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OCR for page 76
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
OCR for page 77
ROLE OF 21ST CENTURY CHEMISTRY IN TRANSPORTATION AND ENERGY 77
OCR for page 78
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
OCR for page 79
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.
Representative terms from entire chapter:
chemical sensing
