Atomic Layer Deposition of TiCxNy films for gate electrode application

Abstract

In the past, poly-Si was used as a gate electrode due to tunable work function, cost-effectiveness and reliability. However as the circuit size decreased and the dielectric layer became thinner, the poly-Si gate electrode caused problems such as gate depletion, high resistivity, and boron penetration. Thus, the metal gate electrode concept has been revived. Since the metal gate electrode does not require doping, it does not have depletion and boron penetration problem. However, pure metal gate has a poor thermal stability and a high reactivity. In order to overcome these problems metal nitride and silicide have been suggested as a new gate material. One of the metal nitride is TiCxNy which is a mixture of TiN and TiC phase. TiC and TiN have some benefits as a gate electrode material. For instance, the resistivity of crystal TiN and TiC is 25 μΩ.cm and 60 μΩ.cm, respectively. It is much lower than poly-Si (≈1,000 μΩ.cm). Also the work function of TiN is 4.7~4.8 eV and it is reported that the effective work function of TiCxNy can be controlled by varying TiN/TiC proportion. The TiN/TiC phase is a ceramic material so its thermal stability is excellent. Current integrated circuit (IC) has become smaller and more complicated. In order to its fine structure, good step coverage, high aspect ratio and excellent uniformity are required to make IC. ALD process meets the requirements of IC manufacturing process, so it has been highlighted. However, ALD process has a limit for controlling the composition due to it uses chemical reaction. Therefore, ‘how to deposit targeting material (TiCxNy) with specific composition’ has become one of the most important issues for gate electrode manufacturing process. In this research, the TiCxNy film for DRAM gate electrode application (low resistivity (< 1,000 μΩ.cm), controllable work function (Δ Vfb ≈ 0.2 V) and enough thermal stability (up to 1,000 oC)) is deposited using ALD process. Precursor and reactive gas evaluation, mechanism confirming and process development are also achieved in the research. As a first step of the research, thermal stability and ALD process of the TDMAT, TEMAT and TDEAT precursor is studied. The decomposition temperature of TDMAT, TEMAT and TDEAT is confirmed by in situ Fourier transform infra-red spectroscopy (in situ FT-IR) as 175 oC, 200 oC, and 250 oC on the SiO2 surface, respectively. In the research, it can be newly confirmed that thermal stability of the precursor relates with its ligand. It can be found in the IR spectra that the ethyl ligand in the precursor shows a better thermal stability than methyl ligand. This result means the ligand thermal stability defines its precursor thermal stability. The thermal stability of the precursor affects ALD process and the film property deposited using ALD process. The precursor decomposition temperature defines ALD window upper limit and the film property is dramatically changed after decomposition temperature. The major change of the film property can be found in film composition. As the precursor decomposes, carbon is incorporated into the film as a residue. Generally, carbon residue gives many disadvantages (increase of film resistivity, degradation of film crystallinity) to the film. However, in this research, the film has more carbon shows lower resistivity. In this research, it is confirmed that the film cannot be stoichiometric structure (Ti: N or C = 1:1) and it has plenty of vacancy in the TiCxNy ALD case. The oxygen easily permeates into vacancy and forms insulating oxide phase. It is proved that more than two third of the carbon in the TiCxNy film deposited using ALD process forms TiC phase and blocks the vacancy that the major cause of the increase of film resistivity. The effect of reactive gas is studied using plasma enhanced ALD (PEALD) process. As shown in precursor evaluation, carbon blocks the vacancy and decreases the film resistivity. To increase carbon content in the film, H2/CH4 PEALD process is suggested and the process is tested with NH3 and H2 PEALD processes. As expected, H2/CH4 PEALD process increases TiC phase in the film and leads a decrease of film resistivity. The film deposited using H2/CH4 PEALD shows lower resistivity than traditional poly-Si (1,000 μΩ.cm) and it is very similar with the film resistivity deposited using NH3 PEALD. However H2/CH4 TiCxNy film shows very different result with NH3 TiN film in the flat band voltage. The flat band voltage of the TiCxNy film is located more negative position than TiN position. It is related with work function of the phases and indicates that threshold voltage of the film can be controlled by reactive gas. The film deposited using H2 PEALD shows much higher resistivity than the other processes due to lot of vacancy and high oxygen concentration. Lastly, the film deposited using H2/CH4 and NH3 PEALD process is annealed to prove thermal stability for DRAM application. The resistivity of the film slightly decreases as annealing regardless of the process due to recrystallization. However, TiN film deposited using NH3 PEALD process more recrystallize than TiCxNy film deposited using H2/CH4 PEALD process due to difference of activation energy for deformation between TiN and TiC phase. Recrystallization of the film also affects film thickness. A decrease of film thickness can be found in TiN film only, but not in TiCxNy film deposited using H2/CH4 PEALD process. However, the rate of decrease is around 10 % and the TiN film thickness is very thin (< 5 nm) to introduce in the manufacturing, so it is affordable to manufacturing. The annealing process decrease C-V hysteresis of the film deposited using H2/CH4 PEALD process due to reduction of C-C phase, but the position of the C-V curve does not change. To confirm the origin of C-V position difference, work function of the film is measured using UPS. The work function of the TiCxNy film is smaller than work function of the TiN film. It clearly shows that the difference of work function leads flat band shift. The result in this paper deals with TiCxNy ALD process from precursor to application and suggests that TiCxNy film deposited ALD process can be used as gate electrode.