Microhotplate-Based High-Speed Polyimide Capacitive Humidity Sensors

Polyimide thin films are locally cured on a sensor chip using MEMS microhotplates as an alternative solution to the conventional thermal curing and evaluated as high-speed humidity sensor materials. The polyimide locally cured at a temperature over 350 degrees C for 1 hour exhibits full imidization. There is no significant difference between the polyimide thin films cured in a conventional convection oven and those cured locally on MEMS microhotplates. The locally cured polyimide humidity sensors with a micro-bridge structure created using front-side etching with XeF2 gas show a sensitivity of 0.77 pF/%RH, a hysteresis of 0.61%RH, and a response time of 2.5 s. They also exhibit a low dielectric loss. In comparison, the sensors with a conventional structure exhibit a time response of 7 s. These results indicate that the locally cured polyimide films may be used as a dielectric material in high speed humidity sensors.