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

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. atoms or molecules ejected Y = incoming ion Introduction Sputtering Target (Z2, m2, T) Elastic Sputtering Electronic Sputtering q Primary excitation Secondary electrons Exciton/Hole Dynamics Linear Cascade Theory (P. Sigmund 1969) 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-2for 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’0 > U0

7. Introduction Linear Cascade Theory Example #1: Si Sigmund’s C0 = 1.8 x 10-16 cm2 C0 = (x0N)-1 Sublimation Energy ~U0 = 4.7 eV 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 10-16 cm2 Water Ice C0 = 1.3 x 10-16 cm2 Sublimation Energy ~U0 = 0.45 eV

9. Introduction 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. 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 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. 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. Computer Modeling Monte Carlo TRIM – He+ (4 keV)  Albite NaAlSi3O8 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. Experimental Methods Total Sputtering Yield for Minerals - Ion microprobe - Interferometry Cambridge A.J.T. Jull et al., NIM 168 (1980) 357 R National Physical Laboratory M.P. Seah et al., SIA 39 (2006) 69 - Mesh replica Virginia Not tested in minerals yet Df - Microgravimetry

16. 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. Experimental Methods Complementary Techniques @ 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. Some Results Secondary ions energy distribution Ar+ (4 kev)  Albite

22. Modeling Instrument Magnetosphere Exosphere + + Yi  Sn/(C0U0) 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