Semiconductor device manufacturing: process – Making field effect device having pair of active regions... – On insulating substrate or layer
Reexamination Certificate
2001-07-05
2003-02-11
Cao, Phat X. (Department: 2814)
Semiconductor device manufacturing: process
Making field effect device having pair of active regions...
On insulating substrate or layer
C438S149000, C438S160000, C257S359000
Reexamination Certificate
active
06518108
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to an electronic device and a method for making the same. More particularly, the invention relates to an electronic device wherein a semiconductor active layer and a gate electrode are in limited relative position from each other and wherein a space between conductor layers and a space between ohmic contact layers are, respectively, controlled, and also to a method for making such a device as mentioned above.
Existing electronic devices which are formed as a switching device relative to an active matrix substrate used in liquid crystal display devices and shutter arrays include reverse staggered thin film transistors (TFT) shown, for example, in
FIGS. 3
to
5
.
(1) With the structure of
FIG. 3
, both a semiconductor active layer
303
consisting of a-Si (i) and an ohmic contact layer
304
consisting of a-Si (n+) have a width greater than a gate electrode
301
. When light is irradiated from the back side of a substrate, the light is permitted to invariably pass to part of the semiconductor active layer
303
. Eventually, the OFF current (I
OFF
) rises with a lowering of S/N sensitivity.
(2) With the structure shown in
FIG. 4
, there is no problem involved in (
1
) above. However, a semiconductor active layer
403
consisting of a-Si (i) directly contacts, at the side face thereof, with a conductor layer
405
made, for example, of Al/Cr, thus presenting the problem that I
OFF
undesirably rises.
(3) With the structure of
FIG. 5
, the problems of (
1
) and (
2
) above can be solved. Nevertheless, an additional step is essential for forming a channel protective layer (i.e. an etch stopper layer) consisting of silicon nitride between a semiconductor active layer
503
and an ohmic contact layer
504
.
As will be apparent from the above, in order to suppress the rise of I
OFF
in case where light is irradiated from the back side of the substrate, the prior art (
3
) can provide the best structure.
In this connection, the prior art techniques (
1
) and (
2
) have the following common problem.
(a) In order to form a source electrode and a drain electrode and also to remove the ohmic contact layer from the semiconductor active layer, a resist mask used to form the source and drain electrodes is employed to process the source and drain electrodes therewith. Thereafter, using this mask, the ohmic contact layer on the semiconductor active layer is processed. Accordingly, where the conductor layer and the ohmic contact layer are successively etched, the ohmic contact layer below the end of the conductor layer is likely to be side etched, thereby causing a gap to be formed. This leaves the residue of an etchant in the gap, thereby degrading TFT characteristics.
On the other hand, the prior art technique (
3
) also has the following problems.
(b) The semiconductor active layer is liable to suffer damages owing to the ion implantation used to form the ohmic contact layer.
(c) The production costs increase because of the additional step of forming the channel protective layer.
(d) Since the channel protective layer is additionally formed, the resultant device is formed as being more stepped, so that electric concentration or short-circuiting is apt to occur at the stepped portion or portions.
SUMMARY OF THE INVENTION
It is therefore an object of the invention to provide an electronic device which can suppress the rise of I
OFF
when light is irradiated from the back side of a substrate without the formation of any channel protective layer.
It is another object of the invention to provide a method for making such an electronic device at reduced production costs.
The above objects can be achieved, according to one embodiment of the invention, by an electronic device of the type which comprises a substrate, a gate electrode formed on one surface of the substrate, and a gate insulating film covering the substrate and the gate electrode therewith, characterized by comprising a semiconductor active layer formed above the gate electrode and having a width smaller than the gate electrode, and a source electrode and a drain electrode formed on the semiconductor active layer through an ohmic contact layer, the source electrode and the drain electrode being, respectively, made of a conductor layer and being in a spaced relation from each other, wherein the ohmic contact layers are, respectively, formed between the semiconductor active layer, and the source electrode and the drain electrode, the space between the source electrode and the drain electrode kept away from each other is wider than the space between the spaced ohmic contact layers, and the substrate is irradiated with light from the other surface on which the gate electrode is not formed.
Preferably, the semiconductor active layer of the electronic device of the invention is made of amorphous silicon, and the ohmic contact layer is made of the semiconductor active layer to which an impurity is added. The conductor layer should preferably be made of a metal such as Al, Ti, Mo or Cu, an alloy thereof, or a compound of the metal.
According to another embodiment of the invention, there is also provided a method for making an electronic device which comprises a substrate, a gate electrode formed on one surface of the substrate, and a gate insulating film covering the substrate and the gate electrode therewith, characterized in that where a semiconductor active layer having a width smaller than the gate electrode is formed just above the gate electrode, the gate insulating film and the semiconductor active layer are continuously formed by a CVD method in such a way that the semiconductor active layer is formed narrower in width than the gate electrode, and a source electrode and a drain electrode both made of a conductor layer are formed through an ohmic contact layer on the semiconductor active layer, that where the ohmic contact layer is formed between the semiconductor active layer and the source electrode and also between the semiconductor active layer and the drain layer, respectively, the ohmic contact layers, the source electrode and the drain layer are continuously formed by a sputtering method, that where a space between the source electrode and the drain electrode spaced from each other is wider than a space between the ohmic contact layers kept away from each other, the source electrode, the drain electrode and the ohmic contact layers are all continuously removed with an etchant, and that a light irradiating means is provided at the other surface of the substrate, where the gate electrode has not been formed, so that light is permitted to pass vertically to the substrate.
Preferably, according to the method of the invention, the etchant has an etching rate for the source electrode and also for the drain electrode higher than an etching rate for the ohmic contact layers.
In the first embodiment of the invention as claimed in claim 1, since the semiconductor active layer whose width is smaller than the gate electrode is formed just above the gate electrode, the rise of I
OFF
can be suppressed on irradiation of light from the other side of the substrate. Moreover, the source electrode and the drain electrode which are, respectively, made of the conductor layer and are formed on the semiconductor active layer through the ohmic contact layer, and the ohmic contact layers are formed between the semiconductor active layer and the source electrode and also between the semiconductor active layer and the drain electrode. By this, the rise of I
OFF
can be prevented.
In addition, the space between the source electrode and the drain electrode is wider than the space between the ohmic contact layers, so that when the multi-layer film consisting of the conductor layers and the ohmic contact layers is etched, the formation of a gap in the ohmic contact layers beneath the end portions of the conductor layers can be avoided.
In a preferred embodiment as claimed in claim 2, the semiconductor active layer is formed of amorphous silicon, so that this layer can be formed at low temperatures of
Fukui Hirofumi
Iwasaki Chisato
Brinks Hofer Gilson & Lione
Cao Phat X.
Le Thao X.
LG. Philips LCD Co. Ltd.
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