Passive wireless integrated humidity sensor based on dual-layer spiral inductors

An integrated humidity sensor capable of passive wireless sensing through inductive coupling is presented. The sensor chip consists of two stacked counter-rotating planar aluminium inductors (0.5 mu m thick, 100 mu m in width, 50 mu m space and 6 mm outer diameter) separated by a maskless polyimide film. The two inductors are inductively and capacitively coupled together to form a resonant tank, whose resonant frequency changes with humidity. This topology eliminates the need of a connection bridge or via as used in the usual inductor-capacitor humidity sensors and offers higher sensitivity due to the inner humidity-sensitive parallel-plate capacitor in contrast with the single-layer self-resonant inductor. Three sensor devices with different inductor turns were fabricated and characterised. Measurements show a sensitivity of 45 kHz/%RH with a centre frequency of 50.5 MHz for sensor A (8 turns), 15.8 kHz/%RH, 22 MHz for sensor B (10 turns) and 65 kHz/%RH, 74 MHz for sensor C (4 turns).