MS482MaterialsCharacterization(재료분석)

LectureNote5:RBS

Byungha ShinDept.ofMSE,KAIST

1

2016FallSemester

CourseInformationSyllabus1. Overviewofvariouscharacterizationtechniques (1lecture)2. Chemicalanalysistechniques (8lectures)

2.1. X-rayPhotoelectronSpectroscopy(XPS)2.2. UltravioletPhotoelectronSpectroscopy(UPS)2.3. AugerElectronSpectroscopy(AES)2.4. X-rayFluorescence(XRF)

3. Ionbeambasedtechniques (4lecture)3.1. RutherfordBackscatteringSpectrometry(RBS)3.2. SecondaryIonMassSpectrometry(SIMS)

4. Diffractionandimagingtechniques (7lectures)4.1. Basicdiffractiontheory4.2. X-rayDiffraction(XRD)&X-rayReflectometry(XRR)4.3. ScanningElectronMicroscopy(SEM)&

EnergyDispersiveX-raySpectroscopy(EDS)4.4. TransmissionElectronMicroscopy(TEM)

5. Scanningprobetechniques (1lecture)5.1. ScanningTunnelingMicroscopy(STM)5.2. AtomicForceMicroscopy(AFM)

6. Summary:Examplesofrealmaterialscharacterization (1lecture)*CharacterizationtechniquesinblueareavailableatKARA(KAISTanalysiscenterlocatedinW8-1)

RBS:RutherfordBackscatteringSpectrometry

RBSaccuratelymeasuresthecompositionanddepthprofileofthinfilms,includinghydrogen.

~

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Quantitative? Yes Destructive? No

Detectionlimits? 0.001– 10at% Lateral resolution(Probesize)? ³ 1 mm

Chemicalbonding? No Depthresolution? 5– 20nm

KeyApplications&InstrumentConfiguration• Determinethicknessandcompositionofthinfilms• Measurehydrogen• Determinefilmdensityfromafilmofknownthickness• Assessdamagetocrystalstructureasaresultofprocessing• Quantificationoffilmsonwholewafers(upto300mm)• Qualify&monitordepositionsystems(alsofab-to-fabcomparisons)

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• MeVionsfromanelectrostaticacceleratorarefocusedonasampleinavacuumchamberforanalysis.

• Typically,2MeVHe++ ionsareused.

PrinciplesKinematicsofelasticscattering

Conservationofenergyandmomentum12𝑀D𝑣F =

12𝑀D𝑣DF +

12𝑀F𝑣FF

v1

v2v

𝑀D𝑣 = 𝑀D𝑣D cos 𝜃 + 𝑀F𝑣F cos𝜙0 = 𝑀D𝑣D sin 𝜃 + 𝑀F𝑣F sin𝜙

ForM1 <M2 ,kinematicfactorK

𝐸D𝐸M=

𝑀FF − 𝑀D

F sinF 𝜃 D/F + 𝑀D cos 𝜃𝑀F + 𝑀D

F

(listedinHandout#5,Append.1)

• ForagivenM1 andM2,smallestK isatq =180o• Differenttypesofatoms,DM2à largestchangeinDE1 whenq =180o• Hence,backscatteringspectrometry,thoughq ~170o inpractice

PrinciplesScatteringCrossSection

Numberofparticlesscatteredinto𝑑Ω = 𝑄 W 𝑁Y W𝑑𝜎 𝜃𝑑Ω W 𝑑Ω

• E0 ofincidentparticleofM1àKM2E0 thatthisparticlepossessesatanyangleq afteranelasticcollisionwithaninitiallystationaryM2

• s(q), howfrequentlysuchacollisionoccuratacertainangleq?

THINTARGET:NS ATOMS/cm2 (=N·t)

(average)scatteringcrosssection[cm2/steradian]

,Q

(inRBS,solidangleW issmall,10-2steradian orless,soaveragecanbeusedinsteadofdifferential)

= 𝑄 W 𝑁Y W1Ω[

𝑑𝜎 𝜃𝑑Ω 𝑑Ω

\W Ω= 𝑄 W 𝑁Y W [

𝑑𝜎 𝜃𝑑Ω 𝑑Ω

\

NumberofparticlesscatteredintothedetectorwithW

differentialscatteringcrosssection

= 𝑄 W 𝑁Y W 𝜎 𝜃 W Ω

PrinciplesScatteringCrossSection

𝜎 𝜃 =𝑞F𝑍D𝑍F4𝐸

F 1sin`𝜃/2

Z1:atomicnumberofincidentparticleZ2:atomicnumberoftargetatomq:elementalchargeE:energyofincidentparticle(forthederivation,readpp.21-24ofHandout#8)

• Withoutconsideringrecoilofthetargetatom(M1 <<M2),i.e.,thetargetatomisstationaryallthetime

• Includingtherecoileffect,

𝜎 𝜃 =𝑞F𝑍D𝑍F4𝐸

F

sina`𝜃2 − 2

𝑀D

𝑀F

F+ ⋯

~4%correctioninthecaseofHe(M1=4)incidentonSi(M1=28)

• Rutherford ScatteringCrossSection,s(q)for1MeV4HeislistedinHandout#5,Appendix2.

PrinciplesDeviationfromRutherfordScattering

• AssumptiontoderiveRutherfordscatteringcrosssection:scatteringduetotherepulsionoftwopositivelychargednucleiofatomicnumberZ1 andZ2

• Meaningthatincidentatompenetrateswellinsidetheorbitaloftheatomicelectronsà closestdistance<Kshellelectronradiusà

𝐸 >𝑞F𝑍D𝑍F𝑎M

, wherea0 isBohrradius.~10keV forHescatteringfromSi~340keV forHescatteringfromAu

• At lowenergy,correctionfromthescreeningshouldbeconsidered. 𝜎fg = 𝜎 𝜃 F

• Athigherenergy,departurefromtheRutherfordscatteringcrosssectionduetotheinteractionoftheincidentparticlewiththenucleusofthetargetatom

~9.6MeVforHeionsincidentonSi

PrinciplesStoppingpower(stoppingcrosssection)

−𝑑𝐸𝑑𝑥 =

2𝜋𝑞`𝑍DF

𝐸 W 𝑁𝑍F W𝑀D

𝑚 ln2𝑚𝑣F

𝐼

EnergylossofMeVlightions(suchasHe)insolids:• electronicenergyloss (interactionwithelectrons,excitedorejected)

• Negligiblenuclearenergyloss

dE/dx:eV/Å(1/r)dE/dx:eV/(µg/cm2),wherer ismassdensity(1/N)dE/dx:eV/(atoms/cm2),whereN isatomicdensity

listedinAppendix3(Handout#5)

N:targetatomconcentration(#/cm3)m:electronmassI:averageexcitationenergyofanelectron

Principles

• Energytransferfromaprojectiletoatargetatominanelastictwo-bodycollisionà conceptofkinematicfactorandcapabilityofmassperception

• Likelihoodofoccurrenceofsuchatwo-bodycollisionàconceptofscatteringcrosssection andcapabilityofquantitativeanalysisofatomiccomposition

• Averageenergylossofanatommovingthroughadensemediumà conceptofstoppingcrosssection andcapabilityofdepthperception

HowtoInterpretRBSData

• Channelnumber=Backscatteringenergy

• Surfaceisontheright(highenergy)greaterdepthstotheleft(lowerenergy)à DE~t

• Heavierelementsproducehigherenergybackscattering.Why?

• Heavierelementsproducelargerpeaksperunitconcentration.Why?

• Shapeofspectrum.Why?

PtonSi

200nmPt

𝜎 𝜃 =𝑞F𝑍D𝑍F4𝐸

F

sina`𝜃2 − 2

𝑀D𝑀F

F+ ⋯

Rutherfordscatteringcrosssection

SifromSi/Ptinterface

(t)

energycorrespondingtoSiatsurface

Ptatsurface

DE

EnergyWidth

𝐾no𝐸M 𝐾np𝐸M

• SurfaceAupeakathigherenergythansurfaceAlpeak

•

t=

∆𝐸rs = [𝑑𝐸𝑑𝑥

t

𝑑𝑥 ≈𝑑𝐸𝑑𝑥vrs

W 𝑡

Att, 𝐸 𝑡 = 𝐸M − ∆𝐸rs = 𝐸M −xyxz{rsW 𝑡

𝐸D

𝐸D = 𝐾no𝐸 𝑡 − t|}~ �

xyxz{�pt

= −𝑡 𝐾noxyxz{rs+ t

|}~ �xyxz{�pt

+ 𝐾no𝐸M

∆𝐸no = 𝐾no𝐸M − 𝐸D = 𝒕[S]

• Surfaceenergyapproximation(for<100nm):xyxz{rs~ xyxz{y�, xyxz{�pt

~ xyxz{�y�

• Meanenergyapproximation:xyxz{rs~ xyxz{y�a

��∆y

, xyxz{�pt

~ xyxz{y��

��∆y

?

DEAu

Q:whichoneislarger,DEAu orDEAu?

DepthProfiles

𝐻�r𝐻Yr

=𝑁�r𝑁Yr

𝜎�r(𝐸M)𝜎Yr(𝐸M)

≅𝑁�r𝑁Yr

𝑍�r𝑍Yr

F

E1

𝐻�r𝐻Yr

=𝑁�r𝑁Yr

𝜎�r(𝐸M)𝜎Yr(𝐸D)

≅𝑁�r𝑁Yr

(𝑍�r/𝐸M)F

(𝑍Yr/𝐸D)F

𝜎 𝜃 =𝑞F𝑍D𝑍F4𝐸

F

sina`𝜃2 − 2

𝑀D𝑀F

F+ ⋯

𝐻�r∆𝐸�r𝐻Yr∆𝐸Yr

=𝑁�r𝑁Yr

𝜎�r𝜎Yr

orbetterapproximation

MeasuringConcentration&Thickness

ComparisonofThreeWSix FilmswithvaryingWconcentrations

ComparisonofThreeTiFilmswithvaryingthickness

2.27meVHe,160° RBS

Si SiO2

Si

O

SiinSiO2

Depth

OatSurface

Depth

SiatSurface

ExampleofRBSSpectrum

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ScatteringGeometryAffectsDepthResolution

Sample

~100°

~160°

GrazingExit

Detector

Normal AngleDetector

Incident He++ IonsBackscattered

He Ion

• Grazingangledetectorimprovesdepthresolutionforthinlayers

ScatteringGeometryAffectsDepthResolution

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EffectofFilmDensityonThicknesses

• FundamentalunitofmeasurementforRBSisatoms/cm2

• Density× thickness=atoms/cm2

• TocalculateafilmthicknessusingRBSaloneonemustassumeafilmdensity• Ifthefilmthicknessisknown(byTEM,SEM,profilometry,etc.),thenthefilm

densitycanbecalculated

2.27meVHe,160° RBS

Thetotalatomsineachfilmareequal(1.13x1018 atoms/cm2)

Bothsamplesproducethisspectrum200nm

density=5.66x1022 Ti atoms/cm3

Si Ti

Si Ti

ChannelNumber0

26

24

22

20

18

16

14

12

10

8

6

4

2

0200 400

Si Ti

400nm

density=2.83x1022Ti atoms/cm3

HydrogenForwardScatteringSpectrometry(HFS)

• AlsocalledForwardRecoilSpectrometry• HeisheavierthanH,sonoHebackscattersfromH(orD)• Hedoesforward scatterHatsignificantenergy• EnergyofrecoilingHismeasured

RBS/HFSAnalysisofSiliconNitrideFilm

RBSspectrum

HFSspectrum

Filmcomposition:Si- 38.4%N- 49.1%H- 12.5%

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Channeling

Conceptualimageofchannelingprocess

Ref.ScientificAmerican

ApplicationsofChanneling

• Quantitativecrystaldamageprofiling– IonImplants– Regrowthofdamagedcrystals– Polishingdamage– Ionetching– Epitaxiallayers– Thicknessofamorphouslayers

• Damagedetectionlimit:1x1015 to1x1017 displacedat/cm2

• Substitutionality ofdopants/impurities

Channeling:CrystalDamage

0

Measurementofdamageincrystalstructure

Disorderincrystalstructureresultsinhigherbackscatteringyield

Region without Region with disorderdisorder

MeV He ions

Energy

Yiel

d

Random

Crystal with disorder

AlignedPerfect crystal

0

Channeling:EpitaxialGrowth

• Depositedatomsareinperfectregistrywiththesubstrate(i.e.,epitaxy)à shadowconesbytheabsorbedatoms

Channeling:Substituionality ofimpurities

Yb-implantedSi

• Yb innotsubstitutionalbutislocatednearthe<110>channels

StrengthsandWeaknesses

• Strengths–Non-destructivedepthprofiling–Quantitativewithoutstandards–Analysisofwholewafers(150,200,300mm),irregularandlargesamples

–Cananalyzeconductorsandinsulators–Canmeasurehydrogen

• Weaknesses–Largeanalysisarea(1mm)–PoorsensitivityforlowZelements–Inmanycases,usefulinformationlimitedtothinfilms(<0.5µm)–Generallynotgoodforbulksamples