Ms. Ekaterina Zoubenko, PhD. Candidate
The introduction of high aspect ratio structures for logic and memory MOSFET devices calls for the development of effective metallization processes. Gate metallization is particularly challenging since it requires not only a conformal deposition of low resistivity metals with high thermal and chemical stability but also control of the effective work function (EWF) on the dielectric. Molybdenum is considered as a promising alternative to the current tungsten-based technology due to its competitive properties and easier deposition.
This research, performed in collaboration with Lam Research Inc., studies Mo-based gate metallization and its modification by annealing in various environments. Two structures are investigated: CVD Mo on top of an ALD tungsten carbo-nitride (WCN) liner and ALD Mo deposited on the gate dielectric. The EWF values on SiO2 and Al2O3 are correlated with the structural and compositional changes of the metal/dielectric interface.
For Mo on WCN liner, EWF changes are correlated with metal composition at the interface: an EWF of 4.8 eV is attributed to a W-rich interface, while Mo diffusion towards the interface reduces the EWF by 0.2 eV. The electrostatic effect resulting from depositing Al2O3 on top of SiO2 of 0.4 eV (Δ), which was previously reported by our group, is observed for the as-deposited stacks. Different interfacial reactions occurring upon annealing, such as MoO3 formation at the ALD Mo/Al2O3 interface but not at the interface with SiO2, lead to Δ elimination. A model of the interfacial processes is discussed in correlation with the EWF trends.