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<p>The reliability of Cu stagger-via interconnects in fan-out chip-on-substrate (FOCoS) packaging is critically affected by electromigration, yet the role of stagger-via geometry remains underexplored. This study investigated the electromigration behavior and failure mechanisms of Cu stagger-via interconnects composed of three vertically stacked but laterally offset microvias, stressed at 0.9 A (6.8 × 10<sup>5</sup> A/cm<sup>2</sup>) and 180°C for up to 2623&#xa0;h until open-circuit failure. Field-emission scanning electron microscope and dual-beam focus ion beam analyses revealed that Cu microvias #1 and #2 maintained superior structural stability, with only limited CuAl<sub>2</sub> intermetallic compound and Kirkendall void formation at the Al pad/Cu microvia #1 interface during downstream electromigration. In contrast, microvia #3 underwent progressive Cu redistribution line transformation, with kinetically favored Cu<sub>3</sub>Sn formation leading to partial Cu microvia consumption and volume-shrinkage-induced void coalescence at failure. Polarity effects were evident, as downstream electromigration enhanced Al diffusion and CuAl<sub>2</sub> growth, while upstream electromigration suppressed IMC formation. Severe Al pad degradation emerged as the primary root cause of electromigration failure. These microstructural changes correlated well with electrical resistance variations. Compared with the stack-via design, the stagger-via architecture significantly delayed IMC formation and improved electromigration resistance, highlighting its promise for enhancing reliability in advanced Cu interconnects.</p>

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Electromigration Reliability and Failure Mechanisms in Novel 3D Cu Stagger-via Interconnects for Advanced High-Density Fan-Out Packaging

  • 期刊论文

  • 发表日期：

    2026年1月7日

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  • Kuan-Ju Shao,
  • Meng-Chun Chiu,
  • Jui-Yi Hu,
  • Chien-Lung Liang,
  • Min-Yan Tsai,
  • Yung-Sheng Lin,
  • Chen-Chao Wang,
  • Chih-Pin Hung,
  • Kwang-Lung Lin

摘要

The reliability of Cu stagger-via interconnects in fan-out chip-on-substrate (FOCoS) packaging is critically affected by electromigration, yet the role of stagger-via geometry remains underexplored. This study investigated the electromigration behavior and failure mechanisms of Cu stagger-via interconnects composed of three vertically stacked but laterally offset microvias, stressed at 0.9 A (6.8 × 105 A/cm2) and 180°C for up to 2623 h until open-circuit failure. Field-emission scanning electron microscope and dual-beam focus ion beam analyses revealed that Cu microvias #1 and #2 maintained superior structural stability, with only limited CuAl2 intermetallic compound and Kirkendall void formation at the Al pad/Cu microvia #1 interface during downstream electromigration. In contrast, microvia #3 underwent progressive Cu redistribution line transformation, with kinetically favored Cu3Sn formation leading to partial Cu microvia consumption and volume-shrinkage-induced void coalescence at failure. Polarity effects were evident, as downstream electromigration enhanced Al diffusion and CuAl2 growth, while upstream electromigration suppressed IMC formation. Severe Al pad degradation emerged as the primary root cause of electromigration failure. These microstructural changes correlated well with electrical resistance variations. Compared with the stack-via design, the stagger-via architecture significantly delayed IMC formation and improved electromigration resistance, highlighting its promise for enhancing reliability in advanced Cu interconnects.