JP2024002279A - Defect inspection method of light permeable plate body - Google Patents

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

The present invention relates to a defect inspection method for a light-transparent plate-like body, and for example, a method for detecting defects in a light-transparent plate-like body such as a synthetic silica glass substrate for a photomask. Regarding.

For example, in the manufacturing process of liquid crystal display devices and plasma display devices, various patterns are formed on a synthetic silica glass substrate, which is a light-transmitting plate, by photolithography to manufacture a photomask.
In the manufacturing process, if a defect such as a foreign object, scratch, or void exists on the photomask, the image formed by the photomask will include the defect, and the image containing the defect will be projected onto the substrate to be processed. be done.
As a result, defects occur in the pattern formed on the substrate to be processed, and the manufacturing yield decreases. In order to prevent this decrease in yield, synthetic silica glass substrates have been inspected for defects, but in order to improve yield, it is necessary to perform inspection with higher precision.

As a defect inspection method for such a glass substrate (light-transmitting plate-like body), an inspection method disclosed in Patent Document 1 (Japanese Unexamined Patent Publication No. 2000-74849) will be described with reference to FIG.
As shown in FIG. 9, the glass substrate defect inspection is performed using, for example, a light source 55 (illumination section) and a CCD camera 56 (imaging section) arranged on the front surface W1 side of the glass substrate W.
When inspecting the front surface W1 of the glass substrate W, the light source 55 disposed on the front surface W1 side of the glass substrate W irradiates the front surface W1 of the glass substrate (solid line part ), the focus of the CCD camera 56 disposed on the glass substrate front surface W1 side is adjusted to the glass substrate front surface W1. The CCD camera 56 receives reflected light from the front surface W1 of the glass substrate, and generates image data using a computer (not shown).

On the other hand, when inspecting the back surface W2 of the glass substrate W, the same light source 55 as used for inspecting the front surface W1 of the glass substrate W is used to irradiate the back surface W2 of the glass substrate (dotted line), and the The focus of the CCD camera 56, which is the same as that used for inspecting the front surface W1, is set on the back surface W2 of the glass substrate. The CCD camera 56 receives reflected light from the back surface W2 of the glass substrate, and generates image data using a computer (not shown).

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 processed by an image processing unit of the computer. (not shown) performs two-dimensional image processing to detect defects on the front surface W1 and the back surface W2 of the glass substrate W.

Japanese Patent Application Publication No. 2000-74849

By the way, in the defect inspection method for the glass substrate (light-transmitting plate-like body) W shown in FIG. 9, when inspecting the back surface W2 of the glass substrate W, the CCD camera 56 is focused on the back surface W2 through the glass substrate W. However, there was a problem in that defects could not be detected with high precision. In other words, the detection accuracy is higher than when defects are detected by inverting the back surface W2 of the glass substrate W, using the back surface W2 as the front surface, and focusing the CCD camera 56 on the front surface. The problem was that it was low.

On the other hand, in order to increase the detection sensitivity of defects on the back surface W2 of the glass substrate W, another set of light sources and a CCD camera are placed on the back surface W2 side of the glass substrate W, and these are used to detect defects on the front surface W2. It is conceivable to perform defect detection on the back surface W2 in the same way as the inspection on W1, but this requires two sets of light sources and a CCD camera, resulting in an increase in cost.

The present inventor has diligently researched a defect inspection method for a light-transmitting plate-like body that can detect defects with high accuracy based on the inspection method for a glass substrate W shown in FIG.
As a result, the focus of the CCD camera is not focused on the back surface of the light-transmitting plate through the light-transmitting plate, but from the front surface of the light-transmitting plate to the back surface. The present invention was completed based on the finding that defects can be detected with high precision by focusing on a position in front of the back surface (back surface).

The present invention has been made under the above circumstances, and uses a light source and an imaging unit (CCD camera, CMOS camera, etc.) disposed on the front surface side of a light-transmitting plate-like body. The light transmittance allows the defect to be detected with high precision by irradiating light from a light source to the light transmitting plate and capturing an image with the focal point of the imaging unit at a position in front of the back surface of the light transmitting plate. An object of the present invention is to provide a method for inspecting defects in a plate-shaped body.

A defect inspection method for a light-transmitting plate-like body according to the present invention, which has been made to solve the above-mentioned problems, involves irradiating the light-transmitting plate-like body with light and detecting scattered light from the defect. A defect inspection method for a light-transmitting plate-like body for inspecting the presence or absence of a defect on the back side of a transparent plate-like body, the inspection preparation step being a step of preparing a plurality of light-transparent plate-like bodies having different thicknesses. , for each of the light-transmissive plate-like bodies having different thicknesses, the focal position of the imaging unit disposed opposite to the front surface of the light-transmissive plate-like body is set to the center of the light-transmissive plate-like body. a step of measuring the brightness of the scattered light by changing it from the front side to the back side; a step of determining the distance from the front surface of a specific position where the brightness of the scattered light is the maximum; and a step of calculating a relational expression between the distance from the front surface of a specific position serving as a value and the thickness of the light-transmitting plate-like body, and the inspection process a step of determining the distance from the front surface of the light-transmitting plate-like object to be inspected to a specific position where the brightness of the scattered light has a maximum value based on the thickness and the above-mentioned relational expression; The present invention is characterized by comprising the step of capturing an image by the imaging unit with a focal point of an imaging unit disposed facing the front surface of the plate-shaped body as the specific position to form a captured image.

In this way, in the inspection preparation process, a relational expression between the distance from the front surface to the specific position and the thickness of the light-transmitting plate-like body is determined, at which the brightness of the scattered light becomes the maximum value. Based on this relational expression, from the thickness of the light-transmitting plate to be inspected, the brightness of the scattered light of the light-transmitting plate to be inspected is the maximum value. Find the distance to the specific position. Then, this specific position is imaged as the focal point 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 if the back surface of the light-transmitting plate is focused.

Further, the light-transmitting plate-like body is a synthetic silica glass body, the thickness of the synthetic silica glass body is t (mm), and the distance from the front surface to the specific position is P (see FIG. 2). In this case, the distance P is expressed as P=0.7844t.
Further, after the focal point of the imaging unit disposed facing the front surface of the synthetic silica glass body is set to P=0.7844t, the error is corrected by the autofocus function of the imaging unit, and the imaging unit The image may be captured and a captured image may be formed.
Note that synthetic silica glass usually has a refractive index of 1.45 to 1.50 (25° C., 1013 hPa) for light with a wavelength of 210 nm to 550 nm.

According to the present invention, using a light source and an imaging unit disposed on the front surface side of a light-transmitting plate-like body, light is irradiated from the light source to the back face of the light-transmitting plate-like body, and the image capturing unit By taking an image with a portion, it is possible to obtain a defect inspection method for a light-transmitting plate-like body that can detect the defect with high accuracy.

FIG. 1 is a schematic diagram schematically showing an inspection apparatus for implementing the inspection method for a light-transmitting plate-like body of the present invention. FIG. 2 is a side view showing the positional relationship between the irradiation section, the imaging section, and the synthetic silica glass substrate in the inspection apparatus of FIG. FIG. 3 is a diagram showing the results of measuring the brightness from the front surface to the back surface of a 6 mm thick light-transmitting plate-like body (synthetic silica glass substrate). FIG. 4 is a diagram showing the results of measuring the brightness from the front surface to the back surface of a light-transmitting plate-like body (synthetic silica glass substrate) having a thickness of 8 mm. FIG. 5 is a diagram showing the results of measuring the brightness from the front surface to the back surface of a 10 mm thick light-transmitting plate-like body (synthetic silica glass substrate). FIG. 6 is a diagram showing the relationship between the thickness of the light-transmissive plate-like body and the distance from the front surface of the light-transmissive plate-like body to a specific position (focus position) P with the highest brightness. FIG. 7 is a defect detection map showing the results of Example 1. FIG. 8 is a defect detection map showing the results of Comparative Example 1. FIG. 9 is a side view for explaining a conventional method for detecting defects on the front and back surfaces of a glass substrate.

Hereinafter, the defect inspection method for a light-transmitting plate-like body according to the present invention will be further described based on embodiments.
The defect inspection method for a light-transmitting plate-like body according to the present invention is for detecting defects such as foreign matter, scratches, and voids in a synthetic silica glass substrate used for, for example, a photomask.
In the following embodiments, a case where the light-transmitting plate-like body is a synthetic silica glass substrate will be described as an example.

(Overview of inspection equipment)
First, an outline of an inspection apparatus for carrying out the defect inspection method for a light-transmitting plate-like body according to the present invention will be explained based on FIG. 1. The inspection apparatus shown in FIG. 1 is an example, and the apparatus for carrying out the defect inspection method for a light-transmitting plate-like body according to the present invention may be any other apparatus as long as it is capable of carrying out the defect inspection method. Also good.

The inspection apparatus 100 shown in FIG. 1 includes a CCD camera (imaging section) 1 disposed on the front surface W1 side of a synthetic silica glass substrate W, which is a substrate to be inspected, facing the synthetic silica glass substrate W; An illumination section 2 arranged around the CCD camera 1 is provided.
The inspection apparatus 100 also includes a movement drive mechanism 5 that moves the synthetic silica glass substrate W in the X direction and the Y direction, and moves the CCD camera 1 in the Z direction.

The range of the imaging field of view by this CCD camera 1 is a small area compared to the area of the front surface W1 of the synthetic silica glass substrate W, and the synthetic silica glass substrate W is ), the CCD camera 1 can obtain imaged 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 section) 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 (synthesis) in front of the back surface of the synthetic silica glass substrate. The distance P (mm) from the front surface of the silica glass substrate can be set.

More specifically, the CCD camera 1 is moved in the Z-axis direction by the moving drive mechanism 5 and stopped at the focal position of the CCD camera 1 (imaging section). Thereafter, the moving drive mechanism 5 moves the CCD camera 1 to each area Ar (Ar1, Ar2 ~Arn), move relatively in the X direction and Y direction so as to face the front of.
Thereafter, the synthetic silica glass substrate W is imaged by the CCD camera 1 for each region Ar (Ar1, Ar2 to Arn) to detect defects.

The inspection apparatus 100 also includes a computer 10 that processes images input from the CCD camera 1, a monitor 11 that displays the processed images, and an input device that inputs the thickness of the synthetic silica glass substrate W, etc. to the computer 10. It is equipped with 12.
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 of the position where the brightness of the scattered light becomes the maximum value and the thickness of the synthetic silica glass substrate, and a calculation 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; It has a display control section 17 that controls output of the image processed by the processing device 16 to the monitor 11.

As shown in FIG. 1, the computer 10 is connected to the CCD camera 1, the moving drive mechanism 5, a rotation drive unit 6 that rotates the illumination unit 2 around the CCD camera 1, and the monitor 11. There is.
When inspecting a synthetic silica glass substrate, when the thickness of the synthetic silica glass substrate to be inspected is inputted from the input device 12, the thickness of the synthetic silica glass substrate to be inspected is input, and then a specification is made from the relational expression stored in the memory 13 that gives the maximum brightness. location is required. Then, the CCD camera 1 is moved by the movement drive mechanism 5 so that the focal point of the CCD camera 1 is at the specific position (focus position).

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

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 and brightness data in the X and Y directions, and area data corresponding to the number of detected pixels.

Note that the brightness measurement performed in the defect inspection preparation described below is performed by processing the image data detected by the CCD camera 1 by the image processing section 16.
Then, the CPU 14 determines the maximum brightness and the specific position where the brightness is the highest, and stores in the memory 13 a 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.

(Defect inspection method)
Next, an embodiment of the defect inspection method for a light-transmitting plate-like body according to the present invention will be described.
In the defect inspection method of the present invention, a plurality of synthetic silica glass substrates with different thicknesses are prepared, and the distance from the front surface of the position where the brightness of scattered light is the maximum value is determined by the thickness of the synthetic silica glass substrate. It is equipped with an inspection preparation process that calculates relational expressions.
Further, in the defect inspection method of the present invention, after the inspection preparation step, the brightness of scattered light in the synthetic silica glass substrate to be inspected is calculated from the thickness of the synthetic silica glass substrate to be inspected based on the above relational expression. The present invention includes an inspection step of determining the distance from the front surface to the position where the maximum value of , and setting the focal point of the imaging unit at the position to take an image of the synthetic silica glass substrate to be inspected.

(Inspection preparation process)
In the inspection preparation process, a plurality of synthetic silica glass substrates with different thicknesses are prepared. For example, synthetic silica glass substrates with a thickness of 6 mm, 8 mm, and 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 of the position where the brightness of the scattered light becomes the maximum value is determined.

Specifically, for each of the synthetic silica glass substrates having different thicknesses, the focal point of an imaging section disposed facing the front surface of the synthetic silica glass substrate is set along the optical axis of the imaging section. The position from the front surface to the back surface of the glass substrate (position in the depth direction) is shifted, and the brightness of the scattered light is measured for each position. Then, the distance from the front surface to a specific position where the brightness of the scattered light reaches its maximum value is determined.

As an example, FIG. 3 shows the results of measuring the brightness at each position from the front surface to the back surface of a 6 mm thick synthetic silica glass substrate. Similarly, FIG. 4 shows the measurement results of the luminance at each position from the front surface to the back surface of a synthetic silica glass substrate (synthetic silica glass substrate) having a thickness of 8 mm. Further, FIG. 5 shows the results of measuring the brightness at each position from the front surface to the back surface of a 10 mm thick synthetic silica glass substrate.

The CCD camera coordinate on the horizontal axis in each figure is the moving distance of the CCD camera from the reference position in the Z direction. The vertical axis is luminance data (255 gradations) extracted from the image data detected by the CCD camera, and the solid line in the figure 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 determined using the minimum number of samples required.
Further, the display "back surface focus" in the figure indicates that the CCD camera is at a position where the focus of the CCD camera is on the back surface of the synthetic silica glass substrate.
Furthermore, the display of "best focus" in the figure indicates the CCD camera focal position where the brightness is highest.

As can be seen from these FIGS. 3 to 5, the CCD camera focal position with the highest brightness is located in front of the back surface of the synthetic silica glass substrate (on the front surface side of the synthetic silica glass substrate).
The position where the brightness is highest relative to the thickness of each glass substrate is 4.6 mm from the front surface of the synthetic silica glass substrate in the case shown in FIG. It is 6.4 mm from the front surface, and in the case shown in FIG. 5, it is 7.6 mm from the front surface of the synthetic silica glass substrate.

Then, a relational expression between the distance from the front surface of the position where the brightness of the scattered light becomes the maximum value and the thickness of the synthetic silica glass substrate is determined.
To give a specific example, as shown in FIG. 6, the horizontal axis is the thickness t (mm) of the synthetic silica glass substrate, the vertical axis is the highest brightness position (focus position) P (mm), and the scattering A relational expression between the distance from the front surface at the position where the luminance of light reaches its maximum value and the thickness of the synthetic silica glass substrate is determined.
From the graph of FIG. 6, P=0.7844t+0.0522 is obtained.
Note that +0.0522 is considered to be the error range here. Further, the coefficient 0.7844 in this equation holds true for synthetic silica glass substrates, and if the material changes, it is necessary to perform this preparatory step again to find the relational equation. Further, the relational expression is determined by the computer 10 of the inspection apparatus, stored in the memory 13, and used for processing in the next inspection process.

(Inspection process)
In the inspection step following the inspection preparation step, the scattered light on the synthetic silica glass substrate to be inspected is determined based on the relational expression P=0.7844t+0.0522 from the thickness of the synthetic silica glass substrate to be inspected. Find the distance from the front surface of the position (specific position) where the brightness is at its maximum value.
That is, the thickness of the synthetic silica glass substrate to be inspected is determined, and from that thickness, the distance from the front surface to a specific position (focus position) P where the brightness is highest is determined.

At this time, 0.0522 in P=0.7844t+0.0522 is determined to be an error, and the focus of the CCD camera is set to P=0.7844t based on P=0.7844t. Then, an image is captured by the CCD camera to form a captured image.
The initial focus of the CCD camera placed opposite the front surface of the synthetic silica glass substrate was set to P = 0.7844t, and then the focus position was corrected using the autofocus function or manual adjustment. It's okay.

Specifically, the synthetic silica glass substrate W is mounted on an XY table. A CCD camera 1 is placed facing the front surface W1 of the synthetic silica glass substrate W, and the focal point of the CCD camera 1 is set to P=0.7844t.
Then, the moving drive mechanism 5 sequentially moves the imaging position in the XY directions so as to scan the entire surface of the synthetic silica glass substrate W, and the CCD camera 1 images each imaging area Ar.

As described above, according to the embodiment of the present invention, 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 , a position in front of the back surface W2 of the synthetic silica glass substrate W, in other words, a position of P=0.7844t from the front surface W1 of the synthetic silica glass substrate W toward the back surface W2.
As a result, it is possible to accurately detect defects such as minute scratches and deposits that conventionally cannot be detected even when focusing on the back surface W2 of the synthetic silica glass substrate W. Furthermore, if the synthetic silica glass (light-transmitting plate-like body) is made of the same material, an appropriate distance P from the front surface to the specific position can be immediately derived based on the above relational expression.

In this embodiment, defect inspection on the back surface W2 of the synthetic silica glass substrate W has been described, but it goes without saying that defect inspection on the front surface W1 of the synthetic silica glass substrate W can also be performed with the configuration shown in FIG. can.
Further, in this embodiment, the synthetic silica glass substrate W has been described as an example, but the method for inspecting defects of a light-transmitting plate according to the present invention is not limited to this, and can be applied to transparent ceramics, transparent resins, etc. It can be applied to light-transmitting plate-like bodies such as.
Further, in this embodiment, the inspection is performed using a CCD camera, but the present invention is not limited to this, and an imaging device such as a CMOS may be used.

The method for inspecting a synthetic silica glass substrate according to the present invention will be further explained 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, which was used as a substrate to be inspected. Defect inspection was performed on this synthetic silica glass substrate. Here, the maximum brightness (reflected light intensity) and defect area (number of pixels exhibiting brightness equal to or higher than an arbitrary threshold value) of arbitrarily selected standard particles were investigated.

(Example 1)
In Example 1, in the apparatus configuration shown in FIG. 1, the entire surface of the synthetic silica glass substrate was scanned and imaged to detect defects. The focus position of the CCD camera was set at P=0.7844t from the front surface of the synthetic silica glass substrate to the back surface, as determined in the above inspection process.

(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 position of the back surface of the synthetic silica glass substrate, and defect detection was performed.

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.

As shown in Tables 1 and 2, it was confirmed that in Example 1, both the detection brightness and defect area were larger than in Comparative Example 1, especially in detecting deposits with small particle diameters of 1.0 μm or less.
Further, as shown in Tables 1 and 2, and FIGS. 7 and 8, the number of detected defects increased significantly in Example 1 compared to Comparative Example 1, confirming that the detection sensitivity was increased.

1 CCD camera (imaging section)
2 Lighting section 5 Movement drive mechanism 10 Computer W Synthetic silica glass substrate W1 Front surface W2 Back surface

Claims (3)

A defect inspection method for a light-transmitting plate-like body, which inspects the presence or absence of defects on the back side of the light-transparent plate-like body by irradiating the light-transparent plate-like body with light and detecting scattered light from defects. hand,
The inspection preparation process
a step of preparing a plurality of light-transmitting plate-like bodies having different thicknesses;
For each of the light-transmitting plate-like bodies having different thicknesses, the focal position of the imaging unit disposed opposite to the front surface of the light-transmitting plate-like body is adjusted to changing from the front side to the back side and measuring the brightness of the scattered light;
a step of determining the distance from the front surface of a specific position where the brightness of the scattered light is at its maximum value;
the step of determining a relational expression between the distance from the front surface of a specific position at which the luminance of the scattered light has a maximum value and the thickness of the light-transmitting plate-like body;
The inspection process is
Based on the thickness of the light-transmitting plate-like body to be inspected and the above relational expression, from the front surface of the light-transmitting plate-like body to be inspected to the specific position where the brightness of the scattered light is the maximum value. a step of finding the distance of
forming a captured image by capturing an image with the focus of the imaging unit disposed facing the front surface of the light-transmitting plate-like body as the specific position;
A method for inspecting defects in a light-transmitting plate-like body, comprising: The light-transmitting plate-like body is synthetic silica glass,
When the thickness of the light-transmitting plate is t (mm), and the distance from the front surface to the specific position is P,
2. The defect inspection method for a light-transmitting plate-like body according to claim 1, wherein the distance P is P=0.7844t. After setting the focal point of the imaging section facing the front surface of the light-transmitting plate-like body to P=0.7844t,
3. The defect inspection method for a light-transmitting plate-shaped body according to claim 2, wherein an error is corrected by an autofocus function of an imaging section, and an image is captured by the imaging section to form a captured image.

Images