Abstract:
A focused ion beam system capable of acquiring surface structure information, internal structure information, and internal composition information about a sample simultaneously from the same field of view of the sample. A method of sample preparation and observation employs such focused ion beam system to accurately set a sample processing position based on information about the structure and composition of the sample acquired from multiple directions of the sample, and to process and observe the sample. The system includes, in order to acquire the sample structure and composition information simultaneously, a secondary electron detector, a transmission electron detector, and an energy dispersive X-ray spectroscope or an electron energy loss spectroscope, and employs a stub having the sample rotating and tilting function. The method includes a marking process.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to focused ion beams, and particularly to a focused ion beam system capable of acquiring surface structure information, internal structure information, and internal composition information about a sample simultaneously. The invention also relates to methods of sample preparation and observation whereby the acquisition of such sample structure and composition information and focused ion beam processing are carried out using a plurality of devices. 
         [0003]    2. Background Art 
         [0004]    As the semiconductor devices become smaller and smaller in size, the electron microscope has become an indispensable tool for structural analysis. It has also become indispensable to use focused ion beam processing for the preparation of a sample for the observation of an electron microscope image. Focused ion beam processing generally involves capturing a scanning ion microscope (SIM) image and designating a position to be processed. Meanwhile, it is becoming increasingly difficult to set a processing position in semiconductor devices, which are becoming increasingly more multi-layered and complicated, with reference to such SIM images alone. This is due to the fact that the SIM images provide only sample surface information. In response, a method has become more common whereby an image obtained by a separate technique is superposed on the SIM image, as disclosed in Patent Document 1 or 2. The separate technique herein includes methods utilizing optical microscope images or CAD. 
         [0005]    Patent Document 1: JP Patent Publication (Kokai) No.2000-223061 A 
         [0006]    Patent Document 2: JP Patent Publication (Kokai) No.7-29535 A 
       SUMMARY OF THE INVENTION 
       [0007]    Nowadays, however, there are semiconductor materials that contain 9 to 10 layers of wiring, and in some cases it is difficult to identify a processing position even by the aforementioned method whereby an image obtained by a separate method is superposed on the material. And now that semiconductor production lines with 70 nm nodes are being established, it is becoming increasingly difficult to identify processing positions with the resolution of an optical microscope. In addition, while CAD information can be applied to semiconductor materials that are manufactured according to design, it cannot be applied to abnormal or deficient portions whose contour is difficult to predict. 
         [0008]    It is therefore an object of the invention to provide a focused ion beam system capable of acquiring surface structure information, internal structure information, and internal composition information simultaneously from the same field of view of the sample. It is another object of the invention to provide a method of sample preparation and observation whereby a sample processing position can be accurately set based on information about the structure and composition of a sample that is acquired from multiple directions using the aforementioned focused ion beam system. 
         [0009]    In accordance with the invention, a focused ion beam system includes: a detector for detecting a secondary electron image based on scanning electron or scanning ion excitation as a detector for obtaining sample surface structure information; a transmission electron detector as a detector for obtaining sample internal structure information; and an energy dispersive X-ray spectroscope or an electron energy loss spectroscope as a detector for obtaining sample composition information. In order to observe the sample from multiple directions, a stub is used that has the sample rotating and tilting functions. Magnification correction between scanning electrons and scanning ions is carried out by comparing a secondary electron image based on scanning electron excitation with a secondary electron image based on scanning ion excitation, both images providing surface structure information about the sample. Superposition of images is facilitated by providing a marking that can be recognized in any observation mode and from any observation direction. 
         [0010]    In accordance with the invention, an appropriate focused ion beam processing position can be set on the sample based on surface structure information, internal structure information, and internal composition information about the sample obtained from multiple directions of the sample extracted from an analysis portion. The sample can then be processed and observed with reference to such processing position. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  shows a schematic diagram of a focused ion beam system according to an embodiment of the invention. 
           [0012]      FIG. 2  shows a schematic procedure for the preparation and observation of a sample using the focused ion beam system shown in  FIG. 1 . 
           [0013]      FIG. 3  shows a detailed procedure for the preparation and observation of a sample using the focused ion beam system shown in  FIG. 1 . 
           [0014]      FIG. 4  shows a schematic diagram of a focused ion beam system according to another embodiment of the invention. 
           [0015]      FIG. 5  shows a detailed procedure for the preparation and observation of a sample using the focused ion beam system shown in  FIG. 4 . 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0016]    In the following, preferred embodiments of the focused ion beam system and method of sample preparation and observation according to the invention will be described with reference to the drawings.  FIGS. 1 to 5  show the embodiments of the invention, throughout which similar parts having basically identical structure or operation are designated with similar reference signs. 
         [0017]    In the focused ion beam system according to each of the embodiments, surface structure information, internal structure information, and internal composition information can be simultaneously acquired from the same field of view of the sample. The system includes a stub that has a rotating function and an tilting function, allowing the sample to be observed from multiple directions. The sample is presumed to have such a thickness as to allow the detection of transmission electrons. In a case where the acquisition of the sample structure and composition information and focused ion beam processing are carried out using a plurality of devices, the individual devices may use a common stub having the rotating function and the tilting function. Images obtained from each of such devices are automatically transferred and superposed. 
         [0018]      FIG. 1  shows a schematic diagram of the focused ion beam system according to an embodiment of the invention. In  FIG. 1 , a focused ion beam system  1  has a focused ion beam processing function enabled by an ion gun  2 , a condenser lens  3 , and scanning coils  4 . The system also has the function to acquire surface structure information, internal structure information, and internal composition information about a sample  6  simultaneously, which is enabled by an electron gun  5 , a condenser lens  3 , and scanning coils  4 . The sample  6  is disposed below the scanning coils  4 . Between the scanning coils  4  and the sample  6 , a detector  7  is disposed for detecting secondary electrons produced by scanning electron or ion excitation. The secondary electron detector  7  is connected to a scanning image display unit  10  via a signal amplifier  8  and an image processing unit  9 . Between the scanning coils  4  and the sample  6 , an EDX detector  11  is also disposed. The EDX detector  11  is connected, via the image processing unit  9 , to the scanning image display unit  10 . The scanning coils  4  are connected to a scanning power supply  12 , to which the scanning image display unit  10  is connected. 
         [0019]    Below the sample  6 , an annular scattered electron detector  13  is disposed for the observation of a dark field scanning transmission electron microscope (DF-STEM) image. The scattered electron detector  13  is connected, via the signal amplifier  8  and the image processing unit  9 , to the scanning image display unit  10 . Below the scattered electron detector  13 , a transmission electron detector  16  is disposed for the observation of a bright field scanning transmission electron microscope (BF-STEM) image. The transmission electron detector  16  can be moved in and out of the optical axis of the electron beam  14 . The transmission electron detector  16  is connected to the signal amplifier  8  and a detector movement control unit  17 . The detector is also connected, via the image processing unit  9 , to the scanning image display unit  10 . Further, below the transmission electron detector  16 , a magnetic prism  18  and an EELS detector  19  are disposed. The EELS detector  19  is connected, via the signal amplifier  8  and the image processing unit  9 , to the scanning image display unit  10 . The scanning image display unit  10  is capable of displaying images of different observation modes in separate windows simultaneously or in the same window in a superposed manner. 
         [0020]    The sample  6  is fixed on the stub  20 , which has the rotating and tilting functions. The movement of the stub  20  is controlled by a stub control unit  21 , which is connected to a CPU processing unit  22 . The CPU processing unit  22  is connected to the lens system of the apparatus and the image processing unit  9 . 
         [0021]    In the thus configured focused ion beam system, an ion beam  15  emitted by the ion gun  2  is focused by the condenser lens  3 . The surface of the sample  6  is scanned with the focused ion beam  15  by the scanning coils  4 . As the ion beam  15  is irradiated on the sample  6 , the sample  6  is sputtered, emitting secondary electrons. The secondary electrons are detected by the secondary electron detector  7 . Based on an image (SIM image) produced from the detection signal, the surface structure of the sample  6  is observed and its processing position is set. 
         [0022]    On the other hand, an electron beam  14  emitted by the electron gun  5  is focused by the condenser lens  3 . The surface of the sample  6  is then scanned with the focused electron beam  14  by the scanning coils  4 . As the electron beam  14  is irradiated on the sample  6 , the sample  6  emits secondary electrons, which are detected by the secondary electron detector  7 . Based on an image (SEM image) produced from the detection signal, the surface structure of the sample  6  is observed. Similarly, a characteristic X-ray emitted by the sample  6  is detected with an EDX detector  11 , and the sample is then observed for internal composition information using an elemental map image (EDX map image) based on the detection signal. The electrons scattered by the sample  6  are detected by the scattered electron detector  13 , and, using an image (dark field STEM image) based on the detection signal, the sample  6  is observed for its internal structure and compositional information. The electrons that passed through the sample  6  are detected by the transmission electron detector  16 , or by the EELS detector  19  after spectrum dispersion by the magnetic prism  18 . Using an image based on a detection signal from the transmission electron detector  16  (bright field STEM image) or an elemental map image from the EELS detector  19  (EELS map image), the internal structure of the sample  6  and its compositional information are observed. 
         [0023]      FIG. 2  schematically shows the procedure for the preparation and observation of a sample using the focused ion beam system shown in  FIG. 1 . In  FIG. 2 , an analysis portion  23  is extracted from the sample  6  by ion beam processing or the like. The sample  6  including the analysis portion  23  is then fixed on the stub  20 , which has the rotating function and the tilting function. The sample  6  is then irradiated with the electron beam  14  or the ion beam  15  from multiple directions so as to acquire its surface structure information, internal structure information, and internal composition information. 
         [0024]      FIG. 3  shows the procedure for the preparation and observation of the sample using the focused ion beam system of  FIG. 1  in detail. In the following, the procedure will be described with reference to  FIG. 3 .
   (1) The processing direction of the sample  6  is aligned with the ion beam direction and an SIM image is captured.   (2) A processing region having a desired shape is set on the SIM image acquired in (1). The thus set processing region  24  is then processed and a marking  25  is provided by making an opening in the sample  6 . By making the opening by FIB (focused ion beam) processing, the marking  25  can be recognized in every observation mode.   (3) The sample  6  is tilted from the ion beam direction to the electron beam direction, and the surface structure information and internal structure information or internal composition information about the sample  6  is simultaneously acquired. The surface structure information is obtained from the SEM image. The internal structure information or the internal composition information is acquired simultaneously from the same field of view in images selected from a dark field STEM image, a bright field STEM image, an EDX map image, and an EELS map image in which the marking  25  can be placed.   (4) Because the multiple images acquired in (3) are acquired simultaneously within the same field of view, they can be easily superposed. The sample  6  is again tilted toward the ion beam direction, and, using the markings  25  of the multiple images that are superposed, the magnification and position of the SIM image are aligned with those of the superposed images. The images that are superposed may be rendered semitransparent or color-coded by the image processing unit  9 .   (5) A processing region  24  is set in the images superposed in (4) such that the analysis portion  23  remains. The processing region  24  thus set is then processed.   (6) The sample  6  is tilted toward the electron beam and rotated by 90°, and the analysis portion  23  is observed and analyzed. If the processing is insufficient, steps (1) to (5) may be repeated.     
         [0031]      FIG. 4  shows a schematic diagram of the focused ion beam system according to another embodiment of the invention. As shown in  FIG. 4 , the system includes a device  26  having the focused ion beam processing function, and a device  27  having the function to acquire sample surface structure, internal structure, and compositional information simultaneously. Thus, the acquisition of sample structure and composition information and focused ion beam processing are carried out by separate devices. 
         [0032]    The device  26  having the focused ion beam processing function includes an ion gun  2 , a condenser lens  3 , and scanning coils  4 . Below the scanning coils  4 , a sample  6  is disposed. Between the scanning coils  4  and the sample  6  is disposed a secondary electron detector  7  for detecting secondary electrons produced by scanning ion excitation. The secondary electron detector  7  is connected, via a signal amplifier  8  and an image processing unit  9 , to a scanning image display unit  10 . The scanning image display unit  10  of the device  26  is connected to the scanning image display unit  10  of the device  27  via a server  28 , via which the images acquired by the device  26  and those acquired by the device  27  are transferred. Each of the scanning image display units  10  is capable of displaying an acquired image and a transferred image in separate windows simultaneously or in the same window in a superposed manner. 
         [0033]    The sample  6  is fixed to a stub  20  which has the rotating and tilting functions. The movement of the stub  20  is controlled by the stub control unit  21  to which the stub  20  is connected. The stub control unit  21  is connected to a CPU processing unit  22 , which is in turn connected to the lens system of the device and to the image processing unit  9 . 
         [0034]    In this device  26 , the ion beam  15  emitted by the ion gun  2  is focused by the condenser lens  3 . With the thus focused ion beam  15 , the surface of the sample  6  is scanned by the scanning coils  4 . As the sample  6  is irradiated with the ion beam  15 , the sample  6  is sputtered, emitting secondary electrons. The secondary electrons are detected by the secondary electron detector  7 , and, using an image (SIM image) based on the detection signal, the surface structure of the sample  6  is observed and a processing position is set. 
         [0035]    The device  27  having the function to acquire sample surface structure, internal structure, and compositional information simultaneously includes an electron gun  5 , a condenser lens  3 , and scanning coils  4 . Below the scanning coils  4 , a sample  6  is disposed. Between the scanning coils  4  and the sample  6 , a secondary electron detector  7  for the detection of secondary electrons produced by scanning ion excitation is disposed. The secondary electron detector  7  is connected, via a signal amplifier  8  and an image processing unit  9 , to a scanning image display unit  10 . Between the scanning coils  4  and the sample  6 , there is also disposed an EDX detector  11 . The EDX detector  11  is connected via the image processing unit  9  to the scanning image display unit  10 . The scanning coils  4  are connected to a scanning power supply  12 , to which the scanning image display unit  10  is connected. Below the sample  6 , an annular scattered electron detector  13  is disposed for the observation of a dark field scanning transmission electron microscope (DF-STEM) image. The scattered electron detector  13  is connected, via the signal amplifier  8  and the image processing unit  9 , to the scanning image display unit  10 . Below the scattered electron detector  13 , a transmission electron detector  16  is disposed for the observation of a bright field scanning transmission electron microscope (BF-STEM) image. The transmission electron detector  16  can be moved into and out of the optical axis of the electron beam  14 . The transmission electron detector  16  is connected to the signal amplifier  8  and to the detector movement control unit  17  and further to the scanning image display unit  10  via the image processing unit  9 . Below the transmission electron detector  16 , a magnetic prism  18  and an EELS detector  19  are disposed. The EELS detector  19  is connected via the signal amplifier  8  and the image processing unit  9  to the scanning image display unit  10 . The scanning image display unit  10  of the device  27  is connected to the scanning image display unit  10  of the device  26  via a server  28 , via which images acquired by the device  26  and those acquired by the device  27  are transferred. The scanning image display unit  10  is capable of displaying the images in different observation modes simultaneously in separate windows or in the same window in a superposed manner. Further, the scanning image display unit  10  is capable of displaying an acquired image and a transferred image simultaneously in separate windows or in the same widow in a superposed manner. 
         [0036]    The sample  6  is fixed to a stub  20  having the rotating and tilting functions. The movement of the stub  20  is controlled by the stub control unit  21  to which the stub is connected. The stub control unit  21  is connected to a CPU processing unit  22 . The CPU processing unit  22  is also connected to the lens system of the device and to the image processing unit  9 . 
         [0037]    In this device  27 , the electron beam  14  emitted by the electron gun  5  is focused by the condenser lens  3 . With the thus focused electron beam  14 , the surface of the sample  6  is scanned by the scanning coils  4 . As the electron beam  14  is irradiated on the sample  6 , secondary electrons are emitted by the sample  6  which are detected by the secondary electron detector  7 . Using an image (SEM image) based on the detection signal, the surface structure observation of the sample  6  is conducted. Similarly, a characteristic X-ray emitted by the sample  6  is detected by the EDX detector  11 , and, using an elemental map image (EDX map image) based on the detection signal, the internal composition information observation of the sample  6  is conducted. The electrons scattered by the sample  6  are detected by the scattered electron detector  13 , and, using an image (dark field STEM image) based on the detection signal, the internal structure and compositional information observation of the sample  6  is conducted. The electrons that passed through the sample  6  are detected by the transmission electron detector  16  or by the EELS detector  19  after spectrum dispersion with the magnetic prism  18 . Then, using an image (bright field STEM image) based on a detection signal from the transmission electron detector  16  or an elemental map image (EELS map image) based on a detection signal from the EELS detector  19 , the internal structure and compositional information observation of the sample  6  is conducted. 
         [0038]      FIG. 5  is a detailed procedure for the preparation and observation of the sample using the focused ion beam system shown in  FIG. 4 . In the following, the procedure will be described with reference to  FIG. 5 .
   (1) In the device  26  having the focused ion beam processing function, the processing direction of the sample  6  is aligned with the ion beam direction, and then an SIM image is captured.   (2) On the SIM image taken in (1), a processing region  24  with a desired shape is set. The thus set processing region  24  is then processed and a marking  25  is provided by making an opening in the sample  6 . By making the opening by FIB processing, the marking can be recognized in every observation mode.   (3) The sample  6 , together with the stub  20 , is moved to the device  27  having the function to simultaneously acquire the surface structure and internal structure or compositional information, and the surface structure information and internal structure information or internal composition information about the sample  6  are acquired simultaneously. The surface structure information is acquired from an SEM image. The internal structure information or the internal composition information is acquired from the same field of view in images selected from a dark field STEM image, a bright field STEM image, an EDX map image, and an EELS map image simultaneously in which the marking can be placed.   (4) Because the multiple images acquired in (3) are acquired from the same field of view simultaneously, they can be easily superposed. The sample  6  is again moved to the device  26  having the focused ion beam processing function. Then, using the markings of the multiple superposed images transferred from the device  27 , which has the function to simultaneously acquire the surface structure and internal structure or compositional information, the magnification and position of the SIM image are aligned with those of the superposed images. The superposed images may be rendered semitransparent or color-coded by the image processing unit  9 .   (5) A processing region  24  is set on the images superposed in (4) such that an analysis portion  23  is left, and the thus set processing region  24  is processed.   (6) The sample  6  is moved to the device  27  having the function to simultaneously acquire the surface structure and internal structure or compositional information. The stub  20  is rotated by 90° and the analysis portion is observed and analyzed. If the processing is insufficient, steps (1) to (5) may be repeated.     
         [0045]    While the invention have been described above with reference to preferred embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in form and detail may be made therein without departing from the spirit and scope of the invention.