Full-field measurements for the mechanics of micrometer-sized structures

As micromachined commercial products with mechanical features today exploit only the integration capabilities microfabrication technologies allow (for mass-production of reliable products), there is room for innovative products making the most of micrometer-sized objects, which are very specific from the mechanical point of view for two reasons:- Their surface/volume ratio is much larger than for the objects mechanical engineers are used to deal with. The consequence is that strong surface couplings have been evidenced, translating changes in the (electro-) chemical environment into mechanical deformation.- the geometric margins (compared to the dimensions) and the material homogeneity resulting from the usual processing techniques are very poor.As a consequence, studying the mechanical behavior of micrometer-sized objects requires to overcome two main barriers:- To identify material constitutive laws at the micrometer scale and to quantify the role of the environment on the behavior;- To model chemo-mechanical couplings when materials are heterogeneous and structures are poorly defined.Moving forward along these two lines requires the development of a dedicated instrumentation and the use of identification techniques suited both to the available experimental data and to the proposed mechanical descriptions. As a consequence: - The full-field measurements techniques proposed within the last few years for the mechanical characterization at the micrometer scale are presented, and the various extensions of scanning microdeformation microscopy toward a quantitative local characterization of anisotropic thin-film materials are detailed.- The various experimental tools developed to measure and control chemo-mechanical surface couplings are first described, and two proposed frameworks for modeling these surface couplings (based on asymptotic analysis and on second-strain gradient elasticity) are detailed

The equilibrium gap method with modeling parameters: identifiability conditions and sensitivity

The use of a Nomarski shear-interferometer with a sinusoidal phase modulation and four integrating buckets allows one to obtain the displacement field of the surface of a micro-cantilever observed in reflection microscopy. One can apply an electrostatic loading to this micro-object, which is first represented as an unknown pressure field. When retrieving both the flexural stiffness field and the load field using the equilibrium gap method, one may model the applied loading as a pure pressure field. This paper intends first to assess the effect of a modeling error, and then to test the identifiability conditions if one uses a parameterized description of the loading to fit measured kinematic data. The influence of the measurement noise on the identified parameters is then semi-analytically derived, and the global identification algorithm is applied to experimental data

Imaging interferometry to measure surface rotation field

International audienceThis paper describes a polarized light imaging interfe-rometer to measure the rotation field of reflecting surfaces. This set-up is based on a home-made prism featuring a birefringence gradient. The arran-gement is presented before focusing on the home-made prism and its manufacturing process. The dependence of the measured optical phase on the rotation of the surface is derived, thus highlighting the key parameters driving the sensitivity. The system's capabilities are illustrated by imaging the rotation field at the surface of a tip-loaded polymer specimen

Charge redistribution in electrochemically-actuated mechanical sensors

International audienceMany proofs of concept studies have established the mechanical sensitivity of functionalized microcantilevers to a large spectrum of target molecules. However, moving to real-life applications also requires the monitored mechanical effect to be highly specific. Moving towards more specificity in cantilever-based sensing, monitoring the mechanical response of electrochemically actuated microcantilevers is then thought to provide a fast, reliable and complementary experimental information to the long-time cantilever bending measurement for the detection of target molecules. Full-field measurements are therefore used to investigate the way the electro-elastic coupling is altered when a microcantilever undergoes decane-thiol adsorption. The proposed technique reveals that the latter results in a charge density redistribution along the cantilever in addition to the local surface passivation. Focusing on the cantilever tip displacement under electrochemical actuation, this redistribution partially compensates the electro-elastic coupling alteration due to the surface passivation, therefore possibly yielding an ambiguous detection result. This effect should be taken into account for the optimal design of specific electrochemically actuated mechanical sensors

Design and fabrication of a multiple-thickness electrochemical cantilever sensor

International audienceThis paper presents a new design and fabrication process of a multiple-thickness electrochemical cantilever sensor, in order to assess the role of the cantilever's thickness on the chemically-induced mechanical effects. Each cantilever can act not only as a functionalized cantilever, but also as an independent working electrode (WE) for electrochemical measurement. The different thicknesses of the silicon nitride layer are achieved by successive masking and reactive ion etching of partially overlapping openings at a low etch rate (10.8 nm/min, 15.8 nm/min, 20.1 nm/min, or 26.5 nm/min). A small-scale thickness difference (<30 nm) is successfully obtained. One advantage of this fabrication process is that the thickness distribution of cantilevers can be altered and broadened by combination of different RIE recipes or modification of the etching time. In addition, the integration of the cantilever chip with a fluidic cell, a printed circuit board (PCB) and a temperature-controlled plate to form a hybrid system is also addressed

Mécanique des objets micrométriques : quelle mesure pour quelle identification?

On s'intéresse au comportement mécanique d'objets de dimensions micrométriques. A mesure que leurs dimensions diminuent, leur surface joue un rôle de plus en plus important dans leur comportement mécanique. Si cette propriété permet d'envisager la conception de capteurs d'un nouveau genre, elle introduit une difficulté expérimentale particulière dans l'étude de leur comportement mécanique : lors d'un essai, l'état (mécanique, électrique, chimique, etc...) de la surface des objets considérés doit être maîtrisé. Or, on ne dispose pas de moyen permettant de définir un état chimique homogène en surface à l'échelle considérée. Par conséquent, toute identification doit prendre en compte le caractère hétérogène des phénomènes en jeu, rendant nécessaire une mesure de champs. En particulier, la prise en compte de l'hétérogénéïté du chargement est cruciale. On décrit donc différentes stratégies d'identification, qui peuvent être classées en deux grandes catégories, suivant qu'elles utilisent les seules grandeurs cinématiques ou qu'elles prennent en compte des informations complémentaires, et qui permettent de s'affranchir de cette difficulté. Une classification de l'ensemble de ces stratégies est alors proposée, proposant un cadre mécanique pour les distinguer et mettant en évidence leurs liens avec des techniques développées à l'échelle macroscopique

Simultaneous measurement of Young's modulus and Poisson's ratio at microscale with two-modes scanning microdeformation microscopy

International audienceIn this paper, we present a technique to simultaneously measure Young's modulus E and Poisson's ratio ν of an isotropic material at local scale in a single experiment. Using several flexural modes of vibration of the scanning microdeformation microscope, it is possible to decouple the contributions of E and ν from the first two resonant frequencies, thereby providing access to both the elastic parameters. The proposed approach is applied to SU8 thin films deposited on silicon substrates and provides values consistent with those from the literature

Multiple wavelengths reflectance microscopy to study the multi-physical behavior of MEMS

International audienceIn order to characterize surface chemomechanical driving micro-electro-mechanical systems (MEMS) behavior, we propose herein a method to simultaneously obtain a full kinematic field describing the surface displacement and a map of its chemical modification from optical measurements. Using a microscope, reflected intensity fields are recorded for two different illumination wavelengths. Decoupling the wavelength-independent and -dependent contributions to the measured relative intensity changes then yields the sought fields. This method is applied to the investigation of the electro-elastic coupling, providing images of both the local surface electrical charge density and the device deformation field

Caractérisation mécanique par sollicitation locale et mesure de champ

Nous proposons d'utiliser un nouveau système d'imagerie interférométrique à lumière polarisée, basé sur un prisme biréfringent « maison », pour mesurer le champ de rotation d'une surface. Ce système est ensuite utilisé en association avec un microscope acoustique à pointe vibrante (SMM : Scanning Microdeformation Microscope) pour mesurer le champ de rotation de la surface, à proximité de la pointe du microscope. Le champ ainsi déterminé est utilisé dans le but de découpler les constantes élastiques issues des mesures faites avec le SMM

Couplage électro-elastique et adsorption : vers une nouvelle instrumentation en chimie analytique

On utilise des poutres de dimensions micrométriques comme capteurs de leur environnement. Toute modification de l'état électrochimique d'une face introduit alors une flexion du levier. On a montré par ailleurs qu'en utilisant un dispositif interférométrique de mesure de champ et une technique d'identification adaptée, on peut construire une modélisation du couplage électro-élastique. On montre ici comment la relation de couplage est modifiée par l'adsorption de molécules neutres à la surface, et on propose d'exploiter cet effet en chimie analytique