The impact of sidewall copper grain condition on thermo-mechanical behaviors of TSVs during the annealing process.

Abstract

With the drastic reduction of the TSV diameter leading to a critical dimension comparable to the Cu-filled grain size, the grain condition strongly influences the thermo-mechanical behavior of the TSV. In this work, the TSV-Cu cross-section with different grain sizes is characterized by EBSD, confirming that the sidewall grain size (0.638-1.580 μm) is smaller compared to other regions (1.022-2.134 μm). A finite element model (FEM) considering copper grains is constructed by using Voronoi diagrams to investigate the effect of sidewall grain size as well as area on the thermo-mechanical behavior during annealing. The material parameters in the FEM are optimized through nanoindentation inversion and considering the mechanical property anisotropy of copper grains. The yield strength σ_y and hardening exponent n of TSV-Cu are 74.6 MPa and 0.514. The simulation results indicate that the protrusion of TSV-Cu after annealing tends to increase initially and then decrease with smaller sidewall grain size and area. The maximum increase in protrusion caused by the two variables can reach 6.74% and 14.6%, respectively, relative to the average grain condition. Additionally, the simulation results were validated by quantifying grain boundaries in TSV-Cu samples with varying grain sizes.