Abstract:
A magnetic domain imaging system is offered which permits application of a strong magnetic field to a specimen. The imaging system includes a transmission electron microscope having an objective lens. The specimen that is magnetic in nature is placed in the upper polepiece of the objective lens. An electron beam transmitted through the specimen is imaged and displayed on a display device. A field application coil assembly for applying a magnetic field to the specimen and two deflection coil assemblies for bringing the beam deflected by the field applied to the specimen back to the optical axis are mounted in the upper polepiece.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a magnetic domain imaging system, i.e., an apparatus permitting one to observe magnetic domains in a magnetic specimen. 
         [0003]    2. Description of Related Art 
         [0004]    From the past, systems for obtaining transmission images of magnetic specimens using a transmission electron microscope have been known. In order to obtain an accurate image of a specimen, it is necessary to cause an electron beam transmitted through the specimen to be focused along the optical axis.  FIG. 3  shows an example of configuration of the prior art instrument of the top entry type, i.e., a magnetic specimen is inserted from top to objective polepieces and held between them. 
         [0005]    In  FIG. 3 , an electron beam  1  is deflected by two stages of deflection coils  2 . An objective lens  8  magnifies a transmission image of the specimen. Indicated by  7  is an objective-lens coil. The objective lens has an upper polepiece  5  and a lower polepiece  6 . A specimen  3  made of a magnetic material is placed in the upper polepiece where no magnetic field is present. Magnetic coils  4  apply a magnetic field to the specimen  3 . The operation of the apparatus constructed in this way is described below. 
         [0006]    The electron beam  1  emitted from an electron gun (not shown) is deflected by the two stages of deflection coils  2  and made to impinge on the specimen  3 . At the same time, the magnetic coils  4  apply the magnetic field to the specimen  3 . As a result, the magnetic domains in the specimen  3  are varied. The direction of magnetization is made different among individual domains. Under this condition, the beam transmitted through the specimen  3  passes through openings  9   a  and  9   b  formed in the polepieces. At this time, the objective lens field between the upper polepiece  5  and the lower polepiece  6  of the objective lens focuses the beam, forming a first transmission image. Then, it enters the imaging system (not shown) where the transmission image of the specimen is magnified in turn. Finally, the beam is focused onto a fluorescent screen or onto the sensitive surface of a CCD camera, thus permitting observation of the magnetic domains in the specimen. 
         [0007]      FIG. 4  shows another example of configuration of the prior art instrument illustrated In JP-A-2007-80724 (paragraphs [0014]-[0022] and  FIGS. 1 and 2 ). In  FIG. 4 , an electron beam  11  is deflected by a first deflector  12 . There is also shown a second deflector  25 . A second deflector coil (excitation coil)  25   a  is wound around the yoke of the second deflector  25 . 
         [0008]    A first principal deflection plane  14  is formed at the position of the first deflector  12 . Indicated by  15  is a second principal deflection plane. A specimen  16  undergoes an inspection by an electron microscope, and is positioned at the front end of a specimen holder  17  of a magnetic field application mechanism having a gap  18  across which a magnetic field is applied. Also shown are an objective lens  19  and an objective-lens coil  27 . 
         [0009]    An objective lens gap  26  is placed in a stage following the specimen  16  and acts to serve a first focusing action immediately under the specimen. A coil  13  is wound around a front-end portion of the specimen holder  17  of the magnetic field application mechanism. The optical axis of the electron beam  11  is indicated by  30 . The operation of the apparatus constructed in this way is as follows. 
         [0010]      FIG. 5  is a schematic diagram illustrating the operation of the apparatus of the structure shown in  FIG. 4 . In  FIG. 5 , the specimen  16  undergoes the first focusing action of the objective lens  19  to form an objective lens image  20 . The electron beam  11  impinges at an angle of incidence  21  on the specimen  16 . 
         [0011]    The electron beam  11  converged by a condenser lens (not shown) travels along the optical axis  30  and is slightly deflected by the first deflector  12  at the first principal deflection plane  14 . On the other hand, the second deflector  25  is placed as close to the specimen  16  as possible. Therefore, the beam  11  is deflected at the second principal deflection plane  15  lying immediately above the specimen  16  and made to impinge at the on-axis center of the specimen  16 . 
         [0012]    In the specimen holder  17  of the magnetic field application mechanism, the specimen  16  is held in the magnetic field gap  18 . Lines of magnetic force produced across the gap  18  apply a magnetic field to the specimen  16 , and deflect the electron beam. A transmission electron image representing variations in the magnetic domains in the specimen due to the application of the magnetic field is focused as an objective lens image  20  by the lens action of the objective lens gap  26 . Then, the objective lens image  20  is magnified by plural stages of focusing systems (not shown) until a desired magnification is produced. Finally, a high-magnification image is formed on an electron-beam dry plate, TV-like detector, or the like. 
         [0013]    This kind of electron microscope permits observation of magnetic domains, the microscope having means to apply a magnetic field mounted within an objective lens, means to deflect and correct an electron beam mounted between the objective lens and an imaging lens, a means for selecting and applying an arbitrary phase of an alternating magnetic field, and a means for exciting an applied magnetic field using a synchronizing signal for image display means (see, for example, JP-A-8-96737 (paragraph [0007] and  FIG. 1 )). 
         [0014]    The above-described prior art satisfies some key points in observing magnetic fields. That is, the magnetic field around the specimen is eliminated. The magnetic field applied to the specimen is controlled. However, there is the problem that it is difficult to make a correction for a greatly deflected electron beam in cases where the magnetic field is applied to the specimen. 
       SUMMARY OF THE INVENTION 
       [0015]    It is an object of the present invention to provide a magnetic domain imaging system permitting application of a strong magnetic field to a specimen. 
         [0016]    A first embodiment of the present invention provides a magnetic domain imaging system for use with a transmission electron microscope including an objective lens having upper and lower polepieces. A magnetic specimen is placed in the upper polepiece. The imaging system is so designed that an electron beam transmitted through the specimen is imaged and displayed on a display device. The imaging system has a field application coil assembly for applying a magnetic field to the specimen and deflection coil assemblies for bringing the electron beam deflected by the field applied to the specimen back to the optical axis. The field application coil assembly and the deflection coil assemblies are disposed within the upper polepiece of the objective lens. 
         [0017]    A second embodiment of the present invention is based on the first embodiment and further characterized in that the deflection coil assemblies are composed of an entrance deflection coil assembly and an exit deflection coil assembly which are disposed ahead of and behind, respectively, the field application coil assembly. 
         [0018]    A third embodiment of the present invention is based on the first or second embodiment and further characterized in that each of the coils of the coil assemblies includes a core made of a magnetic material having small hysteresis. 
         [0019]    A fourth embodiment of the present invention is based on the first embodiment and further characterized in that the deflection coil assemblies produce X and Y deflections and that rotation caused by deflections is corrected. 
         [0020]    A fifth embodiment of the present invention is based on the second embodiment and further characterized in that the field application coil assembly and the entrance and exit deflection coil assemblies are shifted by about 45° with respect to each other in the senses of the X and Y directions. 
         [0021]    A sixth embodiment of the present invention is based on the second embodiment and further characterized in that the distance l between the front end of the exit deflection coil assembly and the opposite inner surface of the upper polepiece and the distance r between the front end of the exit deflection coil assembly and the optical axis satisfy the relationship l&gt;2r. 
         [0022]    A seventh embodiment of the present invention is based on the second embodiment and further characterized in that the distance L 1  between the mutually opposite coils of the entrance deflection coil assembly and the distance L 1  between the mutually opposite coils of the exit deflection coil assembly are set less than a half of the distance L 2  between the mutually opposite coils of the field application coil assembly. 
         [0023]    According to the first embodiment, the deflection coil assemblies are mounted on the entrance and exit sides, respectively, adjacently to the field application coil assembly and, therefore, the amount of correction needed to correct the deflection caused by the field application can be halved. Consequently, the electron beam can be suppressed from greatly deviating from the optical axis. 
         [0024]    According to the second embodiment, deflection of the electron beam due to excitation of the field application coil assembly can be corrected using the entrance and exit deflection coil assemblies. 
         [0025]    According to the third embodiment, the cores of the coils of the deflection coil assemblies are made of a material with small hysteresis. Therefore, a linear relationship can be created between the electrical current flowing through the excitation coils and the resulting magnetic flux. Hence, accurate alignment to the optical axis can be accomplished. 
         [0026]    According to the fourth embodiment, the electron beam can be deflected in two dimensions. In consequence, deflection can be done such that rotation of the beam due to deflection by the previous stage is corrected. 
         [0027]    According to the fifth embodiment, the mutual effects of magnetic fields leaking from the field application coil assembly, entrance deflection coil assembly, and exit deflection coil assembly can be reduced and so accurate axial alignment can be made. 
         [0028]    According to the sixth embodiment, the effects of magnetic fields leaking from the polepieces on the exit deflection coil assembly are reduced. Therefore, the electron beam can be accurately aligned axially. 
         [0029]    According to the seventh embodiment, the distance between the entrance deflection coil assembly and the exit deflection coil assembly is set less than a half of the distance between the mutually opposite coils of the field application coil assembly. This reduces the effects of the magnetic fields produced by the coil assemblies. Consequently, the electron beam can be accurately aligned axially. 
         [0030]    Other objects and features of the invention will appear in the course of the description thereof, which follows. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0031]      FIG. 1  is a schematic vertical cross section of a magnetic domain imaging system according to one embodiment of the present invention; 
           [0032]      FIG. 2  is a perspective view of main parts of a magnetic domain imaging system according to another embodiment of the present invention; 
           [0033]      FIG. 3  is a vertical cross section of a conventional magnetic domain imaging system; 
           [0034]      FIG. 4  is a vertical cross section of another conventional magnetic domain imaging system; and 
           [0035]      FIG. 5  is a schematic diagram illustrating the operation of the conventional system shown in  FIG. 4 . 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiment 1 
       [0036]    Embodiments of the present invention are hereinafter described in detail with reference to the drawings.  FIG. 1  is a vertical cross section showing a magnetic domain imaging system, according to one embodiment of the present invention. In both  FIGS. 1 and 3 , like components are indicated by like reference numerals. Shown in  FIG. 1  are an electron beam  1 , an optical axis  1   a  connecting the centers of optical devices (such as lens deflection systems), an upper polepiece  5  of an objective lens, a lower polepiece  6  of the objective lens, a magnetic specimen  3 , a field application coil assembly  4  having coils mounted on the opposite sides of the specimen  3 , an entrance deflection coil assembly  9  disposed over the field application coil assembly  4 , an exit deflection coil assembly  10  disposed under the field application coil assembly  4 , an opening  2   a  formed in the upper polepiece  5  to permit passage of transmitted electrons, and an opening  2   b  formed in the lower polepiece  6  to permit passage of the transmitted electrons. 
         [0037]    All of the specimen  3 , field application coil assembly  4 , entrance deflection coil assembly  9 , and exit deflection coil assembly  10  are placed in a region surrounded by the upper polepiece. The differences with the system shown in  FIG. 3  are that the components are placed in the region surrounded by the polepieces and that the entrance and exit deflection coil assemblies are mounted on the opposite sides of the field application coil assembly. The operation of the system constructed in this way is described below. 
         [0038]    The electron beam  1  focused by an illumination optical system (not shown) impinges on the specimen  3 . If the field application coil assembly  4  is electrically energized to align the magnetic domains in the specimen  3  or to provide contrast among the domains, the beam  1  hitting the specimen  3  is deflected through  2 θ. Under this condition, the beam  1  will deviate from the optical axis  1   a  by  2 θ. 
         [0039]    Accordingly, the beam impinging on the specimen is deflected by an angle minus θ, or a half of the angle  2 θ by which the beam has been deflected by the applied magnetic field, by means of the entrance deflection coil assembly  9 . As a result, the beam deflected through  2 θ by the field applied to the specimen  3  deviates by the angle θ from the optical axis  1   a  and exits from the specimen. Accordingly, the beam is deflected through the angle minus θ by the exit deflection coil assembly  10  to bring the beam back to the optical axis. Consequently, the beam  1  passes along the optical axis  1   a  and enters the imaging system (not shown). 
         [0040]    In the present invention, the deflection coil assemblies are mounted on the entrance side and on the exit side, respectively, adjacently to the field application coil assembly. Consequently, the amount of correction needed to cancel the deflection caused by the field application is halved on each side. Hence, the beam can be suppressed from deviating off the axis greatly. Furthermore, as the beam  1  passes along the optical axis  1   a , it is possible to observe magnetic domains in the specimen precisely. 
         [0041]    Furthermore, in the present invention, it is the specimen position where the beam is deflected to its greatest extent while the axis of the beam is aligned (i.e., where the strongest deflection field is present). Thus, strong magnetic fields can be applied to the specimen. 
       Embodiment 2 
       [0042]    In the system shown in  FIG. 1 , the cores of the coils of the coil assemblies can be made of a magnetic substance having small hysteresis, such as an iron-nickel soft magnetic material (such as Permalloy). In order to accurately align the beam to the optical axis, there must be a linear relationship between the excitation coil current and the produced magnetic flux. If there is hysteresis, there is no proportional relationship between the excitation coil current and the produced magnetic flux. Accordingly, a proportional relationship can be developed between them by fabricating the cores of the coils from a low-hysteresis material. As a result, accurate alignment with the optical axis can be accomplished. 
       Embodiment 3 
       [0043]    In the structure shown in  FIG. 1 , the entrance deflection coil assembly  9  and exit deflection coil assembly  10  can be made to perform X and Y deflections, respectively. Thus, the electron beam  1  can be deflected in two dimensions. Consequently, the next stage of deflection can be done according to the rotation caused by the deflection. 
       Embodiment 4 
       [0044]      FIG. 2  is a perspective view showing the main portions of a further embodiment of the present invention. In  FIGS. 1 and 2 , like components are indicated by like reference numerals. Shown in  FIG. 2  are entrance deflection coil assembly  9 , field application coil assembly  4 , exit deflection coil assembly  10 , and the specimen  3  surrounded by these coil assemblies. The field application coil assembly  4  is shifted by 45° with respect to the entrance deflection coil assembly  9 . The exit deflection coil assembly  10  is shifted by 45° with respect to the field application coil assembly  4 . In this arrangement, it is possible to reduce the mutual effects of the magnetic fields leaking from the coil assemblies  9 ,  4 , and  10 . Consequently, accurate alignment of the electron beam with the axis can be accomplished. 
       Embodiment 5 
       [0045]    Referring to  FIG. 1 , r is the distance between the exit deflection coil assembly  10  and the optical axis  1   a , and l is the distance between the exit deflection coil assembly  10  and the upper polepiece  5  of the objective lens. This mechanism is so designed that l&gt;2r. This reduces the effects of the magnetic fields leaking from the polepieces on the exit deflection coil assembly  10 . Consequently, the electron beam can be aligned accurately. 
       Embodiment 6 
       [0046]    The distance L 1  between the opposite coils of the entrance deflection coil assembly  9  and the distance L 1  between the opposite coils of the exit deflection coil assembly  10  are set less than a half of the distance between the opposite coils of the field application coil assembly  4 . In  FIG. 1 , the distance between the opposite coils of the entrance deflection coil assembly  9  is L 1 . The distance between the opposite coils of the exit deflection coil assembly  10  is also L 1 . The distance between the opposite coils of the field application coil assembly  4  is L 2 . In this arrangement, the distance L 1  between the opposite coils of the entrance deflection coil assembly  9  or of the exit deflection assembly  10  is set less than a half of the distance L 2  between the opposite coils of the field application coil assembly  4 . 
         [0047]    There is a demand for making the magnetic field produced by the beam deflection coil assemblies  9  and  10  independent of the magnetic field generated by the field application coil assembly  4 . Accordingly, the mutual effects of the magnetic fields set up by the deflection coil assemblies  9 ,  10  and by the field application coil assembly  4  are reduced by arranging the coil assemblies in such a way that the distance L 1  between the opposite coils of the entrance deflection coil assembly  9  or of the exit deflection coil assembly  10  is less than a half of the distance L 2  between the opposite coils of the field application coil assembly  4 . This makes it possible to accurately align the electron beam with the axis. 
         [0048]    As described in detail so far, according to the present invention, deflection coil assemblies are mounted on the entrance side and on the exit side, respectively, adjacently to a field application coil assembly. The amount of correction needed to correct the deflection caused by the magnetic field application is only a half of the deflection angle on each side. Consequently, the electron beam can be suppressed from greatly deviating from the optical axis. The present invention is effective for avoiding the effects of aberrations in cases where magnetic domains are observed while restricting the electron beam. 
         [0049]    Furthermore, in the above embodiments, the coil system consisting of the entrance deflection coil assembly, field application coil assembly, and exit deflection coil assembly can be reduced in size as a whole. 
         [0050]    Having thus described my invention in the detail and particularity required by the Patent Laws, what is desired protected by Letters Patent is set forth in the following claims.