Relative Resistance Chemical Sensors Built on Microhotplate Platforms
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Relative Resistance Chemical Sensors Built on Microhotplate Platforms Joshua L. Hertz, Christopher B. Montgomery, David L. Lahr, and Steve Semancik Chemical Science and Technology Laboratory, National Institute of Standards and Technology, 100 Bureau Dr., MS 8362, Gaithersburg, MD, 20899 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. INTRODUCTION Chemical sensor arrays are desired as inexpensive means to detect a wide range of gaseous analytes. They have the potential to fill vital roles in homeland security, industrial process control, personal safety, and other application areas. At NIST, we have been researching sensors built on micromachined, thermally controlled platform (“microhotplate”) arrays [1,2]. These platforms consist of a silicon oxide membrane that has been grown on a silicon substrate and then, to a large extent, thermally isolated by micromachining. Buried within the membrane is a polysilicon line used for both heating and temperature measurement. On top of the membrane are electrodes for contact to a chemiresistive sensor film. Typically, the film is a metal oxide semiconductor. The purpose of the heater and the high thermal isolation are to enable very rapid, low-power temperature control with good temperature uniformity. Rapidly pulsing the temperature has been shown to create a rich data stream and thus allow differentiation amongst a wide range of analytes, despite a sensor’s broad sensitivity [3]. Generally, sensor arrays must first be “trained” by recording the sensor output upon exposure to a number of relevant analytes and, possibly, interferences. In use, the sensor output is then compared against the recorded training measurements to determine
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