Scaling the MOSFET gate dielectric: From high-k to higher-k? (Invited Paper)

摘要

We discuss options for metal-oxide-semiconductor field-effect transistor (MOSFET) gate stack scaling with thin titanium nitride metal gate electrodes and high-permittivity ('high-κ') gate dielectrics, aimed at gate-first integration schemes. Both options are based on further increasing permittivity of the dielectric stack. First, we show that hafnium-based stacks such as TiN/HfO_2 can be scaled to capacitance equivalent thickness in inversion (T_(inv)) of 10 A and equivalent oxide thickness (EOT) of 6 A by using silicon nitride instead of silicon oxide as a high-κ/channel interfacial layer. This is based on the higher dielectric constant of Si_3N_4 and on its resistance to oxidation. Although the nitrogen introduces positive fixed charges, carrier mobility is not degraded. Secondly, we investigate whether Ti-based 'higher-κ' dielectrics have the potential to ultimately replace Hf. We discuss oxygen loss from TiO_2 as a main challenge, and identify two migration pathways for such oxygen atoms: In addition to well-known down-diffusion and channel Si oxidation, we have newly observed oxygen up-diffusion through theTiN metal gate, forming SiO_2 at the poly-Si contact. We further address the performance of Si_3N_4 and HfO_2 as oxygen barrier layers.