Sophie Ryelandt

Polyimide as a versatile enabling material for microsystems fabrication: surface micromachining and electrodeposited nanowires integration

The interest of using polyimide as a sacrificial and anchoring layer is demonstrated for post-processing surface micromachining and for the incorporation of metallic nanowires into microsystems. In addition to properties like a high planarization factor, a good resistance to most non-oxidizing acids and bases, and CMOS compatibility, polyimide can also be used as a mold for nanostructures after ion track-etching. Moreover, specific polyimide grades, such as PI-2611 from HD Microsystems™, involve a thermal expansion coefficient similar to silicon and low internal stress. The process developed in this study permits higher gaps compared to the state-of-the-art, limits stiction problems with the substrate and is adapted to various top-layer materials. Most metals, semiconductors or ceramics will not be affected by the oxygen plasma required for polyimide etching. Released structures with vertical gaps from one to several tens of μm have been obtained, possibly using multiple layers of polyimide. Furthermore, patterned freestanding nanowires have been synthesized with diameters from 20 to 60 nm and up to 3 μm in length. These results have been applied to the fabrication of two specific devices: a generic nanomechanical testing lab-on-chip platform and a miniaturized ionization sensor.

On the control of the residual porosity in iron aluminides processed by reactive squeeze-infiltration of aluminium into a preform of steel fibres

An original approach for the synthesis of iron aluminides of the type FeAl1-x is presented, based on the use of a squeeze-casting equipment for inducing the reactive infiltration of Al into a porous preform made by sintering continuous Fe-based fibres. The casting is subsequently homogenised by heat treatment in the solid state. Preforms with fibre volume fractions ranging between 40 and 80% (fractions that correspond to stoichiometries close to FeAl and Fe3Al. respectively) are prepared using steel-wool fibres with various diameters. Small fibre diameters and high processing temperatures are found to be beneficial for obtaining, after squeeze casting a low-porosity material with a large amount of uniformly distributed reaction compounds. However, the preform thickness that can be properly infiltrated is limited to about 6 mm. The homogenisation of the intermetallic phase is complete after only 5 h at 1000 degreesC. A major problem to be solved is the heterogeneity of the porosity distribution in the sample after homogenisation. This heterogeneity results from an insufficiently uniform distribution of the fibre volume fraction

Ductility of thin metallic films

Depending on the loading conditions, geometry and material characteristics, the ductility of thin metallic films is controlled either by the resistance to plastic localization or by the resistance to internal damage. New on-chip tensile tests performed on submicron aluminium films show significant strain hardening capacity leading to relatively good resistance to necking, while damage occurs through void nucleation at grain boundaries followed by their growth and coalescence. These results are discussed in the light of several other studies presented in the recent literature in order to unravel the origins of the frequently reported poor ductility of thin metallic films, and the various means existing to improve it.

Multiscale characterisation of the plasticity of Fe-Mn-C TWIP steels

It is known for a long time that mechanical twinning occurring in FCC metal can bring about a very large work hardening rate. It is believed that deformation twins increase the work-hardening rate by acting as obstacles for gliding dislocations. This mode of plastic deformation is active, among others, in the Hadfield steel known for a very long time [1]. Fe-Mn-C grades exhibiting mechanical twinning present a renewed interest for some years since some applications are expected in the automotive industry. Several kinds of TWIP (Twinning Induced Plasticity) steels are now intensively studied.