Processing plasmas in electronegative gas mixtures often produce clusters, ranging in size from a few nanometer to micrometers. Due to their negative charge, they are confined by the sheath electric fields until they become so large that gravity pulls them out of the discharge. Examples are discharges in CF₄ used for plasma etching, and in SiH₄, used for deposition. In these cases negative ions formed by dissociative attachment play an important role in the chemistry leading to cluster formation.
Although there is agreement on the global aspects of the chemistry, details on many processes are lacking. Important questions not aswered concern the attachment of electrons to large molecules and the interaction between large negative ions and (excited) molecules of the parent gas. Results from a one-dimensional model for a radio-frequency discharge in SiH₄/H₂ will be used to illustrate the consequences of various assumptions regarding these basic steps in the chemistry.
Discharges in mixtures containg hydrocarbons also produce clusters, especially when acetylene is present. Various mechanisms have been proposed, including the incorporation of C₂ groups in PAH’s and routes via negative ions, as in the SiH₄ case.
Also in Tokamaks the formation of carbonaceous dust is observed, induced by the erosion of carbon containing tiles in the divertor. The conditions here are quite different from those in a processing discharge, especially regarding the energy of the incoming plasma species and the heat fllux. In ITER and devices beyond ITER hydrogen ion fluxes up to 10²⁴ m^-2s^-1 and power fluxes of 10 MWm^-2 are expected.
The chemistry in these situations (processing discharges and divertors) will be discussed, indicating open questions regarding cluster formation.