Active solid-state devices (e.g. – transistors – solid-state diode – Thin active physical layer which is – Heterojunction
Reexamination Certificate
2001-02-23
2003-02-18
Sherry, Michael (Department: 2829)
Active solid-state devices (e.g., transistors, solid-state diode
Thin active physical layer which is
Heterojunction
Reexamination Certificate
active
06521909
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The present invention relates to thin film transistor devices and methods for producing thereof, and more particularly to a thin film transistor device and a method for producing thereof suitable for a transistor used a poly-crystalline silicon (poly-Si).
The 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.
Mainly, high-temperature poly-crystalline Si has been used for a base thin film employed for forming a conventional thin film transistor. This means that a poly-crystalline Si (poly Si) thin film is formed on a quartz substrate being an insulator substrate by a high-temperature heat treatment at temperature of below or above 900° C., and that 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 crystal and excellent crystallinity, as a channel, so that field effect mobility of 100-150 [cm
2
/Vs], as a value close to that conventional type Si-LSi on a Si substrate having (~500 [cm
2
/Vs], refer to a document, S. M. Sze, Physics of Semiconductor Devices, p. 29, Second Edition, Wiley), can be obtained.
However, the high—temperature poly-Si is necessitated to use the expensive quartz substrate as the insulator substrate capable of withstanding through a high temperature process, since this cost of the substrate has been the main cause of difficulty in a cost reduction of an entire semiconductor device, generalization of use of a TFT has been restricted.
In recent years, in place of the high-temperature poly-Si, a research on low-temperature poly-crystalline Si (hereinafter, referred to as low-temperature poly-Si) has vigorously been carried out. This is the poly-crystalline Si crystallized amorphous Si 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. 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 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 that of the high-temperature poly-Si and the poly-crystalline Si only with large (~50 nm) surface roughness has been formed.
When crystal grain size is small, there are such drawbacks 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 leak 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
/Vs] in case of an electron carrier, is restrained to a degree of up to 50 [cm
2
/Vs] in case of positive hole carrier. With a small mobility like this, since elemental performance cannot reach the required elemental performance, there is such a drawback as that sorts of the 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), whereas the other circuits which are high 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, they are integrated on a printed circuit board as semi-conductor chips utilized a conventional Si-LSI art, this printed circuit board is connected with the glass substrate and must be used.
With such a method, there has been drawbacks as 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 of the entire image display device are brought about. 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 carrier is estimated.
In order to improve these drawbacks, the art to achieve enhancement in performance of a TFT into high level is necessitated by realizing such a poly-crystalline thin film as that current scattering in the grain boundary being restrained, and the surface roughness thereof being 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) 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; an art (for example, Japanese Unexamined Patent Publication H10-41234) 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) for forming columnar poly-crystalline Si layers 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 an-isotropic 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 so far has not been realized.
SUMMARY OF THE INVENTION
All of conventional crystallization methods of a low-temperature poly-Si thin film cannot be said as sufficiently completed arts, for example, when either of the maximum grain size, or surface roughness is taken up, performance of a TFT has not been, as yet, met demand required for a peripheral circuit integrated type liquid crystal display panel. So that these arts cannot sufficiently replace an existing thin film transistor device of low function. Accordingly, a technical theme to realize an image display device having the high performance and a large area with low cost is extremely important.
Thus, a first object of the present invention is, in low-temperature poly-Si being a elemental material of a TFT, to provide a thin film transistor device excellent in characteristics in which a conventional art 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 utilized the thin film transistor device.
In order to achieve the objects described above and as a result of various experiments and inv
Hatano Mutsuko
Park Seong-Kee
Shiba Takeo
Yamaguchi Shin'ya
Antonelli Terry Stout & Kraus LLP
Hitachi , Ltd.
Sarkar Asok Kumar
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