JP2011053085A - Method of detecting lpd on wafer surface - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method of detecting LPDs on a wafer surface for highly accurately detecting LPDs on a wafer surface when detecting the LPDs on the wafer surface by using laser light. <P>SOLUTION: The method of detecting LPDs on a wafer surface includes: a process for obtaining a first dark field image by using light coming from a light source, through a first optical path, into a detector to inspect a wafer surface by using a microscope which can photograph a dark field image, the microscope being equipped with a light source emitting laser light and a detector for detecting light which is laser light applied to the wafer surface from the light source and scattered/irregularly reflected on the wafer surface; a process for using the microscope which can photograph a dark field image to inspect the wafer surface by using light coming from the light source, through an optical path different from the first optical path, into the detector, thereby obtaining at least one dark field image; and a process for detecting the LPDs by using the first dark field image and the at least one dark field image. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a method for detecting LPD (Light Point Defects) such as defects and particles on a wafer surface using a laser beam, and more particularly to a method for detecting LPD with high sensitivity by removing speckles. .

  Conventionally, as a method of accurately evaluating the quality of a wafer by detecting defects existing on the wafer such as a silicon wafer or particles adhering to the wafer, the surface of the wafer is irradiated with laser light, There is known a method of detecting the scattering intensity of laser light using a laser light scattering type surface inspection apparatus, and detecting defects and particles as LPD using the value of the detected scattering intensity.

  Here, the surface of the wafer is not a perfect plane but has a very small roughness, and since the laser light is highly coherent, the laser light is irradiated onto the wafer surface and the scattered light is detected by a detector. When detecting, the light scattered at each point on the wafer surface interferes with each other in an irregular phase relationship caused by the surface roughness, so that not only LPD but also speckle (granular pattern) is included in the detected data. (For example, refer nonpatent literature 1).

  For this reason, in the conventional method of detecting LPD using the scattered intensity of laser light, the number of LPDs on the wafer surface may not be accurately measured due to haze caused by speckle. Therefore, it is usually necessary to detect the LPD by setting the threshold value of the detector in a low sensitivity range that does not include this speckle.

"Laser Handbook (2nd edition)", Laser Society of Japan, April 25, 2005

  Therefore, when detecting the LPD on the wafer surface using laser light, a method for detecting the LPD on the wafer surface capable of detecting the LPD on the wafer surface with high accuracy by reducing the influence of speckle generation is established. It was demanded.

  On the other hand, when detecting the LPD on the wafer surface, the present inventor can take a dark field image, and uses a microscope (dark field microscope) using a laser beam as a light source, and makes the optical path different. It has been found that a dark field image from which speckles have been removed can be obtained by photographing and processing a plurality of dark field images, and the present invention has been completed.

  That is, the present invention aims to advantageously solve the above-mentioned problems, and a method for detecting an LPD on a wafer surface according to the present invention comprises a light source that emits laser light, and a laser beam that is irradiated from the light source onto the wafer surface. Using a microscope capable of taking a dark-field image with a detector that detects light scattered and irregularly reflected on the wafer surface, and incident on the detector from the light source through the first optical path And a step of obtaining a first dark field image by using the microscope, and using the microscope, the light incident on the detector through an optical path different from the first optical path. And inspecting the wafer surface to obtain at least one dark field image, and detecting LPD using the first dark field image and the at least one dark field image. Features. In general, speckles that do not have an entity unlike an actual LPD have different positions (observed) caused by the optical path of light from the light source to the detector, and thus, the first dark field image and If at least one dark field image obtained by photographing the same field with a different optical path from the first dark field image is used, when speckles exist in the dark field image, for example, the position of the speckles in each image Speckle can be removed from the dark field image based on the difference. Therefore, LPD can be detected with high accuracy using a dark field image from which speckles have been removed. Note that the actual LPD has the same position even if the optical path changes.

  Here, the LPD detection method for the wafer surface according to the present invention differs between the first optical path and the optical path different from the first optical path by changing the position of at least one of the light source and the detector. Preferably.

  In the LPD detection method for a wafer surface according to the present invention, it is preferable that the first optical path is different from the optical path different from the first optical path by using detectors having different focal positions.

  Further, according to the method of detecting an LPD on the wafer surface of the present invention, the speckle is removed from the dark field image by multiplying the luminance of the first dark field image by the luminance of the at least one dark field image. Is preferably detected. This is because the speckle can be more effectively removed by multiplying the luminance.

  In the LPD detection method for a wafer surface according to the present invention, the detector is preferably a CCD or a CMOS image sensor.

  According to the present invention, when detecting LPD on a wafer surface using laser light, it is possible to detect the LPD on the wafer surface with high accuracy by reducing the influence of speckle generation.

It is the dark field image of the wafer surface image | photographed using the dark field microscope which used the laser beam as the light source. It is the dark field image which removed the speckles obtained using the dark field image shown in FIG.

  Hereinafter, embodiments of the present invention will be described in detail. The wafer surface LPD (Light Point Defects) detection method of the present invention includes a light source that emits laser light, and light that is scattered and irregularly reflected by the observation target from the light source. A microscope capable of capturing a dark field image (a dark field microscope).

  Here, as the dark field microscope, for example, the wavelength is 680 nm band (LD), 532 nm (YAG second harmonic), 488 nm (Ar Gas), 400 nm band (LD), 355 nm (YAG third harmonic), 266 nm (YAG4 A dark field microscope including a light source that emits laser light (such as a double harmonic) and a CCD or CMOS image sensor as a detector can be used.

  In addition, as a wafer for detecting LPD using the present invention, for example, particles having a mean square roughness (RMS) of 0.1 nm or less and a diameter of 50 nm (diameter in terms of polystyrene latex particles) or less detected as LPD. What has on the surface is mentioned.

  In one example of the LPD detection method for the wafer surface according to the present invention, first, a dark field microscope is used to inspect (photograph) the wafer surface using light incident on the detector from the light source through the first optical path. Then, a first dark field image having speckles is obtained.

  Next, the same wafer surface (same field of view) as the first dark field image was photographed and incident from the light source to the detector through a light path different from the first light path using a dark field microscope. The surface of the wafer is inspected (photographed) using the processed light to obtain at least one dark field image having speckles. Speckle is generated by the light scattered and diffused at each point on the wafer surface interfering with the irregular phase relationship caused by the surface roughness, and since it does not exist, it is detected from the light source. The position generated (observed) differs depending on the optical path of light to the vessel. Accordingly, the speckle position in the at least one dark field image is different from the speckle position in the first dark field image. On the other hand, since the substantial LPD is generated at the same position even if the optical path is changed, the position of the LPD in the at least one dark field image is the same as the position of the LPD in the first dark field image.

Here, as a method of changing the optical path of the light incident on the detector from the light source, for example, the following method may be mentioned.
(1) When obtaining the first dark field image, the optical path is made different by photographing the dark field image of the same field by changing the position of at least one of the light source and the detector. Note that the position of at least one of the light source and the detector may be changed by shifting the optical axis, or may be changed by changing the position of the detector on the same optical axis.
(2) When the first dark field image is obtained, the positions of the light source and the detector remain the same, and the optical paths are made different by photographing the dark field image of the same field using detectors having different focal positions.

  In the LPD detection method for a wafer surface according to the present invention, LPD is detected using the first dark field image and at least one dark field image. Specifically, at least one of the speckle position in the first dark field image captured using the light passing through the first optical path and the light passing through the optical path different from the first optical path. Using the difference between the speckle positions in the two dark field images, a dark field image from which speckles have been removed is obtained to detect LPD. In this way, if the difference between the speckle position in the first dark field image and the speckle position in at least one dark field image obtained by photographing the same field of view with an optical path different from that of the first dark field image is used. Since the dark field image in which the LPD is clearly seen can be obtained by removing the speckle, the LPD can be detected with high accuracy using the dark field image from which the speckle has been removed. Note that the number of at least one dark field image can be any number, but it is necessary to shoot using light passing through different optical paths.

  Here, as a method of obtaining a dark field image from which speckles have been removed using the first dark field image and the at least one dark field image, there is a method of compositing (combining) the dark field images. . More specifically, the brightness of two or more dark field images can be multiplied, added, or subtracted and composited to obtain a dark field image from which speckles have been removed. Since the speckle that does not exist is different for each dark field image, and the existing LPD is at the same position in each dark field image, when the luminance of the dark field image is multiplied, added or subtracted in this way, Since the brightness of the light spot caused by speckle becomes weak and the brightness of the light spot caused by LPD becomes strong, a dark field image from which only speckle is removed can be obtained. When two or more dark-field images are composited, it is preferable to use luminance multiplication from the viewpoint of efficiently removing speckles, and each image may be weighted and composited.

  As described above, when the LPD detection method for a wafer surface according to the present invention is used, when detecting the LPD on the wafer surface using a laser beam, the influence of speckle generation is reduced and the LPD on the wafer surface can be accurately detected. Can be detected.

  In the above embodiment, the case where speckle is present in the dark field image has been described. However, in the LPD detection method for the wafer surface according to the present invention, the LPD is detected by compositing the dark field image without speckle. Also good.

  EXAMPLES Hereinafter, although an Example demonstrates this invention still in detail, this invention is not limited to the following Example.

Example 1
Two dark field images of the surface of the wafer were taken using a dark field microscope equipped with laser light as a light source. Note that the two dark field images were taken using the detectors with different focal positions (with the focus shifted) while the positions of the light source and the detector remain the same. The obtained dark field images A and B are shown in FIGS. In FIGS. 1A and 1B, the large light spot at the center is LPD, and the other small light spots are speckles.
Next, the luminance (256 gradations) at the same point of the dark field image A and the dark field image B is multiplied, and the 65536 gradation luminance obtained by the multiplication is reassigned to 256 gradations to remove speckles. A dark field image was obtained. The obtained dark field image A × B is shown in FIG.

(Example 2)
Two dark field images of the surface of the wafer were taken using a dark field microscope equipped with laser light as a light source. The two dark field images were taken with the light source and the detector at different positions (shifting the optical axis). The obtained dark field images C and D are shown in FIGS.
Next, the luminance (256 gradations) of the same point of the dark field image C and the dark field image D is multiplied, and the luminance of 65536 gradations obtained by the multiplication is reassigned to 256 gradations to remove speckles. A dark field image was obtained. The obtained dark field image C × D is shown in FIG. In FIGS. 1C and 1D, the central large light spot is LPD, and the other small light spots are speckles.

  2 (a) and 2 (b), it can be seen that the LPD on the wafer surface can be detected with high accuracy by obtaining the dark field image from which the speckle has been removed by using the method for detecting the LPD on the wafer surface of the present invention. . The S / N ratio of the dark field image of FIGS. 1A to 1D was 2.5, but the S / N ratio of the dark field image of FIG. The S / N ratio of the dark field image of (b) was 50.

  According to the present invention, when the LPD on the wafer surface is detected using laser light, the LPD on the wafer surface can be detected with high accuracy.

Claims (5)

Using a microscope capable of photographing a dark field image, comprising: a light source that emits laser light; and a detector that detects light that is scattered and irregularly reflected on the wafer surface from the light source emitted from the light source. Inspecting the wafer surface using light incident on the detector through a first light path from a light source to obtain a first dark field image;
Using the microscope to inspect the wafer surface using light incident on the detector through an optical path different from the first optical path from the light source to obtain at least one dark field image;
Detecting LPD using the first dark field image and the at least one dark field image;
A method for detecting LPD on a wafer surface, comprising:   2. The wafer according to claim 1, wherein the first optical path and an optical path different from the first optical path are made different by changing a position of at least one of the light source and the detector. 3. Surface LPD detection method.   2. The LPD detection method for a wafer surface according to claim 1, wherein detectors having different focal positions are used to make the first optical path different from the optical path different from the first optical path. 3.   The LPD is detected by removing speckles from the dark field image by multiplying the luminance of the first dark field image and the luminance of the at least one dark field image. 4. The method for detecting an LPD on a wafer surface according to any one of 3 above.   5. The LPD detection method for a wafer surface according to claim 1, wherein the detector is a CCD or a CMOS image sensor.

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