Nanoindentation characterization of thin film stack structures by finite element analysis and experiments using acoustic emission testing

Abstract

Increased risk of crack formation in the brittle insulating layers of crack-sensitive backend-of-line (BEOL) structures in semiconductor integrated circuits during wafer probing or wire bonding process is detrimental to product reliability. This paper describes the use of a high-resolution, in-situ material testing system that integrates acoustic emission (AE) testing with a nanoindentation system, as well as the finite element analysis (FEA) simulation method, for faster characterization of BEOL structures to optimize the reliability of wafer probing and wire bonding processes. This hybrid testing system is used to determine the critical load of thin film stack structures with Al-Cu input/output (I/O) pads, insulating layers, and the embedded Cu layers. Cracks were induced during the nanoindentation, and scanning electron microscopy (SEM) confirmed the formation and propagation of cracks in the insulating layers below the I/O pad. Finite element analysis (FEA) simulation was carried out to evaluate the contact-related stress distribution and obtain the critical stress by replicating the nanoindentation process to fully understand the critical conditions that lead to brittle fracture in insulating layers. Multiple thin film stack structures with different layer thicknesses are tested by experiment and correlated with simulation in parameter studies. We observed that as the thickness of the insulating layer increases and as the thickness of the embedded copper layer decreases, the critical loads will increase, which is in good agreement with previous studies. We also discovered that the critical loads of structures with thicker metal pads are not always higher than the thinner ones, which means the thicker metal pads do not certainly protect the structures better, as previous studies discovered.