Adsorption-desorption noise in micromechanical resonant structures

In this paper we analyze adsorption-desorption (AD) noise in resonant microelectromechanical structures. This noise is generated by instantaneous differences in the rates of adsorption and desorption of contaminant molecules to and from the resonator surface, which cause mass fluctuations and consequentially frequency fluctuations. Contrary to the usual statistical approach, we used the analogy between AD processes in resonant structures and generation-recombination processes in semiconductors. Starting froth that general analogy and using the theory of Langmuir's isotherm, since it is the most convenient for this case, we derived an exact expression for the AD fluctuations-induced phase noise for the case of one adsorption layer. Our numerical results show that AD induced phase noise is comparable to other noise sources and becomes dominant for very small resonator beam dimensions. The presented theory is applicable to the determination of the limiting performance of various micromechanical resonators and microcantilever sensors and also for the theoretical studying of AD processes in general. In addition, phase noise measurements and their comparison with the theory are convenient for the investigation of catalytic AD processes. (C) 2002 Elsevier Science B.V. All rights reserved.