Annealing-induced interfacial toughening using a molecular nanolayer

Abstract

Self-assembled molecular nanolayers (MNLs) composed of short organic chains and terminated with desired functional groups are attractive for modifying surface properties for a variety of applications. For example, organosilane MNLs are used as lubricants¹, in nanolithography², for corrosion protection³ and in the crystallization of biominerals⁴. Recent work has explored uses of MNLs at thin-film interfaces, both as active components in molecular devices⁵, and as passive layers, inhibiting interfacial diffusion^6,7,8, promoting adhesion^9,10 and toughening brittle nanoporous structures¹¹. The relatively low stability of MNLs on surfaces at temperatures above 350–400 °C (refs 12, 13), as a result of desorption¹⁴ or degradation, limits the use of surface MNLs in high-temperature applications. Here we harness MNLs at thin-film interfaces at temperatures higher than the MNL desorption temperature to fortify copper–dielectric interfaces relevant to wiring in micro- and nano-electronic devices. Annealing Cu/MNL/SiO₂ structures at 400–700 °C results in interfaces that are five times tougher than pristine Cu/SiO₂ structures, yielding values exceeding ∼20 J m^-2. Previously, similarly high toughness values have only been obtained using micrometre-thick interfacial layers^15,16,17. Electron spectroscopy of fracture surfaces and density functional theory modelling of molecular stretching and fracture show that toughening arises from thermally activated interfacial siloxane bridging that enables the MNL to be strongly linked to both the adjacent layers at the interface, and suppresses MNL desorption. We anticipate that our findings will open up opportunities for molecular-level tailoring of a variety of interfacial properties, at processing temperatures higher than previously envisaged, for applications where microlayers are not a viable option—such as in nanodevices or in thermally resistant molecular-inorganic hybrid devices.