Spatiotemporal dynamics of charged species in the afterglow of plasmas containing negative ions - art. no. 036402

The spatiotemporal evolution of charged species densities and wall fluxes d uring the after.-low of an electronegative discharge has been investigated. The decay of a plasma with negative ions consists of two stages. During th e first stage of the afterglow, electrons dominate plasma diffusion and neg ative ions are trapped inside the vessel by the static electric field; the flux of negative ions to the walls is nearly zero. During this stage, the e lectron escape frequency increases considerably in the presence of negative ions, and can eventually approach free electron diffusion. During the seco nd stage of the afterglow, electrons have disappeared, and positive and neg ative ions diffuse to the walls with the ion-ion ambipolar diffusion coeffi cient. Theories for plasma decay have been developed for equal and strongly different ion (T-t) and electron (T-e) temperatures. In the case T-i = T-e , the species spatial profiles are similar and an analytic solution exists. When detachment is important in the afterglow (weakly electronegative gase s, e.g., oxygen) the plasma decay crucially depends on the product of negat ive ion detachment frequency (gamma (d)) and diffusion time (tau (d)). If g amma (d)tau (d)>2, negative ions convert to electrons during their diffusio n towards the walls. The presence of detached electrons results in "self-tr apping" of the negative ions, due to emerging electric fields, and the nega tive ion flux to the walls is extremely small. In the case T-i<<T-e, the sp atiotemporal dynamics is more complicated due to the presence of negative i on density fronts. During the afterglow, although negative ions diffuse fre ely in the plasma core, the negative ion fronts propagate towards the chamb er walls with a nearly constant velocity. The evolution of ion fronts in th e afterglow of electronegative plasmas is important, since it determines th e time needed for negative ions to reach the wall, and thus influence surfa ce reactions in plasma processing.