Semiconductor device manufacturing: process – Coating with electrically or thermally conductive material – Insulated gate formation
Reexamination Certificate
2001-09-05
2004-06-08
Whitehead, Jr., Carl (Department: 2813)
Semiconductor device manufacturing: process
Coating with electrically or thermally conductive material
Insulated gate formation
C438S166000, C438S482000
Reexamination Certificate
active
06746942
ABSTRACT:
This application claims priority from Japanese Patent Application Nos. P2000-269298 filed Sep. 5, 2000, P2000-269274 filed Sep. 5, 2000, and P2000-269261 filed Sep. 5, 2000; the disclosures of which are herein expressly incorporated by reference to the extent permissible by law.
BACKGROUND OF THE INVENTION
The present invention relates to a semiconductor thin film applicable to thin film transistors (TFTs) used for liquid crystal displays, memories, and other electronic devices and a fabrication method thereof; an apparatus for fabricating a single crystal semiconductor thin film; and a method of fabricating a single crystal thin film, a single crystal thin film substrate, and a semiconductor device using the substrate.
As semiconductor thin films such as silicon thin films formed on insulating substrates, there have been known an SOI (Silicon On Insulator) structure, and amorphous silicon thin films or polycrystalline silicon thin films formed on glass substrates which have been practically used for liquid crystal displays.
The SOI structure is often formed by way of various steps including a step of sticking single crystal silicon wafers to each other and a step of polishing them, and since the SOI structure basically uses a single crystal silicon wafer, a substantially perfect single crystal portion of the SOI structure can be typically used for a channel portion of an active device of a thin film transistor (TFT). Accordingly, the device thus fabricated can exhibit good electronic characteristics, for example, a high mobility. The method of fabricating the SOI structure, however, requires various steps, for example, a step of sticking single crystal silicon wafers to each other and a step of polishing them, thereby causing disadvantages that the number of steps is increased to prolong the production time, and also the production cost is raised.
On the contrary, there has been known a method of forming a crystallized silicon thin film by depositing a source gas obtained by mixing hydrogen and SiF
4
to silane gas on a substrate in accordance with a low-pressure CVD process or a plasma CVD process, and a method of forming a crystallized silicon thin film by forming an amorphous silicon thin film as a precursor on a substrate and crystallizing the amorphous silicon thin film. In the former deposition method in which crystallization of silion proceeds along with deposition of the silicon thin film, since the substrate temperature is required to be kept at a relatively high temperature, more specifically, 600° C. or more, the substrate must be made from an expensive material withstanding a high temperature such as quartz. In this method, the use of an inexpensive glass substrate may give rise to a problem that the substrate may be deformed or distorted because of its poor heat resistance. With respect to the latter method, as a process of crystallizing an amorphous silicon thin film formed on a substrate, there has been known a solid-phase growth process of annealing the substrate, on which the amorphous silicon thin film has been formed, for a long time (for example, 20 hr). Such an annealing process, however, has a problem that since it takes a long time, the practical utility is poor and also the production cost is raised. To solve these problems, there has been actively studied and developed a method of crystallizing a non-single crystal thin film by irradiation of laser beams emitted from an excimer laser.
This laser irradiation method involves forming an amorphous silicon thin film or a polycrystalline silicon thin film on a substrate, and heating the thin film by irradiation of laser beams emitted from an excimer laser, thereby crystallizing the thin film. For example, in the case of using an XeCl excimer laser, since an emission wavelength is 308 nm and an absorption coefficient is about 10
6
cm
−1
, the laser energy is absorbed in a region having a depth of about 10 nm from the surface of an amorphous silicon thin film, with a result that the substrate temperature is little raised, and only a portion near the surface of the amorphous silicon thin film is crystallized.
The technique of melting a non-single crystal thin film by irradiation of laser beams emitted from an excimer laser and recrystallizing the melted thin film can grow polycrystalline silicon grains in an amorphous silicon thin film or a polycrystalline silicon thin film; however, it is very difficult to stably control a crystal quality of the thus formed thin film on the basis of the number of shots of laser beams, thereby tending to cause a variation in threshold voltage of a thin film transistor as a final product.
By the way, in the case of using a PECVD (Plasma-Enhanced CVD) system for forming an amorphous semiconductor thin film on a substrate, the film contains hydrogen in an amount of about 2 to 20 atomic %. In this case, the substrate on which the thin film has been formed is put in an electric furnace and is subjected to a degassing treatment, for example, at 420° C. for about 2 hr. The hydrogen concentration in the film is reduced to less than 2 atomic % by the degassing treatment.
Such a degassing (annealing) treatment in an electric furnace for removing hydrogen contained in the film has a problem that since the substrate must be annealed, for example, at 420° C. for about 2 hr, the productivity is degraded, and further, the substrate may be deformed due to heat caused by the degassing treatment and a contaminant from glass may be diffused in the thin film.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a semiconductor thin film which has a performance very higher than that of a related art polycrystalline thin film and is suitable for fabricating a device having stable characteristics and which is fabricated for a sufficiently short time, and a method of and an apparatus for fabricating the semiconductor thin film.
Another object of the present invention is to provide a semiconductor device and a single crystal thin film substrate fabricated by using such a semiconductor thin film having a high crystal quality.
A further object of the present invention is to provide a method of fabricating a semiconductor thin film, which is capable of preventing the explosion of a thin film during fabrication steps and removing hydrogen from the thin film for a short time, and an apparatus for fabricating a single crystal semiconductor thin film.
To solve the above-described problems, the present inventors have found that one of the causes of obstructing the enlargement of sizes of crystal grains in a polycrystalline thin film is dependent on the manner of irradiating the thin film with laser beams, and eventually created an innovative semiconductor thin film quite different from the related art polycrystalline thin film and a fabrication thereof. More specifically, the present inventors have found that a crystallized semiconductor thin film can be formed by crystallizing a non-single crystal thin film by laser irradiation under irradiation conditions such that polycrystalline grains aligned in an approximately regular pattern are formed on the thin film, and heat-treating the thin film with the surface state having projections kept as it is, thereby promoting crystallization of the thin film.
Accordingly, to solve the above-described technical problems, according to a first aspect of the present invention, there is provided a method of fabricating a single crystal thin film, including the steps of: forming a non-single crystal thin film on an insulating base; subjecting the non-single crystal thin film to a first heat-treatment, thereby forming a polycrystalline thin film in which polycrystalline grains are aligned in an approximately regular pattern; and subjecting the polycrystalline thin film to a second heat-treatment, thereby forming a single crystal thin film in which the polycrystalline grains are bonded to each other.
To largely grow a single crystal region in which polycrystalline grains have been bonded to each other, it may be preferred that adjacent polycrystall
Mori Yoshifumi
Nakajima Hideharu
Sakamoto Yasuhiro
Sato Jun-ichi
Usui Setsuo
Schillinger Laura M.
Sonnenschein Nath & Rosenthal LLP
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