Method of making a thin film transistor by overlapping annealing using lasers

The present invention provides a method of making a thin film transistor for driving a liquid crystal display comprising the steps of forming a gate electrode on a glass substrate and forming an insulating layer and an amorphous silicon layer in turn on said glass substrate and said gate electrode, and scanning laser beams on the surface of said amorphous silicon layer with the end portions of the respective scanned laser beams being overlapped. According to the method of making a thin film transistor for driving a liquid crystal display of the present invention, a thin film transistor suitable for HDTV, the field effect mobility of which is high, is achieved. Further, in making a thin film transistor, a separate processing step is not required and the number of processing steps can be reduced because constructional features of a TFT are utilized.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a method of making a thin film transistor 
(TFT) for driving a liquid crystal display (LCD), and more particularly to 
a method of making a TFT adequate to achieve picture quality of high 
resolution by improving recrystalization uniformity of the poly silicon 
layer obtained by laser beam processing. 
2. Description of the Prior Art 
A TFT LCD has the construction that high-integrated TFTs for driving an LCD 
are added thereon and has been developed and used as a main image device 
for portable televisions, wall-hanging type televisions, notebook type 
computers, etc., since it can provide high resolution picture quality. 
FIG. 1 shows the construction of an amorphous silicon TFT, which is the 
kind of TFT used for the above-mentioned TFT LCD. In making such amorphous 
silicon TFT, a metal such as Cr, Ta, Ta alloy, Al, etc. is evaporated on a 
glass substrate 21 and then patterned to form a plurality of gates 22. 
An insulating layer 23 is grown on the gate-formed glass substrate 21, and 
then amorphous silicon 24 is evaporated thereon and patterned. 
On the patterned amorphous silicon 24, an n.sup.+ -type amorphous silicon 
25 is grown and patterned and on the patterned n.sup.+ -type amorphous 
silicon 25, electrode metal 26 is coated, resulting in completion of 
amorphous silicon TFTs. 
The amorphous silicon TFTs constructed as above can be used for driving the 
LCD as mentioned above. That is, as shown in FIG. 4, when voltage is 
applied to a gate electrode a, an electric signal is transmitted from 
source electrode b to drain electrode c, causing LCD d to be charged and 
driven. The characteristics of a TFT are dominated by the characteristics 
of amorphous silicon layer 24. 
Usually, the field effect mobility of an amorphous silicon TFT is in the 
range of 0.5.sup..about. 1.0 cm.sup.2 /visec. And it has been reported 
that a TFT having field effect mobility of 4.sup..about. 5 Cm.sup.2 /visec 
is in development. 
However, an amorphous silicon TFT has a relatively low field effect 
mobility as mentioned above, so that it is inadequate to drive an LCD 
having a large screen or for a high picture quality. 
Furthermore, all manufacturing processes must be carried out below the 
temperature of about 600.degree. C. because of the glass substrate. 
Meanwhile, to improve the field effect mobility of a TFT, a method of 
making a TFT having a polysilicon layer recrystallized by a laser beam 
process has been proposed. 
According to the above method, the temperature distribution is adjusted by 
forming an insulating layer having the configuration shown in FIGS. 2A and 
2B or by adjusting the shape of laser beam as in FIG. 3 without the 
insulating layer in order to effect selective crystallization of the 
amorphous silicon layer. 
In case selective crystallization is carried out by adjusting the 
temperature distributing using the thickness of the insulation layer, as 
shown in FIG. 2A, a crystal core is formed and grown in the part of the 
amorphous silicon layer placed under the convex part of the insulation 
layer owing to the warmth retention effect of the convex part of the 
insulating layer, thereby selective crystal-growing is accomplished and 
optical defects are concentrated in the concave part of the insulation 
layer, or as shown in FIG. 2B, optical defects are formed in the part of 
the amorphous silicon layer above the concave part of the insulation layer 
owing to the heat sink function of the convex part of the insulation layer 
under the amorphous silicon layer. 
As shown in FIG. 3, in case crystallization is carried out by adjusting the 
beam shape by means of an optical system including a filter, a mode 
selector, a beam overlapper, a splitter, a lens, etc., a crystal core is 
formed along the M--M line and grows to either side of the beam, so that 
the active region of the TFT is positioned in the central part of the 
beam. 
However, the method of making a TFT described as above has the disadvantage 
that it is required to form the insulating layer and perform a complicated 
etching process to make the insulation layer uneven prior to laser beam 
scanning as shown in FIG. 2. 
Also, in case the insulation layer is formed too thinly, adjustment of the 
temperature distribution is incomplete, and thus, the process is delayed 
until the layer is formed with insulating thickness of more than 1 mm. 
Furthermore, as shown in FIG. 3, in using an optical system including a 
filter, an aperture, etc., the degree of crystallization may be weakened 
owing to reduction of the power of the laser beam. And since the size of 
the beam is relatively small (.phi.50 .mu.m.sup..about. .phi.10 mm) in 
making a TFT for a large-scaled LCD, laser beam processing time is 
prolonged and the number of overlapping parts (6.sup..about. 8%) between 
beam scan lines increases, as well as making the position adjustment among 
the respective TFTs and the overlapping parts difficult. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a method of making a 
TFT for driving an LCD adequate to achieve LCD picture quality of high 
resolution by improving the characteristics of a poly silicon layer 
through selective crystallization of the polysilicon layer without 
adjusting the shape of the laser beam or etching to make the insulation 
layer uneven. 
It is another object of the present invention to provide a method of making 
a TFT for driving an LCD by which the laser beam is scanned so that the 
energy density distribution of the laser beam is uniform and the number of 
overlapping parts of the laser beam is reduced. 
In order to achieve the above objects, the present invention provides a 
method of making a TFT for driving an LCD comprising the steps of: 
forming a gate electrode on a glass substrate and forming an insulating 
layer and an amorphous silicon layer in turn on said glass substrate and 
said gate electrode; and 
scanning laser beams on the surface of said amorphous silicon layer with 
the end portions of the respective scanned laser beams being overlapped. 
In the step of scanning the laser beams, it is preferred to scan said laser 
beams through an optical system including prisms arranged at predetermined 
intervals so that the overlapped portions of the laser beams pass through 
the prisms. It is also preferred to scan the laser beam through a 
cylindrical lens or to scan the laser beam after applying a protective 
layer on said amorphous layer.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
FIG. 5 shows a method of scanning a laser beam, formation of crystal core 
in an amorphous silicon layer and a transformation process for an 
amorphous silicon layer to polysilicon layer. 
That is, by scanning laser beam (LB) after forming gate 22, insulating 
layer 23 and amorphous silicon layer 24 in turn on glass substrate 21, a 
crystal core is formed in said amorphous silicon layer near the gate metal 
owing to the heat sink effect of the gate metal which is an important 
region of a TFT, and thereby a silicon layer near the gate metal is 
hardened and optical defects are concentrated in a silicon layer between 
the gate metals, so that a TFT of excellent characteristics is achieved. 
In scanning a laser beam (LB) on the surface of amorphous silicon layer 24 
in turn, as shown in FIG. 6, total laser-processed part is completed by 
scanning laser beams with the end portions of the respective scanned laser 
beams being overlapped. 
Mean while, in the above method of scanning laser beam, because it is not 
necessary to regulate laser beam shape, it is possible to use all of the 
power of the laser beam without loss, so that it is possible to enlarge 
beam size with sufficient laser power in crystallization reserved. As 
shown in FIG. 7, on the LCD TFT array part it is possible to enlarge beam 
size using an optical system such as a cylindrical lens, and thereby, even 
for a picture plane of large area, it is possible to reduce the numbers of 
scanning laser beams and of overlap parts between beams (for a 14" picture 
plane, it is possible to reduce the number of scanning the laser beam to 
2.sup..about. 4, thereby to reduce the number of overlap parts to 
1.sup..about. 3). Meanwhile, on the array driving circuit, it may be used 
to scan a beam having a width of 10 mm through lens 12 without 
overlapping. 
Meanwhile, the distribution of energy density of the respective laser beam 
has a shape similar to a trapezoid (that is, flat at the central part of 
the beam and inclined downward at either lateral end parts of the beam), 
and to complement this deficiency of energy at said lateral end parts 
scanning laser beam with the end portions of the respective scanned laser 
beams being overlapped is used by making lateral ends of the respective 
beam overlap each other; the distribution of energy density in this case 
is shown in FIG. 9. However, notwithstanding this overlapping scanning of 
laser beams, the distribution of energy density of laser beams does not 
become even, and the shape of the distribution of specific resistance of a 
poly silicon layer by laser beam scanning becomes very high at overlap 
part y as shown in FIG. 10. 
To overcome this problem, optical system 3 including a plurality of 
regularly spaced prisms 2 can be disposed above TFT test piece 1 so that 
the intensity of the laser beam scanned on overlap part y between beams 
may become nearly equal to the intensity of the laser beam scanned on the 
non-overlap part. 
That is, if the laser beam is scanned through an optical system including 
prisms above the overlap parts, though the energy of the laser beam 
scanned on the overlap part y is lower than the energy of the laser beam 
scanned on part c, because the density of the laser beam rays scanned on 
overlap part y is increased due to the refraction of laser beam rays 
through prisms as shown in FIG. 8B the difference of optical efficiency 
between overlap part y and part c of TFT test piece 1 is compensated. The 
distribution of the poly silicon layer in scanning a laser beam through 
optical system 3 is illustrated in FIG. 10B. 
Angle .theta. and width 1 of prism (2, see FIG. 8A) may be changed 
according to the energy distribution of the laser beam used. Meanwhile, 
the width of the laser beam may be changed according to the permissible 
energy and the focus length of the optical lens used, and thereby it is 
possible to use prisms which are not fixed but are movable according to 
the width of the laser beam respectively. 
In scanning the laser beam, it is possible to scan a laser beam after 
applying a protective layer on an amorphous silicon layer, and a laser 
scanning device may be chosen from among an excimer laser, an argon laser 
and a ruby laser. The gate electrode of a TFT is preferred to be formed 
with Cr, Ta, Ta alloy, Al or a mixed layer of these metals, and in case of 
need, dual-gate construction may be chosen. 
According to the method of making a TFT described above, field effect 
mobility can be increased by transforming an amorphous silicon TFT to a 
poly silicon TFT through a laser process, so that a TFT suitable for a 
picture plane of high resolution like HDTV results. Further, it is 
possible to regulate sub threshold-slop of TFT through regulation of laser 
power and beam area, and thereby it is possible to divide the gray-level 
displayed on LCD. 
Furthermore, the increase of the uniformity of the silicon layer and the 
shortening of the processing time can be achieved by virtue of increase of 
beam area, and a separate processing step is not required and the number 
of processing steps can be reduced because the constructional features of 
a TFT are utilized in crystal core growing. Also, a TFT driving circuit 
can be made simultaneously with making of a TFT, and a TFT with dual gate 
construction can be made. 
Furthermore, improvement of action characteristic of a TFT and easiness of 
processing can be achieved by elimination of the remaining defects and 
impure material in a poly silicon layer in recrystalization by laser beam, 
and the uniformity of picture quality of a TFT LCD can be improved in case 
of recrystallizing with laser beams passing through prisms.