Thin films interfacial adhesion characterization by Cross-Sectional Nanoindentation: Application to pad structures

Abstract

The feature size reduction on IC chips following Moore's law leads to great integration challenge. Among others, the mechanical integrity of pad structures is particularly critical. However, to find suitable containment actions remain tricky, and a better knowledge and characterization of interfaces are then mandatory to face these problems. The Cross-Sectional Nanoindentation (CSN) is a novel method of mechanical characterization, developed by Sanchez et al. [1]. With such method, various interfaces can be characterized, at the micrometer level, in terms of adhesion energy. Its advantages compared to the well-known 4pt bending technique are numerous: a simple and fast sample preparation, direct observation of the crack path, etc. In this paper, the CSN technique is applied to discriminate and characterize the interfaces which compose a typical wire bond pad structure. More precisely, Inter-Metal Dielectric/Metal stacks, describing a pad level are tested by the mean of CSN. The exact failed interface is then determined by SEM views. However, in order to compare the interface to each others, the adhesion energies need to be known. Due to the plastic deformation of the metal during the test, Finite Element Method (F.E.M.) is required. A 2D axisymmetric model, described in [2], is used to reproduce the test. Each stack with their characteristics is simulated and an energetic quantity is calculated. Based on these values, the interfaces are finally ranked according to their mechanical reliability. Additional insights and novel findings from the state of the art are also discussed concerning both experimental and numerical aspects of the method. At last, the ability to discriminate pad structures straightforwardly by CSN is also studied. Crack behavior is investigated by S.E.M. views and a discussion is proposed concerning the most relevant criterion. Future developments concerning this method are finally described.