Active solid-state devices (e.g. – transistors – solid-state diode – Field effect device – Having insulated electrode
Reexamination Certificate
1999-03-12
2002-08-06
Abraham, Fetsum (Department: 2826)
Active solid-state devices (e.g., transistors, solid-state diode
Field effect device
Having insulated electrode
C257S059000
Reexamination Certificate
active
06429483
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention disclosed in this specification relates to a structure of a thin film semiconductor device such as a thin film transistor or the like, and a method for fabricating the same.
2. Description of the Prior Art
There is known a structure for obtaining a display device which has a high display function by using a thin film transistor (TFT) in a liquid crystal display, the display substituting a cathode ray tube. This display is referred to as an active matrix type liquid crystal display device. This active matrix type liquid crystal display device is a display in which a thin film transistor is arranged in each of the pixel electrodes arranged in matrix to provide a high function display. To heighten the display function, the characteristics of the thin film transistor is required to be set to as high as possible.
The thin film transistor used in an active matrix type liquid crystal display device has a problem in that the thin film transistor is required to be formed on a glass substrate. In other words, to use the glass substrate as the substrate, there is a problem in that the substrate is limited in the fabrication process. Not only thin film transistors but also semiconductors needs to be heated to a high temperature (for example, of 800 to 1000° C.) out of the necessity of diffusing impurity into silicon, activating impurity in silicon, and improving the crystallinity of silicon. However, the temperature that can be applied to the glass substrate is generally about 600° C., and various new techniques are required to fabricate a high performance semiconductor device at a temperature level below this. For example, there are such techniques as a technique for irradiating an amorphous silicon film with laser light to crystallize the amorphous silicon film, and a technique for using the laser light irradiation for the diffusion and activation of the impurity. Since the technique for laser light irradiation causes an extremely small thermal damage to the glass substrate, this is an extremely useful technique when the low productivity is permitted.
FIG. 2
is a schematic sectional view of a conventionally known thin film transistor (generally referred to as TFT). What is shown in
FIG. 2
is a thin film transistor which functions to prevent the intrusion of impurity into the active layer from the glass substrate. The active layer comprises a source area
203
, a channel formation area
204
and a drain area
205
. Then, as a gate insulating film
200
, a silicon oxide film or a silicon nitride film is formed. A gate electrode
206
comprises a metal and semiconductors. Further, the whole element is covered with an interlayer insulating film
207
which comprises an appropriate insulator such as a silicon oxide film or the like. Further, a source electrode
208
is taken out from the source area
203
while a drain electrode
209
is taken out from the drain area
205
.
The active layer comprising the source area
203
, the drain area
205
and the channel formation area
204
is formed of crystalline silicon. As the crystalline silicon film, a silicon film formed of the amorphous silicon film crystallized by the laser light irradiation is used. However, there is no technique available for forming a single crystal silicon on the glass substrate. Although the film thus formed has crystallinity, the film has a quality in which a large amount of defects and levels are present. Although the film thus formed has crystallinity, the film has a quality in which defects and levels are present. To reduce the defects and levels in the silicon film, a method for neutralizing a dangling bond (unpaired connectors) of silicon which causes defects and levels by using a hydrogen atom. This holds true of a case in which the active layer is not crystalline silicon and is formed of amorphous silicon.
In this manner, in the silicon semiconductor film formed on the glass substrate, the silicon semiconductor film needs to contain hydrogen. However, when an attempt is made to cause the active layer formed of the silicon semiconductor to contain hydrogen, there is a problem in that hydrogen is diffused into the gate insulating film from the active layer.
On the other hand, in the structure shown in
FIG. 2
, it is not extremely favorable that mobile ions exist in the gate insulating film, the threshold value varies, and a hysterisis is generated in the C-V characteristics. Consequently, containing hydrogen in the active layer is a useful method on the one hand, it is a disadvantageous method on the other in that hydrogen is diffused in the gate insulating film.
SUMMARY OF THE INVENTION
The invention disclosed in this specification is intended to provide a structure of a semiconductor device wherein the active layer formed of a silicon semiconductor is allowed to contain hydrogen, and the hydrogen does not affect other areas and other parts.
A semiconductor device disclosed in this specification primarily comprises, an active layer formed of a silicon film, and a gate insulating film formed on the active layer, wherein a thin film represented by SiO
x
N
y
is formed between the aforementioned active layer and the aforementioned gate insulating film.
In the above structure, examples of the silicon film include an amorphous silicon film and a crystalline silicon film. Examples of the crystalline silicon film include a polycrystalline silicon film, a fine crystal silicon film, an amorphous silicon film partially including a crystal structure and a silicon film having a mixture of a crystal structure and an amorphous structure.
The active layer refers to a semiconductor layer constituting a thin film transistor. Generally the thin film transistor comprises a source/drain area with one conductivity-type and a channel formation area. Further, the active layer includes an offset gate area and a light dope area. When the crystalline silicon film is used, it is desirable that the density of hydrogen contained in the active layer is set to 0.001 to 5 atom %.
Further, in the aforementioned structure, either silicon nitride film or silicon oxide film may be adopted as a base film formed under an active layer. Further, as a base film, a thin film transistor represented by SiO
x
N
y
is further effectively used. A structure for substantially closing hydrogen in the active layer by substantially covering the active layer (in actuality a contact area for the source/drain area is present so that the active layer is not completely covered) with a thin film represented by SiO
x
N
y
formed as a base film.
Further, in the case where a crystalline silicon film is used which contains a metal element which promotes the crystallization of silicon as a silicon film which constitutes an active layer, it is useful to adopt the aforementioned structure. In other words, to form the crystalline silicon film formed by the metal element which promotes the crystallization into a semiconductor with higher electric properties, the aforementioned structure is adopted at the time of hydrogenation to enable further heightening the effect of the hydrogenation. Needless to say, this effect is extremely useful when hydrogen ions are actively contained in the active layer by hydrogen doping or the like.
Further, in the case where nickel is used as a metal element for promoting the aforementioned crystallization, the effect is even more conspicuous. Further an excess amount of the metal element for promoting the crystallization deteriorates the characteristics of semiconductors (which is approximate to the characteristics of the metal). Excessively small amount of the metal element reduces the effect of promoting the crystallization. Consequently, the most appropriate density is 1×10
15
to 1×10
19
cm
−3
.
As metal elements for promoting the crystallization, such elements as Fe, Co, Ru, Rh, Pd, Os, Ir, Pt, Cu, Ag and Au can be used in addition to Ni. What is important about this element for promoting the crystallization of the amorphous silicon is that the element i
Abraham Fetsum
Nixon & Peabody LLP
Robinson Eric J.
Semiconductor Energy Laboratory Co,. Ltd.
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