Active solid-state devices (e.g. – transistors – solid-state diode – Non-single crystal – or recrystallized – semiconductor... – Non-single crystal – or recrystallized – material with...
Reexamination Certificate
2003-06-05
2004-11-30
Lee, Eddie (Department: 2811)
Active solid-state devices (e.g., transistors, solid-state diode
Non-single crystal, or recrystallized, semiconductor...
Non-single crystal, or recrystallized, material with...
C257S066000
Reexamination Certificate
active
06825494
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
This application claims the benefit of Korean Application No. 2002-39495, filed Jul. 8, 2002, in the Korean Intellectual Property Office.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a polycrystalline silicon thin film used in a thin film transistor and a device using the same, and more particularly, to a polycrystalline silicon thin film used in a thin film transistor with silicon grains in a crystal growing direction which is constant and regularized, and a device using a thin film transistor fabricated using the above described polycrystalline silicon thin film.
2. Description of the Related Art
Bonding defects such as atom dangling bonds existing on crystal grain boundaries of polycrystalline silicon included in active channel regions are known to act as traps on electric charge carriers when fabricating a thin film transistor (hereinafter referred to as TFT) using polycrystalline silicon.
Therefore, size, size uniformity, number and position, and direction of crystal grains not only directly or indirectly exert a fatal influence on TFT characteristics such as threshold voltage (Vth), subthreshold slope, charge carrier mobility, leakage current and device stability, but also exert a fatal influence on uniformity of TFTs depending on the position of the crystal grains when fabricating an active matrix display substrate using TFTs.
The number of fatal crystal grain boundaries (hereinafter referred to as “primary” crystal grain boundaries) included in active channel regions of a TFT on the whole substrate of a display device can be equal to or different from each other depending on the size of the crystal grains, the inclination angle &thgr;, the dimension of active channels (length (L) and width (W)) and the position of each TFT on the substrate, as illustrated in FIG.
1
A and FIG.
1
B.
As illustrated in FIG.
1
A and
FIG. 1B
, characteristics of each TFT in a TFT substrate comprising two or more TFTs (type 1 transistor (TR
1
) and type 2 transistor (TR
2
)) having a source/drain direction which is perpendicular to each other have different effects of crystal grain boundaries depending on a degree at which the crystal grain boundaries are perpendicular to the source/drain direction or a degree of inclination to a normal line perpendicular to the source/drain direction, wherein crystal grain boundaries having a fatal influence on characteristics of two shaped TFTs, which are perpendicular to each other, are approximately perpendicular. That is, the size of crystal grains having a fatal influence on TFT characteristics in the type 1 transistor (TR
1
) becomes Gs
1
while the size of crystal grains having fatal influence on TFT characteristics in the type 2 transistor (TR
2
) becomes Gs
2
.
The number of crystal grain boundaries included in active channel regions of each TFT can be varied depending on the size and direction of crystal the grains, and the TFT dimensions. For example, three fatal crystal grain boundaries exist in a type 1 transistor (TR
1
), and two crystal grain boundaries exist in a type 2 transistor (TR
2
) in
FIG. 1A
, while three crystal grain boundaries can be contained in a type 1 transistor (TR
1
), and two crystal grain boundaries can be contained in a type 2 transistor (TR
2
) for the equal crystal grain boundaries and TFT dimensions. Therefore, uniformity of characteristics between TFTs is greatly influenced.
Polycrystalline or single crystalline particles can form large silicon grains on a substrate using sequential lateral solidification (SLS) crystallization technology, as illustrated in FIG.
2
A and FIG.
2
B. It is reported that a TFT fabricated using the large silicon grains can obtain similar characteristics to that of a TFT fabricated using single crystalline silicon.
However, numerous TFTs used in drivers and pixel arrays should be fabricated in order to fabricate an active matrix display.
For example, approximately a million pixels are required in fabricating an active matrix display having super video graphics array (SVGA) resolution, one TFT is required in each pixel in the case of liquid crystal displays (LCD), and two or more TFTs in each pixel are required in a display using an organic light emitting substance, e.g., an organic electroluminescent device.
Therefore, it is impossible to fabricate TFTs by growing a certain number of crystal grains only in one to two million or more active channel regions of each TFT in a certain direction.
In order to supplement these problems, it is disclosed in PCT International Patent No. WO 97/45827 that the amorphous silicon on the whole substrate is converted into polycrystalline silicon, or only selected regions on the substrate are crystallized using SLS technology after depositing amorphous silicon by plasma enhanced chemical vapor deposition (PECVD), low pressure chemical vapor deposition (LPCVD) or sputtering (FIG.
2
A and FIG.
2
B).
The selected regions are also considerably wide regions compared with active channel regions having dimensions of several &mgr;m×several &mgr;m. Furthermore, the size of a laser beam used in SLS technology is approximately several &mgr;m×several &mgr;m so that stepping and shifting of the laser beam or the stage of the laser beam are inevitably required to crystallize amorphous silicon of the whole regions or selected regions on a substrate, wherein misalignment exists between regions on which a laser beam is irradiated. Therefore, the number of “primary” crystal grain boundaries included in numerous active channel regions of a TFT is varied, and the TFT on the whole substrate, in driver regions or in pixel cell regions has unpredictable non-uniformity. The non-uniformity can exert a fatal bad influence on the realization of an active matrix display device.
Furthermore, it is disclosed in U.S. Pat. No. 6,177,391 that a barrier effect of the crystal grain boundaries for the direction of electric charge carriers is minimized (FIG.
3
A), and TFT characteristics being second to single crystalline silicon is obtained accordingly in the case where the direction of active channels is parallel to the direction of crystal grains grown by the SLS crystallization method when fabricating a TFT for an LCD comprising a driver and a pixel array by forming large silicon grains using an SLS crystallization technology while a lot of the crystal grain boundaries in which the TFT characteristics act as a trap for the electric charge carriers exist, and the TFT characteristics are greatly deteriorated in the case where the active channel direction is perpendicular to the crystal grain growing direction (FIG.
3
B).
There are cases where a TFT inside the driver circuit and a TFT inside pixel cell regions usually have an angle of 90° when actually fabricating an active matrix display, wherein uniformity of the device can be improved by fabricating the active matrix display in such a way that a direction of the active channel region is inclined at a growing angle of the crystal grains by 30 to 60° to improve uniformity of characteristics between TFTs while not greatly deteriorating the characteristics of each TFT, as illustrated in FIG.
3
C.
However, there is the probability that fatal crystal grain boundaries are included in the active channel regions as the method also uses crystal grains of a limited size formed by the SLS crystallization technology. Accordingly, the method has problems in that unpredictable non-uniformity causing differences of characteristics between TFTs exist.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a polycrystalline silicon thin film comprising TFTs that are perpendicular to each other to be capable of determining optimum process conditions on the size and the direction of silicon crystal grains and optimum dimensions of active channels to secure TFT characteristics and uniformity required when fabricating a TFT substrate and an active display device by inducing a numerical expression capable of calculating probabi
Lee Ki Yong
Park Ji Yong
So Woo Young
Lee Eddie
Owens Douglas W.
Samsung SDI & Co., Ltd.
Staas & Halsey , LLP
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