Abstract:
A scanning electron microscope includes an electron beam source which emits an electron beam, a beam current controller which controls a beam current of the electron beam, an electron beam converger which converges the electron beam on a surface of a sample, an electron beam scanner which scans the electron beam on the surface of the sample, a table which mounts the sample and moves at least in one direction, a detector which detects a secondary electron or a reflected electron emanated from the sample by the scan of the electron beam, an image former which forms an image of the sample based on a detection value of the detector, an image processor which processes the image formed by the image former. The beam current controller controls the beam current of the electron beam by changing transmittance of the electron beam in an irradiation path of the electron beam.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a continuation of U.S. application Ser. No. 11/319,721, filed Dec. 29, 2005, the contents of which are incorporated herein by reference. 
         [0002]    The present application claims priority from Japanese patent application No. JP 2005-005882 filed on Jan. 13, 2005, the content of which is hereby incorporated by reference into this application. 
     
    
     BACKGROUND OF THE INVENTION 
       [0003]    The present invention relates to a scanning electron microscope (hereinafter, referred to as “SEM: Scanning Electron Microscope”) and an image detecting method thereof for detecting an image of a surface of an observation target by irradiating a convergent electron beam to the observation target such as a semiconductor wafer and detecting electrons emitted from the irradiated position, and particularly relates to a technique effectively applied to an SEM semiconductor wafer inspection apparatus required to take high-magnification images, a review SEM for observing in greater detail defects detected in the semiconductor wafer, and a length measurement SEM for measuring a pattern formed on the semiconductor wafer, etc. 
         [0004]    As a technique that the present inventors have studied, the following techniques are conceivable in, for example, the SEM semiconductor wafer inspection apparatus, and the review SEM, the length measurement SEM, etc. (for example, see Japanese Patent Laid-Open Publication No. 2003-16983). 
         [0005]    Along with miniaturization of the patterns on the semiconductor wafer, control of a front-end manufacture process of semiconductor is becoming more and more difficult; detection of defect, observation, and dimension measurement of pattern width in the optical microscope are becoming difficult; and inspection, review (reexamination), and length measurement are performed based on the images taken by the SEM. 
         [0006]    The SEM emits an electron beam converged on a surface of the observation target, and detects a secondary electron or reflected electron emitted from the irradiated position. When the irradiated position of the electron beam is two-dimensionally moved, a two-dimensional image is taken. 
         [0007]    Expected values of the secondary electron and the reflected electrons then emitted upon irradiation of the electron beam are known to be proportional to a beam current which controls an amount of electron beams to be irradiated. The number of the emitted secondary electrons is not completely the same in the case of the same beam current, a variation occurs in the emitted number, and the variation is proportional to the emitted electron number to one-half power. This variation is generally known to be a main factor of noise of the detected image in SEM. 
         [0008]    Regarding an S/N of detected images, the symbol “S” indicating a signal is proportional to the beam current and the variation which is a noise component is proportional to the beam current to one-half power, so that the S/N is proportional to the beam current to the one-half power. It is known that, for this characteristic, in order to obtain the image with the good S/N, the image has to be detected by use of the beam current which is as large as possible. 
         [0009]    However, it is known that, when the beam current is increased, aberration of an electro-optical system is increased and a beam diameter is increased. Therefore, there has been a problem that high-resolution images cannot be obtained. As a technique for solving this problem, a noise reduction technique which is performed by addition through a frame is known. This is a method in which a plurality of images within the same region are taken and each of the images is used as a frame and the frames are integrated to synthesize a final image. When this method is employed, the S/N is proportional to the number of added frames to the one-half power, while the image taken time increases in proportion to the number of added frames. 
         [0010]    By the SEM having such a characteristic, defects of the semiconductor are reviewed or the dimensions of the semiconductor pattern are measured. However, when such processes are to be performed, two types of images are generally taken, i.e., low-magnification image detection and high-magnification image detection are performed. For example, in the review SEM for reviewing the defects of semiconductor, the defect is enlarged at high magnification and displayed in accordance with defect coordinates outputted by an inspection device for detecting the defects. However, accuracy of the defect coordinates outputted by the inspection device is bad within a visual field of the high-magnification image. Therefore, first, a defect position is specified by comparing the low-magnification defect image with a reference image which is a normal image having the same pattern as that of the defect image, and the defect position is enlarged to obtain the high-magnification image. 
         [0011]    On the other hand, in the length measurement SEM, in order to determine a pattern to be measured, a low-magnification image is similarly detected. The image is taken subsequently at high magnification in order to perform measurement with high accuracy, and line width etc. is measured from the taken image. 
       SUMMARY OF THE INVENTION 
       [0012]    Incidentally, the present inventors have studied the technique of the SEM as described above and, as a result, the following has been apparent. 
         [0013]    That is, in the above-described conventional techniques, the beam current at the low-magnification image detection and the beam current at the high-magnification image detection have to be the same. Therefore, there has been a problem that when a magnification difference between the high-magnification image and the low-magnification image is large, an image taken time of the low-magnification image becomes longer. 
         [0014]    Hereinafter, an example of the review SEM will be described. Although the low-magnification image is used as an image employed for detecting the defects. However, since defect-detection coordinate accuracy of the inspection device with respect to size of the defect is bad, the high-resolution image has to be taken in a large visual field. Even if a pitch of semiconductor pattern is reduced to 65 nm in the future, the size of defect which has to be detected within the visual field thereof is conceived to be about 25 nm, which is sufficiently large when compared with the resolution of a general SEM, for example, with 4 nm. 
         [0015]    Meanwhile, the defect-detection coordinate accuracy in the inspection device is particularly bad in a dark-field inspection device, for example about ±4 μm in accuracy. For this reason, a defect with about 25 m is required to be detected from an 8-μm square visual field. However, when the image is taken by sampling the visual field by 512×512, the defect is detected in about 1.5 pixels, so that a high defect capturing rate cannot be expected. In order to improve the defect capturing rate, a method of, for example, doubling a density of sampling is required. However, in this case, taking the image by the same S/N requires a time that is four times longer. 
         [0016]    On the other hand, when the sampling is performed with a doubled density, i.e., at 1024×1024, the size per pixel becomes about 8 nm which is twice or more of the resolution of the SEM having been described above as an example. Therefore, even when a beam spot diameter is somewhat increased by increasing the beam current, the image can be taken without deteriorating the resolution in the taken image. 
         [0017]    However, there is a problem that, when the same beam current is used for taking the high-magnification image, the image of higher resolution cannot be obtained. For example, when the defect of about 25 nm is to be taken in a size of about one eighth of the image, the image detection in a 200-nm square visual field is required. When this is sampled by 512×512, the size per pixel becomes about 0.4 nm. This is a resolution that the general SEM cannot reach and, moreover, in a state in which the beam current is large, the resolving power is apparently insufficient. 
         [0018]    Thereat, although a method of setting an optimum beam current for each of the low-magnification image and the high-magnification image is conceivable, the conventional method has been incapable of setting the above optimum beam current. 
         [0019]    A reason for that includes a first problem of an occurrence of misalignment of visual fields after being shifted between the small beam current and the large beam current and misalignment of the beam. In the high-resolution SEM, generally, a FE (field-emission) electron gun or a schottky emission electron gun is employed as an electron source. In order to change the beam current by the FE electron gun, the extraction voltage of the FE electron gun is changed. When the extraction voltage is changed herein, an assumed light source position is changed, whereby the visual field misalignment and misalignment in the axis of the electron gun occurs. Therefore, in order to take the good image in an axis-adjusted state, adjusting the alignment of the axis has been required every time the beam current is changed. 
         [0020]    A second problem is a switching time of the current. When shortening an image taken time of the low-magnification image is considered to be a main purpose of beam current switching, the switching of the current has to be carried out at an at least shorter time than the image taken time of the low-magnification image. Generally, the image taken time of the SEM is about 640 ms in terms of 16 frame additions in an image sampling of 512×512. In order to ensure the same S/N in a sampling of 1024×1024, four times the image taken time of the SEM, i.e., 2560 ms is required. Herein, when two images, i.e., a defect image and a reference image which it is known that previously has the same pattern as that of the defect image are taken at the low magnification, it takes 5120 ms. A shortened time in the case where the beam current is increased four times and the low-magnification defect image and reference image are taken 1280 ms becomes 3840 ms, so that the beam current has to be switched within an at least shorter time than 3840 ms. 
         [0021]    However, when the extraction voltage of the FE electron gun is changed, a period of time is generally consumed more than until the beam current is stabilized and the overall image taken time has not been shortened even when the beam current is switched. 
         [0022]    Moreover, the beam current can be changed by controlling a suppressor voltage in the case of using the schottky emission electron gun. Also in this case, however, similarly to the case where the extraction voltage of the FE electron gun is changed, there is a problem that a period of time is consumed until it is stabilized. 
         [0023]    An object of the present invention is to realize both shortening of the image taken time and obtaining of the high-quality image detection in the SEM. 
         [0024]    The above and other objects and novel feature of the present invention will become apparent from the description of the present specification and the accompanying drawings. 
         [0025]    Outlines of representative ones of inventions disclosed in the present application will be briefly described as follows. 
         [0026]    More specifically, an SEM according to the present invention has: a control amount memory means for saving, in advance, an adjustment amount of a gain adjusting means for making a detection value of an electron detector falling within a predetermined range in setting a plurality of beam currents, or a calculation algorithm of the adjustment amount; and an image detecting condition control means for changing an image detecting condition in accordance with a change of the beam current by using the adjustment amount or calculation algorithm of the adjustment amount saved in the control amount memory means. 
         [0027]    In addition, an image detecting method of an SEM according to the present invention includes a first image detecting step of making an electron beam converged into a spot-like shape being scanned on an observation target surface, and converting a secondary electron or reflected electron generated in the scan into an electrical signal so as to form an image; a defect position specifying step of specifying a defect position on the observation target surface based on the image formed in the first image detecting step; a second image detecting step of making an electron beam converged into a spot-like shape being scanned at a defect position specified in the defect position specifying step by using a magnification larger than the magnification in the first image detecting step, and converting a secondary electron or reflected electron generated in the scan into an electrical signal so as to form an image; a first beam current changing step of observing the observation target surface in the second image detecting step by using a beam current smaller than a beam current of the first image detecting step; a first image detecting condition setting step of changing an image detecting condition to a condition set in advance for detecting an image by using the beam current set in the first beam current changing step; a second beam current changing step of setting the beam current employed in the first image detecting step; and a second image detecting condition setting step of setting the image detecting condition employed in the first image detecting step again, wherein the first beam current changing step and the first image detecting condition setting step are performed before the second image detecting step and the second beam current changing step and the second image detecting condition setting step are performed after the second image detecting step. 
         [0028]    According to the SEM and the image detecting method according to the present invention, the electron beam can be switched within a short period of time, and both the short-time low-magnification image detection and the high-resolution high-magnification image detection can be performed. 
         [0029]    These and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0030]      FIG. 1  is a view showing a configuration of an SEM according to an embodiment of the present invention. 
           [0031]      FIG. 2  is a flow chart showing a sequence of the case where defect images are automatically taken in the SEM according to the first embodiment of the present invention. 
           [0032]      FIG. 3  is a flow chart showing a sequence of the case where length measurement of patterns is performed in the SEM according to the first embodiment of the present invention. 
           [0033]      FIG. 4  is a view showing a configuration of an SEM according to another method of the present invention. 
           [0034]      FIG. 5  is a view showing a configuration of an SEM according to another embodiment of the present invention. 
           [0035]      FIG. 6A  is a view showing an energy distribution of spatial frequency in which pixel size of an entire low-magnification image is used as a reference. 
           [0036]      FIG. 6B  is a view showing an energy distribution of spatial frequency in which pixel size in an entire high-magnification image is used as a reference. 
           [0037]      FIG. 7  is a view showing an example of a structure of an aperture stop set in the SEM shown in  FIG. 4 . 
           [0038]      FIG. 8  is a view showing a configuration of an SEM according to another embodiment of the present invention. 
           [0039]      FIG. 9  is a view showing an example of a GUI for inputting image taken conditions in a sequence of a defect image detection shown in  FIG. 2 . 
           [0040]      FIG. 10  is a view showing an example of a GUI for inputting image taken conditions in a sequence of pattern length measurement shown in  FIG. 3 . 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0041]    Hereinafter, embodiments of the present invention will be detailed based on the drawings. Note that, throughout all the drawings for explaining the embodiments, the same members are denoted in principle by the same reference numeral and the repetitive description thereof will be omitted. 
         [0042]    In a semiconductor wafer, patterns are formed in a multi-layered structure through many steps. In order to monitor a manufacture process in a flow of producing the multi-layered structure, a dimensional measurement of a pattern formed per layer, a visual inspection of the patter, and a review (reexamination) of a defect detected in the visual inspection have been performed. 
         [0043]    A recent semiconductor process is increasingly made fine. Therefore, a SEM capable of take an image with resolution higher than that of an optical microscope has been applied as an image detection for performing the process. A review SEM has been widely used as the SEM employed for such a purpose. A main function of the review SEM is to move a visual field to a defect position in accordance with the coordinates of the defect which has been detected in the visual inspection and to take an image of the defect by the SEM. 
         [0044]      FIG. 1  is a view showing a configuration of an SEM according to an embodiment of the present invention. First, an example of the configuration of the SEM according to the present embodiment will be described with reference to  FIG. 1 . 
         [0045]    The SEM of the present embodiment is, for example, a review SEM and is comprised of an electron beam source  101 , condenser lenses  102  and  103 , an electron beam axis adjuster  104 , scanning units  105  and  106 , an objective lens  107 , an E×B (E cross B)  110 , an electron detector  111 , an A/D converter  112 , a memory  113 , an XY stage  114 , an image processing unit  115 , a secondary storage device  116 , a computer terminal device  117 , and an overall control system  118 , etc. 
         [0046]    An operation of the SEM according to the present embodiment will next be described with reference to  FIG. 1 . 
         [0047]    The electron beam source  101  emits an electron beam. The emitted electron beam passes through the condenser lenses  102  and  103 , and then astigmatism and misalignment are corrected by the electron beam axis adjuster  104 . The electron beam is deflected by the scanning units  105  and  106  so as to control a position where the electron beam is irradiated. Then, the electron beam is converged by the objective lens  107  to irradiate an image detecting target  109  of a wafer  108 . As a result, a secondary electron and a reflected electron are emitted from the image detecting target  109 , and the secondary electron and reflected electron are deflected by the E×B  110  and detected by the electron detector  111 . The secondary electron and reflected electron detected by the electron detector  111  are converted into digital signals by the A/D converter  112  and stored in the memory  113 . An XY stage  114  moves the wafer  108 , thereby making it possible to take images at some positions of the wafer  108 . The image processing unit  115  detects a defect position based on the image stored in the memory  113 . A detection method thereof includes a method in which the image at the defect position is compared with an image at a reference position, which it is expected that has the same pattern as that of the image of the defect position, and which detects as a defect a position where a difference between both images exists. The secondary storage device  116  can store the image stored in the memory  113 . The computer terminal device  117  can display the image stored in the secondary storage device  116  or the memory  113 . Also, the user can carry out setting of various operations of the terminal device by performing an input to the computer terminal device  117 . The overall control system  118  controls axis adjustment of the electron beam, deflection of the electron beam performed by the scanning units  105  and  106 , and visual field transfer by movement of the XY stage  114 . 
         [0048]    A sequence of taking an image defect by the SEM according to the present embodiment will next be described with reference to  FIG. 2 .  FIG. 2  is a flow chart showing a sequence of taking the defect image in the SEM shown in  FIG. 1 . The following operations are basically automatically performed in accordance with control executed by, for example, the overall control system  118 , wherein its portion can be manually executed via, for example, the computer terminal device  117 . 
         [0049]    First, a beam current for taking a low-magnification image is set in step S 201 . Then, in step S 202 , alignment of the axis is adjusted for the current, astigmatism is corrected, and a control value for performing this correction is saved in a memory in the overall control system  118 . The correction may be performed manually, or a method disclosed in Japanese Patent Laid-Open Publication No. 2003-16983 may be employed. 
         [0050]    Then, an image is taken in step S 203 . Next, a beam current for taking a high-magnification image is set in step S 204 , the axis at the beam current for taking the high-magnification image is aligned in step S 205 , astigmatism is corrected, and a control amount for performing this correction is saved in the memory in the overall control system  118 . 
         [0051]    In step S 206 , an image is taken by the beam current for taking a high-magnification image. In step S 207 , the position of the image taken in step S 203  and that of the image taken in step S 206  are aligned, and a position misalignment amount is saved in the memory in the overall control system  118 . 
         [0052]    In step S 208 , a defect to take an image is selected. In step S 209 , the XY stage  114  is moved, whereby a reference position corresponding to the selected defect enters the visual field of the SEM. At the same time, the beam current is switched in step S 219  (second beam current changing step), and setting of the electro-optical system for taking the low-magnification image is performed in step S 220  (second image detecting condition setting step). More specifically, a normal control amount of the electro-optical system for taking the low-magnification image is set, and concurrently the alignment of the axis of the beam current for taking the low-magnification image and the control amount for astigmatism correction, which have been saved in step S 202 , are set. 
         [0053]    In step S 210 , a reference image is taken. In step S 211 , the XY stage  114  is moved so that the defect selected in step S 208  enters the visual field of the SEM. 
         [0054]    The image is taken in step S 212  (first image detecting step), and a position of the defect is specified in step S 213  (defect position specifying step) by comparing the image taken in step S 210  and the image taken in step S 212  in step S 213  (defect position specifying step). At the same time, the beam current is varied for high-magnification image in step S 214  (first beam current changing step), and setting of the electro-optical system for taking the high-magnification image is performed in step S 215  (first image detecting condition setting step). More specifically, a normal control amount of the electro-optical system for taking the high-magnification image is set, and the alignment of the axis of the beam current for taking the high-magnification image and the control amount for the astigmatism correction, which have been saved in step S 205 , are set. 
         [0055]    In addition, in step S 216 , based on misalignment amounts of the low-magnification image detection and the high-magnification image detection measured in step S 207 , an electron beam irradiating position where the defect is taken at a center of the visual field is calculated when the electron beam is emitted. 
         [0056]    In step S 217 , an AF (automatic focus) is performed. In step S 218  (second image detection), the high-magnification image is taken. This image is stored in the memory  113  or the secondary storage device  116 . 
         [0057]    In step S 221 , it is checked whether all points of the image are taken. If images of all the points have been taken, the process is ended. If the images of all the points have not been taken, the process returns to step S 208  and the steps S 208  to S 221  are executed again. 
         [0058]    By using the above-described method, the low-magnification images and high-magnification images can be taken without being affected by the misalignment or astigmatism variation caused when the beam current is changed. 
         [0059]    Also, when the beam current of the electron beam source  101  is changed, the position where the electron beam is focused is changed depending on a configuration of the electro-optical system in some cases. Therefore, handling of such cases may be carried out by: performing the AF in step S 202  and step S 205 ; serving the control amount in which the focuses of the respective beam currents coincide with one another; and being corrected by the saved control amount so that the focuses in step S 220  or step S 215  coincide with each other. 
         [0060]    Generally, the electron detector  111  comprises a combination of a scintillator and a photomultiplier in many cases. When the beam current is varied for the low-magnification images and high-magnification images, the number of detected electrons varies, so that adjustment of a gain of the photomultiplier is required. Therefore, the gain of the photomultiplier which enables the respective beam currents to take the good images may be obtained in step S 202  and step S 205  and, at the same time, the gains suitable for the respective beam currents in step S 215  and step S 220  may be set. 
         [0061]    It should be noted that saving previously the control amounts suitable for the respective beam currents has been described, an expression for calculating values of the control amounts may be saved so as to set the control amounts in step S 220  and S 215  based on the expression. For example, as for the gain of the photomultiplier, since it is understood in advance that the beam current is approximately proportional to the number of secondary electrons and reflected electrons generated by the current, a method of making the gain of the photomultiplier inversely proportional to the beam current may be used. 
         [0062]    The image detecting conditions in step S 219  and step S 214  have been described in the description of  FIG. 2 . However, if the beam current for taking the low-magnification image and the beam current for taking the high-magnification image, which are set herein, are configured to be set by the user through the computer terminal device  117 , however, usability is improved. At this time, for example, a GUI (Graphical User Interface) as shown in  FIG. 9  is made to be displayed in the computer terminal device  117 , whereby the image detecting conditions are set. The item “probe current” in  FIG. 9  corresponds to the beam current in the present specification. In addition to the probe current, “Field of View” representing the visual field of taking the image, the number of pixels per image, “image size” indicating, for example, 512×512 or 1024×1024, and “frame integration number” representing the number of times of scanning the beam with respect to the visual field, etc. can be set for each of the high magnification and the low magnification. Moreover, since an acceleration voltage cannot be rapidly varied in general, a common voltage is set herein for the low magnification and the high magnification. 
         [0063]    The sequence of the case where the electro-optical system of  FIG. 1  is applied to an automatic defect image detection of the review SEM has been described in  FIG. 2 . However, a sequence of the case where the present function is applied to a length measurement SEM is shown in  FIG. 3 .  FIG. 3  is a flow chart showing the sequence of the case where the SEM shown in  FIG. 1  is applied to the length measurement SEM. 
         [0064]    In  FIG. 3 , steps S 301  to S 307  are the same as the above-described steps S 201  to S 207  in  FIG. 2  and so their descriptions will be omitted. 
         [0065]    Positions to be subjected to length measurement are selected in step S 308 , and the XY stage  114  is moved in step S 309 , thereby causing length measurement points selected in step S 308  to enter the visual field of the low-magnification image. At the same time, the beam current is switched in step S 310 , and setting of the electro-optical system for detecting the low-magnification image is performed in step S 311 . More specifically, the normal control amount of the electro-optical system for taking the low-magnification image is set, and the alignment of the axis of the beam current for taking the low-magnification image and the control amount for astigmatism correction, which have been saved in step S 302 , are set. 
         [0066]    The image is taken in step S 312 . In step S 313 , the image taken in step S 311  is compared to an image capable of specifying previously stored measurement points, whereby the image detecting point of the high-magnification image is specified. At the same time, the beam current is varied for taking the high-magnification image in step S 314 , and the electro-optical system for taking the high-magnification image is further set in step S 315 . More specifically, the normal control amount of the electro-optical system for taking the high-magnification image is set, and the alignment of the axis of the beam current for taking the high-magnification image and the control amount for astigmatism correction saved in step S 305  are set. 
         [0067]    Also, in step S 316 , based on the misalignment amount of the low-magnification image detection and the high-magnification image detection which are measured in step S 307 , the electron beam irradiation position where the measurement pattern is imaged at a center of the visual field when the electron beam is emitted. 
         [0068]    The AF is performed in step S 317 , the high-magnification image is taken in step S 318 , and a portion in which the pattern is specified is measured from the high-magnification image taken in step S 319 . 
         [0069]      FIG. 10  shows a GUI for setting the beam current and the image detecting conditions in the sequence of  FIG. 3 . In the length measurement SEM, since a portion to be measured is determined in advance, the low-magnification or high-magnification image can be confirmed so as to make a confirmation of whether an expected recipe can be confirmed. A method for setting image detecting conditions other than this is basically the same as that of  FIG. 9 . 
         [0070]    Regarding a relation between the beam current and a spot diameter, chromatic aberration which is a main factor for determining the spot diameter is proportional to the beam current approximately to one-half power by an increase in an open angle of the beam involved in an increase in a beam current value although depending on the beam current. Therefore, in taking the high-magnification image, the beam current is suppressed small to reduce the spot diameter, whereby the high-resolution image is taken. 
         [0071]    On the other hand, in the low-magnification image detection, when a fine defect is to be detected from a wide visual field or a position to be measured is to be accurately determined, the sampling number of images is increased from the general sampling number of 512×512 to, for example, 1024×1024 to perform sampling. In this case, when the beam current is increased by about four times to take the image, a decrease in the S/N due to a decrease in the signal amount per pixel is suppressed. When the beam current is increased by four times, the chromatic aberration is also increased by about two times. However, no problem is caused in the image quality as long as the accordingly enlarged spot diameter is smaller than the pixel size. When the low-magnification image is taken in the above-described manner, the frame integration number can be reduced and the image detecting time can be shortened. 
         [0072]    Note that since the defect position has been already confirmed through the low-magnification image detection, taking the image in the wide visual field is not required at a time of the high-magnification image detection, so that taking the image thought the sampling of 512×512 preferably shortens the image detecting time. 
         [0073]    A problem of shortening the image detecting time in the review SEM and the length measurement SEM is about the beam current switching time. Generally, the switching time takes several tens of seconds in order to switch the current by changing the extraction voltage of the FE electron gun, that is, becomes longer than the shortening of the image detecting time of the case where the beam current is increased. Therefore, instead of changing the extraction voltage of the FE electron gun, a method for changing the beam current by changing the aperture stop of the irradiation path of the electron beam will be described. 
         [0074]      FIG. 4  is a view showing a configuration of an SEM according to the above method. When the SEM shown in  FIG. 4  is compared with the above-described embodiment shown in  FIG. 1 , an aperture stop set  403 , a field stop  404 , and a stop changing means  405  are added and the electron beam source  101  is replaced by an electron beam source  401  and the condenser lens  102  is replaced by a condenser lens  402  and other configurations are the same as those of  FIG. 1 . 
         [0075]    A plurality of stops having different diameters are provided in the aperture stop set  403 , and the stop to be used can be selected by the stop changing means  405 . When the small-diameter stop is used, the beam current becomes small. When the large-diameter stop is used, the beam current becomes large. When the image is to be taken at the low magnification, the large-diameter stop is used to increase the beam current. When the image is to be taken at the high magnification, the small-diameter stop is used to reduce the beam current. The position of the stop is mechanically changed by the stop changing means  405 . However, the switching of the beam current within a range of 100 ms can be realized by virtue of employing the present method. 
         [0076]    Note that although the stops are circular apertures with different diameters in the embodiment of  FIG. 4 , stops having shapes other than the shapes of apertures can be also employed. Such an example includes a mesh (net)-like stop shown in  FIG. 7 . For example, as shown in  FIG. 7 , an aperture stop  701  and mesh-like stops  702  and  703  are provided in the aperture stop set  403 . The meshes provided in the aperture stop set  403  have different electron beam transmittance. When the mesh having high transmittance (for example, stop  703 ) is selected, the large beam current is set. When the mesh having low transmittance (for example, stop  702 ) is selected, the small beam current is obtained. Even when the transmittance is controlled by combining a plurality of apertures instead of the meshes, the same effects can be obtained. 
         [0077]    In the embodiment of  FIG. 4 , an example for changing the beam current by mechanically changing the positions of the stops by the stop changing means  405  has been described. However, in this method, dust generated due to the mechanical movement in the case where the aperture stop set  403  is to be moved at a higher speed may be problematic. Thereat, an embodiment in which stops suitable for corresponding to the beam currents can be electrically selected is a SEM shown in  FIG. 5 . 
         [0078]    When the SEM shown in  FIG. 5  is compared with the above-described embodiment shown in  FIG. 4 , the condenser lenses  402  and  103 , the aperture stop set  403 , the field set  404 , and the stop changing means  405  are replaced by first condenser lenses  501  and  502 , a second condenser lens  503 , an aperture stop set  504 , and deflectors  505 ,  506 ,  507 , and  508 , and other configurations are the same as those of  FIG. 4 . 
         [0079]    The electron beam generated by the electron beam source  401  is converged by the first condenser lenses comprised of a combination of the reference numerals “ 501 ” and “ 502 ”, and is guided to the objective lens  107  through the second condenser lens  503 . Stops with a plurality of apertures are formed in the aperture stop set  504 . In the deflector  505 , a voltage that causes the electron beam to be deflected so that the electron beam enters the stop which realizes the selected beam current is set. Then, a voltage is applied to the deflector  506  in an opposite direction so that the electron beam is incident in a direction orthogonal to the aperture stop set  504 . Also, the deflector  507  and the deflector  508  causes the axis deflected for selecting the stop to return to the original axis of the electro-optical system in accordance with a method similar to that by the combination of the deflector  505  and the deflector  506 . Other configurations are the same as those of the embodiment of  FIG. 1 , so that the description thereof will be omitted. 
         [0080]    In the present method, the beam current can be changed merely by changing voltages of electrodes of the deflectors  505 ,  506 ,  507 , and  508 , so that no dust is generated and switching can be also performed at a high speed. 
         [0081]    The embodiment of the case where the aperture stop set having the plurality of beam diameters is employed has been described above. However, providing the plurality of aperture stops is not required to control the amount of the beam passing through the stop. Such an embodiment is shown in  FIG. 8 . 
         [0082]    In a SEM shown in  FIG. 8 , a stop  801  is disposed instead of the aperture stop set  403  in  FIG. 4 , and a condenser lens  802  is added. A stop amount of the beam at the position of the stop  801  can be adjusted by changing power of an electromagnetic lens of the condenser lens  103 . When the beam width at the stop  801  is smaller than an aperture provided in the stop  801 , the entire electron beam passes through the beam width. When the beam width is larger than the aperture, vignetting is caused and the beam current is suppressed to a low level. The position of the beam path varied in accordance with the power of the condenser lens  103  can be corrected by causing the condenser lens  802  to vary in combination with that. 
         [0083]    A problem for realizing the SEMs shown in  FIGS. 4 ,  5  and  8  is an occurrence of contamination (staining) in the stops. When the beam current is set low by the stops, the “vignetting” at the stops is considerably large compared with the conventional SEMs. Therefore, a gas present in a minute amount in vacuum may be burnt onto and attached to the stops due to an influence of the electron beam, and the quality of the image to be taken may possibly be changed along with time. A means for preventing this includes a method of increasing vacuum degree, and further includes another method of heating the aperture stop sets  403  and  504  so that no contaminant adheres to portions of the stops. 
         [0084]    Incidentally, according to the above-described methods, an image having a high S/N can be obtained in the low-magnification image even if the beam current is large, but an image with a high S/N cannot be obtained in the high-magnification image if the beam current is not made small. However, this does not cause any big problem in practice. 
         [0085]    For example, as for the automatic image detecting sequence of the defect images in the review SEM shown in  FIG. 2 , after the low-magnification images are taken in the step S 212 , the position of the defect has to be detected through comparison of the images in the step S 213 . On the other hand, after the high-magnification image of the defect is taken in the step S 218 , this image is stored in the secondary storage device  116  or the like and then an image of a defect to be the next target is taken. Thereafter, an image processing is performed, whereby the S/N of the high-magnification taken image is improved. In the low-magnification image, although an image processing for improving the S/N, which takes a calculation time, is difficult to be performed before the step S 213  since the overall throughput are lowered. Meanwhile, as for the high-magnification image taken in the step S 218 , this can be implemented. Furthermore, the high-magnification image taken in the step S 218  has the feature of readily performing a processing for improving the S/N. 
         [0086]    This reason will be described with reference to  FIGS. 6A and 6B .  FIG. 6A  shows an energy distribution of spatial frequency, in which pixel size in the entire low-magnification image is used as a reference. Meanwhile,  FIG. 6B  shows an energy distribution of spatial frequency which is the same as that in the high-magnification image. 
         [0087]    Since the low-magnification image and the high-magnification image take the images with the same pattern, the energy image in the high-magnification image is offset to a band with the low spatial frequency as compared with the spatial frequency in which each pixel size is used as a reference. Meanwhile, the reference numerals “ 603 ” and “ 604 ” represent energy distributions of the spatial frequencies of noise. The noise shows a distribution generally called white noise and depending on no particular frequency. Therefore, in the high-magnification image in which the energy distribution of the spatial frequency is previously offset on a lower-frequency band, since components in a high-frequency band of the spatial frequency are suppressed, the S/N can be readily improved. 
         [0088]    On the other hand, in the low-magnification image in which the large energy distribution is offset to a high-frequency band, such a processing is difficult to be performed. 
         [0089]    Note that as a method for improving the S/N by utilizing a difference between the spatial-frequency distribution of noise and a spatial frequency distribution of the target, a method called “wavelet shrinkage” is known. Herein, an advantage of performing an S/N improving processing to the high-magnification image has been described in the sequence of automatic pickup of defect images. However, similarly thereto, since the S/N improving processing including a large calculation amount is performed only to the high-magnification image, the S/N improving processing can be readily performed also in the length measurement SEM. The reason that the S/N improving processing including the large calculation amount can be readily performed to the high-magnification image is as follows. That is, for example in the sequence of  FIG. 2 , in the case of taking the low-magnification image, immediately after the low-magnification image is taken in the step S 212 , specifying the defect position in the step S 213  is required and therefore no spare time is left. However, in the case of taking the high-magnification image, after the high-magnification image is taken in the step S 218 , the stage has to be moved and therefore a spare time is present. 
         [0090]    By using the above-described methods, even if the high-magnification image is taken in a state in which the beam current is small, the taken image can be converted into a good image. Therefore, the image detection can be performed in a state in which the frame addition is small, so that the image detecting time can be shortened. 
         [0091]    Therefore, in the SEMs and the image detecting methods according to the present invention, the electron beam can be switched within a short time, and both the short-time low-magnification image detection and the high-resolution high-magnification image detection can be performed. 
         [0092]    As described above, the invention made by the present inventors has been specifically explained based on the embodiments. However, needless to say, the present invention is not limited to the above-mentioned embodiments and may be variously altered and modified within a scope of not departing from the gist thereof. 
         [0093]    For example, although the review SEM and the length measurement SEM have been described in the above-described embodiments, the present invention is not limited thereto and can be applied to other SEMs. 
         [0094]    The present invention as described above can be applied to SEM semiconductor wafer inspection apparatuses, review SEMs, and length measurement SEMs, etc. 
         [0095]    The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiment is therefore to be considered in all respects as illustrative and not restrictive, and the scope of the invention being indicated by the appended claims rather than by the foregoing description and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.