A distortion-free scanning method for scanning probe microscope

Abstract

The present invention provides a distortion-free scanning method for a scanning probe microscope. The method still retains the traditional line-by-line scanning method, and realizes the bending of the atomic row during the drift process by a variable speed scanning method, thereby correcting the scanned image. The method abandons the complex spiral scanning method, the control signal is simpler, the scanning method is more diverse, and the later correction is more convenient. Compared with the traditional method of calibrating the microscope, the advantages are: no microscope calibration is required, no sample replacement is required, only a one-time scan is required, and the image distortion can be removed later. For different environments, the drift speed of the microscope is also different, and the present invention only requires a one-time scanning imaging.

Description

Technical Field

[0001] The invention belongs to the technical field of scanning probe microscopes, and in particular relates to a distortion-free scanning method of a scanning probe microscope. Background Art

[0002] Scanning probe microscopy (SPM) is a scanning microscope that uses a probe to scan a sample to obtain information about the sample surface. It is a general term for a variety of microscopes, including scanning tunneling microscopes (STM), atomic force microscopes (AFM), scanning magnetic force microscopes (MFM), scanning capacitance microscopes (SCaM), scanning near-field optical microscopes (SNOM), and scanning thermal microscopes (SThM). The reason why a variety of scanning microscopes are collectively referred to as scanning probe microscopes is because of their structural similarities:

[0003] 1. Scanning probe microscopes require a fine stepper motor, which is used to precisely control the scanning head and probe to reach the sample surface.

[0004] 2. Scanning probe microscopes require a scanning head, which is usually made of a piezoelectric tube. Its function is to carry and control the probe to move and scan in the XY plane to detect the surface information of the sample.

[0005] Since the scanning head is usually composed of a scanning tube, the scanning tube will inevitably have various forms of distortion and non-orthogonality during the deformation scanning process, including:

[0006] 1. The scanning tube is bent by applying voltage, so as to scan the sample surface. However, the pressure bending makes it difficult for it to move at a constant speed in a single direction, resulting in distortion of the scanned image.

[0007] 2. Random non-uniformity will occur during the manufacturing process of the scanning tube and the segmentation of the electrodes, resulting in non-orthogonality in the X and Y directions of the scanning tube, causing image distortion.

[0008] 3. Creep, stress release, thermal drift, etc. during the scanning process will cause slow displacement between the probe and the sample, causing the image to drift in a certain direction and thus produce distortion.

[0009] In the actual scanning process, since the scanning image range is usually not large (micrometer level), the distortion caused by the first two reasons is not obvious and can even be ignored. However, the distortion caused by the third reason seriously affects the user's perception of the image (for example, for unknown samples, due to the existence of drift, the atomic arrangement obtained by scanning is not the real atomic arrangement).

[0010] A common method for eliminating distortion is to calibrate the microscope first, that is, scan a sample with a known morphology, and then measure the drift speed of the microscope by comparing the scanned morphology with the actual morphology, and then measure the morphology of the unknown sample and remove the distortion by the measured drift speed. However, this method has obvious defects:

[0011] 1. Calibration of the microscope requires changing the sample.

[0012] 2. The drift of the microscope is different in different environments, so multiple calibrations are required.

[0013] 3. Drift may also be a function of time, so it is necessary to recalibrate once in a while.

[0014] 4. The drift of scanning the calibration sample and scanning the unknown sample may not be consistent (i.e. the sample itself may have drift such as creep and stress release).

[0015] There is also a de-distortion method that does not require calibration, as described in this article: Junting Wang, Jihui Wang, Yubin Hou and Qingyou Lu, Self-manifestation and universal correction of image distortion in scanning tunneling microscopy with spiral scan, Rev. Sci. Instrum. 81, 073705 (2010). This article proposes a spiral scanning method to replace the traditional line-by-line scanning, and then uses software correction to obtain the true atomic spatial arrangement. Figure 1-2 As shown, Figure 1 This is a scan of the original image, which is obviously distorted. The hexagon marked in the image represents the deformed graphite atomic lattice (if the scanned image is not deformed, this hexagon should be a regular hexagon). Figure 2 is the corrected image, where points A and B correspond to the starting points of the original image and the corrected image respectively.

[0016] from Figure 1 It can be seen that the atom in the lower left corner of the image is obviously larger than the atom in the upper right corner, which means that the entire image is drifting toward the lower left corner, causing the atomic row that should have been straight to bend. The image is corrected later by straightening the bent atomic row.

[0017] Although this method can solve the drift problem, it still has certain problems:

[0018] 1. Actual samples are often square, so spiral scanning will miss the edge area of ​​the sample.

[0019] 2. The spiral scanning control signal is relatively complex, and the subsequent correction processing is also complex. Summary of the invention

[0020] In order to solve the above-mentioned problem of SPM image distortion caused by the relative drift between the probe and the sample, a distortion-free scanning method of a scanning probe microscope is proposed.

[0021] The present invention discloses a distortion-free scanning method of a scanning probe microscope, the method comprising the following steps:

[0022] Step 1: Scan the sample line by line, and set the scanning speed to v n+1 =xv n , x is the coefficient of velocity change, set to a positive number not equal to 1, v n is the scanning speed of the nth row, that is, the scanning speed of the next row is x times that of the previous row, and step 2 is executed; or

[0023] The scanning area of ​​the sample is divided into two parts horizontally, area 1 s1 and area 2 s2. The scanning speed of area 1 s1 is constant at v1, and the scanning speed of area 2 s2 is constant at v2, wherein v1 is greater than v2, v1 is greater than 20 lines per second, and v2 can be scanned at a normal speed according to the needs of the occasion. The area of ​​area 1 s1 occupies 1 / 10-1 / 5 of the entire scanning area, and step 4 is executed;

[0024] Step 2: Scan to obtain a variable speed scan image, and observe whether the longitudinal components of the interatomic distances are equal. If they are equal, the variable speed scan image is used as an intermediate scan image. If they are not equal, the variable speed scan image is longitudinally corrected to obtain an intermediate scan image, and step 3 is executed;

[0025] Step 3: Determine whether there is lateral drift. If there is no lateral drift, use the intermediate scan image as the corrected image. If it is curved, if there is lateral drift, correct the intermediate scan image laterally to obtain the corrected image. The corrected image is the final distortion-free image. End;

[0026] Step 4: Observe whether the longitudinal component of the interatomic distance in region s2 is equal to that in region s1. If they are equal, the scanned image of region s2 in the scanned image is used as the intermediate scanned image. If they are not equal, the image of region s2 is longitudinally corrected to obtain the intermediate scanned image, and then execute step 5.

[0027] Step 5. Observe whether the atomic row in area 2 s2 is bent. If not, use the intermediate scan image as the corrected image. If it is bent, correct the intermediate scan image laterally to obtain the corrected image. The corrected image is the final distortion-free image. End.

[0028] Furthermore, in step 2, if the longitudinal components of the interatomic distances are not equal, longitudinal correction is performed based on the minimum value of the longitudinal components.

[0029] Furthermore, in step three, when judging whether there is lateral drift, observe whether the atomic row is bent. If there is no bending, there is no lateral drift. If there is bending, there is lateral drift. The point with the fastest speed when the atomic row arc is scanned is used as the tangent, and the entire image is stretched from the arc to the tangent.

[0030] Furthermore, in the step 4, the scanned image of the area 2 s2 is corrected based on the longitudinal component of the scanned image of the area 1 s1;

[0031] Furthermore, in step 5, the atomic row straight line L1 in the scanned image of area 1 s1 is used as a reference, and the atomic row straight line L2 in the scanned image of area 2 s2 is stretched to be parallel to L1 to remove the lateral distortion. In the scanning method of the present invention, the variable speed scanning or differential scanning method still retains the traditional line-by-line scanning method, and the bending of the atomic row during the drift process is achieved by the variable speed scanning method, thereby correcting the scanned image. This method abandons the complex spiral scanning method, the control signal is simpler, the scanning method is more diverse, and the later correction is more convenient.

[0032] Compared with the traditional method of calibrating microscopes, its advantages are:

[0033] There is no need to calibrate the microscope or change the sample. It only needs to be scanned once and the image can be dedistorted later.

[0034] For different environments (such as different temperatures, different magnetic fields, etc.), the drift speed of the microscope is also different, and the present invention only needs to scan and image once.

[0035] Traditional calibration methods are powerless to solve the different drifts of different samples (such as different stress release and creep during sample bonding), but the present invention can still solve this problem.

[0036] Regarding the method of removing distortion by spiral scanning, the advantages of the present invention are:

[0037] The actual sample is often square, and the spiral scanning will lose the sample edge information, which is sometimes more important. The scanning range of the present invention is still square, and the sample information is more complete.

[0038] The spiral scanning control signal is relatively complex, and the subsequent correction processing is also complex. In comparison, the several control signals of the scanning method of the present invention are much simpler. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1The original image is scanned in a spiral scanning manner.

[0040] Figure 2 This is a corrected image using a spiral scanning method. Points A and B in the figure correspond to the starting points of the original image and the corrected image, respectively.

[0041] Figure 3 This is the actual atomic arrangement of the sample, which appears as a regular hexagon.

[0042] Figure 4 This is a schematic diagram of the situation where drift exists during the scanning process (scanning line by line from top to bottom), and the figure shows drift to the left.

[0043] Figure 5 This is a schematic diagram of the situation where drift exists during the scanning process (scanning line by line from top to bottom), and the figure shows downward drift.

[0044] Figure 6 This is a schematic diagram of the situation where drift exists during the scanning process (scanning line by line from top to bottom), and the figure shows drift toward the lower left.

[0045] Figure 7 This is the atomic image arrangement diagram obtained by scanning in Example 1, and the image shows a drift to the left.

[0046] Figure 8 This is the atomic image arrangement diagram obtained by scanning in Example 1, and the figure shows downward drift.

[0047] Fig. 9 This is the atomic image arrangement diagram obtained by scanning in Example 1, and it is shown in the figure drifting to the lower left.

[0048] Fig.10 A schematic diagram of the tangent line of an image with only lateral drift is shown in Example 1.

[0049] Fig.11 for Fig.10 The image after stretching from an arc to a tangent line.

[0050] Fig.12 This is the atomic image arrangement diagram obtained by scanning in Example 2, and it is shown to drift to the left.

[0051] Fig.13 This is the atomic image arrangement diagram obtained by scanning in Example 2, and the figure shows downward drift.

[0052] Fig.14 This is the atomic image arrangement diagram obtained by scanning in Example 2, and it is shown in the figure drifting to the lower left.

[0053] Fig.15 A schematic diagram of the tangent line of an image with only lateral drift in Example 2 is shown.

[0054] Fig.16 for Fig.15 The image after stretching from an arc to a tangent line.

[0055] Fig.17 This is the atomic image arrangement diagram obtained by scanning in Example 3, and the image shows a drift to the left.

[0056] Fig.18 This is the atomic image arrangement diagram obtained by scanning in Example 3, and the figure shows downward drift.

[0057] Fig.19 This is the atomic image arrangement diagram obtained by scanning in Example 3, and the image shows drifting to the lower left.

[0058] Fig. 20 The image and the circular scanning path obtained by the circular scanning in Example 5. The scanning direction in the figure is from the center position to the periphery. DETAILED DESCRIPTION

[0059] The technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention; it is obvious that the described embodiments are only part of the embodiments of the present invention, rather than all the embodiments, and all other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention without making creative work are within the scope of protection of the present invention.

[0060] As mentioned above, the traditional line-by-line scanning method of SPM has the problem of drift. Figure 3 Shown is the actual surface atomic arrangement order of a sample, assuming it is a regular hexagonal arrangement.

[0061] Figure 4-6 In the case of drift during scanning, the arrow shows the drift direction of the sample relative to the probe and the drift speed is constant. The hexagon is the distorted atomic arrangement. Figure 4-6 It is found that after drift, the atoms are still arranged in rows, that is, the observer will not notice the drift in the image at all. Assuming that we do not know the true atomic arrangement of the sample, we will instinctively think that the result obtained by scanning is the true arrangement, and there will be no way to remove the distortion of the image and restore it to the true image.

[0062] However, no matter what direction the drift is, the drift speed is basically constant. If variable speed scanning is used during the scanning process, such as gradually speeding up or slowing down the scanning speed, the drift will have different effects on different scanning speeds, and the atomic row will bend. The atomic arrangement of the periodic crystal structure must be linear, and the purpose of eliminating distortion can be achieved by correcting the atomic row in the later stage.

[0063] Embodiment 1:

[0064] Slow down scanning

[0065] Figure 7-9 The atomic image arrangement obtained by decelerating scanning is that the first row has the fastest scanning speed, the second row has a scanning speed x times that of the first row (x>1, for example 1.1), the third row has a scanning speed x times that of the second row, and so on. The speed of the next row is always x times that of the previous row. Figure 4 The arrow in the middle shows the drift direction of the sample relative to the probe and the drift speed is constant. During the scanning process, the faster the scanning, the smaller the drift, so Figure 4 In the image, the rows of atoms appear bent.

[0066] The post-distortion removal is divided into two steps:

[0067] 1. If the spacing between atomic rows gradually increases, it means that there is drift in the vertical direction. Take the spacing between the first two rows of atoms as the reference, and correct the spacing between the following atomic rows to the same spacing.

[0068] exist Figure 7 There is no longitudinal drift in the process, and the lateral spacing between atomic rows is always consistent, so there is no need for the first step correction. Figure 8 There is only longitudinal drift. After the first step of correction, the image will be restored to the original image (i.e. Figure 3 ), the atomic rows will no longer bend. Fig. 9 There is both longitudinal drift and lateral drift in the image. After the first correction, the image will become one with only lateral drift, similar to Figure 7 situation.

[0069] 2. After completing the first step, the influence of longitudinal drift is removed. If the atomic row is bent, it means that there is still lateral drift, such as Figure 7 The second step is to remove the lateral drift. At this time, the top point of the atomic line arc is used as the tangent line (such as Figure 10-11 The second step of correction can be completed by stretching the entire image from the arc to the tangent line, and the image is restored to Figure 3 looks like. Fig.10 For an image with only lateral drift, Fig.11 After removing the distortion in the second step, the regular hexagonal arrangement is restored.

[0070] Embodiment 2:

[0071] The deceleration scanning in the first implementation mode is changed to acceleration scanning. That is, the first row has the slowest scanning speed, the second row has a scanning speed x times that of the first row (x<1, for example, 0.9), the third row has a scanning speed x times that of the second row, and so on. The speed of the next row is always x times that of the previous row. Figure 12-14The arrow in the middle shows the drift direction of the sample relative to the probe and the drift speed is constant. During the scanning process, the faster the scanning, the smaller the drift, so Figure 12-14 In the image, the rows of atoms appear bent.

[0072] The post-distortion removal is divided into two steps:

[0073] 1. If the spacing between atomic rows gradually decreases, it means that there is drift in the vertical direction. Take the spacing between the last two rows of atoms as the reference, and correct the spacing between the upper atomic rows to the same spacing.

[0074] exist Fig.12 There is no longitudinal drift in the process, and the lateral spacing between atomic rows is always consistent, so there is no need for the first step correction. Fig.13 There is only longitudinal drift. After the first step of correction, the image will be restored to the original image ( Figure 3 ), the atomic rows will no longer bend. Fig.14 There is both longitudinal drift and lateral drift in the image. After the first correction, the image will become one with only lateral drift, similar to Fig.12 situation.

[0075] 2. After completing the first step, the influence of longitudinal drift is removed. If the atomic row is bent, it means that there is still lateral drift, such as Fig.12 The second step is to remove the lateral drift. At this time, the point at the bottom of the atomic line is used as the tangent line (such as Figure 15-16 The second step of correction can be completed by stretching the entire image from the arc to the tangent line, and the image is restored to Figure 3 looks like. Fig.15 For an image with only lateral drift, Fig.16 After removing the distortion in the second step, the regular hexagonal arrangement is restored.

[0076] Embodiment three:

[0077] In the actual scanning process, the drift speed is not fast. As long as the scanning speed is much greater than the drift speed, the effect of drift on image distortion can be ignored. However, too fast scanning speed will lead to unclear images. Although a slow scanning speed can obtain a clear image, there will be drift. Therefore, image distortion and image clarity are a pair of contradictions.

[0078] To this end, we can use a scanning method that scans fast first and then slows down. Figure 17-19As shown, the arrow indicates the drift direction of the sample relative to the probe and the drift speed is constant. The dashed box is a fast scan (the scanning speed is v1 constant, which is much greater than the drift speed), the image is not clear but the distortion can be ignored; the dashed box is a slow scan (the scanning speed is v2 constant). In this embodiment, "slow scan" is relative to the scanning speed in the dashed box (v1>20 lines / second). When implemented, v2 can be scanned at a conventional speed. The slow scan image is clear but the distortion is obvious.

[0079] The advantage of this scanning method is that it does not require changing the speed for each line, and only two scanning speeds are needed for the entire image. Only the image outside the dotted frame is used for later analysis, and the image is clearer and more stable.

[0080] The post-distortion removal is also done in two steps:

[0081] 1. The first step is to remove the longitudinal distortion. Based on the atomic row spacing d1 obtained by fast scanning, the atomic row spacing d2 in the slow scanning image is compressed to d1, thereby removing the longitudinal distortion.

[0082] 2. The second step is to remove the lateral distortion. Taking the atomic row straight line L1 obtained by fast scanning as the reference, the atomic row straight line L2 in the slow scanning image is stretched to be parallel to L1, thereby removing the lateral distortion.

[0083] After removing the distortion, we use the slow scan part of the image as the final output image for analysis and discard the fast scan part. Therefore, the final image is both clear and distortion-free.

[0084] Embodiment 4:

[0085] On the basis of implementation mode 3, the method is changed to first performing slow scanning and then performing fast scanning. The rest remains unchanged.

[0086] Embodiment five:

[0087] When using circular scanning, the scanning line speed is always v1, and v1 does not need to be very fast. Fig. 20 The figure shows the image and path of the circular scanning. The arrow shows the drift direction of the sample relative to the probe. The figure on the right is a schematic diagram of the scanning path. The scanning line speed remains unchanged at v1. The circular scanning path can also be performed from the outer circle to the inside.

[0088] Since the scanning line speed is constant, the atoms in the center of the image will be scanned in a shorter time, and the distortion can be ignored. The farther the image is extended, the slower the atomic image will be scanned, and the more serious the distortion will be. Therefore, the distortion is shown in the figure, and the atomic row is in an arc. In the later stage, we only need to use the arrangement of the atoms in the center as a reference to correct the rest of the image. (Similar to spiral scanning, but the control signal is simpler).

[0089] The above is only a preferred specific implementation of the present invention; however, the protection scope of the present invention is not limited thereto. Any technician familiar with the technical field can make equivalent replacements or changes according to the technical solution and its improved conception within the technical scope disclosed by the present invention, which should be covered by the protection scope of the present invention.

Claims

1. A method for non-distortion scanning of a scanning probe microscope, characterized in that: The following steps are involved: Step 1: Scan the sample line by line, and set the scanning speed to , is the coefficient of speed change, set to a positive number not equal to 1, is the scanning speed of the nth row, that is, the scanning speed of the next row is x times that of the previous row, and step 2 is executed; or The scanning area of ​​the sample is divided into two parts horizontally. and Region 2 , Region 1 The scanning speed is constant at , Region 2 The scanning speed is constant at ,in Greater than , More than 20 lines per second, area 1 If the area occupies 1 / 10 -1 / 5 of the entire scanning area, proceed to step 4; Step 2: Scan to obtain a variable speed scan image, and observe whether the longitudinal components of the interatomic distances are equal. If they are equal, the variable speed scan image is used as an intermediate scan image. If they are not equal, the variable speed scan image is longitudinally corrected to obtain an intermediate scan image, and step 3 is executed; Step 3: Determine whether there is lateral drift. If there is no lateral drift, use the intermediate scan image as the corrected image. If it is curved, if there is lateral drift, correct the intermediate scan image laterally to obtain the corrected image. The corrected image is the final distortion-free image. End; Step 4: Observe whether the longitudinal component of the interatomic distance in region s2 is equal to that in region s1. If they are equal, the scanned image of region s2 in the scanned image is used as the intermediate scanned image. If they are not equal, the image of region s2 is longitudinally corrected to obtain the intermediate scanned image, and then execute step 5. Step 5: Observe whether the atomic row in the second region s2 is bent. If not, use the intermediate scan image as the corrected image. If it is bent, correct the intermediate scan image laterally to obtain the corrected image. The corrected image is the final distortion-free image. End; In the step 2, if the longitudinal components of the interatomic distances are not equal, longitudinal correction is performed based on the minimum value of the longitudinal components; In step three, when judging whether there is lateral drift, observe whether the atomic row is bent. If there is no bending, there is no lateral drift. If there is bending, there is lateral drift. The point with the fastest speed when the atomic row arc is scanned is used as the tangent, and the entire image is stretched from the arc to the tangent.

2. The method for scanning probe microscope without distortion according to claim 1, characterized in that: In step 4, area 1 The longitudinal component in the scanned image of is taken as a reference, and the scanned image of area 2 s2 is corrected.

3. The method for scanning probe microscope without distortion according to claim 2, characterized in that: In step 5, area 1 Taking the atomic row straight line L1 in the scanned image of area s2 as a reference, the atomic row straight line L2 in the scanned image of area 2 s2 is stretched to be parallel to L1 to remove the lateral distortion.

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