Kinetics of Electroless Copper Plating and Mechanical Properties of Deposits

Abstract

Electroless copper films were prepared from an EDTA bath in which pH was adjusted with three kinds of alkalis (LiOH, NaOH, and KOH). Mechanical properties of the deposits and the kinetics of electroless copper plating were investigated to clarify their mutual relationship. The use of LiOH, among the alkalis tested, gave the best result for the mechanical properties. The electrochemical analysis of the electroless copper deposition based on the mixed potential theory indicates that there is an interaction between the partial anodic and cathodic reactions, which is induced by the adsorption of methylene glycol anion. The polarization technique using a quartz crystal microbalance electrode and the AC impedance method isclose that an improvement of the mechanical properties of copper films is achieved when such an interaction is eliminated by preventing the adsorption of methylene glycol anion through utilizing LiOH and surfactant such as polyethylene glycol as well as elevating the bath temperature. In the field of electroless copper plating that can be ap-plied to printed circuit boards, research and development directed toward improving bath stability and mechanical properties of the deposits has been conducted intensive-ly. 14 In particular, a further improvement in the mechani-cal property is highly required in the process of the fully additive technique, in which the circuit wiring was achieved only by electroless copper plating. As the proper-ties of electroless copper deposits are influenced by various factors, the relationship between the kinetics and the prop-erties of the deposits till remains obscure. The influences of pH, temperature, and stabilizer have been well studied, however, there are few reports on the effects of alkali metal cations except for NaOH. 6 In this study, the mechanical properties of copper de-posits and the kinetics of electroless copper plating were investigated to clarify the mutual relationship for various electroless copper baths containing a different kind of alkali.