1. Laboratory measurements of sputtering and modeling of ion-surface interaction processes
Marcelo Fama
Laboratory for Atomic and Surface Physics
University of Virginia
R.A. Baragiola
R.E. Johnson
SERENA-HEWG Conference - Santa Fe, NM - May 12-14, 2008

2. Outline Motivation Introduction Sputtering Linear Cascade Theory
Sputtering of Compounds
Surface Morphology
Computer modeling
Monte Carlo
Molecular Dynamics
Laboratory simulations
Discussion

3. Motivation A complex scenario Magnetosphere Exosphere
Electron stimulated desorption
Photon stimulated desorption
Thermal desorption
Sputtering induced by charged particles bombardment
Chemical sputtering
Meteoritic impact
- f (Z, m, E, Q)
- Surface Composition
and Morphology
Mercury

4. Introduction Sputtering q Target (Z2, m2, T)
Y =
atoms or molecules ejected
incoming ion
Elastic Sputtering
Electronic Sputtering q
Linear Cascade Theory
(P. Sigmund 1969)
Primary excitation
Secondary electrons
Exciton/Hole Dynamics
Ion beam (Z1, m1, E, Q, q)

5. Introduction Linear Cascade Theory Mono-Atomic Targets
FD: Distribution of deposited-energy
L: Target Parameters
P. Sigmund, Phys. Rev. 184 (1969) 383
Normal Incidence
Sn: Nuclear-stopping cross section (U)
C0  Differential cross section for elastic scattering (B-M)
U0: Surface binding energy
a is an energy-independent function of the ratio between the mass of the target m2 and of the projectile m1
Differential Yield
Maximum at ES = U0 / 2
 ES-2 for ES >> U0

6. Introduction Linear Cascade Theory Limitations Mono-atomic targets
Amorphous materials
It works satisfactorily at intermediate and high energies (> 1keV)
It doesn’t consider local U0
U’ > U0

7. Introduction Linear Cascade Theory Example #1: Si
Sigmund’s C0 = 1.8 x 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
Empirical Fit
4He Si
W. Eckstein & R. Preuss,
J. Nucl. Mater. 320 (2003) 209

8. Introduction Linear Cascade Theory Example #2: H2O (ice)
M. Famá et al., Surf. Sci. 602 (2008) 156
Sigmund’s C0 = 1.8 x cm2
Water Ice C0 = 1.3 x cm2
Sublimation Energy ~U0 = 0.45 eV

9.  Y = Introduction CASSINI
Sputtering 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
 Y =
CASSINI

10. Sputtering of Compounds
Introduction
Sputtering of Compounds
Preferential sputtering
Different binding energies
Recoil implantation
Radiation induced diffusion (segregation)
Surface composition  bulk composition

11. Introduction Surface Morphology Z = h(x,y) P A O YR  c YL(0)
M.A. Makeev & A.L. Barabási, NIM B222 (2004) 316 O
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

12. TRIM - Binary Collision Approximation
Computer Modeling
Monte Carlo
TRIM - Binary Collision Approximation
Equation of Motion q p E
V(r)
q, T p T
Displacement
Energy
Surface
Binding Energy
Lattice
Binding Energy
Heat of
Sublimation
~1-3 eV
~15 eV Semicond.
~25 eV Metals

13. DisplacementEnergy (eV)
Computer Modeling
Monte Carlo
TRIM – He+ (4 keV)  Albite
NaAlSi3O8
DisplacementEnergy (eV)
Surface
Binding
Energy (eV)
Lattice Na 25
1.12 3 Al
3.36 Si 15
4.7 2 O 28
Reliability of a popular simulation code for predicting sputtering yields of solids and ranges of low-energy ions
K. Wittmaack, J. Applied Phys. 96 (2004) 2632

14. Computer Modeling Molecular 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

15. Total Sputtering Yield for Minerals
Experimental Methods
Total Sputtering Yield for Minerals
Cambridge
A.J.T. Jull et al., NIM 168 (1980) 357
- Ion microprobe
- Interferometry R
National Physical Laboratory
M.P. Seah et al., SIA 39 (2006) 69
- Mesh replica
Virginia
Not tested in minerals yet Df
- Microgravimetry

16. Energy Distributions of Sputtered Species
Experimental Methods
Energy Distributions of Sputtered Species +
Time of flight
Electron beams
Low energy plasmas
Penning ionization
Post-ionizing laser
Post-ionization
Argonne National Laboratory
M. J. Pellin (1998)
- Non-radiative deexcitation
- Neutralization
Secondary ions +

17. Complementary Techniques @ Virginia
Experimental Methods
Complementary Virginia
SIMS
X-rays
XPS +
or TOF
Nanosecond laser pulses
(micrometeorite impact) e-
NMS
Quartz Crystal Microbalance (~0.1 ML)
Ultra High Vacuum
(~10-10 Torr)

18. Some Results
XPS

19. Some Results
Thermal depletion of Na

20. Some Results
Depletion of Na due to ion bombardment

21. Secondary ions energy distribution
Some Results
Secondary ions energy distribution
Ar+ (4 kev)  Albite

22. Modeling + + Yi  Sn / (C0 U0) Ei  E / (E + U0)3 Yi+
Instrument
Magnetosphere
Exosphere + +
Yi  Sn / (C0 U0)
Ei  E / (E + U0)3
Yi+
Ei+  exp(-b/E) E / (E + U0)3
f (Z, E) Sn U0 C0
- Surface Composition
- Morphology
Mercury

23. Modeling Mercury boundary conditions Laboratory Simulations Molecular
Dynamics
Sputtering of Minerals
Magnetosphere
Exosphere
simulators
Theory

24. Questions & Suggestions