Chuck Taylor, Richard E. Cavicchi, Steve Semancik
Process Sensing Group, Process Measurements Division, Chemical Sciences and Technology Laboratory
Gas sensing characteristics of metal oxide films are dependent on the preparation method used in their fabrication. To optimize sensing film performance, one must understand how processing parameters influence composition and microstructure, and then correlate these changes with changes in the selectivity, sensitivity and stability of a sensor. We have been using arrays of microhotplates, MEMS devices fabricated with individually addressable heaters and sensing contacts, for both combinatorial studies and gas sensing. The short thermal time constant of the microhotplates makes them excellent microsubstrates for materials research where rapid heating and cooling during deposition are desired (heating rates of 105 - 106 ºC /s are possible). Experiments have been performed using 4- and 16-element arrays as microsubstrates for CVD processing of titanium oxide and tin oxide using the single source precursors titanium(IV) nitrate, titanium(IV) isopropoxide and tin(IV) nitrate. Sensing films have been deposited both at fixed temperatures in the range 100 ºC to 450 ºC, and using variable temperature deposition. Variable temperature deposition was achieved by applying triangle or square waves of varying frequency and amplitude to the heater. Film microstructure was examined by FESEM and its composition measured by EDS. We have observed temperature and concentration dependent switching behavior in the response of sensors to particular analytes, both of which will be helpful for analyte identification.