Design and characterization of a low thermal drift capacitive humidity sensor by inkjet-printing

Small, low-cost and flexible humidity sensors were designed, fabricated by using an inkjet-printing process, and fully characterized. Based on the principles of the capacitor and the ability of a polyimide to absorb humidity, the sensor was fabricated by printing silver interdigitated electrodes on a thin polyimide film of 75 μm thickness. After modeling, the total area of the printed sensor was optimized to be 11.65 mm2. A relative humidity sensitivity of 4.5 fF/%RH and a thermal coefficient of −0.4 fF/°C were measured at 100 kHz, whereas the sensitivity and the thermal coefficient were 4.2 fF/%RH and −0.21 fF/°C, respectively, at 1 MHz. This latter result implies that it could not be necessary to include thermal compensation to use this sensor depending on the required accuracy and the chosen frequency. This work shows a reliable, fast, simple and low-cost manufacturing process to make small humidity sensors with low thermal drift and high temporal stability. These sensors could be easily integrated into inkjet-printed RFID tags for monitoring of environmental humidity in diverse applications.