Glass manufacturing – Processes – Utilizing parting or lubricating layer
Reexamination Certificate
1993-10-05
2001-04-17
Derrington, James (Department: 1731)
Glass manufacturing
Processes
Utilizing parting or lubricating layer
C065S060500, C065S060530, C065S060800, C065S090000, C427S165000
Reexamination Certificate
active
06216491
ABSTRACT:
FIELD OF THE INVENTION
The field of the invention is production of glass panels for use in assembly of a liquid crystal display (LCD) device.
BACKGROUND OF THE INVENTION
Liquid crystal display devices, whether passive or active, customarily embody thin, parallel, spaced glass panels with an intermediate liquid crystal layer.
Initially, soda lime glass panels were used in producing passive LCD devices. It was observed that, when such panels were employed, degradation of the liquid crystal occurred at normal operating temperatures. This was due to sodium ion migration to the glass surface, and an exchange of sodium for hydrogen ions in water contaminating the liquid crystal. To avoid this, it has been proposed to apply a silica film to the surface of the glass panel. This film acts as a barrier layer to stop sodium ion migration from the glass, and thus prevent exposure of the liquid crystal layer to the sodium ions.
Another approach to controlling sodium has been to use a BaO—Al
2
O
3
—B
2
O
3
—SiO
2
glass available from Corning Incorporated as Code 7059. This glass is nominally free of alkali metal oxides. This means that the glass has no intentionally added alkali metal compound in its batch, but may contain up to about 0.1% Na
+
as an impurity. Use of this glass avoids contamination of the liquid crystal.
In the active device, the back panel, or active plane, has thin film transistors formed on the glass by photolithographic steps, together with attached circuitry. The front panel, or color plane, has transparent colored dots or stripes in the case of a full color display. Combined with the inverted design of thin film transistors, Code 7059 glass also provides sufficient protection for the active elements in an active matrix LCD device while the amorphous silicon transistors are being fabricated on the glass at temperatures under 400° C. Sodium contamination would lead to transistor instability over time.
However, use of polysilicon thin film transistors is becoming increasing popular for active matrix LCDs. This practice involves processing temperatures that are substantially higher than the temperatures required for the amorphous silicon, and that approach the strain point of the glass. At these temperatures, sodium, which is present as an impurity, begins to create a problem again. As a result, it again becomes necessary to provide a barrier layer on the glass panel to prevent migration.
Fabrication of the active plane, or active matrix, involves the use of multiple photolithographic steps which require precise alignment. This requires that the panels not only have precise dimensions as formed, but that such precise dimensions be retained during subsequent processing steps. However, these processing steps may involve thermal exposure at or near temperatures where a glass may undergo structural rearrangement and/or dimensional relaxation. Accordingly, it has become common practice to subject glass panels to a compaction process after formation and before further thermal processing.
Compaction involves reheating a glass body to a temperature below the glass softening point, but equal to or above the maximum temperature reached in a subsequent processing step. This achieves structural rearrangement and dimensional relaxation in the glass prior to, rather than during, the subsequent processing. Preliminary compaction is imperative where it is necessary to maintain precise alignment and/or flatness in a glass body during subsequent photolithographic processing, as in the manufacture of flat panel display devices.
It is economically attractive to compact glass sheets in stacks. However, this necessitates interleaving, or separating, adjacent sheets with a release material to avoid sticking. At the same time, it is necessary to maintain the sheets extremely flat, and with an optical-quality surface finish.
The panels used in an LCD device must, of course, be of optical quality. Strict cleanliness is a requirement during all processing. Any marring of the surface, such as surface scratches, indentations, or the like, must be avoided.
Currently, sheets of graphite are inserted between glass panels to serve as a parting agent during the compaction process. They must be removed at completion of the process. This is not only an added step, but on occasion leads to scratches that must be removed by polishing.
It has been proposed in U.S. Pat. No. 5,073,181 (Foster et al.) to substitute a monolayer of submicron silica particles as a parting layer. However, this is also a non-permanent layer that must be removed before further processing. Hence, it could not function as a barrier layer to sodium migration.
It would, therefore, be desirable to apply a permanent surface film on at least one side of a glass panel during formation of the panel, or prior to further processing. This film would have to remain on the panel and not interfere with processing or operation of an LCD display, either passive or active. The film should be inert, transparent and refractory. It should serve as a parting agent to prevent glass adhesion during compaction, and also as a barrier layer to prevent sodium migration which would result in liquid crystal, or thin film transistor, degradation. Finally, it should improve the scratch resistance of the surface. It is a basic purpose of the present invention to provide a method of producing a glass panel for an LCD device that has such features.
SUMMARY OF THE INVENTION
My invention resides in a method of producing a glass panel for a LCD device wherein the glass is nominally alkali-free, that is, may have an alkali metal content no greater than about 0.1% by weight, and wherein the method comprises depositing on at least one surface of a clean panel a continuous, permanent, transparent, barrier layer film from an atmosphere of an atomized, or ionized, inert refractory material, or reactive precursor, the film being greater than 50 nm, but not over 500 nm, in thickness, and, after depositing the transparent film on the clean glass panel, stacking a plurality of the clean glass panels to form a stack of adjacent panels, the total film thickness between each pair of adjacent panels in the stack being greater than loonm and subjecting the stack to a heat treatment to compact the glass, the permanent, barrier layer film on the clean glass panel functioning both as a barrier to sodium ion migration and as a parting agent during the compacting heat treatment.
The invention further resides in a method of compacting a plurality of clean glass panels in a stack wherein the glass is nominally alkali-free, that is, may have an alkali metal content no greater than about 0.1% by weight, and wherein a surface on each glass panel opposes a surface on an adjacent glass panel, the method comprising depositing on at least one surface of each glass panel a continuous, permanent, transparent, barrier layer film from an atmosphere of an atomized, or ionized, inert refractory material, or reactive precursor, the film being greater than 50 nm, but not over 500 nm, in thickness, stacking the filmed panels so that at least one of each pair of adjacent surfaces has a film, and the total film thickness between each pair of adjacent surfaces is greater than 100 nm and subjecting the stack to a compaction thermal cycle, the permanent barrier layer film on the clean glass panel functioning both as a barrier to sodium ion migration and as a parting agent during the compaction thermal cycle.
In preferred embodiments, the inert refractory material deposited is silica, the film is deposited by chemical vapor deposition, preferably while the glass panel is being drawn, and the film is deposited on both sides of the panel.
PRIOR ART
In addition to the art previously mentioned, attention is directed to the following patent literature:
U.S. Pat. No. 4,485,146 (Mizuhashi et al.) discloses a soda-lime glass substrate that contains 10 to 20% of an alkali component in its composition. The patent teaches that the known silica layer is not usually sufficient as a barrier layer. In lieu thereof, the pat
Corning Incorporated
Derrington James
Peterson Milton M.
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