Active solid-state devices (e.g. – transistors – solid-state diode – Non-single crystal – or recrystallized – semiconductor... – Field effect device in non-single crystal – or...
Reexamination Certificate
1997-10-15
2003-07-08
Jackson, Jerome (Department: 2815)
Active solid-state devices (e.g., transistors, solid-state diode
Non-single crystal, or recrystallized, semiconductor...
Field effect device in non-single crystal, or...
C257S066000
Reexamination Certificate
active
06590230
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor device that uses, as an active layer, a semiconductor thin film formed on a base member having an insulating surface and, more specifically, to a thin-film transistor that uses a crystalline silicon film as an active layer.
2. Description of the Related Art
In recent years, the technology for forming a thin-film transistor (TFT) by using a semiconductor thin film (thickness: hundreds to thousands of angstrom) that is formed on a base member having an insulating substrate attract much attention. The thin-film transistor is widely applied to electronic devices such as ICs and electro-optical devices and, in particular, is now being developed rapidly as an switching element of image display devices.
For example, in a liquid crystal display device, it is attempted to apply TFTs to every electric circuit such as a pixel matrix circuit for controlling individual pixel regions that are arranged in matrix form, a driver circuit for controlling the pixel matrix circuit, or a logic circuit (a processor circuit, a memory circuit, or the like) for processing external data signals.
At present, TFTs have been put into practical use that use an amorphous silicon film as an active layer. However, TFTs that use a crystalline silicon film (polysilicon film) are needed for electric circuits that are required to operate even faster, such as a driver circuit and a logic circuit.
The technique disclosed in Japanese Unexamined Patent Publication Nos. Hei. 6-232059 and Hei. 6-244103 is known as a method for forming a crystalline silicon film on a base member. These technique enables formation of a crystalline silicon film that is superior in crystallinity through a heat treatment of 500°-600° C. and about 4 hours by utilizing a metal element (particularly nickel) for accelerating crystallization of silicon.
However, even if a driver circuit is constructed by using such TFTs, it does not completely satisfy the required performance. In particular, it, is still impossible to construct, by conventional TFTs, high-speed logic circuits in which extremely high electrical performance is required to realize high-speed operation and a high breakdown voltage characteristic at the same time.
SUMMARY OF THE INVENTION
Accordingly, to improve the performance of electro-optical devices etc., it is necessary to realize a TFT whose performance is equivalent to that of a MOSFET formed by using a single crystal silicon wafer.
An object of the invention is therefore to provide a thin-film semiconductor device having extremely high performance and a manufacturing method thereof as an breakthrough for enabling further improvement of the performance of electro-optical devices.
As for the reason why the conventional method cannot provide a high-performance TFT as mentioned above, it is considered carriers (electrons or holes) are captured at grain boundaries and, as a result, the field-effect mobility that is one of the parameters indicating the TFT characteristics is prevented from being increased.
For example, dangling bonds of silicon atoms and defect (trap) states exist in a large number at grain boundaries. Carriers traveling through the inside of each crystal are easily trapped by dangling bonds, defect states, or the like when they approach or contact the grain boundaries. Therefore, it is considered that the grain boundaries behave as “malignant grain boundaries” that obstruct carrier movement.
To realize a high-performance semiconductor device as mentioned above, a technique is indispensable that changes the structure of “malignant grain boundaries” to convert them into “benign grain boundaries” for carriers. That is, it can be said that it is important to form grain boundaries at least having a low possibility of capturing carriers, that is, a low possibility of obstructing carrier movement.
Accordingly, the invention provides a manufacturing method of a semiconductor device having an active layer that is a semiconductor thin film, comprising the steps of forming an amorphous silicon film on a base member having an insulating surface; holding a metal element for accelerating crystallization in a given positional relationship with the amorphous silicon film; converting the amorphous silicon film into a crystalline silicon film by a first heat treatment; patterning the crystalline silicon film into an active layer; forming a gate insulating film on the active layer; performing a second heat treatment in an atmosphere containing a halogen element, to thereby remove the metal element from the active layer by gettering and to form a thermal oxidation film at an interface between the active layer and the gate insulating film; and performing a third heat treatment in a nitrogen atmosphere, to thereby improve a film quality and an interface state of the gate insulating film including the thermal oxidation film, wherein grain boundaries in the active layer have directivity and the active layer is a collection of a plurality of needle-like or columnar crystals extending generally parallel with the base member.
If a crystalline silicon film is formed according to the above manufacturing method, a thin film is formed that has an appearance as shown in
FIG. 13
, which is a microscope photograph of a crystalline silicon film as enlarged. As seen from
FIG. 13
, the crystalline silicon film is a collection of a plurality of crystal grains having as large diameters as tens of micrometers to a little larger than 100 &mgr;m. This manufacturing method utilizes, as a means for crystallizing an amorphous silicon film, the technique disclosed in Japanese Unexamined Patent Publication No. Hei. 6-232059.
FIG. 14
is a TEM photograph of a minute region as enlarged of the inside of a crystal grain, which was taken to scrutinize the inside of individual crystal grains shown in FIG.
13
.
That is, the crystalline silicon film according to the invention macroscopically appears like a collection of large grains as shown in
FIG. 13
, actually its inside is a crystal structural body as a collection of a plurality of needle-like or columnar crystals
1401
as shown in FIG.
14
.
In
FIG. 14
, reference numeral
1402
denotes grain boundaries, i.e., boundaries between the needle-like or columnar crystals
1401
. It is seen from the extending direction of the grain boundaries
1402
that the needle-like or columnar crystals
1401
grew generally parallel with each other. In this specification, the term “grain boundaries” means boundaries between needle-like or columnar crystals unless otherwise specified.
In the active layer of the semiconductor device according to the invention, the metal element (principal example: nickel) for accelerating crystallization is gettering-removed by the heat treatment in the atmosphere containing a halogen element, so that the metal element that previously remained at a concentration higher than 1×10
18
atoms/cm
3
is reduced to lower than or equal to 1×10
18
atoms/cm
3
, typically 1×10
14
to 5×10
17
atoms/cm
3
(preferably lower than the spin density). Moreover, the phosphorous gettering method may be used for reducing the concentration of metal element in the semiconductor layer. The technique disclosed in U.S. patent publication Ser. No. 08/623,336 and Japanese Unexamined Patent Publication Hei. 8-340127 by Yamazaki et al. is known as a method for removing the metal element from the crystalline silicon.
It is naturally considered that other metal elements such as Cu, Al, etc. that were introduced by contamination or the like (that is, not introduced intentionally) are also removed by gettering.
At this time, it is expected that dangling bonds of silicon atoms connect to oxygen atoms during the heat treatment, to form the oxide (silicon oxide). It is considered that, as a result, silicon oxide is formed in the regions that were previously “malignant grain boundaries” and substantially functions as grain boundaries.
It is presumed that the thus-formed grain boundaries
1402
are such t
Fukunaga Takeshi
Koyama Jun
Ohtani Hisashi
Yamazaki Shunpei
Fish & Richardson P.C.
Jackson Jerome
Semiconductor Energy Laboratory Co,. Ltd.
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