CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority from Japanese Patent Application No. 2012-065974 filed on Mar. 22, 2012, the entire contents of which are incorporated herein by reference. 
     TECHNICAL FIELD 
     Aspects of the present invention relate to a cross-section processing and observation method of forming a cross-section by a focused ion beam and observing the cross-section by an electron beam. 
     BACKGROUND 
     As a method of analyzing internal structure and defects in a semiconductor device or the like, there is known a cross-section processing and observation method in which a focused ion beam is used for cross-section processing and slicing of a sample to expose a cross-section including a desired structure or a defect, and a scanning electron microscope is used to observe the cross-section. According to this method, an observation target inside the sample can be exposed with pinpoint accuracy, and hence the structure or the defect can be observed quickly. 
     There is disclosed a method of repeatedly performing cross-section processing and cross-section observation and combining a plurality of acquired cross-sectional observation images to construct a three-dimensional image of a region subjected to the cross-section processing (see JP-A-2008-270073). According to this method, a three-dimensional image of an observation target can be constructed. 
     Recently, along with densification and reduction in size of semiconductor devices, a device pattern has become finer, and hence, cross-section processing and observation for a minute observation target have been required. In this case, by setting a gap between one cross-section and another cross-section formed by subjecting the one cross-section to slice processing to be extremely small, that is, by reducing a slice width of a focused ion beam, a minute observation target can be exposed in the cross-section and observed. 
     However, if the slice width is extremely small, it is difficult to measure the width, and hence there has been a problem in reliability of acquired data. For example, it cannot be confirmed whether or not an observation image acquired by a cross-section processing and observation with a slice width of 1 nm is an observation image photographed with an actual slice width of 1 nm. Thus, there has been a problem in that it is difficult to measure an actual shape from the observation image. 
     SUMMARY 
     Aspects of the present invention provide a cross-section processing and observation method and a cross-section processing and observation apparatus, by which a slice width can be measured even when the slice width is minute and highly-reliable observation data can be acquired. 
     According to an aspect of the present invention, there is provided a cross-section processing and observation method in which slice processing on a sample by irradiation of an ion beam to the sample to form a cross-section and acquisition of a cross-sectional image by irradiation of an electron beam to the cross-section are repeatedly performed, the method including: acquiring a surface image by scanning and irradiating a surface of the sample with the ion beam; setting, on the surface image, a first sliced region and a second sliced region for performing the slice processing, the second sliced region being adjacent to the first sliced region and having a longitudinal length obtained by subtracting a slice width of the second sliced region from a longitudinal length of the first sliced region; forming the cross-section by irradiating the first sliced region and the second sliced region with the ion beam; and acquiring the cross-sectional image by irradiating the cross-section with the electron beam. 
     According to another aspect of the present invention, there is provided a cross-section processing and observation apparatus including: an ion beam column configured to irradiate a surface of a sample with an ion beam to form a cross-section to the sample; an electron beam column configured to irradiate the cross-section with an electron beam to acquire an observation image of the cross-section; a detector configured to detect a charged particle generated from the sample; an image forming portion configured to form an observation image of the sample based on a detection signal of the detector; and a sliced region setting portion configured to set, on the observation image of the surface of the sample, a first sliced region and a second sliced region for performing slice processing on the sample, the second sliced region being adjacent to the first sliced region and having a longitudinal length obtained by subtracting a slice width of the second sliced region from a longitudinal length of the first sliced region. 
     Accordingly, the slice width actually subjected to slice processing can be measured from the cross-sectional image. Therefore, the slice width can be measured with high accuracy. 
     According to the cross-section processing and observation method and the cross-section processing and observation apparatus of the present invention, even when the slice width is minute, the slice width can be measured to acquire highly-reliable observation data. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       In the accompanying drawings: 
         FIG. 1  is a configuration diagram of a cross-section processing and observation apparatus according to an exemplary embodiment of the present invention; 
         FIGS. 2A and 2B  are explanatory diagrams of a cross-section processing and observation method according to the exemplary embodiment of the present invention; 
         FIGS. 3A and 3B  are explanatory diagrams of the cross-section processing and observation method according to the exemplary embodiment of the present invention; 
         FIG. 4  is an explanatory diagram of the cross-section processing and observation method according to the exemplary embodiment of the present invention; and 
         FIG. 5  is an explanatory diagram of the cross-section processing and observation method according to the exemplary embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     A cross-section processing and observation method and a cross-section processing and observation apparatus according to an exemplary embodiment of the present invention will be described hereinafter., 
     As illustrated in  FIG. 1 , the cross-section processing and observation apparatus in this exemplary embodiment includes an EB column  1 , a 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 sample processing apparatus further includes a secondary electron detector  4  and a backscattered 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 backscattered electron detector  5  is provided inside the EB column  1 . The backscattered electron detector  5  is capable of detecting backscattered electrons reflected by the sample  7  as a result of the irradiation of the electron beam  8  to the sample  7 . 
     The cross-section processing and observation apparatus further includes a sample stage  6  for placing the sample  7  thereon. The sample stage  6  can be tilted to change an incident angle of the ion beam  9  to the sample  7 . The tilt of the sample stage  6  is controlled by a sample stage control portion  16 . 
     The cross-section processing and observation apparatus further includes an EB control portion  12 , a 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 . The image forming portion  14  forms a backscattered electron image based on a signal for scanning the electron beam  8  sent from the EB control portion  12  and a signal of the backscattered electrons detected by the backscattered electron detector  5 . The display portion  17  is capable of displaying the backscattered electron image. The image forming portion  14  forms data of a 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 a 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. 
     The cross-section processing and observation apparatus further includes an input portion  10  and a control portion  11 . An operator inputs conditions on the apparatus control, such as a beam irradiation condition, 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 cross-section processing and observation apparatus. 
     Description is given to the control of the apparatus. For example, the operator sets an irradiation region of the ion beam  9  based on an observation image displayed on the display portion  17 , such as the backscattered electron image, the SEM image, or the SIM image. The operator inputs, via the input portion  10 , a processing frame for setting the irradiation region on the observation image displayed on the display portion  17 . The processing frame as used herein is a frame indicating a boundary between a region to be irradiated with the ion beam  9  and a region not to be irradiated with the ion beam  9 . When the operator inputs an instruction to start processing to the input portion  10 , a signal indicating the irradiation region and a signal indicating the start of processing are transmitted from the control portion  11  to the FIB control portion  13 , and the FIB control portion  13  radiates the ion beam  9  to the specified irradiation region of the sample  7 . In this manner, the irradiation region input by the operator can be irradiated with the ion beam  9 . 
     The cross-section processing and observation apparatus further includes a sliced region setting portion  15  for setting, on the SIM image, a sliced region for performing slice processing. 
     Additionally, the cross-section processing and observation apparatus further includes a three-dimensional image forming portion  18  for constructing a three-dimensional image of the sample based on acquired SEM images and the slice width. 
     Next, a cross-section processing and observation method in this exemplary embodiment is described. As illustrated in  FIG. 2A , a processing groove  21  is formed in the sample  7  so that a cross-section may be irradiated with the electron beam  8  for observing the cross-section. A surface  7   a  of the sample  7  is scanned and irradiated with the ion beam  9  to form the processing groove  21  by etching processing.  FIG. 2B  is a cross-sectional diagram taken along the line A-A of  FIG. 2A . The processing groove  21  is formed into a slope shape so that a cross-section  7   b  may be scanned and irradiated with the electron beam  8 . The slope shape can reduce the amount of etching and shorten a processing time as compared with the case of forming a box-shape groove. 
     Then, slice processing and observation of a cross-section exposed by the slice processing are repeatedly performed. In other words, a sliced region  22  is scanned and irradiated with an ion beam  9   a  to perform etching processing, and a cross-section exposed by the processing is scanned and irradiated with the electron beam  8  to acquire a SEM image. Next, a sliced region  23  is scanned and irradiated with an ion beam  9   b  to perform etching processing so that the next cross-section is exposed to acquire a SEM image. In this manner, SEM images of a plurality of cross-sections can be acquired at an interval of a width D 1  of the sliced region. Based on the SEM images, the internal structure of the sample  7  can be analyzed. 
     The sliced region is subjected to etching processing by the ion beam  9 , and hence, a portion of the sample within the width D 1  of the sliced region does not appear in the SEM image. Therefore, in order to observe a minute observation target, it is necessary to set the width D 1  of the sliced region to be smaller than the target. However, the set width of the sliced region and an actually etched slice width are not always the same. This is because the etching rate differs depending on the material and structure of a sample to be etched. 
     Therefore, it is necessary to measure the actually etched slice width. The sliced region is scanned and irradiated with the ion beam  9  in a direction perpendicular to the surface  7   a  of the sample  7 , and a SIM image is observed to measure the slice width. However, in the case where the slice width is minute, in particular in the case where the slice width is equal to or smaller than a beam diameter of the ion beam  9 , for example, 5 nm or less, it is difficult to measure the slice width from the SIM image. 
     In view of the above, the sample processing method in this exemplary embodiment uses a slice width measuring method described hereinafter. In the slice width measuring method, as illustrated in  FIGS. 3A and 3B , the sliced region is set and processed so that the length of the sliced region is shorter by the width of the sliced region. Then, a cross-section formed by the processing is observed by SEM. In the SEM observation, minute unevenness in the cross-section can be observed, and hence, a minute step formed in the cross-section and having the same length as the slice width can be observed. In this manner, the slice width can be measured from the SEM image of the cross-section. 
       FIG. 3A  is a SIM image  30  of the surface  7   a  of the sample  7 . A processing groove  31  is formed. In the SIM image  30 , sliced regions  32 ,  33 ,  34 , and  35  are set by the sliced region setting portion  15 . The longitudinal length of each of the sliced regions  33 ,  34 , and  35  is set so as to be a length obtained by subtracting the width of the sliced region from the longitudinal length of a sliced region adjacent thereto on the processing groove  31  side. For example, the sliced region  33  has a length obtained by subtracting a width D 2  of the sliced region  33  from the length of the sliced region  32 . Then, the set sliced regions  32 ,  33 ,  34 , and  35  are scanned and irradiated with the ion beam  9 , to thereby perform cross-section processing and observation. 
       FIG. 3B  is a SIM image  36  after etching processing. A processing groove  37  has a step shape  38  because the sliced regions having different lengths are used. 
       FIG. 4  is a SEM image  40  of a cross-section  37   a  acquired by scanning and irradiation of the electron beam  8 . In the SEM image  40 , a shape  48  resulting from the step shape  38  appears. Based on the width of the shape  48 , the slice width can be measured. For example, the value of a width D 3  measured from the SEM image  40  can be regarded as an actually etched slice width. In this manner, the slice width by which the sample  7  is actually subjected to etching processing by the ion beam  9  can be measured. 
     Next, description is given to a method of constructing a three-dimensional image of a region subjected to cross-section processing, based on SEM images of a plurality of cross-sections acquired by cross-section processing and observation and the actually etched slice width obtained by the slice width measuring method. 
     As illustrated in  FIG. 5 , a plurality of acquired SEM images are arranged at an interval based on the actually etched slice width. Specifically, a SEM image  51  and a SEM image  52  are arranged at an interval of the length obtained by multiplying a slice width D 4  obtained by the above-mentioned slice width measuring method by a display magnification of the SEM images. Other SEM images are arranged in the same way. In this manner, a three-dimensional image of the region subjected to cross-section processing can be constructed.
A cross-section processing and observation method including: acquiring a surface image by scanning and irradiating a surface of a sample with ion beam; setting, on the surface image, a first sliced region and a second sliced region for performing the slice processing, the second sliced region being adjacent to the first sliced region and having a longitudinal length obtained by subtracting a slice width of the second sliced region from a longitudinal length of the first sliced region; forming a cross-section by irradiating the first sliced region and the second sliced region with the ion beam; and acquiring a cross-sectional image by irradiating the cross-section with electron beam.