Method for evaluating the location of defects in the depth direction of a 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

Technical Field

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

Background Art

[0002] X-ray topography (XRT) is a device widely used for observing defects in crystalline materials, particularly semiconductor device materials typified by silicon wafers. XRT has a different measurement principle from X-ray CT, and detects the distortion of a diffraction grating instead of the difference in transmittance. Therefore, it can only be applied to single crystals, but can measure very small defects.

[0003] In the transmission method of XRT, the defect positions of the entire wafer can be known, but depth information of the defects cannot be obtained. Depth information of defects in semiconductor device materials is very important information for determining whether the defects affect device failures. In recent years, in order to obtain depth information by XRT, it has become possible to obtain a cross-sectional topography or a three-dimensional topography obtained by combining a plurality of such cross-sectional topographies. However, in order to obtain a cross-sectional topography, very long-time measurement is required, and even more time is required to obtain three-dimensional data that requires a plurality of such cross-sectional topography information. There are many restrictions on such measurement, such as the use of synchrotron radiation, which is a strong X-ray generator.

[0004] Thus, conventionally, it has been difficult to obtain three-dimensional data by overlapping cross-sectional topography images, but in recent years, methods that enable three-dimensional data of defects have been developed (for example, Patent Document 1). However, even with this method, there are still problems that it takes time to obtain three-dimensional data and the measurement area is narrow.

[0005] Furthermore, in semiconductor device materials, whether the defects that cause device failures are on the front surface or the back surface, which is the device layer, is very important. However, with XRT, it has not been possible to simply evaluate the depth position of defects.

Prior Art Documents

[0006] [Patent Document 1] Japanese Patent Publication No. 2015-105831 [Overview of the Initiative] [Problems that the invention aims to solve]

[0007] This invention has been made in view of the aforementioned problems, and aims to provide a simple method for evaluating the location of defects in the depth direction of a wafer using an X-ray topograph (XRT). [Means for solving the problem]

[0008] The present invention has been made to solve the above problems, and provides a method for evaluating the location of defects 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 the method includes the steps of: inducing X-rays on the front surface from the right and left directions at incidence angles that determine 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 the location of defects on either the front surface or the back surface; and determining the location of other defects in a different depth direction of the wafer from the misalignment between the right-eye image and the left-eye image.

[0009] With this method for evaluating the depth-direction defect location of a wafer, the right-eye and left-eye images will match for defects on the aligned plane, but for defects on a different plane, a discrepancy will occur between the right-eye and left-eye images. This discrepancy allows for the determination that the defect is located at a different depth. This method only requires acquiring two images using the standard X-ray topography (XRT) setup, enabling measurements in a short time. It is also versatile as it does not require special techniques such as increasing the intensity of the synchrotron radiation or narrowing the measurement area. As a result, the depth-direction defect location of a wafer can be evaluated using an extremely simple method.

[0010] Furthermore, in the method of incidenting X-rays onto the surface from the right and left directions at incident angles that result in diffraction conditions, it is preferable to fix the incident angle in either the right or left direction, incident the X-rays to acquire an XRT image, then rotate the wafer by 180°, incident the X-rays again to acquire an XRT image from the other direction, thereby acquiring two XRT images, the right-eye image and the left-eye image. With this method, XRT images can be easily acquired in both the rightward and leftward directions without moving the X-ray generator or detector, making it an extremely simple way to evaluate the location of defects in the depth direction of the wafer.

[0011] Furthermore, it is preferable that the defect location determination step be performed by inverting the black and white of one of the two acquired XRT images and combining them. With this type of defect location determination process, the defect location can be reliably determined by the difference between white and black colors, making it suitable for evaluating the location of defects in the depth direction of a wafer.

[0012] Furthermore, the present invention provides a method for evaluating the location of defects in the depth direction of a wafer using an X-ray topograph (XRT), wherein the wafer has a front and a back surface, and X-rays are incident on the front surface from the right and left directions at incident angles that determine the diffraction conditions, thereby acquiring two XRT images, a right-eye image and a left-eye image, on the back surface, and observing the location of defects in the depth direction as a stereoscopic (3D) image by visually perceiving the two XRT images as a single image.

[0013] With this method of evaluating the depth-direction defect location of a wafer, if two XRT images are attempted to be captured as a single image by visual inspection, defects located at different depths will appear three-dimensional due to the misalignment of the two images. This three-dimensional appearance allows for the determination that the defects are located at different depths. This method only requires acquiring two images using the standard X-ray topography (XRT) method, enabling measurements in a short time. It is also versatile as it does not require special techniques such as increasing the intensity of the synchrotron radiation or narrowing the measurement area. In addition, since the defect location is evaluated visually, there is no need to prepare separate evaluation equipment. As a result, the depth-direction defect location of a wafer can be evaluated using an extremely simple method.

[0014] Furthermore, it is preferable that the wafer is at least one of a single-crystal wafer or a wafer on which a device is formed.

[0015] Such wafers can be suitably applied to methods for evaluating the location of defects in the depth direction of the wafer using X-ray topography (XRT). In particular, for silicon wafers and SiC wafers, the incident angle that constitutes the diffraction condition is known in advance, making them suitable for evaluation methods for evaluating the location of defects in the depth direction of the wafer.

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

[0017] Furthermore, if a device is formed on the wafer, it is easy to evaluate whether a defect that causes device failure is located on the surface (device layer) or the back surface. Since surface defects directly affect device yield on wafers with devices formed on them, even being able to evaluate only surface defects, rather than the entire depth, can help improve yield. [Effects of the Invention]

[0018] As described above, in the method for evaluating the defect position in the depth direction of the wafer of the present invention, it is only necessary to obtain two images in the normal usage of X-ray topography (XRT), and measurement can be performed in a short time. Since it does not require special usage such as strengthening synchrotron radiation or narrowing the measurement area, it is versatile. As a result, the defect position in the depth direction of the wafer can be evaluated by an extremely simple method.

Brief Description of the Drawings

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

Embodiments for Carrying Out the Invention

[0020] As described above, in the method for evaluating a wafer using X-ray topography (XRT), there has been a need for a method that can evaluate the defect position in the depth direction of the wafer in a short time, in a versatile manner, and as a result, by an extremely simple method. After further investigation, the inventors discovered that by irradiating the wafer surface with X-rays from the right and left at incidence angles that determine 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 the location of defects in the depth direction of the wafer in a short time, in a versatile manner, and as a result, in an extremely simple way, thus completing the present invention.

[0021] In other words, the present invention is a method for evaluating the location of defects 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 the method includes the steps of: 1) 2) 3) 3) 4) 4) 4) 4) 4) 4) 4) 4) 4) 4) 4) 4) 4) 4) 4) 4) 4) 4) 4) 4) 4) 4) 4) 4) 4) 4) 4) 4) 4) 4) 4))

[0022] Furthermore, the present invention relates to a method for evaluating the location of defects 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 incident angles that determine the diffraction conditions, thereby acquiring two XRT images on the back surface, a right-eye image and a left-eye image, and observing the location of defects in the depth direction as a stereoscopic (3D) image by visually perceiving the two XRT images as a single image.

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

[0024] First, Figure 5 will be used to explain how defects are observed as images depending on the direction of X-ray incidence. The left side of Figure 5 shows two wafers with different defect locations. The wafer in the upper figure has elliptical and rectangular defects on its bottom surface and a circular defect on its top surface. The wafer in the lower figure has elliptical, rectangular, and circular defects on its bottom surface. The only difference between the upper and lower wafers is the position of the circular defect, which differs not only in its vertical position but also in its horizontal position, with the circular defect on the upper wafer being slightly to the right. The arrows pointing from bottom to top in the same figure indicate the direction of X-ray incidence, with irradiation occurring in a first direction from the lower left to the upper right and a second direction from the lower right to the upper left. X-rays from these two directions are irradiated individually, and separate XRT images are acquired above the wafer. At this time, the images will be called the left-eye image and the right-eye image, analogous to the image observed by a human eye from above.

[0025] Figure 5 shows only one direction (the XRT image from the first direction (left eye image)) in the center, but there is no difference between the image in the upper and lower figures. This is because the position of the circular defect is shifted (it is on the left side of the bottom surface in the lower figure and on the right side of the top surface in the upper figure), and the phenomenon in which the defect on the bottom surface is shifted significantly to the right of the defect on the top surface due to the angle of incidence from the lower left to the upper right results in images where the position of the circular defect matches. In other words, with measurements in only one direction, it is possible that there is no difference in the XRT image even if the defect position is different.

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

[0027] This method utilizes the fact that when images taken from different angles, such as the right eye image and the left eye image, are superimposed, the defect location changes depending on the depth. Conventional XRT images taken from a single direction do not capture the difference due to depth variations.

[0028] One embodiment of the present invention will be described below with reference to Figures 1 and 2. The wafer is positioned with the front surface facing down and the back surface facing up, and two general transmission XRT measurements are taken. As shown in the upper part of Figure 1, X-rays are incident at an oblique angle from the left and right to the same diffraction plane, and measurements are taken accordingly. Because the X-rays are incident at an oblique angle to the sample, the image from the defect obtained will shift in position depending on the depth of the defect. By acquiring the results separately as a left-eye image and a right-eye image and comparing them, it is possible to evaluate the position in the depth direction from the difference in the defect's location.

[0029] In Figure 1, defects near the surface (bottom) (elliptical and rectangular) remain in the same position in both the right-eye and left-eye images, while defects near the back surface (top) (circular) change position. By extracting only the defects whose position changes between the right-eye and left-eye images, we can determine that these are defects near the back surface.

[0030] By inverting the right-eye image of the acquired X-ray topography in black and white, and superimposing it with the left-eye image while aligning the positions of defects (elliptical, rectangular) near the surface (bottom surface), it is possible to highlight only the circular defects on the back surface (top surface) where the defect positions have shifted.

[0031] Figure 2 shows the relative positions of the generator, detector, and sample when obtaining right-eye and left-eye images in a device where the X-ray generator and detector can move freely. To obtain the right-eye image, the generator should be placed on the right side, and to obtain the left-eye image, the generator should be placed on the left side.

[0032] Measurements are performed by incident X-rays from both the left and right sides, but it is desirable to set the incident angles from both sides so that the same diffraction plane can be measured. Specifically, when measuring a silicon wafer of type (100), (400) diffraction or (220) diffraction is often used. When using (400) diffraction, the incident angles are 74.86° from the right and 105.15° from the left, and when using (220) diffraction, the incident angles are 79.36° from the right and 100.64° from the left. However, wafers have an OFF angle, and diffraction rarely occurs at the above incident angles; it is often off by several degrees to several minutes.

[0033] Furthermore, with SiC wafers, measurements are often taken at the (11-20) diffraction plane, where the incident angles are 76.66° from the right and 103.34° from the left.

[0034] Conventional XRT measurements often use monochromatic X-rays, in which case the diffraction angle must be precisely determined. 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, deviations in the diffraction angle are not a problem.

[0035] Furthermore, when taking two measurements, once from the left and once from the right, it is possible to visualize the image in 3D if the measurements are taken on the same diffraction plane. However, it is also possible to compare images taken under different diffraction conditions.

[0036] The image obtained when X-rays are incident from the left is the left-eye image, and the image obtained when X-rays are incident from the right is the right-eye image. However, it is acceptable if the right-eye and left-eye images are swapped, as long as the two XRT images can be compared. If, due to equipment limitations, the X-ray generator and detector cannot be moved to the inversion position, the sample may be rotated 180 degrees and measured at the same angle, and the measured results may also be rotated 180 degrees.

[0037] As described above, one embodiment of the present invention is a method for evaluating the location of defects in the depth direction of a wafer using an X-ray topograph (XRT), wherein the wafer has a front surface (bottom surface) and a back surface (top surface), and the method includes the steps of: 1) 2) 3) 3) 4) 4) 4) 4) 4) 4) 4) 4) 4) 4) 4) 4) 4) 4) 4) 4) 4) 4) 4) 4) 4) 4) 4) 4) 4) 4) 4) 4) 4) 4) 4)) 4)) 5))

[0038] With this method for evaluating the depth-direction defect location of a wafer, the right-eye and left-eye images will match for defects on the aligned surface, but for defects on the back surface (different from the aligned surface), a discrepancy will occur between the right-eye and left-eye images. This discrepancy allows for the determination that the defect is located at a different depth. This method only requires acquiring two images using the standard X-ray topography (XRT) setup, enabling measurements in a short time. It is also versatile as it does not require special techniques such as increasing the intensity of the synchrotron radiation or narrowing the measurement area. As a result, the depth-direction defect location of a wafer can be evaluated using an extremely simple method.

[0039] While we have described the location of defects in the depth direction of the wafer on the surface and back surface, it is also possible to view the inside of the wafer.

[0040] Furthermore, as part of the defect location determination process, the right-eye image of the two acquired XRT images was inverted in black and white and combined with the left-eye image, allowing the circular defect location on the back surface (top surface) to be determined. With this type of defect location determination process, the defect location can be reliably determined by the difference between white and black colors, making it suitable for evaluating the location of defects in the depth direction of a wafer.

[0041] Other embodiments of the present invention will be described below with reference to Figure 3.

[0042] If, due to equipment limitations, the X-ray generator and detector cannot be moved to the inversion position, the right-eye image and left-eye layer can be obtained as shown in Figure 3. 1. Irradiate the right eye with X-rays from the right to obtain an image of the right eye. 2. The wafer is rotated 180° and irradiated with X-rays from the right at the same irradiation angle to acquire an image. Figure 3 shows that when wafers labeled A and B are rotated 180°, the image becomes horizontally flipped. 3. Obtaining the left eye image By rotating the image acquired in step 2 by 180°, it becomes the same as the X-ray topographic image acquired by irradiating from the left without rotating the wafer.

[0043] In this embodiment as well, by inverting the black and white of either the acquired right-eye or left-eye X-ray topographic image and superimposing them, it is possible to highlight the areas where the defect position has shifted, i.e., defects on the surface.

[0044] As described above, the method for evaluating the defect location in the depth direction of the wafer according to this embodiment, which involves incidenting X-rays on the surface from the right and left directions at incident angles that result in diffraction conditions, shows that by fixing the incident angle to the right, incident X-rays to acquire an XRT image (right-eye image), then rotating the wafer 180°, incident X-rays to acquire an XRT image from the other direction, and rotating the acquired image 180°, two XRT images, a right-eye image and a left-eye image, can be obtained.

[0045] With this method, XRT images can be easily acquired in both the rightward and leftward directions without moving the X-ray generator or detector, making it an extremely simple way to evaluate the location of defects in the depth direction of the wafer.

[0046] In this embodiment as well, by treating the acquired right-eye and left-eye images as a single image, it is possible to observe them in stereoscopic vision (3D image), making it easy to distinguish between surface defects and back surface defects.

[0047] Furthermore, in principle, by aligning the defect positions on the X-ray irradiation surface and measuring the displacement of surface defects on the opposite side, it is possible to evaluate only the defects on the X-ray irradiation surface, as aligning the defect positions on the opposite side will shift the defect positions on the X-ray irradiation surface.

[0048] Other embodiments of the present invention will be described below with reference to Figure 4. Alternatively, a three-dimensional image of the defect can be obtained by viewing two images in stereoscopic form. By viewing the right-eye and left-eye images obtained using the same method as described above (by incidenting X-rays onto the surface from the right and left directions at incident angles that determine diffraction conditions, and acquiring two XRT images, a right-eye image and a left-eye image, on the back surface) with the right and left eyes respectively, defects on the surface become visible using methods such as stereoscopic vision or 3D glasses. In Figure 4, a circular defect on the back surface (top surface) of the wafer becomes visible.

[0049] As described above, the method for evaluating the location of defects in the depth direction of a wafer according to this embodiment demonstrates a method for evaluating the location of defects in the depth direction of a wafer, in which a wafer has a front and a back surface, and X-rays are incident on the front surface from the right and left directions at incident angles that determine the diffraction conditions, and two XRT images, a right-eye image and a left-eye image, are acquired on the back surface, and the two XRT images are perceived as a single image by visual inspection, thereby observing the location of defects in the depth direction as a stereoscopic (3D) image.

[0050] With this method of evaluating the depth-direction defect location of a wafer, if two XRT images are attempted to be captured as a single image by visual inspection, defects at different depth locations will appear three-dimensional due to the misalignment of the two images. From this three-dimensional appearance, it can be determined that the defects are at different depth locations (circular defects on the back surface (top surface)). This method only requires acquiring two images using the normal operation of an X-ray topograph (XRT), enabling measurement in a short time. It is also versatile as it does not require special usage such as increasing the intensity of the synchrotron radiation or narrowing the measurement area. In addition, since the defect location is evaluated visually, there is no need to prepare separate evaluation equipment. As a result, the depth-direction defect location of a wafer can be evaluated using an extremely simple method.

[0051] Furthermore, 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. SiC wafers can have complex interwoven crystal defects within the crystal structure, and XRT can be used to examine this situation. Furthermore, since surface defects directly affect device yield on silicon wafers on which devices are formed, XRT can be used to evaluate only surface defects, rather than the entire depth. Furthermore, this method is useful when you want to know the three-dimensional structure of defects within these wafers. [Examples]

[0052] The following provides a detailed explanation of the results of the actual prototype evaluation, using photographs, drawings, and other visual aids.

[0053] (Example 1) First, a carbon film was vapor-deposited on the upper surface of a silicon wafer with a (100) orientation. We then evaluated the scratches formed on the back side due to handling during this process. XRT measurements were performed by incidenting X-rays from the direction shown in (1) in Figure 6, with the surface side as the X-ray irradiation surface. Since the measurement was performed under (400) diffraction conditions, the incident angle was set to 74.86°.

[0054] The apparatus used in this study could not move the X-ray generator to the position in direction (2), so the wafer was rotated 180 degrees and XRT was measured again with the X-ray incident from direction (1). After that, the results of the second measurement were rotated 180 degrees to match the positional relationship of the first measurement. Figure 7 shows a case where 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, and an image was obtained by inverting the colors on one side and superimposing them. In this case, the positions of surface defects were aligned. Defects that could appear as linear cracks on the surface were canceled out and became invisible, and only contact scratches on the back surface remained visible.

[0055] (Example 2) Furthermore, as shown in Figure 8, it is possible to observe the image using stereoscopic vision (3D image) by aligning the round dots on the right and left eye images to form a single image, and this method also allows for obtaining information about the depth of the defect.

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

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

[0058] It should be noted that the present invention is not limited to the above embodiments. The above embodiments are illustrative examples, and any configuration that is substantially identical to the technical idea described in the claims of the present invention and achieves similar effects is included within the technical scope of the present invention.

Claims

1. A method for evaluating the location of defects in the depth direction of a wafer using X-ray topography (XRT), The wafer has a front surface and a back surface, and the process involves incidenting X-rays onto the front surface from the right and left directions at incident angles that determine diffraction conditions, thereby acquiring two XRT images, a right-eye image and a left-eye image, on the back surface. The two acquired XRT images are aligned at the location of defects on either the front or back surface; A defect location determination step that determines other defect locations that are located at different depth positions on the wafer based on the discrepancy between the right-eye image and the left-eye image, A method for evaluating the location of defects in the depth direction of a wafer, characterized by including [a specific component].

2. The method for irradiating the surface with X-rays from the right and left directions at incidence angles that result in diffraction conditions is characterized by fixing the incidence angle in either the right or left direction, irradiating with X-rays to acquire an XRT image, then rotating the wafer by 180°, irradiating with X-rays to acquire an XRT image from the other direction, thereby acquiring two XRT images: the right-eye image and the left-eye image.

3. The defect location determination step is characterized by determining the defect location by inverting one of the two acquired XRT images to black and white and combining them, as described in claim 1 or 2.

4. A method for evaluating the location of defects in the depth direction of a wafer using X-ray topography (XRT), A method for evaluating the location of defects in the depth direction of a wafer, characterized in that 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 incident angles that determine diffraction conditions, two XRT images, a right-eye image and a left-eye image, are acquired on the back surface, and the location of defects in the depth direction is observed as a stereoscopic (3D) image by visually capturing the two XRT images as a single image.

5. The method for evaluating the location of defects in the depth direction of a wafer according to claim 1 or 4, characterized in that the wafer is at least one of a single-crystal wafer or a wafer on which a device is formed.

Images