Optical system and method

Abstract

An optical system comprising an optical component configured to modify electromagnetic radiation and an electric motor configured to actuate the optical component. The electric motor comprises a mover comprising a plurality of magnetic components connected to the optical component. The electric motor comprises a stator unconnected to the mover comprising a plurality of electric components configured to receive an electric current and thereby interact with a magnetic field of the plurality of magnetic elements for contactless actuation of the optical component.

Description

OPTICAL SYSTEM AND METHODCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of EP application 23192387.1 which was filed on August 21, 2023 and which is incorporated herein in its entirety by reference.FIELD

[0002] The present invention relates to an optical system and method. In particular, the present invention relates to the use of an electric motor for contactless actuation of an optical component. The present invention may be utilized in a variety of optical system, including but not limited to lithographic systems, optical measurement systems and substrate processing systems.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. Other lithographic apparatus may use other wavelengths. For example, deep ultraviolet (DUV) lithographic apparatus may utilise ultraviolet radiation having a wavelength of, for example, 365, 248, 193, 157 or 126 nm.

[0005] Lithographic apparatus, amongst other types of optical apparatus such as optical measurement apparatus and substrate processing apparatus, comprise optical components that must be actuated during use in order to perform their functions. It may be desirable to provide an optical system and method which overcomes or mitigates one or more problems associated with the prior art, whether identified herein or elsewhere.SUMMARY

[0006] According to a first aspect of the present disclosure, there is provided an optical system. The optical system comprises an optical component configured to modify electromagnetic radiation. The optical system comprises an electric motor configured to actuate the optical component. The electric motor comprises a mover comprising a plurality of magnetic components connected to the optical component. The electric motor comprises a stator unconnected to the mover comprising aplurality of electric components configured to receive an electric current and thereby interact with a magnetic field of the plurality of magnetic elements for contactless actuation of the optical component.

[0007] Known optical systems comprise Lorentz actuators having flexures that are arranged to keep a pin in line with a motor. The flexures provide a physical connection between the optical component and a frame. This introduces a dynamical stiffness which is disadvantageous for fine and / or rapid actuation of the optical component. The flexures introduce an increasing stiffness with increasing frequency of movement and / or vibration (e.g. natural or resonance frequency) of the optical component. This dynamical stiffness undermines the so-called “zero-stiffness” concept on which Lorentz actuators are based. In addition, known Lorentz actuators are relatively bulky, heavy and have a complex interface with and connection to the optical component, resulting in significant power requirements and complicated control systems in order to operate.

[0008] The optical system of the present disclosure advantageously avoids the need for flexures or any other physical contact between the stator and the mover. This removes the dynamical stiffness associated with known Lorentz actuators and improves fine and / or rapid actuation of the optical component, thereby improving a performance of the optical system compared to known optical systems. The magnetic force control provided by the pluralities of magnetic and electric components advantageously offers a simplified, compact and lightweight actuation mechanism compared to the known Lorentz actuators.

[0009] A number of electric components may be greater than a number of actuation degrees-of- freedom of the optical component.

[0010] It is beneficial to increase the numerical aperture of some optical systems. For example, a lithographic apparatus is typically capable of forming smaller features at greater numerical apertures. Optical systems having increased numerical apertures may be referred to in the art as hyper-numerical aperture (hyper-NA) optical systems. Hyper-NA optical systems may require larger optical components to capture greater angles of incidence of electromagnetic radiation. To maintain control of the dynamical behavior (e.g. high-bandwidth natural frequency or resonance characteristics) of larger optical components, the mass of the optical components may need to be significantly increased (e.g. by increasing a thickness of the optical components). For example, it has been predicted that the mass of a mirror in a hyper-NA lithographic apparatus would need to be about three times greater than the current non-hyper-NA mirror to maintain the same dynamic behavior. Such an increase in mass brings its own problems, and is not deemed to be feasible for all optical systems. For example, the increase in mass may reduce a natural or resonance frequency of the optical component and thereby introduce unwanted vibrations or movements (e.g. higher order frequency modes of vibration and associated deformations of the optical component) that could negatively affect a performance of the optical system.

[0011] The optical system of the present disclosure advantageously provides over-actuation (i.e. a system in which there are a greater number of actuating elements than the system’s actuation degrees- of-freedom) of the optical component. The plurality of magnetic and electric components may be usedto counter the higher-order frequency modes of vibration and associated deformations of the optical component introduced by increasing a size of the optical component for capturing a larger range of angles of incidence of electromagnetic radiation. This allows the mass of the optical component to be kept substantially the same (e.g. a thickness of the optical component does not need to be increased to achieve the same dynamical behavior) in a hyper-NA optical system.

[0012] The optical component may be a mirror. The mover may be connected to a backside of the mirror.

[0013] Mirrors are used to modify electromagnetic radiation (e.g. by changing a propagation direction via reflection) in many optical systems. The mirror may undergo unwanted thermal deformations when interacting with the electromagnetic radiation which may negatively affect a performance of the optical system. The electric motor may advantageously actuate the optical component so as to counter and / or account for the thermal deformations.

[0014] The optical component may take other forms.

[0015] The optical component may be a transmissive optical component. The optical component may be a lens.

[0016] The optical component may be diffractive optical component. The optical component may be a diffraction grating.

[0017] The optical component may be polarizing optical component. The optical component may be a polarizer.

[0018] The optical component may be configured to modify the electromagnetic radiation in one or more ways.

[0019] The optical component may be configured to change a propagation direction of the electromagnetic radiation.

[0020] The optical component may be configured to change a cross-sectional shape of the electromagnetic radiation (e.g. a beam shape).

[0021] The optical component may be configured to change an intensity distribution of the electromagnetic radiation.

[0022] The optical component may be configured to change a phase of the electromagnetic radiation.

[0023] The optical component may be configured to refract the electromagnetic radiation.

[0024] The optical component may be configured to focus the electromagnetic radiation.

[0025] The optical component may be configured to reflect the electromagnetic radiation.

[0026] The optical component may be configured to diffract the electromagnetic radiation.

[0027] The optical component may be configured to polarize the electromagnetic radiation.

[0028] The optical component may be configured to attenuate the electromagnetic radiation.

[0029] The optical component may be configured to scatter the electromagnetic radiation.

[0030] The optical component may be configured to disperse the electromagnetic radiation.

[0031] The optical component may be configured to modify extreme ultraviolet electromagnetic radiation.

[0032] The optical component may be configured to modify deep ultraviolet electromagnetic radiation.

[0033] The optical component may be configured to modify X-ray electromagnetic radiation.

[0034] The optical component may be configured to modify infrared electromagnetic radiation.

[0035] EUV radiation and / or DUV radiation and / or X-ray radiation and / or infrared radiation may be used to perform fine and / or rapid optical processes such as, for example, EUV lithography, DUV lithography, soft X-ray substrate inspection or infrared plasma generation in a liquid produced plasma (LPP) EUV radiation source. Such fine and / or rapid optical processes require correspondingly fine and / or rapid actuation of the optical components therein. The optical system of the present disclosure advantageously an actuation accuracy and speed of the optical component, thereby improving a performance of fine and / or rapid optical processes that utilize the optical system.

[0036] The electric motor may be a planar motor.

[0037] The planar electric motor advantageously improves an accuracy of optical component actuation compared to known optical systems, thereby allowing fine control of higher-order frequency vibrations and associated deformations of the optical component. The planar electric motor advantageously improves a speed of optical component actuation compared to known optical systems, thereby allowing rapid correction of unwanted movements and / or deformations of the optical component.

[0038] The electric motor may be an inverted planar motor.

[0039] The electric motor may be a three-phase electric motor.

[0040] The electric motor may be configured to translate the optical component. The electric motor may be configured to translate the optical component in any direction, e.g. along three perpendicular axes.

[0041] The electric motor may be configured to rotate the optical component. The electric motor may be configured to rotate the optical component in any direction, e.g. about three perpendicular axes.

[0042] The electric motor may be configured to deform the optical component.

[0043] The plurality of magnetic components may be arranged to form a magnetic component array.

[0044] The plurality of electric components may be arranged to form an electric component array.

[0045] Providing magnetic and electric component arrays advantageously improves an accuracy of actuation control of the optical component.

[0046] The plurality of magnetic components may comprise permanent magnets.

[0047] The plurality of electric components may comprise coils.

[0048] The magnetic component array may be a Halbach magnet array.

[0049] The inventors have found a Halbach magnet array to provide improvements in fine and rapid actuation control of the optical component.

[0050] The electric component array may comprise a plurality of layers. The electric components of a first layer may be arranged perpendicularly to the electric components of a second layer.

[0051] The first layer may be configured to provide actuation of the optical component in a first direction and the second layer may be configured to provide actuation of the optical component in a second direction that is perpendicular to the first direction.

[0052] The electric motor may be configured to levitate the optical component.

[0053] Contactless actuation of the optical component through magnetic levitation advantageously avoids the dynamical stiffness associated with known Lorentz actuators, thereby improving an actuation accuracy of the optical components compared to known optical systems.

[0054] The stator may comprise a permanent magnet configured to interact with the magnetic field of the plurality of magnetic components of the mover and thereby apply a lifting force to the optical component.

[0055] Gravity compensation using a permanent magnet advantageously reduces the power requirements needed to levitate the optical component. This in turn reduces a thermal load associated with actuation of the optical component, and reduces unwanted thermal deformations of the optical component, leading to improved performance of the optical system.

[0056] The inventors have realised that actuation of the optical component may not require long- range planar motion that is typically associated with known planar motors. As such, one or more permanent magnets may be introduced to the stator whilst maintaining a necessary range of actuation motion of the optical component. That is, the inventors have realised that the reduced range of movement requires lower powers compared to known planar motors, and that gravity compensation using stator permanent magnets may be introduced which would not be possible for known planar motors.

[0057] The permanent magnet of the stator may be nested within one of the plurality of electric components,

[0058] The permanent magnet of the stator may take the place of one of the plurality of electric components.

[0059] Nesting within or replacing an electric component with a permanent magnet advantageously improves a levitation of the optical component without increasing power usage or compromising the compact arrangement of electric components of the stator.

[0060] The permanent magnet may be situated within a coil.

[0061] The permanent magnet may take the position of a coil in the coil array.

[0062] The permanent magnet of the stator may form part of a stator permanent magnet array arranged to apply a spatially substantially homogenous lifting force to the optical component.

[0063] The stator permanent magnet array improves a levitation of the optical component without increasing power usage or compromising the compact arrangement of electric components of the stator.

[0064] A ratio of permanent magnets to electric components in the stator permanent magnet array may be about one to about three.

[0065] The inventors have found this ratio to be particularly advantageous in improves a levitation of the optical component without increasing power usage or compromising the compact arrangement of electric components of the stator.

[0066] The stator may comprise about two hundred electric components per square meter or more.

[0067] The stator may comprise about forty electric components or more.

[0068] Using such large numbers of electric components advantageously improves over-actuation of the optical component. This improves the ability of the electric motor to counter the higher-order frequency modes of vibration and associated deformations of the optical component.

[0069] The mover may comprise about six hundred magnetic components per square meter or more.

[0070] The mover may comprise about one hundred and sixty magnetic components or more.

[0071] Using such large numbers of magnetic components advantageously improves overactuation of the optical component. This improves the ability of the electric motor to counter the higher- order frequency modes of vibration and associated deformations of the optical component.

[0072] There may about one or more magnetic components per electric component. There may be about four magnetic components or less per electric component.

[0073] The inventors have found these ratios to be particularly advantageous in providing overactuation of the optical component whilst limiting power usage and associated thermal load.

[0074] Each magnetic component may occupy an area of about 0.04 m by about 0.04 m or less.

[0075] The inventors have realised that actuation of the optical component may not require long- range planar motion that is typically associated with planar motors. As such, the size of the magnetic components may be significantly reduced compared to those of known planar motors. Using such small magnetic components advantageously increases a compactness of the mover and contributes to improving an over-actuation of the optical component. This improves the ability of the electric motor to counter the higher-order frequency modes of vibration and associated deformations of the optical component.

[0076] Each electric component may occupy an area of about 0.04 m by about 0.12 m or less.

[0077] The inventors have realised that actuation of the optical component may not require long- range planar motion that is typically associated with planar motors. As such, the size of the electric components may be significantly reduced compared to those of known planar motors. Using such small electric components advantageously increases a compactness of the stator and contributes to improving an over- actuation of the optical component. This improves the ability of the electric motor to counter the higher-order frequency modes of vibration and associated deformations of the optical component.

[0078] A separation distance between neighboring magnetic components may be about 0.04 m or less.

[0079] Providing such a compact arrangement of magnetic components advantageously increases a compactness of the mover and contributes to improving an over- actuation of the optical component. This improves the ability of the electric motor to counter the higher-order frequency modes of vibration and associated deformations of the optical component.

[0080] A separation distance between neighboring electric components may be about 5 mm or less.

[0081] Providing such a compact arrangement of electric components advantageously increases a compactness of the stator and contributes to improving an over-actuation of the optical component. This improves the ability of the electric motor to counter the higher-order frequency modes of vibration and associated deformations of the optical component.

[0082] According to a second aspect of the present disclosure, there is provided a lithographic apparatus arranged to project a pattern from a patterning device onto a substrate. The lithographic apparatus comprises the optical system of the first aspect.

[0083] The optical system may form part of an illumination system.

[0084] The optical system may form part of a projection system.

[0085] The optical system may form part of a radiation source. The optical system may form part of a CO2 beam path in an LPP source.

[0086] The optical system may form part of a metrology system.

[0087] The optical system may form part of a substrate inspection system.

[0088] The optical system may form part of an optical measurement system.

[0089] The optical system may form part of an alignment sensor.

[0090] The optical system may form part of an overlay sensor.

[0091] The optical system may form part of an optical aberration sensor.

[0092] According to a third aspect of the present disclosure, there is provided a method of actuating an optical component. The method comprises connecting a plurality of magnetic components to the optical component. The method comprises arranging a plurality of electric components that are unconnected to the optical component. The method comprises providing an electric current to the plurality of electric components to thereby interact with a magnetic field of the plurality of magnetic elements for contactless actuation of the optical component.

[0093] The method may comprise providing a greater number of electric components than a number of actuation degrees-of-freedom of the optical component.

[0094] The method may comprise levitating the optical component.

[0095] The method may comprise providing a permanent magnet configured to interact with the magnetic field of the plurality of magnetic components and thereby apply a lifting force to the optical component.BRIEF DESCRIPTION OF THE DRAWINGS

[0096] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:Fig. 1 depicts a lithographic system comprising a lithographic apparatus, a radiation source and two electric motors configured to actuate optical components in accordance with the present disclosure.Fig. 2 schematically depicts an optical system comprising an optical component configured to modify electromagnetic radiation and an electric motor configured to actuate the optical component in accordance with the present disclosure.Fig. 3 shows a flowchart of a method of actuating an optical component in accordance with the present disclosure.DETAILED DESCRIPTION

[0097] Fig. 1 schematically depicts a lithographic system comprising a radiation source SO, a lithographic apparatus LA and two electric motors 120 configured to actuate two different optical components 11, 13. 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.

[0098] 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. In the example of Fig. 1, the faceted pupil mirror device 11 is actuated by an electric motor 120. The electric motor 120 comprises a mover 130 comprising a plurality of magnetic components (shown in Fig. 2) connected to the faceted pupil mirror device 11. The electric motor 120 comprises a stator 150 unconnected to the mover 130 comprising a plurality of electric components (shown in Fig. 2) configured to receive an electric current and thereby interact with a magnetic field of the plurality of magnetic elements for contactless actuation of the faceted pupil mirror device 11.

[0099] After being thus conditioned, the EUV radiation beam B interacts with the patterning device MA. As a result of this interaction, a patterned EUV radiation beam B’ is generated. The projection system PS is configured to project the patterned EUV radiation beam B’ onto the substrate W. For that purpose, the projection system PS may comprise a plurality of mirrors 13,14 which areconfigured to project the patterned EUV radiation beam B’ onto the substrate W held by the substrate table WT. The projection system PS may apply a reduction factor to the patterned EUV radiation beam B’, thus forming an image with features that are smaller than corresponding features on the patterning device MA. For example, a reduction factor of four or eight may be applied. Although the projection system PS is illustrated as having only two mirrors 13, 14 in Fig. 1, the projection system PS may include a different number of mirrors (e.g. six or eight mirrors). In the example of Fig. 1, one of the projection system PS mirrors 13 is actuated by an electric motor 120. The electric motor 120 comprises a mover 130 comprising a plurality of magnetic components (shown in Fig. 2) connected to the mirror 13. The electric motor 120 comprises a stator 150 unconnected to the mover 130 comprising a plurality of electric components (shown in Fig. 2) configured to receive an electric current and thereby interact with a magnetic field of the plurality of magnetic elements for contactless actuation of the mirror 13. The electric motors 120 are discussed in greater detail below and with respect to Fig. 2. The lithographic system may comprise further electric motors 120 for contactless actuation of other optical components therein such as, for example, an optical component of the radiation source SO and / or an optical component of an optical measurement system such as, for example, an alignment sensor, a level sensor, an optical aberration sensor and / or an overlay sensor.[000100] The substrate W may include previously formed patterns. Where this is the case, the lithographic apparatus FA aligns the image, formed by the patterned EUV radiation beam B’, with a pattern previously formed on the substrate W.[000101] 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.[000102] The radiation source SO may be a laser produced plasma (LPP) source, a discharge produced plasma (DPP) source, a free electron laser (FEL) or any other radiation source that is capable of generating EUV radiation.[000103] Fig. 2 schematically depicts an optical system 100 comprising an optical component 110 configured to modify electromagnetic radiation (not shown). In the example of Fig. 2, the optical component 110 is a mirror. The mirror 110 may, for example, correspond to one of the illumination system mirrors 10, 11 of the lithographic apparatus LA of Fig. 1. As another example, the mirror 110 may correspond to one of the projection system PS mirrors 13, 14 of the lithographic apparatus LA of Fig. 1. In either of these examples, the mirror 110 may be configured to modify extreme ultraviolet (EUV) electromagnetic radiation as part of an EUV lithographic process. As another alternative, the mirror 110 may correspond to a mirror (not shown) present in the radiation source SO of the lithographic apparatus LA of Fig. 1. In this example, the mirror 110 may be configured to modify infrared radiation as part of plasma and / or EUV radiation generation in a liquid produced plasma (LPP) EUV radiation source. The optical system 100 may form part of an optical measurement system. For example, the optical system 100 may form part of an alignment sensor configured to determine an alignment betweenthe substrate W and the patterning device MA. As another example, the optical system 100 may form part of an overlay sensor configured to determine an overly error between different layers of the substrate W. As a further example, the optical system 100 may form part of an optical aberration sensor such as, for example, an interferometric wavefront sensor configured to determine optical aberrations of the projection system PS and / or the illumination system IL. As a further alternative, the mirror 110 may not form part of a lithographic apparatus, and may instead form part of another optical system such as an optical measurement system for substrate inspection. In this example, the mirror 110 may be configured to modify X-ray (e.g. soft X-ray) electromagnetic radiation as part of a substrate inspection process. In any of these examples, the mirror 110 may be configured to modify electromagnetic radiation by changing a propagation direction of the electromagnetic radiation via reflection. The mirror 110 may be configured to modify the electromagnetic radiation by changing a cross-sectional shape of the electromagnetic radiation (e.g. a beam shape) and / or changing an intensity distribution of the electromagnetic radiation.[000104] The optical system 100 comprises an electric motor 120 configured to actuate the optical component 110. In the example of Fig. 2, the electric motor 120 is an inverted planar motor. The electric motor 120 may, for example, be a three-phase electric motor. The electric motor 120 comprises a mover 130 comprising a plurality of magnetic components 140 connected to the optical component 110. The mover 130 may be formed of a metal such as, for example, aluminum. The mover 130 may be formed of a ceramic material. The plurality of magnetic components 140 may be connected to the mover 130 by an adhesive such as a glue. In the example of Fig. 2, the mover 130 is connected to a backside of the mirror 110. The electric motor 120 comprises a stator 150 unconnected to the mover 130 comprising a plurality of electric components 160 configured to receive an electric current and thereby interact with a magnetic field of the plurality of magnetic elements 140 for contactless actuation of the optical component 110. The stator 150 may be formed of a non-ferromagnetic material such as, for example, aluminum. The plurality of magnetic components 140 is arranged to form a magnetic component array and the plurality of electric components 160 is arranged to form an electric component array.[000105] The magnetic components 140 may comprise a ferromagnetic material such as, for example, iron. In the example of Fig. 2, the magnetic components 140 are permanent magnets, and the permanent magnets 140 are oriented to have alternating polarities. That is, a first magnet 140 of the magnetic component array has its South pole S connected to the optical component 110 and a neighboring magnet 140 of the magnetic component array has its North pole N connected to the optical component 110. The magnets 140 may form a grid array. For example, further rows of magnets 140 not visible in Fig. 2 may extend in a direction parallel to the Y axis. Other arrangements are possible. For example, the magnetic component array may be a Halbach magnet array.[000106] In the example of Fig. 2, the electric components 160 comprise coils configured to receive an electric current. The coils 160 are wound around poles 162. The coils 160 may form a grid array.For example, further rows of coils 160 not visible in Fig. 2 may extend in a direction parallel to the Y axis. The electric component array may generally correspond to the magnetic component array in terms of form and extent. The number of components 140, 160 in each array may differ. A relative positioning of the components 140, 160 of each array may move in and out of phase (i.e. into and out of alignment) along the extents of the mover 130 and the stator 150.[000107] The electric component array may comprise a plurality of layers (not shown). For example, a second layer of coils may be provided on top of the layer of coils 160 shown in Fig. 2. The electric components 160 of a first layer of electric components 160 may be arranged perpendicularly to the electric components 160 of a second layer. The different layers of electric components 160 may be configured to control different aspects of actuations of the optical component 110. For example, a first layer of electric components 160 may be configured to provide actuation of the optical component 110 in a first direction X and a second layer of electric components (not shown) may be configured to provide actuation of the optical component 110 in a second direction Y that is perpendicular to the first direction X.[000108] The number of magnetic components 140 and electric components 160 present in the electric motor 120 may be selected in at least partial dependence upon the size of the optical component 110 and / or the weight of the optical component 110 and / or the extent of actuation movement (e.g. translation and / or rotation and / or deformation) desired. The optical component 110 may have a diameter of about 0.1 m or more. The optical component 110 may have a diameter of about 1 m or less. For example, the optical component 110 may have a diameter of about 0.5 m. The optical component 110 may have a thickness of about 0.05 m or more. The optical component 110 may have a thickness of about 0.2 m or less. For example, the optical component 110 may have a thickness of about 0.1 m. The optical component 110 may have a weight of about 25 kg or more. The optical component 110 may have a weight of about 100 kg or less. For example, the optical component 110 may have a weight of about 50 kg. The electric motor 120 may be configured to translate and / or deform the optical component 110 by a distance of about 0.5 mm or more. The electric motor 120 may be configured to translate and / or deform the optical component 110 by a distance of about 2 mm or less. For example, the electric motor 120 may be configured to translate and / or deform the optical component 110 by a distance of about 1 mm. The electric motor 120 may be configured to to translate and / or deform the optical component 110 by distances that are smaller than a dimension of a single magnetic component 140. The electric motor 120 may be configured to rotate and / or deform the optical component 110 by an angle of about 0.5 mrad or more. The electric motor 120 may be configured to rotate and / or deform the optical component 110 by an angle of about 2 mrad or less. For example, the electric motor 120 may be configured to rotate and / or deform the optical component 110 by an angle of about 1 mrad.[000109] The stator 150 may comprise about two hundred electric components 160 per square meter or more. For example, if the mirror 110 corresponded to one of the mirrors 10, 11, 13, 14 of Fig. 2, the stator 150 may comprise about forty electric components 160 or more. The mover 130 may compriseabout six hundred magnetic components 140 per square meter or more. For example, if the mirror 110 corresponded to one of the mirrors 10, 11, 13, 14 of Fig. 2, the mover 130 may comprise about one hundred and sixty magnetic components 140 or more. There may be about two or more magnetic components 140 per electric component 160. There may be about four or less magnetic components 160 per electric component 160. For example, there may be about three magnetic components 140 per electric component 160. A ratio of magnetic components 140 to electric components 160 may at least partially depend upon a shape and / or an extent of the electric components 160. For example, the electric components 160 may comprise circular coils or elongate elliptical coils. In the example of circular coils, there may be a greater number of magnetic components 140 than electric components 160. In the example of elongate elliptical coils, the number of magnetic components 140 may be substantially equal to the number of electric components 160. Each magnetic component 140 may occupy an area of about 0.04 m by about 0.04 m or less. Each electric component 160 may occupy an area of about 0.04 m by about 0.12 m or less. A separation distance between neighboring magnetic components 140 (e.g. a pitch of the magnetic component array) may be about 0.04 m or less. The pitch of the magnetic component array may be constant. The pitch of the magnetic component array may be substantially equal to a dimension of a single magnetic component 140. A separation distance between neighboring electric components 160 (e.g. a pitch of the electric component array) may be about 5 mm or less. The pitch of the electric component array may be constant. The pitch of the magnetic component array may be different to the pitch of the electric component array.[000110] The number of electric components 160 may be greater than the number of actuation degrees-of-freedom of the optical component 110. For example, if the optical component 110 were to have seven or more degrees-of-freedom (e.g. three translational degrees-of-freedom of forward / back movement, up / down movement and left / right movement, three rotational degrees-of-freedom of yaw, pitch and roll, and one or more deformational degrees-of-freedom) then the number of electric components 160 may be eight or more. This provides over-actuation (i.e. there are a greater number of actuating elements than actuation degrees-of-freedom) of the optical component 110. The plurality of magnetic components 140 and electric components 160 may be used to at least partially account for and / or counter the unwanted effects of higher-order frequency modes of vibration and associated movements of the optical component 110 that may occur during use. Increasing the number of electric components 160 may increase the extent of over-actuation possible. For example, to enable twelve actuation degrees-of-freedom, the electric motor 120 may comprise about forty electric components 160.[000111] The electric motor 120 may be configured to levitate the optical component 110. An electromagnetic force acting between the magnetic components 140 and the electric components 160 may apply a lifting force 180 to the mover 130 in a substantially vertical direction (i.e. parallel to the Z axis) against the force of gravity, thereby contributing to a levitation of the optical component 110 above the stator 150. Alternatively or additionally, the stator 150 may comprise one or more permanentmagnets 170 configured to interact with the magnetic field of the plurality of magnetic components 140 of the mover 130 and thereby apply a lifting force 180 to the optical component 110. In the example of Fig. 2 only two permanent magnets 170 of the stator 150 are shown, though in practice many more may be present. In the example of Fig. 2, the two permanent magnets 170 of the stator 150 are substantially aligned with two magnetic components 140 of the mover 130. This alignment may occur, for example, once every three electric components 160. The permanent magnets 170 are nested within different coils 160. Alternatively or additionally, the permanent magnets 170 of the stator 150 may take the place of one of the plurality of electric components 160. That is, one or more permanent magnets 170 of the stator 150 may be located at positions within the electric component array that may otherwise have been occupied by an electric component 160. The permanent magnets 170 of the stator 150 may form part of a stator permanent magnet array arranged to apply a spatially substantially homogenous lifting force 180 to the optical component 110. A ratio of permanent magnets 170 to electric components 160 in the stator 150 may be about one to about three.[000112] The electric motor 120 may be configured to actuate the optical component 110 in various ways. The electric motor 120 may be configured to translate the optical component 110 in any direction, e.g. along three perpendicular axes X, Y (which is not visible in Fig. 2 as Y is perpendicular to both X and Z axes), Z. The electric motor 120 may be configured to rotate the optical component in any direction, e.g. about three perpendicular axes X, Y, Z. For example, when used to actuate a facetted field mirror device 10 and / or a facetted pupil mirror device 11 such as those shown in Fig. 1, the electric motor 120 may translate and / or rotate the mirror 10, 11 to modify an EUV radiation beam B to have a desired cross-sectional shape and a desired intensity distribution. As another example, when used to actuate a projection system PS mirror 13, 14 such as those shown in Fig. 1, the electric motor 120 may translate and / or rotate the mirror 13, 14 to modify a propagation direction of a patterned EUV radiation beam B’ such that the patterned EUV radiation beam B’ is directed onto the substrate W held by the substrate table WT. The electric motor 120 may be configured to deform the optical component 110. For example, any of the abovementioned mirrors 10, 11, 13, 14 may undergo unwanted thermal deformations when interacting with the EUV radiation which may in turn negatively affect a performance of the lithographic apparatus LA. The electric motor 120 may be configured to deform one or more of said mirrors 10, 11, 13, 14 so as to counter and / or account for the unwanted thermal deformations, and thereby improve a performance of the lithographic apparatus LA.[000113] When the electric components 160 are provided with an electric current, an electromagnetic force may be generated, which in turn may be used to actuate the optical component 110. The electromagnetic force generated may at least partially depend on which electric components 160 receive an electric current and / or the direction of the electric currents provided to the electric components 160 and / or a magnitude of the electric currents provided to the electric components 160. For example, providing a first electric component 160 with a first electric current in a first direction may increase a magnetic field strength in a first direction whilst providing a second electric component 160 with asecond electric current in a second direction may decrease the magnetic field in another direction. This imbalance in the magnetic field may cause the mover 130 to move relative to the stator 150. When the mover 130 moves relative to the stator 150, a relative positioning between the magnetic components 140 and the electric components 160 changes, which in turn effectively adjusts a magnetic field strength experienced between each of the magnetic components 140 and the electric components 160. The electric current provided to the electric components 160 may change as the mover 130 moves with respect to the stator 150. For each electric component 160, a relationship between the direction and magnitude of electric current provided to the electric component 160 and the resulting electromagnetic forces that act on the magnetic components 140 on the mover 130 may be established, e.g. by analytical and / or numerical models. By establishing such a relationship for a variety of different electric currents and electric components 160, any desired electromagnetic force and / or torque may be translated into an electric current by considering an inverse of said relationship. Since the relationship is at least partially position dependent, a distribution of electric currents throughout the electric components 160 may be adjusted according to a position of the mover 130 relative to the stator 150. One or more position sensors (not shown) may be provided to measure a position of the mover 130 relative to the stator 150. Measurements performed by the one or more position sensors may be used to at least partially determine a provision of electric current to the electric components 160 in order to actuate to optical component 110 in a desired manner.[000114] The electric motor 120 may be considered to be a lower power electric motor 120 compared to known planar motors. For example, the electric motor 120 may be configured to provide lower accelerations and / or lower speeds and / or lower ranges of motion of the optical component 110 compared to planar motors that are not used to actuate optical components. For example, a known planar motor may be configured to move a non-optical component having a mass of about 100 kg at accelerations of about 100 ms'2across distances of about 500 mm. In contrast, the electric motor 120 of the present disclosure may be configured to move an optical component having a mass of about 50 kg at accelerations of about 0.1 ms'2across distances of about 1 mm.[000115] Fig. 3 shows a flowchart of a method of actuating an optical component in accordance with the present disclosure. A first step 200 of the method comprises connecting a plurality of magnetic components to the optical component. A second step 210 of the method comprises arranging a plurality of electric components that are unconnected to the optical component. A third step 220 of the method comprises providing an electric current to the plurality of electric components to thereby interact with a magnetic field of the plurality of magnetic elements for contactless actuation of the optical component. The method may comprise an optional step of providing a greater number of electric components than a number of actuation degrees-of-freedom of the optical component. The method may comprise an optional step of levitating the optical component. The method may comprise an optional step of providing a permanent magnet configured to interact with the magnetic field of the plurality of magnetic components and thereby apply a lifting force to the optical component.[000116] The optical component 110 has thus far been described and depicted as being a mirror. However, it will be appreciated that the optical component 110 may take other forms, and that the electric motor may be used to contactlessly actuate and / or levitate any given optical component that requires actuation. The optical component may be a transmissive optical component such as, for example, a lens. The optical component may be diffractive optical component such as, for example, a diffraction grating. The optical component may be polarizing optical component such as, for example, a polarizer.[000117] The optical component 110 has thus far been described and depicted as modifying electromagnetic radiation by reflecting the electromagnetic radiation to change a propagation direction of the electromagnetic radiation. However, it will be appreciated that the optical component 110 may take other forms and may be configured to modify the electromagnetic radiation in other ways. For example, the optical component may be configured to change a propagation direction of the electromagnetic radiation via refraction. The optical component may be configured to change a phase of the electromagnetic radiation. The optical component may be configured to diffract the electromagnetic radiation. The optical component may be configured to polarize the electromagnetic radiation. The optical component may be configured to attenuate the electromagnetic radiation. The optical component may be configured to scatter the electromagnetic radiation. The optical component may be configured to disperse the electromagnetic radiation. In general, the electric motor 120 described herein may be used for contactless actuation of any optical component that is configured to modify electromagnetic radiation in any given way.[000118] Although specific reference may be made in this text to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications. Possible other applications include the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, liquidcrystal displays (LCDs), thin-film magnetic heads, etc.[000119] 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.[000120] Although specific reference may have been made above to the use of embodiments of the invention in the context of optical lithography, it will be appreciated that the invention, where the context allows, is not limited to optical lithography and may be used in other applications, for example imprint lithography. Although specific reference may have been made above to the use of embodiments of the invention in the context of EUV lithography it will be appreciated that the optical system of the present disclosure may form part of other types of lithographic apparatus such as, for example, deepultraviolet (DUV) lithographic apparatus using ultraviolet radiation having a wavelength of, for example, 365, 248, 193, 157 or 126 nm.[000121] 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.[000122] While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. The descriptions above are intended to be illustrative, not limiting. Thus it will be apparent to one skilled in the art that modifications may be made to the invention as described without departing from the scope of the claims set out below.

Claims

CLAIMS1. An optical system comprising: an optical component configured to modify electromagnetic radiation; and, an electric motor configured to actuate the optical component, wherein the electric motor comprises: a mover comprising a plurality of magnetic components connected to the optical component; and, a stator unconnected to the mover comprising a plurality of electric components configured to receive an electric current and thereby interact with a magnetic field of the plurality of magnetic elements for contactless actuation of the optical component.

2. The optical system of claim 1, wherein a number of electric components is greater than a number of actuation degrees-of-freedom of the optical component.

3. The optical system of claim 1 or claim 2, wherein the optical component is a mirror and the mover is connected to a backside of the mirror.

4. The optical system of any preceding claim, wherein the optical component is configured to modify one of: extreme ultraviolet electromagnetic radiation; deep ultraviolet electromagnetic radiation;X-ray electromagnetic radiation; or,Infrared electromagnetic radiation.

5. The optical system of any preceding claim, wherein the electric motor is a planar motor.

6. The optical system of any preceding claim, wherein the plurality of magnetic components is arranged to form a magnetic component array and the plurality of electric components is arranged to form an electric component array.

7. The optical system of claim 6, wherein the magnetic component array is a Halbach magnet array.

8. The optical system of claim 6 or claim 7, wherein the electric component array comprises a plurality of layers, and wherein the electric components of a first layer are arranged perpendicularly to the electric components of a second layer.

9. The optical system of any preceding claim, wherein the electric motor is configured to levitate the optical component.

10. The optical system of any preceding claim, wherein the stator comprises a permanent magnet configured to interact with the magnetic field of the plurality of magnetic components of the mover and thereby apply a lifting force to the optical component.

11. The optical system of claim 10, wherein the permanent magnet of the stator is nested within one of the plurality of electric components, or wherein the permanent magnet of the stator takes the place of one of the plurality of electric components.

12. The optical system of claim 10 or claim 11, wherein the permanent magnet of the stator forms part of a stator permanent magnet array arranged to apply a spatially substantially homogenous lifting force to the optical component.

13. The optical system of claim 12, wherein a ratio of permanent magnets to electric components in the stator permanent magnet array is about one to about three.

14. The optical system of any preceding claim, wherein the stator comprises about two hundred electric components per square meter or more.

15. The optical system of any preceding claim, wherein the mover comprises about six hundred magnetic components per square meter or more.

16. The optical system of any preceding claim, wherein there are between about one and about four magnetic components per electric component.

17. The optical system of any preceding claim, wherein each magnetic component occupies an area of about 0.04 m by about 0.04 m or less.

18. The optical system of any preceding claim, wherein each electric component occupies an area of about 0.04 m by about 0.12 m or less.

19. The optical system of any preceding claim, wherein a separation distance between neighboring magnetic components is about 0.04 m or less.

20. The optical system of any preceding claim, wherein a separation distance between neighboring electric components is about 5 mm or less.

21. A lithographic apparatus arranged to project a pattern from a patterning device onto a substrate comprising the optical system of any preceding claim.

22. A method of actuating an optical component comprising: connecting a plurality of magnetic components to the optical component; arranging a plurality of electric components that are unconnected to the optical component, and providing an electric current to the plurality of electric components to thereby interact with a magnetic field of the plurality of magnetic elements for contactless actuation of the optical component.

23. The method of claim 22, comprising providing a greater number of electric components than a number of actuation degrees-of-freedom of the optical component.

24. The method of claim 22 or claim 23, comprising levitating the optical component.

25. The method of any of claims 22 to 24, comprising providing a permanent magnet configured to interact with the magnetic field of the plurality of magnetic components and thereby apply a lifting force to the optical component.

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