Abstract:
To provide an aberration corrector that guarantees freedom in designing a coma-free plane transfer portion even when the mechanical configuration of the aberration corrector is already decided, and has a flexible adjustment margin regarding the corrector exterior. The aberration corrector causes an electron beam trajectory emanating from a specimen plane (physical surface of objective lens) to be incident in parallel with a multipole lens (HEX 1 18 ), and causes an electron beam trajectory emanating from an objective-lens coma-free plane or a minimum plane of a fifth-order aberration (objective lens center) to form an image on a center plane of a multipole lens of the 4f system. Thus, antisymmetric transfer is performed between two multipole lenses (HEX 1 18,  HEX 2 19 ) to correct a spherical aberration in the 4f system, and a coma-free plane or a minimum plane of a fifth-order aberration is transferred to suppress occurrence of coma aberrations or fifth-order aberrations. (See FIG.  4 )

Description:
CLAIM OF PRIORITY 
       [0001]    The present application claims priority from Japanese patent application JP 2008-092691 filed on Mar. 31, 2008, the content of which is hereby incorporated by reference into this application. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a charged particle beam apparatus, and more particularly to an aberration corrector used in a transmission electron microscope. 
         [0004]    2. Background Art 
         [0005]    Theoretically, spherical aberrations of electron lenses that use rotationally symmetric electromagnetic fields are always a positive value, and even by compensating for a spherical aberration within a range in which these are used, it is not possible to make the spherical aberration 0. Accordingly, until recently spherical aberration had been the biggest factor limiting the resolution of electron microscope apparatuses. 
         [0006]    In contrast, from the latter part of the 1990&#39;s, practical application of spherical aberration correctors using non-rotationally symmetric electron optical systems such as multipole lens has been proceeded. Currently, in 2008, at the practical use level, electron microscopes mounted with spherical aberration correctors are been manufactured commercially. In particular, for transmission electron microscopes (TEM) having an accelerating voltage exceeding 100 kV, in many cases the spherical aberration is the biggest factor that determines the resolution, and the effect of utilizing a spherical aberration corrector is significant. 
         [0007]      FIG. 1  is a view that illustrates the outline of a TEM that is equipped with an aberration corrector. In a TEM mirror body  1 , a spherical aberration corrector  2  is disposed downstream from a specimen  3  in a condition in which an objective lens  4  is sandwiched between the spherical aberration corrector  2  and the specimen  3 . The principal purpose of the spherical aberration corrector  2  is to correct a spherical aberration of the objective lens  4 . The specimen  3  is irradiated with an electron beam  8 . The electron beam  8  is obtained from an electron source  5  (electron sources according to a thermoelectronic emission method, a field emission method, a thermal-field emission method and the like are available), accelerated with a predetermined accelerating voltage in an accelerating tube  6 , and thereafter the degree of convergence of the electron beam  8  is adjusted using one or more condensing lenses  7 . At this time, an electron beam  9  that is scattered at the specimen  3  is formed into an image with the objective lens  4  to obtain a magnified image. When attempting to observe the specimen  3  at a high resolution of 1 nm or less, normally the specimen  3  is arranged in a position in which it is immersed in the magnetic field of the objective lens polepiece (magnetic pole for concentrating a magnetic field), and hence the illustration in  FIG. 1  shows the specimen  3  as being placed inside the objective lens  4 . 
         [0008]    In the process of forming a magnified image, the spherical aberration corrector  2  is arranged directly below the objective lens  4  and compensates for image distortion or blurring that occurs due to a spherical aberration of the objective lens  4 . The obtained magnified image is magnified further at a plurality of image-forming lenses  10  that are arranged further downstream, to thereby form an image on an observation surface  11 . Normally, a fluorescent screen that is illuminated by the electron beam  8  is arranged on the observation surface, and the magnified image can be observed by viewing this directly. It is also possible to capture an image of the magnified image using a film or CCD camera. In this connection,  FIG. 1  illustrates a basic configuration, and other auxiliary devices such as an astigmatic corrector or a deflector that adjusts an electron beam are omitted from the description. 
         [0009]    In the aforementioned TEM, as a spherical aberration corrector that is appropriate for performing a spherical aberration correction, a configuration including two hexapole lenses that is proposed by H. Rose in JP Patent No. 3207196 (hereunder, referred to as “Patent Document 1”) is currently being practically applied by M. Haider et al.  FIG. 2  is a view that illustrates the configuration of the area indicated by a broken-line frame  12  in  FIG. 1  that includes the specimen  3 , the objective lens  4 , and the spherical aberration corrector. In  FIG. 2 , a solid line  13  and a broken line  14  denote electron beam trajectories that emanate from a specimen plane  15  and a coma-free plane  16  (described later), respectively. In  FIG. 2 , the left side is illustrated as the upstream region of the electron microscope, and thus the objective lens  4  and the specimen  3  are shown at the left end in  FIG. 2 . The corrector  2  is disposed on the lower side (right side in  FIG. 2 ) of the objective lens  4  and the specimen  3 . The corrector  2  is broadly divided into a first half portion extending from directly below the objective lens  4  to the front of the first hexapole lens  18 , and the latter half portion below the first half portion. The latter portion is a spherical aberration correction portion (referred to as “4f system  17 ” by H. Rose) that corrects a spherical aberration of the objective lens  4 . The spherical aberration correction portion includes two hexapole lenses HEX 1   18  and HEX 2   19 , and two rotationally symmetric transfer lenses TL 1   20  and TL 2   21  that are sandwiched by the two hexapole lenses HEX 1   18  and HEX 2   19 . 
         [0010]    Although it is known that the hexapole lens  18  has a negative spherical aberration, an aberration such as a three-fold symmetry astigmatic aberration that is unnecessary for correction is also generated at the same time. In such case, it is possible to cancel the three-fold symmetry astigmatic aberration by preparing two of the hexapole lenses  18  and  19  and performing an antisymmetric transfer with the transfer lenses  20  and  21 , to thereby extract only a negative spherical aberration (the astigmatic aberration is cancelled by lenses  20  and  21 ). An adjustment is performed so as to offset the size of the negative spherical aberration with a positive spherical aberration of the objective lens  4  to implement a spherical aberration correction. To establish appropriate transfer conditions, it is necessary to arrange HEX 1   18 , HEX 2   19 , TL 1   20 , and TL 2   21  of the 4f system at the lengths f, 2f, and f as shown in  FIG. 2  based on the focal length f of the transfer lenses. 
         [0011]    In contrast, the first half portion is designed to suppress a first off-axis aberration, that is, a coma aberration, which constitutes the next problem after a spherical aberration in a TEM. A coma-free transfer is realized between the objective lens and the 4f system using the two transfer lenses included in the first half portion. Hereunder, the first half portion is referred to as “coma-free plane transfer portion  22 ”. 
         [0012]    A coma-free plane  16  (plane where coma aberration coefficient becomes 0 in polarity inversion process) of the objective lens  4  itself is formed in the vicinity of a back focal plane of the objective lens  4 . The coma-free plane  16  of the objective lens  4  is transferred to the center plane of HEX 1   18  as the coma-free plane of the 4f system using the two transfer lenses TF 1   23   a  and TF 2   24   a,  to suppress an increase in the coma aberration. In this connection, in order to perform this transfer in a 1:1 ratio, H. Rose stipulates that the lenses in the coma-free plane transfer portion should be disposed at lengths f, 2f, and f as shown in  FIG. 2 , taking the focal length of the transfer lenses as f (see Patent Document 1). 
         [0013]    However, when a condition for disposing lenses in the coma-free plane transfer portion  22  does not comply with the above described 1:1 transfer condition, it is possible to perform a coma-free transfer even outside the restrictive condition of Rose. Conversely, by performing a transfer by imparting a magnification, and not as a 1:1 transfer, there is also the advantage that the effect of the aberration corrector can be adjusted. By focusing on this point, M. Haider et al. disclose a configuration of an improved spherical aberration corrector in JP Patent Publication (Kohyo) No. 2002-510431 B (hereunder, referred to as “Patent Document 2”). 
         [0014]      FIG. 3  illustrates the configuration of the spherical aberration corrector according to Haider. As shown in  FIG. 3 , the latter half 4f portion of the corrector is the same as in the configuration of H. Rose shown in  FIG. 2 . However, the disposition of lenses in the coma-free plane transfer portion is changed so that the TF 1   23   b  and the TF 2   24   b  have differing focal lengths f 1  and f 2 . The respective lenses in the coma-free plane transfer portion at this time are disposed at lengths f 1 , f 1 +f 2 , and f 2 . A magnitude m t  at the transfer lenses TF 1   23   b  and TF 2   24   b  in the coma-free plane transfer portion at this time is represented by the following expression: 
         [0000]    
       
         
           
             
               
                 
                   [ 
                   
                     Expression 
                      
                     
                         
                     
                      
                     1 
                   
                   ] 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     m 
                     t 
                   
                   = 
                   
                     
                       f 
                       1 
                     
                     
                       f 
                       2 
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
         [0015]    In this connection, in the configuration according to H. Rose shown in  FIG. 2 , f 1 =f 2 , that is, m t =1. 
         [0016]    At this time, considering the physical surface of the objective lens, offsetting of a spherical aberration at the spherical aberration corrector and the objective lens is represented by the following expression: 
         [0000]      [Expression 2] 
         [0000]        c   so +( m   o   m   t ) −4   c   sc =0   (2) 
         [0017]    Here, C SO  denotes a spherical aberration (coefficient) of the objective lens, C SC  denotes a spherical aberration of the spherical aberration corrector, and m O  denotes the magnitude of the objective lens. For example, if m t &lt;1 assuming that f 1 &lt;f 2 , the spherical aberration C SO  of the objective lens  4  can be offset with a smaller correction amount (C SC ). Thus, a spherical aberration correction can be adjusted using m t  in this manner. 
         [0018]    [Non-Patent Document 2] “Upper limits for the residual aberrations of a high-resolution aberration-corrected STEM”, M. Haider, S. Uhlemann, J. Zach, Ultramicroscopy vol. 81, p. 163, (2000) 
         [0019]    [Non-Patent Document 2] “Towards sub-0.5 A electron beams”, O. L. Krivanek, P. D. Nellist, N. Dellby, M. F. Murfitt, Z. Szilagyi, Ultramicroscopy vol. 96, p. 229, (2003) 
       SUMMARY OF THE INVENTION 
       [0020]    However, with both the configuration according to H. Rose shown in  FIG. 2  and the configuration according to M. Haider shown in  FIG. 3 , the disposition of lenses in the coma-free plane transfer portion is fixed, and the disposition is restricted by the focal lengths of the respective transfer lenses that are used (f according to the configuration of H. Rose, and f 1  and f 2  according to the configuration of M. Haider). More specifically, in the configuration of H. Rose, f is fixed by the mechanical configuration of the coma-free plane transfer portion, and in the configuration of M. Haider, although f 1  includes some degree of flexibility, when f 1  and the mechanical configuration of the coma-free plane transfer portion are decided, f 2  is decided. This is a restriction (that deprives the apparatus design of flexibility) at the time of considering the specific apparatus configuration. 
         [0021]    Further, the problem that fine adjustment is difficult after assembling the apparatus also arises. More specifically, in the case of both H. Rose and M. Haider, because the optical conditions of the coma-free plane transfer portion are fixed once the apparatus configuration has been decided, for example, in a case in which the coma-free plane of the objective lens is displaced from the design position, or when there is no adjustment margin when attempting to change a usage condition of the objective lens such as the magnification or the specimen position from an initial value in accordance with the observation requirements, strictly speaking, the positions of TF 1  and TF 2  must be shifted in accordance with the displacement from the design values or with the desired optical conditions. 
         [0022]    Further, as described in the foregoing, according to the configuration of M. Haider shown in  FIG. 3  it is possible to impart a magnification at the coma-free plane transfer portion  22  and adjust the balance of offsetting the objective lens spherical aberration with a spherical aberration of the corrector by the method of expression 2. However, if f 1  and f 2  are fixed, then m t  also has a constant value and does not serve as a parameter that can be adjusted in an operating state (although it may be an adjustment parameter at the stage of designing the aberration corrector). With regard to the aberration corrector, there are times when it is necessary to finely adjust a spherical aberration of the aberration corrector in an adjustment process to suppress a parasitic aberration or an aberration of a higher order than a third-order aberration that is ascribable to an imperfection of the corrector itself. Conventionally, according to the correctors of H. Rose shown in  FIG. 2  and M. Haider shown in  FIG. 3 , these problems have been dealt with by directly altering C SC . This means sequentially changing the strength of hexapole lenses HEX 1  and HEX 2  inside the 4f system in accordance with fine adjustment of the aberration corrector. However, a high accuracy is required to combine the mutual axes of the hexapoles, and a change in excitation is liable to cause an axial displacement. Therefore, after finding conditions that are nearest to the appropriate ones, it is desirable to use the apparatus with the conditions fixed as they are with as little movement as possible. If it is possible to keep m t  in expression 2 as a variable adjustment parameter even in the adjustment process, it is convenient for the 4f system since m t  in a transfer lens portion (comprising a spherical lens that is easy to adjust) can be changed while the conditions are kept fixed so that the spherical aberration correction balance represented by expression 2 can be adjusted. 
         [0023]    As described above, in the spherical aberration correctors proposed heretofore, the disposition of lenses is restricted and fixed because of conditions that are necessary for a spherical aberration correction, and thus the degree of freedom in designing a specific apparatus configuration has been limited. Further, after assembling an aberration corrector, the disposition of lenses is fixed and there is no margin left for adjustment. Accordingly in a state in which a corrector is actually being used it is difficult to make an adjustment to the corrector in order to, for example, rectify an error between the design and the manufactured corrector or to correspond to an adjustment according to the actual conditions of use. In particular, since it is difficult to perform a fine adjustment with the 4f system, it is considered that, rather, it is more suitable to use the corrector with fixed conditions. On the other hand, with respect to the coma-free plane transfer portion in the first half of the corrector, it is also considered suitable to provide a flexible adjustment margin with respect to the corrector exterior in order to maintain fixed conditions of the 4f system. For example, as described above, if the transfer magnification m t  in the coma-free plane transfer portion  22  can be made variable, by changing m t  while keeping the conditions fixed without performing a complicated adjustment of the 4f system such as a hexapole lens adjustment, it is also possible to finely adjust the offsetting between the spherical aberrations of the corrector and the objective lens  4 . 
         [0024]    The present invention was made in view of the above described circumstances, and an object of the invention is to provide an aberration corrector that has a flexible adjustment margin with respect to the corrector exterior and that guarantees a degree of freedom in the design of a coma-free plane transfer portion even when the mechanical configuration of the aberration corrector has been already decided. 
         [0025]    To solve the above described problems, an aberration corrector according to the present invention causes an electron beam trajectory that emanates from a specimen plane (physical surface of an objective lens) to be incident parallel to a multipole lens (HEX 1   18 ), and causes an electron beam trajectory that emanates from an objective lens coma-free plane or a minimum plane of a fifth-order aberration (center of objective lens) to form an image on a center plane of a multipole lens of the 4f system. As a result, to perform a spherical aberration correction with the 4f system, an antisymmetric transfer is performed between two multipole lenses (HEX 1   18 , HEX 2   19 ), and transfer of a coma-free plane or a minimum plane of a fifth-order aberration is performed to suppress the occurrence of a coma aberration or a fifth-order aberration. 
         [0026]    More specifically, an aberration corrector according to the present invention is an aberration corrector for a transmission electron microscope that is disposed downstream of an objective lens of a transmission electron microscope and that performs a spherical aberration correction by offsetting a spherical aberration of the objective lens with a negative spherical aberration that is generated at a combination lens including a plurality of multipole lens, comprising: a spherical aberration correction portion that generates a negative spherical aberration; and a transfer portion that is provided between the objective lens and the spherical aberration correction portion and that has a first and a second spherical transfer lens that suppress an occurrence of a coma aberration or a fifth-order aberration. The transfer portion transfers a coma-free plane or a minimum plane of a fifth-order aberration of the objective lens that is formed in the vicinity of a back focal plane of the objective lens to a coma-free plane of the spherical aberration correction portion or a center plane of a first lens (multipole lens located at an upstream position) of the spherical aberration correction portion, and causes an electron beam that is scattered at a specimen and emanates from a specimen plane to be incident in parallel with the spherical aberration correction portion. Further, among the two spherical transfer lenses of the coma-free plane transfer portion, a length between the objective lens and the first spherical transfer lens that is arranged at a nearest position to the objective lens is different to a focal length of the first spherical transfer lens. 
         [0027]    Further, an aberration corrector according to the present invention is an aberration corrector for a transmission electron microscope that is disposed downstream of an objective lens of a transmission electron microscope and that performs a spherical aberration correction by offsetting a spherical aberration of the objective lens with a negative spherical aberration that is generated at a combination lens including a plurality of multipole lens, comprising: a spherical aberration correction portion that generates a negative spherical aberration; and a transfer portion that is provided between the objective lens and the spherical aberration correction portion and that has a first, a second, and a third spherical transfer lens that suppress an occurrence of a coma aberration or a fifth-order aberration, and that enables an adjustment of a transfer magnification to the spherical aberration correction portion from the objective lens by means of the spherical transfer lenses. The transfer portion transfers a coma-free plane or a minimum plane of a fifth-order aberration of the objective lens that is formed in the vicinity of a back focal plane of the objective lens to a coma-free plane of the spherical aberration correction portion or a center plane of a first lens (multipole lens located at an upstream position) of the spherical aberration correction portion, and causes an electron beam that is scattered at a specimen and emanates from a specimen plane to be incident in parallel with the spherical aberration correction portion. Further, among the three spherical transfer lenses of the transfer portion, a length between the objective lens and the first spherical transfer lens that is arranged at a nearest position to the objective lens is different to a focal length of the first spherical transfer lens. 
         [0028]    Other features of the present invention will be apparent from the preferred embodiments of the invention that are described below and from the attached drawings. 
         [0029]    According to the present invention, it is possible to guarantee a degree of freedom in the design of a coma-free plane transfer portion even when the mechanical configuration of an aberration corrector has been already decided, and to provide a flexible adjustment margin with respect to the corrector exterior. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0030]      FIG. 1  is a view that illustrates the configuration of a transmission electron microscope mirror including a spherical aberration corrector. 
           [0031]      FIG. 2  is a view that illustrates the configuration of a conventional aberration corrector for a transmission electron microscope (Rose). 
           [0032]      FIG. 3  is a view that illustrates the configuration of a conventional aberration corrector for a transmission electron microscope (Haider). 
           [0033]      FIG. 4  is a view that illustrates the configuration of an aberration corrector according to a first embodiment of the present invention. 
           [0034]      FIG. 5  is a view that illustrates focal lengths of a transfer lens group and a transfer magnification as advantages of the aberration corrector according to the first embodiment. 
           [0035]      FIG. 6  is a view that illustrates the configuration of an aberration corrector according to a second embodiment of the present invention. 
           [0036]      FIG. 7  is a view that illustrates the relationship between focal lengths of a transfer lens group and a transfer magnification as advantages of the aberration corrector according to the second embodiment. 
           [0037]      FIG. 8  is a view that illustrates actual application conditions of an aberration corrector according to the second embodiment of the present invention. 
       
    
    
     DESCRIPTION OF SYMBOLS 
       [0000]    
       
           1  Transmission electron microscope mirror body including spherical aberration corrector 
           2  Spherical aberration corrector for transmission electron microscope 
           3  Specimen that is observed 
           4  Objective lens 
           5  Electron source 
           6  Accelerating tube 
           7  Converging lens (group) 
           8  Electron beam that irradiates specimen 
           9  Electron beam scattered at specimen 
           10  Projection lens (group) 
           11  Observation surface 
           12  Objective lens and aberration corrector 
           13  Paraxial trajectory of electron beam emitted from specimen plane on optical axis 
           14  Paraxial trajectory of electron beam emitted from coma-free plane of objective lens on optical axis 
           15  Specimen 
           16  Coma-free plane of objective lens 
           17  4f system 
           18  Hexapole lens  1  (HEX 1 ) 
           19  Hexapole lens  2  (HEX 2 ) 
           20  Transfer lens  1  (TL 1 ) inside 4f system 
           21  Transfer lens  2  (TL 2 ) inside 4f system 
           22   a  Coma-free plane transfer portion of H. Rose 
           22   b  Coma-free plane transfer portion of M. Haider 
           22   c  Coma-free plane transfer portion according to first embodiment 
           22   d  Coma-free plane transfer portion according to second embodiment 
           23   a  Transfer lens  1  (TF 1 ) inside coma-free plane transfer portion of H. Rose 
           23   b  Transfer lens  1  (TF 1 ) inside coma-free plane transfer portion of M. Haider 
           23   c  Transfer lens  1  (TF 1 ) inside coma-free plane transfer portion according to first embodiment 
           23   d  Transfer lens  1  (TF 1 ) inside coma-free plane transfer portion according to second embodiment 
           24   a  Transfer lens  2  (TF 2 ) inside coma-free plane transfer portion of H. Rose 
           24   b  Transfer lens  2  (TF 2 ) inside coma-free plane transfer portion of M. Haider 
           24   c  Transfer lens  2  (TF 2 ) inside coma-free plane transfer portion according to first embodiment 
           24   d  Transfer lens  2  (TF 2 ) inside coma-free plane transfer portion according to second embodiment 
       
     
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0071]    An object of the present invention is to improve the resolution in a charged particle beam apparatus. In particular, the present invention relates to an apparatus that compensates for a spherical aberration in a transmission electron microscope. Similarly to the prior art, an aberration corrector according to the present invention can be applied to the charged particle beam apparatus (transmission electron microscope apparatus) shown in  FIG. 1 . 
         [0072]    Hereunder, embodiments of the present invention are described with reference to the attached drawings. However, the following embodiments are merely examples for implementing the present invention, and it should be noted that these embodiments do not limit the technical scope of the present invention. Common components are denoted by the same reference numerals in the drawings. 
       First Embodiment 
       [0073]    First, the principles of the present invention will be described. 
         [0074]    The lens configuration of a coma-free plane transfer portion that can provide an adjustment margin in order to, for example, rectify an error between a design and a manufactured corrector or to correspond to an adjustment according to actual usage conditions while maintaining the coma-free plane transfer conditions is described below. First, the coma-free plane transfer conditions are compiled below. 
         [0075]    Condition 1: An electron beam trajectory  13  emanating from a specimen plane (physical surface of objective lens) is caused to be incident in parallel with the HEX 1   18 . (The electron beam trajectory in question corresponds to the trajectory of an electron beam that is scattered from the specimen. This is a necessary condition for performing an antisymmetric transfer between the HEX 1   18  and the HEX 2   19  in order to perform a spherical aberration correction in a 4f system  17 ). 
         [0076]    Condition 2: An electron beam trajectory  14  emanating from an objective-lens coma-free plane  16  forms an image on a 4f-system coma-free plane (center plane of HEX 1   18 ). (This is a necessary condition for performing a coma-free transfer to suppress the generation of a coma aberration.) 
         [0077]      FIG. 4  illustrates the configuration of an aberration corrector that is based on conditions 1 and 2 above. The relational expression for conditions 1 and 2 is as follows: 
         [0000]    
       
         
           
             
               
                 
                   [ 
                   
                     Expression 
                      
                     
                         
                     
                      
                     3 
                   
                   ] 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   { 
                   
                     
                       
                         
                           
                             l 
                             2 
                           
                           = 
                           
                             
                               f 
                               1 
                             
                             + 
                             
                               f 
                               2 
                             
                           
                         
                       
                     
                     
                       
                         
                           
                             l 
                             3 
                           
                           = 
                           
                             b 
                             2 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
         [0078]    Here, l 1 , l 2 , and l 3  denote lengths between the objective lens and TF 1 , TF 1  and TF 2 , and TF 2  and HEX 1 , respectively, as shown in each drawing. Further, f 1 , and f 2  denote the focal lengths of TF 1   23   c  and TF 2   24   c,  respectively. b 2  denotes a length from the relevant lens as far as a plane at which the TF 2   24   c  transfers the coma-free plane  16 , taking the electron trajectory of condition 2 into consideration, and is represented by the following relation based on the formula of the lens using the aforementioned l 1 , l 2 , and l 3 , and f 1  and f 2 . 
         [0000]    
       
         
           
             
               
                 
                   [ 
                   
                     Expression 
                      
                     
                         
                     
                      
                     4 
                   
                   ] 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     1 
                     
                       f 
                       2 
                     
                   
                   = 
                   
                     
                       
                         1 
                         
                           
                             l 
                             2 
                           
                           - 
                           
                             b 
                             1 
                           
                         
                       
                       + 
                       
                         
                           1 
                           
                             b 
                             2 
                           
                         
                          
                         
                             
                         
                          
                         
                           i 
                           . 
                           e 
                           . 
                           
                               
                           
                            
                           
                             b 
                             2 
                           
                         
                       
                     
                     = 
                     
                       - 
                       
                         
                           
                             f 
                             2 
                           
                            
                           
                             ( 
                             
                               
                                 l 
                                 2 
                               
                               - 
                               
                                 b 
                                 1 
                               
                             
                             ) 
                           
                         
                         
                           
                             f 
                             2 
                           
                           - 
                           
                             ( 
                             
                               
                                 l 
                                 2 
                               
                               - 
                               
                                 b 
                                 1 
                               
                             
                             ) 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
         [0079]    Provided, b 1  denotes a length from the TF 1   23   c  to an intermediate transfer plane of the coma-free plane. 
         [0000]    
       
         
           
             
               
                 
                   [ 
                   
                     Expression 
                      
                     
                         
                     
                      
                     5 
                   
                   ] 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     1 
                     
                       f 
                       1 
                     
                   
                   = 
                   
                     
                       
                         1 
                         
                           l 
                           1 
                         
                       
                       + 
                       
                         
                           1 
                           
                             b 
                             1 
                           
                         
                          
                         
                             
                         
                          
                         
                           i 
                           . 
                           e 
                           . 
                           
                               
                           
                            
                           
                             b 
                             1 
                           
                         
                       
                     
                     = 
                     
                       - 
                       
                         
                           
                             f 
                             1 
                           
                            
                           
                             l 
                             1 
                           
                         
                         
                           
                             f 
                             1 
                           
                           - 
                           
                             l 
                             1 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
           
         
       
     
         [0080]    By combining expression 3, expression 4, and expression 5 to solve f 1  and f 2 , expression 6 is established. 
         [0000]    
       
         
           
             
               
                 
                   [ 
                   
                     Expression 
                      
                     
                         
                     
                      
                     6 
                   
                   ] 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     
                       f 
                       1 
                     
                     = 
                     
                       
                         
                           l 
                           2 
                         
                          
                         
                           ( 
                           
                             
                               2 
                                
                               
                                   
                               
                                
                               
                                 l 
                                 1 
                               
                             
                             + 
                             
                               
                                 l 
                                 2 
                               
                               ± 
                               K 
                             
                           
                           ) 
                         
                       
                       
                         2 
                          
                         
                             
                         
                          
                         L 
                       
                     
                   
                    
                   
                     
 
                   
                    
                   
                     
                       f 
                       2 
                     
                     = 
                     
                       
                         
                           l 
                           2 
                         
                          
                         
                           ( 
                           
                             
                               2 
                                
                               
                                   
                               
                                
                               
                                 l 
                                 3 
                               
                             
                             + 
                             
                               
                                 l 
                                 2 
                               
                               ∓ 
                               K 
                             
                           
                           ) 
                         
                       
                       
                         2 
                          
                         
                             
                         
                          
                         L 
                       
                     
                   
                 
               
               
                 
                   ( 
                   6 
                   ) 
                 
               
             
           
         
       
     
         [0081]    Provided, L t  is the entire length of the coma-free plane transfer portion  22 , that is, expression 7. 
         [0000]      [Expression 7] 
         [0000]        L   t   =l   1   +l   2   +l   3    (7) 
         [0082]    Further, K is a parameter represented by expression 8 below 
         [0000]      [Expression 8] 
         [0000]        K= √{square root over ( l   2   2 −4 l   1   l   3 )}  (8) 
         [0083]    That is, the solution represented by expression 6 is a real solution in a range in which K is a real number, that is, in a case in which expression 9 is satisfied. 
         [0000]      [Expression 9] 
         [0000]      l 2 &gt;2√{square root over ( l   1 l 3 )}  (9) 
         [0084]    Naturally, l 1 , l 2 , and l 3  are positive numbers, and in a range in which expression 9 has a solution with a simple operation, f 1  and f 2  of expression 6 can also be shown to be positive values. 
         [0085]    The foregoing description shows that, while satisfying conditions relating to the coma-free plane transfer portion  22  of conditions 1 and 2, focal lengths f 1  and f 2  of the TF 1  and TF 2  that are effective for arbitrary lens intervals  11 ,  12 , and  13  can be found within a range in which expression 9 has a solution. This result remarkably increases the degree of freedom with respect to the disposition of lenses in the coma-free plane transfer portion at the design stage, and further, after the disposition of lenses has been fixed, it is possible to perform a fine adjustment according to expression 6 even in an operating state. 
         [0086]    The addition of a third transfer lens may also be considered to give the coma-free plane transfer portion  22  an adjustable magnification. Using the increased flexibility obtained by addition of a lens, leeway is generated that makes it possible to control the transfer magnification of the coma-free plane transfer portion  22  while maintaining the coma-free plane transfer conditions. 
         [0087]    Next, a specific example is given and examined based on the above described principles of the invention. In this case, regarding the configuration shown in  FIG. 4 , for example, assuming that l 1 =30 mm and l 3 =45 mm, the values shown in expression 9 and expression 10 are determined. 
         [0000]      [Expression 10] 
         [0000]      l 2 &gt;30√{square root over (6)}≈73.5 mm   (10) 
         [0088]    When l 2  is a value in the range shown by expression 10, the coma-free plane transfer portion  22  has a solution according to expression  6  that establishes the aforementioned conditions 1 and 2. In this connection, expression 10 is, for example, represented with an accuracy of ±1%. 
         [0089]    A graph  25  shown in  FIG. 5  shows the manner in which f 1  and f 2  change with l 1  as a parameter at this time. A broken line and a solid line in the graph depend on whether the upper symbol or the lower symbol of ± plus/minus) and ∓ (minus/plus) in expression 6 is adopted. It is verified that f 1  and f 2  have a positive number solution within the range of expression 10 for either + or −. 
         [0090]    In contrast, as shown in a graph  26  in  FIG. 5 , the transfer magnification m t  differs greatly according to whether the upper symbol or the lower symbol of ± and ∓ in expression  6  is adopted. Accordingly, although in practice the combination of f 1  and f 2  ought to be decided by taking into account this difference in the transfer magnification, normally, in order to obtain an aberration correction more efficiently, it is considered more suitable to select a combination of f 1  and f 2  such that m t  decreases from expression 2, i.e. a combination of f 1  and f 2  that adopts + in expression 6. However, even with a combination of f 1  and f 2  that adopts −, because the coma-free conditions are maintained, by switching these two conditions it is possible to perform a comparative observation with respect to an appropriate spherical aberration correction state and an insufficient correction state by adjusting only the transfer lenses TF 1   23  and TF 2   24  without touching the 4f system  17 . When an observation can be performed by easily switching between an appropriate correction state and an insufficient correction state in this manner, for example, it is possible to confirm the correction status at the stage of adjusting an aberration correction (confirm whether or not an appropriate aberration correction could be executed). Furthermore, it is possible to compensate for a contrast with respect to a long-term structure that, conversely, declines at the time of an appropriate spherical aberration correction by observation of an insufficient correction state. More specifically, when a spherical aberration is corrected, there is a tendency for a large item to become harder to see, although a minute item can be seen clearly. Hence, when it is desired to observe a large item, it is better to select the combination of lower symbols in expression 6 to observe the specimen. By selecting a combination in this manner, it is possible to switch between existence/non-existence of a correction of a spherical aberration while securing a coma free condition. According to the conventional configuration, it is necessary to remove the corrector  2  itself and a coma free condition can not be secured. This is one advantage according to the present invention. 
         [0091]    In this connection, a case in which it is assumed that l 2 =l 1 +l 3 =75 mm under the condition shown in the graph is equivalent to the spherical aberration corrector according to M. Haider (see  FIG. 3 ). Further, when it is further assumed that l 1 =l 3 =l 2 /2, this case is equivalent to the spherical aberration corrector according to H. Rose (see  FIG. 2 ). 
         [0092]    Thus, according to the present invention, the condition (l 2 =l 1 +l 3 , l 1 =l 3 =l 2 /2) should be omitted from expression  6 , and is summarized as l 1 ≠f 1 , l 3 ≠f 2 . 
       Second Embodiment 
       [0093]    As described in the foregoing, since adjustment of the 4f system is difficult, it is desirable that execution of a fine adjustment of a spherical aberration correction is concentrated on transfer lenses of a coma-free plane transfer portion while maintaining the 4f system in a fixed condition. To achieve this, a three-lens configuration is adopted by adding a single lens TF 3   24   d  the transfer lenses that had been a two-lens configuration comprising TF 1  and TF 2 . It is thereby possible to vary the transfer magnification m t  in a zooming manner while maintaining the coma-free transfer conditions. That is, the degree of freedom in setting a transfer magnification can also be increased. 
         [0094]    According to the second embodiment, as shown in  FIG. 6 , the coma-free plane transfer portion  22  comprises lenses in the order of TF 1   27  TF 2   23   d  and TF 3   24   d  between the objective lens  4  and the first hexapole lens  18  of the 4f system. In this case, focal lengths f 1 , f 2 , and f 3  that satisfy the above described conditions 1 and 2 are given by expression 11. 
         [0000]    
       
         
           
             
               
                 
                   [ 
                   
                     Expression 
                      
                     
                         
                     
                      
                     11 
                   
                   ] 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     
                       f 
                       1 
                     
                     = 
                     
                       
                         
                           
                             l 
                             2 
                           
                            
                           
                             ( 
                             
                               
                                 
                                   l 
                                   1 
                                 
                                  
                                 
                                   m 
                                   t 
                                   2 
                                 
                               
                               + 
                               
                                 l 
                                 4 
                               
                             
                             ) 
                           
                         
                         
                           
                             
                               ( 
                               
                                 
                                   l 
                                   1 
                                 
                                 + 
                                 
                                   l 
                                   2 
                                 
                               
                               ) 
                             
                              
                             
                               m 
                               t 
                               2 
                             
                           
                           - 
                           
                             
                               l 
                               3 
                             
                              
                             
                               m 
                               t 
                             
                           
                           + 
                           
                             l 
                             4 
                           
                         
                       
                       = 
                       
                         L 
                          
                         
                             
                         
                          
                         
                           
                             
                               k 
                               3 
                             
                              
                             
                               ( 
                               
                                 
                                   m 
                                   t 
                                   2 
                                 
                                 + 
                                 
                                   k 
                                   4 
                                 
                               
                               ) 
                             
                           
                           
                             
                               m 
                               t 
                               2 
                             
                             - 
                             
                               
                                 k 
                                 2 
                               
                                
                               
                                 m 
                                 t 
                               
                             
                             + 
                             
                               ( 
                               
                                 
                                   k 
                                   3 
                                 
                                 + 
                                 
                                   k 
                                   4 
                                 
                               
                               ) 
                             
                           
                         
                       
                     
                   
                    
                   
                     
 
                   
                    
                   
                     
                       f 
                       2 
                     
                     = 
                     
                       
                         - 
                         
                           
                             
                               l 
                               2 
                             
                              
                             
                               l 
                               3 
                             
                              
                             
                               m 
                               t 
                             
                           
                           
                             
                               
                                 l 
                                 1 
                               
                                
                               
                                 m 
                                 t 
                                 2 
                               
                             
                             - 
                             
                               
                                 ( 
                                 
                                   
                                     l 
                                     2 
                                   
                                   + 
                                   
                                     l 
                                     3 
                                   
                                 
                                 ) 
                               
                                
                               
                                 m 
                                 t 
                               
                             
                             + 
                             
                               l 
                               4 
                             
                           
                         
                       
                       = 
                       
                         
                           - 
                           L 
                         
                          
                         
                             
                         
                          
                         
                           
                             
                               k 
                               2 
                             
                              
                             
                               k 
                               3 
                             
                              
                             
                               m 
                               t 
                             
                           
                           
                             
                               m 
                               t 
                               2 
                             
                             - 
                             
                               
                                 ( 
                                 
                                   
                                     k 
                                     2 
                                   
                                   + 
                                   
                                     k 
                                     3 
                                   
                                 
                                 ) 
                               
                                
                               
                                 m 
                                 t 
                               
                             
                             + 
                             
                               k 
                               4 
                             
                           
                         
                       
                     
                   
                    
                   
                     
 
                   
                    
                   
                     
                       f 
                       3 
                     
                     = 
                     
                       
                         
                           
                             l 
                             3 
                           
                            
                           
                             ( 
                             
                               
                                 
                                   l 
                                   1 
                                 
                                  
                                 
                                   m 
                                   t 
                                   2 
                                 
                               
                               + 
                               
                                 l 
                                 4 
                               
                             
                             ) 
                           
                         
                         
                           
                             
                               l 
                               1 
                             
                              
                             
                               m 
                               t 
                               2 
                             
                           
                           - 
                           
                             
                               l 
                               2 
                             
                              
                             
                               m 
                               t 
                             
                           
                           + 
                           
                             ( 
                             
                               
                                 l 
                                 3 
                               
                               + 
                               
                                 l 
                                 4 
                               
                             
                             ) 
                           
                         
                       
                       = 
                       
                         L 
                          
                         
                             
                         
                          
                         
                           
                             
                               k 
                               2 
                             
                              
                             
                               ( 
                               
                                 
                                   m 
                                   t 
                                   2 
                                 
                                 + 
                                 
                                   k 
                                   4 
                                 
                               
                               ) 
                             
                           
                           
                             
                               
                                 ( 
                                 
                                   1 
                                   + 
                                   
                                     k 
                                     2 
                                   
                                 
                                 ) 
                               
                                
                               
                                 m 
                                 t 
                                 2 
                               
                             
                             - 
                             
                               
                                 k 
                                 3 
                               
                                
                               
                                 m 
                                 t 
                               
                             
                             + 
                             
                               k 
                               4 
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   11 
                   ) 
                 
               
             
           
         
       
     
         [0095]    Provided, it is assumed that the lengths between the objective lens  4  and TF 1   27 , TF 1   27  and TF 2   23   d,  TF 2   23   d  and TF 3   24   d,  and TF 3   24   d  and HEX 1   18 , are l 1 , l 2 , l 3 , and l 4 , respectively, and the second term in each expression is given as follows taking l 1 ≡L as a reference. 
         [0000]      [Expression 12] 
         [0000]      l 1 ≡L 
         [0000]      l 2 =k 2 L 
         [0000]      l 3 =k 3 L 
         [0000]      l 4 =k 4 L   (12) 
         [0096]    As shown in expression 11, since m t  can be left as an independent variable, it is possible to adjust the transfer magnification m t  while maintaining the coma-free transfer conditions. Further, when each of the lenses are arranged at regular intervals (that is, when l 1 =l 2 =l 3 =l 4 =≡L), f 1 , f 2 , and f 3  are given by expression 13. 
         [0000]    
       
         
           
             
               
                 
                   [ 
                   
                     Expression 
                      
                     
                         
                     
                      
                     13 
                   
                   ] 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     
                       f 
                       1 
                     
                     = 
                     
                       L 
                        
                       
                         
                           ( 
                           
                             1 
                             + 
                             
                               m 
                               t 
                               2 
                             
                           
                           ) 
                         
                         
                           
                             m 
                             t 
                             2 
                           
                           - 
                           
                             m 
                             t 
                           
                           + 
                           2 
                         
                       
                     
                   
                    
                   
                     
 
                   
                    
                   
                     
                       f 
                       2 
                     
                     = 
                     
                       
                         - 
                         L 
                       
                        
                       
                         
                           m 
                           t 
                         
                         
                           
                             ( 
                             
                               
                                 m 
                                 t 
                               
                               - 
                               1 
                             
                             ) 
                           
                           2 
                         
                       
                     
                   
                    
                   
                     
 
                   
                    
                   
                     
                       f 
                       3 
                     
                     = 
                     
                       L 
                        
                       
                         
                           ( 
                           
                             1 
                             + 
                             
                               m 
                               t 
                               2 
                             
                           
                           ) 
                         
                         
                           
                             2 
                              
                             
                                 
                             
                              
                             
                               m 
                               t 
                               2 
                             
                           
                           - 
                           
                             m 
                             t 
                           
                           + 
                           1 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   13 
                   ) 
                 
               
             
           
         
       
     
         [0097]    According to expression 13, a positive number solution can be found for f 1 , f 2 , and f 3  when m t &lt;0. As an example, the relation when L=30 mm (expression 13) is plotted in a graph  28  of  FIG. 7 . In particular, in order to finely adjust the magnification, when the behavior in the vicinity of |m t |1 (in this case, m t ≈−1) is expanded and viewed at this point, f 1 , f 2 , and f 3  are each given by expression 14. 
         [0000]    
       
         
           
             
               
                 
                   [ 
                   
                     Expression 
                      
                     
                         
                     
                      
                     14 
                   
                   ] 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     
                       f 
                       1 
                     
                     = 
                     
                       
                         
                           L 
                           2 
                         
                          
                         
                           { 
                           
                             1 
                             + 
                             
                               
                                 δ 
                                  
                                 
                                     
                                 
                                  
                                 m 
                               
                               4 
                             
                           
                           } 
                         
                       
                       + 
                       
                         O 
                          
                         
                           ( 
                           
                             δ 
                              
                             
                                 
                             
                              
                             
                               m 
                               2 
                             
                           
                           ) 
                         
                       
                     
                   
                    
                   
                     
 
                   
                    
                   
                     
                       f 
                       2 
                     
                     = 
                     
                       
                         L 
                         4 
                       
                       + 
                       
                         O 
                          
                         
                           ( 
                           
                             δ 
                              
                             
                                 
                             
                              
                             
                               m 
                               2 
                             
                           
                           ) 
                         
                       
                     
                   
                    
                   
                     
 
                   
                    
                   
                     
                       f 
                       3 
                     
                     = 
                     
                       
                         
                           L 
                           2 
                         
                          
                         
                           { 
                           
                             1 
                             - 
                             
                               
                                 δ 
                                  
                                 
                                     
                                 
                                  
                                 m 
                               
                               4 
                             
                           
                           } 
                         
                       
                       + 
                       
                         O 
                          
                         
                           ( 
                           
                             δ 
                              
                             
                                 
                             
                              
                             
                               m 
                               2 
                             
                           
                           ) 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   14 
                   ) 
                 
               
             
           
         
       
     
         [0098]    Provided, when it is assumed that m t ≡δm t −1, and O(δm 2 ) is a term that is equal to or greater than the square of δm, the influence when δm&lt;&lt;⅘ is small. Accordingly, as an adjustment operation when δm is in the above range, first, in order to obtain m t =−1, expression 15 is taken as the reference state. 
         [0000]      [Expression 15] 
         [0000]      f 1   =L/ 2 
         [0000]        f   2   =L/ 4 
         [0000]        f   3   =L/ 2   (15) 
         [0099]    Since f 2  changes only in the second order or higher with respect to δm, it is found that it is sufficient that f 1  and f 3  are deflected to their respective antisymmetries for every δm/4 at TF 1   23   d  and TF 3   27  while TF 2   24   d  remains fixed. Thus, by applying a condition (arranging lenses at regular intervals) to expression 13 to simplify the expression so as to obtain expression 14, the adjustment operation is simplified. 
         [0100]    The electron beam trajectory in the reference state that results in m t =−1 is shown in the drawing denoted by reference number  29  in  FIG. 8 , and the electron beam trajectories when m t =−0.5 and m t =−1.5 are shown in the drawings denoted by reference numbers  30  and  31 , respectively, in  FIG. 8 . Table 1 shows the value of each parameter at this time. As will be understood from  FIG. 8 , because m t  can not be considered minute when m t =0.5, Table 1 is as follows. 
         [0000]    
       
         
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 m t   
                 f 1  (mm) 
                 f 2  (mm) 
                 f 3  (mm) 
                 t 1  (mm) 
                 t 2  (mm) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 (1) 
                 −1.0 
                 15.0 
                 7.5 
                 15.0 
                 30.0 
                 0.0 
               
               
                 (2) 
                 −0.5 
                 18.8 
                 6.7 
                 13.6 
                 50.0 
                 5.0 
               
               
                 (3) 
                 −1.5 
                 13.9 
                 7.2 
                 17.0 
                 26.0 
                 −9.0 
               
               
                   
               
             
          
         
       
     
         [0101]    Provided, in Table 1, t 1  and t 2  are the lengths from the respective lenses to a plane at which the transfer lenses TF 1  and TF 2  respectively transfer the coma-free plane. 
         [0102]    As described in the foregoing, with respect to the configuration of transfer lenses in a coma-free plane transfer portion, the present invention enables flexible arrangement and electro-optic fine adjustment with a two-lens system. Further, when using three lenses, the present invention provides means that adjusts a spherical aberration correction strength while keeping the 4f system fixed and with a transfer magnification independent thereof. 
         [0103]    In this connection, although restrictive conditions of the transfer lens portion are described with respect to a coma-free transfer in the above description, the present invention can also be applied under a condition which transfers two similar specific planes (planes limited by a specimen plane and a transfer condition; the coma-free plane in the above description). Conditions other than a coma-free plane transfer include, for example, minimization of a fifth-order spherical aberration. In this case, it is sufficient to consider a transfer by substituting a plane that serves as a minimum plane of a fifth-order aberration (for example, center of objective lens) for the coma-free plane in the above description. 
         [0104]    As the spherical aberration correction portion in the above description, a spherical aberration correction portion that uses hexapole lenses based on the configurations of H. Rose and M. Haider is described. However, it is also possible to provide a spherical aberration correction portion that uses other multi-stage multipole lenses instead of using hexapole lenses. In this case also, the consideration for transferring an image from the objective lens to the spherical aberration correction portion with specific restrictive conditions is the same, and it is also the same that the adjustment of a transfer lens portion comprising an ordinary spherical lens is easier than adjustment of a spherical aberration correction portion comprising multiple poles. The conditions in this case are also the same as in the case of hexapole lenses.