## 5 Plasma Precursors
### Ammonia--NH\({}_{3}\)
Hence, while an analysis of the surface chemistry indicated that the nitride layer is effective for reducing the regrowth of the oxide layer, it can have detrimental effects in bonding processes occurring subsequently to the plasma process. Such detrimental effects were noticed even after the devices were heated to temperatures in excess of 350 \({}^{\circ}\)C, which should decompose the nitride layer [137]. The data outlined above indicate that, while ammonia plasmas presented positive results in certain areas, there are other aspects that need to be researched further. Another area that needs to be studied experimentally concerns findings outlined in the literature presenting experimental work benchmarking the NH\({}_{3}\) plasma process with a hydrogen plasma. These findings indicated that the former showed less efficacy in the reduction of \begin{table}
\begin{tabular}{c c c} \hline
**Parameter** & **Summary** & **Parametric Score** \\ \hline \hline \multicolumn{3}{c}{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. Hence, while an analysis of the surface chemistry indicated that the nitride layer is effective for reducing the regrowth of the oxide layer, it can have detrimental effects in bonding processes occurring subsequently to the plasma process. Such detrimental effects were noticed even after the devices were heated to temperatures in excess of 350 \({}^{\circ}\)C, which should decompose the nitride layer [137].  
\begin{tabular}{c c} \hline \hline \multirow{3}{*}{Physical Bombardment} & NH\({}_{3}\) fragments into light species, as outlined above, such as H, NH\({}^{+}\), and NH\({}_{2}^{+}\), upon plasma dissociation. When compared with Ar\({}^{+}\) ions these particles are light and therefore not as capable of physically bombarding surfaces [25]. \\ \hline \hline \multirow{3}{*}{Plasma Reliability for Sensitive Devices (Minimizing Damage)} & Similarly, as with the case with hydrogen plasmas, the conditions need to be monitored to ensure that the hydrogen or nitrogen radicals do not interact negatively with structures, such as bond pads. Such interactions need further study [133]. \\ \hline \hline \multirow{3}{*}{Plasma Safety: Handling Precursors and Emissions Risks} & Ammonia is a pungent and toxic substance that requires specific handling methodologies [138]. \\ \cline{1-1}  & 4/10 \\ \hline \hline \end{tabular}
\end{table} 
Table 20: _Cont._an oxide layer from a copper lead frame [139]. Therefore, further extensive experimental work would be warranted on the use of ammonia for plasma cleaning processes.