A new thumb-sized microscope that works something like a
CD-player, only with microwaves rather than visible light, has been invented by
researchers at the U.S. Department of Energy's Lawrence Berkeley National Laboratory
(Berkeley Lab). Called a Scanning Evanescent Microwave Probe (SEMP), this unique new
instrument can be used to simultaneously characterize critical electronic properties along
with topography in a wide assortment of materials.
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IMAGES
REVEALING THE TOPOGRAPHY OR HEIGHT (TOP) AND THE NONLINEAR ELECTRIC PROPERTIES
(BOTTOM) OF A MATERIAL, WHICH IN THIS CASE IS LiNbO3
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Speaking before the centennial meeting of the American Physical Society, Xiao-Dong
Xiang, a physicist with Berkeley Lab's Materials Sciences Division, described how the SEMP
uses near-field or non-propagating microwaves (as opposed to normal far-field microwaves
such as radar) to measure the electrical impedance of materials with sub-micron
resolution. For the electronics industry, a material's electrical impedance a
measurement of its ability to conduct an alternating current is its most critical
property.
"The SEMP is capable of mapping the complex electrical impedance of any
material," says Xiang. "We chose the lower range microwave frequencies (a few
GigaHertz) because this is the most relevant and best-suited range for most electronic
applications."
By measuring the interaction between evanescent microwaves generated off an ultra
sharp-tipped probe and the surface of a material, Xiang and his colleagues can not only
map electrical impedance across the face of the material, they can at the same time map
the topography of the material's surface, another critical factor for manufacturing chips
and other electronic devices.
The SEMP's sharp-tipped metal probe is connected to a high quality-factor (Q) microwave
resonator equipped with a thin-metal shield. This shielding is specially designed to
screen out all but the evanescent microwaves from being generated at the SEMP's tip. As a
result, when the tip is scanned over a sample, just above the material's surface, only
these evanescent microwaves, with their high spatial resolving power, are free to interact
with the sample.
"This feature is crucial for high resolution quantitative microscopy," says
Xiang. "If both evanescent and propagating microwaves had to be considered and
calculated, as is the case for all other types of microwave probes, the quantitative
microscopy would be impossible." The interaction between the evanescent microwaves
and the sample surface gives rise to a resonant frequency and quality-factor changes in
the resonator that are recorded as signals. Xiang and his colleagues can measure these
change signals and plug the measurements into equations they've developed which translates
the results into a measurement of the sample's complex electrical impedance with a spatial
resolution of 100 nanometers.
The SEMP can be used on conductors and insulators as well as semiconductors. It has
applications in any situation in which there is a need to characterize a material's
electrical properties as a function of electric or magnetic fields, optical illumination,
or temperature variations. The Berkeley Lab researchers have employed "tip-to-sample
distance feedback control techniques" to obtain topographical and electrical
measurements of sample surfaces important for mapping electrical impedance without
contacting the sample surface.
"Since the feedback control ranges from nanometers to microns, the SEMP has a
zoom-out feature that allows it to scan a large area in a short amount of time, and a
zoom-in feature that allows it to scan a small area with high resolution," says
Xiang. "These features make the SEMP a practical tool for industrial
applications."
Working with Xiang on the development of the SEMP were Tao Wei, Chen Gao, Fred Duewer,
and Ichiro Takeuchi. Although the basic technology behind the SEMP has been licensed to
Ariel Technologies, Inc., Xiang and his colleagues are continuing to refine and expand it.
They are currently building a low-temperature version which will allow them to study
superconductors.
Berkeley Lab is a U.S. Department of Energy national laboratory located in Berkeley,
California. It conducts unclassified scientific research and is managed by the University
of California. |