Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
1998-11-20
2001-04-17
Malinowski, Walter J. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S190000
Reexamination Certificate
active
06219126
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to flat panel displays which utilize a pair of spaced-apart substrates having liquid crystal material therebetween and to methods for their assembly.
BACKGROUND OF THE INVENTION
There are several ways of assembling liquid crystal flat panel displays (LCD). For example, the prior art has used a method where the two substrate portions comprising the panel are first fully configured with all required thin film transistors (TFT's), circuit lines and color filters. The two substrate portions (hereinafter sometimes called “substrates”) of the display panel are then aligned with respect to one another at a gap spacing on the order of about 5 microns (e.g. by using 5 micron glass spheres) and are then joined over most of peripheries using a two part pre-mixed high temperature epoxy that requires a temperature of approximately 180° C., applied for several hours to cure or cross-link. After this, the resultant empty panel assembly is placed in a vacuum chamber for removal of air and then placed in a dish of liquid crystal material which is forced into the evacuated panel space by backfilling the chamber with nitrogen gas. This prior art method is described in U.S. Pat. No. 5,029,623, issued on Jul. 9, 1991 to Stefan Brosig.
Any method of assembly that uses a thermal setting epoxy can be problematic because the curing temperature is too high for the liquid crystal material to tolerate without damaging its functional characteristics. For this reason, the assembly of an empty LCD panel from two substrates must precede the step of filling the empty panel with liquid crystal material. However, since the assembled empty LCD panel has a very narrow gap (e.g. 5 to 10 microns) between the two assembled substrates that form the panel, the filling step in vacuum is extremely slow, taking on the order of 3-5 hours.
More recently, a faster fill technique (“one-drop fill”) has been developed wherein one panel substrate receives a filling of droplets of liquid crystal prior to joining it with the second panel substrate. A version of the one-drop fill method is disclosed in U.S. Pat. No. 5,263,888, issued to Teruhisa Ishihara et al on Nov. 23, 1993. Here, the sealant is typically a uv (ultraviolet) setting epoxy where cross-linking requires uv radiation. This one-drop fill method cannot use the thermally setting epoxy but must use instead the uv or other epoxy which cures (sets or cross-links) near room temperature. As will be understood, this uncured epoxy must necessarily come in contact with the liquid crystal because the epoxy forms the peripheral boundary wall for the liquid crystal material. There is mounting evidence that uncured epoxy and other sealants will react with the liquid crystal causing the latter to degrade in its performance as a liquid crystal material. This deleterious effect is especially likely in those regions of the display panel where there is incomplete curing of the uv epoxy because the uv light is unable to reach all portions of the epoxy due to shadows cast by the thin film circuitry that extends out to the periphery of the panel substrates.
SUMMARY OF THE INVENTION
The present invention proposes a number of LCD (liquid crystal display) panel structures and fabrication methods therefor which prevent deleterious mixing of the liquid crystal material with any potentially contaminating sealants such as epoxies or silicones. These are intended to be used with the “1 drop fill” method since this is becoming the preferred method of assembly. Once assembled, even if all the epoxy or other sealant is not hardened or the two components of the epoxy are not fully mixed, the proposed structures of this invention are designed to prevent contact between sealant and liquid crystal, thereby preventing degradation of the crystal over the long term.
Several structures are described that are deposited by vapor deposition or the like, generally using lithographic techniques to produce barriers that separate the liquid crystal from the sealant once the two substrates of the panel are aligned, joined, and the sealant has cured. Some of the proposed structures include a set of double walls with a space in between to receive any overflow of cured or uncured sealant. Another configuration includes a peripherally deposited hard material that is joined to a soft material forming a mechanical flange/O ring type of seal or interlock. The geometries are so chosen that either a uv cured epoxy or a two component cross-linking epoxy using a room or near-room temperature set (cross-linking) or cure can be used. In the latter case, it is proposed to apply the the two-component epoxy formulation such that only one component is applied to each of the two substrates forming the panel, with inter-mixing of the two parts occurring once the two substrates are joined and under pressure. Heat may be applied to accelerate cross-linking so long as the temperature is kept below about 100° C. in order to avoid damaging the liquid crystal material.
Accordingly, the present invention broadly provides a liquid crystal display panel comprising:
a) a pair of mutually spaced-apart substrates, the substrates being spaced apart by a substantially uniform gap, each substrate comprising a central portion and a peripheral portion surrounding the central portion,
b) a preselected quantity of liquid crystal material disposed between the central portions of the pair of substrates,
c) a barrier fillet disposed between the pair of substrates and in the peripheral portions thereof, the barrier fillet substantially surrounding and confining the preselected quantity of liquid crystal material to the central portion, and
d) an adhesive fillet which is disposed between the pair of substrates in the peripheral portions thereof and substantially forming an effective seal therebetween, the adhesive fillet surrounding said barrier fillet.
According to a preferred embodiment, the adhesive fillet comprises epoxy material, while the barrier fillet comprises both i) a first spacer fillet of a uniform thickness that surrounds the preselected quantity of liquid crystal material, and ii) a second spacer fillet of the aforesaid uniform thickness, the second spacer fillet surrounding the first spacer fillet.
Preferably, the barrier fillet (or at least one of said first spacer fillet and said second spacer fillet) comprises a compressible material (e.g. a fully cured silicone elastomer) and is compressed substantially uniformly between the pair of substrates. Moreover, under some circumstances, it may be desirable to provide a third spacer fillet of the aforesaid uniform thickness and surrounding the second spacer fillet, in order to further reduce any risk of migration of uncured epoxy into potentially contaminating contact with the preselected quantity of liquid crystal material.
The present invention also provides a method for fabricating a liquid crystal display panel comprising a pair of substrates mutually spaced-apart by a selected gap, each substrate comprising a central portion and a peripheral portion surrounding said central portion, the method comprising the steps of:
a) depositing a substantially continuous barrier fillet of material in the peripheral portion of at least one of the substrates, the barrier fillet being substantially uniformly at least as thick as said gap,
b) depositing a preselected quantity of liquid crystal material in the central portion of one of the pair of substrates,
c) depositing an adhesive fillet that is thicker than the selected gap in a continuous closed path upon at least one of said peripheral portions, and
d) joining the pair of substrates into substantially gap-spaced adjacency of said peripheral portions.
According to a preferred embodiment, step (c) is carried out by:
i) depositing one component of a two-component epoxy material in a continuous closed path upon the peripheral portion of one of the pair of substrates, and
ii) depositing the other component of the two-component epoxy material in a continuous closed path, corresponding to the continuous clo
International Business Machines - Corporation
Malinowski Walter J.
McGinn & Gibb PLLC
Underweiser, Esq. Marian
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