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Relative Resistance Chemical Sensors Built on Microhotplate Platforms

Published online by Cambridge University Press:  01 February 2011

Joshua L. Hertz
Affiliation:
joshua.hertz@nist.gov, National Institute of Standards and Technology, Chemical Science and Technology Laboratory, 100 Bureau Dr., MS 8362, Gaithersburg, MD, 20899, United States, 301-975-2615
Christopher B. Montgomery
Affiliation:
christopher.montgomery@nist.gov, National Institute of Standards and Technology, Chemical Science and Technology Laboratory, 100 Bureau Dr., MS 8362, Gaithersburg, MD, 20899, United States
David L. Lahr
Affiliation:
david.lahr@nist.gov, National Institute of Standards and Technology, Chemical Science and Technology Laboratory, 100 Bureau Dr., MS 8362, Gaithersburg, MD, 20899, United States
Steve Semancik
Affiliation:
stephen.semancik@nist.gov, National Institute of Standards and Technology, Chemical Science and Technology Laboratory, 100 Bureau Dr., MS 8362, Gaithersburg, MD, 20899, United States
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Abstract

The selectivity, sensitivity, and speed of metal oxide conductometric chemical sensors can be improved by integrating them onto micromachined, thermally-controlled platforms (i.e., microhotplates). The improvements largely arise from the richness of signal inherent in arrays of multiple sensing materials and the ability to rapidly pulse and collect data at multiple temperatures. Unfortunately, like their macroscopic counterparts, these sensors can suffer from a lack of repeatability from sample-to-sample and even run-to-run. Here we report on a method to reduce signal drift and increase repeatability that is easily integrated with microhotplate chemical sensors. The method involves passivating one of a pair of identically-formed sensors by coating it with a highly electrically resistive and chemically impermeable film. Relative resistance measurements between the active and passive members of a pair then provide a signal that is reasonably constant over time despite electrical, thermal and gas flow rate fluctuations. Common modes of signal drift, such as microstructural changes within the sensing film, are also removed. The method is demonstrated using SnO2 and TiO2 microhotplate gas sensors, with a thin Al2O3 film forming the passivation layer. It is shown that methanol and acetone at concentrations of 1 µmol/mol, and possibly lower, are sensed with high reproducibility.

Type
Research Article
Copyright
Copyright © Materials Research Society 2008

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