It is well known that preferential sputtering with binary alloys correlates significantly with chemical binding, but only the sense and not the magnitude of the effect can be understood in these terms. It is argued here that the correlation is, in fact, an indirect one due to bombardment-induced Gibbsian (or similar) segregation. The preferentially removed component is characterized by a composition profile consisting of a one atom-layer-thick spike at the surface; this is the depth from which most sputtered atoms originated so the spike must have near-bulk composition. There is then a severely depleted subsurface region in accordance with Gibbsian segregation equilibrium (or an equivalent effect) and a final return to bulk composition. The reason for the marked correlation with chemical binding is that segregation is governed significantly more often by binding than by the alternatives of size, surface chemistry, an interstitial flux, or long-range ordering.

On the role of Gibbsian segregation in causing preferential sputtering

Recent experiments suggest that weak chemical driving forces play a role in bombarded targets even when, for reasons of extreme energy disparity, it might be difficult to understand why this should be so. With bombardment-induced segregation the driving force, assuming the process to resemble equilibrium segregation, is 0.06–0.52 eV atom −1 . We suggest that the segregation is the result of a certain fraction of the ballistic trajectories terminating near the target surface being followed by one or more low energy, chemically guided steps. With bombardment-induced mixing the driving force, according to Cheng et al., is typically 1.3 eV atom −1 or less. We propose a very similar model to that of segregation, namely ballistic trajectories with terminal steps which are chemically guided. Finally, with bombardment-induced decomposition, as when oxides lose oxygen or sulfates lose sulfur and oxyten, there appears to be a role for what are effectively phase changes in which the system appears to tolerate an energy increase of up to 0.7–0.8 eV atom −1 but not a larger amount. The changes, which resemble those analyzed by Fecht and Johnson, are aided by a number of factors, including amorphization, point-defect accumulation, volatility, diffusional transport and segregation. We tend to avoid in all cases a thermal spike type of argument, if only because the inequality (driving force) ⪆ kT must be obeyed.

Bombardment-induced compositional change with alloys, oxides, oxysalts and halides III. The role of chemical driving forces☆

A summary of the theory of the preferential sputtering of alloys

The effects of segregation on the preferred sputtering of alloy surfaces have been analyzed. The top layer of the surface is regarded as behaving differently from the layers beneath the second layer: segregation and dissolution rate constants between the top and second layers are included in the kinetic equations explicitly. It is shown that the fundamental kinetic equations are of a form similar to that developed by Ho, Webb, Carter and Collins, even though the coefficients of the equations are different. The time-dependent solution of the kinetic equations shows that the segregation rate constant plays an important role in the build-up of the steady state. Similarly to the previous result, the altered layer is shown to have a thickness of the diffusion constant divided by the velocity of the recession of the surface. The steady-state compositions at the top and second layers relative to the bulk composition were found to be a function of three parameters : the surface enrichment factor, the diffusion constant divided by the velocity, and the ratio of the yields of atomic removal from the top layer, a correction factor being needed in a particular boundary condition. The existing experimental data on Cu-Ni, Cu-Au and some other alloys are surveyed critically in terms of the results of the theoretical calculation.

T. Koshikawa

Papers

Ion radiation-enhanced diffusion and segregation in an Au0.56Cu0.44 alloy between − 120°C and room temperature

Changes in gold concentration at the surface of a AuCu alloy sputtered at low temperature