In-plane switching scheme liquid crystal display unit

Details In-plane switching scheme liquid crystal display unit In-plane switching scheme liquid crystal display unit

1999-09-14

2002-09-17

Sikes, William L. (Department: 2871)

Liquid crystal cells, elements and systems

Particular structure

Having significant detail of cell structure only

C349S143000

Reexamination Certificate

active

06452657

ABSTRACT:

BACKGROUND OF THE INVENTION
(a) Field of the Invention
The present invention relates to an in-plane switching scheme liquid crystal display (LCD) unit and, more particularly, to an in-plane switching scheme LCD unit having driving electric field parallel to substrates and achieving a wide view angle while suppressing a color change.
(b) Description of the Related Art
LCD units have advantages of smaller thickness, lower weight and lower power dissipation. Among other LCD units, an active matrix LCD unit wherein each of pixels arranged in a matrix is driven by an active element, such as a thin film transistor (TFT), is expected for use as a high-performance flat panel display unit.
A conventional active matrix LCD unit (AM-LCD) generally includes a twisted-nematic liquid crystal (TN-LC) layer and takes advantage of the electric-optical effect thereof by sandwiching the LC layer between a pair of substrates and applying the LC layer with an electric field substantially perpendicular to the surfaces of the substrates for operation of the LC layer.
U.S. Pat No. 3,807,831 discloses an AM-LCD unit using an in-plane switching scheme wherein the LC layer is operated by a lateral (in-plane) electric field parallel to the substrates. The disclosed LCD unit includes a pair of comb-shape electrodes, with the teeth of both the electrodes being alternately arranged.
Patent Publication JP-B-63-21907 discloses an AM-LCD unit which takes advantage of the electric-optical effect of the TN-LC layer. The disclosed LCD unit also includes a pair of comb-shape electrodes by which parasitic capacitance is reduced between the common electrode and the drain bus lines and between the common electrode and the gate bus lines.
FIG. 1
shows the AM-LCD unit using an in-plane switching scheme, referred to as an IPS-LCD unit hereinafter. The IPS-LCD unit includes a pair of front and rear glass substrates
11
and
12
sandwiching therebetween a LC layer
20
, wherein the second substrate
12
mounts thereon a pair of comb-shape electrodes
70
. By applying a driving voltage between the comb-shape electrodes
70
, a lateral electric field is generated in the direction perpendicular to the extending direction of the teeth of the comb-shape electrodes
70
and parallel to the surfaces of the substrates
11
and
12
. The lateral electric field rotates the orientation of the LC molecules, whereby the transmittance of the LC layer
20
is controlled in each pixel.
In the IPS-LCD unit of
FIG. 1
, the orientations of the LC molecules in each pixel should be determined in a specified direction by the application of the drive voltage for a stable and uniform image of the pixel. This is generally achieved by the configuration of the initial orientation &phgr;
LC0
of the LC molecules, which is somewhat deviated from the direction perpendicular to the direction of the lateral electric field. In other words, the LC layer
20
is subjected to an initial orientation alignment so that the initial orientation &phgr;
LC0
of the LC molecules is somewhat smaller than 90° from the extending direction of the teeth of the comb-shape electrodes
70
.
In the description to follow, angle &phgr; of the direction of the electric field or orientation of the LC molecules is defined from the direction perpendicular to the extending direction of the teeth, with the counter-clockwise rotation as viewed from the front substrate being the positive. The initial orientation &phgr;
LC0
of the LC molecules are generally determined as 45°≦&phgr;
LC0
≦90° for assuring a sufficient contrast while rotating the LC molecules by more than 45°. In the illustrated configuration of
FIG. 1
, the LC molecules are rotated by the driving electric field E
1
in the clockwise direction as viewed from the front substrate or first substrate
11
, as shown by the solid arrow, due to the initial orientation being somewhat deviated in the clockwise direction from the extending direction of the teeth of the electrodes
70
.
If the LCD unit of
FIG. 1
is sandwiched between a pair of polarizing plates having orthogonal polarization axes, the light transmittance T upon application of a driving voltage is expressed by the following equation:
T=
½×sin
2
{2(&phgr;
P
−&phgr;
LC
)}sin
2
(&pgr;·&Dgr;n·d/&lgr;)  (1)
wherein &phgr;
LC
, &phgr;
P
, &Dgr;n, d, and &lgr; are orientation of the LC molecules upon application of the driving voltage, orientation of the passing axis of the polarizing plate disposed on the incident side of the back-light, the birefringence anisotropy of the LC layer, the cell thickness or the thickness of the LC layer, and the wavelength of the back-light, respectively. The orientation &phgr;
A
of the passing axis of the polarizing plate disposed on the light emitting side is expressed by:
&phgr;
A
=&phgr;
P
+90° or &phgr;
A
=&phgr;
P−
90°.
According to equation (1), the transmittance T is controlled by using a driving electric field parallel to the substrates to change the orientation of the LC molecules. If the orientation of the passing axis of one of the polarizing plates is aligned with the orientation of the initial orientation of the LC molecules, i.e., &phgr;
LC0
=&phgr;
P
or &phgr;
LC0
=&phgr;
A
, the LCD layer assumes a dark state upon application of no driving voltage whereas assumes a bright state upon application of the driving voltage. In the latter state, when the orientation of the LC molecules are rotated by 45° by the driving electric field, the LCD unit assumes a brightest state thereof due to a maximum transmittance thereof. Alternatively, a configuration may be employed by changing the arrangement of the polarizing plates so that the LC layer assumes a dark state upon application of the driving voltage.
In the above description, it is stated that all the LC molecules sandwiched between the substrates are rotated by a uniform angle, for simplification of the description. In the display unit using the birefringence or double refraction, the light having a wavelength satisfying the relationship &Dgr;n·d=&lgr;/2 can pass the LC layer most efficiently. Thus, in order to obtain an excellent a white color image or a multi-color image by using a color filter, the birefringence anisotropy &Dgr;n and the thickness “d” of the LC layer are typically controlled so that the central wavelength of the spectrum of the transmitted light is set at 550 nm, i.e., &Dgr;n·d=275 nm. In a practical LCD unit, since the LC molecules disposed at the boundary between the LCD layer and the substrate is relatively firmly fixed to the substrate to assume less rotation, it is preferable that the birefringence anisotropy &Dgr;n and the thickness “d” be designed so that birefringence &Dgr;n·d resides between 280 and 330 nm.
WO91/10936 describes improvement of the view angle characteristics in the IPS-LCD unit using a TN-LC layer, and the IPS-LCD unit having such improved characteristics is expected for use as a large screen monitor.
FIG. 2
shows the view angle dependency of the relationship between the driving voltage and the light transmittance in the improved IPS-LCD unit. The view angles include an azimuth view angle &phgr;
obs
which is defined by a view direction of the observer measured with respect to the direction perpendicular to the extending direction of the teeth, and a polar view angle &thgr;
obs
which is defined by an angle with respect to the perpendicular to the substrates. In
FIG. 2
, curve (I) shows view angles of &thgr;
obs
=0 and &phgr;
obs
=0, and curve II shows &phgr;
obs
=40°, curve III shows &phgr;
obs
=85°, curve IV shows &phgr;
obs
=−50° and curve V shows &phgr;
obs
=−5°, with the polar view angle &thgr;
obs
unchanged.
In the graph, the sample of the LC cell used for measurements has a configuration wherein &phgr;
LC0
=85°, &phgr;
P
=85° and &phgr;
A
=−5°. The pair of electrodes are of comb-shape, wherein the width of the teeth is 5 &mgr;m and the

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