Mechanism of O2-anneal induced Vfb shifts of Ru gated stacks
Introduction
There is a large interest to incorporate metal gates in future MOS devices to eliminate the gate depletion and dopant penetration effect of poly-Si. However, it is well known that the work function (WF) of a metal gate can differ significantly from its vacuum value and can shift significantly after thermal treatment [1]. Many models, including metal induced gate states (MIGS) [2] and dipole formation [3] have been employed to explain the difficulties to control the WF of metal gates. Recently, Schaeffer et al. [4] and Cartier et al. [5] argued that oxygen vacancies in HfO2 may be responsible for the flat-band voltage (Vfb) shift after thermal treatment for noble metal gate based stacks, including Pt, Re and Ru. In this work, we study the mechanism behind the O2-annealing induced Vfb shift for Ru gated stacks in detail. We show that the WF of Ru gated stacks can be modulated by forming a thin RuOx layer at the Ru/dielectric interface in O2 ambient.
Section snippets
Experiment
Large area MOS capacitors were fabricated on 8-in. n-type or p-type substrates. For WF extraction, 15 nm SiO2 was etched to a 3–10 nm wedge (slant SiO2) on a single wafer in some cases. HfO2 (2 nm) was deposited on chemical SiO2 or slant SiO2 by atomic layer deposition (ALD) with an O2 post-deposition anneal (PDA) applied afterwards. Ru was deposited by physical vapor deposition (PVD) or atomic vapor deposition (AVD® [6]). To study the distribution of oxygen in the stack after O2 anneal, some
Results and discussion
Fig. 1 demonstrates the Vfb shift of Ru gated stacks (90 nm Ru /HfO2 here): as compared to the FGA annealed sample, an additional annealing in O2 (500 °C 1 min) shifts the Vfb to a ∼300 mV higher value. To verify that the Ru film was not totally transformed to RuOx during O2 anneal, X-ray photoelectron spectroscopy (XPS) was used to study the distribution of oxygen in Ru after O2 anneal. As shown in Fig. 2, although the Ru surface was oxidized after O2 anneal, the bulk of Ru film remained
Conclusions
In this work, we have found that oxygen anneals on both Ru/HfO2 and Ru/SiO2 gate stacks result in a significant increase in WF. An anneal in 18O2 followed by TOF-SIMS depth profiling indicated the formation of a RuOx layer at the interface between the Ru bulk and the dielectric. The observed WF shift is explained in terms of a change of the material in contact with the dielectric, without the need to resort to an oxygen vacancy model. This investigation showed that the material (even if sub-nm
Acknowledgements
This work was supported by the IMEC industrial affiliation program on high-k/MG. The authors thank Canon Anelva and AIXTRON for Ru deposition and W. Vandervorst for useful discussion.
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