Active solid-state devices (e.g. – transistors – solid-state diode – Non-single crystal – or recrystallized – semiconductor... – Amorphous semiconductor material
Reexamination Certificate
1999-10-26
2001-10-30
Ngô ;, Ngâ ;n V. (Department: 2814)
Active solid-state devices (e.g., transistors, solid-state diode
Non-single crystal, or recrystallized, semiconductor...
Amorphous semiconductor material
C257S072000, C257S350000, C257S408000
Reexamination Certificate
active
06310362
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Industrial Field of the Invention
The present invention relates to a process for fabricating an insulated gate-structured semiconductor device such as a thin film transistor (TFT) or a thin film diode (TFD), comprising a non-single crystal silicon film formed on an insulating substrate such as a glass substrate or on an insulating film formed on various type of substrate. The present invention also relates to a process for fabricating a thin film integrated circuit (IC) to which TFT or TFD is applied, and more particularly, to a thin film integrated circuit (IC) for an active-matrix type liquid crystal displaying unit.
2. Prior Art
Semiconductor devices developed heretofore comprising TFTs on an insulating substrate (such as a glass substrate) include an active matrix-addressed liquid crystal display device whose pixels are driven by TFTs, an image sensor, or a three-dimensional integrated circuit.
The TFTs utilized in those devices generally employ a thin film non-single crystal silicon semiconductor. The thin film non-single crystal semiconductors can be roughly classified into two; one is a type comprising amorphous silicon semiconductor (a—Si), and the other is a type comprising crystalline silicon semiconductors. Amorphous silicon semiconductors are most prevailing, because they can be fabricated relatively easily by a vapor phase process at a low temperature, and because they can be readily obtained by mass production. The physical properties thereof, such as electric conductivity, however, are still inferior to those of a crystalline silicon semiconductor. Thus, to implement devices operating at an even higher speed, it has been keenly demanded to establish a process for fabricating TFTs comprising crystalline silicon semiconductors. Known crystalline semiconductors suitable for the purpose like this include polycrystalline silicon, microcrystalline silicon, amorphous silicon partly comprising crystalline components, and semiamorphous silicon which exhibits an intermediate state between crystalline silicon and amorphous silicon.
Known process for fabricating crystalline thin film silicon semiconductors includes depositing an amorphous semiconductor film by plasma CVD or low pressure CVD, and applying thereto thermal energy for a long duration of time (i.e., thermal annealing) for crystallization.
In general, silicon semiconductors need to be heated to a temperature of 600° C. or higher. More preferably, heating at 640° C. or higher is necessary to further enhance the crystal growth. However, such a high temperature heating has a problem of thermally influencing the substrate. Furthermore, since the heating time required for crystallization was several tens hours or longer, productivity was low. Therefore, it has been demanded to lower the heating temperature and shorten the heating time.
As a means to overcome the aforementioned problems, a process for crystallizing the film by increasing the surface temperature of the film to substantially 800° C. or higher has been developed. The process comprises irradiating an intense light such as an infrared radiation or a visible light for a duration of about 10 to 1,000 seconds to the surface of the film. This process, which is called as lamp annealing or rapid thermal annealing (RTA), is expected to be process for reducing the influence on substrates, because the duration of heating can be extremely shortened.
However, since the film formed by plasma CVD and low pressure CVD contains a lot of hydrogen combined with silicon, the decomposition reaction of hydrogen is mainly caused by RTA owing to the short time of RTA, that is, the crystallization does not sufficiently proceed. Furthermore, there is a problem that hydrogen is ejected to the exterior of the film by the decomposition reaction of hydrogen to degrade the morphology of the film surface. The present invention has been accomplished in the light of the above circumstances. Accordingly, an object of the present invention is to provide a silicon film suitable for forming a semiconductor device and having a sufficiently high crystallinity.
SUMMARY OF THE INVENTION
The process according to the present invention comprises a first step of forming a non-single crystal semiconductor film on a glass substrate and crystallizing the non-single crystal semiconductor film by a thermal annealing and the like to eject hydrogen from the non-single crystal semiconductor film and a second step of heating the non-single crystal semiconductor film by irradiating an intense light thereto (RTA process). Another step of forming, on the surface of said silicon film, an insulating coating which absorbs less than 10% of the intense light used in the second step may be incorporated between the first and the second steps. It is preferred in the present invention that the silicon film obtained by the first step has a low degree of crystallinity, more specifically the degree of crystallinity is 1 to 50%, more preferably 1 to 10%. The first step can be carried out by thermal annealing or other crystallizing methods.
There may be provided a step of patterning the silicon film into at least one island by etching.
As a substrate of this invention, it is preferable to utilize a glass substrate with strain point from 550° C. to 680° C. Specifically, No. 7059 of Corning Co. (strain point 593° C.), No. 1733 of the same (strain point 640° C.), LE30 of HOYA Co. (strain point 625° C.), NA35 (strain point 650° C.) of NH Technoglass Co., NA 45 (strain point 610° C.) of NH Technoglass Co., E-8 of OHARA Co. (strain point 643° C.), OA-2 of Nihon Denki Glass Co. (strain point 625° C.), AN1 (strain point 625° C.) of Asahi Glass Co., AN2 (strain point 625° C.) of Asahi Glass Co. and the like are desirable. However, a glass substrate other than above mentioned can be utilized, too.
Moreover, an insulating film such as silicon oxide, silicon nitride, or aluminum nitride can be formed on the surface of the glass substrate and an amorphous silicon film can be formed on it. In the case that a film of a material with high heat conductivity like aluminum nitride is formed on a glass substrate, the second step above mentioned can be omitted.
In case that the crystallizing in the first step above mentioned is carried out by thermal annealing, temperature and time of the thermal annealing is varied according to the thickness, composition and the like of the semiconductor thin film. In the case of substantially intrinsic silicon semiconductor, 520 to 620° C., for example, 550 to 600° C., and 1 to 4 hours is appropriate. It is preferable if the thermal annealing is performed at a temperature lower than the strain point of the glass substrate.
In addition, it is especially preferable if silicon ions have been implanted in the silicon film by an ion implanting method at a dose of 1×10
14
to 1×10
16
cm
−2
before the above-mentioned thermal annealing, because crystal growth by thermal annealing is suppressed.
Metal element promoting crystallization of the silicon film, e.g. nickel and the like, can be included in the silicon film obtained by the above-mentioned first step of the present invention. As a metal element other than nickel which can promote crystallization like this, Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt, Sc, Ti, V, Cr, Mn, Cu, Zn, Au, and Ag is known. In an amorphous silicon film added with these elements, crystallization proceeds enough even by low temperature short time thermal annealing of 520 to 620° C. and 1 to 4 hours. Crystal growth by RTA later has no effect if the crystallization proceeds excessively. Therefore, in the case of adding these metal elements, it is desirable if time of thermal annealing is shorter, or temperature of thermal annealing is lower, than that of the substantially intrinsic silicon film.
If these metal elements are included in the silicon film, it is also possible to crystallize the silicon film at a lower temperature in the process of RTA later. These metal elements also have an effect of promoting elimination (ejection) of hydrogen from the
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