Sputtered atom transport processes

It is noted that the transport of sputtered atoms can be described in terms of three pressure regimes: low pressure, where no collisions occur during the trajectory of the atom; intermediate pressure, where the atom undergoes perhaps several collisions but does not completely thermalize; and high pressure, where the sputtered atom effectively stops and begins a density-gradient-driven conventional gas-phase diffusion process. The intermediate region is the most complicated to model, given the dependence of the energy on the collision cross-section, the various distributions in energy and angle of the sputtered atoms, and the extended nature of most sputtering sources. Experimental studies reported here have measured the transport probability by observing the distribution of atoms around a chamber following sputtering. The transport is found to be quite dependent on the mass of both the sputtered atom and the background gas, as well as the particle density and geometry of the vacuum system. A strong effect of sputtered-atom-induced gas rarefaction has also been observed. This results in power-dependent transport of sputtered atoms, and as a result may also lead to power-dependent compositional variation in alloy depositions. The general result is that high discharge powers tend to correlate with lower power operation at a significantly lower operating pressure than had been assumed