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Their system is mounted in a large cryogenic cooler, an enclosed container with a helium-3 cooling system, an isotope of a helium molecule that 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 to prevent heat from transmitting in, and to protect the cold from being mitigated by the ambient room temperature.
What
Is Negative-Stiffness Vibration Isolation?
This type of experimentation is not uncommon, with similar experiments underway at a large number of universities. But what is different is the vibration isolation system being used by the Nanostructures Research Group: 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.
"Any
kind of vibration noise in the system makes the 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 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 for 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,"
Dr. Ferry concludes.
Key Advantages The negative-stiffness isolator, a passive isolation approach, 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 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 fundamentally have 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.
"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 1 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, Ph.D., M.S.E.E., B.S.E.E., 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. Dr. Ferry has authored and co-authored several books, including Electronic Materials and Devices (2001) and Semiconductor Transport (2001). Dr. David L. Platus is the inventor of negative-stiffness mechanism vibration isolation systems, and President and Founder of Minus K Technology, Inc. He earned a B.S. and a Ph.D. in Engineering from UCLA, and a diploma from the Oak Ridge School of (Nuclear) Reactor Technology. Dr. Platus holds over 20 patents related to shock and vibration isolation.
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