AVS 45th International Symposium
    Thin Films Division Tuesday Sessions
       Session TF-TuM

Paper TF-TuM9
A Study of Thermally-Defined Gas Sensing Films on Micromachined Arrays

Tuesday, November 3, 1998, 11:00 am, Room 310

Session: Thin Films for Sensing and Data Storage
Presenter: R.W. Walton, National Institute of Standards and Technology
Authors: R.W. Walton, National Institute of Standards and Technology
R.E. Cavicchi, National Institute of Standards and Technology
J.D. Allen, National Institute of Standards and Technology
S. Semancik, National Institute of Standards and Technology
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Film microstructure and interfacial composition of sensing materials can have a profound effect on the performance of solid state gas sensors. We report on an efficient, array-based study of property/performance relationships for conductometric gas sensing films done on micromachined devices called microhotplates. Four-element arrays were used primarily in this study. Two types of lithographic processes were employed to deposit the catalytic metal/semiconducting oxide sensing films; each method utilizes the localized heating and temperature control available for the individual elements of the microhotplate array. A CVD process uses the heater to activate surface reactions for selected-area film deposition. We also introduce a new lithographic process that involves coating the entire array with nitrocellulose and exposing chosen elements by eliminating their coatings with a high temperature pulse. Following deposition (by evaporation, sputtering), lift-off of the nitrocellulose removes unwanted material. In situ monitoring of both deposition processes makes use of built-in electrical contacts for measuring conductance. We demonstrate the power of this array approach for studying film deposition, thermal treatments (sintering), and sensor testing. CVD films of SnO@sub 2@ films were prepared by heating each 50µmx50µm hotplate to 500° C in a flow of tetramethyltin and O@sub @ in Ar. We correlate the effects of catalyst thickness (0-200 Å), annealing (to 700° C), and composition (evaporated Pd and Pt) with gas sensing response from 100° C to 500° C. SEM images show anneal treatments as low as 500° C change the morphology of the catalysts on the oxide grains to produce islands or porous structures that enhance sensitivity to reducing gases.