Abstract:
The present invention provides a charged particle beam apparatus which is provided with a tilting deflector which is disposed between a charged particle source and an objective lens and tilts a charged particle beam, wherein a first optical element includes an electromagnetic quadrupole which generates dispersion to suppress the dispersion which is generated by deflection by the tilting deflector, and a second optical element is composed of a deflector for deflecting the charged particle beam which enters the first optical element or an electromagnetic quadrupole which causes the charged particle beam to generate a dispersion different from the dispersion generated by the first optical element.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates to a charged particle beam apparatus such as a scanning electron microscope, and more particularly to a charged particle beam apparatus which tilts and emits a beam with respect to a specimen or to an ideal optical axis of the beam. 
         [0002]    An inspection device and a measuring device using an electron beam are demanded to have high precision three-dimensional observation function in order to improve a yield of a three-dimensional device. A method of obtaining an image with a stage inclined has been used for three-dimensional observation using an electron microscope. However, it had problems that the reproducibility of throughput and tilt angle was degraded because mechanical movement was needed to tilt a stage or a column. A method is considered to use a deflector to tilt the beam in order to perform a three-dimensional measurement of the device without inclining the stage. 
         [0003]    However, when beam tilt (deflection) is performed by the deflector, a deflection coma aberration and an off-axis chromatic aberration are generated, and beam diameter is increased. To eliminate the deflection coma aberration and the off-axis chromatic aberration which are generated at the time of beam deflection, it is necessary to produce an aberration of the same amount but of opposite sign with respect to the respective aberration generated by the objective lens by another optical element, and the following methods are known. 
         [0004]    JP-A-2000-348658 (U.S. Pat. No. 6,452,175) discloses a method that mutually cancels off-axis chromatic aberrations which are generated before and after the lens by a two-stage deflector which is positioned in the objective lens field and deflects the beam in opposite directions. 
         [0005]    WO01/33603 (U.S. Pat. No. 6,864,482) discloses a technology in which, when shifting a field of view (image shift) is performed by deflecting the beam, the off-axis chromatic aberration of an arbitrary direction generated at the objective lens when an image shift is performed, is corrected by causing chromatic dispersion by a Wien filter which is arranged on the optical axis on the electron source side away from the objective lens. 
         [0006]    JP-A-2001-15055 (U.S. Pat. No. 6,614,026) discloses a method of compensating an off-axis chromatic aberration which is generated when a beam is tilted by causing a chromatic dispersion by an electromagnetic multipole. In addition, it also discloses a method of simultaneously cancelling out the off-axis chromatic aberration and the deflection coma aberration with a hexapole field added to the compensation unit. 
         [0007]    JP-A-2006-54074 (U.S. Pat. No. 7,223,983) discloses a method of compensating chromatic aberration and spherical aberration, which are generated when the beam is tilted, by an aberration corrector by tilting the beam without moving an object point of the aberration corrector which is arranged on the electron source side away from the objective lens. 
         [0008]    JP-A-2006-12664 (U.S. Pat. No. 7,355,174) discloses a method of adjusting to optical conditions under which off-axis chromatic aberration is made equal to 0 by setting the optical conditions so as to make a positional deviation equal to 0 generated when an excitation current is changed. 
       SUMMARY OF THE INVENTION 
       [0009]    According to the compensation method of JP-A-2000-348658 (U.S. Pat. No. 6,452,175), in order to compensate when the beam is tilted, three sizes of tilt angle, off-axis chromatic aberration amount and deflection coma aberration amount are controlled by (the shape of the objective lens and) the strength of two deflectors arranged in the objective lens. Therefore, if there is another factor, such as the axis deviation or the assembling error of the objective lens, which causes a deviation from the ideal compensation state, it is difficult to comply with varying aberration compensation conditions. Further, it is difficult to separate the tilt angle and the generated aberration from each other. 
         [0010]    The technique disclosed in WO01/33603 (U.S. Pat. No. 6,864,482) is a method of correcting an off-axis chromatic aberration which is produced by the objective lens when the beam is tilted by causing a dispersion of the primary electron beam by using an EXB (Wien filter) where an electric field and a magnetic field intersect each other at right angle on a plane perpendicular to the optical axis. Therefore, when a large angle tilting (tilt angle of 10 degrees or more) is performed according to this technique, the deflection coma aberration becomes apparent and the resolution might be degraded. 
         [0011]    In addition, according to the technique disclosed in JP-A-2001-15055 (U.S. Pat. No. 6,614,026), it is possible to correct both of the off-axis chromatic aberration and the deflection coma aberration which are generated at the time of beam tilting. To compensate the above two aberrations by this technology, a hexapole field is newly superimposed on the electromagnetic multipole which is disposed to generate a dispersion. Therefore, the compensation unit shape and the number of power supplies for the electrode control increase, and control becomes complex. (For example, the number of poles required for the compensation unit is estimated according to the structure in an embodiment of JP-A-2001-15055 (U.S. Pat. No. 6,614,026). Here, an electromagnetic dipole, an electromagnetic quadrupole and a hexapole field are superimposed, so that the compensation unit needs 12 poles equal to a least common multiple of 2, 4 and 6.) 
         [0012]    Further, the technique disclosed in JP-A-2006-54074 (U.S. Pat. No. 7,223,983) shows a method for compensating the aberration generated when the beam is tilted by an on-axis aberration corrector. This system considers beam tilting as an extension of the on-axis aberration and can make the aberration correction in all directions but can not avoid from having a correction unit formed of multiple stages and a multipole. 
         [0013]    According to the descriptions of JP-A-2001-15055 (U.S. Pat. No. 6,614,026) and JP-A-2006-54074 (U.S. Pat. No. 7,223,983), the off-axis chromatic aberration and the deflection coma aberration can be compensated but the compensation unit needs hexapole or higher-order pole fields, and complication of control cannot be avoided. Therefore, the optical system structure and its control become complex, and the number of power supplies increase. 
         [0014]    The technique disclosed in JP-A-2006-12664 (U.S. Pat. No. 7,355,174) can correct both of off-axis chromatic aberration and deflection, coma aberration, but it cannot occasionally respond to a beam tilting with a relatively large tilt angle. 
         [0015]    A charged particle beam apparatus aiming to correct both of the off-axis chromatic aberration and the deflection coma aberration by a relatively simple structure is proposed below. 
         [0016]    To achieve the object described above, an embodiment of the invention provides a charged particle beam apparatus comprising an objective lens which focuses a charged particle beam emitted from a charged particle source to impinge a specimen, and a tilting deflector which is disposed between the charged particle source and the objective lens and tilts the charged particle beam, wherein a first optical element includes an electromagnetic quadrupole which causes a dispersion to suppress the dispersion which is generated by the deflection by the tilting deflector, and a second optical element is comprised of a deflector for deflecting the charged particle beam which enters the first optical element or an electromagnetic quadrupole which causes the charged particle beam to generate a dispersion different from the dispersion generated by the first optical element. 
         [0017]    According to the above structure, it becomes possible to provide a charged particle beam apparatus having a simple structure which can suppress the off-axis chromatic aberration and the deflection coma aberration. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]      FIG. 1  is a diagram showing a first embodiment of a scanning electron microscope on which an aberration correction unit including a Wien filter is mounted. 
           [0019]      FIG. 2  is a sectional view of the Wien filter. 
           [0020]      FIG. 3  is an explanatory view of dispersion generated at the time of passage through the Wien filter. 
           [0021]      FIG. 4  is a flowchart showing an optical condition setting process when a beam is tilted. 
           [0022]      FIG. 5  is an explanatory view of a control unit of a beam tilt optical system. 
           [0023]      FIG. 6  is a flowchart showing a distortion correcting process. 
           [0024]      FIG. 7  is a diagram showing a third embodiment of the scanning electron microscope on which the aberration correction unit including the Wien filter is mounted. 
           [0025]      FIG. 8  is a diagram showing a fourth embodiment of the scanning electron microscope on which the aberration correction unit including the Wien filter is mounted. 
           [0026]      FIG. 9  is a diagram showing a fifth embodiment of the scanning electron microscope on which the aberration correction unit including the Wien filter is mounted. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0027]    In this embodiment, an aberration correction unit is configured by combining a pole field of a quadrupole field or less. Specifically, an off-axis chromatic aberration and a deflection coma aberration generated when the beam is tilted are corrected by a deflector (dipole field) and a Wien filter (or a combination of two-stage Wien filters) which has an electromagnetic dipole field and an electromagnetic quadrupole field superimposed. This corrector varies the electromagnetic-field strength of the Wien filter and the excitation amount of the deflector (or strength of the two-stage When filter), so that the off-axis chromatic aberration and the deflection coma aberration generated by the corrector can be adjusted. As a result, it becomes easy to adjust at the time of correction. 
         [0028]    A schematic diagram of a beam tilt optical system of a scanning electron microscope is shown in  FIG. 1 . Primary electrons  12  (electron beam) emitted from an electron source  01  (charged particle source) are converged to a spot P 1  by a condenser lens  02 . The primary electrons  12  the emission angle of which is limited by an objective aperture  03  are further converged by a condenser lens  04 , and enter an aberration correction unit  11 . The optical system exemplified in  FIG. 1  has a deflector  08  for tilting the beam, which is disposed above an objective lens  09 . The electron beam is deflected by the deflector  08  so as to separate from an ideal optical axis (a passing track of the electron beam when the electron beam is not deflected) of the electron beam of the objective lens  09 , so that the beam can be emitted at a prescribed relative angle with respect to the ideal optical axis. 
         [0029]    The aberration correction unit  11  which comprises a two-stage deflector  06  and an electromagnetic quadrupole (Wien filter  07 ) is disposed above the deflector  08 . The principle is described below along a specific operation method of the aberration correction unit. 
         [0030]    The electron beam emitted from the electron source is converged to the spot P 1  by the condenser lens  02  and passes through the objective aperture  03 . Then it is converged to a spot P 2  by the condenser lens  04 . The primary electrons converged to the spot P 2  enter the aberration correction unit  11  to generate a dispersion and a coma aberration according to the energy width of the electrons Subsequently, the electron beam is corrected by the deflector  08  to a track to impinge the specimen  10  at a desired beam tilt angle, and the beam impinges the specimen  10  at the desired tilt angle. Here, the dispersion and the coma aberration generated by the aberration correction unit  11  are used to correct the off-axis chromatic aberration and the deflection coma aberration generated by the objective lens  09  because the beam is tilted. Operations of the deflector and the Wien filter in the aberration correction unit  11  and the generated aberration are described below. 
         [0031]    A shape of the Wien filter  07  is shown in  FIG. 2 .  FIG. 2  is a schematic diagram showing that the Wien filter is projected to a cross section perpendicular to an optical axis  13 . As shown in  FIG. 2 , the Wien filter  07  comprises four electromagnetic poles  51 ,  52 ,  53  and  54 . A coil is wound on each electromagnetic pole, and electrostatic fields E 1  and E 2  and static magnetic fields B 1  and B 2  can be generated by applying voltage and current to the electrode and the coil of each electromagnetic pole. Here, E 1  (B 1 ) is an electric (magnetic) dipole field, and E 2  (B 2 ) is an electric (magnetic) quadrupole field. When the Wien filter operates, a field is generated so that E 1  and B 1  intersect each other at right angle in a plane perpendicular to the optical axis. In this embodiment, the electric field E 1  is taken in the X-axis direction, and the magnetic field B 1  in the Y-axis direction. Considered below is a case where the primary electrons  12  pass through the generated electromagnetic fields (E 1 , B 1 ). The primary electrons  12  are assumed to have a velocity v. The condition under which the primary electrons  12  go straight ahead in the Wien filter is expressed by the following equation. 
         [0000]      [MATH. 1] 
         [0000]        v=E   1 ( z )/ B   1 ( z )  (1)
 
         [0032]    Here, when the primary electrons  12  have low energy (v&lt;E 1 /B 1 ), the primary electrons  12  are bent by receiving a deflection action in the direction of the electric field E 1 , and when the primary electrons  12  have high energy (v&gt;E 1 /B 1 ), they are bent toward the opposite direction. As a result, dispersion is generated in the Wien filter. And, a lens action is simultaneously caused in the electric field (X axis) direction. Of the primary electron beam  12  converged in P 2  by the lens action caused in the X-axis direction, the component which spreads in the electric field (X axis) direction is subjected to a convergence action, and the component which spreads in a magnetic field (Y axis) direction goes straight ahead. As a result, the convergence point of the X-axis direction and the convergence point of the Y-axis direction are formed at different positions. The difference between the convergence point of the X direction and the convergence point of the Y direction caused as described above can be corrected by superimposing a quadrupole field. The quadrupole field strength which is given to correct the deviation between the X direction focused spot and the Y direction focused spot caused by the lens action of the Wien filter is expressed by the following Equation. 
         [0000]    
       
         
           
             
               
                 
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         [0033]    Here, e is the elementary charge and m is the electron mass. Conditions under which the chromatic aberration generated by the addition of the quadrupole field and given by Equation (2) is made equal to 0 are considered below. 
         [0034]    The beam tilt optical system according to this embodiment corrects the off-axis chromatic aberration generated by the objective lens when the beam is tilted by the dispersion generated by the aberration correction unit  11 . To generate only dispersion by the aberration correction unit  11 , it is necessary to make the chromatic aberration generated by the electromagnetic quadrupole field expressed by Equation (2) equal to 0. Conditions under which the chromatic aberration generated by the electromagnetic quadrupole field is made equal to 0 are expressed by Equation (3). 
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         [0035]    Equation (1) represents the conditions under which the primary electrons  12  of a particular velocity go straight ahead in the Wien filter, Equation (2) represents the conditions of the quadrupole field strengths (E 2  and B 2 ) applied in order to correct the deviation between the X direction focused spot and the Y direction focused spot caused when dipole fields E 1 , B 1  are added to the Wien filter, and Equation (3) represents a relational expression of the electric quadrupole field E 2  and the magnetic quadrupole field B 2  to make the chromatic aberration generated when the quadrupole field is applied equal to 0. The electromagnetic field which is superimposed on the Wien filter  07  in the aberration correction unit  11  according to the present invention is determined so as to satisfy Equation (1), Equation (2), and Equation (3). 
         [0036]    A method of generating the deflection coma aberration is described below. The aberration correction unit  11  of the beam tilt optical system according to the present invention is provided with the two-stage deflector  06  to correct the deflection coma aberration generated at the time of beam tilt. The primary electrons  12  are subjected to a positional deviation Δr p2  (an off-axis amount from the ideal optical axis) by the two-stage deflector  06 , and enters the Wien filter  07 . At this time, a dispersion, a secondary aberration and a third aberration are generated by the Wien filter. The generated aberration is classified into five of dispersion, coma aberration, field curvature aberration, astigmatism, and distortion. 
         [0037]    The dispersion can be adjusted by the length in the Z direction and the electromagnetic-field strength of the Wien filter  07 , and the coma aberration is adjusted by the off-axis amount given by the two-stage deflector  06 . The field curvature aberration is adjusted by an excitation amount of the condenser lens  04 , and the astigmatism is adjusted by the quadrupole field strength of the Wien filter  07  or a stigmator  05 . Distortion aberration is given with a positional deviation and an angle deviation of the primary electrons  12  which are emitted from the Wien filter, and the positional deviation and the angle deviation of the electrons emitted from the Wien filter  07  are adjusted by the deflector  08 . By performing the above operation, the aberrations generated by the aberration correction unit  11  become dispersion and deflection coma aberration only, and the above aberrations can be used to correct the off-axis chromatic aberration and the deflection coma aberration which are generated by the objective lens at the time of beam tilt. 
         [0038]    In a case where a dispersion and a coma aberration are generated in the Y-axis direction by this system, the directions of E 1 , E 2 , B 1  and B 2  may be rotated by 90 degrees, and the use of the aberration correction unit according to this system can generate the aberration independently when the beam is tilted in the X-axis direction and the Y-axis direction. 
         [0039]    Embodiments are described in detail below with reference to the drawings. The structure of the first embodiment is shown in  FIG. 1 , a flowchart is shown in  FIG. 4 , and details of a tilt optical system control unit  110  are shown in  FIG. 5 . In Step  001 , excitation conditions for each lens and a specimen potential are given to a condenser lens control unit  101 , an objective lens control unit  105  and a specimen potential setting unit  106  and set by the tilt optical system control unit  110 . (In this embodiment, as an excitation amount of the condenser lens  04 , P 2  is explained referring to the conditions determined at the Wien filter inlet, but an aberration for correcting can be generated by the aberration correction unit  11  on the same principle even when P 2  is set at another place in order to adjust the aperture angle and the current amount.) The tilt angle of the beam is set in Step  002 . At this time, current for operating a deflection coil  8  according to the tilt angle is applied by a deflector control unit  104 . 
         [0040]    In Step  003 , conditions to generate a dispersion for correcting the off-axis chromatic aberration which is generated by the objective lens are calculated by a Wien filter condition setting unit  203  based on the set tilt angle and Equation (1) to Equation (3) to determine the values of E 1 , E 2 , B 1  and B 2  to be applied to the Wien filter. 
         [0041]    In Step  004 , the defocus caused when the electromagnetic field is applied to the Wien filter is adjusted by varying the strength of the condenser lens  04  or the objective lens  09 , and the astigmatism is adjusted by varying the quadrupole field strengths of E 2  and B 2  applied to a stigma coil  05  or the Wien filter  07 . 
         [0042]    In Step  005 , the electromagnetic fields (E 1 , E 2 , B 1  and B 2 ) applied to the Wien filter are adjusted to minimize the positional deviation of the obtained image when the method disclosed in JP-A-2006-12664 (U.S. Pat. No. 7,355,174) or the voltage of the electron source  01  was varied. Thus, the conditions under which the sum of the off-axis chromatic aberration generated by the objective lens and the dispersion generated by the Wien filter becomes 0 are confirmed. After confirming, the conditions of the applied electromagnetic field of the set Wien filter are stored in a Wien filter operation condition recording unit  212 . 
         [0043]    In Step  006 , an exciting current of the same amount but of opposite sign is given to upper and lower deflectors of the two-stage deflector  06  to make parallel movement of the electron beam  12 , thereby defining the origin of the deviation of the Wien filter entry position Δr p2 . When the deviation of the Wien filter entry position Δr p2  is given, a geometric aberration, a field curvature, an astigmatism, a coma aberration, and a distortion aberration are generated according to the Δr p2  in the Wien filter. Here, the origin of the Δr p2  is desired to be at the center of the quadrupole fields (E 2  and B 2 ) applied to the Wien filter  07 , and the center of the quadrupole field is defined as a position where the positional deviation becomes 0 when the strengths of E 2  and B 2  are varied so as to satisfy Equation (3). The excitation condition of the deflector  06  under the conditions that the positional deviation is defined to 0 is stored in a deflector operation condition recording unit  213 . 
         [0044]    In Step  007 , a Δr p2  is newly given on the basis of the Δr p2  which was set in Step  006 . As a result, a geometric aberration, a field curvature, an astigmatism, a coma aberration and a distortion aberration corresponding to the Δr p2  are generated in the Wien filter. The excitation condition for giving the newly given Δr p2  is stored in the deflector operation condition recording unit  213 . 
         [0045]    In Step  008 , the image surface aberration and the astigmatism generated in Step  007  are adjusted by using the lens strength of the condenser lens  04  and the stigma coil  05 . After adjusting, the setting values of the lens and the stigma coil are stored in a lens operation condition recording unit  211 . 
         [0046]    In Step  009 , distortion aberration generated in the aberration correction unit  11  is corrected by the two-stage deflector  08 . Details of the correction method are described in a second embodiment. In Step  010 , the beam is tilted with superimposing the correction amount of the distortion aberration determined in Step  009  on the deflection coil  08 . In Step  011 , an SEM image when the beam was tilted is obtained. 
         [0047]    In Step  012 , a tilt image of the beam is obtained, and it is judged if a desired sharpness degree was obtained from the sharpness degree of the image. If the desired sharpness degree was not obtained, the process returns to Step  007  to give a new Δr p2 . 
         [0048]    In this embodiment, the operation condition stored in the operation condition recording units ( 211 ,  212  and  213 ) are used when the beam is tilted, so that the adjustment procedure performed when the beam is tilted can be made simple. 
         [0049]    As the second embodiment, a method of correcting the distortion aberration generated by the correction unit is described. A flowchart of this embodiment is shown in  FIG. 6 . In Step  101 , the Wien filter is preliminarily adjusted. Specifically, the procedure shown in Step  006  of the first embodiment is used, and as to the center of the quadrupole field, the Δr p2  is set so that the positional deviation becomes 0 when strengths of E 2  and B 2  are varied so as to satisfy Equation (3). In addition, the incidence angle is adjusted, so that the deviation of the primary electron arrival point is made equal to 0 between the cases that electromagnetic field is not applied to the Wien filter and electromagnetic field is applied to the Wien filter, and the operation condition of the Wien filter after adjusting and the operation condition for the deflector  06  are recorded in the Wien filter operation condition recording unit  212  and the deflector operation condition recording unit  213 . 
         [0050]    In Step  102 , an SEM image representing a reference of the arrival point of the primary electrons  12  is obtained. In Step  103 , the deviation of the Wien filter entry position Δr p2  is given to the deflector  06 . In Step  104 , the field curvature caused when the Δr p2  is given is adjusted by varying the lens strength of the condenser lens  04  (or the objective lens  09 ), the astigmatism is adjusted by varying the strength of the stigma coil  05 , and the condition after adjusting is stored in the lens operation condition recording unit  211 . 
         [0051]    in Step  105 , an SEM image when the deflector Δr p2  is given is obtained. In Step  106 , a positional deviation quantity from the reference image obtained in Step  102  is calculated by a positional deviation calculation unit  221  in the distortion aberration measuring unit, and the positional deviation, quantity which is generated when the Δr p2  is given by the deflector  06  is recorded in a positional deviation quantity recording unit  222 . In Step  107 , to correct the positional deviation quantity which is in the positional deviation quantity recording unit  222 , the excitation amount to be given to the deflector  08  is determined and recorded in the deflector operation condition recording unit  213 . In Step  108 , the Δr p2  is varied, and Step  103  to Step  107  are repeated until a desired volume of data is obtained. 
         [0052]    In Step  109 , Step  101  to Step  108  are repeated until the desired data volume is obtained while the strength of the electromagnetic field which is superimposed on the Wien filter is varied to obtain a lens operation condition, a Wien filter operation condition and a deflector operation condition under respective correction conditions. The aberration correction unit  11  and the deflector  08  are operated along the operation condition obtained in this embodiment, so that a size of the dispersion generated by the aberration correction unit  11  is adjusted by the electromagnetic-field strength applied to the Wien filter  07 , and the coma aberration amount generated by the deviation of the entry position Δr p2  given by the deflector  06  can be adjusted. 
         [0053]    By performing the procedure of this embodiment when the SEM main body is adjusted, the parameters of the respective lens and deflector at the time of aberration correction can be formalized to a table. The procedure of setting respective optical parameters when the beam is tilted can be simplified by setting the lens operation condition, the Wien filter operation condition and the deflector operation condition along the correction conditions in the table. 
         [0054]    The aberration correction unit  11  described in the above two embodiments has a feature that it controls the generated amount of the dispersion by the electromagnetic-field strength forming the Wien filter  07  and it controls the coma aberration amount by changing the entry position to the Wien filter by the deflector  06 . According to this system, the emitting position and the angle of the primary electrons  12  emitted from the aberration correction unit  11  are varied by changing the entry position. Therefore, it is necessary to correct the emitting position and angle change by the deflector  08 , and the control becomes complex. 
         [0055]    As a third embodiment, description is made on a beam tilt optical system which can simultaneously generate a dispersion and coma aberration when another stage of the Wien filter is added instead of the deflector  06  without giving a change of entry position Δr p2  to the Wien filter  07 . 
         [0056]      FIG. 7  shows a structure diagram of the beam tilt optical system according to this embodiment. The aberration correction unit  11  of the beam tilt optical system shown in the first and second embodiments is comprised of a combination of the deflector and the Wien filter, but an aberration correction unit  14  according to this embodiment has a feature that it is comprised of a two-stage Wien filter. 
         [0057]    Features of the aberration correction unit  11  and the aberration correction unit  14  are described. When the primary electrons  12  pass through the Wien filter  07  which is disposed in the aberration correction units ( 11  and  14 ), opening components (∝α* 2  and ∝αα*) of the secondary aberration are generated together with the dispersion. Such aberration does the same work as the coma aberration does on the specimen surface. Therefore, the aberration correction unit  11  treats the sum of the opening components of the secondary aberration and the coma aberration generated depending on the entry position Δr p2  to the Wien filter as the coma aberration generated by the aberration correction unit  11 , and adjusts the coma aberration amount generated by the aberration unit  11  by controlling the Δr p2  by the deflector  06 . On the other hand, the aberration correction unit  14  described in this embodiment has a feature that (two) Wien filters having different generated amounts of the dispersion and coma aberration are used, and the voltage condition applied to the respective Wien filter is adjusted to adjust the size of the dispersion and coma aberration generated by the aberration correction unit  14 . 
         [0058]    Here, coma aberration and dispersion adjusting methods are described. When the lens action in the light path is small, the track of the primary electron focused at the object point P 2  expands depending on the distance from the object point. As a general characteristic of the Wien filter, a large dispersion and a secondary opening aberration are generated when the Wien filter is disposed in a position where the above-described primary electrons  12  expand largely. And, a ratio (=ΔW 2 /ΔWdc) between the generated dispersion ΔWdc and the secondary opening aberration ΔW 2  increases according to the expansion of the primary electrons in the Wien filter. Therefore, when the Wien filter is disposed in a place where the primary electrons have a different expansion, an optical element having a difference in the generated dispersion and the secondary aberration is established. 
         [0059]    As shown in  FIG. 7 , this system, is described with reference to an explanatory view of the third embodiment. In  FIG. 7 , the primary electrons  12  are focused at the top end of the Wien filter  07 . At this time, the position of the focused spot is set as P 2 . The primary electrons  12  focused at P 2  expand while receiving the lens action of the Wien filter  07  and enter the Wien filter  15 . In the above case, the expansion of the primary electrons  12  is compared in the Wien filter  07  and the Wien filter  15 , the expansion of the electrons is larger in the Wien filter  15 . Therefore, the ratio (=ΔW 2 /ΔWdc) of aberration generated by the Wien filter  15  becomes larger than the Wien filter  07 . Here, the applied electromagnetic-field strength of the Wien filter  15  is adjusted so as to cancel the secondary opening aberration generated by the Wien filter  07 . Since the secondary opening aberration generated by the Wien filter  07  and the secondary opening aberration generated by the Wien filter  15  cancel to each other, the aberration correction unit  14  does not generate a secondary aberration but causes a dispersion only. In the above embodiment, it was described that the operation was performed under the condition that the secondary aberration was not generated by the aberration correction unit  14 , but in principle, the dispersion and the secondary opening aberration amount generated by the aberration correction unit  14  can be adjusted by adjusting the electromagnetic-field strength applied to the Wien filter  07  and the Wien filter  15 . 
         [0060]      FIG. 8  shows a schematic diagram of a beam tilt optical system provided with a two-stage deflector  06  for passing the center of the Wien filter as a fourth embodiment. An aberration correction, principle of the beam tilt optical system shown in this embodiment is the same as in the third embodiment, and the dispersion and the secondary opening aberration amount generated by the aberration correction unit  14  can be adjusted by adjusting the electromagnetic-field strength applied to the Wien filter  07  and the Wien filter  15 . In addition, the addition of the deflector  06  can make the deviation of the entry position Δr p2  to the aberration correction unit  14  equal to 0. As a result, the aberration generated by the aberration correction unit  14  becomes only the dispersion and the coma aberration, so that it is possible to solve the problem of the positional deviation and angle deviation of the emitted electrons generated when the correction conditions for the aberration generated when the beam is tilted are inputted to the aberration correction unit  14 . 
         [0061]    A fifth embodiment combining plural aberration correction units is described.  FIG. 9  shows a schematic diagram of a beam tilt optical system according to the fifth embodiment. 
         [0062]    Aberration correction units  11  and  14  of this embodiment have a feature in that the dispersion and the coma aberration generated by the aberration correction units can be varied independently. Therefore, additional application of them to the beam tilt optical system that corrects the off-axis chromatic aberration and the deflection coma aberration by a combination of the lens to be rotated and the deflector, makes it easy to comply with changes in compensation conditions when another factor enters, such as an axis deviation or an assembling error of the objective lens, which deviates from the ideal compensation state. 
         [0063]    According to the above embodiment, the off-axis chromatic aberration and the deflection coma aberration generated when the beam is tilted can be corrected by, for example, a combination of the electromagnetic dipole field and the quadrupole field. Since the correction unit has a simple structure, a beam tilted image can be obtained by simple adjustment.