us6573531b1

8/3/2019 US6573531B1

1/17

111111111111111111111111111111111111111111111111111111111111111111111111111US006573531B1(12) United States Patent

1m et al. (10) Patent No.:(45) Date of Patent: US 6,573,531 BlJun. 3, 2003(54) SYSTEMS AND METHODS USING

SEQUENTIAL LATERAL SOLIDIFICATIONFOR PRODUCING SINGLE ORPOLYCRYSTALLINE SILICON THIN FILMSAT LOW TEMPERATURES

JPWOWOWOWO

11 064883 3/1999WO 98/24118 6/1998WO 99/31719 6/19990118854 3/20010118855 3/2001

OTHER PUBLICATIONS

(73) Assignee: The Trustees of Columbia Universityin the City of New York, New York,NY (US)

(75) Inventors: James S. 1m, New York, NY (US);Robert S. Sposili , New York, NY (US);MarkA. Crowder, New York, NY(US)

( * ) Notice: Subject to any disclaimer, the term of thispatent is extended or adjusted under 35U.S.C. 154(b) by 0 days.

S.D. Brotherton et aI., "Infiuence of Melt Depth in LaserCrystallized Poly-Si Thin Film Transistors," 82 J. Appi .Phys. 4086 (1997).J.S. 1m et aI., "Crystalline Si Films for Integrated ActiveMatrix Liquid-Crys ta ls Displays," 21 MRS Bulle tin 39(1996).(List continued on next page.)

Primary E x a m i n e r ~ o n g PhamAssistant Examiner-Wai-Sing Louie(74) Attorney, Agent, or Firm-Baker Botts L.L.P.(57) ABSTRACT

U.S. PATENT DOCUMENTSReferences Cited

FOREIGN PATENT DOCUMENTS1 067 593 A2 1/20012338342 12/1999

System and methods for processing an amorphous siliconthin film sample into a single or polycrystalline silicon thinfilm are disclosed. The system includes an excimer laser forgenerating a plurality of excimer laser pulses of a predetermined fiuence, an energy density modulator for controllablymodulating fiuence of the excimer laser pulses, a beamhomoginizer for homoginizing modulated laser pulses in apredetermined plane, a mask for masking portions of thehomog in iz ed modul at ed l aser pul se s into patternedbeamlets, a sample stage for receivingthe patterned beamletsto effect melting of portions of any amorphous silicon thinfilm sample placed thereon corresponding to the beamlets,t ranslat ing means for control lably trans lating a relativeposition of the sample stage with respect to a position of themask and a computer for controlling the controllable fiuencemodulation of the excimer laser pulses and the controllablerelative positions of the sample stage and mask, and forcoordinating excimer pulse generation and fiuence modulation with the relative positions of the sample stage and mask,to thereby process amorphous silicon thin film sample intoa single or polycrystal line s il icon thin film by sequent ialtranslation of the sample stage relative to the mask andirradiation of the sample by patterned beamlets of varyingfiuence at corresponding sequential locations thereon.

21 Claims, 9 Drawing Sheets

Suzuki et al.Sameshima et al.Noguchi et al. 437/174Tanaka et al. 437/21TanakaSuzukiTreadwell et al. 430/322Shoemaker et al. 219/121.7

11/19909/19926/19965/19984/19999/20009/200011/2001

Appi. No.: 09/390,537Filed: Sep. 3, 1999Int. CI? H01L 29/04U.S. CI 257/45; 257/75; 438/166;438/481; 438/486; 438/488Field of Search 117/37, 43, 44--46,117/54, 56, 73, 74, 904, 923; 257/45, 75;438/149, 166, 479, 481, 486, 488

4,970,546 A5,145,808 A5,529,951 A *5,756,364 A *5,893,990 A6,117,752 A6,120,976 A *6,313,435 Bl *

(21)(22)(51)(52)(58)

(56)

EPGB

19 4

8/3/2019 US6573531B1

2/17

US 6,573,531 BlPage 2

OlliER PUBLICATIONS"Overview of Beam Delivery Systems for Excimer Lasers,"MicrolLas Lasersystem GMBH.K.H. Weiner et aI., "Ultrashallow Junction Formation UsingProjection Gas Immersion Laser Doping (PGILD)," A Verdant Technologies Technical Brief, Aug. 20, 1997.I.W. Boyd, "Laser Processing of Thin Films and Microstructures, Oxidation, Deposition, and Etching of Insulators"(Springer-Verlag Berlin Heidelberg 1987).H. Endert et aI., "Excimer Laser: A New Tool for PrecisionMicromaching," 27 Optical and Quantum Electronics, 1319(1995).Nebel, C.E., "Laser Interference Structuring of A-SI:h"Amorphous Silicon Technology-1996, San Francisco, CA

Apr. 8-12, 1996, Materials Research Society SymposiumProceedings, vol. 420, Pittsburgh, PA.Jeon, J-H et aI., "Two-step laser recrystallization of poly-Sifor effective control of grain boundaries", Journal of NonCrystalline Solids, North-Holland Publishing Company,NL, vol. 266-269, May 2000, pp. 645-649.*Im, J.S. et aI., "Controlled Super-Lateral Growth of SiFilms for Microstructural Manipulation and Optimization,"Phys. Status Solid (a), Applied Research, Berlin, Germany,vol. 166, 1998, pp. 603-617, XP002935002, ISN:0031-8965.* cited by examiner

8/3/2019 US6573531B1

3/17

u.s. Patent Jun. 3, 2003 Sheet 1 of 9 US 6,573,531 Bl

a::> 0o;;t l ! )

8/3/2019 US6573531B1

4/17


AI------, 121

230 I

220TO

~ - - - 4 - COMPUTERL....-__-----J 100

210

,IIII I II ! 1-------.120L-__ -.J

AFIG. 20

' : . . . l -=---210

FIG. 2b

8/3/2019 US6573531B1

5/17


2 3 4 5 6 7 8 9 10LASER BEAM PULSE NUMBER

30 0 - r - - - - - - - - . : ; ~ - . l ~ - * - ~ - ~ - - - - - . : ~ - - - I - - - + - - - - L 250 - t - -+ - - r - - t - - - - -+ - - -+ - - f - - -+ - - - * - " - - l - - - . / . 200 - - - + - - - + - - - + - - - - j f - - - - - + - - t - - - - l - - - J - - I - - - ~ ~ - - - ! 150 - t - -+ - - t - - - - - t - -+ - -+ - - - J - - - -+ - - - - - 1 - -+ - - - l 100 - r - - - - t- - t - - -1- -+--+-- -+--+-- - -1f . - - -+-- - - !- - - -50 - t - - - r - - t - - t - - + - -+ - - -+ - - - + - - - - 1L - -+ - - - I

O - t - - + - - t - - + - - + - - + - - + - - + - - - - 1 - - + - ~o 1

FIG. 3

8/3/2019 US6573531B1

6/17


,----------JI 411 412 II II II 410 420L "-120__________ ---l

FIG. 4530

r-- ---------,I 540II IX ~ II r - ~ 1 2 0L -1

FIG. 50

511 I L __ J :r - - - ' II I I

5 6 0 - 1 - - - 1 : y IL ...I ---120L -1

FIG. 5b

550\

11\ I - r ~ , ----- , 121I i I I /

8/3/2019 US6573531B1

7/17


111 r------------- ,610 620 630 II I 121I r - -., r- -.., .- - -..,I I I I I I II I I I I I II L7 _ -J L_ j -' L_7 _..JI 611 621 631 IL ~ - - 1 2 0- --1

FIG. 6

151

149r - - - - -IIIIII--- _

150/______ ..1-,710 I720 II

II- - - - - - ~

TOCOMPUTER100

FIG. 7

8/3/2019 US6573531B1

8/17

u.s. Patent

810

Jun. 3, 2003

170

Sheet 6 of 9

190

US 6,573,531 Bl

180 820

TO COMPUTER100

FIG. 8

8/3/2019 US6573531B1

9/17


910 912 911 922yh \ v q ~,e .-/ 920 921900 FIG. 90 FIG. 9b933

930 932 931FI G. 9cFLAT, DUE TOSURFACE TENSION

952

950 951FI G. ge

962- - ~ ~ ~960 961

FIG. 9 f

8/3/2019 US6573531B1

10/17

u.s. Patent Jun. 3, 2003 Sheet 8 of 9 US 6,573,531 Bl1000 INITIALIZE HARDWARE

MOVE STAGE TO INITIAL POSITION1010 LOAD SAMPLE 1005

MOVE MASK TO INITIAL POSITION1015 ADJUST FOCUS OF OPTICAL 10201025 SYSTEM, IF NECESSARYSTABILIZE LASER AT DESIREDENERGY LEVEL, REPETITION RATEADJUST ATTENUATION, IF NECESSARYSTART TRANSLATING SAMPLE AT APPROPRIATE SPEED 8 DIRECTION. SPEED=PER-PULSE TRANSLATION DISTANCE XLASER PULSE REPETITION FREQUENCY

1030

103510401050

1055

OPEN SHUTTER

YESCLOSE SHUTTER

1051

STOP TRANSLATING SAMPLE 1060 1066

1070

NO REPOSITION SAMPLE: ; ; ---- f TO NEXT AREA TO

BE CRYSTALLIZEDYESSHUT OFF LASER

SHUT DOWN HARDWARE 10751080 FIG. 10

8/3/2019 US6573531B1

11/17

u.s. Patent Jun. 3, 2003 Sheet 9 of 9 US 6,573,531 Bl1100 INITIALIZE HARDWARE

11101115

1105MOVE STAGE TO INITIAL POSITION

MOVE MASK TO INITIAL POSITION1120

1125

11301135

ADJUST FOCUS OF OPTICALSYSTEM, IF NECESSARYSTABILIZE LASER AT DESIREDENERGY LEVEL, REPETITION RATE

ADJUST ATTENUATION, IF NECESSARYSTART TRANSLATING SAMPLE AT APPROPRIA TE SPEED 8 DIRECTION. SPEED =PER-PULSE TRANSLATION DISTANCE XLASER PULSE REPETITION FREQUENCY1140

1145

1160

MODULATE ATTENUATION IN A PREDEFINED MANNER SO AS TO BESYNCHRONIZED WITH THE LASERPULSES AND THE INSTANTANEOUSP OS IT IO N OF THE SAMPLE

NO

STOP TRANSLATING SAMPLE

1151

1166

NO1165

1170 YESSHUT OFF LASER

REPOSITION SAMPLE>---1 TO NEXT AR EA TOBE CRYSTALLIZED

1175 SHUT DOWN HARDWARE1180 FIG. 11

8/3/2019 US6573531B1

12/17

US 6,573,531 B12

specification, the present invention provides an excimerlaser for generating a plurality of excimer laser pulses of apredetermined fiuence, an energy densi ty modulator forcontrollably modulating fiuence of the excimer laser pulses,5 a beam homoginizer for homoginizing modulated laserpulses in a predetermined plane, a mask for masking portions of the homoginized modulated laser pulses into patterned beamlets, a sample stage for receivingthe patternedbeamlets to effect melting of portions of any amorphous

10 silicon thin film sample placed thereon corresponding to thebeamlets, translating means for controllably translating arelative position of the sample stage with respect to aposition of the mask and a compute r for controlling thecontrollable fiuence modulation of the excimer laser pulses15 and the controllable relative positions of the sample stageand mask, and for coordinat ing excimer pulse generat ionand fiuence modulation with the relative positions of thesample stage and mask, to thereby process amorphoussilicon thin film sample into a single or polycrystalline20 silicon thin film by sequential translation of the sample stage

relative to the mask and irradiation of the sample by patterned beamlets of varying fiuence at corresponding sequential locations thereon.In a preferred arrangement , the excimer laser is a ult ra-

25 viole t exc imer laser, and the energy densi ty modulatorincludes a rotatable wheel , two or more beam attenuatorscircumfcrential ly mounted on the wheel , and a motor, forcontrollably rotat ing the wheel such that each sequent ialpulse emitted by the laser passed through one of the two or30 more beam attenuators. Advantageously, the beam attenuators are capable of producing at least two different levels offiuence attenuation.

In an alternative arrangement, the energy density modulator includes a mult ilayer dialectic plate that is rotatable35 about an axis perpendicular to a path formed by the excimerpulses, in order to variably fiuence modulate the excimerpulses in dependance of an angle formed between theexcimer pulse path and the axis of rotation. A compensatingplate is advantageously provided to compensate for a dia-40 lect ic induced shift in the beam path,

In another alternative arrangement, the energy densitymodulator includes one or more beam attenuators and atranslating stage for controllably translating the one or more45 beam attenuators such that each sequential pulse emitted bythe laser passes through one of the one or more beamattenuators or passes through the energy density modulatorwithout passing through any of the one or more beamattenuators. The translating stage is movable in both a50 direction parallel to a path formed by the excimer pulses and

a direction perpendicular to the path, and the beam attenuators are posi tionable such tha t the excimer pulses pas sthrough one of the one or more beam attenuators or throughnone of the one or more beam attenuators.In still another alternative arrangement, the energy den-sity modulator includes one or more movable beam attenuators being control lably moved such that each sequent ialpulse emitted by the laser passes through one or more of theone or more beam attenuators or passes through the energy

60 density modulator without passing through any of the one ormore beam attenuators.In one pre fe rred arrangement , the transla ting meansincludes a mask translating stage that is translatable in bothorthogonal directions that are perpendicular to a path formed

65 by the homoginized beams, and a translating stage motor forcontrollably translating the mask translating stage in both ofthe translatable directions under control of the computer. In

SUMMARY OF THE INVENTION

NOTICE OF GOVERNMENT RIGHTS

BACKGROUND OF THE INVENTION

1SYSTEMS AND METHODS USING

SEQUENTIAL LATERAL SOLIDIFICATIONFOR PRODUCING SINGLE OR

POLYCRYSTALLINE SILICON THIN FILMSAT LOW TEMPERATURES

The U.S. Government has certain rights in this inventionpursuant to the terms of the Defense Advanced ResearchProject Agency award number N66001-98-1-8913.

An object of the present invention is to provide techniquesfor growing large grained polycrystalline or single crystalsilicon structures using energy-controllable laser pulses.A further object of the presen t invent ion i s to utilizesmall-scale translation of a si licon sample in order to growlarge grained polycrystalline or single crystal silicon structures on the sample. 55Yet another object of the present invention is to providetechniques for growing location controlled large grainedpolycrystal line or single crystal s il icon s tructures whichyield planarized thin silicon films.Yet a further object of the present invention is to providetechniques for growing large grained polycrystal line ors ingle crystal si licon s tructures at low temperatures, forexample at room temperature, and without preheating.A still further object of the present invention is to provide

techniques for coordinated attenuation of laser fiuence.In order to achieve these objectives as well as others thatwill become apparent with r efe rence to the fol lowing

I. Field of the InventionThe present invention relates to techniques for semiconductor processing, and more particularly to semiconductorprocessing which may be performed at low temperatures.II. Description of the Related ArtIn the field of semiconductor processing, there have been

severa l att empts to use lasers to convert thin amorphouss il icon films into polycrystal line films. For example, inJames 1m et aI., "Crystalline Si Films for Integrated ActiveMatrix Liquid-Crys tal Displays, " 11 MRS Bullitin 39(1996), an overview of conventional excimer laser annealingtechnology is presented. In such a system, an excimer laserbeam is shaped into a long beam which is typical ly up to 30em long and 500 microns or g rea te r in width. The shapedbeam is scanned over a sample of amorphous silicon tofacilitate melting thereof and the formation of polycrystalline silicon upon resolidification of the sample.The use of conventional excimer laser annealing technol

ogy to generate polycrystalline silicon is problematic forseveral reasons. First, the polycrystalline silicon generatedin the process is typically small grained, of a randommicrostructure, and having a nonuniform grain sizes, therefore resulting in poor and nonuniform devices andaccordingly, low manufacturing yield. Second, in order toobtain acceptable performance levels, the manufacturingthroughput for producing polycrystalline silicon must bekept low. Also, the process generally requires a controlledatmosphere and preheating of the amorphous silicon sample,which leads to a reduction in throughput rates. Accordingly,there ex ist s a need in the field to generate higher quali typolycrystalline silicon at greater throughput rates.

8/3/2019 US6573531B1

13/17

US 6,573,531 B13 4

which may be Lambda Physik Model LPX315i, generates alaser beam 110 which passes through an energy densitymodulator 120, described with greater particularity below.While the Lambda Physik excimer laser generates an ultra-S viole t beam at a wave leng th of 308 nm, more power fulexcimer lasers or excimer lasers generating beams at otherwavelengths may be utilized. In accordance with the presentinvention, the modulator 120 acts to rapidly change theenergy density of laser beam 110. The excimer laser 110 and

10 energy density modulator 120 are each linked by a standardcomputer interface connection 101 to a computer 100 toeffect control of the energy density modulator 120 in accordance with the timing of laser pulses generated by laser 110.The modulated laser beam 121 is di rected th rough beam15 attenuator and shutter 130, which permit fine control overthe fluence of the modulated laser beam.

The fluence contro ll ed lase r beam 131 is directed toreflecting surface 140 and through telescoping lenses 141and 142 to become incident on reflecting surface 143, where20 it is directed through beam homogenizer 144. The telescoping lenses, which may be two plano-convex lenses 141 and142 or other well known lens configurations, act to shape thelaser beam to match the requirements of beam homogenizer144. The beam homogenizer, whichmay be a Microlas beam25 homogenizer 144, causes the laser beam to gain nearlyuniform fluence in the plane of homogenization. Thehomogenized beam 146 is passed through condensing lens145, reflected by reflecting surface 147 and passed throughfield lens 148, which collimates the beam.

The collimated beam 149 passes through masking system150, which wil l be descr ibed in greater detail below. Patte rned beamlet s 151 are output from the masking system150, reflected by reflecting surface 160, passed through aneye lens 161, reflected off reflecting surface 162 and passed35 through an objective lens 163. Alternatively, the beamletscould be directed to an objective lens 163 without intermediate reflections. The objective lens 162 acts to demagnifyand focus the pat tern beam 151.40 The focused pattern beam 164 is incident on a thin siliconfilm sample 170, such as a film of amorphous or randomlygrained polycrystalline silicon ranging from 100Angstromsto greater than 5000 Angstroms, deposited on a substrate. Inaccordance with the preferred invention, the sample 170 is45 preferably kept at room temperature and does not have to bepre-heated. As further described below, the focused patternbeam 164 is used to laterally solidify the thin silicon sample170 into a single or uniformly grained polycrystalline film.

The silicon film sample 170 is placed on top of a sample50 translation stage 180, to be described in greater detail below,which in turn rests on a granite block 190. The granite block190 is supported by support system 191, 192, 193, 194, 195,196 which is actively controllable to minimize vibration ofthe granite block 190 which may be caused by movement in55 the floor. The granite block must be precision manufacturedto have a flat surface, and preferably is of a high grade suchas Laboratory Grade AA per federal specification GGG-P463c.The support system may be a commercially availablesystem from TechnicalManufacturing Corporation, which is60 pneumatically controlled to inhibit the passing of vibrationsto the granite block 190. Cross supports between legs 191,

192,193, 194, 195, 196 may be includedfor further stability.Referring next to FIGS. 2a and 2b, the energy densitymodulator 120 is described in more detail. FIG. 2a shows a

65 side view of the energy beam modulator 120,which includesa metal wheel 210, motor 220, and a plurality of beamattenuators 230. The motor 220 is a standard stepper motor

BRIEF DESCRIPTION OF THE DRAWINGS

an alternative arrangement, the translating means includesthe sample translat ing stage, and has an X direction translation portion and a Y direction translation portion permitt ing movement in two orthogonal directions that are perpendicular to a path formed by the patterned beamlets andbeing controllable by the computer for controllably translating the sample in both of the translatable directions undercontrol of the computer. The sample translation stage mayadditionally include a Z direction translation portion, forpermitting movement of the sample in a direction parallel tothe path formed by the patterned beamlets. Most preferably,the entire system is mounted on a granite block to stabilizingthe sample from ambient vibration.The present invention also provides methods for processing an amorphous si licon thin film sample into a single orpolycrystalline silicon thin film. In a preferred technique, themethod includes the steps of generat ing a plurali ty ofexcimer laser pulses of a predetermined fluence; controlla

bly modulating the fluence of the excimer laser pulses;homoginizing the modulated laser pulses in a predeterminedplane; masking portions of the homoginized modulated laserpulses into patterned beamlets, irradiating an amorphoussilicon thin film sample with the patternedbeamlets to effectmelting of portions thereof corresponding to the beamlets;and controllably translating the sample with respect to thepatterned beamlets and with respect to the controlled modulat ion to thereby process the amorphous s il icon thin filmsample into a s ingle or polycrystalline silicon thin film bysequential translation of the sample relative to the patternedbeamlets and irradiation of the sample by patterned beamlets 30of varying fluence at corresponding sequential locationsthereon.The accompanying drawings, which are incorporated andconstitute part of this disclosure, illustrate several preferred

embodiment of the invention and serve to explain theprinciples of the invention.

DESCRIPTION OF PREFERREDEMBODIMENTSReferring to FIG. 1, a preferred embodiment of thepresent invention will be described. An excimer laser 110,

FIG. 1 is a functional diagram of a system in accordancewith a preferred embodiment of the present invention;FIG. 2a is an illustrative diagram of an energy densitymodulator suitable for use in the system of FIG. 1;FIG. 2b is an i llustrative diagram taken along sectionA-A" of FIG. 2a;FIG. 3 is a graph showing an illustrative fluence profile ofthe laser beam pulses shaped by the energy density modulator of FIG. 2;FIGS. 4-6 are illustrative diagrams of alternative energydensity modulators suitable for use in the system of FIG. 1;FIG. 7 i s an illustrative diagram of a masking sys temsui table for use in the sys tem of FIG. 1;FIG. 8 is an i llus trat ive diagram of a sample translations tage suitable for use in the sys tem of FIG. 1;FIG. 9 is an il lustrat ive diagram showing the formationand avoidance of a mount in the region where two crystalsmeet;FIG. 10 is a flow diagram illustrating the basic stepsimplemented in the system of FIG. 1; andFIG. 11 is a flow diagram illustrating the steps implemented in the sys tem of FIG. 1 with energy-density modulation.

8/3/2019 US6573531B1

14/17

US 6,573,531 B15 6

will now be described. As shown in FIG. 5a, the embodiment includes a translating stage 510 to which several beamattenuators 520, 530, 540 of varied attenuation are attached.In operation, the trans lating s tage would be pos it ioned by5 computer 100 such that the laser beam 111 does not passthrough any attenuator dur ing the ini tial excimer pulses.Towards the end of a pulse cycle , the trans lating s tage ismoved so that the beam 111 passesthrough attenuations 520,530, 540, to become an increasingly attenuated beam 121.

10 The computer synchronizes the movement of the translatings tage 510 such that each sequent ial excimer pulse passesthrough the center of the respective attenuations. One drawback of this embodiment is that the stage will be positionedto maximally attenuate the first pulse of the immediatelysucceeding pulse cycle. However, this drawback can be

15 overcome if the attenuating stage includes two orthogonaldegrees of f reedom, as shown in FIG. 5b. The beam translators 550 can be positioned to intersect the beam pathwhenattenuation is desired towards the end of a pulse cycle, andmoved out of the way of the beam path 560 prior to initiation20 of a succeeding cycle, as indicated by the y direction, and

translated in the X direction to an initial position while nonattenuated beams are desired.Referring next to FIG. 6, a movable mult i-plate energydensity modulator suitable for use in the system of FIG. 125 will now be described. The embodiment includes severalbeam attenuators, each of which is movable to be positionedeither in the path 610, 620, 630 of incident beam 111 oroutside of the beam path 611, 621, 631. The attenuators maybe movable in a direction perpendicular to the beam path, or30 may be pivoting or rotatable to move in and out of the beampath. In operation, computer 100 positions each attenuatorsuch that the laser beam 111 does not pass through anyattenuator during the initial excimer pulses. Towards the end

of a pulse cycle, the computer causes the attenuators to be35 moved so that the beam 111 passes through one or more

attenuations to become an increasingly attenuated beam 121in accordance wi th the desired modula tion profile. Thecomputer synchronizes the movement of the attenuatorssuch that each sequential excimer pulse passes through the40 center of all attenuators that are placed in the beam path. I tshould be understood that in operation, it may be desirableto pass several excimer pulses through the same attenuation,

or to vary the attenuation scheme in any other manner toachieve the desired attenuation profile.Referring next to FIG. 7, the masking system 150 isdescr ibed in more detai l. Homogenized and columnatedbeam 149, passes through mask 710which contains a patternthereon. The mask 710 may be a chrome or dielectric coatedfused s il ica slab, and should include a pat tern , such as an

50 array of slits or chevrons, which have been etched from thecoating. The mask 710 rests upon an open frame XYtranslation stage 720 which is control led by X and Y axismotors 730 under the direction of computer 100. Movementof the XY t rans lation stage 720 permits crystal growth

55 within the s il icon sample 170, as will be descr ibed below.Alternatively, the mask could rest upon a fixed open framestage, with beam translation being effected by the sampletranslator 180.As described in detail in commonly assignedapplication Ser. No. 09/200,533, filed on Nov. 27, 1996, the60 disclosure of which is incorporated by reference herein, thesl it array mask enables the production of large grainedpolycrystalline silicon having a substantially uniform grainstructure, while the chevron array mask enables the production of locat ion control led, large single crystal s il icon65 regions.Referring next to FIG. 8, the sample translation stage 180is descr ibed in more detai l. The stage may include a l inear

and includes an encoder so that the motor 220 is able toprovide computer 100 with feedback regarding the angularposition of wheel 210, and accordingly, the position of eachbeam attenuator 230. Each beam attenuator 230 is attachedto the whee l 220 to permit rotation thereof. Commerciallyavailable beam at tenuators fabricated from a dielect riccoated piece of silicon dioxide or magnesium floride aresuitable for use as beam attenuators 230.For the laser beam generated by the Lambda PhysikModel LPX315i, which is approximately 1.5x3 em, a suitable wheel210 may be approximately 10-20 em in diameterand include at least 10 beam attenuators 230, each being atleast slightly larger than 1.5x3 em, circumscribing the wheel210. FIG. 2b is a diagram taken along cross sectionA-A' ofFIG. 2a., showing ten beam attenuators 230. The number of

beam attenuators 230 chosen wil l depend on the desiredgrain size, with longer grains requiring more excimer pulsesto manufacture, and accordingly, more beam attenuators.In operation, the energy density modulator 120 and exci

mer laser 110 are operated in a synchronized manner underthe control of computer 100 to achieve the desired attenuation of each laser pulse emitted by the excimer laser 110.For example, if ten excimer pulses are required to properlyirradiate a small region of the silicon sample 170 and theexcimer laser emits laser pulses at 100 Hz, the whee l 210would be rotated at ten revolutions per second, or 600 rpm,in synchronicity with the laser pulse emissions. In thisexample, each laser pulse would be incident on a differentbeam attenuator 230 when each attenuator was in a positionsubstantially corresponding to the beam path. In accordancewith the present invention, the last laser pulses of the pulseset are attenuated in order to planarize the thin silicon filmbeing irradiated. Accordingly, in the foregoing example, thefirst seven beam attenuators 230 would cause no or littlebeam attenuat ion, while the eighth, ninth and tenth beamattenuators would cause increasing fluenee attenuationof theincident beam pulses 111.Thus, as shown in FIG. 3, the energy densi ty modulator120 changes the fluenee profile of the laser beam pulsesemitted by excimer laser 110. I f the eximer laser is emittinglaser beam pulses having a fluence of 300 mJ/cm2 , theenergy density modulator 120 may be set up to freely pa ssthe first s even pulses, to attenuate the e ight pulse to 250mJ/cm2 , the ninth pulse to 200 mJ/cm2 , and to completelyblock the tenth pulse. Of course, the foregoing is merely anexample and those skilled in the art will appreciate that other 45fluence profiles are easily achievable by changing the number of attenuators on wheel 210 and the dielectric coating oneach attenuator.In an alternative embodiment of the energy density modu

1ator 120 shown with reference to FIG. 4, greater flexibilityin configuring the attenuation profile is achieved where theattenuator is coated with a multi-layered dielectric suitablefor variable transmittance depending on the incident angle ofthe incident beam pulse. Thus, a variable beam attenuator410 controlled by step motor 411 is pos it ioned to receiveincident beam pulses 111 and to attenuate the beam inaccordance with the angle theta. The attenuated beam 121 isbrought back onto the beam axis by passing through acompensa ting plate 420 which is moved by motor 412 sothat the beam attenuator 410 and compensating plate 420 areat opposing angles with respect to the beam axis.As with theembodiment shown in FIG. 2, the beam attenuator is rotatedunder the control of computer 100 to synchronize the timingof the lase r beam pulses emi tt ed by the laser 110 and theattenuation caused by the energy density modulator 120.Referring next to FIGS. 5a and 5b, a translational energydensity modulator suitable for use in the system of FIG. 1

8/3/2019 US6573531B1

15/17

US 6,573,531 B17 8

ment with respect to reference features on the sample. Thevarious optical components of the system are focused 1020if necessary. The laser is then stabilized 1025 to a desiredenergy level and reputation rate, as needed to fully melt thesilicon sample in accordance with the particular processingto be carried out. I f necessary, the attenuation of the laserpulses is finely adjusted 1030.Next, translation of the sample is commenced 1035 at apredetermined speed and in a predetermined direction, inaccordance with the desired microstructure of the sample.The shutter is opened 1040 to expose the sample to irradiat ion and accordingly, to commence the sequent ial lateralsolidification process.Sample translation and irradiation continues until that the

15 des ired crystal lization has been competed 1050, 1051, atwhich time the computer closes the shutter and stops translation 1055, 1060. I f other areas on the sample have beendesignated for crystallization, the sample is repositioned1065,1066 and the crystallization process is repeated on thenew area. I f no further areas have been designated forcrystallization, the laser is shut off 1070, the hardware is shutdown 1075, and the process is completed 1080. Of course,if processing of addit ional samples is desired or if thep resent invent ion i s uti lized for ba tch processing, steps1005,1010, and 1035-1065 can be repeated on each sample.FIG. 11 is a flow diagram i llustrat ing the steps imple-mented in the system of FIG. 1 with energy-density modulation. Steps 1100-1140, and 1150-1180 are ident ical tothose described above with reference to FIG. 10 for steps

1000-1040, and 1050-1080. In order to implement energy-density modulation, the attenuation of the excimer laserpulses are modulated 1145 in a predef ined manner so as tobe synchronized with both the timing of the laser pulsesemitted by the laser and the instantaneous position of thesi li con sample being irradiated. In connect ion wi th theforegoing, the ability to vary the rate at which beam attenuators are moved to impact energy density modulation overthe attenuation-modulation cycle may be desirable toachieve greater flexibilityThe foregoing merely i llus trates the principles of theinvent ion. Various modif ica tions and a lterat ions to thedescribed embodiments will be apparent to those skilled inthe art in view of the teachings herein. For example, whilethe foregoing descr ibes a sample si tt ing on a translationstage, it may be advantageous to place the sample within a

45 vacuum chamber or a chamber w ith a controlledatmosphere, such as one housing an inert gas, with thechamber ly ing atop the translation stage. O ther types ofhomogenizers may be ut il ized, such as a fly's eye homogenizer. Instead of using a stacked XY translator, more50 precise translators having additional degrees of freedommay be utilized. Moreover, in order to insure that theexcimerpulses are properly focused on the sample, an activefocusing system may be utilized. It will thus be appreciatedthat those ski lled in the art wil l be able to devise numerous

55 systems and methods which, although not explicitly shownor described herein, embody the principles of the inventionand are thus within the spiri t and scope of the invention.What is claimed is:1. A system for processing an amorphous silicon thin film

60 sample into a single or polycrystalline silicon thin film,comprising:(a) an excimer laser for generating a plurality of excimerlaser pulses of a predetermined fluence;(b) an energy density modulator, opticallycoupled to saidexcimer laser, for controllably modulating said fluenceof said excimer laser pulses emitted by said excimerlaser;

motor/air bearing translation stage, for example an AerotechATS 8000 model stage. Thus, the Aerotech stage includes Xand Y direction translators 810, 820, and is controllable bycomputer 100. A separate Z direction trans lator 830, alsocontrollable by computer 100, is preferably included. The 5silicon sample 170 is rested on the Z direction tram;lator 830i n the path of masked beam 164.In operation, the computer 100 controls the movement ofeither the sample translation stage 180 or the mask translation stage 720 in accordance with the timing of the pulses 10generated by excimer laser 110 to effect the desired crystalgrowth in silicon sample 170. Either the sample 170 is

moved with respe ct to the incident pulse 164, oralternatively, the location of the pulse 164 i s moved wi threspect to the sample 170 through mask translation stage.In order to grow large grained silicon structures, smallscale translation occurs in between each excimer pulse of apulse set until the final pul se of the pulse set has beenabsorbed by the sample 170. As each pulse is absorbed bythe sample, a small area of the sample is caused to melt and 20resolidify into a crystal region initiated by the initialpulse ofa pulse set. Of course, the number of pulses in a pulse se t

will define the size of the grain that can be produced, withmore pulses enabl ing the growth of larger sized crystals.Thl1'S, since crystal structures having lengths varying from 25approximately 0.5 microns to 2 microns may be producedfrom a single pulse , i t should be under stood that crysta lstructures which obtain lengths of lOs of microns may begenerated by a suitable set of pulses. 30In order to avoid surface protrusions near the last place inthe crystal to solidify, the last pu ls es of a pulse set areattenuated. Referring to FIGS. 9a-9c, when a s il icon thinfilm 900 is irradiated with an excimer laser pulse havingsufficient energy to effect complete melt through of the film 35900, a l iquid area 912 is formed in between two sol id areas911,912. Two crystal fronts 921, 922 form and grow into thenarrowing l iquid area 922 until all l iquid s il icon has crystallized as part of crystals 930, 931. Since silicon is denserwhen in the l iquid phase than when in the sol id phase, the 40volume of the silicon film increases as the silicon solidifies,thereby forming a mount 933 where the two crystals 930,931 meet 932.In accordance with the present invention, one or moreattenuated pulses are applied to the thin film, either after thelong polycrystalline or single crystalline silicon structureshave been formed or near the end of the formation of suchstructures. Referring to FIGS. 9d-9f, when an attenuatedlaser pulse is applied to the silicon film 940 in the region ofa mount 941, the top surface of the film is liquified 952,leaving neighboring crystals 950, 951 adjoining at the bottom surface of the film. Since there can be no lateral crystalgrowth of crystals 950, 951, the crystals grow upward 960,961 to form crystal boundary which either has no mount ora far less pronounced mount 962 than mount 941.Referring next to FIGS. 10 and 11, the steps executed bycomputer 100 to control the crystal growth process in

accordance with the present inventi on wi ll now bedescribed. FIG. 10 is a flow diagram illustrating the bas ics teps implemented in the sys tem of FIG. 1. The variouselectronics of the system shown in FIG. 1 are ini tial ized1000 by the computer to initiate the process. A thin siliconfilm sample is then loaded onto the sample translation stage1005. It shou ld be noted that such loading may be eithermanual or robotically implemented under the control of 65computer 100. Next, the sample translation stage is movedinto an ini tial pos it ion 1015, which may include an al ign-

8/3/2019 US6573531B1

16/17

US 6,573,531 B19 10

50

35

beam attenuators such that each sequential pulse emitted by said laser passes through one of said one or morebeam attenuators or passes through said energy densitymodulator without passing through any of said one ormore beam attenuators.9. The system of claim S, wherein said translating stage ismovable in both a direction parallel to a path formed by saidexcimer pulses and a direction perpendicular to said path,and where in said one or more beam attenuators are posi-

10 tionable such that said excimer pulses pass through on e ofsaid one or more beam attenuators or through none of saidone or more beam attenuators.10. The sys tem of claim 1, wherein said energy densitym o du la t or c om p ri se s on e or more m ova bl e b ea mattenuators, coupled to said computer, and being controllably moved such that each sequential pulse emitted by saidlaser passes through one or more of said one or more beamattenuators or passes through said energy density modulatorwithout passing through any of sa id one or more beamattenuators.11. The sys tem of claim 1, further comprising a shutteroptically coupling said beam at tenuator to sa id beamhomogenizer and permitting said attenuated beams to passthrough when placed in an open position and blocking saidattenuating beams when placed in a closed position.12. The system of claim 1, further comprising telescopinglens optics optically coupling said beam attenuator to saidbeam homogenizer and shaping said attenuated beams tomatch said beam homogenizer.13. The system of claim 1, wherein said translating meanscomprises:

(a) a mask translating stage, mechanically coupled to saidmask, and translatable in both orthogonal directionsthat are perpendicular to a path formed by said homogenized beams; and(b) a translating stage motor, mechanicallycoupled to said

mask translating stage and coupled to said computer,for controllably translating said mask translating stagein both of said translatable directions under control ofsaid computer.14. The system of claim 1, wherein said translating meanscomprises said sample translating stage, and wherein saidsample translating stage includes an X direction translationport ion and a Y direction trans lation por tion, each beingcoupled to said computer and to each other, said X and Ydirection translation portions permitting movement in twoorthogonal directions that are perpendicular to a path formedby said patterned beamlets and being controllable by saidcomputer for controllably translating said sample in both ofsaid translatable directions under control of said computer.15. The sys tem of claim 14, wherein said sample translation stage further compr ises a Z d irection transla tionportion, coupled to said computer and to at least one of saidX andY direction translation portions, for permitting movement of said sample in a direction parallel to said pathformed by said patterned beamlets.16. The system of claim 14, further comprising a graniteblock, mechanically coupled to said translating means on aside thereof opposite to a side to which said sample isapplied, for stabilizing said sample from ambient vibration.17. The sys tem of claim 1,

wherein said sequential locations of said sample areirradiated by a sequence of said excimer laser pulses,whereon a first locat ion of said sequential locations isirradiated by a first laser pulse of the sequence of said

excimer laser pulses, andwherein a second locations of said sequential locations isirradiated by a second laser pulse of the sequence of

(c) a beam homogenizer, optically coupled to said energydensity modulator, for homogenizing said modulatedlaser pulses in a predetermined plane;(d) a mask, optically coupled to said beam homogenizer,for masking portions of said homogenized modulated 5

laser pulses into patterned beamlets,(e) a sample stage, optically coup led to said mask, forreceiving said patterned beamiets to effect melting ofportions of any amorphous s il icon thin film sampleplaced thereon corresponding to said beamlets;(1) translating means, coupled to one or more of the groupconsisting of said sample stage and said mask, forcontrol lably trans lating a relat ive pos it ion of saidsample s tage with respect to a pos it ion of said mask;and 15(g) a computer, coupled to said excimer laser, said energydensi ty modulator, and sa id translat ing means, forcontrolling said controllable fluence modulation of saidexc imer laser pulses and said controllable relative 20

positions of said sample stage and said mask, and forcoordinat ing said excimer pulse generat ion and saidfluence modulation with said relative positions of saidsample stage and sa id mask, to thereby proces s sa idamorphous si licon thin film sample into a single or 25polycrystalline silicon thin film by sequential translation of s aid sample stage relative to sa id mask andirradiation of said sample by patterned beamlets ofvarying fluence at corresponding sequential locationsthereon. 302. Th e system of claim 1, wherein said excimer laser is aultraviolet excimer laser for generating ultraviolet excimerlaser pulses.3. The system of claim 1, wherein said energy densi tymodulator comprises:(a) a rotatable wheel;(b) two or more beam attenuators circumferentiallymounted on said wheel; and(c) a motor, mechanically coupled to said wheel andcoupled to said computer, for controllably rotating said 40wheel such that each sequential pulse emitted by saidlaser passed through on e of said two or more beamattenuators.4. The system of claim 3, wherein said two or more beamattenuators are capable of producing at least two different 45levels of fluence attenuation.5. The system of claim 3, wherein at least one of said two

or more beam attenuators does not attenuate beam fluenceand at least one different attenuator causes fiuence attenuation.6. The system of claim 1, wherein said energy densi tymodulator comprises a multilayer dielectric plate, opticallycoupled to sa id exc imer laser and rotatable about an axisperpendicular to a path formed by said excimer pulses, andvariably fiuence modulating said excimer pulses in depen- 55dence of an angle formed between said excimer pulse pathand said axis of rotation.7. The sys tem of claim 6, further comprising a compensating plate, optically coupled to said multilayer dialecticplate, for compensating for translation of said excimer pulse 60path caused by said multilayer dialectic plate.S. The system of claim 1, wherein said energy densi tymodulator comprises:(b) one or more beam attenuators; and(b) a translating stage, mechanically coupled to each of 65said two or more beam translators and coupled to saidcomputer, for controllably translating said one or more

8/3/2019 US6573531B1

17/17

11US 6,573,531 B1

12said excimer laser pulses, an emission of said secondpulse immediately following an emission of said firstlaser pulse.

18. The system of claim 1, wherein said computer coordinates said exc imer pulse genera tion and sa id fiuence 5modulation with said relative positions of said sample stageand said mask during the irradiation of said sample.19. The system of claim 1, wherein said computer coordinates said exc imer pulse genera tion and sa id fiuence

modulation with said relative positions of said sample stageand said mask in real-time.

20. The system of claim 1, wherein the mask includes apat tern which has an array of slits.

21. The system of claim 1, wherein the mask includes apat tern which has an array of chevrons.* * * * *

us6573531b1

Documents