A Novel Differential Capacitive Humidity Sensor on SIW Re-Entrant Cavity Microwave Resonators With PEDOT:PSS Film

This work presents a differential humidity sensor by longitudinally stacking the sensing and referencing substrate integrated waveguide (SIW) re-entrant cavity resonators (RECRs) for relative humidity (RH) detection. Three different technologies, such as air-filled substrate, metal grid holes (METGH) and sensitive material, are integrated to enhance the humidity sensing response. Firstly, introduction of air-filled substrate and METGH can effectively improve the flowability of the humid air, and can reduce the response time. Secondly, PEDOT:PSS is used as a humidity sensitive (HS) material and deposited in the highly concentrated E-field distribution area of sensing SIW RECR, and the modulation of the dielectric constant of HS material by humid air can achieve high humidity sensitivity. Furthermore, the proposed differential structure itself can suppress common-mode perturbations to prevent temperature cross sensitivity. Several different sensors are fabricated and a series of experiments are performed to investigate the dependence of humidity sensing response on the device structure and the deposited amount of HS material. When a specific amount of HS material is deposited in the HS area, excellent humidity sensing performances such as high sensitivity, low hysteresis and fast response can be successfully achieved. Among them, the sensor $\text{B}_{{1}}$ (300-uL HS material and with METGH) demonstrates the best humidity sensing performance in the range of 32.8% to 97.3% RH, and shows high sensitivity of 209.3 kHz/%RH and low hysteresis of 0.592% RH, and the corresponding response time is reduced to 1/6 of that without METGH. Moreover, the sensor $\text{B}_{{1}}$ has excellent suppression on the temperature variation.