Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
1998-08-06
2002-03-12
Sikes, William L. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S058000, C349S149000
Reexamination Certificate
active
06356334
ABSTRACT:
TECHNICAL FIELD
The present invention relates, generally, to liquid crystal display assemblies and, more particularly, relates to miniature liquid crystal display assemblies constructed to reduce residual stresses.
BACKGROUND ART
In the recent past, substantial research and development resources have been directed toward small scale Liquid Crystal Display (LCD) and light valve technologies. These miniature LCD assemblies are typically employed in high resolution projection displays, such as a reflective LCD projectors, SXGA formats (1,280×1,024 pixel resolution) and even HDTV formats (above 1,000 line resolution), or the like.
Briefly, as shown in
FIGS. 1 and 2
, a conventional small scale LCD assembly
20
is illustrated including a die
21
having a pixel array
22
. This pixel array
22
is typically composed of rows and columns of electrically conductive pathways each forming an individual pixel (not shown). Each pixel can be individually changed to an “on” condition by selecting the appropriate row and column of pixel array
22
. Positioned around or concentrated on one end of the pixel array are a plurality of die bond pads
23
which are internally connected to the pixel array
22
to enable operational control thereof. Selection of the appropriate pixel is controlled by control circuitry, either included within the die
21
or external to the die
21
. In either configuration, external control signals may be used to control the functions of the die
21
.
As best viewed in
FIGS. 2 and 3
, a transparent glass plate
24
is typically placed over the die
21
and the pixel array
22
, such that a portion of the glass plate
24
overhangs the die
21
. The glass plate
24
is, usually affixed to die
21
through an adhesive seal
25
which together cooperate to define a sealed volume encompassing the pixel array
22
. This sealed volume is then commonly filled with a solution
26
of Polymer Dispersed Liquid Crystals (PDLC). To facilitate grounding of the glass plate
24
, a conductive coating (not shown) may be deposited over the undersurface
28
thereof.
The die
21
is typically rigidly or semi-rigidly mounted to a substrate
27
for mounting support and heat conductive dissipation for the die. A conductive adhesive
29
(FIG.
3
), such as a conductive epoxy, is generally applied to the undersurface
28
of the die
21
to affix the die directly to the top surface of the substrate
27
. Accordingly, a heat conductive pathway is created directly between the die and the substrate to dissipate heat generated by the die.
The substrate
27
generally includes a plurality of substrate bond pads
30
which are typically wire bonded to the die bond pads
23
through bonding wires
31
. Finally, a glob coating
32
is applied to seal die
21
to substrate
27
. The glob coating
32
(
FIG. 3
) further normally encapsulates the bonding wires
31
and the internal elements of die
21
without obscuring a view of the pixel array
22
through the glass plate
24
.
By activating the appropriate pixels, the corresponding liquid crystals in the PDLC, deposited in sealed volume, are caused to either align or disperse. Upon alignment, light is permitted to pass through the aligned crystals and the adjacent glass plate, thus appearing light in color. In contrast, when the liquid crystals are dispersed, light is prevented from passing therethrough and, hence the glass plate
24
, so that the corresponding pixel appears dark in color.
One important aspect in the proper operation of these small scale LCD or light valve assemblies is the maintenance of proper distance uniformity (preferably about 2-4 &mgr;m) between the pixel array and the undersurface
33
of the glass plate. Variances in the separation of the glass plates may often times cause the pixel array to function improperly or cause operational failure.
Conventional rigid display device constructions, for example, often warp during operation since the substrate
27
, the glass plate
24
and the silicon die
21
are all composed of materials or composites having different coefficients of expansion. The individual components of the LCD assembly, therefore, often expand at different degrees and rates. Further, depending in part upon the construction processes, such as the adhesive curing techniques, significant residual stresses may be induced upon the cell. Eventually, in severe instances, the glass plate
24
may delaminate from the die
21
. At a minimum, these internal stresses cause optical defects such as variations in color uniformity and fringes, and variations in the cell gap thickness may cause optical shadows.
This is especially true since the undersurface
28
of the die
21
is typically rigidly affixed or attached directly to the substrate. For example, when the substrate and the dies are both composed of a silicon material, upon heating, the glass plate expansion tends to negatively bow or warp (
FIG. 4
) at a rate greater than that of the die and substrate. Therefore, upon more extensive high temperature thermal cycling during operation, the glass plate
24
may eventually delaminate from the die
21
to expose the Polymer Dispersed Liquid Crystals (PDLC)
26
.
In contrast, when the die
21
is composed of a silicon material and the substrate
27
is composed of a more conductive material, such as aluminum, upon heating, the substrate expansion tends to positively bow or warp (
FIG. 5
) the substrate at a rate greater than that of the die
21
and glass plate
24
. As viewed in the cross-sectional view of
FIG. 5
, central thinning of the PDLC
26
is caused which result in discoloration and the appearance of optical shadows and nonuniformity.
Moreover, during low temperature conditioning, the glass plate
24
often fractures due to internal stress induced by the substrate, which is then transmitted to the glass through the rigidly mounted die. This is especially problemsome at the regions where the adhesive mounts the die to the substrate, and/or where the glob coating contacts the glass.
Accordingly, there is a need to provide a LCD assembly which minimizes residual stress induced upon the cell.
DISCLOSURE OF INVENTION
The present invention provides a liquid crystal display assembly comprising a display device and a support substrate. The display device includes a die having a pixel array, and a transparent plate positioned over the die. An adhesive seal adhesively couples the die to the transparent plate, which together with the transparent plate and the die cooperate to define a sealed volume therebetween encompassing the pixel array. A liquid crystal material is disposed within the sealed volume. The support substrate is coupled to the transparent plate for support of the display device such that the die is substantially insulated from transmission of residual stresses induced by or acting upon the substrate.
In one aspect of the present invention, the substrate may be mounted to the transparent plate at a single point location, while in another aspect, the substrate may be mounted to the transparent plate proximate a single peripheral edge portion thereof. An adhesive may be positioned between the substrate and the transparent plate for either single point mounting or the single peripheral edge portion mounting.
In another aspect of the present invention, the substrate provides a cavity formed and dimensioned for receipt of the die therein which is of a depth sufficient for non-contact between the die undersurface and the substrate.
In still another aspect of the present invention, a liquid crystal display assembly includes a die having a pixel array, a top surface and an opposite undersurface. A transparent plate is provided coupled to the die through an adhesive seal. The adhesive seal, the transparent plate and the die cooperate to define a sealed volume therebetween encompassing the pixel array, which contains a liquid crystal material disposed therein. A support substrate is coupled to the transparent plate for support of both the transparent plate and the mounted die thereon in a manner substantia
Mathew Ranjan J.
Vikram Seshadri
Beyer Weaver & Thomas
National Semiconductor Corporation
Nguyen Dung
Sikes William L.
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