EUV radiation beam power reduction

Abstract

A method of providing additional EUV radiation exposure of a portion of a die on a substrate at a level that compensates for previously low-exposure EUV radiation power, the method using EUV radiation power incident on a patterning apparatus of a lithographic apparatus comprising a first array of mirrors and a second array of mirrors, the first mirror array is configured to receive EUV radiation and reflect the EUV radiation as radiation beamlets towards the second mirror array, where the method includes rotating the mirrors of the first array such that at least some of the radiation beamlets are incident on the mirrors of the second array at locations where radiation beamlet transmission to the patterned device is reduced.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to EP application 22194917.5 filed on September 9, 2022, the entire contents of which are incorporated herein by reference. Technical Field

[0003] The present invention relates to reducing the power of an EUV radiation beam incident on a patterning device in a lithographic apparatus. Background Art

[0004] A lithographic apparatus is a machine configured to apply a desired pattern to a substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). For example, a lithographic apparatus can project a pattern at a patterning device (e.g., a mask) onto a layer of radiation-sensitive material (resist) disposed on a substrate.

[0005] To project a pattern onto a substrate, a lithographic apparatus may use electromagnetic radiation. The wavelength of the radiation determines the minimum size of features that can be formed on the substrate. A lithographic apparatus using extreme ultraviolet (EUV) radiation with a wavelength in the range of 4 nm to 22 nm (e.g., 6.7 nm or 13.5 nm) may be used to form smaller features on a substrate than a lithographic apparatus using radiation with a wavelength of, for example, 193 nm.

[0006] The lithographic apparatus may be provided with EUV radiation from a radiation source of a type known as a laser produced plasma (LPP) source. In an LPP source, a laser system (such as a CO2 laser) with an optical amplifier generates laser pulses that store energy into a fuel (such as tin (Sn)) provided from a fuel emitter. The fuel may be provided as droplets. The laser energy is stored into the tin to form a tin plasma. During deexcitation and electron-ion recombination in the plasma, EUV radiation is emitted from the plasma.

[0007] LPP EUV radiation sources are highly complex systems, and it can be difficult to control the power of the EUV radiation emitted by the source. For example, if it is desired to temporarily reduce the power of the light emitted from the source, for example to perform die repair, the power supplied to the laser and / or optical amplifier cannot be reduced only slightly and then returned to its original position in order to achieve this. Modifying the power supplied to the laser and / or amplifier in this manner will easily produce undesirable substantial fluctuations in the power of the light delivered to the tin droplets.

[0008] Although other sources of EUV radiation are known, these sources either do not provide sufficiently high power for practical use in EUV lithography apparatus or are otherwise impractical (eg, due to size or cost).

[0009] Typically, it can be difficult to control the power of EUV radiation output from known EUV radiation sources suitable for use in a lithographic apparatus.

[0010] It may be desirable to provide methods and apparatus that overcome or mitigate one or more problems associated with the prior art. Summary of the invention

[0011] According to a first aspect of the invention, there is provided a method for providing additional EUV radiation exposure of a portion of a bare die on a substrate at a level compensating for a previously low exposure of EUV radiation power, the method using EUV radiation power incident on a patterning device of a lithography apparatus, the lithography apparatus comprising a first mirror array and a second mirror array, the first mirror array being configured to receive EUV radiation and reflect the EUV radiation as radiation sub-beams towards the second mirror array, wherein the method comprises rotating the mirrors of the first array so that at least some of the radiation sub-beams are incident on the mirrors of the second array at a position where transmission of the radiation sub-beams to the patterning device is reduced.

[0012] Advantageously, the method reduces the power incident on the patterning device without modifying the power output from the EUV radiation source (which may be difficult to control).

[0013] The radiation sub-beam may be moved from a starting position on the mirror of the second array that provides maximum transmission of the radiation sub-beam.

[0014] The radiation sub-beam may be moved from a starting position on the mirror of the second array at the centre of the mirror of the second array.

[0015] Reduced transmission may be provided in cases where portions of the radiation sub-beam do not fall outside the mirrors of the second array.

[0016] Reduced transmission may be provided in the event that a portion of the radiation sub-beam falls outside the mirrors of the second array.

[0017] Some of the radiation sub-beams may be moved in a first direction across the mirrors of the second array and other of the radiation sub-beams may be moved in a second direction across the mirrors of the second array.

[0018] The second direction may be opposite to the first direction.

[0019] The rotation of the mirrors of the first array may be performed based on a predetermined transmission calibration.

[0020] All mirrors of the first array can be rotated.

[0021] The power of the EUV radiation beam may be reduced without modifying the telecentricity of the EUV radiation beam.

[0022] The transmission of the EUV radiation beam may be reduced, for example, by 50% or more.

[0023] According to a second aspect of the invention, there is provided a lithographic apparatus comprising an irradiation system, the lithographic apparatus comprising a first mirror array and a second mirror array, the first mirror array being configured to receive EUV radiation and reflect the EUV radiation as radiation sub-beams towards the second mirror array, wherein the lithographic apparatus further comprises a controller configured to rotate the mirrors of the first array so that at least some of the radiation sub-beams are incident on the mirrors of the second array at a position where the transmission of the radiation sub-beams of the irradiation system is reduced, and then performing exposure of a portion of a die, the reduced transmission compensating for a previous low exposure of the portion of the die.

[0024] Advantageously, the lithographic apparatus reduces the power delivered by the illumination system without modifying the power output from the EUV radiation source, which may be difficult to control.

[0025] The controller may be configured to rotate the mirrors of the first array such that the radiation sub-beam moves from a starting position on the mirrors of the second array that provides maximum transmission of the radiation sub-beam.

[0026] The controller may be configured to rotate the mirrors of the first array so as to provide reduced transmission such that a portion of the radiation sub-beams falls outside the mirrors of the second array.

[0027] The controller may be configured to rotate the mirrors of the first array such that some radiation sub-beams move across the mirrors of the second array in a first direction and other radiation sub-beams move across the mirrors of the second array in a second direction.

[0028] The second direction may be opposite to the first direction.

[0029] The controller may be configured to rotate all mirrors of the first array.

[0030] The lithographic apparatus may further comprise a memory storing predetermined transmission calibration data as a function of rotation of the mirrors of the first array.

[0031] According to a third aspect of the present invention, there is provided a lithography illumination system, which comprises a first mirror array and a second mirror array, wherein the first mirror array is configured to receive EUV radiation and reflect the EUV radiation as radiation sub-beams toward the second mirror array, wherein the illumination system further comprises a controller configured to rotate the mirrors of the first array so that at least some of the radiation sub-beams are incident on the mirrors of the second array at a position where the transmission of the radiation sub-beams of the illumination system is reduced.

[0032] The controller may be configured to rotate the mirrors of the first array so as to provide reduced transmission such that a portion of the radiation sub-beams falls outside the mirrors of the second array.

[0033] According to a fourth aspect of the present invention, there is provided a computer readable storage medium comprising instructions which, when executed by a processor of a computing device, cause the computing device to perform the method of the first aspect of the present invention.

[0034] According to a fifth aspect of the present invention, there is provided a computing device comprising a processor and a memory, the memory storing instructions which, when executed by the processor, cause the computing device to perform the method of the first aspect of the present invention.

[0035] Features of different aspects of the invention may be combined together. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:

[0037] Figure 1 Schematically depicts a lithography system including a radiation source and a lithography apparatus according to an embodiment of the present invention;

[0038] Figure 2 A radiation source is schematically depicted and an illumination system and patterning device of a lithographic apparatus are schematically depicted;

[0039] Figure 3 schematically depicts a mirror forming part of a second mirror array of an illumination system;

[0040] Figure 4 schematically depicting a radiation sub-beam centrally incident on and partially overlapping a reflector, and also depicting a graph showing the transmission of the reflector; and

[0041] Figure 5 A second mirror array of the illumination system is schematically depicted and an illumination pattern is schematically depicted. DETAILED DESCRIPTION

[0042] Figure 1 A lithographic system is shown 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.

[0043] The illumination system IL is configured to condition the EUV radiation beam B before it is incident on the patterning device MA. In addition, the illumination system IL may include a polygonal field lens device 10 and a polygonal pupil lens device 11. The polygonal field lens device 10 and the polygonal pupil lens device 11 together provide an EUV radiation beam B having a desired cross-sectional shape and a desired intensity distribution. In addition to the polygonal field lens device 10 and the polygonal pupil lens device 11, the illumination system IL may also include other mirrors.

[0044] After being so 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 produced. The projection system PS is configured to project the patterned EUV radiation beam B' onto the substrate W. For this purpose, the projection system PS may include a plurality of mirrors 13, 14 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' so as to form an image with features that are smaller than the 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 Figure 1 Although illustrated in FIG. 1 as having only two mirrors 13 , 14 , the projection system PS may comprise a different number of mirrors (eg six or eight mirrors).

[0045] During scanning exposure, the patterning device MA and support structure MT are moved in the y-direction, and the substrate W and substrate table WT are moved in the opposite y-direction. In this way, a band of EUV radiation passes through an exposure field on the substrate W.

[0046] A relative vacuum, ie a small amount of gas (eg 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.

[0047] Figure 1The radiation source SO shown in the figure is, for example, of a type that may be referred to as a laser produced plasma (LPP) source. The laser system 1, which may, for example, include a CO2 laser, is arranged to store energy via a laser beam 2 into a fuel, such as tin (Sn), provided from, for example, a fuel emitter 3. Although reference is made to tin in the following description, any suitable fuel may be used. The fuel may be, for example, in liquid form and may, for example, be a metal or an alloy. The fuel emitter 3 may include a nozzle configured to direct tin, for example in the form of droplets, along a trajectory toward a plasma formation region 4. The laser beam 2 is incident on the tin at the plasma formation region 4. The laser energy is stored in the tin to form a tin plasma 7 at the plasma formation region 4. During deexcitation and electron-ion recombination in the plasma, radiation is emitted from the plasma 7, the radiation including EUV radiation.

[0048] EUV radiation from the plasma is collected and focused by a collector 5. The collector 5 includes, for example, a near normal incidence radiation collector 5 (sometimes more generally referred to as a normal incidence radiation collector). The collector 5 may have a multilayer mirror structure arranged to reflect EUV radiation (e.g., EUV radiation having a desired wavelength, such as 13.5 nm). The collector 5 may have an elliptical configuration having two focal points. A first of the focal points may be at the plasma formation region 4, and a second of the focal points may be at the intermediate focus 6, as discussed below.

[0049] The laser system 1 may be spatially separated from the radiation source SO. In this case, the laser beam 2 may be transferred 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 beam expanders and / or other optical devices. The laser system 1, the radiation source SO and the beam delivery system may be considered together as a radiation system.

[0050] The radiation reflected by the collector 5 forms an EUV radiation beam B. The EUV radiation beam B is focused at an intermediate focus 6 to form an image of the plasma present at the plasma formation region 4 at the intermediate focus 6. The image at the intermediate focus 6 serves as a virtual radiation source for the illumination system IL. The radiation source SO is arranged so that the intermediate focus 6 is located at or near an opening 8 in an enclosure 9 of the radiation source SO.

[0051] although Figure 1 The radiation source SO is depicted as a laser produced plasma (LPP) source, but the EUV radiation may be produced using any suitable source, such as a discharge produced plasma (DPP) source or a free electron laser (FEL).

[0052] Figure 2The radiation source SO is schematically depicted, the illumination system IL is depicted in more detail, and the patterning device MA is depicted. As further described above, the laser beam 2 is incident on the tin droplets, thereby forming an EUV emitting plasma 7. The EUV radiation is collected by the collector 5, focused by the intermediate focus IF and incident on the multi-faceted field mirror device 10.

[0053] The multifaceted field mirror device 10 includes an array of individually movable mirrors. The multifaceted field mirror device 10 may be referred to as a first array of mirrors or a first array of mirrors. The multifaceted field mirror device 10 is schematically depicted as including four mirrors 10a to 10d. In practice, the multifaceted field mirror device 10 will include more mirrors. These mirrors may be arranged in a two-dimensional array. These mirrors may all be concave (as schematically depicted). In one example, the multifaceted field mirror device 10 may include at least 100 mirrors, may include at least 300 mirrors, and may include at least 500 mirrors. In one example, the multifaceted field mirror device may include up to 300 mirrors, up to 500 mirrors, or a much larger number of mirrors, such as up to 10,000 mirrors, up to 100,000 mirrors, up to 500,000 mirrors, or more. The mirrors, which may be referred to as facets, may be individually movable.

[0054] exist Figure 2 The multi-faceted pupil mirror device 11 is also described in more detail in FIG. The multi-faceted pupil mirror device 11 comprises an array of individually movable mirrors 11a to 11h. In order to avoid Figure 2 Complexity, in Figure 2 Only some of the mirrors are labeled in FIG. The multifaceted pupil mirror device 11 may be referred to as a second array of mirrors or a second mirror array. The mirrors 11 may be referred to as facets. As schematically depicted, the mirrors may be concave.

[0055] exist Figure 2 , the faceted pupil mirror device 11 has twice as many mirrors 11a to 11h as the faceted field mirror device 10. In general, it may be desirable for the faceted pupil mirror device 11 to have more mirrors than the faceted field mirror device 10. This is because the additional mirrors of the pupil mirror device 11 allow for greater flexibility in generating the illumination pupil (as described below). The faceted pupil mirror device may have twice as many mirrors, three times as many mirrors, or some other multiple as the faceted field mirror device.

[0056] Because the mirrors of the polygonal field mirror device 10 and the mirrors of the polygonal pupil mirror device 11 are concave, these mirrors act as lenses. As a result, the polygonal field mirror device 10 and the polygonal pupil mirror device 11 act as an optical system. Working backwards from the patterning device MA, the patterning device MA can be regarded as the field plane, the polygonal pupil mirror device 11 can be regarded as the pupil plane, and the polygonal field mirror device 10 can be regarded as the field plane of the optical system. Therefore, the patterning device MA sees the image of the field facet mirrors of the polygonal field mirror device 10. The illumination pupil of the radiation received by the patterning device MA is determined by the distribution of the radiation incident on the mirrors 11a to 11h of the polygonal pupil mirror device 11.

[0057] The mirrors of the multifaceted field mirror device 10 may each be shaped as an elongated bend of constant width. An image of the elongated bend is formed at the patterning device MA, and as a result, the patterning device is illuminated by radiation shaped as the elongated bend. As schematically depicted, the mirrors 11a to 11h of the multifaceted pupil mirror device 11 may be oriented so that the individual images of the mirrors 10a to 10d of the multifaceted field mirror device 10 overlap each other at the patterning device MA.

[0058] The mirrors of the faceted pupil mirror device 11 receive images of the radiation at the intermediate focus IF. Because the faceted pupil mirror device 11 has more mirrors than the faceted field mirror device 10, only some of the mirrors 11a, 11b, 11g, 11h of the faceted pupil mirror device receive radiation at any given time (as schematically depicted).

[0059] Typically, each mirror of the faceted field mirror device 10 provides radiation which may be referred to as a radiation sub-beam. Each radiation sub-beam may be incident on a different mirror of the faceted pupil mirror device 11.

[0060] The orientation of each mirror of the faceted field mirror device 10 is individually adjustable. This allows EUV radiation to be directed to a desired location at the faceted pupil mirror device 11, thereby forming a desired illumination pupil. For example, a dipole pattern (or a dipole-like pattern) can be formed by directing radiation to two areas on either side of the center of the faceted pupil mirror device 11. This Figure 2 , although in practice more pupil facets will be illuminated when a dipole illumination pattern is formed (see Figure 5 and further related description below).

[0061] Figure 3 Depicted is one mirror 11a of the faceted pupil mirror device 11. As can be seen, the mirror 11a is generally hexagonal (although the mirror may not be a perfect hexagon and, in other embodiments, may have a different shape). Figure 3EUV radiation incident on the mirror 11a is also schematically depicted. As noted above, the mirror 11a sees an image of the intermediate focus IF and thus an image of the EUV radiation emitting plasma 7. For simplicity, EUV radiation (which may be referred to as a beamlet) is incident on the mirror 11a. Figure 3 1 is schematically depicted as a disk. As depicted, the illumination system IL can be configured so that during exposure of the lithographic substrate W, the mirror 10b of the multifaceted field mirror device 10 directs the EUV radiation sub-beam 20 to the center of the mirror 11a of the multifaceted pupil mirror device 11. During normal operation of the lithographic apparatus, this advantageously maximizes the performance of the lithographic apparatus. That is, substantially all of the EUV radiation sub-beams 20 directed by the mirror 10b of the multifaceted field mirror device 10 are reflected by the mirror 11a of the multifaceted pupil mirror device 11 toward the patterning device MA. In general, it is generally desirable to maximize the power of the EUV radiation incident on the lithographic substrate W during operation of the lithographic apparatus LA, as this allows the lithographic exposure to be performed faster, thereby maximizing the throughput of the lithographic apparatus.

[0062] In some cases, it is desirable to be able to provide EUV radiation beam B to the patterning device at a significantly reduced power. One example is when it has been determined that a portion of the die that has been exposed by the lithographic apparatus has not received a sufficiently high dose of EUV radiation. This portion of the die may be provided with additional exposure to EUV radiation at a level of power that compensates for the previous low exposure. The amount of EUV power desired for this compensatory exposure may be significantly less than the conventional exposure power. It may be difficult to control the EUV radiation source SO in order to provide such a low power, because substantial adjustments to the power of the EUV radiation source may cause significant, undesirable fluctuations in the output power.

[0063] In an embodiment of the invention, the power of EUV radiation delivered to the patterning device MA is reduced by moving at least some of the radiation sub-beams so that they are no longer incident on the centers of the mirrors 11a to 11g of the faceted pupil mirror device 11. The radiation sub-beams may partially overlap the mirrors 11a to 11g of the faceted pupil mirror device 11. The radiation sub-beams are moved to be incident on the mirrors of the faceted pupil mirror device 11 at locations where the transmission of the radiation sub-beams to the patterning device MA is reduced. The transmission may be reduced, for example, by 50% or more.

[0064] Figure 4 On the left side the mirror 11a of the faceted pupil mirror device 11 is schematically depicted together with EUV radiation sub-beams 20a to 20c at three different positions. Figure 4On the right hand side is a graph schematically depicting the transmission T of the mirror 11a as a function of the rotational position Ry of the mirror 10b of the multifaceted field mirror device 10, which directs the radiation beam onto the mirror 11a of the multifaceted pupil mirror device 11. Although in this case the rotation of the mirror 10b of the multifaceted field mirror device 10 is about the Y axis, thereby moving the radiation sub-beam in the X direction, the rotation may alternatively be about the Y axis. Rotation about two axes may also be used. The rotation of the mirror 10b of the multifaceted field mirror device 10 causes the radiation sub-beam 20 to translate across the surface of the mirror 11a of the multifaceted pupil mirror device 11. The rotation of the mirror may also be referred to as the tilt of the mirror.

[0065] When the radiation sub-beam 20a is centrally positioned on the mirror 11a of the multi-faceted pupil mirror device 11, the transmission of the mirror is maximum. In this context, the transmission can be regarded as referring to the proportion of radiation incident on the patterning device MA. The maximum transmission is schematically shown as Figure 4 The graph depicts the peak value Ta of the transmission curve.

[0066] When the radiation sub-beam 20b moves so that part of the radiation sub-beam is no longer incident on the reflector 11a, but instead falls outside the reflector, the transmission of the reflector is significantly reduced. This is caused by Figure 4 The reduced transmission Tb on the transmission curve is schematically indicated.

[0067] As the radiation sub-beam 20c moves further away from the centre of the reflector 11a so that a large portion of the radiation sub-beam is not incident on the reflector, the transmission of the reflector 11a is further reduced. Figure 4 The transmission Tc in the transmission curve diagram is schematically indicated.

[0068] If Figure 4 As schematically indicated by the graph in , the transmission of the mirror 11a follows the curve . This curve thus allows the transmission of the mirror 11a to be reduced in a controlled manner via the orientation of the mirror 10b of the multifaceted field mirror device 10 .

[0069] Generally, movement of the radiation sub-beam 20 away from the position providing maximum transmission may reduce the transmission of the mirror 11a. This may apply even if all radiation sub-beams are still incident on the mirror (ie even if the radiation sub-beams do not partially overlap the mirror).

[0070] Figure 4The movement of one radiation sub-beam relative to one mirror 11a of the faceted pupil mirror device 11 is depicted. In practice, as further explained above, a plurality of radiation sub-beams are provided, which are typically directed to different mirrors of the faceted pupil mirror device 11. Thus, in practice, when it is desired to reduce the transmission of the illumination system IL in order to provide a reduced EUV radiation power at the patterning device MA, the plurality of mirrors of the faceted field mirror device 10 may be rotated such that they move the radiation sub-beams to positions on the mirrors of the faceted pupil mirror device 11 where the transmission is reduced.

[0071] In an embodiment, a subset of the radiation sub-beams may be moved to overlap with the mirrors of the faceted pupil mirror device 11 (or to a position where the transmission is reduced). The remaining radiation sub-beams may remain at positions on the mirrors of the faceted pupil mirror device 11 that provide maximum transmission (e.g., at the center of the mirrors). The transmission curves for each mirror of the faceted pupil mirror device 11 as a function of the mirror orientation of the faceted field mirror device 10 may be known and may be stored in a memory in the lithographic apparatus. These known transmission curves allow the transmission of each mirror of the faceted pupil mirror device 11 to be controlled in a known manner.

[0072] Moving only a subset of the radiation sub-beams to partially overlap the pupil facet mirror provides another mechanism for controlling the transmission provided by the illumination system IL. For example, if 10% of the mirrors of the faceted pupil mirror device 11 are moved, this will result in a smaller reduction in transmission than if 20% of the mirrors of the faceted pupil mirror device are moved.

[0073] Moving a subset of the radiation sub-beams to overlap the mirror portions of the faceted pupil mirror device 11 allows the EUV radiation beam transmission to be reduced without modifying the telecentricity of the radiation beam. Figure 5 Schematically illustrated in FIG. Figure 5 The multi-faceted pupil mirror device 11 is schematically depicted as viewed from above. For illustration purposes, a small portion of the mirrors of the multi-faceted pupil mirror device 11 is also depicted. Figure 5 , the mirrors of the faceted field mirror device 10 (not shown) have been oriented so that they direct EUV radiation to form a dipole pattern at the pupil plane of the illumination system IL (i.e., at the faceted pupil mirror device 11). The dipole pattern is schematically depicted as a left-hand pole 30 and a right-hand pole 32.

[0074] Figure 5A faceted pupil mirror device 11 is schematically depicted as viewed from above. For illustration purposes, a few of the mirrors 11a to 11h of the faceted pupil mirror device 11 are depicted (only two are labeled). A dipole pattern consisting of two regions 30, 32 of EUV radiation is formed at the faceted pupil mirror device 11. The dipole pattern is formed by directing EUV radiation to a first region 30 and a second region 32 using the mirrors of the faceted field mirror device 10.

[0075] When it is desired to reduce the power of EUV radiation incident on the patterning device MA, a subset of the mirrors of the faceted field mirror device 10 may be rotated so that the EUV radiation sub-beams move away from the centers of the mirrors of the faceted pupil mirror device 11. As explained above, this reduces the power of the EUV radiation reflected from the mirrors of the faceted pupil mirror device 11, thereby reducing the power of the EUV radiation incident on the patterning device MA. Figure 5 In FIG. 1 , a region 34 is depicted which forms part of the left hand region 30 of the dipole mode. If all EUV radiation sub-beams that are moved to reduce transmission were to lie in this region 34, this would have an undesirable effect as this would significantly modify the illumination pattern. The shape of the illumination pattern would change and the telecentricity of the illumination pattern would change. In order to reduce undesirable modifications of the illumination pattern, the positions of the mirrors of the faceted pupil mirror device 11 to which the radiation sub-beams are moved when the EUV radiation sub-beams are moved to reduce EUV power may be distributed across the illuminated part of the illumination pattern (i.e. the part of the illumination pattern that is not dark).

[0076] In another embodiment, instead of tilting the reflector mirror sets of the multifaceted field mirror device 10, all the reflectors can be tilted (i.e., all the reflectors can be rotated). Figure 4 As depicted in , each mirror of the faceted pupil mirror device 11 has a transmission curve that is known to be a function of the rotation of the mirror (or more than one mirror) of the faceted field mirror device 11. Figure 4 As schematically depicted in FIG, an initial movement of an EUV radiation subbeam away from the center of the mirror of the faceted pupil mirror device 11 (or other position providing maximum transmission) will only cause a small reduction in transmission. In general, the reduction in EUV transmission by rotating all the mirrors of the faceted field mirror device 10 is controllable in a straightforward manner. It can be seen that any desired reduction in transmission required for die exposure can be obtained thereby. The reduction in transmission can be, for example, a reduction of 50% or more.

[0077] The transmission reduction provided by the rotation of the field facet mirror may be stored as a look-up table or other suitable form of data (e.g. in a memory which may form part of the lithographic apparatus). The transmission reduction may be measured via calibration measurements performed for each mirror of the multifaceted field mirror device 10. For each mirror of the multifaceted pupil mirror device 11 which may receive radiation reflected from that mirror in use, a calibration may be performed for that mirror. These calibrations may be combined together to form a data set. Thus, embodiments of the present invention may use a predetermined calibration of the transmission reduction as a function of mirror rotation.

[0078] The controller may control the rotation of the mirrors of the polygonal field mirror device 10. The controller may control the rotation based on data calibrating the reduction in transmission as a function of the mirror rotation.

[0079] When all the mirrors of the polygonal field mirror device 10 are rotated to reduce transmission, the directions of rotation may be randomized, or distributed in some other way. This may result in no significant change in the geometry of the illumination pattern in the plane of the polygonal pupil mirror device 11. If all the mirrors of the polygonal field mirror device 10 were to rotate in the same direction, this would have the effect of causing a lateral shift in the illumination pattern at the polygonal pupil mirror device 11. This would change the telecentricity of the EUV radiation beam incident on the patterning device, and this may be undesirable. The same may apply when the mirror sets of the polygonal field mirror device 10 are rotated to reduce transmission.

[0080] As noted above, when reducing transmission, the rotation directions of the mirrors of the polygonal field mirror device 10 can be distributed, for example, via randomization, so as to avoid cumulative translation of the illumination pattern in the plane of the polygonal pupil mirror device 11. However, despite this, the movement of the EUV radiation sub-beam away from the center (or other maximum transmission position) of the mirrors of the polygonal pupil mirror device 11 may still have some effect on the illumination pattern. Computer modeling of this effect can be performed using data obtained via the calibration measurements mentioned above. The modeling can be iterative or analytical and can seek to minimize the effect of the EUV radiation sub-beam movement on the illumination pattern. The model can, for example, seek to minimize changes in telecentricity, seek to minimize the non-uniformity of the EUV radiation incident on the patterning device MA, and so on.

[0081] Typically, in embodiments of the present invention, a radiation sub-beam is moved on a mirror of the faceted pupil mirror device 11 to a position where the transmission of the radiation sub-beam to the patterning device is reduced. The radiation sub-beam may be moved from a starting position that provides a maximum transmission of the radiation sub-beam. The starting position of the radiation sub-beam may be at the center of the mirror of the faceted pupil mirror device 11. Reduced transmission may be provided in the case where part of the sub-beam does not fall outside the mirror of the faceted pupil mirror device 11. Reduced transmission may be provided in the case where part of the sub-beam falls outside the mirror of the faceted pupil mirror device 11.

[0082] The method according to an embodiment of the present invention can be performed by a computing device. The device may include a central processing unit ("CPU") connected to a memory. The method described herein may be implemented in a code (software), which is stored on a memory including one or more storage media and is arranged to be executed on a processor including one or more processing units. The storage medium may be integrated into the CPU and / or separated from the CPU. The code that may be referred to as an instruction is configured to be acquired from the memory and executed on the processor to perform operations consistent with the embodiments discussed herein. Alternatively, it is not excluded that some or all of the functions of the CPU are implemented in a dedicated hardware circuit device or a configurable hardware circuit device (like an FPGA). Typically, the method can be performed by a processor.

[0083] The computing device may include an input configured to enable a user to input data into a software program running on the CPU. The input device may include a mouse, keyboard, touch screen, microphone, etc. The computing device may also include an output device configured to output measurement results to the user.

[0084] Although specific reference may be made in this text to the use of lithographic apparatus in IC manufacturing, it should be understood that the lithographic apparatus described herein may have other applications. Other possible applications include manufacturing integrated optical systems, guidance and detection patterns for magnetic domain memories, flat panel displays, liquid crystal displays (LCDs), thin film magnetic heads, etc.

[0085] 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 apparatuses. 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 a mask (or other patterning device). These apparatuses may generally be referred to as lithographic tools. Such lithographic tools may use vacuum conditions or ambient (non-vacuum) conditions.

[0086] Although specific reference has been made above to the use of embodiments of the invention in the context of optical lithography, it will be appreciated that the invention is not limited to optical lithography and may be used in other applications, such as imprint lithography, where the context permits.

[0087] Where the context permits, embodiments of the present invention may be implemented in hardware, firmware, software, or any combination thereof. Embodiments of the present 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 a read-only memory (ROM); a random access memory (RAM); a magnetic storage medium; an optical storage medium; a flash memory device; an electrical, optical, acoustic, or other form of propagation signal (e.g., a carrier wave, an infrared signal, a digital signal, etc.), and the like. In addition, firmware, software, routines, instructions may be described herein as performing certain actions. However, it should be understood that such descriptions are merely for convenience, and that such actions are actually generated by a computing device, processor, controller, or other device executing firmware, software, routines, instructions, etc., and when executing firmware, software, routines, instructions, etc., the computing device, processor, controller, or other device may cause an actuator or other device to interact with the physical world.

[0088] Although specific embodiments of the present invention have been described above, it should be understood that the present invention may be practiced in a manner other than that described. The above description is intended to be illustrative rather than restrictive. Therefore, it will be apparent to those skilled in the art that modifications may be made to the described invention without departing from the scope of the clauses listed below.

[0089] 1. A method for reducing EUV radiation power incident on a patterning device of a lithographic apparatus, the lithographic apparatus comprising a first mirror array and a second mirror array, the first mirror array being configured to receive EUV radiation and reflect the EUV radiation as radiation sub-beams towards the second mirror array, wherein the method comprises rotating the mirrors of the first mirror array so that at least some of the radiation sub-beams are incident on the mirrors of the second mirror array at a position where the transmission of the radiation sub-beams to the patterning device is reduced.

[0090] 2. A method according to clause 1, wherein the radiation sub-beam is moved from a starting position on a mirror of the second mirror array that provides maximum transmission of the radiation sub-beam.

[0091] 3. A method according to clause 1 or clause 2, wherein the radiation sub-beam is moved from a starting position on a mirror of the second mirror array at a centre of a mirror of the second mirror array.

[0092] 4. A method according to any preceding clause, wherein reduced transmission is provided such that parts of the radiation sub-beams do not fall outside the mirrors of the second mirror array.

[0093] 5. A method according to any of clauses 1 to 4, wherein the reduced transmission is provided such that a portion of the radiation sub-beam falls outside of the mirrors of the second mirror array.

[0094] 6. A method according to any preceding clause, wherein some of the radiation sub-beams are moved across the mirrors of the second mirror array in a first direction and other of the radiation sub-beams are moved across the mirrors of the second mirror array in a second direction, the second direction being opposite to the first direction.

[0095] 7. A method according to any preceding clause, wherein the rotation of the mirrors of the first mirror array is performed based on a predetermined transmission calibration.

[0096] 8. A method according to any preceding clause, wherein all mirrors of the first mirror array are rotated.

[0097] 9. A lithographic apparatus comprising an illumination system, the illumination system comprising a first mirror array and a second mirror array, the first mirror array being configured to receive EUV radiation and reflect the EUV radiation as radiation sub-beams toward the second mirror array, wherein the lithographic apparatus further comprises a controller configured to rotate the mirrors of the first mirror array so that at least some of the radiation sub-beams are incident on the mirrors of the second mirror array at a position where the transmission of the radiation sub-beams of the illumination system is reduced.

[0098] 10. The lithographic apparatus of clause 9, wherein the controller is configured to rotate the mirrors of the first mirror array such that the radiation sub-beams move from a starting position on the mirrors of the second mirror array that provides maximum transmission of the radiation sub-beams.

[0099] 11. A lithographic apparatus according to clause 9 or clause 10, wherein the controller is configured to rotate the mirrors of the first mirror array so as to provide a reduced transmission such that a portion of the radiation sub-beam falls outside the mirrors of the second mirror array.

[0100] 12. A lithographic apparatus according to any one of clauses 9 to 11, wherein the controller is configured to rotate the mirrors of the first mirror array so that some radiation sub-beams move across the mirrors of the second mirror array in a first direction and other radiation sub-beams move across the mirrors of the second mirror array in a second direction, the second direction being opposite to the first direction.

[0101] 13. The lithographic apparatus of any of clauses 9 to 12, wherein the controller is configured to rotate all mirrors of the first mirror array.

[0102] 14. The lithographic apparatus according to any of clauses 9 to 13, wherein the lithographic apparatus further comprises a memory storing predetermined transmission calibration data as a function of the rotation of the mirrors of the first mirror array.

[0103] 15. A lithography illumination system, comprising a first mirror array and a second mirror array, the first mirror array being configured to receive EUV radiation and reflect the EUV radiation as radiation sub-beams toward the second mirror array, wherein the illumination system further comprises a controller configured to rotate the mirrors of the first mirror array so that at least some of the radiation sub-beams are incident on the mirrors of the second mirror array at a position of the illumination system where transmission of the radiation sub-beams is reduced.

Claims

1. A method of providing additional EUV radiation exposure to a portion of a die on a substrate at an EUV radiation power level that compensates for a previous low exposure, the method using EUV radiation power incident on a patterning device of a lithographic apparatus, the lithographic apparatus comprising a first array of mirrors and a second array of mirrors, the first array of mirrors being configured to receive EUV radiation and reflect the EUV radiation as radiation sub-beams towards the second array of mirrors, wherein the method comprises rotating the mirrors of the first array so that at least some of the radiation sub-beams are incident on the mirrors of the second array at a position where transmission of the radiation sub-beams provided to the patterning device is reduced.

2. The method of claim 1, wherein the radiation sub-beam is moved from a starting position on a mirror of the second array that provides maximum transmission of the radiation sub-beam.

3. A method according to claim 1 or claim 2, wherein the radiation sub-beam moves from a starting position on the mirror of the second array at the centre of the mirror of the second array.

4. A method according to any preceding claim, wherein the reduced transmission is provided in the event that a portion of the radiation sub-beam does not fall outside the reflector of the second array.

5. A method according to any one of claims 1 to 4, wherein the reduced transmission is provided if a portion of the radiation sub-beam falls outside the reflectors of the second array.

6. A method according to any preceding claim, wherein some radiation sub-beams are moved across the mirrors of the second array in a first direction and other radiation sub-beams are moved across the mirrors of the second array in a second direction, the second direction being opposite to the first direction.

7. A method according to any preceding claim, wherein the rotation of the mirrors of the first array is performed based on a predetermined transmission calibration.

8. A method according to any preceding claim, wherein all of the mirrors of the first array are rotated.

9. A method according to any preceding claim, wherein the power of the EUV radiation beam is reduced without modifying the telecentricity of the EUV radiation beam.

10. A lithographic apparatus comprising an illumination system, the illumination system comprising a first array of mirrors and a second array of mirrors, the first array of mirrors being configured to receive EUV radiation and reflect the EUV radiation as radiation sub-beams toward the second array of mirrors, wherein the lithographic apparatus further comprises a controller configured to rotate the mirrors of the first array so that at least some of the radiation sub-beams are incident on the mirrors of the second array at a position that reduces the transmission of the radiation sub-beams provided by the illumination system, and then performing exposure of a portion of a die, the reduced transmission compensating for a previous low exposure of the portion of the die.

11. The lithographic apparatus of claim 10, wherein the controller is configured to rotate the mirrors of the first array such that the radiation sub-beam moves from a starting position on the mirrors of the second array that provides maximum transmission of the radiation sub-beam.

12. A lithographic apparatus according to claim 10 or claim 11, wherein the controller is configured to rotate the mirrors of the first array such that the reduced transmission is provided if part of the radiation sub-beam falls outside the mirrors of the second array.

13. A lithographic device according to any one of claims 10 to 12, wherein the controller is configured to rotate the mirrors of the first array so that some radiation sub-beams move across the mirrors of the second array in a first direction and other radiation sub-beams move across the mirrors of the second array in a second direction, and the second direction is opposite to the first direction.

14. The lithographic apparatus according to any one of claims 10 to 13, wherein the controller is configured to rotate all the mirrors of the first array.

15. The lithographic apparatus according to any one of claims 10 to 14, wherein the lithographic apparatus further comprises a memory storing predetermined transmission calibration data as a function of the rotation of the mirrors of the first array.

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