KR101323035B1 - Polycrystalline wafer inspection method - Google Patents

Abstract

Irradiating the infrared ray 3 toward the irradiation position P1 from the light source 2 arranged so that the optical axis passes through the irradiation position P1 on the polycrystalline wafer 1; The polycrystalline wafer 1 incident from the irradiation position P1 and repeatedly refracted and reflected by crystal grain boundaries and defects inside the polycrystalline wafer 1 and spaced a predetermined distance D from the irradiation position P1 in the plane direction of the polycrystalline wafer 1. Photographing the infrared ray 3 emitted from the photographing position P2 on the camera) with a camera 6 photographing the photographing position P2; On the picked-up image obtained by the camera 6, the inspection method of the polycrystal wafer which includes the process of detecting the defect in the polycrystal wafer 1 by the brightness difference of a defect part and a defect part is provided. According to this inspection method, the crystal shape of the polycrystalline wafer 1 is thin, and the picked-up image which can distinguish clearly the presence of a defect can be obtained, and a defect can be detected easily and reliably.

Description

Inspection method of polycrystalline wafer {POLYCRYSTALLINE WAFER INSPECTION METHOD}

The present invention relates to a method for inspecting defects in polycrystalline wafers, such as polycrystalline silicon wafers for solar cells, by infrared transmission.

Patent document 1 discloses the method of irradiating an infrared ray to a silicon wafer, imaging a transmitted infrared ray with a CCD camera, and detecting defects, such as a microcracker, from the picked-up image at that time by image processing.

Moreover, patent document 2 irradiates infrared rays from the front surface and the back surface of a polycrystal wafer, image | photographs the infrared reflection light from the surface and the infrared transmission light from the back surface by an infrared camera, and compares the image data from the surface / back surface by the result of comparison A method of detecting crack defects inside a polycrystalline wafer is disclosed.

However, in the case where the inspection object is a polycrystalline silicon wafer, according to the general imaging technique of the infrared transmitted light, since the crystal shape along the direction of the crystal, the boundary of the crystal, and its outline is also read as an image, the crystal shape in the process of image processing. It is difficult to identify excessive defects, and it is easy to miss false detections and defects.

Japanese Patent Application Publication No. 2007-258555 Japanese Patent Application Publication No. 2007-218638

An object of the present invention is to make the crystal shape according to the direction of the crystal of the polycrystalline wafer, the boundary of the crystal and the contour thereof thinned during the imaging process, so as to reliably detect a defect in the polycrystalline wafer.

Under the background of the problem described above, the inventor repeated the experiment of irradiating infrared rays to the polycrystalline wafer and observing the transmitted infrared rays. As a result, the following findings were made. In other words, if the infrared rays transmitted through the polycrystalline wafer are directly observed at the infrared irradiation position, the crystal shape of the polycrystalline wafer in the photographed image cannot be thinned. However, if the irradiation position of infrared rays and the observation position of the transmitted infrared rays (i.e., the photographing position by the camera) are separated by a suitable distance, the crystal shape of the polycrystalline wafer can be made thin, and only the brightness of defects in the polycrystalline wafer are other normal portions. Could be different from the brightness of. This invention was completed based on such a discovery.

In order to achieve the above object, according to the present invention, the following is provided. (1) a step of irradiating infrared rays toward the irradiation position from a light source arranged so that the optical axis passes through the irradiation position on the polycrystalline wafer, and entering and exiting from the irradiation position to repeat the refraction and reflection in the inside of the polycrystalline wafer; The step of photographing the infrared rays emitted from the photographing position on the polycrystalline wafer spaced a predetermined distance away from the position in the plane direction of the polycrystalline wafer by a camera photographing the photographing position, and on the photographed image obtained by the camera. And detecting a defect in the polycrystalline wafer by the difference in brightness between the portion and the defective portion.

(2) The said imaging position is set to the surface on the opposite side to the surface of the said polycrystalline wafer in which the said irradiation position is set, The inspection method of the polycrystalline wafer of said (1) characterized by the above-mentioned. (3) The said imaging position is set to the same surface as the surface of the said polycrystalline wafer in which the said irradiation position is set, The inspection method of the polycrystalline wafer of said (1) characterized by the above-mentioned. (4) The light source is a single light source, and the optical axis of the light source is inclined with respect to the surface of the polycrystalline wafer so as to extend from the irradiation position to the imaging position side. The inspection method of the polycrystalline wafer of one base material. (5) The light sources are a plurality of light sources arranged substantially symmetrically with respect to the photographing position, and the optical axis of each of the light sources extends to the photographing position side at each of the irradiation positions to the surface of the polycrystalline wafer. It is inclined at the same inclination angle with respect, The inspection method of the polycrystal wafer as described in any one of said (1)-(3). (6) The light source is a line type light source, the camera is a line sensor type camera, and the camera detects infrared rays collected by a cylindrical lens. The inspection method of the polycrystalline wafer of any one of description). (7) The said light source is a ring-shaped light source which forms a ring-shaped irradiation area, The said camera is an area sensor type camera which makes the inside of a ring-shaped irradiation area into a photographing area, The said camera is a magnifying lens. And detecting the infrared rays collected by the method. The method for inspecting a polycrystalline wafer according to any one of the above (1) to (5).

According to the inspection method of the polycrystalline wafer of the present invention, the infrared rays incident on the polycrystalline wafer from the irradiation position are repeatedly reflected or refracted in the polycrystalline wafer and photographed on the polycrystalline wafer spaced a predetermined distance away from the irradiation position in the plane direction of the polycrystalline wafer. Emitted from the location. By photographing the infrared rays emitted from this photographing position with a camera, a crystal image is thin and a captured image which can clearly distinguish the presence of a defect can be obtained, and defects can be detected easily and reliably. Specifically, when no defect is present in the polycrystalline wafer, the infrared rays are repeatedly reflected or refracted in the polycrystalline wafer, whereby the intensity of the infrared rays reaching the photographing position becomes almost uniform and is hardly affected by the crystal shape. In this case, the photographed image obtained by the camera becomes an image of uniform brightness that does not reflect the crystal shape of the polycrystalline wafer. However, when a defect exists in the polycrystalline wafer, the infrared rays are diffusely reflected by the defect, and the intensity of the infrared rays reaching the imaging position becomes nonuniform. Therefore, on the picked-up image obtained by a camera, a defect appears as an area | region where brightness differs compared with the case where a defect does not exist. As described above, according to the present invention, the photographed image obtained by the camera is hardly influenced by the direction of the crystal of the polycrystalline wafer, the crystal boundary along the boundary of the crystal or its contour, and only the defect has the defect-free portion and the brightness. Since different, the defect in a polycrystal wafer can be detected reliably.

1 is a side view of an optical system for performing a method for inspecting a polycrystalline wafer according to the present invention.
2 is a front view of an optical system for performing a method for inspecting a polycrystalline wafer according to the present invention.
3 is an explanatory diagram of a reflection and refraction situation of infrared rays inside a polycrystalline wafer.
4A is a photographed image photograph of a polycrystalline wafer with infrared rays according to the present invention.
4B is a photographed image photograph of a polycrystalline wafer with infrared rays according to a reference example.
5 is a side view of an optical system for performing a method for inspecting a polycrystalline wafer according to a modification of the present invention.
6 is a side view of an optical system for performing a method for inspecting a polycrystalline wafer according to a modification of the present invention.
7 is a side view of an optical system for performing a method for inspecting a polycrystalline wafer according to a modification of the present invention.
8 is a plan view of an inspection range (observation range) on a polycrystalline wafer.
9 is a side view of an optical system for performing a method for inspecting a polycrystalline wafer according to a modification of the present invention.

1 and 2 show an optical system for performing a method for inspecting a polycrystalline wafer 1 according to the present invention. FIG. 1 is a side view of an optical system showing a state in which an inspection direction (the conveying direction of the polycrystalline wafer 1) A is from right to left, and FIG. 2 is a state in which the inspection direction A faces the front of the drawing in the drawing. It is a front view of the optical system shown.

With reference to FIG. 1, FIG. 2, the optical system for implementing the inspection method of the polycrystal wafer 1 which concerns on this invention is demonstrated. First, from the linear light source 2 arranged on the lower surface side of the polycrystalline wafer 1, the linear infrared ray 3 extending in the direction orthogonal to the conveying direction A of the polycrystalline wafer 1 is subjected to the polycrystalline wafer ( Irradiate toward the line type irradiation position P1 of 1). At this time, the light source 2 is arrange | positioned so that the optical axis of the light source 2 which may pass through the irradiation position P1 may be inclined with respect to the normal line n1 of the surface of the polycrystalline wafer 1. Specifically, the optical axis of the light source 2 is the inclination angle α with respect to the normal line n1 so that the infrared rays 3 emitted from the light source 2 extend from the irradiation position P1 side to the imaging position P2 side.

Such a linear light source 2 can be comprised by linearly arranging many infrared light emitting diodes, or by combining the rod-shaped infrared light source and the light source cover in which the linear slit was formed.

As shown in FIG. 3, the infrared ray 3 incident from the irradiation position P1 repeats reflection and refraction inside the polycrystalline wafer 1 and further reflects on the surface / backside of the polycrystalline wafer 1. Repeat to reach the shooting position P2. Part of the infrared ray 3 that has reached the imaging position P2 is reflected, and part of it is emitted from the surface of the polycrystalline wafer 1 as it is. Among these, the infrared ray 3 emitted from the imaging position P2 is image | photographed by the camera 6 arrange | positioned so that the optical axis 7 may pass through the imaging position P2, and a photographed image can be acquired by the camera 6. . Here, the photographing position P2 is set at a position spaced apart from the irradiation position P1 by a predetermined distance D in the plane direction of the polycrystalline wafer 1.

In the present embodiment, the camera 6 is disposed on the side opposite to the light source 2 with respect to the polycrystalline wafer 1. The optical axis 7 of the camera 6 passes through the photographing position P2 and is perpendicular to the surface of the polycrystalline wafer 1.

As for the wavelength of the infrared ray 3 irradiated linearly, the wavelength suitable for detection of an internal defect, for example, the wavelength range of 0.7 micrometer-2.5 micrometers is preferable. It is also preferable that the camera 6 also has good sensitivity in this wavelength range.

The imaging position P2 is set in the position spaced apart by the predetermined distance D from the irradiation position P1. This distance D is set according to the crystal structure of the polycrystalline wafer 1, its thickness, etc., and is set in the best position where the crystal shape becomes thin.

Moreover, it is preferable that the test | inspection method of this invention targets the polycrystal wafer 1 of thickness 0.1-0.25 mm. As the thickness of the polycrystalline wafer 1 becomes thicker, the infrared ray 3 is refracted, reflected, or absorbed inside the polycrystalline wafer 1, and the intensity of the infrared ray 3 captured by the camera 6 is lowered, thereby providing a clearer captured image. Because you can't get this. When the thickness of the polycrystalline wafer 1 becomes thin, the number of refractions and reflections occurring until the infrared ray 3 reaches the imaging position P2 decreases, and the crystal shape remains in the photographed image obtained by the camera 6.

In addition, the inclination angle α of the optical axis of the light source 2 with respect to the normal line n1 of the surface of the polycrystalline wafer 1 is preferably set in a range of 20 ° or more and 40 ° or less. When the inclination angle α is less than 20 °, the number of refractions / reflections required until the infrared ray 3 reaches the shooting position P2 spaced apart from the irradiation position P1 by a predetermined distance D increases, thereby photographing with the camera 6. It is because the intensity | strength of the infrared rays 3 made into them falls, and a clear photographed image cannot be obtained. On the contrary, when the inclination angle α is larger than 20 °, the number of refractions / reflections required until the infrared ray 3 reaches the imaging position P2 decreases, and the crystal shape remains in the captured image.

Moreover, it is preferable to set the predetermined distance D between irradiation position P1 and imaging | photography position P2 to 1-3 mm. If the predetermined distance D is shorter than 1 mm, the number of refractions / reflections required until the infrared ray 3 reaches the imaging position P2 decreases, and the crystal shape remains in the captured image. If the predetermined distance D is longer than 3 mm, the number of refractions / reflections increases, the intensity of the infrared ray 3 picked up by the camera 6 decreases, and a clear captured image cannot be obtained.

In the inspection method of the polycrystalline wafer 1 of the present invention, the thickness, the inclination angle α, and the predetermined distance D of the polycrystalline wafer 1 described above are described in detail so that a clear photographed image can be obtained while the influence of the crystal pattern is small. Set appropriately within the range.

In the optical system for carrying out the inspection method of the polycrystalline wafer 1 configured as described above, the infrared rays 3 passing through the defect-free region of the polycrystalline wafer 1 have a large number of randomly present crystals. Refraction or reflection is repeated at the crystal direction of the particles or at the boundary of the crystal to reach the shooting position P2. When the infrared rays 3 having repeated random refractions or reflections reach the photographing position P2 spaced apart from the irradiation position P1 by a predetermined distance D, the effects of refraction / reflection on the respective crystal particles cancel each other, The picked-up image picked up by the camera 6 at the picking-up position P2 turns into a linearly picked-up image with uniform brightness.

On the other hand, when the defect 4 is present in the polycrystalline wafer 1, differently from the above, the infrared ray 3 causes diffuse reflection or absorption by the defect 4, so that the captured image photographed at the photographing position P2 The shadow or bright part by the defect 4 is revealed. Since the shadow and the bright part by this defect 4 differ in brightness from the photographed image formed by the infrared ray 3 which passed through the defect-free area mentioned above, the defect 4 is compared by comparing the brightness of both. Can be detected.

By carrying out the above process continuously and repeatedly while conveying the polycrystalline wafer 1 in the transport direction A, a photographed image having an area as shown in FIGS. 4A and 4B can be obtained.

4A and 4B show photographed images of the camera 6 photographing the infrared ray 3 transmitted through the region including the defect 4. In FIG. 4A, a bright image with dark shadows by the infrared rays 3 passing through the defect 4 is formed on the background image of uniform brightness formed by the infrared rays 3 passing through the defect free area. Therefore, the defect 4 can be recognized simply and reliably by detecting the area | region where brightness differs in the background image of uniform brightness. 4A is a picked-up image obtained by setting the polycrystal wafer 1 of thickness 0.2mm as a defect detection object, and setting predetermined distance D = 2mm and inclination-angle (alpha) = 20 degrees.

In the present invention, the photographing position P2 is set at a position spaced apart from the irradiation position P1 by a predetermined distance D = 2 mm in the plane direction of the polycrystalline wafer 1. On the other hand, when the photographing position is set at a position P3 where the predetermined distance D on the extension line of the optical axis of the light source 2 is shorter than 1 mm (see FIG. 1), the image is emitted without sufficiently refracting or reflecting. Since the infrared ray 3 is photographed at the photographing position P3, the photographed image becomes an image affected by the boundary of the crystal. Therefore, even if the picked-up image is formed from the infrared ray 3 which passed through the area | region containing the defect 4, the part affected by the defect 4 will be buried in a crystal form like FIG. 4B, and the defect 4 And crystallization becomes difficult to identify.

FIG. 5 shows two line light sources 2 at the lower side of the polycrystalline wafer 1 at positions symmetrical with respect to the normal line (the optical axis 7 of the camera 6) on the imaging position P2, and each light source 2. ) Is an example of irradiating the line-type infrared ray 3 to two irradiation positions P1 of the polycrystalline wafer 1 in different inclined directions. In addition, in this example, the inclination angle formed by the optical axis of each light source 2 and the surface of the polycrystalline wafer 1 is set to be substantially the same. According to this example, in addition to the above-described effects, the amount of light of the infrared ray 3 that can be detected by the camera 6 increases, so that a bright photographed image is obtained, so that the detection of the defect 4 becomes easy.

6 shows an example in which the infrared rays 3 transmitted through the polycrystalline wafer 1 are collected by the cylindrical lens 8 and the collected infrared rays 3 are detected by the line sensor type camera 6. In this example, the cylindrical lens 8 is arrange | positioned so that the longitudinal direction may follow the line-type infrared rays 3, and the image of the infrared rays 3 is expanded in the conveyance direction of the polycrystalline wafer 1.

In this way, when the infrared ray 3 is enlarged by the lens 8, the detection of the infrared ray 3 by the camera 6 becomes easy, and it is possible to reduce the erroneous detection and missing even in the continuous movement of the polycrystalline wafer 1. It is advantageous in that it is. Here, the lens 8 can be applied to an example in which the light source 2 is single, as shown in FIGS. 1 and 2.

The specific dimensions, the arrangement of the optical system, and the like are set to appropriate values according to the thickness of the polycrystalline wafer 1, the wavelength region of the infrared ray 3, the irradiation angle of the infrared ray 3, the sensitivity of the camera 6, and the like.

Next, FIG. 7 shows an example in which the light source 2 is a ring light source and the camera 6 is an area camera, and the light source 2 and the camera 6 are arranged on different surface sides with respect to the polycrystalline wafer 1. . The ring light source 2 is arranged concentrically with respect to the optical axis 7 of the camera 6. The irradiation position P1 of the light source 2 is given in the position where the luminous flux of the infrared ray 3 which the light source 2 irradiates is largest, and is a circular shape slightly smaller than the circular shape of the light source 2.

According to this example, the imaging position (photographing area) P2 is a detection range by the area camera 6, and the optical axis of the camera 6 from the irradiation position P1 inside the ring-shaped light source 2 as shown in FIG. In the direction of (7), the inside of the circle having a radius as small as the distance D is obtained. In addition, the convex lens 8 for enlargement on the objective lens side of the camera 6 is arrange | positioned as needed. In addition, irradiation position P1 can also be formed by ring-shaped slit.

According to the example of FIG. 7, the infrared ray 3 from the light source 2 enters the inside of the polycrystalline wafer 1 from the circular irradiation position P1, repeats refraction and reflection, and performs circular imaging of the camera 6. The inner side of the position P2 is reached, and the image is captured by the area camera 6.

Since the infrared light 3 is irradiated from the entire direction of the camera 6 toward the irradiation position P1 of the polycrystalline wafer 1 by the ring-shaped light source 2, the defect 4 in the polycrystalline wafer 1 is in either direction. Even when it is difficult to detect from the above, the defect 4 can be detected. Moreover, since the inspection range (observation range) of the polycrystalline wafer 1 can be set to the surface larger than the line type inspection range by employ | adopting the area type camera 6, inspection efficiency improves.

9 shows an example in which the ring light source 2 and the area camera 6 are arranged on the same surface side of the polycrystalline wafer 1. Also in this example, the infrared ray 3 from the light source 2 enters the inside of the polycrystalline wafer 1 from the circular irradiation position P1, repeats refraction and reflection to reach the inside of the circular photographing position P2, It is photographed by the type camera 6.

In addition, when the infrared ray 3 becomes unclear due to reflection on the surface of the polycrystalline wafer 1, the reflected light of the infrared ray 3 is shielded to the camera 6 so that the reflected light is not directly incident on the camera 6. A hood 9 may be provided. Moreover, also in this example, irradiation position P1 can also be formed by ring-shaped slit.

According to the example of FIG. 9, since the irradiation position P1 and the imaging position P2 are on the same plane with respect to the polycrystalline wafer 1, the portion of the defect 4 in the polycrystalline wafer 1 is more than the other normal portion with respect to the infrared ray 3. When it has a strong reflection characteristic, the detection of the defect 4 becomes effective and easy. Furthermore, even if the irradiation position P1 or the imaging position P2 cannot be set on one surface of the polycrystalline wafer 1, the defect 4 can be detected.

Of course, also about the example of FIG. 1, FIG. 2, FIG. 5, and FIG. 6 mentioned above, the linear light source 2 may be arrange | positioned with respect to the polycrystal wafer 1 on the surface of the same side as the camera 6.

In addition, the infrared ray 3 from the line-shaped light source 2, as illustrated by a dashed-dotted line in FIG. 9, uses a light guide such as an optical fiber or an acrylic resin plate, if necessary, of the polycrystalline wafer 1. It is also possible to irradiate the inside of the polycrystalline wafer 1 from at least one end face among four end faces (four side faces).

In this case, according to the examples of Figs. 5, 6, 7 and 9, in the moving process of the polycrystalline wafer 1, one end in the advancing direction or one end in the advancing direction of the polycrystalline wafer 1 Even if the light source 2 or a part of the light source 2 is deviated, the other light source 2 or another part of the light source 2 does not deviate from the end of the polycrystalline wafer 1 during the movement, and then the defect 4 continues. Detection can proceed. Therefore, the defect 4 can be detected also at the end of the polycrystalline wafer 1.

In the above example, the infrared rays 3 are irradiated in the oblique direction toward the irradiation position P1 of the polycrystalline wafer 1. For this reason, in the process of passing the infrared rays 3 through the crystal wafer 1, the chances of refraction and reflection are greater than the irradiation in the vertical direction, so that the infrared rays 3 can not be easily affected by the crystal shape. However, the irradiation direction of the infrared rays 3 can also be set in a substantially vertical direction toward the irradiation position P1 of the polycrystalline wafer 1. Even if it is set in this way, since the infrared ray 3 is reflected by the boundary of many crystal | crystallization, since the infrared ray 3 diffuses also in a vertical direction other than this, a crystal shape is obtained by imaging this diffused infrared ray 3 It is possible to obtain a photographed image that is not affected by.

In addition, the above-mentioned example is irradiating in the state which inclined the infrared ray 3 to the imaging position P2, toward the irradiation position P1 of the polycrystalline wafer 1, and is inclined. For this reason, since many infrared rays 3 face the imaging position P2 via the polycrystalline wafer 1, the quantity of light required at the imaging position P2 can be ensured. However, even if the infrared rays 3 are directed in a direction other than the photographing position P2 via the polycrystalline wafer 1, the amount of light that can be photographed at the photographing position P2 by refraction and reflection in the inside of the polycrystalline wafer 1 and further by diffuse reflection. Since this appears, inspection of the defect 4 is possible in principle.

When the polycrystalline wafer 1 is stopped at the inspection position, the imaging condition is good. On the other hand, when prioritizing the shutter speed, the polycrystalline wafer 1 may be continuously moved. In addition, the attitude | position of the polycrystal wafer 1 may be set to a vertical or inclined state according to an inspection space rather than horizontal. In addition, the present invention is not limited to a silicon wafer, and can also be used for wafers of other polycrystalline structures.

Although this invention was detailed also demonstrated with reference to the specific embodiment, it is clear for those skilled in the art that various changes and correction can be made within the thought and range of this invention. This application is based on the JP Patent application (Japanese Patent Application No. 2009-130725) filed on May 29, 2009, and the Japanese patent application (Japanese Patent Application No. 2009-186304) filed on August 11, 2009. Is incorporated herein by reference.

According to the inspection method of the polycrystalline wafer of this invention, the crystal shape according to the direction of a crystal | crystallization of a polycrystal wafer, the boundary of a crystal | crystallization, and its outline is thin, and the picked-up image which can distinguish clearly the presence of a defect can be obtained, The defect can be reliably detected.

Claims (9)

Irradiating infrared rays toward the irradiation position from a light source arranged such that an optical axis passes through the irradiation position on the polycrystalline wafer;
Repeatedly refracted and reflected due to grain boundaries and defects inside the polycrystalline wafer, incident from the irradiation position, infrared rays emitted from an imaging position on the polycrystalline wafer spaced a predetermined distance from the irradiation position in the plane direction of the polycrystalline wafer. Photographing with a camera photographing the photographing position;
Detecting a defect in the polycrystalline wafer by a difference in brightness between the defect portion and the defect portion on the captured image obtained by the camera,
The predetermined distance is set to a distance at which the photographed image capable of identifying a defect is obtained without leaving a crystal pattern on the photographed image,
And when the thickness of the polycrystalline wafer is 0.1 to 0.25 mm, the predetermined distance is 1 to 3 mm. The method of claim 1,
The photographing position is set on a surface opposite to the surface of the polycrystalline wafer on which the irradiation position is set. The method of claim 1,
The photographing position is set on the same surface as that of the polycrystalline wafer on which the irradiation position is set. 4. The method according to any one of claims 1 to 3,
The light source is a single light source,
The optical axis of the light source is inclined with respect to the surface of the polycrystalline wafer so as to extend from the irradiation position to the imaging position side. 4. The method according to any one of claims 1 to 3,
The light source is a plurality of light sources arranged symmetrically with respect to the shooting position,
The optical axis of each of the light sources is inclined at the same inclination angle with respect to the surface of the polycrystalline wafer so as to extend from the respective irradiation position to the photographing position side. 4. The method according to any one of claims 1 to 3,
The light source is a line type light source,
The camera is a line sensor type camera,
The camera detects the infrared light collected by a cylindrical lens. 4. The method according to any one of claims 1 to 3,
The light source is a ring light source that forms a ring-shaped irradiation area,
The camera is an area sensor type camera having an inside of the ring-shaped irradiation area as a shooting area,
The camera detects the infrared rays focused by the magnifying lens. delete The method of claim 1,
And said light source irradiates said infrared rays toward said irradiation position in a direction inclined in a range of 20 ° or more and 40 ° or less with respect to a normal of the surface of said polycrystalline wafer.

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