Nickel dependence of hydrogen co-deposition and nanoporosity in electrolessly deposited Cu-films

Hydrogen is inevitably produced during electroless Cu plating, it accumulates on the Cu-surface, forms gas bubbles and sticks on the as-growing Cu surface. The encapsulation of these gas-bubbles in the electroless Cu layer results in nanoporous, low ductile and compressively stressed deposits with a high blister tendency, this process being known as "hydrogen embrittlement". In this context, the present paper investigates the nanovoid density in electroless Cu films as a function of Ni0 co-deposition. The Ni0 co-deposition was varied by ranging the Ni2+-concentration in the bath from 0 ppm to 2000 ppm, as well as by using different single stabilizer components at a constant Ni2+-concentration of 400 ppm in the bath. A standardized and optimized sample preparation utilizing focused ion beam(FIB) and scanning electron microscopy (SEM) inspection enables automatic image processing of the SEM-micrographs to quantify the void density of the deposits down to a void size of 10 nm. A minimum addition of 30 ppm Ni2+ in the electroless Cu bath prevents the encapsulation of hydrogen bubbles during Cu growth and leads to solid, nanoscopically defect-free layers, which have been confirmed by transmission electron microscopy. Additionally, deposits with a high Ni0-incorporation up to the miscibility limit of Ni in Cu (4 at. %) are nanoscopically defect-free. In contrast to Ni2+, the addition of single organic additives, commonly used to stabilize the electroless Cu bath, provokes a substantial increase of the nanoporosity of the deposit. The results were discussed in terms of a different Cugrowth mode caused by the co-deposition of Ni0. The results will have implications on the development of low/no-Ni electroless Cu baths, e.g. EDTA-based baths, as well as to applications where the integrity and thermo-mechanical stability of electroless Cu plays a crucial role.