Abstract:
A sample observation method including: placing a sample stage at a first tilt angle with respect to a charged particle beam, and irradiating an observation surface of a sample with the charged particle beam to acquire a first charged particle image; tilting the sample stage to a second tilt angle different from the first tilt angle about a first sample stage axis, and irradiating the observation surface with the charged particle beam to acquire a second charged particle image; tilting the sample stage to a tilt angle at which an area of the observation surface in the acquired charged particle image is larger between the first charged particle image and the second charged particle image; and irradiating the observation surface with the charged particle beam to observe the observation surface.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority from Japanese Patent Application No. 2012-064263 filed on Mar. 21, 2012, the entire contents of which are incorporated herein by reference. 
       TECHNICAL FIELD 
       [0002]    Aspects of the present invention relate to a sample observation method for observing a sample by irradiation of a charged particle beam. 
       BACKGROUND 
       [0003]    A focused ion beam apparatus is known as an apparatus for processing and observing a fine sample such as a semiconductor device. A FIB-SEM composite apparatus is known as an apparatus for observing a sample under processing by a focused ion beam through a scanning electron microscope in real time. 
         [0004]    In the FIB-SEM composite apparatus, in general, an FIB column and an SEM column are arranged so that an angle formed by an irradiation axis of the FIB column and an irradiation axis of the SEM column is approximately between 50 degrees to 60 degrees. With this arrangement, the same region of a sample can be observed by FIB and SEM. 
         [0005]    Due to the reduction in device dimensions in recent years, it has been required to observe a cross-section processed by FIB with a high resolution by SEM. As an apparatus for realizing fine processing by FIB and high resolution observation by SEM, there has been proposed a composite charged particle beam apparatus in which the FIB column and the SEM column are arranged perpendicularly (see JP-A-H06-231720). 
         [0006]    In this apparatus, the cross-section processed by FIB can be observed by SEM from a direction perpendicular thereto. In SEM observation, in general, when an observation surface of a sample is observed from a direction perpendicular thereto, observation can be performed with a high resolution. According to the apparatus described in JP-A-H06-231720, the cross-section processed by FIB is irradiated with an electron beam simultaneously from the direction perpendicular to the cross-section, and hence, SEM observation can be performed with a high resolution. 
         [0007]    As an observation preparation for high resolution SEM observation, it is necessary to adjust a position of the sample so that the observation surface of the sample is perpendicular to an irradiation axis of the electron beam. As a position adjusting method, for example, there is known a method of measuring heights of the sample at a plurality of points in the observation surface, calculating a tilt of the observation surface based on the measurement results, and tilting the sample so as to correct the tilt of the observation surface. In this case, the heights of the sample are measured by tilting the sample and measuring the eucentric height at each measurement point in the observation surface. 
         [0008]    According to this method, however, the sample needs to be moved and tilted many times, and hence, observation preparation time becomes long. Further, a tip of a beam column, a detector, and other components are arranged close together in the vicinity of the sample at the intersection of an ion beam and an electron beam, and hence, the tilt angle of the sample cannot be increased, and thus, the eucentric height cannot be adjusted with high accuracy. Thus, it has been difficult to measure the sample height with high accuracy. 
       SUMMARY 
       [0009]    Aspects of the present invention provide a sample observation method and a charged particle beam apparatus for observing a sample in a manner that an observation surface is disposed perpendicularly to an irradiation direction of a charged particle beam efficiently and accurately. 
         [0010]    According to an exemplary embodiment of the present invention, there is provided a sample observation method for observing an observation surface of a sample by irradiation of a charged particle beam, the method including: placing a sample stage at a first tilt angle with respect to the charged particle beam, and irradiating the observation surface with the charged particle beam to acquire a first charged particle image; tilting the sample stage to a second tilt angle different from the first tilt angle about a first sample stage axis, and irradiating the observation surface with the charged particle beam to acquire a second charged particle image; tilting the sample stage to a tilt angle at which an area of the observation surface in the acquired charged particle image is larger between the first charged particle image and the second charged particle image; and irradiating the observation surface with the charged particle beam to observe the observation surface. 
         [0011]    Accordingly, the observation surface can be observed in a manner that the observation surface is placed to be perpendicular to the charged particle beam. 
         [0012]    According to the sample observation method of the present invention, the observation surface can be disposed perpendicularly to the irradiation direction of the charged particle beam efficiently and accurately, and hence, the observation surface can be observed with a high resolution. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0013]    In the accompanying drawings: 
           [0014]      FIG. 1  is a configuration diagram of a charged particle beam apparatus according to an embodiment of the present invention; 
           [0015]      FIGS. 2A to 2D  are explanatory diagrams of a sample observation method according to the embodiment of the present invention; 
           [0016]      FIGS. 3A to 3D  are explanatory diagrams of the sample observation method according to the embodiment of the present invention; 
           [0017]      FIG. 4  is an explanatory diagram of a sample processing method according to the embodiment of the present invention; and 
           [0018]      FIG. 5  is a flowchart of the sample observation method according to the embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0019]    A sample observation method according to an embodiment of the present invention will be described hereinafter. 
         [0020]    First, a charged particle beam apparatus for performing the sample observation method is described. As illustrated in  FIG. 1 , the charged particle beam apparatus includes an EB column  1 , an FIB column  2 , and a sample chamber  3 . The EB column  1  and the FIB column  2  irradiate a sample  7  accommodated in the sample chamber  3  with an electron beam  8  and an ion beam  9 , respectively. The EB column  1  and the FIB column  2  are arranged so that the irradiation axes thereof are orthogonal to each other on the sample  7 . Note that, a FIB column provided with a gas field ionization ion source may be used instead of the EB column  1 . 
         [0021]    The charged particle beam apparatus further includes a secondary electron detector  4  and a transmission electron detector  5  as charged particle detectors. The secondary electron detector  4  is capable of detecting secondary electrons generated from the sample  7  by irradiation of the electron beam  8  or the ion beam  9 . The transmission electron detector  5  is provided at a position opposed to the EB column  1 . The transmission electron detector  5  is capable of detecting transmitted electrons that have transmitted through the sample  7  and the electron beam  8  that has not entered the sample  7  as a result of the irradiation of the electron beam  8  to the sample  7 . 
         [0022]    The charged particle beam apparatus further includes a sample stage  6  for holding the sample  7 . The sample stage  6  can be tilted or rotated to change an incident angle of the electron beam  8  to the sample  7 . The sample stage  6  is driven by a sample stage driving portion  15 , and the movement of the sample stage  6  is controlled by a sample stage control portion  16 . 
         [0023]    The sample stage driving portion  15  moves the sample stage  6  in three axis directions of the X, Y, and Z directions. The sample stage driving portion  15  tilts the sample stage  6  in a first tilt direction  24  about a first sample stage axis direction  23  parallel to the irradiation axis of the FIB column  2 . Further, the sample stage driving portion  15  tilts the sample stage  6  in a second tilt direction  28  about a second sample stage axis direction  27  orthogonal to both the irradiation axis of the EB column  1  and the irradiation axis of the FIB column  2 . 
         [0024]    The charged particle beam apparatus further includes an EB control portion  12 , an FIB control portion  13 , an image forming portion  14 , and a display portion  17 . The EB control portion  12  transmits an irradiation signal to the EB column  1  to control the EB column  1  to radiate the electron beam  8 . The FIB control portion  13  transmits an irradiation signal to the FIB column  2  to control the FIB column  2  to radiate the ion beam  9 . 
         [0025]    The image forming portion  14  forms a transmission electron image based on a signal for scanning the electron beam  8  sent from the EB control portion  12  and a signal of the transmission electrons detected by the transmission electron detector  5 . The display portion  17  is capable of displaying the transmission electron image. The image forming portion  14  forms data of an SEM image based on the signal for scanning the electron beam  8  sent from the EB control portion  12  and a signal of the secondary electrons detected by the secondary electron detector  4 . The display portion  17  is capable of displaying the SEM image. Further, the image forming portion  14  forms data of an SIM image based on a signal for scanning the ion beam  9  sent from the FIB control portion  13  and a signal of the secondary electrons detected by the secondary electron detector  4 . The display portion  17  is capable of displaying the SIM image. 
         [0026]    The charged particle beam apparatus further includes an input portion  10  and a control portion  11 . An operator inputs conditions on the apparatus control to the input portion  10 . The input portion  10  transmits the input information to the control portion  11 . The control portion  11  transmits a control signal to the EB control portion  12 , the FIB control portion  13 , the image forming portion  14 , the sample stage control portion  16 , or the display portion  17 , to thereby control the operation of the charged particle beam apparatus. 
         [0027]    The charged particle beam apparatus further includes a tilt angle calculating portion  18  and an image processing portion  19 . The tilt angle calculating portion  18  calculates an optimum tilt angle of the sample stage  6  to be described later. For calculating the optimum tilt angle, the image processing portion  19  determines the area of an observation surface in an SEM image by image processing. 
         [0028]    Next, the sample observation method in this embodiment will be described. First, as illustrated in  FIG. 2A , an irradiation region  21  is set to a region including the sample  7 . Then, charged particle image acquisition Si in a flowchart of  FIG. 5  is performed. In other words, the irradiation region  21  is irradiated with the electron beam  8 , and secondary electrons generated from the irradiation region  21  are detected by the secondary electron detector  4 , to thereby acquire a SEM image based on a detection signal of the secondary electron detector  4  and a scanning signal of the electron beam  8 .  FIG. 2B  is an acquired SEM image  22 . When an observation surface  7   a  of the sample  7  is perpendicular to the electron beam  8 , a side surface  7   b  of the sample  7  does not appear in the SEM image  22 . However, since the sample  7  is tilted with respect to the irradiation direction of the electron beam  8 , the SEM image  22  includes the observation surface  7   a  of the sample  7  and the side surface  7   b  of the sample  7 . 
         [0029]    Then, sample stage tilting S 2  is performed. In other words, the sample stage  6  is tilted in the first tilt direction  24  about the first sample stage axis direction  23  so that the observation surface  7   a  is perpendicular to the electron beam  8 .  FIG. 2C  illustrates a state of the sample stage  6  after tilting. 
         [0030]    Next, charged particle image acquisition S 3  is performed.  FIG. 2D  is an acquired SEM image  25 . 
         [0031]    Then, tilt angle calculation S 4  is performed. When the observation surface  7   a  is perpendicular to the electron beam  8 , the area of the observation surface  7   a  in the SEM image becomes larger than that when the observation surface  7   a  is tilted in other directions. Regarding this, the area of the observation surface  7   a  in the SEM image  22  and the area of the observation surface  7   a  in the SEM image  25  are compared. As a result of the comparison, a tilt angle of the sample stage  6  at which the area of the observation surface  7   a  in the acquired SEM image is larger is calculated as an optimum tilt angle. In this case, the observation surface  7   a  in the SEM image  25  has a larger area, and hence the tilt angle of the sample stage  6  at the time of acquiring the SEM image  25  is calculated as the optimum tilt angle. 
         [0032]    Next, sample observation S 5  is performed. In other words, the sample stage  6  is tilted at the optimum tilt angle, and the observation surface  7   a  is placed so as to be perpendicular to the irradiation direction of the electron beam  8 . Then, the observation surface  7   a  is observed by irradiation of the electron beam  8 . In this case, the observation surface  7   a  is observed from the direction perpendicular thereto, and hence the observation can be performed with a high resolution. 
         [0033]    Note that, it is also possible to calculate a more optimum tilt angle in order to adjust the tilt angle of the sample stage  6  more accurately. In other words, in addition to the above-mentioned calculation of the optimum tilt angle, the sample stage  6  is tilted about the second sample stage axis direction  27  perpendicular to the first sample stage axis direction  23 , to thereby calculate a more optimum tilt angle. 
         [0034]    As illustrated in  FIG. 3A , the irradiation region  21  is irradiated with the electron beam  8  to acquire an SEM image.  FIG. 3B  is an SEM image  26 . The observation surface  7   a  is not perpendicular to the irradiation direction of the electron beam  8 , and hence, a side surface  7   c  of the sample  7  appears in the SEM image  26 . Accordingly, the sample stage  6  is tilted about the second sample stage axis direction  27 . 
         [0035]      FIG. 3C  illustrates the tilted state, and  FIG. 3D  is an SEM image  29  acquired in this state. 
         [0036]    Then, the area of the observation surface  7   a  in the SEM image  26  and the area of the observation surface  7   a  in the SEM image  29  are compared. As a result of the comparison, a tilt angle of the sample stage  6  at which the area of the observation surface  7   a  in the acquired SEM is larger is calculated as an optimum tilt angle. In this case, the observation surface  7   a  in the SEM image  29  has a larger area, and hence, the tilt angle of the sample stage  6  at the time of acquiring the SEM image  29  is calculated as the optimum tilt angle. 
         [0037]    The sample stage  6  is tilted at the optimum tilt angle, and the observation surface  7   a  is placed so as to be perpendicular to the irradiation direction of the electron beam  8 . Then, the observation surface  7   a  is observed by irradiation of the electron beam  8 . The tilt angle of the sample stage  6  is adjusted based on the two axes, and the observation surface  7   a  is placed so as to be perpendicular to the electron beam  8  and is observed. Thus, the observation can be performed with a higher resolution. 
         [0038]    Further, since the observation surface  7   a  set at the optimum tilt angle is perpendicular to the irradiation direction of the electron beam  8 , with the use of the ion beam  9  radiated to be orthogonal to the electron beam  8 , another observation surface parallel to the observation surface  7   a  can be formed. 
         [0039]    As illustrated in  FIG. 4 , the sample  7  is irradiated with the ion beam  9 , to thereby perform etching processing so as to peel off the observation surface  7   a.  In this case, the sample  7  is irradiated with the ion beam  9  from the direction perpendicular to the irradiation direction of the electron beam  8 , and hence, an observation surface  7   d  parallel to the observation surface  7   a  can be formed. 
         [0040]    After the formation of the observation surface  7   d,  the observation surface  7   d  can be irradiated with the electron beam  8  from the direction perpendicular thereto without moving the sample stage  6 . Thus, similarly to the observation surface  7   a,  the observation surface  7   d  can be observed with a high resolution. 
         [0041]    Further, observation surfaces  7   e  and  7   f  can be observed with a high resolution by repeatedly performing the cross-section formation by the ion beam  9  and the observation by the electron beam  8 . In this manner, high resolution SEM images of the observation surfaces  7   a,    7   d,    7   e,  and  7   f  can be acquired and subjected to three-dimensional reconstruction, to thereby acquire a high-accurate three-dimensional image of the sample  7 .