Abstract:
Methods and apparatus for increasing the efficiency of a voice coil motor (VCM) are disclosed. According to one aspect of the present invention, a cylindrical and radially symmetric VCM includes a plurality of sets of magnets, and a single coil. The plurality of sets of the magnets are each arranged in an array configuration, and cooperate to form a magnetic field. The coil receives current and has a plurality of windings. A first space is defined within the coil, and the plurality of sets of the magnets are arranged such that a first set of the magnets is positioned within the first space and a second set of the magnets is positioned external to the coil.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This patent application is a divisional of U.S. patent application Ser. No. 10/908,178, filed Apr. 29, 2005, which claims the benefit of U.S. provisional patent application 60/624,243, filed Nov. 2, 2004, which are incorporated by reference in their entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of Invention 
         [0003]    The present invention relates generally to actuators. More particularly, the present invention relates to a radially symmetric voice coil motor which utilizes a magnet array which includes wedge-shaped cylindrical magnets. 
         [0004]    2. Description of the Related Art 
         [0005]    For precision instruments such as photolithography machines which are used in semiconductor processing, factors which affect the performance, e.g., accuracy, of the precision instrument generally must be dealt with and, insofar as possible, eliminated. When the performance of a precision instrument such a wafer scanning stage or a reticle scanning stage is adversely affected, products formed using the precision instrument may be improperly formed and, hence, function improperly. 
         [0006]    Voice coil motors (VCMs) are often used in lithography machines to enable accurate movement of a wafer scanning stage along one axis. A VCM utilizes a magnetic coil that is placed in a magnetic field. When current is applied to the coil, a force is generated between the coil and the magnetic field predominantly in a single degree of freedom. Small VCMs are capable of generating force in one direction with a relatively high efficiency. 
         [0007]    When a VCM is to be used in a system that is sensitive to heat such as a lithography system, excessive heat may cause the accuracy with which a lithography process may be performed to be compromised. In addition, excessive heat may cause any insulating material in coils of a VCM, as well as the copper from which coils are often formed, to melt. The amount of force that may be generated by a VCM is also limited by the amount of current which may be supplied by an amplifier, or by electronics which are used to control the VCM. 
         [0008]    Therefore, what is needed is a method and an apparatus for enabling the force generated by a VCM to be increased without significantly increasing the current usage of the VCM or the heat output of the VCM. 
       BRIEF SUMMARY OF THE INVENTION 
       [0009]    The present invention relates to a voice coil motor (VCM) which uses a magnetic circuit which includes magnets with wedge-shaped cross-sections. According to one aspect of the present invention, a cylindrical and radially symmetric VCM includes a plurality of sets of magnets, and a coil. The plurality of sets of the magnets are each arranged in an array configuration, and cooperate to form a magnetic field. The coil receives current and has a plurality of windings. A first space is defined within the coil, and the plurality of sets of the magnets are arranged such that a first set of the magnets is positioned within the first space and a second set of the magnets is positioned external to the coil. The coil is the only coil associated with the VCM which moves within the magnetic field between the two sets of magnets. 
         [0010]    In one embodiment, the first set of the magnets includes at least a first magnet that is wedge-shaped. In another embodiment, the VCM also includes a magnetic material that physically couples the first set and the second set to enable a flux path created within the VCM to pass between the first set and the second set. 
         [0011]    The use of Halbach arrays of magnets in a VCM generally enables the strength of a magnetic field within the VCM to be increased. By configuring the magnets in Halbach arrays such that at least some of the magnets have a non-rectangular cross-section, e.g., a substantially trapezoidal or triangular cross-section, the strength of the magnetic field in a VCM which includes the Halbach arrays may be further increased, thereby enabling the force generated by the VCM to be increased without significantly increasing the heat produced by the VCM. In some instances, the configuration of the magnets with a non-rectangular cross-section, i.e., the wedge-shaped magnets, is such that a single coil rather than a pair of coils may be used within the VCM. 
         [0012]    According to another aspect of the present invention, a VCM includes a first magnet arrangement, a second magnet arrangement, and a coil arrangement. The first magnet arrangement includes at least a first magnet having a cross-section that is approximately rectangular and a second magnet having a cross-section that is non-rectangular relative to a plane. The first magnet is arranged substantially at a center of the first magnet arrangement and the first magnet and the second magnet are arranged to be in contact. The second magnet arrangement includes at least a third magnet having a cross-section that is approximately rectangular and a fourth magnet having a cross-section that is non-rectangular relative to the plane, where the third magnet is arranged substantially at a center of the second magnet arrangement and the third magnet and the fourth magnet are arranged to be in contact. The coil arrangement is at least partially positioned between the first magnet arrangement and the second magnetic arrangement, and moves relative to the first magnet arrangement and the second magnet arrangement within a magnetic field associated with the first magnet arrangement and the second magnet arrangement. 
         [0013]    In one embodiment, the VCM also includes at least one plate from a magnetic material that is in physical contact with both the first magnet arrangement and the second magnet arrangement. In another embodiment, the coil arrangement includes a single-phase coil. 
         [0014]    According to yet another aspect of the present invention, a VCM includes a first magnet arrangement which includes a first plurality of magnets which each have a cross-section that is non-rectangular. The VCM also includes a second magnet arrangement that includes a second plurality of magnets each having a cross-section that is non-rectangular. The first magnet arrangement and the second magnet arrangement are arranged to cooperate to generate a magnetic field. A coil arrangement of the VCM is at least partially positioned between the first magnet arrangement and the second magnetic arrangement and is arranged to receive current to generate a force between the coil arrangement and the first magnet arrangement and the second magnet arrangement within the magnetic field. 
         [0015]    These and other advantages of the present invention will become apparent upon reading the following detailed descriptions and studying the various figures of the drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0016]    The invention may best be understood by reference to the following description taken in conjunction with the accompanying drawings in which: 
           [0017]      FIG. 1  is a diagrammatic representation of cylindrical Halbach magnet arrays in accordance with an embodiment of the present invention. 
           [0018]      FIG. 2  is a diagrammatic cross-sectional representation of a radially symmetric cylindrical VCM which utilizes a substantially cylindrical wedge Halbach array in accordance with an embodiment of the present invention. 
           [0019]      FIG. 3  is a diagrammatic cross-sectional representation of a radially symmetric cylindrical VCM which utilizes a substantially cylindrical wedge Halbach array in accordance with another embodiment of the present invention. 
           [0020]      FIG. 4  is a diagrammatic cross-sectional block diagram representation of a portion of a single coil and a portion of two wedge Halbach magnet arrays in accordance with an embodiment of the present invention. 
           [0021]      FIG. 5   a  is a block diagram representation of a magnet array, e.g., magnet array  416  of  FIG. 4 , in accordance with an embodiment of the present invention. 
           [0022]      FIG. 5   b  is a diagrammatic representation of a portion of a wedge-shaped magnet, e.g., wedge magnet  414   b  of  FIG. 4 , in accordance with an embodiment of the present invention. 
           [0023]      FIG. 6  is a representation of magnetic field equipotential lines associated with a coil and wedge Halbach magnet arrays, i.e., coil  418  and wedge Halbach magnet arrays  406 ,  416  of  FIG. 4 , in accordance with an embodiment of the present invention. 
           [0024]      FIG. 7  is a cross-sectional block diagram representation of approximately half of a VCM which uses wedge Halbach arrays and a single coil in accordance with an embodiment of the present invention. 
           [0025]      FIG. 8  is a cross-sectional block diagram representation of a VCM which uses wedge Halbach arrays and a single coil in accordance with an embodiment of the present invention. 
           [0026]      FIG. 9   a  is a diagrammatic representation of an overall wedge-shaped cylindrical magnet, e.g., magnet  702   b  of  FIG. 7 , in accordance with an embodiment of the present invention. 
           [0027]      FIG. 9   b  is a diagrammatic representation of an overall wedge-shaped cylindrical magnet, e.g., magnet  702   e  of  FIG. 7 , in accordance with an embodiment of the present invention. 
           [0028]      FIG. 10  is a diagrammatic representation of a photolithography apparatus in accordance with an embodiment of the present invention. 
           [0029]      FIG. 11  is a process flow diagram which illustrates the steps associated with fabricating a semiconductor device in accordance with an embodiment of the present invention. 
           [0030]      FIG. 12  is a process flow diagram which illustrates the steps associated with processing a wafer, i.e., step  1304  of  FIG. 11 , in accordance with an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0031]    Typically, when a higher force is to be provided by a voice coil motor (VCM), the current input into the VCM is increased, and, consequently, the heat generated by the VCM is increased. Increasing current requirements is often impractical, and increased heat associated with the operation of a VCM may adversely affect heat-sensitive systems, e.g., lithography equipment. 
         [0032]    By increasing the magnetic field within a VCM, i.e., by increasing the strength or the magnitude of the magnetic field within a VCM, a higher force may be generated using the VCM, substantially without increasing the amount of heat generated by the VCM and increasing the amount of current used by the VCM. The magnetic field within a VCM may be increased by utilizing a magnet array in the VCM and, further, by varying the shape of magnets in the magnet array. In one embodiment, the magnet array is a substantially cylindrical wedge Halbach array. 
         [0033]    A substantially cylindrical wedge Halbach array is an array of magnets which includes at least one wedge-shaped magnet. A wedge-shaped magnet is generally a magnet which has either a substantially triangular cross-section with respect to at least one plane or a substantially trapezoidal cross-section with respect to at least one plane. When a wedge-shaped magnet has a substantially trapezoidal cross-section with respect to a plane, while two opposite sides of the cross-section are substantially parallel to each other within the plane, the other two sides are not substantially parallel to each other within the plane. 
         [0034]    A VCM which includes a substantially cylindrical wedge Halbach array may have a variety of different configurations. In general, the wedge-shaped magnets are arranged to form an overall cylindrical Halbach array. As shown in  FIG. 1 , an outer Halbach magnet array  360  typically has an overall, hollow cylindrical shape or an overall donut shape. However, the individual magnets of magnet array  360  may be wedge-shaped donuts. Similarly, an inner Halbach magnet array  364  also has an overall hollow, cylindrical shape, though the individual magnets of magnet array  364  may be wedge-shaped donuts. 
         [0035]    The orientation of wedge-shaped magnets and other magnets within the substantially cylindrical wedge Halbach array may be widely varied.  FIG. 2  is a diagrammatic cross-sectional representation of a radially symmetric cylindrical VCM which utilizes a substantially cylindrical wedge Halbach array in accordance with an embodiment of the present invention. A VCM  370 , as shown in cross-section, includes non-magnetic hollow cylinders  374   a,    374   b  which are substantially centered about a central axis  378 . Alternatively, either or both hollow cylinders  374   a,    374   b  may be formed from a magnetic material. 
         [0036]    Magnet arrays  382 ,  392  are also effectively hollow cylinders that are positioned within VCM  370  about central axis  378  such that coils  398   a ,  398   b , having center lines that are substantially coincident with central axis  378 , are positioned between magnet arrays  382 ,  392 . Coils  398   a ,  398   b  are such that the turns or windings of coils  398   a ,  398   b  effectively define a ring or a donut shape for coils  398   a ,  398   b . Hollow cylinder  374   b  and magnet array  392  are positioned within the inner area of the ring or the donut shape defined by coils  398   a ,  398   b . As will be understood by those skilled in the art, coils  398   a ,  398   b  are generally substantially encased in cooling cans which are filled with coolant. However, for ease of illustration, cooling cans have not been shown. 
         [0037]    As shown, coils  398   a ,  398   b  are positioned between magnet arrays  382 ,  392 . Coils  398   a ,  398   b  move relative to magnet arrays  382 ,  392  and, hence, within a magnetic field associated with magnet arrays  382 ,  392  when current is applied. Each magnet array  382 ,  392  includes a plurality of magnets, i.e., permanent magnets. By way of example, magnet array  382  includes magnets  382   a - 382   c  which each have an overall donut shape but each have a substantially non-rectangular, e.g., triangular, cross-section in at least one plane. That is, magnets  382   a - c  are each substantially wedge-shaped donuts. By shaping magnets  382   a - c  as wedges, or with wedge-shaped cross sections, the magnetic field associated with VCM  370  may be enhanced. Magnets  392   a - c  are also each substantially wedge-shaped donuts. 
         [0038]    With reference to  FIG. 3 , a second radially symmetric cylindrical VCM which utilizes a substantially cylindrical wedge Halbach array will be described in accordance with another embodiment of the present invention. A VCM  300 , which is shown in cross-section, includes non-magnetic cylinders  304   a ,  304   b  which are substantially centered about a central axis  308 . It should be appreciated that cylinders  304   a ,  304   b  are each effectively a hollow cylinder with a center that is coincident with central axis  308 . 
         [0039]    Magnet arrays  312 ,  322  are also arranged as hollow cylinders, and are positioned within VCM  300  such that coils  318   a ,  318   b , which each effectively form a hollow, cylindrical shape with center lines that are substantially coincident with central axis  308 , are positioned between magnet arrays  312 ,  322 . Magnet arrays  312 ,  322  are also centered about central axis  308 . Coils  318   a ,  318   b  are such that the turns or windings of coils  318   a ,  318   b  effectively define a ring or a donut shape for coils  318   a ,  318   b.    
         [0040]    Coils  318   a ,  318   b  are positioned between magnet arrays  312 ,  322  and, hence, within a magnetic field associated with magnet arrays  312 ,  322  when current is applied. Each magnet array  312 ,  322  includes a plurality of magnets. By way of example, magnet array  312  includes magnets  312   a - 312   e  which each have an overall donut shape. Magnets  312   a  and  312   e , which make up the ends of magnet array  312 , are substantially block-shaped donuts. That is, magnets  312   a ,  312   e  have substantially rectangular, as for example square, cross-sections in at least one plane. Magnets  312   b - d  are each substantially wedge-shaped donuts, i.e., magnets  312   b - d  each have a substantially triangular or trapezoidal cross-section in one plane. By shaping magnets  312   b - d  as wedges, or with wedge-shaped cross sections, the magnetic field associated with VCM  300  may be enhanced. As shown, magnets  322   b - d  are also each substantially wedge-shaped donuts, while magnets  322   a ,  322   e  are substantially block-shaped donuts. 
         [0041]    While the use of wedge-shaped magnets in a Halbach array within a VCM improves the efficiency of the VCM, i.e., by increasing the strength of the magnetic field within the VCM, the efficiency of a VCM may further be increased by essentially concentrating a magnetic field near a coil of a VCM. In one embodiment, by altering the orientation of magnets associated with a wedge Halbach array of a VCM such that the use of only a single coil in the VCM is possible, the efficiency of the VCM is further enhanced. 
         [0042]    By orienting the magnets in a Halbach array of a VCM such that a magnet with radial magnetization is effectively in the center of the Halbach array, while magnets with longitudinal magnetization are at the ends of the Halbach array, a single coil may be used within the VCM. It should be appreciated that for a Halbach array with a given total magnet height, the height of a single coil is generally greater than or equal to the combined height of two coils used in VCMs, e.g., VCM  300  of  FIG. 3 , with magnet arrays of the given total magnet height. The use of a single coil enables the magnetic field within a VCM to be substantially concentrated through the coil, thereby further increasing the efficiency of the VCM. 
         [0043]      FIG. 4  is a diagrammatic cross-sectional block diagram representation of a portion of a single coil and a portion of two wedge Halbach magnet arrays in accordance with an embodiment of the present invention. Within a VCM, a portion of a coil  418  is positioned such that a first wedge Halbach magnet array  406  is located in a space defined by an inner edge  424  of coil  418  and a second wedge Halbach magnet array  416  is positioned outside an outer edge  426  of coil  418 . 
         [0044]    Both magnet arrays  406 ,  416  include wedge magnets, or magnets which have a cross-section that is either approximately triangular or trapezoidal. Magnet array  406  includes wedge magnets  402   a ,  402   b ,  402   d ,  402   e , whereas magnet array  416  includes wedge magnets  414   a ,  414   b ,  414   d ,  414   e . Magnets  402   c ,  414   c , which are arranged at the center of magnet arrays  406 ,  416 , respectively, are substantially block-shaped, or have approximately rectangular or square cross-sections. Specifically, magnets  402   c ,  414   c  are donut-shaped blocks. The orientation of magnets  402   a - e  within magnet array  406 , and the orientation of magnets  414   a - e  within magnet array  416  are such that a magnetic field is centered approximately along a horizontal centerline  438  of magnet arrays  406 ,  416 . Magnetic field equipotential lines associated with an arrangement of magnet arrays  406 ,  416  and coil  418  as shown will be described below with respect to  FIG. 6 . 
         [0045]    With reference to  FIGS. 5   a  and  5   b , the positioning and shapes of magnets associated with magnet array  406  will be described.  FIG. 5   a  is a block diagram representation of magnet array  416  of  FIG. 4  in accordance with an embodiment of the present invention. As previously described, magnet array  416  includes magnets  414   a - e  which may each, in one embodiment, be permanent magnets. Magnet  414   c , which has a substantially rectangular cross-section, generally has a magnetic field direction that is substantially parallel to a y-axis  502 , while magnets  414   a ,  414   e , which are each shaped as a wedge, have magnetic field directions that are parallel to a z-axis  504 . Magnets  414   b ,  414   d , which are each also shaped as a wedge and have a non-rectangular, e.g., substantially trapezoidal, cross-section generally have associated magnetic field directions that are neither horizontal, e.g., parallel to y-axis  502 , nor vertical, e.g., parallel to z-axis  504 . 
         [0046]    As shown, magnets  414   a ,  414   b ,  414   d ,  414   e  each have a non-rectangular cross-section, e.g., a substantially trapezoidal cross-section, at least in a plane that is defined by y-axis  502  and z-axis  504 . Magnet  414   c  has a rectangular cross-section in the plane that is defined by y-axis  502  and z-axis  504 . 
         [0047]    The relative sizes of each magnet  414   a - e  within magnet array  416  may vary widely. It should be appreciated that the size and shape of each magnet  414   a - e  may also vary. By way of example, as shown in  FIG. 5   b , a portion  414   b ′ of wedge  414   b  is shaped such that in a plane defined by y-axis  502  and z-axis  504 , a cross-section of a wedge portion  414   b ′ is approximately trapezoidal, whereas a cross-section of wedge portion  414   b ′ in a plane defined by z-axis  504  and an x-axis  506  is substantially rectangular. Wedge portion  414   b ′ has at least some edges, as for example edges  540 ,  542 ,  544 , which have some curvature as wedge portion  414   b ′ is a part of a wedge  414   b  which is effectively a wedge-shaped donut. 
         [0048]      FIG. 6  is a representation of magnetic field equipotential lines associated with a coil and wedge Halbach magnet arrays, i.e., coil  418  and wedge Halbach magnet arrays  406 ,  416  of  FIG. 4 , in accordance with an embodiment of the present invention. As shown, equipotential or flux lines  602   a re generally concentrated in the vicinity of coil  418 . Equipotential lines may be arranged to pass substantially through a center of coil  418 , and through magnet block  402   c  of magnet array  406  and through magnet block  414   c  of magnet array  416 . It should be appreciated that only representative equipotential lines  602  have been shown for ease of illustration. 
         [0049]    In general, equipotential lines  602  pass through a medium which allows flux to pass from magnet array  406  to magnet array  416 , and vice versa. The medium that allows flux to pass between magnet array  406  and magnet array  416  is preferably a relatively high permeability magnetic medium, as for example magnetic steel.  FIG. 7  is a cross-sectional block diagram representation of approximately half of a VCM which uses wedge Halbach arrays and a single coil in accordance with an embodiment of the present invention. A portion  700  of a VCM includes a coil  718 , which is effectively shielded by a cooling can  720 . In general, cooling can  720  may be formed from substantially any suitable material. Suitable materials include, but are not limited to, plastic, sheet metal, and carbon fiber. A coolant is typically provided within cooling can  720  such that coil  718 , which has an overall hollow cylindrical shape, is surrounded by the coolant. 
         [0050]    Flux is carried between “inner” magnets  702   a - e  and “outer” magnets  714   a - e,  e.g., from magnets  714   a - e  to magnets  702   a - e,  through magnetic material  770 . Magnetic material  770  may be a material such as magnetic steel or iron. Typically, magnetic material  770  is arranged as plates which are substantially located on top and on the bottom of portion  700  to enable flux to “return” flux from magnets  714   a - e  to magnets  702   a - e.    
         [0051]    Coil  718  is powered by electronics  750  which often include components such as an amplifier and a current supply. In one embodiment, coil  718  is a single-phase coil. When current flows through coil  718 , coil  718  moves relative to magnets  702   a - e,    714   a - e  within a magnetic field associated with magnets  702   a - e,    714   a - e.    
         [0052]    Since flux is conducted through magnets  702   a - e,    714   a - e  and magnetic material  770  which form a magnetic circuit, sides  760  of portion  700  may be formed from a non-magnetic material. The non-magnetic material may generally be a material such as stainless steel, aluminum, ceramic, or plastic. Alternatively, sides  760  may be formed from a magnetic material. 
         [0053]    As mentioned above, portion  700  is a part of a VCM. With reference to  FIG. 8 , a more complete cross-sectional representation of a VCM will be described in accordance with an embodiment of the present invention. A radially symmetric cylindrical VCM  800 , which has a centerline  808  that is parallel to a z-axis  890 , includes portion  700  of  FIG. 7  as well as a portion  810 , which is effectively a mirror image of portion  700 . Coil  718  moves within space  820  and provides a force in a direction along z-axis  890 , and is substantially positioned at a center of VCM  800 . It should be appreciated that in addition to allowing movement along z-axis  890  and providing force in a direction along z-axis  890 , VCM  800  may also permit a slight rotation about z-axis  890 , a y-axis  894 , or an x-axis  892 , as well as slight movement in directions along x-axis  892  and y-axis  894 . 
         [0054]    Magnets within VCM  800  are generally configured as block-shaped or wedge-shaped donuts. By way of example, wedge-shaped magnet  702   b  may generally have a donut shape as shown in  FIG. 9   a , whereas wedge-shaped magnet  702   e  may generally have a donut shape as shown in  FIG. 9   b . The donut shapes are of magnets  702   b ,  702   e  are such that the footprints of magnets  702   b ,  702   e , taken with respect to a plane defined by x-axis  892  and y-axis  894 , are substantially ring-like in shape. 
         [0055]    A VCM which utilizes a substantially cylindrical wedge Halbach array is suitable for a variety of different uses. By way of example, since the heat generated by such a VCM is not significant, and such a VCM provides a significant amount of force, such a VCM is particularly suitable for use as a component within a photolithography apparatus. Referring next to  FIG. 10 , a photolithography apparatus which may use a VCM with a substantially cylindrical wedge Halbach array will be described in accordance with an embodiment of the present invention. A photolithography apparatus (exposure apparatus)  40  includes a wafer positioning stage  52  that may be driven by a planar motor or linear motors (not shown), as well as a wafer table  51  that is coupled to wafer positioning stage  52  by utilizing an actuator spring or other means. The planar motor which drives wafer positioning stage  52  generally uses an electromagnetic force generated by magnets and corresponding armature coils. A wafer  64  is held in place on a wafer holder or chuck  74  which is coupled to wafer table  51 . Wafer positioning stage  52  is arranged to move in multiple degrees of freedom, e.g., between one to six degrees of freedom, under the control of a control unit  60  and a system controller  62 . The movement of wafer positioning stage  52  allows wafer  64  to be positioned at a desired position and orientation relative to a projection optical system  46 . 
         [0056]    Wafer table  51  may be levitated in a z-direction  10   b  by any number of VCMs (not shown), e.g., three voice coil motors. The VCMs may include substantially cylindrical wedge Halbach arrays. Optionally, at least one electromagnetic actuator (not shown) may couple and move wafer table  51  along an x-axis  10   c  or a y-axis  10   a.  The motor array of wafer positioning stage  52  is typically supported by a base  70 . Base  70  is supported to a ground via isolators  54 . Reaction forces generated by motion of wafer stage  52  may be mechanically released to a ground surface through a frame  66 . One suitable frame  66  is described in JP Hei 8-166475 and U.S. Pat. No. 5,528,118, which are each herein incorporated by reference in their entireties. 
         [0057]    An illumination system  42  is supported by a frame  72 . Frame  72  is supported to the ground via isolators  54 . Illumination system  42  includes an illumination source, and is arranged to project a radiant energy, e.g., light, through a mask pattern on a reticle  68  that is supported by and scanned using a reticle stage  44  which includes a coarse stage and a fine stage. The radiant energy is focused through projection optical system  46 , which is supported on a projection optics frame  50  and may be supported the ground through isolators  54 . Suitable isolators  54  include those described in JP Hei 8-330224 and U.S. Pat. No. 5,874,820, which are each incorporated herein by reference in their entireties. 
         [0058]    A first interferometer  56  is supported on projection optics frame  50 , and functions to detect the position of wafer table  51 . Interferometer  56  outputs information on the position of wafer table  51  to system controller  62 . A second interferometer  58  is supported on projection optics frame  50 , and detects the position of reticle stage  44  which supports a reticle  68 . Interferometer  58  also outputs position information to system controller  62 . 
         [0059]    It should be appreciated that there are a number of different types of photolithographic apparatuses or devices. For example, photolithography apparatus  40 , or an exposure apparatus, may be used as a scanning type photolithography system which exposes the pattern from reticle  68  onto wafer  64  with reticle  68  and wafer  64  moving substantially synchronously. In a scanning type lithographic device, reticle  68  is moved perpendicularly with respect to an optical axis of a lens assembly (projection optical system  46 ) or illumination system  42  by reticle stage  44 . Wafer  64  is moved perpendicularly to the optical axis of projection optical system  46  by a wafer stage  52 . Scanning of reticle  68  and wafer  64  generally occurs while reticle  68  and wafer  64  are moving substantially synchronously. 
         [0060]    Alternatively, photolithography apparatus or exposure apparatus  40  may be a step-and-repeat type photolithography system that exposes reticle  68  while reticle  68  and wafer  64  are stationary, i.e., at a substantially constant velocity of approximately zero meters per second. In one step and repeat process, wafer  64  is in a substantially constant position relative to reticle  68  and projection optical system  46  during the exposure of an individual field. Subsequently, between consecutive exposure steps, wafer  64  is consecutively moved by wafer positioning stage  52  perpendicularly to the optical axis of projection optical system  46  and reticle  68  for exposure. Following this process, the images on reticle  68  may be sequentially exposed onto the fields of wafer  64  so that the next field of semiconductor wafer  64  is brought into position relative to illumination system  42 , reticle  68 , and projection optical system  46 . 
         [0061]    It should be understood that the use of photolithography apparatus or exposure apparatus  40 , as described above, is not limited to being used in a photolithography system for semiconductor manufacturing. For example, photolithography apparatus  40  may be used as a part of a liquid crystal display (LCD) photolithography system that exposes an LCD device pattern onto a rectangular glass plate or a photolithography system for manufacturing a thin film magnetic head. Photolithography apparatus  40  may also be used as a part of an immersion lithography system. 
         [0062]    The present invention may be utilized in an immersion type exposure apparatus when suitable measures for a liquid are incorporated. By way of example, PCT patent application WO 99/49504, which is incorporated herein by reference in its entirety, discloses an exposure apparatus in which a liquid is supplied to a space between a substrate such as a wafer and a projection lens system in an exposure process. 
         [0063]    Further, the present invention may be utilized in an exposure apparatus which includes two or more substrates and/or reticle stages. In such an apparatus, the additional stage may be used in parallel or preparatory steps while the other stage may be used for exposing. Exemplary multiple stage exposure apparatuses are described, for example, in Japan patent Application Disclosure No. 10-163099, as well as in Japan patent Application Disclosure No. 10-214783 and its counterparts U.S. Pat. No. 6,341,007, U.S. Pat. No. 6,400,441, U.S. Pat. No. 6,549,269, and U.S. Pat. No. 6,590,634. Each of these references is herein incorporated by reference in its entirety. Other exemplary multiple stage exposure apparatuses are described in Japan patent Application Disclosure No. 2000-505958, as well as in U.S. Pat. No. 5,969,441 and U.S. Pat. No. 6,208,407. Each of these references is herein incorporated by reference in its entirety 
         [0064]    The present invention may also be utilized in an exposure apparatus that has a movable stage which retains a substrate, e.g., a wafer, for exposure, and a stage having various sensor or measurement tools for measuring, as described in Japan patent Disclosure No. 11-135400. As far as is permitted, the disclosure of Japan patent Disclosure No. 11-135400 is incorporated herein by reference in its entirety. 
         [0065]    In addition, the present invention may be utilized in an exposure apparatus that is operated in a vacuum environment. It should be appreciated that suitable measures may need to be incorporated to the present invention to accommodate a vacuum environment for the air, or fluid, bearing arrangements. Such an exposure apparatus may be, but is not limited to being, an EB type exposure apparatus, or an EUVL type exposure apparatus. 
         [0066]    The illumination source of illumination system  42  may be g-line (436 nanometers (nm)), i-line (365 nm), a KrF excimer laser (248 nm), an ArF excimer laser (193 nm), and an F 2 -type laser (157 nm). Alternatively, illumination system  42  may also use charged particle beams such as x-ray and electron beams. For instance, in the case where an electron beam is used, thermionic emission type lanthanum hexaboride (LaB 6 ) or tantalum (Ta) may be used as an electron gun. Furthermore, in the case where an electron beam is used, the structure may be such that either a mask is used or a pattern may be directly formed on a substrate without the use of a mask. 
         [0067]    With respect to projection optical system  46 , when far ultra-violet rays such as an excimer laser is used, glass materials such as quartz and fluorite that transmit far ultra-violet rays is preferably used. When either an F 2 -type laser or an x-ray is used, projection optical system  46  may be either catadioptric or refractive (a reticle may be of a corresponding reflective type), and when an electron beam is used, electron optics may comprise electron lenses and deflectors. As will be appreciated by those skilled in the art, the optical path for the electron beams is generally in a vacuum. 
         [0068]    In addition, with an exposure device that employs vacuum ultra-violet (VUV) radiation of a wavelength that is approximately 200 nm or lower, use of a catadioptric type optical system may be considered. Examples of a catadioptric type of optical system include, but are not limited to, those described in Japan Patent Application Disclosure No. 8-171054 published in the Official gazette for Laid-Open Patent Applications and its counterpart U.S. Pat. No. 5,668,672, as well as in Japan Patent Application Disclosure No. 10-20195 and its counterpart U.S. Pat. No. 5,835,275, which are all incorporated herein by reference in their entireties. In these examples, the reflecting optical device may be a catadioptric optical system incorporating a beam splitter and a concave mirror. Japan Patent Application Disclosure (Hei) No. 8-334695 published in the Official gazette for Laid-Open Patent Applications and its counterpart U.S. Pat. No. 5,689,377, as well as Japan Patent Application Disclosure No. 10-3039 and its counterpart U.S. Pat. No. 5,892,117, which are all incorporated herein by reference in their entireties. These examples describe a reflecting-refracting type of optical system that incorporates a concave mirror, but without a beam splitter, and may also be suitable for use with the present invention. 
         [0069]    Further, in photolithography systems, when linear motors (see U.S. Pat. Nos. 5,623,853 or 5,528,118, which are each incorporated herein by reference in their entireties) are used in a wafer stage or a reticle stage, the linear motors may be either an air levitation type that employs air bearings or a magnetic levitation type that uses Lorentz forces or reactance forces. Additionally, the stage may also move along a guide, or may be a guideless type stage which uses no guide. 
         [0070]    Alternatively, a wafer stage or a reticle stage may be driven by a planar motor which drives a stage through the use of electromagnetic forces generated by a magnet unit that has magnets arranged in two dimensions and an armature coil unit that has coil in facing positions in two dimensions. With this type of drive system, one of the magnet unit or the armature coil unit is connected to the stage, while the other is mounted on the moving plane side of the stage. 
         [0071]    Movement of the stages as described above generates reaction forces which may affect performance of an overall photolithography system. Reaction forces generated by the wafer (substrate) stage motion may be mechanically released to the floor or ground by use of a frame member as described above, as well as in U.S. Pat. No. 5,528,118 and published Japanese Patent Application Disclosure No. 8-166475. Additionally, reaction forces generated by the reticle (mask) stage motion may be mechanically released to the floor (ground) by use of a frame member as described in U.S. Pat. No. 5,874,820 and published Japanese Patent Application Disclosure No. 8-330224, which are each incorporated herein by reference in their entireties. 
         [0072]    Isolaters such as isolators  54  may generally be associated with an active vibration isolation system (AVIS). An AVIS generally controls vibrations associated with forces  112 , i.e., vibrational forces, which are experienced by a stage assembly or, more generally, by a photolithography machine such as photolithography apparatus  40  which includes a stage assembly. 
         [0073]    A photolithography system according to the above-described embodiments, e.g., a photolithography apparatus which may include one or more dual force actuators, may be built by assembling various subsystems in such a manner that prescribed mechanical accuracy, electrical accuracy, and optical accuracy are maintained. In order to maintain the various accuracies, prior to and following assembly, substantially every optical system may be adjusted to achieve its optical accuracy. Similarly, substantially every mechanical system and substantially every electrical system may be adjusted to achieve their respective desired mechanical and electrical accuracies. The process of assembling each subsystem into a photolithography system includes, but is not limited to, developing mechanical interfaces, electrical circuit wiring connections, and air pressure plumbing connections between each subsystem. There is also a process where each subsystem is assembled prior to assembling a photolithography system from the various subsystems. Once a photolithography system is assembled using the various subsystems, an overall adjustment is generally performed to ensure that substantially every desired accuracy is maintained within the overall photolithography system. Additionally, it may be desirable to manufacture an exposure system in a clean room where the temperature and humidity are controlled. 
         [0074]    Further, semiconductor devices may be fabricated using systems described above, as will be discussed with reference to  FIG. 11 . The process begins at step  1301  in which the function and performance characteristics of a semiconductor device are designed or otherwise determined. Next, in step  1302 , a reticle (mask) in which has a pattern is designed based upon the design of the semiconductor device. It should be appreciated that in a parallel step  1303 , a wafer is made from a silicon material. The mask pattern designed in step  1302  is exposed onto the wafer fabricated in step  1303  in step  1304  by a photolithography system. One process of exposing a mask pattern onto a wafer will be described below with respect to  FIG. 12 . In step  1305 , the semiconductor device is assembled. The assembly of the semiconductor device generally includes, but is not limited to, wafer dicing processes, bonding processes, and packaging processes. Finally, the completed device is inspected in step  1306 . 
         [0075]      FIG. 12  is a process flow diagram which illustrates the steps associated with wafer processing in the case of fabricating semiconductor devices in accordance with an embodiment of the present invention. In step  1311 , the surface of a wafer is oxidized. Then, in step  1312  which is a chemical vapor deposition (CVD) step, an insulation film may be formed on the wafer surface. Once the insulation film is formed, in step  1313 , electrodes are formed on the wafer by vapor deposition. Then, ions may be implanted in the wafer using substantially any suitable method in step  1314 . As will be appreciated by those skilled in the art, steps  1311 - 1314  are generally considered to be preprocessing steps for wafers during wafer processing. Further, it should be understood that selections made in each step, e.g., the concentration of various chemicals to use in forming an insulation film in step  1312 , may be made based upon processing requirements. 
         [0076]    At each stage of wafer processing, when preprocessing steps have been completed, post-processing steps may be implemented. During post-processing, initially, in step  1315 , photoresist is applied to a wafer. Then, in step  1316 , an exposure device may be used to transfer the circuit pattern of a reticle to a wafer. Transferring the circuit pattern of the reticle of the wafer generally includes scanning a reticle scanning stage which may, in one embodiment, include a force damper to dampen vibrations. 
         [0077]    After the circuit pattern on a reticle is transferred to a wafer, the exposed wafer is developed in step  1317 . Once the exposed wafer is developed, parts other than residual photoresist, e.g., the exposed material surface, may be removed by etching. Finally, in step  1319 , any unnecessary photoresist that remains after etching may be removed. As will be appreciated by those skilled in the art, multiple circuit patterns may be formed through the repetition of the preprocessing and post-processing steps. 
         [0078]    Although only a few embodiments of the present invention have been described, it should be understood that the present invention may be embodied in many other specific forms without departing from the spirit or the scope of the present invention. By way of example, while the use of a single coil has been described as being suitable for use with a cylindrical wedge Halbach array with wedge-shaped magnets at the ends of the array, it should be appreciated that more than a single coil may also be used with such an array. In one embodiment, two coils may be used in lieu of a single coil. 
         [0079]    While the use of a wedge Halbach array of magnets has been described in terms of being used in a cylindrical VCM, it should be appreciated that an array of magnets which includes at least one wedge-shaped magnet may be used within a variety of different VCMs. In one embodiment, a VCM which utilizes a coil which is substantially shaped as a square tube may utilize arrays of magnets which have an overall rectangular block shape with at least one component magnet being wedge-shaped. 
         [0080]    A VCM may include substantially only a single magnet array with at least one wedge-shaped component magnet. By way of example, an inner magnet ring of a radially symmetric cylindrical VCM may include at least one wedge-shaped component magnet, while an outer magnet ring of the VCM may be formed as a substantially uniform donut-shaped magnet, or may include only component magnets which are not wedge-shaped. Alternatively, an outer magnet ring of a radially symmetric cylindrical VCM may include at least one wedge-shaped component magnet, and an inner magnet ring of the VCM may include a substantially uniform donut-shaped magnet, or may include only component magnets which are not wedge-shaped. 
         [0081]    The parameters associated with a coil that is used in a radially symmetric cylindrical VCM with a cylindrical wedge Halbach array that includes wedge-shaped magnets at the ends of the array may vary widely. By way of example, both the number of turns in a coil as well as the gauge of the wire in the coil may vary. That is, the coil geometry may vary. Typically, as the wire gauge increases, the number of turns in a coil increases, and the resistance associated with the coil increases. Since the force generated by a VCM is proportional to the amount of current and the number of turns in a coil, to maintain the same force with lower current, the number of turns in the coil is increased. However, the voltage provided to the coil is generally increased in order to provide the same electric power, since power is proportional to both current and voltage. As the efficiency of a VCM is dependent upon the orientation of magnets within the VCM and not the number of turns in a coil or the wire gauge associated with the coil, the wire gauge may be selected such that the voltage and the current requirements of the VCM are consistent with any requirements of an amplifier or other electronics associated with the VCM. 
         [0082]    In addition to coil-related parameters, other parameters associated with a VCM in accordance with the present invention may also be widely varied. For instance, the inner and outer radii of the magnets in a cylindrical wedge Halbach array may vary. The positioning of the coil within a VCM may also be varied depending upon the requirements of a particular system. For example, the radial clearance or gap between a magnet array and the coil may vary depending upon the length of a trajectory stroke, such as a stroke in an XY plane, and a maximum stage position error, among other factors. 
         [0083]    Since many magnets, e.g., NdFeB magnets, are anisotropic, it is often preferable to fabricate each radially magnetized ring magnet out of several sections. By way of example, approximately six magnet sections may be used to form a single ring magnet. Generally, the number of magnets included in a wedge Halbach array may vary. In other words, although each wedge Halbach array described above has been described as including five or six magnets, a wedge Halbach array may include fewer or more magnets without departing from the spirit or the scope of the present invention. Therefore, the present examples are to be considered as illustrative and not restrictive, and the invention is not to be limited to the details given herein, but may be modified within the scope of the appended claims. 
         [0084]    This description of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form described, and many modifications and variations are possible in light of the teaching above. The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications. This description will enable others skilled in the art to best utilize and practice the invention in various embodiments and with various modifications as are suited to a particular use. The scope of the invention is defined by the following claims.