Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2001-01-11
2004-03-16
Kim, Robert H. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C501S066000
Reexamination Certificate
active
06707526
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to an alkali-free aluminoborosilicate glass. The invention also relates to uses of this glass.
2. Background of the Invention
High requirements are made of glasses for applications as substrates in flat-panel liquid-crystal (or expressed differently: liquid crystal) display technology, for example in TN (twisted nematic)/STN (supertwisted nematic, or expressed differently: super twisted) displays, active matrix liquid crystal displays (AMLCDs), thin film transistors (TFTs) or plasma addressed liquid crystals (PALCs). Besides high thermal shock resistance and good resistance to the aggressive chemicals employed in the process for the production of flat-panel screens, the glasses should have high transparency over a broad spectral range (VIS, UV) and, in order to save weight, a low density. Use as substrate material for integrated semiconductor circuits, for example in TFT displays (“chip on glass”) in addition requires thermal matching to the thin-film material silicon which is usually deposited on the glass substrate in the form of amorphous silicon (a-Si) at low temperatures of up to 300° C. The amorphous silicon is partially recrystallized by subsequent heat treatment at temperatures of about 600° C. Owing to the a-Si fractions, the resulting, partially crystalline poly-Si layer is characterized by a thermal expansion coefficient of &agr;
20/300
≅3.7×10
−6
/K. Depending on the a-Si/poly-Si ratio, the thermal expansion coefficient &agr;
20/300
may vary between 2.9×10
−6
/K and 4.2×10
−6
/K. When substantially crystalline Si layers are generated by high temperature treatments above 700° C. or direct deposition by CVD processes, which is likewise desired in thin-film photovoltaics, a substrate is required which has a significantly reduced thermal expansion of 3.2×10
−6
/K or less. In addition, applications in display and photovoltaics technology require the absence of alkali metal ions. Sodium oxide levels of less than 1000 ppm (parts per million) as a result of production can be tolerated in view of the generally “poisoning” action due to diffusion of Na
+
into the semiconductor layer.
It should be possible to produce suitable glasses economically on a large industrial scale in adequate quality (no bubbles, knots, inclusions), for example in a float plant or by drawing methods. In particular, the production of thin (<1 mm) streak-free substrates with low surface undulation by drawing methods requires high devitrification stability of the glasses. In order to counter compaction of the substrate during production, in particular in the case of TFT displays, which has a disadvantageous effect on the semiconductor microstructure, the glass needs to have a suitable temperature-dependent viscosity characteristic line: with respect to thermal process and shape stability, it should have a sufficiently high glass transition temperature, i.e. T
g
>700° C., while on the other hand not having excessively high melting and processing (V
A
) temperature, i.e. a V
A
of ≦1350° C.
The requirements of glass substrates for LCD display technology or thin-film photovoltaics technology are also described in “Glass substrates for AMLCD applications: properties and implications” by J. C. Lapp, SPIE Proceedings, Vol. 3014, invited paper (1997), and in “Photovoltaik—Strom aus der Sonne” by J. Schmid, Verlag C. F. Muller, Heidelberg 1994, respectively.
The abovementioned requirement profile is fulfilled best by alkaline earth metal aluminoborosilicate glasses. However, the known display or solar cell substrate glasses described in the following publications still have disadvantages and do not meet the full list of requirements.
Some documents describe glasses containing relatively little or no BaO, e.g. European Patent No. 714 862 B1, International Patent Application No. 98/27019, Japanese Patent No. 10-72237 A and European Patent No. 510 544 B1. Glasses of this type, in-particular those having low coefficients of thermal expansion, i.e. low RO content and high network former content, are very susceptible to crystallization. Furthermore, most of the glasses, in particular in EP 714862 B1 and JP 10-72237 A, have high temperatures at a viscosity of 10
2
dPas.
However, the preparation of display glasses having high levels of the heavy alkaline earth metal oxides BaO and/or SrO is likewise associated with great difficulties owing to the poor meltability of the glasses. In addition, glasses of this type, as described, for example, in DE 37 30 410 A1, U.S. Pat. Nos. 5,116,789, 5,116,787, EP 341 313 B1, EP 510 543 B1 and JP 9-100135 A, have an undesirably high density.
Even glasses having relatively low SrO contents in combination with moderate to high BaO levels have unfavorable viscosity characteristic lines with respect to their meltability, for example the glasses described in JP 9-169538 A, WO 97/11920 and JP 4-160030 A.
Glasses having relatively high levels of light alkaline earth metal oxides, in particular MgO, as described, for example, in JP 9-156953 A, JP 8-295530 A, JP 9-48632 A and DE 197 39 912 C1, exhibit good meltability and have a low density. However, they do not meet all requirements made of display and solar cell substrates with regard to chemical resistance, in particular to buffered hydrofluoric acid, to crystallization stability and to heat resistance. Glasses having low boric acid contents exhibit excessively high melting-temperatures or, as a result of this, excessively high viscosities at the melt and processing temperatures required for processes involving these glasses. This applies to the glasses of JP 10-45422 A and JP 9-263421 A.
Moreover, glasses of this type have a high devitrification tendency when combined with low BaO contents.
In contrast, glasses having high boric acid contents, as described, for example, in U.S. Pat. No. 4,824,808, have insufficient heat resistance and chemical resistance, in particular to hydrochloric acid solutions.
Glasses having a relatively low SiO
2
content do not have sufficiently high chemical resistance either, in particular when they contain relatively large amounts of B
2
O
3
and/or MgO and are low in alkaline earth metals. This applies to the glasses of WO 97/11919 and EP 672 629 A2. The relatively SiO
2
-rich variants of the latter document have only low Al
2
O
3
levels, which is disadvantageous for the crystallization behavior.
The glasses described in Japanese Patent No. 9-12333 A for hard disks are comparatively low in Al
2
O
3
or B
2
O
3
, the latter merely being optional. The glasses have high alkaline earth metal oxide contents and have high thermal expansion, which makes them unsuitable for use in LCD or PV technology.
Federal Republic of Germany Patent No. 196 17 344 C1 (U.S. Pat. No. 5,908,703) and Federal Republic of Germany Patent No. 196 03 698 C1 (U.S. Pat. No. 5,770,535) by the Applicant disclose alkali-free, tin oxide-containing glasses having a coefficient of thermal expansion &agr;
20/500
of about 3.7×10
−6
/K and very good chemical resistance. They are suitable for use in display technology. However, since they must contain ZnO, they are not ideal, in particular for processing in a float plant.
In particular at higher ZnO contents (>1.5% by weight), there is a risk of formation of ZnO coatings on the glass surface by evaporation and subsequent condensation in the hot-shaping range.
Federal Republic of Germany Patent No. 196 01 022 A1 describes glasses which are selected from a very wide composition range and which must contain ZrO
2
and SnO. The glasses, which, according to the examples, contain SrO, tend to exhibit glass defects because of their ZrO
2
level.
Federal Republic of Germany Patent No. 42 13 579 A1 describes glasses for TFT applications having a coefficient of thermal expansion &agr;
20/300
Of <5.5×10
−6
/K, according to the examples of ≧4.0×10
−6
/K. These glasses which have relatively high B
2
O
3
levels and relati
Brix Peter
Peuchert Ulrich
Glas Schott
Kim Robert H.
Nils H. Ljungman & Associates
Schechter Andrew
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