Abstract:
It is to prevent an image drift from occurring caused by a specimen being charged when observing the specimen including an insulating material. 
     A first scan is performed in a predetermined direction on scanning line and in a predetermined sequential direction of scanning lines and a second scan is performed in a scanning direction different from the predetermined scanning direction and in a sequential direction different from the predetermined sequential direction. An image may be created by repeating the process of executing the second scan after executing the first scan and by requiring the arithmetic average of the frames obtained by the second scans. An image may be created by averaging arithmetically at least one frame obtained by the first scan and at least one frame obtained by the second scan.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The invention relates to a charged particle beam irradiation system which irradiates a specimen with charged particles such as electron, and particularly to a charged particle beam irradiation system which analyzes the aspect on the surface of a specimen using a secondary discharged signal. 
         [0003]    2. Description of the Related Art 
         [0004]    In the semiconductor manufacturing industry, a scanning electron microscope (SEM), a CD-SEM for performing critical dimension scanning, and an inspection SEM for inspecting the pattern shape and the like are widely used as an inspection device of a semiconductor device. 
         [0005]    In these days, a highly integrated semiconductor device has been developed rapidly in the semiconductor manufacturing industry. According to this, various kinds of electrically-insulating thin films are formed on a semiconductor device. When electron beam is irradiated on an insulating material, electric charges are accumulated through secondary electron discharge and a specimen becomes charged. When an electric field is produced by the charge of the specimen, the electron beam incident on the specimen is bent and the irradiation position of the electron beam is deflected. 
         [0006]    The amount of electric charges on the specimen is determined by the amount of an incident electron beam, the physical property of an insulating material, the presence or absence of a path having diffusion on electric charges and its resistance, and the like. 
         [0007]    In the electron beam scan, a scanning area usually rectangular is scanned sequentially from one end to the other. The accumulated electric charges within the scanning area are spatially asymmetrical. The electric charges are diffused through a conductive material existing within the scanning area and its vicinity. The accumulated electric charges are accompanied by a time fluctuation of a large time constant. According to the spatial asymmetry and the temporal fluctuation of the accumulated electric charges, the electric field spatially and temporally fluctuates. According to this, the irradiation position of the electron beam fluctuates spatially and temporally. 
         [0008]    In the case of a scanning electron microscope, an image is shifted according to the charge of the specimen. Hereinafter, this phenomenon is referred to as “image drift”. Generally, in the scanning electron microscope, the same area is scanned a plurality of times (each scan is referred to as “frame”) and the obtained images are averaged, to improve the S/N ratio. Therefore when the positions of the images are deflected from each other in the respective frames, the image obtained as the averaging result deteriorates remarkably in resolution. A charged particle beam irradiation system which irradiates a charged particle beam also has the same problem. 
         [0009]    Patent Document 1: Japanese Patent Application Laid-Open (JP-A) No. 2005-142038, 
         [0010]    Non-Patent Document 1: “Charge Control During Photomask Critical Dimension (CD) Metrology” written by David Joy, International SEMATEC Technology Transfer #3114452B-ENG (2004), 
         [0011]    Non-Patent Document 2: John C. Russ: Computer-Assisted Microscopy; Plenum Publishing Corp. New York 1990: pp 40-41 
       SUMMARY OF THE INVENTION 
       [0012]    Various methods for preventing a specimen from being charged have been hitherto known. For example, there is a method of depositing an electric conductor such as metal on a specimen when observing the specimen. A coating device and the like are provided for practical use in order to perform this method. This method for analyzing a specimen, however, may be a destructive inspection and it cannot be adopted to a product inspection in a semiconductor industry. 
         [0013]    In the method disclosed in Japanese Patent Application Laid-Open (JP-A) No. 2005-142038, the scanning order in the scanning area is changed. This method is on the assumption that the electric charges accumulated in the respective scans are diffused at the same time constant as the scanning periodic cycle. This method, however, is defective in efficacy when the diffusion constant of the electric charges becomes some hundreds times longer than the scanning cycle. This happens, for example, when observing a liquid crystal optical element and a photomask. 
         [0014]    In the method disclosed in the “Charge Control During Photomask Critical Dimension (CD) Metrology” written by David Joy, International SEMATEC Technology Transfer #3114452B-ENG (2004), the periphery of a specimen is surrounded by inert gas molecules at the time of electron beam irradiation. This method makes use of such an action that the inert gas ions ionized by the electron beam neutralize the accumulated electric charges of the specimen. In this method, the accumulated electric charges themselves can be removed. In this method, however, the detection efficiency of the secondary electrons much depends on the pressure of the inert gas around the specimen. Therefore in order to obtain a stable image, a fine control on the pressure of the gas is necessary, which is a big problem in installation. 
         [0015]    An object of the invention is to prevent from an image drift caused by an electrically charged specimen when observing the specimen including an insulating material, in a charged particle beam system. 
       SUMMARY OF THE INVENTION 
       [0016]    According to the invention, the charged particle beam irradiation system has a deflector for deflecting a particle beam, an objective lens for converging the irradiating particle beam on the specimen, and a secondary electron signal detector for detecting a secondary electron signal released from the specimen, hence to create an image according to the signal from the secondary electron signal detector. 
         [0017]    According to the invention, a first scan is performed in a predetermined scanning direction on scanning line and a predetermined sequential direction of scanning lines and a second scan is performed in a scanning direction different from the predetermined scanning direction and a sequential direction different from the predetermined sequential direction. 
         [0018]    An image may be created by repeating the process of executing the second scan after executing the first scan and by requiring the arithmetic average of the frames obtained by the second scans. An image may be created by averaging arithmetically at least one frame obtained by the first scan and at least one frame obtained by the second scan. 
         [0019]    According to the invention, it is possible to prevent from an image drift caused by charge of a specimen when observing the specimen including an insulating material. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]      FIG. 1  is a view showing a constitutional example of a charged particle beam irradiation system according to the invention. 
           [0021]      FIG. 2  is a view for use in describing a first example of a method for creating an image according to a scanning electron microscope of the invention. 
           [0022]      FIG. 3  is a view for use in describing a second example of the method for creating an image according to the scanning electron microscope of the invention. 
           [0023]      FIG. 4  is a view for use in describing a third example of the method for creating an image according to the scanning electron microscope of the invention. 
           [0024]      FIG. 5  is a view for use in describing the flow of the processing in the third example of the method for creating an image according to the scanning electron microscope of the invention. 
       
    
    
     DESCRIPTION OF REFERENCE NUMERALS 
       [0000]    
       
           101 : specimen 
           103 : target area 
           103   a , 103   b , 103   c : frame (inverse) 
           104 : scanning direction (inverse) on scanning line 
           105   a ,  105   b : scanning direction (normal) on scanning line 
           106   a ,  106   b ,  106   c : frame (normal) 
           107 : sequential direction (inverse) of scanning lines 
           108 : sequential direction (normal) 
           109 : scanning area 
           110 : asymmetry (inverse) in electric charge distribution 
           111 : asymmetry (normal) in electric charge distribution 
       
     
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0036]    With reference to  FIG. 1 , an example of a charged particle beam irradiation system according to the invention will be described. In this example, the charged particle beam irradiation system is a scanning electron microscope. The scanning electron microscope in the example has an electron source cathode  205 , a first anode  206 , a second anode  207 , a first converging lens  208 , an aperture plate  209 , a second converging lens  210 , an orthogonal field generator  214 , a scanning coil  212 , an objective lens  211  and a specimen support  203 . The specimen support  203  is arranged in a specimen chamber  201 . The specimen chamber  201  is kept in a high vacuum state of about 1×10 −4  Pa by a vacuum exhaust system  222 . 
         [0037]    The scanning electron microscope further has power sources  217 ,  218 ,  219 , and  220  for supplying voltage to be applied to the electron source cathode  205 , the first anode  206 , the second anode  207 , the first converging lens  208 , the second converging lens  210 , and the objective lens  211 . The scanning electron microscope further has a scanning signal power source  221  for supplying saw-tooth current to the scanning coil  212 . 
         [0038]    The scanning electron microscope has a secondary electron signal detector  215 , a signal amplifier  216 , a controller  223 , a drawing device  224 , an image display device  225 , an image storing device  226 , and an image processor  227 . 
         [0039]    A method of obtaining an image of a specimen by the scanning electron microscope will be described in brief. First, a specimen  202  is arranged on the specimen support  203  in the specimen chamber  201 . The electron beam  204  discharged from the electron source cathode  205  is accelerated by the first anode  206  and the second anode  207 , converged by the first converging lens  208 , passing through the aperture plate  209 , converged by the second converging lens  210 , and deflected by the scanning coil  212 . The electron beam  204  is further converged by the objective lens  211 , into a convergent beam having a minute cross sectional diameter, and arrives at the surface of the specimen  202 . 
         [0040]    Although the scanning mechanism of the example uses a magnetic field method using a magnetic field generated by the scanning coil, it can also use an electric field method using an electric field generated by applying voltage to the opposite electrodes. The secondary electron signal (in this case, the secondary electron)  213  generated from the specimen  202  is drawn by the voltage applied between the specimen  202  and the objective lens  211  and proceeds beyond the objective lens  211 . The secondary electron signal  213  is separated from the primary electron beam  204  by the orthogonal field generator  214  and arrives at the secondary electron signal detector  215 . The secondary electron signal  213  is converted into electric signal by the secondary electron signal detector  215 , amplified by the signal amplifier  216 , and sent to the drawing device  224 . The drawing device  224  converts the secondary electron signal  213  into image signal and transfers it to the image display device  225 . Thus, the specimen image is displayed by the image display device  225 . The image signal is further transferred to the image storing device  226 . The image processor  227  performs the averaging processing, detection of a specific shape, detection of an image shift, and measurement of a specific shape dimension on the image information taken in by the image storing device  226 . Namely, the image processor  227  performs the image processing described with reference to  FIG. 2  to  FIG. 5 . All these elements are sequentially controlled by the controller  223 . 
         [0041]    A first example of a method for creating an image according to a scanning electron microscope of the invention will be described with reference to  FIG. 2 . When observing a target area  103  on a specimen  101 , generally, a “view search” for searching the target area  103  is first performed. In the view search, a relatively wide area  102  including the target area  103  is observed at a comparatively low magnification. Namely, scan is performed in the normal scanning direction  105   a , to obtain an image of the wide area  102 . At this time, the positive electric charges produced by the discharge of the secondary electrons are accumulated in the wide area  102 . After the target area  103  is found in the wide area  102 , the target area  103  is observed at a comparatively high magnification. 
         [0042]    In the conventional technique, when observing the target area  103 , a scan is performed in a scanning direction  105   b , and the resultant frames  106   a ,  106   b , and  106   c  are arithmetically averaged, hence to get an image of the target area  103 . At this time, since the scanning direction  105   b  on scanning line and the sequential direction  108  of scanning lines are always constant, there occurs asymmetry  111  in the distribution of the electric charges varying periodically, in the scanning area  109  for observing the target area  103 . The asymmetry  111  in the distribution of the electric charges creates an asymmetric electric field above the scanning area  109  and deflects the incident electron beam. This is one of the reasons for causing an image drift. 
         [0043]    According to the invention, when observing the target area  103 , a preliminary scan is first performed and then a main scan is performed. At first, a first preliminary scan is performed. The preliminary scan and the main scan may be performed in the same number of times. 
         [0044]    The preliminary scan is performed in an inverse scanning direction  104  to the scanning direction  105   b  of the main scan and in an inverse sequential direction  107  to the sequential direction  108  of the main scan. According to this, a first preliminary frame  103   a  is obtained. Next, a first main scan is performed. The main scan is performed in the same way as the conventional technique; in the normal scanning direction  105   b  on scanning line and in the normal sequential direction  108  of scanning lines. Thus, a first main frame  106   a  can be obtained. Next, a second preliminary scan is performed to get a second preliminary frame  103   b . A second main scan is performed to get a second main frame  106   b . Similarly, a third preliminary scan is performed to get a third preliminary frame  103   c . A third main scan is performed to get a third main frame  106   c . Thus obtained main frames  106   a ,  106   b , and  106   c  are averaged arithmetically, hence to obtain an image. 
         [0045]    The preliminary scan produces an asymmetry  110  in the distribution of electric charges periodically changing, in the scanning area  109 . The main scan after the preliminary scan produces an asymmetry  111  in the distribution of electric charges periodically changing, in the scanning area  109 . However, the direction of the asymmetry  110  in the distribution of electric charges caused by the preliminary scan is opposite to the direction of the asymmetry  111  in the distribution of electric charges caused by the main scan. Therefore, two asymmetries  110  and  111  are cancelled out each other. 
         [0046]    When the frames  106   a ,  106   b , and  106   c  obtained by the main scan performed after the preliminary scan are averaged arithmetically, since the two asymmetries  110  and  111  are cancelled out, there occurs no image drift. According to this, it is possible to eliminate an image drift caused by the charge of the specimen. The direction of the asymmetries  110  and  111  in the distribution of electric charges means the direction of the electric field vector. 
         [0047]    When the specimen is an insulator, the attenuation constant of the accumulated electric charges is the same order as the frame formation time. Therefore by overlapping the scans of the opposite directions, the asymmetries of the accumulated electric charges can be cancelled out. The scanning direction can be changed just by reversing the polarity of the saw-tooth current applied to the scanning coil  212  in  FIG. 1 . Namely, it is possible just by a change in the control procedure installed in the controller  223  without adding anything to the original structure of SEM shown in  FIG. 1 . The invention is to provide a device for cutting down the image drift occurring at the time of observing an insulator, at a very low cost. 
         [0048]    A second example of the method for creating an image according to the scanning electron microscope of the invention will be described with reference to  FIG. 3 . A first scan is performed in the inverse scanning direction  302  and in the inverse sequential direction  308 , to obtain a first group of frames  301   a  and  301   b . Next, a second scan is performed in the normal scanning direction  304  and in the normal sequential direction  309 , to obtain a second group of frames  303   a ,  303   b , and  303   c . A third scan is performed in the inverse scanning direction  302  and in the inverse sequential direction  308 , to obtain a third group of frames  301   c  and  301   d . A fourth scan is performed in the normal scanning direction  304  and in the normal sequential direction  309 , to obtain a fourth group of frames  303   d ,  303   e , and  303   f.    
         [0049]    In this example, all the frames  301   a ,  301   b ,  303   a ,  303   b ,  303   c ,  301   c ,  301   d ,  303   d ,  303   e , and  303   f  are arithmetically averaged, hence to obtain an image. The scan in the normal scanning direction  304  and in the normal sequential direction  309  may be performed the same number of times as the scan in the inverse scanning direction  302  and in the inverse sequential direction  308 . 
         [0050]    The asymmetry  306  in the distribution of the electric charges produced in the scanning area  305  by the first and third scans is opposite in direction to the asymmetry  307  in the distribution of the electric charges produced in the scanning area  305  by the second and fourth scans. Accordingly, the asymmetry  306  in the distribution of the electric charges produced by the first and the third scans is cancelled out by the asymmetry  307  in the distribution of the electric charges produced by the second and the fourth scans. Therefore, any image drift does not occur. 
         [0051]    The example of  FIG. 3  is effective especially in the path where the accumulated electric charges on the specimen are diffused, for example, when there exists a film of conductive material with high resistance and the like and the attenuation constant of the accumulated electric charges is small. Also in the example, the intervals of inserting the inverse frames and the number of the frames may be set previously in the controller  223  of  FIG. 1 , which can be realized just by changing the control procedure. 
         [0052]    A third example of a method for creating an image according to the scanning electron microscope of the invention will be described with reference to  FIG. 4 . In the example, a plurality of scans are performed in the normal scanning direction  402  and in the normal sequential direction  403 , to obtain frames  401   a ,  401   b , and  401   c . The scans are performed at predetermined intervals of time. Therefore, the positions of the image element  405  included in the respective frames  401   a ,  401   b , and  401   c  are shifted. Therefore a shift amount between the images included in the frames  401   a ,  401   b , and  401   c  is detected. This is the image drift amount. The method for detecting the shift amount between the images is known. For example, refer to John C. Russ: Computer-Assisted Microscopy; Plenum Publishing Corp. New York 1990: pp 45-46. When the image drift amount exceeds a predetermined value, a method shown in  FIG. 2  and  FIG. 3  is executed. 
         [0053]      FIG. 5  is an example of algorithm for executing the third example of the method for creating an image according to the scanning electron microscope of the invention. At first in Step S 1 , it is checked whether the image drift amount, in other words, the shift amount of the images is equal to or more than a predetermined threshold. When the shift amount of the images is equal to or more than a predetermined threshold, the processing proceeds to Step S 2 , where the scan in the inverse scanning direction on scanning line and in the inverse sequential direction of scanning lines is performed, as shown in  FIG. 2  and  FIG. 3 . When the shift amount of the images is less than the predetermined threshold, the processing proceeds to Step S 3 , where the image creating processing is performed without inversing the scanning direction and the sequential direction. 
         [0054]    When the image magnification is, for example, one hundred thousand times, the image shift amount is about one nanometer/second in the threshold used in Step S 1 . 
         [0055]    In the example, the scan is performed inverting both the scanning direction and sequential direction at 180 degree. However, either the scanning direction or the sequential direction may be inverted. For example, when the image drift has a circular motion on the screen, inversion only in the scanning direction is effective. The conversion angle may be any angle other than 180 degree. Further, a plurality of frames at different angles can be sequentially inserted and a method most suitable to a specimen can be searched for. 
         [0056]    As set forth hereinabove, although the example of the invention has been described, the invention is not restricted to the examples but it is easily understood for those who skilled in the art that various modifications can be made within the scope of the invention disclosed in the appended claims. 
         [0057]    Although in the example, the scanning electron microscope has been mainly used as an example, the invention can be applied to an irradiation device using charged particles other than the electron beam, for example, using kalium ion. Since it can be widely applied to an analysis for an insulator, it can be applied especially to an observation and an inspection device of a liquid crystal optical element, a surface acoustic wave device, and a photomask for semiconductor industry.