G. Stan, N.A. Alderete, C.V. Ciobanu
National Institute of Standards and Technology,
United States
Keywords: multimodal AFM metrology, nanoscale elastic-plastic characterization, materials for hybrid bonding
Summary:
One of the most promising integration techniques in advanced packaging is hybrid bonding, which connects dies and wafers without the use of solder bumps. Hybrid bonding allows for a higher density of interconnects with a pitch scale of than 10 micrometers. Additionally, it facilitates 3D stacking integration, increased bandwidth, and improved overall system optimization. The main steps involved in hybrid bonding are relatively straightforward: aligning two mirrored dielectric-copper patterned surfaces, bonding the dielectric components at room temperature, and then fusing the copper pads at elevated temperatures. However, due to size constraints and differences in thermomechanical properties between the materials used, the heterogeneous integration based on hybrid bonding has not yet reached its full potential or gained widespread market adoption. Therefore, developing measurement methodologies to optimize the hybrid bonding processes is both relevant and timely for the semiconductor industry. To address this gap, we developed a nanoscale elastic-plastic characterization method performed at room temperature on electroplated copper surfaces arranged in hybrid-bonding-ready patterns. This methodology combines several Atomic Force Microscopy (AFM) modes that provide various elastic and plastic characteristics of materials at the nanoscale. With high spatial resolution and in the same selected area of interest, we demonstrated the use of AFM contact resonance along with AFM-based single and multi-step indentation techniques. This multimodal approach allowed us to characterize the mechanical heterogeneity of the material, quantify nanoscopic yield stress statistics, and derive indentation stress-strain curves. From these measurements, we 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. The developed metrology highlights the availability and versatility of using AFM for characterizing nanoscale mechanical properties, which are essential for the development of heterogeneous integration in advanced packaging. These types of measurements and the corresponding data on structures ready for integration are crucial for improving predictive manufacturing for hybrid bonding applications.