U.S. patent application number 17/133708 was filed with the patent office on 2021-04-22 for semiconductor device and semiconductor device production system.
The applicant listed for this patent is Semiconductor Energy Laboratory Co., Ltd.. Invention is credited to Mai AKIBA, Tatsuya ARAO, Koji DAIRIKI, Masahiko HAYAKAWA, Atsuo ISOBE, Chiho KOKUBO, Hidekazu MIYAIRI, Hiroshi SHIBATA, Akihisa SHIMOMURA, Koichiro TANAKA, Shunpei YAMAZAKI.
Application Number | 20210118916 17/133708 |
Document ID | / |
Family ID | 1000005305622 |
Filed Date | 2021-04-22 |
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United States Patent
Application |
20210118916 |
Kind Code |
A1 |
ISOBE; Atsuo ; et
al. |
April 22, 2021 |
SEMICONDUCTOR DEVICE AND SEMICONDUCTOR DEVICE PRODUCTION SYSTEM
Abstract
A semiconductor device production system using a laser
crystallization method is provided which can avoid forming grain
boundaries in a channel formulation region of a TFT, thereby
preventing grain boundaries rom lowering the mobility of the TFT
greatly, from lowering ON current, and from increasing OFF current.
Rectangular or stripe pattern depression and projection portions
are formed on an insulating film. A semiconductor film is formed on
the insulating film. The semiconductor film is irradiated with
continuous wave laser light by running the laser light along the
stripe pattern depression and projection portions of the insulating
film or along the major or minor axis direction of the rectangle.
Although continuous wave laser light is most preferred among laser
light, it is also possible to use pulse oscillation laser light in
irradiating the semiconductor film.
Inventors: |
ISOBE; Atsuo; (Atsugi,
JP) ; YAMAZAKI; Shunpei; (Setagaya, JP) ;
DAIRIKI; Koji; (Tochigi, JP) ; SHIBATA; Hiroshi;
(Higashine, JP) ; KOKUBO; Chiho; (Tochigi, JP)
; ARAO; Tatsuya; (Atsugi, JP) ; HAYAKAWA;
Masahiko; (Atsugi, JP) ; MIYAIRI; Hidekazu;
(Atsugi, JP) ; SHIMOMURA; Akihisa; (Atsugi,
JP) ; TANAKA; Koichiro; (Atsugi, JP) ; AKIBA;
Mai; (Isehara, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Semiconductor Energy Laboratory Co., Ltd. |
Atsugi-shi |
|
JP |
|
|
Family ID: |
1000005305622 |
Appl. No.: |
17/133708 |
Filed: |
December 24, 2020 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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16725210 |
Dec 23, 2019 |
10879272 |
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17133708 |
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16515117 |
Jul 18, 2019 |
10515983 |
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16725210 |
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15897318 |
Feb 15, 2018 |
10361222 |
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16515117 |
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14924100 |
Oct 27, 2015 |
9899419 |
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15897318 |
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12533401 |
Jul 31, 2009 |
9178069 |
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14924100 |
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11600833 |
Nov 17, 2006 |
7582162 |
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12533401 |
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11013539 |
Dec 17, 2004 |
7148507 |
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11600833 |
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10338043 |
Jan 8, 2003 |
6841797 |
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11013539 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01L 27/1255 20130101;
H01L 21/02691 20130101; H01L 29/66757 20130101; H01L 29/78696
20130101; Y10T 117/1004 20150115; Y10T 117/10 20150115; H01L
21/32139 20130101; H01L 21/02686 20130101; B23K 26/0738 20130101;
Y10T 117/1008 20150115; H01L 21/02354 20130101; H01L 27/1274
20130101; H01L 29/78603 20130101; H01L 27/1281 20130101; H01L
27/124 20130101; H01L 21/02356 20130101; H01L 29/78675 20130101;
H01L 21/02675 20130101; Y10S 118/90 20130101; H01L 21/84 20130101;
H01L 27/1218 20130101; H01L 21/2026 20130101; H01L 27/1296
20130101; H01L 27/1214 20130101; H01L 21/02683 20130101; H01L
27/1222 20130101 |
International
Class: |
H01L 27/12 20060101
H01L027/12; H01L 21/02 20060101 H01L021/02; H01L 21/20 20060101
H01L021/20; H01L 21/3213 20060101 H01L021/3213; H01L 21/84 20060101
H01L021/84; H01L 29/66 20060101 H01L029/66; H01L 29/786 20060101
H01L029/786; B23K 26/073 20060101 B23K026/073 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 17, 2002 |
JP |
2002-009266 |
Claims
1. (canceled)
2. An electronic apparatus comprising: a display unit comprising a
semiconductor device comprising: a substrate; a pixel portion over
the substrate, the pixel portion comprising: a surface comprising a
first projection portion and a first depression portion; a
polycrystalline semiconductor film over the surface; a gate
insulating film over the polycrystalline semiconductor film; a gate
electrode over the gate insulating film; an insulating film over
the gate electrode; and an electrode connected to a top surface of
the polycrystalline semiconductor film through an opening in the
insulating film and the gate insulating film; and a driving circuit
over the substrate, wherein the gate electrode and the first
projection portion overlap each other, and wherein the electrode is
in contact with a top surface of the insulating film.
3. The electronic apparatus according to claim 2, wherein a surface
of the polycrystalline semiconductor film comprises a second
projection and a second depression.
4. The electronic apparatus according to claims 2, wherein the
polycrystalline semiconductor film is in contact with an insulating
material at the first projection portion.
5. The electronic apparatus according to claim 2, wherein the
electrode overlaps with the first projection portion and the first
depression portion.
6. The electronic apparatus according to claim 2, wherein a width
of the first projection portion is 300 to 3000 nm.
7. The electronic apparatus according to claim 2, further
comprising at least one of a speaker, an audio input unit, a
battery, and an operating key.
8. An electronic apparatus comprising: a display unit comprising a
semiconductor device comprising: a substrate; a pixel portion over
the substrate, the pixel portion comprising: a surface comprising a
first projection portion and a first depression portion; a
polycrystalline semiconductor film over the surface, the
polycrystalline semiconductor film comprising a channel region, a
first impurity region, and a second impurity region; a gate
insulating film over the polycrystalline semiconductor film; a gate
electrode over the gate insulating film; an insulating film over
the gate electrode; and an electrode connected to a top surface of
the first impurity region through an opening in the insulating film
and the gate insulating film; and a driving circuit over the
substrate, wherein the channel region is between the first impurity
region and the second impurity region, wherein the gate electrode
and the channel region overlap each other, wherein the channel
region and the first projection portion overlap each other, and
wherein the electrode is in contact with a top surface of the
insulating film.
9. The electronic apparatus according to claim 8, wherein a surface
of the polycrystalline semiconductor film comprises a second
projection and a second depression.
10. The electronic apparatus according to claim 8, wherein the
polycrystalline semiconductor film is in contact with an insulating
material at the first projection portion.
11. The electronic apparatus according to claim 8, wherein the
electrode overlaps with the first projection portion and the first
depression portion.
12. The electronic apparatus according to claim 8, wherein a width
of the first projection portion is 300 to 3000 nm.
13. The electronic apparatus according to claim 8, further
comprising at least one of a speaker, an audio input unit, a
battery, and an operating key.
14. An electronic apparatus comprising: a display unit comprising a
semiconductor device comprising: a plastic substrate; a pixel
portion over the plastic substrate, the pixel portion comprising: a
surface comprising a first projection portion and a first
depression portion; a polycrystalline semiconductor film over the
surface; a gate insulating film over the polycrystalline
semiconductor film; a gate electrode over the gate insulating film;
an insulating film over the gate electrode; and an electrode
connected to a top surface of the polycrystalline semiconductor
film through an opening in the insulating film and the gate
insulating film; and a driving circuit over the plastic substrate,
wherein the gate electrode and the first projection portion overlap
each other, and wherein the electrode is in contact with a top
surface of the insulating film.
15. The electronic apparatus according to claim 14, wherein a
surface of the polycrystalline semiconductor film comprises a
second projection and a second depression.
16. The electronic apparatus according to claim 14, wherein the
polycrystalline semiconductor film is in contact with an insulating
material at the first projection portion.
17. The electronic apparatus according to claim 14, wherein the
electrode overlaps with the first projection portion and the first
depression portion.
18. The electronic apparatus according to claim 14, wherein a width
of the first projection portion is 300 to 3000 nm.
19. The electronic apparatus according to claim 14, further
comprising at least one of a speaker, an audio input unit, a
battery, and an operating key.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
[0001] The present invention relates to a semiconductor device
constructed by a semiconductor film that has a crystal structure,
and more specifically, to a semiconductor device using a thin film
transistor whose active layer is formed of a crystalline
semiconductor film obtained through crystal growth on an insulating
surface. The present invention also relates to a semiconductor
device product ion system using laser light.
2. Description of the Related Art
[0002] In recent years, techniques for forming TFTs on a substrate
have made great advancements and applications of TFTs to active
matrix type semiconductor display devices are being developed. In
particular, TFTs formed of polycrystalline semiconductor films
(hereinafter referred to as polysilicon TFT) have higher field
effect mobility (also referred to as mobility) than conventional
TFTs that use amorphous semiconductor films and accordingly can
operate at high speed. Therefore pixels can be controlled by a
driving circuit formed on the same substrate on which the pixels
are formed, instead of a driving circuit external to the substrate
as with the conventional technique.
[0003] Incidentally, for substrates used in semiconductor devices,
a glass substrate is deemed more promising than a single crystal
silicon substrate cost-wise. Glass substrates have poor
heat-resistance and are easily deformed by heat. Therefore, when
forming a polysilicon TFT on a glass substrate, using laser
annealing to crystallize a semiconductor film in order to avoid
thermal deformation of the glass substrate is extremely
effective.
[0004] Laser annealing has characteristics such as remarkable
reduction of processing time compared to an annealing method
utilizing radiant. heating or thermal conductive heating, and a
semiconductor or a semiconductor film is selectively and locally
heated so that a substrate is scarcely thermally damaged.
[0005] Note that the term "laser annealing" herein indicates a
technique or recrystallizing a damaged layer formed on a
semiconductor substrate or in a semiconductor film and a technique
for crystallizing a semiconductor film formed on a substrate. The
term "laser annealing" also includes a technique that is applied to
leveling or improvement of a surface quality of the semiconductor
substrate or the semiconductor film. Applicable laser oscillation
devices are gas laser oscillation devices represented by an excimer
laser, and solid laser oscillation devices represented by a YAG
laser. Such laser oscillation devices are known to heat a surface
layer of a semiconductor by laser beam irradiation for an extremely
short period of time, i.e., about several tens of nanoseconds to
several tens of microseconds so as to crystallize the surface
layer.
[0006] Lasers are roughly divided into two types, pulse oscillation
and continuous wave, by their oscillation methods. Pulse
oscillation lasers are relatively high in output energy and
therefore the size of laser beam can be set to several cm.sup.2 to
increase the mass-productivity. In particular, if the shape of
laser beam is processed by an optical system into a linear shape 10
cm or more in length, a substrate can be irradiated with the laser
light efficiently to increase the mass-productivity even more.
Accordingly, using pulse oscillation lasers to crystallize
semiconductor films have been becoming mainstream.
[0007] In recent years, however, it has been found that the grain
size of crystals formed in a semiconductor film is larger when a
continuous wave laser is used to crystallize a semiconductor film
than when a pulse oscillation laser is used. With crystals of
larger grain size in a semiconductor film, the mobility of TFTs
formed from this semiconductor film is increased while fluctuation
in characteristics between the TFTs due to grain boundaries is
reduced. As a result, continuous wave lasers are now suddenly
attracting attention.
[0008] Trying to form a single crystal semiconductor film on an
insulating surface is not new and a technique called graphoepitaxy
has been devised as a more positive attempt. Graphoepitaxy is a
technique in which a level difference is formed on a surface of a
quartz substrate, an amorphous semiconductor film or a
polycrystalline semiconductor film is formed on the substrate, and
the film is heated by a laser beam or a heater so that an epitaxial
growth layer is formed with the level difference on the quartz
substrate as the nucleus. This technique is disclosed in, for
example, Non-patent Literature 1.
Non-Patent Literature 1
[0009] J. Vac. Sci. Technol., "Grapho-epitaxy of silicon on fused
silica using surface micropatterns and laser crystallization",
16(6), 1979, pp. 1640-1643.
[0010] Another semiconductor film crystallizing technique called
graphoepitaxy is disclosed in, for example, Non-patent Literature
2. The literature is about inducing epitaxial growth of a
semiconductor film by artificially-created surface relief grating
on an amorphous substrate. According to the graphoepitaxy technique
disclosed in Non-patent Literature 2, a level difference is formed
on a surface of an insulating film, a semiconductor film is formed
on the insulating film, and the semiconductor film is subjected to
heating, laser light irradiation, or the like to start epitaxial
growth of crystals of the semiconductor film.
Non-patent Literature 2
[0011] M. W. Geis, et al., "CRYSTALLINE SILICON ON INSULATORS BY
GRAPHOEPITAXY", Technical Digest of International Electron Devices
Meeting, 1979, p. 210.
[0012] Crystalline semiconductor films formed using laser annealing
methods, which are roughly classified into pulse oscillation and
continuous wave, are masses of crystal grains in general. These
crystal grains have varying sizes and are positioned at random, and
it is difficult to specify the position and size of crystal grains
in forming a crystalline semiconductor film. Therefore an active
layer formed by patterning the crystalline semiconductor film into
islands generally have interface between crystal grains (grain
boundaries).
[0013] Unlike the inside of a crystal grain, a grain boundary has
an infinite number of re-combination centers and trap centers due
to an amorphous structure and crystal defects. When carriers are
trapped in these trap centers, the potential of the grain boundary
rises to block carriers and lower the current carrying
characteristic of carriers. Therefore, grain boundaries in an
active layer, in particular, in a channel formation region of a
TFT, seriously affect TFT characteristics by lowering the mobility
of the TFT greatly, by lowering ON current, and by increasing OFF
current since a current flows in grain boundaries. Grain boundaries
also cause fluctuation in characteristic among TFTs that are
intended to have the same characteristic because the characteristic
of a TFT having grain boundaries in its active layer is different
from that of a TFT whose active layer has no grain boundaries.
[0014] Crystal grains obtained by irradiating a semiconductor film
with laser light have varying sizes and are positioned randomly
because of the following reason. It takes time for a liquefied
semiconductor film that has been thoroughly melted by laser light
irradiation to create a solid nucleus. As time passes, an infinite
number of crystal nuclei are generated in the thoroughly melted
region and crystals grow from the crystal nuclei. Since positions
of the crystal nuclei to be generated are at random, they are
distributed unevenly. Crystal growth is stopped as crystal grains
collide against each other. Accordingly, the crystal grains
obtained have varying sizes and are positioned at random.
[0015] Ideally, a channel formation region, which has a great
influence over TFT characteristics, is formed from a single crystal
grain removing adverse effect of grain boundaries. However, prior
art is mostly unsuccessful in forming a crystalline silicon film
with no grain boundaries by laser annealing. Therefore no TFT whose
active layer is formed of a crystalline silicon film crystallized
by laser annealing has succeeded in obtaining characteristics that
rival the characteristics of a MOS transistor manufactured on a
single crystal silicon substrate.
SUMMARY OF THE INVENTION
[0016] The present invention has been made in view of the
above-mentioned problems, and an object of the present invention is
therefore to provide a semiconductor device production system using
a laser crystallization method which can avoid forming grain
boundaries in a channel formation region of a TFT, thereby
preventing grain boundaries from lowering the mobility of the TFT
greatly, from lowering ON current, and from increasing OFF
current.
[0017] The inventors of the present invention have found that, when
a semiconductor film is formed on an insulating film having
depression and projection portions and is irradiated with laser
light, crystal nuclei are generated in the vicinity of edges of the
depression bottom or projection top of the insulating film and
crystal growth starts from the crystal nuclei and proceeds in the
direction parallel to the insulating film. A depression portion
refers to a dented region where no projection portion is
formed.
[0018] This mechanism is explained with reference to FIGS. 1A and
1B. FIG. 1A shows a semiconductor film 11 formed on an insulating
film 10 that has a projection portion 10a. The semiconductor film
11 is melted by laser light irradiation and the heat in the
semiconductor film 11 is released to the insulating film 10. The
heat radiation is efficient where a large area of the semiconductor
film is in contact with the insulating film. For instance, in FIG.
1A, the heat is released to the insulating film more efficiently
and crystal nuclei are formed faster in portions 14 and 15 where
the semiconductor film 11 and the insulating film 10 meet on two
planes that intersect with each other than in portions 12 and 13
where the semiconductor film 11 and the insulating film 10 meet on
one plane. Also, the heat radiation is efficient in a portion of
the insulating film that has a large heat capacitance. For example,
the portion 14, which is in the vicinity of the edge of the
depression portion, is larger in volume of the insulating film
within a certain range and accordingly has larger heat capacitance
than the portion 15, which is in the vicinity of the edge of the
projection portion. Therefore released heat does not stay long in
the portion 14 and heat radiation is more efficient in the portion
14 than in the portion 15. As a result, crystal nuclei are formed
faster in the portion 14 which is near the edge of the depression
portion than in the portion 15 which is near the edge of the
projection portion.
[0019] As time passes, crystal growth starts from the crystal
nuclei formed in the portion 14 near the edge of the depression
portion and proceeds in the direction parallel to the insulating
film. Crystal growth directions in a semiconductor film will be
described with reference to FIG. 1B. FIG. 1B shows a semiconductor
film 11 formed on an insulating film 10 that has two projection
portions 10a and 10b. In the semiconductor film 11, crystal growth
starts from a portion 14 near an edge of a depression portion and
proceeds in the upper and lateral directions indicated by arrows.
The crystal growth started from the portion 14 toward lateral
direction near an edge of a depression portion comes to an end as
it meets crystal growth started from an edge of the adjacent
depression portion 14 halfway, thereby forming a grain boundary
16.
[0020] As described above, crystallization by laser light
irradiation of an insulating film that has a projection portion
makes it possible to control the position at which a grain boundary
is formed to a certain degree. This gives foresight of where grain
boundaries are formed in the semiconductor film at the stage of
designing the shape of the insulating film. In other words, the
present invention can choose where grain boundaries are to be
formed and this makes it possible to place a channel formation
region, preferably an active layer, such that the active layer or
channel formation region includes as few grain boundaries as
possible.
[0021] Specifically, the insulating film is given rectangular or
stripe pattern depression and projection portions. Then a
semiconductor film is formed on the insulating film and is
irradiated with continuous wave laser light along the stripe
pattern depression and projection portions of the insulating film
or along the major or minor axis direction of the rectangular.
Although continuous wave laser light is most preferred among laser
light, it is also possible to use pulse oscillation laser light in
irradiating the semiconductor film. The projection portion in
section in the direction perpendicular to the laser light scanning
direction may be rectangular, triangular, or trapezoidal.
[0022] A grain boundary is formed in the semiconductor film about
the midpoint between edges of adjacent projection portions and
another grain boundary is formed in the semiconductor film about
the midpoint between edges of a depression portion. These grain
boundaries are formed by collision between growing crystals.
Accordingly, the present invention uses as a channel formation
region a portion 17 between one edge of a depression portion and
the midpoint between the one edge and the other edge of the
depression portion, where fewer grain boundaries are formed to give
the portion excellent crystallinity. A portion 18, which is between
one edge of a projection portion and the midpoint between the one
edge and the other edge of the projection portion, has fewer grain
boundaries to give the portion excellent crystallinity and can also
be used as an active layer or a channel formation region. It is not
that the excellent crystallinity portions 17 and 18 have no grain
boundaries. However, the portions 17 and 18 have better
crystallinity even if they have grain boundaries because their
crystal grains are large in size.
[0023] In the present invention, a semiconductor film crystallized
by laser light is patterned to remove a portion of the film around
the midpoint between edges of a depression portion or projection
portion. The remaining portion between one edge of a depression
portion or projection portion and the midpoint between the one edge
and the other edge of the depression portion or projection portion,
which has fewer grain boundaries and therefore has excellent
crystallinity, is used as an active layer of a TFT. This makes it
possible to avoid forming a grain boundary in a channel formation
region of a TFT, thereby preventing grain boundaries from lowering
the mobility of the TFT greatly, from lowering ON current, and from
increasing OFF current . How far from an edge of a depression
portion or projection portion is to be removed by patterning can be
decided at designer's discretion
[0024] In general, laser beam edges and the vicinity thereof are
lower in energy density than the center of the laser beam and a
semiconductor film irradiated with laser beam edges often has poor
crystallinity. It is therefore desirable at the time of laser light
scanning to prevent edges of laser light track from overlapping a
portion that later serves as a channel formation region of a
TFT.
[0025] To achieve this, a semiconductor device production system of
the present invention first stores data of the shape of the
insulating film or semiconductor film viewed from above the
substrate (pattern information) as the data is obtained in the
design stage. From the pattern information and the width of a laser
beam in the direction perpendicular to the laser light scanning
direction, the laser light scanning path is determined so that
edges of the laser light track is prevented from overlapping at
least a portion that serves as a channel formation region of a TFT.
Then the substrate is positioned with a marker as the reference and
the semiconductor film on the substrate is irradiated with laser
light by running it along the scanning path determined.
[0026] The above-mentioned structure makes it possible to at least
run laser light over only portions that need laser light
irradiation, instead of irradiating the entire substrate with laser
light. Therefore time for laser irradiation of portions that do not
need laser light irradiation can be saved to shorten the whole
laser irradiation time and improve the substrate processing speed.
The above-mentioned structure also makes it possible to avoid
damage to a substrate which is caused by irradiating a portion that
does not need laser irradiation with laser light.
[0027] The marker may be formed by directly etching the substrate
with laser light or the like, or may be formed in a part of the
insulating film having depression and projection portions at the
same time the insulating film is formed. Another method of
positioning the substrate is to use an image pickup device such as
a CCD to read the shape of the insulating film or semiconductor
film actually formed, then store it as data in the first storing
means, store in the second storing means the insulating film or
semiconductor film pattern information obtained in the design
stage, and check the data stored in the first storing means against
the pattern information stored in the second storing means.
[0028] By forming a marker in a part of the insulating film or by
using the shape of the insulating film as a marker, one fewer
marker mask is needed and the marker can be formed and positioned
more accurately than when forming it on a substrate by laser light.
As a result, the positioning accuracy is improved.
[0029] In general, the energy density of laser light is not
thoroughly uniform and is varied between different points in a
laser beam. The present invention requires to irradiate at least an
area that serves as a channel formation region, preferably the
entire flat face of a depression portion or the entire flat face of
a projection portion, with laser light having a constant energy
density. Therefore, it is necessary in the present invention to use
a laser beam having such an energy density distribution that makes
a region of the laser beam that has uniform energy density
completely overlap at least an area that serves as a channel
formation region, preferably the entire flat face of a depression
portion or the entire flat face of a projection portion, during
laser light scanning. A shape desirable for a laser beam to meet
the above-mentioned energy density condition would be rectangular,
linear, etc.
[0030] A slit may be used to cut off a portion of a laser beam that
is low in energy density. The use of a slit makes uniform
crystallization possible by irradiating the entire flat face of a
depression portion or the entire flat face of a projection portion
with laser light that has relatively uniform energy density. In
addition, the use of a slit allows a laser beam to partially change
its width in accordance with the insulating film or semiconductor
film pattern information. This reduces limitations in layout of a
channel formation region or active layer of a TFT. The laser beam
width here means the length of a laser beam in the direction
perpendicular to the scanning direction.
[0031] One laser beam obtained by synthesizing laser beams that are
emitted from plural laser oscillators may be used in laser
crystallization. This structure allows low energy density portions
of laser beams to supplement one another.
[0032] After the semiconductor film is formed, the semiconductor
film may be crystallized by laser light irradiation without
exposing the film to the air (for example, noble gas, nitrogen,
oxygen, or other specific gas atmosphere or a reduced pressure
atmosphere is employed). This structure can prevent molecule-level
contaminants in a clean room, such as boron contained in a filter
for enhancing the cleanliness of the air, from mixing in the
semiconductor film during laser light crystallization.
[0033] A conventional semiconductor film crystallization technique
called graphoepitaxy is to induce epitaxial growth of a
semiconductor film by artificially-created surface relief grating
on an amorphous substrate. Graphoepitaxy-relating techniques are
described in Non-patent Literature 2 given in the above and others.
The paper discloses that a graphoepitaxy technique is for forming a
level difference on a surface of an insulating film, forming a
semiconductor film on the insulating film, and subjecting the
semiconductor film to treatment such as heating or laser light
irradiation for epitaxial growth of crystals in the semiconductor
film. As the temperature required for epitaxial growth is
700.degree. C. or higher a glass substrate can not be used due to
poor heat resisting properties. Even when epitaxial growth is
attempted using a quartz substrate, a grain boundary is formed in
the semiconductor film near center of a depression portion or
projection portion of the insulating film. In the present
invention, the crystallinity of an area to form an island is
improved by placing a mask for the island, so that layout of the
island dictates the shape of a depression portion or projection
portion of the insulating film and the position of an edge of a
depression portion or projection portion. Specifically, the shape,
size, and the like of a depression portion or projection portion
are determined such that an island does not overlap an edge of the
depression portion or projection portion or the midpoint between
the edges of the depression portion or projection portion. Using
the insulating film designed in accordance with the layout of the
island, the position of a grain boundary is selectively set. A
portion of the semiconductor film where a grain boundary is
selectively formed is removed by patterning and the remaining
portion, which has relatively good crystallinity, is used as the
channel formation region. The technique disclosed in the present
invention is similar to conventional graphoepitaxy in that a
semiconductor film is formed on an insulating film having a level
difference and the level difference is used to crystallize the
semiconductor film. However, conventional graphoepitaxy does not
include using the level difference to control the position of a
grain boundary and reduce grain boundaries in number in an island,
and therefore is not identical with the present invention despite
the resemblance.
BRIEF DESCRIPTION OF THE DRAWINGS
[0034] In the accompanying drawings:
[0035] FIGS. 1A and 1B are diagrams showing crystal growth
directions in a semiconductor film when irradiated with laser
light;
[0036] FIGS. 2A to 2C are diagrams showing a semiconductor film
being irradiated with laser light;
[0037] FIGS. 3A to 3C are diagrams of islands formed by patterning
a crystallized semiconductor film;
[0038] FIGS. 4A and 4B are diagrams showing the structure of a TFT
that is formed from the islands shown in FIGS. 3A to 3C;
[0039] FIG. 5 is a flowchart of a production system of the present
invention;
[0040] FIG. 6 is a diagram of laser irradiation apparatus;
[0041] FIG. 7 is a diagram of laser irradiation apparatus;
[0042] FIGS. 8A to 8D are diagrams showing a method of forming an
insulating film that has depression and projection portions;
[0043] FIGS. 9A to 9C are diagrams showing a method of forming an
insulating film that has depression and projection portions;
[0044] FIGS. 10A to 10C are diagrams of TFTs formed from islands
that are separated from each other;
[0045] FIGS. 11A and 11B are diagrams showing shapes of an
insulating film that has depression and projection portions;
[0046] FIGS. 12A to 12D are a top view and sectional views of a TFT
that is formed from the insulating film shown in FIG. 11B;
[0047] FIGS. 13A to 13D are diagrams showing a method of
manufacturing a semiconductor device using the present
invention;
[0048] FIG. 14 is a diagram showing a method of manufacturing a
semiconductor device using the present invention;
[0049] FIGS. 15A to 15E are diagrams showing a method of
crystallizing a semiconductor film using a catalytic metal;
[0050] FIGS. 16A and 16B are diagrams showing the energy density
distribution of a laser beam;
[0051] FIGS. 17A and 17B are diagrams showing the energy density
distribution of a laser beam;
[0052] FIG. 18 is a diagram showing the energy density distribution
of a laser beam;
[0053] FIG. 19 is a diagram of an optical system;
[0054] FIGS. 20A to 20C are diagrams of optical systems;
[0055] FIG. 21 is a diagram showing the energy density distribution
in the central axis direction of laser beams overlapped;
[0056] FIG. 22 is a diagram showing the energy difference in
relation to the distance between the centers of laser beams;
[0057] FIG. 23 is a diagram showing the output energy distribution
in the central axis direction of a laser beam;
[0058] FIG. 24 is a diagram showing the structure of a light
emitting device that is an example of a semiconductor device of the
present invention;
[0059] FIG. 25 is a diagram showing a pixel structure in a light
emitting device that is an example of a semiconductor device of the
present invention;
[0060] FIGS. 26A to 26H are diagrams of electronic equipment using
a semiconductor device of the present invention;
[0061] FIG. 27 is a sectional view of TFTs forming a stack
structure;
[0062] FIG. 28 is a diagram showing the energy density distribution
of a laser beam that is obtained by synthesizing two laser
beams;
[0063] FIG. 29 is a diagram showing the energy density distribution
of a laser beam that is obtained by synthesizing four laser
beams;
[0064] FIG. 30 is a diagram showing the energy density distribution
of a laser beam that is obtained by synthesizing four laser
beams;
[0065] FIG. 31 shows the concentration profile of oxygen in a
silicon film crystallized by laser light;
[0066] FIG. 32 shows the concentration profile of nitrogen in a
silicon film crystallized by laser light;
[0067] FIG. 33 shows the concentration profile of carbon in a
silicon film crystallized by laser light;
[0068] FIG. 34 shows the concentration profile of boron in a
silicon film crystallized by laser light; and
[0069] FIGS. 35A to 35C are sectional views of an insulating film
that has depression and projection portions and a semiconductor
film that is formed on the insulating film.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0070] A laser light irradiation method used in the present
invention will be described with reference to FIGS. 2A to 2C.
[0071] First, an insulating film 101 is formed on a substrate 100
as shown in FIG. 2A. The insulating film 101 has projection
portions 101a forming a stripe pattern. How the insulating film is
given depression and projection will be described later in detail.
The insulating film 101 is a silicon oxide film, a silicon
oxynitride film, a silicon nitride film, or the like. Other
insulating films can be used as long as they are insulating films
which can prevent an alkaline metal or other impurities from
entering a semiconductor film subsequently formed, which have
enough heat resistance to withstand the temperature in subsequent
treatment, and which can have depression and projection. The
insulating film 101 may also be a laminate of two or more
films.
[0072] A marker may be formed from a part of the insulating film
101 at the same time the insulating film 101 is formed.
[0073] The material of the substrate 100 has to have enough heat
resistance to withstand the temperature in subsequent treatment.
For example, a quartz substrate, silicon substrate, glass
substrate, metal substrate, or stainless steel substrate with an
insulating film formed on its surface is used as the substrate 100.
The glass substrate is formed of barium borosilicate glass,
alumino-borosilicate glass, or the like. A plastic substrate may
also be used if it has enough heat resistance to withstand the
temperature in subsequent treatment.
[0074] Next, a semiconductor film 102 is formed to cover the
insulating film 101. The semiconductor film 102 can be formed by a
known method (sputtering, LPCVD, plasma CVD, or the like). The
semiconductor film may be an amorphous semiconductor film, a
microcrystalline semiconductor film, or a crystalline semiconductor
film. The semiconductor film may also be formed of silicon or
silicon germanium.
[0075] The semiconductor film 102 also has depression and
projection along the depression and projection of the insulating
film 101. The size of the projection portions 101a of the
insulating film 101 can be set at designer's discretion but the
projection portions have to be thick enough to avoid discontinuity
in the subsequently-formed semiconductor film near edges of the
projection portions. If an active layer is placed in a depression
portion, restrictions in layout of an active layer can be reduced
by setting the depression portion wider than the projection
portion. If an active layer is placed in a projection portion,
restrictions in layout of an active layer can be reduced by setting
the projection portion wider than the depression portion. In this
embodiment, the flat portion of a depression portion is twice wider
than the flat portion of a projection portion or more and the width
of a projection portion is set to 300 to 3000 nm. The height of a
projection portion is set to 30 to 300 nm.
[0076] Next, the semiconductor film 102 is irradiated with laser
light as shown in FIG. 2A to form a semiconductor film (post-LC)
103 with improved crystallinity. The laser light energy density is
low in the vicinity of the edges of a laser beam 104. Therefore a
film irradiated with the laser beam edges has small crystal grains
and a ridge is formed protruding along a grain boundary. Therefore,
the edges of the track of the laser beam 104 is prevented from
overlapping a portion to serve as a channel formation region.
[0077] The laser light scanning direction is set parallel to the
direction of the projection portions 101a as indicated by the
arrow.
[0078] The present invention can employ known lasers. Continuous
wave laser light is desirable but it is considered that pulse
oscillation laser light can also provide the effect of the present
invention to a certain degree. A gas laser or solid-state laser can
be employed. Examples of the gas laser include an excimer laser, an
Ar laser, and a Kr laser. Examples of the solid-state laser include
a YAG laser, a YVO.sub.4 laser, a YLF laser, a YAlO.sub.3 laser, a
glass laser, a ruby laser, an alexandrite laser, a Ti:sapphire
laser, and a Y.sub.2O.sub.3 laser. The solid-state laser employed
is a laser that uses crystals of YAG, YVO.sub.4, YLF, YAlO.sub.3 or
the like doped with Cr, Nd, Er, Ho, Ce, Co, Ti, Yb, or Tm. The
fundamental wave of the laser is varied depending on the material
used for doping, but laser light obtained has a fundamental wave of
about 1 .mu.m. A non-linear optical element is used to obtain
harmonic of the fundamental wave.
[0079] Ultraviolet laser light may also be employed. The
ultraviolet laser light is obtained by using a non-linear optical
element to convert infrared laser light that is emitted from a
solid-state laser into green laser light and then using another
non-linear optical element to convert the green laser light.
[0080] FIG. 2B corresponds to a sectional view taken along the line
A-A' of FIG. 2A before crystallization and FIG. 2C is a sectional
view taken along the line B-B' after crystallization. In the
semiconductor film (post-LC) 103 crystallized by laser light
irradiation, a grain boundary 105 is easily formed around the
center of a depression portion of the insulating film 101. Used in
FIGS. 2A to 2C as an active layer or a channel formation region is
a portion 106 between one edge of a depression portion and the
midpoint between the one edge and the other edge of the depression
portion, where fewer grain boundaries are formed to give the
portion excellent crystallinity. It is not that the excellent
crystallinity portion 106 has no grain boundaries. However, the
portion 106 has better crystallinity even if it has grain
boundaries because its crystal grains are large in size. A portion
between one edge of a projection portion and the midpoint between
the one edge and the other edge of the projection portion has fewer
grain boundaries to give the portion excellent crystallinity, and
this portion too can be used as an active layer or a channel
formation region.
[0081] The semiconductor film 103 after crystallization is then
patterned as shown in FIG. 3A, avoiding the semiconductor film
around the midpoint between edges of a depression portion and near
a projection portion where many grain boundaries are supposedly
formed to be used as channel formation regions. The remaining
portion, between one edge of a depression portion and the midpoint
between the one edge and the other edge of the depression portion,
has excellent crystallinity and is used to form channel formation
regions.
[0082] In this embodiment, the semiconductor film 103 is patterned
so as to partially leave regions near edges of projection portions
or regions near edges of depression portions, projection portions,
and regions around the centers of depression portions. Thus
obtained is the island 108 in which only channel formation regions
are separated as shown in FIG. 3A and which is used as a slit-like
active layer. A sectional view taken along the line A-A' of the
island 108 is shown in FIG. 3B and FIG. 3C is a sectional view
taken along the line B-B' of the island 108. In a portion to serve
as a source region or a drain region, the crystallinity of a
semiconductor film has less influence over TFT characteristics than
in a channel formation region. Therefore using as a source region
or a drain region a portion of a semiconductor film that has rather
poor crystallinity does not present a problem.
[0083] Next, a gate insulating film 110 is formed as shown in FIG.
4A to cover at least a portion of the island 108 that serves as a
channel formation region. Although a portion to serve as a source
region or a drain region is exposed in FIG. 4A, the gate insulating
film 110 may cover the entire island 108.
[0084] Then a conductive film is formed and patterned to form a
gate electrode 111. A sectional view taken along the line A-A' in
FIG. 4A is shown in FIG. 4B. The gate electrode 111 overlaps every
channel formation region.
[0085] Through the above-mentioned manufacturing process, a TFT
having channel formation regions separated from one another is
completed. This structure makes it possible to increase the channel
width in a channel formation region so that the TFT can be driven
while ensuring ON current. As a result, the heat generated can be
released efficiently.
[0086] The description given next is about a production system of
the present invention. FIG. 5 is a flow chart for a production
system of the present invention. First, a mask for an island is
designed and then an insulating film is designed to have
rectangular or stripe pattern depression and projection portions.
One or more channel formation regions are placed on the flat face
of a depression portion or projection portion of the insulating
film. Desirably, a channel formation region is placed avoiding a
region around the midpoint between edges of a depression portion or
projection portion where more grain boundaries are formed than the
rest. The carrier moving direction in a channel formation region
desirably matches the direction of the stripe pattern of the
insulating film, or the direction of the longer sides or shorter
sides of the rectangular of the insulating film. However, they may
be varied intentionally if it suits the use.
[0087] The insulating film may be designed to have a marker as its
part.
[0088] Information relating to the shape of the insulating film
designed (pattern information) is inputted to a computer of laser
irradiation apparatus and stored in storing means of the computer.
The computer decides the laser light scanning path based on the
insulating film pattern information inputted and the width in the
direction perpendicular to the laser beam scanning direction. It is
important in determining the scanning path that the edges of the
laser light track do not overlap the flat face of a depression
portion or projection portion of the insulating film. The computer
may store in its storing means pattern information of an island in
addition to the insulating film pattern information, and may decide
the scanning path so as to prevent the edges of the laser light
track from overlapping the island or a channel formation region of
the island.
[0089] If a slit is used to control the width of a laser beam, the
computer grasps the width of a depression portion or projection
portion of the insulating film in the direction perpendicular to
the scanning direction from the insulating film pattern information
inputted. Then, taking into account the width of a depression
portion or projection portion of the insulating film, the width of
the slit in the direction perpendicular to the scanning direction
is set so as to prevent the edges of the laser light track from
overlapping the flat face of a depression portion or projection
portion of the insulating film.
[0090] After the insulating film is formed on a substrate in
accordance with the designed pattern information, a semiconductor
film is formed on the insulating film. After the semiconductor film
is formed, the substrate is set on a stage of the laser irradiation
apparatus and is positioned. In FIG. 5 illustrates an example in
which the substrate is positioned by detecting the marker with a
CCD camera. A CCD camera refers to a camera using a CCD
(charge-coupled device) as an image pickup device.
[0091] In another method to position the substrate, a CCD camera or
the like is used to detect pattern information of the insulating
film or semiconductor film on the substrate that is set on the
stage, and then the computer checks the pattern information of the
insulating film or semiconductor film actually foamed on the
substrate which is provided by the CCD camera against information
of an insulating film or semiconductor film pattern designed by
CAD.
[0092] Laser light irradiates the semiconductor film by running
along the scanning path determined and crystallizes the
semiconductor film.
[0093] The semiconductor film having its crystallinity enhanced by
the laser light irradiation is patterned to form an island.
Subsequently, a process of manufacturing a TFT from the island
follows. Although specifics of the TFT manufacturing process are
varied depending on the TFT form, a typical process starts with
forming a gate insulating film and forming impurity regions in the
island. Then an interlayer insulating film is formed so as to cover
the gate insulating film and a gate electrode. A contact hole is
formed in the interlayer insulating film to partially expose the
impurity region. A wire is then formed on the interlayer insulating
film to reach the impurity region through the contact hole.
[0094] Next, a description is given with reference to FIG. 6 on the
structure of laser irradiation apparatus used in the present
invention. Reference symbol 151 denotes laser oscillators. Four
laser oscillators are used in FIG. 6 but the number of laser
oscillators in the laser irradiation apparatus is not limited
thereto.
[0095] A chiller 152 may be used to keep the temperature of the
laser oscillators 151 constant. Although the chiller 152 is not
always necessary, fluctuation in energy of laser light outputted
due to a temperature change can be avoided by keeping the
temperature of the laser oscillators 151 constant.
[0096] Denoted by 154 is an optical system, which changes the path
of light emitted from the laser oscillators 151 or manipulates the
shape of the laser beam thereof to collect laser light. In the
laser irradiation apparatus of FIG. 6, the optical system 154 can
also synthesize laser beams of laser light outputted from the
plural laser oscillators 151 by partially overlapping the laser
beams.
[0097] An AO modulator 153 capable of changing the travel direction
of laser light in a very short time may be provided in the light
path between a substrate 156 that is a processing object and the
laser oscillators 151. Instead of the AO modulator, an attenuator
(light amount adjusting filter) may be provided to adjust the
energy density of laser light.
[0098] Alternatively, energy density measuring means 165, namely,
means for measuring the energy density of laser light outputted
from the laser oscillators 151 maybe provided in the light path
between the substrate 156 that is a processing object and the laser
oscillators 151. Changes with time of measured energy density may
be monitored by a computer 16D. In this case, output from the laser
oscillators 151 may be increased to compensate attenuation in
energy density of the laser light.
[0099] A synthesized laser beam irradiates through a slit 155 the
substrate 156 that is a processing object. The slit 155 is
desirably formed of a material that can block laser light and is
not deformed or damaged by laser light. The width of the slit 155
is variable and a laser beam can be changed in width by changing
the width of the slit.
[0100] When laser light emitted from the laser oscillators 151 does
not pass through the slit 155, the shape of the laser beam on the
substrate 156 is varied depending on the laser type and may be
shaped by an optical system.
[0101] The substrate 156 is set on a stage 157. In FIG. 6, position
controlling means 158 and 159 correspond to means for controlling
the position of a laser beam on a processing object. The position
of the stage 157 is controlled by the position controlling means
158 and 159.
[0102] In FIG. 6, the position controlling means 158 controls the
position of the stage 157 in the direction X and the position
controlling means 159 controls the position of the stage 157 in the
direction Y.
[0103] The laser irradiation apparatus of FIG. 6 has the computer
160, which is a central processing unit and at the same time
storing means such as a memory. The computer 160 controls
oscillation of the laser oscillators 151, determines the laser
light scanning path, and controls the position controlling means
158 and 159 to move the substrate to a given position so that a
laser beam runs along the scanning path determined.
[0104] In FIG. 6, the laser beam position is controlled by moving
the substrate. Alternatively, the laser beam position may be moved
by an optical system such as a Galvano mirror. The laser beam
position may also be controlled by moving both the substrate and
the laser beam.
[0105] In FIG. 6, the computer 160 controls the width of the slit
155 so that the laser beam spot width can be changed in accordance
with mask pattern information. The slit is not always
necessary.
[0106] The laser irradiation apparatus may also have means for
adjusting the temperature of a processing object. A damper may also
be provided to prevent reflected light from irradiating a portion
that should avoid laser irradiation since laser light is highly
directional and has high energy density. Desirably, the damper is
absorptive of reflected light. Cooling water may be circulated
inside the damper to avoid a temperature rise of the partition wall
due to absorption of reflected light. The stage 157 may be provided
with means for heating a substrate (substrate heating means).
[0107] If a laser is used to form a marker, a laser oscillator for
a marker may be provided. In this case, oscillation of the laser
oscillator for a marker may be controlled by the computer 160.
Another optical system is needed when the laser oscillator for a
marker is provided in order to collect laser light outputted from
the laser oscillator for a marker. The laser used to form a marker
is typically a YAG laser or a CO.sub.2 laser, but it is needless to
say that other lasers may be employed instead.
[0108] One or more if it is necessary, CCD camera(s) 163 may be
provided for positioning that uses a marker. A COD camera refers to
a camera using a CCD (charge-coupled device) as an image pickup
device.
[0109] Instead of forming a marker, the CCD camera(s) 163 may be
used to recognize the pattern of the insulating film or
semiconductor film for positioning of the substrate. In this case,
insulating film or semiconductor film pattern information by a mask
which is inputted to the computer 160 and the actual insulating
film or semiconductor film pattern information collected by the CCD
camera(s) 163 are checked against each other to grasp the substrate
position information.
[0110] Part of laser light entering the substrate is reflected by
the surface of the substrate and travels back the same light path
it has taken upon entering. This is called return light and has
adverse effects such as changing the output and frequency of the
laser and damaging the rod. In order to remove such return light
and stabilize laser oscillation, an isolator may be provided.
[0111] Although FIG. 6 shows a laser irradiation apparatus
structure which has plural laser oscillators, only one laser
oscillator may be provided. FIG. 7 shows a laser irradiation
apparatus structure which has one laser oscillator. In FIG. 7, 201
denotes a laser oscillator and 202 denotes a chiller. Denoted by
215 is an energy density measuring device, 203, an AO modulator,
204, an optical system, 205, a slit, and 213, a CCD camera. A
substrate 206 is set on a stage 207. The position of the stage 207
is controlled by X-direction position controlling means 208 and
Y-direction position controlling means 209. Similar to the
apparatus shown in FIG. 6, a computer 210 controls operations of
the means of this laser irradiation apparatus. The major difference
between FIG. 7 and FIG. 6 is that there is one laser oscillator in
FIG. 7. Unlike FIG. 6, the optical system 204 only has to have a
function of collecting one laser beam.
[0112] As described above, in the present invention, a
semiconductor film crystallized by laser light is patterned to
remove a portion of the film around the midpoint between edges of a
depression portion or projection portion. The remaining portion
between one edge of a depression portion or projection portion and
the midpoint between the one edge and the other edge of the
depression portion or projection portion, where fewer grain
boundaries are formed to give the film excellent crystallinity, is
effectively used as a channel formation region of a TFT. This makes
it possible to avoid forming a grain boundary in a channel
formation region of a TFT, thereby preventing grain boundaries from
lowering the mobility of the TFT greatly, from lowering ON current,
and from increasing OFF current. How far from an edge of a
depression portion or projection portion is to be removed by
patterning can be decided at designer's discretion.
[0113] The present invention runs laser light so as to obtain at
least the minimum degree of crystallization of a portion that has
to be crystallized, instead of irradiating the entire semiconductor
film with laser light. As a result, time for laser irradiation of
portions that are removed by patterning after crystallization of
the semiconductor film can be saved to greatly shorten the
processing time per substrate.
Embodiment 1
[0114] This embodiment explains how to form an insulating film
having depression/projection.
[0115] At first, a first insulating film 251 is formed on a
substrate 250, as shown in FIG. 8A. Although the first insulating
film 251 uses silicon oxide nitride in this embodiment, this is not
limited to, i.e. an insulating film having a great etching
selective ratio to a second insulating film is satisfactory. In
this embodiment, the first insulating film 251 was formed to a
thickness of 50-200 nm using SiH.sub.4 and N.sub.2O by a CVD
apparatus. Note that the first insulating film may be of a single
layer or a layered structure having a plurality of insulating
films.
[0116] Then, a second insulating film 252 is formed in contact with
the first insulating film 251, as shown in FIG. 8B. The second
insulating film 252 requires a film thickness to a degree that,
when a depression-projection is formed thereon by patterning in a
subsequent process, the depression-projection appears on a surface
of a semiconductor film to be subsequently deposited. This
embodiment forms, as the second insulating film 252, silicon oxide
having 30 nm-300 nm by a plasma CVD.
[0117] Next, a mask 253 is formed as shown in FIG. 8C to etch the
second insulating film 252. This embodiment conducts wet etching at
20.degree. C. using an etchant of a mixture solution containing
7.13% of ammonium hydrogen fluoride (NH.sub.4HF.sub.2) and 15.4% of
ammonium fluoride (NH.sub.4F) (product name: LAL500 by Stella
Chemifa Corporation). This etching forms a projection part 254 in a
rectangular or stripe form. In this specification, a combination of
the first insulating film 251 and the projection part 254 is
considered as one insulating film. And then, the mask 253 is
removed.
[0118] Then, a semiconductor film is formed covering the first
insulating film 251 and projection part 253. Because in the
embodiment the projection part has a thickness of 30 nm-300 nm, the
semiconductor film is desirably given a film thickness of 50-200
nm, herein 60 nm. Incidentally, in case an impurity is mixed
between the semiconductor film and the insulating film, there is a
possibility that bad affection is exerted to the crystallinity of
semiconductor film to increase the characteristic and threshold
voltage variation of the TFT fabricated. Accordingly, the
insulating film and the semiconductor film are desirably formed
continuously. For this reason, in this embodiment, after forming an
insulating film comprising the first insulating film 251 and the
projection part 253, a silicon oxide film is formed in a small
thickness on the insulating film, followed by continuously forming
a semiconductor film 256 without exposure to the air. The thickness
of silicon oxide film, although properly set by the designer, was
given 5 nm-30 nm in this embodiment.
[0119] Incidentally, when etching the second insulating film 252,
the projection part may be etched into a taper form. By making the
projection part in a taper form, a semiconductor film, gate
insulating film or gate electrode is prevented from having
disconnection at a projection-region edge.
[0120] Now, explanation is made on a different way to fault an
insulating film. At first, a first insulating film 261 is formed on
a substrate 260, as shown in FIG. 9A. The first insulating film 261
is formed of silicon oxide, silicon nitride or silicon oxide
nitride.
[0121] In the case of using a silicon oxide nitride film, it can be
formed by mixing Tetraethyl Ortho Silicate (TEOS) and O.sub.2 and
subjecting it to a plasma CVD with discharge under a reaction
pressure of 40 Pa, at a substrate temperature of 300-400.degree. C.
and with a radio frequency (13.56 MHz) power density of 0.5-0.8
W/cm.sub.2. In the case of using a silicon oxide nitride film, it
may be formed by a plasma CVD with a silicon oxide nitride film
formed from SiH.sub.4, N.sub.2O and NH.sub.3 or a silicon oxide
nitride film formed from SiH.sub.4 and N.sub.2O. This is performed
under a forming condition of a reaction pressure of 20-200 Pa and a
substrate temperature of 300-400.degree. C., with a radio frequency
(60 MHz) power density of 0.1-1.0 W/cm.sup.2. Meanwhile, a silicon
oxide nitride hydride film may be used that is to be formed from
SiH.sub.4, N.sub.2O and H.sub.2. A silicon nitride film can be
similarly formed from SiH.sub.4 and NH.sub.3 by a plasma CVD.
[0122] After forming a first insulating film 261 to a thickness of
20-200 nm (preferably 30-60 nm) over the entire surface of the
substrate, a mask 262 is formed by using a photolithography
technique as shown in FIG. 9B. Unwanted regions are removed away to
form a projection part 263 in a stripe or rectangular form. To
remove away unwanted regions, a dry etching process may be used
that uses a fluorine-based gas. Otherwise, a wet etching process
may be used that uses a fluorine-based solution. In the case of
selecting the latter, etching is preferably conducted using a
mixture solution containing 7.13% of ammonium hydrogen fluoride
(NH.sub.4HF.sub.2) and 15.4% of ammonium fluoride (NH.sub.4F)
(product name: LAL500 by Stella Chemifa Corporation). And then, the
mask 262 is removed.
[0123] Then, a second insulating film 264 is formed covering the
projection part 263 and substrate 260. This layer is formed of
silicon oxide, silicon nitride or silicon oxide nitride to a
thickness of 50-300 nm (preferably 100-200 nm), similarly to the
first insulating film 261.
[0124] By the above fabrication process, an insulating film is
formed comprising the projection part 263 and the second insulating
film 264. After forming the second insulating film 264, by
continuously foaming a semiconductor film without exposure to the
air, the impurities in the air are prevented from mixing between
the semiconductor film and the insulating film.
Embodiment 2
[0125] This embodiment explains an example that a semiconductor
film formed on an insulating film in a stripe form is crystallized
by laser light irradiation and thereafter mutually isolated islands
are formed on a surface parallel with an projection-formed
substrate to fabricate TFT using the islands.
[0126] FIG. 10A shows a TFT structure of this embodiment. In FIG.
10A, an insulating film 152 having striped projection parts 151 is
formed on a substrate 150. A plurality of islands 153 are formed,
isolated from one another, on the top surfaces of the projection
parts 151. An gate insulating film 154 is formed in a manner
contacting with the islands 153. Incidentally, although the gate
insulating film 154 in FIG. 10A is formed exposing the regions, to
be made into impurity regions, of the island, it may be formed
covering the entire island 154.
[0127] A plurality of gate electrodes 155 is formed on the gate
insulating film 154 in a manner superposed over a plurality of
islands 153. The plurality of gate electrodes 155 may be mutually
connected depending upon a circuit configuration.
[0128] Note that the sectional view on the line A-A' in FIG. 10A
corresponds to FIG. 10B while the sectional view on the line B-B'
in FIG. 10A corresponds to FIG. 10C. As shown in FIG. 100, each
gate electrode 155 is superposed on a channel region 156 of the
island 153 with gate insulating film 154 sandwiched therebetween.
The channel region 156, in turn, is sandwiched between two impurity
regions 157 included also in the island 153.
[0129] In this embodiment TFT is formed by using an island formed
at the bottom of the depression part. TFT can also be formed by
using an island on top of the projection part.
[0130] This embodiment can be implemented by combining with
Embodiment 1.
Embodiment 3
[0131] This embodiment explains variations of insulating film
forms.
[0132] FIG. 11A shows an embodiment on an insulating film form of
the invention. In FIG. 11A, an insulating film 171 is formed on a
substrate 170 wherein the insulating film 171 has a plurality of
projection parts 172. The projection part 172 is rectangular in
form as viewed from the above. All the projection parts have
respective rectangular longer or shorter sides in a direction
parallel with a scanning direction of laser light shown by the
arrow.
[0133] The projection parts 172 are not necessarily identical to
one another in the width in laser-light scanning direction and the
width perpendicular to the scanning direction. A form of an
insulating film is desirably designed to meet a desired island
form.
[0134] It is not necessary that projection parts of the insulating
film using the present invention being completely striped. It is
need only a portion of the insulating film is striped or
rectangular. FIG. 11B shows an embodiment on an insulating film
form of the invention. In FIG. 11B, an insulating film 181 is
formed on a substrate 180. The insulating film 181 is formed with a
rectangular projection part 182 having slit-like openings as viewed
from the above. In the projection part 182, the slit has a longer
or shorter side in parallel with a scanning direction of laser
light shown by the arrow.
[0135] Explanation is now made on an example of a TFT structure
formed by using the insulating film having slit-like openings shown
in FIG. 11B.
[0136] FIG. 12A shows a top view of the TFT of this embodiment. As
shown in FIG. 12A, this embodiment used an insulating film having a
rectangular projection part 760 having therein slit-like openings.
A semiconductor film is formed covering the projection part 760.
Laser light is scanned, in a direction shown by the arrow, along a
direction of a longer axis of the slit-like opening to crystallize
the semiconductor film. Then, the semiconductor film is patterned
to form an island 761 having an opening formed in the upper surface
of the projection part. The channel region of the island 761 avoids
using the vicinity of a center between edges of depression part to
use a portion having high crystallinity between the
edge-neighborhood of a depression part and the vicinity of a center
between edges of a depression part
[0137] Then, a gate insulating film 762 is formed in a manner
contacting with the island 761. Then, a conductive film is formed
on the gate insulating film 762. By patterning the conductive film,
a gate electrode 763 is formed. The gate electrode 763 is
superposed on a channel region 764 of the island 761 with gate
insulating film 762 sandwiched therebetween. The channel region 764
is sandwiched between the two impurity regions 765 included in the
island 761.
[0138] A first interlayer insulating film 766 is formed covering
the gate electrode 763, island 761 and gate insulating film 762.
The first interlayer insulating film 766 is formed of inorganic
insulator having an effect to prevent a substance, such as alkali
metal, having a bad effect upon TFT characteristics from mixing in
the island 761.
[0139] A second interlayer insulating film 767 is formed of organic
resin on the first interlayer insulating film 766. Openings are
formed, by etching, through the second interlayer insulating film
767, first interlayer insulating film 766 and gate insulating film
762. Through the openings, the interconnections 768, 769 are formed
on the second interlayer insulating film 767, respectively
connecting between the two impurity regions 765 and the gate
electrode 763. Note that the sectional view on the line A-A' in
FIG. 12A is shown in FIG. 12B, the sectional view on the line B-B'
in FIG. 12C and the sectional view on the line C-C' in FIG.
12D.
[0140] In this embodiment, channel regions 764 are formed in
plurality and the channel regions are isolated from each other.
Accordingly, by increasing the channel width of the channel region,
the heat generated by driving the TFT can be efficiently dissipated
while securing on-current.
[0141] In this embodiment, TFT having a channel region formed at
the bottom of the depression part is described though, it can also
be formed by using channel region formed on top of the projection
part.
Embodiment 4
[0142] This embodiment explains a method for manufacturing an
active-matrix substrate using a laser crystallization method of the
invention, by using FIGS. 13 and 14. In this specification, the
substrate forming, on the same substrate, a CMOS circuit and a
pixel region having drive circuit, pixel TFTs and hold capacitances
is referred to as an active-matrix circuit, for ease of
description.
[0143] This embodiment uses a substrate 600 formed of a glass such
as barium borosilicate glass or aluminum borosilicate glass. The
substrate 600 may use a quartz, silicon, metal or stainless steel
substrate formed with an insulating film on a surface thereof.
Otherwise, a plastic substrate may be used that has a heat
resistance to withstand at process temperature in this
embodiment.
[0144] Then, an insulating film of silicon oxide, silicon nitride
or silicon oxide nitride is formed in a thickness of 100-300 nm on
the substrate 600, by the known means (a sputtering, an LPCVD, a
plasma CVD or the like)
[0145] Next, in order to form a large and small thickness regions
in the insulating film, the embodiment forms a resist mask 693 by a
photolithography technique and carries out an etching process on
it. Although the dimension of a step is determined by an etching
amount, the embodiment provides nearly 50-100 nm. For example, to
etch a silicon oxide nitride film having 150 nm by 75 nm, it is
possible to use wet etching using a solution containing hydrogen
fluoride or applying a dry etching using CF.sub.4. In this manner,
an insulating film 601 formed with a projection form is formed. In
this case, the width of a projection part perpendicular to a
scanning direction may be properly determined taking a TFT size
into consideration, preferably a size (in diameter or diagonal
length) of approximately 2-6 .mu.m for the purpose of controlling
the number of crystal-nucleation (FIG. 13A).
[0146] Then, an amorphous semiconductor film 692 is formed in a
thickness of 25-80 nm (preferably 30-60 nm) on the insulating film
601, by the known means (sputter process, LPCVD process, plasma CVD
process or the like) (FIG. 13B). Incidentally, although this
embodiment forms an amorphous semiconductor film, a fine crystal
semiconductor film or crystalline semiconductor film is also
applicable. Otherwise, an amorphous-structured compound
semiconductor film, such as an amorphous silicon-germanium film,
may be used.
[0147] Next, the amorphous semiconductor film 692 is crystallized
by a laser crystallization method. The scanning direction of laser
light is parallel with an extension of the striped projection part
of insulating film 601. Incidentally, where the projection part of
insulating film 601 is rectangular as viewed from the above of the
substrate, the scanning direction of laser light is defined
parallel with a direction of a longer or shorter side of the
rectangle. Specifically, laser light is preferentially irradiated
according to the information about mask inputted to the computer of
the laser irradiation apparatus. Of course, besides the laser
crystallization method, this may be combined with other known
crystallization methods (thermal crystallization method using RTA
or furnace anneal, thermal crystallization method using a metal
element to promote crystallization, or the like). Although the
embodiment shows an example to change a laser beam width to a width
of insulating film perpendicular to a scanning direction by the use
of a slit, the invention is not limited to this, i.e. the slit is
not necessarily required to be used.
[0148] In crystallizing the amorphous semiconductor film, by using
a continuous oscillatable solid laser and a second to fourth
harmonic of basic wave, an increased grain size of crystal can be
obtained. Typically, desirably used is the second harmonic (532 nm)
or third harmonic (355 nm) of an Nd:YVO.sub.4 laser (basic wave:
1064 nm). Specifically, the laser light emitted from a
continuous-oscillation YVO.sub.4 laser is changed into a harmonic
by a nonlinear optical device to obtain a 10W-output laser light.
Meanwhile, there is a method that an YVO.sub.4 crystal and a
nonlinear optical device are inserted in a resonator to emit a
higher harmonic. Preferably, laser light is formed by an optical
system into a rectangular or elliptic form on irradiation plane,
which is irradiated to a subject to be worked. The energy density,
in this case, requires approximately 0.01-100 MW/cm.sup.2
(preferably 0.1-10 MW/cm.sup.2). For irradiation the semiconductor
film is moved at a speed of approximately 10-2000 cm/s relatively
to laser light.
[0149] In laser irradiation can be used a pulse-oscillation or
continuous-oscillation gas laser or solid laser. Gas lasers
includes an excimer laser, an Ar laser and a Kr laser. Solid lasers
include a YAG laser, a YVO.sub.4 laser, a YLF laser, a YAlO.sub.3
laser, a glass laser, a ruby laser, an alexandorite laser, a
Ti:sapphire laser and a Y.sub.2O.sub.3 laser. As the solid laser
can be used a laser using a crystal of YAG, YVO.sub.4, YLF or
YAlO.sub.3 doped with Cr, Nd, Er, Ho, Ce, Co, Ti Yb or Tm. Also, a
slab laser is usable. The laser has a different basic wave
depending upon a doping material, providing laser light having a
basic wave at around 1 .mu.m. The harmonic to basic wave is
available by the use of a non-linear optical device.
[0150] The foregoing laser crystallization forms a crystalline
semiconductor film 694 enhanced in crystallinity (FIG. 13C). In the
crystalline semiconductor film, grain boundaries 695 tend to occur
in the vicinity of center between edges.
[0151] Next, the crystalline semiconductor film 694 enhance in
crystallinity is patterned into a desired form to form crystallized
islands 602-606 (FIG. 13D). Here, a number of grain boundaries in
the islands 602 to 606 can be suppressed by removing the vicinity
of a center between an edge of the depression or projection part
where the grain boundaries 695 tend to occur.
[0152] After the islands 602 to 606 are formed, the islands may be
doped with a minute amount of impurity element (boron or
phosphorus) in order to control the threshold of TFTs.
[0153] Subsequently, a process of manufacturing TFTs from the
islands 602 to 606 follows. Although specifics of the TFT
manufacturing process are varied depending on the TFT form, a
typical process starts with forming a gate insulating film and
forming impurity regions in the islands. Then an interlayer
insulating film is formed so as to cover the gate insulating film
and a gate electrode. A contact hole is formed in the interlayer
insulating film to partially expose the impurity regions. A wire is
then formed on the interlayer insulating film to reach the impurity
regions through the contact hole.
[0154] FIG. 14 is a sectional view of the semiconductor device of
this embodiment. The islands 602 to 606 have channel formation
regions, first impurity regions, and second impurity regions. Each
channel formation region is sandwiched between two first impurity
regions. Each second impurity region is formed between one first
impurity region and one channel formation region. The concentration
of an impurity element that gives one conductivity type is higher
in a first impurity region than in a second impurity region. A gate
insulating film 607 is formed to cover the islands 602 to 606. On
the gate insulating film 607, gate electrodes 608 to 613 are formed
overlapping the channel formation regions and a source signal line
614 is also formed. An interlayer insulating film 615 is formed on
the gate insulating film 607 so as to cover the gate electrodes 608
to 613 and the source signal line 614.
[0155] In a driving circuit 686, formed on the interlayer
insulating film 615 are wires 663 to 667, which are electrically
connected to the impurity regions. In a pixel portion 687, a pixel
electrode 670, a gate wire 669, and a wire 668 are formed. The wire
668 electrically connects the source signal line 614 with a pixel
TFT 684.
[0156] Although not shown in the drawing, the gate wire 669 is
electrically connected to the gate electrodes 611 and 612 of the
pixel TFT 684. The pixel electrode 670 is electrically connected to
a first impurity region of the pixel TFT and is electrically
connected to the island 606, which functions as one of electrodes
constituting capacitor storage 685. In this application, the pixel
electrode and the wires are formed from the same material. However,
the pixel electrode 670 may be formed from a highly reflective
material such as a film mainly containing Al, a film mainly
containing Ag, or a laminate of films containing Al and Ag.
[0157] The driving circuit 686, which has a CMOS circuit composed
of an n-channel TFT 681 and a p-channel TFT 682 and has an
n-channel TFT 683, and the pixel portion 687, which has the pixel
TFT 684 and the capacitor storage 685, can thus be formed on the
same substrate to complete an active matrix substrate. The
capacitor storage 685 is composed of the electrode 613 and the
island 606 with the gate insulating film 607 as dielectric.
[0158] According to the pixel structure of this embodiment, edges
of a pixel electrode overlap a source signal line so that the gap
between pixel electrodes is shielded against light without using a
blocking film.
[0159] Although the structure of an active matrix substrate used in
a liquid crystal display device is described in this embodiment,
the manufacturing process of this embodiment can also be employed
to manufacture alight emitting device. "Light emitting device" is a
generic term for a display panel in which a light emitting element
is formed on a substrate and sealed between the substrate and a
cover member, and for a display module obtained by mounting a TFT
and the like to the display panel. A light emitting element has a
layer (light emitting layer) containing an organic compound that
provides luminescence upon application of electric field
(electroluminescence), as well as an anode layer and a cathode
layer.
[0160] In a light emitting element, a hole injection layer, an
electron injection layer, a hole transporting layer, an electron
transporting layer, or the like may be formed from an inorganic
compound alone or from a material obtained by mixing an inorganic
compound with an organic compound. These layers may be partially
blended with one another.
[0161] The present invention is also applicable to a semiconductor
element of sub-micron level. In the example shown in this
embodiment, an active layer is formed on the bottom of a depression
portion. Alternatively, an active layer may be formed on the top of
a projection portion.
Embodiment 5
[0162] This embodiment explains an example of a combination of a
laser irradiation process and a semiconductor film crystallizing
process using a catalyst when crystallizing a semiconductor film.
In the case of using a catalytic element, desirably used is an art
disclosed in JP-A-130652/1995 and JP-A-78329/1996.
[0163] At first, as shown in FIG. 15A, an insulating film 501
having a projection part 502 is formed on a substrate 500. Then, a
semiconductor film 503 is formed on the insulating film 501.
[0164] Next, a catalytic element is used to crystallize the
semiconductor film 503 (FIG. 15B). For example, in the case of
using the art disclosed in JP-A-130652/1995, a nickel acetate
solution containing 10 ppm nickel by weight is applied onto the
semiconductor film 503 to form a nickel-containing layer 504. After
a dehydrogenation process at 500.degree. C. for 1 hour, thermal
process is carried out at 500-650.degree. C. for 4-12 hours, e.g.
at 550.degree. C. for 8 hours, to form a semiconductor film 505
enhanced in crystallinity. Incidentally, usable catalytic elements
may be an element of germanium (Ge), iron (Fe), palladium (Pd), tin
(Sn), lead (Pb), cobalt (Co), platinum (Pt), copper (Cu) or Gold
(Au), besides nickel (Ni).
[0165] By laser irradiation, a semiconductor film 506 further
enhanced in crystallinity is formed from the semiconductor film 505
crystallized by the heat treatment using Ni. The semiconductor film
506 obtained by laser irradiation contains a catalytic element.
After laser irradiation, carried out is a process to remove the
catalytic element from the semiconductor film 506 (gettering). For
gettering, it is possible to use an art described in
JP-A-135468/1998 or JP-A-135469/1998.
[0166] Specifically, a phosphorus-added region 507 is formed in a
part of a semiconductor film 506 obtained after laser irradiation.
Thermal process is carried out in a nitrogen atmosphere at
550-800.degree. C. for 5-24 hours, e.g. at 600.degree. C. for 12
hours. Then, the phosphorus-added region 507 of the semiconductor
film 506 acts as a gettering site to aggregate the catalytic
element existing in the semiconductor film 506 to the
phosphorus-added region 507 (FIG. 15D).
[0167] Thereafter, the phosphorus-added region 507 of the
semiconductor film 506 is removed by patterning, thereby obtaining
an island 508 reduced in catalytic element concentration to
1.times.10.sup.17 atoms/cm.sup.3 or less, preferably to
approximately 1.times.10.sup.16 atoms/cm.sup.3 (FIG. 15E).
[0168] Incidentally, after applying a solution containing a
catalytic element to the semiconductor film of prior to
crystallization, crystal growth may be by laser light irradiation
instead of SPC.
[0169] This embodiment can be implemented in combination with
Embodiments 1-4.
Embodiment 6
[0170] This embodiment explains the form of a laser beam combined
by superposing together a plurality of laser beams.
[0171] FIG. 16A shows an example of a laser beam form on a subject
to be processed in the case that laser light is oscillated from a
plurality of laser oscillators without a slit and n laser light
energy density distribution in a major-axis. The laser beam shown
FIG. 16A is elliptic in form. Incidentally, in the invention, the
laser beam form of laser light oscillated from the laser oscillator
is not limited to the elliptic. The laser beam form is different
depending on a laser kind and can be formed by an optical system.
For example, the laser light emitted from an XeCl excimer laser
(wavelength 308 nm, pulse width 30 ns) L3308 by Lambda is
rectangular in form having 10 mm.times.30 mm (each, width at half
maximum in beam profile). The laser light emitted from a YAG laser
is circular in form if a rod is cylindrical and rectangular in form
if it is a slab type. By further forming such laser light by an
optical system, a desired size of laser light can be formed.
[0172] The laser light has an energy density distribution
increasing toward an elliptic center O. In this manner, the laser
beam has an energy density in a center axis direction following the
Gaussian distribution, wherein the region is narrow where energy
density is to be determined uniform.
[0173] FIG. 16B shows a laser beam form when the laser light having
a laser beam of FIG. 16A is combined together. Although FIG. 16B
shows the case that four laser-light laser beams are superposed
together to form one linear laser beam, the number of laser beams
superposed is not limited to that.
[0174] As shown in FIG. 16B, the laser beams of laser light are
combined together by partly superposed one over another with their
major axes placed in coincidence, thereby being formed into one
laser beam 360. Note that, hereinafter, a straight line obtained by
connecting the ellipse centers O is assumed to be a center axis of
the laser beam 360.
[0175] FIG. 16B shows the laser-light energy density distribution
in a center-axis y-direction of a combined laser beam. Energy
density is added on in the overlapped areas of the uncombined laser
beams. For example, adding the energy densities E1 and E2 together
of the overlapped beams as shown in the figure, it becomes nearly
equal to a peak value E3 of beam energy density. Thus, energy
density is flattened between the elliptic centers O.
[0176] Incidentally, the addition of E1 and E2 together, ideally,
equals to E3, practically an equal value is not necessarily
obtainable. It is possible for the designer to appropriately set an
allowable range of deviation between the added value of E1 and E2
and the value of E3.
[0177] With the use of a single laser beam, the energy density
distribution follows the Gaussian distribution. Accordingly, it is
difficult to irradiate an even energy density of laser light to the
entire of a semiconductor film contacting with the flat region of
insulating film or a part to be made into an island. However, as
can be seen from FIG. 16B, by superposing together a plurality of
laser light to mutually compensate for the regions low in energy
density, the region having a uniform energy density is broadened
rather than the single use thereof without superposing a plurality
of laser light. This can efficiently enhance the crystallinity of a
semiconductor film.
[0178] FIG. 17 shows an energy density distribution, determined by
computation, on B-B' and C-C' in FIG. 16B. Note that FIG. 17 is
with reference to the region satisfying an energy density of
1/e.sup.2 of a peak value of an uncombined laser beam. When the
uncombined laser beam assumably has a length in minor axis
direction of 37 .mu.m and a length in major axis direction of 410
.mu.m and a center-to-center distance of 192 .mu.m, the energy
densities on B-B' and C-C' have respective distributions as shown
in FIGS. 17A and FIG. 17B. Although the one on B-B' is somewhat
smaller than the one on C-C', these can be considered to be
substantially the same in magnitude. The combined laser beam, in a
region satisfying an energy density of 1/e.sup.2 of a peak value of
an uncombined laser beam, can be considered as linear in form.
[0179] FIG. 18A shows an energy distribution of a combined laser
beam. The region shown at 361 is a region where energy density is
to be determined uniform while the region shown at 362 is a region
having a low energy density. In FIG. 18, it is assumed that the
laser beam has a length in a center axis direction of W.sub.TBW
while the region 361 having a uniform energy density has a length
in a center axis direction of W.sub.max. As W.sub.TBW increases
greater as compared to W.sub.max, the ratio of the region 362
uneven in energy density not to be used in crystallizing a
semiconductor film increases relatively to the region 361 uniform
in energy density to be used in crystallization. The semiconductor
film irradiated only by the region 362 uneven in energy density has
fine crystals, thus being not well in crystallinity. Consequently,
there arises a necessity to define a layout of scanning route and
insulating film depression-projection such that the region of
semiconductor film to be made into an island is not superposed with
only the region 362. This restriction increases furthermore as the
ratio of region 362 to region 361 increases. Accordingly, it is
effective to use a slit to prevent only the region 362 uneven in
energy density from being irradiated to the semiconductor film
formed on the insulating film depression part or projection part,
in respect of decreasing the restriction occurring upon providing a
layout of scanning route and insulating film depression
-projection.
[0180] This embodiment can be implemented by combining with
Embodiments 1 through 5.
Embodiment 7
[0181] This embodiment describes an optical system of laser
irradiation apparatus used in the present invention, and the
positional relation between a slit and the optical system.
[0182] FIG. 19 shows an optical system for synthesizing four laser
beams to obtain one laser beam. The optical system shown in FIG. 19
has six cylindrical lenses 417 to 422. Four laser beams entering
the optical system from the directions indicated by the arrows
separately enter the four cylindrical lenses 419 to 422. Two laser
beams shaped by the cylindrical lenses 419 and 421 reach the
cylindrical lens 417, which modifies the shapes of the laser beams.
The laser beams travel through a slit 424 to irradiate a processing
object 423. On the other hand, two laser beams shaped by the
cylindrical lenses 420 and 422 reach the cylindrical lens 418,
which modifies the shapes of the laser beams. The laser beams
travel through the slit 424 to irradiate the processing object
423.
[0183] The laser beams on the processing object 423 partially
overlap one another for synthesization, thereby forming one laser
beam.
[0184] The focal length and incident angle of each lens can be set
at designer's discretion. However, the focal length of the
cylindrical lenses 417 and 418 which are the closest to the
processing object 423 is set shorter than the focal length of the
cylindrical lenses 419 to 422. For example, the focal length of the
cylindrical lenses 417 and 418 which are the closest to the
processing object 423 is set to 20 mm whereas the focal length of
the cylindrical lenses 419 to 422 is set to 150 mm. In this
embodiment, the lenses are arranged such that laser beams enter the
processing object 423 from the cylindrical lenses 417 and 418 at an
incident angle of 25.degree. and laser beams enter the cylindrical
lenses 417 and 418 from the cylindrical lenses 419 to 422 at an
incident angle of 10.degree.. In order to avoid return light and
irradiate uniformly, the incident angle at which laser light enters
the substrate is kept at an angle larger than 0.degree., desirably,
5 to 30.degree..
[0185] In the example shown in FIG. 19, four laser beams are
synthesized. In this case, four cylindrical lenses respectively
associated with four laser oscillators and two cylindrical lenses
associated with the four cylindrical lenses are provided. The
number of laser beams synthesized is not limited to 4. It is
sufficient if the number of laser beams synthesized is equal to or
more than 2 and equal to or less than 8. When n (n=2, 4, 6, 8)
laser beams are synthesized, n cylindrical lenses respectively
associated with n laser oscillators and n/2 cylindrical lenses
associated with the n cylindrical lenses are provided. When n (n=3,
5, 7) laser beams are synthesized, n cylindrical lenses
respectively associated with n laser oscillators and (n+1)/2
cylindrical lenses associated with the n cylindrical lenses are
provided.
[0186] When five or more laser beams are synthesized, the fifth and
the following laser beams desirably irradiate a substrate from the
opposite side of the substrate, taking into consideration where to
place the optical system, interference, and the like. In this case,
another slit is needed on the opposite side of the substrate. Also,
the substrate has to be transmissive.
[0187] In order to prevent light from traveling back its light path
(return light), the incident angle at which laser light enters the
substrate is desirably kept at an angle larger than 0.degree. and
smaller than 90.degree..
[0188] A plane which is perpendicular to the irradiated face and
which includes a shorter side of the rectangular shape of each beam
before synthesization, or a longer side thereof, is defined as an
incident plane. When the length of the shorter side or longer side
included in the incident plane is given as W, and the thickness of
a substrate which is transmissive of the laser light and which is
set on the irradiated face is given as d, an incident angle .theta.
of the laser light desirably satisfies .theta..gtoreq.arctan (W/2d)
to achieve uniform laser light irradiation. This has to be true in
each laser light before synthesization. If the track of this laser
light is not on the incident plane, the incident angle of the track
projected onto the incident plane is deemed as .theta.. When laser
light enters the substrate at this incident angle .theta.,
interference between light reflected at the front side of the
substrate and reflected light from the back side of the substrate
can be avoided to give the substrate uniform laser beam
irradiation. The premise of the above discussion is that the
refractive index of the substrate is 1. In practice, the refractive
index of the substrate is often around 1.5, and the angle
calculated taken this fact into account is larger than the angle
calculated in the above discussion. However, the energy of a beam
spot is attenuated at its ends in the longitudinal direction and
influence of interference is small in these portions. Therefore
enough interference attenuation effect can be obtained with the
value calculated in the above discussion. The above-mentioned
inequality of .theta. does not apply to those substrates that are
transmissive to a laser beam.
[0189] An optical system of laser irradiation apparatus used in the
present invention can have other structures than the one shown in
this embodiment.
[0190] This embodiment can be combined with Embodiments 1 through
6.
Embodiment 8
[0191] The laser light having a laser beam in an elliptic form has
an energy density distribution perpendicular to a scanning
direction following the Gaussian distribution. Consequently, the
ratio of a low energy density region to the entire is higher as
compared to the laser light having a rectangular or linear laser
beam. Accordingly, in the invention, the laser beam of laser light
is desirably rectangular or linear comparatively uniform in energy
density distribution.
[0192] The representative gas laser, for obtaining a rectangular or
linear laser beam, is an excimer laser while the representative
solid laser is a slab laser. This embodiment explains a slab
laser.
[0193] FIG. 20A shows an example of a laser oscillator structure of
a slab type. The slab-type laser oscillator of FIG. 20A has a rod
7500, a reflection mirror 7501, an output mirror 7502 and a
cylindrical lens 7503.
[0194] In case an excitation light is irradiated to the rod 7500,
laser light travels through a zigzag optical path and emits toward
the reflection mirror 7501 or emission mirror 7502. The laser light
emitted toward the reflection mirror 7501 is reflected thereon and
again enters the rod 7500, then emitting toward the emission mirror
7502. The rod 7500 is of a slab type using a plate-like slab medium
to form a comparatively long rectangular or linear laser beam upon
emission. The emitted laser light, in the cylindrical lens 7503, is
formed smaller in its laser beam form and emitted at the laser
oscillator.
[0195] FIG. 20B shows a slab-type laser oscillator structure
different from that showed in FIG. 20A. In FIG. 20B, a cylindrical
lens 7504 is added to the laser oscillator of FIG. 20A to control a
laser beam length by the cylindrical lens 7504.
[0196] Incidentally, with a coherent length of 10 cm or longer,
preferably 1 m or longer, the laser beam can be reduced in form
furthermore.
[0197] In order to prevent the rod 7500 from excessively rising in
temperature, temperature control means may be provided, e.g.
circulating a cooling water.
[0198] FIG. 20C shows an embodiment of a cylindrical lens form.
7509 is a cylindrical lens of this embodiment fixed by a holder
7510. The cylindrical lens 7509 has a form that a cylindrical
surface and a rectangular flat surface are opposed to each other,
wherein the two generating lines of the cylindrical surface and the
two sides of the opposed rectangle are all in parallel with one
another. The two surfaces, formed by the two lines of cylindrical
surface and the parallel two lines, intersect with the rectangular
flat surface at an angle greater than 0 degree and smaller than 90
degrees. In this manner, the two surfaces formed with the two
parallel sides intersect with the rectangular flat surface at an
angle of smaller than 90 degrees, whereby the focal length can be
shortened as compared to that at 90 degrees or greater. This can
further reduce the form of laser beam and approximate it to a
linear form.
[0199] This embodiment can be implemented by combining with
Embodiments 1 through 7.
Embodiment 9
[0200] This embodiment explains a relationship between a
center-to-center distance of laser beams and an energy density when
laser beams are superposed one over another.
[0201] FIG. 21 shows an energy density distribution of each laser
beam in a center axis direction by the solid line and an energy
density distribution of a combined laser beam by the dotted line.
The energy density value of a laser beam in a center axis direction
of a laser beam generally follows the Gaussian distribution.
[0202] It is assumed that, the uncombined laser beam has a
peak-to-peak distance X when a distance in a center axis direction
is taken as 1 that is satisfying an energy density equal to or
greater than 1/e.sup.2 of a peak value. Meanwhile, in a combined
laser beam, the increase amount of peak value is assumably taken as
Y with respect to an average value of a peak value and valley value
of after combination. FIG. 22 shows a relationship between X and Y
determined on simulation. Note that Y in FIG. 22 is expressed by
percentage.
[0203] In FIG. 22, an energy difference Y is expressed by an
approximate expression as given in the following Equation 1.
Y=60-293X+340X.sup.2 (X: assumed to be greater one of two
solutions) [Equation 1]
[0204] According to Equation 1, it can be seen that X=0.584 may be
provided when obtaining an energy difference of approximately 5%
for example. Incidentally, although ideally Y=0, there is
practically a difficulty in realizing it. There is a need for the
designer to appropriately set an allowable range of energy
difference Y. Although ideally Y=0, it makes the beam spot length
short. Consequently, X is preferably determined considering a
balance with throughput.
[0205] Explanation is now made on an allowable range of Y. FIG. 23
shows an output (W) distribution of YVO.sub.4 laser with respect to
a beam width in a center axis direction in the case the laser beam
has an elliptic form. The hatched region is an output energy range
required to obtain favorable crystallinity. It can be seen that the
output energy of combined laser light is satisfactorily within a
range of 3.5-6 W.
[0206] When the output energy maximum and minimum values of a beam
spot after combination are fallen, to a full limit, within the
output energy range required to obtain favorable crystallinity, the
energy difference Y for favorable crystallinity assumes to be the
maximum. Accordingly, in the case of FIG. 23, the energy difference
Y is .+-.26.3%. It can be seen that favorable crystallinity is to
be obtained provided that the energy difference Y falls within the
foregoing range.
[0207] Incidentally, the output energy range for favorable
crystallinity varies depending upon to what extent crystallinity is
to be determined favorable. Further, because output energy
distribution changes depending on a laser beam form, the allowable
range of energy difference Y is not necessarily limited to the
foregoing value. The designer is required to appropriately define
an output energy range required to obtain favorable crystallinity
and set an allowable range of energy difference Y from an output
energy distribution of a laser to be used.
[0208] This embodiment can be implemented in combination with
Embodiments 1-8.
Embodiment 10
[0209] The present invention can be applied to various
semiconductor devices. A mode of a display panel manufactured in
accordance with Embodiments 1 through 9 will be described with
reference to FIGS. 24 and 25.
[0210] In FIG. 24, a substrate 901 is provided with a pixel portion
902, gate signal side driving circuits 901a and 901b, a data signal
side driving circuit 901c, an input/output terminal portion 908,
and a wire or group of wires 904. A shield pattern 905 may
partially overlap the gate signal side driving circuits 901a and
901b and the data signal side driving circuit 901c, as well as the
wire or group of wires 904 for connecting the driving circuits with
the input/output terminal portion 908. In this way, the area of the
frame region (the region surrounding the pixel portion) of the
display panel can be reduced. An FPC 903 is fixed to the
input/output terminal portion 908.
[0211] The present invention can be used in active elements
constituting the pixel portion 902, the gate signal side driving
circuits 901a and 901b, and the data signal side driving circuit
901c .
[0212] FIG. 25 shows an example of the structure of one pixel in
the pixel portion 902 shown in FIG. 24. The pixel has TFTs 801 to
803, which are a switching TFT, a reset TFT, and a driving TFT,
respectively, for controlling a light emitting element or liquid
crystal element of the pixel.
[0213] These TFTs have active layers 812 to 814. Each of the active
layers is placed between one edge of a depression portion 810 or
811 of the insulating film formed below the active layers and the
midpoint between the one edge and the other edge of the depression
portion. Gate wires 815 to 817 are formed in a layer above the
active layers 812 to 814. A passivation film and a planarization
film are formed on the gate wires. A data line 819, a power supply
line 820, other various wires 821 and 822, and a pixel electrode
823 are formed on the passivation film and the planarization
film.
[0214] This embodiment uses for the TFTs islands formed on the
bottom of depression portions. Instead, islands formed on the top
of projection portions may be used for the TFTs.
[0215] This embodiment can be combined freely with Embodiments 1
through 9.
Embodiment 11
[0216] The semiconductor device equipped with the TFT formed by the
present invention can be applied to various electronic apparatuses.
Examples of the electronic apparatuses are portable information
terminals (electronic books, mobile computers, cellular phones, or
the like), video cameras, digital cameras, personal computers, TV
receivers, cellular phones, projection display apparatuses, or the
like. Specific examples of these electronic apparatuses are shown
in FIGS. 26A to 26G.
[0217] FIG. 26A shows a display apparatus, which is composed of a
case 2001, a support base 2002, a display unit 2003, speaker units
2004, a video input terminal 2005, etc. The display apparatus of
the present invention is completed by using the semiconductor
device of the present invention to the display unit 2003. Since the
light emitting device having the light emitting element is
self-luminous, the device does not need back light and can make a
thinner display unit than liquid crystal display devices. The
display device refers to all display devices for displaying
information, including ones for personal computers, for TV
broadcasting reception, and for advertisement.
[0218] FIG. 26B shows a digital still camera, which is composed of
a main body 2101, a display unit 2102, an image receiving unit
2103, operation keys 2104, an external connection port 2105, a
shutter 2106, etc. The digital still camera of the present
invention is completed by using the semiconductor device of the
present invention to the display unit 2102.
[0219] FIG. 26C shows a notebook personal computer, which is
composed of a main body 2201, a case 2202, a display unit 2203, a
keyboard 2204, an external connection port 2205, a pointing mouse
2206, etc. The notebook personal computer of the present invention
is completed by using the semiconductor device of the present
invention to the display unit 2203.
[0220] FIG. 26D shows a mobile computer, which is composed of a
main body 2301, a display unit 2302, a switch 2303, operation keys
2304, an infrared port 2305, etc. The light emitting device
manufactured in accordance with the present invention can be
applied to the display unit 2302. The mobile computer of the
present invention is completed by using the semiconductor device of
the present invention to the display unit 2302.
[0221] FIG. 26E shows a portable image reproducing device equipped
with a recording medium (a DVD player, to be specific). The device
is composed of a main body 2401, a case 2402, a display unit A
2403, a display unit B 2404, a recording medium (DVD or the like)
reading unit 2405, operation keys 2406, speaker units 2407, etc.
The display unit A 2403 mainly displays image information whereas
the display unit B 2404 mainly displays text information. The light
emitting device manufactured in accordance with the present
invention can be applied to the display units A 2403 and B 2404.
The portable image reproducing device of the present invention is
completed by using the semiconductor device of the present
invention to the display units A 2403 and B 2404.
[0222] FIG. 26F shows a goggle type display (head mounted display),
which is composed of a main body 2501, display units 2502, and arm
units 2503. The goggle type display of the present invention is
completed by using the semiconductor device of the present
invention to the display units 2502.
[0223] FIG. 26G shows a video camera, which is composed of a main
body 2601, a display unit 2602, a case 2603, an external connection
port 2604, a remote control receiving unit 2605, an image receiving
unit 2606, a battery 2607, an audio input unit 2608, operation keys
2609, eye piece portion 2610 etc. The video camera of the present
invention is completed by using the semiconductor device of the
present invention to the display unit 2602.
[0224] FIG. 26H shows a cellular phone, which is composed of a main
body 2701, a case 2702, a display unit 2703, an audio input unit
2704, an audio output unit 2705, operation keys 2706, an external
connection port 2707, an antenna 2708, etc. The light emitting
device manufactured in accordance with the present invention can be
applied to the display unit 2703. If the display unit 2703 displays
white letters on black background, the cellular phone consumes less
power. The cellular phone of the present invention is completed by
using the semiconductor device of the present invention to the
display unit 2703.
[0225] As described above, the application range of the present
invention is so wide that it is applicable to electric apparatuses
of any field. This embodiment can be operated by combining with any
structure shown in Embodiments 1 through 10.
Embodiment 12
[0226] This embodiment describes the structure of a semiconductor
device of the present invention. FIG. 27 is a sectional view of a
semiconductor device of this embodiment.
[0227] A first insulating film 701 having projection portions 701a
and 701b is formed on a substrate 700. A first TFT 702 is formed on
the first insulating film 701. An island of the first TFT 702 is
formed on the first insulating film 701 between one edge of a
depression portion and the midpoint between the one edge and the
other edge of the depression portion. This depression portion is
created by the projection portions 701a and 701b.
[0228] A first interlayer insulating film 703 is formed to cover
the first TFT 702. On the first interlayer insulating film 703, a
first connection wire 705 and a wire 704 are formed. The wire 704
is electrically connected to the first TFT 702.
[0229] A second interlayer insulating film 706 is formed to cover
the wire 704 and the first connection wire 705. The second
interlayer insulating film 706 is an inorganic insulating film. If
the top face of the second interlayer insulating film is polished
by chemical mechanical polishing (CMP), a second insulating film
formed later can have more level surface and the crystallinity of a
semiconductor film to be formed on the second insulating film and
crystallized by laser light can be enhanced.
[0230] A second insulating film 707 is formed on the second
interlayer insulating film 706. The second insulating film 707 has
a projection portion 707a. A second TFT 708 is formed on the second
insulating film 707. An island of the second TFT 708 is formed on
the second insulating film 707 between one edge of a depression
portion and the midpoint between the one edge and the other edge
(not shown in the drawing) of the depression portion. This
depression portion is created by the projection portion 707a.
[0231] A third interlayer insulating film 709 is formed to cover
the second TFT 708. On the third interlayer insulating film 709, a
second connection wire 711 and a wire 710 are formed. The wire 710
is electrically connected to the second TFT 708. An embedded wire
(plug) 712 is formed between the first connection wire 705 and the
second connection wire 711 by Damascene process or the like.
[0232] A fourth interlayer insulating film 713 is foamed to cover
the wire 710 and the second connection wire 711.
[0233] In this embodiment, the first TFT 702 and the second TFT 708
overlap each other with an interlayer insulating film sandwiched
therebetween to form a so-called stack structure. The stack
structure TFTs of this embodiment can be used to build a CPU using
an LSI, memory devices (e.g., SRAM) of various logic circuits, a
counter circuit, a frequency divider circuit, etc.
[0234] This embodiment uses for the TFTs islands formed on the
bottom of depression portions. Instead, islands formed on the top
of projection portions may be used for the TFTs.
[0235] This embodiment can be combined freely with Embodiments 1
through 11.
Embodiment 13
[0236] This embodiment describes the energy density distribution of
a linear laser beam that is obtained by synthesizing plural
elliptical laser beams.
[0237] FIG. 28 shows the energy density distribution in 1/e.sup.2
width of a laser beam obtained by overlapping two elliptical laser
beams each measuring 400 .mu.m in major axis and 40 .mu.m in minor
axis. Measurements in the graph are all in mm (unit). The distance
between centers of adjacent beams is 0.255 mm.
[0238] In addition, FIG. 29 shows the energy density distribution
in 1/e.sup.2 width of a laser beam obtained by overlapping two
elliptical laser beams each measuring 400 .mu.m in major axis and
40 .mu.m in minor axis. Measurements in the graph are all in mm
(unit). The distance between centers of adjacent beams is 0.255
mm.
[0239] In addition, FIG. 30 shows the energy density distribution
in 1/e.sup.2 width of a laser beam obtained by overlapping four
elliptical laser beams each measuring 400 .mu.m in major axis and
40 .mu.m in minor axis. Measurements in the graph are all in mm
(unit). The distance between centers of adjacent beams is 0.215
mm.
[0240] In an elliptical laser beam, the energy density distribution
in the center line direction matches Gaussian distribution. On the
other hand, a laser beam obtained by overlapping plural elliptical
laser beams has an energy density distribution in the center line
direction which forms a waveform above a certain level as shown in
FIGS. 28, 29, and 30. Unlike an elliptical laser beam, it can be
said that the energy density distribution in the center line
direction of a laser beam obtained by synthesizing elliptical laser
beams is relatively uniform and linear.
[0241] The use of such laser beam having a linear energy density
distribution in the present invention makes it possible to form an
island that has uniform crystallinity.
[0242] This embodiment can be combined freely with Embodiments 1
through 12.
Embodiment 14
[0243] This embodiment gives a description on the concentration of
oxygen, nitrogen, carbon, and boron taken into a semiconductor film
irradiated with continuous wave laser light.
[0244] First, an amorphous silicon film is formed to a thickness of
1500 .ANG. on an insulating film that is formed of silicon
oxynitride. A nickel acetate solution is applied to the amorphous
silicon film and the film is heated at 500 to 650.degree. C.
Continuous wave laser light is then used to crystallize the film
and obtain a crystalline silicon film (poly-Si). The laser light
irradiation is conducted in the air in a clean room. A thin oxide
film is naturally formed (natural oxide film) on the surface of the
crystalline silicon film. Then an amorphous silicon film is formed
to cover the crystalline silicon film and the natural oxide
film.
[0245] In this state, secondary ion mass spectroscopy (SIMS) is
performed on the film. The atomic percentage profiles of oxygen,
nitrogen, carbon, and boron are shown in FIGS. 31 to 34,
respectively.
[0246] The oxygen concentration profile in the crystalline or
amorphous silicon film is measured by SIMS and the results are
shown in FIG. 31. The axis of ordinate shows the atomic percentage
of oxygen and the axis of abscissa shows the depth from the sample
surface. The solid line indicates the oxygen concentration of when
laser light irradiation process is carried out, and the dashed line
indicates the oxygen concentration of when laser light irradiation
process is not carried out. The graph also shows the ionic strength
of silicon with the axis of abscissa indicating the depth from the
sample surface. The oxygen concentration after laser light
irradiation is 2.times.10.sup.19 atoms/cm.sup.3 or lower. As FIG.
31 shows, the oxygen concentration in the silicon film is increased
by laser light irradiation.
[0247] The nitrogen concentration profile in the crystalline or
amorphous silicon film is measured by SIMS and the results are
shown in FIG. 32. The axis of ordinate shows the atomic percentage
of nitrogen and the axis of abscissa shows the depth from the
sample surface. The solid line indicates the nitrogen concentration
of when laser light irradiation process is carried out, and the
dashed line indicates the nitrogen concentration of when laser
light irradiation process is not carried out. The graph also shows
the ionic strength of silicon with the axis of abscissa indicating
the depth from the sample surface. The nitrogen concentration after
laser light irradiation is 1.times.10.sup.19 atoms/cm.sup.3 or
lower. As FIG. 32 shows, the nitrogen concentration in the silicon
film is increased by laser light irradiation.
[0248] The carbon concentration profile in the crystalline or
amorphous silicon film is measured by SIMS and the results are
shown in FIG. 33. The axis of ordinate shows the atomic percentage
of carbon and the axis of abscissa shows the depth from the sample
surface. The solid line indicates the carbon concentration of when
laser light irradiation process is carried out, and the dashed line
indicates the carbon concentration of when laser light irradiation
process is not carried out. The graph also shows the ionic strength
of silicon with the axis of abscissa indicating the depth from the
sample surface . The carbon concentration after laser light
irradiation is 5.times.10.sup.18 atoms/cm.sup.3 or lower. As FIG.
33 shows, the carbon concentration in the silicon film is increased
by laser light irradiation.
[0249] The boron concentration profile in the crystalline or
amorphous silicon film is measured by SIMS and the results are
shown in FIG. 34. The axis of ordinate shows the atomic percentage
of boron and the axis of abscissa shows the depth from the sample
surface. The solid line indicates the boron concentration of when
laser light irradiation process is carried out, and the dashed line
indicates the boron concentration of when laser light irradiation
process is not carried out. The graph also shows the ionic strength
of silicon with the axis of abscissa indicating the depth from the
sample surface. In FIG. 34, it seems that the boron concentration
in the silicon film is slightly increased by laser light
irradiation. The boron concentration is below the lowest level
detectable by SIMS before and after laser irradiation anyway and
the boron content in the film is very minute.
Embodiment 15
[0250] This embodiment describes the shape of an insulating film
and a relation between it and the thickness of a semiconductor film
formed on the insulating film.
[0251] FIG. 35A shows an insulating film 950 having projection
portions 950a and a semiconductor film 951 that is formed on the
insulating film. The semiconductor film 951 shown here has already
been crystallized by laser light.
[0252] As shown in FIG. 35A, a thickness Ht of the semiconductor
film 951 on the projection portions 950a is smaller than a
thickness Hb of the semiconductor film 951 on a depression portion
between the projection portions 950a. This is supposedly because
the semiconductor film temporarily melted by laser light
irradiation moves into the depression portion. Therefore it is
considered that the surface of the semiconductor film 951 is
leveled to a certain degree through laser light irradiation.
[0253] FIG. 35B shows an insulating film 960 having projection
portions 960a and a semiconductor film 961 which is formed on the
insulating film and which has a flat surface. In contrast to FIG.
35A where the semiconductor film 951 has depression and projection
on its surface, the surface of the semiconductor film in FIG. 35B
is leveled by laser light irradiation.
[0254] FIG. 35C is a sectional view of an insulating film in which
a width Wt of a projection portion is larger than a width Wb of a
depression portion in the direction perpendicular to the laser
light scanning direction. When a portion of a semiconductor film
that is on a projection portion and has excellent crystallinity is
used as an active layer of a TFT, the width Wt of a projection
portion is preferably larger than the width Wb of a depression
portion as shown in FIG. 35C because this reduces restrictions in
layout of an island.
[0255] This embodiment can be combined freely with Embodiments 1
through 14.
[0256] In the present invention, a semiconductor film crystallized
by laser light is patterned to remove a portion of the film around
the midpoint between edges of a depression portion or projection
portion. The remaining portion between one edge of a depression
portion or projection portion and the midpoint between the one edge
and the other edge of the depression portion or projection portion,
which has fewer grain boundaries and therefore has excellent
crystallinity, is effectively used as an active layer of a TFT.
This makes it possible to avoid forming a grain boundary in a
channel formation region of a TFT, thereby preventing grain
boundaries from lowering the mobility of the TFT greatly, from
lowering ON current, and from increasing OFF current. How far from
an edge of a depression portion or projection portion is to be
removed by patterning can be decided at designer's discretion.
[0257] The present invention runs laser light so as to obtain at
least the minimum degree of crystallization of a portion that has
to be crystallized, instead of irradiating the entire semiconductor
film with laser light. This structure saves time for laser
irradiation of portions that are removed by patterning after
crystallization of the semiconductor film and thereby greatly
shortens the processing time per substrate.
[0258] The crystallinity of a semiconductor film can be enhanced
more efficiently when plural laser beams are overlapped to
supplement one another's low energy density portions than when
using a single laser beam.
[0259] The position of a grain boundary in a semiconductor film
formed on an insulating film may be controlled by forming
depression and projection portions on a substrate itself through
etching, instead of forming depression and projection portions on
the insulating film.
* * * * *