Relationships between crater and sputtered material characteristics in large gas cluster sputtering of polymers: Results from molecular dynamics simulations

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Abstract
This molecular dynamics study focuses on the relationships between the sputtered volume and the crater size and shape as a function of scaled energy, upon a 45° incidence of (Ar)n and (CH4)n clusters on an amorphous solid made of 1.4 kDa polymers [CH3-(CH2)97-CH3]. The cluster sizes were in the range of 10–104 and their kinetic energies, between 2.5 and 15 keV. The craters were satisfactorily approximated by semiellipsoids. First, our results show that the crater shape is a complex function of the projectile composition, number of constituents (nuclearity), and energy. This dependence can be presented as a single “universal” curve by plotting the crater volume, scaled by the projectile nuclearity or mass, versus the projectile energy scaled in the same way. Second, the ratio of the sputter yield volume Yv over the crater volume V varies monotonically with the scaled energy, so that large impact craters are still formed under 0.025 eV/amu bombardment with almost no ejection, but only material displacement on the surface. While the sputtered material originates mostly from the top third of the crater at high scaled energy, the ejection is limited to surface molecules at low energy. This implies that large, slow clusters in addition to softer emission should provide more surface sensitivity for cluster-based molecular analysis. Finally, the relation between the craters and sputtering for ultrathin layers (2–15 nm) on a rigid substrate indicates that a maximum of sputtering efficiency is reached for 4 nm films in the case of 10 keV Ar3000 projectiles at 45° incidence.

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