U.S. patent application number 12/006848 was filed with the patent office on 2008-06-12 for method of and apparatus for manufacturing semiconductor thin film, and method of manufacturing thin film transistor.
Invention is credited to Mitsuru Nakata, Hiromichi Takaoka.
Application Number | 20080135785 12/006848 |
Document ID | / |
Family ID | 33549156 |
Filed Date | 2008-06-12 |
United States Patent
Application |
20080135785 |
Kind Code |
A1 |
Takaoka; Hiromichi ; et
al. |
June 12, 2008 |
Method of and apparatus for manufacturing semiconductor thin film,
and method of manufacturing thin film transistor
Abstract
A method of manufacturing a semiconductor thin film includes (A)
forming an amorphous semiconductor film on a substrate, (B)
irradiating a beam to a surface of the amorphous semiconductor film
such that a predetermined region of the amorphous semiconductor
film is melted and solidified to form a crystallized semiconductor
film, and (C) scanning the beam in a first direction. A second
direction is a direction on the surface of the amorphous
semiconductor film perpendicular to the first direction. A length
along the second direction of a cross section of the beam is
substantially equal to or less than two times a width along the
second direction of the crystallized semiconductor film.
Inventors: |
Takaoka; Hiromichi; (Tokyo,
JP) ; Nakata; Mitsuru; (Tokyo, JP) |
Correspondence
Address: |
MUIRHEAD AND SATURNELLI, LLC
200 FRIBERG PARKWAY, SUITE 1001
WESTBOROUGH
MA
01581
US
|
Family ID: |
33549156 |
Appl. No.: |
12/006848 |
Filed: |
January 7, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10856275 |
May 28, 2004 |
7354810 |
|
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12006848 |
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Current U.S.
Class: |
250/492.22 ;
257/E21.134; 257/E21.413; 257/E29.293 |
Current CPC
Class: |
H01L 21/2026 20130101;
H01L 29/66757 20130101; H01L 29/78675 20130101; H01L 21/0268
20130101; H01L 21/02691 20130101; C30B 13/24 20130101 |
Class at
Publication: |
250/492.22 |
International
Class: |
G21K 5/00 20060101
G21K005/00 |
Foreign Application Data
Date |
Code |
Application Number |
May 30, 2003 |
JP |
156033/2003 |
Claims
1-18. (canceled)
19. An apparatus for manufacturing a semiconductor thin film
comprising: a laser oscillator configured for generating a beam; a
stage on which a substrate is placed; a mask provided on a light
path of said beam and having a plurality of aperture sections
through which said beam from said laser oscillator is irradiated to
a surface of said substrate; a means for scanning said beam in a
first direction; wherein a second direction is a direction on said
surface of said substrate perpendicular to said first direction,
wherein a length along said second direction of a cross section of
said beam on said surface, as a beam length, is substantially equal
to or less than two times a width along said second direction of a
crystal to be formed on said substrate, as a crystal growth
width.
20. The apparatus according to claim 19, wherein said beam length
is substantially equal to or less than said crystal growth.
21. The apparatus according to claim 19, wherein said plurality of
aperture sections are arranged along said second direction.
22. The apparatus according to claim 19, wherein each of said
plurality of aperture sections belongs to any one of a plurality of
aperture groups, and each of said plurality of aperture groups
includes a predetermined number of said aperture sections, wherein
said plurality of aperture groups are arranged in said first
direction, wherein said predetermined number of aperture sections
belonging to said each aperture group are arranged along said
second direction.
23. The apparatus according to claim 22, wherein one of said
plurality of aperture groups includes a first aperture section, and
another of said plurality of aperture groups includes a second
aperture section, wherein a part of said first aperture section
overlaps with a part of said second aperture section along said
first direction.
24. (canceled)
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional of U.S. patent application
Ser. No. 10/856,275 filed May 28, 2004 (pending), which is
incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to a method of manufacturing a
semiconductor thin film, an apparatus for manufacturing a
semiconductor thin film, and a method of manufacturing a thin film
transistor (TFT). More particularly, the present invention relates
to a method of manufacturing a semiconductor thin film in which the
formation of a grain boundary is controlled, and a thin film
transistor having a layer of the semiconductor thin film.
[0004] 2. Description of the Related Art
[0005] As a display of an electronic product such as a personal
computer and the like, a liquid crystal display (LCD) is known and
widely used. In particular, an active matrix liquid crystal display
(AM-LCD) has been rapidly popularized in recent years, because high
quality images can be achieved due to switching devices provided
for respective pixels. In the AM-LCD, a thin film transistor
(referred to as "TFT", hereinafter) is used as a switching device
for controlling a pixel and as a driver IC. In addition to a liquid
crystal display, such a TFT is also used in a
driving-circuit-integrated-type image sensor and a fluorescent
display tube and the like.
[0006] FIGS. 1A to 1D show conventional processes to manufacture
the above-mentioned TFT. At first, as shown in FIG. 1A, an
amorphous silicon film 202 is formed on a surface of a glass
substrate 100, for example. This amorphous silicon film 202 is a
precursor film of a semiconductor layer which will be described
later. Next, as shown in FIG. 1B, a laser light 300 is irradiated
to the surface of the amorphous silicon film 202, and crystal
grains are grown. As a result, a polycrystalline silicon
(poly-silicon) film 102 is formed from the amorphous silicon film
202. Here, the laser annealing is carried out by scanning the laser
light 300 from one end (for example, the left end in FIG. 1B) of
the amorphous silicon film 202 to the other end (the right end in
FIG. 1B).
[0007] Then, as shown in FIG. 1C, a gate insulating film 104 is
formed on the formed poly-silicon film 102. Then, the
channel-doping is performed on a channel region 120 of the
semiconductor layer (poly-silicon film) 102. After that, a gate
electrode 106 is formed on the gate insulating film 104.
[0008] Next, as shown in FIG. 1D, a first inter-layer insulating
film 130 is formed so as to cover the gate electrode 106 and the
gate insulating film 104. Then, contact holes 108a, 110a are formed
to penetrate the first inter-layer insulating film 130 and the gate
insulating film 104. Then, a source electrode 108 connected to the
contact hole 108a and a drain electrode 110 connected to the
contact hole 110a are formed on the first inter-layer insulating
film 130. Then, a second inter-layer insulating film 132 is formed
so as to cover the source electrode 108 and the drain electrode
110. Thus, the TFT is manufactured.
[0009] In recent years, the fineness of an LCD tends to be
increasingly improved, and also the higher performance is required
for an LCD to support the higher resolution moving image.
Therefore, a TFT used to control the pixels is desired to operate
faster. A TFT can operate faster as the mobility of carrier
(electron or hole) in the poly-silicon film 102 increases. However,
if there are a large number of grain boundaries in the poly-silicon
film 102, the mobility of the carrier decreases, which results in a
problem that the mobility of the TFT can not be made faster.
[0010] Therefore, a technique has been proposed, in which the
crystal growth during the laser annealing process is controlled in
order to reduce the number of the grain boundaries in the
poly-silicon film 102 and hence to improve the mobility of the
carrier.
[0011] Japanese Laid Open Patent Application (JP-P-2002-217206)
discloses a technique in which a rectangular laser line beam is
irradiated to form a poly-silicon layer which has crystal grains
extending for a length equal to about half of a width of the laser
line beam.
[0012] FIG. 2 is a schematic picture for explaining this technique.
According to this technique, the rectangular laser line beam with
the size of about 5 .mu.m.times.100 .mu.m is irradiated to an
amorphous silicon layer 303 shown in FIG. 2. At this time, the
profile of the laser beam energy density is trapezoidal as shown on
the left side of FIG. 2. As a result, silicon seed crystals (not
shown) are randomly generated in portions of the amorphous silicon
layer indicated by Y1 and Y2 which correspond to the ends of the
laser line beam. Then, poly-silicon grows from those silicon seed
crystals toward a portion which corresponds to the center of the
laser line beam and is indicated by Y3. The growth of the
poly-silicon stops at the portion Y3. In this way, a poly-silicon
layer 303' (331, 332) is formed, which has crystal grains extending
for the length equal to about half of the width of the laser line
beam. Here, the grain boundaries extend from the portion Y1, Y2 to
the portion Y3.
[0013] According to this technique, it is possible to make the
mobility of the thin film transistor and ON-state current higher by
setting the growth direction of the crystal (poly-silicon) into
alignment with a carrier running direction in the thin film
transistor. However, there is a problem with this technique in that
the formation of the grain boundaries can not be controlled, even
though the growth direction of the crystal can be controlled to
some degree. If the grain boundary crosses the channel portion of
the thin film transistor, the desired mobility of the carrier can
not be obtained. Also, the diameter of the crystal grain is at most
the length equal to the half of the width of the laser line beam.
Thus, there is also a problem with this technique in that
poly-silicon with a large grain diameter can not be formed.
[0014] As another technique, James S. Im et al. reports a technique
in which a narrow beam is scanned to form a giant crystal grain in
the scanning direction (refer to a non-patent document: "Sequential
lateral solidification of thin silicon films on SiO2", Robert S.
Sposili and James S. Im, Appl. Phys. Lett 69 (19) 1996 pp.
2864-2866).
[0015] FIGS. 3A to 3E schematically show the processes according to
this technique. In this technique, the narrow beam 310 shown in
FIG. 3A is made from a pulse laser light by using an appropriate
mask. The narrow beam 310 has a width of 5 .mu.m and a length of
200 .mu.m, and is irradiated to an amorphous silicon film 202. This
narrow beam 310 is to be scanned along the direction E shown in
FIG. 3A, i.e., from one side (indicated by a numeral D in FIG. 3A)
to the opposite side (indicated by a numeral S in FIG. 3A). Thus,
the amorphous silicon film 202 is to be sequentially heated
(annealed) as shown in FIGS. 3B to 3E.
[0016] FIG. 3B shows a situation when the first irradiation of the
narrow beam 310 is finished. At this time, the crystallization
(solidification) of the melted amorphous silicon film 202 starts
from boundaries between "the melted region" and "the non-melted
region" (indicated by two-dot chain lines in FIG. 3B). Here, these
boundaries correspond to the ends of the narrow beam 310 (top and
bottom ends in FIG. 3B). Then, the growth of the crystals begins
from those boundaries toward the center of the melted region. In
this way, the solidified portion becomes a crystallized
poly-silicon film 102. The growth of the crystals stops when they
collide with each other near the center. Thus, a grain boundary B
is formed near the center portion as shown in FIG. 3B. It should be
noted that the crystallization advances in the lateral direction in
FIG. 3B, and a lot of grain boundaries are formed along the
longitudinal direction. These longitudinal grain boundaries will be
described later.
[0017] Next, as shown in FIG. 3C, the irradiation region (the
narrow beam 310) is moved upward, and the second irradiation of the
narrow beam 310 is performed. Here, the displacement of the narrow
beam 310 is 0.75 .mu.m.
[0018] Similarly, the crystallization (solidification) starts from
boundaries between "the melted region" and "the non-melted region"
as shown in FIG. 3D. These boundaries are indicated by two-dot
chain lines in FIG. 3D, and correspond to the ends of the narrow
beam 310. Crystals grow from those boundaries toward the center of
the melted region, and a grain boundary B' is formed near the
center portion as shown in FIG. 3D. At this time, the grain
boundary B formed in the previous step disappears due to the melt.
Also, the melted portion along the lower boundary crystallizes
based on the crystal formed by the previous step (the first
irradiation). Therefore, no lateral grain boundary is formed along
this lower boundary.
[0019] After that, the irradiation region (the narrow beam 310) is
moved and the irradiation of the narrow beam 310 is performed
sequentially. The melt and crystallization of the amorphous silicon
film 202 are repeated in the similar way. Thus, the crystal grains
extending in the longitudinal direction (the scanning direction E)
can be formed as shown in FIG. 3E, and no grain boundary is formed
along the lateral direction (orthogonal to the scanning direction
E).
[0020] Also, Matsumura et al. reports a technique that uses a
light-shield plate in laser annealing (refer to a non-patent
document: "Excimer-Laser-Induced Lateral-Growth of Silicon Thin
Films", Kensuke Ishikawa, Motohiro Ozawa, Chang-Ho Oh and Masakiyo
Matsumra, Jpn. J. Appl. Phys. Vol. 37 (1998) pp. 731-736). FIG. 4A
is a schematic picture for explaining this technique, and FIG. 4B
is a cross sectional view along a dashed line J-J' in FIG. 4A.
[0021] According to this technique, a light-shield plate 400 is
placed in a light path to shade a part of the narrow beam 300A, as
shown in FIGS. 4A and 4B. A part of the narrow beam 300A is
diffracted to be a diffracted beam 310A as shown in FIG. 4B, and
the energy density of this diffracted beam 310A becomes low.
Therefore, the temperature of the amorphous silicon film 202
associated with the diffracted beam 310A becomes lower than that
associated with the directly incoming narrow beam 300A. In other
words, the temperature of the irradiation region on the left side
of FIG. 4B is higher than the temperature on the right side, i.e.,
a temperature gradient is generated as indicated by an arrow in
FIG. 4B. This temperature gradient promotes the growth of the
poly-silicon film 102.
[0022] According to the conventional techniques mentioned above,
the growth direction of the crystal can be controlled to some
degree. However, in the technique disclosed in the above-mentioned
patent document, the formation of the grain boundaries can not be
controlled. There is also a problem with the other conventional
techniques in that the grain boundaries along the scanning
direction can not be controlled.
[0023] FIG. 5 is a schematic picture explaining such a problem in
the conventional technique. In FIG. 5, the longitudinal direction I
denotes the scanning direction. As shown in FIG. 5, crystals grow
along the direction I. However, each crystal can not grow largely
in the direction orthogonal to the direction I, and a lot of grain
boundaries B along the direction I are generated in the
poly-silicon film 102.
[0024] Here, the formation of these grain boundaries B is not
controlled, i.e., the locations and the distribution density of the
grain boundaries B are not controlled. Therefore, there is a
certain distribution in the distances between grain boundaries B
adjacent to each other (each distance corresponds to a width t of
each crystal 102a). The maximum value of the distance is about 1
.mu.m. Even if the movement direction of the carriers in the TFT is
set to the direction I, the movement of the carriers is interfered
by the grain boundaries B which are bent in the lateral direction
(orthogonal to the direction I) or intersect with other grain
boundaries B. This causes the depression of the carrier mobility.
Moreover, the widths t of the respective crystals 102a are not
constant and vary depending on the manufacturing condition.
Furthermore, the number and the directions of the grain boundaries
B also vary. Thus, the carrier mobility and the threshold voltage
of the manufactured TFT vary depending on the manufacturing
condition.
[0025] The following may be considered as a reason for the
formation of the crystals 102a with the various widths t. When the
first irradiation of the laser light 300 is performed on the
amorphous silicon film 202, the temperature gradient is not
generated along the direction orthogonal to the direction I in the
melted amorphous silicon film 202. Therefore, crystal seeds are
formed randomly in the direction orthogonal to the direction I.
Then, the crystals grow based on the crystal seeds and extend along
the direction I as mentioned above. As a result, various crystal
grains with various widths t are formed as shown in FIG. 5.
SUMMARY OF THE INVENTION
[0026] It is therefore an object of the present invention to
provide a method and an apparatus for manufacturing a semiconductor
thin film which can reduce the number of grain boundaries in the
manufactured semiconductor thin film.
[0027] Another object of the present invention is to provide a
method and an apparatus for manufacturing a semiconductor thin film
which can control the positions of grain boundaries.
[0028] Still another object of the present invention is to provide
a thin film transistor in which a mobility is improved and
variation of the mobility is suppressed, and a method for
manufacturing the thin film transistor.
[0029] In an aspect of the present invention, a method of
manufacturing a semiconductor thin film includes (A) forming an
amorphous semiconductor film on a substrate, (B) irradiating a beam
to a surface of the amorphous semiconductor film such that a
predetermined region of the amorphous semiconductor film is melted
and solidified to form a crystallized semiconductor film, and (C)
scanning the beam in a first direction.
[0030] The first direction is a scanning direction and a second
direction is a direction on the surface of the amorphous
semiconductor film perpendicular to the first direction. A "beam
length" is defined by a length along the second direction of a
cross section of the beam. A "crystal growth width" is defined by a
width along the second direction of the crystallized semiconductor
film. The crystallized semiconductor film may include a single
crystal region and a small poly-crystalline silicon region formed
around the single crystal region.
[0031] According to the present invention, the beam length is
substantially equal to or less than two times the crystal growth
width. The beam length can be substantially equal to or less than
the crystal growth width. Preferably, the beam length is equal to
or less than 6 .mu.m.
[0032] In another aspect of the present invention, a method of
manufacturing a semiconductor thin film includes (AA) forming an
amorphous semiconductor film on a substrate, (BB) irradiating a
plurality of beams to a surface of the amorphous semiconductor film
such that predetermined regions of the amorphous semiconductor film
are melted and solidified to form a plurality of crystallized
semiconductor films, and (CC) scanning the plurality of beams in
the first direction. A "beam length" is a length along the second
direction of a cross section of each of the plurality of beams. A
"crystal growth width" is a width along the second direction of
corresponding one of the plurality of crystallized semiconductor
films.
[0033] According to the present invention, the beam length is
substantially equal to or less than two times the crystal growth
width. A plurality of the beam lengths of the plurality of beams
can be substantially equal to or less than a plurality of the
crystal growth widths of the plurality of crystallized
semiconductor films, respectively. Preferably, the beam length of
each beam is equal to or less than 6 .mu.m.
[0034] In the present invention, the plurality of beams are
arranged along the second direction. In this case, an interval
between one of the plurality of beams and an adjacent one of the
plurality of beams is preferably equal to or more than 0.3 .mu.m.
More preferably, the interval is equal to or more than 0.4 .mu.m.
More preferably, the interval is equal to or more than 0.6
.mu.m.
[0035] In the present invention, each of the plurality of beams
belongs to any one of a plurality of beam groups. Thus, each of the
plurality of beam groups includes a predetermined number of the
beams. The plurality of beam groups are arranged in the first
direction. The predetermined number of beams belonging to each beam
group are arranged along the second direction.
[0036] Also, one of the plurality of beam groups includes a first
beam. Another of the plurality of beam groups includes a second
beam. A part of the first beam overlaps with a part of the second
beam along the first direction. Preferably, a width for which the
first beam overlaps with the second beam is equal to or more than
0.7 .mu.m.
[0037] In still another aspect of the present invention, a method
of manufacturing a semiconductor thin film includes (D) forming an
amorphous semiconductor film on a substrate, (E) irradiating a beam
to a surface of the amorphous semiconductor film such that a
predetermined region of the amorphous semiconductor film is melted
and solidified to form a crystallized semiconductor film, and (F)
scanning the beam in the first direction.
[0038] According to the present invention, during the (E)
irradiating step, a temperature distribution of the surface along
the second direction has two gradients near both ends of the
predetermined region. Preferably, the temperature distribution
along the second direction has a trapezoidal shape. A full width at
half maximum (FWHM) of the temperature distribution is
substantially equal to or less than two times the crystal growth
width. The FWHM of the temperature distribution can be
substantially equal to or less than the crystal growth width. Here,
the two gradients along the second direction are generated by
changing amount of the beam along the second direction. Preferably,
the change in the amount of the beam along the second direction is
equal to or more than 460.8 mJ/cm.sup.2 per 1 .mu.m on the
surface.
[0039] In still another aspect of the present invention, an
apparatus for manufacturing a semiconductor thin film has a laser
oscillator composed for generating a beam, a stage on which a
substrate is placed, a mask, and a means for scanning the beam in a
first direction. The mask is provided on a light path of the beam
and has a plurality of aperture sections through which the beam
from the laser oscillator is irradiated to a surface of the
substrate.
[0040] The first direction is a scanning direction and a second
direction is a direction on the surface of the substrate
perpendicular to the first direction. A "beam length" is a length
along the second direction of a cross section of the beam on the
surface of the substrate. A "crystal growth width" is a width along
the second direction of a crystal to be formed on the
substrate.
[0041] According to the present invention, the beam length is
substantially equal to or less than two times the crystal growth
width. The beam length can be substantially equal to or less than
the crystal growth width. Preferably, the beam length is equal to
or less than 6 .mu.m.
[0042] In the apparatus, the plurality of aperture sections are
arranged along the second direction.
[0043] Also, in the apparatus, each of the plurality of aperture
sections belongs to any one of a plurality of aperture groups.
Thus, each of the plurality of aperture groups includes a
predetermined number of the aperture sections. The plurality of
aperture groups are arranged in the first direction. The
predetermined number of aperture sections belonging to each
aperture group are arranged along the second direction. Also, one
of the plurality of aperture groups includes a first aperture
section, and another of the plurality of aperture groups includes a
second aperture section. A part of the first aperture section
overlaps with a part of the second aperture section along the first
direction.
[0044] In still another aspect of the present invention, a method
of manufacturing a thin film transistor includes (a) forming an
amorphous semiconductor film on a substrate, (b) irradiating a beam
to a surface of the amorphous semiconductor film such that a
predetermined region of the amorphous semiconductor film is melted
and solidified to form a crystal, (c) scanning the beam in the
first direction to form a crystallized semiconductor thin film, and
(d) forming a channel region of the thin film transistor by using
the crystallized semiconductor thin film such that carriers move
along the first direction. During the (b) irradiating, the beam
length is substantially equal to or less than two times the crystal
growth width.
[0045] According to the method and the apparatus for manufacturing
the semiconductor thin film in the present invention, the following
effects can be attained. That is, the width of the crystal along
the second direction can be made larger than that of a conventional
semiconductor thin film, because of the temperature gradient
generated along the second direction. Moreover, the positions of
the grain boundaries can be controlled, and also the number of the
grain boundaries can be reduced. Also, the positions and the number
of the crystals can be controlled.
[0046] Furthermore, according to the present invention, a thin film
transistor (TFT) is manufactured by using the single crystal region
generated by the above apparatus and the above method. Thus, the
mobility of the manufactured TFT is improved, because the number of
the grain boundaries is reduced. This implies that a TFT with high
mobility can be obtained according to the present invention.
Moreover, since the number and the directions of the grain
boundaries are controlled and hardly vary, the variation in the
mobility of the TFT can be suppressed.
BRIEF DESCRIPTION OF THE DRAWINGS
[0047] FIGS. 1A to 1D show conventional processes to manufacture a
thin film transistor;
[0048] FIG. 2 is a schematic view for explaining a conventional
technique;
[0049] FIGS. 3A to 3E are schematic views for explaining another
conventional technique;
[0050] FIG. 4A is a schematic view for explaining still another
conventional technique;
[0051] FIG. 4B is a cross sectional view along a dashed line J-J'
in FIG. 4A;
[0052] FIG. 5 is a schematic view for explaining conventional
techniques;
[0053] FIG. 6 shows a configuration of an apparatus for
manufacturing a semiconductor thin film according an embodiment of
the present invention;
[0054] FIG. 7 is a plan view showing an example of a mask in the
apparatus according to the embodiment of the present invention;
[0055] FIG. 8 is a magnified sectional view showing a step of laser
annealing according to the present invention;
[0056] FIG. 9 shows a distribution of laser energy density
according to the embodiment of the present invention;
[0057] FIG. 10 is a plan view showing another example of a mask in
the apparatus according to the present invention;
[0058] FIG. 11A is a plan view showing still another example of a
mask in the apparatus according to the present invention;
[0059] FIG. 11B is a cross sectional view along a line G-G' in FIG.
11A;
[0060] FIG. 12 is a schematic view showing a laser annealing
apparatus used in a first example;
[0061] FIG. 13 is a cross sectional view schematically showing the
structure of a substrate to be processed according to the first
example;
[0062] FIG. 14 is a cross sectional view schematically showing the
structure of a mask in the laser annealing apparatus according to
the first example;
[0063] FIG. 15 is a plan view of the mask in the laser annealing
apparatus according to the first example;
[0064] FIG. 16 shows a result of the SEM observation of the
crystallized film according to the first example;
[0065] FIG. 17 shows results of the SEM observations of the
crystallized films according to a second example;
[0066] FIG. 18 is a schematic view showing the beam cross section
according to a third example;
[0067] FIG. 19A shows a result of the SEM observation of the
crystallized film according to a comparative example;
[0068] FIG. 19B shows a result of the SEM observation of the
crystallized film according to the third example;
[0069] FIGS. 20A to 20C show results of the SEM observations of the
crystallized films according to Examples 4-1 to 4-3,
respectively;
[0070] FIGS. 21A to 21C are schematic pictures showing the
mechanism of the crystal growth according to Examples 4-1 to 4-3,
respectively;
[0071] FIGS. 22A and 22B show results of the SEM observations of
the crystallized films according to Examples 5-1 and 5-2,
respectively;
[0072] FIG. 23 is a schematic picture showing an aperture pattern
of a mask according to a sixth example;
[0073] FIGS. 24A and 24B show results of the SEM observations of
the crystallized films according to Examples 6-1 and 6-2,
respectively;
[0074] FIG. 25A is a plan view schematically showing a
semiconductor thin film with a single crystal region according to a
seventh example; and
[0075] FIG. 25B is a plan view schematically showing a thin film
transistor according to the seventh example.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0076] Embodiments of the present invention will be described below
with reference to the attached drawings.
[0077] FIG. 6 shows a configuration of a laser annealing apparatus,
i.e., an apparatus for manufacturing a semiconductor thin film
according to the present embodiment.
[0078] The laser annealing apparatus has a laser oscillator 60, a
homogenizer 62, a mirror 64, a mask 66, a lens 68, and a process
chamber 70. The laser oscillator 60 is placed outside the process
chamber 70. The process chamber 70 has a window 72, a stage 74 and
a mask 80. The window 72 is provided on the wall surface of the
process chamber 70. The stage 74 and the mask 80 are provided
within the process chamber 70. A substrate 90 with a precursor film
20A is placed on this stage 74.
[0079] A method of manufacturing a semiconductor thin film and an
operation of the laser annealing apparatus are as follows. First, a
precursor film 20A is deposited on a substrate 90. This precursor
film 20A is an amorphous semiconductor film or a poly-crystalline
semiconductor film. For example, this precursor film 20A can be an
amorphous silicon film deposited by a general deposition method
such as a plasma CVD method and the like. As shown in FIG. 6, the
substrate 90 with the precursor film 20A is placed on the stage 74
within the process chamber 70.
[0080] Next, an XeCl excimer laser with a wavelength of 308 nm is
oscillated in a pulse shape by the laser oscillator 60. The laser
light 30A outputted from the laser oscillator 60 is changed to have
a predetermined beam profile through the homogenizer 62. Then, the
laser light 30A is reflected by the mirror 64 to be incident into
the mask 66. Passing through the mask 66, the shape of the laser
light 30A is made into a predetermined beam shape. After adjusted
by the lens 68, the laser light 30A is incident into the process
chamber 70 through the window 72.
[0081] The mask 80 is placed on the light path of the laser light
30A between the window 72 and the precursor film 20A. This mask 80
is used for throttling the laser light 30A, i.e., the "beam cross
section" of the incoming laser light 30A is reduced by this mask
80. Here, the "beam cross section" refers to the area perpendicular
to the beam axis. Then, the throttled laser light 30A is irradiated
to the surface of the precursor film 20A.
[0082] The laser light 30A is scanned over the precursor film 20A.
Here, the stage 74 is designed to be movable in a "first
direction". The first direction is the scanning direction and is
defined by a direction vertical to the paper surface in FIG. 6. A
"second direction" is defined by a direction on the surface of the
precursor film 20A and perpendicular to the first direction, i.e.,
a direction perpendicular to the first direction and the beam axis.
The second direction is also indicated by a direction F in FIG. 6.
The substrate 90 having the precursor film 20A moves in the first
direction together with the stage 74. Thus, the laser light 30A is
scanned in the first (scanning) direction over the precursor film
20A.
[0083] FIG. 7 is a plan view showing an example of the mask 80 in
the process chamber 70. As shown in FIG. 7, the mask 80 is made of
an Al plate, and a rectangular aperture 82 is formed in the center
of the Al plate. As will be described later, this aperture 82 is
placed on the light path of the laser light 30A. The laser light
30A passed through the aperture 82 is irradiated to the precursor
film 20A. Thus, in the mask 80 shown in FIG. 7, the aperture 82 is
associated with a light transmitting section 86, and the other
section is associated with a light shielding section
(non-transmitting section) 84. Here, "aperture length" is defined
by a length of the aperture 82 along the second direction
(direction F). Also, "aperture width" is defined by a width of the
aperture 82 along the first direction (scanning direction).
[0084] FIG. 8 is a magnified sectional view showing a step of laser
annealing. As shown in FIG. 8, the laser light 30A is irradiated to
the precursor film 20A through the aperture 82. Here, a length of
the beam cross section along the second direction (direction F) is
larger than the "aperture length" above the mask 80. Therefore, the
laser light 30A passing through the aperture 82 is diffracted, and
the diffracted laser light 30A is irradiated to a predetermined
area of the precursor film 20A. Consequently, the precursor film
20A is melted in the predetermined area to which the diffracted
laser light 30A is irradiated (the predetermined area is referred
to as an "irradiated area"). After the irradiation of the laser
light 30A is stopped, the melted area is solidified, and hence a
crystallized semiconductor layer is formed.
[0085] FIG. 9 shows the distribution of laser energy intensity,
i.e., the temperature distribution of the precursor film 20A in the
irradiated area at the time of the laser irradiation. The
temperature distribution is a distribution along the second
direction. As shown in FIG. 9, the temperature distribution has a
trapezoidal shape. A high temperature area indicated by a numeral
"H" shown in FIG. 9 corresponds to a high temperature area "H"
shown in FIG. 8. Similarly, low temperature areas "L" on both sides
of the high temperature area "H" in FIG. 9 correspond to low
temperature areas "L" shown in FIG. 8. Since the laser light 30A is
diffracted as shown in FIG. 8, temperature gradients are generated
on both sides of the high temperature area H at the center of the
irradiated area. The distance between the areas with temperature
gradients is in an order of sub-micron to micron.
[0086] Here, "beam length" and "beam width" are defined on the
basis of the cross section of the diffracted laser light 30A. The
"beam width" is defined by a width of the beam cross section along
the first direction (scanning direction). The "beam length" is
defined by a length of the beam cross section along the second
direction (direction F). Also, the "beam length" may be defined by
a FWHM (Full Width at Half Maximum) of the temperature distribution
shown in FIG. 9.
[0087] According to the present invention, at the time of
crystallizing the semiconductor film as mentioned above, the "beam
length" is set to a value equal to or less than about two times a
width along the second direction of the crystal to be formed. As a
result, single-crystal regions extending in the first direction
(scanning direction) can be formed in the precursor film 20A, which
will be shown later.
[0088] FIG. 10 is a plan view showing another example of the mask
80 in the process chamber 70. In this mask 80B, two rectangular
apertures 82B are formed. These apertures 82B are associated with
light transmitting sections 86B, and a section between two
apertures 82B is associated with an interval section 88B. The other
sections are associated with light shielding section 84B.
[0089] Also, the material of the mask 80 is not limited to aluminum
(Al). FIG. 11A is a plan view showing still another example of the
mask 80 in the process chamber 70, and FIG. 11B is a sectional view
along a line G-G' in FIG. 11A. This mask 80c has a substrate made
of a transparent material such as quartz and the like. Further,
non-transparent pattern is formed on a predetermined area of the
substrate. For example, the non-transparent pattern is formed by
patterning a metal film made of, for example, aluminum, molybdenum,
chrome, tungsten silicide and stainless alloy and so on. Also, a
protective film such as a chrome oxide film can be laminated over
the non-transparent pattern. Also, a single-layer or a multi-layer
dielectric film can be used to form the non-transparent pattern.
Thus, light transmitting areas 82C with rectangular shapes and a
light shielding area 84C are formed in the mask.80C as shown in
FIGS. 11A and 11B. These light transmitting areas correspond to
apertures. A part of the laser light 30A transmits these light
transmitting areas 82C to be irradiated on the precursor film
20A.
[0090] Moreover, in the mask 80 mentioned above, the shape of the
aperture (the light transmitting area) 82 is not limited to
rectangle. The shape can be polygon such as triangle, hexagon,
octagon and the like. Also, the shape can be circle, ellipse and
the like. Moreover, the aperture 82 can have a chevron shape. The
step of laser annealing by using a mask 80 with any of these
apertures 82 is similar to that in the case mentioned above. So,
its explanation is omitted.
[0091] Also, a lot of slits with extremely thin widths can be
formed in the mask 80. Or, a lot of holes can be formed
concentrated in the mask 80. These slits or holes are used instead
of the aperture (the light transmitting area) 82. In this case, the
energy of the laser light 30A can be adjusted by changing the
number of slits or the number and density of the holes.
[0092] Moreover, the above-mentioned mask 80 can be provided not
only in the process chamber 70 as in FIG. 6 but also outside the
process chamber 70. That is, the mask 80 can be placed at any
position on the light path from the laser oscillator 60 to the
precursor film 20A. Also, the mask 66 shown in FIG. 6 can be
configured to have the non-transmission area similar to the mask 80
shown in FIGS. 7, 10, 11. In this case, the mask 66 replaces the
mask 80, and plays the same role as the mask 80.
[0093] Furthermore, the laser light 30A used in the laser annealing
is not limited to the XeCl excimer laser. Another excimer laser
such as a KrF laser can be used as the laser light 30A. Also, a
solid laser such as an Nd:YAG laser, an Nd:YLF laser, an
Nd:YVO.sub.4 laser and the like can be used. Further, a gas laser
such as a carbonic acid gas laser, an argon gas laser and the like
can be used.
[0094] A TFT (Thin Film Transistor) can be formed on the surface of
various insulating substrates by using the manufacturing method
according to the present invention. In particular, it is preferable
with regard to the high integration and the high performance to use
an SOI (Silicon On Insulator) in which an oxide film is formed on
the surface of a silicon wafer.
[0095] Next, a plurality of examples of the manufacturing according
to the present invention will be shown below together with
experimental results.
FIRST EXAMPLE
[0096] FIG. 12 is a schematic picture showing a laser annealing
apparatus (an apparatus for manufacturing a semiconductor thin
film) used in the present example. The laser annealing apparatus
has an excimer laser oscillator 60A, mirrors 152, 153, 154, a
homogenizer 155, a mask stage 157, a 1/3 reduction-imaging lens
160, and a process chamber 70. The laser annealing apparatus also
has a mask 66 which is-supported movably by the mask stage 157. The
substrate stage 74 is provided within the process chamber 70, and a
window 162 is provided on the surface of the process chamber
70.
[0097] According to this laser annealing apparatus, a substrate 165
is placed on the substrate stage 74. Then, a beam 150L (XeCl,
wavelength of 308 nm) is outputted from the excimer laser
oscillator 60A. As shown in FIG. 12, the beam 150L is injected into
the process chamber 70 through the mirrors 152, 153, the
homogenizer 155, the mirror 154, the mask 66, the 1/3
reduction-imaging lens 160 and the window 162. Then, the beam 150L
is irradiated onto the substrate 165. Here, the mask stage 157 is
composed of an XY stage, an air bearing, a linear motor and the
like. Thus, the beam 150L can be scanned with high precision by
moving the mask stage 157. It should be noted that the beam 150L
can be scanned by moving the substrate stage 74, although it is
scanned by moving the mask stage 157 in the present laser annealing
apparatus.
[0098] The cross section of the beam 150L is made into a shape
corresponding to a pattern of the aperture of the mask 66. Then,
the beam 150L is irradiated into a filed area. According to the
present invention, a lens with more aperture number (NA) than that
of a usual laser annealing apparatus is used in order to make the
intensity profile of the beam 150L steep at laser edges. Here, the
NA of the lens is set to 0.2. Also, the optical system is adjusted
such that a length with which the energy density of the beam 150L
on the substrate 165 varies from 2% to 98% of its maximum value is
1 .mu.m.
[0099] Next, the substrate 165 will be described below. FIG. 13 is
a cross sectional view schematically showing the structure of the
substrate 165. As shown in FIG. 13, an SiO.sub.2 film 170 is formed
on a glass substrate 165a, and an amorphous silicon (a-Si) film 171
is formed on the SiO.sub.2 film 170 as the precursor film. In this
example, no-alkali glass is used for the glass substrate 165a. More
specifically, the 300 nm-SiO.sub.2 film 170 is deposited on the
glass substrate 165a by using an LP-CVD (Low Pressure Chemical
Vapor Deposition) method. The SiO.sub.2 film 170 is for preventing
the diffusion of impurities from the glass. The a-Si film 171 of 60
nm as the precursor film is formed on this SiO.sub.2 film 170 by
using the same LP-CVD method.
[0100] Next, the mask 66 will be described below. FIG. 14 is a
cross sectional view schematically showing the structure of the
mask 66. In this mask 66, a chrome film 175 is formed on a quartz
substrate 166a. The chrome film 175 plays a role of the light
shielding section (referred to as a shading section, hereinafter).
Thus, a plurality of aperture sections 175a are formed as an
aperture pattern. These aperture sections correspond to light
transmitting sections.
[0101] FIG. 15 is a plan view of the mask 66 showing the aperture
pattern. The aperture pattern has a plurality of unit patterns
arranged repeatedly in one direction. In FIG. 15, one rectangular
aperture 175a is the unit pattern, and a plurality of the
rectangular apertures 175a are arranged in x direction (the second
direction mentioned above). As shown in FIG. 15, the width of the
aperture 175a along the y (first) direction is "a" ("aperture
width"), and the length of the aperture 175a along the x (second)
direction is "b" ("aperture length"). Here, the y (first) direction
is consistent with the scanning direction. Also, the distance
between the adjacent apertures 175a is "c", which is referred to as
an "aperture interval".
[0102] The laser annealing apparatus having the above-mentioned
composition is used to manufacture the semiconductor thin film. The
manufacturing process is carried out under the following experiment
conditions shown in table 1.
TABLE-US-00001 TABLE 1 LASER FLUENCE 550 mJ/cm.sup.2 APERTURE WIDTH
a 12 .mu.m APERTURE LENGTH b 6 .mu.m APERTURE INTERVAL c 1.8 .mu.m
STEP WIDTH 0.2 .mu.m
[0103] As mentioned above, the length with which the energy density
of the beam 150L on the substrate 165 varies from 2% to 98% of its
maximum value (laser fluence: 550 mJ/cm.sup.2) is set to 1 .mu.m.
Therefore, the gradient of the energy density is given by 528
mJ/cm.sup.2/.mu.m.
[0104] Also, the "aperture width" indicates the length "a" in FIG.
15, the "aperture length" indicates the length "b" in FIG. 15, and
the "aperture interval" indicates the length "c" in FIG. 15. The
values a, b, c indicate lengths on the mask 66. Due to the 1/3
reduction-imaging lens 160 (see FIG. 12), the "beam width" and the
"beam length" on the substrate 165 are one third of the "aperture
width a" and the "aperture length b", respectively. More
specifically, the "beam width" is 4 .mu.m and "the beam length" is
2 .mu.m on the a-Si film 171 of the substrate 165. The "step width"
in the table 1 indicates the distance for which the beam 150L moves
between two pulse irradiation. The scanning direction (first
direction) is indicated by an arrow in FIG. 15 (upward).
[0105] After manufacturing the semiconductor thin film, the
Secco-etching is performed on that crystallized film in order to
emphasis grain boundaries. Then, the obtained crystallized film is
observed by using a scanning electron microscope (SEM). FIG. 16
shows a result of the SEM observation of the crystallized film
formed under the conditions shown in the table 1.
[0106] It can be clearly seen from FIG. 16 that a single crystal
region with no grain boundary is formed along the first (scanning)
direction in the center of the image. Here, the image in FIG. 16
shows an area corresponding to one aperture section 175a and two
shading sections, and it is found that the single crystal region is
formed in an area corresponding to the one aperture section 175a.
On the other hand, formed in areas corresponding to the two shading
sections are groups of grain boundaries which extend so as to mark
off the single crystal region. Thus, it is confirmed that it is
possible according to the manufacturing method of the present
invention to form single crystal regions with controlling locations
of the grain boundaries.
SECOND EXAMPLE
[0107] Another experiment is carried out as a second example, in
which the aperture interval c is changed as a variable. The
experimental conditions other than the aperture interval are the
same as those in the first example. In the present experiment, the
aperture interval c on the mask 66 is set to 0 .mu.m, 0.6 .mu.m,
0.9 .mu.m, 1.05 .mu.m, 1.2 .mu.m and 1.8 .mu.m.
[0108] FIG. 17 shows results of the SEM observations of the
crystallized films formed under the conditions with respective
aperture intervals. The aperture intervals used are indicated at
the bottom of respective images, and the scanning direction (first
direction) is indicated by an arrow. As is clearly seen from FIG.
17, the appearances of the crystallized films vary with changing
aperture intervals.
[0109] In the cases when the aperture interval is set to 0 .mu.m or
0.6 .mu.m, the influence of the shading section hardly appears in
the SEM observation. In the case when the aperture interval is set
to 0.9 .mu.m, the difference in crystalline state begins to appear
between the area corresponding to the shading section and the area
corresponding to the aperture section. In the case when the
aperture interval is set to 1.2 .mu.m, it is found that a single
crystal region with no grain boundaries is formed under the
aperture 175a. The width of the single crystal region along the
second direction is about 1 .mu.m. In the case when the aperture
interval is set to 1.8 .mu.m (the same condition as in the first
example), single crystal regions with no grain boundaries are
formed under the aperture 175a with higher probability. The width
of the single crystal region along the second direction is also
about 1 .mu.m.
THIRD EXAMPLE
[0110] Another experiment is carried out as a third example, in
which the shape of the beam shape is changed. The beam shape is
rectangle as shown in FIG. 18. The beam width is a width of the
beam cross section along the scanning direction (x direction in
FIG. 18). The beam length is a length of the beam cross section
along the second direction (y direction). The laser annealing
apparatus similar to that in the first example is used to
manufacture the semiconductor thin film. The experiment conditions
according to the present example are shown in table 2.
TABLE-US-00002 TABLE 2 EXAMPLE COMPARATIVE 3 EXAMPLE LASER
FLUENCE(mJ/cm.sup.2) 480 480 STEP WIDTH(.mu.m) 0.2 0.2 SCAN
DISTANCE(.mu.m) 100 100 APERTURE WIDTH (.mu.m) 21 9.9 APERTURE
LENGTH (.mu.m) 12 270
[0111] In the present example (indicated by Example 3 in table 2),
the laser fluence is 480 mJ/cm.sup.2, and hence the gradient of the
energy density is given by 460.8 mJ/cm.sup.2/.mu.m. The step width
is set to 0.2 .mu.m, and the scan of the beam is carried out for
100 .mu.m (scan distance). Also, the aperture width and the
aperture length are set to 21 .mu.m and 12 .mu.m, respectively. Due
to the lens 160 (see FIG. 12), the beam width and the beam length
on the substrate 165 are one third of the aperture width and the
aperture length, respectively. Thus, the beam width is 7 .mu.m and
the beam length is 4 .mu.m on the a-Si film 171 of the substrate
165.
[0112] For comparison, a comparative experiment is also carried
out. In the comparative experiment (indicated by Comparative
Example in table 2), the laser fluence is 420 mJ/cm.sup.2. The step
width and the scan distance are the same as those in the Example 3.
The aperture width and the aperture length are set to 9.9 .mu.m and
270 .mu.m, respectively. Thus, the beam length is 90 .mu.m on the
the a-Si film 171 of the substrate 165, which is much longer than
that in the Example 3.
[0113] FIGS. 19A and 19B show results of the SEM observations of
the crystallized films according to the comparative example and the
third example, respectively. Here, after Secco-etching treatment,
the crystallized films are observed. In the comparison example, as
shown in FIG. 19A, oblique grain boundaries are generated randomly
in the crystallized film.
[0114] On the contrary, in the third example where the aperture
length is set to 12 .mu.m and hence the beam length is set to 4
.mu.m, as shown in FIG. 19B, a single crystal without random grain
boundaries is formed in the center of the irradiation region
corresponding to the aperture section. The size of the single
crystal is 100 .mu.m in the first direction (scanning direction)
and 2.5 .mu.m in the second direction. Also, a micro
polycrystalline silicon region is formed on both sides of the
single crystal region. The size of the poly-Si region is slightly
less than 1 .mu.m in the second direction. Thus, in the third
example, the "beam length" (4 .mu.m) is less than two times a
"crystal growth width". Here, the "crystal growth width" is defined
as a length along the second direction of the region including the
single crystal region and the micro poly-Si region. As described
above, according to the manufacturing method in the present
invention, it is possible to form a single crystal region with no
grain boundaries which extends in the first direction. A thin film
transistor (TFT) can be made on the single crystal region
associated with the irradiated region. Thus, it is possible to
obtain the TFT with high uniformity and high mobility, because no
grain boundary exists in the channel region of the TFT. This can
solve the problems in the conventional techniques.
[0115] The reason why the single crystal region with no grain
boundary is formed as shown in FIG. 19 is considered to be as
follows. At first, immediately after the start of the beam
scanning, crystal grains with the width of 1 .mu.m or less grow
randomly in the first direction from the regions corresponding to
the beam edges. After that, during the irradiation process, the
temperature gradient along the second direction is generated around
the edges of the beam 150L in the second direction. Due to this
temperature gradient, one of the crystal grains grows not only in
the first direction but also preferentially in the second
direction. As a result, the random oblique grain boundaries do not
appear, and the single crystal region is formed whose size is about
2.5 .mu.m along the second direction.
FOURTH EXAMPLE
[0116] Another experiment is carried out as a fourth example in
order to investigate the beam length dependence of the crystal
growth, in which the beam length is changed as a variable. The
laser annealing apparatus and the mask 66 similar to those in the
first example are used to manufacture the semiconductor thin film.
The experiment conditions according to the present example are
shown in table 3.
TABLE-US-00003 TABLE 3 EXAM- EXAM- EXAM- PLE PLE PLE 4-1 4-2 4-3
LASER FLUENCE(mJ/cm.sup.2) 480 480 480 STEP WIDTH(.mu.m) 0.2 0.2
0.2 SCAN DISTANCE(.mu.m) 100 100 100 APERTURE WIDTH (.mu.m) 21 21
21 APERTURE INTERVAL (.mu.m) 3 3 3 APERTURE LENGTH (.mu.m) 12 18
30
[0117] In the present example, the laser fluence is 480
mJ/cm.sup.2, and hence the gradient of the energy density is given
by 460.8 mJ/cm.sup.2/.mu.m. The step width is set to 0.2 .mu.m, and
the scan distance is set to 100 .mu.m. Also, the aperture width and
the aperture interval are set to 21 .mu.m and 3 .mu.m,
respectively. In Example 4-1, the aperture length is set to 12
.mu.m, and hence the beam length is set to 4 .mu.m. In Example 4-2,
the aperture length is set to 18 .mu.m, and hence the beam length
is set to 6 .mu.m. In Example 4-3, the aperture length is set to 30
.mu.m, and hence the beam length is set to 10 .mu.m. Under these
conditions, the beam 150L is irradiated to the a-Si film 171.
[0118] FIGS. 20A to 20C show results of the SEM observations of the
crystallized films according to the Examples 4-1 to 4-3,
respectively. Here, after Secco-etching treatment, the crystallized
films are observed. In each figure, images at the locations where
the scanning starts (Starting Location) and ends (Ending Location)
are shown. Also, FIGS. 21A to 21C are schematic pictures showing
the mechanism of the crystal growth in the semiconductor thin film
according to the Examples 4-1 to 4-3, respectively.
[0119] As shown in FIG. 20A, when the aperture length is set to 12
.mu.m and the beam length is set to 4 .mu.m, single crystal regions
are formed at the centers of the irradiation regions. The reason
for this result will be as follows. As shown in FIG. 21A, the
temperature gradient along the second direction (direction y in
FIG. 20A) is generated, which suppresses the occurrence of oblique
grain boundaries as explained above. Thus, the single crystal
regions are formed at the centers of the irradiation regions.
[0120] As shown in FIG. 20B, when the aperture length is set to 18
.mu.m and the beam length is set to 6 .mu.m, one straight grain
boundary extending in the first direction (direction x) is formed
only at the center of the irradiation region. The reason for this
result will be as follows. As shown in FIG. 21B, the temperature
gradient along the second direction (direction y in FIG. 20B) is
generated, which suppresses the occurrence of oblique grain
boundaries. Also, the "beam length" is consistent with two times
the "crystal growth width" which is the length along the second
direction of the region including the single crystal region and the
small poly-Si region.
[0121] As shown in FIG. 20C, when the aperture length is set to 30
.mu.m and the beam length is set to 10 .mu.m, grain boundaries
extending in the first direction (direction x) are formed at the
center of the irradiation region. The reason for this result will
be as follows. As shown in FIG. 21C, the temperature gradient along
the second direction (direction y in FIG. 20C) around the beam edge
suppresses the occurrence of oblique grain boundaries to some
extent. However, since the "beam length" becomes longer than two
times the "crystal growth width", the effect of the temperature
gradient along the second direction becomes scarce. That's why
oblique grain boundaries are formed randomly around the center of
the irradiation region.
[0122] It is obvious from the results mentioned above that the
single crystal region can be formed in the a-Si film 171 with
controlling the positions of grain boundaries when the beam length
is set to less than 6 .mu.m, or preferably less than 4 .mu.m. Thus,
it is possible to manufacture the TFT which has high mobility and
high uniformity. It should be noted that the preferable beam length
depends on thickness and a deposition method of the a-Si film 171
as the precursor film, laser fluence of the beam 150L, the
resolution of the optical system and so on. Hence, the beam length
is not limited to 6 .mu.m or 4 .mu.m, and can be set to an
appropriate value according to manufacturing conditions. According
to the present invention for manufacturing the semiconductor thin
film, the "beam length" (4 .mu.m) is set to a value equal to or
less than two times the "crystal growth width". Here, the "crystal
growth width" indicates a length along the second direction of the
region including the single crystal region and the small poly-Si
region.
FIFTH EXAMPLE
[0123] Another experiment is carried out as a fifth example in
order to investigate the aperture interval (beam interval)
dependence of the crystal growth, in which the aperture interval is
changed as a variable. The laser annealing apparatus and the mask
66 similar to those in the first example are used to manufacture
the semiconductor thin film. The experiment conditions according to
the present example are shown in table 4.
TABLE-US-00004 TABLE 4 EXAMPLE 5-1 EXAMPLE 5-2 LASER
FLUENCE(mJ/cm.sup.2) 480 480 STEP WIDTH(.mu.m) 0.2 0.2 SCAN
DISTANCE(.mu.m) 100 100 APERTURE WIDTH (.mu.m) 21 21 APERTURE
LENGTH (.mu.m) 12 12 APERTURE INTERVAL (.mu.m) 30 1.5
[0124] In the present example, the laser fluence is 480
mJ/cm.sup.2, and hence the gradient of the energy density is given
by 460.8 mJ/cm.sup.2/.mu.m. The step width is set to 0.2 .mu.m, and
the scan distance is set to 100 .mu.m. Also, the aperture width and
the aperture length are set to 21 .mu.m and 12 .mu.m, respectively.
Thus, the beam length is set to 4 .mu.m. In Example 5-1, the
aperture interval is set to 30 .mu.m, and hence the beam interval
is set to 10 .mu.m. In Example 5-2, the aperture interval is set to
1.5 .mu.m, and hence the beam interval is set to 0.5 .mu.m. Under
these conditions, the beam 150L is irradiated to the a-Si film
171.
[0125] FIGS. 22A and 22B show results of the SEM observations of
the crystallized films according to the Examples 5-1 and 5-2,
respectively. Here, after Secco-etching treatment, the crystallized
films are observed. It is found by comparing images in FIGS. 22A
and 22B that there exists no conspicuous difference in size of the
single crystal region and crystalline status between the
crystallized films formed under both conditions. Thus, it is
concluded based on the results obtained from the present example
and the second example (see FIG. 17) that a certain length is
necessary for the aperture interval. However, the aperture interval
has no influence on the crystalline status of the crystallized film
formed on the a-Si film 171, if the aperture interval is longer
than the certain length.
SIXTH EXAMPLE
[0126] In the above-mentioned examples, the mask 66 shown in FIG.
15 is used for manufacturing the semiconductor thin film. In this
mask 66 shown in FIG. 15, the plurality of the rectangular
apertures 175a are arranged in the second direction.
[0127] Another experiment is carried out as a sixth example in
order to reduce the number of grain boundaries. Used in the present
example for manufacturing the semiconductor thin film is a laser
annealing apparatus which is similar to that in the first example
but has a different mask 66. FIG. 23 is a schematic picture showing
an aperture pattern of the mask 66 in the sixth example. The mask
66 according to the present example has a plurality of aperture
groups arranged in the first direction (scanning direction;
direction x). Each of the plurality of aperture-groups includes a
plurality of rectangular apertures arranged along the second
direction (direction y). For example, the mask 66 in FIG. 23 has
two aperture groups, a front aperture group and a rear aperture
group. The front aperture group includes three front apertures
arranged along the second direction. The rear aperture group
includes two rear apertures arranged along the second
direction.
[0128] The "aperture width", the "aperture length" and the
"aperture interval" can-be defined in a similar way to the
above-mentioned examples. A "front-rear interval" is defined by an
interval between adjacent aperture groups, i.e., between the front
aperture group and the rear aperture group. Also, as shown in FIG.
23, an "overlap width" is defined by a width along the second
direction for which apertures belonging to adjacent aperture groups
overlap.
[0129] The mask 66 mentioned above is used to manufacture the
semiconductor thin film. The experiment conditions according to the
present example are shown in table 5.
TABLE-US-00005 TABLE 5 EXAMPLE EXAMPLE 6-1 6-2 LASER
FLUENCE(mJ/cm.sup.2) 480 480 STEP WIDTH(.mu.m) 0.2 0.2 SCAN
DISTANCE(.mu.m) 100 100 APERTURE WIDTH (FRONT)(.mu.m) 9 9 APERTURE
LENGTH (FRONT)(.mu.m) 12 12 APERTURE INTERVAL (FRONT)(.mu.m) 3 6
APERTURE WIDTH (REAR)(.mu.m) 9 9 APERTURE LENGTH (REAR)(.mu.m) 12 9
APERTURE INTERVAL (REAR)(.mu.m) 3 9 FRONT-REAR INTERVAL(.mu.m) 15
15 OVERLAP WIDTH(.mu.m) 4.5 1.5
[0130] In the present example, the laser fluence is 480
mJ/cm.sup.2, and hence the gradient of the energy density is given
by 460.8 mJ/cm.sup.2/.mu.m. The step width is set to 0.2 .mu.m, and
the scan distance is set to 100 .mu.m. In Example 6-1, the aperture
width, the aperture length and the aperture interval for each
aperture are set to 9 .mu.m, 12 .mu.m and 3 .mu.m, respectively.
Also, the front-rear interval and the overlap width are set to 15
.mu.m and 4.5 .mu.m, respectively. In Example 6-2, the aperture
width, the aperture length and the aperture interval for each of
front apertures are set to 9 .mu.m, 12 .mu.m and 6 .mu.m,
respectively. The aperture width, the aperture length and the
aperture interval for each of front apertures are set to 9 .mu.m, 9
.mu.m and 9 .mu.m, respectively. Also, the front-rear interval and
the overlap width are set to 15 .mu.m and 1.5 .mu.m, respectively.
Under these conditions, the beam 150L is irradiated to the a-Si
film 171. Here, the beam 150L corresponding to the front aperture
is referred to as a front beam. Also, the beam 150L corresponding
to the rear aperture is referred to as a rear beam. Thus, a
plurality of the front beams belong to a "front beam group", and a
plurality of the rear beams belong to a rear "ream group".
[0131] FIGS. 24A and 24B show results of the SEM observations of
the crystallized films according to the Examples 6-1 and 6-2,
respectively. Here, after Secco-etching treatment, the crystallized
films are observed. In each figure, images at the locations where
the scanning starts (Starting Location) and ends (Ending Location)
are shown.
[0132] In the Example 6-1, the overlap width is 4.5 .mu.m which
means that an overlap width of the beam 150L on the substrate
(referred to as a "beam overlap width") is 1.5 .mu.m. In this case,
as shown in FIG. 24A, the small poly-Si regions formed by the front
beams and regions corresponding to the shading sections between the
front apertures become single crystal regions because the
irradiation of the rear beams is performed. Found in FIG. 24A are
straight grain boundaries, which are formed along the first
direction and are parallel with each other. Obviously, the
positions of these grain boundaries are controlled, and the number
of the grain boundaries are reduced. A TFT can be manufactured by
using the obtained semiconductor film. In the TFT, the channel
layer is formed such that electrons move along the first direction
(a direction along which the single crystal regions are extending).
Thus, it is possible to form the TFT having high mobility and high
uniformity.
[0133] In the Example 6-2, the overlap width is 1.5 .mu.m which
means that an overlap width of the beam 150L on the substrate (a
beam overlap width) is 0.5 .mu.m. In this case, as shown in FIG.
24B, the small poly-Si regions remain between the single crystal
regions extending in the first direction and formed by the front
beams and the rear beams.
[0134] It is found from the results mentioned above that the small
poly-Si region extending in the first direction can be removed by
setting the beam overlap width to a value equal to or larger than
the width along the second direction of the small poly-Si region.
In this example, the beam overlap width should be equal to or more
than 0.7 .mu.m. It should be noted that the number of aperture
groups is not limited to two, and can be more than three.
SEVENTH EXAMPLE
[0135] In the present example, the mobility of two TFTs will be
compared. A first TFT was made from a semiconductor thin film
formed by using a mask with comparatively large aperture 175a. A
second TFT was made from a semiconductor thin film formed by using
a mask with comparatively small aperture 175a.
[0136] In manufacturing the first TFT, the laser annealing
apparatus and the substrate 165 similar to those in the first
example were used. Here, the "aperture length" was set to 270
.mu.m, and hence the "beam length" was set to 90 .mu.m, which did
not meet the requirements of the present invention. Also, the
"aperture width" was set to 9.9 .mu.m, and hence the "beam width"
was set to 3.3 .mu.m. The scan distance was 300 .mu.m. As a result,
a poly-crystalline film with oblique grain boundaries was formed
under those conditions. Then, the first TFT (n-ch TFT) was
manufactured by using this poly-crystalline film as an active
layer. The channel length of the first TFT was 1.4 .mu.m and the
channel width of the first TFT was 1.4 .mu.m. The measured mobility
of the first TFT obtained was 360 cm.sup.2/Vs.
[0137] In manufacturing the second TFT, the laser annealing
apparatus and the substrate 165 similar to those in the first
example were used. Here, the "aperture length" was set to 12 .mu.m,
and hence the "beam length" was set to 4 .mu.m, which met the
requirements of the present invention. Also, the "aperture width"
was set to 21 .mu.m, and hence the "beam width" was set to 7 .mu.m.
The scan distance was 300 .mu.m. As a result, a semiconductor thin
film with a single crystal region was formed under those
conditions.
[0138] FIG. 25A is a plan view schematically showing the
semiconductor thin film with the single crystal region according to
the present example. As shown in FIG. 25A, the single crystal
region 180 was formed in an a-Si region 181. The length of the
single crystal region 180 along the scanning direction was 300
.mu.m corresponding to the scan distance. Also, the width of the
single crystal region 180 along the second direction was 2.5 .mu.m.
Then, an island 182 was formed in the single crystal region 180 as
shown in FIG. 25A. The length of the island 182 along the scanning
direction (indicated by a numeral 1 in FIG. 25A) was 10 .mu.m.
Also, the width of the island 182 along the second direction
(indicated by a numeral w in FIG. 25A) was 1.4 .mu.m.
[0139] Then, as shown in FIG. 25B, a source 187 and a drain 188
were formed on the island 182. Further, a gate electrode 185
crossing over the island 182 was formed through an insulating film
(not shown). Also, contacts 186 were formed so as to be
electrically connected to the source 187 and the drain 188,
respectively. Thus, a field effect transistor is formed. Here, a
channel region is formed such that carriers move along the first
direction. In this way, the second TFT (n-ch TFT) was formed. The
channel length of the second TFT was 1.4 .mu.m and the channel
width of the second TFT was 1.4 .mu.m. The measured mobility of the
second TFT obtained was 520 cm.sup.2/Vs.
[0140] It is obvious from the above comparison that the mobility of
the second TFT is higher than that of the first TFT. That is to
say, the second TFT manufactured by the method according to the
present invention has higher mobility than the first TFT. Thus, a
TFT with high performance can be provided by using the
manufacturing method according to the present invention.
[0141] As described above in detail, according to the method and
the apparatus for manufacturing the semiconductor thin film in the
present invention, the following effects can be attained. That is,
the width of the crystal along the second direction can be made
larger than that of a conventional semiconductor thin film, because
of the temperature gradient generated along the second direction.
Moreover, the positions of the grain boundaries can be controlled,
and also the number of the grain boundaries can be reduced. Also,
the positions and the number of the crystals can be controlled.
[0142] Furthermore, according to the present invention, a thin film
transistor (TFT) is manufactured by using the single crystal region
generated by the above apparatus and the above method. Thus, the
mobility of the manufactured TFT is improved, because the number of
the grain boundaries is reduced. This implies that a TFT with high
mobility can be obtained according to the present invention.
Moreover, since the number and the directions of the grain
boundaries are controlled and hardly vary, the variation in the
mobility of the TFT can be suppressed.
[0143] It will be obvious to one skilled in the art that the
present invention may be practiced in other embodiments that depart
from the above-described specific details. The scope of the present
invention, therefore, should be determined by the following
claims.
* * * * *