Nano-grass polyimide-based humidity sensors

This paper deals with a micro-capacitive-type relative humidity sensor with nano-grass polyimide as a dielectric sensing material. Our humidity sensor achieves key performance indices such as quick response, high sensitivity, and stability enabled by the modification of polyimide into nano-grass morphology where the dimension of a typical individual pillar is 387 nm × 40 nm. A low hysteresis operation is also achieved by integrating a micro-heater in the sensing area. The nano-grass morphology is created with an oxygen plasma treatment on polyimide surface which increases surface to volume ratio by more than 280 times larger than that of a simple flat-film. This amplification of the surface to volume ratio leads to the rapid adsorption and desorption of water into the sensing material. Furthermore the oxygen plasma treatment introduces a carbonyl group that facilitates an enhanced affinity of the polyimide surface to water molecules. XPS analysis is used to confirm the emergence of carbonyl groups as a result of the treatment. The total response time of a nano-grass sensor is 11 s which is improved by about 2.5 times than a normal flat-film sensor. The sensitivity of the nano-grass sensor is 0.08 pF/%RH (relative humidity) which is improved by 8 times compared with the flat-film one. In stability test for 200 h, the signal of the nano-grass sensor is fluctuated ±1.0%RH. Theoretical models employing the Looyenga and Dubinin equations are used to explain the behavior of the nano-grass sensor.