# Atomic Layer Deposition (ALD) of Metal Gates for CMOS
## 2 ALD of the Metal Gates
### ALD of N-Type Metal Gate
The linear growth curve showed no incubation. The layer was amorphous and had low roughness (-0.33 nm by AFM) and low resistivity. The C-V measurement of the MOSCAP with a structure of W/TiN/TiAlC/HfO\({}_{2}\)/SiO\({}_{2}\)/Si revealed no hysteresis, implying low defect density. The TiAlC layers processed with different thickness showed a tunable _EWF_ range from 4.49 eV to 4.79 eV. In a follow-up work [142], we found that the layer deposited at lower temperature such as 200 \({}^{\circ}\)C had metal Ti content and will be oxidized when exposed to air after the deposition. The growth mechanism was proposed as follows as there are a set of reactions during the ALD in parallel: first, the reductive behavior of TMA forms ethane. The reaction between TiCl\({}_{4}\) and TMA, generates Ti metal, which will react with the ethane, forming Ti-CH\({}_{2}\), TiCH=CH\({}_{2}\) and TiC fragments. The Al generated by the decomposition of TMA alloyed in and formed a final TiAlC layer. A comparison study between TiAlC and TiAlN was carries out [140] using three combinations of TiCl\({}_{4}\), TMA and NH\({}_{3}\) pulse separated by N\({}_{2}\) purge for TiAlN ALD and the ALD of TiAlC is the same to the previous works. Table 1 listed the processing sequence and the properties of the layers and Figure 10 displays the _EWF_ of the four work function layers (WFL) in the W/TiN/WFL/HfO\({}_{2}\)/SiO\({}_{2}\)/Si. One can see that the n-type _EWF_ does not only correlate to the Al content, but to the chemical bonds formed. The layers with only Al-N bonds did not show large _EWF_ shift, while that with Ti-C did. The only deficiency of the ALD TiAlC using TiCl\({}_{4}\) and TMA is its relatively small _EWF_ shift. Compared to 0.44 eV for the high performance nMOSFETs, the _EWF_ of the TiAlC from TMA still had room for improvement. In order to further tune the _EWF_, we studied the ALD using TiCl\({}_{4}\) and Triethylaluminum (TEA) [142]. TEA is a metal organic precursor with special \(\beta\)-hydrogen, which could lead to H\({}_{2}\) elimination, and generate Al intermediate [143]. Higher Al doping level was thus expected in the TiAlC deposited by ALD using TEA to replace TMA. The same ALD sequence is used to study the growth behavior at different temperatures between 250 and 350 \({}^{\circ}\)C. We found that the deposition in this temperature range saturated after the pulse time of TEA was over 10 sec, showing the typical self-limiting characteristic of ALD. The deposition rate was two times higher than that of the TMA process. A linear growth at 375 \({}^{\circ}\)C was found without incubation. The layer deposited at 375 \({}^{\circ}\)C has 24 at% Ti, 35 at% Al and 40 at% C. The chemical bonds detected by XPS are Ti-C, Al-C and Al-Al. The C-V curves of the MOSCAP with the same structure to the previous work were used to extract the _EWF_. The _EWF_ of 10 nm TiAlC layer tested in the stack of 75 nm W/2 nm TiN/TiAlC/3 nm HfO\({}_{2}\)/SiO\({}_{2}\)/Si varied with temperature. It decreased from 4.46 eV to 4.24 eV when the growth temperature increased from 250 \({}^{\circ}\)C to 375 \({}^{\circ}\)C. The _EWF_ deposited at 375 \({}^{\circ}\)C decreased along with the TiAlC thickness and \begin{table}
\begin{tabular}{c c c c c} \hline  & **Mode A** & **Mode B** & **Mode C** & **Mode D** \\ \hline Sequence & TiCl/NH\({}_{3}\)/TMA/NH\({}_{3}\) & TiCl/TMA/NH\({}_{3}\) & TiCl/NH\({}_{3}\)/TMA & TiCl/TMA \\ Elements (at\%) & Ti25,Al31,N43 & Ti67,Al4,C3,N25 & Ti76, Al2, C5,N12 & Ti35,Al8,C55 \\ Bonds & Ti-N, Al-N & Tt-N, Al-N, Ti-C & Tt-N, Al-N, Ti-C & Ti-C, Al-C \\ _EWF_ (eV) & \(\sim\)4.79 & \(\sim\)4.74 & \(\sim\)4.68 & \(\sim\)4.5 \\ \hline \end{tabular}
\end{table} 
Table 1: The comparison of the ALD sequence and properties of and TiAlX. Figure 10: _EWF_ comparison between the ALD TAIN and TiAlC. after over 6 nm retained constant. The MOSCAP again had good electrical behavior, including low defect density and low leakage through the high-k dielectric of _EOT_-0.7 nm.