Thermodynamic modeling framework with experimental investigation of the large-scale bonded area and local void in Cu-Cu bonding interface for advanced semiconductor packaging

Abstract

With the increase in computational costs driven by the use of artificial intelligence, enhancing the performance of semiconductor systems while improving efficiency has become an inevitable challenge. Due to the fine pitch limits of micro bumps, bumpless Cu-Cu bonding is emerging as the next-generation core technology. This study aims to analyze the effects of individual temperature and pressure on both large- and local-scale behaviors of material in the Cu-Cu bonding process with experiments and numerical analysis. The motivation of this study is to compensate the deficiencies in reported studies on process optimization, particularly the lack of exploration of the separated effects of temperature and pressure on large- and local-scale Cu-Cu bonding. Furthermore, reports on the thermodynamic modeling of Cu-Cu bonding behavior are not sufficient, making it challenging to find suitable models. Bonding experiments were performed by independently controlling the temperature and pressure using blank Cu films treated by precise chemical mechanical polishing (CMP) processes. The large-scale bonded area under each condition was measured, and transmission electron microscope (TEM) images were captured to observe the patterns of local void formation under various temperature and pressure conditions. In the experiments, it was observed that the temperature increase had a greater impact on the bonded area at a larger scale than the increase in pressure. However, for nanoscale-local voids, an increase in pressure had a more dominant effect. To discuss the experimental results, a thermodynamic modeling framework that considers coupled heat-induced deformation, plastic deformation, and volumetric changes caused by material flux was proposed. The proposed model has been implemented in the user-defined material subroutine (UMAT) of the ABAQUS program for finite element (FE) analysis. Numerical analysis using the proposed model captures the experimental data well. In large-scale simulations, temperature conditions have a significant impact, with plastic deformation being the primary mode of deformation, while the pressure conditions dominate the material flux, making substantial contributions to reducing voids at local-scale. To achieve complete closure of the void, the simulation demonstrated that maintaining a sufficient pressure gradient until the complete closure is required. The study findings provide an explicit understanding of how the temperature and pressure conditions differently affect large-scale bonding and local voids for semiconductor package manufacturing.