Performance and Modeling of a Nanostructured Relative Humidity Sensor
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Performance and Modeling of a Nanostructured Relative Humidity Sensor Mike Taschuk1, John Steele1, and Mike Brett1,2 1 Electrical and Computer Engineering, University of Alberta, 9107 - 116 Street, University of Alberta, Edmonton, AB, Canada T6G 2V4, Edmonton, T6G 2V4, Canada 2 NRC National Institute for Nanotechnology, Edmonton, T6G 2M9, Canada ABSTRACT Capacitive humidity sensors were fabricated using interdigitated electrodes coated with amorphous nanostructured TiO2 thin films grown by glancing angle deposition. The sensor exhibited a large change in capacitance, increasing exponentially from ~ 1 nF to ~ 1 µF for an increase in relative humidity from 2 % to 92 %. A simple model of the capacitive response and dielectric constant of the devices has been explored and compared to the experimental results. From this comparison, it is clear that the magnitude of the device response observed cannot be explained with bulk dielectric constants or literature values. INTRODUCTION Relative humidity (RH) sensors are found in a variety of applications, depending on the sensitivity, range and response time of the sensor technology used [1-3]. The performance of a RH sensor is dependent on the morphological properties of the sensing medium including porosity, surface area, and pore size distribution [1, 2, 4-13]. Glancing angle deposition (GLAD) is a physical vapour deposition technique that is capable of fabricating thin films with an exceptional degree of control over film morphology on the 10 nm length scale [14, 15]. The GLAD technique represents an excellent technique for RH sensing studies. We have recently shown that both capacitive and optical RH sensors utilizing nanostructured metal oxide films produced by GLAD are extremely fast with response times less than 400 ms [16, 17]. Subsecond response times are advantageous for many medical applications [18 – 22]. Alternative techniques have produced fast RH sensors [12, 23, 24], but in some cases this improved response time comes at the cost of a reduced sensitivity [12]. The RH sensors presented in this paper are based on water vapour interaction with TiO2 surfaces. The mechanisms of water vapour adsorption on metal oxide surfaces are widely studied [8, 9, 25 – 28]. The water vapour interaction induces permittivity changes to the metal oxide coating, which are measured as a change in the capacitance or resistance of the device. Orders of magnitude changes in both the capacitance and resistance are commonly reported [4, 8, 11, 16, 29 - 33] and often attributed to water vapour condensing into the pores of the material [1, 4, 11, 29]. However, bulk properties of condensed water cannot account for the large changes in capacitance and resistance. Consider the case of a RH sensor comprised of a TiO2 with a porosity of 50%. The dielectric constant of TiO2 is between 86 and 170 for rutile. The dielectric constant of water is ~
80 for low frequencies. As the RH increases to 100%, water vapour will condense, and fill the remaining 50% of the sensor volume. In this ca
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