Glass substrates for flat panel displays are classified roughly into an alkali glass containing an alkali metal oxide and an alkali free glass containing substantially no alkali metal oxide. An alkali glass substrate is used for e.g. plasma displays (PDP), inorganic electroluminescence displays, or field emission displays (FED), and an alkali free glass substrate is used for e.g. liquid crystal displays (LCD), or organic electroluminescence displays (OLED).
Among them, in a case of e.g. a glass substrate for LCD, e.g. a thin film of a metal or a metal oxide is formed on its surface, and therefore, the following characteristics are required.    (1) To be an alkali free glass containing substantially no alkali metal oxide (for the purpose of preventing deterioration of film properties, caused by diffusion, into a thin film, of alkali metal ions of an alkali metal oxide in a glass substrate.)    (2) To have a high strain point (for the purpose of minimizing deformation or shrinkage of a glass substrate caused by exposing the glass substrate to a high temperature during a step of forming a thin film transistor (TFT))    (3) To have an adequate chemical durability against various chemicals to be used for formation of TFT. Especially, to have a durability against buffered hydrofluoric acid (BHF: hydrofluoric acid+ammonium fluoride) to be used for etching of SiOx or SiNx, chemicals containing hydrochloric acid to be used for etching of ITO (tin-doped indium oxide), various acids (e.g. nitric acid, sulfuric acid) to be used for etching of a metal electrode, or a resist-removing alkaline liquid.    (4) To have no defects (such as bubbles, stria, inclusions, undissolved materials, pits or flaws) which affect on display panels, inside or on the surface of a glass substrate.
In recent years, as an area of a glass substrate for flat panel displays becomes larger, the number of defects per glass substrate becomes large, even when such a glass substrate is one having the same defect density, whereby problems of the decrease in yield have been distinct. Especially, bubbles and undissolved starting materials are mentioned as defects.
Heretofore, a method has been employed, in which bubbles in an alkali free glass are reduced by adding e.g. As2O3 or Sb2O3 as a clarifier for reducing bubbles, to a starting material for an alkali free glass. Though As2O3 or Sb2O3, especially As2O3 is an excellent clarifier in view of releasing bubbles from a molten glass, a load on environment is immerse, and therefore, it is required to avoid its use.
On the other hand, though a sulfate as a clarifier is excellent for an alkali glass since the load on environment is remarkably low as compared with As2O3, an effect of reducing bubbles was not so high as we expected. Accordingly, a method of using a sulfate and a chloride in combination (Patent Document 1) or a method of using a sulfate and a tin oxide in combination (Patent Document 2) has been proposed. However, neither of them was sufficient in a clarification effect and still had a problem that undissolved materials remain in a glass.
Heretofore, it was difficult to lower the viscosity of an alkali free glass, secure the characteristics (strain point and acid resistance) required for e.g. a glass substrate for displays, and reduce bubbles or undissolved materials.    Patent Document 1: JP-A-10-25132    Patent Document 2: JP-A-2004-299947