Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2002-03-07
2003-04-29
Parker, Kenneth (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
Reexamination Certificate
active
06556269
ABSTRACT:
TECHNICAL FIELD
The present invention relates, generally, to liquid crystal display assemblies and, more particularly, relates to reflective liquid crystal display assemblies and interconnection assemblies.
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 high information content, miniature LCD assemblies enable enhanced availability of graphics, data and video information for employment 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.
Reflective LCD projectors, in particular, are highly desirable since they offer the brightness of traditional three-lamp front-projection systems in combination with the high resolution of an LCD panel. At the heart of these optical engines is reflective liquid crystal on crystalline silicon light valve technology which, when combined with sophisticated optical architecture and the appropriate electronic interface, enables very high resolution, high brightness, large screen displays. In one optical engine form, as shown in
FIG. 1
, full spectrum visible light from an illumination source
13
is directed through a sophisticated polarized beam splitter or prism
14
which separates the light into red, blue and green beam components. Each beam component is then directed into a corresponding red, blue and green reflective LCD panel assembly
20
,
20
′ and
20
″, each of which is configured to control the amount of light absorbed and reflected. More specifically, each pixel of the pixel array for each LCD panel regulates the amount of red, green, and blue light, respectively, it reflects back into the prism. Subsequently, the prism
14
reintegrates the shades of corresponding red, blue and green light for each pixel to create a single visible color at one convergent location. Collectively, the reintegrated pixels (i.e., from the red LCD panel
20
, the blue LCD panel
20
′ and the green LCD panel
20
″) create an image which is projected through a lens
15
and onto a screen
16
.
Due in part to the complexity of the optical engine
17
and the precise nature of the alignment between the cooperating light valves (i.e., LCD panel assemblies
20
,
20
′ and
20
″), the structural and optical coupling to the prism
14
is critical to performance. Current mounting techniques, thus, require completely independent coupling to the prism with independent interconnections to the electronic interfaces. As schematically illustrated in FIG.
1
and as shown in
FIGS. 2
an
3
, each LCD panel assembly
20
,
20
′ and
20
″ includes an independent flex circuit device interconnection
18
,
18
′ and
18
″, respectively, electrically coupling a corresponding respective LCD panel to the electronic interface
19
.
One problem associated with the current design of these optical engines is the substantial fabrication costs involved due to the abundance of relatively costly individual interconnections between the LCD panels and the electronic interface. These additional flex circuit device interconnections
18
also increase the space requirements in order to accommodate the individual interconnections. Moreover, manufacture time is increased as well as requiring additional labor resources to connect, position and place the individual flex circuit device interconnections.
Another problem associated with both transmissive and reflective-type LCD panels assemblies is the bowing or warpage of the individual panels caused by residual stresses acting upon the die during operation. This is particularly noticeable in reflective-type LCD panels which have increased flatness requirements due to the nature of the reflective surface of the die. For example, thermal expansion characteristics, as well as lattice mismatching, can generate significant stresses in the underlying substrate material (the silicon), therein causing significant bowing of the mirrored surface. The bowing, which translates to a non-planarity of the surface, causes both (1) a non-uniform thickness of the liquid crystal layer between the bowed reflective surface and the planar transmissive top layer, and (2) variations in the path length of the reflected light from different parts of the element, and of the array. These effects compromise the electro-optic properties of the elements and/or array.
As mentioned, a primary source of these residual stresses originate from the different materials and composites of the LCD panel having different coefficients of expansion. This is best shown in
FIGS. 2 and 3
which illustrate a conventional small scale LCD assembly
20
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
.
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 flex circuit
18
includes a plurality of flex circuit bond pads
30
which are typically wire bonded to the die bond pads
23
through bonding wires
31
. The distal end of flex circuit
18
is coupled to the top surface of substrate
27
. Finally, a glob coating
32
is applied to die
21
, substrate
27
and the distal end of flex circuit
18
. The glob coating
32
(
FIG. 3
) further normally encapsulates the bonding wires
31
and the die and flex circuit bonding pads
23
and
30
without obscuring a view of the pixel array
22
through the glass plate
24
.
As previously indicated, 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 ma
Mathew Ranjan J.
Takiar Hem P.
Beyer Weaver & Thomas LLP
National Semiconductor Corporation
Parker Kenneth
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