HfO2 Gate Stack Engineering by Post-Gate Cleaning Using NF3/NH3 Plasma

HfO₂ has been one of the most important subjects of intensive research in recent years due to its outstanding material properties making it suitable for several industrial applications in the field of semiconductor electronics, especially for the high-k gate stack application. Recently, the fluorine incorporation into the HfO₂ gate stack has been suggested for the sake of defect passivation of the gate stack.^1-2

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 600^oC 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 180^oC 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 SiF_x 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 SiO_x 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).

3. H. Ogawa, et al., Jpn. J. Appl. Phys.,41, 5349 (2002).