Nanostructured Oxide Films for High-Speed Humidity Sensors
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Nanostructured Oxide Films for High-Speed Humidity Sensors John J. Steele, Kenneth D. Harris and Michael J. Brett Department of Electrical and Computer Engineering, University of Alberta Edmonton, Alberta, T6G 2V4, Canada. ABSTRACT Miniaturized thin film humidity sensors were fabricated using nanostructured materials deposited by an advanced technique known as glancing angle deposition (GLAD). These sensors exhibited extremely fast desorption response times of less than 40 ms to steplike changes in humidity. Multiple response time measurements for various initial humidities have shown that the sensors maintain their rapid response at all levels of humidity. INTRODUCTION There are many types of sensors that utilize different sensing mechanisms for various applications in industry and science. Capacitive, impedance, gravimetric and optical techniques are commonly utilized. Of these, the capacitive type sensor accounts for approximately 75% of the commercial market [1]. Capacitive sensors track changes in humidity via changes in the dielectric constant of a hygroscopic material as a result of water adsorption and desorption at different levels of relative humidity. When a hygroscopic material is used as the dielectric of a capacitor, changes in the dielectric constant under varying ambient humidity conditions will result in changes in capacitance. These changes are then monitored and correlated to the ambient humidity. The rate at which successive humidity measurements can be made is limited by the response time of the sensor. Currently, capacitive and impedance type sensors exhibit the fastest response times, which are typically on the order of seconds [2,3]. Although their response times are fast compared to other humidity sensing technologies, there is still a need for faster response. Applications such as industrial process control, where humidity can be a crucial variable affecting the efficiency, quality and yield of a process, and pulmonary disease diagnostics, where multiple measurements are needed as a patient inhales and exhales, would greatly benefit from humidity sensors that have response times in the sub-second range [3,4]. Recently it has been reported that GLAD nanostructured films in a capacitive structure showed nominal response times on the order of 100 ms [5]. In this work we will confirm that GLAD can be used to fabricate high-speed humidity sensors and investigate the performance with interdigitated capacitor structures. GLAD GLAD is a recently developed advanced nano-materials fabrication technique where highly oblique deposition, along with controlled substrate motion results in high surface area, high porosity films, with nanostructures that can be engineered for various applications [6]. Figure 1 shows a typical GLAD apparatus in a physical vapour deposition (PVD) system. Electron-beam evaporation is depicted in the figure 1, but it should be noted that GLAD is a versatile process and can be used in other PVD systems such as sputtering, thermal evaporation and pulsed laser deposition
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