A method of manufacturing a wafer, a wafer, and a semiconductor system

Abstract

The application provides a wafer preparation method, a wafer and a semiconductor system. The wafer preparation method comprises the following steps: placing a first surface of a wafer to be processed upward on a fixing device; placing the fixing device loaded with the wafer to be processed into an X-ray diffractometer to perform phi scanning on the first surface and obtain a first scanning curve; placing a second surface of the wafer to be processed upward on the fixing device to perform phi scanning on the second surface and obtain a second scanning curve; wherein the first scanning curve and the second scanning curve respectively represent the corresponding relationship between a plurality of in-plane scanning angles and a plurality of diffraction intensities; determining the included angle between a specific crystal orientation of the wafer to be processed and the bottom edge of the fixing device based on the first scanning curve and the second scanning curve to obtain a positioning angle; and cutting the wafer to be processed based on the positioning angle to obtain a wafer with a positioning edge. The method provided by the application can improve the positioning edge processing precision of the wafer.

Description

Technical Field

[0001] This invention relates to the field of wafer edge processing technology, and more particularly to a wafer fabrication method, a wafer, and a semiconductor system. Background Technology

[0002] In the field of semiconductor wafer fabrication, the wafer positioning edge (Flat Edge) is a straight structure on the outer edge of the wafer. It is mainly used to mark the crystallographic orientation of the wafer and serves as an important reference for subsequent key processes such as photolithography alignment, ensuring the positioning accuracy during wafer fabrication.

[0003] In the semiconductor device manufacturing process, wafers often lack high-precision positioning references during the early initial processing. To ensure the alignment accuracy of subsequent processes, target positioning edges that are strictly aligned with specific crystal orientations inside the wafer need to be machined at the wafer edge. This process relies on the precise spatial positioning of specific crystal orientations inside the wafer relative to the external support fixture, so as to provide accurate processing coordinates for subsequent dicing equipment.

[0004] Currently, the industry commonly uses X-ray diffraction (XRD) equipment to perform single-sided scanning measurements on wafers to calibrate crystal orientation positions; however, this method causes the final cut target positioning edge to deviate from the actual crystal orientation, reducing the wafer positioning edge processing accuracy.

[0005] It should be noted that the above description of the background technology is only for the purpose of providing a clear and complete explanation of the technical solutions of the present invention and facilitating understanding by those skilled in the art. It should not be assumed that the above technical solutions are known to those skilled in the art simply because they have been described in the background section of the present invention. Summary of the Invention

[0006] The purpose of this invention is to disclose a wafer fabrication method, a wafer, and a semiconductor system, which solves the problem that in the prior art, the crystal orientation position calibrated by single-sided scanning measurement of the wafer using X-ray diffraction equipment leads to a deviation between the final cut target positioning edge and the actual crystal orientation, thus reducing the processing accuracy of the wafer positioning edge.

[0007] To achieve the above objectives, in a first aspect, the present invention provides a method for preparing a wafer, comprising: placing the wafer to be processed with its first surface facing upwards in a fixing device; The fixture holding the wafer to be processed is placed in an X-ray diffractometer to perform a phi scan on the first surface to obtain a first scan curve; The wafer to be processed is placed with its second surface facing upwards on the fixing device to perform a phi scan on the second surface, thereby obtaining a second scan curve; wherein, the first scan curve and the second scan curve are used to represent the correspondence between multiple in-plane scanning angles and multiple diffraction intensities, respectively. Based on the first scanning curve and the second scanning curve, the angle between the specific crystal orientation of the wafer to be processed and the bottom edge of the fixing device is determined to obtain the positioning angle; Based on the positioning angle, the wafer to be processed is cut to obtain a wafer with a positioning edge.

[0008] As a further improvement of the present invention, the step of determining the angle between a specific crystal orientation of the wafer to be processed and the bottom edge of the fixing device based on the first scanning curve and the second scanning curve to obtain the positioning angle includes: The in-plane scanning angle corresponding to the maximum diffraction intensity is extracted from the first scanning curve to obtain the first angle; The in-plane scanning angle corresponding to the maximum diffraction intensity is extracted from the second scanning curve to obtain the second angle; The positioning angle is calculated based on the first angle and the second angle.

[0009] As a further improvement of the present invention, the step of calculating the positioning angle based on the first angle and the second angle includes: Calculate the difference between the second angle and the first angle, and take half of the difference as the positioning angle; The second angle is greater than the first angle.

[0010] As a further improvement of the present invention, the specific crystal orientation is the [10-10] crystal orientation; And / or, the X-ray diffractometer further includes a sample stage for supporting the fixing device; the first angle satisfies: Φ1=α-β; the second angle satisfies: Φ2=α+β; where β is the positioning angle, and α is the angle between the projection of the X-ray emitted by the X-ray diffractometer onto the surface of the sample stage and the bottom edge of the sample stage.

[0011] As a further improvement of the present invention, the step of cutting the wafer to be processed based on the positioning angle to obtain a wafer with a positioning edge includes: The fixing device that holds the wafer to be processed is placed on the worktable of the cutting equipment, and the bottom edge of the fixing device is aligned with the reference direction of the cutting equipment. The worktable is rotated by the positioning angle to align the specific crystal orientation with the cutting direction of the cutting equipment. The wafer to be processed is cut along the cutting direction to form the positioning edge.

[0012] As a further improvement of the present invention, before dicing the wafer to be processed, the method further includes: Repeatedly perform the phi scan operation on the first surface and the second surface to obtain at least two of the positioning angles; Calculate the standard deviation of the at least two positioning angles; When the standard deviation is less than or equal to the set standard deviation threshold, the mean of the at least two positioning angles is calculated as the final positioning angle.

[0013] As a further improvement of the present invention, the scanning range of the first surface phi scan is 0°~60°, and the scanning step size is 0.1° / second; And / or, the scanning range of the second surface subjected to phi scanning is 0°~60°, and the scanning step size is 0.1° / second.

[0014] As a further improvement of the present invention, the wafer to be processed is gallium nitride, the first surface is a gallium surface, and the second surface is a nitrogen surface.

[0015] Secondly, the present invention also provides a wafer, which is prepared by the wafer preparation method described in any one of the first aspects.

[0016] Thirdly, the present invention also provides a semiconductor system, comprising: an X-ray diffractometer for performing phi scans on a first surface and a second surface of the wafer to be processed, respectively, to obtain a first scan curve and a second scan curve; wherein the first scan curve and the second scan curve are respectively used to represent the correspondence between multiple in-plane scanning angles and multiple diffraction intensities; The processing device, coupled to the X-ray diffractometer, is used to determine the angle between a specific crystal orientation of the wafer to be processed and the bottom edge of the fixing device based on the first scanning curve and the second scanning curve, thereby obtaining the positioning angle; A cutting device, coupled to the processing device, is used to cut the wafer to be processed based on the positioning angle to obtain a wafer with a positioning edge.

[0017] Compared with the prior art, the beneficial effects of the present invention are as follows: By placing the first surface of the wafer to be processed upwards on a fixing device, and placing the fixing device containing the wafer to be processed into an X-ray diffractometer to perform a phi scan on the first surface, a first scanning curve is obtained; the wafer to be processed is removed from the fixing device, rotated 180° horizontally so that the second surface faces upwards, and placed back on the same fixing device and fixed to perform a phi scan on the second surface, obtaining a second scanning curve; by analyzing the two scanning curves simultaneously, errors can be eliminated, thereby accurately extracting the positioning angle between a specific crystal orientation and the bottom edge; after obtaining the positioning angle, the wafer to be processed is transferred to a cutting device; during cutting, the edge of the wafer to be processed is cut using the specific crystal orientation of the wafer to be processed as the reference direction, combined with the positioning angle, to form a straight positioning edge. Compared with the prior art method of performing a single phi scan on a single surface of the wafer, the present invention eliminates errors by performing phi scans on the first and second surfaces of the wafer to be processed separately and determining the positioning angle by combining the two scanning curves, thereby improving the processing accuracy of the positioning edge. Attached Figure Description

[0018] Figure 1 This is a schematic diagram of the semiconductor system disclosed in this invention; Figure 2 A schematic diagram showing the placement of the fixing device within the X-ray diffractometer; Figure 3 A top view of a fixture holding a wafer to be processed placed on a sample stage, wherein the first surface of the wafer to be processed faces upward. Figure 4 This is a schematic diagram showing the geometric relationship between the inherent angle and the positioning angle when the sample stage and the wafer rotate synchronously in the first forward direction. Figure 5 This is a schematic diagram showing the geometric relationship between the inherent angle and the positioning angle when the sample stage and the wafer rotate synchronously in opposite directions by a second angle. Figure 6 This is a flowchart of the wafer fabrication method disclosed in this invention; Figure 7 This is a schematic diagram showing the fixing device placed on the cutting equipment; Figure 8 A schematic diagram showing the rotational positioning angle of the worktable of the cutting equipment. Detailed Implementation

[0019] The present invention will now be described in detail with reference to the embodiments shown in the accompanying drawings. However, it should be noted that these embodiments are not intended to limit the present invention. Equivalent changes or substitutions in function, method, or structure made by those skilled in the art based on these embodiments are all within the scope of protection of the present invention.

[0020] The accompanying drawings in this invention are not strictly drawn to scale, and the specific dimensions of each structure can be determined according to actual needs. The drawings described in this invention are merely structural schematic diagrams. The lines shown in the accompanying drawings of this invention can be understood as components with a certain actual thickness.

[0021] Combined with parameters Figure 1 and Figure 2 As shown, the present invention provides a semiconductor system, which includes an X-ray diffractometer 30, a cutting device 40, and a processing device 50.

[0022] The X-ray diffractometer 30 is used to perform phi scans on the first surface 11 and the second surface 12 of the wafer 10 to be processed, respectively, to obtain a first scan curve and a second scan curve; wherein, the first scan curve and the second scan curve are used to represent the correspondence between multiple in-plane scanning angles and multiple diffraction intensities.

[0023] The processing device 50 is coupled to the X-ray diffractometer 30. The processing device 50 is used to determine the angle between a specific crystal orientation 14 of the wafer to be processed 10 and the bottom edge 21 of the fixing device 20 based on the first scanning curve and the second scanning curve, and obtain the positioning angle β.

[0024] The cutting device 40 is coupled to the processing device 50. The cutting device 40 is used to cut the wafer 10 to be processed based on the positioning angle β to obtain a wafer with a positioning edge.

[0025] In one embodiment, the X-ray diffractometer 30 includes an X-ray source (not shown), a sample stage 31, and a detector (not shown), the sample stage 31 being used to support the fixture 20.

[0026] The sample stage 31 is positioned at the center of the X-ray diffractometer 30, and the bearing surface 311 of the sample stage 31 provides a stable support reference for the fixing device 20. The sample stage 31 can be rotated and scanned about an axis Q that passes through the center of the wafer 10 to be processed and is perpendicular to the bearing surface 311 (i.e., in-plane rotation). After the fixing device 20 is placed on the bearing surface 311 of the sample stage 31, the test surface of the wafer 10 to be processed (i.e., the first surface 11 and the second surface 12) remains parallel to the bearing surface 311 of the sample stage 31. This ensures that the geometric relationship when the X-ray 101 is incident on the test surface of the wafer 10 to be processed is determined only by the inherent angle determined by the optical path structure of the X-ray diffractometer 30 (i.e., the angle α between the projection light 102 of the X-ray 101 on the bearing surface 311 and the bottom edge of the sample stage 31) and the actual deflection of the specific crystal orientation 14 of the wafer 10 to be processed relative to the bottom edge 21 (i.e., the positioning angle β). This eliminates the interference of external random factors such as the installation position deviation of the fixing device 20 on the sample stage 31 and the levelness error of the sample stage 31 itself, thereby ensuring that the positioning angle β calculated subsequently can truly reflect the lattice orientation characteristics of the wafer 10 to be processed.

[0027] It is worth noting that both the fixing device 20 and the sample stage 31 are rectangular parallelepipeds, and the bottom edge 21 of the fixing device 20 is parallel to the bottom edge of the sample stage 31.

[0028] During the phi scan, the X-ray source emits X-rays 101 to irradiate the surface of the wafer 10 to be processed. The sample stage 31 rotates continuously around the axis Q at a preset scanning step size. The detector synchronously collects the X-ray diffraction intensity signals at each angle position, thereby obtaining a scanning curve with the in-plane scanning angle as the abscissa and the diffraction intensity as the ordinate.

[0029] When the X-ray diffractometer 30 performs a phi scan on the first surface 11 of the wafer 10 to be processed, the sample stage 31 rotates continuously around axis Q from a starting position of 0° to a final position of 60°, with a scanning step size set to 0.1° / second. The detector synchronously acquires diffraction intensity signals at each angular position to generate a first scan curve. The in-plane scanning angle corresponding to the peak diffraction intensity in this first scan curve is the first angle Φ1. Similarly, when the X-ray diffractometer 30 performs a phi scan on the second surface 12 of the wafer 10 to be processed, the same scanning parameters are used to generate a second scan curve. The in-plane scanning angle corresponding to the peak diffraction intensity in this second scan curve is the second angle Φ2.

[0030] The processing device 50 extracts the first angle Φ1 corresponding to the diffraction peak from the first scanning curve and the second angle Φ2 corresponding to the diffraction peak from the second scanning curve, and calculates the positioning angle β according to the formula β=(Φ2-Φ1) / 2. Since the inherent angle α of the device in this calculation formula is completely canceled out during the subtraction process, the positioning angle β depends only on the lattice orientation characteristics of the wafer 10 to be processed and is not affected by the installation deviation of the X-ray diffractometer 30.

[0031] In some examples, the sample stage 31 is provided with at least one of a positioning pin (not shown), a vacuum adsorption hole (not shown), or a magnetic clamping structure (not shown) for circumferentially limiting and / or normally fixing the fixing device 20, preventing circumferential slippage or tilting of the fixing device 20 during the phi scanning process, and ensuring the repeatability of the peak position of the scanning curve.

[0032] In one embodiment, the processing device 50 is further configured to repeatedly perform the phi scan operation, acquire at least two positioning angles β, calculate the standard deviation σ of the at least two positioning angles β, and when the standard deviation σ is less than or equal to a set standard deviation threshold, calculate the mean of the at least two positioning angles β as the final positioning angle β. For example, the set standard deviation threshold is 0.05°.

[0033] In some examples, the cutting device 40 may be a laser cutting device, and this disclosure does not limit the cutting method.

[0034] In one embodiment, the dicing apparatus 40 includes a worktable (not shown) and a dicing assembly (not shown). The worktable is used to support the fixture 20, on which the wafer 10 to be processed is loaded, and is rotatable about its axis of rotation. After receiving the positioning angle β sent by the processing device 50, the dicing apparatus 40 first aligns the bottom edge 21 of the fixture 20 with the reference direction of the dicing apparatus 40, and then rotates the worktable about its axis of rotation by the positioning angle β, so that the specific crystal orientation 14 of the wafer 10 to be processed is precisely aligned with the dicing direction of the dicing apparatus 40. Subsequently, the dicing assembly cuts the wafer 10 to be processed along the dicing direction, forming a positioning edge on the outer circumferential surface of the wafer.

[0035] In some examples, after the fixing device 20 is placed on the worktable, it can also be locked by means of vacuum adsorption, mechanical clamping or magnetic fixing to prevent displacement during rotation and cutting and ensure the accuracy of the positioning edge formation.

[0036] Please refer to the references. Figures 2 to 8 This invention discloses a specific embodiment of a wafer fabrication method. This method is used to prepare positioning edges aligned with a specific crystal orientation within the wafer. Specifically, it involves performing phi scans on both sides of the wafer and simultaneously establishing two scan curves to determine the positioning angle β, thereby eliminating systematic errors and improving the processing accuracy of the positioning edges.

[0037] Reference Figure 6 as shown, the method for preparing a wafer provided by the present invention includes the following Step S1 to Step S5.

[0038] Step S1: Place the first surface 11 of the to-be-processed wafer 10 upward and position it on the fixing device 20.

[0039] The to-be-processed wafer 10 is a single crystal substrate with a polar structure, and its first surface 11 and second surface 12 have different polar characteristics in crystallography.

[0040] The fixing device 20 is configured to carry and fix the to-be-processed wafer 10, and the fixing device 20 has a straight section (i.e., the bottom edge 21). The fixing device 20 can stably fix the to-be-processed wafer 10 by vacuum adsorption, paraffin bonding or other appropriate methods, so as to avoid displacement of the to-be-processed wafer 10 during the subsequent scanning process.

[0041] Step S2: Place the fixing device 20 loaded with the to-be-processed wafer 10 into the X-ray diffractometer 30 to perform a phi scan on the first surface 11, so as to obtain a first scanning curve.

[0042] The X-ray diffractometer 30 is configured to generate X-rays 101 and perform diffraction analysis on the to-be-processed wafer 10.

[0043] The phi scan refers to an in-plane rotation scan of the to-be-processed wafer 10 around the bottom edge of its surface. During the scanning process, the X-ray diffractometer 30 collects corresponding diffraction intensities at a plurality of in-plane scanning angles (phi angles), so as to obtain a first scanning curve representing the corresponding relationship between the plurality of in-plane scanning angles and the plurality of diffraction intensities. When the in-plane scanning angle is aligned with the specific crystal orientation 14 inside the to-be-processed wafer 10 and the diffraction condition is satisfied, a characteristic peak appears in the diffraction intensity.

[0044] It should be noted that when the wafer 10 to be processed is placed on the fixing device 20, there is usually an angle between its specific crystal orientation 14 and the bottom edge 21 of the fixing device 20. This angle is the positioning angle β that needs to be determined later (for convenience, the line passing through the center of the wafer 10 to be processed and parallel to the bottom edge 21 is referred to as the virtual edge 22). However, in actual measurement, the X-rays 101 generated by the X-ray diffractometer 30 irradiate the surface of the wafer 10 to be processed (i.e., the first surface 11 and the second surface 12) at a set incident angle. This incident angle is determined by the optical path structure of the X-ray diffractometer 30. Correspondingly, the projection light 102 of the X-rays 101 on the bearing surface 311 forms an inherent angle α between it and the bottom edge 312 of the sample stage 31. The inherent angle α is introduced into the measurement result as a fixed offset in each measurement. Therefore, if only a single surface of the wafer 10 to be processed is scanned once using the phi scan, the measured angle is actually the superposition value of the positioning angle β and the inherent angle α.

[0045] In some examples, the specific crystal orientation 14 is the [10-10] crystal orientation. The specific crystal orientation 14 refers to the crystal orientation of the material used in the wafer 10 to be processed, and its specific orientation depends on the material of the wafer 10. When the wafer 10 is made of other semiconductor materials, it is defined accordingly as the crystal orientation of its corresponding material. This invention does not limit the specific type of the specific crystal orientation.

[0046] Step S3: Place the second surface 12 of the wafer 10 to be processed upward on the fixing device 20, and perform a phi scan on the second surface 12 to obtain a second scan curve; wherein, the first scan curve and the second scan curve are used to represent the correspondence between multiple in-plane scanning angles and multiple diffraction intensities.

[0047] The wafer 10 to be processed is removed from the fixing device 20, rotated 180° horizontally so that the second surface 12 faces upward, and then placed back on the same fixing device 20 and fixed. After being rotated, the orientation of the crystal polarity face of the wafer 10 relative to the fixing device 20 is reversed.

[0048] Step S4: Based on the first scan curve and the second scan curve, determine the angle between the specific crystal orientation 14 of the wafer to be processed 10 and the bottom edge 21 of the fixing device 20, and obtain the positioning angle β. Step S4 specifically includes: S41. Extract the in-plane scanning angle corresponding to the maximum diffraction intensity from the first scanning curve to obtain the first angle Φ1.

[0049] The first scanning curve is a relational curve between a plurality of in-plane scanning angles obtained by performing a phi scan on the first surface 11 and the corresponding diffraction intensity. When the to-be-processed wafer 10 is rotated and scanned around the bottom edge of its surface, when the in-plane scanning angle is aligned with the specific crystal orientation 14 inside the to-be-processed wafer 14 and satisfies the diffraction condition, a characteristic peak appears in the diffraction intensity. Therefore, for the first scanning curve, the in-plane scanning angle corresponding to the maximum value of the diffraction intensity is the first angle Φ1.

[0050] The first angle Φ1 reflects the comprehensive angle information measured when the X-ray 101 irradiates the first surface 11, which includes the real deflection of the specific crystal orientation 14 relative to the bottom edge 21 (i.e., the positioning angle β) and the inherent angle between the X-ray 101 and the first surface 11.

[0051] In the state where the first surface 11 faces upward, the inherent angle α and the positioning angle β have a subtractive relationship in the phi scan, so that the measured first angle Φ1 is smaller than the inherent angle α; therefore, the first angle Φ1 can be expressed as: Φ1=α-β (1) Wherein, α is the included angle (i.e., the inherent angle) between the projection light 102 of the X-ray 101 emitted from the X-ray diffractometer 30 on the surface of the sample stage 31 (i.e., the bearing surface 311) and the bottom edge 312 of the sample stage 31.

[0052] Reference Figure 4 As shown, the initial virtual edge 22a is perpendicular to the bottom edge 312 of the sample stage 31; the inherent angle α is formed between the projection light 102 of the X-ray and the bottom edge 312 of the sample stage 31; the positioning angle β is formed between the initial specific crystal orientation 14a and the initial virtual edge 22a. The sample stage 31 and the wafer 10 synchronously rotate by the first angle Φ1, the projection light 102 remains stationary, and the rotated specific crystal orientation 14b is perpendicular to the projection light 102; the included angle between the initial virtual edge 22a and the rotated virtual edge 22b is the first angle Φ1. Draw a perpendicular line E to the projection light 102, and a second included angle α1 is formed between the initial virtual edge 22a and the perpendicular line E; since one side of the second included angle α1 (i.e., the initial virtual edge 22a) is perpendicular to one side of the inherent angle α (i.e., the bottom edge 312), and the other side of the second included angle α1 (i.e., the perpendicular line E) is perpendicular to the other side of the inherent angle α (i.e., the projection light 102), according to the geometric theorem that "if the two sides of two angles are respectively perpendicular, then the two angles are equal", combined with Figure 4It can be seen that both are acute angles, so α1=α; the extension 141b of the rotated specific crystal orientation 14b forms a third included angle α2 with the initial virtual side 22a; the rotated crystal orientation 14b is perpendicular to the projection light 102, and the perpendicular line E is perpendicular to the projection light 102, so the rotated specific crystal orientation 14b and its extension 141b are parallel to the perpendicular line E, and the included angles formed by the two with the same section (i.e., the initial normal 21a) are equal, i.e., α2=α1=α; the third included angle α2 is the exterior angle of the triangle formed by the initial virtual side 22a, the rotated virtual side 22b and the extension 141b, the first angle Φ1 and the positioning angle β are two non-adjacent interior angles of the triangle, according to the exterior angle theorem of triangles, α2=Φ1+β, substituting α2=α into α2=Φ1+β, we get Φ1=α-β.

[0053] It should be noted that the wafer 10 to be processed is clamped onto the sample stage 31 by the fixing device 20. In the clamped state, the bottom edge of the fixing device 20 is collinear with and in the same direction as the bottom edge of the sample stage 31. Since the wafer 10 to be processed is fixed on the fixing device 20, the bottom edge of the surface of the wafer 10 to be processed is also consistent with the bottom edge of the sample stage 31. Therefore, the sample stage 31, the wafer 10 to be processed, and the fixing device 20 rotate synchronously. The angular relationships derived above based on the bottom edge of the fixing device 20 (i.e., the initial virtual edge 22a and the rotated virtual edge 22b) also apply to the bottom edge of the sample stage 31 and the bottom edge of the surface of the wafer 10 to be processed.

[0054] S42. Extract the in-plane scanning angle corresponding to the maximum diffraction intensity from the second scanning curve to obtain the second angle Φ2.

[0055] The second scanning curve is a curve showing the relationship between multiple in-plane scanning angles and corresponding diffraction intensities obtained by performing a phi scan on the second surface 12 after flipping the wafer 10. For the second scanning curve, the in-plane scanning angle corresponding to the maximum diffraction intensity is the second angle Φ2.

[0056] Because the wafer 10 to be processed has a polar structure, after flipping the wafer 10 by 180° so that the second surface 12 faces upward, in the measurement coordinate system of the X-ray diffractometer 30, the deflection direction of the specific crystal orientation 14 relative to the bottom edge 21 after the flip is mirror-symmetrical, so that the positive and negative orientations of the positioning angle β are opposite in the calculation formula of the two measurements. Specifically, the second angle Φ2 is greater than the inherent angle α, and the calculation formula of the second angle Φ2 is as follows: Φ2=α+β (2) Understandably, since the wafer 10 to be processed has a polar structure, and the angle between the X-ray 101 and the surface of the wafer 10 to be processed (i.e., the aforementioned inherent angle α) is determined only by the optical path structure and is independent of the placement orientation of the surface of the wafer 10 to be processed, the positive and negative orientation of the inherent angle α remains unchanged in the calculation formula of the two measurements.

[0057] When the positioning angle β is a positive value, the second angle Φ2 is always greater than the first angle Φ1. This magnitude relationship can also be used as a verification basis for judging the validity of measurement results: if a situation where Φ2 is not greater than Φ1 occurs in actual measurement, it indicates that there may be an abnormality in the measurement process, and the equipment status or operation error needs to be checked.

[0058] Reference Figure 5 as shown, the initial virtual edge 22a is perpendicular to the bottom edge 312 of the sample stage 31; an inherent angle α is formed between the projection light 102 of X-rays and the bottom edge 312 of the sample stage 31; a positioning angle β is formed between the initial specific crystal orientation 14a and the initial virtual edge 22a. The sample stage 31 and the wafer 10 synchronously rotate in opposite directions by a second angle Φ2, while the projection light 102 remains stationary, and the rotated specific crystal orientation 14b is perpendicular to the projection light 102; the included angle between the initial virtual edge 22a and the rotated virtual edge 22b is the second angle Φ2. A perpendicular line F is drawn to the projection light 102, and a second included angle α1 is formed between the initial virtual edge 22a and the perpendicular line F; since one side of the second included angle α1 (i.e., the initial virtual edge 22a) is perpendicular to one side of the inherent angle α (i.e., the bottom edge 312), and the other side of the second included angle α1 (i.e., the perpendicular line F) is perpendicular to the other side of the inherent angle α (i.e., the projection light 102), according to the geometric theorem that "if the two sides of two angles are respectively perpendicular to each other, then the two angles are equal", combined with Figure 5 it can be known that both are acute angles, so α1=α; a third included angle β1 is formed between the rotated virtual edge 22b and the perpendicular line F; since the rotated specific crystal orientation 14b is perpendicular to the projection light 102, and the perpendicular line F is perpendicular to the projection light 102, the rotated specific crystal orientation 14b is parallel to the perpendicular line F, and the included angles formed by the two with the same transversal (i.e., the rotated virtual edge 22b) are equal, that is, β1=β; the perpendicular line F is located between the initial virtual edge 22a and the rotated virtual edge 22b, so Φ2=α1+β1; substituting α1=α and β1=β, we obtain Φ2=α+β.

[0059] S43. Calculate the positioning angle β based on the first angle Φ1 and the second angle Φ2.

[0060] It can be seen from the above analysis that in the first scanning curve and the second scanning curve, the sign of the positioning angle β (i.e., the positive or negative sign in the algebraic expression) is opposite, while the sign of the inherent angle α is the same. Therefore, by jointly analyzing the two scanning curves, the systematic error introduced by the inherent angle α can be eliminated, so as to accurately extract the true included angle between the specific crystal orientation 14 and the bottom edge 21 (i.e., the positioning angle β). This positioning angle β characterizes the actual deflection amount of the specific crystal orientation 14 relative to the bottom edge 21, and provides an accurate angle reference for the subsequent dicing step.

[0061] Furthermore, the difference between the second angle Φ2 and the first angle Φ1 is calculated, and half of this difference is taken as the positioning angle β; where the second angle Φ2 is greater than the first angle Φ1. That is, the formula for calculating the positioning angle β is: β = (Φ2 - Φ1) / 2.

[0062] The above calculation process does not require prior knowledge or calibration of the inherent angle α of the X-ray diffractometer 30, nor does it require the introduction of additional reference samples or calibration procedures. It is simple to operate and highly efficient. During this calculation, the inherent angle α between the X-ray beam 101 and the surface of the wafer 10 to be processed is completely canceled out, eliminating the need for prior calibration or separate measurement. This eliminates the influence of the systematic error introduced by the inherent angle α on the measurement result of the positioning angle β, achieving decoupling of the device's inherent angle α from the measurement result. Through this method, this embodiment only needs to extract the in-plane scanning angle corresponding to the diffraction intensity peak from the two scanning curves to obtain the accurate positioning angle β through calculation. The operation is simple and does not require additional calibration steps, which is beneficial for improving measurement efficiency and the processing accuracy of the positioning edge. While eliminating the influence of the device's inherent angle α, it also ensures the simplicity of the calculation and the reliability of the results.

[0063] Step S5: Based on the positioning angle β, the wafer 10 to be processed is cut to obtain a wafer with a positioning edge.

[0064] After obtaining the positioning angle β, the wafer 10 to be processed is transferred to the dicing equipment 40. During dicing, the edge of the wafer 10 is cut using the specific crystal orientation 14 as a reference direction and in conjunction with the positioning angle β, forming a straight positioning edge. This positioning edge is strictly aligned with the specific crystal orientation 14 inside the wafer 10 and can serve as a crystallographic direction reference for subsequent key processes such as photolithography alignment, ensuring positioning accuracy during wafer processing.

[0065] Through steps S1 to S5, this embodiment utilizes the polar structure characteristics of the wafer 10 to be processed. By performing phi scans on the first surface 11 and the second surface 12 respectively, the sign of the positioning angle β is reversed in the two measurements. Then, by combining the two scan curves, the influence of the inherent angle α between the X-ray 101 and the surface of the wafer 10 to be processed is eliminated, and the positioning angle β between the specific crystal orientation 14 and the bottom edge 21 is accurately determined. Based on this, the cutting and preparation of the positioning edge is completed. Compared with the prior art, which only performs a phi scan on a single surface of the wafer, the angle measured by this method is coupled with the systematic error introduced by the inherent angle of the X-ray 101 incident, which cannot accurately separate the true positioning angle, resulting in limited processing accuracy of the positioning edge. In contrast, this embodiment eliminates the systematic error introduced by the inherent angle α by performing phi scans on the first surface 11 and the second surface 12 of the wafer 10 to be processed respectively and combining the two scan curves to determine the positioning angle, thereby improving the processing accuracy of the positioning edge.

[0066] In some examples, the parameter Figure 7 and Figure 8 As shown, based on the positioning angle β, the wafer 10 to be processed is diced to obtain a wafer with a positioning edge, including: placing the fixing device 20 that holds the wafer 10 to be processed on the worktable 41 of the dicing equipment 40, such that the bottom edge 21 of the fixing device 20 is aligned with the reference direction of the dicing equipment 40 (e.g., ...). Figure 7 Align the Y-axis direction of the mechanical coordinate system of the cutting equipment 40; rotate the worktable 41 by a positioning angle β to align the specific crystal orientation 14 with the cutting direction of the cutting equipment 40; cut the wafer 10 to be processed along the cutting direction to form a positioning edge.

[0067] After completing the in-plane scanning based on X-ray diffraction and calculating the positioning angle β, the fixture 20 carrying the wafer 10 to be processed is transferred to the worktable 41 of the dicing equipment 40. Figure 7 As shown, the worktable 41 is in its initial position (without angle compensation). Using the existing bottom edge 21 on the fixing device 20 as a mechanical positioning reference, it is aligned with the preset reference direction (such as the Y-axis) of the cutting equipment 40. Since the relative position of the fixing device 20 and the wafer 10 has been pre-calibrated, the spatial orientation of the wafer 10 is initially determined when the bottom edge 21 is aligned with the equipment reference. However, due to wafer growth or previous process errors, the specific crystal orientation 14 of the wafer 10 to be processed and the actual cutting path of the cutting equipment 40 ( Figure 7 There is still an angular deviation between the direction indicated by the dashed line 42 and the previous measured positioning angle β.

[0068] To eliminate the above deviation, the worktable 41 is rotated around its axis of rotation by a positioning angle β. For example... Figure 8 As shown, after the rotation is completed, the specific crystal orientation 14 of the wafer 10 to be processed is deflected in space, thereby aligning with the cutting path of the cutting device 40. Figure 8 (Dash line 42) achieves precise parallelism. At this point, the specific crystal orientation 14 is precisely aligned with the cutting direction. In the aligned state, the cutting device 40 moves along the aligned cutting path ( Figure 8 (Dash line 42) The wafer 10 to be processed is processed to form a high-precision positioning edge.

[0069] Since the positioning angle is obtained by scanning with X-ray diffraction on both sides and after offsetting the inherent error of the equipment, the angular deviation between the positioning edge and the specific crystal orientation 14 is extremely small, which can meet the requirements of subsequent semiconductor processes such as photolithography or epitaxy for crystal orientation accuracy.

[0070] In some examples, before dicing the wafer 10 to be processed, the above preparation method further includes: repeatedly performing phi scan operations on the first surface 11 and the second surface 12 to obtain at least two positioning angles β; calculating the standard deviation σ of the at least two positioning angles β; and when the standard deviation is less than or equal to a set standard deviation threshold, calculating the mean of the at least two positioning angles β as the final positioning angle.

[0071] After completing the initial phi scan of the first surface 11 and the second surface 12 and obtaining a positioning angle β, this positioning angle β is not immediately used for subsequent cutting. Instead, the phi scan operation is repeated to obtain multiple positioning angles β. During each repeated scan, the fixing device 20 carrying the wafer 10 to be processed is placed on the sample stage 31 of the X-ray diffractometer 30, and the first surface 11 and the second surface 12 of the wafer 10 to be processed are sequentially scanned in-plane using phi scans. The first angle Φ1 and the second angle Φ2 are extracted respectively, and the corresponding positioning angle β is calculated according to the formula for calculating the positioning angle β. Through multiple repeated scans, at least two positioning angles β can be obtained, such as β1 and β2. Subsequently, the standard deviation σ of the at least two positioning angles β is calculated. The standard deviation σ is used to characterize the dispersion between the positioning angles β obtained from multiple measurements. When the standard deviation σ is less than or equal to a set standard deviation threshold, it indicates that the results of multiple measurements have high consistency and small random fluctuations. In this case, the mean of the at least two positioning angles β can be calculated, and this mean is used as the final positioning angle for subsequent cutting of the wafer 10 to be processed. When the standard deviation σ is greater than the set standard deviation threshold, it indicates that the dispersion between multiple measurement results is large, which may be due to clamping deviation, scanning abnormality, diffraction peak extraction error, or other interference factors. In this case, instead of directly using the mean as the final positioning angle, a supplementary scan is performed to obtain more positioning angles β, and the standard deviation σ is recalculated until the standard deviation σ is less than or equal to the set standard deviation threshold.

[0072] The standard deviation threshold can be set based on the wafer material, crystal orientation type, positioning accuracy requirements, and the measurement repeatability of the X-ray diffractometer 30. For example, in high-precision positioning scenarios, a smaller standard deviation threshold can be set to improve the reliability of the final positioning angle; in scenarios with relatively lower accuracy requirements, the standard deviation threshold can be appropriately relaxed.

[0073] In some examples, the standard deviation threshold is set to 0.05°. Specifically, when the standard deviation σ of at least two positioning angles β obtained from repeated measurements does not exceed 0.05°, it indicates that the consistency between the multiple measurement results is high, and the dispersion of each positioning angle β around its mean is within an acceptable range. At this time, the measurement process can be considered stable and reliable, and the mean of at least two positioning angles β is then calculated as the final positioning angle.

[0074] For example, suppose that three phi scan operations are performed on the same wafer 10 to be processed, obtaining positioning angles β1=15.32°, β2=15.35°, and β3=15.30° respectively. The mean is 15.32°, and the standard deviation σ is approximately 0.025°, which is less than the set threshold of 0.05°. Therefore, the mean of 15.32° can be used as the final positioning angle for subsequent cutting. Conversely, if the positioning angles obtained from the three scans are β1=15.10°, β2=15.50°, and β3=15.20° respectively, the standard deviation σ is approximately 0.21°, which is greater than the set threshold of 0.05°. This indicates that the measurement results have large dispersion, which may be caused by clamping deviation, abnormal diffraction peak extraction, or other interference factors. In this case, the mean should not be used directly, and the cause should be investigated and the scan measurement should be repeated.

[0075] By using the above method, the positioning angle β can be repeatedly measured and its consistency verified before cutting, avoiding positioning angle deviation caused by single measurement abnormalities or random errors, thereby improving the stability and reliability of the final positioning angle, which is beneficial to improving the alignment accuracy between the positioning edge and the specific crystal orientation 14.

[0076] In some examples, the scanning range of the first surface 11 phi scan is 0° to 60°, and the scanning step size is 0.1° / second.

[0077] In some examples, the scanning range of the second surface 12 phi scan is 0° to 60°, and the scanning step size is 0.1° / second.

[0078] When performing an in-plane phi scan on the first surface 11, the sample stage 31 rotates continuously around axis Q from a starting position of 0° to a ending position of 60°, with a scan step size set to 0.1° / second, meaning the sample stage 31 rotates 0.1° per second. The detector (not shown) in the X-ray diffractometer 30 simultaneously acquires X-ray diffraction intensity signals at each angular position. This scan range covers all possible angular intervals of the wafer 10 in the specific crystal orientation 14 within the in-plane direction, ensuring that diffraction peak signals can be completely captured in a single scan and avoiding peak omissions due to insufficient scan range. When performing an in-plane phi scan on the second surface 12, the same scan range and scan step size are used as for the first surface 11, i.e., the scan range is 0°~60°, and the scan step size is 0.1° / second. Maintaining consistent scan parameters helps eliminate additional angular deviations introduced by differences in scan conditions, ensuring that the first angle Φ1 and the second angle Φ2 are obtained under the same measurement reference, thereby improving the accuracy of subsequent positioning angle β calculations.

[0079] In some examples, the wafer 10 to be processed is gallium nitride (GaN), with a first surface 11 being the gallium face and a second surface 12 being the nitrogen face. The GaN wafer 10 is a GaN wafer, belonging to the hexagonal crystal system. The GaN wafer has two surfaces with opposite polarities: one surface terminated by gallium (Ga) atoms, called the gallium face (Ga-face), i.e., the first surface 11; the other surface terminated by nitrogen (N) atoms, called the nitrogen face (N-face), i.e., the second surface 12. Due to the different atomic types, bonding states, and surface energies of the gallium and nitrogen faces, the two surfaces exhibit different diffraction characteristics in the X-ray diffraction response. When the wafer 10 to be processed is scanned in-plane with the first surface 11 (gallium face) facing the X-ray source using an in-plane phi scan, the diffraction peak position acquired by the detector corresponds to the first angle Φ1; when the wafer 10 is flipped to face the second surface 12 (nitrogen face) towards the X-ray source using an in-plane phi scan, due to the reversal of crystal polarity causing a mirror-symmetric transformation of the diffraction geometry, the diffraction peak position acquired by the detector corresponds to the second angle Φ2. Based on the above polarity difference, the first angle Φ1 and the second angle Φ2 satisfy the relationships Φ1=α-β and Φ2=α+β respectively. Thus, the positioning angle β can be accurately extracted by subtracting the two equations, and the influence of the inherent angle α of the equipment can be eliminated.

[0080] It should be noted that this disclosure uses gallium nitride wafers as an example for illustration, but this solution is also applicable to other hexagonal semiconductor materials with polar surfaces, such as silicon carbide (SiC) or aluminum nitride (AlN), and this disclosure does not limit them.

[0081] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail with reference to embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. Example

[0082] This embodiment uses a gallium nitride wafer 10 as an example, with its first surface 11 being a gallium surface and its second surface 12 being a nitrogen surface. The specific crystal orientation 14 is selected as the [10-10] crystal orientation. A bottom edge 21 has been formed on the fixing device 20 to serve as a mechanical positioning reference in the subsequent dicing process.

[0083] Step S1: Place the gallium nitride wafer with its first surface 11 (gallium surface) facing upward in the fixing device 20, so that the outer circumferential surface of the gallium nitride wafer is in contact with the bottom edge 21 of the fixing device 20, thus completing the loading and fixing of the wafer.

[0084] Step S2: Place the fixture 20 containing the gallium nitride wafer onto the sample stage 31 of the X-ray diffractometer 30, with the bottom edge 21 of the fixture 20 facing the preset initial orientation. The sample stage 31 can rotate and scan around an axis Q passing through the center of the gallium nitride wafer and perpendicular to the bearing surface 311.

[0085] Step S3: Perform an in-plane phi scan on the first surface 11 (i.e., the gallium surface). Specifically, the sample stage 31 rotates continuously around axis Q from the starting position of 0° to the ending position of 60°, with the scanning step size set to 0.1° / second. The detector synchronously acquires the X-ray diffraction intensity signals at each angular position and extracts the first angle Φ1 corresponding to the diffraction peak from them.

[0086] Step S4: Remove the fixture 20 from the sample stage 31, flip the gallium nitride wafer so that the second surface 12 (nitrogen surface) faces upwards and reload it into the fixture 20, and then place the fixture 20 back onto the sample stage 31.

[0087] Step S5: Perform an in-plane phi scan on the second surface 12 (i.e., the nitrogen surface). The scanning parameters are consistent with those in step S3, i.e., the scanning range is 0°~60°, the scanning step size is 0.1° / second, and the detector synchronously acquires the X-ray diffraction intensity signals at each angle position, from which the second angle Φ2 corresponding to the diffraction peak is extracted.

[0088] Step S6: Calculate the positioning angle β based on the first angle Φ1 and the second angle Φ2.

[0089] The formula for calculating the positioning angle β is: β = (Φ2-Φ1) / 2.

[0090] In this formula, the inherent angle α of the device is completely canceled out during the subtraction process. Therefore, the positioning angle β depends only on the lattice orientation characteristics of the gallium nitride wafer itself and is not affected by the installation deviation of the X-ray diffractometer 30.

[0091] Step S7: Repeat steps S1 to S6, performing three independent phi scans to obtain three positioning angles β1, β2, and β3. Calculate the standard deviation σ of the three positioning angles. When the standard deviation σ does not exceed the set standard deviation threshold of 0.05°, calculate the mean of the three positioning angles as the final positioning angle.

[0092] For example, the positioning angles obtained from the three scans are β1=15.32°, β2=15.35°, and β3=15.30°, respectively. The mean is 15.32° and the standard deviation σ is about 0.025°, which is less than the set threshold of 0.05°. Therefore, the mean of 15.32° is taken as the final positioning angle.

[0093] Step S8: Transfer the fixture 20 containing the gallium nitride wafer to the worktable 41 of the cutting equipment 40, aligning the bottom edge 21 of the fixture 20 with the reference direction of the cutting equipment 40.

[0094] Step S9: Rotate the worktable 41 around its rotation axis by a positioning angle β (i.e., 15.32°) to precisely align the [10-10] crystal orientation of the gallium nitride wafer with the cutting direction of the cutting device 40.

[0095] Step S10: Cut the gallium nitride wafer along the cutting direction to form a positioning edge on the outer circumference of the wafer. Since the positioning angle β has been accurately obtained through two X-ray diffraction scans on both sides and has been verified by repeated measurements, the final positioning edge has a very small angular deviation from the [10-10] crystal orientation, which can meet the crystal orientation accuracy requirements of subsequent semiconductor processes such as photolithography or epitaxy.

[0096] On the other hand, based on the specific implementation of the wafer fabrication method disclosed in the foregoing embodiments, the present invention also discloses a wafer, which is processed by the wafer fabrication method in the foregoing embodiments. The wafer fabrication method disclosed in this embodiment is as described above and will not be repeated here.

[0097] The wafer is a hexagonal semiconductor wafer, such as a gallium nitride (GaN) wafer, a silicon carbide (SiC) wafer, or an aluminum nitride (AlN) wafer. The wafer has a first surface 11 and a second surface 12, wherein the first surface 11 is a gallium surface and the second surface 12 is a nitrogen surface. The specific crystal orientation 14 is [10-10] crystal orientation.

[0098] The positioning edge is formed on the outer circumferential surface of the wafer, and its position is determined by the positioning angle β in the aforementioned preparation method. Since the positioning angle β is obtained by two in-plane X-ray diffraction scans on both the front and back sides, and the inherent angle α of the equipment is completely canceled out during the calculation, the positioning angle β depends only on the lattice orientation characteristics of the wafer itself and is not affected by the installation deviation of the measuring equipment.

[0099] In some examples, the angular deviation between the positioning edge and the [10-10] crystal orientation does not exceed 0.5°. Due to the extremely high alignment accuracy between the positioning edge and the specific crystal orientation 14, the wafer can be directly used for subsequent semiconductor processes without the need for additional crystal orientation calibration, which helps to improve processing efficiency and product yield.

[0100] The detailed descriptions listed above are merely specific descriptions of feasible embodiments of the present invention, and are not intended to limit the scope of protection of the present invention. All equivalent embodiments or modifications made without departing from the spirit of the present invention should be included within the scope of protection of the present invention.

[0101] Furthermore, it should be understood that although this specification describes embodiments, not every embodiment contains only one independent technical solution. This narrative style is merely for clarity. Those skilled in the art should consider the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

Claims

1. A method for preparing a wafer, characterized in that, include: The wafer to be processed is placed with its first surface facing upwards in the fixing device; The fixture holding the wafer to be processed is placed in an X-ray diffractometer to perform a phi scan on the first surface to obtain a first scan curve; The wafer to be processed is placed with its second surface facing upwards on the fixing device to perform a phi scan on the second surface, thereby obtaining a second scan curve; wherein, the first scan curve and the second scan curve are used to represent the correspondence between multiple in-plane scanning angles and multiple diffraction intensities, respectively. Based on the first scanning curve and the second scanning curve, the angle between the specific crystal orientation of the wafer to be processed and the bottom edge of the fixing device is determined to obtain the positioning angle; Based on the positioning angle, the wafer to be processed is cut to obtain a wafer with a positioning edge.

2. The preparation method according to claim 1, characterized in that, The step of determining the angle between a specific crystal orientation of the wafer to be processed and the bottom edge of the fixing device based on the first scanning curve and the second scanning curve to obtain the positioning angle includes: The in-plane scanning angle corresponding to the maximum diffraction intensity is extracted from the first scanning curve to obtain the first angle; The in-plane scanning angle corresponding to the maximum diffraction intensity is extracted from the second scanning curve to obtain the second angle; The positioning angle is calculated based on the first angle and the second angle.

3. The preparation method according to claim 2, characterized in that, The step of calculating the positioning angle based on the first angle and the second angle includes: Calculate the difference between the second angle and the first angle, and take half of the difference as the positioning angle; The second angle is greater than the first angle.

4. The preparation method according to claim 2, characterized in that, The specific crystal orientation is the [10-10] crystal orientation; And / or, the X-ray diffractometer further includes a sample stage for supporting the fixing device; the first angle satisfies: Φ1=α-β; the second angle satisfies: Φ2=α+β; where β is the positioning angle, and α is the angle between the projection of the X-ray emitted by the X-ray diffractometer onto the surface of the sample stage and the bottom edge of the sample stage.

5. The preparation method according to claim 1, characterized in that, The step of cutting the wafer to be processed based on the positioning angle to obtain a wafer with a positioning edge includes: The fixing device that holds the wafer to be processed is placed on the worktable of the cutting equipment, and the bottom edge of the fixing device is aligned with the reference direction of the cutting equipment. The worktable is rotated by the positioning angle to align the specific crystal orientation with the cutting direction of the cutting equipment. The wafer to be processed is cut along the cutting direction to form the positioning edge.

6. The preparation method according to claim 1, characterized in that, Before dicing the wafer to be processed, the method further includes: Repeatedly perform the phi scan operation on the first surface and the second surface to obtain at least two of the positioning angles; Calculate the standard deviation of the at least two positioning angles; When the standard deviation is less than or equal to the set standard deviation threshold, the mean of the at least two positioning angles is calculated as the final positioning angle.

7. The preparation method according to claim 1, characterized in that, The scanning range of the first surface using the phi scan is 0°~60°, and the scanning step size is 0.1° / second; And / or, the scanning range of the second surface subjected to phi scanning is 0°~60°, and the scanning step size is 0.1° / second.

8. The preparation method according to claim 1, characterized in that, The wafer to be processed is gallium nitride, with the first surface being the gallium surface and the second surface being the nitride surface.

9. A wafer, characterized in that, The wafer is prepared using the wafer preparation method described in any one of claims 1-8.

10. A semiconductor system, characterized in that, include: An X-ray diffractometer is used to perform phi scans on the first and second surfaces of the wafer to be processed, respectively, to obtain a first scan curve and a second scan curve; wherein, the first scan curve and the second scan curve are used to represent the correspondence between multiple in-plane scanning angles and multiple diffraction intensities; The processing device, coupled to the X-ray diffractometer, is used to determine the angle between a specific crystal orientation of the wafer to be processed and the bottom edge of the fixing device based on the first scanning curve and the second scanning curve, thereby obtaining the positioning angle; A cutting device, coupled to the processing device, is used to cut the wafer to be processed based on the positioning angle to obtain a wafer with a positioning edge.

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