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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. [ABSTRACT FROM AUTHOR]

• Modulation on the twinning density and direction of Cu nanowires is achieved. • The thickness of twinning lamella is adjusted by varying the boric acid content. • Waterfall, bevel, and ladder type nt-Cu NWs are obtained by varying the current density. • The NWW growth behaviour diagram is proposed to describe the microstructural transition. • Twin characteristics of Cu NWs significantly impact their oxide morphology. In this study, nanotwinned Cu nanowires (nt-Cu NWs) with varying twin characteristics were fabricated via pulse electrodeposition using a novel H 3 BO 3 based electrolyte, and their impact on the oxidation process was investigated. The density and direction of twin lamellas within nt-Cu NW were adjusted by modifying the concentration of H 3 BO 3 and the current density. Specifically, the average twinning thickness of Cu NWs increases from 26.7 nm to 424.5 nm as the concentration of H 3 BO 3 increases from 0 g/L to 6 g/L. Moreover, three types of nt-Cu NWs with distinct twinning directions, namely waterfall type, bevel type, and ladder type were fabricated by changing the current density from −0.16 ASD to -1.6 ASD (vs Ag/AgCl). A nanowire Winand (NWW) diagram was established to describe the change of twin density and direction based on the difference between the growth behaviour of thin films and nanowires. The thermogravimetric analysis (TGA) and transmission electron microscopy (TEM) were further used to characterize the oxidation process of nt-Cu NWs. It revealed that the oxide thickness was closely related to the thickness of twin lamellas for the ladder type nt-Cu NWs. Also, the waterfall type Cu NWs exhibited a unique bead-chain like oxide morphology and a slower oxidation rate through the impact on the nanoscale Kirkendall effect (NKE). This study provides valuable insights into the controllable fabrication of nt-Cu NWs and Cu oxide NWs. [ABSTRACT FROM AUTHOR]

The copper-copper (Cu-Cu) direct bonding technology was proposed in recent years in order to realize precise preparation and reliable service under high current density for three-dimension (3D) packaging in the post-Moore-era. In this study, Cu-Cu direct bonding was achieved in the ambient atmosphere by utilizing highly (111) oriented copper with nanotwin structure (called (111) nt-Cu). The thermal compression with a pressure of 30 MPa was conducted on a couple of Cu films at 300 and 400 °C in air. The Cu-Cu joints after bonding were further investigated by scanning electron microscopy (SEM), electron back-scattered diffraction (EBSD) and transmission electron microscopy (TEM). Different from the polycrystalline copper, perfect bonding was achieved as confirmed by high-resolution transmission electron microscopy (HRTEM) and electron energy loss spectroscopy (EELS). The shear strength after air bonding was measured as 96.04 ± 5.67 MPa on average, which demonstrated relatively high performance in traditional Cu-Cu bonding. The excellent resistance to oxidation and fast Cu(111) surface diffusivity of (111) nt-Cu guaranteed the bonding process. Above all, the achievement of (111) nt-Cu high performance bonding in the ambient atmosphere should give an insight into developing new interconnected technology. • Novel Cu-Cu direct bonding was achieved in air. • (111) crystal plane can promote diffusion and decelerate oxidation. • No gaps and oxide layers were found at the interface of (111) nt-Cu. • Bonding temperature was as low as 300 °C. • Bonding strength was as high as 96.04 ± 5.67 MPa. [ABSTRACT FROM AUTHOR]