Response time of nanostructured relative humidity sensors

Abstract

Capacitive relative humidity (RH) sensors were fabricated by coating countersunk interdigitated electrode substrates with nanostructured TiO 2 films produced using glancing angle deposition. Areal capacitance increased from ∼ 1 nF cm −2 to ∼ 800 nF cm −2 as relative humidity was increased from ∼ 2 % RH and 95% RH. For films deposited at 81 ° and with a thickness below 4 μ m, response time was ( 162 ± 4 ) ms μ m −1. Response times increased from 64 ms to 1440 ms as film thickness increased from 280 nm to 8.5 μ m. 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 ) ms degree −1 for the adsorption data, and ( − 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.