# Atomic Layer Deposition (ALD) of Metal Gates for CMOS
## 2 ALD of the Metal Gates
### ALD of N-Type Metal Gate  
Compared to the p-type metal gate, the selection of n-type metal gate is more difficult. The first demonstration of a dual-metal gate CMOS using Ti as n-metal gate and Mo as p-metal gate, respectively, together with SiO\({}_{x}\)N\({}_{y}\) as dielectric, was reported in 2002. The device showed negligible gate depletion, remarkably reduced gate leakage and acceptable channel mobility [133]. The TiN/Ti double layer was deposited by sputtering, targeting at a theoretical work function of metal Ti on SiO\({}_{x}\)N\({}_{y}\) of 4.36 eV. The _EWF_ extracted, however, from the C-V measurements is 4.56 eV, near the midgap value for TiN. The complete silicidation of the poly-silicon gate electrode, called FUSI, was also proposed for dual gate application [134]. In the FOSI process, poly-Si is first deposited on SiON and doped by B and As for pMOS and nMOS, respectively. Afterwards, a thick enough Ni layer was deposited on the poly-Si, followed by annealing for silicidation. Two different work functions, -4.5 and -4.9 eV for NMOS and PMOS, respectively, were obtained. This approach attracted much attention at beginning [135], due to its integration simplicity, but was finally abandoned due to the difficulty to achieve low enough \(V_{t}\) devices. Another dual metal gate integration work [136] used PVD Ru as p-type metal and PVD TaC as n-type metal, and HfO\({}_{2}\) as dielectric, demonstrated good device behavior. As mentioned before, the first industrial high-x/metal gate technology for 45 nm node, reposted in 2007, used TiAl alloy as n-type MOSFETs [25]. The capacitor using AlTi n-type metal and HfO\({}_{2}\) showed an ideal C-V curve and n-type _EWF_[45]. In all the above works, the n-type metal gate were deposited by PVD. The first work on ALD of n-type metal was reported in 2007 by [PERSON] using TaC\({}_{y}\) as the metal gate and HfO\({}_{2}\) as the dielectric [137]. The TaC\({}_{y}\) films deposited by PEALD using organic precursor and a carbon containing gas was near-stoichiometric, nitrogen free, and had low oxygen impurities and amorphous carbon content. Excellent electrical properties using this TaC\({}_{y}\)/HfO\({}_{2}\) stack were demonstrated, including work function between 4.77 and 4.54 eV and low fixed charge density (-2-4 \(\times\) 10\({}^{-11}\) cm\({}^{-2}\)). [PERSON] et al. studied the ALD of TiN-TiC mixture using a fine designed precursor and process [138]. The TDMAT was used as precursor for Ti, H\({}_{2}\) and NH\({}_{3}\) as reactants. The sequence of one ALD cycle was designed as 1s TDMAT pulse/3s Ar purge/3s H\({}_{2}\) plasma/1s (H\({}_{2}\)-NH\({}_{3}\)) preflow/3s (H\({}_{2}\)-NH\({}_{3}\)) plasma/3s Ar purge. The temperature range of pure ALD mode deposition was found between 150 \({}^{\circ}\)C and 200 \({}^{\circ}\)C, generating a layer of TiN. The deposition between 200 and 350 \({}^{\circ}\)C formed a mixture of TiC and TiN. The higher the temperature, the higher the TiC content will be. The _EWF_ extracted from C-V measurement of MOSCAPs showed a decreasing trend from 5.02 eV for TiN to 4.8 eV for 18.5% TiC and 4.6 eV for 39% TiC. Such a tunable _EWF_ was promising for dual gate CMOSFETs and multi-\(V_{t}\) applications. The CVD mode for TiC formation seemed a potential risk of causing processing variation. Based on the encouraging results of PVD TiAl n-MOSFET [25] and the n-type _EWF_ of ALD of TaC and TiC, the authors of this review started a series of systematic studies on the ALD of the TiAlC and TaAlC [139]. In 2015, we investigated the thermal ALD of TiAlC using TiCl\({}_{4}\) as Ti precursor and trimethylaluminum (TMA) as Al precursor on ALD HfO\({}_{2}\) surface [139; 140; 141; 142]. One deposition cycle consisted of TiCl\({}_{4}\) pulse, N\({}_{2}\) purge, TMA pulse and N\({}_{2}\) purge. The deposition was studied in the temperature range of 300 \({}^{\circ}\)C to 400 \({}^{\circ}\)C. It was found that the major content of the film deposited at 400 \({}^{\circ}\)C is TiAlC, with 55 at% C, 35 at% Ti, 8 at% Al and small amount of Cl.