An interface dipole model explaining threshold voltage (Vt) tuning in HfSiON gated n-channel field effect transistors (nFETs) is proposed. Vt tuning depends on rare earth (RE) type and diffusion in Si∕SiOx∕HfSiON∕REOx/metal gated nFETs as follows: Sr<Er<Sc+Er<La<Sc<none. This Vt ordering is very similar to the trends in dopant electronegativity (EN) (dipole charge transfer) and ionic radius (r) (dipole separation) expected for a interfacial dipole mechanism. The resulting Vt dependence on RE dopant allows distinction between a dipole model (dependent on EN and r) and an oxygen vacancy model (dependent on valence).
A La2O3 capping scheme has been developed to obtain n-type band-edge metal gates on Hf-based gate dielectrics. The viability of the technique is demonstrated using multiple metal gates that normally show midgap work function when deposited directly on HfSiO. The technique involves depositing a thin interfacial of La2O3 on a Hf-based gate dielectric prior to metal gate deposition. This process preserves the excellent device characteristic of Hf-based dielectrics, but also allows the realization of band-edge metal gates. The effectiveness of the technique is demonstrated by fabricating fully functional transistor devices. A model is proposed to explain the effect of La2O3 capping on metal gate work function.
Metal gate work function enhancement using thin AlNx interfacial layers has been evaluated. It was found that band edge effective work functions (∼5.10eV) can be achieved on hafnium-based high dielectric constant (high-k) materials using the AlNx interfacial layer and TiSiN electrodes. It was also found that the effective work function enhancement by the AlNx interfacial layer increased when the concentration of SiO2 in the gate dielectric was increased. Thus, the enhancement was minimal for HfO2 and maximum for SiO2. A model is proposed to explain these results and a bonding analysis is presented to support the proposed model.
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