Alkali-free aluminoborosilicate glass and its use

The invention relates to an alkali-free aluminoborosilicate glass. The invention also relates to uses of this glass.

Glasses for applications as substrates in liquid crystal flat display technology, e.g. B. in TN (Twisted Nematic) / STN (Super Twisted Nematic) - Displays, Active Matrix Liquid Crystal Displays (AMLCD's), Thin Film Transistors (TFT's) or Plasma Addressed Liquid Crystals (PALC's) make high demands. In addition to high resistance to temperature changes and good resistance to the aggressive chemicals used in the manufacturing process for flat screens, the glasses should have high transparency over a wide spectral range (VIS, UV) and low density to save weight. The use as a carrier material for integrated semiconductor circuits such. B. in TFT displays ("chip on glass") also requires thermal adaptation to the thin film material silicon. This is usually deposited on the glass substrate as amorphous silicon (a-Si) at low temperatures up to 300 ° C. Subsequent heat treatment at temperatures of approx. 600 ° C partially recrystallizes the amorphous silicon. The resulting partially crystalline poly-Si layer is characterized on the basis of the a-Si components by a value of the thermal expansion of α _20/300 ≅ 3.7 x 10^-6/ K. Depending on the ratio of a-Si to poly-Si, the coefficient of thermal expansion α_20/300 between 2.9 x 10^-6/ K and 4.2 · 10^-6/ K vary. If high-temperature treatments above 700 ° C or direct deposition via CVD processes generate largely crystalline Si layers, which is also desirable in thin-film photovoltaics, a substrate with a significantly reduced thermal expansion of up to 3.2 x 10 is required^-6/ K or less required.

The absence of alkali ions is also a requirement for applications in display and photovoltaic technology. Production-related proportions of sodium oxide below 1000 ppm are in view of the generally "poisoning" effect by diffusion of Na⁺ still tolerable in the semiconductor layer.

Suitable glasses should be of sufficient quality on an industrial scale (no bubbles, knots, inclusions), e.g. B. be economically producible on a float system or in drawing processes. In particular, the production of thin (<1 mm) streak-free substrates with low surface ripple using a drawing process requires a high degree of devitrification stability. A shrinkage ("compaction") of the substrate which has a disadvantageous effect on the semiconductor microstructure during manufacture, in particular in the case of TFT displays, can be counteracted by setting a suitable temperature-dependent viscosity characteristic of the glass. With regard to the thermal process and dimensional stability, on the one hand, the melting and processing (V_A) temperatures, ie at a V_A ≤ 1350 ° C, a sufficiently high transformation temperature, ie Tg> 700 ° C.

The requirements for glass substrates for LCD display technology or thin-film photovoltaic technology are also described in "Glass Substrates for AMLCD applications: properties and implications" by JC Lapp, SPIE Proceedings, Vol. 3014, Invited paper (1997) "Photovoltaic power from the sun" by J. Schmid, CF Müller Verlag, Heidelberg 1994.

The requirement profile mentioned is best met by alkaline earth aluminum borosilicate glasses. However, the known glasses for display or solar cell substrates, which are described in the following documents, still have disadvantages and do not meet the entire catalog of requirements:

Numerous documents describe glasses with low MgO and / or CaO contents: JP 9-169 538 A, JP 4-160 030 A, JP 9-100 135 A, EP 714 862 A1, EP 341 313 B1, US 5,374,595, JP 9-48632 A, JP 8-295530 A, WO 97/11919 and WO 97 11920. These glasses do not have the desired meltability, which means very high temperatures at the viscosities 10² dPas and 10⁴ dPas confirm and have relatively high densities. The same applies to the MgO-free glasses of DE 37 30 410 A1, US 5,116,787 and US 5,116,789.

On the other hand, glasses with high contents of MgO, as described in JP 61-123 536 A, show deficiencies with regard to their chemical resistance and their devitrification and demixing behavior.

The glasses of WO 98/27019 are also prone to crystallization with their very low proportions of BaO and SrO.

Glasses with high contents of the heavy alkaline earth oxides BaO and / or SrO, as described in EP 341313 B1, have undesirably high densities and are not easily meltable. This also applies to the glasses of JP 10-72237 A. As can be seen from the examples, the glasses have high temperatures at viscosities 10⁴ dPas and 10² dPas on.

Glasses with low boric acid contents also have melting temperatures that are too high or, because of the process-related predetermined melting and processing temperatures, have too high viscosities. This applies to the glasses from JP 10-45422 A, JP 9-263421 A and JP 61-132536 A. In combination with low BaO contents, such glasses also have a high tendency to devitrification.

In contrast, glasses with high proportions of boric acid, as described, for example, in US Pat. No. 4,824,808, do not show sufficient temperature resistance and chemical resistance, especially with regard to hydrochloric acid solutions.

Even the glasses that have relatively little SiO₂ contain, especially if they contain large amounts of B₂O₃ and / or MgO and are low in alkaline earths, not sufficiently high chemical resistance. This applies to the glasses from WO 97/11919 and EP 672 629 A2. The SiO2-rich variants of the latter document have only low Al₂O₃Shares in what is disadvantageous for the crystallization behavior.

The glasses for hard drives described in JP 9-12333 A are comparatively Al₂O₃- or B₂O₃-arm, the latter component being only an optional one. The glasses are high in alkaline earth oxide and have a high thermal expansion, which makes them unsuitable for use in LCD or PV technology.

DE 42 13 579 A1 describes glasses for TFT applications with thermal expansion coefficients <5.5 x 10^-6/ K, according to the examples ≥ 4.0 x 10^-6/ K. These glasses with relatively high proportions of B₂O₃ at comparatively low SiO₂-Contents are not very chemically resistant, especially not to dilute hydrochloric acid.

DE 196 01 022 A1 describes glasses from a very variable composition range, which are compulsorily ZrO₂ and SnO included. This Al₂O₃- poor glasses tend due to their ZrO₂-Share of glass defects.

DE 196 17 344 C1 and DE 196 03 689 C1 from the applicant are alkali-free tin oxide-containing SiO₂-arm or Al₂O₃poor glasses with a thermal expansion coefficient α_20/300 of about 3.7 · 10^-6/ K and very good chemical resistance. They are suitable for use in display technology. However, since they contain ZnO, they are not ideal for processing on a float system. In particular at higher ZnO contents (> 1.5% by weight) there is a risk of ZnO deposits forming on the glass surface due to evaporation and subsequent condensation in the hot-forming area.

JP 9-156 953 A also relates to alkali-free glasses for display technology, the Al₂O₃-are poor. The temperature resistance of these glasses, as evidenced by the transformation temperatures of the sample glasses, is not sufficient.

In the unexamined Japanese publications JP 10-25132 A, JP 10-114538 A, JP 10-130034 A, JP 10-59741 A, JP 10-324526 A, JP 11-43350 A, JP 10-139467 A, JP 10-231139 A and JP 11-49520 A call very large composition ranges for display glasses, which can be varied with many optional components, to each of which one or more specific refining agents are added. However, these writings do not provide any information as to how glasses with the complete described requirement profile can be obtained in a targeted manner.

It is an object of the invention to provide glasses which meet the stated physical and chemical requirements for glass substrates for liquid crystal displays, in particular for TFT displays, and for thin-film solar cells, in particular based on μc-Si, glasses which have a high Temperature resistance, a processing area favorable to the process and sufficient devitrification stability.

The object is achieved by aluminoborosilicate glasses according to the main claim.

The glass contains between> 58 and 65% by weight SiO₂. At lower contents the chemical resistance deteriorates, at higher proportions the thermal expansion increases and the tendency of the glass to crystallize increases. A maximum content of 64.5% by weight is preferred.

The glass contains> 20 to 25 wt .-% Al₂O₃. Al₂O₃ has a positive effect on the temperature stability of the glass without raising the processing temperature too much. If the content is low, the glass becomes more susceptible to crystallization. A content of at least 20.5% by weight, in particular at least 21% by weight, of Al is preferred₂O₃. A content of at most 24% by weight of Al is preferred₂O₃.

The B₂O₃Content is limited to at most 11.5% by weight in order to achieve a high transformation temperature T._G to achieve. Higher levels would also impair chemical resistance. The B is preferably₂O₃-At most 11% by weight. The B₂O₃Content is more than 6% by weight to ensure good meltability and good crystallization resistance of the glass.

Network-changing alkaline earth oxides are an essential part of glass. Especially by varying their proportions, a coefficient of thermal expansion becomes α_20/300 between 2.8 · 10^-6/ K and 3.6 · 10^-6/ K achieved. The individual oxides are present in the following proportions:

The glass contains 4 to <6.5% by weight of MgO and> 4.5 to 8% by weight of CaO. Rather high proportions of the two components have a positive effect on the desired properties of low density and low processing temperature, while rather small proportions favor the resistance to crystallization and the chemical resistance.

The glass also contains BaO, namely at least 0.5% by weight. The maximum BaO content is limited to less than 5% by weight. This ensures good meltability and keeps the density low.

The glass can also contain up to <4% by weight of the comparatively heavy alkaline earth oxide SrO. The limitation of this optional component to this low maximum content is particularly advantageous for a low density and good meltability of the glass. To improve the crystallization stability, it is preferred that SrO is present , preferably with at least 0.2% by weight.

The total content of BaO and SrO is at least> 3% by weight in order to ensure sufficient crystallization stability.

The glass can contain up to 2% by weight of ZnO, preferably up to <2% by weight of ZnO, ZnO has a framework-loosening function as a network converter, and it has little influence on the thermal expansion than the alkaline earth oxides. It has a similar influence on the viscosity characteristic as B₂O₃. The ZnO content is preferably limited to a maximum of 1.5% by weight, particularly when the glass is processed using the float process. Higher proportions would increase the risk of annoying ZnO deposits on a glass surface, which can result from evaporation and subsequent condensation. can form in the hot forming area.

The glass is alkali-free. Alkali-free is understood here to mean that it is essentially free from alkali oxides, and it can contain impurities of less than 1000 ppm.

The glasses can contain up to 2% by weight of ZrO + TiO₂ included, both the TiO₂Content as well as the ZrO₂Content individually can be up to 2% by weight. ZrO₂ advantageously increases the temperature stability of the glass. However, due to its poor solubility, it increases the risk of ZrO₂-containing melting relics (so-called "zircon nests") in the glass. Therefore, the addition of ZrO is preferred₂ waived. Low levels of ZrO₂, which result from the corrosion of zircon-containing tub material, are unproblematic. TiO₂ advantageously reduces the tendency to solarization, ie the decrease in transmission in the visible wavelength range due to UV-VIS radiation. If the content is more than 2% by weight, complexing with Fe³⁺-Ions that are present in the glass in low contents due to contamination of the raw materials used, color casts occur.

The glasses can contain conventional refining agents in conventional amounts: for example, it can contain up to 1.5% by weight As₂O₃, Sb₂O₃, SnO₂ and / or CeO₂ contain. Also the addition of 1.5 wt .-% Cl^- (e.g. as BaCl₂), F^- (e.g. as CaF₂) or so₄^2- (e.g. as BaSO₄) possible. The sum of aces₂O₃, Sb₂O₃, CeO_2, SnO_2, Cl^-, F^- and so₄^2- however, should not exceed 1.5% by weight.

If the refining agents As₂O₃ and Sb₂O₃ is dispensed with, the glasses can be processed not only with the various drawing processes, but also with the float process.

For example, with regard to a simple batch preparation, it is advantageous that both ZrO₂ as well as on SnO₂ can be dispensed with and yet glasses with the property profile mentioned, in particular with high thermal and chemical resistance and with a low tendency to crystallize, are obtained.

Examples:<ul id="ul0001" list-style="none"><li>Glasses were melted in Pt / Ir crucibles from conventional raw materials, which were essentially alkali-free apart from inevitable impurities, at 1620 ° C. The melt was refined at this temperature for one and a half hours, then poured into induction-heated platinum crucibles and stirred at 1550 ° C. for 30 minutes for homogenization.</li><li>The table shows 11 examples of glasses according to the invention with their compositions (in% by weight on an oxide basis) and their most important properties. The refining agent SnO₂ with a proportion of 0.3% by weight is not listed. The following properties are specified:<ul id="ul0002" list-style="bullet" compact="compact"><li>the coefficient of thermal expansion α _20/300 [10^-6/ K]</li><li>the density ρ [g / cm³]</li><li>the dilatometric transformation temperature T_G [° C] according to DIN 52324</li><li>the temperature at viscosity 10⁴ dPas (referred to as T 4 [° C])</li><li>the temperature at viscosity 10² dPas (referred to as T2 [° C], calculated from the Vogel-Fulcher-Tammann equation</li><li>the refractive index nd</li><li>an acid resistance "HCl" as a weight loss (ablation value) from all-round polished glass plates with the dimensions 50 mm x 50 mm x 2 mm after treatment with 5% hydrochloric acid for 24 hours at 95 ° C [mg / cm²].</li><li>the resistance to buffered hydrofluoric acid "BHF" as a weight loss (removal value) from all sides polished glass plates with the dimensions 50 mm x 50 mm x 2 mm after treatment with 10% NH₄F · HF for 20 min at 23 ° C [mg / cm²].<img file="EP1118596A2_D0001.tif" /><img file="EP1118596A2_D0002.tif" /></li></ul></li></ul>

As the exemplary embodiments make clear, the glasses according to the invention have the following advantageous properties:<ul id="ul0003" list-style="bullet"><li>a thermal expansion α 20/300 between 2.8 · 10^-6/ K and 3.6 x 10^-6/ K, thus adapted to the expansion behavior of amorphous and increasingly polycrystalline silicon.</li><li>with T_G &gt; 700 ° C a high transformation temperature, i.e. a high temperature resistance. This is essential for the lowest possible production-related shrinkage ("compaction") and for the use of the glasses as substrates for coatings with amorphous Si layers and their subsequent tempering.</li><li>with ρ &lt;2,600 g / cm³ a low density</li><li>a temperature at viscosity 10⁴ dPas of maximum 1350 ° C, and a temperature at viscosity 10² dPas of maximum 1720 ° C, which means a suitable viscosity characteristic with regard to hot forming and meltability. The glasses are flat glasses with the various drawing processes, e.g. B. Micro-sheet-down-draw, up-draw or overflow fusion method and in a preferred embodiment if they are free of As₂O₃ and Sb₂O₃ are also producible with the float process.</li><li>high chemical resistance, documented by good resistance to hydrochloric acid and to buffered hydrofluoric acid solution, which makes them sufficiently inert to the chemicals used in the manufacture of flat screens.</li><li>with n_d &lt;1.531 a low refractive index. This property is the physical basis for high transmission.</li></ul>

The glasses have a high resistance to temperature changes and good devitrification stability.

This makes the glasses ideal for use as substrate glass in display technology, especially for TFT displays, and in thin-film photovoltaics.