Observations of particle layers levitated in a radio-frequency sputtering plasma

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Abstract
Submicron carbon particles are grown in an argon radio-frequency (rf) sputtering plasma with graphite electrodes. In situ laser light scattering revealed particles levitated in the plasma in stratified layers, which were often thinner than 0.5 mm. Under some conditions, the number of layers was very large. These layers are grouped into clouds. In a pure argon plasma, there is one cloud near the sheath edge above the lower electrode, while adding SF6 results in additional particle clouds near the upper electrode and in the glow region. The different layers appear to separate particles according to size, with the heaviest in the lowest layer. During several hours of plasma operation, the layers collapsed and grew again into more stratified layers. The influence of parameters, such as gas pressure, rf power, electrode separation, and gas mixture was investigated. Scanning electron microscopy revealed that the particles were multisized spheres with a cauliflowerlike surface. Some of the particles were found to be coagulated, due to collisions within the plasma.

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