Abstract:
A charged-particle beam instrument (such as a transmission electron microscope) which facilitates modifying the diameters of aperture stops installed above and below (on the beam entrance and exit sides) the specimen chamber and exchanging the aperture stops. The instrument has bottom and bottom polepieces forming the specimen chamber, aperture stops each having plural holes, pushing mechanisms for pushing the aperture stops against the polepieces and supporting the stops, and stop drive mechanisms for sliding the aperture stops in a direction perpendicular to the path of the beam in response to a manipulation performed outside the electron optical column. The aperture stops are made of a metal foil or sheet and provide a cover over the opening of at least one beam passage hole in the polepieces that faces into the specimen chamber.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a charged-particle beam instrument, such as a transmission electron microscope (TEM), for observing or analyzing a specimen by irradiating the specimen accommodated in a specimen chamber with an electron beam. More specifically, the present invention relates to a charged-particle beam instrument permitting one to observe the process of a reaction of a specimen with a gas when the specimen is placed in an ambient of the gas. 
         [0003]    2. Description of Related Art 
         [0004]    A charged-particle beam instrument, such as a transmission electron microscope (TEM), is sometimes required to make in situ dynamic observation of a specimen placed in an ambient of a gas. For example, in the field of research on catalysts, dynamic observation of a process in which particles of a catalyst react with a gas and undergo changes plays a great role in improving the catalyst. Furthermore, it is expected that making a dynamic observation to understand how a material is varied by a gas will be helpful to research on corrosion of the material, for example, due to environmental pollution gases and to improvements of the material. Furthermore, in observations of biological specimens, if a specimen is placed in a vacuum, the specimen dries. Therefore, observing a specimen after it has been placed in a given ambient gas is an important technique. 
         [0005]    Where a specimen is placed in an ambient of a gas and observed dynamically with an electron microscope by the prior art technique, the used electron microscope is a specially designed microscope in which the specimen chamber itself of the microscope is a chamber containing a gaseous ambient as described in JP-A-2003-187735. 
         [0006]      FIG. 7  is a vertical cross section of the electron optical column of a conventional transmission electron microscope (TEM) that is one kind of charged-particle beam instrument. 
         [0007]    As shown in  FIG. 7 , the electron optical column indicated by reference numeral  101  is made of a substantially cylindrical outer cylinder  102 . If necessary, the inside of the outer cylinder  102  is partitioned into discrete zones by partition walls. Near the center axis of the electron optical column  101 , the portions constituting a flight path  105  of an electron beam  103  form an axially continuous space that is evacuated to a vacuum. 
         [0008]    An electron source  104  is mounted inside the electron optical column  101  and emits the electron beam  103 . A condenser lens  106 , an objective lens  107 , and other electron lenses for diffusing and converging the electron beam  103  utilizing magnetic or electric fields are incorporated in the electron optical column  101 . Furthermore, an aperture stop  108  for removing unwanted electrons and shaping the electron beam  103  is mounted in the electron optical column  101 . An aperture stop driver  109  for adjusting the position of the aperture stop  108  is also mounted in the column. A specimen holder  111  that is a stage on which a specimen  110  to be observed and analyzed is placed is also mounted in the column. A specimen holder driver  112  for adjusting the position of the specimen holder  111  is also mounted in the column. 
         [0009]    The specimen holder  111  has a front-end portion placed within the specimen chamber  113  that is located within the electron optical column  101 . The specimen  110  is introduced into the specimen chamber  113 , where observation and analysis are performed. Normally, the specimen chamber  113  is formed inside the objective lens  107 . 
         [0010]      FIG. 8  is a vertical cross section showing the configuration of the objective lens  107  of the conventional transmission electron microscope (TEM). The objective lens  107  is made up of a yoke  114 , an excitation coil  115 , and a polepiece assembly  116  inside the outer cylinder  102  as shown in  FIG. 8 . The yoke  114  is made of a material having a high magnetic permeability. The yoke is fabricated by closing the upper and lower ends of a double cylinder. An upper portion of the inner cylinder has been cut out. A beam passage hole  117  is formed in the center of the yoke  114  and extends in the up-and-down direction. The electron beam  103  passes through the passage hole  117 . The excitation coil  115  is received between the two cylinders of the yoke  114 . The coil  115  is fabricated by cylindrically winding many turns of a metal wire, such as a copper wire, coated with an insulator. 
         [0011]    The polepiece assembly  116  of the objective lens is made of a material having a high magnetic permeability, and is shaped substantially cylindrically. The polepiece assembly  116  is made up of a top polepiece  118 , a bottom polepiece  119 , and a spacer  120  made of a nonmagnetic material, such as a copper alloy. Each of the top polepiece  118  and bottom polepiece  119  has a protrusive portion obtained by trimming away a conic top portion. The protrusive portions of the top polepiece  118  and the bottom polepiece  119  are opposite to each other and spaced apart a given distance. The top polepiece  118  and bottom polepiece  119  have beam passage holes  121  and  122 , respectively, permitting passage of the electron beam  103 . The holes  121  and  122  extend in the up-and-down direction. The spacer  120  connects the top polepiece  118  and the bottom polepiece  119  while maintaining the space between them. The polepiece assembly  116  of the objective lens is mounted in an upper cutout portion of the inner cylinder of the yoke  114 . 
         [0012]    In the transmission electron microscope, a specimen  110  is inserted in the specimen chamber  113  and observed or analyzed, the chamber  113  being located between the opposite top polepiece  118  and bottom polepiece  119 . The position at which the specimen  110  is installed can be termed the specimen observation position  123 . Inside the yoke  114 , a specimen stage  124  made of a nonmagnetic material is mounted at a side of the specimen chamber  113 . Plural connection flanges  125  radially extend through the outer cylinder  102  and yoke  114  on the side of the outer periphery of the specimen stage  124 . The specimen holder driver  112  and pipes (not shown) for vacuum pumping are held to the connection flanges  125 . 
         [0013]    The specimen stage  124  is provided with a hole  126  extending through it. The hole  126  is formed to extend toward the specimen observation position  123 . The specimen holder  111  can be introduced via the hole  126  to bring the specimen  110  into the observation position  123  or the observation position  123  can be evacuated to a vacuum. If necessary, a through-hole  127  for the specimen holder  111  is formed in a side portion of the spacer  120 . 
         [0014]    Required connected portions of the components of the objective lens  107  are hermetically sealed with O-rings  128 ,  129 ,  130 ,  131 ,  132 , and  133 . The space around the specimen observation position  123  is maintained as a vacuum. The space including the observation position  123  and maintained as a vacuum is referred to as the specimen chamber  113 . 
         [0015]    In the objective lens  107 , the specimen chamber  113  is evacuated to a vacuum via the through-hole  126  in the specimen stage  124 . The spaces above and below the polepiece assembly  116  of the objective lens are also evacuated to a vacuum with other vacuum pumping piping (not shown). In this case, all vacuum pipes branch off from the same main pipe. The beam passage holes  121  and  122  formed in the top polepiece  118  and bottom polepiece  119  are sufficiently large and so the degree of vacuum is substantially uniform from location to location. 
         [0016]      FIG. 9  is a vertical cross section showing the structures of main portions of an objective lens  107 ′ incorporated in a transmission electron microscope (TEM) having a conventional specimen chamber containing an ambient of a gas. This objective lens  107 ′ has a top polepiece  118  and a bottom polepiece  119  provided with beam passage holes  121  and  122 , respectively, as shown in  FIG. 9 . Aperture stops  135 ,  136 ,  137 , and  138  may be mounted above and below (locations closer to and remote from the specimen  110 ) the beam passage holes  121  and  122 . Each aperture stop is a disk made of a nonmagnetic metal centrally provided with a small orifice. The aperture stops  135  and  136  closer to the specimen  110  are referred to as the first upper aperture stop and the first lower aperture stop, respectively. The aperture stops  137  and  138  further from the specimen are referred to as the second upper aperture stop and the second lower aperture stop, respectively. Each aperture stop is mounted to the top polepiece  118  and bottom polepiece  119  such that the connections are made as hermetic as possible. For example, each aperture stop is bonded with a conductive adhesive. 
         [0017]    In this case, the space formed between the first upper aperture stop  135  and the second upper aperture stop  137  is referred to as the upper intermediate chamber  139 . The space formed between the first lower aperture stop  136  and the second lower aperture stop  138  is referred to as the lower intermediate chamber  140 . The upper intermediate chamber  139  is in communication with the vacuum space located above the polepiece assembly  116  of the objective lens and with the specimen chamber  113  only via small aperture holes  141  formed in the second upper stop  137  and first upper aperture stop  135 . Similarly, the lower intermediate chamber  140  is in communication with the vacuum space located below the polepiece assembly  116  of the objective lens and with the specimen chamber  113  only via small aperture holes  141  formed in the second lower aperture stop  138  and first lower aperture stop  136 . 
         [0018]    An upper differential pumping tube  142  and a lower differential pumping tube  143  extend through the outer cylinder  102 , yoke  114 , specimen stage  124 , and spacer  120  and are connected with the upper intermediate chamber  139  and lower intermediate chamber  140 , respectively. Required portions are kept hermetic with O-rings  144 . 
         [0019]    In the objective lens  107 ′, the pumping system for evacuating the specimen chamber  113  is separate from the upper differential pumping tube  142  and lower differential pumping tube  143  which evacuate the upper intermediate chamber  139  and lower intermediate chamber  140 , respectively. The pumping system for evacuating the specimen chamber  113  is separate from pumping systems for evacuating the spaces located above and below the polepiece assembly  116  of the objective lens. The aperture holes  141  are present among the specimen chamber  113 , upper intermediate chamber  139 , lower intermediate chamber  140 , and the spaces located above and below the polepiece assembly  116  of the objective lens. Because admission and venting of gas are limited, pressure differences can be created among the spaces. Accordingly, in the objective lens  107 ′, a trace amount of arbitrary gas can be introduced into the specimen chamber  113  through the specimen stage  124  such that the degrees of vacuum in the spaces located above and below the polepiece assembly  116  of the objective lens are hardly affected. The specimen  110  can be observed and analyzed under arbitrary gaseous environments. 
         [0020]    The above-described charged-particle beam instrument has the problem that it is difficult to exchange the first upper aperture stop  135  and first lower aperture stop  136 . That is, if the diameter of the aperture hole  141  is varied or if contamination has occurred due to the charged-particle beam, it is necessary to exchange the first upper aperture stop  135  and first lower aperture stop  136 . However, the first upper aperture stop  135  and first lower aperture stop  136  are adhesively bonded to the sides of the top polepiece  118  and bottom polepiece  119  facing the specimen chamber  113 . Therefore, it is impossible to exchange them unless the polepiece assembly  116  of the objective lens is taken out, for example, by disassembling the electron optical column  101 . Consequently, it is difficult to exchange the first aperture stops. In addition, there is the problem that it is impossible to adjust the positions of the first upper aperture stop  135  and first lower aperture stop  136  or to vary the diameter during microscopic examination. 
         [0021]    Additionally, the charged-particle beam instrument has the problem that it is difficult to maintain the accuracy of the positions at which the first upper aperture stop  135  and first lower aperture stop  136  are mounted. In particular, the first aperture stops  135  and  136  are held in position after the polepiece assembly  116  of the objective lens has been taken out of the electron optical column  101 . Hence, it is difficult that their positions within the electron optical column  101  are previously checked and that the first aperture stops are placed in position. 
         [0022]    Moreover, in this charged-particle beam instrument, the first aperture stops  135  and  136  are kept on the passage of the electron beam  103 . Therefore, there is the problem that it is difficult to use the instrument if a method not using the aperture stops  135  and  136  is employed. That is, when it is necessary to remove the first aperture stops  135  and  136 , the polepiece assembly  116  of the objective lens must be taken out, for example, by disassembling the electron optical column  101 . 
       SUMMARY OF THE INVENTION 
       [0023]    Accordingly, the present invention has been proposed in view of the foregoing circumstances. It is an object of the present invention to provide a charged-particle beam instrument, such as a transmission electron microscope (TEM), that facilitates varying the diameters of aperture stops installed above and below (i.e., on the beam entrance side and exit side) a specimen chamber and exchanging the aperture stops. 
         [0024]    A charged-particle beam instrument associated with the present invention is used to observe or analyze a specimen by receiving the specimen in a specimen chamber formed in an electron optical column and irradiating the specimen with an electron beam. According to one embodiment, the charged-particle beam instrument has: top and bottom polepieces disposed on the beam entrance side and exit side, respectively, of the specimen chamber and is provided with beam passage holes, the top and bottom polepieces forming the specimen chamber; an upper aperture stop made of metal foil or sheet having plural holes, the upper aperture stop being located on a side of an opening end of the beam passage hole in the top polepiece that faces into the specimen chamber, the top polepiece being located on the beam entrance side of the specimen chamber; a first pushing mechanism for pushing the upper aperture stop toward the top polepiece and supporting the upper aperture stop, the pushing mechanism stopping from pushing the upper aperture stop toward the top polepiece in response to a manipulation performed outside the electron optical column; an upper stop drive mechanism for sliding the upper aperture stop in a direction perpendicular to a path of the electron beam in response to a manipulation performed outside the electron optical column; a lower aperture stop made of metal foil or sheet having plural holes, the lower aperture stop being located on a side of an opening end of the beam passage hole in the bottom polepiece that faces into the specimen chamber, the bottom polepiece being located on the beam exit side of the specimen chamber; a second pushing mechanism for pushing the lower aperture stop toward the bottom polepiece and supporting the lower aperture stop, the second pushing mechanism stopping from pushing the lower aperture stop toward the bottom polepiece in response to a manipulation performed outside the electron optical column; and a lower aperture stop drive mechanism for sliding the lower aperture stop in the direction perpendicular to the path of the electron beam in response to a manipulation performed outside the electron optical column. 
         [0025]    In the charged-particle beam instrument according to one embodiment of the present invention, the upper and lower aperture stops are stopped from pushing the first and second pushing mechanisms against the polepieces when a manipulation is performed outside the electron optical column. The stop drive mechanisms slide the aperture stops in the direction perpendicular to the path of the electron beam. The beam passage holes can be switched between a mode in which the holes are narrowed and a mode in which the holes are opened without breaking the vacuum in the specimen chamber. That is, the inside of the specimen chamber can be switched between a vacuum state and a desired gaseous ambient by performing an operation outside the electron optical column. 
         [0026]    In this charged-particle beam instrument, one of the holes in the stops can be selected by performing an operation outside the electron optical column. During differential pumping, the conductance can be adjusted. Where the aperture stops are contaminated, the aperture stops can be exchanged or the used aperture stop can be switched. Furthermore, in the charged-particle beam instrument, the positions of the aperture stops can be adjusted accurately by performing an operation outside the electron optical column. 
         [0027]    In addition, in the charged-particle beam instrument, the aperture stops are supported by the stop drive mechanisms. Consequently, the aperture stops can be taken out of the electron optical column without disassembling the column. 
         [0028]    That is, the present invention provides a charged-particle beam instrument, such as a transmission electron microscope (TEM), that facilitates varying the diameters of aperture stops installed above and below (i.e., beam entrance side and exit side) the specimen chamber and exchanging the aperture stops. 
         [0029]    Other objects and features of the invention will appear in the course of the description thereof, which follows. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0030]      FIG. 1  is a vertical cross section of main portions of a charged-particle beam instrument associated with one embodiment of the present invention, showing the structures of the main portions; 
           [0031]      FIG. 2  is a vertical cross section of a specimen chamber formed in the charged-particle beam instrument showing the configuration of the specimen chamber; 
           [0032]      FIG. 3  is a plan view of first aperture stops included in the charged-particle beam instrument associated with the invention, showing the structures of the first aperture stops; 
           [0033]      FIG. 4  is a vertical cross section of main portions of a fourth embodiment of the charged-particle beam instrument showing the structures of the main portions; 
           [0034]      FIG. 5  is a vertical cross section of main portions of a fifth embodiment of the charged-particle beam instrument showing the structures of the main portions; 
           [0035]      FIG. 6  is a vertical cross section of main portions of a sixth embodiment of the charged-particle beam instrument showing the structures of the main portions; 
           [0036]      FIG. 7  is a vertical cross section of the electron optical column of a conventional transmission electron microscope; 
           [0037]      FIG. 8  is a vertical cross section of the objective lens of the conventional transmission electron microscope showing the structure of the objective lens; and 
           [0038]      FIG. 9  is a vertical cross section of main portions of the objective lens of the conventional transmission electron microscope showing the structures of the main portions. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0039]    The preferred embodiments of the present invention are hereinafter described with reference to the accompanying drawings. 
       First Embodiment 
       [0040]    A charged-particle beam instrument associated with the present invention comprises a transmission electron microscope (TEM). The inside of the specimen chamber is differentially pumped to place the specimen into a desired ambient of a gas. Thus, the instrument permits in situ dynamic observation of images of the specimen. In this embodiment, the charged-particle beam instrument associated with the present invention is built as a transmission electron microscope (TEM). 
         [0041]    The charged-particle beam instrument has an electron optical column  101  in the same way as the conventional charged-particle beam instrument already described in connection with  FIG. 7 . The column  101  is made of a substantially cylindrical outer cylinder  102  as shown in  FIG. 7 . The inside of the outer cylinder  102  is partitioned into discrete zones by partition walls as the need arises. Vicinities of the center axis of the electron optical column  101  which are in the flight path  105  of the electron beam  103  form an axially continuous space, which is evacuated to a vacuum. 
         [0042]    An electron source  104  is mounted inside the electron optical column  101  and emits the electron beam  103 . A condenser lens  106  and an objective lens  107  that are electron lenses for diffusing and converging the electron beam  103  utilizing magnetic or electric fields are incorporated in the electron optical column  101 . Furthermore, an aperture stop  108  for removing unwanted electrons and shaping the electron beam  103  is mounted in the electron optical column  101 . An aperture stop driver  109  for adjusting the position of the aperture stop  108  is also mounted in the column. A specimen holder  111  that is a stage on which the specimen  110  to be observed and analyzed is placed is also mounted in the column. 
         [0043]    The specimen holder  111  has a front-end portion disposed inside the specimen chamber  113  located within the electron optical column  101 . The specimen  110  is introduced into the specimen chamber  113 , where observation and analysis are performed. The specimen chamber  113  is formed inside the objective lens  107 . 
         [0044]      FIG. 1  is a vertical cross section of main portions of a charged-particle beam instrument according to one embodiment of the present invention, showing the structures of the main portions. As shown in  FIG. 1 , the objective lens  107  is made up of a yoke  14 , an excitation coil  15 , and a polepiece assembly  16  inside the outer cylinder  102 . The yoke  14  is made of a material having a high magnetic permeability. The yoke is fabricated by closing the upper and lower ends of a double cylinder. An upper portion of the inner cylinder has been cut out. A beam passage hole  17  is formed in the center of the yoke  14  and extends in the up-and-down direction. The electron beam  103  passes through the passage hole  17 . The excitation coil  15  is received between the two cylinders of the yoke  14 . The coil  15  is fabricated by cylindrically winding many turns of a metal wire, such as a copper wire, coated with an insulator. 
         [0045]    The polepiece assembly  16  of the objective lens is made of a material having a high magnetic permeability, and is shaped substantially cylindrically. The polepiece assembly  16  of the objective lens is made up of a top polepiece  18 , a bottom polepiece  19 , and a spacer  20  made of a nonmagnetic material, such as a copper alloy. Each of the top polepiece  18  and bottom polepiece  19  has a protrusive portion obtained by trimming away a conic top portion. The protrusive portions of the top polepiece  18  and the bottom polepiece  19  are opposite to each other and spaced apart a given distance. The top polepiece  18  and bottom polepiece  19  have beam passage holes  21  and  22 , respectively, permitting passage of the electron beam  103 . The holes  21  and  22  extend in the up-and-down direction. The spacer  20  connects the top polepiece  18  and the bottom polepiece  19  while maintaining the given space between them. The polepiece assembly  16  of the objective lens is mounted in an upper cutout portion of the inner cylinder of the yoke  14 . 
         [0046]    In the transmission electron microscope, a specimen  110  is inserted in the specimen chamber  113  and observed or analyzed, the chamber  113  being located between the opposite top polepiece  18  and bottom polepiece  19 . Inside the yoke  14 , a specimen stage  24  made of a nonmagnetic material is mounted at a side of the specimen chamber  113 . Plural connection flanges  25  radially extend through the outer cylinder  102  and yoke  14  on the side of the outer periphery of the specimen stage  24 . The specimen holder driver  112  and tubes (not shown) for vacuum pumping are held to the connection flanges  25 . 
         [0047]    The specimen stage  24  is provided with a hole  26  extending through it. The hole  26  is formed to extend toward the specimen chamber  113 . The specimen holder  111  can be introduced via the hole  26  to introduce the specimen  110  into the specimen chamber  113  or the specimen chamber  113  can be evacuated to a vacuum. A through-hole  27  for the specimen holder  111  is formed in a side portion of the spacer  20 . 
         [0048]    Required portions of the joints of the components of the objective lens  107  are hermetically sealed with O-rings. The space around the specimen chamber  113  is maintained as a vacuum. 
         [0049]    In the objective lens  107 , the specimen chamber  113  is evacuated to a vacuum via the through-hole  26  in the specimen stage  24 . The spaces above and below the polepiece assembly  16  of the objective lens are also evacuated to a vacuum with other vacuum pumping piping (described later). In this case, all vacuum pipes are independent vacuum pumping systems. 
         [0050]      FIG. 2  is a vertical cross section of the specimen chamber of the charged-particle beam instrument associated with the present invention, showing the configuration of the specimen chamber. The objective lens  107  has the polepiece assembly  16  including the top polepiece  18  and bottom polepiece  19 . As shown in  FIG. 2 , the polepieces  18  and  19  have flat portions facing the specimen  110 . The flat portions are referred to as top-polepiece vertex surface  45  and bottom-polepiece vertex surface  46 , respectively. An upper stop-pushing plate  48  forming a first pushing mechanism is placed in intimate contact with the top-polepiece vertex surface  45 . The upper stop-pushing plate  48  is made of a nonmagnetic metal material and assumes a form of a flat plate. The pushing plate  48  is provided with a beam passage hole  47  in a portion corresponding to the beam passage hole  21  in the top polepiece  18 . The beam passage hole  47  in the upper stop-pushing plate  48  is large enough to enable normal usage of the TEM. A pair of handle-like portions  49  extends upwardly and externally from the flat surface of the upper stop-pushing plate  48  in intimate contact with the top-polepiece vertex surface  45 . A first upper stop insertion hole  50  that is a horizontally elongated slit is formed in the side surface of each handle-like portion  49 . A first upper aperture stop  35  is inserted into the first upper aperture stop insertion holes  50 . 
         [0051]    The handle-like portions  49  of the upper stop-pushing plate  48  are connected to the spacer  20  near the top portion of the top polepiece  18  via two pairs of tension springs  51  constituting a second pushing mechanism. The upper stop-pushing plate  48  is pushed into intimate contact with the top-polepiece vertex surface  45  by the tensile force from the tension springs  51 . 
         [0052]    A pair of actuators  52  forming a third pushing mechanism utilizing a piezoelectric device, for example, is mounted between the handle-like portions  49  of the upper stop-pushing plate  48  and the spacer  20  such that almost no electric or magnetic field is produced during operation. When the actuators  52  are not in operation, the upper stop-pushing plate  48  is kept in intimate contact with the top-polepiece vertex surface  45 . During operation of the actuators  52 , respective one ends of the actuators  52  push down their respective handle-like portions  49 , bringing the upper stop-pushing plate  48  out of intimate contact with the top-polepiece vertex surface  45 . As a result, a slight gap is formed between the plate  48  and the vertex surface  45 . The actuators  52  can be operated in a sequence opposite to the above-described sequence of operations. Preferably, the tensile springs  51  and actuators  52  are made of a feebly magnetic material or nonmagnetic conductive material that affects neither the magnetic field produced by the objective lens  107  nor the electron beam  103 . 
         [0053]    The bottom polepiece  19  is similar in structure with the top polepiece  18 . The bottom polepiece  19  and top polepiece  18  are arranged symmetrically with respect to the specimen  110 . That is, a bottom stop-pushing plate  48 ′ is placed in intimate contact with the bottom polepiece vertex surface  46  of the bottom polepiece  19 . 
         [0054]      FIG. 3  is a plan view of the first aperture stops of the charged-particle beam instrument associated with the present invention, showing the structures of the first aperture stops. The first aperture stop  35  is sandwiched between the top-polepiece vertex surface  45  and the upper stop-pushing plate  48 . The first lower aperture stop  36  is sandwiched between the bottom polepiece vertex surface  46  and the lower stop-pushing plate  48 ′. As shown in  FIG. 3 , each of the first aperture stops  35  and  36  is provided with plural aperture holes  41  and shaped like stripes. The first aperture stops  35  and  36  are inserted between the polepiece vertex surfaces  45 ,  46  and the stop-pushing plates  48 ,  48 ′ from the first upper and lower stop insertion holes  50 . The first aperture stops  35  and  36  are made of nonmagnetic metal foil or sheet. The aperture holes  41  in the first aperture stops  35  and  36  are formed along the longitudinal center line. One of the aperture holes  41  is comparable in size with the beam passage holes  47  in the stop-pushing plates  48  and  48 ′ and used as an open hole. The remaining aperture holes  41  can have any arbitrary size but are smaller than the beam passage holes  47  in the stop-pushing plates  48  and  48 ′. According to desired operation conditions, plural hole diameters may be selected for the aperture holes  41 , and the holes  41  may be machined. It is not required that the beam passage holes  41  in the first upper aperture stop  35  be identical in size and number with the beam passage holes  41  in the first lower aperture stop  36 . 
         [0055]    The tensile springs  51 , actuators  52 , and first aperture stops  35 ,  36  are preferably arranged such that they are made parallel as much as possible to the axis of the specimen holder  111  to prevent the operation of the specimen holder  111  consisting of rotating about its axis and tilting. 
         [0056]    As shown in  FIG. 1 , respective one ends of the first upper and lower aperture stops  35  and  36  are connected to first upper and lower aperture stop position-adjusting devices  54  constituting aperture stop drive mechanisms. The first aperture stop position-adjusting devices  54  introduce a connection shaft  56  into the vacuum via a member capable of being deformed while maintaining the airtightness, such as bellows  55  (see  FIG. 1 ). The bellows have front-end portions connected with the first upper and lower aperture stops  35  and  36 . The stop position-adjusting devices  54  are hermetically held to the connection flanges  25 . 
         [0057]    In this charged-particle beam instrument, the first upper and lower aperture stops  35  and  36  can be moved in a direction perpendicular to the path of the electron beam  103  as indicated by the arrow A in  FIG. 3  by manipulating the first aperture stop position-adjusting devices  54  from outside of the enclosure or column  101 . Thus, the positions can be adjusted. 
         [0058]    The charged-particle beam instrument further includes a second upper aperture stop  37  and a second lower aperture stop  38  as shown in  FIG. 2 . The second aperture stops  37  and  38  are cylindrically-shaped members and have front-end portions that are open. The second aperture stops  37  and  38  are inserted in upper and lower differential pumping tubes  42  and in the top and bottom polepieces  18  and  19 , respectively. That is, the differential pumping tubes  42  are tubular and have closed front-end portions. Top and bottom polepiece venting holes  57  extend through side surfaces of the top and bottom polepieces  18  and  19 , respectively. The pumping tubes  42  are inserted in the venting holes  57 , respectively. The venting holes  57  intersect the beam passage holes  21  and  22  in the top and bottom polepieces  18  and  19 , respectively. Hermeticity is maintained between the pumping tubes  42  and polepieces  18 ,  19  by O-rings. The second upper and lower aperture stops  37  and  38  are inserted in the upper and lower differential pumping tubes  42 , respectively. 
         [0059]    The portions of the differential pumping tubes  42  which intersect the beam passage holes  21  and  22  are provided with upper and lower differential pumping tube holes  61  and  62 , respectively, which are large enough to enable normal use of the TEM. Preferably, the differential pumping tube hole  62  on the side of the specimen chamber  113  is maximized in size. 
         [0060]    The second upper and lower aperture stops  37  and  38  have front-end portions inserted in the top and bottom polepieces  18  and  19 , respectively. The other ends of the aperture stops  37  and  38  are closed. The relationship of the outside diameter of the second aperture stops  37  and  38  relative to the inside diameter of the upper and lower differential pumping tubes  42  is so set that the second aperture stops  37  and  38  are slidably fitted in the pumping tubes  42  with a minimum gap therebetween. The second upper and lower aperture stops  37  and  38  are provided with the aperture holes  41  on the sides of the upper holes  61  of the differential pumping tubes, the aperture holes  61  being on the opposite side of the specimen chamber  113 . The second upper and lower aperture stops  37  and  38  are provided with upper and lower intermediate chamber venting holes  63  on the side of the specimen chamber  113 . Preferably, the diameters of the venting holes  63  are maximized. 
         [0061]    The closed other ends of the upper and lower second aperture stops  37  and  38  are brought out of the objective lens  107  as shown in  FIG. 1  and connected with the second upper and lower aperture stop position-adjusting devices  64 , respectively. The positions of the aperture holes  41  in the second aperture stops  37  and  38  can be adjusted by manipulating the second aperture stop position-adjusting devices  64 . The aperture stop position-adjusting devices  64  are connected with connection ports  65  formed in the upper and lower differential pumping tubes  42  while the airtightness is maintained by O-rings  66 . 
         [0062]    Vicinities of the connection ports  65  in the upper and lower differential pumping tubes  42  are connected to the vacuum pipe  68 . Venting holes  67  opening into the vacuum pipe  68  are formed in side surfaces of the second upper and lower aperture stops  37  and  38  on the other side. 
         [0063]    Preferably, the second aperture stops  37  and  38  and the differential pumping tubes  42  are made of a feebly magnetic material or nonmagnetic conductive material that affects neither the magnetic field produced by the objective lens  107  nor the electron beam  103 . 
         [0064]    Where the charged-particle beam instrument is used as a normal transmission electron microscope, the actuators  52  are driven to place the upper and lower aperture stop-pushing plates  48  and  48 ′ at a distance of about tens of micrometers to hundreds of micrometers from the top and bottom polepiece surfaces  45  and  46 . The first upper and lower aperture stops  35  and  36  are opened to make them movable. The first upper and lower aperture stop position-adjusting devices  54  are manipulated to align the open holes in the first upper and lower aperture stops  35  and  36  with the beam passage holes  21  and  22  in the top and bottom polepieces  18  and  19 . After completion of the alignment, the actuators  52  are stopped from being driven. The first upper and lower aperture stops  35  and  36  are again held to the top and bottom polepiece surfaces  45  and  46  by the upper and lower aperture stop-pushing plates  48  and  48 ′, respectively. That is, the first upper and lower aperture stops  35  and  36  are disposed to cover the open ends of the beam passage holes in the top and bottom polepieces  18  and  19  which face into the specimen chamber. 
         [0065]    The second upper and lower aperture stops  37  and  38  are slid within the upper and lower differential pumping tubes  42  outwardly of the objective lens  107  by manipulating the second upper and lower aperture stop position-adjusting devices  64  until the front ends of the second aperture stops  37  and  38  come close to the outer peripheries of the beam passage holes  21  and  22  in the top and bottom polepieces  18  and  19 . 
         [0066]    Because of the operations described so far, the beam passage holes  21  and  22  are opened to a sufficient degree, together with the top polepiece  18  and bottom polepiece  19 . The transmission electron microscope can be used by a normal method of microscopic examination. In this case, the aperture holes  41  can be used as an ordinary aperture stop for cutting out a part of the electron beam  103  or shaping it by moving any one of the first upper and lower aperture stops  35  and  36  by a procedure similar to the foregoing procedure and bringing the aperture holes  41  onto the flight path  105  of the electron beam  103 . 
         [0067]    Where the specimen  110  is observed and analyzed under an ambient of a gas, the gas is supplied into the specimen chamber  113 . The first upper and lower aperture stops  35  and  36  are then moved by a procedure similar to the foregoing procedure to align arbitrary small aperture holes  41  with the beam passage holes  21  and  22 , respectively. At this time, the first upper aperture stops  35  and  36  are pushed into intimate contact with the top and bottom polepiece surfaces  45  and  46 , respectively, by the flat portions of the stop-pushing plates  48  and  48 ′, respectively. Consequently, it is possible to have large pumping resistance (conductance). 
         [0068]    The second upper and lower aperture stops  37  and  38  are slid inwardly of the objective lens  107  and inserted into it within the upper and lower differential pumping tubes  42  by manipulating the second upper and lower aperture position-adjusting devices  64  to align the aperture holes  41  with the centers of the beam passage holes  21  and  22  in the top and bottom polepieces  18  and  19 , respectively. 
         [0069]    The evacuated inside of the electron optical column  101  is divided into the specimen chamber  113 , an upper middle chamber  39  (located between the first upper aperture. stop  35  and the second upper aperture stop  37 ), a lower middle chamber  40  (located between the first lower aperture stop  36  and the second lower aperture stop  38 ), and vacuum chambers located, respectively, above and below the polepiece assembly  16  of the objective lens as shown in  FIG. 2  because of the operations described so far. These chambers are connected to each other with large pumping resistance via the aperture holes  41 . Accordingly, the specimen  110  can be observed and analyzed under an ambient of an arbitrary gas such that the degrees of vacuum in the vacuum chambers located, respectively, above and below the polepiece assembly  16  of the objective lens are hardly deteriorated, even if a trace amount of the arbitrary gas is introduced into the specimen chamber  113 , by adjusting the pumping speeds in the vacuum chambers, respectively. 
       Second Embodiment 
       [0070]    In the charged-particle beam instrument associated with the present invention, the first upper aperture stop  35  and the first lower aperture stop  36  may be plated with a soft metal, such as gold. In this case, the degree of intimateness between the first upper aperture stop  35  and the first lower aperture stop  36  can be improved. Similarly, the degree of intimateness between the top-polepiece vertex surface  45  and the bottom-polepiece vertex surface  46  can be improved. 
       Third Embodiment 
       [0071]    In the charged-particle beam instrument associated with the present invention, the aperture holes  41  formed in the first upper aperture stops  35  and  36  may be all identical in diameter. In this case, where the aperture hole  41  used during microscopic examination is contaminated by the effects of the introduced gas, the used aperture hole  41  may be exchanged in turn by the procedure described above. 
       Fourth Embodiment 
       [0072]      FIG. 4  is a vertical cross section of main portions of a fourth embodiment of the charged-particle beam instrument associated with the present invention, showing the structures of the main portions. As shown in  FIG. 4 , in the charged-particle beam instrument associated with the present invention, driving shafts  70  may be introduced into the electron optical column  101  from obliquely above instead of the actuators  52 , and the upper aperture stop-pushing plate  48  may be manipulated by screws  71  and gears  72  via the driving shafts  70 . The gears  72  are driven by motors  73  disposed outside the electron optical column  101 . Preferably, the motors  73  are spaced from the electron optical column  101  by a large distance where the electron beam  103  is not affected. 
         [0073]    In this case, the gears  72  are rotated by the motors  73 . This, in turn, rotates the driving shafts  70 . The screws  71  cause the driving shafts  70  to move the upper aperture stop-pushing plates  48 . These operations create gaps between the upper aperture stop-pushing plates  48  and the polepiece vertex surfaces  45 ,  46 . Consequently, the first upper aperture stops  35  and  36  are freed. Conversely, the first upper aperture stops  35  and  36  can be held by moving the upper aperture stop-pushing plates.  48  toward the polepiece vertex surfaces  45  and  46  by means of the driving shafts  70 . 
       Fifth Embodiment 
       [0074]      FIG. 5  is a vertical cross section of main portions of a fifth embodiment of the charged-particle beam instrument associated with the present invention, showing the structures of the main portions. As shown in  FIG. 5 , in the charged-particle beam instrument associated with the present invention, a second lower aperture stop  37  may be introduced into the electron optical column  101  from obliquely above it and placed at a position located above the objective lens  107 . In this case, the second lower aperture stop  37  is moved by manipulating the second aperture stop position-adjusting device  64  that is located outside the electron optical column  101  and supports the second lower aperture stop  37 . The second lower aperture stop  37  can be switched between a state in which the aperture stop is located over the beam passage holes  21  and  22  and a state in which the stop is located not over the beam passage holes  21  and  22 . 
       Sixth Embodiment 
       [0075]      FIG. 6  is a vertical cross section similar to the cross section of  FIG. 2 , but showing the structures of main portions of a sixth embodiment of the charged-particle beam instrument associated with the present invention. 
         [0076]    Disk-like or otherwise shaped components are mounted to cover the vertex surfaces  45  and  46  of the top polepiece  18  and bottom polepiece  19 . The component mounted over the top-polepiece vertex surface  45  is referred to as the top-polepiece protective plate  80 . The component mounted over the bottom-polepiece vertex surface  46  is referred to as the bottom-polepiece protective plate  81 . The protective plates  80  and  81  are substantially centrally provided with holes  82  and  83 , respectively, extending through the plates. The holes  82  and  83  permit passage of the electron beam and vacuum pumping. The holes  82  and  83  are comparable in size with the beam passage holes  21  and  22  formed in the top polepiece  18  and bottom polepiece  19 . The holes  82  and  83  are so sized that the pumping resistances in the beam passage holes  21  and  22  are not greatly deteriorated. The polepiece protective plates  80  and  81  are made of a nonmagnetic conductive material. The protective plates  80  and  81  are bonded to the top-polepiece vertex surface  45  and bottom-polepiece vertex surface  46 , for example, with a conductive adhesive such that the joints are made hermetic. 
         [0077]    Because the shapes of the top-polepiece vertex surface  45  and bottom-polepiece vertex surface  46  are determined by electron optics calculations, it is difficult to enhance the degree of intimateness when the first upper aperture holes  35  and  36  are pressed against the polepiece vertex surfaces  45 ,  46  by devising the shapes of the vertex surfaces  45 ,  46 . Furthermore, some restrictions are imposed on the used material. Therefore, it is also difficult to make contrivances to enhance the degree of intimateness, for example, by taking account of the hardness. 
         [0078]    Accordingly, the top-polepiece protective plate  80  and bottom-polepiece protective plate  81  made of a nonmagnetic material, i.e., do not play a role as magnetic poles, are hermetically held to the top-polepiece vertex surface  45  and bottom-polepiece vertex surface  46 . Consequently, the shapes of the surfaces pressed against the first upper aperture stops  35  and  36 , their hardness, and surface treatment can be devised. When the first upper aperture stops  35  and  36  are pressed against the vertex surfaces, the airtightness can be enhanced. At the same time, contamination of the polepiece vertex surfaces  45  and  46  due to introduction of a gas can be prevented. In addition, abrasion and scratching due to fretting of the first upper and lower aperture stops  35  and  36  can be prevented. 
         [0079]    Having thus described my invention with the detail and particularity required by the Patent Laws, what is desired protected by Letters Patent is set forth in the following claims.