Disclosed is a wiring structure and method of forming the structure with a conductive diffusion barrier layer having a thick upper portion and thin lower portion. The thicker upper portion is located at the junction between the wiring structure and the adjacent dielectric materials. The thicker upper portion: (1) minimizes metal ion diffusion and, thereby TDDB; (2) allows a wire width to dielectric space width ratio that is optimal for low TDDB to be achieved at the top of the wiring structure; and (3) provides a greater surface area for via landing. The thinner lower portion: (1) allows a different wire width to dielectric space width ratio to be maintained in the rest of the wiring structure in order to balance other competing factors; (2) allows a larger cross-section of wire to reduce current density and, thereby reduce EM; and (3) avoids an increase in wiring structure resistivity.