Sol-Gel Materials for Gas Phase Sensing Using Microhotplate Arrays
Nancy Ortins Savage, Richard E. Cavicchi, Michael Tarlov, Steve Semancik
Process Measurements Group, Process Measurements Division, Chemical Science and Technology Laboratory
Greg Gillen, Analytical Microscopy Group, Surface and Microanalysis Science Division, Chemical Science and Technology Laboratory
Sol-gel chemistry is used to create suspensions (sols) of small particles of materials such as metal oxides. These suspensions can be dried to a gel or powder or used in the sol form for coatings and thin films. One promising application of sol-gel derived metal oxides is as sensor materials. Their small particle sizes lead to higher gas sensitivities than conventional materials. The results of isothermal sensing measurements show that the conductance of sol-gel films can change by as much as a factor of 700 in the presence of methanol compared to materials made by chemical vapor deposition that have a conductance change of a factor of two.
Our group works with microhotplate arrays (micromachined silicon devices consisting of multiple individually addressable sensing platforms with electrical contact pads and heaters) for testing sensor materials. Considerable effort has been directed toward depositing oxides on the different sensor platforms by chemical vapor deposition. Because CVD is a temperature-initiated process, materials can be selectively deposited on a single platform of a larger array by heating only platforms where one would like to deposit a material. By introducing multiple precursors while heating different platforms an array can be made which has a number of unique materials for sensing.
Sol-gel materials can be easily deposited on the microhotplate
arrays by spin coating. This results in the same material on each of the
sensing platforms of the array. The challenge in working with sol-gel materials
is developing a way to put a different material on each platform of an
array. To reach this end, we are developing a technique to mask the surface
of microhotplate devices using silane monolayers. Organosilanes such as
(tridecafluoro-1,1,2,2-dihydrooctyl) trichlorosilane readily react with
the surface hydroxyl groups on the SiO2 surface of the microhotplate
arrays leaving a hydrophobic surface. Heating specific array platforms
removes the silane monolayer and leaves the underlying hydrophilic SiO2
and electrical contacts exposed. This difference in surface polarity is
used to selectively coat sol-gel films (which are water-based) on designated
microhotplate platforms. Characterization of the microhotplate arrays with
Secondary Ion Mass Spectroscopy (SIMS) has shown how the readily the surface
modification and removal of the organosilanes occurs with temperature.