Paper AS-TuP23
Characterization of a Self-assembled Molecular Nanolayer at Buried Cu-silica Interface

Tuesday, October 19, 2010, 6:00 pm, Room Southwest Exhibit Hall

Integration of metal-dielectric interfaces using molecular nanolayers (MNLs) is attractive for prospective applications such as laminates in high frequency electronics and packaging, nanodevice wiring and composites. Recent works have shown that annealing-induced siloxane bridging can toughen organosilane-functionalized copper-silica interfaces. While strong bonding of the MNL with the under- and over-layers is essential for promoting adhesion, the nature of the MNL structure and bonding, especially at temperatures where the MNLs are known to degrade on bare surfaces, are unclear. But tracking atomic-level intermixing and interfacial phase formation in a sub-nm-layer is an exacting challenge due to difficulties in distinguishing Si atoms in the organosilane MNL from Si atoms in the silica substrate, and obtaining sufficient contrast by electron microscopy. Here, we study organogermane-tailored interfaces using a combination of electron spectroscopy and microscopy, and density functional theory calculations to obtain insights into the interface chemical changes. Our results reveal that annealing decomposes the organic monolayer into an inorganic Cu-O-Si network, leading to interface toughening.

We assembled Benzyl-trichlorogermane (BTCG) on silica to form a 0.7-nm-thick nanolayer. Four-point bending fracture tests on as-prepared Cu/BTCG/SiO₂ sandwiches revealed a low interface toughness of 2.1 J/m², comparable to pristine Cu/SiO₂ structures. However, interfacial toughness increased monotonically with annealing temperature, yielding values as high as 23.3 J/m² for T_anneal­ = 500 ºC. Core-level spectra from silica fracture surfaces show a strong Ge signature for T_anneal ≤ 300 ºC that becomes undetectable for T_anneal ≥ 400 ºC, suggesting Ge transport and destruction of the organic MNL. This result is corroborated by time-of-flight secondary ion mass spectroscopy (SIMS) profiles showing the smearing of the interfacial Ge spike into the silica layer upon annealing. Incorporation of Ge in the silica weakens the Si-O-Si network, leading to intermixing of Si, O and Cu, forming nanoscale islands of rhombohedral CuSiO₃ observable by cross-sectional transmission electron microscopy and X-ray spectroscopy. For pristine Cu/SiO₂ structures there were no changes at the interface and the toughness value was ~ 3 J/m² for T_anneal ≤ 700 °C. Our findings suggest that molecular degradation of the organic MNL to form nanoscopic layer of inorganic metal-oxide-silicon bonds could be an attractive approach for toughening interfaces.