Method of producing a semiconductor device

Details Method of producing a semiconductor device Method of producing a semiconductor device

2000-10-31

2002-08-20

Everhart, Caridad (Department: 2825)

Semiconductor device manufacturing: process

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

On insulating substrate or layer

C438S471000, C438S486000, C438S765000

Reexamination Certificate

active

06436745

ABSTRACT:

BACKGROUND OF THE INVENTION
This invention relates to semiconductor devices such as a thin film transistor (hereinafter referred to as TFT) used for, for example, driving an active matrix liquid display device, and more particularly to a method of producing a semiconductor device having a silicon film crystallized using a so-called catalytic metal element.
Among thin-type, low power consumption liquid crystal display devices, those devices using TFTs for drive elements ensure high performance such as high contrast and high response speed. Therefore, these devices are mainly used for display portions of personal computers (PCs), portable televisions (TVs) and the like, and thus the market scale of TFTs has been expanding markedly.
Some of the TFTs use a CGS (Continuous Grain Silicon) film as a semiconductor for a channel region. As described in JP-A-6-244103, the CGS film is an Si film having excellent crystallinity, which is obtained by depositing a minute amount.of a certain metal element such as Ni on the surface of an amorphous silicon (hereinafter abbreviated to a-Si) film and then conducting a heat treatment thereof. The CGS film ensures low-power consumption and high-speed response compared with the conventional a-Si and polycrystalline silicon (hereinafter abbreviated to p-Si) films. Further, the CGS film has the advantage in that the utilization of its high mobility also permit fabrication of future sheet computers. Thus, the CGS film has been regarded as a promising film that can be used for next-generation liquid crystal display devices.
Incidentally, a CGS film obtained by the above fabrication steps contains atoms of a metal element promoting crystallization. When fabricating a TFT using the CGS film containing the metal element, the metal element acts as an impurity in Si that forms a channel region of the TFT, resulting in the occurrence of energy levels in Si. Therefore, some serious problems such as a change in the threshold voltage of the TFT with time or an increase in OFF current are caused.
In order to solve such problems, a method of removing the metal element is disclosed in JP-A-10-223533. In JP-A-10-223533, parts of the CGS film fabricated are doped with phosphorus (P), a group V element, at a high concentration and then subjected to heat treatment. Thereby, the metal element is gettered to the parts doped with P of the CGS from a region that will become a channel portion of the TFT.
Incidentally, in the method of producing a semiconductor device using the gettering method disclosed in JP-A-10-223533, the element P is selectively introduced into the CGS film and therefore it is necessary to form a mask on the CGS film. Thus, a photolithography step for forming the mask is required. As a result, there is a problem that the number of steps is increased, resulting in an increase in production cost.
Further, after the gettering of the metal element from the channel regions, because the P-doped regions contain the gettered metal element, those regions cannot be used for the fabrication of devices and thus must be removed. As a result, there occurs a limitation in the layout of semiconductor devices such as picture elements, driver elements and the like on the substrate. Accordingly, there occurs a problem that the area of the CGS film required for the production of such semiconductor devices increases, resulting in an increase in the size of the resultant apparatus incorporating those semiconductor devices.
SUMMARY OF THE INVENTION
This invention was made in view of the above problems, and an object of the invention is to provide a method of producing a semiconductor device which method performs the gettering without using any masks to thereby reduce production cost and allows reduction of the size of an apparatus incorporating the semiconductor devices produced by this method.
In order to accomplish the above object, a method of producing a semiconductor device according to the present invention comprises the steps of:
crystallizing an amorphous silicon film or a partially crystalline amorphous silicon film using a catalytic metal element promoting crystallization of silicon to form a crystalline first silicon film;
forming a second silicon film containing a group V element directly on an entire surface of the first silicon film;
subjecting the first silicon film and the second silicon film to a heat treatment to thereby getter the catalytic metal element from the first silicon film to the second silicon film; and
removing the second silicon film to which the catalytic metal element has been gettered.
In the method of producing a semiconductor device of this invention, the gettering of the catalytic metal element from the crystalline first silicon film is performed using the second silicon film directly formed on the whole surface of the first silicon film, and not using parts of the first silicon film itself. That is, the method of the invention does not involve selective injection of the group V element into the first silicon film in the gettering process. Therefore, the gettering process requires no mask for selective injection of the group V element and hence no photolithography step for forming a mask. Therefore, the fabrication steps for a semiconductor device are simplified, whereby the production cost can be reduced.
Further, after removing the second silicon film to which the catalytic metal element has been gettered from the first silicon film, the catalytic metal element as an impurity is substantially not present in the first silicon film, and thus the first silicon film does not include any unusable region. Accordingly, there is no limitation in the layout of devices, such as picture elements and driver elements, so that the size of an apparatus incorporating these semiconductor devices can be reduced.
However, there are two possible problems that are inherent to the method of producing a semiconductor device in this invention, i.e., (A) diffusion of atoms of the group V element into the first silicon film, (B) method of removing the second silicon film containing the group V element. Solutions to these problems will be described below.
(A) Solution to the Diffusion of Group V Element Into the First Silicon Film
In the method of producing a semiconductor device according to this invention, there is a possibility that the heat treatment of the first and second silicon films causes the group V element atoms in the second silicon film to migrate or move to the first silicon film. The group V element that has moved into the first silicon film acts as an impurity. Therefore, when fabricating a TFT as the semiconductor device using the first silicon film containing the group V element, the first silicon film containing the group V element adversely affects the properties of the TFT.
In order to prevent the group V element from diffusing into the first silicon film, the difference in diffusion constant between the group V element and the catalytic metal element is utilized. It is known that the diffusion constant within a silicon film of the group V element greatly differs from that of the catalytic element. For example, phosphorus (P) and nickel (Ni) are now selected as representatives of the group V element and the catalytic metal element, respectively. As the catalytic metal element, at least one element selected from a group consisting of Fe, Co, Ni, Cu, Ru, Rh, Pd, Os, Ir, Pt and Au may be used.
In general, the diffusion constant of P within the silicon film is calculated to be 1.47×10
−27
cm
2
/sec. at 400° C., and 2.80×10
−21
cm
2
/sec. at 600° C., while the diffusion constant of Ni within the silicon film is calculated to be 5.84×10
−16
cm
2
/sec. at 400° C., and 1.06×10
−12
cm
2
/sec. at 600° C. As described above, it turns out that, within the silicon film, the diffusion constant of Ni is larger than that of P by about 10 digits.
Based on the diffusion constants of P and Ni, two simulations are tried: (1) diffusion of P into the first silicon film; (2) gettering of Ni to the second silicon

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