Thin-film transistor elements and methods of making same

Details Thin-film transistor elements and methods of making same Thin-film transistor elements and methods of making same

2000-09-06

2003-05-20

Everhart, Caridad (Department: 2825)

Semiconductor device manufacturing: process

Making field effect device having pair of active regions...

On insulating substrate or layer

C438S161000

Reexamination Certificate

active

06566174

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to thin-film transistor elements and methods of making the same. More particularly, it relates to thin-film transistor element structures used in active matrix type liquid crystal displays, and methods of making the same.
2. Description of the Related Art
In recent years, active matrix type liquid crystal displays in which thin-film transistors (TFTs) using a hydrogenated amorphous silicon film are utilized as switching elements for display picture elements are being produced in large quantities. Especially with the popularization of notebook type personal computers, the demand for liquid crystal displays is growing rapidly and, therefore, an improvement in their productivity is being needed.
Referring to
FIG. 17
, there is shown a sectional view of an inverted staggered type thin-layer transistor element which is commonly used as a switching element for each picture element of a liquid crystal display. First of all, a metal for use as a gate electrode is deposited on a transparent insulating substrate
10
, and patterned into a desired shape to form a gate electrode
11
. Then, a silicon nitride film
12
serving as a gate insulating film, an amorphous silicon film
13
, and an n-doped amorphous silicon film
14
for making ohmic contacts of source-drain regions are successively formed thereon, and n-doped amorphous silicon film
14
and amorphous silicon film
13
are patterned into an island-like shape. Subsequently, a metal for use as source-drain electrodes is deposited and patterned into desired shapes to form source-drain electrodes
15
. Finally, the undesired n-doped amorphous silicon film
14
above the channel is etched together with a part of amorphous silicon film
13
in consideration of a margin. Thus, a thin-film transistor element as illustrated in
FIG. 17
is completed.
As an inverted staggered type thin-film transistor element of this type, Japanese Patent Publication No. 51069/'92 has proposed a thin-film transistor in which an n-doped amorphous silicon film is formed so as to cover the whole surface of the island-like amorphous silicon film and so as to impart an off resistance of not less than 10
9
&OHgr; to the thin-film transistor (i.e., the n-doped amorphous silicon film above the channel is not removed). However, in order to obtain good ohmic contact characteristics, the n-doped amorphous silicon film must have a resistivity of not greater than 10
4
&OHgr;cm. Moreover, in order to achieve an off resistance of not less than 10
9
&OHgr; for a typical thin-film transistor element size as represented by a (channel width)/(channel length) ratio of 10, the thickness of the n-doped amorphous silicon film must be limited to 10 nm or less even if the resistivity thereof is not greater than 10
4
&OHgr;cm. Where it is desired to achieve a satisfactory panel representation by using thin-film transistor elements as driving elements for the picture elements of a liquid crystal display, an off resistance of about 10
10
to 10
11
&OHgr; is actually required. In order to obtain such an off resistance, the n-doped amorphous silicon film must have a thickness of about 0.1 to 1 nm. However, such a very thin n-doped amorphous silicon film involved a problem in that it fails to give good ohmic contact characteristics and hence causes a marked reduction in on-state current.
Moreover, in recent years, technical developments are being made to improve the aperture ratio of each picture element section of a liquid crystal display by using, as protective insulating films, insulating films formed by applying and thermally curing various polymeric materials. In these applied insulating films, a film thickness of about 2 to 3 &mgr;m can be easily obtained, and their relative permittivities are equal to about ½ of that of a conventionally used silicon nitride film. Accordingly, even if the transparent conductive picture element electrodes formed on such an applied insulating film overlap with the data lines and the signal lines, the coupling capacity due to such overlap is very low and, therefore, display defects such as crosstalk are minimized. Thus, it becomes possible to achieve a high aperture ratio while maintaining satisfactory display characteristics. In this respect, an explanation is given below with reference to FIG.
18
.
A metal for use as a gate electrode is deposited on a transparent insulating substrate
10
, and patterned into a desired shape to form a gate electrode
11
. Then, a silicon nitride film
12
serving as a gate insulating film, an amorphous silicon film
13
, and an n-doped amorphous silicon film
14
for making ohmic contacts of source-drain regions are successively formed thereon, and n-doped amorphous silicon film
14
and amorphous silicon film
13
are patterned into a desired island-like shape. Subsequently, a metal for use as source-drain electrodes is deposited and patterned into desired shapes to form source-drain electrodes
15
. Moreover, the undesired n-doped amorphous silicon film
14
above the channel is etched together with a part of amorphous silicon film
13
in consideration of a margin. Thereafter, a protective insulating film (or applied insulating film)
18
is formed over the whole surface. Finally, a transparent conductive picture element electrode
19
is formed so that it is electrically connected to the source electrode through a contact hole. Thus, a thin-film transistor element is completed. This technique has been reported, for example, by Y. Takafuji et al. [SID '93, Digest, p. 383 (1993)] and Jeong Hyun Kim et al. [AM-LCD 96, Digest, p. 149 (1996)].
At present, in order to reduce the prices of liquid crystal displays, it is strongly desired to simplify the thin-film transistor fabrication process and achieve an improvement in the throughput thereof. In particular, inverted staggered type thin-film transistor elements as described above are most widely utilized in liquid crystal displays because of their excellent element characteristics and stability, and a simplification of their fabrication process and an improvement in the throughput thereof are believed to contribute greatly to a reduction in the prices of liquid crystal displays. As described above, in the case of conventional inverted staggered type thin-film transistor elements, it is necessary to etch the undesired n-doped amorphous silicon film above the channel during the fabrication process thereof. To this end, it is difficult to etch the n-doped amorphous silicon film selectively at a high selectivity ratio to the underlying amorphous silicon film. Accordingly, the n-doped amorphous silicon film has been etched together with a part of the underlying amorphous silicon film in consideration of a margin.
However, the amorphous silicon film surface, i.e., back channel interface, having been exposed to an etching gas is severely affected by process damage and hence has a very high surface state density due to defects. Consequently, if the thickness of the amorphous silicon film in the channel region is decreased to about 150 nm or less after etching, the on-state characteristics of the thin-film transistor element will be significantly reduced under the influence of the surface state on the back channel side. For theses reasons, it has been necessary to form an amorphous silicon film having a thickness of as large as about 300 nm.
As described above, conventional inverted staggered type thin-film transistor elements have involved the following two important problems:
(1) It is necessary to etch the undesired n-doped amorphous silicon film above the channel together with a part of the underlying amorphous silicon film in consideration of a margin.
(2) In order to obtain good on-state characteristics, the amorphous silicon film must be thick.
It is believed that these problems raise the costs of liquid crystal displays for the following reasons:
As to the above problem (1), there is only a slight difference in etching selectivity between the n-dop

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