Biomedical Engineering Reference
In-Depth Information
18.1.2.2 Atomic Force Microscope
The atomic force microscope (AFM), also called the scanning force microscopy (SFM), was devel-
oped in 1986, subsequent to the STM
[12]
. Similar in operation to the STM, the AFM involves scan-
ning a sharp tip across a sample surface while monitoring the tip-sample interaction to allow the
reconstruction of the three-dimensional surface topography. A typical AFM can provide resolutions
of the order of 1 nm laterally and 0.07 nm (sub-angstrom) vertically and can image insulators as well
as conductors. UHV AFM resolution is comparable to that available from STM and TEM. An AFM
consists of a sharp tip on the end of a flexible cantilever that is moved across a sample surface by
piezoelectric actuators. The cantilever is typically made from Si or Si
3
N
4
with a tip curvature radius
of a few nanometers and typically has a spring stiffness,
k
, in the range of 10-100 N/m. Displacement
of the tip is recorded by a noncontact laser displacement measurement. A laser light directed onto
the cantilever above the laser tip is recorded on a photodetector area which allows calculation of dis-
placement via signal strength measurement or triangulation. A feedback loop maintains a constant
tip-surface interaction force by vertically moving the scanner to maintain a constant photodetector
difference signal. The distance the scanner moves vertically is recorded at each
x
,
y
position that allows
the surface information to be presented and analyzed. A complicated set of forces can be present at the
tip-sample interaction. For a surface under ambient conditions, when the tip touches the surface a repul-
sive force is present, with the tip at a small distance from the surface attractive forces can be present as
well as van der Waals force and capillary force arising from condensation of water vapor in the contact
area. Operating modes can be roughly classified as contact, noncontact, or dynamic.
In contact mode, the scanning tip is dragged across the sample surface and the tip deflection mon-
itored. Using Hook's law, the force between the tip and the surface is automatically kept constant
during scanning (typically between 0.1 and 100 nN). Lower stiffness cantilevers (spring constant,
k
0.1 N/m) are used in this mode to amplify the deflection signal. Contact mode may not be suit-
able for soft materials which can be easily deformed or damaged, such as for polymer or molecu-
lar imaging. When scanning is performed in the region where the tip is attracted to the surface, the
scanning is termed noncontact mode. In this region, the cantilever bends toward the sample. If an
oscillatory tip displacement is sufficiently large to pass through both regions, the probe experiences
both attractive and repulsive forces. This mode is known as dynamic, intermittent, or tapping mode.
Tapping mode was developed for investigation of soft materials
[13]
. In this mode, the cantilever
oscillates near its resonant frequency and slightly taps the surface during scanning. The tip rapidly
moves in and out of the sample surface with an amplitude that is sufficiently high to overcome adhe-
sion forces so that it stays in contact only for a short fraction of the oscillation period. Depending on
the cantilever type, the frequency typically varies from 50 to 500 kHz, and amplitudes up to 100 nm
are used. The laser spot deflection is used to measure the amplitude of cantilever oscillation and a
feedback loop maintains a constant oscillation amplitude by adjustments to the servo that adjusts the
cantilever height. In addition to the favorable imaging conditions and high resolution, the AFM offers
a variety of new contrast mechanisms which can be used to provide information on differences in
sample friction, adhesion, elasticity, hardness, electric fields, magnetic fields, carrier concentration,
temperature distribution, spreading resistance, and conductivity.
AFM Case Studies
A discussion of surface roughness measurement applied to bioengineering applications and a com-
parison of AFM, stylus profilometry, and optical profilometry has previously been presented
[14]
.
