Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
1998-09-03
2001-01-30
Parker, Kenneth (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S038000, C349S047000
Reexamination Certificate
active
06181398
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to the field of reflective array display devices, and, more particularly, to a novel reflective array structure that provides a novel multi-mirror structure for maximizing aperture ratio while minimizing optical power absorption.
2. Discussion of the Prior Art
In transmissive or reflection display arrays, it is desirable to have the aperture ratio of the cell as high as possible to minimize the amount of illumination required and optical power absorption by the array. A better display results from higher brightness and efficiency.
FIG. 1
illustrates the physical layout of an absorbing gap cell
10
, e.g., 10 &mgr;m pitch, for a LCD reflection display light valve with
FIG. 2
illustrating its equivalent circuit. As shown in
FIG. 2
, in an active matrix array LCD display, each pixel “cell” comprises a (thin-film) transistor
15
, and a capacitance
20
, and other components (not shown) and, may be fabricated using well-known CMOS fabrication techniques.
With more particularity, as shown in
FIGS. 1 and 2
, the absorbing gap cell
10
includes the following important functional layers: a conductive “P
1
” layer (doped polysilicon) providing a control signal to the gate of the transistor
20
row for determining the optical properties of the cell and forming one electrode of capacitor “C
sub
” and having another electrode return via the substrate; a first metal layer “M
1
” for carrying data signals to the source terminal of the active transistor
15
; a top-surface aluminum mirror layer “M
2
” located beneath the liquid crystal material (not shown) and forming one electrode of the liquid crystal display capacitor element C
LC
and having a top plate electrode formed of a transparent conductor such as ITO. Additionally, as part of the fabrication of the absorbing gap cell design, there is an anti-reflecting “AR” layer which forms a capacitance C
AR
with the M
2
and M
1
metal layers.
FIG.
3
(
a
) illustrates a cross-sectional view of the absorbing gap cell
10
of
FIG. 1
taken along line X
1
-X
1
′. FIG.
3
(
b
) illustrates a cross-sectional view of the absorbing gap cell
10
of
FIG. 1
taken along line Y
1
-Y
1
′. As shown in FIGS.
3
(
a
) and
3
(
b
), the cell includes: regions of implanted Silicon, for example, N
+
regions, indicated as region “RX” and forming the gate and drain/source regions for the thin-film transistor
20
;
the P
1
poly-Si conductive layer forming a gate for the transistor and one electrode of capacitance C
sub
with the other electrode formed of the implanted Si (RX layer); the first metallization layer M
1
for carrying data control signals to the source terminal of the active transistor layer RX and providing another end of capacitor C
AR
; the second metallization layer which is an light energy absorber layer “AR”, e.g., formed of a tri-layer composite of titanium-nitride, aluminum, and titanium; and, the third level metallization layer M
2
which is a top-surface aluminum mirror layer located beneath the liquid crystal material (not shown) and providing the liquid crystal cell with reflective optical properties. As shown in FIG.
1
and FIG.
3
(
a
), a contact “CA” is provided for connecting the M
1
layer to the P
1
contact.
As further shown in FIG.
3
(
b
), the titanium-nitride, aluminum, titanium anti-reflective or absorbing layer AR is provided between the M
1
and M
2
layers throughout the cell. The anti-reflective or light absorption is provided by the top titanium nitride layer, with the aluminum core layer providing the conductivity and the titanium underlayer providing the good contact and a barrier between the aluminum and the underlying SiO
2
. This AR layer is held at the top plate electrode potential (connection not shown) and, typically is fabricated at a depth below the M
2
mirror surface that is equal to an integer number of &lgr;/(2*n) for polarized illumination oriented in the normally black mode where &lgr; is the wavelength of the illuminating light. The aluminum mirror M
2
is shown contacting the M
1
metal layer underneath the AR absorbing layer by the provision of via “V
1
”, which may be a tungsten plug, for example, connecting M
1
and M
2
layers. As shown in FIG.
1
and
3
(
a
), a region AR of the AR layer is removed about the via V
1
so as to electrically isolate the AR layer.
In an active matrix array comprising absorbing gap pixel cells (of c-Si technology) shown in
FIG. 1
, the M
2
reflecting mirror surface area covers a fraction of the pixel surface area with exposed gaps “G” remaining within the cell. Disposed underlying the gaps “G” is the AR layer between the mirrors M
1
and M
2
that absorbs illumination energy. Thus, if the illumination directed at the cell is of high enough intensity, then optical power absorbed and heat removal from the array may be a design problem because the light valve array is typically packaged for compactness and accommodating heat sink sizes may expand the packaging, and/or require additional fan cooling which adds system weight and noise. This additionally applies to absorbing gap cells fabricated of p-Si technology which utilize a glass substrate (not shown). These problems are compounded in reducing cell pitch or incorporating binary area weighted mirrors. Thus, the prior art absorbing gap cell design exhibits a decreased aperture ratio, i.e., decreased light reflection efficiency.
It is the case then that an increase in aperture ratio is very desirable as this would reduce the illumination requirements and reduce array power absorption, thereby saving cost.
SUMMARY OF THE INVENTION
The present invention is directed to a reflection display array implementing two or more layers of reflecting front surface mirrors with upper layer mirror(s) having absorbing back surface(s). The mirror surfaces associated with each pixel are electrically connected to the pixel output electrode. The lower mirrors are appropriately positioned in the three dimensions to achieve nearly 100% aperture fill. Thus, the previous absorbing gap has been replaced with mirror(s).
Thus, according to the principles of the invention, there is provided a pixel structure for a reflective LCD display comprising a first layer of reflecting material for reflecting light directed at the cell structure in accordance with a control signal; a second layer of reflecting material disposed above the first layer of reflecting material for reflecting light directed at the cell structure in accordance with the control signal; and, a means for providing control signal to the first and second layers in said the structure for controlling amount of reflection thereof; whereby provision of the first and second layers of reflecting material results in reflective LCD display having substantially increased aperture ratio.
Advantageously, the fabrication of this multi-mirror structure for reflective array displays need not require any additional masks than that used by reflecting cell absorbing gap fabrication technique since the number of metal layers are the same.
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Libsch Frank R.
Sanford James Lawrence
Yang Kei-Hsiung
International Business Machines - Corporation
Parker Kenneth
Scully Scott Murphy & Presser
Shofi, Esq. David M.
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