicinity of the substrate W). For example, in general, a different illumination pupil (or angular distribution of the radiation in an object plane) may be used for each of the plurality of exposures HOa-l lOn. Similarly, in some embodiments of the method 100 shown in Figure 6, the illumination of at least two of the plurality of discrete portions 302a-302n of the illumination region 300 may be carried out (at step 130) using different illumination pupils. In principle, the intensity in the illumination-pupil may be different for each of the plurality of distinct portions 302a-302n of the illumination region 300. That is, in general, a different illumination mode (or angular distribution of the radiation in an object plane) may be used for each of the distinct portions 302a-302n of the illumination region 300.[000151] Advantageously, as now discussed, such embodiments may achieve the discrete focus sampling described above (which can achieve an increased depth of focus relative to an exposure wherein the focal position of the image does not move significantly relative to the substrate W) whilst resulting in an increase of contrast. This is contrary to traditional focus drilling, which typically results in a loss of contrast.[000152] In order to explain this increase in contrast, we consider the case of a lithographic process in which vertical dense lines (i.e. lines extending in the y-direction) having a pitch, p, are printed. In such method, the (aerial) image of the feature formed during each of the plurality of exposures (or while the feature is moved through each of the plurality of discrete portions of the illumination region) is typically a diffraction-limited image. For example, the lithographic process may use EUV exposure radiation (for example having a wavelength of 13.5 nm) and the pitch of the vertical lines may be of the same order, for example 16 nm. Such vertical lines will cause diffraction of the exposure radiation, creating a plurality of diffraction orders that are angularly separated such that, in a pupil plane optically downstream therefrom the diffraction orders are separated in the x-dircction (or the x-dircction, where x is a coordinate in the pupil plane that the x-dircction in the field plane is mapped to). The x-direction (or the x-direction) may be referred to as the shearing direction for such vertical lines.[000153] Figure 12 schematically shows an illumination pupil 500 comprising a plurality of different poles 502, 504, 506, 508, 510 and 512 (regions of the pupil 500 that may be illuminated, for example by the illumination system IL). The numerical aperture of the projection system PS is illustrated by a circle 520.[000154] A first pair of poles 502, 504 are separated by a distance of A / p in the shearing direction (the x-direction), where A is the wavelength of the exposure radiation and p is the pitch of the feature. Furthermore, each of the first pair of poles is a distance of A / 2p from a center 522 of the pupil 500 inthe shearing direction (the x -direction). A second pair of poles 506, 508 are also separated by a distance of A / p in the shearing direction (the x-direction) and are shifted relative to the first pair of poles 502, 504 in the shearing direction by a distance, — d. A third pair of poles 510, 512 are also separated by a distance of A / p in the shearing direction (the x -direction) and are shifted relative to the first pair of poles 502, 504 in the shearing direction by a distance, +d.[000155] In general, a position (for example of one of the poles) in the pupil plane 300 may be expressed using a radial coordinate, or sigma value, <r, which expresses the distance of the position from a center of the numerical aperture of the projection system PS as a fraction of the numerical aperture of the projection system PS. That is, for a position that is a distance r from the center of the numerical aperture of the projection system PS, this sigma value is <r = r / NA. The region of the pupil plane 300 that is accepted by the pupil plane (represented by circle 520) corresponds to all points for which <r < 1. The fraction of the pupil plane within the numerical aperture 520 of the projection system PS that is illuminated may be referred to as the pupil fill ratio.[000156] When forming such a diffraction-limited image, typically, only two diffraction orders may be captured by the numerical aperture of the imaging optics PS (i.e. only two diffraction orders fall within the circle 520): a zeroth order contribution and a first order contribution. The first order diffraction beams are shifted relative to the zeroth order diffraction beam in the pupil plane (a Fourier transform plane to that of the object plane) in a shearing direction by a distance of +A / p, where A is the wavelength of the exposure radiation and p is the pitch of the feature. The first order contribution that is captured by the numerical aperture of the imaging optics typically only comprises one of these: either the + 1st order diffraction beam (for parts of the illumination radiation on a first side of the pupil plane) or the -1st order diffraction beam (for parts of the illumination radiation on a second side of the pupil plane).[000157] For example, consider a first pole 502 of the first pair of poles, which is to the left-hand side of the region 520 of the pupil plane that corresponds to the numerical aperture of the projection system PS. For an example (shown in Figure 12) where the wavelength is 13.5 nm, the pitch of the vertical lines, p, is 16 nm and the numerical aperture of the projection system PS is -0.55, this first pole is at a sigma position of <r = 0.77. The zeroth order diffraction beam generated by this pole will also be on the left-hand side of the circle 520 at a sigma position of <r = 0.77. The first order diffraction beams are shifted relative to the zeroth order diffraction beam in the pupil plane (a Fourier transform plane to that of the object plane) in a shearing direction by a distance of + '-lp. One of these will be shifted to the left (to a sigma position of <r = 2.30) and will not be captured by the projection optics PS and the other one will be shifted to the right so as to be on the right-hand side of the circle 520 at a sigma position of <r = 0.77 (i.e. at the same position as the second pole 504 of the first pair of poles).[000158] Therefore, the two contributions that arise from the first pole 502 of the illumination pupil 300 that (coherently) sum at the wafer to contribute to the image of the vertical lines in the wafer-levelpupil (a pupil plane of the projection system PS) are two positions of +A / 2p relative to a center of the wafer-level pupil plane in the shearing direction (or, equivalently, at sigma positions of <r = +0.77). Therefore, these two (coherent) contributions will be incident on the wafer W at equal and opposite angles and the illumination of the wafer W may be considered to be telecentric. Similarly, the two contributions that arise from the second pole 504 of the illumination pupil 300 that (coherently) sum at the wafer to contribute to the image of the vertical lines in the wafer-level pupil (a pupil plane of the projection system PS) are also at two positions of +2 / 2p relative to a center of the wafer-level pupil plane in the shearing direction (or, equivalently, at sigma positions of <r = +0.77) and can also be considered to illuminate the wafer W telecentrically.[000159] Therefore, as will be discussed further below with reference to Figure 14, the first pair of poles 502, 504 each result in an aerial image that is parallel to the optical axis of the lithographic apparatus LA and therefore the position of the image generated by either of these poles 502, 504 in the shearing direction (x-direction) will, in general, be independent of a position of the image relative to the wafer W (in a direction perpendicular to the wafer W or the "-direction).[000160] However, the patterning device MA (also referred to as a reticle or mask) is not purely 2- dimensional and has a non-zero depth in the "-direction. Therefore, the patterning device MA (also referred to as a reticle or mask) may be considered to be 3-dimensional. As a result of this non-zero depth in the "-direction (or the three-dimensional nature of the patterning device MA), in general a phase difference is introduced between the zeroth and first order diffraction beams generated when the exposure radiation B is scattered from the patterning device MA). In general, this phase difference that is introduced between the zeroth and first order diffraction beams due to the three-dimensional nature of the patterning device (reticle) that defines the feature may be referred to as “three-dimensional mask effects”.[000161] This phase difference results in a relative shift of the images of the features (the vertical lines) formed on the substrate W by each of the two poles 502, 504 of the first pair of poles. This is shown in Figure 13, which shows a portion of the aerial images 532, 534 of the vertical lines corresponding to a single pitch of the lines (i.e. 16 nm) formed by each of the two poles 502, 504 of the first pair of poles as a function of the shearing direction (x-direction) in a plane of best focus. As can be seen in Figure 13, the two aerial images 532, 534 of the vertical lines formed by each of the two poles 502, 504 of the first pair of poles are offset by a relative shift, Ax, of the order of 2.8 nm.[000162] This relative shift, Ax, of the images of the features (the vertical lines) formed on the substrate W by each of the two poles 502, 504 due to these intrinsic three-dimensional mask effects can also be shown in Figure 14, which shows a pattern shift 542, 544, 546, 548, 550, 552 of the image formed by each of the six poles 502, 504, 506, 508, 510, 512 shown in Figure 12 respectively as a function of focus. As discussed above, since the illumination of the wafer W is telecentric, the pattern shifts 542, 544 of the images generated by either of the poles 502, 504 of the first pair of poles (in theshearing direction; x-direction) is independent of focus (i.e. the relative position of the image and the wafer W (in a direction perpendicular to the wafer W or the "-direction). The two individual aerial images both have non-zero pattern shifts and are offset relative to each other by relative shift, Ax.[000163] This relative shift, Ax, results in fading of the overall image formed on the wafer W and to a loss of contrast. Each of the poles 502, 504 of the first pair of poles experiences an image shift of +Ax / 2 respectively due to three-dimensional mask effects. Note that, in general, the image shift experienced by a pole in the pupil 300 due to three-dimensional mask effects is dependent on the position of that pole. In general, the image shift experienced by a pole in the pupil 300 due to three- dimensional mask effects is larger for larger sigma values.[000164] However, for a more general point in the pupil plane the illumination of the wafer will be non-telecentric. For example, the pole 506 will generate a zeroth order contribution at the same position in the wafer-level pupil and a first order contribution (that is captured by the projection optics PS) at the same position as pole 508 in the wafer-level pupil. As a result of these two coherent contributions converging on the wafer W at different angles to the "-direction, the aerial images formed by such portions of the illumination pupil 300 will not be parallel to the optical axis and there will be a focusdependent shift of the image formed on the wafer W. This can be seen in Figure 14 as the pattern shifts 546, 548, 550, 552 of the images formed by each of the four poles 506, 508, 510, 512 shown in Figure 12 (that are not on the telecentric lines 524, 526) are dependent on focus.[000165] It can also be seen from Figure 14 that the pattern shifts 546, 552 of the images of the poles 506, 512 that are farther from the center 522 of the numerical aperture 520 of the projection optics PS than the first pair of poles 502, 504 (or, equivalently, the telecentric lines 524, 526) approach zero for a focus of around 24 nm. The poles 506, 512 that are farther from the center 522 of the numerical aperture 520 of the projection optics PS than the first pair of poles 502, 504 (or, equivalently, the telecentric lines 524, 526) may be referred to as the outer poles 506, 512.[000166] These four poles 506, 508, 510, 512 shown in Figure 12 (that are not on the telecentric lines 524, 526) each result in non-telecentric illumination of the wafer W. Furthermore, a suitable selection of the focal position of the aerial image of each of these poles relative to the wafer W may result in a shift of these aerial images of the feature that at least partially compensates for the image shift (+Ax / 2) due to three-dimensional mask effects.[000167] It can also be seen from Figure 14 that the pattern shifts 548, 550 of the images of the poles 506, 512 that are closer to the center 522 of the numerical aperture 520 of the projection optics PS than the first pair of poles 502, 504 (or, equivalently, the telecentric lines 524, 526) approach zero for a focus of around -24 nm. The poles 508, 510 that are closer to the center 522 of the numerical aperture 520 of the projection optics PS than the first pair of poles 502, 504 (or, equivalently, the telecentric lines 524, 526) may be referred to as the inner poles 506, 512.[000168] The inventors have realized that by using different illumination pupils with different focus values the fading or loss of contrast from intrinsic three-dimensional mask effects can be reducedsignificantly, increasing contrast of the overall image formed on the wafer W. Put differently, by selecting a suitable combination of illumination pupil and focus, or focal position, the pattern shifts of the images from all parts of the illumination pupil can be zero, or close to zero. The new methods 100 shown in Figures 3 and 6 comprise performing a plurality of exposures HOa-llOn wherein a focal position of the image relative to the substrate W is different for at least two of the plurality of exposures HOa-l lOn. Therefore, if a different illumination pupil is used for each of the plurality of exposures 110a-l lOn, a pattern shift for all contributions to each of the plurality of exposures 110a-l lOn can be minimized. In particular, an illumination pupil may be used for each of the plurality of exposures 110a- HOn such that a pattern shift for all contributions to each of the plurality of exposures HOa-l lOn is minimized.[000169] Figure 15 shows an example leaf dipole illumination pupil 560 (with a 24% pupil fill ratio) that may be used for imaging vertical lines. Figure 16 shows an example bar dipole illumination pupil 562 (with a 12% pupil fill ratio) wherein the two bar poles are each centered on one of the telecentric lines 524, 526.[000170] Figure 17A shows a first new half-leaf dipole illumination pupil 564 (with a 12% pupil fill ratio) which corresponds to the portions of the leaf dipole illumination pupil 560 shown in Figure 15 that are farther from the center 522 of the numerical aperture 520 of the projection optics PS than the telecentric lines 524, 526. Figure 17B shows a second new half-leaf dipole illumination pupil 564 (with a 12% pupil fill ratio) which corresponds to the portions of the leaf dipole illumination pupil 560 shown in Figure 15 that are closer to the center 522 of the numerical aperture 520 of the projection optics PS than the telecentric lines 524, 526.[000171] Figure 18 shows the exposure latitude as a percentage as a function of depth of focus for imaging vertical lines with a pitch, p, of 16 nm using a wavelength of 13.5 nm and a projection system with a numerical aperture of -0.55 for three different illumination modes. Exposure latitude is equivalent to contrast so an increase in exposure latitude is equivalent to in increase in contrast.[000172] The first exposure latitude 570 shown in Figure 18 is for the illumination pupil 560 shown in Figure 15. Similarly, the second exposure latitude 572 shown in Figure 18 is for the illumination pupil 562 shown in Figure 16.[000173] The third exposure latitude 574 shown in Figure 18 is for exposure using a method 100 of the type shown in Figures 3 and 6 (and described above) having two subsequent exposures 110a, 110b. A first one of the two exposures 110a is illuminated using the first new half-leaf dipole illumination pupil 564 shown Figure 17A and with a defocus of +17 nm; a second one of the two exposures 110b is illuminated using the second new half-leaf dipole illumination pupil 566 shown Figure 17B and with a defocus of -17 nm. Note that the two subsequent exposures 110a, 110b may be achieved using the an illumination region 300 of the type shown in Figure 7 comprising two distinct portions 302a, 302b that are illuminated, wherein the one of the distinct portions 302a, 302b is illuminated using the first newhalf-leaf dipole illumination pupil 564 shown Figure 17A and the other one of the distinct portions 302a, 302b is illuminated using the second new half-leaf dipole illumination pupil 566 shown Figure 17B.[000174] It can be seen from Figure 18 that although the third exposure latitude 574 uses a (discrete) focus-drilling recipe (which normally leads to decreased contrast) the contrast increases by 12%. Comparison with the. Using both of the pupils 564, 566 shown in Figures 17A and 17B (at an appropriate defocus for each) results in a large depth of focus and may improve an overlapping process window in the case of imaging a pattern containing multiple pitches, as now discussed with reference to Figure 19.[000175] Figure 19 shows the exposure latitude as a percentage as a function of depth of focus when imaging vertical lines with a combination of a first pitch, p1, of 16 nm and second pitch, p2, of 20 nm (again using a wavelength of 13.5 nm and a projection system with a numerical aperture of -0.55) for two different illumination modes. The first exposure latitude 580 shown in Figure 19 is for the illumination pupil 562 shown in Figure 16. The second exposure latitude 582 shown in Figure 19 is for exposure using a method 100 of the type shown in Figures 3 and 6 (and described above) having two subsequent exposures 110a, 110b: a first exposure 110a using the first new half-leaf dipole illumination pupil 564 shown Figure 17A and with a defocus of +17 nm; and a second exposure 110b using the second new half-leaf dipole illumination pupil 566 shown Figure 17B and with a defocus of -17 nm. It can be seen from Figure 19 that the contrast is still improved significantly using the two exposures and the two half-leaf dipole illumination pupils 564, 566. This proposed split-pupil design in combination with different focal positions (for example by using a focus-tilt and only illuminating distinct portions 302a, 302b of an illumination region 300) increases the overlapping process window for lines with pitches of 16 nm and 20 nm considerably.[000176] In some embodiments, pupil-splits may be used in combination with different focal positions (for example by using a focus-tilt and only illuminating distinct portions 302a, 302b of an illumination region 300) that do not deteriorate an overlapping process of a main feature but which have a focus-drilling effect on a second feature.[000177] In some embodiments of the new methods 100 shown in Figures 3 and 6, at least one of the of the plurality of exposures HOa-l lOn may use an illumination pupil that results in non-telecentric illumination of the substrate and which, for the focal position of the image relative to the substrate when that illumination mode is used, results in a shift of the image of the feature that at least partially compensates for an image shift due to three-dimensional mask effects.[000178] In some embodiments of the new methods 100 shown in Figures 3 and 6, at least one of the plurality of exposures 110a- 11 On may use an illumination pupil such that a pattern shift for that contribution to the image of the feature is minimized. In some embodiments of the new methods 100 shown in Figures 3 and 6, the plurality of exposures HOa-l lOn may use illumination pupils that are such that a pattern shift for all contributions to the image of the feature is minimized.[000179] In some embodiments of the new methods 100 shown in Figures 3 and 6, the illumination pupils of the plurality of exposures 110a- 11 On may be such that a contrast of the image of the feature formed on the substrate W is maximized.[000180] In some embodiments of the new methods 100 shown in Figures 3 and 6 two sequential exposures 110a, 110b may be performed. For a first one 110a of the two sequential exposures the illumination pupil may be such that only a portion of the pupil plane less than A / 2p from a center of the pupil plane in the shearing direction is illuminated. Such an illumination pupil may be referred to as an inner pupil. An example of such an inner pupil is shown in Figure 17B. For a second one 110b of the two sequential exposures the illumination pupil may be such that only a portion of the pupil plane greater than A / 2p from the center of the pupil plane in the shearing direction is illuminated.[000181] Some embodiments of the present disclosure relate to a lithographic apparatus that is operable to perform at least one of the methods 100 shown in Figures 3 and 6. The lithographic apparatus may, for example, be generally of the form of the lithographic apparatus LA shown in Figure 1 and described above (and may comprise any of the features thereof).[000182] In some embodiments, the lithographic apparatus LA may comprise an illumination system IL comprising: a first optical component 10; a second optical component 11; and a controller CN. The first optical component 10 may comprise a two-dimensional array of independently movable reflective optical elements which is arranged to receive radiation B from a radiation source SO. The second optical component 11 may comprise a two-dimensional array of independently movable reflective optical elements and may be arranged to receive radiation B from the first optical component 10 and to direct it to an illumination region IR.[000183] The controller CN may be operable to control the first and second optical components 10, 11. In particular, the controller CN may be operable to control the first and second optical components 10, 11 so as to illuminate a plurality of discrete portions (for example two discrete portions 302a, 302b) of the illumination region IR, the plurality of discrete portions being at different positions in a first direction. The first direction may be referred to as a scanning direction of a y-direction. The plurality of discrete portions 302a, 302b may be separated in the first direction. This may result in an optimum contrast performance. Alternatively, in some embodiments, there may not be a strict separation between the discrete portions 302a, 302b.[000184] In some embodiments, each of the independently movable reflective optical elements of the first optical component 10 may comprise a micro-electromechanical system (MEMS) micro-mirror. For such embodiments, the first optical component 10 may be considered to comprise a MEMS micromirror array.[000185] In some embodiments, each of the independently movable reflective optical elements of the second optical component 11 may comprise a micro-electromechanical system (MEMS) micromirror. For such embodiments, the second optical component 11 may be considered to comprise a MEMS micro-mirror array.[000186] Note that embodiments wherein the first optical component 10 and / or the second optical component 11 comprises a MEMS micro-mirror array are particularly beneficial as this may allow significant freedom to: (a) only illuminate a plurality of discrete portions (for example two discrete portions 302a, 302b) of the illumination region IR; and / or (b) illuminate each of the plurality of discrete portions (for example two discrete portions 302a, 302b) of the illumination region IR with a different illumination pupil (angular intensity distribution).[000187] The lithographic apparatus LA may comprising 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.[000188] The lithographic apparatus may 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.[000189] 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.[000190] 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.[000191] 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.[000192] While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. The descriptions above are intended to be illustrative, not limiting. Thus it will be apparent to one skilled in the art that modifications may be made to the invention as described without departing from the scope of the claims set out below.

Claims

CLAIMS1. A method of forming a feature on a substrate, the method comprising: performing a plurality of sequential exposures, each of the plurality of exposures comprising: forming an image of the feature in the vicinity of the substrate; wherein a focal position of the image relative to the substrate is different for at least two of the plurality of exposures; and wherein there is a range of focal positions of the image relative to the substrate that is not used during any of the plurality of exposures and which lies between the focal positions of the image relative to the substrate for two of the plurality of exposures.

2. The method of claim 1 wherein a range of focal positions of the image relative to the substrate during any one of the plurality of exposures is less than a difference between average focal positions of that exposure and an adjacent exposure when the exposures are ordered by the average focal position of the image relative to the substrate.

3. The method of claim 1 or claim 2 wherein each of the plurality of exposures comprises: illuminating a patterning device with radiation; capturing radiation scattered from the patterning device with imaging optics; and projecting the scattered radiation onto the substrate so as to form an image of the patterning device in the vicinity of the substrate.

4. The method of any preceding claim wherein at least two of the plurality of exposures are formed by illuminating the feature with radiation using different illumination pupils.

5. The method of any preceding claim wherein performing the plurality of exposures comprises: illuminating a plurality of discrete portions of an illumination region in an object plane, the plurality of discrete portions being at different positions in a first direction; moving a patterning device having the feature through the illumination region in the first direction; and moving the substrate such that an image of the patterning device is substantially static with respect to the substrate in the first direction; wherein a surface of the substrate is disposed at a non-zero angle to a plane of best focus in a plane containing the first direction.

6. A method of forming a feature on a substrate, the method comprising:illuminating a plurality of discrete portions of an illumination region in an object plane, the plurality of discrete portions being at different positions in a first direction; moving a patterning device having the feature through the illumination region in the first direction; and moving the substrate such that an image of the patterning device is substantially static with respect to the substrate in the first direction; wherein a surface of the substrate is disposed at a non-zero angle to a plane of best focus in a plane containing the first direction.

7. The method of claim 5 or claim 6 wherein an extent in the first direction of any of the plurality of discrete portions of the illumination region in the object plane is less than a separation in the first direction of that discrete portion from any adjacent discrete portions.

8. The method of claim 6 or claim 7 when dependent on claim 6, further comprising: capturing radiation scattered from the patterning device with imaging optics; and projecting the scattered radiation onto the substrate so as to form the image of the patterning device in the vicinity of the substrate.

9. The method of any one of claims 6 to 8 wherein the illumination of at least two of the plurality of discrete portions of the illumination region is carried out using different illumination pupils.

10. The method of any preceding claim when dependent either directly or indirectly on claim 4 or claim 9 wherein at least one of the different illumination pupils results in non-telecentric illumination of the substrate that, for the focal position of the image relative to the substrate when that illumination mode is used, results in a shift of the image of the feature that at least partially compensates for an image shift due to three-dimensional mask effects.

11. The method of any preceding claim when dependent either directly or indirectly on claim 4 or claim 9 wherein at least one of the different illumination pupils is such that a pattern shift for all contributions to the image of the feature is minimized.

12. The method of any preceding claim when dependent either directly or indirectly on claim 4 or claim 9 wherein the plurality of different illumination pupils are such that a contrast of the image of the feature formed on the substrate is maximized.

13. The method of any preceding claim wherein two sequential exposures are performed.

14. The method of claim 13 wherein for a first one of the two sequential exposures the illumination pupil is such that only a portion of the pupil plane less than A / 2p from a center of the pupil plane in the shearing direction is illuminated and wherein for a second one of the two sequential exposures the illumination pupil is such that only a portion of the pupil plane greater than A / 2p from the center of the pupil plane in the shearing direction is illuminated, where A is the wavelength of the exposure radiation and p is a pitch of the feature.

15. The method of claim 14 wherein for each of the first and second sequential exposures the illumination pupil comprises a half-leaf dipole.

16. The method of any preceding claim wherein the number and / or relative intensities of the plurality of sequential exposures performed is optimized so as to maximize a homogeneity of the intensity of radiation over a target depth of focus.

17. The method of any preceding claim wherein the feature comprises an isolated feature.

18. A lithographic apparatus operable to perform the method of any preceding claim.

19. The lithographic apparatus of claim 18 comprising an 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 operable to control the first and second optical components so as to illuminate a plurality of discrete portions of the illumination region, the plurality of discrete portions being at different positions in a first direction.

20. The lithographic apparatus of claim 19 wherein each of the independently movable reflective optical elements of the first optical component comprises a micro-electromechanical system (MEMS) micro-mirror.

21. The lithographic apparatus of claim 19 or claim 20 wherein each of the independently movable reflective optical elements of the second optical component comprises a micro-electromechanical system (MEMS) micro-mirror.

22. The lithographic apparatus of any one of claims 18 to 21 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.

23. The lithographic apparatus of any one of claims 18 to 22 further comprising: 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.

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