Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2002-04-23
2003-12-16
Kim, Robert H. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S005000, C349S144000, C349S191000
Reexamination Certificate
active
06665041
ABSTRACT:
This application incorporates by reference of Taiwan application Ser. No. 090110668, filed on May 3, 2001.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates in general to a liquid crystal on silicon (LCOS), and more particularly to a vertical alignment (VA) mode liquid crystal on silicon capable of forming a single domain.
2. Description of the Related Art
As the market of portable products, e.g., personal digital assistant (PDA), cellular phone, and projector, and large-sized projection television progress, more and more customers require that the resolutions of these portable products or the projection television are to be identical to that of personal computer systems. Liquid crystal on silicon (LCOS) is just enough to fulfil these requirements. Unlike liquid crystal display (LCD) whose the front and rear plates are made of glass, LCOS employs a silicon plate and glass plate between which liquid crystal is filled. The structure of LCOS can provide displays not only meeting the requirement for compactness of portable products, but also having high resolution. The resolution of a display is represented by pixels formed on the plates. The more the pixels a display has, the finer and the resolution the display can show. In addition, LCOS is capable of having its driving circuit manufactured by using semiconductor manufacturing process, e.g., complementary metal oxide semiconductor manufacturing process so that the silicon plate that uses silicon wafer can be manufactured in a standard semiconductor manufacturing production line. Therefore, it is unnecessary to invest additionally in the production equipment while the resolution of the LCOS is higher than that of the LCD, which requires glass-manufacturing process.
Liquid crystal on silicon can be categorized into transmissive type and reflective type while major research and development focus on the reflective type.
FIG. 1A
shows a single pixel on a reflective-type LCOS in a cross-sectional view. The LCOS has a front plate
100
and a rear plate
101
. The rear plate
101
includes a silicon substrate
102
on which a thin film transistor
106
, an opaque layer
107
, and a capacitor
108
are formed. The thin film transistor
106
is used for controlling the operation of the pixel, the opaque layer
107
is used for making the thin film transistor
106
from not being shined so as to avoid misoperation, and the capacitor
108
is used for maintaining the brightness of the pixel. A metal layer
111
is electrically coupled to the thin film transistor
106
and the capacitor
108
while the metal layer
111
is covered with an insulating layer
109
. In addition, a pixel electrode
110
is disposed above the insulating layer
109
and is covered with a reflector
112
. As to the front plate
100
, a glass plate
120
is included and a transparent electrode (indium-tin-oxide electrode)
118
is formed on the glass plate
120
. The front plate
100
and the rear plate
101
are assembled in parallel and the space between them is filled with liquid crystal molecules
115
so as to form a liquid crystal layer
114
. Further, alignment films
113
and
116
for molecular alignment are formed on the reflector
112
and the transparent electrode
118
.
By the above structure, a light signal having brightness corresponding to a voltage applied to the pixel electrode
110
is obtained. When an incident ray (denoted by I, as shown in
FIG. 1A
) that is normal to the liquid crystal layer
114
strikes the glass plate
120
, a reflected ray (denoted by O) is reflected by the reflector
112
. The polarization of the light passing through the liquid crystal layer
114
is modulated by changing the alignment of the liquid crystal molecules
115
that is varying with a voltage applied to the pixel electrode
110
. After that, the reflected ray is processed by the polarizing film (not shown in
FIG. 1A
) formed on the glass plate
120
. In this way, the polarized reflected ray has the brightness corresponding to the voltage applied to the pixel electrode
110
.
FIG. 1B
illustrating an LCOS in a top view. As shown, each of the pixel electrodes
110
is isolated with grooves
124
, wherein the bottoms of the grooves
124
are covered with the alignment film
113
.
To be more specific, when a voltage is applied to the pixel electrodes
110
, the arrangement of the liquid crystal molecules is to be varied so that the light transmission changes. Thus, the LCOS can display images with different brightness such as white, black, and intermediate gray scale. In addition, the molecules of the liquid crystal layer of LCOS panels can be categorized into twisted nematic mode (TN) and vertical alignment mode (VA).
FIGS. 2A-2B
show the operations of liquid crystal molecules in twisted nematic mode when a voltage is not applied or applied to the liquid crystal molecules, respectively. When an electric field is not applied across the alignment films
202
and
204
, the liquid crystal molecules
200
gradually twist layer by layer until the uppermost layer is at a 90° angle to the bottom layer, as shown in FIG.
2
A. When a sufficient electric field is applied, the liquid crystal molecules
200
are to be aligned and parallel to the direction of the electric field, as shown in FIG.
2
B.
FIGS. 3A-3B
show the operations of liquid crystal molecules in vertical alignment mode when a voltage is not applied or is applied to the liquid crystal molecules, respectively. When a voltage is not applied across the alignment films
302
and
304
, the liquid crystal molecules
300
are aligned and perpendicular to the alignment films
302
and
304
, as shown in FIG.
3
A. When a voltage is applied, the liquid crystal molecules
300
, as shown in
FIG. 3B
, are to be twisted by an angle of 90° to the direction of the liquid crystal molecules
300
when the voltage is not applied, while they are parallel to the alignment films
302
and
304
.
As compared with LCOS panels with liquid crystal molecules in twisted nematic mode, LCOS panels with liquid crystal molecules in vertical alignment mode have higher contrast ratios. A twisted nematic LCOS panel can provide a contrast ratio of about 100:1 to 150:1, but a vertical-alignment LCOS panel can provide a contrast ratio of about 400:1 or above. Therefore, the development of LCOS panels with liquid crystal molecules in vertical alignment mode is interested.
Moreover, the liquid crystal layer
114
can be damaged if a voltage in the sane polarity is continuously applied to the pixel electrode
110
. This problem can be avoided by using polarity inversion because the gray levels produced by the LCOS panel is related to the difference between voltages across the liquid crystal layer
114
but not related to the polarities of the voltages. Polarity inversion is a driving method that a voltage of alternate positive and negative is applied to the pixel electrode
110
. With respect to polarity inversion, liquid crystal display driving methods can be categorized into frame inversion, column inversion, and dot inversion. The following is to describe the three driving methods briefly.
FIG. 4A
illustrates the conventional frame inversion driving method for a liquid crystal display (LCD) panel
400
having a number of pixels
401
. The positive sign “+” and negative sign “−”, hereinafter, are indicative of polarities of the voltages applied to the associated pixels. In frame inversion, if positive voltages are applied to all pixels at one time, then negative voltages are applied to them in the next time instant. In this way, voltages in positive and negative polarities are alternately applied to them.
FIG. 4B
illustrates the conventional column inversion driving method for an LCD panel
402
having a number of pixels
403
. In column inversion, polarity inversion occurs on pixels of columns. If positive voltages are applied to a column of pixels, negative voltages are applied to the adjacent column of pixels. In the next time instant, the polarities of voltages applied to the above pixels are i
Liao Bing-Jei
Liu Chung-Yuan
Himax Optoelectronics Corp.
Rabin & Berdo PC
Schechter Andrew
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