Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
1998-03-24
2001-05-01
Parker, Kenneth (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S141000, C349S169000, C349S188000, C349S075000
Reexamination Certificate
active
06226061
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a liquid crystal display device. More specifically, the present invention relates to a liquid crystal display device equipped with switching elements such as thin film transistors (hereinafter referred to as TFTs) which improve visibility properties and display properties of a display screen and make a display not inferior to a display of a CRT (cathode-ray tube) display device.
BACKGROUND OF THE INVENTION
Conventionally, a liquid crystal display (LCD) device using a nematic-type liquid crystal display element has been widely used as a digit-segment-type liquid crystal display device for use in a watch or a electric calculator. Recently it has come to be used as a display device for use in a word processor, a computer, a navigation system, or the like.
Among such LCD devices, in particular, an active matrix LCD device wherein active elements such as TFTs are used as switching elements and pixels are arranged in a matrix form is widely used and known. The LCD device has advantages such as having a drastically reduced thickness (depth), consuming less power, and being easily modified to a full-color display device, as compared with the CRT display device. Therefore, the demand for the LCD device is increasing in various fields such as those of personal computers, various monitors, potable TVs, and cameras. However, since such a conventional LCD device is inferior to the CRT display device in a viewing angle range, luminance, color reproduction, and the like, improvement is eagerly desired in respect to these aspects.
The active matrix LCD device has a transparent active matrix substrate, on which a plurality of pixel electrodes
51
for applying voltages to a liquid crystal layer are arranged in a matrix form as shown in FIG.
11
. As active elements which are switching means for selectively driving the pixel electrodes
51
, thin-film transistors (TFTs)
52
are formed on the substrate and are connected with the pixel electrodes
51
. Further, in the case where the color display is conducted, color filter layers of red, green, blue, and other colors are provided, though not shown, on the active matrix substrate or a counter substrate, in addition to the foregoing arrangement.
Gate electrodes of the TFTs
52
are connected with scanning lines
53
, while source electrodes of the TFTs
52
are connected with signal lines
54
. The scanning lines
53
and the signal lines
54
are provided so as to run beside the pixel electrodes
51
arranged in matrix and orthogonally cross each other. The TFTs
52
are driven in response to input of gate signals through the scanning lines
53
. Upon the driving of the TFTs
52
, data signals (display signals) are supplied to the pixel electrodes
51
through the signals lines
54
and the TFTs
52
.
Furthermore, drain electrodes of the TFTs
52
are connected with the pixel electrodes
51
and additional capacitors
55
. Counter electrodes provided vis-a-vis the additional capacitors
55
with an insulating layer therebetween are connected with common lines
56
. The additional capacitors
55
are intended to hold voltages to be applied to the liquid crystal layer.
In the active matrix LCD device, the liquid crystal layer is provided between the active matrix substrate and the counter substrate vis-a-vis to the active matrix substrate. In other words, the liquid crystal layer is provided between the pixel electrodes
51
on the active matrix substrate and the counter electrodes provided on the counter substrate, thereby constituting a liquid crystal capacitor. The additional capacitors
55
are connected in parallel with the liquid crystal capacitor.
To explain each TFT
52
in more detail, as shown in
FIG. 12
, a gate electrode
62
is formed on a transparent insulating substrate
61
, and a gate insulating film
63
is formed so as to cover the gate electrode
62
. On the gate electrode
62
, a semiconductor film
64
is provided with the gate insulating film
63
interlaminated therebetween. On a center portion of the semiconductor film
64
, there is provided a channel protective layer
65
. On source section sides of the channel protective layer
65
and the semiconductor thin film
64
, a source electrode
66
a
composed of a microcrystal n
+
-silicon layer is formed, while on drain section sides of the same, a drain electrode
66
b
composed of a microcrystal n
+
-silicon layer is formed.
The source electrode
66
a
is connected with a metal layer
67
a
serving as a source wire, while the drain electrode
66
b
is connected with a metal layer
67
b
serving as a drain wire. A surface of the TFT
52
is covered with an interlayer insulating film
68
, and a transparent conductive film serving as the pixel electrode
51
is formed thereon. The pixel electrode
51
is connected with the metal layer
67
b
as the drain wire of the TFT
52
through a contact hole
69
. Further, on the pixel electrode
51
, an alignment film (not shown) for aligning the liquid crystal is uniformly provided substantially throughout a whole display region including marginal portions.
As described above, the interlayer insulating film
68
is provided between the scanning lines
53
and the signal lines
54
on one hand and the transparent conductive films serving as the pixel electrodes
51
on the other hand. Therefore, the pixel electrodes
51
can be laminated on the scanning lines
53
and the signal lines
54
with the interlayer insulating film
68
interlaminated therebetween. Such arrangement is disclosed by, for example, the Japanese Publication for Laid-Open Patent Application No.58-172685/1983 (Tokukaisho 58-172685). With the arrangement, a pixel aperture ratio of the LCD device is enhanced, and disclination of the liquid crystal is suppressed by shielding an electric field due to signals conducted through the signal lines
54
with the use of the interlayer insulating film
68
.
As the interlayer insulating film
68
, an inorganic thin film made of SiN or the like has conventionally been used. The SiN film is formed by, for example, the CVD (chemical vapor deposition) method to a thickness of about 500 nm.
Here, liquid crystal molecules used as the liquid crystal layer have a refractive index anisotropy &Dgr;n, and the liquid crystal molecules are aligned with an inclination with respect to the active matrix substrate and the counter substrate sandwiching the liquid crystal molecules. Therefore, a contrast of a displayed image alters depending on a viewing direction or a viewing angle of an observer, whereby aggravating the viewing angle dependence.
Regarding the foregoing problem, a liquid crystal display method for an LCD device of the twisted-nematic (hereinafter referred to as TN) type, which is particularly often used among the LCD devices of the nematic type, is explained as follows. As shown in
FIG. 13
, when a voltage for a half-tone display is applied to an LCD element
71
of the TN type, each liquid crystal molecule
72
slightly raises an end thereof. Here, a linearly polarized light
75
running in a normal direction of surfaces of substrates
73
and
74
, and linearly polarized lights
76
and
77
running in a direction inclining with respect to the normal direction cross the liquid crystal molecules
72
at different angles. Since the liquid crystal molecules
72
have the refractive index anisotropy &Dgr;n as described above, an ordinary light and an extraordinary light occur when the linearly polarized lights
75
,
76
, and
77
directed in the respective directions pass through the liquid crystal molecules
72
. T he linearly polarized lights
75
,
76
, and
77
are converted to elliptically polarized lights in accordance with phase differences between the ordinary light and the extraordinary light, respectively, thereby causing the viewing angle dependence.
Inside the liquid crystal layer, among the liquid crystal molecules
72
, those around a mid point between the substrates
73
and
74
, those near the substrate
73
, and those near the substrate
74
hav
Conlin David G.
Dike, Bronstein, Roberts and Cushman LLP
Jensen Steven M.
Parker Kenneth
Qi Mike
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