Role of the Reducing Agent in the Electroless Deposition of Copper on Functionalized SAMs

Metallized organic layer constructs have a wide range of technological applications. Electroless deposition is an attractive technique by which to deposit metal overlayers because it is inexpensive and can be performed at low temperatures, compatible with organic materials. Amine borane reducing agents are versatile and are capable of depositing metals, semiconductors, and even insulators. We have investigated the role of amine borane reducing agents in the electroless deposition of copper on -CH₃-, -OH-, and -COOH-terminated SAMs adsorbed on gold using time-of-flight secondary ion mass spectrometry, optical microscopy, and complementary MP2 calculations. Three reducing agents were studied: amine borane, dimethylamine borane, and trimethylamine borane. At pH >9, -COOH-terminated SAMs form copper-carboxylate complexes, which serve as nucleation sites for subsequent copper deposition. The rate of copper deposition is dependent on the strength of the B-N bond of the amine borane reducing agent. Similarly, if the terminal group is nonpolar such as a -CH₃ functionality, then the rate of copper deposition is dependent on the amine borane B-N bond strength. However, in contrast to -COOH-terminated SAMs, copper deposition does not begin immediately. If the terminal group contains polar bonds, such as the C-OH bond of -OH-terminated SAMs, deposition is dominated by the interaction of the reducing agent with the terminal group rather than the relative bond strengths of the amine borane reducing agents.