Semiconductor device manufacturing: process – Formation of semiconductive active region on any substrate – Amorphous semiconductor
Reexamination Certificate
2002-10-22
2004-04-06
Pert, Evan (Department: 2829)
Semiconductor device manufacturing: process
Formation of semiconductive active region on any substrate
Amorphous semiconductor
C438S149000, C438S482000, C438S488000, C438S933000
Reexamination Certificate
active
06716726
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The present invention relates to thin film transistor devices and methods for producing thereof. More particularly, the present invention relates to a thin film transistor device and a method for producing thereof suitable for a transistor using a poly-crystalline silicon (poly-Si)
Thin film transistor devices are utilized mainly for an image display device of a liquid crystal display device (LCD), a plasma display device (PDP) and the like as pixels or thin film transistors (TFT) for driving a peripheral circuit.
High-temperature poly-crystalline Si has been used for a base thin film employed for forming conventional thin film transistors. Poly-crystalline Si (poly Si) thin film is formed on a quartz substrate (i.e., an insulator substrate) by a high-temperature heat treatment at a temperature below or above 900° C., and the poly-crystalline Si of comparatively large grain size (for example, 500-600 nm) is formed.
A TFT formed on the high-temperature poly-crystalline Si (hereinafter, referred to as high-temperature poly-Si) thin film utilizes a Si thin film having a low density in a grain boundary of the crystal and excellent crystallinity, as a channel, so that a field effect mobility can be obtained of 100-150 cm
2
Ns, as a value close to conventional type Si-LSi on a Si substrate having ~500 cm
2
Ns. See S. M. Sze, Physics of Semiconductor Devices, p. 29, Second Edition, Wiley.
However, high-temperature poly-Si uses expensive quartz substrate as the insulator substrate so as to withstand a high temperature process. Since this cost of the substrate has been the main cause of difficulty in cost reduction of the entire semiconductor device, the use of such a TFT has been restricted.
In recent years, rather than the high-temperature poly-Si, research has vigorously been carried out on low-temperature poly-crystalline Si (hereinafter, referred to as low-temperature poly-Si). This poly-crystalline Si crystallized amorphous Si is formed on a low cost glass substrate or a plastic substrate by a plasma CVD method or the like utilizing a zone melting re-crystallization method such as excimer laser annealing. Since the poly-crystalline Si thin film is capable of being formed at low-temperature (~150° C.), there is an advantage that a remarkably inexpensive TFT can be formed.
However, the low-temperature poly-Si up to now is small (~100 nm) in crystal grain size compared with the high-temperature poly-Si. Only poly-crystalline Si with large (~50 nm) surface roughness has been formed.
When crystal grain size is small, there are drawbacks such that a density in the grain boundary of crystal existing in a current path becomes large, and current mobility is lowered through current scattering in the grain boundary thereof.
Further, when the surface roughness is large, a requirement for thickening (~100 nm) a gate insulation film to that amount is generated in order to restrain a gate leakage current. Consequently, since the carrier number induced to the channel by the same gate voltage becomes small, the current mobility is also lowered.
From that reason, in a TFT of a product base utilizing conventional low-temperature poly-Si as an elemental material, the field effect mobility thereof is restrained to a degree of up to 150 cm
2
Ns in case of an electron carrier, and is restrained to a degree of up to 50 cm
2
Ns in case of positive hole carrier. With a small mobility like this, since elemental performance cannot reach the required elemental performance, there is a drawback that elements capable of being formed on the same glass (or plastic) substrate are restricted.
For example, in the case of the image display device, a pixel circuit part which is comparatively low in required performance, can be formed on glass (or plastic). Other circuits which are higher in the required performances such as a source driver, a gate driver, a shift register, and a peripheral controller, since they cannot be formed on the same substrate, are integrated on a printed circuit board as semi-conductor chips utilizing a conventional Si-LSI art. This printed circuit board is connected with the glass substrate.
With such a method, there have been drawbacks such that in addition to small dimensioning (4 in.-10 in.) in screen size depending upon a dimension where the periphery circuit part is mounted, a remarkable increase in cost occurs for the entire image display device. Further, in a power saving image display device, which is promising for a future market, a TFT is indispensable to conduct CMOS (complementary MOS) forming. For that purpose, the requirement for a further increase in performance with respect to the field effect mobility of a positive hole carder is estimated.
In order to improve these drawbacks, enhancement in performance of a TFT into high level is necessitated by realizing such a polycrystalline thin film and that current scattering in the grain boundary is restrained, and the surface roughness thereof is lessened. In order to high-function the low-temperature poly-Si, various arts have been proposed as exemplified hereinafter.
Among them; for example, an art (for example, Japanese Unexamined Patent Publication H7-321339) is provided for forming poly-crystalline Si having an [
111
] axis in a current moving direction, by introducing a metal element for selectively promoting an amorphous Si film formed on the insulator substrate into crystallization and by carrying out respective crystal growth in a direction parallel to a substrate. Further, an art (for example, Japanese Unexamined Patent Publication H10-41234) is provided for forming rectangular poly-crystalline Si having a <
100
>axis in a direction perpendicular to the substrate, and a {
220
} surface in parallel (or at an angle of 45°) to a beam scanning direction by accurately controlling a shape of a laser beam for annealing and a scanning rate of a laser annealing position; and an art (for example, Japanese Unexamined Patent Publication H8-55808) is provided for forming columnar poly-crystalline Si layers trued up of a crystal orientation by forming a first poly-crystalline Si layer on the substrate, by forming a seed crystal having either of typical orientations ({
100
}, {
110
}, and {
111
}) by anisotropic etching and by forming a second poly-crystalline Si layer thereon and the like.
However, in spite of these numerous trials, a TFT with sufficiently high mobility has not been realized so far.
SUMMARY OF THE INVENTION
The conventional crystallization methods of low-temperature poly-Si thin films are not sufficiently complete. For example, when either of the maximum grain size, or the surface roughness is taken up, performance of a TFT has not, as yet, met the demand required for a peripheral circuit integrated type liquid crystal display panel. So, these arts cannot sufficiently replace an existing thin film transistor device of low function. Accordingly, it is important to realize an image display device having high performance and a large area with low cost.
Thus, a first object of the present invention is to provide a thin film transistor device being excellent in characteristics in which conventional arts cannot provide by restraining current scattering in the grain boundary of crystal, by decreasing the surface roughness, and by realizing a poly-crystalline thin film having a crystal structure so as to realize high mobility even for a positive carrier. A second object is to provide a production method by which a thin film transistor device can be easily obtained. A third object is to provide an image display device utilizing the thin film transistor device.
In order to achieve the objects described above and as a result of various experiments and investigations about low-temperature poly-Si for forming a TFT, the inventors have realized a TFT with high mobility by introducing Ge into a poly-Si thin film, by differentiating (for further details, a ratio of Ge composition in a grain boundary of crystal is made larger than
Hatano Mutsuko
Park Seong-Kee
Shiba Takeo
Yamaguchi Shin'ya
Pert Evan
Sarkar Asok Kumar
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