Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2001-01-11
2003-12-30
Kim, Robert H. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C501S066000
Reexamination Certificate
active
06671026
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 nematic) 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. Compaction of the substrate during production, in particular in the case of TFT displays, which has a disadvantageous effect on the semiconductor microstructure, can be countered by establishing a suitable temperature-dependent viscosity characteristic line of the glass: 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. Müller, 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:
Numerous documents describe glasses having low MgO and/or CaO contents: Japanese Patent Application No. 9-169 538 A, Japanese Patent Application No. 4-160 030 A, Japanese Patent Application No. 9-100 135 A, European Patent Application No. 714 862 A1, European Patent Application No. 341 313 B1, U.S. Pat. No. 5,374,595, Japanese Patent Application No. 9-48632 A, Japanese Patent Application No. 8-295530 A, International Application No. 97/11919 and International Application No. 97/11920. These glasses, do not have the desired meltability, as is evident from very high temperatures at viscosities of 10
2
dPas and 10
4
dPas, and have a relatively high density. The same applies to the MgO-free glasses of DE 37 30 410 A1, U.S. Pat. No. 5,116,787 and U.S. Pat. No. 5,116,789.
On the other hand, glasses having high MgO contents, as described in Japanese Patent Application No. 61-123 536 A, are insufficient in terms of their chemical resistance and their devitrification and segregation behavior.
The glasses described in International Application No. 98/27019 contain very little BaO and SrO and are likewise susceptible to crystallization.
Glasses having a high content of the heavy alkaline earth metals BaO and/or SrO, as described in European Patent Application No. 341313 B1, have undesirably high densities and poor meltabilities. The same is true for the glasses of Japanese Patent Application No. 10-72237 A. According to the examples, the glasses have high temperatures at viscosities of 10
4
dPas and 10
2
dPas.
Glasses having low boric acid contents likewise 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 Japanese Patent Application No. 10-45422 A, Japanese Patent Application No. 9-263421 A and Japanese Patent Application No. 61-132536 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 International Application No. 97/11919 and European Patent Application No. 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 Application 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.
DE 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 relatively low SiO
2
contents do not have a high chemical resistance, in particular to diluted hydrochloric acid.
DE 196 01 022 A1 describes glasses which are selected from a very wide composition range and which must contain ZrO
2
and SnO. These low-Al
2
O
3
glasses tend to exhibit glass defects because of their ZrO
2
level.
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 03 689 C1 (U.S. Pat. No. 5,770,535) by the Applicant disclose alkali-free, tin oxide-containing, low-SiO
2
or low-Al
2
O
3
glasses having a coefficient of thermal expansion &agr;
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 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 su
Brix Peter
Peuchert Ulrich
Kim Robert H.
Nils H. Ljungman & Associates
Schechter Andrew
Schott Glas
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