Suspended Heater Embedded Metal Oxide Nanowire Based Gas Sensor Platform

Abstract

Introduction Over the past few decades, different types of gas sensors have been developed based on different sensing materials and methods. Among them, semiconducting metal oxide nanomaterials are suitable for gas sensing applications due to their high electron mobility, high surface-to-volume ratio, high crystallinity, and long-term stability. Considerable efforts have been made to improve the performance indicators of metal oxide gas sensors such as sensitivity, selectivity, reaction and recovery time [1]. However, in recent years, there has been growing demand for miniaturization, low power consumption, low cost and mass production, which are the characteristics required for continuous and systematically integrated monitoring and management in the sensor market [2]. Besides, widely used materials for metal oxide gas detection are wide-bandgap semiconducting oxides such as zinc oxide, tin oxide, gallium oxide or indium oxides, which are operate at elevated temperatures (250–600°C) since enough thermal energy of surface redox reaction is required to overcome the activation energy barrier and increase the reaction kinetics in order to realize sensing measurement. Therefore, additional heater systems are required to metal oxide gas sensor and it may cause high power consumption, high cost or occupying a lot of space. Hearin, we propose a nano-heater integrated metal oxide gas sensor that capable of sensing with low power consumption and fast response and of being fabricated at a wafer level through C-MEMS which facilitated sub-micrometer-level carbon patterning due to a dramatic volume reduction during the polymer pyrolysis. The temperature of the metal oxide, the sensing material, can be heated immediately to 600°C within a low power of 7mW. In addition, the maximum temperature of the metal oxide can be easily controlled according to the applied voltage. Therefore, only a voltage source is needed to heat the sensing material. Method Our sensor platforms as shown in Figure a, were fabricated in several steps. First, eave structures for the selective metal coating on a suspended carbon nanowire were fabricated by oxide etching and isotropic silicon etching. Then, the suspended carbon nanowire was fabricated through C-MEMS consisting of two successive photolithography and polymer pyrolysis [3]. Then, we deposited gold on a carbon wire using evaporation as a heater line. Owing to the eave structure and anisotropic evaporation, the gold layer could be connected only through the suspended wire as shown in Figure b. The next step was the deposition of HfO2 as an insulation layer by Atomic Layer Deposition. In what follows, the ZnO nanowire forest (Figure c) was integrated selectively on the middle of a suspended wire structure via seed layer patterning and a hydrothermal process in an autoclave [4]. Due to the high aspect ratio of the carbon nanowire, heat is hard to be released to both posts, resulting in a uniform temperature distribution over 80% of the total length of the wire. Therefore, we used ZnO nanowire forest as a sensing site by growing in the center of the carbon nanowire selectively where the temperature is kept uniform and within the range of the sensing target temperature according to the simulation results (Figure d). In the last, to obtain the electrical signal of sensing material, we deposited gold once more using an E-beam evaporator. Results and Conclusions We have identified the possibility of gas sensing with heater integrated metal oxide sensor platform. Observing that the resistance of the metal oxide decreased rapidly when a voltage was applied, we speculated that the temperature of the metal oxide would have risen immediately according to the Joule-heating principle. Furthermore, the resistance of the metal oxide decreased continuously as the applied-voltage gradually increased. When we applied 1.2V to a gold heater, the resistance of the ZnO dropped to about 1/20 of its room temperature resistance (Figure e) similar when it is heated by an external heater to about 250℃, which is occasionally used temperature condition in the MOS type gas sensor. Detailed gas sensing results will be presented at the conference.