Nanoscratch evaluation of adhesive strength of Cu/PI films

Abstract

A scratch test has provided a simple, rapid means of assessing the adhesion strengths of thin films on substrates. However, it is not standardized how to detect the interfacial failure. Copper thin films deposited on Polyimide (PI) substrates are used for Flexible Print Circuit (FPC). It is needed to improve and measure the adhesive strength of Cu films on PI substrates to avoid interfacial fractures caused by cyclic bending. In this study, in order to improve the adhesive strength, the surfaces of PI substrates were modified by ion beam using argon or oxygen with a linear ion source and the adhesion strengths of Cu films on PI substrates were measured using nanoscratch tests. Cu thin films were deposed by Physical Vapor Deposition (PVD) using an electron beam. The friction coefficient rapidly decreased and normal displacement rapidly increased at the delaminated point at the same time. The slope of the friction coefficient increased at the delaminated point, whose normal load is called the critical load. The scratched surfaces were observed from films (Cu) side and substrate (PI) side. A transparency of PI substrates allowed us to observe the interfaces between Cu film and PI substrate. The scratched mark observed from substrate side started at just the point where the slope of the friction coefficient changed. As the roughness of the surface of polyimide substrates increased, critical load increased.