HfO2 Gate Stack Engineering by Post-Gate Cleaning Using NF3/NH3 Plasma
In this paper, we present preliminary findings on the fluorine incorporation into the HfO2 gate stack using post-gate dry cleaning technique with NF3/NH3 plasma as an engineering technique to improve the electrical property of the gate stack. For this study, HfO2 films were deposited onto p-type (100) Si wafers by atomic layer deposition (ALD) with H2O and TEMAH sources after removal of the native oxide. Post deposition annealing (PDA) was followed using rapid thermal process (RTP) at 600oC for 30sec in N2 ambient. Subsequently, the HfO2 samples were dry-cleaned using an indirect down-flow plasma system with NF3/NH3 gas mixture. The plasma was generated at 1 Torr with 50sccm of NF3 and 100sccm NH3 at different powers ranging from 30W to 150W for 30sec. Then, in-situ annealing was performed at 180oC for 2min in N2 to remove by-product. The chemical and physical properties of the 8nm-thick HfO2 films were examined using XPS and TEM measurements. Electrical characterizations were carried out with the MOS capacitors of the dry-cleaned HfO2 gate stacks employing TiN metal gate.
Fig.1 shows that fluorine atoms were incorporated into the HfO2 films by NF3/NH3 dry cleaning. The concentration of F was slightly increased and Si-O bonding was decreased with the increase of the plasma power. Any other significant changes in HfO2 chemical composition by the dry cleaning were not observed by the XPS measurements. TEM observations (Fig.2) show that the interfacial layer was suppressed significantly at 150W compared to the sample without dry cleaning. Electrical characterizations of I-V and C-V measurements of the MOS capacitors are summarized in Fig.3. There was no significant change in flat band voltage by the dry cleaning application and its conditions of plasma power. There is a report that the fluorine incorporation led to an oxide capacitance reduction and poor electrical characteristics in HfO2 gate stack.2 Compared to the previous study on the fluorine incorporation into the HfO2 gate stack, our experimental results may indicate that the active fluorine species produced from NF3/NH3 plasma reach to the HfO2/Si interface and break the vulnerable Si-O bonds making volatile SiFx compounds selectively and the degree of the interfacial reaction is increased by the power of the NF3/NH3 plasma.
In conclusion, we have found that a post-gate dry cleaning using the NF3/NH3 plasma can reduce the interfacial low-k SiOx layer without any serious degradation of the electrical properties of the HfO2 gate stack. And we suggest that the interfacial layer of the HfO2 gate stack can be controlled with process parameters of the novel NF3/NH3 dry cleaning process technique.
1. W. C. Wu, et al., J. Electrochem. Soc., 154, H561 (2007).
2. J. C. Lee, et al., Microelectron. Eng., 88, 1417 (2011).