Elimination of the reverse-tilt in high-density reflective LCDs

Details Elimination of the reverse-tilt in high-density reflective LCDs Elimination of the reverse-tilt in high-density reflective LCDs

1998-07-24

2002-10-29

Sikes, William L. (Department: 2871)

Liquid crystal cells, elements and systems

Particular structure

Having significant detail of cell structure only

C349S086000, C349S156000, C349S157000

Reexamination Certificate

active

06473149

ABSTRACT:

BACKGROUND AND SUMMARY OF THE INVENTION
This invention generally pertains to the field of liquid crystal displays (LCDs). In particular, the invention relates to high density reflective LCDs and improving their quality and efficiency.
High density reflective LCDs are generally known.
FIG. 1
shows a partial cross-section of a generic embodiment of a prior art high density reflective LCD. Generally, the LCD is supported by a silicon substrate
10
. Secondary electrodes
12
a
,
12
b
,
12
c
are fabricated on or within substrate
10
. Above substrate
10
is liquid crystal
14
, which is contained by transparent electrode
16
. Above transparent electrode is a second substrate
18
, also transparent.
Fabricated below secondary electrode
12
a
,
12
b
,
12
c
in substrate
10
are electronic circuits
20
a
,
20
b
,
20
c
, which interface with secondary electrodes
12
a
,
12
b
,
12
c
. Such electronic circuits
20
a
,
20
b
,
20
c
are also known in the art, and provide a voltage level at the respective secondary electrode
12
a
,
12
b
,
12
c
that alters the state of the liquid crystal
14
adjacent the electrodes
12
a
,
12
b
,
12
c
. (Such secondary electrodes are referred to as “pixels” by those skilled in the art.)
It is the state of the liquid crystal
14
that determines whether and how much light is transmitted. As noted above, the embodiment shown in
FIG. 1
is a reflective LCD. Thus, polarized light is projected downward through substrate
18
and transparent electrode
16
. If the liquid crystal
14
above the secondary electrode
12
a
, for example, is in a transmissive state, then the polarization of the light is altered as it passes through the liquid crystal
14
and reflected by secondary electrode
12
a
. This change in polarization allows the light to transmit through a polarizer positioned externally (not shown in
FIG. 1
) and, consequentally, the pixel appears bright.
On the other hand, if the liquid crystal
14
above secondary electrode
12
a
is not in a transmissive state, then the polarization of the light incident on secondary electrode
12
a
is unaltered and no light passes the external polarizer. Consequently, the pixel corresponding to electrode
12
a
is dark.
The LCD, of course, is made up of an array of many secondary electrodes (or pixels), such as electrodes
12
b
and
12
c
shown in the cross-section of FIG.
1
. The states of these electrodes, and the corresponding state of the liquid crystal
14
above each one, will determine the state of the corresponding pixel. Also, as described further below, the state of the liquid crystal
14
may also provide for partial transmission of the light, resulting in a lower intensity glow of the respective pixel.
FIG. 2
is a top view of the array of an LCD such as that shown in partial cross-section in FIG.
1
. The pixels of the display corresponding to secondary electrodes
12
a
,
12
b
,
12
c
are marked in FIG.
2
. From this perspective, the driving electronics
20
a
,
20
b
,
20
c
would be beneath the electrodes
12
a
,
12
b
,
12
c
and, because of this, the pixels can be positioned closer together on the substrate, resulting in a high “fill” factor. (Fill is defined as the area of the secondary electrodes or pixels divided by the area of the supporting substrate. In
FIG. 2
, this would be equivalent to the square of the width of an electrode (w) divided by the square of the pitch.) A high density reflective LCD can have a fill factor on the order of 0.9 and higher.
Also visible in
FIG. 2
are a series of “spacer beads.” The spacer posts are not visible in the cross-sectional illustration of
FIG. 1
, but serve to set the liquid crystal cell gap between secondary electrodes
12
a
,
12
b
,
12
c
and common electrode
16
. The spacer beads shown in
FIG. 2
are comprised of a series of plastic beads that are randomly positioned between the substrates
10
,
18
. The spacer posts can be constructed by depositing and patterning an insulating later on the substrate
10
. The beads set the liquid crystal cell gap. The beads can be seen when the display is in operation, so reduction of the number of beads needed to set the liquid crystal gap has been pursued.
Referring to
FIG. 3
, a schematic of the state of the liquid crystal
14
directly above secondary electrode
20
a
is shown as a function of the voltage of the secondary electrode. (The portion of the liquid crystal
14
shown in
FIG. 3
corresponds to the dashed area shown in
FIG. 1.
)
The alignment of the liquid crystal molecules when there is a low voltage (referred to as “0 V”) is shown in cross-section to be tilted with respect to the axis between the secondary electrode
12
a
and the transparent electrode
16
. (If viewed from above, the molecules would form a helical structure.) In the “relaxed” state shown, light is transmitted; thus, the liquid crystal shown in
FIG. 3
is “normally transmissive.”
When a “high” voltage is applied, shown in
FIG. 3
to be 6V or higher, the liquid crystal aligns substantially normal to electrodes
12
a
,
16
, or, equivalently, substantially parallel to the electric field between the electrodes. Such alignment of the liquid crystal corresponds to a “dark state” of the liquid crystal, where little or no light is transmitted.
Referring to
FIG. 4
, the alignment of the liquid crystal is shown for adjacent electrodes both having a “high” voltage magnitude, but where one voltage is positive and one is negative. Adjacent electrodes in this state are referred to as bring in an “inversion mode.”
Such application of opposing voltage is routinely practiced in the LCD arts and is for the purpose of reducing artifacts such as flicker and improving the overall uniformity of the display.
The liquid crystal generally designated as being in the “central” regions of both electrodes
12
a
,
12
b
in
FIG. 4
are similarly aligned to give a dark state, like the alignment corresponding to 6V as shown in FIG.
3
. The normal tilt inclination of the liquid crystal is the same in the central regions above both electrodes
12
a
and
12
b
, even though the potentials of the electrodes are 6V and −6V, respectively.
The region spanning the gap between the electrodes
12
a
,
12
b
is generally designated as the “interpixel region” in FIG.
4
. Moving from the central region of electrode
12
a
through the interpixel region and into the central region of electrode
12
b
, the electric field transitions from +6V in a direction perpendicular to electrodes
12
a
,
16
to −6V in a direction perpendicular to electrodes
12
b
,
16
. As shown in
FIG. 4
, above the interpixel region between the pixels
12
a
,
12
b
this electric field (12V) dominates the alignment of the liquid cystal, forcing it to align parallel to the substrate
10
surface in the interpixel region. (This strong parallel electric field also removes the normal helix-like alignment of the liquid crystal. That and the lack of a reflective surface between pixels
12
a
,
12
b
leads to little or no transmission of light in the interpixel region, as shown in
FIG. 4.
)
As shown, in the interpixel region the liquid crystal tends to align with this relatively strong electric field (approximately 12V, resulting from the composite electric field from electrodes
12
a
,
12
b
). As a result, at the right side of the gap region (i.e., above electrode
12
b
) the liquid crystal tends to tilt opposite its normal tilt inclination. This corresponds to the beginning of the transition of the strong electric field parallel to the electrodes in the interpixel region to a field of −6V perpendicular to electrodes
12
b
,
16
at the central region of electrode
12
b.
Thus, moving from the interpixel region toward the central region of electrode
12
b
, the electric field decreases in magnitude and changes direction, from parallel with respect to the substrate
10
surface to perpendicular with respect to the substrate
10
surface. At a certain point, the influence of the elastic energy of the liquid crystal to align according to its norma

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