Method for measuring the surface roughness of the bevel portion of a notch

Abstract

To provide a measuring method capable of measuring roughness of a bevel part of a straight-line part connecting a deepest position of a notch and an outer periphery of a silicon single crystal wafer.SOLUTION: A measuring method for measuring a surface roughness of a bevel part of a notch of a silicon single crystal wafer by AFM measures the surface roughness by arranging the silicon single crystal wafer so that a straight-line part and a scanning direction of an AFM probe are at right angles and scanning the AFM probe in a thickness direction of the silicon single crystal wafer, when measuring the surface roughness of the bevel part of the straight-line part connecting a deepest position of the notch and an outer periphery of the silicon single crystal wafer.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a method for measuring the surface roughness of the bevel portion of a notch on a silicon single crystal wafer using an AFM (Atomic Force Microscope).

Background Art

[0002] This will be described using FIG. 5. The shape and roughness of the edge 2 of the silicon single crystal wafer 1 (hereinafter simply referred to as the wafer) are one of the important qualities because they affect the yield of the devices taken from the periphery of the wafer. In particular, the starting portion of the bevel portion 3, that is, the boundary between the polished surface and the bevel, is close to the device layer, so the quality is important. However, the quality of this portion is created by the surface polishing process and the edge polishing process. The resulting quality consists of the properties of both the smooth surface polishing surface 4 and the rough edge polishing surface 5, and since the surface has an angle, it is a difficult place to evaluate.

[0003] In addition, the edge polishing process of the notch portion is a more special polishing process. Since the place is narrow and deep, it is difficult to evaluate the shape and roughness of the notch portion. Also, there is a lot of dust generation due to the roughness of the notch portion, and the evaluation of the notch bevel portion, which is the dust generation part, is important. However, since the notch width is as narrow as about 1.5 mm, the measurement difficulty is high, and it has been difficult to measure accurately.

[0004] As prior art, Patent Document 1 and Patent Document 2 describe that the surface roughness of a wafer is measured by AFM and that the roughness with a wavelength of 20 nm to 50 nm affects the breakdown voltage of the oxide film, which is the representative crystal quality of the wafer, but there is no description about the measurement of the surface roughness of the notch portion.

[0005] Also, Patent Document 3 describes a method for measuring the grinding streaks of the notch portion. However, since light is used, there is a problem that only large roughness (grinding streaks) can be detected.

[0006] Furthermore, Patent Document 4 discloses a notch inspection method in which the wafer is held vertically and the notch shape is evaluated using a tactile shape measuring means. This method measures the outer shape of the tip of the notch while rotating the wafer and is not intended to evaluate the roughness of the bevel portion of the notch.

[0007] As described above, no technique has been disclosed for accurately measuring the surface roughness of the bevel portion of a notch.

[0008] When measuring the surface roughness of a wafer, it is easy to determine the roughness of the base shape because it is flat. However, when determining the roughness of the edge, it is necessary to extract only the roughness component while excluding the substrate shape. Quadratic curve fitting is often used to extract the roughness component, but because the edge shape is complex, cubic or higher-order curve fitting is often performed.

[0009] Here, we will explain an example of fitting.

[0010] Figure 6(a) shows the measured surface roughness of the wafer. Although the roughness changes in the vertical direction, the average height within the measurement range (horizontal direction in the figure) remains constant. Figure 6(b) is a schematic diagram of the roughness calculated based on the measurement results. Roughness is represented, for example, by the total difference between the average height and each measured value, i.e., the area of ​​the shaded region in Figure 6(b). This area is often averaged over the measurement range (horizontal length in Figure 6(b)) and expressed as surface roughness Ra in length.

[0011] Next, we will explain the case where there is a slope in which the height gradually decreases, such as in the bevel of an edge, using Figure 7. Figure 7(a) is a schematic diagram in which the difference from the measured value and Figure 7(b) is filled in. In this area, it is clear that the majority of this area is not roughness but the effect of the original shape gradually decreasing, so it is necessary to remove the original shape by fitting.Therefore, as shown in Figure 7(c), a fitting curve 6 is calculated from the original measured value, and by removing the fitting curve component from the measured value to leave only the roughness component, Figure 7(d) is obtained.Since Figure 7(d) is a change in only the roughness component, roughness alone can be evaluated with high accuracy using the same method as explained in Figure 6.

[0012] However, while this method is often effective for line data, fitting can be difficult for surface data. Even with 2D data, fitting is easy if the reference plane slopes uniformly downwards or follows a quadratic curve, but fitting becomes difficult when the reference plane has a complex 2D shape, such as changing in both the X and Y directions, and thus it becomes difficult to determine the roughness.

[0013] This will be explained using Figure 8. Figure 8 shows a schematic diagram of wafer notch 7. The arrows in Figure 8(a) represent the AFM measurement position and scan direction. Measurement at position 8 is easy to fit because the reference plane slopes uniformly downward only in the X direction (Figure 8(b) shows an image of the measurement position), but at position 9, fitting is difficult because changes in the X direction and the Y direction are combined (Figure 8(c) shows an image of the measurement position). [Prior art documents] [Patent Documents]

[0014] [Patent Document 1] Japanese Patent Publication No. 2010-16078 [Patent Document 2] Japanese Patent Publication No. 2013-38435 [Patent Document 3] Japanese Patent Publication No. 2014-85295 [Patent Document 4] Japanese Patent Publication No. 2003-37138 [Overview of the project] [Problems that the invention aims to solve]

[0015] Conventionally, there was a problem in that it was difficult to measure the roughness of the bevel portion (position 9) of the straight line 12 connecting the deepest position 10 of the notch 7 and the outer edge 11 of the silicon single crystal wafer, as shown in position 9 of Figure 8.

[0016] This invention has been made in view of the problems of the prior art described above, and aims to provide a measurement method that can measure the roughness of the bevel portion in the straight line connecting the deepest position of the notch and the outer edge of the silicon single crystal wafer. [Means for solving the problem]

[0017] To solve the above problems, the present invention provides a method for measuring the surface roughness of the bevel portion of a notch in a silicon single crystal wafer, which is a method for measuring the surface roughness of the bevel portion of a notch in a silicon single crystal wafer using an AFM, wherein when measuring the surface roughness of the bevel portion of a straight line connecting the deepest position of the notch and the outer circumference of the silicon single crystal wafer, the silicon single crystal wafer is positioned such that the scanning direction of the AFM probe is perpendicular to the straight line, and the AFM probe is scanned in the thickness direction of the silicon single crystal wafer to perform the measurement.

[0018] With this measurement method, the component of the bevel's shape—the change in curvature (slope)—can be separated into changes parallel to the straight section (tiny changes with almost no slope) and changes perpendicular to the straight section (large changes mainly consisting of slope). The AFM probe scans and measures the changes perpendicular to the straight section (large changes mainly consisting of slope), thus providing the most accurate measurement of the bevel's slope. Therefore, since the component of the bevel's slope included in the measurement can be determined, the other components included in the measurement, namely the roughness component, can be estimated, making roughness measurement possible.

[0019] Further, it is preferable to perform quadratic fitting on the measured values measured by the AFM in the X direction and the Y direction parallel to the surface of the silicon single crystal wafer, respectively.

[0020] By performing such fitting, the inclination of the bevel portion can be easily obtained from the measured values scanned and measured by the AFM probe.

Advantages of the Invention

[0021] In the method for measuring the surface roughness of the bevel portion of the notch of the silicon single crystal wafer of the present invention, the inclination of the bevel portion can be accurately measured, and the inclination component of the bevel portion included in the measured value can be known. Therefore, other components included in the measured value, that is, the roughness component can be estimated, and roughness measurement becomes possible.

Brief Description of the Drawings

[0022] [Figure 1] It is a schematic diagram of a method for measuring a straight portion in an embodiment of the present invention. [Figure 2] It is a schematic diagram of a measurement method in an embodiment of the present invention. [Figure 3] It is a diagram for explaining Example 1 and Comparative Example 1 of the present invention. [Figure 4] It is a diagram showing the result of measuring the roughness of the edge of a wafer as a reference example. [Figure 5] It is a schematic diagram showing the appearance and edge of a general wafer. [Figure 6] It is a schematic diagram showing the surface roughness and measured value of a wafer surface when there is no inclination. [Figure 7] It is a schematic diagram showing the surface roughness and measured value of a wafer surface when there is an inclination. [Figure 8] It is a schematic diagram showing the AFM measurement position and scan direction for a conventional notch. [Figure 9] It is a schematic diagram when measuring the edge by rotating the wafer while fixing the scan direction. [Figure 10]This is a schematic diagram illustrating how to measure the edge by changing the measurement position of the AFM without rotating the wafer. [Figure 11] This is a schematic diagram showing the difference between the deepest part (bottom) of the notch and the straight section. [Modes for carrying out the invention]

[0023] In light of the problems with the prior art described above, the inventor conducted thorough research and devised a measurement method that can measure the roughness of the bevel portion in the straight line connecting the deepest point of the notch and the outer edge of the silicon single crystal wafer. The inventor also devised a method that allows for easy fitting during the measurement process.

[0024] Refer to Figure 9 for explanation. When measuring the roughness of the bevel portion of an edge using AFM, it is easier to pick up irregularities if the direction of the probe's movement (scan direction) is radial (thickness direction) rather than circumferential. The scan direction of the AFM probe is determined by the device, so when measuring an arbitrary edge position on wafer 1, rotate wafer 1 so that the direction of the probe's movement (scan direction) relative to the edge portion of wafer 1 is always constant. For example, if the AFM probe position 13 is on the right side of the wafer, the measurement of the edge portion can always be performed under the same conditions for the probe and wafer by rotating the wafer so that the measurement position is on the right side.

[0025] On the other hand, if the AFM measurement position is changed without rotating the wafer, the measurement positions 14, 15, and 16 and the AFM scan direction will be as shown in Figure 10, and the scan direction of the edge of wafer 1 and the needle will constantly change, making it impossible to measure under the same conditions.

[0026] As shown in Figure 11, when measuring the notch 7 by setting the AFM probe on the right edge of the wafer, the deepest part (bottom) 10 of the notch 7 has a shape where the reference surface for roughness (reference plane) drops in one dimension, making it easy to measure. However, the straight portion 12 of the notch 7 has a shape where the reference plane changes in two dimensions, making it difficult to eliminate the influence of the reference plane. However, as shown in Figure 1, by rotating the wafer 1 by 45 degrees, the shape of the straight portion 12 of the notch 7 changes in a simple one-dimensional way, making it easy to measure.

[0027] When measuring the surface roughness of the beveled edge, the detector position of the device is often kept in one place, and the measurement location is moved to the detector position by rotating the wafer. This allows for comparison of the outer edge of the wafer under the same conditions. For example, if the right edge of the wafer is set as the detection position of the measuring device, the AFM's scanning direction will always be perpendicular to the edge, making it the position where the edge can be detected as the simplest changing shape.

[0028] Similarly, when measuring the surface roughness of the bevel at the deepest point (bottom) of the notch, the measurement is taken with the notch at the 3 o'clock position. The AFM scan direction is perpendicular to the notch bottom shape, making it the position where the notch bottom shape can be detected as the simplest changing shape. This is the same as in the case of the edge described above. However, for the straight section of the notch that is slightly offset from this position, the relationship between the AFM scan direction and the edge shape becomes oblique, making shape fitting difficult.

[0029] Therefore, the inventor reasoned that in order to accurately measure the notch line, it was necessary to position the measurement point so that the shape changes most simply, just as with the edge shape. Thus, instead of setting the notch at the 3 o'clock position for measurement, by shifting it 45 degrees to either the 1:30 or 4:30 position, the inventor discovered that the notch line could be considered to have the same shape as the edge, allowing the same fitting conditions to be applied as for the edge.

[0030] If the notch position is set at the 4:30 position, referring to Figure 2, and assuming AFM probe positions 17, 18, and 19, and the straight sections of the notch 20 and 21, the AFM scan direction will be perpendicular to the straight section 20 at AFM probe position 19, which is located in the straight section 20. The relationship between the notch shape at position 19 and the AFM scan direction is the same as the relationship when measuring the edge described above, and it is the position where the bevel of the notch can be detected as the simplest changing shape.

[0031] Similarly, if the notch position is set to 1:30, the AFM scan direction can be set to a direction perpendicular to the linear portion 21 at the AFM probe position 18 located in the linear portion 21.

[0032] In other words, by setting the notch position to 1:30 or 4:30, measurements can be taken for each of the straight sections 20 and 21 in the same way as for the edges, making fitting easier and allowing for the acquisition of the correct roughness.

[0033] As described above, the method for measuring the surface roughness of the bevel portion of a notch in a silicon single crystal wafer according to this embodiment is a method for measuring the surface roughness of the bevel portion of a notch in a silicon single crystal wafer using an AFM, wherein when measuring the surface roughness of the bevel portion of the straight line portion 12(20, 21) connecting the deepest position of the notch and the outer circumference of the silicon single crystal wafer, the silicon single crystal wafer is rotated and positioned so that the scanning direction of the AFM probe is perpendicular to the straight line portion 12(20, 21), and the measurement is performed by scanning the silicon single crystal wafer in the thickness direction with the AFM probe.

[0034] With this measurement method, the component of the bevel, which is not a component of fine changes in roughness but rather a component of large changes in shape (slope), can be separated into changes in the direction parallel to the straight section 12(20, 21) (tiny changes that contain almost no slope) and changes in the direction perpendicular to the straight section 12(20, 21) (large changes mainly consisting of slope). Since the AFM probe scans and measures the changes in the direction perpendicular to the straight section 12(20, 21) (large changes mainly consisting of slope), the slope of the bevel can be measured most accurately. Therefore, since the component of the bevel's slope included in the measurement value can be determined, the other components included in the measurement value, namely the roughness component, can be estimated, and roughness measurement becomes possible.

[0035] Furthermore, the measured values ​​obtained by AFM are subjected to secondary fitting in both the X and Y directions, which are parallel to the plane of the silicon single-crystal wafer.

[0036] By performing this type of fitting, the inclination of the bevel can be easily determined from the measurements taken by scanning with an AFM probe. [Examples]

[0037] The present invention will be described in detail below with reference to examples and comparative examples, but this is not intended to limit the present invention.

[0038] [Example 1, Comparative Example 1] Figure 3 shows the results of measuring the surface roughness at the notch position of a 300 mm wafer.

[0039] Figure 3 shows Example 1 on the left and Comparative Example 1 on the right, from top to bottom: a photograph of the area around the AFM probe, a schematic diagram of the entire wafer, a schematic diagram of the area around the notch, the measurement results, and the obtained surface roughness (Ra).

[0040] Here, the measurement device used was a Park NX-Wafer, and the measurement area was set to 1 μm × 2 μm.

[0041] First, in Example 1, the surface roughness was measured with the notch at the 1:30 position, so that the straight portion of the notch and the scanning direction of the AFM probe were perpendicular. On the other hand, in Comparative Example 1, the surface roughness was measured with the notch at the 3 o'clock position, so that the straight portion of the notch and the scanning direction of the AFM probe were at an angle (45 degrees).

[0042] After the measurement is complete, a second-order fitting is performed in the X direction (left-right direction in the overall schematic diagram; perpendicular to the straight portion of the notch in Example 1) and a second-order fitting is performed in the Y direction (up-down direction in the overall schematic diagram; parallel to the straight portion of the notch in Example 1) to remove the effect of large-scale undulation changes (slope). The roughness measurement results after this are shown in the measurement results column. In Example 1, the effect of the shape is successfully removed, so the roughness is clearly visible. However, in Comparative Example 1, the effect of the shape is not removed, and the large undulation component of the entire data area becomes dominant, making it difficult to see the finer roughness.

[0043] Despite measuring the straight portion of the same notch, the fitting of the straight portion of the notch was unsuccessful in Comparative Example 1. However, in Example 1, by positioning the notch at 1:30 (so that the straight portion of the notch and the scanning direction of the AFM probe are perpendicular), and performing a secondary fitting in the X direction (perpendicular to the straight portion of the notch) + a secondary fitting in the Y direction (parallel to the straight portion of the notch), the fitting functioned well, and it was found that the roughness of the bevel portion of the straight portion of the notch could be measured.

[0044] [Reference example] As a reference example, Figure 4 shows an example where roughness was measured at the wafer edge instead of the notch bevel. In this case, the notch was set at the 6 o'clock position, and the bevel of the edge at the 3 o'clock position was measured. The fitting worked well, and measurements without waviness components were obtained. It is noteworthy that the measurement results of the notch bevel in Example 1 described above show that the roughness measurement was as accurate as that of this reference example.

[0045] It should be noted that the present invention is not limited to the embodiments described above. The embodiments described above are illustrative, and any configuration that has substantially the same technical idea as described in the claims of the present invention and achieves similar effects is included within the technical scope of the present invention. [Explanation of Symbols]

[0046] 1...Wafer (silicon single crystal wafer), 2...Edge, 3...Bevel, 4...Surface polished surface, 5...Edge polished surface, 6...Fitting curve, 7...Notch, 8, 9, 13, 14, 15, 16, 17, 18, 19...Position, 10...Deepest position (bottom), 11...Outer edge, 12, 20, 21...Straight sections.

Claims

1. A method for measuring the surface roughness of the bevel portion of a notch in a silicon single crystal wafer using an AFM, characterized in that, when measuring the surface roughness of the bevel portion of a straight line connecting the deepest position of the notch and the outer circumference of the silicon single crystal wafer in a top view, the silicon single crystal wafer is positioned such that the scanning direction of the AFM probe is perpendicular to the straight line, and the AFM probe is scanned in the thickness direction of the silicon single crystal wafer to perform the measurement.

2. The method for measuring the surface roughness of the bevel portion of a notch in a silicon single crystal wafer according to claim 1, characterized in that secondary fitting is performed in the X direction and the Y direction parallel to the upper surface of the silicon single crystal wafer, respectively, to the measurement value measured by the AFM.

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