Fundamentals on Bionanotechnologies - Bionanoelectronics: Bioinquiring and Bioinspired Devices - page 18

1.2 Nanotechnologies for Bionanoelectronic Devices

The nanotechnologies for bionanoelectronic devices rely on the technologies used
in electronics to produce complex circuits based on AIII-BV semiconductors and
micrometer and submicrometer-scale Si integrated circuits. Silicon is still the
main material for micro- and nanoscale devices, but carbon allotropes, AIII-BV
semiconductors, and biomolecular assemblies could end its dominance. There are
two main approaches for nanotechnologies: top-down and bottom-up.
In the top-down approach, the major steps are (1) deposition of a single or
multiple layers on a substrate, followed by (2) the transfer onto the layers grown
in the first step of desired patterns, process that involves in turn several steps to
remove unnecessary materials that are not included in the desired pattern. The first
major step is called deposition technique, whereas the second is called lithography
or nanolithography when the desired patterns have nanosized features.
The bottom-up nanotechnology techniques have no counterpart in the electronic
technologies. For example, in the self-assembly technologies, which are mask-
less techniques, the mask manufacturing process characteristic for the top-down
approach is replaced by less expensive specific chemical reactions able to form
desired three-dimensional (3D) patterns consisting of metallic or semiconducting
nanosized materials. However, it is not uncommon to fabricate nanoelectronic
devices, especially biosensors, by a combination of bottom-up and top-down
approaches.

1.2.1 Deposition Techniques for Bionanoelectronic Devices

In this section, we examine briefly the most important deposition techniques for
micro- and nanoscale devices. An extended review with comprehensive references
can be found in Ziaie et al. ( 2004 ).
To start with, the fabrication of SiO 2 by oxidation of silicon is an essential
process at both micro- and nanoscale, although other oxides, such as HfO 2 ,are
increasingly studied. SiO 2 is a dielectric material used to isolate a variety of
metallic electrodes from other conductive substrates and can be deposited on silicon
substrates with thicknesses ranging from few nm up to 2m. In particular, SiO 2 is
encountered in many transistors and micro-electro-mechanical systems (MEMS) or
nano-electro-mechanical systems (NEMS), for example, switches. It is grown in the
presence of oxygen or water at temperatures of about 1;000-1;200 ı C into furnaces
containing a quartz tube, an electrical resistance heater, and a wafer holder. The
thickness of the SiO 2 depends on the temperature and the gas flow.
Another basic technological process in microelectronics, which is associated to
deposition, is semiconductor chemical doping, performed with the aim of changing
dramatically its electrical, optical, or mechanical characteristics. The process of
changing these characteristics is generically referred to as functionalization or