channel atoms to make chips
Technology Research News
A common method for adding a very thin
layer of material to a surface is to condense a vapor of the material,
often a metal, and let its atoms rain down.
An important process in chipmaking
is to use one substance to dope, or chemically alter, a layer of another
in order to change its electrical or optical properties in certain areas.
For example, the wires on a computer chip are made by doping the silicon
in lines to make those areas conduct electricity.
Researchers at the University of Konstanz in Germany have come up with
a technique that allows them to condense two or more substances at once
and to direct the "rainfall" of one of them to specific places on the
surface, effectively doping the layer as it's made.
The key is using standing lightwaves as a set of lenses to focus the atoms
of one of the substances as they fall to the surface. Standing lightwaves
are lightwaves that are reflected back on themselves so that the waves'
crests and troughs remain stationary in space.
Other researchers have used lightwave lenses to focus beams of atoms.
The University of Konstanz researchers have gone a step further by tuning
the frequency of the standing lightwaves to match the resonance frequency
of one of the substances. The frequency-matched substance is focused by
the lightwave lenses while the other substance is mostly unaffected.
"The... force is strongly enhanced when the laser wavelength is extremely
close to the resonance frequency of the deposited material," said Dirk
Jürgens, a graduate student at the University of Konstanz. "Therefore
another material... will interact only very weakly with the light field
and the atomic trajectories are not altered."
This allows the researchers to deposit one of the substances in patterns
and the other in a homogeneous layer all at once.
The researchers have used the method to deposit chromium and magnesium
fluoride onto a surface at the same time. "The dopant material -- in our
case chromium -- [is] focused during deposition, whereas the host material
is deposited homogenously," said Jürgens.
Doping magnesium fluoride with chromium increases its refractive index,
or the angle at which magnesium fluoride bends light. By alternating areas
of doped and pure magnesium fluoride, thereby alternating the refractive
index, the researchers made a photonic bandgap layer that blocked a specific
frequency of light. Photonic bandgap materials are used in optical communications
devices to channel light.
The method allowed the researchers to easily discover that chromium was
not a useful dopant for magnesium fluoride because the refractive index
change was not enough to produce a practical photonic bandgap material,
according to the researchers.
The researchers were also able to use the technique to dope materials
in three-dimensional patterns by changing the position of the lightwave
lenses for successive layers.
This method of changing optical properties using optical-deposition interactions
is "quite remarkable," said Sandip Tiwari, a professor of electrical and
computer engineering at Cornell University and director of the Cornell
Nanofabrication Facility. "It does provide a convenient means to [form]
three-dimensional structures with optical bandgaps. At small dimensions,
this will be an intriguing and, if applicable, convenient technique that
does not rely on complicated lithography and [the] reproduceability issues
associated with it," he said.
The technique could be used for research purposes in the next two to five
years, said Markus K. Oberthaler, head of atom optics research at the
Konstanz Optics Center. "After that, [it] will be used for very specific
applications. It is very unlikely that the method will enter big production
lines. If so, that will be [at the] earliest in 10 to 15 years," he said.
Jürgens' and Oberthaler's research colleagues were Thomas Schulze, Tobias
Müther, Björn Brezger, Tilman Pfau and Jürgen Mlynek of the University
of Konstanz. They published the research in the March 19, 2001 issue of
the journal Applied Physics Letters. The research was funded by the University
of Konstanz, the Konstanz Optics Center and the European Union.
Timeline: 2-5 years; 10-15 years
Funding: University; Government
TRN Categories: Semiconductors; Materials Science and Engineering
Story Type: News
Related Elements: Technical paper, "Structure doping with
light forces," Applied Physics Letters, March 19, 2001
June 13/20, 2001
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