Electromigration Failure Caused by Interdiffusion between Al Trace and Cu Seed Layer in the Microbump in 3D IC

(13:57 + Q&A) Yifan Yao, City University of Hong Kong
From the 2024 IEEE Symposium on Reliability for Electronics and Photonics Packaging
Summary: Three-dimensional integrated circuits (3D ICs) is widely used in consumer electronic products to increase the density of interconnect and enhance the performance of the device. In 3D IC structure, microbumps are used to connect the interposer and chip. However, due to the small size and enhanced current, electromigration (EM) in the microbump is a crucial reliability issue and can result in a circuit failure. However, due to the ultrafine size of the microbump, intermetallic compound (IMC) is easily formed and the microbump will transform into a full-IMC structure during a short period of current stress, which has very high EM-resistance. Herein, we investigated the EM failure mechanism of the full-IMC microbump. Our sample has a Cu-Ni-solder-Ni-Cu structure with a diameter of around 12um. Each microbump is connected by Al traces in a daisy-chain arrangement. We first performed the test at 132C and 1.57×10⁵ A/cm² for 2000 hours. The resistance increases to 8% and stays stable. All of the solder has transformed into IMC, which is Ni3Sn4. At a new condition which is more severe, the sample resistance exhibits an abrupt increase and rises to 60% at 5000 h, indicating a semi-failure. We used focused ion-beam (FIB) to cut a thin piece of the newly-formed IMC at the interface for transmission electron microscopy (TEM) characterization. Elemental mapping shows that the IMC consists of Cu and Al, which reveals interdiffusion between the Al trace and the Cu seed layer during the high current density EM test.

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