## 5 Plasma Precursors
### Argon  
Argon is a noble gas with atomic mass of 39.948 u, is inert, and can be ionized into Ar\({}^{+}\) ions, which are characterized by energetic nature, albeit not exhibiting a high temperature. When in the plasma state, argon does not react strongly with matter on the surface being cleaned and therefore does not chemically alter surface properties, nor does it react or alter the underlying layers on the lead frames. Of course, Ar\({}^{+}\) ions gain significant kinetic energy when accelerated by the sheath potential toward the surface, making them ideal for use in bombardment processes [70]. The literature has indicated that argon plasma is effective in the micro-roughening of the substrate's surface. The reviewed experimental work started with the oxidization of copper lead frames at 175 \({}^{\circ}\)C for 1 h, after which they were then plasma-cleaned with a DC plasma having a 200 V bias [108]. Surface micrographs of a copper lead frame surface before and after plasma argon bombardment clearly showed the ablative nature of the \begin{table}
\begin{tabular}{c c c} \hline
**Parameter** & **Summary** & **Parametric Score** \\ \hline Removal of Organic & Oxygen plasmas generate reactive oxygen species & \\ Contaminants Through & (O\({}^{+}\), O\({}^{+}\), O\({}_{3}\), and O\({}_{2}^{+}\)) that react with organic compounds, & \\ Chemical Reduction & such as hydrocarbons, oils, and organic residues. These & \\ and Oxidation & organic residues are oxidized into volatile by-products, such & \\  & as CO\({}_{2}\), H\({}_{2}\)O and CO which are easily pumped away [14]. & 8/10 \\ \hline Removal of Oxides Through & Oxygen plasmas do not remove oxides; in fact, the opposite & \\ Chemical Reduction & is true, as O\({}_{2}\) plasma oxidizes Cu, Al, or Ag surfaces, & \\  & forming CuO on lead frames [104]. & \\ \hline  & Oxygen (O\({}_{2}\)) plasmas are poor for physical bombardment & \\  & compared with inert gases, like argon (Ar) or xenon (Xe), & \\ Physical Bombardment & since physical sputtering efficiency depends on momentum & \\ Efficacy & transfer from ions to target atoms. Oxygen ions (O\({}^{+}\) and O\({}_{2}^{+}\)) & \\  & are light (16–32 amu) compared with Ar\({}^{+}\) (40 amu) or Xe\({}^{+}\) & \\  & (131 amu), resulting in weak momentum transfer. Oxygen & \\  & plasmas also have high chemical reactivity, which dominates & \\  & over physical effects [105]. & \\ \hline Potential to Cause Damage & Even though lead frame plasma cleaning operates at low & \\ to Devices Being Cleaned & energy levels, oxygen radicals react with Cu lead frames to & \\  & form thin CuO layers. Furthermore, oxygen radicals also & \\ to Devices Being Cleaned & react with Al bond pads to form Al\({}_{2}\)O\({}_{3}\), which has an impact & \\  & on wire bonding [106; 107]. & \\ \hline Plasma Safety: Handling & No particular hazards are envisaged with the use of oxygen. & \\ Precursors and & Oxygen induces other substances to burn and so some care & \\ Emissions Risks & in the use of oxygen needs to be exercised. & \\ \hline \end{tabular}
\end{table} 
Table 12: Grading of parameters for oxygen plasmas. plasma. The experimental process showed that cleaning in argon for a period of 2.5 min produced a fourfold increase in the strength of adhesion [108]. This was attributed to the thinning of the surface oxide, where the literature indicated that the adhesion of moulding compounds to the lead frame is enhanced when the oxide is not thicker than 25 nm. A rapid decrease in adhesion was observed when the oxide thickness varied between 25 and 60 nm [109]. Argon plasmas also impart two further advantages. They act uniformly over the surface being cleaned and show consistency, even on devices having geometries that have difficult-to-reach areas and are effective to remove organic residues on lead frames. Argon plasmas are also non-toxic, and the plasma cleaning does not produce by-products that are harmful to health or the environment [70]. Argon has a first ionization potential of 15.76 eV.