Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
1999-03-29
2001-05-15
Sikes, William L. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S074000, C438S030000, C438S038000, C438S128000, C430S311000, C430S319000
Reexamination Certificate
active
06233033
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to light valves, and in particular, to pixel cell arrays for silicon light valves having pixels whose edges overlap in order to maintain reflectance in inter-pixel regions.
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. 1
shows a cross-sectional view of adjacent thin LC transducer pixel cells in a conventional light valve. 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. Top plate
102
is composed of a translucent material, typically glass. The bottom plate is formed by the 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 trench
118
. Pixel electrodes
112
a
and
112
b
lie on top of an upper intermetal dielectric layer
128
that is one component of interconnect scheme
104
. Interconnect
104
overlies capacitor structures
118
a
and
118
b
formed within underlying silicon substrate
105
. Underlying capacitors
118
a
and
118
b
are in electrical communication with pixel electrodes
112
a
and
112
b
, respectively, through metal-filled vias
140
and middle interconnect metallization layer
124
and lower interconnect metallization layer
122
.
The conventional pixel array described above in
FIG. 1
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 trench
118
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 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 stands in stark contrast to the bright surrounding reflective pixel electrodes. Projection displays can in turn magnify the light reflected from pixel array to such an extent that non-reflective space between pixels is readily observable and may distort the image.
Therefore, there is a need in the art for a pixel array and a process of forming a pixel array where inter-pixel regions exhibit reflectance comparable to reflectance in pixel regions.
Another problem is that the penetration of light into inter-pixel regions can cause distortion of the image displayed by the light valve. Specifically, incident light can travel through a variety of paths in the interconnect and finally enter into the underlying silicon. Penetration of incident light into the silicon substrate induces electrical currents that disturb charge stored in the underlying capacitors. As a result of fluctuation in charge stored in these capacitors, luminance of the pixel cells may change between succeeding write states, causing the image to “flicker.” This flickering reduces image quality, and may cause eye strain in a viewer.
Therefore, there is a need in the art for a pixel array and a process of forming a pixel array that substantially blocks the penetration of incident light through inter-pixel regions into the underlying substrate.
SUMMARY OF THE INVENTION
The present invention provides a pixel array and a process flow for forming an array of pixel cells which features pixel electrodes having overlapping edges. This overlapping electrode configuration precludes absorption of light in inter-pixel regions that could give rise to the appearance of dark lines between bright reflective pixel electrodes.
A process flow for forming a pixel cell array in accordance with a first embodiment of the present invention comprises the steps of forming an intermetal dielectric layer over an interconnect metallization layer, and forming a first dielectric layer over the intermetal dielectric layer. A second dielectric layer is then formed over the first dielectric layer, the second dielectric layer different than the first dielectric layer. Next, a trench photoresist mask is patterned over the second dielectric layer, the trench photoresist mask masking pixel regions and exposing inter-pixel regions. A trench is then created by etching the second dielectric layer and the first dielectric layer in inter-pixel regions to stop on the intermetal dielectric layer. The trench photoresist mask is removed, and a reflective metal layer is formed over the second dielectric layer and within the trench. A pixel photoresist mask is patterned over the reflective metal layer, the pixel photoresist mask masking first alternative pixel regions and exposing second alternative pixel regions. The reflective metal layer and the second dielectric layer are etched in second alternative pixel regions to stop on the first dielectric layer. A third dielectric layer is formed over the reflective metal layer in first alternative pixel regions and over the first dielectric layer in second alternative pixel regions. A second reflective metal layer over the third dielectric layer, the second reflective metal layer having a raised portion in first alternative pixel regions and a lower portion in second alternative pixel regions. A fourth dielectric layer is formed over the over the second reflective metal layer. Finally, chemical mechanical polishing is performed through the fourth dielectric layer, the raised portion of the second reflective metal layer, and the third dielectric layer to stop on the first reflective metal
National Semiconductor Corp.
Sikes William L.
Stallman & Pollock LLP
Vu Quynh-Nhu H.
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