Abstract:
There is provided an electromagnetic lens which includes an electromagnetic coil wound to be rotationally symmetrical about an optical axis of an electron beam, and a pole piece covering the electromagnetic coil, in which: a gap is integrally formed in either one of an inner wall formed at an inner circumference side of the pole piece and a lower end wall formed in an end portion at an emission side of the electron beam, or a boundary portion between the two walls; the inner wall is formed to be thinnest at a portion close to the gap and to gradually become thicker as a distance from the gap increases; and the electromagnetic lens is formed such that a width in a radial direction thereof is more increased as being closer to the gap along with the change of the thickness of the inner wall.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority of the prior Japanese Application No. 2012-273068, filed on Dec. 14 2012, the entire contents of which are incorporated herein by reference. 
     TECHNICAL FIELD 
     The present invention relates to an electromagnetic lens for an electron beam exposure apparatus. 
     BACKGROUND ART 
     In an electron beam exposure apparatuses perform exposure in such a manner that an electron beam is emitted from an electron gun. The electron beam is let through a stencil mask with a rectangular aperture or an aperture having a predetermined pattern shape formed therein. Thereafter the electron beam is reduced in size at 1/20 for example by an electron optical system. Then the electron beam is projected onto a wafer. 
     For reducing the time required for the exposure, there is also proposed a multi-column type electron beam exposure apparatus which includes a plurality of electron beam columns (column cells), each of which a column cell is downsized and includes an electron gun and an electron optical system for projection of an electron beam. The multi-column electron beam exposure apparatus performs exposure in parallel using the plurality of column cells. Thus, the processing speed by the multi-column electron beam exposure apparatus is improved by a factor corresponding to the number of the electron beam columns, with respect to that of an electron beam exposure apparatus which uses a single column cell. 
     For further improving throughput, the number of column cells in the electron beam exposure apparatus is preferably increased, and further downsizing of the electron beam column is demanded.
     PATENT DOCUMENT 1: Japanese Laid-open Patent Publication No. 2001-110351   

     SUMMARY OF INVENTION 
     Problems to be Solved 
     For downsizing of an electron beam column, an outer diameter of an electromagnetic lens for converging an electron beam needs to be reduced. 
     However, the reduction of the outer diameter of the electromagnetic lens results in downsizing of an electromagnetic coil included in the electromagnetic lens and a shortage of the winding number of the electromagnetic coil. As a result, a larger current is required to generate a desired magnetic field. The larger current causes a problem of an increased amount of heat generated by the electromagnetic coil. 
     Moreover, when an electromagnetic lens is formed in a shape elongated so as to increase the winding number of the electromagnetic coil, magnetic saturation occurs in a pole piece covering the electromagnetic coil, thereby failing to generate a desired magnetic field. 
     Therefore, an object is to provide an electromagnetic lens for an electron beam exposure apparatus which can generate a desired magnetic field with the reduced outer diameter of the electromagnetic lens without increasing the amount of generated heat. 
     Means for Solving the Problem 
     According to one aspect, there is provided an electromagnetic lens which includes: an electromagnetic coil wound to be rotationally symmetrical about an optical axis of an electron beam; and a pole piece including an inner wall covering an inner circumference side of the electromagnetic coil, an upper end wall covering the electromagnetic coil at an entering side of the electron beam, a lower end wall covering the electromagnetic coil at an emitting side of the electron beam, an outer wall covering an outer circumference side of the electromagnetic coil, and a gap formed by cutting out at least a part of the inner wall, the lower end wall, and the upper end wall to be rotationally symmetrical about the optical axis. In the electromagnetic lens, a thickness of the inner wall is thinnest at a portion close to the gap and gradually becomes thicker as a distance from the gap increases, and a width of the electromagnetic coil in a radial direction is more increased as being closer to the gap. 
     Effect of the Invention 
     In the electromagnetic lens according to the abovementioned aspect, the inner wall has a thicker thickness at a portion that is away from the gap and is most likely to have a high magnetic flux density in the pole piece. This increases a portion through which the magnetic flux can pass in the inner wall to moderate an increase in the magnetic flux. Moreover, the thickness of the inner wall is gradually changed to allow the magnetic flux to smoothly flow through the inner wall, thereby making it possible to prevent a portion with the high magnetic flux density from being generated. 
     Accordingly, a pole piece having a shape long in the vertical direction can be employed, thereby making it possible to increase the winding number of the electromagnetic coil. 
     As a result, there can be obtained an electromagnetic lens which can generate a desired magnetic field with the reduced outer diameter thereof without increasing the amount of heat generated by the electromagnetic coil. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1A  is a cross-sectional view of an electromagnetic lens according to a prelude, and  FIG. 1B  is a perspective view illustrating the electromagnetic lens in  FIG. 1A  partially cut out. 
         FIGS. 2A to 2D  are views each illustrating a result of the distribution of the magnetic flux density in a pole piece obtained by calculation, for electromagnetic lenses of various shapes. 
         FIG. 3  is a graph illustrating a result of a relation between the spherical aberration coefficient of the electromagnetic lenses having been examined in the prelude and the amount of heat generated by the electromagnetic lenses. 
         FIGS. 4A and 4B  are perspective views illustrating an electromagnetic lens according to a first embodiment. 
         FIG. 5A  is a cross-sectional view of an electromagnetic lens according to an example 1 in the first embodiment,  FIG. 5B  is a cross-sectional view of an electromagnetic lens according to an example 2 in the first embodiment, and  FIG. 5C  is a cross-sectional view of an electromagnetic lens according to a comparative example. 
         FIGS. 6A to 6C  are views each illustrating a result of the distribution of the magnetic flux density in the pole piece obtained by calculation, for the electromagnetic lenses in  FIG. 5A to 5C . 
         FIG. 7  is a block diagram of an electron beam exposure apparatus according to a second embodiment. 
         FIG. 8  is a block diagram illustrating one column cell in  FIG. 7 . 
         FIG. 9A  is a view illustrating an arrangement of a plurality of electromagnetic lenses mounted on the electron beam exposure apparatus in  FIG. 7 , and  FIG. 9B  is a cross-sectional view of the electromagnetic lenses in  FIG. 9A . 
         FIG. 10  is a perspective view illustrating an electromagnetic lens according to a third embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     A prelude will be described prior to explanations of embodiments. 
       FIG. 1A  is a cross-sectional view illustrating an electromagnetic lens according to a prelude, and  FIG. 1B  is a perspective view illustrating the electromagnetic lens in  FIG. 1A  partially cut out. An arrow A in the drawings indicates the emission direction of electron beams. 
     An electromagnetic lens  84  illustrated in  FIGS. 1A and 1B  includes an electromagnetic coil  83  formed to be rotationally symmetrical about an optical axis c of an electron beam EB, and a pole piece  82  covers the surrounding of the electromagnetic coil  83 . 
     The pole piece  82  includes an inner wall  82   b  formed on an inner circumference side portion thereof, an upper end wall  82   c  covering the electromagnetic coil  83  at an entering side of the electron beam EB, a lower end wall  82   d  covering the electromagnetic coil  83  at an emitting side of the electron beam EB, and an outer wall  82   e  covering an outer circumference side of the electromagnetic coil  83 . 
     The electromagnetic lens  84  is an objective lens which converges the electron beam on the surface of a wafer  12  to be irradiated with the electron beam, and includes an annular gap  82   a  formed around the optical axis c in the lower end wall  82   d  to be opposed to the wafer  12 . 
     A magnetic flux leaked from a magnetic pole of the electromagnetic coil  83  passes inside the pole piece  82  formed of a magnetic material is leaked outside from the gap  82   a  and generates a magnetic field above the wafer  12 . An electron beam as an image S 1  formed on an image plane  80  is converged by the magnetic field of the electromagnetic lens  84  while passing through a through-hole of the electromagnetic lens  84  to reach the surface (image plane) of the wafer  12 . Thus, an image S 2  is formed on the surface (image plane) of the wafer  12 . 
     Meanwhile, a value of spherical aberration on the optical axis to determine the minimum size of the image S 2  formed by the electromagnetic lens  84  is in proportion to Csα 3  where Cs [mm] denotes a spherical aberration coefficient of the electromagnetic lens  84  and α [rad] denotes an aperture angle of the electron beam EB. For obtaining the sufficient resolution, it is preferable to decrease the spherical aberration coefficient Cs in such a manner that the magnetic field generated by the electromagnetic lens  84  is localized around the surface of the wafer  12  and increased in strength. 
     For increasing the number of column cells, the electron beam exposure apparatus is required to have a reduced outer diameter of the electromagnetic lens while maintaining a predetermined spherical aberration coefficient Cs. 
     The reduced outer diameter of the electromagnetic lens  84  requires the downsizing of the electromagnetic coil  83 . This poses a problem that the electromagnetic coil  83  is difficult to cool because the amount of generated heat increases due to the limited cross-section area of the winding portion of the electromagnetic coil  83 . 
     To address this, various models with different shapes of the pole piece  82  and the electromagnetic coil  83  were created, the spherical aberration, the magnetomotive force, and the amount of generated heat of the electromagnetic lens  84 , and the distribution of the magnetic flux density in the pole piece  82  were examined. 
       FIGS. 2A to 2D  are views each illustrating a result of the distribution of the magnetic flux density in the pole piece obtained by calculation, for the electromagnetic lenses of various shapes. Note that, the cross sections in  FIG. 2  are illustrated as cross-sectional views in which the electromagnetic lens  84  is taken along a plane in parallel with the optical axis c of the electron beam EB, the bottom side in each view is an optical axis c side, and the direction of the arrow in each view corresponds to the emission direction of the electron beam EB. Moreover, although the cross sections and the distributions of the magnetic flux density of the electromagnetic lens  84  appear rotationally symmetrical about the optical axis c, the other cross section across the optical axis c is not illustrated. 
     The calculation uses the electromagnetic lenses  84  downsized to have an outer diameter of φ60 [mm] and having shapes in  FIGS. 2A to 2D  each including the electromagnetic coil  83  and the magnetic body pole piece  82 , and obtained the magnetomotive force current of the electromagnetic coil  83 , the current density in the magnetic body pole piece, and the like which are required to generate the lens magnetic field strength with which an electron beam at the acceleration voltage of 50 Kev can be converged onto a predetermined image surface by the electromagnetic lens  84 . Moreover, under such conditions, the distribution of the magnetic flux density generated in the electromagnetic lens  84  and the surrounding thereof is obtained, and the spherical aberration coefficient Cs is obtained by the orbit calculation of the electron beam. 
     In the electromagnetic lens in  FIG. 2A , a length of the pole piece  82  in the optical axis direction is set to 31 [mm], which is the longest among those of four examples of  FIGS. 2A to 2D , in order to increase the winding number of the electromagnetic coil  83  without increasing the outer diameter of the electromagnetic lens  84 . 
     A region  91  in the drawing is a region where the magnetic flux density reaches about 2.2 [T] (tesla) which is the saturated magnetic flux density of a magnetic body included in the pole piece  82 , and a diagonally shaded region  92  is a region where the magnetic flux density exceeds 2.2 [T] (tesla) which is the saturated magnetic flux density of the magnetic body. Moreover, the other region with no sign is a region where the magnetic flux density is sufficiently lower than 2.2 [T] (tesla) which is the saturated magnetic flux density. Further, if the region  91  having a possibility of reaching the magnetic saturation, or the magnetically saturated region  92  appears over a whole region of the inner wall  82   b  of the pole piece  82  from the inner side (side close to the optical axis c) to the outer side (side close to the electromagnetic coil  83 ), the magnetic flux leaks from the pole piece  82  to disturb the orbit of the electron beam EB. Accordingly, the magnetomotive force of the electromagnetic coil  83  is limited by the magnetic saturation of the pole piece  82 . 
     In the case in  FIG. 2A , the magnetomotive force of the electromagnetic coil is 3022 [A·T] (ampere-turn), at which the amount of heat generated by the electromagnetic coil is 202 [W] and the spherical aberration coefficient Cs is 10.0 [mm]. 
     Meanwhile,  FIG. 2B  illustrates a calculation result where the length of the pole piece in the optical axis direction is set to 26 [mm] which is shorter than that of the pole piece in  FIG. 2A . 
     In the electromagnetic lens in  FIG. 2B , the magnetic saturation of the pole piece is less likely to occur because of the shorter length of the pole piece. As a result, the magnetomotive force of the electromagnetic coil-is 3438 [A·T] (ampere-turn), and the spherical aberration coefficient Cs decreases to 8.2 [mm]. However, the amount of heat generated by the electromagnetic coil  83  increases to 359 [W] because the electromagnetic coil  83  is downsized to decrease the winding number. 
       FIG. 2C  illustrates a calculation result where the length of the pole piece of the electromagnetic lens is set to 21 [mm] which is shorter than that of the pole piece in  FIG. 2B . The pole piece  82  in  FIG. 2C  is more unlikely to be magnetically saturated than the pole pieces  82  in  FIGS. 2A and 2B . The magnetomotive force of the electromagnetic coil  83  is 3783 [A·T], and the spherical aberration coefficient Cs decreases to 6.7 [mm]. Meanwhile, the winding number of the electromagnetic coil  83  is further reduced compared with those of  FIGS. 2A and 2B , and the magnetomotive force current is increased. Thus, the amount of heat generated by the electromagnetic coil  83  increases to 946 [W]. 
     The electromagnetic lens in  FIG. 2D  has a wider gap than the electromagnetic lens in  FIG. 2C  while having the same length of the pole piece as the electromagnetic lens in  FIG. 2C . 
     In this electromagnetic lens, the magnetomotive force of the electromagnetic coil  83  is 4882 [A·T], and the spherical aberration coefficient Cs decreases to 5.4 [mm]. 
     However, the amount of heat generated by the electromagnetic coil  83  increases to 1850 [W]. 
       FIG. 3  is a graph illustrating a result of a relation obtained between the spherical aberration coefficient of the electromagnetic lenses examined in the prelude and the amount of heat generated by them. 
     As illustrated in  FIG. 3 , it has revealed that the amount of heat generated by the electromagnetic coil abruptly increases as the spherical aberration coefficient of the electromagnetic lens is decreased. 
     For the purpose of solving a problem caused by such an abrupt increase in the amount of generated heat, an idea of embodiments described below has been arrived at. 
     (First Embodiment) 
       FIG. 4A  is a perspective view illustrating an electromagnetic lens  4  according to a first embodiment, and  FIG. 4B  is a perspective view illustrating a pole piece  2  of the electromagnetic lens  4  in  FIG. 4A . Note that  FIGS. 4A and 4B  illustrate them part of which is cut out for illustrating the inner structure. Moreover, a dashed dotted line in the drawings indicates an optical axis c of an electron beam EB which travels in the direction of an arrow in the drawing. 
     As illustrated in  FIG. 4A , the electromagnetic lens  4  according to the embodiment is provided with an electromagnetic coil  3  formed around the optical axis c of the electron beam EB, and the pole piece  2  which covers the electromagnetic coil  3 . The electromagnetic coil  3  and the pole piece  2  are respectively formed to be rotationally symmetrical about the optical axis c. 
     As illustrated in  FIG. 4B , the pole piece  2  is formed of a soft magnetic material having a relatively high saturated magnetic flux density. The pole piece  2  includes an inner wall  2   b  formed at an inner circumference side near the optical axis c, an upper end wall  2   c  formed on an end portion at an entering side of the electron beam EB, a lower end wall  2   d  formed on an end portion at an emitting side of the electron beam EB, and a cylindrical outer wall  2   e  connected to peripheral portions of the upper end wall  2   c  and the lower end wall  2   d . Further, a coil housing unit  2   f  is formed in a portion surrounded by the inner wall  2   b , the upper end wall  2   c , the lower end wall  2   d , and the outer wall  2   e , of the pole piece  2 . 
     Moreover, the pole piece  2  includes a gap  2   a  formed such that a part of the lower end wall  2   d  is cut out in a circular shape around the optical axis c. A magnetic flux is leaked from the portion of the gap  2   a  to generate a magnetic field for causing the electron beam to be converged above a sample (not illustrated) as a target to be irradiated with the electron beam EB. 
     In the pole piece  2  according to the embodiment, the thickness (thickness of a portion T in the drawing) of the inner wall  2   b  in the radial direction of the optical axis c gradually increases as being away from the gap  2   a.    
     This increases the thickness of the inner wall  2   b  at a portion away from the gap  2   a  which is likely to be magnetically saturated in the pole piece  2 , thereby inhibiting magnetic saturation in the inner wall  2   b.    
     Moreover, the thickness of the inner wall  2   b  gradually changes toward the gap  2   a  to allow the magnetic flux to smoothly pass through the inner wall  2   b  and to prevent the density of the magnetic flux from locally increasing. Thus, the magnetic saturation is more unlikely to occur. 
     As a result, even if the length of the pole piece  2  in the optical axis direction is increased, the magnetic saturation is unlikely to occur. Thus, the electromagnetic coil  3  having a larger winding number can be used with the length of the pole piece  2  in the optical axis direction increased. 
     In addition, as illustrated in  FIG. 4A , the electromagnetic coil  3  is formed to have a trapezoidal cross section, and a width of the electromagnetic coil  3  in the radial direction gradually increases as being close to the gap  2   a , in the electromagnetic lens  4 . 
     This allows the electromagnetic coil  3  to be wound in a large area on an inner side (side near the optical axis), compared with a case where the electromagnetic coil  3  is formed in a cylindrical shape having a rectangular cross section. 
     As a result, the magnetomotive force by the electromagnetic coil  3  can be increased while the amount of heat generated by the electromagnetic coil  3  is reduced. Thus, even though the electromagnetic lens  3  is downsized, it is possible to reduce the spherical aberration coefficient Cs while inhibiting the heat generation. 
       FIG. 5A  is a cross-sectional view of an electromagnetic lens  1 A according to a first example in the first embodiment,  FIG. 5B  is a cross-sectional view of an electromagnetic lens  1 B according to an example 2 in the first embodiment, and  FIG. 5C  is a cross-sectional view of an electromagnetic lens  84  according to a comparative example. One of the cross sections of the electromagnetic lens is illustrated in each of the drawings. Moreover, the numeric characters in the drawings indicate sizes [mm] of the respective electromagnetic lenses. 
     The electromagnetic lens  1 A in the example 1 illustrated in  FIG. 5A  has an outer diameter of φ60 [mm]. 
     The electromagnetic lens  1 A has a length in the optical axis direction of 40 [mm] which is longer than that of the electromagnetic lens illustrated in  FIG. 2A . The winding number of the electromagnetic coil  3  in which a conductive wire having a diameter of 0.5 [mm] is used is 893 turns. 
     Moreover, the electromagnetic lens  1 B in the example 2 illustrated in  FIG. 5B  has a reduced outer diameter of φ40 [mm] compared with the outer diameter of the electromagnetic lens  1 A in  FIG. 5A . The winding number of the electromagnetic coil  3  in which a conductive wire having a diameter of 0.5 [mm] is used is 416 turns. 
     Meanwhile, an electromagnetic lens  84  in the comparative example illustrated in  FIG. 5C  has a structure similar to that of the electromagnetic lens in  FIG. 2C , and has an outer diameter of φ60 [mm] and a length in the optical axis direction of 27 [mm]. 
     Next, description will be provided for results of the distribution of the magnetic flux density in the pole piece obtained by calculation under the conditions in which predetermined spherical aberration coefficients Cs for the electromagnetic lenses  1 A,  1 B, and  84  can be obtained. 
       FIGS. 6A to 6C  are views each illustrating a result of the distribution of the magnetic flux density in the pole piece obtained by calculation for the electromagnetic lenses  1 A,  1 B, and  84  in  FIGS. 5A to 5C . 
       FIG. 6A  illustrates the distribution of the magnetic flux density in the pole piece  2  of the electromagnetic lens  1 A (example 1) in  FIG. 5A . In the electromagnetic lens  1 A in the example 1, the spherical aberration coefficient Cs of 7.5 [mm] is obtained when the magnetomotive force of the electromagnetic coil is 3521 [A·T] (ampere-turn), and the amount of generated heat is 162 [W] under these conditions. 
     Moreover, as illustrated in the drawing, a portion where the magnetic flux density exceeds the saturated magnetic flux density is hardly generated in the pole piece  2 . Thus, this result confirms that the magnetic saturation can be effectively inhibited. 
       FIG. 6B  illustrates the distribution of the magnetic flux density in the pole piece  2  of the electromagnetic lens  1 B (example 2) in  FIG. 5B . In the electromagnetic lens  1 B in the example 2, the spherical aberration coefficient Cs of 7.5 [mm] is obtained when the magnetomotive force of the electromagnetic coil is 2983 [A·T] (ampere-turn), the amount of generated heat is 167 [W] under these conditions. 
     As illustrated in the drawing, a portion where the magnetic flux density exceeds the saturated magnetic flux density is only partially generated in the pole piece  2 , and the magnetic flux is not leaked to the optical axis side of the electron beam. 
     The above result confirms that even the electromagnetic lens  1 B in the example 2 with the reduced outer diameter of about 40 [mm] can inhibit the magnetic saturation and prevent the amount of generated heat from increasing. 
       FIG. 6C  illustrates the distribution of the magnetic flux density in the pole piece of the electromagnetic lens  84  (comparative example). The electromagnetic lens  84  in the comparative example has a length of the pole piece in the optical axis direction of 27 [mm] which is shorter than that of the electromagnetic lens  1 A in  FIG. 6A . Regardless of the shorter length, when the magnetomotive force of the electromagnetic coil is 3158 [A·T] (ampere-turn), the region  91  where the magnetic flux density reaches the saturated magnetic flux density is generated over a whole region of the inner wall of the pole piece  2  in the thickness direction. The spherical aberration coefficient Cs in this case remains at 10 [mm], and the amount of generated heat is 202 [W] which is increased compared with those of the electromagnetic lenses  1 A and  1 B in the examples. 
     The above results confirm that, with the electromagnetic lenses  1 A and  1 B according to the embodiment, the desired spherical aberration coefficient Cs can be obtained even if the outer diameter thereof is downsized, and the heat generation by the electromagnetic coil can be effectively reduced. 
     (Second Embodiment) 
       FIG. 7  is a schematic configuration view of a multi-column electron beam exposure apparatus according to a present embodiment. 
     The multi-column electron beam exposure apparatus is roughly divided into an electron beam column  10 , and a control unit  20  which controls the electron beam column  10 . Between them, the electron beam column  10  includes a plurality of, for example 16, equivalent column cells  11  which constitute the overall column. All the column cells  11  are configured to include equivalent units. Below the column cells  11 , a wafer stage  13  on which a wafer  12  of 300 [mm] is mounted is disposed for example. 
     Meanwhile, the control unit  20  includes an electron gun high-pressure power supply  21 , a lens power supply  22 , a digital control unit  23 , a stage drive controller  24 , and a stage position sensor  25 . Among them, the electron gun high-pressure power supply  21  supplies power for driving an electron gun in each of the column cells  11  in the electron beam column  10 . The lens power supply  22  supplies power for driving an electromagnetic lens in each of the column cells in the electron beam column  10 . The digital control unit  23  is an electric circuit which controls deflection outputs from deflectors in each of the column cells  11 , and outputs a high-speed deflection output and the like. The digital control units  23  are prepared as many as the number of the column cells  11 . 
     The stage drive controller  24  moves the wafer stage  13  on the basis of position information from the stage position sensor  25  in such a manner that a desired position on the wafer  12  is irradiated with the electron beam. The abovementioned respective units  21  to  25  are integrally controlled by an integral control system  26  such as a work station. 
       FIG. 8  is a schematic configuration view of each column cell  11  used in the multi-column electron beam exposure apparatus. 
     Each column cell  11  is roughly divided into an exposure unit  100 , and a column cell control unit  31  which controls the exposure unit  100 . Between them, the exposure unit  100  is configured to include an electron beam generation unit  130 , a mask deflection unit  140 , and a substrate deflection unit  150 . 
     In the electron beam generation unit  130 , an electron beam EB generated by an electron gun  101  is subjected to a convergence action by a first electromagnetic lens  102 , and then passes through a rectangular aperture  103   a  in a beam forming mask  103  to shape a cross section of the electron beam EB into a rectangle. 
     Thereafter, an image of the electron beam EB is formed on an exposure mask  110  by a second electromagnetic lens  105  in the mask deflection unit  140 . Further, the electron beam EB is deflected by first and second electrostatic deflectors  104  and  106  to a particular pattern P formed in the exposure mask  110  to shape the cross section thereof into a shape of the pattern P. 
     Further, the exposure mask  110  is fixed to a mask stage  123  in the electron beam column  10 , and the mask stage  123  is movable in the horizontal plane. Accordingly, when the pattern P positioned at a portion out of a deflection range (beam deflection range) of the first and second electrostatic deflectors  104  and  106  is used, the mask stage  123  is moved so that the pattern P is moved into the beam deflection range. 
     Third and fourth electromagnetic lenses  108  and  111  respectively disposed above and below the exposure mask  110  function to form an image of the electron beam EB on a substrate. 
     The electron beam EB passed through the exposure mask  110  is deflected back to the optical axis c due to deflection actions by third and fourth electrostatic deflectors  112  and  113 , and then is reduced in size by a fifth electromagnetic lens  114 . 
     The mask deflection unit  140  is provided with first and second correction coils  107  and  109 , which correct beam deflection aberration generated in the first to fourth electrostatic deflectors  104 ,  106 ,  112 , and  113 . 
     Thereafter, the electron beam EB passes through an aperture  115   a  in a screening plate  115  which constitutes the substrate deflection unit  150 , and is projected on the substrate by first and second projection electromagnetic lenses  116  and  121 . In this manner, the image of the pattern on the exposure mask  110  is transferred onto the substrate at a predetermined reduction ratio, for example, 1/10. 
     The substrate deflection unit  150  provided with a fifth electrostatic deflector  119  and an electromagnetic deflector  120 , and these deflectors  119  and  120  deflect the electron beam EB to allow the image of the pattern on the exposure mask to be projected on a predetermined position of the substrate. 
     In addition, the substrate deflection unit  150  is provided with third and fourth correction coils  117  and  118  for correcting the deflection aberration of the electron beam EB on the substrate. 
     Meanwhile, the column cell control unit  31  includes an electron gun control unit  202 , an electron optical system control unit  203 , a mask deflection control unit  204 , a mask stage control unit  205 , a blanking control unit  206 , and a substrate deflection control unit  207 . Among them, the electron gun control unit  202  controls the electron gun  101  to control the acceleration voltage of the electron beam EB, the beam emission conditions, or the like. Moreover, the electron optical system control unit  203  controls the amounts of currents into the electromagnetic lenses  102 ,  105 ,  108 ,  111 ,  114 ,  116 , and  121  to adjust the magnification or the focal point positions of the electron optical systems configured to include these electromagnetic lenses. The blanking control unit  206  controls the voltage to be applied to a blanking electrode  127  to deflect the electron beam EB having been generated before the start of exposure on the screening plate  115 , thereby preventing the substrate from being irradiated with the electron beam EB before the exposure. 
     The substrate deflection control unit  207  controls the voltage to be applied to the fifth electrostatic deflector  119  and the amount of current into the electromagnetic deflector  120  so that the electron beam EB is deflected on a predetermined position of the substrate. The abovementioned respective units  202  to  207  are integrally controlled by the integral control system  26  such as a work station. 
       FIG. 9A  is a view illustrating the electromagnetic lenses  121  mounted on the multi-column electron beam exposure apparatus in  FIG. 7 , and  FIG. 9B  is a cross-sectional view of one of the electromagnetic lenses  121  in  FIG. 9A . 
     As illustrated in  FIG. 9A , the electromagnetic lenses  121  are disposed in parallel in a housing  14  which houses the respective column cells. The housing  14  has a diameter of about 150 [mm], and a pitch P which is an arrangement interval between the electromagnetic lenses  121  is set depending on the number of the column cells to be disposed. 
     For example, when four (2×2) pieces of column cells are disposed in the housing  14 , the pitch P between the electromagnetic lenses  121  becomes 66 [mm], and when nine (3×3) pieces of column cells are disposed therein, the pitch P becomes 44 [mm]. Moreover, when 16 (4×4) pieces of column cells are disposed in the housing  14  as illustrated in the drawing, the pitch between the electromagnetic lenses  121  becomes approximately 33 [mm]. The outer diameter of the electromagnetic lens  121  is required to be downsized smaller than the pitch P. 
     As illustrated in  FIG. 9B , the electromagnetic lens  121  is positioned and held in the housing  14  via holding plates  15  and  16 . The electromagnetic lens  121  according to the embodiment is provided with the pole piece  2  having the nearly same shape as that of the pole piece  2  of the electromagnetic lens  4  illustrated in  FIG. 4 . Further, the electromagnetic coil  3  having a trapezoidal cross section is disposed in the coil housing unit  2   f  of the pole piece  2 . 
     The electromagnetic lens  121  according to the embodiment is provided with a coolant flow path  2   g  between the electromagnetic coil  3  and the pole piece  2 , and a coolant, for example cooling water, can flow through the coolant flow path  2   g . In addition, the gap  2   a  of the pole piece  2  is filled with a sealing member  5  made of a nonmagnetic material, thereby preventing the coolant from overflowing. 
     An inlet  6   a  is provided at one end of the coolant flow path  2   g , and an outlet  6   b  is provided at the other end of the coolant flow path  2   g . The inlet  6   a  and the outlet  6   b  are connected to a circulation cooling mechanism (not illustrated). The coolant supplied from the circulation cooling mechanism circulates through the coolant flow path  2   g  to cool the electromagnetic coil  3 . 
     The magnetic saturation is unlikely to occur in the electromagnetic lens  121  according to the embodiment described above because the electromagnetic lens  121  is provided with the pole piece  2  similar to the pole piece of the electromagnetic lens illustrated in  FIG. 4 . Thus, even if the outer diameter of the electromagnetic lens  121  is reduced, the length of the pole piece  2  in the optical axis direction is increased to achieve the increased volumetric capacity of the electromagnetic coil  3 , thereby allowing the heat generation by the electromagnetic coil  3  to be reduced. 
     (Third Embodiment) 
       FIG. 10  is a perspective view illustrating an electromagnetic lens  50  according to a present embodiment. Note that, the electromagnetic lens illustrated in the drawing is taken along a plane in parallel with the optical axis c of the electron beam for easy understanding of the inner structure, and is actually formed to be rotationally symmetrical about the optical axis c of the electron beam. Further, an arrow in the drawing indicates the emission direction of the electron beam. 
     As illustrated in the drawing, the electromagnetic lens  50  is provided with an electromagnetic coil  53  formed to be rotationally symmetrical about the optical axis c, and a pole piece  52  which covers the electromagnetic coil  53 . The pole piece  52  according to the embodiment includes a gap  52   a  formed at the central portion in the optical axis direction of an inner wall  52   b  which is formed around the optical axis c. 
     The inner wall  52   b  is formed such that a portion in the vicinity of the gap  52   a  protrudes to the optical axis side. Accordingly, the electromagnetic lens  50  can form a magnetic field having the maximum value at the central portion in the direction of the optical axis c, and can be preferably used for parts other than objective lenses. 
     Moreover, the thickness (thickness of a portion T in the drawing) of the inner wall  52   b  of the pole piece  52  in the radial direction of the optical axis c increases as being away from the gap  52   a . Accordingly, the magnetic saturation is unlikely to occur in the inner wall  52   b.    
     Moreover, in the electromagnetic coil  53 , a portion near the gap  52   a  is formed in a convex shape protruding to the optical axis side of the electron beam along the shape of the inner wall  52   b.    
     In this manner, the portion of the electromagnetic coil  53  near the gap portion is formed in the convex shape along the shape of the pole piece  52  to increase the cross-section area S 0 , thereby making it possible to increase the winding number with the increased cross-section area. 
     In addition, since the electromagnetic coil  53  is formed in the convex shape protruding to the optical axis c, the average length L 0  of one turn of the winding of the electromagnetic coil  53  is shorter than in a case where the electromagnetic coil  53  is formed in a non-convex shape. 
     When the magnetomotive force of the electromagnetic coil  53  is N [A·T] (ampere-turn) and the resistivity of the winding is ρ, the amount of heat generated by the coil unit is in proportion to ρ(L 0 /S 0 ) (N) 2 . In other words, the amount of heat generated by the electromagnetic coil  53  is proportional to the average length L o  of the winding, and inversely proportional to the cross-section area S 0  in the electromagnetic coil  53 , under a condition with a constant magnetomotive force. 
     Accordingly, in the electromagnetic coil  53  according to the embodiment, the portion near the gap  52   a  formed in the convex shape protruding to the optical axis c results in decrease in the winding L 0  and increase in the cross-section area S 0 , thereby allowing the amount of heat generated by the electromagnetic coil  53  to be reduced.