Formation of dense submicronic clouds in low-pressure argon-SiH4 rf reactor : diagnostics and growth processes from monomers to large size particulates

The formation of solid particles in the gas phase is a general problem for PECVD technology when looking at high deposition rates of good quality layers. The present paper report an overview on particle growth and associated plasma phenomena in a specific situation: Argon-Silane RF low pressure discharge.The diagnostics developped for this study include laser multi-angle Mie scattering for particle diameter above 35 nm and excimer induced light emission for nm scale particles. The evolution of the free electron concentration is obtained by microwave diagnostics and electron energy is deduced from OES data. In-situ FTTR absorption spectroscopy add insigths on the gas phase chemistry. Four successive steps are evidenced in the story of particle formation: clusterring of negative ions, formation of dense clouds of nm size crystallites, coalescence to few tens nm diameter particulates and a final growth by deposition of a-Si:H on the particles negatively charged and trapped in the discharge. The gas temperature appears as a sensitive parameter for the initial step of crystallites formation and a drastic transition of plasma properties occurs during the coalescence step of dust formation