Active solid-state devices (e.g. – transistors – solid-state diode – Non-single crystal – or recrystallized – semiconductor... – Field effect device in non-single crystal – or...
Reexamination Certificate
1999-06-18
2001-02-13
Jackson, Jr., Jerome (Department: 2815)
Active solid-state devices (e.g., transistors, solid-state diode
Non-single crystal, or recrystallized, semiconductor...
Field effect device in non-single crystal, or...
C257S075000
Reexamination Certificate
active
06188085
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a thin film transistor and a method of manufacturing thereof, and more particularly, to a thin film transistor of a polysilicon film and a method of manufacturing thereof.
2. Description of the Background Art
A thin film transistor (TFT) is a known semiconductor device used as a load transistor of a highly integrated SRAM and a drive transistor for a liquid crystal panel display. To meet the demand of high performance of devices employing TFTs, significant improvement in the electric characteristics of a TFT itself is desired.
A TFT is generally formed of a polysilicon film. The electric characteristics thereof are greatly affected by the grain boundary in a field region. The localized level depending on a grain boundary acts as a capture trap of carriers and as a generation center of an electron-hole pair. The presence of a grain boundary in a channel region of a TFT results in the capture of carriers to form a potential barrier by which passage of carriers are prevented. This induces a problem of lowering the ON current of a TFT. The presence of a grain boundary in the pn junction at the drain side will cause generation of a great amount of electron-hole pairs, resulting in increase of OFF current in a TFT. Conventionally, the electric characteristics of a TFT was improved by increasing the grain size of polysilicon for the channel to reduce the number of grain boundaries.
FIG. 80
is a sectional view of a conventional TFT for describing a method of manufacturing thereof;
FIG. 81
is a plan view of the conventional TFT of
FIG. 80
; and
FIG. 82
is a perspective view of a TFT formed according to a conventional manufacturing method.
A conventional manufacturing process of a TFT will be described hereinafter with reference to
FIGS. 80-82
.
Referring to
FIG. 80
, a polysilicon layer (not shown) of approximately 1500 Å in thickness is formed by CVD on an insulating film
101
, which is patterned to result in a gate electrode
102
. A gate insulating film
103
of approximately 300 Å in thickness is formed so as to cover gate electrode
102
by CVD. An amorphous silicon layer
104
is formed by CVD on gate insulating film
103
to a thickness of approximately 800 Å. A heat treatment at a temperature condition of approximately 600° C. is applied to solid phase grow an amorphous silicon layer
104
. Thus, polysilicon
105
as shown in
FIG. 81
is formed. Referring to
FIG. 81
, there is a grain boundary
106
at the boundary region of polysilicon
105
. By growing polysilicon
105
according to the above-described process, polysilicon
105
having a grain size of approximately several thousand Å can be formed. Since the grain size of polysilicon formed by CVD is approximately 100 Å, the above-described manufacturing process provides polysilicon
105
of a grain size several ten times thereof.
Then, source/drain regions
111
and
112
, and a field
150
are formed.
FIG. 83
is a graph showing the electric characteristics of a TFT obtained by the conventional manufacturing method of FIG.
82
. Referring to
FIG. 83
, gate voltage is plotted along the abscissa, and drain current is plotted along the ordinate. The drain current plotted along the ordinate shows the measured result of a pattern where 10000 TFTs are connected in parallel, each TFT having a channel length of 1.3 &mgr;m and a channel width of 0.6 &mgr;m. The TFT shown in
FIG. 82
is a solid phase grown poly TFT. It is apparent from the graph of
FIG. 82
that the ON current is one order of magnitude greater than that of a CVD poly TFT manufactured by CVD. Conventionally, the characteristics of a TFT was improved by forming polysilicon
105
increased in grain size by solid phase growing amorphous silicon layer
104
, as shown in FIG.
82
.
However, it is to be noted that the data shown in
FIG. 83
represents the average value of 10000 TFTs. The property of each TFT for all the 10000 TFTs is not necessarily improved.
FIG. 84
is a graph showing the electric characteristics of three single TFTs on the same wafer manufactured according to the process shown in
FIG. 80-82
. It is appreciated from the graph of
FIG. 84
that there is a variation of approximately one order of magnitude in the drain current between each of the three TFTs.
This variation is due to the fact that the crystals are grown in random since there is no selectivity in the solid phase growth when converting amorphous silicon Layer
104
into polysilicon
105
. A TFT with a grain boundary in the channel or a TFT with no grain boundary will be formed in random, resulting in difference in the characteristics of each TFT as shown in FIG.
84
. Although it can be observed that the grain size of the solid phase grown polysilicon
105
according to the process shown in
FIG. 81
is increased in average, the grain is not in uniform, and there are partially extremely small grains. The presence of such small grains in a channel portion of a TFT will degrade the characteristics of a TFT. This is also considered to be the cause of reducing the uniformity of characteristics of each TFT.
Thus, it was difficult to selectively grow polysilicon
105
according to the above-described method of forming polysilicon
105
having a large grain size from an amorphous silicon layer
104
. This results in a problem that there is variation in the characteristics between each TFT.
SUMMARY OF THE INVENTION
In order to solve the above-described problems, an object of the present invention is to provide a method of manufacturing a thin film transistor (TFT) having improved electrical characteristics of each TFT improved and to obtain a plurality of TFTs having similar, i.e. uniform, characteristics.
According to an aspect of the present invention, a method of manufacturing a thin film transistor includes the steps of: forming a gate electrode on an insulating film; forming a gate insulating film to cover the gate electrode; forming a polysilicon film on the gate insulating film; forming amorphous silicon by ion-implanting either silicon or nitrogen into a predetermined region of the polysilicon film to render a portion of the polysilicon film amorphous with the polysilicon film partially remaining; and applying a heat treatment to convert the amorphous silicon into polysilicon with the remaining polysilicon film as a seed crystal.
According to the above-described method of manufacturing a thin film transistor, a polysilicon film serving as a seed crystal can be selectively left by the above-described silicon ion implantation, whereby solid phase recrystallization of the amorphous silicon is carried out in uniform. By forming a thin film transistor using such solid phase recrystallized polysilicon, the characteristics of each thin film transistor is made uniform. Furthermore, the characteristics of each thin film transistor is improved by the recrystallized polysilicon of a large grain size.
According to another aspect of the present invention, a method of manufacturing a thin film transistor includes the steps of: forming a polysilicon film on an insulating film; forming a first mask layer on a first region of the polysilicon film; forming a first amorphous silicon by ion-implanting either silicon or nitrogen into the polysilicon film using the first mask layer as a mask to render the region of the polysilicon film other than the first region amorphous; applying a heat treatment to convert the first amorphous silicon into polysilicon with the polysilicon film of the first region as a seed crystal; forming a second mask layer on a second region of the polysilicon film; forming a second amorphous silicon by ion-implanting either silicon or nitrogen into the polysilicon film using the second mask layer as a mask to render the region of the polysilicon film other than the second region amorphous; and applying a heat treatment to convert the second amorphous silicon into polysilicon with the polysilicon film of the second region as a seed crystal.
According to the abo
Ipposhi Takashi
Maeda Shigenobu
Shimizu Satoshi
Ueno Shuichi
Jackson, Jr. Jerome
McDermott & Will & Emery
Mitsubishi Denki & Kabushiki Kaisha
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