Abstract:
An automatically aligning objective aperture assembly for a CDSEM includes a plate that is moveable in X and Y directions relative to an electron beam generated by the SEM. The plate defines one or more objective apertures. Encoders and motors are provided for affecting movement of the plate in the X and Y directions. An image controller, responsive to an image of a semiconductor wafer feature focused upon by the electron beam, controls the encoders and motors in a manner which affects movement of the plate to automatically align the objective aperture with the electron beam.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to a scanning electron microscope. More particularly the present invention relates to an objective aperture assembly for a critical dimension scanning electron microscope, the objective aperture assembly having an automatically aligning objective aperture.  
       BACKGROUND OF THE INVENTION  
       [0002]     It is important to measure and control the feature size of various patterns during the fabrication of semiconductors. Such a measurement is commonly known in the art as a critical dimension (CD) measurement. A critical dimension scanning electron microscope (CDSEM) is typically utilized for CD measurement of semiconductor features.  
         [0003]     A CDSEM operates by focussing of an electron beam onto a semiconductor wafer or substrate. Backscattered and secondary electrons are generated by the electron beam and collected for measurement purposes. Scanning the electron beam across a feature of interest and detecting the backscattered and secondary electrons allows CD measurement of the feature.  
         [0004]     Production line operators are typically instructed how to operate the CDSEM so that they may accurately perform CD measurements during various stages of semiconductor fabrication. Because the accuracy of a CD measurement depends upon how accurately the electron beam of the CDSEM is focussed onto the feature of interest on the semiconductor wafer, an emphasis is placed during instruction on how to properly focus the electron beam.  
         [0005]     The procedure for focussing the electron beam of the CDSEM generally involves three basic steps: objective aperture alignment, focus adjustment, and astigmatism adjustment. Objective aperture alignment is especially critical because if the objective aperture is not aligned, the image of the feature will move across the display screen during focussing. The objective aperture is typically aligned by focussing the electron beam on a circular feature on a fiducial mark. If the central axis of the aperture is not aligned with the condenser lens of the CDSEM, the image of the feature will move across the display screen during focussing, as stated earlier. The central axis of the aperture may be aligned with the condenser lens using manually adjustable controls that move the aperture in the X and Y directions. The manual controls are adjusted so that the feature stays in about the same area of the display screen during focusing. When this is accomplished, the aperture is considered to be properly aligned.  
         [0006]     Once the aperture alignment has been correctly adjusted, the CDSEM may be operated for many months by merely adjusting electron beam focus and astigmatism. Over time, however, the aperture tends to degrade thereby making proper focus and astigmatism adjustment difficult for the operator. When this occurs, the aperture must be changed and then properly aligned. Since the task of changing and aligning the new aperture is typically not released to the operator of the CDSEM, an equipment engineer must be called to change the aperture and align it manually. The down time resulting from this reduces critical dimension measurement throughput and effectively increases the measuring time per wafer.  
       SUMMARY OF THE INVENTION  
       [0007]     An automatically aligning objective aperture assembly for a CDSEM is disclosed for an SEM. The automatically aligning objective aperture assembly includes a plate that is moveable in X and Y directions relative to an electron beam generated by the SEM. The plate defines one or more objective apertures. Motors are provided for affecting movement of the plate in the X and Y directions. An image controller, responsive to an image of a semiconductor wafer feature focused upon by the electron beam, controls the motors in a manner which affects movement of the plate to automatically align the objective aperture with the electron beam. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]      FIG. 1  is an illustration of a CDSEM according to an embodiment of the present invention.  
         [0009]      FIG. 2A  is a perspective view of an aperture assembly having an automatically aligning objective aperture according to an exemplary embodiment of the present invention.  
         [0010]      FIG. 2B  is an exploded view of an aperture holder, an aperture plate, and aperture support of the aperture assembly of  FIG. 2A .  
         [0011]      FIG. 3  is a functional illustration of the aperture assembly of  FIG. 2A .  
         [0012]      FIG. 4  is an illustration depicting the operation of the automatically aligning objective aperture of the aperture assembly.  
         [0013]      FIG. 5  is a flowchart depicting the steps performed to measure a critical dimension of a semiconductor wafer feature using the CDSEM and the automatically aligning objective aperture of the aperture assembly.  
         [0014]      FIGS. 6A-6C  illustrate aperture detector maps for adjusting aperture focus to align the image point on a center of a circle, wherein the aperture detector map of  FIG. 6A  illustrates a focus center for an aligned aperture position, the aperture detector map of  FIG. 6B  illustrates a focus center for an X-axis shifted aperture position, and the aperture detector map of  FIG. 6C  illustrates a focus center for a Y-axis shifted aperture position. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0015]     A CDSEM according to an exemplary embodiment of the present is shown in  FIG. 1  and denoted by reference numeral  10 . As can be seen, electron gun  12  having electron source  14  directs electron beam  13  into sample chamber  20  containing XY wafer mounting stage  22  which mounts wafer  24 . The electron beam  13  is focussed onto wafer  24  by CDSEM components including first condenser lens  16   a , aperture assembly  30  having an automatically aligning objective aperture, second condenser lens  16   b , and objective lens  18 . Collected from wafer  24  are beams  15   a  and  15   b  of backscattered and secondary electrons which are detected by detectors  19   a  and  19   b . A computer (not shown) performs imaging analysis for CD measurement and a display screen (not shown) allows image data to be viewed by an operator.  
         [0016]      FIGS. 2A and 2B  collectively show an exemplary embodiment of the aperture assembly  30  having the automatically aligning objective aperture. As shown, the aperture assembly  30  may include a main body  31 , a rod-like aperture holder  32  extending from within the main body  31  which mounts on a free end thereof an aperture plate  33  defining one or more objective apertures  34 , typically of the same diameter. The aperture plate  33  may be retained on the free end of the aperture holder  32  by an aperture support  35 . Adjustment knobs  36   a  and  36   b  or like devices are provided for manually moving the rod-like aperture holder  32  (and therefore the aperture plate  33 ) respectively along X and Y axes.  
         [0017]     In one exemplary embodiment of the invention, automatic alignment of the one or more objective apertures  34  may be achieved by providing an X stepper motor  40   a  that is attached to X axis adjustment knob  36   a  by coupling  41   a , and providing a Y stepper motor  40   b  that is attached to Y direction adjustment knob  36   b  by coupling  41   b . As shown in  FIG. 3 , image controller ( FIG. 3 ), which may be integrated with the earlier mentioned computer, actuates X stepper motor  40   a  using X encoder  42   a  and actuates Y stepper motor  40   b  using Y encoder  42   b.    
         [0018]     Referring to  FIG. 4 , the automatically aligning objective aperture  34  of the objective aperture assembly  30  operates as follows. The CDSEM is activated to focus an electron beam  13  onto a feature of semiconductor wafer  24  mounted on wafer mounting stage  22 . The feature (denoted by numeral  70  in  FIGS. 6A-6C ) selected for performing automatic objective alignment is typically circular in shape. To align the X-axis position of one of the selected objective apertures  34 , the image controller  50  generates a first X-axis feedback  70   a  signal which is applied to X encoder  42   a . The X encoder  42   a , in turn, activates X motor  40   a  with an appropriate electrical pulse. Activated X motor  40   a  rotates X-axis control knob  36   a  of aperture assembly  30  using coupling  41   a , thereby moving the aperture plate  33  in the appropriate direction along the X-axis until beginning edge position X 1  of aperture  34  is detected in the image. The image controller  50  generates a second X-axis feedback signal  70   a  which causes the aperture plate  33  to move (via the X encoder  42   a , X motor  40   a , coupling  41   a , and X-axis control knob  36   a  as described above) in the appropriate direction along the X axis until end edge position X 2  of aperture  34  is detected in the image. The image controller  50  then uses the beginning and end aperture edge positions X 1  and X 2  respectively, to identify the X-axis center X 3  of aperture  34  according to the following relationship: 
 
 X   3 =( X   1 + X   2 )/2 
 
 The image controller  50  then generates a third X-axis feedback signal  70   a  which cause the aperture plate  33  to move (via the X encoder  42   a , X motor  40   a , coupling  41   a , and X-axis control knob  36   a  as described above) in the appropriate direction along the X axis to the calculated X-axis center position X 3 . 
 
         [0019]     To align the Y-axis position of objective aperture  34 , the image controller  50  generates a first Y-axis feedback signal  70   b  which is applied to Y encoder  42   b . The Y encoder  42   b , in turn, activates Y motor  40   b  with an appropriate electrical pulse. Activated Y motor  40   b  rotates Y-axis control knob  36   b  of aperture assembly  30  using coupling  41   b , thereby moving the aperture plate  33  in the appropriate direction along the Y axis until beginning edge position Y 1  of aperture  34  is detected in the image. The image controller  50  generates a second Y-axis feedback signal  70   b  which causes the aperture plate  33  to move (via the Y encoder  42   b , Y motor  40   b , coupling  41   b , and Y-axis control knob  36   b  as describe above) in the appropriate direction along the Y-axis until end edge position Y 2  of aperture  34  is detected in the image. The image controller  50  then uses the beginning aperture edge position Y 1  and the end aperture edge position Y 2  to identify the Y-axis center Y 3  of aperture  34  according to the following relationship: 
 
 Y   3 =( Y   1 + Y   2 )/2 
 
 The image controller  50  then generates a third Y-axis feedback signal  70   b  which cause the aperture plate  33  to move (via the Y encoder  42   b , Y motor  40   b , coupling  41   b , and Y-axis control knob  36   b  as described above) in the appropriate direction along the Y axis to the calculated Y-axis center position Y 3 . 
 
         [0020]      FIG. 5  is a flowchart depicting the steps performed to measure a critical dimension (CD) of a semiconductor wafer feature (the feature shown in  FIGS. 6A-6C  for aperture aligning) using a CDSEM comprising an aperture assembly  30  having the automatically aligning objective aperture of the invention. The steps for CD measurement include: step  1 , the automatic objective alignment method of the invention; step  2 , conventional rough focus adjustment; and step  3 , conventional fine focus adjustment (astigmatism adjustment). Note that once the objective aperture has been correctly aligned, CD measurements can be taken for many months by merely performing the rough focus adjustment of step  2  and the astigmatism adjustment of step  3 . Over time, however, the objective aperture tends to degrade thereby making rough focus and astigmatism adjustment difficult for the operator, as the image of feature will tend to move across and/or off the display screen during focussing. When this occurs, the operator must change the objective aperture and perform step  1  according to the earlier described method to properly align the new objective aperture. Once step  1  has been performed, the rough focus method of step  2  is performed. If image of the feature still moves across and/or off the display screen during rough focussing, the objective aperture may have to be aligned again by repeating the automatic objective aperture alignment method of step  1 . After rough focus has been achieved, fine focussing or astigmatism adjustment is performed. If a fine focus is not achieved, it may be necessary to repeat step  2  and then perform step  3  again. Once a fine focus image of the feature is achieved, it may be saved to the image controller  50 .  
         [0021]     While the foregoing invention has been described with reference to the above, various modifications and changes can be made without departing from the spirit of the invention. Accordingly, all such modifications and changes are considered to be within the scope of the appended claims.