A passive wireless integrated humidity sensor based on dual-layer spiral inductors

This paper presents an integrated humidity sensor capable of wireless operation through inductive coupling. The sensor chip consists of two counter-rotating planar aluminum coils (0.5µm thick, 100µm width, 50µm space, 6mm outer diameter and 4 turns) separated by a maskless humidity-sensitive polyimide film. The two inductors are capacitively and inductively coupled together to form an inductor-capacitor (LC) electrical resonator, whose resonant frequency changes with humidity. In contrast with previous work, our proposed design eliminates the need of a connection bridge between the inner and outer terminals of the spiral inductor. Additionally, its inner humidity-sensitive overlap parallel-plate capacitor offers higher sensitivity than the passive humidity sensor based on single-layer self-resonant inductor. An equivalent circuit model is investigated for further understanding of the electromagnetic behavior of the sensor. Measurements show a sensitivity of 65kHz/%RH with a centre frequency of about 74MHz.