Breakdowns in high-k gate stacks of nano-scale CMOS devices

New failure mechanisms associated with breakdown in high-k gate stack consisting of HfO2/SiOx bilayered structure are presented. In addition to dielectric-breakdown-induced epitaxy (DBIE) commonly found in breakdowns in poly-Si/SiOxNy and poly-Si/Si3N4 MOSFETs, grain-boundary and field-assisted breakdowns near the poly-Si edge are found. A model based on breakdown induced thermo-chemical reactions has been developed to describe the physical microstructural damages triggered by breakdown in the high-k gate stack and the associated post-breakdown electrical performance. Some abnormal post-breakdown electrical behaviors such as recouping of the transistor's saturated drain current, percolation resistance and transconductance are found to be common for high-k MOSFETs. Grain-boundary enhanced breakdown in annealed HfO2 films is of critical importance in degrading the reliability of high-k gate stacks.