Fabien Amiot

Position and mass determination of multiple particles using cantilever based mass sensors

Resonant microcantilevers are highly sensitive to added masses and have the potential to be used as mass-spectrometers. However, making the detection of individual added masses quantitative requires the position determination for each added mass. We derive expressions relating the position and mass of several added particles to the resonant frequencies of a cantilever, and an identification procedure valid for particles with different masses is proposed. The identification procedure is tested by calculating positions and mass of multiple microparticles with similar mass positioned on individual microcantilevers. Excellent agreement is observed between calculated and measured positions and calculated and theoretical masses.

Nomarski imaging interferometry to measure the displacement field of micro-electro-mechanical systems

We propose to use a Nomarski imaging interferometer to measure the out- of-plane displacement field of micro-electro-mechanical systems. It is shown that the measured optical phase arises from both height and slope gradients. By using four integrating buckets, a more efficient approach to unwrap the measured phase is presented, thus making the method well suited for highly curved objects. Slope and height effects are then decoupled by expanding the displacement field on a functions basis, and the inverse transformation is applied to get a displacement field from a measured optical phase map change with a mechanical loading. A measurement reproducibility of approximately 10 pm is achieved, and typical results are shown on a microcantilever under thermal actuation, thereby proving the ability of such a setup to provide a reliable full-field kinematic measurement without surface modification.

In Situ, Real Time Monitoring of Surface Transformation: Ellipsometric Microscopy Imaging of Electrografting at Microstructured Gold Surfaces

Surface chemical reactivity is imaged by combining electrochemical activation of a surface transformation process with spatiotemporal ellipsometric microscopy. An imaging ellipsometric microscope is built, allowing ellipsometric images of surfaces with a lateral resolution of ∼1 μm and a thickness sensitivity of ∼0.1 nm in air and 0.4 nm in a liquid. These performances are particularly adapted for using such optical setup as an in situ, real time chemical microscope to observe a chemical surface transformation. This microscope is tested for the monitoring of the electrochemically actuated diazonium grafting of a gold surface. Such reaction is a model system of organic material deposition on a gold surface induced by an electrochemical actuation. Using either plain or physically or chemically structured electrodes, it allows for the characterization of local phenomena associated with the electrografting process. This illustrates its potential to reveal the local (electro)chemical reactivity of surfaces.

Identification of the electroelastic coupling from full multi-physical fields measured at the micrometre scale

Metal coated microcantilevers are used as transducers of their electrochemical environment. Using the metallic layer of these cantilevers as a working electrode allows one to modify the electrochemical state of the cantilever surface. Since the mechanical behaviour of micrometre scale objects is significantly surface-driven, this environment modification induces bending of the cantilever. Using a full-field interferometric measurement set-up to monitor the objects then provides an optical phase map, which is found to originate from both electrochemical and mechanical effects. The scaling of the electrochemically-induced phase with respect to the surface charge density is modelled according to Gouy–Chapman–Stern theory, whereas the relationship between the mechanical effect and the surface charge density is analysed. An identification technique is described to determine a modelling of the electroelastic coupling and to identify the spatial charge density distribution from full-field phase measurements. Minimizing the least-squares gap between the measured phase and a statically admissible phase field, the mechanical effect is found to be charge-driven. The charge density field is also found to be singular on the cantilever edge, and the shear stress versus charge density is found to be non-linear.

Mapping fluxes of radicals from the combination of electrochemical activation and optical microscopy

The coating of gold (Au) electrode surfaces with nitrophenyl (NP) layers is studied by combination of electrochemical actuation and optical detection. The electrochemical actuation of the reduction of the nitrobenzenediazonium (NBD) precursor is used to generate NP radicals and therefore initiate the electrografting. The electrografting process is followed in situ and in real time by light reflectivity microscopy imaging, allowing for spatio-temporal imaging with sub-micrometer lateral resolution and sub-nanometer thickness sensitivity of the local growth of a transparent organic coating onto a reflecting Au electrode. The interest of the electrochemical actuation resides in its ability to finely control the grafting rate of the NP layer through the electrode potential. Coupling the electrochemical actuation with microscopic imaging of the electrode surface allows quantitative estimates of the local grafting rates and subsequently a real time and in situ mapping of the reacting fluxes of NP radicals on the surface. Over the 2 orders of magnitude range of grafting rates (from 0.04 to 4 nm s(-1)), it is demonstrated that the edge of Au electrodes are grafted -1.3 times more quickly than their centre, illustrating the manifestation of edge-effects on flux distribution at an electrode. A model is proposed to explain the observed edge-effect, it relies on the short lifetime of the intermediate NP radical species.

Multiple wavelength reflectance microscopy to study the multiphysical behavior of microelectromechanical systems.

In order to characterize surface chemomechanical phenomena driving microelectromechanical systems 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 electroelastic coupling, providing images of both the local surface electrical charge density and the device deformation field.

Scanning electrochemical microscopy monitoring in microcantilever platforms

The deflection of cantilever systems may be performed by an indirect electrochemical method that consists of measuring the local cantilever activity and deflection in a feedback generation-collection configuration of the SECM. This is illustrated during the electrochemically assisted adsorption of Br onto a gold-coated cantilever, either in its pristine state or previously coated with a thin organic barrier. It is further extended to the adsorption of an antibody in a heterogeneous immunoassay at an allergen-coated microcantilever platform. In both reactions, the cantilever deflection is qualitatively detected from the SECM tip current measurement and a quantitative estimate is obtained through modeling. This electroanalytical strategy provides an alternative approach to standard optical detection. It can overcome some limitations of the optical method by allowing electrochemical characterization of nonconductive cantilevers and appropriate use for closed systems.

Toward massive parallel reading of sensitive mechanical microsensors

We describe a new approach for the parallel reading of the response of micromechanical sensors in array using an interferential imaging method coupled with a multichannel lock-in detection. The mechanical response of each sensor is deduced from the image of their topography. The goal is the detection of stress changes through the measurement of induced topography changes between loaded sensors and reference sensors. A measurement of the out-of-plane displacement field is a sensitive and reliable method to determine the mechanical stress in microcantilevers. In this study gold-covered SiO2 cantilevers have been used as sensors and a displacement measurement sensitivity of 230 pm has been achieved with no averaging. Such a value is equivalent to a stress change sensitivity better than 0.001 MPa for our application. The fields of application extend from DNA biochips to environmental sensors.

Calibration procedures for quantitative multiple wavelengths reflectance microscopy

In order to characterize surface chemo-mechanical phenomena driving micro-electro-mechanical systems (MEMSs) behavior, it has been previously proposed to use reflected intensity fields obtained from a standard microscope for different illumination wavelengths. Wavelength-dependent and -independent reflectivity fields are obtained from these images, provided the relative reflectance sensitivities ratio can be identified. This contribution focuses on the necessary calibration procedures and mathematical methods allowing for a quantitative conversion from a mechanically induced reflectivity field to a surface rotation field, therefore paving the way for a quantitative mechanical analysis of MEMS under chemical loading.

Surface Mechanics and Full-Field Measurements: Investigation of the Electro-Elastic Coupling

Many 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. On the other hand, describing the involved surface effects in the continuum mechanics framework is still challenging. Several attempts to overcome the Stoney’s surface stress failure to satisfy field equations tend to show such a description has to be non-local, so that at least one ‘characteristic length’ parameter has to be used. The consequence is twofold: first, suited modelings have to be developed to describe the surface effects at the cantilever scale; and second, the involved characteristic length is (thermodynamically) connected to the molecular mechanisms at the cantilever surface, and may therefore be a key parameter for the target molecules identification. This requires to experimentally access displacement fields induced by the molecular interactions under scrutiny. A set-up providing mechanical and chemical fields along the cantilever is thus implemented focusing on cases where the cantilever’s surface reacts heterogeneously. The large amount of data obtained using full-field set-ups is redundant from the mechanical point-of-view, and this redundancy is used to identify some of the key parameters describing the mechanical surface effects. Results obtained when studying the electro-elastic coupling in a non-adsorbing case are presented.