## 5 Plasma Precursors
### Ammonia--NH\({}_{3}\)  
The reviewed literature indicated that NH\({}_{3}\) plasmas can be used to create both hydrogen and nitrogen radicals, which can be used to carry out a combined cleaning approach utilizing two plasma gases with different characteristics. Ammonia in a plasma can produce a multitude of species including ions such as N\({}^{+}\),N\({}_{2}^{+}\),N\({}_{3}^{+}\),N\({}_{4}^{+}\)H\({}^{+}\),H\({}_{2}^{+}\),H\({}_{3}^{+}\),H\({}^{-}\),N\({}_{2}\)H\({}^{+}\),NH\({}^{+}\), NH\({}_{2}^{+}\),NH\({}_{3}^{+}\), and NH\({}_{4}^{+}\). A number of vibrational excited and electronically excited states are also created [132]. The literature indicated that ammonia-based plasmas were used to achieve oxide removal, surface roughening characteristics, which can be achieved through the hydrogen component, as well as the passivation layer, which can be achieved through the nitrogen component of the plasma. Through this combination, NH\({}_{3}\) plasma treatment maintained higher uniformity, higher overall surface conditions, and a smooth reduction process [117]. While published studies on ammonia plasma cleaning of lead frames remain scarce, research efforts have instead focused on its applications in advanced packaging technologies, particularly for 3D stacked semiconductor architectures employing copper interconnects in chip-scale packaging. Such plasma systems were found to utilize an RF source to drive the process [133]. Table 20 presents the grading of the parameters relevant to ammonia-based plasmas to outline ammonia's suitability for use in cleaning plasmas. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Parameter** & **Summary** & **Parametric Score** \\ \hline Removal of Organic Contaminants Through Chemical Reduction and Oxidation & Ammonia plasmas need a longer time to remove oxides than comparable cleaning with hydrogen only. The reason for this is that hydrogen plasmas produce more hydrogen species than ammonia plasmas because the H-H dissociation energy is lower [133]. & \\ \hline \hline \end{tabular}
\end{table} 
Table 20: Grading of parameters for ammonia plasmas. \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Summary** & **Parametric Score** \\ \hline Plasma Safety: Precursors and Emissions Risks & Water-based plasmas produce by-products that are non-toxic substances, such as H\({}_{2}\)O, CO\({}_{2}\), and possibly trace H\({}_{2}\)O\({}_{2}\), when reacting with organic residues and oxides [131]. & \\ \hline \hline \end{tabular}
\end{table} 
Table 19: _Cont._The literature relating to plasma processes carried out for devices used for 3D stacked semiconductor products has indicated that ammonia plasmas are less aggressive than pure hydrogen plasma, reducing the risk of excessive surface etching or damage to thin metal layers [133; 134]. In fact, TEM results indicated that void formation on bond pads was less noticeable when an ammonia plasma process was used, compared with a process that used argon. This could have been a contributing factor to another positive result achieved by ammonia plasmas, namely the achievement of lower electrical resistance values in bond pad areas. Hydrogen radicals act to remove the oxides from the surfaces, while the nitrogen component of ammonia plasmas passivates the copper by creating a nitride layer over the copper layer. This has been confirmed in the literature through XPS analysis [117; 135; 136]. Further analysis showed that the surfaces that were treated with ammonia inhibited the growth of oxides when compared with comparable surfaces cleaned with Ar. However, while this layer has been shown to inhibit further oxidation, comparing the results of shear tests carried out on Cu-Cu bonds created after Ar plasma cleaning with shear results of bonds creating after NH\({}_{3}\) cleaning indicated that the former presented significantly higher values.