MOCVD of SnO 2 on Silicon Microhotplate Arrays for Use in Gas Sensing Applications
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tures. For example, the detection of oxygen, CO,, and NOx in the exhaust of automobiles has
become necessary due to environmental concerns. The manufacturing sector also requires quantitative chemical detection for improving the control of the many processes that involve gaseous reactant feeds or by-products. These processes range from burning organic binder during ceramic firing, to baking bread, or processing semiconductor wafers. The sensors required for these applications need to be chemically selective, show long term stability, have fast response speed, and finally have a low cost. What is also needed to meet the demands of such a wide variety of applications is a broad-based technology platform that can then be tailored to the specific case at hand. Our approach to meet these demands has been to develop integrated arrays of conductometric gas sensors. The conductometric sensor is based on the conductance change of an active element when in the presence of the target species. The Taguchi sensor is typical of the discrete conductometric sensors currently available. It consists of a ceramic tube with an internal heater element and coated with an active element, usually a conductive oxide such as Sn0 2 , that is operated at temperatures ~200-500'C.1 These sensors suffer however, from long response times, and poor gas detection specificity. One way to improve the overall selectivity and ability to deal with mixtures, is to use arrays of devices, where the individual elements have sensitivity and selectivity differentiated either by composition or by operating temperature. Pattern recognition algorithms can then be used to translate the observed multiple responses into a unique compositional output. Size and cost become inhibiting factors in fabricating this type of array from discrete components, so we have turned to silicon micromachining techniques to fabricate planar arrays of microhotplates, such as that shown in Figure 1. This prototype microhotplate array consists of 4 elements, each having suspended polysilicon resistive heaters that produce a thermal rise time of -3 msec, a thin film thermometer, and four contact pads for measuring the resistance of the active film. By separately heating individual elements, we can take advantage of the thermally selective nature of the 2 MOCVD process to limit deposition to heated areas, resulting in a maskless deposition process. 231 Mat. Res. Soc. Symp. Proc. Vol. 415 01996 Materials Research Society
Figure 1. a) A 4-element array of microhotplates. Wirebonds are visible at the perimeter ot the image. b) A single microhotplate element. Contact pads are at the corners, and the polysilicon resistor pattern is seen beneath them. This paper will discuss the effectiveness of the MOCVD process when used in conjunction with microhotplate arrays. Particular attention will be given to how control of the MOCVD process can affect the film properties, such as film thickness, grain size, and base conductance, that are important to the gas sensing mechanism. 1 EXPERIMENTAL In this
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