Fluorine was introduced by ion implantation into MOS structures with sputtered oxide. After annealing, its effects on the high-frequency C-V curve and Fowler-Nordheim tunnelling barrier height were studied. An increase of acceptor dopant concentration in the substrate was observed due to the fluorine implantation. The presence of fluorine increased the Fowler-Nordheim tunnelling barrier height for electron injection from both metal and silicon. The role of fluorine is explained by a physical model similar to that used by Ma, where fluorine is assumed to bond with trivalent silicon trap centres in the sputtered oxide.