Patent Publication Number: US-2023140499-A1

Title: Ion implantation method, ion implanter, and method for manufacturing semiconductor device

Description:
RELATED APPLICATIONS 
     The content of Japanese Patent Application No. 2021-176875, on the basis of which priority benefits are claimed in an accompanying application data sheet, is in its entirety incorporated herein by reference. 
     BACKGROUND 
     Technical Field 
     Certain embodiments of the present disclosure relate to an ion implantation method, an ion implanter, and a method for manufacturing a semiconductor device. 
     Description of Related Art 
     In a manufacturing process of a semiconductor device, a process of implanting ions into a semiconductor wafer (also referred to as an ion implantation process) is generally performed in order to change conductivity of a semiconductor, or in order to change a crystal structure of the semiconductor. An apparatus used for the ion implantation process is called an ion implanter. In the ion implantation process, in addition to “uniform implantation” performed so that a two-dimensional dose distribution is uniform on a wafer processing surface, “nonuniform implantation” performed so that the two-dimensional dose distribution is intentionally nonuniform may be required. 
     The two-dimensional dose distribution on the wafer processing surface is controlled by changing at least one of a beam scan speed and a wafer scan speed in accordance with a beam irradiation position on the wafer processing surface. For example, a beam scan speed distribution in a beam scan direction is adjusted to control a one-dimensional dose distribution in the beam scan direction. The beam scan speed distribution for realizing a target one-dimensional dose distribution is calculated, based on a measurement value and a target value of a beam current density distribution in the beam scan direction. In calculating the beam scan speed distribution, a relationship that beam current density is inversely proportional to the beam scan speed is used. 
     SUMMARY 
     According to an embodiment of the present disclosure, there is provided an ion implantation method including generating a first scan beam by performing a reciprocating scan using a spot-like ion beam in a predetermined direction, based on a first scan signal, measuring a beam current of the first scan beam by using a beam measurement device at a plurality of measurement positions different in the predetermined direction, calculating a beam current matrix in which beam current values with respect to a plurality of positions different in the predetermined direction and a plurality of scan command values are set as components, based on a time waveform of the beam current measured by the beam measurement device and a time waveform of the scan command values determined in the first scan signal, calculating a first beam current density distribution of the first scan beam in the predetermined direction by performing time integration on the measured beam current, correcting a value of each component of the beam current matrix, based on the first beam current density distribution, and generating a second scan signal for realizing a target beam current density distribution in the predetermined direction, based on the corrected beam current matrix. 
     According to another embodiment of the present disclosure, there is provided an ion implanter. The ion implanter includes a beam scan unit that generates a first scan beam by performing a reciprocating scan using a spot-like ion beam in a predetermined direction, based on a first scan signal, a beam measurement device configured to measure a beam current of the first scan beam at a plurality of measurement positions different in the predetermined direction, and a control device that generates a scan signal for determining a time waveform of a plurality of scan command values with respect to scan positions in the predetermined direction, based on a measurement obtained by the beam measurement device. the control device is configured to acquire a time waveform of the beam current of the first scan beam measured at the plurality of measurement positions, calculate a beam current matrix in which beam current values with respect to a plurality of positions different in the predetermined direction and the plurality of scan command values are set as components, based on the acquired time waveform of the beam current and the time waveform of the scan command values determined in the first scan signal, calculate a first beam current density distribution of the first scan beam in the predetermined direction by performing time integration on the measured beam current, correct a value of each component of the beam current matrix, based on the first beam current density distribution, and generate a second scan signal for realizing a target beam current density distribution in the predetermined direction, based on the corrected beam current matrix. 
     According to still another embodiment of the present disclosure, there is provided a method for manufacturing a semiconductor device. The method for manufacturing a semiconductor device includes an ion implantation process. The ion implantation process includes generating a first scan beam by performing a reciprocating scan using a spot-like ion beam in a predetermined direction, based on a first scan signal, measuring a beam current of the first scan beam at a plurality of measurement positions different in the predetermined direction, calculating a beam current matrix in which beam current values with respect to a plurality of positions different in the predetermined direction and a plurality of scan command values are set as components, based on a time waveform of the measured beam current and a time waveform of the scan command values determined in the first scan signal, calculating a first beam current density distribution of the first scan beam in the predetermined direction by performing time integration on the measured beam current, correcting a value of each component of the beam current matrix, based on the first beam current density distribution in the predetermined direction, generating a second scan signal for realizing a target beam current density distribution, based on the corrected beam current matrix, generating a second scan beam by performing the reciprocating scan using the spot-like ion beam in the predetermined direction, based on the second scan signal, and irradiating a semiconductor wafer with the second scan beam. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a top view illustrating a schematic configuration of an ion implanter according to an embodiment. 
         FIG.  2    is a side view illustrating a schematic configuration of the ion implanter in  FIG.  1   . 
         FIG.  3    is a diagram schematically illustrating an example of a configuration of a control device. 
         FIG.  4    is a top view schematically illustrating a configuration inside an implantation processing chamber. 
         FIG.  5    is a diagram schematically illustrating a data structure of an implantation recipe. 
         FIGS.  6 A and  6 B  are diagrams schematically illustrating a two-dimensional dose distribution. 
         FIG.  7    is a diagram illustrating an example of a correction function file and a correlation information file. 
         FIG.  8    is a table illustrating an example of the correlation information file. 
         FIG.  9    is a diagram schematically illustrating multiple step implantation. 
         FIG.  10    is a block diagram schematically illustrating a functional configuration of the control device. 
         FIG.  11    is a diagram schematically illustrating a beam current matrix according to the embodiment. 
         FIG.  12    is a view schematically illustrating measurement of a beam current distribution with respect to scan command values. 
         FIG.  13    is a view schematically illustrating measurement of the beam current distribution with respect to positions. 
         FIG.  14    is a graph schematically illustrating a relationship between the beam current matrix and a beam current density distribution. 
         FIG.  15    is a graph illustrating an example of time waveforms of a first scan signal and a beam current. 
         FIG.  16    is a circuit diagram illustrating an example of a configuration of a beam current measurement circuit. 
         FIG.  17    is a graph illustrating an example of a calculation value of the beam current density distribution based on the beam current matrix before correction and an actual measurement value of the beam current density distribution. 
         FIG.  18    is a flowchart illustrating an example of an ion implantation method according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     When a beam size is sufficiently small and a beam current is constant regardless of a position, a simple inversely proportional relationship is established between the beam current density distribution and the beam scan speed distribution. Therefore, the beam scan speed distribution for realizing the target value can be easily calculated. However, as the beam size increases, the simple inversely proportional relationship is less likely to be established. Accordingly, the measurement value of the beam current density distribution obtained by the calculated beam scan speed distribution may significantly deviate from the target value. In this case, it is necessary to repeat measurement and calculation until the beam scan speed distribution for realizing the target value is obtained. Therefore, it takes time for adjustment. In some cases, adjustment of the beam scan speed distribution may fail. An increase for adjustment time or an adjustment failure leads to degraded productivity in the ion implantation process. 
     It is desirable to provide a technique for improving productivity in an ion implantation process. 
     Any desired combination of the above-described components, and those in which components or expressions according to the present disclosure are substituted from each other in methods, devices, or systems are effectively applicable as an aspect of the present disclosure. 
     According to a non-limiting exemplary embodiment of the present disclosure, a technique for improving productivity in an ion implantation process can be provided. 
     Hereinafter, embodiments for implementing an ion implantation method, an ion implanter, and a method for manufacturing a semiconductor device according to the present disclosure will be described in detail with reference to the drawings. In describing the drawings, the same reference numerals will be assigned to the same elements, and repeated description will be appropriately omitted. In addition, configurations described below are merely examples, and do not limit the scope of the present disclosure in any way. 
     Before the embodiments are described in detail, an outline will be described. The present embodiment relates to a technique for controlling a two-dimensional dose distribution of an ion beam used to irradiate a semiconductor wafer, and more particularly relates to a technique for controlling a one-dimensional beam current density distribution in a beam scan direction. The beam current density distribution in the beam scan direction is controlled by adjusting a beam scan speed distribution of a beam scan using a reciprocating ion beam. In the present embodiment, instead of using a simple relationship that an inversely proportional relationship is established between the beam current density distribution and the beam scan speed distribution, a relationship between the beam current density distribution and the beam scan speed distribution will be defined by using a “beam current matrix”. 
     The beam current matrix consists of beam current values I(Vi, Xj)=Iij with respect to scan command values Vi and positions Xj in the x-direction, and represents a set of beam current distributions (that is, beam shapes) in an x-direction of the spot-like ion beam forming a scan beam. A beam current density distribution J (Xj) with respect to the positions Xj in the x-direction is expressed by Equation (1) below. 
     
       
         
           
             
               
                 
                   
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                           I 
                           
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     Here, Δti is a minute time held at the scan command value Vi in the beam scan (that is, a staying time of the scan beam), and is proportional to a reciprocal of a beam scan speed at the scan command value Vi. 
     In the present embodiment, the beam staying time Δti for realizing a target beam current density distribution J (Xj) is calculated by using the beam current matrix Iij, and a scan signal is generated, based on the beam staying time Δti held at the scan command value Vi in the beam scan. The beam current matrix includes all information relating to the beam shape of the spot-like ion beam in the x-direction. Accordingly, even when a beam size (that is, a spot size) of the ion beam in the x-direction is large, the scan signal for realizing the target beam current density distribution J (Xj) can be highly accurately calculated in a short time. 
     In the present embodiment, furthermore, a value of each component Iij of the beam current matrix is corrected, based on a measurement value of the beam current density distribution, and the scan signal for realizing the target beam current density distribution is generated, based on the corrected beam current matrix. By using the corrected beam current matrix, the beam current matrix can conform to the actual measurement value of the beam current density distribution. Since the scan signal is generated, based on the corrected beam current matrix, the scan signal for realizing the target beam current density distribution can be highly accurately calculated in a short time. 
       FIG.  1    is a top view schematically illustrating an ion implanter  10  according to an embodiment, and  FIG.  2    is a side view illustrating a schematic configuration of the ion implanter  10 . The ion implanter  10  is configured to perform an ion implantation process on a surface of a workpiece. For example, the workpiece is a substrate, or for example, the workpiece is a semiconductor wafer. For convenience of description, the workpiece may be referred to as a wafer W in the specification herein. However, this is not intended to limit an implantation process target to a specific object. 
     The ion implanter  10  is configured to irradiate a whole processing surface of the wafer W with the spot-like ion beam by performing a reciprocating scan using the beam in one direction and causing the wafer W to reciprocate in a direction perpendicular to a scan direction. In the specification herein, for convenience of description, a traveling direction of the ion beam traveling along a designed beamline A is defined as a z-direction, and a plane perpendicular to the z-direction is defined as an xy-plane. When the wafer W is scanned with the ion beam, the scan direction of the beam is defined as an x-direction, and a direction perpendicular to the z-direction and the x-direction is defined as a y-direction. Therefore, the reciprocating scan using the beam is performed in the x-direction, and a reciprocating movement of the wafer W is performed in the y-direction. 
     The ion implanter  10  includes an ion generation device  12 , a beamline unit  14 , an implantation processing chamber  16 , and a wafer transport device  18 . The ion generation device  12  is configured to provide the ion beam for the beamline unit  14 . The beamline unit  14  is configured to transport the ion beam from the ion generation device  12  to the implantation processing chamber  16 . The implantation processing chamber  16  accommodates the wafer W serving as an implantation target, and an implantation process of irradiating the wafer W with the ion beam provided from the beamline unit  14  is performed in the implantation processing chamber  16 . The wafer transfer device  18  is configured to load an unprocessed wafer before the implantation process into the implantation processing chamber  16 , and unload a processed wafer after the implantation process from the implantation processing chamber  16 . The ion implanter  10  includes a vacuum system (not illustrated) for providing a desired vacuum environment for the ion generation device  12 , the beamline unit  14 , the implantation processing chamber  16 , and the wafer transfer device  18 . 
     The beamline unit  14  includes a mass analyzing unit  20 , a beam park device  24 , a beam shaping unit  30 , a beam scan unit  32 , a beam parallelizing unit  34 , and an angular energy filter (AEF)  36 , in order from an upstream side of the beamline A. The upstream region of the beamline A means a region closer to the ion generation device  12 , and a downstream side of the beamline A means a side closer to the implantation processing chamber  16  (or a beam stopper  46 ). 
     The mass analyzing unit  20  is provided downstream of the ion generation device  12 , and is configured to select a required ion species from the ion beam extracted from the ion generation device  12  by performing mass analyzing. The mass analyzing unit  20  has a mass analyzing magnet  21 , a mass analyzing lens  22 , and a mass resolving aperture  23 . 
     The mass analyzing magnet  21  applies a magnetic field to the ion beam extracted from the ion generation device  12 , and deflects the ion beam to travel in a specific path in accordance with a value of the mass-to-charge ratio M=m/q (m is mass, and q is charge) of the ions. For example, the mass analyzing magnet  21  applies the magnetic field in the y-direction (−y-direction in  FIGS.  1  and  2   ) to the ion beam so that the ion beam is deflected in the x-direction. Magnetic field intensity of the mass analyzing magnet  21  is adjusted so that the ion species having a desired mass-to-charge ratio M passes through the mass resolving aperture  23 . 
     The mass analyzing lens  22  is provided downstream of the mass analyzing magnet  21 , and is configured to adjust focusing/defocusing power for the ion beam. The mass analyzing lens  22  adjusts a focusing position of the ion beam passing through the mass resolving aperture  23  in a beam traveling direction (z-direction), and adjusts a mass resolution M/dM of the mass analyzing unit  20 . The mass analyzing lens  22  is not an essential component, and the mass analyzing unit  20  may not have the mass analyzing lens  22 . 
     The mass resolving aperture  23  is provided downstream of the mass analyzing lens  22 , and is provided at a position away from the mass analyzing lens  22 . The mass resolving aperture  23  is configured so that a beam deflection direction (x-direction) by the mass analyzing magnet  21  is a slit width direction, and has an opening  23   a  having a shape which is relatively short in the x-direction and relatively long in the y-direction. 
     The mass resolving aperture  23  may be configured so that the slit width is variable for adjusting the mass resolution. The mass resolving aperture  23  may be configured to include two beam shield members that are movable in the slit width direction, and may be configured so that the slit width is adjustable by changing an interval between the two beam shield members. The mass resolving aperture  23  may be configured so that the slit width is variable by selecting any one of a plurality of slits having different slit widths. 
     The beam park device  24  is configured to cause the ion beam to temporarily retreat from the beamline A and to temporarily block the ion beam directed to the implantation processing chamber  16  (or the wafer W) located downstream. The beam park device  24  can be disposed at any desired position in an intermediate portion of the beamline A. For example, the beam park device  24  can be disposed between the mass analyzing lens  22  and the mass resolving aperture  23 . A prescribed distance is required between the mass analyzing lens  22  and the mass resolving aperture  23 . Accordingly, the beam park device  24  is disposed between both of them. In this manner, a length of the beamline A can be shortened, compared to a case where the beam park device  24  is disposed at another position. Therefore, the whole ion implanter  10  can be reduced in size. 
     The beam park device  24  includes a pair of park electrodes  25  ( 25   a  and  25   b ) and a beam dump  26 . The pair of park electrodes  25   a  and  25   b  face each other across the beamline A, and faces in a direction (y-direction) perpendicular to the beam deflection direction (x-direction) of the mass analyzing magnet  21 . The beam dump  26  is provided on the downstream side of the beamline A than the park electrodes  25   a  and  25   b , and is provided away from the beamline A in a facing direction of the park electrodes  25   a  and  25   b.    
     The first park electrode  25   a  is disposed on an upper side of the beamline A in a direction of gravity, and the second park electrode  25   b  is disposed on a lower side of the beamline A in the direction of gravity. The beam dump  26  is provided at a position away to the lower side of the beamline A in the direction of gravity, and is disposed on the lower side of the opening  23   a  of the mass resolving aperture  23  in the direction of gravity. For example, the beam dump  26  is configured to include a portion of the mass analyzing slit  23  where the opening  23   a  is not formed. The beam dump  26  may be configured to be separate from the mass resolving aperture  23 . 
     The beam park device  24  deflects the ion beam by using an electric field applied between the pair of park electrodes  25   a  and  25   b , and causes the ion beam to retreat from the beamline A. For example, a negative voltage is applied to the second park electrode  25   b , based on a potential of the first park electrode  25   a . In this manner, the ion beam is deflected downward from the beamline A in the direction of gravity, and is incident into the beam dump  26 . In  FIG.  2   , a trajectory of the ion beam directed toward the beam dump  26  is indicated by a broken line. The beam park device  24  causes the ion beam to pass toward the downstream side along the beamline A by setting the pair of park electrodes  25   a  and  25   b  to have the same potential. The beam park device  24  is configured to be operable by switching between a first mode in which the ion beam passes to the downstream side and a second mode in which the ion beam is incident into the beam dump  26 . 
     An injector Faraday cup  28  is provided downstream of the mass resolving aperture  23 . The injector Faraday cup  28  is configured to be movable into and out of the beamline A by an operation of an injector drive unit  29 . The injector drive unit  29  moves the injector Faraday cup  28  in a direction (for example, the y-direction) perpendicular to an extending direction of the beamline A. When the injector Faraday cup  28  is disposed on the beamline A as illustrated by a broken line in  FIG.  2   , the injector Faraday cup  28  blocks the ion beam directed toward the downstream side. On the other hand, as illustrated by a solid line in  FIG.  2   , when the injector Faraday cup  28  retreats from the beamline A, the blocking of the ion beam directed toward the downstream side is released. 
     The injector Faraday cup  28  is configured to measure a beam current of the ion beam subjected to mass analyzing by the mass analyzing unit  20 . The injector Faraday cup  28  can measure a mass analyzing spectrum of the ion beam by measuring the beam current while changing the magnetic field intensity of the mass analyzing magnet  21 . The mass resolution of the mass analyzing unit  20  can be calculated using the measured mass analyzing spectrum. 
     The beam shaping unit  30  includes a focusing/defocusing device such as a focusing/defocusing quadrupole lens (Q-lens), and is configured to shape the ion beam having passed through the mass analyzing unit  20  to have a desired cross-sectional shape. For example, the beam shaping unit  30  is configured to include an electric field type three-stage quadrupole lens (also referred to as a triplet Q-lens), which has three quadrupole lenses  30   a ,  30   b , and  30   c . The beam shaping unit  30  adopts the three lens devices  30   a  to  30   c . Accordingly, the beam shaping unit  30  can adjust the ion beam to converge or diverge independently in the x-direction and the y-direction, respectively. The beam shaping unit  30  may include a magnetic field type lens device, or may include a lens device that shapes the beam by using both an electric field and a magnetic field. 
     The beam scan unit  32  is configured to provide reciprocating scan using the beam and is a beam deflection device to perform scanning using the shaped ion beam in the x-direction. The beam scan unit  32  has a scan electrode pair facing in a beam scan direction (x-direction). The scan electrode pair is connected to a variable voltage power supply (not illustrated), and a voltage applied between scanning electrode pair is periodically changed. In this manner, an electric field generated between the electrodes is changed so that the ion beam is deflected at various angles. As a result, a whole scan range is scanned with the ion beam in the x-direction. In  FIG.  1   , the scan direction and the scan range of the beam are indicated by an arrow X, and a plurality of trajectories of the ion beam in the scan range are indicated by a one dot chain line. The beam scan unit  32  may be replaced with another beam scan unit, and the beam scan unit may be configured to serve as a magnet device using a magnetic field. 
     The beam parallelizing unit  34  is configured so that the traveling direction of the ion beam used for the scanning becomes parallel to the trajectory of the designed beamline A. The beam parallelizing unit  34  has a plurality of arc-shaped parallelizing lens electrodes in which an ion beam passing slit is provided in a central portion in the y-direction. The parallelizing lens electrode is connected to a high-voltage power supply (not illustrated), and applies an electric field generated by voltage application to the ion beam so that the traveling directions of the ion beam are parallelized. The beam parallelizing unit  34  may be replaced with another beam parallelizing device, and the beam parallelizing device may be configured to serve as a magnet device using a magnetic field. 
     An acceleration/deceleration (AD) column (not illustrated) for accelerating or decelerating the ion beam may be provided downstream of the beam parallelizing unit  34 . 
     The angular energy filter (AEF)  36  is configured to analyze energy of the ion beam, to deflect ions having necessary energy downward, and to guide the ions to the implantation processing chamber  16 . The angular energy filter  36  has an AEF electrode pair for electric field deflection. The AEF electrode pair is connected to a high-voltage power supply (not illustrated). In  FIG.  2   , the ion beam is deflected downward by applying a positive voltage to the upper AEF electrode and applying a negative voltage to the lower AEF electrode. The angular energy filter  36  may be configured to include a magnet device for magnetic field deflection, or may be configured to include a combination between the AEF electrode pair for electric field deflection and the magnet device for magnetic field deflection. 
     In this way, the beamline unit  14  supplies the ion beam to be used for irradiating the wafer W to the implantation processing chamber  16 . In the present embodiment, the ion generation device  12  and the beamline unit  14  are also referred to as a beam generation device. The beam generation device is configured to generate the ion beam for realizing a desired implantation condition by adjusting operation parameters of various devices constituting the beam generation device. 
     The implantation processing chamber  16  includes an energy slit  38 , a plasma shower device  40 , side cups  42  ( 42 L and  42 R), a profiler cup  44 , and the beam stopper  46 , in order from the upstream side of the beamline A. As illustrated in  FIG.  2   , the implantation processing chamber  16  includes a platen driving device  50  that holds one or more wafers W. 
     The energy slit  38  is provided on the downstream side of the angular energy filter  36 , and analyzes the energy of the ion beam incident into the wafer W together with the angular energy filter  36 . The energy slit  38  is an energy defining slit (EDS) configured to include a slit that is horizontally long in the beam scan direction (x-direction). The energy slit  38  causes the ion beam having a desired energy value or a desired energy range to pass toward the wafer W, and blocks the other ion beams. 
     The plasma shower device  40  is located on the downstream side of the energy slit  38 . The plasma shower device  40  supplies low-energy electrons to the ion beam and a surface of the wafer W (wafer processing surface) in accordance with a beam current amount of the ion beam, and suppresses charge-up caused by accumulation of positive charges on the wafer processing surface which are induded by the ion implantation. For example, the plasma shower device  40  includes a shower tube through which the ion beam passes, and a plasma generating device that supplies electrons into the shower tube. 
     The side cups  42  ( 42 L and  42 R) are configured to measure the beam current of the ion beam during the ion implantation process into the wafer W. As illustrated in  FIG.  2   , the side cups  42 L and  42 R are disposed to be shifted to the left and right (x-direction) with respect to the wafer W disposed on the beamline A, and are disposed at positions where the side cups  42 L and  42 R do not block the ion beam directed toward the wafer W during the ion implantation. The ion beam is used for scanning in the x-direction beyond a range where the wafer W is located. Accordingly, a portion of the beam used for the scanning is incident into the side cups  42 L and  42 R even during the ion implantation. In this manner, the beam current amount during the ion implantation process is measured by the side cups  42 L and  42 R. 
     The profiler cup  44  is a Faraday cup configured to measure the beam current on the wafer processing surface. The profiler cup  44  is configured to be movable by an operation of a profiler driving device  45 , retreats from an implantation position where the wafer W is located during the ion implantation, and is inserted into the implantation position when the wafer W is not located at the implantation position. The profiler cup  44  measures the beam current while moving in the x-direction. In this manner, the profiler cup  44  can measure the beam current over the whole beam scan range in the x-direction. In the profiler cup  44 , a plurality of Faraday cups may be aligned in the x-direction to be formed in an array shape so that the beam currents can be simultaneously measured at a plurality of positions in the beam scan direction (x-direction). 
     At least one of the side cups  42  and the profiler cup  44  may include a single Faraday cup for measuring the beam current amount, or may include an angle measurement device for measuring angle information of the beam. For example, the angle measurement device includes a slit and a plurality of current detectors provided away from the slit in the beam traveling direction (z-direction). For example, the angle measurement device can measure an angle component of the beam in the slit width direction by causing the plurality of current detectors aligned in the slit width direction to measure the beam having passed through the slit. At least one of the side cups  42  and the profiler cup  44  may include a first angle measurement device capable of measuring angle information in the x-direction and a second angle measurement device capable of measuring angle information in the y-direction. 
     The platen driving device  50  includes a wafer holding device  52 , a reciprocating mechanism  54 , a twist angle adjusting mechanism  56 , and a tilt angle adjusting mechanism  58 . The wafer holding device  52  includes an electrostatic chuck for holding the wafer W. The reciprocating mechanism  54  causes the wafer holding device  52  to reciprocate in a reciprocating direction (y-direction) perpendicular to the beam scan direction (x-direction). In this manner, the wafer held by the wafer holding device  52  is caused to reciprocate in the y-direction. In  FIG.  2   , a reciprocating movement of the wafer W is indicated by an arrow Y as an example. 
     The twist angle adjusting mechanism  56  adjusts a rotation angle of the wafer W. The twist angle adjusting mechanism  56  rotates the wafer W around a normal line of the wafer processing surface as a rotation center axis. In this manner, the twist angle adjusting mechanism  56  adjusts a twist angle between an alignment mark provided on an outer peripheral portion of the wafer and a reference position. Here, the alignment mark of the wafer means a notch or an orientation flat provided on the outer peripheral portion of the wafer, and means a mark that serves as a reference for a crystal axis direction of the wafer or an angular position in a circumferential direction of the wafer. The twist angle adjusting mechanism  56  is provided between the wafer holding device  52  and the reciprocating mechanism  54 , and is caused to reciprocate together with the wafer holding device  52 . 
     The tilt angle adjusting mechanism  58  adjusts tilting of the wafer W, and adjusts a tilt angle between the traveling direction of the ion beam directed toward the wafer processing surface and the normal line of the wafer processing surface. In the present embodiment, out of tilt angles of the wafer W, an angle with respect to which the axis in the x-direction is a rotation center axis is adjusted as the tilt angle. The tilt angle adjusting mechanism  58  is provided between the reciprocating mechanism  54  and an inner wall of the implantation processing chamber  16 , and rotates the whole platen driving device  50  including the reciprocating mechanism  54  in an R-direction. In this manner, the tilt angle adjusting mechanism  58  is configured to adjust the tilt angle of the wafer W. 
     The platen driving device  50  holds the wafer W so that the wafer W is movable between an implantation position where the wafer W is irradiated with the ion beam and a transfer position where the wafer W is loaded or unloaded between the platen driving device  50  and the wafer transfer device  18 .  FIG.  2    illustrates a state where the wafer W is located at the implantation position, and the platen driving device  50  holds the wafer W so that the beamline A and the wafer W intersect with each other. The transfer position of the wafer W corresponds to a position of the wafer holding device  52  when the wafer W is loaded or unloaded through a transfer port  48  by a transfer mechanism or a transfer robot provided in the wafer transfer device  18 . 
     The beam stopper  46  is provided on the most downstream side of the beamline A, and is mounted on the inner wall of the implantation processing chamber  16 , for example. When the wafer W does not exist on the beamline A, the ion beam is incident into the beam stopper  46 . The beam stopper  46  is located close to the transfer port  48  that connects the implantation processing chamber  16  and the wafer transfer device  18  to each other, and is provided at a position vertically below the transfer port  48 . 
     The beam stopper  46  has a plurality of tuning cups  47  ( 47   a ,  47   b ,  47   c , and  47   d ). The plurality of tuning cups  47  are Faraday cups configured to measure the beam current of the ion beam incident into the beam stopper  46 . The plurality of tuning cups  47  are disposed with intervals in the x-direction. For example, the plurality of tuning cups  47  are used for easily measuring the beam currents at the implantation positions without using the profiler cup  44 . 
     The side cups  42  ( 42 L and  42 R), the profiler cup  44 , and the tuning cups  47  ( 47   a  to  47   d ) are beam measurement devices for measuring the beam current as a physical quantity of the ion beam, or beam detection units (beam detectors) for detecting the beam current. The side cups  42  ( 42 L and  42 R), the profiler cup  44 , and the tuning cups  47  ( 47   a  to  47   d ) may be beam measurement devices for measuring a beam angle as a physical quantity of the ion beam, or beam detection units for detecting the beam angle. 
     The ion implanter  10  further includes a control device  60 . The control device  60  controls an overall operation of the ion implanter  10 . The control device  60  is realized in hardware by elements such as a CPU and a memory of a computer or a mechanical device, and is realized in software by a computer program or the like. Various functions provided by the control device  60  can be realized by cooperation between the hardware and the software. 
       FIG.  3    is a diagram schematically illustrating an example of a configuration of the control device  60 . The control device  60  includes a processor  90  such as a central processing unit (CPU), a memory  91  such as a read only memory (ROM) and a random access memory (RAM), a storage device  92  such as a hard disk drive (HDD) and a solid state drive (SSD), and a system bus  93  for connecting these. For example, the control device  60  is connected via the system bus  93  to an input device  94  which is a user interface such as a keyboard and a mouse, a display device  95  such as a liquid crystal display, a reading device  96  for reading a program recorded on a storage medium such as a magnetic tape, a magnetic disk, and an optical disk, and a communication interface  97  for acquiring a program by communication via a network  98 . 
     For example, the control device  60  controls an overall operation of the ion implanter  10  in accordance with the program by causing the processor  90  to execute the program stored in the memory  91 . The processor  90  may execute the program stored in the storage device  92 , may execute the program acquired by the reading device  96  from the storage medium, or may execute the program acquired by the communication interface  97  via the network  98 . The memory  91  storing the program may be a volatile memory such as a dynamic random access memory (DRAM) or a non-volatile memory such as an electrically erasable programmable read-only memory (EEPROM), a flash memory, a magnetic resistance memory, a resistance change type memory, and a ferroelectric memory. The non-volatile memory, a magnetic storage medium such as the magnetic tape and the magnetic disk, and an optical storage medium such as the optical disk are examples of a non-transitory and tangible computer-readable storage medium). 
       FIG.  4    is a top view schematically illustrating a configuration inside the implantation processing chamber  16 , and illustrates a state where beam measurement devices disposed inside the implantation processing chamber  16  measure a scan beam SB. An ion beam B is used for a reciprocating scan in the x-direction as indicated by the arrow X, and is incident into the wafer W as the scan beam SB. 
     The ion beam B is used for the reciprocating scan over a scan range C 3  including an implantation range C 1  where the wafer W is located and monitor ranges C 2 L and C 2 R outside the implantation range C 1 . The left and right side cups  42 L and  42 R are respectively disposed in the left and right monitor ranges C 2 L and C 2 R. The left and right side cups  42 L and  42 R can measure the ion beam B used for over-scanning performed on the monitor ranges C 2 L and C 2 R during the implantation process. 
     The profiler cup  44  retreats to a non-scan range C 4 R outside the scan range C 3  during the implantation process. In the illustrated configuration, the profiler driving device  45  is disposed on the right side. During the implantation process, the profiler cup  44  retreats to the non-scan range C 4 R on the right side. In a configuration in which the profiler driving device  45  is disposed on the left side, during the implantation process, the profiler cup  44  may retreat to a non-scan range C 4 L on the left side. 
     The profiler cup  44  is disposed in the implantation range C 1  during a preparation process performed prior to the implantation process, and measures the beam current of the ion beam B in the implantation range C 1 . The profiler cup  44  measures the beam current while moving in the x-direction in the implantation range C 1 , and measures a beam current density distribution of the scan beam SB in the x-direction. The profiler cup  44  measures the beam current at a position of the wafer processing surface by moving in the x-direction along a plane (measurement surface MS) coincident with the wafer processing surface in the implantation process. The profiler cup  44  may measure the beam current density distribution of the scan beam SB in the x-direction in the monitor ranges C 2 L and C 2 R, in addition to the implantation range C 1 . 
     The plurality of tuning cups  47  are disposed in the implantation range C 1 , and measure the beam current of the ion beam B in the implantation range C 1 . The plurality of tuning cups  47  are disposed at positions separated to the downstream side away from the wafer W. The tuning cups  47  do not need to be moved between the implantation range C 1  and the non-scan range C 4 R unlike the profiler cup  44 . Therefore, compared to the profiler cup  44 , the beam current in the implantation range C 1  can be more easily measured by the tuning cups  47 . 
     In the preparation process, beam current measurement values are measured by various Faraday cups provided inside the implantation processing chamber  16 . Specifically, a plurality of the beam current measurement values are measured by using the side cups  42 L and  42 R, the profiler cup  44 , and the plurality of tuning cups  47 . The control device  60  stores a ratio between the acquired beam current measurement values so that the beam current value on the wafer processing surface can be calculated from the beam current measurement values measured by the side cups  42 L and  42 R during the implantation process. Normally, the ratio between the beam current measurement values measured by various Faraday cups depends on a setting of a beam optical system in the beamline unit  14 . Even when the beam current of the ion beam B extracted from the ion generation device  12  slightly fluctuates, the ratio between the beam current measurement values is substantially constant. That is, when the setting of the beam optical system is determined during the preparation process, the ratio between the beam current measurement values during the subsequent implantation process is not changed. Therefore, when the ratio between the beam current measurement values is stored during the preparation process, based on the ratio and the beam current measurement values measured by the side cups  42 L and  42 R, it is possible to calculate the beam current value at the implantation position (that is, the wafer processing surface) where the ions are implanted into the wafer W during the implantation process. 
     During the implantation process, the beam current can be measured at all times by using the side cups  42 L and  42 R. During the implantation process, the beam current cannot be measured at all times and can be only intermittently measured, by using the profiler cup  44  or the tuning cup  47 . Therefore, during the implantation process, a dose of the ions implanted into the wafer processing surface is controlled, based on the beam current measurement values measured by the side cups  42 L and  42 R. When the beam current measurement values measured by the side cups  42 L and  42 R are changed during the implantation process, a dose distribution on the wafer processing surface is adjusted by changing a wafer scan speed vw(y) of the wafer W in the y-direction. For example, when an in-plane uniform dose distribution needs to be realized on a plane of the wafer processing surface, the wafer W is caused to reciprocate at a speed proportional to the beam current measurement value monitored by the side cups  42 L and  42 R. Specifically, when the beam current measurement value to be monitored increases, the wafer scan speed vw(y) becomes faster, and when the beam current measurement value to be monitored decreases, the wafer scan speed vw(y) becomes slower. In this manner, it is possible to prevent the dose distribution on the wafer processing surface from varying due to fluctuations in the beam current of the scan beam SB. 
       FIG.  5    is a diagram schematically illustrating a data structure of an implantation recipe  70 . The control device  60  controls the ion implantation process in accordance with the implantation recipe. The implantation recipe  70  includes basic setting data  71  and detailed setting data  72 . The basic setting data  71  determines an implantation condition indicating essential settings. For example, the basic setting  71  includes setting data of 1) an ion species, 2) beam energy, 3) a beam current, 4) a beam size, 5) a wafer tilt angle, 6) a wafer twist angle, and 7) an average dose. The average dose indicates an in-plane average value of a dose distribution to be implanted into the wafer processing surface. 
     The detailed setting data  72  is set when “nonuniform implantation” is performed to intentionally provide nonuniformity for the dose distribution of the ions to be implanted into the wafer processing surface. The detailed setting data  72  may not be set when “uniform implantation” is performed to provide a uniform pattern for the two-dimensional dose distribution on the wafer processing surface. The detailed setting data  72  includes 8) the two-dimensional dose distribution and 9) a correction data set. For example, the two-dimensional dose distribution is an actual pattern of the two-dimensional nonuniform dose distribution realized on the wafer processing surface WS when the nonuniform implantation is performed. The correction data set is used for performing variable control on the beam scan speed in the x-direction by the beam scan unit  32  and the wafer scan speed in the y-direction by the platen driving device  50 . The correction data set includes a correction function file and a correlation information file for realizing the two-dimensional nonuniform dose distribution. 
       FIGS.  6 A and  6 B  are diagrams schematically illustrating a two-dimensional nonuniform dose distribution  73 .  FIG.  6 A  illustrates the two-dimensional dose distribution that is set to be nonuniform on the circular wafer processing surface WS, and illustrates a magnitude of the dose by using the contrasting density of regions  74   a ,  74   b ,  74   c , and  74   d  on the wafer processing surface WS. In the illustrated example, the dose in the first region  74   a  is the largest, and the dose in the fourth region  74   d  is the smallest. The two-dimensional nonuniform dose distribution  73  is determined, based on an orientation of the wafer W held by the platen driving device  50 . Specifically, the two-dimensional nonuniform dose distribution  73  is determined, based on the beam scan direction (x-direction) and a wafer scan direction (y-direction) when the wafer W is disposed on the platen driving device  50  so as to indicate the wafer twist angle determined in the basic setting data  71 . In the illustrated example, a direction from a center O of the wafer W toward an alignment mark WM is set as the +y-direction. However, a position of the alignment mark WM may vary depending on the wafer twist angle. 
       FIG.  6 B  schematically illustrates a plurality of lattice points  75  for defining the two-dimensional nonuniform dose distribution  73 . For example, the plurality of lattice points  75  are set with equal intervals on the wafer processing surface WS. For example, the two-dimensional nonuniform dose distribution  73  is defined by data that associates each position coordinate at the plurality of lattice points  75  with each of dose amounts at the plurality of lattice points  75 . For example, in a case of a wafer having a diameter of 300 mm, the lattice points  75  of  31   x 31 in which the center O of the wafer processing surface WS is an origin is set, and an interval d 1  between two of the lattice points  75  adjacent to each other is 10 mm. The interval d 1  between the two of the plurality of lattice points  75  adjacent to each other is set so as to be smaller than a beam size of the ion beam B. An example of the beam size of the ion beam B is approximately 20 mm to 30 mm. 
       FIG.  7    is a diagram illustrating an example of correction function files  77  and a correlation information file  78 . Each of the correction function files  77  defines a correction function h(x) determined based on the one-dimensional nonuniform dose distribution in the x-direction. A plurality of the correction function files  77  are defined for one of the two-dimensional nonuniform dose distributions  73 , and six correction function files  77 A,  77 B,  77 C,  77 D,  77 E, and  77 F are determined in the illustrated example. In the plurality of correction function files  77 A to  77 F, the correction function h(x) have mutually different shapes, respectively. For example, the number of the plurality of correction function files  77 A to  77 F which are defined for one of the two-dimensional nonuniform dose distributions  73  is as much as approximately 5 to 10. 
     The correlation information file  78  defines correlation information that associates the two-dimensional nonuniform dose distribution  73  with the plurality of correction function files  77 . The wafer processing surface WS is divided into a plurality of division regions  76 _ 1  to  76 _ 31  (generically referred to as division regions  76 ) in the y-direction. Any one of the plurality of correction function files  77 A to  77 F is associated with any one of the plurality of division regions  76 . Each division width d 2  of the plurality of division regions  76  in the y-direction is the same as the interval d 1  of the lattice points  75 , and is 10 mm, for example. Each center position of the plurality of division regions  76  in the y-direction can correspond to each position of the lattice points  75 . The division width d 2  of the plurality of division regions  76  in the y-direction is set to be smaller than the beam size (for example, 20 mm to 30 mm) of the ion beam. 
     The number of the plurality of correction function files  77  may be smaller than the number of the plurality of division regions  76 . Therefore, at least one of the correction function files  77  may be associated with two or more of the plurality of division regions  76 . In other words, the correction function h(x) determined in one of correction function files  77  may be commonly used for two or more of the plurality of division regions  76 . The correction function h(x) may be normalized so as to be usable in two or more of the plurality of division regions  76 . For example, the correction function h(x) may be normalized such that a maximum value, an average value, or an integral value in the x-direction of the correction function h(x) becomes a predetermined value. As correction coefficients k, the correlation information file  78  stores ratios between respective one-dimensional nonuniform dose distributions D(x) in the plurality of division regions  76  and the respective correction functions h(x) corresponding to D(x). Each of one-dimensional nonuniform dose distributions D(x) in the plurality of division regions  76  corresponds to k·h(x) obtained by multiplying the correction function h(x) by the correction coefficient k. A value of the correction coefficient k tends to become larger in one of the division regions  76  having a relatively high dose, and tends to become smaller in another of the division regions  76  having a relatively low dose. The correction coefficients k are used to control the wafer scan speed in the y-direction. 
       FIG.  8    is a table illustrating an example of the correlation information file  78 . The correlation information file  78  determines ranges in the x-direction and the y-direction where the wafer processing surface WS exists, symbols A to F that identify the correction function files  77 , and values of the correction coefficients k, for each of region numbers “1” to “31” that identifies each of the plurality of division regions  76 . The wafer processing surface WS has a disc shape. Accordingly, as the implant position becomes away from the center O of the wafer processing surface WS in the y-direction, the range in the x-direction where the wafer processing surface WS exists decreases. For example, in the region number “1”, the wafer processing surface WS exists only in a range of ±20 mm in the x-direction with respect to the center O of the wafer processing surface WS, and the wafer processing surface WS does not exist outside the range. On the other hand, in the region number “16” corresponding to the center O of the wafer processing surface WS in the y-direction, the wafer processing surface WS exists in an entire range of ±150 mm in the x-direction which corresponds to the diameter of the wafer processing surface WS. In the illustrated example, each width of the plurality of division regions  76  in the y-direction is a constant value (that is 10 mm). However, the respective widths of the plurality of division regions  76  in the y-direction may be different from each other. 
       FIG.  9    is a diagram schematically illustrating a multiple step implantation. When the nonuniform implantation is realized, the “multiple step implantation” may be performed to practice the ion implantations multiple times with changing the wafer twist angle while fixing the two-dimensional nonuniform dose distribution based on the alignment mark WM of the wafer W as a reference. When the wafer twist angle is changed, the two-dimensional nonuniform dose distribution based on a coordinate system of the ion implanter  10  is rotated together.  FIG.  9    illustrates a case where the ion implantations are performed four times with rotating the wafer twist angle as much as 90 degrees between the respective ion implantations. A first two-dimensional nonuniform dose distribution  73   a  is the same as the two-dimensional nonuniform dose distribution  73  illustrated in  FIG.  6 A  described above. A second two-dimensional nonuniform dose distribution  73   b  is obtained by rotating the first two-dimensional nonuniform dose distribution  73   a  clockwise as much as 90 degrees. Similarly, a third two-dimensional nonuniform dose distribution  73   c  is obtained by rotating the second two-dimensional nonuniform dose distribution  73   b  clockwise as much as 90 degrees, and a fourth two-dimensional nonuniform dose distribution  73   d  is obtained by rotating the third two-dimensional nonuniform dose distribution  73   c  clockwise as much as 90 degrees. Each of the plurality of two-dimensional nonuniform dose distributions  73   a  to  73   d  in the multiple step implantation has a different shape when viewed from the coordinate system of the ion implanter  10  in the x-direction and the y-direction. Therefore, in the multiple step implantation, an individual correction data set is determined for each of the plurality of two-dimensional nonuniform dose distributions  73   a  to  73   d . When the quadrupole step implantation is performed, the implantation recipe  70  includes four detailed setting data  72  corresponding to the four implantation processes. 
       FIG.  10    is a block diagram schematically illustrating a functional configuration of the control device  60 . The control device  60  includes an implantation control unit  61 , a measurement control unit  65 , a beam current matrix generation unit  66 , and a storage unit  67 . Each functional block illustrated in  FIG.  10    schematically illustrates various functions provided by the control device  60 , and illustrates functions realized by causing the processor  90  of the control device  60  to execute a program stored in the memory  91 . A boundary surrounding each functional block is determined in any desired way for convenience of description, and another boundary different from that of the above-described functional block may be determined as long as various functions are appropriately realized. Various functions provided by the control device  60  may be realized by a single device including the processor  90  and the memory  91 , or may be realized by the cooperation of a plurality of devices respectively including the processor  90  and the memory  91 . 
     The implantation control unit  61  controls an operation of the ion implanter  10 , based on an implantation recipe. The implantation control unit  61  includes a beam control unit  62 , a beam scan control unit  63 , and a platen control unit  64 . The measurement control unit  65  controls an operation of the beam measurement device for measuring the beam current, and acquires measurement values measured by the beam measurement device. The beam current matrix generation unit  66  generates a beam current matrix, based on a scan signal and the measurement values measured by the beam measurement device. The storage unit  67  stores the implantation recipe, operation parameters for realizing the implantation recipe. 
     The beam control unit  62  adjusts the operation parameters of various devices constituting the ion implanter  10  to realize implantation parameters determined in a desired implantation recipe. The beam control unit  62  controls the ion species of the ion beam by adjusting a gas type and an extraction voltage of the ion generation device  12 , the magnetic field intensity of the mass analyzing unit  20 , and the like. The beam control unit  62  controls the beam energy of the ion beam by adjusting the extraction voltage of the ion generation device  12 , an application voltage of the beam parallelizing unit  34 , an application voltage of an AD column, an application voltage of the angular energy filter  36 , and the like. The beam control unit  62  controls the beam current of the ion beam by adjusting various parameters such as a gas amount, an arc current, an arc voltage, and an ion source magnet current of the ion generation device  12 , an opening width of the mass resolving aperture  23 , and the like. The beam control unit  62  controls a beam size of the ion beam incident into the wafer processing surface WS by adjusting an operation parameter of a focusing/defocusing device included in the beam shaping unit  30  and the like. 
     The beam scan control unit  63  generates a scan signal that determines a time waveform of a scan command value of the beam scan unit  32 , and controls an operation of the beam scan unit  32 , based on the scan signal. When the beam scan unit  32  is an electric field type, the scan command value corresponds to a scan voltage V applied to the scan electrode pair of the beam scan unit  32 . When the beam scan unit  32  is a magnetic field type, the scan command value corresponds to a magnet current flowing through a magnet device of the beam scan unit  32 . In the present embodiment, a case where the beam scan unit  32  is the electric field type will be described. Regarding that the scan command value has the same meaning as the scan voltage V, the scan command value will be also referred to as the scan command value V. 
     The beam scan control unit  63  controls a beam current density distribution J(x) in the beam scan direction (x-direction) by variably controlling a beam scan speed vb(x) realized by the beam scan unit  32 . The beam scan speed vb(x) in the x-direction is substantially proportional to a change rate dV/dt of the scan command value V with respect to time t. For example, the beam scan control unit  63  decreases the time change rate dV/dt of the scan command value V so that the beam scan speed vb(x) becomes slower at a location where the dose is to be relatively high. For example, the beam scan control unit  63  increases the time change rate dV/dt of the scan command value V so that the beam scan speed vb(x) becomes faster at a location where the dose is to be relatively low. 
     The beam scan control unit  63  generates the scan signal for realizing a target beam current density distribution, based on the beam current matrix generated by the beam current matrix generation unit  66 . For example, the beam scan control unit  63  generates the scan signal for realizing a beam current density distribution proportional to the correction function h(x), based on the correction function h(x) determined in the correction function files  77  included in the implantation recipe and the beam current matrix. Details of the beam current matrix will be described later. 
     The platen control unit  64  generates a speed command value for designating a reciprocating speed of the reciprocating mechanism  54 , that is, a wafer scan speed vw(y) in the y-direction, based on the correlation information file  78 . The platen control unit  64  determines the speed command value so that the wafer scan speed vw(y) becomes slower at a location where the dose is to be relatively high, and the wafer scan speed vw(y) becomes faster at a location where the dose is to be relatively low. For example, the wafer scan speed vw(y) corresponding to a position in the y-direction is set to be proportional to a reciprocal 1/k of each correction coefficient k of the plurality of division regions  76  determined in the correlation information file  78 . 
     The platen control unit  64  may adjust the wafer scan speed, based on the measurement value of the beam current acquired in the implantation process. For example, the platen control unit  64  may adjust the wafer scan speed so as to reduce influence of fluctuations in the beam current in the implantation process, based on the beam current measurement value measured by the side cups  42 L and  42 R. 
       FIG.  11    is a diagram schematically illustrating a beam current matrix  80  according to the embodiment. Components  82  of the beam current matrix  80  are beam current values Iij with respect to an array Vi(i=1 . . . m) of the scan command values V of the beam scan unit  32  and an array Xj(j=1 . . . n) of positions X in the x-direction on the measurement surface MS corresponding to the wafer processing surface. In an example in  FIG.  11   , a row (horizontal direction) of the beam current matrix  80  correspond to the scan command values Vi, and a column (vertical direction) of the beam current matrix  80  correspond to the position Xjs in the x-direction. A row component  84  of the beam current matrix  80  is a beam current distribution I(Vi) with respect to the scan command value Vi when the position Xj in the x-direction is fixed at a specific position. A column component  86  of the beam current matrix  80  is a beam current distribution I(Xj) with respect to the position Xj in the x-direction when the scan command value Vi is fixed at a specific value. 
       FIG.  12    is a view schematically illustrating the measurement of the beam current distribution I(Vi) with respect to the scan command values Vi, and illustrates a method of measuring the row component  84  of the beam current matrix  80  in  FIG.  11   . The beam current distribution I(Vi) with respect to the scan command value Vi is measured in a state where the scan command value Vi of the beam scan unit  32  is changed so that the ion beam B is scanned as indicated by the arrow X, and is obtained by measuring the beam current of the scan beam SB with the profiler cup  44  fixed at a specific measurement position Xp. For example, the beam current distribution I(Vi) with respect to the scan command value Vi represents a distribution shape having a peak in a range  84   a  formed around a specific scan command value Vp and showing 0 in ranges  84   b  and  84   c  sufficiently separated away from the specific scan command value Vp. Here, the specific scan command value Vp corresponds to the scan command value Vi for deflecting the ion beam B to irradiate the specific measurement position Xp on the measurement surface MS coinciding with the wafer processing surface. 
       FIG.  13    is a view schematically illustrating the measurement of the beam current distribution I(Xj) with respect to the positions Xj, and illustrates a method of measuring the column component  86  of the beam current matrix  80  in  FIG.  11   . The beam current distribution I(Xj) with respect to the position Xj is measured in a state where the scan command value Vi of the beam scan unit  32  is fixed at a specific scan command value Vp, and is obtained by measuring the beam current of the ion beam B at a plurality of the measurement positions Xj while moving the profiler cup  44  as indicated by the arrow X. For example, the beam current distribution I(Xj) with respect to the position Xj represents a distribution shape having a peak in a range  86   a  formed around the specific measurement position Xp and showing 0 in ranges  86   b  and  86   c  sufficiently separated away from the specific measurement position Xp. Here, the specific measurement position Xp corresponds to a position Xj in the x-direction which is irradiated with the ion beam B on the measurement surface MS coinciding with the wafer processing surface when the scan command Vi is fixed at the specific scan command value Vp. 
       FIG.  14    is a graph schematically illustrating a relationship between the beam current matrix I(Vi, Xj) and a beam current density distribution J(Xj). In  FIG.  14   , a plurality of the beam current distributions Ii(Xj) obtained at different scan command values Vi are illustrated in an overlapping manner. The beam current density distribution J(Xj) corresponds to a sum of the plurality of beam current distributions Ii(Xj) constituting the beam current matrix I(Vi, Xj), and can be expressed by Equation (2) below. 
     
       
         
           
             
               
                 
                   
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     Here, Δti is a staying time of the scan beam at the scan command value Vi, and is proportional to a reciprocal 1/vi of a beam scan speed vi at the scan command value Vi. For example, beam current density J(Xq) at a specific position Xq in the x-direction is a total value of a plurality of beam current values Ii(Xq) at the position Xq of a plurality of beam current distributions Ii(Xj) obtained at different scan command values Vi in view of the staying time Δti, and can be expressed by Equation (3) below. 
     
       
         
           
             
               
                 
                   
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     Next, a method of generating the beam current matrix I(Vi, Xj) will be described. The beam current matrix  80  in  FIG.  11    is a set of a plurality of row components  84  or a set of a plurality of column components  86 . Accordingly, the beam current matrix  80  can be derived by measuring either the plurality of row components  84  or the plurality of column components  86 . In the present embodiment, the beam current matrix  80  is derived by measuring the plurality of row components  84  from a viewpoint of a measurement time. The shortest time needed to measure the row component  84  at the specific measurement position Xp illustrated in  FIG.  12    corresponds to half of a reciprocating scan time of the scan beam SB. When a scan period of the scan beam SB is 1 kHz, the shortest time needed to measure the row component  84  at one measurement position Xj is 0.5 ms. On the other hand, the shortest time needed to measure the column component  86  in the specific scan command value Vp illustrated in  FIG.  13    corresponds to a time needed to move the profiler cup  44  over the implantation range C 1  in  FIG.  4   , and is approximately 2 seconds, for example. 
     The measurement control unit  65  acquires the plurality of row components  84  by measuring the beam currents of the scan beam SB at the plurality of measurement positions while moving the profiler cup  44 . The scan beam SB serving as a measurement target is subject to reciprocating scan, based on a first scan signal. For example, the first scan signal is configured so that the beam scan speed vb(x) is constant and the time change rate dVi/dt of the scan command value Vi is constant. The first scan signal may be configured so that the beam scan speed vb(x) is not constant and is intentionally nonconstant. A scan beam used for reciprocating scan, based on the first scan signal, is also referred to as a “first scan beam”. 
     For example, the measurement control unit  65  causes the profiler cup  44  to measure the beam current of the scan beam SB at measurement positions which are set at 100 locations with 3 mm intervals in the implantation range C 1  of 300 mm. When measuring the beam current while moving the profiler cup  44 , for example, the measurement control unit  65  acquires the beam current measured in a range of 3 mm, which reaches to positions of ±1.5 mm from a certain measurement position, as a measurement value of the beam current at the certain measurement position. When a movement speed of the profiler cup  44  is 150 mm/s, a time required for passing through an interval of 3 mm between the measurement positions is 20 ms. When the scan period of the scan beam SB is 1 kHz, the ion beam B is used for the reciprocating scan 20 times in 20 ms. Accordingly, the scan beam SB can be measured 20 times each in forward and backward traveling of the scanning, and can be measured 40 times in total in forward and backward traveling. Therefore, in 2 seconds when the profiler cup  44  is moved over the implantation range C 1  of 300 mm, for example, a time waveform of the beam current of the scan beam SB can be measured 40 times at each of the  100  measurement locations, and an average value thereof can be calculated. 
       FIG.  15    is a graph illustrating an example of the time waveforms of the first scan signal and the beam current. An upper part in  FIG.  15    illustrates an example of the time waveform of the first scan signal. A lower part in  FIG.  15    illustrates the time waveform (also referred to as a scan beam shape) of the beam current measured by the profiler cup  44  when the profiler cup  44  is located at the specific measurement position Xp. The scan beam SB crosses the profiler cup  44  at a timing when the scan command value Vi reaches the specific scan command value Vp corresponding to the measurement position Xp of the profiler cup  44 . Accordingly, a scan beam shape I(t) is measured at a timing when the scan command value Vi reaches the specific scan command value Vp. A time width tw of one scan beam shape I(t) corresponds to a time required for the scanning spot-like ion beam B to cross a measurement width (slit width) of the profiler cup  44 . For example, when the measurement width of the profiler cup  44  in the x-direction is 5 mm, a spot size of the ion beam B in the x-direction is 15 mm, and a scan speed of the ion beam B in the x-direction is 800 m/s, the time width tw of one scan beam shape is 25 μs. A circle in the lower part in  FIG.  15    indicates a sampling timing for the beam current measurement. A sampling frequency of the beam current measurement is 0.5 MHz, for example. In the example in  FIG.  15   , the scan beam shape having the time width tw of 25 μs is sampled with a time interval of 2 μs. Accordingly, one scan beam shape I(t) is configured to have 13 sampling values. 
     Various parameters relating to the measurement of the scan beam SB are not limited to numerical values described above as examples, and any other desired numerical value may be adopted. A scan frequency of the scan beam SB is selected from a range of 0.1 Hz to 10 kHz, and is selected from a range of 1 Hz to 5 kHz, for example. The implantation range C 1  is selected from a range of 100 mm to 1,000 mm, and is selected from a range of 150 mm to 450 mm, for example. The implantation range C 1  is preferably selected from a range of 200 mm to 300 mm. For example, the implantation range C 1  may be selected, based on the diameter of the wafer W. The movement speed of the profiler cup  44  is selected from a range of 50 mm/s to 500 mm/s, and is selected from a range of 100 mm/s to 300 mm/s, for example. The interval between the measurement positions is selected from a range of 0.5 mm to 100 mm, and is selected from a range of 1 mm to 10 mm, for example. The spot size of the ion beam B is selected from a range of 5 mm to 500 mm, and is selected from a range of 10 mm to 300 mm, for example. The spot size of the ion beam B is preferably selected from a range of 20 mm to 200 mm. The measurement width of the profiler cup  44  is selected from a range of 1 mm to 30 mm, and is selected from a range of 5 mm to 10 mm, for example. The sampling frequency of the beam current is selected from a range of 10 kHz to 10 MHz, and is selected from a range of 0.1 MHz to 1 MHz, for example. 
     The measurement control unit  65  may cause the profiler cup  44  to reciprocate over the implantation range C 1  so that the measurement positions of the beam current are different between the forward traveling and the backward traveling of the reciprocating movement. For example, the profiler cup  44  may measure the time waveform of the beam current at a plurality of first measurement positions in the forward traveling of the reciprocating movement, and may measure the time waveform of the beam current at a plurality of second measurement positions in the backward traveling of the reciprocating movement. The plurality of first measurement positions and the plurality of second measurement positions may be set to be alternately located in the x-direction. 
     The measurement control unit  65  may correct the time waveform of the beam current, based on a parameter relating to the beam current measurement of the beam measurement device. The measurement control unit  65  may correct the scan beam shape, based on the measurement width in the x-direction of the beam measurement device. The beam measurement device such as the profiler cup  44  measures an average value of the beam current over the measurement width (slit width) in the x-direction. Therefore, the time width tw of one scan beam shape measured by the profiler cup  44  is larger than the time width of the true scan beam shape corresponding to the actual spot size of the ion beam B. In the example in  FIG.  15   , the time width tw of the scan beam shape measured by the profiler cup  44  is 25 μs for the ion beam B having the spot size of 15 mm. However, the time width of the true scan beam shape is 19 μs. 
     For example, the measurement control unit  65  may derive the true scan beam shape by correcting the time waveform of the beam current by using a relationship that the measurement value of the scan beam shape is configured with components each of which is obtained by smoothing a part of the true scan beam shape. The measurement control unit  65  may derive the true scan beam shape by correcting the time waveform of the beam current so as to narrow the time width tw of the measurement value of the scan beam shape. 
     The measurement control unit  65  may correct the time waveform of the beam current, based on a time constant τ=CR of a low-pass filter included in a beam current measurement circuit.  FIG.  16    is a circuit diagram illustrating an example of a configuration of a beam current measurement circuit  100 . The beam current measurement circuit  100  includes a shunt resistor  102 , a filter circuit  104 , an amplifier circuit  106 , and an analog to digital (AD) conversion circuit  108 . The shunt resistor  102  converts a beam current Iin of the ion beam incident into the profiler cup  44  into an input voltage Vin. When a resistance value of the shunt resistor  102  is Rs, Vin=Rs·Iin. The filter circuit  104  is a low-pass filter (LPF) using a resistor R and a capacitor C, and generates an output voltage Vout which is obtained by smoothing the input voltage Vin. A relationship between the input voltage Vin and the output voltage Vout of the filter circuit  104  is expressed as Vin=Vout+CR (dVout/dt). The amplifier circuit  106  is a voltage amplification circuit configured to include an operational amplifier and resistors R 1  to R 4 , and outputs a voltage ß·Vout which is obtained by amplifying the output voltage Vout of the filter circuit  104  with a predetermined amplification factor ß. The A/D conversion circuit  108  samples the output voltage ß·Vout of the amplifier circuit  106  to generate a digital value. The measurement control unit  65  acquires the digital value output from the A/D conversion circuit  108  as the measurement value of the beam current. 
     The measurement value of the beam current acquired by the measurement control unit  65  is proportional to the output voltage Vout of the filter circuit  104 . Accordingly, the measurement value is not proportional to the true scan beam shape Iin(t) measured by the profiler cup  44 . The measurement control unit  65  may derive the true scan beam shape Iin(t) by calculating Vin from Vout, based on a relational expression of Vin=Vout+CR (dVout/dt) by using the time constant τ=CR of the filter circuit  104 . 
     Furthermore, the measurement control unit  65  calculates the beam current density distribution in the x-direction of the first scan beam by performing time integration on the beam current measured by the beam measurement device at each of the measurement positions Xj. An actual measurement value of the beam current density distribution of the first scan beam is also referred to as a “first beam current density distribution J1(Xj)”. For example, the measurement control unit  65  performs time integration on the scan beam shape illustrated in the lower part in  FIG.  15   , and calculates an area occupied by the scan beam shape. In this manner, the measurement control unit  65  calculates an actual measurement value J1(Xp) of the first beam current density at the specific measurement position Xp. The measurement control unit  65  acquires the scan beam shape I(t) by sampling and measuring the time waveform of the beam current measured by the profiler cup  44 , and simultaneously calculates the actual measurement value of the first beam current density distribution J1(Xj) by performing time integration on the time waveform of the beam current. 
     The beam current matrix generation unit  66  calculates a beam current matrix, based on a time waveform I(t) of the beam current at a plurality of measurement positions and a time waveform V(t) of the scan command value determined in the first scan signal. The beam current matrix generation unit  66  converts the time waveform I(t) of the beam current into the beam current distribution I(Vi) with respect to the scan command value Vi, based on the time waveform V(t) of the scan command value, and calculates the beam current distributions I(Vi) at the plurality of measurement positions. In this manner, the beam current matrix generation unit  66  calculates the beam current matrix in which beam current values I(Xj, Vi)=Iij with respect to the plurality of positions Xj and the plurality of scan command values Vi are set as components. 
     The beam current matrix generation unit  66  may calculate components Iij=I(Xd, Vj) of the beam current values with respect to a plurality of supplementary positions Xd different from a plurality of measurement positions Xc, based on components Iij=I(Xc, Vj) of the beam current values with respect to the plurality of measurement positions Xc. For example, the beam current matrix generation unit  66  may perform interpolation to calculate the component Iij=I(Xd, Vj) of the beam current value with respect to a supplementary position Xd located between two adjacent measurement positions Xc1 and Xc2. The beam current matrix generation unit  66  may perform extrapolation to calculate the component Iij=I(Xd, Vj) of the beam current value with respect to the supplementary position Xd located outside the plurality of measurement positions Xc. The number of the plurality of supplementary positions Xd may be approximately the same as the number of the plurality of measurement positions Xc, or may be larger than the number of the plurality of measurement positions Xc. The number of the plurality of supplementary positions Xd may be approximately 2 to 5 times the number of the plurality of measurement positions Xc. 
     The beam current matrix generation unit  66  corrects a value of each component Iij of the calculated beam current matrix, based on the actual measurement value of the first beam current density distribution J1(Xj), and determines a value of each component I′ij of the corrected beam current matrix. The corrected beam current matrix I′ij is obtained by multiplying the beam current matrix Iij before correction by a correction matrix αij, and can be expressed as I′ij=αij·Iij. The correction matrix αij is determined so that a calculation value of the beam current density distribution J′(Xj) calculated based on the corrected beam current matrix I′ij and the actual measurement value of the first beam current density distribution J1(Xj) coincide with each other. The calculation value of the beam current density distribution J′(Xj) calculated based on the corrected beam current matrix I′ij is expressed by Equation (4) below by using the staying time Δt1i of the first scan beam at the scan command value Vi determined in the first scan signal. 
     
       
         
           
             
               
                 
                   
                     
                       J 
                       ′ 
                     
                     ( 
                     
                       X 
                       j 
                     
                     ) 
                   
                   = 
                   
                     
                       ∑ 
                       i 
                     
                       
                     
                       
                         
                           I 
                           ij 
                           ′ 
                         
                         · 
                         Δ 
                       
                       ⁢ 
                       t 
                       ⁢ 
                       
                         1 
                         i 
                       
                     
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
     Ideally, it is desirable that the calculation value of the beam current density distribution J(Xj) calculated by Equation (5) below, based on the beam current matrix Iij before correction and the scan command value Vi determined in the first scan signal coincides with the actual measurement value of the first beam current density distribution J1(Xj). However, according to findings of the inventors, there is a deviation therebetween. 
     
       
         
           
             
               
                 
                   
                     J 
                     ⁡ 
                     ( 
                     
                       X 
                       j 
                     
                     ) 
                   
                   = 
                   
                     
                       ∑ 
                       i 
                     
                       
                     
                       
                         
                           I 
                           
                             i 
                             ⁢ 
                             j 
                           
                         
                         · 
                         Δ 
                       
                       ⁢ 
                       t 
                       ⁢ 
                       
                         1 
                         i 
                       
                     
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
           
         
       
     
     The calculation value of the beam current density distribution J(Xj) expressed by Equation (5) above is calculated supposing that Δti=Δt1i in Equation (1). 
       FIG.  17    a graph illustrating an example of the calculation value of the beam current density distribution J(Xj) calculated based on the beam current matrix Iij before correction and the scan command value Vi determined in the first scan signal, and the actual measurement value of the first beam current density distribution J1(Xj). A vertical axis of the graph illustrates current intensity, and an average value of a whole beam current density distributions is normalized as 1. The calculation value J(Xj) and the actual measurement value J1(Xj) substantially coincide with each other. However, there exist deviations therebetween within a range of approximately ±2% depending on the positions Xj. In order that the calculation value J(Xj) and the actual measurement value J1(Xj) coincide with each other, a correction coefficient α(Xj) according to the position Xj can be defined, and can be expressed as α(Xj)=J1(Xj)/J(Xj). The correction matrix αij is a diagonal matrix having the correction coefficients α(Xj) as diagonal components. 
     Next, a method of generating a scan signal based on the beam current matrix will be described. The beam scan control unit  63  generates the scan signal, based on a target beam current density distribution Jt(Xj) and the corrected beam current matrix I′ij generated by the beam current matrix generation unit  66 . Here, the scan signal for realizing the target beam current density distribution Jt(Xj) is also referred to as a “second scan signal”. The beam scan control unit  63  calculates a beam staying time Δt2i so as to satisfy a relationship in Equation (6) below. 
     
       
         
           
             
               
                 
                   
                     
                       J 
                       t 
                     
                     ( 
                     
                       X 
                       j 
                     
                     ) 
                   
                   = 
                   
                     
                       ∑ 
                       i 
                     
                       
                     
                       
                         
                           I 
                           ij 
                           ′ 
                         
                         · 
                         Δ 
                       
                       ⁢ 
                       t 
                       ⁢ 
                       
                         2 
                         i 
                       
                     
                   
                 
               
               
                 
                   ( 
                   6 
                   ) 
                 
               
             
           
         
       
     
     The beam scan control unit  63  generates the second scan signal, based on the staying time Δt2i of the scan beam at the scan command value Vi. 
     The beam scan control unit  63  can calculate the staying time Δt2i of the scan beam at the scan command value Vi by using an optimization calculation method. For example, the beam staying time Δt2i can be calculated by optimization calculation based on a first evaluation value for evaluating a difference between the target beam current density distribution Jt(Xj) and the beam current density distribution J′(Xj) calculated based on the corrected beam current matrix I′ij and the beam staying time Δt2i. As a first evaluation value E1, for example, a sum of square errors between the target values Jt(Xj) and the calculation values J′(Xj), which is expressed by Equation (7) below, can be used. 
     
       
         
           
             
               
                 
                   
                     E 
                     ⁢ 
                     1 
                   
                   = 
                   
                     
                       ∑ 
                       j 
                     
                       
                     
                       
                         { 
                         
                           
                             
                               J 
                               t 
                             
                             ( 
                             
                               X 
                               j 
                             
                             ) 
                           
                           - 
                           
                             
                               J 
                               ′ 
                             
                             ( 
                             
                               X 
                               j 
                             
                             ) 
                           
                         
                         } 
                       
                       2 
                     
                   
                 
               
               
                 
                   ( 
                   7 
                   ) 
                 
               
             
           
         
       
     
     The beam scan control unit  63  can calculate the beam staying time Δt2i by the optimization calculation in which the first evaluation value E1 is minimized. 
     The beam scan control unit  63  may perform the optimization calculation by combining a plurality of evaluation values. The beam scan control unit  63  may further use a second evaluation value for evaluating a change amount of the time change rate dVi/dt of the scan command value Vi in the second scan signal. For example, the second evaluation value E2 can be defined as a sum of squares of change amounts in the beam staying time Δt2i, and can be expressed by Equation (8) below. 
     
       
         
           
             
               
                 
                   
                     E 
                     ⁢ 
                     2 
                   
                   = 
                   
                     
                       ∑ 
                       i 
                     
                       
                     
                       
                         { 
                         
                           
                             
                               Δ 
                               ⁢ 
                               t 
                               ⁢ 
                               
                                 2 
                                 i 
                               
                             
                             - 
                             
                               Δ 
                               ⁢ 
                               t 
                               ⁢ 
                               
                                 2 
                                 
                                   i 
                                   - 
                                   1 
                                 
                               
                             
                           
                           
                             Δ 
                             ⁢ 
                             t 
                             ⁢ 
                             
                               2 
                               i 
                             
                           
                         
                         } 
                       
                       2 
                     
                   
                 
               
               
                 
                   ( 
                   8 
                   ) 
                 
               
             
           
         
       
     
     The optimization calculation is performed so that the second evaluation value E2 becomes smaller. AS a result, the change rate of the beam staying time Δt2i can be reduced, and a change at the scan command value Vi in the second scan signal can be mitigated. The beam scan control unit  63  can use an evaluation function E in which the first evaluation value E1 and the second evaluation value E2 are combined with each other, and may perform the optimization calculation so that E=E1+w2·E2 is minimized. Here, w2 is a weighting coefficient of the second evaluation value E2 with respect to the first evaluation value E1. 
     The beam scan control unit  63  may further use a third evaluation value for evaluating the beam current amount used to irradiate a partial range of the scan range C 3  of the ion beam B. The third evaluation value E3 can be defined as the sum of squares of the calculation values of the beam current density distribution J′(Xj) for which the positions Xj are located in a specific range (for example, Xa≤Xj≤Xb), and can be expressed by Equation (9) below. 
     
       
         
           
             
               
                 
                   
                     E 
                     ⁢ 
                     3 
                   
                   = 
                   
                     
                       ∑ 
                       
                         a 
                         ≤ 
                         j 
                         ≤ 
                         b 
                       
                     
                       
                     
                       
                         { 
                         
                           
                             J 
                             ′ 
                           
                           ( 
                           
                             X 
                             j 
                           
                           ) 
                         
                         } 
                       
                       2 
                     
                   
                 
               
               
                 
                   ( 
                   9 
                   ) 
                 
               
             
           
         
       
     
     For example, when the implantation range C 1  in  FIG.  4    is set as the specific range, the optimization calculation may be performed so as to maximize the beam current amount in the implantation range C 1  and so as to minimize the beam current amount in the monitor ranges C 2 L and C 2 R. In addition, when an X-range in  FIG.  8    is set as the specific range, the optimization calculation may be performed so as to maximize the beam current amount in a range where the wafer W exists. When the correction function h(x) that reduces the implantation dose in a partial region of the wafer W is defined, the optimization calculation may be performed so as to minimize the beam current amount in the range where the wafer W exists. In addition, a range where the wafer W does not exist may be set as the specific range, and the beam current amount in the range where the wafer W does not exist may be maximized (or minimized). In this manner, the beam current amount in the range where the wafer W exists may be minimized (or maximized). The beam scan control unit  63  can use an evaluation function E in which the first evaluation value E1 and the third evaluation value E3 are combined with each other, and the optimization calculation may be performed so that E=E1+w3·E3 is minimized. Here, w3 is a weighting coefficient of the third evaluation value E3 with respect to the first evaluation value E1. The beam scan control unit  63  may use an evaluation function E in which three evaluation values E1 to E3 are combined with each other, or may perform the optimization calculation so that E=E1+w2·E2+w3·E3 is minimized. 
     The beam scan control unit  63  may generate a plurality of second scan signals, based on the corrected beam current matrix I′ij. The plurality of second scan signals are generated to realize the plurality of target beam current density distributions. For example, the plurality of target beam current density distributions correspond to a plurality of correction functions h(x) for performing two-dimensional nonuniform implantation. 
     The beam scan control unit  63  controls an operation of the beam scan unit  32 , based on the generated second scan signal, and generates the second scan beam. The measurement control unit  65  measures the beam current of the second scan beam by using the beam measurement device. For example, the measurement control unit  65  calculates the actual measurement value of the second current density distribution J2(Xj) of the second scan beam by performing time integration on the measurement value of the beam current measured by the profiler cup  44 . The beam scan control unit  63  compares the actual measurement value of the second current density distribution J2(Xj) with the target beam current density distribution Jt(Xj), and evaluates validity of the second scan signal. For example, when a difference between the actual measurement value J2(Xj) and the target value Jt(Xj) falls within a predetermined range, it is determined that the second scan signal is valid, and when the difference deviates from the predetermined range, it is determined that the second scan signal is not valid. When it is determined that the second scan signal is valid, the implantation control unit  61  irradiates the wafer W with the second scan beam based on the second scan signal to perform the ion implantation process. 
     The measurement control unit  65  may measure the beam current of the second scan beam by using another beam measurement device different from the profiler cup  44 , instead of measuring the beam current of the second scan beam by the profiler cup  44 . The measurement control unit  65  may measure the second current density distribution J2(Xj) of the second scan beam by using another beam measurement device different from the beam measurement device used for measuring the first scan beam. The measurement control unit  65  may measure the second current density distribution J2(Xj) of the second scan beam by using the plurality of tuning cups  47   a  to  47   d . When the plurality of tuning cups  47   a  to  47   d  are used, it is not necessary to move the profiler cup  44  in the x-direction. Therefore, the second current density distribution J2(Xj) can be more quickly and easily measured compared to a case of using the profiler cup  44 . The beam scan control unit  63  may evaluate the validity of the second scan signal, based on the actual measurement value of the second current density distribution J2(Xj) measured by the plurality of tuning cups  47   a  to  47   d.    
     When it is determined that the second scan signal is not valid, the beam scan control unit  63  may regenerate the second scan signal for realizing the target beam current density distribution Jt(Xj). The beam scan control unit  63  may regenerate the second scan signal by changing the target value, instead of using the target beam current density distribution Jt(Xj) as the target value without any change. For example, a value Ju(Xj)=Jt(Xj)+m·ΔJ(Xj) obtained by adding a difference value ΔJ(Xj)=Jt(Xj)−J2(Xj) between the target value Jt(Xj) and the actual measurement value of the second current density distribution J2(Xj) of the second scan beam to the initial target value may be set as a new target value. The second scan signal may be regenerated by using the new target value Ju(Xj). Here, m has a positive value, and is a coefficient for adjusting weighting of the difference value ΔJ(Xj). 
     When it is determined that the second scan signal generated or regenerated by the beam scan control unit  63  is not valid, the beam current matrix generation unit  66  may regenerate the corrected beam current matrix I′ij. For example, when beam conditions such as the beam current and the beam size of the ion beam B are changed, the measurement control unit  65  may remeasure the first scan beam based on the first scan signal, and the beam current matrix generation unit  66  may regenerate the corrected beam current matrix I′ij, based on a re-measurement result. 
     The beam control unit  62  may estimate a beam shape of the spot-like ion beam B in the x-direction, based on the corrected beam current matrix I′ij, and may adjust the beam shape of the ion beam B, based on the estimated beam shape. The column component  86  of the beam current matrix  80  illustrated in  FIG.  11    is the beam current distribution I(Xj) with respect to the position Xj in the x-direction when the scan command value Vi is fixed at the specific value. Therefore, the column component  86  represents the beam shape of the ion beam B in the x-direction without scanning. For example, the column component I′0(Xj) of the corrected beam current matrix at the scan command value Vi=0 may be regarded as the beam shape of the ion beam B. The beam control unit  62  may adjust the beam size, based on the column component I′0(Xj). 
     According to the present embodiment, the second scan signal for realizing the target beam current density distribution Jt(Xj) is generated, based on the corrected beam current matrix I′ij. In this manner, compared to the related art, the valid second scan signal can be highly accurately generated in a short time. According to the method in the related art, in some cases, the valid second scan signal cannot be generated with a single calculation process. In some cases, the valid second scan signal cannot be generated unless the calculation process is performed multiple times while repeating the actual measurement of the beam current density distribution. In addition, when the shape of the target beam current density distribution Jt(Xj) is complicated, according to the method in the related art, the valid second scan signal cannot be generated even if the calculation process is performed multiple times while repeating the actual measurement of the beam current density distribution. Consequently, the generation may fail. On the other hand, according to the present embodiment, even in a case where the method in the related art fails, the valid second scan signal can be generated by performing less number of the calculation processes such as once or twice. As a result, a working time required for generating the second scan signal in a preparation process before the implantation process can be shortened, and productivity in the ion implantation process can be improved. In particular, when the plurality of second scan signals need to be generated for the nonuniform implantation, the working time required in the preparation process can be significantly shortened. 
     Subsequently, an ion implantation method of using the above-described ion implanter  10  will be described. Here, an ion implantation process included in a method for manufacturing a semiconductor device in which an irradiation target of the ion beam B is a semiconductor wafer W will be described. 
       FIG.  18    is a flowchart illustrating an example of an ion implantation method according to an embodiment. The beam scan control unit  63  operates the beam scan unit  32 , based on the first scan signal, and causes the beam scan unit  32  to perform reciprocating scan using the spot-like ion beam B in the predetermined direction (x-direction). In this manner, the beam scan control unit  63  generates the first scan beam (S 10 ). The measurement control unit  65  measures the beam current of the first scan beam at the plurality of measurement positions different in the predetermined direction (x-direction) by using a beam measurement device (S 12 ). The beam current matrix generation unit  66  calculates the beam current matrix in which the beam current values with respect to the plurality of positions different in the predetermined direction (x-direction) and the plurality of scan command values are set as components, based on the time waveform of the beam current measured by the beam measurement device and the time waveform of the scan command values determined in the first scan signal (S 14 ). The measurement control unit  65  calculates the first beam current density distribution in the predetermined direction (x-direction) of the first scan beam by performing time integration on the measured beam current (S 16 ). The beam current matrix generation unit  66  corrects a value of each component of the beam current matrix, based on the first beam current density distribution, and generates the corrected beam current matrix (S 18 ). The beam scan control unit  63  generates the second scan signal for realizing the target beam current density distribution, based on the corrected beam current matrix (S 20 ). The beam scan control unit  63  operates the beam scan unit  32 , based on the second scan signal, and causes the beam scan unit  32  to perform reciprocating scan using the spot-like ion beam B in the predetermined direction (x-direction). In this manner, the beam scan control unit  63  generates the second scan beam (S 22 ). The implantation control unit  61  performs the ion implantation process by irradiating the semiconductor wafer with the second scan beam (S 24 ). 
     Hitherto, the present disclosure has been described with reference to each of the above-described embodiments. However, the present disclosure is not limited to each of the above-described embodiments. The configuration of each of the embodiments may appropriately be combined or replaced with each other. In addition, the combination or the process order in each embodiment can be appropriately rearranged, based on the knowledge of those skilled in the art, and modifications such as various design changes can be added to the embodiments. The embodiments to which the rearrangement or the modifications are added in this way may also be included in the scope of the ion implanter, the ion implantation method, and a method for manufacturing a semiconductor device according to the present disclosure. 
     The embodiments according to the present disclosure may adopt a form of a computer program including one or more computer-readable sequences for describing the methods according to the present disclosure, or may adopt a form of a non-temporary and tangible storage medium (for example, a non-volatile memory, a magnetic tape, a magnetic disk, or an optical disk) in which the computer program is stored. The processor may realize the method according to the present disclosure by executing the computer program. 
     It should be understood that the disclosure is not limited to the above-described embodiment, but may be modified into various forms on the basis of the spirit of the disclosure. Additionally, the modifications are included in the scope of the disclosure.