biostudies-literatureHe CInnovation and Technology Commission (ITF)City University of Hong Kong (CityU)Research Grants Council, University Grants CommitteeResearch Grants Council, University Grants Committee (RGC, UGC)City University of Hong KongInnovation and Technology Commission7095https://www.ebi.ac.uk/biostudies/studies/S-EPMC11330484biostudies-literatureUnknown15(1)Direct copper-to-copper (Cu-Cu) bonding is a promising technology for advanced electronic packaging. Nanocrystalline (NC) Cu receives increasing attention due to its unique ability to promote grain growth across the bonding interface. However, achieving sufficient grain growth still requires a high thermal budget. This study explores how reducing grain size and controlling impurity concentration in NC Cu leads to substantial grain growth at low temperatures. The fabricated NC Cu has a uniform nanograin size of around 50 nm and a low impurity level of 300 ppm. To prevent ungrown NC and void formation caused by impurity aggregation, we propose a double-layer (DL) structure comprising a normal coarse-grained (CG) layer underneath the NC layer. The CG layer, with a grain size of 1 μm and an imNature communicationsNanocrystalline copper for direct copper-to-copper bonding with improved cross-interface formation at low thermal budget.PMC113304849380413ITS/104/22T46-705/23-RHe CCherng SJLu YZhou RMu KTu KNJing SChen CChung CCFeng SPZhou JHuang YTfalseNanocrystalline copper for direct copper-to-copper bonding with improved cross-interface formation at low thermal budget.Direct copper-to-copper (Cu-Cu) bonding is a promising technology for advanced electronic packaging. Nanocrystalline (NC) Cu receives increasing attention due to its unique ability to promote grain growth across the bonding interface. However, achieving sufficient grain growth still requires a high thermal budget. This study explores how reducing grain size and controlling impurity concentration in NC Cu leads to substantial grain growth at low temperatures. The fabricated NC Cu has a uniform nanograin size of around 50 nm and a low impurity level of 300 ppm. To prevent ungrown NC and void formation caused by impurity aggregation, we propose a double-layer (DL) structure comprising a normal coarse-grained (CG) layer underneath the NC layer. The CG layer, with a grain size of 1 μm and an im2024-01-01T00:00:00Z2024 Aug2025-05-18T12:13:51.107Z2025-05-18T12:13:51.107ZS-EPMC113304843915402010.1038/s41467-024-51510-7