Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
1996-08-16
2001-03-27
Sikes, William L. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S041000
Reexamination Certificate
active
06208395
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a reflective liquid crystal display device, particularly relates to a reflective liquid crystal display device having a feature in a light reflection plate thereof.
In the reflective liquid crystal display device, a light having been incident from outside is reflected by a light reflecting plate provided inside of the liquid crystal display device to use the reflected light as a light source for the display, for which reason any back light is not necessary for the light source. It has been considered that the reflective liquid crystal display device with the light reflecting plate is suitable for reductions in dissipation power, thickness and weight rather than a light transmission liquid crystal display. The reflective liquid crystal display device includes a liquid crystal layer, a switching element such as a thin film transistor or a diode, and a light reflecting plate. The liquid crystal layer may comprise either a twisted nematic liquid crystal, a super-twisted nematic liquid crystal, a guest-host liquid crystal or a polymer dispersion liquid crystal.
The reflective liquid crystal display device is required to have a bright and white displaying performance in the light transmission mode. The light reflective performance of the light reflecting plate depends upon various parameters of the surface roughness of the light reflection plate, particularly sloped angles of convex and concave portions constituting the rough surface of the light reflection plate, and the irregularity of the surface roughness.
In the conventional reflective liquid crystal display device, an active matrix driving system is used together with the light reflection plate, wherein thin-film transistors (TFT) or diodes having a metal/insulator/metal structure, for short an MIM structure, are used as a switching element for realizing high line and high quality pictures.
The structure of the conventional reflective liquid crystal display device will be described with reference to FIG.
1
.
The conventional liquid crystal display device comprises top and bottom substrates
1
and
4
which sandwich a liquid crystal layer
11
. The top substrate
1
comprises laminations of a glass substrate
2
and a transparent electrode
3
which is in contact with the liquid crystal layer
11
. The bottom substrate
4
comprises a glass substrate
5
, arrays of thin film transistors
6
with an inverse stagger structure provided on the glass substrate
5
, a polyamide interlayer insulator
7
provided to cover the arrays of thin film transistors
6
and a light reflection plate
10
provided over the polyimide interlayer insulator
7
and under the liquid crystal layer
11
. The light reflection plate
10
has a surface roughness
14
. The light reflection plate
10
is made of a conductive material so that the light reflection plate
10
serves as a pixel electrode. The surface of the polyimide interlayer insulator
7
has a roughness
18
on which the light reflection plate
10
substantially corrugated to have the surface roughness
14
is provided. The polyimide interlayer insulator
7
has contact holes
49
over drain electrodes
9
of the thin film transistors
6
so that the light reflection plate
10
extends not only over the polyimide interlayer insulator
7
but also within the contact holes
49
whereby the light reflection plate
10
made of a conductive material for serving as the pixel electrode is in contact with the drain electrodes
9
of the thin film transistors
6
.
The liquid crystal layer
11
comprises a guest-host liquid crystal which has been injected into a gap between the top and bottom substrates
1
and
4
.
An incident light
12
is transmitted through the glass substrate
2
, the transparent electrode
3
and the liquid crystal layer
3
to the light reflection plate
10
by which the transmitted light is reflected and transmitted through the liquid crystal layer
3
to the substrate
1
and then outputted therefrom. The reflective liquid crystal display device utilizes the reflected light
13
. In order to obtain a sufficient brightness of the screen for the liquid crystal display, it is necessary that lights having been incident in various angles may be reflected in a direction just or almost vertical to the surface of the substrates
1
and
4
. The rough surface
14
of the light reflection plate
10
comprises convex and concave portions which form top and valley portions and sloped portions. If the incident light having been in the direction vertical to the surfaces of the substrates
1
and
4
is reflected at the top and valley portions of the rough surface
14
of the light reflection plate
10
, then the reflected light is transmitted in the direction just or almost vertical to the surfaces of the substrates
1
and
4
. If the incident light having been in a direction tilted from the normal of the surfaces of the substrates
1
and
4
is reflected at the sloped portions of the rough surface
14
of the light reflection plate
10
, then the reflected light is also transmitted in the direction just or almost vertical to the surfaces of the substrates
1
and
4
. The rough surface
14
including the top and valley portions and the sloped portions allows the incident lights having been incident not only in the vertical direction but also in the tilted direction to be reflected and transmitted in just or almost the vertical direction to the surfaces of the substrates
1
and
4
.
The above conventional reflective liquid crystal display device may be fabricated as follows. The descriptions of the fabrication processes for the above reflective liquid crystal display device will hereinafter be made with reference to
FIGS. 2A through 2G
.
With reference to
FIG. 2A
, a gate electrode
15
is formed on the glass substrate
5
.
With reference to
FIG. 2B
, a gate insulation film
16
is formed, which extends over the glass substrate
5
and the gate electrode
15
. A surface of the gate insulation film
16
has a hillock over the gate electrode
15
. A semiconductor layer
17
being doped or undoped with an impurity is formed on an entire surface of the gate insulation film
16
. A surface of the semiconductor layer
17
also has a hillock over the hillock of the gate insulation film
16
.
With reference to
FIG. 2C
, the semiconductor layer
17
is selectively removed by patterning process to leave the same over and in the vicinity of the hillock of the gate insulation film
16
.
With reference to
FIG. 2D
, source and drain electrodes
8
and
9
are formed, wherein the source electrode
8
extends over a left side portion of the remaining semiconductor layer
17
and over the gate insulation film
16
in the vicinity of the left side portion of the remaining semiconductor layer
17
, whilst the drain electrode
9
extends over a right side portion of the remaining semiconductor layer
17
and over the gate insulation film
16
in the vicinity of the right side portion of the remaining semiconductor layer
17
to thereby form a thin film transistor
6
.
With reference to
FIG. 2E
, a polyimide insulation film
7
is formed, which extends over the gate insulation film
16
and over the source and drain electrodes
8
and
9
. A surface of the polyimide insulation film
7
is subjected to a patterning to form a rough surface which comprises convex and concave portions whereby the rough surface comprises top and valley portions and sloped portions.
With reference to
FIG. 2F
, a contact hole
49
is formed in the polyimide insulation film over the drain electrode
9
to expose a part of the drain electrode
9
.
With reference to
FIG. 2G
, a light reflection plate
10
made of a conductive material is formed on the rough surface with the convex and concave portions of the polyimide insulation film
7
as well as in the contact hole
49
so that the light reflection plate
10
is in contact with the drain electrode
9
of the thin film transistor
6
. The light reflection plate
10
is patterned to form a pixel electrode.
In th
Kaneko Setsuo
Kanoh Hiroshi
Mizobata Eishi
NEC Corporation
Nguyen Dung
Sikes William L.
Young & Thompson
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