# The Challenges of Advanced CMOS Process from 2D to 3D
## 3 Process Integration of New Transistor Architecture
### High-\(k\) Dielectric and Metal Gate (HKMG)
#### 3.6.2 Atomic Layer Deposition of Metal Gate  
Continued scaling down of CMOS devices requires high-\(k\) dielectrics and metal electrodes to replace traditional SiO\({}_{2}\) dielectrics and poly-silicon gate electrodes [69; 70]. The replacement of poly-silicon with metal gate is essential in order to prevent the Fermi level pinning effect and poly-silicon depletion [71; 72]. Intel introduced TiAlN and TiN are used as work function metals for nMOS and pMOS, respectively for 45 nm and 32 nm node [39; 40]. With further down scaling of the CMOS device into 22 nm node and beyond, a three-dimensional FinFET structure was introduced into the CMOS in order to suppress the short-channel effects [21]. In such transistors, the aspect ratio of the replacement gate structure became larger, and this made it difficult to fill the trench. Atomic layer deposition (ALD) is widely regarded as the best solution for metal gate deposition because of its excellent conformal step coverage capability [73; 74]. The metal gate materials such as TiN, TiAlN, HfAlC, TaN, HfN, AlN, TiC\({}_{x}\)N\({}_{y}\), TiC-TiN have effective work function of 4.6-5.0 eV, which can be used in pMOS or SOI device [75; 76; 77; 78]. [PERSON] and [PERSON] et al. synthesized a thermally stable triple Ti\({}_{1-x}\)Al\({}_{x}\)N metal gate and studied the effect of Al concentrations in the atomic-layer-deposited Ti\({}_{1-x}\)Al\({}_{x}\)N layers on the effective work function. Tunable effective work function of 4.86-5.04 eV was measured which could be implemented for pMOS [76]. Recently, the impact of using ALD technique on the effective work function of carbon-containing TiN films (TiC-TiN) has been presented. The results demonstrated that the bonding concentration of Ti-C increases at higher deposition temperatures and in contrast the work function of the TiC-TiN metal gate decreases from 5.0 to 4.6 eV. A high work function of 5.0 eV metal gate is suitable for a p-FET application while the midgap work function 4.6 eV is suitable for a fully depleted silicon-on-insulator FET [75]. ALD Hf\({}_{x}\)Al\({}_{y}\)C\({}_{z}\) films using hafnium chloride and trimethylaluminum precursors were studied by [PERSON] et al. [77]. The effective work function in metal-oxide-semiconductor capacitor devices with the Hf\({}_{x}\)Al\({}_{y}\)C\({}_{z}\) layer was quantified to be 4.6 eV which is compatible with an ALD HfO\({}_{2}\) dielectric. Thus, Hf\({}_{x}\)Al\({}_{y}\)C\({}_{z}\) is a metal gate work function material that can be used for the tuning of device threshold voltages (\(V_{th}\)) for anticipated multi-\(V_{th}\) IC devices [78]. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Technology Nodes**} & \multicolumn{4}{c}{**Film Thickness (nm)**} \\ \cline{2-5}  & **Thermal Oxide** & **High-\(k\)** & **TiAl(N)** & **TiN** \\ \hline
45 nm & \(\sim\)1.2 & \(\sim\)1.5 & \(\sim\)2 & \(\sim\)2.1 \\
32 nm & \(\sim\)1.2 & \(\sim\)1.1 & \(\sim\)1.7 & \(\sim\)2 \\
22 nm & \(\sim\)1.1 & \(\sim\)1.0 & \(\sim\)1.2 & \(\sim\)1.4 \\
14 nm & \(\sim\)0.6 & \(\sim\)1.2 & \(\sim\)3.7 & NA \\ \hline \For nMOS, due to the lack of appropriate precursor, it is relatively difficult to acquire n-type work function metal. [PERSON] et al. applied a new ALD TiAl process to demonstrate conformal low \(V_{th}\) bulk FinFET devices [79]. [PERSON] et al. used tert-butylimido tri-diethylamido tantalum and methane/hydrogen reactive gas mixture to obtain ALD TaC\({}_{x}\)N\({}_{y}\) films. By introducing of 1.5-2.5% methane, the TaC\({}_{x}\)N\({}_{y}\) film with lower work function could be obtained. For these films, the lowest effective work function of 4.37 eV could be achieved [80]. [PERSON] et al. introduced TaC\({}_{y}\) film deposited by plasma-enhanced ALD (PEALD) technique to extend the effective work function within 4.54-4.77 eV [81]. For the metal gate, the thermal ALD without plasma damage is more suitable. [PERSON] et al.