# Atomic Layer Deposition (ALD) of Metal Gates for CMOS
## 2 ALD of the Metal Gates
### ALD of P-Type Metal Gate
in 1995 [109]. Using TiCl\({}_{4}\) as precursor and NH\({}_{3}\) as reactant, polycrystalline TiN was deposited on glass at 500 \({}^{\circ}\)C. It was found that the deposition rate is below 0.02 nm/cycle, corresponding to 0.1 monolayers of the (111) direction per cycle. These TiN layers have clorine content less than the detection limit of the Rutherford Back scattering (RBS) and hydrogen impurity less than 0.4 at\(\%\). The oxygen content of the films is about \(3\pm 2\%\) in 1400 A thick film and it is relatively higher in thinner film. The oxygen incorporation may be due to the oxidation of the film after the deposition. Thinner film is easier to oxidize. The oxidation will happen in all the other TiN films, including the films discussed in the following sections. ALD with TiI\({}_{4}\) and NH\({}_{3}\) was also tried based on the consideration on the difference in dissociation energy between Ti-I (D\({}_{0}\) = 296 kJ/mol) and Ti-Cl (D\({}_{0}\) = 429 kJ/mol) [110]. Lower deposition temperature is expected for the ALD with TiI\({}_{4}\)-NH\({}_{3}\) than TiCl\({}_{4}\)-NH\({}_{3}\). The experiments showed that TiN layer could be deposited at 350 \({}^{\circ}\)C. At 500 \({}^{\circ}\)C, the deposition rates of the TiI\({}_{4}\)-NH\({}_{3}\) is slightly higher than the TiCl\({}_{4}\)-NH\({}_{3}\). The film deposited at 350 \({}^{\circ}\)C has 2 at\(\%\) iodine content, and for those deposited at temperatures exceeding 400 \({}^{\circ}\)C, iodine content is below 0.5 at\(\%\). The oxygen content, however, was similar (10 at\(\%\) at 400 \({}^{\circ}\)C) to the films by TiCl\({}_{4}\) -NH\({}_{3}\) process. At 350 \({}^{\circ}\)C, 40 at\(\%\) oxygen was incorporated in the film. As a result, the resistivity of the films decreased as a function of the deposition temperature. The ALD of TiN on Si was studied by [PERSON] et al. [111]. Better quality of TiN was demonstrated at 350 \({}^{\circ}\)C with chlorine content of about 3 at\(\%\). The most important difference between these processes and that of [PERSON] et al. [109] is in NH\({}_{3}\) flow rate. [PERSON] et al. [111] deposited TiN films using the flow rate of 75 sccm, which is nearly 10 times higher than used by [PERSON] et al. [109] This might attribute to the difference in film quality at 350 \({}^{\circ}\)C.  
The above works using Ti halides show a very low deposition rate at the temperature range compatible to CMOS process. To solve the problem, organic metal precursors were tested, based on the experience in MOCVD [112; 113; 114]. With CVD technique, TiN films could be grown at low temperatures with different titanium alkylamides [115]. Although the compounds include the Ti-N bond, external nitrogen source such as NH\({}_{3}\) is still needed since the simple decomposition of the alkylamides is not enough to achieve qualified films [116]. The ALD of TiN have been carried out with Ti[N(C\({}_{2}\)H\({}_{5}\)CH\({}_{3}\))\({}_{2}\)]\({}_{4}\) (tetrakis(ethylmethylamido)titanium, TEMAT) and NH\({}_{3}\) as well [113; 114]. It was found that the self-limited ALD reactions take place in the temperature range of about 150-220 \({}^{\circ}\)C. At higher temperatures, TEMAT decomposed and CVD type reactions became dominant. The growth rate of these films was 10 times higher than in the halide based processes, varying between 0.5-0.6 nm/cycle. This is even higher than the ideal monolayer per cycle (ML/cycle) and was explained by a model of re-chemisorption of the precursor and NH\({}_{3}\). The films showed low impurity level, with only 4 at\(\%\) carbon and 6 at\(\%\) hydrogen incorporation and excellent step coverage above 90\(\%\). [PERSON] et al. studied the ALD of TiN from tetrakis(dimethylamino)titanium (TDMAT) [112]. Both thermal and plasma enhanced processes were studied, with N\({}_{2}\) and NH\({}_{3}\) as reactive gases. A growth rate of 0.06 nm/cycle was achieved with an optimized thermal ALD process, comparable to the ALD using TEMAT. The thermal ALD films had very high impurity level with about 37\(\%\) oxygen and 9\(\%\) carbon.