A critical size scanning electron microscope standard sample, calibration device and calibration method

Abstract

The present invention discloses a critical dimension scanning electron microscope (CD-SEM) standard sample, a calibration device, and a calibration method, comprising: a critical dimension scanning electron microscope standard sample, the standard sample being divided into four different regions S1, S2, S3, and S4, wherein the minimum structural dimensions of the patterns in the four regions are different; each of the four different regions S1, S2, S3, and S4 being further divided into four regions P1, P2, P3, and P4, wherein the patterns are four different shapes; the four different shapes being further divided into upper and lower parts, which are two different pattern types. A calibration device, using the standard sample, is characterized in that it comprises: a sample holding device, a standard sample, a sample to be measured, and a three-dimensional motion platform; the present invention also discloses a critical dimension scanning electron microscope calibration method, which can be used to timely and efficiently calibrate the CD-SEM and achieve measurement of the critical dimensions of the sample to be measured.

Description

Technical Field

[0001] The present invention relates to the technical field of scanning electron microscopes, and in particular to a calibration device and a calibration method for a critical dimension measurement scanning electron microscope. Background Art

[0002] A critical dimension scanning electron microscope (CD-SEM) is an instrument used in the semiconductor manufacturing industry to measure the critical dimensions of patterns fabricated on wafers. With the advancement of semiconductor manufacturing technology, the critical dimensions of patterns on wafers are shrinking, placing increasing demands on CD-SEM measurement accuracy. The basic principle of CD-SEM measurement is that an extremely fine electron beam, focused on a nanometer diameter, is scanned across the sample under test. The intensity of the generated signal electrons (secondary electrons or backscattered electrons) varies with the scanning position of the electron beam. These signal electrons are received by corresponding detectors. The signal changes with position, and the signal processing system forms a two-dimensional grayscale image of the CD-SEM. The grayscale value of a pixel is related to the signal intensity generated at that location, and the size of a pixel corresponds to the actual physical size of a scan point. By analyzing the grayscale profile of a structure in the grayscale image, the number of pixels occupied by the structure is determined. Multiplying the number of pixels by the size of a single pixel yields the critical dimension of the structure.

[0003] To ensure accurate CD-SEM measurements, the CD-SEM needs to be calibrated. This involves determining the actual size of individual dimensions in the image. Typically, CD-SEM calibration involves measuring a pattern with known line width (e.g., lines with a fixed spacing) and measuring the number of pixels occupied by this fixed-pitch pattern in the CD-SEM two-dimensional image to obtain the calibrated pixel size. However, due to the signal generation mechanism of scanning electron microscopes, CD-SEM images vary depending on the size, shape, and type of relief of the observed pattern. Therefore, even if the calibration patterns have the same nominal size, the grayscale profiles they create in the CD-SEM two-dimensional image can differ. In particular, when structures are on the order of hundreds of nanometers or smaller, the grayscale profiles of different shapes can differ significantly at the edges of the structures. Consequently, when the shape of the sample being measured differs from that of the calibration pattern, calibration and measurement errors can occur. While no prior art addresses this issue, the present invention proposes a standard sample that allows for targeted calibration based on the structural shape of the sample being measured.

[0004] The measurement accuracy of a calibrated CD-SEM decreases after a period of use. Therefore, the CD-SEM needs to be recalibrated at regular intervals when the accuracy does not meet the requirements. In the prior art, CD-SEM calibration requires removing the sample to be measured and replacing it with a dedicated calibration sample. This process is relatively cumbersome and it is not easy to detect in a timely manner whether the CD-SEM has deviated from the qualified calibration accuracy. To address this problem, the present invention proposes a calibration device and calibration method for a critical dimension scanning electron microscope. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to overcome the deficiencies of the prior art and provide a critical dimension scanning electron microscope standard sample, a calibration device and a calibration method, in order to improve the efficiency of CD-SEM calibration and achieve the purpose of efficiently measuring the sample to be measured.

[0006] The present invention discloses a critical size scanning electron microscope standard sample, which is characterized by:

[0007] The standard sample is divided into four different areas S1, S2, S3 and S4, and the minimum structural dimensions of the patterns in the four areas are different; each of the four different areas S1, S2, S3 and S4 is divided into four areas P1, P2, P3 and P4, and the patterns therein are four different shapes; the four different shapes are further divided into two parts, upper and lower, which are two different pattern types.

[0008] Optionally, the minimum structural dimensions in the four different regions S1, S2, S3 and S4 are respectively: the minimum structural dimension of the S1 region is on the order of hundreds of nanometers, with a typical dimension of 100 nm; the minimum structural dimension of the S2 region is on the order of several microns, with a typical dimension of 1 μm; the minimum structural dimension of the S3 region is on the order of ten microns, with a typical dimension of 10 μm; the minimum structural dimension of the S4 region is on the order of hundreds of microns, with a typical dimension of 100 μm.

[0009] Optionally, the shapes of the four regional patterns P1, P2, P3 and P4 are respectively: P1 is a line shape; P2 is a square shape; P3 is a cross shape; and P4 is a circle shape.

[0010] Optionally, the four different shapes are divided into two parts, an upper part and an lower part, which are two different pattern types. The pattern type of the upper part is a convex type, and the pattern type of the lower part is a concave type.

[0011] Optionally, the number of regions classified by minimum structural size and pattern shape is not limited to 4, and the standard sample can be divided into M (M≧2) different regions S1, S2, ... S M, the minimum structural size of the patterns in the M regions is different; each of the M regions is further divided into N (N≧2) regions P1, P2, ... P N , the patterns in these N regions have different shapes.

[0012] At the same time, in order to realize the technical solution, the present invention also discloses a critical dimension scanning electron microscope calibration device, which is characterized by comprising: a sample carrying device, a standard sample, and a three-dimensional motion platform;

[0013] The sample carrying device can simultaneously mount the standard sample and the sample to be measured;

[0014] The sample carrying device is mounted on the three-dimensional motion platform;

[0015] At the same time, in order to implement this technical solution, the present invention also discloses a critical dimension scanning electron microscope calibration method, the method comprising:

[0016] Step 1: Control the movement of the three-dimensional motion platform so that the electron beam irradiates the position of the sample to be measured, adjust the focusing current parameters of the electron optical lens barrel, and obtain a clear image of the sample to be measured;

[0017] Step 2: Based on the image obtained in Step 1, the minimum structural size of the sample pattern to be measured is obtained, the pattern shape of the sample to be measured is identified, and the position coordinates X0, Y0, and Z0 of the current sample are recorded;

[0018] Step 3: Based on the minimum structural size, shape, and type information of the sample pattern to be measured obtained in Step 2, find the calibration pattern of the corresponding standard sample and obtain the position information of the calibration pattern;

[0019] Step 4: Based on the standard sample pattern position information obtained in Step 3, control the translation stage to move to the standard sample calibration pattern position in the X and Y directions;

[0020] Step 5: Keep the electron optical lens focusing current parameters unchanged, adjust the position of the translation stage in the Z direction, obtain a clear image of the calibration pattern on the standard sample, measure the critical dimensions of the obtained calibration pattern, and compare them with the standard values ​​of the critical dimensions of the calibration pattern to complete the CD-SEM calibration;

[0021] Step 6: Control the three-dimensional motion platform to move in the X, Y, and Z directions and return to the original position X0, Y0, and Z0 of the sample to be measured;

[0022] Step 7: Acquire an image of the sample pattern to be measured again, measure the critical dimensions of the pattern, and obtain the calibrated measurement dimensions.

[0023] Optionally, when the minimum line widths of the samples to be measured are similar and their shapes and types are the same, only one calibration is required using a standard sample. When imaging the entire sample to be measured, the same electron optical lens parameters can always be used for imaging. At this time, only the Z-direction height adjustment of the three-dimensional motion platform is required to ensure that the sample is within the depth of field of the electron optical lens.

[0024] Optionally, when obtaining a clear image of the standard sample pattern in Step 5, it is further characterized in that: after obtaining one image, the three-dimensional motion platform is controlled to move to obtain multiple images of the same pattern at nearby positions.

[0025] Optionally, when performing critical dimension measurement on the obtained calibration pattern in Step 5, the method is further characterized in that: measurement is performed at multiple locations on an image, and then the measurement results are averaged.

[0026] Optionally, when the critical dimension of the obtained calibration pattern is measured in Step 5, it is further characterized in that: multiple images obtained from the same pattern are measured, and then the average value of the measurement results is calculated. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] The accompanying drawings are part of the present invention and are used to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention but do not constitute improper limitations on the present invention.

[0028] Obviously, the drawings described below are only some embodiments. For those skilled in the art, other drawings can be obtained based on these drawings without inventive efforts. In addition, the drawings are schematic diagrams, and their size ratios are not necessarily the same as the actual design, nor do they limit the present invention in any way. In the drawings:

[0029] Figure 1 This is a schematic diagram of a critical-size scanning electron microscope standard sample provided by the present invention;

[0030] Figure 2 This is a schematic diagram of the size area of ​​a critical size scanning electron microscope standard sample S1 provided by the present invention;

[0031] Figure 3 Schematic diagram of two structural types of a critical-size scanning electron microscope standard sample provided by the present invention;

[0032] Figure 4 This is a schematic diagram comparing the grayscale contours of key dimension measurements of a linear pattern and a circular pattern of the same size;

[0033] Figure 5It is the intention of grayscale contour contrast of two types of critical dimension measurement of linear patterns of the same size, convex and concave;

[0034] Figure 6 This is a schematic diagram of a critical dimension scanning electron microscope calibration device provided by the present invention;

[0035] Figure 7 It is a schematic diagram of the critical dimension scanning electron microscope observation position switching between the sample to be measured and the standard sample;

[0036] Figure 8 This is a flow chart of a critical dimension scanning electron microscope calibration method provided by the present invention;

[0037] It should be noted that these drawings and textual descriptions are not intended to limit the conceptual scope of the present invention in any way, but rather to illustrate the concept of the present invention for those skilled in the art by referring to specific embodiments. DETAILED DESCRIPTION

[0038] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. The following embodiments are used to illustrate the present invention but are not used to limit the scope of the present invention.

[0039] like Figure 1 The figure shows a schematic diagram of a critical size scanning electron microscope standard sample provided by the present invention. Figure 2 and Figure 3 The standard sample is described in detail:

[0040] like Figure 1As shown, the standard sample 10 is divided into four areas, represented by S1, S2, S3 and S4 respectively. The patterns of these four areas have four different minimum structural sizes, among which the minimum structural size of the pattern in the S1 area is on the order of hundreds of nanometers, with a typical size of 100nm; the minimum structural size of the pattern in the S2 area is on the order of several microns, with a typical size of 1μm; the minimum structural size of the pattern in the S3 area is on the order of ten microns, with a typical size of 10μm; the minimum structural size of the pattern in the S4 area is on the order of hundreds of microns, with a typical size of 100μm. The advantage of setting these four calibration patterns of different sizes is that for CD-SEM and other types of electron beam instruments, the control of the electron beam scanning field of view is generally controlled by adjusting the voltage applied to the electric deflector or the current applied to the magnetic deflector coil. For electron microscopes, the actual field of view size is not strictly linearly related to the voltage or current applied to the deflector. Therefore, calibration performed under a larger field of view (low magnification) is not suitable for measurement under a small field of view (high magnification), and vice versa. Therefore, it is necessary to select a calibration sample size that is closest to the size of the sample to be measured based on the size of the sample to be measured. Therefore, the four sizes of standard samples provided by the present invention can meet the calibration requirements when measuring samples of different sizes.

[0041] It should be noted that the standard sample proposed in the present invention can be divided into four regions of different sizes, but the number of regions is not limited to four, and can be two, three, or more than four. According to a characteristic embodiment of the present invention, the number of regions classified by minimum structural size and pattern shape is not limited to four, and the standard sample can be divided into M (M≧2) different regions S1, S2, ... S M , the minimum structural sizes of the patterns in the M regions are different; as long as the basic idea of ​​selecting the size of the calibration sample is based on the size of the sample to be measured, it is within the protection scope of the present invention.

[0042] Combine Figure 2 and Figure 3 , taking the S1 area of ​​the standard sample as an example, the specific structure of the standard sample is introduced in detail. Figure 2 As shown in Figure 1, the patterns in the standard sample S1 area are divided into four different shapes: P1, P2, P3 and P4: P1: line shape; P2: square shape; P3: cross shape; P4: circle shape. These four different shapes are further divided into two different types: convex type and concave type, as shown in Figure 1. Figure 3As shown, the two types of patterns are illustrated using the line shape of P1 as an example. The left part (a) is a top view of the line shape of P1, wherein the upper half area 301 is a convex type and the lower half area 302 is a concave type; the right part (b) is a cross-sectional view of the line shape of P1, which clearly shows the convex type 301a in the upper half area and the concave type 301b in the lower half area. For the S1 area, according to the present invention, the minimum structural size of the pattern in the area is on the order of hundreds of nanometers, with a typical size of 100 nm. Figure 3 Part (b) on the right shows this dimension marked.

[0043] It should be noted that the patterns of the standard samples described above are divided into four different shapes, namely P1, P2, P3 and P4, but the number of pattern shapes is not limited to four and can be two, three or more than four. According to a characteristic embodiment of the present invention, each of the M regions divided according to the minimum critical dimension of the pattern is further divided into N (N≧2) regions P1, P2, ... P N The patterns in these N regions have different shapes. Any basic idea of ​​selecting a calibration sample with a corresponding shape according to the shape of the sample to be measured falls within the scope of protection of the present invention.

[0044] The above uses the linear shape P1 as an example to illustrate the two types of patterns. For the other three shapes P2, P3 and P4, they are also divided into the convex type in the upper half and the concave type in the lower half, just like P1. Figure 4 and Figure 5 The advantages of setting these four shapes and two types of patterns are described below. Figure 4 As shown in the figure, 401a is a linear pattern, 401b is the grayscale profile of its corresponding CD-SEM image, 402a is a circular pattern, and 402b is the grayscale profile of its corresponding CD-SEM image. It can be seen that although the critical dimensions of the two patterns are the same, the grayscale profiles in the CD-SEM image are different. Therefore, there will be differences when using the two patterns to calibrate the CD-SEM. If the shape of the pattern to be measured is the same or similar to that of the calibration pattern, then their grayscale profiles are also the same or similar. Using a standard sample with the same or similar shape as the pattern to be measured for calibration will significantly reduce measurement deviation and improve measurement accuracy. Similar to the effect of different shapes on calibration, two types of concave and convex in the same pattern will also have similar effects, such as Figure 5As shown in the figure, taking the P1 linear pattern as an example, 501a is a convex type, and 501b is the grayscale profile of its corresponding CD-SEM image. It can be seen from the figure that although their critical dimensions are the same, both 100nm, their grayscale profiles in the CD-SEM image are quite different. Therefore, there will be deviations in the CD-SEM calibration due to these two types. Therefore, it is necessary to use the corresponding standard sample for calibration according to the type of pattern to be measured. If the pattern to be measured is a convex type, the pattern selected during CD-SEM calibration should also be a convex type. If the pattern to be measured is a concave type, the calibration pattern selected should also be a concave type, thereby improving calibration and measurement accuracy.

[0045] The present invention also provides a critical dimension scanning electron microscope calibration device, such as Figure 6 As shown, the device includes: a sample carrier 601, a standard sample 602, and a three-dimensional motion platform 603. The sample carrier 601 is mounted on the three-dimensional motion platform 603, on which a standard sample mounting position is set, on which the standard sample 602 is mounted. At the same time, the sample carrier 601 is provided with a mounting position for a sample to be measured, on which a sample to be measured 604 (a wafer sample with a pattern to be measured) is mounted. Figure 7 As shown, 701 is a schematic diagram of a CD-SEM imaging a sample to be measured, and 702 is a schematic diagram of a CD-SEM imaging a standard sample. In the figure, 701a illustrates the electron optical tube of the CD-SEM, and 701b is the converged electron beam emitted from the electron optical tube. The advantage of using the critical dimension scanning electron microscope calibration device provided by the present invention is that, in the prior art, CD-SEM calibration requires removing the sample to be measured and replacing it with a dedicated calibration sample, a cumbersome process and making it difficult to promptly detect whether the CD-SEM has deviated from the acceptable calibration accuracy. Using the calibration device of the present invention, because the standard sample and the sample to be measured are mounted on the same carrier, the CD-SEM can be calibrated conveniently, allowing for timely detection of CD-SEM calibration accuracy issues and the need for recalibration.

[0046] The present invention also provides a critical dimension scanning electron microscope calibration method:

[0047] As discussed above, different sizes of samples to be measured require different sizes of standard samples to be used for calibration; different shapes and types of samples to be measured require different shapes and types of standard samples for accurate calibration; and in industrial production, in order to promptly determine whether the measurement accuracy of the CD-SEM meets the requirements, it is necessary to perform online inspection and real-time calibration of the CD-SEM's measurement accuracy. Using the standard samples and calibration device proposed by the present invention described above, the present invention also proposes a critical dimension scanning electron microscope calibration method:

[0048] Step 1: Control the three-dimensional motion platform to make the electron beam irradiate the sample position to be measured (such as Figure 7 ), adjusting the electron optical lens tube focusing current and other parameters to obtain a clear image of the sample to be measured;

[0049] Step 2: Based on the image obtained in Step 1, obtain the minimum structural size of the sample pattern to be measured, identify the pattern shape of the sample to be measured, and record the position coordinates X0, Y0, and Z0 of the current sample; for example: the minimum structural size of the pattern to be measured is about 80nm, and the pattern shape is a circular, concave (hole) structure;

[0050] Step 3: Based on the minimum structural size, shape, and type information of the sample pattern to be measured obtained in Step 2, find the corresponding calibration pattern of the standard sample and obtain the position information of the calibration pattern. For example, if the minimum structural size of the sample pattern to be measured in Step 2 is 80 nm, the S1 area in the standard sample should be selected. Since the pattern shape is circular, the P4 shape should be selected. Since the pattern has a hole structure, the concave type in the lower half of the P4 shape should be selected.

[0051] Step 4: According to the standard sample pattern position information obtained in step 3, control the translation stage to move to the standard sample calibration pattern position in the X and Y directions (such as Figure 7 (as shown in 702);

[0052] Step 5: Keeping the electron optical lens focusing current and other parameters unchanged, adjust the position of the translation stage in the Z direction to obtain a clear image of the calibration pattern on the standard sample. Measure the critical dimensions of the obtained calibration pattern and compare them with the standard values ​​of the critical dimensions of the calibration pattern to complete the CD-SEM calibration.

[0053] Step 6: Control the three-dimensional motion platform to move in the X, Y, and Z directions and return to the original position X0, Y0, and Z0 of the sample to be measured;

[0054] Step 7: Acquire an image of the sample pattern to be measured again, measure the critical dimensions of the pattern, and obtain the calibrated measurement dimensions.

[0055] In the above method, optionally, when the minimum line width dimensions of the samples to be measured are close and the shapes and types are the same, only one calibration is required using a standard sample. When imaging the entire sample to be measured, the same electron optical lens parameters can always be used for imaging. At this time, only the height adjustment of the three-dimensional motion platform in the Z direction is used to ensure that the sample is within the depth of field of the electron optical lens.

[0056] In the above method, optionally, when obtaining a clear image of the standard sample pattern in Step 5, the three-dimensional motion platform can be controlled to move after obtaining one image to obtain multiple images of the same pattern at nearby positions.

[0057] In the above method, optionally, when measuring the critical dimension of the obtained calibration pattern in Step 5, measurements can be performed at multiple locations on an image, and then the average of these measurement results can be calculated.

[0058] In the above method, optionally, when measuring the critical dimension of the obtained calibration pattern in Step 5, multiple images of the same pattern may be measured, and then the average of the measurement results may be calculated.

[0059] Compared with the prior art, the present invention has the following advantages:

[0060] A standard sample for calibrating a CD-SEM is provided, featuring four different minimum structural dimensions, four different pattern shapes, and two structural types. The corresponding calibration sample pattern can be selected based on the size, shape, and type of the sample to be measured, enabling more accurate CD-SEM calibration. The present invention also provides a calibration device in which the standard sample and the sample to be measured are mounted on the same carrier, enabling convenient online CD-SEM calibration, allowing for timely detection of CD-SEM calibration accuracy issues and the need for recalibration.

[0061] The specific embodiments described above further illustrate the objectives, technical solutions and beneficial effects of the present invention in detail. It should be understood that the above description is only a specific embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A critical dimension scanning electron microscope standard sample, characterized by: The standard sample is divided into four different areas S1, S2, S3 and S4, and the minimum structural dimensions of the patterns in the four areas are different; each of the four different areas S1, S2, S3 and S4 is further divided into four areas P1, P2, P3 and P4, and the patterns therein are four different shapes: P1 is a line shape, P2 is a square shape, P3 is a cross shape, and P4 is a circle shape; the four different shapes are further divided into two parts, upper and lower, which are two different pattern types: the upper part pattern type is a convex type, and the lower part pattern type is a concave type.

2. The standard sample according to claim 1, wherein The minimum structural dimensions in the four different regions S1, S2, S3 and S4 are respectively: the minimum structural dimension in the S1 region is on the order of hundreds of nanometers; the minimum structural dimension in the S2 region is on the order of several microns; the minimum structural dimension in the S3 region is on the order of ten microns; and the minimum structural dimension in the S4 region is on the order of hundreds of microns.

3. The standard sample according to claim 1, wherein The number of regions classified by minimum structural size and pattern shape is not limited to 4. The standard sample is divided into M different regions S1, S2, ... S M , the minimum structural size of the patterns in the M regions is different; each of the M regions is further divided into N regions P1, P2, ... P N , the patterns in these N regions have different shapes; where M≧2, N≧2.

4. A critical dimension scanning electron microscope calibration device, using the standard sample according to claim 1, characterized in that: include: Sample carrying device, standard sample, sample to be measured, three-dimensional motion platform; The sample carrying device is equipped with the standard sample and the sample to be measured at the same time; The sample carrying device is mounted on the three-dimensional motion platform.

5. A critical dimension scanning electron microscope calibration method, applied to the critical dimension scanning electron microscope calibration device according to claim 4, the method comprising: Step 1: Control the movement of the three-dimensional motion platform so that the electron beam irradiates the position of the sample to be measured, adjust the focusing current parameters of the electron optical lens barrel, and obtain a clear image of the sample to be measured; Step 2: Based on the image obtained in Step 1, the minimum structural size of the sample pattern to be measured is obtained, the pattern shape of the sample to be measured is identified, and the position coordinates X0, Y0, and Z0 of the current sample are recorded; Step 3: Based on the minimum structural size, shape, and type information of the sample pattern to be measured obtained in Step 2, find the calibration pattern of the corresponding standard sample and obtain the position information of the calibration pattern; Step 4: Based on the standard sample pattern position information obtained in Step 3, control the translation stage to move to the standard sample calibration pattern position in the X and Y directions; Step 5: Keep the electron optical lens focusing current parameters unchanged, adjust the position of the translation stage in the Z direction, obtain a clear image of the calibration pattern on the standard sample, measure the critical dimensions of the obtained calibration pattern, and compare them with the standard values ​​of the critical dimensions of the calibration pattern to complete the CD-SEM calibration; Step 6: Control the three-dimensional motion platform to move in the X, Y, and Z directions and return to the original position X0, Y0, and Z0 of the sample to be measured; Step 7: Acquire an image of the sample pattern to be measured again, measure the critical dimensions of the pattern, and obtain the calibrated measurement dimensions.

6. According to the method described in claim 5, when the minimum line width dimensions of the samples to be measured are similar and the shapes and types are the same, only one calibration using a standard sample is required. When imaging the entire sample to be measured, the same electron optical lens parameters are always used for imaging. At this time, only the height adjustment of the three-dimensional motion platform in the Z direction is used to ensure that the sample is within the depth of field of the electron optical lens.

7. The method according to claim 5, wherein in step 5, when obtaining a clear image of the standard sample pattern, the method is further characterized by: After acquiring an image, the three-dimensional motion platform is controlled to move and multiple images of the same pattern are acquired at nearby locations.

8. The method according to claim 5, wherein in step 5, when performing critical dimension measurement on the obtained calibration pattern, the method is further characterized by: Measurements are taken at multiple locations on an image and the results of these measurements are averaged.

9. The method according to claim 5, wherein in step 5, when performing critical dimension measurement on the obtained calibration pattern, the method is further characterized by: Multiple images of the same pattern are measured and the results of these measurements are averaged.

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