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Scientific Reports on Micro and Nanosystems
edited by Christofer Hierold
Vol. 32
Sebastian Eberle
Ultra-clean suspended carbon nanotube
gas sensors – concept
for large scale
fabrication and sensor
characterization
1st Edition 2019. XX, 180 pages. € 64,00.
ISBN 978-3-86628-659-7
Abstract
With millions of deaths worldwide
every year due to the adverse health effects from air pollution, a growing
world population and ongoing industrialization, the need for clean air is
becoming ever more pressing. In order to combat air pollution and to raise
necessary awareness the first step is to accurately determine pollutant levels.
As these concentrations are highly fluctuating over time and space, the
requirements on a sensor or sensor network on sensitivity, selectivity,
response and recovery times and especially power consumption are very strict.
Carbon nanotubes, due to their
outstanding physical properties, present themselves as highly interesting
candidates for the targeted application. When individual, suspended CNTs are
operated in field effect transistor devices, sub-ppm detection of NO2 at a power consumption of only a few μW has been demonstrated. While this
impressive performance is rarely achieved by other nanoscale sensors, e.g.
based on Si-nanowires or metal-oxide nanoparticles, the fabrication of these
devices poses a major challenge for its widespread application. Due to their
inherently high sensitivity the final CNT-based sensors’ performance is highly
susceptible to process-contamination, which can be avoided by time-consuming
serial fabrication steps.
Therefore in the first part of this
thesis the impact of automation on the dry-transfer process used to conserve
the CNTs pristine state is investigated. A novel, automated setup capable of
performing translational and rotational alignment and the complex mechanical
transfer of a suspended nanotube with a precision below 1 μm, is presented. An estimate of the
maximum throughput shows an increase over 600 times, theoretically yielding in
almost 1000 successful transfers per hour.
This is followed by the introduction
of a novel, flexible fabrication process for the sensor substrates themselves.
Apart from compatibility with the automated transfer routine the presented
process flow allows for wafer-scale fabrication of the sensor substrates with
readily adjustable gate distances dg from 0.2μm to 2μm and channel lengths lchannel from 2μm to 8 μm. It is demonstrated that by reducing the gate
distances to 0.24μm it is possible to acquire a full CNTFET gate
characteristic within a voltage window of only 1 V, which makes the used
sensors fully compatible with prevalent CMOS technology and low power read-out
circuits. Even though the shift in threshold voltage upon gas adsorption for
larger gate distances is up to 3.2× higher than
for shorter gate distances, this behaviour is
exploited to derive recommendations for determining the optimum gate distance
for the desired application.
Subsequently
the dependence of the sensors response on varying bias conditions is presented.
When sweeping the source drain bias Vds over a range
from 0.1V to 2.2V and the gate bias voltage Vg from −10V to 10V a maximum change in
current is found at a bias conditions of Vds = 0.6V and Vg = −10V under gas
exposure. Furthermore at these bias conditions the conductance shows a maximum,
which is dependent on the gate bias and the applied gas concentration. Through
simulation of the involved electrostatic potentials down to a sub-nm regime,
the potential induced through the measured threshold voltage shift upon gas
adsorption is matched with the potential of a uniform charge distribution. From
this charge distribution it is estimated that a charge equivalent to 650 NO2 molecules is necessary on the tubes’ surface to induce
a shift of DVthr = 1.7V at a concentration
of 10 ppm. Furthermore a phenomenological description of different charge
contributions for the sensor architecture under consideration is given.
Keywords:
Scientific Reports on Micro and Nanosystems
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