Influence of organic surface chemistry on the nucleation of plasma deposited SiOx films

Abstract

The nucleation and film growth of SiOx plasma polymer films as a function of the substrate surface chemistry are analysed by a combination of microscopic, spectroscopic and electrochemical techniques. Self-assembled organothiol monolayers (SAMs) surfaces with different terminating groups (methyl, carboxyl- and trimethoxysilane groups) on Au(111) serve as a model system. Ultra-thin SiOx films with thickness ranging from 0.4 to 1.4 nm were deposited by microwave plasma in a mixture of HMDSO and O2. The changes in surface, interface and thin film chemistry are characterized by PM-IRRAS. Cyclic voltammetry with ferricyanide as a redox system is used to probe the defect density of the bare SAMs and the SiOx-covered SAMs. Furthermore, the evolution of the SiOx surface morphology for increasing film thickness as function of the substrate chemical termination is investigated by AFM. A strong influence of the surface chemistry on the SiOx nucleation and film growth is observed. While the methyl and carboxyl terminated SAMs are degraded during the nucleation leading to defect rich ultrathin films, the trimethoxysilane group protects the aliphatic chain of the SAM and leads to much better barrier properties of the ultra-thin SiOx-films. A mechanistic explanation of the results is provided.