A metal interconnect structure and a method of manufacturing the metal interconnect structure. Manganese (Mn) is incorporated into a copper (Cu) interconnect structure in order to modify the microstructure to achieve bamboo-style grain boundaries in sub-90 nm technologies. Preferably, bamboo grains are separated at distances less than the “Blech” length so that copper (Cu) diffusion through grain boundaries is avoided. The added Mn also triggers the growth of Cu grains down to the bottom surface of the metal line so that a true bamboo microstructure reaching to the bottom surface is formed and the Cu diffusion mechanism along grain boundaries oriented along the length of the metal line is eliminated.