Active solid-state devices (e.g. – transistors – solid-state diode – Non-single crystal – or recrystallized – semiconductor... – Amorphous semiconductor material
Reexamination Certificate
2002-09-06
2004-10-05
Elms, Richard (Department: 2824)
Active solid-state devices (e.g., transistors, solid-state diode
Non-single crystal, or recrystallized, semiconductor...
Amorphous semiconductor material
C257S052000, C257S072000
Reexamination Certificate
active
06800872
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an active matrix substrate for use in an active matrix type liquid crystal display using a thin film transistor (which will be abbreviated to TFT hereinafter) and to a manufacturing method thereof. More particularly, the present invention relates to reduction in production steps of a method for manufacturing an active matrix substrate.
2. Description of the Prior Art
An active matrix liquid crystal display using TFTs has an active matrix substrate on which pixel electrodes and TFTs for controlling a voltage applied to the pixel electrodes are arranged in the form of a matrix. The active matrix liquid crystal display has a structure that a liquid crystal is sandwiched between the active matrix substrate and an opposed substrate and the liquid crystal is driven by a voltage applied between the pixel electrode and the other electrode. In this case, there is a liquid crystal display adopting a vertical electric field system, in which the pixel electrode on the active matrix substrate is constituted by a transparent electrode and a voltage is applied between the pixel electrode and a transparent common electrode formed on the opposed substrate as the other electrode in order to drive the liquid crystal. Alternatively, there is a liquid crystal display adopting a horizontal electric field system, in which the pixel electrode on the active matrix substrate and the common electrode are constituted by a pair of comb-like electrodes and a voltage is applied between these electrodes to drive the liquid crystal. At any rate, the TFT and the pixel electrode must be finely formed on the active matrix substrate, and nowadays the TFT and the pixel electrode are formed by a photolithography technique.
FIG. 1
is a cross-sectional view showing a one-pixel area of a TFT and a pixel electrode having the prior art structure provided on an active matrix substrate. Here, a technique disclosed in Japanese Patent Application Laid-open No. 268353/1998 is illustrated. In manufacture of the TFT and the pixel electrode, the first metal thin film is formed on a transparent insulating substrate
300
. In the first photolithography step (which will be abbreviated to a PR step hereinafter) using photoresist, the first metal thin film is patterned to form a gate electrode
301
and a gate bus line
302
connected to this gate electrode
301
. Subsequently, the first insulating film
303
as a gate insulating film, an intrinsic amorphous silicon film (which will be referred to as an intrinsic a-Si film hereinafter)
304
, and an ohmic contact film
305
consisting of the a-Si film in which impurities are introduced are sequentially superimposed on the insulating substrate
300
.
The ohmic contact film
305
and the intrinsic a-Si film
304
are then patterned in the second PR step. Further, the second metal thin film is deposited, and the second metal thin film is patterned by the third PR step to form a drain electrode
306
connected to a drain bus line, and a source electrode
307
.
Then, the second insulating film
308
is deposited on the entire surface. Further, by the fourth PR step, a pixel contact hole
309
and a drain bus line contact hole (not shown) which pierce the second insulating film
308
and reaches the surface of the source electrode
307
and the drain bus line respectively are opened through the second insulating film
308
. Furthermore, a gate bus line contact hole (not shown) which reaches even to the surface of the gate bus line is opened through the first insulating film
303
and the second insulating film
308
. Moreover, a transparent electrode film, e.g., an ITO (INDIUM TIN OXIDE) film is deposited on the entire surface, and the transparent electrode film is patterned by the fifth PR step to form a pixel electrode
310
connected to the source electrode
307
. In addition, although not illustrated in
FIG. 1
, a gate terminal portion connected to the gate bus line and a drain terminal portion connected to the drain bus line are simultaneously formed.
Such a prior art active matrix substrate requires five PR steps for forming the TFT, the pixel electrode, the gate terminal portion and the drain terminal portion. The five PR steps currently obstruct manufacturing the active matrix substrate at a low price, which is led by eliminating the cost required for the manufacturing steps. Thus, reduction in number of manufacturing steps, especially reduction in number of PR steps is examined. For example, Japanese Patent Application Laid-open No. 218925/1983 proposes a technique for reducing the PR steps to four steps by forming the pixel electrode, the gate electrode and the gate bus line by one PR step. Alternatively, a monthly publication FPD INTELLIGENCE 1999, pp. 31 to 35 or Japanese Patent Application Laid-open No. 2000-66240 discloses a technique for realizing patterning of films in different layers by one PR step by utilizing a half-tone exposure method to thereby reduce a number of the PR steps for the active matrix substrate.
In these improved techniques, the former technique has a process restriction that the gate electrode and the pixel electrode are formed by the same PR step and a structural restriction that the gate electrode and the pixel electrode are formed in the same layer. In particular, the structural restriction that the pixel electrode is formed in a lower layer must largely change the structure of the existing active matrix substrate. Additionally, in this structure, since the side surface of the intrinsic a-Si layer is not covered with a channel protection film but exposed, a liquid crystal layer exists through only a porous film such as a polyimide alignment layer when the liquid crystal display is configured. Further, there occurs a problem that impurities existing in the liquid crystal layer enter the intrinsic a-Si film by diffusion or an electric field to greatly deteriorate the characteristic of the TFT. In this regard, the latter technique has less structural and process restrictions and is particularly advantageous in that the pixel electrode can be manufactured as a layer structure similar to that of the existing active matrix substrate.
According to the latter technique utilizing the half-tone exposure method, as shown in schematic views illustrating a TFT portion of
FIG. 2
, a gate electrode
401
is first formed on an insulating substrate
400
and a gate insulating film
402
, a intrinsic a-Si film
403
, an ohmic a-Si film
404
, and a metal film
405
are then sequentially superimposed thereon as shown in FIG.
2
(
a
). Subsequently, a photoresist film
406
is coated thereon, and then exposed and developed to form a required pattern. At this time, a film thickness of the developed photoresist
406
becomes thinner in an area exposed by half-tone exposure, a part corresponding to a channel area of the TFT, than in a non-exposed area. Then, as shown in FIG.
2
(
b
), the photoresist
406
is used as an etching mask to pattern the metal film
405
, the ohmic a-Si film
404
and the intrinsic a-Si film
403
to form each island of the metal film
405
, the ohmic a-Si film
404
and the intrinsic a-Si film
403
. Thereafter, as shown in FIG.
2
(
c
), when O
2
gas is used to perform ashing of the photoresist
406
, the photoresist in the area subjected to half-tone exposure is removed and the metal film
405
is exposed in this area. Furthermore, as shown in FIG.
2
(
d
), the metal film
405
is etched by using the photoresist
406
subjected to ashing to be formed as a source electrode and a drain electrode. Subsequently, the ohmic a-Si film
404
is etched to form a channel area. Then, by removing the photoresist
406
, the TFT is completed as shown in FIG.
2
(
e
).
As described above, according to the technique disclosed in the latter reference, among the manufacturing steps explained with reference to
FIG. 1
, the second PR step and the third PR step can be combined as one PR step in the manufacturing process. As a result, by using half-tone exposure, the acti
Tanaka Hiroaki
Uchida Hiroyuki
Elms Richard
NEC LCD Technologies Ltd.
Smith Bradley
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