Ludivine Fadel

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.

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.

Coupled determination of gravimetric and elastic effects on two resonant chemical sensors: love wave and microcantilever platforms

The field of chemical microsensors for both gas and liquid sensing has been widely investigated in recent years. Several technologies have been utilized which include love-wave acoustic sensors and silicon microcantilevers. Those structures are both used as chemical sensors by adding a sensitive coating to the device surface. Perturbations of the sensitive coating properties induce frequency drift in both devices, thus making chemical detection possible. Microcantilevers are essentially sensitive to the coating mass changes which modify the resonant frequency of the structure. However, the acoustic wave device is sensitive to all types of propagation perturbations which include mass loading and mechanical properties changes of the coating. One of the difficulties in acoustic sensor field is to separate each contribution from the induced frequency shifts. The aim of this paper is to couple experimental results from microcantilevers and love-wave devices in order to identify and separate the two effects. At last, this coupled study is also interesting for gas and liquid phase detection applications, as it will permit to determine the elasticity evolution during the detection process, i.e. the analyte sorption.

Optimization of the signal to noise ratio of resonant microcantilever type chemical sensors

Resonant microstructures can be used as chemical microsensors, by adding a sensitive coating to the device structure. Those vibrating structures are sensitive to the coating mass changes which modify the natural resonant frequency. The sensitivity, key parameter of such sensor, is proportional to the resonant frequency. But, if detection method consists in frequency measurement with a ring oscillator, the noise signal must be taken into account. A large study of these reflections, combined with a signal to noise ratio optimization, leads us to maximize quality factor to microstructure thickness ratio.

Coupled determination of gravimetric and elastic effects on two resonant chemical sensors: Love wave and microcantilever platforms

The objective of this paper is to couple theoretical and experimental results from microcantilevers and Love-wave acoustic devices in order to identify and separate mass loading effects from elastic effects. This is important in the perspective of sensing applications. For that, a thin-film polymer is deposited on both resonant platforms. It is demonstrated that microcantilevers are essentially mass sensitive. They allow one to determine the polymer layer thickness, which is validated by optical profilometry measurements. Then, taking into account this thickness, theoretical modeling and experimental measurements with Love-wave devices permit one to estimate an equivalent elastic shear modulus of the thin-film polymer at high frequency. Results are interesting if one is to fully understand and optimize (bio)chemical sensor responses.