McMahon for Minus K Technology, Inglewood, Calif.
With its development in 1986, and subsequent
commercial introduction in 1989, the atomic force microscope
(AFM) has become one of the foremost tools for imaging and
measuring materials and cells on the nanoscale. Revealing
sample details at the atomic level, with resolution on the
order of fractions of a nanometer, the AFM is instrumental
for imaging an array of applications, such as defining surface
characterizations, lithography, data storage and manipulation
of atoms and nano-sized structures on a variety of surfaces.
The atomic force microscope uses a sharp tip (probe), with
a radius of curvature on the order of nanometers, attached
to the end of a tiny cantilever used to scan across a sample
surface to image its topography and material properties.
When the tip is brought into proximity of a sample surface,
forces between the tip and the surface lead to a deflection
of the cantilever. This deflection is recorded using, typically,
a laser beam that is reflected from the top surface of the
cantilever to a photo-sensitive detector. The resultant
change of position of the cantilever permits characteristics
such as mechanical, electrostatic, magnetic, chemical and
other forces to be precisely measured by the AFM. These
characteristics are displayed in a three-dimensional surface
profile of the sample (in the X, Y and Z axes}-an advantage
that the AFM can provide compared to other microscopy techniques,
such as the scanning electron microscope (SEM), which delivers
a two-dimensional image of a sample (in the X and Y axis).
Expanding capability, scanning range
Since the release of the first commercial AFM about 25 years
ago, technology advances have been integrated into AFMs
to improve their performance. One of these has been expanding
the AFMs ability to image biological samples in an aqueous
buffer, and provide a range of physical data for the sample-a
key capability that not all microscopy techniques can deliver.
Another has been to increase the imaging speed of AFMs.
Unlike SEMs, which are capable of scanning in near real-time,
conventional AFMs, prior to about five years ago, required
between one and 100 minutes to obtain a high-resolution
image. With the introduction of high-speed AFM systems,
imaging speeds can now be achieved that are three orders
A of magnitude of three times faster than with previous
Since the AFM was designed for imaging rnicro-structures
on the micrometer and nanometer scale, its single scan image
size is somewhat limited to a maximum scanning area of about
l00 x100 micrometers. When researchers require a larger
scale image, they typically use a SEM, which can produce
quality images on a wide range of magnifications and sample
sizes in the X and Y axis. In one pass, the SEM can image
an area on the order of square millimeters, and with a depth
of field of millimeters.
Within the past several years, however, research into AFM
design, conducted by the Paul Hansma Research Group, Department
of Physics, at the Univ.. of California, Santa Barbara,
has demonstrated success with AFM imaging of large-scale
samples at nanoscale resolutions, while extending the range
of the Z-axis. The lab's current technical challenge is
to design an AFM system that can significantly extend the
scan range, while operating at a reasonable imaging speed
with acceptable image resolution and linearity.
The Hansma lab's objective for expanding the scan range
of its latest prototype AFM design is directed toward exploring
the molecular origins of fracture resistance in mineralized
tissues, primarily bone. Because of the limited range of
AFMs, many critical measurements of large-scale bone fractures
have not been produced. Typically, SEM is used to show the
entirety of a bone crack, but it is unable to provide the
detailed information that an AFM is capable of producing,
nor to image bone matrix material properties in a natural
hydrated fracture process.
Bone fracture and fracture surfaces on the microscale and
nanoscale have been imaged with AFM," says Hansma..
"but many microfractures remain beyond the reach of
AFM, such as those created by bone diagnostic instruments
like the reference point indentation (RPI) instrument."
The RPI is an instrument that creates a small indent in
the bone to determine its fracture resistance and gathers
data on the depth and forces of the indent.
"By imaging these indents and the associated rnicrofractures,
we have an opportunity to make real progress in understanding
the nanoscale mechanisms that contribute to fracture resistance,"
The large height difference between the bone surface and
the depth of the fracture is impossible for current AFMs
to image completely; both because of the insufficient range
and the probe length. To image at the extreme depths necessary
in large-scale cracks and deep microcracks, the AFM must
have a Z-range of at least 200 microns and a cantilever
up long enough to probe the crack."
As the vertical movement of the tip was increased, however,
it brought into play more of a possibility for vibration.
This issue was solved with the incorporation of negative-stiffness
Improving vibration isolation
The need for more precise vibration isolation with
atomic force microscopy as becoming more critical
as resolutions continue to bridge from micro to nano.
AFM systems are extremely susceptible to vibrations
from the environment. When measuring very few angstroms
or nanometers of displacement, an absolutely stable
surface must be established for the instrument. Any
vibration coupled into the mechanical structure of
die instrument will cause vertical and/or horizontal
noise and bring about a reduction in the ability to
measure with the highest resolution. The vertical
axis is the most sensitive for AFMs, but they are
also quite sensitive to vibrations in the X and Y
Developed and patented by Minus K Technology; negative-stiffness
isolators provide a unique capability to the field
of AFM research. They employ a completely mechanical
concept in low-frequency vibration isolation, while
achieving a high level of isolation in multiple directions.
In negative-stiffness vibration isolation, vertical-motion
isolation is provided by a stiff spring that supports
a weight load., combined with a negative-stiffness
mechanism. The net vertical stiffness is made very
low without affecting the static load-supporting capability
of the spring. Beam-columns connected in series with
the vertical-motion isolator provide horizontal-motion
isolation. A beam-column behaves as a spring combined
with a negative-stiffness mechanism. The result is
a compact passive isolator capable of very low vertical
and horizontal natural frequencies and very high internal
The result is an isolator that provides 0.5 Hz isolation
performance vertical and 0.5 Hz horizontal, using
a totally passive mechanical system-no air or electricity
A 32-um wide image of a butterfly
wing using UCSB AFM with negative-stiffness vibration
isolation. Image obtained with the support of NIH
grant R01 GM 065354
Negative-stiffness isolators resonate at 0.5 Hz. At this
frequency there is almost no energy present. It would be
very unusual to find a significant vibration at 0.5 Hz.
Vibrations with frequencies above 0.7 Hz (where negative-stiffness
isolators begin isolating) are rapidly attenuated with increase
We llike the vibration isolation to be at 0.5 Hz, which
we can achieve with the negative-stiffness table,' says
Hansma. "Not so much because of the vibrations at that
frequency; which are minimal., but because 0.5 Hz is 16%
more resistant to transmitting vibrations at a building
resonance of 10 or 20 Hz than compared with a resonant frequency
of 2 Hz, which would be found with air tables."
In contrast, air tables, as vibration isolation systems,
deliver limited isolation vertically and less isolation
horizontally. They can make vibration isolation problems
worse since they have a resonant frequency that can match
that of floor vibrations. Air tables will actually amplify;
instead of reduce, vibrations in a typical range of 2 to
Using combined AFM and RP1 techniques, and supported by
passive negative-stiffness vibration isolation system, Hansma's
lab has been able to achieve scan ranges exceeding one millimeter,
an order of magnitude larger in the Z-axis than any commercially
The AFM/RPI system has also proved capable of exploring
die molecular origins of fracture resistance in bone tissues
to more than 1 mm2, with good resolution and linearity.