A powered microheater can be used to localize the deposition of a sensing film onto its associated microhotplate using chemical vapor deposition. When CVD gases are set to flow over an array, only heated micro-hotplates initiate the reactions to deposit a film. Deposition can be stopped simply by turning off the micro-heater.
This self-lithography can be used to deposit different sensing materials on different micro-hotplates. This is advantageous because the process is simple, requires minimal contamination of the surface and avoids the reduced yield associated with multiple photolithography steps, and allows for high-temperature processing of the sensing film to optimize the materials properties.
Electrical contact to the growing film allows conductance measurements during deposition. Electrical contact during growth serves as an in situ characterization of the growth. In the following example, the microheater was switched off and then on several times during the growth. Shown are a log plot and a linear plot of the same data during the growth of a tin oxide film using tetramethyltin, oxygen and argon. The data show the insulator-conductor transition in the growing film, the change in the degree of thermal activation as the thickness increases, and the linear growth of conductance with time when a continuous film has formed. This is expected for a constant growth rate of fixed conductivity material. Note when the heater is switched on, the conductance takes on the value it had just before the heater was switched off.
Varying the flow rate of the precursor is detected in the conductance data.
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