Wireless LC-Type Passive Humidity Sensor Using Large-Area RF Magnetron Sputtered ZnO Films

We report on a ZnO-based LC-type passive humidity sensor (HS) using a scalable, large-area thin-film semiconductor fabrication process. The reported sensor is capable of monitoring relative humidity (RH) remotely. The fabricated sensor is 30 mm in diameter and comprises an LC resonator formed via an octagonal planar inductor and a moisture sensitive interdigitated ZnO capacitor in series. A printed circuit board reader coil, which can sense the sensor output from <;25 mm distance, is also reported. The HS is demonstrated to read 30%-90% of RH by interrogating the change in resonance frequency (f
_R
) of the reader-senor system. The reading resolution was ±2.38% RH and the sensor sensitivity ranged from 53.33 kHz to 93.33 kHz for a 1% change in RH during measurements above 45% RH. Experimental results also showed that the fabricated sensor is operational for a range of 0 °C-75 °C as long as calibration is performed for temperature drifts of ≥±3 °C. The reported results are promising to expedite the deployment of novel inexpensive sensor, for example, in sealed locations to remotely monitor humidity without a need for on-sensor power.