Effect of Ethanolamine Bath Additives in Copper Electroless Deposition on Functionalized Self-Assembled Monolayers: From Area-Selective Deposition to Nanowire Formation

Electroless deposition (ELD) is commonly used in industry to deposit metals and other materials on a wide variety of substrates. Bath additives are often added to deposition baths to control the deposition process and kinetics, as well as deposit properties. We have investigated the role of ethanolamine additives in Cu ELD using dimethylamine borane (DMAB) on -OH- and -CH₃-terminated alkanethiolate self-assembled monolayers (SAMs) using the tape adhesion test, optical microscopy, secondary electron microscopy, X-ray photoelectron spectroscopy, and time-of-flight secondary ion mass spectrometry (TOF SIMS). Three additives were investigated: monoethanolamine, diethanolamine, and triethanolamine. Our data show that these bath additives can be used to control deposit morphologies and adhesion using ELD. Using monoethanolamine or triethanolamine, copper nanowires are formed at the junction of patterned -OH/-CH₃-terminated SAMs. Further, by reducing the concentration of monoethanolamine or triethanolamine in the deposition bath to <0.15 M, the deposited nanowires did not adhere to the SAM substrate, indicating that there is a critical coverage of ethanolamine required for copper film adhesion. Further, we demonstrate that the nanowires can be transferred to other substrates such as silicon. In contrast, using diethanolamine, area-selective deposition is observed in the -CH₃-terminated SAM area. TOF SIMS data suggest that monoethanolamine and triethanolamine adsorb vertically (or at an angle), with the amine terminal group interacting with the -CH₃-terminated SAM and leaving the hydroxyl groups free to interact with the deposition bath components. This causes repulsion of the DMAB reducing agent, hindering deposition on the -CH₃-terminated SAM areas, leading to nanowire formation. In contrast, diethanolamine adsorbs flat on -CH₃-terminated SAMs, and no nanowire formation is observed; rather, area-selective deposition occurs.