Alkali-free aluminoborosilicate glass, and uses thereof

Details Alkali-free aluminoborosilicate glass, and uses thereof Alkali-free aluminoborosilicate glass, and uses thereof

2000-08-21

2002-07-09

Sample, David (Department: 1755)

Compositions: ceramic

Ceramic compositions

Glass compositions, compositions containing glass other than...

C501S056000, C501S064000, C501S067000, C501S069000, C501S070000, C313S493000, C313S636000

Reexamination Certificate

active

06417124

ABSTRACT:

The invention relates to alkali-free aluminoborosilicate glasses. The invention also relates to uses of these glasses.
High requirements are made of glasses for applications as substrates in flat-panel liquid-crystal display technology, for example in TN (twisted nematic)/STN (supertwisted 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 relatively low density &rgr;, preferably ≦2.600 mg/cm
3
. Use as substrate material for integrated semiconductor circuits, for example TFT displays (“chip on glass”) in addition requires thermal matching to the thin-film material a-Si or polysilicon (&agr;
20/300
≈3.7·10
−6
/K) and the absence of alkali metal ions. Sodium oxide contents of less than 1000 ppm as a result of production can be tolerated with respect to the generally “poisoning” action due to diffusion of Na
+
into the semiconductor layer.
Suitable glasses should be capable of being produced economically on a large scale in adequate quality, for example in a float plant or by the drawing method. In particular, the production of thin (<1 mm) streak-free substrates with low surface undulation by the drawing process requires high devitrification stability of the glasses. Compaction of the substrate during production, 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 T
g
, i.e. a glass transition temperature T
g
of at least 650° C., while on the other hand not having excessively high melting and working (V
A
) points, i.e. a V
A
of ≦1330° C.
The demands made of glass substrates for LCD display technology are also described in “Glass substrates for AMLCD applications: properties and implications” by J. C. Lapp, SPIE Proceedings, Vol. 3014, invited paper (1997).
Corresponding requirements in principle are made of glasses for substrates in thin-film photovoltaics, especially based on microcrystalline silicon (&mgr;c-Si).
An essential prerequisite for the commercial success of thin-film photovoltaics over high-cost solar technology based on crystalline Si wafers is the presence of inexpensive high-temperature-resistant substrates.
At present, two different coating processes are known for the production of thin-film solar cells based on crystalline Si. A process which has proven particularly favourable with respect to high layer quality and thus efficiency and high deposition rates is a high-temperature CVD process using inexpensive trichlorosilane as Si source. This process proposes the direct deposition of thin crystalline Si layers and requires the heating of a suitable substrate to 1000° C. or above. The only suitable substrates are then comparatively expensive ceramics, graphite, silicon or similar materials. The use of glass-ceramics, which are likewise expensive, is also being discussed.
As an alternative to this, low-temperature Si deposition processes are being developed which allow the use of the less expensive substrate material glass. One possibility here is the deposition of amorphous silicon at low temperatures of up to 300° C. and, in a subsequent step, the recrystallization of the layers, for example using laser or zone-melting methods, with formation of partly crystalline &mgr;c-Si layers. In order to prevent deformation of the glass plate at the temperatures prevailing in the heating process, a special glass with very high heat resistance which is matched thermally to silicon is necessary, as is the case in glasses having glass transition temperatures T
g
of at least 650° C. As a consequence of the tendency to change over from a-Si to crystalline poly-Si coatings, the highest possible heat resistance of the substrate is also desired for substrates for TFT display technology. Since the thermal expansion of the Si layers decreases with increasing crystallinity thereof, glasses having very low expansion &agr;
20/300
of up to 3.0·10
−6
/K or less are also desired here.
The current development of &mgr;c-Si technology is moving in the direction of a substrate concept, i.e. the support material forms the basis of the solar cells and the incident light does not pass through it. In addition, a development towards a less expensive superstrate arrangement (light passes through the substrate material, no cover glass necessary) is not excluded. In order to achieve high efficiencies, high transparency of the glass in the VIS/UV is then necessary, which means that the use of semi-transparent glass-ceramics, besides the abovementioned cost reasons, proves to be disadvantageous.
For the last-mentioned application (superstrate) and for said applications as display substrates, the quality of the glasses with respect to the number and size of flaws, such as solid inclusions and bubbles, is of great importance since they impair the transparency of the glasses.
The glasses should thus if possible be free from or at least have a low content of bubbles and streaks. In particular of the glasses processed by the float method, a well fined glass can only be obtained with difficulty since the known effective fining agents As
2
O
3
and Sb
2
O
3
cannot be employed owing to their ease of reduction under the conditions of the float bath.
Similar demands are also made of glasses for light bulbs: glasses for halogen lamps must be essentially free from alkali metals since alkali metal ions disrupt the regenerative halogen cycle of the lamp. The glasses must have high thermal stability since high bulb temperatures usually occur in operation. The glasses must be sufficiently stable to devitrification in order to be suitable for tube drawing. For use as lamp envelope glass for light bulbs which contain molybdenum components as electrode or lead material, the thermal expansion of the glasses must be matched to that of molybdenum (&agr;
20/300
=5.0·10
−6
/K) in order that leak-tight, stress-free fusing is achieved between the metal and the glass. Also for this use, the glasses should have the lowest possible bubble content.
The complex property profile outlined above is achieved best by borosilicate glasses from the alkaline earth metal aluminoborosilicate glass sub-family. Commercially available glasses for TFT and AMLCD applications also belong to this glass type. Numerous patent specifications which describe glasses for said uses are also already known:
EP 672 629 A2 and U.S. Pat. No. 5,508,237 describe aluminosilicate glasses for flat-panel displays. They exhibit various composition ranges with various coefficients of thermal expansion. These glasses can allegedly be processed not only by the overflow fusion drawing process, but also by other flat-glass production methods. However, the glasses, in particular, which have a coefficient of thermal expansion matched to polycrystalline Si will have very high working points V
A
, which make them unsuitable for the float process. The visual quality of the glasses will not be high since no method for effective, in particular float-compatible fining is indicated. The fining agents Sb
2
O
3
and As
2
O
3
mentioned by way of example are unsuitable for the float process owing to their ease of reduction. The same applies to the optional glass components Ta
2
O
5
and Nb
2
O
5
. The same applies to the alkali-free glasses for TFT display applications which are described in the specifications U.S. Pat. No. 5,374,595, WO 98/270 19, EP 714 862 A1, EP 341 313 B1 and JP 10-722 37 A. The glasses of JP 8-295530 A, JP-9-48632 A and JP 9-156 953 A likewise cannot be fined effectively.
A similar, albeit lesser deficit is also exhibited b

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