## 5 Plasma Precursors
### Hydrogen  
Hydrogen plasmas are used for the removal of surface oxide layers on many types of surface finishing, including gold plating. The literature has presented studies that were conducted on NiPdAu-plated lead frames treated with an oxygen plasma. These formed a Au\({}_{2}\)O\({}_{3}\) layer after the oxygen plasma process. Subsequently, the lead frames were subjected to a hydrogen plasma process in a batch microwave plasma system, thereby removing the oxide layer [3]. Studies on similar NiPdAu lead frames have shown that the recontamination rate is reduced through the use of hydrogen. This reduction in the recontamination rate was observed to be up to 50% [75]. One may say that hydrogen has a unique ability to remove many types of contaminants from copper-lead frames, including the halogens, hydroxides, and oxides of copper. The equations for some of these reduction processes in copper lead frames can be seen in (6)-(8) [76; 77].  
\[\mathrm{Cu_{2}O+2H}\rightarrow\mathrm{Cu_{2}+H_{2}O} \tag{6}\]  
\[\mathrm{CuO+2H}\rightarrow\mathrm{Cu+H_{2}O} \tag{7}\]  
\[\mathrm{Cu(OH)_{2}+2H}\rightarrow\mathrm{Cu+2H_{2}O} \tag{8}\]  
Apart from removing the oxide layer, hydrogen saturates immobilized radicals, thereby providing a protective layer on metals. Such a protective layer prevents surface oxidation over an extended time period [78].  
Hydrogen's plasma chemistry is quite complex to model and can be approximated by around 26 volume reactions and 6 surface reactions. There are many species involved, such as electrons, molecular hydrogen (H\({}_{2}\)), neutral hydrogen atoms (H), protons (H\({}^{+}\)), hydrogen anions (H\({}^{-}\)), molecular hydrogen ions (H\({}_{2}^{+}\)), trihydrogen cations (H\({}_{3}^{+}\)), and three vibrationally exited states, H\({}_{2}\) (v = 1), H\({}_{2}\) (v = 2), and H\({}_{2}\) (v = 3). Apart from this, there are also four electronically excited states [79; 80; 81; 82; 83].  
The ion which is the most abundant in hydrogen plasmas having a degree of dissociation less than 50% is the H\({}_{3}^{+}\) ion [36]. H\({}_{3}^{+}\) ions are actually the most prevalent ion in hydrogen glow discharge processes and therefore are an important factor in hydrogen-based plasma processes [84]. This can change depending on the specific conditions underpinning the particular plasma process [85].  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Parameter** & **Scores** & **Score Explanation** \\ \hline \multirow{6}{*}{Plasma Safety: Handling Precursors and Emissions Risks} & \multirow{3}{*}{Moderate Scores (4–7) Precursors and Emissions Risks} & Gases that are inert, non-toxic, non-flammable, and whose plasma processing byproducts are also benign, requiring minimal specialized handling or exhaust abatement. \\ \cline{2-3}  & & Gases with some safety considerations (e.g., \\  & & flammability like H\({}_{2}\), oxidizer like O\({}_{2}\), or mild \\ \cline{1-1}  & & precursor toxicity) but manageable with standard \\ \cline{1-1}  & & industrial safety protocols and potentially requiring \\ \cline{1-1}  & & simple exhaust treatment. \\ \cline{1-1}  & & Gases that are highly toxic, corrosive, or pyrophoric as \\ \cline{1-1}  & & precursors, or produce highly hazardous emissions \\ \cline{1-1}  & & (e.g., HF from fluorine plasmas and HCN from \\ \cline{1-1}  & & N\({}_{2}\)/organic reactions) requiring extensive safety \\ \cline{1-1}  & & measures and specialized abatement systems. \\ \hline \hline \end{tabular}
\end{table}
Table 9: _Cont._The main processes underpinning the plasma process will now be explained in the context of hydrogen [86]. These processes hold true for other elemental and molecular plasmas, as will be explained in the other subsections tackling the other substances.