Nanoscale Characterization of Thin Immersion Silver Coatings on Copper Substrates
The rather long history of silver plating, both by electrodeposition and via immersion plating, started with the cyanides. Though such compounds are rather hazardous, they are still used today for many metal plating tasks in the microelectronic industries. New trends, aim at replacing the cyanide. Using new complexing agents and additives in the newer industrial immersion silver baths, there should be less concern that immersion silver processes would be inappropriate or less advantageous than gold in high-tech applications in the microelectronic industries.
Tamás I. Török,* Éva Kun & Dániel Sós
University of Miskolc
Attila Csik, József Hakl, Kálmán Vad, László Kövér, József Tóth & Sándor Mészáros
Institute for Nuclear Research
Hungarian Academy of Sciences
Editor’s Note: This paper is a peer-reviewed and edited version of a presentation delivered at NASF SUR/FIN 2014 in Cleveland, Ohio on June 9, 2014. A printable PDF version is available by clicking HERE.
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.
Keywords: silver, copper, displacement, immersion coating, GDOES, SNMS, depth profiles
The rather long history of silver plating, both by electrodeposition and via displacement reactions (so-called immersion plating), using aqueous solutions of soluble silver compounds, started with the cyanides. Though such compounds are rather hazardous, they are still used today for many metal plating tasks, even in the microelectronic industries. New trends, however, aim at replacing the cyanide anions with other complexing agents (Table 1) in both electroplating and immersion silver plating. In some cases, the latter technique is becoming favored as a surface finishing alternative to protect the surface of conducting copper layers from excessive oxidation, e.g., before lead-free tin base soldering in many applications, such as printed circuit boards (PCB) (Table 2).
Table 1 - Non-cyanide complexing agents proposed to replace cyanide in silver plating baths.1