Structure of a liquid crystal display device and a method of manufacturing same

A liquid crystal display device comprising a substrate; a first metal layer including an aluminum alloy having a first refractory metal with a first melting temperature over the substrate; and a second metal layer including a pure aluminum or an aluminum alloy having a second refractory metal with a second melting temperature lower than the first melting temperature over the first metal layer. The above liquid crystal display device of the present invention prevents the occurrence of hillocks on the aluminum gate metal. A method of manufacturing a liquid crystal display device is disclosed including the steps of forming a first metal layer of an aluminum alloy including a refractory metal having a first melting temperature on a substrate; and forming a second metal layer of a pure aluminum or an aluminum alloy including a refractory metal having a second melting temperature lower than the first melting temperature on the first metal layer.

BACKGROUND OF THE INVENTION 
A. Field of the Invention 
The present invention relates to a liquid crystal display device, and more 
particularly, to a liquid crystal display device in which the generation 
of hillocks is controlled. 
B. Description of the related art 
Thin film transistors (TFT) have been widely used as a switching device for 
switching, in response to an image signal, picture elements (pixel) in an 
active matrix liquid crystal display device. This so called "TFT-LCD" is 
composed of a bottom plate on which thin film transistors and 
corresponding pixel electrodes are formed, a top plate having color 
filters for corresponding pixel electrodes and a common electrode, and 
liquid crystal filled between the top and bottom plates. 
In the thin film transistor (TFT) of the above TFT-LCD, to reduce a line 
delay, an aluminum metal film having a low resistivity is used for a gate 
electrode, a gate line, source/drain electrodes, a pad and the like. 
However, this type of TFTs have a problem in that hillocks are generated 
on the surface of the aluminum metal film due to a compressive stress 
generated between the aluminum metal and an insulating substrate on which 
the aluminum metal is formed. 
The compressive stress is generated because the insulating substrate is 
bent due to the difference in the coefficients of thermal expansion 
between the insulating substrate and the aluminum metal during the 
subsequent high temperature process (e.g., deposition of a gate insulating 
film by plasma enhancement chemical vapor disposition "PECVD" and the 
like). More specifically, since the coefficient of thermal expansion of 
aluminum Al (about 20.times.10.sup.-6 /.degree.C.) is greater than that of 
glass (about 4.times.10.sup.-6 /.degree.C.) which constitutes the 
insulating substrate, the glass substrate inhibits the expansion of the 
aluminum metal film during the high temperature process, causing the 
substrate to bend or bulge. 
When such a compressive stress is applied to the aluminum metal film, 
aluminum atoms in the Al metal film tend to diffuse from the place where 
the stress is high to the place where the stress is low. Then, the 
aluminum atoms rise to the surface of the aluminum metal film to relieve 
the compressive stress, thereby generating the hillock. 
Since the film which is deposited on the aluminum metal film in the 
subsequent process may not go over the sudden rise (or step) of the 
hillock, the metal line is shorted due to enchant in the subsequent 
etching process, deteriorating the process yield. To solve the above 
problem, various approaches have been proposed. For example, FIG. 1 is a 
plan view of a signal line of a conventional liquid crystal display 
device. The signal line is composed of a gate line 1 arranged on a 
substrate, a gate electrode 2 connected to gate line 1 (i.e., composed of 
2a and in a vertical direction, and a pad 3 (i.e., composed of 2c 
connected to gate line 1 in a horizontal direction. 
FIG. 2 is a cross-sectional view of the liquid crystal display of FIG. 1, 
taken along lines A-A', B-B', and C-C'. The cross-section taken along line 
A-A' is that of the gate electrode. The cross-section taken along line 
B-B' is that of the gate line. The cross-section taken along line C-C' is 
that of the pad. An Al metal line 2a, 2b, 2c of a single film structure is 
formed on an insulating substrate 1. An anodic oxide film 3 is formed on 
the surface of each of Al metal lines 2a and 2b (i.e., gate lines 2a and 
2b), and on Al metal line 2c (i.e, pad part 2c) except for an open portion 
thereof. 
Further, a refractory metal 4 is formed on the portion of Al metal line 2c 
which is not covered by anodic oxide film 3. A gate insulating film 5 is 
formed on the entire surface of insulating film 1 on which each Al metal 
line 2a, 2b, 2c is formed except for the portion of Al metal line 2c on 
which the refractory metal is formed. 
FIGS. 3a, 3b, 3c and 3d are cross-sectional views for illustrating a 
conventional method of manufacturing the liquid crystal display of FIG. 1, 
taken along lines A-A', B-B', and C-C'. The cross-section taken along the 
line A-A' is that of the gate electrode. The cross-section taken along 
line B-B' is that of the gate line. The cross-section taken along line 
C-C' is that of the pad. Referring to FIG. 3a, an Al metal is deposited on 
an insulating substrate 1. The Al metal is selectively removed 
photo-lithographically to form a gate electrode 2a, a gate line 2b, and a 
pad 2c constituting Al metal lines. 
Referring to FIG. 3b, a photoresist (PR) film (not shown) is formed on the 
entire surface of insulating substrate 1 on which Al metal line 2a, 2b, 2c 
is formed. Then, the PR film is patterned so as to be left only on Al 
metal line 2c of the pad. Using the patterned PR film as a mask, through 
anodic oxidation, an anodic oxide film 3 of Al.sub.2 O.sub.3 is formed on 
the surface of Al metal lines 2a and 2b of the gate electrode and gate 
line, respectively, and on Al metal line 2c of the pad except for the part 
of the pad on which the PR is left (i.e., open portion of Al metal line 2c 
of the pad). The anodic oxidize film is not formed on the open portion of 
Al metal line 2c of the pad, to maintain the current flow in the electrode 
part performing TAB when the LCD is operated. 
Referring to FIG. 3c, after removing the PR pattern, a refractory metal is 
deposited on the entire surface of insulating substrate 1 on which the Al 
metal line is formed. The refractory metal is photo-lithographically 
removed selectively to form a refractory metal film 4 only on the open 
portion of Al metal line 2c where the pad is exposed. The refractory metal 
is left only on the open portion of the pad to suppress the hillock 
generated on the surface of the Al metal during the subsequent high 
temperature process (e.g., deposition of a gate insulating film by PECVD 
and the like). 
Since no anodic oxide film is formed on the open portion of the pad, the 
hillock is generated on the surface of the Al metal of the pad during the 
subsequent high temperature process. This causes the metal line to be 
shorted due to enchant in the subsequent process, thereby deteriorating 
the yield. Since the refractory metal is left on the open portion of the 
pad, the hillock is suppressed due to the mechanical force applied from 
refractory metal film 4. 
Referring to FIG. 3d, an insulating film is formed on the entire surface. 
Then, a PR film is formed on the entire surface except for refractory 
metal film 4 of the open portion of the pad. Using the PR film patterned 
as such as a mask, a gate insulating film 5 of the pad is removed. Then, 
the patterned photoresist film is removed, thereby forming a pad's open 
metal line 2c without gate insulating film 5 thereover. 
In the above approach, an Al metal having a low resistivity is used as the 
signal line to minimize the line delay, but the hillock generated during 
the high temperature process is not suppressed by the anodic oxide film 
formed on the Al metal. Due to the generation of the hillock, the metal 
line is shorted by the enchant in the subsequent process, thereby 
deteriorating the yield. 
In an alternative approach for suppressing the generation of the hillock on 
the aluminum metal, the gate electrode is formed in a single film 
structure of an aluminum alloy. In such an aluminum alloy, an impurity 
metal such as Ta, Si and Cu is added to an aluminum site in several atomic 
percents to suppress the diffusion of the aluminum, thereby inhibiting the 
generation of the hillock. However, this approach has a drawback in that 
it tends to increase the line resistivity, thus making it inapplicable to 
large area and/or and high definition TFT LCDs. 
For example, FIG. 4 is a cross-sectional view for illustrating another 
conventional method for making the liquid crystal display (shown in 
Japanese Laid-Open patent application 6-104437), taken along lines A-A', 
B-B', and C-C' in FIG. 1. The cross-section taken along line A-A' is that 
of the gate electrode. The cross-section taken along line B-B' is that of 
the gate line. The cross-section taken along line C-C' is that of the pad. 
Referring to FIG. 4, each Al/AlTa metal line 2a, 2b, 2c is formed on an 
insulating substrate 1, and composed of an Al metal film and an AlTa metal 
film stacked on the Al metal film. An anodic oxide film 3 is formed on the 
surface of Al/AlTa metal lines 2a (i,e., gate electrode) and 2b (i.e., 
gate line) and on the sidewalls of Al/AlTa metal line 2c of the pad. A 
gate insulating film 4 is formed over the entire surface of insulating 
substrate 1 on which Al/AlTa metal line 2a, 2b, 2c is formed except for 
the open portion of Al/AlTa metal line 2c of the pad. A refractory metal 5 
is formed on the open portion of Al/AlTa metal line 2c of the pad and on a 
portion of gate insulating film 4. 
FIGS. 5a through 5e are cross-sectional views for illustrating yet another 
conventional method of manufacturing the liquid crystal display taken 
along lines A-A', B-B', and C-C'. The cross-section taken along line A-A' 
is that of the gate electrode. The cross-section taken along line B-B' is 
that of the gate line. The cross-section taken along line C-C' is that of 
the pad. 
Referring to FIG. 5a, an Al metal is deposited on an insulating substrate 
1. An AlTa metal of an Al alloy having a refractory metal added to AlTa is 
deposited on the Al metal. The combined Al metal and AlTa metal film 
structure is selectively removed photo-lithographically, to form a Al/AlTa 
metal line 2 of a double film structure in which the AlTa film is stacked 
on the Al film. 
Referring to FIG. 5b, a photoresist film 6 is formed on the entire surface 
of insulating substrate 1 on which each Al/AlTa metal line 2a, 2b, 2c is 
formed. Then, photoresist film 6 is patterned so as to be left only on the 
top portion of Al/AlTa metal line 2c of the pad. 
Referring to FIG. 5c, using the patterned photoresist film as a mask, 
through anodic oxidation, an anodic oxide film 3 is formed on the surface 
of Al/AlTa metal line 2a, 2b of the gate electrode and gate line, 
respectively, and only on the sidewalls of Al/AlTa metal line 2c so as to 
expose the pad for electrical connection to the outside. An anodic oxide 
film, Al.sub.2 O.sub.3 is formed on the sidewalls of the Al metal film of 
Al/AlTa metal line 2a, 2b, 2c, and AlTaOx is formed on the surface of the 
AlTa metal film of Al/AlTa metal line 2a, 2b, and of the sidewalls of 
metal line 2c. 
Referring to FIG. 5d, a gate insulating film 4 is deposited on the entire 
surface of insulating substrate 1 on which Al/AlTa metal line 2a, 2b, 2c 
is formed. A gate insulating film 4 is removed photo-lithographically only 
from the top portion of Al/AlTa metal line 2c of the pad. 
Referring to FIG. 5e, a refractory metal is deposited on the entire surface 
of gate insulating film 4, and Al/AlTa metal line 2c in which the pad 
portion is exposed. The refractory metal is selectively removed 
photo-litho-graphically, to form a refractory metal film 5 so as to be 
left only on the open portion of Al/AlTa metal line 2c. In the open 
portion of the pad where no anodic oxide film is formed, the generation of 
the hillock is inhibited only by the mechanical force applied thereto by 
the AlTa film during the subsequent high temperature process (e.g., 
deposition of the gate insulating film by PECVD or CVD). To enhance such 
inhibition, refractory metals such as Cr, Mo and Ta are additionally 
deposited to cover the open portion of the pad. 
According to the above approach, where a pure Al is used for the gate 
electrode, and an Al alloy having a refractory metal added to the Al metal 
is formed on the top of the Al electrode, the anodic oxide film and/or 
aluminum alloy (AlTa) over the Al electrode suppresses the hillock 
generated on the surface of the Al electrode during the subsequent high 
temperature process, due to the mechanical force applied to the Al 
electrode. However, the above approach has several drawbacks. 
For example, the structure of the glass substrate/Al film/AlTa film stacked 
in that order does not suppress the Al diffusion caused by the compressive 
stress generated between the glass substrate and the Al film during the 
high temperature process. Therefore, when the mechanical force applied to 
the Al metal by the layers formed over the Al metal is not sufficient 
and/or no anodic oxidate film is formed over the Al metal, the generation 
of the hillock cannot be controlled sufficiently only by the mechanical 
force of the AlTa. It can reduce the number of hillocks, but large size 
hillocks still appear. 
Further, to suppress the generation of the large size hillock, the Al alloy 
deposited on the Al metal is designed to have a thickness (i.e., about 
2.about.3000 .ANG.) greater than that of the Al metal (i.e., about 1000 
.ANG.). However, the deposition of thicker Al alloy having a higher 
resistivity on the Al metal increases the overall line delay. Furthermore, 
since the Al alloy (AlTa) has a high resistivity, it is not suitable for 
AM-LCDs with a large picture size, high aperture ratio or high resolution. 
SUMMARY OF THE INVENTION 
In order to solve the aforementioned problems, it is an object of the 
present invention to provide the structure of a liquid crystal display 
device and a method of manufacturing same, in which a process is 
simplified and a buffer layer is formed between a glass substrate and an 
Al metal film, thereby preventing the hillock. 
To accomplish the object of the present invention, a liquid crystal display 
device, as embodied and broadly defined herein, includes a substrate; a 
first metal layer including an aluminum alloy having a first metal with a 
first melting temperature formed over the substrate; and a second metal 
layer including a pure aluminum or an aluminum alloy having a second metal 
with a second melting temperature lower than the first melting temperature 
formed over the first metal layer. 
To accomplish the object of the present invention, a method of making a 
liquid crystal display device, as embodied herein, includes forming a 
first metal layer including an aluminum alloy having a first metal with a 
first melting temperature over a substrate; and forming a second metal 
layer including a pure aluminum or an aluminum alloy having a second metal 
with a second melting temperature lower than the first melting temperature 
over the first metal layer. 
It is to be understood that both the foregoing general description and the 
following detailed description are exemplary and explanatory only and are 
not restrictive of the invention, as claimed. 
The accompanying drawings, which are incorporated in and constitute a part 
of this specification, illustrate several embodiments of the invention and 
together with the description, serve to explain the principles of the 
invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Reference will now be made in detail to the present preferred embodiments 
of the invention, examples of which are illustrated in the accompanying 
drawings. 
FIG. 6 is a plan view of a liquid crystal display device according to an 
embodiment of the present invention. A gate line 1 is arranged. A gate 
electrode 2 is formed so as to be connected with gate line 1 substantially 
perpendicular to gate line 1. A data line 3 is formed in the direction 
crossing gate line 1 perpendicularly. 
FIGS. 7a through 7c are various cross-sectional views for illustrating a 
method for making the liquid crystal display of FIG. 6 according to an 
embodiment of the present invention, taken along lines A-A', B-B', and 
C-C'. The cross-section taken along line A-A' is that of the thin film 
transistor including gate electrode 2. The cross-section taken along line 
B-B' is that of the gate line. The cross-section taken along line C-C' is 
that of the pad representing data line 3. 
In the structure of the liquid crystal display device of the present 
invention, referring to FIGS. 7b and 7c, each first metal layer 2a, 2b, 2c 
is formed on a substrate 1 and preferably composed of an aluminum alloy. 
In the Al alloy, an impurity is added to an aluminum site including a 
refractory metal having a first melting temperature. On each first metal 
layer 2a, 2b, 2c, a corresponding second metal layer 3a, 3b, 3c, 
respectively, is formed. Each second metal layer 3a, 3b, 3c is preferably 
composed of a pure aluminum, or an aluminum alloy preferably having a 
refractory metal with a second melting temperature lower than the first 
melting temperature. 
Further, a first anodic oxide film 4a is formed on the sidewalls of each 
first metal layer 2a, 2b, 2c; and a second anodic oxide film 4b is formed 
on the surface of each second metal layer 3a, 3b, and also on the surface 
of second metal layer 3c except for an open portion of second metal layer 
3c of the pad. A first insulating film 5 is formed on the entire surface 
of insulating substrate 1 and the entire surface of first and second metal 
layers 2a.about.2c and 3a.about.3c except for the open portion of second 
metal layer 3c of the pad. 
A preferred manufacturing method of the above structure will be described 
hereinafter. Referring to FIG. 7a, each first metal layer 2a, 2b, 2c, 
which is formed on an insulating substrate 1, as embodied herein, is 
preferably composed of an aluminum alloy. The Al alloy preferably 
includes, by about 0.1.about.2 atomic %, a refractory metal having a 
melting temperature greater than about 1500.degree. C. Subsequently, a 
second metal layer 3a, 3b, 3c having a pure aluminum, or an Al alloy 
having a refractory metal with a melting temperature less than about 
1500.degree. C., which is added to Al by about 0.1.about.2 atomic %, is 
formed on first metal layer 2a, 2b, 2c, respectively. Each second metal 
layer 3a, 3b, 3c is preferably stacked on first metal layer 2a, 2b, 2c, 
respectively. 
The above refractory metal of first metal layer 2a, 2b, 2c preferably 
includes one or more of tantalum (Ta), titanium (Ti), molybdenum (Mo), 
tungsten (W), niobium (Nb), zirconium (Zr), and vanadium (V). The above 
refractory metal of second metal layer 3a, 3b, 3c preferably includes one 
or more of silicon and copper. In the structure of the present invention 
as described above, during the subsequent high temperature process, 
although no anodic oxide film is formed over the first/second metal 
layers, no hillock is generated on the surface of the second metal layer. 
Referring to FIG. 7b, first metal layer 2a, 2b, 2c and second metal layer 
3a, 3b, 3c are selectively removed photolithographically at the same time 
using a common mask, thereby patterning concurrently the first/second 
metals layers stacked on top of another constituting a double film 
structure. In other words, an island composed of the second metal film 
layer stacked on the first metal layer is concurrently produced. 
During the subsequent high temperature process (e.g., deposition of an 
insulating film SiO.sub.2, SiNx by PECVD or CVD), although no anodic oxide 
film is formed over the first/second metal layers, no hillock is generated 
on the metal layer. 
Referring to FIG. 7b, a photoresist film is patterned so as to be left only 
on the open pad portion of second metal layer 3c on line C-C'. Through 
anodic oxidation process, first anodic oxide film 4a is formed on the 
sidewalls of each first metal layer 2a, 2b, 2c, and second anodic oxide 
film 4b is formed on the entire surface of each of second metal layers 3a 
and 3b, and second metal layer 3c except for the open portion of second 
metal layer 3c on line C-C'. Each of first and second anodic oxide films 
4a and 4b, as embodied herein, preferably has an angle which is not zero 
with respect to the lateral direction of substrate. 
Referring to FIG. 7 
c, a first insulating film 5 (preferably SiO.sub.2, SiNx) is formed over 
the entire surface of insulating substrate 1 and first and second metal 
layers 2a.about.2c and 3a.about.3c. Then, first insulating film 5 is 
photolithographically removed selectively, thereby exposing the open 
portion of second metal layer 3c of the pad on line C-C'. 
FIGS. 8a.about.8e show cross-sectional views for illustrating another 
method of manufacturing the display of FIG. 6 according to an embodiment 
of the present invention, taken lines A-A', B-B', and C-C'. The 
cross-section taken along line A-A' is that of the thin film transistor. 
The cross-section taken along line B-B' is that of the gate line. The 
cross-section taken along line C-C' is that of the pad. 
Referring to FIG. 8a, a first metal layer 2a, 2b, 2c is formed on an 
insulating substrate 1 and preferably composed of an aluminum alloy. The 
Al alloy preferably has a refractory metal with a melting temperature 
greater than about 1500.degree. C. added to the Al by about 0.1.about.2 
atomic %. Then, a second metal layer 3a, 3b, 3c is formed on first metal 
layer 2a, 2b, 2c, respectively. Each second metal layer 3a, 3b, 3c 
preferably includes a pure aluminum or an Al alloy having a refractory 
metal with a melting temperature less than about 1500.degree. C. added to 
the Al by about 0.1.about.2 atomic %. 
The above refractory metal of first metal layer 2a, 2b, 2c preferably 
includes one or more of tantalum (Ta), titanium (Ti), molybdenum (Mo), 
tungsten (W), niobium (Nb), zirconium (Zr), and vanadium (V). The above 
refractory metal of second metal layer preferably includes one or more of 
silicon copper. Then, first metal layer 2a, 2b, 2c and corresponding 
second metal layer 3a, 3b, 3c are photo-lithographically removed 
selectively at the same time using a common mask, thereby forming a 
stacked metal line having a double film structure. 
Referring to FIG. 8b, a photoresist film (not shown) is formed on the 
entire surface of the resultant structure and patterned so as to be left 
only on the open pad portion of second metal layer 3c on line C-C' (pad 
open region). Then, through anodic oxidation, a first anodic oxide film 4a 
is formed on the sidewalls of each first metal layer 2a, 2b and 2c, and a 
second anodic oxide film 4b is formed on the entire surface of each second 
metal layer 3a, 3b on lines A-A' and B-B', and second metal layer 3c on 
line C-C' except for the open pad portion thereof. 
Each of first and second anodic oxide films 4a and 4b preferably has an 
angle which is not zero with respect to the lateral direction of substrate 
1. Then, a first insulating film 5 (preferably SiO.sub.2 or SiNx formed by 
PECVD or CVD) is formed on the entire surface of insulating substrate 1. 
In this structure, even where first insulating film 5 is not formed over 
anodic oxide film 4a, 4c, no hillock is generated. 
Referring to FIG. 8c, first insulating film 5 is photo-lithographically 
removed selectively, so as to expose the open pad portion of second metal 
layer 3c on line C-C'. 
Referring to FIG. 8d, after sequentially depositing a semiconductor layer 
and a doped semiconductor layer on first insulating film 5, a patterned 
semiconductor layer 6 and a patterned doped semiconductor layer 7 are 
photo-lithographically patterned on first insulating film 5 on line A-A'. 
Then, a metal layer is deposited on the entire surface of the resultant 
structure and patterned photo-lithographically to form a third metal layer 
8 of the source/drain electrodes on doped semiconductor layer 7 and second 
metal layer 3c on line C-C'. 
Referring to FIG. 8e, a second insulating film 9 is formed on the entire 
surface of third metal layer 8 and first insulating film 5 and removed 
photo-lithographically to form a contact hole selectively on the drain 
region of third metal layer 8 and to expose a portion of third metal layer 
8 on line C-C'. Then, a transparent conductive film is deposited on the 
entire surface of the resultant structure and patterned 
photolithographically to form a pixel electrode and a transparent 
electrode 10. The pixel electrode is so formed to be connected through the 
contact hole formed on the drain electrode on line A-A', and transparent 
electrode 10 is so formed on a portion of second insulating film 9 and on 
third metal layer 8 on line C-C'. 
In the structure where the glass substrate, AlTa and Al are stacked in this 
order, the Al diffusion caused by the compressive stress generated due to 
the difference in the coefficients of thermal expansion between 
glass/AlTa/Al is suppressed by Ta. Thus, no hillock is generated. 
Further, since the Al film formed on the AlTa film has the same coefficient 
of thermal expansion (and material characteristics) as that of the AlTa, 
the compressive stress is not generated in the interface between the AlTa 
and Al films during the subsequent high temperature process. Since the 
compressive stress which exists between the glass substrate and the Al 
film is relieved by the AlTa (buffer) layer intermediate the glass 
substrate and the Al film, no hillock on the Al film is generated. 
Therefore, according to the present invention, no hillock is generated on 
the Al film, regardless of whether the mechanical force (by anodic oxide 
film and/or refractory metal) is applied to the Al film or not. This 
feature of the invention has been demonstrated through experiments, 
explained below. FIG. 9 is a graph showing the density variation of the 
hillock versus the thickness of the AlTa buffer layer. The annealing is 
performed for about 30 minutes at the temperature of about 320.degree. C. 
The thickness of the Al on the AlTa buffer layer is about 3000.ANG.. 
Referring to FIG. 9, when the AlTa buffer layer is not used (i.e., AlTa 
thickness is zero in FIG. 9), the hillock is generated to a density of 
about 17.times.10.sup.9 /m.sup.2. When the thickness of the AlTa buffer 
layer is 500 .ANG., the hillock is generated to a density of about 
3.times.10.sup.9 /m.sup.2. When the thickness of the AlTa buffer layer is 
equal to or greater than about 1000 .ANG., no hillock is generated. 
FIG. 10 is a graph showing the density variation of the hillock versus the 
annealing temperature. The thickness of the Al on the AlTa buffer layer is 
about 3000 .ANG.. The annealing is performed for about 30 minutes. In the 
case where there is no AlTa buffer layer, but only a pure aluminum, when 
the annealing temperature is about 130.degree. C., the hillock was 
generated to a density of about 3.times.10.sup.9 /m.sup.2. When the 
annealing temperature is increased to about 240.degree. C., the density of 
the hillock increases to about 7.times.10.sup.9 /m.sup.2. When the 
annealing temperature is further increased to about 300.degree. C., the 
density of the hillock increases to about 15.times.10.sup.9 /m.sup.2. When 
the annealing temperature is yet further increased to about 450.degree. 
C., the density of the hillock increases to about 25.times.10.sup.9 /m2. 
In the case where the AlTa buffer layer having a thickness of 500 .ANG. is 
used, when the annealing temperature is about 300.RTM. C., the hillock is 
generated to a density of about 3.times.10.sup.9 /m.sup.2. When the 
annealing temperature is increased to about 450.degree. C., the density of 
the hillock increases to about 20.times.10.sup.9 /m.sup.2. 
Further, in the case where the thickness of the AlTa buffer layer is 
increased to 1000 .ANG., no hillock is generated. 
FIG. 11 is a graph showing the size variation of the hillock with respect 
to various structures of the gate metal. The condition is that the 
annealing is performed for 30 minutes at a temperature of about 
320.degree. C. In case (A) where a pure aluminum having a thickness of 
about 3000 .ANG. is formed directly on the substrate, the height of the 
hillock is about 0.5 .mu.m. That is, there is a large number of hillocks, 
but the size of each hillock is relatively small. 
In case (B) where the Al film with a thickness of about 1000 .ANG. is 
formed on the substrate and the AlTa having a thickness of about 3000 
.ANG. is formed over the Al, the height of the hillock is about 0.7 .mu.m. 
That is, the number of hillocks is decreased. However, since the amount of 
pressure applied to the Al film by the AlTa buffer layer varies over the 
surface of the Al film, the size of the hillock increases in the where the 
amount of the pressure is relatively weak. 
In case (C) where the AlTa buffer layer having a thickness of about 500 
.ANG. is formed on the substrate and the Al film having a thickness of 
about 3000 .ANG. is formed on the AlTa buffer layer, the height of the 
hillock is about 0.2 .mu.m. In case (D) where the AlTa buffer layer having 
a thickness of about 1000 .ANG. is formed on the substrate and the Al film 
having a thickness of about 3000 .ANG. is formed on the AlTa buffer layer, 
no hillock was generated. 
FIG. 12 is a cross-sectional view illustrating a method of making the 
liquid crystal display of FIG. 6 according to an embodiment of the present 
invention, taken along the lines A-A', A-A', B-B', and C-C'. The process 
sequence of FIG. 12 is partially same as that of FIG. 8a through FIG. 8d. 
Referring to FIG. 12, a transparent conductive film is deposited on the 
entire surface of the resultant structure of FIGS. 8a.about.8d and 
patterned photo-lithographically to form a transparent electrode 10 on a 
portion of drain region 8 on line A-A' and on third metal layer 8 on line 
C-C'. Then, an insulating film 9 is deposited on the entire surface and 
patterned photolithographically to form a second insulating film 9 on the 
entire surface of insulating substrate 1 excluding transparent electrode 
10 on third metal layer 8 on line C-C'. 
FIG. 13 is a cross-sectional view for illustrating yet another method for 
making the liquid crystal display of FIG. 6 according to an embodiment of 
the present invention, taken along lines A-A', B-B', and C-C'. The process 
sequence of FIG. 13 is partially same as that for FIG. 8a.about.8c. A go 
semiconductor layer and a doped semiconductor layer are sequentially 
deposited on the entire surface of the resultant structure of FIGS. 
8a.about.8c, and patterned photolithographically. 
Then, a transparent conductive film is deposited on the entire surface and 
selectively patterned photolithographically to form a transparent 
electrode 10 on the pixel region of a first insulating film 5 on line A-A' 
and a second metal layer 3c on line C-C'. Then, a third metal layer is 
deposited on the entire surface and patterned photolithographically using 
a mask for the source/drain, to form a third metal layer 8 on a portion of 
transparent electrode 10 on the line A-A' and transparent electrode 10 on 
the line C-C'. Then, an insulating film is deposited on the entire surface 
and patterned photo-lithographically to form a second insulating film 9 on 
the entire surface of an insulating substrate 1 except for third metal 
layer 8 on line C-C'. 
The present invention provides several unique features. For example, in the 
structure where the glass substrate, AlTa film and Al film are stacked in 
that order, the Al diffusion caused by the compressive stress generated 
due to the difference in the coefficient of thermal expansion between the 
glass substrate, AlTa film, and Al film is suppressed by Ta. Thus, no 
hillock is generated. 
Further, since the Al film formed on the AlTa film has the same coefficient 
of thermal expansion (and material characteristics) as that of the AlTa 
film, the compressive stress is not generated in the interface between the 
AlTa and Al films during the subsequent high temperature process. Since 
the compressive stress which exists between the glass substrate and the Al 
film is relieved by the AlTa buffer layer intermediate the glass and the 
Al film, no hillock on the Al film is generated. Since no hillock is 
generated, the occurrence of a step in the metal line in the subsequent 
process is decreased. Therefore, the shorting of the metal line caused by 
enchant in the etching process is prevented. 
Furthermore, since the defects such as the hillock and the shorting of the 
metal line are prevented, the yield is improved. Yet further, it is 
possible to simultaneously etch the AlTa/Al stacked on the substrate. Yet 
further, since it is not necessary to form a refractory metal layer such 
as Cr, Mo and the like to prevent the hillock on the open portion of the 
pad, the process is further simplified. Since the generation of the 
hillock is prevented, the shorting in the crossing part of the gate line 
and the data line is prevented. 
Other embodiments of the invention will be apparent to those skilled in the 
art from consideration of the specification and practice of the invention 
disclosed herein. It is intended that the specification and examples be 
considered as exemplary only, with a true scope and spirit of the 
invention being indicated by the followings claims.