Superconducting magnet assembly, planar motor and lithographic apparatus

Abstract

The invention provides a superconducting (SC) magnet assembly for a planar motor, the superconducting magnet assembly comprising:-a two-dimensional (2D) array of superconducting coils, the two-dimensional array of superconducting coils being configured to generate a two-dimensional spatially alternating magnetic field; -a shield arranged on a side of the two-dimensional array of superconducting coils facing a mover of the planar motor during use, the shield being configured to slow down magnetic field variations experienced by the two-dimensional array of superconducting coils; wherein the shield comprises a layer of electrically conductive material, the layer spanning an area substantially covering the two-dimensional array of superconducting coils, the layer of electrically conductive material having a change in thickness across the area, the change in thickness across the area being associated with a geometric parameter of the two-dimensional array of superconducting coils.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to European application 22191098.7 filed on August 18, 2022, and the entire contents of this European application are incorporated herein by reference. Technical Field

[0003] The present invention relates to a superconducting magnet assembly for a planar motor, a planar motor and a lithography apparatus comprising the planar motor. Background Art

[0004] A lithographic apparatus is a machine configured to apply a desired pattern onto a substrate. A lithographic apparatus may be used, for example, in integrated circuit (IC) manufacturing. A lithographic apparatus may, for example, project a pattern (often also referred to as a "design layout" or "design") of a patterning device (e.g., a mask) onto a layer of radiation-sensitive material (resist) disposed on a substrate (e.g., a wafer).

[0005] As semiconductor manufacturing processes continue to advance, the size of circuit elements has been continuously reduced for decades, while the number of functional elements (such as transistors) per device has been steadily increasing, following a trend commonly referred to as "Moore's Law". In order to keep up with Moore's Law, the semiconductor industry is pursuing technologies that enable the creation of smaller and smaller features. In order to project a pattern onto a substrate, a lithographic apparatus can use electromagnetic radiation. The wavelength of this radiation determines the minimum size of the features patterned on the substrate. Typical wavelengths currently in use are 365 nm (i-line), 248 nm, 193 nm, and 13.5 nm. Compared to lithographic apparatus using radiation with a wavelength of, for example, 193 nm, lithographic apparatus using extreme ultraviolet (EUV) radiation with a wavelength in the range of 4 nm to 20 nm (e.g., 6.7 nm or 13.5 nm) can be used to form smaller features on a substrate.

[0006] Typically, the aim is to increase the yield and accuracy of an apparatus such as a lithographic apparatus. In order to increase the yield of a lithographic apparatus, a more powerful motor may be applied to position the substrate and the patterning device. An example of such a motor is a superconducting planar motor. A superconducting motor applies superconducting coils to generate a magnetic field. The magnetic field strength that can be achieved is typically much higher than that of a conventional planar motor, for example, which applies permanent magnets. In order to generate the magnetic field, it is important that the applied superconducting coils are maintained in a superconducting state during operation. It has been observed that in known apparatuses, there may be a risk that the coils lose their superconducting state due to losses induced in the coils. Summary of the invention

[0007] An object of the present invention is to provide a superconducting magnet assembly for a planar motor, whereby it is easier to maintain the superconducting state of the coils of the assembly.

[0008] According to one aspect of the present invention, there is provided a superconducting (SC) magnet assembly for a planar motor, the superconducting magnet assembly comprising:

[0009] - a two-dimensional (2D) array of superconducting coils, the two-dimensional array of superconducting coils being configured to generate a two-dimensional spatially alternating magnetic field;

[0010] - a shield arranged on a side of the two-dimensional array of superconducting coils that faces a mover of the planar motor during use, the shield being configured to mitigate magnetic field variations experienced by the two-dimensional array of superconducting coils;

[0011] wherein the shield comprises a layer of conductive material spanning an area substantially covering the two-dimensional array of superconducting coils, the layer of conductive material having a thickness variation across the area, the thickness variation across the area being associated with a geometric parameter of the two-dimensional array of superconducting coils.

[0012] According to another aspect of the present invention, there is provided a planar motor for use in a lithographic apparatus, the planar motor comprising the superconducting magnet assembly according to the present invention.

[0013] According to another aspect of the present invention, there is provided a lithographic apparatus comprising a planar motor according to the present invention for displacing a substrate. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:

[0015] Figure 1 Depicting a schematic overview of a lithographic apparatus according to the present invention;

[0016] Figure 2 Depiction Figure 1 a detailed view of a portion of a lithographic apparatus;

[0017] Figure 3 schematically depicting a positioning control system;

[0018] Figure 4 depicts a top view of a planar motor having a magnet assembly including permanent magnets;

[0019] Figure 5 A top view depicting a two-dimensional array of superconducting coils;

[0020] Figure 6 depicts a top view of a planar motor having a magnet assembly including a two-dimensional array of superconducting coils;

[0021] Figure 7 Depiction Figure 6 A side view of a planar motor;

[0022] Figure 8 depicts a side view of a planar motor according to the present invention;

[0023] Fig. 9 depicts a first example of a conductive material layer that may be applied as a shield herein;

[0024] Figures 10a to 10c Other examples of conductive material layers that may be applied as shields herein are depicted. DETAILED DESCRIPTION

[0025] In this document, the terms "radiation" and "beam" are used to cover all types of electromagnetic radiation, including ultraviolet radiation (e.g., having a wavelength of 365 nm, 248 nm, 193 nm, 157 nm or 126 nm) and EUV (extreme ultraviolet radiation, e.g., having a wavelength in the range of about 5 nm to 100 nm).

[0026] The terms "reticle", "mask" or "patterning device" as used herein may be broadly interpreted as referring to a general patterning device that can be used to impart a patterned cross-section to an incident radiation beam that corresponds to the pattern to be produced in a target portion of the substrate. In such a context, the term "light valve" may also be used. In addition to classical masks (transmissive or reflective, binary, phase-shifting, hybrid, etc.), other examples of such patterning devices include programmable mirror arrays and programmable LCD arrays.

[0027] Figure 1A lithographic apparatus LA is schematically depicted. The lithographic apparatus LA comprises an illumination system (also referred to as illuminator) IL configured to condition a radiation beam B (e.g. UV radiation, DUV radiation or EUV radiation), a mask support (e.g. mask stage) MT configured to support a patterning device (e.g. mask) MA and connected to a first positioner PM configured to accurately position the patterning device MA according to certain parameters, a substrate support (e.g. wafer stage) WT configured to hold a substrate (e.g. a wafer coated with resist) W and connected to a second positioner PW configured to accurately position the substrate support according to certain parameters, and a projection system (e.g. refractive projection lens system) PS configured to project a pattern imparted to the radiation beam B by the patterning device MA onto a target portion C (e.g. comprising one or more dies) of the substrate W. In an embodiment, the second positioner PW may, for example, comprise a planar motor according to the present invention.

[0028] In operation, the illumination system IL receives a radiation beam from a radiation source SO, for example via a beam delivery system BD. The illumination system IL may include various types of optical components for directing, shaping and / or controlling the radiation, such as refractive, reflective, magnetic, electromagnetic, electrostatic and / or other types of optical components, or any combination thereof. The illuminator IL may be used to condition the radiation beam B to have a desired spatial and angular intensity distribution in a cross-section of the radiation beam B at the plane of the patterning device MA.

[0029] The term "projection system" PS as used herein should be broadly interpreted as covering various types of projection systems suitable for the exposure radiation used and / or for other factors such as the use of immersion liquid or the use of a vacuum, including refractive, reflective, catadioptric, anamorphic, magnetic, electromagnetic and / or electrostatic optical systems, or any combination thereof. Any use of the term "projection lens" herein may be considered synonymous with the more general term "projection system" PS.

[0030] The lithographic apparatus LA may be of a type in which at least a portion of the substrate may be covered by a liquid having a relatively high refractive index (e.g. water) so as to fill the space between the projection system PS and the substrate W - this is also known as immersion lithography. More information on immersion techniques is given in US6952253, which is incorporated herein by reference.

[0031] The lithographic apparatus LA may also be of a type having two or more substrate supports WT (also referred to as a "dual stage"). In such a "multi-stage" machine, the substrate supports WT may be used in parallel and / or a step of preparing a substrate W for subsequent exposure may be performed on a substrate W located on one of the substrate supports WT while another substrate W on another substrate support WT is being used to expose a pattern on the other substrate W.

[0032] In addition to the substrate support WT, the lithographic apparatus LA may also comprise a measurement platform. The measurement platform is arranged to hold sensors and / or cleaning means. The sensors may be arranged to measure properties of the projection system PS or properties of the radiation beam B. The measurement platform may hold a plurality of sensors. The cleaning means may be arranged to clean a part of the lithographic apparatus, for example a part of the projection system PS or a part of a system for providing an immersion liquid. The measurement platform may be moved under the projection system PS when the substrate support WT is away from the projection system PS.

[0033] In operation, a radiation beam B is incident on a patterning device (e.g. a mask) MA held on a mask support MT and is patterned by a pattern (design layout) presented on the patterning device MA. After having traversed the patterning device MA, the radiation beam B passes through a projection system PS which focuses the beam onto a target portion C of the substrate W. With the aid of a second positioner PW and a position measurement system IF, the substrate support WT can be accurately moved, for example, in order to position different target portions C in a focused and aligned position in the path of the radiation beam B. Similarly, the first positioner PM and possibly a further position sensor (the further position sensor not being in position) are arranged in a manner such that the radiation beam B is patterned by a patterning device MA. Figure 1 The patterning device MA may be accurately positioned relative to the path of the radiation beam B using the mask alignment marks M1, M2 and the substrate alignment marks P1, P2. Although the substrate alignment marks P1, P2 occupy dedicated target portions as illustrated, they may be located in spaces between target portions. When the substrate alignment marks P1, P2 are located between target portions C, they are referred to as scribe line alignment marks.

[0034] In order to explain the present invention, a Cartesian coordinate system is used. The Cartesian coordinate system has three axes, namely the x-axis, the y-axis and the z-axis. Each of the three axes is orthogonal to the other two axes. The rotation around the x-axis is called the Rx rotation. The rotation around the y-axis is called the Ry rotation. The rotation around the z-axis is called the Rz rotation. The x-axis and the y-axis define a horizontal plane, while the z-axis is in the vertical direction. The Cartesian coordinate system does not limit the present invention and is only used for explanation. Instead, another coordinate system such as a cylindrical coordinate system can be used to explain the present invention. The orientation of the Cartesian coordinate system can be different, for example, so that the z-axis has a component along the horizontal plane.

[0035] Figure 2 Show Figure 1 A more detailed view of a part of a lithographic apparatus LA. The lithographic apparatus LA may be provided with a base frame BF, a balancing mass BM, a metrology frame MF and a vibration isolation system IS. The metrology frame MF supports the projection system PS. Additionally, the metrology frame MF may support a part of a position measurement system PMS. The metrology frame MF is supported by the base frame BF via the vibration isolation system IS. The vibration isolation system IS is arranged to avoid or reduce the propagation of vibrations from the base frame BF to the metrology frame MF.

[0036] The second positioner PW is arranged to accelerate the substrate support WT by providing a driving force between the substrate support WT and the balancing mass BM. The driving force accelerates the substrate support WT in the desired direction. Due to the conservation of momentum, a driving force is also applied to the balancing mass BM with equal magnitude but in the opposite direction to the desired direction. Typically, the mass of the balancing mass BM is significantly greater than the mass of the moving parts of the second positioner PW and the substrate support WT.

[0037] In an embodiment, the second positioner PW is supported by a balancing mass BM. For example, wherein the second positioner PW comprises a planar motor for suspending the substrate support WT above the balancing mass BM. In another embodiment, the second positioner PW is supported by a base frame BF. For example, wherein the second positioner PW comprises a linear motor and wherein the second positioner PW comprises a bearing, such as a gas bearing, for suspending the substrate support WT above the base frame BF.

[0038] The position measurement system PMS may comprise any type of sensor suitable for determining the position of the substrate support WT. The position measurement system PMS may comprise any type of sensor suitable for determining the position of the mask support MT. The sensor may be an optical sensor, such as an interferometer or an encoder. The position measurement system PMS may comprise a combined system of an interferometer and an encoder. The sensor may be another type of sensor, such as a magnetic sensor, a capacitive sensor or an inductive sensor. The position measurement system PMS may determine the position relative to a reference, such as a measurement frame MF or a projection system PS. The position measurement system PMS may determine the position of the substrate table WT and / or the mask support MT by measuring the position or by measuring the time derivative of the position, such as a velocity or an acceleration.

[0039] The position measurement system PMS may include an encoder system. For example, the encoder system is well known from the U.S. patent application US2007 / 0058173A1 filed on September 7, 2006, which is hereby incorporated by reference. The encoder system includes an encoder head, a grating and a sensor. The encoder system can receive a primary radiation beam and a secondary radiation beam. Both the primary radiation beam and the secondary radiation beam originate from the same radiation beam, i.e., the original radiation beam. At least one of the primary radiation beam and the secondary radiation beam is generated by diffracting the original radiation beam with a grating. If both the primary radiation beam and the secondary radiation beam are generated by diffracting the original radiation beam with a grating, the primary radiation beam needs to have a different diffraction order from the secondary radiation beam. Different diffraction orders are, for example, +1 order, -1 order, +2 order and -2 order. The encoder system optically combines the primary radiation beam and the secondary radiation beam into a combined radiation beam. The sensor in the encoder head determines the phase or phase difference of the combined radiation beam. The sensor generates a signal based on the phase or phase difference. The signal represents the position of the encoder head relative to the grating. One of the encoder head and the grating may be arranged on the substrate structure WT. The other of the encoder head and the grating may be arranged on the metrology frame MF or the base frame BF. For example, a plurality of encoder heads are arranged on the metrology frame MF, and the grating is arranged on the top surface of the substrate support WT. In another example, the grating is arranged on the bottom surface of the substrate support WT, and the encoder head is arranged below the substrate support WT.

[0040] The position measurement system PMS may include an interferometer system. An interferometer system is known from, for example, U.S. Pat. No. 6,020,964, filed on July 13, 1998, which is hereby incorporated by reference. The interferometer system may include a beam splitter, a reflector, a reference reflector, and a sensor. The radiation beam is divided into a reference beam and a measurement beam by a beam splitter. The measurement beam propagates to the reflector and is reflected back to the beam splitter by the reflector. The reference beam propagates to the reference reflector and is reflected back to the beam splitter by the reference reflector. At the beam splitter, the measurement beam and the reference beam are combined into a combined radiation beam. The combined radiation beam is incident on the sensor. The sensor determines the phase or frequency of the combined radiation beam. The sensor generates a signal based on the phase or frequency. The signal represents the displacement of the reflector. In an embodiment, the reflector is connected to the substrate support WT. The reference reflector may be connected to the measurement frame MF. In an embodiment, the measurement beam and the reference beam are combined into a combined radiation beam by replacing the beam splitter with an additional optical component.

[0041] The first positioner PM may comprise a long-stroke module and a short-stroke module. The short-stroke module is arranged to move the mask support MT relative to the long-stroke module with high accuracy within a small moving range. The long-stroke module is arranged to move the short-stroke module relative to the projection system PS with relatively low accuracy within a large moving range. By the combination of the long-stroke module and the short-stroke module, the first positioner PM is able to move the mask support MT relative to the projection system PS with high accuracy within a large moving range. Similarly, the second positioner PW may comprise a long-stroke module and a short-stroke module. The short-stroke module is arranged to move the substrate support WT relative to the long-stroke module with high accuracy within a small moving range. The long-stroke module is arranged to move the short-stroke module relative to the projection system PS with relatively low accuracy within a large moving range. Such a long-stroke module may, for example, comprise a planar motor according to the present invention. By the combination of the long-stroke module and the short-stroke module, the second positioner PW is able to move the substrate support WT relative to the projection system PS with high accuracy within a large moving range.

[0042] The first positioner PM and the second positioner PW are each provided with an actuator to move the mask support MT and the substrate support WT, respectively. The actuator may be a linear actuator that provides a driving force along a single axis (e.g., the y-axis). A plurality of linear actuators may be applied to provide a driving force along a plurality of axes. The actuator may be a planar actuator for providing a driving force along a plurality of axes. For example, the planar actuator may be arranged to move the substrate support WT in 6 degrees of freedom. The actuator may be an electromagnetic actuator comprising at least one coil and at least one magnet. The actuator is arranged to move the at least one coil relative to the at least one magnet by applying an electric current to the at least one coil. The actuator may be a moving magnet type actuator having at least one magnet coupled to the substrate support WT and the mask support MT, respectively. The actuator may be a moving coil type actuator having at least one coil coupled to the substrate support WT and the mask support MT, respectively. The actuator may be a voice coil actuator, a reluctance actuator, a Lorentz actuator or a piezoelectric actuator, or any other suitable actuator.

[0043] The lithographic apparatus LA comprises Figure 3 Schematically depicted in the figure is a position control system PCS. The position control system PCS includes a set point generator SP, a feedforward controller FF and a feedback controller FB. The position control system PCS provides a drive signal to an actuator ACT. The actuator ACT can be an actuator of a first positioner PM or a second positioner PW. The actuator ACT drives a device P, which may include a substrate support WT or a mask support MT. The output of the device P is a position quantity, such as a position or a velocity or an acceleration. The position quantity is measured by a position measurement system PMS. The position measurement system PMS generates a signal, which is a position signal representing the position quantity of the device P. The set point generator SP generates a signal, which is a reference signal representing the desired position quantity of the device P. For example, the reference signal represents the desired trajectory of the substrate support WT. The difference between the reference signal and the position signal forms an input for the feedback controller FB. Based on the input, the feedback controller FB provides at least a portion of the drive signal to the actuator ACT. The reference signal can form an input for the feedforward controller FF. Based on the input, the feedforward controller FF provides at least a portion of the drive signal to the actuator ACT. The feed-forward FF may exploit information about the dynamic characteristics of the device P, such as mass, stiffness, resonant modes and eigenfrequencies.

[0044] Figure 4Schematically shown is a top view of a known planar motor 200, which can be used, for example, in a lithographic apparatus for positioning a substrate support such as the substrate support WT discussed above. The planar motor 200 comprises a magnet assembly 210, which comprises a plurality of permanent magnets 210.1 configured to generate a spatially alternating magnetic field in two directions, for example in the X direction and in the Y direction. The magnets indicated by the grey squares have an opposite magnetic polarization compared to the magnets indicated by the white squares.

[0045] The planar motor 200 also includes a mover 220 having coil assemblies 220.1, 220.2, 220.3 and 220.4, which are configured to generate forces in both the X and Y directions by providing appropriate currents to the coils or coil groups 220.1, 220.2, 220.3 and 220.4 of the coil assembly. In the illustrated embodiment, each coil group includes a group of three coils that can be powered, for example, by a three-phase power supply. It should be noted that the magnet assembly 210 of the planar motor 200 can also be referred to as the stator of the motor, but the magnet assembly 210 does not need to be configured to remain stationary. In particular, the magnet assembly 210 can be arranged as a balancing block.

[0046] As an alternative to using permanent magnets to generate the magnetic field distribution required to operate a planar electromagnetic motor such as planar motor 200, it has been proposed to use superconducting (SC) coils. In such embodiments, a circular or cylindrical coil can be used to generate a magnetic field in the axial direction of the coil when the coil is supplied with an electric current. By a suitable arrangement of the coil and the supply current, a person skilled in the art can obtain a magnet assembly that, when used, generates a magnetic field similar to that generated by Figure 4 The magnetic field generated by the magnet assembly 210 shown in FIG. 2 is a two-dimensional spatial alternating magnetic field. Figure 5 A top view schematically shows a two-dimensional (2D) array 300 of superconducting (SC) coils 310 that can generate a two-dimensional spatial alternating magnetic field when powered. Arrows 320 in the superconducting coils 310 indicate the direction of current flowing in the superconducting coils 310 .

[0047] Figure 6 A top view of a planar motor 600 is schematically shown having an array of superconducting coils 610 as stators, the superconducting coils being configured to generate a two-dimensional spatial alternating magnetic field. In the arrangement shown, the direction of the current in the black coil 610.1 is opposite to the direction of the current flowing in the white coil 610.2. Thus, a two-dimensional spatial alternating magnetic field can be generated. The planar motor 600 also includes a mover 620. In the arrangement shown, the mover has a Figure 420. In particular, the mover has a coil assembly including coils or coil groups 620.1, 620.2, 620.3 and 620.4, which are configured to generate forces in both the X direction and the Y direction by providing appropriate currents to the coils or coil groups 620.1, 620.2, 620.3 and 620.4 of the coil assembly. It can also be noted that a set of currents supplied to the coil group can be configured to control the positioning of the mover relative to the array of superconducting coils 610 in six degrees of freedom.

[0048] Figure 7 Schematically shows Figure 6 A cross-sectional side view of a planar motor 600 is shown. Figure 7 , arrows 710 and 720 indicate force components of the force generated when the coil or coil group of the mover 620 is supplied with a suitable current. In particular, the generated force may, for example, include a vertical component 710 for suspending the mover 620 relative to the stator 610 (i.e., the array of superconducting coils 610) and a horizontal component 720 for displacing the mover 620 in a horizontal plane (i.e., the XY plane) relative to the stator 610. In the illustrated arrangement, the two-dimensional array of superconducting coils 610 is arranged in a cryostat 730, which is configured to maintain the superconducting coils 610 in a superconducting state.

[0049] When such a planar motor 600 is to be applied in a lithographic apparatus, it is necessary to ensure that the magnetic field generated by the two-dimensional array of superconducting coils 610 is maintained in order to achieve accurate positioning of the mover 620, which for example holds the substrate 622. It has been observed that the current-carrying coils of the mover may induce losses in the superconducting coils of the two-dimensional array of superconducting coils 610. The losses can be regarded as caused by changes in the magnetic field as experienced by the two-dimensional array of superconducting coils 610, whereby the magnetic field changes can be attributed to the movement of the current-carrying coils of the mover relative to the two-dimensional array of superconducting coils 610 of the stator or to current changes or current transients in the coils of the mover 620. Due to the losses, there is a risk that one or more coils of the two-dimensional array of superconducting coils 610 or even part of the coils undergo a transition from a superconducting state to a normal conducting state. This transition can also be referred to as a "quench". According to the present invention, measures are proposed to reduce or mitigate the risk of such a quench occurring during motor operation. In particular, according to the invention, it is proposed to arrange a shield configured to mitigate magnetic field variations experienced by the two-dimensional array of superconducting coils 610 during displacement of a mover of a planar motor relative to the two-dimensional array of superconducting coils 610 .

[0050] Figure 8A cross-sectional view of a planar motor 800 according to the invention is schematically shown, the planar motor comprising a magnet assembly 810 according to the invention. In the embodiment shown, the magnet assembly 810 comprises a two-dimensional array of superconducting (SC) coils 810.1 and a shield 810.2. In the embodiment shown, both the two-dimensional array of superconducting coils 810.1 and the shield 810.2 are arranged in a cryostat 830. Depending on the material of the layer used as the shield, the shield may also be arranged outside the cryostat, for example on an outer surface of the cryostat. In an embodiment, the two-dimensional array of superconducting coils 810.1 may be Figure 5 and Figure 6 A two-dimensional array of circular or cylindrical coils shown in . According to the present invention, the two-dimensional array of superconducting coils 810.1 of the magnet assembly 810 is configured to generate a two-dimensional spatial alternating magnetic field. Such a magnetic field can be achieved by supplying a suitable current, typically a direct current of several thousand amperes (kA), to the two-dimensional array of superconducting coils 810.1 of the magnet assembly. The magnetic field generated will generally extend beyond the top surface 830.1 of the cryostat 830 and will have a magnitude superior to the magnetic field strength generated using permanent magnets. In the illustrated embodiment, the planar motor 800 also includes a mover 840 configured to be displaced, for example, in 6 degrees of freedom relative to the magnet assembly 810. In order to perform such operation, the mover 840 may, for example, include one or more coil groups. When the coil group is supplied with current, a force will be generated due to the interaction between the current supplied to the coil of the mover and the magnetic field of the magnet assembly 810. When the mover 840 moves relative to the magnet assembly 810, or when a current change or current transient occurs in one or more coils of the mover 620, the two-dimensional array of superconducting coils 810.1 may experience a change in magnetic field strength. In this article, a shield 810.2 is applied to mitigate the side effects of such magnetic field changes. According to the present invention, the shield 810.2 is arranged on the side of the mover 840 of the two-dimensional array of superconducting coils 810.1 that faces the planar motor 800 during use. In the embodiment shown, the mover 840 is arranged above the magnet assembly 810. In such an embodiment, the shield 810.2 can therefore be arranged above the two-dimensional array of superconducting coils 810.1, that is, arranged between the two-dimensional array of superconducting coils 810.1 and the mover 840. In the embodiment shown, the shield roughly spans the entire area covered by the two-dimensional array of superconducting coils 810.1. In an alternative embodiment, the shield 810.2 can be arranged in the middle or below the two-dimensional array of superconducting coils 810.1.

[0051] According to the present invention, the shield comprises a layer of electrically conductive material, wherein the layer substantially covers an area covered by the two-dimensional array of superconducting coils 810.1 of the magnet assembly 810. According to the present invention, the layer of electrically conductive material further has a thickness variation across the area, the thickness variation being associated with geometrical parameters of the two-dimensional array of superconducting coils.

[0052] In an embodiment, the conductive material layer may be a superconducting (SC) material layer.

[0053] When the shield according to the present invention is applied in a superconducting magnet assembly of a planar motor, the adverse effects of magnetic field variations experienced by the array of superconducting coils can be mitigated. This situation can be understood as follows.

[0054] When the current-carrying coil of the mover 840 causes a change in the magnetic field close to the two-dimensional array of superconducting coils 810.1, the conductive material layer of the shield 810.2 will also experience this magnetic field change. In particular, the magnetic field change will induce a current or current distribution in the conductive material layer (optionally, a superconducting material layer) of the shield 810.2. This current or current distribution will produce a magnetic field that opposes or offsets the magnetic field change. Therefore, the two-dimensional array of superconducting coils 810.1 will experience a lower magnetic field change. Therefore, less loss will be induced in the two-dimensional array of superconducting coils, thereby reducing the risk of quenching of one or more coils of the two-dimensional array of superconducting coils. It can also be pointed out that due to the lower loss in the two-dimensional array of superconducting coils, a lower temperature of the superconducting coils can be maintained and a lower cooling cost can be obtained. For this reason, the superconducting magnet assembly can also be more robust.

[0055] By introducing thickness variations in the layer of conductive material in the shield, improved control of the induced magnetic field in the shield may be obtained.

[0056] In an embodiment, the planar motor 800 according to the present invention further includes a first power source P1 for powering the two-dimensional array of superconducting coils 810.1 and a second power source P2 for powering one or more coil groups of the mover 840 of the planar motor 800. In an embodiment, the first power source P1 may be configured, for example, to supply a direct current to the two-dimensional array of superconducting coils 810.1 of the magnet assembly 810 of the planar motor 800. The second power source P2 may be configured, for example, to provide multiple groups of alternating currents, for example, three-phase currents, to one or more coil groups of the mover 840 of the planar motor 800. The planar motor according to the present invention may also have a control unit CU for controlling the power sources P1 and P2.

[0057] In the following, more details are provided on various embodiments of shielding applied in a superconducting magnet assembly according to the invention, in particular on the applied thickness variations.

[0058] In an embodiment of the present invention, the thickness variation applied to the layer of conductive material spans a thickness between a nominal thickness and zero. In other words, the layer of conductive material need not be a continuous layer of material spanning the area covered by the two-dimensional array of superconducting coils 810.1. Rather, certain parts or portions of the shield may not have conductive material. Alternatively, certain areas or portions may be a layer of conductive material having a reduced thickness or less than the nominal thickness.

[0059] In an embodiment, the thickness variation of the conductive material layer of the shield 810.2 comprises a repetitive pattern. Such a repetitive pattern may, for example, have a periodicity associated with a geometric parameter of the two-dimensional array of superconducting coils 810.1. The periodicity may, for example, be associated with a pitch P of the two-dimensional array of superconducting coils 810.1, which refers to the distance between adjacent coils in the two-dimensional array of superconducting coils 810.1.

[0060] In an embodiment, the repeating pattern comprises a repeating pattern having geometric shapes, each geometric shape being associated with a superconducting coil in the two-dimensional array of superconducting coils 810 . 1 .

[0061] Fig. 9A patterned conductive material layer that can be applied as a shield for a magnet assembly according to the present invention is schematically shown. In the illustrated embodiment, the conductive material layer 900 includes a pattern of circles 910, each circle thus representing a circular conductive material layer. The region in the middle of the circle 910 (i.e., region 920) may be covered with a conductive material layer having different thicknesses, or may not have a conductive material. It should be noted that in an embodiment of the present invention, the conductive material layer will be arranged on a support layer or surface. In the case where the conductive material layer is made of a metal such as copper or aluminum, the support layer or surface may be, for example, a plate made of a non-conductive material. In the case where the conductive material layer is made of a superconducting material, the support layer may be, for example, a thin metal plate, such as an aluminum or copper plate. It should be noted that such a conductive plate may also be patterned, i.e., portions of the plate that are not covered by a superconducting material may be at least partially removed in order to reduce the conductivity between regions of the superconducting material (e.g., a pattern of circles 910). Alternatively, the support layer or surface may also be made of a non-conductive material. In an embodiment of the present invention, the superconducting material layer may be arranged on a layer or surface of a cryostat to serve as a shield. As an example, the patterned superconducting material layer can be arranged, for example, on the inner surface of the top plate of the cryostat, which is arranged, for example, between the magnet assembly and the mover. Alternatively or additionally, the patterned conductive material (e.g., copper or aluminum) layer can be arranged on the outer surface of the top plate of the cryostat. In the illustrated embodiment, the dashed circle 930 represents the outer diameter of the superconducting coil in the two-dimensional array of superconducting coils, for example, arranged under the shield or the conductive material 900 layer of the shield. Therefore, in the illustrated arrangement, the patterned conductive material 900 layer includes a plurality of circular shapes 910, each of which can be associated with a superconducting coil in the two-dimensional array of superconducting coils of the magnet assembly using the shield. In the illustrated arrangement, the circular shape has a diameter slightly smaller than the outer diameter of the associated superconducting coil. In the illustrated arrangement, the circular shape 910 is also aligned with the corresponding superconducting coil in the two-dimensional array of superconducting coils of the magnet assembly to which the shield is applied.

[0062] It should be understood that application of circular shape 910 as a pattern in the conductive material layer of the shield is merely one example of possible shapes and patterns that may be applied and will achieve control of the magnetic field induced in the shield due to changes in the magnetic field caused by movement of the mover relative to a magnet assembly comprising a two-dimensional array of superconducting coils (e.g., coil 810.1).

[0063] Figures 10a to 10c Some other possible patterns of geometries that may be applied to shape the layer of conductive material of the shield are schematically shown.

[0064] Fig.10aAnother example of a pattern that can be applied in a layer of conductive material of a shield for use in the present invention is schematically shown. The pattern includes a plurality of squares 1010 configured to align with a two-dimensional array of superconducting coils. Fig.10a , the dashed outline 1020 represents the outer diameter of the coils in this two-dimensional array of superconducting coils. In a similar manner as indicated above, the area outside the patterned squares 1010 may be covered with a conductive material having a different thickness than the squares 1010 or may contain no conductive material.

[0065] Fig.10b Schematically shows another example of a pattern of a conductive material layer that can be applied to a shield used in the present invention. The pattern includes a plurality of annular structures 1050 arranged to align with a two-dimensional array of superconducting coils. Fig.10b , dashed outlines 1060, 1065 represent the outer and inner diameters of the coils in this two-dimensional array of superconducting coils. In the configuration shown, the ring structure 1050 has an outer diameter that is smaller than the outer diameter of the associated superconducting coil and an inner diameter that is larger than the inner diameter of the associated superconducting coil. Alternatives are also contemplated in which the outer diameter of the ring structure 1050 corresponds approximately to the outer diameter 1060 of the superconducting coil or in which the inner diameter of the ring structure 1050 is equal to or smaller than the inner diameter 1065 of the superconducting coil.

[0066] Fig.10c A further example of a pattern of a layer of conductive material that may be applied to a shield used in the present invention is schematically shown. The pattern comprises a plurality of strips 1070 of conductive material that are aligned with the pattern of a two-dimensional array of superconducting coils of a magnet assembly. Fig.10c In FIG. 1 , the dashed outline 1020 represents the outer diameter of the coils in this two-dimensional array of superconducting coils. In the arrangement shown, the strips are arranged in a braided manner. It may be pointed out that the strips do not have to be arranged in a braided manner. Alternatively, two sets of parallel strips arranged on top of each other may also be applied.

[0067] As mentioned, according to the present invention, the conductive material layer of the shielding member applied in the magnet assembly has a thickness variation across the region, and the thickness variation is associated with the geometric parameters of the two-dimensional array of superconducting coils. As described above, the geometric parameters of the two-dimensional array of superconducting coils may include one or more of the pitch of the two-dimensional array of superconducting coils or the geometric parameters of the coils themselves (such as the inner diameter or outer diameter of the superconducting coils).

[0068] In embodiments, the thickness variation applied to the layer of conductive material may also depend on the distance between the two-dimensional array of superconducting coils and the shield, e.g. Figure 8 The thickness variation may also depend on the applied distance between the coil of the mover and the shield.

[0069] In an embodiment of the invention, the superconducting magnet assembly comprises more than one shield. In an embodiment, the superconducting magnet assembly comprises a plurality of shields, each shield having a different pattern or thickness variation applied. Such an embodiment will provide even more design freedom in shaping the induced magnetic field that counteracts the magnetic field variation.

[0070] The planar motor according to the present invention can be advantageously applied in the lithographic apparatus according to the present invention. In particular, the planar motor can be applied, for example, as the second positioner PW described above.

[0071] Although specific reference may be made herein to the use of lithographic apparatus in IC manufacturing, 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, liquid crystal displays (LCDs), thin film magnetic heads, etc.

[0072] Although specific reference may be made herein to embodiments of the invention in the context of a lithographic apparatus, embodiments of the invention may be used in other devices. Embodiments of the invention may form part of a mask inspection apparatus, a metrology apparatus, or any apparatus that measures or processes an object such as a wafer (or other substrate) or a mask (or other pattern forming device). These apparatuses may generally be referred to as lithographic tools. Such lithographic tools may use vacuum conditions or ambient (non-vacuum) conditions.

[0073] Although the above may have made specific reference to the use of embodiments of the invention in the context of optical lithography, it will be appreciated that the invention is not limited to optical lithography and may be used in other applications, such as imprint lithography, where the context permits.

[0074] Where context permits, embodiments of the present invention may be implemented in hardware, firmware, software, or any combination thereof. Embodiments of the present invention may also be implemented as instructions stored on a machine-readable medium, which may be read and executed by one or more processors. A machine-readable medium may include any mechanism for storing or transmitting information in a form that can be read by a machine (e.g., a computing device). For example, a machine-readable medium may include a read-only memory (ROM); a random access memory (RAM); a magnetic storage medium; an optical storage medium; a flash memory device; an electrical, optical, acoustic, or other form of propagation signal (e.g., a carrier wave, an infrared signal, a digital signal, etc.); etc. In addition, firmware, software, routines, instructions may be described herein as performing certain actions. However, it should be understood that such descriptions are for convenience only, and such actions are actually caused by a computing device, a processor, a controller, or other devices that execute firmware, software, routines, instructions, etc., and when performing such operations, an actuator or other device may be caused to interact with the physical world.

[0075] Although specific embodiments of the present invention have been described above, it will be appreciated that the present invention may be practiced in other ways than those described. The above description is intended to be illustrative rather than restrictive. Therefore, it will be appreciated by those skilled in the art that modifications may be made to the described invention without departing from the scope of the claims set forth below.

Claims

1. A superconducting (SC) magnet assembly for a planar motor, the superconducting magnet assembly comprising: a two-dimensional (2D) array of superconducting coils configured to generate a two-dimensional spatially alternating magnetic field; a shield disposed on a side of the two-dimensional array of superconducting coils that faces a mover of the planar motor during use, the shield being configured to mitigate magnetic field variations experienced by the two-dimensional array of superconducting coils; wherein the shield comprises a layer of conductive material spanning an area substantially covering the two-dimensional array of superconducting coils, the layer of conductive material having a thickness variation across the area, the thickness variation across the area being associated with a geometric parameter of the two-dimensional array of superconducting coils.

2. The superconducting (SC) magnet assembly according to claim 1, wherein: The magnetic field variations are caused by displacement of the mover of the planar motor relative to the two-dimensional array of superconducting coils or by current variations or current transients in a coil or coil group of the mover.

3. A superconducting (SC) magnet assembly according to claim 1 or 2, wherein: The thickness variation spans a thickness between a nominal value and zero.

4. A superconducting (SC) magnet assembly according to claim 1, 2 or 3, wherein: The variation in thickness of the layer of conductive material comprises a repeating pattern.

5. The superconducting (SC) magnet assembly according to claim 4, wherein: The repeating pattern includes a repeating pattern having geometric shapes, each geometric shape being associated with a superconducting coil in the array of superconducting coils.

6. The superconducting (SC) magnet assembly according to claim 5, wherein: The geometric shapes include a circle or a square.

7. A superconducting (SC) magnet assembly according to any one of the preceding claims, wherein: The conductive material layer includes a superconducting material layer.

8. The superconducting (SC) magnet assembly according to claim 1 or 2, wherein: The layer of conductive material comprises a plurality of conductive or superconducting (SC) strips arranged in a weave pattern.

9. A superconducting (SC) magnet assembly according to any one of the preceding claims, wherein: The geometric parameters include a magnetic pitch of the array of superconducting coils, an inner diameter of a coil in the array of superconducting coils, or an outer diameter of a coil in the array of superconducting coils.

10. A superconducting (SC) magnet assembly according to any one of the preceding claims, wherein: The layer of conductive material is disposed on a non-conductive layer or surface.

11. A superconducting (SC) magnet assembly according to any one of the preceding claims, wherein: The conductive material layer comprises a superconducting (SC) material layer, and wherein the superconducting (SC) material layer is arranged on a non-conductive layer or a conductive layer, or on a non-conductive surface or a conductive surface.

12. A superconducting magnet assembly according to any one of the preceding claims, further comprising a cryostat, the two-dimensional array of superconducting coils being arranged inside the cryostat.

13. A superconducting magnet assembly according to any one of the preceding claims, wherein: The shield is disposed inside the cryostat.

14. The superconducting magnet assembly according to claim 13, wherein: The cryostat is configured to cool the two-dimensional array of superconducting coils and the shield to a superconducting state during operation.

15. The superconducting magnet assembly according to claim 13 or 14, wherein: The shield is arranged on an inner surface or an outer surface of the cryostat arranged between the two-dimensional array of superconducting coils and the mover.

16. The superconducting magnet assembly according to claim 13 or 14, wherein: The shield is disposed at least partially intermediate the two-dimensional array of superconducting coils.

17. A planar motor for use in a lithographic apparatus, the planar motor comprising a superconducting magnet assembly according to any one of the preceding claims and a mover, the mover comprising one or more coil groups configured to cooperate with the magnetic field of the two-dimensional array of superconducting coils of the superconducting magnet assembly during use.

18. The planar motor of claim 17, comprising a first power source for powering the two-dimensional array of superconducting coils and a second power source for powering the one or more coil groups of the mover.

19. A lithographic apparatus comprising a planar motor according to any one of claims 17 to 18 for positioning a substrate.

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