Method for manufacturing substrate for photomask, method for manufacturing photomask blank, method for manufacturing photomask, method for reusing substrate for photomask, photomask, substrate for photomask, and photomask blanks

Abstract

This method for manufacturing a substrate for a photomask includes a layer formation step for forming a silicon dioxide layer on a first surface of a quartz glass substrate.

Description

Method for manufacturing a photomask substrate, method for manufacturing a photomask blank, method for manufacturing a photomask, method for reusing a photomask substrate, photomask, photomask substrate, photomask blank

[0001] The present invention relates to a method for manufacturing a photomask substrate, a method for manufacturing a photomask blank, a method for manufacturing a photomask, a method for recycling a photomask substrate, a photomask, a photomask substrate, and a photomask blank. This invention claims priority to Japanese Patent Application No. 2023-192354, filed on November 10, 2023, and the contents of that application are incorporated by reference into this application in designated states where incorporation by reference of documents is permitted.

[0002] Patent Document 1 discloses a photomask blank having a film on a photomask substrate. However, when a photomask substrate having scratches or the like is recycled by a conventional method, the thickness of the photomask substrate is significantly reduced, and the number of times it can be recycled is limited to approximately one. There is a need for a method for repeatedly recycling photomask substrates without changing the thickness of the photomask substrate.

[0003] JP 2016-105158 A

[0004] One aspect of the present invention is a method for manufacturing a photomask substrate, which includes a layer-forming step of forming a silicon dioxide layer on a first surface of a quartz glass substrate. The method also includes a depleting step of grinding and depleting the first surface of the quartz glass substrate, and a layer-forming step of forming a silicon dioxide layer on the depleted first surface.

[0005] Another aspect of the present invention is a method for manufacturing a photomask blank, which comprises forming at least one type of film selected from a light-shielding film, a halftone film, and a phase shift film on a photomask substrate produced by the above-described method for manufacturing a photomask substrate, thereby manufacturing a photomask blank.

[0006] Another aspect of the present invention is a method for manufacturing a photomask, which comprises carrying out fine line circuit processing on a photomask blank produced by the above-described method for manufacturing a photomask blank.

[0007] Another aspect of the present invention is a method for recycling photomask substrates, which involves carrying out the above-described method for manufacturing a photomask substrate, thereby maintaining the thickness of the quartz glass substrate at a predetermined thickness and enabling used photomask substrates to be repeatedly recycled into new photomask substrates.

[0008] Another aspect of the present invention is a photomask substrate having a quartz glass substrate and a silicon dioxide layer formed on a first surface of the quartz glass substrate.

[0009] Another aspect of the present invention is a photomask blank having at least one film selected from a light-shielding film, a halftone film, and a phase shift film on the above-mentioned photomask substrate.

[0010] Another aspect of the present invention is a photomask in which a pattern is formed on the above-mentioned photomask blank.

[0011] FIG. 1 is a cross-sectional view showing a conventional method for manufacturing a photomask substrate. FIG. 1 is a cross-sectional view showing an example of a method for manufacturing a photomask substrate according to the present embodiment. FIG. 1 is a cross-sectional view showing another ...2 is a cross-sectional SEM image of a recess in a quartz glass substrate before smoothing. FIG. 3 is a cross-sectional SEM image of a recess in a quartz glass substrate after smoothing by carbon dioxide laser irradiation. (A) is a cross-sectional SEM image of a recess in a quartz glass substrate before immersion in a buffered hydrofluoric acid aqueous solution, and (B) is a cross-sectional SEM image of a recess in a quartz glass substrate after immersion in a buffered hydrofluoric acid aqueous solution for 5 hours. FIG. 4 is a diagram showing an example of mutual lamination of a silicon dioxide anode monolayer and a cathode monolayer. FIG. 5 is a diagram showing the FTIR spectrum of a plasma CVD silicon dioxide monolayer. (A) is a cross-sectional SEM image of recesses in a quartz glass substrate on which a CVD-silicon dioxide layer was formed after smoothing, and (B) is a cross-sectional SEM image of recesses in a quartz glass substrate on which a CVD-silicon dioxide layer was formed without smoothing. This is a histogram tabulating the number of recesses and recess depths in quartz glass substrates recovered from used photomask substrates. This is a histogram tabulating the number of recesses and recess widths in quartz glass substrates recovered from used photomask substrates.

[0012] An embodiment of the present invention (hereinafter referred to as "the present embodiment") will be described below. The present embodiment is an example for explaining the present invention, and is not intended to limit the present invention to the following content. The present invention can be practiced with appropriate modifications within the scope of its gist.

[0013] Typically, a photomask for a flat panel display (FPD) is a parallel plate substrate made of quartz glass with a first surface and a second surface opposite the first surface polished, on which a thin line pattern of a light-shielding film such as chrome, a halftone film, or a phase shift film such as chrome or molybdenum silicide is formed. Panel manufacturers that produce a variety of liquid crystal displays and organic EL displays use projection exposure machines or proximity exposure machines to transfer and fix the thin line pattern of the photomask onto the glass substrate for the display, thereby manufacturing the displays.

[0014] Similarly, a photomask for semiconductor circuits refers to a parallel plate substrate made of quartz glass with its first and second surfaces polished, on which a thin line pattern of a light-shielding film such as chrome, a half-tone film, or a phase shift film such as chrome or molybdenum silicide is formed. Manufacturers of semiconductor devices use a reduced projection exposure machine to reduce and transfer the thin line pattern of the photomask onto various substrates for semiconductor devices.

[0015] Photomask substrates that have finished being used as photomasks by panel manufacturers and semiconductor device manufacturers are sometimes collected for recycling purposes if they are in a state where there is no visible damage. To recycle collected photomask substrates, the photomask pattern formed on the substrate surface, consisting of a Cr film, MoSi film, etc., is first dissolved and removed by immersion in a solvent. After the photopattern is dissolved and removed, the substrate, with the quartz glass exposed, typically has recesses on its front and back surfaces. Therefore, the substrate cannot be used as a recycled photomask by forming a new photomask in its current state. The recesses must be removed by some means, and the first and second surfaces of the quartz glass substrate must be flat again before forming the photomask.

[0016] The method for manufacturing a photomask substrate according to this embodiment will be described below.

[0017] The method for manufacturing a photomask substrate according to this embodiment includes a layer forming step of forming a silicon dioxide layer on a first surface of a quartz glass substrate. In the layer forming step, the silicon dioxide layer is formed over the entire first surface of the quartz glass substrate. The first surface also has a recess.

[0018] The method for manufacturing a photomask substrate according to this embodiment includes a reducing step of reducing the first surface of a quartz glass substrate by grinding it, and a layer forming step of forming a silicon dioxide layer on the reduced first surface. In the reducing step, the entire first surface is reduced, and in the layer forming step, the silicon dioxide layer is formed on the entire first surface. The first surface also has a recess.

[0019] In the depletion step, the first surface is depleted so that the depth of the recess is 20 μm or less, preferably 10 μm or less, and more preferably 5 μm or less.

[0020] The layer formation process uses at least one of plasma CVD, laser ablation, sputtering, ion plating, and vacuum deposition. Two or more types of layers can also be stacked in the layer formation process. Figure 8 shows an example of an alternating layer stack in which the thickness ratio of the anode monolayer to the cathode monolayer is varied to control stress and adjust the refractive index.

[0021] The method for manufacturing a photomask substrate according to this embodiment includes a layer planarization step of scraping the silicon dioxide layer to reduce the arithmetic mean height Sa (surface roughness; ISO 25178) of the first surface on which the silicon dioxide layer is formed.

[0022] In the layer planarization step, the silicon dioxide layer is polished so that the arithmetic mean height Sa (surface roughness; ISO25178) of the first surface on which the silicon dioxide layer is formed is 1 μm or less, preferably 0.5 μm or less, and more preferably 0.1 μm or less.

[0023] The layer forming step forms a silicon dioxide layer of a first thickness, and the layer planarizing step removes the silicon dioxide layer by a second thickness, where the second thickness is less than or equal to the first thickness.

[0024] The method for manufacturing a photomask substrate according to this embodiment includes a substrate planarization step of grinding the quartz glass substrate to reduce the arithmetic mean height Sa (surface roughness; ISO25178) of the first surface of the quartz glass substrate.

[0025] In the method for manufacturing a photomask substrate according to this embodiment, a layer planarization process can be performed on the first surface on which the silicon dioxide layer is formed until the quartz glass substrate is exposed, and then a substrate planarization process can be performed on the exposed quartz glass substrate.

[0026] In the layer planarization step and the substrate planarization step, the silicon dioxide layer and the quartz glass substrate are polished so that the arithmetic mean height Sa (surface roughness; ISO25178) of the first surface on which the silicon dioxide layer is formed is 1 μm or less, preferably 0.5 μm or less, and more preferably 0.1 μm or less.

[0027] The method for manufacturing a photomask substrate according to this embodiment includes a smoothing step for reducing the arithmetic mean height Sa (surface roughness; ISO 25178) of the inner surface of the recesses, and a silicon dioxide layer is formed in the recesses after the smoothing step.

[0028] In the smoothing step, the arithmetic mean height Sa (surface roughness; ISO25178) of the inner surface of the recess is set to 50 nm or less, preferably 25 nm or less, and more preferably 10 nm or less.

[0029] In the smoothing step, the inner surface of the recess is smoothed with at least one solution selected from the group consisting of an aqueous solution of hydrofluoric acid, an aqueous solution of buffered hydrofluoric acid, and an aqueous solution of a strong alkali.

[0030] In the smoothing step, the inner surfaces of the recesses on the quartz glass substrate are smoothed by at least one of reactive plasma etching, reactive sputtering, and ion beam sputtering.

[0031] In the smoothing step, the inner surface of the recess is smoothed by irradiating the quartz glass substrate with a carbon dioxide gas laser having a wavelength of 9.2 μm or more and 10.8 μm or less. The lower limit of the wavelength of the irradiated gas laser is preferably 9.5 μm, more preferably 9.6 μm. The upper limit of the wavelength of the irradiated gas laser is preferably 10.7 μm, more preferably 10.6 μm.

[0032] In the photomask substrate manufacturing method according to this embodiment, the process of making a used photomask substrate reusable is hereinafter referred to as "refining." The quartz glass substrate before refining is a used photomask from which the photomask pattern has been removed.

[0033] In the method for manufacturing a photomask substrate according to this embodiment, the first surface of the quartz glass substrate before refining has recesses (scratches). The depth of the recesses is not particularly limited, but can be 0.1 μm or more and 500 μm or less. The depth of the recesses is generally 25 μm or less, more generally 15 μm or less, and even more generally 10 μm or less.

[0034] The width of the recess is not particularly limited, but is generally 20 μm or less, more generally 15 μm or less, and even more generally 7 μm or less.

[0035] In the method for manufacturing a photomask substrate according to this embodiment, the arithmetic mean height Sa (surface roughness; ISO 25178) of the inner surfaces of the recesses of the quartz glass substrate before refining is not particularly limited, but is generally 100 nm or less, more generally 70 nm or less, and even more generally 50 nm or less.

[0036] In the method for manufacturing a photomask substrate according to this embodiment, minute cracks (fractures) may occur on the inner surfaces of the recesses of the quartz glass substrate before refining.

[0037] In the method for manufacturing a photomask substrate according to this embodiment, the second surface opposite to the first surface can be subjected to the same treatment as that for the first surface.

[0038] In the method for manufacturing a photomask substrate according to this embodiment, the difference between the thickness of the thickest part of the quartz glass substrate before refining and the thickness of the photomask substrate after manufacturing is 0.2 mm or less, preferably 0.15 mm or less, and more preferably 0.1 mm or less.

[0039] The method for manufacturing a photomask substrate according to this embodiment may further include one or more of a photomask recovery process for recovering used photomasks, and a pattern removal process for removing the photomask pattern from the used photomask to obtain a quartz glass substrate.

[0040] Photomask blanks can be manufactured by depositing at least one film selected from a light-shielding film, a halftone film, and a phase shift film on a photomask substrate produced by the photomask substrate manufacturing method according to this embodiment. Furthermore, photomasks can also be manufactured by forming a pattern on the photomask blanks or by processing fine lines into the photomask.

[0041] By carrying out the method for manufacturing a photomask substrate according to this embodiment, it is possible to maintain the thickness of the quartz glass substrate at a predetermined thickness and repeatedly recycle used photomask substrates into new photomask substrates.

[0042] In the method for manufacturing a photomask substrate according to this embodiment, the quartz glass substrate after refinement may have a recognition portion formed on its periphery so as to be recognized by a desired method. The recognition portion is contained within the quartz glass substrate and includes information for uniquely identifying the photomask substrate, such as a manufacturing number, model number, or serial number. The recognition portion may not only be contained within the quartz glass substrate, but may also be formed between layers or engraved during the mask pattern formation process. For example, the recognition portion may be formed using one selected from a data matrix code, a QR code (registered trademark), a barcode, etc., so that the recognition portion can be recognized by a barcode reader.

[0043] Furthermore, an example of a quartz glass substrate having a recess on its first surface used in this embodiment is one in which a fine line pattern formed on the substrate surface has been removed (pattern removal step), exposing the quartz glass surface. For example, this would be a substrate in which a photomask pattern has been removed from a photomask. A method for removing the pattern may be selected according to the material of the pattern. For example, if the fine line pattern contains chromium, the pattern can be dissolved and removed by immersing the photomask in a sulfuric acid solution.

[0044] Next, the method for manufacturing a photomask substrate according to this embodiment will be specifically described with reference to FIGS.

[0045] <Regarding FIG. 1> FIG. 1 is a cross-sectional view showing a conventional method for manufacturing a photomask substrate.

[0046] In state (1)-1, the fine line pattern formed on the substrate surface has been dissolved and removed, exposing the quartz glass surface. The flatness of the entire surface in state (1)-1 is satisfactory, consistent with the flatness achieved after the previous precision polishing. However, when viewed locally, scratches (concave portions) are present on the surface of the quartz glass substrate without exception. While FIG. 1 shows a single scratch on the first surface as an example, in reality, at least 10 scratches (concave portions) and up to several tens of scratches (concave portions) exist on the quartz substrate in state (1)-1. The depth of the concave portions is generally several μm to several tens of μm. To reuse the quartz glass substrate as a photomask, all of the concave portions must be removed. If a photomask is formed on a quartz glass substrate with concave portions, the fine line pattern will cross the concave portions. This can result in defects of the fine lines at the crossing points, preventing the desired fine line pattern from being transferred to the glass substrate for ICs or displays, resulting in defects in the display area of ​​the semiconductor device or display.

[0047] The quartz glass substrate in state (1)-1 in Figure 1 has recesses (scratches) on its first surface. The inner surface of the recesses may have a rough surface, or may have numerous microcracks (fractures). First, the quartz glass substrate in state (1)-1 is ground across the entire first surface, resulting in state (1)-2 (grinding process). State (1)-2 is a state in which the thickness of the substrate has been reduced to more than the depth of the recesses present on the first surface. This allows the recesses present on the first surface of the substrate to be removed. Next, if recesses also exist on the second surface opposite the first surface, they are ground in the same manner as the first surface (not shown). In state (1)-2 after grinding, the substrate has been reduced in thickness to more than the thickness corresponding to the sum of the depths of the recesses present on the first surface and the second surface.

[0048] The surface roughness of the entire first and second surfaces of the substrate was smooth due to previous precision polishing in state (1)-1, but in state (1)-2, grinding has resulted in an uneven surface, so-called ground glass, which transmits almost no light due to scattering.

[0049] Next, the substrate in state (1)-2 is roughly polished to bring it to state (1)-3 (rough polishing process). Rough polishing smoothes the first and second surfaces, and the parallelism of both surfaces is also adjusted, so the substrate becomes light-transmitting again. However, the surface is not yet flat enough to form a photomask.

[0050] Next, the substrate in state (1)-3 is subjected to precision polishing to complete a photomask in state (1)-4 (precision polishing process). By precision polishing, the first and second surfaces are made flat enough to form a photomask.

[0051] As described above, in the conventional method for manufacturing a photomask substrate, all recesses present on the first and second surfaces of the quartz glass substrate can be removed and the flatness of both surfaces can be restored to the original state, but the thickness of the substrate is significantly reduced. The amount of reduction in thickness per process is greatest during grinding, followed by rough polishing, and least during precision polishing.

[0052] As described above, conventional methods for manufacturing photomask substrates result in significant reduction in the thickness of the substrate. Upper and lower limit tolerances are set for the thickness of photomask substrates, and substrates that exceed these tolerances cannot be used because exposure performance is not guaranteed. Conventional substrate reclamation methods limit the number of times the substrate can be recycled to approximately one time due to the reduction in thickness. If a second reclamation attempt is made, the substrate will fall below the lower limit of the substrate thickness tolerance, making it unusable.

[0053] 2 to 5 are cross-sectional views showing the method for manufacturing a photomask substrate according to this embodiment.

[0054] <Regarding Fig. 2> The quartz glass substrate in state (2)-1 in Fig. 2 is in a state where the fine line pattern formed on the substrate surface has been dissolved and removed, exposing the quartz glass surface. In addition, there is a recess (scratch) on the first surface.

[0055] In FIG. 2, a silicon dioxide layer is formed by CVD on a quartz glass substrate in state (2)-1, resulting in state (2)-2 (layer formation process). The depressions of the recesses are completely filled and repaired by the deposition of the silicon dioxide layer. In this CVD process, it is important to form a silicon dioxide layer thick enough to completely fill the deepest recess among the multiple recesses present on the substrate. By doing so, in state (2)-2 after the CVD process has been performed, all recesses present on the first or second surface of the quartz glass substrate are completely filled with a silicon dioxide layer, and excess silicon dioxide layer is deposited over the entire quartz glass substrate.

[0056] Next, the substrate in state (2)-2 is planarized to state (2)-3 or state (2)-4. In this specification, planarizing the silicon dioxide layer is referred to as the "layer planarization process." One purpose of layer planarization is to remove excess silicon dioxide layer deposited on the first or second surface of the quartz glass substrate. The silicon dioxide layer is planarized so as to fall within the upper and lower tolerance limits of the substrate thickness dimension. Since the interface between the silicon dioxide layer and the quartz glass substrate has already been assimilated to a state where it is completely unrecognizable in state (2)-2, it is not necessary to completely remove the silicon dioxide layer from the first or second surface of the quartz glass substrate in the layer planarization process. As long as it falls within the upper and lower tolerance limits of the substrate thickness dimension, even if the silicon dioxide layer remains, it will not affect the optical performance of the photomask. By planarizing the silicon dioxide layer, the first and second surfaces become flat enough to form a photomask, and the thickness is adjusted appropriately to complete the photomask substrate in state (2)-3 (layer planarization step). Alternatively, the silicon dioxide layer of the substrate in state (2)-2 may be planarized until it reaches the quartz glass substrate before layer formation (layer planarization step), and the partially exposed quartz glass substrate may be further planarized (substrate planarization step) to produce a photomask substrate in state (2)-4.

[0057] <Regarding Figure 3> The quartz glass substrate in state (3)-1 in Figure 3 is in a state where the fine line pattern formed on the substrate surface has been dissolved and removed, exposing the quartz glass surface. The first surface also has recesses (scratches). Unlike the case of Figure 2, the quartz glass substrate in state (3)-1 in Figure 3 has inner surfaces of the recesses with a surface roughness of several tens to several hundreds of nanometers. The inner surfaces of the recesses may also have fine cracks. In Figure 3, the inner surface of the first surface is first subjected to a smoothing treatment (smoothing treatment) to reduce the surface roughness of the inner surfaces of the recesses and remove fine cracks, resulting in state (3)-2 (smoothing step). In state (3)-2 after the smoothing step, the surface with the recesses and cracks has been smoothed, and the surface roughness of the inner surfaces of the recesses is, for example, a smooth surface with a surface roughness of 10 nm or less.

[0058] Next, a silicon dioxide layer is formed on the quartz glass substrate in state (3)-2 by a CVD method, resulting in state (3)-3 (layer formation process). The deep recesses are completely filled and repaired by the deposition of the silicon dioxide layer. It is important in this CVD process to form a silicon dioxide layer thick enough to completely fill the deepest recess among the multiple recesses present on the substrate. By doing so, in state (3)-3 after the CVD process has been performed, all recesses present on the first or second surface of the quartz glass substrate are completely filled with a silicon dioxide layer, and excess silicon dioxide is deposited over the entire quartz glass substrate.

[0059] Next, the substrate in state (3)-3 is planarized to state (3)-4 or state (3)-5. Since the interface between the silicon dioxide layer and the quartz glass substrate has already been assimilated to a state where it is completely indistinguishable in state (3)-3, in the layer planarization step, it is not necessary to completely remove the silicon dioxide layer from the first or second surface of the quartz glass substrate; as long as it is within the upper and lower tolerance limits of the substrate thickness dimension, even if the silicon dioxide layer remains, it will not affect the optical performance as a photomask. By planarizing the silicon dioxide layer, the first and second surfaces become flat enough to form a photomask, and the thickness is adjusted appropriately to complete the photomask substrate in state (3)-4 (layer planarization step). Alternatively, a photomask substrate in state (3)-5 can be manufactured by planarizing the silicon dioxide layer of the substrate in state (3)-3 until it reaches the quartz glass substrate before layer formation, and then further planarizing the partially exposed quartz glass substrate (substrate planarization step).

[0060] <Regarding Figure 4> The quartz glass substrate in state (4)-1 in Figure 4 has a recess (scratches) on the first surface. First, the quartz glass substrate in state (4)-1 is ground entirely over the first and second surfaces using, for example, a grinding wheel, to obtain state (4)-2. State (4)-2 is a state in which the thickness of the first surface side of the substrate has been reduced to a depth greater than the depth of the deepest recess among the multiple recesses present on the first surface, and at the same time, the thickness of the second surface side of the substrate has been reduced to a depth greater than the depth of the recesses present on the second surface. By doing so, the recesses present on the first and second surfaces of the substrate can be removed (depletion process).

[0061] Next, a silicon dioxide layer is formed on the quartz glass substrate in state (4)-2 by the CVD method in the same manner as in FIGS. 2 and 3, to give state (4)-3 (layer formation step).

[0062] Next, the substrate in state (4)-3 is planarized while adjusting the thickness appropriately in the same manner as in FIG. 2, to complete a photomask substrate in state (4)-4 (layer planarization step).

[0063] In addition, for a quartz glass substrate in state (4)-1, the thickness of the substrate may be reduced by a reduction process to a value greater than the depth of the recess, and then the flatness of the substrate surface may be improved by a substrate flattening process (the arithmetic mean height Sa may be reduced) before the layer formation process is carried out.

[0064] 5A and 5B show a quartz glass substrate in state (5)-1 in Fig. 5A, in which the fine line pattern formed on the substrate surface has been dissolved and removed, exposing the quartz glass surface. The first surface also has a recess (scratch).

[0065] In Figure 5, a silicon dioxide layer is formed by CVD on the quartz glass substrate in state (5)-1, resulting in state (5)-2 (layer formation process). The depressions in the recesses are completely filled and repaired by the deposition of the silicon dioxide layer. At this time, the silicon dioxide layer is formed only in the recesses and their surrounding areas.

[0066] Next, the substrate in state (5)-2 is planarized to state (5)-3. The silicon dioxide layer and the quartz glass substrate are planarized so that they fall within the upper and lower tolerance limits of the substrate thickness dimension (layer planarization step, substrate planarization step). Since the interface between the silicon dioxide layer and the quartz glass substrate has already been assimilated to a state where it is completely unrecognizable in state (5)-2, it is not necessary to completely remove the silicon dioxide layer from the first or second surface of the quartz glass substrate in the layer planarization step; as long as it falls within the upper and lower tolerance limits of the substrate thickness dimension, even if the silicon dioxide layer remains, it will not affect the optical performance of the photomask.

[0067] Each of the above steps will be described in detail below.

[0068] 4, the quartz glass substrate is ground using, for example, a grinding stone, etc. Alternatively, the quartz glass substrate may be further ground using, for example, cerium oxide having a particle diameter of about 100 μm as an abrasive.

[0069] <Smoothing Step> In the smoothing step shown in Figure 3, solvent immersion, irradiation with a carbon dioxide laser having a wavelength of 10.6 µm, or plasma dry etching are effective. In solvent immersion, the quartz glass substrate is immersed in buffered hydrofluoric acid, hydrofluoric acid, or a strong alkaline aqueous solution to dissolve and remove the fine irregularities that make up the surface roughness of the inner surface of the recesses, thereby smoothing the inner surface of the recesses. In other words, the surface roughness (arithmetic mean height Sa) of the inner surface of the recesses can be reduced. Examples of strong alkaline aqueous solutions include a sodium hydroxide aqueous solution and a potassium hydroxide aqueous solution.

[0070] In the case of carbon dioxide laser irradiation with a wavelength of 10.6 μm, the concave portions on the front or back surface of the quartz glass substrate are irradiated with the focused light of the carbon dioxide laser, and the concave portions are rapidly heated locally to about 1500 to 1900°C, whereby only the concave portions are locally melted and the inner surfaces of the concave portions are made smooth. In the case of plasma dry etching, the quartz glass substrate is placed in a reduced pressure plasma generating vessel, and CF ４ , C ２ F ６ Fluorocarbon gases such as NF ３ Gas or SF６ A gas is introduced to generate low-pressure plasma by controlling the pressure inside the chamber to a range of 0.1 Pa to 10 Pa. The quartz glass substrate exposed to the active fluorine radicals and ions is etched, and the inner surfaces of recesses, which have many defects, are selectively etched, making the inner surfaces of the recesses smooth.

[0071] <Layer Formation Process> Plasma CVD, thermal CVD, and laser ablation are effective CVD methods in the above-mentioned Figures 2 to 5. In particular, tetraethoxysilane (TEOS) and SiF ４ Gas and O ２ Plasma CVD using a gas mixture can form a silicon dioxide layer at a rate of 1 μm / 10 min or more. Furthermore, it is important to control the CVD conditions during the CVD process so that the difference in refractive index between the quartz glass substrate and the silicon dioxide layer is small, i.e., so that the chemical composition and density of the quartz glass substrate and the silicon dioxide layer are approximately equivalent. This is because a large difference in refractive index between the quartz glass substrate and the silicon dioxide layer can adversely affect exposure performance when used as a photomask. The refractive index difference should be controlled to 0.01 or less, preferably 0.005, and more preferably 0.002 or less, at a wavelength of 365 nm. By forming a silicon dioxide layer with a small refractive index difference on a quartz glass substrate that has been smoothed by a smoothing process, the interface between the quartz glass substrate and the silicon dioxide layer becomes almost indistinguishable. Furthermore, the surface roughness of the recesses is reduced, allowing them to be completely covered with the silicon dioxide layer without any voids ( FIG. 10A ). Furthermore, in order to prevent the silicon dioxide layer covering the recesses from peeling off or falling off semi-permanently, it is preferable to control the CVD conditions in the CVD process to minimize the residual stress in the silicon dioxide layer in advance.

[0072] 2 to 5, the quartz glass substrate or silicon dioxide layer is planarized using, for example, silicon dioxide ultrafine particles with a particle diameter Φ of about several μm as an abrasive (to reduce the arithmetic mean height Sa of the quartz glass substrate or silicon dioxide layer). In the substrate planarization step and layer planarization step, for example, after planarization using an abrasive with larger particles (such as cerium oxide with a particle diameter Φ of about 100 μm), further planarization may be performed using an abrasive with smaller particles (such as silicon dioxide ultrafine particles with a particle diameter Φ of about several μm).

[0073] As described above, in the method for manufacturing a photomask substrate according to this embodiment, all recesses in the quartz glass substrate are repaired, the flatness of the first and second surfaces is maintained, and the thickness of the manufactured photomask substrate is hardly reduced from the thickness of the quartz glass substrate before refining. As long as the substrate is not accidentally hit or dropped and severely damaged, it can be recycled multiple times to almost indefinitely.

[0074] The method for manufacturing a photomask substrate will be specifically described below with reference to examples and comparative examples, but the present invention is not limited to these examples and comparative examples.

[0075] Example 1: Measurement and Analysis of Depressions (Scratches) on Used Photomasks Six used photomask substrates were collected, and the fine line patterns, such as Cr, were dissolved and removed to expose the quartz glass substrate. Using a scratch measurement device (model number: IGS, manufactured by Nikon), the number of depressions on each quartz glass substrate was counted, and the width and depth of each depression were measured. The depth of the depression was measured as the depth of the deepest point. For dot-shaped depressions, the vertical and horizontal widths were measured and averaged to determine the depression width. For line-shaped scratches, the line width was measured and determined to determine the depression width. A total of more than 300 depressions were detected within the exposed areas on the front and back surfaces of the six quartz glass substrates. Histograms of the counted results are shown in Figures 11(A) and (B). Figures 11(A) and (B) reveal that the majority of depressions were 5 μm or less in width and 15 μm or less in depth.

[0076] Example 2: Creation of artificial scratches (recesses) by laser ablation Based on the results of the measurement and analysis of the recesses in the used photomask described above, a recess with a depth of 7 μm and a width of 10 μm was selected as an example of a typical recess size, and a recess of this size was artificially formed on the surface of a quartz glass substrate.

[0077] Laser ablation using a picosecond laser was selected as the method for forming artificial scratches (depressions). Generally, in the photomask manufacturing process, the manufactured photomask is compared with the design drawing, and any areas that do not conform to the drawing are corrected by ablation using a picosecond or femtosecond laser mounted on a pattern correction machine. This correction process not only ablates the fine line pattern, but also commonly leaves ablation marks (depressions) on the quartz glass substrate. Therefore, using a quartz glass substrate on which ablation marks (depressions) have been artificially formed using a short-pulse laser is the most practical method.

[0078] Artificial scratches (depressions) 5 μm deep and 10 μm wide were formed on the surface of a quartz glass substrate (152 mm × 152 mm × 6.35 mm thick) using picosecond laser ablation. There were areas where many dot scratches were formed at a 250 μm pitch, and areas where two line scratches were formed. Note that most scratches present on substrates that are actually recovered are dot scratches, with line scratches being rare. Furthermore, extremely long line scratches that cross the substrate are almost nonexistent. Line scratches that cross the substrate were formed to facilitate evaluation.

[0079] Example 3: Smoothing of recesses by carbon dioxide laser irradiation In order to reduce the surface roughness (arithmetic mean height Sa) of the inner surface of an artificial recess present on a quartz glass substrate having a size of 152 mm × 152 mm × 6.35 mmt and to remove fine cracks, carbon dioxide gas (CO 2 ) with a wavelength of 10.6 μm was irradiated. ２ ) The recess was irradiated with a laser to heat it to the softening point (1700° C.) or higher.

[0080] When irradiating with a carbon dioxide laser, the distance between the substrate and the irradiation optical system was adjusted so that the spot diameter of the laser light was sufficiently larger than the width of the recess, and the temperature of the irradiated area was set to be higher than the softening point of the quartz glass. The irradiation time was set to 5 seconds. Figure 6 shows the results of observation of the cross section of the recess before and after laser irradiation using a scanning electron microscope (SEM). Before carbon dioxide laser irradiation, fine irregularities were observed on the inner surface of the recess (Figure 6(A)), but after irradiation, the fine irregularities disappeared, revealing a smooth surface (Figure 6(B)).

[0081] Example 4: Smoothing of recesses by solvent immersion In order to reduce the surface roughness (arithmetic mean height Sa) of the inner surface of artificial scratches (recesses) present on a quartz glass substrate measuring 152 mm × 152 mm × 6.35 mmt and to remove fine cracks, the substrate was immersed in a 5% NaOH aqueous solution (50°C) or a buffered hydrofluoric acid aqueous solution (room temperature) for 5 hours.

[0082] Figures 7(A) and 7(B) show cross-sectional SEM images of the artificial scratches (depressions) before and after immersion in a buffered hydrofluoric acid solution for five hours. Comparing Figure 7(A) (cross-sectional view of the depression before immersion) and Figure 7(B) (cross-sectional view of the depression after immersion) reveals that chemical etching with buffered hydrofluoric acid completely removed the microcracks on the inner surface of the artificial scratches (depressions) after immersion, and also smoothed the sub-micron-order micro-irregularities on the inner surface of the depressions. This means that the surface roughness of the inner surface of the depressions has been reduced. Since the width of the artificial depressions was approximately doubled by isotropic etching, we also confirmed that by adjusting the immersion time and solvent concentration, the aspect ratio of the depressions can be adjusted to suit the CVD silicon dioxide layer formation.

[0083] Example 5: Formation of silicon dioxide layer by plasma CVD A silicon dioxide layer was formed by plasma CVD using a capacitively coupled low-pressure glow plasma device. TEOS was used as the silicon source and O was used as the oxidizing gas. ２ The silicon dioxide layers were formed under six different conditions: three substrate temperatures of 200°C, 300°C, and 400°C, and two conditions in which the substrate was placed on a cathode electrode and one in which it was placed on an anode electrode. The refractive index and residual stress of the deposited silicon dioxide layers are shown in Table 1.

[0084]

[0085] FTIR spectra of six types of silicon dioxide layers are shown in Figure 9. For comparison, the FTIR spectrum of thermally synthesized quartz glass is described, for example, in G. Lucovsky, J. Vac. Sci. Tech., B5, 530 (1987). It was confirmed that all six types of silicon dioxide layers were in a chemically bonded state equivalent to that of the quartz glass substrate.

[0086] Table 2 shows the results of XPS measurements of the chemical compositions of the six types of silicon dioxide layers. XPS measurements of a quartz glass substrate were also performed for comparison. It was confirmed that all six types of silicon dioxide layers had the same stoichiometric composition as the quartz glass substrate. These results demonstrate that plasma CVD can form silicon dioxide layers with the same chemical bonding state and chemical composition as quartz glass at low temperatures, and that the refractive index can be finely adjusted by adjusting the density (i.e., stress) of the silicon dioxide layer through a combination of substrate temperature and anode / cathode placement. The refining CVD process requires the formation of a silicon dioxide layer with a thickness of at least several micrometers, corresponding to the depth of the recesses. By selecting at least one compressive stress monolayer and one tens of stress monolayer from the six types of silicon dioxide layers shown in Figure 9, Tables 1, and 2, and alternately stacking them to form a multilayer thick film as shown in Figure 8, it was possible to form the desired silicon dioxide thick layer with a thickness of several tens of micrometers while maintaining low stress. When a silicon dioxide layer having a thickness of 13 μm was formed, a high speed of 1.4 μm / 10 minutes was achieved.

[0087]

[0088] Example 6: Refining of Used Quartz Glass Substrates. Using a photomask substrate measuring 152 mm x 152 mm x 6.35 mm (t), artificial scratches (depressions) as described in Example 2 were formed on the surface of the quartz glass substrate using a picosecond laser. Next, the inner surfaces of the artificial scratches (depressions) were smoothed using the smoothing procedure described in Example 3 or Example 4. Next, a 13 μm-thick silicon dioxide layer was formed on the surface of the quartz glass substrate using plasma CVD as described in Example 5. The results of cross-sectional SEM observation of this state are shown in Figure 10(A). It can be easily seen that the artificial scratches (depressions) were completely covered, and the interface between the quartz glass substrate and the silicon dioxide layer was completely indistinguishable and blended. Next, the substrate surface was polished using ultrafine silicon dioxide particles with a particle diameter of approximately several μm as an abrasive to complete the refined substrate.

[0089] Example 7: Photomask fabrication using refined quartz glass substrate and exposure experiment. A chromium binary film was formed on the refined quartz glass substrate by magnetron sputtering, and then an FPD mask pattern was formed using a specified process. This state is called a refined photomask. A mask inspection device was used to confirm that the photomask was actually formed according to the CAD design.

[0090] Next, the fine pattern of the refining photomask was exposed and transferred at a 1 / 5 reduction projection using an i-line exposure machine to form a transfer pattern image on a glass substrate. The transfer pattern was compared with the photomask pattern formed directly above the repaired recesses, and with the photomask pattern formed in an area without any scratches. As a result, regardless of whether or not there were recesses before refining in the area where the photomask pattern was formed, the pattern transferred to the glass substrate matched the pattern on the photomask substrate. That is, the pattern was accurately formed even directly above the repaired recesses. No defects were observed in the transfer pattern image, and conversely, no patterns that did not exist on the photomask appeared on the transfer pattern.

[0091] Table 3 shows the results of SEM measurement of the L / S length of the transferred pattern image. It was confirmed numerically that there was no significant difference in line width between the L / S transferred pattern corresponding to the portion where the recess was repaired (the repaired recess) and the L / S transferred pattern corresponding to the portion where there was no recess originally. Therefore, it was confirmed that the quartz glass substrate recycled by the refining method can function without any problems as a photomask substrate.

[0092]

[0093] Comparative Example 1: Quartz glass substrate regenerated by a refining method without smoothing When smoothing was not performed in the refining step shown in Figure 3, that is, on a quartz glass substrate in which the inner surface of the recesses was rough and had fine cracks on the inner surface, a silicon dioxide layer was formed by plasma CVD, and the substrate surface was polished using ultrafine silicon dioxide particles with a particle diameter Φ of around several μm as an abrasive, thereby completing the regeneration.

[0094] The results of cross-sectional SEM observation of the state after the CVD silicon dioxide layer was formed are shown in Figure 10(B). It can be seen that the CVD silicon dioxide thick layer was unable to completely cover the sub-micron level microscopic irregularities and microscopic cracks on the inner surface of the recess, resulting in the presence of voids at the interface. If voids exist at the interface between the quartz glass substrate (base material) and the CVD silicon dioxide thick layer in the recess, there is a risk of light scattering.

[0095] From the above, it has been found that the method for manufacturing a photomask substrate according to this embodiment can realize semi-permanent recycling of a photomask substrate, which has not been possible with conventional methods.

Claims

1. A method for manufacturing a substrate for a photomask, comprising a layer formation step of forming a silicon dioxide layer on the first surface of a quartz glass substrate.

2. The method for manufacturing a photomask substrate according to claim 1, wherein in the layer formation step, the silicon dioxide layer is formed on the entire surface of the first surface of the quartz glass substrate.

3. A wear reduction process in which the first surface of the quartz glass substrate is scraped and worn down, A method for manufacturing a photomask substrate, comprising a layer formation step of forming a silicon dioxide layer on the first surface that has been worn down.

4. In the wear reduction step, the entire surface of the first surface is worn down. The method for manufacturing a photomask substrate according to claim 3, wherein in the layer formation step, the silicon dioxide layer is formed on the entire surface of the first surface.

5. A method for manufacturing a photomask substrate according to any one of claims 1 to 4, comprising a layer planarization step of reducing the flatness of the first surface on which the silicon dioxide layer is formed by removing the silicon dioxide layer.

6. The first surface has a recess, This includes a smoothing step to reduce the arithmetic mean height Sa of the inner surface of the recess, A method for manufacturing a photomask substrate according to any one of claims 1 to 4, wherein in the layer formation step, the silicon dioxide layer is formed in the recess after the smoothing step.

7. The first surface has a recess, The method for manufacturing a photomask substrate according to any one of claims 3 or 4, wherein the first surface is worn down in the wear reduction step so that the depth of the recess is 20 μm or less.

8. The first surface has a recess, The method for manufacturing a photomask substrate according to any one of claims 1 to 4, wherein the depth of the recess is 0.1 μm or more and 500 μm or less.

9. A method for manufacturing a photomask substrate according to any one of claims 1 to 4, wherein the layer formation step uses at least one of plasma CVD, laser ablation, sputtering, ion plating, and vacuum deposition.

10. The method for manufacturing a photomask substrate according to any one of claims 1 to 4, wherein the layer formation step involves stacking two or more types of layers.

11. The method for manufacturing a photomask substrate according to claim 5, wherein in the layer planarization step, the silicon dioxide layer is scraped off so that the arithmetic mean height Sa of the first surface on which the silicon dioxide layer is formed becomes 1 μm or less.

12. In the layer formation step, the silicon dioxide layer of the first thickness is formed. In the aforementioned layer planarization step, the silicon dioxide layer is removed by a second thickness, The method for manufacturing a photomask substrate according to claim 5, wherein the second thickness is less than or equal to the first thickness.

13. A method for manufacturing a photomask substrate according to claim 5, comprising a substrate planarization step of reducing the flatness of the first surface of the quartz glass substrate by grinding the quartz glass substrate.

14. A method for manufacturing a photomask substrate according to claim 13, comprising performing the layer planarization step on the silicon dioxide layer until the quartz glass substrate is exposed, and then performing the substrate planarization step on the exposed quartz glass substrate.

15. The method for manufacturing a photomask substrate according to claim 13, wherein in the layer planarization step and the substrate planarization step, the silicon dioxide layer and the quartz glass substrate are scraped so that the arithmetic mean height Sa of the first surface on which the silicon dioxide layer is formed becomes 1 μm or less.

16. The method for manufacturing a photomask substrate according to claim 6, wherein in the smoothing step, the arithmetic mean height Sa of the inner surface of the recess is made 50 nm or less.

17. The method for manufacturing a photomask substrate according to claim 6, wherein in the smoothing step, the inner surface of the recess is smoothed with at least one of the following solutions: an aqueous hydrofluoric acid solution, a buffered aqueous hydrofluoric acid solution, or a strong alkaline aqueous solution.

18. The method for manufacturing a photomask substrate according to claim 6, wherein in the smoothing step, the inner surface of the recess is smoothed on the quartz glass substrate by at least one of the following methods: reactive plasma etching, reactive sputtering, and ion beam sputtering.

19. The method for manufacturing a photomask substrate according to claim 6, wherein in the smoothing step, the inner surface of the recess is smoothed by irradiating the quartz glass substrate with a carbon dioxide laser having a wavelength of 9.2 μm or more and 10.8 μm or less.

20. A method for manufacturing a photomask substrate according to any one of claims 1 to 4, wherein the same processing as the first surface is performed on the second surface facing the first surface.

21. A method for manufacturing a photomask substrate according to any one of claims 1 to 4, wherein the difference between the thickness of the thickest part of the quartz glass substrate and the thickness of the photomask substrate is 0.2 mm or less.

22. The method for manufacturing a photomask substrate according to any one of claims 1 to 4, wherein the quartz glass substrate is obtained by removing the photomask pattern from a used photomask.

23. A method for manufacturing a photomask substrate according to any one of claims 1 to 4, further comprising a pattern removal step of removing a photomask pattern from a used photomask to obtain the quartz glass substrate.

24. The method for manufacturing a photomask substrate according to claim 23, further comprising a photomask recovery step for recovering a used photomask.

25. A method for manufacturing photomask blanks, comprising: forming at least one type of film, such as a light-shielding film, a halftone film, or a phase-shift film, on a photomask substrate produced by the method for manufacturing a photomask substrate according to any one of claims 1 to 4; and manufacturing a photomask blank.

26. A method for manufacturing a photomask, comprising performing fine-wire circuit processing on a photomask blank produced by the method for manufacturing a photomask blank described in claim 25, to manufacture a photomask.

27. A method for reusing a photomask substrate, which allows the thickness of the quartz glass substrate to be maintained at a predetermined thickness and enables repeated reproduction of a used photomask substrate into a new photomask substrate, by performing the manufacturing method for the photomask substrate according to any one of claims 1 to 4.

28. A photomask substrate comprising a quartz glass substrate and a silicon dioxide layer formed on a first surface of the quartz glass substrate.

29. The photomask substrate according to claim 28, wherein the quartz glass substrate has a recess on the first surface, and the silicon dioxide layer is formed in the recess.

30. A photomask substrate according to claim 28 or 29, wherein a silicon dioxide layer is formed over the entire surface of the first surface.

31. The photomask substrate according to claim 29, wherein the depth of the recess is 0.1 μm or more and 500 μm or less.

32. A photomask substrate according to any one of claims 28, 29, or 31, wherein the second surface facing the first surface has a silicon dioxide layer.

33. A photomask substrate according to any one of claims 28, 29, or 31, having a recognition portion formed on the periphery of the quartz glass substrate so as to be recognized in a desired manner.

34. The photomask substrate according to claim 33, wherein the recognized portion is one or more of the following: embedded in the quartz glass substrate, formed between layers during the mask pattern formation process, or engraved in a predetermined part of the photomask substrate.

35. A photomask blank having at least one film, a light-shielding film, a halftone film, and a phase-shift film, on a photomask substrate according to any one of claims 28, 29, or 31.

36. A photomask having a pattern formed on a photomask blank as described in claim 35.