Replacement metal gate stack for diffusion prevention

A method of forming a semiconductor structure includes depositing a gate dielectric layer lining a recess of a gate structure formed on a substrate with a first portion of the gate dielectric layer covering sidewalls of the recess and a second portion of the gate dielectric layer covering a bottom of the recess. A protective layer is deposited above the gate dielectric layer and then recessed selectively to the gate dielectric layer so that a top surface of the protective layer is below of the recess. The first portion of the gate dielectric layer is recessed until a top of the first portion of the gate dielectric layer is approximately coplanar with the top surface of the protective layer. The protective layer is removed and a conductive barrier is deposited above the recessed first portion of the gate dielectric layer to cut a diffusion path to the gate dielectric layer.