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.
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Dickson et al. (1997) studied this question.