Flat panel display device comprising an alkaline earth aluminoborosilicate glass substrate

Abstract: This invention is directed to the production of flat panel display devices employing polycrystalline silicon thin film transistors, in which the substrate glasses are essentially free from alkali metal oxides and consist essentially, in mole percent, of: SiO₂ 60-65 MgO 0-4 Al₂O₃ 8-10 SrO 0-12 B₂O₃ 1-4 BaO 0-9 CaO 11-24 CaO+MgO+SrO+BaO 23-28

Description: Related Applications EP-A-528 114 published 24 February 1993 (US-A-5 116 787) is directed to glasses designed for use as substrates in flat panel display devices utilizing polycrystalline silicon thin film transistors. The glasses disclosed therein exhibited strain points higher than 625°C, liquidus temperatures below 1075°C, long term stability against devitrification, liquidus viscosities greater than 1.5x10⁵ poises, and have compositions essentially free from alkali metal oxides and MgO, while consisting essentially, in mole percent, of: SiO₂ 63-68 BaO 4.5-10 Al₂O₃ 7.5-11 CaO+SrO+BaO 14-26 CaO 9.5-16 B₂O₃ 1-7 SrO 0-5 EP-A-527 320 published 17 February 1993 (US-A-5.116.789) is likewise directed to glasses designed for use as substrates in liquid crystal display devices utilizing polycrystalline silicon thin film transistors. The glasses disclosed therein exhibit strain points higher than 675°C, linear coefficients of thermal expansion over the temperature range 25°-300°C between 45-62x10⁻⁷/°C, long term stability against devitrification, liquidus viscosities greater than 1.5x10⁵ poises, and have compositions essentially free from alkali metal oxides and MgO, while consisting essentially, in mole percent, of: SiO₂ 65-75 BaO and/or CaO 0-10 Al₂O₃ 6-10 B₂O₃ 0-5 SrO 15-26 [BaO and/or CAO] +B₂O₃ 0-12 Background of the Invention Glass has been chosen as a substrate in liquid crystal display devices for at least three reasons: first, it is transparent; second, it can withstand the chemical and physical conditions to which it is exposed during display processing; and third, it can be manufactured at reasonable cost in thin sheets with precisely controlled dimensions. Liquid crystal displays are passive displays which are dependent upon external sources of light for illumination. They are fabricated as segmented displays or in one of two basic matrix configurations. The needs of the two types of substrates differ. The first type is intrinsic matrix addressed, relying upon the threshold properties of the liquid crystal material. The second type is extrinsic matrix or active matrix addressed in which an array of diodes, metal-insulator-metal devices, or thin film transistors (TFTs) supplies an electronic switch to each pixel. In both designs, however, two sheets of glass form the structure of the display. Intrinsically addressed liquid crystal displays are fabricated employing thin film deposition at temperatures of about 350°C, followed by photolithographic patterning. Because of the low temperature requirements involved in the process, soda lime silicate glass having a silica barrier layer thereon to prevent migration of Na+ ions has been used extensively as substrates therefor. A higher performance version of intrinsically addressed liquid crystal displays, termed the super twisted nematic, has an added substrate requirement, viz., extremely precise flatness. That requirement has demanded that the soda lime silicate glass employed in those displays be polished. Alternatively, Corning Code 7059 glass, a barium boroaluminosilicate glass marketed by Corning Incorporated, Corning, New York, which is precision formed into sheet requiring no surface polishing utilizing the downdraw fusion pipe, such as is described in US-A-3,338,696 and US-A-3,682,609 has been employed. Extrinsically addressed liquid crystal displays can be subdivided into two categories: the first based upon metal-insulator-metal or amorphous silicon (a-Si) devices; and the second based upon polycrystalline silicon (poly-Si) devices. Devices formed from poly-Si are processed at substantially higher temperatures than those employed with a-Si thin film transistors. Those temperatures have demanded the use of glasses exhibiting higher strain points than soda lime silicate glasses and Corning Code 7059 glass to preclude thermal deformation of the sheet during processing. The lower the strain point of the glass, the greater this dimensional change. A practical solution to that problem is to develop glasses demonstrating high strain points so that the dimensional change is minimized during device processing at temperatures of about 600°C. Contamination of thin film transistors by Na+ ions migrating from the glass substrate is a major concern during processing. That problem has led to the use of a coating on the substrate glass to provide a barrier against migrating of any alkali. Therefore, glasses suitable as candidates for substrates in liquid crystal display devices utilizing poly-Si thin film transistors must comply with the following needs: (1) the glass composition will be essentially free from alkali metal oxides; (2) the glass must be relatively inert to the chemicals used in display processing; and (3) the glass must demonstrate a strain point of at least 625°C and preferably much higher. The glasses disclosed in EP-A-528 114 and in EP-A-527,320 were designed to be used in forming sheet via the downdraw fusion pipe process and, hence, required compositions exhibiting a viscosity at the liquidus temperature greater than about 1.5x10⁵ poises. In DE-A-3730410 there is described a glass substrate for use in liquid crystal display devices which has the for use in liquid crystal display devices which has the following composition in mole % calculated from Examples 1-10. SiO₂ 65.9-67.6 BaO 6.1-8.8 Al₂O₃ 6.3-8.4 SrO 0-3.5 B₂O₃ 5.1-9.2 ZnO 0.9-4.6 CaO 5.7-9.0 CaO+SrO+BaO 12.8-18.6 The amount of B₂O₃ is greater than, and the amount of CaO and the combined amount of CaO+SrO+BaO are less than, those of the glass in the display device according to the present invention. In JP-A-63176332 (Chemical Abstracts, Vol. 109, ref. 195924d) there are described acid resistant aluminosilicate glasses for electronics. The abstract refers to one example which has the following composition in wt % and mole % Wt. % Mole % SiO₂ 66.3 70 Al₂O₃ 16.0 10 CaO 17.7 20 The SiO₂ content is greater than that of the glass used according to the present invention, B₂O₃ is absent and the total CaO+MgO+SrO+BaO is lower. Example 15 of the reference has a composition listed below in terms of mole %. SiO₂ 64.5 ZrO₂ 1.2 Al₂O₃ 9.2 TiO₂ 1.6 B₂O₃ 4.5 P₂O₅ 0.6 CaO 18.4 In this example the B₂O₃ level exceeds the maximum of the glass used according to the present invention and the total CaO+MgO+SrO+BaO is below the minimum of 23% of the glass used according to the invention. In US-A-4180618 there is described the fabrication of electronic devices comprised of a thin film of silicon deposited onto a glass substrate, the glass consisting essentially, in weight percent, of 55-75% SiO₂, 5-25% Al₂O₃, and at least one alkaline earth metal oxide selected from the group in the indicated proportions of 9-15% CaO, 14-20% SrO, and 18-26% BaO. B₂O₃ makes up no part of the composition, and even ignoring the absence of B₂O₃, no working example provided in the patent had a composition coming within that of the composition used according to the present invention. In US-A-4824808 there are described glasses for use as substrates for liquid crystal displays consisting essentially in cation percent, of 52-58% SiO₂, 20-23% B₂O₃, 0-4% MgO, 0-6% CaO, 0-6% SrO, 1-9% BaO, 8-12% MgO+CaO+SrO+BaO, 0-3% ZnO, and 0-1% fining agent. The B₂O₃ contents are very much higher and the CaO contents very much lower than those of the glasses used according to the present invention. Summary of the Invention The present invention was designed to produce glasses demonstrating the high strain points and excellent chemical durability displayed by the glasses of EP-A-528 114 and EP-A-527 320, but which would exhibit lower melting viscosities than those glasses. Such lower melting viscosities would permit the manufacture of precision flat glass sheeting in an economical manner by an alternate forming process, for example, the float process. The lower liquidus viscosities of the glasses render them prone to devitrification when used in the down draw fusion pipe process. We have found a narrow range of glass compositions within the alkaline earth metal aluminoborosilicate system wherein the products exhibit strain points in excess of 675°C, an internal liquidus temperature below 1125°C, a viscosity at the liquidus temperature between 2x10⁴-10⁵ poises, a viscosity of 100 poises at temperatures below 1550°C, and excellent chemical durability as evidenced by a weight loss of less than 1 mg/cm² when immersed for 24 hours in an aqueous 5% by weight HCl solution operating at 95°C. Those compositions are essentially free from alkali metal oxides and consist essentially, expressed in terms of mole percent on the oxide basis, of: SiO₂ 60-65 MgO 0-4 Al₂O₃ 8-10 SrO 0-12 B₂O₃ 1-4 BaO 0-9 CaO 11-24 CaO+MgO+SrO+BaO 23-28 As used herein, essentially free from alkali metal oxides means that no alkali metal-containing material was purposefully included as a batch material. It is very difficult to evaluate in the laboratory the tendency of a glass to devitrify under various operating conditions. As a first approximation, a "liquidus" measurement is used. In reality, however, because of the method used in the laboratory, the measurement is not a true liquidus. Thus, the measuring technique involves placing crushed glass in a platinum boat which is then introduced into a gradient furnace having a temperature spread spanning the range wherein the liquidus is thought to be located. After 24 hours the boat is removed from the furnace, allowed to cool, the glass removed therefrom, thin sections prepared from the glass, and those thin sections examined microscopically. A measurement is made of the maximum temperature at which crystals are observed. The viscosity corresponding to this "liquidus" temperature provides the first estimate as to whether a particular glass is close to acceptability. At least 60% SiO₂ is demanded to assure the desired acid durability to the glasses. Nevertheless, higher SiO₂ contents increase the melting viscosity of the glass so 65% SiO₂ is deemed to be a practical maximum. B₂O₃ imparts greater fluidity to the glasses, both at high and low temperatures. Therefore, at least 1% will be included, but more than 4% adversely impacts the acid durability and the strain point of the glasses. The concentration of Al₂O₃ exerts a significant effect upon the liquidus of the glasses. To maintain a liquidus temperature below 1125°C and a viscosity at the liquidus temperature between 2x10⁴-10⁵ poises, the Al₂O₃ level will be held between 8-10%. At Al₂O₃ contents below 8%, the liquidus temperature can rise and the strain point is reduced. The ratio of the alkaline earth one to another is used to modify the melting and forming properties of the glasses. However, the total molar alkaline earth metal oxide content must be greater than 22% to provide fluidity at melting temperatures. The key principle involved is the capability of the alkaline earth metal ions to increase the slope of the viscosity temperature curve, thereby giving a high strain point with a relatively low melting viscosity. Whereas it is not possible mathematically to precisely convert mole percent to weight percent, the following ranges represent approximations of the inventive glass compositions in terms of weight percent: SiO₂ 53-61 MgO 0-3 Al₂O₃ 12-15 SrO 0-14 B₂O₃ 1-4 BaO 0-12 CaO 12-21 CaO+MgO+SrO+BaO 24-32 Description of Preferred Embodiments Table I below records a group of glass compositions, expressed in terms of parts by weight on the oxide basis, illustrating the compositional parameters of the instant invention. Inasmuch as the sum of the individual constituents totals or very closely approximates 100, for all practical purposes the tabulated values may be deemed to represent weight percent. The actual batch ingredients may comprise any materials, either oxides or other compounds, which, when melted together with the other batch ingredients, will be converted into the desired oxide in the proper proportions. For example, CaCO₃ and H₃BO₃ can provide the source of CaO and B₂O₃, respectively. The batch components were compounded, tumble mixed together thoroughly to assist in preparing a homogeneous melt, and charged into platinum crucibles. After placing lids thereon, the crucibles were moved into a furnace operating at about 1600°C and the batches melted for about 16 hours. Subsequently, the crucibles were removed from the furnace, the melts poured onto steel plates to produce glass slabs having dimensions of about 30cm x 15cm x 1cm, and those slabs introduced immediately into an annealer operating at about 750°C. (The Examples labelled 7059, 1733, and 1724 refer to glasses commercially marketed by Corning Incorporated and are included as comparative examples.) Although the above description is reflective only of research conducted in a laboratory, it must be understood that the inventive glasses are capable of being melted and formed utilizing large scale, commercial glass melting and forming equipment. Also, whereas not employed in the laboratory melts, conventional fining agents, e.g., As₂O₃ and Sb₂O₃, can be incorporated in the batches where deemed desirable. Table I also reports measurements of several physical properties as well as the acid durability of the glasses as determined in accordance with techniques conventional in the glass art. Accordingly, the annealing point (A.P.), the strain point (St.P), the temperature at which the glass exhibits a viscosity of 100 poises (100), and the internal liquidus temperature (Liq.) using a platinum boat are tabulated in °C. Also listed are the viscosity of the glass at the liquidus temperature (Vis.) in terms of 10⁴ poises and an evaluation of the resistance of the glasses to attack by acids as evidenced by measuring the weight loss (W.L.) in terms of mg/cm² after an immersion for 24 hours in a bath of 5% by weight aqueous solution of HCl, the bath operating at a temperature of 95°C. Table II records the exemplary glass compositions of Table I in terms of mole percent. RO represents the total of CaO+MgO+SrO+BaO. Examples 1-6 represent glass compositions coming within the ranges of the present invention and Examples 7-11 are included as comparative glasses to illustrate the criticality of composition control to yield glasses exhibiting the desired properties. As can be observed in the comparative examples, compositions outside of the required ranges do not yield the desired high strain points and/or viscosity relationships. The more preferred composition intervals of the subject inventive glasses consist essentially, in mole percent, of: SiO₂ 61-64 MgO 0-3 Al₂O₃ 8-9 SrO 0-9 B₂O₃ 1-3 BaO 0-7 CaO 12-22 CaO+MgO+SrO+BaO 25-26 Examples 1 and 2 comprise the most preferred glass composition. Whereas the present invention has been described in detail utilizing the inventive glasses as substrates in liquid crystal display devices, it will be understood that can be employed in other flat panel display devices such as electroluminescent displays and plasma displays.

Claims: A flat panel display device comprising a glass substrate utilizing polycrystalline silicon thin film transistors wherein said glass exhibits a strain point over 675°C, a liquidus temperature below 1125°C, a viscosity at the liquidus temperature between 2x10⁴-10⁵ poises, a viscosity of 100 poises at temperatures below 1550°C, and a weight loss of less than 1 mg/cm² after immersion for 24 hours in an aqueous 5% by weight HCl solution at 95°C, said glass being essentially free from alkali metal oxides and consisting essentially, expressed in terms of mole percent on the oxide basis, of: SiO₂ 60-65 MgO 0-4 Al₂O₃ 8-10 SrO 0-12 B₂O₃ 1-4 BaO 0-9 CaO 11-24 CaO+MgO+SrO+BaO 23-28 A liquid crystal display device according to claim 1 wherein said glass consists essentially of: SiO₂ 61-64 MgO 0-3 Al₂O₃ 8-9 SrO 0-9 B₂O₃ 1-3 BaO 0-7 CaO 12-22 CaO+MgO+SrO+BaO 25-26

Inventors: DUMBAUGH WILLIAM HENRY CORNING, DUMBAUGH, WILLIAM HENRY, CORNING INCORPORATED
Applicants: CORNING INC, CORNING INCORPORATED
Application year: 1992
Main classification: C03C3
IPC codes: H01L29, C03C3, G09F9