Liquid crystal cells – elements and systems – Particular excitation of liquid crystal – Electrical excitation of liquid crystal
Reexamination Certificate
2001-07-24
2004-02-03
Ngo, Julie (Department: 2871)
Liquid crystal cells, elements and systems
Particular excitation of liquid crystal
Electrical excitation of liquid crystal
C349S114000, C349S139000
Reexamination Certificate
active
06686977
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to liquid crystal on silicon having pixel cell arrays for silicon light valves and in particular to microdisplays comprising a liquid crystal on silicon.
BACKGROUND OF THE INVENTION
Liquid crystal displays (LCDs) are commonly used in devices such as portable televisions, portable computers, control displays, and cellular phones to display information to a user. LCDs act in effect as a light valve, i.e., they allow transmission of light in one state, block the transmission of light in a second state, and some include several intermediate stages for partial transmission. When used as a high resolution information display, as in one application of the present invention, LCDs are typically arranged in a matrix configuration with independently controlled pixels (the smallest segment of the display). Each individual pixel is signaled to selectively transmit or block light from a backlight (transmission mode), from a reflector (reflective mode), or from a combination of the two (transflective mode).
LCDs are actuated pixel-by-pixel, either one at a time or a plurality simultaneously. A voltage is applied to each pixel mirror electrode and the liquid crystal responds to the voltage by transmitting a corresponding amount of light. In some LCDs an increase in the actuation voltage decreases transmission, while in others it increases transmission. When multiple colors are involved for each pixel, multiple voltages are applied to the pixel at different positions or times depending upon the LCD. Each voltage controls the transmission of a particular color. For example, one pixel can be actuated to allow only blue light to be transmitted while another allows only green. A greater number of different light levels available for each color results in a much greater number of possible combination colors.
Converting a complex digital signal that represents an image or video into voltages to be applied to the pixels of an LCD involves circuitry that can limit the monochrome resolution. The signals necessary to drive a single color of an LCD are both digital and analog. It is digital in that each pixel requires a separate selection signal, but it is analog in that an actual voltage is applied to the pixel to determine light transmission.
Each pixel in the core array of the LCD is addressed by both a column (vertical) driver and a row (horizontal) driver. The column driver turns on an analog switch that connects an analog voltage representative of the video input (control voltage necessary for the desired liquid crystal twist) to the column, and the row driver turns on a second analog switch that connects the column to the desired pixel.
The video inputs to the LCD are analog signals centered around a center reference voltage of typically from about 6.0 to 8.0 volts. This center reference voltage is not a supply or signal from anywhere, rather it is a mathematical entity. Nearly the same as the center reference voltage is a voltage called “VCOM,” which connects to the LCD cover glass electrode, which is a transparent conductive coating on the inside face (liquid crystal side) of the cover glass. This transparent conductive coating is typically Indium Tin Oxide (ITO).
One frame of video pixels are run at voltages above the center reference voltage (positive inversion) and for the next frame the video pixels are run at voltages below the center reference voltage (negative inversion). Alternating between positive and negative inversions results in a zero net DC bias at each pixel.
FIG. 1
shows a cross-sectional view of adjacent 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. These pixel electrodes function therefore as mirrors which reflect the light.
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
. For protection and enhanced reflective characteristics a passivation layer and an alignment layer
116
are deposited on top of the pixel electrodes.
The conventional pixel array described above in
FIG. 1
functions adequately in many applications. However, this design suffers from the disadvantage that such a display can experience image retention if mobile ions can enter and charge the liquid crystal alignment layer with the retained image resulting in significant degradation of a displayed image.
Therefore, there is a need in the art for a pixel array and a process of forming a pixel array where image retention is avoided.
SUMMARY OF THE INVENTION
The invention overcomes the above-identified problems as well as other shortcomings and deficiencies of existing technologies in one embodiment by a liquid crystal on silicon device comprising a mirror layer comprising a plurality of mirror electrodes, a passivation layer formed on the surface of the mirror layer comprising a plurality of openings to the mirror electrodes, and a liquid crystal layer being arranged on top of the alignment layer. Ions trapped in the alignment layer are discharged or electrochemically inverted to an inert state through the opening by means of the pixel electrodes of the mirror layer.
Another embodiment is a semiconductor arrangement within a liquid crystal on silicon device which comprises a substrate having a first and second surface. The substrate comprises a source/drain region which extends from the first surface into the substrate. The arrangement further comprises a dielectric layer deposited on the first surface of the substrate having a surface, a gate region within the dielectric layer, a mirror layer forming a plurality of pixel mirrors extending from the surface of the dielectric layer being electrically coupled with the source/drain region, a passivation layer formed on the surface of the mirror layer comprising an opening to the mirror region, and a liquid crystal alignment layer formed on the passivation layer.
The semiconductor arrangement can further comprise a conductive path reaching from the surface of the source/drain region to the mirror region. The conductive path can be formed by a first via coupling the source/drain region with a metallization layer embedded between the source/drain region and the mirror region and a second via coupling the metallization layer with the mirror region. The passivation layer can be a reflectivity enhancement coating comprising a silicon dioxide layer and a silicon nitride layer. Pluralities of these layers can be formed on top of the structure in an alternating fashion. The opening can be filled with conducting material or with the alignment layer. The conducting material can be one of tungsten, aluminum or wolfram and the opening can comprise a diameter of approximately 1 &mgr;m and have the shape of a circle.
A method of manufacturing a semiconductor arrangement according to another aspect of the present invention comprises the steps of:
forming a semiconductor device having a dielectric layer and a plurality of pixel mirror electrodes arranged on top of the dielectric layer;
forming a passivation layer on top of the pixel mirror electrodes;
etching an opening to the surface of the pixel mirror electrode in the passivation layer.
In one enhancement of the
Frazee Jerome A.
Smith Joseph Terence
Baker & Botts L.L.P.
Ngo Julie
Three-Five Systems, Inc.
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