Probing the Nanoscale Onset of Plasticity in Electroplated Copper for Hybrid Bonding Structures via Multimodal Atomic Force Microscopy

The slowdown of Moore's law has elicited a paradigm shift whereby shrinking of in-plane dimensions is being replaced by 3D-stacking advanced packaging approaches to satisfy the ever-increasing demands for power, performance, area, and cost. Driven by the widespread use of metallic interconnects with submicron pitches, robust metrology for probing mechanical behavior at the nanoscale emerges as a key area of interest in the semiconductor industry. Here, we develop an atomic force microscopy (AFM)-based protocol for characterizing the incipient stages of plasticity and illustrate it on hybrid bonding-ready (prior to bonding) copper pads. We combined AFM's high-resolution imaging with contact resonance and indentation techniques (including single- and multistep indentation) to characterize the mechanical heterogeneity of the material, quantify the nanoscopic yield stress statistics, and derive indentation stress-strain curves. From these measurements, we have clarified the mechanisms of early plasticity and determined the elastoplastic constitutive response of polycrystalline copper, including parameters such as elastic modulus, yield stress, and strain-hardening slope. Besides providing metrology relevant to various length scales, our approach offers a pathway to utilize an industry-standard instrument for characterizing the thermomechanical properties that are essential for the development of semiconductor structures.