JP2003188224A - Method for measuring dislocation pit density in compound semiconductor wafer - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a measuring method of dislocation pit density in a wafer for accurately measuring the dislocation pit density, regardless of surface planarity and the overlapping of dislocation pits. <P>SOLUTION: The measuring method includes a process for obtaining the enlarged monochrome image of a wafer crystal surface that is subjected to etching treatment, a process for obtaining a first binarization image including the contour section of a dislocation pit according to the enlarged monochrome image, a process for obtaining a second binarization image, including the core section of the dislocation pit according to the enlarged monochrome image, a process for extracting a portion, where both the images are overlapped from the obtained first and second images to obtain a third binarization image and for obtaining an independent figure included in the third binarization image for counting, and a process for obtaining dislocation pit density with a counted numerical value as the number of dislocation pits. <P>COPYRIGHT: (C)2003,JPO

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for measuring dislocation pit density (Etch Pit Density: EPD) in a compound semiconductor wafer used as a technique for evaluating crystal defects. In particular, the dislocation pit density of a compound semiconductor wafer can be determined accurately using an image processing technique, and without being affected by the flatness of the wafer surface or the overlap of dislocation pits in the compound semiconductor. It relates to improvement of a measuring method. 2. Description of the Related Art Wafers of compound semiconductors such as GaP and GaAs are widely used as light emitting / receiving devices and electronic devices. In order to use such a compound semiconductor wafer, it is necessary to use a high quality single crystal with few defects. This is because, for example, when a crystal is grown on a GaP wafer obtained from a GaP single crystal ingot grown by a pulling method or the like by an epitaxial growth method or the like, a defect on the GaP wafer propagates to an epitaxial growth layer and deteriorates device characteristics. Because there is. As a technique for evaluating this crystal defect, there is measurement of dislocation pit density (EPD). In other words, since a part where a defect exists has a weaker bonding force between atoms than a part without a defect, when the semiconductor wafer surface is treated with a chemical solution, the defect part is dissolved faster than a part without the defect and becomes a hole (pit) shape. . The holes are called dislocation pits, the chemical used at this time is called an etchant, and this operation is called etching. [0004] By measuring the dislocation pit density (EPD), the crystal defect density in the compound semiconductor wafer can be evaluated. The dislocation pit density (EPD) is measured by enlarging the wafer surface with a microscope or the like so that the dislocation pits can be recognized, and converting the pits present in the enlarged image into the number per unit area. Is being done. As a basic means for counting dislocation pits, there is known a visual method in which a human recognizes dislocation pits by looking at an enlarged image and counts the number of dislocation pits. For example, one field of view has an area of 200 μm × 200 μm,
Counting is performed for 5 to 7 visual fields in the diameter direction of the wafer. The counted number of dislocation pits is converted into the number per unit area (1 cm ² ), and is used as one index for judging crystal quality as the dislocation pit density. Incidentally, it takes about 10 to 15 minutes to measure the dislocation pit density (EPD) per wafer. However, when the wafer is a compound semiconductor,
When the wafer surface enlarged by the microscope is displayed on a CRT monitor, and a line having a side of about 200 to 500 μm is drawn on the image and dislocation pits in the rectangle are counted, the number of pits to be counted exceeds 100. There are many things. In addition, the visual method has a problem in that the work is performed while looking at the CRT monitor, so that the burden on the operator's eyes is great. Therefore, instead of the visual method, a measurement method using an image processing technique by a computer system as described below has been developed. However,
The visual method is highly reliable because the operator measures each dislocation pit while judging its shape, color and the like. Therefore, when developing a measurement method using an image processing technique, it is a problem whether or not the same measurement accuracy as that of the visual method can be obtained. A conventional method for measuring dislocation pits using an image processing technique will be described below. First, there are two illumination methods, a dark field and a bright field, for obtaining an enlarged image of the wafer surface using a reflection microscope. In the case of bright field illumination, dislocation pits appear dark because illumination light falls perpendicular to the measurement surface, and portions that are not dislocation pits appear bright. On the other hand, in the case of dark-field illumination, the illumination light emitted from the periphery of the lens falls slightly obliquely onto the measurement surface, so that a surface having an inclination such that the illumination light just reflects off the lens appears bright. That is, a part of the dislocation pit looks bright, and a part without the dislocation pit looks dark. Further, the shape of the dislocation pit takes a characteristic shape depending on the type of crystal and the plane orientation in the compound semiconductor wafer. For example, FIG. 1 shows an enlarged monochrome image of a reflection microscope using bright field illumination of a GaP (111) etched surface. In FIG. 1, a figure having a triangular pyramid with no corners and a core at the center indicates a dislocation pit. Therefore, in FIG. 1, the dislocation pit portion looks black, and the other portions look white. The measurement of dislocation pits in the conventional method using the image processing technique is performed as follows. That is, for each pixel of the enlarged monochrome image, a reference density (threshold) for classifying the pixel into a white portion and a black portion based on the density gradation is set, and the pixel group constituting the enlarged monochrome image in FIG. When the pixels in the black portion are also extracted, the enlarged monochrome image of FIG. 1 is converted to the image of FIG. The image of FIG. 2 is obtained by extracting the dislocation pit portion from the enlarged monochrome image of FIG. This method is called binarization, and a binarized image as shown in FIG. 2 is called a binarized image. Then, the number of independent figures included in the binarized image is counted by mechanical measurement or visual observation to determine the number of dislocation pits, and this value is converted per unit area to determine the dislocation pit density (EPD). Further, the threshold value is set so that all portions except for dislocation pits are not taken out. In the case of bright-field illumination, if a pixel in a white portion is removed too much, a portion which is a dislocation pit (a pixel in a black portion) cannot be detected. Further, if the pixels in the black part are extracted too much, even the parts that are not dislocation pits (pixels in the white part) will be detected.
Therefore, the threshold is set during this time. The conventional method for measuring dislocation pits utilizing the above-mentioned image processing technique is intended to accurately measure the dislocation pit density when the flatness of the etched wafer surface is good. It is possible to measure. However, when the flatness of the wafer surface is poor, fine irregularities on the wafer surface appear as shades of the enlarged image as shown in FIG. 3, and the threshold value is set so that almost all dislocation pits are detected. In this case, as shown in FIG. 4, even the irregularities are detected, and the dislocation pit cannot be accurately measured. When the number of the dislocation pits is large, the dislocation pits overlap each other, and it is difficult to separate them by image processing, so that the dislocation pit density (EPD) is counted low. He also had points. The present invention has been made in view of such a problem, and an object of the present invention is to utilize an image processing technique and to control the flatness of the surface of a compound semiconductor wafer and the overlap of dislocation pits. It is an object of the present invention to provide a method for measuring a dislocation pit density in a compound semiconductor wafer, which can accurately determine a dislocation pit density (EPD) without being performed. In order to solve this problem, the present inventors have obtained an enlarged monochrome image on the etched surface of the compound semiconductor wafer, and based on the enlarged monochrome image, dislocation pits and irregularities on the wafer surface have been obtained. The method of isolating the dioxins was studied diligently. First, in an enlarged monochrome image of a reflection microscope using bright-field illumination, a core portion near the center of a dislocation pit becomes white as shown in FIG. However, since the uneven portions existing on the wafer surface also become white at the same time, if the threshold of shading is converted to a binarized image according to the core portion of the dislocation pits for the detection of dislocation pits, the uneven portions are also together as described above. There is a problem that it is detected. However, if some sort of filter is applied to the above-mentioned uneven portion or dislocation pit portion so that only the core portion of the dislocation pit can be detected, the uneven portion can be removed from the binarized image. That is, as shown in FIG. 3, the outline of the dislocation pit is observed as a black portion, and the uneven portion is observed as a relatively white portion. Therefore, if the image is converted into a binarized image by adjusting the shading threshold to the black portion of the contour of the dislocation pit, the above-mentioned uneven portion is removed and the portion where the dislocation pit overlaps is separated separately. Although it is not possible to perform counting, the position where it exists can be specified. Therefore,
The binarized image indicating the position of the dislocation pit is superimposed on the core image of the dislocation pit and the binarized image in which the unevenness is detected, and if only the pixel where both exist at the same time is extracted, the dislocation pit is theoretically obtained. Only the core portion can be detected. The present invention has been completed as a result of such technical analysis. That is, the invention according to claim 1 is a compound for exposing crystal dislocations by etching a crystal face of a compound semiconductor wafer and measuring a dislocation pit density from an enlarged image of the etched crystal face. Assuming a method for measuring the dislocation pit density in a semiconductor wafer, a step of obtaining an enlarged monochrome image of the etched crystal plane, and a density higher than a first reference density which is a threshold value from each pixel group constituting the enlarged monochrome image. Or a step of extracting a pixel having a low density and converting it to a binary first image including a contour portion of a dislocation pit, and a second reference density serving as a threshold value from each pixel group constituting the enlarged monochrome image. Extracting pixels having high or low density and converting them into a binarized second image containing a core portion of dislocation pits, and A step of extracting a portion where both images overlap from one image and the second image, converting it into a binarized third image, obtaining an independent figure included in the third image, and counting it, and transposing the counted number A step of obtaining a dislocation pit density as the number of pits. Embodiments of the present invention will be described below. First, as the compound semiconductor wafer to be measured in the present invention, there is a compound semiconductor wafer of GaAs, GaP, InP or the like. Further, the enlarged monochrome image of the etched crystal plane means a monochrome image in which the etching surface is enlarged to a size in which dislocation pits can be recognized by a method such as a reflection microscope or a differential interference microscope. As for the means for counting the independent figures included in the binarized third image, the figures may be counted by mechanical measurement or visual observation, as in the related art. Further, the present invention utilizes an image processing technique. The term “image processing” refers to an operation such as capturing a monochrome image as image data into a computer via an imaging unit such as a video camera and binarizing the image data. It refers to a series of processing that performs processing. The case where the measuring method of the present invention is used in a general system will be described below. First, a video camera is attached to a reflection microscope, and the output side of the video camera is connected to a computer so that an image captured by the video camera can be captured. Next, the wafer to be measured is set on the microscope, and the measurement point is brought into the visual field of the microscope and focused. Next, an enlarged image of the wafer measurement surface is displayed on a monitor of the computer, and the enlarged image is taken into the computer. Then, the enlarged monochrome image is binarized to obtain a binarized first image including a contour portion of the dislocation pit and a binarized second image including a core portion of the dislocation pit, respectively, and After obtaining a binarized third image of a portion where both images overlap from the obtained first image and second image, the independent figures included in the binarized third image are counted, and the value is expressed as a dislocation pit. Convert to EPD and exit. EXAMPLES Examples of wafers having irregularities on the surface and overlapping dislocation pits, which were subjected to EPD measurement by the measuring method according to the present invention, will be specifically described below. First, for a wafer obtained from a GaP single crystal ingot grown by the LEC method, (11)
1) After etching the surface with aqua regia for about 10 minutes, etchant obtained by mixing 400 ml of HF, 600 ml of NO ₃ , 800 ml of H ₂ O, and 1 g of AgNO ₃ at a ratio of 60-70. The (111) plane of the wafer was further etched for 6 to 7 minutes while being heated to ℃. Next, the etched surface is enlarged under the bright field illumination of a reflection microscope while focusing on the amount of illumination and the enlarged image is taken into a computer via an industrial camera and displayed as a monochrome image. 3. FIG. 4 shows a binarized image (a binarized second image) obtained by adjusting the density standard to the core portion of the dislocation pit in FIG.
Shown in From this image, the irregularities are also detected together with the dislocation pits. Next, a binarized image (a first binarized image) in which the contour of the dislocation pit in FIG.
Shown in FIG. 6 shows a binarized image (binary third image) obtained by superimposing the binarized images of FIGS. 4 and 5 and extracting pixels at positions where both are present at the same time. In the binarized third image shown in FIG. 6, an independent figure of the binarized image remains at a position corresponding to the core of the dislocation pit shown in FIG. 3, and the dislocation pit is separated from the uneven portion. Has been detected. Table 1 shows the results of counting the independent figures in each of the binary images shown in FIGS. 4, 5 and 6. FIG.
Since the number of independent figures included in the binarized image (the binarized second image) is 135 and the uneven portion is detected, the number of the independent figure is 7 in the visual example.
The number of independent graphics included in the binarized image (the binarized first image) in FIG. 5 is more than eight, and is smaller than that in the visual example because the dislocation pits overlap each other. However, FIG.
The number of independent graphics included in the binarized image (binarized third image) of FIG. 6 obtained by extracting the image overlapping FIG. 5 and FIG. 6 is 81, which is very close to the number of dislocation pits 78 described in the visual example. Value. That is, by counting the number of independent figures included in the binarized image (binary third image) of FIG. 6, the number of dislocation pits can be obtained with the same level of accuracy as the visual method. Is confirmed. [Table 1] According to the method for measuring dislocation pit density in a compound semiconductor wafer according to the first aspect of the present invention,
A step of obtaining an enlarged monochrome image of the crystal plane subjected to the etching process, and extracting pixels having a higher or lower density than a first reference density serving as a threshold value from each pixel group constituting the enlarged monochrome image to obtain dislocation pits. Converting the image into a binary first image including a contour portion, and extracting and transposing pixels having a density higher or lower than a second reference density serving as a threshold from each pixel group constituting the enlarged monochrome image A step of converting into a binarized second image including a core portion of the pit, and extracting a portion where both images overlap from the obtained first image and second image and converting the same into a binarized third image. Since the method includes the steps of obtaining and counting the independent figures included in the third image and calculating the dislocation pit density by using the counted numerical values as the number of dislocation pits, the method includes the steps of: Has the effect of a dislocation pit density can be accurately measured without being influenced by the overlap of flatness and dislocations pit surface. Further, since this measuring method can be implemented by image processing and its arithmetic processing, it has an effect that it can be applied to automatic measurement of dislocation pit density.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an enlarged monochrome (black and white) image diagram of a reflection microscope with bright field illumination of a GaP (111) etched surface. 2 is a binarized image diagram in which dislocation pits of the enlarged image in FIG. FIG. 3 is an enlarged monochrome image diagram of a reflection microscope by bright-field illumination of an etched surface of a wafer having poor surface flatness and overlapping dislocation pits. 4 is a binarized image (binary second image) in which the core of the dislocation pit of the enlarged image in FIG.
FIG. 5 is a binarized image (a first binarized image) in which the contour of a dislocation pit of the enlarged image in FIG.
FIG. FIG. 6 is a binarized image (binary third image) obtained by extracting a portion where both images overlap from each of the binarized images of FIGS. 4 and 5;
FIG.

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Claims (1)

Claims: 1. A compound semiconductor wafer for etching a crystal plane of a compound semiconductor wafer to expose crystal dislocations and measuring a dislocation pit density from an enlarged image of the etched crystal plane. In the method for measuring the dislocation pit density in the above, a step of obtaining an enlarged monochrome image of the crystal face subjected to the etching treatment, and a step of obtaining a density higher or lower than a first reference density as a threshold value from each pixel group constituting the enlarged monochrome image Extracting pixels having the contours of the dislocation pits and converting them into a binarized first image, and a higher or lower density than a second reference density as a threshold from each pixel group constituting the enlarged monochrome image. A step of extracting a pixel having a density and converting it to a binarized second image including a core portion of a dislocation pit, and the obtained first image and A step of extracting a portion where both images overlap from the two images, converting the image into a binarized third image, obtaining an independent figure included in the third image, and counting the number; and displacing the counted number as the number of dislocation pits. A method for measuring a dislocation pit density in a compound semiconductor wafer, comprising the steps of: determining a pit density.