Sheath structure in electronegative plasmas

Abstract

Some general aspects of the sheath structure in electronegative plasmas are analysed, through an extensive review of the authors' contributions to this subject. An analysis of the positive ion flux as a function of the electric potential, in the region in which neutrality still holds, is shown to be useful to establish the structure of both the sheath and the presheath. Depending on the plasma parameters, the sheath can show the structure of a triple layer in which a region with a net positive charge is followed by a region with net negative charge and finally by the positive ion sheath. However, the electric potential distribution is always a monotonic function in the sheath. In the parameter space region in which the presheath becomes stratified, a separate analysis of the sheath fails to give the correct positive ion flux in the sheath. In any case, the analysis of the sheath gives either the exact value for the positive ion flux or a good approximation and, therefore, a separate analysis of the sheath is also useful in the case of electronegative plasmas. Finally, a criterion to establish when the presheath–sheath structure is governed by the negative ions instead of by the electrons is introduced. This criterion determines when the main properties of the sheath, such as the sheath thickness or the floating potential, begin to be governed by the negative ions.