Radiation imagery chemistry: process – composition – or product th – Radiation modifying product or process of making – Screen other than for cathode-ray tube
Reexamination Certificate
1998-07-24
2001-06-05
McPherson, John (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Radiation modifying product or process of making
Screen other than for cathode-ray tube
C359S891000
Reexamination Certificate
active
06242139
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
This invention relates to an improved method of manufacture of color filters suitable for such uses as flat panel displays.
2. Description of Related Art
Liquid Crystal Displays (LCD) have been used for many years in place of cathode ray tubes (CRT) screens for small and large sized displays. However, LCD usage has been limited to high cost applications (such as expensive laptop computers) due to the high cost of fabrication. Recent improvements have permitted development of large size, high resolution displays which are useful in notebook and desktop computers. Such LCD panels, particularly color LCD panels, are used for flat screen televisions, projection television systems and camcorder view finders, with many more applications anticipated in the future. Such display panels may take two forms: passive matrix and active matrix liquid crystal displays (AMLCDs). Passive matrix displays employ transparent electrodes patterned in perpendicular striped arrays on facing glass plates, that is superimposed one on the other. Red, green and blue color filters on the inner surface of one of the glass plates provide the full color display. The passive matrix display is ostensibly easier to fabricate than AMLCDs, but is much more limited in performance capabilities.
One of the challenges to reducing the cost of fabricating thin film transistor (TFT), also known as an active matrix, displays is the color filter, which can cost up to 25% of the total LCD cost. In these devices, white light passes through a light valve (the TFT LCD) which adjusts the intensity of the light and then the intensity adjusted light passes through a color filter to give the desired color. A pixel is made up of three colors (each with an independent light valve) corresponding to the primary colors. Accordingly, white color results when the filters are full on; and black color results when all of the filters are full off. The resolution and alignment of the color filter must be such that the filter overlays the TFT devices exactly and provide a very clean differentiation between colors of the mask. Furthermore, the color of the elements in the filter must be consistent from one filter to another and be within a narrow color tolerance. Other types of displays, such as plasma filters, can use the color filter embodied in the present invention.
The fabrication of an active matrix liquid crystal display involves several steps. The assembly comprises two glass panels, identified as front and rear panels. In the first step, the front glass panel is prepared, which involves deposition of a color filter element onto a suitable substrate, such as glass. Color filter deposition typically involves depositing a black matrix pattern and three primary (red, green and blue) color patterns within the spaces outlined by the black matrix. The color elements are each typically about 70 to 100 microns in width by 200 to 300 microns in length. These dimensions are typically used for notebook computer applications. The front glass substrate is completed by deposition of a transparent conducting layer over the color filter element.
Although the present invention also is suitable for use in passive liquid crystal displays, it will be described in embodiments of an active display and specifically a thin film transistor (TFT) liquid crystal display. As partially shown in
FIG. 1
, a conventional TFT display
10
comprises an array of cells or pixels A, each cell including a thin film transistor
11
to address the cell by applying a voltage to the cell when the transistor is in its on state and a capacitor
12
which maintains the voltage after the transistor is switched off. The transistor is formed on a glass substrate
13
on the back side of the display
10
and is connected between a column or data electrode
14
and a row electrode
15
and to a display transparent electrode
16
of each pixel, all at the back side of the display
10
. The front side of the display
10
is formed with a continuous common transparent electrode
17
which is spaced apart from and positioned parallel to the transparent display electrode. Both the common electrode
17
and the display electrode
16
are preferably formed of a thin transparent conductive material, such as indium tin oxide (ITO), carried on a glass substrate. Since the display electrode of each pixel is smaller in dimensions than the continuous common electrode, a fringe field results which spreads outward from the pixel or cell edges of the display electrode to the common electrode when voltage is applied across the electrodes. Parallel with the outside of the common electrode
17
and adjacent glass substrate
18
is a polarizer
19
, which is appropriately orientated relative the a polarizer
20
mounted in back of the rear glass substrate
13
. Alignment layers
21
and
22
are disposed on the inner surface of the display and common electrodes
16
and
17
, respectively, and are in contact with a liquid crystal layer
23
, herein twisted nematic liquid crystal molecules with a positive dielectric anisotropy, which is sealed between the two parallel mounted glass substrates carrying the alignment layers
21
and
22
. On the back side of the display
10
is a visible light source (not shown) which irradiates the display
10
through a diffuser
24
. If it is desired to have the display
10
in color, a color filter
25
is disposed adjacent the non-alignment layer side of the common electrode
17
, and contains groups of the three primary colors (red, green, and blue), each one of the primary colors being associated with one of a group of three adjacent pixels A to form a color cell.
To illustrate the environment of the present invention in more detail,
FIG. 2
shows an enlarged cross-section of the layers of a single domain cell or pixel (prior art) of the liquid crystal display taken along line
1
—
1
of FIG.
1
. with switch
26
(representing the TFT in each pixel) open and a voltage is not applied across the liquid crystal layer
23
. In this illustration, the liquid crystal layer comprises twisted nematic liquid crystals with a left-handed twist which is conventionally achieved by using chiral additives.
FIG. 2
diagrammatically shows this LC layer
23
as elongated molecules
28
a
,
28
b
,
28
c
,
28
d
,
28
e
,
28
f
,
28
g
,
28
h
,
28
i
,
28
j
,
28
k
, and
28
l
with molecules
28
a
,
28
b
,
28
c
, and
28
d
being in contact with surface
29
of the front alignment layer
22
and molecules
28
i
,
28
j
,
28
k
, and
28
l
being in contact with surface
30
of the rear or back alignment layer
21
. Molecules
28
a-d
and molecules
28
i-l
are tilted longitudinally away from their respective surfaces
29
,
30
by the same angle a0. Because of the twist angle of the LC molecules, the molecules along the surfaces
29
and
30
are drawn going into and out of the plane of the paper. The bulk molecules, as depicted by
28
e
-
28
h
, are drawn longer since they are oriented more parallel to the plane of the paper. Surface
29
of the front alignment layer
22
is disposed adjacent the transparent electrode
17
, the color filter
25
, which is optional, the glass substrate
18
, and the polarizer
19
in that order. Surface
30
of the rear alignment layer
21
is disposed adjacent the transparent electrode
16
, and the glass substrate
13
, the polarizer
20
, and the diffuser
24
in that order. The light on the back side of the diffuser
24
for irradiating the liquid crystal display panel is not shown. When switch
26
is closed as shown by the dashed line
26
a
and voltage is applied, the molecules
28
a-d
and
28
i-l
on alignment surfaces
29
and
30
which are influenced by the same pre-tilt angle a0 cause the bulk molecules, as shown by the center molecules
28
e-h
, to move in the direction as shown by the dashed arrows
31
.
In a second step, a separate (rear) glass panel is used for the formation of thin film transistors or diodes, as well as metal interconnect lines. Each transistor acts as an on-of
Hedrick Jeffrey Curtis
Lewis David Andrew
Whitehair Stanley Joseph
Beck Thomas A.
International Business Machines - Corporation
McPherson John
Morris Daniel P.
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