The
first AFM was made by meticulously gluing a tiny shard of diamond onto
one end of a tiny strip of gold foil. In the fall of 1985 Gerd Binnig
and Christoph Gerber used the cantilever to examine insulating surfaces.
A small hook at the end of the cantilever was pressed against the
surface while the sample was scanned beneath the tip. The force between
tip and sample was measured by tracking the deflection of the
cantilever. This was done by monitoring the tunneling current tot a
second tip positioned above the cantilever. They could delineate lateral
features as small as 300 Å. The force microscope emerged in this way. In
fact, without the breakthrough in tip manufacture, the AFM probably
would have remained a curiosity in many research groups. It was
Albrecht, a fresh graduate student, who fabricated the first silicon
microcantilever and
measured the atomic structure of boron nitride. Today the tip-cantilever
assembly typically is microfabricated from Si or Si3N4. The era of AFM
came finally when the Zurich group released the image of a silicon (111)
7X7 pattern. The world of surface science knew that a new tool for
surface microscope was at hand. After several years the microcantilevers
have been perfected, and the instrument has been embraced by scientists
and technologists.The force between the tip and the sample surface is very small, usually less than 10-9 N. How to monitor such small forces is another story. The detection system does not measure force directly. It senses the deflection of the microcantilever. The detecting systems for monitoring the deflection fall into several categories. The first device introduced by Binnig was a tunneling tip placed above the metallized surface of the cantilever. This is a sensitive system where a change in spacing of 1 Å between tip and cantilever changes the tunneling current by an order of magnitude. It is straightforward to measure deflections smaller than 0.01 Å. Subsequent systems were based on the optical techniques. The interferometer is the most sensitive of the optical methods, but it is somewhat more complicated than the beam-bounce method which was introduced by Meyer and Amer. The beam-bounce method is now widely used as a
result
of the excellent work by Alexander and colleagues. In this system an
optical beam is reflected from the mirrored surface on the back side of
the cantilever onto a position-sensitive photodetector. In this
arrangement a small deflection of the cantilever will tilt the reflected
beam and change the position of beam on the photodetector. A third
optical system introduced by Sarid uses the cantilever as one of the
mirrors in the cavity of a diode laser. Motion of the cantilever has a
strong effect on the laser output, and this is exploited as a motion
detector. An expanded view of the image at left, and a legend describing
its parts is found here.According to the interaction of the tip and the sample surface, the AFM can be classified as repulsive or Contact mode and attractive or Noncontact mode. Now the Tappingmode shows a prosperous future to image the micro-world.