Fabrication of high-speed polyimide-based humidity sensor using anisotropic and isotropic etching with ICP

A novel high-speed, high-sensitivity capacitive-type humidity sensor is fabricated by using a new microfabrication process involving combination of directional and isotropic etching with inductively coupled plasma (ICP) etcher and a localized curing of polyimide films on a micro-hotplate. The polyimide humidity sensor is found to have a sensitivity of 0.75 pF/%RH, a linearity of 0.995, a hysteresis of 1.32%RH, a time response of 3 s, and a temperature coefficient of 0.22%RH/°C. This high-sensitivity and high-speed device is achieved using the locally-cured polyimide and a sensor structure having many holes and allowing moisture to diffuse through the top and side surfaces of polyimide film.