U.S. patent application number 11/588387 was filed with the patent office on 2007-02-22 for apparatus for fabricating a display device.
Invention is credited to Mutsuko Hatano, Mikio Hongo, Toshihiko Nakata, Mineo Nomoto, Makoto Ohkura, Sachio Uto, Shinya Yamaguchi.
Application Number | 20070041410 11/588387 |
Document ID | / |
Family ID | 31980562 |
Filed Date | 2007-02-22 |
United States Patent
Application |
20070041410 |
Kind Code |
A1 |
Hongo; Mikio ; et
al. |
February 22, 2007 |
Apparatus for fabricating a display device
Abstract
Apparatus for fabricating a display device includes a stage
capable of mounting an insulating substrate of the display device
and moving the insulating substrate, linear scales which detect a
position or moving distance of the substrate, a laser oscillator
which generates continuous-waves laser light, a modulator which
turns ON/OFF the continuous-wave laser light, a beam forming optic
which shapes the continuous-wave laser light passing through the
modulator into a linear or rectangular form, an objective lens
which projects the at least one of the laser light on the
insulating substrate so as to irradiate the insulating substrate
with the laser light. The controller counts signals generated by
the linear scales for every movement of the stage for a given
distance, causes the modulator to turn the generated
continuous-wave laser light in an ON state at time when a position
of the insulating substrate on which the laser light irradiation is
to be started reaches an area on which the laser light is
projected, and causes the modulator to turn the generated
continuous-wave laser light in an OFF state at another time.
Inventors: |
Hongo; Mikio; (Yokohama,
JP) ; Uto; Sachio; (Yokohama, JP) ; Nomoto;
Mineo; (Yokohama, JP) ; Nakata; Toshihiko;
(Hiratsuka, JP) ; Hatano; Mutsuko; (Kokubunji,
JP) ; Yamaguchi; Shinya; (Mitaka, JP) ;
Ohkura; Makoto; (Fuchu, JP) |
Correspondence
Address: |
ANTONELLI, TERRY, STOUT & KRAUS, LLP
1300 NORTH SEVENTEENTH STREET
SUITE 1800
ARLINGTON
VA
22209-3873
US
|
Family ID: |
31980562 |
Appl. No.: |
11/588387 |
Filed: |
October 27, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10628421 |
Jul 29, 2003 |
7129124 |
|
|
11588387 |
Oct 27, 2006 |
|
|
|
Current U.S.
Class: |
372/24 ;
257/E21.134; 257/E21.347; 257/E21.413; 257/E29.003; 359/623;
372/9 |
Current CPC
Class: |
H01L 21/0268 20130101;
H01L 21/02422 20130101; H01L 21/02488 20130101; H01L 27/1285
20130101; H01L 21/2026 20130101; H01L 21/02683 20130101; H01L
29/66757 20130101; H01L 21/02532 20130101; H01L 21/02691 20130101;
H01L 29/04 20130101; H01L 21/02595 20130101; H01L 21/02678
20130101; H01L 21/268 20130101; H01L 21/02686 20130101 |
Class at
Publication: |
372/024 ;
372/009; 359/623 |
International
Class: |
H01S 3/10 20060101
H01S003/10; G02B 27/10 20060101 G02B027/10 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 2, 2002 |
JP |
2002-256533 |
Mar 10, 2003 |
JP |
2003-062938 |
Claims
1. An apparatus for fabricating a display device, comprising: a
stage capable of mounting an insulating substrate of the display
device and moving the insulating substrate; at least one position
sensor which detects at least one of a position and moving distance
of the substrate; a laser oscillator which generates
continuous-waves laser light; a modulator which turns ON/OFF the
continuous-wave laser light generated by the laser oscillator; a
beam forming optic which shapes the continuous-wave laser light
passing through the modulator into at least one of a linear and
rectangular form; an objective lens which projects the at least one
of linear and rectangular laser light on the insulating substrate
so as to irradiate the insulating substrate with the at least one
of linear and rectangular laser light; and a controller which
counts signals generated by the at least one position sensor for
every movement of the stage for a given distance, which causes the
modulator to turn the generated continuous-wave laser light in an
ON state at time when a position of the insulating substrate on
which the laser light irradiation is to be started reaches an area
on which the at least one of linear and rectangular laser light is
projected, and which causes the modulator to turn the generated
continuous-wave laser light in an OFF state at time when another
position of the insulating substrate on which the laser light
irradiation is to be stopped reaches the area; wherein a plurality
of regions on the substrate are irradiated with the continuous-wave
laser light in a state when the substrate is kept in continuous
movement.
2. An apparatus for fabricating a display device according to claim
1, wherein the modulator is an electro-optical modulator.
3. An apparatus for fabricating a display device according to claim
1, wherein the insulating substrate is a substrate with an
amorphous semiconductor film of a granular polycrystalline
semiconductor film formed thereon.
4. An apparatus for fabricating a display device according to claim
1, wherein the continuous-wave laser light is light of a second
harmonics of a laser diode pumped YVO.sub.4 continuous wave
laser.
5. An apparatus for fabricating a display device according to claim
1, wherein a plurality of laser oscillators, a plurality of
modulators, a plurality of beam forming optics, and a plurality of
objective lenses are provided; and wherein a plurality of spots on
the insulating substrate which is mounted on the stage are
simultaneously irradiated with the laser light.
6. An apparatus for fabricating a display device, comprising: a
stage capable of mounting an insulating substrate of the display
device and moving the insulating substrate; at least one position
sensor which detects at least one of a position and moving distance
of the substrate; a laser oscillator which generates
continuous-wave laser light; a modulator which turns ON/OFF the
continuous-wave laser light generated by the laser oscillator; a
beam forming optics which shapes the continuous-wave laser light
passing through the modulator into at least one of a linear and
rectangular form; an objective lens which projects the at least one
of linear and rectangular laser light on the insulating substrate
so as to irradiate the insulating substrate with the at least one
of linear and rectangular laser light; and a controller which
counts signals generated by the at least one position sensor for
every movement of the stage for a given distance, which causes the
modulator to turn the generated continuous-wave laser light in an
ON state at time when a position of the insulating substrate on
which the laser light irradiation is to be started which reaches an
area on which the at least one of linear and rectangular laser
light is projected, and which causes the modulator to turn the
generated continuous-wave laser light in an OFF state at time when
a preset time has elapsed from the start of the laser light
irradiation; and wherein a plurality of regions on the substrate
are irradiated with the continuous-wave laser light in a state when
the substrate is kept in continuous movement.
7. An apparatus for fabricating a display device according to claim
1, wherein the at least one position sensor includes a plurality of
linear scales, each of the linear scales being installed in the
stage.
8. An apparatus for fabricating a display device according to claim
1, wherein the at least one position sensor includes a plurality of
measuring machines, each of the measuring means enabling
measurement using laser light interference.
9. An apparatus for fabricating a display device according to claim
1, wherein the at least one position sensor includes a plurality of
rotary encoders, each of the rotary encoders being installed on an
axis of a motor which drives the stage.
10. An apparatus for fabricating a display device according to
claim 6, wherein the at least one position sensor includes a
plurality of linear scales, each of the linear scales being
installed in the stage.
11. An apparatus for fabricating a display device according to
claim 6, wherein the at least one position sensor includes a
plurality of measuring machines, each of the measuring means
enabling measurement using laser light interference.
12. An apparatus for fabricating a display device according to
claim 6, wherein the at least one position sensor includes a
plurality of rotary encoders, each of the rotary encoders being
installed on the axis of a motor driving the stage.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This is a continuation of U.S. application Ser. No.
10/628,421, filed Jul. 29, 2003. This application relates to and
claims priority from Japanese Patent Application No. 2002-256533,
filed on Sep. 2, 2002 and No. 2003-062938, filed on Mar. 10, 2003.
The entirety of the contents and subject matter of all of the above
is incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to a flat panel display
device, and in particular, to a display device using an insulating
substrate with active elements formed in a band-like
polycrystalline semiconductor film, obtained by reforming an
amorphous or granular polycrystalline semiconductor film formed on
the top surface of the insulating substrate so as to expand crystal
grains into a substantially band-like shape by use of annealing
with laser light (also referred to merely as laser hereinafter)
irradiated thereto to, a process of fabricating the same, and an
apparatus for fabricating the same.
[0004] 2. Description of Related Art
[0005] This type of display device comprises a multitude of data
lines (drain lines for thin-film transistors) juxtaposed so as to
be extended in a direction of the display region of an insulating
substrate on one side (hereinafter referred to also as an active
matrix substrate, and as a thin-film transistor substrate in the
case where thin-film transistors are used as the active elements),
with active elements such as thin-film transistors, thin-film
diodes, and so forth, formed thereon, a multitude of scanning lines
(gate lines for thin-film transistors) juxtaposed so as to be
extended in another direction crossing the direction described as
above, active elements formed of a granular polycrystalline silicon
film (polysilicon film) as a semiconductor film formed on the
active matrix substrate and disposed at cross-over points of the
data lines and the scanning lines, and pixels arranged in a matrix
form, made up of pixel circuits having pixel electrodes driven by
the active elements, respectively. There will be described
hereinafter mainly of a display device using a silicon film as the
semiconductor film and thin-film transistors, which are typical
active elements, as the active elements.
[0006] With a flat panel display device of the present day, pixel
circuits comprising thin-film transistors, respectively, are made
up of a non-crystalline silicon film (hereinafter referred to also
as amorphous silicon film) and a granular polycrystalline silicon
film (hereinafter referred to also as polysilicon film) as a
semiconductor film on top of an insulating substrate, made of
glass, fused glass, etc., serving as an active matrix substrate,
and pixels are selected by switching of the respective thin-film
transistors of the pixel circuits, thereby forming images.
[0007] The thin-film transistor constituting each of the pixel
circuits is driven by a driving circuit (hereinafter referred to
also as driver circuit or driver) mounted on the periphery of the
active matrix substrate. The granular polycrystalline silicon film
described above is a silicon film which crystal grains are small in
diameter as will be described later. Herein, "crystal grains are
small in diameter" means a size so small that, for example, there
exist a multitude of grain boundaries of silicon crystals in an
active layer (or active region) of the thin-film transistor, that
is, within a so-called channel width, thereby causing current
passing through the active layer to cut across the multitude of the
grain boundaries of the silicon crystals without fail.
[0008] If it becomes possible to form the driver circuit for
driving the thin-film transistors of the pixel circuits
concurrently with the thin-film transistors of the pixel circuits,
drastic reduction in production cost and enhancement in reliability
can be expected. However, because the conventional polysilicon
film, which is a semiconductor layer for forming the active layer
of the thin-film transistor, has poor crystallinity (crystal grains
are small in grain size), operation performance (operation
characteristic) represented by mobility of electrons or holes is
low, so that it is difficult to fabricate a circuit of which
high-speed and high function are required. In order to fabricate
the circuit having high-speed and high function, high-mobility
thin-film transistors are required, but to implement this, there is
the need for improving the crystallinity of the polysilicon film.
Improvement on the crystallinity means primarily expansion of the
grain size of the crystal grains or rendering a dimension in one
direction of the crystal grains to be greater than a dimension
thereof, in other directions, so as to be turned into a band-like
or stripe like shape, thereby increasing the dimensions thereof.
Herein, to differentiate from the conventional polysilicon film,
the silicon film as reformed is referred to a band-like polysilicon
film.
[0009] As a method for improving the crystallinity of a polysilicon
film, there has since been known annealing with laser light such as
excimer laser, and so forth. With this method, by irradiating, for
example, excimer laser to an amorphous silicon film formed on top
of an insulating substrate (also referred to merely as a substrate
hereinafter), made of glass, fused glass, etc., the amorphous
silicon film is turned into a polysilicon film, thereby improving
the mobility. However, the polysilicon film obtained by irradiation
with the excimer laser is on the order of several 100 nm in grain
size, and the mobility thereof is on the order of 100 cm.sup.2/Vs,
so that the performance thereof is insufficient for use in the
driver circuit for driving a liquid crystal panel.
[0010] As a method of overcoming the problem, annealing techniques
with the use of continuous-wave laser as described in Non-patent
Document 1. Patent Document 1 has description to the effect that by
maintaining the pulse width of pulse laser light in a range of 1
.mu.s to 100 ms, it is possible to reduce fluctuation in threshold
value of transistor fabricated. Further, description concerning
reformation of a silicon film by irradiation of laser light is
given in Patent Document 2.
[Non-patent Document 1]
[0011] F. Takeuchi et al "Performance of poly-Si TFTs fabricated by
a Stable Scanning CW Laser Crystallization" AM-LCD '01 (TFT4-3)
[Patent Document 1] [0012] JP-A No. 335547/1995 [Patent Document 2]
[0013] JP-A No. 283356/1993
SUMMARY OF THE INVENTION
[0014] With the conventional techniques described in [Non-patent
Document 1] described, by scanning the second harmonics of LD
(laser diode) pumped YVO.sub.4 continuous wave laser on an
amorphous silicon film formed on the surface of a glass substrate
to thereby cause growth of crystal grains to occur, mobility in
excess of 500 cm.sup.2/Vs was obtained. If mobility at this level
is achieved, a driver circuit high in performance can be formed and
a so-called system on panel (or chip on glass: COG) mounting can be
implemented.
[0015] However, according to the conventional techniques described
in [Non-patent Document 1], irradiation is performed by scanning
the whole surface of a region on the substrate, for forming a
driver circuit, with the continuous wave laser, and no
consideration is given to an idea of irradiation of necessary parts
only. Accordingly, a wide region including a part with high
mobility where the driver circuit is formed and the periphery
thereof is continuous irradiated. As a result, after start of laser
irradiation, laser absorbed by a silicon film is converted into
heat, and accumulated in the substrate, so that the silicon melts
and undergoes aggregation by the agency of interfacial tension or
thermal damage occurs to the substrate. In order to solve the
problem, it is sufficient to selectively irradiate laser light to
necessary regions only. Since irradiation is applied to specific
spots on the substrate being moved at a high speed relative to
laser, means for implementing laser irradiation start and stoppage
with high precision have not been known so far. So, this has been
one the pending problems.
[0016] Further, in [Patent Document 1], it has been disclosed that
fluctuation in the threshold value of a transistor produced by
setting a pulse width of pulse laser light to a range of 1 to 100
.mu.s.
[0017] However, no thought has been given to a method of
irradiating laser light to a specific spot on a substrate being
moved at a high speed relative to laser. This has been another
problem to be solved.
[0018] The present invention has been developed in view of the
problem described as above, and a fist object of the invention is
to provide a display device comprising an active matrix substrate
having active elements such as thin-film transistors in a stable
and high quality semiconductor film (silicon film) formed only at
desired position on an insulating substrate. A second object of the
invention is to provide a process of fabricating the display
device, enabling a stable and high quality silicon film to be
formed only at desired position on the insulating substrate. A
third object of the invention is to provide an apparatus for
implementing the process of fabricating the display device.
[0019] To attain the fist object, the insulating substrate (the
active matrix substrate) of the display device according to the
invention comprises a multitude of data lines juxtaposed so as to
be extended in a direction on the top surface of the insulating
substrate, and a multitude of scanning lines juxtaposed so as to be
extended in another direction crossing the direction. There are
also provided pixel circuits having active elements (also referred
to as thin-film transistors) such as thin-film transistors,
disposed in the vicinity of cross-over points of the data lines and
the scanning lines. The thin-film transistors are made of a
band-like polycrystalline silicon film having crystallinity
described and disposed in a matrix form in a display region. The
respective pixels are made up of pixel circuits having pixel
electrodes driven by the active elements, respectively.
[0020] A driving circuit (hereinafter referred to as driver
circuit) is divided and formed at a plurality of spots on at least
one side of the insulating substrate. The active layer (active
region) of the thin-film transistor making up the driver circuit is
obtained by reformation implemented by scanning the continuous-wave
laser light, condensed into a linear form or a rectangle form
extremely longer in the longitudinal direction than in the
transverse direction, along a given direction crossing the
longitudinal direction. This is made up of a poly silicon film
containing crystal grains having no grain boundaries crossing the
direction of current flow, that is, a band-like polycrystalline
silicon film.
[0021] To attain the second object, the process of fabricating the
display device made up of a thin-film transistor substrate
comprises the steps of: taking out continuous-wave laser light at
timing required by use of an electro-optical modulator (hereinafter
referred to also EO modulator); forming the continuous-wave laser
light into linear or rectangular form as described above;
irradiating the laser light thus formed only to necessary portions
including the thin-film transistor part making up the driver
circuit disposed on the outside of the display region, and
periphery thereof out of amorphous silicon film and amorphous
silicon film composed of fine crystal grains, formed on the entire
top surface of the insulating substrate such as glass and so
forth.
[0022] The thin-film transistor substrate with the top surface
thereof facing upward is mounted on the stage making relative
movement in relation to the laser light, and the above-described
necessary portions of the thin-film transistor substrate are
irradiated and scanned by the laser light, thereby implementing
reformation. Such scanning is executed such that a pulse signal
generated by the linear scale and so forth for detecting the
position of the stage following movement of the stage is counted,
and at a time when a position where the thin-film transistor is to
be formed is reached, irradiation of the laser light is started.
Further, the pulse signal is counted, and at a time when a region
where the laser light is to be irradiated is passed, irradiation of
the laser light is stopped. Such operation is repeated while the
stage is kept in constant movement.
[0023] With the invention, while the stage is kept in continuous
movement, only necessary pats can be irradiated with the laser
light by turning the continuous-wave laser light ON/OFF, and parts
where the laser irradiation is not required will not be irradiated,
so that occurrence of melting or aggregation of the silicon film
can be prevented. Also, it is possible to prevent thermal damage to
a glass substrate etc. as the thin-film transistor substrate.
Further, since the start and stoppage of the laser light
irradiation can be controlled on the basis of the position of the
stage, high precision irradiation start and stop is ensured even
there occurs variation in the moving speed of the stage.
[0024] In Patent Document 2, there has been disclosed a
configuration wherein a laser pulse is emitted when a laser
irradiation position is opposed to a laser irradiation position as
a technology to irradiate laser light accurately. In this
technology, no consideration is given to the control adopted by the
present invention, that is, while keeping the stage in movement,
irradiation of the continuous-wave laser light is started at a
specified position and after irradiation for a predetermined time
(predetermined distance), irradiation is stopped.
BRIEF DESCRIPTION OF THE DRAWINGS
[0025] These and other features, objects and advantages of the
present invention will become more apparent from the following
description when taken in conjunction with the accompanying drawing
wherein:
[0026] FIG. 1 is a block diagram for schematically illustrating an
embodiment of an apparatus of fabricating a display device
according to the invention for carrying out an embodiment of a
process of fabricating the display device according to the
invention;
[0027] FIG. 2 is a perspective view of the EO modulator 10 in FIG.
1 for explaining the function thereof;
[0028] FIG. 3 is another perspective view of the EO modulator 10 in
FIG. 1 for explaining the function thereof;
[0029] FIG. 4 is a graph showing a relationship between applied
voltage and transmittance in the EO modulator;
[0030] FIG. 5 is a graph showing a relationship among laser input,
applied voltage, and laser output in the EO modulator;
[0031] FIG. 6 is a plan view for illustrating the glass substrate
as the object for a laser annealing method, which is an embodiment
of a process of fabricating the display device according to the
invention, and an enlarged view of the principal part thereof is
also shown in the figure;
[0032] FIG. 7 is a time chart for illustrating the embodiment of
the process of fabricating the display device, according to the
invention, showing timing of shifting the stage 2 and irradiating
the laser light, respectively;
[0033] FIG. 8 is a plan view showing crystal morphology prior to
the laser annealing according to the embodiment of the process of
fabricating the display device according to the invention;
[0034] FIG. 9 is a plan view showing crystal morphology after the
laser annealing;
[0035] FIG. 10 is a plan view of the glass substrate 1, showing a
relationship in position between regions where the laser annealing
is applied and active regions of the driver circuit;
[0036] FIG. 11 is a plan view of a substrate, showing a
configuration of the thin-film transistors of the driver part
formed by the laser annealing according to the invention;
[0037] FIG. 12 is a schematic representation for illustrating
electronic equipment with the display device of the invention
mounted therein;
[0038] FIG. 13 is a time chart showing timing of shifting a stage
and irradiating laser light, respectively, with reference to a
laser annealing method, which is said embodiment of the process of
fabricating the display device according to the invention;
[0039] FIG. 14 is a schematic representation for illustrating
another embodiment of an apparatus, that is, a laser annealing
apparatus for fabricating the display device according to the
invention;
[0040] FIG. 15 is a schematic representation for illustrating the
optical system in FIG. 14;
[0041] FIG. 16 is a perspective view for illustrating a condensed
state of the laser light, suitable for the laser annealing
according to the invention;
[0042] FIG. 17 is a perspective view for illustrating a laser
irradiation region at the time of the laser annealing according to
the invention;
[0043] FIG. 18 is a plan view of an insulating substrate for
illustrating another embodiment of a process of fabricating the
display device, according to the invention;
[0044] FIG. 19 is a schematic representation for illustrating a
relationship between a stage position according to another
embodiment of the invention and laser output;
[0045] FIG. 20 is a sectional view showing the sectional shape of a
thin-film transistor substrate to which a laser annealing method
according to the present embodiment is applied;
[0046] FIG. 21 is a sectional view showing the sectional shape of a
thin-film transistor substrate to which the laser annealing method
described with reference to FIG. 19A is applied;
[0047] FIG. 22 is a flowchart for illustrating the steps of
fabricating a display device, to which the process of fabrication
according to the invention is applied;
[0048] FIG. 23 is a flowchart for illustrating the step of
annealing according to the invention;
[0049] FIG. 24 is a sectional view of the principal part of a
liquid crystal display panel of a liquid crystal display device
which is an example of an embodiment of a display device according
to the invention for illustrating an example of configuration
thereof;
[0050] FIG. 25 is a sectional view of the principal part of a
liquid crystal display panel of a liquid crystal display device
which is an example of an embodiment of a display device according
to the invention for illustrating another example of configuration
thereof;
[0051] FIG. 26 is a sectional view for illustrating a schematic
configuration of a liquid crystal display device using the liquid
crystal display panel described with reference to FIGS. 24, and
25;
[0052] FIG. 27 is a sectional view of the principal part of a
display panel making up an organic electro-luminescent display
panel, another example of the display device according to the
invention, for illustrating a schematic configuration thereof;
[0053] FIG. 28 is a plan view showing a state of annealing region
at a first scanning according to the invention;
[0054] FIG. 29 is a plan view showing a state of annealing region
at a second scanning according to the invention;
[0055] FIG. 30 is a plan view showing a state of annealing region
upon completion of scanning according to the invention;
[0056] FIG. 31 is a plan view showing regions where transistor can
be formed upon completion of annealing according to the
invention;
[0057] FIG. 32 is a plan view showing a state of annealing region
upon completion of a first scanning according to another embodiment
of the invention;
[0058] FIG. 33 is a plan view showing a state of annealing region
upon completion of a second scanning according to another
embodiment of the invention;
[0059] FIG. 34 is a plan view showing a state of annealing region
upon completion of annealing according to another embodiment of the
invention; and
[0060] FIG. 35 is a schematic representation showing a relative
position among pixel part, peripheral circuit part, and circuit
formed in peripheral part of a panel after laser annealing
according to the invention
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0061] Embodiments of the invention are described in detail
hereinafter with reference to the accompanying drawings.
[0062] FIG. 1 is a block diagram for schematically illustrating an
embodiment of an apparatus of fabricating a display device
according to the invention for carrying out an embodiment of a
process of fabricating the display device according to the
invention. In this case, for an insulating substrate to serve as a
thin-film transistor substrate, a glass substrate is used. A glass
substrate 1 is movable in one direction (X) and in another
direction (Y) crossing the one direction (X) at right angles, and
is placed on a XY.theta. stage 2 (hereinafter referred to merely as
stage) capable of adjusting both the directions (.theta.). The
stage 2 fixedly attached to a platen (not shown) having a vibration
isolation mechanism is provided with linear scales (also referred
to as linear encoders) 3, 4, for detecting coordinates in the X
direction and the Y direction, respectively.
[0063] A laser light irradiation system (an annealing optical
system) for performing reformation of a silicon film comprises a
laser oscillator 6 for oscillating continuous-wave laser light 18,
a shutter 7 for prevention of inadvertent irradiation of the
continuous-wave laser light 18, a beam expander 8 for expanding the
beam diameter of the continuous-wave laser light 18, a continuously
variable transmittance filter 9 for adjusting output (energy) of
the continuous-wave laser light 18, an EO (electro-optical)
modulator 10 for turning the continuous-wave laser light 18 ON/OFF
and modulating the laser light 18 time-wise as necessary, a power
source (driver) 21 of the EO modulator 10, a beam forming optics 11
for compressing the continuous-wave laser light 18 in one direction
to be converted into linear beams, an electromotive rectangular
slit 12 for cutting out only a necessary portion of the
continuous-wave laser light 18 converted into the linear beams, an
objective lens 13 for projecting the continuous-wave laser light 18
passing through the electromotive rectangular slit 12 on the glass
substrate 1, a slit reference light source 14 for checking
irradiation position and shape of the continuous-wave laser light
18, an overhead illuminating light source 15 for shining the
surface of the glass substrate 1, and a CCD camera 16 for
observation of a glass substrate face or detection of alignment
marks at the time of alignment as necessary.
[0064] The laser light irradiation system further comprises a
controller 22 for performing opening/closing of the shutter 7,
adjustment of transmittance of the continuously variable
transmittance filter 9, control of the power source (driver) 21 of
the EO modulator 10, control of the electromotive rectangular slits
12, control of the stage 2, processing of respective signals from
the linear scales 3, 4, processing of images detected by the CCD
camera 16, and so forth. In FIG. 1, as for electrical connection,
there is shown relationship only among the linear scales (linear
encoders) 3, 4, the controller 22, the EO modulator 10, and the
power source (driver) 21.
[0065] For the laser oscillator 6, use is made of one for
generating for oscillating continuous oscillation laser light of
ultraviolet or visible light wavelength, and in particular, one for
generating the second harmonics of LD (laser diode) pumped
YVO.sub.4 continuous wave laser is optimum from the viewpoint of
magnitude of output, stability, and so forth. However, the
invention is not limited thereto, use may be made of harmonics and
so forth of argon laser and YAG laser. The shutter 7 is installed
to prevent inadvertent irradiation of the continuous-wave laser
light 18 in the course of transit of the glass substrate 1,
positioning thereof, and so forth, and is not intended for use to
turn the continuous-wave laser light 18 ON/OFF at the time of laser
annealing. The beam expander 8 is intended to expand the beam
diameter in order to prevent damage from occurring to crystals, in
particular, of Pockels cell, and so forth, constituting the EO
modulator 10 however, in the case of using Pockels cell capable of
withstanding high energy density, use of the beam expander 8 may be
unnecessary.
[0066] The continuous-wave laser light 18 oscillated by the laser
oscillator 6 passes through the shutter 7 in open state, and is
expanded in beam diameter by the beam expander 8 to subsequently
fall on the EO modulator 10. In this stage, the beam diameter is
expanded by the beam expander 8 to a size close to the effective
diameter of the EO modulator 10 taking power resistance of the EO
modulator 10 into account. In the case where the beam diameter of
the continuous-wave laser light 18 oscillated by the laser
oscillator 6 is approximately 2 mm, and the EO modulator 10 with
the effective diameter of 15 mm is used, a suitable ratio of
expansion by the beam expander 8 is from about 3 to 5 times. The
laser light 18 with the beam diameter expanded by the beam expander
8 falls on the EO modulator 10.
[0067] FIG. 2 is a perspective view of the EO modulator 10 in FIG.
1 for explaining the function thereof. FIG. 3 is another
perspective view of the EO modulator 10 in FIG. 1 for explaining
the function thereof. As shown in FIGS. 2 and 3, the EO modulator
10 in this case comprises a Pockels cell 61 (hereinafter also
referred to as a crystal) combined with a polarized beam splitter
62. If the laser light 18 is a linearly polarized light, it is set
such that by applying voltage V1 (normally voltage 0V) to the
crystal 61 via a power source (not shown) of the EO modulator 10,
as shown in FIG. 2, a polarization direction of the laser light 18
passing through the crystal 61 is not rotated to be maintained as
it is, and falls as S polarized light on the polarized beam
splitter 62 to be thereby deflected by 90 degrees. That is, in this
state, since the laser light 18 is sent out after deflection by 90
degrees, the laser light 18 does not fall on the rest of the laser
light irradiation system, and is in OFF state on top of the glass
substrate 1.
[0068] Further, by applying voltage V2 capable of rotating the
direction of polarization of the laser light 18 passing through the
crystal 61 by 90 degrees as shown in FIG. 3, the direction of
polarization of the laser light 18 passing through the crystal 61
is rotated by 90 degrees and falls as P polarized light on the
polarized beam splitter 62, whereupon the laser light 18 passes
through the polarized beam splitter 62, and travels in a straight
line. That is, in this state, since the laser light 18 travels in
the straight line, and falls on the rest of the laser light
irradiation system, the laser light 18 is in ON state on top of the
glass substrate 1.
[0069] FIG. 4 is a graph showing a relationship between applied
voltage and transmittance in the EO modulator. As is evident from
the relationship between voltage applied to the crystal 61 and
transmittance of the laser light 18 passing through the EO
modulator 10, shown in FIG. 4, the transmittance of the laser light
18 passing through the EO modulator 10 can be set suitably between
T1 (normally 0) and T2 (herein, the maximum transmittance, that is,
1) by varying the voltage applied to the crystal 61 between V1
(normally, 0V) and V2. That is, the transmittance of the laser
light 18 passing through the EO modulator 10 can be set suitably
between 0 and 1. In this case, however, it is presumed that
reflection and absorption do not take place on the surface of the
crystal 61 and the polarized beam splitter 62, respectively.
[0070] FIG. 5 is a graph showing a relationship among laser input,
applied voltage, and laser output in the EO modulator. On the basis
of description given with reference to FIGS. 3 and 4, by varying
voltage applied to the crystal 61 to V1, V2, V3, and V1 in stages
while output (input to the EO modulator 10) of the laser light 18
falling on the EO modulator 10 is kept constant, pulse output of
P2, P3 in stages as laser output from the EO modulator 10 can be
obtained as shown in FIG. 5. Herein, the output P2 is found as the
product of the input to the EO modulator 10, P0, and the
transmittance T2 when the voltage V2 is applied and the output P3
is found as the product of the input P0, and the transmittance T3
when the voltage V3 is applied. It is evident that the laser light
18 passing through the EO modulator 10 can be successively varied
by successively varying the voltage applied to the crystal 61, so
that the laser light 18 pulsing and having variation with time can
be obtained.
[0071] Now, it has been described hereinabove that the Pockels cell
61 in combination with the polarized beam splitter 62, is used as
the EO modulator 10, however, in place of the polarized beam
splitter 62, various sheet polarizers can be used. In description
given hereinafter, the Pockels cell 61 in combination with the
polarized beam splitter 62 (or sheet polarizer) will be referred to
as the EO modulator 10.
[0072] Further, besides the EO modulator 10, an AO
(acousto-optical) modulator may be used. However, because the AO
modulator generally has a lower driving frequency as compared with
the EO modulator, there is a possibility of the AO modulator being
unsuitable in the case where fast rising and falling are required
or pulse light with a small pulse width is cut out. Thus, with the
use of a modulator such as the EO modulator 10 or the AO modulator,
in a state where the continuous-wave laser light is constantly sent
out from the laser oscillator 6, it is possible to start
irradiation to an optional spot of irradiation start on a part as
the object of irradiation, and to complete irradiation to an
optional spot of irradiation completion thereof.
[0073] The laser light 18 turned into ON state by the EO modulator
10 is formed into a beam in a desired shape by the beam forming
optics 11. Since an output beam from a gas laser oscillator and a
solid-state laser oscillator, respectively, normally has Gaussian
energy distribution circular in shape, the same cannot be used as
it is for the laser annealing according to the invention. If output
from the oscillator is sufficiently large, by sufficiently
expanding the beam diameter of the laser light, and cutting out a
necessary shape from a relatively uniform portion at the center
thereof, a suitable shape with substantially uniform energy
distribution can be obtained, however, this result in discarding
the peripheral part of the laser light, so that most of energy will
be wasted.
[0074] In order to convert the Gaussian distribution into a uniform
distribution for overcoming the shortcoming, a beam homogenizer is
used as necessary. Otherwise, by condensing the laser light 18 only
in one direction with a cylindrical lens, a linear beam can be
obtained on the surface of the electromotive rectangular slit 12.
In FIG. 1, the cylindrical lens only is shown as the beam forming
optics 11.
[0075] Now, reverting to FIG. 1, operation of the fabrication
apparatus according to the invention is described hereinafter.
Unnecessary peripheral portions of the laser light 18 condensed
into a linear form by the cylindrical lens 11 are cut off by the
electromotive rectangular slit 12 to be thereby formed into a
rectangular form as desired (deemed as linear in macroscopic
terms), which is then projected on the glass substrate 1 by the
objective lens 13 after demagnification. In the case of condensing
the laser light into a linear form by the cylindrical lens 11,
energy distribution in the longitudinal direction remains as
Gaussian while energy distribution on both edges is lower.
[0076] Accordingly, low energy density portions thereof, normally
unsuitable for annealing, are cut off by the electromotive
rectangular slit 12. As a result, by scanning the laser light
condensed in a linear form in the direction of the width thereof,
an annealing process can be satisfactorily applied an entire
scanned part. Assuming that a magnification of the objective lens
13 is M, an image of the electromotive rectangular slit 12, or the
laser light 18 passing through the face of the electromotive
rectangular slit 12 is projected in size corresponding to the
reciprocal of the magnification, that is, 1/M.
[0077] Upon irradiation of the glass substrate 1 with the laser
light 18, the laser light 18 is irradiated to a desired spot at a
pulse while the stage 2 is being shifted in an X-Y plane, but if
defocusing occurs due to asperities, undulation, and so forth,
present on the surface of the glass substrate 1, there occurs
variation in power density of the laser light 18 as condensed and
deterioration in irradiation shape, so that an intended object
cannot be attained. Accordingly, by detecting a focusing position
with an automatic focusing optical system (not shown) to enable
irradiation to be directed to the focusing position all the time,
control is implemented such that the focusing position (projection
position of the face of the electromotive rectangular slit 12) is
in agreement with the glass substrate 1 all the time by driving the
stage 2 in the Z-direction (height direction) or by driving the
optical system in the Z-direction (height direction) in case the
laser light 18 is off the focusing position. The surface of the
glass substrate 1 irradiated with the laser light 18 can be
illuminated by illuminating light from the overhead illuminating
light source 15.
[0078] The surface of the glass substrate 1 is photographed with
the CCD camera 16 so as to be observed with a monitor (not shown).
When observing the surface of the glass substrate 1 during laser
irradiation, a laser cut filter is inserted before the CCD camera
16 to thereby prevent halation from occurring to the CCD camera 16
caused by the laser light 18 reflected from the surface of the
glass substrate 1, resulting in failure to observe, and to prevent
occurrence of damage to the CCD camera 16 in the extreme case.
[0079] Alignment of the glass substrate 1 placed on the stage 2 can
be made along three axes of X, Y, .theta. by driving the stage 2
after taking pictures of the alignment mark provided on the glass
substrate 1, corners of the glass substrate 1, or specific patterns
at a plurality of spots, with use of the objective lens 13 and the
CCD camera 16, and calculating coordinates of respective positions
thereof by performing image processing such as binarization
processing of those pictures, and pattern matching, etc. with the
controller 22, respectively, as necessary.
[0080] In FIG. 1, only one piece of the objective lens 13 is shown,
however, by fitting an electromotive revolver with a plurality of
objective lenses, and changing over those objective lenses as
appropriate, proper use can be made of an optimal one of those
objective lenses, corresponding to the content of processing. More
specifically, use can be made of an objective lens optimal to the
alignment at the time of placing the glass substrate 1 on the stage
2, fine alignment to be executed if needed, laser annealing
processing, observation after the processing, formation of an
alignment mark which will be described later, and so forth,
respectively. Alignment can be provided by installing an optical
system (lens, camera, and illumination device) for exclusive use,
however, use of the optical system for the laser annealing in
common with an optical system for alignment enables detection along
an identical optical axis, thereby enhancing precision of
detection.
[0081] Now, there will be described hereinafter an embodiment of a
process of fabricating a display device, that is, a process of
laser annealing, according to the invention, using the
above-described fabrication apparatus according to the invention.
Herein, a glass substrate 1 as an object for annealing is obtained
by forming an amorphous silicon film (a non-crystalline silicon
film) 40 to 150 nm thick on the top surface of a glass substrate on
the order of 0,3 to 1.0 mm thick through the intermediary of an
insulator thin film, and by scanning across the amorphous silicon
film with excimer laser light before crystallizing the same into a
polysilicon film (polycrystalline silicon film). This is
hereinafter referred to merely as the glass substrate 1 at times.
In this case, the insulator thin film is a SiO.sub.2, or SiN film,
50 to 200 nm thick, or a composite film thereof.
[0082] FIG. 6 is a plan view for illustrating the glass substrate
as the object for a laser annealing method, which is an embodiment
of a process of fabricating the display device according to the
invention, and an enlarged view of the principal part thereof is
also shown in the figure. The glass substrate 1 with the
polysilicon film formed thereon after excimer laser annealing is
placed on the stage 2 in FIG. 1. As shown in FIG. 6, the glass
substrate 1 comprises a display region 101, which is a pixel part,
and driver circuit regions 102, 102', and alignment marks 103, 103'
are formed at least two spots on an outer edge thereof. These
alignment marks 103, 103' may be formed by photo-etching
techniques, however, use of a photo resist step for this purpose
only will result in much wastage.
[0083] Accordingly, the laser light 18 for use in laser annealing
is formed in the shape of, for example, a rectangle longer in the
longitudinal direction and a rectangle longer in the transverse
direction, respectively, in sequence by rotation of the cylindrical
lens 11, and by use of the electromotive rectangular slit 12 to
thereby remove portions of the polycrystalline silicon film, thus
forming cross marks, which are to serve as the alignment marks 103,
103', respectively. Otherwise, dot-like alignment marks may be
formed by ink-jet means and so forth. In these cases, pre-alignment
needs to be provided in corners and so forth of the glass substrate
1.
[0084] While respective positions of the alignment marks 103, 103'
are detected, and the positions are corrected in terms of X, Y,
.theta. (X-axis, Y-axis, .theta.-axis), and subsequently, the
optical system is shifted in the direction indicated by the arrow
in FIG. 6 or the stage 2 is shifted in the direction opposite
thereto in accordance with design coordinates so as to be
relatively scanned, the laser light 18 turned into ON state by the
EO modulator 10 is condensed by the objective lens 13 and is
irradiated. Regions irradiated with the laser light 18 are, for
example, driver circuit parts 102, 102', for driving the respective
pixels, in a stricter sense, thin-film transistor forming regions
of a driver part (parts indicated by 104, 105, 106, 107, 108, 109,
and 110 in the enlarged view in FIG. 6, hereinafter referred to
also as annealing regions). While the glass substrate 1 is
reciprocatively shifted relatively a plurality of times as
necessary, the laser light 18 is sequentially irradiated. Depending
on a configuration of the fabrication apparatus, relative scanning
may be executed by shifting the optical system.
[0085] FIG. 16 is a perspective view for illustrating a condensed
state of the laser light, suitable for the laser annealing
according to the invention. Respective sizes of the annealing
regions 104 through 110 are, for example, 4 mm.times.100 .mu.m, and
these rectangular regions are set at pitches of 250 .mu.m.
Meanwhile, the size of a laser beam irradiated is 500
.mu.m.times.10 .mu.m. That is, as shown in FIG. 16, the laser beam
is formed in the shape of a rectangle (band) 500 .mu.m in the
longitudinal direction and 10 .mu.m in the transverse
direction.
[0086] Suitable energy density of the laser light irradiated at
this time is in a range of about 100.times.10.sup.3 W/cm.sup.2 to
500.times.10.sup.3 W/cm.sup.2, however, the optimum value varies
depending on scanning speed of the laser light, thickness of the
silicon film, whether the silicon film is amorphous or
polycrystalline, and so forth. In the case of using a laser
oscillator with output at 10 W, since the width of a region that
can be annealed by one scanning is 500 .mu.m, it is necessary to
execute one way scanning 8 times or reciprocative scanning 4 times
in order to implement annealing of a required width (4 mm). The
width of the region that can be annealed is dependent on the output
of the laser oscillator 6, so that if the output of the laser
oscillator 6 is sufficiently large, it is possible to irradiate a
larger region, thereby reducing scanning times. Or it is also
possible to render an irradiating laser beam in a shape smaller in
condensed light width or greater in the longitudinal direction.
[0087] FIG. 17 is a perspective view for illustrating a laser
irradiation region at the time of the laser annealing according to
the invention. While relatively shifting the glass substrate 1 at a
speed of 500 mm/s, the glass substrate 1 is irradiated up to a
length 100 .mu.m only at pitches of 250 .mu.m in a manner shown in
FIG. 17. That is, irradiation with the laser light is started at an
irradiation start position, and the stage 2 is relatively shifted
100 .mu.m while maintaining the irradiation with the laser light,
stopping the irradiation with the laser light at an irradiation
completion position.
[0088] Subsequently, at a spot where the stage 2 is shifted 250
.mu.m, irradiation start and irradiation termination are again
executed, which is repeated as many times as necessary. By so
doing, there is formed an annealing region approximately 500
.mu.m.times.100 .mu.m (in a stricter sense, an annealing region 500
.mu.m.times.110 .mu.m if the width of the irradiating laser light
is taken into account) at the pitches of 250 .mu.m, and as
explained in detail later, grain growth occurs in the direction of
scanning with the laser light.
[0089] FIG. 7 is a time chart for illustrating the embodiment of
the process of fabricating the display device, according to the
invention, showing timing of shifting the stage 2 and irradiating
the laser light, respectively. The controller 22 is provided with
counters C1 through C4 (not shown). Herein, there is described a
procedure for irradiating the laser light 18 while relatively
scanning the glass substrate 1 after turning the same ON/OFF with
the EO modulator 10.
[0090] By scanning in the X-direction as indicated by the arrow in
FIG. 6, the laser light 18 is irradiated to 1024 spots for a
distance of 100 .mu.m only at the pitches of 250 .mu.m. Every time
the stage 2 is shifted for a predetermined distance in the
X-direction, the linear scale (the linear encoder) 3 securely
attached to the x-axis of the stage 2 generates one pulse of pulse
signal. If the signal generated is a sinusoidal wave, the same may
be converted into a rectangular wave for use. By counting the pulse
signal, a position of the stage 2 can be detected. In the case of
the linear scale 3 with high precision, one pulse of the pulse
signal is generated by the linear scale 3, for example, for every
0.1 .mu.m in shift amount. In case a pulse interval is large, the
pulse interval can be rendered into smaller intervals through
electrical division.
[0091] The stage 2 requires a given distance (an acceleration
region) to reach a given speed from a stop condition. Assuming that
a stage speed at the time of laser irradiation is 500 mm/s, the
acceleration region on the order of 50 mm is necessary, and
positioning of the stage 2 is made at a position (at Xs in FIG. 7)
not less than 50 mm, as the acceleration region, for example, 60
mm, to the left from the irradiation start position (the left edge
of the annealing region 104 in FIG. 6) before stopping.
[0092] At this point in time, a counter circuit C1 (the counter C1)
for counting the pulse signal from the linear scale 3 according to
a command of the controller 22 starts counting after clearing
counter numbers once and concurrently, starts driving the stage 2.
The counter circuit C1 counts the pulse signal generated following
a shift of the stage 2, and sends out a gate-ON signal (gate ON) at
a point in time when the stage 2 has reached the first irradiation
start position X1, that is, pulse numbers n1 (600000 pulses)
correspond to a shift of 60 mm are counted. The gate-ON signal
opens a gate to the power source 21 of the EO modulator 10,
enabling the signal to be transmitted thereto. By this point in
time, the stage speed has completed acceleration, having reached
the given speed.
[0093] Upon receiving the gate-ON signal, a counter circuit C3 (the
counter C3) sends out an ON signal (EOM-ON) for the power source 21
of the EO modulator, and concurrently, starts counting after
clearing count numbers, thereafter sending out the ON signal to the
power source 21 of the EO modulator every time when pulse numbers
n3 (2500 pulses) corresponding to a pitch of irradiation are
counted. Meanwhile, a timer T1 (timer 1) (not shown) incorporated
in the controller 22.
[0094] Meanwhile, upon receiving the ON signal to the power source
21 of the EO modulator, a counter circuit C4 (the counter C4)
starts counting while clearing count numbers, and sends out an OFF
signal (EOM-OFF) to the power source 21 of the EO modulator at a
point in time when pulse numbers n4 (1000 pulses) corresponding to
the length 100 .mu.m of the annealing region are counted. This
operation is repeated every time when the counter circuit C3 sends
out the ON signal for the power source 21 of the EO modulator.
[0095] During a time period from receipt of the ON signal for the
power source 21 of the EO modulator until receipt of the OFF signal
for the power source 21 of the EO modulator (time required for the
stage 2 to pass over a distance of 100 .mu.m at the stage speed of
500 mm/s), the power source 21 of the EO modulator applies a
voltage for causing the polarization direction of the laser light
18 to be rotated by 90 degrees to the Pockels cell 61. As a result,
the laser light 18 passes though the EO modulator 10 to be
irradiated to the substrate 1 only for time corresponding to time
when the voltage is applied to the Pockels cell 61.
[0096] Meanwhile, upon receiving the gate-ON signal from the
counter circuit C1, a counter circuit C2 (the counter C2) clears
count numbers, counts the OFF signals for the power source 21 of
the EO modulator, send out by the counter circuit C4, and closes
the gate at a point in time when pulse numbers n2 (1024 pulses)
corresponding to the number of the annealing regions are counted.
As a result, the power source 21 of the EO modulator no longer
receives the ON signal for the power source 21 of the EO modulator,
and the OFF signals for the power source 21 of the EO modulator, so
that the power source 21 of the EO modulator stops its
operation.
[0097] In accordance with the procedure described above, the first
laser annealing for the driver circuit region 102 shown in FIG. 6
is completed, however, since the driver circuit region, in
practice, is several mm in width, the glass substrate 1 in whole
cannot be annealed with one scanning. Accordingly, after shifting
the glass substrate 1 in the Y-direction by a given pitch (with the
present embodiment, 500 .mu.m), the above-described procedure is
repeated. By so doing, in a state where the stage 2 is continuously
shifted, the laser light 18 can be irradiated with high precision
without being affected in any way by variation in the stage speed.
However, when scanning is repeated, there are cases where annealed
portions are overlapped in parallel with a scanning direction or
portions not irradiated with the laser light 18 occur. Because
grain growth is disturbed in those portions, it is desirable to
consider a layout design such that no transistor is formed in
portions where scanned parts are overlapped with each other.
[0098] Now, behavior of a polycrystalline silicon thin film upon
irradiation thereof with the laser light 18 is described
hereinafter.
[0099] As previously described, with the present embodiment, the
glass substrate 1 obtained by forming the polycrystalline silicon
thin film on the top surface of the glass substrate by the excimer
laser annealing (that is, reformation) is used as the object for
annealing.
[0100] FIG. 8 is a plan view showing crystal morphology prior to
the laser annealing according to the embodiment of the process of
fabricating the display device according to the invention, FIG. 9
is a plan view showing crystal morphology after the laser
annealing, and FIG. 10 is a plan view of the glass substrate 1,
showing a relationship in position between regions where the laser
annealing is applied and active regions of the driver circuit. The
polycrystalline silicon thin film obtained by the excimer laser
annealing is an aggregate of fine crystal grains 120, 121, not more
than 1 .mu.m (several 100 nm) in grain size as shown in FIG. 8.
Upon irradiation of regions shown in the figure with the laser
light, the fine crystal grains 120 outside of laser irradiation
regions are left intact, however, fine crystal grains (for example,
the crystal grains 121) within the laser irradiation regions melt.
Thereafter, with passage of each of the laser irradiation regions,
the crystal grains 121 are rapidly solidified and
recrystallized.
[0101] Hereupon, melted silicon uses crystal grains remaining on
the periphery of melted parts as seed crystals, and crystals
following the crystal orientation of the seed crystals undergo
growth in the scanning direction of the laser light in accordance
with temperature gradient. At this point in time, since a growth
rate of crystal grains varies depending on the crystal orientation,
only the crystal grains having the crystal orientation with the
fastest growth rate eventually continue crystal growth.
[0102] More specifically, as shown in FIG. 9, growth of a crystal
grain 122 having the crystal orientation with a slow growth rate is
suppressed by growth of crystal grains 124, 126, surrounding the
crystal grain 122, having the crystal orientation with a faster
growth rate, respectively, so that crystal growth thereof stops.
Further, a crystal grain 123 and the crystal grain 124, having the
crystal orientation with an intermediate growth rate, respectively,
continue growth, however, growth thereof is suppressed by growth of
crystal grains 125, 126, having the crystal orientation with a
still faster growth rate, respectively, so that crystal growth
thereof stops before long. Eventually only the crystal grains 125,
126, and a crystal grain 127, having the crystal orientation with
the fastest growth rate, respectively, continue growth. These
crystal grains 125, 126, 127, continuing crystal growth until the
last are individual crystal grains in a strict sense, but have
nearly the same crystal orientation, so that a melted and
recrystallized part can be effectively regarded as a single
crystal.
[0103] By irradiating the laser light to the polycrystalline
silicon thin film as described hereinbefore, only portions
irradiated with the laser light are annealed in the shape of
islands, and only crystal grains having a specific crystal
orientation undergo growth, thereby forming regions of the crystal
grains 125, 126, 127, having properties substantially similar to
those of a single crystal, although in polycrystalline state in a
strict sense. These regions can be effectively regarded as a single
crystal particularly in the direction not crossing the grain
boundaries, that is, in the scanning direction of the laser light.
With the present invention, the silicon film crystallized in this
way is called a band-like polycrystalline silicon film.
[0104] By repeating the procedure described while relatively
scanning the gals substrate 1, and by sequentially irradiating
portions requiring annealing with the laser light, all the
thin-film transistor (thin-film transistor of the driver part)
forming regions of the driver circuit can be converted into regions
of the band-like polycrystalline silicon film having properties
similar to those of the single crystal. Further, because in the
regions having properties similar to those of the single crystal,
the growth of the crystal grains has taken place in a given
direction as shown in FIG. 9, it is possible to cause a direction
of current flow to coincide with a direction of grain growth when
forming the transistors, thereby preventing current from flowing in
such a direction as to cross grain boundaries.
[0105] FIG. 11 is a plan view of a substrate, showing a
configuration of the thin-film transistors of the driver part
formed by the laser annealing according to the invention. That is,
alignment can be made such that portions of a laser irradiation
region 301, made up of the crystal grains having a high growth
rate, respectively, correspond to active layers (or active regions)
302, 303 of the thin film transistors of the driver circuit,
respectively, as shown in FIG. 10. After impurities implantation
process and photo-etching process, regions other than the active
regions 302, 303 are removed, and there are formed a gate electrode
305 through the intermediary of a gate insulating film, a source
electrode 306 and a drain electrode 307, having ohmic connection,
thereby completing a thin-film transistor. In this case, grain
boundaries 304, 304' exist in the active region 303, but since
current flows between the source electrode 306 and the drain
electrode 307, the current does not flow across the grain
boundaries 304, 304', so that mobility equivalent to that in the
case where the active region is effectively made of a single
crystal can be obtained.
[0106] As described above, in the part melted and reacrystallized
by the laser annealing according to the invention, by causing the
direction of current flow to coincide with the direction of crystal
grain boundaries so as not to allow current to flow in such a
direction as to cross the grain boundaries, mobility can be
improved more than twice as compared with a case where annealing
with the excimer laser is simply applied, more specifically, to
more than 350 cm.sup.2/Vs. Such a mobility value is sufficient to
constitute a driver circuit capable of driving liquid crystals at a
high speed.
[0107] Meanwhile, transistors for switching the pixel part
(thin-film transistors of the pixel part) are formed in the region
101 of polycrystalline silicon film, where annealing with the
excimer laser is simply applied. With a polycrystalline film
obtained by the annealing with the excimer laser, crystal grains
are fine, and the crystal orientations thereof are at random, so
that mobility is small as compared with the case of the crystal
grains obtained by the excimer laser annealing according to the
invention, however, such mobility is good enough for the thin-film
transistors for switching the pixel part, that is, the thin-film
transistors of the pixel part.
[0108] In some cases, even an amorphous silicon film can be
sufficient for use in the thin-film transistors of the pixel part.
In such a case, an amorphous silicon thin film is formed on top of
the glass substrate 1, and it is sufficient that the excimer laser
annealing according to the invention is not applied thereto while
the excimer laser annealing according to the invention is applied
to the driver circuit parts only. Silicon initially melted by
irradiation with the laser light 18 is turned into fine
polycrystalline state in the course of solidification thereof, and
crystal grains formed in this stage serve as seed grains, whereupon
growth of crystal grains having various crystal orientations takes
place, however, as with the case where the silicon film in
polycrystalline state, formed by the excimer laser annealing, is
irradiated with the laser light 18, only crystal grains having
crystal orientations with the fastest growth rate eventually
continue crystal growth, thereby forming a polycrystalline silicon
thin film regarded effectively as a single crystal.
[0109] After completion of the laser annealing applied to the
driver circuit region 102 shown in FIG. 6, the driver circuit
region 102' needs annealing. In this case, the glass substrate 1
may be rotated by 90 degrees or a scanning direction may be altered
by 90 degrees. In the latter case, it is necessary to rotate a
beam-forming device (in FIG. 1, the cylindrical lens 11) by 90
degrees and to switch over between the transverse direction and the
longitudinal direction of the rectangular slit. Further, the driver
circuit region 102 shown in FIG. 6 is normally a data driver
circuit region (referred to as a drain driver when thin-film
transistors are used as the active elements) and the driver circuit
region 102' shown in FIG. 6 is a scanning driver circuit region
(referred to as a gate driver when thin-film transistors are used
as the active elements).
[0110] In the case of the glass substrate 1 shown in FIG. 6, if
transistors requiring fast operation can be put together in either
of the driver circuit regions 102, 103, for example, in the driver
circuit region 102, the laser annealing according to the invention
may be applied to the driver circuit region 102 only. In other
words, the active layers (active regions) of the transistors formed
in the driver circuit region 102 are made of polycrystalline
silicon containing crystal grains having no grain boundaries in the
direction of current flow, so that fast operable transistors can be
obtained.
[0111] On the other hand, since thin-film transistors not requiring
so fast operation are formed in the driver circuit region 102', the
active layers (active regions) of the transistors are made up of a
polycrystalline silicon film composed of fine crystal grains,
simply annealed by the excimer laser. In this case, since it
becomes unnecessary to rotate the substrate or the scanning
direction and the direction of the linear beam, and in addition,
regions to be annealed can be reduced, the effect of improvement in
throughput is large.
[0112] FIG. 18 is a plan view of an insulating substrate for
illustrating another embodiment of a process of fabricating the
display device, according to the invention. With the present
embodiment, driver circuit regions as objects for the laser
annealing are put together on one side of an insulating substrate
(glass substrate). As shown in FIG. 18, if a driver circuit region
602 formed on top of a glass substrate 1 can be put together on one
side thereof, outside of a pixel region 601, active layers (active
regions) of all thin-film transistors of a driver part are made up
of a polycrystalline silicon film containing crystal grains having
no grain boundaries in the direction of current flow, so that fast
operable transistors can be obtained. In addition, it becomes
unnecessary to rotate the substrate or a scanning direction and the
direction of a linear beam, so that such a configuration is
preferable from the viewpoint of improvement in throughput.
However, a plurality of alignment marks, for example, alignment
marks 603, 603', shown in the figure, are evidently required.
[0113] Further, with reference to the embodiment previously
described, it has been described that the signals from the linear
scales (linear encoders) installed in the stage 2 are counted for
detecting the position or shift amount of the stage 2. However, the
invention is not limited thereto, and use may be made of output
signals from a measuring machine using laser light interference, a
rotary encoder installed on the axis of a motor driving the stage,
and so forth. A process of fabricating a thin-film transistor
substrate (an active matrix substrate) including the
above-described procedure for reformation of the band-like
polycrystalline silicon film according to the invention can be
summed up in respective flowcharts in FIGS. 22 and 23.
[0114] FIG. 22 is a flowchart for illustrating the steps of
fabricating a display device, to which the process of fabrication
according to the invention is applied. In this case, the steps of
fabricating a liquid crystal display device are described by way of
example. FIG. 23 is a flowchart for illustrating the step of
annealing according to the invention. The respective steps are
indicated by reference numerals P-XX. As shown in FIG. 22, an
insulating film is formed on top of a substrate (P-1), an a --Si
(amorphous silicon) film is formed (P-2), and excimer laser
annealing is applied (P-3) before applying laser annealing
according to the invention (P - 4) only to active layer part of
respective transistors making up a driver circuit, and the
periphery thereof. Details of a process of the laser annealing
according to the invention (P-4) are shown in FIG. 23.
[0115] In FIG. 23, the substrate (glass substrate) with the excimer
laser annealing applied thereto (P-3) is mounted on the stage 2 of
a fabrication apparatus (laser annealing apparatus) according to
the invention, described with reference to FIG. 1 (P-41),
pre-alignment is executed in edge faces or corners of the substrate
(P-42), and alignment marks are formed by laser beam machining
(P-43). After execution of alignment (fine alignment) by detecting
the alignment marks (P-44), laser annealing is applied in
accordance with design data only to the active layer part of the
respective transistors making up the driver circuit, and the
periphery thereof (P-45). If the alignment marks have already been
formed by other means such as a photo resist process and so forth
at the time of mounting the substrate on the laser annealing
apparatus, there is no need for the steps of executing the
pre-alignment (P-42), and forming the alignment marks (P-43). After
repeating the laser annealing until all intended regions are
annealed (P-46), the substrate is transported (P-47).
[0116] Thereafter, as shown in FIG. 22, in the step pf
photo-etching (P-5), only necessary portions of a polycrystalline
silicon film are left intact in the shape of islands on the basis
of the alignment marks 103, 103' or coordinates of an origin point
as calculated from the alignment marks 103, 103'. Subsequently,
after a photo-resist process comprising the step of gate insulating
film formation (P-6), and the step of gate electrode formation
(P-7), there are executed the steps of impurities implantation
(P-8), and activation of implanted regions (P-9), respectively.
Thereafter, after a further photo-resist process comprising the
steps of interlayer insulator formation (P-10), source electrode
and drain electrode formation (P-11), and passivation film
formation (P-12), there are formed the driver circuit and the pixel
part 101, thereby completing a TFT substrate (LCD (panel) step)
(P-13). The alignment marks 103, 103' are used for alignment in at
lest one photo-resist process after the laser annealing according
to the invention. Thereafter, alignment marks newly formed by the
photo-resist process described as above may be used. The thin-film
transistors in FIG. 11 are shown merely by way of example, and the
invention is not limited thereto. It is obvious that thin-film
transistors can have various constructions, and the thin-film
transistors of the invention may be formed in such a manner as to
have constructions without departing from the spirit and scope of
the invention.
[0117] Meanwhile, the transistors for switching the pixel part (the
thin-film transistors of the pixel part) are formed in the region
101 of polycrystalline silicon film, where the annealing with the
excimer laser is simply applied. That is, after a photo-resist
process executed for gate insulating film formation, gate electrode
formation, impurities implantation, activation of implanted
regions, source electrode and drain electrode formation,
passivation film formation, and so forth, on the basis of the
alignment marks 103, 103' or coordinates of an origin point as
calculated from the alignment marks 103, 103', there is completed a
TFT substrate.
[0118] Subsequently, after executing the LCD (panel) step of
forming an alignment layer on the TFT substrate as completed,
overlaying color filters on top of the TFT substrate after a
rubbing step, and sealing in liquid crystal material, and a module
step (P-14) of assembling together with a backlight, there is
completed a liquid crystal display device (so-called system on
panel) with a high-speed driver circuit formed on the glass
substrate thereof.
[0119] With the above-described embodiment of the invention, it has
been described that the polycrystalline silicon thin film formed by
the excimer laser annealing, composed of fine crystal grains, is
used as the object for the laser annealing according to the
invention. When a polycrystalline silicon thin film is formed
directly on the substrate, polycrystalline silicon (Poly-Si) film
formation is substituted for non-crystalline, that is, amorphous
silicon (a --Si) film formation in the flowchart shown in FIG. 22,
so that the step of applying the excimer laser annealing can be
omitted, still obtaining the same advantageous effect as that for
the above-described embodiment.
[0120] FIG. 12 is a schematic representation for illustrating
electronic equipment with the display device of the invention
mounted therein. The display device of the invention can be mounted
in a display part of a TV receiver 401 shown in FIG. 12A, a mobile
phone 402 shown in FIG. 12B, or a notebook PC 403 shown in FIG.
12C. Other applications include a display of various instruments
housed in a dashboard of a car, a display of a potable game device,
a monitor display of a VTR or digital camera, and so on. Further,
the display device of the invention can be used as a display device
using an organic electro-luminescent display panel and other panel
type display devices besides the liquid crystal display device
using a liquid crystal display panel.
[0121] Now, there is described hereinafter another embodiment of a
process of fabricating the display device according to the
invention. FIG. 13 is a time chart showing timing of shifting a
stage and irradiating laser light, respectively, with reference to
a laser annealing method, which is said embodiment of the process
of fabricating the display device according to the invention. There
is described an example where as with the case of the
previously-described embodiment, while scanning in the X-direction
indicated by the arrow in FIG. 6, laser light is irradiated to 1024
spots for a distance of 100 .mu.m only at the pitches of 250 .mu.m.
The embodiment differs from the previously described embodiment in
respect of a procedure for turning the laser light ON/OFF with an
EO modulator while relatively scanning the substrate.
[0122] The linear scale 3 securely attached to the x-axis of the
stage 2 in FIG. 1 generates a pulse signal at a given interval so
as to correspond to a shift of the stage 2 in the X-direction. If
the signal generated is a sinusoidal wave, the same may be
converted into a rectangular wave for use. By counting the pulse
signal, a position of the stage 2 can be detected. In the case of
the linear scale 3 with high precision, one pulse of the pulse
signal is generated by the linear scale 3, for example, for every
0.1 .mu.m in shift amount. In case a pulse interval is large, the
pulse interval can be rendered into smaller intervals through
electrical division.
[0123] The stage 2 requires a given distance (an acceleration
region) to reach a given speed from a stop condition. Assuming that
a stage speed at the time of laser irradiation is 500 mm/s, the
acceleration region on the order of 50 mm is necessary, and
positioning of the stage 2 is made at a position (at Xs in FIG. 7)
not less than 50 mm, as the acceleration region, for example, 60
mm, to the left from the irradiation start position (the left edge
of the annealing region 104 in FIG. 6) before stopping.
[0124] At this point in time, a counter circuit C1 (the counter C1)
for counting the pulse signal from the linear scale 3 according to
a command of the controller 22 starts counting after clearing
counter numbers once and concurrently, starts driving the stage 2.
The counter circuit C1 counts the pulse signal generated following
a shift of the stage 2, and sends out a gate-ON signal (gate ON) at
a point in time when the stage 2 has reached the first irradiation
start position X1, that is, pulse numbers n1 (600000 pulses)
correspond to a shift of 60 mm are counted. The gate-ON signal
opens a gate to the power source 21 of the EO modulator 10,
enabling the signal to be transmitted thereto. By this point in
time, the stage speed has completed acceleration, having reached
the given speed.
[0125] Upon receiving the gate-ON signal (gate ON), a counter
circuit C3 (the counter C3) sends out an ON signal (EOMON) for the
power source 21 of the EO modulator, and concurrently, starts
counting after clearing count numbers, thereafter sending out the
ON signal to the power source 21 of the EO modulator every time
when pulse numbers n3 (2500 pulses) corresponding to a pitch of
irradiation are counted.
[0126] Meanwhile, upon receiving the ON signal of the power source
21 of the EO modulator, a timer T1 (not shown) (timer 1)
incorporated in the controller 22 starts counting time, and sends
out an OFF signal (EOMOFF) to the power source 21 of the EO
modulator at a point in time, with the elapse of time (200 .mu.s)
required for shifting across an anneal distance of 100 .mu.m.
Otherwise, upon receiving the ON signal of the power source 21 of
the EO modulator, a pulse signal having a pulse width corresponding
to time (200 .mu.s) required for shifting across an anneal distance
of 100 .mu.m may be generated. This operation is repeated every
time the ON signal of the power source 21 of the EO modulator is
received.
[0127] During a time period from receipt of the ON signal for the
power source 21 of the EO modulator until receipt of the OFF signal
for the power source 21 of the EO modulator (time required for the
stage 2 to pass over a distance of 100 .mu.m at the stage speed of
500 mm/s), the power source 21 of the EO modulator applies a
voltage for causing the polarization direction of the laser light
18 to be rotated by 90 degrees to the Pockels cell 61. As a result,
the laser light 18 passes though the EO modulator 10 to be
irradiated to the substrate 1 only for time corresponding to time
when the voltage is applied to the Pockels cell 61.
[0128] In this connection, for use as the power source 21 of the EO
modulator, there is available another type capable of applying a
voltage waveform corresponding to a pulse signal waveform by
receiving a pulse signal from outside. In such a case, a pulse
generator may be used in place of the timer T1. That is, the ON
signal of the power source 21 of the EO modulator, generated every
time the pulse numbers n3 (2500 pulses) corresponding to the pitch
of irradiation are counted by the counter circuit C3 is delivered
to the pulse generator to thereby generate a signal with a
predetermined pulse width, that is, a pulse width corresponding to
time required for the laser light passing through the annealing
region (in the case of the present embodiment, the pulse width: 200
.mu.s), and the signal is delivered to the power source 21 of the
EO modulator. By so doing, as with the above-described embodiment,
the laser light can be irradiated to necessary regions on the
substrate 1.
[0129] Meanwhile, upon receiving the gate-ON signal from the
counter circuit C1, a counter circuit C2 (the counter C2) clears
count numbers, counts the OFF signals for the power source 21 of
the EO modulator, sent out by the counter circuit C4, or output
pulses of the pulse generator, and closes the gate at a point in
time when pulse numbers n2 (1024 pulses) corresponding to the
number of the annealing regions are counted. As a result, the power
source 21 of the EO modulator no longer receives the ON signal for
the power source 21 of the EO modulator, and the OFF signals for
the power source 21 of the EO modulator, so that the power source
21 of the EO modulator stops its operation.
[0130] With the present embodiment, the irradiation start position
of the laser light is controlled by a position of the stage,
however, the irradiation completion position of the laser light is
regulated by time elapsed from the start of laser irradiation or a
pulse width of the output pulse of the pulse generator.
Accordingly, if there is variation in the stage speed, there is a
possibility of slight variation of the irradiation completion
position, corresponding to the variation in the stage speed. When
the stage of relatively large mass is being shifted at a high
speed, however, the variation in the stage speed is slight, and an
effect due to the variation in the stage speed is effectively very
small. In case the stage speed varies on the order of .+-.1%, there
is no variation in the irradiation start position, and variation of
the irradiation completion position is to the extent of about of 1
.mu.m, so that no problem occurs in practice.
[0131] In accordance with the procedure described above, the first
laser annealing for the driver circuit region 102 shown in FIG. 6
is completed, however, since the driver circuit region, in
practice, is several mm in width, the glass substrate 1 in whole
cannot be annealed with one scanning. Accordingly, after shifting
the glass substrate 1 in the Y-direction by a given pitch (with the
present embodiment, 500 .mu.m), the above-described procedure is
repeated. By so doing, in a state where the stage 2 is continuously
shifted, the laser light 18 can be irradiated with high precision
without being affected in any way by variation in the stage speed.
However, when scanning is repeated, there are cases where annealed
portions are overlapped in parallel with a scanning direction or
portions not irradiated with the laser light 18 occur. Because
grain growth is disturbed in those portions, it is desirable to
consider a layout design such that no transistor is formed in
portions where scanned parts are overlapped with each other.
Further, a change occurring to the crystal grains of the
polycrystalline silicon thin film upon irradiation thereof with the
laser light 18 is as previously described.
[0132] As with the case of the previously described embodiment, if
on the glass substrate 1 shown in FIG. 6, transistors requiring
fast operation can be put together in either of the driver circuit
regions 102, 103, for example, in the driver circuit region 102,
the laser annealing according to the invention may be applied to
the driver circuit region 102 only. In other words, the active
layers (active regions) of the transistors formed in the driver
circuit region 102 are made of polycrystalline silicon containing
crystal grains having no grain boundaries in the direction of
current flow, so that fast operable transistors can be obtained. On
the other hand, since thin-film transistors not requiring so fast
operation are formed in the driver circuit region 102', the active
layers (active regions) of the transistors are made up of a
polycrystalline silicon film composed of fine crystal grains,
simply annealed by the excimer laser. In this case, since it
becomes unnecessary to rotate the substrate or the scanning
direction and the direction of the linear beam, and in addition,
regions to be annealed can be reduced, the effect of improvement in
throughput is large.
[0133] Or as shown in FIG. 18, if a driver circuit region 602
formed on top of the glass substrate 1 can be put together on one
side thereof, outside of a pixel region 601, active layers (active
regions) of all thin-film transistors of the driver circuit are
made up of a polycrystalline silicon film containing crystal grains
having no grain boundaries in the direction of current flow, so
that fast operable transistors can be obtained. In this case as
well, it becomes unnecessary to rotate the substrate or a scanning
direction and the direction of a linear beam, so that such a
configuration is preferable from the viewpoint of improvement in
throughput. However, a plurality of alignment marks, for example,
alignment marks 603, 603', shown in the figure, are evidently
required.
[0134] Further, with reference to the embodiment previously
described, it has been described that the signals from the linear
scales (linear encoders) installed in the stage 2 are counted for
detecting the position or a shift amount of the stage 2. However,
the invention is not limited thereto, and use may be made of output
signals from a measuring machine using laser light interference, a
rotary encoder installed on the axis of a motor driving the stage,
and so forth.
[0135] Now, there is described hereinafter still another embodiment
of a process of fabricating a display-device according to the
invention. In the previously described embodiment, there has been
shown a case where the annealing regions each approximately 500
.mu.m.times.100 .mu.m are juxtaposed at the pitches of 250 .mu.m.
Herein, another case where the annealing regions are juxtaposed at
closer pitches is described.
[0136] Assuming a case of annealing a region 4 mm in width by
scanning with laser beam 500 .mu.m.times.10 .mu.m by way of
example, an annealing width by one scanning is preferably increased
as much as possible within a permissible range of laser output, and
an annealing length (a size of an annealing region, in the scanning
direction) and a pitch of the annealing region are preferably an
integral multiple of a pixel pitch, respectively. In this case,
assuming that a pixel pitch is 250 .mu.m, one annealing region is
set to 500 .mu.m.times.500 .mu.m.
[0137] First, regions each 500 .mu.m.times.500 .mu.m in size are
irradiated at pitches of 1 mm by a first scanning. In this case, as
shown in FIG. 28, the respective regions 500 .mu.m.times.500 .mu.m
are irradiated with laser light, and annealed regions are formed at
the pitches of 1 mm. In the respective annealed regions 801, 802,
803, there occurs a decrease of film thickness in regions 811, 812,
813, about 10 .mu.m wide, respectively, of an anneal start part,
due to melted silicon being pulled in the scanning direction by
interfacial tension. In respective regions 821, 822, 823, about 10
.mu.m wide, of an anneal termination part, a swell (protrusion) is
formed The respective annealed regions 801, 802, 803, sandwiched
between those regions, are well annealed, forming a pseudo-single
crystal film, respectively.
[0138] Subsequently, by executing annealing at pitches of 1 mm
after shifting an irradiation start position by 500 .mu.m in the
scanning direction while keeping an irradiation region set at 500
.mu..mu.m.times.500 .mu.m as above, regions 804, 805, not annealed
at the time the precious scanning, are annealed, thereby forming
respective annealed regions 500 .mu.m wide as shown in FIG. 29.
However, as previously described, since film thickness is decreased
or a swell is formed in an irradiation start part and irradiation
termination part, respectively, regions unsuitable for formation of
transistors 811, 842, 843, 844, 845, 823, about 10 .mu.m wide,
respectively, are formed at pitches of 500 .mu.m.
[0139] Now, dwelling on the region 842, it can be said that this
part corresponds to an anneal termination part at the first
scanning and a protrusion is formed therein, but at the second
scanning, this part corresponds to an anneal start part, so that
the protrusion is substantially eliminated, but is still unsuitable
for formation of transistors in contrast with a normal region, for
example, 801. Further, dwelling on the region 843, this part
corresponds to an anneal start part at the first scanning, and film
thickness decreases, but this part corresponds to an anneal
termination part at the second scanning and a protrusion is formed
therein, so that this part is unsuitable for formation of
transistors.
[0140] Next, similar annealing is executed by shifting 500 .mu.m in
the direction normal to the scanning direction, which is repeated
until an entire width requiring annealing is annealed. With the
present embodiment, a width 4 mm is to be annealed, scanning of 8
rows, that is, scanning is repeated 16 times.
[0141] As shown in FIG. 30, when laser light is irradiated by
shifting 500 .mu.m in the direction normal to the scanning
direction, in consequence of such operation, irradiation is
duplicated, portions not irradiated are left out, or portions
previously irradiated are subjected to thermal effects at the time
of a successive irradiation, thereby disturbing crystal morphology,
in overlapped parts 851, 852. Accordingly, there are left out the
regions 851, 852, unsuitable for formation of transistors.
[0142] Having taken those into consideration, it can be said that
well annealed regions (that is, pseudo-single crystal regions) each
490 .mu.m.times.490 .mu.m are finally formed at pitches of 500
.mu.m as shown FIG. 31. In the interests of simplicity, it can be
said that pseudo-single crystal silicon films each 490
.mu.m.times.490 .mu.m are formed at pitches of 500 .mu.m on the
glass substrate in such a manner as tiles are stuck thereto. By
designing such that a transistor can be disposed on the respective
tiles of pseudo-single crystal silicon, high performance
transistors can be formed.
[0143] Herein, there has been described a case where annealing is
executed by scanning in the same direction, however, irradiation
may be carried out by setting such that there is a shift of the
irradiation position by 500 .mu.m between scanning in one direction
and scanning in the opposite direction in the case of reciprocative
scanning. In this case, there will be a change in arrangement of
parts with decreased film thickness and parts with a protrusion, as
finally obtained, however, since any thereof is about 10 .mu.m in
width, and is unsuitable for formation of transistors,
pseudo-single crystal regions suitable for formation of transistors
will the same as those shown in FIG. 31.
[0144] Assuming a case where transistors to be formed are to
constitute a driver circuit 981 for signal lines, formed on top of
a glass substrate 980, as shown in FIG. 35, thee are formed 6
circuits 983 each for driving 2 pixels at a pitch of 250 .mu.m,
more exactly, 2 pixels each comprising 1 dot each of R, G, B, that
is, 6 dots, in one pseudo-single crystal region 982 formed at
pitches of 500 .mu.m. Generally, in one pseudo-single crystal
region, circuits are formed at identical pitches to form a circuit
group, and these circuit groups are formed at pitches for forming
the pseudo-single crystal regions. That is, these are configured
such that, on the glass substrate, the circuits having an identical
function for driving respective signal lines are disposed not at an
equal interval throughout one panel, but a plurality of the circuit
groups having an identical function are disposed at identical
pitches.
[0145] Assuming a further embodiment where a region 4 mm in width
is annealed by scanning with laser beam 500 .mu.m.times.10 .mu.m by
way of example, an annealing width by one scanning is preferably
increased as much as possible within a permissible range of laser
output, and an annealing length (a size of an annealing region, in
the scanning direction) and a pitch of the annealing region are
preferably an integral multiple of a pixel pitch, respectively. In
this case as well, assuming that a pixel pitch is 250 .mu.m, one
annealing region is set to 500 .mu.m.times.500 .mu.m, and
irradiation is executed at pitches of 500 .mu.m.
[0146] First, regions each 500 .mu.m.times.490 .mu.m in size are
irradiated at pitches of 1 mm by a first scanning as shown in FIG.
32. In this case, the respective regions 500 .mu.m.times.490 .mu.m
are irradiated with laser light, and annealed regions are formed at
the pitches of 500 .mu.m. In the respective annealed regions 901,
902, 903, 904, 905, there occurs a decrease of film thickness in
regions 911, 912, 913, 914, 915, about 10 .mu.m wide, respectively,
of an anneal start part, due to melted silicon being carries away
by interfacial tension. In respective regions 921, 922, 923, 924,
925, about 10 .mu.m wide, of an anneal termination part, a swell
(protrusion) is formed The respective annealed regions 901, 902,
903, 904, 905, sandwiched between those regions, are well annealed,
forming a pseudo-single crystal film, respectively.
[0147] Further, with the present embodiment, there is left out a
non-irradiated region 10 .mu.m wide between the respective
irradiation regions, however, these regions are necessary regions
in order to temporarily stop crystal growth taking place up to then
to thereby induce new crystal growth, and also, to interrupt
buildup of heat in the substrate due to laser irradiation.
[0148] Next, similar annealing is executed by shifting 500 .mu.m in
the direction normal to the scanning direction, which is repeated
until an entire width requiring annealing is annealed. With the
present embodiment, a width 4 mm is to be annealed, scanning of 8
rows, that is, scanning is repeated 16 times.
[0149] When laser light is irradiated by changing rows, in
consequence of such operation, irradiation is duplicated, portions
not irradiated are left out, or portions previously irradiated are
subjected to thermal effects at the time of a successive
irradiation, thereby disturbing crystal morphology, in overlapped
parts 951, 952, 953, 954, 955, 956, 957, 958, 959, 960.
Accordingly, there are left out the regions about 10 .mu.m wide,
unsuitable for formation of transistors.
[0150] Having taken those into consideration, it can be said that
well annealed regions (that is, pseudo-single crystal regions) each
490 .mu.m.times.470 .mu.m are finally formed at pitches of 500
.mu.m as shown FIG. 31. In the interests of simplicity, it can be
said that pseudo-single crystal silicon films each 490
.mu.m.times.470 .mu.m are formed at pitches of 500 .mu.m on the
glass substrate in such a manner as tiles are stuck thereto. By
designing such that a transistor can be disposed on the respective
tiles of pseudo-single crystal silicon, high-performance
transistors can be formed.
[0151] Herein, there has been described a case where annealing is
executed by scanning in the same direction, however, irradiation
may be carried out by setting such that there is a shift of the
irradiation position by 500 .mu.m between scanning in one direction
and scanning in the opposite direction in the case of reciprocative
scanning. In this case, there will be a change i n arrangement of
parts with decreased film thickness and parts with a protrusion
from row to row, however, since any thereof is about 10 .mu.m in
width, and is unsuitable for formation of transistors,
pseudo-single crystal regions suitable for formation of transistors
will the same as those shown in FIG. 31. In comparison with the
previously described embodiment, the respective pseudo-single
crystal regions become slightly narrower, however, throughput is
increased about twice as much.
[0152] Assuming a case where transistors to be formed are to
constitute a driver circuit 981 for signal lines, formed on top of
a glass substrate 980, as shown in FIG. 35, thee are formed 6
circuits 983 each for driving 2 pixels at a pitch of 250 .mu.m,
more exactly, 2 pixels each comprising 1 dot each of R, G, B, that
is, 6 dots, in one pseudo-single crystal region 982 formed at
pitches of 500 .mu.m. Generally, in one pseudo-single crystal
region, circuits are formed at identical pitches to form a circuit
group, and these circuit groups are formed at pitches for forming
the pseudo-single crystal regions. That is, these are configured
such that, on the glass substrate, the circuits having an identical
function for driving respective signal lines are disposed not at an
equal interval throughout one panel, but a plurality of the circuit
groups having an identical function are disposed at identical
pitches.
[0153] In the foregoing description, there has been described that
an annealing width, annealing length, and a pitch regulate
respective laser annealing regions, however, respective sizes can
be converted into pulse numbers as generated by the linear scales
installed in the stage 2. Accordingly, it is evident that timing
for ON/OFF of the laser light can be implemented by activating at a
point in time when corresponding pulse numbers are counted,
respectively. However, detailed description in this connection is
omitted herein.
[0154] FIG. 14 is a schematic representation for illustrating
another embodiment of an apparatus, that is, a laser annealing
apparatus for fabricating the display device according to the
invention. The apparatus according to the present embodiment
comprises a stage 502 on which a large sized substrate 502 is
placed, a plurality of optical body tubes 503 each provided with a
laser irradiation optical system, adjustment stages 504 for
independently adjusting the respective positions of the optical
body tubes 503, a stand 505 (partly shown in the figure) for
holding the adjustment stage 504, continuous-wave oscillators 506,
laser diode power sources 507 for pumping the respective
continuous-wave oscillators 506, fibers 508 for transmitting pumped
light, and linear scales 509, 510, for detecting the position of
the stage 502.
[0155] FIG. 15 is a schematic representation for illustrating the
optical system in FIG. 14. Inside the respective optical body tubes
503 shown in FIG. 14, there is provided the laser irradiation
optical system comprising a shutter 511, a beam expander 512, a
continuously variable transmittance filter 513, an EO modulator
514, a cylindrical lens 515, a rectangular slit 516, an objective
lens 517, a CCD camera 518, and so forth, as shown in FIG. 15.
Further, an illuminating light device for observation, a reference
light source a monitor for observation, an automatic focusing
optical system, an image processing device, a controller, and so
forth are omitted in FIG. 15, however, the optical system is
basically the same in configuration as that shown in FIG. 1.
Further, the respective functions of components are the same as
those for the annealing apparatus shown in FIG. 1, and detailed
description thereof is omitted herein. The present optical system
differs from that in FIG. 1 in that a plurality of (6 in FIG. 14)
the laser irradiation optical systems are housed in the individual
optical body tubes (represented by 503 in the figure),
respectively, and are fixedly attached on the adjustment stages
(represented by 504 in the figure) capable of independently moving
in the directions of X, Y, and Z, respectively, so that the
respective positions of the optical body tubes (represented by 503
in the figure) is adjustable so as to enable laser light to be
irradiated to an identical spot on respective panels, thereby
enabling laser annealing to be simultaneously applied to a
plurality of spots.
[0156] Now, there is described a laser annealing method using the
laser annealing apparatus described. For a substrate 501, use is
made of a polycrystalline silicon thin film substrate 501 obtained
by forming an amorphous silicon film on the top surface of a glass
substrate 1 shown FIG. 6 through the intermediary of an insulator
thin film, and by transforming the amorphous silicon film into a
polycrystalline silicon film composed of fine crystal grains after
scanning across the amorphous silicon film with excimer laser
light. In this case, the insulator thin film is a SiO.sub.2, SiN
film, or a composite film thereof. A plurality of panels (in FIG.
14, 6 panels on one substrate) is formed on the polycrystalline
silicon thin film substrate.
[0157] First, the polycrystalline silicon thin film substrate 501
is placed on the stage 502. On the polycrystalline silicon thin
film substrate 501, alignment marks (not shown) are formed at
plural spots of regions where the respective panels (in FIG. 14, 6
panels) are to be formed. These alignment marks are normally formed
by photo-etching techniques, however, use of a photo resist step
for this purpose only will result in much wastage. Accordingly,
after execution of approximate alignment by detecting the corners
of the polycrystalline silicon thin film substrate 501, laser light
for use in laser annealing is formed in the shape of, for example,
a rectangle longer in the longitudinal direction and a rectangle
longer in the transverse direction, respectively, by use of the
rectangular slit 516 of the respective optical body tubes 503 to
thereby remove portions of the polycrystalline silicon thin film,
so that a cross mark is sequentially formed at plural spots of the
respective panels, thereby serving as the alignment marks. Or after
positioning the respective optical body tubes at a predetermined
base position, a cross mark may be simultaneously formed at plural
spots of the respective panels, thereby serving as the alignment
marks. Otherwise, dot-like alignment marks may be formed by ink-jet
means and so forth.
[0158] Subsequently, by taking pictures of the alignment marks at
two spots with the CCD camera 518 of one of the optical body tubes
(for example, 503), and detecting the position of the center of
gravity of the respective pictures, the stage 502 is caused to move
along three axes of X, Y, Z, respectively, according to design
coordinates on the basis of the alignment marks, thereby
implementing fine alignment of the polycrystalline silicon thin
film substrate 501. In this case, the CCD camera of the optical
body tube intended for annealing is used for detection of the
alignment marks, but another optical system for alignment may be
installed. In such a case, a plurality of the alignment marks may
be sequentially detected with one optical system, or a plurality of
the alignment marks may be simultaneously detected with a plurality
of optical systems.
[0159] After completion of alignment of the polycrystalline silicon
thin film substrate 501, the stage 502 is moved according to the
design coordinates such that the alignment mark at one spot among
the alignment marks of the respective panels comes into a field of
view of the respective optical body tubes, the pictures of the
alignment marks are taken with the CCD camera 518 of the respective
optical body tubes, and adjustment is made with the respective
adjustment stages 504 such that the center of gravity of the
respective pictures coincides with the center of the respective
fields of view. By so doing, the position of the respective optical
body tubes is adjusted such that an identical spot on the
respective panels formed on the polycrystalline silicon thin film
substrate 501 is irradiated with laser light.
[0160] Thereafter, as previously described, annealing is applied
such that only a portion of a driver circuit forming region of the
respective panels, where an active-layer (active region) is to be
formed, is irradiated with laser light in accordance with design
data. At this point in time, pulse signals generated by the linear
scale 509 or 510, installed in the stage 502, are counted and upon
the stage 502 reaching the position for irradiation with laser
light, the laser light is turned into ON state by the EO modulator
514, and is condensed into a linear form by the cylindrical lens
515, so that unnecessary portions of the laser light are cut off by
the rectangular slit 516 to be thereby condensed by the objective
lens 517 before irradiation.
[0161] Laser energy is adjusted with the continuously variable
transmittance filter 513 as necessary. Further, the signals from
the linear scale 509 or 510 are counted and when the stage 502 is
shifted and passes through a region to be annealed, the laser light
is turned into OFF state by the EO modulator 514, so that only
regions requiring annealing can be accurately irradiated with the
laser light. Timing for irradiation with the laser light is as
preciously described with reference to FIGS. 7 and 13.
[0162] Regions to be irradiated with the laser light are, for
example, active layer parts of the thin-film transistors making up
the driver circuit for driving pixels, and while scanning the
polycrystalline silicon thin film substrate 501 by driving the
stage 502, only necessary parts are sequentially irradiated. At
this point in time, the respective optical body tubes drive the
individual adjustment stages 504 with the optical body tube mounted
thereon in the Z-direction by the agency of an automatic focusing
mechanism (not shown), thereby controlling all the objective lenses
so as to be at a given position in relation to the surface of the
polycrystalline silicon thin film substrate 501.
[0163] If a multitude of small sized panels are arranged on one
glass substrate, a procedure is repeated whereby annealing is
executed for every several panels, and after shifting the glass
substrate by a distance corresponding to a pitch at which the
panels are arranged, annealing is again executed, thereby enabling
all the panels to be annealed. Changes occurring to crystal grains
of the polycrystalline silicon thin film when irradiated with the
laser light are as previously described, and since growth of
crystal grain occurs in the direction of scanning with the laser
light, the polycrystalline silicon thin film can effectively obtain
properties equivalent to those of a single crystal by causing the
direction of current flow to coincide with the direction of crystal
growth when transistors are formed.
[0164] Now, a still further embodiment of the invention is
described hereinafter. With the embodiment described in the
foregoing, it has been described that only regions to be annealed
are irradiated with the laser light. More specifically, for a time
period from the start of movement of the stage 502 until arrival
thereof at an irradiation region, the laser light is kept fully in
OFF state and upon the stage 502 reaching the irradiation region,
irradiation is started at a predetermined output while upon the
stage 502 passing through the irradiation region, the laser light
is turned fully in OFF state. By repeating such an operation, laser
annealing is applied to a plurality of regions. If the laser light
is irradiated by this method, there occurs the following
phenomenon.
[0165] FIG. 20 is a sectional view showing the sectional shape of a
thin-film transistor substrate to which a laser annealing method
according to the present embodiment is applied. As shown in FIG.
20, a polycrystalline silicon thin film 703 formed on a glass
substrate 701 through the intermediary of an insulator film 702
melts the instant that irradiation of continuous-wave laser is
started at a spot of irradiation start, and melted silicon is
pulled in a scanning direction by the agency of interfacial
tension. Accordingly, upon cooling and solidification of the melted
silicon after passing of the laser light, there occurs a portion
705 of the polycrystalline silicon thin film 703, smaller in
thickness. In a region following the portion 705 smaller in
thickness, a portion 704 thereof, having an original film
thickness, is maintained, however, when the continuous-wave laser
is turned into OFF state at a spot of irradiation completion, the
melted silicon pulled in by the agency of interfacial tension cools
down and is solidified on the spot, thereby causing a swell 706 to
occur.
[0166] Thus, the thickness of the silicon film at the spots
irradiation start and irradiation completion differs from that in
other parts, and consequently, characteristics of transistors
formed at these spots are changed from those in other parts, so
that the transistors cannot be disposed at these spots.
Accordingly, there has been necessity of giving consideration so as
not to allow the portion 705 smaller in thickness and the swell 706
to overlap the active layer of the respective transistors making up
the driver circuit. Furthermore, in case the swell 706 at the spot
of irradiation completion is large, the swell 706 cannot be
completely removed in an etching step for leaving out only the
active layer of the respective transistors, causing etching residue
to occur. It has since turned out that there is a problem in that
overlying electrodes and wiring are broken in the worst case, and
reliability deteriorates even if no break occurs thereto. Hence,
the following laser irradiation method is adopted.
[0167] FIG. 19 is a schematic representation for illustrating a
relationship between a stage position according to another
embodiment of the invention and laser output. By changing setting
of the EO modulator 10 in FIG. 1, laser light is irradiated at low
output in a region where no annealing is applied while the laser
light is irradiated at high output in a region where annealing is
to be applied as shown in FIG. 19A. Annealing at a power density of
the laser light, in a range of 100.times.10.sup.3 W/cm.sup.2 to
500.times.10.sup.3 W/cm.sup.2, is suitable, however, in the region
where no annealing is applied, irradiation is executed at a power
density not more than one third the power density described. FIG.
21 is a sectional view showing the sectional shape of a thin-film
transistor substrate to which the laser annealing method described
with reference to FIG. 19A is applied.
[0168] As shown in FIG. 21, since no annealing is applied to
portions of a band-like polycrystalline silicon film 703 formed on
a glass substrate 701 through the intermediary of an insulator film
702, in regions where laser light is irradiated at low output, no
damage occurs to the substrate, so that an adverse effect of a
portion 705' smaller in thickness at a spot of irradiation start
and a swell 706' at a spot of irradiation completion, respectively,
can be alleviated and in a portion irradiated at output suitable
for annealing, growth of crystal grains occurs in a direction of
scanning with the laser light, thereby obtaining a band-like
polycrystalline silicon film of intended film quality.
[0169] Further, as shown in FIGS. 19B or 19C, laser output is
successively increased predetermined time or a predetermined
distance before reaching an annealing region so as to reach output
suitable for annealing upon reaching the annealing region, and
after passing through the annealing region, the laser output is
successively decreased so that at predetermined time or a
predetermined distance later, the laser output becomes not more
than one third the output suitable for annealing or the laser light
is turned into OFF state. As a result, rapid increase in
temperature at a spot of irradiation start as well as a spot of
irradiation completion can be eased and an adverse effect of a
decrease in thickness at the spot of irradiation start and a swell
at the spot of irradiation completion, respectively, can be
alleviated, so that in a portion irradiated at output suitable for
annealing, growth of crystal grains occurs in a direction of
scanning with the laser light, thereby obtaining a silicon film of
intended film quality.
[0170] FIG. 24 is a sectional view of the principal part of a
liquid crystal display panel of a liquid crystal display device
which is an example of an embodiment of a display device according
to the invention for illustrating an example of configuration
thereof. The liquid crystal display panel comprises a first
substrate SUB1, a second substrate SUB2, and a liquid crystal layer
LC sandwiched in a gap therebetween. The first substrate SUB1
corresponds to the active matrix substrate (thin-film transistor
substrate) described with reference to the embodiments in the
foregoing. The first substrate SUB1 is a glass substrate, and on
the top surface, that is, the inner surface thereof, there are
formed a gate electrode GT, an active layer (semiconductor film)
PS1 made up of a band-like polycrystalline silicon film, a source
electrode SD1, a drain electrode SD2. And a pixel electrode PX
connected to the source electrode SD1. Further, reference numerals
G1, PASD (one layer or multiplayer) denote insulating layers, OR11
denotes an alignment layer, and POL1 a polarizer. On the periphery
of the first substrate SUB1, there is formed a driving circuit
(driver circuit) described with reference to FIGS. 6 or 18.
[0171] Meanwhile, the second substrate SUB2 too is a glass
substrate, and on the top surface (inner surface) thereof, there
are formed a color filter CF partitioned with a black matrix BM, an
overcoat layer OC, a common electrode (opposite electrode) ITO and
an alignment layer OR12. Further, reference numeral POL2 denotes a
polarizer. An electric field in a direction normal to the surface
of the substrate is formed between the pixel electrode PX and the
common electrode ITO, and the electric field controls the direction
of the alignment of molecules of liquid crystal composition making
up the liquid crystal layer in such a way as to allow light falling
on the first substrate SUB1 to go out of the second substrate SUB2
or to block the light, thereby displaying images.
[0172] FIG. 25 is a sectional view of the principal part of a
liquid crystal display panel of a liquid crystal display device
which is an example of an embodiment of a display device according
to the invention for illustrating another example of configuration
thereof. The liquid crystal display panel comprises a first
substrate SUB1, a second substrate SUB2, and a liquid crystal layer
LC sandwiched in a clearance therebetween. The first substrate SUB1
corresponds to the active matrix substrate (thin-film transistor
substrate) described with reference to the embodiments in the
foregoing. The first substrate SUB1 is a glass substrate, and on
the top surface, that is, the inner surface thereof, there are
formed a gate electrode GT, an active layer (semiconductor film)
PS1 made up of a band-like polycrystalline silicon film, a source
electrode SD1, a drain electrode SD2. And pixel electrodes PX
connected to the source electrode SD1, arranged in a manner like
the teeth of a comb in a pixel region.
[0173] Opposite electrodes CT are interposed between the pixel
electrodes PX arranged in the manner like the teeth of the
comb.
[0174] Further, reference numerals G1, PASD (one layer or
multiplayer) denote insulating layers, OR11 denotes an alignment
layer, and POL1 a polarizer. On the periphery of the first
substrate SUB1, there is formed a driving circuit (driver circuit)
described with reference to FIGS. 6 or 18.
[0175] Meanwhile, the second substrate SUB2 too is a glass
substrate, and on the top surface (inner surface) thereof, there
are formed a color filter CF partitioned with a black matrix BM, an
overcoat layer OC, and an alignment layer OR12. Further, reference
numeral POL2 denotes a polarizer. An electric field in a direction
normal to the surface of the substrate is formed between the pixel
electrode PX and the common electrode ITO, and the electric field
controls the direction of the alignment of molecules of liquid
crystal composition making up the liquid crystal layer in such a
way as to allow light falling on the first substrate SUB1 to go out
of the second substrate SUB2 or to block the light, thereby
displaying images.
[0176] FIG. 26 is a sectional view for illustrating a schematic
configuration of a liquid crystal display device using the liquid
crystal display panel described with reference to FIGS. 24, and
25.
[0177] With the liquid crystal display device (liquid crystal
display module), a backlight is installed on the backside of a
liquid crystal display panel PNL through the intermediary of an
optical compensatory sheet OPS made up of a diffusion sheet
laminated to a prism sheet, and a shield case SHD, which is an
upper case, is formed integrally with a mold case MDL, which is a
lower case. On the periphery of a first substrate SUB1 of the
liquid crystal panel PNL, there is formed the driving circuit
(driver circuit) as described above.
[0178] The backlight shown in FIG. 26 is a so-called side lamp type
comprising a light source (herein, a cold cathode fluorescent lamp
CFL) disposed on the side edge of an guide light board GLB suitably
made up of an acrylic plate, a reflective sheet RFS, a lamp
reflection sheet LFS, and so forth. However, as a backlight other
than the type as above, there are known a so-called directly
underside type backlight wherein a plurality of light sources are
disposed directly under the backside of a liquid crystal display
panel, or a so-called front-light type disposed in the vicinity of
the surface (visible side) of a liquid crystal display panel, and
so on.
[0179] FIG. 27 is a sectional view of the principal part of a
display panel making up an organic electro-luminescent display
panel, another example of the display device according to the
invention, for illustrating a schematic configuration thereof. The
organic electro-luminescent display panel (abbreviated organic EL)
comprises a first substrate SUB1, and a second substrate SUB2, but
the second substrate SUB2 is a sealing case for protecting various
function films of the first substrate SUB1, as described
hereinafter, from environments, and is made up of not only a glass
substrate but also a metal sheet at times. The first substrate SUB1
corresponds to the active matrix substrate (thin-film transistor
substrate) described with reference to various embodiments in the
foregoing. The first substrate SUB1 is a glass substrate, and on
the top surface, that is, inner surface thereof, there are provided
thin-film transistors made of the band-like polycrystalline silicon
film as reformed by the previously described method.
[0180] Respective pixel circuits of the organic EL panel have at
least the thin-film transistors for switching and the thin-film
transistors for driving, and the thin-film transistors shown in the
figure correspond to the thin-film transistors for driving, the
thin-film transistors for switching being omitted in the figure.
The thin-film transistor comprises a band-like polycrystalline
silicon film PS1, a gate electrode GT, and a source electrode SD.
Further, it comprises an anode AD connected to the source electrode
SD, a luminescent layer OLE, and a cathode CD. Reference numerals
IS (IS1, IS2, IS3), PSV, IL denote an insulating layer,
respectively. There is a case where a desiccant is provide on the
inner surface of the second substrate SUB2. Further, dispositions
of the anode AD and cathode CD are not necessarily limited to those
shown in the figure, and respective polarities may be changed
over.
[0181] With this configuration, current flows between the anode AD
and cathode CD by selection of the thin-film transistors for
driving, whereupon the luminescent layer OLE, interposed between
the anode AD and cathode CD, emits light. The emitted light outgoes
from the side of the first substrate SUB1. There is another type
wherein reflective metal is used for the anode AD, and a
transparent electrode is used for the cathode CD, thereby sending
out the emitted light from the side of the second substrate SUB2 In
such a case, a transparent sheet such as a glass sheet is used for
the second substrate SUB2 (sealing case). The organic EL panel is
housed in an appropriate case or frame to be used as an organic EL
display device (module).
[0182] It is to be pointed out that the invention is not limited to
those configurations as described hereinbefore, and that various
changes and modifications may be made in the invention without
departing from the spirit and scope thereof. Further, it is obvious
that the invention is similarly applicable to a substrate for
various electronic equipment wherein active elements such as
thin-film transistors are formed on an insulating substrate.
[0183] As described hereinbefore, with the process (laser annealing
method) of fabricating the display device according to the
invention and the apparatus (laser annealing apparatus) for
fabricating the same, laser light can be accurately irradiated to a
spot to be irradiated (annealed) without variation in stage speed
while preventing adverse effects on the insulating substrate such
as a glass substrate. Further, since the stage moves at a constant
speed, annealing can be executed under a constant condition
regardless of the position of a spot on the substrate.
[0184] As a result, by causing growth of crystal grains of the
amorphous or polycrystalline silicon thin film to take place in a
desired direction, these can be reformed into the band-like
polycrystalline silicon film composed of large crystal grains in
excess of 10 .mu.m in grain size, so that mobility of the active
elements such as the thin-film transistors, made of the band-like
polycrystalline silicon film, can be drastically improved.
[0185] The active elements such as the thin-film transistors,
formed of a silicon film reformed by the process of the invention,
have performance sufficient for making up the driver circuit in a
display device such as a liquid crystal display device, an organic
EL display, so that a system on panel can be implemented, and
various display devices for a liquid crystal display device
intended for reduction in size and cost.
[0186] While we have shown and described several embodiments in
accordance with our invention, it should be understood that
disclosed embodiments are susceptible of changes and modifications
without departing from the scope of the invention. Therefore, we do
not intend to be bound by the details shown and described herein
but to intend to cover all such changes and modifications as fall
within the ambit of the appended claims.
* * * * *