It was not until long after 1977 that the name nanoelectronics
came into use, but Dr. David K. Ferry was already actively
engaged in developing some of the worlds smallest transistors.
The field, called Ultra-Small Devices at that time in the
later part of the1970s, was in its infancy, and Dr. Ferrys
research team was one of only four select groups around
the world aggressively researching the limits of small electronic
devices.
Recently, Dr. 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 Universitys College of Engineering, Center 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. 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 300 milli-Kelvin,
or 1,000 times below room temperature, about one-half a
degree above absolute zero. The cooler has a vacuum jacket
around it so the heat cant 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. Dr. Ferrys group is utilizing a process called
a piezo-electric sensor. They metalicize 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 Dr. 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 Dr. 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 25 nm is achievable.
4. Professor Michael Kozicki, in the group, has examined
Chemically Enhanced Vapor 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 Dr. 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 1/2 Hz) achieve 93% isolation
efficiency at 2 Hz, 99% at 5 Hz, and 99.7% at 10 Hz.
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 10X 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
10 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.
Source: Minus K (Jim McMahon)
