## 5 Plasma Precursors
### Water  
Water vapor can also be utilized as a process gas to produce oxygen and hydroxyl species [125]. Water-based plasmas are more complex than plasmas using monoatomic elementary gases. The energetic electrons impact the H\({}_{2}\)O molecules and start by ripping away one of the hydrogen atoms by breaking the OH-H bond. The dissociation energy of the OH-H bond is 4.85 eV, which is provided by the collision of the H\({}_{2}\)O molecule with the energetic electron, as shown by Equation (32). Equations (32) and (34) present the results of other high energy collision processes in the aqueous plasma [126,127]. \[\mathrm{H_{2}O+e^{*}\to H+OH+e} \tag{32}\]  
\[\mathrm{H_{2}O+e^{*}\to H_{2}O+2e} \tag{33}\]  
\[\mathrm{H_{2}O^{+}+H_{2}O\to H_{3}O^{+}+OH} \tag{34}\]  
where  
\(\mathrm{e^{*}}\) denotes a high-energy electron. The charged radicals thus formed are highly reactive and ready to react with other matter to produce new species. The OH radical is the most active oxidation component. \begin{table}
\begin{tabular}{c c c} \hline
**Parameter** & **Summary** & **Parametric Score** \\ \hline Removal of Organic & Chlorine and bromine are also very reactive halogens, although to a lesser extent than fluorine. Therefore, these two substances are not ideal for organic contaminant removal, especially since the literature has not indicated any increase in the speed of the reaction with the use of chlorine and bromine when compared with hydrogen or oxygen. Another negative aspect reviewed in literature is the fact that both halogens attack most metals, forming non-volatile chlorides and toxic by-products such as HCl [14]. \\ \hline Removal of Oxides & Chlorine and bromine do not remove oxides, as halogens are themselves oxidizing agents. It is also the case that halogens may create non-volatile chlorides and bromides with metals. \\ \hline Physical Bombardment & Chlorine and bromine are not indicated for plasma bombardment mechanisms as they are too reactive and also their particle masses are relatively light when compared with particles of the noble gases argon and xenon. \\ \hline Plasma Reliability for & Chlorine and bromine can both cause extensive damage to die structures and also to the lead frames. \\ \hline Plasma Safety: Handling & Chlorine and bromine are both toxic chemicals that require specific handling regimes. Furthermore, emissions from the plasma processes of both elements are toxic, thus requiring scrubber systems. \\ \hline \end{tabular}
\end{table} 
Table 18: Grading of parameters for chlorine and bromine plasmas. It can react with electrode materials and form new species with them. Thus, plasma equipment design needs to factor in ways of avoiding sputtering the surface being cleaned with the material forming the electrode [6]. The problem is further exacerbated due to the fact that water condensation on the cool electrodes can cause an arc path to form, thus speeding up the electrode breakdown mechanism. Thus, the electrode must be specifically designed to avoid this through spacing steam preheating, heating of gas lines, and thermal regulation of the electrodes themselves [128; 129]. The quantity of studies discussing H\({}_{2}\)O based plasmas was not copious, although water vapor plasmas have seen growing interest, particularly in fields like sterilization, surface activation for biomaterials, and some specialized cleaning/etching processes. However, the research effort being carried out in the semiconductor manufacturing field has increased in recent years, and even production-grade equipment was identified. H\({}_{2}\)O plasmas have high chemical reactivity and enthalpy. Table 19 presents the grading of the parameters relevant to oxygen plasmas to outline oxygen's suitability for use in cleaning plasmas.