Glass plate, disk-shaped glass, magnetic disk glass substrate, and method for manufacturing glass plate

Abstract

The glass plate of one embodiment is a rectangular plate having a thickness of less than 0.68 mm. In this glass plate, the flatness of a square measurement area of 100 mm per side, said square measurement area being cut out from the center area of the glass plate excluding the edge areas on both of the long side direction and the short side direction of the glass plate, is 30 μm or less. When the measurement area is subjected to a first heat treatment in which the measurement area is maintained at 700°C for 4 hours and then cooled from 700°C to 400°C at a speed of 50°C / hour, the heat shrinkage of the measurement area is 130 ppm or less. When the measurement area is subjected to a second heat treatment in which the measurement area is maintained at Tg-160°C [wherein Tg (°C) stands for the glass transition temperature of the glass plate] for 60 seconds and then cooled to room temperature in air, the resulted amount of change in the flatness of the measurement area is 10 μm or less.

Description

Glass plate, disc-shaped glass, glass substrate for magnetic disk, and method for manufacturing glass plate

[0001] The present invention relates to a glass substrate for a magnetic disk used in a hard disk drive device, a circular glass, a glass plate, and a method for manufacturing the glass plate.

[0002] With the recent rise of cloud computing, many hard disk drives (HDDs) are being used in cloud data centers to increase storage capacity. HDDs use magnetic disks, which have a magnetic layer formed on a circular non-magnetic glass substrate for magnetic disks, as their storage medium. In order to increase the storage capacity of HDDs, it is preferable to increase the recording density of the magnetic disks and to increase the number of thin magnetic disks installed.

[0003] To increase recording density, thermally assisted magnetic recording (HAMR) and microwave-assisted magnetic recording (MAMR) have been considered as recording methods for magnetic disks, in addition to conventional perpendicular magnetic recording. In recent years, heat treatment of magnetic films has been performed to form magnetic recording layers suitable for these recording methods. Heat treatment is performed, for example, by heating a glass substrate at high temperatures after magnetic film formation, or by forming a magnetic film while heating the glass substrate at high temperatures. In this case, the temperature of the magnetic film can reach 700°C or higher, far exceeding 600°C. Because this heat treatment heats the glass substrate along with the magnetic film, glass substrates for magnetic disks are required to have high heat resistance, i.e., a high glass transition temperature (Tg), to prevent thermal deformation.

[0004] As a manufacturing method of a glass plate that is the base material for a glass substrate for a magnetic disk, a press method, a float method, a Furcol method, a Pittsburgh method, a downdraw method, a Colburn method, a redraw method, etc. Among these, the float method, Furcol method, Pittsburgh method, a downdraw method, a Colburn method, and a redraw method are suitable for manufacturing glass substrates for flat panel displays (FPDs) such as liquid crystal displays, because they make it easier to manufacture large-sized glass plates than the press method.

[0005] FPDs use glass substrates equipped with electronic elements such as thin film transistors (TFTs). In the TFT manufacturing process, the glass substrate is heated to high temperatures, which causes thermal shrinkage and dimensional changes. Therefore, when producing glass plates using the above-mentioned methods, such as the float method, the glass plate is slowly cooled while being formed, and the conditions for the slow cooling are adjusted to reduce the residual stress in the glass plate and the thermal shrinkage rate. Furthermore, offline annealing, in which a glass plate cut to a predetermined size from a formed long glass sheet is heat-treated, is known as a method for further reducing the thermal shrinkage rate of the glass plate (Patent Document 1).

[0006] JP 2017-178711 A

[0007] It has been found that when a magnetic disk glass substrate is fabricated from a glass plate having a high glass transition temperature (Tg) produced by the above-mentioned methods such as the float process and a magnetic disk is fabricated using this glass substrate, the glass substrate thermally shrinks and deforms, causing a distortion, due to heat treatment of the magnetic film, resulting in a deterioration in the flatness of the magnetic disk. It has also been found that this deterioration in flatness occurs significantly, particularly when a thin magnetic disk glass substrate is used. Poor flatness of the magnetic disk makes it more likely to flutter in a HDD device, making it difficult to perform stable reading.

[0008] Therefore, an object of the present invention is to provide a glass substrate for a magnetic disk that can suppress deterioration of flatness due to heat treatment for forming a magnetic recording layer of the magnetic disk, and a circular glass, glass plate, and method for manufacturing the glass plate that are used for such a glass substrate for a magnetic disk.

[0009] One aspect of the present invention is a glass plate, the glass plate being a rectangular glass plate having a thickness of less than 0.68 mm, wherein a square measurement region having a side length of 100 mm cut out from a central region of the glass plate, excluding end regions from each end in a short-side direction of the glass plate on the inner side of the glass plate, the end regions being 5 to 20% of the length of the short sides of the glass plate, and end regions from each end in a long-side direction of the glass plate on the inner side of the glass plate, the end regions being 5 to 20% of the length of the long sides of the glass plate, has a flatness of 30 μm or less, and the measurement region has a thermal shrinkage of 130 ppm or less when subjected to a first heat treatment in which the measurement region is maintained at 700° C. for 4 hours and then cooled from 700° C. to 400° C. at a rate of 50° C. / hour, When the glass transition temperature of the glass plate is expressed as Tg (°C), the change in flatness of the measurement area due to a second heating treatment in which the measurement area is maintained at Tg-160°C for 60 seconds and then cooled to room temperature in the atmosphere is 10 μm or less.

[0010] It is preferable that the short side length exceeds 900 mm.

[0011] The glass plate is preferably a portion cut from a long glass sheet formed using any one of the float process, the Frucol process, the Pittsburgh process, the downdraw process, the Colburn process, and the redraw process.

[0012] It is preferable that the difference between the thermal shrinkage amount S1 of the measurement area in the in-plane direction in which the thermal shrinkage rate is the smallest and the thermal shrinkage amount S2 of the measurement area in the direction in which the thermal shrinkage rate is the largest is 1.0 μm or more.

[0013] The glass plate has been annealed to reduce the thermal shrinkage rate, and the glass plate before the annealing treatment has anisotropy in the thermal shrinkage rate, in which the magnitude of the thermal shrinkage rate varies depending on the in-plane direction of the region of the glass plate corresponding to the measured region, and the difference between the amount of thermal shrinkage S1 of the region in the in-plane direction in which the thermal shrinkage rate is minimum and the amount of thermal shrinkage S2 of the region in the in-plane direction in which the thermal shrinkage rate is maximum may be greater than 1.0 μm.

[0014] Another aspect of the present invention is a glass plate, characterized in that the glass plate is a rectangular glass plate having a thickness of less than 0.68 mm, a flatness of 30 μm or less, and two orthogonal sides of 95 to 120 mm each, wherein the glass plate exhibits a thermal shrinkage of 130 ppm or less when subjected to a first heat treatment in which the glass plate is maintained at 700°C for 4 hours and then cooled from 700°C to 400°C at a rate of 50°C / hour, and wherein the change in flatness associated with a second heat treatment in which the glass plate is maintained at Tg-160°C for 60 seconds and then cooled to room temperature in the atmosphere, where Tg is the glass transition temperature of the glass plate (°C), is 10 μm or less.

[0015] It is preferable that the rectangular glass plate is a raw plate that will be used to produce a disk-shaped glass plate having a circular periphery, and that the area of ​​a main surface of the rectangular glass plate is 1.6 times or less the area of ​​the inside of the periphery of the disk-shaped glass plate.

[0016] Another aspect of the present invention is a disk-shaped glass, which has a thickness of less than 0.68 mm, a flatness of 30 μm or less, and a circular outer periphery with a diameter of 95 to 100 mm, characterized in that the disk-shaped glass exhibits a thermal shrinkage of 130 ppm or less when subjected to a first heat treatment in which the glass is maintained at 700°C for 4 hours and then cooled from 700°C to 400°C at a rate of 50°C / hour, and the change in flatness associated with a second heat treatment in which the glass is maintained at Tg-160°C for 60 seconds and then cooled to room temperature in the atmosphere, where Tg (°C) is the glass transition temperature of the disk-shaped glass, is 10 μm or less.

[0017] Another aspect of the present invention is a magnetic disk glass substrate, the magnetic disk glass substrate having a thickness of less than 0.68 mm, a flatness of 30 μm or less, and a diameter of 95 to 100 mm, characterized in that the magnetic disk glass substrate exhibits a thermal shrinkage of 130 ppm or less when subjected to a first heat treatment in which the substrate is maintained at 700°C for 4 hours and then cooled from 700°C to 400°C at a rate of 50°C / hour, and the change in flatness associated with a second heat treatment in which the substrate is maintained at Tg-160°C for 60 seconds and then cooled to room temperature in the atmosphere, where Tg (°C) is the glass transition temperature of the magnetic disk glass substrate, is 10 μm or less.

[0018] It is preferable that the amount of change in circularity due to the first heat treatment is 0.5 μm or less.

[0019] Another aspect of the present invention is a method for manufacturing a glass plate, the method comprising the step of subjecting a glass plate material that serves as the glass plate to an annealing heat treatment, wherein the glass plate is a rectangular plate having a thickness of less than 0.68 mm, wherein a square measurement region having a side length of 100 mm cut out from a central region of the glass plate, excluding end regions from each end in a short-side direction of the glass plate on the inner side of the glass plate, the end regions being 5 to 20% of the length of the short sides of the glass plate, and end regions from each end in a long-side direction of the glass plate on the inner side of the glass plate, the end regions being 5 to 20% of the length of the long sides of the glass plate, has a flatness of 30 μm or less, and a thermal shrinkage of the measurement region is 130 ppm or less when the measurement region is subjected to a first heat treatment in which the measurement region is maintained at 700° C. for 4 hours and then cooled from 700° C. to 400° C. at a rate of 50° C. / hour, When the glass transition temperature of the glass plate is expressed as Tg (°C), the change in flatness of the measurement area due to a second heating treatment in which the measurement area is maintained at Tg-160°C for 60 seconds and then cooled to room temperature in the atmosphere is 10 μm or less.

[0020] Another aspect of the present invention is a method for manufacturing a glass plate, comprising the steps of: annealing a glass plate material that serves as the glass plate; and removing the glass plate from the annealed glass plate material, wherein the glass plate is a rectangular plate having a thickness of less than 0.68 mm, a flatness of 30 μm or less, and two orthogonal sides each having a length of 95 to 120 mm, and wherein the glass plate has a thermal shrinkage of 130 ppm or less when subjected to a first heat treatment in which the glass plate is maintained at 700°C for 4 hours and then cooled from 700°C to 400°C at a rate of 50°C / hour, and wherein the change in flatness of the glass plate in a second heat treatment in which the glass plate is maintained at Tg-160°C for 60 seconds and then cooled to room temperature in the atmosphere is 10 μm or less, where Tg (°C) is the glass transition temperature of the glass plate.

[0021] It is preferable that the rectangular glass plate is a square raw plate that will be used to produce a disk-shaped glass plate having a circular periphery, and that the area of ​​a main surface of the rectangular glass plate is 1.6 times or less the area of ​​the inside of the periphery of the disk-shaped glass plate.

[0022] Another aspect of the present invention is a method for manufacturing a glass plate, the method comprising the steps of: annealing a glass plate material that is to become the disk-shaped glass; and removing the disk-shaped glass from the annealed glass plate material, wherein the glass plate material is a rectangular glass plate, and the disk-shaped glass has a thickness of less than 0.68 mm, a flatness of 30 μm or less, and a diameter of 95 to 100 mm, and exhibits a thermal shrinkage of 130 ppm or less when subjected to a first heat treatment in which the disk-shaped glass is maintained at 700°C for 4 hours and then cooled from 700°C to 400°C at a rate of 50°C / hour, and exhibits a change in flatness of 10 μm or less when subjected to a second heat treatment in which the disk-shaped glass is maintained at Tg-160°C for 60 seconds and then cooled to room temperature in the atmosphere, where Tg (°C) is the glass transition temperature of the disk-shaped glass.

[0023] It is preferable that the rectangular glass plate is square, and the area of ​​the main surface of the rectangular glass plate is 1.6 times or less the area of ​​the inside of the outer periphery of the disk-shaped glass.

[0024] The above-mentioned magnetic disk glass substrate can suppress deterioration of flatness due to heat treatment for forming a magnetic recording layer of a magnetic disk. Furthermore, the above-mentioned glass plate and disk-shaped glass can provide such a magnetic disk glass substrate. Furthermore, the above-mentioned glass plate manufacturing method can provide the above-mentioned glass plate.

[0025] 1A is an external view of a glass plate (large glass plate) according to one embodiment, and FIG. 1B is a plan view illustrating a measurement area of ​​the glass plate. 1A is an external view of a glass plate (segmented glass) according to one embodiment, and FIG. 1B is a plan view of the glass plate showing a portion that will become a disk-shaped glass. 1B is an external view of a disk-shaped glass according to one embodiment. 1C is an external view of a glass substrate for a magnetic disk according to one embodiment.

[0026] Hereinafter, a glass plate, a method for manufacturing a glass plate, a glass disk, and a glass substrate for a magnetic disk according to one embodiment will be described in detail.

[0027] (Large Glass Plate) Fig. 1(a) shows an external view of a glass plate 10 according to one embodiment. Fig. 1(b) shows a plan view illustrating a measurement region 13 of the glass plate 10, which will be described later.

[0028] The glass plate 10 is a rectangular plate having a thickness of less than 0.68 mm.

[0029] When the thickness of the glass plate 10 is less than 0.68 mm, the thickness of a magnetic disk glass substrate (hereinafter also referred to as a glass substrate) made from the glass plate 10 can be reduced when the magnetic disk is made into a magnetic disk, thereby increasing the number of magnetic disks that can be mounted in an HDD device. The thickness of the glass plate 10 is preferably less than 0.61 mm, more preferably less than 0.58 mm. The lower limit of the thickness of the glass plate 10 is not particularly limited, but is, for example, 0.2 mm.

[0030] The short side length of the glass plate 10 preferably exceeds 900 mm. This allows many magnetic disk glass substrates to be produced from the glass plate 10, thereby reducing the manufacturing cost of magnetic disk glass substrates. The long side length of the glass plate 10 may be longer than the short side length, as in the example shown in FIG. 1 , or may be equal to the short side length. That is, the glass plate 10 is rectangular or square. When the glass plate 10 has a short side and a long side, the ratio of the long side length to the short side length (long side length / short side length) is preferably 1.2 or less. By performing the precision annealing process described below on a glass plate material that is the basis for the glass plate 10 having the ratio of 1.2 or less, it is possible to reduce the thermal shrinkage rate and the anisotropy of the thermal shrinkage rate (described below). This is thought to be because a workpiece that is closer to a square is less likely to experience differences in thermal history due to differences in position within the workpiece surface during annealing. In this specification, such glass plates having a short side length of more than 900 mm may be referred to as "large glass sheets." The long side length of the glass plate 10 is preferably 2000 mm or less. If the long side length exceeds 2000 mm, it may be difficult to maintain a uniform temperature inside the furnace during precision annealing, which will be described later.

[0031] The flatness of the measurement area 13 cut out from the central region of the glass plate 10 is 30 μm or less. When the flatness of the measurement area 13 is 30 μm or less, the amount of grinding or polishing required when manufacturing a magnetic disk glass substrate from the glass plate 10 is reduced, allowing for the manufacture of magnetic disk glass substrates with a high yield. Furthermore, when the flatness of the measurement area 13 is 30 μm or less, fluttering is less likely to occur when a magnetic disk manufactured using the glass plate 10 as the base material is rotated at high speed, allowing for stable reading by the head of the reading unit of the HDD device. In particular, when the thickness of the magnetic disk is thin, the rigidity of the glass substrate is low, which may cause deflection that causes fluttering. However, by having a low flatness of the glass plate 10, fluttering can be suppressed even when the plate thickness is thin. In this specification, flatness refers to flatness in accordance with JIS B0621-1984. The flatness can be measured, for example, using an interferometric flatness measuring device and phase measurement interferometry (phase shift method) at a predetermined measurement wavelength (e.g., 680 nm). The flatness of the measurement area 13 is preferably 20 μm or less, more preferably 10 μm or less. Note that the above flatness refers to the flatness of the measurement area 13 before the first or second heat treatment described below is performed. The same applies hereinafter unless otherwise specified.

[0032] The central region 12 of the glass plate 10 refers to a region of the glass plate 10 excluding end regions 11a each having a length Le of 5 to 20% of the length L of the short side 10a of the glass plate 10 from each end in the short side direction of the glass plate 10 on the inner side of the glass plate 10, or end regions 11b each having a length We of 5 to 20% of the length W of the long side 10b of the glass plate on the inner side of the glass plate 10 from each end in the long side direction of the glass plate 10. The length of the central region 12 in the short side direction is Lc, and the length in the long side direction is Wc.

[0033] The measurement area 13 is a square area with sides of 100 mm cut out from the central area 12 of the glass plate 10. The measurement area 13 does not have to be cut out as shown in FIG. 1( b) and can be cut out arbitrarily from the central area 12. The size and shape of the measurement area 13 are close to the size of the glass plate (described later) that will be used to make the magnetic disk glass substrate (the "singulated glass" described later).

[0034] According to the inventor's investigations, it has been found that, when the glass transition temperature of the glass plate 10 is expressed in terms of Tg (°C), the thermal shrinkage of the measurement region 13 is 130 ppm or less when a first heat treatment is performed in which the measurement region 13 is maintained at 700°C for 4 hours and then cooled from 700°C to 400°C at a rate of 50°C / hour, and the change in flatness of the measurement region 13 is 10 μm or less when a second heat treatment is performed in which the measurement region 13 is maintained at Tg-160°C for 60 seconds and then cooled to room temperature in the atmosphere. This results in the following effects: When a magnetic film is heat-treated on a magnetic disk glass substrate made from a glass plate 10 having a thickness of less than 0.68 mm and a flatness of the measurement region 13 of 30 μm or less, thermal shrinkage of the glass substrate is suppressed, thereby suppressing deformation such as bending of the glass substrate due to thermal shrinkage, and as a result, it has been found that deterioration of the flatness of the glass substrate can be suppressed. By achieving this effect, deterioration of the flatness of the glass plate 10 of 30 μm or less is suppressed in the glass substrate after heat treatment of the magnetic film, and fluttering is suppressed when the glass plate 10 is made into a magnetic disk and rotated at high speed.

[0035] For the above reasons, the glass plate 10 of this embodiment has a thermal shrinkage of 130 ppm or less in the measurement area 13 when subjected to a first heat treatment in which the measurement area 13 is maintained at 700°C for 4 hours and then cooled from 700°C to 400°C at a rate of 50°C / hour, and a change in flatness of 10 μm or less in the measurement area 13 when subjected to a second heat treatment in which the measurement area 13 is maintained at Tg-160°C for 60 seconds and then cooled to room temperature in the atmosphere. If the thermal shrinkage of the measurement area 13 when subjected to the first heat treatment exceeds 130 ppm and the change in flatness of the measurement area 13 when subjected to the second heat treatment exceeds 10 μm, deformation of the glass substrate, such as bending during thermal shrinkage, cannot be suppressed, and the flatness of the glass plate deteriorates. Even slight thermal shrinkage can significantly impair a high flatness of 30 μm or less. The conditions for the first and second heat treatments are determined with reference to the processing conditions when performing heat treatment on the magnetic film. The conditions for the first heat treatment are determined from the viewpoint of temperature conditions that allow evaluation of the thermal shrinkage rate during high-temperature, long-term heating, which is presumed to be related to the deterioration of the flatness of the glass substrate during heat treatment of the magnetic film at 600° C. or higher. The conditions for the second heat treatment are determined from the viewpoint of temperature conditions that allow direct evaluation of the amount of deterioration of the flatness of the glass substrate during heat treatment of the magnetic film at 600° C. or higher. This is because L1 is considered to be optimal for energy-assisted magnetic recording (EAMR) such as thermally assisted magnetic recording (HAMR) and microwave-assisted magnetic recording (MAMR). ０ This is because the temperature required to form the magnetic film of this structure can far exceed 600°C and reach 700°C or higher. In this specification, unless otherwise specified, the thermal shrinkage refers to the thermal shrinkage before and after the first heat treatment, and refers to the maximum value of the thermal shrinkage measured in 25 directions parallel to the main surface of the object, passing through the center of the object, and changing in circumferential directions by 7.2 degrees. Note that measuring the thermal shrinkage using the above method makes it possible to measure the thermal shrinkage in all directions (360 degrees), thereby enabling more accurate evaluation of the thermal shrinkage than conventional methods.

[0036] The heating (heating), temperature maintenance, and cooling (cooling) in the first heat treatment are preferably performed continuously in an atmosphere with the same conditions except for the temperature conditions (e.g., in the atmosphere of a single annealing furnace). The temperature increase in the first heat treatment is preferably performed from room temperature (normal temperature) over 2.5 hours. In other words, the substrate to be treated is preferably heated from room temperature to 700°C at a rate of 270°C / hour. The temperature decrease is preferably performed from 700°C to room temperature at a rate of 50°C / hour. In addition, in order to avoid a significant deterioration in the flatness of the substrate to be treated (which may be the measurement area 13 of the glass plate 10, or the glass plate 20, disc-shaped glass 30, or magnetic disk glass substrate 40 described below), it is preferable to perform the first heat treatment by placing the substrate to be treated flat using two setters (described below) sandwiching it from above and below. Avoiding a significant deterioration in flatness enables accurate measurement of the thermal shrinkage. In this case, the size of the setter should be equal to or larger than the substrate to be treated. The thickness of the setter placed on top of the substrate to be treated may be set to a weight that does not interfere with the thermal shrinkage of the substrate to be treated and that allows the flatness to be maintained substantially (for example, maintaining a flatness of 30 μm or less). It goes without saying that a setter with a weight that reduces the thickness of the substrate to be treated is inappropriate. Maintaining the flatness of the substrate to be treated substantially allows the evaluation of the thermal shrinkage rate due to the first heat treatment and the evaluation of the flatness change due to the second heat treatment, which will be described later, to be performed using the same substrate to be treated. The evaluation of the thermal shrinkage rate due to the first heat treatment and the evaluation of the flatness change due to the second heat treatment, which will be described later, may be performed using different substrates to be treated.

[0037] With respect to the second heat treatment, cooling in the atmosphere means allowing the substrate to cool in an atmosphere at room temperature without temperature adjustment to control the cooling rate. The room temperature is, for example, 25°C. The heating and temperature maintenance of the substrate in the second heat treatment are performed, for example, in an atmosphere between heaters in a heating device equipped with two panel-shaped heaters, with the substrate held by a substrate holder. This heating device is modeled after a substrate heating chamber installed in a known single-wafer vacuum film deposition device used for depositing magnetic films on magnetic disks, etc. The substrate is set vertically relative to the ground in a known substrate holder (also called a carrier) for film deposition. Three or four L-shaped leaf spring support members are fixed to the substrate holder (e.g., the substrate holder described in paragraph 0045 and FIG. 4 of JP 2011-117019 A). The tips of the support members are pressed against the outer peripheral edge of the substrate, thereby securing the substrate to the substrate holder by the elasticity of the leaf springs. It can hold not only circular but also rectangular and other shaped substrates in the same way. By adjusting the specifications of the substrate holder, substrates of various shapes can be heated in the same way. It should be noted that the elastic force of the leaf spring from the support member is always applied to the substrate in a direction that bends the substrate, making it more likely to bend than when heated without a support member (for example, when placed flat).

[0038] The thermal shrinkage rate can be determined, for example, by measuring the change in length in the measurement area 13 before and after the heat treatment and calculating it according to the following formula: C (thermal shrinkage rate) = (L ０ -L) / L ０ Here, L ０ is the length before heat treatment, and L is the length after heat treatment. The sign of C is positive if the material shrinks due to heat treatment, and negative if it expands. L ０ and L can be obtained by, for example, making two markings on the surface of the cut-out measurement area 13 and measuring the distance between the two markings before and after the heat treatment. ０and L may be the lengths of the measurement area 13 before and after the heat treatment. These lengths are preferably lengths passing through the center of the measurement area 13. When the measurement object is a disk-shaped glass or a glass substrate for a magnetic disk, the diameter may be used. Furthermore, when evaluating the anisotropy of the thermal shrinkage, for example, lengths (e.g., diameter) in 25 directions that are changed in 7.2 degree increments in the circumferential direction from the center of the measurement object can be used. The thermal shrinkage in 25 directions is measured, and the absolute value of the difference in the thermal shrinkage obtained by subtracting the minimum value from the maximum value can be used as an index of the anisotropy of the thermal shrinkage. Note that the amount of thermal shrinkage (S) may be used instead of the thermal shrinkage (C) to determine the absolute value of the difference between the maximum and minimum values ​​of the amount of thermal shrinkage in the above 25 directions, and this may be used as an index of the anisotropy. S (amount of thermal shrinkage) is calculated by S = (L ０ -L).

[0039] The thermal shrinkage of the measurement area 13 after the first heat treatment is preferably 90 ppm or less, more preferably 50 ppm or less. The change in flatness of the measurement area 13 after the second heat treatment is preferably 7.5 μm or less, more preferably 5 μm or less.

[0040] The glass plate 10 is preferably a portion cut from a long glass sheet formed using any one of the float process, the Fourcol process, the Pittsburgh process, the downdraw process, the Colburn process, and the redraw process. Large-sized glass plates 10 can be obtained from glass sheets formed by these methods, allowing a large amount of individualized glass, which is the basis for magnetic disk glass substrates, to be obtained from the glass plate 10, thereby reducing the manufacturing cost of magnetic disk glass substrates. Furthermore, these methods are advantageous for forming glass sheets with high glass transition temperatures (Tg), thereby reducing the manufacturing cost of glass plates 10 with high glass transition temperatures (Tg). Specific examples of downdraw processes include the slot downdraw process and the overflow downdraw process. Furthermore, it is preferable that at least one main surface of the glass plate 10 is a fire-polished surface. This allows for the omission of some of the grinding and polishing processes of the main surfaces of the substrate, which are generally required when manufacturing magnetic disk glass substrates, and for the reduction of the machining allowance. In other words, it is preferable that at least one of the main surfaces of the glass plate 10 is an unground surface and / or an unpolished surface.

[0041] In a glass sheet obtained by the above-mentioned methods such as the float method, the thickness of both ends in the width direction of the glass sheet, which is orthogonal to the longitudinal direction of the glass sheet (the direction in which the glass flows out of the melting furnace), is usually thicker than that of the central part in the width direction, and therefore the glass plate material that will become the glass plate 10 is cut out from the remaining part of the glass sheet after both ends in the width direction of the glass sheet are cut off. Usually, the glass plate material that will become the glass plate 10 is cut out so that the width direction of the glass sheet coincides with the short side direction or long side direction of the glass plate 10.

[0042] The glass plate 10 may have anisotropy in thermal shrinkage. Thermal shrinkage anisotropy refers to the characteristic that the magnitude of the thermal shrinkage varies depending on various directions within the plane of the main surface of the measurement area 13. According to the inventor's research, if the glass plate 10 has anisotropy in thermal shrinkage, the circularity (JIS B0621-1984) of the glass substrate for a magnetic disk obtained from the glass plate 10 may be deteriorated when the magnetic film is heat-treated. In particular, when the glass plate material from which the glass plate 10 is made is a portion cut out from a glass sheet formed using the above-mentioned method, such as the float method, differences in the thermal shrinkage are likely to occur depending on the in-plane direction of the glass sheet, and anisotropy in the thermal shrinkage is likely to occur. Furthermore, the inventor also found that the direction in which the thermal shrinkage is maximum and the direction in which the thermal shrinkage is minimum are not necessarily perpendicular to each other by 90 degrees within the in-plane direction of the main surface. That is, in the past, when evaluating the anisotropy of thermal shrinkage, the thermal shrinkage rates were measured in two directions: the longitudinal direction of the glass sheet and the width direction of the glass sheet perpendicular to the longitudinal direction, and the difference between them was used as an index of anisotropy. However, it has been found that this method sometimes fails to accurately evaluate the maximum and minimum values ​​of thermal shrinkage, or the difference between them. The reason for this is not entirely clear, but it is generally believed that in methods for continuously producing long glass sheets, such as the float process and down-draw process, glass flowing out of a melting furnace or softening furnace is stretched in the width direction while being pulled in the flow direction, and thus is formed into a glass sheet, and is therefore pulled in an oblique direction that combines the two perpendicular directions. Furthermore, the pulling direction varies depending on the forming conditions and the position within the glass sheet, and also changes over time. In addition, the thermal history varies depending on the position, so it is thought that the direction of maximum and minimum thermal shrinkage, the magnitude of the thermal shrinkage, and other factors change in various ways. Therefore, to accurately evaluate anisotropy, it is necessary to cut out the desired glass from a long glass sheet and examine it in all directions.

[0043] As described above, if the circularity of the outer periphery of a magnetic disk glass substrate deteriorates, the magnetic disk will wobble when rotated at high speed, making fluttering more likely to occur. Therefore, it is very important to accurately grasp the thermal shrinkage values ​​in the directions where the thermal shrinkage is maximum and minimum, and the difference between them, for the magnetic disk glass substrate, the disk-shaped glass substrate from which the disk-shaped glass is made, and even the glass plate from which the disk-shaped glass is made. From the viewpoint of suppressing such deterioration of the circularity of the glass substrate, it is preferable that the difference (absolute value) between the thermal shrinkage C1 in the direction where the thermal shrinkage is minimum and the thermal shrinkage C2 in the direction where the thermal shrinkage is maximum, among the in-plane directions of the measurement region 13, be 10 ppm or less. Furthermore, it is preferable that the difference (absolute value) between the thermal shrinkage S1 of the measurement region in the direction where the thermal shrinkage is minimum and the thermal shrinkage S2 of the measurement region in the direction where the thermal shrinkage is maximum be 1.0 μm or less.

[0044] The glass plate 10 is preferably subjected to an annealing treatment (e.g., "precision annealing" described below) to reduce the thermal shrinkage. The glass plate (the glass plate material from which the glass plate 10 is made) before the annealing treatment in this case has anisotropy in thermal shrinkage, in which the magnitude of the thermal shrinkage varies depending on the in-plane direction of the region of the glass plate corresponding to the measurement region 13. The difference (absolute value) between the thermal shrinkage C1 in the in-plane direction where the thermal shrinkage is smallest and the thermal shrinkage C2 in the in-plane direction where the thermal shrinkage is largest may be greater than 10 ppm. Furthermore, the difference (absolute value) between the thermal shrinkage S1 of the region in the direction where the thermal shrinkage is smallest and the thermal shrinkage S2 of the region in the direction where the thermal shrinkage is largest may be greater than 1.0 μm. Even when such anisotropy in thermal shrinkage exists, the annealed glass plate 10 of this case satisfies the above-described ranges of change in thermal shrinkage and flatness. Therefore, when a magnetic film is heat-treated in a magnetic disk glass substrate obtained from the glass plate 10, deterioration in flatness and roundness is suppressed. The change (deterioration) in the circularity is preferably 0.5 μm or less, and more preferably 0.2 μm or less.

[0045] According to one embodiment, it is preferable that a square measurement region having sides of 100 mm cut out from the entire glass plate 10 including the edge regions 11 a and 11 b of the glass plate 10 has flatness, thermal shrinkage, and a change in flatness that satisfy the above-mentioned ranges, similar to the measurement region 13. More magnetic-disk glass substrates can be produced from such a glass plate 10 than when magnetic-disk glass substrates are produced from the central region 12.

[0046] The glass plate 10 is preferably made of a material such as aluminosilicate glass, soda-lime glass, soda-aluminosilicate glass, aluminoborosilicate glass, or borosilicate glass.

[0047] The glass transition temperature (Tg) of the glass plate 10 is preferably 750°C or higher, more preferably 770°C or higher. A magnetic disk glass substrate made from a glass plate 10 with such a high glass transition temperature (Tg) is less likely to deform at high temperatures, and is therefore highly effective in preventing deterioration of flatness when the magnetic film is heat-treated, for example, at 700°C. The upper limit of the glass transition temperature (Tg) of the glass plate 10 does not need to be particularly set, but is preferably 850°C or lower. If the glass transition temperature (Tg) exceeds 850°C, it may be difficult to form a thin sheet glass. Furthermore, the Young's modulus of the glass plate 10 is preferably 80 GPa or higher. If the Young's modulus is less than 80 GPa, warping due to elastic stress from the support member for holding the substrate may occur when the magnetic film is heat-treated, for example, at 700°C. This, combined with the warping due to the heat treatment, may significantly deteriorate the flatness. If the flatness deteriorates too much, problems such as the substrate falling off the holder during film formation may occur. The average linear expansion coefficient of the glass plate 10 at 100 to 300°C is 45 × 10 －７ / °C or less. －７ If the temperature exceeds 2.65 g / cm 3 / °C, the risk of cracking the substrate may increase when the substrate is rapidly heated or cooled to improve productivity. ３ Preferably, it is 2.60 g / cm or less. ３If the density is too high, the weight increases when the glass substrate is used for a magnetic disk, and the power consumption of the HDD tends to increase.

[0048] (Glass Plate Manufacturing Method) The glass plate 10 described above can be manufactured by a glass plate manufacturing method including an annealing treatment in which a glass plate material that is the base of the glass plate 10 is heated under predetermined conditions. In the following description, the annealing treatment of the glass plate material performed under the predetermined conditions is referred to as "precision annealing." The precision annealing is performed on the glass plate material so that the measurement area 13 of the glass plate 10 satisfies the above-mentioned ranges of the thermal shrinkage rate and the amount of change in flatness.

[0049] According to the inventor's research, as described above, when a small piece of glass is extracted from a conventional glass plate manufactured by the float method or other methods to prepare a glass substrate for a magnetic disk and then heat-treated to form a magnetic film serving as a magnetic recording layer, the glass substrate thermally shrinks and deforms, resulting in a deterioration in the flatness of the magnetic disk. Because glass sheets formed by the float method or other methods are stretched in various directions and rapidly cooled while maintaining a large area, it is difficult to maintain constant stress, temperature, and thermal history across the entire glass sheet, making it difficult to uniformly reduce the thermal shrinkage across the entire glass sheet. This is thought to result in variations in the thermal shrinkage rate due to differences in the in-plane position of the glass sheet. Therefore, when a portion is extracted from the glass sheet and subsequently heated, the amount of thermal shrinkage may be large. That is, when a glass sheet is used as a glass substrate for an FPD, it is used in a large area state, so a single thermal shrinkage value measured across the entire glass sheet is sufficient as long as it is within an acceptable range, and there is no need to consider in-plane variations in the thermal shrinkage. However, because magnetic disk glass substrates are much smaller than FPD glass substrates, it has been found that magnetic disk glass substrates with large thermal shrinkage rates can be produced due to in-plane variations in the thermal shrinkage. In addition, the heat treatment temperature for magnetic films formed on magnetic disk glass substrates has been increasing in recent years, sometimes reaching, for example, 700°C or higher. This temperature is close to the glass transition temperature (Tg) of high-heat-resistant glass substrates. Because the conditions for heat treatment of such magnetic films are much stricter than the conditions (e.g., 350-600°C) under which glass substrates are heated when forming TFTs on FPD glass substrates, it has been found that even a slight thermal shrinkage that is not a problem for FPD glass substrates due to slow cooling during molding can have a significant adverse effect when the magnetic film is heat-treated. That is, it has become clear that the heat treatment of the magnetic film causes the glass substrate to undergo large thermal shrinkage and deformation such that the glass substrate bends during the thermal shrinkage.The inventors have discovered that by performing the above-described precision annealing on the glass plate material that is the basis for the glass plate 10, it is possible to precisely remove the glass plate 10 while maintaining the flatness of the glass plate 10 at a predetermined value or less, so as to prevent in-plane variation in the thermal shrinkage rate; in other words, it is possible to obtain a glass plate 10 in which the above-described changes in the thermal shrinkage rate and flatness of the measurement area 13 are within a predetermined range.

[0050] Furthermore, as the inventors continued their research, they found that, in order to address the above-mentioned problem of the glass substrate deforming so as to bend while thermally shrinking when the magnetic film is heat-treated, the following problem occurs even when a conventionally known annealing process, such as offline annealing, is performed: When a conventional annealing process is performed on a glass plate material that is the source of a large glass sheet, the effects of the annealing (such as the effect of reducing the thermal shrinkage rate) are not uniformly distributed throughout the entire surface of the glass plate material. As a result, even if the thermal shrinkage rate of the large glass sheet as a whole when subjected to the first heat treatment described above is equal to or less than a predetermined value, when multiple rectangular glass sheets, each measuring 95 to 120 mm on a side, are cut out (singlected) from the large glass sheet, some of the individual glass sheets will not have a thermal shrinkage rate equal to or less than the predetermined value, or will warp when the magnetic film is heat-treated to form a glass substrate for a magnetic disk, resulting in variations in the properties of the individual glass sheets. In particular, when the glass plate material from which the glass plate 10 is derived is cut from a glass sheet formed using the above-mentioned methods, such as the float process or downdraw process, it has been found that the above-mentioned variations may occur in magnetic disk glass substrates cut from the central region of the glass sheet, excluding the edge regions near each side. Since it is not possible to directly measure the annealing effect of a narrower region within the central region of a large glass plate, even if a region with a low annealing effect exists, this cannot be detected. The inventors investigated the cause of such variations, even when a typical annealing treatment is performed, and inferred that the main influence is a slight difference in thermal history between the peripheral portion and the central portion of the glass plate during the annealing treatment. Furthermore, they found that the thermal shrinkage rate of the central portion (central region) of a large glass plate cannot be accurately determined unless the desired portion is cut out, as long as the central portion is connected to the peripheral portion (edge ​​region) surrounding it, the peripheral portion may restrict the movement of the central portion, preventing thermal shrinkage of the central portion, or the central portion may shrink excessively due to the thermal shrinkage of the peripheral portion.The inventors have found that glass plate 10 obtained by the above-described precision annealing eliminates variations in the annealing effect, and that variations in the properties of individual pieces of glass (singulated glass) taken out from central region 12 are suppressed. Therefore, as described above, measurement region 13 is cut out from central region 12 of glass plate 10. The sizes of edge regions 11a, 11b, which determine the size of central region 12, are determined from the perspective of suppressing variations in the amount of change in thermal shrinkage and flatness described above among the plurality of individual pieces of glass.

[0051] For the above reasons, precision annealing is performed on the glass plate 10 so that the measurement area 13 of the glass plate 10 satisfies the above-mentioned ranges of thermal shrinkage and flatness change. Precision annealing is preferably performed by heat treatment at Tg-110°C or higher for 4 hours or more, more preferably at Tg-80°C or higher for 4 hours or more. The conditions for such heat treatment are determined with reference to the treatment conditions used when heat treating a magnetic film. The heating (heating), temperature maintenance, and cooling (temperature reduction) in precision annealing are preferably performed continuously in an atmosphere with the same conditions except for the temperature conditions (e.g., the atmosphere in a single annealing furnace). The temperature increase in precision annealing is preferably performed from room temperature (normal temperature) over 2.5 hours. In other words, the glass plate is preferably heated from room temperature to a temperature of preferably Tg-110°C or higher, more preferably Tg-80°C or higher, at a rate of 270°C / hour. The temperature is preferably lowered from a temperature of Tg-110° C. or higher, more preferably from a temperature of Tg-80° C. or higher, to room temperature at a rate of 50° C. / hour.

[0052] The precision annealing is preferably performed using a plate material for annealing treatment (hereinafter referred to as a setter) described below, which allows for efficient production of a glass plate 10 that satisfies the above-mentioned ranges of flatness, thermal shrinkage, and flatness change.

[0053] The setter has a plate-like shape with a pair of main surfaces, and at least one surface is configured to contact a main surface of the glass plate material that will become the glass sheet 10. The main surface of the setter is wider than the main surface of the glass plate material that will become the glass sheet 10, and is large enough to protrude from the entire periphery of the glass plate material. The protruding length is, for example, 5 centimeters or more in the direction away from the center of the glass plate material.

[0054] In order to efficiently obtain a glass plate 10 having a flatness of 30 μm or less, the flatness of the setter is preferably less than 30 μm, more preferably 20 μm or less, and even more preferably 10 μm or less.

[0055] The thermal conductivity of the setter is, for example, 1 to 200 W / (m·K) at 20° C. If the thermal conductivity of the setter is within the above range, when precision annealing is performed on the glass plate material that is the base of the glass plate 10, the glass plate material is likely to be heated and cooled uniformly, and variation in the magnitude of the thermal shrinkage rate after precision annealing depending on the in-plane position of the glass plate 10 can be effectively suppressed.

[0056] Examples of materials for the setter include alumina (Al2O3), silicon carbide (SiC), silicon nitride (Si3N4), zirconia (ZrO2), sialon (Si3N4.Al2O3), steatite, spinel, cordierite, etc. Among these, alumina (Al2O3) and silicon carbide (SiC) are preferably used.

[0057] An example of precision annealing using a setter is a method using two setters and a heat insulating material. According to one embodiment, precision annealing is preferably performed in a state in which a glass plate is sandwiched between two setters having main surfaces larger than the glass plate and surrounded by a heat insulating material disposed in the gap between the setters. The heat insulating material is preferably made of a fibrous material having high heat resistance. As the fibrous material, in addition to rock wool, as described below, inorganic fibers such as ceramic fiber and glass fiber are preferably used. In this example, precision annealing is performed in a state in which the setters and the glass plate are stacked so that one glass plate is sandwiched between two setters having an area larger than the glass plate, and the gap between the two setters adjacent to the side (edge) of the glass plate is further filled with highly heat-resistant rock wool. Here, the two setters have the same shape and are arranged to overlap each other almost perfectly in the thickness direction while sandwiching the glass plate (i.e., without in-plane misalignment). The outer periphery of the setter extends approximately evenly from the glass plate around its entire periphery, leaving a gap between the two setters near the entire edge of the glass plate. Lightly filling the gap with highly heat-resistant rock wool ensures that the entire glass plate is covered with the setter and rock wool, and that the setter's load is applied appropriately and evenly to the main surface of the glass plate. This precision annealing method imposes a lighter weight on the glass plate than precision annealing a laminate formed by alternating multiple setters and multiple glass plates. This prevents the weight of the setter and glass plate from interfering with the in-plane expansion and contraction of the underlying glass plate, thereby contributing to a reduced thermal shrinkage rate regardless of the in-plane position of the glass plate 10. Rock wool's insulating and breathable properties allow it to effectively seal the gap between the setters. As a result, the entire glass sheet can be heated or cooled evenly (heat uniformity), and the thermal shrinkage rate can be reduced regardless of the in-plane position of the glass sheet. Furthermore, the rock wool is used to an extent that it does not interfere with the load applied to the glass sheet by a setter placed on the glass sheet.Therefore, since the load of the setter is applied moderately and evenly to the main surfaces of the glass sheet, deterioration of the flatness of the glass sheet during the precision annealing process can be suppressed, and in some cases, the flatness can be reduced. In other words, the above method can reduce the effect of reducing the flatness of the glass sheet by suppressing the effect of the weight of the setter on the glass sheet, thereby reducing the thermal shrinkage rate of the glass sheet 10 regardless of the position in the plane. The precision annealing described above is not limited to being performed on the original plate material of the glass sheet 10, but can also be performed on the individual glass sheets that will be the original glass sheet 20, as described below.

[0058] The glass plate 10 can be manufactured by the glass plate manufacturing method including the above-described precision annealing.

[0059] (Segmented Glass) Fig. 2(a) shows an external view of a glass plate 20 according to one embodiment. Fig. 2(b) shows a plan view of the glass plate 20, with the portion that will become the disk-shaped glass indicated by a broken line.

[0060] The glass plate 20 is a rectangular plate having a thickness of less than 0.68 mm, a flatness of 30 μm or less, and two orthogonal sides each having a length of 95 to 120 mm. This glass plate is smaller in size than the glass plate (large glass plate) 10 described above, and may be referred to as "singulated glass" in this specification.

[0061] The glass plate 20 is a rectangular plate measuring 95 to 120 mm, and has a size suitable for producing a disk-shaped glass (described below) that serves as a raw material for a magnetic disk glass substrate, and thus for producing a magnetic disk glass substrate, with minimal machining allowance. The glass plate 20 is preferably a square plate. By subjecting the glass plate material that forms the square glass plate 20 to precision annealing, it becomes easier to reduce the thermal shrinkage rate and the anisotropy of the thermal shrinkage rate for the same reasons as above. Note that even when the ratio of the length of the vertical and horizontal sides is slightly different (for example, when the ratio is 0.95 to 1.05), it is still considered within the above-mentioned square range. The thickness of the glass plate 20 is preferably less than 0.61 mm, more preferably less than 0.58 mm. The lower limit of the thickness is not particularly limited, but is, for example, 0.2 mm.

[0062] The thermal shrinkage of the glass plate 20 when subjected to a first heat treatment in which the glass plate 20 is maintained at 700°C for 4 hours and then cooled from 700°C to 400°C at a rate of 50°C / hour is 130 ppm or less. The thermal shrinkage is preferably 90 ppm or less, more preferably 50 ppm or less. Furthermore, when the glass transition temperature of the glass plate 20 is expressed as Tg (°C), the change in flatness of the glass plate 20 caused by a second heat treatment in which the glass plate 20 is maintained at Tg-160°C for 60 seconds and then cooled to room temperature in the atmosphere is 10 μm or less. The change in flatness is preferably 7.5 μm or less, more preferably 5 μm or less.

[0063] The glass plate 20 is a raw plate from which a disc-shaped glass is produced. The area of ​​the main surfaces of the glass plate 20 is preferably 1.6 times or less, more preferably 1.5 times or less, the area of ​​the inside of the outer periphery of the disc-shaped glass. Since the area of ​​the main surfaces of the glass plate 20 is close to the area of ​​the inside of the outer periphery of the disc-shaped glass (the influence of the inner hole is negligible and therefore negligible), the disc-shaped glass is more likely to have the two characteristics of the glass plate 20 described above, namely, the thermal shrinkage rate and the amount of change in flatness. In other words, the characteristics of the disc-shaped glass do not deviate significantly from the characteristics of the glass plate 20 before cutting. This effect is particularly effective in a glass plate 20 produced by performing precision annealing (precision annealing after singulation) on a singulated glass plate material from which the glass plate 20 is produced. That is, because the area of ​​the singulated glass plate is small, the effect of precision annealing easily reaches every corner of the outer periphery of the singulated glass plate, and the variation in the annealing effect depending on the in-plane position is small. Therefore, the effect of suppressing deterioration of flatness when heat-treating the magnetic film in the disk-shaped glass cut from the glass plate 20 is greater. Thus, the closer the shape of the glass plate to be precision annealed is to the shape of a magnetic disk glass substrate, the greater the effect of precision annealing. Note that singulation means obtaining the singulated glass 20 from the large glass plate 10, or obtaining a glass plate of the same dimensions as the singulated glass 20 from the glass plate that is the source of the large glass plate 10.

[0064] The glass plate 20 is obtained, for example, by cutting out the glass plate 10 (which has been precision annealed), or by subjecting a glass plate material that is the source of the glass plate 10 to individual pieces and then precision annealing the pieces.

[0065] The glass plate 20 is preferably subjected to an annealing treatment (e.g., the precision annealing described above) to reduce the thermal shrinkage. The glass plate (the glass plate material from which the glass plate 20 is made) before the annealing treatment has anisotropy in the thermal shrinkage, i.e., the magnitude of the thermal shrinkage varies depending on the in-plane direction of the glass plate. The difference (absolute value) between the thermal shrinkage C1 in the in-plane direction where the thermal shrinkage is smallest and the thermal shrinkage C2 in the in-plane direction where the thermal shrinkage is largest may be greater than 10 ppm. Furthermore, the difference (absolute value) between the thermal shrinkage S1 of the glass plate in the direction where the thermal shrinkage is smallest and the thermal shrinkage S2 of the glass plate in the direction where the thermal shrinkage is largest may be greater than 1.0 μm.

[0066] The glass plate 20 preferably has a difference (absolute value) of 10 ppm or less between the thermal shrinkage C1 in the direction in which the thermal shrinkage is smallest and the thermal shrinkage C2 in the direction in which the thermal shrinkage is largest among the in-plane directions of the glass plate 20. Furthermore, the difference (absolute value) between the thermal shrinkage S1 in the direction in which the thermal shrinkage is smallest and the thermal shrinkage S2 in the direction in which the thermal shrinkage is largest is preferably 1.0 μm or less.

[0067] The glass plate 20 described above is produced, for example, by cutting the glass plate 20 from the large glass plate 10 and dividing it into individual pieces. The cutting method may involve forming cut lines and breaking the glass plate 20 using a known scriber (cutter), or by irradiating the large glass plate 10 with laser light to form defects at regular intervals and then joining the defects to separate the glass plate 20 from the large glass plate 10. The glass plate 20 is preferably cut from the central region 12 of the large glass plate 10. Therefore, it is preferable that at least one of the main surfaces of the glass plate 20 be a fire-polished surface. This can omit some of the grinding and polishing processes typically required for manufacturing glass substrates for magnetic disks, or reduce the machining allowance. In other words, it is preferable that at least one of the main surfaces of the glass plate 20 be an unground and / or unpolished surface.

[0068] The glass plate 20 can be produced, for example, by singulating a pre-precision-annealed glass plate that will become the large glass plate 10, followed by precision annealing. That is, the glass plate 20 can be produced by a glass plate manufacturing method that includes precision annealing the singulated glass plate that will become the glass plate 20. The precision annealing in this method is performed by heating the singulated glass plate that will become the glass plate 20, in a manner similar to the precision annealing described above for the large glass plate 10. The singulated glass plate that will become the glass plate 20 used in this method is, for example, a plate cut from the glass plate that will become the large glass plate 10 without precision annealing, and has approximately the same dimensions and shape as the glass plate 20. According to the inventor's studies, it has been found that by performing precision annealing on the singulated glass plate that will become the glass plate 20, a glass plate 20 can be obtained that has even smaller changes in thermal shrinkage and flatness than a glass plate 20 cut from a precision-annealed large glass plate 10. Therefore, by performing precision annealing on the individualized glass plate material that is the basis for the glass plate 20, a glass plate 20 with even smaller changes in the thermal shrinkage rate and flatness can be obtained.

[0069] (Disc-Shaped Glass) FIG. 3 shows an external view of a disk-shaped glass 30 according to one embodiment.

[0070] The glass disk 30 is, for example, a raw material plate that is the basis for a glass substrate for a magnetic disk. The glass disk 30 has a circular outer periphery. A hole (inner hole) penetrating through the center of the glass disk in the thickness direction is provided, and the glass disk may have an annular shape. However, it does not have to have an inner hole, as in the glass disk 30 of the example shown in FIG. 3 .

[0071] The glass disk 30 has a thickness of less than 0.68 mm and a flatness of 30 μm or less. The glass disk 30 has a thermal shrinkage of 130 ppm or less when subjected to a first heat treatment in which the glass disk 30 is maintained at 700°C for 4 hours and then cooled from 700°C to 400°C at a rate of 50°C / hour. The thermal shrinkage is preferably 90 ppm or less, more preferably 60 ppm or less, and even more preferably 50 ppm or less. When the glass disk 30 has a glass transition temperature Tg (°C), the glass disk 30 has a change in flatness of 10 μm or less when subjected to a second heat treatment in which the glass disk 30 is maintained at Tg-160°C for 60 seconds and then cooled to room temperature in the atmosphere. The change in flatness is preferably 7.5 μm or less, more preferably 6 μm or less, and even more preferably 5 μm or less. The glass disk 30 has a thickness of preferably less than 0.61 mm, more preferably less than 0.58 mm. The lower limit of the plate thickness is not particularly limited, but is, for example, 0.2 mm.

[0072] The disc-shaped glass 30 preferably has a difference (absolute value) of 10 ppm or less between the thermal shrinkage C1 in the direction in which the thermal shrinkage is smallest and the thermal shrinkage C2 in the direction in which the thermal shrinkage is largest among the in-plane directions of the disc-shaped glass 30. The difference (C2 - C1) is more preferably 7 ppm or less, and even more preferably 5 ppm or less. Furthermore, the difference (absolute value) between the amount of thermal shrinkage S1 in the direction in which the thermal shrinkage is smallest and the amount of thermal shrinkage S2 in the direction in which the thermal shrinkage is largest is preferably 1.0 μm or less. The difference (S2 - S1) is more preferably 0.7 μm or less, and even more preferably 0.5 μm or less.

[0073] The disk-shaped glass 30 is obtained, for example, by cutting it from the glass plate 20. The disk-shaped glass 30 can be cut out from the glass plate 20 by, for example, a known scribing method or coring method. The scribing method can be performed using, for example, a diamond scriber, a scribing wheel, a laser, or the like. Therefore, it is preferable that at least one main surface of the disk-shaped glass 30 is a fire-polished surface. This can omit part of the grinding and polishing processes of the main surfaces of the substrate, which are generally required when manufacturing a magnetic disk glass substrate, or reduce the machining allowance. In other words, it is preferable that at least one main surface of the disk-shaped glass 30 is an unground and / or unpolished surface. When the disk-shaped glass 30 is used as a raw plate (intermediate body) from which a magnetic disk glass substrate is to be manufactured, it is preferable that the diameter of the disk-shaped glass 30 be adjusted according to the size of the magnetic disk glass substrate to be finally manufactured. The numerical values ​​and numerical ranges shown below are all examples. When a raw plate is used as the base for a magnetic disk glass substrate having a nominal diameter of 3.5 inches, the outer diameter (diameter) can be 95 to 100 mm. When an inner hole is provided, the inner diameter (diameter) can be 23 to 25 mm. On the other hand, when a raw plate is used as the base for a magnetic disk glass substrate having a nominal diameter of 2.5 inches, the outer diameter (diameter) can be 65 to 70 mm. When an inner hole is provided, the inner diameter (diameter) can be 18 to 20 mm.

[0074] (Magnetic Disk Glass Substrate) Fig. 4 shows an external view of a magnetic disk glass substrate 40 according to one embodiment. The magnetic disk glass substrate 40 shown in Fig. 4 has an inner hole in the center.

[0075] The size of the glass substrate 40 is not limited, but may be, for example, the size of a magnetic disk glass substrate having a nominal diameter of 3.5 inches or 2.5 inches. In the case of a magnetic disk glass substrate having a nominal diameter of 3.5 inches, the outer diameter (diameter) may be, for example, 95 to 100 mm, and the inner hole diameter (diameter) may be, for example, 24 to 26 mm. Specifically, for example, the outer diameter (diameter) may be 95 mm or 97 mm, and the inner hole diameter (diameter) may be, for example, 25 mm. On the other hand, in the case of a magnetic disk glass substrate having a nominal diameter of 2.5 inches, the outer diameter (diameter) may be, for example, 65 to 70 mm, and the inner hole diameter (diameter) may be, for example, 19 to 21 mm. Specifically, for example, the outer diameter (diameter) may be 65 mm or 67 mm, and the inner hole diameter (diameter) may be, for example, 20 mm.

[0076] The magnetic disk glass substrate 40 has a thickness of less than 0.68 mm and a flatness of 30 μm or less. The magnetic disk glass substrate 40 has a thermal shrinkage of 130 ppm or less when subjected to a first heat treatment in which the substrate is maintained at 700°C for 4 hours and then cooled from 700°C to 400°C at a rate of 50°C / hour. The thermal shrinkage is preferably 90 ppm or less, more preferably 60 ppm or less, and even more preferably 50 ppm or less. When the glass transition temperature of the magnetic disk glass substrate 40 is expressed as Tg (°C), the change in flatness associated with a second heat treatment in which the substrate is maintained at Tg-160°C for 60 seconds and then cooled to room temperature in the atmosphere is 10 μm or less. The change in flatness is preferably 7.5 μm or less, more preferably 6 μm or less, and even more preferably 5 μm or less. The magnetic disk glass substrate 40 has a thickness of preferably less than 0.61 mm, more preferably less than 0.58 mm. The lower limit of the plate thickness is not particularly limited, but is, for example, 0.2 mm.

[0077] The magnetic-disk glass substrate 40 preferably has a difference (absolute value) of 10 ppm or less between the thermal shrinkage C1 in the direction in which the thermal shrinkage is smallest and the thermal shrinkage C2 in the direction in which the thermal shrinkage is largest among the in-plane directions of the magnetic-disk glass substrate 40. The difference (C2 - C1) is more preferably 7 ppm or less, and even more preferably 5 ppm or less. Furthermore, the difference (absolute value) between the amount of thermal shrinkage S1 in the direction in which the thermal shrinkage is smallest and the amount of thermal shrinkage S2 in the direction in which the thermal shrinkage is largest is preferably 1.0 μm or less. The difference (S2 - S1) is more preferably 0.7 μm or less, and even more preferably 0.5 μm or less.

[0078] The magnetic disk glass substrate 40 is obtained by a manufacturing method for a magnetic disk glass substrate, which includes, for example, grinding and / or polishing of the main surfaces of the glass disk 30. The manufacturing method for a magnetic disk glass substrate may include, in addition to grinding and / or polishing of the main surfaces of the glass disk 30, processing such as forming a chamfered surface, grinding and / or polishing of the edge surfaces, chemical strengthening, cleaning, etc. Furthermore, when the glass substrate for a magnetic disk 40 is produced from a glass disk 30 that does not have an inner hole, the manufacturing method for a magnetic disk glass substrate may include processing for forming an inner hole in the glass disk 30 (for example, processing by the above-mentioned scribing method or coring method using a core drill).

[0079] According to one example, the manufacturing method of a magnetic disk glass substrate is carried out as follows. That is, chamfered surfaces are formed on the inner and outer peripheral end surfaces of a circular ring-shaped glass disk 30. Next, the main surfaces of the glass disk with the chamfered surfaces formed thereon are ground. In the grinding process, the main surfaces of the glass disk are ground using a grinding member having fixed abrasive grains formed in a sheet form, or a slurry containing free abrasive grains. Next, the main surfaces of the ground glass disk are polished. In the polishing process, polishing is carried out using a polishing pad and a slurry containing free abrasive grains with a grain size smaller than the free abrasive grains used in the grinding process. The polishing process can be divided into multiple processes and carried out using abrasive grains with different grain sizes or polishing pads with different hardnesses.

[0080] When chemical strengthening is performed, it is preferable to perform it before or after the final polishing treatment. The chemical strengthening treatment is performed, for example, by immersing the disk-shaped glass in a molten solution of a mixed salt of multiple types of nitrates. The cleaning treatment involves cleaning the disk-shaped glass with a cleaning solution after chemical strengthening or after the final polishing treatment. Note that a cleaning treatment may be added between the above treatments as needed.

[0081] (Experimental Example 1) To investigate the effects of the present invention, various magnetic disk glass substrates (Conventional Example 1 and Examples 1 to 5) shown in the table below were prepared, and the thermal shrinkage and change in circularity upon the first heat treatment were evaluated, and the degree of deterioration in flatness upon the second heat treatment was evaluated. Regarding the thermal shrinkage, the difference in thermal shrinkage (C2-C1), the difference in thermal shrinkage amount (S2-S1), and the angle between the maximum and minimum directions of the thermal shrinkage were also evaluated to evaluate the anisotropy of the thermal shrinkage. However, when the difference in thermal shrinkage amount (S2-S1) was 0.5 μm or less, the anisotropy was determined to be extremely small, and therefore the angle was not measured.

[0082]

[0083]

[0084] The magnetic disk glass substrates of Conventional Example 1 and Examples 1 to 5 all had the following specifications: aluminosilicate glass with a glass transition temperature (Tg) of 810°C, outer diameter 97 mm, inner diameter 25 mm, plate thickness 0.5 mm, and flatness of the main surfaces 5 μm. The average retardation on the main surfaces of all of the magnetic disk glass substrates produced in the examples was 0.5 nm or less. In other words, the residual stress in the magnetic disk glass substrates produced in the examples is sufficiently small, so it is believed that the residual stress has almost no effect on various evaluations.

[0085] The glass substrates of Conventional Example 1 and Examples 1 to 5 were each produced as follows. (Conventional Example 1) From a glass sheet formed using the overflow downdraw method while slowly cooling, both end portions, which were thicker than the central portion in the width direction, were cut off, and a predetermined region of the glass sheet was cut out from the remaining glass sheet to obtain a rectangular glass plate with a short side of 1000 mm, a long side of 1200 mm, and a thickness of 0.6 mm. The long side direction of the glass plate corresponded to the width direction of the glass sheet. From the obtained glass plate, a square glass plate (singulated glass) with a side of 109 mm was cut out from the central region, excluding end regions 200 mm inward from both ends in the short and long sides. Then, a circular glass plate with a diameter of 99 mm was cut out from the above-mentioned segmented glass by a scribing method. At this time, the area ratio of the segmented glass to the circular glass was approximately 1.54. Thereafter, using known methods, circular hole formation, chamfering, adjustment of outer and inner diameters, edge polishing, grinding and polishing of the main surfaces, cleaning, etc. were performed to obtain a magnetic disk glass substrate with the above specifications. (Example 1) A magnetic disk glass substrate was obtained in the same manner as in Conventional Example 1, except that the above precision annealing was performed on a rectangular glass plate (large plate) with short sides of 1000 mm, long sides of 1200 mm, and a plate thickness of 0.6 mm. (Example 2) A magnetic disk glass substrate was obtained in the same manner as in Conventional Example 1, except that precision annealing was performed on the singulated glass plate. (Example 3) A magnetic disk glass substrate was obtained in the same manner as in Example 2, except that the size of the singulated glass was 106 mm x 106 mm and the area ratio of the singulated glass to the disk-shaped glass was approximately 1.46. (Example 4) A magnetic disk glass substrate was obtained in the same manner as in Example 2, except that the size of the segmented glass was 112 mm × 112 mm and the area ratio of the segmented glass to the disk-shaped glass was about 1.63. (Example 5) A magnetic disk glass substrate was obtained in the same manner as in Example 4, except that the segmented glass was a rectangle having a size of 118.3 mm × 106 mm.

[0086] The precision annealing was performed by placing the glass plate in an annealing furnace with the atmospheric temperature adjusted to 700°C (Tg-110°C) and holding it for 4 hours. More specifically, the temperature was raised to 700°C over 2.5 hours, held at 700°C for 4 hours, and then cooled at a rate of 50°C / hour. At this time, the glass plate was sandwiched between two setters whose main surfaces were wider than the glass plate and whose size extended beyond the entire periphery of the glass plate, with the outer periphery of the setter extending 5 cm beyond the entire periphery of the glass plate. Furthermore, rock wool was lightly packed into the gap between the two setters that contacted the side surfaces of the glass plate, so that the entire glass plate was covered with the setter and rock wool.

[0087] <Measurement of Thermal Shrinkage and Change in Circularity> The glass substrate to be measured was subjected to the following first heat treatment: (First Heat Treatment) The glass substrate was placed in an annealing furnace at room temperature and heated to 700°C, and then maintained at 700°C for 4 hours, followed by cooling from 700°C to 400°C at a rate of 50°C / h.

[0088] The thermal shrinkage was calculated from the change in diameter before and after the first heat treatment in 25 directions passing through the center of the glass plate and spaced at intervals of 7.2 degrees in the circumferential direction around this center, and the maximum value was used as the thermal shrinkage of the glass substrate. The difference in thermal shrinkage (C2-C1) is the difference (absolute value) between the thermal shrinkage C1 in the direction with the smallest thermal shrinkage among the 25 directions and the thermal shrinkage C2 in the direction with the largest thermal shrinkage. The difference in thermal shrinkage (S2-S1) is the difference (absolute value) between the thermal shrinkage S1 in the direction with the smallest thermal shrinkage among the 25 directions and the thermal shrinkage S2 in the direction with the largest thermal shrinkage among the 25 directions.

[0089] The change in circularity was calculated by subtracting the circularity before the first heat treatment from the circularity after the first heat treatment. All values ​​of the change in circularity were absolute values. The circularity was measured using a circularity measuring instrument. In principle, the circularity after the first heat treatment was greater than the circularity before the first heat treatment. The degree of deterioration in circularity was evaluated as follows: A: when the difference in circularity of the outer periphery of the glass substrate measured before and after the first heat treatment was 0.2 μm or less; B: when it was more than 0.2 μm and 0.5 μm or less; and C: when it exceeded 0.5 μm. Grades A and B were evaluated as indicating that the deterioration in circularity was suppressed.

[0090] <Measurement of Flatness Change> The glass substrate to be measured was subjected to the second heating treatment described below. (Second Heating Treatment) The glass substrate was placed in a heating apparatus at room temperature and heated to 650°C (corresponding to Tg-160°C) in 50 seconds. After maintaining the glass substrate at 650°C for 60 seconds, it was removed from the apparatus and allowed to cool naturally to room temperature in the atmosphere. As described above, the heating apparatus used was equipped with two panel heaters arranged parallel to each other at a distance. The glass substrate was attached to a holder (substrate holder) and could be placed upright in the gap between the panel heaters. The holder (substrate holder) with the glass substrate attached was also able to move between the outside of the heating apparatus, which was in the atmosphere, and the inside of the heating apparatus.

[0091] The change in flatness was calculated by subtracting the flatness before the second heat treatment from the flatness after the second heat treatment. Each flatness value and the change in flatness were expressed as absolute values. The degree of deterioration in flatness was evaluated as A when the difference in flatness measured before and after the second heat treatment was 7 μm or less, B when it was more than 7 μm and 10 μm or less, and C when it was more than 10 μm. Of these, A and B were evaluated as having been able to suppress the deterioration of flatness.

[0092] A comparison between Conventional Example 1 and Example 1 shows that by performing precision annealing on the large plate before singulation, the thermal shrinkage rate during the first heat treatment is 130 ppm or less, and the change in flatness associated with the second heat treatment can be suppressed to 10 μm or less. Furthermore, with regard to the anisotropy of the thermal shrinkage rate, the difference in thermal shrinkage rate (C2-C1) is 10 ppm or less, and deterioration of the roundness of the glass substrate can also be suppressed. A comparison between Example 1 and Example 2 shows that precision annealing the glass plate after singulation provides a greater effect of suppressing deterioration of the flatness of the glass substrate compared to precision annealing on the large plate before singulation. Furthermore, a comparison between Examples 2 to 4 shows that by setting the area ratio of the singulated glass to the disk-shaped glass to 1.6 or less, preferably 1.5 or less, the thermal shrinkage rate during the first heat treatment is reduced, and the effect of suppressing the change in flatness associated with the second heat treatment is improved. It can also be seen that the difference in thermal shrinkage (C2-C1) between the anisotropy of the thermal shrinkage rates is reduced, improving the effect of reducing the amount of change in the roundness of the glass substrate. From a comparison between Example 4 and Example 5, it can be seen that the square shape of the segmented glass reduces the thermal shrinkage rate when the first heat treatment is performed, and improves the effect of minimizing the amount of change in flatness associated with the second heat treatment, compared to when the shape of the segmented glass is not square. It can also be seen that the difference in thermal shrinkage (C2-C1) between the anisotropy of the thermal shrinkage rates is reduced, improving the effect of reducing the amount of change in the roundness of the glass substrate.

[0093] In addition, when the conditions for the first heat treatment for measuring the thermal shrinkage of other magnetic disk glass substrates (multiple) manufactured by the method of Conventional Example 1 were changed to increase the temperature from room temperature to 600°C at a rate of 100°C / hour, hold at 600°C for 80 minutes, and then decrease the temperature from 600°C to room temperature at a rate of 100°C / hour, the thermal shrinkage was measured. The results were 20 to 40 ppm, which was significantly smaller than that of Conventional Example 1 when the first heat treatment was performed. This is thought to be mainly due to the fact that both the heating temperature and maintenance time in the thermal shrinkage measurement conditions were relaxed. This shows that the thermal shrinkage value is significantly affected by the heat treatment conditions in the measurement conditions.

[0094] (Experimental Example 2) Twenty magnetic disk glass substrates were manufactured under the conditions of Conventional Example 1, and the thermal shrinkage (maximum value of the thermal shrinkage in the above 25 directions) was measured. The difference (variation) between the maximum and minimum values ​​among the 20 substrates was calculated to be 188 ppm. Similarly, magnetic disk glass substrates under the conditions of Example 1 were prepared, and the difference (variation) between the maximum and minimum values ​​of the thermal shrinkage among the 20 substrates was calculated to be 37 ppm. Similarly, magnetic disk glass substrates under the conditions of Example 2 were prepared, and the difference (variation) between the maximum and minimum values ​​of the thermal shrinkage among the 20 substrates was calculated to be 9 ppm.

[0095] In addition, when the variation in thermal shrinkage (difference between maximum and minimum values) between Conventional Example 1, Example 1, and Example 2 was compared in the same manner as in the above (Experimental Example 2), except that instead of a magnetic disk glass substrate, segmented glass immediately before being cut out from a glass disk was used, the results were generally similar to those of the above (Experimental Example 2). The above results show that (1) precision annealing reduces the variation in thermal shrinkage between segmented glass pieces, and (2) precision annealing reduces the variation in thermal shrinkage when performed on segmented glass rather than on a large sheet of glass.

[0096] The magnetic disk glass substrate, the circular glass plate, the glass plate, and the method for manufacturing the glass plate of the present invention have been described in detail above. However, the present invention is not limited to the above-described embodiments and examples, and various improvements and modifications may be made without departing from the spirit and scope of the present invention.

[0097] REFERENCE SIGNS LIST 10 Glass plate (large glass plate) 10a Short side 10b Long side 11a, 11b End region 12 Central region 13 Measurement region 20 Glass plate (segmented glass) 30 Glass disk 40 Glass substrate for magnetic disk

Claims

1. A rectangular glass plate having a plate thickness of less than 0.68 mm, from each end in the short side direction of the glass plate, an end region having a length of 5 to 20% of the short side of the glass plate is formed inside the glass plate, and from each end in the long side direction of the glass plate, an end region having a length of 5 to 20% of the long side of the glass plate is formed inside the glass plate. The flatness of a square measurement region with a side length of 100 mm cut out from the central region of the glass plate excluding these is 30 μm or less, after maintaining the measurement region at 700 °C for 4 hours, when a first heat treatment of cooling from 700 °C to 400 °C at a rate of 50 °C / hour is performed, the thermal shrinkage rate of the measurement region is 130 ppm or less, when the glass transition temperature of the glass plate is represented by Tg (°C), after maintaining the measurement region at Tg - 160 °C for 60 seconds and then cooling to room temperature in the atmosphere, the change amount of the flatness of the measurement region accompanying the second heat treatment is 10 μm or less, the thermal shrinkage rate of the measurement region is the maximum value of the thermal shrinkage rates measured in 25 directions that are changed by 7.2 degrees in the circumferential direction through the center of the measurement region in the direction parallel to the main surface of the measurement region, a glass plate characterized by this.

2. The glass plate according to Claim 1, having a short side length exceeding 900 mm.

3. The glass plate according to Claim 1, which is a portion cut out from a long glass sheet formed using any one of the float method, the full - cole method, the Pittsburgh method, the down - draw method, the Colburn method, and the redraw method.

4. In the in - plane direction of the measurement region, the difference between the thermal shrinkage amount S1 of the measurement region in the direction where the thermal shrinkage rate is the minimum and the thermal shrinkage amount S2 of the measurement region in the direction where the thermal shrinkage rate is the maximum is 1.0 μm or less, the glass plate according to any one of Claims 1 to 3.

5. The glass plate has been subjected to an annealing treatment to reduce the thermal shrinkage rate, before the annealing treatment, the glass plate has anisotropy in thermal shrinkage rate, where the magnitude of the thermal shrinkage rate varies depending on the in - plane direction of the region of the glass plate corresponding to the measurement region, in the in - plane direction, the difference between the thermal shrinkage amount S1 of the region in the direction where the thermal shrinkage rate is the minimum and the thermal shrinkage amount S2 of the region in the direction where the thermal shrinkage rate is the maximum is greater than 1.0 μm, the glass plate according to Claim 4.

6. A glass plate in the shape of a rectangle with a thickness of less than 0.68 mm, a flatness of 30 μm or less, and the lengths of two orthogonal sides being 95 to 120 mm each, when a first heat treatment is performed in which it is maintained at 700 °C for 4 hours and then cooled from 700 °C to 400 °C at a rate of 50 °C / hour, the thermal shrinkage rate is 130 ppm or less, when the glass transition temperature of the glass plate is represented by Tg (°C), after maintaining it at Tg - 160 °C for 60 seconds and then cooling it to room temperature in the atmosphere, the change amount of flatness accompanying the second heat treatment is 10 μm or less, the thermal shrinkage rate when performing the first heat treatment is the maximum value of the thermal shrinkage rates measured in 25 directions each changed by 7.2 degrees in the circumferential direction through the center of the glass plate in a direction parallel to the main surface of the glass plate, a glass plate characterized by this.

7. The rectangular glass plate is a base plate that becomes the source of a disk-shaped glass having a circular outer periphery, the area of the main surface of the rectangular glass plate is 1.6 times or less the area inside the outer periphery of the disk-shaped glass, the glass plate according to claim 6.

8. A disk-shaped glass having a thickness of less than 0.68 mm, a flatness of 30 μm or less, and a circular outer periphery with a diameter of 95 to 100 mm, when a first heat treatment is performed in which it is maintained at 700 °C for 4 hours and then cooled from 700 °C to 400 °C at a rate of 50 °C / hour, the thermal shrinkage rate is 130 ppm or less, when the glass transition temperature of the disk-shaped glass is represented by Tg (°C), after maintaining it at Tg - 160 °C for 60 seconds and then cooling it to room temperature in the atmosphere, the change amount of flatness accompanying the second heat treatment is 10 μm or less, the thermal shrinkage rate when performing the first heat treatment is the maximum value of the thermal shrinkage rates measured in 25 directions each changed by 7.2 degrees in the circumferential direction through the center of the disk-shaped glass in a direction parallel to the main surface of the disk-shaped glass, a disk-shaped glass characterized by this.

9. Among the 25 directions, the difference between the thermal shrinkage amount S1 in the direction where the thermal shrinkage rate is the minimum and the thermal shrinkage amount S2 in the direction where the thermal shrinkage rate is the maximum is 1.0 μm or less, the disk-shaped glass according to claim 8.

10. A glass substrate for a magnetic disk having a thickness of less than 0.68 mm, a flatness of 30 μm or less, and a diameter of 95 to 100 mm, when a first heat treatment is performed in which it is maintained at 700 °C for 4 hours and then cooled from 700 °C to 400 °C at a rate of 50 °C / hour, the thermal shrinkage rate is 130 ppm or less, When the glass transition temperature of the glass substrate for the magnetic disk is represented by Tg (°C), after maintaining at Tg - 160 °C for 60 seconds and then cooling to room temperature in the atmosphere, the amount of change in flatness associated with the second heat treatment is 10 μm or less. The glass substrate for a magnetic disk, wherein the thermal shrinkage rate when the first heat treatment is performed is the maximum value of the thermal shrinkage rates measured in 25 directions each changing by 7.2 degrees in the circumferential direction through the center of the glass substrate for the magnetic disk in a direction parallel to the main surface of the glass substrate for the magnetic disk. The glass substrate for a magnetic disk according to claim 10, wherein the difference between the thermal shrinkage amount S1 in the direction where the thermal shrinkage rate is minimum and the thermal shrinkage amount S2 in the direction where the thermal shrinkage rate is maximum among the 25 directions is 1.0 μm or less.

12. The glass substrate for a magnetic disk according to claim 10 or 11, wherein the amount of change in roundness associated with the first heat treatment is 0.5 μm or less.

13. A method for manufacturing a glass plate, comprising: a step of annealing the glass plate material that is the source of the glass plate; The glass plate is a rectangular plate with a thickness of less than 0.68 mm, the flatness of a 100 mm square measurement region cut out from the central region of the glass plate excluding the end regions with a length of 5 - 20% of the short side of the glass plate from both ends in the short side direction of the glass plate and the end regions with a length of 5 - 20% of the long side of the glass plate from both ends in the long side direction of the glass plate is 30 μm or less, when the first heat treatment is performed by maintaining the measurement region at 700 °C for 4 hours and then cooling from 700 °C to 400 °C at a rate of 50 °C / hour, the thermal shrinkage rate of the measurement region is 130 ppm or less, when the glass transition temperature of the glass plate is represented by Tg (°C), after maintaining the measurement region at Tg - 160 °C for 60 seconds and then cooling to room temperature in the atmosphere, the amount of change in flatness of the measurement region associated with the second heat treatment is 10 μm or less, the thermal shrinkage rate of the measurement region is the maximum value of the thermal shrinkage rates measured in 25 directions each changing by 7.2 degrees in the circumferential direction through the center of the measurement region in a direction parallel to the main surface of the measurement region, which is a method for manufacturing a glass plate.

14. A method for manufacturing a glass plate, comprising: a step of annealing the glass plate material that is the source of the glass plate; A step of taking out the glass plate from the glass plate material after the annealing treatment, and the glass plate is a rectangular plate with a thickness of less than 0.68 mm, a flatness of 30 μm or less, and the lengths of two orthogonal sides are 95 to 120 mm respectively, after maintaining at 700°C for 4 hours, when subjected to a first heat treatment of cooling from 700°C to 400°C at a rate of 50°C / hour, the thermal shrinkage rate is 130 ppm or less, when the glass transition temperature of the glass plate is represented by Tg (°C), after maintaining at Tg - 160°C for 60 seconds and then cooling to room temperature in the atmosphere, the change amount of flatness accompanying the second heat treatment is 10 μm or less, the thermal shrinkage rate when the first heat treatment is performed is the maximum value of the thermal shrinkage rates measured in 25 directions that are changed by 7.2 degrees each in the circumferential direction passing through the center of the glass plate in a direction parallel to the main surface of the glass plate, a method for manufacturing a glass plate, characterized by this.

15. the rectangular glass plate is a square and is a base plate that becomes a disc-shaped glass having a circular outer circumference, the area of the main surface of the rectangular glass plate is 1.6 times or less the area inside the outer circumference of the disc-shaped glass, the method for manufacturing a glass plate according to claim 13 or 14.

16. A method for manufacturing a disc-shaped glass, including a step of annealing a glass plate material that becomes the disc-shaped glass, and a step of taking out the disc-shaped glass from the glass plate material after the annealing treatment, the glass plate material is a rectangular glass plate, the disc-shaped glass is with a thickness of less than 0.68 mm, a flatness of 30 μm or less, and a diameter of 95 to 100 mm, after maintaining at 700°C for 4 hours, when subjected to a first heat treatment of cooling from 700°C to 400°C at a rate of 50°C / hour, the thermal shrinkage rate is 130 ppm or less, when the glass transition temperature of the disc-shaped glass is represented by Tg (°C), after maintaining at Tg - 160°C for 60 seconds and then cooling to room temperature in the atmosphere, the change amount of flatness accompanying the second heat treatment is 10 μm or less, the thermal shrinkage rate when the first heat treatment is performed is the maximum value of the thermal shrinkage rates measured in 25 directions that are changed by 7.2 degrees each in the circumferential direction passing through the center of the disc-shaped glass in a direction parallel to the main surface of the disc-shaped glass, a method for manufacturing a disc-shaped glass, characterized by this.

17. the rectangular glass plate is a square, The manufacturing method of the disk-shaped glass according to claim 16, wherein the area of the main surface of the rectangular glass plate is 1.6 times or less the area inside the outer periphery of the disk-shaped glass.