Resonant microelectromechanical systems are promising devices for real time and highly sensitive measurements. The sensitivity of such sensors to additional mass loadings which can be increased thanks to the miniaturisation of devices is of prime importance for biological applications. The miniaturisation of structures passes through a photolithographic process and wet chemical etching. So, this paper presents new results on the anisotropic chemical etching of the gallium arsenide (GaAs) crystal used for this application, in several solutions. This paper focuses on the micro/nanostructuration of the sensing surface to increase the sensor sensitivity. Indeed, this active surface will be biofunctionalized to operate in biological liquid media in view of biomolecules detection. Several experimental conditions of etching bath composition, concentration and temperature were examined to obtain a large variety of geometrical surfaces topographies and roughness. According to the orientation dependence of the chemical etching process, the experiments were also performed on various GaAs crystal plates. The bath 1 H3PO4:9 H2O2:1 H2O appeared to be particularly adapted to the fabrication of the GaAs microstructured membrane: indeed, the bath is highly stable, anisotropic, and, as a function of temperature, it allows the production of a large variety of GaAs surface topographies.
Piezoelectric actuators are widespread for the design of micro/nanorobotic tools and microsystems. Studies toward the integration of such actuators in complex micromechatronic systems require the size reduction of these actuators with a large range of performances. Two main fabrication processes are nowadays used for the fabrication of piezoelectric actuators, providing very different behaviors: i) the use of bulk PZT layer, ii) and the use of thin film growth. In this paper, we propose a trade-off between these tow extremes processes and technologies in order to explore new actuators performances. It resulted in the design and fabrication of thick film PZT unimorph cantilevers. They allowed the generation of high performances, both in the static (displacement) and dynamic (first resonance frequency) regimes, in addition to the small sizes. Such cantilevers size are obtained through the wafer scale bonding and thinning of PZT plate onto a SOI wafer. The piezoelectric cantilevers have a PZT layer of 26 µm thickness with a 5 µm thick silicon layer, over a length of 4 mm and a width of 150 µm. The experimental characterization has shown that the static displacements obtained are in excess of 4.8 µm V −1 and the resonance frequency up to 1103 Hz, which are useful for large displacements and low voltage actuators.
GaAs crystal presents some interesting perspectives for resonant biosensors due to its piezoelectric and good mechanical properties and the opportunity to bio-functionalize the surface. Moreover, GaAs can be micromachined by wet etching in several solutions, which constitutes a batch and low-cost process of fabrication. The lateral field excitation (LFE) is used to generate bulk acoustic waves. The main advantage of LFE is the possibility to measure in liquid media, but moreover reduced aging and increased frequency stability are also ensured. In this study, an analytical modelisation is used to determine the orientations of the vibrating membrane and the electric field that give satisfactory metrological performances. Electrical performances are discussed as a function of geometrical parameters. A simulation based on a Finite Element Modelisation is performed in order to optimize the design of the resonant structure. The microfabrication process of the structure is presented. The choice of etchants is discussed in terms of etch rates and surface textures. Several steps of the fabrication of the sensing area structure are shown and characterized. Finally, the active area is fabricated according to the theoretical and experimental results of this study.
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