Thin-film deposition on suspended particles in dusty plasma

The ability of low-pressure plasmas to trap micron and sub-micron particles for indefinite periods of time, coupled with the variety of plasma chemistries than can be run in the plasma, create a unique environment for the synthesis and processing particulate materials and nanocomposites. We have developed a radio-frequency plasma deposition process that allows us to deposit thin films, ranging from ultra-low thicknesses of few nm to upwards of 100 nm. We seed a low-pressure radio-frequency plasma with micron and submicron silica particles and induce surface deposition of plasma-generated polymers produced by the decomposition of various hydrocarbon molecules. The thickness of the films is controlled by the depositionn time, which ranges from few minutes to two hours. We study the kinetics of this process by monitoring the size of the particles (core plus coating) as a function of time. The mean film growth rate is of the order of nm/min and depends on the size of the seed particles. A kinetic-rate argument allows us to attribute this apparent size dependence of the growth rate on the effect of particles in depleting electrons from the plasma. We further observe a wide distribution of deposition rates, which results in some particles being coated much less than others. A population balance model is used to describe this process: assuming particles to be stationary - and thus subject to a local deposition environment - we are able to make predictions for the mean particle size (core + coating) and its variance after a given time spent in the plasma. The experimental results show linear increase of the size and quadratic increase of the variance, in agreement with the proposed model.