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International audience; In this work, the thermo-optical properties of nematic liquid crystals are investigated through localized reflectance spectra measurements performed on a tunable LC-filter. The final aim is to insert such LC in the cavity of a tunable VCSEL device, in which local self-heating due to optical or electrical pumping must be taken into account. We demonstrate that, whereas standard LC E7 is not suited for real device operation, the maximal operating temperature of QYDPLC-36 LC is 95°C, with a birefringence at 1.55 μm higher than 0.18 at 60°C.

International audience; In this work we investigate a tunable 850nm laser based on a hybrid combination of a liquid crystal micro-cell and a half GaAs VCSEL. The target application is optical coherence tomography. The inherent tolerances of the hybrid technology, the presence of metals in the cavity and the need for a pure extraordinary mode lasing make challenging the optical design of this laser. Hence, VELM, a full 3D and vectorial VCSEL optical solver, is utilized to understand loss mechanisms and provide an optimized design.

There is a major interest in replacing cobalt binder in hardmetals (cemented carbides) aiming for materials with similar or even improved properties at a lower price. Nickel is one of the materials most commonly used as a binder alternative to cobalt in these metal-ceramic composites. However, knowledge on mechanical properties and particularly on fatigue behavior of Ni-base cemented carbides is relatively scarce. In this study, the fatigue mechanics and mechanisms of a fine grained WC-Ni grade is assessed. In doing so, fatigue crack growth (FCG) behavior and fatigue limit are determined, and the attained results are compared to corresponding fracture toughness and flexural strength. An analysis of the results within a fatigue mechanics framework permits to validate FCG threshold as the effective fracture toughness under cyclic loading. Experimentally determined data are then used to analyze the fatigue susceptibility of the studied material. It is found that the fatigue sensitivity of the WC-Ni hardmetal investigated is close to that previously reported for Co-base cemented carbides with alike binder mean free path. Additionally, fracture modes under stable and unstable crack growth conditions are inspected. It is evidenced that stable crack growth under cyclic loading within the nickel binder exhibit faceted, crystallographic features. This microscopic failure mode is rationalized on the basis of the comparable sizes of the cyclic plastic zone ahead of the crack tip and the characteristic microstructure length scale where fatigue degradation phenomena take place in hardmetals, i.e. the binder mean free path. (C) 2014 Elsevier Ltd. All rights reserved.

A low cost packaging solution has been successfully implemented on RF MEMS switches. The encapsulation is made at wafer level by thin film packaging, it allows keeping very small footprint for the device and is fully compatible with wire bonding and flip chip assembly. In addition to their highly shrinked size, packaged switches excellent microwave performances and promising behaviors in terms of reliability, especially for redundancy schemes required by space applications.

3D integration technology opens the way of heterogeneous silicon platform, as for example, millimeter-wave functionalities could be integrated in future communication modules. Consequently, copper pillar technology realized at 1st level of interconnection between silicon top dies and silicon interposer must be evaluated in this new frequency range. In this paper, a test vehicle has been designed to assess radio frequency behavior of 3D stacking as insertion losses (IL) up to 40GHz induce by copper pillar interconnection and assembly parameters (pitch, underfilling,...). Copper pillar ground-signal-ground (GSG) vertical transition exhibits IL lower than 0.2dB at 40GHz on high resistive silicon substrate and seems to be the minor contributor of all 3D interconnections as TSV, and RDL. Capillary underfill and GSG transition pitch have been evaluated to have a weak impact. Cu ring designed to limit bleed-out and fillet assures PAD integrity and induces a strong signal reflection above 15GHz. Finally, process assembly robustness of high aspect ratio top die (10x3mm^2) has been assessed with DC measurements.

A reflectarray element with electronic phase control implemented by ohmic MEMS switches is characterized and validated in the X-band. The proposed element is based on two patches aperture-coupled to a microstrip network with a common delay line, forming a sub-array, in order to reduce both cost and manufacturing complexity in large reflectarrays. The electrical length of the line can be modified through the inclusion of a series switch between different segments of the microstrip line. The ohmic electrostatic switch has been designed for RF applications and manufactured on a coplanar line. The transition between the coplanar line of the MEMS and the microstrip delay line has been implemented using gold wires which have been bonded to the printed pads. This connection leads to a high impedance line. The MEMS switches have been characterized using reflection and transmission measurements on microstrip lines for deducing an equivalent circuit, which has been used in the validation of the gathered element with electronic phase control measured using waveguide simulator. The same approach has been applied to evaluate a 2-bit reflectarray element.

In most of the applications where hardmetals are implemented, mechanical contact plays an important role from a design viewpoint, either as direct key parameter (e.g. wear resistance) or as indirect relevant factor (e.g. residual strength associated with contact-induced damage). In this work, the contact response of three cemented carbides WC–Co with different microstructural features is evaluated. The study is performed using spherical indenters (Hertzian contact) and it is focused on: 1) determining the critical loading parameters affiliated to the emergence and evolution of damage; and 2) investigating the relevance of microstructure on the levels of residual strength attained. It is found that strength retention is improved as contact damage mode goes from brittle to quasi-plastic, a transition directly dependent on microstructure. Hence, microstructural design searching for higher damage tolerance (i.e. deformation prevailing over fracture as damage mode) is concluded to be an optimal approach for the development of hardmetals with improved reliability. In practical terms, this is achieved by enhancing toughness through either higher binder content and/or microstructural coarsening, as far as the field application requirements are satisfied.

Massive integration of RF-MEMS switches is still jeopardized due to reliability issues. To overcome this challenge, this contribution presents a novel electrostatically-actuated MEMS switch. This novel switch integrates a new design: on the one hand a dielectric free actuator on order to reduce drastically the dielectric charging; and on the other hand a symmetrical stacking of the silicon nitride suspended bridge in order to have a switch behavior insensitive to thermal stresses. In addition to that, the process flow has been totally reviewed so as to better control the different gap heights and also reduce contact surface contamination. Among others, thermal oxidations and wet etchings processes are used to fabricate the lower parts of the switch, and amorphous silicon is used as sacrificial layer. Finally, these major evolutions have allowed to demonstrate remarkable results in terms of process capability, microwave performances and lifetime.

Dielectric charging and contact degradations are the two major challenges to improve the lifetime of series ohmic electrostatic MEMS switches. This paper details our approach and our main results to overcome the failures due to dielectric charging. We introduce a time predictive model for charge accumulation that we use as a design tool for reliability. The key parameters are listed and a new switch design is proposed to reduce the charging: the dielectric materials between the actuation electrodes are removed as much as possible. This dielectric-free design gives remarkable results in terms of dielectric charging sensitivity: the pull-in voltage decreases a little bit at initial times, and remains steady for longer time. The second major challenge, that remains to be considered, deals with the contact reliability. For that purpose, gold and ruthenium contacts have been investigated under several operating conditions. Gold contacts give good results in cold switching conditions whereas ruthenium suffers from surface contamination which increases quickly the contact resistance. In hot switching conditions, both materials are sensitive to material transfer mechanism, the failure rate being proportional to the open circuit voltage. Nevertheless, ruthenium seems more resistant to a material transfer than gold, and offers performances acceptable for some specific applications. A hermetic packaging at wafer level should allow to improve the contact reliability thanks to an efficient management of the surface contamination issue.

International audience; A 60 GHz cavity-backed antenna array integrated on high-resistivity silicon is demonstrated. The antenna design makes use of Through-Silicon-Vias (TSV), silicon micromachining, and wafer-to-wafer bonding to meet the bandwidth and radiation gain requirements for short-range multi-Gbps communications. The fabrication process is presented. Simulated and experimental results show that the antenna element covers easily the 57–66 GHz standard band with good impedance matching and more than 5 dBi of gain. Several fixed-beam four-element antenna arrays demonstrate the capabilities for beam-steering across a range up to ±60°.

Active control of VCSELs beam properties is a key issue to improve their integration in microsystems. We have designed a micro-optical system that allows for a dynamic displacement of the VCSEL beam. It consists of a polymer microlens associated to a SU-8 membrane vertically moved by means of a thermal actuator. This approach is suitable with laser sources arrays. We present results on optical design demonstrating that a small deflection of the membrane (2μm) could lead to a large displacement of the beam waist vertical position (in the millimetric range). Thermomechanical modelling is performed to evaluate the maximum membrane displacement achievable with this system. Finally, first feasibility results are presented.

The field emission current generated across the actuation pads in ohmic MEMS switches during ON state is shown to constitute an additional source of degradation. Switches with Au/Au and Au/Ru contacts have been subjected to 24 h continuous stress. In both cases the switch ohmic contact resistance and field emission current across actuation pads were monitored simultaneously. The experimental results revealed a negligible degradation in Au/Au contact devices while the Au/Ru contact devices show a fast degradation. The experimental results from Au/Au and Ru/Au contact switches have been compared taking into account the plasma generation in the actuation pads.

We present a low-cost fabrication technique of a polymer-based micro-optical-electrical-mechanical systems (MOEMS) suited for the dynamic focusing of VCSELs (vertical-cavity surface-emitting lasers). A simple method based on an SU(8) double exposure is proposed for the collective integration of small footprint transparent suspended membrane arrays on vertical laser diodes. We demonstrate that this kind of MOEMS can provide under thermal actuation a vertical displacement of around 0.2 ?m W?1?over a maximal range of 8??m. As a wide range of initial gaps between the membrane and the laser source can be chosen, this approach opens new insights into the dynamic control of the VCSEL beam waist position as well as for tunable VCSEL fabrication.