Abstract:
Using, as a detector, a CCD detector having a CCD element to which a scintillator is closely fixed, a backscattered or scanning transmission image is obtained by the following method. The detector is disposed directly under an objective lens to obtain the backscattered electron image. When one point of a specimen is irradiated with an electron beam, backscattered or transmission electrons generated from the specimen collide with the scintillator to form a luminescent pattern. This pattern is detected by the CCD detector, and stored in a memory. This processing is sequentially repeated for each irradiation position to obtain all the patterns in an electron beam scanning range. Arithmetic processing is performed on each pattern to convert it into an image. Usually, image data for one pixel is calculated from one pattern. By sequentially repeating this, a backscattered or transmission electron image in the electronic beam scanning range can be obtained.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to an electron microscopy employing a CCD detector in order to acquire a backscattered electron image, a transmission electron image, or a scanning transmission electron image. 
         [0003]    2. Description of the Related Art 
         [0004]    When an electron beam enters a specimen in an electron microscopy, a part of the electron beam is scattered at a large angle by an atom in the specimen. Then, the scattered electron beam discharges electrons (backscattered electrons) from the surface of the specimen while losing little energy. Another part of the electron beam discharges electrons (transmission electrons) behind the specimen, without being absorbed in the specimen. 
         [0005]    The backscattered electrons can be mainly classified into two types: high angle backscattered electrons scattered at high angles from the surface of the specimen and low angle backscattered electrons scattered at low angles from the surface of the specimen. The high angle backscattered electrons mainly have information on composition distribution, while the low angle backscattered electrons mainly have information on depression and projection. By discriminating these two types of backscattered electrons from each other, higher composition contrast and higher depression and projection contrast can be obtained. 
         [0006]    For example, elastically scattered electrons having energy equal to that of the incident electron beam are mainly scattered at high angles, have information on the vicinity of the surface of the specimen, and include little information on the inside of the specimen. Therefore, a surface composition image of comparatively high resolution can be obtained by discriminating and detecting only the electrons scattering at high angles. 
         [0007]    Various methods are used as the method for such discrimination, including: a method of colliding backscattered electrons with an electrode for low angle and an electrode for high angle, respectively, converting the backscattered electrons into secondary electrons, and detecting the secondary electrons; a method of selectively detecting high angle backscattered electrons and low angle backscattered electrons by use of an aperture; and a method of emphasizing stereoscopic effects by dividing a detecting face of a backscattered electron detector into two to five, and calculating a signal between each of the divided detecting faces. 
         [0008]    On the other hand, the information on the inside of the specimen is reflected in a scanning transmission electron image (STEM image). The information in the STEM image is classified into a bright field image and a dark field image in accordance with a scattering angle of the transmission electron. A bright field image signal includes information on density of the specimen and information on electron diffraction. In the case of the dark field image, different information is visualized depending on a detected angle. Since a heavier element causes the transmission electrons to scatter at larger scattering angles, a difference of the atomic number of the specimen can be visualized. For this, discrimination of the bright field from the dark field is important. Discrimination of the bright field from the dark field has been conventionally performed using a selected area aperture. 
         [0009]    For example, WO2000/19482 has disclosed a method of converting low angle backscattered electrons into secondary electrons, and detecting the secondary electrons while attaining a short working distance. Japanese Patent Application Publication No. 2002-110079 has disclosed a method of adding low angle backscattered electrons, high angle backscattered electrons, which are discriminated from each other and detected, and the secondary electrons at any selected ratio. Japanese Patent Application Publication No. Hei 11-273608 has disclosed a method of selectively detecting high angle backscattered electrons and low angle backscattered electrons using an aperture. Additionally, Japanese Patent No. 3776887 has disclosed a method of changing a range of the scattering angles of the transmission electrons to be detected, by making a position of a transmission electron detector variable. 
       SUMMARY OF THE INVENTION 
       [0010]    Discrimination of the backscattered electrons or the transmission electrons has been performed with such conventional methods. However, such conventional methods cannot offer sufficient discrimination, and can no longer meet needs for an analysis in recent years. 
         [0011]    An object of the present invention is to provide a detector that can perform discrimination of backscattered electrons or transmission electrons with more accuracy, and can detect the backscattered electrons or the transmission electrons as needed by an observer. 
         [0012]    Using, as a detector, a CCD detector having a CCD element to which a scintillator is closely fixed, a backscattered electron image or a scanning transmission image is obtained by the following method. 
         [0013]    The detector is disposed directly under an objective lens to obtain the backscattered electron image. The detector has a structure thin enough to be mounted between the objective lens and a specimen. When one point of the specimen is irradiated with an electron beam, backscattered electrons generated from the specimen collide with the scintillator, thus causing the scintillator to emit light, so that a luminescent pattern is formed on the scintillator. The CCD detector detects this pattern and stores the pattern in a memory as data. This processing is sequentially repeated for each of irradiation positions to acquire all the patterns in an electron beam scanning range. 
         [0014]    Next, arithmetic processing is performed on each of the acquired patterns to convert it into an image. Usually, image data for one pixel is calculated from one pattern. The backscattered electron image in the electron beam scanning range can be obtained by sequentially repeating this arithmetic processing. Distribution of scattering angles of the backscattered electrons is reflected in a shape of the pattern, while the number and energy of the detected backscattered electrons are reflected in luminance. Accordingly, the backscattered electron image in any selected scattering angle range can be easily obtained by extracting and integrating only the data in the selected region on the pattern. 
         [0015]    In the case of the scanning transmission image, the scanning transmission electron image in any selected scattering angle arbitrary range can be similarly obtained by disposing the detector under the specimen. 
         [0016]    According to the present invention, discrimination of the backscattered electrons and the transmission electrons can be performed at any scattering angle. Accordingly, it is possible to obtain the backscattered electron image and the scanning transmission electron image on the basis of discrimination at appropriate scattering angles even when thickness, inclination, and composition of the specimen are changed. Thereby, the backscattered electron image or the scanning transmission electron image can be obtained as needed by the observer. 
         [0017]    Furthermore, once a pattern is acquired, an image having any changed discrimination angle can be obtained from the pattern by calculation. Consequently, it is unnecessary to take the data again, and operability thus improves. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]      FIG. 1  is an explanatory view showing an embodiment of the present invention; 
           [0019]      FIG. 2  is an explanatory view of backscattered electrons and transmission electrons; 
           [0020]      FIG. 3  shows a backscattered electron pattern image; 
           [0021]      FIG. 4  shows a transmission electron pattern image; 
           [0022]      FIG. 5  is a schematic diagram of a structure of a CCD backscattered electron detector; 
           [0023]      FIG. 6  shows a scanning electron microscopic image of a copper mesh; 
           [0024]      FIG. 7  is a distribution map of electrons detected by irradiation of a charged particle beam in vacuum; 
           [0025]      FIG. 8  is a backscattered electron distribution map obtained when irradiating copper with the charged particle beam; 
           [0026]      FIG. 9  is a map of composition information; and 
           [0027]      FIG. 10  shows comparison of backscattered electron distribution information or transmission electron distribution information in different irradiation positions. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0028]    Hereinafter, with reference to the drawings, an embodiment of the present invention will be described in detail. 
         [0029]      FIG. 1  is a schematic diagram of the embodiment of the present invention. 
         [0030]    A primary electron beam  3  is discharged from a cathode  1  by a voltage V 1  applied to the cathode  1  and a first electrode  2 , and is accelerated by a voltage Vacc applied to a second electrode  4  to advance to an electromagnetic lens system at a rear stage. 
         [0031]    The acceleration voltage Vacc and the V 1  are controlled by a high voltage control circuit  20 . The primary electron beam  3  is converged by a first converging lens  5  controlled by a first converging lens control circuit  22 . Here, the specimen irradiation current of the primary electron beam  3  is restricted by an objective lens aperture  6 . In order for the center of the electron beam to pass through a hole center of the objective lens aperture  6 , an electron-beam center-axis adjustment aligner  28 , an aligner control circuit  21 , and an electron beam center adjustment deflector  29  for scanning the electron beam on the objective lens aperture  6  are provided. 
         [0032]    Then, the primary electron beam  3  is converged again by a second converging lens  7  controlled by a second converging lens control circuit  23 , and narrowly focused onto a specimen  15  by an objective lens  10  controlled by an objective lens control circuit  25 . Furthermore, the specimen  15  is two-dimensionally scanned by an upper deflecting coil  8  and a lower deflecting coil  9  connected with a deflection control circuit  24 . 
         [0033]    The specimen  15  is placed on a specimen fine moving device  14  controlled by a specimen fine moving control circuit  27 . Backscattered electrons  18 , which have comparatively higher energy among signals generated from a primary electron beam irradiation point of the specimen  15 , are detected by a backscattered electron detector  11 , and are amplified by an amplifier  16 . Moreover, secondary electrons  47  having lower energy are wound up by a magnetic field of the objective lens  10 . The secondary electrons  47  are detected by a secondary electron detector  12 , while deviation of an optical axis of the primary electron beam  3  is prevented by a crossed electromagnetic field (E×B) unit  13  disposed above the objective lens  10 . Then, the secondary electrons  47  are amplified by an amplifier  17 . 
         [0034]    Transmission electrons  38  that transmit through the specimen  15  are detected by a transmission electron detector  19 , and amplified by an amplifier  37 . The amplifier  17  is controlled by a signal control circuit  26 , and the amplifiers  16  and  17  are controlled by a CCD controller  36 . The various control circuits  20  to  27  and the CCD controller  36  are controlled by a computer  290  that controls the whole apparatus. A signal of the amplified backscattered electrons is shown on a screen of a display unit  30  as an enlarged image of the specimen. In addition, the computer  290  is connected with an image acquiring means  31  for acquiring an observed image displayed on the display unit  30  as image information, an image processing means  32  for performing various kinds of image processing on the observed image, a calculating means  33  for performing various calculations on the basis of a result of the image processing, a memory  34  for storing the observed image and the calculation results, and an input means  35  for inputting observation conditions or the like. 
         [0035]      FIG. 2  shows scattering of the backscattered electrons  18  and the transmission electrons  38 . Contrast of composition information on the specimen is intensely detected from the backscattered electrons  18  the reflection angles of which from a specimen surface are in a high range, while contrast of depression and projection information is intensely detected from the backscattered electrons  18  the reflection angels of which are in a low range. 
         [0036]    In the conventional method, a scintillator, a semiconductor, or a metal electrode that converts backscattered electrons into secondary electrons is used as a detector for discriminating and detecting the low angle backscattered electrons and the high angle backscattered electrons. However, it is difficult for the detector to have a multi-divided structure in any of the above-mentioned cases. Generally, the detector has a two-divided structure. Since a threshold of the scattering angle for discrimination is determined by the structure of the detector, a threshold cannot be selectively set. 
         [0037]    It is also impossible to extract only the backscattered electrons in a selected range of the scattering angles, not even by using an aperture. Since a threshold and a range of extracted angles for discrimination are determined by a diameter of the aperture, the threshold and the range of extracted angles cannot be selectively set. The scattering angle of the backscattered electron changes depending on inclination and composition of the specimen. However, a scattering angle for detection cannot be appropriately set with the conventional method. Accordingly, accurate discrimination is difficult. 
         [0038]    In the present invention, the deflection controlling circuit  24  is controlled in synchronization with control of the CCD backscattered electron detector  11 . Thereby, a pattern image of the backscattered electrons is detected for each irradiation point (irradiation position) of the electron beam by the CCD backscattered electron detector  11 , and recorded in the memory  34 . 
         [0039]      FIG. 3  shows an example of a backscattered electron pattern image  39 . This backscattered electron pattern image  39  corresponds to one pixel of an enlarged image. The image processing means  32  and the calculating means  33  are used to integrate or average only information on a region a of the backscattered electron pattern image  39 , and data for one pixel is obtained. By performing this processing on each backscattered electron pattern image  39  obtained from each irradiation point of the electron beam, an image of only the high angle backscattered electrons is obtained. Similarly, an enlarged image formed only on the basis of information on a region b is an enlarged image of only the low angle backscattered electrons. 
         [0040]    After the backscattered electron pattern image  39  of the specimen surface to be observed is thus recorded, by designating any selected region on the pattern, the designated region on the pattern can be converted into a high angle backscattered electron image or a low angle backscattered electron image. An arbitrary region can also be designated by use of a component element or inclination of the specimen. 
         [0041]      FIGS. 6 to 8  show distribution of a backscattered electron image obtained for each of the irradiation positions, according to the present invention. As shown in  FIG. 6 , a copper mesh is used as the specimen.  FIG. 7  shows a state of distribution of the backscattered electrons which is obtained by the backscattered electron detector when the electron beam is applied in vacuum. The upper diagram shows a distribution image (distribution information) of the backscattered electrons in the irradiation position, and the lower graph shows signal amounts plotted along a white line of the above-mentioned diagram. 
         [0042]      FIG. 8  shows a state of distribution of the backscattered electrons which is obtained by the backscattered electron detector when a copper is irradiated with the electron beam. The upper diagram shows a distribution image (distribution information) of the backscattered electrons in the irradiation position, and the lower graph shows signal amounts plotted along a white line of the above-mentioned diagram. 
         [0043]    The result of the experiments shows that the above-mentioned spot appears in the same position in spite of shifting the irradiation position when copper exists in the irradiation position. 
         [0044]    It turned out that the composition of the irradiation position of the primary electron beam can be identified by using this result of the experiments. Since identification of an incident position is limited to four in a four-divided detector, for example, in the conventional case, the composition cannot be identified even when the composition information is reflected. In the present invention, since distribution information (distribution map) on the backscattered electrons is acquired for every irradiation position, the composition can be accurately identified. 
         [0045]    By retaining information on the spot position of the backscattered electrons in each substance in advance, the composition at each irradiation position can be found. Further, a map on the composition information on the specimen as shown in  FIG. 9  can be also created. 
         [0046]      FIG. 2  shows scattering of the backscattered electrons  18  and the transmission electrons  38 . Density information and electron diffraction information on the specimen  15  are included in a bright field picture signal of the transmission electrons  38  which has a lower scattering angle among the transmission electrons  38  that transmit through the specimen  15 . In the case of a dark field image signal of the transmission electrons  38  which has a higher scattering angle, information visualized is different depending on the detection angle. A heavier element causes the transmission electrons  38  to scatter at a higher scattering angle. 
         [0047]    Discrimination in accordance with the scattering angle of the transmission electron has the same problem as in the case of the backscattered electron since the scattering angle varies in accordance with the thickness or composition of the specimen. Some types of electron microscopy employ a method of shifting the detector on the optical axis and controlling the angle for detection. Such a method, however, has a problem that a mechanism becomes complicated. 
         [0048]    In the present invention, the deflection controlling circuit  24  is controlled in synchronization with control of the CCD backscattered electron detector  11 . Thereby, a pattern image of the transmission electron is detected for each irradiation point (irradiation position) of the electron beam by the CCD backscattered electron detector  11 , and recorded in the memory  34 . 
         [0049]      FIG. 4  shows an example of a pattern image  40  of the transmission electrons. This transmission electron pattern image  40  corresponds to one pixel of an enlarged image. The image processing means  32  and the calculating means  33  are used to integrate or average only information on a region c of the transmission electron pattern image  40 , and data for one pixel is obtained. The light field image is formed by performing this processing on the transmission electron pattern image  40  obtained from each irradiation point of the electron beam. Similarly, the dark field image is an enlarged image formed only by information on a region d. 
         [0050]    It is expected that the contrast of the backscattered electron image or the transmission electron image should be further improved by using discrimination in accordance with luminance of the pattern image in addition to discrimination in accordance with selection of the region in the pattern image. 
         [0051]    As mentioned above, variety of information can be acquired by acquiring and storing the distribution information on (distribution image of) the backscattered electrons or the transmission electrons for every irradiation position. 
         [0052]    For example, as shown in  FIG. 10 , the distribution information is observed, only a signal of a detection position that appears in a spot form is extracted, and a signal of a detection position that has a spread region is removed. Thereby, only the composition information can be extracted with the projection and depression information on the specimen being removed. This can be attained by subtracting the information on a region having an area not less than a predetermined value from the distribution image. Conversely, when only the information having an area not less than the predetermined value is extracted, the information other than the composition information can be extracted. 
         [0053]    With respect to this point, conventionally, while the backscattered electrons or the transmission electrons enter a divided detecting surface, the backscattered electrons or transmission electrons that enter the same detecting surface are all collected as one signal irrespective of the information (depression and projection information, composition information) that the electron has. For that reason, the composition information and the projection and depression information are mixed, and are detected. 
         [0054]    In the present invention, the information that the backscattered electron or the transmission electron has can be isolated with more accuracy. 
         [0055]    Furthermore, as shown in  FIG. 10 , by comparing distribution information (distribution image) for every different irradiation position, only a part common to each distribution image can be extracted, or only the information unique to each distribution image not included in the distribution image of other irradiation position can be extracted. 
         [0056]      FIG. 5  is a schematic diagram of a structure of the CCD backscattered electron detector  11 . 
         [0057]    A scintillator  44  is fixed to a CCD element  46  with a buffer layer  45  in between. An electron beam passing hole  43  for passing the primary electron beam  3  through is provided in a central portion of the detector  11 . The number of pixels of the image that can be captured is approximately 1500 to 1700 wide by approximately 1000 to 1200 long. 
         [0058]    The CCD backscattered electron detector  11  is disposed between the objective lens  10  and the specimen  15  to acquire the backscattered electron image, and under the specimen  15  to acquire the scanning transmission electron image. The CCD backscattered electron detector  11  measures approximately 40 mm wide by 30 mm long by 5 to 10 mm thick, which is small enough to be mounted on. 
         [0059]    In the conventional detector employing the CCD element, the size of the CCD element is approximately 6 μm. For that reason, light generated from the scintillator inevitably enters multiple CCD elements. Accordingly, the light needs to be converged with a lens. The light generated in the scintillator is therefore converged by the lens disposed between the scintillator and the CCD element, and subsequently, the converged light enters the CCD element. Accordingly, the conventional detector employing the CCD element is large-sized, and cannot be disposed between the objective lens and the specimen. 
         [0060]    In the present invention, the size of a detection element of the CCD is 20 μm, which is larger than the conventional one. Accordingly, the CCD backscattered electron detector  11  has a structure in which the light discharged from the scintillator  44  is detected without being converged with a lens. 
         [0061]    The thickness of the scintillator is determined depending on the size of the CCD element. When the scintillator is too thick, the light spread within the scintillator enters multiple CCD elements, so that accuracy of the incident position deteriorates. 
         [0062]    The above-mentioned experiment shows that at least not less than 10 bits of the CCD element is needed in order to obtain accurate observation of the peak as shown in  FIG. 8 . 
         [0063]    The CCD backscattered electron detector  11  may be configured to be movable in an optical axis direction by a moving means  41  according to a purpose. For example, when detecting all dark field signals to be generated, the CCD backscattered electron detector  11  may be brought closer to the specimen  15 . Moreover, for example, when a part of the dark field signals is to be discriminated, the CCD backscattered electron detector  11  may be moved away from the specimen  15  so that a discrimination angle can be set finely. 
         [0064]    Detection elements, such as CCD elements, that are two-dimensionally arranged (for example, arranged in lattice pattern) and that the incident position of the electron is identifiable therein are used as the detection elements for the detector of the backscattered electron and the transmission electron. 
         [0065]    While the above-mentioned embodiment has been described using the CCD detector, the present invention is not limited to this. It is also possible to use a detector of any other form that can identify the incident position of the electron and can be inserted between the specimen and the objective lens. Moreover, it is also possible to use a detector of a type in which multiple detection elements are arranged regularly in a plane (for example, in lattice pattern) for identification of the incident position of the electron. 
       Explanation of Reference Numerals 
       [0066]      1  . . . cathode,  2  . . . first electrode,  3  . . . electron beam,  4  . . . second electrode,  5  . . . first converging lens,  6  . . . objective lens aperture,  7  . . . second converging lens,  8  . . . upper deflecting coil,  9  . . . lower deflecting coil,  10  . . . objective lens,  11  . . . backscattered electron detector,  12  . . . secondary electron detector,  13  . . . crossed electromagnetic-field apparatus,  14  . . . fine moving device,  15  . . . specimen,  16  and  17  . . . amplifier,  18  . . . backscattered electron,  19  . . . transmission electron detector,  20  . . . high voltage control circuit,  21  . . . aligner control circuit,  22  . . . first converging lens control circuit,  23  . . . second converging lens control circuit,  24  . . . deflection controlling circuit,  25  . . . objective lens control circuit,  26  . . . signal control circuit,  27  . . . specimen fine moving control circuit,  28  . . . electron beam central axis adjustment aligner,  30  . . . display device,  31  . . . image acquiring means,  32  . . . image processing means,  33  . . . calculating means,  34  . . . memory,  35  . . . input means,  36  . . . CCD controller,  37  . . . amplifier,  38  . . . transmission electron,  39  . . . backscattered electron pattern image,  40  . . . transmission electron pattern image,  41  . . . moving means,  42  . . . transmission electron detector moving means,  43  . . . electron beam passing hole,  44  . . . scintillator,  45  . . . buffer layer,  46  . . . CCD element,  47  . . . secondary electron,  290  . . . computer,