Copper-Based Nanomaterials for Fine-Pitch Interconnects in Microelectronics

Abstract

Conspectus Nanostructured copper-based materials have emerged as a new generation of robust architectures for realizing high-performing and reliable interconnection in modern electronic packaging. As opposed to traditional interconnects, nanostructured materials offer better compliance during the packaging assembly process. Due to the high surface area-to-volume ratio of nanomaterials, they also enable joint formation by sintering through thermal compression at much lower temperatures compared to bulk counterparts. Nanoporous Cu (np-Cu) films have been employed in electronic packaging as materials that facilitate a chip-to-substrate interconnection, realized by a Cu-on-Cu bonding after sintering. In this Account, we discuss the use of self-supported np-Cu films for low-temperature joint formation. The novelty of this work comes from the incorporation of tin (Sn) into the np-Cu structure, thus ensuring lower sintering temperatures with a goal of producing Cu–Sn intermetallic alloy-based joints between two Cu substrates. The incorporation of Sn is done using an all-electrochemical bottom-up approach that involves the conformal coating of fine-structured np-Cu (initially formed by dealloying of Cu–Zn alloys) with a thin layer of Sn. This Account provides insight on existing technologies for using nanostructured films as materials for interconnects as well as the optimization studies for the Sn-coating processes as a new alternative approach. The applicability of the synthesized Cu–Sn nanomaterials for low-temperature joint formation is also discussed. To realize this new approach, the Sn-coating process is administered by a galvanic pulse plating technique, which is optimized to preserve the porosity in the structure with a Cu/Sn atomic ratio that allows for the formation of the Cu6Sn5 intermetallic compound (IMC). Nanomaterials obtained using this approach are subjected to joint formation by sintering at temperatures between 300 and 200 °C under 20 MPa pressure in forming gas atmosphere. Cross-section characterization of the formed joints postsintering reveals densified bonds with minimal porosity that consist predominantly of the Cu3Sn IMC. Furthermore, these joints are less prone to structural inconsistencies compared to existing joints formed using purely np-Cu. The results presented in this Account provide a glimpse into a facile and cost-effective approach for synthesizing nanostructured Cu–Sn films and illustrate their applicability as new interconnect materials.

New generation copper-based interconnection from nanoporous CuSn alloy film sintered at low temperatures

“…This result aligns with our previous findings in another paper, where similarly generated joints showed no void formation even under extended heat treatment. 15 These images reaffirm the hypothesis that the completion of the Cu 6 Sn 5 conversion to Cu 3 Sn during the sintering process results in joints with a stable Cu 3 Sn IMC content. Consequently, the annealing process does not induce the diffusion of extra Cu or Sn, preventing the creation of a Kirkendall imbalance that could promote potential void formation and growth.…”

“…35 This newly formed joint exhibits a densified configuration alike with observed in previous research. 15 Characterizations and previous observations 15 suggest that during sintering, the high pressure and rapid heating cause Cu from the substrate to diffuse into the np-Cu 6 Sn 5 bonding material, ultimately converting it to Cu 3 Sn. Notably, Cu 3 Sn IMC possesses a lower resistivity of 8.3 μΩ cm compared to Cu 6 Sn 5 (17.5 μΩ cm).…”

“…Interestingly, it has been observed that in the absence of the Cu 6 Sn 5 layer, voiding does not occur in the Cu/Cu 3 Sn interface. 15…”

“…9,14 Nanoporous materials have emerged as the next generation of interconnects, offering unique properties that distinguish them from their bulk counterparts. 15 These properties, such as a high surface area-to-volume ratio, result in a significant melting point reduction vis-a-vis their bulk counterparts. 16 This characteristic enables sintering at considerably lower temperatures during the electronic circuits assembly.…”

A Study of Electroplated Nanoporous Copper Using Aberration-Corrected Transmission Electron Microscopy

“…Therefore, the use of organic additives can be avoided during the sintering process and the density of the sintered joint can be improved [ 11 ]. Dimitrov et al proposed a method of plating a layer of Sn on the surface of nanoporous Cu [ 12 ]. This method can reduce the sintering temperature and improve oxidation resistance.…”