Physical Principles of Electron Microscopy: An Introduction to TEM ...

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Chapter 1
Typically, the STM head is quite small, a few centimeters in dimensions;
small size minimizes temperature variations (and therefore thermal drift) and
forces mechanical vibrations (resonance) to higher frequencies, where they
are more
easily damped.
The STM was developed at the IBM Zurich Laboratory (Binnig et al.,
1982) and earned two of its inventors the 1986 Nobel prize in Physics
(shared with Ernst Ruska, for his development of the TEM). It quickly
inspired other types of scanning-probe microscope, such as the atomic force
microscope (AFM) in which a sharp tip (at the end of a cantilever) is
brought sufficiently close to the surface of a specimen, so that it essentially
touches it and senses an interatomic force. For many years, this principle had
been applied to measure the roughness of surfaces or the height of surface
steps, with a height resolution of a few nanometers. But in the 1990’s, the
instrument was refined to give near-atomic resolution.
Initially, the z-motion of the cantilever was detected by locating an STM
tip immediately above. Nowadays it is usually achieved by observing the
angular deflection of a reflected laser beam while the specimen is scanned in
the x- and y-directions; see Fig. 1-18b. AFM cantilevers can be made (from
silicon nitride) in large quantities, using the same kind of photolithography
process that yields semiconductor integrated circuits, so they are easily
replaced when damaged or contaminated. As with the STM, scanning-force
images must be examined critically to avoid misleading artifacts such as
multiple-tip effects.
The mechanical force is repulsive if the tip is in direct contact with the
sample, but at a small distance above, the tip senses an attractive (van der
Waals) force. Either regime may be used to provide images. Alternatively, a
4-quadrant photodetector can sense torsional motion (twisting) of the AFM
cantilever, which results from a sideways frictional force, giving an image
that is essentially a map of the local coefficient of friction. Also, with a
modified tip, the magnetic field of a sample can be monitored, allowing the
direct imaging of magnetic data-storage media materials for example.
Although is more difficult to obtain atomic resolution than with an STM,
the AFM has the advantage that it does not require a conducting sample. In
fact, the AFM can operate with its tip and specimen immersed in a liquid
such as water, making the instrument valuable for imaging biological
specimens. This versatility, combined with its high resolution and relatively
moderate cost, has enabled the scanning probe microscope to take over some
of the applications previously reserved for the SEM and TEM. However,
mechanically scanning large areas of specimen is very time-consuming; it is
less feasible to zoom in and out (by changing magnification) than with an