Abstract:
The present invention provides an exposure apparatus which forms a pattern on a substrate, the apparatus including an electron optical system configured to guide a charged particle beam onto the substrate, a stage configured to hold the substrate, an electromagnetic actuator configured to drive the stage, a magnetic shield which is placed in the stage so as to surround the electromagnetic actuator, a measurement member configured to measure a position of the stage, a coil member configured to generate a magnetic field on a path of the charged particle beam between the electron optical system and the substrate, and a control member configured to control the coil member so as to reduce a fluctuation of the magnetic field on the path, the magnetic field on the path fluctuating while the stage being driven by the electromagnetic actuator, based on the position of the stage measured by the measurement member.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to an exposure apparatus and a device fabrication method. 
       BACKGROUND ART 
       [0002]    A lithography technique of transferring the pattern of a mask (reticle) onto a substrate such as a wafer is employed to fabricate a semiconductor device. Since the mask used for the lithography technique must have a pattern with an extremely high dimensional accuracy, an electron beam exposure apparatus (charged particle beam exposure apparatus) is used to fabricate this mask. An electron beam exposure apparatus is also used to directly draw a pattern on a substrate without using a mask. 
         [0003]    An electron beam exposure apparatus generally includes an electron gun unit for emitting an electron beam, an electron optical system (charged particle beam optical system) for guiding the electron beam from the electron gun unit onto a substrate, a stage for driving the substrate relative to the electron beam, and a deflector for positioning the electron beam guided on the substrate. 
         [0004]    The electron beam exposure apparatus has extremely high response characteristics to electron beam positioning. Therefore, it is a common practice to provide the apparatus with a feedback control system which measures the orientation or positional shift of the stage and feeds back the measurement result to electron beam positioning by the deflector, instead of enhancing the mechanical control characteristics of the stage. Also, the stage is placed in a vacuum chamber and is designed as a contact type such as a roller guide or a ball screw actuator and made of a non-magnetic material so as not to generate a fluctuation of a magnetic field (magnetic field fluctuation) which adversely affects electron beam positioning. 
         [0005]    On the other hand, to avoid problems such as dust generation and deformation of a contact stage, Japanese Patent No. 4234768 proposes a stage including a non-contact electromagnetic actuator. However, the electromagnetic actuator causes a magnetic field fluctuation, so a magnetic shield surrounds the electromagnetic actuator to achieve high positioning accuracy while reducing any leakage magnetic field fluctuation generated by the stage in Japanese Patent No. 4234768. Nevertheless, in Japanese Patent No. 4234768, as the thrust of the electromagnetic actuator improves to comply with a demand for speeding up the stage, the electromagnetic actuator becomes larger and the leakage magnetic field fluctuation, in turn, becomes larger, so the magnetic shield also becomes larger and thicker. 
         [0006]    Also, Japanese Patent Laid-Open No. 2003-173755 discloses a magnetic field canceller, as shown in  FIG. 9 , as a technique which copes with a leakage magnetic field fluctuation generated by the electromagnetic actuator. The magnetic field canceller shown in  FIG. 9  uses a magnetic field sensor  1010  to detect a leakage magnetic field generated by the electromagnetic actuator, and performs feedback control so as to generate a cancelling magnetic field by a cancelling coil  1020 , based on the detected value. In this case, to generate a cancelling magnetic field by the cancelling coil  1020  based on the value detected by the magnetic field sensor  1010 , the detection range of the magnetic field sensor  1010  must be set in correspondence with the leakage magnetic field (magnetic field fluctuation) generated by the electromagnetic actuator. 
         [0007]    In recent years, as a demand has arisen for a further speedup of a stage and a substrate becomes larger, an electromagnetic actuator and a magnetic shield also become larger. Also, a leakage magnetic field generated by an electron optical system is always present in the space between the electron optical system and the substrate, so, when a magnetic shield made of a high magnetic permeability material moves within the leakage magnetic field, the magnetic field in the space between the electron optical system and the substrate is disturbed, thus generating a magnetic field fluctuation. This magnetic field fluctuation becomes larger (that is, has a larger amplitude) with an increase in size of the magnetic shield. 
         [0008]    Unfortunately, in the prior arts, as the magnetic field fluctuation has a larger amplitude, it falls outside the detection range of the magnetic field sensor. This often makes it impossible to detect a magnetic field fluctuation due to an insufficient detection range. Hence, a magnetic field fluctuation with a large amplitude cannot be cancelled simply by detecting the magnetic field fluctuation by the magnetic field sensor and performing feedback control so as to generate a cancelling magnetic field based on the detected value. In this case, the use of a magnetic sensor having a wide detection range in correspondence with a magnetic field fluctuation with a large amplitude is plausible, but such a magnetic sensor having a wide detection range generally has a low detection resolution and therefore cannot cancel the magnetic field fluctuation with high accuracy. 
       SUMMARY OF INVENTION 
       [0009]    The present invention provides a technique advantageous to reduce a magnetic field fluctuation generated upon driving a stage. 
         [0010]    According to one aspect of the present invention, there is provided an exposure apparatus which forms a pattern on a substrate using a charged particle beam, the apparatus including an electron optical system configured to guide the charged particle beam onto the substrate, a stage configured to hold the substrate, an electromagnetic actuator configured to drive the stage, a magnetic shield which is placed in the stage so as to surround the electromagnetic actuator, a measurement member configured to measure a position of the stage, a coil member configured to generate a magnetic field on a path of the charged particle beam between the electron optical system and the substrate, and a control member configured to control the coil member so as to reduce a fluctuation of the magnetic field on the path, the magnetic field on the path fluctuating while the stage being driven by the electromagnetic actuator, based on the position of the stage measured by the measurement member. 
         [0011]    Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0012]      FIG. 1  is a view showing the arrangement of an exposure apparatus according to an aspect of the present invention. 
           [0013]      FIGS. 2A to 2C  are views for explaining a magnetic field fluctuation on the electron beam path between an electron optical system and a substrate in the exposure apparatus shown in  FIG. 1 . 
           [0014]      FIG. 3  is a graph illustrating an example of the relationship between the position of a stage and the disturbance magnetic field fluctuation on the electron beam path between the electron optical system and the substrate in the exposure apparatus shown in  FIG. 1 . 
           [0015]      FIG. 4  is a block diagram showing a control configuration for reducing a magnetic field fluctuation on the electron beam path between the electron optical system and the substrate in the exposure apparatus shown in  FIG. 1 . 
           [0016]      FIG. 5  is a graph illustrating an example of a first table indicating the relationship between the position of the stage and the disturbance magnetic field fluctuation on the electron beam path between the electron optical system and the substrate in the exposure apparatus shown in  FIG. 1 . 
           [0017]      FIG. 6  is a view illustrating an example of the arrangement of a Helmholtz coil. 
           [0018]      FIG. 7  is a table showing the relationship between the magnetic field on the electron beam path between the electron optical system and the substrate and that at the position of a detection unit in the exposure apparatus shown in  FIG. 1 . 
           [0019]      FIG. 8  is a block diagram showing a control configuration for reducing a magnetic field fluctuation on the electron beam path between the electron optical system and the substrate. 
           [0020]      FIG. 9  is a view showing the arrangement of a magnetic field canceller according to the prior art. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0021]    Preferred embodiments of the present invention will be described below with reference to the accompanying drawings. Note that the same reference numerals denote the same members throughout the drawings, and a repetitive description thereof will not be given. 
         [0022]      FIG. 1  is a view showing the arrangement of an exposure apparatus  1  according to an aspect of the present invention. The exposure apparatus  1  is an electron beam exposure apparatus which is accommodated in a vacuum chamber having its interior maintained in a vacuum atmosphere, and forms a pattern on a substrate using an electron beam (charged particle beam). The exposure apparatus  1  includes an electron optical system  10  which guides an electron beam onto a substrate ST, a stage (substrate stage)  20  which holds the substrate ST, a driving unit  30  which drives the stage  20 , a stage base  40 , and a measurement unit  50  which measures the position of the stage  20 . The exposure apparatus  1  also includes a coil unit  60  which generates a magnetic field on the electron beam path between the electron optical system  10  and the substrate ST, a first power supply  72 , a second power supply  74 , a detection unit  80 , and a control unit  90 . 
         [0023]    The stage  20  includes, on its upper surface, a substrate holder (not shown) for holding the substrate ST, and, on its side surface, a reflecting mirror (not shown) for measuring the position of the stage  20 . A bearing (not shown) is placed between the stage  20  and the stage base  40 , and the driving unit  30  smoothly drives the stage  20  in the X- and Y-axis directions along the upper surface of the stage base  40 . 
         [0024]    The driving unit  30  includes an electromagnetic actuator  32  fixed on the stage  20  and a magnetic shield  34  which surrounds the electromagnetic actuator  32 , and drives the stage  20  in the X- and Y-axis directions perpendicular to the optical path of the electron beam (Z-axis direction). The electromagnetic actuator  32  is formed from, for example, a linear motor, an electromagnet actuator, or a planar motor. The electromagnetic actuator  32  generates an electromagnetic force by energization to generate a thrust in the X- and Y-axis directions between itself and the stage base  40 . The magnetic shield  34  is made of a high magnetic permeability material such as soft iron, and covers members which generate a magnetic field, such as magnets and coils that constitute the electromagnetic actuator  32 . Note that in the stage  20  and driving unit  30 , members other than the electromagnetic actuator  32  are basically made of a non-magnetic material. Therefore, the magnetic shield  34  can reduce (attenuate) most components of a disturbance magnetic field running from the stage  20  and driving unit  30  to the optical path of the electron beam. 
         [0025]    The measurement unit  50  is formed from, for example, a laser interferometer, and measures the position of the stage  20  in the X- and Y-axis directions upon receiving light reflected by the reflecting mirror provided on the stage  20 . Although only a measurement unit (that is, a measurement axis in the X-axis direction) which measures the position of the stage  20  in the X-axis direction is shown in  FIG. 1  as the measurement unit  50 , a measurement unit (that is, a measurement axis in the Y-axis direction) which measures the position of the stage  20  in the Y-axis direction is also disposed. 
         [0026]    The coil unit  60  functions as a magnetic field canceller which reduces (cancels) a fluctuation of a magnetic field (magnetic field fluctuation) on the electron beam path between the electron optical system  10  and the substrate ST in cooperation with, for example, the measurement unit  50 , first power supply  72 , second power supply  74 , detection unit  80 , and control unit  90 . The coil unit  60  includes a first coil  62  which generates a magnetic field having a first amplitude, and a second coil  64  which generates a magnetic field having a second amplitude smaller than the first amplitude, as shown in  FIG. 1 . 
         [0027]    The first coil  62  is formed from, for example, two Helmholtz coils which are aligned with a space between them in the X-axis direction, and the electron beam path is positioned in the middle between these two Helmholtz coils. The first coil  62  can generate a magnetic field in the X-axis direction by energization by the first power supply  72 , and can generate an almost uniform magnetic field within the X-Y plane defined by a cross-section of the electron beam path. The second coil  64  includes two Helmholtz coils which are aligned with a space between them in the X-axis direction, and the electron beam path is positioned in the middle between these two Helmholtz coils, like the first coil  62 . The second coil  64  can generate a magnetic field in the X-axis direction by energization by the second power supply  74 . 
         [0028]    In this embodiment, the number of turns of the first coil  62  is set larger than that of the second coil  64 . Thus, the first coil  62  can generate a magnetic field with a large amplitude (first amplitude) even when the first power supply  72  and second power supply  74  have the same output range. Also, the second coil  64  can generate a magnetic field with a small amplitude (second amplitude) with high accuracy even when the first power supply  72  and second power supply  74  have the same output range or output resolution. 
         [0029]    The detection unit  80  is placed in the space between the electron optical system  10  and the substrate ST, and detects a magnetic field (components of a disturbance magnetic field fluctuation in the X- and Y-axis directions) on the electron beam path between the electron optical system  10  and the substrate ST. The detection unit  80  is formed from a magnetic sensor and covered with a non-magnetic material (for example, aluminum, phosphor bronze, stainless steel, a resin, or ceramics) with less degassing. 
         [0030]    The control unit  90  includes, for example, a CPU and memory and controls the whole (operation) of the exposure apparatus  1 . The control unit  90  determines current values to be supplied to the first coil  62  and second coil  64 , respectively, based on the measurement result obtained by the measurement unit  50  and the detection result obtained by the detection unit  80 , and controls the first power supply  72  and second power supply  74  which energize the first coil  62  and second coil  64 , respectively. As will be described later, the control unit  90  includes a feedforward control system and feedback control system for magnetic fields generated by the coil unit  60 . The feedforward control system feedforward-controls a magnetic field, generated by the first coil  62 , so as to generate a magnetic field which reduces a magnetic field fluctuation generated on the electron beam path upon driving the stage  20 , based on the position of the stage  20  measured by the measurement unit  50 . Also, the feedback control system feedback-controls a magnetic field, generated by the second coil  64 , so as to generate a magnetic field which reduces a magnetic field fluctuation generated on the electron beam path upon driving the stage  20 , based on the magnetic field detected by the detection unit  80 . 
         [0031]    A magnetic field fluctuation (disturbance magnetic field fluctuation) on the electron beam path between the electron optical system  10  and the substrate ST will be described herein with reference to  FIGS. 2A to 2C . The stage  20  is provided with the electromagnetic actuator  32  and the magnetic shield  34  which is made of a high magnetic permeability material, as described above. Note that a leakage magnetic field MF leaked from the electron optical system  10  is always present in the space between the electron optical system  10  and the substrate ST, so the stage  20  is driven in the X- and Y-axis directions within the leakage magnetic field MF. This means that the magnetic shield  34  made of a high magnetic permeability material moves in the X- and Y-axis directions within the leakage magnetic field MF. 
         [0032]      FIGS. 2A to 2C  schematically show a fluctuation in leakage magnetic field MF (its distribution) upon driving the stage  20  in the X-axis direction.  FIG. 2A  shows the leakage magnetic field MF when the stage  20  is positioned in the middle of a moving stroke of the stage  20 . The position (the position in the X-axis direction) of the stage  20  at this time is defined as X=0, and the disturbance magnetic field fluctuation at this time is defined as ΔB=0 assuming the magnetic field value on the electron beam path as a reference. 
         [0033]    A case in which the stage  20  moves from the position X=0 to a position X=Xb (&lt;0) to move the magnetic shield  34  in the negative X-axis direction, as shown in  FIG. 2B , will be considered. In this case, the leakage magnetic field MF is attracted to the magnetic shield  34  which is positioned in the negative X-axis direction relative to the optical axis of the electron optical system  10 . With such a change in leakage magnetic field MF (its distribution), a negative X-axis component is generated in the leakage magnetic field MF, so the disturbance magnetic field fluctuation on the electron beam path becomes ΔB=ΔB×b (&lt;0). 
         [0034]    A case in which the stage  20  moves from the position X=0 to a position X=Xc (&gt;0) to move the magnetic shield  34  in the positive X-axis direction, as shown in  FIG. 2C , will be considered next. In this case, the leakage magnetic field MF is attracted to the magnetic shield  34  which is positioned in the positive X-axis direction relative to the optical axis of the electron optical system  10 . With such a change in leakage magnetic field MF (its distribution), a positive X-axis component is generated in the leakage magnetic field MF, so the disturbance magnetic field fluctuation on the electron beam path becomes ΔB=ΔB×c (&gt;0). 
         [0035]      FIG. 3  is a graph illustrating an example of the relationship between the position of the stage  20  and the amount of disturbance magnetic field fluctuation ΔB on the electron beam path between the electron optical system  10  and the substrate ST.  FIG. 3  shows the position X of the stage  20  on the abscissa and the amount of disturbance magnetic field fluctuation ΔB on the ordinate. Referring to  FIG. 3 , the amount of disturbance magnetic field fluctuation ΔB as a function of the position X of the stage  20  is ΔB=0 when X=0, ΔB=ΔB×b when X=Xb, and ΔB=ΔB×c when X=Xc. The disturbance magnetic field fluctuation generated due to factors associated with both the leakage magnetic field MF and movement of the stage  20  (magnetic shield  34 ) can be expressed as a function of the position of the stage  20 , as shown in  FIG. 3 . Note that if a leakage magnetic field leaked from a permanent magnet which serves as a constituent element of the electromagnetic actuator  32  and is fixed on the stage  20  reaches the electron beam path, and this leads to a magnetic field fluctuation, this magnetic field fluctuation is also generated in correspondence with the position of the stage  20  and therefore can be expressed as a function of the position of the stage  20 . 
         [0036]      FIG. 4  is a block diagram showing a control configuration for reducing (cancelling) a magnetic field fluctuation (distributed magnetic field fluctuation) on the electron beam path between the electron optical system  10  and the substrate ST in the exposure apparatus  1 . Referring to  FIG. 4 , upon driving the stage  20 , the measurement unit  50  measures the position (the position in the X- and Y-axis directions) of the stage  20  and inputs the measurement result to the control unit  90 . 
         [0037]    The feedforward control system of the control unit  90  controls a magnetic field, generated by the coil unit  60  (first coil  62 ), so as to generate a magnetic field which reduces a disturbance magnetic field fluctuation generated upon driving the stage  20 , based on the position of the stage  20  measured by the measurement unit  50 . 
         [0038]    Detailed control by the feedforward control system will be explained herein. The memory of the control unit  90  stores, in advance, a first table indicating the relationship between the position of the stage  20  and the disturbance magnetic field fluctuation on the electron beam path between the electron optical system  10  and the substrate ST. The first table is, for example, a table which shows the position of the stage  20  in the X- and Y-axis directions on the two horizontal axes, and the amount of disturbance magnetic field fluctuation ΔBx on the vertical axis, as shown in  FIG. 5 . Note that the first table can be created by driving the stage  20  while measuring the position of the stage  20  by the measurement unit  50  and detecting an amount of disturbance magnetic field fluctuation ΔBx at each position of the stage  20  by the detection unit  80 , while the coil unit  60  generates no magnetic field. 
         [0039]    The memory of the control unit  90  also stores, in advance, a second table indicating the relationship among the position of the stage  20 , the current value supplied to the first coil  62 , and the magnetic field (magnetic field value) generated by the first coil  62 . The second table is, for example, a table which shows the position of the stage  20  in the X- and Y-axis directions on the two horizontal axes, and the constant of proportionality Kb given by (the current value supplied to the first coil  62 )/(the magnetic field value generated by the first coil  62 ) on the vertical axis. Note that the second table can be created by driving the stage  20  while measuring the position of the stage  20  by the measurement unit  50  to obtain a constant of proportionality Kb while changing the current value supplied to the first coil  62  at each position of the stage  20 . 
         [0040]    The feedforward control system looks up the first table to obtain an amount of disturbance magnetic field fluctuation ΔB corresponding to the position of the stage  20  measured by the measurement unit  50 . The feedforward control system determines, as a magnetic field to be generated by the first coil  62 , a magnetic field which is equal in absolute value and opposite in direction to the amount of disturbance magnetic field fluctuation ΔB corresponding to the position of the stage  20 , and generates a command value used to generate this magnetic field. The feedforward control system looks up the second table to obtain a constant of proportionality Kb corresponding to the position of the stage  20  measured by the measurement unit  50 , and multiplies the constant of proportionality Kb by the above-mentioned command value to obtain a current value to be supplied to the first coil  62 . The feedforward control system inputs the obtained current value as a current command value for the first power supply  72 . The first power supply  72  energizes the first coil  62  based on the current command value from the feedforward control system. Thus, the first coil  62  generates a magnetic field which reduces (cancels) a disturbance magnetic field fluctuation generated on the electron beam path between the electron optical system  10  and the substrate ST upon driving the stage  20 . 
         [0041]    A magnetic field generated on the electron beam path between the electron optical system  10  and the substrate ST by the first coil  62  cancels a disturbance magnetic field fluctuation generated upon driving the stage  20 . However, the magnetic field generated by the first coil  62  sometimes cannot perfectly cancel the disturbance magnetic field fluctuation, thus generating a magnetic field fluctuation residual. 
         [0042]    In this case, the feedback control system of the control unit  90  controls a magnetic field, generated by the coil unit  60  (second coil  64 ), so as to generate a magnetic field which reduces a magnetic field fluctuation residual, based on the magnetic field detected by the detection unit  80 . Detailed control by the feedback control system will be explained herein. While the feedforward control system performs feedforward control, the detection unit  80  detects a magnetic field on the electron beam path between the electron optical system  10  and the substrate ST, and inputs the detection result to the control unit  90 . 
         [0043]    The feedback control system obtains a deviation between the command value used to cancel the disturbance magnetic field fluctuation to zero and the magnetic field (its fluctuation) detected by the detection unit  80 , and multiplies the deviation by a constant of proportionality Kc to obtain a current value to be supplied to the second coil  64 . The feedback control system inputs the obtained current value as a current command value for the second power supply  74 . The second power supply  74  energizes the second coil  64  based on the current command value from the feedback control system. Thus, the second coil  64  generates a magnetic field which reduces (cancels) a magnetic field fluctuation residual, that is, a disturbance magnetic field fluctuation generated on the electron beam path between the electron optical system  10  and the substrate ST upon driving the stage  20 . 
         [0044]    The exposure apparatus  1  according to this embodiment feedforward-controls a magnetic field generated by the first coil  62 , based on the position of the stage  20 , to generate a magnetic field which reduces (cancels) a disturbance magnetic field fluctuation generated upon driving the stage  20 . Therefore, the exposure apparatus  1  can reduce (cancel) a disturbance magnetic field fluctuation generated upon driving the stage  20 , without detecting a magnetic field (its fluctuation) on the electron beam path between the electron optical system  10  and the substrate ST by the detection unit  80 . Also, when a magnetic field fluctuation residual occurs as a magnetic field generated by the first coil  62  cannot cancel a disturbance magnetic field fluctuation, a magnetic field generated by the second coil  64  is feedback-controlled based on the magnetic field detected by the detection unit  80 . Note that the magnetic field fluctuation residual is not a disturbance magnetic field fluctuation (a disturbance magnetic field fluctuation with a large amplitude) corresponding to the position of the stage  20 , but a magnetic field fluctuation which has a small amplitude and is obtained by superposing the disturbance magnetic field fluctuation corresponding to the position of the stage  20  and the magnetic field generated by the first coil  62 . Therefore, the detection unit  80  can detect a magnetic field (that is, a magnetic field fluctuation residual) on the electron beam path between the electron optical system  10  and the substrate ST without suffering from an insufficient detection range. 
         [0045]    In this manner, because the exposure apparatus  1  can accurately reduce (cancel) a magnetic field fluctuation generated on the electron beam path between the electron optical system  10  and the substrate ST upon driving the stage  20 , it can form a pattern on the substrate ST with high accuracy. Hence, the exposure apparatus  1  can provide high-quality devices (for example, a semiconductor device and a liquid crystal display device) with a high throughput and good economical efficiency. These devices are fabricated by a step of exposing a substrate (for example, a wafer or a glass plate) coated with a photoresist (photosensitive agent) using the exposure apparatus  1 , a step of developing the exposed substrate, and subsequent known steps. 
         [0046]    Although the output ranges of magnetic fields generated by the first coil  62  and second coil  64 , respectively, are set in accordance with the numbers of turns of these coils in this embodiment, the present invention is not limited to this. For example, a power supply with an output range wider than that of the second power supply  74  which energizes the second coil  64  may be used as the first power supply  72  which energizes the first coil  62 . It is also possible to set the output ranges of these power supplies in accordance with, for example, the shapes and arrangements of the first coil  62  and second coil  64 , respectively.  FIG. 6  is a view illustrating a Helmholtz coil which includes coils  602   a  and  602   b  that are aligned with a space between them and generates a magnetic field in a direction parallel to the coil axis by energization by a power supply  604 . Letting a be the radius of the coils  602   a  and  602   b , and Z×2 be their distance, the magnetic field B in a direction parallel to the coil axis at the position X is given by: 
         [0000]    
       
         
           
             
               
                 
                   B 
                   = 
                   
                     
                       
                         μ 
                         0 
                       
                       · 
                       I 
                       · 
                       
                         a 
                         2 
                       
                     
                     
                       
                         ( 
                         
                           
                             a 
                             2 
                           
                           + 
                           
                             Z 
                             2 
                           
                         
                         ) 
                       
                       
                         3 
                         / 
                         2 
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
         [0000]    where μ 0  is the magnetic permeability in a vacuum, and I is the current value. As can be seen from equation (1), the magnetic field B decreases as the distance between the coils  602   a  and  602   b  increases. Therefore, the output range of a magnetic field generated by the first coil  62  can be widened by setting the position (the position in the X-axis direction) of the first coil  62  to be closer to the electron beam path than the position (the position in the X-axis direction) of the second coil  64 . The output range of a magnetic field generated by the first coil  62  can also be widened by appropriately changing the radius of the first coil  62  or the distance between the coils. 
         [0047]    In this embodiment, the detection unit  80  detects a magnetic field (its fluctuation) on the electron beam path between the electron optical system  10  and the substrate ST. However, because the detection unit  80  is placed in the space between the electron optical system  10  and the substrate ST, it does not detect an exact magnetic field on the electron beam path between the electron optical system  10  and the substrate ST. Hence, the detection accuracy of the detection unit  80  can be improved by correcting a deviation of the magnetic field fluctuation due to the difference between the electron beam path and the position of the detection unit  80 . Thus, the coil unit (second coil  64 ) can more accurately generate a magnetic field which reduces a disturbance magnetic field fluctuation (that is, a magnetic field fluctuation residual) generated upon driving the stage  20 . 
         [0048]      FIG. 7  is a table showing the relationship between the magnetic field (its value) on the electron beam path between the electron optical system  10  and the substrate ST and that at the position of the detection unit  80 . Let Ba be the magnetic field fluctuation on the electron beam path, and Ba′ be the magnetic field fluctuation at the position of the detection unit  80  at that time. Also, let Bc be the magnetic field on the electron beam path upon supplying a given current value to the coil unit  60 , and Bc′ be the magnetic field at the position of the detection unit  80  at that time. Moreover, let Bd be the disturbance magnetic field fluctuation on the electron beam path upon driving the stage  20 , and Bd′ be the disturbance magnetic field fluctuation at the position of the detection unit  80  at that time. Note that the amount of magnetic field fluctuation Ba on the electron beam path satisfies a relation: Ba=Bd+Bc, and the amount of magnetic field fluctuation Ba′ at the position of the detection unit  80  at that time satisfies a relation: Ba′=Bd′+Bc′. 
         [0049]    The relationship among the magnetic field Bc, the position (x, y) of the stage  20 , and the current I supplied to the coil unit  60  is obtained and stored in the memory of the control unit  90  in advance as a table L. Thus, a magnetic field Bc(x, y, I) corresponding to the position (x, y) of the stage  20  and the current I can be obtained by looking up the table L. 
         [0050]    Also, the relationship among the magnetic field Bc′, the position (x, y) of the stage  20 , and the current I supplied to the coil unit  60  is obtained and stored in the memory of the control unit  90  in advance as a table L′. Thus, a magnetic field Bc′(x, y, I) corresponding to the position (x, y) of the stage  20  and the current I can be obtained by looking up the table L′. 
         [0051]    Moreover, the relationship between the disturbance magnetic field fluctuations Bd and Bd′ and the position (x, y) of the stage  20  is obtained, and a correction coefficient Kbd(x, y)=Bd/Bd′ is stored in the memory of the control unit  90  in advance as a table M. Thus, a correction coefficient Kbd(x, y) corresponding to the position (x, y) of the stage  20  can be obtained by looking up the table M. 
         [0052]    A procedure for obtaining an amount of magnetic field fluctuation Ba on the electron beam path based on the position (x, y) of the stage  20  and the amount of magnetic field fluctuation Ba′ when the stage  20  is at an arbitrary position (x, y) will be described. 
         [0053]    First, a magnetic field Bc′(x, y, I) corresponding to both the position (x, y) of the stage  20  and the current I is obtained by looking up the table L′. From the amount of magnetic field fluctuation Ba′ and magnetic field Bc′, a disturbance magnetic field fluctuation Bd′ is obtained in accordance with: 
         [0000]        Bd′=Ba′−Bc ′( x,y,I )  (2)
 
         [0054]    Next, a correction coefficient Kbd(x, y) corresponding to the position (x, y) of the stage  20  is obtained by looking up the table M. From the disturbance magnetic field fluctuation Bd′ and correction coefficient Kbd(x, y), a disturbance magnetic field fluctuation Bd is obtained in accordance with: 
         [0000]        Bd=Kbd ( x,y )× Bd′   (3)
 
         [0055]    Lastly, a magnetic field Bc(x, y, I) corresponding to the position (x, y) of the stage  20  and the current I is obtained by looking up the table L. From the disturbance magnetic field fluctuation Bd and magnetic field Bc, an amount of magnetic field fluctuation Ba is obtained in accordance with: 
         [0000]        Ba=Bd−Bc ( x,y,I )  (4)
 
         [0056]    In this way, a magnetic field fluctuation on the electron beam path can be obtained with high accuracy by correcting the amount of magnetic field fluctuation Ba′ (that is, the magnetic field detected by the detection unit  80 ) at the position of the detection unit  80  to the amount of magnetic field fluctuation Ba in the electron beam path. 
         [0057]    Although a disturbance magnetic field fluctuation in the X-axis direction is reduced (canceled) in this embodiment, a disturbance magnetic field fluctuation in the Y-axis direction can similarly be reduced by providing the above-mentioned arrangement for the Y-axis direction. 
         [0058]    Also, the first coil  62  and second coil  64  can be replaced with a single coil  66 , and the first power supply  72  and second power supply  74  can be replaced with a single power supply  76 , as shown in  FIG. 8 . Note that the coil  66  has the functions of both the first coil  62  and second coil  64 , and the power supply  76  has a function of energizing the coil  66 . Referring to  FIG. 8 , upon driving the stage  20 , the measurement unit  50  measures the position (the position in the X- and Y-axis directions) of the stage  20  and inputs the measurement result to the control unit  90 . While the feedforward control system performs feedforward control, the detection unit  80  detects a magnetic field on the electron beam path between the electron optical system  10  and the substrate ST, and inputs the detection result to the control unit  90 . 
         [0059]    As described above, the memory of the control unit  90  stores, in advance, a first table indicating the relationship between the position of the stage  20  and the disturbance magnetic field fluctuation on the electron beam path between the electron optical system  10  and the substrate ST. The memory of the control unit  90  also stores, in advance, a second table indicating the relationship among the position of the stage  20 , the current value supplied to the first coil  62 , and the magnetic field (magnetic field value) generated by the first coil  62 . 
         [0060]    The feedforward control system looks up the first table to obtain an amount of disturbance magnetic field fluctuation ΔB corresponding to the position of the stage  20  measured by the measurement unit  50 . The feedforward control system determines, as a magnetic field to be generated by the first coil  62 , a magnetic field which is equal in absolute value and opposite in direction to the amount of disturbance magnetic field fluctuation ΔB corresponding to the position of the stage  20 , and generates a command value used to generate this magnetic field. The feedforward control system looks up the second table to obtain a constant of proportionality Kb corresponding to the position of the stage  20  measured by the measurement unit  50 , and multiplies the constant of proportionality Kb by the above-mentioned command value to obtain a current value to be supplied to the first coil  62 . The feedforward control system inputs the obtained current value as a current command value for the first power supply  72 . 
         [0061]    On the other hand, the feedback control system obtains a deviation between the command value used to cancel the disturbance magnetic field fluctuation to zero and the magnetic field (its fluctuation) detected by the detection unit  80 , and multiplies the deviation by a constant of proportionality Kc to obtain a current value to be supplied to the second coil  64 . The feedback control system inputs the obtained current value as a current command value for the second power supply  74 . 
         [0062]    The power supply  76  energizes the coil  66  based on the sum total of the current command value from the feedforward control system and that from the feedback control system. Thus, the coil  66  generates a magnetic field which reduces (cancels) a disturbance magnetic field fluctuation (including the above-mentioned magnetic field fluctuation residual) generated on the electron beam path between the electron optical system  10  and the substrate ST upon driving the stage  20 . 
         [0063]    In this manner, feedforward control based on the position of the stage  20  measured by the measurement unit  50  prevents generation of a magnetic field fluctuation with a large amplitude on the electron beam path between the electron optical system  10  and the substrate ST so that the amplitude of the magnetic field fluctuation falls within the detection range of the detection unit  80 . After that, feedback control based on the magnetic field detected by the detection unit  80  reduces (cancels) a magnetic field fluctuation with a small amplitude on the electron beam path between the electron optical system  10  and the substrate ST. The control configuration shown in  FIG. 8  is effective when the magnetic field generated by the coil  66  (and the power supply  76 ) has a sufficient output range and resolution and the detection unit  80  has an insufficient detection range. 
         [0064]    While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
         [0065]    This application claims the benefit of Japanese Patent Application No. 2010-145529 filed on Jun. 25, 2010, which is hereby incorporated by reference herein in its entirety.