CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of EP application 17192125.7 which was filed on 20 September 2017 and which is incorporated herein in its entirety by reference.

FIELD

[0002] The present invention relates to a control system for adjusting a wavefront of a beam of radiation. The control system may form part of a radiation source that is suitable for a lithographic apparatus.

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] The wavelength of radiation used by a lithographic apparatus to project a pattern onto a substrate determines the minimum size of features which can be formed on that substrate. A lithographic apparatus which uses EUV radiation, being electromagnetic radiation having a wavelength within the range 4-20 nm, may be used to form smaller features on a substrate than a conventional lithographic apparatus (which may for example use electromagnetic radiation with a wavelength of 193 nm).

[0005] EUV radiation may be produced using a laser-produced plasma (LPP) radiation source. The LPP source may comprise a seed laser configured to provide a laser radiation beam. The laser radiation beam may be amplified and then be transported to a plasma formation location region of the LPP source in order to excite a target to form a plasma in order to generate EUV radiation. The wavefront of the laser radiation beam may collect optical aberrations before reaching the plasma formation region. The optical aberrations may alter the shape of the wavefront of the laser radiation beam in an undesirable way. The shape of the wavefront of the laser radiation beam may affect a conversion efficiency of the LPP source (i.e. the power of EUV radiation generated versus the power required to generate the plasma).

SUMMARY

[0006] According to a first aspect of the invention, there is provided a control system for adjusting a wavefront of a beam of radiation. The control system comprises a first mirror configured to receive the radiation beam along a first propagation direction and reflect the radiation beam along a second propagation direction. The first mirror is further configured to apply a first change to a shape of the wavefront of the radiation beam. The control system also comprises a second mirror configured to reflect the reflected radiation beam along a third propagation direction, the second mirror being further configured to apply a second change to the shape of the wavefront of the radiation beam. The control system also has an actuation system configured to rotate a specific one of the first mirror and the second mirror about an axis, the axis being configured to preserve each of the first propagation direction, the second propagation direction and the third propagation direction.

[0007] The shape of the wavefront of the radiation beam may deviate from a desired shape of the wavefront due to the collection of optical aberrations when the radiation beam travels through the radiation source, which may in turn reduce an efficiency of the radiation source. The control system advantageously controls the shape of the wavefront of the radiation beam enabling an increase in the efficiency of the radiation source. Using mirrors rather than transmissive optics advantageously allows the wavefront of high power (e.g. about 25 kW) laser beams to be controlled, thus making the control system suitable for use with an LPP EUV radiation source. The control system offers an inexpensive and flexible solution which does not require extensive re -design of known radiation sources.

[0008] The actuation system may be further configured to rotate the other one of the first mirror and the second mirror about a second axis, the second axis being configured to preserve each of the first propagation direction, the second propagation direction and the third propagation direction.

[0009] At least one of the first mirror and second mirror may have a spatial reflective structure that corresponds with a Zernike polynomial, the Zernike polynomial having a radial degree that is greater than or equal to two.

[00010] Zernike polynomials may be used to represent different types of optical aberration. Having the spatial reflective structure of a mirror substantially correspond with a Zernike polynomial advantageously enables specific types of optical aberration of the wavefront of the radiation beam to be controlled. Zernike polynomials having a radial degree that is greater than or equal to two are non-flat.

[00011] The Zernike polynomial may be an astigmatic Zernike polynomial.

[00012] It has been found via experiment that astigmatism is an important aberration to consider and control when improving an efficiency of a radiation source. Having the spatial reflective structure of at least one of the first mirror and the second mirror substantially correspond with an astigmatic Zernike polynomial advantageously enables a desired astigmatic change in the shape of the wavefront to be applied to the radiation beam.

[00013] Having the spatial reflective structures of the first and second mirrors substantially correspond with an astigmatic Zernike polynomial advantageously enables a magnitude and an orientation of astigmatic aberration of the wavefront to be controlled, thus allowing greater control of the efficiency of the radiation source.

[00014] The spatial reflective structure may correspond with a Zernike polynomial being superimposed on a quadratic surface, the Zernike polynomial having a radial degree that is greater than or equal to two.

[00015] Some mirrors in known radiation sources have quadratic surfaces. Having the shape of at least one of the first mirror and the second mirror substantially correspond with an astigmatic Zernike polynomial superimposed onto a quadratic surface advantageously enables the divergent or convergent effects of quadratic mirrors to be maintained whilst also allowing such mirrors to be used to control a magnitude and/or an angular position of an aberration (e.g. astigmatism) of the wavefront.

[00016] The quadratic surface may be a paraboloid.

[00017] The control system may further comprise a wavefront sensor, the wavefront sensor being configured to sense a wavefront of the radiation beam and output a wavefront signal indicative of the wavefront sensed.

[00018] The control system may further comprise a processor, the processor being configured to receive the wavefront signal; to determine a first angular position of the first mirror; to determine a second angular position of the second mirror, to determine, under control of the wavefront sensed, of the first angular position and of the second angular position, a desired adjustment of at least one of the first angular position and the second angular position; and to output an adjustment signal indicative of the desired adjustment.

[00019] The control system may further comprise a controller, a controller, the controller being configured to receive the adjustment signal and control the actuation system in dependence on the adjustment signal.

[00020] According to a second aspect of the invention, there is provided an EUV radiation source configured to produce EUV radiation and comprising: a laser system and a fuel emitter. The fuel emitter is configured to provide a fuel target. The laser system is configured to provide a beam of radiation to be incident on the fuel target so as to convert the fuel target into plasma that produces the EUV radiation, The EUV radiation source further comprises a control system for adjusting a wavefront of the beam of radiation, as discussed supra.

[00021] According to a third aspect of the invention there is provided a lithographic system comprising an EUV radiation source as specified above, and a lithographic apparatus. The lithographic apparatus is configured to receive EUV radiation from the EUV source and to use the EUV radiation for projecting a pattern onto a substrate.

[00022] According to a fourth aspect of the invention there is provided a method of controlling a wavefront of a radiation beam, the method comprising receiving the radiation beam along a first propagation direction, reflecting the radiation beam from a first mirror along a second propagation direction and applying a first change to a shape of the wavefront of the radiation beam, reflecting the reflected radiation beam from a second mirror along a third propagation direction and applying a second change to the shape of the wavefront of radiation, and rotating one of the first mirror and the second mirror about an axis such that each of the first propagation direction, the second propagation direction and the third propagation direction are preserved.

[00023] At least one of the first mirror and second mirror may have a spatial reflective structure that corresponds with a Zernike polynomial, the Zernike polynomial having a radial degree that is greater than or equal to two.

[00024] An angular position of at least one of the first mirror and the second mirror may be adjusted such that the wavefront of the radiation beam includes some induced astigmatism.

[00025] As mentioned above, the fuel emitter of the EUV radiation source provides a fuel target, and the laser system of the EUV radiation source provides a beam of radiation to be incident on the fuel target so as to convert the fuel target into plasma that in turn produces the EUV radiation. The fuel emitter may provide the fuel target in the form of a tiny droplet that is launched on a trajectory to the region where the droplet is to be accurately hit by the laser beam. A magnitude and angular position of the induced astigmatism may be selected in dependence on knowledge of the shape of a droplet of fuel that the radiation beam is incident upon.

[00026] Selecting a magnitude and angular position of the induced astigmatism in dependence on knowledge of the shape of a droplet of fuel that the radiation beam is incident upon, is advantageous because it has been found that matching a beam intensity profile of the laser pulse to the shape of the droplet of fuel may increase a conversion efficiency of the LPP radiation source.

[00027] According to a fifth aspect of the invention there is provided a non-transitory computer readable medium carrying computer readable instructions configured to cause a radiation wavefront control system to perform the method discussed above.

[00028] It will be appreciated that one or more features of an aspect of the invention may be combined with one or more features of other aspects of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[00029] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:

Figure 1 schematically depicts a lithographic system comprising a lithographic apparatus and a radiation source comprising a control system for adjusting a wavefront of a beam of radiation according to an embodiment of the invention; Figure 2 schematically depicts a control system for adjusting a wavefront of a beam of radiation according to an embodiment of the invention;

Figure 3, consisting of Figures 3A-C, schematically depicts front views of a first mirror and a second mirror of the control system in three different relative angular positions according to an embodiment of the invention;

Figure 4, consisting of Figures 4A-C, schematically depicts front views of the first mirror and the second mirror shown in Figure 3 using an alternative illustration technique;

Figure 5, consisting of Figures 5A-C, schematically depicts front views of a first mirror and a second mirror of the control system in three different relative angular positions according to an embodiment of the invention; and,

Figure 6, consisting of Figure 6A-C, schematically depicts front views of the first mirror and the second mirror shown in Figure 5 using an alternative illustration technique.

DETAILED DESCRIPTION

[00030] Figure 1 shows a lithographic system including a control system 20 for adjusting a shape of a wavefront of a beam of radiation according to one embodiment of the invention. The lithographic system comprises a radiation source SO and a lithographic apparatus LA. The radiation source SO is configured to generate an extreme ultraviolet (EUV) radiation beam B. 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. The illumination system IL is configured to condition the radiation beam B before it is incident upon the patterning device MA. The projection system PS is configured to project the radiation beam B (now patterned by the mask MA) onto the substrate W. The substrate W may include previously formed patterns. Where this is the case, the lithographic apparatus aligns the patterned radiation beam B with a pattern previously formed on the substrate W.

[00031] The radiation source SO, illumination system IL, and projection system PS may all be constructed and arranged such that they can be isolated from the external environment. A gas at a pressure below atmospheric pressure (e.g. hydrogen) may be provided in the radiation source SO. A vacuum may be provided in illumination system IL and/or the projection system PS. A small amount of gas (e.g. hydrogen) at a pressure well below atmospheric pressure may be provided in the illumination system IL and/or the projection system PS.

[00032] The radiation source SO shown in Figure 1 is 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 radiation beam 2 into a fuel, such as tin (Sn) which is provided from a fuel emitter 3. Although tin is referred to in the following description, any suitable fuel may be used. The fuel may for example be in liquid form, and may for example be a metal or alloy. The fuel emitter 3 may comprise a nozzle configured to direct the fuel, such as tin, e.g. in the form of droplets, along a trajectory towards a plasma formation region 4. The laser radiation beam 2 is incident upon the tin at the plasma formation region 4. The deposition of laser energy into the tin creates plasma 7 at the plasma formation region 4. Radiation, including EUV radiation, is emitted from the plasma 7 during de-excitation and during recombination of ions and electrons of the plasma.

[00033] The EUV radiation is collected and focused by 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 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 focal point may be at the plasma formation region 4, and a second focal point may be at an intermediate focus 6, as discussed below.

[00034] The laser system 1 may be located at a distance from the radiation source SO. Where this is the case, the laser radiation 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 and the radiation source SO may together be considered to be a radiation source. The laser system 1 may, for example, comprise a seed laser, one or more optical amplifiers and a beam delivery system.

[00035] Radiation that is reflected by the collector 5 forms a radiation beam B. The radiation beam B is focused at point 6 to form an image of the plasma formation region 4, which acts as a virtual radiation source for the illumination system IL. The point 6 at which the radiation beam B is focused may be referred to as the intermediate focus. 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.

[00036] The radiation beam B passes from the radiation source SO into the illumination system IL, which is configured to condition the radiation beam. 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 radiation beam B with a desired cross-sectional shape and a desired angular distribution of the intensity of the radiation beam B. The radiation beam B passes from the illumination system IL and is incident upon the patterning device MA held by the support structure MT. The patterning device MA reflects and patterns the radiation beam B. 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. [00037] Following reflection from the patterning device MA the patterned radiation beam B enters the projection system PS. The projection system comprises a plurality of mirrors 13, 14 which are configured to project the radiation beam B onto a substrate W held by the substrate table WT. The projection system PS may apply a reduction factor to the radiation beam, forming an image with features that are smaller than corresponding features on the patterning device MA. A reduction factor of 4 may for example be applied. Although the projection system PS has two mirrors 13, 14 in Figure 1, the projection system may include any number of mirrors (e.g. six mirrors).

[00038] The radiation source SO shown in Figure 1 may include components which are not illustrated. For example, a spectral filter may be provided in the radiation source. The spectral filter may be substantially transmissive for EUV radiation but substantially blocking for other wavelengths of radiation such as infrared radiation.

[00039] A conversion efficiency of the radiation source SO may depend at least in part upon an intensity distribution of the laser radiation beam 2 at the plasma formation region 4. As explained above, the deposition of laser radiation beam energy into the tin creates a plasma 7 at the plasma formation region 4. The intensity distribution of the laser radiation beam 2 at the plasma formation region 4 may depend at least in part upon a wavefront of the laser radiation beam 2. The wavefront of a radiation beam is a surface across which all points have the same phase. The wavefront propagates through the laser system 1. The wavefront of the laser radiation beam 2 may deviate from a desired wavefront due to the collection of optical aberrations such as, for example, astigmatism when the laser radiation beam 2 travels through the laser system 1 and interacts with optical components of the laser system 1. For example, when the laser radiation beam 2 travels from a seed laser (not shown) to the plasma formation region 4, the laser radiation beam 2 may collect optical aberrations which cause the shape of the wavefront of the laser radiation beam 2 to deviate from a desired shape. For example, optical aberrations may arise from imperfect optical components such as mirrors and/or lenses within the laser system 1 , misaligned optical components within the laser system 1 and/or thermally induced variations in one or more optical properties of optical elements (e.g. reflective surfaces of mirrors). The altered shape of the wavefront may result in an undesired intensity distribution in the cross-section of the laser radiation beam 2 at the plasma formation region 4, which may in turn reduce the amount of EUV radiation generated by the radiation source SO. A reduced conversion efficiency of the radiation source SO may negatively affect a throughput of the lithographic apparatus because less radiative energy is available for performing lithographic exposures of target regions of the substrate W. Optical aberrations collected by the laser radiation beam 2 when travelling through the laser system 1 may vary over time (e.g., owing to drift in the optical properties of the components of the laser system 1) and/or may vary from one laser system to another. [00040] Figure 2 schematically depicts a control system 20 for adjusting a wavefront of a beam of radiation according to an embodiment of the invention. The control system 20 comprises a first mirror 21, a second mirror 23 and an actuation system 22. The first mirror 21 is configured to receive the laser radiation beam 25 along a first propagation direction 24a and reflect the laser radiation beam 25 along a second propagation direction 24b. The first mirror 21 is further configured to apply a first change in the shape of the wavefront of the radiation beam 25. The second mirror 23 is configured to reflect the reflected radiation beam 25 along a third propagation direction 24c. The second mirror 23 is further configured to apply a second change in the shape of the wavefront of the radiation beam 25. The radiation beam 25 has been represented using a line for ease of illustration. In practice, the radiation beam 25 has a finite cross- sectional area that is incident across the first mirror 21 and the second mirror 23 (i.e. the radiation beam 25 reflects from an area of the reflective surface of the first mirror 21 and an area of the second mirror 23 rather than reflecting from single points on the first and second mirrors 21, 23). The radiation beam 25 may, for example, have a power of about 25 kW. The actuation system 22 is configured to rotate the first mirror 21 and the second mirror 23 relative to one another about an axis 30 and an axis 34, respectively. The axis 30, 34 is configured to preserve each of the first propagation direction 24a, the second propagation direction 24b and the third propagation direction 24c. The axis 30, 34 may pass through the reflective surface of the first mirror 21 or the second mirror 23. The actuation system 22 may be further configured to rotate the other one of the first mirror 21 and the second mirror 23 about a second axis 30, 34. The second axis 30, 34 is configured to preserve each of the first propagation direction 24a, the second propagation direction 24b and the third propagation direction 24c.

[00041] Accordingly, the control system is configured to adjust the wavefront of the beam of radiation. The control system has a pair of mirrors defining a part of the propagation path of the beam. Each of the mirrors has a profiled reflective surface configured to cause a change in the beam' s wavefront. The mirrors are positioned in such a manner that rotating the mirrors, relative to one another, enables to adjust the wavefront without affecting the propagation path of the beam.

[00042] The following example scenarios can be implemented. The first mirror 21 is rotated about axis 30 to a new angular position whereas the angular position of the second mirror about the axis 34 is kept fixed. As another example, the first mirror 21 is rotated about the axis 30 to a new angular position and the second mirror 23 is rotated about the axis 34 to another new angular position. As yet another example, the angular position of the first mirror 21 about the axis 30 is kept fixed and the second mirror 23 is rotated about the axis 34 to a new angular position.

[00043] The actuation system 22 may, for example, comprise a stepper motor.

[00044] Additional optical components may be present between the first mirror 21 and the second mirror 23. That is, after reflecting from the first mirror 21, the radiation beam 25 may interact with one or more other optical components (e.g. one or more further mirrors, not shown) before reflecting from the second mirror 23. The radiation beam 25 may be incident on the first mirror 21 and/or the second mirror 23 along a propagation direction 24a-b that is non-parallel with a rotation axis 30, 34 of the first mirror 21 and/or the second mirror 23. Alternatively the radiation beam 25 may be incident on the first mirror 21 and/or the second mirror 23 along a propagation direction that is parallel with a rotation axis 30, 34 of the first mirror 21 and/or the second mirror 23.

[00045] The control system 20 may comprise a wavefront sensor 28. The wavefront sensor 28 may, for example, comprise a Shack-Hartmann wavefront sensor. A beam splitter 29 may be provided in the path of the radiation beam 25 after the radiation beam has reflected from the second mirror 23. The beam splitter 29 may be configured to transmit a majority of the radiation beam 25 for use in the LPP radiation source and reflect a minority of the radiation beam 25 towards the wavefront sensor 28. The wavefront sensor 28 may be configured to sense a wavefront of the radiation beam 25 after the radiation beam has reflected from the second mirror 23. The wavefront sensor may then output a signal indicative of the wavefront of the radiation beam 25 as sensed. The signal output by the wavefront sensor 28 may be provided to a processor 27. The processor 27 is configured to receive a signal indicative of the wavefront of the radiation beam 25 as sensed by the wavefront sensor 28. The processor 27 is also configured to receive a signal indicative of the relative angular positions of the first mirror 21 and the second mirror 23. The processor 27 is configured to determine an adjustment of the angular positions of the first mirror 21 and/or the second mirror 23 so as to apply a desired modification to the wavefront of the radiation beam 25. The processor 27 is configured to provide a signal indicative of the adjustment to the controller 26. The controller 26 may be configured to receive the signal from the processor 27 and control the actuation system 22 so as to apply the determined adjustment to the angular position of the first mirror 21 and/or the second mirror 23. By monitoring the wavefront of the radiation beam 25 and rotating the first mirror 21 and/or the second mirror 23, a desired modification may be applied to the wavefront of the radiation beam 25.

[00046] The laser system in an LPP radiation source may be configured to provide a pre-pulse and a main pulse when generating the plasma. The pre-pulse and the main pulse may have similar wavelengths (e.g. about 10 μηι). Alternatively, the pre-pulse and the main pulse may have substantially different wavelengths (e.g. the pre-pulse may have a wavelength of about 1 μηι and the main pulse may have a wavelength of about 10 μηι). The main pulse may have a higher power than the pre-pulse. The pre-pulse may be configured to condition the fuel droplet for receipt of the main pulse, e.g. by means of changing a shape of the fuel droplet. The main pulse may be configured to convert the conditioned fuel droplet into a plasma after the fuel drop has been struck by the pre-pulse. The control system 20 may be located in a section of the LPP radiation source through which a pre-pulse travels but a main pulse does not. This enables the wavefront of the pre-pulse to be controlled without affecting the wavefront of the main pulse. Alternatively, the control system 20 may be located in a section of the LPP radiation source through which both the pre-pulse and the main pulse travel. This enables the wavefront of both the pre-pulse and the main pulse to be controlled. As another alternative, the LPP radiation source may be provided with two control systems 20. A first control system 20 may be located in a section of the LPP radiation source through which a pre-pulse travels but a main pulse does not. A second control system 20 may be located in another section of the LPP radiation source through which both the pre-pulse and the main pulse travel. As yet another alternative, a first control system may be located in a section of the LPP radiation source through which only the pre-pulse travels, and a second control system may be located in another section of the LPP radiation source through which only the main pulse travels. This enables the wavefronts of the pre-pulse and the main pulse to be controlled independently.

[00047] At least one of the first mirror 21 and second mirror 23 may have a spatial reflective structure that corresponds with a Zernike polynomial. That is, the shape of the spatial reflective structure (i.e. the non-flat topography of the portion of the mirror that reflects the radiation beam) of at least one of the first mirror and the second mirror may be considered to substantially correspond with the shape of a Zernike polynomial. The Zernike polynomial may have a radial degree that is greater than or equal to two. The Zernike polynomial may, for example, be an astigmatic Zernike polynomial. An example of first and second mirrors that have shapes that substantially correspond with the shape of an astigmatic Zernike polynomial is shown in Figure 3 and Figure 4. Alternatively, the spatial reflective structure may correspond with a Zernike polynomial being superimposed on a quadratic surface. The Zernike polynomial may have a radial degree that is greater than or equal to two (e.g. an astigmatic Zernike polynomial). The quadratic surface may, for example, be a paraboloid. An example of a second mirror that has a shape that substantially corresponds with an astigmatic Zernike polynomial being superimposed onto a paraboloid is shown in Figure 5 and Figure 6.

[00048] The first mirror and/or the second mirror may be formed from a bulk reflective material (e.g. copper) that has been provided with a spatial reflective structure (i.e. a non-flat topography) on a reflective surface. For example, a surface of a block of copper may be processed in a computer-controlled polishing machine or milling machine. The computer-controlled polishing machine or milling machine may be a freeform optics manufacturing and measurement tool. A coating may be applied to the spatially reflective structure of the first and/or second mirror in order to improve a reflectivity of the first and/or second mirror.

[00049] As mentioned earlier, a wavefront is, in the context of the invention, a virtual surface representing a set of points in the laser radiation (an electromagnetic wave) that all propagate with the same phase. Different wavefronts represent different sets of points, and the different sets are associated with different phases. By definition, the rays representing the radiation run perpendicular to the wavefronts. Assume that at a particular location in the radiation's propagation path, the path lengths of some rays are increased relative to the path lengths of other rays. Then, the points of the same phase on the rays that experience the increased path length will start lagging behind the points of the same phase on the other rays. Accordingly, the shape of the wavefront will start to change accordingly.

[00050] Figure 3, consisting of Figures 3A-C, schematically depicts front views of the first mirror and the second mirror in three different relative angular positions according to an embodiment of the invention. The rows of shaded circles depicted in Figures 3A-C represent both the individual and net effects on the phases of the rays of the radiation beam, and therefore the individual and net effects on the phase, arising from different relative angular positions of the first mirror and the second mirror The left-hand shaded circle 31 in each of Figures 3A-C represents the change in path length, or phase, resulting from reflection by the first mirror. The central shaded circle 32 in each of Figures 3A-C represents the change in path length (or phase) of the radiation resulting from reflection by the second mirror. The right-hand shaded circle 33 in each row represents the net change in the path length (or phase) resulting from reflection by the first mirror and the second mirror. Dark shading indicates a positive change in phase, with darker shading corresponding to a larger positive change in phase. Light shading indicates a negative change in phase, with lighter shading corresponding to a larger negative change in phase. No change in phase is represented by the shading shown in the right-hand circle 33 of Figure 3A. Different areas of different shading in a circle indicate that different changes in phase are applied to rays incident on different areas.

[00051] The shape of the spatial reflective structure (i.e. the non-flat topography of the portion of the mirror that reflects the radiation beam) of each mirror 31, 32 corresponds with the change in phase applied to the rays of the radiation reflecting from the mirror. The spatial reflective structure is configured to change an optical path length of incident rays of radiation by different amounts depending on the area of the spatially reflective structure that the rays were incident upon. Changing the optical path length of different rays of radiation in the radiation beam by different amounts changes a shape of the wavefront of the radiation beam. The distance between a peak of the spatial reflective structure and a valley of the spatial reflective structure (i.e. the peak-to-valley distance) may be selected in dependence on amplitude magnitude of aberration that is to be corrected and/or on a resolution of the control system. The peak-to-valley distance of the spatial reflective structure may be less than or equal to the wavelength of the radiation beam that is to be controlled using the control system. The peak-to-valley distance of the spatial reflective structure may be equal to about half the wavelength of the radiation beam that is to be controlled using the control system. For example, a magnitude of the aberration that is to be corrected may be of the same order of magnitude as the wavelength of the radiation beam. For example, if the radiation beam has a wavelength of 10 μηι then a peak-to-valley distance the spatial reflective structure may be less than or equal to about 10 μηι, e.g. about 5 μηι. As another example, if the radiation beam has a wavelength of about 1 μηι then a peak-to-valley distance of the spatial reflective structure may be less than or equal to about 1 μηι, e.g. about 0.5 μηι.

[00052] In the example of Figure 3, the shape of the spatial reflective structure (i.e. the non-flat topography of the mirror that reflects the radiation beam) of the first mirror 31 substantially corresponds with the shape of an astigmatic Zernike polynomial and the shape of the spatial reflective structure of the second mirror 32 substantially corresponds with the shape of an astigmatic Zernike polynomial. That is, in the example of Figure 3, the first mirror and the second mirror have identical shapes. In Figure 3A, the angular position of the first mirror 31 differs from the angular position of the second mirror 32 by 90°. The change in phase that is applied to the rays of the radiation beam reflecting from the first mirror 31 is cancelled out by the change in phase that is applied to the rays of the radiation reflecting from the second mirror 32. The net change in the phase of the rays of the radiation beam reflecting from the first and second mirrors 31, 32 is therefore zero, as shown by the shaded circle 33 on the right-hand side of Figure 3 A.

[00053] In Figure 3B, the first mirror 31 is rotated counter-clockwise (when viewed straight-on, as indicated by the arrows in Figure 3) relative to its position in Figure 3A. The second mirror 32 is rotated clockwise relative to its position in Figure 3 A. In the example of Figure 3B, the first mirror 31 and the second mirror 32 are rotated by equal amounts in opposite directions. The change in the angular positions of the mirrors 31, 32 results in a non-zero net change in the phase 33 in the radiation beam reflecting from the first and second mirrors. That is, in the example of Figure 3B a magnitude of the net change in phase 33 is controlled by rotating the first mirror 31 and the second mirror 32 in opposite directions. Rotating only one of the mirrors 31, 32 may cause a change in an angular position as well as a magnitude of the net change in phase 33. The largest magnitude of the net change may be achieved by rotating the first mirror

31 and/or the second mirror 32 such that the first mirror 31 and the second mirror 32 are aligned in the same angular position. That is, when the first mirror 31 and the second mirror 32 are in rotational alignment with each other, the sum of the changes in the phase in the radiation beam reflecting from the first and second mirrors 31, 32 is at its greatest value.

[00054] In Figure 3C, the first mirror 31 is rotated clockwise (when viewed straight-on, as shown by the arrows in Figure 3) relative to its position in Figure 3B. The second mirror 32 is also rotated clockwise relative to its position in Figure 3B. In the example of Figure 3C, the first mirror 31 and the second mirror

32 are rotated clockwise by equal amounts. The net change in phase 33 applied to radiation reflecting from the first and second mirrors 31, 32 has the same magnitude as that shown in Figure 3B. However, the angular position of the net change in phase 33 has been rotated clockwise relative its position in Figure 3B. That is, in Figure 3C an angular position of the net change in phase 33 is controlled by rotating the first mirror 31 and the second mirror 32 by equal amounts in the same direction. [00055] Rotating both mirrors 31, 32 by equal amounts in opposite directions alters the magnitude of the net change 33. Rotating both of the mirrors 31, 32 by equal amounts in the same direction alters the angular position of the net change 33. A change in the relative angular positions of the first mirror 31 and the second mirror 32 enables control of the magnitude and/or the angular position of a net change 33 in phase that is to be applied to a radiation beam reflecting from the first and second mirrors. In the example of Figure 3, the shape of the first mirror 31 substantially corresponds with the shape of an astigmatic Zernike polynomial (i.e. Zj ² or Zf). The shape of the second mirror 32 also substantially corresponds with the shape of an astigmatic Zernike polynomial. Both spatial reflective structures of the mirrors 31, 32 being astigmatic results in a net change 33 in phase which is also astigmatic in form. Different forms of net changes 33 may be achieved by using first and second mirrors 31, 32 that have different shapes. For example, the first and second mirrors may have spatial reflective structures that substantially correspond with the shape of a coma Zernike polynomial (i.e. or

[00056] Figure 4, consisting of Figures 4A-C, schematically depicts front views of the first mirror and the second mirror shown in Figure 3 using an alternative illustration technique. In the example of Figure 4, numbers are used to represent change in phase of the rays of a radiation beam reflecting from the first and second mirrors rather than the shading used in Figure 3. In the example of Figure 4, a positive change in phase is represented using a positive sign, e.g. +1, and a negative change in phase is represented using a negative sign, e.g., -1. It will be appreciated that the first and second mirrors have gradual changes in their surface topography and that the distinct boundaries shown in Figure 4 are merely for ease of illustration. For example, the change in the spatial reflective structure of the first mirror 31 shown in Figure 4 A between a "+1" quadrant and a "-1" quadrant is gradual, i.e. like the gradual change depicted in the first mirror 31 depicted in Figure 3A. No change in phase is represented by the number "0". The above discussion of the spatial reflective structures of the first and second mirrors 31, 32, the changes in relative angular positions of the first and second mirrors 31, 32 and the net change 33 in phase applied in relation to Figures 3A-C is equally applicable to Figures 4A-C.

[00057] Figure 5, consisting of Figures 5A-C, schematically depicts front views of the first mirror and the second mirror in three different relative angular positions according to an embodiment of the invention. As was the case in Figure 3, the rows of shaded circles depicted in Figures 5A-C represent both the individual and net effects on the phase arising from different relative angular positions of the first mirror 41 and the second mirror 42. The shape of the spatial reflective structure (i.e. the non-flat topography of the portion of the mirror that reflects the radiation beam) of each mirror 41 , 42 corresponds with the changes in phase applied to radiation reflecting from the mirror. The left-hand shaded circle 41 in each of Figures 5 A-C represents the change in phase resulting from reflection by the first mirror. The inner left-hand shaded circle 42 in each of Figures 5A-C represents the change in phase resulting from reflection by the second mirror. The inner right-hand shaded circle 43a in each of Figures 5A-C represents the net change in an astigmatic aberration resulting from radiation reflecting from the first mirror 41 and the second mirror 42. The right-hand shaded circle 43b in each row represents the net change in a focus aberration resulting from radiation reflecting from the first mirror 41 and the second mirror 42. Dark shading indicates a negative change in phase, with darker shading corresponding to a larger negative change in phase. Light shading indicates a positive change in phase, with lighter shading corresponding to a larger positive change in phase. No change in phase is represented by the shading shown in the inner right-hand circle 43a of Figure 5A.

[00058] In the example of Figure 5, the shape of the spatial reflective structure of the first mirror 41 substantially corresponds with the shape of an astigmatic Zernike polynomial. The shape of the spatial reflective structure of the second mirror 42 substantially corresponds with an astigmatic Zernike polynomial being superimposed on a paraboloid. The application of an astigmatic Zernike polynomial 41 to a paraboloid may, for example, result in a surface that substantially corresponds with the curved surface of a cylinder. The peak-to-valley distance of mirror having a quadratic reflective surface (e.g. a paraboloid) may be significantly larger than the peak-to-valley distance of a Zernike polynomial superimposed onto the mirror. The peak-to-valley distance of the paraboloid may, for example, be about 60 μηι whereas the peak- to-valley distance of the astigmatic Zernike polynomial may, for example, be about 5 μηι (when the radiation beam has a wavelength of about ΙΟμηι). As another example, if the radiation beam has a wavelength of about 1 μηι then a peak-to-valley distance of the spatial reflective structure may be less than or equal to about 1 μηι, e.g. about 0.5 μηι. Quadratic surfaces (e.g. the surfaces of convergent and/or divergent mirrors) are present in known LPP radiation sources and may be modified to superimpose the shape of an astigmatic Zernike polynomial. Superimposing a convex parabolic surface and an astigmatic Zernike polynomial may, for example, result in a surface 42 that substantially corresponds with the curved surface of a cylinder, such as that shown in Figures 5A-C. In Figure 5A, the angular position of the first mirror 41 differs from the angular position of the second mirror 42 such that the net change 43a in an astigmatic aberration is zero. That is, the change in an astigmatic aberration that is applied to a radiation beam reflecting from the first mirror 41 is negated by the change in an astigmatic aberration that is applied to radiation reflecting from the second mirror 42. The net change 43a, 43b in the phases of the rays of the radiation beam reflecting from the first and second mirrors 41, 42 is therefore the application of a spherical wavefront 43b via the shape of the second mirror 42.

[00059] In the example of Figure 5B, the first mirror 41 is rotated counter-clockwise relative to its position in Figure 5A and the second mirror 42 is rotated clockwise relative to its position in Figure 5A. In the example of Figure 5B, the first mirror 41 and the second mirror 42 are rotated by equal amounts in opposite directions. The change in the relative angular positions of the mirrors 41, 42 results in a non-zero net change 43 a in an induced astigmatic aberration of the radiation beam reflecting from the first and second mirrors 41, 42. That is, in the example of Figure 4B a magnitude of the net change 43a in an induced astigmatic aberration is controlled by rotating the first mirror 41 and the second mirror 42 in opposite directions. Rotating only one of the mirrors 41, 42 may cause a change in both a magnitude and an angular position of the net change 43 a in an induced astigmatic aberration to be controlled. The net change in phase also includes the application of a spherical wavefront 43b due to the shape of the spatial reflective structure of the second mirror 42 substantially corresponding with an astigmatic Zernike polynomial being superimposed on a paraboloid. That is, a desired amount of astigmatism has been introduced to the wavefront whilst maintaining the application of a spherical wavefront to the radiation beam reflecting from the first and second mirrors 41 , 42.

[00060] In the example of Figure 5C, the first mirror 41 is rotated clockwise relative to its position in Figure 5B and the second mirror 42 is rotated clockwise relative to its position in Figure 5B. The first mirror 41 and the second mirror 42 are rotated clockwise by equal amounts. The net change 43a in an induced astigmatic aberration of the radiation reflecting from the first and second mirrors 41, 42 has the same magnitude as that shown in Figure 5B. However, the angular position of the net change 43a in an induced astigmatic aberration has been rotated clockwise relative its position in Figure 5B. That is, in Figure 5C an angular position of the net change in an induced astigmatic aberration is controlled by rotating the first mirror 41 and the second mirror 42 by equal amounts in the same direction. The net change in phase also includes the application of a spherical wavefront 43b due to the shape of the spatial reflective structure of the second mirror 42 substantially corresponding with an astigmatic Zernike polynomial being superimposed onto a paraboloid. That is, a controlled magnitude and angular position of astigmatism has been introduced whilst maintaining the application of a spherical wavefront to the radiation beam reflecting from the first and second mirrors 41, 42.

[00061] Figure 6, consisting of Figure 6A-C, schematically depicts front views of the first mirror and the second mirror shown in Figure 5 using an alternative illustration technique. As was the case in Figure 4, numbers are used to represent changes in the phases of the rays in the radiation beam reflecting from the first and second mirrors 41, 42 rather than the shading used in Figure 5. In the example of Figure 6, a positive change in phase is represented using a positive sign, e.g., "+1", and a negative change in phase is represented using a negative sign, e.g., "-1". It will be appreciated that the spatial reflective structures of the first and second mirrors 41, 42 have gradual changes in their surface topography and that the distinct boundaries shown in Figure 6 are merely for ease of illustration. For example, the change in shape of the first mirror 41 shown in Figure 6A between a "+1" quadrant and a "-1" quadrant is gradual, i.e. the gradual change in topography of the first mirror 41 depicted in Figure 5 A. No change in phase is represented by the number "0". The above discussion of the shapes of the first and second mirrors 41, 42, changes in relative angular positions of the first and second mirrors 41, 42, the net change in an induced astigmatic aberration and the application of a spherical wavefront in relation to Figures 5A-C, is equally applicable to Figures 6A-C.

[00062] It will be appreciated that the spatial reflective structure of the first mirror and/or the spatial reflective structure of the second mirror may substantially correspond with the shape of any Zernike polynomial having a radial degree that is greater than or equal to two. For example, the shape of the spatial reflective structure of the first mirror and/or the second mirror may substantially correspond with a Zernike polynomial that represents a coma aberration, a Zernike polynomial that represents a trefoil aberration, etc.

[00063] The control system may be used to apply any desired change in phase of radiation. For example, the radiation wavefront control system may be used to control the wavefront of the radiation beam such that the radiation beam reaches the plasma formation location with a substantially flat wavefront (i.e. the surface consisting of points of equal phase is substantially planar). Alternatively, the control system may be used to control the wavefront of the radiation beam such that the radiation beam reaches the plasma formation location with a desired magnitude and/or angular position of an optical aberration (e.g. astigmatism). Providing the radiation beam with a desired amount of astigmatism may advantageously improve a conversion efficiency of the LPP radiation source. This is because a cross-section of a droplet of fuel as "seen" by one of the laser pulses is not circular but instead is generally elliptical, and better matching the beam intensity profile of the laser pulse to the shape of the droplet of fuel may increase a conversion efficiency of the LPP radiation source.

[00064] The radiation beam may suffer from unwanted aberrations when reflecting from the first and second mirrors. For example, when the radiation beam is incident upon the first mirror and/or the second mirror along a propagation direction that is non-parallel with the rotation axis of the first mirror and/or the second mirror, the radiation beam may suffer from higher order aberrations such as, for example, coma and/or lower order aberrations such as, for example tilt. However, simulations performed using the first and second mirrors shown in Figures 3 and 5 have determined that these unwanted aberrations have negligible amplitude. Misalignment between the first and second mirrors may introduce unwanted aberrations, e.g. tilt.

[00065] The first mirror and/or the second mirror may not need to be added to known LPP radiation sources. One or more flat mirrors that are already present in known LPP radiation sources may be modified such that the mirrors comprise a non-flat reflective surface and such that each mirror is rotatable via an actuation system. Alternatively and/or additionally, one or more mirrors having non-flat reflective surfaces that are already present in known LPP radiation sources may be given the ability to rotate (e.g. by installing an actuation system) in order to enable the mirrors to form part of the control system described herein. Existing mirrors having non-flat reflective surfaces (e.g. mirrors having reflective surfaces that are ellipsoidal, cylindrical, parabolic, etc.) in known LPP radiation sources may have their topographies and/or conic constants modified to apply a different change to shape of the wavefront of the radiation beam. For example, a convex paraboloid mirror may be modified to also include astigmatism, which may result in a mirror having a shape that substantially corresponds with an astigmatic Zernike polynomial being superimposed onto a paraboloid (e.g. the inner left-hand shaded circles 42 in Figure 5). The actuation system may be configured to rotate the first mirror about an axis that passes through the reflective surface of the first mirror. The actuation system may be configured to rotate the second mirror about an axis that passes through the reflective surface of the second mirror. For example, the actuation system may be configured to rotate the first and second mirrors about a central surface normal of the reflective surfaces of the first and second mirrors.

[00066] The term "EUV radiation" may be considered to encompass electromagnetic radiation having a wavelength within the range of 4-20 nm, for example within the range of 13-14 nm. EUV radiation may have a wavelength of less than 10 nm, for example within the range of 4-10 nm such as 6.7 nm or 6.8 nm.

[00067] 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). The invention may be used to control a wavefront of a radiation beam used in such apparatus. These apparatus may be generally referred to as lithographic tools. Such a lithographic tool may use vacuum conditions or ambient (non-vacuum) conditions.

[00068] 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 invention described herein may have other applications. Possible other applications include controlling the wavefront of a radiation beam that is to be used in an inspection tool for inspecting a substrate patterned by a lithographic apparatus.

[00069] 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 disk 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.

[00070] 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.