Picosecond acoustics versus tape adhesion test: Confrontation on a series of similar samples with a variable adhesion

Abstract

A large number of methods have been developed to characterize the adhesion of a thin film to a substrate. Among them, tape test is still one of the most popular methods. Acoustic waves and especially ultra-high frequency acoustic waves are also sensitive to adhesion defects as they affect the way acoustic waves are reflected at such an interface. In this work, we first prepare a series of identical thin-film samples with a variable adhesion at the interface between the film and its substrate. To modulate adhesion of some samples, an ultrathin gold layer is deposited on the substrate before thin-film sputtering. Si+ ion implantation is also used to reinforce locally gold adhesion. The samples are then characterized using two much different techniques: picosecond acoustics (PA) for measuring thickness and acoustic reflection coefficient (R) at the interface and tape test to have an independent evaluation of the adhesion through the peeled area (P). On the samples series, R is found to vary from −0.46 the expected value for a perfectly bonded Ni/Si interface to almost −0.83 not far from −1 which corresponds to delamination. An excellent correlation is found between R and P: high |R| value samples are easily peeled off and P is large. That work demonstrates the capability of PA to perform adhesion test in a totally non-destructive manner and furthermore locally.