Liquid crystal cells – elements and systems – Particular excitation of liquid crystal – Electrical excitation of liquid crystal
Reexamination Certificate
1999-04-23
2002-08-20
Parker, Kenneth (Department: 2871)
Liquid crystal cells, elements and systems
Particular excitation of liquid crystal
Electrical excitation of liquid crystal
C349S042000
Reexamination Certificate
active
06437839
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to light valves used in display devices, and in particular, to integrated circuits using liquid crystal on silicon (LCOS) for creating LCOS display pixels forming high resolution light valves.
2. Description of the Related Art
Liquid crystal displays (LCDs) are becoming increasingly prevalent in high-density projection display devices. These conventional high density projection-type color display devices typically include a light source which emits white light. Dichroic mirrors separate the white light into its corresponding red, green and blue (RGB) bands of light. Each of these colored bands of light is then directed toward a corresponding liquid crystal light valve which, depending upon the image to be projected, either permits or prevents light transmission. Those RGB bands of light which are permitted to be transmitted through the light valves are then combined by dichroic mirrors or a prism. A projection lens then magnifies and projects the image onto a projection screen (Examples of circuit designs and fabrication techniques involving liquid crystal pixel circuits can be found in U.S. Pat. Nos. 5,247,289, 5,461,501 and 5,550,072, the disclosures of which are incorporated herein by reference.)
FIG. 1
illustrates a conventional LCD projection-type imaging system
100
. Imaging system
100
includes a light source
101
. White light is emitted from light source
101
. Once the light hits the prism
103
, the light is separated into its red, green and blue colored bands of light by dichroic filter coatings. Colored light is directed toward LCD light valves
105
. When reflected off light valve
105
, the colored light waves travel back through the prism and through projection lens
107
. Lens
107
magnifies and projects the synthesized color image onto projection screen
109
.
Conventional LCD light valves are formed by confining a thin layer of liquid crystal material between a top plate and a bottom plate. The top plate is a translucent substrate (typically glass) having one large electrode on a surface adjacent to the liquid crystal material. The bottom plate is generally interconnect overlying a storage capacitor structure formed within a silicon substrate.
FIG. 2
illustrates a cross-sectional view of adjacent pixel cell structures lacking a light absorbing layer, that form a portion of a conventional light valve. Portion
200
of the conventional light valve includes a glass top plate
202
bonded to a interconnect
204
by a sealing member (not shown). The sealing member serves to enclose a display area and to separate glass plate
202
from interconnect
204
by a predetermined minute distance. Thus, the light valve has an inner cavity
206
defined by the glass plate
202
and interconnect
204
. Liquid crystal material
211
, such as polymer dispersed liquid crystal (PDLC), is sealed in inner cavity
206
.
Portion
200
of the conventional light valve depicted in
FIG. 2
shows adjacent pixel cells
210
a
and
210
b
having reflective pixel electrodes
212
a
and
212
b
, respectively. Reflective pixel electrodes
212
a
and
212
b
are formed as part of third metalization layer
214
of interconnect
204
. The surfaces of adjacent pixel electrodes
212
a
and
212
b
are covered with a reflecting layer
216
. Reflecting layer
216
serves to reflect away white light incident to the pixel cell as described above in connection with FIG.
1
. Adjacent pixel electrodes
212
a
and
212
b
are electrically coupled to respective storage capacitor structures
218
a
and
218
b
formed in underlying silicon substrate
205
.
During operation of pixel cells
210
a
and
210
b
, driving circuits (not shown) are electrically coupled with storage capacitors
218
a
and
218
b
through row select lines
220
a
and
220
b
formed as part of first metalization layer
222
of interconnect
204
. Storage capacitors
218
a
and
218
b
in turn transmit voltages to pixel cell electrodes
212
a
25
and
212
b
through portions of first, second, and third metalization layers
222
,
224
, and
214
of interconnect
204
.
First metalization layer
222
is electronically isolated from silicon substrate
205
by first intermetal dielectric layer
226
. Second metalization layer
224
is electronically isolated from first metalization layer
222
by second intermetal dielectric layer
225
. Third metalization layer
214
is electronically isolated from second metalization layer
224
by third intermetal dielectric layer
228
.
The selective application of voltage to pixel electrodes
212
a
and
212
b
switches pixel cells
210
a
and
210
b
of light valve
200
on and off. Specifically, a voltage applied to a pixel electrode varies the direction of orientation of the liquid crystal material on the pixel electrode. A change in the direction of orientation of the liquid crystal material at the pixel electrode changes the optical characteristics of the light traveling through the liquid crystal. If the light valve contains twisted nematic crystal, light passes through the light valve unchanged where no voltage is applied to the pixel electrode, and the light is polarized if a voltage is applied to the pixel electrode. If the light valve contains PDLC, light passes through the light valve unchanged where a voltage is applied to the pixel electrode, and light is scattered if no voltage is applied to the pixel electrode.
However, as the densities of such displays have increased, limitations have been encountered. For example, one limitation on this density is the need for a minimum amount of storage capacitance in each pixel. For example, depending upon the frame rate of the display, the decay of the charge on the storage capacitor can lead to instability of the level of grey scale illumination, thereby leading to degradation in the image quality.
Accordingly, it would be desirable to have a technique for increasing individual pixel storage capacitance while maintaining the desired pixel density within the pixel array.
SUMMARY OF THE INVENTION
A liquid crystal pixel circuit in accordance with the present invention provides increased storage capacitance by using multiple storage capacitors with minimal, if any, increase in required circuit area. According to one embodiment, two storage capacitors are integrated in a vertical, or stacked, configuration in which a common, or shared, electrode is used for both capacitors.
In accordance with one embodiment of the present invention, an integrated liquid crystal pixel circuit with a plurality of storage capacitors includes an integrated circuit which includes a semiconductor substrate, a charge terminal, a reference terminal, a pass transistor, a liquid crystal cell and a plurality of storage capacitors. The charge terminal is within the integrated circuit and configured to receive and convey an electrical charge. The reference terminal is within the integrated circuit and configured to receive and convey a reference voltage. The pass transistor is within the integrated circuit, coupled to the charge terminal and configured to receive a control signal and in accordance therewith conduct the electrical charge. The liquid crystal cell is atop the integrated circuit, coupled to the pass transistor and responsive to the electrical charge. The plurality of storage capacitors is within the integrated circuit, coupled in parallel between the pass transistor and the reference terminal and configured to receive and store the electrical charge.
In accordance with another embodiment of the present invention, an integrated liquid crystal pixel circuit with a plurality of storage capacitors includes an integrated circuit which includes a semiconductor substrate, a charge terminal, a reference terminal, a transistor, a liquid crystal cell and first and second capacitors. The charge terminal is within the integrated circuit and configured to receive and convey an electrical charge. The reference terminal is within the integrated circuit and configured to receive and convey a r
National Semiconductor Company
Parker Kenneth
Wildman Harrold Allen & Dixon
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