Atomic Force Microscopy (AFM)
is a powerful form of scanning probe microscopy (SPM) that performs its imaging function by
measuring a local property of the surface being inspected, such as its
height, optical absorption, or magnetic properties.
AFM employs a probe or tip that's positioned very close to surface to
get these measurements. AFM operates in two modes, namely, contact mode
imaging and non-contact mode imaging.
soft cantilevered beam that has a sharp tip at its end, which is brought
in contact with the surface of the sample.
between the tip and the sample causes the cantilever to deflect in
accordance with Hooke's Law, exhibiting a spring constant that typically
0.001 to 100 N/m.
This deflection, as will be explained later, is used by AFM to derive
information about the surface of the sample.
The amount of
is measured by reflecting light from a laser diode off the back of the
beam, and onto a pair or array of position-sensitive photodetectors. The
differences between the reflected light received by the individual photodetectors
indicate the amount of angular deflection of the cantilever at any given
point on the sample. As a cheaper (but less sensitive) alternative to
laser detection, the cantilever deflection may be measured using
piezoresistive AFM probes that serve as a strain gauge system.
to monitor this deflection allows the AFM to create an image of the
sample non-destructively even if the tip is continuously in contact with
the sample. To prevent
the cantilever tip from damaging the surface of the sample, it is
maintained at a constant angular deflection so that the force applied by the tip on the surface is also kept
constant. This is achieved using a feedback mechanism that adjusts the
distance between the tip and the surface to keep the
forces between the tip and the sample typically range from 10-11
to 10-7 N.
feedback mechanism controls
that hold the sample and move it in all
three axes relative to the tip. The z-movement is used for maintaining
the force at the right level, while the x- and y- movements are used for the
raster-scanning the tip over the sample. The resulting map s(x,y)
of the tip's relative distance from the surface at different x-y values
of the scan is then used to form a topographical image of the
scanned area of the sample. Thus, contact-mode imaging is
primarily used for generating images of a sample's
imaging employs a
small piezo element mounted under the cantilever to make it oscillate at
its resonance frequency. When this oscillating cantilever is brought
down to within 10-100 nm from the sample surface, the oscillation gets
interaction forces (Van der Waals, electrostatic, magnetic,
or capillary forces)
tip and the sample.
The changes in the oscillation usually involve
a decrease in resonant frequency, a decrease in amplitude, and a phase
shift. These changes in oscillation characteristics can be used to
generate a map that characterizes the surface of the sample. For instance, amplitude
modulation can provide information about the sample's topography.
Phase shifts can be used to distinguish different surface materials from
Frequency modulation can be used to get information about the sample's
in non-contact imaging is being able to keep the correct tip-to-sample
distance while preventing the tip from touching the surface,
since there is a maximum distance for the inter-atomic forces to become
Furthermore, the tendency of most samples to develop a liquid meniscus
layer in ambient conditions complicates this task.
advantages of AFM
over electron microscopy include the following: 1) it generates true,
3-dimensional surface images; 2) it does not require special sample
treatments that can result in the sample's destruction or alteration;
and 3) it does not require a vacuum environment in order to operate (it
can operate in both air and liquid). On the other hand, its
following: 1) the image size that it provides is much smaller than what
electron microscopes can create; and 2) it is slow in scanning an image,
unlike an electron microscope which does it in almost real-time.
In the semiconductor
industry, AFM is primarily used for the imaging of VLSI cross-sections.
Materials that can be imaged by AFM include metals, polymers,
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