Introduction - Atomic Force Microscopy - page 5

Fig. 1.4. Example of an STM image with atomic resolution. The image shows an atomic-resolution
image of the 5 5 reconstruction of the Si(111) surface. Individual atoms, defects and vacancies are
visible. Reproduced with kind permission from Dr. Randall Feenstra.
Despite the amazing results obtained with STM, the limitation to conducting samples
led the inventors to immediately think about a new instrument that would be able to image
insulating samples. In 1986 Binnig, Quate and Gerber published a paper entitled 'Atomic
Force Microscope' [18, 19]. In that paper they described how they replaced the wire of a
tunnelling probe from the STM with a lever made by carefully gluing a tiny diamond onto
the end of a spring made of a thin strip of gold. This was the cantilever of the first AFM.
Although the first instrument was used only for a few experiments, the results produced
had such great impact that the first instrument now resides in the science museum in
London. The movement of the cantilever was monitored by measuring the tunnelling
current between the gold spring and a wire suspended above it. This set-up was highly
sensitive to the movement of the probe as it scanned along the sample, again moved by
piezoelectric elements. In their paper, Binnig et al . proposed that the AFM could be
improved by vibrating the cantilever above the surface [20]. Thankfully nowadays we
don't have to glue tiny diamonds onto gold levers to carry out AFM, but this first
instrument led to the whole field of AFM. The instrument, and the first image recorded
in AFM, are shown in Figure 1.5.
The AFM caused a revolution. Suddenly, with a relatively cheap and simple instrument,
extremely high-resolution images of nearly any sample were possible. While initial
images, such as that shown in Figure 1.5, did not have as high resolution as STM,
atomic-resolution images were soon reported [21]. Soon after the invention of the AFM,
the gold leaf/diamond combination was replaced by much more reproducible cantilever
manufacture by silicon lithography, which enables the production of more than 400
cantilevers on a single 7-inch wafer [22]. Furthermore, it was quickly realized that simpler
methods than the STM could be used to detect the motion of the cantilever. Nowadays,
most AFMs use a light lever to sensitively detect the motion of the cantilever, this method
is considerably simpler than the STM set-up, allows for larger cantilever motions, and is