This article investigates the breakdown of high-k/metal gate stacks under substrate injection regime. It is shown that it is initiated by a field driven breakdown of the interfacial layer (IL). Furthermore, because it is very sensitive to IL thickness variations, T-BD optimization supposes a control of the high-k deposition process that can induce IL thickness fluctuations. (C) 2009 American Vacuum Society. [DOI: 10.1116/1.3077185]