A traditionally etched tungsten STM probe, sharpened to a 1-nm point after bombarding it with ions. Image: Joseph Lyding |
A simple new improvement to an essential microscope
component could greatly improve imaging for researchers who study the very
small, from cells to computer chips.
Joseph Lyding, a professor of electrical and computer
engineering at the University
of Illinois, led a group
that developed a new microscope probe-sharpening technique. The technique is
described in research published in Nature
Communications.
Scanning probe microscopes provide images of tiny structures
with high resolution at the atomic scale. The tip of the probe skims the
surface of a sample to measure mechanical, electrical, or chemical properties.
Such microscopes are widely used among researchers who work with tiny
structures in fields from nanotechnology to cellular biology.
Laboratories can spend hundreds of thousands of dollars on
an elegant instrument—for example, a scanning tunneling microscope (STM) or an
atomic force microscope (AFM)—yet the quality of the data depends on the probe.
Probes can degrade rapidly with use, wearing down and losing resolution. In
such cases, the researcher then has to stop the scan and replace the tip.
“To put it in perspective, if you had an expensive racecar
but you put bicycle tires on it, it wouldn’t be a very good car,” Lyding said.
To shape tips, researchers shoot a stream of ions at the
tip. The material sputters off as the ions collide with the tip, whittling away
the probe. One day in the laboratory, after yet another tip failure, Lyding had
the simple, novel idea of applying a matching voltage to the tip to deflect the
incoming ions. When a voltage is applied to a sharp object, the electrical
field gets stronger as the point narrows. Therefore, ions approaching the
sharpest part of the electrified tip are deflected the most.
“This causes the ions to remove the material around that
sharp part, not on the sharp part itself, and that makes it sharper,” Lyding
said. “You preserve the point and you sharpen what’s around it.”
Lyding and graduate student Scott Schmucker purchased an
inexpensive ion gun and tested Lyding’s idea. It worked beautifully. STM tips
with a starting radius of 100 nm were honed to a sharp 1-nm point, yielding
extremely high resolution. In addition, the sputtering process works with any
electrically conductive material.
But once the probes are ultra-sharp, what’s to keep them
from wearing down just as quickly as other probes? Lyding and Schmucker then
teamed with U. of
I. chemistry professor Gregory
Girolami and materials science and engineering professor John Abelson, whose
groups had demonstrated coatings for silicon semiconductors made of a material
called hafnium diboride. The coatings are 10 times harder than the metal
usually used to make STM tips, but are also metallic—the key property for the
ion-sputtering process.
The group applied the hafnium diboride coatings to their
probes, sputtered them further, and found that the resulting probes are stable,
durable and excel in the types of microscopy and patterning applications for
which such tips are used.
“Nobody else makes probes with the combination of sharp,
hard and metallic conduction,” said Lyding, who is also affiliated with the Beckman
Institute for Advanced Science and Technology at the U. of I. “You can find one or the other but not all three.
There’s a tremendous demand for that.”
The researchers now are moving to commercialize their tough,
sharp probes. They received a patent and started a company called Tiptek to
begin manufacture. They are also expanding their sharpening technique to
include AFM probes as well as STM, and are developing batch-processing techniques
for higher throughput.
“When people make AFM tips they make them on wafers,
hundreds of tips at a time,” said Lyding. “The methodology that we’re
developing lets us process this entire wafer as a unit so all 400 tips would be
done at the same time.”
