Alkali-free aluminoborosilicate glass, its use and process...

Details Alkali-free aluminoborosilicate glass, its use and process... Alkali-free aluminoborosilicate glass, its use and process...

2000-07-24

2002-10-15

Sample, David (Department: 1755)

Compositions: ceramic

Ceramic compositions

Glass compositions, compositions containing glass other than...

C501S056000, C501S059000, C501S070000, C501S072000, C501S027000, C065S134300

Reexamination Certificate

active

06465381

ABSTRACT:

The invention relates an alkali-free aluminoboro-silicate glass, for example for use in display technology and other applications, and to a process for its preparation.
The basic requirements made on a glass which is to be used in display technology, for example as the front screen for a flat-panel display, have been described in detail by W. H. Dumbaugh, P. L. Bocko and F. P. Fehlner (“Glasses for flat-panel displays” in “High-Performance Glasses”, ed. M. Cable and J. M. Parker, Blackie and Son Limited, Glasgow and London, 1992). The glasses currently available for TFT (thin film transistor) applications have also been reviewed in the article “Advanced glass substrates for flat panel displays” by J. C. Lapp, P. L. Bocko and J. W. Nelson, Corning Research 1994. The quality of the properties which are needed for flat glass substrates and have a decisive effect on the precision of the imaging properties of a system is dictated, on the one hand, directly by the composition of the glass and, on the other hand, by the production, processing and shaping methods and their ability to set particular properties for the glasses, for example thickness profile parameters and planarity parameters, the applicability of the methods being frequently limited in turn by the composition of the glass or by properties of the glass.
Borosilicate glasses play a dominant role in a large number of technically oriented glass applications. In particular, they are distinguished by high stability when subjected to cyclic and differential thermal loads, by low thermal expansion and by good resistance to corrosive reagents and media.
Borosilicate glasses are therefore in principle also of interest for use as substrate glass in display technology, but the display production process, for example for thin-film active matrix liquid crystal displays (TFT-AMLCDs), and the desired application require a very specific property profile of the glasses:
a coefficient of thermal expansion &agr;
20/300
matched to polycrystalline silicon of from 3.0 to 3.8×10
−6
/K; in particular an &agr;
20/300
of between 3.0 and 3.3×10
−6
/K means a good expansion match even up to temperatures of up to 700° C.
a temperature at a viscosity of 10
14.5
dPas of at least 680° C. in order to ensure high thermal processing and dimensional stability of the glass during production, in particular low compaction of the glass in the cooling phase,
an alkali-free glass composition, a maximum alkali metal oxide content of &Sgr; R
2
O=2000 ppm being tolerable, in order to avoid poisoning the microstructured thin-film transistors by alkali metal ions diffusing into the semiconductor layer,
a high high-temperature process stability documented by a transformation temperature Tg of between 710° C. and 780° C.,
sufficient chemical, i.e. hydrolytic, acid and alkali resistance, with respect to the reagents and media used in the microstructuring process,
a very low density, i.e. &rgr;≦2.50 g/cm
3
, in order to keep the overall weight of the display low in view of the trend towards large screen formats.
Furthermore, the visual quality of the glass screens, that is to say the quality in terms of the absence of crystalline inclusions, knots and bubbles, must be very good.
This complex and extensive requirement profile is fulfilled best by borosilicate glasses from the alkaline earth metal aluminoborosilicate glass subfamily. The known commercially available glasses for TFT-AMLCD applications belong to this type of glass; the glasses in the patents or patent applications discussed below are also representatives of this group. However, all currently known glasses for display applications still have disadvantages and do not meet the full list of requirements.
U.S. Pat. No. 5,506,180 describes thermally stable and chemically resistant glasses, amongst other things for use as TFT display glass. On account of the high B
2
O
3
contents of 10% or more and the relatively low SiO
2
levels (46-56%), glasses of this type are not sufficiently resistant to hydrochloric acid. Furthermore, their resistance to solutions containing hydrofluoric acid is only moderate. The matching of the thermal expansion to polycrystalline silicon is insufficient. The processing temperatures V
A
of less than 1150° C. are too low to make it possible to use drawing methods such as the microsheet down-draw method and the overflow fusion method as alternatives to the float process. Furthermore, the specified strain points of 642° C. or less are too low to ensure that there is little compaction.
European Patent EP 510 544 B1 describes alkali-free glasses which can be made by the float process and, amongst other things, are used as a substrate for a variety of displays and photographic masks. The glasses are free of BaO and MgO and have very low levels of B
2
O
3
. However, a disadvantage of these glasses is that their alkaline earth metal levels are high, with CaO levels being at least 10% by weight and SrO levels being at least 11% by weight. Their thermal expansion of 4.5-6.0×10
−6
/K is no longer sufficient to meet the requirements of high-quality TFT display glasses which are matched to polycrystalline silicon.
EP 527 320 B1 describes flat panel display devices having a strontium aluminosilicate glass substrate comprising at least 21 mol % of SrO. The glass compositions for glasses with high devitrification stability appear to be specifically designed to be suitable for being made in the overflow fusion draw method. The density of the glasses and their coefficient of thermal expansion are too high.
Japanese JP 8-295530 A describes alkali-free glass substrates whose stability with respect to hydrochloric acid will be low owing to the high B
2
O
3
content (up to 15% by weight).
PCT application WO 97/11919 also describes alkali-free glass substrates. The glasses comprise relatively little SiO
2
and little or no MgO. They may contain up to 5% by weight each of ZnO and TiO
2
. ZnO can cause glass defects owing to its tendency to evaporate from the glass surface in the float bath and subsequently condense. The possible high TiO
2
content will cause a brown tint in the glasses when conventional raw materials are used, since the Fe
3+
always present in the usual raw materials forms a brown colour complex with Ti
4+
. The same applies to the glass substrates described in WO 97/11920.
European Patent Application EP 714 862 A1 describes alkali-free ZrO
2
- and TiO
2
-free glasses for use in TFT flat panel displays. Aluminosilicate glasses of this type having relatively high SiO
2
contents are highly viscous, making efficient refining extremely problematic. These glasses do not therefore meet the requisite high demands in terms of visual quality. EP 672 629 A2 or U.S. Pat. No. 5,508,237 describe aluminosilicate glasses for flat panel displays. They present a variety of composition ranges with different coefficients of thermal expansion. These glasses are allegedly processable not only by the overflow fusion draw method but also with other flat glass production methods. However, in particular the glasses which have a coefficient of thermal expansion matched to polycrystalline Si have very high processing temperatures V
A
, which make them unsuitable for the float process. As in the case of the glasses described above, the visual quality here is not high, since no way of effective refining, in particular one compatible with the float process, is presented. The refining agents Sb
2
O, and As
2
O
3
mentioned by way of example are unsuitable for the float process because they can be reduced readily. The same is true for the optional glass components Ta
2
O
5
and Nb
2
O
5
.
In the alkali-free glass substrates for TFT-AMLCD from JP 9-48632 A, the visual quality will likewise not be high, since merely SiO
2
, B
2
O
3
, MgO and BaO have to be present in the glass.
German Patent DE 38 08 573 C2 describes alkali-free SrO- and B
2
O
3
-free aluminosilicate glasses which contain SnO
2
, are easy to melt and can be refined at low cost. The glasses exhibit

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