EQCM and EIS characterization of electrochemical deposition of tin from aqueous solution containing pyrophosphate

Abstract

Electroless plating of tin layers with thicknesses of more than 3 μm is becoming an important process in the production of circuit boards and semiconductor. Phosphonates constitute promising complexing agents for autocatalytic tin electrolytes. However, detailed time-resolved investigations or electrochemical impedance spectroscopy (EIS) measurements of tin deposition in this system are lacking. Here, deposition of tin was investigated by electrochemical quartz microbalance (EQCM) and electrochemical impedance spectroscopy. EQCM investigations showed a strong drop in deposition rate within the first ten minutes. Bath parameters had a significant effect on the drop of deposition rate. The results indicated an inhibitive pyrophosphate adsorption on the tin electrode surface which caused the observed drop of deposition rate. Impedance measurements confirmed this assumption. The equivalent circuit applied for the analysis of EIS data, included an increasing adsorbate resistance Rp, which can be related to the thickness of an adsorbed permeable pyrophosphate layer. Impedance measurements at selected frequencies revealed a linear relation between deposition rate and conductance 1/Rp. Subsequently, gluconate substituting for pyrophosphate was tested as complexing agent. Combined EQCM and EIS measurements during deposition using a gluconate containing electrolyte showed a stable rate with an invariant conductance.