Stable, ultra-fast vertical polyimide capacitive-type humidity sensors by CNT gas permeable electrodes and hydrophobic underlayer

Abstract

We developed the stable, ultra-fast vertical polyimide capacitive-type humidity sensor with a response time of 10 ms order using fluorinated polyimide with a multi-walled carbon nanotube (CNT) gas permeable upper electrode. On the basis of the diffusion model, the response time is reduced to less than 0.1s by reducing the hydrophobic partially fluorinated polyimide dielectric layer to ca. 100 nm. Although the sensitivity increases with the reduction of polyimide (inversely proportional) thickness, the recovery time increases and non-linear sensitivity are observed after the exposure to a high-humid air (>80 %RH). The slow recovery and instability were improved by the insertion of ultra-thin perfluoro polymer (Cytop).