Chip device gets to the point
Technology Research News
Images of individual atoms spelling out
words mark a major leap forward in science -- the ability of researchers
to manipulate matter atom by atom.
Being able to work on the level of atoms has opened possibilities
ranging from molecule-size machines to ultrahigh-capacity data storage.
The primary tool of this new trade is the atomic force microscope, an
extremely sharp probe tip moved by a high-resolution positioner.
Researchers from the University of Wisconsin at Madison have made
an inexpensive positioner-on-a-chip that can move small objects, like
the probe tip of an atomic force microscope, to within a third of a nanometer.
A nanometer is the length of 10 hydrogen atoms.
The device could lead to chip-based atomic force microscopes,
and could also provide an inexpensive way to access tiny bits on next-generation
The researchers' microelectromechanical system (MEMS) positioner
has a resolution comparable to today's larger atomic force microscope
positioners but is much cheaper, said Larry L. Chu, the research associate
at the University of Wisconsin at Madison. "It may be possible soon to
make these $20,000 instruments small enough to carry in a shirt pocket
and cheap enough to be disposable," said Chu.
Today's atomic force microscopes use piezoelectric positioners.
Piezoelectric materials are crystals that change shape when electric current
flows through them. It takes a relatively high voltage -- about 100 volts
-- to make a piezoelectric actuator change shape. This makes for expensive
control electronics, said Chu.
The researchers' positioner is driven by electrothermal actuators,
which are strips of semiconductor or metal that bend when an electric
current heats one side more than the other. "The MEMS-based device itself
is cheap to manufacture," said Chu. More than 60 can be made on a four-inch
diameter silicon wafer, he said. "The driving electronics can also be
less expensive since the device uses only 12 volts... and the electronics
can be integrated onto the [chips]."
The positioner uses a capacitive position sensor to measure how
far it has moved. Capacitance is the amount of electric charge an object
can hold. The capacitance of two conductors -- one charged positively
and the other charge negatively -- increases as the conductors move closer
together. The researchers' positioner precisely measures small distances
by sensing the change in capacitance of two adjacent sets of tines, one
fixed and the other attached to the moving actuator.
The researchers carved the device into a silicon wafer using deep
reactive ion etching, a form of lithography similar to standard chipmaking
The first application for the positioner is in microscopes, said
There's also a growing need for higher-resolution positioners
in data storage devices. As bits become smaller, they become more difficult
to track. Precision positioning is necessary for high-density magnetic,
optical and MEMS-based data storage systems, said Chu. "Many laboratories
[are working] on developing a microactuator to assist the positioning
of the head, for example, for hard disk drives," he said.
The researchers' next step is developing control algorithms and
electronics to make the system easier to use, said Chu.
The positioner could be used in prototype microscopy and data
storage applications in one to two years, but practical applications will
take more than three years, said Chu.
Chu's research colleague was Yogesh B. Gianchandani. The research
appeared in the March, 2003 issue of the Journal of Micromechanics and
Microengineering. It was funded by the National Science Foundation (NSF).
Timeline: > 3 years
TRN Categories: Microelectromechanical Systems (MEMS);
Data Storage Technology; Applied Technology
Story Type: News
Related Elements: Technical paper, "A micromachined 2D positioner
with electrothermal actuation and sub-nanometer capacitive sensing," Journal
of Micromechanics and Microengineering, March, 2003
March 12/19, 2003
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