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

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

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

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)

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

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

Introduction Linear Cascade Theory Example #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 Empirical Fit 4He Si W. Eckstein & R. Preuss, J. Nucl. Mater. 320 (2003) 209

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

 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

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

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

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

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

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

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

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 +

Complementary Techniques @ Virginia 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)

Some Results XPS

Some Results Thermal depletion of Na

Some Results Depletion of Na due to ion bombardment

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

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

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

Questions & Suggestions