Fluorine segregation influences the intrinsic reliability of thin gate oxides in poly-Si/SiO2/Si structures. We analyze diffusion and segregation kinetics of F in gate stacks with 5 nm gate oxides using secondary ion mass spectroscopy. Well defined doses of F were introduced by ion implantation. We find that F segregation at interfaces to the gate oxide is diffusion limited with an effective activation energy of 1.4 eV. For F concentrations lower than 5×1015 cm-2, F segregation is beneficial improving, for example, the tails of the Weibull plots and slightly increasing the breakdown field. For F doses higher than 1×1016 cm-2, detrimental consequences were found, degrading the charge to breakdown values by about a factor of 5 after long-term thermal treatments. From ab initio density functional theory pseudopotential calculations for atomic structures and total energies of segregated F, we conclude that the segregation to the interface is driven mainly by the energy difference between Si–O and Si–Si bonds in the interfacial region, by oxygen assisted strain relaxation around Si atoms attacked by F and, possibly, by kinetic bottlenecks in diffusion of network oxygen through SiO2. © 2001 American Institute of Physics.