A microfabricated amperometric moisture sensor

We describe a microfabricated moisture sensor with interdigitated Au or Pt electrodes on a silicon substrate. The sensor active area is covered with a spin-coated, baked-on layer of Nafion® perfluorosulfonate ionomer of submicron thickness. The sensor responds to moisture with a 10–90% rise time of 50–100 ms and a 90–10% fall time of 20–30 ms, faster than any other presently available sensor. The logarithm of the sensor current is related to the cube root of the moisture level at a given temperature. At 23 °C, the sensor easily measures relative humidities as low as 10%. The sensor response at a given absolute humidity level decreases exponentially with increasing temperature. The film is stable up to a temperature of 150 °C, permitting elevated temperature moisture measurement. Since sorbed water is actively decomposed electrolytically, the sensors exhibit negligible hysteresis. Response reproducibility of an individual sensor is <1%, that between identically made sensors is <5%, suggesting mass production techniques without individual calibration will be acceptable for all but the most demanding situation.