Determination of Stress-Strain Properties Combining Small-Depth Nanoindentation and Numerical Simulation

In microelectronic packages, the mechanical material properties, relevant e.g. for questions of thermomechanical reliability, of thin layers and small structures can be determined only approximately via established test methods like tensile or bending tests. The main problem is manufacturing samples which arc sufficiently large to enable handling in the experiment, while having the same material properties as in the application. Nanoindentation tests have the potential to characterize such material properties on a microstructural level and with the same conditions as in the application. Determining Young's modulus and hardness of ductile materials with nanoindentation by using a Berkovich tip is common practice. Several studies have shown that it is difficult to extract stress strain behaviour from nanoindentation experiments with shallow indentation depth [1, 2, 3]. The difficulty is mostly attributed to the Indentation Size Effect (ISE), a phenomenon of non -linearity at low indentation depth. Due to the continuing miniaturization, the challenge is there to determine comprehensive stress-strain behaviour from experimental data that is affected by the ISE. In this study, a method is described where in addition to a Berkovich tip also a spherical tip is used. This spherical tip allows a more homogenous strain distribution at the same indentation depth compared to the Berkovich tip, cf. Fig. 1. Studies showed that the use of spherical indenters to determine stress-strain behaviour has advantages over pyramidal/conical indenters like the Berkovich [2]. The extracted load-depth curves for both geometries are used for an inverse method of material characterization to increase the uniqueness of results.