Active solid-state devices (e.g. – transistors – solid-state diode – Non-single crystal – or recrystallized – semiconductor... – Field effect device in non-single crystal – or...
Utility Patent
1999-01-20
2001-01-02
Hardy, David (Department: 2815)
Active solid-state devices (e.g., transistors, solid-state diode
Non-single crystal, or recrystallized, semiconductor...
Field effect device in non-single crystal, or...
C257S059000, C257S155000, C257S157000, C257S162000, C257S166000
Utility Patent
active
06169292
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a thin film type monolithic semiconductor device which has a plurality of thin film transistors (TFTs). The TFTs fabricated in the present invention are formed both on insulating substrates such as glass or the like and on semiconductor substrates such as single crystal silicon or the like. More particularly, the present invention relates to a semiconductor circuit which has a low speed operating matrix circuit such as a monolithic active matrix type circuit (which is used in a liquid crystal display or the like) and a high speed operating peripheral circuit to drive the matrix circuit.
2. Description of the Prior Art
In recent years, research has been made on semiconductor devices of insulated gate type, the devices having a thin film-like active layer (which is referred to as an active layer) on an insulating substrate. In particular, efforts are concentrated on studying thin film gate type transistors, or so-called thin film transistors (TFTs). The TFTs are formed on a transparent insulating film, and are used for the control of each pixel and in a driving circuit in a display which is formed of a liquid crystal or the like and which has a matrix structure.
Examples of thin film semiconductors which constitute TFTs include amorphous silicon semiconductors and crystalline semiconductors which are crystallized by heating or laser light irradiation of the amorphous silicon semiconductors. The TFTs using these amorphous silicon thin film and crystalline silicon thin film are referred to as amorphous silicon TFTs and crystalline TFTs.
Generally, the field mobility of semiconductors in an amorphous state is small, and therefore cannot be used in TFTs which are required to be operated at a high speed. Therefore, research and development has been carried out in recent years on crystalline TFTs for the fabrication of circuits which have higher performance.
Crystalline semiconductors have large field mobilities, and therefore can be operated at high speed. Since NMOS TFTs and PMOS TFTs are obtained with the crystalline silicon in the same manner, a CMOS circuit can be formed. For example, in an active matrix type liquid crystal display device, display devices with a monolithic structure in which both an active matrix type part and a peripheral circuit (drivers or the like) are constituted with a CMOS crystalline TFTs are known.
FIG. 3
shows a block diagram of a monolithic active matrix circuit used in a liquid crystal display. In a structure shown in
FIG. 3
, a column decoder
1
and a line decoder
2
are provided on a substrate
7
as a peripheral driver circuit. Further, in a matrix area
3
in which a plurality of pixels are arranged in a matrix configuration, a plurality of pixel circuits
4
which comprise transistors and capacitors are formed so that the matrix area and the peripheral circuit are connected with wires
5
and
6
. The TFTs used in the peripheral circuit are required to be operated at a high speed while the TFTs used in the pixel circuit are required to have low current characteristics. These two characteristics are inconsistent to each other in terms of physics. However, the TFTs used in the peripheral circuit and the TFTs used in the pixel circuit are demanded to be formed on the same substrate at the same time.
However, the TFTs fabricated in the same process all exhibit the same characteristics. For example, means of crystallization (so-called laser anneal) can be used to obtain a crystalline silicon. However, in a silicon which has been crystallized by laser crystallization, the TFTs in the matrix area and TFTs in the peripheral driving circuit area exhibit the same characteristics. Therefore, the low leak current characteristics demanded of the pixel circuit and the high field mobility characteristics demanded of the peripheral circuit can coexist with great difficulty. The present invention is intended to solve such a difficult problem.
SUMMARY OF THE INVENTION
As a result of the investigations by the present inventors, it has been made clear that the crystallization of the silicon is promoted by doping an extremely small amount of a metal material to a silicon film in a substantially amorphous silicon so that the temperature of crystallization can be lowered, and the time required of the crystallization is shortened. As a catalyzing material, one or more kinds of elements selected from Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt, Cu, Ag and Au and further a compound of these elements (for example, a silicide) can be used.
Specifically, films, powders, clusters or the like containing these metal elements are allowed to adhere to the amorphous silicon film. Otherwise, these catalyzing elements are introduced into the amorphous silicon film by a method of ion doping process or the like followed by subjecting the film to the heat treatment at 550° C. or lower about 4 hours.
Quite naturally, there is a relation such that the crystallization time is shorter with higher annealing temperature. In addition, there is also a relation such that the crystallization temperature is lower, and the crystallization time is shorter with a higher concentration of the metal element. The investigation of the present inventors has revealed that the concentration of at least one element out of the aforementioned elements is required to be set to 1×10
16
cm
−3
or more to carry out the crystallization in a manner of thermal equilibrium. Further, it has been also made clear that when the concentration becomes 5×10
19
cm
−3
or more, the physical characteristics as a semiconductor material are lost. Thus, the metal element concentration to accelerate the crystallization of silicon is preferably within a range of 1×10
16
cm
−3
to 5×10
19
cm
−3
. Further, it has been also made clear that use of nickel out of the aforementioned metal elements is the most favorable. Incidentally, the concentration of the impurity in this specification is defined as the minimum value measured with the SIMS (second ion mass spectrometer).
Further, it has been also made clear that a domain (which is referred to as a mono-domain area) with a large grain diameter is obtained by heating a sample at 450° C. or higher at the time of the laser light irradiation in a method for obtaining a crystalline silicon thin film by carrying out the crystallization by irradiating an amorphous silicon film with laser light. This mono-domain area has a crystal structure inside of which can be regarded as a substantially single crystal.
No crystal grain boundary exists inside of the mono-domain area. In addition, the mono-domain has point defects that should be neutralized unlike the single crystal silicon wafer. The mono-domain contains 1×10
16
cm
−3
to 1×10
20
cm
−3
of a recombination center neutralizer such as hydrogen or a halogen element which neutralizes the point defect.
In the case where a metal element such as the aforementioned nickel or the like is introduced into a starting film for forming the aforementioned mono-domain area, a mono-domain area with smaller defect concentration can be obtained. In the case where a thin film transistor is fabricated by using the mono-domain area which is formed by the introduction of this metal element, it is possible to obtain a TFT which has a high field mobility and allows the passage of a larger ON current. In other words, it is possible to obtain a TFT which has characteristics required for arranging the TFTs in a peripheral circuit area of a liquid crystal display with an active matrix structure.
Further, it has been made clear that an amorphous silicon TFT can be sufficient as the TFT which is arranged in each pixel in a matrix area because of the problem of the response speed of the liquid crystal (even when the thin film transistor is operated at any high speed, the liquid crystal cannot follow the speed). Since the OFF current is small instead of the fact that the TFT cannot be operated at a high speed, the amo
Teramoto Satoshi
Yamazaki Shunpei
Costellia Jeffrey L.
Fenty Jesse A
Hardy David
Nixon & Peabody LLP
Semiconductor Energy Laboratory Co,. Ltd.
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