Semiconductor device manufacturing: process – Making device or circuit emissive of nonelectrical signal – Including integrally formed optical element
Reexamination Certificate
1998-07-09
2003-11-04
Booth, Richard (Department: 2812)
Semiconductor device manufacturing: process
Making device or circuit emissive of nonelectrical signal
Including integrally formed optical element
C438S164000, C438S166000, C438S486000, C438S487000
Reexamination Certificate
active
06642073
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to thin-film transistors (TFTS) and also to a method of fabricating TFTs. Furthermore, the invention relates to a semiconductor circuit using plural TFTs and also to a method of fabricating such a semiconductor circuit. A thin-film transistor fabricated according to the present invention is formed either on an insulating substrate of glass or the like or on a semiconductor substrate of a single crystal of silicon. Especially, where the invention is applied to a semiconductor circuit comprising a low-speed matrix circuit and a high-speed peripheral circuit for driving the matrix circuit such as a monolithic active-matrix circuit used in a liquid-crystal display or the like, great advantages can be obtained.
BACKGROUND OF THE INVENTION
In recent years, an insulated gate semiconductor device having an active layer (also called an active region) in the form of a thin film has been investigated. Especially, an insulated-gate transistor in the form of a thin film which is known as a TFT has been earnestly investigated. Transistors of this kind are formed on a transparent insulating substrate and used either to control each pixel in a display device such as a liquid-crystal display having a matrix structure or to form a driver circuit. Depending on the material or the state of crystallization of the used semiconductor, they are classified as amorphous silicon TFTs or crystalline silicon TFTs.
Generally, amorphous semiconductors have small field mobilities and so they cannot be used in TFTs which are required to operate at high speeds. Accordingly, in recent years, crystalline silicon TFTs have been investigated and developed to fabricate circuits of higher performance.
Since crystalline semiconductors have higher field mobilities than amorphous semiconductors, the crystalline semiconductors are capable of operating at higher speeds. With respect to crystalline silicon, PMOS TFTs can be fabricated, as well as NMOS TFTs. For example, it is known that the peripheral circuit of an active-matrix liquid-crystal display is composed of CMOS crystalline TFTs similarly to the active-matrix circuit portion. That is, this has a monolithic structure.
FIG. 3
is a block diagram of a monolithic active-matrix circuit used in a liquid-crystal display. A column decoder
1
and a row decoder
2
are formed on a substrate
7
to form a peripheral driver circuit. Pixel circuits
4
each consisting of a transistor and a capacitor are formed in a matrix region
3
. The matrix it region is connected with the peripheral circuit by conductive interconnects
5
and
6
. TFTs used in the peripheral circuit are required to operate at high speeds, while TFTs used in the pixel circuits are required to have a low-leakage current. These are conflicting characteristics in terms of physics but it is necessary that these two kinds of TFTs be formed on the same substrate at the same time.
However, all TFTs fabricated by the same process show similar characteristics. For example, TFTs using crystalline silicon fabricated by thermal annealing, TFTs used in the matrix region, and TFTs in the peripheral driver circuit all have similar characteristics. It has been difficult to obtain a low-leakage current suited for the pixel circuits and a high mobility adapted for the peripheral driver circuit at the same time. It has been possible to solve the above difficulty by using thermal annealing and crystallization using selective laser annealing at the same time. In this case, TFTs fabricated by thermal annealing can be used in the matrix region, whereas TFTs fabricated by laser annealing can be employed in the peripheral driver circuit region. However, the crystallinity of silicon crystallized by laser annealing has quite low homogeneity. Especially, it is difficult to use these TFTs in a peripheral driver circuit which is required to be defect-free.
It is also possible to use crystallization relying on laser annealing in order to obtain crystalline silicon. If a semiconductor circuit is fabricated from this silicon crystallized by laser annealing, TFTs in the matrix region and TFTs in the peripheral driver circuit all have similar characteristics. Accordingly, an alternative method of crystallizing silicon may be contemplated. In particular, TFTs in the matrix region are formed, using thermal annealing. TFTs in the peripheral driver circuit are formed, using laser annealing. However, where the thermal annealing is adopted, the silicon must be annealed at 600° C. for as long as 24 hours, or the silicon must be annealed at a high temperature exceeding 1000° C. In the former method, the throughput drops. In the latter method, the material of the usable substrate is limited to quartz.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a method of fabricating a semiconductor circuit without relying on a complex process or without deteriorating the production yield or the cost.
It is another object of the invention to provide a method of easily mass-producing two kinds of TFTs with minimum variations in the process, one of the two kinds being required to have a high mobility, the other being required to have a low-leakage current.
Our research has revealed that addition of a trace amount of a catalytic material to a substantially amorphous silicon film promotes crystallization, lowers the crystallization temperature, and shortens the crystallization time. Examples of the catalytic material include simple substances of nickel (Ni), iron (Fe), cobalt (Co), and platinum, and silicides thereof. More specifically, a film containing such a catalytic element, particles of the element, or clusters of the element are used to form a first film on or under an amorphous silicon film such that the first film is in intimate contact with the amorphous silicon film. Alternatively, such a catalytic element is implanted into an amorphous silicon film by ion implantation or other method. Then, the film is thermally annealed at an appropriate temperature, typically below 580° C., in a short time within 8 hours. As a result, the amorphous film is crystallized.
Where a film is fabricated from such a catalytic element, the concentration of the element is sufficiently low and so the film is quite thin. To form this film, a method using a vacuum pump such as sputtering or vacuum evaporation can be employed. In addition, a method which can be effected under atmospheric pressure such as spin coating or dipping can be utilized. This atmospheric-pressure method is easy to perform and provides high productivity. In this case, an acetate, a nitrate, an organic salt, or the like containing such a catalytic element is dissolved in an appropriate solvent, and the concentration is adjusted to an adequate value.
When the amorphous silicon film is formed by CVD, the catalytic material is added to the raw material gases. When the amorphous silicon film is formed by physical vapor deposition such as sputtering, the catalytic material may be added to the target or evaporation source for forming a film. Of course, as the anneal temperature rises, the crystallization time decreases. Furthermore, as the concentrations of nickel, iron, cobalt, and platinum are increased, the crystallization temperature drops, and the crystallization time is shortened. Our research has revealed that it is necessary that the concentration of at least one of these elements be in excess of 10
17
cm
−3
, in order to promote crystallization. Preferably, the concentration is in excess of 5×10
18
cm
−3
.
Since all of the aforementioned catalytic materials are not desirable for silicon, it is desired that their concentrations be made as low as possible. Our research has shown that the total concentration of these catalytic materials is preferably not in excess of 1×10
20
cm
−3
. Also, local concentrations (e.g., those at grain boundaries) are preferably not in excess of 1×10
20
cm
−3
.
In the present invention, TFTs which operate at high speeds and are used as TFTs for driving a
Takayama Toru
Takemura Yasuhiko
Zhang Hongyong
Booth Richard
Robinson Eric J.
Robinson Intellectual Property Law Office P.C.
Semiconductor Energy Laboratory Co,. Ltd.
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