Christoph Niederberger

Compression of freestanding gold nanostructures: from stochastic yield to predictable flow

Characterizing the mechanical response of isolated nanostructures is vitally important to fields such as microelectromechanical systems (MEMS) where the behaviour of nanoscale contacts can in large part determine system reliability and lifetime. To address this challenge directly, single crystal gold nanodots are compressed inside a high resolution scanning electron microscope (SEM) using a nanoindenter equipped with a flat punch tip. These structures load elastically, and then yield in a stochastic manner, at loads ranging from 16 to 110 microN, which is up to five times higher than the load necessary for flow after yield. Yielding is immediately followed by displacement bursts equivalent to 1-50% of the initial height, depending on the yield point. During the largest displacement bursts, strain energy within the structure is released while new surface area is created in the form of localized slip bands, which are evident in both the SEM movies and still-images. A first order estimate of the apparent energy release rate, in terms of fracture mechanics concepts, for bursts representing 5-50% of the structures initial height is on the order of 10-100 J m(-2), which is approximately two orders of magnitude lower than bulk values. Once this initial strain burst during yielding has occurred, the structures flow in a ductile way. The implications of this behaviour, which is analogous to a brittle to ductile transition, are discussed with respect to mechanical reliability at the micro- and nanoscales.

Quantitative stress/strain mapping during micropillar compression

Micropillar compression is increasingly used as a method to examine small length-scale mechanical properties since it minimises the strain gradients that are unavoidable during nanoindentation into a flat surface. It also simplifies the data analysis since it is assumed that the compression is uniaxial. But how valid is this assumption when misalignments in the microscale setup are generally unavoidable? In order to investigate this, the stress and strain tensors of the micropillar should be measured during the compression. To accomplish this, electron backscatter diffraction (EBSD) mappings were obtained before, during and after consecutive compressions of a GaAs micropillar inside of a high-resolution scanning electron microscope. Elastic strain and the corresponding stress tensors were determined from the EBSD measurements using a cross-correlation technique. The results show that the von Mises stress compares well to the engineering stress determined from the nanoindenter load–displacement data. Even with rotations of almost 3° at maximum load, the negligible shear components in the strain tensor within the pillar indicates that the assumption of uniaxial compression can be made. Due to its high spatial resolution and strain sensitivity, in situ strain–stress mapping during micromechanical testing is a very promising route for the investigation of deformation phenomena on the sub-micron scale.

Compression of Nanowires Using a Flat Indenter: Diametrical Elasticity Measurement

A new experimental approach for the characterization of the diametrical elastic modulus of individual nanowires is proposed by implementing a micro/nanoscale diametrical compression test geometry, using a flat punch indenter. A 250 nm diameter single crystal silicon nanowire is compressed inside of a scanning electron microscope. Since silicon is highly anisotropic, the wire crystal orientation in the compression axis is determined by electron backscatter diffraction. In order to analyze the load-displacement compression data, a two-dimensional analytical closed-form solution based on a classical contact model is proposed. The results of the analytical model are compared with those of finite element simulations and to the experimental diametrical compression results and show good agreement.

Toward Local Growth of Individual Nanowires on Three-Dimensional Microstructures by Using a Minimally Invasive Catalyst Templating Method

We present a novel minimally invasive postprocessing method for catalyst templating based on focused charged particle beam structuring, which enables a localized vapor-liquid-solid (VLS) growth of individual nanowires on prefabricated three-dimensional micro- and nanostructures. Gas-assisted focused electron beam induced deposition (FEBID) was used to deposit a SiO(x) surface layer of about 10 × 10 μm(2) on top of a silicon atomic force microscopy cantilever. Gallium focused ion beam (FIB) milling was used to make a hole through the SiO(x) layer into the underlying silicon. The hole was locally filled with a gold catalyst via FEBID using either Me(2)Au(tfac) or Me(2)Au(acac) as precursor. Subsequent chemical vapor deposition (CVD)-induced VLS growth using a mixture of SiH(4) and Ar resulted in individual high quality crystalline nanowires. The process, its yield, and the resulting angular distribution/crystal orientation of the silicon nanowires are discussed. The presented combined FIB/FEBID/CVD-VLS process is currently the only proven method that enables the growth of individual monocrystalline Si nanowires on prestructured substrates and devices.

Cyclic loading for the characterisation of strain hardening during in situ microcompression experiments

Abstract Various parameters from fabrication and testing are known to influence the behaviour for microcompression experiments, especially the work-hardening behaviour. In this regard, the most important factor is found to be the availability of unconstrained slip planes. The second-most important factor is the lateral constraint acting on the sample, which usually arises from friction between indenter and pillar in a combination with a laterally stiff indenter setup, which can generate significant grain rotation and deviation from ideal single slip behaviour. The effect of lateral constraints on the strain-hardening rate is demonstrated on single crystals of gold and a novel solution by cyclic loading conditions is suggested, which could provide comparable conditions for different types of indenter hardware. In this work, the cyclic loading method is shown to minimise the influence of lateral constraints and provide more accurate measurements of strain-hardening behaviour than commonly applied microcompression methods by preventing grain rotation due to frictional constraint.