Manufacturing method for synthetic quartz glass substrates

Abstract

The present invention provides a method for manufacturing a synthetic quartz glass substrate, characterized by including an evaluation step for evaluating the depth of the processed alteration layer on the machined surface of the synthetic quartz glass substrate, and a removal step for removing the processed alteration layer based on the evaluation result of the depth of the processed alteration layer. [Effects] By removing the processed alteration layer based on an evaluation of the depth of the processed alteration layer on the machined surface, it becomes possible to remove the processed alteration layer reliably and efficiently, thereby increasing the productivity of synthetic quartz glass substrates.

Description

Technical Field

[0001] The present invention relates to a method for manufacturing a synthetic quartz glass substrate.

Background Art

[0002] Synthetic quartz glass substrates used in semiconductor substrates, optical members, microfluidics, microlens arrays, etc. are made practical by forming through or non-through holes, grooves, steps, etc. by cutting. When performing such cutting, it is common to use tools such as grindstones and drills. At this time, a processed affected layer will occur on the cutting surface. In order to remove this processed affected layer, it is necessary to set a large grinding amount or polishing amount in subsequent grinding processes or polishing processes, etc., which also contributes to reducing productivity.

[0003] Also, usually, for fixed abrasive grain tools used for such cutting, dressing is performed before the start of cutting or during cutting in order to adjust the protrusion amount of the abrasive grains or to create the cutting edges of the abrasive grains. Since the state of the tool after dressing is not always constant, and depending on the life of the fixed abrasive grain tool, there is a risk that a particularly deep processed affected layer will occur or that there will be a large variation in the depth of the processed affected layer.

[0004] Regarding such problems, for example, in Japanese Patent Application Laid-Open No. 2012-035330 (Patent Document 1), in the manufacture of a glass substrate for a magnetic recording medium, a method of suppressing the occurrence of a deep processed affected layer on the cutting surface by making the abrasive grain size distribution width of the abrasive grains contained in the grindstone not more than a predetermined value is disclosed.

[0005] Also, in Japanese Patent Application Laid-Open No. 2012-027976 (Patent Document 2), a method of suppressing the occurrence of a deep processed affected layer on the cutting surface by defining the surface roughness of the cutting surface of a fixed abrasive grain tool is described in the manufacture of a glass substrate for a magnetic recording medium.

Prior Art Documents

Patent Documents

[0006] [Patent Document 1] Japanese Patent Publication No. 2012-035330 [Patent Document 2] Japanese Patent Publication No. 2012-027976 [Overview of the project] [Problems that the invention aims to solve]

[0007] However, the methods described in Patent Document 1 or 2 do not take into account the deep machining-induced layers that occur specifically due to the dressing condition of the grinding wheel or fixed abrasive tool, or the tool life, and variations in the depth of the machining-induced layers. As a result, there is a risk that deep cracks may remain even after the machining-induced layer removal process. Furthermore, measuring the depth of the machining-induced layer requires additional processes such as precision cleaning and etching, which may reduce productivity.

[0008] This invention has been made in view of the above circumstances, and aims to provide a manufacturing method that can reliably and efficiently remove the processed altered layer and produce synthetic quartz glass substrates with high productivity. [Means for solving the problem]

[0009] The inventors of the present invention conducted diligent studies to achieve the above objectives and, as a result, evaluated the depth of the processed alteration layer on the machined surface of a synthetic quartz glass substrate. Based on the evaluation results, they found that by removing the processed alteration layer, the processed alteration layer can be reliably and efficiently removed, thereby increasing the productivity of synthetic quartz glass substrates, leading to the present invention.

[0010] Therefore, the present invention provides the following method for manufacturing a synthetic quartz glass substrate. 1. A method for manufacturing a synthetic quartz glass substrate, comprising an evaluation step of evaluating the depth of the processed alteration layer on the machined surface of the synthetic quartz glass substrate, and a removal step of removing the processed alteration layer based on the evaluation result of the depth of the processed alteration layer. 2. The method for manufacturing a synthetic quartz glass substrate according to claim 1, wherein the evaluation step evaluates whether the arithmetic mean height (Sa), skewness (Ssk), and kurtosis (Sku) on the cut surface of the synthetic quartz glass substrate are at predetermined values, and the removal step is carried out according to the evaluation result. 3. A method for manufacturing a synthetic quartz glass substrate according to 2, wherein a synthetic quartz glass substrate that satisfies the predetermined value in the evaluation step is selected, and the removal step is performed on the synthetic quartz glass substrate that satisfies the predetermined value according to predetermined conditions set in advance. 4. A method for manufacturing a synthetic quartz glass substrate according to 2 or 3, wherein the predetermined values ​​are the arithmetic mean height (Sa) of 2.0 μm or less, the absolute value of the skewness (Ssk) of 0.8 or less, and the kurtosis (Sku) of 5.0 or less. 5. The method for manufacturing a synthetic quartz glass substrate according to 4, wherein the removal step is performed on a synthetic quartz glass substrate that satisfies the predetermined value, with the removal depth set to 30 μm. 6. A method for manufacturing a synthetic quartz glass substrate according to any one of 1 to 5, wherein the removal step includes mirror polishing or wet etching. 7. A method for manufacturing a synthetic quartz glass substrate according to any one of 1 to 6, wherein the depth of the processed altered layer is 30 μm or less. [Effects of the Invention]

[0011] According to the present invention, in the manufacturing of synthetic quartz glass substrates, the depth of the processed alteration layer on the cut surface is evaluated, and the processed alteration layer is removed based on this evaluation. This makes it possible to reliably and efficiently remove the processed alteration layer, thereby increasing the productivity of synthetic quartz glass substrates. [Modes for carrying out the invention]

[0012] The present invention will be described in more detail below. As described above, the method for manufacturing a synthetic quartz glass substrate of the present invention includes an evaluation step for evaluating the depth of the processed altered layer on the cut surface of the synthetic quartz glass substrate, and a removal step for removing the processed altered layer based on the evaluation result of the depth of the processed altered layer.

[0013] In the present invention, the synthetic quartz glass substrate used in the evaluation and removal steps can be obtained by conventional methods. For example, it can be manufactured by forming a synthetic quartz glass ingot, produced by reacting silica raw material compounds such as silane compounds or siloxane compounds with an oxyhydrogen flame, into a desired shape, annealing it, slicing it to a desired thickness, lapping it, and polishing the outer periphery as needed, and then performing rough polishing and fine polishing on the resulting raw material substrate.

[0014] The shape of the synthetic quartz glass substrate can be any shape, such as a square or circular shape, and there are no particular restrictions on its shape or size. For example, for square glass substrates, sizes from 10mm x 10mm to 300mm x 300mm are preferably used. For circular glass substrates, substrates with a diameter of 10 to 300mm are preferably used. The thickness of the synthetic quartz glass substrate is also not particularly limited, but is preferably 0.01mm or more, especially 0.05mm or more, particularly 0.1mm or more, and preferably 300mm or less, particularly 100mm or less, particularly 30mm or less.

[0015] This invention evaluates and removes the depth of the processed alteration layer that occurs during cutting. The target substrate can be a synthetic quartz glass substrate, as described above, that has been cut to give it a shape suitable for various applications.

[0016] In this case, although not particularly limited, it is preferable that the depth of the machined altered layer on the machined surface be 30 μm or less. If the depth of the machined altered layer exceeds 30 μm, the amount of material to be removed in the process of removing the machined altered layer increases, which is disadvantageous in terms of productivity. If the depth of the machined altered layer is 30 μm or less, as will be explained in detail later, the removal of the machined altered layer can be carried out uniformly under conditions that allow for the removal of at least 30 μm, eliminating the need to measure the depth of the machined altered layer each time, thus achieving both quality and productivity.

[0017] Machining of synthetic quartz glass substrates can be performed using a numerically controlled machine tool such as a machining center. Specifically, this can be done using a grinding wheel tool in which diamond abrasive grains, cubic boron nitride abrasive grains, etc., are fixed by electroplating, metal bonding, etc., to the spindle of a numerically controlled machine tool such as a machining center programmed with the desired shape. The grit size of the abrasive grains of the grinding wheel tool is not particularly limited, but from the viewpoint of machining accuracy and productivity, it is preferably #20 or higher, particularly preferably #100 or higher, preferably #5000 or lower, and especially preferably #2000 or lower. The spindle rotation speed of the grinding wheel tool is also not particularly limited, but from the viewpoint of machining accuracy and productivity, it is preferably 100 rpm or higher, particularly preferably 1,000 rpm or higher, preferably 70,000 rpm or lower, and especially preferably 60,000 rpm or lower. Furthermore, while there are no particular restrictions on the cutting feed rate, from the viewpoint of machining accuracy and productivity, it is preferably 1 mm / min or more, particularly preferably 10 mm / min or more, preferably 10,000 mm / min or less, and particularly preferably 1,000 mm / min or less. It is also preferable to use a cutting fluid such as an emulsion, water-soluble, or oil-based fluid for cooling and chip removal during cutting.

[0018] In this invention, for example, with respect to a synthetic quartz glass substrate having a machined surface prepared by the above-described cutting process, the depth of the machined altered layer on the machined surface of the synthetic quartz glass substrate is evaluated before subjecting it to a removal process to remove the machined altered layer.

[0019] In the present invention, methods for evaluating the depth of the processed alteration layer on the machined surface of a synthetic quartz glass substrate include precisely cleaning the substrate to completely remove foreign matter such as cutting dust and cutting fluid, and then measuring it with a laser microscope, or etching it with hydrofluoric acid to isotropically etch the processed alteration layer to a size that is easy to observe, and then measuring it with an optical microscope or laser microscope.

[0020] Here, this evaluation process may be any process that can evaluate the depth of the machined affected layer on the machined surface of the synthetic quartz glass substrate, and there is no particular limitation on the specific method. However, it is preferable to evaluate whether the arithmetic mean height (Sa), skewness (Ssk), and kurtosis (Sku) on the machined surface of the synthetic quartz glass substrate are predetermined values.

[0021] The above-mentioned arithmetic mean height (Sa), skewness (Ssk), and kurtosis (Sku) are values measured using a laser microscope and are defined in ISO 25178-2. As the laser microscope, one can be appropriately selected from conventionally known ones. Specific examples thereof include the Shape Analysis Laser Microscope: VK-X150 (control unit) / VK-X160 (measurement unit) manufactured by Keyence Corporation.

[0022] The arithmetic mean roughness (Sa) is a parameter obtained by three-dimensionally expanding Ra, which is a profile curve (line roughness) parameter, and represents the average of the absolute values of the height differences of each point with respect to the average surface of the surface, and is calculated by the following formula (1).

[0023]

Equation

[0024] Skewness (Ssk) is a parameter representing the symmetry (degree of bias) of the height distribution in surface roughness and is calculated by the following formula (2). Here, Sq is the root mean square height and is a parameter corresponding to the standard deviation of the distance from the average surface.

[0025]

Equation

[0026] Skewness (Ssk) represents the symmetry between concave and convex shapes with respect to a reference height. A skewness value closer to 0 means that the number of sharp concave shapes is equal to the number of sharp convex shapes. Conversely, a skewness value greater than 0 means that the number of sharp convex shapes is more pronounced relative to the reference height, and a skewness value less than 0 means that the number of sharp concave shapes is more pronounced relative to the reference height.

[0027] Kurtosis (Sku) is a parameter that represents the sharpness of the height distribution in surface roughness and is calculated by the following formula (3). Here, Sq is the root mean square height and is a parameter that corresponds to the standard deviation of the distance from the mean surface.

[0028]

number

[0029] Curtosis (Sku) represents the sharpness of the concave (or convex) relative to the reference height. A value of curtosis (Sku) greater than 3 indicates a significant increase in the number of steeply pointed concave (or convex) relative to the reference height, while a value of curtosis (Sku) less than 3 indicates a significant decrease in the number of steeply pointed concave (or convex) relative to the reference height. Furthermore, a curtosis (Sku) of 3 means that the distribution of concave (or convex) sharpness (in particular, the relationship between the number of concave (or convex) and their height) is close to a normal distribution.

[0030] The evaluation step for evaluating the depth of the processed altered layer on the machined surface of the synthetic quartz glass substrate of the present invention preferably involves evaluating whether the arithmetic mean height (Sa), skewness (Ssk), and kurtosis (Sku) on the machined surface of the synthetic quartz glass substrate are at predetermined values, and it is preferable to carry out the removal step according to the evaluation result.

[0031] The predetermined value mentioned above is that the arithmetic mean height (Sa) is preferably 2.0 μm or less, and more preferably 1.8 μm or less. If the arithmetic mean height (Sa) is 2.0 μm or less, it means that there are relatively few extreme recesses (or protrusions) on the machined surface.

[0032] Furthermore, the absolute value of the skewness (Ssk) is preferably 0.8 or less, more preferably 0.6 or less. An absolute value of 0.8 or less of skewness (Ssk) means that there are relatively few extreme recesses (or protrusions) on the machined surface.

[0033] Furthermore, the kurtosis (Sku) is preferably 5.0 or less, more preferably 4.5 or less. A kurtosis (Sku) of 5.0 or less means that the uneven structure of the machined surface tends to be an uneven surface with a relatively gentle slope.

[0034] When the arithmetic mean height (Sa), skewness (Ssk), and curtsis (Sku) of the machined surface of the synthetic quartz glass substrate satisfy the predetermined values, it can be evaluated that there is no uniquely deep machined alteration layer on the machined surface, and the depth of the machined alteration layer is 30 μm or less. Therefore, if these predetermined values ​​are satisfied, the machined alteration layer can be reliably removed by performing a removal process in the next step, the removal process, with settings that assume a removal depth of 30 μm. In this way, by determining whether each value of the arithmetic mean height (Sa), skewness (Ssk), and curtsis (Sku) satisfies the predetermined values, and by uniformly performing the removal process under pre-set conditions based on the evaluation results, the evaluation process and removal process can be carried out simply and efficiently without having to perform complicated evaluation work and set the conditions of the removal process each time.

[0035] In the present invention, as described above, a removal step is performed to remove the processed altered layer generated in the cutting process, based on the evaluation results from the evaluation step.

[0036] The removal step for removing the machined altered layer from the machined surface preferably employs a method that allows for processing (removal) at a constant speed, and is not particularly limited, but mirror polishing using a rotary polishing tool or polishing pad, or wet etching using a glass etching solution are preferably employed.

[0037] In mirror polishing using a rotary polishing tool, for example, the polishing part of the rotary polishing tool can be brought into contact with the cutting surface at a constant pressure and moved at a constant speed. By performing polishing under constant pressure and constant speed conditions, the cutting surface can be uniformly polished at a constant polishing rate. Specifically, the pressure at which the polishing part of the rotary polishing tool contacts the surface is preferably 1 to 1,000,000 Pa, more preferably 1,000 to 100,000 Pa, from the viewpoint of economy and ease of control. The speed is preferably 1 to 10,000 mm / min, more preferably 10 to 1,000 mm / min, from the viewpoint of economy and ease of control. The amount of movement is appropriately set according to the shape and size of the synthetic quartz glass substrate.

[0038] The rotary polishing tool used in this mirror polishing process can be any rotating body whose polishing section is capable of polishing, such as a spindle with a tool chucking section, or a system in which a polishing tool is attached to a rotary tool.

[0039] The material of the polishing tool can be any material that can remove the altered processing layer, such as a GC grinding wheel, WA grinding wheel, diamond grinding wheel, cerium grinding wheel, cerium pad, rubber grinding wheel, felt buff, or polyurethane, and there are no restrictions on the type of material.

[0040] There are no restrictions on the shape of the polishing part of the polishing tool; any shape that can remove the altered layer of material is acceptable, such as a circular or donut-shaped flat disc, a cylindrical shape, a bullet shape, a disc shape, or a barrel shape.

[0041] When polishing by bringing the polishing part of a rotary polishing tool into contact with the machined surface described above, it is preferable to perform the polishing with an abrasive slurry interposed. In this case, examples of abrasive grains include silica, ceria, alundum, white alundum (WA), emery, zirconia, SiC, diamond, titania, and germania, with a particle size of 10 nm to 10 μm being preferred, and these aqueous slurries can be suitably used.

[0042] Furthermore, the relative movement speed of the rotary polishing tool can be selected within the range of 1 to 10,000 mm / min, particularly 10 to 1,000 mm / min, as described above. The rotational speed of the polishing section of the rotary polishing tool is preferably 100 to 10,000 rpm, more preferably 1,000 to 8,000 rpm, and even more preferably 2,000 to 7,000 rpm. If the rotational speed is too low, the processing rate will be slow, and it may take too long to remove the processed altered layer. If the rotational speed is too high, the processing rate will be too fast, or the tool will wear out too quickly, making it difficult to control the process of removing the processed altered layer.

[0043] Furthermore, in mirror polishing using the polishing pad, for example, polishing can be performed by moving either or both the synthetic quartz glass substrate and the polishing pad so that they oscillate relative to each other, while the rotating polishing pad is brought into contact with the synthetic quartz glass substrate at a constant pressure. While a polishing pad impregnated with an abrasive may be used as the rotating polishing pad, it is preferable to perform the process with an abrasive slurry interposed between the pad and the synthetic quartz glass substrate. The material of the polishing portion of the rotating polishing pad can be any material that can remove the workpiece, such as foamed polyurethane, cerium oxide-impregnated polyurethane, zirconium oxide-impregnated polyurethane, nonwoven fabric, suede, rubber, or wool felt, and there are no restrictions on the type of material.

[0044] Examples of abrasive grains used when performing mirror polishing with an abrasive slurry in between include silica, ceria, alundum, white alundum (WA), FO abrasive grains, zirconia, SiC, diamond, titania, and germania, with a particle size of 10 nm to 10 μm being preferred, and these aqueous slurries can be suitably used. Furthermore, methods for pressing the rotating polishing pad against the side surface of the substrate to be polished with constant pressure include using a pressurizing mechanism such as a pneumatic piston or load cell.

[0045] Furthermore, in wet etching using the glass etching solution, it is preferable to use an aqueous solution containing hydrofluoric acid or a fluorinated salt at a concentration of preferably 1 to 60% by mass, more preferably 20 to 50% by mass, as the glass etching solution, and to immerse a synthetic quartz glass substrate in this glass etching solution. Examples of fluorinated compound salts include sodium fluoride and ammonium fluoride. Adding a mixed aqueous solution of hydrofluoric acid and a fluorinated salt, or a surfactant, to the glass etching solution is also effective in improving the uniformity and stability of etching.

[0046] The processing temperature and processing time in this wet etching process vary depending on the concentration of the glass etching solution, but generally, the processing temperature is preferably 10 to 80°C, particularly 20 to 40°C, and the processing time is preferably 10 seconds to 10 hours, particularly 30 seconds to 5 hours.

[0047] In the manufacturing method of the present invention, the removal step is performed based on the evaluation results of the evaluation step. In this case, the evaluation step evaluates whether the arithmetic mean height (Sa), skewness (Ssk), and kurtosis (Sku) on the cut surface of the synthetic quartz glass substrate are at predetermined values, and the removal depth in the removal step can be set according to the evaluation results.

[0048] In this case, in the evaluation step, synthetic quartz glass substrates whose arithmetic mean height (Sa), skewness (Ssk), and crustosis (Sku) satisfy predetermined values ​​can be selected, and the removal step can be performed on these synthetic quartz glass substrates according to predetermined conditions. This allows the removal step to be performed continuously under predetermined conditions, significantly improving production efficiency. For example, if the predetermined values ​​for the arithmetic mean height (Sa), skewness (Ssk), and crustosis (Sku) are set to (Sa) 2.0 μm or less, the absolute value of (Ssk) 0.8 or less, and (Sku) 5.0 or less, the processing alteration layer depth can be evaluated as 30 μm or less. By performing the removal step on the synthetic quartz glass substrate under constant conditions with a removal depth of 30 μm, the processing alteration layer can be reliably and efficiently removed without the need for the complicated work of measuring the processing alteration layer depth and setting the removal depth for each synthetic quartz glass substrate, thereby effectively improving productivity. As mentioned above, the more preferable values ​​for the predetermined values ​​in the evaluation process are Sa: 1.8 μm or less, Ssk absolute value: 0.6 or less, and Sku: 4.5 or less. This allows for more reliable removal of the processed altered layer and further enhances processing reliability.

[0049] Furthermore, in the evaluation process, synthetic quartz glass substrates that do not satisfy the set predetermined values ​​may be subjected to a removal process under conditions that assume a deeper removal depth, for example, or they may be excluded from the removal process if necessary.

[0050] The present invention provides a method for manufacturing a synthetic quartz glass substrate. By forming holes, slits, grooves, steps, etc., in the synthetic quartz glass substrate by cutting, according to its intended use as a semiconductor substrate, optical component, microfluidic, microlens array, etc., and evaluating the processed altered layer on the cut surface, it is possible to remove the processed altered layer generated by cutting simply by setting a removal time that assumes, for example, a processed altered layer depth of 30 μm, thereby increasing productivity. [Examples]

[0051] The present invention will be described more specifically below with reference to examples and comparative examples, but the present invention is not limited to the following examples.

[0052] [Example 1] Sliced ​​synthetic quartz glass raw material was lapped using a planetary motion double-sided lapping machine, then roughly polished using a hard urethane polishing cloth and cerium oxide-based polishing agent in a planetary motion double-sided polishing machine, and finally fine polished using a suede-type polishing cloth and colloidal silica-based polishing agent in a planetary motion double-sided polishing machine to prepare a synthetic quartz glass substrate with an outer diameter of 152 mm square and a thickness of 6.35 mm.

[0053] Next, a non-through hole with a diameter of 50 mm and a depth of 2.5 mm was formed in the synthetic quartz glass substrate by cutting using a machining center. The cutting process was performed using a water-soluble cutting fluid and helical cutting under the following processing conditions.

[0054] <Processing conditions> Cutting tool: Diamond metal mounted wheel (manufactured by Tokyo Diamond Tool Manufacturing Co., Ltd.) Abrasive grain size: #120 Spindle speed: 10,000 rpm Cutting feed rate: 600 mm / min Helical lead: 0.050 mm

[0055] The arithmetic mean height (Sa), skewness (Ssk), and kurtosis (Sku) of the machined surface of a synthetic quartz glass substrate with non-through holes formed by the above machining process were measured using a shape analysis laser microscope in accordance with ISO 25178-2 under the following conditions: Sa: 1.79 μm, Ssk: -0.26, and Sku: 4.56. <Measurement conditions> Shape analysis laser microscope: VK-X150 (control unit) / VK-X160 (measurement unit) (manufactured by Keyence Corporation) Objective lens: 10x Laser wavelength: 658nm Measurement mode: Surface shape mode Measurement pitch: 0.20 μm Measurement range: Rectangle (1056 μm × 1408 μm) Measurement quality: high precision

[0056] When the depth of the altered layer on the machined surface of this synthetic quartz glass substrate was determined using the measurement method described below, it was found that the altered layer depth was 30 μm or less (28.03 μm). It was confirmed that the altered layer could be completely removed with the processing time required to remove 30 μm, which is calculated from the processing rates of mirror polishing and wet etching.

[0057] Determination methods The synthetic quartz glass substrate was precisely cleaned with hot concentrated sulfuric acid, a weakly alkaline surfactant, and pure water, and then dried with IPA. Subsequently, the maximum height (Sz) of the machined surface of the synthetic quartz glass substrate was measured using a shape analysis laser microscope under the following conditions, in accordance with ISO 25178-2. <Measurement conditions> Shape analysis laser microscope: VK-X150 (control unit) / VK-X160 (measurement unit) (manufactured by Keyence Corporation) Objective lens: 10x Laser wavelength: 658nm Measurement mode: Surface shape mode Measurement pitch: 0.20 μm Measurement range: Rectangle (1056 μm × 1408 μm) Measurement quality: high precision

[0058] [Reference example] A synthetic quartz glass substrate was prepared using the same procedure as in Example 1, and a non-through hole with a diameter of 50 mm and a depth of 2.5 mm was formed by cutting it using the same machining center as in Example 1 and the same processing conditions as in Example 1.

[0059] Following the same procedure as in Example 1, the cut surface of the synthetic quartz glass substrate was measured, and the results were Sa: 1.79 μm, Ssk: -1.18, and Sku: 8.27.

[0060] When the depth of the altered layer on the machined surface of this synthetic quartz glass substrate was determined using the same measurement method as in the example, it was found that the altered layer depth was greater than 30 μm (36.74 μm). It was found that the 30 μm of altered layer, calculated from the processing rate of mirror polishing and wet etching, could not be completely removed, resulting in the need for additional processing or quality defects. Therefore, this synthetic quartz glass substrate will either be excluded from the subsequent removal process, or the removal process will be carried out again with a revised processing time.

Claims

1. A method for manufacturing a synthetic quartz glass substrate, comprising an evaluation step for evaluating the depth of the processed alteration layer on the machined surface of the synthetic quartz glass substrate, and a removal step for removing the processed alteration layer based on the evaluation result of the depth of the processed alteration layer.

2. The method for manufacturing a synthetic quartz glass substrate according to claim 1, wherein the evaluation step evaluates whether the arithmetic mean height (Sa), skewness (Ssk), and curtsis (Sku) on the cut surface of the synthetic quartz glass substrate are at predetermined values, and the removal step is performed according to the evaluation result.

3. A method for manufacturing a synthetic quartz glass substrate according to claim 2, wherein a synthetic quartz glass substrate that satisfies the predetermined value is selected in the evaluation step, and the removal step is performed on the synthetic quartz glass substrate that satisfies the predetermined value according to predetermined conditions set in advance.

4. The method for manufacturing a synthetic quartz glass substrate according to claim 2 or 3, wherein the predetermined values ​​are the arithmetic mean height (Sa) of 2.0 μm or less, the absolute value of the skewness (Ssk) of 0.8 or less, and the kurtosis (Sku) of 5.0 or less.

5. A method for manufacturing a synthetic quartz glass substrate according to claim 4, wherein the removal step is performed on a synthetic quartz glass substrate that satisfies the predetermined value, with the removal depth set to 30 μm.

6. The method for manufacturing a synthetic quartz glass substrate according to claim 1, wherein the removal step includes mirror polishing or wet etching.

7. The method for manufacturing a synthetic quartz glass substrate according to claim 1, wherein the depth of the processed altered layer is 30 μm or less.

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