Liquid crystal cells – elements and systems – Nominal manufacturing methods or post manufacturing...
Reexamination Certificate
1999-09-01
2001-11-06
Sikes, William L. (Department: 2871)
Liquid crystal cells, elements and systems
Nominal manufacturing methods or post manufacturing...
C438S275000, C438S294000
Reexamination Certificate
active
06313901
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to light valves, and in particular, to a process for forming a light valve pixel cell utilizing a final rapid thermal anneal (RTA) step.
2. Description of 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
1
B-
1
B′. 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 trench
118
.
Trench
118
is filled with dielectric material
121
. Dielectric material
121
also extends over the surface of the active pixel electrodes, performing a passivation, planarization, and/or LC alignment function.
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
.
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
.
The conventional pixel cell described above in
FIGS. 1A-1B
functions adequately in many applications. However, this design suffers from a number of disadvantages.
One problem is that favorable optical characteristics of the pixel cell can be diminished by specific processing steps utilized during its manufacture. In particular, one important measure of pixel cell performance is reflectance, which determines both brightness and resolution of the image presented. Reflectance of the pixel cell is determined by reflectance of the active pixel electrode, and the reflectance of this electrode can be adversely affected by heat during fabrication of the device.
Therefore, there is a need in the art for a process for fabricating a pixel cell which preserves reflectance of the active pixel electrode.
SUMMARY OF THE INVENTION
The present invention provides a process flow for forming a pixel cell in which the furnace alloy/sintering step is performed prior to formation of the reflective metal layer from which the active electrodes are created. In this manner, the deposited reflective metal layer is spared prolonged exposure to high temperatures that increase metal surface roughness and degrade reflectance. Adequate removal of any remaining charge is ensured at the conclusion of the process flow by the addition of a final rapid thermal anneal step.
A process flow for forming a pixel cell in accordance with one embodiment of the present invention comprises the steps of forming an oxide/silicon interface in a semiconductor workpiece, the oxide/silicon interface including surface state charges. A furnace anneal of the semiconductor workpiece is performed for at least 30 minutes at a temperature of between about 400-450° C., the furnace anneal suppressing the surface state charges. An interconnect structure is formed including an intermetal dielectric layer over the semiconductor workpiece. A plurality of discrete reflective metal pixel electrodes is formed over the intermetal dielectric layer after the furnace anneal step, and a rapid thermal anneal is performed at a temperature of between about 400-450° C. for not longer than 30 seconds after formation of the discrete reflective metal pixel electrodes, the rapid thermal anneal further suppressing the surface state charges. The features and advantages of the present invention will be understood upon consideration of the following detailed description of the invention and the accompanying drawings.
REFERENCES:
patent: 5198371 (1993-03-01), Li
patent: 5330921 (1994-07-01), Yoshida et al.
patent: 5374578 (1994-12-01), Patel et al.
patent: 5706067 (1998-01-01), Colgan et al.
patent: 5764324 (1998-06-01), Lu et al.
patent: 5789318 (1998-08-01), Delfine et al.
patent: 6117737 (2000-09-01), Wang et al.
patent: 6225193 (2000-05-01), Simpson et al.
Wolf, S., “Silicon Processing for the VLSI ERA”, vol. 3, The Submicron Mosfet, pp. 425-429.
Cacharelis, P., et al., “A Reflective-mode PDLC Light Valve Display Technology”, Proceedings of European Solid State Device Research Conference (ESSDERC) pp. 596-599 (1997).
Cacharelis, P., et al., 18.1: An 0.8-&mgr;m EEPROM Technology Modified for a Reflective PDLC Light-Valve Application, SID 97 Digest, pp. 289-292.
National Semiconductor Corporation
Ngo Julie
Sikes William L.
Stallman & Pollock LLP
LandOfFree
Liquid crystal display fabrication process using a final... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Liquid crystal display fabrication process using a final..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Liquid crystal display fabrication process using a final... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2592813