U.S. patent application number 14/732920 was filed with the patent office on 2015-12-03 for laser annealing method and device.
The applicant listed for this patent is SEMICONDUCTOR ENERGY LABORATORY CO., LTD.. Invention is credited to Norihito KAWAGUCHI, Ryusuke KAWAKAMI, Miyuki MASAKI, Kenichiro NISHIDA, Atsushi YOSHINOUCHI.
Application Number | 20150348781 14/732920 |
Document ID | / |
Family ID | 37864915 |
Filed Date | 2015-12-03 |
United States Patent
Application |
20150348781 |
Kind Code |
A1 |
KAWAKAMI; Ryusuke ; et
al. |
December 3, 2015 |
LASER ANNEALING METHOD AND DEVICE
Abstract
A laser annealing method for executing laser annealing by
irradiating a semiconductor film formed on a surface of a substrate
with a laser beam, the method including the steps of, generating a
linearly polarized rectangular laser beam whose cross section
perpendicular to an advancing direction is a rectangle with an
electric field directed toward a long-side direction of the
rectangle or an elliptically polarized rectangular laser beam
having a major axis directed toward a long-side direction, causing
the rectangular laser beam to be introduced to the surface of the
substrate, and setting a wavelength of the rectangular laser beam
to a length which is about a desired size of a crystal grain in a
standing wave direction.
Inventors: |
KAWAKAMI; Ryusuke; (Tokyo,
JP) ; NISHIDA; Kenichiro; (Tokyo, JP) ;
KAWAGUCHI; Norihito; (Tokyo, JP) ; MASAKI;
Miyuki; (Tokyo, JP) ; YOSHINOUCHI; Atsushi;
(Tokyo, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
SEMICONDUCTOR ENERGY LABORATORY CO., LTD. |
Atsugi-shi |
|
JP |
|
|
Family ID: |
37864915 |
Appl. No.: |
14/732920 |
Filed: |
June 8, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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14138273 |
Dec 23, 2013 |
9058994 |
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14732920 |
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13608818 |
Sep 10, 2012 |
8629522 |
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14138273 |
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|
12946051 |
Nov 15, 2010 |
8299553 |
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13608818 |
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11916687 |
Dec 6, 2007 |
7833871 |
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PCT/JP2006/318006 |
Sep 12, 2006 |
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12946051 |
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Current U.S.
Class: |
438/795 |
Current CPC
Class: |
B23K 26/0613 20130101;
H01L 21/02675 20130101; H01L 21/02691 20130101; H01L 21/268
20130101; H01L 21/0268 20130101; H01L 27/1296 20130101; H01L
27/1285 20130101; B23K 26/0732 20130101 |
International
Class: |
H01L 21/02 20060101
H01L021/02; H01L 27/12 20060101 H01L027/12; H01L 21/268 20060101
H01L021/268 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 14, 2005 |
JP |
2005-266607 |
Feb 3, 2006 |
JP |
2006-027096 |
Claims
1. (canceled)
2. A method of manufacturing a semiconductor device comprising:
controlling a first laser oscillator and a second laser oscillator
by a pulse controller; combining a first laser beam from the first
laser oscillator and a second laser beam from the second laser
oscillator to form a combined laser beam; adjusting a shape of the
combined laser beam by an optical system to form an adjusted
combined laser beam; and irradiating a semiconductor film with the
adjusted combined laser beam to increase crystallinity of the
semiconductor film.
3. The method of manufacturing a semiconductor device according
claim 2, wherein a step of adjusting the shape of the combined
laser beam is performed by adjusting a width of the combined laser
beam.
4. The method of manufacturing a semiconductor device according
claim 2, wherein the semiconductor film is included in a transistor
and includes a channel portion.
5. The method of manufacturing a semiconductor device according
claim 2, wherein the first laser oscillator and the second laser
oscillator is controlled by the pulse controller so as to make
output timings of the first laser beam and the second laser beam
different.
6. The method of manufacturing a semiconductor device according
claim 2, wherein the semiconductor film irradiated with the
combined laser beam includes a first crystal grain and a second
crystal grain adjacent to the first crystal grain, wherein a length
of the first crystal grain in a first direction is greater than a
length of the first crystal grain in a second direction, wherein a
length of the second crystal grain in the first direction is
greater than a length of the second crystal grain in the second
direction, and wherein a grain boundary between the first crystal
grain and the second crystal grain extends in the first
direction.
7. The method of manufacturing a semiconductor device according
claim 2, wherein an energy density of the adjusted combined laser
beam is greater than 500 mJ/cm.sup.2.
8. A method of manufacturing a semiconductor device comprising:
controlling a first laser oscillator and a second laser oscillator
by a pulse controller; combining a first laser beam from the first
laser oscillator and a second laser beam from the second laser
oscillator to form a combined laser beam; adjusting a shape of the
combined laser beam by an optical system to form an adjusted
combined laser beam; and irradiating a semiconductor film over a
substrate with the adjusted combined laser beam to increase
crystallinity of the semiconductor film, wherein an advancing
direction of the adjusted combined laser beam is not perpendicular
to a surface of the substrate.
9. The method of manufacturing a semiconductor device according
claim 8, wherein a step of adjusting the shape of the combined
laser beam is performed by adjusting a width of the combined laser
beam.
10. The method of manufacturing a semiconductor device according
claim 8, wherein the semiconductor film is included in a transistor
and includes a channel portion.
11. The method of manufacturing a semiconductor device according
claim 8, wherein the first laser oscillator and the second laser
oscillator is controlled by the pulse controller so as to make
output timings of the first laser beam and the second laser beam
different.
12. The method of manufacturing a semiconductor device according
claim 8, wherein the semiconductor film irradiated with the
combined laser beam includes a first crystal grain and a second
crystal grain adjacent to the first crystal grain, wherein a length
of the first crystal grain in a first direction is greater than a
length of the first crystal grain in a second direction, wherein a
length of the second crystal grain in the first direction is
greater than a length of the second crystal grain in the second
direction, and wherein a grain boundary between the first crystal
grain and the second crystal grain extends in the first
direction.
13. The method of manufacturing a semiconductor device according
claim 8, wherein an energy density of the adjusted combined laser
beam is greater than 500 mJ/cm.sup.2.
14. A method of manufacturing a semiconductor device comprising:
polarizing a first laser beam from a first laser oscillator to form
a first polarized laser beam polarized in a first direction;
polarizing a second laser beam from a second laser oscillator to
form a second polarized laser beam polarized in a second direction;
combining the first polarized laser beam and the second polarized
laser beam to form a combined laser beam whose polarization
direction is alternately changed between the first direction and
the second direction; adjusting a shape of the combined laser beam
by an optical system to form an adjusted combined laser beam; and
irradiating a semiconductor film with the adjusted combined laser
beam to increase crystallinity of the semiconductor film.
15. The method of manufacturing a semiconductor device according
claim 14, wherein a step of adjusting the shape of the combined
laser beam is performed by adjusting a width of the combined laser
beam.
16. The method of manufacturing a semiconductor device according
claim 14, wherein the semiconductor film is included in a
transistor and includes a channel portion.
17. The method of manufacturing a semiconductor device according
claim 14, wherein the first laser oscillator and the second laser
oscillator is controlled by a pulse controller so as to make output
timings of the first laser beam and the second laser beam
different.
18. The method of manufacturing a semiconductor device according
claim 14, wherein the semiconductor film irradiated with the
combined laser beam includes a first crystal grain and a second
crystal grain adjacent to the first crystal grain, wherein a length
of the first crystal grain in a third direction is greater than a
length of the first crystal grain in a fourth direction, wherein a
length of the second crystal grain in the third direction is
greater than a length of the second crystal grain in the fourth
direction, and wherein a grain boundary between the first crystal
grain and the second crystal grain extends in the third
direction.
19. The method of manufacturing a semiconductor device according
claim 14, wherein an energy density of the adjusted combined laser
beam is greater than 500 mJ/cm.sup.2.
20. The method of manufacturing a semiconductor device according
claim 14, further comprising: generating a rectangular laser beam
from the combined laser beam, wherein the first direction is a
long-side direction of the rectangular laser beam, and wherein the
second direction is a short-side direction of the rectangular laser
beam.
21. The method of manufacturing a semiconductor device according
claim 14, further comprising: generating a rectangular laser beam
from the combined laser beam, wherein the first direction is tilted
by 45 degrees from a long-side direction of the rectangular laser
beam and a short side direction of the rectangular laser beam, and
wherein the second direction is tilted by 45 degrees from the
long-side direction of the rectangular laser beam and the short
side direction of the rectangular laser beam.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. application Ser.
No. 14/138,273, filed Dec. 23, 2013, now allowed, which is a
continuation of U.S. application Ser. No. 13/608,818, filed Sep.
10, 2012, now U.S. Pat. No. 8,629,522, which is continuation of
U.S. application Ser. No. 12/946,051, filed Nov. 15, 2010, now U.S.
Pat. No. 8,299,553, which is a continuation of U.S. application
Ser. No. 11/916,687, filed Dec. 6, 2007, now U.S. Pat. No.
7,833,871, which is a 371 of International Application No.
PCT/JP2006/318006, filed Sep. 12, 2006, which claims the benefit of
foreign priority applications filed in Japan as Serial No.
2005-266607 on Sep. 14, 2005 and Serial No. 2006-027096 on Feb. 3,
2006, all of which are incorporated by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Technical Field of the Invention
[0003] The present invention relates to a technique of reforming
amorphous semiconductor film such as a silicon film into a
polycrystalline or monocrystalline semiconductor film by
irradiating a rectangular laser beam onto the amorphous
semiconductor film on a substrate in fabricating a semiconductor
device, and a technique of improving the quality of a
polycrystalline or monocrystalline semiconductor film by
irradiating a rectangular laser beam onto the polycrystalline or
monocrystalline semiconductor film on a substrate. As an original
polycrystalline or monocrystalline semiconductor film whose quality
is to be improved, there is a film prepared by solid-phase growth
or a film prepared by laser annealing. Improvement of the quality
of a polycrystalline or monocrystalline semiconductor film means
(1) increasing the size of crystal grains, (2) decreasing defects
in crystal grains, and (3) crystallization of an amorphous portion
remaining among crystal grains.
[0004] 2. Description of the Related Art
[0005] In a case where a thin film transistor (hereinafter called
"TFT") is formed on a substrate in fabrication of a semiconductor
device, the use of an amorphous semiconductor film, such as an
amorphous silicon film, as a semiconductor layer where TFTs are to
be formed cannot achieve a fast operation due to a lower mobility
of carriers. In this respect, an amorphous silicon film is usually
transformed into a polycrystalline or monocrystalline silicon film
crystallized by laser annealing.
[0006] To transform an amorphous silicon film into a
polycrystalline or monocrystalline silicon film by laser annealing,
a laser beam whose cross section perpendicular to the advancing
direction is a rectangle (hereinafter called "rectangular laser
beam") is often used. A rectangular laser beam is irradiated on an
amorphous silicon film while moving the substrate having the
amorphous silicon film formed thereon in a short-side direction of
the rectangle. A method of forming a polycrystalline or
monocrystalline silicon film with a rectangular laser beam is
disclosed, in Patent Document 1 described below, for example.
[0007] Non-patent Documents 2 and 3 described below show techniques
relevant to the present invention. Those documents describe that
when a polarized laser beam is irradiated onto a solid surface, a
surface electromagnetic wave is excited on the solid surface and
interference of the surface electromagnetic wave with the incident
laser beam generates a standing wave on the solid surface, thereby
forming a micro periodic structure on the solid surface.
[Patent Document 1]
[0008] Japanese Laid-Open Patent Publication No. 2003-347210
"SEMICONDUCTOR DEVICE AND FABRICATION METHOD THEREFOR"
[Non-Patent Document 1]
[0009]
www.nml.co.jp/new-business/SUB2/investigation/ripples/texture.pdf
[Non-Patent Document 2]
[0010] Laser Study Dec. 2000, Vol. 28, No. 12, pp. 824-828
"Incident-Angle Dependency of Laser-induced Surface Ripples on
Metals and Semiconductors"
[Non-Patent Document 3]
[0011] pp. 1384-1401, IEE JOURNAL OF QUANTUM ELECTRONICS. VOL.
QE-22, NO. 8, AUGUST, 1986
[0012] In a process of forming polycrystalline or monocrystalline
silicon by irradiation of a rectangular laser beam, the direction
of,growth of crystal grains is greatly affected by temperature
gradient or energy gradient of the laser beam. As shown in FIGS.
1A, 1B and 1C, the energy of the rectangular laser beam in the
long-side direction is constant, so that a nucleus is generated at
a random position relative to the long-side direction. This results
in growth of the nucleus to a random size.
[0013] The energy distribution of rectangular laser beam in the
short-side direction has a large gradient as shown in FIG. 2.
Because the crystal growth is extremely sensitive to the energy
distribution in the short-side direction, therefore, it is very
difficult to make the crystal size in the short-side direction
uniform. As a result, a variation in crystal size in the short-side
direction becomes greater than a variation in crystal size in the
long-side direction as shown in FIG. 3.
[0014] As apparent from the above, conventionally, a
polycrystalline or monocrystalline silicon film having crystal
grains with an nonuniform size is formed. Accordingly, when TFTs
are formed on the polycrystalline or monocrystalline silicon film,
the performance of the TFTs varies due to a difference in the
number of crystal grains in the channel portion per unit length. As
the size of crystal grains greatly differs between the short-side
direction and the long-side direction, the performance of TFTs
greatly differs between the short-side direction and the long-side
direction. This is because the performance of TFTs becomes lower by
increase in the number of times a carrier moving the channel
portion encounter the crystal grain boundary.
SUMMARY OF THE INVENTION
[0015] It is therefore a first object of the invention to provide a
laser annealing method capable of acquiring a polycrystalline or
monocrystalline semiconductor film comprising crystal grains with a
uniform size in a long-side direction.
[0016] It is a second object of the invention to provide a laser
annealing method and device capable of acquiring a polycrystalline
or monocrystalline semiconductor film comprising crystal grains
with a uniform size in a short-side direction.
[0017] It is a third object of the invention to provide a laser
annealing method and device capable of acquiring a polycrystalline
or monocrystalline semiconductor film comprising crystal grains
with a uniform size between a long-side direction and a short-side
direction.
[0018] To achieve the first object, according to the present
invention, there is provided a laser annealing method for executing
laser annealing by irradiating a semiconductor film formed on a
surface of a substrate with a laser beam, the method including the
steps of:
[0019] generating a linearly polarized rectangular laser beam whose
cross section perpendicular to an advancing direction is a
rectangle with an electric field directed toward a long-side
direction of the rectangle or an elliptically polarized rectangular
laser beam having a major axis directed toward a long-side
direction;
[0020] causing the rectangular laser beam to be introduced to the
surface of the substrate; and
[0021] setting a wavelength of the rectangular laser beam to a
length which is about a desired size of a crystal grain in a
standing wave direction (Claim 1).
[0022] According to the method, a standing wave is generated on the
surface of the semiconductor film by scattered light of an
introduced incident rectangular laser beam at the surface of the
semiconductor film and the introduced incident rectangular laser
beam, making it possible to form a polycrystalline or
monocrystalline semiconductor film comprised of crystals with a
uniform size in the direction of the standing wave.
[0023] That is, a standing wave is generated on the semiconductor
film in the long-side direction which is a polarization direction,
thus producing the periodic energy of the standing wave or a
temperature gradient corresponding thereto. When laser annealing is
performed on an amorphous semiconductor film by this method,
therefore, nucleuses are generated at troughs of the periodic
energy, so that the individual nucleuses grow in a direction of a
higher temperature and those portions where the nucleuses collide
with one another become crystal grain boundaries. As nucleuses
generated at periodic positions are grown by the influence of the
same temperature gradient in the long-side direction, therefore, it
is possible to form a polycrystalline or monocrystalline
semiconductor film comprising crystal grains with a uniform size in
the long-side direction. When laser annealing is performed on a
polycrystalline or monocrystalline semiconductor film by this
method, the crystal is grown by the influence of the periodic
temperature gradient in the long-side direction, thus improving the
quality of a polycrystalline or monocrystalline semiconductor film
such that the sizes of crystal grains in the long-side direction
become uniform. Further, a desired crystal grain size in the
long-side direction can be acquired by selecting the wavelength of
the rectangular laser beam.
[0024] To achieve the second object, according to the present
invention, there is provided a laser annealing method for executing
laser annealing by irradiating a semiconductor film formed on a
surface of a substrate with a laser beam, the method including the
steps of:
[0025] generating a linearly polarized rectangular laser beam whose
cross section perpendicular to an advancing direction is a
rectangle with an electric field directed toward a short-side
direction of the rectangle or an elliptically polarized rectangular
laser beam having a major axis directed toward a short-side
direction; and
[0026] causing the rectangular laser beam to be introduced to the
substrate (Claim 2).
[0027] To achieve the second object, according to the present
invention, there is provided a laser annealing device which
executes laser annealing by irradiating a semiconductor film formed
on a surface of a substrate with a laser beam, including:
[0028] short-side polarized beam generating means that generates a
linearly polarized rectangular laser beam whose cross section
perpendicular to an advancing direction is a rectangle with an
electric field directed toward a short-side direction of the
rectangle or an elliptically polarized rectangular laser beam
having a major axis directed toward a short-side direction, and
causes the rectangular laser beam to be introduced to a surface of
the semiconductor film (Claim 6).
[0029] According to the method and device, a standing wave is
generated on the surface of the semiconductor film by scattered
light of an introduced incident rectangular laser beam at the
surface of the semiconductor film and the introduced incident
rectangular laser beam, making it possible to form a
polycrystalline or monocrystalline semiconductor film comprised of
crystals with a uniform size in the direction of the standing
wave.
[0030] That is, a standing wave is generated on the semiconductor
film in the short-side direction which is a polarization direction
or a standing wave is intensely generated in the major axial
direction of elliptically polarized light, thus producing the
periodic energy of the standing wave or a temperature gradient
corresponding thereto. When laser annealing as performed on an
amorphous semiconductor film by this method and device, therefore,
nucleuses are generated at troughs of the periodic energy, so that
the individual nucleuses grow in a direction of a higher
temperature and those portions where the nucleuses collide with one
another become crystal grain boundaries. As nucleuses generated at
periodic positions are grown by the influence of the same
temperature gradient in the short-side direction, therefore, it is
possible to form a polycrystalline or monocrystalline semiconductor
film comprising crystal grains with a uniform size in the
short-side direction. When laser annealing is performed on a
polycrystalline or monocrystalline semiconductor film by this
method and device, the crystal is grown by the influence of the
periodic temperature gradient in the short-side direction, thus
improving the quality of a polycrystalline or monocrystalline
semiconductor film such that the sizes of crystal grains in the
short-side direction become uniform.
[0031] According to a preferred embodiment of the present
invention, the method includes a step of irradiating a surface of
the semiconductor film on the substrate with the rectangular laser
beam while transferring the substrate in a direction perpendicular
to a long side of the rectangular laser beam,
[0032] wherein an incident angle of the rectangular laser beam to
the semiconductor film is adjusted in such that the incident angle
is increased in a transfer direction of the substrate or a
direction opposite to the transfer direction of the substrate
(Claim 3).
[0033] The crystal grain size in the short-side direction increases
as the incident angle is increased in the transfer direction of the
substrate, whereas the crystal grain size in the short-side
direction decreases as the incident angle is increased in the
opposite direction to the transfer direction of the substrate.
[0034] Therefore, adjusting the incident angle can adjust the
crystal grain size in the short-side direction. For example, the
crystal grain size in the short-side direction can be made about
the same as the size of crystal grains formed in the long-side
direction by adjusting the incident angle.
[0035] To achieve the third object, according to the present
invention, there is provided a laser annealing method for executing
laser annealing by irradiating a semiconductor film formed on a
surface of a substrate with a laser beam, the method including the
steps of:
[0036] generating a polarized rectangular laser beam whose cross
section perpendicular to an advancing direction is a rectangle with
an electric field whose direction is alternately changed to a
long-side direction and a short-side direction of the rectangle;
and
[0037] causing the rectangular laser beam to be introduced to the
surface of the substrate (Claim 4).
[0038] To achieve the third object, according to the present
invention, there is provided a laser annealing device which
executes laser annealing by irradiating a semiconductor film formed
on a surface of a substrate with a laser beam, including:
[0039] a first laser oscillator and a second laser oscillator that
output laser beams;
[0040] a pulse controller that controls the first and second laser
oscillators so as to make laser pulse output timings of the first
and second laser oscillators different from each other;
[0041] first polarization means that transforms the laser beam from
the first laser oscillator to linearly polarized light;
[0042] second polarization means that transforms the laser beam
from the second laser oscillator to linearly polarized light;
[0043] beam combining means that combines the laser beam from the
first laser oscillator and the laser beam from the second laser
oscillator; and
[0044] rectangular beam generating means that turns a laser beam
from the beam combining means to a rectangular laser beam whose
cross section perpendicular to an advancing direction is a
rectangle,
[0045] wherein the first polarization means polarizes the laser
beam in a long-side direction of the rectangle, and the second
polarization means polarizes the laser beam in a short-side
direction of the rectangle (Claim 8).
[0046] According to the method and device, standing waves directed
perpendicular to each other, which is caused by scattered light of
an introduced incident rectangular laser beam at the surface of the
semiconductor film and the introduced incident rectangular laser
beam, are alternately generated on the surface of the semiconductor
film, making it possible to form a polycrystalline or
monocrystalline semiconductor film comprised of crystals with a
uniform size in the direction of each standing wave.
[0047] That is, standing waves are alternately generated on the
semiconductor film in the long-side direction and the short-side
direction which are polarization directions, thus producing the
periodic energy of the standing wave or a temperature gradient
corresponding thereto. When laser annealing is performed on an
amorphous semiconductor film by this method and device, therefore,
nucleuses are generated at troughs of the periodic energy, so that
the individual nucleuses grow in directions of a higher temperature
and those portions where the nucleuses collide with one another
become crystal grain boundaries. As nucleuses generated at periodic
positions are grown by the influence of the same temperature
gradients in the long-side direction and the short-side direction,
therefore, it is possible to farm a polycrystalline or
monocrystalline semiconductor film comprising crystal grains with
uniform sizes in the long-side direction and the short-side
direction. When laser annealing is performed on a polycrystalline
or monocrystalline semiconductor film by this method and device,
the crystal is grown by the influence of the periodic temperature
gradient in the long-side direction and the short-side direction,
thus improving the quality of a polycrystalline or monocrystalline
semiconductor film such that the sizes of crystal grains in the
long-side direction and the short-side direction become
uniform.
[0048] According to a preferred embodiment of the present
invention, an energy density of the rectangular laser beam or a
short-side width of the rectangular laser beam is adjusted to
adjust a size of a crystal grain of a polycrystalline or
monocrystalline semiconductor film to be formed (Claim 5).
[0049] Accordingly, finer adjustment of the crystal grain size is
possible, so that a polycrystalline or monocrystalline
semiconductor film comprising more uniform crystal grains can be
formed.
[0050] To achieve the third object, according to the present
invention, there is provided a laser annealing method for executing
laser annealing by irradiating a semiconductor film formed on a
surface of a substrate with a laser beam, the method including the
steps of:
[0051] generating a first laser beam linearly polarized;
[0052] generating a second laser beam linearly polarized;
[0053] combining the first laser beam and the second laser beam in
such that a polarization direction of the first laser beam and a
polarization direction of the second laser beam become
perpendicular to each other;
[0054] turning the combined laser beam to a rectangular laser beam
whose cross section perpendicular to an advancing direction is a
rectangle; and
[0055] causing the rectangular laser beam to be introduced to the
surface of the substrate (Claim 9).
[0056] Further, to achieve the third object, according to the
present invention, there is provided a laser annealing device which
executes laser annealing by irradiating a semiconductor film formed
on a surface of a substrate with a laser beam, including:
[0057] a first laser oscillator and a second laser oscillator that
output laser beams;
[0058] beam combining means that combines the laser beam from the
first laser oscillator and the laser beam from the second laser
oscillator; and
[0059] rectangular beam generating means that turns a laser beam
from the beam combining means to a rectangular laser beam whose
cross section perpendicular to an advancing direction is a
rectangle, and causing the rectangular laser beam to be introduced
onto the substrate,
[0060] the laser beams from the first and second laser oscillators
being linearly polarized,
[0061] a polarization direction of the laser beam from the first
laser oscillator and a polarization direction of the laser beam
from the second laser oscillator being perpendicular to each other
at a position of incidence to the substrate (Claim 12).
[0062] According to the method and device, standing waves directed
perpendicular to each other, which caused by scattered light of an
introduced incident rectangular laser beam at the surface of the
semiconductor film and the introduced incident rectangular laser
beam, are generated on the surface of the semiconductor film,
making it possible to form a polycrystalline or monocrystalline
semiconductor film comprised of crystals with a uniform size in the
direction of each standing wave.
[0063] That is, standing waves are generated on the semiconductor
film in polarization directions perpendicular to each other, thus
producing the periodic energy of the standing wave or a temperature
gradient corresponding thereto.
[0064] When laser annealing is performed on an amorphous
semiconductor film by this method and device, therefore, nucleuses
are generated at troughs of the periodic energy, so that the
individual nucleuses grow in directions of a higher temperature and
those portions where the nucleuses collide with one another become
crystal grain boundaries. As nucleuses generated at periodic
positions are grown by the influence of the same temperature
gradients produced in directions perpendicular to each other,
therefore, it is possible to form a polycrystalline or
monocrystalline semiconductor film comprising crystal grains with
uniform sizes in the directions perpendicular to each other. As a
result, the crystal grain sizes became uniform between the
long-side direction and the short-side direction.
[0065] Even when laser annealing is performed on a polycrystalline
or monocrystalline semiconductor film by this method and device,
the crystal is grown uniformly by the influence of the periodic
temperature gradient in the directions perpendicular to each other,
resulting in an improvement of the quality of a polycrystalline or
monocrystalline semiconductor film such that the sizes of crystal
grains in the long-side direction and the short-side direction
become uniform.
[0066] To achieve the third object, according to the present
invention, there is provided a laser annealing method for executing
laser annealing by irradiating a semiconductor film formed on a
surface of a substrate with a laser beam, the method including the
steps of:
[0067] generating a circularly polarized rectangular laser beam
whose cross section perpendicular to an advancing direction is a
rectangle; and
[0068] causing the rectangular laser beam to be introduced to the
surface of the substrate (Claim 10).
[0069] Further, to achieve the third object, according to the
present invention, there is provided a laser annealing device which
executes laser annealing by irradiating a semiconductor film formed
on a surface of a substrate with a laser beam, including:
[0070] circularly polarized beam generating means that generates a
circularly polarized rectangular laser beam whose cross section
perpendicular to an advancing direction is a rectangle, and causes
the rectangular laser beam to be introduced to a surface of the
semiconductor film (Claim 13).
[0071] According to the method and device, a standing wave is
generated on the surface of the semiconductor film in the
polarization direction by scattered light of an introduced incident
rectangular laser beam at the surface of the semiconductor film and
the introduced incident rectangular laser beam. Because a
rectangular laser beam is a circularly polarized beam, the standing
wave takes a circular motion on a plane perpendicular to the
advancing direction of light. Accordingly, the periodic energy of
the standing wave or a temperature gradient corresponding thereto
is produced uniformly in every direction on the surface of the
semiconductor film.
[0072] When laser annealing is performed on an amorphous
semiconductor film by this method and device, therefore, nucleuses
are generated at troughs of the periodic energy, so that the
individual nucleuses grow in a direction of a higher temperature
and those portions where the nucleuses collide with one another
become crystal grain boundaries. As nucleuses generated at periodic
positions are grown by the influence of the periodic temperature
gradients produced uniformly in every direction, therefore, it is
possible to form a polycrystalline or monocrystalline semiconductor
film comprising crystal grains with a uniform size in every
direction. As a result, the crystal grain sizes became uniform
between the long-side direction and the short-side direction.
[0073] Even when laser annealing is performed on a polycrystalline
or monocrystalline semiconductor film by this method and device,
the crystal is grown uniformly by the influence of the periodic
temperature gradient produced in every direction, resulting in an
improvement of the quality of a polycrystalline or monocrystalline
semiconductor film such that the sizes of crystal grains in the
long-side direction and the short-side direction become
uniform.
[0074] To achieve the third object, according to the present
invention, there is provided a laser annealing method for executing
laser annealing by irradiating a semiconductor film formed on a
surface of a substrate with a laser beam, the method including the
steps of:
[0075] generating a linearly polarized laser beam;
[0076] turning the linearly polarized laser beam to unpolarized
light;
[0077] turning the unpolarized laser beam to a rectangular laser
beam whose cross section perpendicular to an advancing direction is
a rectangle; and
[0078] causing the rectangular laser beam to be introduced to the
surface of the substrate (Claim 11).
[0079] Further, to achieve the third object, according to the
present invention, there is provided a laser annealing device which
executes laser annealing by irradiating a semiconductor film formed
on a surface of a substrate with a laser beam, including:
[0080] a laser oscillator that outputs a linearly polarized laser
beam;
[0081] unpolarization means that turns the laser beam from the
laser oscillator to unpolarized light; and
[0082] rectangular beam generating means that turns the laser beam
from the unpolarization means to a rectangular laser beam whose
cross section perpendicular to an advancing direction is a
rectangle, and causes the rectangular laser beam to be introduced
onto the substrate (Claim 14).
[0083] While a laser beam output from a laser oscillator is often
linearly polarized, the linearly polarized laser beam is turned
into unpolarized light to be introduced to the substrate according
to the method and device, a standing wave is not produced on the
surface of the semiconductor film on the substrate.
[0084] When laser annealing is performed on the semiconductor film
on the substrate by this method and device, therefore, crystal
grains are generated at random positions, and, what is more, the
crystal grains grow in a random direction, thereby suppressing an
increase in the sizes of crystal grains only in a specific
direction. As a result, the sizes of the crystal grains of the
semiconductor film are generally made uniform, making the crystal
grain size uniform between the long-side direction and the
short-side direction.
[0085] Even when laser annealing is performed on a polycrystalline
or monocrystalline semiconductor film by this method and device,
the crystal grains grow in a random direction, thereby suppressing
an increase in the sizes of crystal grains only in a specific
direction. As a result, the sizes of the crystal grains of the
semiconductor film are generally made uniform, so that the quality
of the polycrystalline or monocrystalline semiconductor film is
improved so as to make the crystal grain size uniform between the
long-side direction and the short-side direction.
[0086] The other objects and advantages of the present, invention
will become apparent from the following description referring to
the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0087] FIGS. 1A, 1B and 1C show the conventional relationships
between an energy density on a substrate which is produced by
irradiation of a rectangular laser beam and the size of crystal
grains to be formed.
[0088] FIG. 2 is a diagram showing an energy distribution in the
long-side direction of a rectangular laser beam in a conventional
art.
[0089] FIG. 3 is a diagram showing the size of crystal grains
acquired by a conventional method.
[0090] FIG. 4 is a structural diagram of a long-side optical system
provided in a laser annealing device according to a first
embodiment of the present invention.
[0091] FIG. 5 is a structural diagram of a short-side optical
system provided in a laser annealing device according to the first
embodiment of the present invention.
[0092] FIGS. 6A and 6B show energy distributions in the long-side
direction of rectangular laser beams.
[0093] FIGS. 7A and 7B show energy distributions in the short-side
direction of rectangular laser beams.
[0094] FIG. 8 is an explanatory diagram of an operation of
transferring a substrate while irradiating a rectangular laser
beam.
[0095] FIGS. 9A, 9B and 9C are diagrams showing the relationships
between an energy distribution in the long-side direction produced
on the surface of a substrate by irradiation of a rectangular laser
beam polarized in the long-side direction, and the size of crystal
grains to be formed.
[0096] FIG. 10 is a status diagram of the size of crystal grains
acquired experimentally by irradiating a rectangular beam polarized
in the long-side direction.
[0097] FIG. 11 is a status diagram of the size of crystal grains
acquired experimentally by irradiating a rectangular beam of a high
energy density polarized in the long-side direction.
[0098] FIGS. 12A, 12B and 12C are diagrams showing the
relationships between an energy distribution in the short-side
direction produced on the surface of a substrate by irradiation of
a rectangular laser beam polarized in the short-side direction, and
the size of crystal grains to be formed.
[0099] FIG. 13 is a diagram showing the size of crystal grains
acquired by irradiation of a rectangular laser beam polarized in
the short-side direction.
[0100] FIGS. 14A and 14B are explanatory diagrams of a case where a
rectangular laser beam polarized in the short-side direction is
introduced obliquely.
[0101] FIG. 15 is a status diagram showing the size of crystal
grains acquired experimentally by irradiation of a rectangular
laser beam polarized in the short-side direction.
[0102] FIG. 16 is a status diagram showing the size of crystal
grains acquired experimentally by irradiation of a rectangular
laser beam of a high energy density polarized in the short-side
direction.
[0103] FIG. 17 is a structural diagram of a laser annealing device
according to a third embodiment to irradiate a substrate with a
rectangular laser beam whole alternately changing the polarization
direction to the long-side direction and the short-side
direction.
[0104] FIGS. 18A and 18B are diagrams for explaining adjustment of
the polarization direction.
[0105] FIG. 19 is a structural diagram of a laser annealing device
according to a fifth embodiment of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0106] First, the principle of the present invention will be
described.
[0107] When a linearly polarized laser beam is introduced to a
silicon substrate, a micro structure which periodically appears in
the polarization direction of the laser beam, i.e., in the
vibration direction of an electric field, is formed. The period of
the periodic micro structure is about the wavelength of the laser
beam introduced to the silicon substrate.
[0108] This phenomenon will be described briefly (see Non-patent
Documents 2 and 3 for more details). A laser beam introduced to a
solid from air is scattered by minute irregularity on the solid
surface, causing a surface electromagnetic wave to be excited
between a solid medium and air. The electric field of the surface
electromagnetic wave and the electric field of the incident laser
beam interfere with each other, generating a standing wave having a
period of the wavelength or so of the laser beam on the solid
surface. Ablation by the standing wave causes a periodic micro
structure to be formed at the solid surface.
[0109] The present invention performs a laser annealing process on
a semiconductor film, such as a silicon film, using the periodic
energy distribution of the standing wave generated by interference
of the surface electromagnetic wave with the incident laser beam.
More specifically, a polycrystalline or monocrystalline
semiconductor film comprising crystal, grains grown to a uniform
size is formed by controlling the growth of the crystal grains
using the periodic energy distribution.
[0110] Preferred embodiments of the present invention will be
described below referring to the accompanying drawings. Same
reference numerals are given to common portions in the individual
drawings to avoid redundant descriptions.
First Embodiment
[0111] FIGS. 4 and 5 show the configuration of a laser annealing
device which performs an annealing process on an amorphous silicon
film on a substrate 1, such as a semiconductor device. The laser
annealing device has an optical system for generating a rectangular
laser beam. The optical system comprises a long-side optical system
2 corresponding to the long-side direction of the rectangular laser
beam and a short-side optical system 4. FIG. 4 shows the structure
of the long-side optical system 2, and FIG. 5 shows the structure
of the short-side optical system 4. Same reference numerals in
FIGS. 4 and 5 indicate optical elements shared by the long-side
optical system 2 and the short-side optical system 4.
[0112] As shown in FIGS. 4 and 5, the laser annealing device has a
laser oscillator (not shown) which outputs a laser beam, a
polarizer 5 which linearly polarizes the laser beam output from the
laser oscillator, and an beam expander 7 which generates a
rectangular laser beam whose cross section perpendicular to an
advancing direction is a rectangle. In the following description,
the long-side direction and the short-side direction of the
rectangular cross section of the rectangular laser beam are simply
called "long-side direction" and "short-side direction",
respectively.
[0113] The beam expander 7 expands the introduced laser beam in the
long-side direction. The laser annealing device further has a
cylindrical lens array 9 to which the laser beam expanded in the
long-side direction is introduced.
[0114] The laser annealing device has a long-side condenser lens 11
which adjusts the long-side directional length of the rectangular
laser beam having passed the cylindrical lens array 9 in the
long-side direction on the substrate 1, and a short-side condenser
lens 12 which condenses the rectangular laser beam having passed
the cylindrical lens array 9 with respect to the short-side
direction on the substrate 1.
[0115] FIG. 6A shows an energy distribution having a width A of a
laser beam in the long-side direction before passing the beam
expander 7, and FIG. 6B shows an energy distribution having a width
A' in the long-side direction at the time of irradiating an
amorphous silicon film. FIG. 7A shows an energy distribution having
a width B of a laser beam in the short-side direction before
passing the beam expander 7, and FIG. 7B shows an energy
distribution having a width B' in the short-side direction at the
time of irradiating an amorphous silicon film. As shown in FIG. 6B,
the energy of the rectangular laser beam at the time of irradiation
is substantially constant in the long-side direction.
[0116] According to a first embodiment, a laser beam is linearly
polarized by the polarizer 5 but the direction of polarization is
in the long-side direction. That is, the electric field of the
rectangular laser beam to be irradiated onto an amorphous silicon
film is directed in the long-side direction. A laser beam may be
linearly polarized by another method, instead of the polarizer 5,
such as reflecting the rectangular laser beam at a glass surface or
the like at a Brewster's angle to be linearly polarized.
[0117] The laser annealing device further has a transfer device
(not shown) which transfers the substrate 1 which has an amorphous
silicon film formed on the surface thereof in an arrow direction in
FIG. 8 at a predetermined speed when the rectangular laser beam is
introduced to the amorphous silicon film by the long-side optical
system 2 and the short-side optical system 4. With the rectangular
laser beam introduced to the surface of the semiconductor film, the
transfer device transfers the substrate in a direction
perpendicular to the long-side direction so that a desired range of
the surface of the semiconductor film can be irradiated with the
rectangular laser beam. The direction indicated by the arrow in
FIG. 8 is perpendicular to the long-side direction and corresponds
to the short-side direction. A direction in which the short side of
a rectangular laser beam is perpendicularly projected on the
surface of the substrate is also simply called "short-side
direction". The transfer device constitutes transfer means.
[0118] A rectangular laser beam may be generated to be irradiated
on an amorphous silicon film by using other adequate optical
systems.
[0119] As a rectangular laser beam polarized in the long-side
direction is irradiated onto the amorphous silicon film on the
substrate 1, a periodic energy distribution is produced in the
long-side direction in correspondence to a standing wave on the
amorphous silicon film. The standing wave is generated by
interference of a surface electromagnetic wave with the rectangular
laser beam. The surface electromagnetic wave is excited by
scattering of the incident rectangular laser beam at minute
irregularity on the surface of the amorphous silicon film. FIG. 9A
shows the periodic energy distribution in the long-side direction
corresponding to the standing wave.
[0120] A periodic temperature distribution is produced at the
amorphous silicon film in correspondence to the periodic energy
distribution. Therefore, nucleuses of crystal grains are generated
at locations which are cooled to the critical temperature of
nucleus generation in the solidification process of molten silicon.
The nucleus generated locations are locations with a lower
temperature, and, specifically, are positions of troughs of the
periodic energy distribution in FIG. 9A as shown in FIG. 9B. From
the nucleus generated locations, the nucleuses grow toward a
higher-temperature surrounding portion, so that the crystals
collide with each other to stop the growth. The crystal collision
locations are crystal grain boundaries. Consequently, crystal
grains are produced at periodic positions dependent on the energy
distribution as shown in FIG. 9C, making the size of the crystal
grains in the long-side direction uniform.
[0121] With the rectangular laser being irradiated, the substrate 1
is transferred in the short-side direction to irradiate the entire
amorphous silicon film with the rectangular laser beam. At this
time, the energy distribution of the laser beam in the long-side
direction does not change with time, so that crystals can be formed
in the whole silicon film at equal intervals in the long-side
direction.
[0122] The energy period of the standing wave becomes about the
wavelength of the rectangular laser beam. Therefore, the desired
crystal grain size, in the long-side direction can be obtained by
selecting the wavelength of the rectangular laser beam to be used
in the irradiation.
[0123] The size of crystal grains to be formed can also be adjusted
by changing the energy density of the rectangular laser beam. FIG.
10 shows crystal grains in polycrystalline or monocrystalline
silicon acquired by irradiating a rectangular beam with a
wavelength of 1 .mu.m and an electric field directed in the
long-side direction onto an amorphous silicon film at an energy
density of 450 to 500 mJ/cm.sup.2. FIG. 11 shows crystal grains in
polycrystalline or monocrystalline silicon acquired by irradiating
a rectangular beam with a wavelength of 1 .mu.m and an electric
field directed in the long-side direction onto an amorphous silicon
film at an energy density greater than 500 mJ/cm.sup.2 The crystal
grain size in the long-side direction is about 1.0 .mu.m in FIG. 10
while the crystal grain size in the long-side direction is about
1.5 .mu.m in FIG. 11. As apparent from the experimental results, as
the energy density is increased, crystal grains with a site greater
than the energy period of the standing wave are acquired.
[0124] Effects similar to or the same as those mentioned above are
also obtained by generating a rectangular laser beam from an
elliptically polarized laser beam whose major axis is directed in
the long-side direction instead of a linearly polarized laser
beam.
Second Embodiment
[0125] A second embodiment of the present invention will be
described next.
[0126] In the second embodiment, a laser annealing device is the
same as that of the first embodiment with a difference lying in
that the polarization direction of the polarizer 5 linearly
polarizes a laser beam output from the laser oscillator such that
an electric field is directed in the short-side direction, and then
a rectangular laser beam is generated by the beam expander 7. In
this manner, a rectangular laser beam with an electric field
directed in the short-side direction is generated to be introduced
to an amorphous silicon film. The short-side polarized beam
generating means is constituted by the laser oscillator which
outputs a laser beam, and the long-side optical system 2 and the
short-side optical system 4 which include the polarizer 5
polarizing the beam in the short-side direction. In a case where
the laser oscillator outputs a laser beam linearly polarized in the
short-side direction, the polarizer 5 can be omitted.
[0127] As shown in FIG. 8, with a rectangular laser beam being
introduced onto the amorphous silicon substrate 1, the substrate 1
is moved in the short-side direction at a predetermined speed as in
the first embodiment. This allows the rectangular laser beam
polarized in the short-side direction to be irradiated onto the
entire amorphous silicon film.
[0128] The rectangular laser beam introduced to the amorphous
silicon film is scattered by minute irregularity on the amorphous
silicon film, thus exciting a surface electromagnetic wave. The
interference of the surface electromagnetic wave with the
introduced incident rectangular laser beam generates a standing
wave at the surface of the amorphous silicon film in the short-side
direction. Therefore, the standing wave has a periodic energy in
the short-side direction. As mentioned above, the energy
distribution of the introduced rectangular laser beam in the
short-side direction becomes as shown in FIG. 7B. The periodic
energy distribution of the standing wave is superimposed on the
energy distribution of the rectangular laser beam in the short-side
direction to become an energy distribution on the amorphous silicon
film. A curve represented by a solid line in FIG. 12A indicates the
energy distribution of the standing wave combined with the energy
distribution of the introduced rectangular laser beam to curve
indicated by a broken line).
[0129] A temperature distribution corresponding to the energy
distribution in FIG. 12A is produced in the short-side direction of
silicon melted by the energy distribution. As shown in FIG. 12B,
crystal nucleuses are produced at positions of troughs of the
energy distribution. Thereafter, the crystal nucleuses grow toward
locations with a higher temperature in the short-side direction,
and locations at which their crystals collide one another to stop
the growth become crystal grain boundaries. As a result,
polycrystalline or monocrystalline silicon comprising crystals with
a uniform size in the short-side direction is formed as shown in
FIG. 12C.
[0130] The energy period of the standing wave becomes about the
wavelength of the rectangular laser beam. Therefore, the short-side
directional size of the crystal grains to be formed becomes the
interval of nodes or loops of the standing wave, i.e., about half
the wavelength of the rectangular laser beam. Therefore, the
desired crystal grain size in the short-side direction can be
acquired by selecting the wavelength of the rectangular laser beam
to be used in irradiation.
[0131] Since the energy of the rectangular laser beam in the
long-side direction is constant as described above referring to
FIG. 6B, crystal nucleuses are produced at random positions in the
long-side direction, thus forming crystal grains grown to random
sizes in the long-side direction. Typically, the size of the
crystal grains grown in the long-side direction becomes several
hundred nanometers or so. The use of the wavelength of several
hundred nanometers or so can make the sizes of the crystal grains
in the long-side direction and the short-side direction
approximately equal to each other. It is therefore preferable to
select the wavelength of the rectangular laser beam such that the
crystal grain size becomes about the long-side directional crystal
grain size of polycrystalline or monocrystalline silicon to be
formed. Accordingly, the crystal grain size as shown in FIG. 13 can
be acquired.
[0132] Further, according to the second embodiment, while
transferring the substrate 1, the rectangular laser beam is
introduced to the amorphous silicon film with the incident angle of
the rectangular laser beam to the amorphous silicon film being
adjusted. Thereby, the short-side directional crystal grain size
can be acquired according to the incident angle. That is, the
short-side directional crystal grain site can be adjusted by
adjusting the incident angle. This will be explained below.
[0133] When the incident angle .theta. is increased in the transfer
direction of the substrate 1 as shown in FIG. 14A, an interval X of
nodes or loops of the standing wave increases as indicated by an
equation 1 where .lamda. is the wavelength of the laser beam.
X = .lamda. 1 - sin .theta. [ Eq . 1 ] ##EQU00001##
[0134] When the incident angle .theta. is increased in the opposite
direction to the transfer direction of the substrate 1 as shown in
FIG. 14B, the interval X of nodes or loops of the standing wave
decreases as indicated by an equation 2 where .lamda. is the
wavelength of the laser beam. The relevant description is given in
Non-patent Document 1.
X = .lamda. 1 + sin .theta. [ Eq . 2 ] ##EQU00002##
[0135] Therefore, adjusting the incident angle of the rectangular
laser beam changes the period of the standing wave, so that
polycrystalline or monocrystalline silicon comprising crystal
grains with the same size as the energy period of the standing wave
in the short-side direction can be formed. In this manner, the size
of crystal grains can be adjusted by adjusting the incident angle
of the rectangular laser beam.
[0136] To adjust the incident angle of the rectangular laser beam,
the optical system side or the substrate side can be tilted. In a
case of tilting the optical system, for example, with the optical
system being integrally constructed, the entire optical system is
tilted by a tilting device. In a case of tilting the substrate
side, the transfer table for transferring the substrate 1 is tilted
by a tilting device. Those tilting devices may be any adequate
publicly known device. The tilting device that tilts the optical
system or the transfer table constitutes incident angle adjusting
means.
[0137] According to the second embodiment, the wavelength of the
standing wave to be generated or the short-side directional crystal
grain size can be adjusted by adjusting the angle at which the
rectangular laser beam is introduced to the amorphous silicon film,
instead of selecting the wavelength of a rectangular laser beam or
in addition to the selection of the wavelength of a rectangular
laser beam.
[0138] The size of crystal grains to be formed can also be adjusted
by changing the energy density of the rectangular laser beam. FIG.
15 shows crystal grains in polycrystalline or monocrystalline
silicon acquired by irradiating a rectangular beam with a
wavelength of 1 .mu.m and an electric field directed in the
short-side direction onto an amorphous silicon film at an incident
angle of 10 degrees to the substrate transfer direction at an
energy density of 450 to 500 mJ/cm.sup.2. FIG. 16 shows crystal
grains in polycrystalline or monocrystalline silicon acquired by
irradiating a rectangular beam with a wavelength of 1 .mu.m and an
electric field directed in the short-side direction onto an
amorphous silicon film at an incident angle of 10 degrees to the
substrate transfer direction at an energy density greater than 500
mJ/cm.sup.2. The crystal grain size in the short-side direction is
about 1.0 .mu.m in FIG. 15 while the crystal grain size in the
short-side direction is about 1.5 .mu.m in FIG. 16. As apparent
from the experimental results, as the energy density is increased,
crystal grains with a size greater than the energy period of the
standing wave are acquired.
Third Embodiment
[0139] A third embodiment of the present invention will be
described next.
[0140] FIG. 17 shows the configuration of a laser annealing device
according to the third embodiment which forms polycrystalline or
monocrystalline silicon by irradiating an amorphous silicon film
with a rectangular laser beam whose cross section perpendicular to
the advancing direction is a rectangle. The laser annealing device
includes a pair of laser oscillators 21, 22, polarizers 24, 25
provided in association with the laser oscillators 21, 22, a
reflecting mirror 27 which reflects a laser beam from the laser
oscillator 21, and a beam splitter 28 which combines laser beams
from the two laser oscillators 21, 22. The combined beam from the
beam splitter 28 is introduced to an optical system similar to or
same as that of the first embodiment shown in FIGS. 4 and 5,
generating a rectangular laser beam. This rectangular laser beam is
introduced to an amorphous silicon film on the substrate 1. FIG. 17
shows only the long-side optical system 2 corresponding to FIG. 4
as indicated by a broken line (the polarizer 5 in FIG. 4 not used);
the short-side optical system 4 is the same as the one shown in
FIG. 5 and is thus omitted. The polarizers 24, 25 constitute
polarization means which may be constituted by other adequate
components. The long-side optical system 2 and the short-side
optical system 4 used in the third embodiment constitute
rectangular laser beam generating means which may be constituted by
other adequate components. The beam splitter 28 and the reflecting
mirror 27 constitute beam combining means which may be constituted
by other adequate components.
[0141] According to the third embodiment, the polarizers 24, 25
linearly polarize the laser beams from the laser oscillators 21,
22, respectively. The polarization direction of the polarizer 24 is
the long-side direction, while the polarization direction of the
polarizer 25 is the short-side direction.
[0142] The laser annealing device according to the third embodiment
further has a pulse controller 29 which controls the laser
oscillators 21, 22 such that the timings of laser pulses output
from the laser oscillators 21, 22 are different from each other.
Therefore, the polarization direction of the laser beam combined by
the beam splitter 28 is alternately changed between the long-side
direction and the short-side direction.
[0143] The laser annealing device further has a transfer device
which transfers the substrate 1 in the short-side direction at a
predetermined speed as in the first embodiment.
[0144] The entire amorphous silicon film is irradiated with a
rectangular laser beam by transferring the substrate 1 in the
short-side direction while introducing the rectangular laser beam
whose electric field direction is alternately changed to the
amorphous silicon film on the substrate 1.
[0145] The long-side directional energy distribution at locations
on the substrate 1 at which the rectangular laser beam with an
electric field direction directed in the long-side direction is
irradiated is the same as the one shown in FIG. 9A, and the
short-side directional energy distribution at locations on the
substrate 1 at which the rectangular laser beam with an electric
field direction directed in the short-side direction is irradiated
is the same as the one shown in FIG. 12A. Therefore, temperature
distributions corresponding to the energy distributions in FIGS. 9A
and 12A are respectively produced in the long-side direction and
the short-side direction of molten silicon. Therefore, nucleuses of
crystal grains are generated at locations which are cooled to the
critical temperature of nucleus generation in the solidification
process of molten silicon. The nucleus generated locations are
positions of troughs of the periodic energy distributions in FIGS.
9A and 12A. Those crystal nucleuses grow in the long-side direction
and the short-side direction to the higher-temperature portions.
The locations at each of which nucleuses collide with each other to
stop the growth are crystal grain boundaries. Consequently,
polycrystalline or monocrystalline silicon comprising crystals with
uniform sizes in the long-side direction and the short-side
direction is formed.
[0146] Effects similar to or the same as those mentioned above are
also obtained by generating a rectangular laser beam using a
circularly polarized laser beam instead of a combined laser beam
whose electric field direction is alternately changed between the
long-side direction and the short-side direction.
[0147] In the third embodiment, the crystal grain size may also be
adjusted by changing the energy density of the rectangular laser
beam.
Fourth Embodiment
[0148] A fourth embodiment of the present invention will be
described next.
[0149] A laser annealing device according to the fourth embodiment
is similar to or the same as the laser annealing device of the
third embodiment shown in FIG. 17.
[0150] However, in the fourth embodiment, the pulse controller 29
may not control the laser oscillators 21, 22 so as to shift the
timings of the laser pulses output from the laser oscillators 21,
22 from each other. That is, while the pulse controller 29 controls
the timings of the laser pulses output from the laser oscillators
21, 22, the laser pulses output from the laser oscillators 21, 22
may overlap each other. The laser oscillators 21, 22 are
constructed to output linearly polarized lights, so that the
polarizers 24, 25 in FIG. 17 can be omitted. For example, the laser
oscillators 21, 22 themselves may output linearly polarized lights;
otherwise, the polarizers 24, 25 in FIG. 17 are respectively
provided in the laser oscillators 21, 22.
[0151] According to the fourth embodiment, the laser annealing
device is set such that the polarization direction of the laser
beam from the first laser oscillator 21 and the polarization
direction of the laser beam from the second laser oscillator 22 are
perpendicular to each other.
[0152] Therefore, standing waves are generated on the amorphous
silicon film of the substrate 1 in polarization directions
perpendicular to each other, and the periodic energy of the
standing wave similar to the one shown in FIG. 9A is produced,
thereby producing a temperature gradient corresponding to this
energy.
[0153] As a result, crystal nucleuses are produced at the positions
of troughs of the periodic energy, the crystal nucleuses grow in a
direction of a higher-temperature portion, and the locations at
which the crystal nucleuses collide with each other become crystal
grain boundaries as in the third embodiment. Therefore, crystal
nucleuses produced at periodic positions are grown by the influence
of the same temperature gradient produced in the directions
perpendicular to each other, so that a polycrystalline or
monocrystalline semiconductor film comprising crystal grains with
uniform sizes in the directions perpendicular to each other can be
formed. As a result, the crystal grain size becomes uniform between
the long-side direction and the short-side direction.
[0154] In the fourth embodiment, polarizers, such as a
half-wavelength plate, which adjust the polarization direction, may
be provided between the beam splitter 28 and the long-side optical
system 2 and the short-side optical system 4. Such a polarizer can
change, for example, the state where the polarization direction of
the laser beam is directed in the long-side direction and the
short-side direction as shown in FIG. 18A to the state where the
polarization direction of the laser beam is tilted by 45 degrees
from the long-side direction and the short-side direction as shown
in FIG. 18B.
Fifth Embodiment
[0155] A fifth embodiment of the present invention will be
described next.
[0156] FIG. 19 shows the configuration of a laser annealing device
according to the fifth embodiment of the present invention which
forms polycrystalline or monocrystalline silicon by irradiating an
amorphous silicon film with a rectangular laser beam whose cross
section perpendicular to the advancing direction is a
rectangle.
[0157] The laser annealing device has the laser oscillator 21
similar to or the same as that of the fourth embodiment, a
quarter-wavelength plate 31 which circularly polarizes a linearly
polarized laser beam from the laser oscillator 21, and the
aforementioned long-side optical system 2 and short-side optical
system 4 (the polarizer 5 in FIGS. 4 and 5 not used) which turn the
laser beam from the quarter-wavelength plate 31 to a rectangular
laser beam. In FIG. 19, the short-side optical system 4 is omitted
for the sake of simplicity.
[0158] The quarter-wavelength plate 31 constitutes circular
polarization means which may be constituted by another adequate
component. The laser oscillator 21, the circular polarization
means, the long-side optical system 2 and the short-side optical
system 4 constitute circularly polarized beam generating means
which may be constituted by other adequate components.
[0159] The laser annealing device with such a configuration causes
a circularly polarized laser beam to be introduced onto an
amorphous silicon film on the substrate 1.
[0160] Accordingly, a standing wave produced on the amorphous
silicon film takes a circular motion on a plane perpendicular to
the advancing direction of light. This makes the periodic energy of
the standing wave or a temperature gradient corresponding thereto
to be produced uniformly in every direction on the surface of the
semiconductor film.
[0161] When laser annealing is performed on an amorphous
semiconductor film by this method and device, therefore, nucleuses
are generated at troughs of the periodic energy, so that the
individual nucleuses grow in the directions to higher-temperature
portions and those portions where the nucleuses collide with one
another become crystal grain boundaries. As nucleuses generated at
periodic positions are grown by the influence of the periodic
temperature gradients uniformly produced in every direction,
therefore, it is possible to form a polycrystalline or
monocrystalline semiconductor film comprising crystal grains with a
uniform size in every direction. As a result, the crystal grain
sizes become uniform between the long-side direction and the
short-side direction.
Sixth Embodiment
[0162] A sixth embodiment of the present invention will be
described next.
[0163] A laser annealing device according to the sixth embodiment
is similar to or the same as the laser annealing device of the
fifth embodiment shown in FIG. 19 except for the quarter-wavelength
plate 31.
[0164] According to the sixth embodiment, the laser annealing
device has a polarization canceling plate which turns a linearly
polarized laser beam from the laser oscillator 21 to unpolarized
light, instead of the quarter-wavelength plate 31 in FIG. 19. The
polarization canceling plate constitutes unpolarization means which
turns linearly polarized light to unpolarized light but which may
be constituted by another adequate component.
[0165] The polarization canceling plate can turn the linearly
polarized laser beam from the laser oscillator 21 to an unpolarized
laser beam. The unpolarized laser beam from the polarization
canceling plate passes through the long-side optical system 2 and
the short-side optical system 4 to be a rectangular laser beam.
Therefore, the unpolarized rectangular laser beam is introduced to
the amorphous silicon film on the substrate 1.
[0166] As a linearly polarized laser beam is introduced to the
substrate 1 after being turned to unpolarized light according to
the sixth embodiment, a standing wave is not produced on the
surface of the amorphous silicon film on the substrate 1.
[0167] Therefore, crystal grains are generated at random positions,
and, what is more, the crystal grains grow in a random direction,
thereby suppressing an increase in the sizes of crystal grains only
in a specific direction. As a result, the sizes of the crystal
grains of the semiconductor film are generally made uniform,
obtaining a uniform crystal grain size between the long-side
direction and the short-side direction.
Other Embodiments
[0168] The present invention is not limited to the above-described
embodiments, and can of course be modified in various manners
without departing from the scope and spirit of the invention. For
example, the crystal grain size may be adjusted by adjusting the
short-side directional shape of a rectangular laser beam. The
adjustment of the short-side directional shape may be carried out
by adjusting the length of the short side of the rectangular laser
beam. This can make the energy gradient in the short-side direction
smaller, thereby suppressing the growth of crystal grains in the
short-side direction. The invention can be adapted not only to an
amorphous silicon film but also to other amorphous semiconductor
films.
[0169] Further, while the above-described embodiments are for a
case where a polycrystalline or monocrystalline semiconductor film
is modified by irradiating a rectangular laser beam to an amorphous
semiconductor film, the quality of a polycrystalline or
monocrystalline semiconductor film may be improved by irradiating a
rectangular laser beam on the polycrystalline or monocrystalline
semiconductor film instead of an amorphous semiconductor film. This
allows crystals to grow in one or both of the long-side direction
and the short-side direction by the influence of the periodic
temperature gradient. Thereby, the quality of the polycrystalline
or monocrystalline semiconductor film is improved so as to make
uniform the size of crystal grains in one or both of the long-side
direction and the short-side direction. In this case, the present
invention can be adapted to an improvement on the quality of a
polycrystalline or monocrystalline silicon film, or other
polycrystalline or monocrystalline semiconductor films. In the case
of the sixth embodiment, however, an increase in the size of
crystal grains only in a specific direction is suppressed to
improve the quality of the polycrystalline or monocrystalline
semiconductor film.
* * * * *
References