Spatiotemporal dynamics of charged species in the afterglow of plasmas containing negative ions

The spatiotemporal evolution of charged species densities and wall fluxes during the afterglow of an electronegative discharge has been investigated. The decay of a plasma with negative ions consists of two stages. During the first stage of the afterglow, electrons dominate plasma diffusion and negative 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 electron escape frequency increases considerably in the presence of negative ions, and can eventually approach free electron diffusion. During the second stage of the afterglow, electrons have disappeared, and positive and negative ions diffuse to the walls with the ion-ion ambipolar diffusion coefficient. Theories for plasma decay have been developed for equal and strongly different ion (T_i) 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 gases, e.g., oxygen) the plasma decay crucially depends on the product of negative ion detachment frequency (γ_d) and diffusion time (τ_d). If γ_dτ_d>2, negative ions convert to electrons during their diffusion towards the walls. The presence of detached electrons results in “self-trapping” of the negative ions, due to emerging electric fields, and the negative ion flux to the walls is extremely small. In the case T_i≪T_e, the spatiotemporal dynamics is more complicated due to the presence of negative ion density fronts. During the afterglow, although negative ions diffuse freely in the plasma core, the negative ion fronts propagate towards the chamber walls with a nearly constant velocity. The evolution of ion fronts in the afterglow of electronegative plasmas is important, since it determines the time needed for negative ions to reach the wall, and thus influence surface reactions in plasma processing.