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JAPAN NANONET BULLETIN
-- 90th Issue -- February 22, 2007
Nanotechnology Researchers Network Center of Japan
Ministry of Education, Culture, Sports, Science and Technology (MEXT)
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IN THIS ISSUE
Nanonet Interview:
"Three-dimensional fine structures created by etching
-- Micro/nano processing using crystal anisotropy --"
Kazuo SATO, Professor, Department of Micro-Nano Systems Engineering,
and Director, Center for Creative Engineering, Graduate School of
Engineering, Nagoya University
Young Researchers' Introduction:
"Development of a supercritical fluid jet technique and its
application for the supersonic jet spectroscopy of bio molecules"
Shun-ichi ISHIUCHI, Assistant Professor, Chemical Spectroscopy
Division, Chemical Resources Laboratory, Tokyo Institute of Technology
Nano Info:
"Japan-US Symposium on Nano systems
- 6th US-Japan Joint Symposium (Oct. 10-12, 2006) -"
-- NANO CALENDAR --
For information on nanotechnology related symposiums and conferences
held in the world,
http://www.nanonet.go.jp/english/calendar/
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NANONET INTERVIEW
Three-dimensional fine structures created by etching
-- Micro/nano processing using crystal anisotropy --
(Issued in Japanese: April 19, 2006)
Kazuo SATO, Professor, Department of Micro-Nano Systems Engineering,
and Director, Center for Creative Engineering, Graduate School of
Engineering, Nagoya University
Prof. Sato, who was researching metal forming, started research on micro
electro mechanical systems (MEMS) in 1983. The technology was not called
MEMS at that time, but the concept was beginning to emerge in the United
States. He says, "For a long time, I thought that machines were
fabricated based on conventional concepts, although they had progressed,
but then, I began to have the feeling that machines could be smaller by
one or two orders of magnitude in size with a substantial change in
processing. Then, I decided to shift my research to MEMS."
Prof. Sato, who studied anisotropy in metal polycrystals as the subject
of his doctoral thesis, thought of using anisotropic etching of single
crystal silicon for fabrication of MEMS devices. He investigated the
anisotropic etching of single crystal silicon in potassium hydroxide
(KOH) and tetramethylammonium hydroxide (TMAH) aqueous solutions using a
hemispherical single crystal specimen with a diameter of 44 mm (Fig. 1).
He says, "Crystal planes with arbitrary orientations appear on the
surface of a hemispherical single crystal specimen. When the specimen
surface is etched, the crystal planes where the etching rate is high are
deeply etched, while the planes that are barely etched remain. When the
shape of the hemispherical specimen is measured with a three-dimensional
measuring instrument before and after etching, an etching rate contour
map (Fig. 2) can be obtained. The accumulated experiments enable us to
predict how to obtain a desired shape using calculations." The database,
ODETTE, and an etching simulation system, MICROCAD, which were developed
through accumulated experiments using different types of etchants, and
temperatures and concentrations of the etchants, have been used by
Japanese companies.
Prof. Sato says, "The experiments using the hemispherical single crystal
silicon specimens dealt with fabrication in the range of millimeters to
micrometers. However, the etching rate contour map that is
experimentally obtained is quite variable according to changes in
etching conditions. We actually found significant differences in the map
pattern between KOH and TMAH. In order to find out how the difference
occurs, knowledge of nanotechnology is required." He has experimentally
confirmed that the orientations of atomic steps that are easily etched
on a crystal plane differ with the types and concentrations of the
etchants. He says, "It has been thought that the etching rate can be
determined by the energy that is required to take an atom out of a
defectless surface. However, taking an atom out causes a defect, from
which an atomic step moves laterally along the surface, and the etching
process proceeds."
Prof. Sato introduced a defect, which becomes a nucleus of atomic steps,
on a (111) silicon plane and conducted an etching experiment by changing
the types and concentrations of the etchants. This defect may be
regarded as a hexagonal seed pit on the (111) plane, of which the
sidewalls are composed of three equivalent atomic steps each with a
single dangling bond that make a triangular shape and another three
atomic steps each with two dangling bonds that make a triangular shape
rotated by 60 degrees. Since the atomic steps with two dangling bonds
are etched faster than the ones with a single dangling bond in a 40% KOH
aqueous solution (9.5M), as predicted from conventional theories, the
seed pit becomes a triangle comprised of the three equivalent atomic
steps each with a single dangling bond in the KOH solution. In contrast,
since the atomic steps with a single dangling bond are etched faster in
a 25% TMAH aqueous solution (2.7M), the seed pit becomes a triangle
comprised of the three equivalent atomic steps each with two dangling
bonds in the TMAH solution. In a highly concentrated 60% KOH (16.7M)
solution, the steps with a single dangling bond are etched faster and in
a low concentrated 10% TMAH (1.1M) solution, the steps with two dangling
bonds are etched faster. Therefore, the seed pit becomes a triangle in
the low concentrated KOH solution and also becomes a triangle that is
rotated by 60 degrees in the highly concentrated TMAH solution (Fig. 4).
This experiment was extremely important, because it showed that the
etching activity depends on crystal plane orientations and changes
largely due to the types and concentrations of the etchants.
Prof. Sato says, "It has been said that only a hydroxyl group in the
etchant controls the etching phenomenon. However, I think positive ions
in the etchant have something to do with it. It is likely that positive
ions stably attach somewhere on a stepped silicon surface, which blocks
the movement of atomic steps. I have just started to research the
interaction between positive ions and surfaces using first-principle
calculations under collaboration with Helsinki University of Technology,
Finland." The critical molar concentration of hydroxyl group, at which
the etching activity is reversed, changes with the etchant. "When the
volume fraction of the positive ions in the etchant is used instead of
the concentration of hydroxyl group, the critical concentration is 10
vol% in both KOH and TMAH solutions. This phenomenon is not understood
yet," says Prof. Sato. Although the diffusion of the ions in the etchant
has not been considered as a factor controlling the etching rate, a
localized diffusion phenomenon in the etchant around the atomic steps
may also be one of the factors. "So, I have been trying to build a
mechanism that enables us to understand the phenomenon in the range of
the atomic level to the millimeter level together with physicists and
chemists," says Prof. Sato.
Hesitation to equipment investment is one of the reasons for the slow
commercialization of MEMS devices. Prof. Sato showed that MEMS devices
could be fabricated utilizing the crystal plane dependence of the
etching rates without expensive equipment. He proposed a combination of
mechanical machining and wet etching for fabricating an arrayed needle
having a high aspect ratio profile. At first, silicon wafer surface is
machined by a dicing saw. Deep grooves are cut in crosswise, resulting
in arrayed column structures on the wafer. Then the total surface is
immersed in a KOH solution. Some fast etching orientations appear on the
column surface after heavy etching, resulting in a sharp needle tip,
while mechanical damages introduced by dicing are entirely removed. Prof.
Sato has fabricated arrayed micro needles that are 200 microns in height
with a 150 microns pitch (Fig. 5) for transdermal drug delivery using
this method.
Prof. Sato also fabricated an on-chip tensile testing device (Fig. 6) to
measure the mechanical properties of micro- and nano-materials used for
MEMS such as single crystal silicon, silicon oxide and silicon nitride
films. He found that when a 100 nm notch is introduced to a Si film
specimen with a width of 50 microns, the fracture strength significantly
decreases by the brittleness. On the other hand, the toughness of Si
increases with increasing temperature up to around 100 degree C. He says,
"Bulk silicon shows plastic behavior at 500 to 600 degree C, while
silicon nanowires show plastic behavior even at room temperature.
Although micrometer-sized materials are brittle, they also show slight
plastic behavior. I think that there are plenty of things to do in the
micro- and nanometer scale regions. I believe that MEMS is essential for
fabricating devices using nanotechnology."
(Interviewer: Kuniko Ishiguro, Cosmopia Inc.)
For more information including figures,
http://www.nanonet.go.jp/english/mailmag/2007/090a.html
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YOUNG RESEARCHERS' INTRODUCTION
Development of a supercritical fluid jet technique and its
application for the supersonic jet spectroscopy of bio molecules
(Issued in Japanese: September 7, 2005)
Shun-ichi ISHIUCHI, Assistant Professor, Chemical Spectroscopy
Division, Chemical Resources Laboratory, Tokyo Institute of Technology
Supersonic jet spectroscopy, which is a type of laser spectroscopy for
studying isolated molecules obtained by the supersonic adiabatic
expansion in a high vacuum, is one of the most ideal tools for molecular
science. This technique is regarded as the next generation of
analytical methods because of its high sensitivity and molecular
selectivity. A comparison of supersonic jet spectroscopy with
conventional solution, or gas-phase spectroscopy is shown in Fig. 1. As
shown in the figure, very sharp and clear spectra can be obtained, which
give detail information of electronic, vibrational and rotational
structure of molecules. In addition, we can trace the molecular
dynamics of chemical reactions and structural reformation of molecular
clusters in real time by using ultrafast lasers.
However, it is difficult to apply this technique to bio molecules and
artificial functional molecules, such as nano-tech molecules, because
the samples must be vaporized before supersonic expansion. In order to
overcome this problem, I have developed a new technique using a
supercritical fluid jet, which can vaporize and jet-cool nonvolatile and
pyrolytic molecules without hard heating. The experimental apparatus is
shown in Fig. 2. In order to keep the vacuum to ~10^-5 Torr during the
expansion of the supercritical fluid jet, high pumping speed turbo
molecular pumps are installed with a very small pinhole nozzle (dim. ~5
microns) or a high pressure pulsed solenoid valve. Supercritical CO2,
which is used most generally, is a non-polar solvent, and thus, its
ability to dissolve polar solutes is very poor. However, it is well
known that mixing of a small amount of polar organic solvents, which are
called entrainers, to supercritical CO2 improves the dissolubility of
polar solutes. It was verified that the very sharp and clear spectra of
the nonvolatile molecules can be observed by admixing an entrainer into
supercritical CO2 without heating to high temperature. By optimizing
the supercritical medium and the entrainer, the supercritical fluid jet
technique can be used with laser spectroscopy to analyze various bio
molecules and nano-tech molecules.
For more information including figures,
http://www.nanonet.go.jp/english/mailmag/2007/090b.html
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NANO INFO:
Japan-US Symposium on Nano systems
- 6th US-Japan Joint Symposium (Oct. 10-12, 2006) -
(Issued in Japanese: October 25, 2006)
Japan's Ministry of Education, Culture, Sports and Technology (MEXT) and
the US National Science Foundation (NSF) have held joint symposiums,
according to their agreement on promoting exchanges of research in the
nanotechnology field. The first symposium was on "Tools and Metrology
for Nanotechnology," the second "Nanotechnology in Advanced Therapy and
Diagnosis," the third "Directed Self-Assembly and Self-Organization,"
the fourth "Nanophotonics: Beyond the Limit of Optical Technology," and
the fifth "Carbon nanotube and its applications." The sixth and latest
joint symposium, on "Nano systems," was held in Tokyo from October 10
to 12, 2006.
Dr. Ken P. Chong, Program Director at NSF, introduced NSF's National
Nanotechnology Initiative (NNI) in his opening speech. He talked about
how NNI has been conducted and "Active nanostructures and nanosystems"
as NNI's program for fiscal 2006. He also explained that NNI's emphasis
for fiscal 2007 will be on "complex nanosystems," "three-dimensional
measurements with excellent temporal resolution in engineering-related
fields" and "convergence of science, engineering and technology at the
nanoscale." The theme of the sixth symposium, "nanosystems," is believed
to have been chosen in consideration of the "systems of nanosystems,"
the third-generation R&D of the four-generation development program
described in NNI's long-term vision.
The first day's theme was "Nano Bio-Chemo Systems," with subtitles "Nano
chemo fluido mechano systems" and "Nano bio-medical application." Prof.
Takehiko Kitamori at the University of Tokyo gave a speech entitled
"Nano Chemical Systems on Micro Chips." He talked about the results of
his research on microsystems and their development of his research into
nanochemical systems, which are quite different from microsystems. Prof.
Narayan Aluru at the University of Illinois also delivered a nano-fluid
related speech entitled "Fluid Transport through Nanometer Channels:
Fundamental Issues and Computational Studies." Prof. Michael Roukes at
the California Institute of Technology gave a speech on "Toward Large-
Scale-Integration of NEMS." Prof. Jerry Bernholc at the North Carolina
State University talked about "Atomic Scale Design of Nanostructures."
Prof. Alex Zettl of the University of California at Berkeley introduced
his research, "Applications of Nanotubes to NEMS."
One of the Japanese speakers, Dr. Taro Uyeda of the National Institute
of Advanced Industrial Science Technology, talked about "Three
Approaches to Assemble Nano-Bio-Machines using Protein Molecular Motors."
Other Japanese speakers included Prof. Gen Hashiguchi of Kagawa
University, who delivered a speech entitled "Micromachined Probes as a
Research Tool for Nanotechnology," Prof. Hiroyuki Fujita at the
University of Tokyo, who talked about "From MEMS to Nano Systems," and
Prof. Yoshinobu Baba at Nagoya University, who spoke about
"Nanobiodevice for Bioscience and Biomedicine."
The key characteristic of the sixth symposium was that while most
Japanese speakers with engineering backgrounds talked about relatively
specific applications of nanotechnology, many of their US counterparts
are physicists who mainly introduced basic research. The difference
between research themes of the Japanese researchers and their US
participants appear to be attributable to the US researchers'
consideration of "convergence of science, engineering and technology at
the nanoscale," one of NSF's key nanotechnology themes for 2007. As Prof.
Zettl indicated clearly in his speech, stressing "efforts to understand
the basic physics for developing practical applications," it appeared
that such an attitude is a key characteristic among US researchers in
proceeding with their research.
Many of the US speakers are engaged in basic research. They sought and
exchanged opinions with others to solve their problems in a casual
atmosphere at the symposium. Soft-spoken scientists considered questions
carefully from time to time, and held discussions with others. Such a
discussion style was very impressive to me, as were Dr. Chong's
technological questions.
(By Takao Kitamura, nanonet)
For more information including figures,
http://www.nanonet.go.jp/english/mailmag/2007/090c.html
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