Excimer laser microlithography at 193 NM

Theses

Thesis/Dissertation Collections

12-1-1994

Excimer laser microlithography at 193 NM

Bruce W. Smith

Follow this and additional works at:

http://scholarworks.rit.edu/theses

This Dissertation is brought to you for free and open access by the Thesis/Dissertation Collections at RIT Scholar Works. It has been accepted for inclusion in Theses by an authorized administrator of RIT Scholar Works. For more information, please contactritscholarworks@rit.edu.

Recommended Citation

by Bruce W. Smith

A thesis submittedin partial fulfillment of the requirements for the degree of Ph.D.

inthe Center for Imaging Science at the Rochester Institute of Technology

December 1994

Signature of Author _

Accepted by __M,-,,-=a:.:...:rk..:...=D-=--"-'--F.=a"-'-ir=ch:..:.;ic:.::1d=---- _

Certificate of Approval

Ph.D. Degree Thesis

The Ph.D. Degree Thesis of Bruce W. Smith has been examined and approved by the

thesis committee as satisfactory for the thesis requirement for the Ph.D. degree in Imaging Science

Dr. Dana Marsh, Thesis Advisor

Dr. Jonathan Arney

Dr. Pantazis Mouroulis

Thesis release pennission

Title of Thesis: EXCIMER LASER MICROLITHOGRAPHY AT 193 NM

Pennission to reproduce this thesis in whole or in part can be obtained by contacting the author, Bruce W. Smith at the following address:

Rochester Institute of Technology Microelectronic Engineering Dept. 82 Lomb Memorial Drive

Rochester, N.Y. 14623-5604

Signature:

Date:

by Bruce W. Smith

Submittedto the

Centerfor_Imaging Science

inpartialfulfillmentoftherequirements forthePh.D. Degree

attheRochester Institute of_Technology

ABSTRACT

As integratedcircuitdevicesare pushedtowardhigher_speeds, higher_{packingdensities,}

andlowerpower_{consumption, the}requirements ofhighresolution optical_lithography

increase. Researchinto _lithographyatthe 193 nm wavelength of anArF excimerlaser

has been conducted. _Usingavariablenumerical aperture refractive_imaging system and a

spectrallynarrowed excimer_laser, lithographicresolutionto 0.25 p.mhas been obtained in

anew 193 nm resist materialbasedon a copolymerization ofa silylated methacrylatewith

Chapter 1. Introductionandsummary 1

Chapter 2. Background

-Optical microlithography 4

2.1 Excimerlasers 6

2.2 Deep-UV_lithographywith excimerlasers 8

2.3 ArF excimer laser_lithography 9

2.4 Photosensitiveresist materialsfor 193 nm 11

Chapter 3. ArFexcimerlaser for193 nm_lithography 16

3.1 UVlasersystems 16

3.1.1 Excimerlasersystems 19

3.1.2 Raregashalogen excimers 22

3.1.3. Excitationschemes 25

3.1.4 Excimerlasersfor IC_lithography 27

3.2 Spectral narrowingofArFexcimerlaser 3 1

3.3 Lasercharacteristics 35

Chapter 4. Opticsfor 193 nm_lithography 46

4.1 Opticalmaterialsfor 193 nm 46

4.1.1 UVproperties offused silica 48

4. 1.2 UVinduced damageto fused silica 50

4. 1.3 Damage_testingoffusedsilicawith 193 nm radiation 53

4.2 193 nm refractive optics 58

4.2.1 Lens requirements anddesign 60

4.2.2 Illumination systemfor 193 nm 61

4.2.3 Refractiveobjectivelens for 193 nm 65

4.2.4 Scalardiffraction_modeling 74

4.2.4. 1 Scalarand vectordiffractionmodels 74 4.2.4.2 Verificationof scalardiffractionmodels 76

4.2.5 Simulated 193 nmlensperformance 77

4.2.6 193 nm_imagingperformance 84

Chapter 5. Phase-shiftmasksfor 193 nm projection_lithography 91

5.1 193 nm resistsfor maskevaluation 98

5.2 Subtractivephase-shift maskprocess 99

5.3 Phase errorsat 193 nm 100

5.4 _Alternating 193 nm phase-shift mask 102

5.5 Attenuated 193 nmphase-shift mask 105

Chapter 6. Optimization, synthesis, and characterization of_P(SI-CMS) 109

as a_crosslinkingnegativeresistfor 193 nm_lithography

6. 1 Trimethylsilylmethylmethacrylateandpoly(trimethylsilylmethyl 110

methacrylate as 193 nm materials

P(SI-CMS)

6.6.2 Gel pointand physical propertiesof_P(SI-CMS) 134

formulations

6.7 Lithographiccharacterizationof_P(SI-CMS) materials 138 6.7.1 Evaluationof2000Ato 5000_{A P(SI-CMS)} 143

films

6.7.2 _{P(SI-CMS)imaging}at 193 nm 144

6.8 OxygenplasmaRTEbehaviorof_P(SI-CMS) 148

6.9 _{P(SI-CMS) summary} 152

Chapter 7. Conclusionsand recommendations 154

Appendix! Comparisonof scalar and vectordiffraction_modelling 157

fordeep-UV _lithography

ALL Scalarsimulations 158

A 1.2. Vectorsimulations 161

Experimentalrun data forscalar simulations 162

Experimenatlrun dataforvector simulations 174

AppendixII. Simulationcodefor 193 nmlensperformance 178

References 186

Figure2. 1. Energylevel diagram fora rare gashalogenexcimerlaser. 7

Figure2.2. Absorptionspectraof atypicalDiazonapthoquinone sensitizer 12

andNovolacresinfor 300nmto450 nm_exposure,togetherwith

principle emissionlines of a_Hgarclamp.

Figure 2.3. UV absorbancespectrum of poly(vinyl phenol). 12

Figure 2.4. UVabsorbancespectrum of poly(methyl_{methacrylate)} and 13

severalmethacrylate co-polymers.

Figure 3.1. Comparisonofaverage output powers availablefromvarious 1 7

primary and_{secondary UV laser} sources.

Figure 3.2. Potential energycurvesforelectronic states ofHe2. 20

Figure 3.3. _Energyflow diagramfor _{electronically}excited excimerrare 22

gas dimers.

Figure 3.4. Typical potential_energycurvefor rare-gashalideexcimerlasers. 24

Figure 3.5. Gain spectrum of a rare gashalogen_{lasingmedium, along}with 28

spectral profiles of afree runningand a_spectrallynarrowed excimer

laser.

Figure 3.6. Excimerspectral_narrowingtechniques: _{a) Littrowgrating}with 30 beamexpander, b)grazingincidencegrating, c)intracavityprisms,

d) intracavityetalons.

Figure 3.7. Transmission characteristics ofcoarseandfine _etalons, together 35

with excimerlasergainbandwidth.

Figure 3.8. Vaporpressure curves forexcimerlasergases andtypical 37

impuritybyproducts.

intracavity

Figure3.11. MaximumtiltangleforetalonFSRvaluesof1 to 150_cm"1, 4 1

?i=193nm,L/W=150.

Figure 3.12. Output spectrumfromArFexcimerlaser: _a)unnarrowed 43

broadband_output,b) narrowed to32pm with coarse_etalon, and

c) narrowedto 3 pmwithfineetalon.

Figure 3.13. Output_intensityfrom ArF excimerlaser: _a)unnarrowed 45 broadband_output,b) narrowed to32pm with coarse_etalon, and

c) narrowedto 3 pm withfineetalon.

Figure 4. 1. Transmittanceof _{CaF2, BaF2,} and_MgF2 . 47

Figure 4.2. StructuralformofSi(0,/2)4. 49

Figure 4.3. _{Bondingenergy}forSi02. 49

Figure 4.4. ProductionofE'centerthrough creation of an oxygen_vacancy 5 1 betweenadjacent silica.

Figure 4.5. ProductionofE'centerthrough hydrogen Si-Sicombination 52

inwet silica.

Figure 4.6. Productionmodefor peroxyradical via reaction withE' center. 53

Figure 4.7. Inducedabsorption at 193 nm and210-220 nmfrom5

mj/cm2

55

ArF excimerlaser_radiation, 150Hz.

Figure 4.8. Inducedabsorptionat 193 nm and210-220 nmfrom 30

mj/cm2

55

ArF excimerlaser_radiation, 150 Hz.

Figure 4.10. Kohlerilluminationfor lithographicsystem. 6 1

Figure 4. 1 la. Illumination uniformityfromcondenserlens systemat 0.60 NA. 64

Figure 4._13a,b, c. Wavefront aberration plots at0.70fieldfordefocus 69

of -0.3, 0.0, and+0.3 microns.

Figure4._{13d, e,} f. Wavefrontaberration plotsatfull field for defocus 70

of_{-0.3, 0.0,} and+0.3 microns.

Figure4. 14. 193 nm projection system configuration. 71

Figure 4. 15a. Two-dimensionalaerial image_intensitydistributions for 79

0.25 micronfeatures; 0.60_{NA, o=0.50, 0.70}field.

Figure 4. 15b. Two-dimensional aerialimage _intensitydistributionsfor 79

0.25 micron_features; 0.60_{NA o=0.50,}full field.

Figure 4.15c. Two-dimensionalaerial image_intensitydistributionsfor 80

0.20micron_features; 0.60_{NA a=0.50, 0.70}field.

Figure 4.15d. Two-dimensionalaerial image_intensitydistributions for 80

0.20micron _features;0.60_{NA c=0.50,}full field.

Figure 4. 16a. One-dimensional aerialimage_intensityplotfor 0.25 micron 81

features; 0.60_NA, o=

0.50, full field position, 0.0, 0.15, 0.3, and

0.45 microndefocus. Imagemodulationis _{95%, 94%, 89%,} and

59%for_{0, 0.15, 0.30,} and0.45 micron defocus.

Figure 4. 16b. One-dimensionalaerial image_intensityplotfor0.20micron 81

features; 0.60_NA a=

0.50, full fieldposition, 0.0, 0.15, 0.3, and 0.45microndefocus. Imagemodulationis_{91%, 89%, 73%,} and

40% for_{0.0, 0.15, 0.30,} and 0.45 microndefocus.

Figure 4. 17. Simulated aerialimage_logslope for193 nmlens _{operating0.30,} 83

0.45, and0.60NAas afunctionoflaser_bandwidth, FWHM. k,=0.5

correspondingto0.332 urn, 0.214 urn, and0.161 urnfor_{0.30, 0.45,} and 0.60NA.

Line/spacepairs rangefrom2.0to 0.2microns at IX.

Figure4.20. MTF for_{imagingsystem, 0.30 NA}a=

0.70. 88

Figure4.21. Effects offlareon 0.25 urn aerialimages for 193 _{nm, cr=0.5,} 90

0.60 NA.

Figure 5.1. Resolutioncondition forcoherentillumination. 91

Figure 5.2. Resolutioncondition for incoherent illumination. 92

Figure 5.3. Schematicof an _alternatingphase-shift mask. 93

Figure 5.4a. Imagelog-slopevs. partial coherencefor_binarymask, 0.2 u.m 96

dense_lines, 193 _nm, 0.45 NA.

Figure 5.4b. Imagelog-slopevs. partial coherencefor_alternatingphase-shift 96

mask, 0.2 u.mdense_lines, 193 _nm, 0.45 NA.

Figure 5.5a. Image log-slopevs. partial coherencefor_binarymask, 0.2 97

urnisolated_lines, 193 nm, 0.45NA

Figure 5.5b. Imagelog-slopevs. partial coherencefor 6%attenuated 97

phase-shift_mask, 0.2urnisolated_lines, 193 _nm, 0.45NA.

Figure 5.6a. AFMmeasurement offused _silica,RTE CHF3etched 2000 A. 101

Figure 5.6b. AFMmeasurement offused_silica, RIECHF3etched 2000_A, 101

10: HF smoothed 100 A.

Figure 5.6c. AFMmeasurementoffused _silica,no etch. 101

Figure 5.7. Imagelog-slopevs. defocus for_alternatingphase-shift _mask, 102

0.2 urndense_lines, 193 _{ran, 0.45NA, o=0.3,} with_5,10, and20

phase-shifterrors.

Figure 5.8. 0.24 u.m_denselines imagedwith_alternating phase-shift mask. 104

lines, nm, 0.45NA o=0.7.

Figure 5.10. 0.30 \xmdense lines imagedwith6%attenuatedphase-shift 107 mask.

Figure 5.11. SEMof0.30micron_{dense lines imaged}with6% attenuating 108

phase-shift mask.

Figure6. 1 Schematicrepresentation of poly(trimethylysilylmethyl 1 1 1

methacrylate).

Figure 6.2. Absorption spectraoftrimethylsilylmethyl methacrylate. 113

Figure 6.3 Schematicrepresentation ofP(SI-CMS). 116

Figure 6.4. Spinspeedcurvefor 90: 10_P(SI-CMS), 14wt./vol. % in EEP. 122

Figure 6.5. Absorbancespectrafor90:10P(SI-CMS). 123

Figure 6.6. Gel curvefor90:10_P(SI-CMS)at40KMW. 125

Figure 6.7. Gel curvefor 90: 10_P(SI-CMS) at 160Kand 120KMW. 126

Figure6.8. Generalized_Floryfunction forpolymerswith alognormal 129

distributionof molecular weightfor bvaluesfrom0to 2.2.

Figure 6.9. Weightfractionof gel vs. _E/Egfor90: 10 SI:CMS. 130

Figure 6.10. Exposurevaluesfor_P(SI-CMS) predictedfrom_Floryfunction 13 1

for 50% filmretention.

Figure 6.11. Absorptionspectrafor_P(SI-CMS)formulations. 134

Figure6.12. Resistthicknessvs. spin speedfor95:5 and 98:2formulations. 135

Figure 6.13. Gel curvesfor95:5 and98:2_P(SI-CMS) formulations. 136

Figure 6.14. Weightfractionof gelvs. _E/Egfor95:5 and 98:2 SLCMS. 137

showingswelling P(SI-CMS)

development.

Figure6. 17a. SEMresultsoflithographicexposureof90: 10material. 144

Figure 6. 17b. SEMresultsoflithographicexposure of90:10material. 144

Figure 6. 18a. SEMresults oflithographicexposure of95:5 material. 145

Figure 6.18b. SEMresults oflithographicexposureof95:5 material. 145

Figure 6. 19a. SEMresults oflithographicexposureof98:2material. 146

Figure 6. 19b. SEMresultsoflithographicexposureof98:2material. 146

Figure 6.20. SEMof0.25 printedfeatures in 4000A98:2P(SI-CMS). 148

Figure 6.21. SEMofbilayerresults. 0.35 mmfeaturesin 2000 A98:2 152

P(SI-CMS), 02RIEtransferredinto 6000 A_Shipley812 resist.

Table 3.1. Operationalparameters ofrare gashalogen lasers. 27

Table4. 1. Experimentallydeterminedband_{gaps and}UV_{cut-off wavelengths} 47

forselected materials.

Table4.3. Condenser lens design forKohlerilluminationin 193 nm system. 62

Table 4.4. Condensersystem illumination_uniformity at0.60NAand0.24NA. 65

Table 4.5 Desired performance criteriafor 193 nmobjectivelens. 65

Table 4.6. Zernike Polynomialtermsfor_{axis, 0.70}zone, and edgefield 68

positions at 193.300 and 193.302 _nm, inwavelengths

Table4.7. Transmissionvalues for illuminationsystematvarious_NAcsettings. 73

Table 6. 1 Requiredresistabsorbancefro optimalresponse. 120

Table 6.2. Propertiesof_P(SI-CMS) formulations. 132

Table 6.3. Absorbanceandoptimumthicknessvaluesfor_P(SI-CMS) 133

formulations.

Table 6.4. Exposureresponse properties of_P(SI-CMS)formulations. 136

Table 6.5. Predictedcontrast _(g)andCMTFvaluesfor_P(SI-CMS)materials 141

atvariousfilmthicknesses.

Table 6.6. _Summaryof required doseto sizevaluesfor_P(SI-CMS) formulations. 147

Asintegratedcircuit featuresizes continueto _shrink, shorter wavelength

projection_lithographytechniquesrequiredevelopment. _{Lithographyutilizing}the 193 nm

wavelength of anArgon Fluorideexcimerlaserhasbeeninvestigatedinthisstudy.

Requirementsintroduced atthisshort wavelength prohibituse of conventional optical

materials, lensdesigns, andresistmaterials_currentlyusedforlonger UVwavelengths ofa

mercuryarclamp. _{Additionally,} spectralrequirementsofa refractiveprojection system

foruse at 193 nmpreventuse of a_{"free-running",}unnarrowedArF excimerlaser

producing a spectralbandwidthontheorder of0. 1 nm. Thisresearchhas leadtothe

developmentofasmallfield experimental 193 nm projection_{lithographysystem,} basedon

arefractivefused silica objective and anArF excimerlaser spectrallynarrowedwithdual

Fabry-Perotetalons. Thisworkhasalsoledto thedevelopmentof a_{new, negative-acting}

193 nmresist material suitableforapplicationinsingle-layerandbi-layerprocesses.

Fewoptical materials aresuitableforusebelow 200nm. Materialsinclude fused

silicaand variousfluoridecrystals. Fused silicawas chosenasthe solematerialforusein

this work, duetoits superiorenvironmental, thermal, and mechanicalstability. Radiation

damageto fused silicaisaconcernatsuchshort_wavelengths, as absorptioncenters and

optical compaction canbe induced with_longtermradiation. Investigations have shown

The 193 nmprojection system utilizesKohler_{illumination,} with control over

partial coherencethroughincorporationof avariableapertureat afirstelement difiuser

source. Theprojectionlens isa 1 mm_field, sixelementallfused silica20: 1 _objective, with

variable numericalaperturefrom0.30to0.60 NA. Spectralrequirements ofthe excimer

source aredictated_bythefused silica_objective, and depend onthenumerical aperture

settingofthe lens. At0.30_NA aspectralbandwidthof26 pm_(FWHM) isrequired from

the source while at0.60 NAabandwidthof7pmisneeded. Requirementsaremet_by

spectrallynarrowinganArFexcimer with a single_FabryPerotetalonfor lensoperation at

0.30to 0.38 NAandtwoetalonsfor lensoperationto 0.60 NA.

Phase shift masktechniqueshavebeeninvestigated forresolution downto 0.24

u.matfocal depthsgreaterthan 1.5 um. _Alternatingphase shiftmaskshave been

fabricated_{using Reactive Ion Etched(RE)}fused silica. An attenuatingphase shiftmask

approachhasbeen developed for 193 nm_{using 6% transmitting}chrome and reactive ion

etchedfused silica. Thisapproachhas beenshownto beaintroduce little additionalmask

designand process complexity.

ResistsusedforlongerwavelengthUV_lithographyare_generallynot suitablefor

193 nm_use, duetoasubstantial absorption increaseof resinmaterialsbelow200nm. A

methacrylate andthechloromethyl_styrene, materialshave been developedwhich are

suitableforuseatthicknessesfrom2400Ato 5500 A. Oxygen RTEtransferof resist

imagestoan_underlyingnovolaclayerviathesiliconcontentofthe copolymer allowsthis

materialtobeusedin bi-layeraswellas singlelayermodes. Lithographicimagingofthis

resist of0.25 u.mlineand space pairshasbeen achieved, usingthe 193 nm projection

Asintegratedcircuit devicesare pushedtowardhigherspeeds, higher packing

densities, andlowerpower _consumption,therequirementsofhigh resolution_lithography

increase. Theadvancementofthe_{64M 256M} and 1G DRAMis_pushingcircuitfeature

resolutiontoward andbelow0.25p.m. Conventional opticallithographic_techniques,

utilizingultravioletlines ofthe_mercuryarc_lamp (downtothe365 nm_i-line)have been

pushedto deliver0.5 u.m_resolution, butresolutionbelowthisisconstrained _byoptical

capabilities and illuminationwavelength. Rayleigh'scriteria _[1] relatestheresolutionlimit

of a projection_imagingsystemto _exposingwavelength_(X), lensnumericalaperture _(NA),

and aprocess coherencefactor (&7):

R_kiX

R~NA

Asseenfrom_{this relationship,} decreasing_exposingwavelengthand_increasinglens

numerical aperture willincreaseresolution attainableinagivensystem. Theadverse

effects of_increasinglens numericalaperture canbeseen when_consideringloss ofdepthof

focus (DOF):

k{k DOF=

integratedcircuitlithography. Toachieve0.25 pmresolutionwith365 nm_mercuryarc

i-line_{lithography (using}arealisticprocesskfactorof0.6forawellcontrolled

development_process)requiresalensNAof0.88. Thispresentsan impractical situation

from alens design_{and manufacture standpoint as well as a process standpoint. Depthof}

focus of suchasystemis+/-_{0.24 pm.}

To achieveilluminationwavelengthsbelowthoseproduced _bythedominant lines

ofthe_Hgarc_lamp, alternative sourceshave beenpursued. Lasers producingradiation in

thedeep-UVregion(150-300_nm)with _{sufficiently high}powerare ofinterest. Theuse of

excimerlasers forIC_lithographyhas beenproposedforseveral years andhas been

demonstrated ina_varietyofexposure systemssince 1982 [2-15]. Featureresolutionto

0.35 pm has beenachieved withKrF 248nm excimerlasersources andresolution of0.25

pm has beeninvestigated for developmentapplications. Resolutionbelow0.25 pmis

difficult_{using 248} nm radiation andconventional _imagingtechniqueswith singlelayer

resist materials. For 0.25 pm resolution_using aKrFexcimerlaser (248 _nm), alensNAof

0.60is_{required, resulting}inadepthoffocusof +/-0.35 pm. Thesenarrow _{focal depths}

put stringent requirements on_{focusingsystems,} photoresist_materials, andprocessing.

Theuse of shorterwavelength_sources, such as afifth-harmonicNd:YAGlaser of213 nm

increased, utilizingwavelengthsasshort as 193 nmpresentsever_increasingproblems with

optical_materials, photosensitive_resists, and masks.

2.1 Excimer Lasers

Theexcimerlaser_{represents a unique class}ofhigh_{power, spatially}incoherent

lasers capableof_operating atwavelengthswellintotheUV. Powerlevelsand efficiencies

produced fromexcimerlaserswell surpassthose of_anyotherUVlaserorlaser-like

source. Thereareseveralclasses ofexcimer_{lasers, only}one of which operates_usinga

trueexcimergas asa_lasing species. Thetermexcimerisa contraction ofthewords

excitedand_dimer; diatomicmolecules suchas_F2fallunderthis classification. Thegeneral

classification of excimer_{laser, though,} includes excited complexesotherthan dimersandis

characterized_byaboundor metastableexcited state and a_{very weakly}bound ground

state.

Excimer lasersystems exhibit alargenumberof electronic statesand _{energy flow}

pathwaysthat provideforpopulationofthemetastableexcited state(Figure2.1).

Reactionsinan excimer mediumallowforhigh efficiencythrough_channeling of_energy

alongkineticpathways [18]. Atthelowest_{energy state,}or ground_state,theredoesnot

stablethan theground state. Relaxationoccursthrougha series of_energypathwaysinan

ordered _energyflow. Thedirecttransitiontotheground stateis_onlypossibleinthis

orderedflowthrough emission of_radiation, withno_possibilityfor lossesfromthermal

crossings.

12

10.

e.

>

6-U u v c ₄.

0-ArF

LASER TRANSITION

193nm

Ion Channel

X State

-i 1 1 I 1

T-0 0.2 0.4 0.6 0.8 1.0

InternuclearSeparationnm

Ar+F

1.2

Inthese_systems, excited rare gashalidemoleculesareformed_bythe _strongmutual

coulombic attraction of_{positivelycharged rare gasions (suchasHe, Ne, Ar, Kr, andXe)}

and_negativelychargedhalogens(suchas_F,Cl,Br,orI). Whenthemolecule is_created,

an_energyminimum exists. Emission of aUVphoton within 10s_{seconds,though,} lowers

themoleculeto theunstableground statewhichcauses separationwithin 10"12seconds.

Populationinversion canbe_easilyachievedthrough electric_discharge, electron_beam, or

optical methods.

2.2 Deep-UV

Lithography

Therare gashalideexcimersofmostinterest for deep-UV_lithographyarethose

whichproduceradiation of_sufficientlylowerwavelengththanpossiblewith a_Hggas

discharge_lamp [20]. Although XeClproduces radiation of308 _ran, it doesnot offer much

improvement inresolution overthe365 nmi-lineof a_Hglamp. _{The KrF excimer,}

operating at_{248 nm,} has been investigated_by several workersforlithographicuse. The

high_efficiencyoftheexcimerlaserallows operation_up to severalhundredwatts.

Additionally,the poor spatialcoherent propertiesoftheexcimerlaserproduce minimal

Thetemporal coherent properties areofmore concernfor_lithographyusing an

excimerlaser. _{In many}applicationsof_lithography,the temporalnatureofan excimer

laseris satisfactory. Theseapplications _mayinclude: _(a) contact and_{proximityprinting,}

(b) projectionsystems_utilizingachromatic reductionlenses designedwith several optical

material_types, or_(c)imagingsystems_usingreflective components only. In UV

applications_involvingreductionprojection_{printing, though,} a narrowbandwidthof near

0.003 nm_mayberequired duetothelimitednumber of materialsavailableforuseas

stableoptical components. Thisrequirement_maybe2 orders of magnitude smallerthan

thatproduced_bya"free-running" excimerlaser. Becauseof_this, means of spectral

line-narrowingmustbe incorporatedintothelaser. Theseapproachesinclude _(a)a

diffraction_gratingwithabeam_{expander, (b)}a_gratingat_{grazingincidence, (c)prisms, (d)}

etalon, and_(e) injection_locking [21]. Thegoal ofline_narrowing isnot_onlyto achieve

proper spectral_bandwidth, butalso toretain power. After narrowingto achievethe

required2 ordersof_magnitude,a75%lossof power_{may be}expected.

2.3 ArF Excimer Laser

Lithography

Excimerlaserprojection lithography_usingthe_{248 nm, KrF} excimerlaser has been

resolutionwith optical_tools, shorterexposure wavelengths such asthe 193 nm

wavelengthof anArF excimerlaserneedtobeutilized. _Imaging at wavelengthsbelow

200nmpresents_{manyproblems,} with

increasingly

radiation. Thelackofsuitableopticalmaterialsforcesspectral constraints upon thelaser

source, requiringoperation at or near2pmforreduction ofchromaticaberration in large

field lenses. _{Consequently,} most effortshave been directedtoward partialor complete

reflection _{systems, eliminating}thedifficulties involvedwiththeline_narrowing of anArF

excimerlaserand _placinglittle demandonthespectralbandwidthofthe source [22-24].

Withtwo-mirror or catadioptric systems(suchastheSchwarzchildorCassegrain

arrangement), central obscuration_maybeundesirablefroma_{frequencyfiltering}

standpoint. _Scanningsystems are_{currentlybeingpursued, requiring}complex mechanical

scanningtechniques to image large fields. Forreduction_printing,mask and substrate

scanningmustbeperformed at different _{rates, creating}challengesfor imageregistration

(whichmustbeheldontheorderof_{0.05 pm)}

Inadditionto the challengesbrought about_byoptical materialtransmission

properties, spectral_{line-narrowing}forArFexcimerlasers becomesmore_challengingthan

for KrF. Thereduced gainforoperation withArFcombineswiththelosses dueto lower

transmissionsoflaser_{optics, decreasingnarrowing}efficiencies. Pulsedurationsof10-20

nsecresultin laseremissionfrom_veryfewround_triplaserpasses. Additional energyloss

purification. Expected efficienciesfora_{highly-narrowed}ArFlaser_{are, therefore,} lower

than thoseforhigher_operatingwavelength rare gas_{halogens, ranging}from 25%efficient

to near 15%.

2.4 _{Photosensitive Resist Materials for 1 93} nm

Singlelayerphotosensitiveresist _technologyismatureforwavelengths downto

365 nm_(Hgi-line),based on_{Diazonapthoquinone/Novalac}

chemistry (Figure2)[25].

Theseresistsystems are_generallynotsuitableforshorter wavelengthsbecause oftheir

lack of_transparencybelow300nm. _Lithographyat248 nmhaspushed_technologyinto

acid-catalyzed resist _chemistryanda class oi_chemicallyamplified resistsbasedon

poly(vinyl_phenol)has been developedto deliverbothpositive-toneand negative-tone

capabilities[26-28]. Althoughthese aromaticpolymersprovide suitable_transparencyfor

wavelengthsnear_{248 run,} theirhighabsorptionat 193 nmconfines exposureto the_top

0.1 pm ofa 1 pmfilm_coating(Figure_{2.3). Poly(methyl methacrylate)(PMMA)} is_highly

transparentatthiswavelength andhasbeenutilizedfor 193 nm_{applications, but its}poor

sensitivityrequiresexposuresof several_Joules/cm2, muchhigherthanthe sensitivitieson

theorder of several

mj/cm2

requiredforsuitable performance (Figure_2.4) [29].

Asalternativesto singlelayerresist_exposure, silicon_containingresistsin bi-layer

schemesand surface_imagingtechniquesweredeveloped for longerwavelength

s2o^cH

Ntjvolac

/ 1

\ \

3i: J

"^i

1

\ \

\ i

i i

^

*

i \

V

1

V... -.

...._..\

250 300 350 400 450nm

Figure 2.2. Absorptionspectraof a typicalDiazonapthoquinonesensitizerandNovalacresinfor300nmto

450nmexposure,togetherwith principle emissionlinesofa_Hgarclamp, (fromA.Reiser,Photoreactive

Polymers,WileyInterscience, 1989,p. 188.

tit! I I s

08

07L III

; !' 1 _{! i} i ii

i i ii

i I

i " II I i I I i i

i > i

- ;

(o)

8700* PMtU

6I0OA _PIMMA-CO-IN) 6000A_{PIMMi-CO-OBM)} <O00l_{P(MM-CO-0eM-CO-MAN)}

248imi

'. I ' ₁ i l

' \ \ \

\ _i Wi

01 I ,_ -> -i

250 J00

wavelength (nmi

Figure4. UVabsorbancespectrum ofpoly(methyl_{methacrylate)}andseveralmethacrylate co-polymers.

(fromT.M.Wolfetal,J.VacSci. Tech.B(5),396(1987)).

resists suitablefor_deepUVexposure include_polysilynes, linearpolysilanes suchas_poly

phenylmethyl_silane, and plasma-deposited polymersfromtetramethylsilane [31-32].

Irradiationofthesematerialsformsalatent imagethat containsincreased amountsof

oxygen. _Patterningcanbeachieved_usinganHBrplasmaor atoluenewet development.

Whencoated ona _suitablythickresin suchas novalacor_polyimide, oxygenetchresistance

ismaintainedinundevelopedareas and patterndelineationcanbeachieved.

Surface_imagingtechnologies_primarilythroughsilylationhave beendevelopedfor

materialistransferredintothebulkofthefilmthrougha plasma etch process [33].

Throughthe exposuredependentincorporationofsiliconintoa resist_surface, negative

imaginghasbeen demonstrated[34]. Anegativetoneimagearisesfrompreferential

incorporationofsilicon(inanorganosilanevapor orliquid_environment) into exposed

areas ofthe resist surface. An₀₂ reactiveionenhanced _(RE)plasma etch willconvert

siliconinto silicon_{dioxide,preventing}etch erosion ofbulkresistbeneaththe silylated

surface. Unexposed areasoftheresist surfaceremain unprotected_bythis _Si02layerand

will etch _{anisotropically in}the₀₂ environment. Positive-tone silylationhas been

achieved_byachemical or radiation _{crosslinkingmechanism,} promoted_by an

acid-catalyzed melamine reaction[35]. Crosslinked surfaceareas prevent silicon

incorporation_duringsilylation and allowforresistremoval _duringan₀₂ RIE.

Although severalbilayerand surface_imaging schemeshave been_introduced,

singlelayerresists aredesirable froma_processing standpoint. Theplasma etch and image

transferrequirementsofthemultilayer schemes_maymakethemimpracticalina

manufacturing environment. Asfeaturesizes_decrease, so doerrorbudgetsand small

variationsin_{processingmay}causelargeresponsesifprocesslatitude is poor. Aclassof

chemicallyamplifiedresistsbasedon methacrylateterpolymers_{has recently been reported,}

whichbehave inapositivemodeuponexposure and subsequent_processing[36]. These

acrylateterpolymers consistofmethylmethacrylate_(MMA), t-butylmethacrylate

highresolution_potential, and environmentalstability. Theirapplicationtointegrated

circuit_processingis_{currentlylimited,} dueto theirpoorresistanceto plasmaetching. A

classofsinglelayernegative resistsfor 193 nm_lithographyis_beingintroducedwiththe

Chapter 3

ArF _{Excimer Laser for 193} nm

Lithography

Excimerlasersare a class ofhigh_{power, efficient,}pulsed lasers_radiatinginthe

short visible andUVregions. The_possibility of abound-freeexcimer system wasfirst

investigatedinthe_{early 1960s}and commercialsystemsbecameavailableinthelate

1970's. Thehigh pulsedpowerand shortwavelengthgenerationoftheselasers has

allowed applicationinsuchareasas_{photochemistry,} diagnosticsofhigh_{densityplasmas,}

biochemical _research, thefabrication ofmicroelectronic_components, ablation ofpolymeric

materials, andthegeneration ofsoftx-raysfor x-ray_holography and ultralargescale

microcircuitfabrication.

3.1 UV Laser Systems

Awide_varietyoflasers andlaser-likesources are capable of_generatingultraviolet

photons. _Primarylaser sources produce UVradiation_byafundamentaltransitionina

lasingspecies(between_{electronic, vibrational,} or rotationallevels). _Secondarysources

produceUVphotons _byshiftingthewavelengthofthe_primarylasersource_usingvarious

frequencyconversionmethods such as_{frequencymixing,} Raman _shifting, orharmonic

generation. Figure 3.1 comparestheaverage output powers availablefromvariousUV

200 240 280

A(nm)

320 360

PrimaryUVlasersourcesinclude _{multiplyionized rare gaslasers,}

singlyionized

metal vapor_sources, and nitrogenlasers. _Multiplyionizedraregas_lasers, such as _Ar(III)

or_Kr(IV)utilizea_{multiplyionized} stateofa noble gas asa_lasingmedium [38]. These

species exhibitlowergainthanbetter_{know singly}ionized rare gaslasers forthe visible/TR,

and require_{high pumping}threshold currentdensities. Outputpowers _mayrangebetween

1 mWto 1Wand efficiencies arebelow 0.01%. _Singlyionizedstatesof certain metals

such as _{copper, gold, silver,} and _cadmium, alsohavetransitionsintheUVthatcanbe

madeto laseunder properconditions [39]. Theselasersrunincw mode and canbemade

very compact,buttheirefficiencies are_verylow. Nitrogenlasers produceUVradiation

usingmolecular nitrogen astheir_lasingspecies. Theselasertypesuse excitation schemes

similarto excimer_lasers,but have low averagepower and poor efficiencies. Typical pulse

energiesareinthefewmilhjoules range.

Secondarylaser-like sourcesutilizetechniques suchasharmonic_generation,

frequencymixing, and stimulatedRaman shifting [40]. RadiationintheUVisproduced

fromthese sources_byprimarywavelength _shiftingthrough_frequencyconversion and

non-linearinteractioninan optical medium. Most ofthese sourcetypes require_verylarge

pumppowers andhavepoor_{efficiencies,making}themunsuitablefor_manyapplications.

Powerlevelsand efficiencies producedfromexcimerlaserswell surpassthoseof

anyotherUVlaser. _Properlydesignedexcimerlasers canhave_extremelyhigh

150nm. It is forthesereasonsthatexcimerlasers have developed _rapidlysincetheir

introduction.

3.1.1 Excimer Laser Systems

Excimersystems are_verycomplexin_nature, dueto thelargenumber ofelectronic

statesevenforthemost_elementarysystems [41]. Figure 3.2 showsthepotential _energy

curvesforthesimplest excimer _{system, He2.} There existsfamilies of_closelynested curves

forthe excitedstates as well aslevel crossings_linking specificlevels. Thesecrossings

providekineticpathways forthepopulation oftheexcimer states. Variousforms of

excimerspecies are_possible,and allhaveincommonthecharacteristic of such _energy

flowpathways. Theexcimermedia_essentiallyacts as an_{energyfunnel, efficiently}

channeling energy asexcited products_{develop spontaneously along}thereaction path.

Examinationofararegas dimersystem(suchas_{Kr2, Xe2,} and _Ar^ shows how such

reactions createhighefficiencies. Figure 3.3 showsthe_energyflowpatternforthe

formationof_{electronically}excitedraregasdimers. The lowest energy state availableisat

thebottomofthe _diagram, correspondingtoground state atoms. Fordimers, an_energy

minimumdoesnot occur at an equilibrium separationfor groundstates, but forexcited

states only. Itfollowsthat atequilibriumthereare_veryfewdimermoleculespresentin

theunstableground _state,butthroughexcitation several possibilities exist. Following

40.000

35.000

30.000

2S.O00

..tf'//

//tf

-vXtf/

//

if

tf

3iJSHi _'s

2p'P-li3_'S

Jp-'Cli'

'S

'X.'

^^ 2'3i"<

ESTIMATEDERROR

<003eV

<010IV

-^

Speculation

-1.0

>

o E

-20 2

-3.0

R (Al

Figure 3.2. Potential_energycurvesforelectronicstatesofHe2. Observedvibrationallevelsare

pathways seeninFigure 3.2 forHe2. Thenature ofthe_pathwayissuchthat direct

transition to thebottomground stateis_onlypossibleinan_{orderlyflow, allowing}forno

thermallyaccessible crossings. Thekineticsofthesystembasedonthis ordered _energy

flow_onlyallow emission of radiationforrelaxationtoground state. Stimulated emission

canbe created whenthis systemis combinedwith population_inversion, whichis _easily

achievedbecause ofthelowpopulation oftheground state. Thepopulation oftheground

statewill remain_low, asmoleculesintheground state move_rapidlyapart and dissociate.

Excimerlasers_{generally fall}into threecategories: raregas, rare gas _halogen,and

metal vaporexcimers[42]. Therearethreesub-classificationsof rare gas _excimers;the

pure noblegas _{excimers, the}rare gas_oxides, andthediatomichalogens. Thesesystems

exhibitadominantchannelfor_populatingtheupperlevelsvia neutral _energytransferfrom

excited rare gas atomsanddimers. Theraregas donors (bothatomsand _dimers)maybe

produced_byan electron beamdischarge. Metalvaporexcimersusea_{Group I, II,} orIII

metal atom astheradiative speciesinadiatomicmolecule. Inplace ofthenoble gas asthe

otheratom ofthe diatomicmolecule canbetaken_{by Group}IImetal _atoms, since_they

INTERNUCLEAR SEPARATIONR "COLD"

ATOMS

Figure 3.3. _{Energyflow diagram for electronically}excitedexcimer raregasdimers. FromCh. K.

Rhodes,ExcimerLasers,(Wiley, 1984).

3.1.2 Rare Gas Halogen Excimers

Althoughboththepure rare gas andmetal vaporexcimershavebeenutilized as

laser systems, most commercial attentionhas beengiventotherare gashalogen_excimers,

introducedin 1975 [43]. Thegeneral structureofthepotential _energycurvesforrare gas

halidesis showninFigure 3.4. Thesetypesoflasersoperatewithtransitionsinmolecules

boundand correlatestotheground state 'S oftherare gas and 2P ofthehalogenatoms at

infinite internuclearseparation. Thistwo-stateground stateis_weaklybound, and

population inversionis_readilyachievedbecausethelower level dissociationtime(~10~12

seconds) ismuchlessthan theupperlevelradiativelifetime(10'9to 10"6 seconds).

Population oftheupper stateRXcanbeachieved_byrecombination oftherare gasions

andthe negativehalogen_ions, sincetheexcited stateisthesameastheionpairR+X". The

positive rare gas andnegativehalogen ions areproduced_bycollisionsbetweenhigh

energy electroncollisions.

Severalrare gas monohalide excimershave beenobservedto emit_radiation, six of

which exhibit stimulatedemission. Themost efficient oftheseexcimerspecies andtheir

wavelengths of emission are: ArFat 193_nm, KrFat248 nm, XeCl at_{308 nm,} andXeF at

351 nm. Theemission spectrum of raregashalides consistsof severalbands. The

strongestbandwhichgivesrisetothelasertransitionsisassignedto the_B(2T)-X(iI.)

transition. At highpressures, thisemissionbandisstructured _dominates, with weaker

broad bands dueto a

^n.-2!!)

and thebandofpeak_energyshifts upward ashighvibrational_energylevelsarerelaxed to

lowerlevels.

Aparameter ofimportance foralaseristhestimulated emission cross_section, or

thecross section-lifetime product. Thisproduct canbeestimatedbyassumingtheband

e LU

Molecularion

(RX)+

Otherexcited states

R +X

ound excited state

R +X

Pumping

Weakly boundground state

Internuclear Distance

Figure 3.4. Typicalpotential_energycurveforrare-gashalideexcimerlasers.

CTv r=(l/4n)(ln

2/n^(X4/cAX),

where _cr,isthestimulatedemission crosssection(in dimension of_area)and _r,isthe

photonlifetime. For_ArF,theeffectivebandwidth_(FWHM)is about 3 nm_{[44], yielding}a

cross section-lifetime product of_{approximately 57} A2ns. _Usinga measuredlifetimeof

10nsyields astimulated emissioncross sectionof_{approximately 6}x

10"16

cm2. The

processof stimulated emission canbecharacterizedby:

W =

where _W2I isthe stimulated emission_probabilityandFisthephotonflux oftheincident

wave. _{Corresponding} cross sections ofother rare gashalogenexcimers areexpectedto be

similar.

3.1.3 Excitation Schemes

Several approachesto_pumping excimerlasers_{exist, including}directelectronbeam

excitation, e-beam controlled electric _discharge, optical_{pumping excitation,} anddirect

highvoltage electricdischargeexcitation. Vigorous_pumpingisneededtoinitiate

reactions,which_invariablyleadsto pulsed operation. _{Most currently} producedexcimer

lasersutilize adirect highvoltage schemebecauseofcompactness and ease ofoperation.

Atypicalexcitationprocess canbe demonstratedfor ArFasfollows:

1. Electronattachmenttakesplace:

e"

+F2->F+_F.

2. Thenegativeions formedcombine with positiveionstogiveanexcited molecule:

Ar+

+_F->(ArF)*.

In spite oftheinvolved _path, reactions suchasthese canbe_veryefficientin_producing

exciteddimermolecules.

Efficientexcitation requires a_spatiallyuniform, highvoltage(20 - 30

kV)

discharge ingaspressures of several atmospheres. Energyistransferredformamain

storage capacitortoelectrodes_typically50to 100cm_long,with a_gap of afew

highpressurepulsed_lasers, and differs_{greatly from}thelongitudinalexcitationfor low

pressure continuous wavelasers such as aHeNe laser. Thebeam dimensionsof an

excimerlaserare_{determinedchieflybytheelectrode geometry. Mostexcimerlasershave}

nearlyrectangularbeamswith largecross _{sections,typically}2cm_{(long) by} 1 cm(wide).

Beamdivergence is_typically7mrad inthe_longdimensionand2 mradinthe short

dimension. These beam characteristics are achievabledueto the _{extremely high}gain

obtained inthe lasermedium. The_keyoperational parameters oftypical excimerlasers

are showninTable 3.1. Excimer_{lasers generally} operatewithoutcontinuousgas

replenishment, but insteadarefilledwith anappropriategas mixture andreplenished when

operationalperformance necessitates.

Comparedto conventional laserssuch asHeNeorAr-ionlasers which utilize

optical resonanceto achieve_radiation, excimerscan producehighenergieswithout

resonance. Excimer lasers,_therefore, exhibit poor spatial coherence and do not produce

speckle, whichmakethem _especiallywellsuitedforlithographicapplications. Speckleis

therandominterference pattern produced_byillumination froma_spatiallycoherent

wavefront. Specklecontrast canbecalculatedfromthe_{relationshipN"I/2,} whereNisthe

numberofindependent, _spatially coherentmodesusedtoilluminatethe object. The

number ofmodesforan excimerlaser_maybeseveral_{thousands, leading}to _verylow

F2(157nm) ArF_{(193 nm) KrF (248 nm) XeCl (308 nm)}

Averagepower_(watts) 0.05 40 100 75

Pulse energy(mj) 6 175 300 200

Repetitionrate_(Hz) 10 400 500 500

Pulsewidth_(ns) 20 15 25 20

Table 3.1. Operationalparametersofrare gashalogen lasers.

Thespectralbandwidthoftheradiation emitted_byalaser determines itstemporal

coherence,which_{may be}expressed_bycoherence_{length, lc,} where :

lc

Foran excimer_laser,the_lasingbandwidthis_typically~3 _ran,whichcorrespondsto a

coherencelengthof- _10-20

pm. An Argon Ion laserhasa spectralbandwidthlessthan

0.0001 ran, and acoherence length> 1 m. For_comparison, aHg-Xegas discharge_lamp

has a coherencelengthof_{approximately 10} pm.

3.1.4 Excimer Lasers for IC

Lithography

In manyapplications oflithography,thetemporalnature of an excimerlaseris

satisfactory. Theseapplications_mayinclude: (a) contactand_{proximityprinting,}(b)

material_types, or _{(c) imaging} systems_usingreflective components only. Inapplications

involvingreduction projection_{printing, though,} anarrowbandwidth of near0.003 nm_may

berequired dueto thelimited number ofmaterials availableforuseas stableoptical

components. Thisrequirement _maybe2 orders ofmagnitudesmallerthanwhata

"free-running"

excimerlaserproduces(seeFigure 3.5). Several methodshave been

utilizedto _effectivelyline-narrowthe output ofan excimerlaser.

Amethod _commonlyusedto narrowthespectral widthoflasers that_normallyemit

inawidebandwidth is injection_locking [45]. Atwo-stageprocess is_used, wherethefirst

stage consistsof alowpower excimer oscillatorinwhichtheoptical_cavityincludesa

stable resonator as well a certain_{intracavity frequencytuning}elements_{(prisms, gratings,}

Spectrally Narrowed

Laser

A A^0.003 nm

%

% %

KrF Qain Spectrum

^\~0.9 nm

* _Free-running Laser

*s AA.~0.3 nm

247.5 248.0 248.5 249.0

WAVELENGTH _(nm)

~1~

249.5

oretalons). Inthesecond _stage, theoutput oftheoscillatorisusedto initiatethe

stimulated emissionof radiationinanexcimerlaseramplifierinwhichtheoptical _cavity

consists ofanunstable resonator. Thesecond stage providespower amplificationforthe

input_beam, while_keepingthenarrow spectrum ofthelatterunchanged. Thissystemis

quitelargeand adds_{substantially}tothesystem expense. Severalothertechniqueshave

been employedtominimize space and capital requirements and are showninFigure 3.6.

Theseapproachesinclude_(a) adiffraction_gratingwitha_{beam expander, (b)} a_gratingat

grazingincidence, (c)prisms, and_(d)etalons [46]. Thegoal ofline_narrowingis not _only

to achieveproper spectral_bandwidth, but alsoto retain power. _{After narrowing}the

required2 ordersof_{magnitude, up}to 90%of_free-runningpower_maybesacrificed.

Excimerlasers have limitstotheir output_stabilityand spatial_{uniformity, along}

with spectralbandwidth and spatial coherence. Therearebothshort and _longterm

considerations_regardingtheoutput _stabilityof an excimer. Inthepresenceofminute

impurities intheexcimer_supplygases, heterogeneous reactionscause _steadydeterioration

ofthegasmixture. Shorttermfluctuationsoflaseroutputrelateto pulse-to-pulse

variationsin output energy. Thesearecaused_bythestatistical nature of_reactions, power

supplyregulation, and gasflow fluctuations. Improvementsinlaser designhaveallowed

&

4^

^

S

T

I

fcM

Figure 3.6. Excimerspectral_narrowingtechniques: a)Littrow gratingwithbeamexpander,b) grazing incidencegrating, c)intracavityprisms,d) intracavityetalons. Opticcomponents are: ₁₎

3.2 Spectral narrowing of anArF laser system

Toachievetherequired spectralbandwidth and power outputfroman ArF

excimer_laser, anefficient method ofline_narrowingwas desired. _Usinghigh-gradefused

silica, multipleprismtechniques_maybe_utilized, butexhibit_{very low dispersion}efficiency.

Utilizingtheprismdispersion equation [47]:

dQ _

2l(dn)

dv a

W

where Ohtotal angulardeviationofthe_beam, tispathlengthofthebeam ineach_prism,

ais beam _size, vislaser_frequency, anddn/dv isthedispersion oftheprism_material,

expected minimumlinewidth canbecalculated as:

Av =

l.lltf^]

ifdiffractionlimited beam divergenceis assumed. _Furthermore, an_{instability factor,D,}

canbedeterminedastheratio of_frequencyfluctuationsto expected linewidth:

For t= ₁

cmforeach oftwo prisms, a= ₁

cm,dn/dv=(dn/dX)xA2 =_6.3 x 10"*

cm"', an expected minimumlinewidthforadualprism approachtolinenarrowing is3.77

cm"1

or0.014nm. Thepredictedinstabilityfactoris2 x 103|d9|. Theperformance ofthis

bandwidthontheorderof afewpicometers. _Anyfluctuation intherear_reflecting mirror

willcontribute _{significantly}to _instability,throughinfluenceofd6.

Gratingtechniques_maybeemployed_, eitherat_grazingincidenceor with abeam

expander,tonarrowbeyondwhat canbeachievedwithprisms. To evaluatethe

performanceof such a_technique, the_gratingequationisconsidered[48]:

^

= d(sinli

sin/2) ,

whered isthe _{gratingpitch,I1}istheincident_angle, and_I2istherefracted angle. Through

differentiation, holding I1 constant,laserlinewidth canbedetermined:

Av =

^p

Ifadiffractionlimited beam isassumed and a_{grating beam}expansionof_l/coslj is_used,

thisequationbecomes:

Av =

2.44^

Foran angle ofincidenceof_89, a2400 cy/mm_grating, and a 1 cmbeam_diameter, an

estimatedlinewidthof0.08

cm"1

or0.0003 nmispossible. Although a_gratingapproach

features ahighdamage_threshold,theloss inpowerdueto_onlymoderate _efficiencylimits

narrowbandwidthperformance.

Spectral narrowingthroughtheuseofFabry-Perot etalonsallows forselective

fixedseparation and _highlyreflective multilayer surfaces. Transmitted_intensitycanbe

estimatedby:

(l+R2-2Rcos(?) '

whereR isthereflection coefficientofthe_coatings, J isthetransmission_{coefficient, <p}=

(47i/X}idcos _6, 6 istheangleof_incidence,nis refractive_index, anddisthesurface

separation. Thefreespectral range_(FSR) measuresthedifference inwavelengths

correspondingto successivepeaksintransmittedenergy:

FSR

ii

and finesse_(F)is defined astheratio oftheseparationof adjacentfringesto their_FWHM,

givenby:

TijR F =

1-R

To spectrallynarrowanArFexcimerlasertotheorder of afew_picometers,

surface separationsare chosentogiveFSRvalues which_will,whendivided_bythe_finesse,

produceFWHMbandwidthscorrespondingto:

AX(FWHM) = jp .

Thisefficiencyoftheetalon_{method, along}withthe_versatilityof_adaptingofelementsinto

gap etalons were chosento minimizedamageandthermaldriftwhichcould resultathigh

laserpowerlevels. The_free-runningbandwidthof anArF excimerlaserisnear300pm.

Adesired bandwidthontheorder of3-7pm was requiredfor highNA_imagingwitha

refractive optical system. Inordertoachieve_this, atwoetalon approach was_taken,

where aninitial coarseetalon would narrowtheoutputto -30pm and asecond fineetalon

would narrowto~3 pm. To accomplish_this, etalon parameters weredeterminedand a set

ofetalonswasfabricatedthroughExitechLimited, Oxford UK. Asshownin Figure_3.7,

thewavelength separationbetweentransmissionpeaks_(FSR)ofthecoarse etalon was

chosentomatchtheFWHMbandwidth ofthebroadunnarrowedlaseroutput. This

enabled _onlyonetransmissionpeak oftheetalonto _exist, and allowsfor_tuningby

changingtheangle ofincidence. The FSRofthefineetalon was chosento matchthe

FWHMofthebandwidth produced_bythecoarse_etalon,where _narrowingcanbe

optimizedthrough_tuning onitstilt angle.

Fused silica air_gapetalons with reflection coefficients ofthemultilayerdielectric

coatings of86% allowedfor finessevalues of20. The lackof_{non-absorbing dielectric}

materials withdifferentindicesof refraction at 193 nm makes multilayercoatingsdifficult

tofabricate. To achieve reflection coefficients above90% (and finessevaluesabove30)

upto 50layersmayberequired, whichwouldleadtohigh internal absorption. FSRfor

thecoarse etalon was specifiedtobe 143

cm"1

or0.532 _ran,which correspondsto aAX

(FWHM) of27pm. FSRforthefineetalon was specifiedto be8

cm"1

which allows_{further narrowing}to aAX_(FWHM)of2 pm. Estimated _intensity

transmittedforthecoarse etalon is near50%of unnarrowed energy. Estimated _intensity

transmittedforthe twoetalon systemisnear 10%.

ETALON

TRANSMISSION

EXCIMER GAIN TRANSITIONBAND

CoarseEtalon ^^"^*s^ _{Free Spectral Range (FSR)}

FSR

n Fine

ft

WAVELENGTH

Figure 3.7. Transmissioncharacteristicsofcoarse andfineetalons,togetherwithexcimerlasergain

bandwidth.

3.3 Laser Characteristics

ALumonicsEX-700excimerlaser operatingat80watts maximum powerwith

KrFgas mixturewas retrofittedforoperationwithArFgas at 193 nm. Thepulse

dimensionsare_{approximately 10}x27 mm(as defined_byelectrode_separation), and

divergence is 1 x3 mrad.

Operationat 193 nm withArFintroducesmorechallenges overKrFoperation, as

transmissionlossesand gas contamination reducesmaximum power and lasergaslifetime.

Theloss oflaser_efficiency overtimefromgaseousreactions canbeattributedto:

1. Lossofthehalogen donorfrom side reactions.

2. Gaseousimpurities formed_bysidereactions ofthehalogenwiththe

walls ofthelaservesseland withtraceimpurities _initiallypresentinthegas

mix,whichcancauselossesthroughphotoabsorption, kineticlosses, and

opticaldegradation.

3. Particulateimpuritiesformedbysputteringandfinedust particles

accumulate on optical surfaces.

Separation ofimpuritiesthrough cryogenic condensation andfineparticlefiltrationcan

reducelosses _duringArF laseroperation.

To effectivelyremove contaminantsformedduringlaseroperationwithout

condensing primarycarriergases, relative vapor pressuredataofthegases needto be

considered. Maximum efficiencyisachievedbycoolingthegasmixture toatemperature

justabovethatwhich would condenselasergases. Inorderto optimize_this,vapor

pressure curves suchasthoseshown inFigure 3.8were utilized. Usinggasmixtures

100,

Vapor

Pressure

(TORR)

0.1

(

-MO -300 -iao -130 -ao -40

Temperature _(C)

J i L

40

Figure 3.8. Vaporpressure curvesforexcimerlasergasesandtypical_impuritybyproducts.

was utilizedfor 193 nm operation. This correspondedto 0. 16% of5%_F2(> _98.0%) in

Ne(>99.995%), 3.45%_{Ar(>99.998%),} andbalance_Ne(>99.995%). Argonpartial

pressurewiththismixtureis 105_Torr, andfluorinepartialpressureis5 Torr. Atthese

pressures, neitherAror_F2willcondenseattemperaturesofliquidnitrogen(-196_C),

whereastypical contaminant will. Cryogenic purificationis, _therefore, simplifiedforArF

operation, since operationcloseto liquidnitrogentemperatureisefficient and no

temperaturecontrolisrequired aswouldbe forKrandXeoperation. A continuously

frontand rear ofthelasercavity. The_{setup is}shownin Figure _3.9, where adiaphragm

circulatingpumppassesthegasmixturefromthelaserthroughafineparticlefilterto

remove_{contaminants,} throughaheatexchangerto _partiallycoolthemixture, througha

liquidnitrogencold_trap, andbacktothelaser. Gaslifetimesmeasured_using a newgas

fill (measured asthe number of100Hzpulsesbefore _energylossto _50%)were28.8

million, or 8hours ofunnarrowed operation.

Coarseandfineetalons placedwithinthe_cavitywereinserted betweenthe

laser_cavity andtherear mirror_byremovingtherear mirror and _replacingitwith a_MgF2

uncoatedblankplate. Amirrorand etalon_{holding assembly}wasfabricatedtoallowthe

rear mirrorto beplaced at adistance of_{approximately 28}cmfromtherear ofthelaser

cavity. Thecoarseetalon was mounted at 15 cmfromthe_cavityandthefine etalon was

mounted at22cm. Allthreeelementsareheldwiththe_abilityto tilt: therear mirrorhas

horizontal andverticaltiltcontrol while each etalonhas verticaltilt controlfortuning. A

schematic ofthesystemisshownin Figure 3.10.

Alignmentwas accomplished_usingaHe-Ne_laser, directed downthemiddleofthe

bore ofthe excimerlaser fromtheoutputend. Therearmirror was aligned sothat the

reflectedspotsfrom its surfacestraveledback downtheboreoftheHeNelaser. Withthe

fineetalon raisedout oftheoptical_{path, the}coarse etalon was aligned sothat italso

Fineparticle

filter

VI

-B-V2

e-Lumonics EX700

X

Rear

Mirror ArFlaser cavity

Front

Mirror

J Circulating \

~\ pump

J

Heatexchanger

Valveto

external_pump X

Liquid

Nitrogen

cold_trap

Figure3.9. Schematicof cryogenicgaspurificationfor ArFoperation of excimerlaser.

Rear

mirror

Removable

shutter

Fineetalon

FSR=8cm-l

Coarseetalon

FSR=143cm-l

MgF2

window

Dischargechamber

Gainmedium

-<

Window

(frontmirror)

Upon achievingthecoarse etalon"normal" _position, itwastilted tothe

approximate position of optimum selectivetransmissionasdetermined by:

e,=

j(91+^)

,

where_0, istheminimumtiltangleoftheetalonto preventreflectionthroughthegain

medium. Maximumtransmissionoccurswhen:

mX = _2ndcosQ

,

and _0; isthedetermined as:

0 - W

9l ~

T

withW_beingthe laseraperture dimensionandL_beingthe_{laser cavity}length. The

minimumtiltangle forthelaseris _{approximately 0.4, using}aLAV of150. Figure3.11

showsthe maximumtilt angleforFSRvaluesfrom 1 to 150 cm"1

at 193 nm_usingaLAV

ratioof150. Maximumangleforthecoarseetalon(143 cm"1) is2. 14 and maximum

angleforthefineetalon (8 cm"1)is 0.54.

Inordertoadjustthetilt angleofthecoarse etalontodeliveroptimum_narrowing,

ahighresolution spectrometer was set_uptomonitorinrealtimethelaser linewidth. The

spectrometerconsists ofavariable aperture slit assembly, aseriesoffourplanemirrorsto

foldthe_beam, aconcave collection mirror situatedtoplacetheslit atits focal lengthof1

max62j

Figure 3.11. MaximumtiltangleforetalonFSRvalues of1to150 cm"1, X=193 nm, L/W-150.

diffract light efficientlyat_relativelyhighorders. The beam illuminatesthegratingatan

angle ofincidence suchthat thedispersed light isdirected back alongthedirectionof

incidentlight. Light dispersed_bythe_gratingisreflected onto alinear array detector

locatedintheimageplane oftheslitobject. Imagecapture allowscharacterization ofthe

excimerbeam. Thespectrometerwastunedforoperationat 193 nmthrough

adjustment

ofthe_gratingangle ofincidence. Usingafusedsilica90mil plate as abeam splitterto

attenuatetheexcimer_beam, output spectra was monitored andcoarse etalontilt angle was

fromthebroadbandArFexcimerisnear300pm. _{Absorption features in}the spectrumare

dueto absorptionbandsofthe4-0 _{Schumann-Runge}systemfor_oxygen, whichare

centered at _{193.1, 193.3,} and 193.5 nm. AnestimatedAXof21pmFWHMforthe 143

cm"1

FSRcoarse etalon comparesto the32pmmeasured fromFigure 3. 12b. Output

poweroftheArF laser dropped from-10watts unnarrowedat 100_{Hz (100 mj/pulse)}to

~2wattsat 100 Hz(20 _mj/pulse), or an 80%loss comparedto anestimated 50% loss.

Alignmentofthefineetalon wasfirst achieved_usingtheHeNe_laser, andthrough

spectrometer_monitoringto achievefinetuning. Figure 3. 12c showsthe spectrum

achievedwithboth etalonsinplace. Abandwidthof3 pmFWHMwas_achieved,

comparedwiththeestimated2 pmFWHM. Apowerlossof95%resultedfrom insertion

ofbothetalonsintothelaser_{cavity, resulting}in5mj/pulseat 100Hz. Although lessthan

the estimated90% loss,thispowerlevel isadequate forexposure of a resist_requiring

<100 mj/cm2

inareduction_imaging system withlow losses.

Theupper limitonthebackground broadbandwidth amplified spontaneous

emission_(ASE)present inthenarrowedoutput canbe determined_{by blocking}therear

mirror withthe etalonsinplace. TheASE, although_present, isnot at alevel high enough

tobedetected_bythespectrometer oraUV energymeter. Theactual broad bandwidth

contribution_during operationis lessthan thisamountdetected, since gain saturation

8 _{5 E a f. % S}

pixel number ( 0 82pm/pixe] )

ArF Excimerwith 1 Etalon

110

--1(0

--140

--i so

--_ 100

---* ID

--60

--40

--2 0

--0

^Jj-.iylllllll llftiUMtlJrf^rr\lrtMtMitrrl\'lMlMMh 'ifAVfrjitMrMtofMitJltM,IMj.iUmtfrilJJuMWMti

pixel number ( 0 82pm/pixel ₎

ArF Excimer 2 Etalons

300

--2 50

--2 00

--| 1 50

-100

-50

--0 -t

r*^.^tnZir-Sb9

pixel number ( 0 82pm/pixe1)

Figure3.12. Outputspectrumfrom ArFexcimerlaser: a)unnarrowedbroadbandoutput,b)narrowed

Figure 3.13 showstheoutput profilesfortheArFlaser_operatingunnarrowed

(Figure 3._13a), withthecoarse etalon(Figure3._13b), andwithboththecoarse andfine

etalons (Figure 3.13c). _Energylevelscorrespondto 100_mj/pulse, 20_mj/pulse, and5

mj/pulse, respectively. Beam_{uniformity is degraded}upon spectralnarrowing, requiring

attention_during designoftheilluminationopticsforthe_imaging system.

Themethod of_{spectrally narrowing}anArF excimerlaserthrough use ofdual

Fabry-Perotetalons has_successfullyproducedlaseroutput with abandwidth downto3

pmFWHM. Throughuseof cryogenic gas_{purification,} outputinthismode isstable_upto

1 hourat 100 Hzbefore gasdepletion. OperationoftheArFlaserwith a single _etalon, at

abandwidthof32 _pm, isstableforseveralhoursat 100 _Hz,which increasesthe_versatility

ofthelaserwhen optical requirements allow(suchaswithlowobjectivelensnumerical

Broadbandoutput

100mj/pulse

Coarseetalonoutput

20mj/pulse

Dualetalon output

5mj/pulse

Figure 3.13. Output_intensityfrom ArFexcimerlaser: _a)unnarrowedbroadbandoutput,b)narrowed

Chapter

4

Optics for 193 nm

Lithography

Optical_lithographybelow300nmismade difficult becauseoftheincrease in

absorptioninoptical materials. Fewtransparentmaterials existbelow_{200 nm, limiting}

designandfabrication_flexibilityin opticalsystems. Refractive projection systems are

possibleatthese short_wavelengths,butrequire considerationofissues_concerning

aberration effects andradiationdamage.

4.1 Optical Materials for 193 nm

Theopticalcharacteristics ofglassesintheUVareimportant when_considering

photolithographic systems_containingrefractive elements. Aswavelengthsbelow250nm

are_utilized, issuesof radiationdamageand changesinglass molecularstructurebecome

additional concerns. Refraction ininsulatorsis limitedbyinterband absorption atthe

materialsband_gapenergy,Eg. For 193 nm_radiation, photon _energyofE-6.4 eVlimits

optical materialstothosewith_relativelylargebandgaps. _{Halide crystals, includingCaF2,}

LiF, BaF2, MgF2, and_NaF, andamorphous _Si02(or fused_silica) arethefewmaterialsthat

possess largeenoughbandgaps andhave suitabletransmissionbelow200 nm. Table4. 1

shows_{experimentally}determined bandgapsandUVcut-off wavelengths of several halide

Material _{r (eV) X=hc/Eg}(nm)

BaF2 CaF2 MgF2 LiF

NaF

SiO,

8.6

9.9

12.2

12.2

11.9 9.6

144 126 102 102 104 130

Table 4.1. Experimentallydetermined bandgapsandUVcut-off wavelengthsforselected materials

(from Gan_Fuxi, OpticalandSpectroscopic_{Properties of}