Failure mechanisms of PVD Ta and ALD TaN barrier layers for Cu contact applications

PVD Ta-based and ALD TaN layers were studied as Cu diffusion barriers on poly-silicon, NiSi and CoSi₂ for Cu contact applications. The effectiveness of nanometer-thick layers, deposited in manufacturing compatible chambers on 200 and 300 mm wafers, is evaluated by detection of Cu-silicidation temperature using high temperature in situ XRD. It is found that Si diffuses into the α-Ta lattice for PVD barriers between 300 and 500 °C, and induces Ta silicidation at 600 °C. The agglomeration of TaSi₂ seems to be responsible for the damage of barrier continuity and cause subsequent Cu-silicidation. The growth of ALD TaN on different surfaces of NiSi was studied by XRF, RBS and XRR. The growth curves show excellent linearity as a function of thickness. TOF-SIMS shows closed layers after 60 ALD cycles. In situ XRD reveals that the failure temperature of 4 nm thick ALD layers is higher than 500 °C. It is found that the failure of 3 and 4 nm ALD TaN layers in Cu/barrier/NiSi stacks is a diffusion controlled process, with an activation energy Q of ∼2.2 eV and a pre-exponential factor D₀ of ∼3.8 × 10⁻³ cm²/s.