Stock material or miscellaneous articles – Composite – Of inorganic material
Reexamination Certificate
1999-06-21
2001-08-21
Jones, Deborah (Department: 1775)
Stock material or miscellaneous articles
Composite
Of inorganic material
C428S215000, C428S216000, C428S426000, C428S428000, C428S546000, C428S698000, C428S702000
Reexamination Certificate
active
06277507
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method for coating an insulating film having the ability to prevent alkali dissolution, on a surface of a glass plate, in particular, a glass plate containing an alkali component such as sodium. The present invention further relates to a glass substrate suitable for use in image displays fabricated through a high-temperature production step, such as liquid-crystal displays, not to mention plasma displays and field emission displays (FED).
BACKGROUND OF THE INVENTION
In producing a liquid-crystal display, a glass is coated with an insulating film of SiO
2
in a thickness of about from 0.04 to 0.15 &mgr;m in order to prevent the alkali components such as sodium and potassium contained in the glass from dissolving in the liquid-crystal cell. Techniques used for forming this SiO
2
film include: a high-frequency sputtering process in which a quartz glass is used as a target; a method comprising applying a solution of an organosilicon compound to a surface of a glass plate and heating the coating to form an SiO
2
film; the CVD process (chemical vapor deposition process) disclosed in JP-A-61-63545 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”) in which a vapor of an organosilicon compound is blown against a heated glass plate; and the technique disclosed in JP-A-60-176947 in which hydrosilicofluoric acid is used. The SiO
2
film coated by either of these methods has electrical insulating properties and also the ability to prevent alkali dissolution, and the glass plates thus coated are in practical use in liquid-crystal displays.
In the process steps for fabricating an STN liquid-crystal display, the maximum temperature used therein is about 350° C. In contrast, in fabricating image displays having an inner space which has a reduced pressure or is vacuum, such as plasma displays and field emission displays, the process steps involve a high-temperature step where a temperature of 500° C. or higher is used, because two spaced glass substrates are united into a panel by sealing the edge parts thereof with a glass frit through sintering.
However, the SiO
2
insulating films coated by the prior art techniques described above are known to have the following drawback. When a glass plate coated with the SiO
2
insulating film is processed at a temperature of 500° C. or higher, for example, in the step of sealing with a glass frit, alkali components contained in the glass, especially sodium ions, diffuse into the insulating film.
In particular, when the heat treatment is conducted at a high temperature for a prolonged time period, the alkali components dissolve in the insulating film and diffuse to the surface of the film. There has hence been a fear that the alkali components present on the surface of the insulating film may exert an adverse influence on the operation of an element to be formed on the glass substrate surface facing the inside of the panel.
An attempt was made to overcome the above problem by increasing the thickness of the SiO
2
film. However, this measure has problems that the coated SiO
2
film cracks probably because of the internal stress within the film, resulting in no improvement in insulating properties, and that formation of the thick film requires much time, resulting in reduced productivity in the coating step of the insulating film.
Namely, the SiO
2
film coated as an alkali dissolution preventive film on a glass plate by any of the prior art techniques has a problem that alkali components contained in the glass thermally diffuse to the surface of the SiO
2
film upon high-temperature heat treatment and the film does not always show sufficient insulating properties.
SUMMARY OF THE INVENTION
Accordingly, one object of the present invention is to provide a method for coating an insulating film.
Another object of the present invention is to provide a glass substrate for image display using the insulating film.
The present invention provides a method for coating an insulating film, which comprises coating an insulating film of one member selected from SiO
2
, a silicon oxynitride, and a silicon nitride on a surface of a substrate by reactive sputtering using one or more silicon targets in oxygen or/and nitrogen in a vacuum apparatus in which an atmosphere having a reduced pressure can be adjusted, such that a multilayered insulating film is formed.
In the preferred embodiment of the above method, the sputtering is reactive sputtering in which two silicon targets respectively disposed on two closely arranged cathodes are used, and a plasma discharge for which one of the cathodes is used as a negative electrode and the other is used as a positive electrode is generated alternately with a plasma discharge for which the one cathode is used as a positive electrode and the other is used as a negative electrode.
The present invention further provides a glass substrate for image display, which comprises a glass plate containing an alkali component and the insulating film coated on a surface of the glass plate by the above-described method, wherein the interface between layers in the insulating film functions to trap the alkali component thermally diffusing from the glass plate.
In one preferred embodiment of the glass substrate for image display, the insulating film has a thickness of 0.5 &mgr;m or larger. By regulating the total thickness of the insulating film to 0.5 &mgr;m or larger, an alkali contained in the glass can be effectively inhibited from dissolving in the insulating film and diffusing to both the surface of the film and a nearby part thereof.
Furthermore, by increasing the number of layers, the amount of the alkali component thermally diffusing to the surface of the insulating film and its nearby part can be reduced further.
In another embodiment of the glass substrate for image display, the insulating film has a thickness of 2 &mgr;m or smaller. This is because even when the total thickness of the insulating film is increased beyond 2 &mgr;m, the amount of a dissolved alkali present on the surface of the insulating film and its nearby part cannot be effectively reduced any more.
REFERENCES:
patent: 4187336 (1980-02-01), Gordon
Anzaki Toshiaki
Ogino Etsuo
Jones Deborah
Miranda Lymarie
Nippon Sheet Glass Co. Ltd.
Sughrue, Mion, Macpeak & Seas, PLLC
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