JP5468515B2 - Observation image acquisition method, scanning electron microscope - Google Patents

Description

  The present invention relates to a method for acquiring an observation image of a sample using a scanning electron microscope.

  Conventionally, a defect analysis of a semiconductor component in which a predetermined shape pattern is regularly and repeatedly arranged has been performed using a scanning electron microscope. In recent years, semiconductor miniaturization has progressed more than the positioning accuracy of the sample stage, and it is becoming difficult to specify a defective position even when a high-magnification scanning electron microscope is used.

  On the other hand, there is a defect that cannot be determined from the appearance, such that the presence or absence of a defect can be determined only after measuring the device characteristics by applying a probe to the defective part. When analyzing such a defect, it is necessary to cut out the defective portion with a focused ion beam or the like. At this time, in order to accurately specify a portion to be cut out from a large number of adjacent identical shape patterns, it is necessary to identify each shape pattern by giving an identification number to each shape pattern. This is because even if device characteristics are measured without specifying which shape pattern has been cut out, there is no significance of inspection.

  Patent Document 1 below describes a method of counting the same shape pattern as described above. In the method described in this document, as a development of the method of counting shape patterns while moving the sample stage, a method of assigning an identification number while enclosing each shape pattern using a rectangular frame on the screen is disclosed. ing. The operator specifies the shape pattern to be cut out while visually confirming the identification number of each shape pattern displayed on the screen. According to this method, there is an advantage in that each shape pattern can be specified by the identification number regardless of the positioning accuracy of the stage.

JP 2000-251824 A

  In the method described in Patent Document 1, when the sample stage is moved to move the visual field range while the charged particle beam is being irradiated, the irradiation position of the charged particle beam is stretched along with the stage movement. The observed image is distorted in the stage moving direction. When counting while assigning an identification number to a shape pattern, each shape pattern is identified while searching for the same shape by pattern matching. Therefore, if the observed image is distorted, the shape pattern cannot be identified. There is a possibility of skipping without counting the shape pattern and proceeding to the next shape pattern.

  If the frame rate is increased much faster than the moving speed of the stage, the distortion of the observed image will be reduced, but the yield of secondary electrons per pixel will also be reduced, and the observed image will be rough, and as a result, the shape pattern can be identified. There is a possibility of disappearing.

  As an operator, the operator wants to move the sample stage quickly in order to complete the work quickly. On the other hand, if the sample stage is moved too quickly, the above-mentioned problems occur. That is, the moving speed of the sample stage and the shape pattern identification accuracy are in a trade-off relationship. In actual work, the movement speed of the sample stage is often lowered with an emphasis on the accuracy of the observation image, which seems to be a factor that lowers work efficiency.

  Although it is conceivable to move the sample stage in the imaging pause section where the charged particle beam is not irradiated, the observation image moves out of frame and is difficult to see.

  The present invention has been made in order to solve the above-described problems, and prevents the observation image from being distorted even if the sample stage is moved during irradiation of the charged particle beam. An object of the present invention is to provide an observation image acquisition technique that can be used.

  In the observation image acquisition method according to the present invention, a scan line in which a shift occurs between frames is specified, and a position correction is performed so that a portion other than the scan line in the observation image is aligned with the position of the shift scan line. .

  According to the observation image acquisition method of the present invention, even if the scanning line is shifted because the stage is moved during irradiation of the charged particle beam, the scanning line is specified by specifying the shift between frames. Can do. By correcting the position of other portions in accordance with the position of the scanning line, the distortion of the observation image can be restored and the shape pattern can be accurately identified. As a result, the stage can be moved even during irradiation with the charged particle beam, so that work efficiency can be improved.

1 is a configuration diagram of a scanning electron microscope 28 according to Embodiment 1. FIG. It is a figure explaining the principle which distortion arises in an observation image by moving the sample stage 26 in the middle of irradiating the charged particle beam 3. FIG. It is a figure explaining the principle in which an observation image becomes unclear. It is a figure explaining the procedure which eliminates distortion by correct | amending the position of an observation image. 10 is a flowchart showing a conventional procedure for generating an observation image while counting a predetermined shape pattern on a sample 11. 5 is a flowchart illustrating a procedure for generating an observation image while the scanning electron microscope counts a predetermined shape pattern on a sample in the first embodiment. It is a figure explaining the observation image acquisition method which concerns on Embodiment 2. FIG. It is a flowchart which shows the conventional procedure which produces | generates the observation image for 1 frame. 10 is a flowchart illustrating a procedure in which the scanning electron microscope 28 generates an observation image for one frame in the third embodiment.

<Embodiment 1>
FIG. 1 is a configuration diagram of a scanning electron microscope 28 according to Embodiment 1 of the present invention. The main body 1 includes a charged particle beam source 2, a first focusing lens 5, a second focusing lens 6, a deflection coil 7, a field moving coil 8, an objective lens 9, a secondary electron detector 10, and a sample stage 26. These are controlled by the control unit 12. The stage control unit 27 controls the movement position of the sample stage 26.

  The control unit 12 includes a charged particle beam control unit 13, an amplifier 15, an image processing unit 16, and a system control unit 14. The system control unit 14 performs integrated control of these components.

  The computer 22 is an operation terminal used by an operator to give instructions to the control unit 12, and includes a keyboard 23, a mouse 24, and a display 25.

  The charged particle beam stop 4, the first converging lens 5, the second converging lens 6, the deflection coil 7, and the objective lens 9 narrow the charged particle beam 3 emitted from the charged particle beam source 2 and irradiate the sample 11. Then, the sample 11 is scanned. The charged particle beam control unit 13 includes components for controlling the charged particle beam 3, such as a high voltage power source, a lens power source, and a deflection amplifier. The system control unit 14 controls the charged particle beam 3 using these components of the charged particle beam control unit 13.

  The system control unit 14 controls the focal position of the charged particle beam 3 on the sample 11 by changing the current supplied to the objective lens 9 using the objective lens driving unit 13b.

  The secondary electron detector 10 detects secondary electrons generated by irradiating the sample 11 with the charged particle beam 3. The amplifier 15 amplifies the secondary electron detection signal detected by the secondary electron detector 10 and outputs the amplified signal to the image processing unit 16.

  The image processing unit 16 includes an A / D converter 17, a pixel integration unit 18, a frame integration unit 19, an image memory 20, and an image transmission unit 21. The A / D converter 17 converts the output of the amplifier 15 into digital data. The pixel integration unit 18 converts the output of the A / D converter 17 into a luminance value for each pixel. The frame integration unit 19 shapes the output of the pixel integration unit 18 as observation image data for each frame and outputs the observation image data to the image transmission unit 21. The image transmission unit 21 transmits the observation image data to the computer 22, and the computer 22 displays the observation image on the screen. The image memory 20 stores observation image data for one or more frames, and superimposes the observation images of subsequent frames as necessary to reduce image roughness.

  The display 25 displays the observation image on the screen. When the stage control unit 27 moves the sample stage 26 or the visual field moving coil 8 changes the position of the charged particle beam 3 irradiated on the sample 11, the observation visual field displayed on the screen of the display 25 moves. When the stage control unit 27 rotates the sample stage 26 or modulates the current of the deflection coil 7, the scanning direction of the charged particle beam 3 rotates, and as a result, the observation field of view displayed on the display 25 rotates. When the stage control unit 27 tilts the sample stage 26, a tilted image of the sample 11 can be obtained.

  FIG. 2 is a diagram for explaining the principle of distortion in the observed image caused by moving the sample stage 26 while the charged particle beam 3 is being irradiated. The scanning electron microscope 28 generally acquires a horizontal scanning line image while scanning the charged particle beam 3 in the horizontal direction of the observation field, and generates a two-dimensional observation image by repeating this from the upper side toward the lower side. Here, the horizontal scanning lines 35 to 39 are illustrated.

  Here, it is assumed that the sample stage 26 has moved in the direction 29 while the charged particle beam 3 is being irradiated. It is assumed that a circular pattern is formed on the sample 11. The initial position of the circular pattern is position 30. As the sample stage 26 moves, the position of the circular pattern moves from the position 30 toward the position 34.

  Assuming that the circular pattern has moved from position 30 to position 31 while scanning the charged particle beam 3 on the horizontal scanning line 36, the scanning line obtained as a result of scanning the circular pattern on the horizontal scanning line 36. The image is on the line segment 36b. Similarly, it is assumed that the scanning line image obtained on the horizontal scanning line 37 is on the line segment 37b, and the scanning line image obtained on the horizontal scanning line 38 is on the line segment 38b.

  When the scanning line images on the line segments 36 b to 38 b are connected, the finally obtained circular pattern observation image has a shape stretched in the direction 29 as an elliptical pattern 40. This is because the irradiation position is stretched along the direction 29 and shifted due to the movement of the sample stage 26 during irradiation of the charged particle beam 3.

  Although the distortion of the observation image can be reduced by increasing the frame rate extremely with respect to the moving speed of the sample stage 26, the yield of secondary electrons per pixel is reduced, resulting in a rough observation image. By applying a recursive filter or the like, it is possible to superimpose a plurality of observation images and suppress the roughness of the observation images. However, in this case, the observation images may become unclear.

  FIG. 3 is a diagram for explaining the principle that an observed image becomes unclear. Here, it is assumed that the sample stage 26 has moved in the direction 41 while observing the observation images accumulated by applying the recursive filter while irradiating the charged particle beam 3. The initial position of the circular pattern on the sample 11 is a position 42. It is assumed that the circular pattern has moved to position 43 after one frame.

  The region 45 has a gradation in which four observation images are superimposed. On the other hand, the region 44 and the region 46 have a gradation in which three observation images are superimposed. Therefore, the contrast between the region 44 and the region 46 is lower than that of the region 45, and the observation image becomes unclear.

  The distortion and unclearness of the observed image explained in FIGS. 2 to 3 are that the scanning position is shifted when the scanning is not completed and all the pixel data is not acquired, that is, the pixel data in the screen is acquired at the same time. It is caused by not being able to do it. This always occurs when the sample stage 26 is moved while the sample 11 is being scanned using the charged particle beam 3. Therefore, in the present invention, the acquired observation image is corrected in the moving direction of the sample stage 26, thereby eliminating distortion and unclearness.

  FIG. 4 is a diagram illustrating a procedure for eliminating distortion by correcting the position of the observation image. Frame 51 is the first observation image, and frame 47 is the second observation image. Here, it is assumed that the sample 11 has moved in the direction 50 when the charged particle beam 3 is scanning on the scanning line 48.

  When scanning is performed on the scanning line 48, if the sample 11 is moved to move the observation field of view, the scanning line 49 on the frame 47 corresponding to the scanning line 52 on the frame 51 is shifted in the direction 50. . As a result, the scanning line 52 is disposed at a position closer to the right side of the frame 51, while the scanning line 49 is disposed at a position closer to the left side of the frame 47. If the observation image is generated in this state, the circular pattern is distorted leftward.

  The system control unit 14 obtains a difference for each scanning line between the frame 47 and the frame 51 by pattern matching. Here, the observation image obtained from the scanning line 52 matches the observation image obtained from the scanning line 49 by pattern matching. However, it can be seen that these positions are shifted by a shift amount 53.

  Therefore, the system control unit 14 corrects the position of the observation image 54 obtained from the scanning line excluding the scanning line 49 according to the position of the scanning line 49 after completing the scanning of the scanning line 49, and The screen is displayed together with 49 observation images. This is shown in the lower part of FIG. Thereby, even when the sample 11 moves during irradiation of the charged particle beam 3, it is possible to prevent distortion of the observation image.

  When the observation image is moved, a defect portion may be generated in the region outside the circular pattern. To prevent this, the charged particle beam is applied to a region wider than the observation range 55 by expecting the defect portion in advance. 3 and the scanning line image should be obtained for the outer region of the circular pattern. Thereby, even if the observation image is moved by the shift amount 53, it is possible to prevent a defective portion from being generated in the observation range 55.

FIG. 5 is a flowchart showing a conventional procedure for generating an observation image while counting a predetermined shape pattern on the sample 11. This flowchart is shown for comparison with the method according to the present invention, which will be described later with reference to FIG. Hereinafter, each step of FIG. 5 will be described.
(FIG. 5: Steps S501 to S502)
The A / D converter 17 converts the secondary electron detection signal detected by the secondary electron detector 10 into digital data (S501). The pixel integration unit 18 converts the output of the A / D converter 17 into a luminance value for each pixel (S502).
(FIG. 5: Steps S503 to S504)
The pixel integration unit 18 stores the luminance value for each pixel in the image memory 20 for each scanning line (S503). The image transmission unit 21 transmits the observation image data for each scanning line to the computer 22, and the computer 22 displays the observation image of the scanning line on the screen (S504). That is, in the conventional method, the observation image is updated for each scanning line.

FIG. 6 is a flowchart illustrating a procedure in which the scanning electron microscope 28 generates an observation image while counting a predetermined shape pattern on the sample 11 in the first embodiment. Hereinafter, each step of FIG. 6 will be described.
(FIG. 6: Steps S601 to S603)
These steps are the same as steps S501 to S503 in FIG.
(FIG. 6: Step S604)
The system control unit 14 pattern matches the observation image of the scanning line currently irradiated with the charged particle beam 3 with the observation image of the same scanning line in the previous frame.

(FIG. 6: Step S605)
The system control unit 14 determines whether the scanning line images match between frames as a result of step S604. If there is a positional deviation between the scanning lines as described in FIG. 4, the process proceeds to step S606. If there is no displacement, the process skips to step S607. Through this step, the system control unit 14 can specify a scanning line having a positional deviation.
(FIG. 6: Step S606)
As described with reference to FIG. 4, the system control unit 14 corrects the position of the observation image of the scanning line excluding the scanning line with the positional deviation in the previous frame so as to match the position of the scanning line with the positional deviation. .
(FIG. 6: Step S607)
The system control unit 14 displays the corrected observation image obtained as a result of step S <b> 606 on the display 25 of the computer 22. Further, the system control unit 14 individually identifies patterns having a predetermined shape appearing on the corrected observation image, assigns an identification number, and displays the screen on the display 25.

<Embodiment 1: Summary>
As described above, the scanning electron microscope 28 according to the first embodiment identifies the scanning line in which the positional deviation occurs between the frames, and the position of the observation image of the scanning line excluding the scanning line is the position. The position is corrected according to the shifted scanning line. As a result, even when the sample 11 moves while the charged particle beam 3 is being applied to the sample 11, the position of the distorted observation image can be corrected according to the moved scanning line position. That is, the distorted observation image is corrected to the original shape in accordance with the moved position, and the shape pattern can be accurately identified.

  In the first embodiment, the pattern matching is performed in the horizontal direction on the assumption that the sample 11 moves in the horizontal direction, but the sample 11 is positioned above and below the scanning line irradiated with the charged particle beam 3. Pattern matching may also be performed on the scanning lines. Thereby, even when the sample 11 moves in the direction perpendicular to the scanning line, the distortion of the observation image can be corrected. The same applies to the following embodiments.

<Embodiment 2>
In the second embodiment of the present invention, processing when the sample 11 moves in an oblique direction with respect to the scanning line while the charged particle beam 3 is being irradiated will be described. Since the configuration of the scanning electron microscope 28 is the same as that of the first embodiment, the following description focuses on the differences.

  FIG. 7 is a diagram for explaining an observation image acquisition method according to the second embodiment. Here, it is assumed that the sample 11 has moved in a direction 50 oblique to the scanning line. The system control unit 14 performs pattern matching between the scanning line 49 and other scanning lines. Here, it is assumed that the scanning line 52 and the scanning line 49 match.

  The system control unit 14 can specify the moving direction 50 of the sample 11 based on the positional deviation between the scanning line 49 and the scanning line 52. Thereafter, the system control unit 14 instructs the charged particle beam control unit 13 so that the scanning direction of the charged particle beam 3 is parallel to the direction 50. The subsequent processing is the same as in the first embodiment.

  As described above, the scanning electron microscope 28 according to the second embodiment is configured such that when the sample 11 moves in an oblique direction with respect to the scanning line while the charged particle beam 3 is being irradiated, The scanning direction of the beam 3 is corrected so as to be parallel to the moving direction of the sample 11. As a result, since the subsequent pattern matching only needs to be performed in the horizontal direction with respect to the scanning line, the processing burden of the system control unit 14 performing pattern matching can be reduced.

<Embodiment 3>
In the above Embodiments 1 and 2, a plurality of frames of observation images can be overlapped and integrated to obtain a single observation image. If the integration process is performed in a state where the observation image is distorted, the observation image may become unclear as described with reference to FIG. 3. However, if the distortion is corrected as described in the first and second embodiments, such a problem is caused. Does not occur.

  Therefore, in the third embodiment of the present invention, an operation example in which integration processing is performed after correcting distortion of an observation image will be described. Since the configuration of the scanning electron microscope 28 is the same as that of the first embodiment, the following description focuses on the points related to the integration process.

FIG. 8 is a flowchart showing a conventional procedure for generating an observation image for one frame. This flowchart is illustrated for comparison with the method according to the present invention described in FIG. 9 described later. Hereinafter, each step of FIG. 8 will be described.
(FIG. 8: Steps S801 to S802)
These steps are the same as steps S501 to S502 in FIG.
(FIG. 8: Steps S803 to S804)
The frame integration unit 19 shapes the output of the pixel integration unit 18 as observation image data for each frame (S803). The image transmission unit 21 transmits the observation image data to the computer 22, and the computer 22 displays the observation image on the screen (S804). That is, in the conventional method, since the scanning line images whose positions are shifted are integrated as they are, the observation image may be unclear as described with reference to FIG.

FIG. 9 is a flowchart illustrating a procedure in which the scanning electron microscope 28 generates an observation image for one frame in the third embodiment. Hereinafter, each step of FIG. 9 will be described.
(FIG. 9: Steps S901 to S906)
These steps are the same as steps S601 to S606 in FIG.
(FIG. 9: Steps S907 to S908)
The frame integration unit 19 integrates the corrected observation image obtained as a result of step S906 by superimposing them at the same position as the observation image of the previous frame. The frame integration unit 19 performs the same processing for all the scanning lines.
(FIG. 9: Step S909)
The system control unit 14 displays the corrected observation image obtained as a result up to step S908 on the display 25 of the computer 22 on the screen. That is, in the method according to the present invention, since the observation images after correcting the positional deviation of the scanning lines are integrated, the observation images having no distortion are superimposed, and blurring as described in FIG. 3 is reduced. be able to.

<Embodiment 3: Summary>
As described above, the scanning electron microscope 28 according to the third embodiment can reduce the roughness of the observation image by superimposing and integrating the corrected observation images. In addition, since the integration process is performed after correction by the method described in the first and second embodiments, blurring as described in FIG. 3 can be suppressed.

<Embodiment 4>
In the first to third embodiments, it has been described that the sample stage 26 is moved by the movement of the sample stage 26 during the irradiation of the charged particle beam 3 and the observed image is distorted. This is because the sample stage 26 is moved to move the observation field of view.

  Similar distortion of the observation image is caused by a change in the irradiation direction of the charged particle beam 3. For example, instead of moving the sample stage 26 in order to move the observation field of view, it is conceivable to change the deflection direction of the charged particle beam 3.

  In any case, the technique according to the present invention can be used as a technique for correcting the distortion generated in the observed image.

<Embodiment 5>
As mentioned above, although embodiment which concerns on this invention was described concretely, it cannot be overemphasized that this invention is not limited to these embodiment, and can be variously changed in the range which does not deviate from the summary.

  In addition, each of the above-described configurations, functions, processing units, etc. can be realized as hardware by designing all or a part thereof, for example, with an integrated circuit, or the processor executes a program for realizing each function. By doing so, it can also be realized as software. Information such as programs and tables for realizing each function can be stored in a storage device such as a memory or a hard disk, or a storage medium such as an IC card or a DVD.

  1: main body, 2: charged particle beam source, 3: charged particle beam, 4: charged particle beam aperture, 5: first focusing lens, 6: second focusing lens, 7: deflection coil, 8: field moving coil, 9 : Objective lens, 10: secondary electron detector, 11: sample, 12: control unit, 13: charged particle beam control unit, 13b: objective lens driving unit, 14: system control unit, 15: amplifier, 16: image processing , 17: A / D converter, 18: Pixel integration unit, 19: Frame integration unit, 20: Image memory, 21: Image transmission unit, 22: Computer, 23: Keyboard, 24: Mouse, 25: Display, 26 : Sample stage, 27: Stage control unit, 29: Moving direction, 30-34: Position of circular pattern, 35-39: Scan line, 40: Ellipse pattern, 42-43: Position, 44-46: Region, 47: One eye frame, 48-49: scanning line, 50: movement direction, 51: first frame 52: a scanning line, 53: shift amount, 54: observation image, 55: observation range.

Claims (6)

A method of obtaining an observation image of the sample by scanning the sample using a charged particle beam,
An irradiation position moving step of moving an observation field of view by moving the irradiation position of the charged particle beam on the sample while scanning the sample with the charged particle beam;
Generating an observation image of the sample for each scanning line on the sample;
Obtaining the observed image for a plurality of frames;
Obtaining a difference between images corresponding to the scanning lines for the plurality of frames;
Identifying a misaligned scan line in which the position of an image corresponding to the scan line is different between the plurality of frames among the scan lines ;
Correcting the position of the observation image corresponding to the scan line excluding the misalignment scan line to the position of the misalignment scan line;
Counting the number of predetermined patterns on the sample using the corrected observation image;
The observation image acquisition method characterized by having. Identifying a shift direction between the plurality of frames based on the difference;
Correcting the scanning direction of the scanning line to be parallel to the displacement direction;
The observation image acquisition method according to claim 1, wherein: The observation image acquisition method according to claim 1, further comprising a step of generating a new observation image by superimposing observation images for the plurality of frames after correction at the same position. The observation image according to claim 1, wherein, in the irradiation position moving step, the irradiation position of the charged particle beam on the sample is moved by moving the position of the stage on which the sample is placed. Acquisition method. The observation image acquisition method according to claim 1, wherein in the irradiation position moving step, the irradiation position of the charged particle beam on the sample is moved by deflecting an irradiation direction of the charged particle beam. A charged particle beam source for irradiating the sample with a charged particle beam;
A stage on which the sample is placed;
A deflector for deflecting the irradiation direction of the charged particle beam;
A detector for detecting electrons emitted from the sample;
An image processing unit for generating an observation image of the sample based on a detection result of the detector;
A control unit for controlling the operation of the stage and the deflector;
With
The controller is
An irradiation position moving step of moving an observation field of view by moving the irradiation position of the charged particle beam on the sample while scanning the sample with the charged particle beam;
Generating an observation image of the sample for each scanning line on the sample;
Obtaining the observed image for a plurality of frames;
Obtaining a difference between images corresponding to the scanning lines for the plurality of frames;
Identifying a misaligned scan line in which the position of an image corresponding to the scan line is different between the plurality of frames among the scan lines ;
Correcting the position of the observation image corresponding to the scan line excluding the misalignment scan line to the position of the misalignment scan line;
Counting the number of predetermined patterns on the sample using the corrected observation image;
A scanning electron microscope characterized in that

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