Barrier layer stack to prevent Ti diffusion

A method for forming a metallic inter-diffusion resistant barrier layer stack for preserving an AlCu interconnect resistivity comprising the steps of:

• providing a semiconductor process wafer comprising a metal interconnect opening formed to extend through a thickness of at least one dielectric insulating layer to form closed communication with an underlying conductive region, said underlying conductive region comprises an AlCu alloy;
• forming a titanium nitride (TiN) barrier layer over the conductive region to contact with the conductive region and line the metal interconnect opening;
• forming at least one sequence of a titanium (Ti) barrier layer followed by a TiN barrier layer to end with an uppermost TIN barrier layer to form a barrier layer stack;
• forming an AlCu layer over the barrier layer stack to fill the metal interconnect opening;
• annealing the AlCu layer at a temperature of about 300° C. to about 450° C.