Laboratory measurements of sputtering and modeling of ion-surface interaction processes

Marcelo Fama

Laboratory for Atomic and Surface PhysicsUniversity of Virginia

R.A. BaragiolaR.E. Johnson

SERENA-HEWG Conference - Santa Fe, NM - May 12-14, 2008

Outline

•Motivation

•IntroductionSputteringLinear Cascade TheorySputtering of CompoundsSurface Morphology

•Computer modelingMonte CarloMolecular Dynamics

•Laboratory simulations

•Discussion

MotivationA complex scenario

•Electron stimulated desorption•Photon stimulated desorption•Thermal desorption•Sputtering induced by charged particles bombardment•Chemical sputtering•Meteoritic impact

Exosphere

Mercury

- f (Z, m, E, Q)- Surface Compositionand Morphology

Magnetosphere

IntroductionSputtering

Ion beam (Z1, m1, E, Q, )

Target (Z2, m2, T)

Y = atoms or molecules ejected

incoming ion

Elastic SputteringLinear Cascade Theory(P. Sigmund 1969)

Electronic SputteringPrimary excitationSecondary electronsExciton/Hole Dynamics

IntroductionLinear Cascade TheoryMono-Atomic Targets

),,( xEFY D : Target Parameters

FD: Distribution of deposited-energy

0024

3

UC

SY n

Sn: Nuclear-stopping cross section (U)

C0 Differential cross section for elastic scattering (B-M)

U0: Surface binding energy

is an energy-independent function of the ratio between the mass of the target m2 and of the projectile m1

Normal IncidenceP. Sigmund, Phys. Rev. 184 (1969) 383

Differential Yield

SSS

S

S

SS

SS EUE

EUY

E

EY

cos

)(

23

0

02

3Maximum at ES = U0 / 2

ES-2 for ES >> U0

•Mono-atomic targets

•Amorphous materials

•It works satisfactorily at intermediate and high energies (> 1keV)

•It doesn’t consider local U0

IntroductionLinear Cascade TheoryLimitations

U’0 > U0

IntroductionLinear Cascade TheoryExample #1: Si

Sigmund’s C0 = 1.8 x 10-16 cm2

C0 = (x0 N)-1

Sublimation Energy ~U0 = 4.7 eV

Ycalc. Yexp.

1 keV H+ 0.11 0.008

4 keV He+ 0.28 0.09

Problem partially solved by M. Vicanek et al., NIM B36 (1989) 124 refine calculation for C0

101 102 103 10410-4

10-3

10-2

10-1

100

Y (

atom

s/io

n)

Energy (eV)

1/

1/

th

thn

EE

EESqY

W. Eckstein & R. Preuss, J. Nucl. Mater. 320 (2003) 209

Empirical Fit

4He Si

IntroductionLinear Cascade TheoryExample #2: H2O (ice)

Sigmund’s C0 = 1.8 x 10-16 cm2

Water Ice C0 = 1.3 x 10-16 cm2

Sublimation Energy ~U0 = 0.45 eV

M. Famá et al., Surf. Sci. 602 (2008) 156

fkTEa

enOH

eY

Y

SSCU

TZmEY

cos1

4

31),,,,(

/

0

1

2

02

0112

10-3 10-2 10-1 100 101 10210-1

100

101

Sp

utt

eri

ng

Yie

ld (

mo

lecu

les/

ion

)

Energy (keV)

H+

He+

N+

O+

Ar+

MD

Model

IntroductionSputtering of ice grains and icy satellites in Saturn's inner magnetosphere, Planetary and Space Science, In Press

R.E. Johnson, M. Famá, M. Liu, R.A. Baragiola, E.C. Sittler Jr, H.T. Smith

3 4 5 6 7 8 9 10 110.01

0.1

1

10

100

Figure 1a

De

nsi

ty (

1/c

m3 )

R (Rs)

Y =CASSINICASSINI

IntroductionSputtering of Compounds

i

iYY

Preferential sputtering

•Different binding energies•Recoil implantation•Radiation induced diffusion (segregation)

Surface composition bulk composition

mAB

mAB

cB

cA UUMMYY 212 )/()/(/

ii

ci cYY /

IntroductionSurface Morphology

A

P

O

Z = h(x,y)

22 )),(()),((arctancos),(, yxhyxhyxhrFrdY yxD

M.A. Makeev & A.L. Barabási, NIM B222 (2004) 316

•Maximum enhancement in the yield ~200%

T.A. Cassidy & R.E. Johnson, Icarus 176 (2005) 499

•Monte Carlo simulations of sputtering within a regolith

YR c YL(0) with 0.2 < c < 1

~15 eV Semicond.~25 eV Metals

Displacement Energy

SurfaceBinding Energy

LatticeBinding Energy

Computer ModelingMonte CarloTRIM - Binary Collision Approximation

Heat ofSublimation

~1-3 eV

pT

, T

pEV(r)

Equation of Motion

Computer ModelingMonte CarloTRIM – He+ (4 keV) Albite

Reliability of a popular simulation code for predicting sputtering yields of solids and ranges of low-energy ionsK. Wittmaack, J. Applied Phys. 96 (2004) 2632

NaAlSi3O8

DisplacementEnergy (eV)

Surface

Binding

Energy (eV)

Lattice

Binding

Energy (eV)

Na 25 1.12 3

Al 25 3.36 3

Si 15 4.7 2

O 28 2 3

10-3 10-2 10-110-3

10-2

10-1

O

Si

Al

YTR

IM (

atom

s/io

n)

YCalc.

(atoms/ion)

Na

1.

iicalc UnY

Computer ModelingMolecular Dynamics

•No assumptions or approximations other than V(r) and Se

•Complete description of the projectile-surface interaction

•Complete description of energy dissipation

•Local surface binding energy, Sn, Tm are naturally included

•Surface topography can be easily considered

Experimental MethodsTotal Sputtering Yield for Minerals

CambridgeA.J.T. Jull et al., NIM 168 (1980) 357

- Ion microprobe- Interferometry

R

National Physical LaboratoryM.P. Seah et al., SIA 39 (2006) 69- Mesh replica

VirginiaNot tested in minerals yet

- Microgravimetry f

Experimental MethodsEnergy Distributions of Sputtered Species

Post-ionization

+

-Electron beams-Low energy plasmas-Penning ionization-Post-ionizing laser

Time of flight

30 )( UE

E

E

b

UE

Eexp

)( 30

Secondary ions+

- Non-radiative deexcitation- Neutralization

Argonne National LaboratoryM. J. Pellin (1998)

Experimental MethodsComplementary Techniques @ Virginia

X-rays

Ultra High Vacuum

(~10-10 Torr)

Quartz Crystal Microbalance (~0.1 ML)

+

SIMS

XPS

NMSe-

or TOF

Nanosecond laser pulses (micrometeorite impact)

Some ResultsXPS

1100 1000 900 800 700 600 500 400 300 200 100 0

5.0x104

1.0x105

1.5x105

Ca2s

Ca2p

K2p

NaKLL

O2s

Na2s

Na1s

Al2p

Si2p

Si2s

Al2s

C1s

O1s

Surface

2.2 x 1017 He/cm2

In

tensi

ty (

cps)

Binding Energy (eV)

OKLL

Al x-rays200 eV Pass Energy

Albite (NaAlSi3O

8)

Some ResultsThermal depletion of Na

0 50 100 150 200 250 300 350 400 4500.20

0.22

0.24

0.26

0.28

0.30

0.32

Na/

Si C

once

ntr

atio

n R

atio

Albite Temperature (0C)

0 1 2 3 4 52

3

4

5

6

7

8

Dukes & BaragiolaFigure 3

Na

(Ato

mic

%)

4 keV He+ Fluence (1015 ions/cm2)

Na/Olivine

= 1 x 1015 cm2

Some ResultsDepletion of Na due to ion bombardment

0 1 2 3 4 52

3

4

5

6

Dukes & BaragiolaFigure 2

Na

(Ato

mic

%)

Na

(Ato

mic

%)

Na

(Ato

mic

%)

x 10-17 cm2Anorthoclase

4 keV He+ Fluence (1017 ions/cm2)

2

3

4

5

6

7

x 10-17 cm2

Albite

1

2

3

4

x 10-18 cm2

Labradorite

Some ResultsSecondary ions energy distribution

0.1 1 10 100102

103

104

105

b = 3

b = 1

b = 0

Na Al Si O Ca AlO

Cou

nts

(ar

b.

unit

s)

Energy (eV)

~ E-2

E

b

UE

Eexp

)( 30

Ar+ (4 kev) Albite

Modeling

Exosphere

Mercury

Magnetosphere

Sn

U0

C0

- Surface Composition- Morphology

f (Z, E)

Yi Sn / (C0 U0)Ei E / (E + U0)3

Yi+

Ei+ exp(-b/E) E / (E + U0)3

+

+

Instrument

Modeling

LaboratorySimulations

MolecularDynamics

Mercuryboundaryconditions

MagnetosphereExospheresimulators

Theory

Sputtering of Minerals

Questions & Suggestions