U.S. patent application number 09/758275 was filed with the patent office on 2001-05-24 for method for crystallizing amorphous silicon thin-film for use in thin-film transistors and thermal annealing apparatus therefor.
This patent application is currently assigned to Seungki Joo. Invention is credited to Joo, Seungki, Kim, Taekyung.
Application Number | 20010001716 09/758275 |
Document ID | / |
Family ID | 22635424 |
Filed Date | 2001-05-24 |
United States Patent
Application |
20010001716 |
Kind Code |
A1 |
Joo, Seungki ; et
al. |
May 24, 2001 |
Method for crystallizing amorphous silicon thin-film for use in
thin-film transistors and thermal annealing apparatus therefor
Abstract
A method for crystallizing an amorphous silicon thin-film is
provided, in which amorphous silicon thin-films on a large-area
glass substrate for use in a TFT-LCD (TFT-Liquid Crystal Display)
are crystallized uniformly and quickly by a scanning method using a
linear lamp to prevent deforming of the glass substrate. The
crystallization method includes the steps of forming an amorphous
silicon thin-film on a glass substrate, and illuminating a linear
light beam on the amorphous silicon thin-film from the upper
portion of the glass substrate according to a scanning method. The
crystallization method is applied to a polycrystalline silicon
thin-film transistor manufacturing method including the steps of
forming an amorphous silicon thin-film on a glass substrate, and
crystallizing the amorphous silicon of the thin-film transistor
according to a scanning method using a linear light beam. In the
scanning illumination of the linear light beam, either one of a
supporting member of the glass substrate and a light source is
relatively moved by a scanning driver apparatus.
Inventors: |
Joo, Seungki; (Seoul,
KR) ; Kim, Taekyung; (Kyungki-do, KR) |
Correspondence
Address: |
McDermott, Will & Emery
600 13th Street, N.W.
Washington
DC
20005-3096
US
|
Assignee: |
Seungki Joo
Seoul
KR
|
Family ID: |
22635424 |
Appl. No.: |
09/758275 |
Filed: |
January 12, 2001 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
09758275 |
Jan 12, 2001 |
|
|
|
09174244 |
Oct 16, 1998 |
|
|
|
Current U.S.
Class: |
438/151 ;
257/E21.134; 257/E21.413; 257/E21.528; 438/156 |
Current CPC
Class: |
H01L 22/26 20130101;
H01L 29/66757 20130101; H01L 21/2026 20130101; H01L 21/02532
20130101; H01L 21/02691 20130101; Y10T 428/24479 20150115; Y10T
428/24562 20150115; H01L 21/02422 20130101; Y10T 428/12819
20150115; H01L 21/02672 20130101; Y10T 428/24512 20150115 |
Class at
Publication: |
438/151 ;
438/156 |
International
Class: |
H01L 021/00; H01L
021/84 |
Claims
What is claimed is:
1. A thermal annealing apparatus for crystallizing an amorphous
silicon thin-film, the thermal annealing apparatus comprising:
supporting means for supporting at least one glass substrate on
which the amorphous silicon thin-film has been formed; a light
source for illuminating a linear light beam to be focused on the
glass substrate from the upper portion of the glass substrate; and
scanning driver means for relatively moving one of the supporting
means and the light source so that the linear light beam can be
illuminated on the silicon thin-film according to a scanning
method.
2. The thermal annealing apparatus according to claim 1, wherein
said supporting means and said scanning driver means are comprised
of a conveyer belt, to successively transfer the glass substrate to
the thermal annealing apparatus.
3. The thermal annealing apparatus according to claim 1, further
comprising a preheater for preheating the glass substrate from the
lower portion of the glass substrate.
4. The thermal annealing apparatus according to claim 3, further
comprising: at least one transmittivity detection sensor for
detecting the transmittivity of the light illuminated from said
preheater to the glass substrate; and a controller for controlling
a scanning velocity of said scanning deriver means based on the
transmittivity data received from said transmittivity detection
sensor.
5. The thermal annealing apparatus according to claim 1, wherein
said controller controls the power of the lamp based on the
received transmittivity data.
6. The thermal annealing apparatus according to claim 4, further
comprising a detection pattern installed along the transfer
direction of the substrate on one side of the glass substrate,
which can allow monitoring of a crystallization process of the
amorphous silicon according to the scanning of the light beam, and
said transmittivity detection sensor detects the transmittivity of
the light received via the detection pattern.
7. The thermal annealing apparatus according to claim 6, wherein
said detection pattern is comprised of an amorphous silicon of a
strip pattern.
8. The thermal annealing apparatus according to claim 1, wherein
said light source comprises: at least one linear lamp disposed in
the direction perpendicular to the scanning direction; and a
focusing unit for focusing the light emitted from the linear lamp
so that the light is converged as a linear light beam on the
amorphous silicon thin-film.
9. An amorphous silicon thin-film crystallization method comprising
the steps of: forming an amorphous silicon thin-film on a glass
substrate; and illuminating a linear light beam on the amorphous
silicon thin-film from the upper portion of the glass substrate
according to a scanning method.
10. The crystallization method according to claim 9, further
comprising the step of forming at least one metal thin-film pattern
for crystallization induction on the amorphous silicon
thin-film.
11. The crystallization method according to claim 10, wherein said
metal thin-film is comprised of one selected from a group composed
of Ni, Fe, Co, Ru, Rh, Pd, Os, Ir, Pt, Sc, Ti, V, Cr, Mn, Cu, Zn,
Au and Ag or an alloy thereof.
12. The crystallization method according to claim 10, further
comprising the step of forming a transparent capping oxide layer on
the metal thin film pattern.
13. A method for manufacturing a polycrystalline silicon thin-film
transistor employing the above crystallization method, comprising
the steps of: forming an amorphous silicon thin-film on a glass
substrate; and crystallizing the amorphous silicon of the thin-film
transistor according to a scanning method using a linear light
beam.
14. The polycrystalline silicon thin-film transistor manufacturing
method according to claim 13, wherein said step of forming the
amorphous silicon thin-film transistor comprises the sub-steps of:
forming an active layer composed of the amorphous silicon on the
glass substrate; forming a gate insulation film and a gate
electrode on the active layer in turn; and depositing a metal
thin-film on the resultant glass substrate, wherein the amorphous
silicon with respect to a channel region positioned in the lower
portion of the gate insulation film is crystallized by a
metal-induced lateral crystallization (MILC) method using the metal
thin-film.
15. The polycrystalline silicon thin-film transistor manufacturing
method according to claim 14, wherein said metal thin-film is
comprised of one selected from a group composed of Ni, Fe, Co, Ru,
Rh, Pd, Os, Ir, Pt, Sc, Ti, V, Cr, Mn, Cu, Zn, Au and Ag or an
alloy thereof.
16. The polycrystalline silicon thin-film transistor manufacturing
method according to claim 14, further comprising the step of
forming a transparent capping oxide layer on the whole surface of
the amorphous silicon transistor.
17. The polycrystalline silicon thin-film transistor manufacturing
method according to claim 13, further comprising the step of
preheating the glass substrate at 400.degree.C. or below before
scanning of the glass substrate due to the linearly heating.
18. The polycrystalline silicon thin-film transistor manufacturing
method according to claim 14, wherein said glass substrate further
comprises a detection pattern installed along the transfer
direction on one side of the glass substrate, which can allow
monitoring of the crystallization process of the amorphous silicon
according to the scanning of the light beam, wherein the
transmittivity of the light received via the detection pattern is
detected to control the scanning velocity and the power of the
light beam.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a method for crystallizing
an amorphous silicon thin-film for use in a thin-film transistor
(TFT) and a thermal annealing apparatus therefor, and more
particularly, to a method for crystallizing an amorphous silicon
thin-film for a TFT in which amorphous silicon thin-films on a
large-area glass substrate for use in a TFT-LCD (TFT-Liquid Crystal
Display) are crystallized uniformly and quickly by a scanning
method using a linear lamp to prevent deforming of the glass
substrate, and a method for fabricating a polycrystalline TFT using
the same, and a thermal annealing apparatus therefor.
[0003] 2. Description of the Related Art
[0004] To enhance driving speed and resolution and improve
productivity through integration of driving circuits, replacement
of amorphous silicon TFT by polycrystalline silicon TFT is being
vividly performed under study. Difficulties confronting when
fabricating a polycrystalline silicon thin-film are to prevent a
deform of glass which is used as a substrate. To do so, amorphous
silicon should be crystallized within a temperature and time at
which the glass substrate is resistant without being deformed.
[0005] A metal-induced lateral crystallization (MILC) method
proposed to overcome the above difficulties can lower an amorphous
silicon crystallization temperature at 500.degree. C. or below and
has advantages using simple equipment and processes compared with
other crystallization methods. In this MILC method, a metal
thin-film such as Ni, Pd and so on, is partially formed on the
interface between the surface of an amorphous silicon thin-film and
a substrate, and is thermally annealed at 500.degree. C. or so, in
such a manner that crystallization proceeds at the portion where
the metal thin-film has been formed and in lateral direction
thereof. A polycrystalline silicon TFT can be fabricated using the
above MILC, in which a device having an excellent electrical
characteristic can be fabricated at 500.degree.C. or below.
[0006] FIG. 1 is a sectional view showing a manufacturing process
of a TFT using the MILC method. As shown, an amorphous silicon
thin-film 10 is formed in the form of an island on the whole
surface of a glass substrate 100. Then, a gate insulation film 12
and a gate electrode 13 are formed in turn. Then, a metal film 14
of Ni is deposited on the whole surface of the substrate including
a source region 10S and a drain region 10D and then annealed, to
thereby crystallize a channel region 10C of the amorphous silicon
thin-film 10 by the MILC method.
[0007] The above method has a shorter thermal processing time than
that of a method for forming a gate electrode after depositing and
crystallizing an amorphous silicon thin-film on the whole surface
of a substrate. Since the above method crystallizes only a channel
region, yield is considerably improved.
[0008] To crystallize an amorphous silicon by thermal annealing at
the state where the metal thin-film is not formed requires thermal
processing of about 30 hours at a temperature of 600.degree.C. or
above. Meanwhile, the above MILC technique shows a crystallization
velocity of 1.6 .mu.m/hr or more at 500 .quadrature..quadrature. or
so, which must be very useful crystallization method. In the MILC
method, when a thermal annealing temperature is 600.degree.C. or
above, the lateral crystallization proceeds more quickly depending
upon the temperature. Thus, the lateral portions of the portion
where the metal thin-film has been formed are crystallized all by
the MILC method.
[0009] Meanwhile, in the case of a next generation large-area glass
substrate, it does not facilitate to implement a furnace thermal
annealing apparatus and it is difficult to enhance productivity
because of a long-term thermal annealing time. For this reason, a
thermal annealing apparatus adopting a number of lamps shown in
FIG. 2 has been proposed.
[0010] FIG. 2 is a sectional view schematically showing a lamp
thermal annealing apparatus which is used for crystallization of an
amorphous silicon thin-film according to the conventional prior
art. As depicted, a bottom layer oxide film 22 is formed on a
substrate 21. A Ni metal layer 24 is formed on the surface of an
amorphous silicon thin-film 23 formed on the oxide film 22. Then, a
process for thermally processing the amorphous silicon thin-film at
high temperature for a second using lamps 29 and cooling it for
five seconds is performed at least once, to thereby crystallize the
amorphous silicon thin-film by the MILC method.
[0011] In the MILC method, only an opaque amorphous silicon
thin-film is heated and crystallized and a transparent glass
substrate is not heated by the lamps, to accordingly prevent a
deformation of the glass substrate. The reason for cooling the
amorphous silicon for five seconds or so is to block the heats of
the heated amorphous silicon from being transferred to the glass
substrate, in order to prevent deforming of the glass substrate due
to the heats transferred from the amorphous silicon to the glass
substrate.
[0012] However, the above method for heating the whole surface of
the substrate is also limited to implement a thermal annealing
apparatus for uniformly heating a large-area glass substrate, such
as a substrate of 600 mm.times.500 mm or larger. As described
above, if all the portions of the substrate are not uniformly
heated, a thermal processing time should be longer in order to
crystallize all the portions of the substrate. Thus, the
temperature of the amorphous silicon may be locally raised, so that
the substrate may be deformed.
SUMMARY OF THE INVENTION
[0013] To solve the above problems, it is an object of the present
invention to provide an amorphous silicon crystallization method
capable of crystallizing an amorphous silicon without deforming a
large-area transparent glass substrate irrespective of the size of
the substrate, employing a continuous process rapid thermal
annealing (RTA) method using light.
[0014] It is another object of the present invention to provide a
method for manufacturing a low-temperature polycrystalline silicon
thin-film transistor capable of greatly improving a crystallization
uniformity and a crystallization velocity by employing both a
continuous process rapid thermal annealing (RTA) method and a
metal-induced lateral crystallization (MILC) method
simultaneously.
[0015] It is still another object of the present invention to
provide a thermal annealing apparatus which is used for
crystallization of an amorphous silicon thin-film for use in a
thin-film transistor (TFT), capable of preventing deformation of a
glass substrate, in which an amorphous silicon thin-film is
uniformly and rapidly crystallized on a large-are glass substrate
for use in a thin-film transistor-liquid crystal display (TFT-LCD)
by a continuous process or scanning method using a linear lamp.
[0016] To accomplish the above object of the present invention,
according to one aspect of the present invention, there is provided
a thermal annealing apparatus for crystallizing an amorphous
silicon thin-film, the thermal annealing apparatus comprising:
supporting means for supporting at least one glass substrate on
which the amorphous silicon thin-film has been formed; a light
source for illuminating a linear light beam to be focused on the
glass substrate from the upper portion of the glass substrate; and
scanning driver means for relatively moving one of the supporting
means and the light source so that the linear light beam can be
illuminated on the silicon thin-film according to a scanning
method.
[0017] According to another aspect of the present invention, there
is also provided an amorphous silicon thin-film crystallization
method comprising the steps of: forming an amorphous silicon
thin-film on a glass substrate; and illuminating a linear light
beam on the amorphous silicon thin-film from the upper portion of
the glass substrate according to a scanning method.
[0018] Also, a method for manufacturing a polycrystalline silicon
thin-film transistor employing the above crystallization method,
comprising the steps of: forming an amorphous silicon thin-film on
a glass substrate; and crystallizing the amorphous silicon of the
thin-film transistor according to a scanning method using a linear
light beam.
[0019] Here, the step of forming the amorphous silicon thin-film
transistor comprises the sub-steps of: forming an active layer
composed of the amorphous silicon on the glass substrate; forming a
gate insulation film and a gate electrode on the active layer in
turn; and depositing a metal thin-film on the resultant glass
substrate, wherein the amorphous silicon with respect to a channel
region positioned in the lower portion of the gate insulation film
is crystallized by a metal-induced lateral crystallization (MILC)
method using the metal thin-film.
[0020] As described above, the thermal annealing apparatus
according to the present invention can locally heat the amorphous
silicon to be crystallized by a scanning method where the substrate
is transported at the state where the linearly focused light of the
lamp is illuminated on the glass substrate. As a result, the
amorphous silicon can be crystallized without deforming a large-are
transparent glass substrate such as a LCD for a TV irrespective of
the size of the substrate and without expanding the size of the
thermal annealing apparatus on a three-dimensional basis.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] The objects and other advantages of the present invention
will become more apparent by describing in detail the structures
and operations of the present invention with reference to the
accompanying drawings, in which:
[0022] FIG. 1 is a sectional view showing a TFT in order to explain
a MILC method;
[0023] FIG. 2 is a sectional view schematically showing a thermal
annealing apparatus which is used for transforming an amorphous
silicon into a polycrystalline silicon according to the
conventional prior art;
[0024] FIG. 3 is a perspective view showing a thermal annealing
apparatus for crystallizing an amorphous silicon thin-film
according to the present invention;
[0025] FIG. 4 is a schematic sectional view showing an automatic
control apparatus for controlling a transportation velocity of the
substrate in the FIG. 3 thermal annealing apparatus;
[0026] FIG. 5A is a sectional view of a test piece to be thermally
annealed;
[0027] FIG. 5B is a graphical view showing the temperature slopes
of a silicon thin-film at the thermal annealing start step in the
cases that a capping oxide film exists or not, respectively;
[0028] FIG. 5C is a graphical view showing the temperature slopes
of a silicon thin-film when a metal-induced crystallization (MIC)
proceeds at the thermal annealing intermediate step in the cases
that a capping oxide film is formed or not, respectively;
[0029] FIG. 5D is a graphical view showing the temperature slopes
of a silicon thin-film when a metal-induced lateral crystallization
(MILC) proceeds at the thermal annealing intermediate step in the
cases that a capping oxide film is formed or not, respectively;
[0030] FIG. 6 is a sectional view of the substrate for explaining
the temperature distribution of the substrate and a graphical view
of a corresponding temperature distribution when an amorphous
silicon thin-film for a TFT according to the present invention is
crystallized;
[0031] FIG. 7 is a graphical view illustrating the temperature
change according to a time when a thermal annealing is performed
according to a scanning method of the present invention;
[0032] FIG. 8 is a graphical view showing the MILC distances at a
maximum thermal annealing temperature in the cases that a capping
oxide film is formed and not, respectively; and
[0033] FIG. 9 is a graphical view showing the transfer
characteristics of the polycrystalline silicon TFTs fabricated by
he conventional art and the present invention, respectively.
DETAILED DESCRIPTION OF THE INVENTION
[0034] A preferred embodiment of the present invention will be
described in detail with reference to the accompanying drawings.
Referring to FIG. 3, a thermal annealing apparatus according to the
present invention includes a halogen linear lamp 35 functioning as
a light source 36 for illuminating a light beam 30 focused in
linear form on a TFT array 41 formed on a glass substrate 40 and an
elliptical reflective mirror 34 for focusing the light illuminated
from the lamp 35 in linear form on the glass substrate 40. A lower
heating portion 31 including a number of halogen preheating lamps
32 is fixedly installed in the lower portion of the light source
36. A conveyer belt 37 is mounted on the lower heating portion 31,
which functions as a transportation unit for transporting a number
of glass substrates 40 continuously. The conveyer belt 37 supports
the substrate 40 and simultaneously moves the substrate in one
direction continuously.
[0035] The thermal source, that is, the light source 36 is combined
with an automatic control apparatus shown in FIG. 4, to thereby
configure a continuous and uniform heating apparatus. In this case,
as the conveyer belt 37 enables the substrate 40 to move
continuously in the arrow mark direction X, the thermal source 36
scans the substrate 40 and performs crystallization of the
substrate 40 without excessively heating the substrate 40. In this
case, the thermal source 36 also moves continuously by a
predetermined distance in the direction opposite to the arrow mark,
to thereby scan the substrate.
[0036] Meanwhile, as shown in FIG. 4, a pair of transmittivity
detection sensors 58A and 58B for detecting transmittivity of the
light are installed at the back and forth of the thermal source 36.
The transmittivity data of each portion obtained from the pair of
the transmittivity detection sensors 58A and 58B is supplied to an
automatic controller 59 for controlling the power of the lamp 35
and the transportation velocity of the conveyer belt 37 using the
transmittivity data.
[0037] The operation of the thermal annealing apparatus according
to the present invention will be described below in more detail
with reference to FIGS. 3 and 4.
[0038] A TFT array 41 including an amorphous silicon to be
crystallized is formed on the glass substrate 40 which is put on
the conveyer belt 37 and transported toward the thermal annealing
apparatus. A detection pattern 42 composed of an amorphous silicon
of an elongate strip pattern, for monitoring crystallization of the
amorphous silicon is formed in one side of the glass substrate 40
along the transportation direction of the substrate.
[0039] The lower heating portion 31 plays a role of preheating the
substrate in advance up to a temperature, e. g. 400.degree.C. or
below in which crystallization of the amorphous silicon does not
occur, in order to shorten a thermal annealing time of the glass
substrate 40. The light beam 30 generated from the linear lamp 35
of the thermal source 36 is linearly focused by a reflective mirror
34 whose inner circumferential portion is elliptical and which is
formed in lengthy direction to cover the lamp 35 and illuminated on
the glass substrate 40. In this state, if the conveyer belt 37 is
driven in the arrow direction X, the glass substrate 40
continuously moves and is subject to a scanning illumination by the
linear light beam 30.
[0040] Thus, it is possible to perform a rapid heating of
80.degree.C./sec or more by adjusting a scanning condition of the
lamp 35. Here, only a heating effect due to conduction of the light
beam becomes a single heating source with respect to the substrate,
since cooling air is supplied to the substrate continuously.
Accordingly, the transparent glass substrate 40 is not, on the one
hand, heated by the light illuminated from the lamp 35 or the lower
preheating lamps 32. On the other hand, the amorphous silicon
thin-film of the TFT array 41 formed on the glass substrate 40
absorbs the energy corresponding to the wavelength of the light
beam 30 illuminated from the lamp 35 and heated locally.
[0041] Meanwhile, at least one lamp 35 can be configured. The array
of the lamps 35 can be adjusted according to the area of the
substrate and the processing conditions in order to obtain a
uniform temperature slope.
[0042] In the thermal annealing process using the light, the glass
substrate 40 transmits the light and so is not heated. Thus, only
the amorphous silicon absorbs the light to be heated. Here, since
the amorphous silicon is transformed into a crystalline silicon
which is transparent, a self-stop process for stop a further
heating is possible.
[0043] Referring to FIGS. 3 and 4, the detection pattern 42 is also
comprised of an amorphous silicon. Accordingly, the amorphous
silicon of the TFT array 41 is transformed into a crystalline
silicon and becomes transparent through a crystallization process
of the amorphous silicon. A reference numeral 42-1 denotes a
portion in which the amorphous silicon is changed into a
crystalline silicon by illumination of the light beam of the lamp
35 and which has become transparent. A reference numeral 42-2 is a
portion of the opaque amorphous silicon which has not been
illuminated yet from the lamp 35.
[0044] During crystallization of the detection pattern 42 due to
illumination of the light beam 30 from the lamp 35, the preheating
lamps 32 in the lower heating portion 31 also illuminate the light
on the substrate 40 to supply an appropriate amount of heat
thereto. The light transmits through the detection patterns 42-1
and 42-2 to then reach the transmittivity detection sensors 58A and
58B. Here, the light L1 transmitting through the transparent
detection pattern 42-1 and the light L2 whose part is reflected
from the detection pattern 42-2 and other part is transmitted
through the opaque detection pattern 42-2 are detected as a
respective different value by the transmittivity detection sensors
58A and 58B according to the difference of the amount of the
transmitted light.
[0045] Then, the automatic controller 59 compares the
transmittivity values read from each portion of the detection
patterns 42-1 and 42-2 by the transmittivity detection sensors 58A
and 58B with a reference value and judges a thermal annealing state
according to the comparison result, to thereby automatically
control the operation of each component of the thermal annealing
apparatus. That is, the power of the lamps 35 and 32, the
transportation velocity of the conveyer belt 37 or the
transportation velocity of the upper thermal source 36 are
automatically controlled according to the transmittivity values
obtained from the detection sensors 58A and 58B. Thus, the
real-time measurement results in the detection sensors 58A and 58B
are fed back to the automatic controller 59, when the silicon
crystallization are not formed uniformly on the glass substrate 40
during the thermal annealing or the uniformity between the
processes is changed. As a result, a relative scanning velocity
between the glass substrate 40 and the thermal source 36 and a
process variable such as a lamp power can be adjusted.
[0046] As the amorphous silicon thin-film is crystallized,
transparency is changed. Thus, if crystallization of the amorphous
silicon thin-film is measured while thermally annealing the
large-area glass substrate 40 one by one, a process condition can
be adjusted in real time. As a result, the present invention can
greatly enhance a crystallization uniformity compared with a
conventional furnace thermal annealing method which thermally
annealing a number of large-area glass substrates all at a
time.
[0047] The conventional furnace thermal annealing apparatus has a
technical limitation since the size of the furnace becomes larger
on a three-dimensional basis as the area of the substrate becomes
larger. However, the present invention can solve the above problem
by constructing a two-dimensional expanded heating apparatus, that
is, lengthening the linear lamp, in order to heat the large-area
glass substrate 40. In this case, a scanning apparatus, that is,
the conveyer belt 37 or a thermal source scanning apparatus (not
shown) is enough if a one-dimensional uniformity of the light beam
illuminated on the glass substrate is maintained although a
substrate area increases. Thus, the thermal annealing apparatus of
the present invention is very advantageous compared with the
conventional thermal annealing apparatus requiring the
two-dimensional uniform temperature. The above advantages are
preferably applied to manufacturing of a large-area flat display
such as a liquid crystal display (LCD) which is used for a notebook
or desktop computer or a large-sized TV.
[0048] Meanwhile, crystallization of the amorphous silicon at high
temperatures requires a sufficient incubation time. If a thermal
process employing a lamp is used for a MILC, crystallization starts
from at a portion where a metal layer is formed and proceeds in
lateral direction before it reaches an incubation time necessary
for crystallization of a portion where the metal layer is not
formed. Here, at least one lamp 35 can be used in order to enhance
a crystallization velocity as described above.
[0049] Referring to FIGS. 5A through 5D, a change of the
temperature slopes of the amorphous silicon thin-film are described
according to a thermal annealing time during thermal annealing
using the lamp, in the cases that a capping oxide layer exists or
not on the whole surface of a test piece where devices are formed
on the glass substrate.
[0050] FIG. 5A is a sectional view of a test piece to be thermally
annealed. FIG. 5B is a graphical view showing the temperature
slopes of a silicon thin-film at the thermal annealing start step
in the cases that a capping oxide film exists or not, respectively.
FIG. 5C is a graphical view showing the temperature slopes of a
silicon thin-film when a metal-induced crystallization (MIC)
proceeds at the thermal annealing intermediate step in the cases
that a capping oxide film is formed or not, respectively. FIG. 5D
is a graphical view showing the temperature slopes of a silicon
thin-film when a metal-induced lateral crystallization (MILC)
proceeds at the thermal annealing intermediate step in the cases
that a capping oxide film is formed or not, respectively.
[0051] Referring to FIGS. 5B through 5D, a dotted line indicates
the temperature of the silicon region where a capping oxide layer
53 composed of SiO2 is deposited on the surface of the transparent
glass substrate 50 to be thermally processed. The transparent oxide
layer 53 has an effect increasing the temperature of the amorphous
silicon thin-film 51. As such, the thermal processing of the
present invention can form a capping oxide layer 53 covering the
upper portion of the amorphous silicon thin-film 51 in order to
enhance the efficiency of the RTA due to the lamp heating. Since
the capping oxide layer 53 is transparent, it plays a role of a
thermal protection layer for assisting the light absorption and
suppressing the thermal discharging, to thereby increase a
temperature rise effect of only an amorphous silicon.
[0052] The capping oxide layer 53 is also {fraction (1/100)}the
thermal conduction compared with a silicon, which transmits the
light of the lamp toward the amorphous silicon thin-film 51 and
prevents the locally heated silicon thin-film from directly
contacting the atmosphere and so being cooled. Thus, the large-area
transparent glass substrate 50 does not reach a transformation
temperature and only an amorphous silicon thin-film 51 which has
been patterned on the substrate 50 is heated up to a temperature
necessary for the metal-induced crystallization.
[0053] Referring to FIG. 5A, the test piece has a structure in
which an opaque amorphous silicon thin-film 51 is patterned in the
form of an island on the transparent glass substrate 50, an opaque
metal thin-film 52 having a thickness of 5 .ANG. through 50 .ANG.
is partially deposited on the upper portion of the amorphous
silicon thin-film 51, and a transparent capping oxide layer 53 of a
thickness of about 3000 .ANG. is deposited on the whole surface of
the test piece.
[0054] Here, the metal thin-film 52 deposited on the upper portion
of the amorphous silicon thin-film 51 acts as a catalyst for
lowering the temperature of crystallization of the amorphous
silicon thin-film 51. The metal thin-film 52 is formed by
depositing a metal material such as Ni, Fe, Co, Ru, Rh, Pd, Os, Ir,
Pt, Sc, Ti, V, Cr, Mn, Cu, Zn, Au and Ag or an alloy thereof on the
amorphous silicon thin-film 51.
[0055] FIG. 5B shows the temperature slopes of a silicon thin-film
at the time when a thermal annealing starts with respect to a test
piece. As indicated, a silicon region A where an opaque amorphous
silicon thin-film 51 and a metal thin-film 52 are deposited absorbs
a more amount of light relatively than a transparent substrate
region and an amorphous silicon region B where the metal thin-film
52 is not deposited, and is heated at higher temperatures. Here,
the temperature as indicated in dotted lines in the case that the
capping oxide layer 53 has been formed is relatively higher than
that as indicated in solid lines in the case that the capping oxide
layer does not exist.
[0056] FIG. 5C shows the temperature slopes of a silicon thin-film
at the thermal annealing intermediate step, in which a
metal-induced crystallization (MIC) proceeds in lateral direction
of the metal thin-film 52. As a result, an amorphous silicon region
A positioned in the lower portion of the metal thin-film is changed
rapidly into a transparent crystalline silicon by a metal-induced
crystallization (MIC), in such a manner that a light absorption
decreases and a temperature falls. Thus, a heat transfer to the
substrate is decreased, to thereby decrease the possibility of
deformation of the substrate.
[0057] FIG. 5D shows the temperature slopes of a silicon thin-film
at the time when a metal-induced lateral crystallization (MILC)
proceeds from an amorphous silicon region A positioned in the lower
portion of the metal thin-film 52 where crystallization has been
completed according to the continuous thermal annealing to an
amorphous silicon region B positioned in the lateral portion of the
amorphous silicon region A. The portion where crystallization has
been completed is changed into a transparent crystalline silicon.
Accordingly, since a light absorption decreases and a temperature
falls, it can be seen that the temperature of the glass substrate
50 is decreased as crystallization of the amorphous silicon
proceeds. In FIG. 5D, an arrow mark Z1 indicates a direction in
which the MILC proceeds.
[0058] FIG. 8 is a graphical view showing the MILC distances at a
maximum thermal annealing temperature in the cases that a capping
oxide film is formed and not, respectively, in which a scanning is
accomplished at a velocity of 1 mm/sec or so.
[0059] As shown in FIG. 8, a test piece where a capping oxide layer
is formed has about five times the crystallization velocity as that
where the capping oxide layer does not exist, which indicates that
a temperature rise effect is obtained by existence of the capping
oxide layer.
[0060] FIG. 6 is a sectional view of the substrate and a graphical
view of a corresponding temperature slope when crystallizing an
amorphous silicon thin-film for a TFT according to the present
invention.
[0061] In the case of the TFT, an amorphous silicon thin-film 72 is
formed and patterned on a glass substrate 700 and then a gate
insulation film 73 and a gate electrode 74 are formed thereon.
Thereafter, a metal thin-film 75, e. g. a Ni thin-film is deposited
in the thickness of 5 .ANG. or more on the whole surface of the
glass substrate 700. The metal thin-film 75 is automatically
aligned only in a source and drain region B1 except for a channel
region A1 and discriminatively formed.
[0062] In this case, a metal which can be used as a metal thin-film
is a metal material such as Fe, Co, Ru, Rh, Pd, Os, Ir, Pt, Sc, Ti,
V, Cr, Mn, Cu, Zn, Au and Ag except for Ni or an alloy thereof.
[0063] Sequentially, a thermal annealing is performed using a
thermal annealing apparatus according to the present invention, in
which the amorphous silicon thin-film 72 is crystallized by the MIC
and MILC methods. In this way, since the gate electrode 74 is
opaque in the case that the substrate 700 on which the gate
electrode 74 has been formed is thermally annealed, heating is
further easily performed by a lamp, to expediate a MILC method.
[0064] That is, at the initial time of the thermal annealing
operation, the amorphous silicon region B1 (e. g. source and drain
regions) covered with the metal thin-film 75 and the amorphous
silicon region A1 (e. g. a channel region) covered with the opaque
gate electrode 74 absorb more light than a transparent substrate
region C1 and are heated at high temperatures as shown as a solid
line. Thereafter, the MIC proceeds and the amorphous silicon region
B1 positioned in the lower portion of the metal thin-film 75 is
changed into a transparent crystalline silicon. As a result, a
light absorption rate is decreased, to thereby cause the
temperature to fall down. The MILC also proceeds from the amorphous
silicon region B1 which has been crystallized in the lower portion
of the metal toward the amorphous silicon region A1 located on the
lateral portion of the amorphous silicon region B1. The portion in
which crystallization has proceeded is changed into a transparent
crystalline silicon. Accordingly, a light absorption rate is
decreased, to thereby cause the temperature to fall down. In the
drawing, an arrow mark Z2 indicates a direction toward which
lateral crystallization proceeds.
[0065] FIG. 7 is a graphical view illustrating the temperature
change of the substrate according to a time when a thermal
annealing is performed according to a scanning method of the
present invention. As shown, in the case of a substrate which has
been preheated at 400.degree.C. or so by a preheating lamp, an
abrupt temperature rise occurs for several seconds, for example,
ten to fifteen seconds when a lamp scans over the substrate. By
doing so, it can be seen that crystallization is completed and then
cooling is performed.
[0066] Thus, deformation of the glass substrate is minimized and a
process condition appropriate for a large-area continuous process
is obtained, by properly adjusting a scanning velocity and a lamp
power.
[0067] FIG. 9 is a graphical view showing the transfer
characteristics of the polycrystalline silicon TFTs fabricated by
he conventional art and the present invention, respectively. The
characteristics of the present invention and the prior art are
measured using a transistor having the structure shown in FIG. 1.
In the present invention, crystallization with respect to an
amorphous silicon thin-film proceeds according to a linear RTA-MILC
method. In the prior art, crystallization proceeds according to a
furnace MILC method using a furnace thermal annealing apparatus. In
this case, when a transistor is thermally annealed according to the
present invention, a scanning velocity is 1 mm/sec, a preheating
temperature of the substrate is 400.degree.C., and the temperature
of a heating line is 700.degree.C. After crystallization has
proceeded according to the present invention and the prior art, the
characteristics with respect to the TFTs obtained via a general
successive process are investigated. First, a drain current [A] is
measured according to the change of the gate voltage with respect
to the TFT whose drain voltage VD is 5V and width/length (W/L) is
10/8 and the thus-obtained transfer characteristic is shown in the
graph of FIG. 9.
[0068] Meanwhile, a threshold voltage [V], a sub-threshold slope
[mV/dec], a field-effect mobility [cm.sup.2/V.multidot.]s, and a
maximum on/off current ratio are shown in the following Table
1.
1 TABLE 1 Item Furnace-MILC RTA-MILC Threshold voltage [V] 1 2.5
Sub-threshold 467 588 slope [mV/dec] Field-effect mobility 120 150
[cm.sup.2/V .multidot. s] Maximum on/off current 2.8E6 4.5E6
ratio
[0069] As can be seen from FIG. 9 and Table 1, the physical
characteristics of the transistor fabricated by crystallizing an
amorphous silicon according to the present invention are
substantially same as those of a conventional polycrystalline
silicon transistor. However, in the case of the on/of current ratio
and the field-effect mobility, it can be seen that the values of
the transistor according to the present invention are greatly
enhanced.
[0070] As described above, the thermal annealing apparatus
according to the present invention can locally heat an amorphous
silicon to be crystallized according to a scanning method where a
substrate is transferred at the state where linearly focused lamp
light is illuminated on a glass substrate. Accordingly, the present
invention can crystallize an amorphous silicon uniformly without
deformation of a large-area transparent glass substrate such as a
LCD for TV irrespective of the size of the substrate and without
extending the size of the thermal annealing apparatus
three-dimensionally.
[0071] Further, in the case where a number of lamps are installed,
an amorphous silicon thin-film can be crystallized at a number of
positions, to thereby enhance a crystallization velocity. Since
crystallization with respect to an amorphous silicon is controlled
individually and in real-time by an automatic control apparatus
including a transmittivity detection sensor, the quality of the
thermally annealed products can be uniformly maintained, to thereby
greatly enhance a yield of large-area LCD products.
[0072] In addition, the method for crystallizing an amorphous
silicon thin-film according to the present invention can locally
illuminate linear light on the thin-film using the thermal
annealing apparatus, to thereby crystallize the amorphous silicon
thin-film uniformly and prevent the substrate from being deformed.
Also, in the case that a capping oxide layer, a crystallization
velocity can be enhanced. Further, in the case that the present
invention is applied to manufacturing of a polycrystalline silicon
transistor, a device having excellent physical characteristics can
be fabricated without deform of the substrate.
[0073] As described above, the present invention has been shown and
described with respect to a preferred embodiment as a particular
example. The present invention is, however, not limited to the
above embodiment, and there are many variations and modifications
by a person skilled in the art without departing from the scope and
spirit of the present invention.
* * * * *