Quantum Tunneling Hygrometer with Temperature Stabilized Nanometer Gap.

We present the design, fabrication and response of humidity sensor based on electrical tunneling through temperature-stabilized nanometer gaps. The sensor consists of two stacked metal electrodes separated by 2.5 nm of vertical air gap. Upper and lower electrodes rest on separate 1.5 um thick polyimide patches with nearly identical thermal expansion but different gas absorption characteristics. When exposed to a humidity change, the patch under the bottom electrode swells but the patch under the top electrode does not, as it is covered with a water-vapor diffusion barrier of about 8 nm of Alumina. The air gap thus decreases leading to increase in the tunneling current across the junction. The gap however is independent of temperature fluctuations as both patches expand or contract by near equal amounts. Humidity sensor action demonstrates an unassisted reversible resistance reduction of about five orders of magnitude when the device is exposed to a relative humidity of 20-90% at a standby DC power consumption of about 0.4 pW. The observed resistance change when subject to a temperature sweep of 25-60 degrees C at 24% RH was 0.0025% of the full device output range.