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Through the use of silicon micromachining, we have developed a microhotplate structure capable of reaching temperatures in excess of 500 °C, onto which thin films have been selectively grown via metalorganic chemical vapor deposition. The microhotplate structure contains surface electrical contacts which permit conductance measurements to be made on films during and after deposition, and therefore presents some unique opportunities for the in situ characterization of growing films as well as for novel gas sensing approaches. We have investigated the deposition of conducting oxides such as SnO2 and ZnO on these microhotplate platforms for gas sensing applications. The conductance of the deposited films has been measured in situ as a function of time, and used in combination with postdeposition thickness measurements to provide insights into the growth rate of the oxide films. Results indicate that our conductance measurements are sensitive, in certain cases, to changes in the film thickness on the order of an angstrom. Conductance oscillations observed during the growth of ZnO thin films have been attributed to variations in the precursor concentration, and were detected as a gas sensor response by the growing films.

Chemical sensors based on a microhotplate platform generally function via a conductometric or calorimetric transduction mechanism. In addition to these mechanisms, a mechanical transduction mechanism is proposed based on the detection of the microhotplate bending due to volume change of functional layers. In this letter, we demonstrate sensing of gaseous hydrogen based on volumetric expansion of Pd∕Y functional layers. In this case, the embedded polysilicon heater element also serves as the piezoresistive strain-detecting element, changing its resistance as the microhotplate bends. This transduction mechanism can be used independently of, or in conjunction with, a simultaneous conductometric or calorimetric mechanism.