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1 '2 Role of 21st Century Chemistry in Transportation and Energy Jiri Janata, Georgia Institute of Technology CHEMICAL SENSORS Chemical sensors, important for environmental and processing applications, are finding increasing utility in national security. National security requires constant vigilance for detection of the myriad chemical and biological agents that could potentially be used against the United States Because not every sensor can monitor the presence of all agents, the most recent and growing trend is to use higher-order chemical sensors and sensor arrays. A chemical sensing array is achieved by obtaining multiple parameter measurements from one information channel in addition to using multiple channels. The information obtained then comes from "information space." This approach is similar to hyphenated techniques, such as gas chromatography-mass spectrometry, which separates components and then finds further information about those components with the second technique. Chemical sensors are key in the transportation industry. Sensors are used to determine the fuel ratio, manage the optimum ratio, and measure the oil quality for pollution control. Chemical sensors monitor tailpipe emissions and catalytic converters. Additionally, sensors aid in chemical diagnostics of oil, transmission fluids, and other performance fluids. Higher-order chemical sensing can alleviate some inherent problems of chemical sensors. For example, a sensor array can mathematically correct for systematic drift. It also provides cross selectivity for elimination of interference. Sensor arrays are produced by microfabrication. Because different portions of the array have different fabrication requirements, convenience dictates that the sensors be fabricated in two parts: the chemical sensing chip with the transducer 76
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ROLE OF 21ST CENTURY CHEMISTRY IN TRANSPORTATION AND ENERGY 77
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78 ENERGY AND TRANSPORTATION that converts the primary interaction into a measurable signal and the data pro- . . cesslng c. alp. Conventional field effect transistors are the building blocks of binary elec- tronics. Worldwide production is 10~7 transistors per year, and per capita use in the United States is 107 transistors per year. The cost per transistor is one micro dollar, and the yield of fabrication is greater than 99 percent. The metalization on the interconnects of the chips was previously aluminum, but it is starting to be switched over to copper. Likewise, the solid-state circuit feature size was 0.1 ,um but is now becoming smaller. Silicone dioxide comprised the top layer of the device. On the other hand, chemical electronics (chemical sensing chip) production was probably less than 10,000 yearly (10-4 per capita) with a cost of $5 per chip. The yield is no better than 60 percent, and the metalization is platinum or gold. The feature size in the XY direction is typically on the micron scale, but it is 100 microns in the Z direction. The top layer is very high quality defect-free silicone nitrate, which is much higher quality than silicone dioxide. The lack of defects, as well as the gold or platinum metalization, allows the chemical electronics chip to function in harsh environments. As features on chips have become smaller, the wafers from which the chips are made have become larger. Because of these trends, the silicon foundries for
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ROLE OF 21ST CENTURY CHEMISTRY IN TRANSPORTATION AND ENERGY 79 fabrication of chemical electronics are scarce. It is difficult to find a facility that has the capability to complete the entire device process. However, to continue with development of the integrated chemical sensing field, these facilities are a necessity. The protection of closed spaces is another area where chemical sensors are of great importance. This concern is very closely related to transportation, since there is a high density of people in closed spaces on airplanes, trains, ships, subways, and other means of transit.