Inh.: Dr. Renate Gorre
Fon: +49 (0)7533 97227
Fax: +49 (0)7533 97228
Scientific Reports on Micro and Nanosystems
edited by Christofer Hierold
Ultra-clean suspended carbon nanotube
gas sensors – concept for large scale
fabrication and sensor characterization
1st Edition 2019. XX, 180 pages. € 64,00.
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.
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