Xavier Maeder

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.

Nanomechanical investigation of thin-film electroceramic/metal-organic framework multilayers

Thin-film multilayer stacks of mechanically hard magnetron sputtered indium tin oxide (ITO) and mechanically soft highly porous surface anchored metal-organic framework (SURMOF) HKUST-1 were studied using nanoindentation. Crystalline, continuous, and monolithic surface anchored MOF thin films were fabricated using a liquid-phase epitaxial growth method. Control over respective fabrication processes allowed for tuning of the thickness of the thin film systems with a high degree of precision. It was found that the mechanical indentation of such thin films is significantly affected by the substrate properties; however, elastic parameters were able to be decoupled for constituent thin-film materials (EITO ≈ 96.7 GPa, EHKUST−1 ≈ 22.0 GPa). For indentation of multilayer stacks, it was found that as the layer thicknesses were increased, while holding the relative thickness of ITO and HKUST-1 constant, the resistance to deformation was significantly altered. Such an observation is likely due to small, albeit sign...

Silicon etch with chromium ions generated by a filtered or non-filtered cathodic arc discharge

Abstract The pre-treatment of substrate surfaces prior to deposition is important for the adhesion of physical vapour deposition coatings. This work investigates Si surfaces after the bombardment by energetic Cr ions which are created in cathodic arc discharges. The effect of the pre-treatment is analysed by X-ray diffraction, Rutherford backscattering spectroscopy, scanning electron microscopy and in-depth X-ray photoemission spectroscopy and compared for Cr vapour produced from a filtered and non-filtered cathodic arc discharge. Cr coverage as a function of ion energy was also predicted by TRIDYN Monte Carlo calculations. Discrepancies between measured and simulated values in the transition regime between layer growth and surface removal can be explained by the chemical reactions between Cr ions and the Si substrate or between the substrate surface and the residual gases. Simulations help to find optimum and more stable parameters for specific film and substrate combinations faster than trial-and-error procedure.

Improved test setup for MEMS mechanical strength investigations and fabrication process qualification

The mechanical stability of silicon MEMS dies is strongly influenced by the microfabrication processes, especially grinding, dicing and etching, which leave characteristic damage (defects, cracks, dislocations…) in the substrate material. Specially designed mechanical tests are used to assess the resistance of micro-structures to monotonic and cyclic loading. We report on the development progress of a micromechanical test bench for reliability assessment of microstructures in 2-, 3- and 4-point bending configurations. Strain distributions and defects in micron-sized silicon devices can be investigated by in-situ testing in combination with high-resolution x-ray diffraction measurements for experimentally assessing the strain distribution.