Characterization of Patterened Galvanic Porous Silicon for On-Chip Combustion

We investigate the nuances of galvanically etched porous silicon that affect combustion. Several porous silicon samples are investigated with high speed imaging, SEM, and gas adsorption porosimetry. Porous silicon characteristics were primarily varied by altering the concentration of hydrofluoric acid in the etch solution. This provided combustion propagation rates through the different porous silicon films that varied across an order of magnitude from 160 1840 m/s. We show with SEM that for samples with a high concentration of hydrofluoric acid, an electropolishing effect occurred that restricts the amount of oxidizer that entered the pores, resulting in diminished reactivity. A different effect was observed if the hydrofluoric acid concentration is low, where the porous silicon film cracks and peels away from the substrate, again showing poor reactivity. We also show that similar effects on that material properties as were observed through variations in solution concentration were also observed with a single etch solution by varying the surface area ratio of backside platinum to frontside silicon. A change in this ratio was made by creating patterns of porous silicon with different surface area ratios on one chip. We show that a larger surface area ratio increased the etch rate, and resulted in a significantly different porous silicon structure. This feature is of concern for applications where porous silicon is patterned on-chip, and must be considered at multiple points of the fabrication process.