Radiation system

Abstract

A radiation system comprising a deformable minor having a reflective surface arranged to receive and reflect a laser beam to form a reflected laser beam, a beam splitter arranged to separate the reflected laser beam into a first portion and a second portion, a sensor, a focusing system configured to form an image of the reflective surface at the sensor using the first portion of the reflected laser beam, the sensor being configured to output beam data representative of the reflected laser beam, means for generating radiation using the second portion of the reflected laser beam, and a controller operable to cause the deformable mirror to deform the reflective surface based on the beam data.

Description

RADIATION SYSTEMCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of EP application 23198294.3 which was filed on 19 September 2023 and which is incorporated herein in its entirety by reference.FIELD

[0002] The present invention relates to a radiation system. In particular, but not exclusively, it relates to a radiation system for use in 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 at a patterning device (e.g., a mask) onto a layer of radiation-sensitive material (resist) provided on a substrate.

[0004] To project a pattern on a substrate a lithographic apparatus may use electromagnetic radiation. The wavelength of this radiation determines the minimum size of features which can be formed on the substrate. A lithographic apparatus, which uses extreme ultraviolet (EUV) radiation, having a wavelength within the range 4-20 nm, for example 6.7 nm or 13.5 nm, may be used to form smaller features on a substrate than a lithographic apparatus which uses, for example, radiation with a wavelength of 193 nm.

[0005] EUV radiation for a lithographic apparatus may be produced by a laser produced plasma (LPP) radiation source. Within an LPP radiation source a laser beam is used to irradiate fuel droplets so as to produce a plasma. The plasma subsequently emits EUV radiation.

[0006] It may be beneficial to control the shape of the laser beam (for example, its irradiance distribution and phase) prior to irradiating the fuel droplets. Such control can be referred to as beam conditioning. By conditioning the beam, the generation of EUV radiation may be improved.SUMMARY

[0007] According to a first aspect of the invention there is provided a radiation system comprising a deformable mirror having a reflective surface arranged to receive and reflect a laser beam to form a reflected laser beam, a beam splitter arranged to separate the reflected laser beam into a first portion and a second portion, a sensor, a focusing system configured to form an image of the reflective surface at the sensor using the first portion of the reflected laser beam, the sensor being configured to output beam data representative of the reflected laser beam, means for generating radiation using the second portion of the reflected laser beam, and a controller operable to cause the deformable mirror to deform the reflective surface based on the beam data.

[0008] Deformation of the deformable mirror can be used to alter the properties of the reflected laser beam. This may be referred to as beam conditioning. The beam can be conditioned to optimize its properties, for example such that they are optimized for the generation of radiation. By using a focusing system to form an image of the deformable mirror at the sensor, the deformable mirror and sensor are conjugated. By positioning the deformable mirror and sensor in conjugate planes, more information can be determined, by the sensor, about the reflected laser beam. In another radiation system wherein the deformable mirror and sensor are in non-conjugated planes, such information cannot be determined without additional components, for example an interferometer.

[0009] Due to the formation of an image of the deformable mirror at the sensor, the beam data can be collected relatively quickly compared to systems with non-conjugate arrangements. As such, a beam conditioning process using this beam conditioning system provides a relatively fast means for conditioning the beam. As a result, beam conditioning can be done in an online process, for example when the second portion of the reflected laser beam is used for the generation of radiation. The first portion of the reflected laser beam may be significantly lower power than the second portion of the reflected laser beam. By splitting a small fraction of the power of the reflected laser beam for measurement at the sensor, information regarding the reflected laser beam may be monitored while a significant proportion of the power of the reflected laser beam is used for the process of generating radiation. That is, the reflected laser beam can be monitored and conditioned in an online process while the second portion thereof is being used. It should be understood that properties of the first portion of the reflected laser beam will be related to or identical to the corresponding properties of the entire reflected laser beam (i.e. the reflected laser beam prior to being separated into portions). Therefore the sensor may be configured to output beam data representative of the reflected laser beam and / or the first portion of the reflected laser beam. The power of the first portion of the reflected laser beam may be lower than the power of the entire reflected laser beam but may have, for example, substantially the same beam profile as the entire reflected laser beam. These properties are illustrative and used for explanatory purposes, but it will be understood that the reflected laser beam and the first and second portions thereof will have any number of different properties and the similarities and differences between the first and second portions and between the first portion and the entire reflected laser beam may be selected based on, for example, the type of beam splitter used.

[0010] The deformable mirror may comprise one or more adjustable reflective portions. The controller being operable to cause the deformable mirror to deform the reflective surface may comprise the controller being operable to adjust at least one of said one or more adjustable reflective portions. The reflective portions can be controlled to deform the reflective surface into any of a range of shapes. The one or more adjustable reflective portions may be, for example, rotatable or movable in any other way.

[0011] The controller being configured to adjust the at least one of the one or more adjustable reflective portions may comprise the controller being configured to receive the beam data, determine, based on the beam data, an adjustment to at least one of the one or more adjustable reflective portionsand provide an instruction to the at least one of the one or more adjustable reflective portions to perform the adjustment. The adjustment may be, for example, a translational or rotational movement of the one or more adjustable reflective portions. The adjustment may be referred to as adjusting, moving, steering, or actuating the adjustable reflective portions.

[0012] The adjustment may be configured to optimise the reflected laser beam for the generation of radiation. The adjustment may cause a change in the reflected laser beam which improves the generation of radiation using the second portion of the reflected laser beam. The adjustment may be configured to optimise the reflected laser beam for the generation of EUV radiation, for example in a laser produced plasma radiation system. Optimizing a beam may be referred to as beam conditioning. Conditioning may comprise adjusting one or more of the: beam profile (also referred to as intensity distribution or spatial intensity profile), beam position (e.g. with respect to the reflective surface), beam centre, average intensity, peak intensity, stability. Conditioning may comprise adjusting the phase profile of the beam. Conditioning may comprise compensating for one or more aberrations in the beam, for example reducing the magnitude of one or more aberrations. The controller may be configured to determine, based on the beam data, one or more properties of the reflected laser beam and the adjustment may be determined at least partially based on the one or more properties.

[0013] The radiation system may further comprise a positioning system for receiving the laser beam from a laser system and directing the laser beam to the reflective surface of the deformable mirror, the positioning system configured to control the position of the laser beam on the reflective surface.

[0014] A controller may be used to control the positioning system or a portion thereof. The controller for controlling the deformable mirror may be used to control the positioning system, or another controller may be used. Beneficially, by controlling the position of the laser beam on the reflective surface, the beam conditioning may be optimized, because for example the conditioning of the beam using the deformable mirror may be more accurately performed when the position of the laser beam is accurately known and / or controlled. Furthermore, by controlling the position of the laser beam on the reflective surface, it can be ensured that the laser beam remains on the reflective surface and at a desired position on the reflective surface.

[0015] The laser beam may be positioned such that not all of the laser beam illuminates the reflective surface (e.g. due to misalignment some of the laser beam may travel past the reflective surface, for example past an outer perimeter of the reflective surface). If the laser beam moves such that it does not illuminate the reflective surface, or interacts with an undesired portion of the reflective surface, this may lead to stray light within the radiation system (i.e. some or all of the laser beam travelling to undesired areas of the radiation system). Stray light is dangerous both to users and to equipment and so it is advantageous to avoid stray light. Stray light can be avoided using a controllable positioning system for receiving the laser beam from a laser system, directing the laser beam to the reflective surface of the deformable mirror and controlling the position of the laser beam on the reflective surface.

[0016] The position of the laser beam on (or with respect to) the reflective surface may be described in terms of a position of a centre of the laser beam or an outer perimeter of the laser beam (wherein the outer perimeter of a laser beam can be described as commonly known by a full-width half-max (FWHM), 1 / e, 1 / e2boundary or similar). The position of the laser beam on the reflective surface may be considered equivalent to the position of the reflected laser beam on the reflective surface, for example because the laser beam and reflected laser beam are equivalent at the point of reflection.A positioning system is particularly useful when used in combination with a deformable mirror because deforming the deformable mirror increases the chance of stray light occurring, for example if a deformation causes the laser beam to be reflected to an undesired area of the radiation system. By providing control of the laser beam on the reflective surface, such stray light following deformation of the deformable mirror can be reduced or avoided.

[0017] The positioning system may receive the laser beam directly or indirectly from the laser system. That is, there may be intervening components (e.g. an amplifier, directing or focusing elements, other beam conditioning apparatus) between the laser system and the positioning system.

[0018] The positioning system may comprise a tilt / tip mirror. A tilt / tip mirror may also be referred to as a steering mirror. A tilt / tip mirror is typically rotatable around two axes such that received radiation may be steered within a plane. The tilt / tip mirror may be actuated by any means, for example piezoelectric means or by a voice coil motor. The positioning mirror may be movable with more degrees of freedom, for example the positioning mirror may be implemented with a hexapod positioner.

[0019] The controller may be further configured to determine, based on the beam data, a current position of the laser beam on the reflective surface of the deformable mirror, determine, based on the current position of the laser beam, a position adjustment for altering the position of the laser beam on the reflective surface of the deformable mirror using the positioning mirror, and provide an instruction to the positioning mirror to perform the position adjustment.

[0020] The determination of the beam data and the position adjustment can be done periodically or continually, for example in a feedback loop, so as to control the position over an extended period of time. This process can be done in an online process, for example during the generation of radiation using the second portion of the reflected laser beam. Such an online position adjustment process is particularly well enabled in combination with the conjugate arrangement of the reflective surface of the deformable mirror and the sensor, because beam data (from which the current position can be determined) is obtained relatively quickly compared to non-conjugate systems. For example, the process may take less than five minutes, for example one or two minutes. As such, the determination and performance of a position adjustment can be performed quickly enough to be used in an online process. This is in contrast to previously used systems, where the process may take thirty minutes or longer, or not be possible.

[0021] The reflective surface may comprise a flexible layer. The one or more adjustable reflective portions may each comprise an actuator in contact with a portion of the flexible layer. The actuatorsmay be in direct or indirect contact with their corresponding portion of the flexible layer. The actuators may therefore exert a force on their corresponding portion of the flexible layer. The flexible layer may be resiliently flexible such that when the force applied by an actuator is removed, the corresponding portion of the flexible layer returns to an original position.

[0022] The actuators may comprise electro-mechanical actuators, piezo-electric actuators and / or electromagnetic actuators. Any other actuation means may additionally or alternatively be used.

[0023] The means for generating radiation may comprise a droplet generator operable to deliver fuel droplets to a target location and means for delivering the second portion of the reflected laser beam to the target location. That is, the radiation system may be an LPP radiation system. The generated radiation may be EUV radiation. The fuel may comprise tin. An LPP radiation system as described herein provides a more stable and controllable laser beam for the generation of radiation, resulting in more effective and efficient radiation generation.

[0024] The radiation system may further comprise an additional focusing system for focusing the second portion of the reflected laser beam at the target location. The deformable mirror may be positioned proximal to the additional focusing system. Proximal is understood to mean along the optical axis of the reflected laser beam, and may not be in a straight line. By positioning the deformable mirror close to the additional focusing system, the risk of stray light is reduced due to the reduction in travel distance between the deformable mirror and the means for generating radiation.

[0025] In this implementation, it may be preferable to provide as many components as possible prior to the deformable mirror. In this way, the deformable mirror is positioned close to the additional focusing system such that the second portion of the laser beam does not travel through a large distance of free space prior to being used. This is advantageous as a reduced free space travel distance reduces the risk of stray light causing damage to people or components. For example, with a large free space travel distance, a small deviation from a desired laser optical path can result in a significant deviation from the optical path at the end of the free space travel distance, because divergence increases with distance. On the other hand, with a small free space travel distance, the same deviation has less length to diverge over, so can be more easily controlled and contained.

[0026] The radiation system may further comprise a laser system configured to generate the laser beam. The laser system may comprise one or more lasers and / or generate one or more laser beams. The laser radiation generated may be, for example, 1 pm and / or 10 1 pm. It should be understood that a laser system generates laser radiation, but that the radiation system also comprises means for generating radiation using said laser radiation.

[0027] The radiation system may further comprise a beam processing apparatus for processing the laser beam. The beam processing apparatus may further condition the beam, for example for the generation of radiation. The beam processing apparatus may be positioned between (where between means along the optical path, not necessarily in a straight line) the deformable mirror and the laser.

[0028] The beam processing apparatus may comprise one or more of a compressor, an amplifier, a beam shaping system. Where the beam conditioning system receives more than one laser beam, the amplifier may increase the power of one or more of said laser beams. Alternatively, each laser beam may be amplified by a separate amplifier. In another arrangement, a laser beam may be provided to multiple amplifiers for increased power amplification. Similarly, a compressor may compress more than one laser beam or multiple compressors may be provided. Similarly, a beam shaping system may shape more than one laser beam or multiple beam shaping systems may be used. The beam shaping system is considered distinct from deformable mirror, that is it provides different shaping to the shaping provided by the deformable mirror.

[0029] The beam processing apparatus may be positioned between the laser system and the deformable mirror. Between is understood to mean between the laser system and the deformable mirror along the optical axis, which is not necessarily in a straight line direction. Beneficially this reduces the travel distance between the deformable mirror and the means for generating radiation, therefore reducing the risk of stray light.

[0030] The reflective surface of the deformable mirror may be further arranged to receive and reflect a second laser beam. The focusing system may be configured to form an image of the reflective surface at the sensor using a first portion of the reflected second laser beam. The means for generating radiation may further use a second portion of the reflected second laser beam to generate radiation. The beam data may be further representative of the reflected second laser beam. That is, the radiation system and the components therein may be configured to generate and / or condition more than one laser beam.

[0031] The second laser beam may have the same or different properties, for example wavelength, compared to the first laser beam. The radiation system may be further configured to generate and / or condition a third laser beam. That is, the reflective surface of the deformable mirror may be further arranged to receive and reflect multiple second laser beams, the focusing system may be configured to form images of the reflective surface at the sensor using a first portion of each of multiple reflected second laser beams, the means for generating radiation further uses a second portion of multiple reflected second laser beams to generate radiation and the beam data is further representative of multiple reflected second laser beams. The multiple laser beams may comprise, for example, a first, second and third pulse for use in shaping, liquefying and / or converting a fuel droplet into a plasma in an LPP source.

[0032] According to a second aspect there is provided a lithographic apparatus comprising the radiation system of the first aspect.

[0033] According to a third aspect there is provided a method a method of conditioning a laser beam, comprising illuminating a reflective surface of a deformable mirror with a laser beam to form a reflected laser beam, the deformable mirror being operable to deform the reflective surface of the deformable mirror, forming an image of the reflective surface at the sensor using a first portion of the reflected laser beam, obtaining, by a sensor, beam data indicating one or more properties of a reflected laser beam reflected from a reflective surface of a deformable mirror, determining, based on the beam data, anadjustment to the deformable mirror, the adjustment configured to condition the reflected laser beam, and providing an instruction, to the deformable mirror, to perform the adjustment. The method of the third aspect can be performed using the radiation system of the first aspect or the lithographic apparatus of the second aspect.

[0034] Deformation of the deformable mirror can be used to alter the properties of the reflected laser beam. This may be referred to as beam conditioning. The beam can be conditioned to optimize its properties, for example such that they are optimized for the generation of radiation.

[0035] The method may comprise separating, for example by a beam splitter, a first portion of the reflected laser beam from the reflected laser beam. The first portion may be used for forming an image of the reflective surface at the sensor. The reflected laser beam may be separated into the first portion and a second portion.

[0036] Determining, based on the beam data, an adjustment to the deformable mirror may comprise determining, based on the beam data, one or more properties of the reflected laser beam, and determining, based on based on the one or more determined properties of the reflected laser beam and a corresponding one or more desired properties of the reflected laser beam, the adjustment to the deformable mirror. The adjustment may be configured to alter the actual properties of the reflected laser beam, represented by the one or more determined properties of the reflected laser beam, such that they are closer to or equal to the desired properties of the reflected laser beam. The adjustment may be, for example, the condition the beam. Bringing the actual properties of the reflected laser beam closer or equal to the desired properties may be considered conditioning or optimizing the beam.

[0037] The one or more properties may comprise a beam profile of the reflected laser beam, a stability of the reflected laser beam and / or a measure of one or more aberrations in the reflected laser beam. The one or more properties may comprise a current position of the reflected laser beam with respect to the reflective surface.

[0038] The method may further comprise controlling a position of the laser beam on the reflective surface of the deformable mirror based on the current position. Beneficially, by controlling the position of the laser beam on the reflective surface, the beam conditioning may be optimized, because for example the conditioning of the beam using the deformable mirror may be more accurately performed when the position of the laser beam is accurately known and / or controlled. Furthermore, by controlling the position of the laser beam on the reflective surface, it can be ensured that the laser beam remains on the reflective surface and at a desired position on the reflective surface. The laser beam may be positioned such that not all of the laser beam illuminates the reflective surface (e.g. due to misalignment some of the laser beam may travel past the reflective surface, for example past an outer perimeter of the reflective surface). If the laser beam moves such that it does not illuminate the reflective surface, or interacts with an undesired portion of the reflective surface, this may lead to stray light within the radiation system (i.e. some or all of the laser beam travelling to undesired areas of the radiation system). Stray light is dangerous both to users and to equipment and so it is advantageous to avoid stray light.Stray light can be avoided using a controllable positioning system for receiving the laser beam from a laser system, directing the laser beam to the reflective surface of the deformable mirror and controlling the position of the laser beam on the reflective surface.

[0039] The position of the laser beam on (or with respect to) the reflective surface may be described in terms of a position of a centre of the laser beam or an outer perimeter of the laser beam (wherein the outer perimeter of a laser beam can be described as commonly known by a full-width half-max (FWHM), 1 / e, 1 / e2boundary or similar). The position of the laser beam on the reflective surface may be considered equivalent to the position of the reflected laser beam on the reflective surface, for example because the laser beam and reflected laser beam are equivalent at the point of reflection. Providing positioning control is particularly beneficial when conditioning the beam with a deformable mirror because deforming the deformable mirror increases the chance of stray light occurring, for example if a deformation causes the laser beam to be reflected to an undesired area of the radiation system. By providing control of the laser beam on the reflective surface, such stray light following deformation of the deformable mirror can be reduced or avoided.

[0040] Controlling the position of the reflected laser beam may comprise determining, based on the current position, an adjustment to a positioning system for altering the position of the laser beam on the reflective surface, and providing an instruction to the positioning system to perform the adjustment.

[0041] Illuminating a reflective surface of a deformable mirror with a laser beam to form a reflected laser beam may comprise illuminating the reflective surface with a laser beam at a first time, wherein at the first time the reflective surface is in a first configuration, deforming the reflective surface from the first configuration to a second configuration, illuminating the reflective surface with the laser beam at a second time wherein at the second time the reflective surface is in the second configuration.

[0042] Obtaining, by the sensor, beam data may comprise obtaining first beam data at the first time and obtaining second beam data at the second time.

[0043] Determining the current position of the reflected laser beam may be based on the first beam data and second beam data.

[0044] As such, the determination of the current position (and any subsequent adjustment) can be performed multiple times. It can be done periodically or continually, for example in a feedback loop, so as to control the position over an extended period of time. This process can be done in an online process, for example during the generation of radiation using the second portion of the reflected laser beam. Such an online position adjustment process is particularly well enabled in combination with the conjugate arrangement of the reflective surface of the deformable mirror and the sensor, because beam data (from which the current position can be determined) is obtained relatively quickly compared to non-conjugate systems. For example, the process may take less than five minutes, for example one or two minutes. As such, the determination and performance of a position adjustment can be performed quickly enough to be used in an online process. This is in contrast to previously used systems, where the process may take thirty minutes or longer, or not be possible.

[0045] The first beam data may represent a measure of an intensity profile and / or a phase profile of the reflected laser beam following reflection from the reflective surface in the first configuration. The second beam data may represent a measure of an intensity profile and / or a phase profile of the of the reflected laser beam following reflection from the reflective surface in the second configuration.

[0046] Determining the current position of the laser beam on the reflective surface of the deformable mirror may comprise determining a difference between a centroid of the intensity profile and / or phase profile of the first beam data and a centroid of the intensity profile and / or phase profile of the second beam data.

[0047] According to a fourth aspect there is provided a method of determining one or more properties of a laser beam for use in generating radiation comprising illuminating a reflective surface of a deformable mirror with a laser beam to form a reflected laser beam, the deformable mirror being operable to deform the reflective surface of the deformable mirror, forming an image of the reflective surface at a sensor using at a first portion of the reflected laser beam, directing a second portion of the reflected laser beam to a target location for generating radiation, obtaining, by the sensor, beam data indicating one or more properties of the reflected laser beam and determining, based on the beam data, one or more properties of the reflected laser beam.

[0048] The method of the fourth aspect can be performed using the radiation system of the first aspect or the lithographic apparatus of the second aspect. Any of the method steps of the fourth aspect may be combined with any of the method steps of the third aspect.

[0049] Deformation of the deformable mirror can be used to alter the properties of the reflected laser beam. This may be referred to as beam conditioning. The beam can be conditioned to optimize its properties, for example such that they are optimized for the generation of radiation. This method of determining one or more properties of the laser beam enables improved optimization of said properties.

[0050] The method may comprise separating, for example by a beam splitter, the reflected laser beam into a first portion and a second portion. The first portion may be used for forming an image of the reflective surface at the sensor. The second portion may be directed to the target location for generating radiation.

[0051] The one or more properties may comprise a current position of the reflected laser beam with respect to the reflective surface. The method may further comprise controlling a position of the laser beam on the reflective surface of the deformable mirror based on the current position. Controlling the position of the reflected laser beam may comprise determining, based on the current position, an adjustment to a positioning system for altering the position of the laser beam on the reflective surface and providing an instruction to the positioning system to perform the adjustment.

[0052] Illuminating a reflective surface of a deformable mirror with a laser beam to form a reflected laser beam may comprise illuminating the reflective surface with a laser beam at a first time, wherein at the first time the reflective surface is in a first configuration, deforming the reflective surface from the first configuration to a second configuration and illuminating the reflective surface with the laserbeam at a second time, wherein at the second time the reflective surface is in the second configuration. Obtaining, by the sensor, beam data may comprise obtaining first beam data at the first time and obtaining second beam data at the second time. Determining the current position of the reflected laser beam may be based on the first beam data and second beam data.

[0053] The first beam data may represent a measure of an intensity profile and / or a phase profile of the reflected laser beam following reflection from the reflective surface in the first configuration. The second beam data may represent a measure of an intensity profile and / or a phase profile of the of the reflected laser beam following reflection from the reflective surface in the second configuration.

[0054] Determining the current position of the laser beam on the reflective surface of the deformable mirror may comprise determining a difference between a centroid of the intensity profile and / or phase profile of the first beam data and a centroid of the intensity profile and / or phase profile of the second beam data.

[0055] The method may further comprise determining, based on the one or more determined properties of the reflected laser beam and a corresponding one or more desired properties of the reflected laser beam, an adjustment to the deformable mirror, the adjustment configured to condition the reflected laser beam and providing the instruction, to the one or more of the adjustable reflective portions, to perform the adjustment.

[0056] The one or more properties may comprise a beam profile of the reflected laser beam, a stability of the reflected laser beam and / or a measure of one or more aberrations in the reflected laser beam.

[0057] The method may further comprise directing a second portion of the reflected laser beam to a target location for generating radiation.

[0058] The method may further comprise providing a fuel droplet at the target location for illumination by the second portion of the reflected laser beam. That is, the method may further comprise generating radiation using LPP. The generated radiation may comprise EUV radiation. The generation of EUV radiation using the methods described herein provides more effective and efficient EUV radiation generation.

[0059] According to a fifth aspect there is provided a computer program comprising instructions to cause the radiation system of the first aspect or the lithographic apparatus of the second aspect to execute the method steps of the third or fourth aspects.

[0060] According to a sixth aspect there is provided a computer readable storage medium having stored thereon the computer program of the fifth aspect.

[0061] Features of different aspects of the invention may be combined together.BRIEF DESCRIPTION OF THE DRAWINGS

[0062] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:Figure 1 depicts a lithographic system comprising a lithographic apparatus and a radiation source;Figure 2 depicts the beam delivery system and laser system of Figure 1;Figures 3A and 3B schematically depict the deformable mirror of Figure 2 in cross-section; Figure 4 schematically illustrates a plan view of the deformable mirror of Figures 3 A and 3B; Figure 5 shows a method of conditioning the laser beam; andFigure 6 shows a method for controlling the position of the laser beam on the reflective surface.DETAILED DESCRIPTION

[0063] Figure 1 shows a lithographic system comprising a radiation source SO and a lithographic apparatus LA. The radiation source SO is configured to generate an EUV radiation beam B and to supply the EUV radiation beam B to the lithographic apparatus LA. The lithographic apparatus LA comprises an illumination system IL, a support structure MT configured to support a patterning device MA (e.g., a mask), a projection system PS and a substrate table WT configured to support a substrate W.

[0064] The illumination system IL is configured to condition the EUV radiation beam B before the EUV radiation beam B is incident upon the patterning device MA. Thereto, the illumination system IL may include a facetted field mirror device 10 and a facetted pupil mirror device 11. The faceted field mirror device 10 and faceted pupil mirror device 11 together provide the EUV radiation beam B with a desired cross-sectional shape and a desired intensity distribution. The illumination system IL may include other mirrors or devices in addition to, or instead of, the faceted field mirror device 10 and faceted pupil mirror device 11.

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

[0066] The substrate W may include previously formed patterns. Where this is the case, the lithographic apparatus LA aligns the image, formed by the patterned EUV radiation beam B’, with a pattern previously formed on the substrate W.

[0067] A relative vacuum, i.e. a small amount of gas (e.g. hydrogen) at a pressure well below atmospheric pressure, may be provided in the radiation source SO, in the illumination system IL, and / or in the projection system PS.

[0068] The radiation source SO shown in Figure 1 is, for example, of a type which may be referred to as a laser produced plasma (LPP) source. A laser system 1, which may, for example, include one or more lasers, is arranged to deposit energy via a laser beam 2 into a fuel, such as tin (Sn) which is provided from, e.g., a fuel emitter 3. Although tin is referred to in the following description, any suitable fuel may be used. The fuel may, for example, be in liquid form, and may, for example, be a metal or alloy. The fuel emitter 3 may comprise a nozzle configured to direct tin, e.g. in the form of droplets, along a trajectory towards a plasma formation region 4. The laser beam 2 is incident upon the tin at the plasma formation region 4. The deposition of laser energy into the tin creates a tin plasma 7 at the plasma formation region 4. Radiation, including EUV radiation, is emitted from the plasma 7 during de-excitation and recombination of electrons with ions of the plasma. The plasma formation region may be referred to as a target region.

[0069] The EUV radiation from the plasma is collected and focused by a collector 5. Collector 5 comprises, for example, a near-normal incidence radiation collector 5 (sometimes referred to more generally as a normal-incidence radiation collector). The collector 5 may have a multilayer mirror structure which is arranged to reflect EUV radiation (e.g., EUV radiation having a desired wavelength such as 13.5 nm). The collector 5 may have an ellipsoidal configuration, having two focal points. A first one of the focal points may be at the plasma formation region 4, and a second one of the focal points may be at an intermediate focus 6, as discussed below.

[0070] Radiation that is reflected by the collector 5 forms the EUV radiation beam B. The EUV radiation beam B is focused at intermediate focus 6 to form an image at the intermediate focus 6 of the plasma present at the plasma formation region 4. The image at the intermediate focus 6 acts as a virtual radiation source for the illumination system IL. The radiation source SO is arranged such that the intermediate focus 6 is located at or near to an opening 8 in an enclosing structure 9 of the radiation source SO.

[0071] The laser beam 2 generated by the laser system 1 is directed to a beam delivery system 15 before being directed to the plasma formation region 4. The beam delivery system 15 directs the laser beam 2 from the laser system 1 to the plasma formation region 4. The beam delivery system 15 is also configured to condition the laser beam 2. The beam delivery system 15 therefore comprises one or more suitable directing mirrors and one or more suitable beam conditioning components, for example a beam shaper, a beam expander, a focusing element.

[0072] The laser system 1 and beam delivery system 15 in the arrangement illustrated in Figure 1 are positioned outside of the enclosing structure 9. In another arrangement, the laser system 1 and / or the beam delivery system 15 are located within the enclosing structure 9 of the radiation source SO. The laser system 1, the radiation source SO and the beam delivery system may together be considered to bea radiation system. The radiation system may comprise one or more amplifiers for increasing the power of one or more of the laser beams generated therein.

[0073] The laser system 1 may comprise one or more lasers, each configured to provide one or more laser beams. The laser(s) of the laser system 1 may include, for example, a CO2 laser. Additionally or alternatively, one or more 1 pm lasers may be used. The laser system 1 may provide pulsed radiation. In a preferred example, the laser system 1 provides, to the plasma formation region 4, a first pulse to shape the fuel droplet from a droplet to a disk shape, a second pulse to rarely the fuel into a disperse ellipsoid shape, and a third pulse to convert the disperse fuel into a plasma. The first, second and third pulses may be generated by one, two or three lasers, each generating a laser beam which travels along the optical path indicated by the laser beam 2 in Figure 1. Each pulse of radiation may be processed by the same beam delivery system 15 prior to arrival at the plasma formation region 4, may be processed by different beam delivery systems (not shown) or may be processed by different portions of the same beam delivery system 15. In this instance, processing of the pulses or beams refers to the delivery of radiation from the laser system 1 to the plasma formation region 4 and to the conditioning of the laser beam.

[0074] Although Figure 1 depicts the radiation source SO as a laser produced plasma (LPP) source, any suitable source such as a discharge produced plasma (DPP) source or a free electron laser (FEL) may be used to generate EUV radiation. The methods and apparatus described herein can be equally applied with reference to a laser system of such radiation sources. Any such radiation source may similarly have a target location which laser radiation may be directed to for the purposes of generating EUV radiation.

[0075] Figure 2 depicts the beam delivery system 15 and laser system 1 described above with reference to Figure 1. The beam delivery system 15 receives, as an input, a laser beam 2 received from the laser system 1. The laser beam 2 is also referred to as a beam 2. The laser system 1 is not part of the beam delivery system 15. The beam delivery system 15 comprises one or more conditioning components for conditioning the beam 2 and one or more positioning components for directing the beam 2 to the plasma formation region 4. Some components may be both conditioning and positioning components, i.e. they both direct and condition the beam 2. The components include a positioning system 20, a deformable mirror 22, a beam splitter 23, a focusing system 24 and a sensor 26. While described here in combination, many of the components may be implemented in a beam delivery system 15 in isolation from others, for example the deformable mirror 22 can be implemented without the positioning mirror 20. Furthermore, the beam delivery system 15 may comprise additional components not explicitly mentioned here.

[0076] The beam 2 is received by the positioning system 20 which directs the beam 2 to the deformable mirror 22. The deformable mirror 22 receives and reflects the beam 2. In particular, the deformable mirror 22 has a reflective surface 22a which lies in a deformable mirror plane D and which generally receives and reflects the beam 2. After being reflected, the laser beam 2 may be referred to as a reflectedlaser beam 2’ or reflected beam 2’. However, it should be understood that the beam 2 and the reflected beam 2’ comprise the same laser beam and that the terms are simply used to distinguish the location along the optical axis being discussed.

[0077] The shape of the reflective surface 22a of the deformable mirror 22 can be changed as described in more detail below and with reference to Figures 3 A, 3B and 4. By changing the shape of the reflective surface 22a (i.e. by deforming the reflective surface 22a), the reflected beam 2’ may be conditioned. For example, one or more properties of the reflected beam 2’ may be changed due to the change in shape of the reflective surface 22a.

[0078] After the beam 2 has reflected from the deformable mirror 22 (and is hence referred to as the reflected beam 2’), a beam splitter 23 is used to split the reflected beam 2’ into a first beam 2a and a second beam 2b (wherein the first beam 2a is a first portion of the reflected beam 2’ and the second beam 2b is a second portion of the reflected beam 2). The first beam 2a is directed to a focusing system 24 which forms an image of the deformable mirror 22 at a sensor 26 using the first beam 2a. In particular, the first beam 2a is detected at a sensor plane S of the sensor 24. The sensor plane S may contain, for example, a charge coupled device (CCD) or other sensing system. The second beam 2b is directed to the plasma formation region 4 such that it can be used to illuminate fuel droplets and thereby enable the generation of EUV radiation.

[0079] Rather than a beam splitter 23, any means of separating a portion of the reflected beam 2’ for direction to the sensor 26 can be used. Additionally, the beam splitter 23 (or other separating means) can be implemented as part of the focusing system 24.

[0080] The focusing system 24 can comprise, for example, one or more focusing elements such as lenses or focusing mirrors. The focusing system 24 is configured to conjugate the deformable mirror plane D and the sensor plane S. As such, the focusing system 24 may also be referred to as a conjugation system. The focusing power of the focusing element(s) of the focusing system 24 are selected, in combination with the optical distance between the deformable mirror plane D and the focusing element(s) and the optical distance between the focusing element(s) and the sensor plane S, such that the planes D, S are conjugated. It will be readily understood that standard optical models can be used to determine an appropriate arrangement given a focusing element with a particular focal length, and vice versa, for example using the mirror formula. Using this arrangement, where the deformable mirror plane D and the sensor plane S are conjugated, an image of the reflective surface 22a is formed at the sensor 26.

[0081] The beam delivery system 15 also comprises a controller 28. The controller is operable to receive data from and / or provide instructions to, one or more of the deformable mirror 22, the sensor 26, and the positioning system 20. The controller may also be operable to perform one or more methods relating to beam conditioning as described in more detail further below. The controller may be implemented remotely from the beam delivery system 15 and be in wired or wireless communication with one or more of the deformable mirror 22, the sensor 26, and the positioning system 20.

[0082] Figures 3 A and 3B schematically depict the deformable mirror 22 in cross-section. A nominal Cartesian coordinate system is shown, showing an x-direction, y-direction and z-direction although it should be understood that this coordinate system is used for illustrative purposes only and the invention may be implemented in a different orientation or arrangement. The deformable mirror 22 comprises a reflective surface 22a. In Figure 3A, the reflective surface 22a extends along the x-direction and is substantially flat in this direction. It can be imagined that the mirror may be generally planar and so can also extend along the y-direction in a similarly flat manner. Of course, the methods described herein may similarly be used on any shape or orientation of mirror.

[0083] The reflective surface 22a is deformable, such that when a force is exerted on a portion of the reflective surface 22a that portion deforms to a new (deformed) position and / or shape. The shape of the reflective surface 22a may be referred to as a configuration. That is, the reflective surface 22a may have a first shape which is referred to as a first configuration, then be deformed to a second shape, which is referred to as a second configuration. The reflective surface 22a can have any number of possible configurations. The reflective surface 22a may be elastically deformable such that, when the force is removed, the portion returns to an undeformed position and / or shape. Alternatively, the portion may be returned to an undeformed position and / or shape by exerting a different (e.g. opposite) force on the portion.

[0084] The reflective surface 22a may be formed from a thin membrane, for example with a thickness on the order of microns or millimetres. The thickness can be selected so as to provide sufficient flexibility to allow for deformation. The thickness may also be selected so as to also provide sufficient mechanical strength to withstand such deformation and / or sufficient thermal robustness to withstand the radiation incident upon it without significant degradation.

[0085] The deformable mirror 22 also has a plurality of actuators 30a-g. In Figures 3A and 3B, seven actuators 30a-g can be seen, extending along an x-direction. The deformable mirror 22 may also have additional actuators extending in the y-direction. A deformable mirror 22 can have any number of actuators, for example one actuator or thirty actuators. Each actuator 30a-g is in contact with a corresponding portion 32a-g of the reflective surface 22a. Each actuator 30a-g is therefore configured to exert a force on a portion of the reflective surface 22a. The amount of force exerted can be varied based on the magnitude by which the actuator is actuator. Each actuator 30a-g is actuatable (e.g. movable or steerable) such that, when actuated, it moves its corresponding portion 32a-g of the reflective surface 22a. By moving one or more portions 32a-g of the reflective surface 22a, the shape of the reflective surface 22a is thereby changed. This change in shape can be referred to as a deformation. That is, the deformable mirror 22 (and the reflective surface 22a thereof) can be deformed by controlling the actuators 30a-g.

[0086] Figure 3B depicts the deformable mirror 22 following actuation of one actuator 30d of the plurality of actuators 30a-g. The actuator 30d has been moved (e.g. by an extension of the actuator 30d) in the positive z-direction. A corresponding portion 32d of the reflective surface 22a is moved by theactuator 30d. In particular, the position of the portion 32d is changed, such that the portion 32d is moved in the positive z-direction. Additionally, regions adjacent to the portion 32d may change in shape, for example forming a sloping segment between the deformed portion 32d and adjacent portions 32c, 32e. Whereas the reflective surface 22a in Figure 3 A was generally flat, the reflective surface 22a in Figure 3B has a protrusion due to the deformation caused by the actuation of an actuator 30d.

[0087] Rather than a protrusion, a concavity may be provided, for example by moving an actuator 30a-g in the negative z-direction. Furthermore, while Figure 3B depicts the formation of a protrusion at the portion 32d by moving the corresponding actuator 30d in the positive z-direction, a protrusion may instead be formed by not actuating actuator 30d but instead actuating adjacent actuators 30c, 30d in the positive z-direction, or by actuating adjacent actuators 30c, 30d by a greater distance d in the positive z-direction compared to the actuation distance of the actuator 30d corresponding to the portion 32d.

[0088] The actuators may be implemented a binary system of being not actuated (e.g. in an initial position, for example at a nominal ‘zero’ point in the z-direction) or being actuated (e.g. moved in the z-direction by a predetermined distance) such that a protrusion on the reflective surface 22a can be formed or removed. Alternatively, the actuators may be implemented with a continuous range of actuation states (e.g. by moving the actuator by a distance d, where d can take any value within a range of values, the range of values corresponding to the range of the actuator and / or the range of motion of the flexible reflective surface 22a) such that the magnitude of the protrusion formed by the actuation of an actuator can be controlled.

[0089] Furthermore, Figure 3B depicts an actuation which comprises translational movement along a single axis. However, actuators may have many degrees of freedom and so actuation can include translation in any direction and / or rotation. The actuation of an actuator be referred to as steering or adjusting the actuator.

[0090] The combination of an actuator and its corresponding portion of the reflective surface 22a may be referred to as an adjustable reflective portion of the deformable mirror 22. By controlling (e.g. adjusting, steering) the adjustable reflective portions of the deformable mirror 22, the shape of the reflective surface 22a can be controlled. Controlling an adjustable reflective portion can be any action which causes the portion to move, thereby inducing a deformation of the reflective surface 22a, for example by adjustment of an actuator.

[0091] Figure 4 schematically illustrates a plan view of the deformable mirror 22 of Figures 3 A and 3B. The mirror 22 is shown from the positive z-direction and so the reflective surface 22a is seen. However, dotted lines indicate the position of thirty one portions 32a-32e’ of the reflective surface 22a which correspond to thirty one actuators (not shown) positioned beneath the reflective surface 22a. The combination of each portion 32a-32e’ of the reflective surface 22a with its corresponding actuator may be referred to as an adjustable reflective portion.

[0092] A range of shapes can be formed by adjusting one or more of the actuators. A single actuator can form or remove a protrusion or concavity at a corresponding portion 32a-32e’ of the reflectivesurface 22a. Additionally or alternatively multiple actuators can cooperate to deform the reflective surface 22a into any of a range of shapes or configurations. The shape may be selected so as to condition the laser beam, for example by compensating for aberrations in the laser beam.

[0093] Methods relating to beam conditioning are described with reference to Figures 5 and 6. The methods can be implemented using the beam delivery system described above. An individual method may not specifically condition the laser beam, but may constitute a step involved in beam conditioning. For example, some methods are methods for determining a property of the laser beam. Such properties may be used in subsequent beam conditioning processes. Each method can be used alone, or in combination with any of the other methods. Furthermore, some of the method steps may be performed in isolation from other method steps, for example as a sub-routine.

[0094] Figure 5 shows a method 500 of conditioning the laser beam. In a first step 501, the deformable mirror 22 is illuminated with a laser beam 2. In particular, the reflective surface 22a of the deformable mirror is illuminated. The laser beam 2 is reflected and is hence referred to as a reflected laser beam 2’.

[0095] In a second step 502, an image of the deformable mirror 22 is formed at the sensor 26. In particular, an image of the reflective surface 22a of the deformable mirror is formed at the sensor. The image is formed due to the conjugate arrangement of the deformable mirror 22 and the sensor 26, aided by the focusing system 24.

[0096] The image could be formed from the entirety of the reflected laser beam 2’, or alternatively it could be formed from only a portion 2a, for example by splitting the beam with a beam splitter 24 as described above. By using the beam splitter 24 (or other beam splitting means), the second beam 2b can be directed to a means for generating radiation, for example the plasma formation region 4, and thereby used to generate radiation. As such, the method 500 comprises an optional step of generating radiation. The generation of radiation can be performed concurrently with the other method steps. Given that the second beam 2’ can be used to generate radiation, the method 500 can be used as a method of conditioning a laser beam for use in generating radiation.

[0097] In a third step 503, beam data is obtained. The beam data is generated by the sensor 26 and contains information regarding the reflected laser beam 2’. Because the reflected laser beam 2’s is merely a reflection of the laser beam 2, the beam data therefore contains information regarding the laser beam 2. The beam data, and the information it contains, may depend on the type of sensor used. For example, an image sensor may be used to detect a spatial intensity profile and / or a wavefront sensor may be used to detect a phase profile. The beam data may be obtained by a controller, for example by the controller 28 of Figure 2.

[0098] It should be understood that, when only a portion 2a of the reflected laser beam 2’ is directed to the sensor 26, the beam data is representative of the first portion 2a of the reflected laser beam 2’. However, the first portion 2a of the reflected laser beam 2’ is representative of the reflected laser beam 2’ because, for example, it is simply a portion of the total intensity of the reflected laser beam 2’ split from the reflected laser beam 2’ and does not otherwise process or change the beam 2’. As such, thebeam data is representative of the first portion 2a of the reflected laser beam 2’ and also the reflected laser beam 2’.

[0099] In a fourth step 504, an adjustment is determined. The aim of the adjustment is to condition the reflected beam 2’. The conditioning may include inducing a change to one or more properties of the reflected beam 2’, compared to the properties of the laser beam 2 prior to reflection, due to the interaction between the laser beam 2 and the deformable mirror 22. The adjustment could be, for example, a steering of one or more adjustable reflective portions of the deformable mirror 22 so as to change the beam profile and / or phase profile of the reflected beam 2’. The adjustment could be, for example, to compensate for aberrations in the reflected laser beam 2’. The adjustment may be determined by a controller, for example by the controller 28 of Figure 2.[000100] Determining the adjustment can comprise determining one or more properties of the reflected laser beam 2’, wherein the adjustment is determined based on the one or more properties.[000101] The one or more properties determined based on the beam data can include: a beam profile, a position of the beam on (e.g. with respect to) the reflective surface 22a, a measure of stability (e.g. positional stability, profile stability and / or intensity stability). Such properties can especially be determined when the beam data comprises a spatial intensity profile of the beam. The one or more properties determined based on the beam data can include: a beam profile, a position of the beam on (e.g. with respect to) the reflective surface 22a, a measure of one or more aberrations (e.g. the presence of one or more aberrations and optionally a quantification of said one or more aberrations), a measure of stability (e.g. positional stability, profile stability and / or aberration stability). Such properties can especially be determined when the beam data comprises a phase profile of the beam. The determined aberrations may be represented with reference to a set of base functions, for example Zemikes. The determined aberrations may be referred to by the classical names associated with each Zemike mode such as coma, tilt, astigmatism etc. Other[000102] The adjustment can be determined based on the determined one or more properties. The adjustment may also be determined based on one or more desired properties of the reflected laser beam 2’, for example the adjustment can be determined based on both the determined and desired properties. An adjustment can be determined which would change the shape of the reflective surface 22a to a shape which would alter the reflected laser beam 2’ properties, and in particular bring the determined properties closer to the desired properties. For example, the determined properties may indicate the presence of an aberration may be. A desirable beam property may be to have no aberrations. An adjustment is therefore determined which would deform the reflective surface 22a into a shape such that a reflected laser beam 2’ reflected from the deformed reflective surface would have reduced aberrations compared to the previously determined aberrations (that is, compared to the reflected laser beam 2’ reflected from an undeformed reflective surface). The shape required to achieve this alteration of the beam properties may be determined empirically or theoretically.[000103] Additionally or alternatively, the desired adjustment may depend on the desired use of the second portion of the reflected laser beam 2’ (for example, the portion of the reflected laser beam 2’ which has not been used to form an image of the deformable mirror 22 at the sensor 26). For example, the second portion of the beam 2’ may be used for the generation of EUV radiation. In this example, it is desirable to condition the beam 2’ for optimal EUV generation. Typically, for the generation of radiation, and in particular the generation of EUV radiation, it is desirable for the beam to have low aberrations, a high stability, and a specific beam profde (e.g. a top-hat profde).[000104] In a fifth step 505, an instruction is provided to the deformable mirror 22 for performing the adjustment. The instruction may be provided by a controller, for example by the controller 28 of Figure 2. The instruction may be an instruction to actuate one or more actuators, e.g. actuators 30a-30e’ of Figures 4. The instruction may include a magnitude of movement and / or a type of movement (e.g. translation, rotation).[000105] A property which may be determined based on the beam data is a current position of the laser beam 2 on the reflective surface 22a of the deformable mirror 22. Above, it has been described how the beam data is representative of the reflected laser beam 2’ and the first portion 2a of the reflected laser beam 2’. However, the beam data is also, at least somewhat, representative of the laser beam 2 itself, prior to reflection. Some properties of the laser beam 2 may be changed upon reflection from the deformable mirror 22, for example the aberration state (i.e. the presence and / or magnitude of one or more aberrations within the beam) may be changed upon reflection due to the shape of the reflective surface 22a. The relative position of the laser beam 2 and the reflective surface 22a, however, are not changed due to reflection. In particular, the position of the laser beam 2 at the instant of reflection is identical to the position of the reflected laser beam 2’ at the instant of reflection, because they are the same beam.[000106] In addition to, or alternatively to, determining an adjustment to the deformable mirror, an adjustment to the positioning mirror 20 can be determined. The purpose of the adjustment to the positioning mirror 20 is to alter the position of the laser beam 2 on the reflective surface 22a. That is, the position of the laser beam 2 on the reflective surface 22a can be controlled based on the beam data, and in particular based on a current position determined using the beam data. Following determination of the current position, an adjustment to the positioning mirror 20 can be determined. Such an adjustment may be referred to as a positioning adjustment. Conversely, an adjustment to the deformable mirror 22 may be referred to as a deforming adjustment. The positioning adjustment may be provided, in the form of an instruction to perform the positioning adjustment, to the positioning mirror 20. As described above, any positioning system may be used instead of specifically a positioning mirror.[000107] By determining the position of the laser beam 2 on the reflective surface 22a, and determining and effecting a positioning adjustment, the laser beam 2 can be kept at a desired position (e.g. centrally) on the reflective surface 22a. This can be particularly beneficial to overcome positional instability of the laser beam 2, due to instabilities in the laser system 1 and / or other components in the radiationsystem, and correct any deviation of the laser beam 2 from a desired position on the reflective surface 22a. For beam conditioning, it is important to keep the laser beam 2 at the desired location on the reflective surface 22a because beam conditioning caused by the shape of the reflective surface 22a may affect the laser beam 2 (and so the reflected laser beam 2’) differently depending on the relative position of the laser beam 2 and reflective surface 22a. For example, to optimise the beam profde in a specific implementation it may be desirable to provide a protrusion at the beam centre. With the laser beam 2 centrally aligned on the reflective surface 22a, such a protrusion may be provided by actuating the central actuator 30d as depicted in Figure 3B. However, if the laser beam 2 has strayed from a central position, the beam centre may actually lie at a different position on the reflective surface 22a and so actuation of the central actuator 30d may lead to a sub-optimal beam conditioning. However, by determining the current position of the laser beam 2 and using the positioning system to return to laser beam 2 to a central position, optimal beam conditioning using the central actuator 30d can be enforced. [000108] In another approach, rather than altering the position of the laser beam 2 on the reflective surface 22a based on the determined current position of the laser beam 2, the deformable mirror 22 may be deformed to compensate for the current position. In the above example, where the laser beam 2 has strayed to a non-central position, the beam centre may now lie at a position corresponding to another actuator, for example actuator 30e. Rather than moving the laser beam 2 using the positioning system, the shape of the deformable mirror 22 may be changed so as to provide a protrusion at the actuator 30e corresponding to the current beam centre’s position, and returning the central actuator 30d to its original (undeformed) position.[000109] It should be understood that the above example, which references the actuation of a single actuator for conditioning the beam, is a simplified example and actual beam conditioning operations may use the actuation of multiple actuators to provide complex shapes of the reflective surface 22a.[000110] Figure 6 depicts a method 600 for controlling the position of the laser beam 2 on the reflective surface 22a. The method 600 uses the deformable nature of the reflective surface 22a to aid in determining the current position of the laser beam 2 on the reflective surface 22a.[000111] In a first step 601, the deformable mirror 22 is illuminated with a laser beam 2. In particular, the reflective surface 22a of the deformable mirror is illuminated. The laser beam 2 is reflected and is hence referred to as a reflected laser beam 2’. In this first step 601, the reflective surface 22a is in a first configuration, i.e. the reflective surface 22a has a first shape. The first step 601 can be said to occur at a first time, wherein the reflective surface 22a is in the first configuration at the first time.[000112] In a second step 602, first beam data is obtained. The obtaining of first beam data is comparable to the obtaining of beam data in the third step 503 of the method 500 of Figure 5, but in this second step 602 of the method 600 of Figure 6, the beam data is obtained at the first time, i.e. when the reflective surface 22a is in the first configuration. As such, the first beam data is representative of the reflected laser beam 2’ following reflection from the reflective surface 22a in the first configuration.[000113] In a third step 603, the deformable mirror 22 is deformed. In particular, the reflective surface 22a is deformed from the first configuration (i.e. a first shape) to a second configuration (i.e. a second shape). The first and second configurations are different.[000114] In a fourth step 604, second beam data is obtained. The obtaining of second beam data is comparable to the obtaining of beam data in the third step 503 of the method 500 of Figure 5, but in this fourth step 604 of the method 600 of Figure 6, the beam data is obtained at the second time, i.e. when the reflective surface 22a is in the second configuration.[000115] In a fifth step 605, the current position of the laser beam 2 on the reflective surface 22a is determined. In particular, the current position is determined based on the first and second beam data. It should be understood that the first and second beam data are taken at different times and so the current position is associated with the first and second times. The first and second times may temporally close so that negligible beam deviation (i.e. change in position) occurs between the first and second time.[000116] In a specific example, the reflective surface 22a is flat in the first configuration and in the second configuration the central actuator 30d is actuated to form a protrusion, for example as illustrated in Figures 3A and 3B. The first beam data obtained comprises a spatial intensity profile and a phase profile of the of the reflected laser beam 2’ at the first time and the second beam data obtained comprises a phase profile of the reflected laser beam 2’ at the second time. The centroid of each profile (that is, the centroid of each of the spatial intensity profile and phase profiles) is determined. The centroids can be determined by any known method, for example calculating the centre of gravity of the profile data. Determining the current position of the laser beam 2’ is performed by comparing the centroid of the spatial intensity profile with the difference between the centroids of the phase profiles at the first and second times. Comparing can include calculating the difference between the centroid of the spatial intensity profile and the difference between the centroids of the phase profiles at the first and second times. Because of the conjugate nature of the reflective surface 22a and sensor 26, both intensity and phase data can be determined quickly and easily and so the determination of the current position using this method is quick.[000117] In a sixth step 606, an instruction is provided change the position of the laser beam 2 on the reflective surface 22a. The instruction may be provided by a controller, for example by the controller 28 of Figure 2. The instruction is provided to a positioning system, for example the positioning mirror 20 of Figure 2. As such, the position of the laser beam 2 on the reflective surface 22a can be controlled. [000118] The method 600 for controlling the position of the laser beam 2 on the reflective surface 22a can be performed multiple times so as to control the position of the laser beam 2 over an extended period of time. The method may be performed as a feedback loop, where the position of the laser beam 2 is periodically or continually determined and adjusted. This can include alternating the shape of the reflective surface 22a between the first and second configuration repeatedly, and collecting multiple sets of first and second beam data. Each set of first and second beam data can then be used to determine an updated current position.[000119] Similarly, the method 500 for conditioning the laser beam described above can be performed multiple times, for example in a feedback loop. In this manner, the deformable mirror 22 and / or positioning mirror 20 can be periodically or continually adjusted so as to periodically or continually condition the beam.[000120] Similar to the method 500 of Figure 5, the method of Figure 6 may also include generating radiation, for example with a second portion 2b of the laser beam 2, where only a first portion 2a of the laser beam 2 has been used to generate the first and second beam data. Controlling the position of the laser beam 2 on the reflective surface 22a can be seen as a method of beam conditioning, because by controlling the position, the beam 2 can be optimally conditioned by the reflective surface 22a for the purposes of generating radiation.[000121] The above methods include providing an instruction to perform an adjustment, for example an adjustment to the deformable mirror or to the positioning system. Alternatively, use of the beam conditioning system described herein provides a means for efficiently and effectively determining one or more properties of the laser beam. Such a method comprises illuminating a reflective surface of a deformable mirror with a laser beam to form a reflected laser beam, the deformable mirror being operable to deform the reflective surface of the deformable mirror, forming an image of the reflective surface at a sensor using at a first portion of the reflected laser beam, obtaining, by the sensor, beam data indicating one or more properties of the reflected laser beam, and determining, based on the beam data, one or more properties of the reflected laser beam.[000122] The actuators described herein can take any form. For example, the actuators may be, for example, piezoelectric or electromagnetic actuators. The force can be exerted due to, for example, the actuators 30a-g being in contact with a portion of the reflective surface 22a, either directly or indirectly. Deformation of the reflective surface can be performed in any other way, for example through noncontact means (e.g. magnetic or using gas flow) or otherwise.[000123] Reference is made herein to the reflection of a laser beam from a reflective surface (for example, the reflection of the laser beam 2 from the reflective surface 22a of the deformable mirror 22). It should be understood that the process of reflection can utilise many different optical mechanisms and that, during a reflection process, radiation may propagate into a material. As such, a reflective surface or should not be interpreted narrowly as an infinitely thin plane, but rather as generally defining the region of an optical component which receives and reflects radiation. Similarly, radiation in a sensor may propagate through a sensor during a sensing process rather than sensing occurring in an infinitely thin plane. The deformable mirror plane D and sensor plane S should therefore be construed as generally indicating the regions of reflecting and sensing rather than being limited to infinitesimally thin planes of matter.[000124] Any of the above apparatus or methods can be used with multiple laser beams, or a laser beam which includes multiple components. For example, in an EUV radiation system three pulsed laser beams may be used so as to provide a first pulse, second pulse, and third pulse to the plasma formation region.These pulses each contribute to shaping the fuel droplet and turning it into a plasma. Each of the pulsed laser beams may be reflected from the deformable mirror and conditioned thereby.[000125] The optical properties to be determined in the methods described herein are mentioned for illustrative purposes only and are non-limiting. Other properties not explicitly described herein may be determined as will be understood to the skilled person.[000126] Although specific reference may be made in this text to embodiments of the invention in the context of a lithographic apparatus, embodiments of the invention may be used in other apparatus. Embodiments of the invention may form part of a mask inspection apparatus, a metrology apparatus, or any apparatus that measures or processes an object such as a wafer (or other substrate) or mask (or other patterning device). These apparatus may be generally referred to as lithographic tools. Such a lithographic tool may use vacuum conditions or ambient (non-vacuum) conditions.[000127] Where the context allows, embodiments of the invention may be implemented in hardware, firmware, software, or any combination thereof. Embodiments of the invention may also be implemented as instructions stored on a machine -readable medium, which may be read and executed by one or more processors. A machine-readable medium may include any mechanism for storing or transmitting information in a form readable by a machine (e.g., a computing device). For example, a machine-readable medium may include read only memory (ROM); random access memory (RAM); magnetic storage media; optical storage media; flash memory devices; electrical, optical, acoustical or other forms of propagated signals (e.g. carrier waves, infrared signals, digital signals, etc.), and others. Further, firmware, software, routines, instructions may be described herein as performing certain actions. However, it should be appreciated that such descriptions are merely for convenience and that such actions in fact result from computing devices, processors, controllers, or other devices executing the firmware, software, routines, instructions, etc. and in doing that may cause actuators or other devices to interact with the physical world.[000128] 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.CLAUSES1. A radiation system comprising: a deformable mirror having a reflective surface arranged to receive and reflect a laser beam to form a reflected laser beam; a beam splitter arranged to separate the reflected laser beam into a first portion and a second portion; a sensor;a focusing system configured to form an image of the reflective surface at the sensor using the first portion of the reflected laser beam, the sensor being configured to output beam data representative of the reflected laser beam; means for generating radiation using the second portion of the reflected laser beam; and a controller operable to cause the deformable mirror to deform the reflective surface based on the beam data.2. The radiation system of clause 1, wherein the deformable mirror comprises one or more adjustable reflective portions and wherein the controller being operable to cause the deformable mirror to deform the reflective surface comprises the controller being operable to adjust at least one of said one or more adjustable reflective portions.3. The radiation system of clause 2, wherein the controller being configured to adjust the at least one of the one or more adjustable reflective portions comprises the controller being configured to: receive the beam data; determine, based on the beam data, an adjustment to at least one of the one or more adjustable reflective portions; and provide an instruction to the at least one of the one or more adjustable reflective portions to perform the adjustment.4. The radiation system of clause 3, wherein the adjustment is configured to optimise the reflected laser beam for the generation of radiation.5. The radiation system of any preceding clause, further comprising a positioning system for receiving the laser beam from a laser system and directing the laser beam to the reflective surface of the deformable mirror, the positioning system configured to control the position of the laser beam on the reflective surface.6. The radiation system of clause 5, wherein the positioning system comprises a tilt / tip mirror.7. The radiation system of clause 5 or 6, wherein the controller is further configured to: determine, based on the beam data, a current position of the laser beam on the reflective surface of the deformable mirror; determine, based on the current position of the laser beam, a position adjustment for altering the position of the laser beam on the reflective surface of the deformable mirror using the positioning mirror; and provide an instruction to the positioning mirror to perform the position adjustment.8. The radiation system of any preceding clause, wherein the reflective surface comprises a flexible layer and wherein the one or more adjustable reflective portions each comprise an actuator in contact with a portion of the flexible layer.9. The radiation system of clause 8, wherein the actuators comprise electro-mechanical actuators, piezo-electric actuators and / or electromagnetic actuators.10. The radiation system of any preceding clause, wherein the means for generating radiation comprises:a droplet generator operable to deliver fuel droplets to a target location; and means for delivering the second portion of the reflected laser beam to the target location.11. The radiation system of clause 10, further comprising an additional focusing system for focusing the second portion of the reflected laser beam at the target location.12. The radiation system of any preceding clause, further comprising a laser system configured to generate the laser beam.13. The radiation system of any preceding clause further comprising a beam processing apparatus for processing the laser beam.14. The radiation system of clause 13, wherein the beam processing apparatus comprises one or more of a compressor, an amplifier, a beam shaping system.15. The radiation system of clause 14, wherein the beam processing apparatus is positioned between the laser system and the deformable mirror.16. The radiation system of any preceding clause, wherein: the reflective surface of the deformable mirror is further arranged to receive and reflect a second laser beam; the focusing system is configured to form an image of the reflective surface at the sensor using a first portion of the reflected second laser beam; the means for generating radiation further uses a second portion of the reflected second laser beam to generate radiation; and the beam data is further representative of the reflected second laser beam.17. A lithographic apparatus comprising the radiation system of any preceding clause.18. A method of conditioning a laser beam, comprising: illuminating a reflective surface of a deformable mirror with a laser beam to form a reflected laser beam, the deformable mirror being operable to deform the reflective surface of the deformable mirror; forming an image of the reflective surface at the sensor using a first portion of the reflected laser beam; obtaining, by a sensor, beam data indicating one or more properties of a reflected laser beam reflected from a reflective surface of a deformable mirror; determining, based on the beam data, an adjustment to the deformable mirror, the adjustment configured to condition the reflected laser beam; and providing an instruction, to the deformable mirror, to perform the adjustment.19. The method of clause 18, wherein determining, based on the beam data, an adjustment to the deformable mirror comprises: determining, based on the beam data, one or more properties of the reflected laser beam; determining, based on based on the one or more determined properties of the reflected laser beam and a corresponding one or more desired properties of the reflected laser beam, the adjustment to the deformable mirror.20. The method of clause 19, wherein the one or more properties comprise a beam profile of the reflected laser beam, a stability of the reflected laser beam and / or a measure of one or more aberrations in the reflected laser beam.21. The method of any of clauses 18 to 20, wherein the one or more properties comprise a current position of the reflected laser beam with respect to the reflective surface.22. The method of clause 21, further comprising controlling a position of the laser beam on the reflective surface of the deformable mirror based on the current position.23. The method of clause 22, wherein controlling the position of the reflected laser beam comprises : determining, based on the current position, an adjustment to a positioning system for altering the position of the laser beam on the reflective surface; and providing an instruction to the positioning system to perform the adjustment.24. The method of any of clauses 21 to 23, wherein: illuminating a reflective surface of a deformable mirror with a laser beam to form a reflected laser beam comprises: illuminating the reflective surface with a laser beam at a first time, wherein at the first time the reflective surface is in a first configuration; deforming the reflective surface from the first configuration to a second configuration; and illuminating the reflective surface with the laser beam at a second time wherein at the second time the reflective surface is in the second configuration; obtaining, by the sensor, beam data comprises: obtaining first beam data at the first time; and obtaining second beam data at the second time; and determining the current position of the reflected laser beam is based on the first beam data and second beam data.25. The method of clause 24, wherein: the first beam data represents a measure of an intensity profile and / or a phase profile of the reflected laser beam following reflection from the reflective surface in the first configuration; and the second beam data represents a measure of an intensity profile and / or a phase profile of the of the reflected laser beam following reflection from the reflective surface in the second configuration.26. The method of clause 25, wherein determining the current position of the laser beam on the reflective surface of the deformable mirror comprises determining a difference between a centroid of the intensity profile and / or phase profile of the first beam data and a centroid of the intensity profile and / or phase profile of the second beam data.27. A method of determining one or more properties of a laser beam for use in generating radiation, the method comprising:illuminating a reflective surface of a deformable mirror with a laser beam to form a reflected laser beam, the deformable mirror being operable to deform the reflective surface of the deformable mirror; forming an image of the reflective surface at a sensor using at a first portion of the reflected laser beam; directing a second portion of the reflected laser beam to a target location for generating radiation; obtaining, by the sensor, beam data indicating one or more properties of the reflected laser beam; and determining, based on the beam data, one or more properties of the reflected laser beam.28. The method of clause 27, wherein the one or more properties comprise a current position of the reflected laser beam with respect to the reflective surface.29. The method of clause 28, further comprising controlling a position of the laser beam on the reflective surface of the deformable mirror based on the current position.30. The method of clause 29, wherein controlling the position of the reflected laser beam comprises: determining, based on the current position, an adjustment to a positioning system for altering the position of the laser beam on the reflective surface; and providing an instruction to the positioning system to perform the adjustment.31. The method of any of clauses 28 to 30, wherein: illuminating a reflective surface of a deformable mirror with a laser beam to form a reflected laser beam comprises: illuminating the reflective surface with a laser beam at a first time, wherein at the first time the reflective surface is in a first configuration; deforming the reflective surface from the first configuration to a second configuration; and illuminating the reflective surface with the laser beam at a second time, wherein at the second time the reflective surface is in the second configuration; obtaining, by the sensor, beam data comprises: obtaining first beam data at the first time; and obtaining second beam data at the second time; and determining the current position of the reflected laser beam is based on the first beam data and second beam data.32. The method of clause 31, wherein: the first beam data represents a measure of an intensity profile and / or a phase profile of the reflected laser beam following reflection from the reflective surface in the first configuration; and the second beam data represents a measure of an intensity profile and / or a phase profile of the of the reflected laser beam following reflection from the reflective surface in the second configuration.33. The method of clause 32, wherein determining the current position of the laser beam on the reflective surface of the deformable mirror comprises determining a difference between a centroid of the intensity profile and / or phase profile of the first beam data and a centroid of the intensity profile and / or phase profile of the second beam data.34. The method of any of clauses 27 to 33, further comprising: determining, based on the one or more determined properties of the reflected laser beam and a corresponding one or more desired properties of the reflected laser beam, an adjustment to the deformable mirror, the adjustment configured to condition the reflected laser beam; and providing the instruction, to the one or more of the adjustable reflective portions, to perform the adjustment.35. The method of any of clause 28 to 34, wherein the one or more properties comprise a beam profde of the reflected laser beam, a stability of the reflected laser beam and / or a measure of one or more aberrations in the reflected laser beam. 36. The method of any of clauses 18 to 34, further comprising directing a second portion of the reflected laser beam to a target location for generating radiation.37. The method of clause 35, further comprising providing a fuel droplet at the target location for illumination by the second portion of the reflected laser beam.38. A computer program comprising instructions to cause the radiation system of any of clauses 1 to 16 or the lithographic apparatus of clause 17 to execute the method steps of any of clauses 18 to 37.39. A computer-readable storage medium having stored there on the computer program of clause 38.

Claims

CLAIMS1. A radiation system comprising: a deformable mirror having a reflective surface arranged to receive and reflect a laser beam to form a reflected laser beam; a beam splitter arranged to separate the reflected laser beam into a first portion and a second portion; a sensor; a focusing system configured to form an image of the reflective surface at the sensor using the first portion of the reflected laser beam, the sensor being configured to output beam data representative of the reflected laser beam; means for generating radiation using the second portion of the reflected laser beam; and a controller operable to cause the deformable mirror to deform the reflective surface based on the beam data.

2. The radiation system of claim 1, wherein the deformable mirror comprises one or more adjustable reflective portions and wherein the controller being operable to cause the deformable mirror to deform the reflective surface comprises the controller being operable to adjust at least one of said one or more adjustable reflective portions.

3. The radiation system of claim 2, wherein the controller being configured to adjust the at least one of the one or more adjustable reflective portions comprises the controller being configured to: receive the beam data; determine, based on the beam data, an adjustment to at least one of the one or more adjustable reflective portions, wherein the adjustment is configured to optimise the reflected laser beam for the generation of radiation; and provide an instruction to the at least one of the one or more adjustable reflective portions to perform the adjustment.

4. The radiation system of any preceding claim, further comprising a positioning system for receiving the laser beam from a laser system and directing the laser beam to the reflective surface of the deformable mirror, the positioning system configured to control the position of the laser beam on the reflective surface, wherein the positioning system comprises a tilt / tip mirror.

5. The radiation system of claim 4, wherein the controller is further configured to: determine, based on the beam data, a current position of the laser beam on the reflective surface of the deformable mirror;determine, based on the current position of the laser beam, a position adjustment for altering the position of the laser beam on the reflective surface of the deformable mirror using the positioning mirror; and provide an instruction to the positioning mirror to perform the position adjustment.

6. The radiation system of any preceding claim, wherein the reflective surface comprises a flexible layer and wherein the one or more adjustable reflective portions each comprise an actuator in contact with a portion of the flexible layer.

7. The radiation system of any preceding claim, wherein the means for generating radiation comprises: a droplet generator operable to deliver fuel droplets to a target location; and means for delivering the second portion of the reflected laser beam to the target location, and further comprising an additional focusing system for focusing the second portion of the reflected laser beam at the target location.

8. The radiation system of any preceding claim further comprising a beam processing apparatus for processing the laser beam, wherein the beam processing apparatus comprises one or more of a compressor, an amplifier, a beam shaping system, wherein the beam processing apparatus is positioned between the laser system and the deformable mirror.

9. The radiation system of any preceding claim, wherein: the reflective surface of the deformable mirror is further arranged to receive and reflect a second laser beam; the focusing system is configured to form an image of the reflective surface at the sensor using a first portion of the reflected second laser beam; the means for generating radiation further uses a second portion of the reflected second laser beam to generate radiation; and the beam data is further representative of the reflected second laser beam.

10. A lithographic apparatus comprising the radiation system of any preceding claim.

11. A method of conditioning a laser beam, comprising: illuminating a reflective surface of a deformable mirror with a laser beam to form a reflected laser beam, the deformable mirror being operable to deform the reflective surface of the deformable mirror;forming an image of the reflective surface at the sensor using a first portion of the reflected laser beam; obtaining, by a sensor, beam data indicating one or more properties of a reflected laser beam reflected from a reflective surface of a deformable mirror; determining, based on the beam data, an adjustment to the deformable mirror, the adjustment configured to condition the reflected laser beam; and providing an instruction, to the deformable mirror, to perform the adjustment.

12. The method of claim 11, wherein determining, based on the beam data, an adjustment to the deformable mirror comprises: determining, based on the beam data, one or more properties of the reflected laser beam, wherein the one or more properties comprise a beam profile of the reflected laser beam, a stability of the reflected laser beam and / or a measure of one or more aberrations in the reflected laser beam; determining, based on based on the one or more determined properties of the reflected laser beam and a corresponding one or more desired properties of the reflected laser beam, the adjustment to the deformable mirror.

13. The method of any of claims 11 to 12, wherein the one or more properties comprise a current position of the reflected laser beam with respect to the reflective surface, the method further comprising controlling a position of the laser beam on the reflective surface of the deformable mirror based on the current position, wherein controlling the position of the reflected laser beam comprises: determining, based on the current position, an adjustment to a positioning system for altering the position of the laser beam on the reflective surface; and providing an instruction to the positioning system to perform the adjustment.

14. The method of claim 13, wherein: illuminating a reflective surface of a deformable mirror with a laser beam to form a reflected laser beam comprises: illuminating the reflective surface with a laser beam at a first time, wherein at the first time the reflective surface is in a first configuration; deforming the reflective surface from the first configuration to a second configuration; and illuminating the reflective surface with the laser beam at a second time wherein at the second time the reflective surface is in the second configuration; obtaining, by the sensor, beam data comprises: obtaining first beam data at the first time; and obtaining second beam data at the second time; anddetermining the current position of the reflected laser beam is based on the first beam data and second beam data, wherein the first beam data represents a measure of an intensity profile and / or a phase profile of the reflected laser beam following reflection from the reflective surface in the first configuration; and the second beam data represents a measure of an intensity profile and / or a phase profile of the of the reflected laser beam following reflection from the reflective surface in the second configuration.

15. The method of claim 14, wherein determining the current position of the laser beam on the reflective surface of the deformable mirror comprises determining a difference between a centroid of the intensity profile and / or phase profile of the first beam data and a centroid of the intensity profile and / or phase profile of the second beam data.

16. The method of any of claims 11 to 15, further comprising directing a second portion of the reflected laser beam to a target location for generating radiation.

17. A computer program comprising instructions to cause the radiation system of any of claims 1 to 9 or the lithographic apparatus of claim 10 to execute the method steps of any of claims 11 to 16.

18. A computer-readable storage medium having stored there on the computer program of claim17.

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