Defect inspection method for a light-transmissive plate-like body

Abstract

To provide a defect inspection method of a light permeable plate body for highly accurately detecting a defect.SOLUTION: An inspection preparation step includes the processes of changing a focal position of an imaging unit arranged so as to be opposed to a front surface of a light permeable plate body toward a rear surface from the front surface of the light permeable plate body for each light permeable plate body with different thicknesses, measuring luminance of scattered light, and obtaining a relational expression between a distance from the front surface at a specific position where the luminance of scattered light becomes a maximum value and the thickness of the light permeable plate body. An inspection step includes the processes of obtaining a distance to the specific position where the luminance of scattered light becomes the maximum value from the front surface of the light permeable plate body being the inspection object on the basis of the relational expression from the thickness of the light permeable plate body being the inspection object, and performing imaging by an imaging unit with the focal point of the imaging unit arranged so as to be opposed to the front surface of the light permeable plate body as the specific position to form a photographed image.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a method for inspecting defects in a light-transmissive plate-like body, and more particularly to a method for inspecting defects in a light-transmissive plate-like body such as a synthetic silica glass substrate for a photomask, which detects defects in the light-transmissive plate-like body.

Background Art

[0002] For example, in the manufacturing process of a liquid crystal display device or a plasma display device, various patterns are formed on a synthetic silica glass substrate, which is a light-transmissive plate-like body, by a photolithography method to manufacture a photomask. In the above manufacturing process, if defects such as foreign matters, scratches, and voids exist in the photomask, the image formed by the photomask becomes an image including the defects, and the image including the defects is projected onto the substrate to be processed. As a result, defects occur in the pattern formed on the substrate to be processed, and the manufacturing yield decreases. To prevent this decrease in yield, conventionally, defects in the synthetic silica glass substrate have been inspected, but in order to improve the yield, it is necessary to perform the inspection with higher accuracy.

[0003] As a method for inspecting defects in such a glass substrate (light-transmissive plate-like body), the inspection method disclosed in Patent Document 1 (Japanese Patent Laid-Open No. 2000-74849) will be described with reference to FIG. 9. As shown in FIG. 9, the inspection of the glass substrate for defects is performed, for example, using a light source 55 (illuminating unit) and a CCD camera 56 (imaging unit) arranged on the front surface W1 side of the glass substrate W. When inspecting the front surface W1 of the glass substrate W, the front surface W1 of the glass substrate is irradiated with the light source 55 arranged on the front surface W1 side of the glass substrate W (solid line portion), and the focus of the CCD camera 56 arranged on the front surface W1 side of the glass substrate is adjusted to the front surface W1 of the glass substrate. The CCD camera 56 receives the reflected light from the front surface W1 of the glass substrate and generates image data by a computer (not shown).

[0004] On the other hand, when inspecting the back surface W2 of the glass substrate W, the back surface W2 of the glass substrate is irradiated with the same light source 55 as used for inspecting the front surface W1 of the glass substrate W (broken line portion), and the focus of the same CCD camera 56 as used for inspecting the front surface W1 of the glass substrate is adjusted to the back surface W2 of the glass substrate. The CCD camera 56 receives the reflected light from the back surface W2 of the glass substrate and generates image data by a computer (not shown).

[0005] Then, the image data obtained by receiving the reflected light from the front surface W1 of the glass substrate W and the image data obtained by receiving the reflected light from the back surface W2 of the glass substrate W are two-dimensionally image-processed in an image processing unit (not shown) of the computer to detect defects on the front surface W1 and the back surface W2 of the glass substrate W.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0007] By the way, in the method for inspecting defects of the glass substrate (light-transmissive plate-like body) W shown in FIG. 9, when inspecting the back surface W2 of the glass substrate W, even if the focus of the CCD camera 56 is adjusted to the back surface W2 through the glass substrate W, there is a problem that defects cannot be detected with high precision. In other words, there is a problem that the detection accuracy is low compared to the case where the back surface W2 of the glass substrate W is inverted, the back surface W2 is used as the front surface, and the focus of the CCD camera 56 is adjusted to the front surface to detect defects.

[0008] On the other hand, in order to increase the detection sensitivity of defects on the back surface W2 of the glass substrate W, it is conceivable to arrange another set of light sources and a CCD camera on the back surface W2 side of the glass substrate W and use them to detect defects on the back surface W2 in the same manner as the inspection of the front surface W1. However, there is a problem that two sets of light sources and CCD cameras are required, resulting in high costs.

[0009] Based on the inspection method of the glass substrate W shown in FIG. 9, the inventor intensively studied a method for inspecting defects of a light-transmissive plate-like body capable of detecting defects with high precision. As a result, it was found that by focusing the CCD camera not on the back surface (rear surface) of the light-transmissive plate-like body through the light-transmissive plate-like body, but on a position in front of the back surface (rear surface) on the way from the front surface to the back surface of the light-transmissive plate-like body, defects can be detected with high precision, and the present invention was completed.

[0010] The present invention has been made under the above circumstances, and by using a light source and an imaging unit (such as a CCD camera or a CMOS camera) arranged on the front surface side of the light-transmissive plate-like body, irradiating the light-transmissive plate-like body with light from the light source, and imaging with the focus of the imaging unit set at a position in front of the back surface of the light-transmissive plate-like body, an object of the present invention is to provide a method for inspecting defects of a light-transmissive plate-like body capable of detecting the defects with high precision.

Means for Solving the Problems

[0011] The method for inspecting defects of a light-transmissive plate-like body according to the present invention made to solve the above problems irradiates the light-transmissive plate-like body with light and detects scattered light from the defects, thereby inspecting the presence or absence of defects on the back surface of the light-transmissive plate-like body. The inspection preparation process includes a step of preparing a plurality of light-transmissive plate-like bodies having different thicknesses, and for each of the light-transmissive plate-like bodies having different thicknesses, changing the focal position of an imaging unit arranged to face the front surface of the light-transmissive plate-like body from the front surface of the light-transmissive plate-like body toward the back surface and measuring the luminance of the scattered light, a step of obtaining the distance from the front surface to a specific position where the luminance of the scattered light becomes the maximum value, and a step of obtaining a relational expression between the distance from the front surface to the specific position where the luminance of the scattered light becomes the maximum value and the thickness of the light-transmissive plate-like body. The inspection process includes a step of obtaining, based on the thickness of the light-transmissive plate-like body to be inspected and from the relational expression, the distance from the front surface of the light-transmissive plate-like body to be inspected to a specific position where the luminance of the scattered light becomes the maximum value, and a step of forming an imaging image by imaging with the imaging unit with the focus of the imaging unit arranged to face the front surface of the light-transmissive plate-like body set as the specific position.

[0012] Thus, in the inspection preparation process, a relational expression between the distance from the front surface to a specific position where the luminance of the scattered light becomes the maximum value and the thickness of the light-transmissive plate-like body is obtained. Then, based on this relational expression and from the thickness of the light-transmissive plate-like body to be inspected, the distance from the front surface of the light-transmissive plate-like body to be inspected to the specific position where the luminance of the scattered light becomes the maximum value is obtained. Then, imaging is performed with this specific position set as the focus of the imaging unit. As a result, it is possible to accurately detect defects such as minute scratches and deposits that cannot be detected even when the focus is on the back surface of the light-transmissive plate-like body.

[0013] Further, when the light-transmissive plate-like body is a synthetic silica glass body, with the thickness of the synthetic silica glass body being t (mm) and the distance from the front surface to the specific position being P (see FIG. 2), the distance P is expressed as P = 0.7844t. Further, after setting the focus of the imaging unit disposed opposite to the front surface of the synthetic silica glass body to P = 0.7844t, the error may be corrected by the autofocus function of the imaging unit, and an image may be captured by the imaging unit to form a captured image. Note that synthetic silica glass usually has a refractive index of 1.45 to 1.50 (25 ° C., 1013 hPa) with respect to light having a wavelength of 210 nm to 550 nm.

Effects of the Invention

[0014] According to the present invention, by using a light source and an imaging unit disposed on the front surface side of the light transmissive plate-like body, irradiating light from the light source to the back surface of the light transmissive plate-like body, and imaging by the imaging unit, it is possible to obtain a method for inspecting defects of a light transmissive plate-like body that can detect the defects with high accuracy.

Brief Description of the Drawings

[0015]

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Figure 9

[0016] Hereinafter, a method for inspecting defects of a light-transmissive plate-like body according to the present invention will be further described based on embodiments. The method for inspecting defects of a light-transmissive plate-like body according to the present invention is for detecting defects such as foreign matters, scratches, and voids in a synthetic silica glass substrate used for, for example, a photomask or the like. In the following embodiments, the case where the light-transmissive plate-like body is a synthetic silica glass substrate will be described as an example.

[0017] (Outline of Inspection Apparatus) First, an outline of an inspection apparatus for implementing the method for inspecting defects of a light-transmissive plate-like body according to the present invention will be described with reference to FIG. 1. Note that the inspection apparatus shown in FIG. 1 is an example, and an apparatus for implementing the method for inspecting defects of a light-transmissive plate-like body according to the present invention may be any other apparatus as long as it can implement the defect inspection method.

[0018] The inspection apparatus 100 shown in FIG. 1 includes a CCD camera (imaging unit) 1 disposed opposite to the front surface W1 side of a synthetic silica glass substrate W, which is a substrate to be inspected, and an illumination unit 2 disposed around the CCD camera 1. The inspection apparatus 100 further includes a movement drive mechanism 5 that moves the synthetic silica glass substrate W in the X and Y directions and moves the CCD camera 1 in the Z direction.

[0019] The range of the imaging field by this CCD camera 1 is a small area with respect to the area of the front surface W1 of the synthetic silica glass substrate W. By moving the synthetic silica glass substrate W in the plane direction (vertical and horizontal directions: X and Y directions), the CCD camera 1 can obtain imaging screen data of the entire front surface W1 of the synthetic silica glass substrate W. Further, by moving the CCD camera 1 along the thickness direction (Z direction) of the synthetic silica glass substrate W, the focal position (focus position) of the CCD camera 1 (imaging unit) can be changed. By changing the focal position (focus position) of the CCD camera 1 (imaging unit), for example, the focal position (focus position) of the CCD camera 1 (imaging unit) can be set to a specific position in front of the back surface of the synthetic silica glass substrate (distance P (mm) from the front surface of the synthetic silica glass substrate).

[0020] More specifically, the CCD camera 1 is moved in the Z-axis direction by the movement drive mechanism 5 and stopped at the focal position (focus position) of the CCD camera 1 (imaging unit). Thereafter, by the movement drive mechanism 5, the CCD camera 1 moves relatively in the X direction and the Y direction so as to face the front of each region Ar (Ar1, Ar2 to Arn) obtained by virtually mesh-dividing the front surface W1 of the synthetic silica glass substrate W into n parts as partially shown in FIG. 1. Thereafter, for each region Ar (Ar1, Ar2 to Arn), the synthetic silica glass substrate W is imaged by the CCD camera 1 to detect defects.

[0021] The inspection apparatus 100 further includes a computer 10 that processes an image input from the CCD camera 1, a monitor 11 that displays the processed image, and an input device 12 that inputs the thickness and the like of the synthetic silica glass substrate W to the computer 10. The computer 10 includes a memory 13 that stores a relational expression between the distance from the front surface of the synthetic silica glass substrate W at the position where the luminance of the scattered light becomes the maximum value and the thickness of the synthetic silica glass substrate, a CPU 14 that performs calculations, a memory 15 for temporarily storing image data captured by the CCD camera 1, an image processing device 16 for performing two-dimensional image processing on the image data temporarily stored in the memory 15, and a display control unit 17 that controls the output of the image processed by the image processing device 16 to the monitor 11.

[0022] As shown in FIG. 1, the computer 10 is connected to the CCD camera 1, the moving drive mechanism 5, the illumination unit 2, the rotation drive unit 6 that rotates the CCD camera 1 around the CCD camera 1, and the monitor 11. When inspecting the synthetic silica glass substrate, when the thickness of the synthetic silica glass substrate to be inspected is input from the input device 12, a specific position where the luminance is maximized is obtained from the relational expression stored in the memory 13. Then, by the moving drive mechanism 5, the CCD camera 1 moves so that the focus of the CCD camera 1 is at the specific position (focus position).

[0023] For the illumination unit 2, for example, a metal halide lamp can be preferably used. As shown in FIG. 2, the irradiation direction of each illumination unit 2 is provided toward the intersection point (referred to as the irradiation position F) between the optical axis direction of the imaging of the CCD camera 1 (the optical axis direction of the lens) and the back surface W2 of the glass substrate W. The angle θ1 formed between the optical axis direction of the CCD camera 1 and the irradiation direction of the illumination unit 2 at this time is preferably 40° to 50°.

[0024] The image processing unit 16 detects surface defects and deposits as bright spots from the image data detected by the CCD camera 1. These bright spots have position data in the XY direction, luminance data, and area data corresponding to the number of detected pixels.

[0025] In addition, the luminance measurement performed in the defect inspection preparation described below is performed by being processed by the image processing unit 16 from the image data detected by the CCD camera 1. Then, the CPU 14 obtains the maximum luminance and the specific position where the maximum luminance is obtained, and the relational expression between the distance from the front surface of the synthetic silica glass substrate W and the thickness of the synthetic silica glass substrate is stored in the memory 13.

[0026] (Defect inspection method) Next, an embodiment of the defect inspection method for the light-transmissive plate-like body according to the present invention will be described. The defect inspection method of the present invention includes an inspection preparation step of preparing a plurality of synthetic silica glass substrates with different thicknesses, and obtaining a relational expression between the distance from the front surface at the position where the luminance of the scattered light becomes the maximum value and the thickness of the synthetic silica glass substrate. Furthermore, after the inspection preparation step, the defect inspection method of the present invention obtains, based on the thickness of the synthetic silica glass substrate to be inspected, the distance from the front surface at the position where the luminance of the scattered light in the synthetic silica glass substrate to be inspected becomes the maximum value, and sets the focus of the imaging unit at the position to image the synthetic silica glass substrate to be inspected, and includes an inspection step.

[0027] (Inspection preparation step) In the inspection preparation step, a plurality of synthetic silica glass substrates with different thicknesses are prepared. For example, synthetic silica glass substrates with a thickness of 6 mm, a thickness of 8 mm, and a thickness of 10 mm are prepared. Then, using the inspection apparatus shown in FIG. 1, for each synthetic silica glass substrate, the distance from the front surface at the position where the luminance of the scattered light becomes the maximum value is obtained.

[0028] Specifically, for each of the synthetic silica glass substrates with different thicknesses, the focus of the imaging unit arranged to face the front surface of the synthetic silica glass substrate is shifted along the optical axis of the imaging unit to a position (position in the depth direction) from the front surface to the back surface of the synthetic silica glass substrate, and the luminance of the scattered light is measured for each position. Then, the distance from the front surface to a specific position where the luminance of the scattered light becomes the maximum value is obtained.

[0029] As an example, FIG. 3 shows the measurement results of the luminance for each position from the front surface to the back surface of a synthetic silica glass substrate with a thickness of 6 mm. Similarly, FIG. 4 shows the measurement results of the luminance for each position from the front surface to the back surface of a synthetic silica glass substrate with a thickness of 8 mm (synthetic quartz glass substrate). Further, FIG. 5 shows the measurement results of the luminance for each position from the front surface to the back surface of a synthetic silica glass substrate with a thickness of 10 mm.

[0030] The horizontal axis CCD camera coordinates in each figure represent the moving distance from the reference position in the Z direction of the CCD camera. The vertical axis represents the luminance data (255 gradations) extracted from the image data detected by the CCD camera. In the figure, the solid line indicates an approximate quadratic curve. By setting the peak value of this approximate quadratic curve as the best focus position, the focus position can be obtained with the minimum necessary number of samples. Also, the display of "back surface focus" in the figure indicates that the focus of the CCD camera is at the position of the CCD camera where it is aligned with the surface of the back side of the synthetic silica glass substrate. Furthermore, the display of "best focus" in the figure indicates that it is the CCD camera focus position with the highest luminance.

[0031] As can be seen from these Figures 3 to 5, the CCD camera focus position with the highest luminance is located in front of (on the front side of the synthetic silica glass substrate) the back side of the synthetic silica glass substrate. For the position with the highest luminance with respect to the thickness of each glass substrate, in the case shown in Figure 3, it is 4.6 mm from the front side of the synthetic silica glass substrate, in the case shown in Figure 4, it is 6.4 mm from the front side of the synthetic silica glass substrate, and in the case shown in Figure 5, it is 7.6 mm from the front side of the synthetic silica glass substrate.

[0032] Then, the relational expression between the distance from the front side of the position where the luminance of the scattered light becomes the maximum value and the thickness of the synthetic silica glass substrate is obtained. Specifically, as an example, as shown in Figure 6, with the thickness t (mm) of the synthetic silica glass substrate on the horizontal axis and the position (focus position) P (mm) with the highest luminance on the vertical axis, the relational expression between the distance from the front side of the position where the luminance of the scattered light becomes the maximum value and the thickness of the synthetic silica glass substrate is obtained. From the graph of this Figure 6, P = 0.7844t + 0.0522 is obtained. Here, +0.0522 is considered to be the error range. Also, the coefficient 0.7844 in this formula holds for a synthetic silica glass substrate. If the material changes, this preparation process needs to be carried out again to obtain the relational expression. The relational expression is obtained by the computer 10 of the inspection device, stored in the memory 13, and used for the processing of the next inspection process.

[0033] (Inspection process) In the inspection process following the inspection preparation process, from the thickness of the synthetic silica glass substrate to be inspected, based on the relational expression P = 0.7844t + 0.0522, the distance from the front surface of the position (specific position) where the luminance of the scattered light in the synthetic silica glass substrate to be inspected becomes the maximum value is obtained. That is, the thickness of the synthetic silica glass substrate to be inspected is obtained, and from that thickness, the distance from the front surface to the specific position (focus position) P where the luminance is the highest is obtained.

[0034] At this time, 0.0522 in P = 0.7844t + 0.0522 is determined as an error, and based on P = 0.7844t, the focus of the CCD camera is set to P = 0.7844t. Then, imaging is performed by the CCD camera to form an imaging image. In addition, after setting the initial focus of the CCD camera arranged to face the front surface of the synthetic silica glass substrate to P = 0.7844t, the autofocus function or manual adjustment may be used to correct the focus position.

[0035] Specifically, the synthetic silica glass substrate W is mounted on the X - Y table. The CCD camera 1 is arranged to face the front surface W1 side of this synthetic silica glass substrate W, and the focus of the CCD camera 1 is set to P = 0.7844t. Then, by the movement drive mechanism 5, the imaging position is sequentially moved in the XY directions so as to scan the entire surface of the synthetic silica glass substrate W, and imaging is performed on each imaging region Ar by the CCD camera 1.

[0036] According to the embodiment of the present invention as described above, when detecting defects on the back surface of the synthetic silica glass substrate W, the focal position of the CCD camera 1 facing the front surface W1 of the synthetic silica glass substrate W is set to a position in front of the back surface W2 of the synthetic silica glass substrate W. In other words, it is set to a position where P = 0.7844t from the front surface W1 to the back surface W2 of the synthetic silica glass substrate W. As a result, it is possible to accurately detect defects such as minute scratches and deposits that could not be detected conventionally even when focusing on the back surface W2 of the synthetic silica glass substrate W. Also, for a synthetic silica glass (light-transmitting plate-like body) made of the same material, based on the above relational expression, the distance P from an appropriate front surface to the specific position can be immediately derived.

[0037] In addition, in this embodiment, the defect inspection of the back surface W2 of the synthetic silica glass substrate W has been described. However, naturally, the defect inspection of the front surface W1 of the synthetic silica glass substrate W can also be performed with the configuration shown in FIG. 1. Also, in this embodiment, the synthetic silica glass substrate W has been described as an example. However, the method for inspecting defects of the light-transmitting plate-like body according to the present invention is not limited to this, and can be applied to light-transmitting plate-like bodies such as transparent ceramics and transparent resins. Furthermore, in this embodiment, the inspection was performed using a CCD camera. However, it is not limited to this, and an imaging device such as a CMOS may be used.

Example

[0038] The inspection method of the synthetic silica glass substrate according to the present invention will be further described based on examples. [Experiment 1] In Experiment 1, particles with particle diameters of 0.5 μm, 1.0 μm, and 2.0 μm were attached to the back surface of a glass substrate with a thickness of 8 mm to obtain a substrate to be inspected. A defect inspection was performed on this synthetic silica glass substrate. Here, for arbitrarily selected standard particles, the maximum luminance (reflected light intensity) and defect area (number of pixels showing luminance equal to or higher than an arbitrary threshold) were examined respectively.

[0039] (Example 1) In Example 1, in the apparatus configuration shown in FIG. 1, imaging was performed by scanning the entire surface of the synthetic silica glass substrate, and defect detection was carried out. The focus position of the CCD camera was set at a position of P = 0.7844t from the front surface to the back surface of the synthetic silica glass substrate as determined in the above inspection process.

[0040] (Comparative Example 1) In Comparative Example 1, in the apparatus configuration shown in FIG. 1, the focus position of the CCD camera was adjusted to the back surface position of the synthetic silica glass substrate, and defect detection was carried out.

[0041] The results of Example 1 are shown in Table 1 and FIG. 7, and the results of Comparative Example 1 are shown in Table 2 and FIG. 8, respectively.

[0042]

Table 1

[0043]

Table 2

[0044] As shown in Table 1 and Table 2, in Example 1, it was confirmed that in the detection of deposits with a particularly small particle size of 1.0 μm or less, both the detection luminance and the defect area were larger than those in Comparative Example 1. Also, as shown in Table 1, Table 2, FIG. 7, and FIG. 8, it was confirmed that the number of detected defects increased significantly in Example 1 compared to Comparative Example 1, and the detection sensitivity was higher.

Explanation of Signs

[0045] 1 CCD camera (imaging unit) 2 Lighting unit 5 Moving drive mechanism 10 Computer W Synthetic silica glass substrate W1 Front surface W2 Back surface

Claims

1. A method for inspecting defects on the back surface of a light-transmissive plate-like body by irradiating the light-transmissive plate-like body with light and detecting scattered light from the defects, comprising: an inspection preparation step including: a step of preparing a plurality of light-transmissive plate-like bodies having different thicknesses; for each of the light-transmissive plate-like bodies having different thicknesses, changing the focal position of an imaging unit disposed opposite to the front surface of the light-transmissive plate-like body from the front surface to the back surface of the light-transmissive plate-like body, and measuring the luminance of the scattered light; a step of obtaining the distance from the front surface to a specific position where the luminance of the scattered light becomes the maximum value; a step of obtaining a relational expression between the distance from the front surface to the specific position where the luminance of the scattered light becomes the maximum value and the thickness of the light-transmissive plate-like body; and an inspection step including: a step of obtaining the distance from the front surface of the light-transmissive plate-like body to be inspected to a specific position where the luminance of the scattered light becomes the maximum value based on the relational expression from the thickness of the light-transmissive plate-like body to be inspected; a step of imaging with the imaging unit by setting the focal point of the imaging unit disposed opposite to the front surface of the light-transmissive plate-like body as the specific position to form an imaging image. A method for inspecting defects on a light-transmissive plate-like body, characterized by comprising the above steps.

2. The light-transmissive plate-like body is synthetic silica glass, and when the thickness of the light-transmissive plate-like body is t (mm) and the distance from the front surface to the specific position is P, the distance P is P = 0.7844t. The method for inspecting defects on a light-transmissive plate-like body according to Claim 1, characterized by this.

3. After setting the focal point of the imaging unit disposed opposite to the front surface of the light-transmissive plate-like body as P = 0.7844t, an error is corrected by the autofocus function of the imaging unit, and an imaging image is formed by imaging with the imaging unit. The method for inspecting defects on a light-transmissive plate-like body according to Claim 2, characterized by this.

Images