Isabelle Dufour

Chemical sensing: millimeter size resonant microcantilever performance

Based on the use of a resonant cantilever, a mass sensitive gas sensor for the detection of volatile organic compounds (VOC) has been developed. Analyte gases are absorbed by a sensitive layer deposited on the cantilever: the resulting mass change of the system implies the cantilever resonant frequency decreases. In this paper, the process technology, based on the use of SOI wafer, is described. To integrate the measurement, piezoelectric and electromagnetic excitations are investigated and for the detection of microcantilever vibrations, piezoresistive measurement is performed. Then, the polymer choice and the spray coating system are detailed. Using various geometrical microcantilevers, the frequency dependence on mass change is measured and allows us to estimate the mass sensitivity (0.06 Hz ng−1). In gas detection the first experiments exhibit the sensor response, then by calculating the partition coefficient (K = 977), the minimum detectable concentration of ethanol is deduced and permits us to estimate the gas sensor resolution (14 ppm). Finally a comparison between millimeter size and micrometer size cantilevers shows the importance of noise in the design of an integrated sensor.

Thermal Excitation and Piezoresistive Detection of Cantilever In-Plane Resonance Modes for Sensing Applications

Thermally excited and piezoresistively detected bulk-micromachined cantilevers vibrating in their in-plane flexural resonance mode are presented. By shearing the surrounding fluid rather than exerting normal stress on it, the in-plane mode cantilevers exhibit reduced added fluid mass effects and improved quality factors in a fluid environment. In this letter, different cantilever geometries with in-plane resonance frequencies from 50 kHz to 2.2 MHz have been tested, with quality factors as high as 4200 in air and 67 in water.

Longitudinal vibration mode of piezoelectric thick-film cantilever-based sensors in liquid media

We report on the fabrication of a self-actuated resonant-microsensor, based on a thick-film piezoelectric cantilever, dedicated to either (bio)chemical detection in gaseous or liquid media or fluid characterization. The aim of this paper is to demonstrate that longitudinal modes can be used in highly viscous environments. Lower levels of fluid-solid interaction in comparison with classical flexural modes are expected from the results of our analytical model of a cantilever oscillating in a fluid. For example, in various fluid ranging from air to a Newtonian fluid of 300 cP viscosity, measured quality factors for the first longitudinal mode range from 300 to 20.

Analytical static modelling and optimization of electrostatic micropumps

Having developed a mechanical law with a simple elementary model of membrane deflection, we demonstrate the static instability of such a system under electrostatic actuation and provide an analytical solution for the membrane position. This allows us to highlight the sticking effect and the hysteresis phenomenon of the pump strike volume due to the nonlinearity of such excitation. We determine a parameter optimization procedure for the dimensions of the electrostatic actuated membrane in order to achieve maximum volume displacement.

Rheological behavior probed by vibrating microcantilevers

The aim of this paper is to demonstrate that vibrating microcantilevers can be used to quantify fluid properties such as density and viscosity. Contrary to classical rheological measurements using microcantilevers, the development of the proposed microrheometer is based on the measurement of fluid properties over a range of vibration frequencies, without necessarily being restricted to resonant phenomena. To this end, an analytical model is implemented and, when combined with measurements, allows the determination of the viscosity as a function of frequency. The preliminary results are encouraging for the development of a useful microrheometer on a silicon chip for microfluidic applications.

Dynamic simulation of an electrostatic micropump with pull-in and hysteresis phenomena

Abstract After presenting the role of micropumps in medical applications and demonstrating that an electrostatic micropump would seem to be the most appropriate, the geometrical and electrical static characteristics of this pump are summarized. Classical dynamic simulations do not take into account the pull-in effect or the hysteresis phenomenon, both of which appear in electrostatic micropumps. The novel principle of dynamic simulation, which includes these two non-linear characteristics, is then outlined; changes in pressure, volume, flow and current are also calculated. The last section displays the advantages associated with this global simulation.

Resonant microcantilever type chemical sensors: analytical modeling in view of optimization

Silicon microcantilevers can be used as microbalances or chemical microsensors if a sensitive layer is deposited on the moving structures. Indeed, the sorption of specific species by the sensitive coating modifies the mechanical properties of the structure and then its fundamental natural frequency. In this paper, analytical expressions of the sensitivities of different structures are obtained in view of optimization of the geometrical parameters for both mass sensors or concentration gas sensors. For chemical sensors, a thin parallelepiped microcantilever with sensitive coating on the whole structure gives good performances especially regarding the sensibility. However, the frequency measurement and the active surface can be improved with a rectangular plate at the free-end of the microcantilever with little influence on the sensitivities.

Characteristics of laterally vibrating resonant microcantilevers in viscous liquid media

The characteristics of microcantilevers vibrating laterally in viscous liquid media are investigated and compared to those of similar microcantilevers vibrating in the out-of-plane direction. The hydrodynamic loading on the vibrating beam is first determined using a numerical model. A semi-analytical expression for the hydrodynamic forces in terms of the Reynolds number and the aspect ratio (beam thickness over beam width) is obtained by introducing a correction factor to Stokes’ solution for a vibrating plate of infinite area to account for the effects of the thickness. The results enable the effects of fluid damping and effective fluid mass on the resonant frequency and the quality factor (Q) to be investigated as a function of both the beam’s geometry and liquid medium’s properties and compared to experimentally determined values given in the literature. The resonant frequency and Q are found to be higher for laterally vibrating microcantilevers compared to those of similar geometry experiencing transv...

The Microcantilever: A Versatile Tool for Measuring the Rheological Properties of Complex Fluids

Silicon microcantilevers can be used to measure the rheological properties of complex fluids. In this paper two different methods will be presented. In the first method the microcantilever is used to measure the hydrodynamic force exerted by a confined fluid on a sphere that is attached to the microcantilever. In the second method the measurement of the microcantilever’s dynamic spectrum is used to extract the hydrodynamic force exerted by the surrounding fluid on the microcantilever. The originality of the proposed methods lies in the fact that not only may the viscosity of the fluid be measured but also the fluid’s viscoelasticity, i.e., both viscous and elastic properties, which are key parameters in the case of complex fluids. In both methods the use of analytical equations permits the fluid’s complex shear modulus to be extracted and expressed as a function of shear stress and/or frequency.

Effect of Coating Viscoelasticity on Quality Factor and Limit of Detection of Microcantilever Chemical Sensors

Microcantilevers with polymer coatings hold great promise as resonant chemical sensors. It is known that the sensitivity of the coated cantilever increases with coating thickness; however, increasing this thickness also results in an increase of the frequency noise due to a decrease of the quality factor. By taking into account only the losses associated with the silicon beam and the surrounding medium, the decrease of the quality factor cannot be explained. In this paper, an analytical expression is obtained for the quality factor, which accounts for viscoelastic losses in the coating. This expression explains the observed decrease of the quality factor with increasing polymer thickness. This result is then used to demonstrate that an optimum coating thickness exists that will maximize the signal-to-noise ratio and, thus, minimize the sensor limit of detection