Semiconductor device manufacturing: process – Formation of semiconductive active region on any substrate – Amorphous semiconductor
Reexamination Certificate
2000-05-23
2002-09-24
Sherry, Michael (Department: 2829)
Semiconductor device manufacturing: process
Formation of semiconductive active region on any substrate
Amorphous semiconductor
C438S479000
Reexamination Certificate
active
06455401
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of fabricating a semiconductor device produced by depositing a non-monocrystal silicon (called Si hereinafter) film on an insulating film formed on an insulating substrate made of glass or the like or a variety of substrates. For example, the method is suitable for making a thin film transistor (TFT), a thin film diode (TFD) and a thin film integrated circuit containing transistors and diodes, especially a thin film integrated circuit for an active liquid crystal display (LCD).
2. Description of the Related Art
Recently, a semiconductor device in which TFTs are mounted on an insulating substrate made of glass or the like, for example an active liquid crystal display and an image sensor in which TFTs are used for activating pixels. has been developed.
Generally, thin film Si semiconductors are used for the TFTs used in the above-mentioned devices. The above-mentioned thin film Si semiconductor comprises two types of semiconductors, these being an amorphous Si semiconductor (a-Si) and a crystalline Si semiconductor. The amorphous Si semiconductor is used most generally because it can be readily produced at low temperatures by a vapor phase process, and is suitable for mass production; however, it is less conductive than a crystalline Si semiconductor.
Therefore, it is strongly desired that a fabricating method of TFTs made of a crystalline Si semiconductor should be established which can hereinafter obtain a higher speed characteristics. As a crystalline Si semiconductor, polycrystalline silicon, microcrystalline silicon, amorphous silicon containing a crystalline component and a semi-amorphous silicon which is in an intermediate condition of a crystal and an amorphous are well known.
As the production method of a thin film crystalline Si semiconductor. the following methods are known:
(1) Crystalline film is deposited directly.
(2) First, amorphous semiconductor film is deposited and next, it is crystallized by the energy of a laser beam.
(3) First, amorphous semiconductor film is deposited and next, it is crystallized by applying thermal energy for a long time (annealing).
However, even formation of a film which has a good semiconductive characteristic on the overall surface of a substrate by the method described in (1) is technically difficult. This method also has a cost problem in that a low-priced glass substrate cannot be used because the film is formed at a temperature of 600° C. or more. Deposition of a film which has a good characteristic at low temperatures by this method is difficult. As crystals grow perpendicularly to the substrate, film formed by this method is not suitable for TFTs which have flat conductivity.
For example, if an excimer laser used most generally at present is used in the method described in (2), this method has the problem that throughput is low because the area on which the laser beam is radiated is small. This method also has another problem in that the stability of the laser is not sufficient to form film evenly on the overall surface of a substrate having a large area. Further, this method requires a substrate to be heated and irradiated by a laser in a vacuum in order to crystallize well. Therefore, this method has the problem that throughput is limited.
The method described in (3) has the advantage that a substrate having a large area can be processed by this method compared with the methods described in (1) and (2). However, this method also requires high temperatures of 600° C. or more to heat a substrate on which amorphous film is formed. If a low-priced glass substrate is used, the heating temperature must be lowered. Especially at present, LCD screens are becoming larger and larger and therefore, a large-sized glass substrate is required to be used for such large screens. If a large-sized glass substrate is used, a significant problem of shrinkage or distortion caused in the heating process essential for producing semiconductor devices and which deteriorates the precision of mask alignment occurs. Especially if a substrate made of No. 7059 glass manufactured by Corning Inc. used most generally at present is used, distortion occurs at a temperature of 593° C. and significant deformation occurs in the prior crystallizing process due to heating. The heating time required for crystallization in the conventional process exceeds 20 to 30 hours and therefore, reduction of the time is also required together with reduction of the heating temperature.
SUMMARY OF THE INVENTION
The object of the present invention is to provide a means of solving the above-mentioned problems. In detail, the object is to provide a process in which good crystallization is obtained at lower heating temperatures, in other words, the effect on the glass substrate is reduced where a method by which film made of amorphous silicon is crystallized by heating is used.
Another object of the present invention is to provide a means of reducing or removing a metal element (a catalytic metal element) added to silicon film to promote crystallization.
DETAILED DESCRIPTION OF THE INVENTION
In the first process according to the present invention, a crystallized silicon film is heated selectively by irradiating strong light on the surface of a Si film crystallized by a metal element which promotes crystallization in an ambient atmosphere containing chloride gas such as hydrogen chloride (HCl), carbon tetrachloride (CCl
4
) and silicon tetrachloride (SiCl
4
), or fluoride gas such as nitrogen trifluoride (NF
3
) and dicarbon hexafluoride (C
2
F
6
) to 10 to 90%. Producing plasma by excitation of a microwave or a high frequency has an effect of promoting the reaction during irradiation with strong-light.
If strong beams, for example light between near infrared rays and visible light, preferably light from 0.5 to 4 &mgr;m in wavelength (for example infrared rays with peaks at 1.3 &mgr;m in wavelength) are irradiated according to the present invention, it is desirable that beams are irradiated only for a relatively short time of approx. from 10 to 1000 seconds and that the surface of silicon film is heated until it is at from 900 to 1200° C. As the light in the above-mentioned wavelength is absorbed by a silicon film and is not absorbed by a substrate substantially, selective heating of the Si film is enabled without having an effect on a substrate if beams are irradiated only for the above-mentioned time.
Visible light, especially light in wavelength of 0.5 &mgr;m or less is absorbed well by intrinsic or substantially intrinsic amorphous silicon and can be converted to heat. Near infrared rays or visible radiations from 0.5 to 4 &mgr;m in wavelength are absorbed effectively by intrinsic or substantially intrinsic crystallized silicon film in which phosphorus or boron is contained only to 10
17
cm
−3
or less and can be converted to heat. On the other hand, far infrared radiation 10 &mgr;m or more in wavelength is absorbed by a glass substrate and can be converted to heat. However, if most light are light 4 &mgr;m or less in wavelength, little light is absorbed by glass. That is, near infrared rays or visible radiations from 0.5 to 4 &mgr;m in wavelength are suitable for heating crystallized Si film formed on a glass substrate selectively.
If ultraviolet rays of which wavelength is shorter than the above-mentioned light are used, they are absorbed not only by Si film but by most substrate materials and therefore, the most suitable time for irradiating light should be shorter. For example, in case of light 248 nm in wavelength, it is desirable that the above-mentioned time is 1 &mgr;sec. or less. If the above-mentioned light is irradiated for a longer time than the above-mentioned time, much light is absorbed by the substrate, which causes deformation of the substrate. As described above, the amount of light must be selected so that the temperature on the surface of the Si film exceeds 1000° C. temporarily by irradiation of light for an extremely short time. The first irradiation cannot
Ohnuma Hideto
Takemura Yasuhiko
Zhang Hongyong
Pert Evan
Robinson Eric J.
Robinson Intellectual Property Law Office PC
Semiconductor Energy Laboratory Co,. Ltd.
Sherry Michael
LandOfFree
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