Atomistic modeling of the sputtering of silicon by electrosprayed nanodroplets

Abstract

The hypervelocity impact of electrosprayed nanodroplets on single-crystal silicon ejects a large number of atoms. Although sputtering by atomic, molecular, and gas cluster ions has been thoroughly studied, the significantly larger size of nanodroplets prevents a straightforward extrapolation of the physics governing the impact of these smaller projectiles. This motivates the present molecular dynamics simulations of nanodroplet impact on silicon, aimed at resolving the mechanisms and the effect of the projectile's size and velocity on sputtering. We find that both collision cascades and thermal sputtering contribute to the overall atom ejection, the former being active during the initial stages of the impact characterized by strong interactions between the molecules of the projectile and the atoms of the target, and the absence of partial thermodynamic equilibrium. In addition, for sufficiently large projectile diameters and impact velocities, conglomerates of atoms are ejected by hydrodynamic forces. The sputtering yield, defined as the average number of target atoms ejected per projectile's molecule, increases monotonically with the kinetic energy of the molecules and, at constant molecular kinetic energy, slightly decreases with projectile diameter as a result of enhanced backscattering of the ejected atoms by the projectile's molecules. For the ionic liquid considered in this study, sputtering is first observed at molecular energies near 12.7 eV and, at the highest energy simulated of 73 eV, the sputtering yield averages to 0.37. V C 2014 AIP Publishing LLC. [http://dx.

The Effect of the Molecular Mass on the Sputtering of Si, SiC, Ge, and GaAs by Electrosprayed Nanodroplets at Impact Velocities up to 17 km/s

“…The forces between the pseudo atoms of the projectiles, and between them and the silicon atoms are calculated with the Ziegler-Biersack-Littmark (1985) potential; in the parameterization of this potential we use the aggregate atomic number of the molecule as the atomic number of the pseudo atom. A more detailed description of the potentials, the thermostat, and other features of the simulations can be found in Saiz and Gamero-Castaño (2014). Figure 3 shows pictures and profilometer measurements of the Ge target, bombarded by both EMI-Im and EAN nanodroplets.…”

“…The maximum sputtering rates for the hard and inert SiC and GaN are 410 and 630 nm/min, much higher than the sputtering rates of ion beams and comparable to typical reacting ion etching rates for Si (Sugiura et al 1986;Cho et al 2000). Molecular dynamics simulations of the impact of EMIIm nanodroplets on single-crystal silicon have shown that the ejection is a combination of knock-on and thermal sputtering, both driven by the initial phase of the impact characterized by intense collisionality (Saiz and Gamero-Castaño 2014). Finally, a study of the effect of the projectile's molecular mass on the sputtering of silicon using six different liquids covering the mass range between 45 and 773 amu has revealed an important dependence on this parameter, the larger the mass the higher the sputtering yield and the roughness of the bombarded surface at fixed impact velocity (Borrajo-Pelaez and Gamero-Castaño 2015).…”

Molecular dynamics of nanodroplet impact: The effect of the projectile’s molecular mass on sputtering

“…The thermostat is maintained on the lateral and bottom faces throughout the impact, to prevent the unphysical reflection of shock waves at the boundaries. 3,9 The projectile is directed towards the target at velocity v P , and the equations of motion are integrated during 70 ps with a timestep of 1 fs. The impact velocities are adjusted so that all projectiles have an identical kinetic energy of 63 keV, and a stagnation pressure of 19 GPa, P 0 = ρv 2 P /2.…”

“…14 The following two arguments justify this simplified model: the projectile molecules are expected to remain largely intact due to the relatively low impact energy per atom (impact simulations of fullerene projectiles have shown that as much as 8.3 eV per C atom is required to promote significant atom dissociation); 15 and previous simulations of nanodroplet impact using the ZBL potential reproduce experimental sputtering yields, as well as the onset and extent of amorphization. 3,16 The impact of each projectile is simulated five times to average results whenever appropriate. We apply the methodology proposed by Samela and Nordlund to identify knock-on atoms produced by direct collisions with projectile molecules, and by collision cascades of Si atoms.…”

“…A significant fraction of this energy is dissipated. 3 In this respect nanodroplets are similar to cluster ions, 4 and differ from smaller atomic projectiles characterized by deep penetration. Electrosprayed nanodroplets are generated as relatively monodisperse beams, their average diameters are controllable from a few to hundreds of nanometers, and are charged near the maximum level set by the Rayleigh limit for liquid droplets.…”