## 5 Plasma Precursors
### Fluorine and Fluorine Compounds  
Gases such as SF\({}_{6}\) and CF\({}_{4}\) can be used to produce fluorine plasmas, as shown in (31) [27].  
\[\mathrm{SF}_{6}+\mathrm{e}\rightarrow\mathrm{SF}_{2}+4\mathrm{F} \tag{31}\]  
These precursors are relatively harmless compounds that allow the safe use of fluorine in lieu of dangerous fluorine compounds, such as HF or elemental halogen [93]. Fluorine plasmas are highly reactive and will readily etch group 14 elements, such as silicon, since fluorine forms volatile fluorides with these elements [119]. No evidence of the use of fluorine plasmas for lead frame cleaning was detected in the literature, probably due to the fact that this most aggressive of species will readily etch the silicon dies, producing SiF, SiF\({}_{2}\), SiF\({}_{3}\), and SiF\({}_{4}\). which are volatile [27]. However, one might consider the use of fluorine for lead frames that do not yet contain dies if any cleaning is needed at that stage as fluorine is used in cleaning applications, such as the cleaning of process chambers. However, the use of such an aggressive element needs to be carefully weighed, as even the equipment needs to be specifically designed, including features such as the use of a remote plasma source to reduce the presence of ions within the process chamber itself [120].  
Table 17 presents the grading of the parameters relevant to fluorine plasmas to outline fluorine's suitability for use in cleaning plasmas.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Parameter** & **Summary** & **Parametric Score** \\ \hline Physical & Nitrogen plasmas primarily generate ions such as N\({}_{2}^{+}\), which have a lower mass (28 amu) compared with Ar\({}^{+}\) ions (40 amu). Additionally, ion yields in nitrogen plasmas are often lower than those in argon plasmas. These two characteristics—lower ion mass and potentially lower ion density—make nitrogen a less effective gas than argon for plasma processes relying mainly on physical bombardment [14]. & \\ \hline Plasma Reliability for & The nitriding process brought about by the nitrogen plasma can have & \\ Sensitive Devices & an effect on characteristics of the bond pad surfaces on both the die & \\ (Minimizing Damage) & and the lead frames. This has an impact on wire bonding processes. & \\ \hline Plasma Safety: & Nitrogen plasmas can produce highly toxic emissions, such as HCN, & \\ Handling Precursors & CN, and NOx, which requires scrubbing through a special & \\ and Emissions Risks & exhaust system. & \\ \hline \hline \end{tabular}
\end{table}
Table 16: _Cont._  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Summary** & **Parametric Score** \\ \hline \hline \multirow{5}{*}{
\begin{tabular}{c} Removal of Organic \\ Contaminants Through \\ Chemical Reduction \\ and Oxidation \\ \end{tabular} } & **F\({}^{*}\)** radicals react with hydrocarbons, breaking C-C/C-H bonds and forming volatile by-products; however, some of the products & \\ \cline{1-1}  & formed are toxic, like hydrogen fluoride, which needs to have a & \\ \cline{1-1}  & scrubbing system installed. & \\ \cline{1-1}  & The risk of polymerization is also a major issue. In plasmas like & \\ \cline{1-1}  & CF\({}_{4}\), if there is not enough fluorine relative to carbon, & \\ \cline{1-1}  & fluorocarbon polymers (-(CF\({}_{2}\))\({}_{n}\)-) can deposit on the surface & \\ \cline{1-1}  & instead of etching. This is the basis of Teflon-like coatings [121]. & \\ \hline \hline \end{tabular}
\end{table}
Table 17: Grading of parameters for fluorine plasmas.  
Due to the reasons mentioned above, especially the high risk of damaging the silicon die and the safety/environmental concerns associated with HF generation, fluorine plasma is generally avoided for lead frame cleaning in standard semiconductor packaging processes.