Solution Depositions of Copper and Copper Sulfide Thin Films: Effects of Thiourea

Electroless deposition is widely used in industrial processes for the metallization of organic layers. It is an attractive technique to deposit metal overlayers because it can be performed at low temperatures, is inexpensive, and is compatible with a wide range of materials. Bath additives are often employed to alter the morphology and properties of a deposited film as well as control the kinetics and mechanisms of the film growth. Thiourea is a common bath additive in electroless deposition and is commonly employed as a leveling agent and brightener to produce more uniform reflective films. We have investigated the role of thiourea on the electroless deposition of copper on −CH₃-, −OH-, and −COOH-terminated self-assembled monolayers (SAMs) adsorbed on gold using dimethylamine borane as a reducing agent. Using X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, optical microscopy, and scanning electron microscopy (SEM), we demonstrate that thiourea decreases the film deposition rate significantly. Further, for all bath pH studied, the deposit composition is strongly dependent on the concentration of thiourea in the bath. At <10^–2 M thiourea, copper films are deposited on all substrates and bath pH studies. At ≥10^–2 M, the deposited layers are copper sulfides. At pH 5 and pH 9, the deposit is composed of CuS for all functionalized SAMs studied, while at pH 12, the deposit is Cu₂S. We propose that the dominant role of thiourea changes with its concentration in the bath, leading to the observed strong dependence of the deposit composition on thiourea concentration. At low thiourea concentrations, it acts as a complexing and reducing agent, while at high concentrations (>10^–2 M), the dominant reaction is thiourea hydrolysis, leading to the formation of copper sulfides. Finally, we demonstrate a simple method to deposit layered Cu/Cu_xS and CuS nanowires by manipulating the thiourea concentration in the bath.