Abstract:
The purpose of the present invention is to be able to acquire high-resolution images in a scanning electron microscope using a combination of a cold cathode (CFE) electron source and a boosting process, even at low accelerating voltage enhancing the current stability of the CFE electron source. A configuration in which a CFE electron source ( 101 ), an anode electrode ( 103 ) at positive (+) potential, and an insulator ( 104 ) for isolating the anode electrode ( 103 ) from ground potential are accommodated within a single vacuum chamber ( 105 ), and an ion pump ( 106 ) and a non-evaporable getter (NEG) pump ( 107 ) are connected to the vacuum chamber ( 105 ), is employed.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a scanning electron microscope with a combination of a CFE electron source and a boosting process. 
       BACKGROUND ART 
       [0002]    PTL 1 discloses a boosting process (a process in which primary electron beam is caused to pass in a SEM column with high accelerating voltage with a sample kept at ground potential and to be decelerated to ground potential immediately before passing an objective lens). A boosting process can realize, with low accelerating voltage, resolution much higher than a case where a single out-lens objective lens in which magnetic field is not immersed on a sample is used. Note that, in a commercial SEM with a boosting process, a schottky electron source (SE electron source) is mainly used. 
         [0003]    PTLs 2 and 3 disclose a cold cathode (CFE: cold field-emission) electron source in which the electron source is not always heated. The current stability (10%/min) of the CFE electron source is extremely worse than that of the SE electron source. A SEM with a CFE electron source generally performs flushing to instantaneously heat the electron source in order to clean the surface of the electron source. Note that, emission current of the electron beam is reduced or varied by absorption and desorption of residual gas around the electron source after flushing. The longer the time required for the emission current to be reduced is, the more current stability of the CFE electron source is improved. Since decrease of emission current depends on the amount of residual gas around the electron source, a technique to further improve current stability by reducing residual gas around the electron source as much as possible with a non-evaporable getter pump in an electron gun chamber including the electron source is disclosed (PTLs 2 and 3). 
         [0004]    PTL 4 conceptually discloses a SEM with a combination of a CFE electron source and a boosting process. 
       CITATION LIST 
     Patent Literature 
       [0005]    PTL 1: JP-A-63-160144 (Japanese Patent No. 2789094) 
         [0006]    PTL 2: JP-A-2009-4112 
         [0007]    PTL 3: JP-A-2007-157682 
         [0008]    PTL 4: JP-A-2013-254736 
       SUMMARY OF INVENTION 
     Technical Problem 
       [0009]    Currently, although an SE electron source is used in a commercial SEM with a boosting process, while current stability of the SE electron source is very high (2%/hr or less), energy width thereof is as wide as 0.6 eV, and resolution characteristics are significantly deteriorated at low accelerating voltage (in particular, at 1 kV or less). On the other hand, energy width of a CFE electron source is 0.3 eV when a tungsten single crystal is used and can be a very advantageous electron source when improvement of resolution characteristics at low accelerating voltage of 1 kV or less is required. 
         [0010]    Then, the inventors have pursued design and development to achieve the world&#39;s first commercial SEM with a combination of a CFE electron source and a boosting process; however, when the inventors combined a CFE electron source and a boosting process, they found that electric potential distribution around an extracting electrode and an anode electrode that form the electron source is more easily changed than a case in which a CFE electron source is not combined with a boosting process and that the amount of residual gas around the electron source becomes extremely large to lower current stability. 
         [0011]    By the way, PTL 4 does not disclose a technical problem unique to a combination of a CFE electron source and a boosting process that the inventors have found (relationship between a cause or generation of residual gas and decrease in current stability). 
         [0012]    In the course of study of a method for solving that unique technical problem newly found, the inventors found that a gas component generated from an insulator holding an anode electrode with positive (+) potential when electron beam is being emitted is not sufficiently removed by an existing ion pump, and found an idea to remove that gas component, by an ion pump and a non-evaporative getter (NEG) pump. Note that, because NEG pump has a disadvantage that it cannot exhaust chemically stable gas such as Ar and CH 4 , current stability is not improved only with a NEG pump. On the other hand, an ion pump has an advantage that it can exhaust Ar and CH 4 . Therefore, the inventors considered that it is important to remove a gas component generated from an insulator holding an anode electrode with positive (+) potential when electron beam is being emitted with a combination of an ion pump and a NEG pump in order to realize higher current stability. 
         [0013]    This idea is not disclosed in PTL 4 described above nor in PTLs 2 and 3. 
         [0014]    For example, although PTL 2 (JP-A-2009-4112) discloses an exhaust structure with a combination of an ion pump and a NEG pump as an example, only around the electron source as exhausted by the NEG pump. In addition, PTL 2 regards the length of vacuum waiting time of the MEG pump as a problem, and the idea of the present invention to intentionally widen an area to vacuum-exhaust to the downstream side is not achieved. 
         [0015]    The paragraphs [0084] to [0085] of PTL 3 (JP-A-2007-157682) describe that a valve  21  or a valve  16  is closed before the system is operated and disclose that the vicinity of an electron gun is exhausted only with a MEG pump during SEM observation. That is, PTL 3 does not show an idea to vacuum-exhaust with a combination of an ion pump and a NEG pump during emission of electron beam as in the present invention. 
       Solution to Problem 
       [0016]    In order to solve the problem described above, a configuration described in the claims is adopted, for example. Although the present specification Includes several methods for solving the problem described above, it adopts a configuration of a scanning electron microscope with a combination of a CFE electron source and a boosting process in which a CFE electron source, an anode electrode with positive (+) potential, and an insulator that separates the electrode from ground potential are housed in one vacuum chamber and an ion pump and a NEG pump are connected to the vacuum chamber. 
         [0017]    In addition, as another example, a configuration of a scanning electron microscope with a combination of a CFE electron source and a boosting process in which a first ion pump and a first NEG pump are connected to a first vacuum chamber that houses a CFE electron source and a second ion pump and a second NEG pump are connected to a second vacuum chamber that houses an anode electrode with positive (+) potential and an insulator that separates the electrode from ground potential is adopted. 
       Advantageous Effects of Invention 
       [0018]    According to the present invention, it is possible to obtain an image with high resolution even at low accelerating voltage with increased current stability of a CFE electron source even when a CFE electron source and a boosting process are used in combination. Other problems, structures, and effects other than those described above will be apparent by the description of the following embodiments. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0019]      FIG. 1  is a cross-sectional view illustrating a device structure of an electron source according to a first example and therearound. 
           [0020]      FIG. 2  illustrates an entire structure of a scanning electron microscope according to the first example. 
           [0021]      FIG. 3  shows attenuation characteristics of emission current emitted from the electron source. 
           [0022]      FIG. 4  is a cross-sectional view illustrating a device structure of an electron source according to a second example and therearound. 
           [0023]      FIG. 5  is a cross-sectional view illustrating a device structure of an electron source according to a third example and therearound. 
           [0024]      FIG. 6  is a cross-sectional view illustrating a device structure of an electron source according to a fourth example and therearound. 
           [0025]      FIG. 7  is a cross-sectional view illustrating a device structure of an electron source according to a fifth example and therearound. 
           [0026]      FIG. 8  is a cross-sectional view illustrating a device structure of an electron source according to a sixth. example and therearound. 
           [0027]      FIG. 9  is a cross-sectional view illustrating a device structure of an electron source according to a seventh example and therearound. 
           [0028]      FIG. 10  is a cross-sectional view illustrating a device structure of an electron source according to an eighth example and therearound. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0029]    In an example, a scanning electron microscope that includes a CFE (cold field-emission) electron source, an extracting electrode for extracting electron from the CFE electron source, an anode electrode with positive (+) potential for accelerating the electron and for outputting the electron to an optical system located at a downstream side, a first insulator for separating the anode electrode from ground potential, a single vacuum chamber including both the CFE electron source and the anode electrode, an ion pump connected to the vacuum chamber, a PEG pump connected to the vacuum chamber, a lens tube housing the CFE electron source, the extracting electrode, the anode electrode, the first insulator, the vacuum chamber, the ion pump, and the NEG (non-evaporative getter) pump, and a sample chamber connected to the lens tube is disclosed. In addition, a scanning electron microscope that further includes a second insulator for separating the extracting electrode from ground potential, and a attaching member with at least one side cave connecting a cylindrical part with outer diameter smaller than inner diameter of the lens tube to an inner space and an outer space of the attaching member, which is the cylindrical attaching member for fixing the extracting electrode to a casing via the second insulator and for housing the anode electrode together with the extracting electrode, is disclosed. In addition, an aspect in which a space where the anode electrode is located is connected to a space where the electron source is located via a side cave formed in the attaching member and a connecting path formed between a cylindrical part of the attaching member and the casing is disclosed. 
         [0030]    In addition, in the example, a scanning electron microscope that includes a CFE (cold field-emission) electron source, an extracting electrode for extracting electron from the CFE electron source, a first vacuum chamber located at an upstream side with respect to the extracting electrode and for housing the electron source, an anode electrode with positive (+) potential for accelerating the electron and outputting the electron to an optical system, located at a downstream side, a first insulator for separating the anode electrode from ground potential, a second vacuum chamber located at a downstream side with respect to the extracting electrode and for housing the anode electrode, a first ion pump connected to the first vacuum chamber, a first NEG pump (non-evaporative getter) connected to the first vacuum chamber, a second ion pump connected to the second vacuum chamber, a second NEG (non-evaporative getter) pump connected to the second vacuum chamber, a lens tube for housing the CFE electron source, the extracting electrode, the first vacuum chamber, the anode electrode, the first insulator, the second vacuum chamber, the first and the second ion pumps, and the first and the second NEG pumps, and a sample chamber connected to the lens tube is disclosed. 
         [0031]    In addition, in the example, the CFE electron source is any of a tungsten single crystal chip, a nanochip, a carbon nanotube chip, and a superconductive chip. 
         [0032]    In addition, in the example, the first insulator is made of ceramics or engineering plastic. 
         [0033]    In addition, in the example, the first insulator is attached to a position where second electron emitted when electron passing the extracting electrode collides with the anode electrode and/or third electron emitted when the second electron re-collides with an anode electrode or an inner wall of the vacuum chamber do not collide therewith. 
         [0034]    In addition, in the example, the anode electrode has a structure for prevent second electron emitted when electron passing the extracting electrode collides with the anode electrode and/or third electron emitted when the second electron re-collides with an anode electrode or an inner wall of the vacuum chamber from colliding with the first insulator, 
         [0035]    In addition, in the example, the scanning electron microscope further includes a heater that can temporarily selectively heat the extracting electrode. 
         [0036]    In addition, in the example, the scanning electron microscope further includes a control electrode provided between the extracting electrode and the anode electrode, and a second insulator for separating the control electrode from ground potential, and the second insulator is attached to a position where second electron emitted when electron passing the extracting electrode collides with the control electrode and/or third electron emitted when the second electron re-collides with the extracting electrode, the anode electrode, and/or an inner wall of the vacuum chamber do not collide therewith. 
         [0037]    In addition, in the example, the scanning electron. microscope further includes a control electrode provided between the extracting electrode and the anode electrode and a second insulator for separating the control electrode from ground potential, and the control electrode has a structure for preventing second electron emitted when electron passing the extracting electrode collides with the control electrode and/or third electron emitted when the second electron re-collides with the extracting electrode, the anode electrode, and/or an inner wall of the vacuum chamber from colliding with the second insulator. 
         [0038]    In addition, in the example, the scanning electron microscope further includes a monochrome meter unit including a second insulator as one component and provided between the extracting electrode and the anode electrode, and the monochrome meter unit has a structure for preventing second electron emitted when electron passing the extracting electrode collides with a non-insulator in the monochrome meter unit and/or third electron emitted when the second electron re-collides with the non-insulator, the extracting electrode, the anode electrode, and/or an inner wall of the vacuum chamber from colliding with the second insulator. 
         [0039]    Hereinafter, embodiments of the present invention will be described with reference to the drawings. Note that, the embodiments of the present invention are not limited to examples of modes described later and may be modified in various ways without departing spirit of technique thereof. 
       (1) FIRST EXAMPLE 
     (1-1) Device Structure 
       [0040]    (Entire structure) 
         [0041]      FIG. 2  illustrates an example of an entire structure of a scanning electron microscope  100  according to the present example. The scanning electron microscope  100  includes a lens tube (column)  10 , a sample chamber  121 , various power sources, and a control unit. As the various power sources described above, for example, a power source unit  122  (accelerating voltage power source  123  and extracting voltage power source  124 ) and a booster voltage power source  126  are prepared. The accelerating voltage power source  123  is connected to an electron source  101 , the extracting voltage power source  124  is connected to an extracting electrode  102 , and the booster voltage power source  1 . 26  is connected to an anode electrode  103  and a booster electrode  144  via a booster control substrate  127 . The control unit includes control a microcomputer/control PC  131 , a monitor display  132 , a keyboard  133 , a mouse  134 , and a detection controller  135 . 
         [0042]    In the case cylinder (column)  10 , an objective movable diaphragm  141  for adjusting beam diameter and the amount of probe current of primary electron beam  111  emitted from the electron source  101  s provided at a downstream side of a condenser lens  108 . The primary electron beam  111  passing the objective movable diaphragm  141  is converged by an objective lens  142  and deflection-scanned by a deflector  143 . The primary electron beam  111  to be deflection-scanned is emitted to an observation area of a sample  151  placed on a stage  150  provided in the sample chamber  121 . Note that, the objective movable diaphragm  141  is provided inside the cylindrical booster electrode  144 . 
         [0043]    Reflected electron  152  and secondary electron  153  generated from the observation area of the sample  151  are detected by a detector  154 . Note that, a detection signal of the detector  154  is provided to the control microcomputer/control PC  131  via the detection controller  135  described above, and displayed on the monitor display  132  as a two-dimensional image via image processing by the control microcomputer/control PC  131 . 
         [0000]    (Configuration around Electron Source) 
         [0044]      FIG. 1  illustrates a device structure or the electron source  101  and therearound.  FIG. 1  illustrates a cross-section of a device structure around the electron source. Many of parts appearing in the lens tube (column)  10  have rotational symmetry with respect to the axis line obtained by extending the electron source  101 . 
         [0045]    The electron source  101  according to the present example is an electron source that does not always heat. Such an electron source is generally referred to as CFE electron source. As the electron source  101 , for example, a tungsten single crystal chip, a carbon nanotube chip, a nanochip with curvature radius of the tip sharpened in nanometer order or made by deposition of electron beam or ion beam on a member, or a superconductor chip is used. Certain. CFE electron sources show characteristics with emission current not attenuated; however, the higher the degree of vacuum therearound is, the more stable emission current is with respect to all types of CFE electron source. 
         [0046]    The description returns to the description of the device structure illustrated in  FIG. 1 . The electron source  101  is attached to the lens tube (column)  10  via an insulator  110 . With this structure, the electron source  101  is separated from ground potential and kept at high voltage (for example, negative potential from 0.1 kV to 30 kV). At a downstream side of the electron source  101 , the extracting electrode  102  is provided. The extracting electrode  102  has a substantially well-shaped or substantially deep bowl-shaped cross-sectional shape, and is attached to a cylinder  115  (attaching member) provided inside the lens tube (column)  10  via an insulator  109 . Note that, the lower end of the cylinder  115  is attached to an insulating flange  104 . 
         [0047]    By attachment to the lens tube (column)  10  via the insulator  109 , the extracting electrode  102  is also separated from ground potential and kept at high voltage. Several side caves  115 A are discretely formed on the side of the cylinder  115 . A sub vacuum chamber (corresponding to the downstream part of differential exhaustion system) surrounded by the cylinder  115 , the extracting electrode  102 , and the anode electrode  103  is connected to a sub vacuum chamber (corresponding to the upstream part of differential exhaustion system) on the side where the electron source  101  is provided via the side cave  115 A. Although the side caves  115 A with the same diameter are formed at the same height positions on the right and left sides of the cylinder  115  in  FIG. 1 , the height position where the side cave  115 A is formed is optional. That is, the height positions of the side caves  115 A may be the same or different with respect to the circumferential direction. In addition, the side caves  115 A may be formed in plural with different height at the same positions on the circle. 
         [0048]    The voltage applied to the extracting electrode  102  (extracting voltage) is generally kept at positive potential of 2 kV to 7 kV with respect to the potential applied to the electron source  101 . Note that, when a nanochip is used as the extracting electrode  102 , the extracting voltage is the order of several hundred volts. The primary electron beam  111  is extracted from the tip of the electron source  101  by this extracting voltage. A diaphragm that allows part of the extracted primary electron beam  111  to pass is formed at the center of the extracting electrode  102  (on axis line of the electron source  101 ). Generally, a heating heater  116  is provided on the outer face of the cylindrical part of the extracting electrode  102  that houses the electron source  101 . The heating heater  116  is controlled to be on while the primary electron beam  111  is not being emitted (while SEM observation is being stopped) and heats the extracting electrode  102 . The gas absorbed on the surface of the extracting electrode  102  is thermally desorbed by this heating. As a result, during SEM observation, the amount of generation of gas is suppressed even if the primary electron beam  111  collides with the extracting electrode  102 . 
         [0049]    The anode electrode  103  is provided at a downstream side of the extracting electrode  102 . The anode electrode  103  is attached to the lens tube (column)  10  via the insulating flange  104  (including an insulator  104 A). The anode electrode  103  is separated from ground potential by the insulating flange  104  and kept at high voltage (accelerating voltage). In the present example to which a boosting process is applied, the anode electrode  103  is generally kept at positive potential of maximum 10 kV. With this accelerating voltage, the primary electron beam  111  passing the diaphragm of the extracting electrode  102  is accelerated and output to an optical system (for example, the condenser lens  108 ) at a downstream side. 
         [0050]    The insulating flange  104  separates a space (electron source chamber) at an upstream side with respect to the insulating flange  104  from a space (intermediate chamber) at a downstream side with respect to the insulating flange  104  except a diaphragm formed at the center thereof (on axis line of the electron source  101 ). Hereinafter, container defining a space (electron source chamber) including the electron source  101  is referred to as vacuum chamber  105 . As described, the electron source  101  and the anode electrode  103  according to the present example are housed in one (or common) vacuum chamber  105 . The vacuum chamber  105  is kept at ground potential. A space between the vacuum chamber  105  (electron source chamber) and the intermediate chamber (space housing the condenser lens  108  and the like) are differentially exhausted via the diaphragm of the insulating flange  104 . 
         [0051]    Two vacuum pumps are used for vacuum exhaustion of the vacuum chamber  105 . One is a main vacuum pump  106  and the other is a sub vacuum pump  107 . The main vacuum pump  106  is a pump that can make the vacuum chamber  105  vacuum state of about 10 −8  hPa alone. For example, an ion pump is used as the main vacuum pump  106 . The sub vacuum pump  107  is a pump used for the purpose of increasing the degree of vacuum of the vacuum chamber  105 . For example, a NEG pump is used as the sub vacuum pump  107 . There may be several main vacuum pumps  106  and several sub vacuum pumps  107 . Although  FIG. 1  illustrates a parallel structure in which the sub vacuum pump  107  is provided at an upstream side of the main vacuum pump  106 , a structure in which the sub vacuum pump  107  and the main vacuum pump  106  are serially arranged is also possible. 
       (1-2) Improvement of Current Stability 
       [0052]    Next, a phenomenon that occurs in the scanning electron microscope according to the present example will be described. As described above, potential of the anode electrode  103  is kept higher than that of the extracting electrode  102 . Therefore, secondary electron (secondary electron or reflected electron)  112  generated when the primary electron beam  111  collides with the anode electrode  103  cannot reach the extracting electrode  102 , and collides with the anode electrode  103  again. Secondary electron  113  is generated by this re-collision. 
         [0053]    The secondary electron  113  collides with the insulator  104 A included in the insulating flange  104  by potential relationship with its surrounding (potential relationship with the extracting electrode  102 , the anode electrode  103 , the vacuum chamber  105 , and the condenser lens  108 ). However, the insulator  104 A has a porous structure, and much gas is absorbed thereon or occluded therein. Therefore, the insulator  104 A emits much gas  114  in the vacuum chamber  105  from, a part where the secondary electron  113  collides with. The generation of the gas  114  is a cause of deterioration of current stability when a CFE electron source is used in combination with a boosting process. 
         [0054]    Note that, the insulator  104 A is an insulator for electrically separating ground potential from booster potential, and as described, absorption gas on the surface and occluded gas inside thereof are discharged when the secondary electron  113  collide therewith. In addition, the surface of the insulator  104 A is charged by collision of the secondary electron  113 . Although the insulator  104 A is generally made of ceramics, it may be made of engineering plastic. The gas  114  is also generated by re-collision of secondary electron generated by re-collision of the secondary electron  113  with the inner wall of the lens tube (column)  10  with the insulator  104 A. 
         [0055]    By the way, in the scanning electron microscope according to the present example, the electron source  101  and the anode electrode  103  are included in one (same) vacuum chamber  105 . Specifically, as illustrated in  FIG. 1 , a space including the electron source  101  is connected to a space including the anode electrode  103  at an upper part near the area where the gas  114  is generated (a position near the side cave  115 A with relatively large caliber formed n the cylinder  115  and far from the diaphragm with relatively small caliber formed in the extracting electrode  102 ). Therefore, most part of the generated gas  114  passes not the diaphragm of the extracting electrode  102  but the side cave  115 A with relatively small resistance with respect to molecule level, and is exhausted by the main vacuum pump  106  and the sub vacuum pump  107 . That is, in the present example, most part of the gas  114  passes through a connecting path formed between the lens tube (column)  10  and the cylinder  115  from the side cave  115 A and is exhausted by the main vacuum pump  106  and the sub vacuum pump  107 . 
         [0056]    As a result, the main vacuum pump  106  and the sub vacuum pump  107  can sufficiently exhaust the gas  114  from the vacuum chamber  105  without increasing the amount of residual gas around the electron source even temporarily. 
         [0057]      FIG. 3  shows attenuation characteristics of emission current immediately after flushing. An attenuation characteristics curve  13  according to the present example shows that the time required for emission current to attenuate from the initial set value to half thereof is about 100 min or more. Note that, an attenuation characteristics curve  11  shows characteristics corresponding to the comparative example 1 described later and an attenuation characteristics curve  12  shows characteristics corresponding to the comparative example 2 described later. 
       COMPARATIVE EXAMPLE 1 
       [0058]    A scanning electron microscope in which a vacuum chamber (including an electron source  101 ) at an upstream side with respect to an extracting electrode  102  and a vacuum chamber (including an anode electrode  103 ) at a downstream side with respect to the extracting electrode  102  are independently provided as different vacuum chambers, and a main vacuum pump is connected to each vacuum chamber. 
       COMPARATIVE EXAMPLE 2 
       [0059]    In addition to the configuration of comparative example 1, a scanning electron microscope in which a sub vacuum pump is connected only to the vacuum chamber (including the electron source  101 ) at an upstream side with respect to the extracting electrode  102 . 
         [0060]    As illustrated in  FIG. 3 , a half-value period of the emission current (current applied to entire primary electron beam  111  discharged from the electron source  101 ) from the initial set value in the comparative example 1 is about 1 min, and a half-value period of emission current from the initial set value in the comparative example 2 is about 10 min, which confirms configuration of the present example is superior. This means that high-intensity stability region can be kept longer in the present example than in the comparative examples 1 and 2. Generally, high-intensity stability region refers to region in which emission current is 50% or more with respect to the original and probe current (emission current passing the diaphragm of the extracting electrode  102 ) is 90% or more with respect to the original. In the high-intensity stability region, a unique noise called CFE noise is low, and probe beam scanning a sample surface is stable at high intensity. Therefore, in the present example, it is possible to keep probe current at high intensity longer than in the comparative examples. 
       (1-3) Overview 
       [0061]    By adopting the configuration of the present example, it is possible to cause emission current or probe current to stably operate to obtain an image with high resolution even if the primary electron beam  111  extracted from the CFE electron source is accelerated by accelerating voltage of 2 kV or less (even if a CFE electron source and a boosting process are combined). Therefore, it is possible to observe with SEM even if the sample is a magnetic body or the shape of the sample is not flat. As a result, convenience or the user is dramatically improved. Note that, although scanning electron microscope has been described in the present example, the microscope structure according to the present example can be applied to a compound charged particle beam device in which an FIB and a SEM co-exist. This is also true for the subsequent examples. 
       (2) SECOND EXAMPLE 
       [0062]    (2-1) Device structure 
         [0063]      FIG. 4  illustrates a device structure of an electron source and therearound that form a scanning electron microscope according to the present example. In  FIG. 4 , the parts that correspond to  FIG. 1  are given the same reference numerals. One difference between  FIG. 4  and  FIG. 1  is that, in  FIG. 4 , an outer periphery of an insulator  109  is directly fixed to an inner wall surface of a lens tube (column)  10  over the entire circumference, and as a result, independent two vacuum chambers are provided. That is, in a scanning electron microscope according to the present example, a vacuum chamber  105 A (including an electron source  101 ) that defines a space at an upstream side with respect to an extracting electrode  102  and a vacuum chamber  105 B (including an anode electrode  103 ) that defines a space at a downstream side with respect to the extracting electrode  102  are independently provided. 
         [0064]    Another difference between  FIG. 4  and  FIG. 1  is that, in  FIG. 4 , main vacuum pump and a sub vacuum pump are provided to each of the vacuum chambers  105 A and  105 B. That is, a main vacuum pump  106  and a sub vacuum pump  107  are provided to the vacuum chamber  105 A, and a main vacuum pump  106 A and a sub vacuum pump  107 A are provided to the vacuum chamber  105 B. In the present example, gas  114  generated by collision of secondary electron  113  with an insulator  104 A can be promptly and sufficiently exhausted via the main vacuum pump  106 A and the sub vacuum pump  107 A provided to the vacuum chamber  1053  that is the same as the generation source. 
         [0065]    In other words, most part of the gas  114  generated while the scanning electron microscope is being used is exhausted by the main vacuum pump  106 A and the sub vacuum pump  107 A connected to the vacuum chamber  105 B at a downstream side. As a result, it is possible to prevent possibility of temporal increase of gas concentration around the electron source  101 . Therefore, an effect similar to that of the scanning electron microscope according to the example 1 (attenuation characteristics curve  13  of  FIG. 3 ) can be obtained. 
       (3) THIRD EXAMPLE 
       [0066]    While a technique to efficiently exhaust the gas  114  generated by collision of the secondary electron  113  with the insulator  104 A from the vacuum chamber has been described in the first and the second examples described above, a structure that can prevent generation of the gas  114  will be described in the present example. 
         [0000]    (3-1) Device structure 
         [0067]      FIG. 5  illustrates a device structure of an electron source and therearound that form a scanning electron microscope according to the present example. In  FIG. 5 , the parts that correspond to  FIG. 1  are given the same reference numerals. That is, in the present example, a structure including both an electron source  101  and an anode electrode  103  in one vacuum chamber  105  is assumed. A difference between  FIG. 5  and  FIG. 1  is an attaching structure of an insulator  117  and the anode electrode  103  to an insulating flange  118  of  FIG. 5 . 
         [0068]    In  FIG. 1 , the insulating flange  104  and the insulator  104 A are integrally formed and the anode electrode  103  is placed to be over the surface of the inner periphery of the insulating flange  104  and the surface of a part of the insulator  104 A. Therefore, most parts of the insulator  104 A are exposed to the outside of the anode electrode  103 . However, in the structure illustrated in  FIG. 5 , the anode electrode  103  is placed on the insulator  117  that is placed on the insulating flange  118 . That is, the anode electrode  103  is separated from the insulating flange  118  with the insulator  117  in between. 
         [0069]    The insulator  117  is tubular, and while the inner face thereof is located to the outside of the diaphragm formed at the center of an insulating flange  104 , the outer face thereof is at least provided to be located inside the outer edge of the anode electrode  103 . Provided, however, that the outer face of the insulator  117  needs to be attached to the position where secondary electron  113  generated at the anode electrode  103  and the like cannot come around as illustrated in  FIG. 5 . For example, if the height of the insulator  117  (z direction) is large and a space between the anode electrode  103  and the insulating flange  118  is large, it is provided so that the outer face of the insulator  117  is separated from the outer edge of the anode electrode  103  as much as possible (located inside) On the other hand, if the height of the insulator  117  (z direction) is small and a space between the anode electrode  103  and the insulating flange  118  is small, it is possible to provide the outer face of the insulator  117  near the outer edge of the anode electrode  103 . 
       (3-2) Summary 
       [0070]    If the microscope structure according to the present example is adopted, it is possible to prevent collision of the secondary electron  113  generated at the anode electrode  103  with the insulator  117  even if primary electron beam  111  extracted from the OFF electron source is accelerated by accelerating voltage of 2 kV or less (even if a CFE electron source and a boosting process are combined). Therefore, as with the example described above, it is possible to cause emission current and probe current to stably operate to obtain an image with high resolution. Moreover, since gas  114  is not generated essentially in the present example, it is possible to use a scanning electron microscope while maintaining conditions around the electron source  101  better than in the example  1 . Note that, the microscope structure according to the present example can al so be applied to the second example ( FIG. 4 ) 
       (4) FOURTH EXAMPLE 
       [0071]      FIG. 6  illustrates a device structure of an electron source and therearound that form a scanning electron microscope according to the present example. In  FIG. 6 , the parts that correspond to  FIG. 5  are given the same reference numerals. A difference between  FIG. 6  and  FIG. 5  is a shape of an anode electrode  103 . The anode electrode  103  according to the present example is different in that a structure forming a cross-section L shape is formed as a member with a skirt shape projecting downward from a lower face thereof so as to extend along the outer peripheral edge of the anode electrode  103 . However, the member with the skirt shape mentioned here does not contact with an insulating flange  118 . The member with the skirt shape functions to make a space formed between the anode electrode  103  and the insulating flange  118  smaller and to make it more difficult for secondary electron  113  to come around to collide with an insulator  117 . 
         [0072]    By adopting the microscope structure according to the present example, it is possible to prevent generation of gas in a vacuum chamber  105  more than in the third example. In addition, since the size of the space can be adjusted by the length of the member with the skirt shape of the anode electrode  103 , it is possible to increase the distance between the anode electrode  103  and the insulating flange  118  by increasing the height of the insulator  117 . The microscope structure according to the present example can also be applied to the second example ( FIG. 4 ). 
       (5) FIFTH EXAMPLE 
       [0073]      FIG. 7  illustrates a device structure of an electron source and therearound that form a scanning electron microscope according to the present example. In  FIG. 7 , the parts that correspond to  FIG. 6  are given the same reference numerals. A difference between  FIG. 7  and  FIG. 6  is a shape of an anode electrode  103 . The anode electrode  103  according to the present example is different from the structure of the fourth example in that the structure forming a cross-section L shape is formed as a wall structure projecting upward from an upper surface thereof so as to extend along the outer peripheral edge of the anode electrode  103 . 
         [0074]    The reason for using the substantially bowl-shaped or substantially deep plate-shaped anode electrode  103  is to prevent secondary electron  113  from coming out of the anode electrode  103 . Also by this means, it is possible to reduce the amount of generation of gas  114  by collision of the secondary electron  113  with an insulator  117  as with the fourth example. Note that, the height of the wall structure can be any height in so far as the secondary electron  113  can be prevented from coming around. 
         [0075]    By adopting the microscope structure according to the present example, it is possible to prevent generation of gas  114  in a vacuum chamber  105  as with the fourth example described above. In addition, since the secondary electron  113  is prevented from coming around by the wall structure, the height of the insulator  117  can be arbitrarily set. The microscope structure according to the present example can also be applied to the second example ( FIG. 4 ). 
       (6) SIXTH EXAMPLE 
       [0076]      FIG. 8  illustrates a device structure of an electron source and therearound that form a scanning electron microscope according to the present example. In  FIG. 8 , the parts that correspond to  FIG. 7  are given the same reference numerals difference between  FIG. 8  and  FIG. 7  is an attaching structure of an anode electrode  103  against an insulating flange  118 . Specifically, it is different in that it is connected to an insulator  117  at an outer face near an upper end of the wall structure projecting upward from an upper face of the anode electrode  103  along the outer peripheral edge and is attached to a cylinder  115  that is attached to the insulating flange  118  via the insulator  117 . In  FIG. 8 , the insulator  117  is attached to the cylinder  115  at a position lower than a side cave  115 A. However, the attaching structure is not limited thereto and may be a structure in which an insulator  109  and the insulator  117  are directly attached to a lens tube (column)  10 . 
         [0077]    Note that, in the present example, the height of the wall structure projecting upward from the upper face of the anode electrode  103  along the outer peripheral edge is higher than that of the fifth example (at least higher than a lower end face of an extracting electrode  102 ). That is, the anode electrode  103  according to the present example has a bowl shape or well shape deeper than that of the fifth example. Since the height of the wall structure formed in the anode electrode  103  sufficiently high, there is almost no possibility that secondary electron  113  comes over the wall structure described above to collide with the insulator  117 . 
         [0078]    Also by adopting the microscope structure according to the present example, it is possible to prevent generation of gas in a vacuum chamber  105  as with the fifth example described above. The microscope structure according to the present example can also be applied to the second example ( FIG. 4 ). 
       (7) SEVENTH EXAMPLE 
       [0079]      FIG. 9  illustrates a device structure of an electron source and therearound that form a scanning electron microscope according to the present example. In  FIG. 9 , the parts that correspond to  FIG. 1  are given the same reference numerals. A difference between  FIG. 9  and  FIG. 1  is that, in  FIG. 9 , a control electrode  120  is provided between an extracting electrode  102  and an anode electrode  103 . The control electrode  120  is a member provided for the purpose of adjusting so that a light source position of primary electron beam  111  is not changed by lens function of the extracting electrode  102  and the anode electrode  103  (so that the light source position is kept constant) to improve control accuracy of the electron optical system. 
         [0080]    However, gas is discharged by collision of electron generated by collision of the primary electron beam  111  passing the extracting electrode  102  with the control electrode  120  and/or electron generated by re-collision of the electron that has collided with the control electrode  120  with a component in an electron gun with the insulator. 
         [0081]    Then a structure for preventing collision of discharge electron with the insulator is adopted for the control electrode  120 . One structure is for preventing collision of discharge electron with an insulator  119  for attaching the control electrode  120 . As this structure, the control electrode  120  according to the present example adopts a structure similar to that of the anode electrode  103  illustrated in  FIG. 8 . That is, a structure in which the control electrode  120  s formed as a bowl-shaped or deep plate-shaped electrode with a diaphragm formed at the center of the bottom thereof and is connected to the insulator  119  at an outer face near an upper edge of the wall structure projecting upward from the upper face of the control electrode  120  so as to extend along the outer peripheral edge is adopted. Note that, the insulator  119  is fixed to a cylinder  115  that is attached to an insulating flange  104 . Also here, the insulator  119  is attached to a position lower than a side cave  115 A so as not to close the side cave  115 A formed in the cylinder  115 . 
         [0082]    Another structure is a structure in which collision of secondary electron  113  emitted from the anode electrode  103  with an insulator  104 A is prevented. As this structure, a structure in which a member with a skirt shape projecting downward from the lower face so as to extend along the outer peripheral edge of the control electrode  120  is provided to the control electrode  120  according to the present example is adopted. However, the structure illustrated in  FIG. 6  or the structure illustrated in  FIG. 7  may be adopted for the anode electrode  103  instead of providing a member with a skirt shape to the control electrode  120 . 
         [0083]    As described above, it is possible to effectively prevent generation of gas  114  in a vacuum chamber  105  by adopting a structure in which the secondary electron  113  does not collide with the insulators  119  and  104 A as in the present example even when the control electrode  120  is provided between the extracting electrode  102  and the anode electrode  103 . Note that, an example of a structure of the control electrode  120  illustrated in  FIG. 9  is merely an example, and any structure is acceptable in so far as it is a structure in which the secondary electron  113  does not collide with the insulator  104 A or the insulator  119 . The microscope structure according to the present example can also be applied to the second example ( FIG. 4 ). 
       (8) EIGHTH EXAMPLE 
       [0084]      FIG. 10  illustrates a device structure of an electron source and therearound that form a scanning electron microscope according to the present example. In  FIG. 10 , the parts that correspond to  FIG. 1  are given the same reference numerals. A difference between  FIG. 10  and  FIG. 1  is that, in  FIG. 10 , a monochrome meter unit  128  is provided between an extracting electrode  102  and an anode electrode  103 . The structure of monochrome meter unit  128  may be any structure in so far as energy width of primary electron beam  111  emitted from an electron source  101  is reduced. 
         [0085]    Energy width can further be reduced by combination of a CFE electron source and the monochrome meter unit  128  as with the effect of reduction of energy width by switching an SE electron source with a CFE electron source. For example, it is possible to obtain an image with much higher resolution than that in the examples described above by low accelerating voltage of 1 kV or less, for example. Note that, the monochrome meter unit  128  may also include an insulating member (insulator) Then, various structures described in other examples described above are adopted to prevent collision of secondary-generated discharge electron with an insulating member inside the monochrome meter unit  128 . 
         [0086]    According to the present example, it is possible to obtain an image with high resolution by accelerating voltage much lower than that in the examples described above. The microscope structure according to the present example can also be applied to the second example ( FIG. 4 ). 
       (9) OTHER EXAMPLES 
       [0087]    The present invention is not limited to configurations of the examples described above, but includes various modifications. For example, the example described above describes one or more of examples in detail in order to clearly describe the present invention, and some of the components that are described may riot necessarily included. In addition, a part of an example can be replaced by a configuration of another example, and a configuration of another example can be added to a configuration of one example. In addition, another configuration can be added to a configuration of each example, one or more configurations of each example can be replaced by other configurations, and one or more configurations of each example can be removed. In addition, control line and information line show those that are deemed necessary for description, but not all control lines and information lines necessary for achieving a product are shown. Actually, it should be understood that almost all configurations are connected to one another. 
       REFERENCE SIGNS LIST 
       [0000]    
       
           10  lens tube (column) 
           11  attenuation characteristics curve (comparative example 1) 
           12  attenuation characteristics curve (comparative example 2) 
           13  attenuation characteristics curve (example 1) 
           101  electron source 
           102  extracting electrode 
           103  anode electrode 
           104  insulating flange 
           104 A insulator 
           105  vacuum chamber 
           105 A vacuum chamber 
           105 B vacuum chamber 
           106  main vacuum pump 
           106 A main vacuum pump 
           107  sub vacuum pump 
           107 A sub vacuum pump 
           108  condenser lens 
           109  insulator 
           110  insulator 
           111  primary electron beam 
           112  secondary electron (secondary electron or reflected electron) 
           113  secondary electron (electron after re-collision) 
           114  gas 
           115  cylinder 
           115 A side cave 
           116  heating heater 
           117  insulator 
           118  insulating flange 
           119  insulator 
           120  control electrode