Semiconductor device manufacturing: process – Making field effect device having pair of active regions... – On insulating substrate or layer
Reexamination Certificate
2001-01-25
2004-08-24
Niebling, John (Department: 2812)
Semiconductor device manufacturing: process
Making field effect device having pair of active regions...
On insulating substrate or layer
C438S151000, C438S161000, C438S164000, C438S166000
Reexamination Certificate
active
06780687
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor device using a semiconductor thin film and its manufacturing method and, in particular, to a thin film transistor (hereinafter referred to as TFT) using a crystalline semiconductor thin film containing silicon.
2. Description of the Related Art
In recent years, a technology for forming a TFT on a glass substrate or the like to constitute a semiconductor circuit has been rapidly advanced. As a typical semiconductor device, there is an active matrix type liquid crystal display (hereinafter referred to as AMLCD) integrated with a driver. The AMLCD integrated with a driver is a monolithic display device having a pixel section and a driver circuit on the same substrate. Further, a system-on panel in which a memory circuit, a clock generating circuit, and the like are built has been developed.
While a TFT in which an amorphous silicon (hereinafter referred to as a-Si) is used in an active layer is employed as a switching device of a pixel in a conventional AMLCD, in the peripheral circuit of the AMLCD integrated with a driver is mainly employed a TFT in which a polycrystalline silicon (hereinafter referred to as poly-Si) having a higher field effect mobility is used as the active layer because the a-Si is not suitable for the peripheral circuit which is required to operate at high speeds.
A conventional poly-Si TFT has a higher field effect mobility than an a-Si TFT. When a variety of circuits are mounted on a system-on panel, however, because the circuits are required to operate at higher speeds, the TFTs employed by the circuits are required to have a field effect mobility higher than the AMLCD integrated with a driver.
Also, even in the AMLCD integrated with a driver, TFTs having a higher field effect mobility are required because it is required to operate at high speeds due to an increase in the number of pixels and to reduce the area of a driver circuit.
Chief among factors determining the field effect mobility of the TFT is a surface dispersion effect. The flatness of an interface between the active layer of the TFT and a gate insulating film produces a large effect on the field effect mobility of the TFT, and as the interface becomes flatter, the effect of the surface dispersion becomes less and thus the field effect mobility becomes larger.
One of currently available methods of producing a crystalline silicon film is a laser crystallization method, and a method of applying an excimer laser to an amorphous silicon film to crystallize it has been known. An amorphous silicon film having a thickness of 10 nm to 150 nm (typically 30 nm to 60 nm) is formed on an insulating substrate by sputtering, CVD, or the like and subsequently irradiated with excimer laser light, whereby the amorphous silicon film is melted, solidified, and crystallized. In the case where the amorphous silicon film contains about 5% or more hydrogen, it is previously dehydrogenated by a heat treatment performed at temperatures of about 400° C. to 500° C. for several hours because hydrogen is explosively removed when it is annealed by a laser.
While the conditions of the laser crystallization are selected by an operator, when the excimer laser is employed, for example, a laser pulse oscillation frequency is 30 Hz and a laser energy density is 100 mJ/cm
3
to 500 mJ/cm
3
(typically, 300 mJ/cm
3
to 400 mJ/cm
3
). A linear laser beam is applied to the whole surface of the substrate, wherein the overlapping ratio of the linear beam is 80% to 98%.
Protrusions (or bumps) called ridges are formed at random on the surface of the film crystallized in this way by the laser beam. It is thought that the protrusions are produced by surface tension waves induced on the surface of Si annealed and melted by the laser. Typically, the protrusions have a thickness two times the thickness of the thin film. The thickness of the thin film is usually 30 nm to 60 nm and thus the protrusions have a height of 30 nm to 60 nm from the surface of the film. The protrusions formed in this way give dispersion to the movements of electrons or holes due to the above-mentioned surface dispersion effect to reduce the field effect mobility of the TFT. The larger the protrusions are, the larger the effects are.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a technology for controlling the arrangement of the protrusions described above to reduce the effect of a surface dispersion on an electric current.
The present invention produces a crystalline semiconductor thin film by melting and solidifying a non-single crystalline semiconductor film such as amorphous, microcrystalline or polycrystalline semiconductor thin film by the use of strong light such as a laser or the like, and is characterized in that protrusions existing on the surface of the crystalline semiconductor thin film are aligned in parallel to the direction of length of a channel, that is, the direction in which an electric current flows to thereby produce an electric current path which is not affected by the surface dispersion caused by the protrusions.
In FIGS.
1
(A) and
1
(B) and FIGS.
2
(A) and
2
(B) are shown the conceptional view of the present invention. FIGS.
1
(A) and
1
(B) are a schematic view of the surface of a semiconductor thin film produced by crystallizing an amorphous silicon film by a conventional laser crystallization technology, whereas FIGS.
2
(A) and
2
(B) are a schematic view of the surface of a semiconductor thin film produced by the present invention, the respective figures showing a schematic view of a channel portion of a TFT. In the conventional technology, since protrusions
1001
are formed at random on the surface of a semiconductor thin film
1003
of a substrate
1004
, a plurality of protrusions interfere with an electric current path
1002
to reduce mobility by the effects of surface dispersion. In the present invention, since the protrusions
1005
are aligned in parallel to an electric current path
1006
, current paths which do not cross the protrusions
1005
are predominant. Such current paths are not affected by the surface dispersion and hence produce high field effect mobility. In other words, the arrangement of the protrusion is such that regions having a larger number of protrusions and regions having no or less number of protrusions appear in turn in a direction orthogonal to the electric current path
1006
. The effect of the present invention may be obtained even if this direction is slightly deviated from the ideal direction, for example, within a range of ±30°, preferably ±15°.
Further, also in the case where a crystalline thin film produced by a thermal crystallization technology or the like is melted at least partly and solidified by strong light such as a laser or the like for the purpose of improving its properties, protrusions are produced as is the case where an amorphous film is melted, solidified and crystallized by strong light such as a laser or the like. In this case, the case where the protrusions existing in the crystalline thin film melted, solidified and re-crystallized are aligned in parallel to the direction in which an electric current flows to produce an electric current path not affected by the surface dispersion effect of the protrusions is also included in the present invention. The essential object of the present invention resides in intentionally aligning protrusions, which are produced when a thin film containing silicon is melted and solidified, in an objective direction, and the present invention is not limited by the property or the sort of a starting film.
A mechanism of producing the protrusions produced when a thin film containing silicon is melted and solidified by strong light such as a laser or the like has not yet completely clarified until now. It is considered true as described above, however, that the protrusions are caused by surface waves produced when the thin film is melted. The present inventor has tried controlling the positions of the pr
Kawasaki Ritsuko
Nakajima Setsuo
Isaac Stanetta
Niebling John
Semiconductor Energy Laboratory Co,. Ltd.
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