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

Abstract

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 (gamma(d)) and diffusion time (tau(d)). If gamma(d)tau(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.

Properties of highly electronegative plasmas produced in a multipolar magnetic-confined device with a transversal magnetic filter

“…Negative ion etching with low energies and low charging potentials is also attractive for low damage processing required in node technologies below 50 nm [30]. So far, despite of good theoretical coverage [31][32][33][34][35] and a large variety of methods for negative ion diagnostic including optical emission [36], photodetachment [37], Langmuir probe [38][39][40][41][42][43], thermal probe [44], propagation of the ion acoustic waves [45][46][47] and Thomson scattering [48] there are still important basic questions related to highly electronegative discharges that need to be answered.…”

GEOMETRICAL ORIGIN OF FERMION FAMILIES IN SU(2) × U(1) GAUGE THEORY

“…In this paper we will follow a different geometric approach to the family problem of particle physics. Basic ideas and methods of this approach have been developed in previous papers [2][3][4][5][6][7] where the emphasis was on cosmological questions, in special the question of the cosmological constant problem. It was noticed however [7] that a natural solution to the family problem could be given along these lines as well.…”

“…[2][3][4][5][6][7] consists of using an alternative volume element Φd 4 x, in addition to the standard one √ −gd 4 x. So a general action of the form…”

Fundamentals and applications of ion–ion plasmas

“…The decay of electronegative plasmas in the afterglow of pulse discharges under a variety of situations has been studied by Kaganovich et al [32]. The time needed for negative ions to reach the wall is of importance for applications based on negative ion extraction.…”