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Laboratorytalk
- April 2007
Edited by the Laboratorytalk editorial team
Better atomic force
microscopy for nanoelectronics
It was not until long after 1977 that the name nanoelectronics
came into use, but David Ferry was already actively engaged
in developing some of the world's smallest transistors.
The field, called ultra-small devices at that time in the
later part of the1970s, was in its infancy, and Dr Ferry's
research team was one of only four select groups around the
world aggressively researching the limits of small electronic
devices.
Today, Ferry heads up the nanostructures research group at
Arizona State University (ASU) in Tempe, a collection of faculty,
staff and students working on research in the regimes of nanolithography,
and the physics of nanostructures and ultra-small semiconductor
devices.
The group is a part of the University's college of engineering,
centre for solid state electronics research, whose alumni
makes up a serious constituency throughout the nanoelectronics
universe, in both industry and academia.
Their current interests lie in the area of quantum dots,
quantum wires, and ultra-small semiconductor devices in a
variety of materials. The group conducts a wide spectrum of
theoretical studies of quantum transport in these very small
devices. For example, they are doing a process called scanning
gate microscopy at low temperatures.
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This involves taking the equivalent of an atomic force
microscope and putting bias on it, and studying the
change in conductance of small semiconductor structures
as they move this bias tip around on a surface.
Their system is mounted in a large cryogenic cooler,
an enclosed container with a helium-3 cooling system,
an isotope of a helium molecule - which is brought down
to 300milli-Kelvin, about one-half a degree above absolute
zero.
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Arizona State University nanostructures group uses
negative-stiffness vibration isolation to eliminate
ultra-low frequencies and improve data in nanoelectronics
AFM research, reports Jim McMahon
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The cooler has a vacuum jacket around it so the heat can't
transmit in, and it protects the cold from being mitigated
by the ambient room temperature. The AFM tip is on a cantilever.
Normally, with the AFM, you just move this cantilever along
the surface then note the change in position as it goes over
topography on the surface. Ferry's group is utilizing a process
called a piezo-electric sensor.
They metalicise the AFM cantilevered tip with a very thin
layer of metal so they can apply a voltage to it. Then use
that voltage to perturb the structure they are looking at.
As the tip moves it creates a voltage across the plane, which
is measured to determine certain mechanical property values.
This is a technique that was developed at Harvard four to
five years ago.
This type of experimentation is not uncommon, similar experiments
are being done by a large number of universities. But what
is not common is the system that the nanostructures research
group is using for vibration isolation: negative-stiffness
vibration isolation, developed by Minus K Technology - which
provides a significantly greater and more stable attenuation
of the critical lower vibration frequencies, and therefore
more reliable accrued data sets.
When measuring a very few angstroms or nanometers of displacement,
you have got to have an absolutely stable surface upon which
to rest your instrument. If you do not, any of that vibration
coupled into the mechanical structure of your instrument will
cause vertical noise, and fundamentally an inability to measure
these kinds of high resolution features.
'Any kind of vibration noise in the system makes that AFM
cantilever tip move, and that gives you bad signals and incorrect
data,' says Ferry. 'We actually went further than most university
applications because we integrated a rather large magnet into
our system, something that Harvard, for example, is just now
putting into their operation.
'The magnet allows us to look at different types of transport.
'We can turn the magnet on and look at the magneto-transport
of the semiconductors. 'It is a quite a different mode of
transport altogether. 'The entire system had to be isolated,
not just the cantilever,' continues Ferry.
'We required an extremely high level of vibration isolation
given our research parameters. 'We are deriving modern electronic
devices from our experiments. 'Future electronic devices are
our interest. 'What we are doing is looking at the conductivity
of materials and then seeing how quantum mechanics fits into
this.
'We study basic physics which has a real application - engineering
- and particularly in the semiconductor industry. 'Our research
covers: (1) Electron beam lithography of quantum dots and
quantum devices, with applications such as quantum ballistic
transport at very low temperatures and high magnetic fields,
as well as the quantum-classical transition, and the role
of quantum effects in real devices at room temperature.
'(2) Magneto-transport studies used to probe the nature of
electron dynamics in semiconductor quantum dots, which are
quasi-zero-dimensional structures whose size is comparable
to the Fermi wavelength of the electrons themselves.
'Magneto-transport studies may be used to probe these phenomena
and to determine the factors which limit electron phase coherence
within the structures. 'Current interest in these devices
is motivated by their potential application in new areas of
technology, such as quantum computing and ultra high frequency
signal processing.
'(3) Surface chemical analysis
performed with a scanning Auger microprobe. 'Under good conditions,
a lateral resolution of about 25nm is achievable. 'And, (4)
Professor Michael Kozicki, in the group, has examined chemically
enhanced vapour etching (CEVE) patterning technique.
'He has used hydrocarbon contamination layers from laboratory
air or vacuum chamber ambients and successfully demonstrated
nanoscale pattern formation in silicon dioxide. 'He has also
developed a nitrogen chamber coupled directly to a UHV STM/AFM
facility for CEVE processing of silicon dioxide resists, and
their use in semiconductor device fabrication.
'Within the nitrogen chamber there is a processing system
for the actual CEVE development. But, the current work with
the scanning probe system is really interesting, and made
possible by the negative-stiffness isolators,' says Ferry.
The negative-stiffness isolator is a passive isolation approach,
and has a key advantage in that it is not powered. It has
no electricity going to it. So, in a site where heat buildup
could be an issue, such as with enclosed cryogenic chambers,
negative-stiffness becomes a highly efficient option.
Negative-stiffness isolators employ a unique - and completely
mechanical - concept in low-frequency vibration isolation.
Vertical-motion isolation is provided by a stiff spring that
supports a weight load, combined with a negative-stiffness
mechanism (NSM).
The net vertical stiffness is made very low without affecting
the static load-supporting capability of the spring. Beam-columns
connected in series with the vertical-motion isolator provide
horizontal-motion isolation. The horizontal stiffness of the
beam-columns is reduced by the beam-column effect. (A beam-column
behaves as a spring combined with an NSM).
The result is a compact passive isolator capable of very
low vertical and horizontal natural frequencies and very high
internal structural frequencies. The isolators (adjusted to
1/2Hz) achieve 93% isolation efficiency at 2Hz, 99% at 5Hz,
and 99.7% at 10Hz. Negative-stiffness isolators provide a
capability quite unique to the field of nanotechnology
- specifically, the transmissibility of the negative-stiffness
isolator.
That is, the vibration that transmits through the isolator
as measured as a function of floor vibrations - which is substantially
improved over active isolation systems. Although active isolation
systems have fundamentally no resonance, their transmissibility
does not roll off as fast as negative-stiffness isolators.
So, at building and seismic frequencies the transmissibility
of active isolators can be ten times greater than negative-stiffness
isolators. This causes substantial adverse measurement and
imaging artifacts in the data. Compared to other laboratory
research instrumentation, the growth of AFM usage has been
quite extensive over the past ten years.
AFM equipment placement has gone through a doubling phase
pretty much every year during the last decade. Since its inception
in 1988, it has continuously proven to be a key tool in moving
nanotechnology research forward.
'More than half of the universities in the United States,
and worldwide, are engaging in nanotechnology research,' says
Dr Ferry. 'In the electronic area, the nanoelectronic side
of it has been going since the late 1960s. 'This is driven
by the fact that in the semiconductor industry all things
are getting smaller and smaller.
'Today, the transistors have critical dimensions down around
25 nanometers. 'And the most critical dimension is the oxide
thickness which is one nanometer. 'When you consider that
you have to control one nanometer vertical thickness over
300 millimeters of lateral dimension, that is a difference
of 10 to the 8th power. 'That defines what modern manufacturing
technology produces.
'The need for effective vibration isolation has never been
greater, and will continue to become more demanding as the
nano-industry progresses'. David K Ferry is a Regents' Professor
of electrical engineering; recipient of the Arizona State
University Graduate Mentor Award, 2001; and recipient of the
IEEE Cledo Brunetti Award, 1999 'for fundamental contributions
to the theory and development of nanostructured devices'.
The IEEE Cledo Brunetti Award was established in 1975 through
a bequest made by the late Cledo Brunetti, who was an executive
of FMC.
The award is presented annually by the IEEE board of directors
on the recommendation of the Technical Field Awards Council
and the Awards Board, for outstanding contributions to miniaturisation
in the electronics arts.
Ferry has authored and co-authored several books including,
Electronic Materials and Devices (2001); Semiconductor Transport
(2001); Transport in Nanostructures (1997); Quantum Transport
in Ultrasmall Devices (1995); and Quantum Mechanics (1995,
2nd Edition 2000).
David Platus is the inventor of negative-stiffness mechanism
vibration isolation systems, and president and founder of
Minus K Technology. He earned a BS and a PhD in engineering
from UCLA, and a diploma from the Oak Ridge School of (Nuclear)
Reactor Technology.
Prior to founding Minus K Technology he worked in the nuclear,
aerospace and defense industries conducting and directing
analysis and design projects in structural-mechanical systems.
He became an independent consultant in 1988. Platus holds
over 20 patents related to shock and vibration isolation.
Minus K Technology was founded in 1993 to develop, manufacture
and market state-of-the-art vibration isolation products based
on the company's patented negative-stiffness-mechanism technology.
Minus K products, sold under the trade name Nano-K, are used
in a broad spectrum of applications including nanotechnology,
biological sciences, semiconductors, materials research, zero-g
simulation of spacecraft, and high-end audio.
The company is an OEM supplier to leading manufacturers of
scanning probe microscopes, micro-hardness testers and other
vibration-sensitive instruments and equipment.
Minus K customers include private companies and more than
200 leading universities and government laboratories in 35
countries.
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