JP6948988B2 - Photomask substrate and its manufacturing method - Google Patents

Description

The present invention relates to a photomask substrate and a method for manufacturing the same, and by devising the shape of a corner portion of the substrate, for example, a photomask substrate and its manufacture capable of improving stability and distinctiveness during handling operations. Regarding the method.

Synthetic silica glass is used as a photomask substrate for photolithography of ICs and LCDs because of its excellent low thermal expansion and light transmission.
This photomask substrate is manufactured by obtaining a block of synthetic silica glass and then slicing, chamfering, and polishing. In this polishing process, the main surface of the substrate is flattened (mirrored), and the side surfaces of the substrate are also trapped by abrasive particles and the like in the irregularities and fine fissures generated by the cutting process. A predetermined polishing process is performed in order to suppress the occurrence of defects such as pinholes, which cause the problem.

In recent years, the demand for quality improvement of photomask substrates has been increasing, and polishing of photomask substrates has been performed not only on the main surface but also on the side surfaces, not just flattening, but also according to the purpose. There is.

One of them is the handleability of the substrate. For example, in a large-sized photomask substrate, there are cases where a heavy substrate slips from the hand during handling and cannot be held firmly. Is known to have (see Patent Document 1).
Further, in the mask blank substrate, the arithmetic average roughness Ra of the end surface of the corner side region is a mirror surface of 0.5 nm or less, and the central region is a rough surface, which facilitates handling during handling and the like. Is known (see Patent Document 2).

JP-A-2010-107613 Special Table 2010/092937

By the way, recently, as with the main surface, the side surface is required to have a uniform surface condition over the entire surface in addition to keeping the surface roughness small.
This is due to the fact that due to the increase in the diameter of the substrate, the influence on the dimensional accuracy and the optical characteristics due to the difference in surface roughness and shape from a part of the side surface portion cannot be ignored.

From this point of view, the technique described in Patent Document 1 is not necessarily suitable for a product in which flatness and a mirror surface are required on the side surface because the side surface is uneven.
Further, in the technique described in Patent Document 2, since regions having different surface roughness exist in one side surface, these may affect the optical characteristics and the dimensional accuracy. Further, since the area for processing the side surface is narrow, if two types of surface roughness areas are created separately, the processing process increases and the cost increases.

In order to solve the above-mentioned problems, the present inventor eliminates the influence of optical characteristics and dimensional accuracy by forming regions having different surface roughness in one side surface, so that the optical characteristics and dimensions can be eliminated. We focused on the corners of the glass substrate for photomasks, which have little effect on accuracy.
Then, while approximating the surface roughness of the corner portion and the surface roughness of the side surface, it is diligently studied to improve the handleability and to easily form the corner portion to improve the handleability, and to conceive the present invention. I arrived.

An object of the present invention is to provide a photomask substrate and a method for manufacturing the same, which improve the handleability of the substrate and maintain optical characteristics and dimensional accuracy.

The photomask substrate according to the present invention made to achieve the above object has two main surfaces facing each other, a side surface formed between the two main surfaces, and two adjacent side surfaces. A photomask substrate made of silica glass including a corner portion formed between the two, wherein the corner portion has an arithmetic average roughness Ra of 0.003 μm or more and 0.015 μm or less, and the corner portion has a corner portion. It is characterized in that the ten-point average roughness Rzjis is a value larger than the ten-point average roughness Rzjis of the side surface.
The arithmetic average roughness Ra and the ten-point average roughness Rzjis are defined by JIS B0601-2001.

In the present invention, the ten-point average roughness Rzjis of the corners of the glass substrate for a photomask, which has little influence on the optical characteristics and dimensional accuracy, is set to a value larger than the ten-point average roughness Rzjis of the side surface. As a result, the corner portion is not slippery, and excellent handleability can be obtained.
By setting the ten-point average roughness Rzjis of the corner portion to a value larger than the ten-point average roughness Rzjis of the side surface, the arithmetic average roughness Ra of the corner portion 4 becomes larger than the arithmetic average roughness Ra of the side surface 3. .. Since the arithmetic average roughness Ra of the side surface is generally 0.010 μm or less, the arithmetic average roughness Ra (maximum value) in the corner portion 4 is 0.015 μm.
On the other hand, when the side surface is in a mirror surface state, the arithmetic mean roughness Ra (minimum value) at the corner portion 4 is 0.003 μm.
As described above, by setting the arithmetic average roughness Ra of the corner portion to 0.003 μm or more and 0.015 μm or less, it is possible to eliminate the influence on the optical characteristics and the dimensional accuracy.

Here, in the corner portion, the ten-point average roughness Rzjis is 0.07 μm or more and 0.15 μm or less, and the difference between the ten-point average roughness Rzjis and the maximum height Rz is 0.02 μm or more and 0.05 μm or less. Is desirable.
The maximum height Rz is defined by JIS B0601-2001, similarly to the arithmetic average roughness Ra and the ten-point average roughness Rzjis.

Further, it is desirable that four corner portions are provided and the arithmetic mean roughness Ra of at least one of the four corner portions is different from the arithmetic mean roughness Ra of the other corner portions.
In this way, when the arithmetic average roughness Ra of at least one of the four corners is configured to be different from the arithmetic average roughness Ra of the other corners, the front and back surfaces of the substrate are used using the corners. , The orientation of the substrate, etc. can be identified.

Further, in the method for manufacturing a photomask substrate according to the present invention in order to solve the above-mentioned problems, two main surfaces facing each other, a side surface formed between the two main surfaces, and the side surface of the side surface. A method for manufacturing a photomask substrate made of silica glass, which comprises a corner portion formed between two adjacent two portions, wherein a side polishing step of pressing a polishing pad against the side surface to polish the corner portion and the corner portion. The arithmetic average roughness Ra of the corner portion is 0.003 μm or more and 0.015 μm or less by adjusting the polishing time in the corner polishing step, which includes a corner polishing step of pressing the polishing pad against the surface. It is characterized in that the ten-point average roughness Rzjis of the corner portion is larger than the ten-point average roughness Rzjis of the side surface portion.
It is desirable that the polishing time in the corner polishing step is half the time required to make the arithmetic average roughness Ra in the corner portion and the arithmetic average roughness Ra in the side surface the same.
According to such a manufacturing method, the photomask substrate according to the present invention can be obtained, and the effect thereof can be exhibited.

According to the photomask substrate of the present invention, it is possible to obtain a photomask substrate having optical characteristics and dimensional accuracy, and since the corners of the substrate are not slippery, good handleability can be obtained.
Further, according to the method for manufacturing a photomask substrate of the present invention, the photomask substrate can be easily manufactured without increasing the processing cost.

FIG. 1 is a plan view schematically showing a photomask substrate according to an embodiment of the present invention. FIG. 2 is a side view schematically showing the photomask substrate shown in FIG. FIG. 3 is a plan view schematically showing one step of a method for manufacturing a photomask substrate according to an embodiment of the present invention. FIG. 4 is a plan view schematically showing one step of a method for manufacturing a photomask substrate according to an embodiment of the present invention.

Hereinafter, a photomask substrate and a method for manufacturing the photomask substrate according to the embodiment of the present invention will be described with reference to the drawings. However, the photomask substrate and the method for manufacturing the photomask substrate according to the embodiment of the present invention are not limited to the embodiments described below. The attached drawings are schematic, and the dimensions and ratios of each element are different from the actual ones. Further, the definitions of the arithmetic mean roughness Ra, the ten-point average roughness Rzjis, and the maximum height Rz used in the following description follow the provisions of JIS B0601-2001 as described above.

FIG. 1 is a plan view schematically showing a photomask substrate according to an embodiment of the present invention, and FIG. 2 is a side view schematically showing a photomask substrate according to an embodiment of the present invention.
The photomask substrate 1 according to the embodiment of the present invention is used as, for example, a photomask substrate for an IC or a photomask substrate for an LCD, and the size varies depending on the application.
For example, in the case of a photomask substrate for an IC, for example, it is formed of synthetic silica glass having a side length L of 152 mm and a thickness D of 6.35 mm. Further, in the case of a photomask substrate for an LCD, for example, the length L of one side thereof is 850 mm (the length L of the other orthogonal sides is 1200 mm), and the substrate is formed of synthetic silica glass having a thickness D of 10 mm. Will be done.

As shown in FIGS. 1 and 2, the photomask substrate 1 has two main surfaces 2 facing each other, a side surface 3 formed between the two main surfaces, and two adjacent side surfaces 3. It is provided with a corner portion 4 formed between them. Further, the ridge line portion between the main surface 2 and the side surface 3 and the corner portion 4 is subjected to C surface processing, and a chamfered surface 5 is formed.
As shown in FIG. 1, the corner portion 4 means a region represented by a curved surface (curved surface) formed between two adjacent side surfaces of the side surface 3 in a plan view.

The main surface 2 is flattened (mirrored) so as to obtain desired optical characteristics. For example, polishing processing is performed so that the arithmetic average roughness Ra is 0.25 nm or less.
The ten-point average roughness Rzjis and the maximum height Rz of the main surface are not particularly limited as long as they are in the range of values obtained when the arithmetic average roughness Ra is 0.25 nm or less.

Further, in order to suppress the occurrence of defects such as pinholes, which cause the polishing particles and the like to be trapped in the irregularities and fine fissures generated by the cutting process, the side surface 3 is generally, for example, an arithmetic mean. Polishing is performed so that the roughness Ra is 0.010 μm or less.
By doing so, it is possible to suppress the risk of supplementing the abrasive grains used during polishing.

The ten-point average roughness Rzjis on the side surface 3 is 0.03 μm or more and 0.05 μm or less, and the difference Δ between the ten-point average roughness Rzjis and the maximum height Rz is 0.002 μm or more and 0.010 μm or less. be. By doing so, it is possible to suppress the risk of trapping abrasive grains.

Further, by setting the arithmetic average roughness Ra of the side surface 3 to 0.010 μm or less and the ten-point average roughness Rzjis of the side surface 3 to 0.03 μm or more and 0.05 μm or less, predetermined optical characteristics and dimensional accuracy can be obtained. ..
Further, since regions having different surface roughness are not formed in one side surface 3, predetermined optical characteristics and dimensional accuracy can be obtained.

Further, the corner portion 4 is also subjected to a predetermined polishing process in order to suppress the occurrence of defects such as pinholes that cause polishing particles and the like to be captured in the unevenness and fine fissures generated by the cutting process. However, the feature of the present invention is that the surface roughness is larger than the surface roughness on the side surface 3.
Specifically, the arithmetic average roughness Ra of the corner portion 4 is slightly larger than the arithmetic average roughness Ra of the side surface 3, and the ten-point average roughness Rzjis of the corner portion 4 is larger than the ten-point average roughness Rzjis of the side surface 3. big.

In this way, while approximating the arithmetic average roughness Ra of the corner portion and the arithmetic average roughness Ra of the side surface, the ten-point average roughness Rzjis of the corner portion 4 becomes larger than the ten-point average roughness Rzjis of the side surface 3. Therefore, the influence on the optical characteristics and the dimensional accuracy can be further reduced.
Further, since the ten-point average roughness Rzjis of the corner portion 4 is formed to be larger than the ten-point average roughness Rzjis of the side surface 3, the corner portion is not slippery and the risk of work error such as dropping can be reduced. ..

Specifically, the arithmetic mean roughness Ra in the corner portion 4 is 0.003 μm or more and 0.015 μm or less.
By setting the arithmetic mean roughness Ra in the corner portion 4 to 0.015 μm or less, defects such as pinholes that cause abrasive particles and the like to be trapped in irregularities and fine fissures generated by cutting are sufficiently satisfied. It can be removed, and the optical characteristics of the entire photomask substrate 1 can be sufficiently ensured.

Since the ten-point average roughness Rzjis of the corner portion is set to a value larger than the ten-point average roughness Rzjis of the side surface, the arithmetic average roughness Ra of the corner portion 4 is larger than the arithmetic average roughness Ra of the side surface 3. Become.
Specifically, when the arithmetic average roughness Ra of the side surface is 0.001 μm, the arithmetic average roughness Ra at the corner portion 4 is 0.015 μm. On the other hand, when the side surface is in a mirror surface state, the arithmetic mean roughness Ra at the corner portion 4 is 0.003 μm.

Further, as described above, the ten-point average roughness Rzjis at the corner portion 4 is polished so as to have a value larger than the ten-point average roughness Rzjis at the side surface 3.
By making the ten-point average roughness Rzjis of the side surface 3 and the corner portion 4 different in this way, it becomes easy to identify the boundary between the side surface 3 and the corner portion 4, and the side surface 3 and the corner portion 4 are separated from each other. It is easy to measure the outer dimensions with the boundary between them as the measurement point.

More preferably, the ten-point average roughness Rzjis at the corner portion 4 is 0.07 μm or more and 0.15 μm or less.
When the ten-point average roughness Rzjis in the corner portion 4 is less than 0.07 μm, for example, as schematically shown by using hatching in FIG. 2, the fine uneven shape of the corner portion 4 can be observed with a CCD camera. It is difficult, and it becomes more difficult to identify the boundary between the side surface 3 and the corner portion 4.
When the ten-point average roughness Rzjis in the corner portion 4 exceeds 0.15 μm, it means that the corner portion 4 has local irregularities at a level that increases the risk of trapping abrasive grains. Not preferable.

Further, the difference Δ between the ten-point average roughness Rzjis and the maximum height Rz in the corner portion 4 is preferably 0.02 μm or more and 0.05 μm or less.
It is not preferable that the difference Δ between the ten-point average roughness Rzjis and the maximum height Rz is less than 0.02 μm because the fine uneven shape at the corner portion 4 is removed.
That is, by setting the difference Δ between the ten-point average roughness Rzjis and the maximum height Rz to 0.02 μm or more, the optical surface accuracy of the entire photomask substrate 1 is not required to be higher than necessary. Sufficient characteristics can be secured.
On the other hand, when the difference Δ between the ten-point average roughness Rzjis and the maximum height Rz in the corner portion 4 exceeds 0.05 μm, the corner portion 4 is locally at a level at which the risk of capturing abrasive grains increases. Unevenness is present, which is not preferable.

Further, if the arithmetic average roughness Ra of at least one corner portion 4 of the corner portions 4 is configured to be different from the arithmetic average roughness Ra of the other corner portions, the corner portions 4 having different surface roughness can be used as a mark. Therefore, the orientation of the photomask substrate 1 can be identified.

(Polishing method)
Next, a method for manufacturing a photomask substrate according to an embodiment of the present invention will be described with reference to FIGS. 3 and 4. 3 and 4 are plan views schematically showing one step of a method for manufacturing a photomask substrate according to an embodiment of the present invention.

First, the photomask substrate 1 in a state where the shape processing is completed is prepared. That is, the photomask substrate 1 in this state is formed between two main surfaces 2 facing each other, a side surface 3 formed between the two main surfaces, and two adjacent side surfaces 3. A corner portion 4 is provided, and a chamfered surface 5 is formed on a ridgeline portion between the main surface 2 and the side surface 3 and the corner portion 4.

The photomask substrate 1 in this state can be obtained by grinding, for example, a plate-shaped synthetic silica glass having a square shape of 152 mm and a thickness of 6.35 mm.
Specifically, the end face and corners of this synthetic silica glass plate-like material are ground using a total-type grindstone in which a mold having the shape of the side surface 3 and the chamfered surface 5 is formed, and the side surface 3, the corner portion 4, and the corner portion 4 are ground. The shape of the chamfered surface 5 is processed.

After that, as shown in FIG. 3, a side polishing step of pressing the polishing pad 6 against the side surface 3 of the photomask substrate 1 to polish the photomask substrate 1 is performed.
The polishing pad 6 is, for example, a smooth disk-shaped polishing pad 6 having a diameter of 350 mm, and while supplying a slurry of cerium oxide, the side surface 3 of the photomask substrate 1 is pressed against the polishing pad 6 with a predetermined force for polishing. .. In addition to cerium oxide, colloidal silica, zirconia, or the like can also be used as the abrasive.
Specifically, a polyurethane resin is used as the polishing pad 6. Further, as the particle size of the abrasive, an average particle size of about 1 μm is used, and the pressing pressure is set to 500 to 1500 g / cm ² .

Further, the polishing time in the side polishing step for each side surface 3 is, for example, 15 seconds for each set of polishing for 4 sets in combination with the forward and reverse of the rotation direction of the polishing pad 6 and the front and back inversion of the photomask substrate 1 (60 seconds in total). ) Do about. The arithmetic average roughness Ra is 0.010 μm or less, the ten-point average roughness Rz is 0.03 μm to 0.05 μm, and the difference Δ between the ten-point average roughness Rzjis and the maximum height Rz is 0.002 μm to 0. Obtain a side surface of 010 μm.
In the polishing of the side surface 3 and the corner portion 4, the arithmetic average roughness Ra and the ten-point average roughness Rzjis can be changed by changing the grain size of the abrasive and the polishing time, but from the viewpoint of manufacturing cost. , It is preferable to use the same polishing pad 6 and polishing agent, and change only the polishing time.

After that, as shown in FIG. 4, a corner polishing step of pressing the polishing pad 6 against the corner portion 4 of the photomask substrate 1 to polish the photomask substrate 1 is performed. The polishing pad 6 and the polishing agent are the same as those used in the side polishing step, and while supplying the polishing agent, the corner portion 4 of the photomask substrate 1 is pressed against the polishing pad 6 with the same force as in the side polishing step. Polishing is performed while rotating the work (photomask substrate 1) 90 degrees in the direction parallel to the main surface of the substrate.

The polishing time in the corner polishing step for each corner portion 4 is, for example, about 15 seconds for each corner portion 4 (total 60 seconds).
The arithmetic mean roughness Ra is 0.003 μm to 0.015 μm, the ten-point average roughness Rzjis is 0.07 μm to 0.15 μm, and the difference Δ between the ten-point average roughness Rzjis and the maximum height Rz is 0.02 μm. Obtain a corner portion of ~ 0.05 μm.

As described above, the arithmetic average roughness Ra in the corner portion 4 is larger than the arithmetic average roughness Ra in the side surface 3, and the ten-point average roughness Rzjis in the corner portion 4 is the ten-point average roughness Rzjis in the side surface 3. It is important to set the polishing conditions appropriately so that the value becomes larger.

Subsequently, the main surface 2 is also flattened (mirrored) so as to have a predetermined arithmetic mean roughness Ra (for example, 0.25 nm or less) so as to obtain desired optical characteristics. Through the polishing method as described above, the photomask substrate according to the embodiment of the present invention is manufactured.

(Example)
Further, the present invention will be described based on the examples shown below. The present invention is not limited to the examples shown below.
(Example 1)
Prepare a plate-shaped synthetic silica glass of 152 mm square and 6.35 mm thick, and grind the end face and corner using a full-mold grindstone in which the shape of the side surface and chamfer surface is molded, and grind the side surface, corner part, and The chamfered surface was processed. The corners were processed into a curved shape with a radius of curvature of 2.5 mm.

Next, a side polishing step of pressing a polishing pad against the side surface to polish the surface was performed. As the polishing pad, a smooth disk-shaped one having a diameter of 350 mm was used, and while supplying a slurry containing cerium oxide having an average particle size of 1 μm, the side surface was pressed against the polishing pad with an air cylinder to perform polishing. The polishing pad 6 was made of polyurethane resin, and the rotation speed of the polishing pad 6 was 400 rpm.

The polishing time in the side surface polishing process for each side surface is 15 seconds for each set (60 seconds in total per side surface) by combining 4 sets of polishing by combining the forward and reverse of the rotation direction of the polishing pad 6 and the front and back inversion of the photomask substrate. rice field.

After that, a corner polishing step was performed in which a polishing pad was pressed against the corner portion for polishing.
The polishing pad and the polishing agent are the same as those used in the side polishing process, and while supplying the polishing agent, the corner portion 4 of the photomask substrate 1 is pressed against the polishing pad 6 with the same force as in the side polishing process. Polishing was performed while rotating the work (photomask substrate 1) 90 degrees in the horizontal direction. The rotation speed of the polishing pad was the same as that of the side polishing step. Further, the polishing time in the corner polishing step for each corner portion was 15 seconds per corner portion, and polishing was performed four times for each corner portion (60 seconds in total).

Further, the main surface was flattened (mirrored) so as to obtain desired optical characteristics (within a predetermined standard range). As described above, the photomask substrate of Example 1 was manufactured.

Then, using a general-purpose surface roughness measuring device, the arithmetic average roughness Ra, the ten-point average roughness Rzjis, and the ten-point average roughness Rzjis are used for the main surface, the side surface, and the corner portion with respect to the photomask substrate of Example 1. Each value of the difference Δ between the ten-point average roughness Rzjis and the maximum height Rz was measured.
As a result, the arithmetic average roughness Ra of the side surface was 0.010 μm, the arithmetic average roughness Ra of the corner portion was 0.014 μm, the ten-point average roughness Rzjis of the corner portion was 0.073 μm, and the ten-point average roughness of the corner portion. The difference Δ between Rzjis and the maximum height Rz was 0.021 μm.

As described above, in the photomask substrate of Example 1, although the arithmetic average roughness Ra of the side surface is 0.010 μm and the arithmetic average roughness Ra of the corner portion is 0.014 μm, the numerical values are close to each other. It was found that the corners have a predetermined ten-point average roughness Rzjis, and the difference Δ between the ten-point average roughness Rzjis and the maximum height Rz takes a predetermined value, so that the corners are hard to slip and have excellent handling characteristics. bottom.
Further, in the photomask substrate of Example 1, the boundary between the side surface and the corner portion can be detected, and the orientation of the substrate is specified by providing a difference in the surface roughness and the uneven shape at the four corner portions. It is also possible.
Although the total polishing time of the side surface and the corner portion is the same, the difference in the arithmetic mean roughness Ra between the side surface and the corner portion is that the side surface abuts perpendicularly to the polishing pad. Therefore, it is considered that the corner portion abuts against the polishing pad while rotating.

(Comparative Example 1)
The polishing time in the corner polishing step for each corner portion was 15 seconds per corner portion, polishing was performed 8 times for each corner portion (120 seconds in total), and other than that, the same as in Example 1 was performed. The photomask substrate of Comparative Example 1 was manufactured. That is, the polishing time of the corner polishing step in Comparative Example 1 was twice the polishing time in Example 1.

As a result of performing the measurement in the same manner as in Example 1, the photomask substrate of Comparative Example 1 had an arithmetic mean roughness Ra of the side surface of 0.010 μm, an arithmetic mean roughness Ra of the corner portion of 0.010 μm, and a corner portion. The ten-point average roughness Rzjis was 0.067 μm, and the difference Δ between the ten-point average roughness Rzjis at the corners and the maximum height Rz was 0.011 μm.
As described above, in order to make the arithmetic average roughness Ra of the side surface and the arithmetic average roughness Ra of the corner portion the same, it is necessary to double the polishing time of the corner portion as compared with the first embodiment. Low efficiency. Further, the corner portion of Comparative Example 1 was slippery as compared with Example 1, and was inferior in handling characteristics. Further, in Comparative Example 1, the boundary between the side surface and the corner portion could not be detected.

From the viewpoint of optical characteristics (reflectance, transmittance, birefringence, etc.), no difference is observed between Example 1 and Comparative Example 1, and the application of the present invention has almost no effect on the optical characteristics. It can be said that.

1 Photomask substrate 2 Main surface 3 Sides 4 Corners 5 Chamfered surface 6 Polishing pad

Claims (5)

For a photomask made of silica glass, which comprises two main surfaces facing each other, a side surface formed between the two main surfaces, and a corner portion formed between two adjacent main surfaces of the side surfaces. It ’s a board,
The corner portion has an arithmetic mean roughness Ra of 0.003 μm or more and 0.015 μm or less.
A photomask substrate characterized in that the ten-point average roughness Rzjis of the corner portion is a value larger than the ten-point average roughness Rzjis of the side surface portion. In the corner portion, the ten-point average roughness Rzjis is 0.07 μm or more and 0.15 μm or less, and the difference between the ten-point average roughness Rzjis and the maximum height Rz is 0.02 μm or more and 0.05 μm or less. The photomask substrate according to claim 1. The photomask according to claim 1 or 2, further comprising four corners, wherein the arithmetic mean roughness Ra of at least one of the four corners is different from the arithmetic mean roughness of the other corners. Board for. For a photomask made of silica glass, which comprises two main surfaces facing each other, a side surface formed between the two main surfaces, and a corner portion formed between two adjacent main surfaces of the side surfaces. It is a method of manufacturing a substrate.
A side polishing process in which a polishing pad is pressed against the side surface to polish the surface,
A corner polishing process in which the polishing pad is pressed against the corner to polish the corner,
Including
By adjusting the polishing time in the corner polishing step, the arithmetic average roughness Ra of the corner portion is 0.003 μm or more and 0.015 μm or less, and the ten-point average roughness Rzjis of the corner portion is ten of the side surface. A method for manufacturing a substrate for a photomask, which is characterized in that the point average roughness is larger than Rzjis. The polishing time in the corner polishing step is half the polishing time required to make the arithmetic average roughness Ra at the corner portion and the arithmetic average roughness Ra at the side surface the same. Item 4. The method for manufacturing a photomask substrate according to Item 4.

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