Damien Fabrègue

MEMS-based microstructures for nanomechanical characterization of thin films

The measurement of mechanical properties of thin films is a major issue for the design of reliable microelectronic devices, microsensors or thin coatings. New simple microstructures actuated through the release of internally stressed long beams made of high temperature, low pressure chemical vapour deposition silicon nitride have been developed to test under uniaxial tension submicron thin film material specimens. The relative displacement between a fixed and a moving cursor is used to determine the strain applied to the specimen. The stress is inferred based on the mismatch strain and Youngs modulus of the silicon nitride actuator beam. By multiplying the tensile test microstructures with different lengths, the full stress-strain curve characterizing the thin material sample is generated from which the elastic stiffness, yield strength, ductility and fracture stress can be extracted. The potential of the method is demonstrated through applications on both brittle and ductile thin films. The Youngs modulus of 238 GPa for a 373 nm thick silicon nitride film is extracted and size effects are observed for the yield strength of pure aluminium with a value of 220 and 550 MPa, respectively, for 373 and 205 nm thick films. An original variant of the procedure based on this new test microstructure for measuring Youngs modulus is also presented.

Core/shell nanoparticles for multiple biological detection with enhanced sensitivity and kinetics

The paper shows the different methods to attach a molecule to detect streptavidin to a dielectric particle made of a rare-earth oxide core and a polysiloxane shell containing fluorescein. First, the detection of streptavidin binding on a biotinylated gold substrate can be achieved in three ways: the shift of the surface plasmon resonance of the substrate and the double luminescence (organic and inorganic) of the core/shell particle. Second, these detections are efficient even after elimination upon thermal annealing of all the undesired molecules that skew the assays. Finally, the particle that ballasts the protein enhances its binding kinetics and increases the localized surface plasmon resonance shift that detects the binding.

Modeling Grain Boundary Motion and Dynamic Recrystallization in Pure Metals

The current study proposes a new approach of modeling discontinuous dynamic recrystallization in pure copper and cobalt based on the inverse analysis of experimental data. This approach comprises two steps: First, the mobility of grain boundaries is determined by a mean-field model in the steady state regime, then in a second step the information collected (mobility, nucleation frequency) is used to determine the mechanical behavior and the grain size change. The nucleation criterion is reformulated in a more general expression, and a new expression of the nucleation frequency with a single empirical parameter is proposed. The model predicts the stress–strain curves and the evolution of mean grain size, and is in good agreement with experimental data for both copper and cobalt. The modeling procedure requires a minimum of initial material parameters and could be especially attractive in the case of complex metals and alloys for which these parameters are unknown.

Direct Copper Bonding for Power Interconnects: Design, Manufacturing, and Test

3-D power module structures allow for better cooling and lower parasitic inductances compared with the classical planar technology. In this paper, we present a 3-D technology that uses an innovative assembly method (direct copper-to-copper bonding). The concept and manufacturing process of this technology is described in detail. An accurate electrical characterization is then performed to compare its performance with that of the classical planar structures.

Assessment of consolidation of oxide dispersion strengthened ferritic steels by spark plasma sintering: from laboratory scale to industrial products

Abstract Oxide dispersion strengthened steels are new generation alloys that are usually processed by hot isostatic pressing (HIP). In this study, spark plasma sintering (SPS) was studied as an alternative consolidation technique. The influence of the processing parameters on the microstructure was quantified. The homogeneity of the SPSed materials was characterised by electron microprobe and microhardness. A combination of limited grain growth and minimised porosity can be achieved on semi-industrial compact. Excellent tensile properties were obtained compared to the literature.

Spark plasma sintering of pure iron nanopowders by simple route

Abstract Pure iron nanopowders have been sintered using the spark plasma sintering technique. Fully dense materials can be achieved by a simple route. The influence of process parameters on density and hardness has been studied. The samples exhibit high hardness characteristic of nanostructured iron. This is confirmed by grain size measurements by X-ray diffraction and TEM. Moreover, when submitted to compression, the sinter exhibits high maximal strength with non-negligible ductility.

Liquid zinc embrittlement of a high-manganese-content TWIP steel

The cracking resistance of a twinning induced plasticity (TWIP) steel with high-manganese content has been studied in relation to the liquid metal embrittlement phenomenon. This phenomenon was investigated by hot tensile tests carried out on electrogalvanised specimens using a Gleeble® simulator at temperatures ranging from 600°C to 1000°C. The results show that this steel can be embrittled by liquid zinc within a limited range of temperature depending on strain rate.

Application of the Spark Plasma Sintering Technique to Low-Temperature Copper Bonding

Planar structures, in which a power die is soldered on a substrate and wirebonds are used to connect the top of the die with the substrate, are limited in terms of thermal management and power density. 3-D packaging techniques have been proposed to overcome these limits. Here, an innovative copper-to-copper bonding solution is presented, that can be used for 3-D packaging. The bonding process is described and the effect of the bonding parameters is investigated. It is found that this technique is compatible with the requirements of power electronic packaging. A test assembly including a silicon power die and ceramic substrates is presented.

Grain growth and static recrystallization kinetics in Co-20Cr-15W-10Ni (L-605) cobalt-base superalloy

The grain size evolution of cold-rolled L-605 cobalt-base superalloy during ultra-rapid annealing is investigated in this paper. Cold-worked specimens undergo static recrystallization, leading to grain refinement or grain coarsening depending on the annealing time and temperature. The kinetics of grain growth is found to be independent of the initial deformation. The evolution of grain size can be simply described by a grain growth model for high temperatures and long annealing times, and the mobility of interfaces is estimated by modelling. Fast annealing treatment process is a very promising technique to customize grain size and enhance mechanical strength. In particular, the reduction of annealing time is an efficient method to produce a refined microstructure through static recrystallization.

Elaboration of Architectured Materials by Spark Plasma Sintering

Spark plasma sintering has been used for decades in order to consolidate a wide variety of materials and permitting to obtain fully dense specimens. This technique has been mainly applied to ceramics. This paper concentrates on an unusual use of spark plasma sintering system: obtaining innovative materials especially architectured ones. Different applications are presented. Firstly, the SPS technique has been used to elaborate nanometers grain size materials or containing nanoscale microstructure. This is possible since the sintering temperature and the holding time are far lower in the SPS compared to other techniques. Then SPS has been used to realize diffusion bonding. In that case again, bonding can be realized at low temperature and for short time. It permits for example to realize bonding between two copper layers which is of a great importance for microelectronic applications. It is worth noting that this bonding can have the same mechanical strength as pure copper even for diffusion time of a few minutes. Secondly, bonding has been also carried out between a metallic layer and a ceramic one. This could lead to design of new layered materials combining interesting properties in terms of mechanical strength but also in terms of electrical resistance. The SPS machine has also been used to obtain porous materials (cobalt alloys or copper) with an adapted microstructure (porosity, tortuosity,). These structures could open new perspectives for biomedical or for microelectronic applications. All these examples lead to a better understanding of the physical processes which happen during spark plasma sintering.