Patent Publication Number: US-2022223372-A1

Title: Charged Particle Beam System

Description:
TECHNICAL FIELD 
     The present disclosure relates to a charged particle beam system, and more particularly to a high resolution scanning electron microscope (SEM) for measuring, inspecting, observing, and analyzing a semiconductor device. 
     BACKGROUND ART 
     In the SEM, a diaphragm plate having a diaphragm hole is disposed on a path of an electron beam emitted from an electron source generally for the purpose of adjusting an amount of electron beam, restricting passage, suppressing aberration, or molding a beam shape (hereinafter, an optical element that exerts such a desired effect on an electron beam is referred to as a “diaphragm”). At this time, when an acceleration voltage of the electron beam irradiated on the diaphragm plate is high, there is a case where the electron beam passes through the diaphragm plate (JP-A-2011-243540 (PTL 1)). Further, a phenomenon is known in which the electron beam irradiated to the diaphragm plate is scattered at an edge of the diaphragm hole (JP-A-6-5499 (PTL 2)). These transmitted electrons (beam) or scattered electrons (beam) have different energies and angular distributions from the electron beam (hereinafter, also referred to as “main beam”) that have passed through the diaphragm hole, and thus, a phenomenon is generated in which the electrons converge on a sample in a trajectory different from an electron trajectory of the main beam (hereinafter, the phenomenon is also referred to as “flare”). This flare causes deterioration of the resolution of the SEM image. 
     CITATION LIST 
     Patent Literature 
     PTL 1: JP-A-2011-243540 
     PTL 2: JP-A-6-5499 
     PTL 3: JP-A-6-163371 
     SUMMARY OF INVENTION 
     Technical Problem 
     The transmitted electrons and scattered electrons generated by irradiating the diaphragm plate and the vicinity of the diaphragm hole with the electron beam show different aspects depending on the acceleration voltage of the electron beam. In other words, since the state of the generated flare differs depending on the acceleration voltage of the electron beam used, an appropriate method capable of minimizing the influence of the flare is required for each acceleration voltage. However, in the technologies of the related art including PTL 1 to PTL 3, there is no disclosure of a solution that can appropriately solve the problem that flares having different states are generated for each acceleration voltage. 
     Solution to Problem 
     According to an aspect of the means for solving the above problems, hereinafter, there is provided a charged particle beam system having a computer system for controlling an acceleration voltage of a charged particle beam emitted from a charged particle source, the system including: a first diaphragm group having first and second diaphragms which are diaphragms that act on the charged particle beam and have different thicknesses; and a first diaphragm switching mechanism for switching the diaphragm in the first diaphragm group, in which the computer system controls the first diaphragm switching mechanism so as to switch from the first diaphragm to the second diaphragm according to an increase or decrease of the acceleration voltage. 
     Advantageous Effects of Invention 
     According to the above-described method or configuration, the influence of flare of which the state differs depending on the acceleration voltage of the electron beam can be minimized, and thus, the resolution deterioration of an SEM image due to flare can be suppressed. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic configuration view of an SEM type length measurement system (charged particle beam system) according to Example 1. 
         FIG. 2  is a relationship diagram of the number of electrons passing through a diaphragm plate, an acceleration voltage of an electron beam, and a plate thickness of the diaphragm plate. 
         FIG. 3  is a relationship diagram of the number of electrons scattered at a side wall portion of a diaphragm hole, the acceleration voltage of the electron beam, and the plate thickness of the diaphragm plate. 
         FIG. 4  is a relationship diagram of the acceleration voltage, the plate thickness of the diaphragm plate, and the total number of transmitted electrons and scattered electrons. 
         FIG. 5  is a schematic diagram of the electron beam irradiated to a beam adjustment diaphragm plate. 
         FIG. 6  is a diagram illustrating calculation results of a current amount generated by the transmitted electrons with respect to the plate thickness of the diaphragm plate, a current amount generated by the scattered electrons, and the total amount of the transmitted electrons and scattered electrons. 
         FIG. 7  is a view illustrating an embodiment of a beam adjustment diaphragm plate unit. 
         FIG. 8  is a schematic configuration view of another SEM type length measurement system (charged particle beam system) according to Example 1. 
         FIG. 9  is a flow chart for creating a diaphragm plate selection table. 
         FIG. 10  is a flow chart for determining the beam adjustment diaphragm plate having an optimum plate thickness and executing positioning adjustment of the diaphragm hole. 
         FIG. 11  is a view illustrating another embodiment of the beam adjustment diaphragm plate unit. 
         FIG. 12  is a schematic configuration view of an SEM type length measurement system (charged particle beam system) according to Example 2. 
         FIG. 13  is a view illustrating an embodiment of a beam cut diaphragm unit. 
         FIG. 14  is a view illustrating a relationship of the beam adjustment diaphragm plate, a beam cut diaphragm plate, and the electron beam. 
         FIG. 15  is a flow chart for creating a beam cut diaphragm hole selection table. 
         FIG. 16  is a flow chart for determining a hole diameter of an optimum beam cut diaphragm hole and executing the positioning adjustment of the diaphragm hole. 
         FIG. 17  is a flow chart when the diaphragm plate selection table and the beam cut diaphragm hole selection table are used. 
         FIG. 18  is a schematic configuration view of an SEM type length measurement system (charged particle beam system) according to Example 3. 
         FIG. 19  is a detailed view of the periphery of a beam adjustment diaphragm plate and an energy filter. 
         FIG. 20  is a flow chart when a diaphragm plate selection table, a beam cut diaphragm hole selection table, and an energy filter applied voltage table are used. 
         FIG. 21  is a diagram illustrating an example of the diaphragm plate selection table. 
         FIG. 22  is a diagram illustrating another example of the diaphragm plate selection table. 
         FIG. 23  is a diagram illustrating an example of the beam cut diaphragm hole selection table. 
         FIG. 24  is a diagram illustrating an example of the energy filter applied voltage table. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, embodiments will be described in detail with reference to the attached drawings. In the attached drawings, there is a case where functionally identical elements are represented by the same number or corresponding number. Further, in the drawings used in the following embodiments, there is also a case where hatching is also added to make the drawings easier to understand in plan views. The attached drawings illustrate embodiments and implementation examples in accordance with the principles of the present disclosure, but these are for the purpose of understanding the present disclosure and are never used to interpret the present disclosure in a limited manner. The description of the present specification is merely a typical example, and does not limit the scope of claims or application examples of the present disclosure in any sense. 
     In the present embodiment, the description is given sufficiently in detail for those skilled in the art to implement the present disclosure, but it is necessary to understand that other implementations and embodiments are also possible and it is possible to change the configuration and structure and replace various elements without departing from the scope and spirit of the technical idea of the present disclosure. Therefore, the following description should not be interpreted while being limited thereto. 
     In the embodiment described below, as an example of a charged particle beam system, an SEM type length measurement system (also referred to as a length measuring SEM) that measures a pattern on a semiconductor wafer using an electron beam will be described as an example, but “Scanning Electron Microscope (SEM)” broadly includes a device for capturing an image of a sample using an electron beam. Other examples of the charged particle beam system include a scanning ion microscope for observing a sample, and a focused ion beam system for processing a sample. Further, other examples of the scanning electron microscope include an inspection device using a scanning electron microscope, a review device, a general-purpose scanning electron microscope, a sample processing device including a scanning electron microscope, and a sample analysis device, and the present disclosure is also applicable to these devices. In addition, in the examples described below, examples of the scanning electron microscope include a system in which the scanning electron microscope is connected by a network and a combined device in which a plurality of sets of scanning electron microscopes are combined. 
     Further, in the embodiment described below, the “sample” will be described as an example of a semiconductor wafer on which a pattern is formed, but the present disclosure is not limited thereto, and a metal, ceramics, a biological sample, or the like may be used. 
     In recent years, semiconductor devices have become finer and three-dimensional in structure. For example, in the integrated circuit technology of a logic IC, due to advances in EUV exposure technology, miniaturization with a line width of 10 nm or less is progressing, and especially for the SEM for measuring the dimensions of fine patterns formed on wafers of semiconductors or the like, higher length measurement accuracy and reduction of machine difference between devices are required. Meanwhile, memory devices such as DRAM or NAND are becoming more three-dimensional in structure, and there is an increasing need for measurement of deep hole and deep groove patterns and the like. As represented by this length measuring SEM, in recent years, in the SEM that measures, inspects, observes, and analyzes a semiconductor device, the acceleration voltage of the electron beam is changed within a wide range of several hundreds of V to several tens of kV in order to meet various needs, and the performance that can obtain a high-resolution SEM image is required. At the same time, there is a demand for an SEM in which there is little change in device performance over time and machine differences between devices. 
     Hereinafter, in particular, a scanning electron microscope capable of stably acquiring a high-resolution SEM image over a long period of time and having a small performance difference will be described in detail with reference to the drawings. 
     EXAMPLE 1 
       FIG. 1  illustrates a schematic configuration of an SEM type length measurement system (charged particle beam system) according to Example 1. 
     An SEM type length measurement system  100  includes a beam irradiation system (also referred to as a capturing tool)  101  and a computer system  133 . The computer system  133  includes an overall control unit  102 , a signal processing unit  103 , an input/output unit  104 , and a storage unit  105 . 
     The electrons emitted from an electron gun  106  in the beam irradiation system  101  are accelerated by an acceleration electrode (not illustrated) and are irradiated to a sample  112  (for example, a wafer on which a pattern is formed) as a primary electron beam  107  (also referred to as an electron beam). The beam irradiation system  101  includes a first focusing lens  108  that focuses the electron beam  107 , and a second focusing lens  109  that further focuses the electron beam that has passed through the focusing lens  108 . The beam irradiation system  101  further includes a deflector  110  that deflects the electron beam  107 , and an objective lens  111  that controls the focusing point (focal point) of the electron beam  107 . The electron beam  107  that has passed through each optical element provided in the beam irradiation system  101  irradiates the sample  112  mounted on the stage  113 . Emitted electrons  114  such as secondary electrons (SE) and backscattered electrons (BSE), which are emitted from the sample by irradiation of the electron beam  107 , are guided in a predetermined direction by the deflector  115  (first secondary electron aligner) for deflecting the emitted electrons. The deflector  115  is a so-called Wien filter, and selectively deflects the emitted electrons  114  in a predetermined direction without deflecting the electron beam. Further, the acceleration voltage of the electron beam  107  can be switched and used within a wide range according to the target sample  112 . 
     The detection diaphragm  116  is capable of angularly discriminating the emitted electrons  114 . The emitted electrons  114  that have passed through the detection diaphragm  116  are guided by the deflector  123  (second secondary electron aligner) to a detector  119  disposed off-axis. The detector  119  has a detection surface at a position where the emitted electrons  114  collide, and for example, the emitted electrons incident on the detection surface are converted into an optical signal by a scintillator provided on the detection surface. This optical signal is amplified by a photomultiplier and converted into an electric signal, which becomes the output of the detector. Further, an energy filter  122  provided immediately before the detector  119  can discriminate the emitted electrons  114  having a passing trajectory in the vicinity of an optical axis. Meanwhile, the detector  121  detects tertiary electrons  120  (secondary electrons of the emitted electrons  114 ) generated by the emitted electrons  114  colliding with the wall surface of the detection diaphragm  116 . 
     Further, in the scanning electron microscope exemplified in  FIG. 1 , a beam adjustment diaphragm plate  130 , which has a function of partially restricting the passage of the electron beam and forming the electron beam, and a blanking deflector  131  for restricting the arrival of the electron beam at the sample  112  by reflecting the electron beam  107  off the optical axis. The beam adjustment diaphragm plate  130  is provided with a diaphragm hole  134  through which the electron beam  107  passes. A Faraday cup  132  blocks the electron beam  107  deflected by the blanking deflector  131 , measures the current flowing into the Faraday cup  132 , and accordingly can obtain the number of electrons incident on the Faraday cup  132  per unit time. From the signal output of the Faraday cup  132 , it is possible to monitor the probe current of the electron beam  107  that irradiates the sample. 
     The optical element provided in the scanning electron microscope as described above is controlled by the overall control unit  102 . 
     The signal processing unit  103  generates an SEM image based on the outputs of the detectors  119  and  121 . The signal processing unit  103  generates image data by storing a detection signal in a frame memory or the like in synchronization with scanning by a scanning deflector (not illustrated). When storing the detection signal in the frame memory, the signal profile (one-dimensional information) and the SEM image (two-dimensional information) are generated by storing the detection signal at a position corresponding to the scanning position of the frame memory. 
     Next, the beam adjustment diaphragm plate  130  used in the present example will be described. 
     In general, the beam adjustment diaphragm plate is made of a metal material having a large number of atoms in order to block the electron beam irradiated to the diaphragm plate part other than the diaphragm hole. Otherwise, in order to make the shape of the diaphragm holes uniform, there is a case where a diaphragm plate formed by a semiconductor process using silicon (Si) as a base material and coated with a heavy metal coating film is used as a beam adjustment diaphragm plate. However, in the technology using such a beam adjustment diaphragm plate of the related art, when the acceleration voltage of the electron beam is changed within a wide range of several hundreds of V to several tens of kV to acquire a high-resolution SEM image, the following problem occurs with the switching of the acceleration voltage. 
     When the energy of the electron beam irradiated to the adjustment diaphragm plate used for beam adjustment is high (when the acceleration voltage of the electron beam is high), electrons that pass through the base material part (the diaphragm plate part other than the diaphragm hole) of the diaphragm plate are generated. The electrons that have passed through the base material part of the diaphragm plate have lower energy than that of the electrons that pass through the diaphragm holes, and are emitted from the lower surface of the adjustment diaphragm plate in random directions.  FIG. 2  is a graph illustrating a relationship of the number of electrons passing through the diaphragm plate, the acceleration voltage of the electron beam, and the plate thickness of the diaphragm plate when the base material of the diaphragm plate is made of the same metal material. The number of electrons transmitted through the base material part of the diaphragm plate increases as the plate thickness of the diaphragm plate becomes thinner, and increases as the acceleration voltage of the electron beam increases. 
     Meanwhile, in addition to the electrons passing through the base material part of the diaphragm plate, there are also electrons scattered at the side wall portion (inner wall portion) of the diaphragm hole. The term “scattering” here refers to electrons of which the traveling direction is changed at the side wall portion of the diaphragm hole, or electrons that are reflected at the side wall portion of the diaphragm hole and travel toward the optical axis (or toward the central axis of the diaphragm hole) of the beam irradiation system. The electrons scattered at the side wall portion of the diaphragm hole have lower energy than that of the electrons passing through the diaphragm hole, and fly from the side wall portion of the diaphragm hole in random directions.  FIG. 3  is a graph illustrating a relationship of the number of electrons scattered at the side wall portion of the diaphragm hole, the acceleration voltage of the electron beam, and the plate thickness of the diaphragm plate when the base material of the diaphragm plate is made of the same metal material. The number of electrons scattered at the side wall portion of the diaphragm hole increases as the plate thickness of the diaphragm plate increases, that is, as the height of the side wall portion of the diaphragm hole increases, and increases as the acceleration voltage of the electron beam increases. 
     The electrons transmitted through the base material part of the diaphragm plate or the electrons scattered at the side wall portion of the diaphragm hole cause the above-described flare and cause deterioration of the resolution of the SEM image. Further, when the electrons transmitted through the base material part of the diaphragm plate reach the Faraday cup, there is a concern that erroneous detection of the probe current value of the electron beam is caused. 
     The above-described problems can be solved by the aspect of examples illustrated in the present disclosure. 
     As described above, the inventors have analyzed in detail the relationship of the number of transmitted electrons, the number of scattered electrons, the acceleration voltage of the electron beam, and the plate thickness of the diaphragm plate, and as a result, as illustrated in  FIG. 4 , the inventors newly found that there are plate thicknesses (for example, t 1 , t 2 , t 3  or the like in  FIG. 4 ) at which the total amount of the number of electrons transmitted through the beam-adjusting diaphragm plate and the number of electrons scattered at the side wall portion of the diaphragm hole of the beam adjustment diaphragm plate (total amount of transmitted electrons and scattered electrons) is minimized. In addition, from the relationship illustrated in  FIG. 4 , the inventors have newly found that the plate thickness of the beam adjustment diaphragm plate increases when the acceleration voltage of the electron beam increases, and the plate thickness of the beam adjustment diaphragm plate decreases when the acceleration voltage of the electron beam decreases, and accordingly, the total amount of transmitted electrons and scattered electrons can be minimized. 
     Hereinafter, a specific calculation method regarding the optimum plate thickness of the beam adjustment diaphragm plate will be described. 
       FIG. 5  illustrates a schematic view of the electron beam  107  irradiated to the beam adjustment diaphragm plate  130 . The electron beam  107  is focused by the first focusing lens  108  at a point between the first focusing lens  108  and the diaphragm plate  130  (above the diaphragm plate  130 ). Hereinafter, this focusing point is also referred to as a crossover. Here, it is assumed that the electron beam  107  focused on a crossover (focusing point)  502  spreads at an emission angle α and irradiates the beam adjustment diaphragm plate  130 , the distance from the crossover  502  to the upper surface of the diaphragm is h, the plate thickness (thickness) of the diaphragm plate  130  is t, the radius of the diaphragm hole  134  is r, and the maximum radius of the electron beam irradiated to the upper surface of the beam adjustment diaphragm plate  130  is r′. At this time, a current amount Is of the electron beam (electron beam scattered to the side wall portion of the diaphragm hole  134 ) irradiated to the side wall portion of the diaphragm hole  134  of the diaphragm plate  130  can be expressed as in Equation 1 by using a radiation angle current density J and angles β and γ illustrated in  FIG. 5 . 
         Is=J π(β 2 −γ 2 )   [Equation 1]
 
     When transformed by using r, h, and t, the following Equation 2 is obtained. 
         Is=J π[{tan −1 ( r/h )} 2 −{tan −1 ( r /( h+t ))} 2 ]  [Equation 2]
 
     In addition, a current value Iu of the electron beam irradiated to the upper surface of the diaphragm plate  130  can be expressed as in Equation 3 by using the radiation angle current density J and the angles α and β illustrated in  FIG. 5 . 
         Iu=J π(α 2 −β 2 )   [Equation 3]
 
     When transformed by using r, r′, and h, the following Equation 4 is obtained. 
         Iu=J π[{tan −1 ( r′/h )} 2 −{tan −1 ( r/h )} 2 ]  [Equation 4]
 
     Meanwhile, the electron beam irradiated to the upper surface of the diaphragm plate  130  decays exponentially with respect to the plate thickness t inside the diaphragm plate  130  and is emitted as a transmitted electron from the lower surface of the diaphragm plate  130 . At this time, a current amount It of the electron beam transmitted through the diaphragm plate  130  can be expressed as in Equation 5 by using the current value Iu and a decay constant λ of the electron beam irradiated to the upper surface of the diaphragm plate  130 . The decay constant λ is a parameter determined by the material of the diaphragm plate  130  and the acceleration voltage of the electron beam. 
         It=Iu ×exp(−λ t )   [Equation 5]
 
     From the relationship of the above-described Equations 1 to 5, by obtaining an intersection between the current amount Is of the electron beam scattered at the side wall portion of the diaphragm hole  134  and the current amount It of the electron beam transmitted through the diaphragm plate  130 , the optimum plate thickness t can be determined. The various parameters described in Equations 1 to 5 are parameters obtained by experiments and calculations. 
       FIG. 6  is an example of a graph in which the optimum plate thickness t of the beam adjustment diaphragm plate  130  is obtained by using the above-described relational expression. The graph of  FIG. 6  illustrates the calculation result when the radiation angle current density J is 250 [A/Sr], the radius r of the diaphragm hole  134  is 10 [μm], the radius r′ from the center axis of the electron beam irradiated to the upper surface of the beam adjustment diaphragm plate  130  is 15 [μm], the distance from the crossover  502  to the diaphragm upper surface is 100 [μm], and the decay constant λ is 0.5, as examples of the parameters obtained from the experiment in which a specific acceleration voltage condition was used. From the calculation result of  FIG. 6 , it is possible to obtain the diaphragm plate thickness such that the total amount of the current amount generated by the transmitted electrons and the current amount generated by the scattered electrons is minimized. It can be seen that the plate thickness of the optimum diaphragm plate under this condition is approximately 7 μm. 
       FIG. 7  illustrates an embodiment related to the beam adjustment diaphragm plate. 
     A beam adjustment diaphragm plate unit  700  illustrated in  FIG. 7  is configured with an XY 2-axis electric stage on which a plurality of diaphragm plates having different plate thicknesses are mounted. A Y stage  702  is attached to a stage base  703 . On the Y stage  702 , an X stage  501  that operates in a direction perpendicular to the Y stage is attached. On the X stage  701 , three diaphragm plates  704 ,  705 , and  706  having three types of plate thicknesses t 1 , t 2 , and t 3  are attached via a diaphragm plate base  709 . The diaphragm plates  704  to  706  are each provided with a plurality of diaphragm holes having different hole diameters. The beam adjustment diaphragm plate unit  700  can move the diaphragm plates  704  to  706  to any position in an X direction  707  and a Y direction  708  by the X stage  701  and the Y stage  702 . Further, the diaphragm plate base  709 , the X stage  701 , the Y stage  702 , and the stage base  703  are configured such that a space is created under the diaphragm plates  704  to  706  in order to ensure a path through which the electron beam  107  passing through each diaphragm hole of the diaphragm plates  704  to  706  reaches the sample  112  (not illustrated). Further, the X stage  701  and the Y stage  702  are operated by an actuator (not illustrated) for driving the stage and an encoder (not illustrated) for position monitoring. 
       FIG. 8  illustrates a schematic configuration of an SEM type length measurement system (charged particle beam system)  800  equipped with the beam adjustment diaphragm plate unit  700  illustrated in  FIG. 7 . Since each component other than the beam adjustment diaphragm plate unit  700  in the charged particle beam system  800  is the same as those in  FIG. 1 , the detailed description thereof will be omitted here. 
       FIG. 9  illustrates a flow chart for creating the diaphragm plate selection table in the charged particle beam system  800  of  FIG. 8 . 
     In the following, the processing of each step will be described with “computer system” as the subject (operating subject), but the overall control unit  102  (or processor) may be used as the subject (operating subject) in the description and “various programs” executed by the computer system may be used as the subject (acting subject) in the description. A part or the entirety of the program may be implemented on dedicated hardware or may be modularized. Various programs may be installed in the computer system by a program distribution server or storage media. 
     The computer system  133  calculates in advance the plate thickness of the optimum diaphragm plate in the acceleration voltage range of the usable electron beam  107  by using the above-described Equations 1 to 5 ( 901 ). From the calculation result, the computer system  133  creates a diaphragm plate selection table  904  illustrating the optimum correspondence relationship between the acceleration voltage and each diaphragm plate ( 902 ), and stores the diaphragm plate selection table  904  in the storage unit  105  ( 903 ). As described in  FIG. 4 , the diaphragm plate selection table  904  is created in a form having the correspondence relationship in which the plate thickness of the beam adjustment diaphragm plate increases when the acceleration voltage of the electron beam increases, and the plate thickness of the beam adjustment diaphragm plate decreases when the acceleration voltage of the electron beam decreases. In the flow chart illustrated in  FIG. 9 , another computer system may execute a step of calculating the plate thickness of the optimum diaphragm plate ( 901 ) and a step of creating the diaphragm plate selection table  904  ( 902 ), and the computer system  133  may store the diaphragm plate selection table  904  in the storage unit  105  ( 903 ). 
       FIG. 21  is a diagram illustrating an example of the diaphragm plate selection table. 
     The diaphragm plate selection table  904  is configured such that the diaphragm plates having the optimum plate thicknesses of t 1 , t 2 , and t 3  correspond to the three types of acceleration voltages 25, 35, and 45 [kV], respectively. For example, the computer system  133  selects the diaphragm plate having the plate thickness t 3  when an acceleration voltage of 45 [kV] is input (set) from the input/output unit  104 . 
       FIG. 22  is a diagram illustrating another example of a diaphragm plate selection table  905 . 
     The diaphragm plate selection table  905  is configured so as to achieve correspondence of each of a diaphragm plate having the plate thickness t 1  when the acceleration voltage is less than 30 [kV] (V&lt;30 [kV]), a diaphragm plate having the plate thickness t 2  when the acceleration voltage is 30 [kV] or more and less than 40 [kV] (30≤V&lt;40 [kV]), and a diaphragm plate having the plate thickness t 3  when the acceleration voltage is 40 [kV] or more (V≥40 [kV]). For example, the computer system  133  selects the diaphragm plate having the plate thickness t 3  when an acceleration voltage of 40 [kV] is input (set) from the input/output unit  104 . 
       FIG. 10  illustrates a flow chart for determining the beam adjustment diaphragm plate having an optimum plate thickness and executing the positioning adjustment of the diaphragm hole, in the charged particle beam system  800  of  FIG. 8 . With respect to the acceleration voltage set (changed) by the input/output unit  104  (for example, from a graphical user interface) ( 1001 ), the computer system  133  refers to the diaphragm plate selection table stored in the storage unit  105  ( 1002 ), and determines the diaphragm plate having the optimum plate thickness from the diaphragm plates  704  to  706  ( 1003 ). In other words, the process executed in the steps  1003  to  1003  can also be called a process in which the computer system  133  performs control to select a beam adjustment diaphragm plate having a thick plate thickness when the acceleration voltage is changed in a direction of increasing with respect to a current value, and to select a beam adjustment diaphragm plate having a thin plate thickness when the acceleration voltage is changed in a direction of decreasing with respect to the current value. Otherwise, the computer system  133  can also be called a system that performs control to select a diaphragm plate having a thick plate thickness from among the diaphragm plates having a thin plate thickness (or select a diaphragm plate having a thin plate thickness from among the diaphragm plates having a thick plate thickness) according to the increase or decrease of the input (set) acceleration voltage. After this, the computer system  133  controls the X stage  701  and the Y stage  702  in the beam adjustment diaphragm plate unit  700 , and performs the positioning adjustment of a desired diaphragm hole in the determined diaphragm plate ( 1004 ). After the positioning adjustment of the diaphragm hole, the computer system  133  confirms whether or not the optical axis deviation is within the specifications ( 1005 ), ends the flow when the optical axis deviation is within the specifications, and performs positioning readjustment of the diaphragm hole when the optical axis deviation is out of the specifications ( 1004 ). 
       FIG. 11  illustrates another embodiment related to the beam adjustment diaphragm plate. 
     A beam adjustment diaphragm plate unit  1100  illustrated in  FIG. 11  has a structure in which a disk-shaped beam adjustment diaphragm plate  1101  having a plurality of diaphragm holes  1107  is connected to a motor  1103  via a rotating shaft  1102 . Further, the motor  1103  is attached to an r-axis electric stage  1104 . The disk-shaped beam adjustment diaphragm plate  1101  has a plurality of regions having different thicknesses (for example, thicknesses t 1 , t 2 , and t 3 ) for each predetermined rotation angle in a θ direction  1106 , and a plurality of diaphragm holes  1107  having different hole diameters are formed in each region having different thicknesses. The disk-shaped beam adjustment diaphragm plate  1101  can be rotated in the θ direction  1106  by the motor  1103 , and the r stage  1104  operates in an r direction  1105  by an actuator (not illustrated) for driving the stage and an encoder (not illustrated) for position monitoring. 
     In the SEM type length measurement system (charged particle beam system)  800  of  FIG. 8 , even when the beam adjustment diaphragm plate unit  1100  illustrated in  FIG. 11  is adopted instead of the beam adjustment diaphragm plate unit  700  illustrated in  FIG. 7 , it is possible to create a diaphragm plate thickness selection table  1110  according to the flow chart described in  FIG. 9 . Similarly, it is also possible to determine the diaphragm hole having the optimum plate thickness according to the flow chart described in  FIG. 10  ( 1003 ). After determining the diaphragm hole having the optimum plate thickness, the computer system  133  controls the motor  1103  and the r stage  1104  in the beam adjustment diaphragm plate unit  1100  to perform the positioning adjustment of the diaphragm hole determined with respect to the optical axis  1108  ( 1004 ), and performs confirmation of the optical axis deviation ( 1005 ). 
     According to the present Example 1, even when the electron beam is changed within a wide acceleration voltage range of several hundreds of V to several tens of kV, it is possible to suppress the influence of flare of which the state differs depending on the acceleration voltage, and to acquire a high-resolution image (SEM image) at each acceleration voltage. 
     EXAMPLE 2 
     In the technology using the beam adjustment diaphragm plate of the related art, when the acceleration voltage of the electron beam is changed within a wide range of several hundreds of V to several tens of kV to acquire a high-resolution SEM image, the following second problem occurs with the switching of the acceleration voltage. 
     In other words, as the acceleration voltage of the electron beam is switched, the current amount of the electron beam irradiated to the beam adjustment diaphragm plate changes, and accordingly, there is a problem that the temperature of the beam adjustment diaphragm plate changes and a change (drift) of the diaphragm plate over time occurs. In particular, when the acceleration voltage of the electron beam is significantly changed, the thermal energy generated by the electron beam irradiated onto the diaphragm plate changes significantly, and the expansion and contraction of the diaphragm plate remarkably occur (for example, in JP-A-6-163371 (PTL 3), such problems are described). For example, when the acceleration voltage increases by ΔV, a heat quantity ΔQ generated per unit time in the beam adjustment diaphragm plate is expressed as illustrated in Equation 6 assuming that the current amount of the electron beam irradiated onto the diaphragm plate is I. 
       Δ Q=I·ΔV    [Equation 6]
 
     In this case, the temperature of the beam adjustment diaphragm plate rises due to the generation of the heat quantity ΔQ, and the diaphragm plate thermally expands. When such a change (drift) of the beam adjustment diaphragm plate over time occurs, an event such as a change in the state of the main beam and a change in the optical axis of the imaging unit  101  occurs, and thus, there is a problem that a change in device performance over time and a difference between the devices occurs. 
     The above-described problems can be solved by the aspect of Example 2 illustrated in the present disclosure. 
       FIG. 12  illustrates a schematic configuration of the SEM type length measurement system (charged particle beam system) according to Example 2. 
     In order to solve the above-described second problem, in an SEM type length measurement system  1200  illustrated in  FIG. 12 , above the beam adjustment diaphragm plate unit  1100  of  FIG. 11  described above, a beam cut diaphragm unit  1300  that blocks a part of the electron beam  107  is newly disposed.  FIG. 12  illustrates an enlarged view of the components of the beam adjustment diaphragm plate  130  and the beam cut diaphragm  1101 , and  FIG. 13  illustrates a schematic view of the structure of the beam cut diaphragm unit  1300 . 
     The beam cut diaphragm unit  1300  illustrated in  FIG. 13  has a structure in which a disk-shaped beam cut diaphragm plate  1301  having a plurality of diaphragm holes  1307  is connected to a motor  1303  via a rotating shaft  1302 . Further, the motor  1303  is attached to an r-axis electric stage  1304 . The disk-shaped beam cut diaphragm plate  1301  has a uniform plate thickness, and a plurality of diaphragm holes  1307  having different hole diameters are formed. The disk-shaped beam cut diaphragm plate  1301  can be rotated in a θ direction  1306  by the motor  1303 , and the r stage  1304  operates in an r direction  1305  by an actuator (not illustrated) for driving the stage and an encoder (not illustrated) for position monitoring. The shape of each of the plurality of diaphragm holes  1307  is circular. 
       FIG. 14  is a view illustrating the relationship of the beam adjustment diaphragm plate  1101  of the beam adjustment diaphragm plate unit  1100 , the beam cut diaphragm plate  1301  of the beam cut diaphragm unit  1300 , and the electron beam  107 . Here, it is assumed that the electron beam  107  focused on a predetermined crossover (focusing point)  1401  by the first focusing lens  108  spreads at an emission angle ϕ and travels toward the sample  112 . At this time, the diaphragm hole  1307  and the diaphragm hole  1107  are combined such that a hole diameter ϕ 2  of the diaphragm hole  1307  of the diaphragm plate  1301  is greater than a hole diameter ϕ 3  of the diaphragm hole  1107  of the diaphragm plate  1101  (such that ϕ 2 &gt;ϕ 3 ), and accordingly, a part of the electron beam  107  can be cut (blocked), and the current amount of the electron beam  107  irradiated to the upper surface of the diaphragm plate  1101  can be reduced. In other words, in the configuration without the beam cut diaphragm plate  1301 , all the electron beams that do not pass through the diaphragm hole  1107  need to be cut by the beam adjustment diaphragm plate  1101 , and thus, the influence of the change in thermal energy (change in current amount) due to the change in acceleration voltage of the electron beam is easily received, and the expansion and contraction of the beam adjustment diaphragm plate  1101  remarkably occur. On the other hand, the newly added beam cut diaphragm plate  1301  cuts (blocks) the electron beam (the electron beam irradiated to the region other than the diaphragm hole  1107 ) irradiated to the upper surface of the beam adjustment diaphragm plate  1101  as much as possible, and accordingly, it is possible to reduce the influence of the change in thermal energy due to the change in acceleration voltage of the electron beam as much as possible, and to reduce the expansion and contraction of the beam adjustment diaphragm plate  1101 . 
     In the beam cut diaphragm plate  1301 , by cutting the electron beam irradiated to the region (the upper surface of the beam adjustment diaphragm plate  1101 ) other than the diaphragm hole  1107  of the beam adjustment diaphragm plate  1101  as much as possible, it is possible to reduce the current amount (thermal energy) of the electron beam  107  irradiated to the upper surface of the diaphragm plate  1101  as much as possible. When the hole diameter ϕ 2  of the diaphragm hole  1107  of the beam adjustment diaphragm plate  1101  to be used is determined, the optimum hole diameter ϕ 3  of the beam cut diaphragm plate  1301  can be obtained, for example, as follows. Assuming that the distance from the crossover  1402  to the upper surface of the beam cut diaphragm plate  1301  is L and the distance from the crossover  1402  to the upper surface of the beam adjustment diaphragm plate  1101  is H, the optimum hole diameter ϕ 3  can be expressed as in Equation 7. 
       ϕ 3 =ϕ 2 ×( L/H )   [Equation 7]
 
     Using Equation 7, the hole diameter ϕ 3  of the optimum diaphragm hole  1307  is obtained in advance according to the hole diameter ϕ 2  of the selectable diaphragm hole  1107 , and the optimum combination of ϕ 2  and ϕ 3  can be stored in the storage unit  105  as a beam cut diaphragm hole selection table  1504 . 
       FIG. 15  illustrates a flow chart for creating the beam cut diaphragm hole selection table in the SEM type length measurement system (charged particle beam system)  1200  of  FIG. 12 . The computer system  133  calculates in advance the hole diameter ϕ 3  of the diaphragm hole  1307  of the optimum beam cut diaphragm plate  1301  with respect to the hole diameter ϕ 2  of the diaphragm hole  1107  of the beam adjustment diaphragm plate  1101  that can be used, by using the above-described Equation 7 ( 1501 ). From the calculation result, the computer system  133  creates the beam cut diaphragm hole selection table  1504  illustrating the optimum correspondence relationship between the hole diameter ϕ 2  and the hole diameter ϕ 3  ( 1502 ), and stores the beam cut diaphragm hole selection table  1504  in the storage unit  105  ( 1503 ). Otherwise, another computer system may execute a step of calculating the hole diameter ϕ 3  of the optimum diaphragm hole  1307  ( 1501 ) and a step of creating the beam cut diaphragm hole selection table  1504  ( 1502 ), and the computer system  133  may store the beam cut diaphragm hole selection table  1504  in the storage unit  105  ( 1503 ). 
       FIG. 23  is a diagram illustrating an example of the beam cut diaphragm hole selection table  1504 . 
     The beam cut diaphragm hole selection table  1504  is configured such that the beam cut diaphragm holes  1307  having the optimum hole diameters of X, Y, and Z correspond to the hole diameters A, B, and C of the three types of beam adjustment diaphragm holes  1107 , respectively. For example, when the beam adjustment diaphragm hole  1107  having the hole diameter A is input (set) from the input/output unit  104 , the computer system  133  selects the beam cut diaphragm hole  1307  having the hole diameter X. 
       FIG. 16  illustrates a flow chart for determining the hole diameter of the optimum beam cut diaphragm hole and executing the positioning adjustment of the diaphragm hole, in the SEM type length measurement system (charged particle beam system)  1200  of  FIG. 12 . With respect to the hole diameter ϕ 2  of the beam adjustment diaphragm hole set (selected) by the input/output unit  104  ( 1601 ), the computer system  133  refers to the beam cut diaphragm hole selection table  1504  stored in the storage unit  105  ( 1602 ), and determines the diaphragm hole having the optimum hole diameter ϕ 3  from among the plurality of diaphragm holes  1307  formed on the beam cut diaphragm plate  1301  ( 1603 ). The computer system  133  controls the motor  1103 , the r stage  1104 , the motor  1303 , and the r stage  1304  in the beam adjustment diaphragm plate unit  1100  and the beam cut diaphragm unit  1300 , and performs the positioning adjustment of the set beam adjustment diaphragm hole and the determined beam cut diaphragm hole ( 1604 ). After the positioning adjustment of the both of the diaphragm holes, the computer system  133  confirms whether or not the optical axis deviation is within the specifications ( 1605 ), ends the flow when the optical axis deviation is within the specifications, and performs positioning readjustment of the diaphragm hole when the optical axis deviation is out of the specifications ( 1604 ). 
     According to the embodiment of the above-described disclosure, even when the acceleration voltage of the electron beam is changed, it is possible to reduce the change over time due to the expansion and contraction of the beam adjustment diaphragm plate, and thus, an SEM with little change in the device performance over time and little difference between the devices can be realized. 
     In the beam cut diaphragm hole selection table  1504  of the above-described embodiment, an example in which a table for determining the hole diameter of the optimum beam cut diaphragm hole with respect to the set hole diameter of the beam adjustment diaphragm hole is created in advance. 
     As another embodiment of the beam cut diaphragm hole selection table, a case is considered in which a table for determining the hole diameter of the optimum beam cut diaphragm hole with respect to the setting of the optical conditions of the SEM type length measurement system  1200  is created in advance. For example, when the hole diameter of the beam cut diaphragm hole is calculated and obtained in advance in which the current amount (thermal energy) of the electron beam irradiated to the upper wall surface of the beam adjustment diaphragm plate  1101  is always constant, with respect to the setting (change) of the acceleration voltage of the electron beam  107 , and is stored in the storage unit  105  as the beam cut diaphragm hole selection table  1700 , even in a case where the setting of the acceleration voltage of the electron beam  107  is changed, it is possible to reduce deformation such as expansion and contraction of the beam adjustment diaphragm plate  1101 . The beam cut diaphragm hole selection table  1700  can be created by the same flow chart as that in  FIG. 15 . 
       FIG. 17  illustrates a flow chart when the diaphragm plate selection table  1110  and the beam cut diaphragm hole selection table  1700  are used in the SEM type length measurement system (charged particle beam system)  1200  of  FIG. 12 . With respect to the acceleration voltage set (changed) by the input/output unit  104  ( 1701 ), the computer system  133  refers to the diaphragm plate thickness selection table  1110  stored in the storage unit  105  ( 1702 ), and determines the diaphragm hole  1107  having the optimum plate thickness from among the plurality of diaphragm holes formed on the diaphragm plate  1101  ( 1703 ). Similarly, with respect to the acceleration voltage of the electron beam set (changed) by the input/output unit  104 , the computer system  133  refers to the beam cut diaphragm hole selection table  1700  stored in the storage unit  105  ( 1704 ), and determines the diaphragm hole having the optimum hole diameter ϕ 3  from among the plurality of diaphragm holes  1307  formed on the beam cut diaphragm plate  1301  ( 1705 ). Since the subsequent steps are the same as those in  FIG. 16 , the detailed description thereof will be omitted here. 
     According to the present Example 2, even when the acceleration voltage of the electron beam is changed (switched), it is possible to suppress the drift over time due to the change in heat quantity of the beam adjustment diaphragm plate, and a length measuring SEM with little change in device performance over time and little difference between the devices can be realized. 
     EXAMPLE 3 
     In the disclosure according to Example 1, an example is illustrated in which the total number of the number of electrons transmitted through the beam adjustment diaphragm plate and the number of electrons scattered at the side wall portion of the diaphragm hole of the beam adjustment diaphragm plate can be minimized by using a combination of beam adjustment diaphragm plates having the optimum plate thickness for the desired acceleration voltage of the electron beam. However, in the disclosure according to Example 1, since these transmitted electrons and scattered electrons cannot be completely excluded, the influence of flare on the resolution of the SEM image cannot be eliminated. 
     Therefore, in the present Example, as illustrated in  FIG. 18 , an energy filter  1801  for cutting the transmitted electrons and scattered electrons is disposed below the beam adjustment diaphragm plate. The SEM type length measurement system (charged particle beam system)  1800  of  FIG. 18  is an example in which the energy filter  1801  is added to the SEM type length measurement system illustrated in  FIG. 12 . 
       FIG. 19  illustrates a detailed view of the periphery of the beam adjustment diaphragm plate  1101  and the energy filter  1801 . The energy filter  1801  is formed in a grid pattern by a non-magnetic metal wire such as copper. Further, any voltage can be applied to the energy filter  1801  by a power source  1903 . Electrons  1901  that have passed through the predetermined diaphragm hole  1107  in the beam-adjusting diaphragm plate  1101  and electrons  1902  that have been scattered at the side wall portion of the diaphragm hole  1107  of the beam adjustment diaphragm plate  101  lose the energy of the electrons themselves in the process of transmission and scattering, and thus, the electrons  1901  and  1902  have lower energy than that of the electrons of the main beam (the beam that has passed through the diaphragm hole  1107  of the beam adjustment diaphragm plate  1101 ). In the present Example, this energy difference is used to discriminate the main beam and the transmitted and scattered electrons. In other words, the computer system  133  applies an appropriate negative voltage to the energy filter  1801  and performs control such that only the transmitted and scattered electrons having lower energy than that of the electrons of the main beam are repelled upward. 
     The optimum negative voltage to be applied to the energy filter  1801  is obtained by calculation or experiment in advance according to the acceleration voltage of the electron beam to be used, and the optimum combination of the acceleration voltage and the applied voltage can be stored in the storage unit  105  in advance as the energy filter applied voltage table  2000 . 
       FIG. 24  is a diagram illustrating an example of the energy filter applied voltage table  2000 . 
     The energy filter applied voltage table  2000  is configured such that the optimum energy filter applied voltages of V 1 , V 2 , and V 3  correspond to the three types of acceleration voltages 25, 35, and 45 [kV], respectively. For example, the computer system  133  selects the applied voltage of the energy filter having the plate thickness V 3  when an acceleration voltage of 45 [kV] is input (set) from the input/output unit  104 . 
       FIG. 20  illustrates a flow chart when the diaphragm plate selection table  1110 , the beam cut diaphragm hole selection table  1700 , and the energy filter applied voltage table  2000  are used in the SEM type length measurement system (charged particle beam system)  1800  of  FIG. 18 . Since the flow of steps  1701  to  1707  is the same as that of  FIG. 18 , the detailed description thereof will be omitted here. After executing the steps  1701  to  1706 , when the optical axis deviation is within the specifications ( 1707 ), with respect to the acceleration voltage set (changed) by the input/output unit  104 , the energy filter applied voltage table  2000  stored in the storage unit  105  is referred to ( 2001 ), and the optimum applied voltage for discriminating only the transmitted and scattered electrons is determined ( 2002 ). 
     According to the present Example 3, since it is possible to further suppress the influence of flare by discriminating between the main beam and the transmitted and scattered electrons, even when the electron beam is changed within a wide acceleration voltage range of several hundreds of V to several tens of kV, it is possible to realize a higher resolution of the image (SEM image) at each acceleration voltage. Further, according to the present Example 3, since it is possible to suppress the drift over time of the beam adjustment diaphragm plate, a length measuring SEM with little change in device performance over time and little difference between the devices can be realized. 
     Above, while the embodiments of the present invention have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. The novel embodiments described herein may be embodied in a variety of other forms, and various omissions, substitutions and changes may be made without departing from the spirit of the invention. Such embodiments or modifications thereof are included within the scope and spirit of the invention, and within the scope equivalent to that of the invention described in the scope of the claims. 
     REFERENCE SIGNS LIST 
       100 : SEM type length measurement system (charged particle beam system) 
       101 : beam irradiation system (capturing tool) 
       102 : overall control unit 
       103 : signal processing unit 
       104 : input/output unit 
       105 : storage unit 
       106 : electron gun 
       107 : electron beam 
       108 : first focusing lens 
       109 : second focusing lens 
       110 : deflector 
       111 : objective lens 
       112 : sample 
       113 : stage 
       114 : emitted electron 
       115 : deflector 
       116 : detection diaphragm 
       119 : detector 
       120 : 
       121 : detector 
       122 : energy filter 
       123 : deflector 
       130 : beam adjustment diaphragm plate 
       131 : blanking deflector 
       132 : Faraday cup 
       133 : computer system