Abstract:
Foreign substances present in a sample chamber are attached to or drawn close to an objective lens and an electrode disposed close to the objective lens by applying a higher magnetic field than when normally used to the objective lens and applying a higher electric field than when normally used to the electrode disposed close to the objective lens. A stage is moved such that the center of an optical axis is located directly above a dedicated stand capable of applying voltage, the magnetic field of the objective lens is turned off, and then the potential difference between the electrode disposed close to the objective lens and an electrode disposed close to the sage is periodically maximized and minimized to thereby forcibly drop the foreign substances onto the dedicated stand capable of applying voltage.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a charged particle beam device for irradiating a sample with a charged particle beam and particularly to a method for removing contaminations or particles in a charged particle beam device capable of removing contaminations or particles present in a vacuum chamber and the charged particle beam device. 
       BACKGROUND ART 
       [0002]    A charged particle beam device represented by a critical dimension scanning electron microscope (CD-SEM) for semiconductor wafer measurement is a device which forms an image based on signals (secondary electrons and/or reflected electrons) obtained by scanning a sample with a charged particle beam. As an object of inspection and measurement using the CD-SEM, there is a semiconductor device. 
         [0003]    Miniaturization of the semiconductor device is advanced for improvement of device performance and circuit performance. Requirement for killer contaminations or particles permitted in the semiconductor device is getting also more severe by the miniaturization of the semiconductor device. As specific examples of contaminations or particles, there are contaminations due to outgas of the semiconductor device itself, minute contaminations or particles brought by the semiconductor device, dust from a sliding portion of the CD-SEM, and the like. 
         [0004]    The contaminations or particles float in a vacuum sample chamber and attach to a stage, an objective lens, a wall of the sample chamber, and the like. If contaminations or particles attach to the semiconductor device being measured, there is a possibility that the yield is reduced, and accordingly management thereof is required. In current circumstances, contaminations or particles are measured periodically and, when they become out of specifications, a part of generation of contaminations or particles is cleaned with ethanol to remove contaminations or particles. 
         [0005]    On the other hand, Patent Literature 1 discloses a method for removing contaminations or particles effectively while suppressing the contaminations or particles from attaching to a sample by turning off or reducing magnetization for an objective lens for focusing an electron beam or by turning off or lowering an applied voltage of a boosting plate for accelerating the electron beam while passing through the objective lens when the sample is removed from beneath the objective lens. 
       CITATION LIST 
     Patent Literature 
       [0006]    Patent Literature 1: JP-B-4945267 (corresponding U.S. Pat. No. 7,626,166) 
       SUMMARY OF INVENTION 
     Technical Problem 
       [0007]    The contaminations or particles can be removed by cleaning with ethanol; however, the sample chamber is required to be exposed to the atmosphere and there is a possibility that the contaminations or particles entering into the sample chamber when the sample chamber is exposed to the atmosphere reattach to the stage, the objective lens, the walls of the sample chamber, and the like and/or float in the sample chamber in a vacuum even if the sample chamber is evacuated after cleaning Further, the down time of several hours is required for exposure to the atmosphere and evacuation, and it is not practical to conduct cleaning with ethanol frequently. 
         [0008]    In the method for turning off or reducing magnetization for the objective lens or turning off or lowering the applied voltage to the boosting plate for accelerating the electron beam while passing through the objective lens as described in Patent Literature 1, forces of attaching the contaminations or particles to the objective lens or the boosting plate are weakened, so that the contaminations or particles fall down freely by the gravity. Accordingly, only when the gravity is greater than the forces of attaching the contaminations or particles, the contaminations or particles are made to fall down and there is a case where the contaminations or particles having large attaching forces cannot be removed sufficiently. 
         [0009]    More specifically, even when the magnetization for the objective lens is turned off or reduced, influence of hysteresis remains and, even when the applied voltage to the boosting plate is turned off or reduced, influence of Coulomb force acting between the contaminations or particles and the boosting plate remains; accordingly, there is a case where the contaminations or particles remain to be attached. When such contaminations or particles are accumulated to form a large lump, it falls down by the gravity even when the objective lens is magnetized; it is difficult to grasp timings of the falls and it is desirable that the contaminations or particles are removed promptly before the contaminations or particles are accumulated. 
         [0010]    The following description is made to a method for removing contaminations or particles in a charged particle beam device and the charged particle beam device with which even the contaminations or particles having large attaching forces are promptly removed while preventing them from falling down on a sample and the like or being scattered. 
       Solution to Problem 
       [0011]    According to an aspect for achieving the above object, there is proposed a charged particle beam device including: an objective lens to focus a charged particle beam emitted from a charged particle source; a control unit to control strength of the objective lens; a vacuum chamber to maintain an atmosphere around a sample to be irradiated with the charged particle beam under vacuum; and a contamination or particle collector to retrieve contaminations or particles on a stage to which a sample is arranged or in a vacuum chamber; wherein the control unit moves the contamination or particle collector or the stage so that the contamination or particle collector is positioned below a beam passage opening of the objective lens, and applies a voltage to an electrode and/or a magnetic pole to generate potential difference between the contamination or particle collector or the stage and the objective lens so as to form the potential difference between the contamination or particle collector and the objective lens or between the stage and the objective lens while the contamination or particle collector is positioned below the beam passage opening of the objective lens. 
         [0012]    Further, according to another aspect for achieving the above object, there is proposed a contamination or particle removing method in a charged particle beam device, for removing contaminations or particles in a vacuum chamber in the charged particle beam device, including the steps of: moving a stage in which a contamination or particle collector or a sample is disposed so that the contamination or particle collector, which retrieves the contaminations or particles, is positioned below a beam passage opening of an objective lens which focuses a charged particle beam; and forming potential difference between the contamination or particle collector or the stage and the objective lens while the contamination or particle collector is positioned below the beam passage opening of the objective lens. 
       Advantageous Effects of Invention 
       [0013]    According to the above configuration, since the contaminations or particles can be removed from the objective lens and the like while the contamination or particle collector is positioned below the beam passage opening of the objective lens, the contaminations or particles can be retrieved while suppressing the possibility that the contaminations or particles are attached to the sample and/or the contaminations or particles spread in the vacuum chamber. 
         [0014]    Other objects, features, and advantages of the invention will become apparent from the following description of the embodiment of the invention taken in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1  It shows a schematic of a scanning electron microscope; 
           [0016]      FIG. 2  It is a flow chart showing collection and retrieval process of contaminations or particles; 
           [0017]      FIG. 3  It shows a sample stage provided with a contamination or particle collector; 
           [0018]      FIG. 4  It shows a schematic of the contamination or particle collector; 
           [0019]      FIG. 5A  It shows movement of contaminations or particles in a contamination/particle collection process and a contamination/particle retrieval process; 
           [0020]      FIG. 5B  It shows movement of the contaminations or particles in the contamination/particle collection process and the contamination/particle retrieval process; 
           [0021]      FIG. 5C  It shows movement of the contaminations or particles in the contamination/particle collection process and the contamination/particle retrieval process; 
           [0022]      FIG. 6  It shows a circuit configuration containing a power supply for generating potential difference between the contamination or particle collector and an objective lens; 
           [0023]      FIG. 7  It shows a schematic of a scanning electron microscope provided with an electrode for controlling an electric field on a sample; 
           [0024]      FIG. 8  It shows an example of a boosting plate for accelerating a beam and an objective lens; 
           [0025]      FIG. 9  It shows an example of an objective lens in which part of a magnetic pole piece is rendered to be an accelerator for accelerating a beam; 
           [0026]      FIG. 10  It shows a schematic of a scanning electron microscope including a contamination or particle collector at the bottom of a sample chamber; 
           [0027]      FIG. 11  It shows an example in which a contamination or particle collector is mounted on a sample stage; 
           [0028]      FIG. 12  It is a flow chart showing the process for retrieving contaminations or particles by introducing a contamination or particle collector into a sample chamber; and 
           [0029]      FIG. 13  It is a timing chart showing change of voltages applied to electrodes around a sample with respect to time. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0030]    A scanning electron microscope provided with a contamination or particle collector is now described hereinafter. In the present embodiment, particularly, an example for controlling a magnetizing current and/or an applied voltage to let an objective lens generate a stronger magnetic field than normally used, to let an electrode disposed around the objective lens generate a greater electric field than normally used, or to let the magnetic field and the electric field become greater than normally used prior to retrieval of the contaminations or particles is described. The stronger magnetic field and electric field than being used for the scanning electron microscope are formed to thereby be able to make the contaminations or particles present in a sample chamber be attached to an objective lens and an electrode disposed around the objective lens or be attracted to the vicinity thereof. The attached or attracted contaminations or particles can be forced to fall down on the contamination or particle collector disposed near a sample stage by forming potential difference between the objective lens or the electrode disposed around the objective lens and an electrode disposed around the stage, so that the contaminations or particles can be removed while suppressing the possibility that the contaminations or particles are attached to a sample or scattered. Further, at this time, the potential difference can be minimized and maximized periodically to thereby promote removal of the contaminations or particles attached to the objective lens and the like and enhance the retrieval efficiency. 
         [0031]    Incidentally, in the embodiment, as described above, the example in which the contaminations or particles once attracted to the vicinity of the objective lens are retrieved by the electric field and/or the magnetic field is mainly described; however, when attachment of the contaminations or particles is remarkable, only retrieval using the electric field may be performed. 
         [0032]    With the above configuration, the contaminations or particles in the sample chamber can be actively gathered without exposing the sample chamber to the atmosphere and the gathered contaminations or particles can be collected with high efficiency. 
         [0033]      FIG. 1  is a schematic diagram illustrating a scanning electron microscope according to the present invention. An electron beam  103  extracted out from an electron source  101  by an extraction electrode  102  and accelerated by an acceleration electrode not shown is squeezed by a condenser lens  104  that is a mode of a converging lens, and then is scanned on a sample  109  one-dimensionally or two-dimensionally by a scanning deflector  105 . The electron beam  103  is decelerated by a negative voltage applied to an electrode included in a sample stage  108  and is focused by lens action of an objective lens  106  to impinge on the sample  109 . 
         [0034]    When the sample  109  is irradiated with the electron beam  103 , electrons  110  such as secondary electrons and backscattered electrons are emitted from an irradiation spot. The emitted electrons  110  are accelerated in the direction toward the electron source by the acceleration action based on negative voltage applied to the sample and collide with a conversion electrode  112  to generate secondary electrons  111 . The secondary electrons  111  emitted from the conversion electrode  112  are caught by a detector  113  and output of the detector  113  varies in accordance with an amount of the caught secondary electrons. Brightness of a display unit not shown is changed in accordance with the output. For example, when a two-dimensional image is formed, a deflection signal to the scanning deflector  105  and the output of the detector  113  are synchronized with each other to thereby form an image of a scanned area. 
         [0035]    Moreover, the scanning deflector  105  may be supplied with a deflection signal for moving the field of view in superposition on a deflection signal for making two-dimensional scanning in the field of view. Deflection by this deflection signal is also called an image shift deflection and the position of the field of view of the electron microscope can be moved without moving the sample using the sample stage or the like. In the present embodiment, an example in which the image shift deflection and the scanning deflection are performed by a common deflector is described; however, the deflector for image shift and the deflector for scanning may be provided separately. 
         [0036]    Incidentally, in the example of  FIG. 1 , there has been described the example in which the electrons emitted from the sample are once converted at the conversion electrode to be detected; however, it is a matter of course that the present invention is not limited to such a configuration and it is possible to adopt a configuration in which an electron multiplier tube or a detection plane of a detector is arranged on trajectories of accelerated electrons, for example. 
         [0037]    A control unit  120  controls each component of the scanning electron microscope and has the functions of forming an image based on the detected electrons and measuring a width of pattern formed on the sample based on the intensity distribution of the detected electrons called a line profile. 
         [0038]    Furthermore, in the scanning electron microscope exemplified in  FIG. 1 , a preliminary exhaust chamber  115  for preliminarily evacuating an atmosphere of the sample at the time that the sample is introduced into a sample chamber  107  and a mini environment  117  for forming clean atmosphere environment are installed. Moreover, vacuum valves  114  and  116  for attaining vacuum sealing are disposed therebetween. 
         [0039]    A positive or negative voltage is applied to an electric-field forming electrode  118  in response to an instruction from the control unit  120 , so that a surface electric field on the sample  109  is controlled. Normally, the electric-field forming electrode  118  is used to form an electric field for pulling up electrons emitted from the sample  109  toward the electron source  101  and/or to form an electric field for retarding back electrons having low energies. Further, a contamination or particle collector  119  is disposed on the sample stage  108 . As exemplified in  FIG. 3 , the contamination or particle collector  119  is disposed outside of a mounting position of the sample  109  on the sample stage  108 . 
         [0040]    The method of retrieving the contaminations or particles within the scanning electron microscope as configured above is now described. Incidentally, the following embodiment is described by taking the scanning electron microscope as an example; however, it is not limited thereto and, for example, it is applicable to retrieval of contaminations or particles of other charged particle beam devices such as a focused ion beam in which an ion beam is used as a probe. 
         [0041]      FIG. 2  is a flow chart showing collection and retrieval process of contaminations or particles. First, the sample  109  is transferred out from the sample chamber  107  and the vacuum valve  114  is closed (Steps S 201  and S 202 ). Next, contaminations or particles present in the sample chamber are attached to the objective lens and the electrode disposed around the objective lens or attracted to the vicinity thereof by forming a stronger magnetic field than normally used to the objective lens and/or applying a stronger electric field than normally used to the electrode disposed around the objective lens (Step S 203 ). The electric field for collecting the contaminations or particles is formed by applying, for example, to the electric-field forming electrode  118  a higher voltage as compared with the case of being used as an electron microscope. When the device is used as an electron microscope, the magnetizing current and the like of the objective lens are controlled in order to narrow the beam; however, in the case of this example, since the magnetic field and the like are generated in order to collect the contaminations or particles, priority is given to the collection efficiency irrespective of the focusing conditions and the like and a stronger electric field or magnetic field is formed as compared with the case where the device is used as an electron microscope. Namely, the collection efficiency of the contaminations or particles can be enhanced by application of a greater voltage and/or supply of a larger current as compared with the case where irradiation is made with the beam. 
         [0042]      FIGS. 5A and 5B  are diagrams showing positions of contaminations or particles  501  before and after collection of the contaminations or particles by forming a strong electric field and the like. As shown in the figures, the contaminations or particles  501  are gathered on the objective lens  106  and/or the electric-field forming electrode  118  by strong magnetization of the objective lens  106  or high voltage application to the electric-field forming electrode  118 . Since the contaminations or particles of metal are attracted to the objective lens and the contaminations or particles of insulator and non-conductor charged with electricity are attracted to the electrode disposed around the objective lens, it is desirable that the strong magnetization of the objective lens and the high voltage application to the electric-field forming electrode are performed together in order to remove the contaminations or particles without omission. 
         [0043]    After the contaminations or particles are attached or attracted to the vicinity, the stage is moved so that the center of the optical axis is positioned right above a dedicated stand to which voltage can be applied (Step S 204 ). In this case, the sample stage  108  is moved so that the contamination or particle collector  119  is positioned right below a passage opening of the objective lens which the electron beam passes through and in which the contaminations or particles are concentrated most. Next, while the contamination or particle collector  119  is positioned below the beam passage opening of the objective lens  106 , the magnetic field of the objective lens  106  is turned off or weakened relatively and an electric field is formed by application of voltage to the electrode disposed around the objective lens, so that the contaminations or particles are forced to fall down on the contamination or particle collector  119 .  FIG. 5C  is a diagram showing the state that the contaminations or particles are made to fall down on the contamination or particle collector  119  by control of the electric field. 
         [0044]    In the case of the present embodiment, after the magnetic field of the objective lens is turned off, the potential difference between the electrode disposed around the objective lens and the electrode disposed around the stage is maximized and minimized periodically to thereby force the contaminations or particles to fall down on the dedicated stand to which voltage can be applied. The reason that the magnetic field of the objective lens is turned off is that the force of attaching the contaminations or particles to the objective lens is weakened and at the same time the contaminations or particles forced to fall down are suppressed from being reattached to the objective lens. 
         [0045]    When the potential difference is generated between the electrode disposed around the objective lens and the electrode disposed around the stage, the potential difference can be generated by both of the voltages or either one of the voltages. For example, when the voltage at the electrode disposed around the objective lens is 5 kV and the voltage at the electrode disposed around the stage is 0 kV, the potential difference is 5 kV. When the voltage at the electrode disposed around the objective lens is 5 kV and the voltage at the electrode disposed around the stage is −5 kV, the potential difference is 10 kV. When the voltage at the electrode disposed around the objective lens is 0 kV and the voltage at the electrode disposed around the stage is −5 kV, the potential difference is 5 kV. That is, the potential difference can be generated by various methods. However, in order to generate high potential difference only by either one, it is necessary to provide a large-scale power supply. Accordingly, it is desirable that higher potential difference is generated by two voltages. 
         [0046]      FIG. 13  is a timing chart showing change of voltages applied to the respective electrodes at the time of retrieval of contaminations or particles with respect to time. Vb is an applied voltage to the electric-field forming electrode  118  and Vr is an applied voltage to a retarding electrode embedded in the sample stage. As exemplified in  FIG. 13 , the applied voltages Vb and Vr are changed in synchronism with each other and controlled so that the potential difference between both the electrodes repeats between the maximum (|Vb|+|Vr|) and the minimum (0V), so that falling down of the contaminations or particles can be further promoted. Incidentally, Vb 1  and Vr 1  in  FIG. 13  are maximum values of the applied voltages at the time that measurement and observation with the electron microscope are made. In this manner, higher voltages exceeding voltage ranges used when it is employed as the electron microscope can be applied to enable enhancement of the certainty that the contaminations or particles are made to fall down. 
         [0047]      FIG. 6  is a diagram illustrating an example of the sample stage including a retarding electrode  502  to which a retarding voltage is applied. The retarding electrode  502  is a mode of the electrode disposed around the stage. Further, in the example of  FIG. 6 , an auxiliary power supply  602  for generation of strong electric field is provided in addition to a usual retarding voltage power supply  601 . As described above, the contaminations or particles can be forced to fall down even by controlling the applied voltage to the retarding electrode  502 ; however, since the potential toward the center of the sample stage  108  becomes higher as viewed from the contamination or particle collector  119 , there is a possibility that the gradient of the potential is formed around the contamination or particle collector  119  and the falling contaminations or particles are deflected. According to the example of  FIG. 6 , at the time of retrieval of the contaminations or particles, by changing over a switch  603  with the control unit  120  and rendering the contamination or particle collector  119  to be at a higher potential, the above-mentioned potential gradient can be removed. 
         [0048]    The electrode disposed around the objective lens is not limited to an electrode disposed toward the electron source compared with the principal surface of the objective lens and facing the sample like the electric-field forming electrode  118 , but it may be substituted with an electrode positioned on the principal surface of the objective lens or below it.  FIG. 7  is a diagram showing an example in which an electric-field forming electrode  702  is provided below the magnetic pole of the objective lens  106 .  FIGS. 8 and 9  are diagrams for explaining an installation example of the electric-field forming electrode  702  in more detail.  FIG. 8  is a diagram showing an example in which a boosting plate  801  for accelerating the electron beam is disposed in the beam passage opening of the objective lens  106 . The electric field can be controlled at the time of retrieval of the contaminations or particles by a voltage applied to the electric-field forming electrode  702  and/or the boosting plate  801 . Further, where  FIG. 9  is a diagram illustrating a construction of the objective lens  106  which is divided into an upper magnetic pole  901  and a lower magnetic pole  902  to accelerate the electron beam by applying a high voltage to the upper magnetic pole  901  selectively, the electric field may be controlled at the time of retrieval of the contaminations or particles by a voltage applied to the upper magnetic pole  901  and/or the electric-field forming electrode  702 . In addition, the electrode or the like for controlling the trajectories of secondary electrons may be used as an electric field control electrode at the time of retrieval of the contaminations or particles. 
         [0049]    The electrode disposed around the stage can be substituted with a different electrode other than the retarding electrode  502  or the contamination or particle collector  119 . A specific example is shown in  FIG. 10 .  FIG. 10  illustrates an example in which an electric-field forming electrode  1003  is disposed at the bottom of a vacuum sample chamber  1002  of an electron microscope including an electron beam column  1001  and the vacuum sample chamber  1002 . However, when the electrode disposed at the bottom of the sample chamber is utilized, a distance between the electrode disposed around the objective lens and it becomes long and, accordingly, a higher voltage is required to force the contaminations or particles to fall down. As described above, the contaminations or particles can be retrieved even with the retarding electrode and the electrode disposed at the bottom of the sample chamber; however, it is necessary to dispose a voltage-applicable dedicated stand on the upper surface of the electrode used. 
         [0050]    The contaminations or particles can be attached to the objective lens or attracted to the vicinity thereof by generating a stronger magnetic field than being used normally in the objective lens. At this time, the stronger the magnetic field is, the higher the efficiency of attachment or attraction of the contaminations or particles is; accordingly, it is desirable to be able to apply a stronger magnetic field to the objective lens. 
         [0051]    The contaminations or particles can be attached to the electrode disposed around the objective lens or attracted to the vicinity thereof by generating a stronger electric field than being used normally in the electrode disposed around the objective lens. At this time, the stronger the electric field is, the higher the efficiency of attachment or attraction of the contaminations or particles is; accordingly, it is desirable to be able to apply a stronger electric field to the electrode disposed around the objective lens. 
         [0052]    When the contaminations or particles are attached or attracted, no voltage is applied to the electrode disposed around the stage. It is because, even when the contaminations or particles are attached to the electrode disposed around the stage or attracted to the vicinity thereof, they cannot be forced to fall down on the voltage-applicable dedicated stand. 
         [0053]    When the contaminations or particles are forced to fall down on the voltage-applicable dedicated stand, the higher the potential difference is, the higher the efficiency for forcing down is; accordingly, it is desirable to be able to generate the larger potential difference between the electrode disposed around the objective lens and the electrode disposed around the stage. 
         [0054]    When the contaminations or particles are forced to fall down, there is a possibility that the contaminations or particles forced to fall down collide against the upper surface of the voltage-applicable dedicated stand and bounce back to thereby float in the sample chamber again; accordingly, the upper surface of the voltage-applicable dedicated stand is preferably formed in a mesh structure so that the collection efficiency is improved. Furthermore, in order to generate the larger potential difference, it is desirable to set the height so that it is positioned nearer to the electrode around the objective lens. At this time, it is necessary to design it so as not to come into contact with and/or interfere with the lens system. 
         [0055]    When the contaminations or particles are forced to fall down, it is desirable that vacuum grease is thinly applied in the voltage-applicable dedicated stand or an electric field and a magnetic field are generated in the voltage-applicable dedicated stand for absorption of the contaminations or particles so that the contaminations or particles collected on the voltage-applicable dedicated stand do not float back in the sample chamber again. At this time, it is necessary to design it so that interference with primary and secondary electrons does not occur. 
         [0056]      FIG. 4  is a diagram illustrating an example of the contamination or particle collector  119 . The contamination or particle collector  119  is formed into a container and can contain or be applied with a small amount of vacuum grease. Further, a mesh electrode  401  is provided inside and a voltage for controlling the electric field at the time of retrieval of the contaminations or particles can be applied. 
         [0057]    The retrieval of the contaminations or particles can be performed using a bare wafer on the stage or the contamination or particle collector which can be introduced from the outside of the sample chamber instead of collecting on the voltage-applicable dedicated stand.  FIG. 11  is a diagram illustrating an example in which a contamination or particle collector  1101  transferred in from the outside of the sample chamber is mounted on the sample stage  108 . The contamination or particle collector having a large surface area can be applied to improve the contamination or particle retrieval efficiency. 
         [0058]      FIG. 12  is a flow chart showing the contamination or particle retrieval process using the contamination or particle collector  1101 . First, a sample is transferred out and, then, the vacuum valve  114  is closed (Steps S 1201  and S 1202 ). Next, the electric field and/or the magnetic field is controlled to collect the contaminations or particles on the objective lens (Step 
         [0059]    S 1203 ) and the vacuum valve  114  is opened to introduce the contamination or particle collector  1101  while maintaining the state (Steps S 1204  and S 1205 ). Thereafter, the vacuum valve  114  is closed and the sample stage  108  is moved so that the contamination or particle collector  1101  is positioned below the beam passage opening of the objective lens (Steps S 1206  and S 1207 ). Next, while the contamination or particle collector  1101  is positioned below the beam passage opening of the objective lens  106 , the magnetic field of the objective lens  106  is turned off or is relatively weakened and the electric field is formed by application of voltage to the electrode disposed around the objective lens, so that the contaminations or particles are forced to fall down on the contamination or particle collector  1101  (Step S 1208 ). Thereafter, the vacuum valve  114  is opened and the contamination or particle collector  1101  is transferred out (Steps S 1209  and S 1210 ). 
         [0060]    By retrieving the contaminations or particles via the process as shown in  FIG. 12 , the contaminations or particles can be removed with high efficiency without leaving the contamination or particle collector in the sample chamber. 
         [0061]    Further, according to the example of  FIG. 11 , while the contamination or particle collector  1101  is mounted on the sample stage  108 , the electric field for retrieval of the contaminations or particles is controlled and the sample stage  108  is moved to scan the movable range of the contamination or particle collector  1101 , so that the contaminations or particles can be removed over the wide range in the sample chamber. However, the operation that a bare wafer and the like are transferred each time the contaminations or particles are forced to fall down may become a cause for reducing the throughput of the device and, accordingly, it is desirable that the contamination or particle removing member is provided in the sample chamber in advance. 
         [0062]    Moreover, the contaminations or particles can be merely forced to fall down to the bottom of the sample chamber or the like without using the contamination or particle collector and the like; however, there is a possibility that the contaminations or particles float back in the sample chamber again when they are merely forced to fall down and, therefore, it is desirable to collect on the voltage-applicable dedicated stand. 
         [0063]    In the actual operation of collection, it is indispensable not to influence the throughput. Specifically, a large number of product lots are processed in the semiconductor manufacturing line and, accordingly, collection must be performed without impeding the processing. 
         [0064]    A specific example is described. In the semiconductor manufacturing line in which a CD-SEM is installed, the product lots are automatically transferred and the CD-SEM can grasp the product lots being waited for processing. Therefore, only when there is no product lot being waited for processing, collection is performed once after processing of the product lots. Further, there is a case where there are not many lots to be processed depending on the semiconductor manufacturing line. In such a case, since the possibility that clean atmosphere can be maintained in the sample chamber is high and it is not necessary to perform collection in each of lots to be processed, it is desirable that a fixed threshold can be set at the timing for collection. For example, when one week elapsed or the number of product lots processed exceeds 500 sheets since the last collection and there is no product lot being waited for processing, collection is performed once after processing of product lots. Moreover, starting or re-starting of the device can be used as a trigger for collection. 
         [0065]    On the other hand, there is a possibility that the product lots having increased outgas for the cause of process or material that is not subjected to cleaning for removal of contaminations or particles are processed in the relationship with the semiconductor manufacturing process. In such case, it is desirable that the collection is performed at regular intervals of measurement or at the time when the degree of vacuum is observed to deteriorate. 
         [0066]    The above description has been made to the embodiment; however, it is apparent for a person having ordinary skill in the art that the present invention is not limited thereto and various alterations and modifications can be made within the spirit of the present invention and the scope of the annexed claims. 
       REFERENCE SIGNS LIST 
       [0000]    
       
           101  . . . electron source 
           102  . . . extraction electrode 
           103  . . . electron beam 
           104  . . . condenser lens 
           105  . . . scanning deflector 
           106  . . . objective lens 
           107  . . . sample chamber 
           108  . . . sample stage 
           109  . . . sample 
           120  . . . control unit