FOR IMMEDIATE RELEASE:                  Mark Bello
March 18, 1997                          (301) 975-3776
                                        mark.bello@nist.gov

                                        TN-6135

           TO MAKE A FINE POINT, NEW NIST INSTRUMENT
                    ANALYZES MICROSCOPE TIPS

     A new research facility at the National Institute of Standards and
Technology accepts tips six at a time, in fact. But the tips are not of
the monetary variety. Rather, they are the super-fine probes of the
scanning microscopes used by researchers to glimpse atoms and molecules
and, increasingly, by companies to inspect products with features and
tolerances fast approaching molecular scales.

     NIST has built a customized, two-in-one instrument to help it solve
key issues that limit the measurement capabilities of scanning probe
microscopes and, as a result, the instruments' usefulness in industrial
and research applications.

     SPMs include scanning tunneling microscopes, atomic force
microscopes and a growing repertoire of related instruments with sharp
tips, often needle-like in shape. They are best known for their ability
to generate three-dimensional images of specimens in atomic detail, but
depending on the design, SPMs also are used to analyze a wide range of
physical and chemical properties, from magnetism to temperature to
hardness.

     Microelectronics firms and others that make products with
ultraminiature, high-precision components are pressing SPMs into
practical service. Worldwide, sales of the instruments are reported to
be growing by more than 15 percent annually and may approach $170
million this year. Yet, unaccounted for and often large variations in
the shape of probe tips and the inability to determine accurately tip
dimensions undermine the instruments' usefulness in high-precision
measurement, inspection and other quality-control applications.

     As a practical consequence, measurements made with one probe tip
may differ significantly from those made with another tip used on the
same microscope. This poses a serious problem when trying to inspect,
for example, electronic devices whose specified dimensions are
equivalent to a string of a few atoms.

     Obstacles to improved SPM accuracy, explains NIST physicist Rick
Silver, co-designer of the new instrument, include measurement
uncertainties stemming from uncontrolled variations in the geometry of
fabricated tips and from dynamic shape and dimensional changes that
occur during scanning. Interactions between tips and samples further
complicate matters, he says.

     In SPM-obtained images, tip and sample appear to merge, distorting
measurements. Such distortions can be corrected, but only to the degree
that geometry and dimensions of the tip are known. Today, this key
information either is lacking, or it is acquired with great difficulty.

     NIST aims to resolve this critical void through a combination of
measurement-focused experiments and computer modeling efforts. The
centerpiece of the research program is an imposing, NIST-built apparatus
that features a scanning tunneling microscope, or STM, and a field-ion
microscope, or FIM. Both microscopes can "see" individual atoms, but,
for FIMs, this level of scrutiny is limited to needle-like specimens
with a radius measuring between 1 nanometer (billionth of a meter) and
about 100 nanometers.

     This is not a serious limitation for the task at hand, since most
SPM tips fall within this range. For probes with blunter tips, however,
characterization will be done with a scanning electron microscope.

     The two microscopes function much like a tag team: Probe tips are
characterized with the FIM and then transferred in ultrahigh vacuum to
the neighboring but physically isolated ultrahigh vacuum STM. With the
STM, researchers will "test drive" the probes on silicon samples with
well-controlled, atomically ordered surfaces, featuring, for example, a
series of nanometer-scale steps or terraces. Like all STMs, this
instrument produces profiles of surfaces by sensing a tunneling
electrical current that flows between atoms on the probe tip and those
on the sample surface. Maintaining the probe at a constant height
relative to the surface maintains the current. Successively scanning the
tip over the surface yields a three-dimensional image with atomic
resolution.

     NIST's combination instrument can accommodate up to six probes,
which are shuttled between microscopes on a rotating carousel. In
addition, the apparatus features a separate ultrahigh vacuum chamber,
where tip and sample surfaces can be heat treated.

     Tips also can be "sculpted" with the FIM. By varying the electric
field in the chamber, researchers can evaporate clusters and even planes
of atoms from probe tips. Silver says that while this capability cannot
be controlled precisely, it will permit modifications useful to the
research effort.

     The tandem arrangement of microscopes will enable measurements of
probe tip geometries and dimensions to be correlated with tip
performance in STMs and other SPMs. One goal of the NIST effort is to
develop methods and tools that enable direct characterization of SPM
tips, which would eliminate much of the measurement uncertainty
resulting from unknown variations in tip geometry. Initially, work will
focus on tungsten and platinum-iridium tips.

     "Just as a good mechanic would not grab a screw driver without
inspecting the size and type of the blade," Silver explains, "SPM users
require basic information on tip geometry so that they can confidently
select tips that are most appropriate for particular jobs."

     Another goal is to prepare the equivalent of a recipe book that
prescribes standard methods for making SPM tips with reproducible
geometries, as well as for modifying tips with etching and field
evaporation techniques. The NIST team also intends to identify cleaning
and transporting methods that minimize the risk of unwanted alterations
of probes.

     Characterization artifacts are another anticipated outcome. Samples
with atomically ordered surfaces and nanometer-sized surface features
would enable SPM users to calibrate their tips and verify their
performance before applying them.

     Information gained through this effort also will be used to refine
and extend NIST's computer modeling work on SPM tips and on
measurement-distorting interactions between tip and sample. This work
already has yielded a "blind reconstruction method" that aids in
determining the real dimensions of features in an SPM-obtained image.
With a model developed by NIST physicist John Villarrubia, an SPM user
can estimate the maximum possible dimensions of a probe tip, thereby
reducing measurement uncertainty.

     Plans call for including the NIST tip characterization software in
the agency's Guide to Available Mathematical Software. GAMS is an
on-line repository of publicly available software for mathematical
modeling and statistical analysis. It can be found on the World Wide Web
at http://gams.nist.gov/gams/.

     As a non-regulatory agency of the Commerce Department's Technology
Administration, NIST promotes U.S. economic growth by working with
industry to develop and apply technology, measurements and standards.

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