High-sensitivity capacitive humidity sensor using 3-layer patterned polyimide sensing film

This paper investigates novel isolation and patterning schemes to increase the sensitivity of a capacitive humidity sensor based on a polyimide sensitive layer. We obtained an optimum sensitivity of 23% using aluminum interdigitated electrodes with fingers of 1 nm width separated by 1 pm, deposited on a first insulating polyimide layer and covered by two more polyimide layers whose upper one features a regular array of holes to increase the active surface area. A mathematical model was developed to optimize the sensitivity regarding water absorption factor, electrodes design and ratio between active and parasitic capacitances. The process was optimized to be fully compatible with usual CMOS-IC processes in order to finally be able to fabricate a humidity smart sensor.