## 3 Plasma Sources
### Microwave (Non-ECR) Plasma Source  
Microwaves are one of the common sources used to excite gas molecules. This type of electromagnetic plasma source is more dominantly used for chemical reaction processes, as this plasma type creates more electrons that the radio frequency type and attains a higher cleaning rate predominantly due to the fact that this plasma type operates at \begin{table}
\begin{tabular}{c c c} \hline \hline
**Parameter** & **References** & **Parametric Score** \\ \hline Energy Distribution and & Sheath dynamics are particularly critical in RF plasma systems as most of the energy is concentrated within this narrow boundary region [33]. The strong electric fields in the sheath accelerate ions toward surfaces, governing essential processes like ion bombardment energy and radical generation. In contrast, the bulk plasma region exhibits relatively uniform energy distribution, with electron temperatures (T\({}_{\text{e}}\)) of 1–5 eV and densities (n\({}_{\text{e}}\)) of 1015–1017 m\({}^{-3}\) under typical processing conditions [34]. & 7/10 \\ \hline Plasma Stability & RF plasmas can experience stability issues due to variations in plasma impedance, especially at low pressures and powers. The interaction between the plasma and the RF power delivery system can lead to instabilities if not properly managed; however, the literature has indicated that advanced power delivery systems having features like variable frequency and real-time impedance measurement can help in detecting and mitigating instabilities to provide stable plasma generation [35]. & 9/10 \\ \hline Plasma Energy Density & RF plasmas have significant sheath regions where most of the energy is concentrated. Therefore, although energy density of RF plasmas is similar to that of MW plasmas, the distribution is not as uniform. Inductively coupled plasmas have higher energy densities than capacitively coupled plasmas [7; 31]. & 7/10 \\ \hline Electron Temperature & In RF plasmas, electrons are heated primarily in the sheath regions and through collisions in the bulk plasma. The electron temperature reached by RF plasmas in moderately sized industrial plant can range between 2 and 10 eV. However, in low-pressure RF plasmas, such as those used in cleaning, the range is normally between 1 and 5 eV [14]. & 6/10 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Grading of parameters for radio frequency plasma source. higher frequencies than RF plasma systems [3]. Normally, microwave systems operate at a frequency of 2.4 \(\pm\) 0.5 GHz, which is the industrial microwave frequency, selected especially to ensure that there is no disturbance of communication channels. There are magnetron tube generators, which are specifically built to operate at this particular frequency with high efficiency. Such magnetron tubes consist of a structure that includes an anode, hot filament cathode, magnets, and an antenna. Resonance at the frequency of operation of the magnetron is achieved at the anode. The power generated by the magnetron is then transferred to the antenna. The microwaves sustain the plasma when the electromagnetic waves manage to impart sufficient energy to the electrons when penetrating into the plasma. Through the combination of electric and magnetic fields, electrons are made to oscillate inside the magnetron and accelerate from the cathode to the anode [36]. The formation of electrons in a microwave plasma source is proportional to the square of the frequency as outlined by Equation (3) [37]. \[\mathrm{N_{e}}=\frac{\mathrm{f^{2}4p^{2}me_{0}}}{\mathrm{e^{2}}} \tag{3}\]  
where  
\(\mathrm{N_{e}}\) is the maximum electron density;  
\(\mathrm{f}\) is the source frequency;  
\(\mathrm{m}\) is the electron mass;  
\(\mathrm{e_{0}}\) is the permittivity of free space. An important parameter in microwave plasma systems is the critical electron density. This is the electron density beyond which the microwaves cannot penetrate the plasma anymore.