Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
1998-06-12
2002-09-17
Parker, Kenneth (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S137000
Reexamination Certificate
active
06452652
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to light valves utilized in display devices, and in particular, to a light valve incorporating a light absorbing thin film stack to prevent penetration of incident light into the underlying silicon substrate.
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.
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 liquid crystal display (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 an 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 metallization 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 metallization layer
222
of interconnect
204
. Storage capacitors
218
a
and
218
b
in turn transmit voltages to pixel cell electrodes
212
a
and
212
b
through portions of first, second, and third metallization layers
222
,
224
, and
214
of interconnect
204
.
First metallization layer
222
is electronically isolated from silicon substrate
205
by first intermetal dielectric layer
226
. Second metallization layer
224
is electronically isolated from first metallization layer
222
by second intermetal dielectric layer
225
. Third metallization layer
214
is electronically isolated from second metallization 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.
In the conventional light valve shown in
FIG. 2
, incident white light can penetrate into interconnect
204
through small gap
230
between adjacent pixel electrodes
212
a
and
212
b.
Incident light wave
232
can enter small gap
230
, refract at corners
234
of the pixel cell electrodes
212
a
and
212
b,
and then reflect off of the second layer of interconnect metallization
224
through a variety of paths until finally penetrating silicon substrate
204
.
Penetration of incident light
232
into silicon substrate
204
can induce unwanted currents that can disturb the charge present in storage capacitors
218
a
and
218
b.
As a result of this fluctuation in charge, the luminance of pixel cells
210
a
and
210
b
may change between succeeding write states, causing the image to “flicker.” The flicker produced by the penetrating light waves reduces image quality, and can cause eye strain in an observer.
Existing devices have addressed this problem by incorporating a simple light absorbing layer in the interconnect region.
FIG. 3
illustrates a cross-sectional view of adjacent pixel cell structures including a simple light absorbing layer, that form a portion of a conventional light valve. The light valve shown in
FIG. 3
is identical to the light valve shown in
FIG. 2
, except that a simple light absorbing layer
350
has been placed within the second intermetal dielectric layer
328
. Simple light absorbing layer is typically composed of a highly optically absorbing material, such as TiN.
FIG. 3
indicates that while most of incident light wave
332
entering narrow gap
330
is absorbed by simple light absorbing layer
350
, some incident light is reflected from the surface of light absorbing layer
350
. This reflected light can travel through interconnect
304
in a variety of paths before penetrating silicon substrate
305
and giving rise to electrical currents within silicon substrate
305
, disturbing charges stored in storage capacitor structures
318
a
and
318
b.
Therefore, a need exists for a light absorbing layer that not only absorbs incident light, but which also prevents reflection of incident light that could ultimately lead to penetration of light into the underlying silicon substrate of the pixel cell.
SUMMARY OF THE INVENTION
The present invention relates to a light absorbing thin film stack which is formed above the silicon substrate of an integrated circuit. This light absorbing thin film stack is designed to block penetration of light into the underlying silicon substrate.
In one embodiment of a light valve in accordance with the present invention, a light absorbing thin film stack is formed within the highest level intermetal dielectric of the interconnect.
The light absorbing thin film stack is formed from a surface layer
National Semiconductor Corporation
Parker Kenneth
Stallman & Pollock LLP
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