Abstract:
A Lorentz actuator provides a force between a first part and a second part of the apparatus, comprising a main magnet system, attached to a first part of the apparatus and providing a first magnetic field; a subsidiary magnet system, attached to the first part and arranged in a Halbach configuration, providing a second magnetic field; and an electrically conductive element attached to a second part of the apparatus and arranged so as to produce a force between the first and second parts of the apparatus by interaction of an electric current carried by the electrically conductive element and the combination of the first and second magnetic fields.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a lithographic projection apparatus.  
           [0003]    2. Description of the Related Art  
           [0004]    The term “patterning device” as here employed should be broadly interpreted as referring to 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. Generally, the pattern will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit or other device (see below). An example of such a patterning device is a mask. The concept of a mask is well known in lithography, and it includes mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. Placement of such a mask in the radiation beam causes selective transmission (in the case of a transmissive mask) or reflection (in the case of a reflective mask) of the radiation impinging on the mask, according to the pattern on the mask. In the case of a mask, the support structure will generally be a mask table, which ensures that the mask can be held at a desired position in the incoming radiation beam, and that it can be moved relative to the beam if so desired.  
           [0005]    Another example of a pattering device is a programmable mirror array. One example of such an array is a matrix-addressable surface having a viscoelastic control layer and a reflective surface. The basic principle behind such an apparatus is that, for example, addressed areas of the reflective surface reflect incident light as diffracted light, whereas unaddressed areas reflect incident light as undiffracted light. Using an appropriate filter, the undiffracted light can be filtered out of the reflected beam, leaving only the diffracted light behind. In this manner, the beam becomes patterned according to the addressing pattern of the matrix-addressable surface. An alternative embodiment of a programmable mirror array employs a matrix arrangement of tiny mirrors, each of which can be individually tilted about an axis by applying a suitable localized electric field, or by employing piezoelectric actuators. Once again, the mirrors are matrix-addressable, such that addressed mirrors will reflect an incoming radiation beam in a different direction to unaddressed mirrors. In this manner, the reflected beam is patterned according to the addressing pattern of the matrix-addressable mirrors. The required matrix addressing can be performed using suitable electronics. In both of the situations described hereabove, the patterning device can comprise one or more programmable mirror arrays. More information on mirror arrays as here referred to can be seen, for example, from United States Patents U.S. Pat. Nos. 5,296,891 and 5,523,193, and PCT publications WO 98/38597 and WO 98/33096. In the case of a programmable mirror array, the support structure may be embodied as a frame or table, for example, which may be fixed or movable as required.  
           [0006]    Another example of a pattering device is a programmable LCD array. An example of such a construction is given in U.S. Pat. No. 5,229,872. As above, the support structure in this case may be embodied as a frame or table, for example, which may be fixed or movable as required.  
           [0007]    For purposes of simplicity, the rest of this text may, at certain locations, specifically direct itself to examples involving a mask and mask table. However, the general principles discussed in such instances should be seen in the broader context of the patterning device as hereabove set forth.  
           [0008]    Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the patterning device may generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising one or more dies) on a substrate (silicon wafer) that has been coated with a layer of radiation-sensitive material (resist). In general, a single wafer will contain a whole network of adjacent target portions that are successively irradiated via the projection system, one at a time. In current apparatus, employing patterning by a mask on a mask table, a distinction can be made between two different types of machine. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion at once. Such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatus, commonly referred to as a step-and-scan apparatus, each target portion is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction (the “scanning” direction) while synchronously scanning the substrate table parallel or anti-parallel to this direction. Since, in general, the projection system will have a magnification factor M (generally&lt;1), the speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. More information with regard to lithographic devices as here described can be seen, for example, from U.S. Pat. No. 6,046,792.  
           [0009]    In a known manufacturing process using a lithographic projection apparatus, a pattern (e.g. in a mask) is imaged onto a substrate that is at least partially covered by a layer of radiation-sensitive material (resist). Prior to this imaging, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book “Microchip Fabrication: A Practical Guide to Semiconductor Processing”, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4.  
           [0010]    For the sake of simplicity, the projection system may hereinafter be referred to as the “lens.” However, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, and catadioptric systems, for example. The radiation system may also include components operating according to any of these design types for directing, shaping or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a “lens”. Further, the lithographic apparatus may be of a type having two or more substrate tables (and/or two or more mask tables). In such “multiple stage” devices the additional tables may be used in parallel or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposures. Dual stage lithographic apparatus are described, for example, in U.S. Pat. No. 5,969,441 and WO 98/40791.  
           [0011]    In an apparatus as described above it is very important that the substrate table, which holds the substrate, and the support structure which may hold the mask is positioned with a very high precision. It is therefore proposed, for example, in U.S. Pat. No. 5,120,034 to use a positioning device comprising a carriage displaceable in at least two coordinate directions with respect to a base and intended for displacement of a table which is displaceable with respect to the carriage in the two coordinate directions wherein the table is coupled to the carriage solely by Lorentz forces of the magnet systems and coil systems in the two coordinate directions.  
           [0012]    A critical issue in the use of Lorentz actuators in lithographic projection apparatus is the amount of heat generated in use by the coils of the Lorentz actuator, especially any heat that is produced by a Lorentz actuator mounted very close to the substrate table or the support structure. Any heat produced must be dissipated in order to prevent temperature changes in the substrate table and the support structure that could lead to losses in the accuracy of the apparatus.  
           [0013]    Lorentz actuators include an electrically conductive element, such as a coil, and a magnet assembly. The magnet assembly produces a magnetic field which interacts with a current flowing in the electrically conductive element to produce a Lorentz force between the electrically conductive element and the magnet assembly in a direction perpendicular to the direction of the current flow and the magnetic field at that point. Typically the magnet assembly is comprised of at least one magnet on either side of the electrically conductive element to produce an approximately uniform magnetic field around the electrical conductor. Lorentz actuators do not include any iron in the coils. The magnet assembly of a Lorentz actuator does, however, comprises a back iron, formed from a material with high magnetic saturation, located on the outward side of the magnets. The back iron is typically required to be large to prevent saturation and it constitutes a substantial part of the mass of the actuator and is a source of loss of efficiency in the motor.  
           [0014]    It is an aspect of the present invention to provide a lithographic projection apparatus with a Lorentz actuator having a back iron of reduced mass but no loss of performance of the actuator.  
           [0015]    This and other aspects are achieved according to the invention in a lithographic apparatus including a radiation system constructed and arranged to supply a projection beam of radiation; a support structure constructed and arranged to support a patterning device, the patterning device constructed and arranged to pattern the projection beam according to a desired pattern; a substrate table that holds a substrate; a projection system constructed and arranged to project the patterned beam onto a target portion of the substrate; and a motor comprising a long stoke motor constructed and arranged to produce a force between a first part of the motor and a machine frame of the apparatus over a long range of movement in at least one direction and a Lorentz actuator constructed and arranged to produce a force between the first part and a second part of the motor over a short range of movement in at least one direction, the second part being connected to the support structure or the substrate; a main magnet system, attached to one of the first and second part of the motor, constructed and arranged to provide a first magnetic field, substantially perpendicular to the direction of the force; an electrically conductive element attached to the other of the one part of the motor and arranged so as to produce the force by interaction of an electric current carried by the electrically conductive element and the first magnetic field; and a subsidiary magnet system, attached to the one part of the motor and arranged in a Halbach configuration, providing a second magnetic field substantially perpendicular to the first magnetic field.  
           [0016]    This arrangement is advantageous since it reduces the size of the back iron required to prevent saturation. The mass of back iron in the actuator will be reduced and will further improve the efficiency of the actuator as the moving mass of the actuator is decreased the force required to produce a given acceleration is decreased. It also increases the k-factor of the actuator (also referred to as the motor constant) which means that the force produced for a given flow of current through the coils is increased. The combination of the effects results in a significant reduction in the current to effect a given acceleration which in turn reduces the amount of heat generated by the coils.  
           [0017]    In an embodiment of the present invention, the subsidiary magnet system comprises a first and a second subsidiary magnet and the main magnet system comprises a main magnet, at least a part of which is located between the two subsidiary magnets. The magnets are oriented such that the magnetic polarization of the first subsidiary magnet is substantially anti-parallel to that of the second subsidiary magnet and the magnetic polarization of the main magnet is substantially perpendicular to those of the two subsidiary magnets.  
           [0018]    In a yet further embodiment of the present invention, the Lorentz actuator comprises two magnet assemblies. The first magnet assembly comprises a first main magnet system sub-assembly and a first subsidiary magnet system sub-assembly and the second magnet assembly comprises a second main magnet system sub-assembly and a second subsidiary magnet system sub-assembly. At least a part of the electrically conductive element is located between the first and second magnet assemblies.  
           [0019]    In a yet further embodiment of the present invention, each magnet assembly comprises first and second main magnets, oriented such that their magnetic polarizations are substantially anti-parallel to each other. The magnet assemblies further comprise first, second and third subsidiary magnets arranged such that at least a portion of the first main magnet is located between the first and second subsidiary magnets and at least a portion of the second main magnet is located between the second and third subsidiary magnets. The electrically conductive element comprises a first part, located between the first main magnet of the first magnet assembly and the first main magnet of the second assembly, and a second part, located between the second main magnet of the first magnet assembly and the second main magnet of the second magnet assembly. The electrically conductive element is arranged such that, when it conducts electric current, the direction of the electric current in the first part is substantially anti-parallel to the direction of the electric current in the second part.  
           [0020]    The invention also relates to a motor for use in a lithographic projection apparatus comprising a long stoke motor for producing a force between a first part of the motor and a machine frame of the lithographic projection apparatus over a long range of movement in at least one direction and a Lorentz actuator for producing a force between the first part and a second part of the motor over a short range of movement in at least one direction, wherein the Lorentz actuator includes a main magnet system, attached to one of the first and second part of the motor, constructed and arranged to provide a first magnetic field, substantially perpendicular to the direction of the force; and an electrically conductive element attached to the other of the one part of the motor and arranged so as to produce the force by interaction of an electric current carried by the electrically conductive element and the first magnetic field; and a subsidiary magnet system, attached to the one part of the motor and arranged in a Halbach configuration, constructed and arranged to provide a second magnetic field substantially perpendicular to the first magnetic field.  
           [0021]    Although specific reference may be made in this text to the use of the apparatus according to the invention in the manufacture of ICs, it should be explicitly understood that such an apparatus has many other possible applications. For example, it may be employed in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal display panels, thin-film magnetic heads, etc. One of ordinary skill will appreciate that, in the context of such alternative applications, any use of the terms “reticle”, “wafer” or “die” in this text should be considered as being replaced by the more general terms “mask”, “substrate” and “target portion”, respectively.  
           [0022]    In the present document, the terms “radiation” and “beam” are used to encompass all types of electromagnetic radiation, including ultraviolet radiation (e.g. with a wavelength of 365, 248, 193, 157 or 126 nm) and EUV (extreme ultra-violet radiation, e.g. having a wavelength in the range 5-20 nm), as well as particle beams, such as ion beams or electron beams. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]    Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings in which:  
         [0024]    [0024]FIG. 1 depicts a lithographic projection apparatus;  
         [0025]    [0025]FIG. 2 a  shows, in cross-section, the configuration of an actuator according to the invention;  
         [0026]    [0026]FIG. 2 b  shows the magnetic field lines calculated for the configuration of FIG. 2 a;    
         [0027]    [0027]FIG. 3 shows the configuration of FIG. 2 a  in perspective; and  
         [0028]    [0028]FIG. 4 shows, in cross-section, the configuration of an alternative actuator according to the present invention. 
     
    
       [0029]    In the Figures, corresponding reference symbols indicate corresponding parts.  
       DETAILED DESCRIPTION  
       [0030]    [0030]FIG. 1 schematically depicts a lithographic projection apparatus according to a particular embodiment of the invention. The apparatus includes a radiation system Ex, IL that supplies a projection beam PB of radiation (e.g. UV or EUV radiation). In this embodiment, the radiation system also comprises a radiation source LA; a first object table (mask table) MT provided with a mask holder for holding a mask MA (e.g. a reticle), and connected to a first positioning device M 1  M 2  to accurately position the mask with respect to a projection system PL; a second object table (substrate table) WT provided with a substrate holder for holding a substrate W (e.g. a resist-coated silicon wafer), and connected to a second position device P 1 , P 2  to accurately position the substrate with respect to the projection system PL; the projection system (“lens”) PL (e.g. a refractive or catadioptric system, a mirror group or an array of field deflectors) to image an irradiated portion of the mask MA onto a target portion C (e.g. comprising one or more dies) of the substrate W. The projection system PL is supported on a reference frame RF. As here depicted, the apparatus is of a transmissive type (i.e. has a transmissive mask). However, in general, it may also be of a reflective type, for example (with a reflective mask). Alternatively, the apparatus may employ another kind of patterning device, such as a programmable mirror array of a type as referred to above.  
         [0031]    The source LA (e.g. an excimer laser, an undulator or wiggler provided around the path of an electron beam in a storage ring or synchrotron, a laser-produced plasma source, a discharge source or an electron or ion beam source) produces a beam PB of radiation. The beam PB is fed into an illumination system (illuminator) IL, either directly or after having traversed a conditioner, such as a beam expander Ex, for example. The illuminator IL may comprise an adjusting device AM for setting the outer and/or inner radial extent (commonly referred to as σ-outer and σ-inner, respectively) of the intensity distribution in the beam. In addition, it will generally comprise various other components, such as an integrator IN and a condenser CO. In this way, the beam PB impinging on the mask MA has a desired uniformity and intensity distribution in its cross-section.  
         [0032]    It should be noted with regard to FIG. 1 that the source LA may be within the housing of the lithographic projection apparatus (as is often the case when the source LA is a mercury lamp, for example), but that it may also be remote from the lithographic projection apparatus, the radiation beam which it produces being led into the apparatus (e.g. with the aid of suitable directing mirrors). The latter scenario is often the case when the source LA is an excimer laser. The current invention encompasses both of these scenarios.  
         [0033]    The beam PB subsequently intercepts the mask MA, which is held on a mask table MT. Having traversed the mask MA, the beam PB passes through the lens PL, which focuses the beam PB onto a target portion C of the substrate W. With the aid of the second positioning device P 1 , P 2  (and interferometer IF), the substrate table WT can be moved accurately, e.g. so as to position different target portions C in the path of the beam PB. Similarly, the first positioning device M 1 , M 2  can be used to accurately position the mask MA with respect to the path of the beam PB, e.g. after mechanical retrieval of the mask MA from a mask library, or during a scan. In general, movement of the object tables MT, WT will be realized with the aid of a long-stroke drive module (coarse positioning) and a short-stroke module (fine positioning). However, in the case of a wafer stepper (as opposed to a step-and-scan apparatus) the mask table MT may just be connected to a short stroke actuator, or may be fixed. A long stroke drive module LS is moved by a planar motor, for example, as described in WO 01/18944. A magnet plate MP is fixed to a machine frame of the lithographic projection apparatus and the long stroke drive module LS is provided with a first set of coils SP 1  that generate a force when a current is sent through the coils SP 1 . The force can levitate and move the long stroke drive module LS along the magnet plate MP. The substrate table WT is moved with respect to the long stroke module drive LS with the aid of Lorenz actuators comprising a second coil SP 2  and a magnet MS. If a current is sent through the secondary coil SP 2  a force can be generated between the long stroke drive module LS and magnets MS provided to the substrate table WT to finely position the substrate table WT. The substrate table WT can be moved up to six degrees of freedom (X, Y, Z, Rz, Ry and Rx) with respect to the long stroke drive module LS.  
         [0034]    The depicted apparatus can be used in two different modes:  
         [0035]    1. In step mode, the mask table MT is kept essentially stationary, and an entire mask image is projected at once (i.e. a single “flash”) onto a target portion C. The substrate table WT is then shifted in the x and/or y directions so that a different target portion C can be irradiated by the beam PB;  
         [0036]    2. In scan mode, essentially the same scenario applies, except that a given target portion C is not exposed in a single “flash”. Instead, the mask table MT is movable in a given direction (the so-called “scan direction”, e.g. the y direction) with a speed v, so that the projection beam PB is caused to scan over a mask image. Concurrently, the substrate table WT is simultaneously moved in the same or opposite direction at a speed V=Mv, in which M is the magnification of the lens PL (typically, M=¼ or ⅕). In this manner, a relatively large target portion C can be exposed, without having to compromise on resolution.  
         [0037]    [0037]FIG. 2 a  shows a cross section of the actuator according to a preferred embodiment of the present invention. The same actuator is shown, in perspective, in FIG. 3. In use, the actuator generates a force in a first direction (or its opposite) which may be used to drive a short-stroke drive module for positioning of the mask table MT or the substrate table WT. In FIG. 2 a  this first direction is a horizontal direction within the plane of the Figure. The actuator comprises a first magnet sub-assembly  1 , a second magnet sub-assembly  11  (MS in FIG. 1) and a coil  21  (SP 2  in FIG. 1). The first and second magnet subassemblies  1 ,  11  define a space between them in a second direction, perpendicular to the first direction. The coil  21 ,  22  is located in this space.  
         [0038]    The combination of the first and second magnet sub-assemblies  1 ,  11  forms a magnet assembly that is mounted on the substrate table WT or the mask table MT to be driven by the actuator. The coil  21  is mounted on the long-stroke module (not shown) or, in the case of a mask table MT of a wafer stepper apparatus that does not have a long-stroke module (as described above), on a fixed portion of the apparatus.  
         [0039]    Although the magnet assembly may alternatively be mounted on the long-stroke module and the coil on the substrate table or the mask table, the present configuration is preferred since it facilitates the provision of power and cooling to the coil.  
         [0040]    The first magnet sub-assembly  1  is composed of a first main magnet  2 , a second main magnet  7 , a first subsidiary magnet  5 , a second subsidiary magnet  4  and a third subsidiary magnet  3 . The second magnet sub-assembly  11  correspondingly has a first main magnet  12 , a second main magnet  17 , a first subsidiary magnet  15 , a second subsidiary magnet  14  and a third subsidiary magnet  13 . In a preferred arrangement, the main magnets are permanent magnets consisting of Ni-coated Vacodym722HR having a magnetic remanence of B r =1.47 Tesla (T) and the subsidiary magnets consist of Ni-coated Vacodym362TP with a magnetic remanence of B r =1.30 Tesla (T) and high coercitive field strength, as produced by Vacuumschmelze GmbH.  
         [0041]    Each of the magnet sub-assemblies is arranged such that the component magnets are adjacent to one another in the first direction, as defined above. In this arrangement, a subsidiary magnet is located on either side of each of the main magnets. For example, the first main magnet  2  of the first magnet sub-assembly  1  is located between the first subsidiary magnet  5  and the second subsidiary magnet  4  of the first magnet sub-assembly. Similarly, the second main magnet  7  of the first magnet sub-assembly  1  is located between the second subsidiary magnet  4  and the third subsidiary magnet  3 . The second magnet subassembly is arranged in a similar fashion such that, in the complete magnet assembly, the corresponding magnets in the two sub-assemblies  1 ,  11  are facing each other.  
         [0042]    Each of the magnet sub-assemblies has a back iron  6 ,  16 , respectively. In each subassembly the back iron is located on the other side of the magnets to the space that is defined between the magnet sub-assemblies. The back iron adjoins each of the main and each of the subsidiary magnets in the sub-assembly and preferably entirely covers the surfaces of the magnets. The back iron also preferably tapers, at least partly, in the direction away from the space between the magnet sub-assemblies. The back iron is preferably formed from CoFe.  
         [0043]    The coil  21  (SP 2  in FIG. 1), having two sides  21   a  and  21   b  (as shown in FIG. 2 a ), is located between the magnet sub-assemblies  1 ,  11  (MS in FIG. 1) and is comprised of orthocyclic windings. The coil is arranged such that, where it is located between the first and second magnet sub-assemblies, the wires that it is composed from are perpendicular to both the first and second directions defined above. As shown in FIG. 2 a , the wires are oriented in a direction perpendicular to the plane of the Figure.  
         [0044]    The magnets in the first and second magnet sub-assemblies  1 ,  11  are oriented such that the magnetic polarizations of the first main magnet  2  in the first magnet subassembly is parallel to the magnetic polarization of the first main magnet  12  of the second magnet sub-assembly and in a direction perpendicular to the first direction, defined above, namely perpendicular to the direction of the force generated by the actuator. The second main magnets  7 ,  17  are oriented such that their magnetic polarizations are parallel to one another and anti-parallel to the magnetic polarizations of the first main magnets  2 ,  12 .  
         [0045]    The subsidiary magnets of the magnet sub-assemblies are arranged in the so-called Halbach configuration. The subsidiary magnets are oriented such that their magnetic polarizations are perpendicular to those of the main magnets. As stated above, each of the main magnets is located between two subsidiary magnets of the magnet sub-assembly. These pairs of subsidiary magnets are oriented such that their magnetic polarizations are anti-parallel to one another. Furthermore, the pairs of corresponding subsidiary magnets in the first and second magnet sub-assemblies (which, as described above, face each other across the separation between the two magnet sub-assemblies), for example the first subsidiary magnet  5  of the first magnet sub-assembly and the first subsidiary magnet  15  of the second magnet sub-assembly, are also arranged such that their magnetic polarizations are anti-parallel to one another.  
         [0046]    The resulting magnetic field that this configuration produces, as shown in FIG. 2 b , regions of approximately uniform magnetic field between the two pairs of main magnets. The two sides of the coil  21   a ,  21   b  are located in these regions. When an electric current is passed through the coil, the current flow through the two sides of the coil is in opposite directions. Therefore, since the directions of magnetic field in the two regions in which the two sides of the coil  21   a ,  21   b  are located are also opposite, the force exerted on the two sides of the coil is in the same direction (perpendicular to both the magnetic field and the current flow).  
         [0047]    [0047]FIG. 4 shows, in cross-section, the configuration of an alternative actuator of the present invention. This configuration comprises two coils  81   a ,  81   b  and  82   a ,  82   b . Therefore the structure of the first and second magnet sub-assemblies  51 ,  71  is different to those depicted in FIG. 2 a . Each magnet sub-assembly is comprised of a first main magnet  57 ,  77 , a second main magnet  55 ,  75 , a third main magnet  53 ,  73 , first subsidiary magnet  56 ,  76 , second subsidiary magnet  54 ,  74  and, as in the previous configuration, a back iron  52 ,  72 .  
         [0048]    The main magnets are oriented such that the magnetic polarization of the first main magnet  57  of the first magnet sub-assembly is parallel to that of the first main magnet  77  of the second magnet sub-assembly, the magnetic polarization of the second main magnet  55  of the first magnet sub-assembly is parallel to that of the second main magnet  75  of the second magnet sub-assembly, and the magnetic polarization of the third main magnet  53  of the first magnet sub-assembly is parallel to that of the third main magnet  73  of the second magnet sub-assembly. As in the configuration of FIG. 2 a , all of the main magnets are oriented such that their magnetic polarizations are perpendicular to the direction of the force produced by the actuator. In addition, the first main magnets  57 ,  77  and the third main magnets  53 ,  73  are oriented such that their magnetic polarizations are anti-parallel to those of the second main magnets  55 ,  75 .  
         [0049]    The first subsidiary magnets  56 ,  76  are located between the first main magnets  57 ,  77  and the second main magnets  55 ,  75  and the second subsidiary magnets  54 ,  74  are located between the second main magnets  55 ,  75  and the third main magnets  53 ,  73 . Consequently, the second main magnets are located between the subsidiary magnets of each magnet sub-assembly.  
         [0050]    The subsidiary magnets of the first magnet sub-assembly  51  are oriented such that their magnetic polarizations are mutually anti-parallel and perpendicular to the magnetic polarization of the second main magnet  55 . The subsidiary magnets of the second magnet sub-assembly are oriented in corresponding fashion and such that the magnetic polarization of the first subsidiary magnet  56  of the first magnet sub-assembly  51  is anti-parallel to the magnetic polarization of the first subsidiary magnet  76  of the second magnet sub-assembly  71  and the magnetic polarization of the second subsidiary magnet  54  of the first magnet sub-assembly  51  is anti-parallel to the magnetic polarization of the second subsidiary magnet  74  of the second magnet sub-assembly  71 . The resulting magnetic field between the main magnets of the two magnet sub-assemblies is approximately uniform.  
         [0051]    The coils are arranged such that the first portion  81   a  of the first coil is located between the third main magnets  53 ,  73 , the second portion  81   b  of the first coil and the first portion  82   a  of the second coil are located between the second main magnets  55 ,  75  and the second portion  82   b  of the second coil is located between the first main magnets  57 ,  77 . When an electric current is passed through the coils  81   a ,  81   b  and  82   a ,  82   b , the directions of the flow of electric current in the second part  81   b  of the first coil and the first part  82   a  of the second coil are mutually parallel and are anti-parallel to the directions of the flow of electric current in the first part  81   a  of the first coil and the second part  82   b  of the second coil  82 . Since the direction of the magnetic field between the second main magnets  55 ,  75  is opposite to the directions of the magnetic field between the first main magnets  57 ,  77  and the third main magnets  53 ,  73 , the forces produced on each of the parts  81   a,    81   b ,  82   a ,  82   b  of the coils (in a direction perpendicular to both the direction of the flow of current and the magnetic field) are in the same direction.  
         [0052]    While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. The description is not intended to limit the invention.