Method for evaluating defect position in depth direction of wafer

Abstract

The present invention is a method for evaluating a defect position in the depth direction of a wafer using X-ray topography (XRT), the wafer having a front surface and a back surface, the method characterized by comprising: a step for causing X-rays to enter the front surface from a right direction and a left direction at incident angles such that a diffraction condition is satisfied, to acquire two XRT images on the back surface of the wafer for a right-eye image and a left-eye image; an alignment step for aligning the acquired two XRT images at a defect position on either the front surface or the back surface; and a defect position determination step for determining another defect position having a different depth direction of the wafer on the basis of a shift between the right-eye image and the left-eye image. Thus, a method for evaluating a defect position in the depth direction of a wafer is provided through a simple method using X-ray topography (XRT).

Description

Method for evaluating defect positions in the depth direction of a wafer

[0001] The present invention relates to a method for evaluating defect positions in the depth direction of a wafer by X-ray topography (XRT).

[0002] X-ray topography (XRT) is a device widely used to observe defects in crystalline materials, especially semiconductor device materials such as silicon wafers. XRT has a different measurement principle from X-ray CT, detecting distortion of the diffraction grating rather than differences in transmittance. Therefore, it can only be applied to single crystals, but it can measure very small defects.

[0003] The transmission method of XRT can determine the defect locations across the entire wafer, but it cannot obtain information such as the depth of the defects. Depth information of defects in semiconductor device materials is extremely important for determining whether the defects will affect device failure. In recent years, XRT has made it possible to obtain cross-sectional topographs and 3D topographs obtained by combining multiple cross-sectional topographs to obtain depth information. However, obtaining a cross-sectional topograph requires an extremely long measurement time, and obtaining 3D data that requires multiple cross-sectional topographs takes even longer. Furthermore, there are many limitations to such measurements, such as the need to use synchrotron radiation, a powerful X-ray generator.

[0004] Thus, it has been difficult to obtain three-dimensional data by superimposing cross-sectional topographic images in the past, but in recent years, methods have been developed that enable obtaining three-dimensional data of defects (see, for example, Patent Document 1). However, even with this method, it still takes time to obtain three-dimensional data, and the measurement area is small.

[0005] Furthermore, in semiconductor device materials, it is extremely important whether defects that cause device failures are located on the front or back surface of the device layer, but XRT was not able to easily evaluate the depth position of defects.

[0006] JP 2015-105831 A

[0007] The present invention has been made in view of the above-mentioned problems, and has an object to provide a method for evaluating defect positions in the depth direction of a wafer by a simple method using X-ray topography (XRT).

[0008] The present invention has been made to solve the above-mentioned problems, and provides a method for evaluating defect positions in the depth direction of a wafer using an X-ray topograph (XRT), the method including the steps of: irradiating X-rays onto the front surface from the right and left directions at incident angles that satisfy diffraction conditions, and acquiring two XRT images, a right-eye image and a left-eye image, on the back surface; aligning the two acquired XRT images at defect positions on either the front surface or the back surface; and determining defect position on the wafer from the deviation between the right-eye image and the left-eye image.

[0009] With this method for evaluating defect positions in the wafer depth direction, the right-eye image and the left-eye image coincide for defects on the aligned surface, but a deviation occurs between the right-eye image and the left-eye image for defects on a surface other than the aligned surface, and this deviation can be used to determine that the defects are located at different positions in the depth direction. This method requires only two images to be acquired using the normal method of using an X-ray topograph (XRT), allowing for measurement in a short time, and is versatile because it does not require special operations such as intensifying the synchrotron radiation or narrowing the measurement area. As a result, it is possible to evaluate defect positions in the wafer depth direction using an extremely simple method.

[0010] Furthermore, the method of irradiating X-rays onto the surface from the right and left directions at angles of incidence that satisfy the diffraction conditions preferably involves fixing the angle of incidence in either the right or left direction, irradiating X-rays to acquire an XRT image, and then rotating the wafer by 180° and irradiating X-rays to acquire an XRT image in the other direction, thereby acquiring two XRT images, the right-eye image and the left-eye image. This method makes it possible to easily acquire XRT images in the right and left directions without moving the X-ray generator or detector, thereby enabling evaluation of defect positions in the depth direction of the wafer in an extremely simple manner.

[0011] Furthermore, the defect position determination step preferably involves performing determination by inverting the black and white of one of the two acquired XRT images and combining them. Such a defect position determination step allows the defect position to be reliably determined based on the difference in color (black or white), and therefore can be suitably applied to a method for evaluating defect positions in the depth direction of a wafer.

[0012] The present invention also provides a method for evaluating defect positions in the depth direction of a wafer using an X-ray topograph (XRT), in which the wafer has a front surface and a back surface, X-rays are incident on the front surface from the right and left directions at angles of incidence that satisfy diffraction conditions, two XRT images, a right-eye image and a left-eye image, are obtained on the back surface, and the two XRT images are visually captured as a single image, thereby providing a method for evaluating defect positions in the depth direction of a wafer, in which defect positions in the depth direction are observed as a stereoscopic (3D image).

[0013] With this method for evaluating defect positions in the depth direction of a wafer, if two XRT images are visually viewed as a single image, defects at different depth positions will appear three-dimensional due to the misalignment of the two images, and this three-dimensional appearance can be used to determine that the defects are at different depth positions. This method requires only two images to be acquired using the normal method of using an X-ray topograph (XRT), enabling measurement in a short time. It is also versatile because it does not require special operations such as intensifying the synchrotron radiation or narrowing the measurement area. In addition, since the defect positions are evaluated visually, there is no need to prepare a separate evaluation device or the like. As a result, it is possible to evaluate defect positions in the depth direction of a wafer using an extremely simple method.

[0014] The wafer is preferably at least one of a single crystal wafer and a wafer on which a device is formed.

[0015] Such wafers can be suitably applied to a method for evaluating defect positions in the wafer depth direction using X-ray topography (XRT). In particular, silicon wafers and SiC wafers have incident angles that are diffraction conditions known in advance, and can be suitably applied to a method for evaluating defect positions in the wafer depth direction.

[0016] Furthermore, in the case of SiC wafers, crystal defects may be complexly intertwined within the crystal, and this method can be applied to confirm this situation.

[0017] Furthermore, for wafers on which devices are formed, it is possible to easily evaluate whether defects that cause device failures are on the surface or backside of the device layer.Since surface defects directly affect device yield in wafers on which devices are formed, being able to evaluate only surface defects rather than the entire depth direction can be useful for improving yield.

[0018] As described above, the method for evaluating defect positions in the wafer depth direction according to the present invention requires only two images to be acquired in the normal manner using an X-ray topograph (XRT), enabling measurement in a short time, and is versatile because it does not require special methods such as intensifying the synchrotron radiation or narrowing the measurement area. As a result, it is possible to evaluate defect positions in the wafer depth direction using an extremely simple method.

[0019] FIG. 1 is a schematic explanatory diagram of one embodiment of a method for evaluating defect positions in the depth direction of a wafer according to the present invention. FIG. 2 is a schematic diagram of a case where two XRT images are acquired by changing the angle of incidence of X-rays. FIG. 3 is a schematic diagram of a case where two XRT images are acquired by rotating the wafer. FIG. 4 is a schematic explanatory diagram of another embodiment of the present invention. FIG. 5 is a schematic diagram showing the difference between when X-rays are incident in one direction and when they are incident in two directions. FIG. 6 is a schematic diagram of the angle of incidence and diffraction that are the diffraction conditions for a wafer. FIG. 7 is a photograph for explaining the effect of Example 1 of the present invention. FIG. 8 is a photograph for explaining Example 2 of the present invention. FIG. 9 is a schematic diagram of a case where a defect penetrates from the front surface to the back surface of a wafer. FIG. 10 is a photograph of a case where complex dislocations are present in a wafer.

[0020] As described above, there has been a demand for a wafer evaluation method using an X-ray topograph (XRT) that is quick, versatile, and therefore extremely simple to use and capable of evaluating defect positions in the wafer depth direction. The inventors of the present invention have therefore conducted extensive research and found that by irradiating X-rays onto the front surface of the wafer from the right and left directions at angles of incidence that satisfy diffraction conditions and acquiring two XRT images, a right-eye image and a left-eye image, on the back surface, it is possible to evaluate defect positions in the wafer depth direction in a quick, versatile, and therefore extremely simple and simple method, thereby completing the present invention.

[0021] That is, the present invention is a method for evaluating defect positions in the depth direction of a wafer using an X-ray topograph (XRT), the method comprising the steps of: irradiating X-rays onto the front surface from the right and left directions at angles of incidence that satisfy diffraction conditions, and acquiring two XRT images, a right-eye image and a left-eye image, on the back surface; aligning the two acquired XRT images at defect positions on either the front surface or the back surface; and determining defect position from the deviation between the right-eye image and the left-eye image, the method for evaluating defect positions in the depth direction of a wafer.

[0022] The present invention also provides a method for evaluating defect positions in the depth direction of a wafer using an X-ray topograph (XRT), wherein the wafer has a front surface and a back surface, and X-rays are incident on the front surface from the right and left directions at angles of incidence that satisfy diffraction conditions, two XRT images, one for the right eye and one for the left eye, are obtained on the back surface, and the two XRT images are visually captured as a single image, thereby allowing the defect positions in the depth direction to be observed as a stereoscopic (3D image).

[0023] The present invention will be described in detail below, but the present invention is not limited thereto.

[0024] First, using Figure 5, we will explain how defect images are observed depending on the X-ray incidence direction. The left side of Figure 5 shows two wafers with different defect locations. The wafer in the top figure has elliptical and rectangular defects on its bottom surface and a circular defect on its top surface. The wafer in the bottom figure has elliptical, rectangular, and circular defects on its bottom surface. The only difference between the top and bottom wafers is the location of the circular defect; the circular defect differs not only in its vertical position but also in its horizontal position, with the circular defect on the wafer in the top figure being located slightly to the right. The arrows pointing from bottom to top in the figure indicate the X-ray incidence direction, with the first direction being from the bottom left to the top right and the second direction being from the bottom right to the top left. X-rays are irradiated in these two directions separately, and respective XRT images are obtained above the wafer. These images are called the left-eye image and the right-eye image, in analogy with the image observed by the human eye from above.

[0025] The center of Figure 5 shows only one direction (XRT image (left eye image) in the first direction), but there is no difference between the upper and lower images. This is due to the shift in the position of the circular defect (it is on the left side of the bottom surface in the lower image, and on the right side of the top surface in the upper image) and the phenomenon in which the defect on the bottom surface is significantly shifted to the right of the defect on the top surface due to the incident angle from the bottom left to the top right, resulting in images in which the positions of the circular defects match. In other words, when measuring in only one direction, it is possible that there will be no difference in the XRT images even if the defect positions are different.

[0026] The right side of Figure 5 shows two directions (both the XRT image in the first direction (left-eye image) and the XRT image in the second direction (right-eye image)). The left-eye image is the same as before, but the position of the circular defect in the right-eye image is significantly different between the upper and lower images. In other words, by measuring in two directions, a difference in the defect position will appear in the image in at least one direction. Conversely, this means that if a difference in the defect position appears in the image, then when the lower surface is used as the reference, it will be clear that there is a defect on a different surface above, and it will be possible to determine the position of the defect in a different depth direction.

[0027] This method utilizes the fact that when images from different angles, such as the right eye image and the left eye image, are superimposed, the defect position changes depending on the depth position.Normally, XRT images taken from a single direction cannot capture differences due to differences in depth.

[0028] An embodiment of the present invention will be described below with reference to Figures 1 and 2. A wafer is placed with its front surface facing downward and its back surface facing upward, and a typical transmission XRT is used to measure the wafer twice. As shown in the upper diagram of Figure 1, X-rays are incident obliquely on the same diffraction plane from the left and right directions, and measurements are taken separately. Because the X-rays are incident obliquely on the sample, the resulting images of defects are displaced depending on the depth of the defect. By comparing the results as left-eye and right-eye images, it is possible to evaluate the depth position based on the difference in defect position.

[0029] In Figure 1, defects (oval, rectangular) near the front surface (bottom surface) do not change position between the right-eye and left-eye images, but defects (circular) near the back surface (top surface) do change position. By extracting only defects whose position changes between the right-eye and left-eye images, it can be determined that they are defects near the back surface.

[0030] The right-eye image of the acquired X-ray topograph is inverted and superimposed on the left-eye image while aligning the positions of defects (oval, rectangular) near the front (bottom) surface, making it possible to highlight only the area where the defect position has shifted, i.e., the circular defect on the back (top) surface.

[0031] Figure 2 shows the positional relationship between the generator, detector, and sample when obtaining right-eye and left-eye images in a device in which the X-ray generator and detector are freely movable. When obtaining a right-eye image, the generator should be placed on the right, and when obtaining a left-eye image, the generator should be placed on the left.

[0032] In this way, measurements are performed by irradiating X-rays from both the left and right sides, but it is desirable to set the incidence from both sides so that the same diffraction plane is measured. Specifically, when measuring a (100) silicon wafer, measurement is often performed using either (400) diffraction or (220) diffraction. When using (400) diffraction, the incidence angle is 74.86° from the right and 105.15° from the left, while when using (220) diffraction, the incidence angle is 79.36° from the right and 100.64° from the left. However, wafers have an off-angle, and diffraction rarely occurs at the above incidence angles; in many cases, the angle is off by several degrees to several minutes.

[0033] Furthermore, in the case of SiC wafers, measurements are often made on the (11-20) diffraction plane, in which case the angle of incidence from the right is 76.66° and the angle of incidence from the left is 103.34°.

[0034] Ordinary XRT measurements are often performed using monochromatic X-rays, in which case the diffraction angle must be determined precisely. However, recently, XRT using non-monochromatic X-rays has also become available, in which case the diffraction angle does not need to be so precise. Specifically, as long as an XRT image can be obtained, there is no problem even if the diffraction angle deviates.

[0035] Furthermore, when measuring twice, once from the left and once from the right, if the measurements are taken on the same diffraction surface, it is possible to visually see the image in three dimensions, but it is also possible to compare images under different diffraction conditions.

[0036] The image when X-rays are incident from the left is called the left eye image, and the image when X-rays are incident from the right is called the right eye image, but there is no problem if the right eye image and left eye image are swapped as long as the two XRT images can be compared. If the X-ray generator and detector cannot be moved to the inversion position due to equipment restrictions, the sample can be rotated 180 degrees, measured at the same angle, and the measurement results can be rotated 180 degrees.

[0037] As described above, one embodiment of the present invention is a method for evaluating defect positions in the depth direction of a wafer using an X-ray topograph (XRT), in which the wafer has a front surface (bottom surface) and a back surface (top surface), and includes the steps of: irradiating X-rays onto the front surface (bottom surface) from the right and left directions at angles of incidence that satisfy diffraction conditions to acquire two XRT images, a right-eye image and a left-eye image, on the back surface (top surface); an alignment step of aligning the two acquired XRT images at defect positions on the front surface; and a defect position determination step of determining other defect positions in different depth directions of the wafer from the deviation between the right-eye image and the left-eye image, thereby demonstrating that defect positions on the back surface (top surface) of the wafer in different depth directions can be determined.

[0038] With this method for evaluating defect positions in the depth direction of a wafer, the right-eye image and the left-eye image coincide for defects on the aligned front surface, but a deviation occurs between the right-eye image and the left-eye image for defects on the back surface, which is different from the aligned surface, and this deviation can be used to determine that the defects are located at different positions in the depth direction.This method only requires acquiring two images using the normal method of using an X-ray topograph (XRT), allowing measurement in a short time, and is versatile because it does not require special operations such as intensifying the synchrotron radiation or narrowing the measurement area.As a result, it is possible to evaluate defect positions in the depth direction of a wafer using an extremely simple method.

[0039] Although the defect positions in the depth direction of the wafer have been described on the front and back surfaces, it is also possible to view the inside of the wafer.

[0040] In addition, as a defect position determination process, the right-eye image of the two acquired XRT images was inverted and combined with the left-eye image, thereby enabling the location of a circular defect on the back surface (top surface). This defect position determination process can reliably determine the defect position based on the difference in color (black or white), and therefore can be suitably applied to a method for evaluating defect positions in the depth direction of a wafer.

[0041] Another embodiment of the present invention will now be described with reference to FIG.

[0042] If the X-ray generator and detector cannot move to the inversion position due to equipment limitations, right-eye and left-eye images can be obtained as shown in Figure 3. 1. Irradiate X-rays from the right direction to obtain a right-eye image. 2. Rotate the wafer 180 degrees and irradiate X-rays from the right direction at the same irradiation angle to obtain an image. Figure 3 shows that when the wafers marked A and B are rotated 180 degrees, they appear to be reversed left to right. 3. Obtaining a left-eye image By rotating the image obtained in 2 by 180 degrees, it becomes the same as the X-ray topographic image obtained by irradiating the wafer from the left direction without rotating it.

[0043] In this embodiment, too, by inverting the black and white of either the acquired right-eye image or left-eye image and overlaying them, it is possible to highlight the area where the defect position has shifted, i.e., the defect on the surface.

[0044] As described above, the method for evaluating defect positions in the depth direction of a wafer according to this embodiment has been shown to be a method of irradiating X-rays onto the surface from the right and left directions at angles of incidence that satisfy diffraction conditions, in which the angle of incidence is fixed to the right, X-rays are irradiated to obtain an XRT image (right-eye image), the wafer is then rotated by 180°, X-rays are irradiated to obtain an XRT image in the other direction, and the obtained image is then rotated by 180°, thereby obtaining two XRT images, a right-eye image and a left-eye image.

[0045] With this method, XRT images can be easily acquired in the right and left directions without moving the X-ray generator or detector, making it possible to evaluate the defect position in the depth direction of the wafer in an extremely simple manner.

[0046] In this embodiment, too, the acquired right-eye and left-eye images can be viewed as a single image, making it possible to observe the image in stereoscopic vision (3D image), and it is possible to easily distinguish between surface defects and back-surface defects.

[0047] In principle, it is not only possible to evaluate only the surface defects on the opposite side of the X-ray irradiated surface by aligning the defect positions on the surface irradiated with X-rays and determining the positional deviation of the surface defects on the opposite side of the X-ray irradiated surface, but also possible to evaluate only the defects on the surface irradiated with X-rays by aligning the defect positions on the surface opposite to the X-ray irradiated surface, since the positions of the defects on the surface irradiated with X-rays will be shifted.

[0048] Another embodiment of the present invention will be described below with reference to FIG. 4 . As an alternative method, a stereoscopic image of a defect can be obtained by stereoscopically viewing two images. The right-eye image and the left-eye image obtained by the same method as in the above-described embodiment (X-rays are incident on the front surface from the right and left directions at angles of incidence that satisfy the diffraction conditions, and two XRT images, a right-eye image and a left-eye image, are obtained on the back surface) are viewed with the right and left eyes, respectively, and the defect on the front surface is made to stand out using a method such as stereoscopic viewing or 3D glasses. In FIG. 4 , a circular defect on the back surface (top surface) of the wafer is made to stand out.

[0049] As described above, in the method for evaluating defect positions in the depth direction of a wafer according to this embodiment, the wafer has a front surface and a back surface, and X-rays are incident on the front surface from the right and left directions at angles of incidence that satisfy the diffraction conditions, two XRT images, one for the right eye and one for the left eye, are obtained on the back surface, and the two XRT images are visually captured as a single image, thereby providing a method for evaluating defect positions in the depth direction of a wafer, in which defect positions in the depth direction are observed as a stereoscopic image (3D image).

[0050] With this method for evaluating defect positions in the depth direction of a wafer, if two XRT images are visually viewed as a single image, defects at different depth positions will appear three-dimensional due to the misalignment of the two images, and this three-dimensional appearance can be used to determine that the defect is a circular defect at a different depth position (a circular defect on the back surface (top surface)). This method requires only two images to be acquired using the normal method of using an X-ray topograph (XRT), allowing for measurement in a short time. It is also versatile because it does not require special operations such as intensifying the synchrotron radiation or narrowing the measurement area. In addition, since the defect positions are evaluated visually, there is no need to prepare a separate evaluation device or the like. As a result, it is possible to evaluate defect positions in the depth direction of a wafer using an extremely simple method.

[0051] The present invention is not limited to silicon wafers, but can also be applied to SiC wafers and silicon wafers on which devices are formed. In SiC wafers, crystal defects may be complexly intertwined within the crystal, and this situation can be confirmed. Furthermore, in silicon wafers on which devices are formed, surface defects directly affect device yield, so XRT can be used to evaluate only surface defects rather than the entire depth direction. Furthermore, this method is effective when it is desired to know the three-dimensional structure of defects in these wafers.

[0052] The results of the actual prototype evaluation will be explained in detail below using photographs, drawings, etc.

[0053] Example 1 First, a carbon film was vapor-deposited on the top surface of a silicon wafer with a (100) surface orientation. Scratches formed on the back surface due to handling during this process were evaluated. X-rays were incident on the front surface from the direction (1) shown in Figure 6, and XRT measurement was performed. Since the measurement was performed under (400) diffraction conditions, the incident angle was set to 74.86°.

[0054] Since the X-ray generator of the equipment used this time could not be moved to the position in the direction of (2), the wafer was rotated 180 degrees and the XRT was measured again with the radiation incident from the direction of (1). The second measurement result was then rotated 180 degrees to achieve the same positional relationship as the first measurement. Figure 7 shows a case in which the image taken in (1) was used as the right-eye image and the image taken in (2) was used as the left-eye image, with one side inverted and the other side superimposed. In this case, the surface defect position was aligned. The defects on the surface, which could be seen as linear cracks, cancel each other out and become invisible, leaving only the contact scratches on the backside visible.

[0055] (Example 2) Furthermore, as shown in Figure 8, it is also possible to visually observe the image in stereoscopic vision (3D image) by aligning the round dots on the right-eye image and the left-eye image to capture them as a single image, and this method can also be used to obtain defect depth information.

[0056] The image in Figure 8 is the same as that shown in Figure 7, but by overlapping the circles on the image, a stereoscopic view can be obtained. This data is the result when there are defects on the front and back surfaces of a Si wafer, but when defects occur not only on the front and back surfaces of a crystal but also inside the crystal, such as slips in Si crystals or defects in SiC, this stereoscopic view (3D image) method is more effective because it can also reveal intermediate defects in the crystal bulk.

[0057] Furthermore, we will add an example where stereoscopic vision is effective. Figure 9 shows a schematic diagram of a defect that penetrates from the front surface to the back surface of the wafer, along with an example of stereoscopic vision. Figure 10 shows an example of a case where complex dislocations exist inside the crystal. In cases like Figures 9 and 10, stereoscopic vision makes it easier to grasp the overall picture of the defect.

[0058] The present invention is not limited to the above-described examples, and any other configurations that are substantially identical to the technical ideas described in the claims of the present invention and that provide similar effects are included within the technical scope of the present invention.

Claims

1. A method for evaluating the defect positions in the depth direction of a wafer using X-ray topography (XRT), comprising: a step of obtaining two XRT images of a right-eye image and a left-eye image on the back surface by irradiating the front surface of the wafer, which has a front surface and a back surface, with X-rays at incident angles that satisfy the diffraction condition from the right and left directions; an alignment step of aligning the two obtained XRT images at the defect positions on either the front surface or the back surface; a defect position determination step of determining other defect positions where the positions in the depth direction of the wafer are different from the shift between the right-eye image and the left-eye image; The method for evaluating the defect positions in the depth direction of a wafer, characterized by including the above steps.

2. The method for irradiating the front surface with X-rays at incident angles that satisfy the diffraction condition from the right and left directions is to fix the incident angle in either the right or left direction, irradiate the X-rays to obtain an XRT image, then rotate the wafer by 180°, irradiate the X-rays, and obtain an XRT image in the other direction, thereby obtaining two XRT images of the right-eye image and the left-eye image. The method for evaluating the defect positions in the depth direction of a wafer according to Claim 1, characterized by this.

3. The defect position determination step is to determine by inverting the black and white of either one of the two obtained XRT images and synthesizing them. The method for evaluating the defect positions in the depth direction of a wafer according to Claim 1 or Claim 2, characterized by this.

4. A method for evaluating the defect positions in the depth direction of a wafer using X-ray topography (XRT), comprising: a step of obtaining two XRT images of a right-eye image and a left-eye image on the back surface by irradiating the front surface of the wafer, which has a front surface and a back surface, with X-rays at incident angles that satisfy the diffraction condition from the right and left directions, and observing the defect positions in the depth direction as a stereoscopic view (3D image) by visually grasping the two XRT images as one image. The method for evaluating the defect positions in the depth direction of a wafer, characterized by this.

5. The wafer is at least one of a single-crystal wafer and a wafer on which a device is formed. The method for evaluating the defect positions in the depth direction of a wafer according to Claim 1 or Claim 4, characterized by this.