Metal-gate/high-k dielectric stack engineering by atomic layer deposition: Materials issues and electrical properties

Scaling of silicon devices beyond the 65nm node requires the replacement of poly-Si/SiO2 gate stack with a metal gate/high-k dielectric stack that can potentially reduce the off-state leakage current without compromising device performance. In this study, we have evaluated HfO2 as the gate dielectric material and TaN as the metal gate electrode deposited using the Atomic Layer Deposition (ALD) technique. ALD provides us with the unique capability of growing high quality thin films with excellent control over the film thickness, stoichiometry and conformality. Nanoscale HfO2 films were deposited by ALD on Si substrates using two different precursor chemistries---HfCl4 and Tetrakis (Diethylamido) Hafnium (TDEAH) with H2O as the oxidant. A systematic study of the physical and electrical properties of the HfO2 films derived using the two different chemistries revealed that the magnitude and sign of the fixed charge in the dielectric varied depending on the precursor used for the deposition. Our results indicate that Cl impurities, left behind by the HfCl4 precursor during deposition, significantly impact the electrical properties of the chloride-HfO 2.; Tantalum nitride thin films were deposited by a remote plasma-enhanced ALD (PEALD) method using a novel metal organic precursor---isopropylimino tris(ethylmethylamino) tantalum (IPTEMT). It was identified that the as-deposited tantalum nitride film was the dielectric Ta3N5 phase. High temperature in-situ anneals performed in the TEM column crystallized the ALD tantalum nitride film at 850°C into the stoichiometric cubic TaN phase. The phase transformation from dielectric Ta3N5 to metallic TaN is achieved by out-diffusion of excess nitrogen atoms from the Ta3N5 film during the high temperature anneal. TaN was evaluated as a potential gate electrode material both on SiO2 and HfO2 gate dielectrics. The impact of high temperature anneals on the mobility of hydroxyl and oxygen impurities in the stack and its effect on the thickness of the interfacial layer was studied in detail. A novel low temperature process was identified to engineer the TaN/HfO2 gate stack using a reactive titanium metal overlayer to getter the excess nitrogen from Ta3N5.