Description

Due to the flexibility they offer in the selection of the end groups attached to the substrate and film materials, self-assembled monolayers (SAMs) composed of very short (nanometer-long) aligned polymer chains have been proposed as a unique way to tailor the electrical, thermal and mechanical properties of interfaces. This combined experimental and computational study aims at shedding some light on the impact of the SAMs on the failure properties of a gold film/silicon substrate interface. In this study, we investigate SAMs with methyl (‑CH3) and mercapto (‑SH) terminated functional groups. In the experimental component of the project, we adopt a noncontact laser-based spallation technique to measure the failure strength of a silicon/SAM/gold system. This method consists in converting the thermal energy imparted by a very short (ten nanoseconds-long) Yag laser pulse to an absorbing layer placed on the back side of the substrate into a compressive acoustic pressure pulse that propagates through the substrate toward the interface of interest. Upon reflection from the free surface of the film, the pulse loads the film/substrate in tension, leading to its spallation failure. Preliminary results show a strong dependence of the failure strength of the interface on the choice of SAM. Detailed AFM and XPS analyses performed on the postspallation surfaces provide information on the roughness profile and chemical composition of the failure surface. On the modeling side, molecular dynamics (MD) simulations, based on the ReaxFF model, are used to investigate the separation characteristics and interfacial mechanical behavior between SAM and a thin gold film on a silicon substrate. Although these MD simulations predict the experimentally observed four-fold increase in the failure strength of SH-terminated SAMs compared with their CH3-terminated counterpart, the predicted values are between one and two orders of magnitude higher than those observed experimentally. To address this gap between experimental and simulated strength values, we investigate the key role played by the roughness of the substrate and of the film, using a multiscale approach that combines the cohesive model derived from the MD simulations and a continuum model of the bending response of the film. A substantial drop in the effective strength of the SAM-enhanced film/substrate interface is predicted for relatively small values of the roughness.

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Molecular tailoring of thin film/substrate interfaces using self-assembled monolayers

Due to the flexibility they offer in the selection of the end groups attached to the substrate and film materials, self-assembled monolayers (SAMs) composed of very short (nanometer-long) aligned polymer chains have been proposed as a unique way to tailor the electrical, thermal and mechanical properties of interfaces. This combined experimental and computational study aims at shedding some light on the impact of the SAMs on the failure properties of a gold film/silicon substrate interface. In this study, we investigate SAMs with methyl (‑CH3) and mercapto (‑SH) terminated functional groups. In the experimental component of the project, we adopt a noncontact laser-based spallation technique to measure the failure strength of a silicon/SAM/gold system. This method consists in converting the thermal energy imparted by a very short (ten nanoseconds-long) Yag laser pulse to an absorbing layer placed on the back side of the substrate into a compressive acoustic pressure pulse that propagates through the substrate toward the interface of interest. Upon reflection from the free surface of the film, the pulse loads the film/substrate in tension, leading to its spallation failure. Preliminary results show a strong dependence of the failure strength of the interface on the choice of SAM. Detailed AFM and XPS analyses performed on the postspallation surfaces provide information on the roughness profile and chemical composition of the failure surface. On the modeling side, molecular dynamics (MD) simulations, based on the ReaxFF model, are used to investigate the separation characteristics and interfacial mechanical behavior between SAM and a thin gold film on a silicon substrate. Although these MD simulations predict the experimentally observed four-fold increase in the failure strength of SH-terminated SAMs compared with their CH3-terminated counterpart, the predicted values are between one and two orders of magnitude higher than those observed experimentally. To address this gap between experimental and simulated strength values, we investigate the key role played by the roughness of the substrate and of the film, using a multiscale approach that combines the cohesive model derived from the MD simulations and a continuum model of the bending response of the film. A substantial drop in the effective strength of the SAM-enhanced film/substrate interface is predicted for relatively small values of the roughness.