EP1888473A2

EP1888473A2 - Glass substrates for flat screens

Info

Publication number: EP1888473A2
Application number: EP06794442A
Authority: EP
Inventors: Emmanuel Lecomte
Original assignee: Saint Gobain Glass France SAS; Compagnie de Saint Gobain SA
Current assignee: Saint Gobain Glass France SAS; Compagnie de Saint Gobain SA
Priority date: 2005-05-27
Filing date: 2006-05-18
Publication date: 2008-02-20
Also published as: FR2886288B1; JP2008542164A; FR2886288A1; KR20080017013A; CN101184701A; WO2007000540A2; WO2007000540A3; US20080194394A1

Abstract

The invention relates to a glass substrate whose chemical composition comprises the following components within limits defined thereafter and expressed in percentages by weight: 58-72 % by weight SiO2, 0,8-3 by weight TiO2, 2-15 by weight B2O3, 10-25 by weight AI2O3, 5-12 by weight CaO, 0-3 by weight MgO, 0-6 by weight BaO, 0-4 by weight SrO, 0-3 by weight ZnO, and 0-1 by weight R2O.

Description

GLASS SUBSTRATES FOR FLAT SCREENS

The present invention relates to glass substrates suitable for use in the manufacture of flat screens and having aluminosilicate-type compositions containing low levels of alkaline oxides. Flat screens can be produced by different technologies, among which the main ones are PDP (Plasma Display Panel) and LCD (Liquid Crystal Display) technologies. These two technologies require the use of glass substrates, but impose extremely different properties on these substrates, so that their chemical composition must be specifically adapted to each of them.

LCD technology implements manufacturing processes in which thin glass sheets are used as substrates for the deposition of thin film transistors by techniques used in the semiconductor industry for electronics, among which the techniques high temperature deposition, photolithography, etching by etching. Many requirements in terms of properties of the glass stem from these processes, in particular as regards their mechanical, chemical and thermal resistance.

Given the high temperatures used for the deposition of thin layers of silicon, the thermal stability of the glass is essential to avoid any deformation. A lower annealing temperature of at least 600 ^° C. and even 65 ^° C. is then required. This temperature is commonly called "Strain point" and corresponds to the temperature at which the glass has a viscosity equal to 10 ¹⁴ ' ⁵ poises. A low coefficient of expansion is also necessary to avoid too large a variation of the dimensions of the glass substrate as a function of temperature. A good agreement between the coefficient of expansion of silicon and that of glass is however essential to avoid the generation of mechanical stresses between glass and silicon. The glass substrate of the coefficient of expansion should be between 25 and 35.10 ^"-7 / ° C, measured in the temperature range 25-300 ⁰ C.

The etching used for etching silicon must not degrade the glass substrate, and especially its surface. These attacks being carried out by acids, it is essential that the glass substrate has a very high resistance to acid corrosion.

Given the constant increase in the size of flat screens, it is also important that the weight of the substrate is minimized, which translates the glass used into a requirement of low density (density). The low density, in the same way as the Young's modulus, also plays a role in avoiding the deflection of large substrates and thus facilitating the handling of said substrates during all the steps of the screen manufacturing process.

Some properties of glass are also important in the industrial feasibility of glass substrates. In particular, a high temperature viscosity that is too high would have consequences in economic terms since it would increase energy expenditure and reduce the life of glass melting furnaces. It is also essential that the glass does not devitrify at too high a temperature (the liquidus temperature must therefore be limited) and / or with high crystallization speeds, as this would impair the feasibility of forming in the form of flat glass sheets.

Compositions partially meeting these specifications are known from patent applications WO 00/32528 and US 2004/43887 and consist mainly of silica (SiO ₂ ), alumina (Al ₂ O ₃ ), boron oxides. (B ₂ O ₃ ) and calcium (CaO). These glasses are free of alkaline oxides and comprise low levels, advantageously zero divalent oxides other than calcium oxide. The Young's modulus values obtained, however, are insufficient and range from 60 to 70GPa.

The present invention aims to improve the compositions described in the aforementioned documents by increasing their Young's modulus, while maintaining good properties in terms of density, thermal stability and coefficient of expansion. The invention also aims to provide economic compositions in terms of cost resulting from the raw materials and the amount of energy to be supplied for the manufacture of glass substrates.

The subject of the invention is a glass substrate having a chemical composition comprising the following constituents within the limits defined below expressed in percentages by weight:

SiO ₂ 58 to 72

TiO ₂ 0.8 to 3

B ₂ O ₃ 2 to 15

AI ₂ O ₃ 10 to 25

CaO 5 to 12

MgO 0 to 3

BaO 0 to 6

SrO 0 to 4

ZnO 0 to 3

R ₂ O 0 to 1

R ₂ O designates the alkaline oxides (mainly oxides of sodium, potassium and litihium).

Silica (SiO ₂ ) is an essential element of the vast majority of industrial glasses. It is a formative element of the vitreous network, which influences all the properties of the glass. Too small amounts of silica (below 58%) would result in both a deterioration of glass stability vis-à-vis devitrification, too low resistance to acid corrosion, too much density and a coefficient of dilation too high. It is preferable that the silica content is greater than or equal to 60%, even 62% and even 63%. On the other hand, too high levels (above 72%) result in an unacceptable increase in viscosity, making the melting process difficult. glass. The silica content of the glasses according to the invention is therefore advantageously less than or equal to 70%, even 68% and even 66%.

Titanium oxide (TiO ₂ ) is an essential element of the composition of the glass substrates according to the present invention. The inventors have demonstrated the strong influence of this oxide on the increase of the Young's modulus. Even if this influence begins to be felt for values of 0.8%, contents greater than 1%, or even greater than or equal to 1, 2% are preferred. Too high levels result in a decrease in the light transmission of the glass, accompanied by unacceptable yellowing. The titanium oxide content must therefore be less than or equal to 3%, and advantageously less than or equal to 2%. The addition of a very small amount of cobalt oxide (CoO), advantageously of the order of 5 to 50 ppm, especially 10 to 20 ppm, may contribute to reducing the yellowing. US Pat. No. 5,851,939 discloses strontium oxide-rich, calcium oxide-poor, titanium oxide-rich glass compositions for improving acid resistance, but does not disclose the positive influence of this oxide on increase in Young's modulus.

Boron oxide (B ₂ O ₃ ) is also a network forming element, which contributes to the decrease of liquidus temperature, density and coefficient of expansion. It also has the advantage over silica of reducing the viscosity at high temperature and thus to facilitate melting of the glass. The glass substrates according to the invention therefore comprise at least 2% of boron oxide, and advantageously at least 6%, or even 8%, 9.5% and even 10%. Too high levels of boron oxide, however, have a negative impact on the cost of the raw materials used and on the Strain point. For these reasons, the boron oxide content must be less than or equal to 15%, and advantageously 13% or even 12%.

Alumina (AI ₂ O ₃ ) increases the Strain point and the Young's modulus. Its content is therefore advantageously greater than or equal to 12% or even 14%. However, a high alumina content has the disadvantage of greatly increasing the viscosity at high temperature, and decreasing the resistance to corrosion in acidic medium and the resistance of the glass to the devitrification (especially by increasing the temperature of liquidus). The alumina content of the glasses according to the invention is therefore advantageously less than or equal to 22%, even 20% and even 18%. An alumina content of between 14 and 15% is a good compromise. Lime (CaO) is essential to reduce the viscosity of glass at high temperatures. Its content is therefore greater than or equal to 5%, preferably greater than or equal to 6%, even 7% or 8%, and even 8.5%. Too high a content is however detrimental to obtaining a low coefficient of expansion. A content of less than or equal to 10% is preferred. The glasses according to the invention make it possible to use a high content of CaO, in particular strictly greater than 8%, without observing degradation of the resistance in an acidic medium, as could be expected from the above-mentioned US Pat. No. 5,851,939. .

According to a preferred embodiment making it possible to obtain a very good compromise between a high-point strain and a low coefficient of expansion, a boron oxide content greater than or equal to 11%, even 12% and less than or equal to 13% is associated with a lime content less than or equal to 9% and greater than or equal to 6% or even 7%.

Magnesia (MgO) is an optional element of the present invention. Its beneficial influence on the Young's modulus is unfortunately compensated by a rapid degradation of the devitrification properties resulting in an increase in liquidus temperature and crystallization rates. The MgO content is therefore preferably less than or equal to 2%, or even less than 1% and even 0.5%. According to a preferred embodiment, the glasses do not contain magnesium oxide, with the exception of impurities that are difficult to avoid (less than 0.1%).

The oxides of barium (BaO) and of strontium (SrO) have a detrimental influence on the density of the glass, which leads to advantageously limiting the content of one and / or the other to 3% or less, in particular 2 %, or even 1% or even 0.5% or 0.1%. The glasses according to the invention advantageously do not contain oxides of strontium and / or barium, with the exception of unavoidable impurities. Zinc oxide (ZnO) is also advantageously absent from the glass compositions according to the invention, because of undesirable reactions when the glass sheet is produced by the "float" process, in which the glass is poured onto a bath of water. molten tin under a reducing atmosphere. The reducing conditions necessary to avoid the oxidation of the tin bath lead indeed, for the glasses containing too high ZnO contents, a reduction of this metal zinc oxide which forms a haze on the glass sheet.

The glasses according to the invention preferably do not contain zirconium oxide, zinc oxide, strontium oxide and barium oxide. The alkaline oxides (R ₂ O collectively refer to these oxides, among which are found the oxides of sodium, potassium and lithium) must be limited to very low levels, preferably less than 0.5% and even 0.1 %, 0.05% or 0.01%. Nil quantities of alkaline oxides (with the exception of traces from raw materials) are clearly preferred. The alkaline oxides tend to migrate towards the glass surface and considerably degrade the semiconducting properties of the silicon deposited on the substrate.

According to a preferred embodiment, the glass substrate according to the invention has a chemical composition comprising the following constituents within the limits defined below expressed in percentages by weight:

SiO ₂ 60 to 70

TiO ₂ 1, 2 to 2

B ₂ O ₃ 6 to 13

AI ₂ O ₃ 12 to 18 CaO 5 to 10

MgO 0 to 3

BaO, SrO, ZnO <0.1

R ₂ O <0.1

The glass substrates according to the invention may contain other elements than those listed above. These may be finely introduced refining agents or other oxides, usually introduced unintentionally as impurities and not substantially modifying the way in which the substrates according to the invention solve the technical problem involved. In general, the impurity content of the glasses according to the invention is less than or equal to about 5% and even 3%, or even 2% or 1%.

The glass compositions according to the invention preferably comprise chemical agents intended for the refining of the glass, that is to say the elimination of gaseous inclusions contained in the mass of glass during the melting step. The refining agents used are, for example, oxides of arsenic or antimony, halogens such as fluorine or chlorine, tin or cerium oxide, sulphates, or a mixture of such compounds. The combination of tin oxide and chlorine has proved particularly effective and is therefore preferred in the context of the present invention. The compositions according to the invention advantageously do not contain oxides of arsenic or antimony, because of their high toxicity.

The glass substrates according to the invention may also contain small quantities of other oxides such as zirconium oxide or rare earth oxides such as lanthanum or yttrium, but generally do not contain them, with the exception of traces originating from impurities contained in the raw materials or from the dissolution of elements contained in the refractory materials constituting the glass melting furnace. In the case where the glasses according to the invention contain zirconium oxide (ZrO ₂ ), which may be useful for further improving the Young's modulus, its content is not greater than 3%, or even 2%, because the liquidus temperature is strongly affected by the presence of this oxide.

The glass substrates according to the invention preferably have a lower expansion coefficient than or equal to 35.10 ^"7 / ° C or ^{33.10" 7} / ° C. Their Strain point is advantageously greater than or equal to 63O ⁰ C, and even at 65O ⁰ C. Thanks to the use of titanium oxide, the Young's modulus of the substrates according to the invention is generally greater than 70GPa or even 72GPa . The temperature corresponding to the viscosity at which the glass is formed, ie about 10,000 Poises, temperature denoted "Tlog4" is preferably less than or equal to 135O ⁰ C. The invention also relates to a continuous process for obtaining the substrates according to the invention comprising the steps of melting in a glass furnace a vitrifiable mixture of suitable composition, and of forming a glass sheet by spilling on a bath of molten tin (float process). The melting temperature is advantageously less than 1700 ^° C., or even 165 ^° C.

The invention finally relates to a flat screen, in particular of the LCD type ("liquid-crystal display") or OLED ("organic light emitting diodes"), comprising a glass substrate according to the invention.

The advantages of the invention are illustrated by the following non-limiting examples, shown in Tables 1 and 2.

Comparative example C1 is a representative glass of the teaching of the application WO 00/32528, similar to Examples 10 to 13 and 17 of this same application, and corresponding to an LCD screen composition used industrially. Examples 1 to 3 correspond to the teaching of the present invention.

Table 1 indicates, in addition to the chemical composition expressed in percentages by mass, the following physical properties:

the Young's modulus, expressed in GPa, measured according to the ASTM C 1259-01 standard,

- the coefficient of expansion between 25 and 300 ⁰ C, measured according to standard NF B30-103, expressed as 10 ^"-7 / ° C,

- the density or "density" (in g.cm ^"3 ), measured according to the so-called" Archimedes "method, - the" Strain point ", expressed in ⁰ C, approximately corresponding to the temperature at which the viscosity is 10 ¹⁴ ' ⁵ poles (10 ¹³ ' ⁵ Pa.s), measured according to standard NF B30-105,

the upper annealing temperature, called "Annealing point", expressed in ^O C, approximately corresponding to the temperature at which the viscosity is 10 ¹³ Poises (10 ¹² Pa.s), measured according to the NF B30-105 standard, the temperature at which the viscosity is 10 ⁴ Poises (10 ³ Pa.s), denoted T (log4), the latter being measured according to ISO 7884-2 and approximately corresponding to the viscosity at which the glass is poured onto the bath molten metal during the float process, the optical transmission for a wavelength of 400 nm measured on a 1 mm thick glass sample,

Table 1

The comparison between the examples according to the invention and the comparative example demonstrates the beneficial effect of titanium oxide on the increase of the Young's modulus, the other properties remaining largely unchanged.

Table 2 shows other examples of compositions according to the invention.

Table 2

Claims

1. Glass substrate, characterized in that it has a chemical composition comprising the following constituents within the limits defined below expressed in percentages by weight:

SiO ₂ 58 to 72

TiO ₂ 0.8 to 3

B ₂ O ₃ 2 to 15 AI ₂ O ₃ 10 to 25

CaO 5 to 12

MgO 0 to 3

BaO 0 to 6

SrO 0 to 4 ZnO 0 to 3

R ₂ O 0 to 1 R ₂ O designating the alkaline oxides

2. Glass substrate according to claim 1, comprising titanium oxide (TiO ₂ ) in a mass content of between 1.2% and 2%.

3. Glass substrate according to one of the preceding claims, comprising silicon oxide (SiO ₂ ) in a mass content greater than or equal to 62%.

4. The glass substrate according to one of the preceding claims, comprising boron oxide (B ₂ O ₃ ) in a mass content greater than or equal to 9.5%.

5. Glass substrate according to one of the preceding claims, comprising alumina (Al ₂ O ₃ ) in a mass content of between 12 and 18%.

6. The glass substrate according to one of the preceding claims, comprising lime (CaO) in a mass content greater than or equal to 7%.

7. A glass substrate according to one of the preceding claims, comprising lime (CaO) in a weight content of between 7 and 9%, and boron oxide (B ₂ O ₃₎ in a content of between 11 and 13%.

8. Glass substrate according to one of the preceding claims, characterized in that it does not include magnesia (MgO).

9. Glass substrate according to one of the preceding claims, comprising barium oxide (BaO) in a content not exceeding 3%.

10. Glass substrate according to one of the preceding claims, comprising strontium oxide (SrO) in a content of not more than 3%.

1. Glass substrate according to one of the preceding claims, characterized in that it does not comprise oxides of barium (BaO) and / or strontium (SrO).

12. Glass substrate according to one of the preceding claims, comprising cobalt oxide in a mass content of between 5 and 50ppm.

13. The glass substrate according to claim 1, having a chemical composition comprising the following constituents within the limits defined below expressed in percentages by weight:

SiO ₂ 60 to 70

TiO ₂ 1, 2 to 2

B ₂ O ₃ 6 to 13

AI ₂ O ₃ 12 to 18

CaO 5 to 10

MgO 0 to 3

BaO, SrO, ZnO <0.1

R ₂ O <0.1 14. A continuous process for obtaining the substrates according to one of claims 1 to 13 comprising the steps of melting in a glass furnace a batch of suitable composition, and forming a sheet of glass by spilling on a bath of molten tin. Flat screen comprising a glass substrate according to one of claims 1 to 13.