JP2000258348A - Defect inspection apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a defect inspection apparatus by which an object to be inspected can be inspected with high accuracy without being affected by an irregularity in the film thickness (an irregularity in the color) of an interfering film even when the interfering film is executed to the object to be inspected. SOLUTION: An illumination optical system which illuminates a face to be inspected is provided. In addition, an imaging optical system comprising an imaging element which images the illuminated face to be inspected is provided. In addition, a variable-wavelength means by which light from the face, to be inspected, imaged by the imaging element is changed selectively into band light comprising different center wavelengths is provided. In addition, an image creation means by which pixel data satisfying a prescribed luminance condition is extracted regarding a pixel corresponding to every image obtained by the imaging element by changing a wavelength and which craetes an image constituted of the extracted image data is provided. In addition, a defect detecting means which detects the defect of the face, to be inspected, on the basis of the created image is provided.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a defect inspection apparatus for inspecting a defect on a surface of a test object such as a semiconductor wafer or liquid crystal glass.

[0002]

2. Description of the Related Art A semiconductor wafer illuminated by illumination light is picked up by an image pickup device, and the picked-up image is image-processed. The chip pattern is formed on a test object. Has been proposed. For example, it is detected by comparing two adjacent patterns based on captured image data and performing a difference process on the two patterns.

[0003]

Since the pattern formed on the semiconductor wafer is formed by various manufacturing processes while applying a resist film, an oxide film, and the like, it is extremely difficult to perform an inspection after each manufacturing process. is important. However, when inspecting a pattern formed on or in a coherent film (especially an oxide film), if the film has uneven thickness, unevenness in color will occur in a captured image. If the color unevenness is too large, it cannot be distinguished from a defect such as a pattern, a scratch, or dust, and this has a problem that inspection accuracy is affected.

The present invention has been made in view of the above-mentioned problems of the prior art,
It is an object of the present invention to provide a defect inspection apparatus capable of performing an inspection with high accuracy without being affected by uneven film thickness (uneven color).

[0005]

Means for Solving the Problems In order to solve the above problems, the present invention is characterized by having the following configuration.

(1) In a defect inspection apparatus for inspecting a defect on an inspection surface of a test object, an illumination optical system for illuminating the inspection surface, an imaging optical system having an image sensor for imaging the illuminated inspection surface, Wavelength variable means for selectively changing light from the inspection surface imaged by the image sensor to band light having a different center wavelength, and a corresponding pixel of each image obtained by the image sensor by changing the wavelength. Image creating means for extracting pixel data satisfying a predetermined luminance condition, and creating an image composed of the extracted pixel data; and defect detecting means for detecting a defect on an inspection surface based on the created image. And the following.

(2) In the defect inspection apparatus of (1),
The pixel data that satisfies the predetermined luminance condition is pixel data having a maximum value or a minimum value of luminance for a pixel corresponding to a position of each image obtained by changing the wavelength,
The image creating means creates an image composed of pixel data of either the maximum value or the minimum value of the luminance, or an image composed by selecting both pixel data.

(3) The defect inspection apparatus of (1) further comprises a normalization unit that normalizes the luminance of the image with respect to the wavelength based on the spectral characteristic of each wavelength changed by the wavelength variable unit and the sensitivity characteristic of the image sensor. Image forming means for generating an image based on the normalized image.

(4) In the defect inspection apparatus of (1),
The band light of each wavelength changed by the wavelength variable means is a narrow band determined so as to detect the interference characteristic of the film applied to the inspection surface with required accuracy.

(5) In the defect inspection apparatus of (1),
The center wavelength of the band light to be changed by the wavelength variable means is determined such that the predetermined luminance condition is detected with necessary accuracy with respect to the interference characteristics of the film applied to the inspection surface. And

(6) The defect inspection apparatus of (1) is an apparatus for inspecting an inspection object having a repetitive pattern on an inspection surface, wherein a certain first inspection surface corresponding to a pixel of the image sensor is provided.
Moving means for moving the position pattern and the second position pattern so that the positional relationship is substantially the same; the image creating means forms two images based on the image obtained by moving the moving means; Created, and the defect detecting means detects a defect based on a comparison between the two created images.

[0012]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a diagram showing a configuration of a defect inspection apparatus according to the present invention.

Reference numeral 1 denotes a halogen lamp as a light source of an illumination optical system, and 2 denotes a rotating plate having a plurality of wavelength selection filters 2a and an opening for white illumination. The wavelength selection filter 2a is for selectively converting white light emitted from the halogen lamp 1 into band light having a different center wavelength, and a plurality of wavelength selection filters for changing the center wavelength of the band light at predetermined intervals. It is prepared. In this embodiment, the center wavelength range is 450 n
m to 750 nm, and classified into 19 types based on a method for determining the interval between the wavelengths described later. Further, the bandwidth of each wavelength is set to a narrow enough band to detect the interference characteristic of the film applied to the inspection object with required accuracy. That is, if the bandwidth of the wavelength is widened, the intensity change of the interference light becomes too gentle and the inspection accuracy described later deteriorates, so that the bandwidth of each wavelength is set so that the intensity change with respect to the center wavelength falls within an allowable error. Is stipulated.

The rotating plate 2 is driven to rotate by a motor 3 so that a desired wavelength selection filter 2a and an opening are selectively arranged in the optical path. The light that has passed through the wavelength selection filter 2a or the opening is diffused by the diffusion plate 4, and becomes diffused illumination light having sufficiently uniform luminance. The light emitted from the diffusion plate 4 is made substantially parallel by the collimator lens 5, and then illuminates the wafer W mounted on the XY stage 6. The collimator lens 5 has a diameter slightly larger than the inspection surface of the wafer W to be inspected.

The wafer W illuminated by the illumination optical system is C
An image is captured by an imaging optical system including the CD camera 10 and the collimator lens 5 described above. The specularly reflected light from the wafer W illuminated by the illumination optical system is converged by the collimator lens 5 and an image of almost the entire surface of the wafer W is formed on the CCD camera 10 having an image sensor. Note that the rotating plate 2 having the above-described wavelength selection filter 2a and the like may be arranged on the imaging optical system side (in front of the camera 10).

An image signal from the camera 10 is sent to an image processing unit 2.
Input to 0. The image processing unit 20 performs predetermined processing such as A / D conversion on the image signal from the camera 10 and captures the image signal, and then performs necessary preprocessing such as noise removal and sensitivity correction of the image sensor to perform image processing for defect detection. I do. An image memory 20a stores an image from the CCD camera 10. 2
1 is an image display unit, 22 is a stage drive unit for moving the XY stage 6, and 23 is a control unit for controlling the entire defect apparatus.

Next, the principle of obtaining an image which is not affected by the thickness unevenness of a film having interference will be described.

The intensity of interference light obtained when illuminating a coherent film depends on the film thickness and the wavelength of illumination light.
Assuming that the intensity of light reflected on the surface of the film is A, the intensity of light reflected on the back surface of the film is B, the refractive index of the film is n, the film thickness is h, and the wavelength of the illumination light is λ, The strength C is

(Equation 1) Given by That is, the intensity C of the interference light changes sinusoidally with the change in the film thickness h (see FIG. 2).

(Equation 2) Becomes Further, the period of the intensity change of the interference light changes depending on the wavelength λ.

As can be seen from FIG. 2, when the film thickness is different, the brightness becomes different when observed at the same wavelength, but when observed at different wavelengths, the maximum value (or the minimum value) of the intensity of the interference light at each wavelength is observed. Focusing on (2), portions having different film thicknesses have the same brightness (same interference intensity). Therefore,
By using a plurality of wavelengths and creating an image composed of pixels satisfying a predetermined luminance condition such as a maximum value or a minimum value (those taking into account an allowable error as described later), Can obtain an image having a luminance (brightness) that does not depend on the difference between the two. However, the brightness obtained when the wavelength is changed depends on the spectral characteristics of the selected wavelength and the sensitivity of the imaging side. The luminance of the illumination light and the sensitivity on the camera side are corrected in advance so that the detected luminance is almost the same (this is referred to as normalization of the image luminance with respect to the wavelength change).

In using a plurality of wavelengths (wavelengths λ1, λ2,... Λn), the interval between the wavelengths is determined so that the maximum value (or minimum value) of the interference light is detected with the required accuracy. This is done, for example, as follows.

As shown in FIG. 2, a diagram of the interference light intensity for a certain wavelength λ1 is created. A horizontal line is drawn from the height of the maximum value to a height lower by an allowable error, and the positions r1 and r2 of the horizontal axis at which this and the intensity curve intersect are determined. The ratio (r2 / r1) is calculated from the positions of r1 and r2, and the ratio of each adjacent wavelength is equal to (or less than) the ratio of r1 and r2 based on the previously determined wavelength λ1. , Λ2, λ3 ...
Determine λn.

Each wavelength determined by this method (wavelength λ
1, λ2, λ3) are shown in the relationship between interference intensity and film thickness in FIG. It can be seen that the waveform formed by synthesizing the waveform showing the largest value for each film thickness between the film thicknesses h1 and h2 is within the allowable error with respect to the maximum value of the waveform. That is, it is shown that the maximum value of the intensity of the interference light can be detected with the required accuracy in the range of the film thickness h1 to h2.

When the film thickness is small, the interference intensity may not be maximized at a usable wavelength, but even in this case, if there is a minimum value, the minimum value is changed based on the same concept. Should be determined.

Next, the inspection operation in this embodiment will be described. Here, a description will be given mainly of a defect inspection when a chip pattern is formed on an oxide film formed on a wafer surface.

At the time of inspection, the image brightness is normalized in advance with respect to the wavelength change. This can be done as follows. First, instead of a wafer, a reference plate having a uniform reflection characteristic is illuminated while sequentially changing the wavelength as in the inspection. For each wavelength changed, the reflected light from the reference plate is received by the camera 10,
The luminance information for each wavelength is obtained from the output from the camera 10 (performed by the image processing unit 20). From the luminance information of each wavelength, for example, based on the luminance information of a certain wavelength,
The luminance correction data for each wavelength is obtained and stored so that the luminance information of the other wavelengths becomes substantially the same. When an image for each wavelength is obtained at the time of inspection, by correcting the brightness of each image based on the brightness correction data for each wavelength, each image can be evaluated under the same conditions. .

The actual inspection operation will be described. The control unit 23 drives the motor 3 to sequentially arrange 19 types of wavelength selection filters 2 a in the illumination light path of the halogen lamp 1 and illuminates the wafer W mounted on the XY stage 6.
The image processing unit 20 loads 19 image data of the wafer W captured by the camera 10 into the image memory 20a in synchronization with the selection of the wavelength of the illumination light under the control of the control unit 23.

When all the images have been captured, the image processing section 20 normalizes the brightness of each image data based on the brightness correction data obtained in advance. Thereafter, for the pixels corresponding to the position of each image data (pixels at the same position), those having the maximum value or the minimum value of the luminance in consideration of the allowable error as described above are extracted, and the extracted pixel data at each position is extracted. To create one image composed of As a result, it is possible to obtain an image in which a difference in brightness due to uneven thickness of a coherent film is eliminated. Created image (first created image)
Are stored in the image memory 20a.

In the defect inspection which will be described later, a detection process is performed in pixel units. Therefore, when one image is created, an image composed of only the maximum value or the minimum value of the luminance is not used. One image may be created from the pixel for which is selected and the pixel for which the minimum value is selected. When a pattern is formed in the film, the difference between the film thickness of the pattern portion and the film thickness of the portion without the pattern becomes large, so that the portion where the maximum value of the luminance cannot be obtained is on the minimum value side. Do what you choose.

When the first created image is stored in this way, the control unit 23 connected to the image processing unit 20 operates so that the positional relationship between the first position pattern and the second position pattern on the wafer coincides with each other. Then, the wafer W is relatively moved with respect to the imaging optical system to prepare for obtaining the second created image. This movement is performed, for example, by driving the stage driving unit 22 such that the stage driving unit 22 is shifted by a positive multiple of the repetition period with respect to the arrangement direction of the pattern of the chips formed on the wafer W.
The Y stage 6 is moved. In this case, the image data from the camera 10 is processed to determine the arrangement direction of the chip pattern, and the movement may be performed based on the data of the arrangement direction and the pattern pitch (this is known).

After moving the wafer W, in the same manner as when the first created image was obtained, a difference in brightness due to uneven film thickness was eliminated based on each image data captured by changing the illumination wavelength. One image is created (a second created image is obtained). When obtaining the first created image and the second created image, imaging before and after the movement of the wafer W may be performed first, and image processing may be performed later collectively. When the first created image and the second created image are obtained, the image processing unit 2
0 performs defect detection by comparing the two image data.

Defect detection of a repetitive pattern can be performed by a method of performing differential processing between images of adjacent patterns (pattern matching). In an image obtained by imaging the entire wafer, the camera 10 detects the pattern on the wafer. Since the pixels of the imaging device are coarse, moire occurs in the captured image. Therefore, in the difference processing using one image data, a pseudo defect is detected due to the influence of moire.
By using two pieces of image data in which the positional relationship between the pattern at the first position and the pattern at the second position on the wafer as described above is used, the effect of moire is eliminated and only the original defect is detected. can do. Hereinafter, the reason will be described with reference to FIG.

FIG. 4A shows CC before the wafer is moved.
FIG. 4B is a diagram schematically illustrating a positional relationship between patterns of two adjacent chips 1 and 2 with respect to a pixel of the D camera, and FIG. 4B is a diagram illustrating a luminance signal of image data at that time. . (C) and (d) are diagrams schematically showing the image after the movement (similar to the case where the optical system is relatively moved). Here chip 1
It is assumed that there is a defect pattern 50 in the pattern on the side. FIG. 4C shows the positional relationship of the pixels with respect to the pattern of the chip 1 in FIG. 4A shifted by one chip pitch.
The pattern of the chip 2 and the positional relationship between the pixels are the same. Therefore, the same moiré is generated in the image data of the pattern of the chip 1 in FIG. 4B and the image data of the pattern of the chip 2 in FIG. 4D, respectively. If the difference processing is performed, FIG.
As shown in (e), only the defect data 50 'remains,
Defect inspection without generation of pseudo defects due to moire or the like can be performed.

For defect detection of a test object having no repetitive pattern (for example, inspection for defects such as scratches and dust on a wafer before forming a pattern), one image data (wavelength is changed as described above) (A first created image obtained by the above), the detection can be performed. However, when there is a characteristic difference between pixels of the image pickup device of the CCD camera and this becomes a pseudo defect and affects the detection sensitivity, the following is performed. Just do it. That is, comparison processing of two pieces of image data (the first created image and the second created image obtained by changing the wavelength as described above) moved by a positive multiple of the amount of movement of the pixels of the image sensor is performed. Do.
Alternatively, a comparison process is performed between pre-stored defect-free reference image data and image data obtained as an inspection target. In this way, the defect inspection can be performed with high sensitivity without being affected by the characteristic difference between pixels.

The inspection which is strongly affected by the film thickness unevenness has been described above. However, in the case of the defect inspection which is not affected by the film thickness unevenness, the opening of the rotary plate 2 is formed without using the wavelength selection filter 2a. It is sufficient to arrange the optical path and detect defects based on an image obtained by white illumination light from the halogen lamp 1.

[0035]

As described above, according to the present invention,
Even for a test object provided with a coherent film, a highly accurate defect inspection can be performed without being affected by the film thickness unevenness (color unevenness).

[Brief description of the drawings]

FIG. 1 is a diagram showing a configuration of a defect inspection apparatus according to the present invention.

FIG. 2 is a diagram showing the relationship between the intensity of interference light obtained when illuminating a coherent film and the thickness and wavelength.

FIG. 3 shows, for each of the wavelengths (wavelengths λ1, λ2, λ3) determined so that the maximum value of the interference light is detected with the required accuracy.
FIG. 3 is a diagram showing the relationship between interference intensity and film thickness.

FIG. 4 is a diagram for explaining a method of detecting only an original defect by comparing a pattern at a first position and a pattern at a second position while eliminating the influence of moiré.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Halogen lamp 2 Rotating plate 2a Wavelength selection filter 5 Collimator lens 10 CCD camera 20 Image processing unit 20a Image memory 23 Control unit W Wafer

Claims (6)

[Claims] 1. A defect inspection apparatus for inspecting a defect on an inspection surface of a test object, comprising: an illumination optical system for illuminating the inspection surface; an imaging optical system having an image sensor for imaging the illuminated inspection surface; Wavelength variable means for selectively changing light from the inspection surface imaged by the image sensor to band light having a different center wavelength, and for a corresponding pixel of each image obtained by the image sensor by changing the wavelength, An image creating unit that extracts pixel data satisfying a predetermined luminance condition and creates an image composed of the extracted pixel data; and a defect detecting unit that detects a defect on an inspection surface based on the created image. A defect inspection apparatus comprising: 2. The defect inspection apparatus according to claim 1, wherein the pixel data that satisfies the predetermined luminance condition includes a pixel corresponding to a position of each image obtained by changing a wavelength.
The pixel data is a maximum value or a minimum value of the luminance, and the image creating means is configured by selecting an image composed of one of the pixel data of the maximum value or the minimum value of the luminance, or the pixel data of both of them. A defect inspection apparatus for creating an image. 3. The defect inspection apparatus according to claim 1, further comprising: a normalization unit that normalizes an image brightness with respect to a wavelength based on a spectral characteristic of each wavelength changed by the wavelength variable unit and a sensitivity characteristic of the image sensor. A defect inspection apparatus comprising: means for generating an image based on a normalized image. 4. The defect inspection apparatus according to claim 1, wherein the band light of each wavelength changed by said wavelength variable means is determined so as to detect the interference characteristic of a film applied to the inspection surface with required accuracy. A defect inspection device characterized by a narrow band. 5. The defect inspection apparatus according to claim 1, wherein the center wavelength of the band light changed by the wavelength variable means is determined by the predetermined luminance condition with respect to an interference characteristic of a film formed on the inspection surface. A defect inspection device characterized in that it is determined to be detected with high accuracy. 6. A defect inspection apparatus according to claim 1, wherein the defect inspection apparatus inspects an inspection object having a repetitive pattern on an inspection surface, wherein the pattern at a first position on the inspection surface with respect to the pixels of the image sensor is the same as the defect inspection device. Moving means for moving so that the positional relationship with the two-position pattern becomes substantially the same; the image creating means creates two images based on the image obtained by moving by the moving means; A defect inspection apparatus, wherein the detection means detects a defect based on a comparison between the two created images.