Integrated experimental-numerical characterization of thickness and loading condition effects on mechanical properties in copper thin films

Abstract

Copper (Cu) thin films are critical interconnect materials in advanced semiconductor interconnect and packaging applications, where mechanical reliability under loading conditions relevant to semiconductor manufacturing and packaging becomes increasingly important with continued device scaling. However, the coupled effects of film thickness, loading condition, and microstructure on the mechanical behavior of substrate-supported Cu films remain insufficiently understood. In this study, the thickness- and loading-condition-dependent mechanical properties of Cu thin films are systematically investigated using nanoindentation experiments integrated with finite element analysis (FEA)–based inverse modeling. Cu films with thicknesses of 0.1 μm, 0.5 μm, and 0.9 μm were characterized to evaluate Young's modulus, hardness, effective fracture toughness, and yield strength. The results show that Young's modulus increases from 72 GPa to 117 GPa while hardness decreases from 3.5 GPa to 2.1 GPa with increasing thickness, indicating a transition toward bulk-like behavior. At a peak load of 500 mN, the effective fracture toughness of 1.116 ± 0.156 MPa√m and 3.085 ± 0.593 MPa√m were obtained for the 0.1 and 0.5 μm films, respectively, whereas no measurable surface cracking was observed for the 0.9 μm film. For the 0.1 μm film, the effective fracture toughness decreased from 2.004 ± 0.200 MPa√m to 1.116 ± 0.156 MPa√m as the peak load increased from 100 mN to 500 mN, while no measurable cracking occurred at 20 mN. These observations demonstrate that the measured indentation-derived fracture response is sensitive to both film thickness and the applied indentation loading condition. Yield strength, extracted via inverse FEA, decreases from 500 MPa to 300 MPa with increasing thickness, qualitatively consistent with concurrent grain coarsening and Hall–Petch expectations. These results demonstrate the capability of the integrated experimental–numerical approach to consistently characterize the effective mechanical properties of substrate-supported Cu thin films. In addition, preliminary trend-based grain-size correlations are developed to relate mechanical properties to microstructural features, providing a practical first-order engineering correlation framework for materials optimization and reliability assessment in advanced semiconductor packaging.