Axially-resolved study of highly ionized physical vapor deposition

The evolution of the ionization of sputtered aluminum is examined in an inductively coupled plasma. The experimental configuration consists of a conventional magnetron with an immersed, rf induction coil positioned between the target and the downstream substrate. Measurement of the number of aluminum atoms and ions as a function of distance from a magnetron target reveals a decreasing neutral density and a nearly constant ion density. The neutral metal density is well described by a simple diffusion model with a source term near the target while the metal ion density is approximately that of a diffusion-dominated, volume-generated plasma. These two spatial distributions result in a monotonically increasing ionization fraction versus distance from the target. Evidence suggests that the ionized physical vapor deposition method requires that sputtered species be nearly thermalized before ionization is probable. (C) 1997 American Vacuum Society.