Electroless Deposition of Cobalt by Coordination and Reduction of Palladium Catalyst by Surface Amines Groups

Abstract

Solution-deposited palladium on amine-terminated self-assembled monolayers (SAMs) is a well-characterized catalyst and adhesion layer combination for electroless metallization of dielectric films. A reducing agent is typically added to the deposition bath or a sensitizer such as tin is co-deposited producing relatively thick Pd layers. Thinner Pd deposits would enable barrier seed layers for filling < 10 nm wide gaps in patterned dielectric films with metal. In this work, we eliminated the reducing agent from the deposition bath and worked at pH < 2 to deposit monomeric Pd(2+) species and show that the amine groups terminating the SAM reduce Pd(2+) to Pd(0). The amount of Pd deposited depended on the coverage of the two types of amines on the SAM. The adsorption of PdCl42- ions in solution on protonated amine groups (–NH3+) is well known. Our data suggest that the nonprotonated amine groups (–NH2), which coexist with –NH3+, chemically reduce the Pd(2+) ion to Pd metal by oxidizing to the amine radical cation (–NH2•+). Pd bonds to and covers the –NH2•+ groups in the process depositing around a monolayer of Pd from solution on the SAM-covered silicon oxide surface. The Pd layer served as a catalyst for solution deposition of cobalt films on the surface using a reducing agent under oxygen-free conditions. The cobalt deposited initially as islands that grew together into a closed film with good adhesion.