Sub-Second Humidity Sensing based on Nanostructured Narrow-Bandpass Optical Filters
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0915-R07-01
Sub-Second Humidity Sensing based on Nanostructured Narrow-Bandpass Optical Filters John J. Steele, Andy C. van Popta, Matthew M. Hawkeye, Jeremy C. Sit, and Michael J. Brett Electrical and Computer Engineering, University of Alberta, 9107 116 Street, Edmonton, Alberta, T6G 2V4, Canada ABSTRACT An optical-based humidity sensor with a sub-second response time was fabricated from a nanostructured titanium dioxide thin film. A refractive index profile designed to yield a narrowbandpass optical interference filter was obtained through nanoscale porosity variations produced by glancing angle deposition (GLAD). Under varying humidity conditions the transmittance spectrum of the filter shifts due to effective index changes of the porous structure resulting from adsorption/desorption of water vapor. In the following we will show that this device is highly sensitive, exhibits minimal hysteresis, and is extremely fast. The adsorption and desorption response times were measured to be 270 ms and 160 ms, respectively. INTRODUCTION Current research in humidity sensing is focused on improving properties including sensitivity, durability, reliability, range of humidity and temperature operation, hysteresis, size, and cost. Of particular importance is the response time of a humidity sensor, which is defined as the time required for 90% of the signal change to occur when the sensor is exposed to a step-like change in humidity. While response times on the order of several seconds are considered to be high-speed with current technology [1-3], applications such as the diagnosis of pulmonary diseases, monitoring of atmospheric conditions using radiosondes and dropsondes, management of patients undergoing anesthesia and critical care medicine, and various industrial process controls would benefit from further decreased response times [2-5]. In this paper we report the fabrication and characterization of a highly porous nanostructured thin film prepared by GLAD as an optical interference filter for sub-second humidity sensing. GLAD is a physical vapor deposition (PVD) technique that can fabricate highly structured thin film coatings with morphology engineered on the 10 nm size scale. Optical filters fabricated with GLAD exploit the birefringence (linear or circular) resulting from film morphology and/or the dependence of the effective index of refraction on porosity [6, 7]. Our device incorporates periodic nanoscale porosity variations throughout the thickness of a titanium dioxide GLAD film to generate an index profile designed to yield a narrow-bandpass optical interference filter. The transmittance spectrum of the filter is sensitive to changes in humidity due to effective index changes resulting from water vapor adsorption/desorption. In the following we describe how the sensor is designed, built and tested, and highlight its high sensitivity, minimal hysteresis and extremely rapid response.
EXPERIMENTAL DETAILS Glancing Angle Deposition A schematic of an electron beam PVD setup for depositing GLAD films is shown in figu
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