Radiation imagery chemistry: process – composition – or product th – Imaging affecting physical property of radiation sensitive... – Making named article
Reexamination Certificate
1999-03-04
2001-10-23
Duda, Kathleen (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Imaging affecting physical property of radiation sensitive...
Making named article
C430S020000, C430S313000
Reexamination Certificate
active
06306561
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 light valves which utilize sublithographic isolation based upon the formation of dielectric spacer structures.
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 in 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
.
FIGS.
2
AA-
2
DB illustrate the steps of the conventional process for forming an array of pixel cells in a light valve. For purposes of convention, all FIGS.
2
_A illustrate a top view of the pixel cell, all FIGS.
2
_B illustrate a cross-sectional view of the pixel cell along line A-A′ of the FIG.
2
_A.
FIGS.
2
AA-
2
AB illustrate the starting point for the conventional process for fabricating a thin LC transducer pixel cell. Starting structure
200
is created by forming an upper intermetal dielectric layer
212
over a lower interconnect metallization layer
214
. A central portion of upper intermetal dielectric layer
212
is then etched to form via
216
. A liner (not shown) typically composed of a Ti/TiN layer combination, is then formed on the walls of via
216
, and via
216
is filled with metal (typically CVD Tungsten). Excess metal is then removed from the surface of upper dielectric layer
212
, typically by a combination of etching and chemical-mechanical polishing (CMP).
FIGS.
2
BA-
2
BB illustrate formation of the metal pixel electrode in accordance with the conventional process. Metal pixel electrode layer
206
is formed over the entire surface of the pixel cell.
FIGS.
2
CA-
2
CB illustrate patterning of a photoresist mask
207
over pixel electrode layer
206
. FIGS.
2
DA-
2
DB show the etching of regions of pixel electrode layer
206
unmasked by photoresist
207
, to form a plurality of intersecting trenches
218
, followed by stripping of photoresist mask
207
. Intersecting trenches
218
in turn define a plurality of discrete pixel cell electrodes
230
.
Fabrication of the thin LC transducer pixel cell is completed by forming an alignment surface (not shown) for the LC material positioned on top of the pixel electrode. Forming this alignment surface is a two step process. First, a dielectric film (typically polyimide) is deposited on top of the pixel electrode. Second, the dielectric film is scored by a rubbing wheel, which traverses the surface of the pixel cell and gouges the alignment surface in a uniform direction. Liquid crystal material is then placed within the cell, and a top glass plate is secured to the tops of the support pillars.
The conventional fabrication process described above in FIGS.
2
AA-
2
DB is adequate to produce functional thin LC transducer pixel cells. However, the conventional process flow suffers from a number of disadvantages.
One problem is that the step of defining and etching the inter-pixel trenches requires an additional masking step. This masking step carries with it penalties in terms of additional defects and higher cost.
Another problem is that minimum spacing between adjacent pixels is dictated by the limits of resolution of the photolithographic process employed. This limitation is manifested in the step of patterning the mask to form the trenches in the pixel electrode layer, as shown above in FIGS.
2
CA-
2
CB. For example, in a 0.3 &mgr;m linewidth photolithographic technology, the distance across trench
218
of FIGS.
2
DA-DB could be no less than 0.3 &mgr;m.
The closer together the pixels of the array, the better the array will perform. This is particularly true with respect to minimizing leakage of incident light between pixel electrodes through the interconnect and into the underlying substrate, as there is less inter-pixel spacing to allow the passage of incident light. Close proximity between pixels also reduces unwanted optical artifacts attributable to interruption in the continuous pixel array backplane by topography associated with inter-pixel isolation structures.
Therefore, there is a need in the art for a process of forming an array of pixel cells where inter-pixel spacing is not constrained by photolithographic parameters, thereby enabling formation of arrays having greater pixel cell densities and enhanced resolution and clarity.
SUMMARY OF THE INVENTION
The present invention provides a process flow for forming an array of pixel cells that does not rely upon photolithography to define inter-pixel isolation. Instead, adjacent pixels of the array are electrically insulated from one another by dielectric spacer structures formed by the deposition and etching of a dielectric layer conforming to raised features of a first metal layer. The thickness of the sidewall spacers is determined by
Duda Kathleen
National Semiconductor Corporation
Stallman & Pollock LLP
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