Abstract:
The conventional detection technique has the following problems in detecting interference fringes: (1) Setting and adjustment are complex and difficult to conduct; (2) A phase image and an amplitude image cannot be displayed simultaneously; and (3) Detection efficiency of electron beams is low. The invention provides a scanning interference electron microscope which is improved in detection efficiency of electron beam interference fringes, and enables the user to observe electric and magnetic information easily in a micro domain of a specimen as a scan image of a high S/N ratio under optimum conditions.

Description:
CLAIM OF PRIORITY 
   The present invention claims priority from Japanese application JP 2004-357539 filed on Dec. 10, 2004, the content of which is hereby incorporated by reference on to this application. 
   FIELD OF THE INVENTION 
   The present invention relates to an electron beam apparatus, such as an electron microscope, which measures an electromagnetic field in a matter or vacuum using interference of electron beams. 
   BACKGROUND OF THE INVENTION 
   The electron holography, or the electron interference microscopy, is a technique of quantitatively measuring an electromagnetic field in a matter or vacuum by measuring a phase shift of an electron beam caused by a specimen, and specifically a technique in which an electron beam generated in an electron source is splitted into a plurality of electron beams by an electron biprism, a splitted electron beam is made to enter the specimen, and the electron beam having transmitted through the specimen is detected, whereby an interference image is acquired. Such a scanning interference electron microscope is disclosed in, for example, Japanese Patent Application Laid-Open No. 8-45465 and Japanese Patent Application Laid-Open No. 9-134687. 
   The electron beam holography method is classified in terms of its system into an interference electron microscopy of the scanning transmission electron microscope (STEM; Scanning Transmission Electron Microscope) type and an interference electron microscopy of the transmission electron microscope (TEM; TranSmission Electron Microscope) type. The interference electron microscopy of the STEM type has the following merits as compared with the interference electron microscopy of the TEM type: (1) The STEM type interference electron microscopy can display a phase image on-line and real-time; (2) It can display simultaneously an analytical image, such as detection of a characteristic X-ray etc. generated by scanning illumination of an electron beam, and an interference image; and (3) Since a spatial resolution is determined by a spot size of a focused electron beam, controllability of spatial resolution is excellent; and the like. 
   An electromagnetic field in the specimen can be estimated by measuring the amount of phase shift of interference fringes by image analysis of a detected interference image, namely the amount of positional shift between positions of constructive interference and of destructive interference. As a technique of measuring the amount of phase shift of interference fringes, for example, there is the method of Leuthner et al. In addition, in the invention disclosed in Japanese Patent Application Laid-Open No. 9-134687, the amount of phase shift is calculated with the method of Leuthner et al. In the Leuthner&#39; method (Th. Leuthner, H. Lichte, and K-H. Herrmann: “STEM-Holography Using the Electron Biprism” Phys. Stat. Sol. A 116, 113. (1989)), a phase image of the specimen is acquired by detecting an electron beam having passed through a grating-type slit with an electron beam intensity detector, and converting an intensity signal of the detected electron beam into phase information. Hereafter, the Leuthner&#39;s method will be explained in detail using  FIG. 2A  and  FIG. 2B . 
     FIGS. 2A and 2B  are schematic diagrams each showing a comparative relation among interference fringes of electron beams, a slit, and an electron beam intensity detector. In  FIGS. 2A and 2B , the reference numeral  46  denotes a slit and  50  denotes an electron beam intensity detector. The numerals  48  and  49  each denote interference fringes of the electron beams which reach the slit.  FIG. 2A  corresponds to a case where an aperture of the slit coincides with a position of constructive interference and  FIG. 2B  corresponds to a case where the aperture of the slit coincides with a position of destructive interference. The vertical axis of the interference fringes  48  and  49  corresponds to the intensity of the electron beams. When performing the method of Leuthner et al., first a direction of the interference fringes and a direction of the slit are set in the same direction. Usually, the apparatus user observes the image of interference fringes by visual inspection, and manually adjusts the direction of the interference fringes obtained, the direction of the slit apertures, and a position of the grating-type slit itself. 
   When the interference fringes are detected in a state where the direction of the interference fringes agrees with the direction of the slit, the intensity of the detected electron beam varies depending on positions of constructive interference and of destructive interference relative to the slit. In the case of  FIG. 2A , the amount of the electron beams passing through the slit  46  becomes a maximum, and in the case of  FIG. 2B , the amount of the electron beams passing through the slit  46  becomes a minimum. Therefore, if the amount of the electron beams detected with the detector  50  is normalized using its maximum and minimum, the amount of the detected electron beams could be converted to a cosine of the amount of phase shift. That is, when the amount of the electron beams of the interference fringes passing through the slit  46  assumes a maximum, the phase shift by the specimen is 0□}2π□Λn, and when the amount of the electron beams of the interference fringes passing through the slit  46  assumes a minimum, the phase shift by the specimen is π□}2□Λn. Generally, a direction of the apertures of the slit  46  and a position of the slit  46  are so adjusted that detected constructive interference and destructive interference assume detection intensities of those formed under the condition that there is no specimen or both of the splitted electron beams pass through a vacuum. Therefore, it becomes possible to display an image having phase information of the specimen by displaying the amount of the electron beams having passed through the slit  46  which is normalized to be a value between a maximum and a minimum as a cosine of the amount of phase shift or further converting the value so obtained into the amount of phase shift between zero and π. 
   SUMMARY OF THE INVENTION 
   An S/N ratio of a scanning phase information image obtained with the scanning interference electron microscope becomes higher with increasing intensity of the detected electron beam intensity. Therefore, it is essential to make the electron interference fringes enter a detector effectively in order to achieve a clear scan image. The conventional scanning interference electron microscope using the method of Leuthner et al. has the following problems.
     (1) Setting and adjustment are complex and difficult to do.   (2) Simultaneous display of a phase image and an amplitude image cannot be performed.   (3) The detection efficiency of the electron beams is low.   

   The above (1) problem arises from a fact that a relative direction between the slit and the interference fringes and positions thereof are adjusted manually. Specifically, adjustment to equalize a spacing of apertures of the slit and a spacing of interference fringes and make directions of both spacings agree with each other is done by observing the interference fringes magnified by about 1000 times with imaging lenses with an eye using a fluorescent screen and moving the position and direction of the slit manually. Since the magnification weakens the intensity of interference fringes, the adjustment requires skills and experience and accurate adjustment is difficult. 
   Regarding a problem described in the above (2), since only one detector is used, it is essential to select and display either the amplitude image corresponding to a normal electron microscope image or the phase image, thus simultaneous display being impossible. If the observer is enabled to observe simultaneously structure information obtained from the normal electron microscope image and electromagnetic field information obtained from the phase image, it will give the observer an extra convenience. 
   The above (3) results from a fact that, since electron beams passing through the slit are allowed to enter the detector, a part of the electron beam blocked by the slit is not used. Since the electron beams blocked the slit cannot be used effectively, there is a limit in improving detection sensitivity or detection precision. Although it is possible to capture the whole image of the interference fringes, namely, to detect all the electrons, and process them with a high-speed processor, a time to transfer data to the processor, a time required for arithmetic computing, a time to transfer the data to memory, etc. will become huge, which deprives the STEM type interference electron microscopy of its advantage that a phase image can be displayed real-time. The present invention has its object to provide a scanning interference electron microscope which is easy to set up and adjust and yet highly sensitive. 
   The present invention solves the above-mentioned problems by detecting interference fringes of electron beam with an electron beam detector that consists of one pair of multi pixels. That is, an output of this detector is a 1-dimensional interference fringe image such that a value of each pixel is an integration value of 2-dimensional pixels along one 1-dimensional direction. Moreover, in the present invention, by mounting this detector on an externally controllable rotationable stage, a magnification of the interference fringes and a rotation direction of the detector are automatically adjusted, so that the interference fringes can be detected under conditions of highest efficiency. 
   According to the present invention, the apparatus can detect interference fringes of the electron beams with an asymmetric 2-dimensional detector with integration capability, and adjust them at high-speed and easily, thereby being able to detect them under optimum conditions. Therefore, a scan image of a high S/N ratio can be obtained. Moreover, the use of one pair of detectors enables simultaneous display of the amplitude image and the phase image. Furthermore, unlike the conventional microscopes, this microscope uses no slit, and accordingly the whole electrons constituting the interference fringes can be used, achieving high detection efficiency. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a diagram explaining a method of carrying out the present invention. 
       FIGS. 2A and 2B  are diagrams explaining a detection technique that is being carried out conventionally.  FIG. 2A  corresponds to a case where an aperture of the slit coincides with a position of constructive interference.  FIG. 2B  corresponds to a case where the aperture of the slit coincides with a position of destructive interference. 
       FIG. 3  is a flowchart explaining procedures of optimally adjusting the direction of interference fringes and the direction of a detector in the present invention. 
       FIG. 4  explains procedures of optimally adjusting the direction of interference fringes and the direction of a detector in the present invention. 
       FIG. 5  is a diagram explaining variation of spacing and contrast of interference fringes with magnification. 
       FIG. 6  is a diagram explaining a detection principle in the present invention. 
       FIG. 7  is a diagram explaining a display method in the present invention. 
       FIG. 8A  is a diagram explaining a step of one embodiment of semiconductor dopant profile observation in the present invention. 
       FIG. 8B  is a diagram explaining a step of the one embodiment of semiconductor dopant profile observation in the present invention. 
       FIG. 8C  is a diagram explaining a step of the one embodiment of semiconductor dopant profile observation in the present invention. 
       FIG. 8D  is a diagram explaining a step of the one embodiment of semiconductor dopant profile observation in the present invention. 
       FIG. 9A  is a diagram explaining a step of one embodiment of magnetic domain observation in a magnetic thin film in the present invention. 
       FIG. 9B  is a diagram explaining a step of the one embodiment of magnetic domain observation in the magnetic thin film in the present invention. 
       FIG. 9C  is a diagram explaining a step of the one embodiment of magnetic domain observation in the magnetic thin film in the present invention. 
       FIG. 9D  is a diagram explaining a step of the one embodiment of magnetic domain observation in the magnetic thin film in the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   First Embodiment 
   In this embodiment, an example where the present invention is applied to STEM will be described.  FIG. 1  shows an example of a configuration of an STEM of this embodiment. The STEM of this embodiment comprises an electron gun  38 , an illumination system  39 , a specimen chamber  40 , an imaging system  41 , a detection system  42 , a control system, etc., as a rough breakdown of the STEM. 
   The electron gun  38  of the STEM of this embodiment consists of an electron source  1 , a first anode  2 , a second anode  3 , an acceleration anode  4 , etc. The illumination system  39  consists of an electron biprism  7 , a condenser aperture  8 , a first condenser lens  19 , a second condenser lens  10 , a scanning coil  11 , an objective lens pre-field  13 , an objective lens post-field  16 , etc. In addition, although not illustrated specifically, the electron biprism  7  is equipped with an electron biprism fine positioning system  35 , and is made movable by this. The imaging system  41  and the detection system  42  consist of a secondary electron detector  12 , the stigmator  19 , single or multiple imaging lenses  20 , a detector  23 , a rotationable stage  24 , a rotation mechanism  37  of the rotationable stage, etc. In addition to the secondary electron detector  12 , the STEM may be equipped with a reflected electron detector. 
   The control system consists of a CPU  25 , memory  26 , a display  27 , D/A converters  28 ,  29 , a signal processor  32 , etc. The D/A converter  28  is connected with constituents of the electron optical system and the imaging system through signal transmission line, and a control signal from the CPU  25  is transferred to each constituent through the signal transmission line. Moreover, although not illustrated in the figure, the display  27  is equipped with information input means, such as a keyboard and a mouse, and the system user enters desired information into the system using the information input means as an input interface. There is a case where the CPU  25 , the D/A converter  28 , and the signal processor  32  may be housed in a single enclosure, each as a part of a control computer. The reference numerals  30  and  31  denote a specimen stage fine positioning system and a specimen stage fine positioning sensor, both of which are attached to the rotationable stage  24 . The numeral  37  denotes the rotation mechanism of the rotationable stage for moving the detector  23  and the rotationable stage  24  used in the present invention and replacing the detector  23  to another detector. The numeral  15  denotes a specimen stage, which is movable in an X-Y plane and in a Z-direction by stage drive means indicated by an arrow. 
   First, operations of the electron optical system will be explained. The electron biprism  7  is inserted between the electron source  1  and a first condenser lens  9  and a voltage is applied to this, whereby the electron beam is splitted into two which apparently come from two virtual electron sources  6 . The two splitted electron beams are focused with the first condenser lens  9 , a second condenser lens  10 , and further the objective lens pre-field  13 , respectively, to form two micro spots  14  on a plane of a specimen  15 . At this time, the two beams are so adjusted that one of the two micro spots transmits through the specimen and the other passes through a vacuum in proximity to the specimen. Note here that a distance of separation of the two micro spots becomes larger in proportion to a voltage applied to the electron biprism  7 . 
   In this embodiment, the voltage applied to the electron biprism  7  and the voltage applied to an electron beam deflection coil  11  are gang controlled in response to a magnification of an image to be observed. Although this gang control is automatically done by the CPU  25 , naturally the both voltages can be set manually. This gang control is realized by setting up the voltage of the electron biprism in such a way that the distance of separation of the two micro spots on the plane of the specimen which is calculated with both a deflection angle of the electron beam by the electron biprism and the electron optical system assumes either a comparative value of the spot size of the focused electron beam in each magnification power multiplied by a predetermined multiplier or an absolute value in proportion to an inverse of each magnification value. 
   Now, the electron beam having transmitted through the specimen and the electron beam passing though a vacuum overlap on an arbitrary plane below the specimen to generate interference fringes  17 , which is magnified with the imaging lenses  20 . The magnified interference fringes are recorded by the detector  23  disposed on an observation plane  22 . Generally, if the electron beam is scanned with the electron beam deflection coil  11 , the whole interference fringes will move. However, if the object plane of the imaging lenses  20  is adjusted to be on a pivot plane  18  of the electron beam, the interference fringes will not move even with electron beam scanning. Here, the “pivot plane” means an electron optics plane that remains immovable even when the electron beam is scanned at a fulcrum of deflection of the electron beam. 
   Here, the imaging lenses  20  may be of one stage or a combination of multi-stage lenses according to resolution of the detector. In this embodiment, the detector  23  is placed on the externally controllable rotationable stage  24 . Moreover, the electron biprism  7 , the detector  23 , and the rotationable stage  24  can be removed from the passage of the electron beam with the help of the fine positioning system  35  and the rotationable stage movement mechanism  37 , respectively, so that these components do not hinder operations of the system as a normal scanning transmission electron microscope. Naturally, this microscope can also be used as a special purpose apparatus of the scanning interference electron microscope. Furthermore, the use of the stigmator  19  consisting of a multipole is desirable because an image of the interference fringes is compressed in a direction parallel to the interference fringes and the intensity of the electron beams is enhanced. 
   Next, a method for observing the interference fringes by using the STEM shown in  FIG. 1  will be explained using  FIG. 3 . First, in Step  300 , the specimen is placed and held on the specimen stage  15  and carried into the vacuum chamber. Next, in Step  301 , a predetermined voltage is applied to the acceleration anode  4  to accelerate the electron beam generated in the electron source  1 . In Step  302 , field emission current is pulled out by applying suitable voltages to the first anode  2  and the second anode  3 . 
   In Step  303 , the electron biprism fine positioning system  35  is driven to move the electron biprism  7  to a predetermined position. In Step  304 , the electron biprism is adjusted by using a rotational mechanism of the electron biprism and a rotational mechanism of the specimen so that the edge of the specimen becomes parallel to the direction of the electron biprism. 
   In Step  305 , the monitor  27  shows a screen used to specify observation magnification, and the apparatus user enters the observation magnification into the apparatus by input means, such as a GUI and a keyboard. The CPU  25  determines a voltage to be applied to the biprism and transfers it to the electron biprism  7  based on the entered observation magnification. The applied voltage determined by the CPU  25  is converted into an analog control signal by the D/A converter  28 , and inputted into an unillustrated drive power supply for the electron biprism. Then, Step  306  is executed. 
   Next, in Step  307 , adjustment of the magnifying lens and the stigmator  19  is executed. That is, the magnification of the imaging lens  20  is suitably adjusted, the interference fringes  17  on the pivot plane  18  are converted into interference fringes  21  suitably magnified, and further these interference fringes  21  are compressed in a direction parallel to the fringes by adjusting the stigmator  19 . 
   In Step  308 , formation of the interference fringes  21  is completed in this way, and in Step  309 , the interference fringes  21  are grabbed by the apparatus through the detector  23 . Since the present invention uses one pair of detectors, what is inputted into the apparatus is a 1-dimensional image  33  or a 1-dimensional image  34 . In Step  310 , the inputted interference-fringes image is 1-dimensional-Fourier transformed by the processor  32 . From the results, the CPU  25  finds a current rotational speed and a peak position of the rotationable stage  24 , namely, a spatial frequency giving a peak and a peak intensity, and stores them in the storage device  26 . The rotation angle of the rotationable stage  24  is obtained by inputting an output of the specimen stage fine positioning sensor  31  provided in the rotationable stage  24  into the CPU  25  through the A/D converter  29 . 
   In Step  311 , whether or not the current rotation angle of the rotationable stage  24  is optimum is evaluated. That is, peak intensities corresponding to the respective rotation angles which cover up to 180 degrees are called from the storage device  26 , and whether or not a peak intensity corresponding to the current rotation angle is a maximum among them is evaluated. If data corresponding to rotation angles which cover up to 180 degrees is not obtained or if the current rotation angle is not optimum, the flow proceeds to Step  312 , where the rotation angle is varied by a previously set angle. This operation is done by the CPU  25  by inputting a signal to the specimen stage fine positioning system  30  provided in the rotationable stage  24  through the D/A converter  28 . Here, the flow returns to Step  310  again, and Step  311  and Step  312  are repeated until the rotation angle becomes optimum. 
   If the rotation angle of the detector is determined optimum, whether or not the magnification is optimum is determined in Step  313 . That is, the peak positions stored in Step  310  are scanned over a range of previously set spatial frequencies, and whether or not the peak position gives a maximum peak intensity among them is evaluated. If either of the two criteria is not satisfied, the flow goes back to Step  307 , where the magnification of the magnifying lens is varied by a previously set value, and Steps  308  to  313  are repeated. 
   If it is determined that the magnification is optimum in Step  313 , the flow proceeds to Step  314 , where the specimen is observed and the observation is finished in Step  315 .  FIG. 4  is a diagram showing the imaging system and a main part of the control system of the STEM of  FIG. 1 , and the operation flow shown in  FIG. 3  is executed by constituents shown in  FIG. 4 . In  FIG. 4 , the drawing-out reference numeral  63  denotes interference fringes of the electron beam that passes through imaging lenses  62  and reaches a rotationable stage  66 . On the rotationable stage  66 , one pair of asymmetric 2-dimensional detectors with integration capability  64 ,  65  are placed and held. Here the “asymmetric 2-dimensional detector with integration capability” means a detector which is made up of a 2-dimensional array of multi-pixels such that a ratio of the number of pixels in one dimension and the number of pixels in the other dimension is equal to or more than two and a value obtained by integrating values of pixels along a dimension having a smaller number of pixels is outputted as a value of each pixel being arrayed along the dimension having a larger number of pixels. This function may be realized with hardware or may be realized with software. 
   The asymmetric 2-dimensional detectors with integration capability  64 ,  65  are each made up of a large number of electron sensing elements, wherein signals detected by the elements are integrated in a direction along a direction of integration sequentially and is outputted finally as a 1-dimensional image. In this embodiment, the output signal from the asymmetric 2-dimensional detector with integration capability  64  and the output signal from the asymmetric 2-dimensional detector with integration capability  65  are intended to be used for phase detection and for amplitude detection, respectively, and they are designated by symbols P and A in the figure, respectively. 
   The asymmetric 2-dimensional detectors with integration capability  64 ,  65  are connected with signal transmission lines  67 ,  68 , respectively, being connected to a signal processor  69 . A signal which transmits through the transmission line  67  is a signal for phase detection and a signal which transmits through the transmission line  68  is a signal for amplitude detection, and they are designated by DP and DA in  FIG. 4 , respectively. The signal passing through the signal processor  69  is finally inputted into a CPU  70 , subjected to a predetermined operational processing, and subsequently displayed by display means  76 . A D/A converter  71  is provided in order to convert the rotation angle information of the rotationable stage  66  from the CPU  70  into an analog control signal and transfer it to a fine rotation mechanism  73  for the rotationable stage  66 , and also serves for a stigmator  61  and the imaging lenses  62 . An A/D converter  72  is provided in order to convert a signal from a sensor  74  for detecting rotation of the rotationable stage into digital data which the CPU  70  can process. 
   Next, a position adjustment flow of the detector will be explained in detail. Prior to observation of the specimen, it is necessary to form interference fringes first on the detector placed in the center of this scanning interference microscope, i.e., on the electron optical axis. This can be done by mechanical adjustment of the imaging lenses and adjustment of the electron beam deflection coil built in the illumination system. After this was completed, it is necessary to adjust the detector so that the interference fringes may be formed along a longitudinal direction of the detector used in the present invention. (It is necessary to adjust relatively a direction of the interference fringes and a direction of the asymmetric 2-dimensional detector with integration capability.) Here, the direction of the interference fringes of the electron beams and the direction of the detector are defined as follows. That is, the interference fringes of the electron beams are of a pattern in which an intense part and a weak part of the intensity of the electron beams are repeated in a 1-dimensionally direction in a sinusoidal manner. The 1-dimension direction in concern is defined as a direction of the interference fringes. The direction of the asymmetric 2-dimensional detector with integration capability is defined as its longitudinal direction. Adjusting the direction of the interference fringes and the direction of the detector thus defined can be achieved precisely by performing procedures as described below. These procedures will be explained using  FIG. 4  similarly. 
   First, the interference fringes  63  are made to be incident on the detector  64  and the detector  65 , under appropriate conditions. The 1-dimensional image signal DA  67  which is an output of the phase detecting detector  64  or the 1-dimensional image signal DP  68  which is an output of the amplitude detecting detector  65  is subjected to 1-dimensional fast Fourier transform, namely converted to a spatial frequency spectrum, by the signal processor  69 . When the spectrum is displayed on the display  76 , if the direction of the interference fringes and the direction of the detector agree with each other, a clear peak  77  is observed in the spectrum. The signal processor  69  may be realized with hardware using a special board, or may be realized by executing software of Fourier transform on the CPU  70 . 
   In line with this, rotational angle detection means, such as the angle sensor  74 , is provided in the rotationable stage  66 , and a rotational angle of the rotationable stage  66  counting from the start of rotation is outputted as an angle signal, which is inputted into the CPU  70  though the A/D converter  72 . The CPU  70  displays a phase signal inputted from the signal converter  69  on the display means  76  in synchronization with the angle signal from the A/D converter  72 . Then, data showing a dependence of the peak intensity of the spectrum on the rotational angle of the rotationable stage, as shown in a graph  78 , on the display means  76 . Observing the height of the peak while rotating the detector, the peak intensity assumes a maximum when the angle of the rotation agrees with a best matched direction. The angles at which the peak intensity becomes maximums are determined as optimum arrangement angles of the asymmetric 2-dimensional detectors with integration capability  64 ,  65 , respectively. Determination of the optimum arrangement angle may be selected by the apparatus user, or the apparatus may control adjustment of rotation angle so that the optimum peak is automatically selected. 
   In the case where the apparatus user itself selects the optimum peak, the apparatus is so controlled that, when rotation of the rotationable stage is ended in a range of horizontal direction of the graph  78 , the apparatus becomes a state of waiting an entry from the user. When the apparatus becomes the state of waiting an entry, the apparatus allows the apparatus user to select a peak which is considered optimum from the graph  78  displayed on the display screen  76  with input means, such as a mouse and as key board, entering information of the optimum peak into the apparatus. The information of the optimum peak entered by the apparatus user is transferred to the CPU  70 , and the CPU  70  reads a rotation angle of the optimum peak from the display image (graph  78 ) based on the inputted information and forwards the information of the optimum angle to the D/A converter  71 . The optimum angle is fed back to the fine rotation mechanism  73  installed in the rotationable stage  66 . The control means of the rotationable stage rotates the rotationable stage based on the angle information fed back thereto, and optimizes the arrangement angle of the asymmetric 2-dimensional detectors with integration capability  64 ,  65 . 
   In the case where the apparatus optimizes the arrangement angle of the asymmetric 2-dimensional detectors with integration capability in a fully automatic manner, the CPU  70  automatically reads an optimum peak from the graph  78  and feeds it back to the fine rotation mechanism  73  for the rotationable stage through the D/A converter  71 . In this case, it is not necessary to control the apparatus to be in the state of waiting for the user&#39;s entry after the end of the rotation of the rotationable stage; automatic reading of the optimum peak may be started just after the end of the rotation. It is needless to say that the apparatus needs to be adjusted in advance so that the center of the detector coincides with the center of the interference fringes before the adjustment of the optimum arrangement angle of the interference fringes described above. 
   Note here that, since the interference fringes are integrated in a direction parallel to the interference fringes in the case of the asymmetric 2-dimensional detector with integration capability used in this embodiment, a clear peak can be obtained only when the direction of the interference fringes agrees with the direction of the detector in a highly precise manner; therefore, the direction of the two can be adjusted precisely. Moreover, integration of the interference fringes enhances the ratio excellently, and consequently the directions can be adjusted further accurately. By such procedures, it becomes possible to bring the direction of the interference fringes and the direction of the detector into agreement with each other with high precision. 
   A next important adjustment subject is adjustment between a fringe spacing of the interference fringes, or a magnification of the interference fringes, and a pixel size of the detector.  FIG. 5  shows several electron interference fringes formed under fixed conditions recorded on a high-resolution film while varying only the magnification. A film whose resolution allows interference fringes having a fringe spacing of about 3 μm at a minimum to be recorded was used. When the magnification is so reduced that the interference fringe spacing becomes 33 μm, 13 μm, 9 μm, 5.5 μm, and 3.8 μm on the film, an exposure time necessary to achieve the same optical density becomes smaller as 240 sec, 120 sec, 60 sec, 8 sec, and 4 sec, respectively. Observing a profile ( FIG. 5 , right row) obtained by integrating the interference fringes recorded under these conditions in a direction parallel to the fringe of the interference fringes, it is found that a highest contrast is achieved with an interference fringe spacing of 5.5 μm and an exposure time of 8 sec. As shown in this example, it is preferable to record the interference fringes with as small a magnification as possible. However, when the interference fringe spacing comes close to a resolution limit of a detector (in this case, the film), the contrast blurs because constructive interference and destructive interference cannot be recorded. 
   The above fact teaches that in order to detect the interference fringes, it is recommended to magnify the interference fringes so as to have a best fringe spacing which complies with the resolution of the detector. So, in this embodiment, the magnifying lens  62  and the stigmator  61  are controlled by the CPU  70  and the D/A converter  71 , as shown in  FIG. 4 , and a position and the height of the peak in the 1-dimensional Fourier conversion of the interference fringes are adjusted by the same procedures as was used in adjusting a direction of the detector. The adjustment is done by the following procedures. 
   First, the magnifying lens  62  is so adjusted that the electron beam is incident on the center of the detector under a condition that the spacing of the interference fringes  63  is sufficiently large as compared to a size of a pixel of the detector  64  or detector  65 . Then, the stigmator  61  is so adjusted that the interference fringes are compressed in a direction parallel to the interference fringes. After that, the direction of the detectors is optimized with respect to the direction of the interference fringes by the procedures described above. At this time, a peak value in a spatial frequency spectrum of the interference fringes under the optimum conditions is stored in a storage device  75 . Next, the magnification of the magnifying lens  62  is made small, the same procedures are repeated, and a peak in the spectrum corresponding to a current value of the magnifying lens  62  is stored sequentially. Subsequently, the current value of the magnifying lens  62  is plotted on the horizontal axis and the peak value in the spectrum is plotted on the vertical axis. Since the peak value in the spectrum becomes a maximum at an optimum magnification of the magnifying lens  62 , the magnifying lens  62  and the stigmator  61  are set to this condition and the adjustment is finished. 
   By the above procedures, the interference fringes can be detected under the optimum conditions. Naturally, these adjustment procedures can be put in a program and be performed automatically. It goes without saying that the procedures of matching the direction of the interference fringes described above can be realized by finely tuning a mechanism for rotating the electron biprism in a plane vertical to the direction of the electron beam, except for the rotation of the detector. 
   Now, procedures of obtaining both the amplitude image and the phase image simultaneously after setting detection of the interference fringes to be under the optimum conditions in this way will be explained using  FIG. 6 . Note that, in  FIG. 6 , a rectangular pattern that is hatched is a conceptual image of a digital output signal. First, for the amplitude image, an output signal by the asymmetric 2-dimensional detector with integration capability is obtained in the absence of specimen or under a condition that both of the two spots splitted by the electron biprism pass through a vacuum on the plane of specimen and stored in the storage means, which is designated as D A-in    80 . Next, under a condition that one of the spots transmits through the specimen and the other passes through a vacuum, an output signal D A-in    79  of the asymmetric 2-dimensional detector with integration capability is obtained. The two output signals are added by an adder  81  to obtain an output signal, which is designated as D A-n+A0-in    82 . 
   Further, this is integrated for all the pixels to obtain an output signal, which is designated as I A-out    83 . This output signal I A-OUT    83  is equivalent to an amplitude image of a normal electron microscope. In this embodiment, in parallel to the acquisition of the amplitude image, a phase image is obtained simultaneously using another asymmetric 2-dimensional detector with integration capability. That is, under conditions that there is no specimen or the two spots splitted by the electron biprism both pass through a vacuum on the plane of specimen as in the case of the acquisition of the amplitude image, an output signal of the asymmetric 2-dimensional detector with integration capability is obtained and recorded as D P0-in   85 . 
   Next, under conditions that one of the spots transmits through the specimen and the other spot passes through a vacuum, an output signal D P-in    84  of the asymmetric 2-dimensional detector with integration capability is acquired, and the two signals are added by an adder  86  to obtain a 1-dimensional image D P-in+P0-in    87 . Using a processor  89  which keeps values equal to or larger than a certain threshold of the pixels among the pixels constituting the 1-dimensional image D P-in+P0+in    87  and sets the values of other pixels to zero, a 1-dimensional image D P-OUT    90  composed of values of the pixels each having a value equal to or larger than the certain threshold is obtained. Here, for the threshold, a 1-dimensional image D P-TH    88  that is set arbitrarily by the user may be used. Alternatively, a 1-dimensional image D P-TH    88  each of whose pixels has an average value of the output signal I A-OUT    83  of the amplitude image. Each pixel value of the 1-dimensional image D P-OUT    90  thus set up is integrated over all the pixels to obtain an output signal, which is designated as I P-OUT    91 . The two kinds of output signals I A-OUT    83  and I P-OUT    91  obtained in the above may be displayed, as they are, as the amplitude image and the phase image on the screen, respectively. Alternatively, as shown in the bottom of  FIG. 6 , another signal I P-NORMALIZED    92  may be generated from the two signals and displayed as a new phase image. Here, a computing equation used to covert the signal is given by the following expression.
 
 I   P-NORMALIZED =( I   P-OUT   −I   A-OUT ) /I   A-OUT 
 
This signal I P-NORMALIZED    92  becomes an output signal of the image corresponding to the cosine of a phase. Naturally, the output signal may be further converted to obtain an output signal of an image that corresponds to a value of the phase. Incidentally, the storage device, the adder  86 , and the processor  89  correspond to the storage device  26 , the CPU  25 , and the signal processor  32 , respectively, in the configuration of the STEM shown in  FIG. 1 .
 
   In this way, in this embodiment, the amplitude image and the phase image can be acquired simultaneously. In order to display the two images simultaneously, the two may be displayed independently on the screen of the display. Alternatively, a signal I A-OUT    94  of the amplitude is brought into correspondence with a Lightness value, as shown in  FIG. 7 , a signal I P-OUT    95  corresponding to the cosine of a phase or a signal obtained by further converting it into a phase value is brought into correspondence with a Hue value of the HLS color model, and this is converted to the RGB model with a converter  96  to be displayed in a display  97 . Thus, the phase image and amplitude image are simultaneously displayed, overlaying one image on the other in the same display. The simultaneous display of the two images makes possible for the user to observe both a structure which is recognizable from the amplitude image and a potential distribution or a magnetic field distribution of the sample which is recognizable from the phase image, thereby making it easy to observe the both images being correlated with each other. 
   Second Embodiment 
     FIGS. 8A to 8D  show another example of this embodiment. In this example, this embodiment is applied to dopant profile evaluation of a semi-conductor transistor. First, a voltage is applied to the electron biprism in the absence of specimen, the interference fringes magnified with an imaging lens is made to be incident on the asymmetric 2-dimensional detector with integration capability. After that, the electron beams are deflected with a deflection coil, and thereby the electron beams take an arrangement as shown in  FIG. 8A . Here, one of two splitted electron beam spots  98 ,  99  is adjusted to pass through a vacuum and the other is adjusted to transmit through the specimen. Next, the two splitted electron beam spots are scanned in a direction of scanning  100 , and at each predetermined scanning distance, the interference fringes are acquired.  FIG. 8B  shows a comparative relation between the specimen and the electron beam spot at the time when the electron beam is scanned as far as the central portion of a semiconductor thin film specimen. Further the scanning is continued so as to complete the scanning of the electron beam as far as a desired range ( FIG. 8C ), and subsequently an image corresponding to a dopant profile in an area  107  can be obtained from an image which corresponds to sequentially acquired cosine values of phases of the electron beam or values obtained by converting them into phases. 
   Third Embodiment 
     FIGS. 9A to 9D  show further another example of the embodiment. In this, the STEM is applied to magnetic domain structure evaluation of a magnetic thin film. First, in the absence of specimen, a voltage is applied to the electron biprism and interference fringes magnified with an imaging lens are made to be incident on the asymmetric 2-dimensional detector with integration capability. After that, by deflecting the electron beam with a deflection coil, the electron beams take an arrangement as shown in  FIG. 9A . Here, two splitted electron beam spots  108 ,  109  are so adjusted that one of them passes through a vacuum in proximity to the specimen and the other transmits through the specimen. 
   Next, while the two splitted electron beam spots are being scanned in a direction of scanning  110 , the interference fringes are acquired at each predetermined scanning distance.  FIG. 9B  shows a comparative relation between the specimen and an electron beam spot at a time when the electron beam is scanned as far as the central part of a semiconductor thin film specimen. Further the scanning is continued so as to complete the scanning of the electron beam as far as a desired range ( FIG. 9C ), and subsequently contour line displays  119 ,  120  which correspond to a magnetic domain structure  113  in a magnetic thin film  112  and a stray magnetic field in a vacuum  111  in proximity to the specimen can be obtained. Note that, in this embodiment, the apparatus outputs cosine values of phases rather than values obtained by converting the cosine values of phases into phases, whereby a display corresponding to magnetic lines of force can be obtained directly. 
   The present invention relates to a scanning interference electron microscope used for evaluation of electric and magnetic characteristics of a micro domain.