# Atomic Layer Deposition (ALD) of Metal Gates for CMOS
## 3 Conclusions  
A review on the ALD of metal gate for both NMOSFETs and PMOSFETs based on the state-of-the-art of the ongoing research was presented. The discussion starts with the introduction to the necessities of technical transition from poly-Si/SiO\({}_{2}\) to high-\(\kappa\)/Metal gate and the demand for the ALD of metal gates due to the special challenge the 3D device such as FinFETs and GAA-FETs has to face. The concepts of _EOT_ and _EWF_, and the methods to experimentally extract them are discussed by quotation of the theory on the energy band structure. The difference in the _EWF_ target of the conventional planar CMOSFETs and the 3D devices such as FinFETs and GAA-FETs are discussed as the starting point of material selection. The research works on the ALD of the p-type and the n-type work function metals are cited to tease out the logic of the technical evaluation and to figure out the solution for future generations.  
Following conclusions could be highlighted:  
1. Based on the accumulation of the experimental data, the industry has found the high-\(\kappa\)/metal gate solution for the planar CMOSFETs, with ALD HfO\({}_{2}\) as the gate dielectric, PVD TiAl as the n-type metal gate and PVD TiN as the p-type metal gate, in a replacement gate process. The successful development of the dual metal gate technology seems to originate from the encouraging C-V measurement results by [PERSON] et al. [45], showing a 1 V separation between the MOSCAP using PVD TiAl and PVD TiN.
2. The ALD of the three element metal candidates, Ru, Pt and W, has attracted much attention. After great efforts, the deposition technique has not yet matured for the application of the p-type metals.
3. The ALD using TiCl\({}_{4}\) as precursor for Ti and NH\({}_{3}\) as reactant could deposited TiN with p-type _EWF_. While the TiN deposited by organic metal precursors with NH\({}_{3}\) has n-type _EWF_ and that by PVD has a _EWF_ near the midgap of Si.
4. The TiAlC and TaAlC deposited by TEA has a wide _EWF_ tuning range from 4.65 eV to 4.26 eV, by varying the thickness. It is the Al-C and Al-Al bonds that induce these large _EWF_ shifts. The Al-N bands does not have the _EWF_ shift capability.
5. The ALD W deposited by WF\({}_{6}\) and SiH\({}_{4}\) is an ideal filling metal for pMOSFETs, which shows not only excellent fillability, but also strong tensile stress helpful to enhance the channel mobility.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline  & \multicolumn{2}{c}{**TiAlC**} & \multicolumn{2}{c}{**TaAlC**} \\ \hline Precursors & TiCl\({}_{4}\) + TMA & TiCl\({}_{4}\) + TEA & TaCl\({}_{5}\) + TMA & TaCl\({}_{5}\) + TEA \\ \hline Composition at\% & Ti35, Al8, C55 & Ti24, Al34, C40 & Ta 40, Al10, C44 & Ta38, Al32, C38 \\ Chem. bonds & Ti-C, Al-C & Ti-C, Al-C, Al-Al & Ta-C, Al-C & TaC, Al-C, Al-Al \\ Temp range (\({}^{\circ}\)C) & 300–400 & 250–375 & 300–400 & 250–375 \\ _EWF_ (thick.) & 4.49 eV–4.79 eV & 4.44 eV–4.28 eV & 4.74 eV–4.49 eV & 4.65 eV–4.26 eV \\ _EWF_ (temp) & 4.73 eV–4.53 eV & 4.46 eV–4.24 eV & 4.60 eV–4.54 eV & 4.46 eV–4.26 eV \\ \hline \hline \end{tabular}
\end{table}
Table 2: Comparison of the properties of the TiAlC and TaAlC layers.  
* (6) For GAA-FETs in future generations, an ideal dual metal scheme could be all ALD stacks, with structure of W/TiN/TiAlC/HfO\({}_{2}\)/SiO\({}_{2}\)/Si for n-type FETs, and W/TiN/HfO\({}_{2}\)/SiO\({}_{2}\)/Si for p-type FETs, or replacing the TiAlC with TaAlC. The wide _EWF_ tuning range of the Al-containing carbides might enable also the application for multi-\(V_{t}\) devices.  
Writing--original draft preparation, C.Z. and J.X.; writing--review and editing, C.Z. and J.X.  
This research received no external funding.  
The authors declare no conflict of interest.