Quantifying adsorbed water monolayers on silicon MEMS resonators exposed to humid environments

This study investigated the influence of temperature and humidity on the adsorbed water layer on micron-scale monocrystalline silicon (Si) films in air, using a Si-MEMS kHz-frequency resonator. Both temperature and relative humidity induced a reversible change in resonant frequency, attributed to the temperature-dependent properties of Si and to a change in adsorbed water layer. The excellent precision in resonant frequency measurement (0.02 Hz, or 0.5 ppm) allowed precise calculation of the changes in adsorbed water layer thickness over the specimen surface. The increase in water thickness with relative humidity was a function of temperature and could not be described with simple multimolecular adsorption theories such as the BET theory. A likely explanation is the presence of hydrocarbon contaminants on the Si surface. Guidelines are provided to accurately measure the influence of temperature and relative humidity on the adsorbed water layer thickness on micron-scale Si surfaces, using this technique.