Seedless micropatterning of copper by electroless deposition on self-assembled monolayers

Abstract

A novel approach for direct electroless (EL) patterning of copper films without catalysts has been developed. In a special electroless solution employed in the present study, the formation of copper particles was observed and the negatively charged surfaces were confirmed by zeta potential measurements. The conditions of the solution (ionic strength, pH, temperature, additives etc.) greatly affected the composition of the resulting precipitates. Results showed that pure copper films formed in EL solutions with pH higher than 7.3. A self-assembled monolayer (SAM) terminated with either NH₂ or OH headgroups was employed as a template to enhance the selective adhesion of copper particles. As expected, many particles were deposited on the positive NH₂-SAM by electrostatic attraction, while few particles were deposited on the negative OH-SAM due to repulsive interaction. The measurement results obtained by AFM, QCM, XRD and optical microscopy confirmed the selective formation of copper films on the NH₂-SAM. Subsequently, micropatterns of copper films with high resolution were successfully produced. The deviation in the thickness of the deposit reached 110 nm between the NH₂-SAM and OH-SAM after 90 min soaking in the EL solution. Current–voltage characteristics revealed relatively high resistivity (0.3 mΩ cm) of the formed copper film on the NH₂-SAM, which can be ascribed to the poor connectivity among particles.