Copper is the most common interconnecting material in the field of microelectronic packaging, which is widely used in advanced electronic packaging technologies. However, with the trend of the miniaturization of electronic devices, the dimensions of interconnectors have decreased from hundreds of microns to tens of or even several microns, which has brought serious reliability issues. As a result, nanotwinned copper (nt-Cu) has been proposed as a potential candidate material and is being certified progressively. Firstly, the physical properties of nt-Cu have been widely studied. Notably, the higher thermal stability and oxidation resistance of the (111) texture causes nt-Cu to maintain excellent physical properties under high-temperature serving conditions. Secondly, recent works on the electrolyte and electroplating processes of nt-Cu on wafer substrates are summarized, focusing on how to reduce the thickness of the transition layer, improve the twin density, and achieve complicated pattern filling. Thirdly, nt-Cu can effectively eliminate Kirkendall voids when it serves as UBM or a CuP. Additionally, the high (111) texture can control the preferred orientation of interfacial intermetallic compounds (IMCs) at the Cu–Sn interface, which should be helpful to improve the reliability of solder joints. nt-Cu has superior electromigration resistance and antithermal cycling ability compared to ordinary copper RDLs and TSVs. Above all, nt-Cu has attracted much attention in the field of microelectronic packaging in recent years. The preparation–performance–reliability interrelationship of nt-Cu is summarized and displayed in this paper, which provides a solid theoretical basis for its practical applications.

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Research Progress of Electroplated Nanotwinned Copper in Microelectronic Packaging

In the advanced microelectronic packaging, Cu/Sn joints require high-quality diffusion barriers to inhibit Cu–Sn intermetallic compounds (IMCs) growth and improve the interface brittleness. Cobalt-Phosphorous (Co–P) alloy coatings are ideal candidates due to their good wettability, promising diffusion resistance and low interfacial brittleness. In this work, the microstructural evolution of IMCs between crystalline/amorphous Co–P coatings and lead-free solders during the solid-state diffusion were systemically investigated. The microstructure, phase distribution, and grain characteristics of coatings and IMCs were carefully characterized by SEM, EPMA, EBSD, and TEM. It was found that the consumption rate of crystalline Co–P, 21.2 nm/h, was significantly slower than that of amorphous Co–P, 96.4 nm/h, and exhibited an excellent diffusion resistance, which was attributed to the nanocrystalline structure and the P-rich layer which performed the diffusion barriers for Sn and Cu atoms. Compared with Ni, amorphous Co–P coatings could better improve the interfacial brittleness with soft and ductile Co–Sn compounds (hardness of 3.0 GPa) growing with the rapid diffusion of Co, and the toughened Co–Sn–P attributing to the diffusion of Sn. Kinetic analysis showed that the growth rate-controlling process of Co–Sn IMCs was jointly controlled by interfacial reaction and volume diffusion, and the effect of the interfacial reaction is more pronounced in the amorphous system, which leads to coarse Co–Sn grains that are beneficial to the softness and ductility of IMCs. Crystalline and amorphous Co–P coatings can be applied to the surface treatment for die pads and print-circuit-board pads according to their own characteristics, respectively. 

Microstructural evolution of intermetallic compounds between crystalline/amorphous Cobalt-Phosphorous coatings and lead-free solders

Crystallinity of Co-P coating on microstructure and mechanical properties of Co-Sn intermetallic compounds at Sn-Ag/Co-P/Cu Interface


 Twin boundaries are planar defects between two domains exhibiting mirror symmetry. Nano-twinned metallic materials contain numerous twin boundaries in parent grains exhibiting submicrometer twin spacing. Owing to their unique mechanical and electrical properties, nano-twinned metals have been studied extensively. Although the mechanical strength of the metal can be drastically increased by shrinking grains, nanocrystalline metals lose their ductility (i.e., the strength–ductility tradeoff), and their electrical conductivity is considerably lowered owing to electron scattering at dense grain boundaries. However, nano-twinned metallic materials can overcome these limitations and exhibit excellent strength, ductility, and electrical conductivity. In this paper, the structure and properties of nano-twinned Cu films are reviewed, and direct current and pulse electrodeposition for forming twin boundaries in Cu films and controlling the twin structure and thickness are summarized. Furthermore, the applications of nano-twinned Cu materials for fabricating electronics are presented.

Electrodeposition of Nano-Twinned Cu and their Applications in Electronics

Strategies for solder joints with high shear strength, slow interfacial consumption, ductile intermetallic compounds using hybrid crystalline-amorphous Co-P coatings

Effects of amorphous CoW and NiW barrier layers on the evolution of Sn/Cu interface

Nanotwinned copper micro-cone array fabricated by pulse electrodeposition for low-temperature bonding

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Effects of amorphous CoW and NiW barrier layers on the evolution of Sn/Cu interface

Nanotwinned copper micro-cone array fabricated by pulse electrodeposition for low-temperature bonding