U.S. patent application number 11/147897 was filed with the patent office on 2005-10-13 for method for crystallizing semiconductor with laser beams.
This patent application is currently assigned to Fujitsu Limited. Invention is credited to Ohki, Koichi, Sasaki, Nobuo, Uzuka, Tatsuya.
Application Number | 20050227504 11/147897 |
Document ID | / |
Family ID | 29424260 |
Filed Date | 2005-10-13 |
United States Patent
Application |
20050227504 |
Kind Code |
A1 |
Sasaki, Nobuo ; et
al. |
October 13, 2005 |
Method for crystallizing semiconductor with laser beams
Abstract
Laser beams emitted by a plurality of laser sources are divided
into a plurality of sub-beams, which are irradiated onto selected
portions of an amorphous semiconductor on a substrate to
crystallize the amorphous semiconductor. A difference in diverging
angles between the laser beams is corrected by a beam expander. The
apparatus includes a sub-beam selective irradiating system
including a sub-beam dividing assembly and a sub-beam focussing
assembly. Also, the apparatus includes laser sources, a focussing
optical system, and a combining optical system. A stage for
supporting a substrate includes a plurality of first stage members,
a second stage member disposed above the first stage members, and a
third stage member 38C, rotatably disposed above the second stage
to support an amorphous semiconductor.
Inventors: |
Sasaki, Nobuo; (Kawasaki,
JP) ; Uzuka, Tatsuya; (Shinjuku, JP) ; Ohki,
Koichi; (Mitaka, JP) |
Correspondence
Address: |
Patrick G. Burns, Esq.
GREER, BURNS & CRAIN, LTD.
Suite 2500
300 South Wacker Drive
Chicago
IL
60606
US
|
Assignee: |
Fujitsu Limited
Kawasaki-shi
JP
|
Family ID: |
29424260 |
Appl. No.: |
11/147897 |
Filed: |
June 8, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11147897 |
Jun 8, 2005 |
|
|
|
10436673 |
May 13, 2003 |
|
|
|
Current U.S.
Class: |
438/795 ;
438/166; 438/486; 438/487; 438/798 |
Current CPC
Class: |
H01L 21/02422 20130101;
B23K 2101/006 20180801; B23K 2101/40 20180801; H01L 29/78672
20130101; C30B 13/24 20130101; H01L 21/02683 20130101; B23K 26/0676
20130101; H01L 27/1285 20130101; B23K 26/0884 20130101; H01L
21/02691 20130101; C30B 35/00 20130101; H01L 29/6675 20130101; B23K
26/08 20130101; G02B 27/145 20130101; H01L 21/02532 20130101; G02B
19/0014 20130101; G02B 19/0057 20130101; B23K 26/067 20130101; G02B
27/144 20130101 |
Class at
Publication: |
438/795 ;
438/798; 438/487; 438/486; 438/166 |
International
Class: |
H01L 021/26; H01L
021/20 |
Foreign Application Data
Date |
Code |
Application Number |
May 17, 2002 |
JP |
2002-143032 |
May 17, 2002 |
JP |
2002-143097 |
May 17, 2002 |
JP |
2002-143070 |
Claims
1-12. (canceled)
13. A method for crystallizing a semiconductor, comprising the step
of: irradiating laser beams emitted by a plurality of laser sources
onto a semiconductor layer on a substrate through a focussing
optical system to melt and crystallize the semiconductor layer;
wherein said plurality of laser beams are irradiated onto the
substrate without overlap, scan the semiconductor layer in parallel
to each other, and are positioned so that their molten tracks
overlap each other.
14-19. (canceled)
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a method and an apparatus
for crystallizing a semiconductor.
[0003] 2. Description of the Related Art
[0004] A liquid crystal display device includes an active matrix
drive circuit which includes TFTS. Also, a system liquid crystal
display device includes an electronic circuit including TFTs in the
peripheral regions around the display region. Low-temperature
polysilicon is suitable for forming TFTs for the liquid crystal
display device and TFTs for the peripheral region of the system
liquid crystal display device. Also, low-temperature polysilicon is
applied to pixel driving TFTs for an organic EL display, an
electronic circuit for the peripheral region of the organic EL
display and the like. The present invention relates to a method and
an apparatus for crystallizing a semiconductor, using a CW laser
(continuous wave laser), for producing TFTs with low-temperature
polysilicon.
[0005] In order to form TFTs of the liquid crystal display device
with low-temperature polysilicon, in the prior art, an amorphous
silicon layer is formed on a glass substrate, and the amorphous
silicon layer on the glass substrate is irradiated by an excimer
pulse laser to crystallize the amorphous silicon. Recently, a
crystallization method has been developed wherein the amorphous
silicon layer on the glass substrate is irradiated by a CW solid
laser to crystallize the amorphous silicon.
[0006] In crystallization of silicon by means of the excimer pulse
laser, mobility is in the order of 150 to 300 (cm.sup.2/Vs) but, on
the other band, in crystallization of silicon by means of the CW
laser, mobility in the order of 400 to 600 (cm.sup.2/Vs) can be
realized, this being particularly advantageous in forming TFTs for
electronic circuits in the peripheral region of the system liquid
crystal display device.
[0007] In crystallizing silicon, the silicon layer is scanned by a
laser beam. In this case, the substrate having the silicon layer is
mounted on a movable stage, and the scanning is performed while the
silicon layer is moved with respect to the fixed laser beam. As
shown in FIG. 19, in the excimer pulse laser scanning, scanning can
be performed with a laser beam having, for example, a beam spot "X"
of 27.5 mm.times.0.4 mm, and the area scan speed is 16.5 cm.sup.2/s
when the beam width is 27.5 mm and the scan speed is 6 mm/s.
[0008] On the other hand, as shown in FIG. 20, in the CW solid
laser scanning, scanning can be performed with a beam spot "Y" of,
for example, 400 .mu.m.times.20 .mu.m, and when scanning is
performed at a scan speed of 50 cm/s, an acceptable crystallization
melt width is 150 .mu.m and the area scan speed is 0.75 cm.sup.2/s.
In this manner, crystallization by means of a CW solid laser,
polysilicon of excellent quality can be obtained but there is the
problem that the throughput is low. Also, it is possible to perform
scanning at the scan speed of 2 m/s, in which case the area scan
speed is 5 cm.sup.2/s. However, the mobility of the polysilicon
thus attained is low.
[0009] In crystallization by means of a CW solid laser, because the
output of a stable CW laser is relatively low, even if the scan
speed is increased, there is the problem that the area scan speed
is low and throughput does not increase sufficiently.
[0010] In addition, if scanning is performed by the CW laser with a
laser power of, for example, 10 W, the width "Y" of a beam spot of
approximately 400 .mu.m, and a scan speed of 50 cm/s, an effective
melt width with a beam spot of 400 .mu.m, at which acceptable
crystallization can be attained, would be 150 .mu.m, therefore the
area scan speed is 0.75 cm.sup.2/s. In this manner, in
crystallization by means of a CW solid laser, although polysilicon
of excellent quality can be attained, there is still the problem of
low throughput.
[0011] Further, as shown in FIG. 29, in the prior art, the movable
stage supporting the substrate having the silicon layer comprises a
Y-axis stage 1, an X-axis stage 2, a rotatable stage 3, and a
vacuum chuck 4. Usually, the Y-axis stage 1, which is in the
lowermost position, has a large high-speed structure that is highly
mobile, and the X-axis stage 2, which is positioned above the
Y-axis stage 1, has a relatively small and less mobile structure.
The Y-axis stage 1 which is in the lowermost position takes the
load of all of the upper components. A substrate including an
amorphous semiconductor is secured the vacuum chuck 4, a laser beam
is irradiated onto the amorphous semiconductor while the movable
stages are moved, and the amorphous semiconductor is crystallized
by being molten and hardened to form polysilicon.
[0012] With the excimer pulse laser, because the beam spot formed
is relatively large, a high area scan speed can be achieved.
However, with a CW solid laser, because the beam spot formed is
extremely small, the area scan speed is quite low. Therefore,
crystallization by means of a CW solid laser can achieve excellent
quality polysilicon, but has low throughput.
[0013] In order to improve the throughput of crystallization by
means of laser scanning, the substrate having the silicon layer
must be moved reciprocally at the highest possible speed. In other
words, the substrate is accelerated from a stationary state,
continues to move at a constant speed while being scanned with
laser, and thereafter is decelerated to a stationary state. Then,
the substrate is moved in the opposite direction, at which time the
substrate is accelerated, moves at a constant speed, and is
decelerated to a stationary state. Laser scanning is executed while
this reciprocal movement of the substrate is repeated.
[0014] In order to effectively perform high speed scanning, it is
necessary to increase the acceleration/deceleration of the high
speed Y-axis stage 1. However, if the acceleration is increased,
the shock of acceleration is increased, and this shock is in
proportion to the product of the acceleration and the weight of the
loads supported by the stage. A large shock will vibrate the
optical system for emitting the laser beam, shifting the adjustment
thereof, thus putting the optical system out of focus and moving
the focusing position, making stable crystallization
unattainable.
[0015] In the prior art, because the Y-axis stage 1 which moves at
high speed supports the load of all the other stage components, and
the weight of this load is large, the acceleration thereof cannot
be sufficiently increased and the substrate cannot be accelerated
to a high speed in a short time.
[0016] Further, the rotatable stage 3 is used to correct
dislocation of the rotation position of the substrate having the
silicon layer, and can be rotated within the range of approximately
10 degrees. In order to rotate the substrate having the silicon
layer 90 degrees, it is necessary to remove the substrate from the
vacuum chuck 4 and reattach the substrate to the vacuum chuck 4.
Consequently, in the prior art, 90 degree rotation of the substrate
having the silicon layer is not performed.
SUMMARY OF THE INVENTION
[0017] The object of the present invention is to provide a method
and an apparatus for crystallizing a semiconductor that can
increase throughput even in the case where a CW solid laser is
used.
[0018] A method for crystallizing a semiconductor, according to the
present invention, comprises the steps of dividing a laser beam
emitted by a laser source into a plurality of sub-beams, and
selectively irradiating the sub-beams onto an amorphous
semiconductor on a substrate, wherein laser beams emitted by a
plurality of laser sources are simultaneously irradiated onto the
semiconductor and a difference between diverging angles of the
plurality of laser beams is corrected.
[0019] Also, an apparatus for crystallizing a semiconductor,
according to the present invention, comprises at least one laser
source, beam dividing means for dividing a laser beam emitted by
the laser source into a plurality of sub-beams, at least one
focussing optical system for focusing the sub-beams on an amorphous
semiconductor on a substrate, a moving mechanism for changing a
distance between at least two spot positions of the sub-beams
formed by the focussing optical system, first mirrors for directing
a laser beam to the focussing optical system, and second mirrors
provided in the focussing optical system to receive the sub-beams
reflected by the first mirrors, wherein the sub-beams between the
first mirrors and the second mirrors are parallel to a direction of
movement of the moving mechanism.
[0020] In these structures, throughput can be improved by
simultaneously irradiating a plurality of sub-beams. In the display
region of the display device, the TFT portions are limited compared
to the surface area of the pixels so, in light of the fact that it
is not necessary to crystallize the entire display region,
throughput can be further increased by selectively irradiating the
sub-beams onto only those portions that must be crystallized.
Although those portions that are not irradiated by the beam remain
an amorphous semiconductor, they are eliminated when the TFTs are
separated and, therefore, they pose no problem if they are left as
amorphous semiconductors.
[0021] Next, a method for crystallizing a semiconductor, according
to the present invention, comprises the step of irradiating laser
beams emitted by a plurality of laser sources onto a semiconductor
layer on a substrate through a focussing optical system to melt and
crystallize the semiconductor layer, wherein the plurality of laser
beams are irradiated onto the substrate without overlap, scan the
semiconductor layer in parallel to each other, and are positioned
so that their molten tracks overlap each other.
[0022] Also, a method for crystallizing a semiconductor, according
to the present invention, comprises the step of irradiating laser
beams emitted by a plurality of laser sources onto a semiconductor
layer on a substrate through a focussing optical system to melt and
crystallize the semiconductor layer, wherein a plurality of beam
spots formed by the laser beams emitted by the laser sources at
least partially overlap each other.
[0023] Further, an apparatus for crystallizing a semiconductor,
according to the present invention, comprises first and second
laser sources, a focussing optical system, and a combining optical
system for guiding laser beams emitted by the first and second
sources to the focussing optical system, wherein the combining
optical system comprises a .lambda./2 plate disposed after the
first laser source, a beam expander disposed after at least one of
the first and second laser sources, and a polarizing beam splitter
for combining laser beams emitting by the first and second laser
sources.
[0024] In these structures, by irradiating laser beams emitted by
the first and second laser sources onto an amorphous semiconductor
on a substrate through the focussing optical system, the irradiated
beam spot can be increased in size. By increasing the size of the
beam spot, the melt width increases and, therefore, even if the
necessary scan speed is constant in order to attain high quality
polysilicon, the area scan speed is high. Hence, polysilicon of
excellent quality can be attained with a high throughput.
[0025] Next, an apparatus for crystallizing a semiconductor,
according to the present invention, comprises a laser source, a
stage for supporting a substrate including an amorphous
semiconductor, and an optical focussing system, wherein the stage
comprises a plurality of first stage members disposed in parallel
and movable simultaneously in a first direction, a second stage
member disposed above the first stage members and movable in a
second direction perpendicular to the first direction, and a third
stage member rotatably disposed above the second stage member,
whereby a laser beam emitted by the laser source is irradiated onto
a semiconductor on a substrate fixed to the third state member
through the optical focusing system to melt and crystallize the
semiconductor.
[0026] In this structure, in the stage for supporting the substrate
including the amorphous semiconductor, a plurality of first stage
members are disposed at the lowermost position and support the
second stage member and the third stage member. The second stage
member can be moved at high speed. Consequently, it is not
necessary for the high speed movable second stage member to support
the plurality of first stage members, and therefore the load
thereon is small. The plurality of first stage members move
simultaneously and support the second stage member without bending,
which is long, because it moves at high speed. Accordingly, the
second stage member can be a high-speed member and the
crystallization throughput can be improved.
[0027] Also, a method for crystallizing a semiconductor, according
to the present invention, comprising the step of irradiating a
laser beam onto a semiconductor on a substrate having a display
region and a peripheral region around the display region to melt
and crystallize the semiconductor, performing crystallization of
the peripheral region in a first scanning direction, after rotating
a rotatable stage supporting the substrate by 90 degrees,
performing crystallization of the peripheral region in a second
scanning direction perpendicular to the first scanning direction,
and performing crystallization of the display region in a third
scanning direction parallel to a direction along which sub-pixel
regions of the three primary colors of pixels are arranged.
[0028] In this structure, crystallization of the peripheral region
and crystallization of the display region can be continuously
performed, and overall crystallization throughput can be
improved.
BRIEF DESCRIPTION OF THE DRAWINGS
[0029] The present invention may be more fully understood from the
description of preferred embodiments of the invention set forth
below, together with the accompanying drawings, wherein:
[0030] FIG. 1 is a schematic sectional view showing a liquid
crystal display device according to an embodiment of the present
invention;
[0031] FIG. 2 is a schematic plan view showing a glass substrate of
FIG. 1;
[0032] FIG. 3 is a schematic plan view showing a mother glass for
making the glass substrate of FIG. 2;
[0033] FIG. 4 is a flowchart showing the process of forming the
TFTs on the glass substrate and the TFTs of the peripheral
region;
[0034] FIG. 5 is a flowchart showing the content of the
crystallizing step of FIG. 4;
[0035] FIG. 6 is a perspective view showing an example of
selectively irradiating the amorphous silicon layer in the display
region on the glass substrate with sub-beams;
[0036] FIG. 7 is a view showing the optical device for adjusting
the beam spot of the sub-beam;
[0037] FIG. 8 is a view showing the CW laser oscillators and a
sub-beam selective irradiation system;
[0038] FIG. 9 is a view showing a sub-beam selective irradiation
system forming sixteen sub-beams;
[0039] FIG. 10 is a plan view showing a specific example of the
sub-beam focus assembly of FIG. 9;
[0040] FIG. 11 is a front view showing the sub-beam focus assembly
of FIG. 10;
[0041] FIG. 12 is a side view showing the sub-beam focus assembly
of FIG. 10;
[0042] FIG. 13 is a view showing the relationship between the
sub-beams and the scan pitch;
[0043] FIG. 14 is a view showing the relationship between two glass
substrates and a plurality of sub-beams;
[0044] FIG. 15 is a view showing an example of an arrangement of
sub-beams;
[0045] FIG. 16 is a view showing an example of an arrangement of
sub-beams;
[0046] FIG. 17 is a view showing a TFT arrangement and laser
scanning in order to explain the principle of the present
invention;
[0047] FIG. 18 is a view showing a modified example of the sub-beam
assembly of FIGS. 8 to 12;
[0048] FIG. 19 is a view illustrating a prior art crystallizing
method with an excimer pulse laser;
[0049] FIG. 20 is a view illustrating a prior art crystallizing
method with a CW laser;
[0050] FIG. 21 is a perspective view showing a step of
crystallizing a semiconductor layer by means of a laser beam
according to a further embodiment of the present invention;
[0051] FIG. 22 is a view showing a laser device used in
crystallizing the semiconductor of the peripheral region;
[0052] FIG. 23 is view showing a modified example of the laser
device;
[0053] FIG. 24 is a view showing an example of beam spots;
[0054] FIG. 25 is a view showing an example of beam spots;
[0055] FIG. 26 is a view showing a step of crystallizing a
semiconductor layer by means of a laser beam according to a further
embodiment of the present invention;
[0056] FIG. 27 is a perspective view showing a movable stage
supporting a glass substrate having an amorphous silicon layer
thereon;
[0057] FIG. 28 is a view showing the operation of a laser scan;
and
[0058] FIG. 29 is a perspective view showing a prior art movable
stage.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0059] Embodiments of the present invention will be explained
hereunder with reference to the drawings.
[0060] FIG. 1 is a schematic sectional view showing the liquid
crystal display device according to an embodiment of the present
invention. The liquid crystal display device 10 comprises a pair of
opposing glass substrates 12 and 14 and a liquid crystal 16
inserted therebetween. Electrodes and alignment layers can be
provided on the glass substrates 12 and 14. One of the glass
substrates 12 is a TFT substrate, and the other glass substrate 14
is a color filter substrate.
[0061] FIG. 2 is a schematic plan view showing the glass substrate
12 of FIG. 1. The glass substrate 12 has a display region 18 and a
peripheral region 20 around the display region 18. The display
region 18 includes a large number of pixels 22. In FIG. 2, one
pixel 22 is shown in a partial enlargement. The pixel 22 includes
three primary color sub-pixel regions R, G and B, and a TFT 24 is
formed in each of the sub-pixel regions R, G and B. The peripheral
region 20 has TFTs (not shown), the TFTs in the peripheral region
20 being arranged more densely than the TFTs 24 of the display
region 18.
[0062] The glass substrate 12 of FIG. 2 forms a 15" QXGA liquid
crystal display device and has 2048.times.1536 pixels 22. 2048
pixels are arranged in the direction in which the three primary
color sub-pixel regions R, G and B are arranged (horizontally), so
that the number of sub-pixel region R, G and B is 2048.times.3.
1536 pixels are arranged in a direction perpendicular (vertically)
to the direction in which the three primary color sub-pixel regions
R, G and B are arranged (horizontally). In the process of the
semiconductor crystallization, laser scanning is performed in
directions parallel to sides of the peripheral region 20, and laser
scanning is carried out in the display region 18 in the directions
indicated by the arrows A and B.
[0063] The reason for this is that, because the TFTs 24 are densely
arranged in the direction of the arrows A and B and sparsely
arranged in the direction perpendicular to the direction of the
arrows A and B, and on the mother glass, which is substantially
square, the number of laser scans required in the A/B direction is
less, therefore throughput is higher.
[0064] FIG. 3 is a schematic plan view showing a mother glass 26
for making the glass substrate 12 of FIG. 2. The mother glass 26
encompasses a plurality of the glass substrates 12. In the example
shown in FIG. 3, one mother glass 26 includes four glass substrates
12, but one mother glass 26 can include more than four glass
substrates 12.
[0065] FIG. 4 is a flowchart showing the process of forming the
TFTs 24 of the glass substrate 12 and the TFTs of the peripheral
region 20. In step S1, an insulating layer and an amorphous silicon
layer are formed on the glass substrate. In step S2, the amorphous
silicon layer is crystallized to form polysilicon. In step S3, the
TFTs are separated, leaving the necessary silicon portions such as
those silicon portions that are to become TFTs and the like, and
eliminating the unnecessary polysilicon and amorphous silicon layer
portions. In step S4, gate electrodes, drain electrodes, interlayer
insulating layers, and contact holes are formed. In step S5, an
insulating layer and an ITO layer are formed, and the glass
substrate 12 is completed. The ITO layer becomes pixel electrodes
for forming the pixels 22.
[0066] FIG. 5 is a flowchart showing the content of the
crystallization step S2 of FIG. 4. A CW laser (continuous wave
laser) oscillator 30 is used in the crystallization step S2. The
laser beam output from the CW laser oscillator 30 is supplied to a
peripheral region irradiation system 32 and a sub-beam selective
irradiation system 34 one after another. Firstly, the laser beam is
focussed and irradiated onto the amorphous silicon of the
peripheral region 20 of the glass substrate 12 to melt and harden
the amorphous silicon to crystallize it into a polysilicon. Then,
the sub-beam is selectively focussed and irradiated onto the
amorphous silicon 36 of the display region 18 of the glass
substrate 12 to melt and harden the amorphous silicon to
crystallize it into a polysilicon.
[0067] Since the TFTS of the peripheral region 20 are arranged more
densely than the TFTs 24 of the display region 18, high quality
polysilicon is required in the peripheral region. In the peripheral
region irradiation system 32, the peripheral region 20 is
irradiated with a relatively high power laser beam from the Cw
laser oscillator 30 at a relatively low scan speed. If used in the
example described above, scanning is performed with a beam width of
250 .mu.m and a scanning speed of 40 cm/s, giving an area scan
speed of 1 cm.sup.2/s.
[0068] On the other hand, as the TFTs 24 of the display region 18
do not require polysilicon of higher quality, in the sub-beam
selective irradiation system 34, a laser beam from the CW laser
oscillator 30 is divided into sub-beams, to be described later, and
the display region 18 is irradiated by these sub-beams at a
relatively high scanning speed. By this means, the overall
throughput is improved and high quality polysilicon is attained in
the regions that require it.
[0069] FIG. 6 is a view showing an example of selectively
irradiating the amorphous silicon layer of the display region 18 on
the glass substrate 12 with a plurality of sub-beams SB emitted by
the sub-beam selective irradiation system 34. The plurality of
sub-beams SB are divided from laser beam output from the CW laser
oscillator 30 to form beam spots at predetermined intervals.
Numeral 36 denotes the amorphous silicon layer formed on the glass
substrate 12, and the glass substrate 12 is fixed to a XY stage 38
by means of a vacuum chuck of the XY stage.
[0070] The sub-beams SB are arranged so as to form beams spots in
stripe-shaped portions 40 on the amorphous silicon layer 36 that
includes positions where TFTs 24 are to exist, and the XY stage 38
moves (scans) in the directions of the arrows A and B. The
remaining stripe-shaped portions 42 of the amorphous silicon layer
36 are not irradiated. That is, the stripe-shaped portions 40 of
the amorphous silicon layer 36 are selectively irradiated with the
sub-beams SB.
[0071] FIG. 7 is a view showing the optical system for adjusting
the beam spots of the sub-beams SB. This optical system comprises a
mirror 44 for diverting the optical path of the sub-beam SB, a
substantially semicircular cylindrical lens 46, a substantially
semicircular cylindrical lens 48 arranged perpendicularly to the
lens 46, and a convex lens 50. By means of this optical system, the
beam spot of the sub-beam SB is formed into ellipses.
[0072] FIG. 8 is a view showing a plurality of CW laser oscillators
30 and 30a, and the sub-beam selective irradiation systems 34. A
half-mirror 51 is arranged in front of the CW laser oscillator 30
so that the laser beam LB emitted by the CW laser oscillator 30 is
divided into two sub-beams SB by the half-mirror 51. One sub-beam
SB passing through the half-mirror 51 is further divided into two
sub-beams SB by another half-mirror 52. Numeral 53 indicates a
mirror. The other sub-beam SB reflected by the half-mirror 51 is
further divided into two sub-beams SB by another half-mirror 54. In
this manner, the laser beam LB emitted by the CW laser oscillator
30 is divided into four sub-beams SB.
[0073] An independently adjustable shutter 55 and an independently
adjustable ND filter 56 are arranged in each of the optical paths
of the sub-beams SB. The shutters 55 can interrupt the sub-beams SB
as necessary. The ND filters 56 can adjust the power of the
sub-beams SB.
[0074] Further, mirrors 57 are arranged in order to divert the
horizontal sub-beams SB upwards in a vertical direction, and
mirrors 58 are arranged in order to divert the vertical sub-beams
SB sideways in a horizontal direction. The mirrors 58 divert the
sub-beams SB parallel to the glass substrate 12 at different
heights. The horizontal sub-beams SB are diverted downwards in a
vertical direction by focussing units 59, condensed by the
focussing units 59, and irradiated onto the amorphous silicon layer
36 in predetermined beam spots.
[0075] Each focussing unit 59 includes the mirror 44, the lens 46,
the lens 48 and the convex lens 50 shown in FIG. 7, these optical
components forming one unit. The focussing units 59 are movable
within an allowable range in the direction indicated by the arrow
C. Beam profilers 60 are arranged on the optical axis on each of
the focussing units 59. The beam profilers 60 correct the focus
positions of the respective sub-beams SB. Also, the beam profilers
60 can detect the focus positions of the respective sub-beams
SB.
[0076] Between the half-mirror 51 and the ND filters 56, the four
sub-beams SB are arranged at equal intervals parallel to each other
within a horizontal plane parallel to the glass substrate 12.
Between the mirrors 57 and the focussing units 59, the sub-beams SB
are arranged at equal intervals parallel to each other within a
vertical plane perpendicular to the glass substrate 12. The glass
substrate 12 having the amorphous silicon layer 36 is moved
(scanned) in the direction A/B perpendicular to this vertical
plane.
[0077] The area scan speed in the sub-beam selective irradiation
system 34 is defined by the number of sub-beams.times.the scan
speed.times.intervals between the stripe-shaped portions 40 of the
amorphous silicon layer 36. For this reason, it is preferable to
divide the laser beam LB into a plurality of sub-beams SB, and to
increase the number of laser oscillators 30, so that sufficient
power necessary for crystallization is provided and the number of
sub-beams is increased.
[0078] In FIG. 8, another laser oscillator 30a is arranged parallel
to the laser oscillator 30 and, with this laser oscillator 30a,
optical components (half-mirrors, mirrors, focussing units, and the
like, not shown in the drawing) identical to the optical components
belonging to the laser oscillator 30 are provided, so that it can
form a further four sub-beams SB. In this case, eight sub-beams SB
are all arranged at equal intervals parallel to each other within
the same horizontal plane.
[0079] A beam expander 79 is arranged between the laser oscillator
30a and the first half-mirror 51a. The beam expander 79 adjusts the
diverging angle of the laser beam LB. In other words, if there is
an inconsistency between the diverging angles of the simultaneously
radiated plurality of laser beams LB of the laser oscillators 30
and 30a, there is a case in which one laser beam LB (sub-beam SB)
is focussed by the focussing optical system, the focus of the other
laser beam LB (sub-beam SB) will not match, and therefore, by
adjusting the diverging angle of the laser beam LB, the focuses of
both of the laser beams LB will match. The beam expander 79 may
also be arranged in the optical path of the other laser beam LB.
Also, two of them can be arranged as one in each of the optical
paths of both laser beams LB.
[0080] FIG. 9 is a view showing a sub-beam selective irradiation
system 34 adapted to form sixteen sub-beams SB. This sub-beam
selective irradiation system 34 includes four laser oscillators 30,
two sub-beam dividing assemblies 62, and two sub-beam focussing
assemblies 64. Two of the laser oscillators 30 corresponding to the
two laser oscillators 30 and 30a of FIG. 8. One sub-beam dividing
assembly 62 divides laser beams LB output from two of the laser
oscillators 30 and 30a into eight sub-beams SB, and includes the
optical components corresponding to those arranged between the
half-mirror 51 and the ND filters 56 of FIG. 8. One sub-beam
focussing assembly 64 is optically connected to one sub-beam
dividing assembly 62, and includes the optical components
corresponding to those from the mirrors 57 to the focussing units
59 of FIG. 8.
[0081] FIG. 10 is a plan view showing a specific example of the
sub-beam focussing assembly 64 of FIG. 9, FIG. 11 is a front view
showing the sub-beam focussing assembly 64 of FIG. 10, and FIG. 12
is a side view showing the sub-beam focussing assembly 64 of FIG.
10. In FIG. 10 to FIG. 12, eight mirrors 57 and 58 and eight
focussing units 59 are mounted to a frame 64F. Each focussing unit
59 is attached to the frame 64F by means of an electrically driven
stage 59S, and is movable within an allowable range in the
direction indicated by the arrow C in FIG. 8.
[0082] Where the peripheral region irradiation system 32 of FIG. 5
is used, the optical components from the half-mirror 51 to the
focussing unit 59 of FIG. 8 are removed and optical components of
the peripheral region irradiation system 32 are set in the position
of the half-mirror 51.
[0083] In the above structure, the intervals of the TFTs 24 is
equal to the pitch of the pixels 22. According to the present
invention, the area scan speed is improved in proportion to the
pixel pitch and the number of sub-beams. Also, the smaller the size
of the TFTs 24, the more the surface area that requires melting can
be reduced, therefore the number of sub-beams can be increased.
Under the condition that the pixel pitch does not need to be
excessively reduced, as far as the display be seen by the human
eye, the size of the TFTs 24 can be reduced with advances in
miniaturization techniques. As a result, crystallization can be
selectively performed only on those portions where it is necessary,
without supplying energy to areas that do not require it, so the
throughput of the crystallization process can be improved, and an
energy-saving process can be realized.
[0084] In an example, the size of the TFTs 24 is such that a
channel length is approximately 4 .mu.m and a channel width is
approximately 5 .mu.m. The fluctuation of the XY stage that can
perform high-speed scanning at 2 m/s is in the order of a maximum
of .+-.10 .mu.m, and therefore the width of the sub-beams SB is at
least 25 .mu.m, and preferably 30 .mu.m, with the consideration of
allowance for other factors. The need for increasing the channel
width can be easily achieved by the layout in which the channel
width is arranged parallel to the scanning direction.
[0085] The melt width (the width at which the stripe portions 40 of
the amorphous silicon layer 36 are melted) changes according to the
scanning speed, the thickness of the silicon, the laser power,
irradiation focusing lenses, and the like. In the case where the
depth of the amorphous silicon layer 36 is 150 nm and an optical
system with the lenses of F=200 mm and F=40 mm in combination, that
can attain an elliptical beam spot, is used, and laser scanning is
performed perpendicular to the long axis of the ellipse, an
effective melt width of 30 .mu.m can be attained. Consequently,
even with the power loss accompanied by the division of the laser
beam LB, if 2 W or more power can be supplied to the divided
sub-beams SB, the necessary melt width of 30 .mu.m can be
maintained. The laser used is a Nd:YV04 continuous wave solid
laser.
[0086] With respect to 10 W laser oscillation, the laser power
values after division into four sub-beams are 2.3 W, 2.45 W, 2.45 W
and 2.23 W, all over 2W. It is believed that the deviation in power
values of ten to twenty percent of the sub-beams SB is due to
deviation in the characteristics of the mirrors and half-mirrors.
Due to these values, power at the ND filter 56 is somewhat
attenuated, so that the power values of the four sub-beams SB are
all a uniform 2.2W.
[0087] In FIG. 9, the 16 sub-beams SB are power adjusted by the ND
filters 56 so that all 16 sub-beams SB are adjusted to have the
same power value of 2.1 W. Because the diverging angles of the
beams from different laser oscillators differ, the focussing
positions also differ, so in order to correct this, the beam
expander is provided immediately after the laser beam output from
the laser oscillator and, by correcting the diverging angle
thereof, the same focus positions can be attained. However, if the
dislocation of a focus position does not differ significantly, a
melt width of the same size can be achieved and no significant
problems are caused even if crystallization is performed with the
focus position dislocated.
[0088] In the glass substrate 12 of FIG. 2, the width of the
peripheral regions is approximately 2 mm. Crystallization is
performed using 16 sub-beams SB on the glass substrate 12 of a 15"
QXGA display device. The size of the pixels 22 is 148.5 .mu.m
square. Consequently, the RGB sub-pixel size is 148.5
.mu.m.times.49.5 .mu.m. In order to reduce the number of scans and
increase overall throughput, scanning is performed perpendicular to
the side of 148.5 .mu.m (the direction along which the RGB
sub-pixels are arranged). The arrangement of the 16 sub-beams SB at
148.5 .mu.m intervals is not possible due to the size of the
optical system. The irradiating lenses of each of the focussing
units 59 are arranged at intervals of 30 mm, and are movable within
the range of .+-.4 mm by means of the electrically driven stage 59S
with respect to the direction in which they are arranged.
[0089] As 30 mm/148.5 .mu.m=202.02, a row of 202 TFTs 24 (the
stripe portions 40 of the amorphous silicon layer 36) exists
between two focussing units 59.
[0090] The interval between the first, endmost irradiating lens and
the second irradiating lens is consequently 202.times.148.5
.mu.m=29997 .mu.m=30000-3.
[0091] The interval between the first, endmost irradiating lens and
the third irradiating lens is 202.times.148.5 .mu.m.times.2 =59994
.mu.m=30000.times.2-6.
[0092] The interval between the first, endmost irradiating lens and
the fourth irradiating lens is 202.times.148.5 .mu.m.times.3 =89991
.mu.m=30000.times.3-9.
[0093] The interval between the first, endmost irradiating lens and
the fifth irradiating lens is 202.times.148.5 .mu.m.times.4 =119988
.mu.m.
[0094] The interval between the first, endmost irradiating lens and
the sixth irradiating lens is 202.times.148.5 .mu.m.times.5 =149985
.mu.m.
[0095] The interval between the first, endmost irradiating lens and
the seventh irradiating lens is 202.times.148.5
.mu.m.times.6=179982 .mu.m.
[0096] The interval between the first, endmost irradiating lens and
the eighth irradiating lens is 202.times.148.5 .mu.m.times.7
=209979 .mu.m.
[0097] The interval between the first, endmost irradiating lens and
the ninth irradiating lens is 202.times.148.5 .mu.m.times.8 =239976
.mu.m.
[0098] The interval between the first, endmost irradiating lens and
the tenth irradiating lens is 202.times.148.5 .mu.m.times.9 =269973
.mu.m.
[0099] The interval between the first, endmost irradiating lens and
the eleventh irradiating lens is 202.times.148.5
.mu.m.times.10=299970 .mu.m.
[0100] The interval between the first, endmost irradiating lens and
the twelfth irradiating lens is 202.times.148.5
.mu.m.times.11=329967 .mu.m.
[0101] The interval between the first, endmost irradiating lens and
the thirteenth irradiating lens is 202.times.148.5
.mu.m.times.12=359964 .mu.m.
[0102] The interval between the first, endmost irradiating lens and
the fourteenth irradiating lens is 202.times.148.5
.mu.m.times.13=389961 .mu.m.
[0103] The interval between the first, endmost irradiating lens and
the fifteenth irradiating lens is 202.times.148.5
.mu.m.times.14=419958 .mu.m.
[0104] The interval between the first, endmost irradiating lens and
the sixteenth irradiating lens is 202.times.148.5
.mu.m.times.15=449955 .mu.m=30000.times.15-45.
[0105] Accordingly, each irradiating lens is finely adjusted from a
designed average position to 3 .mu.m in the minus direction in the
case of the second irradiating lens, 6 .mu.m in the minus direction
for the third irradiating lens, . . . , 45 .mu.m in the minus
direction for the sixteenth irradiating lens. Thus, the sub-beams
are focused on the respective TFT regions. In this state, the
sub-beams are irradiated with the output of the laser oscillators
30 of 10 W and the scanning speed of 2m/s. Irradiation is performed
with each sub-beam SB of the power of 2 W.
[0106] FIG. 13 is a view showing the relationship between the
sub-beams SB and scan intervals. As shown in FIG. 13, the sub-beams
SB are arranged at intervals "a", which is (3 mm-3 .mu.m). The
interval between TFTs 24, i.e. the scan interval "b", is 148.5
.mu.m. Scanning is performed while the XY stage 38 is moved
reciprocally in the direction indicated by the arrows A and B. In
other words, after the XY stage 38 moves in the direction of the
arrow A, the XY stage then moves 148.5 .mu.m in the direction
perpendicular to the arrows A and B, then moves in the direction of
the arrow B, then again moves 148.5 .mu.m in the direction
perpendicular to the arrows A and B. This operation is repeated. In
FIG. 13, although each of the sub-beams SB is shown as scanning
four times, in the example being explained here, each sub-beam SB
scans 202 times.
[0107] In one scan in one scanning direction, the 16 sub-beams SB
crystallize stripe portions 40 of the amorphous silicon layer 36 at
the interval for 202 pixels. Next, in the reverse scan, the 16
sub-beams SB crystallize the adjacent stripe portions 40 of the
amorphous silicon layer 36 at the interval for 202 pixels. In 101
reciprocal scans (i.e. 202 scans), portions corresponding to
202.times.16=3332 pixels can be scanned. In this case, the area
scan speed is 148.5 .mu.m.times.2 m/s=47.5 cm.sup.2/s.
[0108] However, in the glass substrate 12 in this example, the
number of pixels in the vertical direction is only 1536.
Consequently, in the next example to be explained, not 16, but
rather 8 sub-beams SB are used. Since
1536=202.times.7+122=122.times.8+80.times.7, scanning is performed
122 times with eight beams, with the remaining 80 scans performed
by seven sub-beams SB. In this case, the eighth sub-beam SB is cut
off by the shutter 55 after the 122nd scan.
[0109] Since, in this example, the device has 16 sub-beams SB, as
well as scanning and crystallizing one glass substrate 12 with
eight sub-beams SB, scanning and crystallization of the adjacent
glass substrate 12 on the mother glass 26 (FIG. 3) can be performed
by the remaining eight sub-beams SB. However, in order to do this,
it is preferable that the distance between the end of the pixels of
the current glass substrate 12 and the nearest end of the pixels of
the adjacent glass substrate 12 be an integral multiple of the
pixel pitch. Alternatively, the positions of the pixels 22 of all
of the glass substrates 12 on the mother glass 26 are preferably
arranged on a mesh having a uniform pixel pitch.
[0110] FIG. 14 is a view showing the relationship between two glass
substrates 12a and 12b on the mother glass 26 and a plurality of
sub-beams SB8 and SB9. Sub-beam SB8 is the eighth sub-beam SB among
eight sub-beams SB for crystallizing the glass substrate 12a, and
sub-beam SB9 is the first sub-beam SB among eight sub-beams SB for
crystallizing the glass substrate 12b.
[0111] When the eighth sub-beam SB8 has finished 122 scans, it is
stopped by the shutter 56. The length of the scan region of the
remaining 80 scans which the eighth sub-beam SB8 is capable of
scanning is 148.5 .mu.m.times.80=11.880 mm. If this distance is the
same as the distance between the last pixel of the glass substrate
12a and the first pixel of the adjacent glass substrate 12b, the
ninth to sixteenth sub-beams SB can be used to crystallize the
adjacent glass substrate 12b without wastage. In other words, when
the first sub-beam SB scans the first pixels of the glass substrate
12a, the ninth sub-beam SB scans the first pixels of the glass
substrate 12b. Where the peripheral region 20 of 2 mm of the glass
substrate 12 exists, the gap (L) of (11.880-2.times.2=7.88 mm) can
be provided between the two glass substrates 12a and 12b.
[0112] In the present apparatus, as the movable region of .+-.4 mm
relative to the average position is provided for each sub-beam SB,
irregularities that can be canceled by this movable range can be
accommodated, but the need of adjustment relative to the adjacent
glass substrate one by one is complicated, and this process is
time-consuming, so it is preferable for the positions of the pixels
of all of the panels of the mother glass substrate to be arranged
on a mesh having a uniform pixel pitch.
[0113] FIG. 14 shows an imaginary mesh M at the pixel pitch of the
pixels on the mother glass. Designing the mother glass so that the
arrangement of the pixels on the plurality of glass substrates 12a
and 12b is the same as the imaginary mesh M, which is drawn with
the pixel pitch of the mother glass, is preferred.
[0114] Stopping this type of single sub-beam SB temporarily occurs
due to its relationship to the pixel pitch, the size of the glass
substrates 12, the average positions of the sub-beams SB, and the
number of sub-beams SB. In the case of a large glass substrate 12,
it should be clear that 16 sub-beams SB can be used more
effectively.
[0115] FIG. 15 is a view showing an example of the arrangement of
the sub-beams SB. To increase effectiveness, it is preferable to
reduce the pitch between sub-beams SB. However, because of the
limit to which the lenses and mirrors can be miniaturized, there is
a limit to which the pitch of the sub-beams SB can be reduced.
Under this limitation, in order to reduce the pitch, the sub-beam
SB irradiating system can be arranged in not one row, but a
plurality of rows as shown in FIG. 15, at the same intervals but
staggered. Arranging the system in a plurality of rows in this
manner, the distance at which the XY stage can move at a uniform
high speed increases at the same rate that the number of rows
increases over the width of the mother glass, therefore throughput
decreases somewhat.
[0116] FIG. 16 is a view showing an example of the sub-beam SB
arrangement. In overcoming this problem in two rows, although
arranging the sub-beam irradiating system of the two rows by
displacing the positions of both sub-beams SB is the same, this can
also be achieved by arranging each row in the foremost position of
the mother glass when the stage has finished moving at a uniform
high speed, as shown in FIG. 16. Naturally, the sub-beam
irradiating systems of a plurality of rows can also be arranged at
these positions.
[0117] FIG. 17 is a view explaining the principle of the present
invention. FIG. 18 is a view showing modified example of the
sub-beam focussing assembly of FIGS. 8 to 12.
[0118] When annealing the panel surface of the amorphous silicon
panel by laser, if the entire panel surface is annealed all over,
too much time is required. If the TFTs 24 are sporadically
scattered as in FIG. 17, it is permissible to anneal only the
stripe portions 40 that include the TFTs 24, and there is no need
to anneal the entire surface.
[0119] In scanning to anneal the panel surface by laser beam, there
are a method of moving the laser beams (sub-beams) while the panel
is fixed, and a method of moving the panel while the laser beams
(sub-beams) are fixed. The present invention can be applied to
either method.
[0120] As the laser annealing with a single laser beam takes too
much time, it is desired to increase the number (n) of the laser
beams, so that "n" laser beams lead to 1/n time, and therefore a
plurality of laser beams (n beams) are used. As shown in FIG. 17,
the TFTs 24 are regularly arranged at pitch PTR, but this pitch PTR
varies according to the products. The apparatus of the present
embodiment can be adapted for different pitches.
[0121] This is illustrated further in FIG. 18. Where annealing with
a plurality (four in FIG. 18) of laser beams (sub-beams SB), the
panel must be irradiated with sub-beams SB at equal intervals. This
mechanism will be explained using the four beam example of FIG.
18.
[0122] The four sub-beams SB are turned 90 degrees using optical
path conversion mirrors 58, so that the sub-beams SB run parallel
to the direction of movement C of the stage shown in the drawing
(left-right movement in FIG. 18). Next, the sub-beams SB are turned
90 degrees using optical path conversion mirrors 44, so that the
sub-beams SB pass through the exact center of the lens units LU
shown in the drawing (lenses 46, 48 and 50 in FIG. 7). The mirrors
44 and lens units LU are located in the focussing units 59. The
focussing unit 59 is mounted on the guide 59G (manual stage) and
the electrically driven stage 59S, so that when the electrically
driven stage 59S moves (left-right movement in the drawing), the
entire focussing unit 59 moves left or right. When the electrically
driven stage 59S moves (left-right movement in the drawing), the
entire focussing unit 59 moves left or right, so that the laser
beam (sub-beam SB) always passes through the exact center of the
lens unit LU.
[0123] With this mechanism, the interval between the outgoing laser
beam passing through the lens unit LU and the next outgoing laser
beam passing through the next lens unit LU (laser beam pitch PLB1)
can be adjusted. The interval between other laser beams can be
similarly adjusted using the same method as for the laser beam
pitch PLB1.
[0124] Next, the method of annealing the panel surface having TFTs
at the transistor pitch PTR as shown in FIG. 17 with a plurality of
(four in FIG. 18) laser beams (sub-beams SB) having the structure
of FIG. 18, without waste or loss will be described.
[0125] The transistor pitch PTR is usually in the order of 100
.mu.m (this differs according to the product being produced, as
already been described). For example, the case where the PTR is 90
.mu.m and the initial laser beam pitch is 20 mm will be
specifically described. As 20 mm/90 .mu.m=222.22 . . . , the
integer 222 is attained by rounding off. 222.times.90 .mu.m=19.98.
Thus, if the laser beam pitches PLB1 to PLB4 are made 19.98 mm,
four transistor rows with a laser pitch of 19.98 mm can be annealed
in one scan. Next, after moving the panel with respect to the laser
beam group 90 .mu.m perpendicularly to the laser scanning
direction, and again performing a laser scan, the next four
transistor rows can be annealed. When laser scanning is thereafter
performed another 220 times (scanning has already been performed
twice, thus the total number of scans is 222), 222.times.4
transistor rows are annealed without duplication or loss. A region
of 222.times.4.times.90 .mu.m=19.98 mm.times.4=approximately 80 mm
can be annealed without waste or failure. Next, after moving the
panel with respect to the laser beam group 80 mm perpendicularly to
the laser scanning direction, if annealing is performed by the same
procedure, a panel of any size can be annealed without duplication
or loss.
[0126] The present embodiment provides a means for annealing
without waste or loss even when laser annealing a panel having a
variety of transistor pitches, by setting the laser beam pitch at
an integral multiple of the transistor pitch with a structure that
can adjust the laser beam pitch. A system of using a plurality of
laser beams when laser annealing an amorphous silicon panel or the
like using a laser has already been proposed. The present
embodiment provides a method that can anneal using a plurality of
laser beams and can respond to transistor pitches that vary
according to the product and are scattered on the surface of the
panel, and by arranging the interval between a plurality of laser
beams at an integral multiple of the transistor pitch, provides a
means that can anneal effectively and without waste.
[0127] As explained above, according to the present invention,
throughput can be increased even when using a CW solid laser.
[0128] Next, a further embodiment of the present invention will be
explained. This embodiment includes the fundamental features
described with reference to FIGS. 1 to 4.
[0129] FIG. 21 is a view showing a step of crystallizing an
amorphous silicon layer (semiconductor layer) by means of a laser
beam. The amorphous silicon layer 36 is formed on a glass substrate
12 with an insulating layer of SiO.sub.2 or the like arranged
therebetween, and the glass substrate 12 is fixed to an XY stage 38
by a vacuum chuck or a mechanical stopper of the stage 38. A laser
beam LB is irradiated onto the amorphous silicon layer 36, and the
XY stage 38 is moved in a predetermined direction, so that scanning
is performed. Initially, the laser beam is focussed and irradiated
onto the amorphous silicon layer 36 of the peripheral region 20 of
the glass substrate 12, to melt and harden the amorphous silicon
layer to crystallize the amorphous silicon layer into polysilicon.
Then, the laser beam is focussed and irradiated onto the amorphous
silicon layer 36 of the display region 18 of the glass substrate
12, to melt and harden the amorphous silicon layer to crystallize
the amorphous order of operation silicon layer into polysilicon.
The reason for this is that, in the case where the laser scanning
is performed in an intersecting manner, the crystallinity of the
intersecting portions when crystallization is performed firstly
with a strong laser light corresponding to that as for the
peripheral region and then with a weak laser light corresponding to
that for the display region is identical to the crystallinity of
the peripheral region when the strong laser light is used, but the
crystallization by the strong laser light is insufficient if the
laser light is irradiated in the reverse order. This is because
light absorption is less if the amorphous silicon is partially
crystallized to a certain degree.
[0130] As the TFTs of the peripheral region 20 are arranged more
densely than the TFTs 24 of the display region 18, high quality
polysilicon is required. Consequently, laser scanning of the
peripheral region 20 is performed with a laser beam of relatively
high power at a relatively low scanning speed and, as the TFTs 24
of the display region 18 do not require polysilicon of a higher
quality, scanning is performed with a relatively low power laser
beam (or by sub-beams divided from the laser beam) at a relatively
high scanning speed.
[0131] FIG. 22 is a view showing a laser device 70 used for
crystallizing the semiconductor of peripheral region 20. The laser
device 70 is used with the XY stage 38 of FIG. 5 for
crystallization. The laser device 70 comprises two laser sources
(continuous wave (CW) laser oscillators) 71 and 72, a common
focussing optical system 73, and a combining optical system 74 for
guiding laser beams LB outgoing from the two laser sources 71 and
72 to the focussing optical system 73.
[0132] The focussing optical system 73 comprises a substantially
semicircular cylindrical lens 75, a substantially semicircular
cylindrical lens 76 arranged perpendicular to the lens 75, and a
convex lens 77. The beam spots of the laser beams LB are formed in
the elliptical shape by the focussing optical system 73.
[0133] The combining optical system 74 comprises a .lambda./2 plate
78 disposed after the first laser source 71, a beam expander 79
disposed after the second laser source 72, and a polarizing beam
splitter 80 for combining outgoing laser beams LB from the first
and second laser sources 71 and 72. Numeral 81 denotes a
mirror.
[0134] The laser beams LB outgoing from the laser sources 71 and 72
are combined by the combining optical system 74, and are irradiated
onto the amorphous semiconductor 36 of the glass substrate 12
through the focussing optical system 73 to crystallize the
amorphous semiconductor 36. The beam expander 79 adjusts the
diverging angle of the laser beam LB. In other words, if there is a
deviation between the diverging angles of the laser beams LB, there
is a case where one laser beam LB is focussed by the focussing
optical system 73 but the focus of the other laser beam LB may not
match, and therefore, it is intended that the focusses of the two
laser beams LB are matched, by adjusting the diverging angle of the
laser beam LB by means of the beam expander 79. The beam expander
79 may also be arranged in the optical path of the other laser beam
LB. Also, two beam expanders can be arranged in both the optical
paths of the laser beams LB.
[0135] The laser beams LB emitted by the first and second laser
sources 71 and 72 are vertically linearly polarized, and the laser
beam LB emitted by the first laser source 71 has its plane of
polarization rotated 90 degrees by the .lambda./2 plate 78 and is
horizontally linearly polarized. Consequently, the laser beam LB
output from the first laser source 71 and passing through the
.lambda./2 plate 78, and the laser beam LB output from the second
laser source 72 are guided into the polarizing beam splitter 80,
and the two laser beams LB are directed to the amorphous
semiconductor 36 in a substantially superposed manner. The change
in the state of linear polarization is illustrated in more detail
in FIG. 23.
[0136] Each laser beam LB passes through the focus optical system
73 to form an elliptical beam spot. As shown in FIG. 24, the
individual beam spots of the laser beams LB are superposed, and the
beam spots of the combined laser beams LB form a cocoon shaped beam
spot BS. This can be achieved by slightly displacing the angle of
any one of the mirrors 81, for example. In other words, the laser
beams LB outgoing from the of laser sources 71 and 72 form
elliptical beam spots, respectively, and the elliptical beam spots
mutually overlap in the direction of their long axes.
[0137] In this example, the SiO.sub.2 layer is formed at the
thickness of 400 nm on the glass substrate 12 by plasma CVD, and
the amorphous silicon 36 is formed thereon by plasma CVD to a
thickness of 100 nm. The laser used is a continuous wave Nd:YV04
solid laser. In one example, where a single laser source is used, a
400 .mu.m.times.20 .mu.m beam spot is formed at the laser power of
10 W. If scanning is performed using a single laser, with a laser
width of 400 .mu.m and a scan speed of 50 cm/s, the area scan speed
of 200 cm.sup.2/s can be achieved. Also, within the laser
irradiation width of 400 .mu.m, the 150 .mu.m wide stripe portion
of the amorphous semiconductor 36 is well melted and crystallized,
and exhibits a flow type grain boundary. Once the TFTs are formed
in the polysilicon region made from this flow type grain boundary,
a high mobility characteristic of 500(cm.sup.2/Vs) can be
attained.
[0138] The combined beam spot of the laser beams outgoing from the
two laser sources 71 and 72, as shown in FIG. 22, is 600
.mu.m.times.20 .mu.m. When the laser scan is performed at the laser
power of 10 W and the spot width of 600 .mu.m, scan speed of 50
cm/s, the 350 .mu.m wide stripe portion of the amorphous
semiconductor 36 is particularly well melted and crystallized
within the laser irradiation width of 600 .mu.m, and attains a flow
type grain boundary. The high quality crystallized stripe portion
with a width of 350 .mu.m is twice the width of the high quality
crystallized stripe portion with a width of 150 .mu.m using a
single laser. In other words, by means of the compound heating of
the two beam spots, the beam spot size and effective melt width
(high quality crystallization width) can be enlarged.
[0139] FIG. 23 is view showing a modified example of the laser
device 70. The laser device 70A of FIG. 23 includes two units of
optical system. Each unit of optical system includes the same
components as those of the laser device 70 of FIG. 22. The optical
system of the first unit uses the same numbers as FIG. 22 to
indicate the same optical components, with the subscript "a"
attached, while the optical system of the second unit uses the same
numbers as FIG. 22 to indicate the same optical components, with
the subscript "b" attached. The beam expander 79 can be provided as
appropriate.
[0140] The optical systems of the two units are arranged in close
proximity, and the beam spots BS created by the optical systems of
the two units are arranged so that they are shifted in both the
direction perpendicular to and the direction parallel to the
scanning direction. In this structure, each of 350 .mu.m effective
melt width regions is arranged so that the scan track overlaps by
50 .mu.m and the effective melt width is 650 .mu.m.
[0141] FIG. 25 is a view showing another example of beam spots.
Three beam spots BS are arranged so that they are shifted in both
the direction perpendicular to, and the direction parallel to, the
scanning direction. The three beam spots are all irradiated onto
the substrate, while shifted in the scanning direction and without
overlapping. However, the three beam spots are arranged such that
they scan the semiconductor layer in parallel and overlap with each
other when seen in the scanning direction so that their melt widths
overlap each other. Also, more than three beam spots can be
arranged so that they are shifted in both the direction
perpendicular to, and the direction parallel to, the scanning
direction.
[0142] As explained above, according to the present invention,
throughput can be increased even when using a CW fixed laser.
[0143] Next, a further embodiment of the present invention will be
explained. This embodiment includes the fundamental features
described with reference to FIGS. 1 to 4. FIG. 26 is a view showing
a step of crystallizing an amorphous silicon layer (semiconductor
layer) 36 by means of a laser beam. The amorphous silicon layer 36
is formed on a glass substrate 12, with an insulating layer of
SiO.sub.2 or the like therebetween, and the glass substrate 12 is
fixed to a movable stage 38 by a vacuum chuck or a mechanical
stopper of the stage. A laser beam LB output from a laser source
(continuous wave (CW) laser oscillator) 30 passes through a concave
lens 31, is reflected by a mirror 44, passes through a focussing
optical system, and is irradiated onto the amorphous silicon layer
36. The focussing optical system comprises a substantially
semicircular cylindrical lens 46, a substantially semicircular
cylindrical lens 48 arranged perpendicular to the lens 46, and a
convex lens 50. The beam spot of the laser beam LB passing through
the convex lens 50 is formed into an elliptical shape.
[0144] The laser beam LB is irradiated onto the amorphous silicon
layer 36, and the movable stage 38 is moved in a predetermined
direction, so that the laser scanning is performed. Firstly, the
laser beam LB is focussed and irradiated onto the amorphous silicon
36 of the peripheral region 20 of the glass substrate 12, to melt
and harden the amorphous silicon to crystallize into polysilicon.
Then, the laser beam is focussed and irradiated onto the amorphous
silicon 36 of the display region 18 of the glass substrate 12 to
melt and harden the amorphous silicon to crystallize it into
polysilicon.
[0145] As the TFTs of the peripheral region 20 are arranged more
densely than the TFTs 24 of the display region 18, high quality
polysilicon is required. Consequently, laser scanning of the
peripheral region 20 is performed with a laser beam of relatively
high power at a relatively low scanning speed and, as the TFTs 24
of the display region 18 do not require polysilicon of a higher
quality, scanning is performed with a relatively low power laser
beam (or by sub-beams divided from the laser beam) at a relatively
high scanning speed.
[0146] FIG. 27 is a perspective view showing the movable stage 38
supporting the glass substrate 12 having the amorphous silicon
layer 36. The movable stage 38 comprises first stage members 38A
arranged in parallel and moves in a first direction P, Q
synchronously, a second stage member 38B disposed above the first
stage members 38A and moves in a second direction R, S
perpendicular to the first direction, and a third stage member 38C
rotatably disposed above the second stage 38B. The third stage
member 38C has a vacuum chuck 38D for securing the amorphous
semiconductor 36 of the glass substrate 12. The third stage member
38C (the rotatable stage) can rotate within the angular range of
110 degrees.
[0147] In the movable stage 38, the first stage members 38A are
disposed at the lowermost position and support the second stage
member 38B and the third stage member 38C. The second stage member
38B is large and long, has a greater stroke, and can move at high
speed. Consequently, the second stage member 38B which is movable
at high speed is not required to support the first stage members
38A, and therefore the load on the second stage member 38B is
small. The first stage members 38A move simultaneously and support
the second stage member 38B without bending. Accordingly, the
second stage member 38B can be driven at a higher speed, by which
the throughput of crystallization can be improved.
[0148] FIG. 28 is a view showing a laser scanning operation.
Firstly, laser scanning of the peripheral region 20 is performed.
In the laser scanning of the peripheral region 20, (1)
crystallization of the areas of the peripheral region 20 running
parallel to the first scanning direction P, Q is performed, (2)
next, after the third stage member 38C (the rotatable stage)
supporting the glass substrate 12 is rotated 90 degrees,
crystallization of the areas of the peripheral region 20 running
parallel to the second scanning direction R, S perpendicular to the
first scanning direction P, Q is performed. Then, (3) the display
region 18 is crystallized in a third scanning direction A, B
parallel to the direction in which sub-pixel regions of the three
basic colors of the pixels 22 are arranged.
[0149] The reason for this order of operation is that, in the case
where the crystallizing scanning is performed over a plurality of
panels and scanning intersecting portions occur, the crystallinity
of the intersecting portions when crystallization is performed
firstly with a high energy density laser light of the peripheral
region and then with a weak laser light corresponding to that of
the display region is identical to that when the crystallization is
performed with a strong laser light, but the crystallization by the
strong laser light is insufficient if the laser light is irradiated
in the reverse order. This is because the absorption of light is
less if the amorphous silicon is partially crystallized to a
certain degree, compared to the amorphous state. An additional
reason for performing the operation in the order is that scanning
in the same direction can be continued.
[0150] That is, initially, laser scanning of two shorter sides
among four sides of the peripheral region 20 of the glass substrate
12 is performed, then laser scanning of two longer sides among four
sides of the peripheral region 20 of the glass substrate 12 is
performed. In the scanning of the two shorter sides, the shorter
sides of the glass substrate 12 are positioned perpendicular to the
second stage member 38B, and the second stage member 38B is
reciprocally moved in the first scanning direction P, Q together
with the glass substrate 12. The second stage member 38B is driven
so as to move in one direction P, and while this movement, the
second stage member 38B is accelerated from a stationary position,
laser scanning is performed with the second stage member 38B in a
constant speed state, and the second stage member 38B is
decelerated and stopped after it has passed the laser scanning
region. Then, after the first stage members 38A is moved a minute
amount in the direction perpendicular to the first scanning
direction P, Q, the second stage member 38B is driven so as to move
in the opposite direction Q. At that time, the second stage member
38B is accelerated, moves at a constant speed, and is decelerated.
While repeating this reciprocal movement, laser scanning is
performed so that the end portions of the irradiated regions
overlap each other.
[0151] Subsequently, the third stage member 38C (rotatable stage)
is rotated 90 degrees and the long sides of the glass substrate 12
are positioned parallel to the second stage member 38B. Laser
scanning of the two long sides is performed in the second scanning
direction R, S. Scanning of the two long sides is performed while
repeating the reciprocal movement in the same manner as for the
short sides.
[0152] After this, laser scanning of the display region 18 is
performed in the third scanning direction A, B. As the third
scanning direction A, B is parallel to the second scanning
direction R, S, the third stage member 38C (rotatable stage) is
supported in the same rotational position as that when the two long
sides of the peripheral region 20 are scanned. After the first
stage members 38A are moved to initial positions in the direction
perpendicular to the second scanning direction R, S, the second
stage member 38B is driven so as to move reciprocally in the third
scanning direction A, B.
[0153] Between reciprocal movements of the second stage member 38B,
the first stage members 38A are moved by a minute amount in the
direction perpendicular to the second scanning direction R, S. The
amount of movement of the first stage members 38A during laser
scanning of the display region 18 is greater than the amount of
movement of the first stage members 38A during laser scanning of
the peripheral region 20. In other words, laser scanning of the
display region 18 is performed at a larger pitch than that of laser
scanning of the peripheral region 20. Also, laser scanning of the
display region 18 is performed at a higher scanning speed than
laser scanning for the peripheral region 20. Further, laser
scanning of the display region 18 is performed at a lower laser
power than laser scanning of the peripheral region 20. Further
still, the number of scans when crystallization of the display
region 18 is performed in the third scanning direction A, B,
parallel to the direction in which the three primary color
sub-pixel regions of the pixels 22 are arranged, is significantly
less than the number of scans when crystallization of the display
region 18 is performed in a direction perpendicular to the
direction in which the three primary color-sub-pixel regions of the
pixels 22 are arranged (perpendicular to the direction A, B),
therefore the laser scanning time can be shortened.
[0154] In this manner, by positioning the high precision first
stage members 38A at the bottom, and positioning the high speed
second stage member 38B thereabove, the weight of the load on the
high speed second stage member 38B can be reduced. Simultaneously,
the long second stage member 38B can be supported by the plurality
of first stage members 38A so that the second stage member 38B can
be supported without bending. The plurality of first stage members
38A are driven in synchronization. Thus, when the high speed second
stage member 38B is accelerated and decelerated the acceleration
can be increased and the time taken for movements other than for
laser scanning can be shortened. By enabling the third stage member
38C (rotatable stage) to rotate within the range of 110 degrees,
after mounting the glass substrate 12 in the vacuum chuck 38D,
crystallization of the peripheral region 20 and crystallization of
the display region 18 can be continuously performed. Thus,
according to the present invention, the throughput of
crystallization can be improved.
[0155] In the example, the SiO.sub.2 layer is formed at a thickness
of 400 nm on the glass substrate 12 by plasma CVD, and the
amorphous semiconductor 36 is formed thereon by plasma CVD to a
thickness of 100 nm. The laser used is a continuous wave Nd:YV04
solid laser. In one example, the laser is 10 W and forms a 400
.mu.m.times.20 .mu.m beam spot. If scanning is performed using a
single laser source, with a laser width of 400 .mu.m and a scan
speed of 50 cm/s, an area scan speed of 200 cm.sup.2/s can be
achieved. Also, within the laser irradiation width of 400 .mu.m,
the 150 .mu.m wide stripe portion of the amorphous semiconductor 36
is well melted and crystallized, and exhibits a flow type grain
boundary. Once the TFTs are formed in the polysilicon region made
from this flow type grain boundary, a high movement characteristic
of 500(cm.sup.2/Vs) can be attained.
[0156] As described above, according to the present invention,
throughput can be increased even in a case where a CW fixed laser
is used.
* * * * *