Properties of ALD TaxNy films as a barrier to aluminum in work function metal stacks.

Atomic layer deposited (ALD) tantalum nitride (Ta_xN_y) is evaluated as a barrier against aluminum inside gate metal stacks of metal-oxide-semiconductor field effect transistor (MOSFET) devices. When deposited on hygroscopic oxides, like HfO₂, amorphous tantalum nitride (a-Ta_xN_y) is obtained, while deposition on Si or TiN results in polycrystalline Ta₃N₅. The low conductivity of both phases is not attractive for gate metal applications; however, a-Ta_xN_y is crystallized to bixbyite Ta₂N₃ at 500 °C, improving its conductivity to ∼130 Ω⁻¹ cm⁻¹. For thicknesses below 10 nm, crystallization did not happen, but thin a-Ta_xN_y barriers still obtain conductivity improvements to ∼500 Ω⁻¹ cm⁻¹ when Al diffuses into the film. In metal gate stacks, a-Ta_xN_y screens the low work function of ALD TiAl more effectively than TiN. A barrier thickness reduction of 50% is achieved for n-MOSFET devices with an effective work function at 4.2–4.3 eV and low gate leakage. Slower diffusion of Al into Ta_xN_y is observed by secondary ion mass spectroscopy; however, the cause of EWF lowering as a result of Al diffusion could not be confirmed. Instead, restoration of high EWF after removal of TiAl occurs, enabling an NMOS-first process integration with the use of 1 nm thin Ta_xN_y barriers. [ABSTRACT FROM AUTHOR]