Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2000-06-02
2002-07-23
Sikes, William L. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S142000
Reexamination Certificate
active
06424397
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATION
This application claims the priority benefit of Taiwan application serial no. 89101566, filed Jan. 29, 2000.
BACKGROUND OF THE INVENTION
1. Field of Invention
The present invention relates to a method of forming a wide-viewing angle (WVA) liquid crystal display (LCD). More particularly, the present invention relates to a method of forming a wide-viewing angle, multi-domain vertical alignment (MVA), thin film transistor (TFT), liquid crystal display.
2. Description of Related Art
Liquid crystal display (LCD) has many advantages over other conventional types of displays including high picture quality, small volume occupation, lightweight, low voltage driven and low power consumption. Hence, LCD is widely used in small portable televisions, mobile telephones, video recording units, notebook computers, desktop monitors, projector televisions and so on. LCD gradually replaces conventional cathode ray tube (CRT) as a mainstream display unit. The biggest drawbacks of LCD are, however, its narrow viewing angle and its relatively high price.
At present, a number of propositions for manufacturing wide-viewing angle LCD is in the developing stage. The most widely adopted technique is the so-called pixel cutting method, or automatic domain formation (ADF). By controlling molecular orientation of the liquid crystal, a single pixel is divided into several domains so that the director of liquid crystal molecules in different domains has different tilt directions. Hence, viewing angle of the LCD is increased.
FIGS. 1A and 1B
are side views showing the operation of a conventional multi-domain vertical alignment LCD. This type of LCD is proposed by Fujitsu Co. Ltd. of Japan in 1998.
FIG. 1A
shows the state of liquid crystal molecules inside the LCD when no external electric field is present or the electric field presence is lower than a threshold value. The color filter (CF) included glass panel
100
and the thin film transistor included glass panel
102
is parallel to each other. Protrusion elements
104
and
104
are formed on the inner surface of both the glass panel
100
and the glass panel
102
. Negative type liquid crystal molecules
108
are vertically aligned between the glass panels
100
and
102
constituting a liquid crystal layer
110
. Those liquid crystal molecules
108
close to the protrusion elements
104
and
106
tilt in specific direction due to local effects and resulting in pre-tilts.
FIG. 1B
shows the state of liquid crystal molecules inside the LCD when an electric field above a threshold value is present. Due to the strong electric field, orientation of the negative type liquid crystal molecules
108
is changed such that director of the molecules is aligned in a direction vertical to the electric field. Liquid crystal molecules
108
near the middle portion of the liquid crystal layer
110
are pre-tilted and the electric field fringing the protrusions
104
and
106
is non-uniform. Hence, within the same pixel, molecules on each side of a protrusion will tilt oppositely and have different molecular alignment. The protrusions
104
and
106
within a pixel divide the pixel into two or more domains. In other words, a multi-domain pixel is formed and viewing angle of LCD is improved.
FIG. 2A
is a schematic top view showing one of the pixels of a second type of conventional multi-domain vertical alignment LCD.
FIG. 2B
is a cross-sectional view along line
2
B—
2
B of FIG.
2
A.
As shown in
FIG. 2B
, the structure includes two glass panels
200
and
202
running parallel to each other. A liquid crystal layer
204
is formed between the glass panels
200
and
202
. The structure is very similar to the one in
FIG. 1A
in that the inner surface of the upper glass panel
200
has protrusions
206
thereon. A transparent electrode
208
is formed on the inner surface of the lower glass panel
202
. The transparent electrode
208
further includes some slits
210
that serve as virtual protrusion. The protrusion
206
and the slit
210
are alternately positioned.
As shown in
FIG. 2A
, the single pixel structure has a data line
212
and a scan line
214
around the periphery of the transparent electrode
208
. The data line
212
and the scan line
214
are connected to the source terminal
218
a
and the gate terminal
218
c
of a thin film transistor (TFT)
218
respectively. The drain terminal
218
b
of the thin film transistor
218
is connected to the transparent electrode
208
. Control signals are transmitted to the source terminal
218
a
and gate terminal
218
c
of the thin film transistor
218
via the data line
212
and the scan line
214
respectively. Orientation of liquid crystal molecules inside each pixel is changed to display an image by employing an active matrices drive. The common line
216
that serves as an electrode for the storage capacitor C
s
is located between the lower glass panel
202
and the transparent electrode
208
. Moreover, the common line
216
passes out through the mid-portion of the transparent electrode
208
. Through the alternately positioned protrusion
206
and slit
210
on the inner surface of different glass panels of a pixel, the pixel is divided into four different domains so that viewing angle of the LCD is increased.
The protrusions are formed by spin-coating a layer of photoresist material over the glass panel, and then performing photolithographic operation using a photomask. Hence, to form protrusion elements on both the upper and the lower glass panel, two photolithographic operations have to be conducted. However, uniformity and pitch distances between protrusion elements are difficult to control using the conventional method.
In addition, the upper and the lower glass panels must be meticulously aligned when they are assembled to form a LCD. Since both the upper and the lower glass panel have protrusion elements or one with protrusion elements and other with slits, any misalignment of the glass panels is likely to affect brightness of the LCD. Occasionally, the entire LCD module may have to be scrapped due to protrusion element misalignment. Hence, process window for aligning glass panels is tight. Furthermore, since the electric field around the protrusion elements and the slits of the glass panels are weaker than other transparent electrode regions, liquid crystal molecules above these regions may not re-orient themselves in the presence of a strong pixel voltage. Therefore, these regions become permanently dark. In other words, the protrusion element regions and the slit regions will occupy a portion of the light passing area within the pixel to form a dark region. Consequently, aperture ratio of a pixel is reduced leading to inferior pixel quality.
In general, the transparent glass panel of a thin film transistor LCD normally has non-transparent metal electrodes. These metal electrodes can be utilized as a self-aligned photomask in a backside exposure (BSE) method for producing a multi-domain vertical alignment LCD.
FIGS. 3A through 3D
are schematic cross-sectional views showing the steps for producing the glass panel of a multi-domain vertical alignment LCD using a conventional backside exposure method.
As shown in
FIG. 3A
, a glass panel
300
having a non-transparent metal electrode
302
thereon is provided. A photoresist layer
304
is formed over the front face of the glass panel
300
so that the metal electrode
302
is also covered. As shown in
FIG. 3B
, ultraviolet light is shone from the backside of the glass panel
300
onto the photoresist layer
304
using the metal electrode
302
as a photomask. Here, no other photomask is used. If the photoresist layer
304
is formed from a positive type of photoresist material, protruded sections
306
that covers the metal electrodes
302
as shown in
FIG. 3C
are formed after backside exposure, development and baking. On the other hand, if the photoresist layer
304
is formed from a negative type of photoresist material, trenches
308
that exposes the metal electrodes
302
as shown in
FIG.
Chi Mei Optoelectronics Corp.
J.C. Patents
Nguyen Hoan
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