Resonant humidity sensors using industrial CMOS-technology combined with postprocessing

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Abstract
The authors report a novel sensor for the detection of relative humidity (RH) in an industrial CMOS process. It is based on silicon-oxide resonators coated with thin polyimide films. The sensors have been postprocessed by micromachining and polyimide deposition. The resonators are excited electrothermally with polysilicon resistors; their vibrations are detected by the piezoresistive effect of the same material. The moisture uptake of the polyimide increases linearly the mass of the resonant system and lowers its resonance frequency. A sensitivity of 270 Hz/100% RH has been obtained for a resonating beam at 16 kHz. The hysteresis is below 2% and the non-linearity below 0.5%. The system is very sensitive to the driving power since a slight heating of the moisture absorbing area makes the sensor effect disappear. This phenomena has been investigated with an infrared camera.

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