Active solid-state devices (e.g. – transistors – solid-state diode – Non-single crystal – or recrystallized – semiconductor... – Amorphous semiconductor material
Reexamination Certificate
1999-12-02
2001-08-21
Flynn, Nathan (Department: 2826)
Active solid-state devices (e.g., transistors, solid-state diode
Non-single crystal, or recrystallized, semiconductor...
Amorphous semiconductor material
C257S072000, C349S041000, C349S042000
Reexamination Certificate
active
06278130
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a thin film transistor (TFT) and a method of fabricating the same and more particularly, to a TFT and its fabricating method having Metal Induced Crystallization (MIC) region crystallized by an MIC process and a Metal Induced Lateral Crystallization (MILC) region crystallized by an MILC process wherein the boundary between the MIC and MILC regions is located outside a channel region of the TFT.
2. Discussion of Related Art
A method of crystallizing amorphous silicon using heat treatment at a low temperature after a certain kind of a metal layer has been deposited on the amorphous silicon is known as an MIC process. The MIC process is beneficial due to the low temperature crystallization of amorphous silicon. However, the MIC process has not been applied to electronic devices because of an inflow of metal into the thin film of crystallized silicon formed underneath the metal layer, which causes the intrinsic characteristics of amorphous silicon to deteriorate.
A different method of crystallizing amorphous silicon by MILC has been proposed to address this problem.
FIGS. 1A-1D
show examples of schematic cross-sectional views for explaining an MILC process.
Referring to
FIG. 1A
, an amorphous silicon layer
11
to be crystallized is formed to a thickness of 1000 Å on an insulating layer (not shown). Metal patterns, such as nickel patterns
13
, are formed on the amorphous silicon layer
11
.
Referring to
FIG. 1B
, the amorphous silicon layer
11
is crystallized by cooling-down the layer II after heat is applied to the nickel patterns
13
at a temperature of 350-500° C. On the regions “A” of the amorphous silicon layer
11
having the nickel patterns
13
thereon, nickel silicide
14
is formed as the nickel in the nickel patterns
13
reacts with the amorphous silicon in the layer
11
. The nickel silicide
14
becomes a seed for crystallization and promotes the crystallization of the amorphous silicon layer
11
. The “A” regions crystallized directly by the nickel silicide
14
become the MIC regions.
Referring to
FIG. 1C
, the boundaries of the “A” regions having crystallized silicon function as a new seed for crystallization and cause lateral crystallization of silicon in the region “B”. Since the region “B” has no seed of crystallization and has not been solidified yet, the lateral crystallization of silicon is performed by the MIC regions, which have been completely crystallized. That is, the region “B” becomes the MILC region as the crystallization by the nickel silicide
14
is induced in the lateral direction of the MIC region.
FIG. 1D
shows a cross-sectional view of a crystallized silicon layer having MIC and MILC regions. Generally, the MILC regions have less metal contamination, superior crystals and less coarseness in the crystallized surface thereof, than the MIC regions. Thus, the MILC regions are more suitable to function as channel regions for
FIGS. 2A
to
2
D show a method of fabricating a channel region of a thin film transistor using an MILC process according to a related art.
Referring to
FIG. 2A
, an amorphous silicon layer as an active layer
21
is deposited on an insulation substrate
20
having a buffer film (not shown) on its upper part, and the active layer
21
is patterned by photolithography and etching. A gate insulation layer
22
and a gate electrode
23
are formed on the active layer
21
by conventional processes.
Referring to
FIG. 2B
, a nickel layer
24
is formed to a thickness of 20 Å by sputtering nickel on the entire surface of the formed structure. Then a source region
21
S and a drain region
21
D are formed at portions of the active layer
21
by heavily doping the entire surface of the formed structure with impurities. Between the source and drain regions
21
S and
21
D, a channel region
21
C is formed on the substrate
20
.
Referring to
FIG. 2C
, amorphous silicon in the active layer
21
is crystallized by heating the panel
20
at a temperature of 350-500° C. Then the source and drain regions
21
S and
21
D on which the nickel layer
24
has been formed become the MIC regions having silicon crystallized by an MIC process. The channel region
21
C without the nickel layer
24
formed directly thereon, becomes the MILC region where silicon has been crystallized by an MILC process. Impurities are activated in the source and drain regions
21
S and
21
D during the heat treatment as amorphous silicon is crystallized in the active layer
21
.
In the thin film transistor fabricated by the above-described method according to the conventional art, the channel region
21
C has boundaries defined by the crystalline structure of silicon in the MIC regions facing that of silicon in the adjacent MILC region. Since the boundary between the MIC region and the MILC region is located at the junction where the source or drain region meets the channel region, an abrupt difference in the crystal structure appears in the junction and the metal from the MIC region contaminates the adjacent MILC region. Consequently, a trap is formed at such junctions as soon as the TFT is turned on which causes unstable channel regions and deteriorates the characteristics of the thin film transistor.
SUMMARY OF THE INVENTION
Accordingly, the present invention is directed to a thin film transistor and its fabricating method that substantially obviate one or more of the problems due to limitations and disadvantages of the related art.
Additional features and advantages of the invention will be set forth in the description, which follows and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve the above-noted and other advantages and in accordance with the purposes of the present invention, as embodied and broadly described, a transistor includes an MILC (metal-induced lateral crystallization) region formed on a substrate with a semiconductor material and including a channel region; and a plurality of MIC (metal-induced crystallization) regions formed on the sides of the MILC region with a semiconductor material, wherein at least one boundary between the MILC region and one of the MIC regions is located outside the channel region.
In another aspect of the present invention, a method of fabricating a transistor comprises the steps of forming an active layer on a substrate; forming a gate electrode on a portion of the active layer; forming source and drain regions in the active layer using the gate electrode as a mask; forming a metal layer on portions of the active layer, excluding portions of the active layer adjacent the gate electrode; and crystallizing the active layer.
In another aspect of the present invention, a method of fabricating a transistor comprises the steps of forming an active layer on a portion of a substrate; forming a gate electrode on a portion of the active layer; forming a metal layer on the active layer excluding portions of the active layer adjacent the gate electrode; forming a source and drain region in the active layer under the metal layer; and crystallizing the active layer.
In another aspect of the present invention, a method of fabricating a transistor comprises the steps of forming an MILC (metal-induced lateral crystallization) region on a substrate using a semiconductor material, the MILC region including a channel region; and forming a plurality of MIC (metal-induced crystallization) regions formed on sides of the MILC region using a semiconductor material, wherein at least one boundary between the MILC region and one of the MIC regions is located outside the channel region.
In another aspect of the present invention, an LCD comprises a substrate; a first transistor formed on the substrate, the first transistor having an MILC (metal-induced lateral crystallization) region
Ihn Tae-Hyung
Joo Seung-Ki
Flynn Nathan
Forde Remmon R.
Long Aldridge & Norman
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