A method of forming an interconnect structure, comprising forming a first interconnect layer (123) embedded in a first dielectric layer (118), forming a dielectric tantalum nitride barrier (150) by means of atomic layer deposition on the surface of the first interconnect (123), depositing a second dielectric layer (134) over the first interconnect (123) and the barrier (150) and etching a via (154) in the dielectric layer (134) to the barrier (150). The barrier (150) is then exposed to a treatment through the via (154) to change it from the dielectric phase to the conductive phase (180) and the via (154) is subsequently filled with conductive material (123).