Multi-beam electronic scanning

Abstract

The multi-beam electronic scanning system uses swath scanning. The system includes an electron emission source configured to emit an illumination beam. The illumination beam is split into multiple electron beams by a beam splitter lens array. The system also includes an electronic deflection system configured to deflect each of the electron beams along two different axes in multiple directions, including a first direction. Finally, a swath stage is used to move the specimen at a constant velocity in a second direction parallel to the first direction.

Description

[Technical Field]

[0001] The present disclosure relates generally to the field of wafer inspection systems, and more particularly to multi-beam scanning techniques. [Background technology]

[0002] REFERENCE TO RELATED APPLICATIONS This application claims the benefit of a prior application, U.S. Provisional Application No. 63 / 129,559, filed February 22, 2020, by Tomas Plettner et al., which is incorporated herein by reference in its entirety for all purposes.

[0003] Generally, the semiconductor manufacturing industry involves highly complex techniques for producing integrated circuits using semiconductor materials that are deposited and patterned on a substrate, such as silicon. With increasing levels of circuit integration and shrinking sizes of semiconductor devices, the manufactured devices are becoming increasingly sensitive to defects; that is, the defects that cause device failure are becoming smaller and smaller. Devices generally need to be free of defects prior to shipment to end users or customers.

[0004] Conventional electronic scanning uses a simple single-beam swath technique. However, single-beam scanning systems are slow and inefficient because only a single beam is used to scan a large sample. Therefore, some scanning approaches involve multi-column techniques. However, the use of multi-column techniques creates new problems. For example, multi-column scanning systems utilize multiple columns of electron beams. However, each column must be spaced apart from another column due to the space requirements for each column. Furthermore, because each column requires its own components, using a multi-column system is much more costly and complicated. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] U.S. Patent Application Publication No. 2018 / 0031498 Summary of the Invention [Problem to be solved by the invention]

[0006] Therefore, there is a need for an improved electronic scanning system that addresses the above-mentioned problems. [Means for solving the problem]

[0007] The following presents a simplified summary of the disclosure in order to provide a basic understanding of certain embodiments of the disclosure. This summary is not an extensive overview of the disclosure, and it does not identify key / critical elements of the disclosure or delineate the scope of the disclosure. Its sole purpose is to present some concepts disclosed herein in a simplified form as a prelude to the more detailed description that is presented later.

[0008] One aspect of the present disclosure is directed to a system. The system includes an electron emitter source configured to emit an illumination beam. The system also includes a beam splitter lens array configured to split the illumination beam into multiple electron beams. The system further includes an electron deflection system configured to deflect each of the electron beams in multiple directions, including a first direction, along two different axes. Finally, the system includes a swath stage configured to move the specimen at a constant velocity in a second direction parallel to the first direction.

[0009] Another aspect of the present disclosure is directed to a method and a computer-readable medium storing instructions for performing the method. The method includes first emitting an illumination beam from an electron emitter source. Next, the method includes splitting the illumination beam into multiple electron beams using a beam splitter lens array. Third, the method includes deflecting each of the electron beams along two different axes in multiple directions, including a first direction, via an electron deflection system. Finally, the method includes moving the sample on a swath stage at a constant velocity in a second direction parallel to the first direction.

[0010] In some embodiments, the electron deflection system is further configured to deflect each electron beam along a first direction for a predetermined length of time or a predetermined scan distance. In some embodiments, the electron deflection system is further configured to move the electron beam in a third direction orthogonal or substantially orthogonal to the first direction after the electron deflection system has deflected the electron beam for a predetermined length of time or to the end of the predetermined scan distance. In some embodiments, the electron deflection system is further configured to move the electron beam in a fourth direction parallel to the second direction after the electron beam has moved in the third direction. In some embodiments, the electron deflection system is configured to write scan lines in a group of stacked rectangles from a perspective of the sample, each rectangle including a predetermined number of scan lines. In some embodiments, each rectangle represents a scan area of ​​the sample, and each line adjacent to another line is written in the opposite direction to the other line. In some embodiments, the rectangles are written as parallelograms from a perspective of the electron deflection system to account for the constant velocity of the swath stage.

[0011] These and other aspects of the present disclosure are further described below with reference to the drawings. [Brief explanation of the drawings]

[0012] [Figure 1] 1 illustrates an exemplary single beam system with a conventional swath, according to an embodiment of the present disclosure. [Figure 2] 1 illustrates an exemplary multi-column system with a conventional swath, according to an embodiment of the present disclosure. [Figure 3A] FIG. 1 illustrates a multi-beam system with conventional swathing, according to an embodiment of the present disclosure. [Figure 3B] FIG. 1 is a top view of a conventional multi-beam system with swaths, according to an embodiment of the present disclosure. [Figure 4A] FIG. 1 illustrates a multi-beam system with parallel swaths according to an embodiment of the present disclosure. [Figure 4B] FIG. 1 illustrates a top view of a multi-beam system with parallel swaths according to an embodiment of the present disclosure. [Figure 5A] 1 illustrates one swath unit from the perspective of the sample, according to an embodiment of the present disclosure. [Figure 5B] 1 illustrates one swath unit from the perspective of a deflection system according to an embodiment of the present disclosure. [Figure 6] 1 illustrates scan line writing for a multi-beam system using conventional swathing, according to an embodiment of the present disclosure. [Figure 7] 1 illustrates scan line writing for a multi-beam system using parallel swathing, according to an embodiment of the present disclosure. [Figure 8] 1 illustrates an example of a multi-beam electronic scanning system according to an embodiment of the present disclosure. [Figure 9] FIG. 1 is a flow diagram illustrating a method for parallel electronic scanning according to an embodiment of the present disclosure. [Figure 10] 1 illustrates an example computer system according to an embodiment of the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0013] In the following description, numerous specific details are set forth to provide a thorough understanding of the present disclosure. The present disclosure may be practiced without some or all of these specific details. In other instances, well-known process operations have not been described in detail in order to not unnecessarily obscure the present disclosure. While the present disclosure will be described in conjunction with specific embodiments, it will be understood that it is not intended to limit the disclosure to the embodiments.

[0014] Defects in wafers can be detected using high-resolution distributed probe inspection tools in the form of an SEM with one or more distributed miniature columns, multiple atomic force microscope (AFM) probes, near-field microwave tools with multiple probes, ion beam probes, or multiple proximal optical probes.

[0015] Conventional electronic scanning typically involves a single beam, a single-axis deflector, and a swath stage that holds a sample, e.g., a wafer. FIG. 1 illustrates an exemplary single-beam system 100 with a conventional swath, according to an embodiment of the present disclosure. As shown in FIG. 1, a single electron beam 102 is emitted from a source and deflected by an electron deflector 104. In some embodiments, the electron deflector 104 comprises a single-axis deflection system. As shown in FIG. 1, the deflection system 104 deflects the beam 102 in a direction 112 that is orthogonal to a direction of movement 114 of a swath stage 106 that holds a sample 108. In some embodiments, the swath stage 106 moves at a constant velocity. In some embodiments, the velocity is on the order of a few millimeters per second. In some embodiments, to account for the constant velocity, the deflection system 104 actually deflects a line 116 in a diagonal direction that is substantially orthogonal to the stage direction 114. Here, "substantially" refers to a vector component that is 50% or more of the sum of the vector components. For example, a first vector that is perfectly orthogonal to a base vector and a second vector that forms a 45-degree angle with the base vector are still considered to be substantially orthogonal because the second vector can be decomposed into two vectors of equal magnitude, one parallel to the base vector and one perpendicular to it. However, a third vector that forms a 22.5-degree angle with the base vector is not considered to be substantially orthogonal because the component parallel to the base vector is twice as large as the component perpendicular to it.

[0016] The single beam system 100 of FIG. 1 can "write" long scan lines. "Scanning a line" refers to deflecting the electron beam to strike the sample and then moving the beam in one direction along the line. After striking either edge 110 of the line (which can be the edge of the stage or sample, or a predetermined stopping point for the line), the deflection system retracts the electron beam to the starting position and begins writing or scanning again. While the single beam system 100 allows for a larger write-to-retrace time ratio, a single beam may not provide sufficient throughput and therefore may not be the most efficient system for electronically scanning a sample. Attempting to scan an entire sample with a single beam system is similar to painting a wall with a single paintbrush bristle.

[0017] In some embodiments, a multi-column approach may address some of the deficiencies of system 100, as a single electron beam system may be insufficient. FIG. 2 illustrates an exemplary multi-column system 200 with conventional swathing, according to an embodiment of the present disclosure. System 200 is similar to system 100, except that there are multiple electron beam columns writing simultaneously. Thus, system 200 includes two different electron sources, each emitting a single beam, such as beams 202 and 222. Each column also includes its own deflector, such as deflectors 204 and 224 for write line 216. Note that FIG. 2 illustrates two separate sections of the same swath stage 206 holding the same sample 208. However, separate sections of the stage are shown in FIG. 2 because each column is far away from the other columns. This is because each column requires additional hardware, such as deflectors, and there is not enough space for multiple columns to fit next to each other if the columns are spaced closely together. Furthermore, having multiple copies of the same hardware is more expensive from a manufacturing and assembly standpoint, which leads to inefficiencies. While multi-column system 200 is an improvement over single beam system 100, scanning a sample with a multi-column system is similar to painting a wall with multiple individual paint brushes with only a single bristles per paint brush.

[0018] As mentioned above, single-beam and multi-column systems have several drawbacks. One approach to addressing these drawbacks is to use a multi-beam system, such as that described in U.S. Pat. No. 8,362,425, the entire contents of which are incorporated herein by reference. Multi-beam systems allow multiple beams to scan areas of a sample that are close to each other. However, because the beams in a multi-beam system are positioned very closely together, conventional swaths using a multi-beam system result in a very small write-to-retrace time ratio. This small ratio reduces the efficiency of the writing / scanning system.

[0019] FIG. 3A illustrates a multi-beam system 300 with conventional swathing, in accordance with an embodiment of the present disclosure. The multi-beam system 300 is similar to the single-beam system 100, except that an illumination beam is split into multiple beams 302 using a beam splitter. In addition, an electronic deflector 304 deflects each of the multiple beams 304 onto a sample 308 positioned on a swath stage 306. As in the system 100, the swath stage 306 moves at a constant velocity in a direction 314. In conventional swathing, the deflection system 304 writes a single line for each beam 304 in a direction 312 that is orthogonal or substantially orthogonal to the direction 314. However, because the beams 304 are positioned closely together, the width of the written "line" is much narrower than the "line" written in FIG. 1. Therefore, the ratio of writing time to return time is much smaller, thereby leading to inefficient writing or scanning.

[0020] FIG. 3B is a top view of a conventional swath multi-beam system 300 according to an embodiment of the present disclosure. The perspective in FIG. 3B is looking straight down from the emitter's perspective onto the sample 308 and stage 306. The beams 302 trace a straight line from the sample's perspective. As with system 100, a deflection system deflects each beam 302 at an angle that accommodates the constant velocity of the stage 306. As shown in FIG. 3B, each beam 302 writes its respective line in parallel or simultaneously with the other beams. Also, as can be seen in FIG. 3B, retrace times are more pronounced in this configuration due to the shorter width of the scan lanes.

[0021] Because conventional swath scanning using multi-beam systems results in a low write-to-fly ratio, other approaches can be considered. For example, multi-beam systems can utilize step-and-scan technology. In some embodiments, a step-and-scan multi-beam system uses a dual-axis deflector to scan a square frame on a sample positioned on a stationary stage. Then, after a square frame is scanned, the stage moves a predetermined amount to allow the deflection system to scan an adjacent square frame. This approach utilizes a dual-axis deflection system and enables two-dimensional scanning. However, this technique has two drawbacks. First, the scanned square area can be very large before encountering problems with the electronics and optics. If the square frame is too large, noise and distortion will occur. Second, because the sample and stage have real mass, starting the stage from a dead stop, then stopping the stage, and waiting for the stage to come to rest, including vibrations from the movement, creates a delay on the order of several milliseconds. This delay is very costly from an electronic scanning perspective. Therefore, a multi-beam system that can address this deficiency would be highly beneficial.

[0022] 4A illustrates a multi-beam system 400 with parallel swathing, according to an embodiment of the present disclosure. Similar to system 300, an emitter source emits an illumination beam that is split into multiple beams 402. An electronic deflector 404 then deflects the beam 402 onto a sample 408 on a swath stage 406 that moves in a direction 414. In some embodiments, the deflector 404 includes a dual-axis deflection system configured to deflect the beam 402 in a direction 412 that is parallel to the direction 414. While the electronic deflection system 304 writes lines in a direction substantially perpendicular to the direction 314, the electronic deflection system 404 writes frames in the direction 412 that are comprised of lines parallel to the direction 414. In some embodiments, each frame includes a predetermined amount of lines and has predetermined dimensions based on the arrangement of the beams 402. For example, if the system includes an array of 10 x 10 beams spaced at a 10 μm pitch, the frame width "x" is approximately 1 μm, and the length "y" can be as long as the scanning electronics and optics support, which in this case is 10 μm. Assuming a pixel size of 5 nm, a frame line has 2000 pixels in the "y" direction and 200 pixels in the "x" direction, which corresponds to 200 scan lines. Because the scan lines within a frame are much longer in length than the orthogonal lines in system 300, the ratio of write time to retrace time is much greater in system 400. Deflection system 404 is also configured to stack the current frame 418 on top of the previously written frame 416 to scan the entire sample 408. In some embodiments, the "fast" axis of the deflector is in the y-axis (e.g., parallel to the stage motion) and the "slow" axis is in the x-axis. In some embodiments, the electronic raster scanning functions are no different in form from those found in other typical SEMs; they are shaped as a sawtooth function with a characteristic linear ramp region and a return time where the sawtooth function is reset. In other embodiments, the electronic raster scanning function will be configured to raster scan in a serpentine motion rather than a sawtooth motion, so that each line adjacent to another line is written in the opposite direction to the other line. In such embodiments, the return time is further reduced.

[0023] 4B is a top view of a multi-beam system 400 with parallel swaths, according to an embodiment of the present disclosure. Like FIG. 3B, FIG. 4B is a top view from the perspective of the emitter source. Each rectangular frame is written by a beam 402. The system 400 is configured to stack a current frame 418 on top of a previous frame 416. Like system 300, each beam 402 writes a line simultaneously with the other beams 402.

[0024] 5A shows one swath unit from the perspective of the sample, according to an embodiment of the present disclosure. Frame 518 appears as a rectangle from the perspective of the sample because the deflection system accounts for the movement of the stage. FIG. 5B shows one swath unit from the perspective of the deflection system, according to an embodiment of the present disclosure. Frame 518 resembles a parallelogram, accounting for the movement of the stage in direction 514.

[0025] In some embodiments, because the stage is continuously moving, each line must counter the shift the stage accumulated over the time a line was written. In such embodiments, the actual video frame is not a rectangular frame, but rather a parallelogram, whose tilt precisely counters the stage swath velocity. In such embodiments, this would produce a rectangular image frame. In other embodiments, the scanning electronics actually scan a rectangle. However, in such embodiments, the actual image frame is a tiled parallelogram instead of a rectangle. Systems incorporating image frames that are parallelograms must include modifications to image analysis algorithms to account for parallelogram-shaped pixels, since standard image analysis algorithms typically assume square pixels.

[0026] According to various embodiments, the width of the written line or frame varies depending on the physical arrangement of the multi-beam array. In some embodiments, the 2D array of beams is essentially folded into a single dimension by movement of the swath stage. FIG. 6 illustrates scan line writing for a multi-beam system using conventional swathing in accordance with an embodiment of the present disclosure. As shown in FIG. 6, beams 622 and 624 are physically located vertically below beam 602, but movement of the swath stage effectively changes the 2D arrangement into a single row of beams. The width of line 616 is limited and / or defined by the physical arrangement of the multi-beams. FIG. 7 illustrates scan line writing for a multi-beam system using parallel swathing in accordance with an embodiment of the present disclosure. The arrangement of the multi-beams is the same as in FIG. 6. The width of each rectangular frame 716 and 718 is defined by the arrangement of beams 702, 722, and 742. As in FIG. 6, movement of the swath stage effectively changes the 2D arrangement of beams into a single row of beams. As can be seen from Figures 6 and 7, parallel swaths result in much more efficient scan lines.

[0027] 8 illustrates an example of a multi-beam electronic scanning system according to an embodiment of the present disclosure. System 800 includes an electron emitter source 850. Electron emitter source 850 emits an illumination beam 852. System 800 also includes a beam splitter lens array 840 configured to split illumination beam 852 into multiple electron beams 802. Multiple beams 802 are then deflected onto a sample 808 by an electron deflector 804. In some embodiments, electron deflector 804 is a scanning deflector configured to scan the sample using the methods described above.

[0028] 9 is a flow diagram illustrating a method for parallel electronic scanning according to an embodiment of the present disclosure. Method 900 begins with emitting an illumination beam from an electron emitter source (902). The illumination beam is then split into multiple electron beams using a beam splitter lens array (904). Next, each of the electron beams is deflected in multiple directions along two different axes via an electronic deflection system (906). In some embodiments, the multiple directions include a first direction. Finally, the sample is moved on the swath stage at a constant velocity in a second direction (908). In some embodiments, the second direction is parallel to the first direction.

[0029] In some embodiments, the electron deflection system is further configured to deflect each electron beam along a first direction for a predetermined length of time or a predetermined scan distance. In some embodiments, the electron deflection system is further configured such that after the electron deflection system has deflected the electron beam for a predetermined length of time or to the end of the predetermined scan distance, the electron deflection system moves the electron beam in a third direction that is orthogonal or substantially orthogonal to the first direction. In some embodiments, the electron deflection system is further configured such that after the electron beam has moved in the third direction, the electron deflection system moves the electron beam in a fourth direction that is parallel to the second direction. In some embodiments, the electron deflection system is configured to write scan lines in a group of stacked rectangles from a perspective of the sample, each rectangle including a predetermined number of scan lines. In some embodiments, each rectangle represents a scan area of ​​the sample, and each line adjacent to another line is written in the opposite direction to the other lines. In some embodiments, each line is scanned in the same direction. In some embodiments, the rectangles are written as parallelograms from a perspective of the electron deflection system at a constant velocity of the swath stage.

[0030] 10 illustrates an example of a computer system according to embodiments of the present disclosure. According to certain embodiments, a system 1000 suitable for implementing certain embodiments of the present disclosure includes a processor 1001, a memory 1003, an interface 1011, and a bus 1015 (e.g., a PCI bus or other interconnect fabric). When operating under the control of appropriate software or firmware, the processor 1001 is responsible for implementing applications such as an operating system kernel, a containerized storage driver, and one or more applications. Various specially configured devices may also be used in place of or in addition to the processor 1001. The interface 1011 is typically configured to send and receive data packets or data segments over a network.

[0031] Specific examples of supported interfaces include Ethernet interfaces, Frame Relay interfaces, Cable interfaces, DSL interfaces, Token Ring interfaces, etc. Additionally, a variety of very high speed interfaces may be provided, such as Fast Ethernet interfaces, Gigabit Ethernet interfaces, ATM interfaces, HSSI interfaces, POS interfaces, FDDI interfaces, etc. Generally, these interfaces may include ports suitable for communication with an appropriate medium. In some cases, they may also include a separate processor, possibly volatile RAM. The separate processor may control communication-intensive tasks such as packet switching, media control and management, etc.

[0032] According to various embodiments, system 1000 is a computer system configured to implement the electronic scanning system as described herein. In some embodiments, processor 1001 is configured to execute all steps of the methods described above, as well as any functions or processes necessary to implement the above or the processes described above. In some implementations, one or more of the computer components may be virtualized. For example, a physical server may be configured in a localized environment or a cloud environment. A physical server may implement one or more virtual server environments in which the electronic scanning system executes. While a specific computer system is described, it should be recognized that various alternative configurations are possible. For example, modules may be implemented on separate devices connected to the computer system.

[0033] Certain embodiments of the present disclosure presented herein generally address the field of semiconductor inspection and process control and are not limited to the hardware, algorithmic / software implementations and architectures, and use cases summarized above.

[0034] Although the foregoing disclosure has been described in some detail for purposes of clarity of understanding, it will be apparent that certain changes and modifications may be practiced within the scope of the appended claims. It should be noted that there are many alternative ways of implementing the processes, systems, and apparatus of the present disclosure. Accordingly, the present embodiments should be considered illustrative rather than restrictive, and the disclosure is not limited to the details given herein.

Claims

1. 1. A system comprising: an electron emission source configured to emit an illumination beam; a beam splitter lens array configured to split the illumination beam into a plurality of electron beams; an electronic deflection system configured to deflect each of the electron beams in a plurality of directions, including a first direction, along two different axes; a swath stage configured to move the specimen at a constant velocity in a second direction parallel to the first direction while scan lines are written by the electron beams, the scan lines being written by each electron beam parallel to the second direction in a rectangular pattern, a length of the rectangular pattern along the first direction being greater than a width of the rectangular pattern; A system comprising:

2. 10. The system of claim 1, wherein the electron deflection system is further configured to deflect each electron beam along the first direction over a predetermined length of time or a predetermined scan distance.

3. 3. The system of claim 2, wherein the electronic deflection system is further configured to move the electron beam in a third direction orthogonal or substantially orthogonal to the first direction after deflecting the electron beam to the end of a predetermined length of time or a predetermined scan distance.

4. 4. The system of claim 3, wherein the electron deflection system is further configured to move the electron beam in a fourth direction parallel to the second direction after the electron beam has moved in the third direction.

5. 2. The system of claim 1, wherein the electronic deflection system is configured to write scan lines in a group of stacked rectangles from a viewpoint of the specimen, each rectangle including a predetermined number of scan lines.

6. 6. The system of claim 5, wherein each said rectangle represents a scanned area of ​​the sample, and wherein a line adjacent to another line is written in an opposite direction to the other line.

7. 7. The system of claim 6, wherein the rectangle is written as a parallelogram from the perspective of the electronic deflection system and a constant velocity of the swath stage.

8. 1. A method comprising: emitting an illumination beam from an electron emission source; splitting the illumination beam into a plurality of electron beams using a beam splitter lens array; deflecting each of the electron beams via an electronic deflection system in a plurality of directions including a first direction along two different axes; moving the specimen on a swath stage at a constant velocity in a second direction parallel to the first direction while writing scan lines with the electron beams, the scan lines being written by each electron beam parallel to the second direction in a rectangular pattern, a length of the rectangular pattern along the first direction being greater than a width of the rectangular pattern; A method for providing the above.

9. 9. The method of claim 8, wherein the electron deflection system is further configured to deflect each electron beam along the first direction over a predetermined length of time or a predetermined scan distance.

10. 10. The method of claim 9, wherein the electronic deflection system is further configured to move the electron beam in a third direction orthogonal or substantially orthogonal to the first direction after deflecting the electron beam to the end of a predetermined length of time or a predetermined scan distance.

11. 11. The method of claim 10, wherein the electron deflection system is further configured to move the electron beam in a fourth direction parallel to the second direction after the electron beam has moved in the third direction.

12. 9. The method of claim 8, wherein the electronic deflection system is configured to write scan lines in a group of stacked rectangles from a viewpoint of the specimen, each rectangle including a predetermined number of scan lines.

13. 13. The method of claim 12, wherein each rectangle represents a scanned area of ​​the sample, and a line adjacent to another line is written in an opposite direction to the other line.

14. 14. The method of claim 13, wherein the rectangle is written as a parallelogram from the perspective of an electronic deflection system and a constant velocity of the swath stage.

15. A computer readable medium storing instructions for causing a computer processor to perform a method, the method comprising: emitting an illumination beam from an electron emission source; splitting the illumination beam into a plurality of electron beams using a beam splitter lens array; deflecting each of the electron beams via an electronic deflection system in a plurality of directions including a first direction along two different axes; moving the specimen on a swath stage at a constant velocity in a second direction parallel to the first direction while writing scan lines with the electron beams, the scan lines being written by each electron beam parallel to the second direction in a rectangular pattern, a length of the rectangular pattern along the first direction being greater than a width of the rectangular pattern; 13. A computer-readable medium comprising:

16. 16. The computer-readable medium of claim 15, wherein the electronic deflection system is further configured to deflect each electron beam along a first direction for a predetermined length of time or a predetermined scan distance.

17. 17. The computer-readable medium of claim 16, wherein the electronic deflection system is further configured to move the electron beam in a third direction orthogonal or substantially orthogonal to the first direction after deflecting the electron beam to the end of a predetermined length of time or a predetermined scan distance.

18. 20. The computer-readable medium of claim 17, wherein the electron deflection system is further configured to move the electron beam in a fourth direction parallel to the second direction after the electron beam has moved in the third direction.

19. 16. The computer-readable medium of claim 15, wherein the electronic deflection system is configured to write scan lines in a group of stacked rectangles from a viewpoint of the specimen, each rectangle including a predetermined number of scan lines.

20. 20. The computer-readable medium of claim 19, wherein each said rectangle represents a scanned area of ​​the sample, and a line adjacent to another line is written in an opposite direction to the other line.

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