We propose and study an integrated refractive index sensor which is based on a plasmonic slot cavity integrated on silicon-on-insulator wafers. In this device, the guided mode isvertically coupled to the cavity which is a metal-dielectric-metal waveguide and is separated from a photonic wire waveguide by a silicon dioxide spacer. We perform an in-depth study that links the geometrical parameters to the coupling and sensitivity. The strong coupling from the dielectric waveguide to the plasmonic slot waveguide cavity allows a localchange in refractive index to be detectable with a high sensitivity of around 600 nm/RIU in a femto-liter volume. These results are obtained with three-dimensional time domain simulations made with the CST Microwave Studio. The sensing performance of the devices are presented and compared to the practical needs to achieve localized biomolecular sensing