Porte-à-faux | EPFL Graph Search

Une installation ou système est dit en porte-à-faux lorsqu'un élément est soutenu par une partie qui est elle-même au-dessus du vide, c'est-à-dire sans support immédiat en dessous de l'élément en « porte-à-faux ». En ingénierie structurelle, en architecture, dans l'aéronautique et plusieurs autres domaines techniques, on utilise également le synonyme cantilever emprunté à l'anglais. Dans le langage courant, dire d'une installation qu'elle est en porte-à-faux évoque un risque de déséquilibre ou de rupture si elle est trop chargée, comme elle repose sur la solidité des supports qui la retiennent et que l'ensemble de l'installation n'est pas à l’aplomb de son point d’appui. En architecture Le porte-à-faux est également le nom architectural donné au départ d'une partie supérieure d'un bâtiment par rapport à sa base. L'un des plus importants porte-à-faux architecturaux à ce jour () est celui du Palais de la culture et des congrès de Lucerne (Kultur und Kongresshaus), réalisé

Fabrication and characterisation of ultrasensitive monocrystalline silicon cantilevers with integrated magnetic tips

Alain Dysli

During this project, ultrasensitive cantilevers designed for nuclear magnetic resonance applications were fabricated and characterised. The cantilevers are 500, 340 or 200 μm long, 10 or 20 μm wide and 500 or 340 nm thick and integrate a thin film of cobalt at the free end. This thin-film can be magnetised. Their fabrication requires novel techniques such as silicon-on-insulator technology, Al protective layer deposition, high anisotropic dry etching or HF-vapour release. A dynamic and a static model of the cantilevers were elaborated. The models describe the behaviour of the cantilevers when actuated by an external magnetic field in the air and in vacuum.

2006

Study of Silicon Nitride resonators for optomechanics and security applications

Alberto Barulli, Tom Larsen

In this work the design, fabrication and characterization of silicon nitride nano-mechanical resonators is presented, starting from silicon nitride thin films deposited on Si wafers according to ten different recipes. The properties of such devices will depend on design and material properties. As demonstrated in the previous work, silicon nitride material properties (such as stoichiometry, stress, roughness, refractive index, Young’s modulus) are strongly influenced by the deposition conditions (temperature, pressure, gas ratio and total gas flow). The long term goal of this project is to achieve the highest performances in NEMS resonators, and so high quality factors, finding out the best combination between deposition condition and device design. Most of the project is based on the microfabrication of membranes, cantilevers and beams. Two fabrications runs are performed: the purpose of the first fabrication run is to complete all the necessary trainings needed to use the tools in the cleanroom; the issues encountered during the first run are the starting point for the second fabrication run whose process flow is partially modified and optimized. In the last part of the thesis, some measurements about cantilevers deflection are shown. These measurements are carried out by using an optical profiler providing a 3D surface profile of devices.

2017

A novel microfabrication platform for hybrid multilayer MEMS

Nahid Hosseini

In Atomic force microscopy (AFM), the tip-sample interaction force can be measured through two primary detection techniques: optical beam detection (OBD) and electrical (self-sensing) readout. Compared to the optical method, the convenience of the self-sensing readout AFM measurements comes at the cost of higher force noise. In the self-sensing method, there is a trade-off between reducing the force noise and maintaining the cantilever characteristics (e.g. resonance frequency, spring constant, quality factor, and planar dimension) within the practical limits. The core of my research was the development of hybrid multilayer self-sensing cantilevers with up to one order-of-magnitude better force sensitivity than state-of-the-art silicon self-sensing cantilevers. Thanks to a material engineering approach combined with non-standard fabrication methods, the developed cantilevers are designed such that a polymer core is sandwiched between two hard thin films. The multilayer self-sensing cantilevers are designed to be thick and soft, thus combining increased deflection sensitivity with low spring constants, and hence increasing the force sensitivity. The high force sensitivity of the hybrid multilayer cantilevers is accompanied by a high detection bandwidth in AC modes. This originates from having a viscoelastic material as the main structural layer, which causes low quality factor and hence high tracking bandwidth. In terms of the imaging speed, the multilayer cantilevers show four times faster response compared to their silicon counterparts. In addition, the hermetically sealed self-sensing multilayer cantilevers can be deployed for various scanning probe microscopy (SPM) applications in liquid as well as in air and vacuum with additional coatings. For even further increase of the deflection sensitivity, newly developed high-gauge factor strain sensors can be incorporated to the multilayer cantilevers governed by their adaptable process flow. As a proof of concept, I show that atomically thin MoS2 piezoresistors can be incorporated into SU8 cantilevers. However, the MoS2 piezoresistors have very high resistance, which has an adverse effect on the force noise of the cantilevers. One common strategy to alleviate this high resistance is doping the MoS2 piezoresistors. In this work, I show that SU8 can act as a structural cantilever layer as well as an n-type doping source and an encapsulation solution for the MoS2 piezoresistors. In addition to the force resolution and the tracking ability, the quality and the repeatability of any AFM image is also correlated with the cantilever tip shape (sharpness) and durability. SU8 cantilevers have shown very good tracking ability but polymers are subjected to high wear-rate as a tip material. In the scope of my research, I have also developed fabrication recipes to integrate sharp, low wear-rate, silicon nitride tips into the pure SU8 cantilevers as well as the polymer-core multilayer cantilevers. Furthermore, to extend the ease of use and versatility of AFM, a closed-loop scanner based on a sidewall piezoresistive displacement sensor is presented. Such a closed loop scheme compensates the piezotube scanner nonlinearities, namely hysteresis and creep. This closed-loop system reshapes the piezotube drive signal through our developed FPGA-based Proportional-Integral (PI) controller.

EPFL2019

A novel microfabrication platform for hybrid multilayer MEMS

Nahid Hosseini

EPFL2019