# Atomic Layer Deposition (ALD) of Metal Gates for CMOS
## 2 ALD of the Metal Gates
### ALD of P-Type Metal Gate  
The top-ranking candidates for p-type metal gate included refractory element metals such as Ru, Pt and W. It is interesting to note that all these three metals are also among the very limited number of metals which could be deposited by ALD. Ru and RuO\({}_{\rm X}\): the ALD of Ru was widely studied [63; 64; 65; 66]. Because of its high work function (\(\sim\)4.7 eV) and conductive oxides, it is expected to be suitable to applications as p-type metal gate electrodes for SrTiO\({}_{\rm X}\) MIM capacitor for DRAM [67; 68; 69] and seed layer of Cu plating for back end of line process of CMOS [70; 71; 72]. A nice summary on the precursors available for the ALD of Ru and the deposition reactions can be found in the reference [72]. The precursors used for Ru are mainly cyclopentadienyl compounds such as RuCp\({}_{2}\) (Cp = cyclopentadienyl) and Ru(EtCp)\({}_{2}\) (Et = ethyl), and \(\beta\)-diketonate compounds like Ru(od)\({}_{2}\) (od = 2,4-octanedionato) and Ru(thd)\({}_{3}\) (thd = 2,2,6,6-tetramethyl-3,5-heptanedionato) [67; 68]. The mechanisms of the processes are based either on oxidation of the ruthenium precursor by oxygen or on reduction by NH\({}_{3}\) plasma. Although so much effort has been made, the quality of the ALD Ru layer does not seem qualified for work function metal application. Up to now, no matter which precursor used, the ALD of the Ru all have remarkable incubation cycles, inducing surface roughness [72]. They are probably matured for the application of the MIM capacitors and interconnections, but not for the work function layer of the metal gate. Platinum: ALD of Pt has also been widely studied in the last 20 years because of its excellent chemical and thermal stability and conductivity for potential applications such as the electrodes for ultra-high \(\kappa\) in the cell of DRAM and those for ferroelectric dielectric in FERAM [67; 68; 69; 73; 74; 75; 76]. Its high work function (5.6 eV) and low resistivity (10.5 \(\upmu\Omega\)-cm) also attracted the attention for the application as p-type metal gates [77]. The ALD of platinum has been studied using two different kinds of compounds, MeCpPtMe\({}_{3}\) (Me = methyl) [67], and Pt(acac)\({}_{2}\) (acac = acetylacetonato), as precursors, and O\({}_{2}\), pure or in air, and H\({}_{2}\), as reactant [67; 78]. For the ALD using MeCpPtMe\({}_{3}\), the precursor would dissociate when chemically absorbed on and reacted with the O in previously deposited layer. For that using Pt(acac)\({}_{2}\), both oxygen and hydrogen could react with the precursor, the ALD with hydrogen, however, could only grow a very poor film at low deposition rate. [PERSON] studied the mechanism of the ALD using MeCpPtMe\({}_{3}\)[79], and indicated that the ALD behavior is relative not only to the reaction between the precursor and O\({}_{2}\), but also to the catalytic nature of the Pt film, based on their observation of the gas products generated. The mechanism was described as a series of reactions in sequence; the combustion and dehydrogenation reactions in MeCpPtMe\({}_{3}\) pulse on the O covered surface, formation of a carbonaceous passivation of the surface, and the combustion of the carbonaceous layer in O pulse. It was believed that it was the carbonaceous passivation which attributes to the self-limitation and the temperature dependence of the deposition rate. [PERSON]'s work studied the electrical performance of the Pt metal gate in the stacks with Al\({}_{2}\)O\({}_{3}\), ZrO\({}_{2}\) and SiO\({}_{2}\)[76; 77]. They found that the _EWF_ of the ALD Pt strongly depended on the dielectric, 4.76 eV for ZrO\({}_{2}\), 5.22 eV for Al\({}_{2}\)O\({}_{3}\) and 5.52 eV for SiO\({}_{2}\). The MOS capacitors using the ALD Pt on ZrO\({}_{2}\) and Al\({}_{2}\)O\({}_{3}\) also showed good defect density control and leakage reduction. A concern on the application of Ru and Pt as the work function metal might lie in the difficulty of wet clean.