Functional Iron Oxide (Fe2O3) Nanoparticles Based Gas Sensor Operating at High Temperature
Authors: Huang Yu-Chun , Liao Shih-Chieh , Chuang Ricky Wenkuei , Hsueh Ting-Jen 
Published: February 2020

The highly responsive gas sensor based on a metal-oxide-semiconductor (MOS) configuration which is conformally coated with iron oxide (Fe2O3) nanoparticles is hereby designed, fabricated, and systematically characterized for this study. The iron oxide nanoparticles are prepared by mixing altogether the respective amounts of FeCl2-4H2O, FeCl3-6H2O, and NaOH in terms of different molar ratios. The iron oxide overcoated gas sensor thus fabricated has been electronically probed in an ecnclosed chamber under a high temperature condition during which the different gases are fed in individually and selectively. The presence of a perticular gas is duly sensed by monitoring the changing magnitude of the current extracted from the sensor. The enhanced sensitivity of the MOS-based sensor overlaid with the Fe2O3 nanoparticles is attributed to a large surface area typically associated with nanoscale particles. The fabrication of MOS-based sensor is entirely CMOS compatible and can be summarized as follows. First, the 400μm-thick silicon wafer is first RCA cleaned and then both sides of the wafer are coated with 800nm-thick oxide layer using high pressure oxidation furnace. Next, a series of photolithographic steps followed by electron-beam-evaporated titanium lift-off is implemented to realize the heater pattern. Afterward, the resultant heater pattern is overcoated with a 400nm-thick oxide layer using plasma-enhanced chemical vapor deposition (PECVD). In order to open up an area for the subsequent deposition of iron oxide nanoparticles-embedded material while leaving the titanium heater protected by the same oxide layer, necessary photolithographic and buffered oxide etching (BOE) steps are performed. Finally, the interdigitated electrodes are deposited followed by injecting the viscous Fe2O3 nanoparticles-embedded solution over the electrodes. Once the sensor is successfully fabricated, the very sensor is placed on a temperature-controlled sample platform maintained at a desired temperature within an enclosed chamber. The entire sensor characterization is performed at a temperature of 300 degree Celsius. The operating mechanism of the sensor is intimately dependent on the oxidation-reduction reaction between the injected gas and Fe2O3 nanoparticles-embedded sensing film. As can be shown in the follwoing figure, the ethanol (C2H5OH) gas vapor is functioned as a reducing agent, while nitrogen dioxide (NO2) gas, on the other hand, is behaved as a oxidizing agent instead. As ethanol gas vapor is injected, the oxygen ions of the ethanol produced at high temperature is converted back to the neutral oxygen atom while releasing the captured electrons back to to the iron oxide sensing film, thereby causing the detected current to go up. On the other hand, nitrogen dioxide is behaved as an oxidizing agent instead by oxidizing the Fe2O3 film and grabbing the electrons away, thereby causing the detected current to drop in response to the presence of the NO2 gas vapor. Consequently, the reversing current-versus-time patterns associated with C2H5OH and NO2, as shown in the follwing figure, are thus detected, which verify the different oxidation-reduction mechanisms associated with these two different gases. The optimization in the design of the iron oxide nanoparticles coated sensor in order to enhance the sensitivity and the response time in the detection of the different gases is currently being pursued and the subsequent experimental results obtained will be reported in the upcoming 235th ECS conference.

Figure 1