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 diffusion6,7,8, promoting adhesion9,10 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 layers15,16,17. 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.

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Figure 1: Effect of thermal annealing on the interface toughness of Cu/MPTMS/SiO 2 and Cu/SiO 2 structures.
Figure 2: Core-level spectra from fracture surfaces.
Figure 3: High-resolution core-level spectra from fracture surfaces, and a schematic illustration of the primary toughening mechanism.
Figure 4: Calculated system energy as a function of molecular stretching for different combinations of siloxane and hydrogen bonds at the MPTMS/SiO 2 interface.

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Acknowledgements

This work was supported by the US National Science Foundation, the US-Israel Binational Science Foundation, an Alexander von Humboldt fellowship, and New York state through the Interconnect Focus Center.

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Correspondence to Ganapathiraman Ramanath.

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Gandhi, D., Lane, M., Zhou, Y. et al. Annealing-induced interfacial toughening using a molecular nanolayer. Nature 447, 299–302 (2007). https://doi.org/10.1038/nature05826

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