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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.

Abstract-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.

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The current trend in increasing microcantilever resonant frequencies leads to relatively low quality factors for microcantilevers resonating in a gas medium. In order to increase the quality factor of such high-frequency microcantilevers, an approach that takes into account the different types of losses is provided to specify appropriate values of microcantilever aspect ratios (length to thickness, width to thickness). Conventional values of aspect ratios result in low quality factors, while those proposed here lead to slightly higher values of quality factor.

A gas sensor based on the use of a resonating microcantilever has been realized by using a polymer sensitive coating. From the theoretical study of the microcantilever sensitivity, it has been deduced that the sensitivity is enhanced when the resonant frequency or the sensitive coating thickness are increased. The sensitive coating thickness influence has then been verified experimentally by using PEUT (polyetherurethane) as sensitive coating for ethanol detection. From these measurements, some drawbacks are shown: the coating thickness increase leads to a sensor response time increase and a frequency noise increase which worsens the limit of detection. Conclusions are then made about the sensitive coating optimization depending on application constraint considerations.

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