Abstract:
An electron beam apparatus has an optical axis, an electron beam source for generating an electron beam directed along the optical axis, and a magnetic field lens having an axis coincident with the optical axis for focusing the electron beam onto a sample which is subjected to a negative voltage so that secondary electrons are emitted from the sample. The magnetic field lens has a conductive cylinder surrounding a part of the optical axis to permit the passage therethrough of an electron beam from the electron beam source. A first detector detects secondary electrons emitted by the sample in a direction away from the optical axis and is disposed at a position generally confronting the conductive cylinder. A second detector is disposed over the conductive cylinder. A Wien filter deflector deflects secondary electrons emitted by the sample toward and for detection by the second detector. The Wien filter deflector is disposed on the optical axis and between the conductive cylinder and the second detector.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an electron beam apparatus. 
     2. Description of Related Art 
     In order to perform inspection or observation of the shape of a fine pattern electronic device, conventionally various electron beam devices, such as a scanning electron microscope, have been used. In particular, there has been a high demand for high resolution observation accompanying the fact that electronic devices have become ultrafine in recent years. With this type of electron beam device capable of high-resolution observation, there are cases where a potential is applied to a sample in order to increase image quality at low accelerating voltages. Therefore, when a potential Vs is applied to a sample, secondary electrons then have substantially the energy Vs and the proportion of secondary electrons on the electron source side traveling in a straight line along the optical axis is therefore high. As a result, there is a problem where the efficiency with which secondary electrons are detected falls. This tendency is therefore particularly strong for electrons coming from the bottoms of holes in a semiconductor wafer. 
     In order to resolve this problem, in Japanese Laid-open Patent Publication No. Hei. 5-258703, there is disclosed an electron beam scanning method and system thereof that deflects secondary electrons using a Wien filter so as to detect secondary electrons while reducing the effect on incident electrons. At the Wien filter, the electric field and the magnetic field are arranged so as to be orthogonal with respect to each other. Of the force acting on electrons incident in a direction from the electron gun towards the sample, force due to the electric field is −eE, and force due to the magnetic field is −e(v×B) constituting the vector sum. The forces acting on the incident electrons when these forces are equal are therefore balanced and incident electrons are therefore not deflected. However, the direction of travel of secondary electrons coming from the sample is opposite to the direction of incident electrons with respect to the Wien filter. The direction of force due to the magnetic field is therefore opposite to that of the incident electrons. The forces of the electric field and the magnetic field therefore act on the secondary electrons in the same direction and therefore only the secondary electrons are deflected. 
     In addition to the methods proposed in Japanese Laid-open Patent Publication Hei. 5-258703, methods are proposed in Japanese Patent Publication No. 3136353 and Japanese Laid-open Patent publication No. Hei. 10-214586 where a plurality of secondary electron detectors are provided along the direction of the optical axis, with the sum of a plurality of detector signals then being used to improve the efficiency with which secondary electrons are detected. 
     However, in the invention disclosed in Japanese Laid-open Patent Publication Hei. 5-258703, the secondary electron detector is installed away from the beam that is on the optical axis. It is therefore necessary to deflect secondary electrons through an angle greater than that of a detection surface of a following scattered electron detector using two deflectors functioning as a Wien filter in order to detect the secondary electrons. The intensity of the electromagnetic field of the Wien filter is therefore large, and the influence of Wien filter aberrations on the beam therefore easily becomes large which can easily prove detrimental to image resolution. 
     Further, a large apparatus capable of generating a large electromagnetic field for the deflectors functioning as the Wien filter is required in order to deflect the secondary electrons through a large angle. In addition to making the apparatus manufacturing costs high, this also makes the electron beam apparatus as a whole large, which results in an expensive product. 
     With the invention disclosed in Japanese Patent No. 3136353, the secondary electrons pass through the hole in the upper side detector meaning that there are secondary electrons that are not detected at the upper side detector. This means that detection efficiency is not sufficient. Further, a deflector is provided in order to perform beam scanning on the sample surface but the amount of secondary electrons deflected depends on the strength of the deflector and there is therefore a further problem in that it is easy for the center of the screen to become dark. 
     In the invention proposed in Japanese Laid-open Patent Publication No. 10-214586, the electron-gun-side axis and the objective lens axis are separated just by a prescribed distance. When this prescribed distance is fixed, the intensity of the beam deflector is fixed because the electron gun axis and the objective lens axis are in line with each other. It has therefore been difficult to set the strength of the deflectors appropriately so as to obtain a sufficient secondary electron signal. There is also a further problem of inconvenience in that this is detrimental to flexibility during lens barrel adjustment. On the other hand, a mechanism for, for example, moving the electron gun axis horizontally etc. is required when it is wished to vary a prescribed distance between the electron gun-side axis and the objective lens axis. This makes the structure of the electron beam apparatus complex, causes the manufacturing cost of the apparatus to rise, and makes the operation of the electron beam apparatus complex. 
     A secondary electron detector having a hole allowing a beam to pass is used as a secondary electron detector and a Micro Channel Plate (MCP) is given as this kind of detector. However, MCPs have a short lifespan compared to the scintillator/photo-multiplier detectors usually used and are therefore expensive. In the case where a secondary detector having a hole allowing this beam to pass is used as the second detector, the secondary electrons are incident to the detection surface in the shape of a spot and there is the fear that this will cause the detection surface to deteriorate rapidly. 
     Further, a secondary electron detector having a hole allowing a beam to pass encompasses the detection electrode to which the negative potential is applied at the inside of the hole. There are therefore parts between the detection surface and the electrode where detection cannot be achieved. In particular, it is necessary to make the strength of the beam deflector substantial in order to surpass portions that cannot detect and deflect the beam as far as the detection surface. There is therefore the problem that deflection aberrations are increased and image resolution is caused to deteriorate. 
     SUMMARY OF THE INVENTION 
     In order to resolve the aforementioned problems, according to the present invention, there is provided an electron beam apparatus equipped with a magnetic field lens arranged on the same axis as an optical axis and configured in such a manner that an electron beam from an electron beam source advancing along the optical axis and being incident to the magnetic field lens focused onto a sample subjected to a negative voltage, with electrons emitted from the sample as a result of the sample being irradiated with the electron beam being detected, wherein a conductive cylinder is arranged so as to cover part of the optical axis in such a manner as to permit the passage of an electron beam from the electron beam source, and, a first detector for detecting emitted electrons of the emitted electrons that do not pass through the cylinder; a second detector for detecting emitted electrons of the emitted electrons that do pass through the cylinder; and a Wien filter deflector for increasing the emitted electrons detected by the second detector for emitted electrons that pass through the cylinder, provided between the cylinder and the second detector; are provided within the magnetic field lens. 
     According to the present invention, at least one of the first detector or the second detector is a scintillator detector, and there is provided a reflecting plate positioned further to the side of the electron beam source than the cylinder at the outside of the cylinder. 
     According to the present invention, an electron beam apparatus is provided where a negative voltage is applied to the cylinder and the reflecting plate, or only to the reflecting plate. 
     According to the present invention, there is provided an electron beam apparatus where the voltage applied to the cylinder and the reflecting plate, or only to the reflecting plate is the same voltage as is applied to the sample or is a larger negative voltage. 
     According to the present invention, detection signals outputted from the first detector and the second detector may be composite signals. 
     According to the present invention, there is provided an electron beam apparatus where the first detector is comprised of a plurality of detectors, with it being possible for each of the plurality of detectors to independently detect each detected signal and form composite signals. 
     According to the present invention, an electron beam apparatus is provided where the first detector may comprise a plurality of scintillator detectors. 
     According to the present invention, there is provided an electron beam apparatus equipped with a magnetic field lens arranged on the same axis as an optical axis and configured in such a manner that an electron beam from an electron beam source advancing along the optical axis and being incident to the magnetic field lens is focused onto a sample subjected to a negative voltage, with electrons emitted from the sample as a result of the sample being irradiated with the electron beam being detected, wherein, using a two-stage deflection system, an electron beam coming from the electron beam source is made incident in such a manner as to be inclined with respect to the optical axis by a deflector comprising a magnetic field or an electro-magnetic field, the electron beam is made incident to the electromagnetic field lens after being deflected by the deflector, emitted electrons emitted from the sample and passing through the magnetic field lens are detected by the first detector, and emitted electrons not detected by the first detector are deflected by the deflector so as to be detected by the second detector. 
     According to the present invention, at least one of the first detector or the second detector is a scintillator detector, and there is provided a reflecting plate positioned further to the side of the electron beam gun than the cylinder at the outside of the cylinder. 
     According to the present invention, an electron beam apparatus is provided where a negative voltage is applied to the cylinder and the reflecting plate, or only to the reflecting plate. 
     According to the present invention, there is provided an electron beam apparatus where the negative voltage applied to the cylinder and the reflecting plate, or only to the reflecting plate is the same voltage as is applied to the sample or is a larger negative voltage. 
     According to the present invention, detection signals outputted from the first detector and the second detector may be composite signals. 
     According to the present invention, there is provided an electron beam device where the first detector is comprised of a plurality of detectors, with it being possible for each of the plurality of detectors to independently detect each detected signal and form composite signals. 
     According to the present invention, an electron beam apparatus is provided where the first detector may comprise a plurality of scintillator detectors. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view of an example of an embodiment of a single pole-piece lens with an electrostatic bipotential lens of the present invention provided in an electron beam apparatus (not shown). 
         FIG. 2  shows an example of the trajectory of the secondary electrons coming from the sample. 
         FIG. 3  is a graph showing results for example trajectories calculated for secondary electrons when respective negative potentials are applied to the first cylinder and the first reflecting plate. 
         FIG. 4  is a cross-sectional view of a further embodiment of a single pole-piece lens with an electrostatic bipotential lens of the present invention. 
         FIG. 5  is a cross-sectional view of a still further embodiment of a single pole-piece lens with an electrostatic bipotential lens of the present invention. 
         FIG. 6  is a cross-sectional view of another embodiment of a single pole-piece lens with an electrostatic bipotential lens of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments of the present invention will be described in more detail below with reference to the drawings. 
       FIG. 1  is a cross-sectional view of an example of an embodiment of a single pole-piece lens with an electrostatic bipotential lens  10  of the present invention provided in an electron beam apparatus (not shown) . The single pole-piece lens with an electrostatic bipotential lens  10  is configured as an objective lens of the electronic apparatus, such as a scanning electron microscope apparatus, used in scanning and observation of the shapes of fine pattern electronic devices, and subjects an electron beam  1  advancing along an optical axis X from an electron gun (not shown) to the focusing action of a magnetic field and superimposes a decelerating electric field with the magnetic field so as to reduce the chromatic aberration coefficient of the lens in order to provide an electrostatic magnetic objective lens capable of narrowly focusing the electron beam  1  onto the sample  2  so as to enable observation of high-resolution images. 
     The single pole-piece lens with an electrostatic bipotential lens  10  is formed with an excitation coil  12  provided at a magnetic circuit  11 , with the magnetic circuit  11  being formed integrally with a yoke  111  and magnetic pole  112 . The yoke  111  is provided at the side of advancing of the electron beam  1  advancing along the direction of the optical axis X. An overhanging portion  111 B L-shaped in cross-section is integrally formed so as to extend radially outwards at an end edge  111   a  of a cylindrical body  111 A forming a cylindrical path Y through-which the electron beam  1  passes. The excitation coil  12  is housed within an annular space formed by the body  111 A and the overhanging portion  111 B. 
     On the other hand, the magnetic pole  112  is formed as a truncated pot-shape, with a large diameter opening edge  112 A constituting one end of the magnetic pole  112  being fixed to the body  111 A of the yoke  111 . The magnetic pole  112  is partitioned into an upper magnetic pole  113  far away from the sample  2  and a lower magnetic pole  114  close to the sample  2 . The two magnetic poles  113  and  114  are then connected via an insulator  115  that is an electrically insulating material. 
     The insulator  115  is a cylindrical member corresponding to the dimensional shape of the large diameter opening edge  112 A of the magnetic pole  112 , with the lower magnetic pole  114  being formed integrally with the large diameter opening edge  112 A of the magnetic pole  112  via the insulator  115 . The top surface  113   a  of the upper magnetic pole  113  is configured so as to be positioned closer to the sample  2  than the insulator  115  provided between the upper magnetic pole  113  and the lower magnetic pole  114 . A through-hole  112 B for allowing the electron beam  1  to pass is therefore provided on the same axis as the optical axis X by the top surface  113   a  of the upper magnetic pole  113  and a top surface  114   a  of the lower magnetic pole  114 . 
     A magnetic field for focusing can therefore be formed within the path Y when a current flows in the excitation coil  12  because the single pole-piece lens with an electrostatic bipotential lens  10  is configured in the above manner. An electron beam  1  advancing towards the single pole-piece lens with an electrostatic bipotential lens  10  along the optical axis X from an electron gun (not shown) constituting an electron beam source can therefore be subjected to focusing action due to the magnetic field when passing through the path Y. 
     A negative potential VL (for example, −1 kV) is applied by a high-voltage supply  31  to the lower magnetic pole  114  and a negative potential VS (for example, −1 kV) is applied by a high-voltage supply  32  to the sample  2 . A decelerating electric field can therefore be generated between the upper magnetic pole  113  and the lower magnetic pole  114  and it is therefore possible to superimpose this decelerating electric field on the focusing magnetic field due to the single pole-piece lens with an electrostatic bipotential lens  10 . 
     In order to detect secondary electrons that are electrons emitted from the sample  2  due to the electron beam  1  approaching the single pole-piece lens with an electrostatic bipotential lens  10  along the optical axis X, first and second detectors  41  and  42  are provided at upper and lower stages at the yoke  111  of the single pole-piece lens with an electrostatic bipotential lens  10 . The first detector  41  is provided at the lower part, and a hollow first cylinder  51  encompassing part of the optical axis X is provided at the optical axis X of the first detector  41 . A hollow second cylinder  52  encompassing part of the optical axis X is also similarly provided at the optical axis X of the second detector  42 . 
     A donut-shaped first reflecting plate  61  is provided perpendicularly with respect to the optical axis X at the upper end of the first cylinder  51  and a donut-shaped second reflecting plate  62  is also provided perpendicularly with respect to the optical axis at the upper end of the second cylinder  52 . A negative potential VT is applied to the first cylinder  51  and first reflecting plate  61  by a high-voltage power supply  33  and a negative potential VU is applied to the second cylinder  52  and the second reflecting plate  62  by a high-voltage power supply  34 . 
     A Wien filter  7  constituting a deflector is provided between the first reflecting plate  61  and the second reflecting plate  62  so that the center of the Wien filter  7  overlaps with the optical axis X. When the forces due to the electric field and the magnetic field acting on incident electrons are equal, i.e. when E=u×B or E/B=u, the incident electrons are not deflected. These are Wien conditions, and a deflector utilizing these conditions is a Wien filter. 
     The single pole-piece lens with an electrostatic bipotential lens  10  has the configuration described above. After passing through the space within the second cylinder  52 , the Wien filter  7 , and the space within the first cylinder  51  in that order, the electron beam  1  incident along the optical axis X coming from the electron gun (not shown) is focused so as to be incident to the sample  2  due to the action of the magnetic field formed in the vicinity of the top surface  114   a  of the single pole-piece lens with an electrostatic bipotential lens  10 . The electron beam  1  is made to scan the sample  2  by the scanning deflectors  16  and  17  but in  FIG. 1 , the strength of the scanning deflectors  16  and  17  is zero, i.e. the electron beam irradiates a point corresponding to the center of the screen. 
     An example of the trajectory of the secondary electrons  8  coming from the sample  2  is shown in FIG.  2 . Some of the secondary electrons  8  collide with the first reflecting plate  61  and secondary electrons  8  emitted from the first reflecting plate  61  are detected by the first detector  41 . Secondary electrons  8  passing through the first cylinder  51  are deflected in the direction of the arrow as shown by the dotted line by the Wien filter  7  and are detected by the second detector  42 . Further, some of the secondary electrons  8  deflected by the Wien filter  7  collide with the second reflecting plate  62  and secondary electrons  8  emitted from the second reflecting plate  62  are detected by the second detector  42 . 
     When the amount of current of the incident electron beam  1  is large, the amount of secondary electrons emitted from the sample  2  becomes large and drops in the number of secondary electrons  8  due to dirt on the first and second reflecting plates  61  and  62  can no longer be ignored. However, the trajectory of the secondary electrons  8  can be changed by applying a negative voltage to the first and second cylinders  51  and  52  and the first and second reflecting plates  61  and  62  using the high-voltage power supplies  33  and  34 . 
     For example, when a voltage of −1 kV that is the same as the sample potential and is taken as the negative potential VT and a potential of −1 kV taken as the negative potential VU are applied to the first cylinder  51  and the first reflecting plate  61 , secondary electrons  8  emitted from the sample can be detected by the first detector  41  without hardly colliding with the first cylinder  51  and the first reflecting plate  61 . Namely, if a negative voltage that is the same as or larger than the voltage applied to the sample is applied to the first and second cylinders  51  and  52  and the first and second reflecting plates  61  and  62 , collisions of the secondary electrons  8  with the first and second reflecting plates  61  and  62  can be prevented and soiling of the first and second reflecting plates  61  and  62  can also be prevented. 
     A graph is shown in  FIG. 3  of example calculated trajectories for the case when a negative potential VT of −1 kV and a negative potential VU of −1 kV are applied to the first cylinder  51  and the first reflecting plate  61 , respectively. According to the graph shown in  FIG. 3 , it can be seen that secondary electrons  8  out putted from the sample  2  are detected by the first detector  41  without hardly colliding with the first cylinder  51  and the first reflecting plate  61  at all. 
     If the diameters of the first and second cylinders  51  and  52  are therefore small at around 2 to 3 mm, most of the secondary electrons  8  can be made to face the first detector  41  and the second detector  42  by the first and second reflecting plates  61  and  62 . The force required to deflect the secondary electrons  8  using the Wien filter  7  can therefore be made small, i.e. the electromagnetic field strength of the Wien filter  7  can be made small. In this way, the influence on the incident electron beam  1  can be made small and lowering of the image resolution can be prevented. It is therefore possible to achieve sufficient angular deflection of the secondary electrons  8  by applying negative voltages to the first and second cylinders  51  and  52  and the first and second reflecting plates  61  and  62 . It is also possible to prevent the first and second reflecting plates  61  and  62  from becoming soiled and therefore improve detection efficiency. 
       FIG. 4  is a cross-sectional view of a further embodiment of a single pole-piece lens with an electrostatic bipotential lens of the present invention. At a single pole-piece lens with an electrostatic bipotential lens  10 A shown in  FIG. 4 , an electron beam  1  from an electron gun (not shown) is deflected by deflector  13  of the deflectors  13  and  14  located at the two upper and lower stages and differs from the single pole-piece lens with an electrostatic bipotential lens  10  shown in  FIG. 1  in that the electron beam  1  is incident to the deflector  15  constructed from a magnetic field or electromagnetic field at an angle with respect to the optical axis due to the deflector  14 . The same numerals are used for the sections in  FIG. 4 , that correspond to those in  FIG. 1 , and the descriptions for those sections are omitted. 
     Here, the strength of the scanning deflectors  16  and  17  is zero, i.e. the electron beam is an electron beam irradiating a point at the center of the screen. Further, the deflector  15  is only configured using a magnetic field but is by no means limited in this respect and may also be constructed from an electromagnetic field or a Wien filter. 
     The electron beam  1  incident to the deflector  15  is deflected by the deflector  15  and is incident to the sample  2  along the optical axis X. The secondary electrons  8  from the sample  2  are detected as shown in the case in FIG.  1 . The deflector  15  is only configured from a magnetic field rather than an electric field but in this case also, as in the case shown in  FIG. 1 , the secondary electrons  8  are subjected to a force in a direction opposite to that of the incident electron beam  1 , are deflected in the direction of the first detector  41  and are detected by the first detector  41 . Secondary electrons  8  passing through the first cylinder  51  are then deflected in the direction of the second detector  42  and are detected by the second detector  42 . 
     In this embodiment, as the strength of the force E due to the electric field and the force B due to the magnetic field is not that of the existing Wien conditions, the incident electron beam  1  is deflected within the path Y of the single pole-piece lens with an electrostatic bipotential lens  10 . The electron beam  1  passing through the deflector  15  is therefore incident in a manner perpendicular to the objective lens and it is necessary for the electron beam  1  to be incident to the deflector  15  after being deflected by a deflector provided above the deflector  15 . In the case of a configuration where an electron beam  1  is deflected by a deflector provided at the upper part taken as one stage so as to be incident to the deflector  15 , it is necessary to offset the electron gun axis and the objective lens axis in advance. The configuration of the single pole-piece lens with an electrostatic bipotential lens  10 A and the operation of offsetting the axes in advance are therefore both complex. In this embodiment, after the electron beam  1  is deflected by the second stage deflectors  13  and  14 , the electron beam  1  is incident to the deflector  15 . It is therefore not necessary to perform the operation of offsetting the axes and the configuration and operation of the single pole-piece lens with an electrostatic bipotential lens  10 A can therefore be simplified. 
     Here, when E/B=v/2, compared with conditions for the deflection angle of the secondary electrons  8  that are the same as the case of Wien filter conditions, picture quality is better than in the case for Wien conditions. Further, when E/B=0, i.e. in the case of a deflector using only magnetic field force with the force of the electric field at zero, as with the case of E/B=v/2, better picture quality is obtained than for under the Wien conditions. 
     In this embodiment, deflectors  13  and  14  are provided in addition to the scanning deflectors  16  and  17 . However, the deflectors  13  and  14  can be omitted by superimposing the deflection due to the deflectors  13  and  14  with that of the scanning deflectors  16  and  17 . 
       FIG. 5  shows a still further embodiment of a single pole-piece lens with an electrostatic bipotential lens of the present invention. A single pole-piece lens with an electrostatic bipotential lens  10 B shown in  FIG. 5  differs from the single pole-piece lens with an electrostatic bipotential lens  10 A shown in  FIG. 4  in that two scintillator detectors  41 A and  41 B are provided on axes symmetrical to the optical axis X in place of the first detector  41  (refer to FIG.  4 ). Signals from the two scintillator detectors  41 A and  41 B are then synthesized. The same numerals are used for the sections in  FIG. 5 , that correspond to those in  FIG. 4 , and the descriptions for those sections are omitted. 
     According to this embodiment, a surface image emphasized for unevenness can be obtained even for in-lens SEM with the sample  2  being placed within a strong lens magnetic field because signals from the two scintillator detectors  41 A and  41 B are synthesized. 
       FIG. 6  shows another embodiment of a single pole-piece lens with an electrostatic bipotential lens of the present invention. A single pole-piece lens with an electrostatic bipotential lens  10 C shown in  FIG. 6  differs from the single pole-piece lens with an electrostatic bipotential lens  10 B shown in  FIG. 5  in that a second detector  42 C, second cylinder  52 C and second reflecting plate  62 C are provided on the side of the electron source from the deflectors  13  and  14  and the scanning deflectors  16  and  17 . The same numerals are used for the sections in  FIG. 6 , that correspond to those in  FIG. 5 , and the descriptions for those sections are omitted. 
     According to this embodiment, a distance L between the second detector  42 C and the deflector  15  becomes large and an angle of deflection α of the secondary electrons  8  therefore becomes small because the second detector  42 C, the second cylinder  52 C and the second reflecting plate  62 C are placed on the side of the electron source from the deflectors  13  and  14  and the scanning deflectors  16  and  17 . The amount of deflection at the second cylinder  52 C is therefore d=L tan α. The angle α can therefore be made small to obtain the same value for d as for the case where the distance L is small. The strength of the deflector  15  can therefore be made small and detrimental effects on image quality can therefore be prevented. 
     According to the present invention, as described above, in an electron beam apparatus equipped with a magnetic field lens arranged on the same axis as an optical axis and configured in such a manner that an electron beam from an electron beam source advancing along the optical axis and being incident to the magnetic field lens focused onto a sample subjected to a negative voltage, with electrons emitted from the sample as a result of the sample being irradiated with the electron beam being detected, a conductive cylinder is arranged so as to cover part of the optical axis in such a manner as to permit the passage of an electron beam from the electron beam source, a first detector for detecting emitted electrons of the emitted electrons that do not pass through the cylinder; a second detector for detecting emitted electrons of the emitted electrons that do pass through the cylinder; and a Wien filter deflector for increasing the emitted electrons detected by the second detector for emitted electrons that pass through the cylinder, provided between the cylinder and the second detector; are provided within the magnetic field lens. It is therefore possible to make the force required to deflect the discharged electrons small, i.e. to make the electromagnetic field strength of a Wien filter deflector small, so that substantially all electrons emitted from the sample go towards the first detector and the second detector. In this way, the influence imparted upon the incident electron beam is small, it is possible to prevent image resolution from falling, the efficiency with which emitted electrons are detected is increased, and picture quality can be improved. Further, the same extent of negative potential can be applied to the cylinders etc. as to the sample. This prevents detection efficiency from being reduced due to soiling of the reflecting plates.