Alkali-free glass and flat panel display

An alkali-free glass consisting essentially of, by wt %, from 58.4 to 66.0% of SiO.sub.2, from 15.3 to 22.0% of Al.sub.2 O.sub.3, from 5.0 to 12.0% of B.sub.2 O.sub.3, from 0 to 8.0% of MgO, from 0 to 9.0% of CaO, from 3.0 to 12.5% of SrO, from 0 to less than 2.0% of BaO, and from 9.0 to 18.0% of MgO+CaO+SrO+BaO and having a strain point of at least 640.degree. C.

BACKGROUND OF THE INVENTION 
 1. Field of the Invention 
 The present invention relates to alkali-free glasses which are suitable as 
 substrate glasses for various displays or photomasks and which are 
 essentially free from alkali metal oxides and can be formed by float 
 process, and flat panel displays employing them. 
 2. Discussion of Background 
 Heretofore, the following properties have been required for substrate 
 glasses for various displays, particularly for the ones intended to form a
 thin film of a metal or oxide on their surface. 
 (1) They are essentially free from alkali metal oxides. If an alkali metal 
 oxide is contained, the alkali metal ion tends to diffuse in the thin 
 film, whereby the film properties will deteriorate. 
 (2) They have high strain points, so that when they are exposed to a high 
 temperature during the process for forming a thin film, deformation of the
 glass and shrinkage due to stabilization of the glass structure are 
 suppressed to the minimum levels. 
 (3) They have adequate chemical durability against various reagents to be 
 used for forming semiconductors. Particularly, they have durability 
 against hydrofluoric acid used for etching SiO.sub.x or SiN.sub.x or 
 against a buffered hydrofluoric acid (BHF) containing ammonium fluoride 
 and hydrofluoric acid as the main components. 
 (4) They have no internal or surface defects (such as bubbles, striae, 
 inclusions, pits or scratch marks). 
 Heretofore, Corning code 7059 glass is widely employed as a substrate glass
 for various displays or photomasks. However, this glass has the following 
 deficiencies for displays. 
 (1) The strain point is as low as 593.degree. C., and preliminary heat 
 treatment to reduce the shrinkage of glass has to be carried out prior to 
 a process for preparing the displays. 
 (2) The amount of elution into hydrochloric acid used for etching a metal 
 electrode or a transparent conductive film (such as ITO) is substantial, 
 and the eluted substance tends to recrystallize during the process for 
 preparing displays, whereby it becomes difficult to prepare the displays. 
 In addition to the above requirements, the following two points have been 
 additionally required along with the trend for large sized displays in 
 recent years. 
 (1) The above-mentioned code 7059 glass has a density of 2.76 g/cc, and a 
 glass having a smaller density is required to meet a requirement for light
 weight. 
 (2) The above code 7059 glass has a coefficient of thermal expansion of 
 46.times.10.sup.-7 /.degree. C., and a glass having a smaller coefficient 
 of thermal expansion is required to increase the rate of temperature 
 increase for the preparation of displays and thus to increase the 
 throughput. 
 With respect to B.sub.2 O.sub.3, JP-A-1-160844 discloses a glass product 
 containing from 20 to 23 cation % of B.sub.2 O.sub.3, but the amount of 
 B.sub.2 O.sub.3 is so large that the strain point is not sufficiently 
 high. JP-A-61-281041 discloses a product containing from 0.1 to 4 wt % of 
 B.sub.2 O.sub.3, JP-A-4-175242 discloses a product containing from 0.1 to 
 5 mol % of B.sub.2 O.sub.3, and JP-A-4-325435 discloses a product 
 containing from 0 to 3 wt % of B.sub.2 O.sub.3. However, in each of them, 
 the amount of B.sub.2 O.sub.3 is so small that the durability against BHF 
 is not sufficient. 
 With respect to BaO, JP-A-4-325434 discloses a glass product containing 
 from 10 to 20 wt % of BaO, JP-A-63-74935 discloses a product containing 
 from 10 to 22 wt % of BaO, and JP-A-59-169953 discloses a product 
 containing from 15 to 40 wt % of BaO. However, in each of them, BaO is so 
 large that the coefficient of thermal expansion is large, and the density 
 is high. 
 With respect to MgO, JP-A-61-132536 discloses a glass product containing 
 from 6.5 to 12 wt % of MgO, JP-A-59-116147 discloses a product containing 
 from 5 to 15 wt % of MgO, JP-A-60-71540 discloses a product containing 
 from 5 to 17 wt % of MgO, and JP-A-60-42246 discloses a product containing
 from 10 to 25 mol % of MgO. However, such glass containing a large amount 
 of MgO tends to undergo phase separation. 
 With respect to CaO, JP-A-63-176332 discloses a glass product containing 
 from 11 to 25 wt % of CaO, JP-A-58-32038 discloses a product containing 
 from 7 to 20 mol % of CaO, JP-A-2-133334 discloses a product containing 
 from 8 to 15 wt % of CaO, JP-A-3-174336 discloses a product containing 
 from 7 to 12 wt % of CaO, JP-A-6-40739 discloses a product containing from
 10 to 12 wt % of CaO, and JP-A-5-201744 discloses a product containing at 
 least 18 cation % of CaO. However, if CaO is contained in a large amount, 
 the coefficient of thermal expansion tends to be too large. 
 With respect to Al.sub.2 O.sub.3, JP-A-61-236631 discloses a glass product 
 containing from 22.5 to 35 wt % of Al.sub.2 O.sub.3, but the amount of 
 Al.sub.2 O.sub.3 is so large that elution to a chemical reagent such as 
 hydrochloric acid is substantial. 
 With respect to P.sub.2 O.sub.5, JP-A-61-261232 and JP-A-63-11543 disclose 
 those containing P.sub.2 O.sub.5. However, they are not desirable, since 
 they tend to deteriorate the semiconductor properties of thin films. 
 Further, glass having a strain point of at least 640.degree. C. and a 
 relatively small coefficient of thermal expansion is disclosed in 
 JP-A-4-160030 or JP-A-6-263473. However, such glass contains a substantial
 amount of BaO as an essential element, whereby it is difficult to satisfy 
 the requirements for low density and small thermal expansion coefficient 
 simultaneously. Accordingly, it does not fully satisfy the demand of the 
 age for large sized panels. 
 It is an object of the present invention to solve the above drawbacks and 
 to provide alkali-free glasses which have strain points of at least 
 640.degree. C. and small coefficients of thermal expansion and small 
 densities and are free from forming of turbidity by BHF and which are 
 excellent in the durability against reagents such as hydrochloric acid, 
 are easy to melt and shape and can be formed by float process. 
 SUMMARY OF THE INVENTION 
 The present invention provides an alkali-free glass consisting essentially 
 of, by wt %, from 58.4 to 66.0% of SiO.sub.2, from 15.3 to 22.0% of 
 Al.sub.2 O.sub.3, from 5.0 to 12.0% of B.sub.2 O.sub.3, from 0 to 8.0% of 
 MgO, from 0 to 9.0% of CaO, from 3.0 to 12.5% of SrO, from 0 to less than 
 2.0% of BaO, and from 9.0 to 18.0% of MgO+CaO+SrO+BaO and having a strain 
 point of at least 640.degree. C. 
 Also, the present invention provides an alkali-free glass consisting 
 essentially of, by wt %, from 58.4 to 66.0% of SiO.sub.2, from 15.3 to 
 22.0% of Al.sub.2 O.sub.3, from 5.0 to 12.0% of B.sub.2 O.sub.3, from 0 to
 6.5% of MgO, from 0 to 7.0% of CaO, from 4.0 to 12.5% of SrO, from 0 to 
 less than 2.0% of BaO, and from 9.0 to 18.0% of MgO+CaO+SrO+BaO and having
 a strain point of at least 640.degree. C. 
 DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
 The alkali-free glasses of the present invention are essentially free from 
 alkali metal oxides (such as Na.sub.2 O or K.sub.2 O). Specifically, the 
 total amount of alkali metal oxides is preferably not higher than 0.5 wt 
 %, more preferably not higher than 0.2 wt %. 
 Now, the reasons for defining the compositional ranges of the respective 
 components as mentioned above, will be described. 
 If the content of SiO.sub.2 is too small, it tends to be difficult to 
 increase the strain point sufficiently, and the chemical durability tends 
 to deteriorate and the coefficient of thermal expansion tends to increase.
 Preferably, it is at least 59.0 wt %. If it is too large, the melting 
 property tends to be poor, and the liquidus temperature tends to increase.
 Preferably, it is at most 65.0 wt %, more preferably at most 62.7 wt %. 
 Al.sub.2 O.sub.3 suppresses phase separation of glass, reduces the 
 coefficient of thermal expansion and increases the strain point. If its 
 content is too small, no adequate effects can be obtained. Preferably, it 
 is at least 15.3 wt %. If it is too large, the melting property of glass 
 tends to be poor. Preferably, it is at most 21.1 wt %. 
 B.sub.2 O.sub.3 serves to prevent formation of turbidity by BHF and is 
 effective to lower the coefficient of thermal expansion and the density 
 without increasing the viscosity at a high temperature. If its content is 
 too small, the BHF property tends to deteriorate. Preferably, it is at 
 least 5.2 wt %. If it is too large, the acid resistance tends to 
 deteriorate, and the strain point tends to be low. Preferably, it is at 
 most 10.2 wt %. 
 MgO is not essential but may be contained, as it lowers the coefficient of 
 thermal expansion without lowering the strain point, among alkaline earth 
 metal oxides. If its content it too large, turbidity by BHF or phase 
 separation of glass tends to occur. Preferably, it is at most 7.0 wt %, 
 preferably at most 6.5 wt %, more preferably at most 6.0 wt %, most 
 preferably at most 3.1 wt %. 
 CaO is not essential, but by its incorporation, it is possible to improve 
 the melting property of glass. On the other hand, if it is too much, 
 coefficient of thermal expansion tends to be large, and the liquidus 
 temperature tends to be high. Preferably, it is at most 8.0 wt %, more 
 preferably at most 7.0 wt %, most preferably at most 6.6 wt %. 
 SrO is essential since it serves to suppress phase separation of glass and 
 is a relatively useful component against turbidity by BHF. Preferably, it 
 is at least 3.5 wt %, more preferably at least 4.0 wt %, most preferably 
 at least 4.6 wt %. If the content is too large, the coefficient of thermal
 expansion tends to increase. Preferably, it is at most 12.1 wt %. 
 BaO is a component which is effective for suppressing phase separation of 
 glass, for improving the melting property and for suppressing the liquidus
 temperature. However, if its content is too large, the density tends to be
 high, and the coefficient of thermal expansion tends to increase. With a 
 view to reducing the density and the coefficient of thermal expansion, its
 content is preferably not higher than 1.8 wt %, more preferably, it is not
 (substantially) contained beyond the amount contained as an impurity. 
 If the total amount of MgO+CaO+SrO+BaO is too small, the melting tends to 
 be difficult. If it is too large, the density tends to be high. 
 Preferably, it is at most 17.4 wt %. 
 In recent years, TFT of polysilicon type has been proposed and employed 
 against TFT of amorphous silicon type which has already been 
 commercialized and used for liquid crystal display devices. TFT of 
 polysilicon type has merits such that (1) since the mobility of transistor
 can be increased, the control time per pixel can be shortened, whereby 
 high precision of LCD can be accomplished, (2) it becomes possible to 
 mount driving ICs along the periphery of the picture surface. On the other
 hand, however, intensive heat treatment (such as at a temperature of from 
 500 to 600.degree. C. for few hours) is required in the process for its 
 preparation. At such a high temperature, impurities in glass tend to 
 diffuse into TFT to increase leak current, deteriorate TFT characteristics
 and make it difficult to prepare high precision TFT. The most problematic 
 among such impurities is phosphorus. Therefore, in the present invention, 
 P is preferably controlled to be at most 20 atomic (cation) ppm, since it 
 tends to bring about a drawback that phosphorus diffuses into TFT by the 
 heat treatment to increase leak current and deteriorate the TFT 
 characteristics. 
 In the glass of the present invention, in addition to the above components,
 ZnO, SO.sub.3, F, Cl and SnO.sub.2 may be incorporated in a total amount 
 of at most 5 mol %, in order to improve the melting property, the clarity 
 and the forming property of the glass. 
 Further, if PbO, As.sub.2 O.sub.3 or Sb.sub.2 O.sub.3 is contained, a 
 number of steps will be required for treatment of glass cullet. Therefore,
 it is preferred that no such a component is incorporated except for an 
 amount unavoidably included as an impurity. 
 Thus, a glass composition according to a preferred embodiment of the 
 present invention consists essentially of, by wt %, from 59.0 to 62.7% of 
 SiO.sub.2, from 15.3 to 21.1% of Al.sub.2 O.sub.3, from 5.2 to 10.2% of 
 B.sub.2 O.sub.3, from 0 to 6.0% of MgO, from 0 to 6.6% of CaO, from 4.6 to
 12.1% of SrO, and from 10.9 to 17.4% of MgO+CaO+SrO+BaO. 
 A glass composition according to another preferred embodiment of the 
 present invention consists essentially of, by wt %, from 59.0 to 62.7% of 
 SiO.sub.2, from 15.3 to 21.1% of Al.sub.2 O.sub.3, from 5.2 to 10.2% of 
 B.sub.2 O.sub.3, from 0 to 9% of MgO, from 0 to 8% of CaO, from 3.0 to 
 6.0% of SrO and from 10.9 to 17.4% of MgO+CaO+SrO+BaO. 
 The glass of the present invention has a strain point of at least 
 640.degree. C. The strain point is preferably at least 650.degree. C. The 
 coefficient of thermal expansion is preferably less than 
 40.times.10.sup.-7 /.degree. C., more preferably at least 
 27.times.10.sup.-7 /.degree. C. and less than 40.times.10.sup.7 /.degree. 
 C., most preferably at least 30.times.10.sup.-7 /.degree. C. and less than
 40.times.10.sup.-7 /.degree. C. The density is preferably less than 2.60 
 g/cc, more preferably less than 2.55 g/cc, most preferably less than 2.50 
 g/cc, particularly less than 2.45 g/cc. 
 In order to make a density lower than 2.40 g/cc, there is provided 
 preferably an alkali-free glass consisting essentially of, by wt %, from 
 58.4 to 66.0% of SiO.sub.2, from 15.3 to 22.0% of Al.sub.2 O.sub.3, from 
 5.0 to 12.0% of B.sub.2 O.sub.3, from more than 6.5 to 8.0% of MgO, from 0
 to 9.0% of CaO, from 3.0 to less than 4.0% of SrO and from 9.0 to 18.0% of
 MgO+CaO+SrO+BaO, and having a strain point of at least 640.degree. C. The 
 amount of MgO is more preferably from 6.6 to 7.5% and the amount of CaO is
 more preferable from 0 to 2.5%. 
 Also, the glass of the present invention is suitable to be used for various
 display substrates and photomask substrates, and usually there are 
 conveying steps in the production processes of various displays and 
 photomasks. If these substrates are bent down by their own weights during 
 the production processes, unpreferable production troubles are sometimes 
 caused. In order to make this bending small, it is favorable to use a 
 glass having a large Young's modulus. Thus, it is preferable to make the 
 Young's modulus of the glass of the present invention at least 7,400 
 kgf/mm.sup.2, preferably 7,500 kgf/mm.sup.2. 
 In order to make the Young's modulus at least 7,400 kgf/mm.sup.2, there is 
 provided preferably an alkali-free glass consisting essentially of, by 
 weight, from 58.4 to 66.0% of SiO.sub.2, from 15.3 to 22.0% of Al.sub.2 
 O.sub.3, from 5.0 to 12.0% of B.sub.2 O.sub.3, from 0 to 8.0% of MgO, from
 more than 7.0 to 9.0% of CaO, from 3.0 to 12.5% of SrO, from 0 to less 
 than 2.0% of BaO and from 9.0 to 18.0% of MgO+CaO+SrO+BaO. 
 Further, the glass of the present invention is suitable for forming by a 
 float process, since the forming temperature (the temperature at which the
 viscosity becomes 10.sup.4 poise) in the float process forming is not 
 lower than the liquidus temperature. 
 The glass of the present invention can be prepared, for example, by the 
 following method. Namely, raw materials of the respective components which
 are commonly used, are blended to obtain a desired composition, which is 
 continuously introduced into a melting furnace, and melted at a 
 temperature of from 1500 to 1600.degree. C. This molten glass is formed 
 into a sheet having a predetermined thickness by a float process, and the 
 sheet is annealed and then cut. A glass sheet thus obtained may be used as
 one of the pair of substrates constituting a cell for flat panel displays.