Chip device gets to the point
By
Eric Smalley,
Technology Research NewsImages 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 disk drives.
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 processes.
The first application for the positioner is in microscopes, said Chu.
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
Funding: Government
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
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March 12/19, 2003
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