| Biomechanics, an emerging science, refers to the mechanical characterization of biological tissues. Recent work published in this field demonstrate the role of mechanical processes and properties on the biological tissues functionalities, and especially at the microscopic scale (cell biomechanics).;In parallel, the fast developments observed these last years in microtechnologies lead to interesting research possibilities. The family of MEMS3 devices for instance introduces a new potential of interaction with the microscopic world. The integration of this technology in the field of cellular biomechanics is thus a natural choice.;In that context, this work aims to design a 3-axis microforce sensor to measure biological tissues deformations at the microscopic scale. The MEMS device, fabricated on SOI4 wafers, is based on piezoresistive and capacitive force transductions. It can be used as an actuator at least in one direction.;This thesis describes the design, fabrication and test of the 3-axis system. A 1-axis prototype, exclusively capacitive, is first realized and acts as the foundation of the 3-axis device. The 1-axis force sensor, tested on the [0 -- 350muN] range shows a sensitivity in the order of 4.85mV/muN (G=2000) and a resolution of 1.24mu N (linearity until 100muN).;Biomechanical data acquisition is however quite challenging. This requires appropriate measurement tools (for forces, strain, ...) to cope with the biological sample and environment constraints (biocompatibility, size, anisotropy, ...).;A new 3-axis geometry is then proposed to improve the direction decoupling efficiency of 2-axis capacitive sensors presented in publications and add a third detection axis. The decoupling is achieved using a "two frames" geometry and piezoresistors implanted in a configuration only sensitive to an out-of-plane loading. The three transducers performances are analysed individually. Tested on a range of 250muN, the sensors show a linear behaviour on the whole forces domain in the out-of-plane axis (piezoresistors) and until 100muN in the in-plane direction (electrostatic combs). The piezoresistive and capacitive transducers are characterized by sensitivities of 0.93mV/muN (g=400) and 6.35mV/muN (G=500) respectively (on the linear part), with resolutions of 7mu N and 0.161muN. The dynamical behaviour of the sensor allows its use above the kHz. The cross-talk sensitivities of each transducer are evaluated to 1-5% of their axis sensitivity (decoupling).;The work presented in this thesis demonstrates the feasability of a 3-axis MEMS force sensor based on capacitive (in-plane sensing) and piezoresistive (out-of-plane sensing) detection. The proof of concept refers to the fabrication and performances (sensitivity, resolution, decoupling) of the proposed design.;Keywords : force sensor, MEMS, capacitive transduction, piezoresistive transduction, electrostatic combs, piezoresistors, biomechanics, biosensor, soft tissues, Deep Reactive Ion Etching - DRIE, SOI (Silicon on Insulator), 3-axis.;3MicroElectroMechanical Systems. 4Silicon on Insulator. |
