Electromagnetic linear motor

Abstract

Embodiments relate to an electromagnetic linear motor comprising a coil assembly comprising a plurality of coils wound about respective cores, a magnet assembly having a first surface facing the coil assembly, and an opposite second surface, the magnet assembly being configured to co-operate with the coil assembly thereby generating, during use, an electromagnetic force to displace the coil assembly relative to the magnet assembly in a direction of travel, or to displace the magnet assembly relative to the coil assembly in the direction of travel, wherein the magnet assembly comprises a plurality of magnets arranged adjacent to one another in the direction of travel; and a plurality of magnetic flux density sensors, wherein the magnetic flux density sensors are configured for measuring a magnetic flux density generated by an interaction between the coil assembly and the magnet assembly.

Description

ELECTROMAGNETIC LINEAR MOTORCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of US provisional application 63 / 770,053 which was filed on 11 March 2025 and which is incorporated herein in its entirety by reference.FIELD

[0002] Embodiments relate to an electromagnetic linear motor and to a linear motor system. The present embodiments further relate to a positioning device and to an apparatus comprising an electromagnetic linear motor.BACKGROUND

[0003] A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus includes so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.

[0004] In a lithographic apparatus, but also in other apparatus like metrology apparatus and inspection apparatus, one or more actuators are used. An actuator may comprise one or more electromagnetic coils. For example linear actuators, typically Lorentz actuators, comprise a coil assembly with flat wire copper coils. A flat wire copper coil is an electromagnetic coil and is often manufactured by winding a single copper wire on and around a core with multiple windings, or turns. Generally copper wire coils are made with the intention to maximize the copper density or achieving high tolerance. A disadvantage of these high density copper wire coils is a limited power capability.

[0005] Linear motors, or linear motion motors, can provide much higher power density than Lorentz motors. This may result in higher acceleration, improved energy efficiency, and potentially improved cost. A disadvantage of linear motors, in particular for high performance applications, e.g. a reticle / mask positioning device or substrate positioning device in a lithographic apparatus, is the ‘cogging’vibrational disturbance created by the linear motor design.

[0006] Feed forward and / or feedback systems may provide a significant improvement, but are considered insufficient on their own to sufficiently reduce the ‘cogging’ vibrational disturbance, in particular for high performance applications, e.g. reticle stages in a lithographic apparatus.

[0007] Taking the reticle stage as example, force and acceleration based feedback systems rely on measured responses of the physical reticle stage components, which typically do not allow control bandwidths above a few 100 Hz due to the inherent vibrational modes within the mechanical assemblies.

[0008] Force feedback may place challenging constraints on the mechanical stage design, since force sensors must be placed in load paths that are typically critical for securing robust and high stiffness interfaces in a high precision mechatronic system.SUMMARY

[0009] It is desirable to provide an improved linear motor, or linear motor system, wherein the ‘cogging’ vibrational disturbance can be reduced.

[0010] In an aspect, an electromagnetic linear motor comprises a coil assembly comprising a plurality of coils wound about respective cores, a magnet assembly having a first surface facing the coil assembly, and an opposite second surface, the magnet assembly being configured to co-operate with the coil assembly thereby generating, during use, an electromagnetic force to displace the coil assembly relative to the magnet assembly in a direction of travel, or to displace the magnet assembly relative to the coil assembly in the direction of travel, wherein the magnet assembly comprises a plurality of permanent magnets arranged adjacent to one another in the direction of travel, a plurality of magnetic flux density sensors, wherein the magnetic flux density sensors are configured for measuring a magnetic flux density generated by an interaction between the coil assembly and the magnet assembly.

[0011] In an aspect, a linear motor system comprises an electromagnetic linear motor and a controller for controlling the electromagnetic linear motor at least partially based on a magnetic flux density measured by one or more magnetic flux density sensors.

[0012] In an aspect, a positioning device comprises an object table and an electromagnetic linear motor.

[0013] In an aspect, an apparatus comprises an electromagnetic linear motor.

[0014] In an aspect, a method for controlling an electromagnetic linear motor comprises a coil assembly and a magnet assembly, the method comprising measuring a magnetic flux density generated by an interaction between the coil assembly and the magnet assembly, and controlling the electromagnetic linear motor at least partially based on the measured magnetic flux density.BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Embodiments will now be described, by way of example only, with reference to the accompanying schematic drawings in which corresponding reference symbols indicate correspondingparts.

[0016] Figure 1 depicts a lithographic apparatus according to an embodiment.

[0017] Figure 2 schematically shows a perspective view of an embodiment of an electromagnetic linear motor according, and of an embodiment of linear motor system.

[0018] Figure 3 schematically shows a perspective view of another embodiment of an electromagnetic linear motor, and of an embodiment of linear motor system.

[0019] Figure 4 schematically shows a cross-sectional view of section A as shown in Figure 3 according to an embodiment.

[0020] Figure 5 schematically shows a partial cross-sectional view of an embodiment of an electromagnetic linear motor, wherein magnetic flux density sensors are provided on different parts thereof.

[0021] Figure 6 schematically shows a top view of an embodiment of a magnet assembly comprising a plurality of magnetic flux density sensors, and a controller.

[0022] Figure 7 schematically shows a top view of another embodiment of a magnet assembly comprising a plurality of magnetic flux density sensors.

[0023] Figure 8 schematically shows a top view of another embodiment of a magnet assembly comprising a plurality of magnetic flux density sensors.

[0024] Figures 9 to 12 schematically show cross-sectional views of an electromagnetic linear motor with different configurations of magnetic flux density sensors located on the magnet assembly according to an embodiment.

[0025] Figure 13 shows a schematic of an embodiment of an electromagnetic linear motor and a linear motor system according to an embodiment.DETAILED DESCRIPTION

[0026] Figure 1 schematically depicts a lithographic apparatus 41 according to one. The apparatus includes an illumination system (illuminator) IL configured to condition a radiation beam B (e.g. UV radiation or any other suitable radiation), a mask support structure (e.g. a mask table) MT constructed to support a patterning device (e.g. a mask) MA and connected to a first positioning device PM configured to accurately position the patterning device in accordance with certain parameters. The apparatus also includes a substrate table (e.g. a wafer table) WT or "substrate support" constructed to hold a substrate (e.g. a resist-coated wafer) W and connected to a second positioning device PW configured to accurately position the substrate in accordance with certain parameters. The apparatus further includes a projection system (e.g. a refractive projection lens system) PS configured to project a pattern imparted to the radiation beam B by patterning device MA onto a target portion C (e.g. including one or more dies) of the substrate W.

[0027] The lithographic apparatus may be of a type having one (single stage), two (dual stage) or more substrate tables or "substrate supports" (and / or two or more mask tables or "mask supports"). Insuch “multiple stage” machines the additional tables or supports may be used in parallel, or preparatory steps may be carried out on one or more tables or supports while one or more other tables or supports are being used for exposure.

[0028] The lithographic apparatus may also be of a type wherein 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 fdl a space between the projection system and the substrate. An immersion liquid may also be applied to other spaces in the lithographic apparatus, for example, between the mask and the projection system. Immersion techniques can be used to increase the numerical aperture of projection systems. The term “immersion” as used herein does not mean that a structure, such as a substrate, must be submerged in liquid, but rather only means that a liquid is located between the projection system and the substrate during exposure.

[0029] Referring to Figure 1, the illuminator IL receives a radiation beam from a radiation source SO. The source and the lithographic apparatus may be separate entities, for example when the source is an excimer laser. In such cases, the source is not considered to form part of the lithographic apparatus and the radiation beam is passed from the source SO to the illuminator IL with the aid of a beam delivery system BD including, for example, suitable directing mirrors and / or a beam expander. In other cases the source may be an integral part of the lithographic apparatus, for example when the source is a mercury lamp. The source SO and the illuminator IL, together with the beam delivery system BD if required, may be referred to as a radiation system.

[0030] The illuminator IL may include an adjuster AD configured to adjust the angular intensity distribution of the radiation beam. Generally, at least the outer and / or inner radial extent (commonly referred to as <j-outcr and o-inncr. respectively) of the intensity distribution in a pupil plane of the illuminator can be adjusted. In addition, the illuminator IL may include various other components, such as an integrator IN and a condenser CO. The illuminator may be used to condition the radiation beam, to have a desired uniformity and intensity distribution in its cross-section.

[0031] The radiation beam B is incident on the patterning device (e.g., mask MA), which is held on the mask support structure (e.g., mask table MT), and is patterned by the patterning device. Having traversed the mask MA, the radiation beam B passes through the projection system PS, which focuses the beam onto a target portion C of the substrate W. With the aid of the second positioning device PW and position sensor IF (e.g. an interferometric device, linear encoder or capacitive sensor), the substrate table WT can be moved accurately, e.g. so as to position different target portions C in the path of the radiation beam B. Similarly, the first positioning device PM and another position sensor (which is not explicitly depicted in Figure 1) can be used to accurately position the mask MA with respect to the path of the radiation beam B, e.g. after mechanical retrieval from a mask library, or during a scan. In general, movement of the mask table MT may be realized with the aid of a long-stroke module (coarse positioning) and a short-stroke module (fine positioning), which form part of the first positioning device PM. Similarly, movement of the substrate table WT or "substrate support" may be realized using a long-stroke module and a short-stroke module, which form part of the second positioner PW. In the case of a stepper (as opposed to a scanner) the mask table MT may be connected to a short-stroke actuator only, or may be fixed. Mask MA and substrate W may be aligned using mask alignment marks Ml, M2 and substrate alignment marks Pl, P2. Although the substrate alignment marks as illustrated occupy dedicated target portions, they may be located in spaces between target portions (these are known as scribe-lane alignment marks). Similarly, in situations in which more than one die is provided on the mask MA, the mask alignment marks may be located between the dies.

[0032] The first positioner PM and the second positioner PW each are provided with an actuator to move respectively the mask support MT and the substrate support WT. The actuator may be a linear actuator to provide a driving force along a single axis, for example the y-axis. Multiple linear actuators may be applied to provide driving forces along multiple axis. The actuator may be a planar actuator to provide a driving force along multiple axis. For example, the planar actuator may be arranged to move the substrate support WT in 6 degrees of freedom. The actuator may be an electro -magnetic 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 electrical current to the at least one coil. The actuator may be a moving-magnet type actuator, which has the at least one magnet coupled to the substrate support WT respectively to the mask support MT. The actuator may be a moving -coil type actuator which has the at least one coil coupled to the substrate support WT respectively to the mask support MT. The actuator may be a voice-coil actuator, a reluctance actuator, a Lorentz-actuator or a piezo-actuator, or any other suitable actuator.

[0033] Turning to Figures 2 and 3 embodiments are shown of a linear motor system 100 comprising an electromagnetic linear motor 1 and a controller 11. A linear motor, or electromagnetic linear motor 1 , also known as a linear motion motor, may have a planar or a tubular configuration. The embodiments shown are of the electromagnetic linear motor 1 being a planar electromagnetic linear motor 1. Figure 2 shows an embodiment comprising a magnet assembly 5 and a coil assembly 2. Figure 3 shows a double and mirrored configuration of the electromagnetic linear motor 1 comprising a first magnet assembly 5A and a first coil assembly 2A, and a second magnet assembly 5B and a second coil assembly 2B.

[0034] The electromagnetic linear motor 1 comprises a coil assembly 2. The coil assembly 2 comprises a plurality of coils 3 wound about respective cores 4. The cores 4 may for example be ferromagnetic cores or plastic cores, i.e. magnetic or non-magnetic.

[0035] The electromagnetic linear motor 1 comprises a magnet assembly 5. The magnet assembly 5 has a first surface 6 facing the coil assembly 2, and an opposite second surface 7. The magnet assembly 5 has a length measured in the direction of travel 8, a height measured from the first surface 6 to the second surface 7, and a width 18 measured in a direction perpendicular to the length and height.

[0036] In the double mirrored configuration of Figure 3 the second surface 7A of the first magnet assembly 5 A faces the second surface 7B of the second magnet assembly 5B.

[0037] Generally there is an air gap 22 extending between the first surface 6 of the magnet assembly 5 and the coil assembly 2. The magnet assembly 5 is configured to co-operate with the coil assembly 2 thereby generating, during use, an electromagnetic force to displace the coil assembly 2 relative to the magnet assembly 5 in a direction of travel 8, or to displace the magnet assembly 5 relative to the coil assembly 2 in the direction of travel 8. The magnet assembly 5 comprises a plurality of permanent magnets 9 arranged adjacent to one another in the direction of travel 8. In the figures the direction of travel 8 is the Y-direction.

[0038] Section A of Figure 3 is shown as schematic cross-section in Figure 4. This is to show that the electromagnetic linear motor 1 comprises a plurality of magnetic flux density sensors 10. The magnetic flux density sensors 10 are not visible in Figures 2 and 3, but are present. Figure 4 also shows the air gap 22. The magnetic flux density sensors 10 are configured for measuring a magnetic flux density generated by an interaction between the coil assembly 2 and the magnet assembly 5.

[0039] The magnetic flux density sensors 10 form a sensor array 19. The sensor array 19 may have a length 20 equal to at least a distance 21 between two cores 4. The sensors 10 may be provided on the coil assembly and / or on the magnet assembly. Regarding the coil assembly, the sensors may be arranged on one or more core and / or on one or more coils. If the sensors are for example only provided on the coils, the sensors forming the sensor array are located at a distance from each other of at least the distance between two cores of the coil assembly. The array 19 comprises one or more rows 25 of sensors 10.

[0040] The magnetic flux density sensor typically has a housing part 23 and a sensing part 24, as schematically shown in Figure 6. The housing part 23 may have dimensions that are greater than a desired distance 25 between sensing parts 24 of adjacent sensors 10. Therefore, the sensors 10 may be provided in two or more rows 25 to allow for the ‘sensing part’ of adjacent sensors 10, between alternating rows 25, to be spaced closer to each other in the direction of travel 8. This can improve the resolution of the measured magnetic flux density.

[0041] The magnetic flux density sensors 10 are in communication with, or connected to, a controller 11. The controller 11 is configured for controlling the electromagnetic linear motor 1 at least partially based on a magnetic flux density measured by one or more magnetic flux density sensors 10.

[0042] During use, or operation, of the electromagnetic linear motor 1, a magnetic flux is generated by an interaction between the coil assembly 2 and the magnet assembly 5. A method for controlling the electromagnetic linear motor 1, or the linear motor system 100, may comprises measuring a magnetic flux density generated by an interaction between the coil assembly 2 and the magnet assembly 5, and controlling the electromagnetic linear motor 1 at least partially based on the measured magnetic flux density. Controlling of the electromagnetic linear motor 1 may for example comprise adjusting a driving current through the coils 3 of the coil assembly 2 via a feedback control loop and / or a feed forward control loop. If for example the measured magnetic flux density is different from what is demanded by the controller, the driving current through the coils 3 can be changed accordingly, thereby reducing‘cogging’ vibrational disturbances. A power supply, not shown, may supply current to the coils 3 of the coil assembly 2.

[0043] The magnetic flux density sensors 10 can be placed outside the load paths that are typically critical for securing robust and high stiffness interfaces in a high precision mechatronic system. In other words, no force sensors 10 are needed that are to be incorporated into the mechanical design of, in the case of a reticle stage, the reticle stage. The electromagnetic control loop, as for example schematically shown in Figure 13, may operate at much higher bandwidths, e.g. >lkHz, than the bandwidth capability of mechanical based feedback systems, because it would be less destabilized by the mechanical vibration modes of, in case of a reticle stage, the reticle stage. Figure 13 shows magnetic flux density sensors 10, e.g. multi -axis sensors 10, arranged on the magnet assembly 5, which may be the moving or stationary part of the linear motor. High frequency changes in the magnetic flux, or magnetic flux density, as seen by these sensors 10 would translate into high frequency forces, e.g. vibrations, injected into the magnet assembly 5 and the part of the reticle stage the magnet assembly 5 is attached to. An exemplary feedback loop could be designed to adjust the current in the coils 3 of the coil assembly 2 in such a way that high frequency changes in the magnetic flux, or magnetic flux density, at the sensor locations are minimized, or at least reduced. The magnetic flux density sensors 10 may be in communication with, or connected to the coil assembly 2 via an amplifier with a closed loop control based on the measured magnetic flux density.

[0044] The controller 11 may be configured for controlling two or more coils 3 independently based on a magnetic flux density measured by the magnetic flux density sensors 10. This allows for a finer control of the individual coils 3, leading to less excitation of internal vibrating modes along the length of for example the magnet assembly 5, the length being measured in the direction of travel 8. This is because several magnetic flux density sensors 10 could be distributed along substantially the entire length of the electromagnetic linear motor 1 and used to control the magnetic field along the length of the linear motor, or at least part of the length of the linear motor. By contrast, a set of force / acceleration sensors 10 produce feedback signals that average over much of the local magnetic disturbances experienced by the linear motor and are less capable of correcting for them.

[0045] Magnetic flux density sensors 10 may be arranged on the magnet assembly 5 and / or on the coil assembly 2. In figures 4, 6 to 12 the magnetic flux density sensors 10 are arranged on the magnet assembly 5. Figure 5 schematically shows an exemplary embodiment wherein the magnetic flux density sensors 10 are arranged on the coil assembly 2, in particular on an end surface of the cores 4. Figure 5 shows two coils 3 wound around two respective cores 4. The magnetic flux density sensor may also, or instead, be provided on the wound coils 3. The magnet assembly 5, or at least part thereof, is shown as a single rectangular block.

[0046] Magnetic flux density sensors 10 may be arranged between the magnet assembly 5 and the coil assembly 2, as for example shown in figures 4 and 5. Most of the magnetic flux is concentrated in the region between the surfaces of the magnet assembly and the coil assembly, and may thereforeprovide a suitable location for measuring the magnetic flux. Said region comprises an air gap 22, also called magnetic air gap.

[0047] Magnetic flux density sensors 10 may be arranged on the first surface 6 of the magnet assembly 5, as for example shown in figures 4 to 12.

[0048] At least one magnetic flux density sensor may be provided on at least two magnets 9 of the magnet assembly 5, wherein in particular at least two sensors 10 are provided on at least two magnets 9. As can be seen in for example figures 6 to 8 the magnetic flux density sensors are arranged along the travel direction 8, here the Y-direction. As the flux variation occurs in the Y-direction, no multiple sensors need to be provided along the X-direction. For linear motor motion systems, the magnetic flux density distribution in the airgap 20 varies along the travel direction 8, or Y -direction. In order to get more accurate force estimations, based on the measured magnetic flux density, it is desired to capture the mentioned variation.

[0049] Magnetic flux density sensors 10 may be provided on at least two adjacent magnets 9 of the magnet assembly 5, as for example shown in figures 4, 6 and 7.

[0050] Magnetic flux density sensors 10 may be arranged on the second surface 7 of the magnet assembly 5 and / or on a leading end surface 12 of the magnet assembly 5 and / or on a trailing end surface 13 of the magnet assembly 5. Figures 4, 10 and 12 show embodiments, wherein magnetic flux density sensors 10 are arranged on the first surface 6, the second surface 7, the leading end surface 12 and the trailing end surface 13 of the magnet assembly 5. In figures 4 and 12 the magnetic flux density sensors 10 in particular extend entire an entire circumference of the magnet assembly 5, wherein in Figure 10 the magnetic flux density sensors 10 are arranged only on a section of the first surface 6 and second surface 7. Figure 9 shows an embodiment, wherein magnetic flux density sensors 10 are arranged only on the first surface 6 of the magnet assembly 5, in particular from a leading end to a trailing end of the magnet assembly 5. Figure 11 shows an embodiment, wherein magnetic flux density sensors 10 are arranged on the first surface 6, the leading end surface 12 and the trailing end surface 13 of the magnet assembly 5.

[0051] One of the magnet assembly 5 and the coil assembly 2 is the stator, and the other of the magnet assembly 5 and the coil assembly 2 is the mover. In the embodiments of figures 4, 6 to 12 the plurality of magnetic flux density sensors 10 are arranged on the mover, wherein the magnet assembly 5 is the mover. In the embodiment of Figure 5 the magnetic flux density sensors 10 are arranged on the coil assembly 2, which is the stator, whereas the magnet assembly 5 is the mover. The magnetic flux density sensors may be provided on the mover and / or on the stator. One of the reasons for having sensors on the mover as opposed to the stator is that the mover is usually smaller in size compared to the stator (especially for linear actuators). So in order to use fewer sensors for the system, providing the sensors on the mover may be more efficient. However, there are advantages of putting sensors on the stator, e.g. a lower number of cables that have to be supplied to the moving stage.

[0052] The plurality of magnetic flux density sensors 10 may comprise a first set of unidirectionalflux density sensors 10 for measuring a tangential or normal flux density in the direction of travel 8, and / or a second set of unidirectional flux density sensors 10 for measuring a tangential or normal flux density in a direction normal to the first surface 6 of the magnet assembly 5.

[0053] The plurality of flux density sensors 10 may comprise bidirectional flux sensors 10 for measuring a tangential or normal flux density in the direction of travel 8 and the other of the tangential or normal flux density in a direction normal to the first surface 6 of the magnet assembly 5.

[0054] At least a plurality of the magnetic flux density sensors 10 may be spaced equidistantly along at least a section of the length 16 of the magnet assembly 5 and / or the coil assembly 2, the length being measured along the direction of travel 8. For example, in Figure 4 the magnetic flux density sensors 10 are spaced equidistantly along the entire length of the first surface 6 of the magnet assembly 5. Hence, magnetic flux density sensors 10 may be arranged along substantially the entire length 16 of the magnet assembly 5.

[0055] Magnetic flux density sensors 10 may be spaced apart between 1-5 mm in the direction of travel 8, although smaller or greater spacings are possible. A spacing of 1-5 mm may provide a good trade-off between resolution and number of sensors 10 used.

[0056] Magnetic flux density sensors 10, in particular all magnetic flux density sensors 10, may be located in a middle region 17 with respect to a width 18 of the magnet assembly 5. The middle region 17 is considered the middle third part of the width.

[0057] The electromagnetic linear motor 1 may be used in a positioning device comprising an object table and the electromagnetic linear motor 1.

[0058] The electromagnetic linear motor 1 may be used in an apparatus comprising the electromagnetic linear motor 1. The apparatus may be a lithographic apparatus. The electromagnetic linear motor 1, or linear motor system 100, may however be used in apparatus other than a lithographic apparatus.

[0059] 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, such as 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. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms “wafer” or “die” herein may be considered as synonymous with the more general terms “substrate” or “target portion", respectively. The substrate referred to herein may be processed, before or after exposure, in for example a track (a tool that typically applies a layer of resist to a substrate and develops the exposed resist), a metrology apparatus and / or an inspection apparatus. Where applicable, the disclosure herein may be applied to such and other substrate processing tools. Further, the substrate may be processed more than once, for example in order to create a multi-layer IC, so that the term substrate used herein may also refer to a substrate that already contains multiple processed layers.

[0060] 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 or wafer 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.

[0061] 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 may be used in other applications, for example imprint lithography, and where the context allows, is not limited to optical lithography. In imprint lithography a topography in a patterning device defines the pattern created on a substrate. The topography of the patterning device may be pressed into a layer of resist supplied to the substrate whereupon the resist is cured by applying electromagnetic radiation, heat, pressure or a combination thereof. The patterning device is moved out of the resist leaving a pattern in it after the resist is cured.

[0062] The terms “radiation” and “beam” used herein encompass all types of electromagnetic radiation, including ultraviolet (UV) radiation (e.g. having a wavelength of or about 365, 248, 193, 157 or 126 nm) and extreme ultra-violet (EUV) radiation (e.g. having a wavelength in the range of 5-20 nm), as well as particle beams, such as ion beams or electron beams.

[0063] The term “lens”, where the context allows, may refer to any one or combination of various types of optical components, including refractive, reflective, magnetic, electromagnetic and electrostatic optical components.

[0064] The term “reticle”, “mask” or “patterning device” as employed in this text may be broadly interpreted as referring to a generic patterning device that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate. The term “light valve” can also be used in this context. Besides the classic mask (transmissive or reflective, binary, phase-shifting, hybrid, etc.), examples of other such patterning devices include a programmable mirror array and a programmable LCD array.

[0065] While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. For example, the invention may take the form of a computer program containing one or more sequences of machine-readable instructions describing a method as disclosed above, or a data storage medium (e.g. semiconductor memory, magnetic or optical disk) having such a computer program stored therein. Other aspects of the invention are set-out as in the following numbered clauses.1. An electromagnetic linear motor comprising:a coil assembly comprising a plurality of coils wound about respective cores;a magnet assembly having a first surface facing the coil assembly, and an opposite second surface, the magnet assembly being configured to co-operate with the coil assembly thereby generating,during use, an electromagnetic force to displace the coil assembly relative to the magnet assembly in a direction of travel, or to displace the magnet assembly relative to the coil assembly in the direction of travel, wherein the magnet assembly comprises a plurality of permanent magnets arranged adjacent to one another in the direction of travel; anda plurality of magnetic flux density sensors, wherein the magnetic flux density sensors are configured for measuring a magnetic flux density generated by an interaction between the coil assembly and the magnet assembly.2. The electromagnetic linear motor according to clause 1, wherein the plurality of magnetic flux density sensors is configured to be connected to a controller for controlling the electromagnetic linear motor at least partially based on a magnetic flux density measured by one or more of the magnetic flux density sensors.3. The electromagnetic linear motor according to any one of the preceding clauses, wherein one of the magnet assembly and the coil assembly is the stator, and the other of the magnet assembly and the coil assembly is the mover, wherein the plurality of magnetic flux density sensors is arranged on the mover and / or the stator.4. The electromagnetic linear motor according to any one of the preceding clauses, wherein the plurality of magnetic flux density sensors is arranged between the magnet assembly and the coil assembly.5. The electromagnetic linear motor according to anyone of the preceding clauses, wherein the plurality of magnetic flux density sensors is arranged on the first surface of the magnet assembly.6. The electromagnetic linear motor according to any one of the preceding clauses, wherein at least one of the magnetic flux density sensors is provided on at least two magnets of the magnet assembly, wherein in particular at least two of the magnetic flux density sensors are provided on at least two magnets respectively.7. The electromagnetic linear motor according to any one of the preceding clauses, wherein at least one of the magnetic flux density sensors are provided on at least two adjacent magnets of the magnet assembly.8. The electromagnetic linear motor according to any one of the preceding clauses, wherein the magnetic flux density sensors are arranged on the second surface of the magnet assembly and / or on a leading end surface of the magnet assembly and / or on a trailing end surface of the magnet assembly.9. The electromagnetic linear motor according to any one of the preceding clauses, wherein the plurality of flux density sensors comprises a first set of unidirectional flux density sensors for measuring a tangential or normal flux density in the direction of travel, and / or a second set of unidirectional flux density sensors for measuring a tangential or normal flux density in a direction normal to the first surface of the magnet assembly.10. The electromagnetic linear motor according to any one of the preceding clauses, wherein the plurality of flux density sensors comprises bidirectional flux density sensors for measuringa tangential or normal flux density in the direction of travel and the other of the tangential or normal flux density in a direction normal to the first surface of the magnet assembly.11. The electromagnetic linear motor according to any one of the preceding clauses, wherein at least three of the magnetic flux density sensors are spaced equidistantly along at least a section of the length of the magnet assembly and / or the coil assembly, the length being measured along the direction of travel.12. The electromagnetic linear motor according to any one of the preceding clauses, wherein the magnetic flux density sensors are spaced apart between 1-5 mm in the direction of travel.13. The electromagnetic linear motor according to any one of the preceding clauses, wherein magnetic flux density sensors are arranged along substantially the entire length of the magnet assembly or the entire length of the coil assembly.14. The electromagnetic linear motor according to any one of the preceding clauses, wherein the magnetic flux density sensors form a sensor array, the sensor array having a length equal to at least a distance between two cores.15. A linear motor system comprising an electromagnetic linear motor according to any one of the preceding clauses and a controller for controlling the electromagnetic linear motor at least partially based on a magnetic flux density measured by one or more of the magnetic flux density sensors.16. The linear motor system according to clause 15, wherein the sensors form a sensor array, the sensor array having a length in the direction of travel equal to at least a distance between two cores, wherein the controller is configured for controlling two or more coils independently based on a magnetic flux density measured by one or more of the magnetic flux density sensors.17. A positioning device comprising an obj ect table and an electromagnetic linear motor according to any one of clauses 1 - 14.18. An apparatus comprising an electromagnetic linear motor according to any one of the clauses 1 - 14.19. The Apparatus according to clause 18, wherein the apparatus is a lithographic apparatus, a metrology apparatus, or an inspection apparatus.20. A method for controlling an electromagnetic linear motor comprising a coil assembly and a magnet assembly, the method comprising:measuring a magnetic flux density generated by an interaction between the coil assembly and the magnet assembly, andcontrolling the electromagnetic linear motor at least partially based on the measured magnetic flux density.21. The method according to clause 20, comprising providing an electromagnetic linear motor according to any one of clauses 1-14 or a linear motor system according to clause 15 or 16.

[0066] 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 departingfrom the scope of the claims set out below.

Claims

CLAIMS1. An electromagnetic linear motor comprising:a coil assembly comprising a plurality of coils wound about respective cores;a magnet assembly having a first surface facing the coil assembly, and an opposite second surface, the magnet assembly being configured to co-operate with the coil assembly thereby generating, during use, an electromagnetic force to displace the coil assembly relative to the magnet assembly in a direction of travel, or to displace the magnet assembly relative to the coil assembly in the direction of travel, wherein the magnet assembly comprises a plurality of permanent magnets arranged adjacent to one another in the direction of travel; anda plurality of magnetic flux density sensors, wherein the magnetic flux density sensors are configured for measuring a magnetic flux density generated by an interaction between the coil assembly and the magnet assembly.

2. The electromagnetic linear motor according to claim 1, wherein the plurality of magnetic flux density sensors is configured to be connected to a controller for controlling the electromagnetic linear motor at least partially based on a magnetic flux density measured by one or more of the magnetic flux density sensors.

3. The electromagnetic linear motor according to any one of the preceding claims, wherein one of the magnet assembly and the coil assembly is the stator, and the other of the magnet assembly and the coil assembly is the mover, wherein the plurality of magnetic flux density sensors is arranged on the mover and / or the stator.

4. The electromagnetic linear motor according to any one of the preceding claims, wherein the plurality of magnetic flux density sensors is arranged between the magnet assembly and the coil assembly.

5. The electromagnetic linear motor according to anyone of the preceding claims, wherein the plurality of magnetic flux density sensors is arranged on the first surface of the magnet assembly.

6. The electromagnetic linear motor according to any one of the preceding claims, wherein at least one of the magnetic flux density sensors is provided on at least two magnets of the magnet assembly, wherein in particular at least two of the magnetic flux density sensors are provided on at least two magnets respectively.

7. The electromagnetic linear motor according to any one of the preceding claims, whereinat least one of the magnetic flux density sensors are provided on at least two adjacent magnets of the magnet assembly.

8. The electromagnetic linear motor according to any one of the preceding claims, wherein the magnetic flux density sensors are arranged on the second surface of the magnet assembly and / or on a leading end surface of the magnet assembly and / or on a trailing end surface of the magnet assembly.

9. The electromagnetic linear motor according to any one of the preceding claims, wherein magnetic flux density sensors are arranged along substantially the entire length of the magnet assembly or the entire length of the coil assembly.

10. The electromagnetic linear motor according to any one of the preceding claims, wherein the magnetic flux density sensors form a sensor array, the sensor array having a length equal to at least a distance between two cores.

11. A linear motor system comprising an electromagnetic linear motor according to any one of the preceding claims and a controller for controlling the electromagnetic linear motor at least partially based on a magnetic flux density measured by one or more of the magnetic flux density sensors.

12. The linear motor system according to claim 11, wherein the sensors form a sensor array, the sensor array having a length in the direction of travel equal to at least a distance between two cores, wherein the controller is configured for controlling two or more coils independently based on a magnetic flux density measured by one or more of the magnetic flux density sensors.

13. A positioning device comprising an obj ect table and an electromagnetic linear motor according to any one of claims 1 - 10.

14. An apparatus comprising an electromagnetic linear motor according to any one of the claims 1 - 10.

15. A method for controlling an electromagnetic linear motor comprising a coil assembly and a magnet assembly, the method comprising:measuring a magnetic flux density generated by an interaction between the coil assembly and the magnet assembly, andcontrolling the electromagnetic linear motor at least partially based on the measured magnetic flux density.

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