Patent Publication Number: US-8987682-B2

Title: Specimen positioning device, charged particle beam system, and specimen holder

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a specimen positioning device, charged particle beam system, and specimen holder. 
     2. Description of Related Art 
     In electron microscopy, in a case where a specimen is observed at high (such as atomic scale) magnification or where a specimen is analyzed at high accuracy, external disturbing vibrations present problems. Such external disturbing vibrations are produced, for example, by vibrations of a floor on which the instrument is mounted. In electron microscopy, the instrumentation is vibrated by external disturbing vibrations. As a result, problems such as vibrations of electron microscope images are caused (see, for example, JP-A-2010-113810). 
     In an electron microscope, the specimen is placed in position by a specimen positioning device. For example, JP-A-2010-157491 discloses a specimen positioning device comprising a specimen holder for supporting a specimen, an X-axis drive mechanism for translating the specimen in the X-direction, and a Y-axis drive mechanism for translating the specimen in the Y-direction. 
     The X-axis drive mechanism set forth in JP-A-2010-157491 has a lever that supports the position of a specimen along the X-axis while preventing the specimen holder from being drawn in by atmospheric pressure. The specimen is moved in the X-direction by rotating the lever. That is, the X-axis drive mechanism places the specimen in position in the X-direction by placing the specimen holder in position. 
     The Y-drive mechanism set forth in JP-A-2010-157491 has a shifter provided with a through-hole in which the specimen holder is accommodated. The specimen is moved in the Y-direction by rotating the shifter about the center of a spherical bearing. That is, with this Y-axis drive mechanism, the specimen is placed in position in the Y-direction by placing the specimen holder, shifter, and spherical bearing in position. 
     SUMMARY OF THE INVENTION 
     One object associated with some aspects of the present invention is to provide a specimen positioning device, charged particle beam system, and specimen holder capable of reducing the effects of external disturbing vibrations. 
     (1) A specimen positioning device associated with the present invention is for use in or with a charged particle beam system including a specimen chamber and has: a base provided with a hole in operative communication with the specimen chamber in the charged particle beam system; a specimen holder including a first portion and a second portion and movably mounted in the hole; and a first portion support portion that supports the first portion in the specimen chamber. The first portion of the specimen holder has a specimen holding portion capable of holding a specimen. The second portion of the specimen holder supports the first portion via a resilient member. 
     In this specimen positioning device, the first portion having the specimen support portion can be placed in position independently of the second portion. Accordingly, an object or member that should be placed in position can be reduced in weight as compared with the case where the whole specimen holder is placed in position. Consequently, the effects of external disturbing vibrations can be reduced. 
     (2) In one feature of this specimen positioning device associated with the present invention, there may be further provided a first portion drive mechanism for moving the first portion of the specimen holder via the first portion support portion. 
     (3) In a further feature of this specimen positioning device associated with the present invention, the first portion drive mechanism may have: a driving portion for rotationally driving a shaft portion; and a motion converter for converting a rotary motion of the shaft portion into a linear motion and transmitting the motion to the first portion support portion. 
     (4) In an additional feature of this specimen positioning device associated with the present invention, the shaft portion may be disposed in an opening that is in operative communication with the specimen chamber. A magnetic fluid seal may be mounted between the shaft portion and an inner surface of the opening. 
     In this specimen positioning device, the magnetic fluid seal provides sealing between the shaft portion and the inner surface of the opening without using an O-ring. In consequence, outward drift which would be normally caused by an O-ring can be prevented. 
     (5) In a still other feature of this specimen positioning device associated with the present invention, there may be further provided: a base support portion by which the base is rotatably supported; a base drive mechanism for rotating the specimen holder via the base; and a magnetic fluid seal mounted between the base support portion and the base. 
     In this specimen positioning device, the magnetic fluid seal provides sealing between the base support portion and the base without using an O-ring. In consequence, outward drift which would be normally caused by an O-ring can be prevented. 
     (6) In a yet other feature of this specimen positioning device associated with the present invention, there may be further provided an O-ring which is mounted in the second portion of the specimen holder and which seals between this second portion and an inner surface of the hole. 
     In this specimen positioning device, if the first portion moves, the O-ring does not slide. Consequently, outward drift which would be normally caused by an O-ring can be prevented. 
     (7) In a still other feature of this specimen positioning device associated with the present invention, the specimen holder may have a third portion connected to the second portion of the specimen holder, and the third portion may have a width greater than the diameter of the hole. 
     In this specimen positioning device, the third portion can be held against the base by a force created by the pressure difference between the pressure inside the specimen chamber and atmospheric pressure. Therefore, the force exerted on the specimen holder by the pressure difference between the pressure inside the specimen chamber and atmospheric pressure can be borne by the base. Accordingly, movement of the specimen holder due to variations in atmospheric pressure can be suppressed. Thus, drift of the specimen can be suppressed. 
     (8) A charged particle beam system associated with the present invention includes a specimen positioning device associated with the present invention. 
     This charged particle beam system can reduce the effects of external disturbing vibrations because the system includes the specimen positioning device associated with the invention. 
     (9) A specimen holder associated with the present invention is for use with a charged particle beam system and has: a first portion including a specimen holding portion capable of holding a specimen; and a second portion that supports the first portion via a resilient member. 
     In this specimen holder, the first portion including the specimen holding portion can be placed in position independently of the second portion. Accordingly, an object or member that should be placed in position can be reduced in weight as compared with the case where the whole specimen holder is placed in position. Consequently, the effects of external disturbing vibrations can be reduced. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic cross portion of a specimen positioning device associated with a first embodiment of the present invention. 
         FIG. 2  is an enlarged view of a part of  FIG. 1 . 
         FIG. 3  is a schematic view of a specimen holder of the specimen positioning device shown in  FIG. 1 . 
         FIG. 4  is a schematic cross portion of a universal coupling of the specimen positioning device shown in  FIG. 1 . 
         FIG. 5  is a diagram illustrating outward drift. 
         FIG. 6  is a diagram illustrating a vibration model of a system having one degree of freedom. 
         FIG. 7  is a schematic view of a specimen holder of a specimen positioning device associated with a modification of the first embodiment. 
         FIG. 8  is a cross-portional view taken on line VIII-VIII of  FIG. 7 . 
         FIG. 9  is a vertical cross portion of a charged particle beam system associated with a second embodiment of the present invention. 
     
    
    
     DESCRIPTION OF THE INVENTION 
     The preferred embodiments of the present invention are hereinafter described in detail with reference to the drawings. It is to be understood that the embodiments described below do not unduly restrict the contents of the present invention delineated by the claims and that not all the configurations described below are essential constituent components of the invention. 
     1. First Embodiment 
     1.1. Configuration of Specimen Positioning Device 
     The configuration of a specimen positioning device associated with a first embodiment of the present invention and used in or with a charged particle beam system is first described with reference to some drawings. The specimen positioning device is generally indicated by reference numeral  100  in  FIG. 1  that is a schematic cross portion of the specimen positioning device. FIG.  2  is an enlarged view of the vicinities of the front end of a specimen holder  20 . In  FIGS. 1 and 2 , X-, Y-, and Z-axes are shown to cross each other perpendicularly at an origin O. 
       FIGS. 1 and 2  show the state in which the specimen positioning device  100  is in use i.e., the specimen holder  20  is mounted to a base  10 . 
     The specimen positioning device  100  contains a specimen holder associated with the present invention. It is now assumed that the contained specimen holder is the specimen holder  20 . 
     As shown in  FIG. 1 , the specimen positioning device  100  contains the base  10 , specimen holder  20 , and a front end support portion  40 . Furthermore, the specimen positioning device  100  can contain an X-axis drive mechanism  50 , a Y-axis drive mechanism  60 , a base drive mechanism (hereinafter may also be referred to as the rotational drive mechanism)  70 , and a flange  80 . In the first embodiment, the specimen positioning device  100  is used in a transmission electron microscope. 
     The specimen positioning device  100  can move a specimen S into a desired position in a specimen chamber  1  and hold the specimen there. In particular, the specimen positioning device  100  can support the specimen S by the specimen holder  20  and move the specimen S in the specimen chamber  1  in the X- and Y-directions by the drive mechanisms  50  and  60 . Furthermore, the specimen positioning device  100  can tilt the specimen S by the rotational drive mechanism  70 . 
     The specimen chamber  1  of the transmission electron microscope can be maintained in a depressurized state by pumping down the chamber  1  by a well-known vacuum pump (not shown). 
     The sample S is introduced into the specimen chamber  1  by the specimen holder  20 . The specimen S in the chamber  1  is irradiated with an electron beam. The specimen chamber  1  is a space surrounded by an electron optical column  2 . In the illustrated example, the electron beam travels through the column  2  parallel to the Z-axis. 
     The specimen holder  20  is movably mounted in a hole  12  formed in the base  10 . Thus, the specimen holder  20  can be attached to and detached from the base  10 . The specimen holder  20  can be mounted in the hole  12  of the base  10  by inserting the holder  20  into the hole  12 . The specimen holder  20  has a first portion (hereinafter may also be referred to as the separating front end portion)  22 , a second portion (hereinafter may also be referred to as the shaft portion)  24 , and a third portion (hereinafter may also be referred to as the grip portion)  26 . 
     The separating front end portion  22  has a specimen holding portion  23  capable of holding the specimen S. The specimen holding portion  23  is mounted, for example, at the front end of the separating front end portion  22 . The specimen holding portion  23  may hold the specimen S, for example, by tightening screws (not shown) or hold down the specimen S using an annular spring (not shown). When the specimen holder  20  is mounted in the hole  12  of the base  10 , the specimen holding portion  23  is disposed in the specimen chamber  1 . 
     When the specimen holder  20  is mounted in the hole  12  of the base  10 , the shaft portion  24  is disposed in the hole  12  of the base  10 . An O-ring  36  is mounted over the shaft portion  24 . In particular, a groove  25  is formed in the outer surface of the shaft portion  24 . The O-ring  36  is mounted in this groove  25 . A hole extending along the X-axis as shown in  FIG. 2  is formed in the front end of the shaft portion  24 . A part of the separating front end portion  22  is inserted in this hole. 
     As shown in  FIG. 1 , the grip portion  26  is fixedly connected to the shaft portion  24 . The grip portion  26  has a width W (in the example of  FIG. 1 , the length taken along the Y-axis) greater than the diameter of the hole  12  in the base  10 . Therefore, when the specimen holder  20  is mounted in the hole  12  of the base  10 , the grip portion  26  is not inserted in the hole  12  of the base  10  but pressed against the base  10  by a force created by the pressure difference between the pressure inside the specimen chamber  1  and atmospheric pressure. That is, the grip portion  26  is held against the base  10  by the force created by the pressure difference between the pressure inside the specimen chamber  1  and atmospheric pressure. In the illustrated example, the grip portion  26  is pressed against an end surface  14  of the base  10 . 
     The separating front end portion  22  and the shaft portion  24  are connected together via a resilient member  34  to permit the front end portion  22  and shaft portion  24  to be moved independently. In the illustrated example, the resilient member  34  is a spring. No restrictions are placed on the structure of the resilient member  34  as long as the front end portion  22  and shaft portion  24  can be moved independently. For instance, the resilient member  34  is made of rubber. One end of the resilient member  34  is connected to the shaft portion  24 , while the other end is connected to the separating front end portion  22 . The resilient member  34  biases the front end portion  22  away from the shaft portion  24  (in the illustrated example, the −X-direction). That is, the resilient member  34  is a compression spring. In the illustrated example, the front end portion  22  is pressed against a slider  42  of the front end support portion  40  by the resilient member  34 . Therefore, the front end portion  22  can be moved independently of the shaft portion  24  by moving the front end support portion  40 . The force generated by the resilient member  34  is set to be smaller than the force of atmospheric pressure applied to the area sealed by the O-ring  36 . That is, the force generated by the resilient member  34  is set to be smaller than the load applied to the specimen holder  20  by the pressure difference between the pressure inside the specimen chamber  1  and atmospheric pressure. 
     The O-ring  36  is in contact with the inner surface of the hole  12  in the base  10  which defines the hole  12  to provide hermetic sealing between the base  10  and the specimen holder  20 . As the shaft portion  24  moves, the O-ring  36  slides in the hole  12  of the base  10 . The O-ring  36  is made of a viscoelastic material such as rubber. 
       FIG. 3  schematically shows the specimen holder  20 , and in which the specimen holder  20  has been removed from the base  10 . 
     In the specimen holder  20 , the shaft portion  24  and the grip portion  26  are fixed. The separating front end portion  22  and the shaft portion  24  are held by pins  32 , the resilient member  34 , and grooves  35  when the specimen holder  20  has been removed from the base  10 . Accordingly, the front end portion  22 , shaft portion  24 , and grip portion  26  are not separate from each other but are integral with each other when the specimen holder  20  has been removed from the base  10 . 
     More specifically, the pins  32  fixed to the separating front end portion  22  are pressed against the inner surfaces  35   a  of the V-shaped grooves  35  formed in the shaft portion  24  by the resilient member  34 , thus holding the front end portion  22 . This prevents the front end portion  22  from coming off the shaft portion  24  when the specimen holder  20  is removed from the base  10 . The number of the pins  32  held to the front end portion  22  is four, for example. The shaft portion  24  is provided with the four grooves  35  in a corresponding manner to the four pins  32 . 
     As shown in  FIG. 1 , when the specimen holder  20  is inserted in the hole  12  of the base  10 , the grip portion  26  is pressed against the end surface  14  of the base  10  by the difference between the pressure inside the specimen chamber  1  hermetically sealed by the O-ring  36  and atmospheric pressure. Consequently, the shaft portion  24  and grip portion  26  are held. At this time, the separating front end portion  22  is pressed and held against the slider  42  of the front end support portion  40  by the resilient member  34 . Since the resilient member  34  is compressed, the pins  32  separate from the inner surfaces  35   a  of the grooves  35 . As a result, the separating front end portion  22  separates from the shaft portion  24 . That is, the front end portion  22  can be moved independently of the shaft portion  24 . The front end portion  22  is provided with grooves  37  that are wider than the diameter of positioning pins  43  mounted on the slider  42 . The positioning pins  43  and grooves  37  cooperate to inhibit rotation of the front end portion  22  relative to the slider  42 . In the illustrated example, the positioning pins  43  and grooves  37  constrain the degree of freedom of rotation of the front end portion  22  about the X-axis. 
     The base  10  has the hole  12  in communication with the specimen chamber  1 . In the illustrated example, the hole  12  extends through the base  10  in the X-direction. For example, the hole  12  is cylindrical in shape. The central axis (not shown) of the hole  12  is parallel to the X-axis. The specimen holder  20  is inserted in the hole  12 . Under this condition, motions of the shaft portion  24  of the specimen holder  20  in the Y- and Z-directions are restricted. A pin (not shown) is mounted on the shaft portion  24  and located closer to the grip portion  26  than the O-ring  36 . A groove wider than the thickness of the pin is machined in the inner surface of the hole  12  of the base  10  and extends up to the end surface  14  parallel to the X-axis. This restrains rotation of the shaft portion  24  about the X-axis. 
     In the specimen chamber  1 , the front end support portion  40  supports the separating front end portion  22 . The support portion  40  is configured including the slider  42  and a rotor  44  as shown in  FIG. 2 . 
     The slider  42  is linearly movably held to the rotor  44  via a linear guide  45 . Motions of the slider  42  other than linear motion along the linear guide  45  relative to the rotor  44  are inhibited by the linear guide  45 . In the illustrated example, motions of the slider  42  other than linear motion along the X-axis relative to the rotor  44  are inhibited by the linear guide  45 . 
     The linear guide  45  is configured including guide grooves  45   a  and plural balls  45   b  captively and rollably held in the guide grooves  45   a . In the illustrated example, the guide grooves  45   a  extend along the X-axis. The balls  45   b  are disposed between the slider  42  and rotor  44 . Since the balls  45   b  roll, the slider  42  can move linearly while experiencing a small resistance. The linear guide  45  constrains motions of the slider  42  other than the linear motion. In the illustrated example, the linear guide  45  constrains degrees of freedom of the slider  42  other than linear motion along the X-axis. 
     The rotor  44  is securely mounted to a stator  82  via a universal coupling  46 . The rotor  44  can rotate within an angular range of several degrees about the origin O that is the center of rotation of the rotor  44 .  FIG. 4  is a schematic cross-sectional view of the universal coupling  46 , taken on line IV-IV of  FIG. 2 . The universal coupling  46  is configured including a ring  47 , bearings  48   a ,  48   b , and screws  49 . 
     Each of the rotor  44  and stator  82  is secured to the ring  47  with the screws  49  via a pair of bearings  48   a  or  48   b . Since the bearings  48   a  and  48   b  are preloaded, the rotor  44  can rotate without rattling. In the illustrated example, the ring  47  is captively held in the rotor  44  as viewed in the Y-direction. Also, the ring  47  is captively held in the stator  82  as viewed in the Z-direction. 
     In the universal coupling  46 , the ring  47  and rotor  44  can rotate about the Z-axis relative to the stator  82 . Furthermore, the rotor  44  can rotate relative to the ring  47  about the Y-axis. Accordingly, the rotor  44  is free to rotate about the origin O. The ring  47  is hollow and the shaft portion  24  extends through the ring  47 . 
     No restrictions are imposed on the structure of the universal coupling  46  as long as the rotor  44  can be rotatably supported by the stator  82  via the universal coupling. 
     The stator  82  is fixedly mounted to the base  10 . The stator  82  and base  10  are integrally supported by bearings  83   a  and  83   b  so as to be rotatable about the X-axis. In the illustrated example, the bearings  83   a  are mounted between the stator  82  and the electron optical column  2 . An outer ring for the bearings  83   a  is securely mounted to the electron optical column  2 . The bearings  83   b  are mounted between the base  10  and the flange  80 . An outer ring for the bearings  83   b  is fixedly secured to the flange  80 . When the stator  82  rotates about the X-axis, the specimen holder  20  and the specimen S rotate about the X-axis. 
     The flange  80  supports the base  10 . That is, the flange  80  functions as a base support portion. A magnetic fluid seal  84  provides sealing between the flange  80  and the base  10 . Consequently, if the base  10  turns, the specimen chamber  1  can be maintained in a depressurized state. 
     The X-axis drive mechanism  50  is configured including a motor  52 , a motor shaft  53 , a coupling  54 , a magnetic fluid seal  55 , another coupling  56 , and a motion converter  59  for converting a rotary motion into a linear motion. 
     The motor  52  rotationally drives the motor shaft  53 . In this embodiment, the motor  52  is shown as one example of a driving portion for rotationally driving the motor shaft  53 . The driving portion is not restricted to a motor as long as the motor shaft can be rotationally driven. The motor  52  is securely mounted to a plate  85 , which in turn is held to a worm wheel  74 . 
     The motor shaft  53  is connected to the motion converter  59  via the coupling  54 , magnetic fluid seal  55 , and coupling  56 . The motor shaft  53  is disposed in an opening  15  formed in the base  10 . The opening  15  is partitioned into a space in communication with the outside and another space in communication with the specimen chamber  1  by the magnetic fluid seal  55 . 
     The magnetic fluid seal  55  is mounted between the inner surface of the opening  15  formed in the base  10  and a motor shaft  63  and seals between the inner surface of the opening  15  and the motor shaft  63 . The inner surface of the opening  15  is a surface of the base  10  that defines the opening  15 . The magnetic fluid seal  55  permits rotary motion of the motor shaft  53  to be introduced into the space in communication with the specimen chamber  1  while maintaining the specimen chamber  1  in a depressurized state. 
     The coupling  56  can expand and contract. Also, the coupling  56  can rotate through a quite small angle. The coupling  56  can absorb linear motion of a ball screw  58 , as well as rotation of the motion converter  59  caused by rotation of the rotor  44 . Consequently, if a ball screw  58  rotates and moves axially (i.e., along the X-axis), the rotating force can be transmitted. Furthermore, if the rotor  44  and ball nut  57  are rotated about the origin O by operation of the Y-axis drive mechanism  60  and a Z-axis drive mechanism (not shown), the rotating force can be transmitted. 
     A ball coupling for transmitting rotation via balls held in an outer hub groove and in an inner hub groove, a helical coupling having a cylindrical material provided with a helical slit, a bellows coupling using a bellows, or the like can be used as each of the couplings  54  and  56 . 
     The motion converter  59  is configured including the ball nut  57  and the ball screw  58 . 
     The ball nut  57  is secured to the rotor  44 , and is in threaded engagement with the ball screw  58 . 
     The ball screw  58  has a spherical front end in contact with the slider  42 . A tension spring (not shown) is mounted between the slider  42  and the rotor  44  such that the contact surface is preloaded. Preloading is provided between the ball screw  58  and the ball nut  57  to prevent the posture of the shaft of the ball screw  58  from varying if the ball screw  58  rotates. 
     When the motor  52  rotates, the motor shaft  53  rotates, moving the ball screw  58  in its axial direction (along the X-axis). Consequently, the slider  42  in contact with the front end of the ball screw  58  is moved linearly along the linear guide  45 . That is, the ball nut  57  and ball screw  58  convert the rotary motion of the motor shaft  53  into a linear motion and transmits it to the slider  42 . 
     The X-axis drive mechanism  50  can move the separating front end portion  22  (and the specimen S) linearly along the linear guide  45 . In the illustrated example, the X-axis drive mechanism  50  moves the front end portion  22  parallel to the X-axis. When the rotor  44  has rotated through a given angle θ from the illustrated position, the sense of the linear guide  45  also varies. Therefore, the X-axis drive mechanism  50  can translate the front end portion  22  to an axis that is tilted by the given angle θ relative to the X-axis. 
     The Y-axis drive mechanism  60  includes a motor  62 , a motor shaft  63 , a coupling  64 , a magnetic fluid seal  65 , another coupling  66 , and a motion converter  69  for converting a rotary motion into a linear motion. 
     The motor  62  rotationally drives the motor shaft  63 . The motor  62  is securely mounted to the plate  85 . In this embodiment, the motor  62  is shown as a driving portion for rotationally driving the motor shaft  63 . The driving portion is not restricted to a motor as long as the driving portion can rotationally drive the motor shaft. 
     The motor shaft  63  is connected to the motion converter  69  via the coupling  64 , magnetic fluid seal  65 , and coupling  66 . The motor shaft  63  is disposed in an opening  16  formed in the base  10 . The opening  16  is partitioned into a space in communication with the outside and a space in communication with the specimen chamber  1  by the magnetic fluid seal  65 . 
     The magnetic fluid seal  65  is mounted between the inner surface of the opening  16  formed in the base  10  and the motor shaft  63 . The magnetic fluid seal  65  seals between the inner surface of the opening  16  and the motor shaft  63 . The inner surface of the opening  16  is a surface of the base  10  that defines the opening  16 . Rotary motion of motor shaft  63  can be introduced into the space in communication with the specimen chamber  1  while the specimen chamber  1  is kept in a depressurized state by the magnetic fluid seal  65 . 
     The coupling  66  can expand and contract. The coupling  66  can absorb linear motion of the motor shaft  63 . Consequently, if a ball screw  68  rotates and moves in its axial direction (along the X-axis), the rotating force can be transmitted. 
     A ball coupling, helical coupling, bellows coupling, or the like can be used as each of the couplings  64  and  66 . 
     The motion converter  69  is configured including the ball nut  67  and the ball screw  68 . 
     The ball nut  67  is securely fixed to the stator  82  and in threaded engagement with the ball screw  68 . 
     The ball screw  68  has a spherical front end in contact with the rotor  44 . A spring  88  is mounted to preload the contact surface. Preloading is provided between the ball screw  68  and the ball nut  67  to prevent the posture of the shaft of the ball screw  68  from varying if the ball screw  68  rotates. 
     When the motor  62  rotates, the motor shaft  63  rotates, moving the ball screw  68  in its axial direction (i.e., along the X-axis). Consequently, the rotor  44  in contact with the front end of the ball screw  68  is rotated about the Z-axis. That is, the ball nut  67  and ball screw  68  convert the rotary motion of the motor shaft  63  into a linear motion and transmits it to the rotor  44 . As a result, the separating front end portion  22  rotates about the Z-axis. 
     The Y-axis drive mechanism  60  can rotate the separating front end portion  22  about the Z-axis. A motion caused by rotation of the front end portion  22  can be regarded as a motion in the Y-direction if the movement is quite small. Accordingly, the Y-axis drive mechanism  60  can move the specimen S nearly along the Y-axis. 
     The specimen positioning device  100  can further include a Z-axis drive mechanism (not shown) which is similar in configuration with the Y-axis drive mechanism  60 . This Z-drive mechanism has a ball screw that is in contact with the rotor  44  at a position which is angularly spaced by 90 degrees about the X-axis from the position at which the ball screw  68  of the Y-axis drive mechanism  60  is in contact with the rotor  44 . Accordingly, the posture of the rotor  44  is determined by three points, i.e., the contact point between the rotor  44  and the ball screw  68  of the Y-axis drive mechanism  60 , the contact point between the rotor  44  and the ball screw of the Z-drive mechanism, and the center of rotation (origin). 
     The rotational drive mechanism  70  is configured including a worm gear  72 , the worm wheel  74 , and a motor (not shown). The worm wheel  74  is securely mounted to the base  10 . The motor is fixedly secured to the flange  80 . The motor rotationally drives the worm gear  72 . When the worm gear  72  turns, the worm wheel  74  rotates, rotating the base  10  about the X-axis. Consequently, the stator  82  secured to the base  10  rotates integrally with the base  10 , thus rotating the specimen holder  20  about the X-axis. 
     1.2. Operation of Specimen Positioning Device 
     The operation of the specimen positioning device  100  according to the first embodiment and for use with a charged particle beam system is next described by referring to  FIGS. 1-3 . 
     The manner in which the specimen S is placed in position in the X-direction is described. In the specimen positioning device  100 , the specimen S is placed in position in the X-direction by the X-axis drive mechanism  50 . In particular, the motor  52  rotates the motor shaft  53 , thus rotating the ball screw  58 . Since the ball nut  57  is held to the rotor  44 , rotation of the ball screw  58  in engagement with it moves the ball screw  58  in the X-direction. The slider  42  moves along the linear guide  45 . Consequently, the separating front end portion  22  moves along the linear guide  45 . In the illustrated example, the front end portion moves in the X-direction. The specimen S can be placed in position in the X-direction. 
     In the specimen positioning device  100 , if the specimen S is moved in the X-direction by the X-axis drive mechanism  50 , the specimen chamber  1  can be maintained in a depressurized state by the magnetic fluid seal  55  mounted between the inner surface of the opening  15  and the motor shaft  53 . In this way, in the specimen positioning device  100 , it is possible to seal between the inner surface of the opening  15  and the motor shaft  53  by the magnetic fluid seal  55  without using an O-ring. Hence, outward drift due to an O-ring can be prevented. 
     Outward drift is now described by referring to  FIG. 5 , which illustrates outward drift and does not directly correspond to the structure of the present embodiment shown in  FIG. 1 . In  FIG. 5 , an X-motor M 2  is shown to move straightly rather than rotate for the sake of convenience. A specimen holder is disposed so as to extend through atmosphere and through a vacuum. A vacuum seal is provided by an O-ring mounted on the outer surface of the specimen holder. When the specimen holder moves straightly, the O-ring slides. When the X-motor M 2  is driven in a direction indicated by the arrow A (hereinafter referred to as the direction A) (e.g., in the +X-direction), the specimen holder, M 4 , is moved linearly in the direction A while supported by a bearing M 8  mounted on a specimen holder support portion M 6 . At this time, both a force P, which is created by the pressure difference between the pressure inside the specimen chamber and atmospheric pressure, and the frictional resistance force F of an O-ring mounted on the specimen holder M 4  act on the specimen holder M 4  in a direction indicated by the arrow B (hereinafter referred to as the direction B) (e.g., in the −X-direction) opposite to the direction A. The rigidity (springiness) M 10  of an X-feeding mechanism exists between the X motor M 2  and the bearing M 8 . Accordingly, the position of the specimen holder M 4  is determined by the amount of feed applied by the X motor M 2  and also by the balance among the force P, the frictional resistance force F of the O-ring, and the rigidity M 1  of the X-feeding mechanism. If the frictional resistance force F of the O-ring does not exist, the force P and the force created by the flexure of the rigidity M 10  of the X-feeding mechanism balance each other out. Therefore, the amount of feed applied by the X motor M 2  agrees with the amount of motion of the specimen holder M 4 . On the other hand, if the frictional resistance force F of the O-ring exists, the frictional resistance force is exerted in the direction opposite to the direction of feed. Therefore, the amount of motion of the specimen holder M 4  is smaller than the amount of feed applied by the X motor M 2 . 
     It is known that stress relaxation occurs in a viscoelastic material (such as rubber) of the O-ring. When the specimen holder M 4  moves, the O-ring is deformed. This induces a stress in the O-ring. This stress decreases gradually with the elapse of time due to stress relaxation. Accordingly, flexure produced in the rigidity M 10  of the X-feeding mechanism by the frictional resistance force F of the O-ring decreases with time. That is, the specimen holder M 4  is moved in the direction A by the X motor M 2  and continues to move in the same direction. This is observed as a post-motion drift. Furthermore, stress accumulated in the O-ring is also relieved as the contact surface of the O-ring continues to slip slowly. This relief similarly produces post-motion drift. 
     As described previously, in the specimen positioning device  100 , sealing can be provided between the inner surface of the opening  15  and the motor shaft  53  by the magnetic fluid seal  55  without using an O-ring. What is placed in position is the separating front end portion  22 . Since neither the shaft portion  24  nor the O-ring  36  moves, post-motion drifts attributed to an O-ring can be prevented. 
     What is placed in position by the X-axis drive mechanism  50  is the separating front end portion  22 . Therefore, the member placed in position can be reduced in weight as compared with the case where the whole specimen holder is placed in position. For example, in the case of a drive mechanism for moving a specimen in the X-direction by rotary motion of a lever mechanism (not shown) as shown in FIG. 1 of JP-A-2010-157491, the member constituting the lever may be relatively long and have low bending rigidity. In the X-axis drive mechanism  50 , any elements constituting a lever do not exist. Thus, higher rigidity can be provided. 
     The manner in which the specimen S is placed in position in the Y-direction is next described. In the specimen positioning device  100 , the specimen S is placed in position in the Y-direction by the Y-axis drive mechanism  60 . In particular, the ball screw  68  is rotated by rotating the motor shaft  63  by means of the motor  62 . At this time, the ball nut  67  in threaded engagement is held to the stator  82  and so the ball screw  68  moves straightly in the X-direction. The rotor  44  in contact with the screw rotates about the Z-axis. Consequently, the separating front end portion  22  can be regarded as if it moved in the Y-direction within a quite small region. Thus, the specimen S can be placed in position in the Y-direction. 
     In the specimen positioning device  100 , if the specimen S is moved by the Y-axis drive mechanism  60  as if it went in the Y-direction, the specimen chamber  1  can be maintained in a depressurized state by the magnetic fluid seal  65  mounted between the inner surface of the opening  16  and the motor shaft  63 . Since sealing can be provided between the inner surface of the opening  16  and the motor shaft  63  in the specimen positioning device  100  by the magnetic fluid seal  65  without using an O-ring in this way, post-motion drifts due to an O-ring can be prevented. 
     What are placed in position by the Y-axis drive mechanism  60  are the separating front end portion  22  and the front end support portion  40 . Accordingly, the weight of members placed in position can be reduced as compared with the case where a specimen holder having an inseparable front end is placed in position. Furthermore, the length of the ball screw  68 , taken from the ball nut  67  to the point at which the ball screw  68  is in contact with the rotor  44 , can be shortened. Furthermore, the distance from this contact point to the specimen S (specimen holding portion  23 ) taken in the Y-direction can be reduced. Hence, the rigidity of the mechanism for moving the separating front end portion  22  including the rotor  44  and the ball screw  68  can be enhanced. The effects of external disturbing vibrations can be reduced. In addition, thermal drift of the specimen caused by thermal expansion due to temperature variations can be suppressed by reducing these distances. 
     The specimen S is driven in the Z-direction by the Z-axis drive mechanism that is similar in configuration with the Y-axis drive mechanism  60  and so a description thereof is omitted. 
     The manner in which the specimen S is placed in position around the X-axis is next described. In the specimen positioning device  100 , the specimen S is placed in position around the X-axis by the rotational drive mechanism  70 . In particular, when the worm gear  72  is rotated by a motor (not shown), the direction of rotation is converted by the worm wheel  74 , so that the base  10  rotates around the X-axis. Consequently, the specimen holder  20  rotates. The specimen S can be placed in position around the X-axis. 
     In the specimen positioning device  100 , if the specimen S is rotated by the rotational drive mechanism  70 , the specimen chamber  1  can be maintained in a depressurized state by the magnetic fluid seal  84  mounted between the base  10  and the flange  80 . In this way, it is possible to seal between the base  10  and the flange  80  by the magnetic fluid seal  84  without using an O-ring. As a consequence, post-motion drifts due to an O-ring can be prevented. 
     The specimen positioning device  100  and the specimen holder  20  have the following features. 
     The specimen positioning device  100  has: the base  10  provided with the hole  12  in operative communication with the specimen chamber  1 ; the separating front end portion  22  having the specimen holding portion  23  capable of holding the specimen S; the specimen holder  20  which includes the shaft portion  24  supporting the separating front end portion  22  via the resilient member  34  and which is movably mounted in the hole  12 ; and the front end support portion  40  supporting the separating front end portion  22  in the specimen chamber  1 . Thus, the front end portion  22  can be placed in position independently of the shaft portion  24 . Therefore, the member (separating front end portion  22 ) to be placed in position can be made lighter in weight than, for example, where the whole specimen holder is placed in position. Consequently, the specimen positioning device  100  can be made more immune to external disturbing vibrations. 
     The relationship between weight reduction and external disturbing vibrations is now described.  FIG. 6  shows a vibration model of a system having one degree of freedom. In  FIG. 6 , it is assumed that a mass M is connected to a base B via a spring Sp and a damper D and has a displacement of a. When the base B is forced to vibrate at a frequency f (in Hz) with a displacement of a 0 , a relative displacement |a 0 −a| between the base B and the mass M is given by 
                              a   0     -   a       a   0            =       λ   2             (     1   -   λ     )     2     +       (     2   ⁢   ξλ     )     2                   (   1   )               
where λ is the ratio of the frequency f to the natural frequency f n  of the system and ξ is the damping ratio of the system having one degree of freedom.
 
     In a region where the ratio λ is sufficiently smaller than unity, the relative displacement is in proportion to λ 2 . In a general accurate measurement instrument, transmission of high frequencies can be suppressed by vibration isolators. Therefore, frequency components on the order of Hz are problematic. These frequencies on the order of Hz are sufficiently lower than the natural frequency of the apparatus and so the requirement that the ratio λ is sufficiently lower than unity is satisfied. Where the frequency f of forced vibrations is constant, the relative displacement is inversely proportional to the square of the natural frequency f n  of the system. That is, the relative displacement is given by 
     
       
         
           
             
               
                 
                   
                     
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                             a 
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                           - 
                           a 
                         
                         
                           a 
                           0 
                         
                       
                        
                     
                     ≈ 
                     
                       λ 
                       2 
                     
                   
                   = 
                   
                     
                       ( 
                       
                         f 
                         
                           f 
                           n 
                         
                       
                       ) 
                     
                     2 
                   
                 
               
               
                 
                   ( 
                   2 
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     Therefore, in order to reduce the effects of external disturbing vibrations by reducing the relative displacement, it is necessary to increase the natural frequency f n  of the system. For this purpose, it is important to increase the spring constant of the spring Sp or to reduce the mass M. In the specimen positioning device  100 , the member that is placed in position can be reduced in weight and, therefore, the natural frequency of the system can be raised as described previously. Hence, the effects of external disturbing vibrations, i.e., the amount of relative displacement, can be reduced. 
     The specimen positioning device  100  contains the X-axis drive mechanism  50 , Y-axis drive mechanism  60 , and Z-axis drive mechanism (not shown) for moving the separating front end portion  22  via the front end support portion  40  and so the separating front end portion  22  can be moved in the X-, Y-, and Z-directions. 
     In the specimen positioning device  100 , the X-axis drive mechanism  50  contains the motor  52  for rotationally driving the motor shaft  53  and the motion converter  59  for converting the rotary motion of the motor shaft  53  into a linear motion and transmitting it to the front end support portion  40  (slider  42 ). The motor shaft  53  is disposed in the opening  15  in communication with the specimen chamber  1 . The magnetic fluid seal  55  is mounted between the motor shaft  53  and the inner surface of the opening  15 . Consequently, in the specimen positioning device  100 , sealing can be provided between the motor shaft  53  and the inner surface of the opening  15  by the magnetic fluid seal  55  without using an O-ring. As a result, post-motion drifts due to an O-ring can be prevented. 
     In the specimen positioning device  100 , the Y-axis drive mechanism  60  contains the motor  62  for rotationally driving the motor shaft  63  and the motor converter  69  for converting the rotary motion of the motor shaft  63  into a linear motion and transmitting it to the front end support portion  40  (rotor  44 ). The motor shaft  63  is disposed in the opening  16  in communication with the specimen chamber  1 . The magnetic fluid seal  65  is mounted between the motor shaft  63  and the inner surface of the opening  16 . Consequently, in the specimen positioning device  100 , sealing can be provided between the motor shaft  63  and the inner surface of the opening  16  by the magnetic fluid seal  65  without using an O-ring. Hence, post-motion drifts due to an O-ring can be prevented. The Z-axis drive mechanism (not shown) is configured similarly. 
     In the specimen positioning device  100 , the rotational drive mechanism  70  includes the bearings  83 ,  83   b  by which the base  10  is rotatably supported, the flange  80 , and the magnetic fluid seal  84  mounted between the flange  80  and the base  10 . The rotational drive mechanism  70  rotates the specimen holder  20  via the base  10 . Consequently, in the specimen positioning device  100 , sealing can be provided between the flange  80  and the base  10  by the magnetic fluid seal  84  without using an O-ring. Thus, post-motion drifts due to an O-ring can be prevented. 
     Since the specimen positioning device  100  is configured including the O-ring  36  which is mounted in the shaft portion  24  and which in contact with the inner surface of the hole  12  in the base  10 , hermetic sealing can be provided between the base  10  and the specimen holder  20 . If the separating front end portion  22  moves and rotates, the O-ring  36  does not slide. Therefore, post-motion drifts due to an O-ring can be prevented. 
     In the specimen positioning device  100 , the specimen holder  20  has the grip portion  26  connected to the shaft portion  24 . The grip portion  26  has a width W greater than the diameter of the hole  12 . Consequently, the grip portion  26  can be held to the base  10  by a force created by the pressure difference between the pressure inside the specimen chamber  1  and atmospheric pressure. Accordingly, if atmospheric pressure varies, the specimen holder  20  can be suppressed from moving and thus drift of the specimen S due to atmospheric pressure variations can be suppressed. 
     The specimen holder  20  is configured including the separating front end portion  22  having the specimen holding portion  23  capable of holding the specimen S and the shaft portion  24  supporting the front end portion  22  via the resilient member  34  and so the front end portion  22  can be placed in position independently of the shaft portion  24 . Accordingly, the member to be placed in position can be reduced in weight as compared with the case where the whole specimen holder is placed in position. This can reduce the effects of external disturbing vibrations. 
     The specimen holder  20  is so designed that when it is withdrawn, the pins  32  and the grooves  35  hold the separating front end portion  22  and the shaft portion  24  together to prevent them from separating from each other. Therefore, the specimen holder  20  can be easily inserted and withdrawn without such undesirable separation. 
     In the specimen positioning device  100 , the specimen holder  20  has the shaft portion  24  and so a member constituting a Y-axis rotational drive mechanism can be mounted in the shaft portion  24  as described later in connection with a modification (described later). Furthermore, electrical wiring can be laid in the shaft portion  24  and the specimen S in the specimen chamber  1  can be heated. Additionally, optical fiber can be laid in the shaft portion  24  and the specimen S in the specimen chamber  1  can be illuminated with light. A flexible tube can be inserted into the shaft portion  24 , and a fluid can be supplied into the separating front end portion. The specimen surrounded by the fluid and reactions of the specimen with the fluid can be observed. 
     1.2. Modification of Specimen Positioning Device 
     A modification of the specimen positioning device  100  is next described.  FIG. 7  schematically shows a specimen holder  20   a  of the specimen positioning device  100  associated with a modification of the first embodiment.  FIG. 8  is a schematic cross portion of the specimen holder  20   a  of the specimen positioning device  100  associated with the modification of the first embodiment, the cross portion being taken on line VIII-VIII of  FIG. 7 . Those members or components of the specimen holder  20   a  associated with this modification which are similar in function with their respective counterparts of the above-described specimen holder  20  are indicated by the same reference numerals as in the above-cited figures and a detail description thereof is omitted. 
     The specimen holder  20   a  is configured including a Y-axis rotational drive mechanism  210  for rotating or tilting the specimen S in the specimen chamber  1  about an axis Yd that is parallel to the Y-axis and passes through the center of the specimen holding portion  23 . 
     The Y-axis rotational drive mechanism  210  is configured including a motor shaft  212 , an O-ring  214 , a coupling  216 , a feed screw  217 , a shaft  218 , a bell crank  219 , and a tilting table  220 . 
     The motor shaft  212  is mounted in the shaft portion  24  and connected to the feed screw  217  via the coupling  216 . 
     Rotation is given to the motor shaft  212  by a motor (not shown). In the motor shaft  212 , the O-ring  214  is mounted between the inner surface of the shaft portion  24  and the motor shaft  212  to seal between them. 
     The coupling  216  is extensible and can absorb linear motion of the feed screw  217 . 
     The feed screw  217  is rotated by the motor shaft  212  and moves in its axial direction (in the X-direction within the specimen chamber  1 ). The front end of the feed screw  217  is in contact with the shaft  218 . 
     The shaft  218  is mounted between the feed screw  217  and the bell crank  219 . When the feed screw  217  moves in its axial direction, the shaft  218  moves in the same direction. The end of the shaft  218  is in contact with one arm  219   a  of the bell crank  219 . 
     The bell crank  219  is mounted in the separating front end portion  22  and has arms  219   a  and  219   b  extending in different directions. The bell crank  219  is rotatably held by a pin  219   c . When the shaft  218  moves in its axial direction, the bell crank  219  rotates about the pin  219   c . This rotation turns the tilting table  220  that is in contact with and held by a pin  221  secured to the arm  219   b.    
     The tilting table  220  has the specimen holding portion  23 , and is rotatably held to the separating front end portion  22  by a pair of pins (not shown) for aligning the center of the table with the axis Yd. The tilting table  220  rotates about the axis Yd passing through the specimen holding portion  23 . When the tilting table  220  rotates, the specimen S supported to the specimen holding portion  23  rotates or tilts about the axis Yd. 
     Since the specimen holder  20   a  is configured including the Y-axis rotational drive mechanism  210 , the specimen S can be rotated or tilted about the axis Yd. If a rotation is made about the axis Yd, post-motion drifts can be prevented by adopting a magnetic fluid seal instead of the O-ring  214 . 
     2. Second Embodiment 
     A charged particle beam system associated with a second embodiment of the present invention is next described by referring to  FIG. 9  showing the configuration of the system. The charged particle beam system, generally indicated by reference numeral  1000 , is configured including a specimen positioning device associated with the present invention. It is now assumed that the charged particle beam system  1000  is configured including the specimen positioning device  100  that is shown to be simplified in  FIG. 9 . 
     As shown in  FIG. 9 , the charged particle beam system  1000  is configured including an electron beam source  1001 , an illuminating lens  1002 , the specimen positioning device  100 , an objective lens  1004 , an intermediate lens  1005 , a projector lens  1006 , an imager  1008 , and an electron optical column  2 . It is now assumed that the charged particle beam system  1000  is a transmission electron microscope (TEM). The charged particle beam system  1000  may also be a scanning transmission electron microscope (STEM). 
     The electron beam source  1001 , illuminating lens  1002 , objective lens  1004 , and projector lens  1006  are housed in the electron optical column  2 . The interior of the column  2  is pumped down by a vacuum pumping system (not shown). 
     The electron beam source  1001  accelerates electrons released from a cathode and releases an electron beam EB. A well-known electron gun can be used as the electron beam source  1001 . 
     The illuminating lens  1002  is disposed behind the electron beam source  1001  and operates to direct the electron beam EB generated by the electron beam source  1001  at a specimen S. The illuminating lens  1002  is configured, for example, including condenser lenses (not shown). 
     The specimen S is held by the specimen holder  20  in the specimen chamber  1 . The specimen S is placed in position within the specimen chamber  1  by the specimen positioning device  100 . 
     The objective lens  1004  is disposed behind the illuminating lens  1002 . The objective lens  1004  is a first stage of lens for focusing the electron beam EB transmitted through the specimen S. 
     The intermediate lens  1005  is disposed behind the objective lens  1004 . The projector lens  1006  is disposed behind the intermediate lens  1005 . The image focused by the objective lens  1004  is further magnified by the intermediate lens  1005  and projector lens  1006  and imaged on the imager  1008 . 
     The imager  1008  has a detector for detecting the electron beam EB. For example, the detector is a CCD camera having a two-dimensional array of CCDs. The imager  1008  detects an electron microscope image and delivers information about the electron microscope image. 
     In the illustrated example, the charged particle beam system  1000  is mounted on a pedestal  1012  via vibration isolators  1010 . The pedestal  1012  is mounted on a floor FL. 
     Since the charged particle beam system  1000  is configured including the specimen positioning device  100 , the effects of external disturbing vibrations generated by vibrations of the floor FL can be reduced. 
     A case in which the specimen positioning device  100  is applied to a transmission electron microscope has been described. The specimen positioning device  100  can also be applied to charged particle beam systems other than a transmission electron microscope. Examples of charged particle beam system include electron microscope, focused ion beam system, and electron beam exposure system. 
     The present invention embraces configurations substantially identical (e.g., in function, method, and results or in purpose and advantageous effects) with the configurations described in the embodiments of the invention. Furthermore, the invention embraces configurations described in the embodiments and including portions which have non-essential portions replaced. In addition, the invention embraces configurations which produce the same advantageous effects as those produced by the configurations described in the embodiments or which can achieve the same objects as the configurations described in the embodiments. Further, the invention embraces configurations which are similar to the configurations described in the embodiments except that well-known techniques have been added. 
     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.