JP2003307499A - Defect observation method for substrate - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a defect inspection method that eliminates the need for polishing a wafer end face for allowing laser beams to enter in a wafer-like transparent sample in face of the problems of the conventional technique, eliminates the need for cutting a sample, enables a defect in the wafer state on an object to be inspected to be observed, and can put in a line industrially. <P>SOLUTION: The defect-observing apparatus comprises: a means for applying laser beams 31 for observation from the surface of a wafer-like object 5 to be inspected; and an observation means 37 for observing an object 3 to be inspected that is illuminated by the injection means. The defect-observing apparatus is sued for observing a defect or a foreign object in the object 5 to be inspected. The defect-observing apparatus further discriminates a surface defect due to the surface scratch of the object to be inspected or dust adhesion on the surface and the like from an internal defect existing inside the object to be inspected. <P>COPYRIGHT: (C)2004,JPO

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention detects foreign matter and defects in a test object such as a crystal used for a semiconductor substrate or an optical element substrate and obtains information on the foreign matter or defect. And a defect observation apparatus for the same. 2. Description of the Related Art Conventionally, as a technique for obtaining information on a foreign substance or a defect in a test object such as a crystal, a laser beam having a transparent wavelength is applied to a measurement material, and the laser beam is applied from the side of a sample. There is known a laser tomography method for observing, with a microscope or the like, internal scattered light from a main surface which is incident and perpendicular to the incident surface.
(Reference 1 Kazuo Moriya, Applied Physics 55, 6, 542 (198
6)) Ultraviolet laser scattering tomography has also been developed which utilizes the fact that the scattering intensity due to defects and the like increases in inverse proportion to the fourth power of the wavelength. (Reference 2 Hajime Komura et al. Proce
^{edings of the 9 th Sony Research Forum} , 364 (199
9)) The laser beam is incident from the end face and can pass through the wafer because the refractive index is higher than air and light is refracted and passes through the inside of the sample as a thin beam (Reference 1). However, when the wafer size is large, there is a problem that the light cannot be incident because the mode diameter of the entrance end face and the exit end face becomes larger than the wafer thickness. In other words, for a relatively small sample such as a sample piece, the laser beam is incident from the entrance surface by performing optical polishing on the entrance, exit surface and observation surface, and scattered light can be observed from the main surface orthogonal to the entrance surface. It is difficult to apply the above-described method to a large-area wafer-like specimen having a small sample thickness. Further, the wafer is subjected to a so-called beveling process in which the periphery is rounded in the thickness direction to prevent chipping due to handling and the surface is roughened. Can not. For this reason, it is necessary to optically polish the end face. However, it is difficult to optically polish the end face because of its small thickness, and it is not desirable because it becomes a destructive inspection. SUMMARY OF THE INVENTION In view of the problems of the prior art, it is an object of the present invention to eliminate the need to polish a wafer end surface because a laser beam is incident on a wafer-shaped transparent sample. It is an object of the present invention to provide a defect inspection method that enables a test object to observe a defect in a wafer state without cutting, and that can be industrially put into a line. [0005] In order to solve the above-mentioned problems, a thin wafer-shaped sample can be obtained by arranging a wafer at an angle with respect to an incident laser beam and observing with a microscope from a direction perpendicular to the laser beam. It was found that only internal defects could be observed. That is, the present invention includes illumination means for illuminating the test object with the observation laser beam, and observation means for observing the test object illuminated by the observation laser light. Using a defect observation device used for observing foreign matter, the test object has two optically polished opposing main surfaces, and enters an observation laser beam light obliquely to the incident side main surface vertical, A defect observation method is characterized in that observation is performed by the observation means from the direction of the emission-side main surface. As shown in FIG. 2, a wafer-like test object 5 is arranged at an angle of 45 degrees with respect to a laser beam 1. A part of the laser beam hitting the wafer incident side main surface 6 is Fresnel reflected (partial reflection due to a difference in refractive index between air and the object to be detected).
Is reflected by the optical axis 2 in the 90-degree direction. The specimen 5
Is partially reflected along the optical axis 3 by Fresnel reflection on the emission-side main surface 7. On the other hand, the laser light that has passed through the emission surface 7 propagates along the optical axis 1. When observed from the direction 21 on the Fresnel reflection side, strong Fresnel reflection light becomes stray light and does not catch scattered light from weak internal defects. When observing from the 90-degree direction 20 on the transmission side of the laser beam in order to avoid the influence of the stray light, it was found that scattering by the defect can be observed without being affected by the stray light. [0008] Surface scratches and deposits existing on the laser beam cross section on the entrance surface 6 and the exit surface 7 are strong scattering spots 12, 1.
Yields 3. The strong scattered light further illuminates the attachments and flaws (eg, 15 and 16 in the figure) on a remote surface outside the cross section of the laser beam and is observed as a bright spot on the microscope screen. On the other hand, point-like defects and line-like defects existing inside the test object do not shine due to the scattered light from these scattering spots 12 and 13, and the defect 10 crosses the internal defect in the beam cross section of the laser beam. It is observed only as a bright spot. FIGS. 3A and 3A are schematic diagrams of scattered images observed from a direction 20 at 90 degrees with respect to the laser beam propagation axis 1 in the direction of the exit side main surface 7 of the test object in FIG. b) and (c). A laser beam is incident from the left side of the figure. At the position where the test object exists, the image shown in FIG. Scattered image 12 due to processing scratches, scratches, etc. on the incident surface 12 in the cross section of the laser beam, and scattered image 1 due to defects and scratches on the exit surface
3, and bright spots 16 and 15 due to scratches and the like existing on the entrance / exit surface illuminated by the scattered lights of 12 and 13 are observed. On the other hand, there is no bright spot in the line passing the laser beam. Next, the test object is moved downward by Δy in the figure (FIG. 3B). In this case, the bright spots 16 and 15 due to processing scratches and scratches existing on the surface are shifted downward by Δy from the position on the screen while maintaining the same relative position.
On the other hand, a point-like defect, a linear defect, or a planar defect 11 existing inside the test object is observed as a luminescent spot 10 only when the laser beam crosses the cross section of the laser beam. When the test object is further moved, the position where the laser beam crosses in the case of a line or planar defect changes. Therefore, as shown in FIG. It turned out to move dimensionally. The one-dimensional position of the luminescent spot 10 has information in the depth direction of the detector, and by reading the one-dimensional position in the linear laser beam 1, it is possible to three-dimensionally measure the defect distribution in the detector. it can. In consideration of the difference in the appearance of a luminescent spot due to a scratch or a scratch existing on the surface and the appearance of a defect existing inside the detection object, the present inventors further determine these automatically and measure them. Was devised. FIGS. 4 (a), 4 (b) and 4 (c) show schematic diagrams of the principle of the automatic discrimination method. First, as shown in FIG.
The image at the step is captured as digital information by image capture, and the step moving distance Δ in the y direction
intensity distribution A on two lines separated by a distance corresponding to y
(n) and B (n) profile computer
It is recorded in the storage area of the microcomputer as an intensity distribution replaced with 0 and 1 information by the binarization processing. Next, as shown in FIG. 4 (b), the test object is moved in the y direction by a movement amount of Δy, and an image of n + 1 steps is similarly captured. Similarly, intensity distribution A on two lines
The intensity distributions of (n + 1) and B (n + 1) are read into a computer and converted into an intensity distribution replaced with 0 and 1 information by a binarization process. Next, the intensity distribution of the binarized A (n) one step before in the memory on the microcomputer and the intensity distribution of B (n + 1) taken in this time are compared and calculated.
A channel position that is 0 on A (n) and 1 on B (n + 1) is detected. This operation is n steps (Fig. 4 (a))
N + 1 steps (FIG. 4)
This corresponds to determining whether a bright spot exists on the B line in (b)). Specifically, B (n + 1) -A (n) for each image sensor channel other than the masked area
If +1 becomes the channel number (address of pixel)
Is detected and the address is stored. The channel number corresponds to the position information of the internal defect in the x-axis direction. Next, by moving the object by Δy in the y direction and repeating the same process, it becomes possible to map the defect distribution inside the object. FIG. 5 is a flowchart showing these steps. FIG. 6 is a schematic view showing a defect observation apparatus using laser tomography according to an embodiment of the present invention. As shown in the figure, this defect observation apparatus includes a means for causing a wafer-like test object 5 to enter from the surface with an observation laser beam 31 and an observation means 37 for observing the test object 3 illuminated thereby. Which is used for observing a defect or a foreign matter in the test object 5. The defect observation apparatus can further distinguish a surface defect caused by a scratch on the surface of the test object or adhesion of dust on the surface, and an internal defect existing inside the test object. As a wafer-shaped specimen 5, a Z having a thickness of 1 mm
Cut 4 inch Lithium Neobate (LiNbO3) Wafer 5
Is used. The wafer 5 is mounted on an XY stage 36 driven by pulse motors 34 and 35. The incident laser light source 31 has a wavelength of 532 nm and an output of 100.
An mW green laser (MDS-1000, manufactured by Mitsui Chemicals) is used. The laser light source 31 is fixed on the heat-dissipating mount table 30, is emitted in the horizontal direction, and is condensed by the collimator lens 32 and the condensing lens 33 so that the mode radius at the condensing point is about 50 μm. The lens position is adjusted so as to connect the focal position to the object center. Driving range 200 mm as xy stage
Is driven by a stage controller (Suruga Seiki: MCS-10). The drive stage 36 is fixed obliquely at 45 degrees with respect to the optical axis 1. With enlargement (× 2
00) The CCD camera 47 is fixed on an xz-axis micrometer (not shown) and is adjusted so that the beam position can be seen. The image from the CCD is 256 images using an image capture board (MTPIC-MN manufactured by μ Technica).
The image is taken into a microcomputer as a digital image of gradation. The image is captured in synchronization with the driving of the XY stage 36. FIG. 7 (a) (b), (c)
2 shows images captured at each step after the stage position is moved by 200 μm in the y-axis direction. Referring to FIG. 7A, strong spots 12 and 13 due to surface flaws, scattering, and the like are observed on the wafer entrance and exit surfaces. In addition, dust and bright spots 15 on the surface apart from the cross section of the laser beam illuminated by the strong scattering spot are also observed. Spot 12,1
The spot 10 seen between 3 is a linear defect existing inside the crystal. When the wafer is further moved by 200 μm, this spot moves and is observed as shown in FIG. If it is further moved by 200 μm, it moves as shown in FIG. It can be seen that such a defect is a line or a planar defect existing inside the wafer and penetrates above and below the wafer. FIG. 1 shows the defect state of a partial area when the internal defect state is mapped over the entirety of the 4-inch wafer by automatic scanning by the microcomputer. As described above, according to the present invention, even in the case of a large and thin wafer-shaped measurement object, only the internal defect state can be observed without polishing the wafer edge.
By linking the automatic moving device, it has become possible to quickly observe the internal defect state of the entire wafer. Further, the present invention is not limited to the oxide crystal, and the laser light source is not limited to the green laser. For example, in the case of a compound semiconductor wafer such as GaAs, it goes without saying that a defect state can be observed as in the case of using an Nd: YAG laser that oscillates at a transparent wavelength of 1.06 μm. In addition, a compound semiconductor such as InP can be effectively observed on a defect by using a fiber laser oscillating at a wavelength of about 1.58 μm or an LD light source amplified by a fiber amplifier.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a view showing a defect according to the measuring method of the present invention. FIG. 2 is a diagram showing a state of measurement according to the present invention. FIG. 3 is a diagram showing an outline of a scattering image. FIG. 4 is a schematic diagram showing the principle of an automatic discrimination method. FIG. 5 is a flowchart showing a defect distribution. FIG. 6 is a schematic view showing an example of a defect observation device used in the present invention. FIG. 7 is an observation diagram of scattered light. [Explanation of Signs] 1, 2, 3 ... Optical axis, 10 ... Defect 1
2,13 ... Scattering spots 15,16 ... Deposits and scratches on the surface

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Claims (1)

Claims: 1. An illumination device for illuminating a test object with an observation laser beam, and an observation device for observing the test object illuminated by the observation laser beam. A defect observing device used for observing a defect or a foreign substance in an inspection object, wherein the inspection object has two optically polished opposing main surfaces, and is inclined with respect to a vertical main surface on an incident side. A defect observation method, wherein a beam light is incident and observed by the observation means from the direction of the main surface on the emission side.