Abstract:
Disclosed are a positioning system and a control method therefor. In one preferred form, the positioning system includes a stage which is going to be positioned, a magnetic material member provided on the stage, electromagnets disposed at opposite sides of the magnetic material plate, a detector for detecting an electric current value of a coil which is a component of the electromagnets, and a position detector for detecting a position of the stage. The center position of the magnetic material member between the electromagnets can be determined on the basis of a relation between the position of the stage and the electric current value of the coil as the magnetic material member is moved relative to the electromagnets.

Description:
FIELD OF THE INVENTION AND RELATED ART  
       [0001]     This invention relates generally to a positioning system having a magnetic bearing and to a method of controlling the same. More particularly, the invention is directed to a positioning system suitably usable in a semiconductor exposure apparatus or an inspection apparatus, for example, for accurately positioning an exposure original such as a mask or a reticle, an article to be exposed such as a semiconductor wafer, or an article to be inspected, at a predetermined position.  
         [0002]     Exposure apparatuses used in the manufacture of semiconductor devices include those called a stepper and those called a scanner. The stepper is an apparatus which is arranged so that an image of a pattern of a reticle is projected onto a semiconductor wafer by a projection lens in a reduced scale while the wafer which is being placed on a stage system is moved stepwise, whereby the pattern image is photoprinted on different zones of a single wafer.  
         [0003]     On the other hand, the scanner is an apparatus which is arranged so that a wafer placed on a wafer stage and a reticle placed on a reticle stage are relatively moved relative to a projection lens and, during the scan motion, slit-like exposure light is projected by which a reticle pattern is projected onto a wafer. The stepper and the scanner will be the mainstream of exposure apparatuses, in the point of resolution and overlay precision.  
         [0004]     In recent years, for further improvements in speed and precision of stages, stage systems having a rough-motion stage and a fine-motion stage are being used. In such stage systems, the rough-motion stage moves with a large stroke, while the fine-motion stage moves with a small stroke as compared with the rough-motion stage.  
         [0005]     Japanese Laid-Open Patent Application, Publication No. 2003-218188 proposes use of an electromagnetic coupling (joint) between a rough-motion stage and a fine-motion stage, for transferring a force between these stages. Japanese Laid-Open Patent Application, Publication No. 2004-030616 proposes measuring a gap between each electromagnet of an electromagnetic coupling and a magnetic material plate, by use of a gap sensor.  
         [0006]     In the positioning system having an electromagnetic coupling such as described above, the clearance between each electromagnet of the electromagnetic coupling and the magnetic material plate may affect the performance of the positioning system. More specifically, if, for example, the magnetic material plate is not positioned exactly at the center between opposed electromagnets but the plate is deviated toward one of them, electrical loads of a control unit such as a driving voltage or a driving current for a driver which is related to one electromagnet having a wider clearance, or an electric power consumption thereof, for example, will become larger, and finally the required specification will not be satisfied.  
         [0007]     On the other hand, if a sensor for measuring the gap is used as disclosed in Japanese Laid-Open Patent Application, Publication No. 2004-030616, it would result is increases of weight, size and cost. Also, there would be many restrictions in regard to the design.  
         [0008]     The sensor may be omitted by a method in which a fine-motion stage (magnetic material plate) is brought into contact with each electromagnet and the gap is measured on the basis of the movement distance. In this method, however, there is a possibility that the electromagnet and the fine-motion stage are worn away and, in a worst case, they are broken.  
         [0009]     There is another problem that different machines may have different gaps due to the mounting precision for the magnetic material plate and the electromagnets. This means that, even if the rough-motion stage and the fine-motion stage are moved to their relative central positions on the basis of measurement of the stage position, the magnetic material plate may not always be positioned exactly at the center between the electromagnets.  
       SUMMARY OF THE INVENTION  
       [0010]     It is accordingly an object of the present invention to provide a positioning system by which, regardless of differences of machines with respect to the gap between an electromagnet and a magnetic material plate, the magnetic material plate can be positioned accurately at the center between opposed electromagnets, such that increases in electrical loads in the control can be suppressed effectively.  
         [0011]     It is another object of the present invention to provide a control method for such positioning system, a magnetic bearing device, or a control method for such magnetic bearing device.  
         [0012]     It is a further object of the present invention to provide an exposure apparatus or a device manufacturing method which is based on such positioning system or a control method therefor as described above.  
         [0013]     In accordance with an aspect of the present invention, to achieve at least one of these objects, there is provided a positioning system, comprising: a stage which is going to be positioned; a magnetic material member provided on said stage; electromagnets disposed at opposite sides of said magnetic material plate; a detector for detecting an electric current value of a coil which is a component of said electromagnets; and a position detector for detecting a position of said stage; wherein a center position of said magnetic material member between said electromagnets is determined on the basis of a relation between the position of said stage and the electric current value of said coil as said magnetic material member is moved relative to said electromagnets.  
         [0014]     In accordance with another aspect of the present invention, there is provided an apparatus, comprising: a first member; a second member; a magnetic material member provided on said first member; electromagnets provided on said second member and being disposed at opposite sides of said magnetic material member; a detector for detecting an electric current value of a coil which is a component of said electromagnets; and a position detector for detecting a position of at least one of said first member and said second member; wherein a center position of said magnetic material member between said electromagnets is determined on the basis of a relation between the position detected by said position detector and the electric current value of said coil as said magnetic material member is moved relative to said electromagnets.  
         [0015]     In accordance with a further aspect of the present invention, there is provided a method of controlling an apparatus having a magnetic material member being provided on a first member, and electromagnets provided on a second member and being disposed at opposite sides of the magnetic material member, said method comprising the steps of: changing a relative position of the first and second members; detecting the relative position of the first and second members; detecting an electric current value of a coil which is a component of the electromagnets; and determining a center position of the magnetic material member between the electromagnets, on the basis of a relation between the relative position and the electric current value.  
         [0016]     These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]      FIG. 1  is a perspective view of a stage system having a positioning and moving mechanism, according to an embodiment of the present invention.  
         [0018]      FIG. 2  is a plan view of the stage system of the  FIG. 1  embodiment.  
         [0019]      FIG. 3  is a diagrammatic view of a magnetic flux feedback control system for an electromagnetic coupling which is provided in the stage system of the  FIG. 1  embodiment.  
         [0020]      FIG. 4  is a schematic and plan view of an electromagnetic coupling (joint).  
         [0021]      FIGS. 5A-5C  are schematic and plan views, respectively, of an electromagnetic coupling having a pair of opposed electromagnets, wherein  FIG. 5A  shows a state in which a fine-motion stage (electromagnet target) is placed at its initial position X,  FIG. 5B  shows a stage in which the fine-motion stage is moved from the initial position, along X direction and by +α, and  FIG. 5C  shows a state in which the fine-motion stage is moved from the initial position, along X direction and by −α.  
         [0022]      FIG. 6A  is a graph showing driving electric current values of E-shaped electromagnets XL and XR in  FIGS. 5A-5C  at respective gap magnitudes, and  FIG. 6B  is a graph showing the results of approximating the electric current values of  FIG. 6A  by a linear function, and illustrating that the point of intersection between the driving currents corresponding to the gaps of XL and XR is at the point where the gaps have the same magnitude.  
         [0023]      FIG. 7  is a flow chart for explaining the gap registering operation in the stage system of the  FIG. 1  embodiment.  
         [0024]      FIG. 8  is a perspective view of a magnetic bearing device according to a second embodiment of the present invention.  
         [0025]      FIG. 9  is a perspective view wherein a Y stage in the  FIG. 8  embodiment is removed for better understanding of the structure of the magnetic bearing device.  
         [0026]      FIG. 10  is a schematic view of an exposure apparatus according to a third embodiment of the present invention.  
         [0027]      FIG. 11  is a flow chart for explaining sequential operations of device manufacturing processes. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0028]     Preferred embodiments of the present invention will now be described with reference to the attached drawings.  
       Embodiment 1  
       [0029]      FIG. 1  illustrates a wafer stage of an exposure apparatus, and  FIG. 2  is a plan view of it. In recent years, for further improvements in moving speed of the stage, a stage system wherein a rough-motion stage ( 11  and  15 ) and a fine-motion stage ( 14 ) are provided separately, such as shown in  FIG. 1 , is used.  
         [0030]     A base table  12  has a mirror-finished top surface, and a guide  13  has a mirror-finished side surface. A Y stage  11  is guided in Y direction along these surfaces, by means of a static bearing (not shown) . An X stage  15  is guided in X direction along the top surface of the base table  12  and the side surface of the Y stage  11 , by means of a static bearing (not shown). The static bearing between the Y stage  11  and the X stage  15  is arranged so as to sandwich the Y stage.  
         [0031]     A Y linear motor coil comprises Y linear motor stators  21   a  and  21   b,  each being constituted by a coil, as well as Y linear motor movable elements  23   a  and  23   b  each being constituted by a magnet. The Y stage  11  is coupled to the Y linear motor by means of Y linear motor connecting plates  25   a  and  25   b . The Y stage  11  can be positioned at a desired position in the Y direction, by means of this Y linear motor and a control system (not shown). Similarly, the X linear motor  27  includes X linear motor stators (not shown) provided on the Y stage  11  as well as X linear motor movable elements (not shown) . The X stage  15  can be position-controlled with respect to X direction, by means of the X linear motor  27  and a control system (not shown).  
         [0032]     There is a fine-motion stage  14  mounted on the X stage  15 . Provided between the X stage  15  and the fine-motion stage  14  are fine-motion X linear motors  31   a  and  31   b , fine-motion Y linear motors  32   a  and  32   b , and fine-motion Z linear motors  33   a ,  33   b ,  33   c , and  33   d , for producing driving forces in X, Y and Z directions, respectively. By means of these linear motors and a control system (not shown), the fine-motion stage  14  can be controllably positioned with respect to six axes (i.e., X axis, Y axis, Z axis and rotational directions about these axes). In  FIG. 1 , illustration of the fine-motion linear motors is partially omitted, for simplicity of the illustration.  
         [0033]     The X stage  15  is provided with a measuring system  3  (which may be a laser interferometer, for example) for measuring stage movement amounts in X and Y directions. The fine-motion stage  14  is provided with a measuring system  4  (which may be a laser interferometer, for example) for measuring the movement amounts of the fine-motion stage  16  in the six-axis directions described above. These measuring systems are used for the positioning control of the stages, respectively. Here, the movement amount of the X stage  15  in the Y direction may be measured by measuring the movement amount of the Y stage.  
         [0034]     Furthermore, between the X stage  15  and the fine-motion stage  14 , there is a force coupling mechanism (hereinafter, “electromagnetic coupling mechanism”) for producing an attraction force on the basis of the function of electromagnets. This mechanism includes E-shaped electromagnets  41   a ,  41   b ,  42   a  and  42   b , an E-shaped electromagnet fixing member  16 , targets (magnetic material members)  43   a ,  43   b ,  44   a  and  44   b , and a target fixing member  17 . Here, the E-shaped electromagnets  41   a  and  41   b  are electromagnets arranged to produce a force in X direction, while the electromagnets  42   a  and  42   b  are electromagnets arranged to produce a force in Y direction. The targets  43   a  and  43   b  are targets effective to produce a force in X direction, while the targets  44   a  and  44   b  are targets effective to produce a force in Y direction.  
         [0035]     The acceleration and deceleration force of the fine-motion stage  14  during acceleration and deceleration of the fine-motion stages  11  and  15  can be transmitted from the X stage  15  to the fine-motion stage  14  through the electromagnetic coupling mechanism, by feed-forwarding a force (command value) calculated from the acceleration and the mass of the fine-motion stage  14  to the E-shaped electromagnets  41   a ,  41   b ,  42   a  and  42   b . With this feed-forward of the acceleration and deceleration force through the E-shaped electromagnets  41   a ,  41   b ,  42   a  and  42   b  and so on, the fine-motion linear motors  31   a ,  31   b ,  32   a ,  32   b ,  33   a ,  33   b ,  33   c  and  33   d  do not need a large force during acceleration and deceleration.  
         [0036]     The force control of the E-shaped electromagnets  41   a ,  41   b ,  42   a  and  42   b  described above is based on magnetic flux feedback such as shown in  FIG. 3 . In  FIG. 3 , the electric current flowing through a driving coil  51  of the E-shaped electromagnet produces a magnetic flux at the electromagnetic coupling, and a force which is proportional to the square of this magnetic flux is produced as an attracting force. An induced voltage is measured by using a search coil  52  provided at the E-shaped electromagnetic coupling. This induced voltage corresponds to a change in time of the magnetic flux when the electromagnetic coupling is actuated. The feedback circuit operates to integrate this induced voltage with time, by using an integrator  53 , and a difference (magnetic flux error) between the detected magnetic flux and the magnetic flux command which is proportional to the force command is calculated. A gain  54  is applied to the obtained difference, and the resultant is outputted to an electromagnetic coupling driving amplifier  55  as a command.  
         [0037]     A magnetic resistance (reluctance) at an electromagnetic coupling (electromagnet) as schematically shown in  FIG. 4  can be expressed by equation (1) below, with a magnetic resistance R m [AT/Wb] (or [1/H]), a gap ξ [m], a sectional area A[m 2 ], lengths L 1 , L 2  and L 3 [m], an absolute permeability of vacuum μ 0 [H/m]=4π*10 −7 , and μ=μ 0 +μ s [H/m] (in case of ferro silicon, μ s =7000).  
             Rm   =           2   ⁢     L   1       +     L   2     +     2   ⁢     L   3           2   ⁢   μ   ⁢           ⁢   A       +         5   ⁢   ξ       2   ⁢   μ   ⁢           ⁢   A       ⁢           [     AT   /   Wb     ]               (   1   )             
 
 Although the magnetic resistance R m  is expressed by the sum of magnetic resistances of the magnetic material (first term) and the gap (second term), since the magnetic permeability μ of the magnetic material is very large, the magnetic resistance of the gap ξ becomes dominant for the resistance. Further, the magnetic resistance R m  can be considered as a linear function of the gap ξ, taking the first term as an offset. On the other hand, the magnetic flux ø can be expressed by using an electric current I drv  flowing through a driving coil of N drv  turns as well as the magnetic resistance R m , as follows.  
             ϕ   =       N   drv     *         I   drv       R   m       ⁢           [   Wb   ]               (   2   )             
 
 Where the magnetic flux control based on the magnetic flux feedback described above is carried out, the current I drv  as well is controlled in accordance with the change of R m  so that the magnetic flux ø takes a predetermined value. Hence, 
 
I drv ∝R m    (3) 
 
 is given. Therefore, from equations (1) and (3), it is concluded that the relation between the driving current I drv  and the gap ξ is linear. Thus, if the driving current when the gap is zero is denoted by “b”, the following relation is obtained. 
 
 I   drv   =a*ξ+b    (4) 
 
         [0038]     In  FIG. 4 , denoted at  56  is a target, and denoted at  57  is an E-shaped electromagnet. Denoted at  58  is a driving coil. These correspond to the targets  44   a ,  44   b ,  43   a  and  43   b  and the E-shaped electromagnets  41   a ,  41   b ,  42   a  and  42   b  in  FIG. 2 .  
         [0039]     The detection sequence is as follows. The relative center of the rough-motion stage and the fine-motion stage as measured by using an interferometer is taken as an initial position, and either the rough-motion stage or the fine-motion stage is shifted in X direction from the initial position by a predetermined amount (1st step). Then, the largest amplitude value of the electric current that flows through the driving coil (hereinafter, this current will be referred to as “driving current”) as the rough motion stage is moved stepwise at the thus shifted position, is measured (2nd step) . Here, in order to assure that a predetermined magnetic force is produced, the stepwise motion is carried out under the same conditions of largest velocity and acceleration, etc. This operation is repeated several times, while changing the amount of shift.  
         [0040]     In  FIGS. 5A-5C , denoted at  59   a  is an E-shaped electromagnet at the XL side, and denoted at  59   b  is an E-shaped electromagnet at the XR side. Denoted at  60  is a target. These correspond to the targets  44   a ,  44   b ,  43   a  and  43   b  and the E-shaped electromagnets  41   a ,  41   b ,  42   a  and  42   b  in  FIG. 2 .  
         [0041]     As shown in  FIGS. 5A-5C , if the fine-motion stage is shifted in the positive direction from the initial position X, the clearance at the XR side is narrowed (the driving current value is lowered), while the clearance at the XL side is widened (the driving current value is raised) ( FIG. 5B ). If the stage is shifted in the negative direction, the result is reversed ( FIG. 5C ) . From the measured values of them ( FIG. 6A ), approximation is carried out as in equation (4) in accordance with the least square method, for example. Then, the point of intersection of driving currents of the opposed electromagnetic couplings is carried out ( FIG. 6B ) (3rd step). It is seen that this point of intersection is the very point where the gaps have the same size. The distance from the current position of the fine-motion stage to the center is calculated, and the stage is moved to there (4th step). This operation may be carried out before start of scan (before stage driving) and/or during the idling state, by which the movable member can be placed to maintain the gaps of the same magnitude.  
         [0042]      FIG. 7  is the flow chart for explaining the procedure described above.  
       Embodiment 2  
       [0043]     The present invention is applicable not only to a positioning system but also to a magnetic bearing device. The second embodiment is an example wherein the invention is applied to a magnetic bearing device. Here, explanation of the structural portion similar to that of the first embodiment will be omitted.  
         [0044]      FIG. 8  is a perspective view of a Y stage  11  and an X stage  15  such as shown in  FIG. 1 . In  FIG. 8 , the Y stage  11  has magnetic material plates  18 , and the X stage  15  has four electromagnets  19 .  
         [0045]      FIG. 9  clearly illustrates the relation between the magnetic material plates  18  and the electromagnets  19  of  FIG. 8 .  
         [0046]     As seen in  FIGS. 8 and 9 , the X stage  15  is guided to be moved along the side surface of the Y stage  11 , by means of the four electromagnets and two magnetic material plates  18 , provided between the side surface of the Y stage  11  and the side plate of the X stage  15 . The two magnetic material plates  18  are fixed to the side surfaces of the Y stage  11 , respectively, and the four electromagnets are fixed to the two side plates of the X stage (each pair of electromagnets being fixed to one side plate) . The magnetic material plate  18  and corresponding two electromagnets are disposed opposed to each other, without contact to each other.  
         [0047]     Even in a case where electromagnets are used as a magnetic bearing, as in this example, any difference in clearance between the magnetic material member and each of opposed electromagnets can be reduced similarly, like the first embodiment.  
       Embodiment 3  
       [0048]      FIG. 10  shows an exposure apparatus for device manufacture, having a positioning system such as described hereinbefore as a wafer stage or a reticle stage.  
         [0049]     This exposure apparatus can be used for manufacture of microdevices having a fine pattern formed thereon, such as semiconductor devices (semiconductor integrated circuits, for example), micromachines, or thin-film magnetic heads, for example. In this exposure apparatus, exposure light (which may include visible light, ultraviolet light, EUV light, X-ray, electron beam, and charged particle beam, for example) as an exposure energy supplied from an illumination system unit  501  is projected onto a semiconductor wafer (substrate) W through a reticle (original), by means of a projection lens  503  (which may include refractive lens, reflective lens, catadioptric lens system, and charged particle lens, for example), whereby a desired pattern is produced on the substrate which is placed on a wafer stage  504 . In such exposure apparatus, as the wavelength of used exposure light is shortened, exposure operation has to be carried out in a vacuum ambience.  
         [0050]     A wafer (substrate) W is held on a chuck which is mounted on the wafer stage  504 , and a pattern of the reticle R (original) mounted on a reticle stage  502  is transferred in a reduced scale onto different regions on the wafer W by means of the illumination system unit  501 , in accordance with a step-and-repeat method or a step-and-scan method.  
         [0051]     It should be noted that the stage system of the first embodiment can be used as the wafer stage  504  or the reticle stage  502 .  
       Embodiment 4  
       [0052]     Next, an embodiment of a microdevice manufacturing method which uses an exposure apparatus of the third embodiment described above, will be explained.  
         [0053]      FIG. 11  is a flow chart for explaining the overall procedure for production of microdevices such as semiconductor chips (IC or LSI), liquid crystal panels, CCDs, thin film magnetic heads, micromachines, etc.  
         [0054]     Step 1 is a design process for designing a circuit of a semiconductor device. Step 2 is a process for making a mask on the basis of the circuit pattern design.  
         [0055]     On the other hand, Step 3 is a process for preparing a wafer by using a material such as silicon. Step 4 is a wafer process which is called a pre-process wherein, by using the thus prepared mask and wafer, a circuit is formed on the wafer in practice, in accordance with lithography. Step 5 subsequent to this is an assembling step which is called a post-process wherein the wafer having been processed at step 4 is formed into semiconductor chips. This step includes an assembling (dicing and bonding) process and a packaging (chip sealing) process. Step 6 is an inspection step wherein an operation check, a durability check an so on, for the semiconductor devices produced by step 5, are carried out. With these processes, semiconductor devices are produced, and finally they are shipped (step 7).  
         [0056]     More specifically, the wafer process at step 4 described above includes: (i) an oxidation process for oxidizing the surface of a wafer; (ii) a CVD process for forming an insulating film on the wafer surface; (iii) an electrode forming process for forming electrodes upon the wafer by vapor deposition; (iv) an ion implanting process for implanting ions to the wafer; (v) a resist process for applying a resist (photosensitive material) to the wafer; (vi) an exposure process for printing, by exposure, the circuit pattern of the mask on the wafer through the exposure apparatus described above; (vii) a developing process for developing the exposed wafer; (viii) an etching process for removing portions other than the developed resist image; and (ix) a resist separation process for separating the resist material remaining on the wafer after being subjected to the etching process. By repeating these processes, circuit patterns are superposedly formed on the wafer.  
         [0057]     While the invention has been described with reference to the structures disclosed herein, it is not confined to the details set forth and this application is intended to cover such modifications or changes as may come within the purposes of the improvements or the scope of the following claims.  
         [0058]     This application claims priority from Japanese Patent Application No. 2004-337357 filed Nov. 22, 2004, for which is hereby incorporated by reference.