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BACKGROUND OF THE INVENTION
The present invention relates to a method for fabricating a thin film
transistor, and more particularly to a method for fabricating a thin film
transistor to be used as a switching element of a liquid crystal display
device, enabling an easy fabrication and an improvement in device
characteristic by use of a self-alignment.
Generally, thin film transistor-liquid crystal display (TFT-LCD) devices
include a bottom plate formed with TFTs and pixel electrodes and a top
plate formed with color filters and common electrodes. A liquid crystal is
filled in a space defined between the upper plate and the lower plate.
Polarizing plates for linearly polarizing visible rays are attached to
opposed surfaces of the plates comprised of, for example, glass
substrates, respectively.
FIG. 1a is a circuit diagram of an equivalent circuit of a general TFT-LCD
array having the above-mentioned arrangement. FIG. 1b is a circuit diagram
of an equivalent circuit of a unit pixel of the array shown in FIG. 1a.
As shown in FIG. 1a, the TFT-LCD device includes a plurality of gate signal
lines G.sub.1 to G.sub.n each arranged between neighboring pixel regions
in one direction, a plurality of data signal lines D.sub.1 to D.sub.n each
arranged between neighboring pixel regions in a direction perpendicular to
the direction of the gate signal lines, and a plurality of thin film
transistors Q.sub.11 to Q.sub.nn each disposed at each corresponding pixel
region and adapted to apply data voltage from each corresponding one of
the data lines D.sub.1 to D.sub.n to each corresponding pixel electrode
and liquid crystal in accordance with a signal from each corresponding one
of the gate signal lines G.sub.1 to G.sub.n.
In each unit pixel of this TFT-LCD device, a capacitor C.sub.STO and an
additional capacitor C.sub.LC are provided which are formed by virtue of
the TFT serving as the switching element of the unit pixel and the liquid
crystal present between the upper and lower plate electrodes.
In operation of the TFT-LCD device having the above-mentioned arrangement,
a gate signal voltage is selectively applied to the TFT which is the
switching element of each unit pixel. When the TFT receives the gate
signal voltage, it is turned on so that data voltage carrying image
information can be applied to the corresponding pixel electrode and the
liquid crystal via the TFT for 2 hours.
As data voltage is applied to the TFT of each unit pixel, the arrangement
of liquid crystal molecules is changed, resulting in a change in optical
properties. As a result, an image is displayed.
In order to obtain high quality images in this TFT-LCD device, the display
area for displaying an image, namely, the aperture ratio or the opening
ratio should be large. Furthermore, the leakage current from the TFTs
should be minimized.
For improving the aperture ratio, the area occupied by the TFT of each unit
pixel should be reduced. This is because the region where the TFT of each
unit pixel is formed can not display any image.
The data voltage which is applied to the pixel electrode of each unit pixel
and the liquid crystal via the corresponding TFT has to be maintained for
a predetermined time by the capacitors C.sub.STO and C.sub.LC provided by
both the pixel electrode and the liquid crystal even when no gate signal
voltage is applied.
In an ideal case, the total charge amount in the capacitors provided by the
pixel electrode and the liquid crystal is maintained until a next signal
is applied to the TFT which is at its turn-off state. In practical cases,
however, a leakage of current occurs at the TFT. When such a leakage
current is not sufficiently reduced, a distortion of liquid crystal
voltage may occur, resulting in an occurrence of a flicker phenomenon.
Consequently, construction of TFTs is very important to achieve an
improvement in aperture ratio and a decrease in leakage current both
required for obtaining high quality images in TFT-LCD devices.
In other words, as the number of pixels is increased for obtaining a higher
definition and a higher resolution in TFT-LCD devices, the dimension of
each TFT should be reduced. Furthermore, the leakage current should be
negligibly small.
Recently, research has been actively conducted to minimize the leakage
current in small TFTs.
A conventional method for fabricating a TFT will be described in
conjunction with FIGS. 2a to 2e.
This conventional method is used for fabricating an etch stopper type TFT.
In accordance with this method, an opaque metal layer made of Al, Ta or Cr
is formed on an insulating transparent substrate 1 to provide a gate
electrode 2, as shown in FIG. 2a. Over the entire exposed surface of the
resulting structure, a gate insulating film 3, an amorphous silicon layer
4 and etch stopper layer 5 are sequentially deposited by a plasma enhanced
chemical vapor deposition (PECVD) process. Thereafter, a photoresist film
9 is coated over the etch stopper layer 5.
Subsequently, the photoresist film 9 is subjected to a hard baking at a
temperature of 110.degree. C. Using the gate electrode 2 as a mask, the
resulting structure is subjected at the lower surface of the substrate 1
to a back light exposure by use of the self-alignment technique, as shown
in FIG. 2b.
In this back light exposure, the positive photoresist film 9 is etched by a
developer at its portion receiving light beams while partially remaining
at its portion not receiving light beams because of the opaque gate
electrode 2, namely, disposed just above the gate electrode 2. The
remaining photoresist portion. Serves as a photoresist pattern.
At this time, the backward incident light beams are refracted inward of the
gate electrode 2 at edges of the gate electrode 2 due to their scattering
and diffraction phenomenons. As a result, the photoresist pattern has a
dimension smaller than that of the pattern of the gate electrode 2.
Using the patterned photoresist film 9 as a mask, the etch stopper layer 5
is selectively removed at its exposed portion, as shown in FIG. 2c. At
this time, the overlap length .DELTA.L between the gate electrode 2 and
the etch stopper layer 5 is proportional to the energy of the incident
light. For example, the overlap length .DELTA.L is less than 1 .mu.m at
the incident light energy of 0.5 J/cm.sup.2.
Thereafter, an, amorphous silicon layer 6 doped with high concentration
n-type impurity ions and a metal layer 7 are sequentially deposited over
the entire exposed surface of the resulting structure, as shown in FIG.
2d.
The high concentration n-type amorphous silicon layer 6 and the metal layer
7 are selectively removed at their portions disposed over the etch stopper
layer 5 so as to form source and drain electrodes 7a and 7b, as shown in
FIG. 2e. Thus a TFT is obtained.
The operation of the TFT fabricated in accordance with the conventional
method will now be described.
When a voltage not lower than the threshold voltage is applied to the gate
electrode 2, a channel is formed at the interface between the amorphous
silicon layer 4 and the gate insulating film 3, thereby causing the source
and the drain to be electrically communicated with each other.
However, this conventional method has the following problems.
In the TFT which is used as the switching element in LCD devices, as shown
in FIG. 3, a channel is generally formed at the interface between the gate
insulating film and the amorphous silicon (a-Si) layer. As a result, where
no overlap is present between the gate electrode and the source/drain
electrodes, an offset region is formed between the amorphous silicon layer
and the source electrode, thereby causing the TFT not to operate. On the
contrary, where the overlap length is excessively large, the TFT is
enlarged in dimension, thereby resulting in a decrease in aperture ratio.
In addition, a parasitic capacitance may be present between the gate
electrode and the source/drain electrode. When the TFT is turned off, such
a parasitic capacitance affects the liquid crystal voltage due to its
capacitive coupling. As a result, the liquid crystal voltage is varied by
.DELTA.V, thereby resulting in a degradation in picture quality.
It is, accordingly, preferred that the overlap length between the gate
electrode and the source/drain electrode is one to two .mu.m.
In the fabrication of TFTs in accordance with the conventional method, a
back light exposure is carried out by utilizing the self-alignment
technique under a condition that the single gate insulating film 3 has
been formed and the gate electrode 2 is used as a mask. In this back light
exposure, light beams are refracted inward of the gate electrode 2 at
edges of the gate electrode 2 due to their scattering and diffraction
phenomenons, as mentioned above. As a result, the overlap length of not
less than one .mu.m can not be obtained, even though the pattern of the
photoresist film 9 is smaller than the pattern of the gate electrode 2. To
obtain an increased overlap length, the light exposure should be performed
using high energy for a long time.
However, such a light exposure shortens the life of the exposure equipment
and lengthens the time of the exposure process step. As a result, the
yield is decreased.
Since only the etch stopper layer is etched by the self-alignment technique
in accordance with the conventional method, the amorphous silicon layer
serving as an active layer of the TFT has a width larger than that of the
gate electrode. As a result, back light enters the amorphous silicon layer
in the operation of the TFT-LCD device, thereby causing electrons to be
excited in the amorphous silicon layer. This causes an increase in leakage
current.
In particular, where the conventional method is used in the fabrication of
LCDs for over head projectors requiring the quantity of light being forty
times or above as large as those of LCDs for office automation, the
leakage current is more increased while the ON/OFF ratio of TFT is
decreased. As a result, a flicker phenomenon occurs, resulting in a
degradation in LCD performance.
SUMMARY OF THE INVENTION
Therefore, an object of the invention is to solve the above-mentioned
problems encountered in the prior art and to provide a method for
fabricating a TFT capable of adjusting an overlap length up to two .mu.m
or above by use of a full self alignment and reducing the width of a
semiconductor layer to the width of a gate electrode or below, providing
improved in TFT-LCD performance and a simplified fabrication.
In accordance with the present invention, this object can be accomplished
by providing a method for fabricating a thin film transistor, comprising
the steps of: forming a gate electrode on an insulating transparent
substrate; stacking a plurality of gate insulating films having different
refractive indices, in the order of lower refractive index, over the
entire exposed surface of the resulting structure after the formation of
said gate electrode, and then sequentially depositing a semiconductor
layer, an etch stopper layer and a photoresist film, in this order, over
the entire exposed surface of the resulting structure after the formation
of said gate insulating films; subjecting the resulting structure to a
back light exposure using said gate electrode as a mask and then to a
development for patterning said photoresist film so that the gate
electrode can be overlapped by a predetermined overlap length with each of
a source electrode and a drain electrode to be formed at a subsequent
step; selectively etching said etch stopper layer using said patterned
photoresist film as a mask; removing the patterned photoresist film and
then sequentially depositing a high concentration n type doped
semiconductor layer and a metal layer over the entire exposed surface of
the resulting structure; and selectively removing the respective portions
of said high concentration n-type doped semiconductor layer and said metal
layer disposed over the patterned etch stopper layer to form said source
electrode and said drain electrode.
BRIEF DESCRIPTION OF THE DRAWINGS
Other objects and aspects of the invention will become apparent from the
following description of embodiments with reference to the accompanying
drawings in which:
FIG. 1a is a circuit diagram of an equivalent circuit of a general TFT-LCD
array;
FIG. 1b is a circuit diagram of an equivalent circuit of a unit pixel of
the array shown in FIG. 1a;
FIGS. 2a to 2e are sectional views respectively illustrating a conventional
method for fabricating a TFT;
FIG. 3 is a schematic sectional view explaining problems encountered in
TFTs fabricated in accordance with the conventional method;
FIGS. 4a to 4f are sectional views respectively illustrating a method for
fabricating a TFT in accordance with a first embodiment, of the present
invention;
FIG. 5 is a schematic view showing a path of light passing through
different mediums;
FIG. 6 is a table illustrating refractive indices of various insulating
materials;
FIG. 7 is a sectional view illustrating a method for fabricating a TFT in
accordance with a second embodiment of the present invention;
FIGS. 8a to 8d are sectional views respectively illustrating a method for
fabricating a TFT in accordance with a third embodiment of the present
invention;
FIGS. 9a to 9d are sectional views respectively illustrating a method for
fabricating a TFT in accordance with a fourth embodiment of the present
invention; and
FIGS. 10a to 10f are sectional views respectively illustrating a method for
fabricating a TFT in accordance with a fifth embodiment of the present
invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIGS. 4a to 4f are sectional views respectively illustrating a method for
fabricating a TFT in accordance with a first embodiment of the present
invention.
In accordance with the first embodiment, an opaque metal layer made of Al,
Cr, Ta or Ti is formed on an insulating transparent substrate 11 and then
patterned to provide a gate electrode 12, as shown in FIG. 4a.
Over the entire exposed surface of the resulting structure, a first gate
insulating film 13 having a high refractive index and a second gate
insulating film 14 having a low refractive index are sequentially
deposited, as shown in FIG. 4b. Preferably, the first gate insulating film
13 has a refractive index of more than two whereas the second gate
insulating film 14 has a refractive index of not more than two. By
referring to FIG. 6 showing refractive indices of various insulating
materials, Ta.sub.2 O.sub.5 and TiO.sub.2 exhibiting a refractive index of
more than two may be used for the first gate insulating film 13 while
Al.sub.2 O.sub.3, SiO.sub.2 and SiO.sub.x N.sub.y may be used for the
second gate insulating film 14. By virtue of this difference in the
refractive index between the gate insulating films 13 and 14, it is
possible to obtain an overlap length of about two .mu.m, as will be
described hereinafter.
Where the first gate insulating film 13 is comprised of a Ta.sub.2 O.sub.5
insulating film formed by the anodic oxidation, its refractive index can
be adjusted within a range of 2 to 2.7, depending on the used condition.
Subsequently, a semiconductor layer 15, an etch stopper layer 16 and a
photoresist film 17 are sequentially deposited over the second gate
insulating film 14. The semiconductor layer 15 may be made of polysilicon
or amorphous silicon. The etch stopper layer 16 is made of SiN.sub.x.
Using the gate electrode 12 as a mask, the resulting structure is then
subjected to a back light exposure by use of the self-alignment technique,
as shown in FIG. 4c. In this back light exposure, incident light beams are
refracted inward of the gate insulating films 13 and 14.
FIG. 5 shows a path of light passing through different mediums. An optical
path defined in two mediums having different refractive indices n.sub.1
and n.sub.2 can be represented by the following equation according to
Snell's law:
n.sub.1 Sin.theta..sub.1 =n.sub.2 Sin.theta..sub.2 (1)
wherein,
.theta..sub.1 : travel angle of light passing through the medium of n.sub.1
;
.theta..sub.2 : travel angle of light passing through the medium of
n.sub.2.
In case of n.sub.1 >n.sub.2, .theta..sub.1 is smaller than .theta..sub.2.
On the contrary, .theta..sub.1 is greater than .theta..sub.2 in case of
n.sub.1 <n.sub.2.
Accordingly, where the first gate insulating film 13 and the second gate
insulating film 14 are made of a material having a high refractive index
and a material having a low refractive index, respectively, as mentioned
above, incident light beams at the back light exposure step are refracted
inward of the gate electrode 12 at the portions "a" of first gate
insulating film 13 disposed at edges of the gate electrode 12 due to their
refraction phenomenon. Also at the interface portions "b" between the
first gate insulating film 13 and the second gate insulating film 14, the
incident light beams are more refracted inward of the gate electrode 12.
As a result, the photoresist film 17 is exposed to the light such that an
increased overlap length is obtained.
Thereafter, the light-exposed photoresist film 17 is developed and patter
so that it remains only over the gate electrode 12 to form a photoresist
film pattern, as shown in FIG. 4d. Using the patterned photoresist film 17
as a mask, the etch stopper layer 16 is then selectively removed at its
portion exposed after the patterning of the photoresist film 17.
Thereafter, the photoresist film 17 is removed.
Over the entire exposed surface of the resulting structure, a semiconductor
layer 18 doped with high concentration n-type impurity ions and a metal
layer 19 are sequentially deposited, as shown in FIG. 4e.
The high concentration n-type doped semiconductor layer 18 and the metal
layer 19 are selectively removed at their portions disposed over the etch
stopper layer 16 so as to form source and drain electrodes 19a and 19b, as
shown in FIG. 4f. Thus a TFT is obtained.
On the other hand, FIG. 7 is a schematic sectional view illustrating a
method for fabricating a TFT in accordance with a second embodiment of the
present invention. This second embodiment is similar to the first
embodiment, except for the use of a triple gate insulating film structure
including three gate insulating films having different refractive indices.
In FIG. 7, elements corresponding to those in FIGS. 4a to 4f are denoted
by the same reference numerals.
In accordance with the second embodiment, an opaque metal layer made of Al,
Cr, Ta or Ti is formed on an insulating glass substrate 11 and then
patterned to provide a gate electrode 12.
Over the gate electrode 12, a first gate insulating film 20 made of
Ta.sub.2 O.sub.5 or TiO.sub.2 having a refractive index of more than two
is formed by the anodic oxidation process. Over the entire exposed surface
of the resulting structure, a second gate insulating film 21 made of a
material such as SiO.sub.2 having a refractive index of one to two and a
third gate insulating film 22 made of a material having a refractive index
of greater than one but less than that of the second gate insulating film
are sequentially deposited. Subsequently, a semiconductor layer 15, an
etch stopper layer 16 and a photoresist film 17 are sequentially deposited
over the third insulating film 22. Using the gate electrode 12 as a mask,
the resulting structure is then subjected to a back light exposure by use
of the self-alignment technique in a manner as described in conjunction
with FIG. 4c. Thereafter, a development is carried out for patterning the
photoresist film 17. Subsequent steps for fabricating a TFT are the same
as those of the first embodiment.
The thickness of each of the gate insulating films 20, 21 and 22 is not
less than 1,000 .ANG..
Alternatively, the third gate insulating film 22 having the refractive
index of greater than one may have a thickness of less than 1,000 .ANG.,
while the first gate insulating film 20 having the refractive index of
more than two and the second gate insulating film 21 having the refractive
index of one to two have a thickness of not less than 1,000 .ANG.. Even in
the latter case, the same effect as that in the former case can be
obtained after the back light exposure. In the latter case, it is
preferable to use a SiO.sub.2 film as the second gate insulating film
having the refractive index of one to two and the thickness of not less
than 1,000 .ANG. and a SiN.sub.x film as the third gate insulating film
having the refractive index of one to two and the thickness of less than
1,000 .ANG..
FIGS. 8a to 8d are sectional views respectively illustrating a method for
fabricating a TFT in accordance with a third embodiment of the present
invention. This method utilizes the self-alignment technique for achieving
a back exposure and the photolithography and etching process for achieving
a simultaneous patterning of both an etch stopper layer and a
semiconductor layer. In FIGS. 8a to 8d, elements corresponding to those in
FIGS. 4a to 4f are denoted by the same reference numerals.
In accordance with the third embodiment, an opaque metal layer is formed on
an insulating transparent substrate 11 and then patterned to provide a
gate electrode 12, as shown in FIG. 8a. Over the entire exposed surface of
the resulting structure, a first gate insulating film 13 having a high
refractive index and a second gate insulating film 14 having a low
refractive index are sequentially deposited. Subsequently, a semiconductor
layer 15, an etch stopper layer 16 and a photoresist film 17 are
sequentially deposited over the second gate insulating film 14. The
semiconductor layer 15 may be made of polysilicon or amorphous silicon.
The etch stopper layer 16 is made of SiN.sub.x. Using the gate electrode
12 as a mask, the resulting structure is then subjected to a back light
exposure by use of the self-alignment technique in a manner as described
in conjunction with FIG. 4c. Thereafter, a development is carried out for
patterning the photoresist film 17 such that the photoresist film 17 can
have a sufficient overlap.
Using the patterned photoresist film 17 as a mask, both the etch stopper
layer 16 and the semiconductor layer 15 are then selectively subjected to
a taper etching so as to remove their portions exposed after the
patterning of the photoresist film 17, as shown in FIG. 8b, at a taper
angle of not more than 20.degree. for example. Of course, a vertical
etching may be used for the removal of the exposed portions of the layers
15 and 16. Thereafter, the photoresist film 17 is removed.
The taper etching is achieved by wet etching the etch stopper layer 16
using a buffered oxide etchant (BOE) solution and then dry etching the
semiconductor layer 15 using an etching gas of CF.sub.4 +O.sub.2 or
C.sub.2 ClF.sub.5 +O.sub.2. Where the semiconductor layer 15 is comprised
of an amorphous silicon layer, the taper etching may be achieved at a
taper angle of not more than 20.degree. using an etching gas of C.sub.2
ClF.sub.5 : O.sub.2 =5 : 4.
Alternatively, both the etch stopper layer 16 and the semiconductor layer
15 are patterned by the dry etching process. Where the etch stopper layer
16 and the semiconductor layer 15 are comprised of a SiN.sub.x layer and
an amorphous silicon layer, respectively, they can be simultaneously
taper-etched using an etching gas of C.sub.2 ClF.sub.5 : SF.sub.6 :
O.sub.2 =6 : 4 : 3.
Thereafter, a high concentration n-type doped semiconductor layer 18 and a
metal layer 19 are sequentially deposited over the entire exposed surface
of the resulting structure, as shown in FIG. 8c. The high concentration
n-type doped semiconductor layer 18 and the metal layer 19 are selectively
removed at their portions disposed over the etch stopper layer 16 so as to
form source and drain electrodes 19a and 19b, as shown in FIG. 4f. Thus a
TFT is obtained.
In this a TFT fabricated in accordance with the third embodiment, the
semiconductor layer 15 serving as an active layer of the TFT has a width
smaller than that of the gate electrode 12.
FIGS. 9a to 9d fare sectional views respectively illustrating a method for
fabricating a TFT in accordance with a fourth embodiment of the present
invention. In accordance with this method, the taper etching shown in
FIGS. 8a and 8b is used for patterning both an etch stopper layer and a
semiconductor layer. An exposed portion of the semiconductor layer is
implanted with high concentration n-type impurity ions to form a high
concentration n-type doped semiconductor layer. In accordance with this
method, a silicide layer is also formed to reduce a contact resistance at
an interface between the high concentration n-type doped semiconductor
layer and a metal layer subsequently deposited. In FIGS. 9a to 9d,
elements corresponding to those in FIGS. 4a to 4f are denoted by the same
reference numerals.
In accordance with the fourth embodiment, an opaque metal layer is formed
on an insulating substrate 11, and then patterned to provide a gate
electrode 12, as shown in FIG. 9a. Over the entire exposed surface of the
resulting structure, a first gate insulating film 13 having a high
refractive index and a second gate insulating film 14 having a low
refractive index, a semiconductor layer 15, an etch stopper layer 16 and a
photoresist film 17 are sequentially deposited. Using the gate electrode
12 as a mask, the resulting structure is then subjected to a back light
exposure by use of the self-alignment technique. Thereafter, a development
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