Advances toward reliable high density Cu-Cu interconnects by Cu-SiO2 direct hybrid bonding

Cu-SiO₂ direct hybrid bonding is considered as one of the most promising approaches for matching the needs of three dimensional integrated circuits (3D-IC). In this paper we present the results of a complete morphological, electrical and reliability study conducted on four-layer copper structures realized by Cu-SiO₂ direct hybrid bonding. Ultra-fine 7 μm pitch, 3 μm × 3 μm pads daisy chains with up to 30 160 connections are bonded with submicron accuracy, matching the interconnection needs in upcoming 3D circuits. A focus is paid on the influence of the temperature of annealing on the bonding quality and electrical performances. Thanks to SEM, FIB/SEM tomography, AFM, TEM and TEM-EDX analysis, the morphology of the bonded structures is described. Electrical parametric tests exhibit very low resistance structures, with high functional yields and low variability. State-of-the-art contact resistivity is obtained for 200 and 400 °C treatments. Unbiased HAST, temperature cycling and temperature storage tests demonstrate excellent reliability performances. Finally, an electromigration resistance comparative study is conducted on bonded copper lines with TaN/Ta and TiN diffusion barriers.