Response time of nanostructured relative humidity sensors

Capacitive relative humidity (RH) sensors were fabricated by coating countersunk interdigitated electrode substrates with nanostructured TiO2 films produced using glancing angle deposition. Areal capacitance increased from ∼1∼1 nF cm−2 to ∼800∼800 nF cm−2 as relative humidity was increased from ∼2∼2% RH and 95% RH. For films deposited at 81°81° and with a thickness below 4 μm, response time was (162±4162±4) ms μm−1. Response times increased from 64 ms to 1440 ms as film thickness increased from 280 nm to 8.5 μm. The linear dependence of response time with film thickness indicates that device response time is dominated by surface adsorption. Response time decreased with increasing deposition angle, with a slope of (−15.2±1.6−15.2±1.6) ms degree−1 for the adsorption data, and (−17.3±2.5−17.3±2.5) ms degree−1 for the desorption data. The optimum operating range of the sensors depends on deposition angle, and can be tuned to different ranges to match application needs.