# Atomic Layer Deposition (ALD) of Metal Gates for CMOS
## 2 ALD of the Metal Gates
### ALD of P-Type Metal Gate
These noble metals are chemically inert and any contamination of them on the backside of the Si wafers needs a very harsh wet process to be removed. When being used as the metal gate, CMP process will inevitably induce backside contamination. This might explain why the two metals, after extensive investigations, have never been employed in front end of line of the CMOS production. Tungsten: The ALD of W is the most successful case among elemental metal ALD processes. Thanks to sound study on the CVD of W, inorganic precursor WF\({}_{6}\), and reducing reactants, silane or borane compound and their reaction mechanisms have been well known. The first study on the ALD of W was reported by [PERSON] et al. in 2000 [80]. They studied the ALD on SiO\({}_{2}\) using WF\({}_{6}\) and Si\({}_{2}\)H\({}_{6}\) and demonstrated a linear growth of W at temperatures below 350 \({}^{\circ}\)C, without any incubation. The mechanism of the deposition was explained as the self-limited reaction in that Si\({}_{2}\)H\({}_{6}\) strips fluorine from WF\({}_{\rm x}\) species and the resulting SiH\({}_{\rm y}\)F species are subsequently removed by the next WF\({}_{6}\) exposure. [PERSON] et al. [81] further studied the nucleation of the above deposition. Slightly different from the previous work, an incubation of five cycles is found, believed to attribute to the nucleation on the OH-group finalized SiO\({}_{2}\) surface. The ALD-W films deposited by using diborane, B\({}_{2}\)H\({}_{6}\), as a reducing agent of WF\({}_{6}\) was reported in 2002 by [PERSON] et al. [82] and further studied by [PERSON] et al. [83; 84]. The ALD-W process using silane, SiH\({}_{4}\), was also extensively investigated, based on the similar reaction mechanism consideration [85; 86]. All the above works were mainly targeting the application as the seed layer of CVD for W contact plug. [PERSON] et al. studied the ALD W on TiN using SiH\({}_{4}\) and B\({}_{2}\)H\({}_{6}\) as the metal for filling the high aspect ratio gap of the replacement gate [87; 88]. A good gap filling capability and electrical performance was demonstrated with MOSCAP and pMOSFETs. The comparison of the properties between the ALD W using SiH\({}_{4}\) and B\({}_{2}\)H\({}_{6}\) for the replacement gate filling in the pMOSFETs revealed that the former, which is crystalline and has a much larger tensile stress (-2.4 GPa), could give superior electrical properties compared to the B\({}_{2}\)H\({}_{6}\)-generated layer. ALD W is less favorable for being used as the work function layer of the metal gate, due to its near midgap work function (-4.6 eV) [89], the usage as filling metal for all-ALD gate stack is without doubt promising. TiN: TiN was first introduced in the 1970s, deposited by reactive sputtering using Ti target and reactive gases such as NH\({}_{3}\) and N\({}_{2}\). Its excellent hardness, density, melting point and gold-like appearance attracted so much attention for the applications as abrasion-resistive or decorative coating. Quite unusual, such a nitride film has a high electrical conductivity. In semiconductor industry, it has been used as an inorganic antireflective coating for lithography [90], hardmask for low-\(\kappa\) patterning [91; 92], and diffusion barrier for tungsten contact and Cu interconnection [93; 94]. As mentioned before, TiN was successfully used as a p-metal gate in the first generation high-\(\kappa\)/metal gate technology. In fact, TiN is an ideal candidate for the metal gate because of its excellent thermal stability in contact with HfO\({}_{2}\), low resistivity and compatibility to conventional CMOS process [95; 96; 97]. Since conformality is difficult to achieve by PVD, extensive exploration of CVD techniques for TiN deposition has been carried out over the last 20 years, with variations such as PECVD [98; 99], low-pressure CVD (LPCVD) [100; 101], atmosphere pressure CVD (APCVD) [102; 103], metalorganic CVD (MOCVD) [104; 105] and low pressure metalorganic CVD [106; 107]. ALD of TiN was first explored by [PERSON] et al. in 1988 [108] and by [PERSON] et al.