U.S. patent application number 13/596693 was filed with the patent office on 2013-01-10 for uniform large-grained and grain boundary location manipulated polycrystalline thin film semiconductors formed using sequential lateral solidification and devices formed thereon.
Invention is credited to Mark A. Crowder, James S. Im, Robert S. Sposili.
Application Number | 20130009074 13/596693 |
Document ID | / |
Family ID | 23542857 |
Filed Date | 2013-01-10 |
United States Patent
Application |
20130009074 |
Kind Code |
A1 |
Im; James S. ; et
al. |
January 10, 2013 |
UNIFORM LARGE-GRAINED AND GRAIN BOUNDARY LOCATION MANIPULATED
POLYCRYSTALLINE THIN FILM SEMICONDUCTORS FORMED USING SEQUENTIAL
LATERAL SOLIDIFICATION AND DEVICES FORMED THEREON
Abstract
Methods for processing an amorphous silicon thin film sample
into a polycrystalline silicon thin film are disclosed. One method
includes generating a sequence of excimer laser pulses,
controllably modulating each pulse to a predetermined fluence,
homoginizing each modulated pulse in a predetermined plane, masking
portions of each homoginized pulse with a pattern of slits to
generate a sequence of fluence controlled pulses of line patterned
beamlets, each slit in the pattern of slits being sufficiently
narrow to prevent inducement of significant nucleation in region of
a silicon thin film sample irradiated by a beamlet corresponding to
the slit, irradiating an amorphous silicon thin film sample with
the sequence of fluence controlled slit patterned beamlets to
effect melting of portions corresponding to each fluence controlled
patterned beamlet pulse, and controllably sequentially translating
a relative position of the sample with respect to each of the
fluence controlled pulse of slit patterned beamlets.
Inventors: |
Im; James S.; (New York,
NY) ; Sposili; Robert S.; (New York, NY) ;
Crowder; Mark A.; (New York, NY) |
Family ID: |
23542857 |
Appl. No.: |
13/596693 |
Filed: |
August 28, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12567414 |
Sep 25, 2009 |
8278659 |
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13596693 |
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12419821 |
Apr 7, 2009 |
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12567414 |
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11744493 |
May 4, 2007 |
7679028 |
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12419821 |
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11141815 |
Jun 1, 2005 |
7319056 |
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11744493 |
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10294001 |
Nov 13, 2002 |
7029996 |
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11141815 |
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09390535 |
Sep 3, 1999 |
6555449 |
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10294001 |
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PCT/US96/07730 |
May 28, 1996 |
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09390535 |
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09200533 |
Nov 27, 1998 |
6322625 |
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PCT/US96/07730 |
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Current U.S.
Class: |
250/453.11 |
Current CPC
Class: |
H01L 27/1285 20130101;
H01L 21/0268 20130101; H01L 21/02686 20130101; H01L 29/04 20130101;
G03F 7/70041 20130101; H01L 21/02532 20130101; H01L 27/1296
20130101; B23K 26/0626 20130101; B23K 26/066 20151001; H01L
21/02691 20130101; B23K 26/0622 20151001; G03F 7/70725
20130101 |
Class at
Publication: |
250/453.11 |
International
Class: |
H01L 21/268 20060101
H01L021/268 |
Goverment Interests
NOTICE OF GOVERNMENT RIGHTS
[0002] The U.S. Government has certain rights in this invention
pursuant to the terms of the Defense Advanced Research Project
Agency award number N66001-98-1-8913.
Claims
1. A system for processing a silicon thin film into a
polycrystalline thin film, comprising: (a) an excimer laser for
generating a plurality of excimer laser pulses of a predetermined
fluence; (b) a mask having one or more slits of a predetermined
width, optically coupled to said excimer laser, for masking each of
said laser pulses to generate one or more laser beamlets
corresponding to each of said laser pulses, such that each beamlet
has a shape defined by a length and a width, said width being
determined by said predetermined slit width; (c) a sample stage,
optically coupled to said mask and adapted for translation and for
holding said silicon thin film, for receiving said one or more
laser beamlets for each of said homogenous laser pulses to effect
melting of a portion of said silicon thin film corresponding to
said shape of said laser beamlet; and (d) a computer, coupled to
said excimer laser, for controlling relative positions of said
sample stage and said mask, and for coordinating said excimer pulse
generation with said relative positions of said sample stage and
said mask, to thereby process said silicon thin film into a
polycrystalline silicon thin film by sequential translation of said
sample stage relative to said mask and irradiation of said thin
film by said one or more laser beamlets for each of said
homogenized laser pulses at corresponding sequential locations
therein.
2. The system according to claim 1, wherein said mask is adapted
for translation.
3. The system according to claim 1, further comprising a beam
homogenizer for homogenizing said laser pulses in a predetermined
plane, said homogenized laser pulses each having a substantially
predetermined size.
4. The system according to claim 1, wherein said computer is
adapted to controllably modulate said predetermined fluence of said
excimer laser pulses.
5. The system according to claim 1, further comprising a computer
readable medium storing instructions executable by said computer
for: (f) irradiating said silicon thin film with said laser beamlet
to effect melting of a first portion of said silicon thin film
corresponding to said shape of said predefined slit pattern in said
mask; and (g) based on dimensions of said mask, translating at
least one of said thin film and said excimer laser pulses relative
to the other so as to reach a second location.
6. The system of claim 1, wherein said length corresponds to said
predetermined size of said homogenized laser pulses.
7. An apparatus for making a laterally extending crystalline region
in a film of semiconductor material on a substrate having at least
a surface region comprising a material which is different from said
semiconductor material and which is inert with respect to crystal
growth in said film, comprising: (a) a pulsed radiation beam source
for providing a plurality of radiation beam pulses; (b) a beam mask
adapted to receive said plurality of laser beam pulses and for
defining a plurality of masked laser beam pulses, each having an
intensity pattern for irradiating said film; (c) a sample
translation stage optically coupled to said beam mask, and adapted
for holding said substrate while at least a portion of said film is
irradiated by said masked laser beam pulses and for translating
said substrate in a lateral direction with respect to said masked
laser beam pulses from a first position to at least a second
position; wherein when said sample translation stage is in said
first position, a first portion of said semiconductor film is
positioned to be irradiated by a first of said masked laser beam
pulses so as to melt said semiconductor material in a first portion
throughout its thickness, and when said sample translation stage is
in said second position, a second portion of said semiconductor
film is positioned to be irradiated by a second of said masked
laser beam pulses so as to melt said semiconductor material in a
second portion throughout its thickness, and wherein said second
portion overlaps said first portion such that at least one
semiconductor crystal formed in said first portion as a result of
said irradiating in said first position and subsequent
solidification is in both said first portion and said second
portion.
8. The apparatus of claim 7, wherein said sample translation stage
is further adapted for translating said substrate in a lateral
direction with respect to said masked laser beam pulses from said
second position to at least a third position; wherein when said
sample translation stage is in said third position, a third portion
of said semiconductor film is positioned to be irradiated by a
third of said masked laser beam pulses so as to melt said
semiconductor material in a third portion throughout its thickness,
and wherein said third portion overlaps said second portion such
that at least one semiconductor crystal formed in said second
portion as a result of said irradiating in said second position and
subsequent solidification is in both said second portion and said
third portion.
9. The apparatus of claim 7, wherein said radiation beam pulses
comprise laser beam pulses.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional of U.S. application Ser.
No. 12/567,414, filed Sep. 25, 2009, which is a continuation of
U.S. application Ser. No. 12/419,821, filed Apr. 7, 2009, which is
a continuation of U.S. application Ser. No. 11/744,493, filed May
4, 2007, which has issued as U.S. Pat. No. 7,679,028, which is a
divisional of U.S. application Ser. No. 11/141,815, filed Jun. 1,
2005, which has issued as U.S. Pat. No. 7,319,056, which is a
continuation of U.S. application Ser. No. 10/294,001, filed Nov.
13, 2002, which has issued as U.S. Pat. No. 7,029,996, which is a
continuation of U.S. application Ser. No. 09/390,535, filed Sep. 3,
1999, which has issued as U.S. Pat. No. 6,555,449, which is a
continuation-in-part of International Application PCT/US96/07730,
filed May 28, 1996, and which is also a continuation-in-part of
U.S. application Ser. No. 09/200,533, filed Nov. 27, 1998, which
has issued as U.S. Pat. No. 6,322,625. The entire disclosures of
each of the aforementioned priority applications are herein
incorporated by reference in their entireties.
BACKGROUND OF THE INVENTION
[0003] I. Field of the Invention
[0004] The present invention relates to techniques for
semiconductor processing, and more particularly to semiconductor
processing which may be performed at low temperatures.
[0005] II. Description of the Related Art
[0006] In the field of semiconductor processing, there have been
several attempts to use lasers to convert thin amorphous silicon
films into polycrystalline films. For example, in James Im et al.,
"Crystalline Si Films for Integrated Active-Matrix Liquid-Crystal
Displays," 11 MRS Bullitin 39 (1996), an overview of conventional
excimer laser annealing technology is presented. In such a system,
an excimer laser beam is shaped into a long beam which is typically
up to 30 cm long and 500 micrometers or greater in width. The
shaped beam is scanned over a sample of amorphous silicon to
facilitate melting thereof and the formation of polycrystalline
silicon upon resolidification of the sample.
[0007] The use of conventional excimer laser annealing technology
to generate polycrystalline silicon is problematic for several
reasons. First, the polycrystalline silicon generated in the
process is typically small grained, of a random microstructure, and
having a nonuniform grain sizes, therefore resulting in poor and
nonuniform devices and accordingly, low manufacturing yield.
Second, in order to obtain acceptable performance levels, the
manufacturing throughput for producing polycrystalline silicon must
be kept low. Also, the process generally requires a controlled
atmosphere and preheating of the amorphous silicon sample, which
leads to a reduction in throughput rates. Accordingly, there exists
a need in the field to generate higher quality polycrystalline
silicon at greater throughput rates. There likewise exists a need
for manufacturing techniques which generate larger and more
uniformly microstructured polycrystalline silicon thin films to be
used in the fabrication of higher quality devices, such as flat
panel displays.
SUMMARY OF THE INVENTION
[0008] An object of the present invention is to provide techniques
for producing uniform large-grained and grain boundary location
controlled polycrystalline thin film semiconductors using the
sequential lateral solidification process.
[0009] A further object of the present invention is to form
large-grained and grain boundary location manipulated
polycrystalline silicon over substantially the entire semiconductor
sample.
[0010] Yet another object of the present invention is to provide
techniques for the fabrication of semiconductors devices useful for
fabricating displays and other products where the predominant
orientation of the semiconductor grain boundaries may be
controllably aligned or misaligned with respect to the current flow
direction of the device.
[0011] In order to achieve these objectives as well as others that
will become apparent with reference to the following specification,
the present invention provides methods for processing an amorphous
silicon thin film sample into a polycrystalline silicon thin film
are disclosed. In one preferred arrangement, a method includes the
steps of generating a sequence of excimer laser pulses,
controllably modulating each excimer laser pulse in the sequence to
a predetermined fluence, homoginizing each modulated laser pulse in
the sequence in a predetermined plane, masking portions of each
homoginized fluence controlled laser pulse in the sequence with a
two dimensional pattern of slits to generate a sequence of fluence
controlled pulses of line patterned beamlets, each slit in the
pattern of slits being sufficiently narrow to prevent inducement of
significant nucleation in region of a silicon thin film sample
irradiated by a beamlet corresponding to the slit, irradiating an
amorphous silicon thin film sample with the sequence of fluence
controlled slit patterned beamlets to effect melting of portions
thereof corresponding to each fluence controlled patterned beamlet
pulse in the sequence of pulses of patterned beamlets, and
controllably sequentially translating a relative position of the
sample with respect to each of the fluence controlled pulse of slit
patterned beamlets to thereby process the amorphous silicon thin
film sample into a single or polycrystalline silicon thin film.
[0012] In a preferred arrangement, the masking step includes
masking portions of each homoginized fluence controlled laser pulse
in said sequence with a two dimensional pattern of substantially
parallel straight slits spaced a predetermined distance apart and
linearly extending parallel to one direction of said plane of
homoginization to generate a sequence of fluence controlled pulses
of slit patterned beamlets. Advantageously, the translating
provides for controllably sequentially translating the relative
position of the sample in a direction perpendicular to each of the
fluence controlled pulse of slit patterned beamlets over
substantially the predetermined slit spacing distance, to the to
thereby process the amorphous silicon thin film sample into
polycrystalline silicon thin film having long grained,
directionally controlled crystals.
[0013] In an especially preferred arrangement, the masking step
comprises masking portions of each homoginized fluence controlled
laser pulse in the sequence with a two dimensional pattern of
substantially parallel straight slits of a predetermined width,
spaced a predetermined distance being less than the predetermined
width apart, and linearly extending parallel to one direction of
the plane of homoginization to generate a sequence of fluence
controlled pulses of slit patterned beamlets. In this arrangement,
translating step comprises translating by a distance less than the
predetermined width the relative position of the sample in a
direction perpendicular to each of the fluence controlled pulse of
slit patterned beamlets, to the to thereby process the amorphous
silicon thin film sample into polycrystalline silicon thin film
having long grained, directionally controlled crystals using just
two laser pulses. In one exemplary embodiment, the predetermined
width is approximately 4 micrometers, the predetermined spacing
distance is approximately 2 micrometers, and the translating
distance is approximately 3 micrometers.
[0014] In an alternative preferred arrangement, the masking step
comprises masking portions of each homoginized fluence controlled
laser pulse in the sequence with a two dimensional pattern of
substantially parallel straight slits spaced a predetermined
distance apart and linearly extending at substantially 45 degree
angle with respect to one direction of the plane of homoginization
to generate a sequence of fluence controlled pulses of slit
patterned beamlets. In this arrangement, the translating step
provides for controllably sequentially translating the relative
position of the sample in a direction parallel to the one direction
of the plane of homoginization over substantially the predetermined
slit distance, to thereby process the amorphous silicon thin film
sample into polycrystalline silicon thin film having long grained,
directionally controlled crystals that are disoriented with respect
to the XY axis of the thin silicon film.
[0015] In yet another preferred arrangement, the masking step
comprises masking portions of each homoginized fluence controlled
laser pulse in the sequence with a two dimensional pattern of
intersecting straight slits, a first group of straight slits being
spaced a first predetermined apart and linearly extending at
substantially 45 degree angle with respect to a first direction of
the plane of homoginization, and a second group of straight slits
being spaced a second predetermined distance apart and linearly
extending at substantially 45 degree angle with respect to a second
direction of the plane of homoginization and intersecting the first
group at substantially a 90 degree angle, to generate a sequence of
fluence controlled pulses of slit patterned beamlets. The
corresponding translating step provides for controllably
sequentially translating the relative position of the sample in a
direction parallel to the first direction of the plane of
homoginization over substantially the first predetermined slit
spacing distance, to thereby process the amorphous silicon thin
film sample into polycrystalline silicon thin film having large
diamond shaped crystals.
[0016] In still another alternative arrangement, the masking step
comprises masking portions of each homoginized fluence controlled
laser pulse in the sequence with a two dimensional pattern of
sawtooth shaped slits spaced a predetermined distance apart and
extending generally parallel to one direction of the plane of
homoginization to generate a sequence of fluence controlled pulses
of slit patterned beamlets. In this arrangement, the translating
step provides for controllably sequentially translating the
relative position of the sample in a direction perpendicular to
each of the fluence controlled pulse of slit patterned beamlets
over substantially the predetermined slit spacing distance, to the
to thereby process the amorphous silicon thin film sample into
polycrystalline silicon thin film having large hexagonal
crystals.
[0017] In a modified arrangement, an alternative technique for
processing an amorphous silicon thin film sample into a
polycrystalline silicon thin film using a polka-dot pattern is
provided. The technique includes generating a sequence of excimer
laser pulses, homoginizing each laser pulse in the sequence in a
predetermined plane, masking portions of each homoginized laser
pulse in the sequence with a two dimensional pattern of
substantially opaque dots to generate a sequence of pulses of dot
patterned beamlets, irradiating an amorphous silicon thin film
sample with the sequence of dot patterned beamlets to effect
melting of portions thereof corresponding to each dot patterned
beamlet pulse in the sequence of pulses of patterned beamlets, and
controllably sequentially translating the sample relative to each
of the pulses of dot patterned beamlets by alternating a
translation direction in two perpendicular axis and in a distance
less than the super lateral grown distance for the sample, to
thereby process the amorphous silicon thin film sample into a
polycrystalline silicon thin film.
[0018] The accompanying drawings, which are incorporated and
constitute part of this disclosure, illustrate a preferred
embodiment of the invention and serve to explain the principles of
the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] FIG. 1 is a functional diagram of a system for performing
the lateral solidification process preferred to implement a
preferred process of the present invention;
[0020] FIG. 2a is an illustrative diagram showing a mask having a
dashed pattern;
[0021] FIG. 2b is an illustrative diagram of a crystallized silicon
film resulting from the use of the mask shown in FIG. 2a in the
system of FIG. 1;
[0022] FIG. 3a is an illustrative diagram showing a mask having a
chevron pattern;
[0023] FIG. 3b is an illustrative diagram of a crystallized silicon
film resulting from the use of the mask shown in FIG. 3a in the
system of FIG. 1;
[0024] FIG. 4a is an illustrative diagram showing a mask having a
line pattern;
[0025] FIG. 4b is an illustrative diagram of a crystallized silicon
film resulting from the use of the mask shown in FIG. 4a in the
system of FIG. 1;
[0026] FIG. 5a is an illustrative diagram showing irradiated areas
of a silicon sample using a mask having a line pattern;
[0027] FIG. 5b is an illustrative diagram showing irradiated areas
of a silicon sample using a mask having a line pattern after
initial irradiation and sample translation has occurred;
[0028] FIG. 5c is an illustrative diagram showing a crystallized
silicon film after a second irradiation has occurred;
[0029] FIG. 6a is an illustrative diagram showing a mask having a
diagonal line pattern;
[0030] FIG. 6b is an illustrative diagram of a crystallized silicon
film resulting from the use of the mask shown in FIG. 6a in the
system of FIG. 1;
[0031] FIG. 7a is an illustrative diagram showing a mask having a
sawtooth pattern;
[0032] FIG. 7b is an illustrative diagram of a crystallized silicon
film resulting from the use of the mask shown in FIG. 7a in the
system of FIG. 1;
[0033] FIG. 8a is an illustrative diagram showing a mask having a
crossing diagonal line pattern;
[0034] FIG. 8b is an illustrative diagram of a crystallized silicon
film resulting from the use of the mask shown in FIG. 8a in the
system of FIG. 1;
[0035] FIG. 9a is an illustrative diagram showing a mask having a
polka-dot pattern;
[0036] FIG. 9b is an instructive diagram illustrating mask
translation using the mask of
[0037] FIG. 9a;
[0038] FIG. 9c is an illustrative diagram of a crystallized silicon
film resulting from the use of the mask shown in FIG. 9a in the
system of FIG. 1 using the mask translation scheme shown in FIG.
9b;
[0039] FIG. 9d is an illustrative diagram of an alternative
crystallized silicon film resulting from the use of the mask shown
in FIG. 9a in the system of FIG. 1 using the mask translation
scheme shown in FIG. 9b; and
[0040] FIG. 10 is a flow diagram illustrating the steps implemented
in the system of FIG. 1.
DESCRIPTION OF PREFERRED EMBODIMENTS
[0041] The present invention provides techniques for producing
uniform large-grained and grain boundary location controlled
polycrystalline thin film semiconductors using the sequential
lateral solidification process. In order to fully understand those
techniques, the sequential lateral solidification process must
first be appreciated.
[0042] The sequential lateral solidification process is a technique
for producing large grained silicon structures through small-scale
unidirectional translation of a silicon sample in between
sequential pulses emitted by an excimer laser. As each pulse is
absorbed by the sample, a small area of the sample is caused to
melt completely and resolidify laterally into a crystal region
produced by the preceding pulses of a pulse set.
[0043] A particularly advantageous sequential lateral
solidification process and an apparatus to carry out that process
are disclosed in our co-pending patent application entitled
"Systems and Methods using Sequential Lateral Solidification for
Producing Single or Polycrystalline Silicon Thin Films at Low
Temperatures," filed concurrently with the present application and
assigned to the common assignee, the disclosure of which is
incorporated by reference herein. While the foregoing disclosure is
made with reference to the particular techniques described in our
co-pending patent application, it should be understood that other
sequential lateral solidification techniques could readily be
adapted for use in the present invention.
[0044] With reference to FIG. 1, our co-pending patent application
describes as a preferred embodiment a system including excimer
laser 110, energy density modulator 120 to rapidly change the
energy density of laser beam 111, beam attenuator and shutter 130,
optics 140, 141, 142 and 143, beam homogenizer 144, lens system
145, 146, 148, masking system 150, lens system 161, 162, 163,
incident laser pulse 164, thin silicon film sample 170, sample
translation stage 180, granite block 190, support system 191, 192,
193, 194, 195, 196, and managing computer 100 X and Y direction
translation of the silicon sample 170 may be effected by either
movement of a mask 210 within masking system 150 or by movement of
the sample translation stage 180 under the direction of computer
100.
[0045] As described in further detail in our co-pending
application, an amorphous silicon thin film sample is processed
into a single or polycrystalline silicon thin film by generating a
plurality of excimer laser pulses of a predetermined fluence,
controllably modulating the fluence of the excimer laser pulses,
homoginizing the modulated laser pulses in a predetermined plane,
masking portions of the homoginized modulated laser pulses into
patterned beamlets, irradiating an amorphous silicon thin film
sample with the patterned beamlets to effect melting of portions
thereof corresponding to the beamlets, and controllably translating
the sample with respect to the patterned beamlets and with respect
to the controlled modulation to thereby process the amorphous
silicon thin film sample into a single or polycrystalline silicon
thin film by sequential translation of the sample relative to the
patterned beamlets and irradiation of the sample by patterned
beamlets of varying fluence at corresponding sequential locations
thereon. The following embodiments of the present invention will
now be described with reference to the foregoing processing
technique.
[0046] Referring to FIGS. 2a and b, a first embodiment of the
present invention will now be described. FIG. 2a illustrates a mask
210 incorporating a pattern of slits 220. The mask 210 is
preferably fabricated from a quartz substrate, and includes either
a metallic or dielectric coating which is etched by conventional
techniques to form a mask pattern, such as that shown in FIG. 2a.
Each slit 220 is of a breadth 230 which is chosen in accordance
with the necessary dimensionality of the device that will be
fabricated on the sample 170 in the particular location that
corresponds to the slit 220. For example, the slits 220 should be
approximately 25 micrometers across to fabricate a 25 micrometer
semiconductor device, or in the case of a multi-part device, a
channel in a device, in sample 170. The width 240 of the slit 220
is preferably between approximately two and five micrometers in
order to be small enough to avoid nucleation in sample 170 and
large enough to maximize lateral crystal growth for each excimer
pulse. It should be understood that although FIG. 2a illustrates a
regular pattern of slits 220, any pattern of slits could be
utilized in accordance with the microstructures desired to be
fabricated on film 170.
[0047] In accordance with the present invention, the sample 170 is
translated with respect to the laser pulses 164, either by movement
of masking system 150 or sample translation stage 180, in order to
grow crystal regions in the sample 170. When the sample 170 is
translated in the Y direction and mask 210 is used in masking
system 150, a processed sample 250 having crystallized regions 260
is produced, as shown in FIG. 2b. The breadth 270 of each
crystallized region will be approximately equal to the breadth 230
in the mask 210. The length 280 of each region will be
approximately equal to the distance of Y translation effected by
movement of the masking system 150 or translation stage 180, and as
with the breadth, should be chosen in accordance with the final
device characteristics. Each crystal region 260 will consist of
polysilicon with long and directionally controlled grains.
[0048] Referring next to FIGS. 3a and b, a second embodiment of the
present invention will now be described. FIG. 3a illustrates a mask
310 incorporating a pattern of chevrons 320. The breadth 320 of
each chevron side will determine the size of the ultimate single
crystal region to be formed in sample 170. When the sample 170 is
translated in the Y direction and mask 310 is used in masking
system 150, a processed sample 350 having crystallized regions 360
is produced, as shown in FIG. 3b. Each crystal region 360 will
consist of a diamond shaped single crystal region 370 and two long
grained, directionally controlled polycrystalline silicon regions
380 in the tails of each chevron.
[0049] While the embodiments described with reference to FIGS. 2
and 3 are advantageous to generate spatially separated devices on
silicon sample 170, at least some of the silicon sample 170 is not
utilized in the final semiconductor. In order to facilitate a more
flexible configuration of devices that can be developed on the
semiconductor sample 170, the following preferred embodiments will
now be described.
[0050] Referring to FIGS. 4a and b, a third embodiment of the
present invention will now be described. FIG. 4a illustrates a mask
410 incorporating a pattern of slits 420. Each slit 410 should
extend as far across on the mask as the homogenized laser beam 149
incident on the mask permits, and must have a width 440 that is
sufficiently narrow to prevent any nucleation from taking place in
the irradiated region of sample 170. The width 440 will depend on a
number of factors, including the energy density of the incident
laser pulse, the duration of the incident laser pulse, the
thickness of the silicon thin film sample, and the temperature and
conductivity of the silicon substrate. For example, the slit should
not be more than 2 micrometers wide when a 500 Angstrom film is to
be irradiated at room temperature with a laser pulse of 30 ns and
having an energy density that slightly exceeds the complete melt
threshold of the sample.
[0051] When the sample 170 is translated in the Y direction and
mask 410 is used in masking system 150, a processed sample 450
having crystallized regions 460 is produced, as shown in FIG. 4b.
Each crystal region 460 will consist of long grained, directionally
controlled crystals 470. Depending on the periodicity 421 of the
masking slits 420 in sample 410, the length of the grains 470 will
be longer or shorter. In order to prevent amorphous silicon regions
from being left on sample 170, the Y translation distance must be
smaller than the distance 421 between mask lines, and it is
preferred that the translation be at least one micron smaller than
this distance 421 to eliminate small crystals that inevitably form
at the initial stage of a directionally controlled polycrystalline
structure.
[0052] An especially preferred technique using a mask having a
pattern of lines will next be described. Using a mask as shown in
FIG. 4a where closely packed mask lines 420 having a width 440 of 4
micrometers are each spaced 2 micrometers apart, the sample 170 is
irradiated with one laser pulse. As shown in FIG. 5a, the laser
pulse will melt regions 510, 511, 512 on the sample, where each
melt region is approximately 4 micrometers wide 520 and is spaced
approximately 2 micrometers apart 521. This first laser pulse will
induce the formation of crystal growth in the irradiated regions
510, 511, 512, starting from the melt boundaries 530 and proceeding
into the melt region, so that polycrystalline silicon 540 forms in
the irradiated regions, as shown in FIG. 5b.
[0053] In order to eliminate the numerous small initial crystals
541 that form at the melt boundaries 530, the sample 170 is
translated three micrometers in the Y direction and again
irradiated with a single excimer laser pulse. The second
irradiation regions 551, 552, 553 cause the remaining amorphous
silicon 542 and initial crystal regions 543 of the polycrystalline
silicon 540 to melt, while leaving the central section 545 of the
polycrystalline silicon to remain. As shown in FIG. 5c, the crystal
structure which forms the central section 545 outwardly grows upon
solidification of melted regions 542, 542, so that a directionally
controlled long grained polycrystalline silicon device is formed on
sample 170.
[0054] Referring to FIGS. 6a and b, a fourth embodiment of the
present invention will now be described. FIG. 6a illustrates a mask
610 incorporating a pattern of diagonal lines 620. When the sample
170 is translated in the Y direction and mask 610 is used in
masking system 150, a processed sample 650 having crystallized
regions 660 is produced, as shown in FIG. 6b. Each crystal region
660 will consist of long grained, directionally controlled crystals
670.
[0055] As with the embodiment described above with respect to FIGS.
4a and b, the translation distance will depend on the desired
crystal length. Also, the process described with reference to FIGS.
5a-c could readily be employed using a mask as shown in FIG. 6a,
having 4 micrometer wide lines 620 that are each spaced apart by 2
micrometers. This embodiment is especially advantageous in the
fabrication of displays or other devices that are oriented with
respect to an XY axis, as the polycrystalline structure is not
orthogonal to that axis and accordingly, the device performance
will be independent of the X or Y coordinates.
[0056] Referring next to FIGS. 7a and b, a fifth embodiment of the
present invention will now be described. FIG. 7a illustrates a mask
710 incorporating offset sawtooth wave patterns 720, 721. When the
sample 170 is translated in the Y direction and mask 710 is used in
masking system 150, a processed sample 750 having crystallized
regions 760 is produced, as shown in FIG. 7b. Each crystal region
760 will consist of a row of hexagonal-rectangular crystals 770. If
the translation distance is slightly greater than the periodicity
of the sawtooth pattern, the crystals will be hexagons. This
embodiment is beneficial in the generation of larger silicon grains
and may increase device performance.
[0057] Referring next to FIGS. 8a and b, a sixth embodiment of the
present invention will now be described. FIG. 8a illustrates a mask
810 incorporating a diagonal cross pattern 821, 822. When the
sample 170 is translated in the Y direction and mask 810 is used in
masking system 150, a processed sample 850 having crystallized
regions 860 is produced, as shown in FIG. 8b. Each crystal region
860 will consist of a row of diamond shaped crystals 870. If the
translation distance is slightly greater than the periodicity of
the pattern, the crystals will be squares. This embodiment is also
beneficial in the generation of larger silicon grains and may
increase device performance.
[0058] Referring next to FIGS. 9a-d, a seventh embodiment of the
present invention will now be described. FIG. 9a illustrates a mask
910 incorporating a polka-dot pattern 920. The polka-dot mask 910
is an inverted mask, where the polka-dots 920 correspond to masked
regions and the remainder of the mask 921 is transparent. In order
to fabricate large silicon crystals, the polka-dot pattern may be
sequentially translated about the points on the sample 170 where
such crystals are desired. For example, as shown in FIG. 9b, the
polka-dot mask may be translated 931 a short distance in the
positive Y direction after a first laser pulse, a short distance in
the positive X direction 932 after a second laser pulse, and a
short distance in the negative Y direction 933 after a third laser
pulse to induce the formation of large crystals. If the separation
distance between polka-dots is greater than two times the lateral
growth distance, a crystalline structure 950 where crystals 960
separated by small grained polycrystalline silicon regions 961 is
generated, as shown in FIG. 9c. If the separation distance is less
or equal to two times the lateral growth distance so as to avoid
nucleation, a crystalline structure 970 where crystals 980 are
generated, as shown in FIG. 9d.
[0059] Referring next to FIG. 10, the steps executed by computer
100 to control the crystal growth process implemented with respect
to FIG. 9 will be described. FIG. 10 is a flow diagram illustrating
the basic steps implemented in the system of FIG. 1. The various
electronics of the system shown in FIG. 1 are initialized 1000 by
the computer to initiate the process. A thin silicon film sample is
then loaded onto the sample translation stage 1005. It should be
noted that such loading may be either manual or robotically
implemented under the control of computer 100. Next, the sample
translation stage is moved into an initial position 1015, which may
include an alignment with respect to reference features on the
sample. The various optical components of the system are focused
1020 if necessary. The laser is then stabilized 1025 to a desired
energy level and reputation rate, as needed to fully melt the
silicon sample in accordance with the particular processing to be
carried out. If necessary, the attenuation of the laser pulses is
finely adjusted 1030.
[0060] Next, the shutter is opened 1035 to expose the sample to a
single pulse of irradiation and accordingly, to commence the
sequential lateral solidification process. The sample is translated
in the X or Y directions 1040 in an amount less than the super
lateral grown distance. The shutter is again opened 1045 to expose
the sample to a single pulse of irradiation, and the sample is
again translated in the X or Y directions 1050 in an amount less
than the super lateral growth distance. Of course, if the sample
was moved in the X direction in step 1040, the sample should be
moved in the Y direction in Step 1050 in order to create a
polka-dot. The sample is then irradiated with a third laser pulse
1055. The process of sample translation and irradiation 1050, 1055
may be repeated 1060 to grow the polka-dot region with four or more
laser pulses.
[0061] Next, if other areas on the sample have been designated for
crystallization, the sample is repositioned 1065, 1066 and the
crystallization process is repeated on the new area. If no further
areas have been designated for crystallization, the laser is shut
off 1070, the hardware is shut down 1075, and the process is
completed 1080. Of course, if processing of additional samples is
desired or if the present invention is utilized for batch
processing, steps 1005, 1010, and 1035-1065 can be repeated on each
sample.
[0062] The foregoing merely illustrates the principles of the
invention. Various modifications and alterations to the described
embodiments will be apparent to those skilled in the art in view of
the teachings herein. For example, the thin silicon film sample 170
could be replaced by a sample having pre-patterned islands of
silicon film. Also, the line pattern mask could be used to grow
polycrystalline silicon using two laser pulses as explained with
reference to FIGS. 5a-c, then rotated by 90 degrees and used again
in the same process to generate an array of square shaped single
crystal silicon. It will thus be appreciated that those skilled in
the art will be able to devise numerous systems and methods which,
although not explicitly shown or described herein, embody the
principles of the invention and are thus within the spirit and
scope of the invention.
* * * * *