Abstract:
A two-dimensional electric motor having a moving magnet and positionable in six degrees of freedom. The electric motor has a coil array and a magnet array. The magnet array has a smaller width than the coil array in the moving magnet embodiment. The invention&#39;s process of achieving motion of a coil array with respect to a magnet array in six degrees of freedom includes providing an electrical current distribution to a coils. The motion is controlled in a first direction and a vertical direction between a portion of the coils and a portion of the magnet array. The electrical current distribution has two wavelike components having a same period but approximately ninety degrees out of phase. The invention&#39;s positioning devices do not require air bearings. Instead, the suspension of the support member by magnetic attraction to the frame or by interaction of the coil array and the magnet array replace the air bearings. The invention&#39;s electric motors and positioning devices should be useful in positioning wafers during semiconductor manufacture.

Description:
TECHNICAL FIELD 
     This invention relates generally to electric motors and more particularly to two-dimensional electric motors. 
     BACKGROUND ART 
     Electric motors are used in a variety of electrical equipment. For example, linear electric motors produce electrical power propelling an armature in one dimension. Wafer stages utilize linear electric motors to position a wafer during photolithography and other semiconductor processing. 
     A typical one-dimensional linear electric motor has a magnet track with pairs of opposing magnets facing each other. (Copending U.S. Ser. No. 09/059,056, entitled “Linear Motor Having Polygonal-Shaped Coil Unit” filed on Apr. 10, 1998, by Hazelton et al. discusses one-dimensional linear electric motors and is incorporated herein by reference in its entirety.) Within spaces between the pairs of opposing magnets, an armature moves. The armature has windings of a conductor which are connected to an electrical current. When the electrical current is turned on, the electric current interacts with the magnetic fields of the magnet pairs to exert force on the armature, causing the armature to move. When the armature is attached to a wafer stage, the wafer stage experiences the same force as and moves in concert with the armature. 
     In a multiphase motor, the armature has various windings grouped into phases. The electric currents are selected applied to the phase groups to create a more efficient motor. As the armature moves within the magnet track as current is applied to a first group of coils, the first group moves out of its optimal position between the pairs of magnets. Then, it becomes more efficient to apply current to a second group of windings. More phase groups are theoretically more efficient since a more even application of force and utilization of power input is maintained. However, each additional phase group complicates a timing of the applied current to the various phase groups. Presently, three-phase motors and armatures have gained favor in balancing these considerations. 
     Linear two-dimensional motors are also used in manufacturing. (U.S. Pat. No. 4,654,571, entitled “Single Plane Orthogonally Moveable Drive System,” issued to Hinds on Mar. 31, 1987 (“Hinds”) and U.S. Pat. No. 4,535,278, entitled “Two-Dimensional Precise Positioning Device for Use in a Semiconductor Manufacturing Apparatus,” issued to Asakawa on Aug. 13, 1985 discuss two-dimensional linear electric motors and are incorporated herein by reference in their entireties.) The motors are two-dimensional in that they have two-dimensional arrays of magnets and armatures instead of magnet tracks and one-dimensional armatures. However, the magnet arrays and two-dimensional armatures may move with respect to each other in more than two dimensions depending upon the design. Conventional two-dimensional linear motors typically have an array of magnets and an armature having one or more coils on one side of the array of magnets. 
     U.S. Pat. No. 5,623,853, entitled “Precision Motion Stage with Single Guide Beam and Follower Stage,” issued to Novak et al. on Apr. 29, 1997 and U.S. Pat. No. 5,528,118, entitled “Guideless Stage With Isolated Reaction Stage,” issued to Lee on Jun. 18, 1996 discuss examples of semiconductor fabrication equipment and are incorporated herein by reference in their entireties. 
     When attached to part of a two-dimensional linear motor, a platform can be moved in two or more dimensions by the motor. For example, a wafer stage in semiconductor processing equipment may be attached to an armature or magnet array of a two-dimensional motor and the two-dimensional motor would control positioning of the wafer stage. 
     When used to position a platform, conventional two-dimensional electric motors do not smoothly and accurately position the platform. Presently, coils in the two-dimensional electric motors move with respect to the magnets. As exemplified in U.S. Pat. No. 4,654,571, entitled “Single Plane Orthogonally Moveable Drive System” issued to Hinds on Mar. 31, 1987, referenced above and incorporated herein by reference in its entirety, cables and hoses are attached to the coil assembly. The cables are for electrical current and the hoses may be used to carry coil cooling fluid or air supply. Unfortunately, the hoses and cables impede free motion of the coil assembly. If the hoses could be eliminated, the stability of motion of the motor and positioning of the platform would be improved. 
     Also, conventional technology relies upon cumbersome stacked arrangements to achieve six degrees of freedom movement of the platform. The six degrees of freedom include three translational and three rotational degrees of freedom. (Richard P. Feynman, Robert B. Leighton, and Matthew Sands,  The Feynman Lectures on Physics,  Addison-Wesley, 1962, discusses translational and rotational motion and degrees of freedom and is incorporated herein by reference in its entirety.) Unfortunately, many designs obtain six degrees of freedom by essentially stacking multiple two dimensional and/or one dimensional motors which move only in two dimensions within a plane. (U.S. Pat. No. 5,623,853, entitled “Precision Motion Stage with Single Guide Beam and Follower Stage” issued to Novak et al. on Apr. 29, 1997, discusses examples of such stacked arrangements and is incorporated herein by reference in its entirety.) For example, a platform may be propelled back and forth in one dimension under the control of linear electric motors. The linear electric motors are part of a holder which holds the platform. In turn, a second holder holds that entire holder and platform arrangement via joint connections and propels it back and forth in a second dimension by another set of linear electric motors. Additional degrees of motion may be provided by voice coil motors which are attached to these holders. 
     These types of stacked arrangements have a few drawbacks. Each additional holder enabling more degrees of freedom also adds mass requiring additional power for the electric motors to move the platform. Also, the complicated joint connections degrade accuracy of positioning of the platform and build-in resonant frequencies. 
     The platforms need a better electric motor to position them. The improved electric motor would eliminate the air hoses and position the platform in multiple degrees of freedom without the cumbersome stacked arrangements. 
     SUMMARY OF THE INVENTION 
     The invention features a moving magnet array or a moving coil array electric motor. The electric motor includes a magnet array and a coil array. The magnet array has a magnet array width and also a first period in a first direction and a second period in a second direction. The coil array has a coil array width which is larger than the magnet array width. The coil array interacts with the magnet array to provide motion of the magnet array relative to the coil array in the first direction and the second direction, and a third direction away from the coil array. Although the present invention is described in terms of a moving magnet array electric motor, the electric motor may be modified to be a moving coil array electric motor wherein the coil array moves relative to the magnet array. 
     The invention also features a process of achieving motion of a coil array with respect to a magnet array in six degrees of freedom. In some embodiments, the process includes positioning a coil of a periodic coil array in proximity to and overlapping a magnet of a periodic magnet array. The process also includes controlling a separation between a portion of the coil array and a portion of the magnet array. The controlling is achieved by the interaction of current in the coil and a magnetic field associated with the magnet. 
     In some embodiments, the method includes providing an electrical current distribution to a coil to control movement of the coils with respect to a magnetic field array. The motion is controlled in a first direction and a vertical direction between a portion of the coils and a portion the magnet array. The electrical current distribution has two wavelike components having a same period but approximately ninety degrees out of phase. The coils are distributed in a first direction with a coil period of approximately half the same period, and magnets in the magnet array are distributed in the first direction with magnet period of approximately 4/3 the coil period. 
     The invention also features positioning devices. The positioning device has a support member, a magnet array, and a coil array. In some embodiments, the magnet array is on the support member and has a magnet array width. The magnet array and the coil array are part of an electric motor. The electric motor is capable of positioning the support member in at least three degrees of freedom. In some embodiments, the coil array and the magnet array interact to suspend the support member. In other embodiments, the positioning device also includes a frame, and the support member is magnetically attracted to the frame to suspend the support member. 
     An advantage of the invention&#39;s moving magnet electric motor and corresponding positioning device is that they provide more stable positioning than conventional moving coil electric motors. The moving magnet electric motor does not require wire connections or cooling hoses to the moving part of the motor. Conventional two-dimensional electrical motors are “moving coil” motors having wires and cooling hoses connected to a moving coil array. Instead, the invention&#39;s electric motor has a moving magnet array. By eliminating the wires and hoses, positioning devices using the moving magnet array are more stable than conventional moving coil platforms. 
     Another advantage of the invention&#39;s positioning devices is that some embodiments do not require air bearings. Instead, the suspension of the support member by magnetic attraction to the frame or by interaction of the coil array and the magnet array replace the air bearings. 
     An advantage of one embodiment in the invention&#39;s method is that the method permits six degree of freedom positioning of the support member attached to either the electric motor&#39;s coil array or magnet array. The method appropriately commutates coils instead of using stacked stages or limited movement adaptations of one dimensional motors. Thus, the invention has a smoother more precise movement and a wider range of movement than conventional six degree of freedom positioning apparatuses. The method applies to both moving magnet and moving coil embodiments of the coil array. 
     The invention&#39;s electric motors and positioning devices should be useful in environments requiring precise and wide ranges of positioning. In particular, the invention should be particularly useful in positioning wafers during semiconductor manufacture. 
     These and other objects, features, and advantages of the invention will become readily apparent to those skilled in the art upon a study of the following drawings and a reading of the description of the invention below. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A-1C illustrate components of a moving coil electric motor in accordance with the present invention. 
     FIG. 2A illustrates a moving magnet electric motor in accordance with the present invention. 
     FIG. 2B illustrates a magnet array of the moving magnet electric motor in FIG.  2 A. 
     FIGS. 3A and 3B illustrate an embodiment of a process of achieving relative motion of a portion of coil array with respect to a portion of magnet array in an X direction and a Y direction in accordance with the present invention. 
     FIGS. 4A-4E illustrate embodiments of a process of achieving motion of a coil array with respect to a magnet array in accordance with the present invention. 
     FIG. 5 diagrammatically illustrates a side cross-section of a positioning device utilizing an embodiment of the electric motor illustrated in FIG.  2 A. 
     FIGS. 6A and 6B diagrammatically illustrate side cross-sections of embodiments of positioning devices utilizing electric motors and suspended support members. 
     FIGS. 7A and 7B diagrammatically illustrate side cross-sections of embodiments of positioning devices. 
     FIG. 8 shows a side view of an example of a photolithography system using the electric motor of the present invention. 
    
    
     DESCRIPTION OF THE INVENTION 
     For background material, the reader is directed to the following standard textbooks all of which are incorporated herein by reference in their entireties:  Permanent - Magnet DC Linear Motors,  A. Basak, Clarendon Press, 1996; R. P. Feynman, R. B. Leighton and M. Sands,  Feynman Lectures on Physics,  Addison-Wesley, 1962;  Fundamentals of Physics,  Second Edition, Extended Version, Revised Printing, David Halliday and Robert Resnick, John Wiley &amp; Sons, 1986;  Brushless Permanent - Magnet Motor Design,  D. C. Hanselman, McGraw-Hill, 1994;  Design of Brushless Permanent - Magnet Motors,  J. R. Hendershot, Jr. and T. J. E. Miller, Magna Physics Publishing and Clarendon Press, 1994; E. M. Purcell,  Electricity and Magnetism,  McGraw-Hill 1965. Additional background material may be found in U.S. Pat. Nos. 5,196,745, entitled “Magnet Positioning Device”, issued to Trumper on Mar. 23, 1993; 4,535,278, entitled “Two-Dimensional Precise Positioning Device for Use in a Semiconductor Manufacturing Apparatus”, issued to Asakawa on Aug. 13, 1985; 4,654,571, entitled “Single Plane Orthogonally Moveable Drive System” issued to Hinds on Mar. 31, 1987, referenced above; and 5,334,892, entitled “Positioning Device for Planar Positioning” issued to Chitayat on Aug. 2, 1994 all of which are incorporated herein by reference in their entireties. 
     FIGS. 1A-1C illustrate components of a moving coil electric motor  100  in accordance one aspect of the present invention. FIG. 1A shows a coil array  110  attached to a platform  112 . FIG. 1B shows a top view of a magnet array  120 , and FIG. 1C shows a side view of the magnet array  120 . During operation of the moving coil electric motor  100 , a face  114  of the coil array  110  is in proximity to the top of the magnet array  120  shown in FIG.  1 B. 
     Magnets in the magnet array  120  and coils in the coil array  110  are periodically distributed in two directions. In some embodiments, a coil period  116  in one direction approximately equals a coil period  118  in a second direction. The periods of the coil array  110  are related to periods of the magnet array  120 . The first coil period  116  is a approximately three-fourths of a first magnet period  122 , and the second coil period  118  is approximately three-fourths of a second magnet period  124 . 
     A total number of coils in the coil array  110  is a multiple of 4. As will be shown below in the discussion of FIG. 3A, principles of operation for movement in an X direction and a Y direction use four adjacent, approximately identically shaped coils. To generate forces about a Z direction perpendicular to X and Y directions, the coil array  110  has at least two sets of four adjacent coils. The coil array embodiment  110  as shown in FIG. 1A has four sets of four adjacent coils. 
     Although all the coils are shown to have approximately the same shape, the shape may vary between different embodiments of the coil array  110 . Preferably, a coil  130  covers as much of an area of one coil period in both X and Y directions as possible. A rectangular profile or outline of the coil  130  achieves this objective. Of course, when the periods  116  and  118  are approximately equal, the outline or profile preferably approximates a square. 
     In some embodiments, the coil  130  is wrapped about a magnetically impermeable post  132 . The magnetically impermeable post  132  facilitates creating a nearly uniform magnetic field on a face of the coil  130 . In contrast, a magnetically permeable post would focus a magnetic field created by the coil  130  and produce an uneven field distribution over the outline of the coil  130 . 
     In some embodiments, a backing panel  134  is on one side of the coil array  110 . The backing panel  134  may comprise a magnetically permeable material, such as iron, or a magnetically impermeable material. A magnetically permeable backing panel  134  increases the permanent magnetic flux through the coils and thus increases the performance. The magnetically permeable backing panel  134  also adds mass to the coil array  110 . Therefore, a greater force must be developed by the moving coil electric motor  100  to move the coil array  110 . 
     In some embodiments, the electric motor includes an air bearing  138  separating the coil array  110  and the magnet array  120 . Construction and usage of an air bearing are known to those skilled in the art. Hinds referenced above teaches an example of an air bearing. When the air bearing separates the coil array  110  and the magnet array  120 , the coil array  110  and/or the magnet array  120  may be potted with any suitable material, such as with epoxy, or covered by a flat plate made of, for example, ceramic, composite or metal, to form essentially flat surfaces. The essentially flat surfaces improve performance of the air bearing in separating or levitating the coil array  110  and magnet array  120  relative to each other. In some embodiments, the air bearing positions the coil array  110  and the magnet array  120  at a neutral position about which the coil array  110  and the magnet array  120  can move relative to each other in three degrees of freedom. 
     FIGS. 1B and 1C illustrate an embodiment of the magnet array  120 . As noted above, the magnet array  120  has periods  122  and  124  in the X and the Y directions, respectively. The magnets alternate in polarity in both the X and the Y directions. However, along any diagonal of the X and the Y directions, the magnets are all of one polarity. The magnets in the magnet array  120  all have the same flux, but, of course, alternate in polarity. In some embodiments, a magnetically permeable backing panel  136  is attached to the magnets. The backing panel  136  completes flux paths between adjacent magnets of opposite polarities to increase the magnetic flux of each polarity. 
     FIG. 2A illustrates a moving magnet electric motor  200  in accordance with another aspect of the present invention. The electric motor  200  has a coil array  210  and a magnet array  220 . Unlike the moving coil electric motor  100  as shown in FIGS. 1A and 1B, a coil array width  213  is larger than a magnet array width  223  in an X direction, and a coil array width  215  is larger than a magnet array width  225  in a Y direction. 
     Apart from the overall width relationships to the magnet array  220 , the coil array  210  has the same structure as the coil array  110  in the moving coil electric motor  100 . The coil array  210  has a period  216  in the X direction and a period  218  in the Y direction. Each coil  230  in the coil array  210  has approximately the same shape and a rectangular coil profile is preferred. 
     In some moving magnet electric motor embodiments, as in the moving coil electric motor  100 , an air bearing (not shown) separates the coil array  210  from the magnet array  220 . When the air bearing separates the coil array  210  and the magnet array  220 , the coil array  210  and/or the magnet array  220  may be potted or covered with a flat plate to form essentially flat surfaces. The same potting and cover plate materials suitable for the moving coil electric motor  100  are suitable for the electric motor  200 . As in the moving coil electric motor  100 , the essentially flat surfaces improve performance of the air bearing in separating or levitating the coil array  210  and magnet array  220  relative to each other. In some embodiments, the air bearing positions the coil array  210  and the magnet array  220  at a neutral position about which the coil array  210  and the magnet array  220  can move relative to each other in three degrees of freedom. 
     FIG. 2B illustrates the magnet array  220  in more detail. The magnet array  220  is similar to the magnet array  120  of the moving coil electric motor  100 . The magnet array  220  has a period  222  in the X direction and a period  224  in the Y direction. The magnets in the magnet array  220  alternate in polarity in both the X and the Y directions. However, diagonals of the X and the Y directions have only one polarity. The magnets are placed on a magnetically permeable backing  236 . 
     However, unlike the magnet array  120  of the moving coil electric motor  100 , magnets  240  on the edge of the magnet array  220  have fractional fluxes, such as one-quarter, one-half, or three-quarters or any other suitable fraction, compared with magnets  242  on the interior of the magnet array  220 . For example, the edge magnet  240 , which is not at a corner of the magnet array  220 , has half a magnetic flux of the interior magnet  242 . All edge magnets  240  which are not on corners in the magnet array  220  shown in FIG. 2B have half the flux of the interior magnets  242 . Corner magnets  244 ,  246 ,  248  and  250  each have one-quarter the flux of the interior magnets  242 . The fractional fluxes for the edge magnets  240  and corner magnets  244 ,  246 ,  248  and  250  complete flux paths with each other and with the interior magnets  242  while simultaneously minimizing fringe magnetic fields at the edges of the magnet array  220 . 
     The fractional flux magnets are an important part of the invention. Without them, the fringe fields at the magnet array edges degrade performance of the moving magnet electric motor. 
     The moving magnet embodiment  200  is generally preferable to the moving coil electric motor  100  when used in positioning devices because the magnet array  220  does not require electrical current connections. Wires connected to the coil array  110  of the moving coil electric motor  100  may interfere with the motion of the coil array  110  with respect to the magnet array  120 . In addition, when coil cooling is required, cooling hoses in coil array  110  may interfere with the motion of the moving coil array  110 . 
     As in the moving coil electric motor  100 , the magnet array  220  may move in six degrees of freedom with respect to the coil array  210 . Each of magnet array widths  223  and  225  is larger than twice the corresponding coil periods  216  and  218  to cover four coils. This permits motion in not only the X and the Y directions, but also rotation about the Z direction perpendicular to the X and the Y directions to achieve three of the six degrees of freedom. The other three degrees of freedom are obtained by forces in the Z direction in some embodiments. The Z direction forces create relative motion of the coil array  210  and the magnet array  220  in the Z direction, about the X direction, and about the Y direction. 
     Referring now to FIGS. 3A and 3B, the interactions of currents in the coils and magnets in the magnet arrays for both the moving coil and the moving magnet electric motors  100  and  200  are described. The description of FIGS. 3A and 3B describes the motion in the X and the Y directions. FIGS. 3A and 3B also show how the embodiments  100  and  200  obtain rotational motion, such as about the Z direction. FIGS. 4A-4D describe commutation of the coils in the coil arrays  110  and  210  to obtain motion in the Z direction thereby providing degrees of freedom in the Z direction and about the X directions and Y directions. 
     FIG. 3A is a diagrammatic representation of a plan view of a section of either moving coil or moving magnet electric motor  100  or  200 . FIG. 3B is a side cross-sectional view of a portion of the electric motor section shown in FIG.  3 A. FIG. 3A shows part of the subarray of the magnetic poles  120 ,  220  and a set of four coils  400 ,  402 ,  404 , and  406  which are part of the coil array  110 ,  210 . In the embodiment shown in FIG. 3A, each coil has first and second periods  116  and  118  in the X and Y directions, respectively, approximately equaling ¾ of first and second magnet periods  122  and  124 , respectively. 
     By appropriately commutating currents flowing in the coils  400 ,  402 ,  404 , and  406 , force is generated between the coil array  110 ,  210  and the magnet array  120 ,  220 . For example, in the position shown in FIG. 3A, a counter-clockwise current  434  through coil  400  will interact with the magnetic field to exert a force on the coil array  110 ,  210  in a direction as indicated by arrow  420 , by the right-hand rule and/or the Lorentz force laws. 
     In a two-phase commutation scheme in the direction  420 , the coils  400  and  402  are commutated. For example, in the position shown in FIG. 3, the counter-clockwise current  434  through the coil  400  is at a maximum while the coil  402  has no current flowing through it. As the coil  400  moves in the direction  420  and its center  400   c  approaches the next magnetic pole  424 , the counterclockwise current  434  through the coil  400  will approach zero. As the coil  400  moves in the direction  420 , the coil  402  also moves in the direction  420 , and its center  402   c  moves to location  428 . 
     To maintain the force in the direction  420 , the current in the coil  402  is commutated to flow in a counter-clockwise direction. The current in the coil  402  increases sinusoidally to its maximum. When the center  402   c  of coil  402  coincides with the location  428 , the current in the coil  402  will be at its maximum. 
     Similarly, current can be commutated to flow in direction  430  about the coil  404  to exert a force on the coil array  110 ,  210  in a direction  432 . As in the movement in the direction  420 , the coils  402  and  404  may be commutated in the same fashion as the coils  400  and  402  to provide a continuous force in the direction  432 . As with the coils  400  and  402 , the commutation of the coils  402  and  404  is a two phase commutation. 
     With respect to coil  406 , magnets  440 ,  442 ,  444 , and  446  are symmetrically positioned about its center  406   c.  The magnets  440 ,  442 ,  444 , and  446  create canceling forces upon the coil  406  provided the coil  406  is symmetric about its center  406   c.  Thus, unlike the other three coils  400 ,  402 , and  404 , the coil  406  cannot generate a force upon the coil array  110 ,  210  in the position shown in FIG.  3 A. 
     Only a few examples of commutation have been described. Clearly, as will be appreciated by those skilled in the art, many other commutations may be applied to the coils  400 ,  402 ,  404 ,  406  and the other coils in the coil array  110 ,  210  to achieve force and motion in X and Y directions. By providing at least two sets of four coils and by simultaneously generating forces in both the directions  420  and  432 , the electric motors  100 ,  200  orients the coil array  110 ,  210  with respect to the magnet array  120 ,  220  about the Z axis out of the plane of FIG.  3 A. 
     Referring now to the cross-sectional side view of FIG. 3B showing the coils  400  and  402  of either moving coil or moving magnet electric motor, the commutation of the coils to generate a force in the Z direction will now be described. As shown, coil  402  comprises coil portions  402   a  and  402   b  about its center  402   c.  Similarly, coil  400  comprises coil portions  400   a  and  400   b  about its center  400   c.  The coil  402  experiences nonvertical components of magnetic flux density B when the magnet is at off-center locations relative to a coil portion, such as coil portions  402   a,    402   b.  A current is applied to the coil  402  which flows through coil portion  402   a  in a direction into the plane of FIG.  3 B and through coil portion  402   b  in a direction out of the plane of FIG.  3 B. Nonvertical components of the magnetic flux density B interact with the current flowing through the coil portions  402   a  and  402   b  to produce forces F 402a  and F 402b , respectively, in the Z direction. 
     In a two-phase commutation scheme to exert force on the coil array in the Z direction relative to the magnet array, the coils  400  and  402  are commutated. For example, in the position shown in FIG. 3B, the current through the coil  402  is at a maximum while the coil  400  has no current flowing through it. When the coil  402  is moved in the X direction and its coil portion  402   a  approaches magnetic pole  426 , the current through the coil  402  will approach zero. As the coil  402  moves in the X direction, the coil  400  also moves in the X direction and its center portion  400   c  moves toward a position directly below magnet pole  424 . 
     To maintain the force in the Z direction, the current in the coil  400  is commutated which flows through coil portion  400   a  in a direction out of the plane of FIG.  3 B and through coil portion  400   b  in a direction into the plane of FIG.  3 B. The current applied to coil  400  will be approximately 90° out of phase relative to the current applied to coil  402 . The current in the coil  400  increases sinusoidally to its maximum. When the center  400   c  of coil  400  is directly below magnet pole  424 , the current in the coil  400  will be at its maximum. 
     Only a few examples of commutation have been described. Clearly, as will be appreciated by those skilled in the art, many other commutations may be applied to the coils  400 ,  402 ,  404 ,  406  and the other coils in the coil array  110 ,  210  to achieve force and motion of the coil array  110 ,  210  with respect to the magnet array  120 ,  220  in six directions: in the X, Y, Z directions and rotation about the X, Y, Z directions. 
     FIGS. 4A-4E illustrate embodiments of a process or method  401  of achieving motion of the coil arrays  110  and  210  with respect to the magnet arrays  120  and  220 . FIG. 4A shows a flow chart describing an embodiment of the method or process  401  beginning at a start procedure  403 . Procedure  405  positions a periodic coil array, such as coil arrays  110  and  210 , with respect to a periodic magnet array, such as magnet arrays  120  and  220 . Procedure  407  determines the position of the coils relative to the magnet array. Procedure  409  energizes the coils according to the commutation scheme. Procedure  411  determines whether the coil position has changed. If the coil position has changed, the method  401  returns to procedure  407  to determine the position of the coils relative to the magnet array. If the coil position has not changed, the method  401  ends at a procedure  413 . 
     Procedure  409  essentially controls a separation between a portion of the periodic coil array and a portion of the periodic magnet array by applying a current to one of the appropriate coils such that the appropriate coil interacts with a magnetic field associated with one of the magnets in the magnet array and generates Lorentz force. Multiple coils distributed in two directions in the periodic coil array are provided with appropriate currents to achieve separations between these other coils and other portions of the magnet array. 
     In this way, the method or process  401  achieves motion of the periodic coil array with respect to the periodic magnet array in a Z direction, separating the periodic magnet array and the periodic coil array, and further achieves rotation about the X and the Y directions. When these three degrees of freedom are combined with the three degrees of freedom illustrated in FIG. 3, the process or method  401  provides six degrees of freedom of motion of the periodic coil array with respect to the periodic magnet array. 
     FIGS. 4B and 4D illustrate side schematic views of a two-dimensional electric motor having a periodic coil array such as coil array  110  or  210  and a periodic magnet array such as magnet array  120  or  220 . As with the periodic magnet arrays described with reference to FIGS. 1B and 2B, the periodic magnet array  120 ,  220  alternates in polarity in the X direction. The coils in the coil array  110 ,  210  are grouped into two phases, A-phase coils and B-phase coils. FIGS. 4B and 4D shows two A-phase coils, A 1  and A 2 , and two B-phase coils, B 1  and B 2 . The two B-phase coils and the two A-phase coils in the section of the electric motor illustrated in FIGS. 4B and 4D are positioned such that the B 1  coil is between the A 1  and A 2  coils and the A- 2  coil is between the B 1  and B 2  coils. 
     FIG. 4C shows a periodic wave-like current distribution I x  applied to a coil, such as coil A 1 , as a function of the position of the coil along the X direction. The current distribution I x  is applied to achieve forces and motion of the coil relative to the magnet array  120 ,  220  in the X direction. FIG. 4B illustrates the direction of the current flow through the coils A 1 , A 2  and B 1  and B 2  as well as the resultant forces F exerted between the coils and the magnet array as a result of the interaction between the current and the magnetic field. Note that although the generated forces F contain Z direction components, these Z direction force components cancel out within the array of coils such that the forces and motions of the coils relative to the magnet array in the X direction are independent of those in the Z direction. 
     Positive and negative current inputs, as indicated by “x” on the I x -X current distribution graph, are part of an electrical current distribution in the X direction on the A 1  and A 2  coils. By the right-hand rule and/or the Lorentz force laws as described with reference to FIG. 3B, the current inputs generate forces between the A 1  and A 2  coils and the magnet array in the X direction. Alternatively, a periodic distribution such as a sine, triangle or square wave can also generate forces between the coils A 1  and A 2  and the magnet array and hence generate a force between the coil array and the magnet array in the X direction. Additionally, the sine wave shows a way of modulating the applied current signal in time as the coil A 1  moves in the X direction to maintain a constant force in the X direction. Of course, in that case the sine wave represents a position-varying current applied to the coil A 1  that results in a time varying current. 
     FIG. 4E shows a periodic wave-like current distribution I z  applied to a coil, such as coil A 1 , as a function of the position of the coil along the Z direction. The current distribution I z  is applied to the coil to achieve forces and motion of the coil relative to the magnet array  120 ,  220  the periodic coil array  110 ,  210  in the Z direction. FIG. 4D illustrates the direction of the current flow through the coils A 1 , A 2  and B 1  and B 2  as well as the result forces F exerted between the coils and the magnet array as a result of the interaction between the current and the magnetic field. Note that although the generated forces F contain X direction components, these X direction force components cancel out within the array of coils such that the forces and motions of the coils relative to the magnet array in the Z direction are independent of those in the X direction. 
     Positive and negative current inputs applied to the B 1  and B 2  coils produce separating forces in the Z direction between the periodic magnet array  120 ,  220  and the periodic coil array  110 ,  210  by the right-hand rule and/or the Lorentz force law, as described with reference to FIG.  3 B. As shown in FIG. 4E, a sine wave current distribution as a function of the X direction having a crest corresponding with the positive current input and a trough corresponding with the negative current input can also be used to generate separating Z forces. However, the sine wave of FIG. 4E is 90° out of phase with respect to the sine wave distribution of FIG.  4 C. 
     The method or process  401  outlined in FIGS. 4A-4E provides six degrees of freedom of motion between the magnet array  120 ,  220  and the coil array  110 ,  210 . Clearly, other portions in addition to the portion illustrated in FIGS. 4B and 4D may be similarly commutated by currents and current distributions as illustrated in FIG. 4C and 4E to produce differing magnitudes of X and Z forces on various portions of the electric motor. Since these various portions are separated by distances in the X direction and/or Y direction, rotational force about the Z direction and Y direction and linear force in the X direction can be produced. An identical analysis of forces acting upon coils distributed in the Y direction as in FIGS. 4B-4E implies rotational force about the X direction and Z direction and linear force in the Y direction can be achieved. Thus, the invention process or method  401  provides six degrees of freedom of motion for the electric motor having a periodic coil array and a periodic magnet array. 
     The electric motors  100  and  200  commutated by currents such as those illustrated in FIGS. 4C and 4D may be included in positioning devices. FIGS. 5,  6 A- 6 B, and  7 A- 7 B diagrammatically illustrate positioning devices incorporating motors of the present invention. 
     FIG. 5 diagrammatically illustrates a side cross-section of a positioning device  500  utilizing an embodiment of the motor  200  illustrated in FIG.  2 A. In the particular environment shown in FIG. 5, the positioning device  500  positions a wafer  502  in relation to a photolithography lens  504 . This arrangement typically occurs in semiconductor processing of the wafer  502 . The positioning device  500  has a frame  506  made of a magnetically permeable material such as iron. The frame is attached to a body  508  in some embodiments. 
     A support member  514  supports the wafer  502  and is attached to various magnets. The support member  514  may be any suitable device for supporting the wafer  502 . For example, the support member  514  may include a vacuum chuck or an electrostatic wafer chuck. Levitation or support magnets  510  and  512  on a same side of the support member  514  as the wafer  502  are magnetically attracted to portions of the frame  506 . The periodic magnet array  220  of the electric motor  200  may be attached to the support member  514  on a side opposite the side supporting the wafer  502 . 
     The support member  514  and hence the wafer  502  are positionable in three or six degrees of freedom. The magnetic flux of the levitation magnets  510  and  512  can be appropriately chosen to produce an attractive force between the levitation magnets  510  and  512  and the frame  506  to offset a combined weight of the levitation magnets  510  and  512 , the wafer  502 , the support member  514 , and the periodic magnet array  220 . Where a magnetic backing plate (not shown) is provided on one side of the coil array, the attractive force of the levitation magnets  510  and  512  may be necessary to counter the force between the magnetic coil array backing plate and the magnet array  220 . The interaction of the levitation magnets  510  and  512  with the magnetically permeable material in the frame  506  suspends or levitates the support member  514  motionless in a neutral position. Therefore, the interaction of the levitation magnets  510  and  512  with the frame  506  replaces an air bearing levitation of the support member  514 . The process or method  401  described with reference to FIGS. 4A-4C commutates coils in the periodic coil array  210  to position the support member in six degrees of freedom via interaction of the periodic coil array  210  with the periodic magnet array  220 . 
     The support member  514  may be made from a variety of materials. In particular, the support member may be made of a magnetically impermeable material such as a ceramic material. A magnetically impermeable material will generally be lighter and require less force to support than an a magnetically permeable material. In addition, magnetically impermeable materials do not interfere with the magnetic interactions suspending and moving the support member  514 . Alternatively, the support member  514  may be made from a combination of a magnetically impermeable material and a magnetically permeable material such as iron and ceramic. 
     FIGS. 6A and 6B diagrammatically illustrate embodiments of positioning devices  600  and  650  utilizing electric motors to suspend support members. In FIG. 6A, the positioning device  600  is a moving magnet electric motor having the magnet array  220  attached to the support member  602 . The positioning device  600  has a wafer  502  beneath a support member  602 . The magnet array  220  is on top of the support member  602  on a side of the support member  602  opposite a side contacting the wafer  502 . The magnet array  220  interacts with the coil array  210  to provide motion in three or six degrees of freedom depending upon the commutation. The method  401  described above with respect to FIGS. 4A-4E are directly applicable to the positioning device  600  to position the support member and hence the wafer  502  in six degrees of freedom with respect to the coil array  210 . In some embodiments of the positioning device  600 , the coil array  210  will have a magnetically permeable backing  604  interacting with magnets in the magnet array  220  to suspend the support member  602  in a neutral position. 
     The positioning device  650  illustrated in FIG. 6B utilizes the moving coil electric motor. The coil array  110  is attached to a support member  606 . The wafer  502  is on one side of the support member  606  and the coil array  110  is on the opposite side of the support member  606 . The magnet array  120  is positioned near the coil array  110 . By appropriate commutation of the coils in the coil array  110 , the support member  606  and hence the wafer  502  may move with respect to the magnet array  120  in either three or six degrees of freedom. 
     FIG. 7A diagrammatically illustrates side cross-section of a winged embodiment  700  of positioning device  600 . Magnet arrays  720   a,    720   b,    720   c,  and  720   d  are attached to a support member  702 . The support member  702  supports the wafer  502  on one side of the support member  702 . The magnet arrays  720   a  and  720   c  are attached to the same side of the support member  702  and the magnet arrays  720   b  and  720   d  are attached to an opposite side of the support member  702 . The magnet arrays interact with corresponding coil arrays  710   a,    710   b,    710   c,    710   d  to provide motion in three or six degrees of freedom depending upon the commutation of the coils in the coil arrays  710   a,    710   b,    710   c,    710   d.  As depicted, the coil array  710   a  is above the magnet array  720   a,  and the coil array  710   c  is above the magnet array  720   c.  The coil arrays  710   a  and  710   c  are attached to a magnetically permeable backing  730  in some embodiments. In addition, the coil array  710   b  is beneath the magnet array  720   b  and the coil array  710   d  is beneath the magnet array  720   d.  The coil arrays  710   b  and  710   d  are attached to a backing  740 . Backing  740  may be made of a magnetically impermeable material. 
     FIG. 7B diagrammatically illustrates side cross-sections of winged embodiments  750  of positioning device  650 . Coil arrays  760   a,    760   b,    760   c,  and  760   d  are attached to a support member  752 . The support member  752  supports the wafer  502  on one side of the support member  752 . The coil arrays  760   a  and  760   c  are attached to the same side of the support member  752  and the coil arrays  760   b  and  760   d  are attached to an opposite side of the support member  752 . The coil arrays interact with corresponding magnet arrays  770   a,    770   b,    770   c,    770   d  to provide motion in three or six degrees of freedom depending upon the commutation of the magnets in the magnet arrays  770   a,    770   b,    770   c,    770   d.  As depicted, magnet coil array  770   a  is above the coil array  760   a,  and the magnet array  710   c  is above the coil array  760   c.  The magnet arrays  770   a  and  770   c  are attached to a magnetically permeable backing  780  in some embodiments. In addition, the magnet array  770   b  is beneath the coil array  760   b  and the magnet array  770   d  is beneath the coil array  760   d.  The magnet arrays  770   b  and  770   d  are attached to a backing  790 . Backing  790  may be made of a magnetically impermeable material. 
     In either of the winged embodiments  700 ,  750  shown in FIGS. 7A and 7B, the embodiments  700 ,  750  may be utilized to adjust the distance between the support member  702 ,  752  and the backing  730 ,  780  and between support member  702 ,  752  and the backing  740 ,  790  such a static balance equilibrium may be achieved, given the mass of the moving array portion  720 ,  760  of the respective embodiments. Thus, the provision of support or levitation magnets may not be necessary in such embodiments. 
     The electric motors of the present invention may be used with a lithography system such as shown and described in, for example, U.S. Pat. No. 5,528,118, “Guideless Stage With Isolated Reaction Stage,” issued to Lee on Jun. 18, 1996, referenced in the Background section, which is incorporated herein by reference in its entirety. FIG. 8 shows a side view of an example of a photolithography system  800  using the electric motor  200  of the present invention. Although the photolithography system  800  is described as utilizing a moving magnet electric motor  812 , the photolithography system may be adapted to utilize a moving coil electric motor or other variations of the moving magnet electric motor. 
     The lithography system  800  generally comprises an illumination system  802  and a moving magnet electric motor  812  for wafer support and positioning. The illumination system  802  projects light through a reticle  806  which is supported by and scanned using stage  810 . The pattern on the reticle  806  is generally a circuit pattern for a semiconductor device. The reticle stage is supported by a frame  832 . The light is focused through a lens  804  supported on a body  826  which is in turn connected to the ground through a support  828 . The lens  804  is also connected to the illumination system  802  through frames  830 ,  832 , and  834 . The light exposes a layer of photoresist on a substrate such as a wafer  808 . 
     The wafer  808  is supported by and scanned using a stage  820  which is in turn supported and positioned by the moving magnet electric motor  812 . The electric motor  812  comprises a moving magnet array  814  and a fixed coil array  818 . The wafer stage  820  is supported by air bearings  816  on a plate  836 . The wafer stage system, including the backing plate  822 , is connected to the body  826  through the frame  824 . It is to be understood that the photolithography system may be different than the one shown herein without departing from the scope of the invention. 
     Although only a few exemplary embodiments of this invention have been described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the exemplary embodiments without materially departing from the novel teachings and advantages of this invention. For example, although depicted as being planar, the arrays of magnetic poles and coils can have either constant or varying curvature in one or two-dimensions as in cylindrical, toroidal, and spherical arrangements of magnetic poles and coils. For cylindrical arrangements, latitudinal and longitudinal directions may be defined, for example, in standard cylindrical coordinates with corresponding diagonal directions, and parallel arrays and coils lie on parallel surfaces. Accordingly, all such modifications are intended to be within the scope of the following claims.