Surface tension enhancement of TRIM sputtering yields for liquid metal targets

A critical part of all models of physical sputtering is the magnitude of the attractive interaction of a surface atom with the bulk, the surface binding energy, which is often equated to the heat of sublimation, independent of temperature and regardless of the state of aggregation of the target. Sputtering occurs when target particles which are accelerated by atomic collisions caused by an incident energetic projectile, reach the surface with enough energy to overcome the surface binding energy. This surface binding energy influences sputtering in two ways: (1) it acts as a barrier which must be overcome by the escaping atoms and so affects the sputtering yield, and (2) its directional properties influence the directions taken by the ejected target atoms. Lower surface binding energies increase the sputtering yield, while higher surface binding energies reduce the sputtering yield. The thermodynamics of liquid surfaces is used to determine the temperature dependence of surface tension and thus the binding energy of atoms in the liquid surface. Liquid lithium and gallium have surface tensions that follow the same universal function of T/Tc observed for most simple atomic liquids. The decrease of surface binding energy with increasing target temperature is calculated, and the TRIM code is then used to determine the resulting increase in sputtering yield with temperature as functions of the incident angle for these targets under bombardment by low energy deuteron projectiles.