In order to compare their gas sensing properties two kinds of sensors based on silicon cantilevers of similar characteristics have been fabricated: On one side we fabricated gravimetric gas sensors based on silicon cantilevers acting as resonators. The active layers consisted of polymer films deposited on top of the cantilevers. Sensors were maintained oscillating at their natural resonance frequency with electronic circuitry also developed in this work. Basically they consist of mass-spring mechanical resonators in which the mass increment due to gas sorption in the polymer provokes a shift on the resonance frequency. The output signal is a sinusoidal voltage extracted directly from the oscillator, and the amount of gas absorbed is related to the frequency of this output signal. The second type of sensors consisted of capacitors in which one electrode is a silicon cantilever and the other is a fixed metallic electrode fabricated parallel to the silicon cantilever. The silicon cantilever of these devices is covered with the same polymer films as for the resonators. The sensing principle in this case relies on the bending produced by the internal mechanical stress induced by the absorption of the gas in the polymeric layer. In these devices the signal is obtained by measuring the capacitance between the two plates of the capacitor, in this case the out coming signal was the current of the capacitor: an amplitude modulated signal. The gas response of both types of sensors have been characterized and a comparison is presented in this paper.
In 1985 Gerd Binnig and Calvin F. Quate from Stanford and Christopher Gerber from IBM (Zurich) designed the Atomic Force Microscope (AFM), since then a big interest arose around one of its main components; micromachined cantilevers. During all these years, authors have employed these tiny devices to sense multiple physical and chemical parameters through diverse transduction principles. Micromachined cantilevers offer many different transduction mechanisms: force sensing, bimetallic effect, mass loading, medium viscoelasticity, thermogravimetry, stress sensing, and more. Along with all these transduction mechanisms, a great variety of detection methods can be employed with cantilever-based sensors as for example: optical detection, resonance frequency measurement, piezoelectric integrated resistors, etc. The design and fabrication process of a micromachined Silicon capacitive gas sensor are described in this paper. Design and testing of the interface circuitry are also shown with preliminary results.