In order to start a discharge, some charged particles are needed to start the
process of ionization and excitation of gas particles. This is provided to a large extent
by weak background radiation from radioactive minerals where some ionized
molecules and free electrons exist, and to a less extent from cosmic rays. By applying
an electric field these electrons gain enough energy for further ionization and
generation of secondary electrons, which in turn contributes to creating a so-called
Townsend discharge with a relatively low, but avalanching, current (Fig. 2).
If the current increases enough, exceeding the breakdown field for the gas, a glow
discharge is obtained. Figure 3 shows a photo of an Ar discharge for a sputtering
system with a Ta sputter source (target). In order to sustain the plasma, regeneration
of electrons is necessary [42]. This is often the most important means of maintaining
the plasma as electrons lose their energy to the gas in ionizing processes, and are
eventually lost to the surrounding chamber walls. Electrons also lose energy by elastic
collisions, transforming energy to neutral species thereby putting them in an excited
state, i.e., not all collisions result in ionization and secondary electrons generation
[41]. As a result, a range of kinetic energies is obtained for the electron population,
often described mathematically by a Maxwell-Boltzmann, a Bi-Maxwellian, or a
Druyvesteyn distribution. More on distributions is presented in section 2.3.