This is a re-post of an article by Dave Christie of Advanced Energy, posted Sep. 2, 2014:
In the magnetron sputtering process, the desired glow discharge mode is sustained by secondary emission of electrons induced by ion impact at the target surface—as an individual process. “An individual process” means that one ion incident on the surface results in emission of some number of secondary electrons (the ion impact being primary) with some probability. These secondary electrons perform bulk ionization of process gas neutrals by electron impact  and possibly sequential secondary processes such as Penning ionization and multiple body collisions. The undesired cathodic arc discharge mode is sustained by explosive emission of ions and electrons from small craters on the target surface in what could be considered a collective process . It is a collective process because the current flowing in the arc provides heat to the arc spot, which, in turn, causes melting and explosive emission of a “collection” of target material atoms. In the cathodic arc mode, target material macro-particles are also explosively emitted from arc craters, often landing on the substrate, resulting in product yield issues.
Electrical discharge devices with the ability to operate in arc and glow regimes have been studied for some time now. Early work on the transition from the glow to the cathodic arc mode showed the importance of oxide on the target surface for sustaining an arc [3-7]. When ultra-pure noble gases were used, it was essentially impossible to sustain an arc. In some experiments, the argon gas was purified in situ with the arc operating. When a high level of purity was attained, the arc mode discharge ceased and only a glow discharge was possible. This result was attributed to formation of oxides on the surface. When the gas was purified, the oxides were eventually removed by the arc. The motivation for this work was likely to understand how to make arc sources work better. Now we are interested in keeping sputtering processes out of the arc regime. This early work at least suggests the importance of process gas and target material purity in metal sputtering processes, and perhaps sets the expectation that target arcing could develop when reactively sputtering oxides. The challenge for power supply developers continues to be innovation in arc detection and arc response to minimize arc energy and, ideally, arc rate.
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