## 5 Plasma Precursors
### Oxygen  
Oxygen produces a plasma that reacts with contaminants such as organic modules to form oxides such as CO\({}_{2}\) and H\({}_{2}\)O. On the negative side, oxygen plasma does promote the metal oxidation of the lead frame surface [3]. Similarly to what has been explained for hydrogen, these reactions involve dissociation, excitation, and ionization, with the most important reactions being presented in Equations (24)-(29). The excitation energies involved going up to 20 eV for the dissociative ionization of oxygen presented in (24) [103].  
\[\mathrm{e}+\mathrm{O}_{2}\rightarrow\mathrm{O}+\mathrm{O}+\mathrm{e} \tag{24}\]  
\[\mathrm{e}+\mathrm{O}_{2}\rightarrow\mathrm{O}+\mathrm{O}^{+}+2\mathrm{e} \tag{25}\]  
\[\mathrm{e}+\mathrm{O}_{2}\rightarrow\mathrm{O}_{2}^{\mathrm{s}}+\mathrm{e} \tag{26}\]  
\[\mathrm{e}+\mathrm{O}\rightarrow\mathrm{O}^{+}+\mathrm{e} \tag{27}\]  
\[\mathrm{e}+\mathrm{O}_{2}\rightarrow\mathrm{O}_{2}^{\mathrm{+}}+2\mathrm{e} \tag{28}\]  
\[\mathrm{e}+\mathrm{O}\rightarrow\mathrm{O}^{+}+2\mathrm{e} \tag{29}\]  
The most reactive species is O\({}^{+}\), followed by O\({}_{2}^{+}\). Both are more reactive than excited oxygen species, followed by atomic and molecular oxygen, respectively [14]. Oxygen interacts with organic substances on the lead frames' surfaces. The by-products produced during an oxygen plasma process are CO\({}_{2}\) and H\({}_{2}\)O. An example of an oxidative breakdown reaction brought about through the oxygen-plasma interaction is shown in (30). Similarly,  
\begin{table}
\begin{tabular}{c c c} \hline
**Parameter** & **Summary** & **Parametric Score** \\ \hline Physical Bombardment Efficacy & Hydrogen ions are light, so physical sputtering is less efficient than that with Ar\({}^{+}\) or other heavier ions [98,99]. & \\ \hline Plasma Reliability for Sensitive Devices (Minimizing Damage) & Hydrogen has the potential to diffuse into metals and causing embrittlement. However, the literature has shown that hydrogen plasma can be used without damage under controlled conditions with low ion energies, such as those found in semiconductor lead frame cleaning systems. The literature has indicated that hydrogen plasma cleaning systems with ion energies below 10 eV are generally safe for silicon dies if properly optimized. At such low energies, hydrogen ions (H\({}^{+}\), H\({}_{2}^{+}\), and H\({}_{3}^{+}\)) and radicals (H\({}^{*}\)) primarily perform chemical cleaning (removing organics and oxides) with minimal physical sputtering or lattice damage. & 7/10 \\  & However, hydrogen diffusion into sensitive structures (gate oxides and metal interconnects) can still occur if exposure is prolonged or temperatures are high, hence why exposure needs to be limited and temperatures kept low [100, 101, 102]. & \\ \hline Plasma Safety: Handling Precursors and Emissions Risks & H\({}_{2}\) is flammable and therefore proper plasma system and process design would be required. However, the by-products emanating from the plasma process are safe molecules, such as H\({}_{2}\)O and CH\({}_{4}\), which can be emitted by regular exhaust systems. No specific scrubber systems are required with plasmas that use hydrogen. & 6/10 \\ \hline \end{tabular}
\end{table}
Table 11: Cont.  
as discussed for hydrogen, the by-products need to be removed to ensure a constant reaction rate with high efficiency.  
\[\mathrm{C_{x}H_{y}+O^{*}\to CO_{2}+H_{2}O} \tag{30}\]  
Table 12 presents the grading of the parameters relevant to oxygen plasmas to outline oxygen's suitability for use in cleaning plasmas.