Nanotechnology-Enabled Sensors

202 Chapter 4: Nano Fabrication and Patterning Techniques
4.9.7 Ion Implantation
Ion implantation is a process by which ions of a material are implanted
into another solid. Energies of the ions are typically in the range from 10 to
500 keV. Although energies in the range 1 to 10 keV may also be utilized,
they have the drawback of lower penetration depths, only a few nanometers
or less. Ion implantation is used in semiconductor device fabrication
and in metal finishing, as well as various applications in materials science
research.
As a result of the ion implanting, the physical properties of the solid
changes. Additionally, the implanted ions can also introduce a chemical
change in the sample, as the implanted ions may differ from the constituents
of the substrate. Structural changes can also occur, as the implanted
ions can alter or damage the sample’s crystal structure. Additionally, ion
implantation can be used for atomic mixing at the substrate’s surface.
By ion implanting, surface morphology of nanostructured thin films can
be precisely controlled and altered for sensing applications. In addition, using
this method, single atoms of certain materials can be implanted onto
the surface of the substrate which can be used as catalysts.
As an example, the controlled implantation of single ions into a silicon
substrate with energy of sub-20-keV has been demonstrated 96 , displaying
potential application in the development of quantum computers.
4.9.8 Etching: Wet and Dry
In order to form the patterns, the electrodes and structures in transducers
and sensors, it is necessary to etch the deposited thin films. In addition, the
quality of sensitive layers can very much depend on the etching process.
There are several important parameters in any etching process:
1. Etching rate: The rate of etching can greatly affect the quality of the
produced structures. For the fabrication of electrode patterns generally
the faster etch the better as it reduces the fabrication time, however,
fast etching can also lead to deformities, particularly in the
edges of the structures.
2. Uniformity: In fabrication of sensors and transducers, we need to obtain
a consistent uniformity across a sample during the etching process.
Some parts may be etched more than others for many reasons,
such as non-uniform concentration of the etchant solution, heat, and
contaminants in the sample. The etching environment should be
properly controlled to circumvent such problems.