NIST Nanomechanical Properties Group researchers have developed a scanning probe microscopy technique that allows the stress state at buried interfaces in nanostructures to be determined. The method was applied in the investigation of the mechanical properties of as-grown and oxidized Si nanowires (NWs) as a function of wire diameter. The wires were grown and oxidized by NIST Metallurgy Division researchers. From contact-resonance atomic force microscopy (CR-AFM) measurements, the effect of the compressive stress at the Si-SiO2 interface was revealed in the diameter dependence of the elastic modulus of Si NWs oxidized at 900 °C and 1000 °C, with diameters in the range 30 nm to 90 nm. A modified core-shell model that includes the interface stress developed during oxidation captured the observed diameter dependence. The magnitude of the compressive stress, about a few gigapascals, as well as the width of the stressed transition region at the Si-SiO2 interface, about a nanometer, agree with those reported in simulations and experiments. Diameter-independent values were observed for the elastic modulus of as-grown and fully oxidized Si NWs. For these NWs, the elastic moduli determined from CR-AFM measurements are in the same range as those found in tensile tests on single crystal Si NWs and three-point bending tests on amorphous SiO2 NWs.
As building blocks for nanoscale devices, Si NWs are distinctly remarkable as they possess exceptional electrical, optical, mechanical, piezoelectric, and thermoelectric properties and bring the previous decades' knowledge of silicon technology into the fabrication process. Measuring and controlling the mechanical properties of nonplanar Si-SiO2 interfaces generated during fabrication are critical in the development of Si nanostructures for Si-based integrated circuits and other applications. The great advantage of the method developed in this investigation is that it provides local stress evaluation at nanoscale locations, thus enabling developers of devices utilizing Si and other NWs to optimize processing and design for new and improved device performance and reliability.
The work has been published online: "Compressive Stress Effect on the Radial Elastic Modulus of Oxidized Si Nanowires," G. Stan, S. Krylyuk, A.V. Davydov, and R.F. Cook, Nano Letters DOI: 10.1021/nl100062n (2010).
Additional Technical Details:
- This effort is part of the Nanoscale Stress Measurements and Standards Project