Active solid-state devices (e.g. – transistors – solid-state diode – Non-single crystal – or recrystallized – semiconductor... – Amorphous semiconductor material
Reexamination Certificate
1999-04-15
2002-04-23
Prenty, Mark V. (Department: 2822)
Active solid-state devices (e.g., transistors, solid-state diode
Non-single crystal, or recrystallized, semiconductor...
Amorphous semiconductor material
C257S066000, C257S347000
Reexamination Certificate
active
06376860
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor device having a TFT (thin film transistor) provided on an insulating substrate such as glass, and a method of manufacturing the semiconductor device.
2. Discussion of the Related Art
As the semiconductor having the TFT formed on the insulating substrate made of glass or the like, there have been known an active liquid crystal display device, an image sensor and the like, which use the TFT for driving a pixel.
A thin film silicon semiconductor is generally used for the TFT used in these devices. The thin film silicon semiconductor is roughly classified into the amorphous silicon semiconductor (a-Si) type and the crystalline silicon semiconductor type. The amorphous silicon semiconductor is most generally used because the manufacturing temperature is low, it can be relatively readily manufactured by a vapor phase method, and the mass productivity is sufficient. However, since the physical properties of the amorphous silicon semiconductor is inferior to the crystalline silicon semiconductor such as the electrical conductivity or the like, there is a strong demand to establish a method for manufacturing the TFT formed of the crystalline silicon semiconductor for the purpose of obtaining the higher speed characteristics in the future. As the crystalline silicon semiconductor, there have been known non-single crystalline silicon semiconductors such as polycrystalline silicon, microcrystalline silicon, amorphous silicon containing crystal components, semi-amorphous silicon having an intermediate state between the crystal property and the amorphous property, and the like. Hereinafter, the non-single crystalline silicon semiconductors having these crystal properties are called a crystalline silicon.
As a method of obtaining the thin film silicon semiconductors with these crystal properties, there have been known the following methods.
(1) A crystalline film is directly formed at the time of film formation.
(2) The energy of a laser illumination is applied to an amorphous semiconductor film which has been previously formed to provide the crystal property.
(3) A heat energy is applied to an amorphous semiconductor film which has been previously formed to provide the crystal property.
However, in the method (1), it is technically difficult to uniformly form a film having the excellent semiconductor physical properties all over the upper surface of a substrate. Further, since the film forming temperature is high, that is, 600° C. or more, there rises such a problem in costs that an inexpensive glass substrate cannot be used. In the method (2), in the case of an example of an excimer laser which is most generally used now, there rises a problem that a through-put is low because a laser beam radiated area is small. Furthermore, the stability of the laser beam is insufficient to uniformly treat the entire upper surface of a large-area substrate, whereby it strongly seems as if this method is the technique for the coming generation. In the method (3), there is advantageous in that this method can cope with the large-area of the substrate in comparison with the methods (1) and (2). However, a high temperature of 600° C. or more is required as a heating temperature, and taking the inexpensive glass substrate used into consideration, it is necessary to further decrease the heating temperature. In particular, the current liquid-crystal display unit advances to a large screen, and for that reason, it is necessary to use a large-scale glass substrate likewise. When such a large-scale glass substrate is used, there rises a serious problem that the contraction or strain of the substrate in the heating process essential to the semiconductor manufacture makes the accuracy in mask matching or the like deteriorate. In particular, in the case of the 7059 glass which is most generally used now, the temperature of the strain point is 593° C., whereby the conventional heat crystallization method causes the substrate to be largely deformed. Moreover, in addition to the temperature problem, since the current process requires heating time of several tens hour or more necessary for crystallization, it is also necessary to shorten the heating time.
SUMMARY OF THE INVENTION
The present invention has been made to eliminate the above-mentioned problems, and an object of the invention is to provide a process of lowering a temperature necessary for crystallization and reducing a time therefor in a method of manufacturing a thin film formed of a crystalline silicon semiconductor by application of a method of crystallizing a thin film formed of an amorphous silicon by heating. The crystalline silicon semiconductor prepared by a process of the present invention has the physical properties not lower than those prepared by the conventional process, and applicable even to the active layer region of a TFT.
The inventors have conducted the following experiments in the above-mentioned method of forming an amorphous silicon semiconductor film by the CVD method or the sputtering method to crystallize the film thus formed by heating, and considered the experiment result.
First, the mechanism of forming the amorphous silicon film on a glass substrate to crystalize the film by heating has been investigated. As a result, it has been observed that the crystal growth started from an interface between the glass substrate and the amorphous silicon, then developed into the columnar shape perpendicular to the front surface of the substrate when it has the thickness of a certain degree.
It is considered that the above-mentioned phenomenon is caused by the fact that a crystalline nucleus forming a base of the crystal growth (the source forming a base of the crystal growth) exists in the interface between the glass substrate and the amorphous silicon film and the crystal grows from the nucleus. Such a crystalline nucleus is considered to be of a bit of impure metallic element which exists on the surface of the substrate or the crystalline component of the glass surface (it is considered that the crystalline component of silicon oxide exists on the surface of the glass substrate as called the crystallized glass).
Therefore, it was considered that the temperature of crystallization can be lowered by more positively introducing the crystalline nucleus, and for the purpose of confirming the effect, a bit of other metals was formed on the substrate, and a thin film of the amorphous silicon was then formed thereon. Thereafter, the experiment of crystallization by heating was conducted. As a result, it was confirmed that, in the case of forming several metals on the substrate, the temperature of crystallization was lowered, and it was expected that there occurred crystal growth which had the foreign matter as the crystalline nucleus. Therefore, the mechanism of a plurality of impurity metals which could lower the temperature has been investigated in more detail.
The crystallization can be classified into two stages, that is, an initial nucleus production and the crystal growth from the nucleus. The speed of the initial nucleus production was observed by measuring a time until fine crystals occurred in the dot pattern at a given temperature. That time was reduced in any cases of the thin films forming the above impurity metals, and the effect of lowering the temperature of crystallization when the crystalline nucleus was introduced was confirmed. Further, the growth of a crystal particle after nucleus production was investigated while changing the heating time. As a result, though this was beyond all expectations, it was observed that even the speed of crystal growth after the nucleus production was remarkably increased in the crystallization of the amorphous silicon thin film formed on the film of a certain metal. This is beyond all expectations. This mechanism will be described in more detail later.
In any case, it was ascertained that, in the case of forming a thin film made of amorphous silicon on a film containing a bit of metal of a certain k
Mitanaga Akiharu
Ohtani Hisashi
Teramoto Satoshi
Nixon & Peabody LLP
Prenty Mark V.
Robinson Eric J.
Semiconductor Energy Laboratory Co,. Ltd.
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