A hybrid microsensor system consisting of an array of microhotplate reactors/sensors and a complimentary surface acoustic wave (SAW) sensor is being developed for selective detection of fluorocarbon compounds using tetrafluoroethane (R134A) as a model compound.  The microhotplate sensors simultaneously detect and decompose the analyte while the SAW depends on exposure to the hydrolysis products of the R134A for its sensing mechanism.  This paper focuses on comparing the microhotplate sensing and catalytic reaction mechanisms.

The microhotplate devices, shown in Fig. 1, were fabricated in the clean room at the University of Maine's Micro Instruments and Systems Laboratory and each consists of a suspended sandwich of multiple SiO₂/SiN layers that separate two platinum circuit element layers: 1) an embedded serpentine heater and 2) surface electrical contacts.  Since microhotplates are thermally isolated from the substrate by silicon micromachining, they can be heated and cooled over a range of more than 400 °C at rates of over 10⁶ °C per second. Semiconducting SnO₂ and catalytically active platinum are selectively deposited on the microhotplates using chemical vapor deposition with precursors tin tetrabutoxide and platinum acetylacetonate.  Measuring the electrical response of the material over a range of operating temperatures is a way to improve the selectivity of metal oxide sensors that is commonly referred to as temperature programmed sensing (TPS).

In TPS the reaction kinetics control the surface concentrations of species such as oxygen and hydroxyl.  These surface concentrations significantly affect the electrical conductivity of the sensing film.  The rates depend on the sensor material as well as the temperature.  The sensor sensitivity and selectivity of SnO₂ (with and without Pt catalyst doping) are correlated to the decomposition rate of R134A measured using a microreactor catalyst characterization system.