GLASS FOR INFORMATION RECORDING MEDIUM SUBSTRATE AND GLASS SUBSTRATE

Abstract: Disclosed is SiO 2 -Al 2 O 3 -RO (RO = divalent metal) glass having high specific elastic modulus of 36 x 10 6 Nm/kg or more or high Young's modulus of 110 GPa or more, high transition temperature of 700°C or higher (high heat resistance). The glass exhibits excellent surface smoothness (surface roughness Ra < 9Å) and high strength, which can provide glass substrate with high strength. Since glass substrates being composed of the glass having excellent heat resistance and flatness, the substrates can be subjected to heat treatment for improvement of magnetic layer characteristics without any decomposition of the substrate and realize smaller flying height of magnetic head, i.e., higher recording density. The glasses exhibiting high specific elastic modulus and high strength enables to realize thinner magnetic discs and prevent breakdown of magnetic discs.

Description: Field of the Invention The present invention relates to glass and glass substrate suitably used for substrates of information recording media such as magnetic discs and optical discs, heat resistant substrates of low temperature polycrystalline silicon liquid crystal display devices, which are expected as next generation LCDs, substrates for various electric and electronic components and the like. In particular, it relates to glass suitable for substrates of information recording media and the like, which exhibits high specific elastic modulus and/or Young's modulus and high heat resistance and, when used as a substrate, realizes excellent surface smoothness. Description of the Related Art Major components of magnetic storage devices cf electronic computers and the like are a magnetic recording medium and a magnetic head for reconstruction of magnetically recorded information. Flexible discs and hard disks have been known as magnetic recording media. As substrates for hard discs (magnetic discs), for example, aluminum substrates, glass substrates, ceramic substrates, carbon substrates and the like have been known. In practical use, however, an aluminum substrate or a glass substrate is mainly used according to the intended size and the use thereof. Recently, flying height of magnetic heads is markedly reduced as hard disc drivers for notebook personal computers are made smaller and their magnetic recording density made higher. Accordingly, extremely high precision has been demanded for the surface smoothness of magnetic disc substrates. However, it is difficult to produce smooth surface more than a certain level of precision with an aluminum alloy. That is, even though it is polished by using highly precise abrasives and processing apparatuses, the polished surface may suffer from plastic deformation because of the low hardness of the alloy. Even if the aluminum alloy is plated with nickel-phosphorous, the surface roughness Ra cannot be made 2 nm (20Å) (angstrom) or less. In addition, as hard disk drivers are made smaller and thinner, a further smaller thickness of substrates for magnetic discs is also strongly desired. However, it is difficult to produce such a thin disc with an aluminum alloy having a certain strength defined by specification of hard disk drivers because of low strength and stiffness of aluminum alloy. Therefore, glass substrates for magnetic discs having high strength, high stiffness, high impact resistance and high surface smoothness have been developed. Because glass substrates have excellent surface smoothness and mechanical strength, they have been paid much attention as substrates for present and future use. For example, as such glass substrates, chemically tempered glass substrates whose surfaces are strengthened by the ion exchange technique, crystallized glass substrates subjected to crystallization treatment, alkali-free glass substrates which do not substantially contain alkaline substances and the like have been known well. For example, as a chemically tempered glass substrate, Japanese Patent Unexamined Publication No. Hei 1-239036 (referred to as Reference 1 hereinafter) discloses a glass substrate for magnetic recording media strengthened by subjecting to ion exchange treatment a glass material for the substrate containing, indicated in terms of % by weight, 60-70% of SiO 2 , 0.5-14 % of Al 2 O 3 , 10-32% of R 2 O where R is an alkali metal, 1-15% of ZnO and 1.1-14% of B 2 O 3 . As a crystallized glass, Japanese Patent Unexamined Publication No. Hei 7-187711 (referred to as Reference 2 hereinafter) discloses a glass substrate for magnetic recording media containing, indicated in terms of % by weight, 50-65% of SiO 2 , 18-25% of CaO, 6-11% of Na 2 O, 6-12% of K 2 O, 0-2.5% of Al 2 O 3 and 5-9% of F and containing kanasite as main crystals. U.S. Patent No. 5,391,522 (referred to as Reference 3 hereinafter) discloses a crystallized glass substrate for magnetic discs containing 65-83% of SiO 2 , 8-13% of Li 2 O, 0-7% of K 2 O, 0.5-5.5% of MgO, 0-5% of ZnO, 0-5% of PbO, (provided that MgO + ZnO + PbO is 0.5-5%), 1-4% of P 2 O 5 , 0-7% of Al 2 O 3 and 0-2% of As 2 O 3 + Sb 2 O 3 and containing microcrystalline particles of Li 2 O·2SiO 2 as main crystals. As an alkali-free glass, Japanese Patent Unexamined Publication No. Hei 8-169724 (referred to as Reference 4 hereinafter) discloses a glass substrate for magnetic discs having a composition containing, indicated in terms of % by weight, 35-55% of SiO 2 + Al 2 O 3 , 0-10% of B 2 O 3 , 40-60% of CaO + BaO, provided that CaO ≧ 5%, 0-10% of ZnO + SrO + MgO, 0-5% of TiO 2 , 0-5% of ZrO 2 , 0-1% of As 2 O 3 and/or Sb 2 O 3 . EP-A-0 710 627 describes a glass-ceramic material used for rigid disc substrates for use in magnetic memory disc devices. These glass-ceramic articles are prepared by heat treatment of precursor glass bodies. Recent HDDs (hard disk drivers) have been required to have higher recording capacity to meet higher performance of personal computers, and a smaller and thinner disc substrate, smaller flying height of magnetic heads and higher revolution speed of discs have been required to meet smaller size and higher performance of personal computers. It is expected that the thickness of 6.35 cm (2.5-inch) diameter disc substrates should become thinner from the present thickness 0.635 mm to a thickness of 0.43 mm, or even 0.38 mm. In addition, for recent higher recording density of 8.89 cm (3.5)-inch hard discs for servers and higher data processing speed, requirement for stiffness of substrate materials becomes increasingly severer, and conventional aluminum substrates seem to almost reach their limits of performance. It is expected that further higher capacity and smaller size of hard discs will be sought in future. Therefore, smaller thickness, higher strength, more excellent surface smoothness, higher impact resistance and the like of substrates for magnetic recording media will be further strongly demanded. However, as disc substrates become thinner, they become more likely to suffer deflexion and warp. On the other hand, as higher recording density is sought, lower flying height of magnetic heads and higher revolution speed of magnetic discs are further sought yet, and such deflexion and warp of substrates may cause breakdown of magnetic discs. However, if thickness of conventional glass substrates is made thinner than currently used, the problems due to the deflexion and warp mentioned above will become unacceptably marked and thus thinner discs cannot be realized. Degree of deflexion and warp of substrates can be evaluated from specific elastic modulus (= Young's modulus/specific gravity) or Young's modulus of substrate material. Materials of higher specific elastic modulus are required for suppressing the problems of the deflexion and warp of substrates made with a smaller thickness. Further, materials of higher Young's modulus are required for suppressing the problems of the deflexion of substrates to be rotated at a high speed. The above situation may be further explained as follows. That is, with recent improvements on smaller size, higher capacity and higher speed of HDDs, it is expected that the thickness of 8.89 cm (3.5)-inch discs currently used of 0.8 mm will be made smaller to 0.635 mm, and 0.635 mm of current 6.35 cm (2.5)-inch discs to 0.43 mm, or even to 0.38 mm. Revolution speed of substrates is also expected to be made faster from the current maximum speed of 7200 rpm to 10000 rpm, or even to 14000 rpm. As substrates for such magnetic recording media become thinner, they become more likely to suffer deflexion, undulation and warp, and it is expected that, as the revolution speed becomes higher, stress loaded on the substrates (force exerted by wind pressure caused by rotation of discs) will become larger. Based on the theory of dynamics, the deflexion W of a disc receiving load of P per unit area is represented by the following formula: W ∞ Pa 4 h 3 E wherein a represents an outer diameter of disc, h represents a thickness of substrate and E represents Young's modulus of disc material. In static state, force loaded on the disc is the gravitation alone, and the deflexion W is represented by the following formula: W ∞ hda 4 h 3 E = da 4 h 2 E = a 4 h 2 G wherein d represents a specific gravity of disc material and G is a specific elastic modulus of disc material (= Young's modulus/specific gravity). On the other hand, supposing that the gravitational force is balanced by centrifugal force and can be ignored in rotating state of disc, force loaded on the disc may be considered only wind pressure caused by the rotation of the disc. The wind pressure is represented as a function of disc revolution speed and said to be proportional to the second power of the speed. Accordingly, the deflexion W when the disc is rotating in a high speed is represented by the following formula: w ∞ ( rpm ) 2 a 4 h 3 E Therefore, in order to suppress the deflexion W of substrate to be rotated at a high speed, a material of high Young's modulus E is required. According to the present inventors' calculation, when the thickness of 6.35 cm (2.5)-inch substrate is made smaller from 0.635 mm to 0.43 mm, and the thickness of 3.5-inch substrate from 0.8 mm to 0.635 mm, a substrate material having a specific elastic modulus higher than that of conventional materials should be required. Further, when the current revolution speed of 8.89 cm (3.5)-inch high-end substrates of 7200 rpm is made faster to prospective 10000 rpm, an aluminum substrate having Young's modulus of around 70 GPa cannot meet such a high speed, and a new substrate material having a further higher Young's modulus should be required. As the specific elastic modulus or Young's modulus of substrate material becomes higher, not only stiffness of substrates becomes higher, but also impact resistance and strength of substrates become higher. Therefore, a glass material having high specific elastic modulus and high Young's modulus is strongly desired in the field of HDD production. There are further properties of substrates for magnetic recording media required for realizing higher recording density other than the specific elastic modulus and Young's modulus. One of those is high heat resistance, and another is high surface smoothness. In order to obtain higher recording density of magnetic recording media, it is necessary to enhance magnetic characteristics such as magnetic coercive force of magnetic layer (magnetic recording layer). While coercive force of magnetic layer may vary depending on the kind of magnetic material used, coercive force of the magnetic material may be enhanced by heat-treatment even if the same material is used. Therefore, separating from development of new magnetic materials, it may be desirable to treat a magnetic layer formed on a substrate at a higher temperature in order to obtain higher coercive force using a conventional material. It is also possible to obtain higher recording density by making flying height of magnetic heads smaller. Therefore, the flying height of magnetic heads will be further made smaller in future. To realize smaller flying height of magnetic heads, good smoothness of disc surfaces and hence good smoothness of substrate surfaces are required. The chemically tempered glass disclosed in Reference 1 has a glass transition point of around 500°C. However, to improve coercive force of magnetic layer, heat treatment at a temperature higher than 500°C is effective. Accordingly, heat resistance of the chemically tempered glass of Reference 1 itself is insufficient. Chemically tempered glasses are generally made by providing an ion exchanged layer with alkali metal ions on surfaces of the glasses. However, when a magnetic layer is formed on the surface of a chemically tempered glass and heat-treated, the ions in the ion exchanged layer may disadvantageously migrate to the magnetic layer and adversely affect it. The migration of the alkali metal ions to the magnetic layer is more activated as the temperature becomes higher. To suppress the migration of alkali metal ions, heat treatment at a lower temperature is desirable. Thus, when chemically tempered glass substrates are used, it is difficult to improve magnetic characteristics by heat treatment at a high temperature and it is difficult to obtain magnetic recording media having high coercive force. The above chemically tempered glass has a specific elastic modulus of about 30 × 10 6 Nm/kg and Young's modulus of about 80 GPa, and hence exhibits poor stiffness. Therefore, it cannot be used for 3.5-inch high-end disc substrates and thinner disc substrates. Moreover, chemically tempered glass substrates have stress layers on both surfaces, and these stress layers may cause deflexion when the stress layers do not have uniform and equivalent stress. Therefore, it is difficult to realize smaller flying height of magnetic heads and high-speed rotation with chemically tempered glass. The conventional crystallized glasses such as those disclosed in References 2 and 3 exhibit excellent heat resistance because they do not show transition. However, glass substrates for magnetic recording media are required to have more excellent surface smoothness as higher recording density is attempted. This is because higher recording density of magnetic recording media requires a smaller flying height of magnetic heads. However, because crystallized glass contains many microparticles, they hardly afford substrates of a surface roughness (Ra) of 1 nm (10Å) or less. As a result, substrates have poor surface smoothness and surface configuration of discs is degraded. An unevenness control layer, for example, is formed on substrates to prevent a magnetic head from being absorbed to magnetic discs. However, it is difficult to control surface homology of such an unevenness control layer provided on a substrate of crystallized glass. The alkali-free glass disclosed in Reference 4 has a high transition temperature as high as 730°C at most. However, it has a specific elastic modulus of only 27-34 × 10 6 Nm/kg and Young's modulus of around 70-90 GPa, and hence it can no way meet the demand of further thinner magnetic disc substrates. As a substrate of excellent heat resistance, the carbon substrate disclosed in Japanese Patent Unexamined Publication No. Hei 3-273525 (referred to as Reference 5 hereinafter) can be mentioned. However, the carbon substrate has a specific elastic modulus of around 15-19 × 10 6 Nm/kg and hence it is inferior to glass in mechanical strength. Therefore, it can hardly meet the demand for a thinner substrate required for the production of smaller magnetic discs. In addition, carbon substrates have many surface defects and hence it is difficult to realize higher recording density with them. Thus, no oxide glass which has high specific elastic modulus or Young's modulus, exhibits high heat resistance and excellent surface smoothness (surface roughness ≦ 0.5 nm (5Å)) and can be produced in a large scale at low cost is currently found in the market. Even the SiO 2 -Al 2 O 3 -MgO glasses, which is well known as commercially available oxide glass of high Young's modulus, have a Young's modulus of around 80-90 GPa at most. Therefore, an object of the present invention is to provide a novel glass material satisfying high strength, high impact resistance, high specific elastic modulus, high heat -resistance and high surface smoothness required for the production of smaller and thinner substrates for information recording media of higher recording density in future. More specifically, the object of the present invention is to provide a glass substrate having a specific elastic modulus of 36 × 10 6 Nm/kg or more and glass transition temperature of 700°C or higher, not containing microcrystalline particles, and exhibiting high surface smoothness (surface roughness Ra of 0.9 nm (9 Å) or less). A further object of the present invention is to provide a glass substrate having a Young's modulus of 110 GPa or more, preferably a glass transition temperature of 700 °C or higher, not containing microcrystalline particles, and exhibiting high surface smoothness (surface roughness Ra of 0.9 nm (9Å) or less). Degree of deflexion and warp of disc substrates can be estimated from specific elastic modulus (= Young's modulus/specific gravity) of the material composing the substrate. In order to suppress deflexion and warp of thinner substrates to the extent that such problems do not occur, materials with higher specific elastic modulus are required. However, in a certain glass composition, effects added to specific elastic modulus by glass components have not been well known. An object of the present invention is to provide a novel glass exhibiting higher specific elastic modulus than those presently known by establishing theoretical relation between specific elastic modulus and glass composition, focussing on SiO 2 -Al 2 O 3 -RO glasses (wherein R is a bivalent metal) which are preferred as substrates for information recording media such as magnetic discs, and investigating effects of glass components to specific elastic modulus. A further object of the present invention is to provide substrates for information recording media using the above glass and information recording media using the above substrate. SUMMARY OF THE INVENTION Therefore, in order to provide glass materials having a specific elastic modulus G of 36 × 10 6 Nm/kg or more or a Young's modulus of 110 GPa or more, the present inventors have designed novel glass compositions based on the theoretical calculation suggested by themselves and conducted various experiments and researches. As a result, it was found that novel glass which has a high specific elastic modulus and/or high Young's modulus not obtained so far, excellent surface smoothness and high heat resistance and producible in a large scale at a low cost can be obtained by using components greatly contributing to the improvement of Young's modulus such as Al 2 O 3 , Y 2 O 3 , MgO, TiO 2 and rare earth metal oxides in a large amount. In addition, with respect to SiO 2 -Al 2 O 3 -RO glasses, new glass materials exhibiting higher specific elastic modulus than those presently known have been found. Based on this finding, the present invention has been completed. The present invention is described hereinafter. Glass for substrates having a specific elastic modulus G of 36 × 10 6 Nm/kg or more (hereinafter referred to Glass (1)). Glass containing, as oxides constituting the glass, SiO 2 : 25-52%, Al 2 O 3 : 5-35%, MgO: 15-45%, Y 2 O 3 : 0-17%, TiO 2 : 0-25%, ZrO 2 ; 0-8%, CaO: 1-30%, provided that Y 2 O 3 + TiO 2 + ZrO 2 + CaO: 5-30% and B 2 O 3 + P 2 O 5 : 0-5%, in molar %, and having a specific elastic modulus of 36 × 10 6 Nm/kg or more (hereinafter referred to Glass (2)). Glass having a composition containing, as oxides constituting the glass, SiO 2 : 25-50%, Al 2 O 3 : 10-37%, MgO: 5-40%, TiO 2 : 1-25%, in molar %, and having a specific elastic modulus of 36 × 10 6 Nm/kg or more (hereinafter referred to Glass (3)). Glass having a composition containing, as oxides constituting the glass, SiO 2 : 25-50%, Al 2 O 3 : 20-40%, CaO: 8-30%, Y 2 O 3 : 2-15%, in molar %, and having a specific elastic modulus of 36 × 10 6 Nm/kg or more (hereinafter referred to Glass (4)). A substrate for information recording media composed of the above glass material and a magnetic disc comprising said substrate and magnetic layer thereon. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic cross-sectional view of a magnetic disc 1 comprising a glass substrate 2, on which an unevenness control layer 3, underlying layer 4, magnetic layer 5, protective layer 6 and lubricating layer 7 are provided in this order. DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be further explained hereinafter. The term "glass" used in the present invention means glass which does not substantially contain crystal grains, and does not mean those called as crystallized glass or glass ceramics containing at least 20% of crystal grains. Glass (1) Glass (1) is a glass for substrate characterized in that it has a specific elastic modulus G of 36 × 10 6 Nm/kg or more. When the specific elastic modulus G is less than 36 × 10 6 Nm/kg, a substrate made of the glass exhibits severe deflexion, and, for example, a substrate of such glass having a thickness of 0.43 mm or less required for magnetic recording media discs of the next generation may exhibit a maximum deflexion of 1.4 µm or more. As a result, flying stability of magnetic heads cannot be obtained and hence reconstruction of recorded information cannot be performed stably. To obtain a substrate exhibiting a maximum deflexion of 1.25 µm or less, glass having a specific elastic modulus G of 37 × 10 6 Nm/kg or more is preferred. Glass having a specific elastic modulus of 42 × 10 6 Nm/kg or more is more preferred because such glass can afford a substrate which exhibits a maximum deflexion of 1.4 µm or less even if it is made as a substrate of a thickness of 0.38 mm or less to meet the demand of further thinner magnetic discs. While a specific elastic modulus as high as possible is preferred, practical value thereof is about 45 × 10 6 Nm/kg or less. Glass (1) is glass whose surface roughness (Ra) can be made 0.9 nm (9Å) or less, in addition to having a specific elastic modulus G of 36 × 10 6 Nm/kg or more. Higher surface smoothness enables a smaller flying height of magnetic heads, which is required for higher density of magnetic discs. A surface roughness (Ra) of 0.9 nm (9Å) or less can realize a flying height smaller than the conventional flying height. For realizing further higher density of magnetic discs, glass whose surface roughness (Ra) can be made 0.5 nm (5Å) or less is preferred. Glass (1) is glass having a transition temperature of 700°C or more, in addition to having a specific elastic modulus G of 36 × 10 6 Nm/kg or more and/or being possible to have surface roughness (Ra) of 0.9 nm (9 Å) or less. Because of a transition temperature of 700°C or higher, heat resistance of substrate higher than that of the conventional one can be obtained, in addition to the reduced deflexion. Thus, a magnetic disc having improved magnetic characteristics such as coercive force can be provided. As specific examples of glass (1) having the above characteristics, glasses (2), (3) and (4) can be mentioned. These glasses are oxide glasses comprising cations having a small ionic radius, strong chemical bonding force and high packing density in the glass structure in order to satisfy the characteristics of glass (1). Glass (2) Glass (2) is a composition constituted mainly for obtaining a high specific elastic modulus, and has a specific elastic modulus G of 36 × 10 6 Nm/kg or more. A glass having specific elastic modulus G of 36 × 10 6 Nm/kg or more can afford a substrate exhibiting reduced deflexion. For example, even when it is made into a substrate having a thickness of 0.43 mm or less required for magnetic recording media discs of the next generation, it may exhibit a maximum deflexion of 1.4 µm or less. As a result, excellent flying stability of magnetic heads can be obtained and hence reconstruction of recorded information can be performed stably. To obtain a substrate exhibiting a maximum deflexion of 1.25 µm or less, glass having a specific elasticmodulus G of 37 × 10 6 Nm/kg or more is preferred. Glass having a specific elastic modulus of 42 × 10 6 Nm/kg or more is more preferred because such glass can afford a substrate which exhibits a maximum deflexion of 1.4 µm or less even if it is made into a substrate of a thickness of 0.38 mm or less to meet the demand of further thinner magnetic discs. While a specific elastic modulus as high as possible is preferred, practical value thereof is about 45 × 10 6 Nm/kg or less. Glass (2) can have a surface roughness (Ra) of 0.9 nm (9Å) or less. Higher surface smoothness enables a smaller flying height of magnetic heads, which is required for higher density of magnetic discs. A surface roughness (Ra) of 0.9 nm (9Å) or less can realize a flying height smaller than the conventional flying height. For realizing further higher density of magnetic discs, a surface roughness (Ra) of 0.5 nm (5Å) or less is preferred. Glass (2) is glass having a transition temperature of 700°C or higher. Because of a transition temperature of 700°C or higher, a substrate having heat resistance higher than that of the conventional one, in addition to the reduced deflexion, can be obtained. Thus, a magnetic disc having improved magnetic characteristics such as coercive force can be provided. SiO 2 acts to form the network structure of glass and is a component for improving stability of glass structure, i.e., enhancing crystallization stability against devitrification. Further, SiO 2 in combination with an intermediate oxide such as Al 2 O 3 can enhance mechanical properties of glass necessary for substrates for magnetic recording media such as strength and stiffness and also improve heat resistance of glass. However, oxide glass containing more than 52% of SiO 2 as a main component of the glass no longer exhibits a specific elastic modulus exceeding 36 × 10 6 Nm/kg, and therefore the content of SiO 2 is suitably 52% or less. On the other hand, a SiO 2 content less than 25% significantly degrades the crystallization stability of glass, and sufficiently stable glass suitable for large scale production cannot be obtained with such a content. Therefore, the lower limit of the SiO 2 content is 25%. Accordingly, the content of SiO 2 is suitably in the range of 25-52%, preferably in the range of 30-50%. Al 2 O 3 is very important as a component for imparting high heat resistance and high durability to glass and also as a component for enhancing stability of glass structure and stiffness together with SiO 2 . In particular, when Al 2 O 3 is introduced into glass to substitute SiO 2 , Al 2 O 3 enters into the skeletal structure of glass and markedly enhance Young's modulus and heat resistance of glass as a skeletal structure-forming component. That is, Al 2 O 3 is a component essential for enhancing Young's modulus and improving heat resistance. However, a content of Al 2 O 3 less than 5% cannot sufficiently improve Young's modulus of glass. When the content of Al 2 O 3 exceeds 35%, MgO, which is a component contributing to improvement of specific elastic modulus of glass, cannot be introduced in a sufficient amount and hence melt characteristics of glass at a high temperature is also degraded. Therefore, the content of Al 2 O 3 is suitably in the range of 5-35%, preferably in the range of 7-32%. MgO is a component introduced for enhancing stiffness and strength of glass and improving melt characteristics of glass at a high temperature. It also contributes to improvement of crystallization stability and homogeneity of glass. In particular, when the content of Al 2 O 3 is less than 20%, it is preferred to introduce a large amount of MgO in order to maintain high specific elastic modulus of glass. However, sufficiently stable glass suitable for large scale production cannot be obtained with a MgO content exceeding 45%. On the other hand, if the content of MgO is less than 15%, Young's modulus of glass tends to be lowered. Therefore, the content of MgO is suitably in the range of 15-45%, preferably in the range of 22-40%. Y 2 O 3 is a component introduced for enhancing crystallization stability of glass and improving durability and melt characteristics of glass at a high temperature. In particular, introduction of a small amount of Y 2 O 3 markedly contributes to enhancement of specific elastic modulus of glass and improvement of glass homogeneity. However, while Y 2 O 3 improves Young's modulus of glass, too much amount of Y 2 O 3 steeply increases the specific gravity of glass and hence disadvantageously tends to degrade the specific elastic modulus of the glass. Therefore, the content of Y 2 O 3 is suitably 17% or less, preferably 15% or less. To obtain distinct effects of the addition of Y 2 O 3 , the content of Y 2 O 3 is preferably 0.5% or more. TiO 2 acts as both of a glass skeletal structure-forming component and a modifying component. It lowers high temperature viscosity, improves melt characteristics of glass, enhances structure stability and improves durability. By introducing TiO 2 as a glass component, Young's modulus of glass can be markedly improved without significantly increasing specific gravity of glass. In particular, in glass containing large amounts of MgO, Al 2 O 3 and the like, TiO 2 improves melt characteristics at a high temperature and crystallization stability of glass and is surely expected to enhance specific elastic modulus of glass in combination with other oxides such as MgO and Al 2 O 3 . However, when too much of TiO 2 is introduced, glass tends to show phase separation and hence disadvantageously degrade crystallization stability and homogeneity of glass. Therefore, the content: of TiO 2 is suitably 25% or less, preferably 20% or less, To obtain distinct effects of the addition of TiO 2 , the content of TiO 2 is preferably 1% or more. CaO is a component introduced for enhancing stiffness and strength of glass and improving melt characteristics at a high temperature like MgO. CaO, like MgO, also contributes to improvement of crystallization stability of glass and homogeneity of glass. As described above, when the content of Al 2 O 3 is less than 20%, it is preferred to introduce a large amount of MgO to maintain high specific elastic modulus of glass. In such a case, CaO is a component introduced mainly for improving melt characteristics at a high temperature and crystallization stability of glass. However, glass having crystallization stability suitable for large scale production cannot be obtained with a content of CaO exceeding 30%. Therefore, the content of CaO is suitably 30% or less, preferably 27% or less. In order to obtain distinct effects of the addition of CaO, the content of CaO is preferably 2% or more. ZrO 2 is a component introduced mainly for enhancing durability and stiffness of glass. Addition of ZrO 2 in a small amount improves heat resistance of glass and also enhances crystallization stability against devitrification. However, the content of ZrO 2 exceeds 8%, melt characteristics of glass at a high temperature is remarkably degraded, and surface smoothness of glass deteriorates and specific gravity increases. Therefore, the content of ZrO 2 is suitably 8% or less, preferably 6% or less. In order to obtain distinct effects of the addition of ZrO 2 , the content of ZrO 2 is preferably 0.5% or more. Y 2 O 3 + TiO 2 + ZrO 2 + CaO is suitably in the range or 1-30%. These components are for contributing to enhancement of Young's modulus of glass and enhancement of crystallization stability. When the total amount of these components is less than 1%, Young's modulus of glass tends to be lowered and crystallization stability of glass tends to be degraded. On the other hand, these components increase specific gravity of glass, and hence introduction in a large amount may lower the specific elastic modulus of glass. Therefore, the total content of Y 2 O 3 + TiO 2 - ZrO 2 + CaO is suitably in the range of 1-30%, preferably in the range of 5.5-27%. P 2 O 5 and B 2 O 3 are components added for controlling melt characteristics of glass at a high temperature. For example, introduction of P 2 O 5 and B 2 O 3 in a small amount does not substantially affect on specific elastic modulus of class but significantly lower high temperature viscosity of glass. Therefore, it is very effective for facilitating melting of glass. For improvement of melt characteristics of glass and control of crystallization stability and physical characteristics of glass, the total amount of B 2 O 3 + P 2 O 5 is suitably 5% or less, preferably 3.5% or less. In order to obtain distinct effects of the addition of B 2 O 3 and P 2 O 5 , the total content is preferably 0.5% or more. As 2 O 3 and Sb 2 O 3 are components added as degassing agents in order to obtain homogenous glass. By adding As 2 O 3 or Sb 2 O 3 or both in a suitable amount selected depending on high temperature viscosity of glass, more homogenous glass can be obtained. However, if too much of the degassing agents is added, specific gravity of glass is increased and specific elastic modulus tends to be lowered. In addition, they may react with and damage a platinum crucible for melting. Therefore, the content is suitably 3% or less, preferably 2% or less. In order to obtain distinct effects of the addition of the degassing agent, the content is preferably 0.2% or more. The other components such as V 2 O 5 , Cr 2 O 3 , ZnO, SrO, NiO, CoO, Fe 2 O 3 , CuO etc. may be added for controlling melt characteristics at a high temperature, physical properties and the like of glass. For example, by adding a small amount of a colorant such as V 2 O 5 , Cr 2 O 3 , CuO and CoO to glass, infrared ray absorbing property can be imparted to the glass and heating treatment of magnetic layer by irradiating with a heat lamp can be effectively performed. For improving melt characteristics of glass and controlling crystallization stability and physical properties of glass, the total amount of ZnO + SrO + NiO + CoO + FeO + CuO + Fe 2 O 3 + Cr 2 O 3 + B 2 O 3 + P 2 O 5 + V 2 O 5 is suitably 5% or less, preferably 4% or less. Other than the components mentioned above, addition of Fe 2 O 3 and the like which are sometimes contained as impurities of starting material and a clarifier for glass such as Cl, F and SO 3 in an amount of 1% or less does not substantially degrade the intended physical characteristics of the glass according to the present invention. The above-mentioned glass is alkali-free glass which does not substantially contain alkali substances. Therefore, when a film is formed on a substrate made of this glass, any alkali substances do not migrate into the film and hence the film is not adversely affected. Glass (3) Glass (3) is a composition constituted mainly for obtaining a high specific elastic modulus, and the glasses have a specific elastic modulus G of 36 × 10 6 Nm/kg or more. Glass having specific elastic modulus G of 36 × 10 6 Nm/kg or more can afford a substrate exhibiting reduced deflection. For example, even when it is made into a substrate having a thickness of 0.43 mm or less required for magnetic recording media discs of the next generation, it may exhibit a maximum deflection of 1.4 µ m or less. As a result, excellent flying stability of magnetic heads can be obtained and hence reconstruction of recorded information can be stably performed. To obtain a substrate exhibiting a maximum deflection of 1.25 µm or less, glass having a specific elastic modulus G of 37 × 10 6 Nm/kg or more is preferred. Glass having a specific elastic modulus of 42 × 10 6 Nm/kg or more is more preferred because such glass can afford a substrate which exhibits a maximum deflection of 1.4 µm or less even if it is made into a substrate of a thickness of 0.38 mm or less to meet the demand of further thinner magnetic discs. While a specific elastic modulus as high as possible is preferred, practical value-thereof is about 45 × 10 6 Nm/kg or less. Glass (3) can have a surface roughness (Ra) of 0.9 nm (9Å) or less. Higher surface smoothness enables a smaller flying height of magnetic heads, which is required for higher density of magnetic discs. A surface roughness (Ra) of 0.9 nm (9Å) or less can realize a flying height smaller than the conventional flying height. For realizing further higher density of magnetic discs, a surface roughness (Ra) of 0.5 nm (5Å) or less is preferred. Glass (3) is glass having a transition temperature of 700°C or higher. Because of a transition temperature of 700°C or higher, a substrate having heat resistance higher than that of the conventional one, in addition to the reduced deflection, can be obtained. Thus, a magnetic disc having improved magnetic characteristics such as coercive force can be provided. SiO 2 acts as an oxide for forming the network structure of glass and is a component for improving stability of glass structure, i.e., enhancing crystallization stability against devitrification. Further, SiO 2 in combination with an intermediate oxide such as Al 2 O 3 can enhance mechanical properties of glass necessary for substrates for magnetic recording media such as strength and stiffness and also improve heat resistance of glass. However, glass containing more than 50% of SiO 2 cannot contain a large amount of Al 2 O 3 which is a component contributing to improvement of impact resistance and mechanical strength of glass. Therefore, in order to obtain glass having a high specific elastic modulus, the upper limit of the SiO 2 content is suitably 50%. On the other hand, a SiO 2 content less than 25% significantly degrades the crystallization stability of glass and sufficiently stable glass suitable for large scale production cannot be obtained with such a content. Therefore, the lower limit of the SiO 2 content is suitably 25%. Accordingly, the content of SiO 2 is suitably in the range of 25-50%, preferably in the range of 30-49%. Al 2 O 3 is very important as a component for imparting high heat resistance and high durability to glass and also as a component for enhancing stability of glass structure and stiffness together with SiO 2 . In particular, when Al 2 O 3 is introduced into glass to substitute SiO 2 , Al 2 O 3 enters into the skeletal structure of glass and markedly enhance Young's modulus and heat resistance of glass as a skeletal structure-forming component. That is, Al 2 O 3 is a component essential for enhancing Young's modulus and improving heat resistance. However, when MgO is used with a content of 25% or less in order to further enhance flexural strength and impact resistance, a content of Al 2 O 3 less than 10% cannot sufficiently improve Young's modulus of glass and hence a desired specific elastic modulus cannot be obtained. When the content of Al 2 O 3 exceeds 37%, melt characteristics of glass at a high temperature is degraded, and hence homogenous glass cannot obtained and crystallization stability of glass is degraded. Therefore, the upper limit of the content of Al 2 O 3 is suitably 37%. The content of Al 2 O 3 is suitably in the range of 10-37%, preferably in the range of 11-35%. MgO is a component introduced for enhancing stiffness and strength of glass and improving melt characteristics of glass at a high temperature. It also contributes to improvement of crystallization stability and homogeneity of glass. In particular, when Al 2 O 3 , which is a component for greatly improving Young's modulus of glass, is introduced in a large amount, MgO is preferably used for improving stability of glass structure as well as lowering melt characteristics at a high temperature to facilitate melting of glass. However, sufficiently stable glass suitable for large scale production containing a large amount of Al 2 O 3 for enhancing impact resistance and strength of glass cannot be obtained with a MgO content exceeding 40%. On the other hand, with a content of MgO of less than 5%, glass exhibiting sufficient stability and high specific elastic modulus cannot be obtained. Therefore, the content of MgO is suitably in the range of 5-40%, preferably in the range of 7-35%. TiO 2 acts as both of a glass skeletal structure-forming component and a modifying component. It lowers high temperature viscosity, improves melt characteristics of glass and enhances structure stability and durability. By introducing TiO 2 as a glass component, Young's modulus of glass can be markedly improved without significantly increasing specific gravity of glass. In particular, in glass containing a large amount of Al 2 O 3 , TiO 2 improves melt characteristics at a high temperature and crystallization stability of the glass and is surely expected to enhance specific elastic modulus of the glass in combination with Al 2 O 3 . However, when the content of TiO 2 exceeds 25%, glass tends to show phase separation and hence crystallization stability and homogeneity of glass tend to be disadvantageously degraded. On the other hand, addition of TiO 2 of 1% or more markedly improves melt characteristics of glass at a high temperature. Therefore, the content of TiO 2 is suitably in the range of 1-25%, preferably in the range of 2-20%. Y 2 O 3 is a component introduced for improving Young's modulus, enhancing crystallization stability of glass and improving durability and melt characteristics at a high temperature of glass. In particular, when a large amount of Al 2 O 3 is introduced for enhancing flexural strength, impact resistance and the like, Y 2 O 3 exerts excellent effect as a melting aid for Al 2 O 3 . For example, when Al 2 O 3 of 25% or more is introduced into glass, homogenous glass can be obtained by adding Y 2 O 3 . However, since Y 2 O 3 is relatively expensive, a smaller content is preferred from economical point of view. In addition, while a proper amount of Y 2 O 3 greatly contributes to enhancement of specific elastic modulus of glass, when the content of Y 2 O 3 exceeds 17%, increase of specific gravity overwhelms increase of Young's modulus of glass, and hence the addition can no longer contribute to improvement of specific elastic modulus of the glass. Therefore, the content of Y 2 O 3 is suitably in the range of 0-17%, preferably in the range of 1-15% depending on the introduced amount of Al 2 O 3 . CaO is a component capable of enhancing stiffness and strength of glass and improving melt characteristics of glass at a high temperature like MgO. CaO also contributes to improvement of crystallization stability of glass and homogeneity of glass. When a large amount of Al 2 O 3 is introduced as a component greatly contributing to improvement of Young's modulus of glass, it is preferred to introduce CaO to improve stability of glass structure and to lower high temperature viscosity to facilitate melting. When the content of CaO exceeds 25%, glass containing a large amount of Al 2 O 3 for enhancing impact resistance and strength of glass and having crystallization stability suitable for large scale production cannot be obtained. Therefore, the upper limit of the content of CaO is suitably 25%. In order to obtain distinct effects of the addition of CaO, the content of CaO is preferably 2% or more. ZrO 2 is a component introduced mainly for enhancing durability and stiffness of glass. Addition of a small amount of ZrO 2 improves heat resistance of glass and also enhances crystallization stability against devitrification. However, when the content of ZrO 2 exceeds 8%, melt characteristics of glass at a high temperature is markedly degraded, and surface smoothness of glass deteriorates and specific gravity increases. Therefore, the content of ZrO 2 is suitably 8% or less, preferably 6% or less. In order to obtain distinct effects of the addition of ZrO 2 , the content of ZrO 2 is preferably 0.5% or more. As 2 O 3 and Sb 2 O 3 are components added as degassing agents in order to obtain homogenous glass. By adding As 2 O 3 or Sb 2 O 3 or both in a suitable amount selected depending on high temperature viscosity of glass, more homogenous glass can be obtained. However, if the amount of the degassing agents is too much, specific gravity of glass is increased and specific elastic modulus tends to be lowered. In addition, they may react with and damage a platinum crucible used for melting. Therefore, the content is suitably 3% or less, preferably 2% or less. In order to obtain distinct effects of the addition of the degassing agent, the content is preferably 0.2% or more. The other components such as P 2 O 5 , V 2 O 5 , B 2 O 3 , Cr 2 O 3 , ZnO, SrO, NiO, CoO, Fe 2 O 3 , CuO etc. may be added for controlling melt characteristics at a high temperature and physical properties and the like of glass. For example, addition of a small amount of P 2 O 5 does not substantially affect specific elastic modulus of glass but significantly lower high temperature viscosity of glass, and hence it is effective for facilitating melting of glass. By adding a small amount of a colorant such as V 2 O 5 , Cr 2 O 3 , CuO and CoO to glass, infrared ray absorbing property can be imparted to the glass and heating treatment of magnetic layer by irradiation with a heat lamp can be effectively performed. For improving melt characteristics of glass at a high temperature and controlling physical and thermal properties of glass, the total amount of ZnO + SrO + NiO + CoO + FeO + CuO + Fe 2 O 3 + Cr 2 O 3 + B 2 O 3 + P 2 O 5 + V 2 O 5 is suitably 5% or less. Other than the components mentioned above, addition of Fe 2 O 3 and the like which are sometimes contained as impurities of starting material and a clarifier for glass such as Cl, F and SO 3 in an amount of 1% or less does not substantially degrade the intended physical characteristics of the glass according to the present invention. When Li 2 O is contained in the glass, chemical tempering treatment by ion exchange can be performed to enhance strength of the glass. On the other hand, when the glass is alkali-free glass not containing Li 2 O, any alkali substances do not migrate into a film formed on a substrate and hence the film is not adversely affected. Glass (4) Glass (4) is a composition constituted mainly for obtaining a high specific elastic modulus, and the glasses have a specific elastic modulus G of 36 × 10 6 Nm/kg or more. Glass having specific elastic modulus G of 36 × 10 6 Nm/kg or more can afford a substrate exhibiting reduced deflection. For example, even when it is made into a substrate having a thickness of 0.43 mm or less required for magnetic recording media discs of the next generation, it exhibits a maximum deflection of 1.4 µm or less. As a result, excellent flying stability of magnetic heads can be obtained and hence reconstruction of recorded information can be stably performed. To obtain a substrate exhibiting a maximum deflection of 1.25 µm or less, glass having a specific elastic modulus G of 37 × 10 6 Nm/kg or more is preferred. Glass having a specific elastic modulus of 42 × 10 6 Nm/kg or more is more preferred because such glass can afford a substrate which exhibits a maximum deflection of 1.4 µm or less even if it is made into a substrate of a thickness of 0.38 mm or less to meet the demand of further thinner magnetic discs. While a specific elastic modulus as high as possible is preferred, practical value thereof is about 45 × 10 6 Nm/kg or less. Glass (4) can have a surface roughness (Ra) of 0.9 nm (9Å) or less. Higher surface smoothness enables a smaller flying height of magnetic heads, which is required for higher density of magnetic discs. A surface roughness (Ra) of 0.9 nm (9Å) or less can realize a flying height smaller than the conventional flying height. For realizing further higher density of magnetic discs, a surface roughness (Ra) of 0.5 nm (5Å) or less is preferred. Glass (4) is glass having a transition temperature of 700°C or higher. Because of a transition temperature of 700°C or higher, a substrate having heat resistance higher than that of the conventional one, in addition to the reduced deflection, can be obtained. Thus, a magnetic disc having improved magnetic characteristics such as coercive force can be provided. SiO 2 acts as an oxide for forming the network structure of glass and is a component for improving stability of glass structure, i.e., enhancing crystallization stability against devitrification. Further, SiO 2 in combination with an intermediate oxide such as Al 2 O 3 can enhance mechanical properties of glass necessary for substrates for magnetic recording media such as strength and stiffness and also improve heat resistance of glass. However, oxide glass of CaO-Al 2 O 3 -SiO 2 system containing more than 50% of SiO 2 no longer exhibits a specific elastic modulus exceeding 36 × 10 6 Nm/kg, and therefore the content of SiO 2 is suitably 50% or less. On the other hand, a SiO 2 content less than 25% significantly degrades the crystallization stability of glass and sufficiently stable glass suitable for large scale production cannot be obtained with such a content. Therefore, the lower limit of the SiO 2 content is 25%. Accordingly, the content of SiO 2 is suitably in the range of 25-50%, preferably in the range of 30-50%. Al 2 O 3 is very important as a component for imparting high heat resistance and high durability and also as a component for enhancing stability of glass structure and stiffness together with SiO 2 . In particular, when Al 2 O 3 is introduced into glass to substitute SiO 2 , Al 2 O 3 enters into the skeletal structure of glass and markedly enhance Young's modulus and heat resistance of glass as a skeletal structure-forming component. That is, Al 2 O 3 is a component essential for enhancing Young's modulus and improving heat resistance. However, a content of Al 2 O 3 less than 20% cannot sufficiently improve Young's modulus of glass. On the other hand, when the content of Al 2 O 3 exceeds 40%, melt characteristics of glass at a high temperature is degraded and hence homogenous glass cannot obtained, and crystallization stability of glass is also degraded. Therefore, the content of Al 2 O 3 is suitably in the range of 20-40%, preferably in the range of 21-37%. CaO is a component for enhancing stiffness and strength of glass and improving melt characteristics at a high temperature. It also contributes to improvement of crystallization stability of glass and homogeneity of glass. In particular, when a large amount of Al 2 O 3 is introduced as a component greatly contributing to improvement of Young's modulus of glass, it is necessary to introduce CaO to improve stability of glass structure and to lower high temperature viscosity to facilitate melting of glass. However, when the content of CaO is less than 8%, crystallization stability of glass is markedly degraded. On the other hand, when the content of CaO exceeds 30%, Young' s modulus of glass tends to be lowered. Therefore, the content of CaO is suitably in the range of 8-30%, preferably in the range of 10-27%. Y 2 O 3 is a component introduced for improving Young's modulus, enhancing crystallization stability of glass and improving durability and melt characteristics of glass at a high temperature. In particular, when a large amount of Al 2 O 3 is introduced for enhancing Young's modulus of glass, Y 2 O 3 exerts excellent effect as a melting aid for Al 2 O 3 . For example, when Al 2 O 3 of 25% or more is introduced into glass, homogenous glass can be obtained by adding Y 2 O 3 as a melting aid. However, since Y 2 O 3 is relatively expensive, its content is preferably a relatively small amount, i.e., 15% or less depending on the properties required for glass. On the other hand, the content of Y 2 O 3 is too small, melt characteristics of glass at a high temperature is degraded and specific elastic modulus of glass is lowered. Therefore, the lower limit of the content of Y 2 O 3 is suitably 2%. Accordingly, the content of Y 2 O 3 is suitably in the range of 2-15%, preferably in the range of 3-12%. MgO is a component for enhancing stiffness and strength of glass and improving melt characteristics of glass at a high temperature. It also contributes to improvement of crystallization stability and homogeneity of glass, and improves specific elastic modulus. This component may optionally be added as desired. However, when the content of MgO exceeds 20%, the essential component, CaO, cannot be introduced in a large amount and thus crystallization stability tends to be lowered. Therefore, the upper limit of the content of MgO is suitably 20%. In order to obtain distinct effects of addition of MgO, its content is preferably 5% or more. TiO 2 acts as both cf a glass skeletal structure-forming component and a modifying component. It lowers high temperature viscosity, improves solubility of glass and enhances structure stability and durability. By introducing TiO 2 as a glass component, Young's modulus of glass can be markedly improved without significantly increasing specific gravity of glass. However, if too much amount of TiO 2 is introduced into CaO-Al 2 O 3 -SiO 2 system oxide glass, the glass tends to show phase separation and crystallization stability and homogeneity are disadvantageously degraded. Therefore, the content of TiO 2 is suitably 25% or less, preferably 20% or less. In order to obtain distinct effects of addition of TiO 2 , its content is preferably 1% or more. Li 2 O is a component mainly for lowering high temperature viscosity of glass to facilitate melting. In particular, when the Al 2 O 3 content is high, addition of small amount of Li 2 O is very effective for obtaining homogenous glass. However, if its content is too high, durability of glass is degraded and Young's modulus tends to be lowered. Therefore, the content of Li 2 O is suitably 15% or less, preferably 12% or less. In order to obtain distinct effects of the addition of Li 2 O, the content of Li 2 O is preferably 1.5% or more. As 2 O 3 and Sb 2 O 3 are components added as degassing agents in order to obtain homogenous glass. By adding As 2 O 3 or Sb 2 O 3 or both in a suitable amount selected depending on high temperature viscosity of glass, more homogenous glass can be obtained. However, if the amount of the degassing agents is too much, specific gravity of glass is increased and specific elastic modulus tends to be lowered. In addition, they may react with and damage a platinum crucible for melting. Therefore, the content is suitably 3% or less, preferably 2% or less. In order to obtain distinct effects of the addition of the degassing agent, the content is preferably 0.2% or more. The other components such as P 2 O 5 , V 2 O 5 , B 2 O 3 , Cr 2 O 3 , ZnO, SrO, NiO, CoO, Fe 2 O 3 , CuO etc. may be added for controlling melt characteristics at a high temperature and physical properties and the like of glass. For example, addition of a small amount of P 2 O 5 does not substantially affect specific elastic modulus of glass but significantly lower high temperature viscosity of glass and therefore melting of glass is facilitated. By adding a small amount of a colorant such as V 2 O 5 , Cr 2 O 3 , CuO and CoO to glass, infrared ray absorbing property can be imparted to the glass and heating treatment of magnetic layer by irradiation with a heat lamp can be effectively performed. For controlling physical and thermal properties of glass, the total amount of ZnO + SrO + NiO + CoO + FeO + CuO + Fe 2 O 3 + Cr 2 O 3 +B 2 O 3 + P 2 O 5 + V 2 O 5 is suitably 5% or less. Other than the components mentioned above, addition of Fe 2 O 3 and the like which are sometimes contained as impurities of starting material and a clarifier for glass such as Cl, F and SO 3 in an amount of 1% or less does not substantially degrade the intended physical characteristics of the glass according to the present invention. Substrate for information recording media Substrates for information recording media of the present invention characterized by being composed of one of the glasses (1)-(4) mentioned above. Examples of information recording media includes magnetic recording media, and examples of magnetic recording media includes magnetic discs such as hard discs. Size and shape of the substrate can be selected in view of its use. The substrate of the present invention is characterized by exhibiting high surface smoothness. High surface smoothness means 0.9 nm (9 Å) or less, preferably 0.5 nm (5Å) or less of surface roughness (Ra). Since distance between magnetic head and magnetic disc can be reduced by a magnetic disc using a substrate with high surface smoothness, higher recording density is obtainable. Production method The glass and the glass substrate of the present invention is not particularly limited, and can be produced by a conventional production method. For example, glass materials of a given composition can be melted by the high temperature melting method, i.e., melted in air or inert gas atmosphere, homogenized by bubbling, addition of degassing agent, stirring or the like and molded into plate glass by well-known press method, down draw method or the like. Then, substrates for magnetic recording media of a desired size and shape can be obtained from the plate glass by processing such as cutting and polishing. In the polishing, surface roughness (Ra) 0.9 nm of (9Å) preferably 0.3-0.5 nm (3-5Å) can be obtained by wrapping or polishing with polishing powder of cerium oxide or the like. Because the glass of the present invention is excellent in the heat-resistance, surface smoothness, chemical resistance, optical properties and mechanical strength, it can be suitably used for substrates of information recording media such as magnetic discs, glass substrates for magnet optical discs, glass substrates for optoelectronics such as those for optical discs, heat resistant substrates for low temperature polycrystalline silicon liquid crystal display devices, which are expected as next generation LCD, substrates for various electric and electronic components or the like. Magnetic disc A magnetic disc (hard disk) comprising a substrate composed of the glass of the present invention described above and at least a magnetic layer formed on a main surface of the substrate will be explained hereinafter. As layers other than the magnetic layer, underlying layer, protective layer, lubricating layer, unevenness control layer and the like are optionally formed depending on functions of the disc. These layers can be formed by various thin film-forming techniques. Material for the magnetic layer is not particularly limited. For example, in addition to Co magnetic layers, ferrite magnetic layers, iron-rare earth metal magnetic layers and the like can be mentioned. The magnetic layer may be either for horizontal magnetic recording or vertical magnetic recording. Specific examples of the magnetic layer include, for example, those containing Co as a main component such as CoPt, CoCr, CoNi, CoNiCr, CoCrTa, CoPtCr and CoNiCrPt, CoNiCrTa, CoCrPtTa, CoCrPtSiO and the like. The magnetic layer may be consisted of multiple layers comprising a non-magnetic layer for noise reduction separating magnetic layers. The underlying layer of the magnetic layer may be selected depending on the nature of the magnetic layer. For example, the underlying layer may be those comprising one or more of non-magnetic metals such as Cr, Mo, Ta, Ti, W, V, B and Al, oxides, nitride, carbides and the like of those metals. For a magnetic layer comprising Co as the main component, an underlying layer of pure Cr or Cr alloy is preferred for improving magnetic characteristics. The underlying layer is not limited to a monolayer, and may be composed of identical or nonidentical multiple layers. For example, the underlying layer may be a multi-layer underlying layer such as Al/Cr/CrMo and Al/Cr/Cr. The unevenness control layer for preventing absorption of magnetic disc to magnetic head may be provided between the substrate and the magnetic layer or on the magnetic layer. Because surface roughness of the disc is properly controlled by the unevenness control layer, the magnetic disc is prevented from being absorbed to the magnetic disc and hence a highly reliable magnetic disc can be provided. Various materials and production methods for the unevenness control layer have been known and they are not particularly limited. For example, the material of the unevenness control layer may be one or more metals selected from Al, Ag, Ti, Nb, Ta, Bi, Si, Zr, Cr, Cu, Au, Sn, Pd, Sb, Ge, Mg and the like, alloys thereof, oxides, nitrides, carbides thereof and the like. For the ease of production, those produced from metals containing Al as a main component such as pure Al, Al alloys, Al oxides and Al nitrides are preferred. For good head stiction, surface roughness of the unevenness forming layer is preferably Rmax of 5-30 nm (50-300Å), more preferably Rmax of 10-20 nm (100-200Å). When the Rmax is less than 5 nm (50 Å), the disc surface is nearly flat, and hence the magnetic head and the disc are absorbed to each other. This may disadvantageously cause damage of the magnetic head and the magnetic disc, and head crash. On the other hand, when the Rmax exceeds 30 nm (300Å), glide height becomes larger and recording density is disadvantageously lowered. Unevenness may be provided on the surface of the glass substrate by a texturing treatment such as etching treatment and irradiation of laser lights instead of providing the unevenness control layer. The protective layer may be, for example, a Cr layer, Cr alloy layer, carbon layer, zirconia layer, silica layer or the like. These protective layers can be successively formed by an inline sputtering apparatus together with the underlying layer, the magnetic layer and the like. These protective layers may have either monolayer structure or multilayer structure comprising identical or different layers. Another protective layer may be provided on or instead of the protective layer explained above. For example, a silicon oxide (SiO 2 ) layer may be formed on the protective layer mentioned above by applying tetraalkoxysilane diluted in an alcoholic solvent, in which colloidal silica is further dispersed, and sintering the applied layer. This layer functions as both a protective layer and as an unevenness control layer. While various kinds of layers have been proposed as the lubricating layer, it is generally formed by applying a liquid lubricating agent, perfluoropolyether, diluted in a solvent such as freons by dipping, spin coating, spraying or the like and subjecting the coated layer to a heat treatment as required. EXAMPLES The present invention will be further explained with reference to the following examples. The glass compositions given in Examples 1-61 which are examples of glasses (1)-(4) are shown in Tables 1-5. As the starting materials of these glasses, SiO 2 , Al 2 O 3 , Al(OH) 3 , MgO, CaCO 3 , Y 2 O 3 , TiO 2 , ZrO 2 , Li 2 CO 3 and the like were weighed into 250-300 g portions according to the given compositions shown in Tables 1-5 and mixed sufficiently to provide formulated batches. Each of them was charged in a platinum crucible and melted in air for 3 to 5 hours at 1550°C. After the melting, the glass melt was cast into a carbon mold having a size of 180 × 15 × 25 mm or  67 × 5 mm, left to cool to the glass transition temperature, immediately transferred into an annealing furnace, annealed in the glass transition temperature range for about 1 hour and left to cool to room temperature in the furnace. The resulting glasses did not contain crystals deposited which can be observed by a microscope. Glass pieces having a size of 180 × 15 × 25 mm were polished into pieces having a size of 100 × 10 × 10 mm or 10 × 10 × 20 mm and used as samples for measurements of Young's modulus, specific gravity and DSC. Glass discs of 67 x thickness of 5 mm were polished into discs of 65 x thickness of 0.5 mm and used as samples for measurement of surface roughness. The plate glass pieces of 10 × 1 × 20 mm were ground into 150 mesh powder, and 50 mg of the resulting powder were charged into a platinum pan and subjected to the DSC measurement using MAC-3300 DSC apparatus. The measurement of Young's modulus was achieved by the ultrasonic method using samples of 100 × 10 × 10 mm. Values of surface roughness, specific gravity, Young's modulus, specific elastic modulus and transition temperature obtained in the measurements for the glasses of Examples 1-61 are shown in Tables 1-5 together with the glass compositions. The obtained glasses were cut into discs and their main surfaces were polished with cerium oxide to afford magnetic disc substrates having a radius of outer circular periphery of 32.5 mm, radius of inner circular periphery of 10.0 mm and thickness of 0.43 mm. The results of deflection measurement of the obtained discs are also shown in Tables 1-5. For comparison, compositions and characteristics of the ion exchanged glass substrate described in Japanese Patent Unexamined Publication No. Hei 1-239036 and the glass substrate described in Japanese Patent Unexamined Publication No. Hei 7-187711 are shown in Table 5 as Comparative Examples 1 and 2. As seen from the results shown in Tables 1-5, the glasses of Examples 1-61 have a high glass transition temperature and hence have sufficiently high heat resistance enough to bear desired heat treatment (usually at a temperature of 700°C or lower) . In particular, they exhibits high glass strength characteristics such as Young's modulus and/or specific elastic modulus. Therefore, when they are used as substrates for magnetic recording media, they are not likely to exhibit warp or walking even when they are rotated at a high speed, and hence they can meet the demand of further thinner substrates. In addition, they can have so excellent flatness as to be polished to a surface roughness (Ra) of 0.5 nm (5Å) or less, and therefore they can realize smaller flying height. Furthermore, the glasses of Examples 1-61 also exhibit reduced deflection. Therefore, the glasses of the present invention are useful as glass substrates for magnetic recording media. On the other hand, while the chemically tempered glass substrate of Comparative Example 1 is excellent in surface smoothness and flatness, it is substantially inferior to the glass substrates of the present invention in heat resistance and strength characteristics such as specific elastic modulus. Therefore, when it is used for producing magnetic recording media, it cannot be subjected to sufficient heat treatment for the magnetic layer to obtain high coercive force and hence magnetic recording media having high coercive force cannot be provided. Moreover, such a glass having a low specific elastic modulus of around 30 × 10 6 Nm/kg suffers severe warp and deflection when it is made into substrates and therefore it cannot be used for thinner substrates. The crystallized glass substrate of Comparative Example 2 is significantly inferior to the glasses of the present invention as to specific elastic modulus and surface smoothness. In particular, the surface smoothness of the substrate is degraded by relatively large crystal particles and hence higher recording density cannot be achieved. The glasses of the present invention have high Young's modulus, high specific elastic modulus and high heat resistance, and therefore they are extremely useful as substrates for magnetic discs. Method for producing hard disc As shown in Fig. 1, a magnetic disc 1 comprises a glass substrate 2 made of the glass of the above Example 1, on which unevenness control layer 3, underlying layer 4, magnetic layer 5, protective layer 6 and lubricating layer 7 are provided in this order. Each layer will be explained in detail. The substrate 1 was a disc having an outer circular periphery radius of 32.5 mm, inner circular periphery radius of 10.0 mm and thickness of 0.43 mm, whose main surfaces were subjected to precision polishing so that they should have surface roughness Ra of 0.4 nm (4 Å) and Rmax of 4 nm (40Å) The unevenness control layer is a thin AlN layer of 5-35% nitrogen content having average roughness of 5 nm (50Å) and surface roughness Rmax of 15 nm (150 Å.) The underlying layer is a thin layer of CrV composed of Cr: 83 at% and V: 17 at% having a thickness of about 60 nm (600Å.) The magnetic layer is a thin layer of CoPtCr composed of Co: 76 at%, Pt: 6.6 at%, Cr: 17.4 at% having a thickness of about 30 nm (300Å.) The protective layer is a carbon thin layer having a thickness of about 10 nm (100Å.) The lubricating layer is a layer having a thickness of 0.8 nm (8Å), which was formed by applying perfluoropolyether on the carbon protective layer by spin coating. The method for producing magnetic discs will be explained hereinafter. The glass of Example 1 was cut into a disc having an outer circular periphery radius of 32.5 mm, inner circular periphery radius of 10.0 mm and thickness of 0.5 mm and the both main surfaces were subjected to precision polishing so that they should have surface roughness Ra of 0.4 nm (4Å) and Rmax of 4 nm (40Å) to afford a glass substrate for magnetic discs. Subsequently, the above glass substrate was placed on a substrate holder and transferred into a charging chamber of inline sputtering apparatus. Then, the holder on which the glass substrate was placed was transferred to a first chamber where an Al target was etched and sputtering was performed at a pressure of 4 mtorr and substrate temperature of 350°C in an atmosphere of Ar + N 2 gas (N 2 = 4%). As a result, an AlN thin layer having surface roughness Rmax of 150Å and thickness of 50Å (unevenness forming layer) was provided on the glass substrate. The holder on which the glass substrate having the formed AlN layer was placed was then serially transferred into a second chamber provided with a CrV target (Cr: 83 at%, V: 17 at%) and a third chamber provided with a CoPtCr target (Co: 76 at%, Pt: 6.6 at%, Cr: 17.4 at%) successively, and thin layers were formed on the substrate. Sputtering was performed at a pressure of 2 mtorr and substrate temperature of 350°C in an Ar atmosphere, thereby a CrV underlying layer having a thickness of about 600 Å and CoPtCr magnetic layer having a thickness of about 300 Å were formed. The substrate having the formed unevenness control layer, underlying layer and magnetic layer was then transferred to a fourth chamber provided with a heater for heat treatment. The fourth chamber had an inner atmosphere of Ar gas ((pressure: 2 mtorr) and the heat treatment was performed. The substrate was then transferred into a fifth chamber provided with a carbon target, and a carbon protective layer having a thickness of about 100Å was formed under the same condition as used for forming of the CrV underlying layer and the CoPtCr magnetic layer except that the layer was formed in an atmosphere of Ar + H 2 gas (H 2 = 6%). Finally, the substrate after forming the carbon protective layer was taken out from the above inline sputtering apparatus, and a lubricating layer having a thickness of 8 A was formed by applying perfluoropolyether on the carbon protective layer by dipping to produce a magnetic disc. The present invention was explained by referring to the preferred examples, but the present invention is not limited to the above examples. Utility in Industry By using glass of the present invention, a glass substrate having high specific elastic modulus of 36 × 10 6 Nm/kg or more or high Young's modulus of 110 GPa or more, high transition temperature of 700°C or higher (high heat resistance), excellent surface smoothness (surface roughness Ra < 0.9 nm (9Å) and high strength can be provided. Because the glass of the present invention has excellent heat resistance, it can be subjected to heat treatment necessary for improving magnetic layer characteristics without causing deformation of substrates made from it. Therefore, it can have excellent flatness and hence achieve smaller flying height of magnetic head, i.e., higher recording density. Moreover, it exhibits high specific elastic modulus and high strength, it can realize thinner magnetic discs and prevent breakdown of magnetic discs. Furthermore, it can be stably obtained as glass and easily produced in an industrial scale. Therefore, it can be surely expected to be used as economical glass for substrates of next generation magnetic recording media.

Claims: Glass for substrates having a specific elastic modulus G of 36 × 10 6 Nm/kg or more. The glass of claim 1 whose surface roughness (Ra) can be made 0.9 nm (9 Å) or less. The glass of claim 1 which has a transition temperature of 700°C or higher . The glass of claim 1 having a composition, as oxides constituting the glass, of 25-52 mole-% SiO 2 , 5-35 mole-% Al 2 O 3 , 15-45 mole-% MgO, 0-17 mole-% Y 2 O 3 , 0-25 mole-% TiO 2 , 0-8 mole-% ZrO 2 , 1-30 mole-% CaO, provided that Y 2 O 3 + TiO 2 + ZrO 2 + CaO is 5-30 mole-% and B 2 O 3 + P 2 O 5 is 0-5 mole-%. The glass of claim 4 further containing 0-3 mole-% As 2 O 3 + Sb 2 O 3 and 0-5 mole-% ZnO + SrO + NiO + CoO + FeO + CuO + Fe 2 O 3 + Cr 2 O 3 + B 2 O 3 + P 2 O 5 +V 2 O 5 . The glass of claim 1 having a composition, as oxides constituting the glass, of 25-50 mole-% SiO 2 , 10-37 mole-% Al 2 O 3 , 5-40 mole-% MgO, and 1-25 mole-% TiO 2 The glass of claim 6 further containing 0-17 mole-% Y 2 O 3 , 0-8 mole-% ZrO 2 , 0-25 CaO mole-%, 0-3 mole-% As 2 O 3 + Sb 2 O 3 and 0-5 mole-% ZnO + SrO + NiO + CoO + FeO + CuO + Fe 2 O 3 + Cr 2 O 3 + B 2 O 3 + P 2 O 5 + V 2 O 5 . The glass of claim 1 having a composition containing, as oxides constituting the glass, of 25-50 mole-% SiO 2 , 20-40 mole-% Al 2 O 3 , 8- 30 mole-% CaO, 2-15 mole-% Y 2 O 3 . The glass of claim 8 further containing 0-20 mole-% MgO, 0-25 mole-% TiO 2 , 0-12 mole-% Li 2 O, 0-3 mole-% As 2 O 3 + Sb 2 O 3 and 0-5 mole-% ZnO + SrO + NiO + CoO + FeO + CuO +Fe 2 O 3 + Cr 2 O 3 + B 2 O 3 + P 2 O 5 + V 2 O 5 . The glass of any of claims 1-9 wherein the glass exhibits a Young's modulus of 110 GPa or more.

Inventors: TORATANI HISAYOSHI, ZOU XUELU, TORATANI, HISAYOSHI, ZOU, XUELU, TORATANI, Hisayoshi, Hoya Corporation, ZOU, Xuelu, Hoya Corporation
Applicants: HOYA CORP, HOYA CORPORATION
Application year: 1997
Main classification: C03C3
IPC codes: C03C3, G11B5