Thiol based self assembly molecular layer for reliable Cu-epoxy interface

This paper focuses on the use of new hydrophobic thiol based molecular as adhesion promoter for copper-epoxy interfacial adhesion improvement. The motivation of this study is the long term interfacial adhesion reliability under moisture environment. In order to solve this problem, our group introduced the use of thiol-based molecules (SAM P) having hydrophobic characteristic as adhesion promoter through solution deposition. The rationale is to reduce the moisture uptake and hinder the diffusion into the interface. Long alkyl chains structure is intended to provide hydrophobic characteristic of the interface. A reactive hydroxyl group which can achieve covalent bonds with the epoxy system is designed as an end group. This study aims at deriving new hydrophobic SAM layer to tackle the moisture reliability problem of the Cu-epoxy interface. Systematic study on the SAM P treated interface is conducted. SEM and XPS are applied for surface analysis. DI water contact angle using sessile drops is used to quantify the hydrophobicity of the interface. To evaluate the interfacial adhesion with SAM P treatment, copper jigs bonded with an epoxy underfill are tested by the tapered double cantilever beams (TDCB) setup. The interfacial toughnesses (GIC) are recorded of both fresh samples and samples after 85°C/85%RH humidity chamber preconditioning. With the hydrophobic treated interfaces, our preliminary results demonstrated that GIC of the interface changes slightly from 117.7±29.4 Jm⁻² to 118.3±7.0Jm⁻² for the fresh samples and moisture preconditioning ones respectively. The results are compared with samples obtained from previous SAM treatment. The results show that with the SAM P treatment, the interface is resistant to moisture environment. The test results will help to establish the guidelines in formation of SAM for a reliable copper-epoxy interface.