Active solid-state devices (e.g. – transistors – solid-state diode – Non-single crystal – or recrystallized – semiconductor... – Amorphous semiconductor material
Reexamination Certificate
2001-02-26
2002-12-31
Meier, Stephen D. (Department: 2822)
Active solid-state devices (e.g., transistors, solid-state diode
Non-single crystal, or recrystallized, semiconductor...
Amorphous semiconductor material
C257S059000
Reexamination Certificate
active
06501095
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The present invention relates to a thin film transistor device.
Mainly high-temperature poly-Si has been used for a base thin film being employed for forming a thin film transistor (TFT: Abbreviation of Thin Film Transistor) used conventionally for the thin film transistor device, mainly for an image display device.
This means that poly-crystalline Si is formed on a quartz substrate being an insulator substrate by a high-temperature annealing process at temperature of below or above 900° C., and that the poly-crystalline Si of comparatively large grain size (500-600 nm) is formed. A TFT formed on this high-temperature poly-Si thin film utilizes a Si thin film having a low density in grain boundary thereof and excellent crystallinity, as a channel, field effect mobility of electron of the TFT of 100-150 [cm
2
/Vs] which is a value close to that (~500 [cm
2
/Vs], S. M. Sze, Physics of Semiconductor Devices, P. 29, Second Edition, Wiley) of single crystalline Si, can be obtained.
However, the high-temperature poly-Si thin film is necessitated to use an expensive quartz substrate capable of withstanding a high temperature process as the insulator substrate, since this cost of substrate has been the main cause of difficulty in a cost reduction of entire device, generalization of use of the TFT has been restricted.
Of late, in place of the high-temperature poly-Si thin film, research in a low-temperature poly-Si thin film has actively been carried out. This is a poly-crystalline Si thin film crystallized an amorphous Si thin film formed on a low cost glass substrate or a plastic substrate by a plasma CVD method or the like utilizing a zone melting recrystallization method such as excimer laser annealing.
With the use of this method, since the poly-crystalline Si thin film is capable of being formed at low temperature(~150° C.) , there is an advantage that remarkably inexpensive TFT can be formed. However, compared with the high-temperature poly-Si thin film, the low-temperature poly-Si thin film can form only the poly-crystalline Si thin film with small grain size and a random crystal orientation.
Since when crystal grain size is small, a density in a grain boundary thereof existing in a current path becomes large, or when a crystal orientation is random, a trapping state density in grain boundary becomes relatively large, in both cases any way, characteristics of a transistor are worsend.
Owing to this, in a TFT as a product, using a conventional low-temperature poly-Si thin film as an elemental material, field effect mobility thereof is restricted to an extent up to 150[cm
2
/Vs]. With such small mobility, since an element cannot reach speed practically necessitated, there is such a drawback that sorts of element to be formed on the same glass (or plastic) substrate are restricted.
For example, such an inconvenience is generated that in a case of an image display device, a pixel matrix can be formed on a glass (or plastic) substrate, whereas the circuits such as a source driver, a gate driver, a shift register, a peripheral controller are formed on a conventional printed circuit board, the former must be connected to the board with a cable terminal. With such a method, there has been such drawbacks as that in addition to smallness (4 in.-10 in.) in screen size, an increase in cost of entire device are brought about.
In order to improve drawbacks as described above, arts for an increase in crystal grain size and for truing up a position of crystal grain and a crystal orientation are necessitated. So far, various arts has been proposed in order to form a low-temperature poly-Si thin film into large sizing and for controlling a position and a crystal orientation of crystal grain.
An art for forming poly-crystalline Si thin film having a [111] axis in a current moving direction by introducing a metal element for selectively promoting an amorphous Si-thin film formed on the insulated substrate for crystallization and by carrying out crystal growth in a direction parallel to the substrate (for example, Japanese Unexamined Patent Publication H 7-321339), an art for forming rectangular-shaped poly-crystalline Si thin film having a <100> axis in a direction perpendicular to the substrate and a {220} surface in parallel (or 45° C.) to a beam scanning direction by controlling a shape of laser beam for annealing and a scanning rate of annealing position (for example, Japanese Unexamined Patent Publication H 10-41234), and an art for forming a columnar poly-crystalline Si layer trued up a crystal orientation by forming a first poly-crystalline Si layer on a substrate, forming a seed crystal having any one of typical orientation ({100}, {110}, {111}) by an anisotropic etching, and by forming a second polycrystalline Si layer thereon (for example, Japanese Unexamined Patent Publication H 8-55808) and the like. However, in spite of these numerous trials, a TFT having sufficient high mobility has not so far be realized.
SUMMARY OF THE INVENTION
In the crystallization method described above, either of them is not a sufficiently completed art, the maximum grain size which can be achieved has been about 2 &mgr;m, and the size is not enough. With this value of 2 &mgr;m, it is far behind the practical size of about 8 &mgr;m of a thin film transistor required for a liquid crystal display panel of large screen, further, dispersion of characteristics between elements due to positional deviation of crystal grains cannot be restrained.
Thereby, with such arts described above, it is not possible to construct a thin film transistor device that can entirely replace the existing thin film transistor device of a low function. This is because these arts can not realize the most stable lattice structure in a Si crystal when it is brought into contact with a substrate, it is an essential limit to be determined by a strain at the interface between Si and a substrate, and not to be depended on a deposition method of thin film or an annealing method.
Accordingly, in order to realize an image displayed device with the high performance and the large area at low cost, an object of the present invention is to realize a TFT having high mobility by providing such arts as that low-temperature poly-Si thin film becoming an elemental material of TFT can be large sized (quasi single-crystal) in a trued up state to a crystal orientation having the most stable lattice structure in consideration of a strain at the interface with the substrate, and that the positions of crystal of the low-temperature poly-Si thin film can be controlled.
In order to achieve the object described above, researchers of a thin film transistor device of the present invention paid attention to a fact that a {110} surface of a crystal composed of either one or their mixed crystal selected from a group of C, Si, Ge, Sn and Pb which are IV group crystal is the surface having smallest dangling bond density, and a high mobility TFT is realized by minimizing strain energy at the substrate interface, and by forming a channel with a crystal grain of large grain size and of controlled crystal orientation obtained through crystal growth by selecting crystal orientation having a growth length equivalent to a channel length.
Hereinafter, characteristic items of a thin film transistor device of the present invention will be specifically enumerated.
(1) This is a thin film transistor device having a insulator substrate, a poly-crystalline thin film formed on the insulated substrate, and a transistor composed of a source, a drain, a channel and a gate formed on the poly-crystalline thin film, and is characterized in that the poly-crystalline thin film is formed from a crystal composed of either one crystal or their mixed crystals selected from a group of C, Si, Ge, Sn and Pb being a IV group crystal, and is formed from one piece or a plurality of pieces of crystal grains having a <110> axis at an angle of 0-5° relative
Hatano Mutsuko
Kimura Yoshinobu
Park Seong-Kee
Shiba Takeo
Yamaguchi Shin'ya
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