Solution Chemistry of Alkaline Formaldehyde-Containing Electroless Copper Plating Systems and Kinetics of Electroless Copper Deposition

Abstract

Electroless copper plating process using solutions containing formaldehyde as reducing agent is known from the middle of this century and are widespread in the practice up to now. However, the investigation of this system remains actual because of its complexity, e. g. alkaline medium, hydrated form of formaldehyde (methanediol), Cu(II) complex compounds with different ligands, additives which improve the properties of the coatings obtained and/or protect from the reduction of copper(II) in the solution bulk. Moreover, various equilibria are possible including deprotonation of methanediol, Cu(II) complex formation, additional interactions between components of the solution.The aims and tasksof this work were to investigate or define more precisely equilibria in conventional alkaline solutions of electroless copper deposition as well as influence of the nature of Cu(II) ligands on the copper deposition process. This research report deals with the results obtained by means of dc-polarography, vis-spectrophotometry, voltammetry, potentiometry, pH-metry, 1H and 13C NMR spectroscopy, scanning electron microscopy, X-ray diffractometry and gravimetry.The composition, stability and the values of the diffusion coefficient of Cu(II) complex compounds with ligands (EDTA, DTPA, pyridine-2,6-dicarboxylic acid and 4-hydroxypyridine-2,6-dicarboxylic acid, Quadrol, L(+)-, D(–)- and DL(-/+)-tartrate, glycerol, saccharose and OH- ions) which are used or usable in alkaline electroless copper deposition solutions were established. Formation of mixed complex compounds between Cu(II) complexonates andK4Fe(CN)6, as well as between Cu(II) tartrate complexes and methanediol was investigated. The interaction of methanediol with L(+)-tartrate alongside with the influence of the ionic strength and cation nature on the deprotonation of methanediol in alkaline solutions were studied.The kinetic data on electroless copper deposition from solutions containing above-mentioned ligands are presented and the influence of the ligand nature on the process of electroless copper deposition is discussed.