New characterization methods for monitoring small resonant frequency variation: Experimental tests in the case of hydrogen detection with uncoated silicon microcantilever-based sensors

The uncoated silicon microcantilever (USMC) operated in the dynamic mode is a new concept in the field of microcantilever-based chemical sensors. Due to the absence of a sensitive layer, this kind of microsensor can only be used for specific applications where it is known that only one chemical species may be varying in concentration, such as monitoring hydrogen release in radioactive waste disposal facilities. Usually, the relative variation of the USMC resonant frequency expected for low concentrations (≤2%) of hydrogen in nitrogen is below 50ppm. As a result, the measurement of both the resonant frequency, fr, and the quality factor, Q, by classical methods, based on the gain spectrum (resonant peak and -3dB bandwidth), is not sufficiently accurate. In this paper, new measurement methods for monitoring fr and Q variations are proposed: (1) variation of gain and phase at fixed frequencies and (2) polynomial approximations of gain and phase spectra. The performance study of these characterization methods shows that monitoring fr by using phase linearization yields the best signal-to-noise ratio (e.g., 100 at 0.6% of H2 in N2), with 0.02% as a limit of detection for hydrogen.