Time-dependent dielectric breakdown in poly-Si CVD HfO₂ gate stack

In this paper, we present a comprehensive study on long-term reliability of CVD HfO₂ gate stacks with n⁺-poly-Si gate electrodes. The area dependence and temperature acceleration (25-150°C) of TDDB, defect generation rate, and critical defect density of CVD HfO₂ gate stacks are studied. Results show that 10 year lifetime of HfO₂/n⁺-poly-Si gate stack (EOT=14.5Å) is projected for Vg=-2.0 V @25°C and Vg=-1.56 V @150°C. This excellent reliability characteristics of HfO₂ gate stack is mainly attributed to the thicker physical thickness of HfO₂, resulting in significant reduction of tunneling leakage current by a factor of 10³∼10⁴ while maintaining comparable Weibull slope factor. In addition, the critical defect density of HfO₂ gate stack is comparable to SiO₂ with similar physical thickness. However, considering the cumulative impact of temperature acceleration at 150°C, scaling of an effective gate oxide area of 0.1 cm² and a maximum allowed fraction of failures of 0.01%, the maximum allowed operating voltage is projected to be only ∼0.85 V for HfO₂/poly-Si gate stack with EOT=14.5Å.