FIELD 
   The present invention relates to an actuator assembly and a lithographic apparatus comprising such an actuator assembly. 
   BACKGROUND 
   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 that instance, 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 include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, 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. 
   For scanning or stepping, the patterning device and/or the substrate need to be moveably supported. In a conventional lithographic apparatus, a patterning support and/or a substrate table are supported by an actuator assembly which includes a first and a second actuator part. In a conventional actuator assembly, the first actuator part includes two magnets for generating a magnetic field in a gap between the magnets. A second actuator part includes a coil, which coil is positioned in the gap between the magnets in order to position the coil in the magnetic field. Thus, the magnets of the first actuator part are positioned on opposite sides of the coil of the second actuator part. 
   Since the magnets of the first actuator part are positioned on opposite sides of the second actuator part, assembling and mounting of the actuator assembly may be difficult. If the actuator assembly malfunctions, it may be necessary to disassemble the actuator assembly, i.e. dismounting at least one of the actuator parts, for inspection, repair or replacement. Thereafter, the assembly has to be assembled and mounted again. Disassembling and assembling an actuator assembly in a lithographic apparatus is even more complicated than the initial assembling and mounting, since the actuator assembly is then surrounded by a number of other parts of the lithographic apparatus, limiting the working space. Disadvantageously, it may therefore be needed to design the lithographic apparatus such that extra space around the actuator assembly is provided for easy accessibility, inspection and fast repair. 
   SUMMARY 
   Embodiments of the invention include an actuator assembly, which is easy to assemble and disassemble, and to mount and dismount. 
   According to an embodiment of the invention, there is provided a lithographic apparatus arranged to transfer a pattern from a patterning device onto a substrate, wherein at least one of the patterning device and the substrate is moveably supported by an actuator assembly, the actuator assembly including a first actuator part moveably positionable with respect to a second actuator part, the first actuator part including a plurality of magnets for generating a magnetic field; and the second actuator part including an electrical conductor positionable in the magnetic field for conducting an electrical current in a second direction through the magnetic field; the magnetic field being shaped to be, at least at the location of the electrical conductor, in a first direction substantially perpendicular to the second direction of the electrical current for generating a force on the electrical conductor in a first degree of freedom perpendicular to a plane defined by the first and second directions, wherein the first actuator part, including all magnets thereof, and the second actuator part are moveable away from each other in the first direction. 
   According to an embodiment of the invention, there is provided an actuator assembly including a first actuator part moveably positionable with respect to a second actuator part, the first actuator part including a plurality of magnets for generating a magnetic field; and the second actuator part including an electrical conductor positionable in the magnetic field for conducting an electrical current in a second direction through the magnetic field; the magnetic field being shaped to be, at least at the location of the electrical conductor, in a first direction substantially perpendicular to the second direction of the electrical current for generating a force on the electrical conductor in a first degree of freedom perpendicular to a plane defined by the first and second directions, wherein the first actuator part, including all magnets thereof, and the second actuator part are moveable away from each other in the first direction. 


   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings in which corresponding reference symbols indicate corresponding parts, and in which: 
       FIG. 1  depicts a lithographic apparatus according to an embodiment of the invention; 
       FIG. 2A  shows a schematic side view of a conventional actuator assembly; 
       FIG. 2B  shows a schematic sectional view of the conventional actuator assembly of  FIG. 2A ; 
       FIG. 3A  schematically shows a side view of an actuator assembly according to an embodiment of the present invention; 
       FIG. 3B  shows the side view of  FIG. 3A  schematically indicating a magnetic field generated in the actuator assembly; 
       FIG. 3C  schematically shows a side view of an actuator assembly according to another embodiment of the present invention; 
       FIG. 4  illustrates a principle of generating a gravity compensation force using Halbach magnets; 
       FIG. 5A  schematically shows an actuator assembly in accordance with an embodiment of the present invention; 
       FIG. 5B  shows a sectional view of the embodiment of  FIG. 5A ; 
       FIG. 5C  shows the side view of  FIG. 5A  schematically indicating a magnetic field generated in the actuator assembly; 
       FIG. 5D  schematically shows a side view of an actuator assembly in accordance with another embodiment of the present invention; and 
       FIG. 5E  schematically shows a side view of an actuator assembly in accordance with an embodiment of the present invention. 
   


   DETAILED DESCRIPTION 
     FIG. 1  schematically depicts a lithographic apparatus according to one embodiment of the invention. The apparatus includes an illumination system (illuminator) IL configured to condition a radiation beam B (e.g. UV radiation), a support structure (e.g. a mask table) MT constructed to support a patterning device (e.g. a mask) MA and connected to a first positioner PM configured to accurately position the patterning device in accordance with certain parameters, and 
   a substrate table (e.g. a wafer table) WT constructed to hold a substrate (e.g. a resist-coated wafer) W and connected to a second positioner PW configured to accurately position the substrate in accordance with certain parameters. The apparatus also 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. 
   The illumination system may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic or other types of optical components, or any combination thereof, for directing, shaping, or controlling radiation. 
   The support structure supports, i.e. bears the weight of, the patterning device. It holds the patterning device in a manner that depends on the orientation of the patterning device, the design of the lithographic apparatus, and other conditions, such as for example whether or not the patterning device is held in a vacuum environment. The support structure can use mechanical, vacuum, electrostatic or other clamping techniques to hold the patterning device. The support structure may be a frame or a table, for example, which may be fixed or movable as required. The support structure may ensure that the patterning device is at a desired position, for example with respect to the projection system. Any use of the terms “reticle” or “mask” herein may be considered synonymous with the more general term “patterning device.” 
   The term “patterning device” used herein should be broadly interpreted as referring to any device that can be used to impart a radiation beam with a pattern in its cross-section such as to create a pattern in a target portion of the substrate. It should be noted that the pattern imparted to the radiation beam may not exactly correspond to the desired pattern in the target portion of the substrate, for example if the pattern includes phase-shifting features or so called assist features. Generally, the pattern imparted to the radiation beam will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit. 
   The patterning device may be transmissive or reflective. Examples of patterning devices include masks, programmable mirror arrays, and programmable LCD panels. Masks are well known in lithography, and include mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. An example of a programmable mirror array employs a matrix arrangement of small mirrors, each of which can be individually tilted so as to reflect an incoming radiation beam in different directions. The tilted mirrors impart a pattern in a radiation beam which is reflected by the mirror matrix. 
   The term “projection system” used herein should be broadly interpreted as encompassing any type of projection system, including refractive, reflective, catadioptric, magnetic, electromagnetic and electrostatic optical systems, or any combination thereof, as appropriate for the exposure radiation being used, or for other factors such as the use of an immersion liquid or the use of a vacuum. Any use of the term “projection lens” herein may be considered as synonymous with the more general term “projection system”. 
   As here depicted, the apparatus is of a transmissive type (e.g. employing a transmissive mask). Alternatively, the apparatus may be of a reflective type (e.g. employing a programmable mirror array of a type as referred to above, or employing a reflective mask). 
   The lithographic apparatus may be of a type having two (dual stage) or more substrate tables (and/or two or more mask tables). In such “multiple stage” machines 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 exposure. 
   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 fill 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 are well known in the art for increasing 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 liquid is located between the projection system and the substrate during exposure. 
   Referring to  FIG. 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. 
   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 σ-outer and σ-inner, 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. 
   The radiation beam B is incident on the patterning device (e.g., mask MA), which is held on the 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 positioner 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 positioner PM and another position sensor (which is not explicitly depicted in  FIG. 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 positioner PM. Similarly, movement of the substrate table WT 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 M 1 , M 2  and substrate alignment marks P 1 , P 2 . 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. 
   The depicted apparatus could be used in at least one of the following modes: 
   Step mode: the mask table MT and the substrate table WT are kept essentially stationary, while an entire pattern imparted to the radiation beam is projected onto a target portion C at one time (i.e. a single static exposure). The substrate table WT is then shifted in the X and/or Y direction so that a different target portion C can be exposed. In step mode, the maximum size of the exposure field limits the size of the target portion C imaged in a single static exposure. 
   Scan mode: the mask table MT and the substrate table WT are scanned synchronously while a pattern imparted to the radiation beam is projected onto a target portion C (i.e. a single dynamic exposure). The velocity and direction of the substrate table WT relative to the mask table MT may be determined by the (de-)magnification and image reversal characteristics of the projection system PS. In scan mode, the maximum size of the exposure field limits the width (in the non-scanning direction) of the target portion in a single dynamic exposure, whereas the length of the scanning motion determines the height (in the scanning direction) of the target portion. 
   Another mode: the mask table MT is kept essentially stationary holding a programmable patterning device, and the substrate table WT is moved or scanned while a pattern imparted to the radiation beam is projected onto a target portion C. In this mode, generally a pulsed radiation source is employed and the programmable patterning device is updated as required after each movement of the substrate table WT or in between successive radiation pulses during a scan. This mode of operation can be readily applied to maskless lithography that utilizes programmable patterning device, such as a programmable mirror array of a type as referred to above. 
   Combinations and/or variations on the above-described modes of use or entirely different modes of use may also be employed. 
     FIG. 2A  schematically illustrates a conventional actuator assembly  2  for moving a pattern support or a substrate table. The actuator assembly includes a generally C-shaped first actuator part  4  of which two arms are indicated by reference numerals  4 A and  4 B. The arms  4 A and  4 B are connected to each other in a manner not shown in  FIG. 2A . Each arm  4 A,  4 B is provided with two magnets  10 ,  8  respectively, thereby generating a magnetic field between each pair of magnets  8 ,  10  essentially in the directions of the arrows drawn in each of the magnets  8 ,  10 . 
   Between the two arms  4 A,  4 B of the C-shaped first actuator part, a second actuator part  6  is positioned. The second actuator part  6  is provided with a coil  12  of which a first branch and a second branch are positioned in the magnetic field between the magnets  8 ,  10 . A current  14  may be generated in the coil  12 . The magnetic field may be guided and locally enforced by magnetized field-shaping elements  18 , e.g. Halbach magnets. 
     FIG. 2B  shows a sectional view through the line B—B in  FIG. 2A . The coil  12  is positioned in a magnetic field  16  generated by the magnets  10 . The orientation of the magnetic field  16  is opposite for each branch of the coil  12 . Also, the orientation of the current  14  through each branch of the coil  12  is opposite to each other. Therefore, an orientation of each force  20 , generated in each branch of the coil  12  due to the current  14  through magnetic field  16 , is the same. Hence, the coil  12  and the magnets  10 ,  8  may move with respect to each other due to the generated force  20 . 
   The position of the second actuator part  6  in between the arms  4 A,  4 B of the first actuator part results in complicated assembling and/or mounting a lithographic apparatus and, if the actuator assembly ever malfunctions, disassembling and thereafter assembling again may be even more complicated. Further, the opposing magnets may generate considerable attracting forces, which need to be compensated. 
   The actuator assembly  2  includes additional actuating devices (not shown) that are configured to position the second actuator part  6  with respect to the first actuator part  4  and, depending on the orientation of mounting, for compensating the force of gravity. These additional actuating devices make the actuator assembly even more complicated to assemble and make the actuator assembly relatively expensive due to the additional components. 
     FIG. 3A  schematically illustrates an actuator assembly  22  according to an embodiment of the present invention. The actuator assembly  22  includes a first actuator part  24  and a second actuator part  26 . The first actuator part is provided with two magnets  28  and with magnetized field-shaping elements  34 A such as Halbach magnets therebetween to guide and enforce the magnet flux. The magnets  28  may be permanent magnets or any other kind of magnets such as electromagnets, for example, generating a magnetic field essentially in the directions of the arrows drawn in each of the magnets  28 . 
   The second actuator part  26  is provided with a coil  30  having two branches. Each branch is positioned in the magnetic field generated by the magnets  28  such that if a current  32  is generated in the coil  30 , a force is generated between each pair of a magnet  28  and a branch of the coil  30 . To generate a force similar to the force generation as explained in relation to  FIG. 2B , the magnetic flux needs to be directed perpendicular to the current  32  similar to the magnetic flux generated in the prior art actuator assembly  2  of  FIG. 2A . Contrary to the assembly of  FIG. 2A , in the embodiment of the invention shown in  FIG. 3A , the first actuator part  24  and the second actuator  26  part have substantially only one side facing the other actuator part. Thus, the first actuator part  24  is moveable away from the second actuator part  26  in a direction substantially perpendicular to a plane of the side without requiring complicated handling. 
   Referring to  FIG. 3B , the magnetic flux  36  may be shaped using magnetized field-shaping elements  34 B. The magnetized field-shaping elements  34 B guide the magnet flux such that a generated magnetic flux becomes more concentrated in the branches of the coil  30  which is similar to the magnetic flux generated between the magnets  8 ,  10  in the prior art actuator assembly of  FIG. 2A . Thus, no magnets are needed at the opposite side of the coil  30 , and hence the first actuator part is not required to have a C-shape. Each actuator part thus has substantially only one side opposing the other actuator part, instead of one actuator part having two opposing sides each facing the other actuator part, like in the prior art. In  FIGS. 3A–B , a second actuator assembly  37  is configured to support and move the first actuator part  24  and the second actuator part  26  of the actuator assembly  22  over relatively long distances. 
     FIG. 3C  illustrates an actuator assembly  22  according to an embodiment of the present invention. Contrary to the embodiment illustrated in  FIG. 3A , the magnetized field-shaping elements  34 B are provided on the first actuator part  24 . The embodiment of  FIG. 3A , however, may be preferred as is explained in relation to  FIG. 4  hereinafter. 
     FIG. 4  shows how a net upwards force  48  can be obtained using attractive and repellent forces from three magnets  40 ,  42  and  44 . In  FIG. 4 , triangular arrowheads indicate a direction of magnetic flux. The enclosed magnet  42  is attracted by the magnetic flux  46  leaving top of magnet  40  and entering the left side of magnet  42 . The flux  46  leaving magnet  42  and the flux  46  entering the top of magnet  44  generate an attracting force. At the bottom of magnets  40  and  44  the magnetic flux  46  generates repellent forces between the magnets  40  and  42  and between the magnets  42  and  44 . The magnetic flux  46  entering the bottom of magnet  40  is repelled by the magnetic flux  46  entering the left side of magnet  42 . The flux  46  leaving magnet  42  at the right side is repelled by the flux  46  leaving the bottom of magnet  44 . So, by this configuration of magnets  40 ,  42 ,  44 , there is an upward attracting force at the top and a repellent upward force at the bottom of magnet  42 . The resulting force  48  may be advantageously be employed for compensating a gravity force in the embodiments of  FIGS. 3A and 3B . For example, in  FIG. 3A , the magnetized field-shaping elements  34 B provide the function of magnets  40  and  44  shown in  FIG. 4 , and magnetized field shaping elements  34 A ( FIG. 3A ) provide the function of the magnet  42  in  FIG. 4 . 
   Referring to  FIGS. 3A and 3B , the first actuator part  24  is positioned between two magnetized field-shaping elements  34 B. The magnetic flux  36  is generated such that the first actuator part  24  tends to move to a predetermined position with respect to the second actuator part  26  as explained above in relation to  FIG. 4 . However, if the first and the second actuator part  24 ,  26  are substantially vertically positioned with respect to each other, the force of gravity exerted on the first actuator part  24  may counteract. In such a case, said force of gravity exerted on the first actuator part  24  may at least partly be compensated by the magnetized field-shaping elements  34 A and  34 B. 
     FIG. 5A  illustrates an actuator assembly according to a further embodiment of the present invention. The actuator assembly  22  includes a first actuator part  24  and a second actuator part  26 . The first actuator part  24  includes two magnets  28  and magnetized field-shaping elements  34 . The second actuator part  26  includes a coil  30  and field-shaping elements  34 . The first actuator part  24  may hover above the second actuator part  26  as described above. 
   In the embodiment of the invention shown in  FIG. 5A , the second actuator part  26  is further provided with a second coil  50  that is configured to conduct a second electrical current  52 . Shielding material  54 , such as mu-metal, for shielding the magnetic field is positioned between a first and a second branch of the coil  50 . The shielding material  54  prevents that an undesired force is generated when a current flows through the lower branch of the coil  50 , and the shielding material  54  prevents that a magnetic field generated by the magnets  28  is disturbed by any other nearby structure. Shielding material may also be present at other locations of the actuator assembly  22  or in the surroundings of the actuator assembly  22 . 
   Functioning of the actuator assembly  22  of  FIG. 5A  is described in relation to  FIGS. 5B and 5C . Referring to  FIG. 5C , a magnetic field  36  is generated by the magnets  28  and guided by the field-shaping elements  34  such that a vertical magnetic flux is present in the area of the branches of the coil  30 . As illustrated in  FIG. 5B  and as above described in relation to  FIG. 2B , a horizontal force  38  may be generated by generating a current  32  through the coil  30 . The second coil  50  is positioned such that a horizontally directed magnetic flux is present in the area of an upper coil branch and no magnetic flux is present in the area of a lower coil branch, e.g. enforced by shielding material  54 . In another embodiment of the invention illustrated in  FIG. 5D , the second branch of the coil  50  may be positioned essentially outside the magnetic field  36 . 
   Due to the orientation of the magnetic flux in the area of the upper branch of the coil  50 , and the orientation of the current  52  through the branch of the coil  50 , a vertical force  56  may be generated. Thus, in this embodiment, the first actuator part  24  and the second actuator part  26  may be controlled to move horizontally and vertically with respect to each other. 
   As shown in  FIG. 5C , the direction of the magnetic flux  36  ( FIG. 5C ) varies in a plane below a plane defined by coil  30 . The variation of the direction of the magnetic flux may advantageously be employed for generating vertical forces. A further embodiment of the invention including a first and a second coil  50 ,  51  in the plane is illustrated in  FIG. 5E . In  FIG. 5E  the first coil  50  has a first branch  50 A and a second branch  50 B and the second coil  51  has a first branch  51 A and a second branch  51 B. The first branches  50 A and  51 A are positioned next to each other such that the direction of the magnetic flux is substantially identical for said branches  50 A and  51 A. The second branches  50 B and  51 B are positioned such that the direction of the magnetic flux is substantially identical for said branches  50 B and  51 B and substantially opposite to the direction of the magnetic flux at the position of the first branches  50 A and  51 A. 
   A direction of a current  52 ,  53  flowing through the first branches  50 A,  51 A, respectively, is substantially identical. Likewise, a direction of the current  52 ,  53  flowing through the second branches  50 B,  51 B, respectively, is substantially identical and substantially opposite to the direction of the current  52 ,  53  through the first branches  50 A and  51 A. In the configuration illustrated in  FIG. 5E , the forces generated in the four branches  50 A,  50 B,  51 A and  51 B all have substantially identical directions, and thus enable to control the first actuator part  24  and the second actuator part  26  to move vertically with respect to each other. 
   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 tool and/or an inspection tool. 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. 
   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. 
   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. 
   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. 
   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. 
   The descriptions above are intended to be illustrative, not limiting. Thus, it will be apparent to one skilled in the art that modifications may be made to the invention as described without departing from the scope of the claims set out below.
An actuator assembly includes a coil moveably positionable in a magnetic field generated by a magnet assembly, each of the coil and the magnet having substantially only one side facing the other. A magnetized field-shaping element is provided for shaping the magnetic field such that the magnetic field is substantially perpendicular to the direction of the electrical current for generating a force in a first degree of freedom. Thus, the coil may easily be mounted, since it does not need to be enclosed by magnet poles.