Nanotechnology-Enabled Sensors

204 Chapter 4: Nano Fabrication and Patterning Techniques
Bosch Company which first developed the method. The DRIE involves alternating
different gas compositions during etching.
In the Bosch process initially a sample with a patterned surface is placed
in the chamber. Once inside, the following steps are carried out: (a) A
highly reactive gas is introduced to the chamber, which isotropically etches
the substrate for just a few seconds, (b) a polymeric passivation layer is
then conformally deposited over the whole surface which generally takes a
few seconds, (c) through the sputtering process, ions remove the passivation
layer from the bottom of the previously etched trench, but not from
the sides, (d) the process is then iteratively repeated, starting with the introduction
of the highly reactive gas. This time, the sidewalls are covered
by the passivation layer, so introducing the reactive gas, for a short period,
ensures that only the bottom of the trench will be etched. Continuing these
steps several times allows high aspect ration structures to be obtained, and
therefore etching is preferential in the vertical direction. 97,98 Such high aspect
ratio structures are very useful in the fabrication of devices such as
sensors, actuators, micro-fluidic systems, and scanning probe tips.
Another dry etching method is sputter etching. It is very similar in principle
to the sputtering deposition process that was described previously in
this chapter. However, the difference is that in this process, the substrate is
now subjected to the ion bombardment instead of the target material.
Vapor phase etching is another dry etching method. The sample to be
etched is placed inside a chamber in which gases are subsequently introduced
and the gas molecules chemically react with the surface of the sample.
The two most common vapor phase etching technologies are silicon
dioxide etching using hydrogen fluoride (HF) and silicon etching using
xenon diflouride (XeF2). The byproducts condense on the surface and interfere
with the etching process which have to be taken care of. 99
4.10 Summary
Different methods for the synthesis of nano-materials in the form of
nano-particles, one-dimensional nanostructures, and nanostructured thin
films were presented in this chapter. In addition, the most common techniques
for the development of nano-resolution patterns were described.
These methods and techniques are utilized for the fabrication of
nanotechnology enabled sensors which will be presented in Chaps. 6 and
7, with Chap. 6 focusing on inorganic sensors. The complementary methods
which can be used for the development of organic nanomaterials for