A new approach to thermal-spike sputtering with ions and laser pulses

The interactions of ion beams and laser pulses with solid or liquid surfaces have much in common, though different levels of importance. One recognizes processes which could be termed ballistic, thermal, electronic, and defect induced. These similarities have already been reviewed adequately so we propose here to concentrate only on thermal processes, in particular thermal sputtering due to normal vaporization, which are currently the most controversial and the least understood processes. Thermal sputtering, perhaps better termed thermal-spike sputtering, involves the target response to a transient high temperature. We will argue that the basic response of essentially all systems can be quantified by using modern values of the free energy of formation, ΔfG0, to deduce vapor pressures for all possible processes for a given substance (e.g. we consider 14 processes with Al2O3 and 21 with WO3), and then introducing these vapor pressures into the Hertz–Knudsen equation. This equation has the approximate form: flux≈psv(2πmkBT)−1/2 particles per m2s, where psv is the equilibrium (or saturated) vapor pressure and m is the particle mass. The equation can then be evaluated for suitable temperatures (here we choose 2600 and 4000 K) and it can be shown whether the number of atom layers leaving the surface in a given time (here we choose 10 ps to 10 ns) is atomically small or large. The relevant target area for a laser pulse is well defined (namely the pulse area) and therefore need not be specified. But for an incident ion the target area is similar to ∼π(Δy)2, i.e. the area defined by the lateral standard deviation of the incident ion, and we therefore (somewhat arbitrarily) take the ion area as λ2, literally the area of one ion, and compensate by identifying the ion time scale as 10 ps to 1 ns instead of the more realistic 1–100 ps. This is permitted because the Hertz–Knudsen equation is linear in both area and time.