Development of CeO2 based High Performance MEMS Oxygen Gas Sensor

Abstract

A gas sensor is a device that is used to measure the concentration of gas in its vicinity. It can also be used as a leak detector to detect a gas leak or other emissions. Extensive research is being carried out on gas sensor in designing miniaturized and cost e ective sensors that possess the required characteristics of high sensitivity, selectivity and stability with respect to a speci c application. Fast and unambiguous analysis of human surroundings will be in the near future inseparable part of public health, security and life quality control. Semiconductor metal oxide gas sensors stand out among the other types of sensors because of their simplicity and low cost. In the present work, we developed scalable, high sensitivity, fast response and low operating temperature CeO2 thin lm based oxygen sensors. A systematic investigation has been carried out to develop the high performance oxygen sensor which includes the optimization and integration of sensor lm and micro-heaters. CeO2 thin lms of di erent thicknesses ranging from 90 nm to 340 nm have been deposited at 400oC using RF magnetron sputtering on Al2O3 substrates. Characterization techniques such as Ellipsometry, XRD, XPS and AFM have been used to characterize the CeO2 lms for their thickness, structural, compositional/chemical and surface morphological properties. From XRD and XPS data, it has been observed that all the lms are polycrystalline and with thickness more than 195 nm are stoichiometric. It has also been observed that the resistivity of the lms depends on the texture coe cient of (200) plane of CeO2. It has been found that 260 nm thick lm has high textured coe cient of (200) plane which shows minimum electrical resistivity and maximum sensitivity towards the oxygen gas. The CeO2 lm with an optimum thickness of 260 nm has shown very high sensitivity (12.6), fast response time ( 10 s) and recovery time (15 s) at a low operating temperature of 400oC, which are the best values reported till date in case of undoped CeO2 thin lm based sensors. The response time of CeO2 based sensor may be reduced further by increasing the conductivity of the CeO2 lms with appropriate dopants. v A novel technique was used for the synthesis of CeO2-HfO2 mixed oxide thin lms using RF sputtering. The mixed oxide lms showed better sensing performance in comparison with pure CeO2 lms. The Hf atomic concentration was controlled varying the size and number of HfO2 pellets to achieve the best sensing performance. The CeO2-HfO2 mixed oxide sensor with 10-11% of Hf concentration showed best sensitivity ( 15), response time (8 s) and recovery time (10 s) at a low operating temperature of <400oC reported till date. From XRD and XPS data, it was understood and concluded that the best sensing characteristics of CeO2-HfO2 mixed oxide lm with 10-11% atomic concentration of Hf can be attributed to the existence of a highly reactive plane (200) with the highest surface energy and a strongly reduced surface with oxygen vacancy formation due to the presence of Ce3+ ions and HfOx, x<2 on the surface of the mixed oxide lm. The sensor performance is reproducible without any drift in the base line resistance. Microheaters play a crucial in MEMS gas sensor technology. Several microheater designs have been studied, however new heater patterns and designs are required to achieve excellent temperature uniformity and low power consumption. Here in this work, the area of the heater is optimized in order to increase the resistance by adopting novel designs / geometries. The single meander shape was taken as a reference design. After several modi cations, iterations and optimizations, two di erent geometrical structures namely Perforated Type 1 and Type 2 Platinum microheaters of dimension 500 m x 500 m were designed and analyzed using FEM based software COMSOL. The simulated results show the temperature being distributed uniformly across the entire structure in both the designs. The designed microheaters were fabricated and characterized thermally and electrically and showed excellent temperature uniformity and the power consumed to obtain the temperature of 400oC is nearly between 1.14 to 1.44 W which is considerably lower than reported values in the literature. The fabricated heaters were integrated into a gas sensor and the device was tested for oxygen gas. The sensing results were found to be in good agreement with the results obtained using a conventional heater. Readout circuits are circuits used to convert the sensed signal, such as vi voltage, current, resistance etc. or changes in it into a more convenient form of the same or di erent type of signal for further processing. A highly e cient 3-stage op-amp based readout circuit is designed to measure the dynamic change of the sensing lm resistance. The three stages are namely : constant current source, bu er ampli er and feedback ampli er. The 3 ampli er con guration with a constant current source is used to measure the change in resistance and voltage is measured across the resistance under test. The real-time simulation results show that the circuit is highly e cient and linear.