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During the last decade, the scanning probe microscope (SPM) has developed to
a point where it is now an accessible, easy-to-use nanotechnology research tool.
The original scanning tunneling microscope has evolved into many new
instruments, the first being the atomic force microscope (AFM), which have
been used to observe nearly every physical property of a surface that it is
possible to measure.
The first applications of SPMs included the measurement of surface texture and
step heights and, after 1980, basic research in surface science; exploration in
engineering, physical, and life sciences; and nanoscale process development. We
now have a host of instruments and imaging modes. Lateral force microscopy, a
materials sensing mode, measures the lateral or twisting deflections of the
probe arising from forces parallel to the surface. If a surface is perfectly flat but
has an interface between two different materials, it is often possible to image
the change in surface properties such as ‘stickiness’, composition, and chemical
makeup. Other sensing modes include the electrostatic force microscope and
the magnetic force microscope. In a modern AFM, many of these operating
modes are integral to the instrument system.
In the early 1990s, the AFM was an expensive, technically challenging
instrument. The research focus was on the microscope itself rather than the
results that it could achieve. If a researcher wanted to examine a sample, he or
she had to seek out an AFM expert and schedule time for an all-day project.
This expert would be known for ‘doing AFM’: “I do AFMs. What do you need to
look at?” Although being able to operate the AFM was a valuable skill, there
were undoubtedly many good ideas that were not pursued because of the
complications involved. Today, although the AFM is still evolving, the focus is on
the results. In addition to incredible images, today’s AFMs provide precise and
accurate measurements. Applications range from nanoscience and
biotechnology to process development and control. For example, AFMs are
essential for DVD production quality control.
When the cost drops and an instrument becomes easy to use, it moves from a
service organization to the individual worker, which opens up creativity and a
vast array of new possibilities. Two centuries ago, an optical microscope was a
rarity; its effective use required training and skill. Now it is a commonplace
laboratory tool that is easily used with minimal training. Similarly, the AFM has
developed (in a couple of decades rather than centuries) from a complex,
expert-only instrument to an intuitive, PC-driven, tabletop R&D and
manufacturing support tool. We may have lost the unique, esoteric appeal
(along with the concomitant expense) of a specialist instrument, but we have
gained an easy-to-use nanotechnology manufacturing and research tool.
In spite of this trend, scanning probe instruments are often still marketed as
expensive, high-end instruments for the well-equipped research laboratory. This
is not the case. Commercial systems are available that provide a ‘means to an
end’ at about half the cost of their high-end counterparts. The ‘SPM for
everyone’ is possible because of developments in computer control and position
feedback. Precision motion control for these instruments has improved and
modern AFMs use calibration sensors to control probe positioning. As a result,
step height measurements in the semiconductor industry can be made to within
a nanometer. Various data collection modes are available including continuous,
vibrating, step-and-repeat, and lateral force scanning. Probe tips have progressed
from electroformed tungsten to chemically deposited, single crystal silicon
pyramids and carbon nanotubes. Intuitive software, based on recent advances in
gaming graphics and digital image manipulation, can simultaneously generate
vivid, three-dimensional images of surface topography and physical properties.
The promise of nanoengineering is driving the development of a number of
AFM-related technologies. Combining an AFM with force-feedback control will
permit the ‘feeling’ of the shape of molecules and surface atoms. Glass or
inexpensive (and disposable) plastic probes will have shapes and sizes tailored to
the required resolution. By imaging precisely patterned samples called ‘tip
characterizers’, it will be easy to tell when a probe is broken or defective.
Atomic-scale reference standards will become available. Arrays of probe tips
operating in tandem will speed up data acquisition and, perhaps, be used as
molecular-scale random access memory elements. Other future applications
include dip-pen lithography – literally writing at the molecular scale; the direct
manipulation of atoms and biomolecules; and, ultimately, single-atom computer
memories. It is risky to predict the products that may evolve from these
technologies. After all, when the laser was invented, no one could have guessed
all its applications.
Paul West is chief technology officer of Pacific Nanotechnology, Inc., California, USA.
SPM for everyone
OPINION
...Paul West
March 200464