Nanoscale characterization of thin immersion silver coatings on copper substrates

Microelectronic-grade copper foils were immersion silver plated in a home-made non-cyanide alkaline silver nitrate thiosulfate solution and in two commercially available industrial baths via contact reductive precipitation. The concentration depth profiles of the freshly deposited silver layers were afterwards analyzed at nanoscale resolution by means of secondary neutral mass spectrometry (SNMS) and glow discharge optical emission spectroscopy (GDOES). The thickness of the deposited silver layers obtained was in the range of 50 to 150 nm, depending on the parameters of the immersion procedure. Slight contamination of sulfur from the thiosulfate bath was detectable. Traces of chromium and sodium could be observed as well around the interface between the copper substrate and silver deposit. The results also indicate that storage for longer time in air, especially at higher than ambient temperatures, induces a kind of aging effect in the deposited layer, changing its composition. The samples were also analyzed by x-ray photoelectron spectroscopy (XPS) to identify the chemical state of the silver.