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International audience

International audience

RF MEMS (Radio Frequency Micro Electro Mechanical System) switches are promising devices but their gold-on-gold contacts, assimilated for this work to a sphere / plane contact, represent a major reliability issue. A first step towards failure mechanism understanding is the investigation of the contact metal microstructure evolution under static and cyclic loading. After static and cyclic loading of sputtered gold thin films under spherical indentation, high resolution Electron Back Scatter Diffraction (EBSD) is used to investigate contact area. Grain rotation against {111} fiber texture of 1 μm thick sputtered gold thin film is a signature of plastic deformation. Grain rotation is observed above 1.6 mN under static loading by a spherical diamond indenter with 50 μm tip radius. A heterogeneity in grain rotation is observed corresponding to a more important plastic deformation in the middle of the indent than at the edge. A 30° Grain rotation is observed for a half million mechanical cycles under 300 μN load by a spherical gold tip (20 μm radius) due to cyclic work hardening. The same test in hot switching mode induces a grain growth in the contact area. Therefore thermal effects occurring during hot switching are underlined

International audience; Inkjet-printing of interconnects is a maskless technology that has attracted great interest for printed electronics and packaging applications. Gemalto is expecting by motivated and developing skills and knowledge in this area to be at the forefront of European Security innovation and to answer to a continuous market pressure for higher security, lower cost and more secure complex systems. With an increasing need for flexible and mass deliveries of advanced secure personal devices such as: electronic passports, ID cards, driver licenses, other smartcards, e-documents and tokens. EMSE is seeing in these new developments an exciting brand new area of research situated between material science and electronics. In this frame, deposit and pattern creation for chip interconnection require specific behaviors which have to be scientifically understood to pursue industrial harmonious implementation. Both groups collaborated on inkjet-printed electronic routing from deposition to sintering and characterization, using collaborative means provided on Micro-PackS platform.

International audience; With the recent development of printed electronics on flexible substrate, urge for low temperature sintering process has emerged. In the frame of temperature sensitive substrates like plastics (PET, ABS, PC, etc.) conventional convective oven cannot be suitably used to achieve near‐bulk electrical resistivity. Consequently, several alternative sintering technologies have been reported and tested. In this present work, the prospective advantage of variable frequency sintering is studied for selective and fast sintering of metal inks. This variable frequency mode (VFM) improves the distribution of the electromagnetic field within the cavity and impedes arcing phenomena that could happen on metal surface. Inkjet-printed devices were processed using a VFM mode with a central frequency of 6.425 GHz with a bandwidth of 1.15 GHz and a sweep rate of 0.1 second has been used. A solid calibration step is required for temperature measurements and control during the process. The procedure includes a camera-based observation coupled with a pyrometer and an optic fiber. A reliable determination of the substrate and ink emissivity is compulsory to tailor the temperature feedback so as to not overcome the substrate's glass transition. The direct impact of microwave has been analyzed comparatively to conventional sintering assisted sintering where a thermally conductive susceptor or matrix is exploited. A huge shrink in electrical resistivity is particularly observable for samples prepared with conventional rapid thermal annealing (RTA), while those prepared in VFM quickly exhibit resistivities below 8 μΩ.cm, about 6 time the silver bulk resistivity. The electrical resistivity shrinkage is mainly originating from the contribution of the grain size. Through a strong coupling between microwave and silver nanoparticle, selective sintering of metallic parts is achieved on low temperature substrates enabling very low electrical resistivity without damaging the plastic substrate. These promising results open ways to successful implementation of microwave sintering in industrial process for flexible electronic application.

As an alternative technology to replace lithographic process, direct printing of conductive nanofluids can thus be used to fabricate high resolution patterns, such as antennas and conductive redistribution lines. In the past few years, the use of metallic nanoparticles has undergone major development in microelectronics thanks to sintering achievements. To preserve electronic components functionalities, thermal budget need to be carefully considered, and thus a limited sintering temperature below 300°C should be used when printing nanoparticles. Nonetheless, the absence of any compacting step requires an exploration of novel sintering approaches to achieve a sufficient coalescence. In this frame, several sintering processes (thermal oven, rapid thermal annealing, selective microwave and laser curing) have been investigated to optimize the annealing of those thin films. The aim of this work is to analyze the induced microstructure of the above-mentioned sintering processes, and to determine how electrical and mechanical characteristics are consequently impacted. According the sintering technique and process parameters, an average grain size up to one micron can been achieved from initial mean particle size of 40nm, without exceeding a temperature range of 300°C. The microstructure characterization has been performed using electron backscatter diffraction (EBSD) in a field emission gun scanning electron microscope. Microstructural evolution of various sintered inkjet-printed films has thus been correlated with electrical resistivity and with the Young's modulus determined by nanoindentation. A strong link between those three parameters was highlighted during experiments giving credit to either a surface or a fully volumetric sintering, according the process. Sintering is then mainly triggered respectively by surface mass transfer or by grain boundary diffusion. Particular interests are shown for selective sintering especially when using microwaves and laser. Silver thin-films with an electrical resistivity three times higher than the bulk can be reached in a few minutes for a Young's modulus of about 50 GPa. In the meantime, the convective rapid thermal processing exhibits a various electrical resistivity and grain size. A Young's modulus even close to the bulk can be obtained when using a fast temperature ramp at 150 °C/sec for 15 minutes. This microstructural tailoring of printed silver nanoparticles were successfully implemented in the post-process step to achieve 600 nm thick films with very low resistivity and improved mechanical stiffness.

Nanoparticles have attracted tremendous interest for their thermodynamic size effect which is particularly valuable to reduce their sintering temperature. This asset is exploited for the fabrication of flexible electronic devices using printing technologies, especially interconnection features. Those printed interconnections require both a low electrical resistivity and high mechanical properties which are largely correlated with microstructure. In this paper, the combined effects of the substrate crystalline orientation and the sintering condition have been demonstrated to have a significant impact on microstructures. Silver nanoparticles with a mean diameter of 30 nm were inkjet-printed on {100} silicon substrates coated with either amorphous Si3N4 or with strongly textured {111} Ti/Au. The crystallographic texture and grain size of those printed films have been investigated after being sintered up to 500°C at 50°C/s using Electron Back-Scattered Diffraction. The results shows that a {111} fiber texture is developed above 300°C only on oriented films through out-of-plane diffusion. Electrical resistivity shows no dependency on texture since it is mainly affected by grain size. An optimal value of 3.4 μΩ.cm is achieved at 300°C using a fast sintering ramp of 10°C/s. Regarding the Young's modulus of nanofilms, nanoindentation measurements exhibited an oscillation between 38 GPa for smaller crystallites to 81 GPa for the biggest, which is compatible with wire bonding process.

International audience; The authors have investigated if silver nanoparticles can possibly be used for direct writing of conductive patterns. In this paper, a colloidal suspension based on 80 nm silver nanoparticles has been printed with an automated drop-on-demand prototype printer. Prior to any printing of patterns, Differential Scanning Calorimetry (DSC) is investigated as a mean to follow the sintering activation phenomenon according thermal treatment. DSC experiments have thus been performed jointly with SEM observations to optimize the curing behaviour of ink by activating coalescence mechanisms. Printed test vehicles have been tested to validate previous DSC/SEM results. Features as fine as 90 μm wide and 250 nm thick have been successfully printed on silicon nitride substrate. Thermal treatment entailed evident microstructural evolution – with final grain size up to 5 μm – coupled with great reduction of electrical resistivity down to 8 μOhm.cm (i.e. five times the bulk resistivity of silver).

International audience; A series of in-situ tensile tests was performed at various low temperatures to characterize the mechanical properties and the fracture processes of the 16MND5 steel: the damage was observed with a SEM and the internal stresses in the ferritic phase were determined by XRD.

International audience; A specific in-situ tensile device has been set up in an X-Ray diffractometer. Stress can be measured at low temperature during loading in the baintic steel and in the ferritic phase. A two-scale modeling has been proposed and evaluated on three levels, means stress in the baintic steel, stress in the ferritic phase and stress heterogeneities in the ferritic phase. Simulations are in general agreement with experiments.

Atomic force microscopy images supply precise information on real surface topography, but an automated method allowing accurate description of roughness through an asperity based approach is necessary for tribological applications. The present paper describes an original selective image analysis algorithm enabling confident indexation and analysis of surface asperities. First, the original image undergoes noise filtering followed by edge detection and thresholding. In the resulting binary image, clusters of asperities are separated by a selective treatment depending on objects' size and convexity. Finally, a regrowth of identified objects is applied to ensure their accurate three-dimensional shape estimation using a second order polynomial least square approximation. The identification performance of the method is hereby verified on an artificial image and illustrated with surface images of sputtered gold thin film alone and with an additional layer of ruthenium.