PORTLAND, Ore. IBM Corp. will demonstrate what it claims is the world's first nanoscale MRI capable of imaging structures as small as 103 atoms.
Called magnetic resonance force microscopy (MRFM), the new technique takes a page from atomic-force microscopy to overcome the sensitivity limitations of a conventional MRI. Using a magnetic-force tip capable of 1.4 million Tesla/meter fields rather than conventional induction coils, IBM's MRFM is said to be capable of spatial resolutions of better than 90 nm, compared to 3 microns for the best conventional MRIs.
IBM also said it was able to image volumes as small as 650 zeptolitres, 60,000 times smaller than the previous record holder.
"Our overall reason for developing this technique is to see below surfaces in three dimensions," said Dan Rugar, manager of Nanoscale Studies at IBM's Almaden Research Center (San Jose, Calif.). The third dimension is "where you'll find all the dopants and interfaces and everything else that goes into a transistor."
Besides imaging the internal structure of semiconductors, the IBM team also hopes to use its MRFM technique to image the atomic structure of biological molecules, a nanoscale application not possible with traditional MRI techniques.
"We are still a long ways from being able to see below surfaces at the atomic scale, by this work is a milestone," Rugar claimed. "We have shown that with our technique you can use the MRI approach on a much finer nanoscale, so we are on our way to the atomic scale."
The technique uses a cantilever similar to that used in an atomic-force microscope.
|A tiny cantilever is used to scan a sample past a fixed magnet in the presence of RF pulses.|
A sample rides on the cantilever which is scanned past a fixed magnet tip measuring 20 nm in diameter. Magnetic resonance is induced in the sample by an RF coil, which reversed the spins of the atoms in the sample, inducing a vibration in the cantilever that was measured with a laser.
"Our techniques uses the same type of cantilever tip used for atomic-force microscopy, which is not what convention MRI uses," said IBM researcher John Mamin. "Instead, we are sensing the magnetic forces between a nanoscale magnetic tip and the nuclear magnetism from the sample. In essence, we are feeling the force between a magnet and the sample, and that force applies a small vibration in the cantilever, which we read off using a laser."
The MRFM's key enabler is an extremely high field: 1.4 million Tesla/meter. The magnetic tips were fabricated in silicon, then coated with a thin film of magnetic material. An 8-nm layer of iron was first deposited to isolate the magnet from the silicon, followed by a 100-nm-thick film of cobalt-iron and, finally, a top layer of 10 nm of ruthenium.
Next, the IBM researchers will seek to improve the sensitivity of the MRFM. Their goal is to image individual molecules and atoms for both semiconductor and medical applications.
|A sample was scanned past the fixed magnetic tip in the presense of an RF coil, inducing resonances measured as vibrations in the cantilever.|