The effect of nickel on the strain evolution in chemical copper films

Chemical (or electroless) copper films are deposited from an electrolyte on palladium-activated insulating substrates in order to construct electrical interconnects for electronic components. These films provide the electrical contacts that are required for subsequent galvanic copper plating. As smoother substrates are required for advanced applications, achieving sufficient film adhesion becomes more difficult. Nickel, initially added to the electrolytes in order to improve deposition speed, has increasingly become important to promote good film adhesion. Film stress strongly affects film adhesion. Here we studied the effect of nickel addition on the internal strain of the film during and after electroless deposition by monitoring the strain of the Cu crystals with X-ray diffraction. Films without nickel tend to have exponential relaxation of the film strain after the deposition. For a 2000 wt. ppm Cu electrolyte, about 30 wt. ppm Ni was required in order to prevent this from occurring. Films with higher nickel content have a columnar structure with reliable and constant tensile strain during and after the electroless deposition.