Abstract
We present the design, fabrication and response of a 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 μm 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 ~8 nm of Al2O3. 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 Rmax/Rmin ~105 when the device is exposed to 20–90 RH% at a standby DC power consumption of ~0.4 pW. The observed resistance change when subject to a temperature sweep of 25–60 ° C @24% RH was ~0.0025% of the full device output range.
Highlights
Humidity sensors can be realized using various technologies[1,2,3,4,5]
Micro-cantilevers have been used as capacitive type humidity sensors
In this work we present a new type of microfabricated humidity sensor that is able to provide large output range and a low temperature dependence
Summary
Humidity sensors can be realized using various technologies[1,2,3,4,5]. Kharaz and Jones were one of the first to report optical methods of humidity measurement[6]. Capacitive humidity sensors display linear operation and can accurately measure from 0–100% relative humidity with an output range typically ~30% of the default capacitance These devices consume very little electrical power and are relatively cheap to fabricate[3]. In this work we present a new type of microfabricated humidity sensor that is able to provide large output range and a low temperature dependence. The device utilizes the expansion of a polymer that swells when exposed to humidity as in the capacitive device, but it produces a resistive output that measures the polymer expansion through tunneling current across a humidity dependent, thermally stabilized nanogap. The tunneling current changes many orders of magnitude providing similar output as the resistive type device with a low temperature dependence. This section includes a discussion of the sorption kinetics of the device and analysis of the absorption-desorption phenomenon
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