Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
1998-03-24
2003-05-20
Chowdhury, Tarifur R. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S086000, C349S155000, C349S190000, C349S187000
Reexamination Certificate
active
06567147
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention disclosed in the present specification relates to a liquid crystal display using thin film transistors and, more particularly, to a liquid crystal display in which pixel switching circuits and peripheral driver circuits are integrally formed on the same substrate.
2. Description of the Prior Art
In recent years, techniques for fabricating semiconductor devices such as thin film transistors (TFTs) consisting of semiconductor thin films formed on cheap glass substrates have developed rapidly, because there is an increasing demand for active matrix type liquid crystal displays.
An active matrix type liquid crystal display has several tens to millions of pixel regions arranged in a matrix-like form and at least one TFT is disposed at each pixel. Electric charge going into and out of each pixel electrode is controlled by the switching function of each TFT.
The fundamental structure of the active matrix type liquid crystal display having TFT arrays is now described by referring to FIG.
1
. Firstly, FIG.
1
(A) is a cross-sectional view taken perpendicular to the substrates of the liquid crystal display. This cross section is equal to the cross sectional view along the broken line A-A′ of FIG.
1
(B).
A substrate
101
is transparent to light, and an insulating film (not shown) is deposited on the surface of the substrate. Reference numerals show an active layer
102
, a gate electrode
103
, a data line
104
, a drain electrode
105
, an interlayer insulating film
106
, a black matrix
107
, pixel electrodes
108
comprising a transparent conducting film, and an orientation film
109
.
The whole substrate having the constitution described above and having such TFT arrays thereon is hereinafter referred to as the active matrix substrate. In FIG.
1
(A), only one pixel is shown, however, in practice, the active matrix substrate is constituted by pixel regions containing several tens to millions of pixel switching TFTs (referred to as pixel TFTs) and by peripheral driver circuit regions including a plurality of TFTs for driving the pixel TFTs.
On the other hand, reference numeral
110
shows a substrate having transparency,
111
shows a counter electrode consisting of a transparent conducting film, and
112
shows an orientation film. The whole of this substrate opposite to the active matrix substrate is referred to as a counter substrate.
As shown in FIG.
2
(A), the active matrix substrate
203
and the counter substrate
204
are performed by orientation treatment to orient the molecules of the liquid crystal material in a given direction. Then, granular spacers
201
are uniformly dispersed on the active matrix substrate
203
to control the cell gap, or cell spacing, between the active matrix substrates
203
and the counter substrate
204
. Thereafter, a sealing material
202
is printed. The sealing material
202
acts as an adhesive to mate the substrates each other and also as a sealant to prevent the liquid crystal material injected into the cell between the substrates from leaking from the substrate. Also, reference numeral
205
shows a driver circuit regions and
206
shows a pixel electrode. The direction of rubbing is indicated by
207
.
FIG. 3
is a cross-sectional view of an active matrix substrate. As shown in
FIG. 3
, since granular spacers
301
are uniformly dispersed on the whole active matrix substrate
302
to control the cell gap, the spacers
301
exist in a peripheral driver circuit region
304
, as well as in a pixel region
303
. Usually, a pixel TFT
305
does not greatly differ in size from a driver circuit TFT
306
. However, a black matrix covering the pixel TFTs
305
and pixel electrodes consisting a transparent conducting film are formed in the pixel region
303
. Also, if the liquid crystal display is of the reflection type, reflective electrodes are formed in the pixel region
303
. Furthermore, connecting wirings are formed in the driver circuit region
304
to constitute a CMOS circuit driving the pixel TFTs
305
. Hence, the pixel region
303
differs in height above the surface of the substrate from the driver circuit region
304
.
It is assumed that the pixel region is at a greater height above the substrate surface than the driver circuit region. The spacers are present in the driver circuit region, as well as in the pixel region. If granular spacers of almost uniform size are positioned in these two kinds of regions, the height above the substrate surface differs according to the spacer position. When, h
p
and h
d
are directed to the height of the top surface of each spacer located in the pixel region and in the driver circuit region, respectively, it can be seen that a difference in size between the pixel and driver circuit regions gives rise to a height difference h=h
p
; h
d
.
Then, as shown in FIG.
4
(A), the active matrix substrate
401
is mated with the counter substrate
402
. Subsequently, a liquid crystal material
406
is injected between the active matrix substrate and the counter substrate, thereby, the entrance hole
403
is sealed with a sealant material (FIG.
4
(B)). In this way, an active matrix type liquid crystal display having such a structure as shown in FIG.
1
(A) is manufactured.
However, the liquid crystal display having such a structure as described above suffers from the following disadvantages.
Because of the height difference h due to the difference in size between the pixel region
404
and the driver circuit region
405
, when the active matrix substrate
401
is mated with the counter substrate
402
, it is impossible to make the cell gap uniform. This results in cell thickness nonuniformities. In addition, as shown in FIGS.
4
(A) and
4
(B), the counter substrate
402
is distorted. Because of these cell thickness nonuniformities and distortion of the counter substrate
402
in the liquid crystal display device, various defects are developed as follows; the image displayed on the liquid crystal display becomes nonuniform and also, interference patterns appear on the top surface of the counter substrate
402
.
Where the driver circuit region is at a greater height than the pixel region above the substrate surface, extra force is applied to the spacers dispersed on the driver circuit region due to the height difference h when the active matrix substrate is mated with the counter substrate. Consequently, the driver circuit TFTs that are more complex in structure than the pixel TFTs are damaged not a less. As a result, the manufacturing yield is adversely affected.
Where granular spacers
115
are present on the pixel region as shown in FIG.
1
(B), the orientation of the molecules of the liquid crystal material is disturbed near the spacers
115
and so disturbance of the displayed image (disclination)
116
may be observed.
Because of the height difference h described above, when the active matrix substrate is mated with the counter substrate, excess force is applied to the spacers dispersed on the driver circuit TFTs. Thus, the driver circuit TFTs that are more complex in structure than the pixel TFTs are damaged not a less. Consequently, the manufacturing yield is deleteriously affected.
Where the cell gap is controlled using the conventional granular spacers as mentioned above, a good display may not be provided due to various factors.
Also, where a liquid crystal display is manufactured as a commercial product or as a prototype, the cell gap would be generally set to about 5 to 6 &mgr;m, regardless of the pixel pitch
117
. In the future, liquid crystal panels with high definition will be required, and so the pixel pitch
117
will tend to be further fine.
For example, a projection type Liquid Crystal Display (projection) must have small panel size. Moreover, it must be designed to display an image with the high definition as possible because the image is projected onto a screen. Accordingly, in the future it will be necessary to manufacture LCDs with pixel pitches not higher than 40 &mgr;m, preferably 30 &mg
Chowdhury Tarifur R.
Robinson Eric J.
Robinson Intellectual Property Law Office P.C.
Semiconductor Energy Laboratory Co,. Ltd.
LandOfFree
Liquid crystal display device and method of fabricating the... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Liquid crystal display device and method of fabricating the..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Liquid crystal display device and method of fabricating the... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3077125