Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
1999-05-26
2002-07-23
Sikes, William I. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S139000, C349S142000, C349S043000, C349S051000, C349S113000
Reexamination Certificate
active
06424399
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an active matrix substrate and a liquid crystal display apparatus, and to a method for producing the same. More particularly, the present invention relates to a liquid crystal display apparatus having excellent continuity of contact holes and excellent image characteristics and to an active matrix substrate capable of realizing such a liquid crystal display apparatus, and to a method for producing the same.
2. Description of the Related Art
Recently, the application of a liquid crystal display apparatus to a word processor, a lap-top personal computer, a pocket TV set, etc. has been increasing rapidly. Liquid crystal display apparatuses of the reflection type have been particularly attracting much attention since they display an image by reflecting the incident light from outside and do not require a back light. As a result, the power consumption is low and the apparatus can be made to be thin and light-weight.
Conventionally, TN (Twisted Nematic) mode and STN (Super Twisted Nematic) mode are employed in the reflection type liquid crystal display apparatus. Since these modes require a polarizing plate, one half of the light intensity of the natural light is inevitably not used for display and, therefore, the display becomes dark.
In order to overcome this problem, display modes in which all of the natural light beams are effectively used have been suggested. An example of such display modes is phase transition type guest-host mode (D. L. White and G. N. Taylor: J. Appl. Phys. Vol. 45, pp. 4718, 1974; referred to as White publication hereinafter). In this display mode, a cholesteric-nematic phase transition phenomenon by electric field is used. Also suggested is a reflection type multi-color display where micro color filters are incorporated in the phase transition type guest-host mode (for example, refer to Tohru Koizumi and Tatsuo Uchida. Proceedings of the SID. Vol. 29/2, pp. 157. 1988).
In order to obtain a brighter display in such display modes which do not require the polarizing plate, it is necessary to increase the intensity of the incident light scattering in the direction perpendicular to the display screen for all incident angles. To accomplish such, it is necessary to produce a reflecting plate having optimal reflecting characteristics. In the above-mentioned White publication, a description is provided to obtain such a reflecting plate as follows. The surface of a substrate made of glass or the like is roughed by a grinding agent. Then, after a certain period of time, the substrate is etched via hydrofluoric acid to form an uneven surface, and then a thin film of silver is formed on the uneven surface.
However, since the uneven surface is formed by scraping the glass substrate with the grinding agent, it is difficult to form a uniform uneven surface. Reproducibility in consistently forming the uneven surface is also poor.
FIG. 21A
is a plan view of a matrix substrate
2
having thin film transistors
1
(referred to as TFT hereinafter) which are switching devices used in the active matrix mode, and
FIG. 21B
is a cross-sectional view of the matrix substrate
2
illustrated in
FIG. 21A
taken along the B—B line. The matrix substrate includes a plurality of gate bus lines
3
made of chromium, tantalum or the like provided in parallel to each other on the insulating matrix substrate
2
made of glass or the like, and a gate electrode
4
which is branched from the gate bus line
3
. The gate bus line
3
functions as a scanning line.
As illustrated in
FIG. 21B
, a gate insulating film
5
made of silicon nitride (SiN
x
), silicon oxide (SiO
x
) or the like is formed on the entire surface of the substrate
2
a
, covering the gate electrode
4
. Formed on the portion of the gate insulating film
5
on the gate electrode
4
is a semiconductor layer
6
which is made of amorphous silicon (referred to as a-Si hereinafter), polycrystalline silicon, CdSe, etc. Formed on both sides of the semiconductor layer
6
are n
+
- or p
+
-contact layers
11
made of a-Si, polycrystalline silicon, CdSe, etc. Furthermore, as illustrated in
FIG. 22
, the gate insulating film
5
is formed on the entire surface of the substrate
2
a
except the portions on the input terminals
3
a
of the gate bus lines
3
.
As illustrated in
FIG. 21B
, a source electrode
7
made of titanium, molybdenum, aluminum, etc. is formed and stacked on one side of the semiconductor layer
6
. Formed and stacked on the other side of the semiconductor layer
6
is a drain electrode
8
which is also made of titanium, molybdenum, aluminum, etc. in a similar manner as the source electrode
7
. A pixel electrode
9
made of a transparent conductive film such as ITO (Indium Tin Oxide) is formed and stacked at the edge of the drain electrode
8
opposite to the semiconductor layer
6
.
As illustrated in
FIGS. 21A and 21B
, a source bus line
10
which crosses the gate bus line
3
with the gate insulating film
5
interposed therebetween is connected to the source electrode
7
. The source bus line
10
functions as a signal line. The source bus line
10
is also formed of a similar metal as the source electrode
7
. The gate electrode
4
, the gate insulating film
5
, the semiconductor layer
6
, the source electrode
7
and the drain electrode
8
constitute the TFT
1
, which has a function as a switching device.
When the matrix substrate
2
having TFTs
1
as illustrated in
FIGS. 21A
,
21
B and
22
is applied to a reflection type liquid crystal display apparatus, it is necessary to form the pixel electrodes
9
of a metal having a light reflecting property such as aluminum, silver, etc. and to form the uneven surface on the gate insulating film
5
. Generally, it is not desirable to form the uneven surface on the gate insulating film
5
since it has negative effects on the device forming processes. Furthermore, it is difficult to uniformly form the tapered uneven surface on the insulating film
5
which is made of an inorganic material.
Japanese Laid-Open Patent Publication No. 56-94386 to Yazawa et al. (referred to as Yazawa publication 1 hereinafter) discloses a method for increasing the intensity of light scattering in the direction perpendicular to the display screen, where a metal thin film layer having an uneven surface is used as a reflecting plate of a liquid crystal display apparatus, and also describes methods for producing the metal thin film layer listed as (1), (2) and (3) below.
(1) A method where a metal thin film layer is formed on the substrate by evaporation or sputtering under a particular condition, and a metal thin film having an uneven surface is obtained.
(2) A method where a metal thin film layer formed on the substrate by evaporation or sputtering is subjected to heat treatment and recrystallization to obtain a metal thin film layer having an uneven surface. For example, when aluminum or aluminum alloy is used as a material for the metal thin film layer, since the melting point of the material is 660° C., the recrystallization is carried out in the temperature range of 100° C. to 600° C. This recrystalization is responsible for the rearrangement of atoms within the metal thin film, which results in the metal thin film layer having an uneven surface.
(3) A method where, as illustrated in
FIG. 23
, an alloy thin film layer
63
formed on the substrate
2
by evaporation or sputtering is subjected to heat treatment so that precipitations
64
precipitate, and then the portion of the alloy thin film layer
63
proximate to the surface is removed by etching. For example, when the alloy thin film layer
63
obtained by mixing 2 weight % of silicon in aluminum is heated in a N
2
environment at 400° C. for 20 minutes, an intermetallic compound of aluminum and silicon having a particle diameter of about 0.2 to 1.0 &mgr;m precipitates as the precipitation
64
. For example, when the alloy thin film layer
63
of 1.0 &mgr;m thickness is subjected to precipitation treatment and then the 0.2
Nakamura Hisakazu
Shimada Yasunori
Taniguchi Koji
Edwards & Angell LLP
Neuner George W.
Ngo Julie
Sharp Kabushiki Kaisha
Sikes William I.
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