Pixellated devices such as active matrix liquid crystal...

Details Pixellated devices such as active matrix liquid crystal... Pixellated devices such as active matrix liquid crystal...

2002-01-22

2004-08-10

Deo, Duy-Vu (Department: 1765)

Semiconductor device manufacturing: process

Making device or circuit emissive of nonelectrical signal

Including integrally formed optical element

C438S030000, C438S066000

Reexamination Certificate

active

06773941

ABSTRACT:

This invention relates to pixellated devices such as active matrix liquid crystal displays, and particularly to methods of fabricating the substrate with the active matrix circuit, known as the active plate, used in the manufacture of such devices.
An active matrix liquid crystal display (AMLCD) typically comprises an active plate and a passive plate between which liquid crystal material is sandwiched. The active plate comprises an array of transistor switching devices, typically with one transistor associated with each pixel of the display. Each pixel also has a pixel electrode on the active plate to which a signal is applied for controlling the display output of the individual pixel.
FIG. 1
shows the electrical components which make up the pixels of one known example of active plate of an AMLCD. The pixels are arranged in rows and columns. The row conductor
10
of a pixel is connected to the gate of the TFT (thin film transistor)
12
, and the column electrode
14
is coupled to the source. The liquid crystal material provided over a pixel electrode of the pixel effectively defines a liquid crystal cell
16
which is connected between the drain of the transistor
12
and a common ground plane
18
. An optional pixel storage capacitor
20
is connected between the drain of the transistor
12
and the row conductor
10
associated with an adjacent row of pixels.
For transmissive displays, a large area of the active plate is at least partially transparent, and this is required because this type of display is illuminated by a back light. In these display devices, the pixel electrode must be transparent, whereas row and column conductors are usually formed as metallic lines which are opaque. Metallic layers, such as chromium, aluminium, alloys or multilayer structures are used for the row and column conductors because of the high conductivity, which improves the device performance. The conductivity of the lines (usually the column lines) to which the pixel drive signals are applied is particularly important in large displays, because a sizeable voltage drop occurs over the length of the line, making it impossible to drive uniformly all pixels along the line (column).
A problem with the use of metallic column conductors is that separate deposition and lithographic procedures are required to form the column conductors and the pixel electrodes. The pixel electrodes must be transparent, and are typically formed from a transparent conductive oxide film. It is well known that the lithography steps in the manufacturing process are a major contributing factor to the expense of the manufacturing process. Each lithographic step can be considered to reduce the yield of the process, as well as increasing the cost.
The conventional manufacturing process for the active plate of an LCD is a five mask process. With reference to the bottom gate TFT LCD active plate shown in
FIG. 2
, the process steps, each requiring a separate mask definition, are:
(i) defining the gate
22
(which is part of the row conductor) over the substrate
21
;
(ii) defining the amorphous silicon island (which overlies a gate dielectric
23
that covers the entire structure), comprising a lower intrinsic layer
24
and an upper doped contact layer
26
;
(iii) defining the metallic source
28
, drain
30
and column electrode
32
;
(iv) defining a contact hole
34
in a passivation layer
36
which covers the entire substrate; and
(v) defining the transparent pixel electrode
38
which contacts the drain
10
through the hole
34
.
The capacitor shown in
FIG. 1
may simply be formed from the gate dielectric by providing an area of overlap of one pixel electrode with a portion of the row/gate conductor of the adjacent row.
There have been various proposals to reduce the number of lithography steps, and thereby the mask count, of the manufacture process in order to reduce cost and increase yield.
For example, it has been proposed to form the column conductors from the same transparent conductive oxide film as the pixel electrode, so that these components of the pixel structure can be deposited and patterned together. Additional measures can result in a two mask process, and this is explained with reference to the bottom gate TFT LCD active plate shown in FIG.
3
. The process steps, each requiring a separate mask definition, are:
(i) defining the gate
22
(and row conductors); and
(ii) defining the transparent column electrode
32
(which also forms the TFT source
28
) and the pixel electrode
38
(which also forms the TFT drain
30
).
The definition of the semiconductor island
24
,
26
can be achieved by a self-aligned process using the gate
22
, for example by using light exposure through the substrate. Of course, the semiconductor could equally be formed with a third mask step (between steps (i) and (ii) above). In the periphery of the array, the gate dielectric
23
is etched away using a low-precision stage, to allow contact to the gate lines at the periphery of the display.
In this structure, the high resistivity of the transparent conductive oxide film used for the column lines prevents the use of the structure in large (TV-sized) displays.
For this reason, there are further proposals to treat the column conductor area of the layer to increase the conductivity, whilst not affecting the transparency of the pixel electrode. For example, the article “Conductivity Enhancement of Transparent Electrode by Side-Wall Copper Electroplating”, J. Liu et al, SID 93 Digest, page 554 discloses a method of enhancing the conductivity by electroplating a copper bus to the sidewall of the metal oxide column line. The process involves an incomplete etching process to leave metal oxide residues, which act as seeds for the copper growth. The process is complicated and difficult to control. In addition, the copper bus will surround the source and drain electrodes, and there is a risk of shorts between the source and drain resulting from fast lateral copper growth when forming the bus. The copper bus around the source and drain electrodes also influences the channel length of the TFT and therefore makes the TFT characteristics less predictable.
WO 99/59024 discloses a method for enhancing the conductivity of a transparent electrode by providing patterned metallic layers adjacent to the transparent electrodes.
There is still a need for a simple process for increasing the conductivity of a transparent metal oxide layer, such as ITO, without increasing dramatically the complexity of the process. Such a process will find for example application in active matrix LCD manufacture but will also be useful for other technologies where mask count reduction could be achieved if a transparent conductive layer could be made more conductive in certain areas without losing the transparency in others. This may be of benefit in, for example, polymer LED displays and large area image sensors.
According to a first aspect of the invention, there is provided a method of fabricating an active plate comprising pixel electrodes and associated address lines formed from a transparent conductive material, which method comprises:
providing a transparent conductive material layer and a metal layer in succession over a substrate,
depositing and patterning a shielding layer into a configuration corresponding to the desired pattern of the transparent conductive layer required for the pixel electrodes and the address lines, the shielding layer being formed in a manner such that an etching property of the shielding layer at regions corresponding to the pixel electrodes differs from that at the regions corresponding to the address lines,
subjecting the shielding layer to an etching process using the difference in properties so as to remove the regions of the shielding layer corresponding to the pixel electrodes while leaving portions of the shielding layer at the regions corresponding to the address lines,
and thereafter removing the portions of the metal layer at the regions corresponding to the pixel electrodes. The remaining portions of the shielding layer may su

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