Low Resistance and High Electromigration Lifetime of Cu-To-Cu Joints Using (111)-Oriented Nanotwinned Copper

Jing-Ye Juang,K N Tu,Kai Cheng Shie,Chih Chen,Yu Jin Li

doi:10.5185/amlett.2021.081654

Abstract

Cu-to-Cu joints of 30 mm in diameter were fabricated using (111)-oriented nanotwinned copper at 300 °C for 20 min in N2 ambient. The joints possess excellent electrical properties. The average resistance and specific contact resistivity are 4.1 mΩ and 3.98 × 10 -8 Ω·cm 2 , respectively for an as-fabricated Cu joint. With a second step annealing at 400 °C, the resistance can be reduced to 3.27 mΩ due to grain growth across the joint interface. There is 50% resistance reduction compared to SnAg solder joints with the same diameter. The electromigration lifetime for Cu-to-Cu joints is at least 750 times longer than solder joints.

Highlights

Solder microbumps have been adopted for vertical interconnects between stacked chips in 3D IC technology, because of the advantage of low melting point and selfalignment process [1,2,3,4]
After the first bonding process, the mean resistance for a Cu joint is 4.14 mΩ, and the contact resistivity is 3.98 × 10-8 Ω·cm2, which is the product of a joint resistance and contact area
The Cu joints have 34% resistance reduction compared to the SnAg solder joints

Summary

Introduction

Solder microbumps have been adopted for vertical interconnects between stacked chips in 3D IC technology, because of the advantage of low melting point and selfalignment process [1,2,3,4]. Cu-to-Cu direct bonding has emerged to be the best solution for the ultra-fine pitch packaging, because Cu microbumps can be fabricated below 1 μm and it has excellent electrical and thermal properties [13]. We fabricated (111)-oriented nanotwinned Cu (nt-Cu) microbumps with 30 μm in diameter, and performed a two-step Cu-to-Cu direct bonding technique to achieve excellent bonding and grain growth to eliminate bonding interfaces.

Results

Conclusion