Hydrophobic self assembly molecular layer for reliable Cu-epoxy interface

This paper discusses the use of a hydrophobic thiol based compound as adhesion promoter for copper (Cu)-epoxy interface. The motivation of this study is the long term reliability of the interface. In order to solve the moisture related reliability problem at the interface, thiol based molecules having hydrophobic characteristic (SAM N) is introduced at the interface. The rationale is to reduce the moisture uptake and hinder the diffusion into the interface. The thiol compound adsorbs onto a Cu substrate to form a self assembly molecular layer (SAM) through solution deposition. The SAM structure was designed to form covalent bonds with epoxy system while hydrophobicity was realized with long alkyl chains. The hydrophobicity of the surface treatment and wettability to epoxy adhesive was characterized using contact angle measurements with DI water and liquid epoxy. The wetting properties were shown independent of the treatment time. The results demonstrate selective wetting properties of the treatment. Water does not wet the treated surfaces (>110deg) but the surfaces achieve good wetting with epoxy (<10deg). Interfacial adhesion of the specimens prepared from modified substrates was bonded to epoxy and evaluated through a TDCB fracture test. In the reliability test, the G_IC of the interface drops slightly from 45.7plusmn5.5 Jm^-2 with freshly prepared specimens to 40.0plusmn11.1 Jm^-2 with the moisture pre-conditioned ones. This indicates that with the SAM N treatment, the interfacial integrity is retained in long term service. The paper also discusses the hydrophobic behavior of the SAM N surface which enables long time storage of the pre-treated substrate under ambient environment. The paper introduces a new thiol based coupling agent which provides reliable Cu-epoxy interfaces.