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6 T echnology
Electronic Circuits with Single Molecule Components
Colin Mann, Physics
Professor Philip Collins
Electron Beam Lithography
ABSTRACT
The use of carbon nanotube field-effect transistors (CNTFETs) as chemical or biological
sensors is the focus of many research groups in nanotechnology. CNTFET sensors have
been demonstrated with extraordinary sensitivities ranging from parts per million down to
parts per trillion. These high sensitivities could allow a broad range of new applications in
the detection of, for example, toxins, regulated substances, industrial gas leaks, and
chemical warfare agents . The immediate goal of this project is to fabricate devices with a
particular architecture in order to study the mechanisms of CNTFET sensors. We would
like to determine whether CNTFET chemical response is a result of the interaction
between the nanotube and the test chemical or the junction between the nanotube and
electrodes and the test chemical. My role in the project is centered around the fabrication
of devices for such an experiment, specifically, the creation of “windows” over portions of
nanotubes using electron beam lithography.
A layer of polymethyl methacrylate (PMMA)
approximately 200 nm thick is placed over the
chip using the spin coater on the left. The
PMMA acts as a barrier during the experiment,
covering the electrodes from exposure to the
test chemical. Once the window is in place this
allows for the chemical to have sole interaction
with the nanotube and not the electrodes.
SEM
Carbon Nanotube Sensors
The design of our devices allows us to create defects
in specific locations on the nanotube. Due to the high
sensitivity of nanotubes, covering up the electrodes
and contacts allows us to determine where the
sensitivity lies.
Once the nanotube is oxidized
(eletrically broken), we can recover the conductivity by
depositing metal at the defect site. The image below
shows such a defect site and it’s response before and
after deposition on the graph.
Spin Coater
CARBON NANOTUBE BACKGROUND
Carbon nanotubes are an ordered arrangement of carbon atoms in two dimensional
hexagonal arrays, wrapped into seamless tubes. This gives rise to unique electronic and
mechanical characteristics. Some important aspects of carbon nanotubes are:
Structure :
Diameter :
Length :
Electrical :
Strength :
analogous to a layer of graphite wrapped around to make a seamless
cylinder
single-walled with diameters around 1 nanometer (approximately 6 carbon
atoms)
Up to many centimeters.
Can be metallic or semi-conducting
High tensile strength but flexible
Procedure
•Coat chip in PMMA
•Using software written in our lab to
manipulate the electron beam of the SEM,
expose necessary regions of PMMA
•Remove the exposed PMMA by placing the
chip in a developer solution (MIBK:IPA)
Carbon Nanotube Transistors
On
Application of an electric field to the gate in a
carbon nanotube circuit turns the device on
and off like a transistor, as shown in the graph
at left.
This behavior is due to an increase or
decrease in the conductivity of hole carriers in
the nanotube. The Fermi level EF is defined as
the energy of the highest occupied electronic
state in the valence band. EF is the energy
state at which hole carriers conduct. When a
Off
positive gate voltage is applied EF is raised
and the hole conduction is suppressed,
turning the nanotube ‘off.’ A negative gate
voltage increases the number of positive charge carriers, lowering EF and increasing the
nanotube’s conductivity, thus turning it ‘on.’
PMMA /
SiO2
Chemical reaction caused by the energy delivered
from the electron beam to the PMMA.
Image taken in SEM after development. White marks
are windows where PMMA is no longer present.
Additional Uses
Above: AFM image
of paladium coated
nanotubes.
Below: SEM image
of PMMA windows.
Sample Fabrication
In preparing a suitable sample for experimentation, one typical end result is shown in the
cartoon below. This is achieved by first growing nanotubes from a catalyst. Once the
nanotubes are placed on the chip, electrodes are laid down using photolithography, this
allows for electrical probing of a device to locate a possible connection between electrodes
and a nanotube. Once a connection is found, the
chip can be used for experimentation. A layer of
polymer is laid down over the device. Then, using
electron beam lithography, a small window is
opened up over the nanotube to allow
experimentation on the nanotube without
interference at the junction between the electrodes.
For a different experiment we exposed the electrodes,
leaving only a thin layer of PMMA over the center of the
tube. We can then deposit metal in these areas thus
modifying the contacts, changing the conductivity between
the electrodes and the nanotube. The nanotube below the
PMMA is unaffected by the deposition and maintains it’s
conductivity.
www.physics.uci.edu/%7Ecollinsp · [email protected]
www.research.calit2.net/students/surf-it2006 ·
www.calit2.net
SWCNT
Single walled carbon
nanotube underneath
a layer of PMMA.
Nanotube is visible
through the windowed
region.
Acknowledgements
Collins Research Group: Dr. Philip Collins, Dr. Jaan
Mannik, Brett Goldsmith, John Coroneus, Alex
Kane, Bucky Khalap, Steve Hunt, Danny Wan, and
Phil Haralson.