Plasma Models

In this chapter we begin to develop more accurate descriptions of the same plasma reactors we considered before, which go beyond the circuit models used so far. The information we can extract from circuit models is useful in some contexts but is very limited. We begin with a discussion of a parallel-plate rf discharge, which is normally operated at higher neutral pressures than the other reactors we consider. The behavior of the plasma in inductively coupled plasmas will be outlined next. Ambipolar diffusion is introduced, to help in the description of the ICP. The ECR discharge is made more difficult to describe by the presence of the magnetic field, which steers electrons almost exclusively along the magnetic field lines but which impedes ions only moderately on their way to the wall. ECR plasmas are considered next. Calculations of the electron and ion distribution functions are the last major topic in this chapter. Before discussing the rf discharge we will describe a type of experiment that can provide a very useful basis for comparison with more complicated plasmas. These experiments involve “swarms” of electrons subjected to an electric field that is constant in space and time.

Swarm Measurements

Swarm experiments [66] involve a so-called swarm of electrons with a low enough density so that their charge does not (significantly) alter the electric field. The conditions for this to occur were discussed in the section on plasma electrostatics.