Schematic process flow for the formation of hollow needles and cuplike structures, (a) the formation of the silicon membrane in desired places, followed by patterning the microrings, (b) the magnified view of rings on the silicon membrane, (c) vertical etching of Si, and (d) deposition of Cr using a rotating small angle deposition method to coat the outer sides of the recessed craters while inner parts and especially the inner bottom remains uncoated. (e) By continuing the vertical etching one can arrive at hollow structures. It must be noted that all steps in this process flow belong to features, which are created on the silicon membrane.
(a) SEM images of a silicon surface where cuplike structures have been created. (b) A top view of the structures indicating the high aspect ratio features.
(a) SEM image of a silicon surface with miniaturized cuplike structures. (b) A closer view of the features with nanometric walls and height. Inset shows the presence of walls with 82 nm width. (c) Partial cross sectional view of the fabricated hollow structures.
SEM images of a silicon needle, (a) prior to thermal oxidation and (b) after thermal oxidation in a dry oxide furnace. The change in the roundness of the top surfaces of the side-walls before and after oxidation is clearly observed in part (c).
(a) The SEM image of the backside of the Si membrane where the microneedles are placed on its front side. (b) The image with a higher magnification to observe the holes. Inset shows a processed hole with a diameter of . (c) The evolution of small holes from backside with typical diameter around as determined from the top circular rings.
Schematic presentation of liquid-stimulated capacitance measurement. Inclusion of a water droplet into the backside cavity of the sample would yield in the penetration of liquid into the little holes, increasing the effective permittivity which in turn leads to an increase in the capacitance value.
The wetting of a water droplet on a Cr-coated surface, indicating a 90° angle.
Capacitance voltage characteristics of the liquid-stimulated capacitor. (a) C–V with and without liquid. (b) The magnified C–V results in the presence of water indicating a small valley, which could be due to the creation of a depletion region in the silicon surface similar to a metal-oxide-semiconductor structure. (c) The effect of slight tilting of the water-holding sample on the capacitance, indicating a significant rise from 10 to 13 pF.
The variation of the capacitance with respect to the tilting angle. At angles higher than 10°, a sharp rise in the capacitor is observed which does not follow the monotonic increase with the angle. This rise could be due to the capillary drawing of the water through the hole. Inset magnifies the near linear regime of operation of the device.
The abnormal capacitance voltage behavior of the device at higher angles. The value of the measured capacitance is around 100 pF which is believed to be due to a water overflow due to the capillary force.
Article metrics loading...
Full text loading...