Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
1999-05-24
2003-06-10
Sikes, William L. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S113000, C349S038000, C349S039000
Reexamination Certificate
active
06577362
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to light valves, and in particular, to a light valve pixel cell possessing enhanced storage capacitance.
2. Description of the Related Art
Liquid crystal displays (LCDs) are becoming increasingly prevalent in high-density projection display devices. These display devices typically include a light source which passes light through a light valve.
One of the methods for producing colors in a liquid crystal display is to sequentially project light having a wavelength corresponding to a primary color onto a single light valve. Color sequential light valves create a spectrum of color within the range of the human perception by switching between a set of discrete primary colors. Typically, red, green, and blue are the primary tri-stimulus colors used to create the remaining colors of the spectrum.
Specifically, during projection of each primary color, the light intensity is modulated such that combination of the intensities of the primary colors in sequence produces the desired color. The frequency of switching between the primary wavelengths by the light valve should be sufficiently rapid to render discrete primary states indistinguishable to the human eye.
Two factors dictate the minimum frequency necessary for switching. The first factor is the ability of the human eye to detect the discrete primary colors (e.g., red, green, blue). At slower than ideal switching speeds, the human eye will detect a flicker and the primaries may not blend.
The second factor determining the frequency of switching is the video refresh rate. During display of video images, the individual frames must be refreshed at frequencies undetectable to the human eye.
The net frequency of switching demanded by the combination of sequential color blending and video refreshing is beyond the capabilities of light valves that utilize thick (>1 &mgr;m) liquid crystal (LC) transducers. However, thin (<1 &mgr;m) liquid crystal transducers have been successfully fabricated. These thin LC transducers demonstrate adequate color sequential blending at video refresh rates. One example of such a thin LC transducer pixel cell structure is disclosed in U.S. Pat. No. 5,706,067, to Colgan et al.
In general, the conventional thin LC transducer pixel cells possess enhanced responsiveness due to the decreased volume of liquid crystal material between the top and bottom plates. A smaller volume enables the liquid crystal to shift orientation more quickly and in response to a lower applied voltage.
FIG. 1A
shows a plan view of adjacent thin LC transducer pixel cells in a conventional light valve.
FIG. 1B
shows a cross-sectional view of the adjacent pixel cells of
FIG. 1A
across line A-A′. Light valve portion
100
comprises adjacent pixel cells
110
a
and
110
b
having liquid crystal (LC) material
111
sandwiched within gap
106
between a top plate and a bottom plate. The top plate is composed of a translucent material, typically glass. The underside of the top plate is coated with optically transparent and electrically conducting material, typically indium-tin oxide (ITO). This conductive layer serves as a passive electrode for the active pixels below. This passive electrode layer also typically bears a polyimide layer, which is scored to provide an anchoring alignment for the LC material
111
.
The bottom plate of the pixel cell is formed by the active reflective metal pixel electrodes
112
a
and
112
b
of adjacent pixels
110
a
and
110
b,
respectively. Pixel electrodes
112
a
and
112
b
are separated and electrically isolated by trenches
118
.
A reflectance enhancing coating (REC)
116
is formed over pixel electrodes
112
a
and
112
b.
REC
116
consists of optically transparent dielectric films
116
a whose thickness and composition are specifically tailored to generate constructive interference of light reflected by electrodes
112
a
and
112
b.
The function and creation of such a REC is described in detail in co-pending U.S. patent application Ser. No. 08/872,013 (“the '013 application”) entitled “REFLECTANCE ENHANCING THIN FILM STACK”, filed Jun. 7, 1997 and hereby incorporated by reference. The '013 application describes one particularly effective embodiment of a REC that consists of alternating silicon oxide and silicon nitride films. Therefore, the REC shown in
FIG. 1B
includes two sets of oxide-nitride films.
Pixel electrodes
112
a
and
112
b
lie on top of an upper intermetal dielectric layer
128
that forms a component of interconnect scheme
104
. Interconnect
104
overlies capacitor structures
120
a
and
120
b
formed within underlying silicon substrate
105
. Capacitor structures
120
include a dielectric layer
162
formed over a double diffused drain (DDD) region
160
created within silicon substrate
105
. Capacitor structures
120
further include a polysilicon contact component
164
formed over dielectric layer
162
.
Storage capacitors
120
a
and
120
b
are in electrical communication with pixel electrodes
112
a
and
112
b,
respectively, through metal-filled vias
140
, middle interconnect metallization layer
124
, and lower interconnect metallization layer
122
. Storage capacitors
120
a
and
120
b
are controlled by MOS switching transistors
142
a
and
142
b,
respectively. MOS switching transistors
142
a
and
142
b
are also formed in underlying silicon substrate
105
, and are electrically isolated from adjacent semiconducting devices by trench isolation structures
144
.
Operation of the conventional pixel cell is described below in conjunction with
FIGS. 1A-1B
and FIG.
1
C.
FIG. 1C
depicts a circuit diagram representing the electrical behavior of conventional pixel cell
110
a.
At the commencement of a write cycle for pixel
110
a,
gate
146
a
of MOS switching transistor
142
a
receives a select voltage (V
select
) through first portion
122
a
of lower interconnect metallization
122
. V
select
activates MOS switching transistor
142
a,
permitting a video voltage signal (V
video
) appearing at drain
148
a
of MOS switching transistor
142
a
from second portion
122
b
of lower interconnect metallization
122
to be transmitted to capacitor structure
120
a
through channel
150
a
and source
152
a
of transistor
142
a
. V
video
is in turn transmitted through interconnect
104
to active electrode
112
a,
causing overlying LC material
111
to exhibit a particular transmission.
The light valve then addresses the next pixel cell
110
b.
The V
select
voltage is no longer applied to gate
146
a
of MOS switching transistor
142
a,
and the V
video
voltage is no longer applied to drain
148
a.
However, V
video
is maintained on active pixel electrode
112
a
by storage capacitor
120
a,
until the next write cycle occurs.
FIG. 1C
shows that there are actually two capacitive components present in conventional pixel cell
110
a.
The first capacitive component is storage capacitor
120
a
created by DDD
160
, dielectric layer
162
, and polysilicon element
164
. The second capacitive component of conventional pixel cell
110
a
is formed by combination of REC
116
and the LC material
111
itself, which form a dielectric between active electrode
112
a
and the overlying passive electrode.
The conventional pixel cell described above in
FIGS. 1A-1C
functions adequately in many applications. However, this design suffers from a number of disadvantages.
One problem is that light incident to array
100
may penetrate through gap between adjacent pixel electrodes
112
a
and
112
b.
Intermetal dielectric layer
128
below trench
118
is substantially transparent to this incident light, which next encounters middle interconnect metallization layer
124
. Metallization layer
124
likely bears an anti-reflective coating as a result of prior photolithographic steps. As a result, light incident to inter-pixel regions is absorbed rather than reflected, and is perceived by a viewer as a dark line. This dark inter-pixel region contrasts with the br
National Semiconductor Corporation
Qi Mike
Sikes William L.
Stallman & Pollock LLP
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