## 2 Plasma Physics
### Bounded Plasmas Which Are Not Thermally Driven  
Plasmas can either be driven by thermal energy (as is the case in stars) or by discharge processes. As an example in the case of many plasma cleaning systems, the plasma is created through energy received via electromagnetic fields. These fields accelerate charge carriers and cause collisions between the particles which collide constantly transferring energy to each other. This energy transfer occurring through the collisions that occur within  
Figure 1: Flow chart showing section flow adopted for this paper.  
the plasma can ionize particles as well as break down molecular bonds if the colliding particles have sufficient energy [6]. This will be discussed further in this paper.  
It should be explicitly noted that the plasmas employed in cleaning applications are typically confined systems, where the ionized gas is spatially constrained using either physical boundaries (such as vacuum chamber walls) or electromagnetic fields. This confinement serves multiple critical functions, such as maintaining plasma density at optimal levels for surface interactions, preventing unwanted discharge spreading, and enhancing control over the reactive species distribution. The confinement strategy directly impacts key process parameters, including plasma uniformity, radical generation efficiency, and, ultimately, the cleaning effectiveness across treated surfaces. Different confinement approaches ranging from simple geometric configurations to advanced magnetic field arrangements are selected based on specific application requirements, whether for precision semiconductor cleaning or large-scale industrial surface treatment [7]. Confined plasmas present a stratified discharge structure with dark regions on the border of the plasma envelope. These areas are known as the plasma sheath, while the central region, which is the bright area, radiates visible radiation due to the interaction of charged (electrons and ions) and neutral particles. The light emission is due to the collisions that occur between the electrons and the neutral particles. Such collisions cause the particles to reach an excited state. Relaxation from the excited state to the ground state results in light emission.  
The central region of the plasma is separated from the walls of the reactor by the sheath regions which are the dark regions at the border zone of the plasma. The sheath regions are dark as the emission of light in these regions is lower than that in the bulk plasma area. The reason for this is that the electron density in the sheath areas is lower than that in the bulk quasi-neutral zone. The quasi-neutral zone, at the core of the plasma, consists of an ensemble of free electrons and positive ions. The charges mathematically cancel each other out and the overall charge of the zone is in equilibrium [5]. The quasi-neutral zone is therefore the region where the electron density (n\({}_{\text{e}}\)) is equal to the ion density (n\({}_{\text{i}}\)), while in the sheath ne is less than n\({}_{\text{i}}\)[7]. The reason for having a lower electron density in the sheath is mostly due to the fact that electrons are 1000 to 10,000 times lighter than ions. This results in the electrons' ability to escape from the plasma at a much faster speed than the ions into the wall [8; 9].  
Effectively, the volume bounding the wall surface (inner pre sheath layer) has a very low charge while the outer sheath region has an excessive ionic population. When using DC sources, the sheath region dimensions do not change with time; however, in the case of other sources such as RF systems, the sheath expands and contracts with the radio frequency cycle [7]. The ionization rate in industrial process chambers is normally less than 0.001%, while in the solar core, due to the enhanced temperatures and pressures, it would be close to 100% [10].