Molecular Simulation Study on Sputter-Deposited Copper-Titanium UBM for Wafer-Level Packaging

With the continuous increase in functionality and feature size of heterogeneous integrated module, the three-dimensional integration technology has emerged
^[12]
. The interconnection between the convex points is crucial for achieving chip three-dimensional stacking, and the preparation of high-reliability micro-convex points is of great significance for the further development of microelectronic packaging technology
^[13]
. One of the key steps in the fabrication of wafer convex points is the UBM (Underball Metal) process. During the preparation of the UBM thin film, a chemical reaction occurs between the Cu film and the Si substrate, resulting in the formation of copper silicide compounds, which can lead to a decrease in the reliability of the Cu film
^[14]
. Titanium, as an adhesion layer and barrier layer, enhances the adhesion between Cu and Si and reduces the diffusion of Cu into Si. Currently, research on Ti substrates and Cu films is limited to experimental studies
^[15]
, and the mechanism of the interaction between Ti substrates and Cu films is not yet clear. In this study, a deposition model was established using molecular dynamics, with Ti as the substrate and Cu as the deposited atoms, to simulate the physical vapor deposition (PVD) process of Cu atoms deposited on the Ti substrate
^[16]
. The study investigated the effects of incident energy, substrate temperature, and incident angle on the growth of Cu atomic deposition on the Ti substrate. The changes in film surface roughness and layer coverage during the process were specifically analyzed. The research showed that during the deposition sputtering process, incident energy is the most critical factor. When the incident energy is 0.1 eV, the film grows in an island-like structure; when the incident energy is 10 eV, the film grows in a layered structure. At a substrate temperature of 300K, in the 0th layer at the interface between the film and the substrate, only a few atoms undergo diffusion, with Cu atoms accounting for 0.05 and Ti atoms accounting for 0.12 in the 0th and 1st layers, respectively. When the temperature is raised to 500K, the degree of interface mixing increases, and the strength of interface mixing further increases, with Cu atoms accounting for 0.32 and Ti atoms accounting for 0.3 in the 0th and 1st layers, respectively. The impact of incident angle is relatively small, with lower incident angles resulting in better film quality. When the incident angle is 0°, the root mean square roughness is 2.056A, and when the incident angle is increased to 45°, the root mean square roughness is 2.4003A, indicating an increase in surface roughness. However, at this point, the Cu film surface exhibits a “groove” shape, which reduces the film quality. The research results have deepened the understanding of the mechanism of the interaction between the Ti substrate and Cu film in the UBM process, and provided valuable theoretical references and basis for optimizing process parameters.