U.S. patent application number 12/129142 was filed with the patent office on 2008-11-27 for image display device and method for manufacturing the same.
Invention is credited to Mutsuko HATANO, Kiyoshi OUCHI, Seongkee PARK, Takeshi SATO, Mitsuharu TAI.
Application Number | 20080290344 12/129142 |
Document ID | / |
Family ID | 37462232 |
Filed Date | 2008-11-27 |
United States Patent
Application |
20080290344 |
Kind Code |
A1 |
TAI; Mitsuharu ; et
al. |
November 27, 2008 |
Image Display Device And Method For Manufacturing The Same
Abstract
An image display device manufactured by using a polycrystalline
semiconductor film. The polycrystalline semiconductor film is
composed of crystal grains with a region free from crystal grain
boundaries of at least 2 .mu.m in width and at least 3 .mu.m in
length, small crystal grain boundary groups each composed of three
or more crystal grain boundaries arranged substantially in parallel
to each other and with an interval of not greater than 100 .mu.m
are included in a part of the region, and the small crystal grain
boundary groups are partially eliminated.
Inventors: |
TAI; Mitsuharu; (Kokubunji,
JP) ; HATANO; Mutsuko; (Kokubunji, JP) ; SATO;
Takeshi; (Kokubunji, JP) ; PARK; Seongkee;
(Yeongchen, KR) ; OUCHI; Kiyoshi; (Kodaira,
JP) |
Correspondence
Address: |
ANTONELLI, TERRY, STOUT & KRAUS, LLP
1300 NORTH SEVENTEENTH STREET, SUITE 1800
ARLINGTON
VA
22209-3873
US
|
Family ID: |
37462232 |
Appl. No.: |
12/129142 |
Filed: |
May 29, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11441021 |
May 26, 2006 |
7384810 |
|
|
12129142 |
|
|
|
|
Current U.S.
Class: |
257/64 ;
257/E21.111; 257/E21.134; 349/43; 438/30 |
Current CPC
Class: |
H01L 21/2026 20130101;
Y10S 438/949 20130101; H01L 27/1285 20130101; H01L 21/02691
20130101; H01L 21/02675 20130101; H01L 27/1296 20130101 |
Class at
Publication: |
257/64 ; 349/43;
438/30; 257/E21.111 |
International
Class: |
H01L 31/036 20060101
H01L031/036; G02F 1/136 20060101 G02F001/136; H01L 21/00 20060101
H01L021/00 |
Foreign Application Data
Date |
Code |
Application Number |
May 26, 2005 |
JP |
2005-153734 |
Claims
1. An image display device manufactured by using a polycrystalline
semiconductor film, wherein the polycrystalline semiconductor film
is composed of crystal grains with a region free from crystal grain
boundaries of at least 2 .mu.m in width and at least 3 .mu.m in
length, small crystal grain boundary groups each composed of at
least three crystal grain boundaries arranged substantially in
parallel to each other and with an interval of not greater than 100
.mu.m are included in a part of the region, and the small crystal
grain boundary groups are partially eliminated.
2. An image display device manufactured by using a polycrystalline
semiconductor film, wherein the polycrystalline semiconductor film
is composed of crystal grains with a region free from crystal grain
boundaries of at least 0.2 .mu.m in width and at least 2 .mu.m in
length, and in most of points where three crystal grain boundaries
cross one another, three angles formed between the adjacent crystal
grain boundaries are not smaller than 90.degree..
3. An image display device manufactured by using a polycrystalline
semiconductor film, wherein the polycrystalline semiconductor film
is composed of crystal grains with a region free from crystal grain
boundaries of at least 2 .mu.m in width and at least 3 .mu.m in
length, and a plurality of protrusions are disposed on the surface
of the polycrystalline semiconductor film.
4. An image display device manufactured by using polycrystalline
semiconductor film, wherein, in the polycrystalline semiconductor
film as set forth in claim 3, intervals between adjacent
protrusions are not shorter than 0.10 .mu.m and not longer than
0.80 .mu.m.
5. An image display device manufactured by using a polycrystalline
semiconductor film, wherein, in the polycrystalline semiconductor
film as set forth in claim 3, a density of the protrusions is not
lower than 5 pieces/.mu.m.sup.2 and not higher than 20
pieces/.mu.m.sup.2.
6. An image display device manufactured by using a polycrystalline
film, wherein, in the polycrystalline semiconductor film as set
forth in claim 3, the protrusions are distributed on virtual lines
extending in the same direction.
7. An image display device manufactured by using a polycrystalline
semiconductor film, wherein, in the polycrystalline semiconductor
film as set forth in claim 6, a distance between the virtual lines
is not shorter than 0.10 .mu.m and not longer than 0.80 .mu.m.
8. An image display device manufactured by using a polycrystalline
semiconductor film, wherein, in the polycrystalline semiconductor
film as set forth in claim 3, groups of the protrusions formed on
crystal grain boundaries and groups of the protrusions formed in
the regions where there is no crystal grain boundary are mixed
together.
9. An image display device manufactured by using a polycrystalline
semiconductor film, wherein, in the polycrystalline semiconductor
film as set forth in claim 3, an average value of a difference in
height in the film is not smaller than 2 .mu.m and not larger than
50 .mu.m.
10. An image display device manufactured by using a polycrystalline
semiconductor film, wherein the polycrystalline semiconductor film
is composed of crystal grains with a region free from crystal grain
boundaries of at least 0.2 .mu.m in width and at least 3 .mu.m in
length, and each boundary connecting the points where three crystal
grain boundaries cross one another has windings.
11. An image display device manufactured by using a polycrystalline
semiconductor film, wherein, in the polycrystalline semiconductor
film as set forth in claim 10, each of the windings is composed of
a set of curves having an arc length of not greater than 1 .mu.m
and a curvature radius of not greater than 20 .mu.m.
12. An image display device manufactured by using a polycrystalline
semiconductor film, wherein, in the polycrystalline semiconductor
film as set forth in claim 1, crystal grains with a crystal
orientation of [100] in a direction normal to the film surface are
included at an area ratio of at least 20%, crystal grains with a
crystal orientation of [111] in a direction normal to the film
surface are included at an area ratio of not greater than 10%, and
crystal grains with crystal orientations [110], [100] and [111]
indirection normal to the film surface are included and a ratio of
the area occupied by the crystal grains with each crystal
orientation is not greater than 50%.
13. An image display device manufactured by using a polycrystalline
semiconductor film, wherein, in the polycrystalline semiconductor
film as set forth in claim 2, crystal grains with crystal
orientation of [110] in a direction normal to the film surface are
included at an area ratio of at least 20%, crystal grains with a
crystal orientation of [100] in a direction normal to the film
surface are included at an area ratio of at least 20%, crystal
grains with a crystal orientation of [111] in a direction normal to
the film surface are included at an area ratio of not greater than
10%, and crystal grains with crystal orientations of [110], [100]
and [111] in direction normal to the film surface are included, and
a ratio of the area occupied by the crystal grains with each
crystal orientation is not greater than 50%.
14. An image display device manufactured by using a polycrystalline
semiconductor film, wherein, in the polycrystalline semiconductor
film as set forth in claim 3, crystal grains with a crystal
orientation of [110] in a direction normal to the film surface are
included at an area ratio of at least 20%, crystal grains with a
crystal orientation of [100] in a direction normal to the film
surface are included at an area ratio of at least 20%, crystal
grains with a crystal orientation of [111] in a direction normal to
the film surface are included at an area ratio of not greater than
10%, and crystal grains with crystal orientations of [110], [100]
and [111] in direction normal to the film surface are included, and
a ratio of the area occupied by the crystal grains with each
crystal orientation is not greater than 50%.
15. An image display device manufactured by using a polycrystalline
semiconductor film, wherein, in the polycrystalline semiconductor
film as set forth in claim 1, crystal grains with crystal
orientations [110], [100] and [111] in direction of the major axis
of the polycrystalline semiconductor film are included, the crystal
grains with a crystal orientation of [100] in a direction of the
major axis are included at an area ratio of at least 30%, and a
ratio of the area occupied by the crystal grains with each crystal
orientation is not greater than 70%.
16. An image display device manufactured by using a polycrystalline
semiconductor film, wherein, in the polycrystalline semiconductor
film as set forth in claim 2, crystal grains with crystal
orientations of [110], [100] and [111] in a direction of the major
axis are included, crystal grains with a crystal orientation of
[100] in a direction of the major axis are included at an area
ratio of at least 30%, and a ratio of the area occupied by the
crystal grains with each crystal orientation is not greater than
70%.
17. An image display device manufactured by using a polycrystalline
semiconductor film, wherein, in the polycrystalline semiconductor
film as set forth in claim 3, crystal grains with crystal
orientations of [110], [100] and [111] in direction of the major
axis are included, crystal grains with a crystal orientation of
[100] in a direction of the major axis are included at an area
ratio of at least 30%, and a ratio of the area occupied by the
crystal grains with each crystal orientation is not greater than
70%.
18. An image display device manufactured by using a polycrystalline
semiconductor film, wherein, in the polycrystalline semiconductor
film as set forth in claim 1, crystal grains with a crystal
orientation [110] in a direction of the minor axis of the
polycrystalline semiconductor film are included at an area ratio of
at least 25% and not greater than 50%, crystal grains with a
crystal orientation of [100] in a direction of the minor axis are
included at an area ratio of not greater than 30%, and crystal
grains with a crystal orientation of [111] in a direction of the
minor axis are included at an area ratio of not greater than
30%.
19. An image display device manufactured by using a polycrystalline
semiconductor film, wherein, in the polycrystalline semiconductor
film as set forth in claim 2, crystal grains with a crystal
orientation [110] in a direction of the minor axis of the
polycrystalline semiconductor film are included at an area ratio of
at least 25% and not greater than 50%, crystal grains with a
crystal orientation of [100] in a direction of the minor axis are
included at an area ratio of not greater than 30%, and crystal
grains with a crystal orientation of [111] in a direction of the
minor axis are included at an area ratio of not greater than
30%.
20. An image display device manufactured by using a polycrystalline
semiconductor film, wherein, in the polycrystalline semiconductor
film as set forth in claim 3, crystal grains with a crystal
orientation [110] in a direction of the minor axis of the
polycrystalline semiconductor film are included at an area ratio of
at least 25% and not greater than 50%, crystal grains with a
crystal orientation of [100] in a direction of the minor axis are
included at an area ratio of not greater than 30%, and crystal
grains with a crystal orientation of [111] in a direction of the
minor axis are included at an area ratio of not greater than
30%.
21. An image display device, comprising a pixel circuit of an image
display portion, a drive circuit for driving the pixel circuit, and
an integrated circuit group or processing and converting an
external input signal to a signal needed for image display and for
transferring the signal to the drive circuit, and the pixel
circuit, the drive circuit and the integrated circuit group are
disposed on the same insulating substrate wherein: each of the
circuits is composed of a device including a plurality of thin film
transistors having a semiconductor film as an active layer; and a
part of the thin-film transistor active layer to constitute the
drive circuit and the integrated circuit group grows crystal grains
by scanning of a first energy beam in a direction along the
scanning and is turned to polycrystalline, a first crystal grain is
composed of a polycrystalline semiconductor film by decreasing
defects within the crystal grain by a second high-speed heat
treatment, and an other part of the thin-film transistor active
layer is composed of a polycrystalline semiconductor film turned to
polycrystalline by the second high-speed heat treatment.
22. An image display device according to claim 21, wherein a part
of the thin-film transistors to constitute the drive circuit and
the integrated circuit group has an electric field effect mobility
of at least 300 cm.sup.2/Vs and is driven at a standard voltage of
not greater than 5 V, and an other part of the thin-film
transistors has an electric field effect mobility of 1 cm.sup.2/Vs
to 100 cm.sup.2/Vs and are driven at standard voltage of at least
5V.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This application is a continuation of U.S. application Ser.
No. 11/441,021, filed May 26, 2006, the contents of which are
incorporated herein by reference.
FIELD OF THE INVENTION
[0002] The present invention relates to an image display device and
a method for manufacturing the same, and particularly relates to an
image display device such as a liquid crystal display, an organic
EL (Electro-Luminescence) display, an inorganic EL display, an EC
(Electro-Chromic) display or the like, using semiconductor thin
film transistors manufactured in a low-temperature process and for
constituting drive circuits (drivers) and their peripheral circuits
for use in the image display device, and a method for manufacturing
the image display device.
DESCRIPTION OF THE BACKGROUND ART
[0003] Thin film transistors (TFT) whose channels are formed out of
polycrystalline semiconductors have been developed as pixels and
pixel drive circuit devices in image display devices such as active
matrix type liquid crystal displays or organic/inorganic EL
displays, or image sensors. The polycrystalline semiconductor TFTs
are advantageous due to their high driving capacity as compared
with other drive circuit devices. Thus, peripheral drive circuits
can be mounted on one and the same glass substrate together with
pixels. As a result, it can be expected to customize circuit
specification, to reduce the cost due to simultaneous progress of a
process for designing pixels and a process for forming the pixels,
or to increase the reliability due to avoidance of mechanical
vulnerability in a connection portion between a drive LSI and each
pixel. The displays will be also referred to as display devices or
image display devices herein.
[0004] Polycrystalline semiconductor TFTs for a liquid crystal
display are formed on a glass substrate in order to reduce the
cost. In a process for forming the TFTs on the glass substrate, the
heat resistance temperature of the glass defines the process
temperature. An ELA (Excimer Laser Annealing) method for melting
and recrystallizing a precursor semiconductor layer with an excimer
laser has prevailed as a method for forming a high-quality
polycrystalline semiconductor thin film without thermally damaging
the glass substrate. The driving capacity of a polycrystalline
semiconductor TFT obtained in this method is improved to be 100 or
more times as high as that of a TFT whose channel is made of an
amorphous semiconductor and which has been used in a background-art
liquid crystal display. It is therefore possible to mount a part of
circuits such as drivers on the glass substrate. In order to mount
higher-performance integrated circuits, however, it is necessary to
realize polycrystalline semiconductor TFTs having higher driving
capacity.
[0005] As a method for forming polycrystalline semiconductor TFTs
having higher driving capacity, for example, there is a method in
which a semiconductor thin film is scanned relatively with a CW
laser beam continuously outputting energy or a pulsed laser beam
with a longer time than that in the ELA method, so that the
semiconductor thin film is irradiated with an energy beam
outputting energy continuously or for a fixed time, as disclosed in
Non-Patent Document 1 or Non-Patent Document 2. When the melting
time of a semiconductor layer is prolonged, crystal grains are
grown in the laser scanning direction. Thus, it is possible to
obtain a polycrystalline semiconductor thin film having a large
crystal grain size and a uniform grain width and having a flat
surface. Hereinafter, growth of crystal grains substantially in the
laser scanning direction will be also referred to as lateral
growth.
[0006] As disclosed in Non-Patent Document 3 or Non-Patent Document
4, repetitious multistage irradiation with a pulsed laser beam with
a short time as in the ELA method may be performed with the
irradiation position being shifted while scanning with the laser.
Thus, crystal grains produced by laser irradiation in the first
stage are used as nuclei so that crystals are grown laterally. When
the crystals are connected with one another, it is possible to
obtain a polycrystalline semiconductor thin film having a large
crystal gain size in the laser scanning direction and a uniform
grain width.
[0007] Polycrystalline semiconductor TFTs manufactured in these
background-art method shave driving capacity in their N channels
two or three or more times as high as that of polycrystalline
semiconductor TFTs manufactured by an excimer laser. Thus, more
peripheral drive circuits can be mounted on one and the same glass
substrate together with pixels.
[0008] Non-Patent Document 1: International Electron Devices
Meeting (Washington D.C., 2001) pp. 747-751
[0009] Non-Patent Document 2: Society For Information Display
International Symposium Digest 2002 pp. 158-161
[0010] Non-Patent Document 3: Society For Information Display
International Symposium Digest 2004, pp. 868-871
[0011] Non-Patent Document 4: IEEE Electron Device Letters vol. 19
1998 pp. 306-308
[0012] Patent Document 1: JP-A-2004-22648
[0013] In a polycrystalline semiconductor thin film grown laterally
as typified by Non-Patent Documents 1 to 4, crystal grain
boundaries are formed substantially in parallel with the growing
direction. On the other hand, in a polycrystalline semiconductor
thin film formed in the ELA method according to the background art,
crystal grain boundaries have random directions. The crystal grain
boundaries behave as inhibitors of electric conduction. When the
current direction is parallel to the laterally growing direction in
the former polycrystalline semiconductor thin film, the crystal
grain boundaries are no longer primary inhibitors of electric
conductions but show a good electric conduction characteristic.
However, the crystal grains are not single crystals in a strict
sense but contain many crystal defects such as dislocations,
stacking defects, point defects, etc. Of these crystal grains,
there are some serving as inhibitors of electric conductions.
[0014] As crystal grain boundaries often observed in a
polycrystalline semiconductor thin film grown laterally, small
crystal grain boundary groups each composed of three or more linear
crystal grain boundaries arranged in parallel with one another and
in an interval not larger than 100 nm are distributed. According to
the ELA method, the effect of crystal grain boundaries inhibiting
electric conduction is so great that these small crystal grain
boundary groups as inhibitors do not manifest themselves. However,
in a polycrystalline semiconductor thin film grown laterally, the
small crystal grain boundary groups become one of main factors in
deterioration of the characteristic of polycrystalline
semiconductor TFTs and in increase of variation among the devices.
For this reason, the performance of the polycrystalline
semiconductor TFTs is not equal to the performance of
mono-crystalline semiconductor TFTs, so that circuits the
polycrystalline semiconductor TFTs can constitute are limited.
[0015] In order to mount drive circuits or peripheral circuits
(hereinafter also referred to as peripheral drive circuits) on one
and the same glass substrate together with pixels, TFTs having
different specifications have to be formed on the same glass
substrate together. For example, in a liquid crystal display
device, TFTs constituting a pixel circuit thereof have to satisfy
properties of a low leak current and a high withstand voltage. On
the other hand, TFTs constituting a peripheral drive circuit for
processing an external input signal and converting the signal into
an analog signal are requested to have a high on-current, a low
threshold value, a low variation and a steep rising edge. There is
no polycrystalline semiconductor thin film for supplying TFTs
satisfying these specifications simultaneously. It is therefore
necessary to produce polycrystalline semiconductor thin films
having different film qualities on one and the same glass
substrate.
[0016] As a method in which polycrystalline semiconductor thin
films having difference film qualities are produced on one and the
same glass substrate, there is a method including a first
irradiation step in which a silicon material is irradiated with a
CW laser and a second irradiation step in which at least a part of
the silicon material is irradiated with a pulsed laser beam in an
oxygen-containing atmosphere, wherein the film surface is oxidized
and formed into a thin film by abrasion in the first and second
irradiation steps, as disclosed in Patent Document 1. According to
this method, it is supposed that two kinds of polycrystalline
semiconductor thin films having different film thicknesses can be
produced on one and the same glass substrate. In this method,
however, the polycrystalline semiconductor thin film obtained in
the first irradiation step is not reformed so that the inhibitors
of electric conduction as described above cannot be removed. It is
also difficult to control the film thickness by abrasion. In
addition, in order to obtain the effect of reduction in leak
current according to the aforementioned Patent Document 1, it is
more effective to shield an active layer from backlight by use of a
light shielding film or the like than to reduce the film thickness
of the polycrystalline semiconductor thin film so as to control the
effect.
SUMMARY OF THE INVENTION
[0017] An object of the present invention is to provide a
polycrystalline semiconductor thin film grown laterally, in which
crystal defects in crystal grains and small crystal grain boundary
groups are reduced, and a polycrystalline semiconductor TFT having
driving capacity high enough to form a high-performance integrated
circuit by use of the polycrystalline semiconductor thin film.
Another object of the present invention is to provide a method in
which polycrystalline semiconductor thin films having different
film qualities are formed together on one and the same glass
substrate so that TFTs having different specifications are formed
for pixel circuits and peripheral drive circuits respectively. A
further object of the present invention is to provide an image
display device manufactured by use of the method.
[0018] Of a precursor semiconductor layer, only a region where TFTs
for forming high-performance circuits will be disposed is
irradiated with a first energy beam so that crystal grains are
grown laterally and made polycrystalline. Further, a second rapid
thermal treatment is performed all over the surface of a glass
substrate so as to reduce defects in the crystal grains in the
region made polycrystalline by the irradiation with the energy
beam, while making the precursor semiconductor layer
polycrystalline in the other region so as to obtain a
polycrystalline semiconductor thin film lower in quality than the
polycrystalline region grown laterally.
[0019] TFTs formed in the low-quality polycrystalline region are
low in on-current. However, the leak current in the TFTs is low due
to high resistance, so that the withstand voltage can be increased.
According to the manufacturing method of the present invention,
high-performance circuit TFTs having properties of a high
on-current, a low threshold value, a low variation and a steep
rising edge, and TFTs having properties of a low leak current and a
high withstand voltage can be formed on the same glass substrate.
Thus, a high-quality image display device can be manufactured by
use of this method.
[0020] According to the present invention, it is possible to obtain
TFTs having higher performance than polycrystalline semiconductor
TFTs obtained in the background-art laterally growing method, so
that peripheral drive circuits having more functions than in the
background art can be formed on one and the same glass substrate
together with pixel circuits.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] FIG. 1A is a conceptual view showing a state in which only a
region where TFTs constituting a high-performance circuit will be
disposed in a precursor semiconductor layer is irradiated with a
first energy beam so as to be poly-crystallized while growing
crystal grains laterally;
[0022] FIG. 1B is a conceptual view showing a method for further
performing a second rapid thermal treatment upon the laterally
grown and poly-crystallized region so as to obtain a high-quality
polycrystalline semiconductor thin film in which defects in crystal
grains have been reduced while simultaneously obtaining a
low-quality polycrystalline semiconductor thin film capable of
forming low-leak-current and high-withstand-voltage devices in the
other region;
[0023] FIG. 2A is a schematic view showing a case where an output
beam of a solid or gas CW laser or a temporally (temporarily)
pulsed beam of the output beam of the CW laser is used as a first
energy beam LSR while a pulsed energy beam of an excimer laser or
RTA is used for a second rapid thermal treatment RTT;
[0024] FIG. 2B is a view for explaining an example where the first
energy beam LSR and the second rapid thermal treatment RTT are
carried out by one and the same apparatus;
[0025] FIG. 2C is a view for explaining another example where the
first energy beam LSR and the second rapid thermal treatment RTT
are carried out by one and the same apparatus;
[0026] FIG. 3 is an explanatory view illustratively showing a
scanning electron microscope image of a plane of a laterally grown
polycrystalline semiconductor film;
[0027] FIG. 4 is an explanatory view illustratively showing a
scanning electron microscope image of a plane of the laterally
grown polycrystalline semiconductor film including small crystal
grain boundary groups;
[0028] FIG. 5 is an explanatory view illustratively showing a
scanning electron microscope image of a plane of a high-quality
polycrystalline semiconductor film;
[0029] FIG. 6A is an explanatory view illustratively showing a
transmission electron microscope image of a plane of a laterally
grown polycrystalline semiconductor film PSI when a long-time
pulsed laser beam is selected as the energy beam LSR in the same
manner as in FIG. 3;
[0030] FIG. 6B is an explanatory view illustratively showing a
transmission electron microscope image of a plane of a high-quality
polycrystalline semiconductor film PSIHQ when irradiation with a
low-energy-density excimer laser beam is selected as a method of
the rapid thermal treatment in the same manner as in FIG. 5;
[0031] FIG. 7 is a conceptual view showing definitions of a triple
point and three angles of the triple point;
[0032] FIG. 8 is a bird's-eye view in which a result of analysis of
a surface of the high-quality polycrystalline semiconductor film by
AFM is visualized;
[0033] FIG. 9 is an explanatory view illustratively showing a
transmission electron microscope image of a cut plane of the
high-quality polycrystalline semiconductor film perpendicular to
the laterally growing direction;
[0034] FIG. 10 is a conceptual view of a crystal grain boundary
having windings;
[0035] FIG. 11 is an explanatory view illustratively showing a
scanning electron microscope image of a plane of a low-quality
polycrystalline semiconductor film;
[0036] FIG. 12 is a graph showing a relationship between the
crystal grain size of a polycrystalline semiconductor film
irradiated with an excimer laser beam and the energy density of the
excimer laser beam;
[0037] FIG. 13A is a view for explaining a region which has been a
laterally grown polycrystalline semiconductor film PSI;
[0038] FIG. 13B is an explanatory view illustratively showing a
surface scanning electron microscope image of a surface of a
low-quality polycrystalline semiconductor film PSILQ;
[0039] FIG. 14A is a view for explaining a region which has been a
laterally grown polycrystalline semiconductor film PSI;
[0040] FIG. 14B is a view for explaining a region of the
low-quality polycrystalline semiconductor film PSILQ;
[0041] FIG. 15 is a conceptual view showing circuits for an image
display device formed on a glass substrate;
[0042] FIG. 16 is a schematic view of an embodiment where an image
display device according to the present invention is applied to a
liquid crystal display device;
[0043] FIG. 17 is an exploded perspective view for explaining an
example of a configuration of an organic EL display device using an
image display device according to the present invention;
[0044] FIG. 18 is a plan view of the organic EL display device in
which constituent parts shown in FIG. 17 are integrated;
[0045] FIG. 19 is a view showing an example of an application to an
image display portion of a monitor for use in a personal computer
or a TV set;
[0046] FIG. 20 is a view showing an example of an application to an
image display portion of a cellular phone MOB;
[0047] FIG. 21 is a view showing an example of an application to an
image display portion of a digital portable terminal PDA; and
[0048] FIG. 22 is a view showing an example of an application to a
view finder portion of a projector CAM.
DETAILED DESCRIPTION OF THE INVENTION
[0049] The best modes for carrying out the present invention will
be described below in detail with reference to the drawings and in
conjunction with embodiments.
First Embodiment
[0050] FIG. 1A is a conceptual view showing the concept that only a
region where TFTs for forming a high performance circuit will be
disposed in a precursor semiconductor layer is irradiated with a
first energy beam so that crystal grains are made polycrystalline
while being grown laterally. FIG. 1B is a conceptual view showing a
method for performing a second rapid thermal treatment on the
laterally grown and poly-crystallized region so as to obtain a
high-quality polycrystalline semiconductor thin film in which
defects in crystal gains have been reduced by performed, while
simultaneously obtaining a low-quality polycrystalline
semiconductor thin film which can form devices with a low leak
current and a high withstand voltage in the other region.
[0051] As shown in FIG. 1A, an insulating film UCL serving as an
undercoat is formed on a glass substrate GLS, and a precursor
semiconductor film PCF is formed thereon. The precursor
semiconductor film PCF may be an amorphous semiconductor film
formed by CVD (Chemical Vapor Deposition), a film made
polycrystalline by irradiating the whole surface of the amorphous
semiconductor film with an excimer laser beam, or a polycrystalline
semiconductor film produced in another method (for example, formed
by CVD). The precursor semiconductor film PCF is irradiated with an
energy beam (typically a laser beam) (in this embodiment, a laser
beam is used) LSR. The beam is moved (scanned) in a crystal growing
direction LSD so as to grow crystals in the scanning direction
(laterally). Thus, a laterally grown polycrystalline semiconductor
film PSI having a large crystal grain size in the laser scanning
direction, having a uniform grain width and having a flat surface
is produced.
[0052] Methods for obtaining the laterally grown polycrystalline
semiconductor film PSI include a method in which a beam output from
a solid or gas CW laser or a temporally pulsed beam of the beam
output from the CW laser is scanned in the crystal growing
direction LSD, and a method in which a position to be irradiated is
irradiated with a beam from a light source for outputting energy,
for example, from a pulse laser such as an excimer laser, for about
several tens of nanoseconds in a plurality of stages while the
output beam is shifted in the crystal growing direction LSD, so as
to grow crystals in such a manner that polycrystals generated in a
previous stage of irradiation are extended in the shifting
direction LSD in the next stage.
[0053] In addition to these methods, thermal energy may be applied
to a precursor semiconductor film doped with a catalyst metal such
as Ni so as to obtain a laterally grown polycrystalline
semiconductor film. Alternatively, the distribution of intensity of
a pulsed beam from an excimer laser or the like may be modulated on
a plane of a sample two-dimensionally, or the precursor
semiconductor film or a sample structure lying thereunder may be
devised to provide a thermal distribution in the crystal growing
direction LSD on the precursor semiconductor film. Thus, a
laterally grown polycrystalline semiconductor film can be obtained.
Here, description will be made about an embodiment of the method in
which a beam output from a solid or gas CW laser or a temporally
pulsed beam of the beam output from the CW laser is scanned in the
crystal growing direction.
[0054] In order to produce the laterally grown polycrystalline
semiconductor film PSI, the conditions of irradiation with the
energy beam LSR are important. The laterally growing speed to
accelerate the growth of crystals in the scanning direction is
about several meters per second. The beam width and the scanning
speed of the energy beam are defined by the laterally growing
speed. When the beam width of the energy beam is about 10 .mu.m and
the scanning speed is several hundreds of millimeters per second,
the lateral growth is accelerated.
[0055] In this event, pay attention to one place in an irradiated
region MSI. Then, it is supposed that the time when the energy beam
will stay in the place will be several hundreds of nanoseconds to
several hundreds of microseconds, and the melting time of the
semiconductor layer will be substantially equal to the staying time
of the energy beam. A semiconductor oxide film or a semiconductor
nitride film formed by CVD is typically used as the insulating film
UCL serving as an undercoat.
[0056] Next, as shown in FIG. 1B, a rapid thermal treatment RTT is
performed on the laterally grown polycrystalline semiconductor film
PSI and the precursor semiconductor film PCF so as to perform
reduction of defects in crystal grains of the laterally grown
polycrystalline semiconductor film PSI and poly-crystallization of
the precursor semiconductor film PCF concurrently. Thus, a
high-quality polycrystalline semiconductor film PSIHQ and a
low-quality polycrystalline semiconductor film PSILQ are obtained.
In this event, the rapid thermal treatment RTT may be performed in
a method in which irradiation with an excimer laser beam is
performed once or a plurality of times or in a RTA (Rapid Thermal
Annealing) method, or by use of the same apparatus as the apparatus
for generating the energy beam LSR.
[0057] There is no problem when the whole of the sample can be
treated at a time in the rapid thermal treatment RTT. However,
there may be a limit in the range of the treatment. In such a case,
it will be go well if the rapid thermal treatment RTT is performed
in a plurality of stages while shifting in a desired direction RTD
as shown in FIG. 1B. In this event, the direction RTD does not have
to be parallel to the direction LSD, but a desired direction may be
selected as the direction RTD.
[0058] FIGS. 2A-2C are schematic views showing the variation per
time of the energy density of the first energy beam, that of the
second rapid thermal treatment, and the conditions of the sample.
FIGS. 2A-2C include graphs showing the variation per time of the
energy density of the first energy beam LSR and that of the second
rapid thermal treatment RTT with respect to one point MPT on a
semiconductor layer SAMP to which the method according to the first
embodiment is applied. The left diagram inserted in the upper part
of each graph shows the condition of the semiconductor layer SAMP
near the point MPT at a time T1 when the first energy beam LSR
passes the point MPT. The right diagram inserted in the upper part
of each graph shows the condition of the semiconductor layer SAMP
near the point MPT at a time T2 when the second rapid thermal
treatment RTT passes the point MPT.
[0059] FIG. 2A is a schematic view showing a case where a beam
output from a solid or gas CW laser or a temporally pulsed beam of
the beam output from the CW laser is used as the first energy beam
LSR, and a pulsed energy beam such as an excimer laser beam or RTA
is used as the second rapid thermal treatment RTT. The staying time
DURA of the first energy beam is just as described above. The
treatment time DURB of the second rapid thermal treatment RTT
depends on the method adopted. In the case of ELA, the treatment
time DURB is several nanoseconds to several tens of
nanoseconds.
[0060] There is no upper limit in an interval INTVL between the two
thermal treatments if the interval INTVL is not smaller than 0
second. The semiconductor layer at the point MPT is in liquid phase
at the time T1 and the time T2. The liquid phases at the time T1
and the time T2 are included in liquid phase regions LSRA and RTTA
respectively. However, when the second rapid thermal treatment RTT
is RTA, the region RTTA becomes a solid phase region. The phase of
the semiconductor layer at the point MPT at a time T3 in the
interval INTVL is a solid phase. The directions LSD and RTD do not
have to be parallel, as described previously.
[0061] FIGS. 2B and 2C are explanatory views showing examples where
the first energy beam LSR and the second rapid thermal treatment
RTT are performed by use of one and the same apparatus. Also in
this event, the variations per time of the energy densities in FIG.
2A can be reproduced if a control system and an optical system are
devised. From the point of view of throughput or the like, it is
however conceived that the examples of FIGS. 2B and 2C are
preferred.
[0062] In these examples, one and the same beam is divided by an
optical system so as to be spatially divided into the first energy
beam LSR and an energy beam for the second rapid thermal treatment
RTT, with which a sample SAMP is irradiated. In the same manner as
in FIG. 2A, there is no upper limit in an interval INTVL between
the two thermal treatments if the interval INTVL is not smaller
than 0 second. The semiconductor layer at one point MPT is in
liquid phase at a time T1 and a time T2. The liquid phases at the
time T1 and the time T2 are included in liquid phase regions LSRA
and RTTA respectively. When a spatially dividing distance DEV is
large as in FIG. 2B, the phase of the semiconductor layer at the
point MPT will be a solid phase at a time T3 in the interval INTVL.
When the distance DEV is small as in FIG. 2C, the phase will be a
liquid phase. The directions LSD and RTD do not have to be
parallel, but in view from the system configuration, it is desired
that the directions LSD and RTD are parallel.
[0063] In either example, the energy density RTTF of the rapid
thermal treatment RTT is set to be lower than the energy density
LSRF of the energy beam LSR. If the energy density of the rapid
thermal treatment RTT is high, a semiconductor thin film MSI2 in
the irradiated region will melt perfectly so that the laterally
grown polycrystalline semiconductor film PSI will perfectly lose
its crystal history made before the rapid thermal treatment RTT.
Thus, the semiconductor thin film MSI2 will be a low-quality
polycrystalline semiconductor film.
[0064] As for the optimum energy density in the rapid thermal
treatment RTT, the conditions are set as follows. When an excimer
laser or the same apparatus as the apparatus for generating the
energy beam LSR is selected, the conditions are set so that at
least the surface portion of the semiconductor thin film MSI2 in
the region irradiated with the beam will melt due to the treatment.
When RTA is selected, the conditions are set so that at least the
surface portion of the semiconductor thin film MSI2 will be
solid.
[0065] A method of repeating a known oxidization and film-formation
step and a photo-lithography step may be used as the method for
forming thin film transistors using the high-quality
polycrystalline semiconductor film PSIHQ and the low-quality
polycrystalline semiconductor film PSILQ described in FIG. 1A or 1B
as active layers.
[0066] TFTs using the high-quality polycrystalline semiconductor
film PSIHQ as their active layer have high-performance properties,
and there is a small variation among the devices. It is therefore
possible to use the TFTs for forming high-performance circuits. On
the other hand, TFTs using the low-quality polycrystalline
semiconductor film PSILQ as their active layer have high resistance
so as to be low in leak current. Deterioration can be suppressed
even if a high voltage is applied to such a TFT. It is therefore
possible to use the TFTs for forming pixel circuits. According to
the method of this embodiment, a system having more functions than
in the background art can be constructed on one and the same glass
substrate together with image circuits.
[0067] FIG. 3 is an explanatory view illustratively showing a
scanning electron microscope image of a plane of the laterally
grown polycrystalline semiconductor film PSI when a long-time
pulsed laser beam is selected as the energy beam LSR. The energy
beam moves in the direction LSD on the sample surface relatively to
the sample. In this event, the energy beam may be moved, or the
sample may be moved in a direction opposite to the direction LSD.
Crystal grain boundaries GBLG are formed to approach parallelism to
the moving direction LSD of the energy beam.
[0068] The laterally grown polycrystalline semiconductor film PSI
is characterized in that at least 60% thereof in area ratio is
occupied by crystal grains LGG having a region free from crystal
grain boundary 0.2 .mu.m or more wide and 3 .mu.m or more long. The
crystal grain boundaries are substantially flat. From analysis by
AFM (Atomic Force Microscope), it has been proved that the
difference in height in the laterally grown polycrystalline
semiconductor film PSI is not larger than 5 nm.
[0069] FIG. 4 is an explanatory view illustratively showing a
scanning electron microscope image of a plane of another region of
the laterally grown polycrystalline semiconductor film PSI when a
long-time pulsed laser beam is selected as the energy beam LSR in
the same manner as in FIG. 3. Small crystal grain boundary groups
FGBS each consisting of three or more crystal grain boundaries
arranged substantially in parallel to one another and in an
interval not larger than 100 nm from one another are distributed
among large crystal grains LGG.
[0070] FIG. 5 is an explanatory view illustratively showing a
scanning electron microscope image of a plane of the high-quality
polycrystalline semiconductor film PSIHQ when irradiation with a
low-energy-density excimer laser beam is selected as a method of
rapid thermal treatment for reduction of defects in crystal grains
of the laterally grown polycrystalline semiconductor film PSI and
poly-crystallization of the precursor semiconductor film PCF. The
small crystal grain boundary groups FGBS shown in FIG. 4 are
reformed into small crystal grain boundary groups DFGBS as shown in
FIG. 5. After the rapid thermal treatment, a large number of sites
DFGB1 where linear boundaries disappear halfway or sites DFGB2
where the linear boundaries are disconnected intermittently are
produced in each small crystal grain boundary group FGBS. This is
because the small crystal grain boundary groups FGBS disappear due
to the rapid thermal treatment. Thus, the crystallinity is
improved.
[0071] FIG. 6A is an explanatory view illustratively showing a
transmission electron microscope image of a plane of the laterally
grown polycrystalline semiconductor film PSI when a long-time
pulsed laser beam is selected as the energy beam LSR in the same
manner as in FIG. 3. FIG. 6B is an explanatory view illustratively
showing a transmission electron microscope image of a plane of the
high-quality polycrystalline semiconductor film PSIHQ when
irradiation with a low-energy-density excimer laser beam is
selected as a method of the rapid thermal treatment in the same
manner as in FIG. 5.
[0072] In the crystal grains of the laterally grown polycrystalline
semiconductor film PSI, there are transcrystalline defects such as
dislocations, twins, vacancies, etc. as well as stacking defects
SF. When the laterally grown polycrystalline semiconductor film PSI
is irradiated with a low-energy-density excimer laser beam, the
stacking defects SF disappear like the regions DSF1 or are
disconnected like the regions DSF2 so that the crystallinity is
improved. The number density of the other transcrystalline defects
is also reduced by the irradiation with the low-energy-density
excimer laser beam, so that the crystallinity is improved.
[0073] FIG. 7 is a conceptual view showing definitions of a triple
point and three angles of the triple point. In the laterally grown
polycrystalline semiconductor film PSI described in FIG. 4 and the
high-quality polycrystalline semiconductor film PSIHQ described in
FIG. 5, there are a large number of points (triple points) TPP
where three crystal grain boundaries GBLG intersect. As shown in
FIG. 7, pay attention to three angles ANGA, ANGB and ANGC between
two adjacent crystal grain boundaries GBLG in each triple point. In
the laterally grown polycrystalline semiconductor film PSI, the
upstream angle ANGA of all the triple angles with respect to the
beam scanning direction LSD is an acute angle not smaller than 70%
and not larger than 90.degree.. On the other hand, in the
high-quality polycrystalline semiconductor film PSIHQ, many triple
points have obtuse angles of 90.degree. or more in all the three
angles ANGA, ANGB and ANGC. Of all the triple points, 60% or more
correspond to this case.
[0074] The high-quality polycrystalline semiconductor film PSIHQ in
FIG. 5 is also characterized in that there are a large number of
protruding structures (hereinafter referred to as protrusions) BMP
on the surface thereof. The protrusions depend on the irradiation
conditions of the excimer laser beam. Under the optimum conditions
to improve the crystallinity of the laterally grown polycrystalline
semiconductor film PSI, 80% or more of distances among the
protrusions are not shorter than 0.10 .mu.m and not longer than
0.80 .mu.m. The density thereof ranges from 5 to 20
pieces/.mu.m.sup.2. From the AFM analysis, it has been proved that
70% or more of peak-to-valley heights of the protrusions range from
2 nm to 50 nm.
[0075] FIG. 8 is a bird's-eye view in which irregularities in the
surface of the high-quality polycrystalline semiconductor film
PSIHQ are analyzed by AFM, and the result of the analysis are
visualized. In FIG. 8, pay attention to one protrusion, and draw a
line connecting the protrusion with adjacent two protrusions. When
the protrusion to pay attention to is changed and the line drawing
is repeated, it is understood that the protrusions BMP are aligned
on virtual lines VLA and VLB running substantially in parallel with
the beam scanning direction LSD. Though the distance between the
virtual lines depends on the irradiation conditions of the excimer
laser beam, the virtual lines are arranged at an interval not
shorter than 0.10 .mu.m and not longer than 0.80 .mu.m. There is a
backbone-like rise on the virtual line VLA, but there is no rise on
the virtual line VLB. From the sectional analysis with the
transmission electron microscope image, there are crystal grain
boundaries on some virtual lines, but there is no crystal grain
boundary on the other virtual lines.
[0076] FIG. 9 is an explanatory view illustratively showing a
transmission electron microscope image of a section of the
high-quality polycrystalline semiconductor film PSIHQ. FIG. 9
corresponds to a section where the sample is cut perpendicularly to
the beam scanning direction LSD in FIG. 5 or 8. In FIG. 9, there is
a crystal grain boundary GBLG in each protrusion BMPA, but there is
no crystal grain boundary in each protrusion BMPB.
[0077] FIG. 10 is a schematic view of a crystal grain boundary
having windings. Compare a crystal grain boundary GBLG of the
laterally grown polycrystalline semiconductor film PSI in FIG. 4
with a crystal grain boundary GBLG of the high-quality
polycrystalline semiconductor film PSIHQ in FIG. 5. It is
understood that the crystal grain boundary WGBLG of the latter has
windings between two triple points TPP as shown in the schematic
view of FIG. 10. From the analysis of the scanning electron
microscope image of the plane, the crystal grain boundary WGBLG
having windings is composed of a set of curves whose arc lengths
LARC are not longer than 1 .mu.m and whose curvature radii RCUR are
not longer than 20 .mu.m. The reference sign SECT in FIG. 10
designates a sector using each constituent curve as its arc and
having a curvature radius equal to that of the constituent
curve.
[0078] The reason why all the three angles ANGA, ANGB and ANGC of
each triple point are obtuse angles not smaller than 90.degree. and
the reason why the protrusions BMPB having no crystal grain
boundary or the crystal grain boundaries WGBLG having windings are
formed are estimated as follows.
[0079] That is, through the rapid thermal treatment based on
irradiation with an excimer laser beam with low energy density, the
laterally grown polycrystalline semiconductor film PSI is once
melted so that atoms constituting the semiconductor thin film are
rearranged. In this event, defects in crystal grains are reduced,
while unmelted crystal grains forming the laterally grown
polycrystalline semiconductor film PSI serve as nuclei so that
crystal grains grow again. Crystal grains derived from one and the
same crystal grain collide with each other so as to form a
backbone-like rise or a protrusion. However, the orientations of
the crystal grains are equal to each other. Thus, the crystal
grains are merged without forming any gain boundary. Crystal grains
grown from crystal nuclei derived from different crystal grains
form not only a backbone-like rise or a protrusion but also a
crystal grain boundary due to a large difference in crystal
orientation therebetween. (Of them, crystal grains having
orientations close to each other are merged.)
[0080] Near each triple point, crystal grains growing from crystal
nuclei derived from three different crystal grains collide with one
another so as to form the triple point. Since the crystal grains
growing isotropically collide with one another, all the three
angles ANGA, ANGB and ANGC of the triple point tend to form angles
close to 120.degree.. Each crystal grain boundary is located in a
position where crystal grains growing from crystal nuclei derived
from two different crystal grains collide with each other. The
crystal grain boundary depends on the positions of crystal nuclei
where they are crystallized again, but the positions of the crystal
nuclei are random. As a result, it is conceived that the crystal
boundary has windings.
[0081] Thus, the high-quality polycrystalline semiconductor film
PSIHQ is characterized in that crystals have been reformed through
the rapid thermal treatment without losing the history of good
crystal grains of the laterally grown polycrystalline semiconductor
film PSI.
[0082] Of crystal orientations normal to the film surface of the
high-quality polycrystalline semiconductor film PSIHQ obtained by
EBSD (Electron Back Scattered Diffraction) analysis and electron
diffraction analysis using a transmission electron microscope, each
of the orientations [110] and [100] occupies an area ratio of 20%
or higher, and the are ratio of the orientation [110] is higher
than that of the orientation [100]. There is no orientation whose
area ratio exceeds 50%. On the other hand, the area ratio of the
orientation [111] is not higher than 10%.
[0083] As for crystal orientations in the crystal growing direction
when the high-quality polycrystalline semiconductor film PSIHQ
grows laterally, crystal grains of the orientation [100] are
included with an area ratio of 30% or higher. The ratio of the
crystal grains of the orientation [100] is higher than that of any
of the other two crystal orientations. However, there is no crystal
orientation whose area ratio exceeds 70%.
[0084] As for crystal orientations in the minor axis direction of
crystal grains of the high-quality polycrystalline semiconductor
film PSIHQ, that is, crystal orientations perpendicular to the
growing direction of crystals when the crystals grow laterally, and
parallel to the film surface, crystal grains of the orientation
[110] are included with an area ratio of 25% or higher. The ratio
of the crystal grains of the orientation [110] is higher than that
of any of the other two crystal orientations. However, the area
ratio does not exceed 50%. Each area ratio of the other two crystal
orientations does not exceed 30%.
[0085] These results are substantially equivalent to the results of
the laterally grown polycrystalline semiconductor film PSI which
has not been subjected to the rapid thermal treatment. That is,
there is no big change in crystal orientations between before and
after the rapid thermal treatment, but the history of crystals of
the laterally grown polycrystalline semiconductor film PSI survives
in the high-quality polycrystalline semiconductor film PSIHQ. Each
crystal orientation is mentioned here as the total sum of areas
occupied by grains which can be regarded as identical because their
orientations are within .+-.20.degree. from a reference direction
of the crystal orientation.
[0086] FIG. 11 is an explanatory view illustratively showing a
scanning electron microscope image of a surface of the low-quality
polycrystalline semiconductor film PSILQ obtained by
poly-crystallization of the precursor semiconductor film PCF when
irradiation with a low-energy-density excimer laser beam is
selected as the method of the rapid thermal treatment. Crystal
grains forming the low-quality polycrystalline semiconductor film
PSILQ have a small grain size of about 20 nm. Thus, a
high-resistance polycrystalline semiconductor film can be
obtained.
[0087] FIG. 12 is a graph showing a qualitative relationship
between the grain size of crystal grains and the energy density of
the excimer laser. When an excimer laser is used for
poly-crystallization of a silicon film, the grain size of crystal
grains depends on the energy density. The scales of axes depend on
the film thickness and the kind of a light source. Therefore, the
scales are set by given units. With increase in energy density EF,
the crystal grain size RG increases gradually. As the energy
density approaches one energy density EFMAX, the crystal grain size
RG increases sharply. After the crystal grain size RG reaches a
maximum value RGMAX at the energy density EFMAX, the crystal grain
size RG decreases sharply.
[0088] When irradiation with a low-energy-density excimer laser
beam is selected as the method of the rapid thermal treatment,
there is an optimum energy density range EFMOD. FIGS. 13A and 13B
are views illustratively showing scanning electron microscope
images of a plane of a polycrystalline semiconductor film
irradiated with energy density EFUF lower than the optimum energy
density range EFMOD in FIG. 12. FIG. 13A is an explanatory view
illustratively showing a surface scanning electron microscope image
of the surface of a region which has served as the laterally grown
polycrystalline semiconductor film PSI. FIG. 13B is an explanatory
view illustratively showing a surface scanning electron microscope
image of the surface of the low-quality polycrystalline
semiconductor film PSILQ. After the rapid thermal treatment,
occurrence or disappearance of intermittently disconnected sites
DFGB2 can be confirmed in a small crystal grain boundary group.
Thus, the crystallinity is improved, but the frequency of the
improvement is low and the effect thereof is low. The crystal grain
size in the low-quality polycrystalline semiconductor film PSILQ is
not larger than 10 nm.
[0089] FIGS. 14A and 14B are explanatory views illustratively
showing scanning electron microscope images of a plane of a
polycrystalline semiconductor film irradiated with energy density
EFOF higher than the optimum energy density range EFMOD in FIG. 12.
FIG. 14A is an explanatory view illustratively showing a scanning
electron microscope image of the plane of a region which has served
as the laterally grown polycrystalline semiconductor film PSI. FIG.
14B is an explanatory view illustratively showing a scanning
electron microscope image of the plane of a region serving as the
low-quality polycrystalline semiconductor film PSILQ.
[0090] It is understood from FIG. 14A that the history of crystal
grains of the laterally grown polycrystalline semiconductor film
PSI has been deleted perfectly under the irradiation conditions.
That is, the crystallinity is not improved at all, but the quality
thereof deteriorates. From FIG. 14B, the crystal grain size in the
low-quality polycrystalline semiconductor film PSILQ is not smaller
than 0.3 .mu.m. When TFTs are produced out of this film, the field
effect mobility is not lower than 100 cm.sup.2/Vs. The TFT
characteristic is improved, but the resistance of the film becomes
so low that the leak current increases.
[0091] The optimum energy density EFMOD shown in FIG. 12 depends on
the film thickness and the kind of the light source. The optimum
energy density EFMOD may be substantially set so that the
low-quality polycrystalline semiconductor film PSILQ will have a
surface shape similar to that in FIG. 11. That is, when irradiation
with a low-energy-density excimer laser beam is selected as the
method of the rapid thermal treatment, the energy density is
selected so that the low-quality polycrystalline semiconductor film
PSILQ will have a crystal grain size of about 10 nm to 100 nm. When
irradiation with the excimer laser beam is selected with such an
energy density, the high-quality polycrystalline semiconductor film
PSIHQ and the low-quality polycrystalline semiconductor film PSILQ
can be produced simultaneously and most efficiently.
[0092] Assume that irradiation with a long-time pulsed laser beam
is selected as the irradiation with the energy beam LSR while
irradiation with a low-energy-density excimer laser beam is
selected as the method of the rapid thermal treatment for reducing
defects in crystal grains of the laterally grown polycrystalline
semiconductor film PSI and poly-crystallizing the precursor
semiconductor film PCF. In this case, the field effect mobility of
N-type TFTs produced out of the high-quality polycrystalline
semiconductor film PSIHQ depends on the existence of termination
processing of crystal grain boundaries and the method for
processing of the interface between the gate insulating film and
the low-quality polycrystalline semiconductor film PSILQ. The
average value of the mobility reaches 300 cm.sup.2/Vs or higher. On
the other hand, the field effect mobility of N-type TFTs produced
out of the low-quality polycrystalline semiconductor film PSILQ
depends on the existence of termination processing of crystal grain
boundaries and the method for processing of the interface between
the gate insulating film and the low-quality polycrystalline
semiconductor film PSILQ. The average value of the mobility ranges
from 1 cm.sup.2/Vs to 100 cm.sup.2/Vs.
[0093] Assume that a beam other than the long-time pulsed laser
beam is selected as the energy beam LSR while irradiation with a
low-energy-density excimer laser beam is selected as the method of
the rapid thermal treatment for reducing defects in crystal grains
of the laterally grown polycrystalline semiconductor film PSI and
poly-crystallizing the precursor semiconductor film PCF. In this
case, the high-quality polycrystalline semiconductor film PSIHQ
formed also has a similar surface shape.
[0094] When irradiation with a low-energy-density excimer laser
beam is selected as the method of the rapid thermal treatment for
reducing defects in crystal grains of the laterally grown
polycrystalline semiconductor film PSI and poly-crystallizing the
precursor semiconductor film PCF, the surface shape of the
low-quality polycrystalline semiconductor film PSILQ depends on the
precursor semiconductor film PCF. When the precursor semiconductor
film PCF is an amorphous semiconductor film or a polycrystalline
semiconductor film composed of small-crystal-size crystal grains
produced in another method (e.g. formed into a film by CVD), the
low-quality polycrystalline semiconductor film PSILQ has a surface
shape similar to that shown in FIG. 11. When the precursor
semiconductor film PCF is a film poly-crystallized by irradiation
all over an amorphous semiconductor film with an excimer laser
beam, the low-quality polycrystalline semiconductor film PSILQ has
a surface shape where the history of the surface shape of the
precursor semiconductor film PCF survives.
[0095] When RTA is selected as the method of the rapid thermal
treatment for reducing defects in crystal grains of the laterally
grown polycrystalline semiconductor film PSI and poly-crystallizing
the precursor semiconductor film PCF, the laterally grown
polycrystalline semiconductor film PSI is reformed under the solid
phase due to a problem of the heat resistance of the insulating
substrate. In this case, there is little difference between the
surface shape of the high-quality polycrystalline semiconductor
film PSIHQ and the surface shape of the laterally grown
polycrystalline semiconductor film PSI, but defects in crystal
grains are reduced.
[0096] The features of the semiconductor thin films according to
the present invention described above can be summarized as follows.
(1) A region occupying at least 60% of a semiconductor thin film in
area ratio is composed of crystal grains with a region free from
crystal grain boundaries 0.2 .mu.m or more wide and 3 .mu.m or more
long, while small crystal grain boundary groups each composed of
three or more crystal grain boundaries arranged substantially in
parallel and at intervals of 100 nm or less are included in parts
of the region, and the small crystal grain boundary groups
disappear partially.
(2) A major part of a semiconductor thin film is composed of
crystal grains with a region free from crystal grain boundaries 0.2
.mu.m or more wide and 3 .mu.m or more long, while in most of
points where three crystal grain boundaries cross one another, all
of the three angles formed between two of the three crystal grain
boundaries are not smaller than 90.degree.. (3) A region occupying
at least 60% of a semiconductor thin film in area ratio is composed
of crystal grains with a region free from crystal grain boundaries
0.2 .mu.m or more wide and 3 .mu.m or more long, while in at least
60% of points where three crystal grain boundaries cross one
another, all of the three angles formed between two of the three
crystal grain boundaries are not smaller than 90.degree.. (4) A
major part of a semiconductor thin film is composed of crystal
grains with a region free from crystal grain boundaries 0.2 .mu.m
or more wide and 3 .mu.m or more long, while a plurality of
protrusions are disposed on the surface of the polycrystalline
semiconductor film. (5) A region occupying at least 60% of a
semiconductor thin film in area ratio is composed of crystal grains
with a region free from crystal grain boundaries 0.2 .mu.m or more
wide and 3 .mu.m or more long, while a plurality of protrusions are
disposed on the surface of the polycrystalline semiconductor film.
(6) In the paragraph (5), the intervals between adjacent two of the
protrusions are not shorter than 0.10 m and not longer than 0.80
.mu.m. (7) In the paragraph (5), at least 80% of the intervals
between adjacent two of the protrusions are not shorter than 0.10
.mu.m and not longer than 0.80 .mu.m. (8) In the paragraph (5), the
density of the protrusions is not lower than 5 pieces/.mu.m.sup.2
and not higher than 20 pieces/.mu.m.sup.2. (9) In the paragraph
(5), the protrusions are distributed on virtual lines extending in
one and the same direction. (10) In the paragraph (9), the distance
between the virtual lines is not shorter than 0.10 .mu.m. (11) In
the paragraph (9), the distance between the virtual lines is not
shorter than 0.10 .mu.m and not longer than 0.80 .mu.m. (12) In the
paragraph (5), groups of the protrusions formed on crystal grain
boundaries and groups of the protrusions formed in regions where
there is no crystal grain boundary are mixed. (13) In the paragraph
(5), the average value of the difference in height in the film is
not smaller than 2 nm and not larger than 50 nm. (14) In a
polycrystalline semiconductor film whose major part is composed of
crystal grains with a region free from crystal grain boundaries 0.2
.mu.m or more wide and 3 .mu.m or more long, each grain boundary
connecting adjacent points where three crystal grain boundaries
cross one another has windings. (15) In a polycrystalline
semiconductor film whose major part is composed of crystal grains
with a region free from crystal grain boundaries 0.2 .mu.m or more
wide and 3 .mu.m or more long, each grain boundary connecting
adjacent points where three crystal grain boundaries cross one
another has windings each composed of a set of curves having an arc
length of 1 .mu.m or less and a curvature radius of 20 .mu.m or
less. (16) In any one of the paragraphs (6) through (15), crystal
grains with a crystal orientation [110] normal to the film surface
of the polycrystalline semiconductor film are included with an area
ratio of 20% or higher, and crystal grains with a crystal
orientation [100] likewise are included with an area ratio of 20%
or higher. (17) In the paragraph (16), crystal grains with a
crystal orientation [111] normal to the film surface are included
with an area ratio of 10% or lower. (18) In any one of the
paragraphs (6) through (15), crystal grains with crystal
orientations [110], [100] and [111] normal to the film surface of
the polycrystalline semiconductor film are included, and the ratio
of the area occupied by the crystal grains with each crystal
orientation is not higher than 50%. (19) In any one of the
paragraphs (6) through (15), crystal grains with a crystal
orientation [100] in the major axis direction of the
polycrystalline semiconductor film are included with an area ratio
of 30% or higher. (20) In any one of the paragraphs (6) through
(15), crystal grains with crystal orientations [110], [100] and
[111] in the major axis direction of the polycrystalline
semiconductor film are included, and the ratio of the area occupied
by the crystal grains with each crystal orientation is not higher
than 70%. (21) In any one of the paragraphs (6) through (15),
crystal grains with a crystal orientation [110] in the minor axis
direction of the polycrystalline semiconductor film are included
with an area ratio of 25% or higher. (22) In any one of the
paragraphs (6) through (15), crystal grains with a crystal
orientation [110] in the minor axis direction of the
polycrystalline semiconductor film are included with an area ratio
of 50% or lower, crystal grains with a crystal orientation [100]
likewise are included with an area ratio of 30% or lower, and
crystal grains with a crystal orientation [111] likewise are
included with an area ratio of 30% or lower.
[0097] A drive circuit and a part of the aforementioned integrated
circuit group of an image display device can be arranged out of
thin film transistors formed out of any one of the polycrystalline
semiconductor films according to the paragraphs (5) to (22).
[0098] The drive circuit and a part of the integrated circuit group
can be arranged out of thin film transistors formed out of thin
film transistors formed out of any one of the polycrystalline
semiconductor films according to the paragraphs (5) to (22), while
active layers of the other thin film transistors are produced out
of a polycrystalline semiconductor film formed out of crystal
grains whose crystal grain size ranges from about 10 nm to about
100 nm.
[0099] FIG. 15 is a schematic plan view showing the outline of a
circuit for an image display device formed on a glass substrate
SUB1, for the sake of explanation of a second embodiment of the
present invention. Here, description will be made using an active
matrix substrate for a line-sequential display as the glass
substrate SUB1 by way of example. A pixel region DSP is provided in
a major part of the circuit formed on the glass substrate SUB1.
Pixels PXL arrayed in a matrix in the pixel region DSP are provided
near intersections between data lines DL and gate lines GL. Each
pixel PXL is constituted by TFTs serving as a switch and a pixel
electrode. This embodiment shows a double-gate configuration in
which the switch is constituted by two TFTs, but the embodiment
also includes a single-gate configuration and a multi-gate
configuration.
[0100] A drive circuit region where circuits for supplying driving
signals to a large number of pixels PXL formed in the pixel region
DSP are formed is disposed outside the pixel region DSP. A shift
register DSR, a digital-analog converter DAC, a level shifter DLS,
buffers BF and sampling switches SSW are disposed on one long side
(upper side in FIG. 15) of the pixel region DSP. The shift register
DSR serves to make the digital-analog converter DAC read digitized
display data sequentially. The digital-analog converter DAC outputs
the digitized display data as a gradation voltage signal. The level
shifter DLS amplifies the gradation voltage signal from the
digital-analog converter DAC so as to obtain a desired gradation
voltage. The sampling switches SSW inverts the polarity of the
gradation voltage between adjacent pixels.
[0101] A shift register GSR and a level shifter GLS for opening
gates of the pixel electrodes PXL sequentially are disposed on one
short side (left side in FIG. 15) of the pixel region DSP. An
interface IF, a gradation signal generator SIG, a clock signal
generator CLG, etc. are disposed in the peripheries of the
aforementioned circuits. The interface IF imports image data sent
from a system LSI (host) into the display circuit and performs
signal conversion on the image data. The clock signal generator CLG
generates a clock signal for controlling timing of each
circuit.
[0102] Of these circuits, the peripheral drive circuits including
the interface IF, the clock signal generator CLG, the drain-side
shift register DSR, the gate-side shift register GSR and the
digital-analog converter DAC have to be fast enough to process
digital signals. In addition, the peripheral drive circuits have to
be driven with a low voltage for the sake of saving of power. On
the other hand, each pixel PXL is a circuit for applying a voltage
to a liquid crystal so as to modulate the transmittance of the
liquid crystal. In order to secure gradation, the pixel PXL has to
be driven with a high voltage. Further, in order to hold the
voltage for a fixed time, each switching transistor has to have a
low leak current.
[0103] The drain-side level shifter DLS, the gate-side level
shifter GLS, the buffers BF and the sampling switches SSW located
between the low-voltage driven circuit group and the high-voltage
driven circuit group have to be driven with a voltage high enough
to send a high-voltage analog signal to each pixel.
[0104] In this manner, TFTs having a plurality of directly-opposed
specifications have to be mounted together in order to manufacture
the image display circuits on the glass substrate SUB1. To that
end, the high-quality polycrystalline semiconductor film PSIHQ is
used in portions of the interface IF, the clock signal generator
CLG, the drain-side shift register DSR, the gate-side shift
register GSR and the digital-analog converter DAC. The range where
the high-quality polycrystalline semiconductor film is used is
designated by the reference sign SX. The low-quality
polycrystalline semiconductor film PSILQ is used in the other
circuits.
[0105] Due to the aforementioned thin film transistors, the fast
circuits which have been mounted as another LSI chip outside an
image display portion mounted on a glass substrate in the
background art can be mounted on one and the same glass substrate
together with the image display portion. Thus, the LSI chip cost
can be saved, and a non-pixel region in the peripheral portion of a
panel can be cut down. In addition, customization of the circuits
which have been performed in the step of designing and producing
the LSI chip in the background art can be performed in the step of
manufacturing the panel.
[0106] FIG. 16 is an exploded perspective view for schematically
explaining an embodiment where an image display device according to
the present invention is applied to a liquid crystal display
device. A plurality of pixel electrodes PXL disposed in a matrix,
circuits DSR and GSR for inputting a display signal to the pixel
electrodes, and a circuit group CIR required for image display are
formed on a glass substrate (active matrix substrate or thin film
transistor substrate) SUB1 which is one of substrates. An
orientation film LO is applied to the glass substrate SUB1 as a top
layer by a printing method. The other glass substrate (opposed
substrate or color filter substrate) SUB2 applied with a color
filter CF, an opposed electrode ITO and an orientation film LO is
bonded. Liquid crystals LIQ are charged between the opposed
orientation films LO by vacuum injection, and the liquid crystals
are sealed off by a sealant SEA.
[0107] After that, polarizing plates DEF are bonded, and a
backlight BK is attached. Thus, a liquid crystal display device is
completed. Although a liquid crystal display device using a color
filter substrate is described here by way of example, the
embodiment can be also applied to a liquid crystal display device
where a color filter is formed on the active matrix substrate
side.
[0108] Further, an organic EL display device can be manufactured
using the active matrix substrate manufactured in the embodiment of
the manufacturing method according to the present invention. FIG.
17 is an exploded perspective view for explaining an example of the
configuration of the organic EL display devices as the second
embodiment of the image display device according to the present
invention. FIG. 18 is a plan view of the organic EL display device
in which the constituent parts shown in FIG. 17 have been
integrated. Organic EL devices are formed on the pixel electrodes
provided on the aforementioned active matrix substrate SUB1. The
organic EL devices are composed of a lamination in which a hole
transport layer, a light emission layer, an electron transport
layer, a cathode metal layer, etc. have been deposited in turn from
the surface of each pixel electrode.
[0109] A sealant is disposed around a pixel region PAR of the
active matrix substrate SUB1 where such a lamination has been
formed, and the pixel region PAR is sealed with a sealing substrate
SUBX or a sealing can. According to this organic EL display device,
a display signal is supplied from an external signal source to a
drive circuit region DDB through a printed circuit board PLB. The
display device is integrated by a shield frame SHD serving as an
upper casing and a lower casing CAS. Thus, an organic EL display
device is formed.
[0110] In active matrix drive for the organic EL display device, a
high-performance pixel circuit is essentially used for provision of
a high-quality image because the organic EL devices are driven in a
current-drive emission system. It is desired to use a pixel circuit
of CMOS thin film transistors. Each thin film transistor circuit to
be formed in the drive circuit region has to be fast to attain high
definition. The active matrix substrate SUB1 in this embodiment has
performance high enough to satisfy these requests. The organic EL
display device using the active matrix substrate according to the
embodiment is one of display devices showing the features of the
embodiment to the full.
[0111] FIGS. 19 to 22 show examples of applications of image
display devices according to the present invention. FIG. 19 is a
front view showing an example of an application where a liquid
crystal display device LIQMON according to the present invention is
mounted in an image display portion of a monitor MON for use in a
personal computer or a TV set.
[0112] FIG. 20 is a front view showing an example of an application
where a liquid crystal display device LIQMON according to the
present invention is used in an image display portion of a cellular
phone MOB.
[0113] FIG. 21 is a front view showing an example of an application
where a liquid crystal display device LIQMON according to the
present invention is used in an image display portion of a digital
portable terminal PDA.
[0114] FIG. 22 is a front view showing an example of an application
where a liquid crystal display device LIQMON according to the
present invention is used in a view finder portion of a video
camera CAM.
[0115] In addition to the aforementioned applications, image
display devices according to the present invention can be used in
image display portions of digital still cameras, projectors,
on-vehicle navigation systems, etc.
[0116] The present invention is not limited to the active matrix
substrate of the image display device, or not limited to the
configurations stated in the claims and the configurations
described in the embodiments. Various changes can be made on the
invention without departing from the technical idea thereof. For
example, the present invention can be applied to various
semiconductor devices.
[0117] In order to heighten the added value of a TFT-driven
display, it is essential to mount a high-performance processing
circuit. To that end, it is necessary to improve the crystallinity
of a polycrystalline semiconductor serving as an active layer,
while it is desired to keep a polycrystalline semiconductor in an
active layer portion of a pixel circuit as it is in the background
art. The present invention is to provide a method for manufacturing
an image display device without introducing another apparatus
newly. Accordingly, the applicability of the present invention is
high.
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